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Browse Prior Art Database

Use of Frequency Shifting in Optical Recording

IP.com Disclosure Number: IPCOM000050019D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 53K

Publishing Venue

IBM

Related People

Cohen, MS: AUTHOR [+3]

Abstract

This article describes a method for writing information by using the beam not to make a physical hole in a film, but rather to irreversibly shift the resonant frequency of an optical cavity.

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Use of Frequency Shifting in Optical Recording

This article describes a method for writing information by using the beam not to make a physical hole in a film, but rather to irreversibly shift the resonant frequency of an optical cavity.

The usual technique for writing in optical storage involves burning small holes in a film by means of a focused laser beam. It is desirable to perform the writing operation with a laser beam of as low energy as possible, in order to make use of inexpensive lasers. On the other hand, in many cases materials of low melting points are undesirable because they are unstable over long periods of time, thus leading to short shelf life of the medium.

To illustrate the concept of this article, the structure of

Fig. 1 may be examined. Here a film stack has been constructed of films of appropriate thicknesses and optical constants so that destructive interference between the bottom thick Al layer and the top thin Al layer takes place. Under these conditions only about 7.3 percent of the incident beam is reflected; the rest is absorbed in either the top or bottom Al film. Thus, if no information had previously been written, the read beam output would be interpreted as a string of zeros, corresponding to low reflected light levels. TABLE I

OPTICAL CHARACTERISTICS OF FIGURE 1

Transmitted: approx. 0 Lambda=514.8 nm

Reflected: 0.073 Optical Constants

Absorbed Al: n=0.691, K=-5.29

base Al: 0.122 MgF(2): n=1.38

Absorbed CeO(2): n=2.2

Active Al: 0.804

0.999

Suppose now that a higher intensity write beam irreversibly changed the refractive index of the intermediate layer (MgF(2) in this case). The conditions for destructive interference would then be affected; i.e., the resonant frequency of the cavity would be shifted. This condition is illustrated in Fig. 2 where the reflectivity is plotted as a function of refractive index change. It is seen that a change in refractive index of less than 2 percent from the original value of 1.38 causes more than an order of magnitude increase in reflectivity, so that upon readout previously written ones would show high reflectivity. TABLE II

DATA PERTAINING TO FIGURE 2

Effect on Reflectivity of Changes in Refractive Index of

MgF(2) Layer in Stack: 143.8 nm CeO(2)/13 nm Al/511.6 nm

MgF(2)/250 nm Al/Fused Quartz Substrate. Al: n=0.691,

K=5.29; MgF(2): n=1.38; CeO(2): n=2.2...