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The problem of automatic modulation format identification (MFI) is one of the main challenges in adaptive optical systems. In this work, we investigate MFI in super-channel optical networks. The investigation is conducted by considering the classification of seven multiplexed channels of 20 Gbaud, each with six commonly used modulation formats, including polarization division multiplexing (PDM)-BPSK, PDM-QPSK, and PDM-MQAM with (M = 8, 16, 32, 64). The classification performance is assessed under different values of optical signal-to-noise ratio (OSNR) and in the presence of channel interference, channel chromatic dispersion, phase noise, and $1^{st}$ polarization mode dispersion (PMD). Furthermore, the effect of fiber nonlinearity on the MFI accuracy is investigated. A well-established machine learning algorithm based on histogram features and a convolutional neural network has been used in this investigation. Results indicate that accurate identification accuracy can be achieved within the OSNR range of practical systems and that the MFI accuracy of side subcarriers outperforms that of middle subcarriers at a fixed value of OSNR. The results also show that the MFI accuracy of PDM-16QAM and PDM-64QAM are affected more by channel interference than the other modulation formats, especially when the ratio of the subcarrier bandwidth to subcarriers spacing is $\geq$ 1.4. Finally, laboratory experiments have been conducted for validation purposes. The experimental results were found in good agreement with those achieved by simulation.

Metal-dielectric thin film structure metamaterials can be designed to absorb solar light radiation over a wide spectral band. By using Kirchhoff's law, metal-dielectric thin film metamaterials are investigated for obtaining high equilibrium temperatures under direct solar light radiation at normal incidence. It is found that among all metal-dielectric thin film structures, the dielectric layer on metal (DLM) surface structure is the optimal structure for producing the highest thermal equilibrium temperature under direct solar optical radiation.

To figure out the energy attenuation of micro/nanofibers (MNFs) more flexibly and conveniently, a backpropagation neural network (BPNN) is proposed to forecast the output intensity of rhodamine B (RhB) doped polymer microfibers (PMFs). According to the diameter, doping concentration, and propagation distance (L), we realize the L-dependence of output energy predictions for the excitation light (IE) and fluorescence (IF) of the doped PMFs. Hundreds of propagation distance-intensity data pairs acquired from dozens of RhB doped PMFs are used for the BPNN training. The prediction ability of the model is evaluated by the root-mean-square error (RMSE), the mean absolute percentage error (MAPE), and R2. The output intensity prediction performance of BPNN is compared with the traditional exponential-fitting (EF) method. The prediction results indicate that the two-hidden-layer network with one and seventeen neurons respectively provides the best performance. After training, BPNN gives a good intensity prediction for both the IE (RMSE = 3.16×10−2, MAPE = 7.3%, and R2 = 0.9802) and the IF (RMSE = 0.91×10−2, MAPE = 0.89%, and R2 = 0.9696) from the output end of the PMF with different diameters and doping concentrations. The energy losses of the two kinds of light from different doped PMFs are also calculated based on the predicted values, which are similar to the ones obtained from the EF method. The approach based on the BPNN prediction for the energy attenuation of the PMFs shows superiority in flexibility and applicability toward the traditional methods, which could promote the optimal design of the MNF devices and the practical application.

We present a detailed simulation study on plasmonic-organic hybrid electro-optic modulators based on coupled symmetric or asymmetric plasmonic slots. An electro-optic polymer is exploited as an active material, and the device is compatible with a silicon photonics platform. The proposed device operates at 1550 nm wavelength, typical of data center or long-haul telecommunication systems. The device performance in terms of area, plasmonic losses, optical bandwidth, intrinsic modulation bandwidth and energy dissipation are comparable to already proposed Mach-Zehnder solutions, but with potentially better extinction ratio, coupling losses due to photonic-plasmonic transitions, and flexibility in exploiting, without any performance penalty, asymmetric slots to shift the ON and OFF states bias. Finally, the bias dependence of the modulation chirp is investigated, comparing through and cross-coupling configurations.

We developed a digital electro-optical phase-locked-loop (EO-PLL) utilizing a repetitive control, allowing the use of very nonlinear tunable lasers for frequency-modulated continuous-wave (FMCW) light detection and ranging (LIDAR) applications. This approach allows reducing the phase error of continuously repeated frequency chirps to virtually zero by using information of previous chirps and additionally guaranties sub-second settling times. With this method, much higher nonlinearities in the laser frequency modulation slope γ are tolerable than with common real-time analog PLLs. Furthermore, we applied a Goertzel algorithm generalized to non-integer multiples of the fundamental frequency to achieve a mean measurement precision of 5.41 µm and 16.35 µm at a distance from the interferometer baseline of 3 m and 10.3 m, respectively.

Trapped ions are excellent candidates for quantum computing and quantum networks because of their long coherence times, ability to generate entangled photons as well as high fidelity single- and two-qubit gates. To scale up trapped ion quantum computing, we need a Bell-state analyzer on a reconfigurable platform that can herald high fidelity entanglement between ions. In this work, we design a photonic Bell-state analyzer on a reconfigurable thin-film lithium niobate platform for polarization-encoded qubits. We optimize the device to achieve high fidelity entanglement between two trapped ions and find >99% fidelity. Apart from that, the directional coupler used in our design can achieve any polarization-independent power splitting ratio which can have a rich variety of applications in the integrated photonic technology. The proposed device can scale up trapped ion quantum computing as well as other optically active spin qubits, such as color centers in diamond, quantum dots, and rare-earth ions.

In recent years, Kramers-Kronig (KK) receiver has been widely studied because of its simple structure and the ability of digital signal processing (DSP) to recover complex value signals. One practical structure is detecting the optical signal with an AC-coupled photodetector. However, the direct current (DC) component of the photocurrent will be lost, in which case the KK algorithm ceases to be effective. To restore the lost DC component, we propose a simple data-aided method to estimate the DC component by the information of signal mean power stored in the designed data-aided sequences. We introduce a correction coefficient to evaluate the accuracy of our DC estimation by comparing it to the conventional DC sweeping method. Furthermore, the coefficient can be used to combat the DC estimation deviation. We carry out both simulations and experiments to study the performance of the proposed method. The results show that the DC component can be effectively recovered and the system performance is close to the results of the DC sweeping method. At the same time, in the case of low optical signal-to-noise ratio, optimization of correction coefficient can effectively improve the system performance.

The cross phase (CP) opens up a new horizon for the generation and measurement of vortex beams, and the conversion between Hermite-Gaussian (HG) and Laguerre-Gaussian (LG) modes can be done with a CP. However, the conversion plane is in the far plane, and the width cannot be adjusted. In this article, we present a more convenient and practical mode converter between HG and LG modes based on CP combined with a lens. The feasibility of the converter for the conversion between two modes is theoretically analyzed. The conversion can be accomplished in the determined plane and the width of converted beam can be controlled by adjusting the parameter of converter. Proof-of-concept experiment is carried out to verify theoretical analysis and the experimental results are agree well with theoretical simulations. The transfer of orbital angular momentum (OAM) between two modes after passing through the converter is also analyzed. From the perspective of OAM conservation, the required parameter conditions for the converter are discussed. In principle, such a converter as proposed here might even be applied to microwave, millimeter wave, and terahertz OAM fields.

We propose a novel indoor optical wireless communications (OWC) system using few-mode based beam shaping integrating time-slot coding (TSC) scheme to serve multiple users simultaneously. We simulated an indoor OWC system incorporating single-sideband (SSB) 16 quadrature amplitude modulation (16-QAM) modulated signals in a multi-user environment employing few-mode based beam shaping with single-photodetector (PD) and multi-PD reception with maximum ratio combining (MRC), respectively. The results show that for both single-PD and multi-PD receptions, the bit-error-rate (BER) performance can be significantly improved at different positions within the cell and the size of the effective coverage (BER−3) can be increased by 4 times by employing few-mode based beam shaping scheme at 1:1.79 power ratio between LP01 mode and LP11 mode, compared to conventional OWC systems without few-mode based beam shaping scheme. In addition, the system tolerance to signal overlapping can also be enhanced with the proposed few-mode based beam shaping. The optimal power ratios of LP01 mode and LP11 mode when achieving the maximum effective coverage under 7% and 20% HD-FEC thresholds for single-PD and multi-PD reception have also been investigated. Furthermore, the tolerance to the interval delay with 0.23 and 0.28 symbols can be achieved in single-PD and multi-PD reception, respectively.

Flexible Pulse Amplitude Modulation (FlexPAM) consists of the use of non-standard PAM formats, such as PAM-5 or PAM-6, to increase the granularity of the link capacity for a given baud rate. In this paper, we demonstrate FlexPAM systems as a scalable solution for future data center interconnects, introduce a symbol-to-bit mapping that minimizes the effect of intensity noise in the performance of non-power of two number of levels formats, and present experimental and simulation results of an unamplified FlexPAM transmission system over 1 km of single-mode fiber. 20 GBaud transmission was successfully demonstrated experimentally for modulation formats ranging from OOK up to PAM-16 using an integrated InP-Si3N4 laser. In addition, a simulation to closely replicate the experiment was used to determine the system requirements to achieve transmission rates of up to 200 Gbps on a single wavelength, at a baud rate of 53.5 Gbaud with component bandwidth limitations of 30 GHz.

An optical fiber sensor is proposed and experimentally demonstrated for simultaneous measurement of high temperature and transverse load. The sensor is designed by splicing a silica capillary tube (SCT) to a single-mode fiber (SMF) and then heating the SCT to form an air bubble. SCT simultaneously functions as a Fabry Perot (F-P) microcavity and an anti-resonant reflection waveguide. These sensing mechanisms lead to high contrast sensitivity values of temperature and transverse load (1.89pm/°C,190.48pm/N and 24.93pm/°C, 16.68pm/N). Remarkably, the temperature sensitivity of this sensor is eleven times higher than that of the current fiber-tip micro-cavity (2.1 pm/°C). The sensor is also more advantageous in quantitative production and practical application due to its low cost, ease of fabrication, good mechanical strength, and convenient reflection probe.

InGaN light-emitting diodes (LEDs) grown with an asymmetric multiple quantum well (MQW) are proposed for use in an optical link with an avalanche photodiode (APD) based receiver. In contrast to the high photoresponse of red AlGaInP LEDs in APDs, the proposed blue LEDs provide improved light output and enhanced system bandwidth for directed line-of-sight optical links passing through a 100-cm-long water tank. This improvement is due to the nonuniform carrier distribution within the InGaN MQWs being mitigated by using a thin GaN barrier near the n-GaN to facilitate hole transport capacity. In addition, bandwidth degradation resulting from APD module saturation can also be avoided by using these blue LEDs, successfully establishing a 300 Mbit/s LED-based underwater data link. The proposed InGaN LEDs (zero bias) under illumination exhibit a peak responsivity of 0.133 at λ = 370 nm, an ultraviolet (UV)-to-visible rejection ratio of 4849 and a 3-dB cut-off frequency of 33.3 MHz. Using violet UV laser diodes and the proposed LEDs respectively as the optical transmitter and receiver, an underwater optical link (L = 100 cm) with a data transmission rate of up to 130 Mbit/s and a bit error rate of 4.2 × 10−9 is also demonstrated.

The use of nanometre fiso interferometer to measure heterodyne interference signals at the same time for phase and phase element detection and phase‐locked frequency multiplication counting is employed here. After a certain logic mixing, a set of measurement results with a certain degree of redundancy can be obtained; the test results can be measured and resolved. The rate is better than 0.1 nm, and the measurement range is not limited by the λ/2 period, thus overcoming the previous problems that is the measurement range of the phase measurement method is limited to one period and the resolution of the phase‐locked frequency multiplication counting method is not easy to improve.

An optical fiber probe composed of a fiber Bragg grating (FBG) and a reflective S fiber taper (SFT) with graphene oxide (GO) film, for simultaneous relative humidity (RH) and temperature measurement, was proposed and experimentally demonstrated in this paper. The SFT is made into reflection type by depositing silver mirror at the fiber end face, which is highly sensitive to surrounding refractive index (RI). The GO nanosheets, as a two-dimensional hydrophilic nano material, is coated on the fiber surface, acting as a RH-to-RI converter. The simultaneous RH and temperature measurements are realized by monitoring the characteristic wavelength shifts of the SFT and FBG in the reflection spectrum. Experimental results show that the RH sensitivity of the probe is 161.1 pm/%RH during the range of 30%RH∼90%RH, and the temperature sensitivity is 14.1 pm/°C during the range of 10 °C∼50 °C. The proposed probe is compact, flexible, and easy to be fabricated, which has potential application for precise localized measurement in medical and biochemical fields.

We propose a frequency modulation (FM) non-coherent detection (NCD) scheme, attempting to suppress the mode partition noise (MPN) in fiber-optic communication systems with the Fabry-Perot (FP) semiconductor laser as the light source. The system comprises a FM transmitter with a directly modulated FP laser and a semiconductor optical amplifier, standard single mode fiber, and a NCD receiver with an optical slope filter as the FM to intensity modulation (IM) signal convertor placed in front of a conventional photodetector. In this configuration, the MPN is converted into a random frequency deviation by the transmitter and is reduced by the slope filter after fiber transmission. The FM signal is therefore less noisy after being converted back into the IM signal. With optimized parameter selections, our simulation result shows that the FM-NCD system allows a direct transmission span of 40 km for the 10 Gbps signal at 1577 nm, and 120 km for the 25 Gbps signal at 1310 nm, respectively, as opposed to a span of only 3 km and 10 km achievable by a conventional FP laser driven IM direct detection (DD) system for the corresponding signals at the corresponding wavelengths. Remarkably, our simulation result also shows that the performance of the FM-NCD system even surpasses that of a directly modulated distributed feedback laser driven IM-DD system, in which the maximum span is around 15 km and 60 km for 10 Gbps and 25 Gbps signals at 1577 nm and 1310 nm, respectively.

Optical vortices (OVs) have received much attention in recent years and are widely explored in communication applications. The on-chip vortex emitter/modulator is preferred when it comes to photonic integration and chip-scale communication. However, the existing emitters/modulators suffer from either narrow bandwidths or extensibility problems, which actually degrade their ability to improve communication capacity. Here, we propose a metasurface OV emitter based on wave vector synthesis for vortex generation and modulation. A thorough theoretical analysis of the OV generation by two approximate methods was carried out, which suggests the superiority of Wave Vector method over Phase method. Afterwards, the wave vector manipulation implemented by Phase gradient metasurfaces was studied for controlling the topological charges of generated OVs. Finite element method simulations showed that, in the topological charge range of −6 to 6 and the wavelength range of 1.45 μm to 1.55 μm, the effective vortex emission efficiency of the proposed emitter is around 45%. The on-chip vortex generation and control by wave vector manipulation is a very promising technology for high-capacity inter-chip optical communication.

Metasurfaces, which are ultrathin planar artificial materials flexibly controlling the phase, amplitude, and polarization of the light, have facilitated the high integration and miniaturization of hyperspectral imaging systems. However, the pushbroom hyperspectral imaging systems based on metasurfaces usually face the large smile and keystone, which reduces the spectral uniformity and the fidelity of the spectral information. Moreover, the employment of many metasurfaces would cause manufacturing difficulties. In this paper, a planar hyperspectral imager only using two metasurfaces is proposed to achieve a spectral resolution of 3.5 nm in 850-1000 nm band with the near diffraction-limited focusing capability, and its maximum of smile and keystone reach to 124.4 μm and 33.6 μm for the image plane size of 4.75 mm × 1.35 mm and pixel size of 26 μm × 26 μm, respectively. This imager can provide a promising avenue for portable environmental monitoring systems and wearable electronic consumer products.

Time delay signature (TDS) is a key issue that affects the security performance of optical chaos communication. Currently, the autocorrelation function (ACF) and delayed mutual information (DMI) are two crucial methods to extract the TDS. Several strategies have been proposed to resist statistical analysis (i.e., ACF and DMI). However, a dynamic inverse modeling method has been proposed to identify TDS by utilizing the strong nonlinear inversion capability of deep learning, which brings about new threat. To resist this analysis, we propose a stratagem that one can increase the nonlinear dimensionality of the chaos system at the transmitter and concurrently reduce the dimensionality of transmitted signals, leading to “deficiency of data dimensionality”. A three-dimensional chaotic system is designed as the transmitter and only one-dimensional signal is transmitted for communication. Furthermore, a hybrid receiver is designed. The nonlinear model of the response system is realized by cooperation of an analog electro-optical link and a neural network in digital domain. The architecture not only destroys the possibility of TDS extraction but also preserves all necessary information required for chaos synchronization with low-complexity implementation.

In this article, a photonic scheme based on dual-drive dual-parallel Mach-Zehnder modulator (DD-DPMZM) to generate switchable dual-band dual-chirp microwave waveforms is proposed and experimentally demonstrated. In the scheme, the switching among two single-band dual-chirp signals and a dual-band dual-chirp signal can be readily realized by simply adjusting the bias voltages on the modulator. Theoretical analysis on the operation of the approach is presented. Full experimental verification on the generation of switchable dual-chirp signals in the S-band and C-band has been successfully demonstrated. The presented scheme features in the simple structure, easy tunability and reconfigurability, which has potential applications in multi-band radar.

Visible light communication (VLC) has been recognized as a promising technology for handling the continuously increasing quality of service and connectivity requirements in modern wireless communications, particularly in indoor scenarios. In this context, the present work considers the integration of two distinct modulation schemes, namely spatial modulation (SM) with space time block codes (STBCs), aiming at improving the overall VLC system reliability. Based on this and in order to further enhance the achievable transmission data rate, we integrate quasi-orthogonal STBC (QOSTBC) with SM, since relaxing the orthogonality condition of OSTBC ultimately provides a higher coding rate. Then, we generalize the developed results to any number of active light-emitting diodes (LEDs) and any $M$-ary pulse amplitude modulation size. Furthermore, we derive a tight and tractable upper bound for the corresponding bit error rate (BER) by considering a simple two-step decoding procedure to detect the indices of the transmitting LEDs and then decode the signal domain symbols. Notably, the obtained results demonstrate that QOSTBC with SM enhances the achievable BER compared to SM with repetition coding (RC-SM). Finally, we compare STBC-SM with both multiple active SM (MASM) and RC-SM in terms of the achievable BER and overall data rate, which further justifies the usefulness of the proposed scheme.

Integrated optofluidic biosensors can fill the need for sensitive, amplification-free, multiplex single molecule detection which is relevant for containing the spread of infectious diseases such as COVID-19. Here, we demonstrate a rapid sample-to-answer scheme that uses a field programmable gate array (FPGA) to enable live monitoring of single particle fluorescence analysis on an optofluidic chip. Fluorescent nanobeads flowing through a micro channel are detected with 99% accuracy and particle concentrations in clinically relevant ranges from 3.4x104 to 3.4 × 106/ml are determined within seconds to a few minutes without the need for post-experiment data extraction and analysis. In addition, other extract salient experimental parameters such as dynamic flow rate changes can be monitored in real time. The sensor is validated with real-time fluorescence detection of single bacterial plasmid DNA at attomolar concentrations, showing excellent promise for implementation as a point of care (POC) diagnostic tool.

Terahertz vortex beams need to be developed for supporting new applications in fundamental sciences. In this paper, a novel and feasible scheme is presented for generation of helically microbunched electron beam for the emission of terahertz (THz) vortex pulses. In the scheme, a seed laser pulse with longitudinal and azimuthal modulation is firstly produced by optical beating of dual-vortex pulses. Through the laser-electron interaction, such a complex profile of laser can be converted to a periodically azimuth-dependence energy modulation in the electron beam, which finally forms a helical microbunching structure in the beam after a dispersion section. Then, this electron beam is applied to emit THz vortex beams and its quantum number can be continuously tuned by changing the topological charges of the optical laser pulses. This approach is suited to conventional accelerator system consisted of a modulator and a chicane. We expect this presentation can access to new researches in THz science.

Optical tunneling is an effect extremely sensitive to wavelength changes. In this article, we use this effect to design a new type of biosensor. The tunneling effect is excited by long range surface plasmon resonance, the energy is concentrated in the sensing medium in the form of standing waves, which lead a very high wavelength sensitivity and quality factor. By using COMSOL for finite element analysis, for the sensor structure of BK7/Cytop /Al2O3/Ag/Al2O3/sensing media/air, a quality factor of 10296 RIU-1 is achieved under angle modulation, a sensitivity of 170,000 nm/ RIU and a quality factor of 11333 RIU-1 are obtained under wavelength modulation. At the same time, the advantage of this structure is that there is no excessive requirement on the refractive index of the material, and a good sensing effect can be achieved even if the material is cheap and easy to prepare. After optimization, this structure is suitable for sensing media with a refractive index of 1.335-1.375, which can be applied to most liquid solutions.

This study proposes a simplified Alamouti-type space-time coding (STC), improving the performance of image sensor communication (ISC) using a rotary LED transmitter. The rotary LED transmitter was developed to increase the data rate of ISC using afterimages of LED lights. The transmitter simultaneously causes the LEDs to blink and rotates them around a vertical axis. Owing to the movement of the blinking LEDs that occurs within the exposure time of the camera, multiple blinking states are captured as afterimages, thus increasing the amount of information that can be received per image. However, with increasing communication distance, the size of the LED light captured on the image sensor decreases. In this case, it is difficult to distinguish each LED blinking state, leading to a degradation of the demodulation performance. To overcome this problem, the proposed STC encodes adjacent angular afterimages as symbol pairs and transmits these symbol pairs using two symbol times. In addition, we simplified the data decoding process by using normalized LED luminance values. We evaluate the demodulation performance of the proposed method through experiments. Compared with conventional coding methods, the proposed STC requires no channel estimation and significantly improves the demodulation performance.

Phase-only optical image security technology Based on one-time pad is an encryption and hiding method by introducing random key through phase modulation, which can improve the security of the system to a great extent. The application of traditional phase-only optical image encryption and hiding (POIEH) method is often limited by the amount of data and the quality of decrypted image. In this paper, a single exposure POIEH method using deep learning (DL) is proposed. The original image and corresponding encrypted hidden interferogram acquired with POIEH system are constructed to the train datasets for the learning of an end-to-end designed U-net, then the corresponding relationship between the encrypted hidden interferogram and the reconstructed image is learned and only single-frame encrypted hidden interferogram is needed to perform the reconstruction of POIEH. This method can realize real-time image encryption and hiding and high-quality decryption with only one interferogram. Simulation results and performance analysis show that the proposed method has higher security, stronger generalization ability and robustness.

Optical fiber sensors are of splendid strength for electrical field intensity sensor due to characteristics including the immunity to electromagnetic interference, lightweight, high sensitivity, and large bandwidth. In this paper, we proposed an electric field intensity sensor based on Mach-Zehnder interferometer (MZI) based liquid crystal (LC) filled photonic crystal fiber (PCF) embedded in optical fiber ring laser (FRL). The air hole of PCF combines the LC and fiber core together. When LC is introduced into air holes, it can maintain the waveguide based on external parameters. The photonic bandgap effect significantly improved the sensitivity between light and external electrical field intensity. Thanks to the FRL demodulation, a high signal to noise ratio (SNR) spectrum about 35 dB is obtained. Besides, in comparison with traditional LC-PCF structures, the sensitivity of ours is as high as 1.1 nm/Vrms which is about twice than traditional sensors. At the same time, the stability of proposed sensor was verified which fluctuation was 0.15 nm around 2.5 hours. Therefore, our structure is expected to practical applications in remote electric field monitor and such electric modulate electro-optical devices.

Superresolution microscopy is of great interests in modern optics and photonics applications. Subdiffraction focusing lens, that has superresolution focal spot with local spatial frequency greater than the maximum Fourier component, has attracted intensive attentions in past decade, because of the significant capability of unlabeled far-field superresolution. Here, we propose a dielectric subdiffraction focusing metalens based on the depletion of Bessel beams. Different from traditional binary wavefront modulation elements, this metalens is designed according to the reverse engineering of subdiffraction focal field, which is constructed from the depletion of a fundamental Bessel beam. With the independent amplitude and phase control ability of dielectric metasurface, this metalens enables breaking the diffraction limit and can produce a focal spot with full width at half maximum of 0.3λ/NA. This work may provide a platform for the design of integrated and compact photonic devices to control the light propagation dynamics and spin angular momentum associated with circular polarization.

Visible light communications (VLC) techniques are viewed as promising candidates for the upcoming B5G&6G mobile wireless networks. Due to the broadcast nature of VLC channel, physical layer security (PLS) techniques have been considered and explored to improve the confidentiality of VLC transmission. Nevertheless, almost all current designs focus on application scenarios including sufficient transmitters and fail to match the scenarios with limited, even just two transmitters. For addressing this challenge, in this article, multiple optical beam switching scheme is proposed. Unlike conventional Lambertian beam based technique paradigm, the given scheme tentatively utilize two asymmetry non-Lambertian beams to configure each VLC transmitter. Numerical results show that, for the typical center legitimate receiver, compared with about 0.92 bps/Hz average secrecy capacity (SC) of the conventional Lambertian beam configuration, up to 5.55 bps/Hz average SC gain is acquired through the proposed beam switching scheme. Moreover, this potential gain will be enhanced to about 5.86 bps/Hz when three candidate non-Lambertian beams are available for each transmitter.

Microwave photonic pulse generation has intrinsic high speed and large bandwidth, and it can overcome the sampling-rate limitation in the electrical domain. Herein, we propose a partly temporally-overlapped photon echo scheme to generate microwave pulses with a high data transfer rate and precision pulse width. The partly overlapped chirped control pulse has the same chirped rate segments but different time delays. The time delay between the segments and the number of segments can be adjusted to achieve a controllable multipulse photon echo train. We developed a theoretical model based on the photon echo process and performed simulations to obtain nonlinear chirped periodic spectral grating in a Tm3+:YAG crystal and gave the time delay provided by the spectral grating to the probe pulse at different instantaneous frequencies. We further generated pulse code modulation signals to verify the performance of the target signal. The proposed signal-generation scheme has potential applications in microwave photonics and modern radar systems.

The Fourier transform method is adopted to analyze random quasi-phase matched (RQPM) nonlinear frequency conversion problems in polycrystalline materials. By treating the material as random signals of effective nonlinear coefficient (deff) along the beam path, fast Fourier transform (FFT) operations directly produce the generated field amplitude versus spatial frequency mismatch (Δk/2π), which relates to all the possible RQPM nonlinear interactions. Thus, the RQPM bandwidth is intuitive from the wavelength-dependent results. More importantly, the Fourier transform treatment greatly decreases the computational amount by orders compared with the traditional step-by-step integration method. The quantitative simulation of RQPM difference frequency generation (DFG) with three variable wavelengths is realized for the first time.

A highly secure and reliable optical orthogonal frequency division multiplexing passive optical network (OFDM-PON) based on the chaos encryption inside polar code scheme is proposed. The 3-dimensional Chua's circuit model is adopted to realize chaotic encryption during polar encoding with low complexity. The three chaotic sequences generated by Chua's circuit model are used to encrypt information index bit matrix, frozen bits (during polar encoding) and subcarrier sequence, respectively. The BER and security performance of the optical transmission system are enhanced by our method. A 70 Gb/s polar encoded encrypted 16 quadrature amplitude modulation OFDM signal transmission over a 2 km seven-core fiber is further experimentally demonstrated. The experimental results show that the polar encoded encrypted 16 QAM-OFDM obtains a 2.1 dB gain in receiver sensitivity at a BER of 10−3 under the same bit rate compared to the normal 16 QAM-OFDM without FEC. Moreover, the key space of the proposed scheme is 1084, which is large enough to guard against any malicious attacks from illegal ONUs effectively. The combined superiority of BER and security performance enable such OFDM-PON based on chaos encryption inside polar code, to have a high prospect of application in low-cost and reliable optical access system.

This study proposes a broadband absorber based on graphene and one-dimensional photonic crystal (1DPC) to achieve magnetically tunable broadband absorption in the terahertz (THz) frequency range. The performance is analyzed using the 4 × 4 transfer matrix method, demonstrating that the proposed absorber operates in a broadband range of 3.34 to 4.68 THz for left-handed circularly polarized (LCP) waves with an absorption greater than 90%. The localized THz wave produces absorption peaks, achieves broadband with high absorption, and is tuned dynamically by varying the magnetic field. This work has potential for many applications, such as broadband filters, anti-radar stealth, and sensors.

In this paper, a new method for pesticide detection based on porous silicon microcavities was proposed. First, the aptamer of acetamiprid was fixed in the functionalized porous silicon microcavity. Then, the complementary strand of the aptamer modified with graphene quantum dots was hybridized with the aptamer, and the effective refractive index of the porous silicon microcavity increased. When acetamiprid was added, it bound to the aptamer due to its stronger specificity. The aptamer separated from the complementary strand of the coupled graphene quantum dots and bound to acetamiprid, resulting in a decrease in the effective refractive index, and its central wavelength of reflection spectrum was blue shifted, and the blueshift increased with the increase of acetamiprid concentration. The relationship between the change in central wavelength and the concentration of acetamiprid was analysed, and the detection limit of acetamiprid was 151 nm.

In this paper, an environment-robust polarization-based phase-shift dynamic demodulation method for optical fiber acoustic sensor is proposed. Through optical lever effect of polarization low-coherence interference and amplitude scaling differential cross multiplication (AS-DCM) algorithm, acoustic-source induced interference phase of the acoustic sensor can be demodulated in real-time with high precision. The maximum relative phase demodulation error was only 0.694% when the cavity length had an offset ∼4 μm. The maximum cavity length demodulation error was keeping less than 3.988 nm under the optical power reduced ten times, which demonstrated an ultra-stable demodulation for the largest permitted optical power attenuation range, to our best knowledge. The proposed method has a capability of tolerating large optical power attenuation and large cavity length random offset, providing an environment-robust way for optical fiber acoustic sensor working in extreme environments.

In this paper, a novel method for simultaneous measurement of rotation speed and vibration based on self-mixing speckle interference (SMSI) has been demonstrated. In view of the autocorrelation characteristic of SMSI, the time delay of laser incident during one rotation period is determined, and the rotation speed of the object is calculated according to the mathematical relationship. At the same time, the signal square-wave conversion and the global maximum fringe-counting method are proposed to measure the vibration displacement subject to the rotation. The theory and signal processing means are introduced in detail, and a series of experiments employing computer disk at different positions with various rotation speeds indicate that the proposed method achieves a simple, efficient, and accurate multi-parameter measurement.

Small target detection in high strength clutter background is in great in remote imaging system, a new improved spatio-temporal filtering was proposed in this paper. Firstly, traditional anisotropy filtering has poor suppression effect in strength edge contour region, so a new diffusion filtering function proposed in paper. According to the analysis with difference of each component of the image, a new anisotropy diffusion function is constructed in this paper. When the difference of background and target is small, this algorithm will give in large diffusion coefficient to filter most background clutter and retain target signal well which achieves background prediction better. Secondly, because the traditional spatiotemporal filter algorithm cant follow the motion object in the fixed search pipe diameter what will make lose the target detection, a new weight constraint function of adaptive change of the search diameter in this paper is built which can change the search diameter with the moving of target, and improve the detection accuracy. Finally, experiments show that compared with traditional algorithms and detected in different scenes, this method can enhance small target detection effectively.

Coherent state quantum communication is attracting extensive attention for its great value in terrestrial and satellite information networks. This paper proposes a new non-orthogonal quantum multi-user (MU) iterative detection scheme. At the receiving end, we develop a composite quantum receiver with multi-stage measurement instrument and soft iterative MU detector. Furthermore, based on a three-dimensional factor graph, a nonlinear Poisson-Gaussian noise limited coherent state iterative MU parallel interference cancellation (PIC) algorithm is derived. Our results demonstrate that this system can well support MU communication and achieve good performance. Quantitatively, for the 8 users scenario, the BER of $10^{-6}$ can be obtained when the photon number is 19. Further numerical results show that the system we designed exhibits superior robustness against imperfections (e.g., quantum mode mismatch and quantum thermal noise).

Internet of everything is being build up rapidly, which causes much higher requirements towards the flexibility of communication resources, especially the adaptivity of the fronthaul to various transmission distances and capacities. In this paper, we proposed a low-cost and spectrum efficient 5G fronthaul broadband connection solution that combines hierarchical modulation technology and delta-sigma modulation. The proposed scheme can double the number of accesses without occupying extra wavelengths. Moreover, such architecture can flexibly allocate the SNR of a group of remote radio units for different user requirement of transmission rate and reaches. Simulation results show that the proposed scheme can increase the access distance of the fronthaul network by 23% and the capacity by 16.7%. The feasibility of the solution is verified through a proof-of-concept experiment. After 20 km transmission, the OFDM-64QAM and -256QAM signal with a center frequency of 3.5 GHz and a bandwidth of 500 MHz can meet the EVM requirements of 8% and 3.5%, respectively. And when the ${R_{LM}}$ of the hierarchical PAM4 signal is 0.6, the ROP required by the most-significant bit branch can be reduced by ∼1.5 dB.

In turbid water, target reflected lidar signal is easily buried in “clutter” due to scattering. We proposed an underwater laser ranging model applying an optical vortex in the receiver to separate the scattering clutter from the target reflected signal. A spiral phase plate (SPP) was placed in front of the detector to convert the target reflected Gaussian beam to an optical vortex. While the scattering clutters were not able to be converted due to the loss of spatial coherence. Therefore, the target reflected signal and scattering clutters were spatially separated by the SPP. An underwater laser detection simulation using Zemax software was carried out. The numerical simulation showed that when the SPP was placed in front of the receiver, the contrast of signal to clutter of the detection was improved by blocking the scattering clutters which had a Gaussian distribution at the center on the detection plane. The effects of beam divergence and orbital angular momentum (OAM) order to the lidar performance were also analyzed.

The Shack-Hartmann wavefront sensor produces incorrect wavefront measurements when some sub-spots are weak and missing. In this paper, a method is proposed to predict the centroids of these sub-spots for the Shack-Hartmann wavefront sensor based on the deep neural network. Using the centroid information of present sub-spots, the method is able to predict the absent sub-spots’ positions. The feasibility and effectiveness of this method are verified by a large number of numerical simulations. The method is applied to wavefront measurement of light with non-uniform near-field intensity. The simulation results show that the proposed method is of great help to improve the measurement accuracy and the Strehl ratio of the focal spot. For wavefronts outside of the training sample, the proposed method shows good generalization and adaptability. In addition, the experiment results demonstrate that the proposed method can predict the missing sub-spots’ centroid displacements accurately even though a large proportion of sub-spot is lost randomly.

Fourier ptychographic microscopy (FPM), an emerging computational imaging technology, can realize wide-field and high-resolution simultaneously and thus achieves great success in biomedical fields. The computational imaging process, however, also accumulates various but inevitable noises into measurements. Conventional deep based reconstruction methods simply ignore the existence of noise and integrate all features from measurements for prediction, which severely degrades the quality of reconstructed images. To mitigate this issue, we present a novel network, dubbed DeUnet, that adaptively filters out noise during reconstruction through a new cross-level channel attention mechanism. Specifically, DeUnet comprises of three key stages: encoding, denoising and decoding. The encoding stage abstracts hierarchical features from input measurements. The denoising stage learns to re-calibrate channel-wise feature responses from multiple levels in a collaborative manner. It selectively highlights informative channels while suppressing noisy ones by exploiting long-range dependencies among features across multiple resolutions. The decoding stage integrates denoised features to reconstruct final intensity and phase images. Extensive experiments on both the simulated and real data well demonstrate the effectiveness of DeUnet in suppressing noise. In particular, DeUnet can produce reconstruction results of higher quality and more valuable details, especially for phase images, compared to previous state-of-the-arts.

Scattering media, such as biological tissues and turbid liquids, scatter light randomly and introduce several challenges when imaging objects behind them. The transmission matrix (TM) describes the relation between the input and output of a beam transmitted through a medium, which can be used to reconstruct a target located behind a scattering medium. However, the current TM methods cannot easily retrieve the phase distribution of objects inside or behind a scattering medium. In this work, a compressive sensing (CS) method to identify the TM of a scatter contained in an imaging system was investigated. By calibrating the TM, the phase information of the object can be retrieved quantitatively. This method allows one to retrieve multilevel and dynamic phase objects behind different scatters. The influence of the calibration parameters on the reconstruction quality was investigated in detail. The proposed method, featuring noninterference measurements of the TM and exploiting a large field of view, can be used in phase imaging applications.

To solve the matching problems caused by the large intensity difference between the multi-source images and the nonlinear radiation distortion, we present a multi-source image matching approach that considers the orientation of the phase sharpness. First, the scale-space of the image pyramid was constructed, and the phase consistency of the image frequency domain was solved to obtain the maximum moment feature, and the KAZE operator is used to extract the feature points. Next, the Log-Gabor even symmetric filter was used to perform Fourier transform, and the improved local phase sharpness feature and phase orientation feature were constructed respectively to replace the gradient amplitude and gradient direction feature of the image. A descriptor of local phase sharpness orientation (LPSO) was established through the log-polar description template, and finally the Euclidean distance was used to measure the similarity to obtain the corresponding points. Multiple sets of typical multi-source images were used as data sources, and the LPSO algorithm was compared with SIFT, LGHD, RIFT and HAPCG algorithms in datasets. Evaluation of the algorithm performances indicates that the proposed method is more accurate and robust in the task of multi-source image matching.

The design and analysis of large arrays of nanoscale scattering elements using existing computational tools is particularly difficult. Such difficulty mainly arises from large number of unknowns and the way existing CAD tools handle the analysis of complex nanostructures. In this work, a simple, reliable and fast analytical tool is introduced based on the surface impedance generating operator method, which not only reduces the number of unknowns through surface discretisation of the scatterers but can also be implemented in a parallel process, reducing simulation time. Circular and linear arrays of nanowires are designed using the proposed method to have a directive pattern and to be deployed in a rake‐shaped optical probe head. The far‐field scattering patterns of the arrays are investigated. In addition, the effects of array configuration as well as the number of elements and their properties on the directivity and beamwidth of the scattering patterns are also evaluated. Finally, high‐resolution directive scanning probes made of a circular array of 31 gold nanowires and a linear array of 15 elements are designed at 800 nm with less than 3° beamwidth.

In this work, we fabricated and characterized 4 × 4 parallel flip-chip AlGaN-based micro-LED arrays with varied mesa diameters of 120 µm, 100 µm, 80 µm, and 60 µm. The reported micro-LED arrays have a maximum bandwidth of 380 MHz and a peak wavelength of ∼275 nm. It is found that the electrical and optical characteristics of AlGaN-based micro-LED arrays show strong size dependence for ultraviolet communication (UVC). The differential resistance increases from 28.8 Ω to 112 Ω, the external quantum efficiency (EQE) is increased by ∼30%, and the bandwidth doubles as the diameter of individual micro-LED decreases from 120 µm to 60 µm. Our research proves that tailoring the mesa size of parallel flip-chip AlGaN-based micro-LED arrays can further enhance its bandwidth and promote its application in UVC.

This paper investigates the ergodic capacity performance of hybrid free space optics (FSO)/ radio frequency (RF) system by employing an adaptive-combining-based switching scheme. In the adaptive combining scheme, the FSO and RF links are combined using the maximal-ratio combining (MRC) at the output, when the instantaneous signal-to-noise ratio (SNR) of the FSO link drops below a predefined threshold SNR value. The FSO link experiences the atmospheric turbulence, which is modeled using the generalized Malaga distribution, non-zero boresight misalignment or pointing errors, and atmospheric attenuation, under two types of detection techniques, i.e. heterodyne detection (HD) and intensity modulation/direct detection (IM/DD). Similarly, the fading of the RF link is characterized using the generalized $\kappa$–$\mu$ distribution. In particular, we derive the probability density function (PDF) of the instantaneous SNR of the adaptive combining scheme. Capitalizing on the SNR statistics, we obtain the unified closed-form ergodic capacity expressions for the adaptive-combining-based hybrid FSO/RF system. In addition, the asymptotic expressions for the ergodic capacity of the system at the high-SNR region are presented. All the derived analytical expressions are validated by using Monte-Carlo simulations. Numerical results and discussions are provided to compare the ergodic capacity performance of the adaptive-combining-based hybrid FSO/RF system with the existing system models such as single-link FSO, MRC-based hybrid FSO/RF, and hard-switching-based hybrid FSO/RF systems.

Adaptive optics systems will encounter challenges under strong atmospheric turbulence conditions. Recently, a modified approach based on plenoptic sensor has attracted considerable attention. However, after wavefront correction under strong turbulence, we found that the existing wavefront reconstruction algorithm has difficulty in accurately reconstructing the residual aberrations. Therefore, we proposed a local threshold checkerboard algorithm to improve the accuracy of wavefront reconstruction by thresholding specific areas of central sub-apertures of the plenoptic sensor. In order to demonstrate the effectiveness of our algorithm, we designed and built an experimental platform. The platform included a deformable mirror, which we were able to adjust to simulate several different aberration conditions. Afterwards the direct-gradient method was used for correction of the residual aberrations, which were reduced to less than 0.1λ(RMS), demonstrating the effectiveness of our algorithm.

The orbital angular momentum (OAM) holography has been developed and experimentally demonstrated the capability of holographic multiplexation and high-security encryption. However, the helical phase of an OAM mode can only be encoded into one hologram associated with a target image, and this severely limits the information capacity in practical applications. The modulated orbital angular momentum (MOAM) holography is proposed by imposing multiple modulation phase modes onto one OAM mode. Three essential properties, including the MOAM-preservation, MOAM-selectivity and MOAM-multiplexation are investigated. This method can significantly enhance the holographic information capacity and it has broad prospects for optical encryption and beam manipulations.

An all fiber linearly polarized narrow linewidth pulsed fiber laser operating at 1016 nm is demonstrated in a master oscillator power amplification (MOPA) configuration. Core-pumped double cladding ytterbium-doped large mode area fiber amplifiers are proposed as the pre-amplifiers to reduce the nonlinearity and the ASE. The laser exhibits 48.6 W average output power, ∼18 kW peak power, and 0.28 nm linewidth.

In the fields of optics and photonics, phase-only spatial light modulators (SLMs) play an increasingly important role in wave-front engineering. However, the SLMs are subject to wavefront distortion arising from the imperfection in the birefringence effect and physical structure of modulators. This paper presents a simple self-interference phase calibration method applicable to liquid-crystal SLM. We build a interferometric imaging system basing on the Pancharatnam-phase-shifting to measure the phase distribution of a light beam coming from SLMs. Two types of phase modulation errors of SLMs can be characterized in the measurement process: the erroneous gamma curve and shape aberration. The former belongs to dynamic phase distortion and is measured through a four-step Pancharatnam phase shifting interference, which allows a one-shot recording of interference pattern via a polarization camera; the latter represents static phase distortion and is extracted from the interference between light waves coming from different regions of the SLM panel by using Zernike polynomial fitting. Our method has the advantages of high-precision pixel-wise phase correction and robustness against environmental disturbance, and thus can facilitate the applications of SLM in the optical field manipulation.