TONE-BURST MODULATION COLOR CENTER LASER SPECTROSCOPY
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Date
1983
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Publisher
Ohio State University
Abstract
The technique of tone-burst modulation has been developed by $Pickett^{1}$ as a general scheme for high sensitivity microwave absorption spectroscopy. This method has been used very successfully by Woods and co-$workers^{2}$ for the detection and study of transient species in DC glow discharge. We report in this paper the extension of the tone-burst technique to tunable infrared laser absorption spectroscopy (2.3-3.3 $\mu$m) using a color center laser and an extracavity $LiTaO_{3}$ phase modulator. The tone-burst consists of a $\sim$50 watt RF ``tone"" (100-400 MHz) which is amplitude modulated at 100 KHz with a depth of 100\%, and is applied to the phase modulator. The infrared laser radiation which traverses the modulator crystal is then composed the original infrared carrier plus sidebands which are switched on and off at 100 kHz and are separated from the carrier by the tone frequency. Simple lock-in signal processing at the 100 kHz burst rate then provides high sensitivity. By observing several R, P, and Q branch NO $v = 0 \rightarrow 2$ overtone transitions, a minimum detectable absorption of .05\% has been determined. This is limited by power fluctuations resulting from standing waves in the infrared path; thus the laser and detector noise limits have not yet been reached, and further improvements in sensitivity are anticipated. We have also observed several OH v = 0$\rightarrow 1$ transitions in a DC glow discharge through $H_{2}O$. The $R(3/2)_{2\pm}$ transitions at $3663.7 cm^{-1}$ were observed with $S/N > 100$ under the following conditions: 500 mTorr $H_{2}O$, 30 mA/cm discharge current density, 100 msec time constant, 40 see scan, and 400 MHz tone frequency. The linewidth is broader than thermal Doppler width due to unresolved hyperfine structure. The adaptation of tone-burst modulation to tunable infrared absorption spectroscopy is a simple but powerful technique, since it results in high sensitivity with conventional instrumentation (e g., lock-in amplifiers). $^{1}$ H. M. Pickett, Applied Optics, 19, 2745 (1980) $^{2}$ C. S. Gudeman and R. C. Woods, Phys. Rev. Lett. 48. 1344 (1982) $^{*}$ Supported by NSF Grant \# CHE 8207307.
Description
Author Institution: Department of Chemistry, University of California