JP4918865B2 - Laser wavelength control device, gas concentration measurement device, laser wavelength control method, and gas concentration measurement method - Google Patents

Description

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

  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.
Here, in the frequency modulation method, since the line width of the laser beam is smaller than the absorption line width of the gas, it is necessary to match the absorption wavelength of the gas with the emission wavelength of the semiconductor laser. As this method, there is a method using a reference gas cell in which the same component as the gas to be measured is enclosed (Patent Document 2).

FIG. 8 is a plan view showing a schematic configuration of a gas concentration measuring apparatus in a conventional frequency modulation method.
In FIG. 8, the light source unit accommodates a semiconductor laser module 121, a reference gas cell 122, and a photo detector 123, and a Peltier element 28 to which a cooling fin 27 is attached is arranged on the bottom surface of the case body 26 of the light source unit. It is installed. Here, the semiconductor laser module 121 is provided with a semiconductor laser that emits frequency-modulated laser light from both sides, and an optical cable 125 including a connector 125a is extended to be emitted from the semiconductor laser. Light is emitted to the atmosphere of the measurement target gas via the optical cable 125.

The reference gas cell 122 is disposed on the optical path behind the semiconductor laser, and the laser light that has passed through the reference gas cell 122 is received and detected by a photo detector 123 disposed behind the reference gas cell 122. .
Then, the temperature of the semiconductor laser is controlled by the Peltier element 28 while referring to the laser beam that has passed through the reference gas cell 122, and the emission wavelength of the semiconductor laser is controlled so that the ratio of the second harmonic wave to the fundamental wave is maximized. This makes it possible to match the absorption wavelength of the gas with the emission wavelength of the semiconductor laser.
Japanese Patent Laid-Open No. 7-151681 JP 2001-235418 A

However, in the conventional frequency modulation method, when the reference gas cell 122 is used to match the gas absorption wavelength and the emission wavelength of the semiconductor laser, the component of the measurement target gas varies depending on the environment, or the measurement target gas and the reference gas cell 122. Since the temperature of the gas enclosed in the gas is different, it becomes difficult to completely match the measurement target gas and the component and absorption wavelength of the gas enclosed in the reference gas cell 122, resulting in a decrease in measurement accuracy. there were.
Accordingly, an object of the present invention is to provide a laser wavelength control device, a gas concentration measurement device, and a laser wavelength control capable of accurately matching the absorption wavelength of a gas and the emission wavelength of a semiconductor laser without depending on the measurement environment. It is to provide a method and a gas concentration measurement method.

In order to solve the above-described problem, according to the laser wavelength control apparatus of claim 1, a laser element that emits laser light, a frequency modulation unit that modulates the frequency of the laser light with a fundamental wave, and a measurement target gas A gas cell in which the same gas is sealed, branching means for branching the frequency-modulated laser light to the measurement object gas side and the gas cell side, and a laser beam branched to the measurement object gas side 1 light detection section, a first fundamental wave component detection section for detecting a fundamental wave component from the laser light detected by the first light detection section, and detected by the first light detection section A first second-harmonic component detector that detects a second-harmonic component from the laser beam, a second photodetector that detects the laser beam branched to the gas cell side, and the second photodetector. Detecting the fundamental wave component from the detected laser beam A fundamental wave component detection unit, a second second harmonic component detection unit that detects a second harmonic component from the laser light detected by the second light detection unit, and a laser that has passed through the measurement target gas side A first amplitude ratio calculation unit for calculating an amplitude ratio between a fundamental wave component detected from light and a second harmonic component; a fundamental wave component detected from laser light transmitted through the gas cell side; and a second harmonic component; A second amplitude ratio calculation unit for calculating an amplitude ratio of the gas, and an amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or an amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side A temperature setting unit configured to set the temperature of the laser element based on any one of the measurement object gas side when the wavelength modulation is performed with reference to a wavelength shifted from the absorption peak wavelength of the measurement object gas The amplitude ratio between the fundamental wave component and the second harmonic wave component or the gas cell Based on one of the amplitude ratio between the fundamental wave component and second harmonic component side, characterized in that it comprises a drive current control unit for controlling the driving current of the laser element.

  According to the laser wavelength control apparatus of claim 2, the temperature setting unit includes an amplitude ratio between a fundamental wave component on the measurement target gas side and a second harmonic component, or a fundamental wave component on the gas cell side and 2. The temperature of the laser element is set so that the amplitude ratio with the harmonic component is maximized, and the drive current control unit is shifted by the same shift amount from the absorption peak wavelength to the long wavelength side and the short wavelength side. So that the amplitude ratio between the fundamental wave component and the second harmonic component on the gas side to be measured or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side when frequency modulation is performed with reference to The drive current is controlled.

According to the gas concentration measuring apparatus of claim 3, a laser element that emits laser light, a frequency modulation unit that modulates the frequency of the laser light with a fundamental wave, and the same gas as the measurement target gas are enclosed. A gas cell; branching means for branching the frequency-modulated laser light into the measurement target gas side and the gas cell; a first light detection unit for detecting the laser light branched into the measurement target gas; A first fundamental wave component detector for detecting a fundamental wave component from the laser light detected by the first light detector; and a second harmonic component from the laser light detected by the first light detector. A fundamental wave from a first second harmonic wave component detection unit to be detected, a second light detection unit to detect a laser beam branched to the gas cell side, and a laser beam detected by the second light detection unit A second fundamental wave component detector for detecting a component; A second harmonic component detection unit that detects a second harmonic component from the laser beam detected by the second optical detection unit, a fundamental wave component detected from the laser beam that has passed through the measurement target gas side, and 2 A first amplitude ratio calculating unit for calculating an amplitude ratio with a harmonic component, and a second amplitude for calculating an amplitude ratio between the fundamental wave component and the second harmonic component detected from the laser light transmitted through the gas cell side. The laser element such that the ratio calculation unit and the amplitude ratio between the fundamental wave component and the second harmonic component on the gas to be measured side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side are maximized. A temperature setting unit for setting the temperature of the gas to be measured, and the gas to be measured when frequency-modulated with reference to wavelengths shifted by the same shift amount from the absorption peak wavelength of the gas to be measured to the long wavelength side and the short wavelength side, respectively Amplitude ratio between fundamental wave component and double wave component Is a drive current control unit that controls the drive current of the laser element so that the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side matches, and when the frequency modulation is performed with reference to the absorption peak wavelength And a gas concentration calculation unit that calculates the concentration of the gas transmitted by the laser beam based on the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side.

According to the gas concentration measuring apparatus of the fourth aspect, the gas cell is a band-pass optical filter configured so that the absorption wavelength matches that of the measurement target gas.
According to the laser wavelength control method of claim 5, the step of branching the laser beam frequency-modulated with the fundamental wave to the measurement target gas side and the gas cell side in which the same gas as the measurement target gas is enclosed And the step of making the branched laser light incident on the measurement target gas and the gas cell, the step of detecting the measurement target gas and the laser light transmitted through the gas cell, respectively, and the detection target gas side being detected A step of extracting a fundamental wave component and a second harmonic component from the laser beam; a step of extracting a fundamental wave component and a second harmonic component from the laser beam detected on the gas cell side; A step of calculating an amplitude ratio between a fundamental wave component and a second harmonic component, and a step of calculating an amplitude ratio between the fundamental wave component extracted on the gas cell side and the second harmonic component. The laser light is generated so that the amplitude ratio between the fundamental wave component and the second harmonic component on the gas to be measured side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side is maximized. A step of setting the temperature of the laser element, and frequency modulation with reference to wavelengths shifted by the same shift amount from the absorption peak wavelength to the long wavelength side and the short wavelength side within the range of the absorption line width of the measurement target gas. Drive current of the laser element so that the amplitude ratio between the fundamental wave component and the second harmonic component on the gas to be measured at the time or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side coincides. And a step of controlling.

  Further, according to the gas concentration measuring method of claim 6, the step of branching the laser light frequency-modulated with the fundamental wave into the measuring object gas side and the gas cell side filled with the same gas as the measuring object gas; , The step of entering the branched laser light into the measurement target gas and the gas cell, the step of detecting the measurement target gas and the laser light transmitted through the gas cell, respectively, and the laser detected on the measurement target gas side A step of extracting a fundamental wave component and a second harmonic component from the light; a step of extracting a fundamental wave component and a second harmonic component from the laser light detected on the gas cell side; and a fundamental extracted on the gas to be measured side Calculating an amplitude ratio between the wave component and the second harmonic component; calculating an amplitude ratio between the fundamental wave component extracted on the gas cell side and the second harmonic component; Laser element for generating the laser beam so that the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side is maximized When the frequency is modulated with reference to wavelengths shifted by the same shift amount from the absorption peak wavelength to the long wavelength side and the short wavelength side within the range of the absorption line width of the gas to be measured. The driving current of the laser element is controlled so that the amplitude ratio between the fundamental wave component and the second harmonic component on the gas side to be measured or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side coincides. And the concentration of the measurement target gas transmitted by the laser light based on the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when frequency-modulated with reference to the absorption peak wavelength The Characterized in that it comprises the step of leaving.

  As described above, according to the present invention, the temperature of the semiconductor laser is set so that the emission wavelength of the semiconductor laser matches the absorption peak wavelength of the measurement target gas, and the wavelength shifted from the absorption peak wavelength is used as a reference. By controlling the drive current of the semiconductor laser while performing wavelength modulation, the gas to be measured is measured even when the component of the gas to be measured varies depending on the environment or the absorption wavelength of the gas to be measured varies depending on the temperature. Therefore, the measurement accuracy of the gas concentration can be improved without depending on the measurement environment.

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 54 that modulates the frequency of the laser light emitted from the laser unit 57 with a fundamental wave, and the laser light emitted from the laser unit 57 are converted into parallel beams. A laser unit 57 on which a collimating lens 56 and a laser element are mounted is provided. A semiconductor laser can be used as the laser element, and the thermistor for detecting the temperature of the laser element and a Peltier element for adjusting the temperature of the laser element can be mounted on the laser unit 57.

  The laser unit 57 is provided with a gas cell filled with the same gas as the measurement target gas, and an optical system for branching the frequency-modulated laser light into the measurement target gas side and the gas cell side, and a laser transmitted through the gas cell. A light detection unit for detecting light is provided. In addition, the transmission unit substrate 54 is equipped with an amplitude ratio calculation unit that calculates the amplitude ratio between the fundamental wave component and the second harmonic component of the laser light transmitted through the gas cell.

  Further, on the receiving side of the gas concentration measuring apparatus, a condensing lens 60 that condenses the laser light that has passed through the measurement target gas, a light detection unit 61 that detects the laser light that has passed through the measurement target gas, and the measurement target gas are transmitted. A receiving unit substrate 62 for calculating the amplitude ratio between the fundamental wave component and the second harmonic component of the laser beam is provided. As the light detection unit 61, for example, a photodiode can be used.

  Here, the transmission unit substrate 54, the collimating lens 56 and the laser unit 57 are accommodated in the housing 58, and the condenser lens 60, the light detection unit 61 and the reception unit substrate 62 are accommodated in the housing 59. Then, flanges 52a and 52b are attached to partition walls 51a and 51b such as pipes through which a measurement target gas flows such as a flue by a method such as welding. The housing 58 in which the transmission unit substrate 54, the collimating lens 56, and the laser unit 57 are accommodated is attached to the flange 52a so as to be separated from the inside of the pipe by the wedge window 55a, and the condensing lens 60, the light detection unit. The housing 59 in which 61 and the receiving unit board 62 are accommodated is attached to the flange 52b so as to be separated from the inside of the pipe by the wedge window 55b.

  Then, the laser light is emitted from the laser unit 57 and converted into a parallel beam by the collimator lens 56 while the output of the laser element is frequency-modulated at the center frequency fc and the modulation frequency fm, and then the partition wall through the wedge window 55a. The measurement target gas between 51a and 51b is transmitted. The laser light that has passed through the measurement target gas is absorbed at a wavelength corresponding to the absorption line of the gas molecule of the measurement target gas, and then enters the condenser lens 60 through the wedge window 55b. Is condensed on the light detection unit 61. When laser light is incident on the light detection unit 61, the light detection unit 61 converts the laser light into an electrical signal, and the electrical signal is sent to the reception unit substrate 62. Then, when the electrical signal converted by the light detection unit 61 is sent to the reception unit substrate 62, the amplitude ratio between the fundamental wave component and the second harmonic component of the laser light is calculated by the reception unit substrate 62. Based on the amplitude ratio between the fundamental wave component and the second harmonic component when the laser light is frequency-modulated, the concentration of the measurement target gas between the partition walls 51a and 51b can be calculated.

  Here, the transmitter substrate 54 sets the temperature of the laser element so that the wavelength of the laser light emitted from the laser unit 57 matches the absorption peak wavelength of the gas to be measured between the partition walls 51a and 51b. By controlling the drive current of the laser element while performing wavelength modulation based on the wavelength shifted from the absorption peak wavelength, the absorption wavelength of the gas to be measured between the partition walls 51a and 51b and the laser element mounted on the laser unit 57 The emission wavelength can be matched.

Further, the frequency-modulated laser light is branched to the gas cell side, passes through the gas cell, and then enters the light detection unit on the transmission side. When the electrical signal converted by the light detection unit on the transmission side is sent to the transmission unit substrate 54, the amplitude ratio between the fundamental wave component and the second harmonic component of the laser light transmitted through the gas cell is transmitted to the transmission unit substrate 54. Is calculated.
The transmitter substrate 54 sets the temperature of the laser element so that the wavelength of the laser light emitted from the laser unit 57 matches the absorption peak wavelength of the gas cell, and then changes the wavelength shifted from the absorption peak wavelength. By controlling the drive current of the laser element while performing reference wavelength modulation, the absorption wavelength of the gas cell and the emission wavelength of the laser element mounted on the laser unit 57 can be matched.

FIG. 2 is a block diagram showing a schematic configuration of a gas concentration measuring apparatus according to an embodiment of the present invention.
In FIG. 2, a semiconductor laser 41 and a temperature setting unit 42 are mounted on the laser unit 57 of FIG. For example, a thermistor that detects the temperature of the semiconductor laser 41 and a Peltier element that adjusts the temperature of the semiconductor laser 41 can be used as the temperature setting unit 42. Further, the laser unit 57 in FIG. 1 is provided with a gas cell 44 in which the same gas as the measurement target gas is sealed, and a beam spiriter for branching the frequency-modulated laser light to the measurement target gas side and the gas cell 44 side. 43 and a light detector 45 for detecting the laser light transmitted through the gas cell 44 is provided. The gas cell 44 may be a band-pass optical filter configured so that the absorption wavelength matches that of the measurement target gas.

  1 includes a laser drive unit 11 for injecting a drive current into the semiconductor laser 41, a frequency modulation unit 12 for modulating the laser light emitted from the semiconductor laser 41 with a fundamental wave, and the semiconductor laser 41. A drive current control unit 13 is provided for controlling the drive current injected into the. Further, the transmission unit substrate 54 of FIG. 1 includes a fundamental wave component detection unit 14 that detects a fundamental wave component from the laser light detected by the light detection unit 45, and 2 from the laser light detected by the light detection unit 45. A second harmonic component detection unit 15 that detects a harmonic component and an amplitude ratio calculation unit 16 that calculates an amplitude ratio between the fundamental wave component and the second harmonic component of the laser light detected by the light detection unit 45 are provided. Yes.

  In addition, the receiving unit substrate 62 of FIG. 1 includes a fundamental wave component detection unit 21 that detects a fundamental wave component from the laser light detected by the light detection unit 61, and 2 from the laser light detected by the light detection unit 61. A second harmonic component detection unit 22 that detects a harmonic component, an amplitude ratio calculation unit 23 that calculates an amplitude ratio between the fundamental wave component and the second harmonic component of the laser light detected by the light detection unit 61, and a light detection unit A gas concentration calculation unit 24 that calculates the concentration of the measurement target gas based on the amplitude ratio between the fundamental wave component and the second harmonic component detected at 61 is provided.

  Then, the frequency modulation unit 12 controls the laser driving unit 11 so that the output of the semiconductor laser 41 is frequency-modulated at the center frequency fc and the modulation frequency fm, so that the frequency-modulated laser light is emitted from the semiconductor laser 41. Emitted. The laser light emitted from the semiconductor laser 41 is branched by the beam spiriter 43, and one light beam branched by the beam spiriter 43 passes through the measurement target gas and is branched by the beam spiriter 43. The other light beam transmitted through the gas cell 44.

  Then, the laser light that has passed through the measurement target gas enters the light detection unit 61 after receiving absorption corresponding to the absorption lines of the gas molecules of the measurement target gas. When the laser light is incident on the light detection unit 61, the light detection unit 61 converts the laser light into an electrical signal, and the fundamental wave component detection unit 21 and the second harmonic wave component detection unit 22 double the fundamental wave component of the laser light. Each wave component is extracted. Then, the fundamental wave component and the second harmonic component of the laser light extracted by the fundamental wave component detection unit 21 and the second harmonic component detection unit 22 are sent to the amplitude ratio calculation unit 23, and the fundamental wave component of the laser light and 2 After the amplitude ratio with the harmonic component is calculated by the amplitude ratio calculation unit 23, it is sent to the temperature setting unit 42.

  On the other hand, the laser light transmitted through the gas cell 44 is incident on the light detection unit 45 after receiving absorption corresponding to the absorption line of the gas cell 44. When the laser light is incident on the light detection unit 45, the light detection unit 45 converts the laser light into an electrical signal, and the fundamental wave component detection unit 14 and the second harmonic wave component detection unit 15 double the fundamental wave component of the laser light. Each wave component is extracted. Then, the fundamental wave component and the second harmonic component of the laser light extracted by the fundamental wave component detection unit 14 and the second harmonic component detection unit 15 are sent to the amplitude ratio calculation unit 16, and the fundamental wave component of the laser beam and 2 After the amplitude ratio with the harmonic component is calculated by the amplitude ratio calculation unit 16, it is sent to the temperature setting unit 42.

  Here, when the amplitude ratio between the fundamental wave component and the second harmonic component of the laser light is calculated by the amplitude ratio calculation units 16 and 23, the temperature setting unit 42 can change the temperature of the semiconductor laser 41. Then, the temperature setting unit 42 acquires the relationship between the amplitude ratio between the fundamental wave component and the second harmonic wave component on the measurement target gas side or gas cell 44 side and the temperature of the semiconductor laser 41, and the measurement target gas side or gas cell 44 side The temperature of the laser element can be set so that the amplitude ratio between the fundamental wave component and the second harmonic wave component becomes maximum.

  When the temperature of the semiconductor laser 41 is set so that the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or gas cell side 44 is maximized, the drive current control unit 13 The drive current of the semiconductor laser 41 is controlled so that the wavelength is shifted from the absorption peak wavelength λc to the longer wavelength side. The frequency modulation unit 12 controls the laser driving unit 11 so that the output of the semiconductor laser 41 is frequency-modulated with reference to the wavelength λL shifted from the absorption peak wavelength λc to the long wavelength side, thereby changing the wavelength λL. As a reference, wavelength-modulated laser light is emitted from the semiconductor laser 41. The laser light emitted from the semiconductor laser 41 is branched by the beam spiriter 43, and one light beam branched by the beam spiriter 43 passes through the measurement target gas and is branched by the beam spiriter 43. The other light beam transmitted through the gas cell 44.

  Then, the laser light that has passed through the measurement target gas enters the light detection unit 61 after receiving absorption corresponding to the absorption lines of the gas molecules of the measurement target gas. When the laser light is incident on the light detection unit 61, the light detection unit 61 converts the laser light into an electrical signal, and the fundamental wave component detection unit 21 and the second harmonic wave component detection unit 22 double the fundamental wave component of the laser light. Each wave component is extracted. Then, the fundamental wave component and the second harmonic component of the laser light extracted by the fundamental wave component detection unit 21 and the second harmonic component detection unit 22 are sent to the amplitude ratio calculation unit 23, and the fundamental wave component of the laser light and 2 After the amplitude ratio with the harmonic component is calculated by the amplitude ratio calculator 23, it is sent to the drive current controller 13.

  On the other hand, the laser light transmitted through the gas cell 44 is incident on the light detection unit 45 after receiving absorption corresponding to the absorption line of the gas cell 44. When the laser light is incident on the light detection unit 45, the light detection unit 45 converts the laser light into an electrical signal, and the fundamental wave component detection unit 14 and the second harmonic wave component detection unit 15 double the fundamental wave component of the laser light. Each wave component is extracted. Then, the fundamental wave component and the second harmonic component of the laser light extracted by the fundamental wave component detection unit 14 and the second harmonic component detection unit 15 are sent to the amplitude ratio calculation unit 16, and the fundamental wave component of the laser beam and 2 After the amplitude ratio with the harmonic component is calculated by the amplitude ratio calculator 16, it is sent to the drive current controller 13.

  Further, the drive current control unit 13 controls the drive current of the semiconductor laser 41 so as to be shifted from the absorption peak wavelength λc to the short wavelength side. Then, the frequency modulation unit 12 controls the laser driving unit 11 so that the output of the semiconductor laser 41 is frequency-modulated with reference to the wavelength λs shifted from the absorption peak wavelength λc to the short wavelength side, thereby changing the wavelength λs. As a reference, wavelength-modulated laser light is emitted from the semiconductor laser 41. The laser light emitted from the semiconductor laser 41 is branched by the beam spiriter 43, and one light beam branched by the beam spiriter 43 passes through the measurement target gas and is branched by the beam spiriter 43. The other light beam transmitted through the gas cell 44.

  Then, the laser light that has passed through the measurement target gas enters the light detection unit 61 after receiving absorption corresponding to the absorption lines of the gas molecules of the measurement target gas. When the laser light is incident on the light detection unit 61, the light detection unit 61 converts the laser light into an electrical signal, and the fundamental wave component detection unit 21 and the second harmonic wave component detection unit 22 double the fundamental wave component of the laser light. Each wave component is extracted. Then, the fundamental wave component and the second harmonic component of the laser light extracted by the fundamental wave component detection unit 21 and the second harmonic component detection unit 22 are sent to the amplitude ratio calculation unit 23, and the fundamental wave component of the laser light and 2 After the amplitude ratio with the harmonic component is calculated by the amplitude ratio calculator 23, it is sent to the drive current controller 13.

  On the other hand, the laser light transmitted through the gas cell 44 is incident on the light detection unit 45 after receiving absorption corresponding to the absorption line of the gas cell 44. When the laser light is incident on the light detection unit 45, the light detection unit 45 converts the laser light into an electrical signal, and the fundamental wave component detection unit 14 and the second harmonic wave component detection unit 15 double the fundamental wave component of the laser light. Each wave component is extracted. Then, the fundamental wave component and the second harmonic component of the laser light extracted by the fundamental wave component detection unit 14 and the second harmonic component detection unit 15 are sent to the amplitude ratio calculation unit 16, and the fundamental wave component of the laser beam and 2 After the amplitude ratio with the harmonic component is calculated by the amplitude ratio calculator 16, it is sent to the drive current controller 13.

When the wavelength of the laser beam is shifted from the absorption peak wavelength λc to the long wavelength side and the short wavelength side, the wavelength shift amount Δλ is Δλ = λc−λL = λc− within the range of the absorption line width of the measurement target gas. It is preferable to set so as to be λs.
The drive current control unit 13 then determines the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell 44 side when the wavelength is modulated with respect to the wavelength λL, and the gas cell when the wavelength is modulated with respect to the wavelength λs. When the amplitude ratio between the fundamental wave component on the 44 side and the second harmonic component is received from the amplitude ratio calculation unit 16, the fundamental wave component on the gas cell 44 side and the second harmonic component when the wavelength is modulated with reference to the wavelength λL, Can be compared with the amplitude ratio between the fundamental wave component on the gas cell 44 side and the second harmonic component when wavelength modulation is performed with reference to the wavelength λs.

  The drive current control unit 13 then determines the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell 44 side when the wavelength is modulated with respect to the wavelength λL, and the gas cell when the wavelength is modulated with respect to the wavelength λs. While controlling the drive current of the semiconductor laser 41 until the amplitude ratio between the fundamental wave component on the 44 side and the second harmonic component matches, the gas cell 44 side when the wavelength is modulated with respect to the wavelength λL is used. The comparison of the amplitude ratio between the fundamental wave component and the second harmonic component and the amplitude ratio between the fundamental wave component on the gas cell 44 side and the second harmonic component when wavelength modulation is performed with reference to the wavelength λs can be repeated.

  The amplitude ratio between the fundamental wave component on the gas cell 44 side and the second harmonic wave component when the wavelength is modulated with respect to the wavelength λL, and the fundamental wave component on the gas cell 44 side when the wavelength is modulated with respect to the wavelength λs When the amplitude ratio with the second harmonic component matches, the drive current control unit 13 sets the drive current of the semiconductor laser 41 so as to be wavelength-modulated with reference to the absorption peak wavelength λc where the amplitude ratios match. can do.

  Alternatively, the drive current control unit 13 may perform the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when the wavelength modulation is performed with the wavelength λL as a reference, and the wavelength modulation with the wavelength λs as a reference. When the amplitude ratio between the fundamental wave component on the measurement target gas side and the second harmonic component is received from the amplitude ratio calculator 23, the fundamental wave component on the measurement target gas side is doubled with respect to the wavelength λL as a reference. The amplitude ratio with the wave component can be compared with the amplitude ratio between the fundamental wave component on the gas side to be measured and the second harmonic component when wavelength modulation is performed with reference to the wavelength λs.

  Then, the drive current control unit 13 determines the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when the wavelength modulation is performed with the wavelength λL as a reference, and the wavelength modulation with the wavelength λs as a reference. Measurement target when wavelength modulation is performed with reference to the wavelength λL while controlling the drive current of the semiconductor laser 41 until the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side matches. The comparison of the amplitude ratio between the fundamental component on the gas side and the second harmonic component and the amplitude ratio between the fundamental component on the gas side to be measured and the second harmonic component when the wavelength is modulated with reference to the wavelength λs is repeated. be able to.

  Then, the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when the wavelength is modulated with respect to the wavelength λL, and the fundamental wave on the measurement target gas side when the wavelength is modulated with respect to the wavelength λs. When the amplitude ratio of the component and the second harmonic component match, the drive current control unit 13 drives the drive current of the semiconductor laser 41 so that the wavelength is modulated with reference to the absorption peak wavelength λc so that the amplitude ratios match. Can be set.

  For example, the laser driving unit 11 drives the semiconductor laser 41 with a current of 30 mA ± 5 mA when wavelength-modulating the laser light with the absorption peak wavelength λc as a reference, and 40 mA when wavelength-modulating the laser light with the wavelength λL as a reference. When the semiconductor laser 41 is driven with a current of ± 5 mA and the laser light is wavelength-modulated with reference to the wavelength λs, the semiconductor laser 41 can be driven with a current of 20 mA ± 5 mA.

  Then, the gas concentration calculation unit 24 calculates the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when the wavelength modulation is performed with the wavelength λL as a reference, and the wavelength modulation with the wavelength λs as a reference. A fundamental wave component and a second harmonic component on the measurement target gas side when wavelength modulation is performed with reference to an absorption peak wavelength λc such that the amplitude ratio between the fundamental wave component on the measurement target gas side and the second harmonic component matches. Based on the amplitude ratio, the concentration of the measurement target gas between the partition walls 51a and 51b can be calculated.

  As a result, the temperature of the semiconductor laser 41 is set so that the emission wavelength of the semiconductor laser 41 coincides with the absorption peak wavelength λc of the measurement target gas, and wavelength modulation based on the wavelength shifted from the absorption peak wavelength λc is performed. By controlling the drive current of the semiconductor laser 41 while performing the measurement, the absorption peak wavelength of the measurement target gas can be obtained even when the component of the measurement target gas varies depending on the environment or the absorption wavelength of the measurement target gas varies depending on the temperature. It becomes possible to match λc and the emission wavelength of the semiconductor laser 41, and it is possible to improve the measurement accuracy of the gas concentration without depending on the measurement environment.

FIG. 3 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. 3, it is assumed that the output of the semiconductor laser 41 is frequency-modulated with a center frequency fc and a modulation frequency fm, and the measurement target gas or gas cell 44 is irradiated. Here, since the absorption line of the gas to be measured or the gas cell 44 is substantially a quadratic function with respect to the modulation frequency, this absorption line serves as a discriminator, and the light detection units 45 and 61 have the modulation frequency fm. A double frequency component (double wave component) is obtained. Here, since the modulation frequency fm may be an arbitrary frequency, for example, when the modulation frequency fm is selected to be about several kHz, it is possible to perform advanced signal processing using a digital signal processing device (DSP) or a general-purpose processor. Become.

  In order to cancel the influence of the attenuation amount of the laser beam due to the distance between the semiconductor laser 41 and the light detection units 45 and 61 by the frequency modulation method, the output of the semiconductor laser 41 is modulated simultaneously with the frequency modulation. Amplitude modulation may be performed at the frequency fm, and amplitude modulation can also be performed by applying frequency modulation to the output of the semiconductor laser 41. Then, the fundamental wave component by amplitude modulation can be estimated by performing envelope detection in the light detection units 45 and 61, respectively, and the ratio of the amplitude of the fundamental wave component and the amplitude of the second harmonic component is phase-synchronized. Thus, it is possible to obtain values proportional to the concentration of the measurement target gas without depending on the distance between the semiconductor laser 41 and the light detection units 45 and 61.

FIG. 4 is a diagram illustrating a method for controlling the center frequency of laser light according to an embodiment of the present invention.
In FIG. 4, when the measurement target gas is irradiated with the output of the semiconductor laser 41 frequency-modulated with the center frequency fc and the modulation frequency fm, the absorption line of the measurement target gas or the gas cell 44 is approximately a quadratic function with respect to the modulation frequency. Therefore, the light detection units 45 and 61 can obtain a frequency component (double wave component) twice the modulation frequency fm.

  When the amplitude ratio calculation unit 16 or 23 calculates the amplitude ratio between the fundamental wave component and the second harmonic component of the laser light, the temperature setting unit 42 can change the temperature of the semiconductor laser 41. Then, the temperature setting unit 42 acquires the relationship between the amplitude ratio between the fundamental wave component and the second harmonic wave component on the measurement target gas side or gas cell 44 side and the temperature of the semiconductor laser 41, and the measurement target gas side or gas cell 44 side By setting the temperature of the laser element so that the amplitude ratio between the fundamental wave component and the second harmonic wave component becomes maximum, the wavelength of the semiconductor laser 41 can be matched with the absorption peak wavelength λc.

  When the temperature setting unit 42 sets the temperature of the semiconductor laser 41 so that the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or gas cell 44 side is maximized, the drive current control unit 13 shows the case where the frequency modulation KL and Ks are performed with reference to the wavelengths λL and λs shifted by the same shift amount Δλ on the long wavelength side and the short wavelength side within the range of the absorption line width of the measurement target gas, respectively. The drive current of the semiconductor laser 41 can be controlled so that the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or gas cell 44 side matches.

  Then, the drive current control unit 13 modulates the wavelength with reference to the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or gas cell 44 side when the wavelength is modulated with the wavelength λL as a reference, and the wavelength λs. Drive current of the semiconductor laser 41 so as to be wavelength-modulated with reference to the absorption peak wavelength λc so that the amplitude ratio of the fundamental wave component and the second harmonic component on the measurement object gas side or gas cell 44 side coincides with each other. Can be set. Then, the gas concentration calculation unit 24 determines the measurement target gas based on the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when the wavelength modulation Kc is performed with the absorption peak wavelength λc as a reference. The concentration can be calculated.

  Here, the amplitude ratio between the fundamental wave component of the laser beam and the second harmonic component when the wavelength is modulated with respect to the wavelength λL is 2 and the fundamental wave component of the laser beam when the wavelength is modulated with respect to the wavelength λs. When the amplitude ratio with the harmonic component is larger, the emission wavelength set by the temperature setting unit 42 is shifted to the longer wavelength side with respect to the absorption peak wavelength λc. By reducing the drive current of 41, the emission wavelength of the semiconductor laser 41 can be made closer to the absorption peak wavelength λc by shifting the emission wavelength of the semiconductor laser 41 to the short wavelength side.

  Further, the amplitude ratio between the fundamental wave component and the double wave component of the laser light when the wavelength is modulated with respect to the wavelength λL is twice the fundamental wave component of the laser light when the wavelength is modulated with respect to the wavelength λs. When the amplitude ratio is smaller than the wave component, the emission wavelength set by the temperature setting unit 42 is shifted to the short wavelength side with respect to the absorption peak wavelength λc. By increasing the drive current, the emission wavelength of the semiconductor laser 41 can be made closer to the absorption peak wavelength λc by shifting the emission wavelength of the semiconductor laser 41 to the longer wavelength side.

FIG. 5 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. 5, the emission wavelength of the semiconductor laser 41 becomes longer as the drive current increases. For this reason, the emission wavelength of the semiconductor laser 41 can be adjusted by controlling the drive current of the semiconductor laser 41.
FIG. 6 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. 6, the emission wavelength of the semiconductor laser 41 becomes longer as the temperature increases. For this reason, the emission wavelength of the semiconductor laser 41 can be adjusted by controlling the temperature of the semiconductor laser 41.

FIG. 7 is a diagram showing a relationship between a light emission wavelength and a light reception voltage when a laser beam is irradiated to a gas cell in which HCN is sealed according to an embodiment of the present invention. P1 shows a waveform when there is a gas cell, and P2 shows a waveform when there is no gas cell.
In FIG. 7, each gaseous gas molecule has its own light absorption spectrum. When the gas cell 44 is irradiated with the laser light, the laser light is absorbed at the wavelength of the light absorption spectrum unique to the gas molecule, so that the light reception voltage by the light detection unit 45 decreases at the position of the wavelength.
As shown in FIGS. 5 and 6, since the emission wavelength of the semiconductor laser 41 changes depending on the drive current or temperature, the emission wavelength of the semiconductor laser 41 is controlled by controlling the drive current or temperature of the semiconductor laser 41. It can be matched with the absorption peak wavelength λc.

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 block diagram showing a schematic structure of a gas concentration measuring device concerning one embodiment of the present 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 center frequency control method of the laser beam 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 figure which shows the relationship between the light emission wavelength when a laser beam is irradiated to the gas cell with which HCN enclosed with one Embodiment of this invention was irradiated, and a light reception voltage. It is a top view which shows schematic structure of the gas concentration measuring apparatus in the conventional frequency modulation system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Laser drive part 12 Frequency modulation part 13 Drive current control part 14, 21 Fundamental wave component detection part 15, 22 Double wave component detection part 16, 23 Amplitude ratio calculation part 24 Gas concentration calculation part 41 Semiconductor laser 42 Temperature setting part 43 Beam splitter 44 Gas cell 45, 61 Photodetector 51a, 51b Bulkhead 52a, 52b Flange 54 Transmitter substrate 55a, 55b Wedge window 56 Collimator lens 57 Laser unit 58, 59 Housing 60 Condensing lens 62 Receiver substrate

Claims (6)

A laser element that emits laser light;
A frequency modulation unit for frequency-modulating the laser beam with a fundamental wave;
A gas cell filled with the same gas as the measurement target gas;
Branching means for branching the frequency-modulated laser light to the gas to be measured side and the gas cell side;
A first light detection unit for detecting a laser beam branched to the measurement target gas side;
A first fundamental wave component detection unit that detects a fundamental wave component from the laser light detected by the first light detection unit;
A first second harmonic component detection unit that detects a second harmonic component from the laser light detected by the first light detection unit;
A second light detection unit for detecting a laser beam branched to the gas cell side;
A second fundamental wave component detection unit for detecting a fundamental wave component from the laser light detected by the second light detection unit;
A second second harmonic component detection unit that detects a second harmonic component from the laser light detected by the second light detection unit;
A first amplitude ratio calculator that calculates an amplitude ratio between a fundamental wave component and a second harmonic component detected from the laser light transmitted through the measurement target gas side;
A second amplitude ratio calculator for calculating an amplitude ratio between a fundamental wave component and a second harmonic component detected from the laser light transmitted through the gas cell side;
The temperature of the laser element is set based on either the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side. A temperature setting unit to
The amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or the fundamental component on the gas cell side when wavelength modulation is performed with reference to a wavelength shifted from the absorption peak wavelength of the measurement target gas And a drive current control unit that controls the drive current of the laser element based on one of the amplitude ratios of the second harmonic component and the second harmonic component. The temperature setting unit may be configured such that the amplitude ratio between the fundamental wave component and the second harmonic component on the gas to be measured side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side is maximized. Set the temperature of
The drive current control unit doubles the fundamental wave component on the measurement target gas side when frequency-modulated with reference to wavelengths shifted by the same shift amount from the absorption peak wavelength to the long wavelength side and the short wavelength side, respectively. 2. The laser wavelength control device according to claim 1, wherein the drive current is controlled so that an amplitude ratio with a wave component or an amplitude ratio between a fundamental wave component on the gas cell side and a second harmonic component coincides. A laser element that emits laser light;
A frequency modulation unit for frequency-modulating the laser beam with a fundamental wave;
A gas cell filled with the same gas as the measurement target gas;
Branching means for branching the frequency-modulated laser light to the gas to be measured side and the gas cell side;
A first light detection unit for detecting laser light branched to the measurement target gas side;
A first fundamental wave component detection unit that detects a fundamental wave component from the laser light detected by the first light detection unit;
A first second harmonic component detection unit that detects a second harmonic component from the laser light detected by the first light detection unit;
A second light detection unit for detecting a laser beam branched to the gas cell side;
A second fundamental wave component detection unit for detecting a fundamental wave component from the laser light detected by the second light detection unit;
A second second harmonic component detection unit that detects a second harmonic component from the laser light detected by the second light detection unit;
A first amplitude ratio calculator that calculates an amplitude ratio between a fundamental wave component and a second harmonic component detected from the laser light transmitted through the measurement target gas side;
A second amplitude ratio calculator for calculating an amplitude ratio between a fundamental wave component and a second harmonic component detected from the laser light transmitted through the gas cell side;
The temperature of the laser element is set so that the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side is maximized. A temperature setting unit;
The fundamental wave component and the second harmonic component on the measurement target gas side when frequency-modulated with reference to wavelengths shifted by the same shift amount from the absorption peak wavelength of the measurement target gas to the long wavelength side and the short wavelength side, respectively. A drive current control unit that controls the drive current of the laser element so that the amplitude ratio of the fundamental wave component on the gas cell side and the amplitude ratio of the second harmonic component coincide with each other,
Gas concentration calculation for calculating the concentration of the gas transmitted by the laser beam based on the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when frequency modulation is performed with respect to the absorption peak wavelength And a gas concentration measuring device.   4. The gas concentration measuring apparatus according to claim 3, wherein the gas cell is a band-pass optical filter configured so that an absorption wavelength coincides with a measurement target gas. Branching a laser beam frequency-modulated with a fundamental wave to a measurement target gas side and a gas cell side in which the same gas as the measurement target gas is sealed;
Making the branched laser light incident on the gas to be measured and the gas cell;
Detecting each of the measurement target gas and the laser beam transmitted through the gas cell;
Extracting a fundamental wave component and a second harmonic component from the laser light detected on the measurement target gas side;
Extracting a fundamental wave component and a second harmonic component from the laser light detected on the gas cell side;
Calculating an amplitude ratio between a fundamental wave component and a second harmonic component extracted on the measurement target gas side;
Calculating an amplitude ratio between a fundamental wave component extracted on the gas cell side and a second harmonic component;
Laser element that generates the laser beam so that the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side is maximized Setting the temperature of the
Basics on the measurement target gas side when frequency modulation is performed with reference to wavelengths shifted by the same shift amount from the absorption peak wavelength to the long wavelength side and the short wavelength side within the range of the absorption line width of the measurement target gas. And a step of controlling the drive current of the laser element so that the amplitude ratio between the wave component and the second harmonic component or the amplitude ratio between the fundamental wave component on the gas cell side and the second harmonic component coincides with each other. And a laser wavelength control method. Branching a laser beam frequency-modulated with a fundamental wave to a measurement target gas side and a gas cell side in which the same gas as the measurement target gas is sealed;
Making the branched laser light incident on the gas to be measured and the gas cell;
Detecting each of the measurement target gas and the laser beam transmitted through the gas cell;
Extracting a fundamental wave component and a second harmonic component from the laser light detected on the measurement target gas side;
Extracting a fundamental wave component and a second harmonic component from the laser light detected on the gas cell side;
Calculating an amplitude ratio between a fundamental wave component and a second harmonic component extracted on the measurement target gas side;
Calculating an amplitude ratio between a fundamental wave component extracted on the gas cell side and a second harmonic component;
Laser element for generating the laser beam so that the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side or the amplitude ratio between the fundamental wave component and the second harmonic component on the gas cell side is maximized Setting the temperature of the
Basics on the measurement target gas side when frequency modulation is performed with reference to wavelengths shifted by the same shift amount from the absorption peak wavelength to the long wavelength side and the short wavelength side within the range of the absorption line width of the measurement target gas. Controlling the drive current of the laser element so that the amplitude ratio between the wave component and the second harmonic component or the amplitude ratio between the fundamental wave component on the gas cell side and the second harmonic component coincides;
Calculating the concentration of the measurement target gas transmitted by the laser beam based on the amplitude ratio between the fundamental wave component and the second harmonic component on the measurement target gas side when frequency-modulated with respect to the absorption peak wavelength. A gas concentration measurement method comprising:

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