The 50 most-cited articles in Optics Letters, 2022

High-efficiency GaN-based green LEDs are of paramount importance to the development of the monolithic integration of multicolor emitters and full-color high-resolution displays. Here, the InGaN quantum well with gradually varying indium (In) content was proposed for improving the performance of GaN-based green LEDs. The InGaN quantum well with gradually varying In content not only alleviates the quantum-confined Stark effect (QCSE), but also yields a low Auger recombination rate. Consequently, the gradual In content green LEDs exhibited increased light output power (LOP) and reduced efficiency droop as compared to constant In content green LEDs. At 60 A/cm 2 , the LOPs of the constant In content green LEDs and the gradual In content green LEDs were 33.9 mW and 55.2 mW, respectively. At 150 A/cm 2 , the efficiency droops for the constant In content green LEDs and the gradual In content green LEDs were 61% and 37.6%, respectively. This work demonstrates the potential for the gradual In content InGaN to replace constant In content InGaN as quantum wells in LED devices in a…

An H-shaped acoustic micro-resonator (AmR)-based quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor is demonstrated for the first time. The H-shaped AmR has the advantages of easy optical alignment, high utilization of laser energy, and reduction in optical noise. The parameter of the H-shaped AmR is designed based on the standing wave enhancement characteristic. The performance of the H-shaped AmR-based QEPAS sensor system and bare quartz tuning fork (QTF)-based sensor system are measured under the same conditions by choosing water vapor (H2O) as the target gas. Compared with the QEAPS sensor based on a bare QTF, the detection sensitivity of the optimal H-shaped AmR-based QEPAS sensor exhibits a 17.2 times enhancement.

A non-reciprocity sensor based on a layered structure with multitasking is proposed, which realizes biological detection and angle sensing. Through an asymmetrical arrangement of different dielectrics, the sensor obtains non-reciprocity on the forward and backward scales, thus achieving multi-scale sensing in different measurement ranges. The structure sets the analysis layer. Injecting the analyte into the analysis layers by locating the peak value of the photonic spin Hall effect (PSHE) displacement, cancer cells can accurately be distinguished from normal cells via refractive index (RI) detection on the forward scale. The measurement range is 1.569∼1.662, and the sensitivity (S) is 2.97 × 10 −2 m/RIU. On the backward scale, the sensor is able to detect glucose solution with 0∼400 g/L concentrations (RI = 1.3323∼1.38), with S = 1.16 × 10 −3 m/RIU. When the analysis layers are filled with air, high-precision angle sensing can be achieved in the terahertz range by locating the incident angle of the PSHE displacement peak; 30°∼45°, and 50°∼65° are the detection ranges, and the…

Conventional defect modes in all-dielectric 1D photonic crystals (PCs) are polarization-insensitive. This poses a great challenge in achieving high-performance polarization selectivity. In this Letter, we introduce a defect layer into a 1D PC containing hyperbolic metamaterials to achieve an anomalous defect mode with polarization-sensitive characteristics. As the incident angle increases, such a defect mode remains almost unshifted under transverse magnetic polarization, while strongly shifting toward shorter wavelengths under transverse electric polarization. The polarization-sensitive characteristics of the defect mode can be well explained by the Fabry-Perot resonance condition. Assisted by the polarization-sensitive defect mode, wide-angle polarization selectivity with an operating angle width up to 54.8° can be realized. Our work provides a route to designing wide-angle linear polarizers using simple 1D structures, which would be useful in liquid crystal display and Q-switched lasers.

We demonstrate 0.034 dB/m loss waveguides in a 200-mm wafer-scale, silicon nitride (Si3N4) CMOS-foundry-compatible integration platform. We fabricate resonators that measure up to a 720 million intrinsic Q resonator at 1615 nm wavelength with a 258 kHz intrinsic linewidth. This resonator is used to realize a Brillouin laser with an energy-efficient 380 µW threshold power. The performance is achieved by reducing scattering losses through a combination of single-mode TM waveguide design and an etched blanket-layer low-pressure chemical vapor deposition (LPCVD) 80 nm Si3N4 waveguide core combined with thermal oxide lower and tetraethoxysilane plasma-enhanced chemical vapor deposition (TEOS–PECVD) upper oxide cladding. This level of performance will enable photon preservation and energy-efficient generation of the spectrally pure light needed for photonic integration of a wide range of future precision scientific applications, including quantum, precision metrology, and optical atomic clocks.

A high-rate continuous-variable quantum key distribution (CV-QKD) system based on high-order discrete modulation is experimentally investigated. With the help of the novel system scheme, effective digital signal processing (DSP) algorithms and advanced analytical security proof methods, the transmission results of 5.059 km, 10.314 km, 24.490 km, and 50.592 km are achieved for 1 GBaud optimized quantum signals. Correspondingly, the asymptotic secret key rates (SKRs) are 292.185 Mbps, 156.246 Mbps, 50.491 Mbps, and 7.495 Mbps for discrete Gaussian (DG) 64QAM, and 328.297 Mbps, 176.089 Mbps, 51.304 Mbps, and 9.193 Mbps for DG 256QAM, respectively. Under the same parameters, the achieved SKRs of DG 256QAM is almost same as ideal Gaussian modulation. In this case, the demonstrated high-rate discrete-modulated CV-QKD system has the application potential for high-speed security communication under tens of kilometers.

Chaotic optical communication technology is considered as an effective secure communication technology, which can protect information from a physical layer and is compatible with the existing optical networks. At present, to realize long-distance chaos synchronization is still a very difficult problem, mainly because well-matched hardware cannot always be guaranteed between the transmitter and receiver. In this Letter, we introduce long short-term memory (LSTM) networks to learn a nonlinear dynamics model of an opto-electronic feedback loop, and then apply the trained deep learning model to generate a chaotic waveform for encryption and decryption at the transmitter and receiver. Furthermore, to improve the security, we establish a deep learning model pool which consists of different gain trained models and different delay trained models, and use a digital signal to drive chaos synchronization between the receiver and transmitter. The proposed scheme is experimentally verified in chaotic-encrypted 56-Gbit/s PAM-4 systems, and a decrypted performance below 7%FEC threshold (BER =…

An acoustic microresonator (AmR) based in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS) sensor with a line interaction mode is proposed for what is believed to be the first time. The interaction area for the acoustic wave of the proposed AmR, with a slotted sidewall, is not limited to a point of the quartz tuning fork (QTF) prongs, but extends along the whole plane of the QTF prongs. Sixteen types of AmRs are designed to identify the best parameters. Water vapor (H2O) is chosen as the analyte to verify the reported method. The results indicate that this AmR for IP-QEPAS with a line interaction mode not only provides a high signal level, but also reduces the thermal noise caused by the laser directly illuminating the QTF. Compared with standard IP-QEPAS without an AmR, the minimum detection limit (MDL) is improved by 4.11 times with the use of the technique proposed in this study.

In this Letter, we experimentally demonstrate the first real-time transparent fiber–THz–fiber 2 × 2 multiple-input multiple-output (MIMO) transmission system with a record line rate of 125.516 Gbps at 360–430 GHz based on photonic remote heterodyning, hybrid optoelectronic down-conversion, and commercial digital coherent modules. The 103.125-Gbps net data rate using dual-polarization quadrature phase-shift keying (DP-QPSK) modulation is successfully transmitted over two spans of 20-km standard single-mode fiber (SSMF) and 60-cm wireless distance under 15% soft-decision forward error correction (SD-FEC) for a pre-FEC bit error ratio (BER) threshold of 1.56 × 10 −2 (post-FEC BER < 10 −15 ). The optical signal to noise ratio (OSNR) margin and the stability of the transmission system are extensively investigated. To the best of our knowledge, this is the first time to realize >100-Gbps real-time transparent fiber–THz–fiber link transmission at beyond the 350-GHz band, making it a promising scheme to pave the way towards a practical seamless integration of a fiber–THz–fiber link to the…

In this Letter, a sensitive light-induced thermoelastic spectroscopy (LITES)-based trace gas sensor by exploiting a super tiny quartz tuning fork (QTF) was demonstrated. The prong length and width of this QTF are 3500 µm and 90 µm, respectively, which determines a resonant frequency of 6.5 kHz. The low resonant frequency is beneficial to increase the energy accumulation time in a LITES sensor. The geometric dimension of QTF on the micrometer scale is advantageous to obtain a great thermal expansion and thus can produce a strong piezoelectric signal. The temperature gradient distribution of the super tiny QTF was simulated based on the finite element analysis and is higher than that of the commercial QTF with 32.768 kHz. Acetylene (C 2 H 2 ) was used as the analyte. Under the same conditions, the use of the super tiny QTF achieved a 1.64-times signal improvement compared with the commercial QTF. The system shows excellent long-term stability according to the Allan deviation analysis, and a minimum detection limit (MDL) would reach 190 ppb with an integration time of 220 s.

We numerically investigate the pulsating dynamics of pure-quartic solitons (PQSs) in a passively mode-locked fiber laser. The bifurcation diagrams show that the PQS can alternate between the stable single soliton and pulsating regimes multiple times before transiting into the chaotic state. This multi-alternation behavior can be attributed to energy redistribution across the central part and the oscillating tails of the PQS, which is caused by an imperfect counterbalance between self-phase modulation (SPM)-induced and fourth-order dispersion (FOD)-induced phase shifts. Soliton creeping behavior can be observed during the pulsating process, accompanied by periodic asymmetric temporal profiles and central wavelength shifts of the PQS. These findings give new insights into the dynamics of PQSs in fiber lasers.

Mamyshev oscillators produce high-performance pulses, but technical and practical issues render them unsuitable for widespread use. Here we present a Mamyshev oscillator with several key design features that enable self-starting operation and unprecedented performance and simplicity from an all-fiber laser. The laser generates 110 nJ pulses that compress to 40 fs and 80 nJ with a grating pair. The pulse energy and duration are both the best achieved by a femtosecond all-fiber laser to date, to our knowledge, and the resulting peak power of 1.5 MW is 20 times higher than that of prior all-fiber, self-starting lasers. The simplicity of the design, ease of use, and pulse performance make this laser an attractive tool for practical applications.

area µLEDs with various mesa shapes (circular, square, and stripes). Appropriate external quantum efficiency (EQE) can yield internal quantum efficiency (IQE) which decreases with increasing peripheral length of the mesas. However, light extraction efficiency (ηe) increased with increasing mesa periphery. We introduce analysis of Jpeak (the current at peak EQE) since it is proportional to the non-radiative recombination. Etching the sidewalls using tetramethylammonium hydroxide (TMAH) increased the peak EQE and decreased the sidewall dependency of Jpeak. Quantitatively, the TMAH etching reduced non-radiative surface recombination by a factor of four. Hence, shrinking µLEDs needs an understanding of the relationship between non-radiative recombination and ηe, where analyzing Jpeak can offer new insights.

Oligomer metasurfaces have attracted a lot of attention in recent years because of their ability to drive strong resonance effects. In this work, by perturbing the symmetry of the structure, we find that there are a large number of resonance modes in the oligomer metasurfaces associated with the optical bound states in the continuum (BICs) near the communication wavelength. When the positions of two nanodisks of the hexamer oligomers are moved along the x- or y-directions at the same time, the mirror symmetry is broken, and an electric quadrupole BIC and three magnetic dipole BICs are excited. The results of near-field distribution of three-dimensional nanodisks and far-field scattering of multiple dipoles in each quasi-BIC reveal that the four BICs present different optical characteristics. It is noted that the method of symmetry breaking by moving the position of nanodisks can accurately control the asymmetric parameter of symmetry-protected BICs, which provides a route for the realization of ultrahigh quality (Q)-factor oligomer metasurfaces in experiment.

Chaotic optical communication has attracted much attention as a hardware encryption method in the physical layer. Limited by the requirements of chaotic hardware synchronization, fiber transmission impairments are restrictedly compensated in the optical domain. There has been little experimental demonstration of high-speed and long-distance chaotic optical communication systems. Here, we propose a method to overcome such limitations. Using a deep-learning model to realize chaotic synchronization in the digital domain, fiber transmission impairments can be compensated by digital-signal processing (DSP) algorithms with coherent detection. A successful transmission of 30 Gb/s quadrature phase-shift keying messages hidden in a 15 GHz wideband chaotic optical carrier was experimentally demonstrated over a 340-km fiber link. Meanwhile, the chaotic receiver can be significantly simplified without compromising security. The proposed method is a possible way to promote the practical application of chaotic optical communications.

Periodically poled lithium niobate (PPLN) structures on a chip enable efficient second-order nonlinear optical effects, benefiting from the tight light confinement and the utilization of d 33 . Here, we report a broadband second-harmonic (SH) generation in a step-chirped PPLN waveguide on X-cut lithium niobate on insulator (LNOI). The high fidelity of the poling period is demonstrated over the whole length of 7 mm using a non-destructive technique of piezoresponse force microscopy. The SH signal was continuously observed in the step-chirped PPLN waveguides while scanning the wavelength of the pump laser from 1550 nm to 1660 nm. The SH conversion efficiency was measured to be 9.6 % W -1 cm -2 at 1642 nm. This work will benefit wavelength conversions of light sources with wideband spectra.

In this Letter, we propose high-resolution ghost imaging (GI) through complex scattering media using temporal correction. We provide evidence that the theoretical description about GI based on spatially correlated beams is still incomplete and cannot work in complex scenarios. We complete the description of temporal correction of beam correlations in GI. The optical experiments demonstrate that high-resolution ghost images can always be retrieved by using the rectified temporally corrected beam correlation algorithm even in complex, dynamic, and highly strong scattering environments where conventional GI cannot work. By using the proposed method, the quality of the retrieved ghost images through complex scattering media can be enhanced effectively as the number of realizations increases, which cannot be achieved by conventional GI. The established general framework provides optical insights beyond the current understanding of GI, and the rectified theory and experimental results would represent a key step toward applications of GI over a wide range of free-space wave propagation…

Existing nonlinear-optic implementations of pure, unfiltered heralded single-photon sources do not offer the scalability required for densely integrated quantum networks. Additionally, lithium niobate has hitherto been unsuitable for such use due to its material dispersion. We engineer the dispersion and the quasi-phasematching conditions of a waveguide in the rapidly emerging thin-film lithium niobate platform to generate spectrally separable photon pairs in the telecommunications band. Such photon pairs can be used as spectrally pure heralded single-photon sources in quantum networks. We estimate a heralded-state spectral purity of >94% based on joint spectral intensity measurements. Further, a joint spectral phase-sensitive measurement of the unheralded time-integrated second-order correlation function yields a heralded-state purity of ( 86 ± 5 ) % .

The strength of interactions between photons in a χ (2) nonlinear optical waveguide increases at shorter wavelengths. These larger interactions enable coherent spectral translation and light generation at a lower power, over a broader bandwidth, and in a smaller device: all of which open the door to new technologies spanning fields from classical to quantum optics. Stronger interactions may also grant access to new regimes of quantum optics to be explored at the few-photon level. One promising platform that could enable these advances is thin-film lithium niobate (TFLN), due to its broad optical transparency window and possibility for quasi-phase matching and dispersion engineering. In this Letter, we demonstrate second harmonic generation of blue light on an integrated thin-film lithium niobate waveguide and observe a conversion efficiency of η 0 = 33, 000%/W- cm 2 , significantly exceeding previous demonstrations.

A multicore fiber Bragg grating (MC-FBG) array shape sensor is a powerful tool for a variety of applications. However, the efficient fabrication of high-quality MC-FBG arrays remains a problem. Here, we report for the first time, to the best of our knowledge, a new method of directly writing FBG arrays in a seven-core fiber (SCF) through the protective coating using femtosecond laser auto-positioning point-by-point technology, which is accomplished by image recognition and micro-displacement compensation. An MC-FBG array consisting of 140 individual FBGs with a grating length of 2 mm was successfully inscribed into seven cores of a 440 mm-long SCF. Each core contained 20 wavelength-division-multiplexed (WDM) FBGs with wavelengths ranging from 1522.11 nm to 1579.28 nm. In other words, the MC-FBG array consisted of 20 WDM nodes with an interval of 2 cm along the fiber, and each node contained seven identical FBGs integrated in parallel into the fiber cross-section. Moreover, the fabricated MC-FBG array exhibited a strong orientation dependence in bend sensing, with a maximum…

The recent advancements in higher-order topology have provided unprecedented opportunities in optical device designs and applications. Here, we propose a new, to the best of our knowledge, method to realize rainbow trapping based on higher-order topological corner modes (HOTCMs), which are constructed by two configurations of breathing kagome photonic crystals with distinct topological phases. Interestingly, the HOTCMs localized at corners with different geometric configurations are found to be frequency dispersive and thus initiate the possible application in realizing rainbow trapping. By designing a polygon structure containing several configurations of corners, we demonstrate that the HOTCMs can be excited with the frequency sequence locked to the corner order (clockwise/anticlockwise direction) in the polygon. The reported HOTCMs provide a new mechanism to realize multiple-frequency trapping, which may find potential applications in future integrated photonics.

The concept of "flat optics" is quickly conquering different fields of photonics, but its implementation in quantum optics is still in its infancy. In particular, polarization entanglement, strongly required in quantum photonics, is so far not realized on "flat" platforms. Meanwhile, relaxed phase matching of "flat" nonlinear optical sources enables enormous freedom in tailoring their polarization properties. Here we use this freedom to generate photon pairs with tunable polarization entanglement via spontaneous parametric downconversion (SPDC) in a 400-nm GaP film. By changing the pump polarization, we tune the polarization state of photon pairs from maximally entangled to almost disentangled, which is impossible in a single bulk SPDC source. Polarization entanglement, together with the broadband frequency spectrum, results in an ultranarrow (12 fs) Hong-Ou-Mandel effect and promises extensions to hyperentanglement.

In this Letter, an effective integrated waveform generation method where an orthogonal frequency division multiplexing (OFDM)-based non-orthogonal multiple access (NOMA) communication signal combined with a linear frequency modulation (LFM) radar signal is proposed and experimentally demonstrated in a W-band communication and radar detection system. The proposed method can generate communication and radar signals simultaneously. In addition, multiple user communication and multi-target radar detection can be realized. Furthermore, for the OFDM-NOMA signal, discrete Fourier transform (DFT) precoding is used to keep the optimal power ratio consistent on different subcarriers, thus, the communication performance can be effectively improved. The experimental results show that the distance estimation error is 1.2 cm for radar signals. Meanwhile, after transmission over 0.8-m free space, the total aggregate rate for two users with the OFDM-NOMA signal is 3.125 Gb/s under the bit error rate of 8.5 × 10 −4 .

Chip-scale optical tweezers, which are usually implemented in a planar format without using bulky diffractive optical elements, are recognized as a promising candidate to be integrated with a lab-on-a-chip system. However, traditional chip-scale optical tweezers are often static and allow for only one type of manipulation functionality since the geometrical parameters of the tweezers are fixed. Herein, we introduce a new, to the best of our knowledge, class of on-chip optical tweezers for diverse types of manipulation of micro-particles. Utilizing both the propagation phase and Pancharatnam-Berry phase, we experimentally demonstrate the spin-dependent trapping, moving, and circling of micro-particles with the transfer of optical gradient force and orbital angular momentum to particles. We further show that the spin angular momentum of the output beam provides an additional degree of freedom to control the spinning rotation of particles. This new type of optical tweezers paves the way for multifunctional and dynamical trapping and manipulation of particles with a lab-on-a-chip system.

A simple plastic optical fiber (POF) based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The sensor consists of a series of V-grooves along the POF and a side-polish structure at the other side of the fiber. The V-groove structure can alter the SPR excitation angle and act as a mode filter, effectively enhancing the SPR effect and narrowing the SPR wavelength width. After coating a layer of thermosensitive material-polydimethylsiloxane (PDMS) film on half part of the fiber probe, a dual-parameter sensor probe is obtained for RI and temperature measurement. Experimental results show the RI sensitivity of the prepared probe can reach 1546 nm/RIU in the RI range of 1.335-1.37 RIU and the temperature sensitivity is -0.83 nm/°C in the temperature range of 20-80°C. The sensor is simple in structure and low cost, and has potential applications in the biochemical sensing fields.

Squeezed light is a key quantum resource that enables quantum advantages for sensing, networking, and computing applications. The scalable generation and manipulation of squeezed light with integrated platforms are highly desired for the development of quantum technology with continuous variables. In this Letter, we demonstrate squeezed light generation with thin-film lithium niobate integrated photonics. Parametric down-conversion is realized with quasi-phase matching using ferroelectric domain engineering. With sub-wavelength mode confinement, efficient nonlinear processes can be observed with single-pass configuration. We measure 0.56 ± 0.09 dB quadrature squeezing (∼2.6 dB inferred on-chip). The single-pass configuration further enables the generation of squeezed light with large spectral bandwidth up to 7 THz. This work represents a significant step towards the on-chip implementation of continuous-variable quantum information processing.

Photonics is currently undergoing an era of miniaturization thanks in part to two-dimensional (2D) optical metasurfaces. Their ability to sculpt and redirect optical momentum can give rise to an optical force, which acts orthogonally to the direction of light propagation. Powered by a single unfocused light beam, these lateral optical forces (LOFs) can be used to drive advanced metavehicles and are controlled via the incident beam's polarization. However, the full control of a metavehicle on a 2D plane (i.e. forward, backward, left, and right) with a sign-switchable LOF remains a challenge. Here we present a phase-gradient metasurface route for achieving such full control while also increasing efficiency. The proposed metasurface is able to deflect a normally incident plane wave in a traverse direction by modulating the plane wave's polarization, and results in a sign-switchable recoil LOF. When applied to a metavehicle, this LOF enables a level of motion control that was previously unobtainable.

Heterogeneous integration of III-V active devices on lithium niobate-on-insulator (LNOI) photonic circuits enable fully integrated transceivers. Here we present the co-integration of InP-based light-emitting diodes (LEDs) and photodetectors on an LNOI photonics platform. Both devices are realized based on the same III-V epitaxial layers stack adhesively bonded on an LNOI waveguide circuit. The light is evanescently coupled between the LNOI and III-V waveguide via a multiple-section adiabatic taper. The waveguide-coupled LEDs have a 3-dB bandwidth of 40 nm. The photodetector features a responsivity of 0.38 A/W in the 1550-nm wavelength range and a dark current of 9 nA at -0.5 V at room temperature.

Ultrahigh-Q chiroptical resonance metasurfaces based on merging bound states in the continuum (BICs) are investigated and numerically demonstrated. The destruction of C2 symmetry results in the leakage of BICs into quasi-BICs, and a chiral quasi-BIC is obtained by oblique incidence or continuous destruction of the mirror symmetry of the structure. Due to the significant topological properties of merging BICs, the Q factor (over 2 × 105) of the chiral resonance peak obtained is much higher than that of the previous work. Moreover, the proposed structure is easy to fabricate because no additional out-of-plane asymmetry is introduced. The proposed scheme is of importance in chiral biosensing applications.

A very-high-bandwidth integrated silicon microring modulator (MRM) designed on a commercial silicon photonics (SiP) platform for C-band operation is presented. The MRM has a 3 dB electro-optic (EO) bandwidth of over 67 GHz and features a small footprint of 24 µm × 70 µm. Using the MRM, we demonstrate intensity modulation-direct detection (IM-DD) transmission with 4-level pulse amplitude modulation (PAM-4) signaling of over 100 Gbaud. By utilizing the optical peaking effect and negative chirp in the MRM, we extend the transmission distance, which is limited by the fiber-dispersion-induced frequency fading. Using a standard single-mode fiber (SSMF) for transmission across distances of up to 2 km, we measured the data transmission of 100 Gbaud PAM-4 signals with a bit error rate (BER) under the general 7% hard-decision forward-error correction (HD-FEC) threshold. The MRM enables an extended transmission distance for 100 Gbaud signaling in the C-band without dispersion compensation.

Bright-field imaging and edge imaging can extract different characteristics from objects, and therefore play important roles in image processing and pattern recognition. Here, we propose a fast, convenient, and electrically driven adjustable scheme to achieve tunable edge-enhanced images based on computing metasurfaces. The computing metasurface can perform spatial differential operation as optical waves propagate through it. This optical differential operation is polarization-dependent, thus any desirable contrast can be realized by the interplay between two orthogonal polarization components. By regulating the external voltages applied on the liquid-crystal phase plate, different phase retardances between two orthogonal polarization components are introduced; this allows us to quickly switch between the bright-field image and the edge image.

Semiconductor saturable absorber mirrors (SESAMs) have been regarded as a revolutionary technology for ultrafast mode-locked lasers, producing numerous landmark laser breakthroughs. However, the operating wavelength of existing SESAMs is limited to less than 3 µm. In this study, we create a 3-5 µm mid-infrared (MIR) SESAM by engineering an InAs/GaSb type-II superlattice. Bandgap engineering and the strong coupling between potential wells in a superlattice enable a broadband response of saturable absorption in the 3-5 µm spectral range. Using the fabricated SESAM, we realize a SESAM mode-locked Er:ZBLAN fiber laser at 3.5 µm, which delivers MIR ultrashort pulses with high long-term stability. The breakthrough of SESAM fabrication in the MIR will promote the development of MIR ultrafast coherent sources and related application fields.

Circular dichroism (CD) is required in the applications of biological detection, analytical chemistry, etc. Here, we numerically demonstrated large-range switchable CD by controlling the phase change of Ge2Sb2Te5 (GST) in a zigzag array. At the amorphous state of GST (a-GST), the strong and dual-waveband CD effects are realized via the selective excitations of electric, magnetic, and toroidal resonances. With the transition from a-GST to crystalline state GST, CD strengths are tailored dynamically in large ranges. In detail, the CD magnitudes change by about 0.93 and the modulation depths exceed 94% at dual wavebands. The strong CD effects and large-range switch capability in the GST-based metasurfaces will boost the development of active chiroptical devices.

Terahertz waves are electromagnetic waves located at 0.1-10 THz, and terahertz imaging technology can be applied to security inspection, biomedicine, non-destructive testing of materials, and other fields. At present, terahertz images have unclear data and rough edges. Therefore, improving the resolution of terahertz images is one of the current hot research topics. This paper proposes an efficient terahertz image super-resolution model, which is used to extract low-resolution (LR) image features and learn the mapping of LR images to high-resolution (HR) images, and then introduce an attention mechanism to let the network pay attention to more information features. Finally, we use sub-pixel convolution to learn a set of scaling filters to upgrade the final LR feature map to an HR output, which not only reduces the model complexity, but also improves the quality of the terahertz image. The resolution reaches 31.67 db on the peak signal-to-noise ratio (PSNR) index and 0.86 on the structural similarity (SSIM) index. Experiments show that the efficient sub-pixel convolutional neural…

A mask-based lensless camera optically encodes the scene with a thin mask and reconstructs the image afterward. The improvement of image reconstruction is one of the most important subjects in lensless imaging. Conventional model-based reconstruction approaches, which leverage knowledge of the physical system, are susceptible to imperfect system modeling. Reconstruction with a pure data-driven deep neural network (DNN) avoids this limitation, thereby having potential to provide a better reconstruction quality. However, existing pure DNN reconstruction approaches for lensless imaging do not provide a better result than model-based approaches. We reveal that the multiplexing property in lensless optics makes global features essential in understanding the optically encoded pattern. Additionally, all existing DNN reconstruction approaches apply fully convolutional networks (FCNs) which are not efficient in global feature reasoning. With this analysis, for the first time to the best of our knowledge, a fully connected neural network with a transformer for image reconstruction is…

The nonlinear propagation of ultrashort pulses in optical fibers depends sensitively on the input pulse and fiber parameters. As a result, the optimization of propagation for specific applications generally requires time-consuming simulations based on the sequential integration of the generalized nonlinear Schrödinger equation (GNLSE). Here, we train a feed-forward neural network to learn the differential propagation dynamics of the GNLSE, allowing emulation of direct numerical integration of fiber propagation, and particularly the highly complex case of supercontinuum generation. Comparison with a recurrent neural network shows that the feed-forward approach yields faster training and computation, and reduced memory requirements. The approach is generic and can be extended to other physical systems.

High-energy, ultrafast, short-wavelength infrared laser sources with high average power are important tools for industrial and scientific applications. Through the coherent combination of four ultrafast thulium-doped rod-type fiber amplifiers, we demonstrate a Tm-doped chirped pulse amplification system with a compressed pulse energy of 1.65 mJ and 167 W of average output power at a repetition rate of 101 kHz. The system delivers 85 fs pulses with a peak power of 15 GW. Additionally, the system presents a high long- and short-term stability. To the best of our knowledge, this is the highest average output power short wavelength IR, mJ-class source to date. This result shows the potential of coherent beam combining techniques in the short wavelength infrared spectral region for the power scalability of these systems.

Tunable focusing is a desired property in a wide range of optical imaging and sensing technologies but has tended to require bulky components that cannot be integrated on-chip and have slow actuation speeds. Recently, integration of metasurfaces into electrostatic micro-electromechanical system (MEMS) architectures has shown potential to overcome these challenges but has offered limited out-of-plane displacement range while requiring large voltages. We demonstrate for the first time, to the best of our knowledge, a movable metasurface lens actuated by integrated thin-film PZT MEMS, which has the advantage of offering large displacements at low voltages. An out-of-plane displacement of a metasurface in the range of 7.2 μ m is demonstrated under a voltage application of 23 V. This is roughly twice the displacement at a quarter of the voltage of state of the art electrostatic out-of-plane actuation of metasurfaces. Using this tunability, we demonstrate a varifocal lens doublet with a focal shift of the order of 250 μ m at the wavelength 1.55 μm. The thin-film PZT MEMS-metasurface is a…

Providing physical layer security at the lowest network layer in fiber-optic communication systems is a technical challenge worldwide. Here, we propose and experimentally demonstrate a pure hardware optical encryption scheme based on temporal spreading and self-feedback phase encryption for high-speed and long-distance physical-layer secure optical communication. A record high bit-rate-distance product of 6400 Gb/s km is successfully achieved by the secure transmission of a 32 Gb/s on-off-keying modulated confidential signal over a 200 km optical fiber link. The demonstrated scheme is fully compatible with conventional optical transmission systems and can be operated in a pluggable manner, which may pave a new path to ultra-high-speed physical-layer secure optical communication in the future.

Hyperfine wind structure detection is important for aerodynamic and aviation safety. Pulse coherent Doppler wind LIDAR (PCDWL) is a widespread wind remote sensing method with tunable spatial and temporal resolutions. However, meter scale and sub-second resolution are still challenging for PCDWL. This is because of the constraints among short laser pulse duration, spectral broadening, detection accuracy, and real-time processing. In this Letter, to further improve the spatial and temporal resolution of PCDWL, we optimize the optical design of a nanosecond fiber laser and telescope and adopt a new, to the best of our knowledge, algorithm called the even-order derivative peak sharpening technique. During the experiment, all-fiber PCDWL with spatial and temporal resolutions of 3 m and 0.1 s, respectively, is demonstrated. Two-day continuous observation of the wakes of the Chinese high-speed train shows detailed hyperfine wind structures. This is similar to a computational fluid dynamics simulation.

In this Letter, we present a Stokes imaging-based method to restore objects and enhance image contrast in turbid water. In the system, a light source illuminates the objects with two orthometric polarization states; based on a new Stokes decomposition model, the recorded images are converted to Stokes maps and subsequently restored to a clear image, free of reflections and scattered lights. A mathematical model has been developed to explain the Stokes decomposition and how the undesired reflections and scattered lights are rejected. Imaging experiments have been devised and performed on different objects, e.g., metals and plastics, under different turbidities. The results demonstrate enhanced image quality and capability to distinguish polarization differences. This new, to the best of our knowledge, method can be readily applied to practical underwater object detection and potentially realize clear vision in other scattering media.

Volumetric additive manufacturing (VAM) enables rapid printing into a wide range of materials, offering significant advantages over other printing technologies, with a lack of inherent layering of particular note. However, VAM suffers from striations, similar in appearance to layers, and similarly limiting applications due to mechanical and refractive index inhomogeneity, surface roughness, etc. We hypothesize that these striations are caused by a self-written waveguide effect, driven by the gelation material nonlinearity upon which VAM relies, and that they are not a direct recording of non-uniform patterning beams. We demonstrate a simple and effective method of mitigating striations via a uniform optical exposure added to the end of any VAM printing process. We show this step to additionally shorten the period from initial gelation to print completion, mitigating the problem of partially gelled parts sinking before print completion, and expanding the range of resins printable in any VAM printer.

Closed-form, analytical expressions for higher-order, multi-charged spatiotemporal optical vortices (STOVs) propagating in free space or non-dispersive media are provided. We consider two relevant and complementary situations where the multi-charged STOV spreads freely, and where it is focused. Previously reported multi-charged STOV breakup upon spreading is merely an effect of diffraction, not an instability effect. The focused STOV reverses the sign of its topological charge upon passage through the focus. While the spreading STOV carries transverse orbital angular momentum, the ideally focused STOV from the far field does not.

We propose and experimentally study a coherent optical secure transmission system based on one dual-polarization in-phase and quadrature modulator (IQM). One beam of the polarized light is used to generate broadband chaos by configuring a nonlinear opto-electronic oscillator while the other beam carries the encrypted signal. The encrypted signal is obtained through sequential encryption of the analog and digital chaos. The mutual mask of the hybrid chaotic signals can effectively enhance the security performance. Moreover, by varying the encryption depth of analog and digital vectors, the transmission performance can be flexibly adjusted. A commercial dual-polarization IQM could simultaneously generate a chaotic signal and a load message, which provides a high-integration solution. A fast independent component analysis (ICA) algorithm is adopted to compensate for the rotation of state of polarization (RSOP). 60 Gb/s encrypted quadrature phase shift keying (QPSK) signal transmission over 100 km single-mode fiber is realized, and the decrypted bit error rate (BER) performance is below…

We experimentally demonstrate two types of programmable, low-threshold, optically controlled nonlinear activation functions, which are challenging to realize in photonic neural networks (PNNs). These devices rely on on-chip integrated Ge-Si photoelectric detectors and silicon electro-optical switches, and they generate rectified linear unit (ReLU) or sigmoid functions with arbitrary slopes without additional electrical processing. Both devices function at an extremely low threshold of 0.2 mW. The embedding of these nonlinear activation functions into convolutional neural networks facilitates the attainment of high inference accuracies of up to 95% when applied to Modified National Institute of Standards and Technology (MNIST) handwritten digit-classification tasks. The devices are suitable for low-power PNNs with an arbitrary number of propagation layers in photonic-computing chips.

Two-stage multipass-cell compression of a fiber–chirped-pulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107 W average power, 1.07 mJ pulses at 100 kHz centered at 1030 nm with excellent spatial beam quality ( M 2 = 1.1, Strehl ratio S = 0.98), pointing stability (2.3 µrad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360 mrad in the frequency range from 10 Hz to 50 kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation.

The advent of imagers with integrated linear polarization selectivity opens new opportunities for researchers interested in the polarization properties of biological tissues. In this Letter, we explore the mathematical framework necessary to obtain common parameters of interest: azimuth; retardance; and depolarization with reduced Mueller matrices that can be measured with the new instrumentation. We show that in the case of acquisition close to the tissue normal, simple algebraic analysis of the reduced form of the Mueller matrix yields results very close to those obtained with more complex decomposition algorithms applied to a complete Mueller matrix.

We report a high-power single-mode mid-infrared (MIR) pulse delivery system via anti-resonant hollow-core fiber (HCF) with a record delivery distance of 108 m. Near-diffraction-limited MIR light was transmitted by HCFs at wavelengths of 3.12–3.58 µm using a tunable optical parametric oscillator (OPO) as the light source. The HCFs were purged beforehand with argon in order to remove or reduce loss due to parasitic gas absorption (HCl, CO 2 , etc.). The minimum fiber loss values were 0.05 and 0.24 dB/m at 3.4–3.6 µm and 4.5–4.6 µm, respectively, with the 4.5–4.6 µm loss figure representing, to the best of our knowledge, a new low loss record for a HCF in this spectral region. At a coupling efficiency of ∼70%, average powers of 592 mW and 133 mW were delivered through 5 m and 108 m of HCF, respectively. Assuming the 120-ps duration of the MIR pulses remained constant over the low-dispersion HCF (theoretical maximum: 0.4 ps/nm/km), the corresponding calculated peak powers were 4.9 kW and 1.1 kW.

A broadband source-driven resonant fiber-optic gyroscope (RFOG) can reduce coherence-related noise, thus achieving a better sensitivity with a much simpler configuration than the traditional system with a coherent source. Its detection sensitivity, however, is still limited by the excess relative intensity noise (RIN) of the broadband source. In this paper, the RIN error mechanism in this broadband source-driven RFOG is revealed and countermeasures are presented. We demonstrate that the use of a high-finesse fiber-optic ring resonator and a high-frequency modulation-demodulation technique can reduce the RIN-induced error. It is indicated that the optimal modulation parameters can provide a RIN-induced error reduction of 6.1 dB, allowing the broadband source-driven RFOG to operate near the shot-noise-limited theoretical sensitivity. With the optimal high-frequency modulation-demodulation technique, an angle random walk of 0.0013°/√h is achieved with a 200-m-long fiber-optic ring resonator of 7.6 cm diameter. This is the best result reported to date, to the best of our knowledge, for…

Phase shifters are key components of large-scale photonic integrated circuits. For the lithium niobate-on-insulator (LNOI) platform, thermo-optic phase shifters (TOPS) have emerged as a more stable and compact alternative to common electro-optic phase shifters (EOPSs), which are prone to anomalous behavior and drifting at low frequencies. Here, we model and experimentally characterize the influence of geometry on the performance of metal strip TOPSs. Compared to EOPSs, a 10-fold reduction of the voltage-length product is measured and bandwidths beyond 100 kHz are demonstrated, while keeping the footprint as low as 0.04 mm 2 . This shows the potential of TOPSs as small-scale building blocks for stable tuning and switching in LNOI photonic circuits.