| dc.description.abstract | The demand for highly sensitive infrared spectroscopic measurements has been a strong motive towards developing high-sensitivity instruments. This demand ranges from the environmental and atmospheric necessities to study, analyze, and predict the concentrations of various greenhouse gases, to medical applications in diagnosis based on the contents of breath samples. Newly developed infrared laser sources with record output powers and ultra-broad spectral coverage opened the door for high accuracy broadband absorption measurements. My work in this thesis is devoted to the field of laser absorption spectroscopy in the mid-infrared spectral region.
My research in this work is divided into two main parts. In the first part, an overview of the field is presented, and the material is designed to form the basis of a graduate course in this field based on the most recent mid-infrared laser sources, detection methods, and a selected set of spectroscopic techniques. This course is intended for teaching graduate students and senior undergraduate students at Kuwait University. The course is mainly on instrumentation, methods, and applications of infrared laser spectroscopy with a particular focus on the novel optical frequency combs and their applications.
The outline of this dissertation also serves as an outline for the course, and the course material is based on Chapter 2, Chapter 3, and Chapter 4 with their detailed subsections. Chapter 1 is an introduction to absorption spectroscopy and its potential applications. In Chapter 2, a review of infrared laser spectroscopy concepts including the principles of molecular absorption spectroscopy and the infrared spectral region is provided. Different sources and generation techniques of infrared light are also discussed in Chapter 2, and a historical overview of photodetection and its concepts is presented. Chapter 3 covers the concepts behind many experimental elements used in laser spectroscopy.
Frequency combs are powerful broadband sources and are excellent for obtaining high resolution absorption spectra of gases. This work presents an experimental demonstration of two powerful spectroscopic techniques based on frequency comb laser sources emitting in the infrared region for chemical sensing applications. This part experimental is represented in Chapter 4.
The first technique is cavity ring-down spectroscopy (CRDS) which is a powerful spectroscopic technique that provides highly sensitive measurements. Furthermore, with an infrared frequency comb laser source it can perform spectroscopic absorption measurements over a broad bandwidth, allowing the detection of various greenhouse gases. Second, we investigate and measure methane concentration in ambient air in the laboratory based on 250 MHz mid-infrared optical combs. We used a multipass cell to enhance the light-matter interaction length to 580 m thus the sensitivity was improved by orders of magnitude. With the dual comb technique, we measured a concentration of 1.5 ppmv for methane in the air in 80 ms with a high sensitivity 7.6×10-7 cm-1. | en |
