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One of the main problems in mobile communication systems is long-range impassable links or some specific atmospheric conditions under which Radio Frequency (RF) connection becomes easily disrupted. One way to solve this problem is by consuming more power or adding processing complexity, but a small user mobile phone cannot deserve more complexity or power supply. Another way is to use an access point that amplifies received signal via short-range RF link and forwards it to via long-range Free Space Optical (FSO) link. In this paper, a novel multi-user multi-hop hybrid FSO / RF communication system is presented. This structure consists of two main parts. The first part establishes the connection between the mobile users and the source base station, and the second part establishes the connection between the source and the destination base stations. In the first part, mobile users want to connect to the source base station via a long-range link; therefore, an amplify and forward relay is used for communication establishment. In the second part, the source and the destination base stations are connected via a multi-hop hybrid parallel FSO / RF link with demodulating and forward relaying. The FSO link in moderate to strong and in saturated regimes is assumed at Gamma–Gamma and Negative Exponential atmospheric turbulence, respectively, also the effect of pointing error is considered, and RF link has Rayleigh fading. New closed-form exact and asymptotic expressions are derived for the Outage Probability and Bit Error Rate (BER) of the proposed structure. Derived expressions are verified with MATLAB simulations. The proposed structure has advantages of FSO, RF, relay-assisted, and multi-user systems at the same time. Results indicate that it has low dependence on the number of users and the number of relays. Therefore, it is suitable for areas with varying population and long-range links. This structure offers independent performance without additional power consumption, processing latency, and complexity. The innovations and contributions of this paper, which are first introduced in the multi-hop hybrid FSO / RF hybrid structure, include the novelty of the proposed structure, the presentation of new exact and asymptotic mathematical solutions, use of multi-user scheme in a multi-hop FSO / RF structure, BER analysis, using amplify and forward as well as decode and forward protocols, taking into account a wide range of atmospheric turbulence with the effect of pointing error, using opportunistic selection schemes at each relay.

The hybrid-order Poincare sphere (HPS) was proposed to describe high-order vector vortex beams. In this paper, we theoretically study the intensity and longitudinal component phase distribution of tightly focusing HPS beams within a homogeneous and isotropic medium. We analyse the phase and demonstrate the orbital angular momentum (OAM) state distribution of the longitudinal component. Arbitrary OAM states can be obtained by manipulating the order of HPS beams, which comprise two orthogonally circularly polarised vortex beams with topological charges m and n . Meanwhile, the OAM states of the longitudinal component are destroyed when m = − n ; otherwise, they split into two parts with topological charges m + 1 and n − 1 . The focus intensity distribution is petal-shaped if the two topological charges have opposite signs, where the number of intensity lobes in the focal plane is m − n + 2 . Moreover, the ratio of the maximum intensities of the longitudinal and total fields depends on the sign and value of the two topological charges, m and n . Finally, we investigate a generalised case of HPS beams, and demonstrate that the focal intensity and phase distribution are also influenced by the coefficients. The results obtained in this paper may facilitate a new application of HPS beams in various fields.

In order to improve the adaptability of spatial light modulator diffraction analysis method to the research object, Collins–Huygens integral is introduced into spatial light modulator (SLM) diffraction analysis. This paper takes two kinds of hollow vortex light waves as examples to discuss the practicability of the simulation method. When the spatial light modulator is used to control the light wave front, the method can greatly improve the adaptability of various types of wave front as simulation objects. In particular, the diffraction of two different types of “holograms” is analyzed, one is CGH that produces hypergeometric beams with extremely wide spatial spectrum, and the other is CGH that produces Laguerre—Gaussian beams with limited spatial bandwidth. In this paper, the phase extraction method in numerical simulation calculation is discussed in detail, and the diffraction characteristics of phase mask and amplitude mask, such as missing order phenomenon, are verified by the numerical method in this paper. Through the simulation, we find that the method proposed in this paper is an effective simulation analysis method. Its applications range from generating light beams with angular momentum to microscopes to trapping neutral atoms. The method is also extended to other beam shaping components using SLM.

Perfect channel estimation is a complex task with high power consumption and cost; in addition, requiring pilot transmission reduces the data rate. So, it is not favourable especially in mobile communication systems. The aim of this paper is to design (a new, low cost and low complexity) deep learning based channel estimator for free space optical (FSO) communication. In order to have a better understanding, this paper goes deeper through the problem, and presents different new deep learning based FSO systems, in which deep learning is used as detector, joint constellation shaper and detector, channel estimator, joint channel estimator and detector, joint constellation shaper and channel estimator and detector. For comparison with conventional systems, the outstanding QAM modulation, perfect channel estimation and maximum likelihood detection is applied. Considering wide range of atmospheric turbulences, from weak to strong by Gamma–Gamma model, symbol error rate performance of the proposed structures is investigated. Results indicate that the proposed deep learning based channel estimation technique, despite its less complexity, cost and power consumption provides close enough performance to the perfect channel estimation. It should be noted that the proposed structure does not need pilot sequence, hence, it has higher data rate than perfect channel estimation. The performance of the proposed deep learning based structures does not change with atmospheric turbulence variation. Furthermore, they are low cost, low complexity, with favourable performance. Accordingly, they could be good choices especially for mobile communication systems. Because the transceiver of these systems is a small mobile phone that should have low cost, complexity, and power consuming.

This paper has been withdrawn by the author due to a crucial sign error in equation 1., Comment: This paper has been withdrawn

Self-seeding schemes, consisting of two undulators with a monochromator in between, aim to reduce the bandwidth of SASE X-ray FELs. We recently proposed to use a new method of monochromatization exploiting a single crystal in Bragg transmission geometry for self-seeding in the hard X-ray range. The obvious and technically possible extension is to use such kind of monochromator setup with two (or more) crystals arranged in a series to spectrally filter the SASE radiation at two (or more) closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet (or multiplet) spectral lines. Exploitations of such mode of operation involve any situation where there is a large change in cross-section over a narrow wavelength range. In this paper we consider the simultaneous operation of the LCLS hard X-ray FEL at two closely spaced wavelengths. We present simulation results for the LCLS baseline, and we show that this method can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band, i.e. within 10 eV. An interesting aspect of the proposed scheme is a way of modulating the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating “mode-beat” component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. Thus, at saturation one obtains FEL-induced modulations of energy loss and energy spread in the electron bunch at optical frequency. These modulations can be converted into density modulation at the same optical frequency with the help of a weak chicane installed behind the baseline undulator. Powerful coherent radiation can then be generated with the help of an optical transition radiation (OTR) station, which have important applications. In this paper we briefly consider how the doublet structure of the XFEL generation spectra can be monitored by an optical spectrometer. Furthermore, the OTR coherent radiation pulse is naturally synchronized with the X-ray pulses, and can be used for timing the XFEL to high power conventional lasers with femtosecond accuracy for pump-probe applications.

In this paper, we address the issue of faster connection establishment in a large vertically stacked optical Banyan (VSOB) network. The best known global routing algorithm, which turns an N × N crosstalk-free VSOB network into a rearrangeably non-blocking one, has time complexity O ( N log N ). This is quite large compared to O ( N log N ) time complexity of a single plane banyan network, which is a self-routing network with very high blocking probability. For a large size of switching network this O ( N log N ) time complexity may result unacceptably long delay. Therefore, an optical network with very low blocking probability and O ( N log N ) time complexity will be useful. Previously proposed Plane Fixed Routing (PFR) algorithm has O (log N ) time complexity but results in higher than 2% blocking probability with zero-crosstalk constraint for a network as large as 4096 × 4096 at full load. In this paper, first we propose the pruning of VSOB networks that reduces the hardware cost by almost 30%. The networks can still use the PFR algorithm and results in the same blocking probability. However, we show that the blocking probability can be reduced dramatically while keeping the optimum time complexity O (log N ) by allowing only a small amount of crosstalk. Then, we propose a new kind of switching networks in which extra regular banyan planes have been added with the pruned VBOS (P-VSOB) networks. Necessary routing algorithms, namely, PFR_RS and PFR_LS show that this new switching network can reduce the blocking probability to very low value even with zero-crosstalk constraint while keeping the hardware cost 3almost the same as for P-VSOB networks. Both these algorithms also have time complexity O ( N log N ).

In this paper, we present a new approach for the exact calculation of band structure in one-dimensional periodic media, such as photonic crystals and superlattices, based on the recently reported differential transfer matrix method (DTMM). The media analyzed in this paper can have arbitrary profile of refractive index. We derive a closed form dispersion equation using differential transfer matrix formalism, and simplify it under the assumptions of even symmetry and real-valued wavenumber. We also show that under normal incidence both TE and TM modes must have the same band structure. Several numerical test cases are also studied and discussed.

The motion of an electromagnetic wave, through a classically-forbidden region, has recently attracted renewed interest because of its implication with regard to the theoretical and experimental problems of superluminality. From an experimental point of view, many papers provide an evidence of superluminality in different physical systems. Theoretically, the problem of a passage through a forbidden gap has been treated by considering plane waves at oblique incidence into a plane parallel layer of a medium with a refractive index smaller than the index of the surrounding medium, and also confined (Gaussian) beams, still at oblique incidence. In the present paper the case of a Bessel beam is examined, at normal incidence into the layer (Secs. II and III), in the scalar approximation (Sec. IV) and by developing also a vectorial treatment (Sec. V). Conclusions are reported in Sic. VI.

The paper presents a comprehensive analysis of the statistical properties of the radiation from a self-amplified spontaneous emission (SASE) free electron laser operating in linear and nonlinear mode. The investigation has been performed in a one-dimensional approximation assuming the electron pulse length to be much larger than a coherence length of the radiation. The following statistical properties of the SASE FEL radiation have been studied in detail: time and spectral field correlations, distribution of the fluctuations of the instantaneous radiation power, distribution of the energy in the electron bunch, distribution of the radiation energy after the monochromator installed at the FEL amplifier exit and radiation spectrum. The linear high gain limit is studied analytically. It is shown that the radiation from a SASE FEL operating in the linear regime possesses all the features corresponding to completely chaotic polarized radiation. A detailed study of statistical properties of the radiation from a SASE FEL operating in linear and nonlinear regime has been performed by means of time-dependent simulation codes. All numerical results presented in the paper have been calculated for the 70 nm SASE FEL at the TESLA Test Facility being under construction at DESY.

Traditional snapshot hyperspectral imaging systems generally require multiple refractive-optics-based elements to modulate light, resulting in bulky framework. In pursuit of a more compact form factor, a metasurface-based snapshot hyperspectral imaging system, which achieves joint optimization of metasurface and image processing, is proposed in this paper. The unprecedented light manipulation capabilities of metasurfaces are used in conjunction with neural networks to encode and decode light fields for better hyperspectral imaging. Specifically, the extremely strong dispersion of metasurfaces is exploited to distinguish spectral information, and a neural network based on spectral priors is applied for hyperspectral image reconstruction. By constructing a fully differentiable model of metasurface-based hyperspectral imaging, the front-end metasurface phase distribution and the back-end recovery network parameters can be jointly optimized. This method achieves high-quality hyperspectral reconstruction results numerically, outperforming separation optimization methods. The proposed system holds great potential for miniaturization and portability of hyperspectral imaging systems.

In this paper, a V-shape photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance (SPR) effect is proposed. With the full vector finite element method, the V-shape photonic crystal fiber polarization filter is designed, and the coupling characteristics between the core mode and surface plasmon polariton mode are analyzed. The simulation results show that the fiber parameters have significant effects on the shift and strength of the SPR wavelength. Moreover, the losses of the core modes along the Y and X polarization directions are 68904 dB/m and 858 dB/m at the SPR wavelength of 1550 nm, respectively. Finally, the extinction ratio and error-tolerant rate of extinction ratio are also analyzed. The proposed V-shape PCF polarization filter can achieve good filtering effect in the communication band and is easy to integrate with existing optical fiber communication and sensing systems.

In this paper, we report a low-cost multi-mode AlGaAs laser diode pumped broadly-tunable solid-state Tm:YLF laser at room-temperature operating in two different regimes: multi-color and Q-switching. A low-cost multi-mode AlGaAs laser diode, driving 3 W power at 780 nm, was used to excite the 3% Tm 3 + -ion doped YLF laser. In the continuous-wave regime, 650 mW of output power was obtained at 1942 nm with 41% power efficiency (indicates high cross-relaxation). The output wavelength can be tuned between 1831 and 2031 nm with a birefringent filter (BRF). To the best of our knowledge, this range is the broadest tuning range obtained from a Tm:YLF laser in 2 μ m band. Furthermore, multi-color laser operations were observed by translating the crystal inside the cavity (without BRF) i.e. three two-color, two three-color and one four-color were obtained. As far as we know, these results are the first demonstration of three- and four-color 2 μ m laser. Additionally, Q-switching regime were obtained by adding a saturable absorber into the cavity. In this regime, the repetition frequencies can be arranged between 16 and 100 kHz with a maximum of 2.7 W peak power and 2.39 μ J pulse energy by translating the saturable absorber along the stability region.

The technologies of two-dimensional sound source localization based on diaphragm extrinsic Fabry–Perot interferometer (EFPI) fiber microphone array have been studied in this paper. The fiber microphone array consists of three EFPI. Self-calibrated intensity demodulation technology with simple structure and low cost is adopted to demodulate the sound signals. Using effective sound signal extraction technology to find the starting point and end point of sound signal, combining with time differences of arrival (TDOA) positioning technology, high-precision sound source location is successfully realized. The average positioning error of this system is no larger than 4.82%. The system has the advantages of low cost, high practicability and wide application scenarios.

Optical wireless technology provides a future-proof solution for indoor communication due to its unparallel broadband features. Different from other applications, indoor communication networks are sensitive to the cost and complexity. Indoor beam-steered infrared wireless communication system (IWC) based on wavelength division device, e.g. arrayed waveguide grating is attractive due to its low cost. However, the traditional wavelength tunable transmitter is expensive. In this paper, we propose a low-cost wavelength tunable transmitter for an indoor beam-steered IWC system. The key device of such transmitter is a simple and low cost Fabry–Perot(F–P) cavity laser. Different from the external-modulator based solutions, the proposed transmitter provides gain rather than insertion loss, which further simplifies the system design and improves the system reliability. The optimal central offset in spectrum between the modulated signal and the wavelength division devices is also investigated to improve the system performance. A 4.5 Gb/s indoor IWC system incorporating 3.2-km single mode fiber and 1.1-m free space link is experimentally demonstrated.

In this paper, we numerically investigate the generation of parabolic pulse (PP) in a silicon waveguide taper with simultaneous variations of the group-velocity dispersion and nonlinearity along the propagation direction. The design of such a waveguide taper is based on the condition of self-similar propagation of the PP. When Gaussian, hyperbolic secant, and super-Gaussian pulses are propagated inside the waveguide taper designed, the evolution processes under the ideal condition are analyzed. Then the PP generation from Gaussian input is shown when higher-order dispersion, higher-order nonlinearity, linear loss, two-photon absorption, free-carrier absorption, and free-carrier dispersion are taken into account. Moreover, the influences of the initial chirp, pulse width, peak power, and waveguide length on the PP generation are further discussed. It is demonstrated that high-quality PP can be obtained with a mismatch parameter as low as 1 . 3 × 1 0 − 3 , in the designed silicon waveguide taper.

International audience; This paper proposes a new scheme for the generation and transmission of a MIMO-OFDM signal over optical fiber using a single local oscillator (LO) at 15 GHz. This technique is based on subcarrier multiplexing and Optical Carrier Suppression (OCS) modulation using both Dual-Parallel Mach–Zehnder Modulator (DP-MZM) and Dual-Drive Mach–Zehnder Modulator (DD-MZM). DP-MZM is used to multiplex the MIMO-OFDM signals while DD-MZM is used to generate the 90 GHz signal by quadrupling. The realistic and global simulation of the complete system is performed to predict the behavior of a Radio over Fiber (RoF) system prior to its realization. The optical and wireless channels are based on Single Mode Fiber (SMF) and Triple-S-and-Valenzuela (TSV) models, respectively. We have exploited the allocated 7 GHz in the 60 GHz band for unlicensed use and we have successfully achieved 50km SMF followed by 3m wireless link at 70Gb/s. In comparison with traditional methods, the technique proposed combines better performance at relatively low-cost.

Optical microscopy is believed to be an efficient method for identifying layer number of two-dimensional (2D) materials. However, since illuminants, cameras and their parameters are different from lab to lab, it is impossible to identify layer numbers merely by comparing a given optical image with standard or calculated images under standard conditions. Here we reported an image reconstruction method, converting raw optical images acquired by arbitrary illuminants and cameras into reconstructed images under the condition of specified illuminant and specified camera. After image reconstruction, the color differences of each layer number roughly equaled those calculated under the specified condition. By comparing the color differences in the reconstructed image with those calculated under the specified condition, the layer numbers of 2D materials in our lab and published papers, including MoS2, WS2, and WSe2, were unambiguously identified. This study makes optical microscopy a precise method to identify layer numbers of 2D materials on known substrates.

In this paper, deep learning as a novel algorithm is proposed to reduce the noise of the fringe patterns. Usually, the training samples are acquired through experimental acquisition, but these data can be easily obtained by simulations in the proposed algorithm. Thus, the time cost used for the whole training process is greatly reduced. The performance of the proposed algorithm has been demonstrated through the analysis on the simulated and real fringe patterns. It is obvious that the proposed algorithm has a faster calculation speed compared with existing denoising algorithm, and recovers the fringe patterns with high quality. Most importantly, the proposed algorithm may provide a solution to other denoising problems in the field of optics, such as hologram and speckle denoising.

[EN] In this paper, we evaluate experimentally and analyze theoretically and by simulation the demultiplexing of LP01 and LP11 modes in a 850 nm few mode optical link when a standard single-mode coupler theoretically designed for 1550 nm operation is used. The simulation analysis indicates a feasible conversion efficiency higher than 98% for the LP01 mode and 100% efficiency for LP11 mode. These results are experimentally validated considering different off-the-shelf optical couplers configured in 99:1, 90:10, 80:20 and 60:40 coupling ratios. Experimental analysis of the optical couplers demonstrate adequate functionality in correspondence with the theoretical analysis, exhibiting a good near-field pattern and modal profile for the LP01 and LP11 modes. The mode purity observed was 98.4% for the LP01 mode, and higher than 90% for the LP11 mode. The 90:10 optical coupler configuration shows best mode purity with the minimal insertion losses., This work was supported by the Ministerio de Economia y Competitividad (MINECO/FEDER), Spain (grant numbers TEC2015-70858-C2-1-R and RTC-2014-2232-3).

In this paper, a compact and low-cost structured illumination microscope (SIM) based on a 2 × 2 fiber coupler is presented. Fringe illumination is achieved by placing two output fiber tips at a conjugate Fourier plane of the sample plane as the point sources. Raw structured illumination (SI) images in different pattern orientations are captured when rotating the fiber mount. Following this, high resolution images are reconstructed from no-phase-shift raw SI images by using a joint Richardson–Lucy (jRL) deconvolution algorithm. Compared with other SIM setups, our method provides a much shorter illumination path, high power efficiency, and low cost.

At present three main mechanisms of optical propagation with periodically focused modes in the chains of dielectric spherical particles with a refractive index near n = 1.59 are considered in the literature: evanescent whispering gallery mode coupling, photonic nanojet-induced modes, and the propagation of transverse magnetic modes under conditions close to Brewster angle geometry. In this paper we show that quasi-periodical focusing modes exist in a range of near unity refractive indices for chains of dielectric particles in both spherical and cubic forms. We also show that such a medium can be considered a gradient index waveguide with a parabolic law of refractive index variation.

In this paper, a simultaneous three-mode silicon (de)multiplexer using a trident coupler and a 3 × 3 multimode interference (MMI) is presented. The fundamental (TE0), first-order (TE1), and second-order (TE2) modes at the input are successfully demultiplexed and converted to three uniform fundamental modes (TE0) at the output ports. The device is designed and optimized by numerical simulation using three-dimensional beam propagation method together with effective index method. The three-mode (de)multiplexing is achieved over the whole C-band with small insertion loss ( 0 . 9 dB) and crosstalk ( − 17 dB). The proposed device also features with a small footprint ( 5 μ m × 400 μ m ) and a large fabrication tolerance against chip-size variations while maintaining the desired optical performance.

In this paper we study phase synchronization in large arrays of weakly coupled single mode semiconductor laser diodes. We show that if the coupling topology is chosen appropriately, the laser array exhibits robust phase synchrony (including chaotic and non-chaotic phase synchrony). Furthermore, one can define coupling topologies that lead to chaotic anti-phase synchronization. To the best of our knowledge, chaotic anti-phase synchronization has not been observed in large arrays of coupled nonlinear oscillators. When diodes are coupled via a decayed non-local coupling scheme, the leading spatial mode can be stable. This leads to an almost-perfect phase synchronous state where the phases are synchronized, but the system is not set exactly on the synchronization manifold. This almost-perfect phase synchronous state is robust to noise and frequency disorder and can be realized under periodic (fixed-intensity limit cycle) continuous-wave and chaotic behavior. The presented result is an example of the broader phenomenon of linear transverse mode selection taking place in a coupled oscillator system with nonlinear dynamics.

We carry out a model study on the interaction of a V -type three-level emitter with two quantized cavity modes which are weakly driven by two classical fields. The emitter may be an atom or a molecule with nondegenerate upper levels in general. The lifetimes of the two upper levels are assumed to be much longer than the lifetime of the cavity photons. We calculate the two-time second order coherence function, namely Hanbury Brown–Twiss function g ( 2 ) ( τ ) where τ is the time delay between the two modes. We analyze the photon–photon correlations between the two cavity modes in terms of g ( 2 ) ( τ ) . The variation of g ( 2 ) ( τ ) as a function of τ exhibits collapse and revival type oscillations as well as quantum beats for relatively short τ while in the limit τ → ∞ , g ( 2 ) ( τ ) ≃ 1 . We further show that the two cavity field modes are entangled when g ( 2 ) ( 0 ) > 2 . We develop a dressed state picture with single photon in each mode to explain the results and use the negativity of the partial transpose of the reduced field density matrix to show the entanglement between field modes. The model presented in this paper may be useful for generating entangled photon pairs, and also manipulating photon–photon correlations with a cavity QED set up.

There have been active studies on optimized dithering techniques to improve 3D shape measurement quality with image sensor and defocusing projector. These techniques can be classified into intensity-based optimization technology and phase-based optimization technology. However, those phase-based optimization methods are sensitive to the amount of defocusing while intensity-based optimization methods cannot reduce phase errors efficiently. This paper presents an optimization framework. This framework combines structural similarity index measurement and intensity residual error optimization. By applying this optimization framework to patches and tiling the best patch, high quality fringe pattern can be generated for three-dimensional measurement. Both simulation and experimental results show that this proposed algorithm can achieve phase quality improvements and it is robust to various defocusing levels.

Laser striking is a growing safety concern for pilots and aircraft passengers and there is a great need to develop a wide-angle and shift-free laser filter and protection device. This paper presents one of such design that can effectively block out a Class 3B 532 nm green laser with an Optical Density (OD) of 1.88+ for all polarisation states over a wide range of incident angles up to 85 degree. The design consists of anti-reflection layer, active blocking and substrate layers. The central active blocking layer is designed to be a metamaterial composite media made of silver nanoparticles (AgNP) organised in a three-dimensional (3D) primitive hexagonal Bravais lattice configuration and embedded in a high-index dielectric of Zinc Sulphide (ZnS). The designed filter can be placed onto a number of devices ranging from personnel goggles to aircraft windows. The work may lead to the realisation of next generation laser protection devices that are currently sought by major aerospace companies and government defence agencies.

Surface Plasmon Resonance (SPR) biosensors are widely used for real-time label-free detection in medical diagnostics, pharmaceutical researches and food safety. Although there is a growing interest in miniaturization of biosensors for self-detection and diagnostics at out of laboratory, the performance of conventional metal SPR sensors is still limited. In this paper, we propose graphene–MoS 2 – metal hybrid structures based plasmon sensors under the best minimum light intensity approach, which represents the performance analysis in case of the lowest reflected light strength. It is demonstrated that the metal thickness can be reduced from 55 nm to 32 nm and 37 nm meanwhile the performance of the background sensor can be improved by 87% and 13% with the 4 additional MoS2 and graphene layers, respectively. We show that MoS2 based SPR devices provide much better sensitivity performance than graphene based devices. Our results reveal the another promising property of MoS2: The sensitivity of SPR sensors can be greatly increased with a few number of MoS2 within the angular SPR system while reducing the size of the device, especially for particular applications such as detecting a single molecule and biosensing at low biomolecule concentration. Furthermore, we show that the equivalent optical properties of multilayered nanostructures also depend on the layer thickness which is a novel knowledge for the next studies on 2D material based SPR plasmonic devices.

Vanadium dioxide (VO2) undergoing phase transition is known alters the polarization state of light in reflection owing to large changes in complex refractive indices. While this effect is promising for optical modulation applications, the usual VO2 films on dielectric substrates tend to offer limited tunability for polarization modulation. In this paper, we show that metallic under-layers greatly enhance the performance by widening the spectral range and include visible wavelengths, by increasing the polarization modulation amplitude, and by widening the range of workable incidence angles. The imaginary part of the refractive index in the metallic layer is found to increase the relative phase shifts between s- and p-components of polarization as well as increasing the reflectance.

In this paper, based on a canonical quantization scheme, we study the effect of the relativistic motion of an excited atom on its decay rate in the presence of absorbing and dispersive media. For this purpose, we introduce an appropriate Lagrangian and describe the center-of-mass dynamical variables by the Dirac field. We obtain the Hamiltonian of the system in a multipolar form and calculate the motion equations of the system in the Schrodinger picture. We find that the decay rate and the quantum electrodynamics level shift of the moving atom can be expressed in terms of the imaginary part of the classical Green tensor and the center-of-mass velocity of the atom.

Photoacoustic (PA) and ultrasonic (US) imaging are promising techniques for imaging biological tissue and diagnosing internal organs. This paper presents a real-time PA and US dual-modality imaging system for early gastric cancer (EGC) based on a clinical US transducer. It has the advantage that only the PA excitation is inside the body but the US imaging system is outside the body, which makes the system less invasive for EGC detection and will be a potential tool for clinical use. The first experiments produced real-time PA images of blood vessel phantoms. Cross-sectional 2D PA images and reconstructed 3D PA images of the blood vessel phantoms demonstrate the system can clearly identity tissue structure. The experiments show that the system has a high axial spatial resolution but a relatively low lateral spatial resolution. The second experiments combining US and PA imaging, an artificial slightly elevated tumor and three artificial superficial flat tumors embedded inside the pig stomach mucosa ex vivo, demonstrate that the system is able to detect EGC accurately.

The nonlinear Schrodinger equation is the basis of the traditional stability analysis of nonstationary guided waves in a nonlinear three-layer slab structure. The stationary (independent of the propagation distance) solutions of the nonlinear Schrodinger equation are used as ”initial data” in this analysis. In the present paper, we propose a method to investigate the dependence of these solutions on the experimental parameters and discuss their stability with respect to the parameters. The method is based on the phase diagram condition (PDC) and compact representation (in terms of Weierstrass’ elliptic function and its derivative) of the dispersion relation (DR). The problem’s parameters are constrained to certain regions in parameter space by the PDC. Dispersion curves inside (or at boundaries) of these regions correspond to possible physical solutions of Maxwell’s equations as ”start” solutions for a traditional stability analysis. Numerical evaluations of the PDC, DR, and power flow including their parameter dependence are presented.

High-speed photonic networks using digital signal processing (DSP) techniques are flourishing nowadays to meet the high-bandwidth requirements of modern bandwidth-thirsty applications in a cost-effective manner. However, the additional latency introduced by DSP is hindering the latency-critical applications. In this paper, a FPGA-based real-time low-latency four-level pulse amplitude modulation (PAM-4) receiver including digital adaptive equalization (DAE) is designed and implemented by using a latency-reducing parallel architecture. The DSP-introduced latency in the receiver end is analyzed in detail. As for DAE parallel implementation, a novel re-allocation scheme is proposed to cope with the issue of the dependency of the output on the successive input samples, and a look-ahead computation technique is introduced to improve the adaptive update efficiency. A real-time PAM-4 receiver is demonstrated in an experimental fiber link with 2.5 Gbit/s data rate for the performance evaluation. Compared with offline processing with MATLAB, the BER performance has little deterioration at 7% FEC limit of 1 × 10−3. With the help of the proposed deep-parallel technique, the DSP-introduced latency is reduced to 0.4μs on average, which better meets the requirements of latency-sensitive user cases in 5G networks. Furthermore, the real-time PAM-4 receiver could be flexibly reconfigured for various scenarios with low-latency requirements, and the latency-efficient parallel technique as well as the latency analysis method can also be extended to high-speed hardware implementation for data rates up to 100 Gbit/s or more.

In this paper, to solve the problem of uneven distribution of far-field intensity of Flash Lidar, a method of amplitude modulation for optical phased array (OPA) is proposed, which makes the far-field intensity distribution close to the uniform rectangular distribution. We use the impulse response sequence of Finite Impulse Response (FIR) digital filter designed by the window functions to modulate the amplitude of the OPA. This kind of uniform intensity distribution can simultaneously detect a wide range with the same intensity. The modulation sequence can also be extended to two-dimensional situation. In the case of considering the radiation pattern of optical waveguide, the amplitude modulation sequence can also be modified to obtain the far-field pattern that is close to the uniform rectangular distribution.

In this paper, we proposed an ultra-high amplitude-sensitive multi-slot sub-wavelength Bragg grating (MS-SW BG) waveguide sensor on silicon on insulator (SOI) platform with a thick silicon waveguide (WG) and analyzed the design and performances. The MS-SW BG sensor with a thick waveguide supports multi-transverse modes for the first and second modes, whose stopbands (SBs) have different center wavelengths and bandwidths to form a mid-transmission band (MTB) between these SBs. By adjusting sensor parameters, the MTB can be made sharp and steep, and these spectral profiles shift in response to a refractive index change in the surrounding medium. Due to the sharp spectral feature of the MTB, ultra-high amplitude-sensitive performance can be realized. The sensor length is as short as about 10 μ m for a grating period count of 21. With an additional increase of the period count the transmission peak count in the MTB region increases, and the shortest wavelength peak among them exhibits steeper slope which is suitable for higher sensitivity. The appearance of the MTB is explained by coupled mode theory and band structure approach. The structural design is investigated from viewpoints of WG thickness, WG width, duty ratio of the grating and so on to demonstrate an ultra-high amplitude sensitivity of 5000 RIU−1 with transmission loss limit up to 3 dB.

The sodium laser guide star (sLGS) adaptive optics (AO) system has become an essential component for large ground-based optical/infrared telescope. The performance of the AO system can be significantly hindered if the on-sky spot profile of its sLGS is degraded by the turbulence along the laser launching uplink path. One effective method to overcome this problem is to perform real-time “pre-correction” on the laser before it is launched onto the sky to counter this turbulence effect especially for the lower altitude where turbulence is stronger. The “by-product” Rayleigh backscatter generated by large molecule and aerosols at low altitude when projecting sLGS is a perfect candidate for real time detection of low altitude turbulence. It is therefore important to evaluate whether this “by-product” of a sodium laser guide star could achieve suitable performance for wavefront sensing. In this paper, we attempt to answer the question regarding to the achievable photon return of such Rayleigh backscattered plume. A comparison between results from our field test and theoretical model of MSISE-90 was presented. We showed that the greatest differences is less than 20% which hints the applicability of using the defocused Rayleigh plume as wavefront sensing signal for different turbulence strengths. The result also showed that in case of strong (Fried parameter ( r 0 ) of 5 cm) or moderate ( r 0 of 10 cm) turbulence, the optimum center for range gating is 9 km, and the corresponding value of range gate depths are 3 km, 1.1 km, 0.56 km and 0.28 km when subaperture sizes are 3 cm, 5 cm, 7 cm and 10 cm, with the maximum pulse repetition frequency at 1500 Hz.

So far, as an optimal design method for 2D and 3D design of optical waveguide devices, topology optimization based on a function expansion method has been reported. In this paper, we improve the conventional function-expansion-based topology optimization method to design three-dimensional photonic devices with structural variation in the depth direction. In order to design realistic 3D optical waveguide devices, we consider a layered structure and design each layer by using function-expansion-based topology optimization method. In this approach, undesired floating structures are avoided and layer thickness can be optimized. The validity of this method is verified through design examples of a polarization splitter and rotator (PSR) and a crossing waveguide which have two-layer structure.

In this paper, we demonstrate a pulse energy-governed nonlinear self-switching of femtosecond pulses at wavelength of 1700 nm in a highly nonlinear high index contrast dual-core fibre. The fibre structure is composed of two microdimension cores and a solid cladding made of two thermally matched soft glasses developed in-house. A double switching behaviour under monotonic increase of the input pulse energy from 1 nJ to 3 nJ was observed at 35 mm fibre length in its anomalous dispersion region. A spatial intensity distribution measurement identified a switching contrast of 16.7 dB, and the registered spectra of the two output channels revealed a broadband switching behaviour within 150 nm. The obtained results indicate a novel type of solitonic switching and represent a significant progress in comparison to the previous experimental works and has a high application potential.

In this paper, a refractive index biosensor is proposed which employs Si photonic nanorings coupled to ring-shaped plasmonic nanohole arrays (PNA) on a hybrid substrate to facilitate sensing of different glucose concentrations in water solution. The proposed structure can be used as a single-mode refractive index biosensor with a high sensitivity of 1278 nm/RIU, a figure of merit (FoM) of 168.1 RIU−1, and good detection accuracy of 0.13 nm−1. The sensitivity and FoM values of the proposed structure are highest in comparison with many reported structures in recent years, while it uses a simple structural topology. The effects of various structural parameters on the reflection spectrum and sensing performance are comprehensively surveyed using the 3D FDTD method. Using the proposed structure, the strength of the interaction between the applied light and the analyte is effectively enhanced and the propagation loss is relatively reduced. Hence, the designed structure is an appropriate candidate for ultrasensitive biosensing applications.

In this paper, a dynamically switchable dual-band absorber based on electromagnetically induced reflection (EIR) in metal-graphene hybrid metamaterial is proposed. The unit cell of metamaterial absorber has three layers. The top layer consists of two split-ring resonators (SRRs) and two orthogonal cut-wires (CWs) with different lengths. Due to the low reflection dips in EIR structure and good absorption of graphene, two absorption bands accompanied with more than 90% absorptions are obtained under y-polarized and x-polarized incident waves, respectively. The power loss densities at the absorption points clearly reveal the absorption positions of the metamaterial absorber. In addition, the electric field, magnetic field and surface current distributions explain the reason of high absorption. According to the surface currents at four resonant points, we draw the schematic diagrams of multipole moments, which are confirmed by multipole scattering theory. The tunable absorption performance of metamaterial absorber can be observed by tuning the Fermi level of graphene. The metamaterial absorber exhibits excellent wide polarization angle behavior and shows great prospects in multi-band metamaterial absorber under oblique incidence. The proposed metamaterial absorber expands the development of metamaterial and shows great potential for multifunctional metamaterial absorber.

In this paper, we report a high-power single-frequency all-fiber amplifier at 1064 nm based on cascaded hybrid active fibers. The cascaded gain fiber design, where a piece of active fiber with lower dopant concentration is spliced before the highly doped gain fiber, is proposed to effectively alleviate the thermal loads of high-power amplifier. By virtue of hybrid pump at 976 nm and 915 nm to increase the threshold of mode instability (MI) effect, an output power of 435 W is achieved from the Yb 3 + -doped fiber with large core diameter of 30 μ m. The beam quality is well maintained with a M2 of 1.40 even at the maximum laser power.

A transmitted optical fiber sensing system based on orthogonal demodulation algorithm is used for hydrogen detection in this paper. WO3/Pt fabricated by sol–gel method was used as sensing material. WO3 film reacts with hydrogen by the catalysis of Pt and refractive index of the sensing film changed, by measuring the transmitted optical power the concentration of hydrogen can be detected. Orthogonal demodulation algorithm was used to demodulate the weak transmitted optical intensity signals. Compared with the traditional ratio-type transmitted optical fiber hydrogen sensing system, the noise reduced by 28 times. The limit of detection is 495ppm and with good repeatability and stability. In order to improve the sensitivity of the system, three pieces of sensing film were used to increase the interaction length of the sensing material with hydrogen and the sensitivity increased by 2.592 times. With the orthogonal demodulated technique and multi pieces of sensing film, high sensitivity, fast response and low cost hydrogen sensing system was implemented and is very promising for the monitoring of hydrogen leakage in the field of hydrogen production, transportation, and storage.

This paper proposes a numerical simulation of an optical frequency comb scheme at 1550 nm based on a dual-frequency laser pump in the AlGaAs-on-insulator waveguide. Optical frequency comb with a 5 dB spectral bandwidth of approximately 125 nm is obtained within a 0.08-m normal-dispersion AlGaAs waveguide based on pulse compression, self-phase modulation, and optical wave breaking. The influences of power difference and frequency spacing of the dual-frequency lasers on the optical frequency comb are investigated along with the improvements in comb flatness due to pulse shaping.

Speckle modulation in the laser beam projection scenarios such as adaptive optics, remote sensing, and active imaging has been a long-standing challenge that causes intensity fluctuation and further degrades the performance of the system. Speckle statistical characteristics coupled with the feature of the laser beam intensity distribution can be obtained by analyzing the speckle variation through a single point detector or an image sensor. A single point detector has the advantage in high sampling rate but may lose speckle spatial information, while the image sensor has a high spatial resolution but slow sampling rate. In this paper, we defined a dynamic speckle metric based on a linear array detector to estimate the encircled energy of the far-field spot on the diffuse target, which well balanced the trade-off between sampling rate and spatial resolution. The potential of the metric is analyzed using a physics-based speckle simulation and experimental verification. Both results agree with each other, showing that the metrics based on the speckle-field spatiotemporal spectrum analysis are monotonically dependent on the target focused-spot size.

In this paper, a new saturable absorber (SA) based on Lanthanum Hexaboride (LaB6) is used to produce Q-switched and mode-locked laser pulses. In constructing the SA, the prepared LaB6 powder is dispersed in polyvinyl alcohol (PVA) solution before the mixture is dried to form a thin film. A small piece of the film is placed between two fiber ferrules to form a LaB6 PVA SA and added to a laser cavity of an erbium-doped fiber laser (EDFL) to produce Q-switched and mode-locked pulses. The Q-switched laser operates at 1531 nm. The pulse width of the pulses can be varied between 4.32 to 2.46 μ s with a corresponding repetition rate of 62.0 to 89.6 kHz when the pump power is increased from 45.4 to 97.6 mW. At the maximum output power of 4.31 mW, the highest pulse energy of 48.1 nJ was obtained. A passively mode locked EDFL was also successfully realized with LaB6 SA by adding 100 m long single mode fiber inside the cavity. A stable mode-locked laser operating at 1559.1 nm was achieved at the threshold pump power of 102.8 mW with a pulse rate of 1.88 MHz. The pulse energy of 5.6 nJ and output power of 10.54 mW were obtained at a maximum pump power of 144.6 mW. To the best of our knowledge, this is the first demonstration of utilizing LaB6 material as SA to produce pulses in the 1.5 μ m region.

Metasurfaces exhibit promising capacities in the manipulation of electromagnetic (EM) waves, as to diverse dimensions like amplitude, phase, and polarization. Despite many methods have been used to improve regulatory capabilities of EM waves, the efficiency and working bandwidth has yet to be developed. In this paper, an ultra-wideband and high-efficiency reflective polarization converter metasurface (PCM) is proposed. Both theoretical analysis and simulation results show that the polarization conversion ratio (PCR) is above 89% in the ultra-wideband from 2.28 THz to 6.75 THz, and the relative bandwidth reaches 99%. Moreover, the multi-bit digital coding metasurface elements are designed by using Pancharatnam–Berry (PB) phase principle. The coding metasurface arrays are designed by using different coding sequences, which can be used to realize the free adjustment of scattering angle of the reflected EM waves. In addition, the radar cross section (RCS) reduction performances of the designed metasurface array antennas based on different coding elements are studied. The results present that it can significantly reduce the RCS by −20 dB over a wide frequency range from 2.1 THz to 6.9 THz. The proposed metasurface has great application potential in manipulation of terahertz wave and contributes to RCS reduction.

Low-cost tunable lasers have broad application prospects in optoelectronics. In this paper, a laser device based on fingerprint-textured cholesteric liquid crystal (CLC) was designed and fabricated, in which a narrow linewidth single-mode laser output on the side of the device was realized. The planar helical structure of the “fingerprint-texture” of CLC was obtained by the combined effect of externally applied electric field and substrate surface anchoring. Under the pumping light of a Nd:YAG solid-state pulsed second harmonic laser with the wavelength of 532 nm, the lasing emission wavelength of our CLC device changes non-monotonically with gradually increasing electric field, which first shifting from 568.5 nm to 609.2 nm and then shifting from 609.2 nm to 586.2 nm. And the observed spectral tuning of the lasing emission was reproducible and reversible. The analysis shows that the output laser radiation was the result of the Bragg reflection feedback provided by the fingerprint-texture.

In this paper, an asymmetric directional coupler (DC) based on a GaAs–CaF2 hybrid plasmonic waveguide (HPW) and a GaAs nanowire (NW) is designed for the mid-infrared polarization beam splitter (PBS). The mode characteristics of the GaAs–CaF2 HPW and GaAs NW-based PBS are simulated by using the finite element method (FEM). With the appropriate widths of the HPW and NW, the TE polarization mode in the HPW and NW is phase-matched, whereas the TM polarization mode is phase mismatch. Hence, the TE mode undergoes a strong coupling to the GaAs NW while the TM mode pass through the GaAs–CaF2 HPW directly. The results show that the PBS achieves the extinction ratio (ER) of 19.78 dB and insertion loss (IL) of 1.64 dB for the TE mode, ER of 7.78 dB and IL of 2.64 dB for the TM mode at the operating wavelength of 3.5 μ m. The PBS based on an asymmetric DC paves the way to achieve integrated waveplates, which has potential applications in the detection and analysis of airborne molecules and the optical communication techniques with large signal transmission capacity.

As a new proposed spectrometer, the filter-based spectrometer uses a group of filters with known transmittance to modulate the spectrum of an incident light beam. After employing a post-processing algorithm, the spectrum of the beam can be reconstructed. For such spectrometers, the transmittance curve characteristics of the filters have a crucial impact on the spectrum reconstruction results. To better capture the details of the spectrum curve of the incident light beam, we propose a new nano-structure spectrometer by introducing gold nano-pillars to the photonic crystal (PC) filters. Because of the surface plasmon resonance effect of the gold nano-pillars, narrow bands will be generated on the transmittance curve of the new filters, which can improve the spectral reconstruction capability of the filters. In this paper, we first briefly introduce the principle of the filter-based spectrometer. Then, the transmittance curves of the filters are simulated by multiple random Gaussian functions according to the waveform of the transmittance curve of practical filters. And the simulated transmittance curves are used to reconstruct the spectrum of a beam to study their anti-noise capability. Next, new filters are designed by introducing gold nano-pillars to the PC filters. And their transmittance curves are calculated by the finite-difference time-domain (FDTD) approach. Finally, the spectrum reconstruction results are compared with those of the PC filters. The simulation results show that the introduction of gold nano-pillars can significantly improve the spectrum reconstruction ability of the PC filters.

This paper proposes a theoretical scheme to synthesize white laser source based on the cascaded sum frequency generation (CSFG) model with stimulated Raman adiabatic passage (STIRAP). In the CSFG model, the signal laser, intermediate laser and output laser correspond to red, green and blue (RGB) lasers respectively. The results show that in general the red laser is completely converted to the blue laser, and there is no apparent intermediate green laser. Moreover, under the counterintuitive order, the pump parameters have a wider adjustment range of system conversion efficiency than the coupling delay parameters. We synthesize the white laser source through the following steps. First, we change the pump intensity to roughly adjust the conversion efficiency of the system, and then vary the coupling delay parameters to fine-tune RGB lasers output power ratio. Compared with the scheme based on frequency conversion with Stark-chirped rapid adiabatic passage to synthesize white laser source, the RGB lasers output of the new scheme can achieve better stability. The results of the investigation can provide a theoretical foundation for the research of novel optical devices.