High density near field readout over 50 GB capacity using a solid immersion lens with high refractive index

We have already reported a high density near field readout system with a GaN blue laser of 405 nm wavelength and an optical pick-up of an effective numerical aperture (NA) of 1.4. A readout performance of the near field readout system depends on the optical parameters, namely the NA of a solid immersion lens (SIL) and the laser wavelength. With a relative high refractive index glass material, we can design a SIL with higher NA. In this study, by using a glass with the refractive index of 2.08, we developed the optical pick-up with the effective NA of 1.8. We could observe a clear eye pattern for 50 GB capacity ROM in a φ120 mm single side disc.