An increasing density of data stored on an optical disk requires better writing and reading control, more sophisticated data processing, and smaller laser beam stylus. An optical head can produce a smaller beam stylus by using a shorter wavelength laser diode and/or by increasing the numerical aperture (NA) of an objective lens. While the usage of a shorter wavelength is clearly curtailed by availability of short wavelength diodes, the upper limit on the NA may not be determined by constraints associated with the lens design and manufacturing. Particularly, when the recording surface of an optical disk is protected with a substrate, the upper NA limit may be dependent on disk tolerances and on the capability of a drive servo control. When the solid immersion lens (SIL) technique is used, the upper NA limit depends more on the optical system design ingenuity and on the magnitude of the refractive index n of a SIL lens. In the SIL case the disk without substrate suffers with far less flaws. Our interest here will be in optical heads working with substrate furnished disks displaced more than one millimeter from a biaspheric objective lens.
Ivan Q. Prikryl, Ivan Q. Prikryl,
"How high an NA is too high?", Proc. SPIE 3864, Joint International Symposium on Optical Memory and Optical Data Storage 1999, 38643F (28 June 1999); doi: 10.1117/12.997671; https://doi.org/10.1117/12.997671