Refractive index and loss changes produced by current injection in InGaAs(P)-InGaAsP multiple quantum-well (MQW) waveguides

We have theoretically estimated the carrier-induced changes in the refractive index δn and the optical loss δα produced by the injection of free carriers in InGaAs(P)-InP multiple quantum-well (MQW) optical waveguides. MQW structures are specially designed for the tuning layer in carrier injection type tunable laser diodes, such as tunable twin-guide laser diode (TTG-LD), at the lasing wavelength λ of 1.55 μm. Anomalous dispersion and plasma effect for δn and IVBA (intervalance band absorption) for δα were included. A valance-band mixing, an overlap integral of electron-hole, and confinement factors of carriers in the well were considered to include quantum-size effect of carriers in δn and δα. Predictions of δn are in reasonably good agreement with the experimental results. Systematic analysis shows that the following InGaAs(P)(well)-InGaAsP (barrier) MQW structures ace promising in order to obtain a large δnat a given injection current density J: 1) well materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.55 μm, 2) barrier materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.4 μm, 3) as many as possible number of wells, typically about 15.δα is also estimated by calculating the carrier distributions in MQW structures and by fitting experimental data of bulk and MQW waveguides. The maximum well number is limited by the increase of optical loss. According to these results, we have found that 1.55-μm InGaAsP (well 1.8-nm thick)/1.40 μm InGaAsP (barrier 14-nm thick) MQW, with the well number of around 15, will be the optimum tuning layer structure for 1.55-μm band tunable LD´s. Over 10 nm continuous tuning range, with moderate output power, is expected for TTG-LD´s