Semiconductor Laser Cavity Engineering for Coherent and Low-coherence Light Emission

Abstract

A typical semiconductor laser consists of a gain material and a resonant cavity. The electrically or optically pumped gain material generates light of a particular wavelength or wavelength range while the laser cavity provides feedback for the stimulated light emission. The shape of the resonant cavity determines beam directionality and spectral properties of the laser. In this work, we explore several new approaches to the cavity design for high power Quantum Cascade (QC) superluminescent (SL) emitters, and highly efficient semiconductor diode micro-lasers.

Smooth and broadband spectral emission of mid-infrared (mid-IR) superluminescent light source has potential applications in spectroscopy and high resolution three dimensional imaging system known as optical coherence tomography (OCT). Although QC lasers have been undergoing rapid development since their invention in 1994, a high power QCSL device did not exist until this thesis. This is because, unlike superluminescent semiconductor diodes, obtaining high power amplified spontaneous emission (ASE) from QC devices is dicult as a result of the very short non-radiative carrier life time (on the order of ps) of the intersubband transition at the core of QC devices. To obtain high power superluminscence in the mid-IR range, we employ a novel cavity formed by the combination of a 17 tilted cleaved facet and a wet etched rounded and sloped facet with a single layer anti-reflection coating, to suppress lasing in QC devices. Our achievement of more than 10 mW superluminescence power can enable the realization of mid-IR broadband absorption spectroscopy and OCT.

We also explore a major effort to reduce power consumption of diode microdisk lasers. Microdisk lasers are usually made from circularly symmetric resonators. They can operate with ultra-low threshold currents because of the high quality (Q) factor whispering gallery modes. However, these modes have low output power and lack directional emission. On-chip applications of these devices demand higher output power, even lower power consumption, and possibly directional emission. We attempt to address these requirements by selectively pumping the current in the microdisk diode lasers to choose a maximally power efficient mode that may never turn on in a uniformly pumped cavity.