| Quantum well semiconductor diode lasers have been predicted to exceed the performance of their non-quantum well, or bulk-laser counterparts in several of the operating characteristics, including modulation bandwidth. Present state-of-the-art quantum well lasers have only approached or slightly exceeded bandwidths exhibited by bulk lasers, failing to fulfill their projected performance. This thesis is an investigation of the effects of carrier transport on the modulation dynamics of quantum well semiconductor diode lasers. In particular, the limitations on the direct modulation bandwidth of quantum well lasers imposed by the very nature of being quantum well devices, rather than non-quantum well, or bulk devices, are investigated. A theoretical model, motivated by recent experiments observing anomalous behavior in quantum well lasers, is developed to describe the interaction between the electrical carriers (holes and electrons) and the laser's optical signal. The motion of carriers within the device as they traverse from their point of injection at the laser electrical contact to the sites for generation of laser light via stimulated emission in the quantum well(s) is detailed, and the resulting influence on the modulation bandwidth is derived. In particular, the time required for transfer, or capture, of electrons from the separate confinement heterostructure region to the quantum well(s) is shown to modify the modulation bandwidth from the standard, two-pole form exhibited by conventional, bulk, semiconductor lasers to a three-pole form. A novel experimental technique, Wavelength-Selective Optical Modulation, is developed and employed to address the implications of the model. This technique is capable of uniquely ascertaining the characteristic time constant describing carrier transport, the effective capture time, independent of any other phenomena limiting the modulation bandwidth of these lasers. The experimental technique is further capable of delineating the relative contributions of diffusion and quantum capture, as outlined by the model. Experiments on 1.6 $mu$m multiple quantum well Fabry-Perot lasers are performed. The effective capture time is determined, and found to be in strong agreement with the theoretical model, as well as with measurements for the effective capture time previously reported in the literature.;The influence of carrier transport on the modulation response of quantum well semiconductor lasers at frequencies in the vicinity of the cavity round-trip time, the mode-locking frequency, is examined for three cases of interest, active modelocking under homogeneous bias conditions, active modelocking under inhomogeneous bias conditions, and passive modelocking. The results are compared to the performance of bulk lasers to assess the advantages and disadvantages of quantum well lasers for use in ultra-high frequency applications. Intermodulation distortion, an important parameter in optical transmission systems, is reevaluated for quantum well lasers, taking into account the role of carrier capture. The resulting expressions are compared to the intermodulation distortion exhibited by bulk lasers. A summary of the analyses and experiments performed is presented and conclusions on the role of carrier transport in the performance of quantum well lasers are drawn. |
