single frequency laser interferometer
figure 1 shows the working principle of a single frequency laser interferometer. The beam emitted from the laser is divided into two paths by the beam splitter after beam expansion and collimation, and reflected from the fixed mirror and the movable mirror respectively to meet on the beam splitter to produce interference fringes. When the movable mirror moves, the light intensity changes of the interference fringes are converted into electric pulse signals by photoelectric conversion elements and electronic circuits in the receiver, which are input into a reversible counter to calculate the total number of pulses after shaping and amplification, and then the computer calculates λ as the laser wavelength (n is the total number of electric pulses) according to the calculation formula [356-11]
, and calculates the displacement L of the movable mirror. When using single-frequency laser interferometer, the surrounding atmosphere is required to be in a stable state, and all kinds of air turbulence will cause DC level changes and affect the measurement results.
schematic diagram of single-frequency laser interferometer
dual-frequency laser interferometer
Figure 2 shows the working principle of dual-frequency laser interferometer. An axial magnetic field of about .3 Tesla is applied to the He-Ne laser. Due to Zeeman splitting effect and frequency pulling effect, the laser produces left-handed and right-handed circularly polarized light with two different frequencies: 1 and 2. After passing through a quarter-wave plate, it becomes two linearly polarized lights perpendicular to each other, and then it is divided into two paths by a beam splitter. One path passes through the polarizer 1 and becomes a reference beam with frequencies f1-f2. The other path is divided into two paths after passing through the polarizing beam splitter: one path becomes a beam containing only f1, and the other path becomes a beam containing only f2. When the movable mirror moves, the beam containing f2 becomes a beam containing F2 Δf after being reflected by the movable mirror, Δ F is the additional frequency generated by the Doppler effect when the movable mirror moves, and the sign indicates the moving direction (Doppler effect is proposed by Austrian C.J. Doppler, that is, the frequency of the wave will change when the wave source or receiver moves). This light beam and the light beam only containing f1 reflected by the fixed mirror pass through the polarizing plate 2 to form the measuring light beam of F1-(F2 Δ f). The measuring beam and the reference beam pass through their respective photoelectric conversion elements, amplifiers and shapers, and then enter the subtracter for subtraction, and the output is an electric pulse signal containing only+/-δ F.. After counting by a reversible counter, the displacement of the movable mirror can be obtained by equivalent conversion (multiplying by 1/2 laser wavelength) by an electronic computer. Dual-frequency laser interferometer uses frequency change to measure displacement. This displacement information is contained in the frequency difference between f1 and f2, and it is insensitive to DC level change caused by light intensity change, so it has strong anti-interference ability. It is often used to verify the coordinate accuracy of length measuring machine, CMM, mask aligner and machining center, and can also be used as the measurement system of length measuring machine and high-precision CMM. With the corresponding accessories, high-precision straightness measurement, flatness measurement and small angle measurement can also be carried out.