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 /spl delta/n and the optical loss /spl delta//spl 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 /spl lambda/ of 1.55 /spl mu/m. Anomalous dispersion and plasma effect for /spl delta/n and IVBA (intervalance band absorption) for /spl delta//spl alpha/ 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 /spl delta/n and /spl delta//spl alpha/. Predictions of /spl delta/n 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 /spl delta/nat a given injection current density J: 1) well materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.55 /spl mu/m, 2) barrier materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.4 /spl mu/m, 3) as many as possible number of wells, typically about 15./spl delta//spl 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-/spl mu/m InGaAsP (well 1.8-nm thick)/1.40 /spl 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-/spl mu/m band tunable LD's. Over 10 nm continuous tuning range, with moderate output power, is expected for TTG-LD's.<
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