Laser type gas analyzer and laser type gas analysis method

Abstract

To provide a laser type gas analyzer for suppressing deviation of laser light emitting wavelength control.SOLUTION: A laser type gas analyzer for measuring gas concentration of gas to be measured existing in a measurement object space by the wavelength variable laser spectroscopy and the wavelength modulation light spectroscopy includes a light emitting section having a laser element and a modulation light generation section, and a light reception section having a light reception signal processing section. The light reception signal processing section includes a phase adjustment section having a function of detecting the phase of an input signal of lock-in detection and a function of fixedly adjusting a phase difference between the input signal and a reference signal to a desired value, the light emitting section acquires an absorption peak position from an X wave or a Y wave of a lock-in detection waveform in which a phase difference between an input signal and a reference signal is adjusted by the light reception signal processing section, or a waveform obtained by combining the X wave and the Y wave, performs a laser temperature setting such that an absorption peak wavelength matches predetermined timing of wavelength sweeping, and performs light emitting wavelength control of a laser element.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to laser-based gas analyzers and methods. [Background technology]

[0002] Patent Document 1 discloses a laser gas analyzer that measures the gas concentration of a measurement target gas present in a measurement target space by wavelength tunable laser spectroscopy and wavelength-modulated optical spectroscopy. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-106742 A Summary of the Invention [Problem to be solved by the invention]

[0004] In an environment where a measurement is performed using a laser gas analyzer, for example, if the ambient temperature changes, the lock-in detection waveform may be distorted, which may cause deviation in the laser emission wavelength control.

[0005] The present disclosure provides a laser-based gas analyzer that reduces deviations in laser emission wavelength control. [Means for solving the problem]

[0006] According to one aspect of the present disclosure, there is provided a laser gas analyzer for measuring a gas concentration of a target gas present in a target space by wavelength-variable laser spectroscopy and wavelength-modulated optical spectroscopy, the laser gas analyzer comprising: a laser element for emitting laser light in a wavelength band including an optical absorption wavelength of the absorption spectrum of the target gas; a laser element driving unit for supplying a driving current to the laser element so that the wavelength is swept in the wavelength band including the optical absorption wavelength of the absorption spectrum of the target gas and modulated by a modulation frequency; and a laser element temperature regulator for controlling the temperature of the laser element; a light receiving element for receiving the laser light that has passed through the target space; and a detection signal output from the light receiving element for extracting a signal component of the modulation frequency or a frequency that is a multiple of the modulation frequency. and a received light signal processing unit consisting of an input signal phase detection unit that detects the phase of an input signal for lock-in detection, and a reference signal phase setting unit that adjusts the phase difference between the input signal and a reference signal to a desired constant value, and the light emission unit obtains an absorption peak position from an X wave, a Y wave, or a waveform that combines the X wave and the Y wave of the lock-in detection waveform obtained by adjusting the phase difference between the input signal and the reference signal by the received light signal processing unit, and sets a laser temperature so that the absorption peak wavelength coincides with a predetermined timing of wavelength sweeping, thereby controlling the emission wavelength of a laser element. Effect of the Invention

[0007] According to the laser gas analyzer of the present disclosure, deviations in the control of the laser emission wavelength can be suppressed. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing the configuration of a laser gas analyzer according to this embodiment. [Diagram 2] FIG. 2 is a diagram showing a schematic configuration of a modulated light generating section and a received light signal processing section in the laser gas analyzer according to this embodiment. [Diagram 3]FIG. 3 is a diagram showing a schematic configuration of a lock-in detection unit in the laser gas analyzer according to this embodiment. [Figure 4] FIG. 4 is a diagram showing a time chart of signal waveforms and synchronization signals in the laser gas analyzer according to this embodiment. [Diagram 5] FIG. 5 is a diagram showing an example of a method for detecting the phase of an input signal in the laser gas analyzer according to this embodiment. [Figure 6] FIG. 6 is a diagram showing an example of a phase shift of a multiplication waveform due to gas absorption in a laser gas analyzer. [Figure 7] FIG. 7 is a diagram showing the relationship between the output signal and the phase in dual-phase lock-in detection in a laser gas analyzer. [Figure 8] FIG. 8 is a diagram showing an example of a change in detection waveform due to a phase difference between an input signal and a reference signal in a laser gas analyzer. [Figure 9] FIG. 9 is a diagram showing an example in which the detection position of an absorption peak is shifted due to a phase shift of the waveform in a laser gas analyzer. [Figure 10] FIG. 10 is a diagram showing an example of a phase-adjusted lock-in detection waveform in the laser gas analyzer according to this embodiment. [Figure 11] FIG. 11 is a diagram showing an example of reducing an error in the absorption peak acquisition position by using an XY average waveform in the laser gas analyzer according to this embodiment. [Figure 12] FIG. 12 is a diagram showing an example of canceling baseline distortion by an XY average waveform in the laser gas analyzer according to this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments will be described with reference to the accompanying drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0010] In addition, in the description of the specification and drawings of each embodiment, components having substantially the same or corresponding functional configurations may be denoted by the same reference numerals to avoid redundant explanation. In addition, in order to facilitate understanding, the scale of each part in the drawings may differ from the actual scale.

[0011] In the directions of parallel, right-angle, orthogonal, horizontal, vertical, up-down, left-right, front-back, etc., deviations are permitted to the extent that they do not impair the effects of the embodiment. The shape of the corners is not limited to right angles and may be rounded. Parallel, right-angle, orthogonal, horizontal, and vertical may include approximately parallel, approximately right-angle, approximately orthogonal, approximately horizontal, and approximately vertical, respectively.

[0012] For example, "approximately parallel" means that even if two lines or two surfaces are not completely parallel to each other, they can be treated as parallel to each other as long as it is within the range of manufacturing tolerance. As with "approximately parallel," "approximately right angle," "approximately perpendicular," "approximately horizontal," and "approximately vertical" are also intended to correspond to the respective terms as long as the relative positional relationship between the two lines or two surfaces is within the range of manufacturing tolerance.

[0013] <Laser gas analyzer according to this embodiment> The laser gas analyzer according to the present embodiment is a laser gas analyzer that measures the gas concentration of a measurement target gas present in a measurement target space by wavelength-tunable laser spectroscopy and wavelength-modulated optical spectroscopy. The laser gas analyzer according to the present embodiment includes a light-emitting unit and a light-receiving unit.

[0014] The light emitting section in the laser gas analyzer according to this embodiment has a laser element that emits laser light in a wavelength band that includes the optical absorption wavelengths in the absorption spectrum of the gas to be measured. The light emitting section in the laser gas analyzer according to this embodiment also has a modulated light generating section. The modulated light generating section in the laser gas analyzer according to this embodiment has a laser element driving section that supplies a driving current to the laser element so that the wavelength is swept in a wavelength band that includes the optical absorption wavelengths in the absorption spectrum of the gas to be measured and is modulated by the modulation frequency. Furthermore, the modulated light generating section in the laser gas analyzer according to this embodiment has a laser element temperature adjusting section that controls the temperature of the laser element.

[0015] The light receiving section in the laser gas analyzer according to this embodiment has a light receiving element that receives the laser light that has passed through the measurement target space. The light receiving section in the laser gas analyzer according to this embodiment also has a light receiving signal processing section. The light receiving signal processing section in the laser gas analyzer according to this embodiment has a light receiving signal acquiring section that acquires a signal component of the modulation frequency or a frequency that is a multiple of the modulation frequency from the detection signal output from the light receiving element. The light receiving signal processing section in the laser gas analyzer according to this embodiment also has an arithmetic processing section that performs lock-in detection on the acquired signal and analyzes the measurement target gas based on the amplitude of the obtained lock-in detection waveform.

[0016] The light receiving signal processing unit in the laser gas analyzer according to this embodiment includes a phase adjustment unit having a function of detecting the phase of the input signal of the lock-in detection and a function of adjusting the phase difference between the input signal and the reference signal to a desired constant value. The light emitting unit in the laser gas analyzer according to this embodiment obtains the absorption peak position from the X wave, Y wave, or a waveform combining the X wave and the Y wave of the lock-in detection waveform in which the phase difference between the input signal and the reference signal is adjusted by the light receiving signal processing unit, and controls the emission wavelength of the laser element by setting the laser temperature so that the absorption peak wavelength coincides with a predetermined timing of the wavelength sweep.

[0017] 1 is a diagram showing the configuration of a laser gas analyzer 1 which is an example of a laser gas analyzer according to the first embodiment. The laser gas analyzer according to the first embodiment will be described using the laser gas analyzer 1.

[0018] Each gas molecule has a unique light absorption wavelength and an absorption spectrum that indicates the absorption intensity. Laser light is light with a narrow spectral line width at a specific wavelength. In a laser gas analyzer, a laser element emits laser light at a light absorption wavelength that is absorbed by a gaseous gas molecule, which is a measurement target gas, and the measurement target gas absorbs the laser light. The laser gas analyzer detects the presence or absence of a measurement target gas based on the amount of absorption of the laser light at the light absorption wavelength. In addition, the laser gas analyzer can detect the concentration of the measurement target gas because the amount of absorption of the laser light at the light absorption wavelength is proportional to the concentration of the measurement target gas.

[0019] The laser gas analyzer 1 measures the concentration of a specific gas contained in the gas flowing between the walls 50a and 50b. Furthermore, the laser gas analyzer 1 can detect the absence of gas if the gas concentration is 0 or a predetermined value or less. In other words, the laser gas analyzer 1 can detect the presence or absence of gas.

[0020] The laser gas analyzer 1 includes a light emitting unit 10, a light receiving unit 20, and a communication line 30. The communication line 30 communicates between the light emitting unit 10 and the light receiving unit 20 by electrical signals. Note that a communication unit such as a wireless or optical communication unit may be used instead of the communication line 30. The laser gas analyzer 1 can use the communication unit using the communication line, wireless, or optical communication.

[0021] The laser gas analyzer 1 emits detection light 40 from the light emitting unit 10. The detection light 40 is projected into a measurement target space between the walls 50a and 50b.

[0022] When detection light 40 is projected into the space to be measured, a portion of the amount of detection light 40 is absorbed by the specific gas. The remaining part of detection light 40 projected from light emitter 10 that is not absorbed, i.e., transmitted light, is incident on light receiver 20. Light receiver 20 detects the amount of the incident transmitted light. Light receiver 20 determines the concentration of the specific gas from the detected amount of transmitted light.

[0023] Next, each part will be described in detail.

[0024] [Light emitting part 10] The optical function of the light-emitting unit 10 will be described. The central wavelength of a specific absorption spectrum absorbed by the gas to be measured is assumed to be λ. The light-emitting unit 10 emits detection light 40 toward the light-receiving unit 20 while modulating the wavelength at the central wavelength λ and wavelengths around it. The detection light 40, which is a substantially parallel light, passes through the light-emitting unit window plate 14 and propagates inside the walls 50a, 50b, i.e., into the space through which gas including the gas to be measured flows.

[0025] The light-emitting section 10 includes at least a modulated light generating section 11 , a laser element 12 , a collimating lens 13 , a light-emitting section window plate 14 , and a light-emitting section container 15 .

[0026] (Modulated light generating unit 11) The modulated light generating unit 11 is a signal processing and current driving circuit. The light emitting unit 10 needs to irradiate laser light according to the light absorption characteristics of the gas to be measured. In addition, the light emitting unit 10 needs to convert the laser light into frequency-modulated modulated light. Therefore, the modulated light generating unit 11 supplies a driving current signal to the laser element 12 for emitting laser light according to the light absorption characteristics of the gas to be measured as frequency-modulated modulated light.

[0027] (laser element 12) The laser element 12 emits light at a central wavelength λ and wavelengths around it. The laser element 12 is, for example, a distributed feedback (DFB (Distributed Feedback)) laser. The laser element 12 may also be, for example, a VCSEL (Vertical Cavity Surface Emitting Laser Diode). Furthermore, the laser element 12 may also be, for example, a distributed Bragg Reflector (DBR) laser.

[0028] The emission wavelength of the laser element 12 can be variably controlled by the drive current and temperature. Therefore, the temperature is controlled so that the central wavelength of the laser light emitted by the laser element 12 becomes the central wavelength of the absorption spectrum of the gas to be measured. In addition, the drive current is controlled so that the wavelengths around the central wavelength of the laser light are swept over time. Furthermore, a sine wave having an appropriate wavelength modulation amplitude and frequency is superimposed on the drive current so that the measurement can be performed with high sensitivity by wavelength modulation spectroscopy.

[0029] The light emitting point of the laser element 12 is disposed near the focal point of the collimator lens 13. The light emitted from the laser element 12 is incident on the collimator lens 13 while diffusing, and is converted into detection light 40 which is a substantially parallel light.

[0030] (Collimating lens 13) The collimating lens 13 is made of a material having high transmittance at the central wavelength λ and its surrounding wavelengths. The collimating lens 13 converts the detection light 40 emitted from the laser element 12 into a substantially parallel light. By converting the detection light 40 into a parallel light by the collimating lens 13, the light emitter 10 can transmit the detection light 40 to the light receiver 20 while suppressing loss due to diffusion.

[0031] In this embodiment, the collimator lens 13 is used, but the present invention is not limited to the collimator lens. For example, a parabolic mirror may be used instead of the collimator lens.

[0032] (Light emitting window panel 14) The light-emitting section window plate 14 is provided so as to close a hole made in a part of the light-emitting section container 15. The light-emitting section window plate 14 is located in the optical path of the detection light 40, and while transmitting the detection light 40, it prevents gas in the measurement target space from entering the inside of the light-emitting section 10. The light-emitting section window plate 14 prevents the optical elements, optical components, and electric / electronic circuits, etc., stored inside from directly coming into contact with the gas. The light-emitting section window plate 14 protects the optical elements, optical components, and electric / electronic circuits, etc., stored inside from directly coming into contact with the gas.

[0033] (Light emitting container 15) The light-emitting part container 15 contains optical elements, optical components, electric and electronic circuits, etc. The light-emitting part container 15 isolates the optical elements, optical components, electric and electronic circuits, etc. contained therein from the outside air. The light-emitting part container 15 protects the optical elements, optical components, electric and electronic circuits, etc. contained therein from wind, rain, dust, dirt, etc.

[0034] 1, holes are provided in walls 50a and 50b of a pipe or the like through which a gas containing a gas to be measured flows, and flanges 51a and 51b are fixed to the holes by welding or the like.

[0035] The light-emitting unit 10 includes an optical axis adjustment flange 52a on the light-emitting unit container 15. The optical axis adjustment flange 52a is attached to the flange 51a so as to be mechanically movable. The position of the light-emitting unit 10 can be adjusted by attaching it to the flange 51a via the optical axis adjustment flange 52a. The optical axis adjustment flange 52a adjusts the emission angle of the detection light 40. The optical axis adjustment flange 52a, together with the optical axis adjustment flange 52b, adjusts the detection light 40 emitted from the light-emitting unit 10 so that it is received by the light-receiving unit 20 with the maximum amount of light.

[0036] [Light receiving section 20] Next, the light receiving unit 20 will be described. The light receiving unit 20 receives the detection light 40 transmitted through the light receiving unit window plate 24, and analyzes the amount of light absorbed by the measurement target gas. The light receiving unit 20 at least includes a received light signal processing unit 21, a light receiving element 22, a condenser lens 23, the light receiving unit window plate 24, and a light receiving unit container 25.

[0037] (Light receiving signal processing unit 21) The received light signal processing unit 21 is a signal processing and light receiving circuit that calculates the gas concentration of the measurement target gas based on the received detection light 40. The received light signal processing unit 21 processes the signal obtained by converting the detection light 40 into an electrical signal by the light receiving element 22 to calculate the gas concentration of the measurement target gas. More specifically, the received light signal processing unit 21 extracts a frequency component that is twice the modulation frequency of the detection light 40 by a lock-in amplifier from the signal obtained by converting the detection light 40 into an electrical signal by the light receiving element 22. The received light signal processing unit 21 then calculates the gas concentration of the measurement target gas from the extracted component.

[0038] (Photodetector 22) The light receiving element 22 can be selected to be sensitive to the central wavelength λ and wavelengths around it.

[0039] (Condenser lens 23) The condenser lens 23 is made of a material with high transmittance at the central wavelength λ and its surrounding wavelengths. The condenser lens 23 condenses the detection light 40 onto the light receiving element 22, making it possible to obtain a high signal intensity. The detection light 40 is incident on the light receiving element 22, whose light receiving surface is disposed near the focal point of the condenser lens 23. Although the condenser lens 23 is used in this embodiment, a parabolic mirror, a doublet lens, a diffractive lens, or the like may also be used instead of the condenser lens 23.

[0040] The light receiving signal from the light receiving element 22 is sent as a detection signal to the light receiving signal processing unit 21. The light receiving signal processing unit 21 processes the detection signal to calculate the gas concentration.

[0041] (Light receiving window plate 24) The light-receiving unit window plate 24 is provided so as to close a hole made 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 detection light 40, and while transmitting the detection light 40, it prevents gas in the measurement target space from entering the inside of the light-receiving unit 20. The light-receiving unit window plate 24 prevents the optical elements, optical components, electric and electronic circuits, etc. stored inside from directly contacting the gas. The light-receiving unit window plate 24 protects the optical elements, optical components, electric and electronic circuits, etc. stored inside from directly contacting the gas.

[0042] (Light receiving container 25) The light-receiving unit container 25 contains optical elements, optical components, electric and electronic circuits, etc. The light-receiving unit container 25 isolates the optical elements, optical components, electric and electronic circuits, etc. contained therein from the outside air. The light-receiving unit container 25 protects the optical elements, optical components, electric and electronic circuits, etc. contained therein from wind, rain, dust, dirt, etc.

[0043] The light receiving unit 20 includes an optical axis adjustment flange 52b on the light receiving unit container 25. The optical axis adjustment flange 52b is attached to the flange 51b so as to be mechanically movable. The position of the light receiving unit 20 can be adjusted by attaching it to the flange 51b via the optical axis adjustment flange 52b. The optical axis adjustment flange 52b adjusts the incident angle of the detection light 40. The optical axis adjustment flange 52b, together with the optical axis adjustment flange 52a, adjusts the detection light 40 emitted from the light emitting unit 10 so that the maximum amount of light is received by the light receiving unit 20.

[0044] <Processing in the modulated light generating unit 11 and the received light signal processing unit 21> Next, a method for determining the modulation conditions (amplitude, frequency) of the modulated light will be described. Fig. 2 is a diagram showing a schematic configuration of a modulated light generating unit 11 and a received light signal processing unit 21 in a laser gas analyzer 1, which is an example of the laser gas analyzer according to this embodiment.

[0045] [Modulated light generating unit 11] The modulated light generating section 11 includes a laser element driving section 110 and a laser element temperature adjusting section 120 .

[0046] The laser element driving unit 110 outputs a driving current Idr to the laser element 12 for controlling the wavelength of the laser light (detection light 40) emitted by the laser element 12. The laser element driving unit 110 includes a current control circuit 111 and a digital-to-analog converter DAC112. A current control value MVI set by the detection parameter setting unit 310 is converted to a control value MVIa by the digital-to-analog converter 112. The converted control value MVIa is then input to the current control circuit 111. The current control circuit 111 outputs a driving current Idr to the laser element 12 based on the input control value MVIa.

[0047] The laser element temperature adjustment unit 120 adjusts the temperature of the laser element 12 to control the wavelength of the laser light (detection light 40) emitted by the laser element 12. The laser element temperature adjustment unit 120 includes a temperature control circuit 121 and a digital-to-analog converter DAC122. A temperature control value MVT set by the detection parameter setting unit 310 is converted to a control value MVTa by the digital-to-analog converter 122. Then, the converted control value MVTa is input to the temperature control circuit 121. The temperature control circuit 121 outputs a control signal Tdr for adjusting the temperature of the laser element 12 based on the input control value MVTa.

[0048] The laser element 12 includes, for example, a Peltier element. The laser element temperature regulator 120 regulates the temperature of the laser element 12 by controlling the current supplied to the Peltier element included in the laser element 12.

[0049] The laser element 12 emits detection light 40 which is modulated light according to the drive current Idr and control signal Tdr set by the detection parameter setting unit 310 .

[0050] It is desirable that the digital-analog converter 112 used in the modulated light generating unit 11 has low distortion. For example, in a laser gas analyzer such as that described in Patent Document 1, the concentration is detected by measuring the harmonic components (generally the second harmonic) of the modulation frequency, utilizing the fact that the modulated laser light is distorted due to absorption by the gas to be measured. Therefore, a low-distortion analog oscillator that oscillates at a specific frequency is often used. On the other hand, in recent years, DAC elements have become more highly accurate and less distorted in fields such as high-resolution audio. And, devices that achieve a low distortion rate that is significantly lower than the total harmonic distortion rate (about -80 to -90 dB) of conventional low-distortion analog oscillators are on the market (for example, Asahi Kasei's AK4499EQ, total harmonic distortion rate -125 dB). By using a high-precision DAC with a low distortion rate as described above, it is possible to drive a laser with an arbitrary waveform while providing the low-distortion modulation performance required for gas concentration measurement. In addition, by using a high-precision DAC with a low distortion rate as described above, it is possible to suppress the circuit from becoming large-scale.

[0051] [Light receiving signal processing unit 21] The received light signal processing unit 21 includes a received light signal acquiring unit 210 and an arithmetic processing unit 220 .

[0052] The received light signal acquiring section 210 includes an amplifier section 211, a filter section 212, a filter section 213, an amplifier section 214, and an analog-to-digital converter (ADC) 215.

[0053] The light receiving signal acquiring section 210 amplifies the light receiving signal Sm from the light receiving element 22 in the amplifier section 211 according to the intensity of the detection light 40. The amplification factor in the amplifier section 211 is set by the detection parameter setting section 310. The amplifier section 211 outputs a signal Sm1 obtained by amplifying the light receiving signal Sm to the filter section 212.

[0054] Next, the received light signal acquisition unit 210 extracts a frequency signal twice the modulation frequency using the filter characteristics set by the detection parameter setting unit 310 in the filter units 212 and 213 having filter elements whose filter characteristics can be digitally controlled. The filter unit 212 is a high-pass filter. The filter unit 213 is a low-pass filter. The filter unit 212 outputs a signal Sm2 obtained by filtering the signal Sm1 to the filter unit 213. The filter unit 213 outputs a signal Sm3 obtained by filtering the signal Sm2 to the amplifier unit 214.

[0055] Next, the received light signal acquisition section 210 amplifies the extracted signal in the amplifier 214. The amplifier 214 outputs to the analog-digital converter 215 a signal Sm4 obtained by amplifying the signal Sm3.

[0056] Next, the received light signal acquisition section 210 converts the signal Sm4 into an input signal DS, which is a digital signal, by the analog-digital converter 215. The received light signal acquisition section 210 outputs the converted input signal DS, which is a digital signal, to the arithmetic processing section 220.

[0057] The arithmetic processing unit 220 includes a lock-in detection unit 221, a peak detection unit 222, and a concentration calculation unit 223. The arithmetic processing unit 220 acquires a lock-in detection signal LS having a frequency component twice the modulation frequency by the lock-in detection unit 221. The arithmetic processing unit 220 then detects a peak in the intensity of the lock-in detection signal LS by the peak detection unit 222. The arithmetic processing unit 220 also processes the lock-in detection signal LS by the concentration calculation unit 223 to calculate the gas concentration.

[0058] It is preferable that the filter element used in the received light signal processing unit 21 is capable of digitally controlling the filter characteristics flexibly in the high frequency band from 1 to 150 kHz. The filter element used in the received light signal processing unit 21 is preferably, for example, MAX263 / 264, MAX267 / 268, etc. manufactured by MAXIM. By using a programmable filter element, it becomes possible to extract a double frequency signal according to the set frequency of the modulated light only by software control using a common circuit configuration.

[0059] <Lock-in detection in the laser gas analyzer according to this embodiment> Next, a phase adjustment method for lock-in detection in the laser gas analyzer according to this embodiment will be described. Fig. 3 is a diagram showing a schematic configuration of a lock-in detection unit 221 in the laser gas analyzer 1, which is an example of the laser gas analyzer according to this embodiment.

[0060] The lock-in detection section 221 includes an input signal phase detection section 221a, a reference signal phase setting section 221b, a signal generation section 221c, multipliers 221d and 221f, and low-pass filters 221e and 221g.

[0061] The light receiving signal acquiring section 210 extracts and amplifies a frequency component twice the acquired modulation frequency in the light receiving signal Sm. The lock-in detection section 221 acquires an input signal DS obtained by analog-to-digital conversion of the signal Sm4 from the light receiving signal acquiring section 210.

[0062] The input signal phase detector 221a extracts a reference phase θ1 of a frequency component that is twice the modulation frequency from the input signal DS.

[0063] The reference signal phase setting unit 221b sets the phase θ2 of the reference signal to have a constant phase difference Δθ with respect to the reference phase.

[0064] Fig. 4 is a diagram showing a time chart of signal waveforms and synchronization signals in the laser gas analyzer 1, which is an example of the laser gas analyzer according to this embodiment. Fig. 4 shows a time chart of signal waveforms of each part of the modulated light generating unit 11 and the received light signal processing unit 21, and a trigger signal that synchronizes the two. The horizontal axis in Fig. 4 typically shows time, and the vertical axis typically shows intensity, etc. Tm in Fig. 4 indicates the measurement period.

[0065] Fig. 4(a) shows a trigger signal for synchronizing the operations of the light-emitting unit 10 and the light-receiving unit 20. Fig. 4(b) shows a current signal for driving the laser element 12. Fig. 4(c) shows a received light signal detected by the light-receiving element 22. Fig. 4(d) shows a lock-in input signal input to the lock-in detection unit 221. Fig. 4(e) shows a lock-in detection output waveform output from the lock-in detection unit 221.

[0066] The input signal phase detection unit 221a acquires the reference phase θ1 of the frequency component twice the modulation frequency by fitting calculation or the like, assuming that the rising or falling edge of the trigger signal in each measurement period Tm is the reference time t=0, and that the input signal DS is a sine wave signal with a frequency twice the modulation frequency and a reference phase θ1 at time t=0.

[0067] An example of detecting a reference phase from an input signal including a moisture absorption waveform is shown in Fig. 5. Fig. 5 is a diagram showing an example of a method for detecting a phase from an input signal in a laser gas analyzer 1, which is an example of the laser gas analyzer according to this embodiment.

[0068] The input signal phase detection unit 221a calculates the reference phase θ1 using data from a region not including gas absorption. The reason why the input signal phase detection unit 221a calculates the reference phase θ1 using data from a region not including gas absorption will be described. FIG. 6 is a diagram showing an example of a phase shift of a multiplication waveform due to gas absorption in the laser gas analyzer 1, which is an example of the laser gas analyzer according to this embodiment. As shown in FIG. 6, in a region where gas absorption occurs, a signal component of gas absorption is superimposed on the modulated signal, causing a phase shift. Therefore, the input signal phase detection unit 221a uses data from a region not including gas absorption, avoiding a region where a signal component of gas absorption is superimposed on the modulated signal, causing a phase shift.

[0069] Next, the reference signal phase setting unit 221b determines the phase difference Δθ between the reference phase θ1, which is the reference phase calculated by the input signal phase detection unit 221a, and the phase θ2, which is the reference signal. 2 +Y 2 ) and phase difference Δθ. FIG. 7 is a diagram showing the relationship between the output signal and phase in two-phase lock-in detection in a laser, which is an example of a laser gas analyzer according to this embodiment. The in-phase component (X wave) and quadrature phase component (Y wave) output from the lock-in amplifier and the absolute value R=√(X 2 +Y 2 7 shows the relationship between the absolute value R and the phase difference Δθ. Therefore, by calculating the absolute value R, the amplitude of the signal can be obtained regardless of the phase difference Δθ.

[0070] The phase difference Δθ varies depending on the phase delay characteristics of the filter circuit, the temperature characteristics of the parts, the tolerances, etc. The shapes of the X wave and the Y wave also vary with the change in the phase difference Δθ. The waveform of the lock-in output when the phase difference Δθ varies will be described. FIG. 8 is a diagram showing an example of a change in the detection waveform due to the phase difference between the input signal and the reference signal in the laser gas analyzer 1, which is an example of the laser gas analyzer according to this embodiment.

[0071] In each diagram in Fig. 8, the horizontal axis indicates time, and the vertical axis indicates the voltage of the lock-in output signal. Fig. 8(a), (b), (c), (d), (e), (f), (g), and (h) show the waveforms of the X wave and the Y wave when the phase difference with respect to the reference signal is 0, 1 / 4π, 1 / 2π, 3 / 4π, π, 5 / 4π, 3 / 2π, and 7 / 4π, respectively.

[0072] When the phase difference Δθ between the input signal and the reference signal shown in FIG. 5 is changed, the detection waveform changes as shown in FIG. 8. Even if the phase difference Δθ between the input signal and the reference signal is changed, the signal amplitude is preserved as described above. Therefore, gas concentration calculation can be performed without being affected by the change in phase. On the other hand, in a laser gas analyzer, an absorption peak is obtained from the X wave or Y wave of the lock-in detection waveform, and the emission wavelength of the laser element is controlled by setting the laser temperature so that the absorption peak wavelength is aligned with the center of the wavelength sweep. For example, when the detection waveform is deformed due to a phase shift caused by a change in the filter characteristics caused by a change in the gain of the received signal amplifier or an effect of a change in the ambient temperature at the actual measurement site, the absorption peak itself may not be detected, or the absorption peak may be falsely detected.

[0073] FIG. 9 is a diagram showing an example of a shift in the detection position of an absorption peak due to a phase shift of the waveform. FIG. 9 shows an example of a shift in the detection position of an absorption peak due to a phase difference Δθ. When attempting to extract an absorption peak position from the maximum or minimum value of the X-wave in the wavelength sweep range, the absorption peak can be accurately captured with the waveform of FIG. 9(a). On the other hand, the amplitude of the X-wave is small and inverted in the waveform of FIG. 9(b). When the amplitude of one waveform is small and the waveform is inverted as in FIG. 9(b), the absorption peak may be erroneously detected. As a result of erroneous detection of the absorption peak, a deviation in the laser wavelength control may occur.

[0074] FIG. 10 is a diagram showing an example of a phase-adjusted lock-in detection waveform in the laser gas analyzer 1, which is an example of the laser gas analyzer according to this embodiment. As shown in FIG. 10, the laser gas analyzer 1 adjusts the phase difference Δθ between the reference signal and the input signal to a constant value so that the absorption peaks of the X wave and the Y wave appear in the same direction and the amplitudes of both waveforms are equal. Specifically, the laser gas analyzer 1 sets the phase θ2 of the reference signal so that the phase difference Δθ is 1 / 4π or 5 / 4π from the relationship shown in FIG. 7. In other words, the laser gas analyzer 1 sets the phase θ2 of the reference signal to θ1+1 / 4π or θ1+5 / 4π.

[0075] The laser gas analyzer 1 sets the phase θ2 of the reference signal so that the phase difference Δθ becomes 1 / 4π or 5 / 4π, thereby making it possible to suppress erroneous detection of an absorption peak.

[0076] In addition, in the laser gas analyzer according to this embodiment, the average waveforms of the X wave and the Y wave may be acquired. Fig. 11 is a diagram showing an example of reducing the error of the absorption peak acquisition position by the XY average waveform. By doing as shown in Fig. 11, the signal amplitude can be always secured with respect to the phase shift, and the erroneous detection of the absorption peak can be suppressed. This may be combined with the above-mentioned phase setting.

[0077] Fig. 12 is a diagram showing an example of canceling baseline distortion by the XY average waveform in the laser gas analyzer according to this embodiment. The lock-in detection waveform of the laser gas analyzer may have a baseline distortion (slope) as shown in Fig. 12. The baseline distortion (slope) is considered to occur because the wavelength sweep pitch and modulation amplitude are not constant in the wavelength sweep range due to the change in wavelength relative to the driving current of the laser element and the nonlinearity of the circuit. If there is a baseline distortion (slope), as shown in the enlarged view of Fig. 12, when an attempt is made to measure an absorption wavelength with low absorbance or a gas with a minute concentration, this baseline becomes a major cause of measurement error because the amplitude of the absorption signal is small.

[0078] For example, if the original amplitudes of the X wave and the Y wave are Ax and Ay, respectively, and the amplitude distortion amounts due to the superposition of the baseline distortion are dAx and dAy, respectively, an error e1 as shown in Equation 1 occurs.

[0079]

number

[0080] For example, as shown in FIG. 10, when the phase difference Δθ is adjusted so that Ax=Ay and dAx=dAy, the error becomes e2 as shown in Equation 2.

[0081]

number

[0082] The X wave and Y wave with adjusted phase difference have baseline distortion that appears inverted relative to the midpoint level of both signals. Therefore, the baseline distortion can be cancelled by taking the average waveform of the X wave and Y wave with adjusted phase difference. By cancelling the baseline distortion and analyzing the gas to be measured based on the amplitude of the average waveform, measurement errors caused by the effects of baseline distortion due to the laser element characteristics can be reduced.

[0083] A laser gas analyzer is disclosed in, for example, Patent Document 1. The laser gas analyzer of Patent Document 1 performs detection by wavelength modulation spectroscopy. A tunable laser element emits a laser beam whose wavelength is swept by a driving current and modulated at a specific frequency. A light receiving element detects the laser beam emitted from the tunable laser element, and a lock-in amplifier performs lock-in detection of the signal at twice the modulation frequency, and calculates the gas concentration from the amplitude of the lock-in detection waveform. The laser gas analyzer of Patent Document 1 is suitable for measuring trace gases because the signal-to-noise ratio is improved by lock-in detection.

[0084] When the composition of multiple gases present in the measurement space is fixed, the amplitude of the lock-in detection wavelength obtained by the absorption of the measurement gas is a function of the wavelength modulation amplitude, and a maximum value exists. Therefore, when calibrating the standard gas, the signal-to-noise ratio can be maximized by adjusting the wavelength modulation amplitude so that the amplitude of the lock-in detection waveform becomes a maximum. Then, the gas concentration can be calculated based on the correspondence relationship (proportionality, etc.) between the gas concentration of the measurement gas and the amplitude of the lock-in detection waveform.

[0085] A lock-in amplifier has a problem that if a phase shift occurs between the reference signal and the input signal (signal to be measured), the output drops compared to when the phases are perfectly aligned, so phase adjustment is very important to obtain an accurate output. However, this phase shift occurs when the circuit contains a filter that causes a phase delay, and it also varies depending on the temperature characteristics and tolerances of the parts, so it cannot be dealt with simply by adding a phase compensation circuit. For this reason, two-phase lock-in amplifiers are widely used. In a two-phase lock-in amplifier, a reference signal with a phase difference of 90° is multiplied in addition to the normal reference signal, and the output obtained through a low-pass filter becomes a low-frequency signal of the in-phase component (X wave) and quadrature-phase component (Y wave) of the input signal, and by calculating the absolute value and tangent of the obtained measurement value, the amplitude of the input signal can be obtained without phase adjustment, and the phase difference Δθ between the input signal and the reference signal can be measured.

[0086] In 2f wavelength modulation spectroscopy using a lock-in amplifier, the emission wavelength of the laser is controlled by detecting the absorption peak of the measurement gas contained in the lock-in detection waveform and setting the laser temperature so that the absorption peak wavelength coincides with the specified timing of the laser wavelength sweep. When using a two-phase lock-in amplifier, the absorption peak of the X wave or Y wave is detected to control the wavelength. When the absorption peak of the X wave or Y wave is detected to control the wavelength, the absorption waveform of the wavelength control waveform may be inverted or the amplitude may be crushed depending on the degree of deviation of the phase difference between the input signal and the reference signal. In addition, when the gas concentration is low and the baseline slope of the waveform is relatively large, the absorption peak cannot be detected correctly, and errors or malfunctions may occur in the laser wavelength control.

[0087] According to the laser gas analyzer of this embodiment, the phase difference of the reference signal is adjusted to correctly detect the absorption peak. Also, according to the laser gas analyzer of this embodiment, the deviation of the laser emission wavelength control can be suppressed by correctly detecting the absorption peak. According to the laser gas analyzer of this embodiment, the deviation of the laser emission wavelength control is suppressed, so that the gas concentration can be measured without causing errors or malfunctions in the laser wavelength control.

[0088] The laser gas analyzer according to the present embodiment can be applied to exhaust gas (de-tester) of internal combustion engines of cars, ships, etc. The laser gas analyzer according to the present embodiment can be applied to disaster prevention (explosive gas detection, toxic gas detection, new building material combustion gas analysis) and chemical analysis (oil refining plants, petrochemical plants, gas generating plants). The laser gas analyzer according to the present embodiment can be applied to environmental measurement (ground concentration, tunnel concentration, parking lot, building management), plant growth, and various physical and chemical experiments.

[0089] Although the laser gas analyzer has been described above by way of an embodiment, the present invention is not limited to the above embodiment. Various modifications and improvements, such as combinations or substitutions with part or all of other embodiments, are possible within the scope of the present disclosure. [Explanation of symbols]

[0090] 1. Laser gas analyzer 10 Light emitting unit 11 Modulated light generation section 12 Laser element 13 Collimating lens 14 Light emitting part 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. Receiver container 30 Communication lines 40 Detection Light 50a Wall 50b wall 51a, 51b flange 52a, 52b Optical axis adjustment flange 110 Laser element driver 111 Current control circuit 112 Digital-to-Analog Converter 120 Laser element temperature control section 121 Temperature control circuit 122 Digital-to-Analog Converter 210 Light receiving signal acquisition unit 211, 214 Amplification section 212, 213 Filter section 215 Analog-to-Digital Converter 220 Processing section 221 Lock-in detector 221a Input signal phase detection unit 221b Reference signal phase setting unit 221c Signal Generator 221d, 221f multiplier 221e, 221g low pass filters 222 Peak detector 223 Concentration calculation section 310 Detection parameter setting section

Claims

1. A laser gas analyzer for measuring a gas concentration of a measurement target gas present in a measurement target space by wavelength tunable laser spectroscopy and wavelength modulation optical spectroscopy, a laser element that emits laser light having a wavelength band that includes an optical absorption wavelength of the absorption spectrum of the measurement target gas; a modulated light generating unit including a laser element driving unit that supplies a driving current to the laser element so that the wavelength is swept in a wavelength band including the optical absorption wavelength of the absorption spectrum of the measurement target gas and modulated by a modulation frequency, and a laser element temperature adjusting unit that controls the temperature of the laser element; A light emitting portion having a light receiving element that receives the laser light that has passed through the measurement target space; a light receiving signal processing unit including a light receiving signal acquiring unit that acquires a signal component of the modulation frequency or a frequency that is a multiple of the modulation frequency from the detection signal output from the light receiving element, and an arithmetic processing unit that performs lock-in detection on the acquired signal and analyzes the measurement target gas based on the amplitude of the obtained lock-in detection waveform; A light receiving unit having Equipped with the received light signal processing unit includes an input signal phase detection unit that detects a phase of an input signal that is subjected to lock-in detection, and a reference signal phase setting unit that adjusts a phase difference between the input signal and a reference signal to a desired constant value; The light emitting unit obtains an absorption peak position from the X wave, Y wave, or a waveform obtained by combining the X wave and the Y wave of the lock-in detection waveform obtained by adjusting the phase difference between the input signal and the reference signal in the light receiving signal processing unit, and sets the laser temperature so that the absorption peak wavelength coincides with a predetermined timing of the wavelength sweep, thereby controlling the emission wavelength of the laser element. Laser gas analyzer.

2. The light receiving signal processing unit adjusts the lock-in detection waveforms X wave and Y wave so that their absorption peaks are in the same direction and the amplitudes of both waveforms are balanced, and analyzes the gas to be measured based on the amplitude of an average waveform obtained by averaging the X wave and Y wave to cancel baseline distortion.

2. The laser gas analyzer according to claim 1.

3. A laser gas analysis method for measuring a gas concentration of a measurement target gas present in a measurement target space by wavelength tunable laser spectroscopy and wavelength modulation optical spectroscopy, comprising: (a) emitting detection light from a laser element such that the detection light includes a wavelength band including the optical absorption wavelength of the absorption spectrum of the gas to be measured, the wavelength of the detection light is swept in the wavelength band including the optical absorption wavelength of the absorption spectrum of the gas to be measured, and the detection light is modulated by a modulation frequency; (b) receiving the detection light that has passed through the measurement target space with a light receiving element; (c) acquiring a signal component of the modulation frequency or a frequency that is a multiple of the modulation frequency from the detection signal output from the light receiving element, performing lock-in detection on the acquired signal, and analyzing the measurement target gas based on the amplitude of the acquired lock-in detection waveform; Including, The step (c) comprises: detecting a phase of an input signal by lock-in detection; adjusting a phase difference between the input signal and a reference signal to a desired constant value; Including, The step (a) comprises: acquiring an absorption peak position from an X wave, a Y wave, or a combined waveform of the X wave and the Y wave of a lock-in detection waveform in which the phase difference between an input signal and a reference signal is adjusted; A step of controlling the emission wavelength of the laser element by setting a laser temperature so that the absorption peak wavelength coincides with a predetermined timing of the wavelength sweep; Including, Laser-based gas analysis method.

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