JP5045480B2 - Gas concentration measuring device and gas concentration measuring method - Google Patents

Description

  The present invention relates to a gas concentration measuring apparatus and a gas concentration measuring method, and is particularly suitable for application to a method of measuring a gas concentration using a frequency-modulated laser beam.

It is known that each gaseous gas molecule has its own light absorption spectrum, and the attenuation at the center frequency of the absorption line of the gas molecule is proportional to the gas concentration. For this reason, the gas concentration can be estimated by irradiating the gas with semiconductor laser light having an oscillation frequency that matches the center frequency of the absorption line of gas molecules, and measuring the attenuation of the laser light at that time ( Patent Document 1).
The two-wavelength difference method and the frequency modulation method are developed from this principle. In the two-wavelength difference method, since the oscillation frequency of the semiconductor laser is a signal on the order of THz, complicated signal processing cannot be performed. On the other hand, the frequency modulation method has an advantage that signal processing can be performed in a baseband region of several kHz.

  In this frequency modulation method, the output of the semiconductor laser is frequency-modulated at the center frequency fc and the modulation frequency fm, and irradiated to the measurement target gas. Note that frequency modulation is to make the drive current supplied to the semiconductor laser sinusoidal. In the semiconductor laser, the wavelength of the emitted light varies depending on the temperature and current. Therefore, the wavelength of the emitted light from the semiconductor laser can be modulated by making the drive current supplied to the semiconductor laser sinusoidal. Here, since the absorption line of the gas to be measured has a substantially quadratic function with respect to the modulation frequency, this absorption line serves as a discriminator and has a frequency component (double wave) that is twice the modulation frequency fm. Component) is obtained. The higher the concentration of the gas to be measured, the greater the amount of absorption of the laser beam. Therefore, the intensity of the second harmonic component is also increased, and the concentration of the gas to be measured is measured by detecting the second harmonic component. Can do. In the following description, the modulation frequency fm is also called 1f, the second harmonic frequency of the modulation frequency fm is also called 2f, and the second harmonic component is also called a 2f component.

FIG. 11 is a block diagram showing a schematic configuration on the receiving side of a conventional gas concentration measuring apparatus.
In FIG. 11, on the receiving side of the gas concentration measuring apparatus, a photodiode 111 that detects laser light emitted from a semiconductor laser, an IV conversion circuit 112 that converts a current output from the photodiode 111 into a voltage, An amplifier 113 that amplifies the voltage output from the −V conversion circuit 112, a bandpass filter 114 that extracts a second harmonic component of the modulation frequency from the signal output through the amplifier 113, and a second harmonic frequency of the modulation frequency fm. By using the 2nd frequency signal generator 115 and 2f reference signal that generate the 2f reference signal, the detector 116 that extracts the 2f component from the output of the bandpass filter 114, and an unnecessary high frequency from the output of the detector 116 A low-pass filter 117 for removing components and an amplifier 118 for amplifying the voltage output from the low-pass filter 117 are provided. It is.

And, for example, when measuring the concentration of NH ₃ gas, when the frequency-modulated laser light passes through NH ₃ gas, after being attenuated corresponding to the amount of absorption by NH ₃ gas, it enters the photodiode 111, Laser light is detected by the photodiode 111. The current detected by the photodiode 111 is converted into a voltage by the IV conversion circuit 112, amplified by the amplifier 113, and then the second harmonic component of the modulation frequency is extracted by the band pass filter 114. , And input to the detector 116. Then, the 2f reference signal generated by the double frequency signal generation unit 115 and the output from the bandpass filter 114 are combined by the detector 116, so that the 2f component is extracted from the output from the bandpass filter 114. The The 2f component extracted by the detector 116 is amplified by the amplifier 118 after an unnecessary high frequency component is removed by the low-pass filter 117, and is output as a 2f component detection signal.
On the other hand, as a modulation method on the transmission side, it is difficult to control the modulation center wavelength so that it always matches the absorption wavelength. Therefore, while modulating at a frequency of 1f with a predetermined amplitude width, the center wavelength of the modulation width In some cases, a wavelength scanning method is employed in which sweeping is performed by gradually changing.

FIG. 12A shows a waveform of a driving current for driving a conventional semiconductor laser, and FIG. 12B shows the emission of laser light when the semiconductor laser is driven with the driving current of FIG. It is a figure which shows a wavelength. Since the light emission amount of the semiconductor laser is substantially proportional to the drive current, the waveform of the light emission amount is the same as the waveform of FIG.
In FIG. 12, in the wavelength scanning method, for example, the wavelength can be scanned in a range of about 0.5 nm. Here, when there is an absorption spectrum of another type of gas B in the immediate vicinity of an absorption spectrum of a certain gas A, a wavelength scan is performed using one semiconductor laser, so that two types of gases A and B can be detected. The concentration can be detected.

Here, when the gas A is in a low concentration state and the gas B is in a high concentration state, a large difference occurs in the signal level detected by the photodiode 111 in FIG. It becomes impossible to accurately detect the concentration of.
For example, the concentration of hydrogen chloride HCl having an absorption spectrum at 1747.2 nm and the concentration of water vapor H ₂ O having an absorption spectrum at 1747.1 nm are detected using one semiconductor laser. Since hydrogen chloride is very easily adsorbed by moisture and the apparent concentration changes depending on the amount of moisture, detecting the concentration of water vapor together with the concentration of hydrogen chloride is useful for accurately estimating the concentration of hydrogen chloride.

In this case, hydrogen chloride is usually measured at a low concentration of 10 ppm to 500 ppm in the full scale range, and water is usually measured at a high concentration of several vol% to several tens vol%. In such a state, although the amount of light absorption in each absorption spectrum is 10 times or more larger in hydrogen chloride, the actual amount of light absorption is several tens to 100 times larger in water vapor.
As a result, if the concentration of water vapor with a large amount of light absorption is detected, the signal-to-noise ratio (SNR) at the time of detecting hydrogen chloride with a small amount of light absorption deteriorates, and the detection accuracy of the concentration of hydrogen chloride with a small amount of light absorption is improved. If it is increased, the detection signal of water vapor having a large absorbance is saturated.

For this reason, in order to be able to detect the concentrations of both the gas A in the low concentration state and the gas B in the high concentration state with high accuracy, both the low concentration detection circuit and the high concentration detection circuit are provided. In some cases, a method of optimizing the circuit gain may be adopted.
Alternatively, in the wavelength scan of FIG. 12B, a method of switching the gain of the detection circuit at the boundary timing between the time t1 when the gas A is absorbed and the time t2 when the gas B is absorbed may be used. .
Japanese Patent Laid-Open No. 10-142148

However, in the method of providing both the low concentration detection circuit and the high concentration detection circuit, there is a problem that the circuit configuration becomes large and the cost is increased.
In the method of switching the gain of the detection circuit, after switching the gain of the detection circuit, the characteristics of the filter circuit and the like greatly vary due to the transient response, and it takes time until the operation of these circuits is stabilized. For this reason, it is necessary to increase the interval between the time t1 at which the gas A is absorbed and the time t2 at which the gas B is absorbed, and it is necessary to lengthen the scan time. was there.

As a method for accurately detecting the concentrations of both the gas A in the low concentration state and the gas B in the high concentration state, the gas A in the low concentration state has a frequency twice the modulation frequency 1f. A method of detecting the 2f component possessed by the synchronous detection method and detecting the gas B in a high concentration state from the envelope of the 1f modulation waveform is also conceivable.
However, in this method, a peak hold circuit is used to detect the envelope. However, if the response of the peak hold is improved, the fluctuation of the peak hold waveform from the peak of 1f to the next peak increases, and the peak hold waveform If the peak hold time constant is increased in order to stabilize, the change in the envelope cannot be followed.
Accordingly, an object of the present invention is to use one semiconductor laser to stably detect the concentrations of high-concentration gas and low-concentration gas while suppressing an increase in circuit scale, and to suppress deterioration in responsiveness. It is to provide a gas concentration measuring apparatus and a gas concentration measuring method that can be performed.

  In order to solve the above-described problem, according to the gas concentration measuring apparatus of claim 1, a wavelength scanning unit that scans a wavelength of a laser beam emitted from a semiconductor laser, and a laser beam emitted from the semiconductor laser A frequency modulation unit that performs frequency modulation, a timing switching control unit that switches timing of the frequency modulation so that the frequency modulation is performed within a part of a period during which scanning of the wavelength of the laser beam is performed, and the frequency modulation A light detecting unit that detects a laser beam that has been scanned without a wavelength and a laser beam that has undergone a wavelength scan with frequency modulation, and a modulation of a light reception signal detected by the light detecting unit A double frequency signal generator for generating a double frequency reference signal having a frequency twice the frequency, and the double frequency signal generator generated by the double frequency signal generator By multiplying the harmonic frequency reference signal with the received light signal that has been subjected to wavelength scanning with frequency modulation, a detection unit that extracts a second harmonic component of the received light signal and the detection unit extracts the second harmonic component. Based on the second harmonic component of the received light signal, the concentration of the gas having the smaller absorption amount of at least two kinds of gases is calculated, and the received light is scanned with the wavelength without the frequency modulation. And a gas concentration calculating unit that calculates the concentration of the gas having the larger light absorption amount based on the signal.

  According to the gas concentration measuring method of claim 2, the step of scanning the wavelength of the laser light emitted from the semiconductor laser, and the scanning of the wavelength of the laser light within a part of the period during which the scanning is performed. Performing frequency modulation of laser light, detecting laser light that has been scanned for wavelength without frequency modulation, and laser light that has been scanned for wavelength with frequency modulation, and the detection Generating a second frequency reference signal having a frequency twice the modulation frequency of the received light signal, and multiplying the second frequency reference signal by the received light signal that has been subjected to wavelength scanning with the frequency modulation. A step of extracting a second harmonic component of the received light signal by combining, and at least based on the extracted second harmonic component of the received light signal Based on the step of calculating the concentration of the gas having the smaller amount of light absorption among the types of gases, and the received light signal that has been scanned for the wavelength without the frequency modulation, of the at least two types of gases And a step of calculating the concentration of the gas having the larger light absorption amount.

  As described above, according to the present invention, the gas having a smaller amount of light absorption can detect the concentration from the second harmonic component of the received light signal, while the gas having a larger amount of light absorption can be detected. The concentration can be detected without extracting the second harmonic component of the received light signal. This eliminates the need for a circuit such as a detector to detect the concentration of the gas with the higher absorbance, and optimizes the circuit gain so that the gas with the lower absorbance can be detected well. However, it is possible to prevent the detection signal of the gas having the larger light absorption from being saturated. As a result, even when one semiconductor laser is used, it is possible to stably detect the concentrations of the high-concentration gas and the low-concentration gas while suppressing an increase in circuit scale, and to suppress deterioration in responsiveness. It becomes possible.

Hereinafter, a gas concentration measuring apparatus according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view showing a schematic configuration of a gas concentration measuring apparatus according to an embodiment of the present invention.
In FIG. 1, on the transmission side of the gas concentration measuring apparatus, a transmitter substrate that performs frequency modulation within a part of the period during which scanning is performed while scanning the emission wavelength of the laser light emitted from the laser unit 17. 14, a collimating lens 16 for converting laser light emitted from the laser unit 17 into a parallel beam and a laser unit 17 on which a laser element is mounted are provided. A semiconductor laser can be used as the laser element, and the laser unit 17 can be equipped with a thermistor for detecting the temperature of the laser element and a Peltier element for adjusting the temperature of the laser element.

  Further, on the receiving side of the gas concentration measuring apparatus, a condensing lens 20 that condenses laser light that has passed through the measurement target gas, a light detection unit 21 that detects laser light that has passed through the measurement target gas, and the measurement target gas are transmitted. The receiver substrate 22 calculates the concentration of the low-concentration gas from the second harmonic frequency of the laser beam and calculates the concentration of the high-concentration gas on the basis of the light-receiving signal that has been scanned for the wavelength without frequency modulation. Is provided. For example, a photodiode can be used as the light detection unit 21.

  Here, the transmission unit substrate 14, the collimating lens 16, and the laser unit 17 are accommodated in the housing 18, and the condenser lens 20, the light detection unit 21, and the reception unit substrate 22 are accommodated in the housing 19. Then, flanges 12a and 12b are attached to partition walls 11a and 11b such as pipes through which a measurement target gas flows such as a flue by a method such as welding. The housing 18 in which the transmission unit substrate 14, the collimating lens 16 and the laser unit 17 are housed is attached to the flange 12a so as to be separated from the inside of the pipe by the wedge window 15a, and the condensing lens 20, the light detection unit. The housing 19 in which the housing 21 and the receiver board 22 are accommodated is attached to the flange 12b so as to be separated from the inside of the pipe by the wedge window 15b.

  Then, while the emission wavelength of the laser light emitted from the laser unit 17 is scanned, the laser light is frequency-modulated at the center frequency fc and the modulation frequency fm within a part of the period during which the emission wavelength is scanned. Then, laser light whose frequency modulation is intermittently performed while the emission wavelength is scanned is emitted from the laser unit 17 and converted into a parallel beam by the collimator lens 16, and then the partition walls 11a and 11b through the wedge window 15a. Two or more kinds of measurement target gases having different types are permeated. And the laser beam which permeate | transmitted 2 or more types of measuring object gas from which a kind differs receives absorption of the wavelength respectively corresponding to the absorption line of the gas molecule of measuring object gas, Then, the condensing lens 20 is passed through the wedge window 15b. And is condensed on the light detection unit 21 by the condenser lens 20. When the laser light is incident on the light detection unit 21, the light detection unit 21 converts the laser light into an electrical signal, and the electrical signal is sent to the reception unit substrate 22. Then, when the electrical signal converted by the light detection unit 21 is sent to the reception unit substrate 22, by multiplying the light reception signal by a second harmonic frequency reference signal having a second harmonic frequency of the modulation frequency, Based on the received light signal obtained by extracting the second harmonic component, calculating the concentration of the low concentration gas based on the extracted second harmonic component of the received light signal, and scanning the wavelength without frequency modulation. Thus, the concentration of the high concentration gas can be calculated.

FIG. 2 is a diagram for explaining the principle of measuring the gas concentration by the frequency modulation method according to one embodiment of the present invention.
In FIG. 2, it is assumed that the output of the semiconductor laser is frequency-modulated with a center frequency fc and a modulation frequency fm, and the measurement target gas is irradiated. Here, since the absorption line of the gas to be measured has a substantially quadratic function with respect to the modulation frequency, this absorption line serves as a discriminator, and the light detection unit 21 has a frequency twice as high as the modulation frequency fm. A component (second harmonic component) is obtained. Since the modulation frequency fm may be any frequency, for example, when the modulation frequency fm is selected to be several kHz, it is possible to perform advanced signal processing using a digital signal processing device (DSP) or a general-purpose processor. .

  In order to cancel the influence of the attenuation amount of the laser beam due to the distance between the semiconductor laser and the light detection unit 21 by the frequency modulation method, the output of the semiconductor laser is modulated at the same time as the amplitude at the modulation frequency fm. Modulation may be performed, and amplitude modulation can also be performed by applying frequency modulation to the output of the semiconductor laser. Then, the fundamental wave component by amplitude modulation can be estimated by performing envelope detection in the light detection unit 21, and the ratio of the amplitude of the fundamental wave component and the amplitude of the second harmonic component is synchronized in phase, A value proportional to the concentration of the measurement target gas can be obtained without depending on the distance between the semiconductor laser and the light detection unit 21.

FIG. 3 is a diagram showing the relationship between the drive current and the emission wavelength of the semiconductor laser according to one embodiment of the present invention.
In FIG. 3, the emission wavelength of the semiconductor laser becomes longer as the drive current increases. For this reason, the emission wavelength of the semiconductor laser can be adjusted by controlling the driving current of the semiconductor laser.
FIG. 4 is a diagram showing the relationship between the temperature and the emission wavelength of the semiconductor laser according to one embodiment of the present invention.
In FIG. 4, the emission wavelength of the semiconductor laser becomes longer as the temperature increases. For this reason, the emission wavelength of the semiconductor laser can be adjusted by controlling the temperature of the semiconductor laser.

FIG. 5 is a block diagram showing a schematic configuration on the transmission side of the gas concentration measurement apparatus according to one embodiment of the present invention.
In FIG. 5, a semiconductor laser 36 is provided on the transmission side of the gas concentration measuring device, and the semiconductor laser 36 includes a thermistor 37 that detects the temperature of the semiconductor laser 36 and a Peltier element 38 that adjusts the temperature of the semiconductor laser 36. It is installed.
Further, by driving the Peltier element 38 based on the temperature detected by the thermistor 37, a temperature control unit 35 for controlling the temperature of the semiconductor laser 36, and a wavelength for scanning the wavelength of the laser light emitted from the semiconductor laser 36. A wavelength scanning drive signal generator 31 for generating the scanning signal S1, a high frequency modulation signal generating unit 32 for generating a high frequency signal S2 for frequency modulating the laser light emitted from the semiconductor laser 36, and scanning of the wavelength of the laser light are performed. A switching control unit 40 for switching the frequency modulation timing so that frequency modulation is performed within a certain period, a switch 39 for turning on / off the output of the high frequency modulation signal generating unit 32, the wavelength scanning signal S1 and the high frequency signal S2. The synthesizer 33 synthesizes the half-wave signal based on the signal obtained by intermittently synthesizing the high frequency signal S2 with the wavelength scanning signal S1. Current control unit 34 for controlling a current for driving the body laser 36 is provided. Note that a triangular wave generation circuit can be used as the wavelength scanning drive signal generator 31. From the viewpoint of stability, a circuit combining a digital counter and a DA converter may be used.

FIG. 6 is a diagram illustrating an example of an absorption spectrum of NH ₃ gas.
In FIG. 6, NH ₃ gas has an absorption wavelength band peak near 1512 nm. Therefore, the wavelength scanning drive signal generator 31 absorbs the laser light in the NH ₃ gas by changing the drive current of the semiconductor laser 36 so that the emission wavelength of the frequency-modulated laser light scans around 1512 nm. And the concentration of NH ₃ gas can be measured. In addition, the temperature control unit 35 can set the temperature of the semiconductor laser 36 in advance so that the concentration of the NH ₃ gas is detected at the central portion during wavelength scanning by driving the Peltier element 38.

Here, when the NH ₃ gas is in a high concentration state, the switching control unit 40 turns off the output of the high frequency modulation signal generation unit 32, and the semiconductor laser 36 based on a signal in which the high frequency signal S2 is not synthesized with the wavelength scanning signal S1. Can be driven. When the NH ₃ gas is in a low concentration state, the switching control unit 40 turns on the output of the high frequency modulation signal generation unit 32, and the semiconductor laser 36 is based on the signal obtained by synthesizing the high frequency signal S2 with the wavelength scanning signal S1. Can be driven.

FIG. 7 is a block diagram showing a schematic configuration on the receiving side of the gas concentration measuring apparatus according to one embodiment of the present invention.
In FIG. 7, on the receiving side of the gas concentration measuring device, a photodiode 41 that detects laser light emitted from a semiconductor laser, an IV conversion circuit 42 that converts a current output from the photodiode 41 into a voltage, An amplifier 43 that amplifies the voltage output from the −V conversion circuit 42, a bandpass filter 44 that extracts a second harmonic component of the modulation frequency from the signal output through the amplifier 43, and a second harmonic frequency of the modulation frequency fm. The second frequency signal generator 45 that generates the 2f reference signal having the 2f reference signal, the detector 46 for extracting the 2f component from the output of the bandpass filter 44, and the unnecessary high frequency from the output of the detector 46. A low-pass filter 47 that removes components, an amplifier 48 that amplifies the voltage output from the low-pass filter 47, and a received light amount output from the amplifier 43 No. and AD converter 49 and control processor 50 which performs a process for calculating the concentration of the gas for converting the 2f component detection signal output from the amplifier 48 into a digital signal are provided.
Here, the control processor 50 calculates the concentration of the high concentration gas based on the received light amount signal output from the amplifier 43 and the concentration of the low concentration gas based on the 2f component detection signal output from the amplifier 48. Can be calculated.

FIG. 8 is a diagram illustrating a gas concentration measurement method using a wavelength scanning method according to an embodiment of the present invention. Here, the wavelength scanning signal S1 of FIG. 5 includes a constant signal 73 in which the intensity of the wavelength scanning signal S1 is kept constant, a wavelength sweep signal 72 in which the intensity of the wavelength scanning signal S1 increases linearly in a trapezoidal shape, It can be constituted by a zero signal 74 in which the intensity of the wavelength scanning signal S1 is zero. In the example of FIG. 8, HCl gas and H ₂ O gas are taken as an example as a plurality of measurement target gases.

When measuring the concentrations of the HCl gas and the H ₂ O gas, the temperature control unit 35 in FIG. 5 drives the Peltier element 38, thereby causing the HCl gas and the H ₂ O gas in the vicinity of the center of the wavelength scanning signal S 1. The temperature of the semiconductor laser 36 is set in advance so that the gas concentration is detected.
Then, the wavelength scanning drive signal generator 31 of FIG. 5 keeps the intensity of the wavelength scanning signal S1 constant to be equal to or higher than the threshold current of the semiconductor laser 35 in order to stabilize the light emission of the semiconductor laser 36, The constant signal 73 of the wavelength scanning signal S1 is output to the synthesizer 33, and the switching control unit 40 turns on the switch 39, so that the high frequency modulation signal generating unit 73 receives the high frequency signal S2 composed of a sine wave of about 10 kHz. To the synthesizer 33.

Then, the constant signal 73 of the wavelength scanning signal S1 and the high frequency signal S2 are combined by the combiner 33, and the current of the semiconductor laser 36 is based on the combined signal of the constant signal 73 of the wavelength scanning signal S1 and the high frequency signal S2. Is controlled, the laser light is frequency-modulated with a sine wave of about 10 kHz around the wavelength λ ₁ that is not absorbed by HCl gas and H ₂ O gas.

When the laser beam is frequency-modulated passes through the HCl gas and the H _{2 O} gas, and enters the photodiode 41 in FIG. 7 without being absorbed by the HCl gas and the H _{2 O} gas, the laser beam by the photodiode 41 Is detected. The current detected by the photodiode 41 is converted into a voltage by the IV conversion circuit 42, amplified by the amplifier 43, and then a second harmonic component of the modulation frequency is extracted by the band pass filter 44. , And input to the detector 46. Then, the 2f reference signal generated by the double frequency signal generation unit 45 and the output from the bandpass filter 44 are combined by the detector 46, whereby the 2f component is extracted from the output from the bandpass filter 44. The The 2f component extracted by the detector 46 is amplified by the amplifier 48 after unnecessary high-frequency components are removed by the low-pass filter 47, and is output to the AD converter 49 as a 2f component detection signal. .

Here, since the laser light frequency-modulated around the wavelength λ ₁ is not absorbed by the HCl gas and the H ₂ O gas, a frequency component twice the high-frequency signal S 2 is extracted from the output of the IV conversion circuit 42. The output from the detector 46 is constant.
Next, the wavelength scanning drive signal generator 31 in FIG. 5 linearly increases the intensity of the wavelength scanning signal S1 in a trapezoidal shape in order to scan the absorption wavelength band of H ₂ O gas. While outputting the wavelength sweep signal 72 of S1 to the synthesizer 33, the switching control unit 40 turns off the switch 39, so that the high frequency signal S2 composed of a sine wave of about 10 kHz generated by the high frequency modulation signal generation unit 32 is generated. The output to the synthesizer 33 is blocked. When measuring the concentrations of HCl gas and H ₂ O gas, the wavelength scanning drive signal generator 31 can generate the wavelength scanning signal S1 so that the line width of about 0.2 nm can be scanned.

The wavelength sweep signal 72 of the wavelength scanning signal S1 is output to the current controller 34 without being synthesized with the high frequency signal S2 by the synthesizer 33, and the semiconductor laser 36 is based on the wavelength sweep signal 72 of the wavelength scanning signal S1. Is controlled to emit laser light whose wavelength is scanned in the range from wavelength λ ₂ to wavelength λ ₃ so as to include the absorption wavelength band of H ₂ O gas.

When the laser beam wavelength was scanned passes through the H _{2 O} gas, after being attenuated corresponding to the absorption by the H 2 _O gas, and enters the photodiode 45 in FIG. 7, the laser by the photodiode 41 Light is detected. The current detected by the photodiode 41 is converted into a voltage by the IV conversion circuit 42, amplified by the amplifier 43, and then sent to the AD converter 49 as a received light amount signal to be converted into digital data. Is done. The received light amount signal converted into digital data is sent to the control processor 50, and the concentration of H ₂ O gas is calculated based on the received light amount signal converted into digital data.

Since the laser beam wavelength was scanned in the range of the wavelength lambda ₂ so as to include an absorption wavelength band of the H 2 _O gas to the wavelength lambda ₃ is receiving an attenuation corresponding to absorption by the H _{2 O} gas, I The output from the −V conversion circuit 42 has a value corresponding to the absorption waveform by the H ₂ O gas.
Next, in order to scan the absorption wavelength band of HCl gas, the wavelength scanning drive signal generator 31 in FIG. 5 further increases the intensity of the wavelength scanning signal S1 into a trapezoidal shape, thereby linearly scanning the wavelength scanning signal S1. Is output to the synthesizer 33, and the switching control unit 40 turns on the switch 39 so that the high frequency modulation signal generation unit 32 outputs the high frequency signal S2 to the synthesizer 33 from a sine wave of about 10 kHz. To do.

Then, the wavelength sweep signal 72 of the wavelength scanning signal S1 and the high frequency signal S2 are synthesized by the synthesizer 33, and the semiconductor laser 36 is based on the synthesized signal of the wavelength sweep signal 72 of the wavelength scanning signal S1 and the high frequency signal S2. Is controlled so that the laser light is frequency-modulated with a sine wave of about 10 kHz around the range from the wavelength λ ₃ to the wavelength λ ₄ so as to include the absorption wavelength band of HCl gas.

  When the frequency-modulated laser beam passes through the HCl gas, the laser beam is attenuated corresponding to the amount absorbed by the HCl gas, and then enters the photodiode 41 shown in FIG. 7. The laser beam is detected by the photodiode 41. . The current detected by the photodiode 41 is converted into a voltage by the IV conversion circuit 42, amplified by the amplifier 43, and then a second harmonic component of the modulation frequency is extracted by the band pass filter 44. , And input to the detector 46. Then, the 2f reference signal generated by the double frequency signal generation unit 45 and the output from the bandpass filter 44 are combined by the detector 46, whereby the 2f component is extracted from the output from the bandpass filter 44. The The 2f component extracted by the detector 46 is amplified by the amplifier 48 after unnecessary high frequency components are removed by the low-pass filter 47, and sent to the AD converter 49 as a 2f component detection signal. In this way, it is converted into digital data. Then, the 2f component detection signal converted into digital data is sent to the control processor 50, and the concentration of HCl gas is calculated based on the 2f component detection signal converted into digital data.

Here, since the laser light frequency-modulated around the range from the wavelength λ ₃ to the wavelength λ ₄ so as to include the absorption wavelength band of the HCl gas is attenuated corresponding to the amount of absorption by the HCl gas, the detector 46 The output of is a value corresponding to the absorption waveform by HCl gas.
As a result, it is possible to detect the concentration of the HCl gas having the smaller amount of light absorption from the double wave component of the light reception signal, while the H ₂ O gas having the larger amount of light absorption is twice the light reception signal. It is possible to detect the concentration without extracting the wave component. For this reason, a circuit such as a detector is not required to detect the concentration of the H ₂ O gas with the higher absorbance, and the circuit gain is optimized so that the HCl gas with the lower absorbance can be detected well. Even in this case, it is possible to prevent the detection signal of the H ₂ O gas having the larger light absorption from being saturated. As a result, even when one semiconductor laser 36 is used, the concentration of the high-concentration gas and the low-concentration gas can be stably detected and the deterioration of responsiveness can be suppressed while suppressing an increase in circuit scale. Is possible.

FIG. 9A is a diagram showing the waveform of the drive current (or light emission amount) of the semiconductor laser according to one embodiment of the present invention, and FIG. 9B is the drive current of FIG. It is a figure which shows the waveform of the light-receiving light quantity light-received with the photodiode when it drive | operated.
In FIG. 9A, the drive current of the semiconductor laser 36 in FIG. 5 has a waveform in which the high frequency signal S2 is intermittently superimposed on the wavelength sweep signal 72 of the wavelength scanning signal S1, and the waveform of the emitted light quantity of the semiconductor laser 36. This is the same as the waveform in FIG.
Then, when the laser beam having the light emission quantity in FIG. 9A passes through the gas flow path, as shown in FIG. 9B, the laser beam is attenuated corresponding to the light absorption amount of the specific wavelength. Then, the attenuated laser light is received by the photodiode 41 of FIG. 7, and the concentration of the high-concentration gas is calculated based on the received light amount signal of the region R1, and based on the 2f component detection signal of the region R2. The concentration of the low concentration gas can be calculated. −

FIG. 10 is an enlarged view showing the time axis of the RA portion in FIG.
In FIG. 10, the wavelengths at which no light absorption occurs are points A and B, the wavelengths at which light absorption occurs are point N, and the times at points A, B, and N are t _A , t _B , t _N , and point A, respectively. If the voltage values of the received light quantity signals at the wavelengths of _B , _N and _N are y _A , y _B , and y _N , respectively, the voltage value V _N corresponding to the amount of light absorption is obtained by the following equation (1). Can do.
_{(T N -t A) (y} A -y B) / (t B -t A) + y A -y N (1)
Since the voltage value V _N is proportional to the gas concentration, the gas concentration can be detected by obtaining the voltage value V _N from the RA portion in FIG. 9B.

  As described above, according to the embodiment of the present invention, the light amount waveform of the received light signal is measured by performing the wavelength sweep without performing the 1f modulation for the detection of the high concentration gas, and the wavelength sweep for the detection of the low concentration gas. 1f modulation is performed and 2f component is measured by synchronous detection, so that it is possible to prevent the processing circuit from becoming complicated, responsiveness slowing down, and waveform fluctuations from increasing. By simply adding a simple configuration, it is possible to stably detect the concentrations of the high-concentration gas and the low-concentration gas without using a plurality of semiconductor lasers.

It is sectional drawing which shows schematic structure of the gas concentration measuring apparatus which concerns on one Embodiment of this invention. It is a figure explaining the measurement principle of the gas concentration by the frequency modulation system which concerns on one Embodiment of this invention. It is a figure which shows the relationship between the drive current which concerns on one Embodiment of this invention, and the light emission wavelength of a semiconductor laser. It is a figure which shows the relationship between the temperature which concerns on one Embodiment of this invention, and the light emission wavelength of a semiconductor laser. It is a block diagram which shows schematic structure of the transmission side of the gas concentration measuring apparatus which concerns on one Embodiment of this invention. NH ₃ is a diagram showing an example of the absorption spectrum of the gas. It is a block diagram which shows schematic structure of the receiving side of the gas concentration measuring apparatus which concerns on one Embodiment of this invention. It is a figure which shows the measuring method of the gas concentration by the wavelength scanning system which concerns on one Embodiment of this invention. FIG. 9A is a diagram showing a waveform of the drive current of the semiconductor laser according to one embodiment of the present invention, and FIG. 9B is a photo when the semiconductor laser is driven with the drive current of FIG. It is a figure which shows the waveform of the received light quantity received by the diode. It is a figure which expands and shows the time-axis of the RA part of FIG.9 (b). It is a block diagram which shows schematic structure of the receiving side of the conventional gas concentration measuring apparatus. FIG. 12A shows a waveform of a driving current for driving a conventional semiconductor laser, and FIG. 12B shows the emission of laser light when the semiconductor laser is driven with the driving current of FIG. It is a figure which shows a wavelength.

Explanation of symbols

11a, 11b Bulkhead 12a, 12b Flange 14 Transmitter board 15a, 15b Wedge window 16 Collimate lens 17 Laser unit 18, 19 Housing 20 Condensing lens 21 Photodetector 22 Receiver board 31 Wavelength scanning drive signal generator 32 High frequency modulation signal Generation unit 33 Synthesizer 34 Current control unit 35 Temperature control unit 36 Semiconductor laser 37 Thermistor 38 Peltier element 39 Switch 40 Switching control unit 41 Photo diode 42 I / V conversion circuit 43, 48 Amplifier 44 Band pass filter 45 Double wave frequency signal Generator 46 Detector 47 Low-pass filter 49 AD converter 50 Control processor

Claims (2)

A wavelength scanning unit that scans the wavelength of laser light emitted from the semiconductor laser;
A frequency modulation unit for frequency modulating laser light emitted from the semiconductor laser;
A timing switching control unit that switches the timing of the frequency modulation so that the frequency modulation is performed within a period of time during which the wavelength of the laser beam is scanned;
A light detection unit that detects the laser light that has been scanned for wavelength without frequency modulation and the laser light that has been scanned for wavelength while accompanied by frequency modulation;
A second harmonic frequency signal generator for generating a second harmonic frequency reference signal having a frequency twice as high as the modulation frequency of the received light signal detected by the light detector;
By multiplying the second harmonic frequency reference signal generated by the second harmonic frequency signal generator with the received light signal that has been subjected to wavelength scanning with frequency modulation, the second harmonic component of the received light signal is obtained. A detector for extracting
Based on the second harmonic component of the received light signal extracted by the detection unit, the concentration of the gas having the smaller absorption amount of at least two kinds of gases is calculated, and the wavelength is not accompanied by the frequency modulation. A gas concentration measuring apparatus comprising: a gas concentration calculating unit that calculates a concentration of a gas having a larger amount of light absorption based on the received light signal. Scanning the wavelength of the laser light emitted from the semiconductor laser;
Performing frequency modulation of the laser light within a period of time during which scanning of the wavelength of the laser light is performed;
Detecting a laser beam that has been scanned with a wavelength without accompanying frequency modulation and a laser beam that has been scanned with a wavelength while involving frequency modulation; and
Generating a second frequency reference signal having a frequency twice the modulation frequency of the detected light reception signal;
Extracting a second harmonic component of the received light signal by multiplying the received light signal that has been scanned with a wavelength with the frequency modulation by the second harmonic frequency reference signal;
Calculating a concentration of a gas having a smaller amount of light absorption of at least two kinds of gases based on a second harmonic component of the extracted received light signal;
Calculating a concentration of a gas having a larger light absorption amount among the at least two kinds of gases based on a light reception signal that has been scanned for a wavelength without frequency modulation. Gas concentration measurement method.

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