Nonlinear Polarization Switching in an InGaAs/GaAs Quantum Well Amplifier

Nonlinear self-switching is an important step toward all-optical signal processing and passive mode-locking of lasers. In nonlinear self-switching, the power-dependent coupling of energy between two channels is caused by the imbalance of nonlinear gain/absorption and/or refractive-index between the two channels. In this paper, we report the experimental results of nonlinear polarization switching in an InGaAs/GaAs single quantum well(QW) GRINSCH optical amplifier. The amplifier consists of a 10 nm InGaAs well. On either side of the well, it includes a 40 nm GaAs barrier, an about 200 nm graded-index separate confinement layer with AlxGa1-xAs (x drops from 0.4 to 0), and an Al0.4Ga0.6As p/n type cladding. It lased at the threshold current 120 mA with 20 % duty cycle at 974 nm. The amplifier had a length about 300 μm and a ridge 50 μm wide for lateral guiding. The electro-luminescence in the TE (TM) mode was centered near 975 (940) nm, indicating the relative energy state of the heavy and light hole subbands. In the experiment, we used a Ti:Sapphire laser (Mirra 900) for femtosecond and cw optical signals. After the signals passed through the amplifier, their polarization status and power distributions between the TE and TM directions were recorded, as shown in Fig. 1.