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UBC Theses and Dissertations

Equivalent-circuit modeling of quantum-well lasers and fibre-optic communications channels Tsou, Benjamin Pen-Cheng


Lasers, photodetectors, optical amplifiers, and optical fibres are the key components in lightwave communications systems. The availability of accurate models for these devices allows designers to predict the performance of lightwave communications systems prior to their implementation. Since many of the components in such communications systems are still purely electronic in nature, the development of equivalent-circuit models for the optical devices enables the entire telecommunications link to be simulated using one of the well-developed circuit simulators from the electronic industry, e.g., SPICE. Suitable models for quantum-well lasers, optical fibres, and erbium-doped fibre amplifiers (EDFA's) are developed here, and then used with equivalent-circuit models of a photodiode, a transimpedance amplifier, and an electric filter to produce a complete equivalent-circuit model for a lightwave communications channel. The main contribution of the thesis is a SPICE model for the quantum-well laser, which takes into account, for the first time, dynamical carrier effects due to the presence of gateway states at the quantum well. These states play important roles when considering carrier transport through a long separate-confinement heterostructure (SCH) and carrier capture and release via these states at the quantum well. Coulomb-enhanced gain due to many-body effects is extended beyond the usual treatment by accounting for transitions between multiple sub-bands. This correction to the free-carrier gain improves the fit between simulated and experimental data of the 3dB modulation response as a function of optical power. A compact analytical expression for the material gain, as a function of carrier density in the quantum well, is proposed for use in the model for self-consistent calculation in SPICE. The laser model is validated by comparing results for the small signal modulation response of lasers with different SCH lengths to experimental data. The number of fitting parameters is minimized by ensuring that, whenever possible, the parameters have traceable physical origins. Two SPICE models are considered for the optical fibre: one based on the finiteimpulse- response (FIR) filter, and one on the infinite-impulse-response (UR) filter. Speed improvements on existing F IR models are achieved by reducing the number of nodes to two through the usage of behavioural current sources connected in parallel. The investigation of the IIR filter representation revealed stability problems during simulations, which may limit the usefulness of this model. The equivalent-circuit model for the inline optical amplifier is obtained from a single ordinary differential equation description of the E D FA. The circuit models for the quantum-well laser, fibres, and E D F A are combined with standard models for a photodetector and an amplifier, and with a Laplace-transform representation of a filter to simulate an entire optical communications channel. It is believed that this is the first time this has been accomplished in a totally S P I C E simulation. The full model is used to examine the sensitivity of overall system performance to changes i n specific system and laser parameters, such as data bit rate, fibre length, dispersion parameter, linewidth enhancement factor, gain compression factor, Coulomb enhancement, and various time constants associated with the carrier dynamics in the QW.

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