Laser Gas Analyzer

Abstract

To provide a laser type gas analyzer capable of highly accurately measuring the concentration of a specific gas included in the gas to be measured.SOLUTION: One embodiment of the present invention is a laser type gas analyzer 1 comprising: a modulating light generation unit 11 that supplies a drive current to a laser element 12; a light receiving element 22 that receives a laser light 30; a filter unit 125 that extracts, from detection signals outputted from the light receiving element 22, an integral multiple of the modulation frequency of the laser light 30 that is wavelength modulated; a waveform analysis unit 135 that calculates amplitude information on unnecessary regions that do not include absorption of a gas to be measured, using some of the detection signals before subtraction that are outputted from an AD converter 128; a subtraction circuit 126 that subtracts the amplitude information on the unnecessary regions from the detection signals outputted from the filter unit 125; and a measurement unit 130 that performs gas analysis on the detection signals after subtraction that is outputted from the AD converter 128 by way of the subtraction circuit 126.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a laser gas analyzer that analyzes the presence and concentration of various target gases in a target space. [Background technology]

[0002] In a laser gas analyzer, a laser element emits laser light at an optical absorption wavelength that is absorbed by the target gas (a gas molecule), causing the target gas to absorb the laser light. The presence or absence of the target gas is detected based on the amount of laser light absorbed at that optical absorption wavelength. In addition, a laser gas analyzer can also detect the concentration of the target gas, because the amount of laser light absorbed at that optical absorption wavelength is proportional to the concentration of the target gas. It is necessary to select and analyze only a specific target gas from among the many gases present in the target space. Therefore, from the optical absorption wavelengths of the target gas and other gases in the target space, an optical absorption wavelength that is absorbed only by the target gas but not by other gases is selected.

[0003] Among these, wavelength modulation spectroscopy is commonly used for measuring trace gases, as described in Patent Document 1, for example. In wavelength modulation spectroscopy, a tunable laser light source emits laser light whose wavelength is swept by a drive current and modulated at a specific frequency. The laser light is detected by a photodetector, and a lock-in detection unit performs lock-in detection on the detection signal at an integer multiple of the modulation frequency. The gas concentration can then be calculated based on a proportional or other correspondence relationship between the gas concentration of the measurement target gas and the amplitude information of the lock-in detection waveform. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2012-177612 A Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, the detection signal having a frequency that is an integral multiple of the modulation frequency corresponding to the gas concentration of the gas to be measured is ideally a signal derived only from the absorption of the gas. However, the actual detection signal contains unnecessary signals that are unrelated to the gas absorption signal and have the same frequency as the gas absorption signal due to the nonlinearity of the laser diode and distortion in the signal processing circuit. As a result, the gas absorption signal becomes relatively small within the detection signal, resulting in insufficient resolution of the gas absorption signal and an accompanying decrease in measurement accuracy.

[0006] Furthermore, in recent years, there has been an increasing need to be able to analyze trace amounts of target gases on the ppm level, which has led to an increasing lack of resolution in the gas absorption signal.

[0007] Therefore, the present invention has been made to solve the above problems, and its object is to provide a laser gas analyzer that reduces the influence of unnecessary components and improves the resolution of the detection signal and measurement accuracy. [Means for solving the problem]

[0008] One aspect of the present invention is a laser gas analyzer for performing gas analysis of a measurement target gas present in a measurement target space, comprising: a laser element for emitting laser light in a wavelength band including an optical absorption wavelength of the absorption line spectrum of the measurement target gas; a modulated light generating unit for supplying to the laser element a drive current generated so that the wavelength is repeatedly swept and modulated in the wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas; a light receiving element for receiving the laser light that has passed through the measurement target; and a light receiving unit for receiving a detection signal output from the light receiving element, the light receiving element receiving a frequency that is an integer multiple of the modulation frequency of the wavelength-modulated laser light. an AD converter that performs analog-to-digital conversion on the detection signal output from the filter; a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a portion of the detection signal before subtraction that is output from the AD converter; a subtraction circuit that subtracts the amplitude information of the unnecessary region from the detection signal output from the filter; and a measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detection at an integer multiple of the modulation frequency of the subtracted detection signal that is output from the AD converter via the subtraction circuit.

[0009] One aspect of the present invention is characterized in that a DA converter that performs digital-to-analog conversion of amplitude information of the unnecessary region is provided between the waveform analysis unit and the subtraction circuit, and the subtraction circuit performs subtraction processing on the digital-to-analog converted amplitude information.

[0010] One aspect of the present invention is a laser gas analyzer for performing gas analysis of a measurement target gas present in a measurement target space, comprising: a laser element for emitting laser light in a wavelength band including an optical absorption wavelength of the absorption line spectrum of the measurement target gas; a modulated light generating unit for supplying to the laser element a drive current generated so that the wavelength is repeatedly swept and modulated in the wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas; a light receiving element for receiving the laser light that has passed through the measurement target; and a light receiving unit for receiving a detection signal output from the light receiving element, the light receiving element receiving a frequency that is an integer multiple of the modulation frequency of the wavelength-modulated laser light. an AD converter that performs analog-to-digital conversion on the detection signal output from the filter unit; a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a portion of the detection signal before subtraction that is output from the AD converter; a subtraction circuit that subtracts the amplitude information calculated by the waveform analysis unit from the drive current generated by the modulated light generation unit; and a measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detection of the subtracted detection signal output from the AD converter at an integer multiple of the modulation frequency.

[0011] One aspect of the present invention is characterized in that a DA converter is provided between the subtraction circuit and the laser element, which performs digital-to-analog conversion of the drive current subtracted by the subtraction circuit, and the digital-to-analog converted drive current is supplied to the laser element. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a laser gas analyzer that can analyze a gas to be measured with high precision. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is an overall configuration diagram of a laser gas analyzer according to an embodiment of the present invention; [Figure 2] FIG. 2 is a signal processing block diagram of the laser gas analyzer according to the first embodiment. [Figure 3]FIG. 3(a) is a conceptual diagram of the waveform of the drive current, and FIG. 3(b) is a conceptual diagram of the waveform of the lock-in detection signal. [Figure 4] FIG. 10 is an enlarged view of a lock-in detection signal. [Figure 5] These are waveform diagrams of detection signals before analog-to-digital conversion in a conventional laser gas analyzer, where Fig. 5(a) is the detection signal when there is no gas absorption, Fig. 5(b) is a conceptual diagram of an enlarged portion of Fig. 5(a), and Fig. 5(c) is the detection signal when there is gas absorption (without subtraction processing). [Figure 6] This is a lock-in detection signal obtained by lock-in detection of the detection signal in FIG. 5(c). [Figure 7] These are detection signals before analog-to-digital conversion in a laser gas analyzer according to this embodiment. Figure 7(a) is the detection signal after subtraction when there is no gas absorption, and Figure 7(b) is the detection signal after subtraction when there is gas absorption. [Figure 8] This is a lock-in detection signal obtained by lock-in detection of the detection signal after subtraction in FIG. 7(b). [Figure 9] FIG. 3 is a diagram showing a signal processing flow of the laser gas analyzer according to the first embodiment. [Figure 10] FIG. 10 is a signal processing block diagram of a laser gas analyzer according to a second embodiment. [Figure 11] FIG. 10 is a diagram showing a signal processing flow of the laser gas analyzer according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, a laser gas analyzer according to the present embodiment will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiment, and can be modified appropriately within the scope of the present invention.

[0015] <Outline of laser gas analyzer> 1 is a diagram showing the overall configuration of a laser gas analyzer according to an embodiment of the present invention. As shown in FIG. 1, the laser gas analyzer 1 includes a light-emitting unit 10, a light-receiving unit 20, and a communication line 40.

[0016] The laser gas analyzer 1 analyzes a target gas present in a target space. In the laser gas analyzer 1, a laser beam 30 emitted from a light-emitting unit 10 is irradiated onto the target gas flowing through the interior (target space) of walls 50a and 50b that form the flow path of the target gas. The laser beam 30 passes through the target gas and enters a light-receiving unit 20, and the concentration of a specific gas can be determined from the detected light intensity. If the gas concentration is 0 or below a predetermined value, it can be detected that the gas is absent, and therefore the presence or absence of the gas can also be detected.

[0017] The light-emitting unit 10 and the light-receiving unit 20 are detachably attached to walls 50a, 50b that form the flow path of the gas to be measured. The walls 50a, 50b are walls such as flue walls through which a specific gas flows, and each has a hole drilled therein. The flanges 51a, 51b are fixed to the holes by welding or the like. The optical axis adjustment flanges 52a, 52b provided on the light-emitting unit 10 and the light-receiving unit 20 are mechanically detachably attached to these flanges 51a, 51b. The light-emitting unit 10 and the light-receiving unit 20 are positioned opposite each other across the walls 50a, 50b, but their positions can be adjusted by the optical axis adjustment flanges 52a, 52b.

[0018] The optical axis adjusting flange 52a adjusts the emission angle of the laser beam 30, and the optical axis adjusting flange 52b adjusts the incident angle of the laser beam 30. The optical axis adjusting flanges 52a and 52b allow the laser beam 30 emitted from the light emitting unit 10 to be received by the light receiving unit 20 with the maximum amount of light.

[0019] [Light-emitting unit 10] The light-emitting unit 10 will now be described. As shown in Fig. 1, the light-emitting unit 10 is configured to include a modulated light generating unit 11, a laser element 12, a collimating lens 13, a light-emitting unit window plate 14, a light-emitting unit container 15, and an optical axis adjusting flange 52a. As shown in Fig. 1, the modulated light generating unit 11, the laser element 12, and the collimating lens 13 are disposed inside the light-emitting unit container 15. The light-emitting unit container 15 isolates the built-in components from the outside air to protect them from wind, rain, dust, dirt, and the like.

[0020] The modulated light generating unit 11 generates a drive current that is generated so that the wavelength is repeatedly swept and modulated in a wavelength band that includes the optical absorption wavelength of the absorption line spectrum of the measurement target gas. The modulated light generating unit 11 then supplies the drive current for emitting modulated laser light to the laser element 12. This makes it possible to irradiate modulated light that is wavelength-modulated in accordance with the absorption characteristics of the measurement target gas for gas concentration analysis.

[0021] The laser element 12 emits light at the central wavelength (hereinafter referred to as "λ1") of a specific absorption line spectrum absorbed by the gas to be measured and at wavelengths around that central wavelength. The laser element 12 variably controls the emission wavelength by controlling the drive current and temperature.

[0022] The temperature of the laser element 12 is controlled so that the central wavelength of the emitted light coincides with the central wavelength λ1 of the absorption line spectrum of the gas to be measured. The laser light 30 emitted from the laser element 12 is controlled by a drive current supplied from the modulated light generating unit 11 so as to sweep wavelengths around the central wavelength of the absorption line spectrum of the gas to be measured over time, and is further modulated by superimposing an appropriate sine wave so that high-sensitivity measurement can be performed by wavelength modulation spectroscopy (WMS). Wavelength modulation spectroscopy is also known as 2f detection.

[0023] The laser element 12 used is not particularly limited, but may be, for example, a DFB laser diode (Distributed Feedback Laser Diode), a VCSEL (Vertical Cavity Surface Emitting Laser), or a DBR laser diode (Distributed Bragg Reflector Laser Diode).

[0024] The collimating lens 13 is made of a material that has high transmittance at the central wavelength λ1 of the absorption line spectrum of the gas to be measured and at wavelengths around that wavelength. The collimating lens 13 converts the laser light 30 into a substantially parallel light, which can be transmitted to the light receiving unit 20 while suppressing loss due to diffusion.

[0025] The light emitting point of the laser element 12 is located near the focal point of the collimator lens 13. The light emitted from the laser element 12 is diffused and enters the collimator lens 13, where it is converted into laser light 30, which is substantially parallel light. Note that in this embodiment, the collimator lens 13 is used as the parallel light converting unit, but this is not intended to be limited to a collimator lens. For example, a parabolic mirror can be used as the parallel light converting unit instead of the collimator lens 13.

[0026] The laser light 30, which is a substantially parallel beam, passes through the light-emitting unit window plate 14 and propagates inside the walls 50a, 50b, i.e., into the space where gas containing the target gas is present. The light-emitting unit window plate 14 is provided to close a hole drilled in part of the light-emitting unit container 15. The light-emitting unit window plate 14 is located in the optical path of the laser light 30, and while allowing the laser light 30 to pass through, it prevents gas containing the specific target gas from entering the light-emitting unit 10. This prevents the components arranged inside the light-emitting unit container 15 from coming into direct contact with the gas, protecting the components inside the light-emitting unit container 15.

[0027] [Light receiving section 20] The light receiving unit 20 will now be described. The light receiving unit 20 is configured to include a light receiving signal processing unit 21, a light receiving element 22, a condenser lens 23, a light receiving unit window plate 24, and a light receiving unit container 25. The light receiving unit container 25 houses the light receiving element 22, optical components, and electrical and electronic circuits, and isolates them from the outside air to protect them from wind, rain, dust, dirt, and the like.

[0028] The light receiving unit 20 receives the laser light 30 that has passed through the light receiving unit window plate 24 and analyzes the light absorbed due to the absorption characteristics of the target gas. The light receiving unit window plate 24 is provided to cover a hole drilled in a part of the light receiving unit container 25. The light receiving unit window plate 24 is located in the optical path of the laser light 30 and transmits the laser light 30 while preventing gas, including the specific target gas, from entering the interior of the light receiving unit 20. This prevents the components arranged inside the light receiving unit 20 from direct contact with the gas, thereby protecting the interior. The laser light 30 is focused by the condenser lens 23 and enters the light receiving element 22. Note that although the condenser lens 23 is used in this embodiment, a parabolic mirror, a doublet lens, a diffractive lens, or the like may be used instead of the condenser lens 23.

[0029] The light receiving element 22 receives the laser light 30 that has passed through the gas to be measured. A light receiving element having sensitivity at the central wavelength λ of the absorption line spectrum of the gas to be measured and its surrounding wavelengths can be selected. The light receiving signal from the light receiving element 22 is sent to the light receiving signal processing unit 21 as an electrical signal.

[0030] The condenser lens 23 is made of a material that has high transmittance at the central wavelength λ1 of the absorption line spectrum of the gas to be measured and at wavelengths around it. The condenser lens 23 condenses the laser beam 30 onto the light receiving element 22, thereby obtaining a high signal intensity.

[0031] The light receiving signal processing unit 21 processes the electrical signal received by the light receiving element 22 to calculate the gas concentration. It performs lock-in detection of the harmonics of the modulation frequency of the wavelength-modulated laser light 30 and calculates amplitude information of the detected waveform, enabling highly sensitive gas detection.

[0032] 1, the communication line 40 is configured to connect the light-emitting unit 10 and the light-receiving unit 20. Electrical signals are used for communication between the light-emitting unit 10 and the light-receiving unit 20. Instead of the communication line, a communication unit such as a wireless or optical communication unit may be used.

[0033] <Description of Blocks Constituting the Laser Gas Analyzer of the First Embodiment> Hereinafter, the laser gas analyzer according to the first embodiment will be described with reference to FIG.

[0034] Fig. 2 is a signal processing block diagram of the laser gas analyzer 1 according to the first embodiment. In the block diagram of Fig. 2, among the light emitting unit 10 and light receiving unit 20 of the laser gas analyzer 1 shown in Fig. 1, the modulated light generating unit 11 and the light receiving signal processing unit 21 are particularly described in detail, but it is assumed that the laser gas analyzer 1 also includes components that are normally included in the laser gas analyzer 1 even if they are not shown in Fig. 2.

[0035] 2, the light emitting unit 10 includes a modulated light generating unit 11, a laser element 12, and a laser element temperature control circuit 112. The modulated light generating unit 11 includes a wavelength sweep / modulation current setting unit 113 and a DA converter 114.

[0036] The wavelength sweep / modulation current setting unit 113 controls the drive current of the laser element 12 so that the wavelength of the laser light 30 emitted by the laser element 12 is swept near the absorption line of the central wavelength λ1 of the absorption line spectrum of the measurement target gas, and so that the wavelength is modulated with a predetermined signal. The wavelength sweep / modulation current setting unit 113 also selects a drive method for the wavelength sweep / modulation current setting unit 113 based on a command from the control unit 160 of the light receiving unit 20. For example, the drive current is repeatedly turned on and off, thereby controlling the laser light 30 to be repeatedly turned on and off. At this time, the wavelength is repeatedly swept at a predetermined lighting timing. In this case, it is preferable to set the modulation frequency of the sine wave to be higher than the wavelength sweep frequency.

[0037] The DA converter 114 converts the digital signal into an analog signal, and therefore the DA converter 114 converts the drive current sent from the wavelength sweep and modulation current setting unit 113 into a digital signal and sends the converted signal to the laser element 12.

[0038] The laser element temperature control circuit 112 controls and stabilizes the output and wavelength of the laser element 12 at constant levels. Since the output and wavelength of the laser element 12 vary with temperature, the laser element temperature control circuit 112 controls the temperature at a constant level so that the output and wavelength do not vary with changes in the ambient temperature. The laser element temperature control circuit 112 is also controlled based on commands from the control unit 160 of the light receiving unit.

[0039] The laser element 12 emits laser light 30 whose wavelength is swept and modulated by the swept drive current so as to cross the entire absorption line of the central wavelength λ1 of the absorption line spectrum of the gas to be measured. The laser light 30 is modulated by superimposing a sine wave on the drive current.

[0040] Laser light 30 incident on light receiving element 22 shown in FIG. 2 is partially absorbed by the gas to be measured, but is composed of the remaining light that is not absorbed by the gas to be measured, that is, transmitted light.

[0041] The light receiving element 22 is an element sensitive to the wavelength of the laser light 30, and can be selected as appropriate for the light receiving element 22, such as a photodiode, depending on the wavelength and signal strength of the laser light 30. At this time, the light receiving element 22 may also receive light emitted from a space where gas is present. Furthermore, if the light receiving element 22 is a photodiode, a dark current will be generated. The sweep repetition period is selected to be short so that fluctuations in the light receiving signal due to these factors will be sufficiently longer than the period of the laser light 30 sweep repetition.

[0042] 2, the received light signal processing unit 21 includes an IV conversion circuit 122, a high-pass filter 123, a first amplifier circuit 124, a band-pass filter 125, a subtraction circuit 126, a second amplifier circuit 127, an AD converter 128, a measurement unit 130, a waveform analysis unit 135, a DA converter 150, and a control unit 160. The measurement unit 130 includes a lock-in detection unit 131, a peak / bottom calculation unit 133 for calculating a signal corresponding to the gas concentration from the lock-in detection signal, and a gas concentration calculation correction unit 134.

[0043] The IV conversion circuit 122 is a circuit that converts the current signal from the light receiving element 22 into a voltage signal. For example, if the light receiving element 22 is a photodiode, a transimpedance amplifier that amplifies the current from the photodiode while converting it into a voltage can be selected. Here, under conditions where the laser light 30 is least attenuated, i.e., under conditions where there is no dust or the like on the optical path, amplification may be performed as appropriate by an amplifier circuit (not shown) to the extent that the signal does not saturate.

[0044] The high-pass filter 123 removes the DC component contained in the detection signal from the IV conversion circuit 122. The detection signal from the IV conversion circuit 122 generally contains a DC component. The DC component is caused, for example, by light emitted from a space where gas exists. Furthermore, if the light-receiving element 22 is a photodiode, the DC component is also caused by dark current generated in the photodiode. Even if these DC components fluctuate, their time constant is sufficiently longer than the period of repeated sweeps of the laser light 30. In other words, because the signal is of low frequency, the DC component is removed by the high-pass filter 123, resulting in a waveform that crosses the reference voltage of 0 V.

[0045] The cutoff frequency of the high-pass filter 123 is selected so that the repetition frequency (the reciprocal of the repetition period) of the laser light 30 and the frequency of the wavelength sweep and modulation signal of the laser light 30 are within the pass band of the high-pass filter 123. As a result, the on / off of the laser light 30 and the wavelength sweep and modulation signal of the laser light 30 pass through almost unchanged.

[0046] The reason why the high-pass filter 123 is provided immediately after the IV conversion circuit 122 is that if the signal is amplified to a DC signal by the first amplifier circuit 124 described below, the absorption signal by the gas that is effective for measurement becomes relatively small and the detection sensitivity decreases under poor measurement conditions, for example, when the light emitted from the space where the gas exists is strong, the dark current is large, and the transmittance of the laser light 30 is low. To prevent this situation, the high-pass filter 123 is provided immediately after the IV conversion circuit 122 to remove the DC component in advance.

[0047] The signal wave from the high-pass filter 123 mainly includes a turn-on / turn-off signal for the laser beam 30 and a wavelength sweep / modulation signal for the laser beam 30. Of these, the wavelength sweep / modulation signal wave when the laser beam 30 is turned on fluctuates because the laser beam 30 is scattered and attenuated due to, for example, fluctuations in the amount of dust coexisting in a space where gas exists. This signal fluctuation due to scattering and attenuation of the wavelength sweep / modulation signal wave is not wavelength-dependent within the wavelength sweep / modulation range of the laser beam 30, and passes through the high-pass filter 123.

[0048] The first amplifier circuit 124 amplifies the detection signal that has passed through the high-pass filter 123 at an appropriate amplification factor without saturating the signal.

[0049] The bandpass filter 125 constitutes a filter unit that extracts a frequency that is an integer multiple of the modulation frequency of the wavelength-modulated laser light 30 from the detection signal output from the light receiving element 22. The bandpass filter 125 extracts, for example, a frequency signal that is twice the modulation frequency (hereinafter referred to as a "2f signal").

[0050] In the wavelength modulation method, gas concentration is detected based on a signal that is an integer multiple of the modulation signal as the gas absorption signal. As described above, for example, a 2f signal is used, but this signal is much smaller than the wavelength sweep and modulation signal of the laser light 30. This allows the 2f signal to be extracted from the bandpass filter 125 and amplified at an appropriate gain without saturating the signal by an amplifier circuit. After conversion to a digital signal, it can be used to accurately detect gas concentration.

[0051] The second amplifier circuit 127 amplifies the detection signal output by the band-pass filter 125 at an appropriate amplification factor without saturating the signal. As will be described later, in this embodiment, the subtraction circuit 126 removes unnecessary components from the detection signal output by the band-pass filter 125. However, before the removal of the unnecessary components, the signal strength of the unnecessary components is high, so the amplification factor is set low to prevent range over. After the unnecessary components are removed, the amplification factor is set high to make the gas absorption signal as strong as possible and send the signal to the AD converter 128. The amplification process is controlled by the control unit 160.

[0052] The AD converter 128 converts the detection signal output from the bandpass filter 125 into an AD signal. The AD converter 128 converts the analog signal sent from the second amplifier circuit 127 into a digital signal. As shown in FIG. 2, the signal branches into two branches: one from the AD converter 128 to a lock-in detection unit 131 and the other from the AD converter 128 to a waveform analysis unit 135. The digital signal is sent to either the lock-in detection unit 131 or the waveform analysis unit 135. The AD converter 128 selects an element with an appropriate sampling speed so that the modulation component can be sufficiently detected. For example, if the laser modulation component is 50 kHz, the 2f detection method detects a frequency component of 100 kHz, which is twice that frequency. Therefore, an AD conversion element with a sampling speed of, for example, 1 MHz or higher is selected so that these frequency components can be sufficiently detected.

[0053] The waveform analysis unit 135 calculates amplitude information (unnecessary components) of an unnecessary region that does not include absorption by the measurement target gas using a part of the detection signal output from the AD converter 128. A signal having a phase and frequency synchronized with the 2f signal according to the calculated amplitude information is sent to the DA converter 150.

[0054] The DA converter 150 is provided between the waveform analysis unit 135 and the subtraction circuit 126, and performs DA conversion on the signal output from the waveform analysis unit 135. This converts the amplitude information of the unnecessary region calculated by the waveform analysis unit 135 from digital to analog, and sends the converted information to the subtraction circuit 126.

[0055] The subtraction circuit 126 subtracts the amplitude information of the unnecessary region calculated by the waveform analysis unit 135 from the detection signal output from the band-pass filter 125. In this way, based on the calculation result of the waveform analysis unit 135, the signal corresponding to the amplitude information of the unnecessary region that does not include the absorption of the gas to be measured is removed, and the signal intensity of the unnecessary component can be reduced to a level equal to or lower than the gas absorption signal.

[0056] The lock-in detector 131 performs phase detection (lock-in detection) at a frequency twice the modulation frequency in the wavelength sweep / modulation current setting unit 113 included in the signal from the second amplifier circuit 127 as a reference.

[0057] The peak / bottom calculation unit 133 calculates peaks and bottoms to calculate a signal corresponding to the gas concentration from the lock-in detection signal. The gas concentration calculation correction unit 134 detects, calculates, and corrects the gas concentration from the signal processed by the lock-in detection unit 131 and the peak / bottom calculation unit 133. The processing by the gas concentration calculation correction unit 134 is controlled by the control unit 160.

[0058] The control unit 160 controls the wavelength sweep / modulation current setting unit 113, the laser element temperature control circuit 112, the second amplifier circuit 127, the waveform analysis unit 135, the lock-in detection unit 131, and the gas concentration calculation correction unit 134 based on information about the gas to be measured.

[0059] <Gas analysis calculation processing> 3(a) is a conceptual diagram of the waveform of the drive current a1, and FIG. 3(b) is a conceptual diagram of the waveform of the lock-in detection signal b1. The drive current a1 shown in FIG. 3(a) is generated by the modulated light generating unit 11 so that the wavelength is repeatedly swept and modulated in a wavelength band including the center wavelength λ1 of the absorption line spectrum of the measurement target gas. FIG. 3(b) corresponds to the waveform after detection by the lock-in detection unit 131 in FIG. 2.

[0060] Fig. 4 is an enlarged view of the lock-in detection signal b1 shown in Fig. 3(b). As shown in Fig. 3(b) and Fig. 4, it is desirable that the lock-in detection signal b1 has a waveform with an extremum based on the absorption line of the measurement target gas component.

[0061] The difference D between the bottom and peak of the signal intensity of the lock-in detection signal b1 shown in Figure 4 correlates with the gas concentration. Therefore, by performing calibration in advance using standard gases set to each concentration, the difference D can be detected to measure the gas concentration.

[0062] <Problems with conventional technology> In gas analysis of a target gas, ideally, the detection signal (2f signal) that is an integer multiple of the modulation frequency corresponding to the gas concentration of the target gas is a signal derived solely from the absorption of the target gas. However, in reality, due to the nonlinearity of the laser diode and distortion in the signal processing circuit, an unnecessary 2f signal (unwanted component) with the same frequency as the gas absorption signal that is unrelated to the gas absorption signal is included. If this unnecessary component is larger than the gas absorption signal, the amplification factor of the amplifier circuit is determined so that the unnecessary component is not saturated, and the gas absorption signal becomes relatively small.

[0063] Fig. 5 is a waveform diagram of a detection signal before analog-to-digital conversion in a conventional laser gas analyzer, Fig. 5(a) is a detection signal when there is no gas absorption, and Fig. 5(b) is a conceptual diagram of an enlarged portion of Fig. 5(a). Unlike the configuration of this embodiment shown in Fig. 2, the conventional laser gas analyzer does not include the subtraction circuit 126 and waveform analysis unit 135, and the detection signal from the bandpass filter 125 shown in Fig. 2 is sent directly to the lock-in detection unit 131 via the AD converter 128.

[0064] Ideally, if there is no gas absorption in the laser light 30, no detection signal (2f signal) is generated from the bandpass filter 125, and the signal intensity is 0. However, in reality, as shown in Figures 5(a) and 5(b), there is an unnecessary 2f signal that is unrelated to the gas absorption signal.

[0065] FIG. 5(c) is a waveform diagram of a conventional detection signal when gas absorption occurs in the laser light 30. Region (I) in FIG. 5(c) is a signal in the region where the laser light 30 absorbs the gas to be measured. On the other hand, region (II) in FIG. 5(c) is a signal in the unwanted region that does not include gas absorption. If the gas absorption signal is smaller than the signal in this unwanted region, the detection signal waveform will be as shown in FIG. 5(c), and the amplification factor in the second amplifier circuit 127 before input to the AD converter 128 will be set to a level that does not saturate the signal in the unwanted region. As a result, the gas absorption signal cannot be sufficiently amplified.

[0066] Figure 6 shows the lock-in detection signal after conventional lock-in detection. As shown in Figure 6, in the lock-in detection signal, the difference D in signal intensity becomes relatively small in the region (III) that includes gas absorption. This causes a problem in that sufficient resolution cannot be obtained for the gas absorption signal, resulting in reduced measurement accuracy.

[0067] 2, in this embodiment, a waveform analyzer 135 and a subtraction circuit 126 are incorporated after the bandpass filter 125, and the waveform analyzer 135 calculates amplitude information of an unnecessary region where there is no gas absorption, and the subtraction circuit 126 subtracts this amplitude information of the unnecessary region from the detection signal output from the bandpass filter 125. In this way, the amplitude information of the unnecessary region is controlled to be reduced to a level equal to or lower than the gas absorption signal. This process enables gas analysis of the measurement target gas to be measured with high accuracy.

[0068] <Subtraction Processing in the Present Embodiment> In this embodiment, as shown in FIG. 2, the detection signal output from the band-pass filter 125 passes through the subtraction circuit 126 without being subjected to subtraction processing, and is then converted from analog to digital by the AD converter 128 before being sent to the waveform analysis unit 135.

[0069] At this time, the waveform diagram of the detection signal from the bandpass filter 125 before subtraction, when gas absorption is present, is similar to, for example, FIG. 5(c). As described above, region (I) in FIG. 5(c) is a signal in a region including gas absorption, and region (II) is a signal in an unnecessary region not including gas absorption. The waveform analyzer 135 calculates amplitude information of the unnecessary region (II) in FIG. 5(c), and outputs a signal having a phase and frequency synchronized with the 2f signal corresponding to the calculated amplitude information from the DA converter 150. Then, the subtraction circuit 126 shown in FIG. 2 subtracts the signal from the DA converter 150 from the signal from the bandpass filter 125, thereby removing the amplitude information of the unnecessary region (II) in FIG. 5(c).

[0070] Figure 7 is a waveform diagram of the detection signal before analog-to-digital conversion in the laser gas analyzer of this embodiment, where Figure 7(a) is the detection signal after subtraction when there is no gas absorption, and Figure 7(b) is the detection signal after subtraction when there is gas absorption.

[0071] As shown in FIG. 7(a), since unnecessary components have been removed from the signal of the bandpass filter 125, in the absence of gas absorption, the signal intensity before AD conversion is sufficiently small, approximately zero.

[0072] On the other hand, Figure 7(b) shows the signal intensity when gas absorption is present. However, because unnecessary components have been subtracted, the gas absorption signal can be sufficiently amplified to avoid saturation, and as is clear from comparison with Figure 5(c), a detection signal with an extreme value can be obtained.

[0073] Fig. 8 shows a lock-in detection signal obtained by lock-in detection of the detection signal after subtraction in Fig. 7(b). The lock-in detection signal obtained in this embodiment has a large difference D in signal intensity compared to the conventional example in Fig. 6. Obtaining such a large difference D makes it possible to obtain high resolution for the gas absorption signal and improve the accuracy of gas analysis.

[0074] <About unnecessary ingredients> The amplitude information (unnecessary components) of the unnecessary region not containing gas absorption shown in (II) of FIG. 5(c) will be described.

[0075] In this embodiment, prior to gas analysis, for example, during factory adjustment prior to shipping, the laser drive current in the wavelength sweep / modulation current setting unit 113 and the setting values ​​of the laser element temperature control circuit 112 are adjusted using the gas to be measured (standard gas) so that the gas absorption waveform is generated approximately at the center of the laser sweep interval. This allows the control unit 160 to store the regions (time regions) within the sweep interval (within a predetermined sweep time) where the gas absorption waveform exists and the regions where the gas absorption waveform does not exist. For example, to perform wavelength sweep / modulation current setting and temperature control based on the output results from the lock-in detection unit 131 to the gas concentration calculation correction unit 134, information from the lock-in detection unit 131 and the gas concentration calculation correction unit 134 can be collected in the control unit 160, and information on unwanted components can be transmitted from the control unit 160 to the waveform analysis unit 135.

[0076] <Explanation of Flowchart Using Laser Gas Analyzer of First Embodiment> FIG. 9 is a flowchart using the laser gas analyzer of the first embodiment. As shown in step S01 of FIG. 9, the modulated light generating unit 11 generates a drive current. The modulated light generating unit 11 supplies the generated drive current to the laser element 12 so that the wavelength is repeatedly swept and modulated in a wavelength band including the center wavelength λ1 of the absorption line spectrum of the gas to be measured. The wavelength sweep / modulation current setting unit 113 constituting the modulated light generating unit 11 selects a drive method for the wavelength sweep / modulation current setting unit 113 and the laser element temperature control circuit 112 based on a command from the control unit 160. An example of the drive current is shown in FIG. 3(a).

[0077] The DA converter 114 shown in FIG. 2 converts the digital signal sent from the wavelength sweep / modulation current setting unit 113 into an analog signal and sends it to the laser element 12 .

[0078] 9, laser element 12 emits laser light 30 in a wavelength band including the central wavelength λ1 of the absorption line spectrum of the gas to be measured, which passes through the gas-containing space to be measured and is received by light-receiving element 22. The signal received by light-receiving element 22 is processed by IV conversion circuit 122, high-pass filter 123, and first amplifier circuit 124, and then transmitted to band-pass filter 125.

[0079] As shown in step S04 of FIG. 9, the band-pass filter 125 extracts a frequency signal that is an integer multiple of the modulation frequency, for example, a frequency signal that is twice the modulation frequency (2f signal).

[0080] As shown in step S05 of FIG. 9, the AD converter 128 shown in FIG. 2 performs AD conversion on the 2f signal output from the band-pass filter 125.

[0081] Next, as shown in step S06 of Fig. 9, the detection signal AD converted in step S05 is branched depending on whether it is before or after subtraction. That is, as shown in Fig. 2, subtraction circuit 126 is provided before AD converter 128, and if this subtraction circuit 126 has performed a predetermined subtraction process (after subtraction [YES]), the process proceeds to step S09, and if the subtraction circuit 126 has not performed a predetermined subtraction process (before subtraction [NO]), the process proceeds to step S07.

[0082] In step S07, waveform analysis unit 135 calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas. That is, in step S07, waveform analysis unit 135 calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a part of the detection signal output from bandpass filter 125. As already explained, in this embodiment, the unnecessary region shown in (II) of Figure 5(c) can be known in advance, and therefore amplitude information of the unnecessary region can be calculated.

[0083] The amplitude information signal of the unnecessary region calculated by the waveform analysis unit 135 is converted into digital form by the DA converter 150 and sent to the subtraction circuit 126 .

[0084] 9, the subtraction circuit 126 subtracts the amplitude information calculated by the waveform analysis unit 135 from the 2f signal output from the band-pass filter 125 in step S04. As a result, the signal intensity of the amplitude information of the unnecessary region not containing gas absorption in the detection signal output from the band-pass filter 125 can be reduced to a level equal to or lower than the gas absorption signal, and for example, the detection signal shown in FIG.

[0085] After the subtraction process, the process returns to step S05, where the subtracted detection signal is converted into an analog signal, and then in step S06, since the subtraction process has been completed, the process proceeds to step S09. In step S09, the concentration is calculated by the measuring unit 130 shown in Fig. 2 based on the detection signal output from the band-pass filter 125. The setting of the amplification factor for the second amplifier circuit 127 from the control unit 160 can be changed in various ways depending on whether the subtraction process has been performed or not.

[0086] <Block diagram of a laser gas analyzer according to a second embodiment> A signal processing block diagram of a laser gas analyzer according to a second embodiment of the present invention is shown in Fig. 10. In the following explanation, only the parts that differ from the first embodiment will be explained.

[0087] 10, the subtraction circuit 115 is provided between the wavelength sweep / modulation current setting unit 113 and the laser element 12. The subtraction circuit 115 subtracts amplitude information calculated by the waveform analysis unit 135 from the drive current signal output from the wavelength sweep / modulation current setting unit 113.

[0088] A DA converter 114 is provided between the subtraction circuit 115 and the laser element 12. The signal subjected to subtraction processing by the subtraction circuit 115 is converted from digital to analog by the DA converter 114. The converted digital to analog signal is sent to the laser element 12.

[0089] In the second embodiment, a signal corresponding to the data obtained by the waveform analysis unit 135 is sent to the light-emitting unit 10, and the signal is subtracted by the subtraction circuit 115 from the signal of the wavelength sweep / modulation current setting unit 113, thereby removing unnecessary components in the light-emitting unit 10. Other operations and the resulting detection signals are the same as those in the first embodiment.

[0090] In the laser gas analyzer 1 of the first embodiment, the subtraction process is performed on the light-receiving unit 20 side, so the influence of noise on the transmission path is small and gas concentration detection can be performed with high accuracy. On the other hand, in the laser gas analyzer 2 of the second embodiment, unlike the first embodiment, the DA converter 150 in the light-receiving unit 20 is not required and the subtraction process can be performed by digital signal processing using a microcomputer or FPGA pre-installed in the light-emitting unit 10, so the number of parts can be reduced and it is advantageous in terms of circuit size.

[0091] 11 is a flowchart of signal processing in a laser gas analyzer according to the second embodiment of the present invention. In the following explanation, only the parts that are different from the first embodiment will be explained.

[0092] In the second embodiment, after step S07, the subtraction circuit 115 subtracts the amplitude information (unnecessary components) calculated by the waveform analysis unit 135 from the driving current signal output from the wavelength sweep / modulation current setting unit 113 in step S01 (step S10). This removes amplitude information (unnecessary components) in the region not containing gas absorption from the driving current signal output from the wavelength sweep / modulation current setting unit 113. Furthermore, the signal that has been subjected to the subtraction process by the subtraction circuit 115 in the DA converter 114 is DA converted. Then, the process proceeds to step S02.

[0093] The digital-to-analog converted signal is sent to laser element 12 and then to step S02. Thereafter, the process proceeds from step S02 to step S06 to step S09, where the concentration of a specific gas contained in the measurement target gas is analyzed. Note that the setting of the amplification factor for second amplifier circuit 127 from control unit 160 can be changed in various ways depending on whether subtraction processing is performed or not. [Industrial Applicability]

[0094] The laser gas analyzer of the present invention is ideal for measuring and controlling combustion exhaust gases from boilers, waste incineration plants, etc. It is also useful for steel gas analysis (blast furnaces, converters, heat treatment furnaces, sintering (pellet plants), coke ovens), fruit and vegetable storage and aging, biochemistry (microorganisms) (fermentation), air pollution (incinerators, flue gas desulfurization and denitrification), exhaust gases from internal combustion engines of automobiles and ships (detester), disaster prevention (explosive gas detection, toxic gas detection, combustion gas analysis of new building materials), plant cultivation, chemical analysis (oil refineries, petrochemical plants, gas generating plants), environmental applications (ground concentration, tunnel concentration, parking lots, building management), and various physical and chemical experiments. [Explanation of symbols]

[0095] 1. Laser gas analyzer 10 Light-emitting part 11 Modulated light generation unit 12 Laser element 13 Collimating lens 14 Light-emitting window plate 15. Light-emitting container 20 Light receiving section 21 Light receiving signal processing section 22 Photodetector 23 Condenser lens 24 Light receiving window plate 25 Light receiving unit container 30 Laser light 40 Communication lines 50a, 50b walls 51a, 51b flanges 52a, 52b Optical axis adjustment flange 112 Laser element temperature control circuit 113 Wavelength sweep and modulation current setting section 114 DA converter 115 Subtraction Circuit 122 IV conversion circuit 123 High Pass Filter 124 Amplifier Circuit 125 Bandpass Filter 126 Subtraction Circuit 127 Amplifier Circuit 128 AD converters 131 Lock-in detection unit 132 Detection frequency setting section 133 Peak-bottom calculation unit 134 Gas concentration calculation unit 135 Waveform analysis section 150 DA converter 160 control section

Claims

1. A laser gas analyzer that performs gas analysis of a measurement target gas present in a measurement target space, a laser element that emits laser light in a wavelength band that includes the optical absorption wavelength of the absorption line spectrum of the measurement target gas; a modulated light generating unit configured to supply to the laser element a drive current generated so that the wavelength is repeatedly swept and modulated in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas; a light receiving element that receives the laser light that has passed through the object to be measured; a filter unit that extracts a frequency that is an integer multiple of a modulation frequency of the wavelength-modulated laser light from the detection signal output from the light receiving element; an AD converter that converts the detection signal output from the filter unit into an analog-to-digital signal; a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a part of the detection signal before subtraction output from the AD converter; a subtraction circuit that subtracts amplitude information of the unnecessary region from the detection signal output from the filter unit; a measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detection of the subtracted detection signal output from the AD converter via the subtraction circuit at an integer multiple of the modulation frequency.

2. Between the waveform analysis unit and the subtraction circuit, a DA converter that performs digital-to-analog conversion of amplitude information of the unnecessary region; 2. The laser gas analyzer according to claim 1, wherein the subtraction circuit performs subtraction processing on the amplitude information that has been digital-to-analog converted.

3. A laser gas analyzer that performs gas analysis of a measurement target gas present in a measurement target space, a laser element that emits laser light in a wavelength band that includes the optical absorption wavelength of the absorption line spectrum of the measurement target gas; a modulated light generating unit configured to supply to the laser element a drive current generated so that the wavelength is repeatedly swept and modulated in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas; a light receiving element that receives the laser light that has passed through the object to be measured; a filter unit that extracts a frequency that is an integer multiple of a modulation frequency of the wavelength-modulated laser light from the detection signal output from the light receiving element; an AD converter that converts the detection signal output from the filter unit into an analog-to-digital signal; a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a part of the detection signal before subtraction output from the AD converter; a subtraction circuit that subtracts amplitude information calculated by the waveform analysis unit from the drive current generated by the modulated light generation unit; a measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detection of the subtracted detection signal output from the AD converter at an integer multiple of the modulation frequency.

4. Between the subtraction circuit and the laser element, a DA converter that performs digital-to-analog conversion of the drive current that has been subjected to the subtraction processing by the subtraction circuit; The digital-to-analog converted drive current is supplied to the laser element.

4. The laser gas analyzer according to claim 3.

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