Experimental demonstration of a free space optical wireless video transmission system based on image compression sensing algorithm.

The wireless transmission of video data mainly entails addressing the massive video stream data and ensuring the quality of image frame transmission. To reduce the amount of data and ensure an optimal data transmission rate and quality, we propose a free-space optical video transmission system that applies compressed sensing (CS) algorithms to wireless optical communication systems. Based on the Artix-7 series field programmable gate array (FPGA) chip, we completed the hardware design of the optical wireless video transceiver board; the CS image is transmitted online to the FPGA through Gigabit Ethernet, and the video data is encoded by gigabit transceiver with low power (GTP) and converted into an optical signal, which is relayed to the atmospheric turbulence simulation channel through an attenuator and a collimating mirror. After the optical signal is decoded by photoelectric conversion at the receiving end, the Camera-Link frame grabber is d; thus, the image is collected, and it is reconstructed offline. Herein, the link transmission conditions of different algorithm sampling rates, optical power at the receiving end, and atmospheric coherence length are measured. The experimental results indicate that the encrypt-then-compress (ETC) type algorithm exhibits a more optimal image compression transmission reconstruction performance, and that the 2D compressed sensing (2DCS) algorithm exhibits superior performance. Under the condition that the optical power satisfies the link connectivity, the PSNR value of the reconstructed image is 3-7 dB higher than that of the comparison algorithm. In a strong atmosphere turbulence environment, the peak signal-to-noise ratio (PSNR) of the corresponding reconstructed image under different transmission rates at the receiving end can still exceed 30 dB, ensuring the complete reconstruction of the image.

Optical adaptive power control based on atmospheric channel reciprocity for mitigating turbulence disturbances in free-space optics communication.

A continuous time-domain adaptive power model of transmitter optical and control algorithm based on atmospheric turbulence channel reciprocity are explored for mitigating the free-space optical communication (FSOC) receiver optical intensity scintillation and bit error rate (BER) deterioration. First, a transmitter optical adaptive power control (OAPC) system architecture using four wavelength optical signals based on atmospheric turbulence channel reciprocity is proposed, and electronically variable optical attenuator (EVOA) and erbium-doped fiber amplifier (EDFA) are employed as the main OAPC units for power adaptation. Moreover, a reciprocity evaluation model for gamma-gamma (G-G) continuous-time signals is generated using the autoregressive moving average (ARMA) stochastic process, which takes into account the delay time and system noise, and a reciprocity-based OPAC algorithm is proposed. Numerical simulations were also performed to analyze the signal reciprocity characteristics under different turbulence, noise, and sampling time mismatch at both ends, as well as the scintillation index (SI) performance under OAPC system operation. Simultaneously, the time-domain signals of continuous quadrature amplitude modulation -16 (QAM-16) and QAM-32 real states are fused with the gamma-gamma (G-G) reciprocal turbulence continuous signals to analyze the probability density function (PDF) and bit error ratio (BER) performance after OAPC correction. Finally, a 64 Gpbs QAM-16 OPAC communication experiment was successfully executed based on an atmospheric turbulence simulator. It is shown that the OAPC correction is carried out using reciprocity at millisecond sampling delay, the light intensity scintillation of the communication signal can be well suppressed, the signal-to-noise ratio (SNR) is greatly improved, the suppression is more obvious under strong turbulence, the overall BER reduction is greater than 2.8 orders of magnitude with the OAPC system, and this trend becomes more pronounced as the received power increases, even reach 6 orders of magnitude in some places. This work provides real time-domain continuous signal samples for real signal generation of communication signals in real turbulence environments, adaptive coding modulation using reciprocity, channel estimation, and optical wavefront adaptive suppression, which are the basis of advanced adaptive signal processing algorithms.

Research on one-to-two-point FSO system based on liquid crystal variable retarder cascade polarization grating.

We experimentally demonstrated a one-to-two-point free-space optical communication (FSO) system based on non-mechanical beam servo device in the laboratory. After the initial pointing, two sets of liquid crystal variable retarder cascaded polarization gratings perform non-mechanical beam servo and realized switching or working simultaneously of two communication links. The non-mechanical beam steerer had four diffraction fields; each can achieve beam steering with a 3.72° field and 30.77 µrad resolution, and the system emission efficiency was higher than 77%. The corresponding switching times of links at 2, 4, and 10 Hz were 46.7, 43.8, and 42.1 ms, respectively. In the quasistatic condition, the sensitivities of the two links under the data rate of 10.3125 Gbps were -23.18 and -23.01 dBm, respectively, indicating the service transmission capability of the multi-node beam control system.

Design of achromatic annular folded lens with multilayer diffractive optics for the visible and near-IR wavebands.

In this paper, the annular folded lens (AFL) is applied to the realization of a miniaturized system for the visible and near-IR spectrums (0.45-1.1µm). In order to correct the chromatic aberration, a hybrid AFL is designed with the multilayer diffractive optical element (MLDOE) in which the substrate materials are precision molded glasses. We propose a new design method of the MLDOE to improve the polychromatic integral diffraction efficiency (PIDE) that makes it suitable for the optical path of the AFL. By comparing the characteristic angle weighted PIDE (CAW-PIDE), the optimal microstructure heights of the MLDOE can be obtained, and the effect of diffraction efficiency on image quality can be minimized for the entire incident angle range. The design results show that the ratio of total length to the focal length is only 0.332, and comprehensive modulation transfer function considering the diffraction efficiency is larger than 0.26 at 166 lp/mm. This study can provide a new idea for designing a broadband, miniaturized, and low-cost imaging system.

Tunable multiwavelength Tm-doped fiber laser based on the multimode interference effect.

A simple multiwavelength Tm-doped fiber laser at the 2 µm band based on multimode interference (MMI) is proposed and experimentally demonstrated. In this scheme, a 4 m Tm-doped single-mode fiber is pumped by a 1568 nm laser, and a single-mode-multimode-single-mode (SMS) fiber structure is used as an MMI filter in which the multimode fiber is used to tune the laser. Laser operation of up to three wavelengths is obtained based on the MMI filter. The wavelengths can be tuned by adjusting the polarization controller and rotating the multimode fiber in the SMS structure, and the tuning region is about 24 nm, i.e., 1892-1916 nm. The side-mode suppression ratio of the laser is about 54 dB. The 3 dB linewidth is less than 0.04 nm. Peak fluctuation at each wavelength is analyzed, and the results show that the power fluctuation is less than 3 dB around the average power.

Frequency switched narrow linewidth microwave signal photonic generation based on a double-Brillouin-frequency spaced fiber laser.

A simple photonic approach to generate microwave frequency switched microwave signal is proposed and experimentally demonstrated. In this scheme, a Brillouin fiber laser with double-Brillouin-frequency spacing is used. The Brillouin ring configuration suppresses incoming Brillouin pump and even-order Stokes signals in the cavity. In addition, it also allows propagation of the odd-order Brillouin Stokes signals from configuration to output coupler. A dual-wavelength optical signal is heterodyned at the high-speed photodetector to produce a microwave signal. Frequency switched microwave signals, at 10.75 and 21.39 GHz, respectively, can be obtained through adjusting the polarization controller (PC) and loss of the variable optical attenuator (VOA).

Stereoselective construction of fluorinated indanone derivatives via a triple cascade Lewis acid-catalyzed reaction.

A one-pot three-component cascade reaction proceeds by way of a Lewis acid-catalyzed Knoevenagel condensation/Nazarov cyclization/electrophilic fluorination sequence to afford fluorinated 1-indanone derivatives in moderate to good yields with high diastereoselectivities.

Highly enantioselective Michael addition of aromatic ketones to nitroolefins promoted by chiral bifunctional primary amine-thiourea catalysts based on saccharides.

[reaction: see text] A new class of thiourea catalysts have been developed which integrate saccharide and primary amine moieties into one small organic molecule. These simple catalysts are shown to be highly enantioselective for direct Michael addition of aromatic ketones to a range of nitroolefins (up to 98% ee).