| Absorption of light radiation in the mid-infrared spectra is widely used to detect the existence and the concentration of gaseous specimen for its simplicity and insensitivity to the environmental conditions of the absorbing species. The absorption is due to the vibration-rotational bands of molecule featuring the "fingerprint" of the certain molecule. This thesis is dedicated to developing technique for sensitive detection of the trace gas specimen. It can be divided into two parts.; In the first part I develop a theoretical description and experimental demonstrations of frequency modulation (FM) spectroscopy using conventional semiconductor laser commercially available for telecommunication. The experimental results verify the theoretically predicted performance by showing an excellent agreement between the two.; Since the progress of spectroscopy owes greatly to the development of excitation source, in the second part of my thesis, I will focus on a new type of laser, the quantum cascade (QC) laser. Developed by researchers at Bell Labs and Lucent Technologies, the QC laser works on a principle quite different from that of conventional semi-conductor laser. It has some unique features of broad band tunability, single mode distributed feedback and mid- and far-infrared spectrum. A brief illustration on how the QC laser works and experimental characterizations of QC laser under injection current modulation with Fabry-Perot interferometer will be given to probe the possible application in spectroscopy in the future. |
