Distributed-feedback (DFB) lasers support a wide range of applications including fiber-optic and free-space optical communications, sensing and measurement, military warfare, and manufacturing and metrology. The desire for a reduction in size, weight, power, and cost (SWAP-C) for each of these applications continues to aggressively drive integrated- and silicon-photonic circuit development. Since DFB lasers based on uniform refractive-index diffraction gratings are naturally dual-mode, a variety of techniques have been introduced to impose single-mode operation, including the currently ubiquitous λ/4-grating phase-shift technique. This grating phase shift, however, creates an undesirable peaky-power profile within the laser cavity resulting in nonlinear behavior ultimately limiting single-mode operation above threshold. Seeking to specifically address the limitations associated with λ/4-phase shifted lasers, this dissertation introduces the following three scientific advancements: 1. Introduces two novel single-mode DFB lasing concepts based on new physical principles 2. Derives parameterized mathematical models and their closed-form analytic solutions for each concept 3. Predicts excellent lasing performance at threshold for one lasing concept while avoiding the detrimental issues associated with a peaky-power profile One of the two single-mode DFB lasing concepts places a passive photonic waveguide in proximity to a uniform-grating DFB lasing structure to form direct-Bragg coupling, exchange-Bragg coupling, and evanescent coupling among the optical modes of the structure’s two waveguides. Waveguide loss, gain, wavenumbers, and the semiconductor nonlinearity along with the lasing structure’s coupling coefficients are the fundamental quantities of the structure. The waveguide-wavenumber detuning is used to position the exchange-Bragg photonic bandgap (PBG), suppressing the degenerate mode associated with the active-waveguide direct-Bragg PBG resulting in single-mode lasing. The second introduced single-mode DFB lasing concept makes use of a negative-index material (NIM) waveguide placed in proximity to an active positive-index material (PIM) waveguide. The electric field in the NIM waveguide has a Poynting vector oriented in the direction opposite to its wave vector and, when evanescently coupled to the PIM waveguide, yields distributed feedback and an associated PBG without using a diffraction grating. Unlike the uniform-grating DFB laser, the NIM-PIM laser has a mode spectrum defined by the difference in waveguide wavenumbers, yielding single-mode operation for waveguides whose wavenumbers never match. A set of newly introduced coupled-mode equations (CMEs) whose solutions generate closed-form parameterized analytical expressions describe each lasing concept and capture structure behaviors. The fourth-order uniform-grating-based lasing structure CMEs are solved using a custom developed mathematical method whereas, solutions for the second-order metamaterial-based lasing structure CMEs are arrived at using traditional eigenvalue-analysis techniques. For both structures, the coupled-mode equations and derived solutions are entirely new to the literature. The uniform-grating based lasing solution predicts a peak gain margin of αL = 1.05 and associated longitudinal power flatness of F = 0.017, surpassing the performance of the industry-standard λ/4-shifted DFB laser of maximum αL = 0.735 with the associated longitudinal power flatness of F = 0.215. Moreover, this dissertation predicts that high-performance single-mode lasing occurs in spite of the introduction of evanescent coupling and, in some cases, marginal amounts can actually help to improve upon the longitudinal power flatness. The impact of exchange-Bragg coupling and secondary-lasing mode competition is also revealed and mitigated. The required dual waveguide geometry of the lasing structure is compelling for both III-V and heterogeneous III-V-on-silicon devices. The predicted performance motivates further research of these novel lasing concepts while the established mathematical models are tools to aid in this future work.
Microsystems Engineering (Ph.D.)
Department, Program, or Center
Microsystems Engineering (KGCOE)
Parsian K. Mohseni
Tennant, Bryce A., "Single-Mode Distributed-Feedback Lasing In Coupled Dissimilar Photonic Waveguides" (2023). Thesis. Rochester Institute of Technology. Accessed from
RIT – Main Campus