Photomultiplier tube protection system for dual optical receiving channels of water depth detection laser radar

Abstract

The invention discloses a photomultiplier tube protection system for dual optical receiving channels of a water depth detection laser radar. Through photomultiplier tube gating technology, an STM32 single-chip microcomputer is used as a main control module, a high-speed AD acquisition module, and based on dual optical receiving channels, a photomultiplier tube protection system is designed. The system comprises step 1: respectively calculating the laser echo receiving power ratio of different optical receiving channels; step 2: the AD sampling module collects the laser echo signal and performs peak value judgment on the data, and then transmits the peak value information to the main control module; step 3: after the main control module collects the echo signal strength information, the photomultiplier tube gating control is respectively performed according to the received echo signal strength and the calculated echo receiving efficiency ratio of different optical receiving channels. If a saturated echo signal appears, the photomultiplier tube is stopped by the photomultiplier tube gating control, and the external laser power is adjusted to achieve multiple protections of the photomultiplier tube.

Description

Photomultiplier tube protection system for dual optical receiving channels of water depth detection laser radar Technical Field

[0001] The present invention relates to the field of laser radar, and in particular to a photomultiplier tube protection system for detecting water depth detection laser echo signals. Background Art

[0002] At present, in the reception of water depth detection laser radar echo signals, photomultiplier tubes are usually used to detect laser echo signals. Photomultiplier tubes are a kind of detector that converts weak light signals into electrical signals. The principle is that when light particles pass through the incident window to reach the photocathode surface, the photons on the photocathode surface are stimulated to emit photoelectrons into the vacuum. The photoelectrons are multiplied and amplified by the electron multiplier. Finally, the secondary electrons emitted by the final multiplier are output through the anode to convert the light signal into an electrical signal. As a highly sensitive photosensitive component, its working state may be saturated or even damage the photomultiplier tube due to strong light such as laser incident on the photomultiplier tube. The commonly used protection method is to use a mechanical shutter to block strong light, but its reaction speed is slow and its durability is low. Therefore, an electronic gating function is usually designed in the photomultiplier tube to control the photoelectrons by adjusting the voltage between the electrodes, thereby achieving a high gated extinction rate at high speed, effectively preventing the anode current from exceeding the maximum rated value of the photomultiplier tube. During the operation of the laser radar, strong laser echo signals are sometimes inevitably generated. If the strong echo signals are not processed accurately and quickly, irreversible damage to the photomultiplier tube will be caused.

[0003] Patent application number CN2023103729506 discloses a photomultiplier tube protection device for a satellite-borne space particle detector. The device achieves protection by designing a physical protection groove and adapting and installing the photomultiplier tube, wrapping the photomultiplier tube in a physical protection manner, but the invention cannot protect the internal circuit of the photomultiplier tube.

[0004] Patent application number CN206505004U discloses a protective device for a photomultiplier tube. The device has a physical structure of a block at the light inlet of the photomultiplier tube. The block can block or expose the light inlet by horizontal movement to prevent the photomultiplier tube from being damaged by long-term exposure to strong light, thereby extending the life of the photomultiplier tube. However, the rate of strong light blocking by physical blocks is slow, and the life of the protective device is also short. At the same time, the device requires the probe to block the light inlet of the photomultiplier tube in the non-detection state, and the protection state cannot be adjusted in the working state.

[0005] The patent application number CN106712758B discloses a control circuit for a gated photomultiplier tube, which controls the photocathode of the photomultiplier tube and the voltage of the first three multiplication stages to control the working state of the photomultiplier tube through the photomultiplier tube gating technology. By adjusting the switch and duration of the gated circuit, the photomultiplier tube can avoid the strong interference pulse or white light background signal that is first reached during the detection, thereby achieving effective detection of weak signals to be measured. However, the invention can only control the working state of the photomultiplier tube, and cannot analyze the current working environment to perform real-time control and adjustment of the opening and closing of the photomultiplier tube, and cannot achieve protection of the photomultiplier tube.

[0006] The patent application number CN201063436Y discloses a photomultiplier tube protection device, which includes a sensor and a control circuit. In the invention, a photoelectric switch is used to control the high-voltage circuit of the photomultiplier tube. When the photoelectric switch detects that the light intensity reaches the threshold, the working voltage of the photomultiplier tube is immediately cut off to prevent the photomultiplier tube from being damaged when strong light appears. At the same time, the maximum leakage light intensity can be set. Only when the light intensity is lower than this, the high-voltage circuit will be connected to make the photomultiplier tube work. However, the invention has a relatively simple light intensity judgment circuit, and cannot fully analyze the echo signal intensity. The judgment accuracy of the working environment is low, and the current working environment cannot be adjusted.

[0007] In 2019, Shi Chao and others from the Institute of Plasma Physics, Chinese Academy of Sciences published an article titled "Design of EAST-NBI Photomultiplier Tube Interlock Protection System" in Nuclear Electronics and Detection Technology, proposing a protection method and designing the photomultiplier tube protection part into two types: external protection and self-protection. External protection is to control the trigger signal of the interrupted beam when the intensity of the stray particle light signal exceeds the set value, and cut off the injection of the neutral beam injector; self-protection is a power-off protection mechanism to prevent sudden high-power light from causing damage to the photomultiplier tube. However, the paper did not effectively use the design mechanism of the photomultiplier tube for gate control for protection, but simply turned it off, and was unable to autonomously adjust the current working environment so that the system can work safely, but simply turned off the light.

[0008] Although the above inventions and papers have designed protection methods for photomultiplier tubes from different perspectives, they have not analyzed the receiving efficiency of photomultiplier tubes in the corresponding working environment from the perspective of the optical receiving system. They have failed to design relevant protection systems for different receiving efficiencies, failed to reasonably utilize the speed advantage and durability advantage of the gating function of the photomultiplier tube itself, and failed to reasonably utilize the intensity information of the collected optical signal to regulate the light source signal. There is also a lack of a photomultiplier tube protection system specifically for water depth detection laser radar. In response to the above problems, the present invention proposes a photomultiplier tube protection system for dual optical receiving channels of water depth detection laser radar, designs different gating modes for photomultiplier tubes for different optical receiving channels, mainly analyzes the receiving efficiency and correlation of different optical receiving channels in the system, controls its working state through the corresponding gating mode and realizes laser power feedback adjustment, which not only prevents the generation of strong echo signals from damaging the photomultiplier tube, but also realizes the active control of the laser intensity in the working environment of the photomultiplier tube, thereby realizing the protection of the photomultiplier tubes of the dual optical receiving channels of the water depth detection laser radar. Summary of the invention

[0009] The invention discloses a photomultiplier tube protection system based on dual optical receiving channels. The system is mainly composed of a laser, dual optical receiving channels, a No. 1 photomultiplier tube and a back-end processing circuit, a No. 2 photomultiplier tube and a back-end processing circuit, a high-speed AD acquisition module, and a main control module.

[0010] The protection system analyzes the optical receiving efficiency of different optical receiving channels, and the high-speed AD acquisition module monitors the peak value of the echo signal and transmits it to the main control module. The main control module collects and analyzes the peak signal transmitted by its high-speed AD acquisition module. When the laser echo signal is oversaturated or about to be saturated, the photomultiplier tube is closed by gate control and the laser energy is reduced. At the same time, according to the calculated optical receiving efficiency ratio of different optical receiving channels, for the photomultiplier tube connected to the strong light channel with strong laser echo receiving ability, the main control module will first calculate and judge according to the calculated receiving efficiency ratio based on the echo peak data of the weak light channel with weak laser echo receiving ability currently collected by the high-speed AD acquisition module, and judge whether there is a risk of damage to the photomultiplier tube in the current working environment of the strong light channel. When it is judged that the current environment is safe, a gate signal is input to the photomultiplier tube connected to the strong light channel to allow it to enter the working state. When it is judged that there is a risk of damage, the laser energy is also reduced, so as to achieve the effect of pre-protection. The protection system can thus prevent the photomultiplier tube from being damaged by output saturation and high-power laser.

[0011] The present invention discloses a photomultiplier tube protection system for dual optical receiving channels of a water depth detection laser radar, which specifically comprises the following steps:

[0012] First, calculate the laser echo receiving power ratio of different optical receiving channels. First, calculate the output current of the photomultiplier tube. The output current of the photomultiplier tube can be calculated using formula (1):

[0013]

[0014] In the formula Represents the current output by the photomultiplier tube; Represents the sensitivity of the photocathode surface of the photomultiplier tube; Represents the light power that the photomultiplier tube can receive after the light is attenuated in the environment; Represents the photomultiplier tube bias voltage.

[0015] The received echo power of the optical receiving channel can be expressed by formula (2):

[0016]

[0017] In the formula, is the bottom echo power, is the backscatter power of water.

[0018] The received power of the backscattered laser echo signal from the bottom and water body can be calculated using formulas (3) and (4) respectively:

[0019]

[0020]

[0021] In the formula, is the seabed echo signal power, It's the depth of water. is the laser peak power, is the bottom reflectivity, is the receiving field of view aperture area, is the optical receiving channel receiving efficiency, It is the angle between the propagation direction of the laser after it enters the seawater and the vertical direction. is the equivalent flight altitude, is the water absorption coefficient, is the water backscattering coefficient, is the field of view loss factor, is the laser pulse width, is the refractive index of water, is the volume scattering function Take the value at 180°.

[0022] According to the design of the optical receiving channel, the laser radar echo signal receiving efficiency ratio of different optical receiving channels is calculated by combining formulas (1), (2), (3) and (4).

[0023] Second, the echo signal received by the optical receiving channel is transmitted to the high-speed AD acquisition module through the photomultiplier tube and the back-end processing circuit. The AD sampling module determines the peak value of the collected data by designing the peak module and transmits the peak information to the main control module.

[0024] Among them, the attenuator in the back-end circuit attenuates the photomultiplier tube acquisition signal to the threshold range of the AD sampling module. The attenuator voltage conversion can be expressed by formula (5):

[0025]

[0026] In the formula, represents the input voltage, represents the output voltage, is the attenuation amount.

[0027] The AD sampling module collects voltage in the range of -0.85v to +0.85v, and the conversion bit number is 14 bits. The corresponding analog-to-digital conversion formula can be expressed by formula (6):

[0028]

[0029] In the formula, To collect voltage, is the acquisition voltage range, The converted value after acquisition. A peak identification module is designed in the high-speed AD acquisition module to match the laser pulse cycle, record the converted value after acquisition in each clock cycle of the high-speed AD acquisition module, and compare it with the value of the previous clock cycle to record the maximum value. When comparing the complete laser pulse cycle, the maximum value obtained is transmitted to the main control module of the photomultiplier tube protection system through the serial port as the echo signal strength information of the protection system.

[0030] Third, after the main control module collects the echo signal strength information, it pre-calculates the echo signal strength that can be collected by the No. 2 photomultiplier tube and the back-end processing circuit connected to the strong light channel in the current working state according to the calculated echo reception efficiency ratio of different optical receiving channels. When it is judged that it can work normally, it inputs the gated start signal to the No. 2 photomultiplier tube to make it work normally. When it is judged that there is a risk of damage to the photomultiplier tube, it prohibits the input of the gated start signal and reduces the laser energy. At the same time, the current echo signal strength collected by the No. 1 photomultiplier tube and the back-end processing circuit, the No. 2 photomultiplier tube and the back-end processing circuit is detected in real time. If a saturated echo signal appears, the gated start signal is stopped, and the external laser intensity is adjusted to achieve multiple protections for the photomultiplier tube.

[0031] Through the system designed by the present invention, corresponding gating methods can be designed for different optical receiving channels, and photomultiplier tube gating control can be performed according to the current working environment. At the same time, the laser power and laser intensity can be adjusted to achieve photomultiplier tube protection for the dual optical receiving channels of the water depth detection laser radar. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The steps of the photomultiplier tube protection system design method of the present invention are as follows:

[0033] Figure 2 The structure diagram of the photomultiplier tube protection system of the present invention is

[0034] Figure 3 This is the working flow diagram of the photomultiplier tube protection system of the present invention. Specific implementation scheme

[0035] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, preferred embodiments are given below and the specific implementation of the present invention is further described in detail in conjunction with the accompanying drawings.

[0036] Combination Figure 2 ,This case is composed of laser, dual optical receiving channels, photomultiplier tube No. 1 and back-end processing circuit, photomultiplier tube No. 2 and back-end processing circuit, high-speed AD acquisition module, and main control module.

[0037] Laser. The laser repetition frequency is 2000Hz. It can be connected to the main control through RS232 serial communication protocol, and the laser power can be adjusted through the main control module.

[0038] Dual optical receiving channels. The optical receiving channel receives the laser echo signal through the split-field mirror and enters the No. 1 photomultiplier tube and the back-end processing circuit and the No. 2 photomultiplier tube and the back-end processing circuit. Combined with the mechanical structure, the main aperture is 100mm, the focal length is 200mm, the split-field mirror is a reflector with a central opening, the diameter is 10mm, located at the focus of the main mirror, the central opening is 1.5mm, the receiving image surface of the detector is directly 8mm, the optical receiving channel corresponding to the No. 1 photomultiplier tube in the system has a receiving field angle of 0 to 5mrad as a weak light channel, and the optical receiving channel corresponding to the No. 2 photomultiplier tube has a receiving field angle of 5 to 40mrad as a strong light channel.

[0039] Photomultiplier tube No. 1 and back-end circuit. Hamamatsu H11526-20-ON normally open photomultiplier tube is used as photomultiplier tube No. 1. The photomultiplier tube No. 1 and the back-end circuit receive the small beam echo laser signal from the optical receiving channel and convert it into an electrical signal. The photomultiplier tube is a normally open type. After the gate signal is input, the photomultiplier tube can be controlled to close. The back-end processing circuit is connected to the attenuator and the transmission circuit is connected to the AD sampling module. It is characterized by a small receiving field of view and low detection sensitivity.

[0040] Photomultiplier tube No. 2 and back-end circuit. Hamamatsu H11526-20-OF normally closed type is used as photomultiplier tube No. 2. The photomultiplier tube No. 2 adopts the back-end circuit to receive the small beam echo laser signal from the optical receiving channel and convert it into an electrical signal. The photomultiplier tube is a normally closed type. After the gate signal is input, the photomultiplier tube can control the photomultiplier tube to perform photoelectric conversion. The back-end processing circuit is also connected to the attenuator and the transmission circuit to the AD conversion module. It is characterized by a large receiving field of view and high detection sensitivity.

[0041] High-speed AD acquisition module. It is implemented using FPGA and AD9208 analog-to-digital conversion chip. Channel 1 and channel 2 in this module receive the electrical signals transmitted from photomultiplier tube No. 1 and back-end circuit and photomultiplier tube No. 2 and back-end circuit respectively and convert them into digital signals. A peak module is designed in the module program to match the laser pulse cycle, record the converted value collected in each clock cycle of the high-speed AD acquisition module, and compare it with the value of the previous clock cycle to record the maximum value. When comparing the complete laser pulse cycle, the maximum value obtained is transmitted to the main control module of the photomultiplier tube protection system through the serial port as the echo signal strength information of the protection system. The maximum value collected is transmitted to the main control module through serial communication.

[0042] Main control module. The STM32 microcontroller is used as the main control module to receive the peak value collected by the high-speed AD acquisition module to determine whether there is a breakdown risk in the current photomultiplier tube sampling peak value, thereby controlling the opening and closing of the photomultiplier tube gate and controlling the laser power.

[0043] Combination Figure 3 The protection steps of the dual-channel optical receiving channel photomultiplier tube protection system of the water depth detection laser radar are as follows:

[0044] First, the laser echo receiving power ratio of different optical receiving channels is calculated respectively. By analyzing the receiving aperture area of ​​different optical receiving channels, the receiving field power of the two channels under the same conditions is calculated. The receiving field power of the two optical receiving channels is calculated by formulas (1), (2), (3), and (4).

[0045] Second, the collected signal is transmitted to the high-speed AD acquisition module through the back-end circuit, where the value obtained after sampling can be calculated by formulas (5) and (6). According to formula (6), the acquisition range of the AD acquisition module is 1.7V, and it can be calculated that the maximum conversion value after acquisition is 35000. After the high-speed AD acquisition module makes a peak judgment, it is transmitted to the main control module through serial communication. The data returned by the AD sampling module is collected and the safety threshold is set to 34000. According to the laser repetition frequency of 2000Hz, within a laser light emission cycle, the conversion value after acquisition in each clock cycle of the high-speed AD acquisition module is recorded and compared with the value of the previous clock cycle, and the maximum value is recorded. The maximum value is then transmitted to the main control module.

[0046] Third, the main control module performs gate control of photomultiplier tube No. 1 and photomultiplier tube No. 2 respectively according to the calculated echo receiving efficiency ratio and the collected peak data, and reduces the laser power when necessary.

[0047] Combination Figure 3, describing the photomultiplier tube protection system involved in the present invention. The working state of photomultiplier tube No. 1 is a normally open photomultiplier tube. After the system is working, the receiving optical path normally receives the laser echo signal. After the photomultiplier tube realizes the photoelectric conversion, the high-speed AD acquisition module performs signal acquisition and processing. The sampling channel 1 in the high-speed AD acquisition module receives the laser echo data currently received by photomultiplier tube No. 1, and obtains the maximum echo peak data after conversion through the peak module, and transmits the current peak back to the main control module. The main control determines whether the current working environment is in a safe state and whether the strong laser echo will damage the photomultiplier tube. When it is determined that the current working state is safe, that is, the echo peak value is less than the set safety threshold, photomultiplier tube No. 1 works normally. When the echo peak value is higher than the set safety threshold, a gate signal is output to photomultiplier tube No. 1, as a normally open photomultiplier tube, a gate signal is input to photomultiplier tube No. 1, and the photomultiplier tube stops working. When the No. 1 photomultiplier tube is in normal working state, the received power ratio of the two optical field receiving channels is calculated to calculate the echo signal strength collected by the No. 2 photomultiplier tube and the back-end circuit in the current environment. The main control module outputs a gate signal, and the No. 2 photomultiplier tube is a normally closed photomultiplier tube. After the gate signal is output, the photomultiplier tube starts to work. If there is a risk of damage, the main control module controls to reduce the laser power. After the gate signal is output to the No. 2 photomultiplier tube, according to the high-speed AD acquisition module channel 2, the data currently collected by the No. 2 photomultiplier tube receiving optical path is received, and the current peak value is returned to the main control. The main control judges whether the current working state has a risk of damage to the photomultiplier tube through the returned peak value. When the returned peak value is greater than the set safety threshold, the gate signal is immediately output to turn off the photomultiplier tube and reduce the laser power. The dual protection of the photomultiplier tube is achieved through the gate control of the photomultiplier tube and the adjustment of the laser power.

[0048] The above embodiments are only used to illustrate the present invention, but not to limit the present invention. A person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also belong to the scope of the present invention, and the patent protection scope of the present invention shall be defined by the claims.

[0049] The technical contents not described in detail in the present invention are all known technologies.

Claims

1. Photomultiplier tube protection system for dual optical receiving channels of water depth detection laser radar, characterized by The following steps are taken: First, the laser echo receiving power ratio of different optical receiving channels is calculated respectively. First, the output current of the photomultiplier tube is calculated. The output current of the photomultiplier tube can be calculated using formula (1): In the formula Represents the current output by the photomultiplier tube; / > Represents the sensitivity of the photocathode surface of the photomultiplier tube; / > Represents the light power that the photomultiplier tube can receive after the light is attenuated in the environment; / > represents the bias voltage of the photomultiplier tube; the echo power received by the optical receiving channel can be expressed by formula (2): In the formula, is the bottom echo power, / > is the backscattering power of the water body; the receiving power of the backscattered laser echo signal from the bottom and water body can be calculated using formulas (3) and (4) respectively: In the formula, is the seabed echo signal power, / > is the water depth, / > is the laser peak power, / > is the bottom reflectivity, / > is the receiving field aperture area, / > is the optical receiving channel receiving efficiency, / > is the angle between the propagation direction of the laser and the vertical direction after it enters the seawater, / > is the equivalent altitude, / > is the water absorption coefficient, / > is the water backscattering coefficient, / > is the field of view loss factor, / > is the laser pulse width, / > is the refractive index of water, / > is the volume scattering function / > Take the value at 180°; according to the design of the optical receiving channel, combine formulas (1), (2), (3), and (4) to calculate the laser radar echo signal receiving efficiency ratio of different optical receiving channels; Second, the echo signal received by the optical receiving channel is transmitted to the high-speed AD acquisition module through the photomultiplier tube and the back-end processing circuit. The AD sampling module judges the peak value of the collected data by designing the peak module and transmits the peak value information to the main control module. Among them, the attenuator in the back-end circuit attenuates the photomultiplier tube acquisition signal to the threshold range of the AD sampling module. The attenuator voltage conversion can be expressed by formula (5): In the formula, represents the input voltage, / > represents the output voltage, / > is the attenuation; the voltage range of the AD sampling module is -0.85v to +0.85v, the conversion bit number is 14 bits, and the corresponding analog-to-digital conversion formula can be expressed by formula (6): In the formula, To collect voltage, / > is the voltage range of the acquisition, / > For the converted values ​​after acquisition, a peak identification module is designed in the high-speed AD acquisition module to match the laser pulse cycle, record the converted values ​​after acquisition in each clock cycle of the high-speed AD acquisition module, and compare it with the value of the previous clock cycle to record the maximum value. When comparing the complete laser pulse cycle, the maximum value obtained is transmitted to the main control module of the photomultiplier tube protection system through the serial port as the echo signal intensity information of the protection system; Third, after the main control module collects the echo signal strength information, it pre-calculates the echo signal strength that can be collected by the No. 2 photomultiplier tube and the back-end processing circuit connected to the strong light channel in the current working state according to the calculated echo reception efficiency ratio of different optical receiving channels. When it is judged that the normal working state can be carried out, the gated start signal is input to the No. 2 photomultiplier tube to make it work normally. When it is judged that there is a risk of damage to the photomultiplier tube, the input of the gated start signal is prohibited and the laser energy is reduced. At the same time, the current echo signal strength collected by the No. 1 photomultiplier tube and the back-end processing circuit, the No. 2 photomultiplier tube and the back-end processing circuit is detected in real time. If a saturated echo signal appears, the input of the gated start signal is stopped, and the external laser intensity is adjusted to achieve multiple protections for the photomultiplier tube.

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