Modeling and simulation of wavelength-tunable laser diodes for WDM system

Wavelength tunable semiconductor laser diodes are one of the most important devices in wavelength division multiplexing (WDM) optical communication systems. There has been intensive research effort to extend the tuning range of laser diodes, in order to increase the transmission capacity and to add other system functions to the existing optical systems and networks. Since the laser fabrication process is complicated and expensive, modeling and simulation become increasingly important to reduce development time and cost. The theme of this dissertation is to develop efficient and accurate models and simulation techniques for laser diodes in general and tunable lasers in particular.; A comprehensive hierarchical model framework is developed and validated, which spans from the material models to the longitudinal waveguiding structures commonly used for tunable lasers. A series of models are established to simulate the intrinsic refractive index and refractive index changes caused by carrier injection, thermal effects and applied electric field. An optical gain model for strained multiple quantum well (MQW) materials is developed based on accurate band structure calculations. Comparisons with experimental data for some of these material models are made and show good agreement.; A comprehensive traveling wave model is developed, which takes into account of spatial-hole-burning, spontaneous random noise as well as material effects in laser diodes with complex longitudinal configurations. Especially, thermal effects are incorporated into the traveling wave model for the first time. With the help of the new model, the various thermal time constants during the large signal modulation of a DFB laser observed experimentally are explained. To improve the computational efficiency and the application versatility of the conventional traveling wave approach, we have proposed and demonstrated a digital filter approach, which combines the standing wave and the traveling wave models. This novel idea makes the traveling wave model more versatile and efficient, especially in simulation of laser diodes integrated with long optical waveguide structures.; Another major contribution of this work is the applications of the models and the simulation techniques for several novel tunable lasers. As the first example, the sampled grating assisted wavelength tunable laser is investigated. In this category, two kinds of devices are discussed. One is the DBR (Vernier ruler type) tunable laser; the other is DFB type tunable laser cascaded with a co-directional filter as wavelength selection element. This work is the first comprehensive simulation based on a rigorous numerical model. Secondly, we modeled and simulated a wavelength tunable multi-section gain-coupled DFB cascade. The interaction between the adjacent section is considered, and some key design parameters are optimized. In the design, the CW working condition and the tuning mechanism are demonstrated. Further, the condition and the characteristics of short pulse generation resulting from multi-mode nature of this complex DFB cascade is also identified.