Laser sensors can be classified into four main types based on their working material.
1. Solid-state lasers: These lasers use a solid as their working medium. Commonly used types include ruby lasers, neodymium-doped yttrium aluminum garnet lasers (YAG lasers), and neodymium glass lasers. They have roughly the same structure and are characterized by their small size, robustness, and high power. Neodymium glass lasers have the highest pulse output power, reaching tens of megawatts.
2. Gas Lasers: These use gas as their working medium. Various gas atomic, ionic, metal vapor, and gas molecular lasers are currently available. Commonly used ones include carbon dioxide lasers, helium-neon lasers, and carbon monoxide lasers. They resemble ordinary discharge tubes in shape and are characterized by stable output, good monochromaticity, and long lifespan, but they have relatively low power and conversion efficiency.
3. Liquid lasers: These can be further divided into chelate lasers, inorganic liquid lasers, and organic dye lasers. Among them, the most important is the organic dye laser, whose biggest feature is that the wavelength is continuously tunable.
4. Semiconductor lasers: These are a relatively new type of laser, with gallium arsenide lasers being the most mature. They are characterized by high efficiency, small size, light weight, and simple structure, making them suitable for use on aircraft, warships, tanks, and for infantry to carry. They can be used to make rangefinders and sights. However, they have relatively low output power, poor directionality, and are greatly affected by ambient temperature.
Working principle of laser sensors
When a laser sensor operates, a laser emitting diode first emits a laser pulse at the target. After reflection from the target, the laser light scatters in all directions. Some of the scattered light returns to the sensor receiver, where it is received by the optical system and imaged onto an avalanche photodiode. An avalanche photodiode is an optical sensor with internal amplification capabilities, allowing it to detect extremely weak light signals and convert them into corresponding electrical signals. A common application is the laser rangefinder, which determines the target distance by recording and processing the time it takes for a light pulse to travel from emission to reception. Laser sensors must measure the transmission time with extreme precision because the speed of light is so fast.
For example, the speed of light is approximately 3*10^8 m/s. To achieve a resolution of 1 mm, the electronic circuitry of the time-of-transmission ranging sensor must be able to distinguish the following extremely short time intervals:
0.001m/(3*10^8m/s)=3ps
Resolving a time interval of 3 ps is an excessively high requirement for electronic technology, making it too costly to achieve. However, modern laser rangefinders cleverly circumvent this obstacle, utilizing a simple statistical principle—the averaging rule—to achieve a resolution of 1 mm while maintaining a fast response time.
