Design of laser source for fiber point diffraction interferometer

The fiber point-diffraction interferometer is introduced for measuring spherical surface with high precision. The interferometer must use special laser source, the coherence length of which is no more than 10 cm. The laser source at present can not meet the demand and the research of laser source design is performed in this paper. The coherence length of laser source in experiment is inconsistent with that computed with general theory, which restrict the research progress. In this paper, Mutual Coherence Function is introduced to describe the coherence of optical field. Based on the spectral characteristic of laser source, the mode of Multi-Longitudinal Mode laser is built. Through numerical calculation, it is discussed that how coherence degree is related with Longitudinal Mode number, spectral half width and resonant cavity length. Some important conclusions are draw. For example, Multi-Longitudinal Mode laser source has periodic coherence degree no matter how many longitudinal modes it contains, and the period of laser coherence degree is always double resonant cavity length. The method for designing short coherence length laser is put forward based on the conclusions and its coherence is measured in experiment. The result shows that the coherence length reaches 5 cm and meets the need of the interferometer.