Abstract
A frequency-shift interference fringe locking system was established to enhance the contrast of holographic grating profile and to decrease the phase shift of exposure interference fringe caused by the external environment. According to the composition and the working principle of the system, the moire pattern occurrence condition in the measuring system and its dependence on the phase change of interference fringe were analyzed and the selection method for photodetectors was given. On the basis of the system accuracy, the digital control system was self-designed including A/D conversion digit. A optical frequency shifting method was used to regulate the fringe phase, and an acousto-optic modulator was taken to correct the frequency shifter of the interference fringe movement in real time. The experimental results illustrate that the system can offer a sampling frequency of 5 kHz and can restrain the interference fringe drift and low-frequency moving below 10 Hz. The 3 value of the phase shift is below 0.12 rad, which means its phase is within 0.02 interference fringe period. The system can lock the phase of the exposure interference fringe effectively in real time, and can satisfy the requirements of the holographic grating lithography better.
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