We demonstrate 1.8 W of average output power with 350 fs of pulse duration from a diode-pumped mode-locked rotary Nd:glass disk laser. In this laser with rotating Nd:glass disk structure, we use a Nd:glass disk with a diameter of 50 mm, which disperses heat to a wide area, to extend the upper limit of pumping power of Nd:glass for high output average power. We achieve passively soliton mode-locking for obtaining femtosecond pulses based on a semiconductor saturable absorber mirror. To our best knowledge, this is the highest average power reported from a diode-pumped femtosecond Nd:glass laser oscillator. The results are significant for power scaling of femtosecond Nd:glass lasers.

In this paper, a novel spectral-efficient coherent radio-over-fiber (RoF) link with linear digital phase demodulation is proposed and experimentally demonstrated. At the transmitter, to make an efficient use of the optical power and spectra, an intensity-modulated optical signal serving as the optical reference signal and a phase-modulated optical signal are polarization-multiplexed on a single optical carrier. At the receiver, the two optical signals are coherently detected with an optical local oscillator (OLO) and demodulated free of laser phase fluctuation through digital signal processing. Owing to simple and linear digital phase demodulation, an RF input signal is linearly demodulated from the optical phase without approximations and preconditions, which preserves the linearity of the phase-modulated RoF optical link from the transmitter end to the receiver end. The proposed scheme is experimentally verified by 25-km single-mode-fiber (SMF) transmission of two 16-QAM microwave vector signals at 2 GHz and 2.4 GHz, both with a symbol rate of 50 Msymbol/s. The transmission performance in term of error vector magnitude (EVM) is evaluated. Additionally, 25-km SMF transmission of the phase-modulated input signal with a spurious-free dynamic range (SFDR) of 112.8 dB·Hz 2/3 is obtained.

To meet the simultaneous requirement of high sensitivity and high measurement range is an uphill task in the development of fiber Bragg grating (FBG) strain sensor. A unique scheme of highly sensitive FBG strain sensor is proposed based on nonlinear four wave mixing (FWM) with a high measuring range. In this paper, both the edges of FBG reflection spectrum are used without increasing the number of sources. The FWM products generated are tuned to the reflection edges of the FBG. The signals reflected from the edges of the FBG are then utilized in the subsequent FWM process to enhance the sensitivity of the system. The proposed scheme can provide a strain measurement range of $2310\,\mu \epsilon $ with strain sensitivity of 91.21 dBm/nm.

In this paper, we report a unique multi-channel Photonic Crystal Fibre (PCF) sensor based on Surface Plasmon Resonance (SPR) structure comprising of silver and gold doped plasmonic layers for multi-analyte sensing applications. We deployed a Full Vectorial Finite Element Method (FV-FEM) to investigate the sensitivity performance of the proposed PCF sensor. The SPR sensor is fully optimised to ensure propagation features, such as confinement loss, resonance condition, resolution and sensitivity are investigated within various optimised design parameters. According to spectral sensitivity analyses, 2500 nm/RIU and 3083 nm/RIU with 4 × 10 −5 RIU and 3.2 × 10 −5 RIU resolutions are obtained for Channel 1 (Ch1) (x-polarized) and Channel 2 (Ch2) (y-polarized), respectively.

Existing literature studying the access point (AP)-user association problem of heterogeneous radio-optical networks either investigates quasi-static network selection or only considers vertical handover (VHO) dwell time from optical to radio. The quasi-static assumption can result in outdated decisions for highly mobile scenarios. Solely focusing on the optical to radio handover ignores the importance of dwell time for VHO from radio to optical. In this paper, we propose a flexible and holistic framework, that runs a self-optimizing algorithm at the centralized coordinator (CC). This CC resides in the LTE eNodeB and controls the handover parameters of all the visible light communication (VLC) APs under the coverage of the LTE eNodeB. Based on Q-learning approach, the algorithm optimizes the time-to-trigger ( $TTT$ ) values for VHO between LTE and VLC. Case studies are performed to validate the considerable gain in terms of average throughput by optimizing $TTT$ s. We evaluate the impact of learning parameters on the optimal throughput and convergence speed through trace-driven simulations. The simulation results reveal that the Q-learning based algorithm improves the average throughput of mobile device by 25% when compared to the fixed $TTT$ scheme. Furthermore, this algorithm is capable of self-optimizing handover parameters in an online manner.

A highly-efficient tunable 1.6 μm MgO:PPLN-OPO, which we believe to be the first one pumped by a diode-end-pumped Raman laser, is demonstrated. A good beam-quality of actively Q-switched Nd:YVO 4 /YVO 4 Raman laser at 1176 nm and a good mode matching of the whole system resulted in a high-performance OPO. The highest Raman-to-signal conversion efficiency was 49.5% at 1638.8 nm when the incident Raman power was 2 W. The maximum output power of 3.2 W at 1663.5 nm with the pulse-width of 2.85 ns was obtained pumped by 7.57-W Raman laser. Meanwhile, the linewidth was less than 0.3 nm and a high beam quality with M 2 factor of 1.92 was obtained.

Narrow-linewidth silicon-based lasersplay an important role in the field of silicon photonics. In this paper, we have proposed a high performance narrow-linewidth silicon-based Er silicate laser, based on strip-loaded DFB waveguide with a hybrid pump and signal co-resonant cavity. The saturated output power can reach over 90 mW at 1535 nm, with a maximum power conversion efficiency of 66%. The pump threshold is about 22 mW, and the laser linewidth is also as narrow as about 755 Hz. The results provide a new way for future scale integrated ultra-narrow-linewidth silicon-based lasers application.

Semi-polar micro-LEDs have gain increasing interests due to the advantages of polarization control and quantum efficiency improvement. In this work, a novel semi-polar (20–21)-plane micro-LEDs array has been designed and manufactured. In comparison with c-plane micro-LEDs, semi-polar micro-LEDs indicate better electrical and optical performance. The relative EQE of semi-polar micro-LEDs remains at 62% under the injected current density of 775.6 A/cm 2 , which indicates a reduced efficiency droop due to less polarization in MQWs. It has been further proved by a significant reduction of 55% in emission peak blue-shift under the injected current density from 11.1 A/cm 2 to 775.6 A/cm 2 . In addition, the carrier recombination dynamics and spatial light distribution of semi-polar micro-LEDs with different pixel sizes have been studied. Fast recombination lifetime in smaller size semi-polar micro-LEDs indicates a promising way to be used as a high modulation bandwidth light source. Stable and uniform light distribution in a wider range of spatial azimuths further supports for the semi-polar micro-LEDs as a strong candidate for the applications of high-resolution display and high-speed visible light communication.

A vanadium dioxide-based multilayer metamaterial is proposed with bifunctional properties of absorption and polarization conversion. When vanadium dioxide is in the metallic state, the designed system behaves as a single-band absorber which is composed of a vanadium dioxide disk-shaped array, a silica spacer, and a vanadium dioxide continuous film. The performance of absorption can be tuned by changing either the diameter of disk or the thickness of silica. The design of this absorber is robust against incident polarization and incident angle. The proposed single-band absorber may be generally applied for plasmonic detection, cavity resonator, and optical band-stop filter. When vanadium dioxide is in the insulating state, the designed system behaves as a cross polarization converter which mainly consists of a one-dimensional metallic strip-shaped array, a silica spacer, and a metallic continuous film. The designed metamaterial can convert a linear plane wave into its corresponding cross-polarized wave with the efficiency of >90% in the frequency of 2.0-3.0 THz. The physical mechanism of polarization conversion is explained by a simple picture. The proposed metamaterial could be a potential candidate for the modern device of polarization control.

In this paper, we demonstrate the feasibility of realizing the extraordinary optical transmission (EOT) and the collimated beaming effect through InSb grating, which has a subwavelength slit surrounded by a finite array of grooves at both sides of the surface. Firstly, we investigate the transmission properties of the structure by changing the temperature and doping concentration of InSb, and the optimized transmission can reach almost 95%. Besides, it is verified that the transmission is mainly controlled by the pattern on the input corrugation with the variable dimension parameters. At last, the properties of the beaming intensity and focal length are thoroughly analyzed with the variation of the number and depth of the grooves on the output corrugation. These results provide a new plasmonic solution for controlling light in a variety of integrated optical components in the terahertz range.

A novel approach to realize broadband microwave spectrum sensing based on photonic RF channelization and compressive sampling (CS) is proposed. The photonic RF channelization system is used to slice the input broadband signal into multiple sub-channel signals with narrow bandwidth in parallel and thus the rate of pseudo-random binary sequence (PRBS) and the bandwidth of the MZM for CS can be largely decreased. It is shown that a spectrally sparse signal within a wide bandwidth can be captured with a sampling rate far lower than the Nyquist rate thanks to both photonic RF channelization and CS. In addition, the influence of the non-ideal filtering of the photonic channelizer is evaluated and a novel approach based on measuring twice is proposed to overcome the problem of frequency aliasing induced by the non-ideal filtering. It is demonstrated that a system with 20 Gbit/s PRBS and 2.5 GS/s digitizer can be used to capture a signal with multiple tones within a 40 GHz bandwidth, which means a sampling rate 32 times lower than the Nyquist rate.

We study analytically and numerically the possibility of vortex modes propagation over planar dielectric rectangular waveguides, and consider the problem of waveguide geometry optimization for the support of vortex modes. The results show, that theoretically rectangular waveguides can provide transmission of quasi-TE and quasi-TM modes with high purity states of orbital angular momentum (OAM) in the dominant field component. However, only for the quasi-generate mode of azimuthal order ±1 the constituent eigenmodes can propagate in a phase-matched regime, and the vortex modes of higher azimuthal orders can propagate only with a certain beat length. Moreover, as the target azimuthal order increases, the normalized power of the corresponding OAM state in the modal superposition decreases. The analytical predictions have been verified by numerical electromagnetic simulations of silicon nitride waveguides providing field distributions and OAM spectra of the corresponding modal superpositions.

In the absence of random mode mixing (RMM), linearization of the cross-mode modulation (XMM) under pump-probe configuration is in general complicated because the evolution of the pump light depends on the local mode decomposition of itself. In this work, general derivation of the Hamiltonian of the XMM system without RMM is carried out and discussed under two interesting scenarios where the pump evolution can be effectively linearized, leading to spatial dependent Hamiltonian of the probe light. Representative evolutions of pump and probe light over transmission are investigated with the assistance of Poincaré sphere based on the Hamiltonian approach. Our results are benchmarked against the results given by precession equations examined by Lin and Agrawal and numerical simulations using nonlinear coupled mode equations (CMEs). By highlighting the eigenstates as well as eigenvalues of the system, our approach provides an intuitive yet powerful approach to understand the XMM nonlinear problems and to dynamically manipulate the spatial profiles of the probe light.

The low-dimensional nonlinear optical materials can provide a cost-effective and feasible way to realize the broadband optoelectronic devices. Here, we investigated the broadband nonlinear optical absorption of antimony thin film, and fabricated a broadband passive photonic diode with antimony thin film to achieve the all-optical nonreciprocal function with optical nonreciprocity factor about 5 dB at 780 nm and 2 dB at 1560 nm, respectively. The experimental results show the potential applications of antimony-based materials in broadband passive photonic diodes, and may make inroads for the cost-effective and reliable broadband optoelectronic devices.

We experimentally investigated the third-order nonlinear optical response of absolute ethanol around 1550 nm. The real and imaginary parts of third-order susceptibilities are distinguished by Z-scan method, which demonstrated that absolute ethanol exhibited saturable absorption and self-defocusing behaviors with nonlinear refractive index n 2 ∼−10 −14 m 2 /W. In addition, the experimental results via self-phase modulation method also verified the comparable nonlinear optical response of absolute ethanol. This work is expected to provide insights for the nonlinearities of organic solvents, and may broaden the potential of cost-effective nonlinear optoelectronic devices.

A new type of circular Airy beams is demonstrated by using a bandpass filter, in order to enhance the field intensity at the focal point of light beam, based on the angular spectrum representation and the ray optics picture. The specific ring and the key spatial frequency corresponding to the focal point are obtained, via the ray optics picture, to make the initial field concentrate near the specific ring in the real space. It is found that the field intensity at the focal point of the new type circular Airy beams is better enhanced than the common circular Airy beams and the modified circular Airy beams, and the enhancement effect can be tuned via the central spatial frequency, apodization steepness and width of the bandpass filter. The enhanced field intensity gives a very strong gradient force and may find applications on optical manipulation.

Niobium nitride (NbN) nanowires have a high repetition rate and efficiency, making them ideal for superconducting nanowire single photon detectors (SNSPDs). However, it is difficult to fabricate NbN arrays over large areas, which is critical for various applications. This paper describes a 4 × 4 NbN SNSPD array (16 pixels) and optical coupling with a 300-μm-diameter multimode fiber using beam compression technology. This is the first NbN SNSPD coupled with such large-diameter fibers. The designed pixels are positioned as closely as possible (pixel filling factor about 98.5%), almost without dead area between them. This results in a system efficiency of 46% and a quantum efficiency of 94.5% for photons (λ = 1064 nm) coupled from multimode fibers. An intrinsic time resolution of less than 69 ps can be obtained. The proposed high-performance single photon detector is suitable for satellite laser ranging. Furthermore, the proposed system is feasible for large SNSPD arrays with NbN, paving the way for the development of efficient photon cameras with NbN nanowires.

Brillouin optical time domain analyzer (BOTDA) assisted by optimized support vector machine (SVM) algorithm for accurate temperature extraction is presented and experimentally demonstrated. Three typical intelligent optimization algorithms, particle swarm optimization algorithm, genetic algorithm and firefly algorithm are explored to optimize the SVM parameters. The performances of optimized SVM algorithms for temperature extraction are investigated in both simulation and experiment under different conditions for Brillouin gain spectrum collection, resulting in the significant enhancement of sensing accuracy. In particular, the extraction accuracy (i.e., smaller root mean square error value) of temperature information is improved about 4 °C compared with the conventional SVM when the signal-to-noise ratio (SNR) as low as 2.5 dB and 40-ns pump pulse width are adopted in the experiment. In addition to the enhanced accuracy with good robustness, the optimized algorithms have faster processing speed than the curve fitting method, over 20-times improvement. That makes the optimized algorithms become a very promising candidate for high performance BOTDA sensors in the future.

A new optical fiber twist sensor with temperature compensation mechanism is designed and experimentally demonstrated. The main structure of this twist sensor is a torsion perturbation point cascaded with single mode-multimode-single mode (SMS) fiber structure. The experimental results show that the twist sensor has ability to distinguish the twist direction with sensitivities of 9.5 pm/° and 34.7 pm/° at the chosen dips in the range from −100° (CCW) to 140° (CW). Furthermore, the temperature response characteristic of the sensor is also researched through experiment. Temperature sensitivities of 71.8 pm/°C and 65.2 pm/°C can be obtained at the same dips in the range from 20 °C to 75 °C. In addition, the temperature's influence can be compensated with post-measurement calculation through the cross-matrix. The theoretical resolutions of twist and temperature simultaneous measurement are ±0.084° and ±0.32 °C, respectively.

We propose a novel absolute calibrate method for digital holographic microscopy with the sequential shift method using Chebyshev polynomials. We separate the object phase and the aberrations by sequential shifting the sample twice in vertical plane of the optical axis. The aberrations phase is then calculated using the high order Chebyshev polynomials. The correct phase is obtained by subtracting the aberrations from the original phase containing the aberration. This method can compensate for the complex aberrations including high-order aberrations without changing the traditional optical system. Meanwhile, it can effectively protect the medium and high frequency information of the specimen in the phase image. Numerical simulation and experimental results demonstrate the availability and advantages of the absolute calibrate method.

We propose a practical approach to identify plasmonic absorption profile on back focal plane (BFP) of surface plasmon microscopy (SPM). Compared to previous morphology approaches, the proposed one features: i) it is applicable in BFP images with non-concentricity; ii) the algorithm of Fourier Correlation Analysis (FCA) maximizes SPP while minimizes random coherent noises, which makes it extremely suited for BFP images with significant low-quality; (iii) it takes much less time for one identification process and the entire identification can be operated automatically.

Integrated zoom and image stabilization system based on deformable mirrors (DMs) has advantages of miniaturization, rapid response and low energy consumption. Integrating the two capabilities on one DM-based imaging system poses considerable challenges. First, limited DM stroke will result in limited changeable aberration correction value for zoom and image stabilization. Second, the DM-based off-axis imaging system suffers a simple relative movement between optical axis and objects caused by carrier vibration may result in complicated optical system aberrations. To address these challenges, a reasonable assignment of the changeable aberration correction value for zoom and image stabilization is needed. Image stabilization and aberration correction in DM-based off-axis imaging system, should be considered when translation or rotation occurs. To troubleshoot the assignment issues of the correcting aberrations, we propose a graded optimization method which comprises two steps. To troubleshoot the image stabilization and aberration correction issues, we introduce vectorial ray tracing method and image point freezing principle to transform the aberration correction problem into an optimization problem of image coordinates. With the proposed method, we construct practical integrated freeform surfaces of DMs for a space camera with three mirrors. The sagittal heights of designed DM surface profiles are limited within their available stroke.

In this study, we proposed an innovative concept of integrating a polarized color-separating backlight with a color-filter-free LC panel for enhancing performance of the curved LCD system. We design a two-level folded backlight to exploit the light sources LEDs with color enhancement by quantum-dot technology to produce color beams emerging at separate angles with a linearly polarizing state. The integrated LC panel focuses the separating color beams onto the corresponding subpixels with no need of the color filter; redirects the color beams at respective angles into the normal and reforms their angular shapes to be identical for attaining good color uniformity in the viewing cone. We established a system model for simulation to evaluate its performance. According to the simulation results, it performs the high optical efficiency of 33.75% that is about four times the traditional LCD, and displays color gamut of 94.1% NTSC (CIE 1931). It also provides the illuminance uniformity of 90% above and good color uniformity in both the spatial space and angular space of the viewing cone with horizontal and vertical angular widths of 100 o and 75 o , respectively. Furthermore, the simulation results demonstrate that the design can apply to a 55-inch curved LCD and further extend the maximum diagonal size up to 88 inches while keeping slim volume with a minimal thickness of 6.5 mm.

We investigate optical property of polarization-maintaining fiber taper (PMFT) for tunable multi-wavelength laser generation in a fiber ring cavity. A panda-type polarization fiber is fused and tapered into a fiber taper with a diameter of about 2.5 μm. Optical and thermal properties of the PMFT are investigated theoretically and experimentally. As combined with a polarization dependent isolator, tunable multi-wavelength fiber laser operation is achieved by controlling of polarization-dependent loss in the fiber ring cavity. The laser wavelength number can be switched from 1 to 4. The 3 dB bandwidths of laser spectra are less than 0.05 nm for all wavelength numbers, with a maximum OSNR of about 54 dB. For single-wavelength laser operation near 1561.66 nm, the measured wavelength and power fluctuations are < 0.02 nm and 0.4 dB, respectively.

Single-photodiode-per-polarization coherent receivers (SCRs) can preserve all of the merits of the conventional digital coherent receivers (DCRs) such as supporting chromatic dispersion (CD) compensation and polarization multiplexing, whilst have much lower optical complexity. But the overall algorithm complexity of the SCRs is relatively higher because an extra dedicated field reconstruction algorithm (FRA) is required in addition to the standard algorithms used in DCRs. The heavy computation burden introduced by the FRA poses a major obstacle to reduce the DSP chip size and power consumption. In this paper we propose a method to reduce the overall algorithm complexity of the SCR by utilizing several techniques, including utilizing a strong local oscillator (LO) to mitigate the signal-signal beat interference (SSBI), a pseudo-single-side-band signal to recovery the signal field and a new FRA to reuse the operations in the CD compensation algorithm to realize field reconstruction. Other than reducing the overall algorithm complexity, the new method also make the SCR more robust to laser frequency offset compared with the existing heterodyne detection based FRA. Numerical simulations and experiments are presented to demonstrate the merits of new method with respect to the existing ones.

An optical back propagation (OBP) technique using Raman pumped dispersion compensation fibers (DCF) is investigated to compensate for nonlinear impairments in WDM systems in real time. The proposed inline OBP module consists of an optical phase conjugator, amplifiers and a Raman pumped DCF. In order to suppress the nonlinear effects of the transmission fibers exactly, the power in the backpropagation fiber should increase exponentially with distance. This can be approximately achieved by using forward/backward Raman pumping of the dispersion compensating fiber (DCF). We introduce two configurations to realize the OBP. In this paper, we show that the OBP with forward/backward pumping provides 2.45 dB Q-factor gain compared to single-channel digital back propagation (DBP) when transmission distance is 1500 km for a WDM system with QAM-64. To minimize the variation of effective gain coefficient of the Raman pumped DCF as a function of distance, bidirectional pumping scheme which can provide the signal power profile closest to that required by the ideal OBP condition is proposed. The bidirectional pumping scheme provides a superior performance over forward/backward pumping and wideband DBP (i.e., DBP is applied on the entire WDM signal). Our numerical simulation results show that the bidirectional pumping scheme provides 7.6 dB and 5 dB advantage in Q-factor as compared to single-channel DBP and wideband DBP, respectively at a transmission distance of 5000 km. The maximum achievable reach of a long haul WDM system can be enhanced by $225\%$ using bidirectional pumping scheme as compared to wideband DBP.

To overcome the frequency-dependent nonlinear characteristics of a directly modulated laser (DML) in a multi-level optical transmission, the spectrum-split digital pre-distortion (SS-DPD) is proposed and experimentally demonstrated to mitigate the frequency-dependent nonlinear impairments. The SS-DPD is employed in a 5 Gb/s PAM-4 optical transmission using a DML over a 5 km standard single-mode fiber at 1550 nm. We experimentally compared the SS-DPD with the DPDs based on the Volterra series and the memory polynomial (MP). The results demonstrate that the SS-DPD improved approximately 1 dB in terms of the receiver sensitivity compared with the MP-DPD. Moreover, the SS-DPD achieves 0.4 dB receiver sensitivity gain compared with the Volterra DPD.

High-speed optical communication systems may suffer from a combination of impairments such as memory effect and nonlinear behavior of the optoelectronic components. Nonlinear digital pre-distortion (DPD) is one of the well-known technique to alleviate these effects. As typical implementation of Volterra-based DPD is considered complex and consumes high power, more efficient orthogonal-based Volterra series representation has been proposed. Previous works offered ways to perform efficient grading of the most dominant dimensions based on the combination of the dimensions variances and the signal projection. Here, it is shown that normalization of the data dynamic range further improves this method and decreases significantly the number of required dimensions. Using normalization combined with the previous methods, maximizes the DPD performance by means of error vector magnitude (EVM) and bit error rate (BER), while minimizing the DPD complexity in the terms of required series dimensions. Extensive simulation and lab measurements indicate a potential saving of up to 87% in the number of dimensions with a negligible performance penalty.

Dimmable optical orthogonal frequency division multiplexing (D-OFDM) is challenging to design for visible light communications, due to the high peak-to-average power ratio (PAPR) of OFDM signals and limited dynamic range of LEDs. Current D-OFDM schemes focused on the time domain and spatial domain designs of OFDM signals. This paper proposes a novel dynamic subcarrier activation based OFDM (DSA-OFDM) scheme. In DSA-OFDM, the number of activated subcarriers, as well as the signal's DC bias and the number of activated LEDs, is adjusted to provide both flexible dimming control and reliable communication. A generalized index modulation in both spatial domain and frequency domain is designed to select the activated subcarriers and activated LEDs. To show the influence of frequency domain design analytically, the closed-form expressions of the PAPR and the clipping noise of the DSA-OFDM signals are derived, and the optimized signal form is obtained accordingly. Simulation and numerical results show that the proposed scheme can outperform conventional dimming control schemes at various illumination levels in terms of the bit-error rate performance.

We develop a real-time discrete multi-tone (DMT) transceiver based on field programmable gate array (FPGA) chips for a single chip silicon-substrate light-emitting diode (LED) based underwater visible light communication (UVLC). On-chip resource usages are analyzed and discussed. To improve bit error rate (BER) performance, a novel channel estimation technique utilizing hybrid inter-symbol frequency-averaging (Inter-SFA) and intra-symbol frequency-averaging (Intra-SFA) is proposed and investigated. The real-time DMT transceiver is experimentally verified in a silicon substrate blue LED-based UVLC system with a 1.2 m underwater link. By using the enhanced channel estimation, a gross bit rate of 2.34 Gbit/s real-time DMT signal over 1.2 m underwater transmission can be achieved with the BER of 3.5 × 10 −3 . What's more, multiple-symbol interleaved Reed-Solomon (RS) codes are employed to further improve BER performance. The real-time measured post-FEC BER of the DMT-UVLC with multiple-symbol interleaved RS (255, 191) codes can be improved by more than six orders of magnitude. As a result, error-free (less than 1 × 10 −9 ) transmission is observed in our real-time experiment. Furthermore, 1.485 Gbit/s 720p high-definition video underwater transmission is successfully demonstrated. To the best of our knowledge, it is the first time to demonstrate a real-time LED-based UVLC system with DMT modulation beyond Gbit/s.

Free space optical (FSO) communication has emerged to provide line of sight connectivity and higher throughput over unlicensed optical spectrums. Cognitive radio (CR), on the other hand, can utilize the radio frequency (RF) spectrum and allow a secondary user (SU) to share the same spectrum with the primary user (PU) as long as the SU does not impose interference on the PU. Owing to the potential of these emerging technologies, to provide full spectrum efficiency, this paper focuses on the mixed CR RF-FSO transmission scheme, where RF communication is employed at one hop followed by the FSO transmission on the other hop in a dual-hop decode-and-forward (DF) configuration. To quantify the performance of the proposed scheme, closed form error probability is derived over Rayleigh/Exponentiated Weibull (EW) fading distributions by considering the statistical and instantaneous feedback channel of the primary network. We also employed an asymptotic analysis to illustrate the diversity gain of the overall system. We believe that the proposed scheme can be applicable to the 5G+ networks where an unlicensed university student connects to the home computer with the aid of an FSO path.

In this paper, we propose a novel method to generate real signal for Visible Light Communication (VLC) systems without using traditional hermitian symmetry on the data symbols obtained using M-ary Pulse Amplitude Modulation (PAM) which is named as Auxiliary PAM (Ax-PAM). We mathematically analyse this method to generate a real signal with tunable energy using auxiliary symbols at the transmitter and the corresponding receiver. Simulation results for Bit Error Rate (BER) show better performance over conventional PAM Discrete Multi-Tone (PAM-DMT) and Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) even under clipping distortion and also demonstrate the tunable energy for the proposed scheme. Furthermore, the proposed scheme is implemented on a VLC test bed designed using Universal Software defined Radio Peripheral (USRP). The experimental results for estimated Signal to Noise Ratio (SNR) and achieved BER for Ax-PAM outperforms PAM-DMT and ACO-OFDM.

In the underwater visible light communication (VLC) system, the single-photon avalanche diode (SPAD) can expand communication distance. However, the output signals from practical SPADs under dead time limit are not Poisson distributed. In this paper, the generalized likelihood block detection (GLBD) receiver is developed for practical SPAD-based underwater VLC system with on–off keying modulation. The proposed receiver can detect the data sequence without any prior knowledge of the channel and the background radiation. Correspondingly, a fast search algorithm for GLBD receiver is also proposed. In addition, a block coding scheme is utilized to solve the error floor problem. Simulation results show that the bit error rate (BER) performance of the proposed receiver is closer to the BER low bound compared to the existing receiver. Moreover, the fast search algorithm reduces the computational complexity without any performance loss.

We investigate temperature-dependent carrier transfer and efficiency droop on AlGaN-based deep ultraviolet light-emitting diodes. The Shockley-Read-Hall (SRH) recombination and carrier leakage are highly associated with the poor thermal stability. The existence of Auger recombination and carrier leakage is identified by the m-power method. A modified ABC model with an additional term f ( n ) related to carrier leakage is employed to analyze the evolution of multiple recombination mechanisms. The SRH process strongly suppresses both Auger recombination and carrier leakage at low currents. At high currents, the latter two processes are responsible for the efficiency droop and exhibit an anti-correlation upon temperature.

We present a new method based on Weierstrass factorization to optimize broadband perfect absorber (BPA) made of metal-dielectric-metal elements, which is an efficient and general approximation to calculate the absorption of these subwavelength structures. With the resonant wavelengths estimated by semi-analytical equations, we design a planar BPA tiling three subunits in one unit cell and a vertical tapered BPA stacking 20 pairs of metal-dielectric layers in one unit cell. Both BPAs shows almost over 90% and wide angle absorption in the concerned range. This method can be an alternative to the traditional full vector methods in initial design.

Fano-like lineshapes in whispering-gallery modes (WGMs) microresonators are of critical for many actual applications, such as high-sensitivity sensors, slow light, and optical switches. In this paper, we theoretically and experimentally demonstrate dynamic transmission lineshapes, including Lorentzian lineshapes, Fano-like lineshapes and gain-like lineshapes, with a simple system, where a surface nanoscale axial photonic (SNAP) microbottle is coupled to the transition of a tapered fiber by carefully choosing tapered fiber diameters. Controlled and robust coupling with a clean and almost equidistant spectrum for different axial modes is achieved while maintaining contact between the resonator and the taper. Our device offers five similar dynamic transmission lineshapes arranged in order simultaneously, demonstrating stable tuning and a high number of potential degrees of freedom in contrast to other coupling systems for single- or double-coupled microresonators. By using coupled-mode theory, these transmission lineshapes are fitted to explain these experiment observations. We also explore a tunable transmission spectra obtained by increasing the powers of the input laser. Our approach hold unique potential in sensitivity-enhanced sensing, quantum information processing, and all-optical switching.

A femtosecond soliton erbium-doped fiber ring laser with the symmetrical saturable absorber (SA) of graded index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) is proposed and proved. Based on the nonlinear multimode interference (NL-MMI) effect of SA, the fiber laser achieved femtosecond soliton mode-locking operation and repetition rate tuning. The duration of soliton was 364 fs at 1562.5 nm and repetition rate was tunable from 10.29 MHz to 763.36 MHz. In the structure of SA, a short step index multimode fiber (SIMF) was inserted between two graded index multimode fibers (GIMFs) to generate more high-order modes, adjust the self-focusing length, and improve operation stability of mode-locked fiber laser. The SA also has the advantages of immunity to the external environment variation, high damage threshold and inoxidizability, which make the laser operate stably for a long time.

The growing popularity of the mini-camera is posing a serious threat to privacy and personal security. Disguised as common tools in rooms, these devices can become undetectable. Moreover, conventional active laser detection systems often fail to recognize them owing to their small lens size, weak reflectivity, and the influence of interference targets. In this paper, a method for building a laser active detection system for mini-cameras is proposed. Using a monostatic optical system and a deep learning classification algorithm, this anti-camera system can detect mini-cameras accurately in real time. This article describes the system components including its optical design, core components and image processing algorithm. The capability of the system for detecting mini-cameras and identifying interference is also experimentally demonstrated. This work successfully overcomes the limit of mini-camera detection using deep learning methods in active laser detection systems.

A high-power all-fiber supercontinuum (SC) source in a highly germania-doped fiber (GDF) pumped by a picosecond thulium-doped master oscillator power amplifier (MOPA) system is presented. By further optimizing the length of the GDF, the long-wavelength edge of the wideband flat SC can be broadened to 3 μm. The flat SC has an average output power of 11.62 W and a 3 dB bandwidth of 750 nm. The power proportion beyond 2400 nm is as high as 55.3%. To our knowledge, this SC average output power is the highest from a highly GDF.

Image fusion integrates complex information about a target scene from multiple sensors into a single image. The fused image can further be utilized for human perception or different machine vision tasks. In the case of infrared and visible images, infrared images have the advantage of capturing thermal radiation intensity, whereas visible images are superior in gradient texture. In order to effectively fuse thermal intensity of infrared image and texture advantage of visible image, we propose a novel fusion method based on L0 decomposition and intensity mask. The proposed method first acquires base and detail layers of images (visible & infrared) using L0 decomposition. Next, an intensity mask is obtained using the basic global thresholding method on base layers of infrared image. The layers (base layers and detail layers) and visible images are divided images into three parts by the use of intensity mask, namely, mask-base layers, mask-detail layers, and texture-background. The first and second parts effectively achieve intensity blending, whereas the third part achieves the fused image with a clear gradient texture. The proposed method shows superior performance when compared with five state-of-the-art methods (on publicly available databases).

Implementing any linear transformation matrix through the optical channels of an on-chip reconfigurable multiport interferometer has been emerging as a promising technique for various fields of study, such as information processing and optical communication systems. Recently, the use of multiport optical interferometric-based linear structures in neural networks has attracted a great deal of attention. Optical neural networks have proven to be promising in terms of computational speed and power efficiency, allowing for the increasingly large neural networks that are being created today. This paper demonstrates the experimental analysis of programming a $4\times 4$ reconfigurable optical processor using a unitary transformation matrix implemented by a single layer neural network. To this end, the Mach-Zehnder interferometers (MZIs) in the structure are first experimentally calibrated to circumvent the random phase errors originating from fabrication process variations. The linear transformation matrix of the given application can be implemented by the successive multiplications of the unitary transformation matrices of the constituent MZIs in the optical structure. The required phase shifts to construct the linear transformation matrix by means of the optical processor are determined theoretically. Using this method, a single layer neural network is trained to classify a synthetic linearly separable multivariate Gaussian dataset on a conventional computer using a stochastic optimization algorithm. Additionally, the effect of the phase errors and uncertainties caused by the experimental equipment inaccuracies and the device components imperfections is also analyzed and simulated. Finally, the optical processor is experimentally programmed by applying the obtained phase shifts from the matrix decomposition process to the corresponding phase shifters in the device. The experimental results show that the optical processor achieves 72 $\%$ classification accuracy compared to the 98.9 $\%$ of the simulated optical neural network on a digital computer.

In this paper, leveraging the resonant phase and the geometry phase simultaneously, we demonstrated a low scattering lens antenna, which can work at both transmission and reflection mode. Different from the traditional microwave lens which only realizes wavefront shaping in half-space, the proposed metasurface lens not only squeezes the transmitted divergent beam to improve the gain of the conventional circular-polarized antenna, but also scatters the electromagnetic wave to maintain a low reflection property in broadband. The proposed lens antenna working in full space may provide a new approach for an integrated communication system.

A dual-frequency absorber with periodic double-layer graphene ring arrays separated by dielectric spacer is proposed. Coupled-mode theory and finite difference time domain method are used to analyze the perfect absorption and the absorption mechanism. Moreover, the influences of structural parameters and chemical potential of graphene on the peak positions have been investigated. Benefiting from chemical potential can be controlled by external bias voltage, thus the proposed structure can be used as a dynamically tunable absorber. Such simple absorber has potential applications in optical storage devices and frequency-selective detectors for terahertz regime.

Microwave photonic technologies have been introduced for achieving broadband radio-frequency signal measurement. However, few of the proposed schemes mention the low-power radio-frequency signal detection, which stringently limits their practical applications in certain areas. In this paper, we designed and demonstrated a wideband low-power radio-frequency signal measurement system with optoelectronic oscillator. Here, the unknown radio-frequency signal matched the potential oscillation mode is allowable to be detected, amplified and estimated. The key component in the tunable optoelectronic oscillator is a silicon nitride micro-disk resonator with a very high Q-factor, which is utilized to achieve frequency selection as a microwave filter. A frequency measurement system range from 1 ∼ 20 GHz with radio frequency power as low as −105 dBm, measurement errors of ±375 MHz and the maximum gain of 61.7 dB were realized experimentally

To address the problem that the traditional multi-focus images fusion methods cannot fully use of spatial context information. A novel image segmentation method for multi-focus image fusion is proposed. This method consists of two main steps, using PSPnet combined, and the region optimization using ConvCRFs for multi-focus image fusion. PSPnet is utilized to extract the focused region of the source image, and ConvCRFs is used to optimize the segmentation map to obtain a refined map. Finally, 20 couples of color multi-focus images are employed as experimental datasets, and the contrast results show that the proposed method has a better fusion visual effect than the other state-of-the-art in subjective and objective point of view.

We demonstrate a chip-scale germanium-silicon optical phased array (OPA) fabricated on a CMOS-compatible platform capable of 2D beam steering in the mid-infrared wavelength range. The OPA included a specially designed grating emitter waveguide array with uniform emission intensity along the mm -length waveguide propagation to realize very sharp instantaneous field-of-view (IFOV) and wide beam-steering total-field-of-view (TFOV). The experimental results indicated lateral beam-steering TFOV up to 12.7° by phase-tuning the waveguide array and longitudinal TFOV up to 12° by wavelength tuning. The 3-dB beam divergence is 3.08° × 0.18°. The demonstrated OPA architecture can employ wafer-scale fabrication and integration while supporting sensing and imaging applications in the mid-infrared spectral range.

In this paper, we present the architecture and the experimental characterization of an improved version of a previously developed 32 × 32 Single Photon Avalanche Diodes (SPADs) and Time to Digital Converters (TDCs) array, and two new arrays (with 8 × 8 and 128 × 1 pixels) with the additional capability of actively gating the detectors with sub-nanosecond rise time. The arrays include high performance SPADs (0.04 cps/μm 2 , 50% peak PDE) and provide down to 410 ps Full-Width at Half-Maximum (FWHM) single shot precision and excellent linearity. We developed a camera to exploit these imagers in time-resolved, single-photon applications.

Optical crosstalk is one of major problems limiting the performance of focal plane arrays based on single photon avalanche diodes (SPADs). In this work, for the first time the crosstalk characteristics of a 4H-SiC SPAD linear array are studied, which is based on a real-time dual channel data acquisition method. It is found that SiC SPAD array exhibits similar crosstalk probability compared with those of InGaAs-based SPAD arrays, which can be effectively reduced by trench isolation between adjacent pixels or by lowering overbias. The average time required for forming a crosstalk event is determined between 7 and 10 ns. As a result, active time-gated operation scheme can be applied for reducing crosstalk.

We demonstrate superconducting nanowire single photon detector (SNSPD) for the wavelength at around 2 μm. The linewidth of the NbN nanowire is squeezed to 56 nm to increase the intrinsic response efficiencies at longer wavelengths such as 2 μm. Serially connecting avalanche architecture is applied to increase the signal-to-noise ratio (SNR) of the response signal. Further, the optical cavity is optimized to improve the absorption of the device. A silica single-mode fiber is adopted to introduce photons to the SNSPD at a temperature of 2.25 K using a Gifford–McMahon cryocooler. The SNSPD exhibits detection efficiencies of 58%, 67%, and 63% at wavelengths of 1550, 1700, and 2000 nm, respectively, with dark count rate of ∼12 kcps, which is reduced to 2 cps when the attached fiber pigtail is all placed inside the 40-K cryostat. The detection efficiency at 2000 nm is 2.5 times greater than that of the best previously developed detector with an efficiency of 25%. Our SNSPD is promising for practical applications in molecular science and earth meteorology.

In this paper, we proposed an ultra slim optical zoom system with Alvarez freeform lenses. A 3X optical zoom system with two pairs of Alvarez lenses was developed. Alvarez lenses are fabricated by precise injection molding and the movable elements are actuated by voice coil motors. A slim camera module was fabricated with a size of 25 mm(width)×25 mm(length)×6 mm(height). The zooming and imaging capabilities of this Alvarez zoom system are demonstrated experimentally. The prototype exhibits a promising future for space-constrained application such as mobile phone camera module, wearable imaging system and endoscopic system.

A unique type of refractive index (RI) sensor is proposed using a dual Mach-Zehnder interferometer (MZI) structure based on side-hole fibers (SHFs). The MZI structure contains two single-mode fibers (SMFs), two coreless fiber (CLF) sections and an SHF section, which are spliced together in the order of SMF-CLF-SHF-CLF-SMF. The SMFs and the CLFs enable light lead in/out and beam splitting/combining, respectively. As a special feature of the structure, one hole of SHF is exposed for liquid filling and to form two MZIs as well. Three types of sensors are fabricated, namely S1, S2 and S3. Numerical simulation and experimental studies have been conducted to characterize the sensing performance. The RI sensitivity of the S1 using the ∼550 μm long SHF section reaches 14,000 nm/RIU. When a shorter SHF section is used in S2, the detectable RI range is broadened due to larger FSR. When the closed hole of SHF in S3 is filled with liquid to introduce the Vernier effect, the sensitivity can be further enhanced to over 44,000 nm/RIU (i.e., Refractive Index Unit), which corresponds to the detection limit at the level of 1.0 × 10 −5 RIU. This sensor design is original and easy to package, which gives it potential for label-free biochemical analyses.

In this paper, we demonstrated an all-fiber bidirectional ultrafast thulium-doped fiber laser at 2 μm with a single-wall carbon nanotube presented as saturable absorber. We successfully obtained bidirectional mode-locking operations with pulse repetition frequency adjustable from 35 to 122 MHz by shortening the cavity length. Meanwhile, with the reduction of the intracavity dispersion, the number of Kelly sidebands was significantly decreased and the output energy was more concentrated on the soliton pulse. Besides, by manipulating the pump power and polarization controller, the two results of bidirectional mode-locked pulses with the same repetition frequency and differential repetition frequency were observed. We found that the repetition rate difference between the clockwise and counterclockwise pulse trains was adjustable by changing the pump power or controlling the intracavity polarization. When the pulse repetition frequency was 35 MHz and 122 MHz, the adjustment range of repetition rate difference was 764–922 Hz and 540–2000 Hz, respectively. It is believed that this novel laser source can support free-running dual optical comb spectroscopy to detect important air gases like H 2 O or CO 2 in the future.

This paper proposes a novel key distribution system based on the measurement of the bit error rate (BER) in a fiber channel. By carrying out the loopback BER measurement at both the transmitting and the receiving end, the BER is quantified and codified to generate consistency keys. Alice and Bob respectively encrypt the signal with the help of the public key base at the transmitting end. At the receiving end the data are decrypted and then the BER is measured. The security of generated keys in the system is enhanced by the use of the encryption base and the randomness of the channel. Additionally, noise sources are added to the channel so that the random noise conceals signals and Eve cannot eavesdrop the useful information in it. Due to the randomness of the channel noise, the generated keys have good randomness. This system at a high key generation rate is compatible with the existing communication device and its measurement is simple. The 10G bit/s-200 km coherent optical communication system is used to measure the BER of the security feature information of the extracted channel. The experimental results show that the key distribution system obtains a consistency rate of 98% with the key generation rate 2 Mbps, both of which have been significantly improved.

We present a simultaneous radio-frequency (RF) and high-speed baseband signal transmission using an electrically superimposed method over a graded-index silica multimode fiber (GI-MMF). To show the feasibility of the method, we experimentally demonstrate simultaneous transmission of electrically superimposed 28-GHz RF and 28-Gbit/s 4-level pulse amplitude modulation (PAM-4) baseband signals at a wavelength of 850 nm over a 50-m GI-MMF. Moreover, to evaluate the scalability of the method, we demonstrate simultaneous transmission of dual-channel, electrically superimposed 28-GHz RF and 14-Gbit/s non-return-to-zero on-off keying baseband signals at 850 nm and electrically superimposed 14-GHz RF and 14-Gbit/s PAM-4 baseband signals at 1550 nm over the 50-m GI-MMF. These results show that the presented method is useful for effectively utilizing the transmission band of transmitters and existing short-reach transmission systems.

Direct detection (DD) is one of the most attractive solutions for short and medium reach optical fiber transmission in comprehensive consideration of digital signal processing (DSP), power consumption, and cost. Various single side band (SSB) DD techniques have been widely investigated to detect and process the complex signal. In this paper, we propose a novel approach which uses only high-speed digital input/outputs (IO) to generate both quadrature phase shift keying (QPSK) signal and virtual carrier signal. A transmission of 4 × 32 Gb/s QPSK signal over 1200-km standard single mode fiber (SSMF) in block-wise manner has been successfully demonstrated. Only a single polarization in-phase-and-quadrature (IQ) modulator and one single photodiode (PD) are used for each channel. Compared with conventional DSP of coherent detection, the DSP in the proposed scheme is also simplified that only fixed frequency compensation is required without additional phase noise elimination. The overall system complexity has been greatly reduced.

An improved particle filter algorithm based on Cam-Shift algorithm applied to outdoor visible light communication (outdoor-VLC) is presented in this paper. In the outdoor-VLC system, accurately and completely tracking and extracting the target signal source Light Emitting Diode (LED) area is the premise for realizing communication. However, few existing studies pay attention to it. In the dynamic outdoor environment, there will be a lot of different environmental interferences (such as background interference, similar object interference, etc.), which greatly increases the difficulty of tracking and extracting the target signal source LED area. Therefore, in this paper, an improved tracking algorithm is proposed to solve the problem of how to track and extract the target signal source LED area accurately, stably and in real time in the outdoor-VLC system under various environmental interferences, so as to increase the feasibility of practical application of VLC system in outdoor scenes. This improved algorithm combines the particle filter algorithm and Cam-Shift algorithm originally. Experimental results show that the proposed algorithm has good accuracy, robustness and real-time performance under the environment of multiple interference factors. Accordingly, the proposed algorithm can be applied to the outdoor-VLC system with various environmental interferences, and can realize the actual first step of communication in VLC system based on image sensor well, laying a foundation for feature extraction, data transmission and other subsequent steps.

The free space optical communication (FSOC) signals often exhibit a much larger dynamic range and much lower OSNR than optical fiber communication signals, due to the large-scale relative motion of the terminals, change of atmospheric channel conditions and large transmission loss. For this reason flexible digital coherent receivers (DCR) capable of adapting to different channel conditions have attracted much attention in the FSOC field. As timing recovery is indispensable for DCRs, the development of a timing recovery algorithm (TRA) capable of processing such FSOC signals is of particular interest in FSOC field. In this paper we evaluate the performance of Gardner's timing error detector (TED) and propose a new blind TED with a lower computation complexity and more stable output characteristics. Based on the proposed TED, we design a feedback parallel TRA suitable for the FSOC signals and implement it in a FPGA to evaluate its real-time performance. Numerical simulations and experiments illustrate the merits of the proposed TRA based on new TED with respect to the TRA based on Gardner's TED.

A novel optical frequency-hopping system based on distributed feedback laser integrated with an electroabsorption modulator is proposed and demonstrated. In the proposed system, a user's data are split into segments and modulated onto multiple optical frequency shift keying carries, therefore, a single wavelength of optical carrier only carries partial user's data. By this way, the security of optical communication system is improved. In this paper, we demonstrate an error free transmission through 32-km single mode fiber and 8-km dispersion compensation fiber with 2.5 Gb/s hopping rate and 10 Gb/s data rate. Meanwhile, we also compare and analyze the performance of the proposed system with different hopping rate and data rate.

Traditional soft or hard decisions ignoring the correlation between symbols removing linear and nonlinear damage directly may lead to lack of information and result in system performance degradation. In order to reduce the performance degradation of the system caused by the lack of information, we propose a method that use the Gaussian mixed model to consider the influence of the correlation between consecutive symbols for underwater visible light communication system. The experimental results show that the performance of the system is greatly improved after the correlation between consecutive symbols is considered. The more consecutive symbols there are, the greater the performance will be. When the forward error correction (FEC) threshold is met, the Q factor of Gaussian mixture models(GMM) that join three adjacent symbols is 1.19 dB greater than without the GMM clustering algorithm. The highest data rate that can be achieved exceeds 1.5 Gbps with GMM.

In this work, the influences of thermal annealing and chemical passivation on the optical and electrical properties of ultraviolet light-emitting-diode (UV-LED) were investigated. The electroluminescence (EL) intensities of the LEDs under KOH treatment and thermal annealing increased by 48% and 81%, respectively compared to as-fabricated LED under current level of 10 mA. Cathodoluminescence (CL) mapping of UV-LEDs confirmed no variation of the density of the non-radiative recombination centers after surface treatments, and no obvious change in surface morphology was identified due to lacking of energy for surface atom migration. However, Raman spectroscopy indicates a relaxation of compressive strains inside the thin film after both thermal and chemical treatments, and conductive atomic force microscopy (c-AFM) also illustrated reduced leakage current after KOH passivation, which are responsible for the improved luminescence properties of UV-LEDs.