System optimization of an all-silicon IQ modulator : achieving 100 Gbaud dual polarization 32QAM

We experimentally demonstrate the highest, to the best of our knowledge, reported net rate in a SiP IQ modulator. At 100 Gbaud 32QAM (quadrature amplitude modulation), and assuming 20% FEC (forward error correction) overhead, we achieved a dual polarization net rate of 833 Gb/s. This record was achieved by adapting digital signal processing to the challenging pattern dependent distortion encountered in the nonlinear and bandwidth limited regime. First the Mach Zehnder modulator (MZM) operating point (trading off modulation efficiency and 3 dB bandwidth) and linear compensation (electrical and optical) are jointly optimized. Next, the key application of nonlinear preand post-compensation are explored. We show that nonlinear processing at the transmitter, in our case an iterative learning control (ILC) method, is essential as post-processing alone could not achieve reliable communications at 100 Gbaud. Nonlinear post-compensation algorithms pushed the performance under the FEC threshold with the introduction of structured intersymbol interference in post processing and a simple one-step maximum likelihood sequence detector. We provide detailed descriptions of our methodology and results

Modeling Multi-wavelength Pulse Profiles of Millisecond Pulsar PSR B1821-24

PSR B1821$-$24 is a solitary millisecond pulsar (MSP) which radiates multi-wavelength pulsed photons. It has complex radio, X-ray and $\gamma$-ray pulse profiles with distinct peak phase-separations that challenge the traditional caustic emission models. Using the single-pole annular gap model with suitable magnetic inclination angle ($\alpha=40^\circ$) and viewing angle ($\zeta=75^\circ$), we managed to reproduce its pulse profiles of three wavebands. It is found that the middle radio peak is originated from the core gap region at high altitudes, and the other two radio peaks are originated from the annular gap region at relatively low altitudes. Two peaks of both X-ray and $\gamma$-ray wavebands are fundamentally originated from annular gap region, while the $\gamma$-ray emission generated from the core gap region contributes somewhat to the first $\gamma$-ray peak. Precisely reproducing the multi-wavelength pulse profiles of PSR B1821$-$24 enables us to understand emission regions of distinct wavebands and justify pulsar emission models.Comment: Accepted for publication in Ap

Fast Synchronization Method of Comb-Actuated MEMS Mirror Pair for LiDAR Application

MEMS-based LiDAR (micro-electro–mechanical system based light detection and ranging), with a low cost and small volume, becomes a promising solution for the two-dimensional (2D) and three-dimensional (3D) optical imaging. A semi-coaxial MEMS LiDAR design, based on a synchronous MEMS mirror pair, was proposed in our early study. In this paper, we specifically reveal the synchronization method of the comb-actuated MEMS mirror pair, including the frequency, amplitude, and phase synchronization. The frequency sweeping and phase adjustment are simultaneously implemented to accelerate the MEMS mirror synchronization process. The experiment is set up and the entire synchronization process is completed within 5 s. Eventually, a one-beam MEMS LiDAR system with the synchronous MEMS mirror pair is set up and a LiDAR with a field of view (FOV) of 60°, angular resolution of 0.2°, and frame rate of 360 Hz is obtained. The experimental results verify the feasibility of the MEMS mirror synchronization method and show a promising potential application prospect for the MEMS LiDAR system

Trellis-Coded Modulation-Enabled Probabilistic Shaping With Simplified Viterbi Decoder for Bandwidth-Limited IMDD Systems

In this article, trellis-coded modulation-enabled probabilistic shaping (TEPS) scheme with a simplified Viterbi decoder is proposed. Trellis-coded modulation (TCM) is embedded in the probabilistic shaping (PS) technique to improve the performance of PS signals in bandwidth-limited intensity modulation and direct detection (IMDD) systems. Further, to reduce the calculation complexity of TCM decoding, the constellation partition decision-assisted (CPDA)-Viterbi decoder is proposed which decides the received signal before decoding to reduce the branches. To verify the performance of the proposed scheme, a 32-GBaud TEPS-four-level pulse-amplitude modulation (PAM4) experimental system with 10G-class O-band directly-modulated laser (DML) is built over a 20-km standard single-mode fiber (SSMF) transmission link. The experimental results show that TEPS-PAM4 achieved up to 1.80-dB higher receiver sensitivity than PS-PAM4 at 7% forward error correction (FEC) limit with the same net rate. Moreover, with the multiplications reduced by up to 45.7% compared with the Viterbi decoder, TEPS-PAM4 with CPDA-Viterbi decoder has up to 1.50-dB higher receiver sensitivity limit than PS-PAM4 at 7% FEC