Characterization of the contribution of CGH fabrication error to measurement uncertainty in null test

Aspheric surface acts an important role in modern optical systems. The null test utilizing a computergenerated hologram (CGH) is fundamental for precision test of aspheres. Its measurement accuracy mainly depends on the fabrication precision of CGH. However, it is not easy to exactly characterize the fabrication error of the CGH as well as its contribution to measurement uncertainty. In this paper, a new CGH wavefront error evaluation method is presented. The CGH fabrication errors such as duty-cycle error, etching depth inhomogeneity, pattern distortion, etc., are related to the fringe spacing based on elaborate measurement of the CGH microstructures. A scanning white-light interferometer and a high-precision two-axis stage are employed to sample the microstructure at a series of designed locations on the CGH. When the fabrication error is modeled through experiments, it can then help to realize rapid measurement of any other CGHs with significantly reduced number of sampling. The second step is then modeling the contribution of CGH fabrication error to measurement uncertainty according to the scalar diffraction theory. Meanwhile, the wavefront error induced by CGH fabrication error can also be characterized through ray-tracing in lens design software. The fabrication error is incorporated into the discrete phase data of CGH surface and its contribution to the final measurement uncertainty is evaluated through simulations.