Because the laser has excellent time coherence, its coherence distance can reach several kilometers, so since the advent of the laser, laser interferometer using laser as a light source has been paid attention to, and its application range has been continuously expanded, and laser interferometer technology has also continued With development, various forms of laser interferometers have emerged.
Although there are various forms of laser interferometers, in principle, they can be attributed to two basic types: single-frequency laser interferometers and heterodyne laser interferometers.
The laser interferometer usually used for length measurement uses the Machelson interferometer system. Figure 1 is a schematic diagram of the principle of a single-frequency laser interferometer. The beam splitter BS divides the laser into 2 beams, one beam is directed to the fixed mirror R, and the other beam is directed to move. Mirror M, when the moving mirror M moves, the reflected light from R and M converge at O to cause interference. Due to the additional phase shift nature of the metal film of the beam splitter, the phase difference of the signals received by the photodetectors D1 and D2 is 90°, It is used to distinguish the direction of the counter.
Figure 2 is a schematic diagram of the principle of a heterodyne laser interferometer. The coaxial dual-frequency laser whose polarization directions are perpendicular to each other is divided into two parts by the beam splitter BS, and the reflected part is received by the photodetector D1 via the analyzer P1 as the reference signal of the system; The transmission part is decomposed at the polarization beam splitting prism PBS according to the polarization direction, one-way points to the fixed mirror R, the frequency is f 1; the other waypoints to the moving mirror M, the frequency is f 2. When the moving mirror M moves, the return light produces a Doppler frequency shift Δf, and the two light beams f2+Δf and f1 are combined in the polarization beam splitter PBS, and the analyzer P2 placed at 45° is received by the photodetector D2.
It can be seen from the table that the measurement information Δf of the heterodyne laser interferometer is superimposed on a fixed frequency difference (f 2-f 1), which belongs to the AC system, while the measurement information Δf of the single-frequency laser interferometer is superimposed on a DC The components (E21+E22) belong to the DC system, which is the essential difference between the two types of laser interferometers.
Polarization beam splitting prisms are mainly used in interferometers for error correction of CNC machine tools and various optical instruments that require polarization or analyzer.