The 50 most-cited articles in Optics Letters, 2019

Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10 dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03-0.1 dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7 dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently…

In this Letter, for the first time, to the best of our knowledge, we propose a digital holographic reconstruction method with a one-to-two deep learning framework (Y-Net). Perfectly fitting the holographic reconstruction process, the Y-Net can simultaneously reconstruct intensity and phase information from a single digital hologram. As a result, this compact network with reduced parameters brings higher performance than typical network variants. The experimental results of the mouse phagocytes demonstrate the advantages of the proposed Y-Net.

The capability to overcome Kirchhoff's law of thermal radiation provides new opportunities in energy harvesting and thermal radiation control. Previously, design towards demonstrating such capability requires a magnetic field of 3 T, which is difficult to achieve in practice. In this work, we propose a nanophotonic design that can achieve such capability with a far more modest magnetic field of 0.3 Tesla, a level that can be achieved with permanent magnets. Our design uses guided resonance in low-loss dielectric gratings sitting on a magneto-optical material, which provides significant enhancement on the sensitivity to the external magnetic field.

We propose and demonstrate a novel physical, secure high-speed optical communication scheme based on synchronous chaotic spectral phase encryption (CSPE) and decryption (CSPD). The CSPE is performed by a module composed of two dispersion components and one phase modulator (PM) between them, and the CSPD is carried out by a twin module with reverse dispersions and inverse PM driving signals. The PM driving signals of the CSPE and CSPD modules are privately synchronized chaotic signals that are independently generated by local external-cavity semiconductor lasers subject to common injection. The numerical results indicate that with the CSPE, the original message can be encrypted as a noise-like signal, and the timing clock of the original message is efficiently hidden in the encrypted signal. Based on the private synchronization of the chaotic PM driving signals, only the legal receiver can decrypt the message correctly, while the eavesdropper is not able to intercept a useful message. Moreover, the proposed scheme can also support secure symmetric bidirectional high-speed WDM…

of our femtosecond pump source with a 30-dB signal-to-noise ratio.

We demonstrate an approach for automated Kerr comb generation in the normal group-velocity dispersion (GVD) regime. Using a coupled-ring geometry in silicon nitride, we precisely control the wavelength location and splitting strength of avoided mode crossings to generate low-noise frequency combs with pump-to-comb conversion efficiencies of up to 41%, which is the highest reported to date for normal-GVD Kerr combs. Our technique enables on-demand generation of a high-power comb source for applications such as wavelength-division multiplexing in optical communications.

An ultra-high sensitive light-induced thermoelastic spectroscopy (LITES) sensor based on a resonant high Q-factor quartz turning fork (QTF) and a Herriot multipass cell was demonstrated for the first time, to the best of our knowledge. The performance of LITES and widely used tunable diode laser absorption spectroscopy (TDLAS) were experimentally investigated and compared at the same conditions. Carbon monoxide (CO) was chosen as the analyte to verify the reported sensors’ performance. With a minimum detection limit (MDL) of 470 ppb for 60 ms integration time and a noise equivalent absorption (NEA) coefficient of 2.0×10−7 cm−1 Hz−1/2, and a MDL of 17 ppb with an optimum integration time of 800 s, the reported LITES sensor showed a superior sensing capability compared with a TDLAS sensor and a conventional quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor.

We numerically solve a generalized nonlinear Schrödinger equation and find a family of pure-quartic solitons (PQSs), existing through a balance of positive Kerr nonlinearity and negative quartic dispersion. These solitons have oscillatory tails, which can be understood analytically from the properties of linear waves with quartic dispersion. By computing the linear eigenspectrum of the solitons, we show that they are stable, but that they possess a nontrivial internal mode close to the radiation continuum. We also demonstrate evolution into a PQS from Gaussian initial conditions. The energy-width scaling of PQSs differs strongly from that for conventional solitons, opening up possibilities for PQS lasers.

An optical diffractive neural network (DNN) can be implemented with a cascaded phase mask architecture. Like an optical computer, the system can perform machine learning tasks such as number digit recognition in an all-optical manner. However, the system can work only under coherent light illumination, and the precision requirement in practical experiments is quite high. This Letter proposes an optical machine learning framework based on single-pixel imaging (MLSPI). The MLSPI system can perform the same linear pattern recognition task as DNN. Furthermore, it can work under incoherent lighting conditions, has lower experimental complexity, and can be easily programmable.

fiber is pumped in-band using an erbium-doped fiber laser at 2.83 μm made in-house and connected through a fusion splice. Two fiber Bragg gratings directly written in the Dy-doped fiber form the 3.24 μm laser cavity to provide a spectrally controlled laser output. This substantial increase of output power in the 3.0-3.3 μm spectral range could open new possibilities for applications in spectroscopy and advanced manufacturing.

In this Letter, we report on a simple, but highly sensitive sensor based on two intrinsic Fabry-Perot interferometers (FPIs) inscribed in a standard single-mode optical fiber. A brief theoretical study on the Vernier effect is presented, in which a simulation of the sensitivity magnification factor dependence on the FPI's length is performed. Based on the simulation results, the FPIs were fabricated using a custom micromachining setup that integrates a near-infrared femtosecond laser and a motorized XYZ platform. Using the Vernier effect, sensitivities of 145 pm/με and 927 pm/°C were obtained for strain and temperature, respectively. The sensor's performance combined with its versatile and customizable configuration allows real-time and in situ strain and temperature monitoring under harsh environments.

We demonstrate octave-spanning supercontinuum generation in unpoled lithium niobate waveguides, which are engineered to possess anomalous dispersion and pumped by a turn-key femtosecond laser centered at 1560 nm. Tunable dispersive waves and strong phase-matched second-harmonic generation are both observed by controlling the widths of the waveguides. The major features of the experimental spectra are reproduced by numerical modeling of the generalized nonlinear Schrödinger equation, which can be used to guide waveguide designs for tailoring the supercontinuum spectrum. Our results identify a path to a simple and integrable supercontinuum source in lithium niobate nanophotonic platform and will enable new capabilities in precision frequency metrology.

, a 1.78 pm/V resonance tunability near 1550 nm wavelength is demonstrated.

Chaotic optical communications were originally proposed to provide high-level physical layer security for optical communications. Limited by the difficulty of chaos synchronization, there has been little experimental demonstration of high-speed chaotic optical communications, and point to multipoint chaotic optical networking is hard to implement. Here, we propose a method to overcome the current limitations. By using a deep-learning-based scheme to learn the complex nonlinear model of the chaotic transmitter, wideband chaos synchronization can be realized in the digital domain. Therefore, the chaotic receiver can be significantly simplified while still guaranteeing security. A successful transmission of 32 Gb/s messages hidden in a wideband chaotic optical carrier was experimentally demonstrated over a 20 km fiber link. We believe the proposed deep-learning-based chaos synchronization method will enable a new direction for further development of high-speed chaotic optical communication systems and networks.

Here we study the possibility to achieve polarization-independent optical absorption in stacked anisotropic 2D material nanostructures (NSs). Focusing on black phosphorus, we demonstrate that by crossly stacking even-layered NSs, surface plasmons resonant in the two lattice directions are complementary excited, leading to polarization-independent absorption at any layer distance. This property is numerically validated using full electromagnetic simulations and theoretically predicted by a two-particle model. Our proposal can open up the possibility of anisotropic 2D materials to develop polarization-independent plasmon devices such as sensors and absorbers.

We demonstrate ultra-narrow linewidth fixed wavelength hybrid lasers composing a semiconductor gain chip and extended silicon nitride Bragg grating. Fabricated ultra-low κ Bragg gratings provide a narrow bandwidth and high side-lobe suppression ratio. A single-wavelength 1544 nm hybrid extended-distributed Bragg reflector laser with 24 mW output power and a Lorentzian linewidth of 320 Hz is demonstrated, providing a high-performance light source for on- and off-chip applications.

A multifocal metalens is proposed based on the optical metasurface consisting of subwavelength gratings etched on silver film. The larger transmission of grating makes the proposed metalens have high focusing efficiency, and the exquisite design of the metasurface enables the metalens to focus the light at multiple spots with the controllable intensity ratio. The intensity ratio of focal spots is controlled by adjusting grating structures. The numerical simulations give the good presentation, and the experiment measurement provides the favorable verification for the performance of the proposed multifocal metalens in light focusing and beam splitting. The advantages of the multifocal metalens, including simple design, compact structure, high efficiency, and the controllable focusing, are a benefit to its applications in optical integration and micromanipulation.

Perfect (optical) vortex (PV) beams are fields whose radial profile is mooted to be independent of orbital angular momentum (OAM). To date, the best experimental approximation of these modes is obtained from passing Bessel-Gaussian beams through a Fourier lens. However, the OAM-dependent width of these quasi-PVs is not precisely known and is often understated. We address this here by deriving and experimentally confirming an explicit analytic expression for the second moment radius of quasi-PVs. We show that this width scales in proportion to ℓ in the best case, the same as most "regular" vortex modes, albeit with a much smaller proportionality constant. This Letter will be of interest to the large community who seek to use such structured light fields in various applications, including optical trapping, tweezing, and communications.

Octave-spanning frequency combs have been successfully demonstrated in Kerr nonlinear microresonators. These microcombs rely on both engineered dispersion, to enable generation of frequency components across the octave, and on engineered coupling, to efficiently extract the generated light into an access waveguide while maintaining a close to critically coupled pump. The latter is challenging, as the spatial overlap between the access waveguide and the ring modes decays with frequency. This leads to strong coupling variation across the octave, with poor extraction at short wavelengths. Here, we investigate how a waveguide wrapped around a portion of the resonator, in a pulley scheme, can improve the extraction of octave-spanning microcombs, in particular at short wavelengths. We use the coupled-mode theory to predict the performance of the pulley couplers and demonstrate good agreement with experimental measurements. Using an optimal pulley coupling design, we demonstrate a 20 dB improvement in extraction at short wavelengths compared to straight waveguide coupling.

The existence and stability of stable bright solitons in one-dimensional (1D) fractional media with a spatially periodical modulated Kerr nonlinearity (nonlinear lattice), supported by the recently introduced nonlinear fractional Schrödinger equation, are demonstrated by means of the linear-stability analysis and in direct numerical simulations. Both 1D fundamental and multipole solitons (in forms of dipole and tripole ones) are found, which occupy one or three cells of the nonlinear lattice, respectively, depending on the soliton’s power. We find that the profiles of the predicted soliton families are impacted intensely by the Lévy index α, and so are their stability. The soliton families are stable if α exceeds a threshold value, below which the balance between fractional-order diffraction and the spatially modulated focusing nonlinearity will be broken.

Silica optical fiber was drawn from a three-dimensional printed preform. Both single mode and multimode fibers are reported. The results demonstrate additive manufacturing of glass optical fibers and its potential to disrupt traditional optical fiber fabrication. It opens up fiber designs for novel applications hitherto not possible.

Chip-based soliton frequency combs have been demonstrated on various material platforms, offering broadband, mutually coherent, and equally spaced frequency lines desired for many applications. Lithium niobate (LN), possessing both second- and third-order optical nonlinearities, as well as integrability on insulating substrates, has emerged as a novel source for microcomb generation and controlling. Here we demonstrate mode-locked soliton microcombs generated around 2 μm in a high-Q z-cut LN microring resonator. The intracavity photorefractive effect is found to be still dominant over the thermal effect in the 2 μm region, which facilitates direct accessing soliton states in the red-detuned regime, as reported in the telecom band. We also find that intracavity stimulated Raman scattering is greatly suppressed when moving the pump wavelength from the telecom band to 2 μm, thus alleviating Raman-Kerr comb competition. This Letter expands mode-locked LN microcombs to 2 μm, and could enable a variety of potential applications based on LN nanophotonic platform.

We report on silicon ablation with a 20 W GHz amplified femtosecond laser source. This novel laser delivers burst energies up to 400 μJ, providing flexible intra-pulse repetition rates of 0.88 or 3.52 GHz, up to 200 pulses with ∼350 fs pulse duration. High-efficiency, high-quality ablation can be achieved through optimally determining the number of pulses, intra-pulse repetition, and average pulse energy within a burst. Due to such optimization, we demonstrate a specific ablation rate of 2.5 mm3/min/W with a burst containing 200 pulses at 0.88 GHz, which is the highest one reported so far for fs laser ablation, to the best of our knowledge. GHz ablation is sensitive to the selection of laser parameters. We conceptually discuss the contributions of the pulses within a burst to heat-accumulation-based incubation and material ablation.

Distributed acoustic sensing (DAS) technology based on Rayleigh backscattering is experiencing a rapid development and leading itself into wider applications because of the unique capability of measuring sound and vibrations at all points along the sensing fiber. However, most implementations of DAS provide the position of detected sources as a function of distance within the one-dimensional axial space along the sensing fiber. A DAS system with the capability of two-dimensional (2D) and three-dimensional (3D) acoustic source localization in air is demonstrated that uses array signal processing to deal with the spatial correlation of the information measured by optical fiber. Preliminary work has demonstrated 2D acoustic source localization for multi-targets with a narrowband signal source of the same frequency and 3D position for a moving narrowband acoustic source. The results establish a new method which opens up new areas of applications of DAS such as location and identification for static, dynamic, and multiple targets in air or water.

Two-dimensional materials such as hexagonal boron nitride (h-BN), graphene, and transition metal dichalcogenides have drawn great attention in various fields of photonics and electronics. Among them, h-BN has recently emerged as a promising material platform to study integrated quantum photonics due to its ultrabright quantum light emission capabilities. However, the fundamental optical properties of h-BN have not yet been investigated in the visible and near-infrared (NIR) spectrum thoroughly. In this Letter, we report the refractive indices of h-BN thin films in the visible to NIR range. To the best of our knowledge, this is the first experimental observation of h-BN birefringence. Accurate parameters of refractive indices enable more precise design of h-BN-based photonic devices in the integrated photonics platforms.

We analyzed the propagation characteristics of the intensity of a vortex beam after it passes through a twisting phase. It was found that the doughnut-like intensity pattern of a vortex beam would separate into several bright and dark fringes. The number of dark fringes between two bright spots is equal to the topological charge (TC) of the vortex beam. Meanwhile, the intensity pattern varies with the sign of the TC. Based on this property, we proposed a convenient method to measure the TC of a vortex beam by observing its intensity pattern after passing through a twisting phase. This detection technique is mainly based on the use of a twisting phase, and the effect of parameters in the twisting phase is demonstrated and clearly studied. By choosing proper parameters in the twisting phase, the separation speed of a vortex beam's intensity could be controlled in the experiment. The experimental results are in good agreement with the theoretical analyses.

We demonstrate the generation of few-cycle deep ultraviolet pulses via frequency upconversion of 5-fs near-infrared pulses in argon using a laser-fabricated gas cell. The measured spectrum extends from 210 to 340 nm, corresponding to a transform-limited pulse duration of 1.45 fs. We extract from a dispersion-free second-order cross-correlation measurement a pulse duration of 1.9 fs, defining a new record in the deep ultraviolet spectral range.

Carbon dioxide ( C O 2 ) is a gas vital for life on Earth. It is also a waste product of human activities and is widely used in agriculture and industry. Its accurate sensing is therefore of great interest. Optical sensors exploiting the mid-infrared light absorption of C O 2 provide high selectivity, but their large size and high cost limit their use. In this Letter, we demonstrate C O 2 gas sensing at 4.2 µm wavelength using an integrated silicon waveguide, featuring a sensitivity to C O 2 of 44% that of free-space sensing. The suspended waveguide is fabricated on a silicon-on-insulator substrate by a single-lithography-step process, and we route it into a mid-infrared photonic circuit for on-chip-referenced gas measurements. Its demonstrated performance and its simple and scalable fabrication make our waveguide ideal for integration in miniaturized C O 2 sensors for distributed environmental monitoring, personal safety, and medical and high-volume consumer applications.

We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6-18 μm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 μm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-μm pulses in GaSe.

We present a state-of-the-art compact high-energy mid-infrared (mid-IR) laser system for TW-level eight-cycle pulses at 7 μm. This system consists of an Er:Tm:Ho:fiber MOPA which serves as the seeder for a ZGP-based optical parametric chirped pulse amplification (OPCPA) chain, in addition to a Ho:YLF amplifier which is Tm:fiber pumped. Featuring all-optical synchronization, the system delivers 260 mJ pump energy at 2052 nm and 16 ps duration at 100 Hz with a stability of 0.8% rms over 20 min. We show that chirp inversion in the OPCPA chain leads to excellent energy extraction and aids in compression of the 7 μm pulses to eight optical cycles (188 fs) in bulk BaF2 with 93.5% efficiency. Using 21.7 mJ of the available pump energy, we generate 0.75 mJ energy pulses at 7 μm due to increased efficiency with a chirp inversion scheme. The pulse quality of the system’s output is shown by generating high harmonics in ZnSe which span up to harmonic order 13 with excellent contrast. The combination of the passive carrier-envelope phase stable mid-IR seed pulses and the high-energy 2052 nm…

:fluoride fiber laser emitting around 3.1 μm based on the nonlinear polarization evolution technique in a ring configuration, using in-band pumping at 2.8 μm. Transform-limited and self-starting mode-locked pulses as short as 828 fs with a center wavelength around 3.1 μm and repetition rates up to 60 MHz are obtained. In the single-pulse regime, a maximum average output power of 204 mW is measured, corresponding to a peak power of 4.2 kW and a pulse energy of 4.8 nJ. This first demonstration, to the best of our knowledge, of a femtosecond mode-locked fiber laser emitting directly beyond 3 μm paves the way for frequency comb synthesis in the molecular fingerprint region.

We report on entering a new era of mid-IR femtosecond lasers based on amplification in a relatively new gain chalcogenide medium, Fe:ZnSe. Our hybrid all-solid-state laser system is based on direct pulse amplification of femtosecond seed from three-stage AGS-based-optical parametric amplification (OPA) in a Fe:ZnSe laser crystal optically pumped by a Cr:Yb:Ho:YSGG Q-switched nanosecond laser. The development of the pump source with output energy up to 90 mJ operating at a 10 Hz repetition rate regime and highly efficient grating compressor (80%) provides 3.5-mJ 150-fs femtosecond pulses centered at 4.4 μm. Diode-pumped Er:YAG/Er:YLF lasers make it possible to increase the beam quality and repetition rate of the proposed laser system up to 100 Hz. Focusing such a laser radiation into the ∼3λ beam diameter allows us to reach a focus laser intensity up to 1016 W/cm2 which is only an order of magnitude lower than a relativistic intensity of 1017 W/cm2 and enough to drive strong nonlinear optics in mid-IR. We show as a proof-of-principle experiment the generation of four-octave spanning…

The interaction of subcycle pulses with quantum systems is considered when the pulse duration becomes much smaller than the timescales of electron oscillations. We show analytically that the interaction process in this case is governed by the electric pulse area. The efficient nonresonant excitation of quantum systems by subcycle pulses with a high degree of unipolarity is demonstrated. The results are confirmed by direct numerical solution of multilevel Bloch equations.

Optical twists are the rotation of light structures along the beam axis, which can be caused by the quadratic twist phase of a partially coherent field. Here, we introduce a new class of partially coherent beams whose spectral density and degree of coherence tend to rotate during propagation. Unlike the previously reported twisted Gaussian Schell-model beams, this family of rotating beams is constructed without the framework of rotationally invariant cross-spectral density functions. Thus, these beams have different underlying physics and exhibit distinctive twist effects. It is shown that such beams can undergo a twist of more than 90 deg, providing larger degrees of freedom for flexibly tailoring the beam twist. Our results may pave the way toward synthesizing rotating beams for applications in optics and, in particular, inspire further studies in the field of twist phase proposed 25 years ago.

Laser remote sensing represents a powerful tool that enables the accurate measurement of the speed of moving targets. Crucially, most sensing techniques are two-dimensional (2D) and only enable direct determination of the speed along the line of sight. Here we put forward a novel three-dimensional technique that enables the direct and simultaneous measurement of both the longitudinal and angular speed of cooperative targets. This technique is based on the use of complex vector light beams, whose polarization and spatial degree of freedom are coupled in a non-separable way. We present evidence of our technique by performing a proof-of-principle experiment.

In this Letter, to the best of our knowledge, we report a new method to generate uniform large-scale optical focus arrays (LOFAs). By identifying and removing undesired phase rotation in the iterative Fourier transform algorithm (IFTA), our approach rapidly produces computer-generated holograms of highly uniform LOFAs. The new algorithm also shows a faster compensation of system-induced LOFA intensity inhomogeneity than the conventional IFTA. After only three adaptive correction steps, we demonstrate LOFAs consisting of O(103) optical foci with an intensity uniformity greater than 98%.

Solitons, ubiquitous in nonlinear sciences, are wavepackets which maintain their characteristic shape upon propagation. In optics, they have been observed and extensively studied in optical fibers. The spontaneous generation of a dissipative Kerr soliton (DKS) train in an optical microresonator pumped with continuous wave (CW) coherent light has placed solitons at the heart of optical frequency comb research in recent years. The commonly observed soliton has a "sech"-shaped envelope resulting from resonator cubic nonlinearity balanced by its quadratic anomalous group velocity dispersion (GVD). Here we exploit the Lagrangian variational method to show that CW pumping of a Kerr resonator featuring quartic GVD forms a pure quartic soliton (PQS) with Gaussian envelope. We find analytical expressions for pulse parameters in terms of experimentally relevant quantities and derive an area theorem. Analytical predictions are validated with extensive numerical simulations and apply also to fiber-based and spatial Kerr resonators. Broader bandwidth with flatter spectral envelope of a PQS,…

An integral imaging-based 2D/3D convertible display system is proposed by using a lens-array holographic optical element (LAHOE), a polymer dispersed liquid crystal (PDLC) film, and a projector. The LAHOE is closely attached to the PDLC film to constitute a projection screen. The LAHOE is used to realize integral imaging 3D display. When the PDLC film with an applied voltage is in the transparent state, the projector projects a Bragg matched 3D image, and the display system works in 3D mode. When the PDLC film without an applied voltage is in the scattering state, the projector projects a 2D image, and the display system works in 2D mode. A prototype of the integral imaging-based 2D/3D convertible display is developed, and it provides 2D/3D convertible images properly.

Metal–organic framework (MOF) glasses are characterized by the possession of both inorganic and organic components, linked in a continuous network structure by coordination bonds. To the best of our knowledge, the optical properties of MOF glasses have not been reported until now. In this work, we prepared a transparent bubble-free bulk MOF glass, namely, the ZIF-62 glass (ZnIm2−xbImx), using our newly developed hot-pressing technique, and measured its optical properties. The ZIF-62 glass has a high transmittance (up to 90%) in the visible and near-infrared wavelength ranges, which is comparable to that of many oxide glasses. Using the Becke line nD method, we found that the ZIF-62 glass exhibits a refractive index (1.56) similar to most inorganic glasses, though a lower Abbe number (∼31).

Quantum key distribution (QKD) allows distant parties to exchange cryptographic keys with unconditional security by encoding information on the degrees of freedom of photons. Polarization encoding has been extensively used for QKD along free-space, optical fiber, and satellite links. However, the polarization encoders used in such implementations are unstable, expensive, and complex and can even exhibit side channels that undermine the security of the protocol. Here we propose a self-compensating polarization encoder based on a lithium niobate phase modulator inside a Sagnac interferometer and implement it using only commercial off-the-shelf (COTS) components. Our polarization encoder combines a simple design and high stability reaching an intrinsic quantum bit error rate as low as 0.2%. Since realization is possible from the 800 to the 1550 nm band using COTS devices, our polarization modulator is a promising solution for free-space, fiber, and satellite-based QKD.

In this Letter, we introduce a new class of abruptly autofocued and rotated circular chirp Pearcey Gaussian vortex beams (AARCCPGVBs) which tend to abruptly autofocused circular chirp Pearcey vortex beams or chirp Gaussian vortex beams by adjusting the spatial distribution factors. Different from other rotated beams [Opt. Lett.31, 694 (2006) OPLEDP0146-959210.1364/OL.31.000694 and Opt. Lett.31, 2199 (2006)OPLEDP0146-959210.1364/OL.31.002199], the AARCCPGVBs are autofocused abruptly, maintain a low rotating speed before the focal point, and rotate abruptly and quickly in the focal point. Further, the position of the focal point in the propagating direction can also be controlled by adjusting the chirp factor.

By arranging two pairs of high-index dielectric disks into a unit cell, a novel, to the best of our knowledge, terahertz metamaterials sensor integrated with a microfluidic channel is proposed. With the introduction of a new way of symmetry breaking in the unit cell, the strong toroidal dipole response with ultrahigh-Q is excited and investigated, which is related to the existence of the trapped mode. The simulation results show that the calculated quality factor and the corresponding figure of merit (FoM) of this sensor can reach 3189 and 515, respectively. These advantages allow for the proposed structure to have potential applications in high-performance gases, liquids, and biological materials sensing.

The requirement of a large number of samplings limits the performance of ghost imaging for moving objects. Conventionally, tracking and imaging of the moving objects are done independently; thus, sequential clear images of the moving target during its evolution are required. In this Letter, we propose to obtain the displacement of the object via cross correlation between sequential unclear rough images. Then, a high-quality image of the moving object can be reconstructed gradually during its evolution. Our method works well for translating and rotating objects.

A hybrid Tamm plasmonic system is proposed to investigate light manipulation at near-infrared frequency. The numerical results reveal that two remarkable absorption peaks are generated due to the different types of resonant modes excited in the structure, which can be well explained theoretically by guided-mode resonance (GMR) and Tamm plasmon polaritons. It is found that the electromagnetic energy can be easily trapped in different parts of the structure. More importantly, strong interaction between the two modes can be achieved by adjusting the structure period or incident angle, resulting in obvious mode hybridization and exhibiting unique energy-transfer characteristics. In addition, the active modulation of GMR-based absorption can be controlled in a continuous type by tuning the polarization angle or in a jump type by adjusting the chemical potential of graphene. This work should be useful for developing many high-performance optoelectronic devices, including sensors, modulators, detectors, etc.

Diffraction puts a fundamental limit on the distance over which a light beam can remain focused. For about 30 years, several techniques to overcome this limit have been demonstrated. Here, we propose a reflective optics, namely, the axiparabola, that allows to extend the production of "diffraction-free" beams to high-peak-power and broadband laser pulses. We first describe the properties of this aspheric optics. We then analyze and compare its performances in numerical simulations and in experiments. Finally, we use it to produce a plasma waveguide that can guide an intense laser pulse over 10 millimeters.

Accurately and quickly obtaining the three-dimensional (3D) shape information of objects has become increasingly important in various scientific fields. However, simultaneously achieving the high-resolution and high-speed 3D shape measurement of unknown objects remains challenging in practice. In this Letter, we propose a novel variant shifting-phase method for 3D optical measurement. Based on a digital fringe projection system, the method performs a point-wise high-resolution measurement of objects by projecting only four intensity-coded patterns. We can retrieve the wrapped phases and their corresponding fringe orders simultaneously from these four patterns, and do not require any pre-acquired information of the object. The experimental results successfully demonstrate the effectiveness of the easy-to-operate method.

Due to the latest progress in image sensor manufacturing technology, the emergence of a sensor equipped with an RGGB Bayer filter and a directional polarizing filter has brought significant advantages to computer vision tasks where RGB and polarization information is required. In this regard, joint chromatic and polarimetric image demosaicing is indispensable. However, as a new type of array pattern, there is no dedicated method for this challenging task. In this Letter, we collect, to the best of our knowledge, the first chromatic-polarization dataset and propose a chromatic-polarization demosaicing network (CPDNet) to address this joint chromatic and polarimetric image demosaicing issue. The proposed CPDNet is composed of the residual block and the multi-task structure with the costumed loss function. The experimental results show that our proposed methods are capable of faithfully recovering full 12-channel chromatic and polarimetric information for each pixel from a single mosaic image in terms of quantitative measures and visual quality.

In this Letter, we investigate and compare the gain and noise figure characteristics of bismuth (Bi)-doped fiber amplifiers configured in both single and double signal pass implementations. A maximum gain of 25 dB and a noise figure of 4 dB is measured at 1360 nm in the single pass configuration for -23 dBm input signal power, whereas in the double pass configuration the gain of the amplifier is improved significantly by 14 dB allowing us to achieve a gain of 39 dB with a noise figure of 5 dB. To the best of our knowledge, this is the highest gain reported to date using Bi-doped fiber as a gain medium. Furthermore, we also study the gain and noise figure dependency on pump power, signal power, and pump wavelength for the double pass amplifier configuration. We observed similar gain and noise figure performance in the double pass configuration to that of the single pass configuration but with the benefit of less pump power and a shorter length of the Bi-doped fiber.

Fluorescence molecular tomography (FMT) is a promising and noninvasive in vivo functional imaging modality. However, the quality of FMT reconstruction is limited by the simplified linear model of photon propagation. Here, an end-to-end three-dimensional deep encoder-decoder (3D-En-Decoder) network is proposed to improve the quality of FMT reconstruction. It directly establishes the nonlinear mapping relationship between the inside fluorescent source distribution and the boundary fluorescent signal distribution. Thus the reconstruction inaccuracy caused by the simplified linear model can be fundamentally avoided by the proposed network. Both numerical simulations and phantom experiments were carried out, and the results demonstrated that the 3D-En-Decoder network can greatly improve image quality and significantly reduce reconstruction time compared with conventional methods.

We demonstrate a 102 W middle infrared ZnGeP2 (ZGP) optical parametric amplifier (OPA) pumped by a 2097-nm Q-switched Ho:YAG laser at a pulse repetition frequency of 10 kHz. The seed middle infrared laser was produced by a ZGP optical parametric oscillator. Its average power was 28.4 W pumped by a 50 W 2097-nm laser. By thermal lens compensation, the beam factor M2 reduced from 3.1 to 2.1. When the incident Ho pump power was 120 W, the middle infrared ZGP OPA yielded the maximum average output power of 102 W and slope efficiency of 61.7%. The overall optical conversion efficiency of 60% from Ho to middle infrared was obtained for the whole middle infrared laser system. In addition, at the maximum average output power, the beam quality factors of the middle infrared ZGP OPA were measured to be about 2.7 and 2.8 for horizontal and vertical directions, respectively.