Trace gas sensor based on quartz tuning fork enhanced laser photoacoustic spectroscopy

A compact photoacoustic gas sensor based on a quartz tuning fork and fiber-coupled distributed feedback (DFB) diode laser for detection of trace gas at atmospheric pressure has been developed. The sensor performance was evaluated by detection of water vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of 1.68×10⁻⁸ cm⁻¹ W/Hz^1/2 was achieved. Influence of different acoustic microresonators and sample pressure on the sensor performance, and the characterization of the sensor response time were investigated. Approaches to improve the current sensor performance are discussed.     

Methods for controlling of the laser-induced absorption in a BTO crystal by using of cw-laser radiation

The iron-atom concentration distribution as well as the gas-phase temperature was measured via laser-induced fluorescence (LIF) during iron-oxide nanoparticle synthesis in a low-pressure hydrogen/oxygen/argon flame reactor using ironpentacarbonyl (Fe(CO)₅) as precursor. Temperature measurements based on multi-line NO-LIF imaging are used to correct for temperature-dependent ground-state populations. The concentration measurement is calibrated based on line-of-sight absorption measurements. The influence of the precursor on the flame is observed at precursor concentrations larger than 70 ppm as the flame front moves closer to the burner surface with increasing Fe(CO)₅ concentration.     

Flexible laser diode trace gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10⁻⁷ cm⁻¹ in a compact 1-cm³ ringdown cavity volume.     

Investigation of heavy-metal accumulation in selected plant samples using laser induced breakdown spectroscopy and laser ablation inductively coupled plasma mass spectrometry

Single-pulse Laser-Induced Breakdown Spectroscopy (LIBS) and Laser-Ablation Inductively Coupled Plasma Mass-Spectrometry (LA-ICP-MS) were applied for mapping the silver and copper distribution in Helianthus Annuus L. samples treated with contaminant in controlled conditions. For Ag and Cu detection the 328.07 nm Ag(I) and 324.75 nm Cu(I) lines were used, respectively. The LIBS experimental conditions (mainly the laser energy and the observation window) were optimized in order to avoid self-absorption effect in the measured spectra. In the LA-ICP-MS analysis the Ag 107 and Cu 63 isotopes were detected. The capability of these two analytical techniques for high-resolution mapping of selected trace chemical elements was demonstrated.     

Remote filament-induced fluorescence spectroscopy from thin clouds of smoke

Remote filament-induced fluorescence spectroscopy is used to probe a cloud of smoke, produced from burning mosquito coils, located at a distance of 25 m from the laser source and LIDAR detector. CN, CH and C₂ molecular fragments were identified in the sample. We demonstrate that temporally gated measurement is an efficient technique to easily suppress spectral contaminations, such as white light and atmospheric N₂ fluorescence.     

Self-mixing interference effects in tunable diode laser absorption spectroscopy

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon^™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detectors.     

Infrared laser-spectroscopic analysis of <Superscript>14</Superscript>NO and <Superscript>15</Superscript>NO in human breath

We report on monitoring of nitric oxide (NO) traces in human breath via infrared cavity leak-out spectroscopy. Using a CO sideband laser near 5 μm wavelength and an optical cavity with two high-reflectivity mirrors (R=99.98%), the minimum detectable absorption is 2×10⁻¹⁰ cm⁻¹ Hz^1/2. This allows for spectroscopic analysis of rare NO isotopologues with unprecedented sensitivity. Application to simultaneous online detection of ¹⁴NO and ¹⁵NO in breath samples collected in the nasal cavity is described for the first time. We achieved a noise-equivalent detection limit of 7 parts per trillion for nasal ¹⁵NO (integration time: 70 s).     

Performance evaluation of a near infrared QEPAS based ethylene sensor

A sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was evaluated for the detection of trace levels of ethylene at atmospheric pressure using a fiber coupled DFB diode laser emitting in the 1.62 μm spectral range. A noise-equivalent QEPAS signal of ∼4 ppm C₂H₄ was achieved for a 0.7 s data acquisition time using wavelength-modulation with a second-harmonic detection scheme on the strongest C₂H₄ absorption peak at 6177.14 cm⁻¹ with an average optical power of ∼15 mW. Improved detection sensitivity of 0.5 and 0.3 ppm C₂H₄ (1σ) was demonstrated using longer averaging time of 70 and 700 s, respectively. Important characteristics for the QEPAS based sensor operation in real-world conditions are presented, particularly the influence of external temperature variations. Furthermore, the response time of the ethylene sensor was measured in different configurations and it is shown that the QEPAS technique can provide a response time in a few seconds range even without active gas flow.     

Development of an optical parametric generator with pulsed dye amplification for high-resolution laser spectroscopy

An injection-seeded optical parametric generator (OPG), coupled with three pulsed dye amplification (PDA) stages, was shown to produce tunable, narrow linewidth laser radiation. The OPG was composed of a pair of beta barium borate (β-BBO) crystals and pumped by the third harmonic (355 nm) output of a seeded Nd:YAG laser. The OPG was injection-seeded at the idler wavelength (824 nm) using an external cavity diode laser (ECDL) with a mode-hop-free tuning range of 20 GHz. Using the PDA stages, the OPG output signal (624 nm) was amplified to 19 mJ/pulse, while maintaining a spectral linewidth of approximately 160 MHz at full-width-half-maximum (FWHM) which was within a factor of 2 of the Fourier limit. A system of lenses and apertures was used to minimize amplified spontaneous emission (ASE) in the PDA stages. Using the OPG/PDA system, two-photon laser-induced fluorescence measurements of atomic oxygen were performed by sum-frequency-mixing the 624-nm beam with the third harmonic output of the seeded Nd:YAG laser to generate approximately 1 mJ/pulse of ultraviolet radiation near 226 nm. Voigt line shapes were found to be in good agreement with oxygen atom spectra in atmospheric-pressure, laminar, counter-flow flames; the magnitude of Doppler and collisional broadening was approximately the same. The measured O-atom concentration profile was found to compare well with that calculated using an opposed-flow flame code.     

Frequency stabilization of CO laser using RF optogalvanic Lamb-dip

The Lamb dip of CO rovibrational transition is detected by a room temperature extracavity RF optogalvanic cell and employed to stabilize the frequency of a CO laser. The S/N ratio of optogalvanic signal is about 2000  at optical power < 1 W. The relative depth of Lamb dip is 2.3%. The S/N ratios of first and third harmonic demodulated saturation signals are about 40  and 10  , respectively. The CO laser is stabilized using the first harmonic demodulated signal, and the frequency stability is better than 300 kHz. Concurrently, the influences of operational parameters, which include the coil current, partial pressures of gas mixture, are investigated. A simple model for the influence of coil current is presented, and further improvements are addressed as well.     

High-resolution infrared polarization spectroscopy and?degenerate four wave mixing spectroscopy of methane

High-resolution infrared polarization spectroscopy (IR-PS) and degenerate four wave mixing (IR-DFWM) spectroscopy of methane using a diode-seeded modeless laser (DSML) system are reported. Mid-infrared radiation around 3.3 μm is generated by difference frequency mixing of the single-mode output of the DSML around 0.634 μm with the frequency-doubled output of a single-mode Nd:YAG pump laser at 0.532 μm. Polarization spectroscopy signals in the forward geometry were generated in methane at around 5 Torr pressure. IR-PS spectra were recorded with a typical signal-to-noise ratio of 150:1 with methane pressures of at least 1 Torr. The line shape of the IR-PS signals was analysed to measure pressure broadening induced by nitrogen buffer gas yielding a value of 6.3±1.5 MHz Torr⁻¹. IR-DFWM spectra of methane were generated in the counter-propagating pump geometry yielding Doppler-free signals with signal-to-noise ratios of typically 650:1. Signals were obtained at methane pressures down to less than 10 mTorr. A comparison of IR-PS and IR-DFWM is made indicating that IR-DFWM has some advantages over IR-PS in this spectral region in terms of sensitivity, signal-to-noise ratio and ease of use. The results illustrate the utility of the DSML for high-resolution nonlinear spectroscopy in the mid infrared.     

Novel resonance profiling and tracking method for photoacoustic measurements

Resonant photoacoustic (PA) detection is widely used in several atmospheric and industrial monitoring applications due to its high sensitivity and short response time. However, unexpected changes in the acoustic resonance frequency of a PA cell caused by sudden changes either in the composition or the temperature of the sample gas can largely diminish the precision of the PA measurement. This paper describes a novel method for tracking such changes in resonance frequency. Besides improving the measurement precision, the introduced CHIrped modulation for Resonance Profiling (CHIRP) method has the additional advantage of maintaining the fast response time of the PA system without using any additional hardware components. The minimum detectable water vapor concentration, depending on the modulation bandwidth of the CHIRP, was found to be 0.3–0.5 ppm in nitrogen buffer gas. The applicability of the CHIRP method was demonstrated in PA measurements in a buffer gas with varying composition, which are typical in, e.g., industrial monitoring applications.     

A narrow linewidth and frequency-stable probe laser source for the <Superscript>88</Superscript>Sr<Superscript>+</Superscript> single ion optical frequency standard

In this paper, we describe in detail a narrow linewidth and frequency-stable laser source used to probe the 5s ² S _1/2–4d ² D _5/2 clock transition of the ⁸⁸Sr⁺ optical frequency standard. The performance of the laser system is investigated with studies of its frequency drift rates and with high resolution spectra of the ⁸⁸Sr⁺ clock transition. The observed short-term drift rates are typically in the range of 10 to 23 mHz/s, and the current long-term drift rate is 13.9(3) mHz/s. The laser stability, after subtraction of linear drifts, reaches 5×10⁻¹⁶ at an averaging time of 3000 s. This high level of stability is attributed for the most part to stabilization of the reference cavity at the temperature where the coefficient of linear thermal expansion crosses zero. An upper bound for the laser linewidth is given by the observation of a Fourier-transform limited resonance of 4.3 Hz (Δν/ν=1×10⁻¹⁴) on the ⁸⁸Sr⁺ clock transition. The effective averaging time during the linewidth measurements was about 100 s.     

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2³S state atoms. We analyze schemes for measuring the two-photon 2³S →4³S interval, and for resonant two-photon excitation to high Rydberg states, 2³S →3³P →n³S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1¹S or 2¹S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 1¹S →2¹S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1¹S, 2¹S and 2³S states are all similar, small, and positive.     

Considerations about Z-scan sensitivity improvement: theory versus experiments

This paper reports an experimental demonstration of the improvement of the Z-scan technique’s sensitivity. It is shown that this sensitivity can be multiplied by a factor equal to almost 400 with the help of simple binary diffractive elements. Such a possibility was actually predicted theoretically in one of our previous papers. In this study, the interpretation is investigated in a wider context taking into account the definition of the signal normalisation as introduced by Z-scan and the well-known eclipsing Z-scan (EZ-scan) experiments. In particular, advantages and drawbacks are compared, by looking at the normalised or the unnormalised aperture transmission.     

Monitoring laser cleaning of titanium alloys by probe beam reflection and emission spectroscopy

The energy relaxation of electrons in InN epilayers is investigated by excitation- and electric field-dependent photoluminescence (PL). From the high-energy tail of PL, we determine the electron temperature of the hot carriers. It was found that the electron temperature variation can be explained by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The LO-phonon lifetime is fitted to be 0.89 ps, which is higher than the theoretical phonon lifetime. This deviation is attributed to the presence of the non-equilibrium hot-phonon effects. PACS 78.55.Cr; 78.66.Fd; 61.66.Fn; 78.20.Jq; 63.20.kd     

Wigner Kirkwood ℏ-expansion of the density matrix in inhomogeneous superfluid Fermi systems

The generalized density matrix of superfluid inhomogeneous Fermi systems is expanded in powers of up to order ². This constitutes the generalisation of the Wigner Kirkwood -expansion of the density matrix of normal fluid systems to the pairing case.One of the authors (P.S.) is very grateful to D. Gogny for contributions in an early stage of this work several years back. He also acknowledges fruitful discussions with M. Centelles and X. Vinas.     

Local crystallization in an oxyfluoride glass doped with Er3+ ions using a continuous argon laser

Local crystalline formation in erbium doped oxyfluoride glass has been obtained under a cw Argon laser irradiation up to 1.8 W pumping power. By exciting at 514 nm, the emission from 800 nm and 850 nm corresponding to the ⁴S_3/2(²H_11/2)→⁴I_13/2 electronic transitions have been analyzed both inside and outside the irradiated area. The changes in the emission spectra indicate that the high power Ar laser irradiation has resulted in a localized desvitrification process. The temperature dependence of the fluorescence intensity ratio of the 800 nm and 850 nm emission bands has been used to determine the temperature of the irradiated zone. Moreover, the average lifetime of the ⁴S_3/2(²H_11/2) thermalized levels have been measured as a function of the excitation spot position. An important decrease is observed at the irradiated area. These results confirm that a localized cristalline phase has been created by the laser action.     

Sensitive detection of ammonia and ethylene with a pulsed quantum cascade laser using intra and interpulse spectroscopic techniques

Spectroscopic concentration measurements of ammonia and ethylene were done with a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm⁻¹. An astigmatic Herriot cell with 150 m path length was employed, and we compare the results from experiments using inter- and intrapulse techniques, respectively. The measurements include the detection of ammonia in breath with these methodologies. In the interpulse technique, the laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with an external current ramp resulting in a ∼0.3 cm⁻¹ frequency scan. A standard amplitude demodulation technique was implemented for extracting the absorption line, thus avoiding the need for a fast digitizer or a gated integrator. In the intrapulse technique, a linear frequency down-chirp is used for sweeping across the absorption line. A 200 ns long current pulse was used for these measurements which resulted in a spectral window of ∼1.74 cm⁻¹ during the down-chirp. The use of a room temperature mercury-cadmium-telluride detector resulted in a completely cryogen free spectrometer. We demonstrate detection limits of ∼3 ppb for ammonia and ∼5 ppb for ethylene with less than 10 s averaging time with the intrapulse method and ∼4 ppb for ammonia and ∼7 ppb for ethylene with the interpulse technique with an integration time of ∼5 s.     

Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra

Cavity ringdown (CRD) spectroscopy, with its high sensitivity, provides a novel way to perform continuous-wave (cw) stimulated Raman gain (SRG) spectroscopy, rather than by conventional optically detected coherent Raman techniques. Tunable cw laser light at ∼1544 nm is used to probe ringdown decay from a rapidly-swept, high-finesse optical cavity containing a gas-phase sample of interest and itself located inside the cavity of a cw single-longitudinal-mode Nd:YAG ring laser operating at ∼1064.4 nm. This approach is used to measure cw SRG spectra of the ν ₁ fundamental rovibrational Raman band of methane gas at ∼2916.5 cm⁻¹. The resulting SRG-CRD resonances have ringdown times longer than in the off-resonance case, in contrast to the usual shorter ringdown times arising from absorption and other loss processes. Previously reported noise-equivalent sensitivities have been substantially improved, by using a second ringdown cavity to facilitate subtraction of infrared-absorption background signals. Moreover, by employing a ringdown cavity in the form of a ring, the SRG-pump and CRD-detected Stokes beams can co-propagate uni-directionally, which significantly reduces Doppler broadening.