Research On Suppression Of Coherent Noise In Coherent Imaging System By Space Light Modulation

Optical interference testing is one of the necessary part during the high precision optical component processing. It is decisive in determining the quality of optical component. Commercial interferometer uses laser as light source to get long coherence length. This method makes optical interference testing much easier. However, this method also has its disadvantage. The diffusive light in optical system and the scattering light which induced by high frequency error, dust and imperfection of the surface are the main disadvantages. Signal to noise ratio(SNR) will decrease because of these disadvantages. To suppress the coherent noise and increase SNR, people usually modulate the coherence feature of the light source to decrease the coherence length. In.this paper, based on a real-time control dynamic optical component, we did research in using spatial light modulation to suppress coherent noise in coherent-imaging system.This paper did analytical research in testing optical path optical field transmission and testing optical path image plane optical field acquisition based on statistical optics. We compared optical field coherence length change before and after local phase modulation and its influence to noise of the imaging system. Theoretical analysis shows that, when the stand deviation(σ) of modulation field phase fluctuation is stable, the system SNR decreases and then increases with increasing of modulation frequency; When the modulation frequency keeps unchanged, system SNR increases with the increasing of stand deviation(σ) of modulation field phase fluctuation. To prove the theoretical analysis, we designed an experiment setup. This system is based on the Twyman-Green interferometer, and we induced a spatial light modulator in main optical path to do local phase modulation to the optical field. Analysis of SNR of the interference pattern was done after the measurement. The experimental results show that, to verify the theoretical derivations. To sum up, to get high visibility and SNR, we should modulate the system under 30hz refresh frequency and modulation pattern whose the stand deviation of modulation field phase fluctuation σs=0.25π.