Micro Holographic Recording Characteristic With Defocused Micrograting

The recording of micro holographic storage is performed by the superposition of two tightly focused counter-propagating beams. The micrograting is affected by misalignment in the z -direction during the recording and read-out processes. Thus, the diffraction efficiency (DE) can be reduced by the defocusing of the micrograting. In this paper, we investigate the recording characteristics of a defocused micrograting. An optical system with a numerical aperture (NA) of 0.60 and a wavelength of 532 nm was designed for micro holographic recording. The recording tolerance was formulated by using vector diffraction theory. The recording tolerance was determined by using the Mahajan's Strehl ratio (SR) of the DE. The DE was dramatically reduced at the ±0.4λ defocusing position and we determined that the ±0.4λ was the recording tolerance in the defocusing direction. In addition, we propose a precise detection method to obtain a focus error signal (FES) within the recording tolerance range. Finally, the numerical results of a defocused micrograting were experimentally verified.