Absorption spectroscopy using quantum cascade laser

Characteristic molecular and atomic absorption spectra in the middle infrared region are extensively used for chemical analysis and spectroscopic detection of gases. The absorption of electromagnetic radiation in this so-called "fingerprint" spectral region is due to the vibration-rotational bands of molecular structures. Various techniques have been developed for high resolution and high sensitivity absorption spectroscopic detection. In this dissertation, we have developed a sensitive method based on wavelength modulation techniques by using a distributed feedback Quantum Cascade Laser operating at room temperature.;Quantum cascade (QC) lasers, invented by Bell Laboratories, Lucent Technologies, are built out of quantum semiconductor structures grown by molecular beam epitaxy and designed by band structure engineering. Since it is a unipolar semiconductor laser based on transitions within the conduction band using electrons only as carriers, the emission wavelength is not constrained by the bandgap, but determined entirely by quantum confinement. Thus, it can be designed to have a wavelength from the mid-infrared to the submillimeter wave region in the same heterostructure material. A distributed feedback quantum cascade (QC DFB) laser can be designed and fabricated to emit light with high power in the middle infrared region under single mode operation at room temperature for spectroscopic applications.;QC DFB lasers near 8 mm were used with wavelength modulation techniques in this dissertation to explore the detection limit of trace gas absorbance under room temperature operation. Using nitrous oxide (N2O) as an absorbing species under single pass operation with an interaction length of 0.1 m, the noise equivalent absorbance of 5 x 10-5 /√Hz was measured. This was equivalent to a detection limit of 0.25 ppm-m/√Hz normalized by length for the strongest absorption line (1.1 x 10-19 cm/molecule) of N2O in the region. Under multi-pass operation with an interaction length of 36 meters, the noise equivalent absorbance of 6 x 10-6 /√Hz was achieved. To our knowledge, it is the lowest noise equivalent absorbance reported using a QC DFB laser at room temperature. This corresponded to a detection limit of 4.43 ppm-m/√Hz normalized by length for the second strongest absorption line (8.8 x 10-6 cm/molecule) of N2O in the region. Since the second strongest line is 12.5 times weaker than the strongest one, it was equivalent to 0.35 ppm-m/√Hz for the strongest line. This result was comparable but slightly higher than its counterpart under single-pass gas operation. We believe this difference was due to excess laser linewidth and multi-pass cell throughput. This can be further confirmed by laser linewidth measurement and optical delay measurement of the multi-pass cell.