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.
Interferometric imaging technology requires accurate acquisition of the modulus, phase, and spatial frequency coordinate information of the complex coherence when reconstructing the target image. Due to the inherent jitter of optical fibers, it is relatively difficult to accurately obtain the phase information. Using the continuous hybrid input–output algorithm, the target image can be reconstructed when only the modulus and spatial frequency coordinate information of the complex coherence are known. However, in the actual imaging process, there are always certain errors in the measurement of the modulus and spatial frequency coordinate information, which affects the quality of the reconstructed image. Based on the theoretical calculation formula of the complex coherence modulus, we analyzed the factors affecting the measurement results of the complex coherence modulus and built an indoor testbed for experimental verification. Finally, through theoretical simulation, the impact of the error in the measurement of the complex coherence modulus on the quality of the reconstructed image was analyzed, proving the necessity of accurate measurement of the complex coherence modulus. The experimental and simulation results have important guiding significance for the design of subsequent interferometric imaging systems and experimental schemes.
Motion error compensation is a crucial aspect of processing inverse synthetic aperture light detection and ranging data. Motion phase error occurs mainly due to the relative motion between the target and the system, as well as vibration of either the system or the target, which significantly affects the image quality of optical synthetic aperture radar. Since spatial targets usually have a non-cooperative motion state with a high degree of motion parameter uncertainty, accurate estimation of cross-range phase error becomes challenging due to the presence of envelope tilt effect. We propose an adaptive compensation method that handles motion errors of maneuvering targets by estimating and compensating various types of errors introduced by the target motion process. Using the geometric and signal models to analyze error components, a compensation model is established that uses envelope contrast and image entropy as fitness functions. The bat algorithm is employed to solve this error model. Simulation and outdoor experimental results demonstrate that the proposed algorithm offers higher accuracy and better stability compared to traditional optimization algorithms.
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