Development of optical gap sensor application to near field recording

For applying near-field recording (NFR) technology to optical storage devices for the next generation, it is positively necessary to maintain a small air gap under about 100 nm. We design an apparatus to measure the air gap between the CISD type SIL [1] and the interface of dielectric substrate of the disk. And it consists of a prism, a polarizer and an analyzer. The air gap including the far-field as well as the near-field range is determined by measuring the intensity of polarized reflectance light. Through the Fresnel equation and Jones matrices, a mathematical model is established to understand the characteristics of a system according to design parameters. We can change measurement ranges/resolutions by adjusting an incident angle into the air interface. Experimental results for some specific cases are in good agreement with simulated ones and demonstrate the possibility as a new optical gap detector.