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Wall-climbing hexapod robot as a bionic robot has become a focus for extensive research, due to a wide range of practical applications. The most contribution of this paper is to analyze the kinematics and stability of a wall-climbing hexapod robot, so as to provide a theoretical basis for the stable walking and control of the robot on the wall. Firstly, the kinematics model of the wall-climbing hexapod robot is established based on the D-H method. Then, in order to keep the robot from tipping over, the stability of the wall-climbing hexapod robot is analyzed in depth, obtaining the critical condition which makes the robot to tip over. Afterward, the kinematics simulation of the wall-climbing hexapod robot is operated to analyze motion performances. Finally, the experiments are used to validate the proposed kinematics model and stability. The experimental results show that the kinematics model and stability condition of the wall-climbing hexapod robot are correct.

In this letter, we report a cascaded surfaceilluminated Germanium-on-Silicon (Ge-on-Si) avalanche photodiode (APD) array with a low dark current. The photodetector array cascades seven Ge-on-Si APDs through interconnecting the charge regions to form photomultiplier tubesiPMTjC which increases the effective light absorption area compared to the single APD scheme. To the best of our knowledge, this is the first time that Ge-on-Si PMT has been reported. The optimum response of single-electrode (2 pixels) Ge-on-Si APD is 2.4 A/W with a dark current of 100 nA, and the optimum bandwidth is 52 MHz. PMT has a responsivity of approximately 3 times that of a singleelectrode Ge-on-Si APD for the same dark current (100 nA). With low dark current, our PMT has a better optical amplification effect than the parallel Ge-on-Si PD. We have also further tested the current tuning properties of the Ge-on-Si APD arrays with different applied Ge voltages. The test demonstrates the feasibility of cascading three-electrode APD arrays. A cascaded Ge-on-Si APD array with good photodetection properties at low-light conditions has been demonstrated which provides guidance for the design of large-scale Ge-on-Si APD arrays for light detection and ranging application.

The effect of the free carrier effect is more significant in the mid-infrared band when compared with the 1310 band and 1550 band. In this paper, we propose a cascaded Mach-Zehnder interference (MZI) structure to improve the extinction ratio (ER) of the modulator in the mid-infrared band. The cascaded compensation method is to add the next-stage equal-arm MZI device to the two phase shifters of the major MZI. The output light intensity of the two phase shifters can be maintained at the same level by adjusting the output loss of both the equal-arm MZI. With the cascaded compensation method, the simulated ER of the optical modulator is increased from 36 dB to 55 dB under −4 Vbias while the device still maintains a low insertion loss (IL) of 12.5 dB. Through the cascaded compensation method, the modulation depth of the modulator at −2 V, −4 V, −6 V, and −8 V are 58 dB, 53 dB, 57 dB, and 59 dB, respectively. Meanwhile, the dynamic ER is 9.2 dB at a data rate of 40 Gbps, which is 4.5 dB higher than that of the original one.