Design and performance issues for complex-coupled distributed feedback semiconductor lasers

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 1998.Complex-coupled distributed feedback (CC-DFB) lasers, which are characterized by resonant periodic refractive index and gain, have been a source of great interest due to their many desirable features. Some of these are high gain margin, high single-mode yield, spectral properties that are insensitive to facet reflections and external feedback, wide temperature range, and potential for narrow linewidth and low chirp. The presence of these last two features depends on the functional dependence of the periodic gain on the carrier density in the active region. Lasers with negative differential coupling (i.e. with the periodic gain decreasing with an increasing carrier density) have been shown to offer the potential for very low linewidth and chirp. Until now, though, no CC-DFB lasers with negative differential coupling had been developed. We propose a laser that we numerically predict will exhibit negative differential coupling. With the realization of negative differential coupling, another phenomenon, known as stationary points, may be achievable. The optical field of a laser operating at a stationary point is predicted to be insensitive (to first-order) to perturbations in the carrier density. The possibility of designing a CC-DFB laser operating at a stationary point is examined, and a stability analysis is presented which shows that many of these should be highly stable. Design issues aimed at simplifying the fabrication of CC-DFB lasers are discussed, as well. To eliminate the regrowth step, a surface grating with the contact evaporated on top is employed. Deep surface gratings provide both large separation of the optical mode from the cap layer and suppression of unwanted diffraction orders. By evaporating germanium into the grooves of the grating, it is shown that the phase and magnitude of κ can both be controlled over a very wide range nearly independently. Techniques for fabricating these lasers using holographic grating definition and chemically-assisted ion beam etching were developed, and DFB lasers fabricated with this procedure are reported