Below-bandgap linear and nonlinear optics waveguiding characteristics of a weakly index-guided semiconductor laser

We report the reduced linear-optics waveguiding efficiency for the signals around 1 560nm as the injection current of a GaAs/A1GaAs multiple quantum well laser diode (lasing wavelength at 840 nm) with a ridge-loading waveguide configuration increased. The decreased waveguide transmission and the more expanded mode profile indicated the variation ofthe effective refractive index distribution in the lateral dimension with injection current. The variation was partially due to carrier diffusion into the regions outside the ridge. Also, enhancements of two-photon and three-photon absorption coefficients with injected carrier density at 1560 nm in the same laser diode device were observed. The results were obtained by calibrating 130 fsec laser transmission data through the laser diode. Such enhancements were mainly attributed to the existence of hole states in the first heavy-hole subband during carrier injection. With the hole states, two-photon absorption in the TE polarization finds the other transition path for a higher transition rate. Such hole states are not so effective to the two-photon transition in the TM polarization.