JP7753933B2 - Laser Gas Analyzer - Google Patents

Description

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

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

Of these, wavelength modulation spectroscopy is commonly used to measure trace gases, as described in Patent Document 1, for example. In wavelength modulation spectroscopy, the wavelength is swept using a drive current, and a tunable laser light source emits laser light modulated at a specific frequency. This laser light is detected by a photodetector, and the detection signal is locked in at an integer multiple of the modulation frequency by a lock-in detection unit. The gas concentration can then be calculated based on the proportionality or other correspondence between the gas concentration of the gas being measured and the amplitude information of the lock-in detection waveform.

JP 2012-177612 A

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

In addition, in recent years, there has been an increasing need to be able to analyze trace amounts of target gases in the ppm range, which has led to an increasing lack of resolution in the gas absorption signal.

The present invention was made to solve the above problems, and its purpose is to provide a laser gas analyzer that reduces the influence of unnecessary components and improves the resolution of the detection signal and measurement accuracy.

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

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

One aspect of the present invention is a laser gas analyzer that performs gas analysis of a target gas present in a target space, comprising: a laser element that emits laser light in a wavelength band that includes the optical absorption wavelengths of the absorption line spectrum of the target gas; a modulated light generation unit that supplies the laser element with a drive current generated so that the wavelength is repeatedly swept and modulated in the wavelength band that includes the optical absorption wavelengths of the absorption line spectrum of the target gas; a light receiving element that receives the laser light that has passed through the target space; and a detection signal output from the light receiving element that receives a frequency that is an integer multiple of the modulation frequency of the wavelength-modulated laser light. the modulation frequency of the detection signal output from the AD converter is calculated based on a lock-in detection signal obtained by lock-in detection of the subtracted detection signal output from the AD converter at an integer multiple of the modulation frequency.

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

The present invention provides a laser gas analyzer that can analyze the gas to be measured with high precision.

1 is an overall configuration diagram of a laser gas analyzer according to an embodiment of the present invention; FIG. 2 is a signal processing block diagram of the laser gas analyzer according to the first embodiment. FIG. 3A is a conceptual diagram of the waveform of the drive current, and FIG. 3B is a conceptual diagram of the waveform of the lock-in detection signal. FIG. 10 is an enlarged view of a lock-in detection signal. 5(a) and 5(c) are waveform diagrams of detection signals before analog-to-digital conversion in a conventional laser gas analyzer, in which FIG. 5(a) is a detection signal when there is no gas absorption, FIG. 5(b) is a conceptual diagram of an enlarged portion of FIG. 5(a), and FIG. 5(c) is a detection signal when there is gas absorption (without subtraction processing). This is a lock-in detection signal obtained by lock-in detection of the detection signal of FIG. 5(c). 7A and 7B show detection signals before analog-to-digital conversion in a laser gas analyzer according to this embodiment, where FIG. 7A shows the detection signal after subtraction when there is no gas absorption, and FIG. 7B shows the detection signal after subtraction when there is gas absorption. This is a lock-in detection signal obtained by lock-in detection of the detection signal after subtraction in FIG. 7(b). FIG. 2 is a diagram showing a signal processing flow of the laser gas analyzer according to the first embodiment. FIG. 10 is a signal processing block diagram of a laser gas analyzer according to a second embodiment. FIG. 10 is a diagram showing a signal processing flow of a laser gas analyzer according to a second embodiment.

The laser gas analyzer according to this embodiment will be described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiment, and can be modified as appropriate within the scope of its gist.

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

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

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

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 amount of light.

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

The modulated light generating unit 11 generates a drive current that repeatedly sweeps and modulates the wavelength in a wavelength band that includes the optical absorption wavelength of the absorption line spectrum of the gas being measured. The modulated light generating unit 11 then supplies the drive current to the laser element 12 to emit modulated laser light. This allows gas concentration analysis to be performed by irradiating modulated light that is wavelength-modulated according to the absorption characteristics of the gas being measured.

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

The laser element 12 is temperature-controlled so that its central emission wavelength is the central wavelength λ1 of the absorption line spectrum of the gas being measured. Furthermore, 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 sweep wavelengths around the central wavelength of the absorption line spectrum of the gas being measured over time. Furthermore, the laser light is modulated by superimposing an appropriate sine wave to enable highly sensitive measurements using wavelength modulation spectroscopy (WMS). Wavelength modulation spectroscopy is also known as 2f detection.

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).

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 that wavelength. The collimating lens 13 converts the laser light 30 into approximately parallel light, allowing it to be transmitted to the light receiving unit 20 while minimizing loss due to diffusion.

The light-emitting point of the laser element 12 is located near the focal point of the collimating lens 13. The light emitted from the laser element 12 is diffused and enters the collimating lens 13, where it is converted into laser light 30, which is substantially parallel light. Note that, although this embodiment will be described assuming that the collimating lens 13 is used as the parallel light conversion unit, this is not intended to be limiting. For example, a parabolic mirror could also be used as the parallel light conversion unit instead of the collimating lens 13.

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

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

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

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

The focusing lens 23 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 that wavelength. The focusing lens 23 focuses the laser light 30 onto the light-receiving element 22, thereby obtaining a high signal intensity.

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

As shown in FIG. 1, the communication line 40 is configured to connect the light-emitting unit 10 and the light-receiving unit 20. Communication is performed between the light-emitting unit 10 and the light-receiving unit 20 using electrical signals. Alternatively, a communication unit such as a wireless or optical communication unit may be used instead of the communication line.

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

Figure 2 is a signal processing block diagram of the laser gas analyzer 1 according to the first embodiment. The block diagram of Figure 2 provides a detailed explanation of the light-emitting unit 10 and light-receiving unit 20 of the laser gas analyzer 1 shown in Figure 1, particularly the modulated light generating unit 11 and the light-receiving signal processing unit 21. However, even if not shown in Figure 2, it is assumed that the laser gas analyzer 1 also includes components that are normally included.

As shown in FIG. 2, the light-emitting unit 10 is configured to include a modulated light generating unit 11, a laser element 12, and a laser element temperature control circuit 112. The modulated light generating unit 11 is configured to include a wavelength sweep/modulation current setting unit 113 and a DA converter 114.

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

The DA converter 114 converts digital signals into analog signals. Therefore, the DA converter 114 converts the drive current sent from the wavelength sweep/modulation current setting unit 113 into an analog signal and sends it to the laser element 12.

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

The laser element 12 emits laser light 30 whose wavelength is swept and modulated by the swept drive current so that it crosses the entire absorption line of the central wavelength λ1 in the absorption line spectrum of the gas being measured. The laser light 30 is modulated by superimposing a sine wave on the drive current.

As shown in Figure 2, the laser light 30 incident on the light receiving element 22 is partially absorbed by the gas being measured, but consists of the remaining light that is not absorbed by the gas being measured, i.e., transmitted light.

The light-receiving element 22 is an element sensitive to the wavelength of the laser light 30, and can be selected as appropriate based on the wavelength and signal strength of the laser light 30, such as a photodiode. In this case, the light-receiving element 22 may also receive light emitted from a space containing gas. Furthermore, if the light-receiving element 22 is a photodiode, dark current will be generated. The sweep repetition period is selected to be short so that fluctuations in the light-receiving signal due to these factors are sufficiently longer than the period for repeating the sweep of the laser light 30.

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

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

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

The cutoff frequency of the high-pass filter 123 is selected so that the repetition frequency (the inverse of the repetition period) of the laser light 30 and the frequency of the wavelength sweep and modulation signal of the laser light 30 are within the passband of the high-pass filter 123. As a result, the on/off of the laser light 30 and the wavelength sweep and modulation signal of the laser light 30 pass through with almost no change.

The reason for providing a high-pass filter 123 immediately after the IV conversion circuit 122 is that if the signal is amplified to a DC signal by the first amplifier circuit 124 (described below), under poor measurement conditions (for example, when the light emitted from the space containing the gas is strong, the dark current is large, and the transmittance of the laser light 30 is low), the absorption signal due to the gas that is effective for measurement will become relatively small, reducing detection sensitivity. To prevent this from happening, the high-pass filter 123 is provided immediately after the IV conversion circuit 122 to remove the DC component in advance.

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

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

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

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

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

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

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

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

The subtraction circuit 126 subtracts the amplitude information of the unnecessary region calculated by the waveform analysis unit 135 from the detection signal output from the bandpass filter 125. In this way, based on the calculation results of the waveform analysis unit 135, signals corresponding to the amplitude information of the unnecessary region that does not include absorption of the gas to be measured are removed, and the signal intensity of the unnecessary components can be reduced to a level below the gas absorption signal.

The lock-in detection unit 131 performs phase detection (lock-in detection) at twice the frequency based on the modulation frequency in the wavelength sweep/modulation current setting unit 113 contained in the signal from the second amplifier circuit 127.

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

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

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

Figure 4 is an enlarged view of the lock-in detection signal b1 shown in Figure 3(b). As shown in Figures 3(b) and 4, it is desirable that the lock-in detection signal b1 has a waveform with an extremum based on the absorption line of the gas component being measured.

The difference D between the bottom and peak of the signal intensity of the lock-in detection signal b1 shown in Figure 4 correlates with the gas concentration. Therefore, by calibrating in advance using standard gases set to each concentration, it is possible to detect the difference D and measure the gas concentration.

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

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

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

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

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

In this embodiment, as shown in FIG. 2, the waveform analysis unit 135 and subtraction circuit 126 are incorporated after the bandpass filter 125, and the waveform analysis unit 135 calculates amplitude information for the unnecessary region where there is no gas absorption. The subtraction circuit 126 subtracts this amplitude information for the unnecessary region from the detection signal output from the bandpass filter 125. This controls the amplitude information for the unnecessary region to be reduced to a level below the gas absorption signal. This process enables highly accurate gas analysis of the gas to be measured.

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

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

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

As shown in Figure 7(a), by removing unnecessary components from the signal of the bandpass filter 125, in the absence of gas absorption, the signal strength before AD conversion is sufficiently small, almost zero.

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

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

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

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

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

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

Next, as shown in steps S02 and S03 of Figure 9, laser element 12 emits laser light 30 in a wavelength band including the central wavelength λ1 of the absorption line spectrum of the gas to be measured, which passes through the gas-containing measurement space and is received by light-receiving element 22. The signal received by light-receiving element 22 is processed by IV conversion circuit 122, high-pass filter 123, and first amplifier circuit 124, and then sent to band-pass filter 125.

As shown in step S04 of Figure 9, the bandpass filter 125 extracts a frequency signal that is an integer multiple of the modulation frequency, for example, twice the modulation frequency (2f signal).

As shown in step S05 of Figure 9, the AD converter 128 shown in Figure 2 performs AD conversion on the 2f signal output from the bandpass filter 125.

Next, as shown in step S06 of Figure 9, the process branches depending on whether the detection signal AD converted in step S05 is before or after subtraction. That is, as shown in Figure 2, a subtraction circuit 126 is provided before the AD converter 128. If the subtraction circuit 126 is performing the specified subtraction process (after subtraction [YES]), the process proceeds to step S09. If the subtraction circuit 126 is not performing the specified subtraction process (before subtraction [NO]), the process proceeds to step S07.

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

The amplitude information signal of the unnecessary region calculated by the waveform analysis unit 135 is converted to analog by the DA converter 150 and sent to the subtraction circuit 126.

In step S08 of Figure 9, the subtraction circuit 126 subtracts the amplitude information calculated by the waveform analysis unit 135 from the 2f signal output from the bandpass filter 125 in step S04. This reduces the signal intensity of the amplitude information in the unnecessary region that does not contain gas absorption in the detection signal output from the bandpass filter 125 to a level below the gas absorption signal, thereby obtaining, for example, the detection signal shown in Figure 7(b).

After the subtraction process, the process returns to step S05, where the subtracted detection signal is AD converted. Then, in step S06, since the subtraction process has been completed, the process proceeds to step S09. In step S09, the concentration is calculated by the measurement unit 130 shown in FIG. 2 based on the detection signal output from the bandpass filter 125. The gain setting for the second amplifier circuit 127 from the control unit 160 can be changed in various ways depending on whether or not subtraction process has been performed.

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

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

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

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

In the laser gas analyzer 1 of the first embodiment, the subtraction process is performed on the light-receiving unit 20 side, which reduces the impact of noise on the transmission path and enables highly accurate gas concentration detection. On the other hand, in the laser gas analyzer 2 of the second embodiment, unlike the first embodiment, the DA converter 150 in the light-receiving unit 20 is not required, and the subtraction process can be performed by digital signal processing using a microcomputer or FPGA pre-installed in the light-emitting unit 10, which reduces the number of parts and is advantageous in terms of circuit size.

Figure 11 is a flowchart of signal processing for a laser gas analyzer according to a second embodiment of the present invention. The following explanation will focus on only the differences from the first embodiment.

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

The digital-to-analog converted signal is sent to the laser element 12 and then to step S02. Subsequently, the process proceeds from step S02 to step S06, and then to step S09, where the concentration of a specific gas contained in the gas to be measured is analyzed. The gain setting for the second amplifier circuit 127 from the control unit 160 can be changed in various ways depending on whether or not subtraction processing is performed.

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

1 Laser gas analyzer 10 Light emitter 11 Modulated light generation unit 12 Laser element 13 Collimator lens 14 Light emitter window plate 15 Light emitter container 20 Light receiver 21 Light-receiving signal processing unit 22 Light receiver element 23 Condenser lens 24 Light receiver window plate 25 Light receiver container 30 Laser light 40 Communication line 50a, 50b Wall 51a, 51b Flange 52a, 52b Optical axis adjustment flange 112 Laser element temperature control circuit 113 Wavelength sweep/modulation current setting unit 114 DA converter 115 Subtraction circuit 122 IV conversion circuit 123 High-pass filter 124 Amplification circuit 125 Band-pass filter 126 Subtraction circuit 127 Amplification circuit 128 AD converter 131 Lock-in detection unit 132 Detection frequency setting unit 133 Peak/bottom calculation unit 134 Gas concentration calculation unit 135 Waveform analysis unit 150 DA converter 160 Control unit

Claims (4)

A laser gas analyzer that performs gas analysis of a measurement target gas present in a measurement target space,
a laser element that emits laser light in a wavelength band that includes the optical absorption wavelength of the absorption line spectrum of the measurement target gas;
a modulated light generating unit configured to supply to the laser element a drive current generated so that the wavelength is repeatedly swept and modulated in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas;
a light receiving element that receives the laser light that has passed through the object to be measured;
a filter unit that extracts a frequency that is an integer multiple of a modulation frequency of the wavelength-modulated laser light from the detection signal output from the light receiving element;
an AD converter that converts the detection signal output from the filter unit into an analog-to-digital signal;
a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a part of the detection signal before subtraction output from the AD converter;
a subtraction circuit that subtracts amplitude information of the unnecessary region from the detection signal output from the filter unit;
a measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detection of the subtracted detection signal output from the AD converter via the subtraction circuit at an integer multiple of the modulation frequency. Between the waveform analysis unit and the subtraction circuit,
a DA converter that performs digital-to-analog conversion of amplitude information of the unnecessary region;
2. The laser gas analyzer according to claim 1, wherein the subtraction circuit performs subtraction processing on the amplitude information that has been digital-to-analog converted. A laser gas analyzer that performs gas analysis of a measurement target gas present in a measurement target space,
a laser element that emits laser light in a wavelength band that includes the optical absorption wavelength of the absorption line spectrum of the measurement target gas;
a modulated light generating unit configured to supply to the laser element a drive current generated so that the wavelength is repeatedly swept and modulated in a wavelength band including the optical absorption wavelength of the absorption line spectrum of the measurement target gas;
a light receiving element that receives the laser light that has passed through the object to be measured;
a filter unit that extracts a frequency that is an integer multiple of a modulation frequency of the wavelength-modulated laser light from the detection signal output from the light receiving element;
an AD converter that converts the detection signal output from the filter unit into an analog-to-digital signal;
a waveform analysis unit that calculates amplitude information of an unnecessary region that does not include absorption of the measurement target gas using a part of the detection signal before subtraction output from the AD converter;
a subtraction circuit that subtracts amplitude information calculated by the waveform analysis unit from the drive current generated by the modulated light generation unit;
a measurement unit that performs gas analysis based on a lock-in detection signal obtained by lock-in detection of the subtracted detection signal output from the AD converter at an integer multiple of the modulation frequency. Between the subtraction circuit and the laser element,
a DA converter that performs digital-to-analog conversion of the drive current that has been subjected to the subtraction processing by the subtraction circuit;
The digital-to-analog converted drive current is supplied to the laser element.
4. The laser gas analyzer according to claim 3.