J. Eur. Opt. Society-Rapid Publ. 22, 6( 2026) 51
However, a special scenario that needs to be considered is when the photon moves underneath the target. As shown in Figure 4, for the vertical detection, the covered extent can be described approximately as
8
x 2 �L C 2; L
C
>< 2
; ð16Þ y 2 ½ �R 2; R 2 Š
>: z > H þ d w
where R 2 = d img tan( h cov / 2), d img = z + H( n w �1) is distance from the detector’ s image position P img to the circle center P C, h cov = 2sin �1( R C d img). When the photon enters this area, the detectable trajectory is blocked by the target, and the photon can no longer make contributions to the signal.
2.5 Interaction with the target
Noise is an influential factor of detection, which mainly includes background signal of solar radiance and detector noise. N b represents the background signal and is given by [ 70 ] kph 2 FOV
N b ¼ I b Aktg oe g qe; ð17Þ
4hc v
where I b is the solar spectral radiance reflected from atmosphere and the water’ s surface, A is receiver aperture area, Dk is the bandwidth of the detector’ s filter, Dt is the sample time, g qe is the quantum efficiency of the detector, g oe is the optical efficiency of the lidar system, k is the laser wavelength, c v is the speed of light in vacuum, h is the Planck constant. The signal-to-noise ratio of the target signal can be calculated by [ 70 ] 8 >< dN ¼
>: SNR ¼
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N st þ N sw þ N b þ N d
pffiffiffiffi m ðN st � N sw Þ dN
; ð18Þ
where N st is the lidar returned photoelectrons with target detection, N sw is the lidar returned photoelectrons without target( only water detection), N d is the dark count signal of the detector, and m is the number of the laser shots that need to be integrated.
3 Scanning oceanic lidar system
The airborne linear scanning oceanic lidar system( SOL) for underwater target detection is shown in Figure 5. Wechose a 532.1 nm Nd: YAG laser as the light source for strong water penetration. The laser operates reliably over a temperature range of 0 – 40 ° C. Under laboratory conditions at 25 ° C, both the output power and pulse energy exhibit stability better than 2 %, effectively reducing variations in pulse width and beam quality induced by environmental temperature fluctuations. The photomultiplier tubes( PMT) from the H10720-01 series manufactured by
Fig
. 5.( a) The schematic diagram of SOL system, the laser beam path is in green. Ap is short for aperture stop, CL is short for collimating lens, BF is short for band filter, FL is short for focusing lens, and PBS is short for polarizing beam splitter.( b) SOL is deployed on the UAV.
Table 1. Parameters of SOL system.
Parameter |
Description |
Transmitter |
Wavelength |
532.1 nm |
Pulse energy |
0.1 mJ |
Laser repetition rate |
2000 Hz |
Pulse width |
1.9 ns |
Beam divergence |
2.4 mrad |
Receiver |
Telescope diameter |
50 mm |
Maximal field of view |
106 mrad |
Optical efficiency |
0.72 |
Detector |
PMT |
Detector efficiency |
0.13 |
Sampling rate |
1 GHz |
Hamamatsu Photonics are chosen as the detector for good sensitivity and wide dynamic range. The PMT exhibits high cathode radiant sensitivity in the green spectral band and operates reliably over a temperature range of 5 – 50 ° C. Owing to its low dark current, the detector is capable of responding to photon-level signals. The hardware parameters are shown in Table. 1.
In the receiving subsystem, a Fresnel lens is selected as the telescope. The telescope has a rear focal length of 60.5 mm and an optical transmittance of up to 96 %. The telescope collects the echoes and focuses them onto the rear