Multi-valued MO recording and multi-wavelength readout

We propose the application of the multi-wavelength readout (MWR) method to multi-valued magneto-optical (MO) recording. Optimal layer structures of the MO disk for obtaining a high signal-to-noise ratio (SNR) were deduced from the calculation of interference in the multi-layer. The optimized medium: substrate/SiN(150 nm)/TbFeCo(8 nm)/SiN(180 nm)/TbFeCo(45 nm)/PtCo(46 nm) was sputtered on a polycarbonate substrate. The optical characteristics, the linear combination factors and the carrier to noise ratio (CNR) were measured. These characteristics lead to improved CNR of 52 dB in each MO layer by using MWR for λ₁=630 and λ₂=780 nm in wavelength, for 1.5 μm in mark length and 3.0 μm in mark period. Since this value of CNR is the same as that of conventional two-level MO recording, each reproduced signal from the recording bi-layer of the optimized MO disk can be detected without loss in CNR by using MWR