Frequency Modulation Multiplexing for Simultaneous Detection of Multiple Gases by use of Wavelength Modulation Spectroscopy with Diode Lasers.

Modulation frequency multiplexing provides a straightforward method, analogous to television or radio broadcasting, for performing simultaneous detection of multiple gases by use of wavelength modulation spectroscopy with diode lasers. When fiber-optic coupled lasers are used, our approach guarantees that all beams transit the same optical path and impinge on the same detector. Each laser is modulated at a different frequency and the detector output is processed by a set of lock-in amplifiers, one for each laser, to measure the absorbance encountered by each laser.

Frequency modulation and wavelength modulation spectroscopies: comparison of experimental methods using a lead-salt diode laser.

Wavelength modulation spectroscopy (WMS) and one-tone and two-tone frequency modulation spectroscopy (FMS) are compared by measuring the minimum detectable absorbances achieved using a mid-IR lead-salt diode laser. The range of modulation and detection frequencies spans over 5 orders of magnitude. The best results, absorbances in the low-to-mid 10(-7) range in a 1-Hz bandwidth, are obtained by using high-frequency WMS (10-MHz detection frequency) and are limited by detector thermal noise. This sensitivity can provide species detection limits well below 1 part per billion for molecules with moderate line strengths if multiple-pass cells are used. High-frequency WMS is also tested by measuring the absorbance due to tropospheric N(2)O at 1243.795 cm(-1). WMS at frequencies < 100 kHz is limited by laser excess (1/f) noise. Both of the FMS methods, which require modulating the laser at frequencies >/= 150 MHz, give relatively poor results due to inefficient coupling of the modulation waveform to the laser current. The re ults obtained agree well with theory. We also discuss the sensitivity limitations due to interference fringes from unintentional étalons and the effectiveness of étalon reduction schemes.

Diode laser measurements of trace concentrations of ammonia in an entrained-flow coal reactor.

We used a single-mode Pb-salt diode laser to quantify in situ the amount of ammonia generated during pyrolysis of pulverized coal in an entrained-flow reactor at 1225 K. The combination of wavelength modulation spectroscopy, a Herriott style multiple pass cell, rapid wavelength scanning (to eliminate noise due to turbulence and vibration) and a novel etalon fringe suppression technique provided a minimum detectable absorbance of 2 x 10(-6) (SNR = 1, 1-Hz bandwidth) corresponding to 0.04-ppm ammonia at 1225 K. This is a 4-order-of-magnitude improvement over CO(2) laser based ammonia detectors and is approximately 2000 times more sensitive than electrochemical detection methods (for equal integration times).

Dual-modulation laser line-locking scheme.

Dual modulation laser line locking, useful for long-term trace species monitoring, achieves wavelength stability <0.1 ppm and rejects baseline drift in the measured absorbance3000-fold.

Stimulated Raman scattering with a tightly focused pump beam.

We predict and experimentally measure the transverse spatial shapes, threshold, and ratio of Stokes to anti-Stokes radiation generated by stimulated Raman scattering of a focused Gaussian pump beam. The eigenmodes and the threshold of the scattered radiation are found to depend critically on the confocal parameter of the pump beam and the background dispersion.