Research on the fiber point-diffraction interferometer for spherical figure measurement

The primary restriction on the precision of spherical figure measurement is the imperfectness of the reference spherical wavefront. Using the nearly perfect spherical wavefront diffracting from a single mode fiber as the reference, the accuracy of spherical figure measurement can be greatly improved. To get good contrast of interference fringe, the extraneous interference should be eliminated, and the intensity of the reference beam must match to the measuring beam at the same time. Using the short coherence length laser source can avoid most of the extraneous interference. The principles through stretching the resonant cavity length to shorten the coherence length are discussed; the effects are validated by constructing a Twymann-Green interferometer using the cavity length tunable YAG solid state laser. Calculations on light intensity show that only through controlling the attenuation of the measuring beam, can it match to the reference beam. Coating the fiber tips with semi-metallic film can substantially improve the contrast of the interference fringe. Comparing to the measurement results of ZYGO interferometer, the single interference pattern collected by our experimental fiber PDI apparatus is analyzed and the major error sources are also discussed.