Coherent lidar uses coherent detection technology to probe target information. The echo signal is mixed coherently with a local oscillator signal to detect the target using the heterodyne detection method. The beat frequency signal is then analyzed to obtain the velocity and distance information of the target. However, a coherent detection system is affected by various factors, such as lasers, phase modulation, transmission distance, and atmospheric turbulence, during the detection process. This leads to the introduction of phase disturbances into the echo signal, consequently compromising the coherence of the coherent detection system, which significantly affects the detection capability of the coherent lidar. Maintaining system coherence is crucial to ensure high detection sensitivity and precision in coherent detection systems. Therefore, research on the coherence of coherent detection systems is crucial. By studying the coherence of a coherent detection system, we can determine the coherence length or time of the coherent lidar, thereby obtaining the maximum detection range and maximum coherent integration time of the coherent detection system. Additionally, the coherence of a coherent detection system should be restored by compensating for the effects of phase disturbances. Through phase compensation, the coherence of a coherent detection system can be restored, thereby enhancing its detection performance of coherent detection system.

This paper evaluates the overall coherence of a coherent detection system using the Strehl ratio and calculates the coherence time of the system based on the Strehl ratio. By designing a coherent detection system with two lasers as separate local oscillators and transmitting laser sources, velocity detection is simulated under the condition of the super-coherence length in actual detection. An acousto-optic frequency shifter and attenuator are employed to simulate the Doppler frequency and signal attenuation after transmission over distances exceeding the coherence length. Moreover, phase compensation for both internal and external phase disturbances within the coherent detection system is performed using phase measurements and an iterative phase-estimation algorithm. Phase measurement compensation utilizes a part of the local oscillator laser and the transmitting laser as the reference signal for coherent heterodyning to monitor the phase disturbances in the echo signal. Subsequently, in the digital domain, the phase disturbances measured from the reference signal are used to compensate for the phase of the echo signal, thereby eliminating the influence of the phase disturbances caused by the laser frequency drift. The iterative phase estimation algorithm performs phase estimation through phase perturbations of time delay and atmospheric turbulence and compensates for them in the digital domain using an iterative method to obtain the optimal algorithmic compensation results. In the experimental system used in this study, the iterative phase estimation algorithm compensated only for the effect of the time-delay phase.

This paper systematically analyzes the uniform influence of various components of a coherent laser detection system (laser, modulator, distance, etc.) on the coherence of the system. The Strehl ratio is used to characterize the coherence degradation and coherence time (coherence length) of the coherent detection system. The simulation results verify that the coherence and detection accuracy of the coherent detection system are affected by the laser phase, phase modulation, and distance (Fig. 3, 4). This study simulates a coherent detection system with a super-coherence length using two lasers (Fig. 6) and evaluates the coherence time of the coherent detection system with a super-coherence length using the Strehl ratio (Fig. 8). The detection probability of the coherent detection system is 52%, the signal-to-noise ratio is 10.1 dB, and the velocity accuracy is 11.4 cm/s with a coherence time of 0.7 ms and echo signal power of 11 fW. After phase compensation (Fig. 7), the detection probability increases to 74%, the signal-to-noise ratio increases to 19.8 dB, and the velocity accuracy increases to 0.16 cm/s. Better detection performance is achieved with longer integration times (Fig. 10,11,12).

During the detection process, coherent detection systems are affected by various factors, such as laser sources, phase modulation, transmission distance, and atmospheric turbulence. These factors can introduce phase disturbances into the echo signal, leading to the degradation of coherence in the coherent detection system. This severely affects the detection capability of the coherent lidar. To simulate velocity detection in a coherent detection system with a super-coherence length, this study designs a structure with two lasers as separate local oscillators and transmitting laser sources. Acousto-optic frequency shifters and attenuators are used to simulate echo signal conditions in an actual detection environment. Utilizing two compensation methods, phase measurement and a phase-estimation iterative algorithm, can effectively solve problems such as decreased coherence and detection capability caused by phase disturbances and enable coherent detection systems to detect target signals beyond the coherence length. The synthesis shows that coherent lidar still has the characteristics of high sensitivity and high detection accuracy with a super-coherence length, which is significant to providing a feasible experimental basis for the target detection of long distances and weak signals with super-coherence lengths.