Design of underwater laser imaging range gating synchronization control circuit

Abstract: In underwater laser imaging systems, the complex underwater environment has a great impact on laser transmission. In order to more effectively realize the distance gating function, the design of the synchronous control circuit uses a high-performance Altera Stratix III series FPGA. The circuit is divided into two modules: distance delay and gate delay. The laser ranging idea is innovatively added to the distance delay module to solve the problem that the distance delay time is difficult to accurately obtain, and output accurate gating pulses while considering various delays. Simulation results show that the distance delay time and ICCD gate delay time can be accurate to 2 ns, with a maximum error of 1 ns.
Keywords: underwater laser imaging; distance gating; synchronous control; laser ranging

Underwater laser imaging technology is developed based on the transmission "window" of blue-green laser in the water. The laser emits pulsed laser to the underwater target, measures the signal reflected from the target, and obtains the image information of the target. Due to the high resolution and long working distance of the blue-green laser imaging system, it can be widely used in underwater monitoring, marine biological telemetry and oil extraction in addition to military applications, so it is very meaningful to study it. Due to the presence of dissolved substances and suspended bodies underwater, the water is turbid, and the backscattering effect is serious, resulting in a sharp decline in imaging quality. In order to effectively overcome the influence of backscattering, distance-gated imaging technology is often used.
The underwater distance-gated imaging system is mainly composed of a narrow pulse laser, a synchronous control device, and a gated ICCD camera.
The synchronous control technology mainly uses a synchronous control device to complete the synchronous operation of the pulse laser and the ICCD camera, and realizes the distance gating function by accurately controlling the opening and closing of the ICCD gate. In order to achieve effective synchronization, this design uses a high-performance FPGA to generate nanosecond-level strobe pulses to select the ICCD camera, and incorporates the idea of ​​ranging into the circuit. For imaging of underwater targets at fixed and variable distances, the distance delay time and gate delay time can be precisely controlled, which improves the stability and accuracy of the circuit and meets the design requirements.

1 The influence of water's optical properties on imaging
The optical properties of water include water's absorption and scattering properties. The degree of water's absorption of light is different in different spectral regions, and it has obvious selectivity. Studies have shown that the spectral transmission window of coastal seawater is 520 nm. In the experiment, Nd:YAG pulse lasers are commonly used, and the output wavelength is 1.064μm. After Q-switching and frequency doubling, 532 nm green light is obtained. Since water's absorption of light causes the loss of light energy, the power of the laser should be appropriately increased for imaging of targets at a slightly longer distance.
The scattering of water includes the scattering of the water itself and the scattering caused by suspended particles in the water. Pulsed lasers are often used in underwater imaging systems. The laser pulses will be affected by scattering during underwater transmission. Since the photons of the same beam of light have different transmission paths in the water body, transmission delay is caused, which is manifested as pulse broadening in the time domain. Pulse broadening has a greater impact on single-pulse range-gated imaging. One of the requirements for range gating is that the gate delay time is equal to the laser pulse width. The laser is broadened during underwater transmission, so it is necessary to know the specific degree of broadening. The small angle approximation method and the phenomenological method are used for estimation respectively, but they both have limitations and are only of reference value for practical applications.

2 Principle of underwater distance gating synchronous control
The distance gating technology is to correctly coordinate the pulse laser and the gated ICCD camera in timing. According to the difference between the time when the scattered light returns and the time when the target reflected light returns, the camera's gating moment is set exactly when the target reflected signal just reaches the ICCD camera, and imaging is performed after gating. The principle diagram of underwater synchronous control distance gating imaging is shown in Figure 1. The laser emits a strong short pulse, and through beam expansion, it illuminates the entire target or the key feature parts of the target. The laser reflected by the target returns to the ICCD camera. When the laser pulse is on the way back and forth, the ICCD camera gate is closed, which can block the backscattered light. When the reflected light reaches the ICCD camera, the gate is opened to allow the useful signal from the target to enter the ICCD camera. The gate delay time is consistent with the reflected laser pulse, which can greatly reduce the impact of backscattering and improve the signal-to-noise ratio of the echo signal.

3 Underwater distance gated laser imaging synchronization control device
3.1 Design ideas
In underwater laser imaging systems, due to the severe absorption underwater, ultrashort laser pulses are usually used. Because the frame scanning cycle is tens of milliseconds, it is impossible to synchronize with the laser. Therefore, a gated image intensifier is installed in front of the CCD camera to achieve nanosecond gating. In order to generate nanosecond gated pulses, a high-frequency clock signal is required. Considering that high-frequency circuits are easily interfered by external noise and the internal delay must be small, FPGA is used to complete the design.
Theoretically, assuming that the distance delay time is T, the distance from the laser pulse to the target underwater and from the target back to the receiver is L, the relative refractive index underwater is n, and the speed of light is C, then T=nL/C can be obtained; the gate delay preset value is the laser pulse width. However, due to the complex underwater environment, it is difficult to obtain an accurate relative refractive index value. At the same time, from the previous analysis, it can be seen that the laser pulse will be broadened when transmitted underwater, so the accurate distance delay and gate delay cannot be directly calculated.

This design proposes a solution to the above problem, and its working principle diagram is shown in Figure 2. When working, the laser first emits a pulsed laser beam to the underwater target. The signals detected by the two PIN tubes are used as the start signal and end signal of the gating, respectively. The count value can be intuitively seen through the digital tube in the circuit. When the laser beam is emitted for the second time, the distance delay measured for the first time is used as the initial count value. The response time of the PIN tube, the delay of the trigger circuit and the delay of the image intensifier drive circuit should also be considered to output the gating pulse in advance. Since the laser pulse is widened during the underwater transmission process, the gate delay time setting is based on the laser pulse width before transmission, and the gate delay count initial value is continuously increased. When a satisfactory gating image is obtained, the system is reset.
3.2 Design process
According to the above design ideas, in the implementation process, a laser with a pulse width of 6 ns and a repetition frequency of 1 kHz was selected, an image intensifier with an effective strobe pulse width of less than or equal to 40 ns, an IC-CD connected to the CCD through a light cone coupling method, and an Ahera Stratix III series FPGA with a rate of up to 500 MHz. It is a low-power and high-performance FPGA with a step size of up to 2 ns. The schematic diagram of the synchronous control is shown in Figure 3. The design contains two modules. The first module realizes the functions of ranging and timing counting and outputs the gate opening pulse. The second module realizes the timing function to generate the gate closing pulse.

When the laser emits the first pulse laser beam, a small part of the light is received by the PIN tube after being split by the beam splitter. The light signal is formed into a high-level signal by the trigger circuit, and the ranging function of the first module is started. One counter counts up while the other counter does not work temporarily. When the light reflected from the target just reaches the receiving end, the PIN tube placed at the receiving end receives the signal and generates a signal to stop ranging through triggering. The ranging result can be seen intuitively through the digital tube.
When the laser pulse emits the second pulse laser beam, the high-level signal generated by the trigger circuit continues to start the first module. One of the counters starts to count down with the first count value as the initial value. When the counter is reduced to zero, a high-level pulse is output as a strobe pulse to open the ICCD strobe gate, and the other counter will do the second ranging work and count up as the initial value of the next strobe count.
When the strobe pulse arrives, the second module is started at the same time. The initial count value set by the program control is used as the starting value, and the counter counts down. When the count is reduced to zero, a high-level pulse is output to close the ICCD strobe gate. The strobe process ends once and waits for the arrival of the next pulse.
3.3 Simulation and Analysis
This design simulation is completed in Ahera's development software QuartersⅡ8.0, and the module is edited using Verilog language. This paper simulates two modules separately, and the simulation results are shown in Figure 4. The clock cycle is 2 ns, pulse is the trigger pulse, cnt is the count value of a counter in the module, and cntl is the count value of another counter in the module. o_pulse is the strobe pulse. The program assumes that the measured distance delay time is 8 cycles. As can be seen from Figure 4, cnt and cntl continuously realize the ranging and timing functions. When the first trigger pulse arrives, cnt counts to complete the ranging function and cnt1 does not work. When the trigger pulse reflected by the target is received, cnt realizes the timing function and cntl completes the ranging work. This function is implemented by the state machine method used in the program. This simulation also takes into account the trigger delay and ICCD power-on delay. These delays can be measured experimentally. Here, it is assumed that the delay is one cycle. If the delay is not considered, a power-on pulse will be output when cnt is equal to 0. As can be seen from Figure 4, the pulse is output one cycle in advance to achieve precise timing.

Figure 5 is a simulation diagram of the gate delay module. As can be seen from Figure 5, Pulse is the power-on pulse, cntl is the gate delay timing, and when the pulse comes, the counter starts timing. In the simulation, the delay problem of pulse transmission underwater is taken into account, so the initial value of the gate delay count is used as a reference. Every time a pulse comes, the initial count value is increased by 1 until a satisfactory gating image is obtained. In order to be more precise, the blocking assignment method can be used in the program to produce a small delay.
This design unifies the preset initial value and timing, simplifies the manual operation in the actual process, and the gating work will be automatically completed once the pulse laser beam is emitted. The design uses fewer logic resources, which can greatly reduce the signal transmission delay and greatly improve the stability of the circuit.

4 Conclusions
The range gating technology can reduce the background noise caused by backscattering, improve the signal-to-noise ratio of the imaging system, and greatly improve the imaging quality, but it requires strict timing control to be effectively realized. Through the analysis of the imaging quality in the underwater environment, the problems existing in underwater laser imaging are explained. In order to improve the accuracy of synchronous control, a high-performance FPGA is used to generate nanosecond-level gating pulses to gating ICCD cameras. In traditional synchronous control circuits, preset and counting are often separated, and the theoretical calculated value is used as the delayed count value, etc., which will result in large errors in complex environments.
This design introduces the idea of ​​ranging into the device, which can automatically image targets of unknown distances and targets of uncertain distances, and is widely used in underwater target detection and imaging systems.