K.P. Petrov

Room-temperature mid-infrared laser sensor for trace gas detection

Design and operation of a compact, portable, room-temperature mid-infrared gas sensor is reported. The sensor is based on continuous-wave difference-frequency generation (DFG) in bulk periodically poled lithium niobate at 4.6 mum, pumped by a solitary GaAlAs diode laser at 865 nm and a diode-pumped monolithic ring Nd:YAG laser at 1064.5 nm. The instrument was used for detection of CO in air at atmospheric pressure with 1 ppb precision (parts in 10(9), by mole fraction) and 0.6% accuracy for a signal averaging time of 10 s. It employed a compact multipass absorption cell with a 18-m path length and a thermoelectrically cooled HgCdTe detector. Precision was limited by residual interference fringes arising from scattering in the multipass cell. This is the first demonstration of a portable high-precision gas sensor based on diode-pumped DFG at room temperature. The use of an external-cavity diode laser can provide a tuning range of 700 cm(-1) and allow the detection of several trace gases, including N(2) O, CO(2), SO(2), H(2) CO, and CH(4).

Detection of CO in air by diode-pumped 4.6-μm difference-frequency generation in quasi-phase-matched LiNbO 3

Detection of CO, N(2)O, and CO(2) in ambient air was performed with a room-temperature cw IR source based on difference-frequency generation in periodically poled LiNbO(3). The source was pumped by a seeded highpower GaAlAs amplif ier at 860 nm and a diode-pumped monolithic Nd:YAG ring laser at 1064 nm. The IR output was tunable between 2160 and 2320 cm(-1) without crystal rotation. The CO detection sensitivity is extrapolated to 5 ppb m/ radicalHz if limited by IR intensity noise.

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS2. IR frequency tuning between 3076 and 3183 cm−1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)−1/2.

Continuous-wave tunable 8.7-μm spectroscopic source pumped by fiber-coupled communications lasers

Tunable narrow-band cw difference-frequency generation at 8.7 microm was demonstrated in silver gallium selenide (AgGaSe(2)) at room temperature. The crystal was pumped by an injection-seeded Er/Yb-codoped fiber amplifier at 1.554 microm and a fiber-coupled diode-pumped monolithic ring Nd:YAG laser at 1.319 microm. The difference-frequency output was used for high-resolution spectroscopy of sulfur dioxide (SO(2)).

Fast sensitive trace gas detection with a portable solid-state mid-infrared laser sensor

The purpose of this work was to construct a portable solid-state gas sensor based on tunable laser difference frequency generation (DFG), and to achieve detection limits of < 10 ppb (parts in 10/sup 9/, by mole fraction) for several atmospheric trace gases. Earlier feasibility tests indicated that this can be done with the use of periodically poled lithium niobate (PPLN). Its quasi-phase-matching properties can be tailored for 2 to 5 /spl mu/m DFG with near-infrared diode lasers. This, along with large nonlinearity and good optical quality, makes PPLN the ideal mixing material for DFG applications targeted at species like CO, N/sub 2/O, CO/sub 2/, SO/sub 2/, H/sub 2/CO and CH/sub 4/. Reported herein are the design, construction, and performance testing of an all-solid-state DFG sensor for carbon monoxide. The sensor employs a 750 mW diode-pumped monolithic Nd:YAG ring laser at 1064.5 nm (signal), and a 100 mW Fabry-Perot diode laser at 865 nm (pump).

Application of a diode-laser-based CW tunable IR source to methane detection in air

This work was done to develop a diode-laser-based technique for sensitive atmospheric trace detection gases such as CH/sub 4/, CO, N/sub 2/O, and NO. Spectroscopic detection of methane in the fundamental, and overtone stretch vibration bands using tunable infrared lasers has been reported. The /spl nu//sub 3/ band of methane near 3.2 /spl mu/m includes its strongest known molecular transition and therefore is better suited for sensitive detection. The band is accessible by either conventional spectroscopy or with Ar/sup +/-dye laser difference-frequency generation, the carbon monoxide overtone laser, the helium-neon laser near 3.39 /spl mu/m, lead-salt diode lasers, and color-center lasers. However, each one of these mid-infrared laser sources suffers from its own specific practical drawbacks such as large physical size, lack of portability, high cost, high power consumption, poor tunability, or the need for cryogenic cooling. In the work, detection of the methane in natural air (1.8 ppmv) was performed using diode-laser-pumped cavity-enhanced CW tunable difference-frequency generation (DFG) near 3.2 /spl mu/m.

Tunable mid-infrared laser-based gas sensors: new technologies and applications

This work describes the design and performance characteristics of room-temperature mid-infrared diode-pumped sensors capable of either single or multicomponent real-time trace gas detection in ambient air. The difference-frequency-generation based sensor technology employs periodically poled lithium niobate crystals with multiple grating periods pumped by two single-frequency solid-state lasers and a thermoelectrically cooled HgCdTe infrared detector.

Environmental monitoring of chemical species

Design, development, and application of novel all-solid- state spectroscopic gas sensors is reported. These compact instruments employ mid-infrared difference-frequency generation in a nonlinear optical medium pumped by room- temperature semiconductor lasers, and are capable of real- time selective measurement of trace gases in ambient air with better than 1 ppb precision (part per billion, by mole fraction).

Measurement of /sup 13/CH/sub 4///sup 12/CH/sub 4/ ratios in air using diode-pumped 3.3 /spl mu/m difference-frequency generation in PPLN

Methane has a significant effect on the radiative balance of the troposphere and stratosphere because it has a strong absorption at 7.66 /spl mu/m where carbon dioxide and water absorb only weakly. Accurate measurements of the abundance of methane are thus of considerable interest to climatologists. Previously, we demonstrated a spectrometer capable of producing fast, accurate measurements of the methane mixing ratios in natural air samples. This system demonstrated a relative accuracy of less than 10/sup -9/ mol/mol (1 ppb by volume), which is comparable to what can be achieved using non-spectroscopic methods. This system employed bulk periodically poled lithium niobate pumped by a solitary diode laser at 808 nm and a diode pumped monolithic ring Nd:YAG laser at 1064 nm, and a multi-pass absorption cell with 18 m pathlength. We generated approximately 1 /spl mu/W of infrared radiation at 3019 cm/sup -1/ using 21 mW from a grating-tuned extended-cavity diode laser at 806 nm and 382 mW from a diode-laser-pumped Nd:YAG laser at 1064 nm to pump a periodically poled lithium niobate crystal. Because this system is compact, uses all solid-state lasers, and could be made transportable, it has potential applications for field measurements where rapid, nondestructive concentration and isotope ratio measurements are needed. Even though it is not competitive with state-of-the-art nonoptical instruments, this system can measure /sup 13/CH/sub 4///sup 12/CH/sub 4/ ratios at CH/sub 4/ concentrations of only 1.6/spl times/10/sup -6/ mol/mol without gas processing.

Tunable mid-infrared source pumped by fiber-coupled communications lasers

than 1 ppb, and methane mole fractions were assigned relative to the CMDL methane scale. Testing was performed in two parts. In the first part, two cylinders (identification numbers 30516 and 37057) were supplied with their methane mole fractions. The second part was a blind test; the two authors performing the spectroscopic measurements (KPP and SW) were not told the methane mole fractions in two other cylinders (64040 and 30482). A summary of data obtained for all samples analyzed in the experiment is given in Table 1. The combined uncertainty of the spectroscopic measurements over 1 minute was less than 1 ppb for a typical ambient methane mixing ratio of 1700-1900 ppb. The uncertainty was limited by residual interference fringes in the multi-pass cell. Without reference gas, the uncertainty of 20 ppb was limited by the longterm stability and reproducibility of the spectroscopic baseline. We have demonstrated a precise spectroscopic application of diode-pumped differencefrequency generation. Diode-pumped DFG in PPLN233 can effectively replace lead-salt diode laser^^,^ and gas lasers in spectroscopic and trace gas monitoring applications requiring low-noise, single-frequency sources, continuously tunable