REFRACTIVE-INDEX AND LOSS CHANGES PRODUCED BY CURRENT INJECTION IN INGAAS(P)-INGAASP MULTIPLE-QUANTUM-WELL (MQW) WAVE-GUIDES

We have theoretically estimated the carrier-induced changes in the refractive index delta n and the optical loss delta alpha 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 lambda of 1.55 mu m. Anomalous dispersion and plasma effect for delta n and IVBA (intervalance band absorption) for delta alpha were included, A valance-band mixing, an overlap integral of electron-hole, and confinement factors of carriers in the web were considered to include quantum-size effect of carriers in delta n and delta alpha. Predictions of delta n are in reasonably good agreement with the experimental results, Systematic analysis shows that the following InGaAs(P)(well)-InGaAsP (barrier) MQW structures are promising in order to obtain a large delta n at a given injection current density J: 1) well materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.55 mu m, 2) barrier materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.4 mu m, 3) as many as possible number of wells, typically about 15. delta alpha 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-mu m InGaAsP (well 1,8-nm thick)/1.40-mu m InGaAsP (barrier 14-nm thick) MQW, with the well number of around 15, will be the optimum tuning layer structure for 1.55-mu m band tunable LD's, Over 10 mn continuous tuning range, with moderate output power, is expected for TTG-LD's.