Gas absorption spectroscopy system and gas absorption spectroscopy method

Abstract

To improve the measuring sensitivity of a component contained in a gas sample without increasing the cost required in the introduction of a gas absorption spectroscopic system.SOLUTION: The gas absorption spectroscopy system includes a resonator, a light source, a driving device, a control device, and a detector. The resonator includes a first mirror and a second mirror. The light source irradiates the resonator with laser light. The driving device changes a length between the first mirror and the second mirror. The control device controls the drive device. The detector outputs a detection signal corresponding to the detected light to the control device. The drive device sweeps at least one of the first mirror and the second mirror in a wave shape around a sweep center, and adjusts a length between the sweep center and the second mirror to a length between the first mirror and the second mirror when the detection signal is acquired in response to the acquisition of the detection signal by the control device.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present disclosure relates to gas absorption spectroscopy systems and methods, and more particularly to improving the sensitivity of measuring a target component in a gas contained in a cell. [Background technology]

[0002] Cavity ring-down absorption spectroscopy (CRDS) is known as one type of gas absorption spectroscopy. CRDS is a measurement method that determines the concentration of a target component in a gas with high sensitivity by using a resonator (cavity) containing a high-reflectivity mirror to increase the effective optical path length for light absorption by the gas. Information on gas absorption spectroscopy devices using CRDS is disclosed, for example, in "Study on Highly Efficient Measurement Technology for Trace Moisture in Gases," Koji Hashiguchi, AIST Metrology Standards Report, Vol. 9, No. 2, October 2015 (Non-Patent Document 1) and "Development of a Low-Temperature Cavity Ring-Down Spectrometer for the Detection of CarBon-14," ADMcCartt, Stanford University, July 2014 (Non-Patent Document 2).

[0003] In CRDS, the concentration of a target component in a gas stored in a cell can be measured based on the ring-down signal obtained when the resonator is in a resonant state. To improve the measurement sensitivity of a target component in a gas, it is necessary to detect multiple ring-down signals and integrate them.

[0004] A known method for adjusting the cavity length to achieve a resonant state is to fix the laser frequency and sweep the cavity mirror in a triangular waveform, as disclosed in, for example, "Mid-infrared continuous wave cavity ring-down spectroscopy of a pulsed hydrocarbon plasma," Dongfeng Zhao, Joseph Guss, Anton J. Walsh, and Harold Linnartz, Chemical Physics Letters, Volume 565, pp. 132-137, 2013 (Non-Patent Document 3) and "CRDS Measurement Data Acquisition in Supersonic Expansion," M. Masat and O. Votava, WDS'11 Proceedings of Contributed Papers, Part II, pp. 204-207, 2011 (Non-Patent Document 4). In this method, the cavity length changes in a triangular waveform over time, and a ring-down signal can be obtained when the cavity length satisfies a predetermined condition.

[0005] In CRDS, measurement sensitivity can be improved by extending the time during which the resonance state occurs per unit time. As a method for maintaining the resonance state and extending the time during which the resonance state occurs per unit time, “Spectroscopic detection of radiocarbon dioxide at parts-per-quadrillion sensitivity”, Iacopo Galli, Saverio Bartalini, Riccardo Ballerini, Marco Barucci, Pablo Cancio, Marco De Pas, Giovanni Giusfredi, Davide Mazzotti, Naota Akikusa, and Paolo De Natale, Optica 3, 385-388, 2016 (Non-Patent Document 5) discloses the Pound-Drever-Hall (PDH) method, which controls the laser frequency based on the light first reflected by a mirror on the laser's incident side. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] "Study on Highly Efficient Measurement Technology for Trace Moisture in Gases," Koji Hashiguchi, AIST Metrology Standards Report Vol. 9, No. 2, October 2015. [Non-patent document 2] “Development of a low-temperature cavity Ring-Down Spectrometer for the detection of CarBon-14”, ADMcCartt, Stanford University, July 2014. [Non-patent document 3] “Mid-infrared continuous wave cavity ring-down spectroscopy of a pulsed hydrocarbon plasma”, Dongfeng Zhao, Joseph Guss, Anton J. Walsh, Harold Linnartz, Chemical Physics Letters, Volume 565, 132-137, 2013. [Non-patent document 4] “CRDS Measurement Data Acquisition in Supersonic Expansion”, M. Masat and O. Votava, WDS'11 Proceedings of Contributed Papers, Part II, 204-207, 2011. [Non-patent document 5] “Spectroscopic detection of radiocarbon dioxide at parts-per-quadrillion sensitivity”, Iacopo Galli, Saverio Bartalini, Riccardo Ballerini, Marco Barucci, Pablo Cancio, Marco De Pas, Giovanni Giusfredi, Davide Mazzotti, Naota Akikusa, and Paolo De Natale, Optica 3,385-388,2016. Summary of the Invention [Problem to be solved by the invention]

[0007] By using the PDH method disclosed in Non-Patent Document 5, it is possible to increase the time during which the resonance state occurs per unit time and improve the measurement sensitivity. However, the PDH method requires not only a detector for detecting the ring-down signal but also a detector for detecting the light first reflected by the mirror on the laser incident side, which may increase the cost of introducing a gas absorption spectroscopy device.

[0008] The present disclosure has been made in consideration of these circumstances, and its purpose is to improve the measurement sensitivity of components contained in a gas sample in a gas absorption spectroscopy system that measures gas components using CRDS, without increasing the cost of introducing the system. [Means for solving the problem]

[0009] A gas absorption spectroscopy system according to a first aspect of the present disclosure is a gas absorption spectroscopy system for measuring a target component in a gas contained in a cell. The gas absorption spectroscopy system includes a resonator including a first mirror and a second mirror arranged inside the cell so that light reflects between them, a light source that irradiates the resonator with laser light, a drive device that changes the length between the first mirror and the second mirror, a control device that controls the drive device, and a detector that detects light extracted from the resonator and outputs a detection signal corresponding to the detected light to the control device. The drive device causes at least one of the first mirror and the second mirror to sweep in a wave-like manner around a sweep center, and in response to the control device acquiring a detection signal, adjusts the length between the sweep center and the second mirror to the length between the first mirror and the second mirror when the detection signal is acquired.

[0010] A gas absorption spectroscopy method according to a second aspect of the present disclosure is a gas absorption spectroscopy method for measuring a target component in a gas contained in a cell using a resonator. The resonator includes a first mirror and a second mirror arranged inside the cell so that light is reflected between them. The gas absorption spectroscopy method includes the steps of irradiating the resonator with laser light from a light source, sweeping at least one of the first mirror and the second mirror in a wave-like manner around a sweep center, acquiring a detection signal from the resonator, and, in response to acquiring the detection signal, adjusting the length between the sweep center and the second mirror to the length between the first mirror and the second mirror when the detection signal was acquired. [Effects of the Invention]

[0011] According to the present disclosure, in a gas absorption spectroscopy system that measures gas components using CRDS, it is possible to improve the measurement sensitivity of components contained in a gas sample without increasing the cost of introducing the system. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram schematically illustrating the configuration of a gas absorption spectroscopy system. [Figure 2] FIG. 10 is a diagram for explaining mode frequencies. [Figure 3] 10A and 10B are diagrams for explaining a method of adjusting the resonator length so as to achieve a resonance state according to a comparative example. [Figure 4] 10A and 10B are diagrams for explaining a method for acquiring a ring-down signal according to a comparative example. [Figure 5] FIG. 3 is a diagram for explaining the sweep of the actuator according to the first embodiment. [Figure 6] FIG. 4 is a diagram for explaining a method for acquiring a ring-down signal according to the first embodiment. [Figure 7] FIG. 10 is a diagram for explaining the number of ring-down signals that can be acquired per unit time. [Figure 8] 4 is a flowchart showing gas absorption spectroscopy according to the first embodiment. [Figure 9] FIG. 10 is a block diagram schematically showing the overall configuration of a gas absorption spectroscopy system according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0014] [Device configuration] 1 is a block diagram schematically illustrating an overall configuration of a gas absorption spectroscopy system according to a first embodiment of the present disclosure. Referring to FIG. 1, the gas absorption spectroscopy system 100 is a spectroscopy system that measures optical absorption by a target component contained in a gas to be measured (sample gas) by cavity ring-down absorption spectroscopy (CRDS).

[0015] The gas absorption spectroscopy system 100 includes a laser light source 10, an AOM (Acousto-Optic Modulator) 20, a cell 30, a resonator 40, a mirror driver 50, a photodetector 60, and a controller .

[0016] The laser light source 10 irradiates the resonator 40 with laser light. The laser light source 10 is configured to be able to change the oscillation frequency of the laser light in accordance with commands from a controller 70. Specifically, the laser light source 10 includes a distributed feedback quantum cascade laser (QCL) 11 and a laser driver 12. The QCL 11 has a central oscillation frequency of, for example, 2200 cm -1 The QCL 11 emits laser light of about 4.5 μm wavelength. The laser driver 12 supplies a drive current to the QCL 11 in accordance with a command from the controller 70. By changing the drive current to the QCL 11, the oscillation frequency of the QCL 11 can be adjusted to 0.2 cm / s. -1 It can be swept to a certain extent.

[0017] The AOM 20 is provided in the optical path between the laser light source 10 and the resonator 40. The AOM 20 is an optical switch (switching device) that quickly switches between irradiating and blocking laser light from the laser light source 10 to the resonator 40 in accordance with a command from the controller 70. When an on command to irradiate light is applied from the controller 70 to the AOM 20, the AOM 20 goes into an on state in which laser light from the laser light source 10 is output to the resonator 40. When an off command to block light is applied from the controller 70 to the AOM 20, the AOM 20 goes into an off state in which laser light from the laser light source 10 is not output to the resonator 40.

[0018] The cell 30 is a container capable of sealingly holding the sample gas, and has, for example, a cylindrical shape. An inlet pipe 31 for introducing the sample gas before the start of measurement and an outlet pipe 32 for discharging the sample gas after the measurement are connected to the cell 30. An inlet valve 33 is provided on the inlet pipe 31. An outlet valve 34 is provided on the outlet pipe 32. The opening and closing of the inlet valve 33 and the outlet valve 34 can be controlled by a controller 70.

[0019] The resonator 40 is provided between the AOM 20 and the photodetector 60. In the first embodiment, the resonator 40 is a Fabry-Perot optical resonator. A pair of mirrors 41 and 42 are provided inside the resonator 40. The mirrors 41 and 42 are arranged facing each other so that light is reflected between them inside the resonator 40. Each of the mirrors 41 and 42 has a concave surface to facilitate satisfying the stability conditions of the resonator 40. Each of the mirrors 41 and 42 has a high reflectivity (for example, approximately 99.9%) so that light leaking outside the resonator 40 is extremely weak. The resonator length of the resonator 40 (the distance between the mirrors 41 and 42 in the optical axis direction) is, for example, approximately 450 mm. The number of mirrors arranged inside the resonator 40 is not limited to two, and may be three or more. That is, the resonator may be a resonator in which the mirrors are arranged so that light is reflected between them, or a resonator in which the mirrors are arranged in a ring shape so that light is reflected in one direction.

[0020] In the first embodiment, the resonator length of resonator 40 is the distance between mirror 41 and mirror 42 in the direction connecting mirror 41 and mirror 42 (optical axis direction). Hereinafter, this resonator length will be represented as L1. Resonator length L1 is, for example, 30 cm.

[0021] 1, both mirrors 41 and 42 are concave mirrors. However, both mirrors 41 and 42 do not have to be concave mirrors. At least one of mirrors 41 and 42 needs to be a concave mirror. For example, one of mirrors 41 and 42 may be a concave mirror and the other a plane mirror.

[0022] The mirror driving device 50 drives the mirrors 41 and 42 that constitute the resonator 40 in accordance with commands from the controller 70. In this embodiment, the mirror driving device 50 includes a pair of actuators provided to correspond to the pair of mirrors 41 and 42. Each actuator is a piezoelectric element (piezoelectric element) with a doughnut-shaped hole for passing light. The piezoelectric element 51 displaces the mirror 41 in the optical axis direction. Similarly, the piezoelectric element 52 displaces the mirror 42 in the optical axis direction.

[0023] When the voltage applied to the piezoelectric element 51 is changed linearly, the mirror 41 is displaced linearly. Therefore, to sweep the mirror 41 in a triangular waveform, a voltage with a triangular waveform is applied to the piezoelectric element 51. The same applies to the piezoelectric element 52. The control of the piezoelectric elements 51 and 52 will be described in detail later.

[0024] The photodetector 60 is a photodetector such as a photodiode or an image sensor. The photodetector 60 detects the weak light extracted from the mirror 42 of the resonator 40 as output light of the resonator 40, and outputs a signal (detection signal) indicating the detection result to the controller 70. For example, a liquid nitrogen-cooled InSb (indium antimony) detector or an MCT detector can be used as the photodetector 60.

[0025] The controller 70 includes a processor 71 such as a CPU (Central Processing Unit) or FPGA (Field-Programmable Gate Array), a memory 72 such as a ROM (Read Only Memory) and RAM (Random Access Memory), and an input / output port (not shown).

[0026] The controller 70 controls each device constituting the gas absorption spectroscopy system 100. Specifically, the controller 70 outputs a command to the laser driver 12 to scan the oscillation frequency of the laser light, and outputs the above-mentioned on or off signal to the AOM 20. The controller 70 outputs a command to the inlet valve 33 to introduce the sample gas into the resonator 40, and outputs a command to the outlet valve 34 to exhaust the sample gas to the outside of the resonator 40. The controller 70 applies voltages to the piezoelectric elements 51 and 52 to displace the mirrors 41 and 42. The controller 70 also performs various data processing operations to calculate the concentration (absolute concentration) of the target component contained in the sample gas based on the detection signal from the photodetector 60.

[0027] The controller 70 may be configured by dividing it into two or more units for each function. For example, the controller 70 may be divided into a unit that controls each device and a unit that executes various data processing.

[0028] [Measurement principle of cavity ring-down absorption spectroscopy (CRDS)] The measurement principle of cavity ring-down absorption spectroscopy in gas absorption spectroscopy system 100 will be briefly described below. Generally, resonance occurs when the frequency of the irradiated laser light and the length of the resonator satisfy the resonance condition. Hereinafter, the frequency of the laser light irradiated onto resonator 40 will be referred to as the "laser frequency," and the frequency of the laser light that can cause resonance by resonator 40 will be referred to as the "mode frequency."

[0029] Fig. 2 is a conceptual diagram for explaining mode frequencies. As shown in Fig. 2, there are a plurality of mode frequencies at a predetermined frequency interval. Hereinafter, the interval between two adjacent mode frequencies among the plurality of mode frequencies will be referred to as the "free spectral range" (FSR).

[0030] The resonance condition is that twice the length L of the resonator is an integer multiple of the wavelength λ of the laser light. Therefore, the resonator 40 is in a resonant state when the following formula (1) is satisfied.

[0031] 2L=qλ (1) In formula (1), q is an integer.

[0032] Here, the relationship between the wavelength λ of the laser light and the laser frequency ν is expressed by the following equation (2) using the speed of light c.

[0033] c=λν (2) Therefore, from equations (1) and (2), the resonance condition is expressed by the following equation (3).

[0034] ν=qc / 2L (3) There are multiple v's that satisfy this condition, and each frequency is a mode frequency of the resonator. Also, from equation (3), the FSR, which is the interval between two adjacent mode frequencies among the multiple mode frequencies, is expressed as c / 2L.

[0035] When the laser frequency does not match any of the mode frequencies, no optical power is stored in the resonator 40. On the other hand, when the laser frequency matches any of the mode frequencies, optical power is stored in the resonator 40.

[0036] The controller 70 determines whether the power of the laser light has been sufficiently accumulated in the resonator 40 based on the output signal from the photodetector 60 (the output light from the resonator 40). When the output light from the resonator 40 reaches a predetermined threshold, the controller 70 determines that the power of the laser light has been sufficiently accumulated in the resonator 40 and outputs an OFF signal to the AOM 20. This causes the AOM 20 to block the light input to the resonator 40. The light accumulated in the resonator 40 then travels back and forth between the mirrors 41 and 42 many times (typically thousands to tens of thousands of times). As this light travels back and forth between the mirrors 41 and 42, it gradually attenuates due to losses caused by reflection leakage from the mirrors 41 and 42 and absorption by the target component in the sample gas. Therefore, the output light from the resonator 40 leaking from the mirror 42 gradually attenuates. In CRDS, the distance (effective optical path length) that light travels through the sample gas is lengthened using the resonator 40, so that even if the light absorption by the target component is extremely small, the light absorption can be detected.

[0037] The controller 70 acquires the output signal of the photodetector 60 after the light input to the resonator 40 is blocked by the AOM 20 as a "ring-down signal," and calculates the decay time constant of the acquired ring-down signal as a "ring-down time." The controller 70 calculates the concentration of the target component contained in the sample gas from the calculated ring-down time.

[0038] The controller 70 acquires the output signal of the photodetector 60 at intervals of, for example, 0.2 μsec, and calculates the ring-down time from the acquired output signal of the photodetector 60. When no gas component that absorbs laser light is present inside the resonator 40, the ring-down time is a decay time constant of the resonator 40, and therefore is a roughly constant value. On the other hand, when a gas component that absorbs laser light is present inside the resonator 40, the ring-down time is a value that varies depending on the concentration of the gas component. Utilizing this feature, the concentration of the target component can be quantified.

[0039] [Comparative Example] In CRDS, after light (laser light) is accumulated in the resonator 40, the light input to the resonator 40 is blocked by the AOM 20, and the attenuation of the light leaking out of the resonator 40 after the light is blocked is measured by the photodetector 60. The time constant (ring-down time) of the light attenuation is calculated from the measured data, and the concentration of the target component contained in the gas inside the resonator 40 is measured.

[0040] CRDS can be used to analyze isotopic molecules by utilizing the fact that different wavelengths of infrared light are absorbed by the isotopes that make up the molecules. For example, radioactive carbon isotopes are the only long-lived radioactive nuclides among the isotopes of elements. 14 C is used as an environmental tracer. 14 By measuring the abundance ratio of C, it is possible to determine whether the organic resource is biomass derived from plants or fossil fuels. 14 C is also used as a biological tracer. In pharmaceutical development, some of the carbon in a compound is 14 C-labeled compounds are administered to living organisms and accumulated in the blood, urine, feces, and organs. 14 By measuring the concentration of C, it is possible to analyze the in vivo kinetics of the administered compound. 14 The C isotope ratio is very low. 14 To measure C, it must be distinguished from other isotopes of carbon. 14 It is necessary to detect C with high sensitivity. Thus, there is a need to improve the measurement sensitivity of components contained in gas samples in CRDS.

[0041] In CRDS, the concentration of a target component in a gas is derived based on a ring-down signal obtained when the resonator 40 is in a resonant state. The resonator 40 is in a resonant state when the frequency of the irradiated laser and the resonator length L1 satisfy the resonance condition. To improve the measurement sensitivity of a target component in a gas, it is necessary to detect multiple ring-down signals and integrate them.

[0042] If the cavity length L1 is even slightly different from the length required for resonance, it is not possible to obtain a ring-down signal. For example, if the cavity length L1 changes due to a temperature change during measurement, the cavity 40 will no longer be in a resonant state. Therefore, even if the laser frequency is fixed, it is necessary to continue adjusting the cavity length L1 during CRDS measurement to maintain the resonant state.

[0043] As a method for adjusting the resonator length L1 so that the resonator 40 is in a resonant state, for example, a method of sweeping the mirror 41 of the resonator 40 in a triangular wave shape is known, as disclosed in Non-Patent Documents 3 and 4. In this method, the resonator length L1 changes in a triangular wave shape, and a ring-down signal can be obtained at the timing when the resonator length L1 satisfies a predetermined condition.

[0044] 3 is a diagram for explaining a method for adjusting the resonator length L1 according to a comparative example. When formula (3) is rearranged for the resonator length L, the following formula (4) is obtained.

[0045] L=qc / 2ν (4) Therefore, when the laser frequency ν is fixed, by changing the cavity length L1 by at least c / 2ν, which is the length corresponding to 1 FSR, there exists a position of the mirror 41 where the resonance state occurs at least once during the sweep.

[0046] 3, in the adjustment method according to the comparative example, mirror 41 is swept at a constant frequency over a width equal to or greater than the length corresponding to 1 FSR. By doing so, there is a position of mirror 41 that enters a resonant state at least once during the sweep. By sweeping mirror 41 as described above, a resonant state occurs in one or more resonant modes regardless of the laser frequency. Therefore, even if the conditions for entering a resonant state change during measurement, a ring-down signal can be obtained at least once during the sweep of mirror 41.

[0047] 4 is a diagram for explaining the timing at which a ring-down signal is acquired in a comparative example. As shown in FIG. 4, if mirror 41 is swept with a width equal to or greater than the length corresponding to 1 FSR, a ring-down signal can be acquired regardless of the laser frequency.

[0048] In the above-described method for adjusting the cavity length L1, if the sweep width is set to a value less than the length corresponding to 1 FSR, there may be laser frequencies that do not resonate, so it is necessary to sweep the mirror 41 with a width equal to or greater than the length corresponding to 1 FSR. As a result, the sweep width cannot be narrowed, and the number of ring-down signals that can be obtained per unit time is limited.

[0049] As a method for improving the ring-down signal that can be obtained per unit time, there is the PDH method, as described in Non-Patent Document 5. In the PDH method, the light that is first reflected by the mirror on the incident side is detected by a detector on the laser incident side. The detected signal can be used to adjust the laser frequency to maintain the resonance state.

[0050] The PDH method requires not only a detector for detecting the ring-down signal but also a detector for detecting the light first reflected by the mirror on the laser's incident side, which can increase the cost of introducing a gas absorption spectroscopy device. Therefore, there is a need to increase the time in the resonance state per unit time and improve the measurement sensitivity of the target component contained in the gas sample without increasing the cost of introducing a gas absorption spectroscopy system.

[0051] [Gas absorption spectroscopy system according to the present disclosure] Therefore, in the gas absorption spectroscopy system 100 according to the first embodiment, in response to the acquisition of a ring-down signal by the controller 70, the controller 70 displaces the mirror 42 so that the distance between the sweep center of the mirror 41 and the mirror 42 becomes equal to the distance between the mirrors 41 and 42 when the ring-down signal was acquired. Therefore, the distance between the sweep center of the mirror 41 and the mirror 42 becomes equal to the resonator length that is most recently in a resonant state. This makes it possible to narrow the sweep width and increase the time during which the resonant state is achieved per unit time.

[0052] Specifically, the sweep width in the first embodiment may be a width corresponding to the difference between the cavity length at which the most recent resonance occurs and the cavity length at which the resonance occurs due to changes in measurement conditions (such as temperature) that occur during the sweep. This difference is generally smaller than the length corresponding to 1 FSR.

[0053] Therefore, the gas absorption spectroscopy system 100 according to the first embodiment can narrow the sweep width and speed up the sweep frequency, thereby increasing the ring-down signal that can be acquired per unit time, thereby improving the measurement sensitivity of components contained in a gas sample by CRDS.

[0054] Furthermore, according to the first embodiment, unlike the PDH method, a photodetector on the laser incident side is not required, so the measurement sensitivity of CRDS can be improved without increasing the cost of introducing a gas absorption spectroscopy device.

[0055] The manner in which the gas absorption spectroscopy system 100 acquires the ring-down signal is described below.

[0056] <Determining the initial sweep center> The gas absorption spectroscopy system 100 first determines the position of the initial sweep center of the mirror 41. The position of the initial sweep center may be determined in advance depending on the type of target component in the sample gas, or may be determined after the mirror 41 and / or the mirror 42 are displaced and a ring-down signal is acquired.

[0057] In the following, a case will be described in which the mirror 41 is displaced and the position where the ring-down signal is first acquired is set as the initial sweep center of the mirror 41.

[0058] The gas absorption spectroscopy system 100 controls the laser driver 12 so that the sample gas to be measured is irradiated with laser light having a laser frequency ν, while the cell 30 is filled with the sample gas.

[0059] The controller 70 causes the AOM 20 to cut off the laser light at a predetermined timing, and then the controller 70 applies a voltage to the piezoelectric element 51 to displace the mirror 41.

[0060] When the mirror 41 is displaced and the laser frequency v matches the resonance condition of the resonator 40, the photodetector 60 acquires a ring-down signal. The controller 70 acquires the voltage value V1 of the piezoelectric element 51 when the laser frequency v matches the resonance condition of the resonator 40. The gas absorption spectroscopy system 100 determines the position of the mirror 41 at this time as the initial sweep center of the mirror 41.

[0061] <Feedback control for mirror 42> Next, the gas absorption spectroscopy system 100 sweeps the mirror 41 and displaces the mirror 42 in response to the acquisition of the ring-down signal. Fig. 5 is a diagram for explaining the feedback control performed on the mirror 42 when the gas absorption spectroscopy system 100 acquires the ring-down signal in the first embodiment.

[0062] The controller 70 applies a triangular waveform voltage to the piezoelectric element 51, centered on the initial sweep center of the mirror 41, to sweep the mirror 41 in a triangular waveform. The sweep width of the mirror 41 at this time may be less than the length corresponding to 1 FSR, for example, the length corresponding to 0.01 FSR. The frequency at which the mirror 41 is swept is 100 Hz to 500 Hz.

[0063] When a ring-down signal is acquired while the mirror 41 is being swept, the controller 70 acquires the voltage value V2 of the piezo element 51 at the timing when the ring-down signal is acquired.

[0064] The difference between the sweep center of mirror 41 and the position of mirror 41 when the ring-down signal is acquired corresponds to voltage value V3, which is the difference between voltage value V1 and voltage value V2. Voltage value V3 is applied to piezo element 52 to displace mirror 42.

[0065] Voltage V3 is applied to piezo element 52, and feedback control is performed to displace mirror 42, thereby resetting the sweep center of mirror 41. The length between mirror 42 and the reset sweep center of mirror 41 matches the length between mirror 41 and mirror 42 when the mirror most recently entered a resonant state.

[0066] The controller 70 continues sweeping the mirror 41 and feedback controlling the mirror 42 until scanning at the laser frequency v is completed. Scanning at the laser frequency v is completed, for example, when a predetermined number of ring-down signals have been acquired or when a predetermined time has elapsed since the start of measurement.

[0067] 6 is a diagram illustrating the results of acquiring a ring-down signal according to the first embodiment. As shown in FIG. 6, by displacing mirror 42, the distance between mirror 41 and the sweep center of mirror 42 can be adjusted to the distance between mirrors 41 and 42 that most recently reached a resonant state. Therefore, the sweep width of mirror 41 only needs to be a width that corresponds to the change in resonant frequency that occurs between the acquisition of one ring-down signal and the acquisition of the next ring-down signal. As a result, as shown in FIG. 6, the number of ring-down signals detected per unit time by photodetector 60 can be increased.

[0068] As described above, adjusting the length between the sweep center of mirror 41 and mirror 42 to the length between mirrors 41 and 42 that are most recently in a resonant state is performed, for example, by adjusting the voltage value applied to piezoelectric element 52.

[0069] After completing the scan at the laser frequency v, the gas absorption spectroscopy system 100 changes the laser frequency and acquires the ring-down signal again, and calculates the concentration of the target component contained in the sample gas based on the spectrum data obtained by measuring the laser frequencies.

[0070] FIG. 7 is a diagram illustrating the number of ring-down signals that can be acquired per unit time in CRDS measurements. In FIG. 7, the plot indicates the timing at which the ring-down signals are acquired. The horizontal axis of the diagram shown in FIG. 7 indicates the modulation frequency of the laser, and the vertical axis indicates the voltage value applied to the piezoelectric element 51. FIG. 7 shows the number of ring-down signals that can be acquired per unit time in each of the method described in the first embodiment and the method described in the comparative example.

[0071] As shown as a comparative example in FIG. 7, when mirror 41 is swept over a width equal to or greater than the length corresponding to 1 FSR, a voltage of 5 V is applied to piezoelectric element 51. In the first embodiment, a voltage of 0.05 V is applied to piezoelectric element 51. Therefore, the sweep width can be narrowed in the first embodiment compared to the comparative example. As a result, in the first embodiment, the speed at which mirror 41 is swept can be increased (the number of reciprocating movements per unit time can be increased) compared to the comparative example, and the number of ring-down signals that can be obtained per unit time is approximately 100 times greater.

[0072] [Flowchart for gas absorption spectroscopy measurements] The following describes the flow of processing related to gas absorption spectroscopy performed by the controller 70. FIG. 8 is a flowchart showing gas absorption spectroscopy performed by the controller 70. In one implementation example, the processing of FIG. 8 is called from a main routine by starting an application program for gas absorption spectroscopy in the controller 70. In the first embodiment, adjusting the length between the sweep center of mirror 41 and mirror 42 to the length between mirrors 41 and 42 that has reached the most recent resonance state is performed by adjusting the voltage value applied to piezoelectric element 52.

[0073] 8, in step S10, controller 70 receives information for identifying the type of target component in the sample gas. For example, the measurer can input the type of target component by operating an input device (not shown) such as a keyboard or a mouse.

[0074] The frequency range of the laser light used for measurement is predetermined to be near the absorption peak of the target component in accordance with the type of target component in the sample gas. Furthermore, the scanning conditions of the mirror 41 (the sweep width and sweep frequency of the mirror 41) are determined for each type of target component. These predetermined measurement conditions are stored in the memory 72 in the controller 70.

[0075] In step S12, the controller 70 reads out the scanning frequency range of the laser light according to the target component from the memory 72. The controller 70 also reads out the scanning conditions of the mirrors 41 and 42 according to the target component from the memory 72.

[0076] In step S14, the controller 70 opens the inlet valve 33 while the outlet valve 34 is closed, thereby introducing the sample gas into the cell 30. The cell 30 is provided with a pressure sensor (not shown) that measures the internal pressure of the cell 30. When the internal pressure measured by the pressure sensor reaches a predetermined value, the controller 70 closes the inlet valve 33. This causes the cell 30 to be filled with the sample gas.

[0077] In step S16, the controller 70 sets the oscillation frequency (laser frequency) v of the laser light in the laser light source 10 and irradiates the laser light onto the resonator 40. More specifically, a correspondence relationship between the laser frequency v and the drive current to the QCL 11 required to oscillate the QCL 11 at the laser frequency v is determined in advance. By referencing this correspondence relationship, the controller 70 outputs a command to the laser driver 12 to output a drive current corresponding to the desired laser frequency v.

[0078] In step S18, the controller 70 determines the initial sweep center of the mirror 41. The initial sweep center may be determined in advance depending on the type of target component in the sample gas, or may be determined in response to the acquisition of a ring-down signal after displacing the mirror 41 and / or the mirror 42.

[0079] In step S20, the controller 70 sweeps the mirror 41 around the determined sweep center with the sweep width and sweep frequency read out in step S12. During this time, the controller 70 causes the AOM 20 to cut off the laser light at a predetermined timing. The controller 70 acquires the voltage value V1 of the piezo element 51 when the mirror 41 is positioned at the sweep center.

[0080] In step S22, the controller 70 checks whether or not a ring-down signal has been acquired by the photodetector 60. If the controller 70 determines that the photodetector 60 has acquired a ring-down signal (YES in step S20), it acquires the voltage value V2 of the piezo element 51 at the timing when the ring-down signal was acquired, and proceeds to step S22; if not (NO in step S20), it repeats the processing of step S22.

[0081] In step S24, the controller 70 determines whether scanning of the laser frequency v is complete. Whether scanning of the laser frequency v is complete is determined, for example, based on the number of acquired ring-down signals and the elapsed time since the start of the sweep. If scanning of the laser frequency v is complete (YES in step S24), the controller 70 proceeds to step S26; if not (NO in step S24), the controller 70 proceeds to step S28.

[0082] In step S26, the controller 70 determines whether scanning with the other laser frequency has been completed. If scanning with the other laser frequency has been completed (YES in step S26), the controller 70 proceeds to step S30; if not (NO in step S26), the controller 70 proceeds to step S32.

[0083] In step S28, the controller 70 displaces the mirror 42 to change the sweep center so that the distance between the sweep center of the mirror 41 and the mirror 42 becomes the resonator length in a resonant state. Specifically, the controller 70 applies a voltage value V3, which is the difference between the voltage values ​​V1 and V2, to the piezo element 52 to displace the mirror 42. Thereafter, the controller 70 returns the process to step S20.

[0084] In step S30, the controller 70 calculates the ring-down time τ of the sample gas at the laser frequency ν based on the ring-down signal at each laser frequency ν measured in step S20, and creates an absorption spectrum of the sample gas.

[0085] In step S32, the controller 70 increments the laser frequency v by a predetermined scanning width Δv, and returns the process to step S16. Note that the scanning manner of the laser frequency v is not particularly limited. Decrements may be used instead of increments, and the scanning width Δv does not have to be a fixed width.

[0086] In step S34, the controller 70 calculates the absolute concentration (number density N) of the target component in the sample gas. For example, the peak frequency can be determined by curve fitting the absorption spectrum, and the number density N can be calculated from the absorption coefficient α at the peak frequency.

[0087] In step S36, the controller 70 opens the exhaust valve 34 and uses a vacuum pump (not shown) installed downstream of the exhaust valve 34 to exhaust the sample gas from the cell 30. This completes the series of processes. Thereafter, the controller 70 returns the process to the main routine.

[0088] In the first embodiment, in response to acquisition of a ring-down signal, the length between the sweep center of mirror 41 and mirror 42 is adjusted to meet the condition for achieving a resonance state. This narrows the sweep width of mirror 41, making it possible to increase the sweep speed and increase the ring-down signal that can be acquired per unit time. This improves the measurement sensitivity in CRDS measurements.

[0089] Furthermore, according to the first embodiment, it is possible to increase the ring-down signal that can be acquired per unit time without using a photodetector on the laser incident side, thereby improving the measurement sensitivity of CRDS without increasing the cost of introducing a gas absorption spectroscopy device.

[0090] In the first embodiment, it has been described that feedback control of mirror 42 is performed in response to controller 70 acquiring a ring-down signal, but feedback control may also be performed on mirror 41. In this case, gas absorption spectroscopy system 100 does not need to include piezoelectric element 52. Controller 70 performs two types of control on piezoelectric element 51: control to sweep mirror 41 in a triangular wave shape, and feedback control to align the sweep center with a position where a resonance state occurs.

[0091] Furthermore, in the first embodiment, it has been described that feedback control of mirror 42 is performed each time controller 70 acquires a ring-down signal, but the timing at which feedback control is performed is not limited to this. For example, after acquiring ten ring-down signals, the deviations between the position of mirror 41 and the mirror sweep center at the timing at which each ring-down signal was acquired may be added up, and feedback control of mirror 42 may be performed at the timing at which the tenth ring-down signal is acquired.

[0092] [Embodiment 2] In the first embodiment, a configuration has been described in which a Fabry-Perot type resonator 40 including two mirrors 41 and 42 is used. In the second embodiment, a configuration will be described in which a ring type optical resonator including three mirrors is used.

[0093] Fig. 9 is a block diagram schematically showing the overall configuration of a gas absorption spectroscopy system 200 according to the second embodiment. Referring to Fig. 9, the gas absorption spectroscopy system 200 differs from the gas absorption spectroscopy system 100 according to the first embodiment (see Fig. 1) in that the gas absorption spectroscopy system 200 includes a resonator 80 instead of the resonator 40. Note that in Fig. 9, a sample gas introduction / exhaust mechanism provided in the cell 30 is not shown to avoid cluttering the drawing.

[0094] The resonator 80 includes three mirrors 81 to 83 arranged inside the cell 30. The laser light irradiated onto the resonator 80 is repeatedly reflected in the order mirror 81, mirror 82, mirror 83, mirror 81, mirror 82, mirror 83, and so on. The mirrors 81 and 82 are flat mirrors. The mirror 83 is a concave mirror. The distance between the mirrors 81 and 82 is equal to the distance between the mirrors 83 and 82. This distance is referred to as the "resonator length L2."

[0095] The mirror 83 is provided with a piezoelectric element 90. The piezoelectric element 90 displaces the mirror 83 in accordance with a command from the controller 70. This makes it possible to change the resonator length L2. Note that the piezoelectric element 90 does not have a doughnut-shaped hole.

[0096] Other configurations of gas absorption spectroscopy system 200 than those described above are equivalent to the corresponding configurations of gas absorption spectroscopy system 100 according to embodiment 1. Furthermore, the gas absorption spectroscopy method according to embodiment 2 is also equivalent to the method according to embodiment 1 (see FIG. 8), and therefore detailed description will not be repeated.

[0097] Mirror 81 corresponds to the "first mirror" according to the present disclosure. Mirror 82 corresponds to the "third mirror" according to the present disclosure. Mirror 83 corresponds to the "second mirror" according to the present disclosure.

[0098] In the second embodiment as well, the piezoelectric element 90 provided on the mirror 83 is controlled to sweep the mirror 83, and the controller 70 adjusts the cavity length L2 in response to acquisition of a ring-down signal so that the center of the sweep of the mirror 83 is in a resonant state. As a result, as in the first embodiment, the mirror 83 is swept around the position where the resonance state is achieved, so that the sweep frequency can be increased and the ring-down signal that can be acquired per unit time can be increased. Therefore, according to the second embodiment as well, in a gas absorption spectroscopy system that measures gas components by CRDS, the measurement sensitivity of components contained in a gas sample can be improved without increasing the cost of introducing the system.

[0099] [Aspect] It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0100] (Item 1) A gas absorption spectroscopy system according to one embodiment is a gas absorption spectroscopy system for measuring a target component in a gas contained in a cell, comprising: a resonator including a first mirror and a second mirror arranged so that light reflects between them inside the cell; a light source that irradiates laser light onto the resonator; a drive device that changes the length between the first mirror and the second mirror; a control device that controls the drive device; and a detector that detects light extracted from the resonator and outputs a detection signal corresponding to the detected light to the control device, wherein the drive device causes at least one of the first mirror and the second mirror to sweep in a wave-like manner around a sweep center, and in response to the control device acquiring the detection signal, adjusts the length between the sweep center and the second mirror to the length between the first mirror and the second mirror when the detection signal is acquired.

[0101] According to the gas absorption spectroscopy system described in paragraph 1, in a gas absorption spectroscopy system for measuring gas components, the measurement sensitivity of components contained in a gas sample can be improved without increasing the cost of introducing the system.

[0102] (Item 2) In the gas absorption spectroscopy system described in Item 1, the detection signal may be detected by the detector when the frequency of the laser light matches the resonance frequency of the resonator.

[0103] According to the gas absorption spectroscopy system described in paragraph 2, the length between the sweep center of the first mirror and the second mirror is adjusted based on the ring-down signal obtained when the frequency of the laser light matches the frequency of the resonator.

[0104] (Item 3) In the gas absorption spectroscopy system described in item 1 or 2, the second mirror may be configured to be displaceable, and the driving device may displace the second mirror to adjust the length between the sweep center and the second mirror.

[0105] According to the gas absorption spectroscopy system described in the third aspect, the first mirror is swept, and the second mirror is subjected to feedback control in response to the acquisition of the ring-down signal.

[0106] (Item 4) In the gas absorption spectroscopy system described in any one of Items 1 to 3, the driving device may include a first actuator that displaces the first mirror and a second actuator that displaces the second mirror.

[0107] In the gas absorption spectroscopy system described in Section 4, the positions of the two mirrors are displaced by an actuator that converts an electrical signal into physical motion.

[0108] (Item 5) In the gas absorption spectroscopy system described in item 4, each of the first actuator and the second actuator may be a piezoelectric element.

[0109] In the gas absorption spectroscopy system described in item 5, the positions of the two mirrors are displaced by a piezoelectric element included in the actuator.

[0110] (Item 6) In the gas absorption spectroscopy system described in any one of Items 1 to 5, the driving device may sweep the first mirror with a width smaller than a length corresponding to a free spectral range (FSR), which is the interval between two adjacent mode frequencies.

[0111] In the gas absorption spectroscopy system described in Section 6, the mirror is swept with a width smaller than the length corresponding to 1 FSR, so the mirror can be moved at a high speed.

[0112] (Item 7) In the gas absorption spectroscopy system described in any one of Items 1 to 6, the wave shape may be a triangular wave shape.

[0113] In the gas absorption spectroscopy system described in paragraph 7, the mirror is swept in a triangular wave shape. (Item 8) In the gas absorption spectroscopy system described in any one of Items 1 to 7, the driving device may sweep the first mirror at a frequency of 100 to 500 Hz.

[0114] According to the gas absorption spectroscopy system described in item 8, the mirror is swept at a frequency of 100 to 500 Hz.

[0115] (Item 9) In the gas absorption spectroscopy system described in any one of Items 1 to 8, the resonator may further include a third mirror, and the distance between the third mirror and the first mirror may be equal to the distance between the first mirror and the second mirror.

[0116] According to the gas absorption spectroscopy system described in paragraph 9, the measurement sensitivity of components contained in a gas sample can be improved in a ring-shaped optical resonator including three mirrors without increasing the cost of introducing the system.

[0117] (Item 10) A gas absorption spectroscopy method according to one embodiment is a gas absorption spectroscopy method for measuring a target component in a gas contained in a cell using a resonator, the resonator including a first mirror and a second mirror arranged so that light is reflected between them inside the cell, and the gas absorption spectroscopy method may include the steps of irradiating laser light from a light source onto the resonator, sweeping at least one of the first mirror and the second mirror in a wave-like manner around a sweep center, acquiring a detection signal from the resonator, and, in response to acquiring the detection signal, adjusting the length between the sweep center and the second mirror to the length between the first mirror and the second mirror when the detection signal was acquired.

[0118] According to the gas absorption spectroscopy system described in paragraph 10, in a gas absorption spectroscopy system for measuring gas components, the measurement sensitivity of components contained in a gas sample can be improved without increasing the cost of introducing the system.

[0119] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims. Furthermore, it is intended that each technique in the embodiments can be implemented alone or, if necessary, in combination with other techniques in the embodiments to the extent possible. [Explanation of symbols]

[0120] 10 laser light source, 11 quantum cascade laser, 12 laser driver, 30 cell, 31 inlet tube, 32 outlet tube, 33 inlet valve, 34 outlet valve, 40, 80 resonator, 41, 42, 81, 82, 83 mirror, 50 mirror drive device, 51, 52, 90 piezoelectric element, 60 photodetector, 70 controller, 71 processor, 72 memory, 100, 200 gas absorption spectroscopy system.

Claims

1. A gas absorption spectroscopy system for measuring a target component in a gas contained in a cell, comprising: a resonator including a first mirror and a second mirror arranged inside the cell so that light reflects between them; a light source that irradiates the resonator with laser light; a driving device that changes the length between the first mirror and the second mirror; a control device that controls the drive device; a detector that detects the light extracted from the resonator and outputs a detection signal corresponding to the detected light to the control device; The drive device is At least one of the first mirror and the second mirror is swept in a wave shape around a sweep center; and wherein, in response to the control device acquiring the detection signal, the control device adjusts the length between the sweep center and the second mirror to the length between the first mirror and the second mirror when the detection signal was acquired.

2. 2. The gas absorption spectroscopy system of claim 1, wherein the detection signal is detected by the detector when the frequency of the laser light matches the resonant frequency of the resonator.

3. the second mirror is configured to be displaceable; 3. The gas absorption spectroscopy system according to claim 1, wherein the driving device displaces the second mirror to adjust the length between the sweep center and the second mirror.

4. The drive device is a first actuator for displacing the first mirror; 3. The gas absorption spectroscopy system of claim 1, further comprising: a second actuator for displacing the second mirror.

5. The gas absorption spectroscopy system of claim 4 , wherein each of the first actuator and the second actuator is a piezoelectric element.

6. 3. The gas absorption spectroscopy system according to claim 1, wherein the driving device sweeps the first mirror with a width smaller than a length corresponding to a free spectral range (FSR), which is the interval between two adjacent mode frequencies.

7. 3. The gas absorption spectroscopy system according to claim 1, wherein the wave shape is a triangular wave shape.

8. 3. The gas absorption spectroscopy system according to claim 1, wherein the driving device sweeps the first mirror at a frequency of 100 to 500 Hz.

9. the resonator further includes a third mirror; 3. The gas absorption spectroscopy system of claim 1, wherein the distance between the third mirror and the first mirror is equal to the distance between the first mirror and the second mirror.

10. A gas absorption spectroscopy method for measuring a target component in a gas contained in a cell using a resonator, comprising: the resonator includes a first mirror and a second mirror arranged to reflect light between each other within the cell; The gas absorption spectroscopy method includes: irradiating the resonator with laser light from a light source; Sweeping at least one of the first mirror and the second mirror in a wave shape around a sweep center; obtaining a detection signal from the resonator; and adjusting, in response to acquiring the detection signal, the length between the sweep center and the second mirror to the length between the first mirror and the second mirror when the detection signal was acquired.

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