Semiconductor laser diodes are are used in numerous applications. Laser diodes must be coupled to some form of external optical system for them to be any use, this can be achieved in many ways, but a the coupling usually results in some of the light be-ing reflected back into the laser's facet. The laser is extremely susceptible to external stimuli, this is of course why they can so easily be modulated and are ideal for opti-cal communication systems, but this also means that the unintended reflections cause dramatic changes in the laser's dynamics. A consequence of this effect has meant that considerable efforts have been made in to research of dynamics of laser diodes subject to optical feedback. The usual approach to a theoretical study of laser diodes is to make some assump-
tions about the laser's output and parameters. However, every assumption removes some of the potential power of the model, this is done to reduce the computational re-quirements, which would otherwise be prohibitive. One fundamental assumption made is the light emitted from the laser is at one frequency, said to operate in the single mode limit, however it has been observed that most devices are actually multimode. The ap-parent and well acknowledged conundrum is addressed in this work. The computational needs are met through the use of a purpose built cluster from commodity components. The model used is a recast version of the 1980's Coupled Cavity approach developed by Ebeling and Coldren. The main results of this work are summarised here:
• A new spatial harmonic coupled cavity model is presented, offering a a single continuous variable of a complex frequency.
• By varying the external reflectivity and thus changing the level of reflected light into the cavity a catastrophic loss of mode discrimination occurs.
• Variation of sub-wavelengths of the external cavity induce a modulation in the mode lifetime, and at loss of mode discrimination this modulation does not occur.
• The critical point of loss of mode discrimination varies linearly with external cavity length. As such it is possible to define a short cavity limit, within which only certain phenomena occur, because at longer cavity lengths the feedback level is so small that it prohibitive.
• Large external cavities see the dielectric medium of the laser diode as a perturba-tion in the cold cavity limit. At a certain level of feedback all the modes can be regarded as compound cavity modes, rather than laser modes and external cavity modes.
• Laser diodes subject to optical feedback in the long cavity limit are seen as a dielectric perturbation to the external cavity modes. The addition of optical gain however, in the laser diode can overcome this effect and the laser diode's mode can regain dominance.
A Beowulf cluster was developed for this work, and through the use of Java as the cluster Middle-ware is used to perform all the numerical simulations using legacy Fortran code.