Cavity-enhanced Absorption Research Articles

Cavity-enhanced saturated absorption spectroscopy of the (30012) - (00001) band of 12C16O2.

The (30012) ← (00001) band of 12C16O2 in the 1.6 μm region is used for satellite observation of carbon dioxide in the Earth's atmosphere. Here, we report a Doppler-free spectroscopy study of this band with comb-locked wavelength-modulated cavity-enhanced absorption spectroscopy. Frequencies of 18 transitions with the rotational quantum numbers up to 42 were determined with sub-kHz accuracy, corresponding to a fractional uncertainty at the 10-12 level. With this precision, we revealed an anomalous decrease of the line shift and an increase of the line broadening for the Lamb dips of CO2 in the low-pressure regime compared to values obtained from Doppler-limited spectra at higher pressures.

Sideband cavity absorption readout (SideCAR) with a robust frequency lock.

We present a simple, continuous, cavity-enhanced optical absorption measurement technique based on high-bandwidth Pound-Drever-Hall (PDH) sideband locking. The technique provides a resonant amplitude quadrature readout that can be mapped onto the cavity's internal loss rate and is naturally compatible with weak probe beams. With a proof-of-concept 5-cm-long Fabry-Perot cavity, we measure an absorption sensitivity ∼10-10cm-1/Hz from 30 kHz to 1MHz, and a minimum value of 6.6×10-11cm-1/Hz at 100 kHz, with 38 µW collected from the cavity's circulating power.

Absolute determination of chemical kinetic rate constants by optical tracking the reaction on the second timescale using cavity-enhanced absorption spectroscopy.

We report a new spectroscopic platform coupled to an atmospheric simulation chamber for the direct determination of chemical rate constants with high accuracy at a second time-scale resolution. These developed analytical instruments consist of an incoherent broadband cavity enhanced absorption spectrometer using a red light emitting diode (LED) emitting at ∼662 nm (LED-IBBCEAS) associated with a multipass cell direct absorption spectrometer (MPC-DAS) coupled to an external cavity quantum cascade laser (EC-QCL) operating in the mid-infrared region at approximately 8 μm (EC-QCL-MPC-DAS). Spectrometers were employed to investigate the NO3-initiated oxidation of four selected volatile organic compounds (VOCs) for the determination of the corresponding rate constants with a dynamic range of 5 orders of magnitude (from 10-11 to 10-16 cm3 molecule-1 s-1). Rate constants of (6.5 ± 0.5) × 10-15, (7.0 ± 0.4) × 10-13, and (5.8 ± 0.5) × 10-16 cm3 molecule-1 s-1 for propanal, isoprene and formaldehyde, respectively, were directly determined by fitting the measured concentration-time profiles of NO3 and VOCs (measured using a proton transfer reaction time-of-flight mass spectrometer, PTR-ToF-MS) to chemical models based on the FACSIMILE simulation software (version 4.2.50) at 760 torr and 293 ± 2 K. The obtained rate constants are in good agreement with the most recent recommendations of the IUPAC (International Union of Pure and Applied Chemistry). In addition, a rate constant of (2.60 ± 0.30) × 10-11 cm3 molecule-1 s-1 for the oxidation of 2-methoxyphenol by NO3 radicals was first determined using the absolute kinetic method. Compared to the mostly used indirect relative rate method, the rate constant uncertainty is reduced from ∼20% to ∼12%. The results demonstrated the high potential of using modern spectroscopic techniques to directly determine the chemical reaction rate constants.

Trace Measurements of Ethylene Oxide Using Cavity-enhanced Absorption Spectrometry near 3066 cm–1

Ethylene oxide (EtO) is a key carcinogen that is widely used in chemical manufacturing and biotechnology industries. Recent work has suggested that permissible exposure limits for EtO be reduced from 1–5 ppm to sub-ppb levels. Such new standards will require new methodologies that are capable of measuring EtO with the requisite precision. In this paper, we demonstrate a new analyzer based on cavity-enhanced absorption spectrometry that utilizes a broad EtO absorption feature near 3066 cm-1. A fit function is developed that includes water, methane, and EtO absorbances and accounts for absorption both inside and outside the cavity. A methane standard is used to determine the cavity gain factor, and the EtO absorbance spectrum is empirically determined. The final system shows excellent linearity from 0 – 909 ppb EtO (R2 ~0.9999) with a measurement precision of better than ±1 ppb (1s, 60 seconds) that improved to ±0.5 ppb (1s, 15 minutes). Deliberate ambient EtO releases demonstrate the instrument’s utility in rapidly detecting hazardous conditions. Further work will include improving the measurement precision and directly comparing the system to EPA Method TO-15.

Mid-infrared cavity-enhanced absorption sensor for ppb-level N2O detection using an injection-current-modulated quantum cascade laser

We report a mid-infrared cavity-enhanced absorption sensor for atmospheric nitrous oxide (N2O) detection using a continuous-wave distributed-feedback quantum cascade laser (DFB-QCL) at 4.5 µm. The QCL beam is coupled to a short (78 mm) Fabry-Pérot (F-P) optical cavity, which consists of two plano-concave dielectric mirrors with a reflectivity of 99.84%. The Pound-Drever-Hall technique is used to lock the QCL to the optical cavity by directly modulating the injection current of the QCL at 4 MHz. Our mid-infrared gas sensor achieves a minimum detection limit of 0.32 ppb at 50 s integration time. We demonstrate a proof-of-concept absorption spectral measurement of ambient air. Our study provides a promising way of developing compact and sensitive gas sensors for environmental monitoring.

Glyoxal in a nocturnal atmosphere measured by incoherent broadband cavity-enhanced absorption spectroscopy

Glyoxal in a nocturnal atmosphere measured by incoherent broadband cavity-enhanced absorption spectroscopy

Frequency-based dispersion Lamb-dip spectroscopy in a high finesse optical cavity.

Frequency-based cavity mode-dispersion spectroscopy (CMDS), previously applied for Doppler-limited molecular spectroscopy, is now employed for the first time for saturation spectroscopy. Comparison with two intensity-based, cavity-enhanced absorption spectroscopy techniques, i.e. cavity mode-width spectroscopy (CMWS) and the well-established cavity ring-down spectroscopy (CRDS), shows the predominance of the CMDS. The method enables measurements in broader pressure range and shows high immunity of the Lamb dip position to the incomplete model of saturated cavity mode shape. Frequencies of transitions from the second overtone of CO are determined with standard uncertainty below 500 Hz which corresponds to relative uncertainty below 3 × 10-12. The pressure shift of the Lamb dips, which has not been detected for these transitions in available literature data, is observed.

Study of a Mode Separation Due to Polarization Existing in a Cavity-Enhanced Absorption Spectroscopy.

A special phenomenon of resonance mode separation is observed during the study of a high sensitivity folded-cavity enhanced absorption spectroscopy for the measurement of trace gases. The phenomenon affects the measurement of gas absorption spectrum in the cavity. This resonant mode separation phenomenon of the resonant cavity is different from the resonant modes previously observed in linear-cavity enhanced absorption spectroscopy systems. To explore the mechanism of this phenomenon, a series of hypotheses are proposed. The most likely reason among these hypotheses is based on the different reflectance properties of the plane mirror at the fold of the cavity for S-polarized light and P-polarized light. Based on the matrix calculation method, the different reflectance and phase shift of the plane mirror for S-polarized light and P-polarized light are analyzed theoretically, and the results are in better agreement with the phenomena observed in the experiment. Finally, in order to eliminate the resonant mode separation phenomenon, line polarizers were added. By improving the system, the cavity enhanced absorption spectrum of residual water vapor in the cavity was successfully measured, and a minimum detectable absorption coefficient of αmin = 7.6 × 10−9 cm−1 can be obtained in a single laser scan of 10 s.

Nitrous acid (HONO) emission factors for diesel vehicles determined using a chassis dynamometer

Vehicle exhaust gases are important sources of nitrous acid (HONO). In this study, HONO in diesel vehicle exhaust was measured by incoherent broadband cavity-enhanced absorption spectroscopy using a chassis dynamometer system. The mean HONO concentrations in exhaust gases emitted by passenger cars and light-duty trucks were high when the after treatment devices were not fully working during the warming up period. The HONO/NOx ratio is a good index of HONO formation. The HONO/NOx ratios were 9.7 × 10−3–18.1 × 10−3, and were higher than what we found in a previous study. The estimated HONO emission factors were 7.71–64.70 mg (kg fuel)−1, and were lower than were found in previous studies. The results indicated that the frequency particulate matter is removed from a diesel particle filter affects the HONO concentration in the emitted gases and the HONO emission factor.

Polarization impedance measurement cavity enhanced laser absorption spectroscopy.

We present a theoretical overview and experimental demonstration of a continuous-wave, cavity-enhanced optical absorption spectrometry method to detect molecular gas. This technique utilizes the two non-degenerate polarization modes of a birefringent cavity to obtain a zero background readout of the intra-cavity absorption. We use a double-pass equilateral triangle optical cavity design with additional feed-forward frequency noise correction to measure the R14e absorption line in the 30012←00001 band of CO2 at 1572.655 nm. We demonstrate a shot noise equivalent absorption of 3 × 10-13 cm-1 Hz-1/2.

Intercomparison of IBBCEAS, NitroMAC and FTIR analyses for HONO, NO<sub>2</sub> and CH<sub>2</sub>O measurements during the reaction of NO<sub>2</sub> with H<sub>2</sub>O vapour in the simulation chamber CESAM

Abstract. We report on applications of the ultraviolet-light-emitting-diode-based incoherent broadband cavity-enhanced absorption spectroscopy (UV-LED-IBBCEAS) technique for optical monitoring of HONO, NO2 and CH2O in a simulation chamber. Performance intercomparison of UV-LED-IBBCEAS with a wet chemistry-based NitroMAC sensor and a Fourier transform infrared (FTIR) spectrometer has been carried out on real-time simultaneous measurement of HONO, NO2 and CH2O concentrations during the reaction of NO2 with H2O vapour in CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber). The 1σ (signal-to-noise ratio (SNR) = 1) detection limits of 112 pptv for NO2, 56 pptv for HONO and 41 ppbv for CH2O over 120 s were found for the UV-LED-IBBCEAS measurement. On the contrary to many set-ups where cavities are installed outside the simulation chamber, we describe here an original in situ permanent installation. The intercomparison results demonstrate that IBBCEAS is a very well suitable technique for in situ simultaneous measurements of multiple chemically reactive species with high sensitivity and high precision even if the absorption bands of these species are overlapped. It offers excellent capacity for non-invasive optical monitoring of chemical reactions without any perturbation. For the application to simulation chambers, it has the advantage to provide a spatially integrated measurement across the reactor and hence to avoid point-sampling-related artefacts.

Frequency stabilization of a quantum cascade laser by weak resonant feedback from a Fabry-Perot cavity.

Frequency-stabilized mid-infrared lasers are valuable tools for precision molecular spectroscopy. However, their implementation remains limited by complicated stabilization schemes. Here we achieve optical self-locking of a quantum cascade laser to the resonant leak-out field of a highly mode-matched two-mirror cavity. The result is a simple approach to achieving stable frequencies from high-powered mid-infrared lasers. For short time scales (<0.1ms), we report a linewidth reduction factor of 3×10-6 to a linewidth of 12 Hz. Furthermore, we demonstrate two-photon cavity-enhanced absorption spectroscopy of an N2O overtone transition near a wavelength of 4.53 µm.

Monitoring of endogenous nitric oxide exhaled by pig lungs during ex-vivo lung perfusion

In the context of organ shortage for transplantation, new criteria for better organ evaluation should be investigated. Ex-vivo lung perfusion (EVLP) allows extra-corporal lung re-conditioning and evaluation, under controlled parameters of the organ reperfusion and mechanical ventilation. This work reports on the interest of exhaled gas analysis during the EVLP procedure. After a 1 h cold ischemia, the endogenous gas production by an isolated lung of nitric oxide and carbon monoxide is simultaneously monitored in real time. The exhaled gas is analysed with two very sensitive and selective laser spectrometers developed upon the technique of optical-feedback cavity-enhanced absorption spectroscopy. Exhaled gas concentration measured for an ex-vivo lung is compared to the corresponding production by the whole living pig, measured before euthanasia. On-line measurements of the fraction of nitric oxide in exhaled gas (FENO) in isolated lungs are reported here for the first time, allowing to resolve the respiratory cycles. In this study, performed on 9 animals, FENO by isolated lungs range from 3.3 to 10.6 ppb with a median value of 4.4 ppb. Pairing ex-vivo lung and pig measurements allows to demonstrate a systematic increase of FENO in the ex-vivo lung as compared to the living animal, by a factor of 3 ± 1.2. Measurements of the fraction of carbon monoxide in exhaled gas (FECO) confirm levels recorded during previous studies driven to evaluate FECO as a potential marker of ischemia reperfusion injuries. FECO production by ex-vivo lungs ranges from 0.31 to 2.3 ppm with a median value of 0.8 ppm. As expected, these FECO values are lower than the production by the corresponding whole pig body, by a factor of 6.9 ± 2.7.

Development of an off-axis cavity-enhanced absorption spectroscopy system with open-path configuration for gas sensing

A compact off-axis cavity-enhanced absorption spectroscopy system with open-path configuration is developed by using a narrow-band CW laser as light source. The open-path configuration can avoid absorption cell wall losses and sampling induced artifacts. The absolute reflectivity of the cavity mirrors is determined to be 99.8% by a simple and effective method, and an effective optical path length of 315.66 m is realized based on a 45 cm long high optical finesse open-path cavity. In order to confirm the ability of the system, a series of measurements of CO2 concentrations are performed by targeting a CO2 line at 5001.49 cm−1. According to the time series measurements of ambient CO2, a sensitivity is 7.819 ppm while the averaging time is 1 s, and an optimum sensitivity of 193 ppb can be achieved with an averaging time of 300 s. And two 24 h’ field continuous measurements are performed to demonstrate the stability of the reported system.

Portable, low cost and sensitive cavity enhanced absorption (CEA) detection.

Absorption is a widely used technique for a range of different applications. It has lower sensitivity than many other techniques such as fluorescence which has 100 to 1000 times higher sensitivity than absorption. Optical cavity approaches have been developed where the light passes back and forth, within the sample, between two high reflectivity mirrors to increase the pathlength and sensitivity. These approaches have not yet, however, been widely used for analytical applications and for point-of-care diagnostics. Here we show a portable cavity enhanced absorption (CEA) spectrometer and a low cost point-of-care (POC) reader with CEA detection with mechanical elements fabricated using 3D printing. The CEA spectrometer can be used in both single pass and multi-pass cavity enhanced mode to provide measurements in the visible region that are very sensitive and over a wide dynamic range. The CEA mode was shown for Rhodamine B dye to increase the pathlength 57.8 fold over single pass measurements and an LOD of 7.1 × 10-11 M. The cost of the CEA POC reader was reduced by use of narrow band LEDs, photodiodes and removal of fibre optic coupling and with a 14 fold increase in the pathlength over conventional single pass microplate readers. The CEA POC reader was demonstrated for immunoassay of C-Reactive Protein (CRP), Procalcitonin (PCT) and Interleukin 6 (IL-6), towards a three biomarker panel to aid the diagnosis of sepsis. The CEA POC reader can be integrated with wireless connectivity for cloud based data sharing. We show here the potential for the wider use of optical cavity approaches where there is a need for sensitive absorption measurements and also for low cost point-of-care diagnostics.

NO&lt;sub&gt;2&lt;/sub&gt; gas detection based on standard sample regression algorithm and cavity enhanced spectroscopy

Cavity-enhanced absorption spectroscopy is a highly sensitive trace gas measurement technology, and the algorithm for retrieving gas concentrations is critical. The absorption cross-section is traditionally used to retrieve the concentration. However, the absorption cross-section used in the fitting process is affected not only by the response function of the instrument and the light source, but also by temperature and pressure. The uncertainty of the absorption cross-section will influence the accuracy of the result. Therefore, in order to eliminate the measurement error introduced by the uncertainty of the absorption cross-section and the instrument response function, a concentration regression algorithm based on the absorption spectrum of the standard sample is proposed. The process of concentration inversion is optimized. The absorption spectrum of standard gas is used to fit the unknown spectrum. In order to verify the effectiveness of the algorithm, the incoherent cavity enhanced absorption spectroscopy (IBBCEAS) system based on a blue light-emitting diode (LED) operating at 440 nm is established to analyze the absorption spectrum of NO&lt;sub&gt;2&lt;/sub&gt;; and the fitting effect, measurement accuracy and stability are compared with the counter parts from the traditional absorption cross-section method. In the experiment, the measured reflectance of the cavity mirror is 99.915%. Compared with the conventional absorption cross-section regression algorithm, the standard sample regression algorithm proposed in this paper shows good results, in which the measurement accuracy is increased by about quadruple. The Allan deviation shows that a detection limit of 5.3 ppb can be achieved at an integration time of 360 s. Finally, the performance of the experimental system is evaluated by measuring the NO&lt;sub&gt;2&lt;/sub&gt; with different concentrations prepared by standard samples. The result shows good agreement with the theoretical value, which indicates that the improved spectral analysis algorithm is feasible and reliable for gas detection. This method can be used not only to measure NO&lt;sub&gt;2&lt;/sub&gt;, but also to detect other gases, which shows great potential applications in monitoring the industrial emissions, atmospheric chemistry and exhaled breath analysis.

A dual-channel incoherent broadband cavity-enhanced absorption spectrometer for sensitive atmospheric NOx measurements.

We describe and characterize a dual-channel incoherent broadband cavity-enhanced absorption spectrometer (DC-IBBCEAS) for the sensitive measurements of NOx: the sum of nitrogen monoxide (NO) and nitrogen dioxide (NO2) in the atmosphere. The instrument employs two 1 m long optical cavities, with air being extracted from a common sampling line. The first channel (cavity-1) spans 340-380 nm and the second channel (cavity-2) spans the 405-460 nm spectral range, both measuring NO2 simultaneously. High absorption cross-sections of NO2 in both channels are effectively utilized for its sensitive quantification. NO is quantified by titrating it with ozone to NO2 in channel-2, where the difference of NO2 measured from that in channel-1 corresponds to the NO concentration in the sampled air. The instrument offers 1-ppb detection sensitivity for both NO and NO2 with a maximum possible uncertainty of ∼9%. The use of close yet different spectral regions in the two channels readily extended measurements to a broader range without compromising its sensitivity to NOx quantification. This would extend the DC-IBBCEAS applicability to simultaneously monitor interfering species with significant absorption cross-sections in the region in either channel, such as glyoxal (CHOCHO), methylglyoxal (CH3COCHO), and nitrous acid (HONO).

Near-infrared optical-feedback linear cavity-enhanced absorption spectroscopy

Near-infrared optical-feedback linear cavity-enhanced absorption spectroscopy

A cavity-enhanced ultraviolet absorption instrument for high-precision, fast-time-response ozone measurements

Abstract. The NASA Rapid Ozone Experiment (ROZE) is a broadband cavity-enhanced UV (ultraviolet) absorption instrument for the detection of in situ ozone (O3). ROZE uses an incoherent LED (light-emitting diode) light source coupled to a high-finesse optical cavity to achieve an effective pathlength of ∼ 104 m. Due to its high sensitivity and small optical cell volume, ROZE demonstrates a 1σ precision of 80 pptv (parts per trillion by volume) in 0.1 s and 31 pptv in a 1 s integration time, as well as an e-fold time response of 50 ms. ROZE can be operated in a range of field environments, including low- and high-altitude research aircraft, and is particularly suited to O3 vertical-flux measurements using the eddy covariance technique. ROZE was successfully integrated aboard the NASA DC-8 aircraft during July–September 2019 and validated against a well-established chemiluminescence measurement of O3. A flight within the marine boundary layer also demonstrated flux measurement capabilities, and we observed a mean O3 deposition velocity of 0.029 ± 0.005 cm s−1 to the ocean surface. The performance characteristics detailed below make ROZE a robust, versatile instrument for field measurements of O3.

Experimental observation of the ν2+4ν3 bands of HD16O and HD18O between 14975 and 15275 cm−1

The ν2+4ν3 combination bands of HD16O and HD18O were measured using Fourier transform-incoherent broadband cavity-enhanced absorption spectroscopy (FT-IBBCEAS) with a spectral resolution of 0.08 cm−1. The ro-vibrational lines of these bands were assigned through comparison with ab-initio molecular lines from the Tomsk variational calculation database (http://spectra.iao.ru/). For HD18O and HD16O in total 114 and 141 strong lines were assigned in the region between 14975.3 cm−1 and 15243.3 cm−1 and between 14998.5 cm−1 and 15274.7 cm−1, respectively. While the very satisfactory agreement of line intensities was used for line assignments, a systematic average discrepancy of ~0.305 cm−1 in line positions was identified between the measured lines of HD18O and the theoretically predicted lines from the Tomsk database. Similarly for HD16O, an approximate wavenumber difference of ~0.361 cm−1 was observed. The wavenumber accuracy of the Fourier transform cavity enhanced absorption spectrometer was confirmed on basis of concurrently measured H216O spectra in the region between 15254.2 cm−1 and 15376.9 cm−1 and corroborated the systematic shifts of the ab initio data. A few lines of the ν1+4ν2+2ν3 bands of HD16O and HD18O were also identified. The data are compared and discussed on basis of existing literature data.