Effect of wavelength change in microholographic recording

In microholographic recording, expensive laser diodes having no spectrum broadening (single mode) and no wavelength variation are used. On the other hand, in conventional optical disk systems, cheap laser diodes having spectrum broadening (multimode) and wavelength variation are used. It is a great advantage if the laser diodes for conventional optical disk systems can be used for microholographic recording. Therefore, the effect of wavelength change in microholographic recording was investigated through a numerical simulation. The laser diodes were modeled so that the full width at 1/e2 maximum of the spectrum was 0.8 nm and the center wavelength was 405 nm. The numerical aperture of the objective lenses was 0.85 and the thickness of the recording medium was 300 μm. The diffraction efficiency of the diffracted beam from a microhologram was calculated using the coupled wave theory and the following results were obtained. The diffraction efficiency decreased by three orders of magnitude by replacing single-mode laser diodes with multimode laser diodes, which makes it necessary to enhance the readout signal. The tolerance of the optical path length difference between the signal and reference beams was -50 ~ 110 μm, which makes it necessary to adjust the optical path length difference. The tolerance of the wavelength variation was 405 ± 0.5 nm, which makes it necessary to select the laser diodes. The conclusion was that it is not practical to use the laser diodes for conventional optical disk systems for microholographic recording.