Abstract

To increase the diffraction efficiency of a binary computer-generated hologram (CGH), a phase- only binary hologram of which index distribution is modified by using an ion exchange method is proposed. The glass substrate is immersed at an elevated temperature around the glass transition temperature in a molten salt containing ions that increase the refractive index of the glass. The ions are diffused only through the window area of the mask in the glass substrate. Theoretically we expect the increase of the diffraction efficiency as a factor of 4 in an optimum case. To verify the theory, we produced the Lee-type phase-only Fourier transform CGH with 64 × 64 sample points. The phase difference is ~0.8π. Due to interferometric observation of a sliced sample cross section, the diffusion depth of ion migration is estimated as ~15 μm. Scattering noise is very low in the first orders and the zero order in the reconstructed plane. The diffraction efficiency of the phase-only CGH is 3.8 times as large as the conventional one.

© 1988 Optical Society of America