US-12618950-B2 - Photomultiplier tube protection system with dual optical receiving channels for bathymetry LiDAR

Abstract

A photomultiplier tube protection system with dual optical receiving channels for bathymetry LiDAR is designed, through a photomultiplier tube gating technology, based on dual optical receiving channels, main control module with STM32 single chip microcomputer and, high-speed AD sampling module. The system includes: calculating laser echo receiving power ratios of different optical receiving channels, respectively; acquiring, by AD sampling module, laser echo signal, and performing peak determination on acquired data, and transmitting peak information to the main control module; and collecting, by the main control module, echo signal intensity information, performing photomultiplier tube gating control according to the received echo signal intensity and the calculated echo receiving efficiency ratios of different optical receiving channels, and stopping the photomultiplier tube through photomultiplier tube gating control if saturated echo signal occurs, and adjusting external laser device power, thus achieving multiple protection of the photomultiplier tube.

Inventors

• Guoqing Zhou
• Zhong'ao Wang

Assignees

• GUILIN UNIVERSITY OF TECHNOLOGY

Dates

Publication Date

20260505

Application Date

20240808

Claims (1)

1. 1 . A method for operating a photomultiplier tube protection system with dual optical receiving channels for a bathymetry LiDAR, the method comprising the following operations: step 1: calculating laser echo receiving power ratios of different optical receiving channels respectively; wherein, an output current of a photomultiplier tube is calculated using Formula (1): I I = P p * S p * G s ( 1 ) wherein I I denotes the output current of the photomultiplier tube, S p denotes sensitivity of a photocathode surface of the photomultiplier tube, P p denotes received optical power of the photomultiplier tube after light attenuated in environment, and G s denotes a bias voltage of the photomultiplier tube; a received echo power of the optical receiving channel is expressed by Formula (2); P p = P b ( h ) + P s ( h ) ( 2 ) wherein P b (h) is water-bottom echo power, and P s (h) is water-body backscattering power; backscattered laser echo signal receiving powers of water bottom and water body are calculated by Formula (3) and Formula (4), respectively: P b ( h ) = P i ⁢ ρ b π ⁢ ∑ η ⁢ cos 2 ⁢ θ w ( H + h ) 2 ⁢ e [ - 2 ⁢ ( a + b b ) cos ⁢ θ w ] ⁢ F ⁡ ( h ) ( 3 ) P s ( h ) = P i ⁢ c ⁢ τ p 2 ⁢ n ⁢ β ⁢ ∑ η ⁢ cos 2 ⁢ θ w ( H + h ) 2 ⁢ e [ - 2 ⁢ ( a + b b ) c ⁢ o ⁢ s ⁢ θ w ] ⁢ F ⁡ ( h ) ( 4 ) wherein P b (h) is seabed echo signal power, h is a water depth, P i is laser peak power, ρ b is water-bottom reflectivity, Σ is an aperture area of a receiving field, η is receiving efficiency of the optical receiving channel, θ w is an included angle between a propagation direction and a vertical direction after laser enters seawater, H is an equivalent flight altitude, a is an absorption coefficient of the water body, b b is a backscattering coefficient of the water body, F(h) is a field loss factor, τ p is a laser pulse width, n is a refractive index of the water body, β π is a value when a volume scattering function β(θ) is 180°; according to a design of the optical receiving channel, LiDAR echo signal receiving efficiency ratios of different optical receiving channels are calculated in combination with Formula (1), Formula (2), Formula (3) and Formula (4); step 2: transmitting an echo signal received by the optical receiving channel to a high-speed AD acquisition module after passing through the photomultiplier tube and a back-end processing circuit, performing peak determining on acquired data by an AD sampling module designing a peak module, and transmitting peak information to a main control module; wherein, an attenuator in the back-end circuit is configured to attenuate a signal acquired by the photomultiplier tube to a threshold range acquired by the AD sampling module, and voltage conversion of the attenuator is expressed by Formula (5); 20 ⁢ lg ⁡ ( U i U o ) = B ( 5 ) wherein U i denotes an input voltage, U o denotes an output voltage, and B is an attenuation amount; an acquisition voltage of the AD sampling module ranges from −0.85 v to +0.85 v, a number of conversion bits is 14, and a corresponding analog-to-digital conversion formula is expressed by Formula (6): V i V o = D o 2 16 - 1 ( 6 ) wherein V i is the acquisition voltage, V o is the range of the acquisition voltage, D o is a converted value after acquisition; a peak determination module is designed in the high-speed AD acquisition module to match a laser pulse period, to record the converted value after acquisition in each clock cycle of the high-speed AD acquisition module, and compare the converted value with a value in a previous clock cycle to record a maximum value; when all values in a complete laser pulse period are compared, a resultant maximum value is transmitted to the main control module of the photomultiplier tube protection system through a serial port as echo signal intensity information of the protection system; and step 3: after receiving the echo signal intensity information, according to the calculated echo receiving efficiency ratios of different optical receiving channels, by the main control module, performing a precalculation on an echo signal intensity acquired by a second photomultiplier tube and a second back-end processing circuit, connected to the strong light channel, in a current operating state; when it is determined that the photomultiplier tube is able to operate in a normal operating state, inputting a gating start signal to the second photomultiplier tube to make the second photomultiplier tube operate as normal; when it is determined that the photomultiplier tube is in danger of damage, prohibiting the gating start signal from being input, and turning down laser energy; wherein, during operation, current echo signal intensities acquired by a first photomultiplier tube and a first back-end processing circuit as well as the second photomultiplier tube and the second back-end processing circuit are detected in real time, and when a saturated echo signal occurs, the input of the gating start signal is stopped, and an external laser intensity is adjusted to achieve multiple protection of the photomultiplier tube.

Description

CROSS-REFERENCE TO RELATED APPLICATION This patent application claims the benefit and priority of Chinese Patent Application No. 202311809784.8 filed with the China National Intellectual Property Administration on Dec. 26, 2023, the disclosure of which is incorporated by reference herein in its entirety as part of the present application. TECHNICAL FIELD The present disclosure relates to the field of LiDARs (Light Detection and Rangings), and in particular to a photomultiplier tube protection system with dual optical receiving channels for a bathymetry LiDAR. BACKGROUND At present, in the reception of echo signals with a bathymetry LiDAR, photomultiplier tube is usually used to detect laser echo signals. The photomultiplier tube is a detector for converting weak optical signals into electrical signals. The operating principle is that when photons reach the photocathode surface through an incident window, photons on the photocathode surface are excited to release photoelectrons into the vacuum, and the photoelectrons are multiplied and amplified by the electronic dynodes. Finally, the secondary electrons emitted by the terminal dynode are output through the anode to convert the optical signals into electrical signals. As a high-sensitivity light-sensing component, the operating state of the photomultiplier tube may be affected by an incidence of strong light such as laser, which may saturate the system output, and even damage the photomultiplier tube. At present, the conventional protection method is to block strong light using a mechanical shutter, but its response speed is slow, and the durability is short. Therefore, an electronic gating function is usually designed in the photomultiplier tube, which is used to control photoelectrons by adjusting the voltage between electrodes, thus achieving high gated extinction rate at a high speed state and effectively preventing anode current from exceeding the maximum rating of the photomultiplier tube. In the operating process of LiDAR, sometimes it is unavoidable to produce strong laser echo signals. If the strong echo signals cannot be processed accurately and quickly, the photomultiplier tube would be irreversibly damaged. Patent application No. CN2023103729506 has disclosed a photomultiplier tube protection device for a space-borne particle detector. A physical protection groove is designed for installing a photomultiplier tube in an adaptive manner, such that the device wraps the photomultiplier tube by means of physical protection to achieve protection effect. However, the internal circuits of the photomultiplier tube cannot be protected. Patent application No. CN206505004U has disclosed a protection device for a photomultiplier tube. The device extends the service life of photomultiplier tube by designing a physical structure of a block at a light inlet of the photomultiplier tube. The block can selectively shield or expose the light inlet by being moved horizontally to prevent the photomultiplier tube from being damaged due to long-term contact with strong light. However, the effect of strong light shielding by the physical block is low, and the service life of the protection device is also shortened. Meanwhile, the device requires a probe to shield the light inlet of the photomultiplier tube in a non-detection state, and a protection state cannot be adjusted in the operating state. Patent application No. CN106712758B has disclosed a control circuit of a gated photomultiplier tube. The photocathode and dynode voltages of the first three stages of the photomultiplier tube are controlled by the photomultiplier tube gating technology, so as to control the operating state of the photomultiplier tube. By adjusting the switch and duration of a gating circuit, the photomultiplier tube can be avoided from the strong interference pulse or white background signal reached first in detection, thus achieving effective detection of a weak signal to be detected. However, only the operating state of the photomultiplier tube can be controlled, and the current operating environment cannot be analyzed to control and adjust the opening and closing of the photomultiplier tube in real time, the photomultiplier tube cannot even be protected. Patent application No. CN201063436Y has disclosed a photomultiplier tube protection device, including a sensor and a control circuit. A photoelectric switch is used to control a high-voltage circuit of the photomultiplier tube. When the photoelectric switch detects that a light intensity reaches a threshold, an operating voltage of the photomultiplier tube is immediately cut off to prevent the photomultiplier tube from being damaged when strong light appears. Meanwhile, the maximum light leakage intensity can be set, and the high-voltage circuit can be turned only in a case of lowering than the light intensity so that the photomultiplier tube can operate. However, a circuit for determining the light intensity is relatively simple, the echo si