MULTIPLEXED CHIRPED PULSE QUANTUM CASCADE LASER MEASUREMENTS OF AMMONIA AND OTHER SMALL MOLECULES...

MULTIPLEXED CHIRPED PULSE QUANTUM CASCADE LASER

MEASUREMENTS OF AMMONIA AND OTHER SMALL MOLECULES

Craig Picken, David Wilson, Nigel Langford and Geoffrey Duxbury

Department of Physics, University of Strathclyde, Glasgow , United

Kingdom

14N and 15N Isotopologuesof the 4 Band of Ammonia

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Figure 1. Fourier transform spectrum of the Q branch region of the 4

band of 15NH3 recorded using a Bomem DA003 spectrometer. The pathlength was 10 cm and the gas pressure ca. 10 Torr. The resolution was0.02 cm−1. (a) Overview of the entire Q branch, showing the location ofthe QC laser spectra. (b) Expanded view of the band center showing thetotal tuning range of the frequency chirped QC spectrometer.

Rapid passage structure visible on the chirped frequency spectra induced by DFB QC laser 61. A 2000 ns top hat pulse has been used to drive the laser. The laser base temperature was−20 ｡ C. The path

length in the astigmatic Herriot cell is 62 m.



Figure 2. Examples of the rapid passage structure visible on the chirped frequency spectra induced by DFB QC laser 61. A 2 s (2000 ns) top hat pulse has

been used to drive the laser. The laser base temperature was −20 ｡ C. The path length in the

Astigmatic Herriot cell is 62 m.

Spectrum (I) 0.49 Torr, mainly 15NH3,drive voltage of 8.5 V, laser repetition frequency 2 kHz. Spectrum (ii) drive voltage of 9 V, laser repetition frequency of 5 kHz, (a) 0.24 Torr, mainly 15NH3 and (b) 0.24Torr. mainly 14NH3

Laser 62 Nitrous Oxide1500 ns



a) Transmission spectra of a section of the 1 band of nitrous oxide recorded at low i, green, and high ii, red, chirp rate conditions. The etalon fringe pattern corresponding to trace ii is also shown in order to Demonstrate the non-linear frequency chirping.

b) Complete spectral region coveredby superimposing the low and high chirp rate scans of a). Nitrousoxide lines are labeled.

(b) Complete spectral region coveredby superimposing the low and high chirp rate scans of a. Nitrous oxide lines are labeled.



The effect of the rapid variation of the frequency downchirp on spectral pattern recognition.

(a) The variation of thefrequency downchirp obtained with the aid of the etalon fringe spacings. (b) An effect of the

nonlinear chirp is to greatly distort the patterns of the vibration–rotation lines of a molecular

spectrum. A spectrum of 14NH3 looks very different when using

the time (i) or the wavenumber scales (ii) for plotting the

spectrum over the tuning range.





The effect of the rapid variation of the frequency downchirp on spectral pattern recognition. (a) The variation of the frequency downchirp obtained with the aid of the etalon fringe spacings. (b) An effect of the nonlinear chirp is to greatly distort the patterns of the vibration–rotation lines of a molecular spectrum.

Laser 62 Ammonia



The effects of nitrogen broadening on the raQ.7,1. and the raQ.6,0. lines as a function of the pressure of added nitrogen. The chirp rates are ca. 9 MHz ns-

1, solid line, and ca. 69 MHz ns-1, dashed line. (a) Ammonia pressure 4.5 mtorr: nitrogen added to a total pressure of (b) 4.2 torr, (c) 12 Torr



The effects of nitrogen broadening on the raQ.7,1. and the raQ.6,0. lines as a function of the pressure of added nitrogen. The chirp rates are ca. 9 MHz ns-1, solid line, and ca. 69 MHz ns-1, dashed line. (c) 12 torr, (d) 24 torr. The other experimental factors are as

Figure 9



A comparison of part of the spectrum of 16O12C18O, obtained via a sample of

nominally fully 18O substituted carbon dioxide, with the same lines recorded in

the presence of 2.5 torr water. The weak rapid passage gain signals of 16O12C18O are completely quenched in the presence of only a very small pressure of water vapour. Top spectrum no water, carbon dioxide pressure 2.8 torr; bottom spectrum, heavy line water only, 2.5 torr ; bottom spectrum. light line, 16 torr total pressure.



Summary

• In this paper we show examples of the use of multiplexed chirped lasers to allow overlapping spectra to be recorded. Among the effects seen are rapid passage effects caused by the rapid down chirp of the pulsed lasers.

References• Quantum Cascade Lasers: diagnostics to non-linear optics. Journal of Modern Optics. Vol.

56, Nos. 18–19, 20 October–10 November 2009, 2034–2048. Geoffrey Duxbury *, Nigel Langford , Kenneth Haya and Nicola Tasinato

• Nicola Tasinato, Geoffrey Duxbury*, Nigel Langford, and Kenneth G. Hay “An investigation of collisional processes in a Dicke narrowed transition of water vapor in the 7.8 m spectral region by frequency down-chirped quantum cascade laser spectroscopy.” J. Chem. Phys. 132, 044316, 2010)

• Nicola Tasinato, Kenneth G. Hay, Nigel Langford, Geoffrey Duxbury*and David Wilson, “Time dependent measurements of nitrous oxide and carbon dioxide collisional relaxation processes by a frequency down-chirped quantum cascade laser: Rapid passage signals and the time dependence of collisional processes”, J. Chem. Phys. 132, 164301 2010)

• Craig Picken, Nigel Langford and Geoffrey Duxbury*,”Measurements and analysis of 14NH3 and 15NH3 and a range of other light molecules”.

MULTIPLEXED CHIRPED PULSE QUANTUM CASCADE LASER MEASUREMENTS OF AMMONIA AND OTHER SMALL MOLECULES...

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