Near field writing on a phase change optical disc

Ukita et al. (Appl. Opt. vol. 28, pp. 4360-4365, 1989) proposed a near field optical recording system, which is comprised of a laser diode whose emitting aperture is brought very close to the optical disc´s data surface (an airgap between the laser and the disc is smaller than the laser´s wavelength) and therefore no optical components are needed. We have studied phase change (PC) optical disc structures suitable for near field recording arrangements that we call direct semiconductor laser read/write systems. The airgap and the recording structure of the optical disc forms an extremely short external cavity (ESEC) at the front facet of the semiconductor laser. In the external cavity, a part of the light emitted by the laser is reflected from the disc and coupled back into the laser cavity. The coupled light affects the laser´s operating wavelength, optical output power and forward bias voltage. These changes in the laser´s operational characteristics, in turn, affect the magnitude and the spatial distribution of light absorbed into the PC layer. Our main goal is to determine how the optical design of the disc structure (as well as the thickness of the airgap) influences the absorption of light in the PC layer in a direct semiconductor read/write system.