Due to the characteristics of photon-counting LIDAR, there exists range walk error (RWE) when the intensity of the signal fluctuates. In this paper, an effective method to rectify underwater RWE was proposed. The method allows the separation of signal detections from noise detections, and based on a prior model, the method can compensate for RWE. An underwater experiment verified its feasibility and results showed RWE of three parts in a plane was reduced from 75mm to 7mm, from 45mm to 3mm and from 5mm to 0mm, respectively, even when the rate of backscatter photons reached 4.8MHz. The proposed correction method is suitable for high precision underwater photon-counting 3D imaging application, especially when the signal intensity varies sharply.

To implement high sensitivity, good anti-interference and low data rate simultaneously in optical heterodyne system, a photon counting heterodyne mechanism is proposed in this letter. Due to the stochastic behavior, a single photon avalanche diode (SPAD) is used as both a photon collector and a sparse sampler. On the one hand, it allows to reduce the power of the intermediate frequency (IF) signal to 1 picowatt or lower. On the other hand, it enables to generate fewer data samples while recovering the spectrum based on compressive sampling (CS). Compared with direct FFT method that recovers the spectrum by FFT directly, the CS method uses the photon arrival sequence as a measurement to recover the spectrum of the IF signal and suppress other frequency components. Therefore, a higher signal-to-noise ratio (SNR) can be implemented. Experimental results show that the SNR of the CS method is approximately 15 dB higher than that of the direct FFT method. In addition, the photon counting heterodyne achieve better anti-interference capability than classical heterodyne.

Backscatter has significant influence on detection efficiency for underwater lidar, especially for coaxial photon-counting lidar using a Geiger-mode avalanche photodiode. In this paper, based on our underwater coaxial photon-counting lidar structure and volume scatter function, a detection model with consideration of backscatter and refraction indices is proposed. Using this detection model, analysis of the detection efficiency is conducted. It reveals that in an underwater environment, higher pulse energy or a closer target range is not necessarily helpful for a higher target detection probability, which is vastly different from our traditional concepts. For example, the detection probability for a 5 m target would be 0.76 using a 200 pJ pulse and 0.55 using a 1000 pJ pulse for our coaxial photon-counting lidar. Monte Carlo simulation is conducted to verify our model and analysis, and some practical methods for improving the target detection probability in an underwater environment are proposed.

Photon-counting LIDAR has effective ability to detect significantly weak echo. However, for imaging application, it typically needs a long acquisition time to obtain sufficient counts to reconstruct clear depth image, which obviously limits imaging efficiency and prevents its wide deployment. In this paper, we proposed a low acquisition time photon-counting imaging method based on two single element SPADs and pixel scanning structure. Compared to fast imaging with SPAD array, the method has low requirement on laser energy and can obtain depth image with arbitrary pixel resolution. Experiment result demonstrates its feasibility and a dynamic target can be clearly captured by the system with our specifically designed reconstruction algorithm. We also detailly investigated the working conditions for the method. It reveals more than 2 echo photons per pulse would be enough to reconstruct clear depth image even when noise is intensive. Restricted to our 10 KHz laser source, the result only exhibited 1 frame per second(fps) imaging with pixel resolution of 100*100. If a common 10 MHz laser is adopted, 512*512 pixels resolution images can be captured at the rate of nearly 40 fps.