Laser absorption spectroscopy using lead salt and quantum cascade tunable lasers

A new class of analytic instruments based on the detection of chemical species through their spectroscopic absorption 'fingerprint' is emerging based on the use of tunable semiconductor lasers as the excitation source. Advantages of this approach include compact device size, in-line measurement capability, and large signal-bandwidth product. To realize these advantages will require the marriage of laser devices with broad tunability in the infrared spectral range with sophisticated signal processing techniques. Currently, commercial devices based on short wavelength telecommunications type lasers exist but there is potential for much more versatile instruments based on longer wavelength operation.; This thesis is divided into two parts. In the first part I present a theoretical analysis and experimental characterization of frequency and wavelength modulation spectroscopy using long wavelength infrared tunable lasers. The experimental measurements were carried out using commercially available lead salt lasers and excellent agreement is found between theoretically predicted performance and experimental verification.; The lead salt laser has several important drawbacks as a source in practical instrumentation. In the second part of the thesis I report on the use of the quantum cascade (QC) laser for use in sensitive absorption spectroscopy. The QC laser is a new type of tunable device developed at Bell Laboratories. It features broad infrared tunability, single mode distributed feedback operation, and near room temperature lasing. Using the modulation techniques developed originally for the lead salt lasers, the QC laser was used to detect N{dollar}sb2{dollar}O and other small molecules with absorption features near 8 {dollar}mu{dollar}m wavelength. The noise equivalent absorption for our measurements was {dollar}5times 10sp{lcub}-5{rcub}{dollar}/{dollar}sqrt{lcub}Hz{rcub}{dollar} which corresponds to a detection limit of {dollar}sim{dollar}0.25 ppm-m/{dollar}sqrt{lcub}Hz{rcub}{dollar} for N{dollar}sb2{dollar}O. The QC laser sensitivity was found to be limited by excess amplitude modulation in the detection channel most likely caused by frequency chirp. Signal size was also reduced by excess laser linewidth. Both problems are amenable to improvement and the detection sensitivity could be improved by two orders of magnitude before quantum detection limits set in.