Phase imaging for photorefractive holographic gratings with dual-wavelength digital holography

In this paper, the phase-type grating recorded in a Fe:Cu:LiNbO3 crystal is measured by dual-wavelength digital holography. In the experiment, a volume hologram, which is recorded in a 3-mm-thick Fe:Cu:LiNbO3 crystal by interference of two recording beams at the wavelength of 532 nm, is reconstructed to be imaging by dual-wavelength digital holography. Two lasers of the different wavelengths 660 nm and 671 nm are used to obtain a larger beat wavelength. Each laser output, which is spatially-filtered and collimated, is split into a reference and object beams in an interferometer setup based on the Mach-Zehnder configuration. In dual-wavelength phase unwrapping, two individual phase images are obtained by using each wavelength, respectively, and the phase image of beat wavelength is obtained by subtracting one single wavelength phase image from the other and then adding 2 pi whenever the resultant value is less than zero. In the final synthetic image, the discontinuities are removed after reduce the noise of the beat wavelength phase image. Thus, a 3D surface profile of the phase grating is obtained.