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Cavity enhanced spectroscopy –
Recent developments and new challenges
Abstracts of presentations
Optical feedback CEAS in a ring cavity for the detection of )2 isopologues in air
Optical feedback cavity enhanced absorption spectroscopy (OF CEAS) has been demonstrated by coupling a distributed feedback diode laser to a ring cavity. Frequency selected light decaying from the ring cavity is retro-reflected, inducing a counter-propagating intracavity beam, and providing optical feedback to the laser. At specific laser-to-cavity distances, all cavity mode frequencies return to the diode laser with the same phase, allowing spectra to be accumulated across the range of frequencies of the current-tuned laser. OF CEAS has been used to measure very weak oxygen isotopologue (16O18O and 16O17O) absorptions in ambient air at wavelengths near 762 nm using the electric-dipole forbidden O2 A-band. A bandwidth reduced minimum detectable absorption coefficient of 2.2 × 10-9 cm-1 Hz-1/2 is demonstrated.
Cavity ring down measurements of the line parameters of water rotation-vibration transitions
Low uncertainty line intensities and line shape parameters were measured for 15 rotation-vibration transitions of water vapor near 1.39 um. High-resolution absorption spectra of water vapor diluted in nitrogen were acquired using the frequency-stabilized cavity ring-down spectrometer at the National Institute of Standards and Technology . The NIST primary standard humidity generator  was used to produce a stable and accurately known amount of water vapor in a nitrogen carrier gas stream. A combination of semiclassical line shape models, which take into account nitrogen-induced collisional broadening and shifting, as well as the collisional narrowing and the speed-dependence of collisional effects, with multi-spectrum fits of spectra measured for different gas pressures allowed us to determine a set of line shape parameters which were linear with pressure. The relative combined standard uncertainties of our reported line intensities are smaller than 0.4% in most cases. We also discuss the influence of the choice of line shape model on fitted line intensities and line shape parameters. Our experimental results are compared to experimental and theoretical data available in the literature. Agreement between our experimental intensity measurements and those derived by recent ab initio calculations of the dipole moment surface of water vapor is within 1.5% .
 J. T. Hodges and D. Lisak, Appl. Phys. B 85, 375 (2006)
 J. T. Hodges and G. E. Scace, Micro 24, 59 (2006).
 D. Lisak, D. K. Havey, J. T. Hodges, Phys. Rev. A 79, 052507 (2009).
Cavity ring-down spectroscopy using waveguides
Cavities of acceptable finesse can be made very simply and inexpensively from strands of optical waveguide, such as standard telecom single-mode
fiber or specialized multi-mode optical fibers.
Light can be trapped into these fiber cavities either by using mirrors (such as fiber Bragg Gratings) or by connecting the two fiber ends to form a loop.
A fiber optic-based sensor head can then be used to introduce optical losses which depend on an external stimulus.
Using these concepts our group and others have demonstrated sensors for absorption, refractive index, pressure, temperature, strain, etc. In this
presentation I will mention some of these devices, but focus on (1) absorption measurements in small volumes of liquid and (2) on refractive
index measurements of polymer matrices that are used for solid phase microextraction.
Stefan Persijn and Adriaan van der Veen
Gas measurements using a versatile spectrometer and the comparison with Hitran and PNNL database
Dutch Metrology Institute,
A cavity ringdown spectrometer based on a widely tunable optical parametric oscillator (2.7-3.5 µm) is presented which is capable of detecting gases down to (sub) part per billion levels. Among others, ethane, methane, acetone, ethanol, and formaldehyde spectra were measured and compared with PNNL and Hitran database. The spectrometer is used for the IMERA+ project ‘Breath analysis as a diagnostic tool for early disease detection’
Helen Waechter, Klaus Bescherer, Jack A. Barnes, Christoph J. Duerr, Richard D. Oleschuk, Hans-Peter Loock
UV-wavelength fiber-loop CRDS for microfluids
In analytical separation systems, such as high performance liquid chromatography (HPLC), microfluidic devices or capillary electrophoresis (CE), compounds need to be detected with high sensitivity in small amounts of liquids. Fiber-loop cavity ring-down spectroscopy is an interesting alternative to standard UV-detectors as it combines the high sensitivity of cavity ring-down with very small sample volumes in the nanoliter range. The technique is an analogue to gas phase cavity ring-down spectroscopy, the cavity consists of an optical fiber loop with a small gap to insert a sample. Instead of monitoring the intensity decay in the cavity, the light intensity is modulated sinusoidally and the optical decay constants are obtained by comparing the phase difference of the light entering and exiting the loop.
Our setup operates at UV-wavelengths as many compounds have strong absorption features in this spectral region. It consists of a diode laser (wavelength 405 nm) and a low loss fiber loop with an interface to insert samples. An interface has been built that allows injecting laser light into the loop and intersects the fiber loop with a sample flow at the same time. The actually probed sample volume is then only 6.0 nL. Experiments with tartrazine (yellow food dye) solutions were performed by injecting samples with concentrations in the micromolar range. The detection limit was determined to be 0.11 cm-1, which corresponds to an concentration of 5 µM. Similar measurements were performed on a protein (myoglobin) and on a pharmaceutical drug provided by Eli Lilly. The results are in good agreement with each other.
Furthermore, very first measurements of microparticles (polystyrene spheres, diameter 5.17 µm) were performed and single particle detection was obtained. The size of the particles used in this work is similar to those of E. coli cells, thus applications of the system in the field of biotechnology and biochemistry can be considered.
Abstracts of Posters
Agata Cygan, Katarzyna Bielska, Daniel Lisak, Piotr Masłowski, Szymon Wójtewicz, Ryszard S. Trawiński, Roman Ciuryło
Instytut Fizyki, Uniwersytet Mikołaja Kopernika, Grudziądzka 5/7,
Line intensities and line shape coefficients measurements of oxygen B-band using frequency stabilized cavity-ring down spectroscopy technique
Cavity ring-down spectroscopy (CRDS) is a modern absorption spectroscopy technique which is used in high precision measurements of atoms and molecules spectra of trace gases. The Continuous-Wave CRDS (CW-CRDS) method is based on recording decays of single-mode, continuous-wave probe laser light injected into the ring-down cavity. Knowing decay time constant and using Beer-Lambert law we can determine absorption of the probe. This technique allows us to obtain spectra which are insensitive to the probe laser power. Moreover, locking the resonance cavity to the reference frequency of the He-Ne laser (stabilized to about 1 MHz) significantly increases its stability and makes spectra insensitive to the laser frequency fluctuations. High spectral resolution of the spectrometer was obtained thanks to the possibility of the He-Ne laser frequency tuning by the AOM and it is only limited by the stability of the He-Ne laser. In our studies the Frequency-Stabilized CW-CRDS (FS-CRDS) spectrometer is used to investigate line shapes and line intensities of several transitions in the oxygen B-band (λ = 689 nm). Till now we examined six transitions in the oxygen B-band at pressures up to 30 Torr of O2. Obtained signal-to-noise ratio of measured spectra exceeded 1000:1. Relative standard uncertainties of the measured line intensities were lower than 0.6% in most cases and collisional self-broadening coefficients were determined with relative uncertainties of order of a few percent. Moreover, it was found that collisional shift is smaller than our experimental errors in the case investigated here. Our experimental data are compared to data available in the literature (like the HITRAN database) and they might be used both in basic research and various applications. In particular in Earth’s atmosphere satellite monitoring line intensities should be determined with uncertainty of order of 0.3% to allow detection of variations in concentration of various trace gases. As one can see the FS-CW-CRDS spectrometer gives a chance to fulfill this condition.
Roberto Grilli1, Luca Ciaffoni2, Gus Hancock2, Robert Peverall2, Grant A. D. Ritchie2, Andrew. J. Orr-Ewing1
1School of Chemistry,
2Department of Chemistry, Physical and Theoretical Chemistry
Mid-IR ethene detection using difference frequency generation in a quasi-phase matched LiNbO3 waveguide
A periodically poled LiNbO3 waveguide has been used to produce up to 200 W of mid-infrared light around 3081 cm-1 with a wide tunability range of >35 cm-1. Two commercial near-infrared diode lasers at 1.064 m (pump) and 1.583 m (signal) are mixed in a nonlinear optic crystal to achieve difference frequency generation. The 48 mm long direct-bonded quasi-phase matched periodically poled LiNbO3 waveguide shows a conversion efficiency of 12.3 %/W. The radiation sits in an important window of the mid-IR spectral region, where a large number of fundamental vibrations of several hydrocarbons occur. Applications in trace gas detection have been demonstrated for ethene, using multi-pass absorption coupled with wavelength modulation spectroscopy, and cavity enhanced absorption spectroscopy with a lock-in detection scheme to reach minimum absorption coefficients of 8 × 10-9 and 1.6 × 10-8 cm-1Hz-1/2 (2), respectively. Preliminary results of phase-shift cavity ring-down spectroscopy (PS-CRDS) measurements will also be presented.(note 1 & 2)
1. L. Ciaffoni, R. Grilli, G. Hancock, A. J. Orr-Ewing, R. Peverall, G. A. D. Ritchie, “3.5-μm high-resolution gas sensing employing a LiNbO3 QPM-DFG waveguide module,” Appl. Phys. B 94, 517-525 (2009).
2. R. Grilli, L. Ciaffoni, G. Hancock, R. Peverall, G. A. D. Ritchie, A. J. Orr-Ewing, “Mid-IR ethene detection using difference frequency generation in a quasi-phase matched LiNbO3 waveguide”, submitted (July 2009).
Detection of Acetylene using Amplified Fibre-Loop Ringdown Spectroscopy
Cavity ring down spectroscopy (CRDS) has been used for over 20 years in high resolution and ultrasensitive gas absorption spectroscopy. Here we present an absorption detector for small sample volumes based on CRDS where the optical cavity is made from fibre optic waveguides and the light source is a continuous wave (cw) diode laser. The fibre-loop CRD system is designed for detection of gas through overtone absorption in the telecom region at 1300-1500 nm. This is done by increasing the ratio of desired loss (extinction caused by the sample), to undesirable loss (due to waveguides or solvents) through amplification of the ringdown signal using an erbium doped fibre amplifier (EDFA). This device has a 6 cm gas cell, a round trip time of 750 ns, and an effective path length of at least 4 m when used to measure the P(11), P(13) and P(15) peaks of acetylene at 1531.59 nm, 1532.83 nm and 1534.1 nm respectively. It is predicted through calculations that by replacing the sample cell with photonic crystal fiber the sample volume will be lowered and the effective path length will be increased.
Magnetic Field Effects Monitored using CEAS
CEAS has been employed to monitor magnetic field effects on the yield of a radical pair reaction occurring within the solution-phase.
Magnetic fields are known to affect the chemistry of molecules via the radical
pair mechanism. There is now increasing evidence that magnetoreception in birds and other animals occurs by means of the radical pair mechanism. In this context, cavity-based techniques may provide a highly sensitive method for monitoring magnetic field effects via measurement of radical absorption. As a demonstration of this, the CEAS-measured absorbance of thionine radicals has been monitored as a function of an applied magnetic field strength; from initial studies, it is clear that CEAS offers significant benefits in terms of miniaturisation of radical systems used to study magnetic field effects.
B. Rudolph, P.Zimmermann and S.Schippel
– optische Beschichtungen GmbH,
Characterization of customer specific low-loss optical components for the UV-VIS-NIR spectral range
LAYERTEC produces high quality optical components for
laser applications in the wavelength range from the VUV (157nm) to the NIR
(~4µm). Some of our most sophisticated products are low loss mirrors, i.e.
mirrors with a reflectivity R>99.99%, for the
The talk gives an overview about our coating processes and about the characterization procedures for superpolished substrates and low loss optics. The focus is set on the CRD setups at LAYERTEC which are used for process optimization as well as for quality control. Moreover, we introduce a broadband CRD setup on the basis of an optical parametric oscillator (OPO) which is currently under construction and which will enable us to characterize mirrors in the wavelength range from 300nm to 1700nm. Examples for CRD measurements of broadband mirrors and thin film polarizers will be presented.
S. Welzel1,3,*, P.B. Davies2, R. Engeln3, J. Röpcke1
Mid-infrared chemical sensing using resonant optical cavities and quantum cascade lasers
For many years the combination of cavity enhanced techniques with light sources in the infrared spectral range, i.e. the molecular fingerprint region, has been challenging, because of the lack of suitable radiation sources. The situation has changed with the invention of quantum cascade lasers (QCLs) which provide the necessary power and tunability.
Since the application of pulsed QCLs for cavity ring-down spectroscopy is hampered by the inherent frequency chirp of the lasers , an alternative approach employing continuous-wave (cw) QCLs for cavity enhanced absorption spectroscopy (CEAS) has been evaluated. Focussing on a straightforward experimental setup a linear, unstabilised and unlocked cavity has been employed.
The sensitivity achieved with a ~ 0.5 m cavity (0.3 l) and a thermoelectrically (TE) cooled cw QCL emitting at 7.66 µm (1300 cm‑1) was mainly limited by incomplete averaging over the residual cavity resonances. Increasing the cavity mirror separation to 1.30 m and a slight dither impressed on the QCL current led to a better signal-to-noise ratio. The Allan minimum was observed after about 40 s integration time and determined by drift effects in the system. Thus a sensitivity of 1 10‑8 cm‑1Hz‑1/2 was achieved being equivalent to a minimum detectable N2O number density of 1.5 108 cm‑3 (i.e. 12 ppb at 50 Pa).
Apart from potential field applications, where cryogen free spectrometers of small volume are highly desirable, this performance and the feasibility of working at low pressure conditions are especially of interest for in-situ plasma diagnostics. In this case sampling to an external multi pass cell is not an option. Additionally, the spatial resolution may be increased in comparison to conventional in‑situ multiple pass configurations. Alternatively, this approach facilitates high resolution studies of complex spectra of jet-cooled molecules or complexes .
Examples for sensing of CH4 and N2O in laboratory air (7.66 µm) and the discrimination between the active and passive zone in an Ar/O2(/N2) microwave plasma by means of NO absorption features (5.49 µm) respectively, will be presented. Finally, the potential for detecting complex spectra such as phosgene (Cl2CO) at 5.49 µm, by means of CEAS is demonstrated.
S. Welzel1,3,*, S. Glitsch1, P.B. Davies2, R. Engeln3, J. Röpcke1
Phosgene detection using cavity enhanced absorption spectroscopy based on quantum cascade lasers
Chemical sensing in the mid-infrared spectral range (3 ‑ 20 µm), where the fundamental absorption features of many molecular species are located, is of increasing interest for a variety of applications such as trace gas detection for environmental purposes, clinical diagnosis or workplace safety, plasma diagnostics or plasma process control. Cavity enhanced techniques can thereby increase the sensitivity due to increased absorption path lengths whilst the sampling volume remains relatively small and the residence time is kept short. On the other hand the gain in absorption path length enables measurements at low-pressure conditions to be performed which is of special interest in low-pressure plasma diagnostics or for detecting e.g. jet-cooled molecules or complexes in order to increase the spectral resolution and thus the selectivity .
A linear, unstabilised and unlocked cavity (~ 0.5 m, 0.3 l) formed by two mirrors of ~ 99.8 % reflectivity has been combined with a continuous wave (cw) quantum cascade laser (QCL) operated at room temperature to detect traces of phosgene (Cl2CO in N2). The fundamental bands are usually located at wavelengths longer than 11 µm (< 900 cm‑1)  and are therefore difficult to measure using the emission of commercially available cw QCLs. Thus the 1 band (centred around 1828 cm‑1 ) was selected and was observed in a spectral window between 1816 cm‑1 and 1829 cm‑1 by tuning the heat sink temperature of the QCL. Since the assignment of individual lines is difficult  effective absorption cross sections were determined and used for the detection of a few parts per million (ppm) at reduced background pressures (< 50 mbar). The detection limit was about 5 1011 cm‑3 (at 5 mbar).
 Y. Xu, X. Liu, Z. Su, R. M. Kulkarni, W.S. Tam, C. Kang, I.Leonov, L. D'Agostino Proc. SPIE 7222, 722208 (2009).
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