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Curr. Opt. Photon. 2023; 7(4): 457-462

Published online August 25, 2023 https://doi.org/10.3807/COPP.2023.7.4.457

Copyright © Optical Society of Korea.

Quantitative Measurement of Ethane Using Mid-infrared Cavity Ring-down Spectroscopy

Yonghee Kim , Byung Jae Chun, Lim Lee, Kwang-Hoon Ko, Seung-Kyu Park, Taek-Soo Kim, Hyunmin Park

Quantum Optics Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea

Corresponding author: *yonghee922@kaeri.re.kr, ORCID 0009-0005-8232-6278

Received: April 26, 2023; Revised: June 7, 2023; Accepted: June 7, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Quantitative measurement of trace ethane is important in environmental science and biomedical applications. For these applications, we typically require a few tens of part-per-trillion level measurement sensitivity. To measure trace-level ethane, we constructed a cavity ring-down spectroscopy setup in the 3.37 μm mid-infrared wavelength range, which is applicable to multi-species chemical analysis. We demonstrated that the detection limit of ethane is approximately 300 parts per trillion, and the measured concentration is in agreement with the amounts of the injected sample. We expect that these results can be applied to the chemical analysis of ethane and applications such as breath test equipment.

Keywords: Cavity ring-down spectroscopy, Ethane, Mid-infrared, Quantitative measurement

OCIS codes: (280.1545) Chemical analysis; (300.6340) Spectroscopy, infrared; (300.6390) Spectroscopy, molecular

Ethane (C2H6) is one of the most abundant hydrocarbon volatile organic compounds (VOCs) in the atmosphere, with average concentrations of approximately 1,000 parts per trillion (ppt) for the Northern Hemisphere [1]. Over the last few decades, many quantitative studies of trace ethane have been conducted in various fields such as environmental science [14] and biomedical applications [512].

Atmospheric ethane is one of the major greenhouse gases, and its origin lies in human activities such as natural gas leakage during production and transportation, biofuel use, and burning of biomass [2]. There is now growing interest in the concentration of ethane and its relation to climate change. Information on atmospheric ethane concentrations, such as regional distribution, seasonal oscillation, and annual changes, offers opportunities to better understand the earth system and its reflection of human activities [13]. Also, ethane is related to various chemical reactions in the atmosphere such as the formation of tropospheric ozone [4].

From a clinical point of view, exhaled human breath contains thousands of VOCs, and many studies have shown a correlation between concentrations of VOCs and various diseases [1317]. A breath test is a non-invasive diagnostic method of diseases by measuring specific molecules in the exhaled breath. It is known that ethane in human breath is produced by oxidation damage of cellular membranes, in a process known as oxidation stress [5]; This is related to a wide range of diseases such as lung cancer [6, 7], chronic obstructive pulmonary disease (COPD) [8], cystic fibrosis [9], asthma [10] and vitamin E deficiency [11]. Intensive studies have been conducted on ethane as a potential biomarker for diagnosing diseases and monitoring oxidation stress [512].

To measure the quantity of ethane and its delicate changes in air or human breath, measurement techniques with precision below several tens of ppt is required. Currently, gas chromatography coupled to mass spectrometry (GC-MS) is a standard method for analyzing the composition and concentration of trace-level samples. However, the GC-MS method has some disadvantages such as a complex sampling procedure, long measurement time and high cost. Therefore, in specific applications for which the chemical species and measurement range are limited, other measurement technologies such as electrochemical sensors [13, 15] and optical methods [16, 17] have been suggested. Among these, optical methods using molecular spectra have distinct advantages compared to GC-MS, such as high sensitivity, real-time measurement, simple sampling procedures, and relatively low cost. Thus, these methods are expected to be widely used in various fields such as clinical applications based on exhaled human breath analysis [16, 17].

In the case of ethane measurement, the mid-infrared wavelength region is generally used because an ethane molecule has strong absorption lines near the 3.3 μm wavelength range. To measure the trace levels of ethane, various spectroscopic techniques such as Fourier transform infrared (FTIR) spectroscopy [2], tunable diode laser absorption spectroscopy (TDLAS) using a multi-pass cell [5, 6, 12], off-axis integrated cavity output spectroscopy (OA-ICOS) [18, 19] and cavity ring-down spectroscopy (CRDS) [7, 20, 21] have been reported. These methods have shown detection limits of 70 to 500 ppt with 1 second measurement time and further demonstrate the potential of optical methods for clinical applications.

In this paper, we focus on the CRDS method to achieve several tens of ppt level sensitivity for the quantitative measurement of ethane molecules. We demonstrate ethane measurement using the 3.37 μm mid-infrared wavelength region (corresponding to 2,967.0 to 2,969.0 cm−1), which can be applicable to multi-species chemical analysis.

In section II, we briefly introduce the CRDS method, the wavelength selection for this study, and details of the experimental setup. In section III, we present experimental results on ethane measurement and discuss system sensitivity. Finally, we summarize results and discuss future work in section IV.

2.1. Cavity Ring-down Spectroscopy

CRDS is a highly sensitive quantitative measurement technique using the decay time (or ring-down time) of light in an optical cavity. This technique, which was first demonstrated at 1988 [22], has been widely used to analyze small amounts of optical absorption.

When a laser is spatially mode matched to a cavity mode and injected into a high finesse optical cavity with length L and two cavity mirrors with reflectivity R, laser light reflects back and forth inside the cavity and the corresponding cavity modes build up a free spectral range (FSR), c/2nL, where c is the speed of light, n is the index of refraction inside the cavity, and L is the cavity length. If the laser injection stops, the stored energy inside the cavity is exponentially decayed by cavity loss mechanisms. The ring-down time is defined as the time at which the intensity of the cavity mode decays to 1/e-times the initial values [17, 23].

For an empty cavity, cavity loss depends on only cavity mirror reflectivity R. In this case, ring-down time τ0 is expressed by

τ0=Lc1R

Cavity loss increases in the presence of target molecules inside the cavity due to the light absorption of target molecules. Then, ring-down time τ become wavelength dependent

τλ=Lc1R+Nσλl

where N is the density of target molecules, σ(λ) is the absorption cross-section of target molecules at a given wavelength λ, and l is the length of the sample inside the cavity, typically L for gas samples. Therefore, if we know the σ(λ), we can directly determine the concentration of target molecules from the difference between the reciprocal of the ring-down time in the presence of a sample inside the cavity and the ring-down time of the empty cavity, 1/τ(λ) − 1/τ0.

Notably, CRDS is very sensitive to injected laser beam alignment; However, the method is free from laser power fluctuation, in principle, because the ring-down time is independent from the laser power. This offers CRDS the unique capability of conducting trace-level sample measurements, which can lead to the development of robust commercial equipment.

2.2. Wavelength Selection for Ethane Measurement

Ethane has a strong absorption band in the mid-infrared wavelength region of 3.31–3.38 μm (approximately 2,960.0 to 3,020.0 cm−1) as shown in Fig. 1(a) [24, 25].

Figure 1.Ethane absorption spectrum in (a) the 3.3 μm mid-infrared wavelength range and (b) the wavelength region selected in this study. These data were extracted from [24, 25].

The largest two absorption lines of ethane are 2,983.28 cm−1 and 2,996.87 cm−1 (corresponding to 3.3520 μm and 3.3368 μm, respectively). Most previous studies [5, 12, 18, 19, 21] have used the wavelength region near these lines, depending on the light source and interference by methane or water molecules.

In this study, we selected the 3.37 μm (2,967.0 to 2,969.0 cm−1) wavelength region, as shown in Fig. 1(b), which is different from the typical region used for ethane measurement. The first candidate absorption lines were at approximately 2,967.5 cm−1, of which the σ(λ) are approximately 10 times smaller than the maximum values at 2,983.28 cm−1. There are also absorption lines of water, acetone and methane near these lines, inside the laser wavelength scanning range. Relatively small σ(λ) and interference from other chemicals are disadvantageous for ethane measurement; However, they are advantageous for simultaneous measurement of multi-species VOCs. We selected this wavelength region at the expense of the measurement sensitivity of ethane because our ultimate goal was to simultaneously analyze multiple VOCs.

However, at this time, we can use only a 503.21 parts-per-million (ppm) ethane sample, which is a too high concentration to measure using the suggested line. Thus, we used two different ethane absorption lines at λ1 = 2,967.840 and λ2 = 2,967.828 cm−1, of which the σ(λ) are approximately 10 and 25 times smaller than that of the suggested line [σ(λ1) = 2.48 × 10−19 and σ(λ2) = 1.01 × 10−19 cm−2, respectively] in this experiment.

2.3. CRDS Setup

We set up the CRDS experimental apparatus for ethane measurement as shown in Fig. 2, with a light source, acousto-optic modulator (AOM), optical fiber for spatial mode filtering, mode matching optics, CRDS cell, photo-detector, and data acquisition system.

Figure 2.Schematics of CRDS experimental setup for ethane measurements. The mid-infrared laser beam is represented by a solid red line. Data acquisition and control system are not shown. CRDS, cavity ring-down spectroscopy; LD, laser diode; OI, optical isolator; λ/2, half-wave plate; AOM, acousto-optic modulator; FP, fiber port; M, mirror; L, lens; I, iris; PD, photodetector.

The light source was required to be in the 3.37 μm range and the wavelength can be tunable. Nowadays, the desired conditions can be easily achieved using a quantum cascade laser (QCL) or inter-band cascade laser (ICL). We used a 7-mW distributed feedback ICL laser diode (DFB ICL; Nanoplus Nanosystems and Technologies GmbH, Meiningen, Germany).

To measure the ring-down time, we needed to switch off the laser beam after the build-up of cavity mode. We used an AOM (AGM-402B9M; IntraAction Corp., IL, USA) for fast optical switching. The optical rise time of AOM is 116 ns, sufficient to measure ring-down time on the order of μs.

When we directly coupled the laser beam to the cavity after the AOM, the coupling efficiency was not sufficient to reach the proper sensitivity because the laser beam profile was deformed. To improve the laser spatial mode, we coupled the laser beam to a single-mode zirconium fluoride fiber (P3-23Z-FC-1; ThorLabs, NJ, USA). After passing through the fiber, a TEM00 mode laser beam was obtained with power of 1.5 mW, which was adequate for CRDS experiments.

Mode matching optics, consisting of three plano-convex lenses, was used to match the laser spatial beam mode and cavity fundamental mode. We designed the system such that the first and second lenses controlled the beam size and the third lens controlled the beam divergence.

The CRDS cell was 50 cm long, and the optical cavity was constructed with two identical concave mirrors with reflectivity higher than 99.95% and curvatures of 1,000 mm (Layertec, Mellingen, Germany) at each end of the cell. The FSR of the cavity was calculated at 299.79 MHz. To scan the cavity modes, we needed to change the length of the cavity. This was achieved by controlling the modulation amplitude and phase of three piezoelectric actuators (PE-4; ThorLabs) independently at one of the cavity mirrors. This was necessary to measure the ring-down time in all experimental conditions while sweeping the laser wavelength.

The photodetector required proper detection sensitivity and bandwidth. We used an HgCdTe photovoltaic detector with a preamplifier (PIP-US-LS; Vigo Photonics, Ozarow Mazowiecki, Poland) that had high sensitivity in the mid-infrared range.

To obtain the cavity ring-down signal, we switched off the laser beam when the Fabry-Perot signal of the cavity was above a certain triggering value and then recorded the decay signal. Next we obtained the ring-down time by exponential fitting of the measured signal. Optical switching control was performed using a delay/pulse generator (DG535; Stanford Research System, CA, USA). A data acquisition system containing a data fitting process, number of averaging, and time interval between data points, was programmed by using Labview.

3.1. System Performance

We measured the ring-down time for the empty cavity of the experimental setup described in section II. Typical ring-down time data is shown in Fig. 3(a). Each measurement took 5 seconds for 16 averaged cavity decay signals in this experiment.

Figure 3.Ring-down time measurement for an empty cavity. (a) Typical ring-down time measurement. This data shows 16 averaged cavity decay signals and fitting results. (b) Repeated measurement of ring-down time.

As a result of repeating the measurements 1,000 times, the ring-down time for the empty cavity was measured and found to have an average value of 18.239 μs with a standard deviation of 0.198 μs [Fig. 3(b)].

The value of ring-down time calculated using Eq. (1), with cavity mirror reflectivity of 0.9995, is 3.336 μs. The measured ring-down time corresponds to a cavity mirror reflectivity of 0.9999. This was reproducible in several experimental conditions.

From Eqs. (1)–(2), we can estimate measurement sensitivity using the minimum detectable values for reciprocals of ring-down time. We obtained an average absorption coefficient from an empty cavity of 1.828 × 10−6/cm, and the standard deviation was 2.249 × 10−8/cm. The signal-to-noise ratio (SNR) was 81.299. We can consider the minimum detectable quantity to be equal to the standard deviation.

If we consider our first candidate line for ethane measurements at approximately 2,967.5 cm−1, with σ(λ) = 2.59 × 10−18 cm2/molecule, the minimum detectable concentration N is 8.68 × 109 cm−3, which can be converted to 322 ppt.

To improve system performance, including the measurement sensitivity and acquisition rate, we can consider the following in future work.

First, if we consider only ethane measurement, we can easily achieve higher sensitivity by using absorption lines with large σ(λ), because the measurement sensitivity directly depends on the σ(λ) of selected absorption lines. For example, in the case of using the line at 2,983.28 cm−1, which has the largest σ(λ) = 2.275 × 10−17 cm2/molecule, we can achieve sensitivity of 36.64 ppt by just changing the laser diode.

Second, we can decrease the noise level of the cavity decay signal by increasing the average number of acquisitions [26, 27]. In this case, since a longer measurement time will be required, we will have to improve both the system stability and the acquisition rate. For large average numbers of acquisitions in particular, every decay signal will need to be as identical as possible. This can be achieved by improving cavity alignment, laser frequency stabilization, cavity temperature stabilization, and vibration isolation. Furthermore, in this experiment, measurement time was limited to a few seconds by the intrinsic parameters of the oscilloscope (DS6104; Rigol, Oregon, USA) such as communication time and acquisition time. However, in principle, the acquisition rate can reach the cavity mirror position scan rate, which was 200 Hz in this experiment. If we optimize the data acquisition system, we can decrease the measurement time to below one second.

3.2. Measurement of Ethane Sample

We measured ring-down times under identical conditions for an empty cavity and for a cavity in which an ethane sample was injected. We used a certificated ethane sample with a 503.21 ppm concentration (AirKorea Co., Yeoju, Korea). After sample injection, the pressure of the cavity was measured at 0.27 torr. This means that the number of ethane molecules inside the cavity was the same as that of a 178.77 parts-per-billion (ppb) ethane sample at 1 atm. If we ignore pressure broadening effects, the measurement condition can be considered as effectively an ethane concentration of 178.77 ppb.

The measurement results are presented in Fig. 4. The measured spectrum of ethane shows good agreement with the reference data [25]. As mentioned in section 2.2, we focused on the two ethane absorption lines at λ1 and λ2 because our ethane sample was too dense for measurement using absorption lines that have a large σ(λ).

Figure 4.Measured ethane spectrum. The data represent the ring-down time for an empty cavity (black dots) and for an ethane sample (red dots). The blue line shows an absorption cross-section of ethane extracted from [25].

From the ring-down time data for the two absorption lines, we calculated the concentration of ethane and compared it to the effective concentration from pressure. The measured ethane concentrations showed consistency, with two different absorption lines (212.9 ppb for the λ1 absorption line and 246.1 ppb for the λ2 absorption line). These values are quite close to the value estimated using pressure, 178.77 ppb, in spite of various experimental errors.

For quantitative measurement, accurate absorption cross-section data of ethane is required. In this study, however, we used the absorption cross-section data taken from [25], which shows broadening effects due to different measurement conditions. In addition, to improve measurement accuracy, we will need to consider measurement using a sample with a lower concentration and the pressure uncertainty inside the cavity introduced by the sample injection and pumping process.

We measured ethane in the 3.37 μm mid-infrared region using the CRDS method. The configured CRDS setup showed a minimum detectable absorption coefficient of 2.249 × 10−8/cm, corresponding to a 322 ppt detection limit, with an absorption line at 2,967.5 cm−1. We demonstrated the ethane measurements using a certificated sample. Quantitative calculations from the measured data were consistent and showed agreement with the amount of injected sample. These results showed the possibility of using CRDS for trace gas analysis, including for atmospheric ethane measurement and breath test applications.

In addition, we expect that the CRDS system can be used as a multi-species VOCs analyzing system applicable to breath test equipment in the future.

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request. Ethane spectrum data in Fig. 1 and Fig. 4 were extracted from [24, 25].

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Article

Research Paper

Curr. Opt. Photon. 2023; 7(4): 457-462

Published online August 25, 2023 https://doi.org/10.3807/COPP.2023.7.4.457

Copyright © Optical Society of Korea.

Quantitative Measurement of Ethane Using Mid-infrared Cavity Ring-down Spectroscopy

Yonghee Kim , Byung Jae Chun, Lim Lee, Kwang-Hoon Ko, Seung-Kyu Park, Taek-Soo Kim, Hyunmin Park

Quantum Optics Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea

Correspondence to:*yonghee922@kaeri.re.kr, ORCID 0009-0005-8232-6278

Received: April 26, 2023; Revised: June 7, 2023; Accepted: June 7, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Quantitative measurement of trace ethane is important in environmental science and biomedical applications. For these applications, we typically require a few tens of part-per-trillion level measurement sensitivity. To measure trace-level ethane, we constructed a cavity ring-down spectroscopy setup in the 3.37 μm mid-infrared wavelength range, which is applicable to multi-species chemical analysis. We demonstrated that the detection limit of ethane is approximately 300 parts per trillion, and the measured concentration is in agreement with the amounts of the injected sample. We expect that these results can be applied to the chemical analysis of ethane and applications such as breath test equipment.

Keywords: Cavity ring-down spectroscopy, Ethane, Mid-infrared, Quantitative measurement

I. INTRODUCTION

Ethane (C2H6) is one of the most abundant hydrocarbon volatile organic compounds (VOCs) in the atmosphere, with average concentrations of approximately 1,000 parts per trillion (ppt) for the Northern Hemisphere [1]. Over the last few decades, many quantitative studies of trace ethane have been conducted in various fields such as environmental science [14] and biomedical applications [512].

Atmospheric ethane is one of the major greenhouse gases, and its origin lies in human activities such as natural gas leakage during production and transportation, biofuel use, and burning of biomass [2]. There is now growing interest in the concentration of ethane and its relation to climate change. Information on atmospheric ethane concentrations, such as regional distribution, seasonal oscillation, and annual changes, offers opportunities to better understand the earth system and its reflection of human activities [13]. Also, ethane is related to various chemical reactions in the atmosphere such as the formation of tropospheric ozone [4].

From a clinical point of view, exhaled human breath contains thousands of VOCs, and many studies have shown a correlation between concentrations of VOCs and various diseases [1317]. A breath test is a non-invasive diagnostic method of diseases by measuring specific molecules in the exhaled breath. It is known that ethane in human breath is produced by oxidation damage of cellular membranes, in a process known as oxidation stress [5]; This is related to a wide range of diseases such as lung cancer [6, 7], chronic obstructive pulmonary disease (COPD) [8], cystic fibrosis [9], asthma [10] and vitamin E deficiency [11]. Intensive studies have been conducted on ethane as a potential biomarker for diagnosing diseases and monitoring oxidation stress [512].

To measure the quantity of ethane and its delicate changes in air or human breath, measurement techniques with precision below several tens of ppt is required. Currently, gas chromatography coupled to mass spectrometry (GC-MS) is a standard method for analyzing the composition and concentration of trace-level samples. However, the GC-MS method has some disadvantages such as a complex sampling procedure, long measurement time and high cost. Therefore, in specific applications for which the chemical species and measurement range are limited, other measurement technologies such as electrochemical sensors [13, 15] and optical methods [16, 17] have been suggested. Among these, optical methods using molecular spectra have distinct advantages compared to GC-MS, such as high sensitivity, real-time measurement, simple sampling procedures, and relatively low cost. Thus, these methods are expected to be widely used in various fields such as clinical applications based on exhaled human breath analysis [16, 17].

In the case of ethane measurement, the mid-infrared wavelength region is generally used because an ethane molecule has strong absorption lines near the 3.3 μm wavelength range. To measure the trace levels of ethane, various spectroscopic techniques such as Fourier transform infrared (FTIR) spectroscopy [2], tunable diode laser absorption spectroscopy (TDLAS) using a multi-pass cell [5, 6, 12], off-axis integrated cavity output spectroscopy (OA-ICOS) [18, 19] and cavity ring-down spectroscopy (CRDS) [7, 20, 21] have been reported. These methods have shown detection limits of 70 to 500 ppt with 1 second measurement time and further demonstrate the potential of optical methods for clinical applications.

In this paper, we focus on the CRDS method to achieve several tens of ppt level sensitivity for the quantitative measurement of ethane molecules. We demonstrate ethane measurement using the 3.37 μm mid-infrared wavelength region (corresponding to 2,967.0 to 2,969.0 cm−1), which can be applicable to multi-species chemical analysis.

In section II, we briefly introduce the CRDS method, the wavelength selection for this study, and details of the experimental setup. In section III, we present experimental results on ethane measurement and discuss system sensitivity. Finally, we summarize results and discuss future work in section IV.

II. EXPERIMENTAL SETUP

2.1. Cavity Ring-down Spectroscopy

CRDS is a highly sensitive quantitative measurement technique using the decay time (or ring-down time) of light in an optical cavity. This technique, which was first demonstrated at 1988 [22], has been widely used to analyze small amounts of optical absorption.

When a laser is spatially mode matched to a cavity mode and injected into a high finesse optical cavity with length L and two cavity mirrors with reflectivity R, laser light reflects back and forth inside the cavity and the corresponding cavity modes build up a free spectral range (FSR), c/2nL, where c is the speed of light, n is the index of refraction inside the cavity, and L is the cavity length. If the laser injection stops, the stored energy inside the cavity is exponentially decayed by cavity loss mechanisms. The ring-down time is defined as the time at which the intensity of the cavity mode decays to 1/e-times the initial values [17, 23].

For an empty cavity, cavity loss depends on only cavity mirror reflectivity R. In this case, ring-down time τ0 is expressed by

τ0=Lc1R

Cavity loss increases in the presence of target molecules inside the cavity due to the light absorption of target molecules. Then, ring-down time τ become wavelength dependent

τλ=Lc1R+Nσλl

where N is the density of target molecules, σ(λ) is the absorption cross-section of target molecules at a given wavelength λ, and l is the length of the sample inside the cavity, typically L for gas samples. Therefore, if we know the σ(λ), we can directly determine the concentration of target molecules from the difference between the reciprocal of the ring-down time in the presence of a sample inside the cavity and the ring-down time of the empty cavity, 1/τ(λ) − 1/τ0.

Notably, CRDS is very sensitive to injected laser beam alignment; However, the method is free from laser power fluctuation, in principle, because the ring-down time is independent from the laser power. This offers CRDS the unique capability of conducting trace-level sample measurements, which can lead to the development of robust commercial equipment.

2.2. Wavelength Selection for Ethane Measurement

Ethane has a strong absorption band in the mid-infrared wavelength region of 3.31–3.38 μm (approximately 2,960.0 to 3,020.0 cm−1) as shown in Fig. 1(a) [24, 25].

Figure 1. Ethane absorption spectrum in (a) the 3.3 μm mid-infrared wavelength range and (b) the wavelength region selected in this study. These data were extracted from [24, 25].

The largest two absorption lines of ethane are 2,983.28 cm−1 and 2,996.87 cm−1 (corresponding to 3.3520 μm and 3.3368 μm, respectively). Most previous studies [5, 12, 18, 19, 21] have used the wavelength region near these lines, depending on the light source and interference by methane or water molecules.

In this study, we selected the 3.37 μm (2,967.0 to 2,969.0 cm−1) wavelength region, as shown in Fig. 1(b), which is different from the typical region used for ethane measurement. The first candidate absorption lines were at approximately 2,967.5 cm−1, of which the σ(λ) are approximately 10 times smaller than the maximum values at 2,983.28 cm−1. There are also absorption lines of water, acetone and methane near these lines, inside the laser wavelength scanning range. Relatively small σ(λ) and interference from other chemicals are disadvantageous for ethane measurement; However, they are advantageous for simultaneous measurement of multi-species VOCs. We selected this wavelength region at the expense of the measurement sensitivity of ethane because our ultimate goal was to simultaneously analyze multiple VOCs.

However, at this time, we can use only a 503.21 parts-per-million (ppm) ethane sample, which is a too high concentration to measure using the suggested line. Thus, we used two different ethane absorption lines at λ1 = 2,967.840 and λ2 = 2,967.828 cm−1, of which the σ(λ) are approximately 10 and 25 times smaller than that of the suggested line [σ(λ1) = 2.48 × 10−19 and σ(λ2) = 1.01 × 10−19 cm−2, respectively] in this experiment.

2.3. CRDS Setup

We set up the CRDS experimental apparatus for ethane measurement as shown in Fig. 2, with a light source, acousto-optic modulator (AOM), optical fiber for spatial mode filtering, mode matching optics, CRDS cell, photo-detector, and data acquisition system.

Figure 2. Schematics of CRDS experimental setup for ethane measurements. The mid-infrared laser beam is represented by a solid red line. Data acquisition and control system are not shown. CRDS, cavity ring-down spectroscopy; LD, laser diode; OI, optical isolator; λ/2, half-wave plate; AOM, acousto-optic modulator; FP, fiber port; M, mirror; L, lens; I, iris; PD, photodetector.

The light source was required to be in the 3.37 μm range and the wavelength can be tunable. Nowadays, the desired conditions can be easily achieved using a quantum cascade laser (QCL) or inter-band cascade laser (ICL). We used a 7-mW distributed feedback ICL laser diode (DFB ICL; Nanoplus Nanosystems and Technologies GmbH, Meiningen, Germany).

To measure the ring-down time, we needed to switch off the laser beam after the build-up of cavity mode. We used an AOM (AGM-402B9M; IntraAction Corp., IL, USA) for fast optical switching. The optical rise time of AOM is 116 ns, sufficient to measure ring-down time on the order of μs.

When we directly coupled the laser beam to the cavity after the AOM, the coupling efficiency was not sufficient to reach the proper sensitivity because the laser beam profile was deformed. To improve the laser spatial mode, we coupled the laser beam to a single-mode zirconium fluoride fiber (P3-23Z-FC-1; ThorLabs, NJ, USA). After passing through the fiber, a TEM00 mode laser beam was obtained with power of 1.5 mW, which was adequate for CRDS experiments.

Mode matching optics, consisting of three plano-convex lenses, was used to match the laser spatial beam mode and cavity fundamental mode. We designed the system such that the first and second lenses controlled the beam size and the third lens controlled the beam divergence.

The CRDS cell was 50 cm long, and the optical cavity was constructed with two identical concave mirrors with reflectivity higher than 99.95% and curvatures of 1,000 mm (Layertec, Mellingen, Germany) at each end of the cell. The FSR of the cavity was calculated at 299.79 MHz. To scan the cavity modes, we needed to change the length of the cavity. This was achieved by controlling the modulation amplitude and phase of three piezoelectric actuators (PE-4; ThorLabs) independently at one of the cavity mirrors. This was necessary to measure the ring-down time in all experimental conditions while sweeping the laser wavelength.

The photodetector required proper detection sensitivity and bandwidth. We used an HgCdTe photovoltaic detector with a preamplifier (PIP-US-LS; Vigo Photonics, Ozarow Mazowiecki, Poland) that had high sensitivity in the mid-infrared range.

To obtain the cavity ring-down signal, we switched off the laser beam when the Fabry-Perot signal of the cavity was above a certain triggering value and then recorded the decay signal. Next we obtained the ring-down time by exponential fitting of the measured signal. Optical switching control was performed using a delay/pulse generator (DG535; Stanford Research System, CA, USA). A data acquisition system containing a data fitting process, number of averaging, and time interval between data points, was programmed by using Labview.

III. RESULTS AND DISCUSSION

3.1. System Performance

We measured the ring-down time for the empty cavity of the experimental setup described in section II. Typical ring-down time data is shown in Fig. 3(a). Each measurement took 5 seconds for 16 averaged cavity decay signals in this experiment.

Figure 3. Ring-down time measurement for an empty cavity. (a) Typical ring-down time measurement. This data shows 16 averaged cavity decay signals and fitting results. (b) Repeated measurement of ring-down time.

As a result of repeating the measurements 1,000 times, the ring-down time for the empty cavity was measured and found to have an average value of 18.239 μs with a standard deviation of 0.198 μs [Fig. 3(b)].

The value of ring-down time calculated using Eq. (1), with cavity mirror reflectivity of 0.9995, is 3.336 μs. The measured ring-down time corresponds to a cavity mirror reflectivity of 0.9999. This was reproducible in several experimental conditions.

From Eqs. (1)–(2), we can estimate measurement sensitivity using the minimum detectable values for reciprocals of ring-down time. We obtained an average absorption coefficient from an empty cavity of 1.828 × 10−6/cm, and the standard deviation was 2.249 × 10−8/cm. The signal-to-noise ratio (SNR) was 81.299. We can consider the minimum detectable quantity to be equal to the standard deviation.

If we consider our first candidate line for ethane measurements at approximately 2,967.5 cm−1, with σ(λ) = 2.59 × 10−18 cm2/molecule, the minimum detectable concentration N is 8.68 × 109 cm−3, which can be converted to 322 ppt.

To improve system performance, including the measurement sensitivity and acquisition rate, we can consider the following in future work.

First, if we consider only ethane measurement, we can easily achieve higher sensitivity by using absorption lines with large σ(λ), because the measurement sensitivity directly depends on the σ(λ) of selected absorption lines. For example, in the case of using the line at 2,983.28 cm−1, which has the largest σ(λ) = 2.275 × 10−17 cm2/molecule, we can achieve sensitivity of 36.64 ppt by just changing the laser diode.

Second, we can decrease the noise level of the cavity decay signal by increasing the average number of acquisitions [26, 27]. In this case, since a longer measurement time will be required, we will have to improve both the system stability and the acquisition rate. For large average numbers of acquisitions in particular, every decay signal will need to be as identical as possible. This can be achieved by improving cavity alignment, laser frequency stabilization, cavity temperature stabilization, and vibration isolation. Furthermore, in this experiment, measurement time was limited to a few seconds by the intrinsic parameters of the oscilloscope (DS6104; Rigol, Oregon, USA) such as communication time and acquisition time. However, in principle, the acquisition rate can reach the cavity mirror position scan rate, which was 200 Hz in this experiment. If we optimize the data acquisition system, we can decrease the measurement time to below one second.

3.2. Measurement of Ethane Sample

We measured ring-down times under identical conditions for an empty cavity and for a cavity in which an ethane sample was injected. We used a certificated ethane sample with a 503.21 ppm concentration (AirKorea Co., Yeoju, Korea). After sample injection, the pressure of the cavity was measured at 0.27 torr. This means that the number of ethane molecules inside the cavity was the same as that of a 178.77 parts-per-billion (ppb) ethane sample at 1 atm. If we ignore pressure broadening effects, the measurement condition can be considered as effectively an ethane concentration of 178.77 ppb.

The measurement results are presented in Fig. 4. The measured spectrum of ethane shows good agreement with the reference data [25]. As mentioned in section 2.2, we focused on the two ethane absorption lines at λ1 and λ2 because our ethane sample was too dense for measurement using absorption lines that have a large σ(λ).

Figure 4. Measured ethane spectrum. The data represent the ring-down time for an empty cavity (black dots) and for an ethane sample (red dots). The blue line shows an absorption cross-section of ethane extracted from [25].

From the ring-down time data for the two absorption lines, we calculated the concentration of ethane and compared it to the effective concentration from pressure. The measured ethane concentrations showed consistency, with two different absorption lines (212.9 ppb for the λ1 absorption line and 246.1 ppb for the λ2 absorption line). These values are quite close to the value estimated using pressure, 178.77 ppb, in spite of various experimental errors.

For quantitative measurement, accurate absorption cross-section data of ethane is required. In this study, however, we used the absorption cross-section data taken from [25], which shows broadening effects due to different measurement conditions. In addition, to improve measurement accuracy, we will need to consider measurement using a sample with a lower concentration and the pressure uncertainty inside the cavity introduced by the sample injection and pumping process.

IV. CONCLUSION

We measured ethane in the 3.37 μm mid-infrared region using the CRDS method. The configured CRDS setup showed a minimum detectable absorption coefficient of 2.249 × 10−8/cm, corresponding to a 322 ppt detection limit, with an absorption line at 2,967.5 cm−1. We demonstrated the ethane measurements using a certificated sample. Quantitative calculations from the measured data were consistent and showed agreement with the amount of injected sample. These results showed the possibility of using CRDS for trace gas analysis, including for atmospheric ethane measurement and breath test applications.

In addition, we expect that the CRDS system can be used as a multi-species VOCs analyzing system applicable to breath test equipment in the future.

DISCLOSURES

The authors declare no conflicts of interest.

DATA AVAILABILITY

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request. Ethane spectrum data in Fig. 1 and Fig. 4 were extracted from [24, 25].

ACKNOWLEDGMENT

This work was supported by the KAERI Institutional Program (Project No. 524430-23).

FUNDING

KAERI Institutional Program (Project No. 524430-23).

Fig 1.

Figure 1.Ethane absorption spectrum in (a) the 3.3 μm mid-infrared wavelength range and (b) the wavelength region selected in this study. These data were extracted from [24, 25].
Current Optics and Photonics 2023; 7: 457-462https://doi.org/10.3807/COPP.2023.7.4.457

Fig 2.

Figure 2.Schematics of CRDS experimental setup for ethane measurements. The mid-infrared laser beam is represented by a solid red line. Data acquisition and control system are not shown. CRDS, cavity ring-down spectroscopy; LD, laser diode; OI, optical isolator; λ/2, half-wave plate; AOM, acousto-optic modulator; FP, fiber port; M, mirror; L, lens; I, iris; PD, photodetector.
Current Optics and Photonics 2023; 7: 457-462https://doi.org/10.3807/COPP.2023.7.4.457

Fig 3.

Figure 3.Ring-down time measurement for an empty cavity. (a) Typical ring-down time measurement. This data shows 16 averaged cavity decay signals and fitting results. (b) Repeated measurement of ring-down time.
Current Optics and Photonics 2023; 7: 457-462https://doi.org/10.3807/COPP.2023.7.4.457

Fig 4.

Figure 4.Measured ethane spectrum. The data represent the ring-down time for an empty cavity (black dots) and for an ethane sample (red dots). The blue line shows an absorption cross-section of ethane extracted from [25].
Current Optics and Photonics 2023; 7: 457-462https://doi.org/10.3807/COPP.2023.7.4.457

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