Magneto-Optical Devices for Optical Integrated Circuits

Abstract

Magneto-optical materials have two unique properties, which make them important for a variety of optical applications. The first property is non-reciprocity. The time inverse symmetry is broken in magneto-optical materials. Therefore, properties of magneto-optical materials are different for two opposite directions of light propagation and optical nonreciprocal devices like the optical isolator and the optical circulator can be fabricated only by utilizing magneto-optical materials. The second important property of the magneto-optical materials is a memory function. If the material is ferromagnetic, the data can be memorized by means of two opposite directions of the residual magnetization. Both the reading and writing of such memory can be done by magneto-optical effect. The optical isolator is an important component of optical networks. It is transparent in one direction and blocks light in opposite direction. Due to the imperfect matching between optical components in the network, the unwanted back reflection always exists and it severely disturbs the network performance. To avoid this, the optical components have to be protected by an optical isolator. Also, the isolator is important to cut the back-travelling amplified spontaneous emission in the case of serially-connected amplifiers. Today there is a big demand to integrate all optical components into an opto-electronics chip. In fact, the isolator is one of few components, which have not yet been integrated into commercial chips. It is because of difficulties to integrate magneto-optical materials into a semiconductor-made chip. To solve this, we proposed to use (Cd,Mn)Te as a magnetooptical material for such isolator. The (Cd,Mn)Te exhibits a huge Faraday effect and can be grown on a semiconductor substrate. For (Cd,Mn)Te waveguide grown on GaAs substrate we achieved a high Faraday rotation of 2000 deg/cm, a high isolation ratio of 27 dB, a low optical loss of 0.5 dB/cm, and a high magneto-optical figure-of-merit of 2000 deg/dB/kG in a wide 25-nm wavelength range (Debnath et al., 2007). These values meet or exceed similar values of commercial discrete isolators. We predicted theoretically (Zaets & Ando, 1999) and proved experimentally (Zayets & Ando, 2005) the effect of non-reciprocal loss in hybrid semiconductor/ferromagnetic metal waveguides. This effect can be utilized for new designs of waveguide optical isolator. Because the structure of this isolator is similar to that of laser diode, such a design is beneficial for the integration. The bistable laser diode with non-reciprocal amplifier was proposed to be used for high-speed optical logic (Zayets & Ando, 2001).