Optical memories using optical discs such as compact discs (CD) and DVDs, optical cards and the like as media are utilized as optical information recording/reproducing device. In order to realize larger capacity for recording information, an optical information recording/reproducing device for recording/reproducing information on/from a multi-layer information recording medium including a plurality of recording layers so as to be capable of three-dimensional recording as shown in FIG. 14 is disclosed in non-patent literature 1.
An optical disc 121 is such an information recording medium that recording layers 101a to 101d using a urethane-urea copolymer material as a photon-mode recording material and intermediate layers 102a to 102c using PVA (polyvinyl alcohol) films and PMMA (polymethyl methacrylate) films are alternately formed on a transparent substrate 104 of glass.
Light 122a emitted as parallel light from a recording light source 120a including a Ti sapphire laser and having a wavelength of 790 nm and a large peak power passes through a beam splitter 118a and has the beam diameter thereof expanded by a beam expander 123. Expanded light 108 passes through a beam splitter 118b and is focused (convergent light 107) on the desired recording layer 101c of the multi-layer optical disc 121 by an objective lens 106 to be recorded as a recording pit 105 utilizing a nonlinear phenomenon such as a two-photon absorption process.
Light 122b emitted as parallel light from a reproducing light source 120b including a He—Ne laser and having a wavelength of 0.6328 μm and a small peak power is focused on the recording pit 105 of the desired recording layer 110c by the objective lens 106 in the same manner as above (convergent light 107). The reflected light is bent in an X-axis direction by the beam splitter 118b and focused by a detection lens 111, passes through a pinhole 114 arranged at a focal position of the detection lens 111 and is detected by a photodetector 119, whereby a signal can be reproduced.
However, the Ti sapphire laser is used for the recording light source and the He—Ne laser is used for the reproducing light source in non-patent literature 1, which leads to the enlargement of the device and higher cost. Thus, it is practically desirable to use small-size semiconductor lasers as both recording and reproducing light sources, and the use of the semiconductor lasers enables the miniaturization and cost reduction of the optical information recording/reproducing device. However, if the semiconductor laser light sources are used on conditions similar to those for normal optical disc drives such as DVD drives, light quantity necessary for nonlinear recording cannot be obtained since there is a limit in peak power as compared to the Ti sapphire laser.
In nonlinear recording such as two-photon absorption recording, multiphoton absorption recording and plasma absorption recording, the sensitivity of an information recording medium nonlinearly increases as the peak power of recording light focused on the information recording medium increases since a nonlinear characteristic is utilized. For example, in the two-photon absorption recording, recording sensitivity has a square characteristic (n-th power characteristic in n-photon recording), wherefore recording sensitivity increases to the fourfold of the square if the peak power of recording light doubles. Accordingly, in order to use a semiconductor laser having a limit in the peak power of emitted light as compared to the Ti sapphire laser, it is desirable to introduce a focused spot of the recording light focused on a recording layer to an objective lens as efficiently as possible without deteriorating optical characteristics of the focused spot (without making the effective focused spot size of the recording light too much larger than the focused spot size of reproducing light). However, in order to obtain a light quantity necessary for nonlinear recording, the effective focused spot size of the recording light has to be set larger than the focused spot size of the reproducing light if semiconductor laser light sources are used on conditions similar to those for a normal optical disc drive such as a DVD drive, wherefore the spot diameter of the reproducing light and that of the recording light cannot be balanced.
[Non-Patent Literature 1]
    “Three-Dimensional Optical Memory Using a Femtosecond Laser” by Yoshimasa Kawada, Optronics No. 11, pp. 138-142 (2001).