Dr. Chen Xu Profile

Dr. Chen Xu

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Proceedings Article | 4 December 2024 Paper

Phase noise measurement of laser sources in coherent lidar system

Dengfeng Liu, Yutang Li, Chen Xu, Xi Luo, Kai Jin, Kai Wei

Proceedings Volume 13283, 1328346 (2024) https://doi.org/10.1117/12.3037209

KEYWORDS: Phase measurement, LIDAR, Interference (communication), Laser frequency, Fourier transforms, Ranging, Optical coherence, Signal to noise ratio, Receivers, Laser sources

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In coherent lidar systems, the phase noise of the laser source determines the maximum operation range and performance. To measure the laser phase noise of the lidar system, we studied three related indicators: the field spectrum, the frequency-modulation noise spectrum, and the phase-error variance. We established a delayed self -homodyne laser phase noise measurement system with an optical coherent receiver, and completed the measurement of phase noise in a MHz-level-linewidth laser. In addition, we proposed an indicator named effective coherent accumulation period (ECAP) of the coherent lidar system, in order to evaluate the coherent accumulation effect. The ECAP of the laser under test is calibrated, within the measured phase noise lev and tens-of-meters operation range. We experimentally verified the impact of effective coherent accumulation period in two typical coherent lidar systems. In FMCW ranging lidar, laser phase noise gradually accumulates with the increase of coherence accumulation time, resulting in a decrease in ranging accuracy. In micro-Doppler sensing lidar, this factor leads to a reduction in signal-to-noise ratio and spectrum broadening. The experimental results of this article can provide a reference for the model selection of laser sources in coherent lidar and the evaluation of coherent accumulation performance.

Proceedings Article | 30 April 2024 Paper

Motion compensation imaging algorithm for inverse synthetic aperture LiDAR of complex moving targets

Jian Li, Kai Jin, Chen Xu, Anpeng Song, Shengqian Wang

Proceedings Volume 13156, 1315616 (2024) https://doi.org/10.1117/12.3018208

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The inverse synthetic aperture LiDAR (ISAL) system demonstrates excellent performance in various application scenarios, including long-range target imaging and recognition. However, due to the micrometer scale of the ISAL system carrier, compensating for the motion error of high-speed complex motion targets becomes challenging. Linear frequency modulation (LFM) signals are not enough to describe these targets accurately, as they can be better represented by high-order polynomial phase signals. Nevertheless, the presence of second-order and higher phase components can greatly affect the coherent synthesis between pulses. To tackle this issue, we propose a non-searching fast cubic phase signal estimation algorithm based on the fourth-order instantaneous autocorrelation function. This algorithm aims to estimate the second-order and third-order coefficients of the rotational error signals using the non-uniform Fourier transform. By accounting for advection compensation, we model the phase of the signal of the complex motion target as the third-order form of the slow time. The Fourier transform of the fourth-order instantaneous autocorrelation function presents challenges, such as the nonlinear coupling between the slow time and delay time, as well as the nonuniform sampling of the delay time. We suggest utilizing the nonuniform Fourier transform to complete the two-dimensional transformation and estimate the obtained second-order and third-order coefficients. In comparison to the traditional range doppler (RD) algorithm, our proposed algorithm achieves better image point focusing and enhances the energy aggregation of scattering points by approximately four times. Simulation results validate the effectiveness of this algorithm.

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