Selective mode coupling in microring resonators for single mode semiconductor lasers

Arbabi, Amir

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https://hdl.handle.net/2142/45565 Copy

Description

Title

Selective mode coupling in microring resonators for single mode semiconductor lasers

Author(s)

Arbabi, Amir

Issue Date

2013-08-22T16:48:00Z

Director of Research (if dissertation) or Advisor (if thesis)

Goddard, Lynford L.

Doctoral Committee Chair(s)

Goddard, Lynford L.

Committee Member(s)

• Carney, Paul S.
• Coleman, James J.
• Dallesasse, John M.
• Wasserman, Daniel M.

Department of Study

Electrical & Computer Eng

Discipline

Electrical & Computer Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• On-chip mirrors
• Semiconductor lasers
• Micro-resonators
• Monolithic integration

Abstract

Single mode semiconductor laser diodes have many applications in optical communications, metrology and sensing. Edge-emitting single mode lasers commonly use distributed feedback structures, or narrowband reflectors such as distributed Bragg reflectors (DBRs) and sampled grating distributed Bragg reflectors (SGDBRs). Compact, narrowband reflectors with high reflectivities are of interest to replace the commonly used DBRs and SGDBRs. This thesis presents our work on the simulation, design, fabrication, and characterization of devices operating based on the coupling of degenerate modes of a microring resonator, and investigation of the possibility of using them for improving the performance of laser diodes. In particular, we demonstrate a new type of compact, narrowband, on-chip reflector realized by selectively coupling degenerate modes of a microring resonator. For the simulation and design of reflective microring resonators, a fast and accurate analysis method is required. Conventional numerical methods for solving Maxwell's equations such as the finite difference time domain and the finite element method (FEM) provide accurate results but are computationally intense and are not suitable for the design of large 3D structures. We formulated a set of coupled mode equations that, combined with 2D FEM simulations, can provide a fast and accurate tool for the modeling and design of reflective microrings. We developed fabrication processing recipes and fabricated passive reflective microrings on silicon substrates with a silicon nitride core and silicon dioxide cladding. Narrowband single wavelength reflectors were realized which are 70 times smaller than a conventional DBR with the same bandwidth. Compared to the conventional DBR, they have faster roll-off, and no side modes. The smaller footprint saves real estate, reduces tuning power and makes these devices attractive as in-line mirrors for low threshold narrow linewidth laser diodes. Self-heating caused by material absorption changes the temperature and material refractive indices. The change in the refractive indices of the core and cladding of a reflective microring changes its reflection peak wavelength and the phase of its reflectivity. Therefore, fluctuations in the laser power lead to fluctuations in the phase of the reflectivity of the reflective microring and can affect the laser linewidth. We theoretically and experimentally studied the dynamics of self-heating in microring resonators and showed that the thermal dynamics can be modeled by a transfer function with two poles and one zero. A small signal model for reflection and transmission of a reflective microring was also derived and was validated by comparing it to measurement data. To predict the characteristics of a semiconductor laser with a reflective microring mirror, we derived the small signal rate equations. These rate equations predict that the laser will be stable when it operates at moderate output powers on the long wavelength side of the reflection peak of a narrowband microring reflector. The modifications of the laser's chirp response and linewidth due to the self-heating effect are also presented. Monolithic fabrication of passive mirrors and gain sections requires an integration platform that provides both active and passive waveguides. We propose a new integration platform which does not involve epilayer regrowth and keeps the confinement factor relatively high in the active sections. Our epilayer design, fabrication process, and characterization results of lasers with passive reflectors fabricated using this platform are presented. Microring resonators by themselves can be used as laser cavities. One of the main issues with their application as a laser cavity is their mode spectra; they have closely spaced modes with very similar quality factors. Furthermore, at each resonant wavelength there are two degenerate modes with the same quality factor. We introduce a novel method to engineer the quality factors of microring modes by coupling two counterpropagating modes together using a second order grating. We present evidence of lasing mode selectivity in a microring laser using this technique.

Graduation Semester

2013-08

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http://hdl.handle.net/2142/45565

Copyright and License Information

Copyright 2013 Amir Arbabi

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