Princeton University

School of Engineering & Applied Science

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

Nyan Aung
Engineering Quadrangle J401
Thursday, May 7, 2015 - 10:00am to 11:30am

A typical semiconductor laser consists of a gain material and a resonant cavity. The pumped gain material generates light of a particular wavelength range while the laser cavity provides feedback for stimulated emission. The shape of the cavity determines beam directionality and spectral properties of the laser. This dissertation focuses on semiconductor laser waveguides and cavity designs to tailor the properties of the laser for various applications.
In this thesis, we study optical losses in Quantum Cascade (QC) lasers arising from intrinsic mechanisms such as free carrier absorption and intentional extrinsic mechanisms such as low mirror reflectivity. The latter is especially valuable in realizing high power QC superluminescent (QCSL) emitter, a semiconductor light source that can be used in mid-infrared optical coherence tomography (OCT). Obtaining high power superluminescence from QC devices is difficult due to inefficient spontaneous emission. By employing 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, we effectively suppress lasing and demonstrate the first milliwatt range QCSL emitters. Our achievement of more than 10 mW power is a three orders of magnitude increase from the previous reported value in literature, enabling QCSL emitters based mid-IR OCT.
Another specific topic addressed by waveguide and cavity designs of both QC lasers and diode lasers is the selective electrical pumping of microdisk lasers. Microdisk lasers are typically made from circularly symmetric resonators and desired for on-chip applications due to their compactness, low power dissipation, and potential for ultrafast modulation. However, as serious drawbacks for practical applications, they have low output power and lack directional emission. We overcome these drawbacks by selectively pumping a mode optimized for low threshold and high output power in a deformed cavity. Despite being impeded by the lateral current spreading in the semiconductor, we excite such a mode in a quadrupole-shaped microdisk diode laser by employing a 2 μm wide triangular shaped electrical contact. With this cavity and contact geometry, we demonstrate the first room temperature milliwatt level microdisk laser with directional emission and low-threshold, continuous wave operation.