Introduction

In magneto-optical (MO) data storage systems the readout of recorded information is achieved by means of a focused beam of polarized light. Conventional systems today utilize a linearly polarized beam, whereas some suggested alternative methods rely instead on circular or elliptical polarization for readout.

Although the Jones calculus is the standard vehicle for analyzing the polarization properties of optical systems, we believe the alternative approach used in this chapter provides a better, more intuitive explanation for the operation of the readout system. The chapter begins by introducing the two basic states of circular polarization: right (RCP) and left (LCP). It then proceeds to show that the other two polarization states, linear (LP) and elliptical (EP), may be constructed by superposition of the two circular states. The sections that follow describe the actions of a quarter-wave plate (QWP) and a polarizing beam-splitter (PBS) on the various states of polarization. These two optical elements are of primary importance in detection schemes aimed at probing the state of polarization of a beam.

The classical scheme of differential detection for MO readout will be analyzed in section 6.4, first in its simple form for detecting the polarization rotation angle and then, with the addition of phase-compensating elements, for detecting ellipticity as well. In section 6.5 we describe an extension of the differential detection scheme. This extended scheme is used for spectral characterization (i.e., measurement of the wavelength-dependence of the Kerr angle and the ellipticity) of MO media.