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.