JP2014142299A - Gas concentration measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas concentration measurement device that can measure gas concentration with high accuracy by reducing interference noise.SOLUTION: The gas concentration measurement device comprises: a super luminescent diode 7 that generates super luminescent diode light; a drive section 4a that causes the super luminescent diode 7 to be driven; a sample cell 2 that introduces gas and makes the super luminescent diode light incident; a grating 8 that is provided on an output side of the sample cell 2, and spectrally separating the super luminescent diode light; a photodiode array 9 that receives the super luminescent diode light spectrally separated by the grating 8; and a computation process section 5 that computes concentration of the gas on the basis of a light reception output of the photodiode array 9.

Description

  The present invention relates to a gas concentration measuring apparatus for measuring a gas concentration using a semiconductor laser.

  The wavelength tunable diode laser spectral absorption method (TDLAS) is a method that can detect the minute absorption of the measurement gas with high sensitivity by modulating the frequency of the laser beam and measure the concentration of the measurement gas.

  As an apparatus for measuring the concentration of a measurement gas, a technique described in Patent Document 1 is known. FIG. 5 is a configuration diagram of a conventional gas concentration measuring apparatus described in Patent Document 1. In FIG. The gas concentration measuring apparatus shown in FIG. 5 includes a wavelength tunable semiconductor laser 1, a flue 17, a photodiode (PD) 3, a laser driving unit 4, and an arithmetic processing unit 5.

  The laser drive unit 4 drives the semiconductor laser 1 and modulates the oscillation wavelength of the semiconductor laser 1 at a constant period. The semiconductor laser 1 generates laser light whose oscillation wavelength is modulated at a constant period, and guides the generated laser light to the flue 17. The flue 17 encloses the measurement gas and guides the laser light from the semiconductor laser 1 to the photodiode 3 through the measurement gas.

  The laser light that has passed through the flue 17 in which the measurement gas is sealed is absorbed by the measurement gas and reduced according to the gas concentration. The photodiode 3 includes a light receiving element, and detects the amount of transmitted light that is absorbed and reduced by the measurement gas in the flue 17. The arithmetic processing unit 7 is, for example, a lock-in amplifier, and measures the concentration of the measurement gas in the flue 17 by synchronously detecting the double frequency of the modulation signal within the transmitted light amount detected by the photodiode 3.

  FIG. 6 is a diagram showing a waveform of a laser driving current of a conventional gas concentration measuring apparatus. As shown in FIG. 6, the conventional gas concentration measuring apparatus changes the oscillation frequency of the semiconductor laser 1 by gradually changing the magnitude of the drive current of the semiconductor laser 1, and further the oscillation wavelength of the semiconductor laser 1. Is modulated at a constant period.

  As another general gas absorption spectroscopic measurement, there is a method of obtaining a spectrum by irradiating an object with white light and decomposing the transmitted light for each wavelength with a grating or the like. For example, a halogen lamp is used as a white light source, and laser light is modulated by a chopper circuit or the like.

JP 2010-216959 A

  However, the coherence length of a normal semiconductor laser is several tens of centimeters or more, and light that has been reflected multiple times by an optical system such as a lens enters the detection surface, so that the light that has directly passed through the optical system and the optical system have been reflected multiple times. Interference with light. For this reason, when measuring a low-concentration gas, the intensity greatly fluctuates on the detection surface with respect to the measurement gas signal, S (signal) / N (noise) deteriorates, and the gas concentration cannot be measured with high accuracy. It had the problem that.

  In addition, even if you try to subtract the intensity fluctuation on the detection surface in the background (measurement in the absence of measurement gas), if a temperature change or vibration occurs, the optical path difference and the number of multiple reflections will be different. The strength at the time fluctuates irregularly. That is, since the light interference state changes with changes in temperature and vibration, it is difficult to subtract in the background.

  Further, in order to reduce the interference noise, a mechanism for vibrating the laser element is required, which complicates the configuration.

  An object of the present invention is to provide a gas concentration measuring apparatus capable of measuring gas concentration with high accuracy without reducing interference noise and requiring a mechanism for vibrating a laser element.

  In order to solve the above problems, a gas concentration measuring apparatus according to the present invention introduces a superluminescent diode that generates superluminescent diode light, a drive unit that drives the superluminescent diode, and a gas. And a gas introducing unit that receives the superluminescent diode light, a spectroscopic unit that is provided on an input side or an output side of the gas introducing unit, and that spectrally separates the superluminescent diode light; A light-receiving element that receives the superluminescent diode light, and an arithmetic processing unit that calculates the concentration of the gas based on the light-receiving output of the light-receiving element.

  According to the present invention, the superluminescent diode has a coherence length of several tens of μm and is sufficiently shorter than the coherence length of the semiconductor laser, so that interference noise can be greatly reduced and the laser element is vibrated. The gas concentration can be accurately measured without requiring a mechanism.

It is a block diagram of the gas concentration measuring apparatus of Example 1. It is a figure which shows the waveform of the SLD drive current of the gas concentration measuring apparatus of Example 1. FIG. It is a figure which shows the spectrum of SLD of the gas concentration measuring apparatus of Example 1. FIG. It is a block diagram of the gas concentration measuring apparatus of Example 2. It is a block diagram of the conventional gas concentration measuring apparatus. It is a figure which shows the waveform of the laser drive current of the conventional gas concentration measuring apparatus.

  Embodiments of a gas concentration measuring apparatus according to the present invention will be described below in detail with reference to the drawings.

  FIG. 1 is a configuration diagram of a gas concentration measuring apparatus according to the first embodiment. The gas concentration measuring apparatus of Example 1 shown in FIG. 1 includes a super luminescent diode (hereinafter abbreviated as SLD) 7, a sample cell 2, an SLD driving unit 4a, a grating 8, a photodiode array 9, and an arithmetic processing unit. 5 is provided.

  The SLD driving unit 4a drives the SLD 7 to modulate the wavelength. The SLD 7 is driven by the SLD drive unit 4a and generates super luminescent diode light (hereinafter abbreviated as SLD light). The SLD 7 has a coherence length of several tens of μm and is sufficiently shorter than the coherence length of the semiconductor laser.

  The sample cell 2 constitutes a gas introduction part, encloses the gas, and receives the SLD light from the SLD 7. The grating 8 enters the SLD light transmitted through the sample cell 2, diffracts the SLD light, and emits it.

  The photodiode array 9 constitutes a light receiving element and includes a plurality of photodiodes. The photodiode array 9 receives the SLD light diffracted by the grating 8 with the plurality of photodiodes.

  The arithmetic processing unit 5 is, for example, a lock-in amplifier, and detects a highly sensitive signal by synchronously detecting the signal of the photodiode array 9 at the same frequency as the SLD 7, and calculates the gas concentration based on the detection signal. To do.

  Next, the operation of the gas concentration measuring apparatus according to the first embodiment configured as described above will be described.

  First, the SLD drive unit 4a gives an SLD drive current having a frequency f as shown in FIG. As a result, the SLD light emitted from the SLD 7 is accompanied by intensity modulation of the frequency f and is incident on the sample cell 2. The sample cell 2 guides the SLD light from the SLD 7 to the grating 8 through the measurement gas. The grating 8 wavelength-disperses the SLD light transmitted through the sample cell 2 and guides it to the photodiode array 9.

  The SLD light of each wavelength incident on the photodiode array 9 is light having a reduced intensity reflecting the gas absorption spectrum and concentration. The arithmetic processing unit 5 detects the signal by synchronously detecting the signal of the photodiode array 9 at the same frequency as that of the SLD 7, and calculates the gas concentration based on the detection signal.

As the method, for example, the spectrum when there is no gas to be absorbed is used as a reference spectrum, and the spectrum that has absorbed the gas is divided by this reference spectrum, the transmittance is obtained. From Lambert-Beer's law, log (T) =-εcl
Is required. T is the transmittance, ε is the absorption coefficient, c is the gas concentration, and l is the optical path length. If ε and l are known, the gas concentration can be determined. Further, by using the SLD 7, a plurality of gas concentrations can be measured.

  In the SLD 7 having a peak wavelength of an emission spectrum of about 1550 nm and a spectrum width of about 100 nm, for example, H20 (1512 nm), NH3 (1531 nm), C2H2 (1534 nm), CO2 (1572 nm), CO (1583 nm) can be measured. )and so on. FIG. 3 shows an example spectrum of SLD7.

  In the SLD 7 having an emission spectrum peak wavelength of about 1400 nm and a spectrum width of about 60 nm, examples of gas species that can be measured include H20 (1395 nm), N 2 O (1400 nm), and CH 4 (1387 nm).

  In the SLD 7 having an emission spectrum peak wavelength of about 1300 nm and a spectrum width of about 60 nm, examples of gas species that can be measured include N 2 O (1283 nm), CH 4 (1324 nm), and O 2 (1274 nm).

  According to the gas concentration measuring apparatus according to the first embodiment, the SLD 7 has a coherence length of several tens of μm and is sufficiently shorter than the coherence length of the semiconductor laser, so that interference noise can be greatly reduced.

  Further, since the change in the state of light interference is small, it is easy to process as a background. Therefore, the gas concentration can be accurately measured. The spectral width of the SLD 7 is several tens of nm, the spectral width of the semiconductor laser is several tens of pm, and the width of scanning the semiconductor laser is about 1 mm.

  Therefore, since the spectral width of the SLD 7 is wider than the scanning width of the semiconductor laser, it becomes possible to acquire absorption lines in a wavelength band that could not be measured conventionally, and a plurality of gases can be measured. It has higher brightness than a blackbody furnace, halogen lamp, and infrared heater, and has a good S / N, and can measure gas concentration.

  FIG. 4 is a configuration diagram of a gas concentration measuring apparatus according to the second embodiment. 4 has a Fourier transform infrared spectrophotometer (FTIR) instead of the grating 8 as compared with the gas concentration measuring apparatus of the first embodiment shown in FIG. It is characterized by. A Fourier transform infrared spectrophotometer (FTIR) is a device that uses an interferometer 14 and measures a spectrum in a non-dispersive and constant wavelength region.

  The gas concentration measuring apparatus according to the second embodiment shown in FIG. 4 includes an SLD 7, a sample cell 2, an SLD driving unit 4a, an interferometer 14, a photodiode 3, and an arithmetic processing unit 5. The interferometer 14 is provided between the SLD 7 and the sample cell 2, and includes a beam splitter 11, a fixed mirror 12, and a movable mirror 13.

  The beam splitter 11 splits the incident SLD light into reflected light and transmitted light. The fixed mirror 12 includes a fixed reflecting mirror, reflects the SLD light from the beam splitter 11, and returns it to the beam splitter 11. The movable mirror 13 is composed of a movable reflecting mirror, reflects the SLD light from the beam splitter 11, and returns it to the beam splitter 11.

  The beam splitter 11 synthesizes the SLD light reflected from the fixed mirror 12 and the SLD light reflected from the moving mirror 13 and guides it to the sample cell 2. The SLD light emitted from the sample cell 2 is interference light. The photodiode 3 receives the interference light from the sample cell 2.

  The arithmetic processing unit 5 can acquire a spectrum displaying each frequency component on the horizontal axis by performing Fourier transform on the interference light from the photodiode 3. The arithmetic processing unit 5 calculates the gas concentration based on the acquired spectrum.

As the method, for example, the spectrum when there is no gas to be absorbed is used as a reference spectrum, and the spectrum that has absorbed the gas is divided by this reference spectrum, the transmittance is obtained. From Lambert-Beer's law, log (T) =-εcl
Is required. T is the transmittance, ε is the absorption coefficient, c is the gas concentration, and l is the optical path length. If ε and l are known, the gas concentration can be determined. Moreover, gas concentration can be measured by using SLD7.

  As described above, according to the gas concentration measuring apparatus according to the second embodiment, by using the interferometer 14 instead of the grating 8 and assembling the Fourier transform infrared spectrophotometer, the wavelength band that could not be measured conventionally is used. Absorption lines can also be acquired, and the concentration of a plurality of gases can be measured.

  In the gas concentration measuring apparatus according to the second embodiment, the same effect as that of the gas concentration measuring apparatus according to the first embodiment can be obtained.

  The gas concentration measuring apparatus according to the present invention can be used for a gas analyzer.

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Sample cell 3 Photodiode 4 Laser drive part 4a SLD drive part 5 Arithmetic processing part 6 Laser beam 7 SLD
8 Grating 9 Photodiode array 12 Fixed mirror 13 Moving mirror 14 Interferometer 16 SLD light
17 Flue

Claims (5)

A super luminescent diode that generates super luminescent diode light; and
A drive unit for driving the superluminescent diode;
A gas introduction part for introducing a gas and entering the superluminescent diode light;
A spectroscopic unit that is provided on the input side or output side of the gas introduction unit, and that splits the superluminescent diode light; and
A light receiving element that receives the superluminescent diode light split by the spectroscopic unit;
An arithmetic processing unit that calculates the concentration of the gas based on the light reception output of the light receiving element;
A gas concentration measuring device comprising:   2. The gas concentration measuring apparatus according to claim 1, wherein the spectroscopic section is formed of a diffraction grating that splits the superluminescent diode light from the gas introduction section.   2. The gas concentration measuring apparatus according to claim 1, wherein the spectroscopic unit includes an interferometer that splits the superluminescent diode light from the superluminescent diode. The interferometer includes a beam splitter that splits the superluminescent diode light from the superluminescent diode;
A fixed mirror that reflects the superluminescent diode light from the beam splitter and guides it to the gas inlet through the beam splitter;
A movable mirror that reflects the super luminescent diode light from the beam splitter and guides it to the gas inlet through the beam splitter;
The gas concentration measuring device according to claim 3, comprising:   5. The gas concentration measuring apparatus according to claim 1, wherein the gas introduction unit is a sample cell that encloses the gas and receives the superluminescent diode light. 6.

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