Two-tone Frequency Modulation Spectroscopy Research Articles

Recent Developments in Modulation Spectroscopy for Methane Detection Based on Tunable Diode Laser

In this review, methane absorption characteristics mainly in the near-infrared region and typical types of currently available semiconductor lasers are described. Wavelength modulation spectroscopy (WMS), frequency modulation spectroscopy (FMS), and two-tone frequency modulation spectroscopy (TTFMS), as major techniques in modulation spectroscopy, are presented in combination with the application of methane detection.

Two-tone frequency modulation saturation spectroscopy for tunable frequency offset locking of a single laser

The two-tone frequency modulation spectroscopy (TTFMS) is widely used for gas trace detection with its low noise absorption signal. In this paper we propose and implement the TTFMS scheme for frequency offset locking a single diode laser to an atomic resonance line by using its dispersion signal. The TTFMS theory is first discussed under the assumption that the intermediate modulation frequency is comparable to the linewidth of the absorption feature, and the dependence of the TTFMS absorption and dispersion features on the intermediate modulation frequency and modulation index are investigated theoretically. Based on a fiber-coupled electro-optic modulator (EOM) with two-tone modulation, we experimentally demonstrated the performance of the frequency offset locking method. The result shows a short-term frequency stability of the frequency offset locked laser reached around 1.1 × 10−11, with an averaging time of 2 s. This method can find wide applications in fields requiring widely tunable frequency offset locking a single laser to the atomic resonance line, like precision spectroscopy and Raman optics for atom interferometers.

Measurement of the scalar polarizability of the indium6p1/2state using two-step atomic-beam spectroscopy

We have completed a measurement of the Stark shift within the $^{115}$In $6s_{1/2} \rightarrow 6p_{1/2}$ excited-state transition using two-step laser spectroscopy in an indium atomic beam. Combining this measurement with recent experimental results we determine the scalar polarizability, $\alpha_{0}$, of the $6p_{1/2}$ state to be $7683 \pm43 \,a_{0}^{3}$ in atomic units, a result which agrees very well with recent theoretical calculations. In this experiment, one laser, stabilized to the $5p_{1/2} \rightarrow 6s_{1/2}$ 410~nm transition, was directed transversely to the atomic beam, while a second, overlapping laser was scanned across the 1343~nm $6s_{1/2} \rightarrow 6p_{1/2}$ transition. We utilized two-tone frequency-modulation spectroscopy of the infrared laser beam to measure the second-step absorption in the interaction region, where the optical depth is less than 10$^{-3}$. In the course of our experimental work we also determined the hyperfine splitting within the $6p_{1/2}$ state, improving upon the precision of an existing measurement.

Two-tone frequency-modulation spectroscopy in off-axis cavity

As opposed to a conventional optical resonator, an off-axis-aligned cavity is able to transmit without distortion radiation modulated at a frequency even far above the cavity bandpass. This allows us to implement a simple spectroscopic technique that combines the cavity path-length enhancement of integrated cavity output spectroscopy (ICOS) and the noise reduction associated with radio-frequency modulation (FM). An FM-ICOS spectrometer is demonstrated for the first time using a two-tone modulation technique. The performance is compared to the traditional ICOS by examining the acetylene absorption at 1543.77 nm. A signal-to-noise ratio improvement by a factor 3.5 is found with our proof-of-concept setup. Larger improvements are expected in a more optimized setup.

Two-tone frequency modulation spectroscopy for ambient-air trace gas detection using a portable difference-frequency source around 3 μm

We report a portable, widely tunable, mW-power mid-infrared spectrometer based on difference-frequency generation in a periodically poled lithium-niobate crystal, realized in a compact and robust design. The analytical performance for real-time monitoring of natural-abundance trace gases in ambient air is evaluated, pointing out the possibility of field applications. In a direct-absorption scheme, a minimum detectable concentration of 3 ppb Hz-1/2 is demonstrated around 3.3 μm for methane at atmospheric pressure. The sensitivity is further improved by using a two-tone frequency modulation spectroscopy technique that provides an enhancement of a factor of 100 in the signal-to-noise ratio, thus yielding a minimum absorption coefficient of 5.3×10-9 cm-1 Hz-1/2.

Detection of HCl and HF by TTFMS and WMS

In this work we discuss on a compact spectrometer based on DFB diode lasers for detection of chloridric and fluoridric acids. HCl and HF concentrations are determined through optical absorption of the P(4) line ( λ = 1.7 μm) and the R(3) line ( λ = 1.3 μm), respectively. Both lines belong to first overtone vibrational bands and their linestrengths are 7.8 × 10 −21 cm/molecule for HCl and 2.8 × 10 −20 cm/molecule for HF. We chose these lines for their relative high intensities and because they are quite far from water vapour lines which represent the main interfering gas for trace-gases analysis. To detect these species we used two different high frequency modulation techniques: two-tone frequency modulation spectroscopy ( f 1 = 800 MHz and f 2 = 804 MHz) was used for HCl while for HF we followed a simpler approach based on wavelength modulation spectroscopy ( f = 600 kHz). We demonstrate that the two techniques provide comparable detection limit of about 80 ppbV at atmospheric pressure. Positive testing of our spectrometer makes it suitable for in situ measurements of exhaust gases coming from waste incinerators.

Long-path monitoring of NO_2 with a 635 nm diode laser using frequency-modulation spectroscopy

In situ monitoring of traffic-generated nitrogen dioxide (NO2) emissions using long-path absorption spectroscopy is reported. High-sensitivity detection of NO2 is achieved by employing two-tone frequency-modulation spectroscopy at a visible absorption band using a tunable high-power diode laser operated around 635 nm. A real-time absorption spectrometer is accomplished by repetitively applying a rectangular current pulse to the diode-laser operating current, allowing detection of isolated NO2 absorption lines. A detection limit of 10 microg/m3 for NO2 at atmospheric pressure using a 160 m absorption path is demonstrated. Continuous monitoring of NO2 over a road intersection at peak traffic is performed.

Nitrogen- and water-broadening coefficient measurements in the [formula omitted] 000–000 band of HO 2 using high-resolution diode laser two-tone frequency modulation spectroscopy

The line shape profiles of the A ˜ 2 A ′ ← X ˜ 2 A ″ 000–000 band of hydroperoxy radical (HO 2) have been measured in the presence of nitrogen and water using laser photolysis/cw near infrared two-tone frequency modulation spectroscopy. On the basis of the theory described by Avetisov and Kauranen [Appl. Opt. 35 (1996) 4705], the absorption line shapes were well represented by the Voigt profile. The nitrogen- and water-broadening coefficients for the 7020.766 cm −1 16 1, 16 ← 16 0, 16 F 1 line of HO 2 were estimated to be 0.101 ± 0.013 and 0.216 ± 0.133 cm −1 atm −1, respectively. The large value of water-broadening coefficient suggests the long range interaction between HO 2 with water.

Tunable diode lasers and two-tone frequency modulation spectroscopy applied to atmospheric gas analysis

In this paper, we discuss the main features of a class of gas analyzers based on spectroscopic techniques with the aim of realizing fully automated systems which can be used for practical purposes. The technique we deal with is based on semiconductor diode laser as sources, emitting in the near infrared region of the electromagnetic spectrum, and the two-tone frequency modulation spectroscopy as the detection technique. We will describe the main features of a typical device and two particular apparata for industrial and biological applications.

Frequency-modulation spectroscopy with blue diode lasers

Frequency-modulation spectroscopy provides ultrasensitive absorption measurements. The technique is especially adaptable to diode lasers, which can be modulated easily, and has been used extensively in the near-infrared and infrared spectral regions. The availability of blue diode lasers now means that the accessible wavelength region can be increased. We successfully demonstrate wavelength-modulation spectroscopy and two-tone frequency-modulation spectroscopy for the weak second resonance line of potassium at 404.8 nm and for the transition at 405.8 nm in lead, starting from the thermally populated 6(p)(2 3)P(2) metastable level. Information on the modulation parameters is obtained with a fitting procedure. Experimental signal-to-noise ratios at different absorption levels are compared with theoretical signal-to-noise ratios and show good agreement. Detection sensitivities of 2 x 10(-6) and 5 x 10(-6) for wavelength and two-tone frequency-modulation spectroscopy, respectively, for a 120-Hz bandwidth are demonstrated.

Development of an IR tunable diode laser absorption spectrometer for trace humidity measurements at atmospheric pressure

The development of a laser diode absorption spectrometer that uses a strong water vapor absorption at 1393 nm is reported. Three spectroscopic techniques were compared in approximately 0.4 m of laboratory air, namely, frequency modulation, wavelength modulation, and two-tone frequency modulation spectroscopy. The first two techniques use a single-frequency modulation at 9.2 GHz and 1 kHz, respectively, generated either by a phase modulator operating at 9.2 GHz or injection current modulation at 1 kHz. The two-tone method requires modulation at two frequencies, in this case 9.19 and 9.21 GHz. It is shown that the two-tone method should provide the highest sensitivity for a trace moisture detection system.

Detection of H 2S at the ppm level using a telecommunication diode laser

Detection of traces of H 2S in air at the ppm level using a Distributed-Feed-Back (DFB) semiconductor diode laser spectrometer is reported. Two-tone frequency modulation spectroscopy is employed to investigate overtone transitions of H 2S near λ = 1580 nm and a sensitivity of 4 ppm·m for the 1577.32 nm component of the ν 1+ ν 2+ ν 3 band is achieved. This result improves the optical detection limit of H 2S in air by one order of magnitude. Self- and air-broadening coefficients at 25°C are determined with direct absorption spectroscopy.

High-resolution absorption measurements by use of two-tone frequency-modulation spectroscopy with diode lasers

The capability of two-tone frequency-modulation spectroscopy (TTFMS) in deriving spectral line-shape information was investigated. Two oxygen A-band transitions at 760 nm were selected, and the Voigt profile and two different collisionally narrowed line profiles were employed in their analysis. By means of a least-squares fitting procedure, we obtained accurate information regarding transition strengths and pressure-induced broadening, shift, and narrowing coefficients. Both TTFMS and direct absorption line shapes were modeled with deviations as small as 0.3% over a wide pressure range by use of the collisionally narrowed line profiles. Line parameters measured with TTFMS showed excellent agreement with the parameters measured with direct absorption. The experimental technique used constant-current fast-wavelength scanning, which improved measurement accuracy.

Two-tone frequency-modulation spectroscopy for quantitative measurements of gaseous species: theoretical, numerical, and experimental investigation of line shapes

The capability of retrieving spectral information from line shapes recorded by two-tone frequency-modulation spectroscopy (TTFMS) is investigated. A TTFMS theory accounting for dispersion and nonlinear distortion of diode laser frequency modulation response is presented. The adequacy of the theory for a detailed modeling of line shapes recorded with high resolution is examined. An extensive error analysis of line parameters (i.e., width, intensity, and line center) retrieved by a nonlinear least-squares fitting procedure is made. Plots of residual errors with characteristic signatures that are due to incorrectly assigned modulation parameters and choice of line profile are presented. In least-squares fits to experimental oxygen data with a Voigt profile influence from collisional(Dicke) narrowing is clearly exhibited, and when we used a collisionally narrowed line profile deviations of the model were reduced to less than 0.2%. We demonstrate that accurate quantitative measurements by TTFMS over a wide range of concentrations, temperatures, and pressures are possible.

Two-tone frequency modulation spectroscopy from laser light scattered off a hard target

One can apply two-tone frequency modulation spectroscopy techniques to the detection of gas-phase species by using laser light scattered from hard targets. High sensitivities are demonstrated, with a minimum detectable absorption of 10(-4) possible with a simple apparatus. The effects of laser speckle on the FM signal are described, and we show that the detection signal-to-noise ratio can be improved by collecting an increased number of speckle cells.

Design of an open path near-infrared diode laser sensor: application to oxygen, water, and carbon dioxide vapor detection

An open path diode laser sensor was constructed with near-infrared diode lasers and two-tone frequency-modulation spectroscopy. The sensor incorporates several novel features (such as digital signal-processing algorithms, a computerized line-locking routine, and discontinuous wavelength scanning) that are important in a field instrument. The sensor was used to monitor oxygen, water, and carbon dioxide in the near-infrared spectral range. For oxygen, an absorbance detection sensitivity of 2 × 10(-6) in a 10-Hz bandwidth was demonstrated with a GaALAs laser at 760.56 nm. The stability of the sensor was 0.1% over a period of 10 h when an absorbance of 6 × 10(-3) was monitored.

Tomographic imaging of fluid flows by the use of two-tone frequency-modulation spectroscopy

Diode laser absorption measurements obtained with two-tone frequency-modulation spectroscopy (TTFMS) are combined with tomography to produce spatially resolved quantitative images of fluid flows. The high sensitivity, good accuracy, and high stability of TTFMS absorption measurements permit optical-absorption tomography to be performed with low noise on extremely weakly absorbing objects. The method is demonstrated on a small oxygen flow with a GaAlAs diode laser operating near 760 nm and with an absorption sensitivity of 1:10(6).

Determination of absorption line parameters using two-tone frequency-modulation spectroscopy with diode lasers

Accurate determination of line parameters from weak absorption using two-tone frequency-modulation spectroscopy (TTFMS) with a GaAlAs diode laser is demonstrated. The air-broadening of three oxygen A-band lines is determined, using a least-squares fit. For a peak absorption of 1.3 × 10 -3 the deviation between the experimental and the fitted spectrum is less than 1%. The fitting procedure is successfully tested on absorptions as small as 4.5 × 10 -5

Relative Vapor Pressure Measurements Using a Frequency-Modulated Tunable Diode Laser, a Tool for Water Activity Determination in Solutions

Two-tone frequency modulation spectroscopy (TTFMS) is applied to the determination of water activity using an 820-nm GaAlAs diode laser. By measuring the relative light absorption of water vapor over a solution and over a pure water reference, the water activity in the solution is determined. Fast, nonintrusive, and selective measurements are demonstrated. The detection limit for measuring a relative water vapor pressure change was 0.3%. The application of laser modulation spectroscopy to the determination Of water activity is discussed, and suggestions on increasing the sensitivity and accuracy are presented.

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.