Monolithic Integration of Semiconductor Waveguide Optical Isolators with Distributed Feedback Laser Diodes

Monolithically InP-based photonic integrated circuits, where more than two semiconductor optoelectronic devices are integrated in a single InP substrate, have long history of research and development. Representatives of these InP-based photonic integrated circuits are, electroabsorption modulator integrated distributed feedback laser diodes (DFB LDs) (Kawamura et al., 1987, H. Soda et al., 1990) and arrayed waveguide grating (AWG) integrated optical transmitters and receivers (Staring et al., 1996, Amersfoort et al., 1994). Recently, dense wavelength division multiplexing (DWDM) optical transmitters and receivers have been reported with large-scale photonic integrated circuits having more than 50 components in a single chip (Nagarajan et al., 2005). However optical isolators have been one of the most highly desired components in photonic integrated circuits in spite of their important roles to prevent the backward reflected light and ensure the stable operation of LDs. Although commercially available “free space” optical isolators are small in size and high optical isolation (>50dB) with low insertion loss (<0.1dB) is already realized, they are composed of Faraday rotators and linear polarizers, which are not compatible with InP based semiconductor LDs. Especially, Faraday rotators are based on magneto-optic materials such as rare earth iron garnets, and they are quite incompatible with InP based materials. Monolithically integrable semiconductor waveguide optical isolators are awaited for reducing overall system size and the number of the assembly procedure of the optical components. Also, such nonreciprocal semiconductor waveguide devices could enable flexible design and robust operation of photonic integrated circuits. To overcome these challenges, we have demonstrated monolithically integrable transverse electric (TE) and transverse magnetic (TM) mode semiconductor active waveguide optical isolators based on the nonreciprocal loss (Shimizu & Nakano, 2004, Amemiya et al., 2006), and reported 14.7dB/mm optical isolation at λ=1550nm (Shimizu & Nakano, 2006). In this chapter, we report monolithic integration of a semiconductor active waveguide optical isolator with distributed feedback laser diode (DFB LDs).