High repetition frequency pulse laser ranging technology

So far, the laser rangefinder has been updated to the third generation. The first-generation laser rangefinder was mainly used in aerospace and military equipment. The first-generation laser rangefinder system composed of a photomultiplier tube (PMT) detector and an infrared gem laser was large in size and harmful to the human eye. , very power-consuming and many other shortcomings, it was gradually replaced by the second generation laser ranging system around the 1970s. The laser used in the second-generation laser ranging system is a near-infrared neodymium laser based on Nd:YAG laser, and the echo detector is an avalanche photodiode (APD) or a PIN photodiode, which makes the ranging system It has gradually begun to develop in the direction of small size and low power consumption. However, the issue of human eye safety is still a major difficulty in the ranging system at this time. The third-generation laser ranging system began to consider human eye safety issues. The laser wavelengths emitted by the system are all in the human eye safety band, and most of them use erbium glass solid lasers.

Figure 1 Internal composition of early ruby laser

With the development and increasing maturity of electronic technology, laser ranging technology is currently developing rapidly in the direction of small size, light weight, low price and simple structure.

Laser performance is one of the important factors affecting the ranging performance of pulse laser ranging systems. At present, there are many types of lasers. Among them, solid-state lasers have compact structure, high power, and easy use, and are suitable for industrial processing, medical and military fields; gas lasers have many types, high conversion efficiency, and wide wavelength range, and are suitable for precision testing and measurement tools; liquid lasers are stable Poor performance, the output laser wavelength can be adjusted within a wide range, and is widely used in laser medicine, photobiochemistry and other fields; chemical lasers directly convert chemical energy into light energy, with high output energy and wave bandwidth, suitable for field operations without power supply ; Semiconductor lasers are low-cost and easy to modulate, and are suitable for communication light sources, lidar, etc.

Laser selection requires comprehensive consideration of cost performance and technical indicators, and the most cost-effective laser should be selected based on meeting the design indicator requirements. Technical indicators mainly include laser repetition frequency, peak power, center wavelength, etc. Among them, the laser repetition frequency mainly affects the ranging speed. The higher the peak power, the farther the distance will be measured. The laser wavelength must comprehensively consider the attenuation of the laser propagation medium and other characteristics. The gain medium of the erbium glass laser is Er glass, and the output wavelength is 1535 nm. It is widely used in human eye safety ranging. Currently, it can achieve an energy output of 100-400 μJ in China, a pulse width of 5 ns, and a repetition frequency of more than 1MHz. Erbium glass lasers offer low power consumption, high peak power, narrow pulse width, compact size, and do not require temperature control. It has extremely high stability and can be used in extreme temperature environments of -45℃~60℃.

Figure 2 Working principle of erbium glass laser

The miniaturized eye-safe laser rangefinder is mainly composed of three parts: laser transmitting module, laser receiving system and signal processing system.

The laser emission unit of the pulse laser ranging system is mainly divided into three sub-modules, namely laser drive module, laser and collimated emission module. The laser is a key part that affects the performance of the ranging system. The laser driving module mainly designs the driving circuit according to the characteristics of the laser, and sets the laser repetition frequency and narrow pulse in the control program according to the design requirements. The collimated emission module includes a collimated emission lens. The laser emitted by the laser has a divergence angle, so the emission lens needs to be collimated and corrected before being emitted to the target to reduce energy divergence and increase the system ranging distance.

In order to ensure that the laser signal emitted by the detection light laser can be coupled to the photodetector with maximum efficiency after being reflected by the target, the structure of the signal receiving module can be roughly divided into the following three parts: The first part is the receiving lens, which ensures Weak echo signals can be collected to the focus of the lens to the greatest extent, thereby ensuring the strength of the signal; the second part is the receiving lens barrel, which is tapered in design, which can effectively isolate stray light; the third part is the signal The receiving base, this part is designed to ensure that the photodetector is located at the focus of the lens, thereby ensuring that as much light signal as possible is received.

Photoelectric Detector is one of the main components of the system receiving module. Its main task in the ranging process is to convert the received weak light signal into an electrical signal. Photodetectors mainly include photoconductive devices such as avalanche diodes, photodiodes, and photomultiplier tubes, among which avalanche diodes and photodiodes are the most common. The echo signal receiving unit of the pulse laser ranging system takes the detector as the core. The optical receiving module focuses the optical signal onto the surface of the detector. The detector induces a photocurrent and converts the optical signal into an electrical signal. The electrical signal passes through the amplification and shaping circuit. , and then the signal processing system TDC receives the echo signal and performs data processing to measure the time of flight to obtain accurate distance information.

Figure 3 Principle of pulse laser ranging

With the continuous improvement of pulse laser ranging technology, its size continues to shrink and its price gradually decreases. It has been gradually used in lidar, building surveying, etc. Its effect also has obvious advantages compared with other ranging technologies. At present, the maximum measurement distance of small ranging modules can reach more than 10km, and the ranging accuracy can reach within ±1m.