Optical path layout and moving mirrors of wavemeter based on Michelson interferometer

Abstract

General wavemeters based on Michelson interferometer only have a moving arm, which cann't more multiply optical paths' differences, and is unable to avoid dispersion from a beamsplitter. Commonly, the moving mirror driven by a direct current motor and a ball screw have some disadvantage, such as heavy weight, unstable motion. In the paper, a better optical layout, and configuration and a driving method of moving mirrors are proposed. A newly optical paths layout of a wavemeter based on Michelson Interferometer is present, including two moving mirrors for forming optical paths' differences, a beamsplitter for splitting a light into a transmitted light and a reflected light, two reflectors, and a reference laser. It has two moving arms and can eliminate dispersion from the beamsplitter. According to Doppler effect, how to form the interference fringes in the photodiodes is analyzed and formulated. The Doppler effect appears with motion of the moving mirrors. Consequently, alternately dark and bright interference fringes are generated, then received and converted into the electronic signals by the photodiodes. It is concluded that the electronic signals involves the wavelength of a light and the velocity of the moving mirror by investigating the Doppler effect. The structure of the moving mirrors is clarified. The moving mirrors are made of the two pyramid prisms which are placed symmetrically on the driving motor. A controlling system for keeping the moving mirrors in constant velocity is designed. In order to make frequencies of electronic signals from interference fringes stable, the moving mirrors must move in a uniform speed. The voice coil motor (VCM) drags the moving mirror to and fro. VCM in uniform motion is realized by an optical-mechanical-electrical closed-loop feedback system. The Doppler frequency difference of the reference laser is the standard of the system. The PID controller comprising parallel proportional-integral-differential operational circuit regulates the velocity of VCM.