Joel A. Silver

Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods

A variety of frequency-modulation methods for high-sensitivity absorption detection of gas-phase species has evolved in recent years. The distinctions among these methods are mostly semantic. The mathematical derivations for wavelength-modulation spectroscopy and one- and two-tone frequency-modulation spectroscopies are presented; a common terminology is used to permit a comprehensive comparison of predicted detection sensitivities. Applying this formalism, I compare the optimum detection sensitivities of these different methods for a typical laser system, using the same parameters. As long as residual amplitude modulation is minimized by proper adjustment of the detection phase angle, high-frequency wavelength modulation and one- and two-tone frequency-modulation methods all achieve approximately the same sensitivities. The choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth, and the detection bandwidth. It is shown that excess laser noise cannot always be excluded from consideration, even at megahertz detection frequencies. Also, detection at harmonics of the modulation or beat frequency may present certain advantages in minimizing residual amplitude-modulation noise.

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 Concentration and Temperature in Microgravity Combustion

Diode laser absorption spectroscopy provides a direct method of determinating species concentration and local gas temperature in combustion flames. Under microgravity conditions, diode lasers are particularly suitable, given their compact size, low mass and low power requirements. The development of diode laser-based sensors for gas detection in microgravity is presented, detailing measurements of molecular oxygen. Current progress of this work and future application possibilities for these methods on the International Space Station are discussed.

Quantitative species measurements in microgravity flames with near-IR diode lasers

Absolute concentrations of water vapor are measured in microgravity (µ-g), nonpremixed methane, and propane jet flames with diode-laser wavelength modulation spectroscopy. These experiments are performed in the 2.2-s µ-g drop facility at the NASA Lewis Research Center. Abel inversion methods are used to determine time-dependent radial profiles from eight line-of-sight projections across the flames. At all measured heights above the nozzle, water vapor spatial distributions in µ-g flames are much wider than their 1-g counterparts. Radial growth of the water signal continues throughout the drop, verifying earlier suggestions that a steady state is not reached during the duration of the test, despite a quasi-steady flame shape. Large amounts of water vapor are observed at larger radii, at odds with visual (video) observations and numerical predictions.

Laser‐induced fluorescence determination of internal‐state distribution of OH produced by H+NO2 in crossed molecular beams

The relative populations of rotational states in the v=0 and v=1 vibrational states of OH produced in the reaction of H+NO2→OH+NO in crossed molecular beams were measured by laser‐induced fluorescence. The excited vibrational state v=1 was found to be produced at 1.3±0.3 times the rate of v=0. High degrees of rotational excitation were also observed. The rotational‐state distributions were analyzed in information theoretic terms. A linear surprisal was found, which yielded a rotational surprisal parameter λROT=−2.69±0.57 for v=0.

Simple dense-pattern optical multipass cells

Multiple-pass optical cells with dense spot patterns are useful for many applications, especially when the cell volume must be minimized relative to the optical path length. Present methods to achieve these dense patterns require expensive, highly precise astigmatic mirrors and complex alignment procedures. This work describes a new, simpler, and less demanding mirror system, comprising either a pair of cylindrical mirrors or one cylindrical and one spherical mirror.

Near‐infrared diode laser airborne hygrometer

We describe a new laser‐based hygrometer for ambient water vapor monitoring which uses a fiber‐optic coupled, near‐infrared diode laser in conjunction with high frequency wavelength modulation spectroscopy. The instrument operates unattended, uses little power, can be extremely compact, and exhibits high detection sensitivity. This instrument was flown on a KC‐135 aircraft for a period of six months and measured frost points at altitudes between 10 000 and 40 000 ft. The water detection sensitivity corresponds to a volume mixing ratio of 8 ppm V. Further improvements including the use of newly designed near‐infrared lasers are expected to realize frost points below −100 °C.

Direct measurement of the radiative lifetime of the A2Σ+(υ′ = 0, K′ = 1, J′ = 32) state of OH and OD

Abstract The radiative lifetime of the A 2 Σ + (υ′ = 0, K ′ = 1, J ′ = 3/2) state of OH and OD has been directly measured by following the decay of fluorescence excited by light from a frequency doubled dye laser. Stern-Volmer extrapolation of the results to zero pressure gave τ(OH) = 788 ± 13 ns and τ(OD) = 754 ± 12 ns.

Measurement of atomic sodium and potassium diffusion coefficients

The gaseous diffusion coefficients for sodium and potassium atoms have been measured as a function of temperature in a flow tube. Analysis is based on a simple relationship between diffusion and the observed alkali decay rate for the case of very reactive walls. The resulting values for the diffusion coefficient in units of cm2 s−1 at 1 atm are: The results agree well with previous measurements and the comparison with theoretical expectations is discussed.

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.