The Preliminary Study On Dual Frequency Laser Interferometer With Mid-frequency Difference

Dual-frequency laser interferometer is a widely used precision measurement instrument. The key element of this interferometer is a dual-frequency laser, which produces two lights with both different frequency and different polarization, and whose wavelength is used as basic scale for length measurement. Presently dual-frequency laser interferometers at home and abroad use the laser with frequency difference below 3MHz, called little frequency difference, as its light resource. And the frequency difference of the birefringence dual-frequency laser is above 40 MHz, called big frequency difference. Recently we have successfully invented He-Ne laser with controllable frequency difference ranging from 3 to 40 MHz called mid-frequency difference to distinguish from those two mentioned above. Because of the little frequency difference of light resource, the top measurement speed of present interferometers is not more than 1 m/s. While the interferometer employing the new laser as light resource is expected to smoothly work with measurement speed of 4 m/s or even higher, and this paper is trying to realize it.This paper mainly contains two parts. In one part we analyze the optical characters of the dual-frequency laser interferometer with mid-frequency difference, and then we construct the optical system. In another part, the mid-frequency difference heterodyne signal processing, especially the integral detection method is discussed. In detail, concerning the optical part, we put a λ/2 glass into the optical path to modulate the polarization direction of these two lights, construct optical system according to this idea, discuss all possible phenomena, design adjusting steps, and finally we observe ideal frequency shifting. Dealing with the signal processing, we design and test an optic-electronic transferring and pre-amplifying circuit, with which we get perfect frequency beat signal, and discuss two kinds of integral detecting methods based on CPLD. One kind is the opposite and alternative pulse eliminating method, and as part of this method a kind of totally digital opposite and alternative pulses eliminating circuit is put forward, which takes effect and needs further improvement in real application. Another kind is the big integer minus method, which is to count two pulses respectively and then one minus another in computer, and a step-by-step minus method is put forward to reduce the range of the counter. The final integral detecting method may process signal below 50MHz without cumulate error. The main function modules of interferometer have been tested and run well, and simple measurement may be executed if they are connected together properly.