Mitigation of coherent noise and spectral interference in integrated and free-space mid-infrared laser spectrometers

Abstract

This dissertation studies different types of coherent intensity noise in mid-infrared laser-based spectrometers. We focus on noises that are unpredictable, highly dynamic, and contain frequency components that cannot be readily distinguished from the signal. We investigate the varying causes of these noises through the development of three spectroscopic systems, and in each case targeted solutions are proposed and implemented to extend the sensing capability and stability.

In Chapter 2, an integrated sensing platform for methane leak detection in the natural gas industry is presented. The integrated waveguide-based platform introduces spurious optical scattering, which causes time-varying coherent interference features that cannot be distinguished from the spectroscopic signal. An adaptive computational noise mitigation approach and hardware implementations are proposed, tested and demonstrated to improve the system stability. In Chapter 3, we discuss challenges with sensing in a combustion environment, where the spectral signal for an important radical species is contaminated not only by time-varying coherent interference features but also by spectrally interfering absorption from nontargeted molecules. This challenge was overcome by employing Faraday rotation spectroscopy implemented with a new digital balancing methodology, which suppresses unwanted coherent interference as well as spectral interference from nontargeted chemical species to allow selective detection of radicals. In chapter 4 we present a mid-infrared dual-comb spectrometer using quantum cascade laser frequency combs. In this case, destabilization of the comb coherence results in spectroscopic retrieval noise. As these devices are still under development, we begin this work by studying the behaviors of these novel combs. We proposed several coherence stabilization methodologies including a versatile optical feedback scheme and frequency stabilization schemes to improve comb coherence and enable practical implementations of a field deployable dual-comb system.