The 100 most-cited articles in Optics Express, 2022

This article proposed a metastructure device that can realize polarization conversion (PC) and absorption function switching in the terahertz (THz) range based on the photoconductivity effect. The photoconductance is formed by exposing silicon to different intensities of light, then the PC and absorption function can be switched. At the same time, the absorption bandwidth is expanded by inserting air resonant cavities into the dielectric substrate, changing the thickness of the dielectric locally, and cutting rectangular slots at the metal bottom plate. When the device works as a polarization converter, linear-to-linear PC with a polarization conversion rate of over 90% at 0.96-1.47 THz can be achieved, and its relative bandwidth is 42%. And when the silicon conductivity is fixed at 3500 S/m through illuminating, the device switches to an ultra-broadband absorber with over 90% absorption at 0.75-1.73 THz and a relative bandwidth of 79%. The designed device can be applied efficiently in many fields, such as electromagnetic cloaking and communication.

The purpose of this article is to propose an optical fiber sensor probe based on the localized surface plasma resonance (LSPR) technique for the detection of creatinine in aquaculture. The sensing probe is functionalized through the use of gold nanoparticles (AuNPs), niobium carbide (Nb 2 CTx) MXene, and creatinase (CA) enzyme. The intrinsic total internal reflection (TIR) mechanism is modified to increase the evanescent field intensity using a heterogeneous core mismatch and tapering probe structure (i.e., convex fiber-tapered seven core fiber-convex fiber (CTC) structure). Strong evanescent fields can stimulate AuNPs and induce the LSPR effect, thereby increasing probe sensitivity. The specific recognition is enhanced by Nb 2 CTx MXene adsorbing more active CA enzymes. The developed sensor probe has a sensitivity and limit of detection of 3.1 pm/µM and 86.12 µM, respectively, in the linear range of 0-2000 µM. Additionally, the sensor probe's reusability, reproducibility, stability, and selectivity were evaluated, with satisfactory results obtained with impact for areas like food…

We present a photonics topology optimization (TO) package capable of addressing a wide range of practical photonics design problems, incorporating robustness and manufacturing constraints, which can scale to large devices and massive parallelism. We employ a hybrid algorithm that builds on a mature time-domain (FDTD) package Meep to simultaneously solve multiple frequency-domain TO problems over a broad bandwidth. This time/frequency-domain approach is enhanced by new filter-design sources for the gradient calculation and new material-interpolation methods for optimizing dispersive media, as well as by multiple forms of computational parallelism. The package is available as free/open-source software with extensive tutorials and multi-platform support.

Laser cavity-soliton microcombs are robust optical pulsed sources, usually implemented with a microresonator-filtered fibre laser. In such a configuration, a nonlinear microcavity converts the narrowband pulse resulting from bandwidth-limited amplification to a background-free broadband microcomb. Here, we theoretically and experimentally study the soliton conversion efficiency between the narrowband input pulse and the two outputs of a four-port integrated microcavity, namely the 'Drop' and 'Through' ports. We simultaneously measure on-chip, single-soliton conversion efficiencies of 45% and 25% for the two broadband comb outputs at the 'Drop' and 'Through' ports of a 48.9 GHz free-spectral range micro-ring resonator, obtaining a total conversion efficiency of 72%.

In this paper, we theoretically and numerically demonstrate a polarization-controlled and symmetry-dependent multiple plasmon-induced transparency (PIT) in a graphene-based metasurface. The unit cell of metasurface is composed of two reversely placed U-shaped graphene nanostructures and a rectangular graphene ring stacking on a dielectric substrate. By adjusting the polarization of incident light, the number of transparency windows can be actively modulated between 1 and 2 when the nanostructure keeps a geometrical symmetry with respect to the x-axis. Especially, when the rectangular graphene ring has a displacement along the y-direction, the number of transparency windows can be arbitrarily switched between 2 and 3. The operation mechanism behind the phenomena can be attributed to the near-field coupling and electromagnetic interaction between the bright modes excited in the unit of graphene resonators. Moreover, the electromagnetic simulations obtained by finite-difference time-domain (FDTD) method agree well with the theoretical results based on the coupled modes theory (CMT). In…

In this paper, a wideband photomultiplier tube (PMT)-based underwater wireless optical communication (UWOC) system is proposed and a comprehensive experimental study of the proposed PMT-based UWOC system is conducted, in which the transmission distance, data rate, and attenuation length (AL) is pushed to 100.6 meters, 3 Gbps, and 6.62, respectively. The receiver sensitivity at 100.6-meter underwater transmission is as low as -40 dBm for the 1.5-Gbps on-off keying (OOK) modulation signal. To the best of our knowledge, this is the first Gbps-class UWOC experimental demonstration in >100-meter transmission that has ever been reported. To further minimize the complexity of channel equalization, a sparsity-aware equalizer with orthogonal matching pursuit is adopted to reduce the number of the filter coefficients by more than 50% while keeping slight performance penalty. Furthermore, the performance of the proposed PMT-based UWOC system in different turbidity waters is investigated, which shows the robustness of the proposed scheme. Thanks to the great sensitivity (approaching the quantum…

Single-shot fringe projection profilometry (FPP) is essential for retrieving the absolute depth information of the objects in high-speed dynamic scenes. High-precision 3D reconstruction using only one single pattern has become the ultimate goal in FPP. The frequency-multiplexing (FM) method is a promising strategy for realizing single-shot absolute 3D measurement by compounding multi-frequency fringe information for phase unwrapping. In order to solve the problem of serious spectrum aliasing caused by multiplexing schemes that cannot be removed by traditional spectrum analysis algorithms, we apply deep learning to frequency multiplexing composite fringe projection and propose a composite fringe projection deep learning profilometry (CDLP). By combining physical model and data-driven approaches, we demonstrate that the model generated by training an improved deep convolutional neural network can directly perform high-precision and unambiguous phase retrieval on a single-shot spatial frequency multiplexing composite fringe image. Experiments on both static and dynamic scenes…

The scattering of electromagnetic waves by resonant systems is determined by the excitation of the quasinormal modes (QNMs), i.e. the eigenmodes, of the system. This Review addresses three fundamental concepts in relation to the representation of the scattered field as a superposition of the excited QNMs: normalization, orthogonality, and completeness. Orthogonality and normalization enable a straightforward assessment of the QNM excitation strength for any incident wave. Completeness guarantees that the scattered field can be faithfully expanded into the complete QNM basis. These concepts are not trivial for non-conservative (non-Hermitian) systems and have driven many theoretical developments since initial studies in the 70's. Yet, they are not easy to grasp from the extensive and scattered literature, especially for newcomers in the field. After recalling fundamental results obtained in initial studies on the completeness of the QNM basis for simple resonant systems, we review recent achievements and the debate on the normalization, clarify under which circumstances the QNM basis…

Delayed self-heterodyne interferometry is a commonly used technique for the measurement of laser linewidth. It typically requires the use of a very long delay fiber when measuring narrow linewidth (especially linewidths in the kHz-range) lasers. The use of long fibers can result in system losses and the introduction of 1/f noise that causes spectral line broadening. In this paper, we present a calculation method for processing the output of a delayed self-heterodyne setup using a short length of delay fiber, to determine laser linewidth. The method makes use of pairs of data points (corresponding to adjacent maxima and/or minima) in the signal generated from the self-heterodyne setup to determine the laser linewidth. Here, the power ratio or amplitude difference of the signal at these data points is of importance. One of the key benefits of this method is that it avoids 1/f noise which would otherwise be introduced into the measurement through the application of long fibers. The experimental results highlight that the method has a high calculation accuracy. Furthermore, the capacity…

We investigate optical transmission of an ultra-wideband signal in a standard single mode fiber. Using a near continuous optical bandwidth exceeding 157 nm across the S-, C- and L-bands, we combine doped-fiber amplifiers covering S, C and L-bands with distributed Raman amplification to enable high-quality transmission of polarization division multiplexed (PDM)-256-quadrature-amplitude modulation (QAM) signals over a 54 km standard single-mode fiber. We receive 793 × 24.5 GBd signals from 1466.34 nm to 1623.57 nm and measure a data rate estimated from the generalized mutual information (GMI) of 256.4 Tb/s and an LDPC decoded throughput of 244.3 Tb/s. The measured data rates exceed the highest previously measured in a single mode fiber, showing the potential for S-band transmission to enhance achievable data rates in optical fibers.

Since its first demonstration in graded-index multimode fibers, spatial beam self-cleaning has attracted a growing research interest. It allows for the propagation of beams with a bell-shaped spatial profile, thus enabling the use of multimode fibers for several applications, from biomedical imaging to high-power beam delivery. So far, beam self-cleaning has been experimentally studied under several different experimental conditions. Whereas it has been theoretically described as the irreversible energy transfer from high-order modes towards the fundamental mode, in analogy with a beam condensation mechanism. Here, we provide a comprehensive theoretical description of beam self-cleaning, by means of a semi-classical statistical mechanics model of wave thermalization. This approach is confirmed by an extensive experimental characterization, based on a holographic mode decomposition technique, employing laser pulses with temporal durations ranging from femtoseconds up to nanoseconds. An excellent agreement between theory and experiments is found, which demonstrates that beam…

The Pancharatnam-Berry (PB) phase can be used to control the phase of circularly polarized electromagnetic waves. However, there are few studies on the modulation of dual-circularly polarized multi-beam using the transmissive coding metasurface. A scheme of spin-controlling multi-beam by transmissive coding metasurface is proposed for dual-circular polarization simultaneously. The transmissive coding metasurface (TCMS) can transmit linearly polarized incidence into multi-beam with orthogonally circular polarization. The phase distribution is designed based the convolution theorem, and the elements of metasurface conforming to the PB phase are arranged according to the phase distribution. In order to compensate the emitting spherical waves into plane waves and realize the transmissive waves with dual-circular polarization, an interesting scheme of elements in different regions with different rotating phase are presented based on the principle of phase compensation. TCMS can transmit linearly polarized waves into two left-hand circularly polarized (LHCP) beams and two right-hand…

Metasurfaces-subwavelength arrays of phase-shifting elements-present new possibilities for polarization optics and polarimetry. In particular, a periodic, polarization-sensitive metasurface diffraction grating can enable full-Stokes imaging polarimetry with a single polarization-sensitive component. In this work, we show that a suitably-designed metasurface grating can serve as a polarimetric "attachment" to an existing intensity-only imaging system, converting it into one capable of full-Stokes imaging polarimetry. Design rules and tradeoffs governing this adaptation are described and demonstrated using a machine vision imaging system as an example.

Mid-infrared fiber sources, emitting between 2.5 µm and 5.0 µm, are interesting for their great potential in several application fields such as material processing, biomedicine, remote sensing and infrared countermeasures due to their high-power, their diffraction-limited beam quality as well as their robust monolithic architecture. In this review, we will focus on the recent progress in continuous wave and pulsed mid-infrared fiber lasers and the components that bring these laser sources closer to a field deployment as well as in industrial systems. Accordingly, we will briefly illustrate the potential of such mid-infrared fiber lasers through a few selected applications.

Ultra-high quality (Q) factor resonances derived from the bound states in the continuum (BICs) have drawn much attention in optics and photonics. Especially in meta-surfaces, they can enable ultrasensitive sensors, spectral filtering, and lasers because of their enhanced light-matter interactions and rare superiority of scalability. In this paper, we propose a permittivity-asymmetric all-dielectric meta-surface, comprising high-index cuboid tetramer clusters with symmetric structural parameters and configuring periodically on a glass substrate. Simulation results offer dual-band quasi-BICs with high Q values of 4447 and 11391, respectively. Multipolar decomposition in cartesian and electromagnetic distributions are engaged to analyze the physical mechanism of dual quasi-BIC modes, which reveals that they are both governed by magnetic quadrupole (MQ) and in-plane toroidal dipole (TD). The polarization-insensitive and scalable characteristics are also investigated. Additionally, we appraise the sensing performances of the proposed structure. As an example, our work supports an…

A surface plasmon resonance (SPR) sensor comprising photonic crystal fiber (PCF) is designed for magnetic field and temperature dual-parameter sensing. In order to make the SPR detection of magnetic field and temperature effectively, the two open ring channels of the proposed sensor are coated with gold and silver layers and filled with magnetic fluid (MF) and Polydimethylsiloxane (PDMS), respectively. The sensor is analyzed by the finite element method and its mode characteristics, structure parameters and sensing performance are investigated. The analysis reveals when the magnetic field is a range of 40-310 Oe and the temperature is a range of 0-60 °C, the maximum magnetic field sensitivity is 308.3 pm/Oe and temperature sensitivity is 6520 pm/°C. Furthermore, temperature and magnetic field do not crosstalk with each other's SPR peak. Its refractive index sensing performance is also investigated, the maximum sensitivity and FOM of the left channel sensing are 16820 nm/RIU and 1605 RIU -1 , that of the right channel sensing are 13320 nm/RIU and 2277 RIU -1 . Because of its high…

We propose a three-axis closed-loop optically pumped magnetometer with high sensitivity. The closed-loop magnetometer has a three-axis sensitivity of approximately 30 fT/Hz 1/2 using two orthogonal laser beams for pumping and probing the alkali metal atoms. In the closed-loop mode, the dynamic range is improved from ±5 nT to ±150 nT. The bandwidth is increased from about 100 Hz to over 2 kHz with 10 kHz modulation fields in x - and y -axes and another 6 kHz modulation field along the z -axis. Compared with single-axis or dual-axis magnetometers, the proposed magnetometer not only provides the direction and magnitude of the magnetic field but also has high robustness in a challenging environment. The magnetometer has applications in biomagnetic measurements, magnetic resonance imaging, and fundamental physics.

Atomic, molecular and optical (AMO) visible light systems are the heart of precision applications including quantum, atomic clocks and precision metrology. As these systems scale in terms of number of lasers, wavelengths, and optical components, their reliability, space occupied, and power consumption will push the limits of using traditional laboratory-scale lasers and optics. Visible light photonic integration is critical to advancing AMO based sciences and applications, yet key performance aspects remain to be addressed, most notably waveguide losses and laser phase noise and stability. Additionally, a visible light integrated solution needs to be wafer-scale CMOS compatible and capable of supporting a wide array of photonic components. While the regime of ultra-low loss has been achieved at telecommunication wavelengths, progress at visible wavelengths has been limited. Here, we report the lowest waveguide losses and highest resonator Qs to date in the visible range, to the best of our knowledge. We report waveguide losses at wavelengths associated with strontium transitions in…

Satellite-derived bathymetry (SDB) has an extensive prospect in nearshore bathymetry for its high efficiency and low costs. Atmospheric correction and bathymetric modeling are critical processes in SDB, and examining the performance of related algorithms and models will contribute to the formulation of reliable bathymetry strategies. This study explored the effectiveness of three general atmospheric correction algorithms, namely Second Simulation of a Satellite Signal in the Solar Spectrum (6S), Atmospheric correction for OLI 'lite' (ACOLITE), and QUick Atmospheric Correction (QUAC), in depth retrieval from Landsat-8 and Sentinel-2A images using different SDB models over Ganquan Island and Oahu Island. The bathymetric Light Detection and Ranging (LiDAR) data was used for SDB model training and accuracy verification. The results indicated that the three atmospheric correction algorithms could provide effective corrections for SDB. For the SDB models except log-transformed band ratio model (LBR) and support vector machine (SVM), the impact of different atmospheric corrections on…

Defects in the optical lens directly affect the scattering properties of the optical lens and decrease the performance of the optical element. Although machine vision instead of manual detection has been widely valued, the feature fusion technique of series operation and edge detection cannot recognize low-contrast and multi-scale targets in the lens. To address these challenges, in this study, an improved YOLOv5-C3CA-SPPF network model is proposed to detect defects on the surface and inside of the lens. The hybrid module combining the coordinate attention and CSPNet (C3) is incorporated into YOLOv5-C3CA for improving the extraction of target feature information and detection accuracy. Furthermore, an SPPF features fusion module is inserted into the neck of the network model to improve the detection accuracy of the network. To enhance the performance of supervised learning algorithms, a dataset containing a total of 3800 images is created, more than 600 images for each type of defect samples. The outcome of the experiment manifests that the mean average precision (mAP) of the…

Single-pixel imaging is a technique that can reconstruct an image of a scene by projecting a series of spatial patterns on an object and capturing the reflected light by a single photodetector. Since the introduction of the compressed sensing method, it has been possible to use random spatial patterns and reduce its number below the Nyquist-Shannon limit to form a good quality image but with lower spatial resolution. On the other hand, Hadamard pattern based methods can reconstruct large images by increasing the acquisition measurement time. Here, we propose an efficient strategy to order the Hadamard basis patterns from higher to lower relevance, and then to reconstruct an image at very low sampling rates of at least 8%. Our proposal is based on the construction of generalized basis vectors in two dimensions and then ordering in zigzag fashion. Simulation and experimental results show that the sampling rate, image quality and computational complexity of our method are competitive to the state of the art methods.

Polarization is an important characteristic of electromagnetic wave. Due to novel optical properties, graphene-based anisotropic structure is widely used to control polarization state of electromagnetic wave. In this work, four graphene-based meta-atoms are designed to regulate polarization state of terahertz wave by changing Fermi energy level of graphene. When Fermi energy level is 0.01 eV, cross-polarized wave is emitted by four meta-atoms with phase difference of 90° at 1.18 THz, and the corresponding polarization conversion ratio reaches ∼90%. When Fermi energy level is adjusted to 0.70 eV, linear phase gradient will disappear, and cross-polarized wave almost disappears. Using four selected elements, three dynamic metasurfaces are designed for controlling wavefront of reflected beam, and they are gradient metasurface, metalens, and vortex beam generator. The designed metasurfaces successfully combine wavefront control and polarization manipulation, and greatly improve the ability to control electromagnetic wave. Our designs may have many potential applications, such as…

A functionally tunable and absorption-tunable terahertz (THz) metamaterial absorber based on vanadium dioxide (VO 2 ) and graphene is proposed and verified numerically. Based on phase transition properties of VO 2 and tunability of graphene, the switching performance between ultra-broadband and narrow-band near-perfect absorption can be achieved. We simulate and analyze the characteristics of the constructed model by finite element analysis. Theoretical calculations show that when VO 2 is in the metallic state and the graphene Fermi energy is 0 eV, the designed absorber can perform ultra-broadband absorption. The absorber achieves greater than 95% absorption in the 2.85 - 10THz range. When VO 2 is in the insulating state and the graphene Fermi energy is 0.7 eV, more than 99.5% absorption can be achieved at 2.3 THz. The absorption rate can be tuned by changing the conductivity of VO 2 and the Fermi energy of graphene. Moreover, the proposed absorber displays good polarization insensitivity and wide incident angle stability. The design may have potential applications in terahertz…

In this paper, we introduce a novel kind of abrupt autofocusing beams namely the generalized circular Airy derivative beams (CADBs) as an extension of circular Airy beam (CAB). The propagation dynamics of the CADBs is examined theoretically. Our results show that the CADBs exhibit stronger autofocusing ability than the CAB under the same condition. The physical mechanism of the abruptly autofocusing of the CADBs is interpreted by mimicking the Fresnel zone plate lens. Here, the abruptly autofocusing ability is described by a ratio K = I fm / I 0 m where I fm and I 0 m correspond to the maximum intensities in the focal and the source planes, respectively. As an example, the K -value of the circular Airyprime beam (CAPB, the first-order Airy derivative beam) is about 7 times of that of the CAB. In addition, the CAPB have narrower FWHM (full width at half maxima) in the focus position than the CAB, and the focal spot size of the CAPB is smaller than that of the CAB. Furthermore, we establish an optical system involving a phase-only spatial light modulator to generate the CAPB and…

A multifunctional design based on vanadium dioxide (VO2) metamaterial structure is proposed. Broadband absorption, linear-to-linear (LTL) polarization conversion, linear-to-circular (LTC) polarization conversion, and total reflection can be achieved based on the insulator-to-metal transition (IMT) of VO2. When the VO2 is in the metallic state, the multifunctional structure can be used as a broadband absorber. The results show that the absorption rate exceeds 90% in the frequency band of 2.17 - 4.94 THz, and the bandwidth ratio is 77.8%. When VO2 is in the insulator state, for the incident terahertz waves with a polarization angle of 45°, the structure works as a polarization converter. In this case, LTC polarization conversion can be obtained in the frequency band of 0.1 - 3.5 THz, and LTL polarization conversion also can be obtained in the frequency band of 3.5 - 6 THz, especially in the 3.755 - 4.856 THz band that the polarization conversion rate is over 90%. For the incident terahertz waves with a polarization angle of 0°, the metamaterial structure can be used as a total…

Augmented reality head-mounted displays (AR-HMDs) based on diffractive waveguides have been a challenging and rewarding research topic focusing on near-eye displays. The size of the exit pupil and uniformity of the image illuminance are two important factors that affect the display performance of the diffractive waveguide. In this paper, a novel method for optimizing high uniformity of two-dimensional (2D) diffractive waveguide is proposed. A straight-line 2D surface relief grating (SRG) waveguide with divided grating regions is designed. An illuminance uniformity evaluation model of the energy propagation process is established, and non-sequential ray tracing is utilized to optimize the diffraction efficiency of multi-regions grating to achieve illuminance uniformity distribution. Then, the uniformity distribution of the diffraction efficiency in different fields of view (FOVs) is realized by combining the particle swarm optimization (PSO) algorithm and rigorous couple wave analysis (RCWA) to optimize the grating structural parameters, which further ensures the uniformity of the…

Visible light communication (VLC), combining wireless communication with white lighting, has many advantages. It is free of electromagnetic interference, is rich in spectrum resources, and has a gigabit-per-second (Gbps) data rate. Laser diodes (LDs) are emerging as promising light sources for high-speed VLC communication due to their high modulation bandwidth. In this paper, we demonstrate a red/green/blue (R/G/B) LDs based VLC system with a recorded data rate of 46.41 Gbps, employing discrete multitone (DMT) and adaptive bit-loading technology to achieve high spectral efficiency (SE). The emission characteristics and transmission performance of R/G/B-LDs are discussed. The optimal data rates of R/G/B-LDs channels are 17.168/14.652/14.590 Gbps, respectively. The bit-error-ratio (BER) of each channel satisfies the 7% forward-error-correction (FEC) threshold (3.8×10−3) and greatly approaches the channel Shannon limit.

We propose and demonstrate purely optical feedback cooling of neutral nanoparticles in an optical lattice to an occupation number of 0.85 ± 0.20. The cooling force is derived from the optical gradients of displaced optical lattices produced with two sidebands on the trapping laser. To achieve highly accurate position observations required for cooling near the ground state, we reduce the laser intensity noise to a relative power noise of 6×10−8/Hz in a frequency band of 30 kHz to 600 kHz. We establish a reproducible method for neutralizing nanoparticles at high vacuum via a combination of discharging and irradiating an ultraviolet light. Our results form an important basis for the investigation of quantum mechanical properties of ultracold nanoparticles and are also useful for precision measurements with neutral nanoparticles.

Most chiral metamaterials and metasurfaces are designed to operate in a single wavelength band and with a certain circular dichroism (CD) value. Here, mid-infrared chiral metasurface absorbers with selective CD in dual-wavelength bands are designed and demonstrated. The dual-band CD selectivity and tunability in the chiral metasurface absorbers are enabled by the unique design of a unit cell with two coupled rectangular bars. It is shown that the sign of CD in each wavelength band can be independently controlled and flipped by simply adjusting the geometric parameters, the width and the length, of the vertical rectangular bars. The mechanism of the dual-band CD selection in the chiral metasurface absorber is further revealed by studying the electric field and magnetic field distributions of the antibonding and bonding modes supported in the coupled bars under circularly polarized incident light. Furthermore, the chiral resonance wavelength can be continuously increased by scaling up the geometric parameters of the metasurface unit cell. The demonstrated results will contribute to…

Infrared (IR) stealth with thermal management is highly desirable in military applications and astronomy. However, developing selective IR emitters with properties suitable for IR stealth and thermal management is challenging. In this study, we present the theoretical framework for a selective emitter based on an inverse-designed metasurface for IR stealth with thermal management. The emitter comprises an inverse-designed gold grating, a Ge 2 Sb 2 Te 5 (GST) dielectric layer, and a gold reflective layer. The hat-like function, which describes an ideal thermal selective emitter, is involved in the inverse design algorithm. The emitter exhibits high performance in IR stealth with thermal management, with the low emissivity (ɛ 3–5 µm =0.17; ɛ 8–14 µm =0.16) for dual-band atmospheric transmission windows and high emissivity (ɛ 5–8 µm =0.85) for non-atmospheric windows. Moreover, the proposed selective emitter can realize tunable control of thermal radiation in the wavelength range of 3–14 µm by changing the crystallization fraction of GST. In addition, the polarization-insensitive…

Infrared quantum absorption spectroscopy is one of the quantum sensing techniques, by which the infrared optical properties of a sample can be estimated through visible or near infrared photon detection without need for infrared optical source or detector, which has been an obstacle for higher sensitivity and spectrometer miniaturization. However, experimental demonstrations have been limited to wavelengths shorter than 5 µm or in the terahertz region, and have not been realized in the so-called fingerprint region of 1500–500 cm−1 (6.6 to 20 µm), which is commonly used to identify chemical compounds or molecules. Here we report the experimental demonstration of quantum Fourier-transform infrared (QFTIR) spectroscopy in the fingerprint region, by which both absorption and phase spectra (complex spectra) can be obtained from Fourier transformed quantum interferograms obtained with a single pixel visible-light detector. As demonstrations, we obtained the transmittance spectrum of a silicon wafer at around 10 µm (1000 cm−1) and complex transmittance spectrum of a synthetic fluoropolymer…

The evolution of ultrafast laser technology hinges partially on the understanding of the soliton nonlinear dynamics. Recently, the concept of pure-quartic soliton (PQS) that arises from the balance of pure negative fourth-order dispersion (FOD) and nonlinearity was proposed to generate high peak power pulse. Herein, we investigate the generation of dissipative pure-quartic soliton (DPQS) in a fiber laser, which is balanced among the positive FOD, nonlinearity, gain and loss. The DPQS features the shape-preserving propagation despite the asymmetrical temporal profile at higher pulse energy. It is found that the asymmetrical temporal profile of DPQS is resulted from the mismatching of the phase shift profiles caused by self-phase modulation and FOD. Moreover, it is demonstrated that the DPQS possesses a higher energy-scaling ability compared to conventional dissipative soliton, owing to the nonlinear relationship between the pulse energy and pulse duration. These findings demonstrated that the employment of positive FOD could be a promising way for manipulation of optical pulse as…

In this paper, a hollow-core anti-resonant fiber (HC-ARF) based light-induced thermoelastic spectroscopy (LITES) sensor is reported. A custom-made silica-based HC-ARF with length of 75 cm was used as light medium and gas cell. Compared to a traditional multi-pass cell (MPC), the using of HC-ARF is advantageous for reducing the sensor size and easing the optical alignment. A quartz tuning fork (QTF) with a resonant frequency of 32766.20 Hz and quality factor of 12364.20 was adopted as the thermoelastic detector. Acetylene (C2H2) and carbon monoxide (CO) with absorption lines located at 6534.37 cm−1 (1530.37 nm) and 6380.30 cm−1 (1567.32 nm) were chosen as the target gas to verify such HC-ARF based LITES sensor performance. It was found that this HC-ARF based LITES sensor exhibits excellent linearity response to the analyte concentrations. The minimum detection limit (MDL) for C2H2 and CO detections were measured as 4.75 ppm and 1704 ppm, respectively. The MDL for such HC-ARF based LITES sensor can be further improved by using a HC-ARF with long length or choosing an absorption line…

We introduce end-to-end inverse design for multi-channel imaging, in which a nanophotonic frontend is optimized in conjunction with an image-processing backend to extract depth, spectral and polarization channels from a single monochrome image. Unlike diffractive optics, we show that subwavelength-scale "metasurface" designs can easily distinguish similar wavelength and polarization inputs. The proposed technique integrates a single-layer metasurface frontend with an efficient Tikhonov reconstruction backend, without any additional optics except a grayscale sensor. Our method yields multi-channel imaging by spontaneous demultiplexing: the metaoptics front-end separates different channels into distinct spatial domains whose locations on the sensor are optimally discovered by the inverse-design algorithm. We present large-area metasurface designs, compatible with standard lithography, for multi-spectral imaging, depth-spectral imaging, and "all-in-one" spectro-polarimetric-depth imaging with robust reconstruction performance (≲ 10% error with 1% detector noise). In contrast to neural…

Underwater images captured by optical cameras can be degraded by light attenuation and scattering, which leads to deteriorated visual image quality. The technique of underwater image enhancement plays an important role in a wide range of subsequent applications such as image segmentation and object detection. To address this issue, we propose an underwater image enhancement framework which consists of an adaptive color restoration module and a haze-line based dehazing module. First, we employ an adaptive color restoration method to compensate the deteriorated color channels and restore the colors. The color restoration module consists of three steps: background light estimation, color recognition, and color compensation. The background light estimation determines the image is blueish or greenish, and the compensation is applied in red-green or red-blue channels. Second, the haze-line technique is employed to remove the haze and enhance the image details. Experimental results show that the proposed method can restore the color and remove the haze at the same time, and it also…

Mode-division multiplexing (MDM) technique based on few-mode fibers (FMFs) can achieve multiplicative growth in single-fiber capacity by using different linearly polarized (LP) modes or mode groups as spatial channels. However, its deployment is seriously impeded because multiple-input multiple-output digital signal processing (MIMO-DSP) with huge computational load must be adopted to combat intermodal crosstalk for long-haul FMF transmission. In this paper, we present an intermodal-MIMO-free MDM transmission scheme based on weakly coupled multiple-ring-core FMF, which achieves ultralow distributed modal crosstalk (DMC) so that the signal in each LP mode can be independently received by single-LP-mode MIMO-DSP even after hundreds-of-kilometer transmission. Evaluation method for the required DMC levels is proposed and different transmission reaches are investigated by simulation. By adopting an improved method for quantitative DMC measurement, we show that the required DMC level for long-haul transmission is feasible. Finally, we experimentally demonstrate 1800-km LP01/LP02…

Narrow-linewidth lasers are important to many applications spanning precision metrology to sensing systems. Characterization of these lasers requires precise measurements of their frequency noise spectra. Here we demonstrate a correlated self-heterodyne (COSH) method capable of measuring frequency noise as low as 0.01 Hz2/Hz at 1 MHz offset frequency. The measurement setup is characterized by both commercial and lab-built lasers, and features low optical power requirements, fast acquisition time and high intensity noise rejection.

Fiber Bragg grating (FBG) sensors have been widely applied in various applications, especially for structural health monitoring. Low cost, wide range, and low error are necessary for an excellent performance FBG sensor signal demodulation system. Yet the improvement of performance is commonly accompanied by costly and complex systems. A high-performance, low-cost wavelength interrogation method for FBG sensors was introduced in this paper. The information from the FBG sensor signal was extracted by the array waveguide grating (AWG) and fed into the proposed cascaded neural network. The proposed network was constructed by cascading a convolutional neural network and a residual backpropagation neural network. We demonstrate that our network yields a vastly significant performance improvement in AWG-based wavelength interrogation over that given by other machine learning models and validate it in experiments. The proposed network cost-effectively widens the wavelength interrogation range of the demodulation system and optimizes the wavelength interrogation error substantially, also…

Fringe projector profilometry (FPP) is an important three-dimensional (3D) measurement technique, especially when high precision and speed are required. Thus, theoretical interrogation is critical to provide deep understanding and possible improvement of FPP. By dividing an FPP measurement process into four steps (system calibration, phase measurement, pixel correspondence, and 3D reconstruction), we give theoretical analysis on the entire process except for the extensively studied calibration step. Our study indeed reveals a series of important system properties, to the best of our knowledge, for the first time: (i) in phase measurement, the optimal and worst fringe angles are proven perpendicular and parallel to epipolar line, respectively, and can be considered as system parameters and can be directly made available during traditional calibration, highlighting the significance of the epipolar line; (ii) in correspondence, when two sets of fringes with different fringe orientations are projected, the highest correspondence precision can be achieved with arbitrary orientations as…

Modulation-based control and locking of lasers, filters and other photonic components is a ubiquitous function across many applications that span the visible to infrared (IR), including atomic, molecular and optical (AMO), quantum sciences, fiber communications, metrology, and microwave photonics. Today, modulators used to realize these control functions consist of high-power bulk-optic components for tuning, sideband modulation, and phase and frequency shifting, while providing low optical insertion loss and operation from DC to 10s of MHz. In order to reduce the size, weight and cost of these applications and improve their scalability and reliability, modulation control functions need to be implemented in a low loss, wafer-scale CMOS-compatible photonic integration platform. The silicon nitride integration platform has been successful at realizing extremely low waveguide losses across the visible to infrared and components including high performance lasers, filters, resonators, stabilization cavities, and optical frequency combs. Yet, progress towards implementing low loss, low…

We propose, both numerically and theoretically, a uniform model to investigate the plasmonically induced transparency effect in plasmonic metamaterial consisting of dual-layer spatially separated borophene nanoribbons array. The dynamic transfer properties of light between two borophene resonators can be effectively described by the proposed model, with which we can distinguish and connect the direct and indirect coupling schemes in the metamaterial system. By adjusting the electron density and separation of two borophene ribbons, the proposed metamaterials enable a narrow band in the near-infrared region to reach high transmission. It provides a new, to the best of our knowledge, platform for optoelectronic integrated high-performance devices in the communication band.

The effect of atomic-layer deposition (ALD) sidewall passivation on the enhancement of the electrical and optical efficiency of micro-light-emitting diode (µ-LED) is investigated. Various blue light µ-LED devices (from 5 × 5 µm 2 to 100 × 100 µm 2 ) with ALD-Al 2 O 3 sidewall passivation were fabricated and exhibited lower leakage and better external quantum efficiency (EQE) comparing to samples without ALD-Al 2 O 3 sidewall treatment. Furthermore, the EQE values of 5 × 5 and 10 × 10 µm 2 devices yielded an enhancement of 73.47% and 66.72% after ALD-Al 2 O 3 sidewall treatments process, and the output power also boosted up 69.3% and 69.9%. The Shockley-Read-Hall recombination coefficient can be extracted by EQE data fitting, and the recombination reduction in the ALD samples can be observed. The extracted surface recombination velocities are 551.3 and 1026 cm/s for ALD and no-ALD samples, respectively.

Orbital angular momentum (OAM) mode multiplexing provides a new strategy for reconstructing multiple holograms, which is compatible with other physical dimensions involving wavelength and polarization to enlarge information capacity. Conventional OAM multiplexing holography usually relies on the independence of physical dimensions, and the deep holography involving spatial depth is always limited for the lack of spatiotemporal evolution modulation technologies. Herein, we introduce a depth-controllable imaging technology in OAM deep multiplexing holography via designing a prototype of five-layer optical diffractive neural network (ODNN). Since the optical propagation with dimensional-independent spatiotemporal evolution offers a unique linear modulation to light, it is possible to combine OAM modes with spatial depths to realize OAM deep multiplexing holography. Exploiting the multi-plane light conversion and in-situ optical propagation principles, we simultaneously modulate both the OAM mode and spatial depth of incident light via unitary transformation and linear modulations,…

Learning-based computer-generated holography (CGH) has shown remarkable promise to enable real-time holographic displays. Supervised CGH requires creating a large-scale dataset with target images and corresponding holograms. We propose a diffraction model-informed neural network framework (self-holo) for 3D phase-only hologram generation. Due to the angular spectrum propagation being incorporated into the neural network, the self-holo can be trained in an unsupervised manner without the need of a labeled dataset. Utilizing the various representations of a 3D object and randomly reconstructing the hologram to one layer of a 3D object keeps the complexity of the self-holo independent of the number of depth layers. The self-holo takes amplitude and depth map images as input and synthesizes a 3D hologram or a 2D hologram. We demonstrate 3D reconstructions with a good 3D effect and the generalizability of self-holo in numerical and optical experiments.

We investigate the propagation and interaction dynamics of the optical dark bound solitons for the defocusing Lakshmanan-Porsezian-Daniel equation, which is a physically relevant generalization of the nonlinear Schrödinger equation involving the higher-order effects. Explicit N-dark soliton solutions in the compact determinant form are constructed via the binary Darboux transformation method. Bound states of the dark solitons are discussed when the incoherent solitons have the same velocity. We find an interesting phenomenon that dark soliton molecules and double-valley dark solitons (DVDSs) can be obtained by controlling the interval of the bound state dark solitons, and abundant interaction modalities between them can be formed. Moreover, dark soliton molecules always undergo elastic interactions with other solitons, while interactions for the DVDSs are usually inelastic, and special parameter conditions for elastic interaction of DVDSs through asymptotic analysis are obtained. Numerical simulations are employed to verify the stability of the bound state dark solitons. Analytical…

Thanks to the development of machine learning and deep learning, data-driven pattern recognition based on neural network is a trend for Φ-OTDR system intrusion event recognition. The data-driven pattern recognition needs a large number of samples for training. However, in some scenarios, intrusion signals are difficult to collect, resulting in the lack of training samples. At the same time, labeling a large number of samples is also a very time-consuming work. This paper presents a few-shot learning classification method based on time series transfer and cycle generative adversarial network (CycleGAN) data augmentation for Φ-OTDR system. By expanding the rare samples based on time series transfer and CycleGAN, the number of samples in the dataset can finally meet the requirement of network training. The experimental result shows that even when the training set has two minor classes with only two samples, the average accuracy of the validation set with 5 classification tasks can still reach 90.84%, and the classification accuracy of minor classes can reach 79.28% with the proposed…

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been proposed for distributed vibration sensing purpose over recent years. Emerging applications, including seismic and hydroacoustic wave detection, demand accurate low-frequency vibration reconstruction capability. We propose to use the direct-detection Φ-OTDR configuration to achieve quantitative demodulation of external low-frequency vibrations by phase-shifted dual-pulse probes. Simultaneous pulsing and phase shifting modulation is realized with a single acousto-optic modulator to generate such probes, relaxing the need for an additional optical phase modulator. In the experiments, vibrations with frequency as low as 0.5 Hz are successfully reconstructed with 10 m spatial resolution and 35 dB signal-to-noise ratio. Excellent linearity and repeatability are demonstrated between the optical phase demodulation results and the applied vibration amplitudes. The proposed method is capable of quantitative demodulation of low-frequency vibrations with a cost-effective system configuration and high computation efficiency,…

Narrow linewidth and fast-chirped frequency are essential in frequency-modulated continuous-wave lasers. We introduce a laser that meets these requirements by coupling a distributed feedback laser with an external high-Q microring resonator, where a bulky stacked piezoelectric chip is attached to the resonator for fast tuning. The laser demonstrates an ultranarrow intrinsic linewidth of 22 Hz in the self-injection-locked state. Actuated by the bulky piezoelectric chip, the maximum triangular actuation bandwidth can reach 100 kHz. The driving voltage is filtered to avoid a resonant mechanical mode, obtaining the minimum residual linearity error at 10 kHz with a 4.2 GHz tuning range. A light detection and ranging system was set up for a proof-of-concept experiment, demonstrating a high detection precision with standard deviations of 2.7 and 4.0 cm for targets at 15 and 30 m, respectively.

The photonics-based technology has the advantages of wide bandwidth in millimeter wave (mm-wave) communication and radar sensing systems. In the present work, we propose a novel joint communication and radar sensing functions system based on photonics at the W-band. In the proposed system, the broadband linear frequency modulated (LFM) signal and high-speed M-quadrature amplitude modulation (MQAM) signal are simultaneously obtained by heterodyning two free-running external cavity lasers (ECLs). Based on this system, a communication rate of 78 Gbit/s and a radar with a 5-GHz bandwidth is achieved. This is a good solution to incorporate a high-speed communication and high-resolution radar sensing functions system.

We propose a simple scheme to generate quantum entanglement and one-way steering between distinct mode pairs in a generic cavity magnomechanical system, which is composed of a microwave cavity and a yttrium iron garnet sphere supporting magnon and phonon modes. The microwave cavity is pumped by a weak squeezed vacuum field, which plays an important role for establishing quantum entanglement and steering. It is found that when the magnon mode is driven by the red-detuned laser, the maximum entanglement between cavity mode and phonon mode and the maximum phonon-to-photon one-way steering can be effectively generated via adjusting the ratio of two coupling rates. While under the much weaker magnomechanical coupling, the quantum entanglement and one-way steering between cavity mode and magnon mode can be achieved, where the steering direction is determined merely by the relative dissipation strength of the cavity to the magnon mode. More interestingly, we reveal that the robustness to the temperature for entanglement and steering between any mode pairs can be evidently enhanced by…

Sensor-less adaptive optics based on stochastic parallel gradient descent (SPGD) is effective for the compensation of atmospheric disturbances in coherent free-space optical communication systems. However, SPGD converges slowly and easily falls into local extremes. Combining adaptive moment estimation and SPGD, we propose the AdamSPGD algorithm for efficient wavefront correction. Theoretical analysis and numerical simulations demonstrate that AdamSPGD can significantly increase the convergence speed, robustness, and dynamic ability, thereby more efficiently suppress the negative effects of atmospheric turbulence on mixing efficiency, bit error rate, and outage probability. Experimental results show that AdamSPGD reduces ∼50% of iterations. The improved performances make the proposed algorithm suitable for SLAO to improve the quality of optical communications.

For the first time, to the best of our knowledge, the design and characterization of a 3 × 1 fused fiber combiner based on multimode step-index fluoroindate optical fibers (InF 3 ) has been performed. Several efforts to develop a well-consolidated normalization procedure and a fabrication protocol have been required due to the low melting temperature and the mechanical properties of fluoroindates. Fabrication results demonstrate repeatability and absence of crystallization. Therefore, the described fabrication process paves the way for manufacturing fluoroindate devices. The electromagnetic design of the combiner is carried out through modal investigation and beam propagation method by computing the transmission efficiency. The experimental results agree with the simulation and demonstrate the device feasibility to operate in the mid-infrared spectral range.

In this work, an integrated liquid-crystal-based phase modulator operating at visible wavelengths was developed and experimentally demonstrated. A visible-light silicon-nitride-based 300-mm-wafer foundry platform and a liquid-crystal integration process were developed to leverage the birefringence of liquid crystal to actively tune the effective index of a section of silicon-nitride waveguide and induce a phase shift over its length. The device was experimentally shown to achieve a 41π phase shift within 4.8 Vpp for a 500-µm-long modulator, which means that a 2π phase shifter would need to be only 24.4 µm long. This device is a compact and low-power solution to the challenge of integrated phase modulation in silicon nitride and paves the way for future low-power small-form-factor integrated systems at visible wavelengths.

In this paper, an all-sapphire fiber-optic Fabry-Perot (F-P) pressure sensor is proposed. The sapphire pressure-sensitive diaphragm with low surface roughness is fabricated by MEMS wet etching. The direct bonding process is adopted to bond the sapphire-sensitive diaphragm and substrate together. And the sapphire fiber is adopted to be the lead-in fiber to ensure the sensor's resistance to high temperature. The performance of the sensor is tested within a pressure range of 0.1∼5 MPa and within the temperature range from room temperature to 1200°C. Experimental results show that the sensor could work stably at the temperature of 1200°C. The pressure sensitivity reaches up to 15nm/MPa. The nonlinearity of the sensor is 0.96% FS (full scale), and the relative resolution reaches 0.12%FS. The all-sapphire F-P sensor could be used for high-pressure testing in a high-temperature environment.

A Tm,Ho:CALGO laser passively mode-locked by a GaSb-based SESAM generated pulses as short as 52 fs at a central wavelength of 2015 nm with a broad spectral bandwidth of 82 nm (full width at half maximum) owing to the combined gain profiles of both dopants for σ-polarized light. The average output power reached 376 mW at a repetition rate of 85.65 MHz. In the continuous-wave regime, the laser was power scaled up to 1.01 W at 2080.6 nm with a slope efficiency of 32.0%, a laser threshold of 155 mW and π-polarized emission. Polarized spectroscopic properties of Ho 3+ ions in singly doped and codoped CALGO crystals were revisited to explain the observed laser performance.

The magnetic field gradient within optical pumping magnetometers (OPMs) suppresses sensitivity improvement. We investigated the effects of the magnetic field gradient along the x-, y-, and z-axes on the limiting factors of magnetometers under extremely low magnetic field conditions. We modified the magnetic field gradient relaxation model such that it can be applied to atoms in the spin exchange relaxation free (SERF) regime. The gradient relaxation time and spin polarizations, combined with fast spin-exchange interaction, were determined simultaneously using the oscillating cosine magnetic field excitation and amplitude spectrum analysis method. During the experiments, we eliminated the errors caused by the temperature and pumping power, and considered different isotope spin exchange collisions in naturally abundant Rb during the data analysis to improve the fitting accuracy. The experimental results agreed well with those of theoretical calculations and confirmed the accuracy of the improved model. The contribution of the transverse magnetic field gradient to the relaxation of the…

Due to the topological charge-independent doughnut spatial structure as well as the association of orbital angular momentums, perfect vortex beams promise significant advances in fiber communication, optical manipulation and quantum optics. Inspired by the development of planar photonics, several plasmonic and dielectric metasurfaces have been constructed to generate perfect vortex beams, instead of conventional bulky configuration. However, owing to the intrinsic Ohmic losses and interband electron transitions in materials, these metasurface-based vortex beam generators only work at optical frequencies up to the visible range. Herein, using silicon nitride nanopillars as high-efficiency half-wave plates, broadband and high-performance metasurfaces are designed and demonstrated numerically to directly produce perfect vortex beams in the ultraviolet region, by combining the phase profiles of spiral phase plate, axicon and Fourier transformation lens based on geometric phase. The conversion efficiency of the metasurface is up to 86.6% at the design wavelength. Moreover, the influence…

Thin-film lithium niobate (TFLN) is an emerging platform for compact, low-power nonlinear-optical devices, and has been used extensively for near-infrared frequency conversion. Recent work has extended these devices to mid-infrared wavelengths, where broadly tunable sources may be used for chemical sensing. To this end, we demonstrate efficient and broadband difference frequency generation between a fixed 1-µm pump and a tunable telecom source in uniformly-poled TFLN-on-sapphire by harnessing the dispersion-engineering available in tightly-confining waveguides. We show a simultaneous 1-2 order-of-magnitude improvement in conversion efficiency and ∼5-fold enhancement of operating bandwidth for mid-infrared generation when compared to equal-length conventional lithium niobate waveguides. We also examine the effects of mid-infrared loss from surface-adsorbed water on the performance of these devices.

The emerging all-dielectric platform exhibits high-quality ( Q ) resonances governed by the physics of bound states in the continuum (BIC) that drives highly efficient nonlinear optical processes. Here we demonstrate the robust enhancement of third-(THG) and fifth-harmonic generation (FHG) from all-dielectric metasurfaces composed of four silicon nanodisks. Through the symmetry breaking, the genuine BIC transforms into the high- Q quasi-BIC resonance with tight field confinement for record high THG efficiency of 3.9 × 10 −4 W −2 and FHG efficiency of 4.8 × 10 −10 W −4 using a moderate pump intensity of 1 GW/cm 2 . Moreover, the quasi-BIC and the resonantly enhanced harmonics exhibit polarization-insensitive characteristics due to the special C 4 arrangement of meta-atoms. Our results suggest the way for smart design of efficient and robust nonlinear nanophotonic devices.

Topological photonics has expanded our understanding of electromagnetic wave propagation and unraveled new methods of electromagnetic wave shaping. Among the various topological photonic systems, valley photonic crystal (VPC) is a highly versatile platform for constructing interfaces that supports unidirectional edge state to enable the robust topological transport of light. Although silicon VPC waveguides has demonstrated the lossless propagation of terahertz (THz) waves through multiple sharp bends, existing designs are mostly based on the standard zigzag-interface. Here, we explore interfacial topological photonics by designing a composite interface junction for an air-slot-like VPC waveguide with a 60° bend and an unchanging bearded-interface to allow the topologically protected propagation of THz waves over a 20% relative bandwidth. We employ particle swarm optimization to design the geometry of a bearded-interface silicon VPC waveguide to achieve a 42% relative bandgap and a topological edge state spanning a usable 25% relative bandwidth. Our work illustrates the prospects and…

Electro-optic (EO) modulators with a high modulation bandwidth are indispensable parts of an optical interconnect system. A key requirement for an energy-efficient EO modulator is the low drive voltage, which can be provided using a standard complementary metal oxide semiconductor circuity without an amplifying driver. Thin-film lithium niobate has emerged as a new promising platform, and shown its capable of achieving driverless and high-speed EO modulators. In this paper, we report a compact high-performance modulator based on the thin-film lithium niobate platform on a silicon substrate. The periodic capacitively loaded travelling-wave electrode is employed to achieve a large modulation bandwidth and a low drive voltage, which can support a driverless single-lane 100Gbaud operation. The folded modulation section design also helps to reduce the device length by almost two thirds. The fabricated device represents a large EO bandwidth of 45GHz with a half-wave voltage of 0.7V. The driverless transmission of a 100Gbaud 4-level pulse amplitude modulation signal is demonstrated with a…

Virtual reality head-mounted displays (VR-HMDs) are crucial to Metaverse which appears to be one of the most popular terms to have been adopted over the internet recently. It provides basic infrastructure and entrance to cater for the next evolution of social interaction, and it has already been widely used in many fields. The VR-HMDs with traditional aspherical or Fresnel optics are not suitable for long-term usage because of the image quality, system size, and weight. In this study, we designed and developed a large exit pupil diameter (EPD), compact, and lightweight VR-HMD with catadioptric optics. The mathematical formula for designing the catadioptric VR optics is derived. The reason why this kind of immersive VR optics could achieve a compact size and large EPD simultaneously is answered. Various catadioptric forms are systematically proposed and compared. The design can achieve a diagonal field of view (FOV) of 96° at -1 diopter, with an EPD of 10 mm at 11 mm eye relief (ERF). The overall length (OAL) of the system was less than 20 mm. A prototype of a compact catadioptric…

This paper presents and demonstrates the three logic processing levels based on complementary photonic crystal logic devices through photonic integrated circuit modeling. We accomplished a set of logic circuits including AND, OR, NAND, NOR, XOR, FAN-OUT, HALF ADDER, and FULL ADDER based on photonic crystal slab platforms. Furthermore, we achieved efficient all-optical logic circuits with contrast ratios as high as 5.5 dB, demonstrated in our simulation results, guaranteeing well-defined output power values for logic representations; a clock-rate up to 2 GHz; and an operating wavelength at λ ≈ 1550 nm. Thus, we can now switch up for high computing abstraction levels to build photonic integrated circuits rather than isolated gates or devices.

This paper proposes a novel and perfect absorber based on patterned graphene and vanadium dioxide hybrid metamaterial, which can not only achieve wide-band perfect absorption and dual-channel absorption in the terahertz band, but also realize their conversion by adjusting the temperature to control the metallic or insulating phase of VO 2 . Firstly, the absorption spectrum of the proposed structure is analyzed without graphene, where the absorption can reach as high as 100% at one frequency point ( f = 5.956 THz) when VO 2 is in the metal phase. What merits attention is that the addition of graphene above the structure enhances the almost 100% absorption from one frequency point ( f = 5.956 THz) to a wide frequency band, in which the broadband width records 1.683 THz. Secondly, when VO 2 is the insulating phase, the absorption of the metamaterial structure with graphene outperforms better, and two high absorption peaks are formed, logging 100% and 90.7% at f 3 = 5.545 THz and f 4 = 7.684 THz, respectively. Lastly, the adjustment of the Fermi level of graphene from 0.8 eV to 1.1 eV…

In this paper, the optical trapping of multiple particles based on a rotationally-symmetric power-exponent-phase vortex beam (RSPEPVB) was introduced and demonstrated. Based on the theories of tight focusing and optical force, the optical force model of RSPEPVB was established to analyze the optical trapping force of tightly focused RSPEPVB. Then, an experimental setup of optical tweezer, by utilizing the RSPEPVB, was built to demonstrate that the optical tweezer of RSPEPVBs can achieve the optical trapping of multiple particles, and the number of captured particles is equal to the topological charge l of RSPEPVB, which shows that the RSPEPVBs can achieve multi-particles trapping with controllable number. Moreover, compared to vortex beam, the captured particles by RSPEPVB will not rotate around the circular light intensity distribution. The results will provide a new option for optical trapping of multiple particles in biomedicine, laser cooling and so on.

The joint radar and communication (JRC) system providing both large-capacity transmission and high-resolution ranging will play a pivotal role in the next-generation wireless networks (e.g., 6G and beyond) and defense applications. Here, we propose and experimentally demonstrate a novel photonics-assisted millimeter-wave (mm-wave) JRC system with a multi-Gbit/s data rate for communication function and centimeter-level range resolution for radar function. The key is the design of the intermediate-frequency (IF) JRC signal through the angle modulation of the linear frequency modulation (LFM) radar carrier using orthogonal frequency division multiplexing (OFDM) communication signal, inspired by the idea of constant-envelope OFDM (CE-OFDM). This IF angle-modulated waveform facilitates the broadband photonic frequency (phase)-multiplying scheme to generate mm-wave JRC signal with multiplied instantaneous bandwidth and phase modulation index for high-resolution LFM radar and noise-robust CE-OFDM communication. It is with fixed low power-to-average power ratio to render robustness against…

We propose a graphene metamaterial consisting of several layers of longitudinally separated graphene nanoribbon array embedded into gain-assisted medium, demonstrating electromagnetically induced transparency-like spectra. Combined with finite-difference time-domain simulations, the transfer matrix method and temporal coupled-mode theory are adopted to quantitatively describe its transmission characteristics. These transmission characteristics can be tuned by altering the gain level in medium layer and the Fermi energy level in graphene. Additionally, it is the incorporation between gain medium and graphene nanoribbons with optimized geometrical parameters and Fermi energy level that the destructive interference between high order graphene plasmonic modes can be obtained, suggesting drastic phase transition with giant group delay and ultra-high group index up to 180 ps and 104, respectively. Our results can achieve efficient slow light effects for better optical buffers and other nonlinear applications.

Sapphire optical fiber has the ability to withstand ultrahigh temperatures and high radiation, but it is multimoded which prevents its use in many sensing applications. Problematically, Bragg gratings in such fiber exhibit multiple reflection peaks with a fluctuating power distribution. In this work, we write single-mode waveguides with Bragg gratings in sapphire using a novel multi-layer depressed cladding design in the 1550 nm telecommunications waveband. The Bragg gratings have a narrow bandwidth (<0.5 nm) and have survived annealing at 1000°C. The structures are inscribed with femtosecond laser direct writing, using adaptive beam shaping with a non-immersion objective. A single-mode sapphire fiber Bragg grating is created by writing a waveguide with a Bragg grating within a 425 µm diameter sapphire optical fiber, providing significant potential for accurate remote sensing in ultra-extreme environments.

Increasing data traffic and bandwidth-hungry applications require electro-optic modulators with ultra-wide modulation bandwidth for cost-efficient optical networks. Thus far, integrated solutions have emerged to provide high bandwidth and low energy consumption in compact sizes. Here, we review the design guidelines and delicate structures for higher bandwidth, applying them to lumped-element and traveling-wave electrodes. Additionally, we focus on candidate material platforms with the potential for ultra-wideband optical systems. By comparing the superiority and mechanism limitations of different integrated modulators, we design a future roadmap based on the recent advances.

In this work, an ultra-wideband flexible radar absorber with low infrared emissivity for a radar-infrared bi-stealth application utilizing multilayer patterned graphene is proposed. The proposed absorber consists of three layers of graphene films with different patterns, flexible substrates, lightweight foam, and a ground layer. The flexible graphene films, rather than the conventional lumped resistors, are adopted as omnidirectional resistors to achieve dual polarization and flexibility. On the top of the absorber, an infrared shielding layer (IRSL) consists of patterned Indium tin oxide (ITO) separated by a thin foam layer. Due to the low-pass characteristics and the high filling ratio of the top ITO layer, the infrared emissivity of the whole structure is reduced effectively while the radar absorption property is slightly affected. As a result, the 90% absorption band is from 1.96 GHz to 20.72 GHz (fractional bandwidth 165.4%), with a low infrared emissivity of about 0.35. Besides, a miniaturized unit is achieved with the period of 0.079 λl at the lowest absorption frequency, and…

A high sensitivity optical fiber gas pressure sensor based on the enhanced Vernier effect is proposed. The sensor is composed of a fiber Fabry-Perot interferometer (FPI) and Mach-Zehnder interferometer (MZI). Since the interference fringes of FPI and MZI drift in the opposite direction with the change of gas pressure, when their free spectral ranges are similar, the enhanced Vernier effect is formed after their cascading. Compared with the traditional Vernier effect gas pressure sensor, the enhanced Vernier effect gas pressure sensor realizes much higher sensitivity gas pressure measurement without complex manufacturing process or desensitized reference interferometer. The experimental results show that the sensitivity of the enhanced Vernier effect sensor is 241.87 nm/MPa. In the two traditional Vernier effect gas pressure sensors formed by cascading FPI and MZI, the sensitivity of sensor is 63.02 nm/MPa and 171.26 nm/MPa, respectively. Compared with the two traditional Vernier effect sensors, the sensitivity of the enhanced Vernier effect sensor is increased by 3.8 times and 1.4…

Thermal control coating for spacecraft based on thermochromic film attracts increasing interest due to their ability of self-adaptive emittance switch and less resource consuming compared with traditional thermal control coatings. However, practical applications of thermochromic film for spacecraft are constrained by the low infrared emittance at a high temperature and narrow emittance tunability. In this work, a thermochromic film with simple structure, nearly perfect infrared emission and large emittance tunability is proposed for the application of spacecraft thermal control. The thermochromic film is a VO 2 -based superposed Fabry-Perot (FP) multilayer film, which is constructed by encapsulating three thin VO 2 layers in four lossless BaF 2 spacer on the Al substrate. The infrared emittance and emittance tunability of the superposed FP film is dramatically enhanced by the three superposed VO 2 -BaF 2 -Al FP resonances at wavelengths of 9, 15 and 20 µm, respectively. For VO 2 layers under metallic state, the spectral normal emittance of the superposed FP film is close to unity in…

The exceptional resonances excited by symmetry-protected quasi-bound states in the continuum (QBICs) have provided significant potential in high-sensitive sensing applications. Herein, we have proposed a type of metal-insulator-metal (MIM) absorbers supported by QBIC-induced resonances, and the ideal Q-factors of QBIC-induced resonances can be enhanced up to 10 5 in the THz regime. The coupled mode theory and the multipole scattering theory are employed to thoroughly interpret the QBIC-induced absorption mechanism. Furthermore, the refractive index sensing capacities of the as-presented absorbers have been investigated, where the maximum values of the sensing sensitivity and figure of merit (FOM) can reach up to 187 GHz per refractive index unit and 286, respectively. Therefore, it is believed that the proposed absorbers enabled by QBIC-induced resonances hold promising potential in a broad range of highly demanding sensing applications.

Visible-infrared compatible camouflage is significant to enhance the equipment survivability through counteracting the modern detecting and surveillance systems. However, there are still great challenges in simultaneously achieving multispectral camouflage with high transmittance in visible, low emissivity in the atmospheric windows and high emissivity in the non-atmospheric window, which can be attributed to the mutual influence and restriction within these characteristics. Here, we proposed an optically transparent infrared selective emitter (OTISE) composed of three Ag-ZnO-Ag disk sub-cells with anti-reflection layers, which can synchronously improve the visible transmittance and widen absorption bandwidth in the non-atmospheric window by enhancing and merging resonance response of multi-resonators. Test results reveal that low emissivity in infrared atmospheric windows, high emissivity in the 5-8 µm non-atmospheric window and high optical transparency have been obtained. In addition, the radiative flux of OTISE in 3-5 µm and 8-14 µm are respectively 34.2% and 9.3% of that of…

A photonic crystal fiber utilizing surface plasmon resonance (PCF-SPR) sensor based on refractive index (RI) control of magnetic fluid (MF) is designed. The air holes of the sensor are arranged in a hexagonal shape, and the optical field transmission channels on both sides of the central air hole can effectively confine the energy of the optical field. We use MF as the sensing medium, and coat the inner wall of the central air hole with gold. It can effectively stimulate the SPR effect to achieve the purpose of magneto-refractive modulation. We study the sensing characteristics of the proposed sensor by finite element analysis. The results show that the highest refractive index sensitivity reaches 19520 nm/RIU in the RI range of 1.42-1.435 and the maximum figure of merit (FOM) is 374.3 RIU-1. In addition, the magnetic field and the temperature response characteristics of the designed sensor are also investigated. In the magnetic field range of 50-130 Oe, the magnetic field sensitivity is 590 pm/Oe. In the temperature range of 24.3-144.3 °C, the temperature sensitivity is only -29.7…

We propose and implement a high-bandwidth white-light visible light communication (VLC) system accomplishing data rate of 2.805 Gbit/s utilizing a semipolar blue micro-LED. The system uses an InGaN/GaN semipolar (20-21) blue micro-LED to excite yellow phosphor film for high-speed VLC. The packaged 30 μm 2 × 4 blue micro-LED array has an electrical-to-optical (EO) bandwidth of 1042.5 MHz and a peak wavelength of 447 nm. The EO bandwidth of the white-light VLC system is 849 MHz. Bit error rate (BER) of 2.709 × 10−3 meeting the pre-forward error correction (FEC) threshold is accomplished by employing a bit and power loaded orthogonal frequency division multiplexing (OFDM) signal. The proposed white-light VLC system employs simple and inexpensive yellow phosphor film for white-light conversion, complex color conversion material is not needed. Besides, no optical blue filter is employed in the white-light VLC system. The fabrication of the InGaN/GaN semipolar (20-21) blue micro-LED is discussed, and its characteristics are also evaluated.

Holographic, multimode fibre (MMF) based endoscopes envision high-quality in-vivo imaging inside previously inaccessible structures of living organisms, amongst other perspective applications. Within these instruments, a digital micro-mirror device (DMD) is deployed in order to holographically synthesise light fields which, after traversing the multimode fibre, form foci at desired positions behind the distal fibre facet. When applied in various imaging modalities, the purity and sharpness of the achieved foci are determinant for the imaging performance. Here we present diffraction-limited foci, which contain in excess of 96% of optical power delivered by the fibre which, to the best of our knowledge, represents the highest value reported to date. Further, we quantitatively study the impact of various conditions of the experimental procedure including input polarisation settings, influence of ghost diffraction orders, light modulation regimes, bias of the calibration camera and the influence of noise.

We propose a linear laser triangulation measurement system using Scheimpflug calibration based on the Monte Carlo optimization strategy. A Scheimpflug inclination camera calibration model is introduced in the measurement system for improving the image definition in small-range measurements with a large depth-of-field. To address the nonlinear optimization problem between the instrument resolution and measurement range, the Monte Carlo method is adopted to determine the optimal optical parameters (scattering angle, Scheimpflug angle, and focus length) in a practical measurement system. Furthermore, we experimentally constructed the measurement system to demonstrate the measurement precision by measuring a standard step block (measurement range 15 mm). The performance parameters of the maximum measurement error, maximum standard deviation, and linearity are obtained as ±7 μm, 0.225 μm, and 0.046%, respectively. Finally, the proposed measurement system based on the Monte Carlo optimization strategy is promising for high-precision measurements in industrial applications and provides…

We propose a method based on neural network training algorithms for the design of diffractive neural networks - with the aim to perform advanced laser beam shaping in the NIR/VIS spectrum for laser materials processing. The method enables the efficient design of systems including multiple cascaded diffractive optical elements (DOEs) and allows the simultaneous optimization for complex (intensity and phase) target field distributions in multiple target planes. The multi-target boundary condition in the optimization method offers great potential for advanced laser beam shaping.

Subwavelength chiral metamaterials with tunable geometries and compositions are essential to advance the development of chiral biochemical samples detection. Here, we report a spatial symmetry breaking chiral terahertz (THz) metamaterial structure with stacked layers of L-shape arranged gold disks as the periodic unit cell. The chiroptical response can be adjusted on-demand by manipulating the number of stacking layers and the twisted angle of the periodic unit between adjacent array layers. We reveal that the chiroptical response originates from the optical resonances of the gold disks and the adjacent gold disks array layers via experiments and numerical simulation analysis. Furthermore, we find that this chiral metamaterial can realize label-free detection of proline in biological samples and label-free enantio-discrimination of chiral molecules. The change of the analyte concentration can also regulate the transmission circular dichroism (TCD) intensity of the chiral metamaterials. Our results not only provide new ideas into the design of functional chiral metamaterials, but…

We present a near-eye display featuring a triple-channel waveguide with chiral liquid crystal gratings. Our triple-channel waveguide is capable of dividing one field of view into three through both the polarization orthogonality and angular separation. To illustrate its principle, a k -space diagram, which takes into account the aspect ratio of field of view, is depicted. Our results demonstrate that its diagonal field of view reaches 90°, eye relief is 10 mm, exit pupil is 4.9 × 4.9 mm 2 , transmittance is 4.9%, and uniformity is 89%.

Terahertz (THz) metamaterials for rapid label-free sensing show application potential for the detection of cancer biomarkers. A novel flexible THz metamaterial biosensor based on a low refraction index parylene-C substrate is proposed. The biomarkers are modified on non-metal areas by a three-step modification method that simplifies the modification steps and improves the modified effectivity. Simulation results for non-metal modification illustrate that a bulk refractive index sensitivity of 325 GHz/RIU is achieved, which is larger than that obtained for the traditional metal modification (147 GHz/RIU). Meanwhile, several fluorescence experiments proved the uniform modification effect and selective adsorption capacity of the non-metal modification method. The concentration of the carcinoembryonic antigen (CEA) biomarkers for breast cancer patients tested using this THz biosensor is found to be consistent with results obtained from traditional clinical tests. The limit of detection reaches 2.97 ng/mL. These findings demonstrate that the flexible THz metamaterial biosensor can be…

As a three-dimensional topological phase of matter, Weyl semimetals possess extremely large gyrotropic optical response in the mid-infrared region, leading to the strong chiral anomaly. This study proposes a circular polarizer design with a double-WSM-layer structure. It is theoretically shown that the proposed polarizer possesses a high circular polarization efficiency and high average transmittance in the wavelength region from 9 µm to 15 µm at incidence angles up to 50°. The modified 4 × 4 matrix method is used to calculate the circularly polarized transmittance of Weyl semimetals in thin-film or multilayer structures. The temperature dependence on the transmittance is also examined to demonstrate the flexibility of the proposed polarizer in a varying temperature environment. This study reveals the technological prospect that Weyl semimetals are promising candidates for high-performance circular polarizers in infrared spectroscopy and polarimetry.

Coherent Ising machine (CIM) is a hardware solver that simulates the Ising model and finds optimal solutions to combinatorial optimization problems. However, for practical tasks, the computational process may be trapped in local minima, which is a key challenge for CIM. In this work, we design a CIM structure with a spiking neural network by adding dissipative pulses, which are anti-symmetrically coupled to the degenerate optical parametric oscillator pulses in CIM with a measurement feedback system. We find that the unstable oscillatory region of the spiking neural network could assist the CIM to escape from the trapped local minima. Moreover, we show that the machine has a different search mechanism than CIM, which can achieve a higher solution success probability and speed-up effect.

We report on the generation of high energy, high power pulses in a tabletop diode-pumped Tm:YLF-based laser system, which delivers amplified pulse energies up to 108 J, as well as GW peak power performance when seeded with nanosecond duration pulses. Furthermore, the high power and efficiency capabilities of operating Tm:YLF in the multi-pulse extraction (MPE) regime were explored by seeding the experimental setup with a multi-kHz burst of pulses exhibiting a low individual pulse fluence, resulting in a 3.6 kW average power train of multi-joule-level pulses with an optical-to-optical efficiency of 19%.

The design of tunable and multifunctional metastructures (MSs) is currently a trend in the terahertz (THz) field. Based on the characteristic that thermal excitation can cause the phase transition of vanadium dioxide (VO 2 ), a MS that concentrates both cross-polarization conversion and absorption functions is proposed in this paper, and switching two functions can be achieved by controlling the temperature. At high temperatures (68°C), the proposed MS exhibits a narrow-band absorption function in the range of 0.67 THz-0.95 THz. When the temperature drops below 68°C, VO 2 is in the insulated state, and the structure can be considered as a polarization converter. Simulation results indicate that the broadband cross-polarization conversion can be realized in 0.69 THz-1.38 THz with a polarization conversion ratio above 90% and a relative bandwidth of 66.7%. This paper analyzes the amplitude, phase, and surface current distributions under the polarization conversion function, as well as the impedance, power loss distributions, and equivalent circuits under the absorption function. In…

A terahertz switchable metasurface with the function of absorption and polarization conversion is proposed. It consists of metal pattern layer - dielectric layer - VO2 layer - dielectric layer - metal pattern layer, and the photosensitive silicon is embedded in the metal pattern. When VO2 is in insulated state, the metasurface behaves as a linear polarization converter. The polarization conversion rate (PCR) is more than 90% at two frequency bands of 1.64 THz ∼ 1.91 THz and 2.35 THz ∼ 2.75 THz. The polarization converter has good asymmetric transmission ability. Moreover, the polarization conversion performance can be dynamically controlled by changing the conductivity of the photosensitive silicon. When VO2 is in metallic state, the metasurface becomes a terahertz bidirectional absorber, which exhibits different absorption properties under TE and TM waves with the maximum absorptance reaching to 100%. In addition, the absorption of TE and TM terahertz waves can be controlled at the specific frequency by changing the conductivity of photosensitive silicon. We also explore the…

The proper selection of signal source structure and parameters is one of the effective methods to suppress the random disturbance of underwater channel and enhance the performance of link communication. In this paper, the bit error probability and average capacity of a weakly turbulent absorbing seawater link with a perfect Laguerre-Gaussian beam are studied. The bit error rate of orbital angular momentum (OAM) channel under on-off key modulation is derived. The average capacity model of the optical wireless communication link is established on the basis that the OAM channel formed by vortex carrier of topological charge m is symmetric channel. The relationship between OAM channel capacity, carrier wavelength and seawater absorption is numerically analyzed by seawater spectral absorption coefficient, and it is concluded that in the range of "window transmission wavelength" and long channel, the conclusion of the longer signal source wavelength is beneficial to the performance of non-absorptive turbulent channel is no longer valid. Some other numerical results are worked out to show…

When a bulk solid is irradiated by an intense laser pulse, transition dipole moments (TDMs) between different energy bands have an important influence on the ultra-fast dynamic process. In this paper, we propose a new all-optical method to reconstruct the k-dependent TDMs between multi-bands using a crystal high-order harmonic generation (HHG). Taking advantage of an obvious separation of bandgaps between three energy bands of an MgO crystal along the <001 > direction, a continuous harmonic spectrum with two plateaus can be generated by a two-color laser pulse. Furthermore, the first harmonic platform is mainly dominated by the polarization between the first conduction band and the valence band, and the second one is largely attributed to the interband HHG from the second conduction band and the valence band. Therefore, the harmonic spectrum from a single quantum trajectory can be adopted to map TDMs between the first, second conduction bands, and the valence one. Our work is of great significance for understanding the instantaneous properties of solid materials in the strong laser…

We demonstrated a high output power distributed-Bragg-reflector (DBR) laser integrated with semiconductor optical amplifier (SOA) for the frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) system. In order to acquire higher output power, different from the conventional SG-DBR laser, the front mirror in this work is a section of uniform grating to get higher transmissivity. Therefore, the output power of the laser reaches 96 mW when the gain current and SOA current are 200 mA and 400 mA, respectively. Besides, we fabricated a spot size converter (SSC) at the laser output port to enhance the fiber coupling efficiency, which reached 64% coupled into the lensed fiber whose beam waist diameter is 2.5 μm. A tuning range of 2.8 nm with free spectral range (FSR) of 0.29 nm and narrow Lorentzian linewidth of 313 kHz is achieved. To realize distance and velocity measurement, we use the iterative learning pre-distortion method to linearize the frequency sweep, which is an important part of the FMCW LiDAR technology.

We propose a physics-assisted deep neural network scheme in Fourier ptychographic microscopy (FPM) using untrained deep neural network priors (FPMUP) to achieve a high-resolution image reconstruction from multiple low-resolution images. Unlike the traditional training type of deep neural network that requires a large labelled dataset, this proposed scheme does not require training and instead outputs the high-resolution image by optimizing the parameters of neural networks to fit the experimentally measured low-resolution images. Besides the amplitude and phase of the sample function, another two parallel neural networks that generate the general pupil function and illumination intensity factors are incorporated into the carefully designed neural networks, which effectively improves the image quality and robustness when both the aberration and illumination intensity fluctuation are present in FPM. Reconstructions using simulated and experimental datasets are demonstrated, showing that the FPMUP scheme has better image quality than the traditional iterative algorithms, especially for…

Vernier-effect has been widely employed in interferometer-based optical fiber sensors to improve the sensitivities greatly. However, the influence of the Vernier-effect on detection limit (DL) that is more important for evaluating the actual performance of the sensor has not been discussed. Two gas pressure fiber sensors (a typical Fabry-Perot interferometer-based sensor and a Vernier sensor) are used to compare the DL of them by experiments. Both the theoretical analysis and the experimental results show that, though the Vernier-effect magnifies the spectrum shift sensitivity, it also magnifies the value of the smallest detectable wavelength shift. As a result, the actual DL of the sensor is not improved by employing the Vernier-effect. If the contrast ratio of the Vernier envelope is not optimized enough for most of the reported sensors, the DL can even degenerate greatly.

Aside from ambient light noise, shot noise, and linear/nonlinear effects, strong low-frequency noise (LFN) severely affects the signal quality in LED-based visible light communication (VLC) systems, which hinders the implementation of data-driven end-to-end (E2E) deep learning approaches in real LED-VLC systems. We present a deep learning-based autoencoder to deal with this challenge. A novel modeling strategy is proposed to bypass the influence of the LFN and other low signal-to-noise ratio data when training the channel model of our E2E framework. The deep learning-based autoencoder then embeds the differentiable channel model and learns to combat the majority of channel impairments. In the E2E LED-VLC experiment, 1.875 Gbps transmission is achieved under the 7% HD-FEC threshold, 0.325 Gbps faster than the baseline. The E2E framework is robust to signal bias and amplitude variations, implying dimming support in the indoor environment.

The 2 µm wavelength band has proven to be a promising candidate for the next communication window. Wavelength-division multiplexing (WDM) transmission at 2 µm can greatly increase the capacity of optical communication systems. Here, we experimentally demonstrate a high-performance silicon photonic flat-top 8-channel WDM (de)multiplexer based on cascaded Mach-Zehnder interferometers for the 2 µm wavelength band. A three-stage-coupler scheme is utilized to provide passbands and reduce channel crosstalk, and 11 thermo-optic phase shifters have allowed active compensation of waveguide phase errors. The fabricated device shows low insertion loss (< 0.9 dB), channel crosstalk (< 20.6 dB) and 1-dB bandwidth of 2.3 nm for operating wavelength ranging from 1955nm to 1985nm. The demonstrated (de)multiplexer could potentially be used for WDM optical data communication in the 2 µm spectral band.

A VO 2 -assisted temperature-controlled multifunctional metamaterial polarization converter with large asymmetric transmission (AT) is proposed by introducing a gold-VO 2 grating. The converter can be switched between reflection mode and transmission mode by controlling the phase transition. When VO 2 is in the metallic state, the converter works in reflection mode, converting the incident forward/backward linearly/circularly polarized waves into the cross-polarized waves, and the broadband polarization conversion rates (PCRs) can reach 90% with relative bandwidth of up to 91.1% and 87.5%, respectively; when VO 2 is in the insulating state, the converter shows giant AT effect for circularly polarized waves at 0.64 THz and 1.28 THz. The multifunctional polarization converter holds great potential in the fields of communication and imaging, which provides a new way to design optical devices such as polarizers, isolators.

Diode laser self-injection locking (SIL) to a whispering gallery mode of a high quality factor resonator is a widely used method for laser linewidth narrowing and high-frequency noise suppression. SIL has already been used for the demonstration of ultra-low-noise photonic microwave oscillators and soliton microcomb generation and has a wide range of possible applications. Up to date, SIL was demonstrated only with a single laser. However, multi-frequency and narrow-linewidth laser sources are in high demand for modern telecommunication systems, quantum technologies, and microwave photonics. Here we experimentally demonstrate the dual-laser SIL of two multifrequency laser diodes to different modes of an integrated Si3N4 microresonator. Simultaneous spectrum collapse of both lasers, as well as linewidth narrowing and high-frequency noise suppression , as well as strong nonlinear interaction of the two fields with each other, are observed. Locking both lasers to the same mode results in a simultaneous frequency and phase stabilization and coherent addition of their outputs.…

In this work, we present a study of bright-bright mode electromagnetically induced transparency based on carbon nanotube films terahertz metasurface consisting of an array of two asymmetric split rings. Under the excitation of terahertz wave, the electromagnetically induced transparency window can be obviously observed. The simulation results agree with the theoretical results. The formation mechanism of the transparent window in bright-bright mode electromagnetically induced transparency is further analyzed. Moreover, the sensing performance of the proposed terahertz metasurface is investigated and the sensitivity can reach to 320 GHz/RIU. To verify the slow light characteristics of the device, the group delay of the terahertz metasurface is calculated and the value is 2.12 ps. The proposed metasurface device and the design strategies provide opportunities for electromagnetically induced transparency applications, such as sensors, optical memories, and flexible terahertz functional devices.

A chromatic confocal measurement method based on a phase Fresnel zone plate (FZP) is described. Strong dispersion of FZP results in significant axial focal shift. The axial dispersion curve is close to linear within a certain wavelength range determined by the quantitative calculation using the vectorial angular spectrum theory. A 11.27 mm diameter phase FZP with a primary focal length of 50 mm was processed using standard photolithography technology and used as the dispersive objective in a homemade chromatic confocal measurement system. The calibrated axial measurement range exceeds 16 mm, the axial resolution reaches 0.8 µm, and the measurement accuracy of displacement is better than 0.4%. This chromatic confocal sensor has been practically used in the measurement of step height, glass thickness, and 3D surface profile. The proposed method has the obvious characteristics of simplicity, greatly reduced cost and superior performance. It is believed that this sensing method has broad application prospects in glass, coating, machinery, electronics, optics and other industries.

We report detailed experimental data aiming for rigorous investigation of Tm:YLF laser performance, especially with a focus on tuning behavior. Continuous-wave (cw) lasing performance of Tm:YLF crystals with thulium dopings in the 2-6% range is investigated under diode and Ti:Sapphire pumping at 792 nm and 780 nm, respectively. While employing the c-axis, we have achieved cw lasing thresholds below 20 mW, laser output power up to 1.42 W, and laser slope efficiencies as high as 70% with respect to absorbed pump power. The passive loss of the Tm:YLF crystal is estimated to be as low as 0.05% per cm, corresponding to a crystal figure of merit above 10000. Via employing this low-loss crystal and a 2-mm thick off-surface optical axis birefringent filter (BRF) with strong sideband rejection, a record cw tuning range covering the 1772-2145 nm interval is demonstrated (except a small gap between 1801-1815nm region). Detailed lifetime and emission cross section measurements have been performed to explain the observed performance, and strategies for further performance enhancement are…

We experimentally demonstrate an integrated visible light modulator at 532 nm on the thin-film lithium niobate platform. The waveguides on such platform feature a propagation loss of 2.2 dB/mm while a grating for fiber interface has a coupling loss of 5 dB. Our fabricated modulator demonstrates a low voltage-length product of 1.1 V·cm and a large electro-optic bandwidth with a roll-off of -1.59 dB at 25 GHz for a length of 3.3 mm. This device offers a compact and large bandwidth solution to the challenge of integrated visible wavelength modulation in lithium niobate and paves the way for future small-form-factor integrated systems at visible wavelengths.