Evaluation of the sphericity measurement uncertainty for the fiber point diffraction interferometer

Due to the imperfectness of the reference wavefront produced by standard lens, the spherical figure measurement precision of the traditional interferometer is limited. The fiber point diffraction interferometer (FPDI) provides a method to produce the nearly perfect reference wavefront for the spherical figure measurement. The diverging wavefront diffracting from the fiber tip can serve as an ideal reference for the concave spherical surface measurement. However, it can not serve for the convex surface measurement until an auxiliary positive lens is introduced. This paper presents the different FPDI configurations for the concave and convex spherical surface measurement. The measurement uncertainty of the FPDI comes from the following factors: the spherical figure error of the reference wavefront diffracting from the fiber tips; the flatness of the fiber tips; the phase shifting error of PZT; the tilting error introduced by adjustment during measurement; the environmental influences such as temperature, vibration, and air turbulence. In all of these factors, the flatness of the fiber tips mostly affects the measurement uncertainty. The estimation of the expanded uncertainty is about λ/95 for the concave and λ/40 (λ=532nm) for the convex spherical figure measurement by FPDI.