JP4905106B2 - 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 method of using the reference gas cell 122 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 gas sealed in the reference gas cell 122 Since the temperatures are different, there is a problem that it becomes difficult to completely match the component of the gas to be measured and the gas contained in the reference gas cell 122 and the absorption wavelength, resulting in a decrease in measurement accuracy.

Further, the corrosive gas such as HCl and HF has a problem that the sealing equipment becomes expensive and it is difficult to seal the gas in the reference gas cell 122.
Furthermore, in the method using the reference gas cell 122, the number of parts increases, so that there is a problem that the operation of the semiconductor laser becomes unstable due to the return light and the measurement accuracy is lowered.
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, a laser wavelength control device according to claim 1, a laser element that emits laser light, a frequency modulation unit that frequency-modulates the laser light with a fundamental wave, and the frequency modulation. A light detection unit that detects the laser beam that has been detected, a fundamental wave component detection unit that detects a fundamental wave component from the laser beam detected by the light detection unit, and a laser beam that is detected from the laser light detected by the light detection unit. A second harmonic component detection unit for detecting a harmonic component; an amplitude ratio calculation unit for calculating an amplitude ratio between the fundamental wave component and the second harmonic component detected from the laser beam; and the fundamental wave component and the second harmonic wave. A temperature setting unit that sets the temperature of the laser element based on an amplitude ratio with the component; and the fundamental wave component when wavelength modulation is performed with reference to a wavelength shifted from the absorption peak wavelength of the measurement target gas; Based on the amplitude ratio with the harmonic component Characterized in that it comprises a drive current control unit for controlling the drive current of the laser element.

  According to the laser wavelength control apparatus of claim 2, the temperature setting unit sets the temperature of the laser element so that an amplitude ratio between the fundamental wave component and the second harmonic wave component is maximized, The drive current control unit has an amplitude ratio between a fundamental wave component and a second harmonic component 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. The drive current is controlled so as to match.

According to the gas concentration measuring apparatus of claim 3, the laser element that emits laser light, the frequency modulation unit that modulates the frequency of the laser light with a fundamental wave, and the light that detects the frequency-modulated laser light. A detection unit; a fundamental wave component detection unit that detects a fundamental wave component from the laser light detected by the light detection unit; and a double that detects a second harmonic component from the laser light detected by the light detection unit. A wave component detection unit; an amplitude ratio calculation unit that calculates an amplitude ratio between the fundamental wave component and the second harmonic component detected from the laser beam; and an amplitude ratio between the fundamental wave component and the second harmonic component is maximized. A temperature setting unit for setting the temperature of the laser element, and frequency modulation with reference to wavelengths shifted from the absorption peak wavelength of the measurement target gas to the long wavelength side and the short wavelength side by the same shift amount, respectively. Fundamental wave component and double wave formation And a drive current control unit that controls the drive current of the laser element so that the amplitude ratio matches the amplitude ratio between the fundamental wave component and the second harmonic component when frequency-modulated with reference to the absorption peak wavelength. And a gas concentration calculation unit for calculating the concentration of the gas transmitted by the laser beam.

  According to the laser wavelength control method of claim 4, the step of making the laser beam incident on the measurement target gas while performing frequency modulation with the fundamental wave, the step of detecting the laser beam that has passed through the measurement target gas, Extracting a fundamental wave component and a second harmonic component from the detected laser beam; calculating an amplitude ratio between the fundamental wave component and the second harmonic component extracted from the laser beam; and the fundamental wave component The step of setting the temperature of the laser element that generates the laser light so that the amplitude ratio between the second harmonic component and the second harmonic component is maximized, and the long wavelength side from the absorption peak wavelength within the range of the absorption line width of the measurement target gas And the drive power of the laser element so that the amplitude ratio of the fundamental wave component and the second harmonic wave component when frequency-modulated with reference to the wavelength shifted by the same shift amount to the short wavelength side is the same. Characterized in that it comprises a step of controlling.

  According to the gas concentration measurement method of claim 5, the step of making the laser beam incident on the measurement target gas while performing frequency modulation with the fundamental wave, the step of detecting the laser beam that has passed through the measurement target gas, Extracting a fundamental wave component and a second harmonic component from the detected laser beam; calculating an amplitude ratio between the fundamental wave component and the second harmonic component extracted from the laser beam; and the fundamental wave component; A step of setting a temperature of a laser element that generates the laser light so that an amplitude ratio with a second harmonic component is maximized; and a long wavelength side from an absorption peak wavelength within a range of an absorption line width of the measurement target gas; The drive current of the laser element is set so that the amplitude ratio between the fundamental wave component and the second harmonic wave component when frequency-modulated with reference to the wavelength shifted by the same shift amount to the short wavelength side is the same. And a step of calculating a concentration of a measurement target gas transmitted by the laser light based on an amplitude ratio between a fundamental wave component and a second harmonic component when frequency modulation is performed with reference to the absorption peak wavelength; It is characterized by providing.

  As described above, according to the present invention, the temperature of the laser element is set so that the wavelength of the laser light 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 laser element while performing wavelength modulation, the reference gas cell can be used 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. Without using it, it is possible to match the absorption peak wavelength of the gas to be measured and the emission wavelength of the laser element, and it is possible to improve the measurement accuracy of the gas concentration 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 a Peltier element for adjusting the temperature of the laser element can be mounted on the laser unit 57.

  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.

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. In addition, as the temperature setting part 42, a Peltier device can be used, for example. 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.

  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. It is emitted and permeates the measurement target gas. 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.

  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 unit 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 component of the laser beam and the temperature of the semiconductor laser 41, and the amplitude between the fundamental wave component and the second harmonic component of the laser beam. The temperature of the laser element can be set so that the ratio is maximized.

  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 of the laser beam is maximized, the drive current control unit 13 causes the wavelength of the laser beam to reach the absorption peak wavelength λc. The drive current of the semiconductor laser 41 is controlled so as to be shifted from 1 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 and passes through the measurement target gas.

  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.

  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 and passes through the measurement target gas.

  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.

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.
Then, the drive current control unit 13 determines the amplitude ratio between the fundamental wave component and the second harmonic component of the laser beam when wavelength-modulated with the wavelength λL, and the laser beam when wavelength-modulated with the wavelength λs as a reference. When the amplitude ratio between the fundamental wave component and the second harmonic wave component is received, the amplitude ratio between the fundamental wave component and the second harmonic wave component of the laser light when the wavelength is modulated with reference to the wavelength λL, and the wavelength λs are referenced. As a comparison, the amplitude ratio of the fundamental wave component and the second harmonic component of the laser light when wavelength modulation is performed is compared.

  Then, the drive current control unit 13 determines the amplitude ratio between the fundamental wave component and the second harmonic component of the laser beam when wavelength-modulated with the wavelength λL, and the laser beam when wavelength-modulated with the wavelength λs as a reference. The fundamental wave component of the laser light when the wavelength is modulated 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 wave component becomes equal. The comparison is repeated between the amplitude ratio of the second harmonic component and the amplitude ratio of the fundamental component and the second harmonic component of the laser light when the wavelength is modulated with reference to the wavelength λs.

  Then, 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, and the double of the fundamental wave component of the laser light when the wavelength is modulated with the wavelength λs as a reference. When the amplitude ratio with the wave 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 that matches the amplitude ratio.

  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 wave component of the laser beam when wavelength-modulated with the wavelength λL and the laser beam when wavelength-modulated with the wavelength λs as a reference. Based on the amplitude ratio between the fundamental wave component and the second harmonic component when wavelength-modulated with reference to the absorption peak wavelength λc such that the amplitude ratio between the fundamental wave component and the second harmonic component coincides, the partition wall 51a , 51b, the concentration of the measurement target gas 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, 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, the reference gas cell is not used. The absorption peak wavelength λc of the measurement target gas and the emission wavelength of the semiconductor laser 41 can be matched, and the measurement accuracy of the gas concentration can be improved 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 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 61 has a frequency twice as high as the modulation frequency fm. A component (second harmonic 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 unit 61 by the frequency modulation method, the output of the semiconductor laser 41 is modulated at the same time as the modulation frequency fm. Amplitude modulation may be performed by the above, 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 at the light detection unit 61, and by taking the phase ratio of the ratio of the amplitude of the fundamental wave component and the amplitude of the second harmonic component, A value proportional to the concentration of the measurement target gas can be obtained without depending on the distance between the semiconductor laser 41 and the light detection unit 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 that is frequency-modulated with the center frequency fc and the modulation frequency fm, the absorption line of the measurement target gas becomes a substantially quadratic function with respect to the modulation frequency. Therefore, the light detection unit 61 can obtain a component having a frequency twice the modulation frequency fm (double wave component).

  When the amplitude ratio calculation unit 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 component of the laser beam and the temperature of the semiconductor laser 41, and the amplitude between the fundamental wave component and the second harmonic component of the laser beam. By setting the temperature of the laser element so that the ratio becomes maximum, the wavelength of the semiconductor laser 41 can be matched with the absorption peak wavelength λc.

  When the temperature of the semiconductor laser 41 is set by the temperature setting unit 42 so that the amplitude ratio between the fundamental wave component and the second harmonic component of the laser beam is maximized, the drive current control unit 13 2 times the fundamental wave component when frequency modulation KL and Ks are performed with reference to wavelengths λL and λs shifted by the same shift amount Δλ on the long wavelength side and short wavelength side within the range of the absorption line width The drive current of the semiconductor laser 41 can be controlled so that the amplitude ratio with the wave component matches.

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

  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 is irradiated with 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 61 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 21 Fundamental wave component detection part 22 2nd harmonic component detection part 23 Amplitude ratio calculation part 24 Gas concentration calculation part 41 Semiconductor laser 42 Temperature setting part 51a, 51b Partition 52a, 52b Flange 54 Transmitter board 55a, 55b Wedge window 56 Collimator lens 57 Laser unit 58, 59 Housing 60 Condensing lens 61 Light detector 62 Receiver board

Claims (5)

A laser element that emits laser light;
A frequency modulation unit for frequency-modulating the laser beam with a fundamental wave;
A light detector for detecting the frequency-modulated laser light;
A fundamental wave component detector that detects a fundamental wave component from the laser light detected by the light detector; and
A second harmonic component detection unit for detecting a second harmonic component from the laser light detected by the light detection unit;
An amplitude ratio calculation unit for calculating an amplitude ratio between a fundamental wave component and a second harmonic component detected from the laser beam;
A temperature setting unit that sets the temperature of the laser element based on the amplitude ratio of the fundamental wave component and the second harmonic wave component;
Drive current for controlling the drive current of the laser element based on the amplitude ratio between the fundamental wave component and the second harmonic wave component when wavelength modulation is performed with reference to the wavelength shifted from the absorption peak wavelength of the measurement target gas A laser wavelength control device comprising: a control unit. The temperature setting unit sets the temperature of the laser element so that the amplitude ratio between the fundamental wave component and the second harmonic component is maximized,
The drive current control unit has an amplitude ratio between a fundamental wave component and a second harmonic component 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 apparatus according to claim 1, wherein the drive current is controlled so that the two coincide with each other. A laser element that emits laser light;
A frequency modulation unit for frequency-modulating the laser beam with a fundamental wave;
A light detector for detecting the frequency-modulated laser light;
A fundamental wave component detector that detects a fundamental wave component from the laser light detected by the light detector; and
A second harmonic component detection unit for detecting a second harmonic component from the laser light detected by the light detection unit;
An amplitude ratio calculation unit for calculating an amplitude ratio between a fundamental wave component and a second harmonic component detected from the laser beam;
A temperature setting unit that sets the temperature of the laser element so that the amplitude ratio between the fundamental wave component and the second harmonic wave component is maximized;
The amplitude ratio between the fundamental wave component and the second harmonic wave component when the frequency is modulated with reference to the wavelength 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 is the same. A drive current control unit for controlling the drive current of the laser element,
A gas concentration calculation unit that calculates the concentration of the gas transmitted by the laser beam based on an amplitude ratio between a fundamental wave component and a second harmonic component when frequency-modulated with respect to the absorption peak wavelength. A gas concentration measuring device. Injecting laser light into the gas to be measured while modulating the frequency with the fundamental wave;
Detecting laser light transmitted through the measurement target gas;
Extracting a fundamental wave component and a second harmonic component from the detected laser beam;
Calculating an amplitude ratio between a fundamental wave component and a second harmonic component extracted from the laser beam;
Setting the temperature of the laser element that generates the laser beam so that the amplitude ratio between the fundamental wave component and the second harmonic component is maximized;
The fundamental wave component and the second harmonic wave 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 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 with the component matches. Injecting laser light into the gas to be measured while modulating the frequency with the fundamental wave;
Detecting laser light transmitted through the measurement target gas;
Extracting a fundamental wave component and a second harmonic component from the detected laser beam;
Calculating an amplitude ratio between a fundamental wave component and a second harmonic component extracted from the laser beam;
Setting the temperature of the laser element that generates the laser beam so that the amplitude ratio between the fundamental wave component and the second harmonic component is maximized;
The fundamental wave component and the second harmonic wave 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 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 with the component matches;
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 wave component when frequency-modulated with respect to the absorption peak wavelength. Gas concentration measurement method.

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