Gas absorption spectrometer and gas absorption spectroscopy method

A gas absorption spectrometer is provided that is capable of measuring a gas concentration and the like with high accuracy even when a measurement target gas is mixed with a gas other than the measurement target gas. A gas absorption spectrometer includes: a wavelength tunable light source; a light source control unit to change a wavelength of the light source; a gas cell to have a measurement object gas introduced thereto, the measurement object gas containing a measurement target gas and a mixed gas other than the measurement target gas; a photodetector to detect intensity of light having passed through the gas cell; a spectrum preparation unit to prepare an absorption spectrum of the measurement object gas from a change in the light intensity relative to the change in the wavelength; a physical quantity measurement unit to measure at least one of a temperature and a concentration of the measurement target gas based on an absorption spectrum of the measurement target gas; a gas concentration measurement unit to measure a concentration of the mixed gas; and a physical quantity correction unit to correct at least one of the temperature and the concentration of the measurement target gas measured by the physical quantity measurement unit based on the concentration of the mixed gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas absorption spectrometer and a gas absorption spectroscopy method to measure the concentration, the temperature, and the like of a measurement target gas based on the laser light absorption spectrum of the gas. The gas absorption spectrometer and the gas absorption spectroscopy method are applicable to, for example, non-contact rapid measurement of a gas concentration and a gas temperature in the automotive industry and also to a wide variety of fields, such as measuring a gas in a high-temperature high-pressure environment like a combustion gas in a plant furnace.

2. Description of the Related Art

Wavelength modulated spectroscopy (WMS) is conventionally known as gas absorption spectroscopy to measure laser light with a photodetector by irradiating a measurement target gas with laser light. In WMS, while a wavelength of laser light is swept, the wavelength is sinusoidally modulated in a period sufficiently shorter than the sweep period (i.e., at a sufficiently high frequency f). Then, it is disclosed that detection of a harmonic at a frequency f (generally, second harmonic, 2f) by the photodetector allows measurement of gas absorption with high sensitivity (JP 2011-196832 A, Reid, J. and Labrie, D.; “Second-harmonic detection with tunable diode lasers-comparison of experiment and theory,” Appl. Phys. B 26, 203-210 (1981); J. A. Silver, “Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods,” Appl. Opt. 31 (1992), 707-717; and G. B. Rieker, J. B. Jeffries, and R. K. Hanson, “Calibration-free wavelength modulation spectroscopy for measurements of gas temperature and concentration in harsh environments,” Appl. Opt., submitted 2009). While a lock-in amplifier is generally used for detection of a harmonic, a method is also proposed in which a detector signal is directly subjected to digital sampling for FFT analysis, thereby performing synchronous detection at 2f (T. Fernholz, H. Teichert, and V. Ebert, “Digital, phase-sensitive detection for in situ diode-laser spectroscopy under rapidly changing transmission conditions,” Appl. Phys. B 75, 229-236 (2002)).

As just described, conventional WMS has good sensitivity and robustness (ease of measurement) and is thus considered preferred for application to industrial gas absorption spectrometers, whereas it has a problem of a decrease in measurement accuracy of a gas concentration, a gas temperature, and the like in rapid measurement because it is difficult to increase the wavelength modulation frequency of the laser and it is also difficult to accurately measure the wavelength modulation width at high wavelength modulation frequencies.

To solve such problems, a method is proposed in which an absorption spectrum of a measurement target gas is obtained from a change in light intensity relative to a change in a wavelength of the laser, the absorption spectrum is approximated by an approximate polynomial, and an nth order differential spectrum of the absorption spectrum is formed based on the factor in each term of the approximate polynomial to measure the temperature, the concentration, the pressure, and the like of the measurement target gas based on the nth order differential spectrum (Japanese Patent No. 5983779).

The gas absorption spectrometer and the gas absorption spectroscopy method described in Japanese Patent No. 5983779 allows measurement of the gas concentration, the gas temperature, and the like with high accuracy even in rapid measurement. However, the measurement target gas is often mixed with other gases depending on the measurement environment and such a case presents a problem that the absorption spectrum thus obtained itself is altered and it is not possible to accurately measure (i.e., it causes a measurement error in) the gas concentration, the gas temperature, and the like of the measurement target gas.

SUMMARY

The present invention has been made in view of such circumstances, and it is an object thereof to provide a gas absorption spectrometer and a gas absorption spectroscopy method that are capable of measuring a gas concentration and the like with high accuracy even when a measurement target gas is mixed with other gases.

To achieve the above problems, a gas absorption spectrometer of the present invention includes: a wavelength tunable light source; a light source control unit configured to change a wavelength of light emitted from the light source; a gas cell configured to have a measurement object gas introduced thereto, the measurement object gas containing a measurement target gas and a mixed gas other than the measurement target gas; a photodetector configured to detect intensity of light emitted from the light source and having passed through the gas cell; a spectrum preparation unit configured to prepare an absorption spectrum of the measurement object gas from a change in the light intensity relative to the change in the wavelength by the light source control unit; a physical quantity measurement unit configured to measure at least one of a temperature and a concentration of the measurement target gas based on an absorption spectrum of the measurement target gas; a gas concentration measurement unit configured to measure a concentration of the mixed gas; and a physical quantity correction unit configured to correct at least one of the temperature and the concentration of the measurement target gas measured by the physical quantity measurement unit based on the concentration of the mixed gas.

The “wavelength” herein uniquely corresponds to a “wave number” and it is also possible to establish a similar configuration using the “wave number”.

The “mixed gas” herein means a gas other than the measurement target gas in a measurement system and is a concept including multiple types of gases.

The gas absorption spectrometer may be configured that the physical quantity measurement unit includes: a polynomial approximation unit configured to approximate the absorption spectrum by an approximate polynomial in a wavelength width range equivalent to a wavelength modulation width of WMS at each point of the wavelength; and a differential spectrum preparation unit configured to prepare a second order differential spectrum of the absorption spectrum based on a factor in each term of the approximate polynomial at each point, and at least one of the temperature and the concentration of the measurement target gas is measured based on the second order differential spectrum of the measurement target gas. In this case, the gas concentration measurement unit may measure the concentration of the mixed gas based on a second order differential spectrum of the mixed gas.

The gas concentration measurement unit may be disposed in the gas cell and have a concentration sensor configured to measure the concentration of the mixed gas.

The physical quantity correction unit may have an approximate expression representing a change in peak height of the second order differential spectrum of the measurement target gas relative to the concentration of the mixed gas and may be configured to correct at least one of the temperature and the concentration of the measurement target gas measured by the physical quantity measurement unit using a correction value obtained by the approximate expression. In this case, the approximate expression is a power function.

The physical quantity correction unit desirably has a correction value representing relationship between the concentration of the mixed gas and a peak height of the second order differential spectrum of the measurement target gas and is desirably configured to correct at least one of the temperature and the concentration of the measurement target gas measured by the physical quantity measurement unit using the correction value. In this case, the correction value may be determined based on a change in Lorentz broadening coefficient (Air) relative to the concentration of the mixed gas.

The physical quantity correction unit may be configured to have a database to store the correction value therein.

The physical quantity measurement unit may measure at least one of the temperature and the concentration of the measurement target gas using a peak height of the second order differential spectrum.

The gas absorption spectrometer may include a pressure sensor configured to detect a pressure of the measurement object gas, wherein the physical quantity correction unit corrects at least one of the temperature and the concentration of the measurement target gas measured by the physical quantity measurement unit based on the pressure of the measurement object gas and the concentration of the mixed gas.

The approximate polynomial may be an approximate quadratic polynomial.

The gas absorption spectrometer may be a cavity ring-down absorption spectrometer.

From another aspect, a gas absorption spectroscopy method of the present invention includes: irradiating a measurement object gas containing a measurement target gas and a mixed gas other than the measurement target gas with light with a tunable wavelength; detecting intensity of light having passed through the measurement object gas; preparing an absorption spectrum of the measurement object gas from a change in the light intensity relative to a change in the wavelength; measuring at least one of a temperature and a concentration of the measurement target gas based on an absorption spectrum of the measurement target gas; measuring a concentration of the mixed gas; and correcting at least one of the temperature and the concentration of the measurement target gas based on the concentration of the mixed gas.

As described above, the gas absorption spectrometer and the gas absorption spectroscopy method of the present invention are capable of measuring gas concentration and temperature with high accuracy even when a measurement target gas is mixed with other gases because at least one of the temperature and the concentration of the measurement target gas measured by the physical quantity measurement unit based on the concentration of the mixed gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed description is given below to embodiments of the present invention with reference to the drawings. In the drawings, an identical sign is given to identical or equivalent parts not to repeat the description.

Configuration of Gas Absorption Spectrometer

FIG. 1is a diagram illustrating a schematic configuration of a gas absorption spectrometer according to an embodiment of the present invention. As illustrated inFIG. 1, a gas absorption spectrometer1in the present embodiment includes: a gas cell11to have a measurement object gas G introduced thereto, the measurement object gas G containing a measurement target gas TG and a mixed gas MG other than the measurement target gas TG; a laser light source12disposed on one side across the gas cell11; a photodetector13disposed on the other side; a light source control unit14to control the laser light source12; a pressure sensor15to detect a pressure of the measurement object gas G in the gas cell11; and a measurement unit20connected to the photodetector13and the pressure sensor15to quantify output of the photodetector13and the pressure sensor15for storage and to perform various calculations.

The gas cell11is a component made of, for example, glass or a resin that is disposed between the laser light source12and the photodetector13and configured to transmit light from the laser light source12.

The laser light source12is wavelength tunable in a range covering absorption wavelengths of at least the measurement target gas TG and the mixed gas MG and is configured with, for example, quantum cascade laser (QCL) or injection current controlled wavelength-tunable diode laser.

The light source control unit14is a device that controls the laser light source12to sweep (change) the wavelength of the laser light source12between predetermined minimum and maximum wavelengths.

The photodetector13is a device that detects the intensity of light emitted from the laser light source12and having passed through the gas cell11and is configured with, for example, a photodiode with spectroscopic sensitivity in a wavelength tunable range of the laser light source12. The photodetector13is connected to the measurement unit20, and information (electrical signal) on the light intensity detected by the photodetector13is output to the measurement unit20.

The pressure sensor15is a sensor that that has a sensor surface, not shown, exposed in the gas cell11to detect the pressure of the measurement object gas G. The pressure sensor15is connected to the measurement unit20, and information (electrical signal) on the pressure detected by the pressure sensor15is output to the measurement unit20.

The measurement unit20has an A/D converter25and an analysis unit26. The A/D converter25is a device that digitizes the information on the light intensity output from the photodetector13and the information on the pressure output from the pressure sensor15for storage in in a storage device, not shown. The analysis unit26is a device that mathematically calculates data obtained by the A/D converter25(data in the storage device) to measure the temperature and the concentration of the measurement target gas TG (described later in detail).

Procedure for Measuring Temperature and Concentration of Measurement Target Gas TG

FIG. 2is a flow chart illustrating a procedure for measuring the temperature and the concentration of the measurement target gas TG using the gas absorption spectrometer1according to the present embodiment. As illustrated inFIG. 2, in the measurement procedure of the present embodiment, first, the light source control unit14controls the laser light source12to emit laser light at a predetermined minimum wavelength (step S1), and the wavelength is sequentially changed to be swept to the maximum wavelength (step S2). Since passing through the measurement object gas Gin the gas cell11, the light from the laser light source12is subjected to absorption at wavelengths in accordance with the measurement object gas G (i.e., measurement target gas TG and mixed gas MG) at this point. Then, the laser light having passed through the measurement object gas G is subjected to detection of the intensity by the photodetector13. Subsequently, the information on the light intensity output from the photodetector13and the information on the pressure output from the pressure sensor15are digitized by the A/D converter25and sent to the analysis unit26to be stored in the storage device, not shown. Then, the analysis unit26obtains the spectral profile of the measurement object gas G based on the change in the light intensity information (step S3(spectrum preparation unit)).

FIG. 3is a diagram illustrating an example of the spectral profile obtained at step S3, where the measurement target gas TG is assumed to be H2O and the mixed gas MG to be CO2. While the profile inFIG. 3is simulated from a spectral profile of H2O and CO2obtained from the HITRAN 2008 database for an H2O concentration of 1(%), a CO2concentration of 10(%), a temperature of 300 (K), and a pressure of 1 (atm), the spectral profile obtained in steps S1-S3is actually used. As illustrated inFIG. 3, in the present embodiment, the measurement target gas TG (H2O) and the mixed gas MG (CO2) are introduced into the gas cell11, and thus in the spectral profile obtained at step S3, an absorption peak of the measurement target gas TG (H2O) appears around a wave number of 6955.2 (cm−1) and an absorption peak of the mixed gas MG (CO2) appears around a wave number of 6955.7 (cm−1).

In the analysis unit26, based on the data of the spectral profile, mathematical calculation described later is further performed. More specifically, the spectral profile obtained at step S3is subjected to calculation using an approximate polynomial for rapid and simple processing equivalent to second order detection of WMS to measure various physical quantities of the measurement target gas TG.

In this context, in the present embodiment, a range [v−a′<v<v+a′] of a width2a′ about the center of each pointvon a wave number axis in the spectral profile obtained at step S3is considered to be represented by an approximate polynomial:
[Math 1]
τ(v)=b0+b1(v−v)+b2(v−v)2+b3(v−v)3+ . . .  (1).
The characters “v” and “v” denote wave numbers, “τ” denote a profile of a transmission spectrum, and “a′” denotes a modulation amplitude. Since an overline is not available due to the limitation of the electronic filing, underlined expression such as “v” is used while “v” means the overlined “v” in the expression (1). Then, an nth order differential of the expression (1) is obtained as

[Math⁢⁢2]dn⁢τ⁡(v)dvnv=v_=bn.(2)
Here, generally, the spectral profile of an nth order harmonic obtained by synchronous detection in WMS is approximately represented by the following expression (Equation 8 in Reid, J. and Labrie, D.; “Second-harmonic detection with tunable diode lasers-comparison of experiment and theory,” Appl. Phys. B 26, 203-210 (1981)):

[Math⁢⁢3]Hn⁡(ν_)≈21-π⁢τ⁡(v)n!⁢αn⁢dn⁢τ⁡(v)dvnv=v_,n≥1,(3)
and from the expressions (2) and (3),

[Math⁢⁢4]Hn⁡(ν_)≈21-nn!⁢an⁢bn,n≥1(4)
is obtained. Then, it is known that, to calculate a WMS signal to the wave number v in the spectrum obtained at step S3, the wave number range [v−a′<v<v+a′] is fit by the least squares method or the like (i.e., polynomial approximated) and the factors b0, b1, b2, b3, . . . are obtained (i.e., the factors of the approximate polynomial are obtained), and further v is sequentially changed to prepare the profiles of the factors b1 and b2 obtained by fitting, which are equivalent to WMS profiles of 1f and 2f (Japanese Patent No. 5983779).

In the present embodiment, by polynomial approximating in the spectrum obtained at step S3(step S4(polynomial approximation unit)), obtaining factors of the approximate polynomial (step S5), and preparing the profile of the factor b2, a second order differential curve (second order differential spectrum) is prepared (step S6(differential spectrum preparation unit)).FIG. 4is a diagram illustrating an example of the second order differential curve (second order differential spectrum) prepared by the method in the present embodiment.

Then, in the measurement procedure of the present embodiment, based on the second order differential curve obtained at step S6, the temperature and the concentration of the measurement target gas TG are calculated (step S7(physical quantity measurement unit)). Specifically, since correlation between the peak height of the second order differential curve obtained at step S6and the concentration of the measurement target gas TG is known (Japanese Patent No. 5983779), the concentration of the measurement target gas TG is measured by measuring the peak height of the second order differential curve. Since it is also known that a ratio of the magnitude of two absorption peaks of the second order differential curve changes with a change in the temperature of the measurement target gas TG (Japanese Patent No. 5983779), the temperature of the measurement target gas TG is measured by measuring the ratio of the magnitude of the two absorption peaks of the second order differential curve. The information on the pressure detected by the pressure sensor15is stored as the pressure of the measurement target gas TG.

As just described, the absorption spectrum of the measurement target gas TG is obtained from the change in the light intensity relative to the change in the wavelength of the laser light source12, the absorption spectrum is approximated by the approximate polynomial, and a second order differential spectrum of the absorption spectrum is prepared based on the factor in each term of the approximate polynomial, thereby allowing calculation of the temperature and the concentration of the measurement target gas TG from the second order differential spectrum. However, as a result of keen examinations by the present inventor, it is found that the peak height of the spectral profile obtained at step S3varies when the measurement object gas G contains the mixed gas MG other than the measurement target gas TG as in the present embodiment.

FIGS. 5A-5Care spectral profiles of the measurement target gas TG (H2O) where the measurement target gas TG is assumed to be H2O and the mixed gas MG to be CO2.FIG. 5Ais a profile for a mixed gas MG (CO2) concentration of 5%,FIG. 5Bis a profile for a mixed gas MG (CO2) concentration of 7%, andFIG. 5Cis a profile for a mixed gas MG (CO2) concentration of 10%. As seen from comparison ofFIGS. 5A-5C, it is understood that the higher the concentration of the mixed gas MG, the less the absorption of the measurement target gas TG (H2O).

As just described, when the peak height of the spectral profile obtained at step S3varies, the peak height of the second order differential spectrum obtained at step S6also varies, thereby not allowing accurate calculation of the temperature and the concentration of the measurement target gas TG from the second order differential spectrum obtained at step S6. To solve such a problem, in the present embodiment, the concentration of the mixed gas MG is calculated based on the second order differential curve obtained at step S6(step S8(gas concentration measurement unit)), thereby correcting the temperature and the concentration of the measurement target gas TG obtained at step S7(step S9(physical quantity correction unit)).

Specifically, at step S8, the absorption peak of the mixed gas MG (CO2) is detected (i.e., the peak at a wave number of approximately 6955.7 (cm−1) is detected) from the second order differential curve obtained at step S6and the peak height is measured to calculate the concentration of the mixed gas MG. Then, an amount of correction of the concentration and the temperature of the measurement target gas TG is calculated from the calculated concentration of the mixed gas MG, and the amount of correction is added or multiplied to the temperature and the concentration of the measurement target gas TG obtained at step S7to correct the temperature and the concentration of the measurement target gas TG (step S9).FIG. 6is a graph illustrating relationship between the concentration of the mixed gas MG (CO2) and the amount of correction (amount of change in the peak height of the second order differential spectrum) of the measurement target gas TG (H2O). As illustrated inFIG. 6, the amount of correction of the measurement target gas TG (H2O) (amount of change in the peak height of the second order differential spectrum) may be generally expressed by a power function. Accordingly, in the present embodiment, using an approximate expression of the characteristic curve illustrated inFIG. 6, the amount of correction of the measurement target gas TG (H2O) is obtained from the concentration of the mixed gas MG calculated by step S8.

The temperature and the concentration of the measurement target gas TG corrected at step S9are displayed on a display device, not shown, together with the pressure data stored at step S7.

As just described, the gas absorption spectrometer1according to the present embodiment approximates the absorption spectrum of the measurement target gas TG by the approximate polynomial, prepares a second order differential spectrum of the absorption spectrum based on the factor in each term of the approximate polynomial, and calculates the temperature and the concentration of the measurement target gas TG based on the second order differential spectrum. Accordingly, it is possible to rapidly and simply measure various physical quantities of the measurement target gas. Even when the measurement target gas TG is mixed with the mixed gas MG, the concentration and the temperature of the measurement target gas TG are corrected from the concentration of the mixed gas MG to allow measurement with high accuracy.

While the above embodiment of the present invention has been described, the present invention is not limited to the configuration, the specific numerical configuration, and the like in the present embodiment and is allowed to be variously modified within the scope of the technical idea of the present invention.

For example, while the description is made in the present embodiment assuming that the measurement target gas TG is H2O and the mixed gas MG is CO2, the present invention is not limited to such a configuration and is applicable to various measurement target gases TG and mixed gases MG. Moreover, the mixed gas MG is not limited to one type of gas, and the present invention is also applicable to a case of containing a plurality of mixed gases MG.

While the gas absorption spectrometer1according to the present embodiment is described that it calculates the temperature and the concentration of the measurement target gas TG, the gas absorption spectrometer1is not limited to such a configuration and may be configured to measure at least one of the temperature and the concentration of the measurement target gas TG.

While the gas absorption spectrometer1according to the present embodiment is described that it corrects the temperature and the concentration of the measurement target gas TG based on the concentration of the mixed gas MG, it may correct the temperature and the concentration of the measurement target gas TG based on the concentration and the pressure of the mixed gas MG.

While the description at step S8in the present embodiment is made that the concentration of the mixed gas MG is described to be calculated from the second order differential curve obtained at step S6, the present invention may be configured to, for example, dispose a concentration sensor in the gas cell11to measure the concentration of the mixed gas MG using the concentration sensor.

While the amount of correction of the measurement target gas TG is obtained using the approximate expression representing the relationship between the concentration of the mixed gas MG and the amount of correction of the measurement target gas TG at step S9in the present embodiment, the present invention is not limited to such a configuration and a correction value may be obtained from a database (by a so-called look up table (LUT)) that is prepared to obtain the correction value from, for example, relationship between the concentration of the mixed gas MG and the peak height of the second order differential spectrum of the measurement target gas TG.

For preparation of the database, it is also possible to prepare the correction value from numerical calculation. In theoretical expressions (5) and (6) of the absorption spectrum:

When a single pass cell or a multipass (multiple reflection) cell is used, the measurement procedure described with reference toFIG. 2is applicable to, not only for the case of using WMS as the gas absorption spectroscopy, but also the case of using an optical resonator cell such as cavity ring-down absorption spectroscopy (CRDS).

Modifications

FIG. 7is a diagram illustrating a modification of the gas absorption spectrometer in the present embodiment and illustrates a measurement system for the case of using CRDS as the gas absorption spectroscopy. A gas absorption spectrometer (cavity ring-down absorption spectrometer)100in the present modification includes: an optical resonator110to have a measurement object gas G introduced thereto, the measurement object gas G containing a measurement target gas TG and a mixed gas MG other than the measurement target gas TG; a laser light source112and an optical switch111disposed on one side across the optical resonator110; a photodetector113disposed on the other side; a light source control unit114to control the laser light source112; a pressure sensor115to detect a pressure of the measurement object gas G in the optical resonator110; and a measurement unit120connected to the photodetector113and the pressure sensor115to quantify output of the photodetector113and the pressure sensor115for storage and to perform various calculations.

The optical resonator110is a Herriott cell that is disposed between the optical switch111and the photodetector113to resonate light from the laser light source112. The optical resonator110is provided with two highly reflective mirrors110aand110b. The two mirrors110aand110bare disposed to face each other and configured to confine the light from the laser light source112incident on the optical resonator110between the two mirrors110aand110bto extend an effective optical path length. The optical resonator110in the present modification is also provided with a function as a gas cell to have the measurement object gas G introduced thereto, the measurement object gas G containing the measurement target gas TG and the mixed gas MG other than the measurement target gas TG. While the optical resonator110in the present modification is configured with the two mirrors110aand110b, a ring resonator configured with three or more mirrors may be used.

The optical switch111is a device that is disposed between the laser light source112and the optical resonator110to turn on/off (i.e., to transmit or block) the light from the laser light source112.

The laser light source112is, similar to the laser light source12in the above embodiment, wavelength tunable in a range covering absorption wavelengths of at least the measurement target gas TG and the mixed gas MG.

The light source control unit114is a device that controls the laser light source112to sweep (change) the wavelength of the laser light source112between predetermined minimum and maximum wavelengths.

The photodetector113is a device that detects intensity of light emitted from the optical resonator110, and the information (electrical signal) on the light intensity detected by the photodetector113is output to the measurement unit120.

The pressure sensor115is a sensor that has a sensor surface, not shown, exposed in the optical resonator110to detect the pressure of the measurement object gas G. The information (electrical signal) on the pressure detected by the pressure sensor115is output to the measurement unit120.

The measurement unit120has an A/D converter125and an analysis unit126. The A/D converter125is a device that digitizes the information on the light intensity output from the photodetector113and the information on the pressure output from the pressure sensor115for storage in a storage device, not shown. The analysis unit126is a device that mathematically calculates data obtained by the A/D converter125(data in the storage device) to measure the temperature and the concentration of the measurement target gas TG.

As described above, the light emitted from the laser light source112is incident, through the optical switch111, on the optical resonator110to have the measurement object gas G introduced thereto. When the frequency of the light incident on the optical resonator110coincides with the mode frequency of the optical resonator110, optical power is stored in the optical resonator110. When the light from the laser light source112is blocked by the optical switch111while the optical power is sufficiently stored in the optical resonator110, the light stored in the optical resonator110reciprocates in the resonator110from thousands to tens of thousands of times during which the light is attenuated due to the absorption of the measurement object gas G in the optical resonator110. Accordingly, by measuring the attenuation of the light leaked from the optical resonator110by the photodetector113to obtain a time constant (ring-down time) of the light attenuation, it is possible to obtain an absorption coefficient of the measurement object gas G at the frequency of light incident on the optical resonator110at that point (i.e., frequency of the light emitted from the laser light source112). Then, the light source control unit114sweeps the frequency of the light of the laser light source112, thereby obtaining the absorption spectrum of the measurement object gas G similar to the gas absorption spectrometer1in the above embodiment. The absorption coefficient α of the measurement object gas G is obtained by the following expression (7):

In the expression (7), “c” denotes speed of light, “τ” denotes a ring-down time while the measurement object gas G is in the optical resonator110, and “τ0” denotes a ring-down time while the measurement object gas G is not in the optical resonator110or the absorption by the components in the measurement object gas G is negligible.

As just described, employment of CRDS as the gas absorption spectroscopy allows significant extension of the effective length of the light transmitting the measurement object gas G and it is thus possible to detect very slight absorption of the measurement object gas G.