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Title: Development of a laser system for high resolution non-linear infrared spectroscopy
Author: Richard, Katherine Louise
ISNI:       0000 0001 3516 0088
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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The Diode-Seeded Modeless Laser (DSML) is based on the pulsed amplification of a narrowband tunable diode laser using a modeless dye laser amplifier. The motivation behind its design is the need for a laser system for quantitative measurements of combustion parameters using non-linear optical techniques. Such a system needs to produce a high power, single longitudinal mode, tunable output; a stringent set of requirements and unavailable commerically. This thesis describes the improvements made to the DSML, specifically a system was designed and fabricated which provides synchronous frequency scanning of both the external-cavity diode laser and the modeless laser amplifier. The system developed in this work, SOPEC (Scanning by Optical Pickup Error Correction), actively monitors and corrects wavelength mismatchs between the diode laser and the modeless laser amplifier down to 0.0001 nm (0.003 cm-I) improving on the existing scanning system which was able to maintain synchronicity for only rv 1cm-I. This active control thus provides synchronous scanning over a range limited only by the diode laser tuning range. In order to verify the effectiveness of SOPEC, a wide absorption scan was taken of molecular iodine over a total of 60cm-I, in five short scans of rv2cm-I wide separated by 15cm-I. In addition a single scan was taken spanning rv 9 em-I. The improvements to the DSML now enable it to produce up to 40 mJ in a 4 ns pulse. The current fundamental output possesses a Fourier Transform limited bandwidth of 165 MHz (0.0055 em-I) over the wavelength ranges 623 - 639 nm (7 nm or 21Ocm-I). Difference frequency mixing with second harmonic Nd:YAG at 532 nm produces 1mJ, rv 3 ns pulse of mid-infared radiation from 3.18 - 3.36 /lm. The line width of the mid-infrared radiation was estimated to be rv 360MHz (0.012cm-1 ). The first application of the DSML employing SOPEC to non-linear spectroscopy was polarization spectroscopy (PS) of molecular iodine at rv 635 nm (15748 em-1) in a cell at 1Torr. Both co- and counterpropagating geometries gave spectra exhibiting good stability in both intensity and wavelength; signal to noise ratios of over 700 were obtained. The spectra obtained using the counterpropagating geometry, although not completely resolving the hyperfine components, clarified the underlying structure sufficiently to determine whether the J level was odd or even. The results demonstrated the highest resolution achieved so far using a pulsed laser system. Applying the DSML and SOPEC to the mid-infrared (rv 3020 cm-1), absorption spectroscopy of V3 band of methane in a low pressure cell at 10 Torr was peformed. The first application of the DSML to non-linear spectroscopy in the mid-infrared was demonstrated using polarization spectroscopy of methane in a low pressure cell, with signal to noise ratios of up to 1400. A high resolution scan of a single peak in the Q-branch at 6 Torr was obtained with a line width of 0.018 cm-1, showing the high stability of the DSML in both the itensity and wavelength. This is the highest resolution, pulsed, infrared PS spectra obtained to date. A scan over three of the most closely separated peaks showed that the DSML is capable of completely resolving features separated by rv 0.02 cm-1. The DSM~'s stability was tested by measuring pressure broadening of methane by nitrogen (0 - 100 Torr). 05 The measured line broadening (0.00021 ± 0.00014 em-1 /Torr) was in agreement with literature values. The results presented in this thesis demonstrate the improved DSML, employing SOP~}o be a powerful tool for high resolution spectroscopy and for obtaining quantitative measurements using non-linear optical techniques.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Oxford, 2007 Qualification Level: Doctoral
EThOS ID:  DOI: Not available