Southwest Sciences Inc

Dual-modulation laser line-locking for wavelength modulation spectroscopy

U. S. Patent 5,969,825, issued October 19, 1999

Wavelength modulation spectroscopy using tunable diode lasers is a fast and accurate way to measure trace gas concentrations. However, because the laser wavelength can be tuned just by slight changes in the temperature of the laser, practical devices require some method of ensuring that the laser is operating at the best wavelength.

The invention by David Bomse and Joel Silver of Southwest Sciences uses two modulations of the laser wavelength, one at high frequency (say 1 MHz, a million modulation cycles per second) and one at lower frequency (say 10 kHz or 10,000 cycles per second). Two modulations reduce the effects of spurious background signals due to etalons. Most of the laser light is sent to the gas measuring part of the apparatus. If the target gas is present, the transmitted light acquires power modulation at the modulation frequencies (10 kHz and 1 MHz) and related frequencies (20 kHz, 2 MHz, 2.01 MHz, 1.99 MHz). Calling the big frequency F and the smaller frequency f, the new frequencies are at m*F + n*f, and m and n are signed integers. The concentration of gas is measured by demodulating the received power signal, first at m*F, then at n*f. When the sum of m + n is an even number, this gives a signal with a maximum at the center of the absorption line. The amplitude depends on the concentration of gas as well as details of the line shape and the strength of modulation at f and F.

At the same time, part of the beam is sent through a reference gas--usually the same gas to be sensed--and it also acquires power modulation at the set of frequencies m*F + n*f. The reference channel uses a second detector and processing electronics to demodulate the signal at 3*F. Low pass filtering yields an error signal proportional to the temperature error. Standard line locking methods are used to drive this signal to zero by adjusting the laser current or temperature. To guarantee that the system is operating correctly, the concentration of gas in the reference path is also measured, using a filter set to n*f, using an odd value for n, and and RMS-DC converter. Only when the reference gas can be detected is a zero value for the error signal meaningful.

Contact Information

Southwest Sciences, Inc.
1570 Pacheco St., Suite E-11, Santa Fe, NM 87505
tel. (505) 984-1322/ fax (505) 988-9230



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