Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for the possible application in cavity-enhanced absorption spectroscopy. In order to estimate the noise level of an absorbance baseline, the triplet of frequency modulated light, i.e. the central laser frequency and the two sidebands, were transmitted through both the V-cavity in open air and a 1.5-cm long optical cell placed behind the cavity output mirror and filled with acetylene (C₂H₂) at low pressure. The performance of the setup was evaluated from the measured relative intensity noise on the cavity output (normalised by the bandwidth) and the frequency modulation absorption signals induced by C₂H₂ absorption in the 1.5-cm cell. From these data, we estimate that the noise-equivalent absorption sensitivity of 2.1 × 10^?11 cm^?1 Hz^?1/2—by a factor of 11.7 above the shot-noise limit—can be achieved for C₂H₂ absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM₀₀ resonances.     

The absorbance enhancement investigation of multiple gas absorption lines based on ICFRL

The absorbance enhancement of multiple gas absorption lines is investigated by simulations and experiments in this paper. The relationship between cavity loss and gas absorbance of different absorption lines is simulated based on rate equation. For the purpose of using simple arithmetic mean instead of weighted arithmetic mean in calculating gas concentration by multiple absorption lines, the absorbance equality method is proposed and simulated. To verify the simulations, some experiments were carried out based on the intra-cavity fiber ring laser (ICFRL) gas sensing system. The experimental results of absorbance enhancement extremely match to the simulation results. More than 40 C₂H₂ absorption lines can be observed and a minimum detectable C₂H₂ concentration of 200 ppm is acquired according to the experiments.     

Diode laser absorption spectra of HCCD and HCCD at 6500 cm

The absorption spectra of H¹²C¹³CD and H¹³C¹²CD have been observed at high resolution between 6480 and 6610 cm⁻¹ using an external cavity diode laser. The strong 2ν₁ band has been observed for each species using a sample enriched in deuterium at natural abundance of ¹³C. Rotational analyses reveal bands of both species to be essentially unperturbed. Centers of unblended lines are determined with an accuracy of approximately 10 MHz.     

Fiber laser intracavity absorption spectroscopy for in situ multicomponent gas analysis in the atmosphere and combustion environments

Intracavity absorption spectroscopy with a broadband Er³⁺-doped fiber laser is applied for the measurements of several molecular species revealing quantitative information about the gas concentration, temperature and chemical reactions in flames. The spectral range of measurements extends from 6200 cm⁻¹ to 6550 cm⁻¹ with the proper choice of the fiber length and by moving an intracavity lens. With a pulsed laser applied in this experiment, the sensitivity to absorption corresponds to an effective absorption path length of 3 km assuming the cavity is completely filled with the sample. For a cw laser, the effective absorption path length is estimated to be 50 km. Absorption spectra of various molecules such as CO₂, CO, H₂O, H₂S, C₂H₂ and OH were recorded separately in the cell and/or in low-pressure methane and propane flames. The presented measurements demonstrate simultaneous in situ detection of three molecular products of chemical reactions at different flame locations. Variation of the relative strengths of OH absorption lines with the temperature enables the estimation of the local flame temperature. The sensitivity of this laser does not depend on the broadband cavity losses and it can be used for in situ measurements of absorption spectra in hostile environments such as contaminated samples, flames or combustion engines. The presented technique can be applied for various diagnostic purposes, such as in environmental, combustion and plasma research, in medicine and in the determination of stable isotope ratios.     

Near-infrared laser based cavity ringdown spectroscopy for applications in petrochemical industry

A simple, economic diode laser based cavity ringdown system for trace-gas applications in the petrochemical industry is presented. As acetylene (C₂H₂) is sometimes present as an interfering contaminant in the gas flow of ethylene (ethene, C₂H₄) in a polyethylene production process, an on-line monitoring of such traces is essential. We investigated C₂H₂–C₂H₄ mixtures in a gas-flow configuration in real time. The experimental setup consists of a near-infrared external cavity diode laser with an output power of a few mW, standard telecommunication fibers and a home-made gas cell providing a user-friendly cavity alignment. A noise-equivalent detection sensitivity of 4.5×10^-8 cm^-1 Hz^-1/2 was achieved, corresponding to a detection limit of 20 ppbV C₂H₂ in synthetic air at 100 mbar. In an actual C₂H₂–C₂H₄ gas-flow measurement the minimum detectable concentration of C₂H₂ added to the C₂H₄ gas stream (which may already contain an unknown C₂H₂ contamination) increased to 160 ppbV. Moreover, stepwise C₂H₂ concentration increments of 500 ppbV were resolved with a 1-min time resolution and an excellent linear relationship between the absorption coefficient and the concentration was found. PACS 07.07.Df; 42.62.Fi; 82.80.Gk     

C2H2 Overtones Near 12300 cm-1 Revisited with a Very Sensitive Cavity Ring-down Spectrometer

利用单模连续的钛宝石激光器, 构建了一台光腔衰荡光谱仪, 其可探测的最小吸收可达1.8×10^-10/cm. 该光谱仪被用来记录C₂H₂分子在12240～12350 cm^-1的泛频光谱. 与在同一波段测量的已报到的CRDS和激光腔内吸收光谱仪结果比较,本测量同时具有更好的灵敏度和精度. 由此,获得了乙炔分子在12290.12、12311.82和12350.61 cm^-1附近高泛频谱带更准确的振转参数     

Laser spectroscopy of ethylene with waveguide CO2 and N2O lasers

Fifty Doppler-broadened absorption lines of ethylene have been measured within large profiles CO₂ or N₂O lase lines. These laser lines are produced by a high pressure waveguide laser and have a full width between 200 and 900 MHz. Eleven absorption lines, the more intense ones, have been assigned to the ν₇ band of C₂H₄. The other absorption lines must belong to hot bands or to the ν₇ band of H₂¹²C¹³CH₂.     

Application of a continuous-wave tunable erbium-doped fiber laser to molecular spectroscopy in the near infrared

Development of a continuous-wave tunable fiber laser-based spectrometer for applied spectroscopy is reported. Wide wavelength tunability of an erbium-doped fiber laser (EDFL) was investigated in the near-infrared region of 1543–1601 nm. Continuous mode-hop free fine frequency tuning has been accomplished by temperature tuning in conjunction with mechanical tuning. The overall spectroscopic performance of the EDFL was evaluated in terms of frequency tunability along with its suitability for molecular spectroscopy. High-resolution absorption spectra of acetylene (C₂H₂) were recorded near 1544 nm with a minimum measurable absorption coefficient of about 3.5×10^-7 cm^-1/Hz^1/2 for direct absorption spectroscopy associated with a 100-m long multipass cell. Detections of C₂H₂ at different concentration levels were performed as well with high dynamic detection range varying from 100% purity to sub ppmv using cavity ring down spectroscopy. A 3σ-detection-limited minimum detectable concentration (MDC) of 400 ppbv has been obtained by using the transition line P_e(22) of the ν₁+ν₃+ν₅ ¹(Π_g)-ν₅ ¹(Π_u) hot band near 1543.92 nm with a detection bandwidth of 2.3 Hz. This corresponds to a minimum detectable absorption coefficient of 6.6×10^-11 cm^-1/Hz^1/2. The sensitivity limit could be further improved by almost one order of magnitude (down to ∼60 ppbv) by use of the P_e(27) line of the ν₁+ν₃(Σ_u ⁺)-0(Σ_g ⁺)combination band near 1543.68 nm. PACS 42.55.Wd; 42.62.Fi; 07.57.Ty; 07.88.+y     

Generation of a reference frequency comb by cascaded four-wave mixing enhanced by Raman amplification

The generation of standard reference frequencies close to the ITU channels is essential for the frequency calibration of DWDM systems. This paper describes the generation of a reference frequency comb based on the combination of a cascaded four-wave mixing in two semiconductor optical amplifiers and Raman amplification in a dispersion-shifted optical fiber. As a result we have achieved a stable frequency comb with 36 reference lines separated by a constant frequency spacing of 177 GHz. The seed of the comb is combination of two narrow-linewidth semiconductor lasers which are locked to two absorption lines of the acetylene (¹²C₂H₂).     

High-sensitivity CRDS absorption spectroscopy of acetylene between 5851 and 6341 cm−1

The absorption spectrum of acetylene has been recorded at room temperature (297 K) using high-sensitivity cavity ring-down spectroscopy (α_min ～ 5×10^?11 cm^?1) in the 5851 and 6341 cm^?1 interval corresponding to a region of very weak absorption. A list of about 10,700 absorption features with estimated absolute line intensities was constructed. The smallest intensities are of the order of 5×10^?29 cm molecule^?1. The line list includes about 2500 absorption lines of ethylene present at the ppm level in the acetylene sample and identified on the basis of a high-resolution Fourier transform spectrum specifically recorded. A total of more than 2700 lines of ¹²C₂H₂ were rovibrationally assigned in comparison with accurate predictions provided by a global effective operator model. Overall, the present effort adds about 2260 new assignments to the set of about 500 assigned transitions available in the literature. The new assignments correspond to 45 new bands and 17 already-known bands, for which additional J lines were assigned. Spectroscopic parameters were derived for the upper vibrational levels from a band by band fit of the line positions (typical root mean square deviation values are of the order of 0.001 cm^?1). A few of the analysed bands were found to be affected by rovibrational perturbations, which are discussed. The new data will be valuable to refine the parameters of the global effective Hamiltonian and dipole moments of ¹²C₂H₂.     

A high-stability semiconductor laser system for a <Superscript>88</Superscript>Sr-based optical lattice clock

We describe a frequency-stabilized diode laser at 698 nm used for high-resolution spectroscopy of the ¹S₀–³P₀ strontium clock transition. For the laser stabilization we use state-of-the-art symmetrically suspended optical cavities optimized for very low thermal noise at room temperature. Two-stage frequency stabilization to high-finesse optical cavities results in measured laser frequency noise about a factor of three above the cavity thermal noise between 2 Hz and 11 Hz. With this system, we demonstrate high-resolution remote spectroscopy on the ⁸⁸Sr clock transition by transferring the laser output over a phase noise-compensated 200-m-long fiber link between two separated laboratories. Our dedicated fiber link ensures a transfer of the optical carrier with frequency stability of 7×10⁻¹⁸ after 100 s integration time, which could enable the observation of the strontium clock transition with an atomic Q of 10¹⁴. Furthermore, with an eye toward the development of transportable optical clocks, we investigate how the complete laser system (laser+optics+cavity) can be influenced by environmental disturbances in terms of both short- and long-term frequency stability.     

Design and Analysis of a Gas Sensor Based on a Ring Multi-Wavelength Fiber Laser in Near Infrared

Numerical model of a gas sensor based on the ring multi-wavelength fiber laser is discussed to detect gas such as methane (CH₄), which has multi-line absorption in near infrared. The schematic of the gas sensor is given, and its detecting principle of multiple windows is demonstrated in detail. The detecting methods based on the differential method and the second harmonic wave method in the sensor are analyzed, and the optimal information syncretions are used to process the information of the detected gas. The analyzing results show the sensor has advantages over the traditional sensor, and can be expanded to detect other gas such as acetylene (C₂H₄).     

Selective optothermal detection of NO2 and H2O with a frequency-tuned CO waveguide laser

The results are reported of the CO-laser optothermal (OT) detection of impurity gases when their absorption spectra overlap with those of an interfering gas. The influence of the latter was avoided using low gas pressures corresponding to a maximum of the OT sensitivity. Frequency tuned in the 5.2–6.3 m wavelength range, ¹²C¹⁶O and ¹³C¹⁶O waveguide lasers were used. The fine frequency tuning at 490 MHz was achieved for 150 laser transitions of both molecules. The OT sensitivity was estimated by NO₂ detection in the presence of water vapor. The minimal detectable concentration proved to be 60 ppb at P _19–18(14) transition of a ¹²C¹⁶O laser for NO₂ and 75 ppb on P _12–11(13) transition of a ¹³C¹⁶O laser for H₂O.     

DCOOD optically pumped by a 13CO2 laser: new terahertz laser lines

In this work we report on new optically pumped THz laser lines from deuterated formic acid (DCOOD). An isotopic ¹³CO₂ laser was used for the first time as a pump source for this molecule, and a Fabry–Perot cavity was used as a THz laser resonator. Optoacoustic absorption spectra were used as a guide to search for new THz laser lines. We could observe six new laser lines in the range from 303.8 μm (0.987 THz) to 725.1 μm (0.413 THz). The lines were characterized according to wavelength, relative polarization, relative intensity, and optimum working pressure. The transferred lamb-dip technique was used to measure the frequency absorption transition for both of these laser lines. Furthermore, we also present a catalogue of all THz laser lines generated from DCOOD.     

Femto-Fourier transform-cavity enhanced absorption spectroscopy in a supersonic expansion

Proofs of principle spectra of C₂H₄, N₂O and C₂H₂, including H¹²C¹³CH in natural abundance, are reported, recorded in the 1.6?µm range in an Ar supersonic expansion using femto-Fourier transform–cavity enhanced absorption spectroscopy. The effective absorption pathlength in the jet-cooled sample is up to 78?m and the optimal S/N is over 2300. The data processing is detailed. Saturation effects are reported for the C₂H₂ bands.     

Optimization of the mode matching in pulsed cavity ringdown spectroscopy by monitoring non-degenerate transverse mode beating

A simple and reliable method is presented for optimizing the mode matching of a laser beam to the high-finesse cavity used in pulsed cavity ringdown spectroscopy (CRDS). The method is based on minimizing the excitation of higher-order transverse cavity modes through monitoring the non-degenerate transverse mode beating which becomes visible with induced cavity asymmetry caused by slight misalignment. No additional instrument is required other than a pinhole aperture, thus this method can be applied for CRDS experiments in the whole wavelength range. Measurements of the CRDS absorption spectrum of acetylene (C₂H₂) near 571 nm demonstrate that the mode-matching optimization improves the sensitivity of pulsed CRDS. Received: 22 October 2001 / Revised version: 16 January 2002 / Published online: 14 March 2002     

Detection of C2H2 and HCl using mid-infrared degenerate four-wave mixing with stable beam alignment: towards practical in situ sensing of trace molecular species

A stable and convenient optical system to realize the forward phase-matching geometry for degenerate four-wave mixing (DFWM) is demonstrated in the mid-infrared spectral region by measuring DFWM signals generated in acetylene (C₂H₂) and hydrogen chloride (HCl) molecules by probing the fundamental ro-vibrational transitions. IR laser pulses tunable from 2900 cm^?1 to 3350 cm^?1 with a 0.025 cm^?1 linewidth were obtained using a laser system composed of an injection seeded Nd:YAG laser, a dye laser, and a frequency mixing unit. At room temperature and atmospheric pressure, a detection limit of 35 ppm (～ 9.5×10¹⁴ molecules/cm³) for C₂H₂ was achieved in a gas flow of a C₂H₂/N₂ mixture by scanning the P(11) line of the (010(11)⁰)–(0000⁰0⁰) band. The detection limit of the HCl molecule was measured to be 25 ppm (～6.8×10¹⁴ molecules/cm³) in the same environment by probing the R(4) line. The dependences of signal intensities on molecular concentrations and laser pulse energies were demonstrated using C₂H₂ as the target species. The variations of the signal line shapes with changes in the buffer gas pressures and laser intensities were recorded and analyzed. The experimental setup demonstrated in this work facilitates the practical implementation of in situ, sensitive molecular species sensing with species-specific, spatial and temporal resolution in the spectral region of 2.7–3.3 μm (3000–3700 in cm^?1), where various molecular species important in combustion have absorption bands.     

Sensitive intracavity photoacoustic measurements with a CO2 waveguide laser

A photoacoustic intracavity configuration is presented; a resonant photoacoustic cell excited in its first longitudinal mode is placed inside the cavity of a CO₂ waveguide laser. Due to the high laser power and the sharp intracavity focus, saturation effects occur in the excitation and relaxation process of absorbing C₂H₄ molecules. A more optimal configuration is applied to measure the C₂H₄ emission of several Rumex species. A detection sensitivity of 6 ppt (parts per trillion) C₂H₄ is reached, equivalent to a minimal detectable absorption of 1.8×10^–10 cm^–1.     

Acetylene spectra with a tunable diode laser: (ν4 + ν5)0+−ν41fQ branches of 12C2H2 and 12C13CH2

The rotational structure of the Q branches of the (ν₄ + ν₅)⁰⁺?ν₄^1f bands of ¹²C₂H₂ and ¹²C¹³CH₂ at 13.7 μm has been observed in a natural sample of acetylene by using a tunable diode laser as a source in a high-resolution infrared grating spectrometer equipped with a precision grating drive. Altogether 23 lines from J = 6 to 28 for ¹²C₂H₂ and 15 lines from J = 6 to 20 for ¹²C¹³CH₂ have been identified. The observed full width at half maximum of the resolved lines of these Q branches is very close to the calculated Doppler width. Molecular constants ν₀ + B″, B′ ? B″ ? 2D″, D′ ? D″, and H′ ? H″ have been derived from the measured line positions of the rotational structure.     

Infrared laser spectroscopy of the CN free radical in a methane-nitrogen-hydrogen plasma

Many Pand R-branch lines of the fundamental bands of the ¹²C¹⁴N and ¹³C¹⁴N radicals in their ground (²Σ⁺) electronic state have been measured at high resolution by diode laser absorption spectroscopy. The radicals were generated in a 2.45 GHz planar microwave plasma in methane with varying proportions of N₂ and H₂. From a fit to the spectra the origins of the fundamental bands of the two isotopomers were determined to be 2042.42104(84) cm^?1 and 2000.084 70(30) cm^?1. The main product detected in the plasma by diode laser spectroscopy is HCN, with concentrations about three orders of magnitude higher than CN.