Detailed explanation of the light source of the phase photoelectric rangefinder
The light sources of the phase range finder mainly include gallium arsenide (GaAs) diodes and helium-neon (He-Ne) gas lasers. The former is generally used in short-range range finders, and the latter is used in medium and long-range range finders. The following is an introduction to these two light sources.
(1) Gallium arsenide (GaAs) diode
A gallium arsenide (GaAs) diode is a crystal diode. Like a common diode, it also has a junction inside, as shown in Figure 4-5. Its forward resistance is small and its reverse resistance is large. When a strong current is injected in the forward direction, infrared light with a wavelength between 0.72 and 0.94m will emerge from the junction, and the intensity of the emitted light will vary with the magnitude of the injected current, so it can be simply changed by changing the feed current Modulating the output of light intensity is the so-called "direct current modulation". This is very meaningful for the rangefinder to be used as a light source, because it can directly modulate the light intensity, and there is no need to equip a modulator with a complex structure and high power consumption. In addition, compared with other light sources, gallium arsenide diode light source has the advantages of small size, light weight, firm structure and no fear of vibration, which is conducive to miniaturization and portability of the range finder.
(2) Helium-Ne (He-Ne) gas laser
A helium-neon gas laser consists of a discharge tube, an excitation power supply and a resonant cavity. The discharge tube is a crystal tube with an inner diameter of several millimeters. The tube is filled with a mixed gas of helium and neon. The length of the tube varies from a few centimeters to tens of centimeters. The longer the tube, the higher the output power. Optically precision machined Brewster windows are installed at both ends of the tube. The excitation power can generally use DC, AC or high-frequency power discharge methods. Currently, the DC power discharge method is most used, and its advantage is that the laser output is stable. The resonant cavity is composed of two spherical mirrors, one of which is totally reflective and the other partially transparent. Its transmittance is 2%, that is, the reflectivity is still 98%.
The helium atom in the discharge tube, under the excitation of the excitation power supply, continuously jumps to a high energy level. When it collides with the neon atom, the energy is continuously transferred to the neon atom, so that the neon atom continuously jumps to a high energy level, and it returns to the high energy level. to the base level. At the same time, under the excitation of photons, the neon atoms at the high energy level are stimulated to radiate back to the base energy level, and new photons are produced at this time. Generally speaking, most photons will leap out through the tube wall or be absorbed by the tube wall, and only the photons along the axis of the tube wall will be reflected back and forth between the two mirrors, resulting in continuous radiation and amplification of light.
The Brewster window is a highly polished crystal plate, and the angle between the normal of the window surface and the axis of the tube is called the Brewster angle. This angle varies with the material of the window, in the case of crystal windows it is approximately equal to 56o. When the light wave is incident on the window along the axis of the tube, the component of the electric vibration of the light wave along the paper surface (indicated by the arrow in the figure) will be completely transmitted without being reflected; while the component along the direction perpendicular to the paper surface (indicated by the arrow in the figure) The black dots) are reflected, so that the remaining light is linearly polarized light vibrating along the paper. Afterwards, this kind of light runs back and forth in the resonant cavity, because the newly born photons of stimulated radiation have the same vibration direction as the original photons, that is to say, the accumulated light is always linearly polarized light vibrating along the direction of the paper, Therefore, whenever they pass back and forth through the Brewster window, almost all of them pass through with little loss of light.
The laser equipped with a Brewster window directly outputs linearly polarized light, so that the photoelectric modulator group does not need a polarizer, thus avoiding the incident light of the general modulator, which causes about 50% loss of light intensity due to passing through the polarizer Defects. Therefore, the maximum range of the range finder equipped with the above laser can reach 40-50km.
The laser emitted by the helium-neon gas laser has very stable frequency and phase, high directivity, and continuous emission, so it is widely used in laser ranging, collimation, communication and holography. However, the helium-neon gas laser also has its disadvantages, that is, the efficiency is very low, and the ratio of its output power to input power is only one thousandth. Therefore, the laser output power on the laser rangefinder is only about 2-5mW.
