Huang, Xuan; (1999) Monolithically integrated quantum confined Stark effect tuned semiconductor lasers. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The thesis describes the design and fabrication of a monolithically integrated GaAs/AlGaAs multiple quantum well (MQW) structure two-section tunable laser, using the quantum confined Stark effect (QCSE) as the tuning mechanism. In contrast to carrier induced effect (CIE) tuned lasers, the QCSE tuned laser provides intrinsically uniform optical frequency modulation (FM) sensitivity over the modulation bandwidth of the device. First, through ridge waveguide modelling and QCSE induced refractive index change analysis, a suitable two-section ridge waveguide laser structure, having the optimised number of quantum wells, is designed. Second, as the tuning mechanism requires an applied electric field, a reverse biased p-i-n structure needs to be integrated with the forward biased laser gain section; therefore, possible isolation schemes to isolate the gain and tuning sections electrically while retaining a single optical cavity are investigated. Wet and dry etches, H+ and O+ implantation to achieve electrical isolation are studied. Third, since it is always desirable to have low residual intensity modulation for a frequency tuned laser, a study on impurity-free vacancy diffusion techniques has been carried out to investigate suitable methods of producing a transparent tuning section. A fabrication process to achieve ~10nm band-gap detuning between gain and tuning section, while maintaining good laser performance, is described. Finally, as laser parasitic parameters limit achievable laser modulation bandwidth, low parasitic laser structures have been designed. An air-bridged laser built on a semi-insulating substrate and a polyimide planarised laser built on an N+ substrate are fabricated successfully, with their tuning section band gap blue shifted relative to that of the gain section. Good CW single mode operation with less than l0MHz linewidth is achieved. FM response uniformity within ±3dB over the bandwidth 30kHz to 6GHz has been achieved, with an intensity modulation index less than 0.1 for 5.5GHz peak optical frequency deviation. The fabricated band gap shifted low parasitic tunable lasers have been characterised extensively, to assess the static and dynamic laser performance and identify the factors influencing pure QCSE tuning. A comparison study has been carried out to compare the tuning merits of this laser and traditional CIE tuned lasers. Further improvements are suggested to achieve better tuning performance and the design for a QCSE tuned laser based on the InP/InGaAsP material system is outlined.

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