Feedback Cavity-enhanced Absorption Spectroscopy Research Articles

Dual laser based optical feedback cavity enhanced absorption spectroscopy by polarization division multiplexing

This article presents a dual laser-based optical feedback cavity-enhanced absorption spectroscopy technique using polarization division multiplexing. The technique involves two laser beams with orthogonal polarization that enter a V-shaped cavity from two injection directions. This allows for independent control of feedback phase and eliminates the need for an optical component to combine the beams. The two lasers are detected separately at the two ends of the cavity. Additionally, a differential process is proposed to mitigate residual interference between the two lasers. By employing this technique, simultaneous detection of CH4, CO2 and H2O is achieved, resulting in a detection sensitivity of approximately 2 × 10−11 cm−1. Despite using a moderate cavity finesse in our spectrometer, the minimal detectable concentration for the gases is comparable to that of superior gas analyzers using cavity ringdown spectroscopy (CRDS).

Comparison of optical feedback cavity enhanced absorption spectroscopy and gas chromatography for the measurement of the main components and impurities in biogas, landfill gas, biomethane and carbon dioxide streams

In this study, we evaluated the performances of a custom-built optical feedback cavity enhanced absorption spectroscopy (OFCEAS) instrument for the determination of the composition of energy gases, focusing on methane and carbon dioxide as main components, and carbon monoxide as impurities, in comparison with the well-established, validated, and traceable gas chromatographic method. A total of 115 real sample gases collected in biogas plants or landfills were analyzed using with both techniques over a period of 12 months. The comparison of the techniques showed that the virtual model which allows the measurement, needs to be optimized using real samples of varied compositions. The OFCEAS measurement technique was found to be capable of measuring both the main components and a trace component in different matrices; to within a 2% measurement uncertainty (higher than the gas chromatograph/thermal conductivity detector (GC/TCD) method). The OFCEAS method exhibits a very fast response, does not require daily calibration, and can be implemented online. The agreements between the OFCEAS technique and the GC/TCD method show that the drift of the OFCEAS instruments remains acceptable in the long term as long as no change is made to the virtual model. Matrix effects were observed, and those need to be taken into consideration when analyzing different types of samples.

Fiber pigtailed DFB laser-based optical feedback cavity enhanced absorption spectroscopy with a fiber-coupled EOM for phase correction.

A novel technique for performing fiber pigtailed DFB laser and linear Fabry-Pérot cavity based optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) is proposed. A fiber-coupled electro-optic modulator (f-EOM) with x-cut y-propagation LiNbO3 waveguide is employed, instead of PZT used in traditional OF-CEAS, to correct the feedback phase, which improves the compactness and applicability of OF-CEAS. Through the efficient and real-time control of the feedback phase by actively changing the input voltage of the f-EOM, a good long-term stability of the signal has been achieved. Consequently, a detection sensitivity down to 7.8×10-10 cm-1, better than the previous by PZT based OF-CEAS, has been achieved over the integration time of 200 s, even by use of a cavity with moderate finesse of 2850.

Interference suppression method in optical feedback-cavity enhanced absorption spectroscopy technology

In this paper, an efficient method of suppressing interference is presented in an optical feedback-cavity enhanced absorption spectroscopy (OF-CEAS) system. The Ariy function is used to analyze the interference signal in the transmission cavity mode signal. It is found that the interference signal in system originates from multiple reflections of the beam in the mirror, which is verified by replacing three kinds of cavity front mirrors with different thickness values. The result obtained by the Ariy function is used as a background signal, and the absorption spectrum signal can be obtained by making its difference from the absorption signal of the measured gas. This method effectively avoids the frequency error caused by the inability to measure the background signal and the absorption signal at the same time in the OF-CEAS system. Finally, the absorption characteristics of acetylene gas at 1.53 μm are measured. Based on the signal-to-noise ratio, the detection sensitivity of the system is evaluated to be 7.143 × 10<sup>–8</sup> (1<i>σ</i>). Experiments show that this method is effective in improving the detection sensitivity of OF-CEAS system.

Measurements of the water vapor continuum absorption by OFCEAS at 3.50 µm and 2.32 µm

Measurements of the water vapor absorption cross-sections at two spectral points of the 2.1 µm and 4.0 µm transparency windows are performed by optical feedback cavity enhanced absorption spectroscopy (OFCEAS).The self-continuum cross-section, CS, is measured for temperature values of 30 and 47°C (303 and 320 K) at the 2853 cm−1 spectral point, corresponding to the lowest opacity region of the 4.0 µm transparency window. The CS values derived from the pressure squared dependence of the self-continuum, are found consistent with previous CEAS measurements in the considered window but significantly smaller than measurements by Fourier transform spectroscopy (FTS). The CS temperature dependence is discussed in relation with FTS measurements at high temperature.Foreign-continuum cross-sections, CF, are newly obtained from OFCEAS spectra of moist air in flow regime at the 4302 cm−1 spectral point of the low energy edge of the 2.1 µm window. After subtraction of the monomer and self-continuum contributions, CF values are derived from the linear variation of the foreign-continuum absorption with the product of the water vapor and air partial pressures. The measurements were performed for temperature values of 34 and 47°C (307 and 320 K) and no significant temperature dependency was observed. The present CF value at 4302 cm−1 is gathered with previous CEAS measurements at seven spectral points of the 2.1 µm window. This consistent set of CF values is used to derive from a polynomial fit, the empirical frequency dependence of CF(ν) over the 4250-5000 cm−1 range. Overall, the semi-empirical MT_CKD_3.5 values of CF are significantly underestimated in the centre of the considered window.

Optical feedback linear cavity enhanced absorption spectroscopy.

A simple and universal technique for performing optical feedback cavity enhanced absorption spectroscopy with a linear Fabry-Pérot cavity is presented. We demonstrate through both theoretical analysis and experiment that a diode laser can be sequentially stabilized to a series of cavity modes without any influence from the direct reflection if the feedback phase is appropriately controlled. With robust handling of the feedback phase and help from balanced detection, a detection limit of 1.3 × 10-9 cm-1 was achieved in an integration time of 30 s. The spectrometer performance enabled precision monitoring of atmospheric methane (CH4) concentrations over a time period of 72 h.

Application of Near-Infrared Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) to the Detection of Ammonia in Exhaled Human Breath.

The qualitative and quantitative analysis to trace gas in exhaled human breath has become a promising technique in biomedical applications such as disease diagnosis and health status monitoring. This paper describes an application of a high spectral resolution optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) for ammonia detection in exhaled human breath, and the main interference of gases such as CO2 and H2O are approximately eliminated at the same time. With appropriate optical feedback, a fibered distributed feedback (DFB) diode laser emitting at 1531.6 nm is locked to the resonance of a V-shaped cavity with a free spectral range (FSR) of 300 MHz and a finesse of 14,610. A minimum detectable absorption coefficient of αmin = 2.3 × 10−9 cm−1 is achieved in a single scan within 5 s, yielding a detection limit of 17 ppb for NH3 in breath gas at low pressure, and this stable system allows the detection limit down to 4.5 ppb when the spectra to be averaged over 16 laser scans. Different from typical CEAS with a static cavity, which is limited by the FSR in frequency space, the attainable spectral resolution of our experimental setup can be up to 0.002 cm−1 owing to the simultaneous laser frequency tuning and cavity dither. Hence, the absorption line profile is more accurate, which is most suitable for low-pressure trace gas detection. This work has great potential for accurate selectivity and high sensitivity applications in human breath analysis and atmosphere sciences.

Collision-induced absorption and electric quadrupole transitions of N2 by OF-CEAS near 4.0 µm and CRDS near 2.1 µm

Highly sensitive cavity enhanced laser techniques are used for accurate measurements of the absorption of dinitrogen (N2) at two spectral points of the fundamental and first overtone bands. Spectra of pure N2 and of a N2/O2 mixture were recorded near 2491 and 4720 cm−1 by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) and Cavity Ring Down Spectroscopy (CRDS), respectively. The noise level of the OF-CEAS and CRDS spectra corresponds to a minimum detectable absorption, αmin, of about 10-9 and 10-11 cm−1, respectively. Measurements involve both the N2 collision-induced absorption (CIA) bands and the very weak 2–0 S(10) and S(11) electric quadrupolar transitions. Accurate CIA binary coefficients, BN2−N2 and BN2−O2, are derived from the pressure-squared dependence of the baseline level of the spectra recorded at pressures below 1 atm. These CIA values are compared to previous determinations from high pressure spectra (up to 90 atm) and to recent classical molecular dynamics simulations (CMDS). An overall good agreement is obtained although a significant deviation is observed for the first overtone CIA near 4720 cm−1. The line position and intensity (on the order of a few 10−30 cm/molecule) of the 2–0 S(10) and S(11) electric quadrupolar transitions are determined experimentally for the first time and compared to theoretical values provided in the HITRAN2016 database.

Nitric Oxide Analysis Down to ppt Levels by Optical-Feedback Cavity-Enhanced Absorption Spectroscopy.

Monitoring nitric oxide at the trace level is required in a large range of applications. We report on a trace gas analyzer optimized for nitric oxide measurements by Optical Feedback Cavity Enhanced Absorption Spectroscopy with an interband cascade laser at 5.3 µm. The short response time of the instrument allows for reaching the level of 50 ppt in only 180 ms. Its stability enables averaging up to 12 min to reach a detection limit of 0.9 ppt. Absolute concentration calibration requires to account for the optical saturation effect that results from the intense absorption line intensity addressed here, in the mid infrared region, in contrast to instruments that are operating in the near infrared region.

Accurate measurements and temperature dependence of the water vapor self-continuum absorption in the 2.1 μm atmospheric window

In spite of its importance for the evaluation of the Earth radiative budget, thus for climate change, very few measurements of the water vapor continuum are available in the near infrared atmospheric windows especially at temperature conditions relevant for our atmosphere. In addition, as a result of the difficulty to measure weak broadband absorption signals, the few available measurements show large disagreements. We report here accurate measurements of the water vapor self-continuum absorption in the 2.1 μm window by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) for two spectral points located at the low energy edge and at the center of the 2.1 μm transparency window, at 4302 and 4723 cm(-1), respectively. Self-continuum cross sections, CS, were retrieved with a few % relative uncertainty, from the quadratic dependence of the spectrum base line level measured as a function of water vapor pressure, between 0 and 16 Torr. At 296 K, the CS value at 4302 cm(-1) is found 40% higher than predicted by the MT_CKD V2.5 model, while at 4723 cm(-1), our value is 5 times larger than the MT_CKD value. On the other hand, these OF-CEAS CS values are significantly smaller than recent measurements by Fourier transform spectroscopy at room temperature. The temperature dependence of the self-continuum cross sections was also investigated for temperatures between 296 K and 323 K (23-50 °C). The derived temperature variation is found to be similar to that derived from previous Fourier transform spectrometer (FTS) measurements performed at higher temperatures, between 350 K and 472 K. The whole set of measurements spanning the 296-472 K temperature range follows a simple exponential law in 1/T with a slope close to the dissociation energy of the water dimer, D0 ≈ 1100 cm(-1).

Optical feedback cavity enhanced absorption spectroscopy: effective adjustment of the feedback-phase

Optical-feedback cavity enhanced absorption spectroscopy (OF-CEAS) is a very sensitive technique for the detection of trace amounts of gaseous absorbers. The most crucial parameter in an OF-CEAS setup is the optical phase of the light fed back into the laser source, which is usually controlled by the position of a piezo driven mirror. Various approaches for the analysis of the cavity transmitted light with respect to feedback-phase are presented, and tested on simulated phase and frequency dependent cavity transmission. Finally, we present the performance of a digital signal processor based regulator—employing one of these approaches—in a real OF-CEAS experiment. The results of the simulation show that several algorithms are well suited for the task of control signal generation. They confirm also that with the presented approach, a mode by mode correction of the feedback-phase is possible. Consequently, a regulatory bandwidth of 37 Hz was achieved. This maximum control frequency was limited by the piezo system.

Optical feedback cavity enhanced absorption spectroscopy with diode lasers

An optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) apparatus has been developed, using a distributed feedback diode laser centred close to a wavelength of 1596 nm. Light from a V-shaped optical cavity is allowed to feed back to the laser diode, injection seeding the laser at the cavity mode frequencies, and bringing about linewidth narrowing and frequency locking to the cavity resonance. The OF-CEAS technique's expediency and sensitivity have been demonstrated, first on measurements of aerial water absorptions close to 6260.8 cm(-)(1), and then on two carbon dioxide absorptions close to 6261.7 cm(-)(1) which may be used to measure the concentrations of (13)CO(2) and (12)CO(2) isotopologues in a CO(2) sample. Employing a cavity of moderate finesse (F approximately 3500), an acquisition time-reduced minimum detectable absorption coefficient of 5.8 x 10(-)(9) cm(-)(1) s(1/2) has been determined.

Extensive characterization of the optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) technique: ringdown-time calibration of the absorption scale

The paper describes in detail an implementation of the highly sensitive optical feedback cavity enhanced absorption spectroscopic (OF-CEAS) technique. A Brewster-angle cavity has been used to enable resonant optical feedback injection. For the first time, an OF-CEAS-adapted processing allowing calibration of the cavity-enhanced spectra in absolute absorption units has been experimentally verified through the combination of ringdown-time and mode-amplitude measurements. The experimental system performance has been evaluated by measuring very weak oxygen B-band transitions near 688 nm. Absorption spectra covering the range of 1.35 cm-1 have been averaged over the 5-s measurement time at the minimum detectable loss of 3×10-10 /cm within the measurement dynamic range greater than four orders of magnitude. A detailed signal-to-noise ratio analysis estimates the measurement spectral precision as being 3 MHz (short time scale averaging).

Optical–feedback cavity–enhanced absorption: a compact spectrometer for real–time measurement of atmospheric methane

We report on the application of the new technique of optical–feedback cavity–enhanced absorption spectroscopy to the real–time quantitative measurement of tropospheric methane traces from an airplane using a compact, low cost instrument based on a telecommunication–type diode laser operating close to room temperature. Methane concentration is obtained by fitting the absorption line centered at 1658.96 nm (6026.23 cm-1) which belongs to the first overtone transition of the CH stretch vibration. The measurement rate is about 30 Hz, but the response time is limited to about 0.3 s by the gas flow in the measurement cell. The instrument provides the absolute ambient methane concentration accurate to ±1% (±20 ppb) without need for a periodic calibration. This is demonstrated by a hands–off comparison with a self–calibrating chromatographic setup during 10 days. The observed measurement stability can be extrapolated to much longer time periods. With respect to the short–term performance (minutes) fast concentration changes at the level of 1 ppb can be detected, and we believe this performance can be extended to the long term. Finally, a laboratory comparison with a lead–salt mid–infrared diode laser multipass spectrometer (operating close to 3028 cm-1 at liquid nitrogen temperature) demonstrates a similar performance.