Alan C. Stanton

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.

Measurements in the HCl 3 ← 0 band using a near-IR InGaAsP diode laser

Hydrogen chloride gas is measured by absorption in the second overtone band near 1.2microm using an InGaAsP diode laser. The strength and air-broadening coefficient of the H(35)Cl P(3) line are measured. The line strength is ~18% higher than the previous grating spectrometer measurement and suggests that the commonly used experimental value for the 3 ?0 HCl vibrational moment may be low. High-frequency two-tone FM detection is also used in this study to measure trace concentrations of HCl. Using a multimode laser, the sensitivity limit is 3-parts per million HCl in air at 50 Torr, corresponding to a minimum detectable fractional absorption (multimode) of 4 x 10(-6). Optimization of the detection method should permit real-time measurement of concentrations below 0.1 ppm. The low cost and convenient operating characteristics of InGaAsP diode lasers make them attractive for many applications in high-resolution near-IR spectroscopy and trace species detection.

Measurement of nitric oxide with an antimonide diode laser

An antimonide diode laser operating near 2.65 mum was used to measure absorption lines of NO gas in the first overtone band. A blended line pair of NO that is sufficiently free of interference from H(2) O to permit the selective detection of NO under reduced pressure conditions was identified. With wavelength-modulation spectroscopy, a rms noise level equivalent to an absorbance of 3.2 x 10(-5) was achieved at a measurement integration time (for a single spectral data point) of 0.1 s. The corresponding detection sensitivity (signal-to-noise ratio of 2) for NO in air at reduced pressure was ~15 ppm m (ppm is parts in 10(6)). Antimonide diode lasers show substantial promise for gas-sensing applications because they can gain access to relatively strong absorption lines of several gases of environmental interest at operating wavelengths at which cryogenic cooling is not required.

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).

Two-tone optical heterodyne spectroscopy using buried double heterostructure lead–salt diode lasers

Two-tone optical heterodyne detection using single mode buried double heterostructure lead-salt diode lasers is studied. Residual amplitude modulation with these lasers is found to be substantially smaller than has been noted in previous work with mesa stripe lasers, permitting improved sensitivity for measurement of small optical absorption. Minimum detectable absorption well below 10(-5) is measured at a laser power of 3 microW, in good agreement with signal-to-noise calculations using the experimental values of the FM and AM modulation indices. Optical heterodyne detection with two-tone modulation at 145 and 155 MHz is found to be nearly an order of magnitude more sensitive than conventional second harmonic detection at kilohertz frequencies. In addition to improved detection sensitivity, the double heterostructure lasers offer the important advantages of single mode operation above liquid nitrogen temperature.

Airborne measurements of humidity using a single-mode Pb–salt diode laser

Instrumentation for the measurement of ambient humidity, incorporating a liquid nitrogen-cooled tunable lead-salt laser and a reduced pressure multipass absorption cell, was installed on the Lockheed L-1011 advanced research aircraft and tested on four flights. Humidity measurements taken over altitudes ranging from 0.76 to 10.7 km were compared to results obtained with a commercial dew point hygrometer. The flight tests showed generally good agreement between the two devices to within the estimated accuracies of the instruments. At altitudes above 8 or 9 km the hygrometer is not reliable, whereas the diode laser system exhibited very good sensitivity at the highest altitudes attained. A time response of better than 0.1 s was demonstrated. We believe that a compact diode laser instrument can ultimately measure atmospheric water vapor concentrations over a wide altitude range with an accuracy of better than 3%.

Diode laser spectroscopy for on-line chemical analysis

Diode laser spectroscopy provides exceptional sensitivity and selectivity for real-time characterization of reacting systems and gas streams. High frequency wavelength modulation techniques achieve species detection limits that are routinely in the ppm range and can reach sub-ppb levels under favorable conditions. Narrow laser linewidths guarantee selective detection of key species even in the presence of myriad other components. Diode laser spectroscopy is also relatively immune from interference by black body radiation or chemiluminescence. Prototype diode-laser based systems have been demonstrated successfully for trace gas detection in turbulent, high temperature particle-laden streams, for oxygen quantitation in flames, for free radical characterization in a plasma etching reactor and for greenhouse gas flux measurements in air. We also discuss the availability of laser wavelengths, compatibility with fiber optics, cost safety and expectations for new laser development.

Applications of diode laser spectroscopy to environmental and industrial process monitoring

Visible/near-infrared diode lasers are well-suited for use as spectroscopic light sources in detection of a wide variety of gases by optical absorption. The high spectral resolution of these devices permits the selective detection of targeted species, while their characteristics of low cost, room temperature operation, and compatibility with fiber optics make them attractive for instrument development. A partial list of industrially or environmentally significant gases that may be measured by near-IR diode laser spectroscopy includes oxygen, water vapor, methane, acetylene, carbon monoxide, carbon dioxide, hydrogen halides, ammonia, hydrogen sulfide, and nitrogen oxides. This paper describes recent work at Southwest Sciences in development of diode laser-based instrumentation for industrial or environmental monitoring applications. Instrumentation utilizing a 1.393 micrometers DFB diode laser for measurement of trace moisture contamination in high purity process gases is described. In addition, recent laboratory studies to characterize the performance of new types of diode lasers in gas sensing applications are discussed, including vertical cavity surface emitting lasers in the 650 to 960 nm region and antimonide-based lasers in the 2.6 micrometers region.

Measuring atmospheric methane and water vapour using near-infrared diode lasers

Single-frequency near-infrared diode lasers are used to measure atmospheric methane and water vapor. Using high-frequency wavelength modulation methods, sensitive instrumentation with fast time response are designed. Communications lasers operating near 1310 nm probe weak overtone transitions of both molecules; lasers with custom wavelengths at present lack sophisticated packaging, but can achieve much higher sensitivity. We describe two field-tested instruments: an automated, airborne hygrometer with a sensitivity of 8 ppm (by volume) with a one second averaging time, and a fast response methane sensor with a sensitivity of 65 ppb. Improvements to these instruments are outlined, and the effects of laser nonlinearities are noted.

Spectroscopic Techniques For Characterization Of Gas Phase Species In Plasma Etching And Vapor Deposition Processes

This paper presents a review of techniques for spectroscopic characterization of mile gas pnase species involved in vapor depositon and plasma etching, two processes of great importance in the semiconductor industry. Descriptions of the apparatus requirements and capabilities of diode laser absorption and dye laser resonance fluorescence detection techniques are given. In addition, band strength and other spectroscopic data for selected molecules are used to give estimates of the detection sensitivity for various species.