Traditional lidar sensitivity is not up to the requirements in detecting small long-range targets with weak echo signals. Due to the limitation by the background noise of sunlight and complex climate, the high sensitivity detection cannot be realized by a photon counting lidar with Geiger-mode Avalanche Photodiode Detector (Gm-APD). In this paper, a new system is proposed to improve the performance of photon counting lidar ranging based on two-dimensional modulation. The new system uses two modulation methods to modulate the transmitted signal, which are chirp modulation and photon orbital angular momentum modulation. Chirped modulation is used to adjust the transmitted signal waveform in the frequency domain, and the detection performance improvement of Gm-APD can be improved with response probability correction. Photon orbital angular momentum modulation is used to adjust the transmitted signal waveform in the spatial domain, which is used to filter out background noise. The simulation results show that two-dimensional modulation is independent and do not interfere with each other. Comparing with the traditional lidar, the detection probability increased by 52%; the SNR can be increased by up to 115%, the range accuracy can be up to two-thirds of the traditional range accuracy (means increased by 32%). The new system achieves high sensitivity detection in complex environment.
Traditional lidar is not effective in detecting small long-range targets with weak echo signals. Geiger-mode Avalanche Photodiode (Gm-APD) single photon detector has super high sensitivity. By adding photon orbital angular momentum modulation module, a photon-counting chirped amplitude modulation lidar with high sensitivity ranging function is designed. Based on the characteristics of photon orbital angular momentum space transmission, a special demodulation method is used to realize the spatial separation of noise and signal. The above scheme achieves the goal of high sensitivity ranging detection. The simulation results show that the scheme can improve the signal-to-noise ratio of the system effectively and realize the high-sensitivity ranging function of small target in long distance.
Lidar based on Geiger-mode Avalanche Photodiode Detector (Gm-APD), also called Gm-APD Lidar for short, has the advantages of the ultra-high sensitivity and ranging accuracy, and therefore it is widely used in the weak signal detection over a long distance. Time-Correlated Single Photon Counting (TCSPC) is a more commonly used signal processing method of Gm-APD Lidar. However, after each avalanche response, Gm-APD needs a certain time to quench avalanche current, which is called the dead time. In the dead time, Gm-APD can't response any signal. This will result in the uneven response by Gm-APD, and the response probability of the front of the echo pulse signal is higher than that of the back of the echo pulse signal. The peak of photon counting results will deviate from the real peak of the echo signal, and this deviation will become larger with the increase of the echo pulse width. In many application environments (for example, underwater, battlefield smoke, fog and dust, etc.), the broadening effect of the echo pulse signal is obvious, and this will seriously impact the ranging accuracy of Gm-APD Lidar. In this paper, an improved method uses the multi-gate detection to response the complete waveform of the echo pulse signal, and thus improves the ranging accuracy of GmAPD due to obtaining more accurate echo pulse peak.