JP5370248B2 - Gas analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure the molecule number density of a component to be measured in sample gas in a wide dynamic range in terms of the molecule number density. <P>SOLUTION: A gas analyzing apparatus comprises a modulation part 16 in which a drive current to a wavelength variable laser device 6 is modulated by a modulation frequency fa. As a signal processing part which processes signals from a light receiving part 8 to detect laser beams made to pass through a sample cell 2, a first signal processing part 18 to extract the signals obtained by frequency 2&times;fa, and a second signal processing part 20 to extract the signals the frequency of which is less than fa are provided. When the molecule number density of the component to be measured is a low molecule number density to which a harmonic synchronization detection method can be applied based on the signals from these signal processing part 18, 20, an arithmetic part 22 executes arithmetic operation using the harmonic synchronization detection method, based on the signals from the first signal processing part 18, and when the molecule number density of the component to be measured is a high molecule number density to which the harmonic synchronization detection method cannot be applied, the arithmetic part 22 executes the arithmetic operation using a direct absorption spectrum method. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to a gas analyzer that measures the molecular number density of a measurement target component in a sample gas by utilizing absorption of laser light.

  In recent years, laser absorption spectroscopy utilizing absorption of laser light has been proposed as a method for measuring the molecular number density of a specific gas in a gas (see, for example, Patent Document 1). This method analyzes a laser beam transmitted by irradiating a laser beam having a predetermined frequency to a sample cell into which a sample gas is introduced, and determines the measurement target component from the degree of absorption of the measurement target component in the sample gas. The molecular number density is derived. Since this apparatus is a non-contact type in which the light receiving unit that is a sensor does not come into contact with the sample gas, there are advantages that measurement can be performed without disturbing the field of the sample and that response time is extremely short. A wavelength tunable laser device is used as a laser light source to select a measurement frequency for a measurement target component in the sample gas.

  Here, a general theory of infrared absorption spectroscopy using laser light will be described. Here, a case where a trace amount of water vapor molecule number density in nitrogen gas is measured will be described as an example.

The relationship between the detected light intensity of the laser beam and the water vapor molecule number density is expressed by the following formula (1) from the Lambert-Beer law. I ₀ (ν) is the light intensity when water molecules are not absorbed at the frequency ν, and I (ν) is the transmitted light intensity at the frequency ν. Further, c is the number density of water molecules, l is the length of the optical path passing through the component to be measured, S is the absorption line intensity at a predetermined frequency ν, and K (ν) is the absorption characteristic function.

When the sample gas is at atmospheric pressure, the absorption characteristic function K (ν) is expressed by equation (2) by the Lorentz profile. γ _L is the half width of the absorption spectrum, and is determined by the type of sample gas, temperature and pressure. ν ₀ is the center frequency of the absorption spectrum.

From the above formulas (1) and (2), the following formula (3) is established.

  If a wavelength tunable laser device whose oscillation frequency width is extremely narrower than the line width of the absorption spectrum, for example, a DFB (Distributed Feedback) type semiconductor laser, is used, measurement is performed at each frequency ν without using a separate spectrometer. Is possible.

The absorption intensity I (ν ₀ ) at the center frequency ν ₀ can be expressed by the equation (4) where ν = ν ₀ in the equation (3).

On the other hand, in the infrared absorption by water molecules under a very low total pressure region (a high vacuum region where the total pressure of the component to be measured is lower than 1 [Torr]), the absorption spectrum width is the Lorentz profile described above. Compared to the spread of, the width is reduced to a fraction of a few to a few tenths. In this total pressure region, the absorption characteristic width is mainly determined by the Doppler effect. The absorption characteristic function K (ν) in this case is expressed by the following equation (5) (Gauss function). In the equation (5), γ _ED is called the Doppler width and depends on the center frequency, molecular weight and temperature of the absorption spectrum.

In this case, the following expression (6) is established from the expressions (1) and (5), and by setting ν = ν _{0 in the} expression (6), the absorption intensity I (ν ₀ ) at the center frequency ν ₀ is It can be expressed by the following equation (7).

In general infrared absorption spectroscopy using laser light, the absorption intensity I ₀ (ν ₀ ) or I (ν ₀ ) at the center of the absorption line is measured from the above equation (4) or (7). The amount of the component to be measured in the sample gas is calculated.

  Further, as a gas analysis method using laser light, it is called absorption spectroscopy (hereinafter referred to as harmonic synchronization detection method) by harmonic detection (Harmonic Detection) for detecting a second harmonic component or the like of an absorption spectrum waveform. There are detection methods (see, for example, Patent Document 2 and Non-Patent Document 1). The harmonic synchronous detection method is known as a particularly sensitive detection method among infrared absorption spectroscopy, and is an effective detection method when the amount of light absorption of a measurement target component is very small.

The harmonic synchronization detection method will be described based on Non-Patent Document 1. When the amount of light absorption of the component to be measured is very small as in the case where harmonic detection is necessary, the Lambert-Beer law (1) can be approximated by the following (8).

In order to perform harmonic detection, it is necessary to modulate the frequency of the light applied to the measurement target component. When the modulation amplitude of the sine wave signal for frequency modulation is a and the frequency is ω, the frequency of light at time t is defined by the following equation (9).

In the second harmonic detection, a signal component corresponding to the double frequency 2ω is extracted by synchronous detection from the detection signal from the light receiving unit. The second-order harmonic detection signal intensity Signal (ν) at the frequency ν has a relationship represented by the following equation (10) when the amount of light absorption is very small. Therefore, the relationship of the following equation (11) is obtained from the equation (8). In the equation (11), const is a proportionality constant and varies depending on the sensitivity of the detector and the synchronization detection circuit. The method for determining the proportionality constant const is determined by measuring a gas having a known molecular number density in advance, such as a gas whose molecular number density has been calculated by the measurement based on the above equation (1).
Signal (ν) ∝ (I ₀ (ν) −I (ν)) (10)

Japanese Patent Laid-Open No. 5-099845 JP 2002-184767 A

Webster (CRWebster), “Infrared Laser Absorption: Theory and Applications in Laser Remote Chemical Analysis”, Wiley ( Wiley), New York, 1988

  While the harmonic synchronization detection method is highly sensitive, accurate measurement can be performed only under the condition that the approximation of the above equation (8) holds. Therefore, although an accurate detection result can be obtained when the measurement target component has a low molecular number density, an accurate result cannot be obtained when the measurement target component has a high molecular number density.

Also in laser absorption spectroscopy, a method of directly measuring I ₀ (ν ₀ ) and I (ν ₀ ) and obtaining the molecular number density of the component to be measured from the equations (4) and (7) (hereinafter referred to as the direct absorption spectrum method). ) Is used for the measurement of samples containing water with a relatively high molecular number density, such as the number of water molecules in the atmospheric environment, whereas the harmonic synchronous detection method is a special technique used in semiconductor manufacturing lines. It is used in fields such as the measurement of the trace density of water molecules in gas. Further, since both measurement methods have different detection circuit configurations, each is configured as a separate measurement device, and there is no measurement device that can be used for both measurement methods.

  Therefore, if the molecular number density of the measurement target component suddenly decreases when measuring with a device using the direct absorption spectrum method, or conversely during measurement with the measurement device of the harmonic synchronous detection method In the case where the molecular number density suddenly increases, accurate measurement results cannot be obtained in any case.

  Therefore, the present invention makes it possible to measure the molecular number density of the component to be measured in the sample gas with a wide dynamic range, and to measure the switching of the measurement method even when the molecular number density suddenly changes. An object of the present invention is to provide a gas analyzer capable of continuously measuring without interrupting the above.

  A gas analyzer according to the present invention includes a sample cell in which a sample gas is circulated, a laser irradiation unit that irradiates the sample cell with a laser beam having a specific optical frequency in which a measurement target component in the sample gas has absorption, A light source driving unit that applies a driving current for irradiating the laser light to the laser irradiating unit, and the driving current that is applied from the light source driving unit to the laser irradiating unit is modulated with a modulation frequency of a first frequency fa. A modulation unit, a light receiving unit that receives the laser light that has passed through the sample cell, and a harmonic detection by a harmonic synchronous detection method by synchronously detecting a light detection signal of the light receiving unit at a frequency that is an integral multiple of the modulation frequency A first signal processing unit for detecting a signal intensity Signal (ν), and a light detection signal of the light receiving unit is captured without passing through the first signal processing unit, and a frequency component equal to or higher than the first frequency fa by a frequency filter; A second signal processing unit that detects the light intensity signal I (ν) at the specific optical frequency, and the harmonic signal intensity Signal (ν) detected by the first signal processing unit and the second A calculation unit that takes in the light intensity signal I (ν) detected by the signal processing unit and calculates the molecular number density c of the measurement target component in the sample gas.

Then, the calculation unit sets the reference light intensity signal when the laser light is not absorbed by the measurement target component at the specific optical frequency as I ₀ (ν), and the harmonic signal intensity Signal (ν) And a reference light intensity signal I ₀ (ν), a first calculation means for calculating a measurement target component molecule number density c in the sample gas, the light intensity signal I (ν) and the reference light intensity signal I ₀ (ν). The second calculating means for calculating the measurement target component molecule number density c in the sample gas by the direct absorption spectrum measurement method.

  In the present invention, the calculation unit always takes in the signal extracted by the first signal processing unit and the signal extracted by the second signal processing unit, and the molecular number density of the measurement target component, specifically, the absorption line of the measurement target component The number density of molecules to be measured or the direct absorption spectrum method (second calculation means) obtained by the harmonic synchronous detection method (first calculation means) according to the signal intensity at a specific optical frequency, preferably the center frequency. ) To select one of the molecular number densities of the components to be measured. As described above, the harmonic synchronous detection method is highly sensitive, but linearity mismatch occurs when the molecular number density is high. On the other hand, the direct absorption spectrum method can be measured even at a high molecular number density although the sensitivity is lower than that of the harmonic synchronous detection method. The gas analyzer according to the present invention uses the same laser control system for the above two measurement methods, a signal for use in the harmonic synchronous detection method from a single received light signal, and a signal for use in the direct absorption spectrum method. Are extracted in parallel, and the molecular number density of the component to be measured by an appropriate measuring means is adopted for the molecule number density of the component to be measured based on these signals. This compensates for the shortcomings of both detection methods and enables measurement in a wide molecular number density range as well as continuous measurement.

The calculation unit may output the measurement results obtained by both the calculation means at the same time, and is equipped with a switching means to automatically select the one with higher accuracy according to the molecular number density of the measurement target component in the sample. You may make it output. Such a switching means has a low molecular number suitable for measurement by the harmonic synchronous detection method in which the molecular number density of the component to be measured in the sample is based on the light intensity signal I (ν) and the reference light intensity signal I ₀ (ν). Judgment whether the molecular number density is suitable for the measurement by the direct absorption spectrum measurement method with the molecular number density higher than that, or the molecular number density suitable for the measurement by the harmonic synchronous detection method When the result is obtained, the calculated molecular number density c by the first calculating means is output, and when the determination result that the molecular number density is suitable for the measurement by the direct absorption spectrum measuring method is obtained, the calculating is performed by the second calculating means. The output is switched so as to output the molecular number density c.

The first form of the switching means is means that can determine whether or not an approximation in equation (8) holds, and any means that can make such determination may be used. Specifically, one of ln (I ₀ (ν) / I (ν)) or (I ₀ (ν) −I (ν)) / I ₀ (ν) is always measured, When the value of is less than or equal to a preset threshold value, a determination result that the molecular number density is low suitable for measurement by the harmonic synchronous detection method is obtained, and when the difference is larger than the set ratio, the direct absorption spectrum measurement method is used. A determination result that the molecular number density is suitable for measurement is obtained.

A second form of the switching means is means for using a difference I ₀ (ν) −I (ν) between the reference light intensity signal I ₀ (ν) and the light intensity signal I (ν). Specifically, when the difference I ₀ (ν) −I (ν) is smaller than a preset threshold value, the determination result that the molecular number density is suitable for measurement by the harmonic synchronous detection method is obtained, When the difference is larger than the set value, the determination result that the molecular number density is suitable for the measurement by the direct absorption spectrum measurement method is obtained. The setting value for determination is determined according to the measuring device.

As a first form of the light source drive unit, a drive current applied to the laser irradiation unit is set so that the optical frequency of the laser light emitted from the laser irradiation unit becomes a specific optical frequency in the absorption region of the measurement target component. And fixing the optical frequency of laser light. In this case, it is necessary to separately measure the reference light intensity signal I ₀ (ν) when the laser light is not absorbed by the measurement target component at a specific optical frequency. ₀ ([nu) and a reference light intensity signal I _{0 (ν)} storage unit storing. Then, using the first operation means and the second computing means the reference light intensity signal I ₀ reference beam intensity signal as (ν) I ₀ (ν) see held in the holding unit light intensity signal I ₀ ([nu) value To do.

The center frequency (ν ₀ ) of the absorption line of the measurement target component is most preferable as the specific optical frequency in which the optical frequency of the laser light is in the absorption range of the measurement target component. In that case, the absorption becomes the largest, so that the measurement with the best S / N (signal to noise) ratio can be performed. In order to set the optical frequency of the laser light to the center frequency (ν ₀ ) of the absorption line of the measurement target component, the light source drive unit needs to strictly control the drive current for driving the laser irradiation unit. A light source driving unit with good performance is required. Even if it is difficult to strictly set the optical frequency of the laser beam to the center frequency (ν ₀ ) of the absorption line of the measurement target component, the present invention is not limited to a range that allows a decrease in the S / N ratio. The purpose of the period can be achieved. Therefore, in the present invention having the first form of the light source driving unit, not only the case where the optical frequency of the laser light is set to the center frequency (ν ₀ ) of the absorption line of the measurement target component, but also the center frequency ( This includes the case where the optical frequency is deviated from (ν ₀ ).

The 2nd form of a light source drive part does not fix the optical frequency of a laser beam to the specific optical frequency of the absorption line of a measuring object component. In this case, a laser wavelength scanning current generation unit is provided that changes the laser drive current applied to the laser irradiation unit so that the laser irradiation unit oscillates to laser light having an optical frequency that is not absorbed by the measurement target component. The second signal processing unit transmits the transmitted light intensity at the optical frequency that does not receive the laser light when the laser wavelength scanning current generation unit oscillates the laser beam having the optical frequency that is not absorbed by the measurement target component. And a portion for calculating a reference light intensity signal I ₀ (ν) at a specific optical frequency corresponding to the case where there is approximately no light absorption by the measurement target component. In this case, since it is not necessary to strictly set the optical frequency of the laser light to the center frequency (ν ₀ ) of the absorption line of the measurement target component, the demand for the performance of the light source driving unit is eased. Furthermore, since the reference light intensity signal I ₀ (ν) can be obtained in the second signal processing unit, it is not necessary to measure the reference light intensity signal I ₀ (ν) in advance.

  A preferred example of the laser wavelength scanning current generator generates a sawtooth wave having a period of the second frequency fb lower than the first frequency fa.

In order to be able to detect the reference light intensity signal I ₀ (ν) at the same time even when the light source driving unit of the first form for fixing the optical frequency of the laser light emitted from the laser irradiation unit is provided. As a form, a beam splitter is arranged between the sample cell and the laser irradiation unit, and laser light emitted from the laser irradiation unit by the beam splitter is incident on the sample cell and laser not incident on the sample cell. Branch to light. A reference light receiving unit is further provided for receiving the laser beam branched by the beam splitter and not incident on the sample cell. In that case, the second signal processing unit blocks the detection signal from the reference light receiving unit by using a frequency filter to block the first frequency fa component or more, detects the light intensity at a specific optical frequency that appears after the processing, There is provided a portion which becomes a reference light intensity signal I ₀ (ν) at a specific optical frequency corresponding to the case where there is no light absorption by the measurement target component.

  While the harmonic synchronous detection method is highly sensitive, linearity mismatch occurs when the molecular number density is high. On the other hand, although the direct absorption spectrum method has a relatively low sensitivity, it can be measured even at a high molecular number density. Since the gas analyzer according to the present invention measures these two measuring means in parallel from a single received light signal using the same laser control system, it compensates for the disadvantages of both detection methods and expands the density range of the number of molecules to be measured. In addition, it is possible to maintain measurement continuity.

  In the present invention, the type of the specific gas to be measured is not particularly limited, but it is effective for measuring the molecular number density of water in the gas to be measured because it is effective for those having a large change in molecular number density. .

It is a schematic block diagram which shows 1st Example. It is a block diagram which shows an example of a structure of the signal processing system | strain in 1st Example. It is a block diagram which shows the calculating part in 1st Example. It is a wave form diagram which shows the signal in FIG. 2A. It is a flowchart which shows the signal processing operation and arithmetic processing operation in 1st Example. It is a block diagram which shows 2nd Example. It is a block diagram which shows the calculating part in 2nd Example. ~ It is a wave form diagram which shows the signal in FIG. 4A. It is a graph which shows an example of the photon detection signal extracted by the 1st signal processing part in the 2nd example. It is a graph which shows an example of the photon detection signal extracted by the 2nd signal processing part in the 2nd example. It is a flowchart which shows the signal processing operation | movement and arithmetic processing operation in 2nd Example. It is a block diagram which shows 3rd Example. It is a block diagram which shows the calculating part in 3rd Example.

[Example 1]
A first embodiment of the present invention will be described with reference to FIGS. 1 and 2A and 2B. The gas analyzer of this embodiment is a moisture measuring device that measures the molecular number density of moisture in the measurement target component.

  As shown in FIG. 1, the gas analyzer of this embodiment includes a sample cell 2 arranged in a substantially horizontal direction on a gas flow path for flowing a sample gas downward from above. Two reflecting mirrors 10 a and 10 b facing each other are provided at the left and right opening ends of the sample cell 2. A part of one reflecting mirror 10a is provided with a transparent window 10c made of, for example, quartz glass so that only light can pass through. The transparent window 10c is sandwiched between the sample cell 2 and a substantially sealed structure. An optical chamber 4 that is an atmospheric pressure atmosphere is installed. In the optical chamber 4, a wavelength tunable laser device 6 as a laser irradiation unit is accommodated, and a light receiving unit 8 is also accommodated. In the optical chamber 4, moisture that is an interfering component is removed by a dehumidifying agent, purge gas, or the like, and its molecular number density is reduced to a negligible level, sealed so that the state can be maintained, and outside air does not enter. The inside of the optical chamber 4 is at the same atmospheric pressure as the outside air. The “substantially sealed structure and substantially atmospheric pressure atmosphere” refers to a state in which the moisture removal state in the optical chamber 6 can be maintained.

  In the example of FIG. 1, the transparent window 10c is used for both the incidence and emission of the laser beam to the sample cell 2, but a configuration in which transparent windows are separately provided for incidence and emission is also possible. .

The wavelength tunable laser device 6 is a DFB laser, for example, and can generate laser light having an optical frequency ranging from the near infrared region to the mid infrared region. As the wavelength tunable laser device 6, a laser other than the DFB type laser can be used. The wavelength tunable laser device 6 is driven by a drive current from the light source drive unit 12. The light source drive unit 12 supplies a drive current to the wavelength tunable laser device 6 so as to generate laser light having the same frequency as the center frequency ν ₀ of the absorption line of the measurement target component from the wavelength tunable laser device 6. The light source driving unit 12 is controlled by the control unit 14. The drive current given from the light source drive unit 12 to the wavelength tunable laser device 6 is modulated by the modulation unit 16 at the modulation frequency fa. The modulation unit 16 will be described later.

  The laser light from the wavelength tunable laser device 6 is irradiated into the sample cell 2, and after being reflected by the reflecting mirrors 10a and 10b a plurality of times, returns to the optical chamber 4 from the transparent window 10c of the reflecting mirror 10a, and the light receiving unit 8 It is comprised so that it may inject into. The laser beam is absorbed by various components in the measurement target component when passing through the gas flow path. The light receiving unit 8 includes a photodiode as a light receiving element, and outputs the light amount of each frequency incident on the photodiode as an electric signal. The signal output from the light receiving unit 8 is subjected to predetermined processing by the first signal processing unit 18 and the second signal processing unit 20, and is taken into the calculation unit 22.

In the first signal processing unit 18, the detection signal from the light receiving unit 8 is synchronously detected using the clock signal with the frequency 2 fa provided from the modulation unit 16 as a reference signal, and the signal with the frequency 2 fa is extracted. Of course, the frequency 2fa means twice the frequency of fa. The intensity of the signal extracted by the first signal processing unit 18 is Signal (ν ₀ ). On the other hand, the second signal processing unit 20 extracts a signal having a frequency less than fa. The intensity of the signal extracted by the second signal processing unit 20 is I (ν ₀ ). The signal intensity Signal (ν ₀ ) extracted by the first signal processing unit 18 is a signal for performing measurement by the harmonic synchronous detection method, and the signal I (ν ₀ ) extracted by the second signal processing unit 20 is It is a signal for performing measurement by the direct absorption spectrum method.

The calculation unit 22 includes a first calculation unit 23-1 that calculates the molecular number density of the measurement target component using the harmonic synchronization detection method based on the signal extracted by the first signal processing unit 18, and a second signal. Second calculation means 23-2 for calculating the molecular number density of the measurement target component using the direct absorption spectrum method based on the signal extracted by the processing unit 20, and a signal when there is no light absorption by the measurement target component Reference light intensity signal I ₀ (ν ₀ ) holding unit 23-3 including a memory for holding intensity (reference light intensity signal) I ₀ (ν ₀ ), and any calculation means according to the molecular number density of the measurement target component A switching means 25 for determining whether to use the output of 23-1 or 23-2 is provided, and the molecular number density of the measurement target component is calculated.

Details of an example of the signal processing system of the embodiment will be further described with reference to FIGS. 2A, 2B, and 2C. In this example, the light source driver 12 in FIG. 1 includes an offset DC voltage generator 24, an adder 26, and a voltage / current for adjusting the optical frequency of the oscillating laser light to the center frequency ν ₀ of the absorption line of the measurement target component. The converter 28 is configured. The offset DC voltage generator 24 generates a voltage signal 24S such that the laser light having the center frequency of the absorption line of the measurement target component is emitted from the wavelength tunable laser device 6, and the voltage / current conversion is performed via the adder 26. Connected to the device 28. The adder 26 adds the modulation wave of the frequency fa from the modulation unit 16 to the voltage signal 24S from the offset DC voltage generation unit 24, and the voltage signal 24S modulated at the frequency fa through the adder 26 is voltage / It is converted into a drive current by the current converter 28 and supplied to the wavelength tunable laser device 6.

The modulation unit 16 includes a 2fa clock generation unit 30 that generates a clock signal with a frequency 2fa, a DC voltage generation unit 32 that generates a modulation amplitude control DC voltage, and a frequency divider that converts the clock signal with a frequency 2fa into a clock with a frequency fa. 34, a multiplier 36, and a band pass filter (BPF1) 38 having a transmission range of frequency fa ± several hundreds Hz for extracting a signal in the vicinity of the frequency fa. The modulation amplitude control DC voltage may be an arbitrary voltage, but if it is too large, the influence of other optical frequency components appears strongly in the signal intensity I (ν ₀ ) detected by the second signal processing unit 20 described later. As a result, accurate measurement cannot be performed. In the modulation unit 16, the clock signal having the frequency 2fa generated by the 2fa clock generation unit 30 is converted into a clock signal having the frequency fa by the frequency divider 34, and the clock signal is converted into a modulation amplitude control DC voltage by the multiplier 36. After being multiplied, the band pass filter 38 converts it into a sine wave. The bandpass filter 38 is connected to the adder 26 via a phase shifter 39, and the signal voltage from the bandpass filter 38 is phase-adjusted by the phase shifter 39 and then the voltage from the offset DC voltage generator 24. Is added to

  In this embodiment, the phase shifter 39 is arranged immediately after the modulation unit 16. However, for example, if phase detection is performed in front of the first signal processing unit 18 or the like, it is possible to perform synchronous detection. Can be placed anywhere.

  The light receiving unit 8 includes a photodiode (PD) 40 and a current / voltage converter 42. The photodiode 40 receives the laser light reflected in the sample cell 2 a plurality of times and returned to the optical chamber 4 again, and outputs the incident intensity as an electric signal. The current / voltage converter 42 converts the signal current from the photodiode 40 into a voltage and outputs the voltage to the first signal processing unit 18 and the second signal processing unit 20.

The first signal processing unit 18 includes a synchronous detection unit 44 including a multiplier, a low-pass filter (LPF1) 46, and an A / D converter (ADC1) 48. As shown in the signal waveform of FIG. 2C, the detection signal 42S from the light receiving unit 8 includes a frequency 2fa and other harmonic components in addition to the signal of the frequency fa. The synchronous detection unit 44 multiplies the clock signal of the frequency 2fa from the 2fa clock generation unit 30 of the modulation unit 16 to invert half of the waveform of the frequency 2fa component, and removes the frequency component other than the frequency 2fa to obtain the signal 44S. Become. By passing this signal 44S through the low-pass filter 46, the peak signal intensity Signal (ν ₀ ) 46 of the harmonic component of the frequency 2fa is extracted. The signal strength Signal (ν ₀ ) is converted into a digital signal by the A / D converter 48 and then taken into the arithmetic unit 22.

The second signal processing unit 20 includes a low pass filter (LPF) 50 and an A / D converter 52. The low-pass filter 50 receives the signal 42S from the light receiving unit 8, and only the fluctuation component of the absorption peak passes as the signal 50S. The signal 50S is the signal intensity I (ν ₀ ). As the low-pass filter 50, for example, if the response speed is 1 second, a filter having a time constant of about 0.1 seconds is appropriate. The signal intensity I (ν ₀ ) is converted into a digital signal by the A / D converter 52 and then taken into the calculation unit 22.

As described above, the calculation unit 22 calculates the number of molecules of the measurement target component by the harmonic synchronous detection method based on the signal intensity Signal (ν ₀ ) from the first signal processing unit 18 using the above-described equation (11). Based on the first calculation means 23-1 for calculating the density c and the signal intensity I (ν ₀ ) from the second signal processing unit 20, the above-described equation (7) is used to directly measure the component to be measured by the absorption spectrum method. Second calculating means 23-1 for calculating the molecular number density c is provided. In this embodiment, the reference light intensity signal I ₀ (ν ₀ ) when light is not absorbed by the measurement target component is measured in advance in this embodiment, and the reference light intensity signal I ₀ (ν ₀ ) holding unit 23-3 of the calculation unit 22 is measured. To keep.

An example of signal processing and calculation processing of this gas analyzer will be described below with reference to the flowchart of FIG. A signal from the light receiving unit 8 that has received the light that has passed through the sample cell 2 is input to the first signal processing unit 18 and the second signal processing unit 20 and subjected to predetermined processing (step S1). The first signal processing unit 18 extracts a signal intensity Signal (ν ₀ ) of frequency 2 × fa for the harmonic synchronization detection method, and the second signal processing unit 20 outputs a signal having a frequency less than the frequency fa for the direct absorption spectrum method. Intensity I (ν ₀ ) is extracted. The signal strengths extracted by these signal processing units 18 and 20 are taken into the calculation unit 22 (step S2).

In the calculation unit 22, the switching means 25 takes in the reference light intensity signal I ₀ (ν ₀ ) from the reference light intensity signal I ₀ (ν ₀ ) holding unit 23-3 and takes in the signal intensity Signal taken in from the first signal processing unit 18. Using (ν ₀ ) and I (ν ₀ ) taken from the second signal processing unit 20, ln (I ₀ (ν ₀ ) / I (ν ₀ )) or (I ₀ (ν ₀ ) −I (ν ₀ )) / I ₀ (ν ₀ ) is calculated (step S3). The switching means 25 outputs the result of calculating the molecular number density c of the component to be measured by the harmonic synchronous detection method using the expression (10) by the first calculation means 23-1 from the calculation result or the second calculation. The means 23-2 determines whether to output the result of calculating the molecular number density c of the component to be measured by the direct absorption spectrum method using the equation (7) (step S4).

As an example of the determination by the switching means 25, ln (I ₀ (ν ₀ ) / I (ν ₀ )) or (I ₀ (ν ₀ ) −I (ν ₀ )) / I ₀ (ν ₀ ) is set in advance. When it is less than the threshold, the first synchronous means 23-1 uses the equation (11) to determine that the molecular number density of the component to be measured is a low molecular number density that can be measured by the harmonic synchronous detection method. The result of calculating the molecular number density c of the measurement target component is output (step S5). Conversely, if ln (I ₀ (ν ₀ ) / I (ν ₀ )) or (I ₀ (ν ₀ ) −I (ν ₀ )) / I ₀ (ν ₀ ) is greater than or equal to a preset threshold, Assuming that the molecular number density is higher than the molecular number density to which the wave synchronous detection method can be applied, the result of calculating the molecular number density c of the measurement target component by the direct absorption spectrum method is output by the second computing means 23-2. (Step S6).

In addition, the determination method about whether to use the harmonic synchronous detection method or the direct absorption spectrum method is a threshold value in which (I ₀ (ν ₀ ) −I (ν ₀ )) is set in advance in addition to the above determination method. The determination method may be such that the harmonic synchronous detection method is used when it is less than the threshold value, and the direct absorption spectrum method is used when it is equal to or greater than the threshold value.

  In this embodiment, both the first calculation means 23-1 and the second calculation means 23-2 execute the calculation, and the calculation result selected by the switching means 25 is output. Only the calculation means selected by 25 may execute and output the calculation process for calculating the molecular number density, and the calculation means not selected may not execute the calculation process. In this case, there is an advantage that the load on the calculation unit 22 can be reduced.

[Example 2]
Next, a second embodiment will be described with reference to FIGS. The overall configuration of the gas analyzer of this embodiment is the same as that shown in FIG. 1, but the signal processing system is partially different in this embodiment because the optical frequency of the laser beam is scanned. The following description will focus on differences from the first embodiment.

In this embodiment, the oscillation frequency of the laser beam is scanned by changing the drive current of the wavelength tunable laser device 6 with a frequency fb lower than the frequency fa, for example, with a sawtooth wave. In order to scan the oscillation frequency of the laser beam, it is not necessary to match the oscillation frequency of the laser beam with high accuracy to the center frequency of the absorption line of the measurement target component as in the first embodiment, and it is continuously scanned many times. Therefore, the S / N ratio can be improved by taking the average value of the signals. In addition, since the optical frequency of the laser beam is scanned, the signal intensity of the light receiving unit 8 when the optical frequency of the laser beam is a frequency that does not receive the measurement target component can be measured. The signal intensity when the light frequency is the center frequency of the absorption line of the measurement target component can be calculated as a predicted value of the reference light intensity signal I ₀ (ν ₀ ) in the absence of the measurement target component. It is not necessary to previously measure the reference light intensity signal I ₀ (ν ₀ ) as in FIG.

  In this embodiment, in addition to the offset DC voltage generator 24, the light source driver 12 generates a laser wavelength scanning voltage that generates a sawtooth scanning voltage having a frequency fb for generating a laser wavelength scanning current. A section 25 is further provided, and the adder 27 is configured to add a sawtooth scanning voltage to the voltage signal from the offset DC voltage generation section 24. The signal 24S generated from the offset DC voltage generator 24, the signal 25S generated from the laser wavelength scanning voltage generator 25, and the signal 27S formed by adding these signals are signals having the waveforms shown in FIG. 4c. .

In addition to the low-pass filter (LPF) 50 and the A / D converter (ADC2) 52, the second signal processing unit 20a includes a bandpass filter (BPF3) 54 having a transmission band for extracting a signal near the frequency fb, An A / D converter 56 (ADC3) is added. The low-pass filter (LPF) 50 and the A / D converter (ADC2) 52 extract the signal intensity I (ν ₀ ) of the absorption peak, which is a DC component signal, as in the first embodiment. The bandpass filter (BPF3) 54 is suitable for measuring an absorption spectrum appearing at a frequency fb period. The signal extracted by the bandpass filter (BPF3) 54 is converted into a digital signal by the A / D converter 56. Are taken into the calculation unit 22a.

  In the first signal processing unit 18a, unlike the low-pass filter (LPF1) 46 of the first embodiment, the low-pass filter (LPF2) 46a needs to extract a signal in the vicinity of the frequency fb. It has a frequency.

The first computing means 23-1a in the computing unit 22a extracts the signal strength Signal (ν ₀ ) based on the scanned signal from the first signal processing unit 18a, and further extracts the extracted signal strength Signal (ν ₀ ) and Based on I ₀ (ν ₀ ) extracted by the second calculation means 23-2a, the molecular number density c of the measurement target component is calculated by the harmonic synchronous detection method using the above-described equation (11). The second computing means 23-2a extracts the signal intensities I (ν ₀ ) and I ₀ (ν ₀ ) from the two scanned signals from the second signal processor 20a, and the extracted signal intensities I (ν ₀ ). Based on the above and I ₀ (ν ₀ ), the molecular number density c of the measurement target component is calculated by the direct absorption spectrum method using the above-described equation (7). The first computing means 23-1a and the second computing means 23-2a always perform computations, and the switching means 25 performs signal strengths I (ν ₀ ) and I ₀ (ν ₀ ) from the second computing means 23-1a. The calculation unit 23-1a or 23-2a determines whether to output the calculated molecular number density c of the calculation means 23-1a or 23-2a.

The operation of this embodiment will be described.
In the adder 26, the signal 39S of the modulation frequency fa is added to the sawtooth signal 27S of the frequency fb from the adder 27, and the drive signal 26S as shown in FIG. 4D is applied to the voltage / current converter 28. The By controlling the laser oscillation by this drive signal, the generated laser light is a sawtooth wave of frequency fb, the wavelength is scanned around the oscillation frequency ν _0, and is modulated at the frequency fa. The scanning frequency fb is considerably lower than the modulation frequency fa, and is preferably several Hz to several hundred Hz. The modulation frequency fa is generally about several kHz to several MHz. As described above, the sample gas is irradiated with the laser beam to which the sine wave modulated at the frequency fa is added. Here, it is desirable that the amplitude of the sine wave having the frequency fa is sufficiently smaller than the amplitude of the sawtooth wave having the frequency fb. As in the first embodiment, if it is too large, the influence of other optical frequency components appears strongly in the signal intensity I (ν ₀ ) detected by the second signal processing unit 20a, and accurate measurement cannot be performed.

  Laser light absorbed by the measurement target component in the sample cell is detected by the photodiode 40 of the light receiving unit 8 and is subjected to signal processing in the first signal processing unit 18a and the second signal processing unit 20a.

The signals extracted by the low-pass filter 50 and the band-pass filter 54 in the second signal processing unit 20a become signals shown as signals 50S and 54S in FIG. 4D, respectively. The signal 50S corresponds to the signal intensity I (ν ₀ ), which is the same as in the first embodiment. The signal 54S is a signal that repeatedly appears at the frequency fb. These signals are digitally converted by the respective A / D converters 52 and 56, and taken into the arithmetic unit 22a and added to become a signal shown as a signal (50S + 54S) in FIG. 4D.

In the first signal processing unit 18a, the detection signal 42S from the light receiving unit 8 is multiplied by the clock signal of the frequency 2fa from the 2fa clock generation unit 30 of the modulation unit 16 by the synchronous detection unit 44, and half the waveform of the frequency 2fa component. Is inverted, and frequency components other than the frequency 2fa are removed, and a signal 44S is obtained. By passing this signal 44S through the low-pass filter 46a, a signal 46aS including the peak signal intensity Signal (ν ₀ ) of the harmonic component of the frequency 2fa is extracted. These signals 42s, 44s and 46as have waveforms as shown in FIG. 4E. As in the first embodiment, synchronous detection is performed at a frequency of 2fa. However, in this embodiment, the wave number ν of the laser beam is scanned, so that the signal passing through the low-pass filter 46a appears as a waveform with a frequency fb period. . The signal 46aS is converted into a digital signal by the A / D converter 48 and then taken into the arithmetic unit 22a.

FIG. 5 is an example of a waveform of a signal extracted by the first signal processing unit 18a, and FIG. 6 is an example of a waveform of a signal extracted by the second signal processing unit 20a. 5 and 6, the horizontal axis represents the difference (ν−ν ₀ ) between the optical frequency ν of the laser beam and the center frequency ν ₀ of the absorption line of the measurement target component, and the vertical axis represents the detection signal intensity. It is. When the relationship with the signal 26s for scanning the laser wavelength is shown, as shown in FIG. 4F, these signals are repeatedly obtained at the period of the sawtooth frequency fb. As described above, since a signal having a waveform as shown in FIGS. 5 and 6 can be repeatedly obtained at a period of the frequency fb, the S / N ratio can be increased by the averaging process.

As shown in FIG. 5, the first calculation means 23-1a in the calculation unit 22a has a signal intensity as a height from the lower peak to the upper peak in the waveform of the signal extracted by the signal processing unit 1. Extract Signal (ν ₀ ).

On the other hand, the second calculation means 23-2a in the calculation unit 22a extracts I (ν ₀ ) as the signal intensity at the center frequency of the absorption line of the measurement target component in FIG. 6, and based on the signal intensity around this waveform. The reference light intensity signal I ₀ (ν ₀ ) is extracted by creating an approximate line. Therefore, all of I ₀ (ν ₀ ), I (ν ₀ ), and Signal (ν ₀ ) can be obtained by a single measurement performed by scanning the frequency of the laser beam. The calculation process of the molecular number density c of the measurement target component based on I ₀ (ν ₀ ), I (ν ₀ ), and Signal (ν ₀ ) is the same as in the first embodiment.

The switching means 25 takes in the signals I ₀ (ν ₀ ) and I (ν ₀ ) from the second computing means 23-2a, and in either computing means 23-1a or 23-2a by the same determination as in the first embodiment. Determine whether to output the number density calculation result. In this case, the calculation of the molecular number density c from the signal intensity Signal (ν ₀ ) by the first calculation means 23-1a, and the numerator from the signals I ₀ (ν ₀ ) and I (ν ₀ ) by the second calculation means 23-2a. The calculation of the number density c is always executed, and when selected from the switching means 25, the molecular number density calculation result is output.

  Even in this embodiment, the second signal processing unit 20 a may be configured by one low-pass filter 50 and one A / D converter 52. However, in that case, since the low-pass filter 50 needs to pass a signal having the frequency fb for laser wavelength scanning, it needs to be a low-pass filter having a cutoff frequency of about several kHz.

An example of signal processing and calculation processing of the gas analyzer of this embodiment will be described below with reference to the flowchart of FIG. A signal from the light receiving unit 8 that has received the light that has passed through the sample cell 2 is input to the first signal processing unit 18a and the second signal processing unit 20b and subjected to predetermined processing, as shown in FIGS. Data indicating the waveform is input to the calculation unit 22a (step S11). The first computing means 23-1a extracts the signal intensity Signal (ν ₀ ), and the second computing means 23-2a extracts the signals I (ν ₀ ) and I ₀ (ν ₀ ) (step S12).

In the computing unit 22a, the switching means 25 uses the signals I (ν ₀ ) and I ₀ (ν ₀ ) from the second computing means 23-2a to make ln (I ₀ (ν ₀ ) / I (ν ₀ )). Or (I ₀ (ν ₀ ) −I (ν ₀ )) / I ₀ (ν ₀ ) is calculated, and based on the calculation result, any of the arithmetic means 23-1a or 23-2a is calculated in the same manner as in the first embodiment. It is determined whether to output the molecular number density calculation result at (step S13). When the switching means 25 designates the first calculation means 23-1a, the first calculation means 23-1a outputs the molecular number density c of the measurement target component calculated by the harmonic synchronous detection method using the equation (11). (Step S14) When the switching means 25 designates the second computing means 23-2a, the second computing means 23-2a calculates the molecular number density of the component to be measured calculated by the direct absorption spectrum method using the equation (1). c is output (step S15).

[Example 3]
Next, a third embodiment of the gas analyzer will be described with reference to FIGS. 8A and 8B. In this embodiment, a part of the laser light emitted from the wavelength tunable laser device 6 is branched from the laser light irradiated to the sample cell 2 by the beam splitter 7, and the light receiving unit 8 b side is passed without passing through the sample cell 2. Configure to lead to. The light receiving unit 8b is added to the photodiode 40a for detecting the laser light that has passed through the sample cell 2 and the current / voltage converter 42a that converts the current signal of the photodiode 40a into a voltage signal, and the inside of the sample cell 2 A photodiode 40b and a current / voltage converter 42b for detecting laser light that is not allowed to pass are provided, and laser light that has passed through the sample cell 2 and laser light that is not allowed to pass are detected simultaneously by separate detectors. Since the photodiode 40b detects laser light that does not pass through the sample cell 2, this detection signal is used as a detection signal when there is no absorption by the measurement target component, that is, a reference light intensity signal.

  The second signal processing unit 20b includes a low-pass filter for processing a signal from the photodiode 40b in addition to a low-pass filter (LPF1) 50 and an A / D converter (ADC2) 52 for processing a signal from the photodiode 40a. (LPF2) 60 and A / D converter (ADC3) 62 are provided. The current / voltage converter 42a is connected to the low-pass filter (LPF1) 50 via the amplifier 58a, and the current / voltage converter 42b is connected to the low-pass filter (LPF2) 60 via the amplifier 58b. The amplifiers 58a and 58b are for adjusting so that the signals from the photodiodes 40a and 40b can be compared as equivalent signals without the influence of attenuation or the like when the laser light is reflected by the reflecting mirrors 10a and 10b. It is.

Other configurations are the same as those of the first embodiment shown in FIGS. 2A and 2B, but the reference light intensity signal I ₀ (ν ₀ ) is obtained as a signal from the photodiode 40b. There is no need to provide an I ₀ (ν ₀ ) holding unit for holding the reference light intensity signal I ₀ (ν ₀ ). With this configuration, the first signal processing unit 18 extracts Signal (ν ₀ ), the second signal processing unit 20 b extracts I (ν ₀ ) and I ₀ (ν ₀ ), and the calculation unit 22 b performs the same operations as in the first embodiment. Similarly, calculation of the molecular number density c of the measurement target component by the harmonic synchronous detection method and calculation of the molecular number density c of the measurement target component by the direct absorption spectrum method are performed by switching by the switching means 25 and output.

  In the above embodiment, the gas analyzer according to the present invention is applied to the measurement of the molecular number density of water in the component to be measured, but the present invention is applied to the measurement of any gas molecular number density other than water. be able to.

2 Sample cell 4 Optical chamber 6 Wavelength tunable laser device 8, 8b Light receiving unit 10a, 10b Reflector 12 Light source driving unit 14 Control unit 16 Modulating unit 18 First signal processing unit 20, 20a, 20b Second signal processing unit 22, 22a , 22b arithmetic units 23-1, 23-1a first arithmetic means 23-2, 23-2a second arithmetic means 24 offset DC voltage generator 25 switching means 26 adder 28 voltage-current converter 30 clock generator 32 DC voltage Generator 34 Divider 36 Multiplier 38, 46 Band pass filter 39 Phase shifter 40, 40a, 40b Photodiode 42, 42a, 42b Current / voltage converter 44 Synchronous detection unit 48, 52 A / D converter 50 Low pass filter

Claims (8)

A sample cell for circulating a sample gas;
A laser irradiation unit that irradiates the sample cell with a laser beam having a specific optical frequency in which the measurement target component in the sample gas has absorption;
A light source driving unit that applies a driving current for irradiating the laser beam to the laser irradiating unit;
A modulation unit that modulates the drive current applied from the light source drive unit to the laser irradiation unit with a modulation frequency of a first frequency fa;
A light receiving unit that receives the laser light that has passed through the inside of the sample cell;
A first signal processing unit that synchronously detects a light detection signal of the light receiving unit at a frequency that is an integral multiple of the modulation frequency, and detects a harmonic signal intensity Signal (ν) by a harmonic synchronous detection method;
The light detection signal of the light receiving unit is captured without passing through the first signal processing unit, the frequency component above the first frequency fa is blocked by a frequency filter, and the light intensity signal I (ν ) Detecting a second signal processing unit;
The harmonic signal intensity Signal (ν) detected by the first signal processing unit and the light intensity signal I (ν) detected by the second signal processing unit are taken in, and the number of molecules of the measurement target component in the sample gas An operation unit for calculating the density c,
The computing unit is
When the laser light is not absorbed by the measurement target component at the specific optical frequency, the reference light intensity signal is I ₀ (ν), and the harmonic signal intensity Signal (ν) and the reference light intensity signal I ₀ a first calculation means for calculating the molecular number density c of the component to be measured in the sample gas from (ν);
A second calculating means for calculating the molecular number density c of the component to be measured in the sample gas by the direct absorption spectrum measurement method from the light intensity signal I (ν) and the reference light intensity signal I ₀ (ν);
Whether the molecular number density of the component to be measured in the sample is a low molecular number density suitable for the measurement by the harmonic synchronous detection method from only the light intensity signal I (ν) and the reference light intensity signal I ₀ (ν) Judgment was made whether the molecular number density was suitable for the measurement by the direct absorption spectrum measurement method at a higher molecular number density, and the judgment result was obtained that the molecular number density was suitable for the measurement by the harmonic synchronous detection method. When the molecular weight density calculated by the first calculating means is output, and the determination result that the molecular number density is suitable for the measurement by the direct absorption spectrum measurement method is obtained, the calculated molecular number density c by the second calculating means is obtained. Switching means for switching the output so as to output
Gas analyzer equipped with. The switching means detects harmonic synchronization when ln (I ₀ (ν) / I (ν)) or (I ₀ (ν) −I (ν)) / I ₀ (ν) is less than a predetermined value. The determination result is low molecular number density suitable for measurement by the method, and when the value is larger than the predetermined value, it is determined that the molecular number density is suitable for measurement by the direct absorption spectrum measurement method. The gas analyzer according to claim 1 , which obtains a result. When the difference between the light intensity signal I (ν) and the reference light intensity signal I ₀ (ν) is not more than a preset value, the switching means has a molecular number density suitable for measurement by the harmonic synchronous detection method. the determination results obtained and the difference is the gas analyzer according to claim 1 is intended to obtain a determination result that the molecule number density suitable for measurement by direct absorption spectroscopy when greater than the set value . The light source driving unit sets a driving current to be applied to the laser irradiation unit such that the frequency of the laser light emitted from the laser irradiation unit becomes the specific frequency,
The arithmetic unit includes a reference light intensity signal I ₀ ([nu) reference light intensity signal I ₀ for holding ([nu) holding portion when the laser beam is not subjected to absorption by the measurement target component in the specific frequency And
Using said first arithmetic means and the reference light intensity signal I ₀ (ν) see held in the holding unit light intensity signal I ₀ (ν) value and the second computation means as the reference light intensity signal I ₀ (ν) The gas analyzer according to any one of claims 1 to 3 . The gas analyzer according to claim 4 , wherein the specific frequency is a center frequency (ν ₀ ) of an absorption line of the measurement target component. A laser wavelength scanning current generation unit that changes a laser drive current applied to the laser irradiation unit so that the laser irradiation unit oscillates to a laser beam having a frequency that is not absorbed by the measurement target component;
The second signal processing unit also has a transmitted light intensity at a frequency at which the laser irradiation unit generates no laser light having a frequency at which the laser irradiation unit does not receive absorption by the measurement target component by the laser wavelength scanning current generation unit. measured, the specific reference light intensity signal I ₀ of the optical frequency ([nu) 3 from claim 1, further comprising a part for calculating the equivalent when there is no light absorption by approximately measurement target component from the result The gas analyzer according to any one of the above. The gas analyzer according to claim 6 , wherein the laser wavelength scanning current generator generates a sawtooth wave having a second frequency fb period lower than the first frequency fa. A beam splitter that is arranged between the sample cell and the laser irradiation unit, and splits the laser beam emitted from the laser irradiation unit into a laser beam that enters the sample cell and a laser beam that does not enter the sample cell;
A reference light receiving unit that receives the laser beam that is branched by the beam splitter and does not enter the sample cell;
The second signal processing unit is configured to block the detection signal from the reference light receiving unit by using a frequency filter to block the first frequency fa component or more, and detect the light intensity at the specific optical frequency that appears after the processing, The gas analysis according to any one of claims 1 to 3 , further comprising a portion that serves as a reference light intensity signal I ₀ (ν) at the specific frequency corresponding to a case where light is not absorbed by a measurement target component. apparatus.

Images