JP2013164315A - Laser gas analysis device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser gas analysis device capable of reducing a processing time for fitting.SOLUTION: The laser gas analysis device comprises: a laser drive unit 2 that generates a driving signal for driving a wavelength variable semiconductor laser 1; a waveform processing unit 6 that, by using a modulation signal, synchronously detects output of light detection units 4 and 5 receiving the output light of the wavelength variable semiconductor laser through a gas cell 3, thereby extracting harmonic components of the output signal of the light detection units and performing waveform processing; and a concentration calculation unit 7 that calculates gas concentration on the basis of the waveform thus obtained. The laser drive unit generates as a driving signal a signal obtained by superposing the modulation signal on a sweep signal and synchronizing with the cycle of the sweep signal to switch a wavelength amplitude of the modulation signal to two levels of large and small. The waveform processing unit comprises: a gas identifying unit for identifying a gas component existing in the gas cell on the basis of the harmonic component of the output signal; and a waveform separation unit for separating the waveform of harmonic components of the output signal by each gas component by using identified gas component information.

Description

  The present invention relates to a laser type gas analyzer that measures the concentration of gas in a gas cell using a wavelength tunable semiconductor laser.

  In recent years, laser absorption spectroscopy using a wavelength tunable laser has been disclosed as a method for measuring the concentration of a specific gas in a gas (Patent Document 1).

  A gas analyzer using laser absorption spectroscopy includes a laser emission unit having a wavelength variable semiconductor laser having an absorption wavelength band of a measurement target gas as a central wavelength, a laser driving unit for driving the wavelength variable semiconductor laser, and a temperature of the wavelength variable semiconductor laser. The temperature control unit that adjusts the gas, the gas cell filled with the measurement target gas, the light detection unit that measures the light intensity of the laser light that has passed through the gas cell, and the waveform of the harmonic component are extracted by synchronously detecting the output of the light detection unit And a concentration calculation unit for calculating the gas concentration based on the waveform obtained by the synchronous detection unit.

  The oscillation wavelength of the tunable semiconductor laser is controlled by the drive current and temperature. FIG. 13 is a diagram showing a laser drive waveform that is generally used. The drive waveform is a harmonic signal (lower left of FIG. 13) set to a frequency of about 10 to 1000 times the sweep signal to a sweep signal (waveform on the upper left of FIG. 13) that sweeps an arbitrary absorption wavelength band of the measurement target gas. A waveform on the right side of FIG. 13 is obtained by superimposing a waveform (hereinafter referred to as a modulation signal).

  The light intensity of the laser output light emitted by this drive waveform is attenuated by the gas present in the gas cell, and the laser output light is received by the light detection unit. The received signal is synchronously detected at a frequency twice that of the modulation signal by the synchronous detection unit, whereby a second harmonic component (hereinafter referred to as 2f spectrum) is obtained.

    Since the attenuation amount of the laser light intensity corresponds to the absorption spectrum of the measurement target gas, the intensity of the 2f spectrum obtained by the synchronous detection unit has a correlation with the gas concentration. For this reason, by preparing a calibration curve in advance using the 2f spectrum obtained from the standard gas, the gas concentration can be calculated from the measured 2f spectrum with ppm level accuracy.

  For example, Non-Patent Document 1 is known as a related art.

Japanese Patent Laid-Open No. 5-99845

LST43-15 Proceedings of 43rd Meeting on Lightwave Sensing Technology (103-110.June 2009)

  However, as shown in Patent Document 1, if a gas other than the measurement target gas (hereinafter referred to as interference gas) exists in the gas cell to be measured, the acquired 2f spectrum interferes with the 2f spectrum of the measurement target gas. The spectrum is convolved with the 2f spectrum of the gas, and the gas concentration cannot be easily calculated from the intensity of the 2f skull. For this reason, it is desired to separate the waveform of the 2f spectrum of the measurement target gas and the interference gas from the acquired 2f spectrum by fitting.

  However, since fitting is performed using data of all interference gas components assumed to exist in the gas cell, there is a problem that it takes a considerable time.

  An object of the present invention is to provide a laser type gas analyzer that can further shorten the processing time of fitting.

  In order to solve the above-described problems, a laser gas analyzer according to the present invention oscillates in the absorption wavelength band of a measurement target gas and outputs a laser beam, and a drive for driving the wavelength tunable semiconductor laser. A laser drive unit that generates a signal, a light detection unit that receives output light of the wavelength tunable semiconductor laser through a gas cell, and an output of the light detection unit by synchronously detecting the output of the light detection unit with a modulation signal In the laser-type gas analyzer, comprising: a waveform processing unit that extracts a harmonic component of a signal and performs waveform processing; and a concentration calculation unit that calculates a gas concentration based on the waveform obtained by the waveform processing unit. The unit generates, as the drive signal, a signal obtained by superimposing the modulation signal on the sweep signal and switching the wavelength swing of the modulation signal between large and small in synchronization with the cycle of the sweep signal. The waveform processing unit uses the gas specifying unit for specifying a gas component existing in the gas cell based on the harmonic component of the output signal, and the gas component information specified by the gas specifying unit, to output the output. And a waveform separation unit for separating a waveform into each gas component in a harmonic component of the signal.

  According to the present invention, the laser driving unit is a signal in which the modulation signal is superimposed on the sweep signal and the signal in which the wavelength swing of the modulation signal is switched between large and small in synchronization with the cycle of the sweep signal. The gas specifying unit specifies the gas component present in the gas cell based on the harmonic component of the output signal, and the waveform separating unit uses the gas component information specified by the gas specifying unit to output the output signal. Waveform separation into each gas component in the harmonic component. Therefore, since fitting is performed only with the measurement target gas and the interference gas component existing in the gas cell, the fitting processing time can be further shortened.

1 is a configuration diagram of a laser type gas analyzer of Example 1. FIG. It is a figure which shows the example of the laser drive signal of the laser type gas analyzer of Example 1. FIG. 3 is a flowchart illustrating an operation of a waveform processing unit of the laser type gas analyzer of Example 1. It is a wave form diagram which shows the waveform processing flow of the laser type gas analyzer of Example 1. FIG. FIG. 3 is a diagram illustrating a sample gas used for simulation of the laser type gas analyzer of Example 1. It is a figure which shows the relationship between 2f spectrum of the laser type gas analyzer of Example 1, and a modulation signal. It is a figure which shows the 2f spectrum of the sample A acquired by the wavelength fluctuation width of the modulation signal of the laser type gas analyzer of Example 1. It is a figure which shows 2f spectrum of the sample A acquired by the large wavelength fluctuation width of the modulation signal of the laser type gas analyzer of Example 1. It is a figure which shows the example of fitting of the sample A of the laser type gas analyzer of Example 1. FIG. It is a figure which shows 2f spectrum of the sample B acquired by the small wavelength fluctuation width of the modulation signal of the laser type gas analyzer of Example 1. It is a figure which shows 2f spectrum of the sample B acquired by the large wavelength fluctuation width of the modulation signal of the laser type gas analyzer of Example 1. It is a figure which shows the example of fitting of the sample B of the laser type gas analyzer of Example 1. FIG. It is a figure which shows the laser output waveform of the conventional laser type gas analyzer.

  Embodiments of a laser gas analyzer according to the present invention will be described below in detail with reference to the drawings. In the laser type gas analyzer of the present invention, the laser driver switches the wavelength amplitude of the modulation signal between large and small, and the waveform processing unit acquires the 2f spectrum of each wavelength amplitude, respectively. Since the 2f spectrum has high wavelength resolution, each 2f spectrum is separated. Thereby, the interference gas component which exists in a gas cell can be specified by acquiring the peak position of each spectrum.

  Further, since the spectrum intensity of the 2f spectrum when the wavelength amplitude is large is increased, a 2f spectrum with good S (signal) / N (noise) can be obtained. The obtained 2f spectrum is characterized by performing waveform separation by fitting using information on interference gas components specified when the wavelength swing is small. The details will be described below.

  FIG. 1 is a configuration diagram of a laser gas analyzer according to the first embodiment. A laser gas analyzer shown in FIG. 1 includes a wavelength tunable semiconductor laser 1, a laser driver (LD driver) 2, a gas cell 3, a photodiode 4, a signal amplification circuit 5, a waveform processing unit 6, a concentration calculation unit 7, and a gas temperature monitor. 8. A pressure monitor 9 and a database 10 are provided.

  The tunable semiconductor laser 1 oscillates in the absorption wavelength band of the measurement target gas and outputs laser light, and is a DFB-LD, DFB-QCL (quantum cascade laser), or the like. The measurement target gas is NH3, NO, NO2, SO2, HCL, H2O, CO, CO2, O2, or the like.

  The laser driver (LD driver) 2 generates a drive signal for driving the wavelength tunable semiconductor laser 1, outputs the drive signal to the wavelength tunable semiconductor laser 1, and generates a trigger signal synchronized with the frequency of the modulation signal. Output to the waveform processing unit 6.

  The gas cell 3 is filled with the measurement target gas, and the laser light from the wavelength tunable semiconductor laser 1 passes therethrough. The photodiode 4 receives the laser beam that has passed through the gas cell 3 and converts the received light signal into a current.

  The signal amplifying circuit 5 amplifies the light-receiving signal that has undergone current conversion by the photodiode 4 and then performs current-voltage conversion, and further digitally converts the voltage-converted signal. The gas temperature monitor 8 measures the temperature in the gas cell 3. The pressure monitor 9 monitors the pressure in the gas cell 3. The database 10 stores position information of the absorption spectrum of the interference gas.

  The waveform processing unit 6 generates a second harmonic component (2f spectrum) by synchronously detecting the output signal from the signal amplifying circuit 5 with a second harmonic of the frequency of the modulation signal, and a gas temperature monitor 8 and a pressure monitor 9. The 2f spectrum is waveform-separated by referring to the monitor value and the position information of the absorption spectrum of the interference gas in the database 10.

  The concentration calculation unit 7 measures the intensity of each 2f spectrum separated by the waveform processing unit 6 and converts the intensity of each 2f spectrum into a gas concentration using a calibration curve prepared in advance with a standard gas.

  The drive signal of the LD driver 2 is a signal obtained by superimposing the modulation signal (high frequency) on the sweep signal, and the wavelength amplitude of the modulation signal (amplitude from the minimum value to the maximum value) in synchronization with the cycle of the sweep signal. Consists of a signal that is switched in two stages, large and small.

  FIG. 2 is a diagram illustrating an example of a laser drive signal of the laser gas analyzer according to the first embodiment. As shown in FIG. 2A, the laser drive signal switches the amplitude range of the modulation signal between two levels, large and small, for each cycle of the sweep signal. Alternatively, as shown in FIG. 2 (b), the laser drive signal may be switched in two steps of a large and small wavelength swing of the modulation signal every certain period (X cycle). Alternatively, the laser drive signal may be operated with a modulation signal having a small wavelength amplitude only at the time of activation, and may be switched to a modulation signal having a large wavelength amplitude after a certain period.

  FIG. 3 is a flowchart illustrating the operation of the waveform processing unit of the laser gas analyzer according to the first embodiment. FIG. 4 is a waveform diagram illustrating a waveform processing flow of the laser gas analyzer according to the first embodiment. Steps S1 to S5 in FIG. 3 correspond to FIGS. 4A to 4E, respectively.

  The operation of the laser gas analyzer according to the first embodiment will be described with reference to FIGS. Here, detailed processing of the LD driver 2 and the waveform processing unit 6 which are features of the first embodiment will be described.

  First, the LD driver 2 drives the wavelength tunable semiconductor laser 1 with a laser drive signal in which the wavelength amplitude of the modulation signal is switched between two levels of large and small for each period of the sweep signal as shown in FIG. For this reason, the waveform processing unit 6 takes in the output signal when the wavelength swing of the modulation signal is large and the output signal when the wavelength swing of the modulation signal is small from the signal amplification circuit 5.

  Next, detailed processing of the waveform processing unit 6 will be described. First, a second-order harmonic component (2f spectrum) is generated by synchronously detecting the output signal from the signal amplification circuit 5 with a second harmonic of the frequency of the modulation signal (step S1 in FIG. 3). At this time, as shown in FIG. 4A, a 2f spectrum generated with a small wavelength swing of the modulation signal and a 2f spectrum generated with a large wavelength swing of the modulation signal are obtained.

  Further, the 2f spectrum generated at each wavelength amplitude of the modulation signal is extracted (step S2 in FIG. 3, FIG. 4B). Further, a plurality of 2f spectra extracted at each wavelength amplitude of the modulation signal are added and averaged (step S3 in FIG. 3, FIG. 4C).

  Further, the gas component is specified using the 2f spectrum when the wavelength swing of the modulation signal is small (step S4 in FIG. 3, FIG. 4D). The process of step S4 corresponds to the gas specifying unit of the present invention.

  Finally, using the identified gas component information, the gas temperature information from the gas temperature monitor 8, and the pressure information from the pressure monitor 9, the waveform is separated in the 2f spectrum when the wavelength swing of the modulation signal is large ( Step S5 in FIG. 3, FIG. 4 (e)). The processing in step S5 corresponds to the waveform separation unit of the present invention.

  Since the present applicant verified the effect of the present embodiment by simulation, this will be described below.

FIG. 5 is a diagram illustrating the sample gas used for the simulation of the laser gas analyzer according to the first embodiment. As shown in FIG. 5, the measurement target gas in the gas cell was NH 3, and the absorption spectrum used was 4837.33 cm ⁻¹ . A case where CO 2 and H 2 O exist in the interference gas existing in the gas cell was designated as sample A, and a case where only H 2 O was present in the interference gas present in the gas cell was designated as sample B. The information of each absorption spectrum used hitran data. The pressure was 100 kPa and the gas temperature was 600K.

FIG. 6 is a diagram illustrating the relationship between the 2f spectrum and the modulation signal of the laser gas analyzer according to the first embodiment. M in FIG. 6 is a modulation index. This modulation index m is given by the following equation as the ratio of the full width at half maximum Δν _c (0.0512 cm ⁻¹ ) of the absorption spectrum of NH 3 that is the measurement target gas and the wavelength amplitude of the modulation signal.

m = a / (Δν _c / 2)
Here, a is the wavelength amplitude of the modulation signal, and Δν _c is the full width at half maximum of the absorption spectrum. From FIG. 6, the intensity of the 2f spectrum increases near m = 2.2 (S / N increases). I understand that. In addition, the spectrum width of the 2f spectrum becomes narrower as the modulation index m becomes smaller. That is, it can be seen that the wavelength resolution is improved.

Accordingly, the setting conditions for each wavelength amplitude are set as follows with respect to the full width at half maximum Δν _c (0.0512 cm ⁻¹ ) of the absorption spectrum of NH 3 that is the measurement target gas.

This time, m = 2.2 was adopted as a value that can achieve S / N satisfying the device specifications. Therefore, from m = 2.2 = Amax / (Δν _c / 2), the set value Amax when the wavelength swing of the modulation signal is large is 0.1126 cm ⁻¹ .

Further, this time, m = 0.6 was adopted as a value that can achieve the wavelength resolution that enables the gas component existing in the gas cell to be specified. From m = 0.6 = Amin / (Δν _c / 2), the set value Amin when the wavelength amplitude of the modulation signal is small is 0.0307 cm ⁻¹ .

(About sample A)
FIG. 7 is a diagram showing a 2f spectrum of sample A acquired with a small wavelength swing of the modulation signal of the laser type gas analyzer of Example 1. FIG.

From FIG. 7, in the range of 4837 cm ^{−1 to} 4847.6 cm ⁻¹ , the absorption spectra exist at (1) 4837.25 cm ⁻¹ , (2) 4837.33 cm ⁻¹ , and (3) 4837.5 cm ⁻¹ . I understand that. Therefore, it can be seen that the interference gas components present in the gas cell are H2O and CO2.

FIG. 8 is a diagram showing a 2f spectrum of Sample A acquired with a large wavelength swing of the modulation signal of the laser gas analyzer of Example 1. FIG. With respect to the 2f spectrum shown in FIG. 8, NH 3 (487.33 cm ⁻¹ ), interference gas components H 2 O (4837.29 cm ⁻¹ 4837.50 cm ⁻¹ ), and CO 2 (4837.24 cm ⁻¹ ) at four points. Perform waveform separation by fitting.

The absorption spectrum of each gas is expressed as a Lorentz profile at 100 kPa. Thereby, the 2f spectrum function of each gas used for fitting can be represented by Formula (1) (described in Non-Patent Document 1).

Here, 2fsignal (v, vo, X) represents the shape of the 2f spectrum. If the 2f spectrum obtained by the equation (1) is 2fsignal ^* , the regression equation used for the fitting can be expressed by the equation (2).

2fsignal ^* is a combination of the gas components in FIG. 8, and can be expressed as a sum of the sum of the gas components as shown in the equation (2) and the error ε.

  Therefore, by deriving aXNH3, bXH2O, b'XH2O, cXCO2 that minimizes the error ε in the equation (2), the waveform of each gas component can be separated.

Specifically, the setting value was HWHM / π * 0.7 in aXNH3 = NH3 (4837.33 cm ⁻¹ ). HWHM is a half value (Δν _c / 2) of the full width at half maximum. HWHM / π * 0.175 in bXH 2 O = H 2 O (4837.29 cm ⁻¹ ), HWHM / π * 0.175 in b′XH 2 O = H 2 O (4837.50 cm ⁻¹ ), and cXCO 2 = CO 2 (4837.24 cm ^{− It was set to} HWHM / π * 3 in ¹ ).

  By changing the coefficients 0.7, 0.175, 0.175, and 3 in each of aXNH3, bXH2O, b′XH2O, and cXCO2, the error ε is minimized, so that aXNH3, bXH2O, b′XH2O, and cXCO2 are Can be derived.

  FIG. 9 is a diagram illustrating a fitting example of the sample A of the laser gas analyzer according to the first embodiment. FIG. 9 shows the acquired 2f spectrum, NH 3, H 2 OA, H 2 OB, CO 2, and the added 2 f spectrum.

  With the fitting of FIG. 9, the spectrum of each gas can be separated, and the concentration of each gas can be calculated by the concentration calculation unit 7 based on the separated spectrum.

(About sample B)
FIG. 10 is a diagram showing a 2f spectrum of Sample B acquired with a small wavelength swing of the modulation signal of the laser gas analyzer of Example 1. FIG. From Figure ^10, in a range of 4837cm ^-1 ~4837.6cm -1, absorption spectrum, ⁽²⁾ 4837.3cm -1, it is found to be present in (3) 4837.5cm ^-1. Therefore, it can be seen that the interference gas component present in the gas cell is H2O.

FIG. 11 is a diagram illustrating a 2f spectrum of Sample B acquired with a large wavelength swing of the modulation signal of the laser gas analyzer of Example 1. FIG. The 2f spectrum shown in FIG. 11 is subjected to waveform separation by fitting at three points of NH 3 (4837.33 cm ⁻¹ ) and interference gas component H 2 O (4837.29 cm ⁻¹ 4837.50 cm ⁻¹ ).

Similarly to the processing of sample A, if the 2f spectrum obtained is 2fsignal ^* , the regression equation used for fitting can be expressed by equation (3).

2fsignal ^* can be represented by the sum of the total sum of the gas components as shown in Expression (3) and the error ε.

  Therefore, by deriving aXNH3, bXH2O, b'XH2O that minimizes the error ε in the equation (3), the waveform of each gas component can be separated.

Specifically, the setting value was HWHM / π * 0.7 in aXNH3 = NH3 (4837.33 cm ⁻¹ ). HWHM is a half value (Δν _c / 2) of the full width at half maximum. HWHM / π * 0.175 in bXH 2 O = H 2 O (4837.29 cm ⁻¹ ) and HWHM / π * 0.175 in b′XH 2 O = H 2 O (4837.50 cm ⁻¹ ).

  By changing the coefficients 0.7, 0.175, and 0.175 in each of aXNH3, bXH2O, and b′XH2O, aXNH3, bXH2O, and b′XH2O can be derived by minimizing the error ε.

  FIG. 12 is a diagram illustrating a fitting example of the sample B of the laser gas analyzer according to the first embodiment. In FIG. 12, the acquired 2f spectrum, NH3, H2OA, H2OB, and the added 2f spectrum are shown.

  With the fitting of FIG. 12, the spectrum of each gas can be separated, and the concentration of each gas can be calculated by the concentration calculation unit 7 based on the separated spectrum.

  As described above, according to the laser gas analyzer of the first embodiment, the LD driver 2 is a signal in which the modulation signal is superimposed on the sweep signal as the drive signal, and the wavelength of the modulation signal is synchronized with the cycle of the sweep signal. A signal in which the amplitude is switched between large and small is generated, and a gas identifying unit in the waveform processing unit 6 identifies a gas component present in the gas cell based on the harmonic component of the output signal, and the waveform processing unit 6 The waveform separation unit performs waveform separation into each gas component in the harmonic component of the output signal using the gas component information specified by the gas specifying unit.

  That is, the LD driver 2 switches the wavelength amplitude of the modulation signal between large and small, the waveform processing unit 6 acquires 2f spectra of each wavelength amplitude, and the 2f spectrum when the wavelength amplitude is small is the wavelength resolution. Is high, each 2f spectrum is separated. Thereby, the interference gas component which exists in a gas cell can be specified by acquiring the peak position of each spectrum.

  Further, since the spectrum intensity of the 2f spectrum when the wavelength fluctuation width is large increases, a 2f spectrum with good S / N can be obtained. The obtained 2f spectrum is subjected to waveform separation by fitting using information on the interference gas component specified when the wavelength amplitude is small. Therefore, since fitting is performed only with the measurement target gas and the interference gas component existing in the gas cell, the fitting processing time can be further shortened.

  The laser type gas analyzer according to the present invention can be used for a gas analyzer.

1. Wavelength tunable semiconductor laser, 2. LD driver, 3. Gas cell, 4. Photodiode, 5. Signal amplification circuit, 6. Waveform processing section, 7. Concentration calculation section, 8. Gas temperature monitor, 9. Pressure monitor, 10 Database.

Claims (3)

A wavelength tunable semiconductor laser that oscillates in the absorption wavelength band of the measurement target gas and outputs a laser beam, a laser drive unit that generates a drive signal for driving the wavelength tunable semiconductor laser, and an output light of the wavelength tunable semiconductor laser as a gas cell A light detection unit that receives light via the light detection unit, a waveform processing unit that performs waveform processing by extracting a harmonic component of the output signal of the light detection unit by synchronously detecting the output of the light detection unit with a modulation signal, and In a laser type gas analyzer having a concentration calculation unit that calculates a gas concentration based on a waveform obtained by a waveform processing unit,
The laser driving unit generates, as the driving signal, a signal in which a modulation signal is superimposed on a sweep signal and a signal in which the wavelength swing of the modulation signal is switched between large and small in synchronization with the cycle of the sweep signal,
The waveform processing unit, based on a harmonic component of the output signal, a gas specifying unit for specifying a gas component present in the gas cell;
Using the gas component information identified by the gas identifying unit, a waveform separating unit that separates the waveform into each gas component in the harmonic component of the output signal;
A laser type gas analyzer characterized by comprising:   2. The laser type gas analyzer according to claim 1, wherein the waveform processing unit separates the waveform into each gas component based on gas temperature information of the measurement target gas and pressure information of the measurement target gas. The gas specifying unit specifies a gas component present in the gas cell based on a harmonic component of the output signal when the wavelength swing of the modulation signal is small,
3. The laser type according to claim 1, wherein the waveform separation unit separates the waveform into each gas component in a harmonic component of the output signal when the wavelength swing of the modulation signal is large. Gas analyzer.

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