Laser gas analyzer

Abstract

To provide a laser type gas analyzer that can reduce a measurement error by appropriately setting a modulation condition according to gas to be measured.SOLUTION: A laser type gas analyzer, which performs gas analysis on gas to be measured present in a space to be measured, comprises a laser element (12), a modulation light generation section (11), a light receiving element (22), and a light receiving signal processing section (21). The modulation light generation section uses a digital-to-analog converter (112) to output a driving current so as to perform wavelength sweeping by a wavelength modulation amplitude and wavelength modulation frequency that are arbitrarily set. The light receiving signal processing section includes filter circuits (212, 213) capable of changing a filter characteristic corresponding to the arbitrarily set wavelength modulation frequency by digital control, and uses the filter circuits to extract a frequency component of an integral multiple of the wavelength modulation frequency.SELECTED DRAWING: Figure 3

Description

[[Technical Field]]

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

[0002] In the gas analysis of a measurement target gas disclosed in Patent Document 1, detection is performed by wavelength modulation spectroscopy. Specifically, a wavelength-tunable laser light source emits laser light whose wavelength is swept by a drive current and modulated at a specific frequency, a photodetector detects the laser light, a lock-in amplifier performs lock-in detection on a signal at a multiple of the modulation frequency, and a gas concentration is calculated from the amplitude of the lock-in detection waveform. [[Prior Art Literature]] [[Patent Literature]]

[0003] [[Patent Document 1]] Japanese Unexamined Patent Publication No. 2017-106742 [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0004] However, due to the influence of noise, an ideal lock-in detection waveform cannot be obtained, the signal-to-noise ratio decreases, and high-precision gas concentration detection cannot be achieved.

[0005] Accordingly, the present invention has been made to solve the above problem, and provides a laser gas analyzer capable of reducing measurement errors caused by noise. [[Means for Solving the Problem]]

[0006] The present invention relates to a laser gas analyzer for performing gas analysis of a target gas present in a measurement target space, comprising: a laser element that emits laser light in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the target gas; a modulated light generation unit that sweeps the wavelength in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the target gas and supplies a drive current to the laser element so that the wavelength is modulated; a light receiving element that receives the laser light that has passed through the measurement target space; and a lock-in function that locks into the detection signal output from the light receiving element at a frequency that is an integer multiple of the wavelength modulation frequency. The device comprises a light receiving unit having a light receiving signal processing unit that analyzes the target gas based on the amplitude of the detected lock-in detection waveform, the modulated light generation unit outputs the drive current to perform wavelength sweep using an arbitrarily set wavelength modulation amplitude and wavelength modulation frequency using a digital-to-analog converter, and the light receiving signal processing unit includes a filter circuit that can digitally change the filter characteristics corresponding to the arbitrarily set wavelength modulation frequency, and the frequency components at integer multiples of the wavelength modulation frequency are extracted using the filter circuit.

[0007] One aspect of the present invention is characterized in that a detection parameter setting unit is provided that can communicate with both the modulated light generation unit and the light receiving signal processing unit, and the wavelength modulation amplitude and wavelength modulation frequency set in the detection parameter setting unit are transmitted to the digital-to-analog converter, and the wavelength modulation frequency is transmitted to the filter circuit.

[0008] One aspect of the present invention is characterized in that the wavelength modulation amplitude that maximizes the amplitude of the lock-in detection waveform when swept with a plurality of wavelength modulation amplitudes, and the wavelength modulation frequency that minimizes the variation in the measured gas concentration when swept with a plurality of wavelength modulation frequencies are set accordingly.

[0009] One aspect of the present invention is characterized in that the central wavelength of the laser light is adjusted to be the central wavelength of the absorption spectrum of the gas to be measured, and subsequently the wavelength modulation amplitude and the wavelength modulation frequency are set. [Effects of the Invention]

[0010] According to the present invention, the modulation conditions for the wavelength modulation amplitude and wavelength modulation frequency of the modulated light during gas concentration measurement can be arbitrarily set, and the filter circuit can be digitally controlled to appropriately set modulation conditions suitable for the gas being measured. This makes it possible to reduce measurement errors due to noise and improve the accuracy of gas concentration calculation. [Brief explanation of the drawing]

[0011] [Figure 1] This is an overall configuration diagram of the laser gas analyzer according to this embodiment. [Figure 2] This is a waveform diagram of a lock-in detection signal. [Figure 3] This is a diagram showing the configuration of the modulated light generation unit and the light receiving signal processing unit according to this embodiment. [Figure 4] This flowchart shows the procedure for determining the modulation conditions of the modulated light according to this embodiment. [Figure 5] This graph shows the relationship between wavelength modulation amplitude and the amplitude of the lock-in detection waveform. [Figure 6] The gas concentration was measured using wavelength modulation frequencies of 48 kHz and 6 kHz. Figures 6(a) to 6(f) show graphs of the overall gas concentration measurement results (Figures 6(a) and 6(d)), a magnified view of the zero point (Figures 6(b) and 6(e)), and a magnified view of the span point (Figures 6(c) and 6(f)). [Figure 7] This is an experimental example showing the difference in lock-in detection waveforms obtained with wavelength modulation frequencies of 48 kHz and 6 kHz. Figure 7(a) shows the waveform obtained by flowing zero gas, and Figure 7(b) shows the waveform obtained by flowing span gas. [Figure 8] This graph shows the difference between lock-in detection waveforms obtained with wavelength modulation frequencies of 48 kHz and 6 kHz, as shown by the Fourier transform results. [Modes for carrying out the invention]

[0012] The laser gas analyzer according to an embodiment of the present invention will be described in detail below with reference to the attached drawings. However, the present invention is not limited to the embodiments described below, and can be modified as appropriate without altering its essence.

[0013] <Overall configuration diagram of a laser gas analyzer> Figure 1 is an overall configuration diagram of a laser gas analyzer according to an embodiment of the present invention. As shown in Figure 1, the laser gas analyzer 1 includes a light-emitting unit 10 and a light-receiving unit 20.

[0014] The laser gas analyzer 1 analyzes the target gas present in the measurement area. The laser gas analyzer 1 emits laser light (also called modulated light or detection light) 30 from the light-emitting unit 10, which irradiates the target gas flowing inside the walls 50a and 50b that constitute the gas pipe (the measurement area). The laser light 30 that has passed through the target gas is incident on the light-receiving unit 20, and a specific gas concentration can be determined from the detected amount of light. Furthermore, if the gas concentration is 0 or below a predetermined value, the absence of gas can be detected, and therefore, the presence or absence of gas can also be detected.

[0015] The light-emitting unit 10 and the light-receiving unit 20 are detachably attached to walls 50a and 50b that constitute a gas pipe. Walls 50a and 50b are walls of pipes or the like where a specific gas is present, and each has a hole drilled in it. Flanges 51a and 51b are fixed to these holes by welding or the like. Optical axis adjustment flanges 52a and 52b provided on the light-emitting unit 10 and the light-receiving unit 20 are mechanically detachably attached to these flanges 51a and 51b. The light-emitting unit 10 and the light-receiving unit 20 are positioned opposite each other with walls 50a and 50b in between, but their positions can be adjusted using the optical axis adjustment flanges 52a and 52b.

[0016] The optical axis adjustment flange 52a adjusts the emission angle of the laser beam 30, and the optical axis adjustment flange 52b adjusts the incidence angle of the laser beam 30. The optical axis adjustment flanges 52a and 52b ensure that the laser beam 30 emitted from the light-emitting unit 10 is received by the light-receiving unit 20 with the maximum light intensity.

[0017] [Light-emitting unit 10] The light-emitting unit 10 will be described. As shown in Figure 1, the light-emitting unit 10 comprises 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 Figure 1, the modulated light generating unit 11, the laser element 12 and the collimating lens 13 are arranged inside the light-emitting unit container 15. The light-emitting unit container 15 isolates each built-in component from the outside air to protect them from wind, rain, dust, dirt and the like.

[0018] The modulated light generating unit 11 generates a drive current that is generated such that the wavelength is repeatedly swept and modulated in a wavelength band including the light absorption wavelength of the absorption line spectrum of the measurement target gas. Then, the modulated light generating unit 11 supplies the drive current for emitting the modulated laser light to the laser element 12. Thereby, for gas concentration analysis, wavelength-modulated modulated light can be irradiated in accordance with the light absorption characteristics of the measurement target gas.

[0019] The laser element 12 emits light at the center wavelength λ1 of a specific absorption line spectrum absorbed by the measurement target gas and wavelengths in the vicinity thereof. The laser element 12 variably controls the emission wavelength by means of drive current and temperature control.

[0020] The laser element 12 is temperature-controlled such that the emission center wavelength thereof matches the center wavelength λ1 of the absorption line spectrum of the measurement target gas. Further, the laser light 30 emitted from the laser element 12 is controlled by the drive current supplied from the modulated light generating unit 11 to temporally sweep the wavelength around the center wavelength of the absorption line spectrum of the measurement target gas. Furthermore, the laser light 30 is 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 called 2f detection method.

[0021] 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).

[0022] 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 being measured, and at wavelengths around it. The collimating lens 13 converts the laser light 30 into nearly parallel light, allowing it to be transmitted to the light receiving unit 20 while suppressing loss due to diffusion.

[0023] The light-emitting point of the laser element 12 is positioned near the focal point of the collimating lens 13. The light emitted from the laser element 12 diffuses as it enters the collimating lens 13 and is converted into laser light 30, which is approximately parallel light. In this embodiment, the collimating lens 13 is used as the parallel light conversion unit, but this is not intended to limit the invention to a collimating lens. For example, a parabolic mirror can be used instead of the collimating lens 13 as the parallel light conversion unit.

[0024] The laser beam 30, which is nearly parallel, passes through the light-emitting window plate 14 and propagates into the interior of the walls 50a and 50b, that is, into the space where the gas containing the gas to be measured exists. The light-emitting window plate 14 is provided so as to create a hole in a part of the light-emitting container 15 and seal it. The light-emitting window plate 14 is in the optical path of the laser beam 30 and allows the laser beam 30 to pass through while preventing the gas containing the specific gas to be measured from entering the interior of the light-emitting unit 10. As a result, each component placed inside the light-emitting container 15 does not come into direct contact with the gas, and each component inside the light-emitting container 15 is protected.

[0025] [Light receiving section 20] The light-receiving unit 20 will now be described. The light-receiving unit 20 is composed of a light-receiving signal processing unit 21, a light-receiving element 22, a light-collecting 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, and dirt.

[0026] The light-receiving unit 20 receives laser light 30 that has passed through the light-receiving unit window plate 24 and analyzes the light absorbed by the gas to be measured according to its absorption characteristics. The light-receiving unit window plate 24 is provided by making a hole in a part of the light-receiving unit container 25 and sealing it. The light-receiving unit window plate 24 is located in the optical path of the laser light 30, allowing the laser light 30 to pass through while preventing gas containing the specific gas to be measured from entering the interior of the light-receiving unit 20. As a result, the components placed inside the light-receiving unit 20 do not come into direct contact with the gas, thus protecting the interior. The laser light 30 is focused by the focusing lens 23 and incident on the light-receiving element 22. In this embodiment, a focusing lens 23 is used, but a parabolic mirror, a doublet lens, or a diffractive lens can be used instead of the focusing lens 23.

[0027] The photodetector 22 receives the laser light 30 that has passed through the gas to be measured. A photodetector with sensitivity at the central wavelength λ of the absorption line spectrum of the gas to be measured, and at the surrounding wavelengths, can be selected. The received signal from the photodetector 22 is sent as an electrical signal to the photodetector signal processing unit 21.

[0028] The focusing lens 23 is made of a material with high transmittance at the central wavelength λ1 of the absorption line spectrum of the gas being measured, and at wavelengths around it. Because the laser light 30 is focused onto the photodetector 22 by the focusing lens 23, a high signal intensity can be obtained.

[0029] The light-receiving signal processing unit 21 processes the electrical signal received by the light-receiving element 22 to calculate the gas concentration. By locking in and detecting the harmonics of the modulation frequency of the wavelength-modulated laser light 30, and calculating the amplitude information of the detected waveform, highly sensitive gas detection is possible.

[0030] When the composition of multiple gases present in the measurement area 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 a standard gas, the wavelength modulation amplitude can be adjusted so that the amplitude of the lock-in detection waveform is at its maximum, thereby maximizing the signal-to-noise ratio.

[0031] Figure 2 shows the waveform of the lock-in detection signal. As shown in Figure 2, the lock-in detection signal has an extremum based on the absorption line of the gas component being measured. The range of width W represents the detection signal due to gas absorption.

[0032] The bottom-to-peak difference D of the signal intensity in the lock-in detection signal shown in Figure 2 correlates with the gas concentration. Therefore, by calibrating with standard gases set in advance for each concentration, the gas concentration can be measured by detecting the difference D. The width W of the lock-in detection signal can be used, for example, to perform correction calculations depending on the type of gas.

[0033] <Description of the blocks constituting the modulated light generation unit 11 and the light receiving signal processing unit 21 of this embodiment> Figure 3 is a diagram showing the configuration of the modulated light generation unit 11 and the light receiving signal processing unit 21 according to this embodiment. As shown in Figure 3, the modulated light generation unit 11 includes an LD drive unit 110 and an LD temperature control unit 120. The laser element (LD) 12 irradiates laser light according to the drive current supplied from the LD drive unit 110. The laser element 12 is also temperature-controlled to a constant temperature by the LD temperature control unit 120. By controlling the temperature to a constant level, fluctuations in the output and wavelength of the laser element 12 can be suppressed.

[0034] The LD drive unit 110 comprises a digital-to-analog converter (DAC) 112 and an LD current control circuit 111. The LD temperature control unit 120 comprises a digital-to-analog converter (DAC) 112 and an LD temperature control circuit 121.

[0035] The set values ​​for controlling the drive current and temperature are set in the detection parameter setting unit 310. These set values ​​are then converted from digital to analog values ​​by the digital-to-analog converters 112 and 122, and the converted control values ​​are input to the LD current control circuit 111 and the LD temperature control circuit 121, causing the laser element 12 to emit modulated light according to the set values. As shown in Figure 3, the light receiving signal processing unit 21 is configured to include a light receiving signal acquisition unit 210 and an arithmetic processing unit 220.

[0036] The light-receiving signal acquisition unit 210 comprises a pre-amplifier circuit 211, a high-pass filter 212, a band-pass filter 213, a post-amplifier circuit 214, and an analog-to-digital converter (ADC) 215. The arithmetic processing unit 220 comprises a digital lock-in detection unit 221, a digital low-pass filter 222, and a density calculation unit 223.

[0037] In the preamplifier circuit 211, the light received from the photodetector 22 is amplified according to the intensity of the laser light 30 and sent to the high-pass filter 212 and the band-pass filter 213.

[0038] The high-pass filter 212 and the band-pass filter 213 are filter circuits having filter elements whose filter characteristics can be digitally controlled. The wavelength modulation frequency is transmitted from the detection parameter setting unit 310 to the filter circuits. The filter circuits can digitally change the filter characteristics corresponding to the wavelength modulation frequency, and using these filter circuits, they extract frequency signals that are integer multiples (for example, 2 times) of the wavelength modulation frequency from the detection signal output from the photodetector 22.

[0039] The filter circuit that extracts frequencies that are integer multiples of the modulation frequency of the laser light 30 is configured to include at least a bandpass filter 213, and the use of a high-pass filter 212 is optional. However, by using the high-pass filter 212, the DC component can be removed in advance, a wide dynamic range can be secured for the extracted harmonic components, and amplification can be performed within the input range of the subsequent filter, thereby preventing the absorption signal by the gas from becoming relatively small and maintaining high detection sensitivity. Therefore, it is desirable to connect the high-pass filter 212 immediately after the preamplifier circuit 211.

[0040] The signal extracted by the bandpass filter 213 is amplified by the subsequent amplification circuit 214, converted into a digital signal by the analog-to-digital converter 215, and sent to the arithmetic processing unit 220.

[0041] In the arithmetic processing unit 220, the aforementioned double-frequency signal is passed through the digital lock-in detection unit 221 and the digital low-pass filter 222 to acquire a lock-in detection signal, and the concentration calculation unit 223 processes the detection signal to calculate the gas concentration. The gas concentration can be calculated from the amplitude of the lock-in detection signal shown in Figure 2 using an existing method.

[0042] <Conventional challenges and the background leading to this embodiment> In 2f wavelength modulation spectroscopy, it is known that the wavelength modulation amplitude at which the amplitude of the lock-in detection waveform is maximized is approximately 2.2 times the full width at half maximum of the absorption peak of the gas being measured, and at this point, the signal-to-noise ratio can be maximized.

[0043] Furthermore, laser elements and photodetectors contain frequency-dependent noise components, such as 1f noise in the 1-10kHz range. By modulating with harmonics, these noise effects can be reduced, allowing for the measurement of minute signals.

[0044] Therefore, theoretically, if an ideal lock-in detection waveform is obtained, the signal-to-noise ratio can be maximized through calculations. However, in reality, an ideal lock-in detection waveform cannot be obtained due to the influence of noise. Noise includes circuit noise, optical interference noise, and environmental noise. For example, factors such as the nonlinearity between the wavelength and current of the laser element, the light intensity and current, wavelength changes due to self-heating in response to current, and modulation wave distortion due to the temporal response of wavelength changes to current changes prevent the acquisition of an ideal lock-in detection waveform.

[0045] In Patent Document 1, with the aim of increasing the signal-to-noise ratio, the light-receiving signal processing unit pre-registers correction information and corrects the pressure dependence of the gas concentration based on the amplitudes of multiple waveforms obtained by lock-in detection corresponding to multiple wavelength modulation amplitudes, and the correction information.

[0046] However, conventionally, the modulation conditions (wavelength modulation amplitude and wavelength modulation frequency) of the modulated light during gas concentration measurement were not appropriately set to suit the target gas, either by switching or by using fixed values. In particular, a fixed value was used for the wavelength modulation frequency, and the filter element used in the light receiving signal processing unit used an analog circuit applied to this fixed value. Thus, conventionally, the circuit configuration was not designed to appropriately set the modulation conditions (wavelength modulation amplitude and wavelength modulation frequency) to suit the target gas and to improve the signal-to-noise ratio.

[0047] Therefore, as a result of diligent research, the inventors have invented a laser gas analyzer having the following configuration (1)(2) in order to increase the signal-to-noise ratio and reduce measurement errors due to noise.

[0048] (1) The modulated light generation unit 11 has a function to output a drive current to sweep the wavelength using a digital-to-analog converter 112 at an arbitrarily set wavelength modulation amplitude and wavelength modulation frequency. (2) The light receiving signal processing unit 21 is equipped with filter circuits (212, 213) that can digitally change the filter characteristics corresponding to an arbitrarily set wavelength modulation frequency, and has the function of extracting frequency components at integer multiples of the wavelength modulation frequency using the filter circuits.

[0049] <Specific description of the characteristic configuration of the laser gas analyzer of this embodiment> In the above configuration (1), the wavelength modulation amplitude and wavelength modulation frequency can be arbitrarily set in the detection parameter setting unit 310. In this embodiment, a digital-to-analog converter 112 that can convert the arbitrarily set wavelength modulation amplitude and wavelength modulation frequency from a digital signal to an analog signal is incorporated into the LD drive unit 110.

[0050] To achieve the above, it is desirable that the digital-to-analog converter (DAC) 112 has low distortion. For example, in the laser gas analyzer described in Patent Document 1, the distortion that occurs when modulated laser light is absorbed by the gas being measured is utilized, and the concentration is detected by measuring the harmonic component of the modulation frequency (generally the second harmonic). For this reason, a low-distortion analog oscillator that oscillates at a specific frequency was often used. However, since the variation in gas concentration measurements is due to the wavelength modulation frequency, fixing it to a specific frequency sometimes made it impossible to minimize the variation in gas concentration measurements. Furthermore, the wavelength modulation amplitude was adjusted by switching so that the lock-in detection waveform had the maximum amplitude, making fine-tuning difficult.

[0051] On the other hand, in recent years, advancements have been made in the precision and low distortion of DAC elements in fields such as high-resolution audio, and some have achieved distortion rates significantly lower than the total harmonic distortion rate (-80 to -90 dB) of conventional low-distortion analog oscillators (for example, Asahi Kasei's AK4499EQ, total harmonic distortion rate -125 dB). Therefore, for example, a high-precision DAC with such low distortion rate (total harmonic distortion rate below -80 to -90 dB) is used as the digital-to-analog converter 112 in this embodiment. Furthermore, sampling frequency and resolution can be cited as indicators of high-precision DACs, and although not limited, the range can be from the typical high-resolution sampling frequency / resolution (192 kHz / 24 bit) to the sampling frequency / resolution of the Asahi Kasei AK4499EQ mentioned above (768 kHz / 32 bit). By using such a high-precision DAC, it becomes possible to drive the laser element 12 with modulation conditions consisting of arbitrarily set wavelength modulation amplitude and wavelength modulation frequency while possessing the low-distortion modulation performance necessary for gas concentration measurement. Furthermore, this avoids the need for large-scale circuitry.

[0052] Therefore, in this embodiment, the wavelength modulation amplitude and wavelength modulation frequency as modulation conditions for the laser light 30 can be arbitrarily set, and the laser element 12 can be driven to perform wavelength sweeping according to the arbitrarily set wavelength modulation conditions. The optimal values ​​for the wavelength modulation amplitude and wavelength modulation frequency will be described later.

[0053] In configuration (2), the light receiving signal processing unit 21 is equipped with filter circuits (212, 213) that can digitally change the filter characteristics corresponding to the wavelength modulation frequency set in the modulated light generation unit 11.

[0054] Conventionally, the wavelength modulation frequency was a fixed value, so a filter circuit with filter characteristics corresponding to the fixed wavelength modulation frequency was incorporated. However, in this embodiment, the wavelength modulation frequency, which is arbitrarily set in the detection parameter setting unit 310, is sent to the filter circuits (212, 213). Therefore, a filter circuit is incorporated in which the filter characteristics can be changed as appropriate according to the wavelength modulation frequency by digital control.

[0055] In this way, the filter characteristics of the filter circuits (212, 213) can be changed (by changing the programming) based on a wavelength modulation frequency arbitrarily set according to the gas being measured, allowing for fine-grained settings and a circuit configuration that improves the signal-to-noise ratio.

[0056] In this embodiment, it is desirable that the filter element used in the light receiving signal processing unit 21 be capable of flexibly digitally controlling its filter characteristics in the range of 1 to 150 kHz. In this embodiment, the high-pass filter 212 and the band-pass filter 213 obtain information on the wavelength modulation frequency set in the modulated light generation unit 11 from the detection parameter setting unit 310, and digitally change the filter characteristics corresponding to the wavelength modulation frequency, thereby extracting a frequency component at twice the wavelength modulation frequency through the band-pass filter 213.

[0057] Applicable filter elements in this embodiment include, for example, MAXIM's MAX263 / 264 or MAX267 / 268. By using such programmable filter elements, it becomes possible to extract a double-frequency signal corresponding to the set frequency of the modulated light using only software control with a common circuit configuration.

[0058] In this embodiment, the wavelength modulation amplitude and wavelength modulation frequency as modulation conditions for the laser light 30 can be arbitrarily set, so the modulation conditions can be finely adjusted to increase the signal-to-noise ratio according to the gas being measured.

[0059] <Regarding the setting of wavelength modulation amplitude and wavelength modulation frequency> Next, the settings for wavelength modulation amplitude and wavelength modulation frequency will be explained. For wavelength modulation amplitude, it is preferable to set it to the amplitude that maximizes the amplitude of the lock-in detection waveform when sweeping with multiple wavelength modulation amplitudes. Furthermore, for wavelength modulation frequency, it is preferable to set it to the frequency that minimizes the variation in the measured gas concentration when sweeping with multiple wavelength modulation frequencies.

[0060] While not limited to this method, one way to set the wavelength modulation amplitude is to use the span gas of the gas being measured to obtain an approximate curve graph from the relationship between multiple wavelength modulation amplitudes and the lock-in detection amplitude using the least squares method, and then set the wavelength modulation amplitude that maximizes the lock-in detection amplitude from that graph. Alternatively, one can select the wavelength modulation amplitude that maximizes the amplitude of the lock-in detection waveform from the measured values ​​of multiple wavelength modulation amplitudes for which the lock-in detection waveform has been obtained. The span gas is used to calibrate the scale value near the maximum scale value of the measurement range and, while not limited to this, includes the gas being measured at concentrations of 80% to 100% of the measurement range. This value is also specified in JIS B 7993 2008, to which laser gas analyzers conform.

[0061] Furthermore, although not limited, the wavelength modulation frequency can be determined from the variation in the measured gas concentration. To measure the gas concentration, the wavelength modulation frequency is arbitrarily set, and while emitting laser light from the laser element 12, a standard gas of the target gas is passed through it, and the gas concentration over time is measured. At this time, although the wavelength modulation amplitude of the laser light is not limited, for example, the wavelength modulation amplitude that maximizes the amplitude of the lock-in detection waveform set above is used.

[0062] Both zero gas and span gas are used as standard gases. For example, the zero gas and span gas are flowed alternately, and the gas concentration is measured. The zero gas is used to calibrate the smallest scale division of the measurement range, and its concentration is 0%. To measure the gas concentration, a lock-in detection waveform is obtained, and the concentration waveform is calculated. In laser gas analyzers, the scale from 0% to 100% is calibrated using traceable (i.e., known concentration) zero gas and span gas, and the concentration is measured from the amplitude ratio of the lock-in detection waveform.

[0063] The maximum variation in measured gas concentration can be determined from both the case where zero gas is flowing and the case where span gas is flowing. Then, for example, the average of the measurement variation when zero gas is flowing and the measurement variation when span gas is flowing can be used as the measurement variation of gas concentration at that wavelength modulation frequency. Note that measurements may be taken by flowing both zero gas and span gas, or by flowing only zero gas.

[0064] Then, an approximate curve graph can be obtained using the least squares method from the relationship between multiple wavelength modulation frequencies and the variation in measured gas concentrations, and the wavelength modulation frequency that minimizes the variation in measured gas concentrations can be set from that graph. Alternatively, the wavelength modulation frequency that minimizes the variation in measured gas concentrations can be selected from the measured values ​​of each of the multiple wavelength modulation frequencies for which the variation in measured gas concentrations has been determined. In Figure 6, described later, the experiment was conducted using only two wavelength modulation frequencies. However, in reality, it is possible to select wavelength modulation frequencies from the approximation curve by taking more wavelength modulation frequencies, or to select wavelength modulation frequencies by taking multiple frequencies, such as the fundamental frequency and its 2 to the power of n multiples. During actual gas measurements, wavelength sweeping is performed using the wavelength modulation conditions determined as described above, and the target gas is analyzed based on the amplitude of the resulting lock-in detection waveform.

[0065] In this embodiment, before determining the wavelength modulation conditions, it is preferable to adjust the laser beam's center wavelength to match the center wavelength of the absorption spectrum of the gas being measured. The laser element 12's emission wavelength can be variably controlled by the drive current and temperature. Therefore, the laser element 12 is temperature-controlled so that its emission center wavelength matches the center wavelength of the absorption line spectrum of the gas being measured. The drive current is also controlled so that the wavelengths around the center wavelength of the absorption line spectrum of the gas being measured are swept over time. At this time, the modulation conditions for the wavelength modulation amplitude and wavelength modulation frequency are provisionally determined, for example, based on past adjustment records from when the gas being measured was measured. As a result, a sine wave with provisional wavelength modulation amplitude and frequency is superimposed on the drive current.

[0066] Thus, in this embodiment, it is preferable to set the sweep current and temperature control so that the central wavelength of the laser light becomes the central wavelength of the absorption spectrum of the gas to be measured, and to determine the above-mentioned modulation conditions (wavelength modulation amplitude and wavelength modulation frequency) under these setting conditions.

[0067] <Flowchart for determining modulation conditions> Next, the procedure for determining the modulation conditions (wavelength modulation amplitude and wavelength modulation frequency) of the laser light 30 in this embodiment will be explained using the flowchart in Figure 4.

[0068] In step ST1 of Figure 4, the laser center wavelength is adjusted. First, the target gas, whose concentration is known (e.g., equivalent to the span concentration), is passed through the calibration pipe. In this state, provisional modulation conditions for wavelength modulation amplitude and wavelength modulation frequency are set based on past adjustment results, and a lock-in detection waveform is obtained. Then, using this lock-in detection waveform, the sweep current and temperature control are set so that the center wavelength of the laser light becomes the center wavelength of the absorption line spectrum of the target gas.

[0069] In step ST2 of Figure 4, the modulation amplitude condition is continuously changed based on the setting conditions of step T1 of Figure 4. It is preferable to use a spanned gas as the gas to be measured. Then, the modulation amplitude condition that maximizes the amplitude of the lock-in detection waveform is obtained.

[0070] Figure 5 is a conceptual diagram showing the relationship between wavelength modulation amplitude and the amplitude of the lock-in detection waveform. As shown in Figure 5, the amplitude of the lock-in detection waveform is determined from multiple wavelength modulation amplitudes, and an approximate curve graph like the one shown in Figure 5 is obtained, for example, using the least squares method. Then, the wavelength modulation amplitude 'a' that maximizes the amplitude of the lock-in detection waveform is determined from Figure 5.

[0071] Next, in step ST3 of Figure 4, under the conditions of the central wavelength of the laser light obtained in step ST1 of Figure 4 and the wavelength modulation amplitude a obtained in step ST2 of Figure 4, gas concentration measurements are performed by changing the wavelength modulation frequency, and the wavelength modulation frequency that minimizes the variation in the measured gas concentration is obtained. In this step ST3, it is preferable to use at least zero gas as the standard gas. If a stable supply of span gas can be provided, span gas can also be used. Alternatively, both zero gas and span gas may be used.

[0072] Figure 6 is a graph showing the relationship between wavelength modulation frequency and gas concentration measurements. In each figure in Figure 6, the horizontal axis represents time and the vertical axis represents gas concentration. Figures 6(a), (b), and (c) show the overall measurement results, a magnified measurement figure near the zero point, and a magnified measurement figure near the span point, when the modulation frequency is 48 kHz. Figures 6(d), (e), and (f) show the overall measurement results, a magnified partial measurement figure near the zero point, and a magnified partial measurement figure near the span point, when the modulation frequency is 6 kHz.

[0073] In this experiment, ammonia (NH3) was used as the target gas. The wavelength modulation amplitude of the laser light was fixed to wavelength modulation amplitude a shown in Figure 5, and the wavelength modulation frequency was set to 48 kHz or 6 kHz. Then, zero gas (0% concentration) and span gas (80% to 100% concentration) were alternately passed through the system, and the gas concentration was measured. The gas concentration was measured using a laser gas analyzer, calibrated from 0% to 100% using traceable (i.e., known concentration) zero gas and span gas, and the concentration was measured from the amplitude ratio of the lock-in detection waveform.

[0074] As shown in Figures 6(b) to (f), measurement variations in gas concentration are observed at both the zero point and the span point. Measurement variations can be determined by the difference between the maximum and minimum measured values ​​within each range of the zero point and the span point. At this time, since the measurement is unstable and factors of measurement error other than noise are likely to be large at the measurement start point, measurement end point, and the switching point between the zero point and the span point, it is preferable to determine the measurement variation near the approximate center of each range of the zero point and the span point, as shown in Figures 6(b) to (f). As shown in Figures 6(a) to (f), the measurement variability at both the span point and the zero point was reduced when the wavelength modulation frequency was set to 48 kHz.

[0075] Figure 7(a) shows an example of the difference between lock-in detection waveforms obtained with wavelength modulation frequencies of 48 kHz and 6 kHz, and is a waveform obtained when zero gas is flowed, while Figure 7(b) shows a waveform obtained when span gas is flowed.

[0076] As shown in Figure 7(a), it was found that setting the wavelength modulation frequency to 48 kHz reduces the influence of noise components caused by optical interference superimposed on the lock-in detection waveform, resulting in smaller periodic fluctuations, compared to setting the wavelength modulation frequency to 6 kHz.

[0077] Figure 8 is a graph showing the difference between lock-in detection waveforms obtained with wavelength modulation frequencies of 48 kHz and 6 kHz, as shown by the Fourier transform results. Specifically, it is the Fourier transform of the lock-in detection waveform in Figure 7(b), and it can be seen that the optical interference noise component at frequencies close to the absorption waveform (around 1400 Hz) has been reduced.

[0078] Therefore, in step ST3 of Figure 4, the wavelength modulation frequency is set to 48 kHz based on the above measurement results. Note that the wavelength modulation amplitude and wavelength modulation frequency settings shown in Figures 5 to 8 are merely examples. Figures 7 and 8 demonstrate that noise reduction was achieved through verification using the actual lock-in detection waveform, and it is sufficient to determine the measurement variation by wavelength modulation frequency as shown in Figure 6. As can be seen from Figures 7(a) and 7(b), the undulation of the lock-in detection waveform is clearly shown by measuring through zero gas, so when determining the measurement variation by wavelength modulation frequency as shown in Figure 6, it is preferable to use zero gas at least. Span gas can also be used to determine the measurement variation at both the zero point and the span point, and for example, these can be averaged and the wavelength modulation frequency can be set from the averaged measurement variation. In addition, although two points were used for the measurement variation in Figure 6, in reality, the optimization of the wavelength modulation frequency can be performed with greater accuracy by further increasing the number of measurement points. Alternatively, the wavelength modulation frequency that minimizes the measurement variation among the individual measurement variations obtained from each wavelength modulation frequency may be determined, or, following Figure 5, an approximation curve between the wavelength modulation frequency and the measurement variation may be obtained to determine the wavelength modulation frequency that minimizes the measurement variation.

[0079] Next, in step ST4 of Figure 4, when measuring the actual gas, the laser element is driven so that the wavelength is swept and modulated within the wavelength band containing the optical absorption wavelength of the absorption line spectrum of the gas to be measured, under the various conditions obtained in steps ST1 to ST3. Then, the concentration analysis of the gas to be measured is performed based on the amplitude of the lock-in detection waveform obtained in response to the received signal output from the photodetector. In this embodiment, the filter circuit filter characteristics are changed by digital control to correspond to the set wavelength modulation frequency.

[0080] As described above, the laser gas analyzer of this embodiment can improve the signal-to-noise ratio by optimizing the modulation conditions (amplitude, frequency) of the laser modulated light, thereby reducing measurement errors due to noise and enabling accurate calculation of gas concentrations.

[0081] In step ST2 shown in Figure 4, the wavelength modulation amplitude that maximizes the amplitude of the lock-in detection waveform (wavelength modulation amplitude a in Figure 5) is set, and in step ST3, the wavelength modulation frequency that minimizes the variation in gas concentration measurement (48 kHz in the experiment in Figure 6) is set. As a result, the indication fluctuation (variation in gas concentration measurement) when measuring the actual gas was reduced by approximately 1.4 times (concentration variation without ST2 and ST3 / concentration variation in this embodiment) compared to when steps ST2 and ST3 were not performed and the provisional wavelength modulation amplitude and wavelength modulation frequency in step ST1 were used. Thus, it was found that in this embodiment, the signal-to-noise ratio can be increased and measurement errors due to noise influence can be reduced. [Industrial applicability]

[0082] The laser gas analyzer of the present invention is ideal for measuring and controlling combustion exhaust gases in boilers, waste incineration, and other applications. It is also useful as an analyzer for various other applications, including gas analysis for steelmaking [blast furnaces, converters, heat treatment furnaces, sintering (pellet equipment), coke ovens], fruit and vegetable storage and aging, biochemistry (microorganisms) [fermentation], air pollution [incinerators, flue gas desulfurization / denitrification], exhaust gas from internal combustion engines of automobiles and ships (de-testing), disaster prevention [explosive gas detection, toxic gas detection, combustion gas analysis of new building materials], plant cultivation, chemical analysis [petroleum refining plants, petrochemical plants, gas generation plants], environmental applications [surface concentration, tunnel concentration, parking lot, building management], and various physical and chemical experiments. [Explanation of Symbols]

[0083] 1: Laser gas analyzer 10: Light-emitting part 11: Modulated light generation unit 12: Laser element 13: Collimating lenses 14: Light-emitting window panel 15: Light-emitting part container 20: Light receiving part 21: Light receiving signal processing unit 22: Photodetector 23: Focusing lens 24: Light-receiving window plate 25: Light-receiving container 30: Laser light 110: LD drive unit 111: LD current control circuit 112: Digital-to-analog converter 120:LD temperature control section 121: LD Temperature Control Circuit 122: Analog converter 210: Light receiving signal acquisition unit 211: Pre-amplification circuit 212: High-pass filter 213: Bandpass filter 214: Post-stage amplifier circuit 215: Analog-to-digital converter 220: Arithmetic Processing Unit 221: Digital lock-in detection unit 222: Digital low-pass filter 223: Concentration calculation section 310: Detection parameter setting unit

Claims

1. A laser gas analyzer that performs gas analysis of a target gas present in a space to be measured, A laser element that emits laser light in a wavelength band that includes the optical absorption wavelength of the absorption line spectrum of the gas to be measured, A modulated light generation unit that sweeps the wavelength in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the gas to be measured and supplies a drive current to the laser element so that the wavelength is modulated, A light-emitting section having, A light-receiving element that receives the laser light that has passed through the space to be measured, A light-receiving signal processing unit performs analysis of the target gas based on the amplitude of a lock-in detection waveform obtained by lock-in detection at a frequency that is an integer multiple of the wavelength modulation frequency with respect to the detection signal output from the light-receiving element. It comprises a light-receiving section having, The modulated light generation unit outputs the drive current to perform wavelength sweeping using an arbitrarily set wavelength modulation amplitude and wavelength modulation frequency, using a digital-to-analog converter. The light receiving signal processing unit includes a filter circuit capable of digitally changing the filter characteristics corresponding to the arbitrarily set wavelength modulation frequency, and the laser gas analyzer is characterized in that frequency components at integer multiples of the wavelength modulation frequency are extracted using the filter circuit.

2. A detection parameter setting unit is provided that can communicate with both the modulated light generation unit and the light receiving signal processing unit. The laser gas analyzer according to claim 1, characterized in that the wavelength modulation amplitude and the wavelength modulation frequency set in the detection parameter setting unit are transmitted to the digital-to-analog converter, and the wavelength modulation frequency is transmitted to the filter circuit.

3. The laser gas analyzer according to claim 1 or 2, characterized in that the wavelength modulation amplitude that maximizes the amplitude of the lock-in detection waveform when swept with a plurality of wavelength modulation amplitudes, and the wavelength modulation frequency that minimizes the variation in the measured gas concentration when swept with a plurality of wavelength modulation frequencies are set accordingly.

4. The laser gas analyzer according to claim 3, characterized in that the central wavelength of the laser light is adjusted to be the central wavelength of the absorption spectrum of the gas to be measured, and subsequently the wavelength modulation amplitude and the wavelength modulation frequency are set.

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