Design and fabrication of CGH for aspheric surface testing and its experimental comparison with null lens

Abstract

Computer-generated hologram (CGH) is an effective way to compensate wavefront in null test of aspheric surfaces and freeform surfaces. Our strategies of CGH design and fabrication for optical testing are presented, and an experiment demonstrating the compensation results of CGH and null lens is also reported. In order to design complex CGH, software was developed, with which we can design a CGH including three sections: main section for compensating wavefront in null test, alignment section for adjusting the relative position between CGH and interferometer, and fiducial section for projecting fiducial marks around the optics under test. The design result is represented in GDS II format file which could drive a laser-direct-writer-machine to fabricate a photomask. Then, a 1:1 replication process is applied to duplicate the patterns from photomask to a parallel optical substrate whose surface is error better than λ/60 rms. Finally, an off-axis aspheric surface was tested with CGH and null lens respectively. The test result with CGH (0.019λrms) is almost the same as the result with null lens (0.020λ rms). This experiment also demonstrated that fiducial marks projected by CGH can be used to guide the alignment of the optics and measurement of its off-axis distance.

Effect of masking phase-only holograms on the quality of reconstructed images

“…Compared with amplitude hologram, phase-only hologram modulates the phase of an incident beam for high diffraction efficiency [1]. It has been widely used in various applications such as holographic displays [2][3][4], holographic optical switches [5,6], wavefront compensations [7], and holographic optical tweezers [8,9]. Researches have been done to develop optimized algorithms to calculate phaseonly holograms for specific required target patterns or images, by balancing the image quality and computation power needed.…”