Gas Sensor

Abstract

This gas sensor comprises a plurality of resonators (4a, 4b, 4c) having different resonator lengths and simultaneously generating a plurality of laser beams from exit light from one gain medium (3), the laser beams having different wavelengths. Light receiving elements (9a, 9b, 9c) detect the plurality of laser beams. Inside the plurality of resonators (4a, 4b, 4c), the plurality of laser beams are caused to interfere with a measured gas (10).

Description

[Technical Field]

[0001] The present disclosure relates to gas sensors. [Background technology]

[0002] Gas molecules have a specific wavelength light absorption spectrum, so the gas type is determined by causing a laser beam with a narrow spectral linewidth to interfere with the gas and detecting the absorption of the laser beam by the gas (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent No. 3304846 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional gas sensors use multiple lasers with different emission wavelengths to detect multiple gas species, which leads to the problem of large size of the gas sensor. Also, lasers are expensive, which leads to increased costs.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its object is to provide a small, inexpensive gas sensor. [Means for solving the problem]

[0006] The gas sensor according to the present disclosure includes a gain medium, a plurality of resonators having different resonator lengths and configured to simultaneously generate a plurality of laser beams having different wavelengths from the light emitted from the gain medium, and a light-receiving element configured to detect the plurality of laser beams, and is characterized in that the plurality of laser beams are made to interfere with a gas to be measured inside the plurality of resonators. [Effects of the Invention]

[0007] In this disclosure, by using multiple resonators with different resonator lengths, it is possible to obtain laser beams of multiple wavelengths even with a single gain medium. Multiple laser beams are made to interfere with the gas under measurement inside the multiple resonators, and the absorption of the multiple laser beams by the gas under measurement is detected, thereby enabling the determination of multiple gas types. Since only one expensive gain medium is required, and only one set of components, such as a power supply circuit for driving the gain medium, is required, a small and inexpensive gas sensor can be realized. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram showing a gas sensor according to a first embodiment; [Figure 2] FIG. 1 is a diagram showing absorption spectra of main gases. [Figure 3] This is an extracted diagram of the absorption spectrum of methane gas from Figure 2. [Figure 4] FIG. 10 is a diagram showing a gas sensor according to a second embodiment. [Figure 5] FIG. 10 is a diagram showing a gas sensor according to a third embodiment. [Figure 6] FIG. 10 is a diagram showing a modified example of the gas sensor according to the third embodiment. [Figure 7] FIG. 10 is a diagram showing a gas sensor according to a fourth embodiment. [Figure 8] FIG. 4 is a diagram showing the light intensity detected by a light receiving element. DETAILED DESCRIPTION OF THE INVENTION

[0009] A gas sensor according to an embodiment will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repeated description may be omitted.

[0010] Embodiment 1 FIG. 1 is a diagram showing a gas sensor according to a first embodiment. A gain medium 3 is disposed between a mirror 1 and multiple semi-transparent mirrors 2a, 2b, and 2c. The mirror 1 and the semi-transparent mirror 2a are like a pair of mirrors and form a resonator 4a. Similarly, the mirror 1 and the semi-transparent mirror 2b form a resonator 4b, and the mirror 1 and the semi-transparent mirror 2c form a resonator 4c. However, the distances between the mirror 1 and the semi-transparent mirrors 2a, 2b, and 2c are different from one another. Therefore, the mirror 1 and the multiple semi-transparent mirrors 2a, 2b, and 2c form multiple resonators 4a, 4b, and 4c having different resonator lengths.

[0011] The gain medium 3 is a semiconductor optical amplifier or an Er-doped fiber that amplifies the intensity of the light passing through it. When a voltage is applied to the gain medium 3, it emits broad natural light with a wide wavelength range. The optical waveguide 5 branches the light emitted from the gain medium 3 into multiple beams. The optical waveguide 5 is an optical fiber or the like, and may be a waveguide formed on a substrate made of Si or SiO2. The optical waveguide 5 and the gain medium 3 may be directly connected, or a lens or an MMI (Multimode Interferometer) coupler may be inserted between them. The light emitted from the optical waveguide 5 into air is emitted at a certain angle due to the refractive index difference. Lenses 6a, 6b, and 6c convert the multiple beams emitted from the optical waveguide 5 into parallel collimated beams and provide them to the multiple resonators 4a, 4b, and 4c, respectively.

[0012] When broad light is introduced into the resonator, laser light with a wavelength corresponding to the resonator length is generated. Therefore, multiple resonators 4a, 4b, and 4c with different resonator lengths simultaneously generate multiple laser light beams with different wavelengths from the light emitted from the gain medium 3. Each laser light beam has a narrow spectral linewidth. Note that if the resonator lengths are similar and the oscillation wavelengths are close, interference may occur and stable laser light may not be obtained.

[0013] A gas cell 7 made of transparent glass or the like is inserted inside the multiple resonators 4a, 4b, and 4c. The gain medium 3, the resonators 4a, 4b, and 4c, and the optical system including lenses 6a, 6b, and 6c are hermetically sealed in a case, and the inside of the gas cell 7 is exposed to the outside. The lenses 6a, 6b, and 6c and lenses 8a, 8b, and 8c may also function as partitions to the gas cell 7. The windows of the resonators 4a, 4b, and 4c may also serve as two parallel surfaces of the gas cell 7.

[0014] Lenses 8a, 8b, and 8c converge the multiple laser beams that have passed through semi-transparent mirrors 2a, 2b, and 2c onto the input surfaces of light-receiving elements 9a, 9b, and 9c. If there is an optical waveguide in front of the light-receiving elements 9a, 9b, and 9c, lenses 6a, 6b, and 6c adjust the beam diameter to converge the light onto the optical waveguide. The light-receiving elements 9a, 9b, and 9c each detect the multiple laser beams.

[0015] A gas cell 7 is supplied with a gas 10 to be measured, and multiple laser beams are made to interfere with the gas 10 inside the resonators 4a, 4b, and 4c. If the gas 10 to be measured contains a gas component having an absorption spectrum corresponding to the wavelength of the laser beam, the intensity of the laser beam decreases.

[0016] The detector 11 detects the absorption of the laser beams by the measurement gas 10 from the outputs of the light-receiving elements 9a, 9b, and 9c, thereby determining the type and concentration of the measurement gas 10. Specifically, the gas type can be determined from the wavelength of the laser beam with reduced intensity, and the gas concentration can be determined from the amount of change in the intensity of the laser beam.

[0017] The intensities of laser light of multiple wavelengths are detected by a photodetector when the measurement gas 10 is not present and air is introduced into the gas cell 7. The detection results of the photodetectors 9a, 9b, and 9c are stored in advance in the memory unit 12 as reference data. The detection unit 11 compares the detection results of the photodetectors 9a, 9b, and 9c with the reference data to determine the gas type and gas concentration of the measurement gas 10. Alternatively, the laser light intensity may be detected using a standard sample whose gas type and gas concentration are known, and the detected intensity may be recorded as reference data. In this case, the measurement gas 10 is introduced into the sensor, and the detected intensity of the laser light is compared with the reference data to calculate the gas type and gas concentration.

[0018] Figure 2 shows the absorption spectra of major gases. Figure 3 shows an extracted absorption spectrum of methane gas from Figure 2. Figure 2 shows the general band in which the absorption spectrum exists, but in reality, as shown in Figure 3, multiple fine absorption spectra exist within the band.

[0019] As described above, in this embodiment, by using multiple resonators 4a, 4b, and 4c with different resonator lengths, laser light of multiple wavelengths can be obtained even with a single gain medium 3. Multiple laser light beams are made to interfere with the measured gas 10 inside the multiple resonators 4a, 4b, and 4c, and absorption of the multiple laser light beams by the measured gas 10 is detected, thereby making it possible to determine multiple gas types. Since only one expensive gain medium 3 is required and only one set of components such as a power supply circuit for driving the gain medium 3 is required, a small and inexpensive gas sensor can be realized.

[0020] The gas sensor according to this embodiment is used, for example, as an odor sensor. The resonator lengths of the resonators 4a, 4b, and 4c are set so as to generate laser light of a wavelength corresponding to the gas to be detected. If there are three resonators 4a, 4b, and 4c, three types of gases, for example, ammonia, carbon dioxide, and nitrous oxide, can be detected. Furthermore, if five types of gases including methane and hydrogen chloride are to be detected, five resonators should be used.

[0021] The oscillation wavelength of the laser light is determined by the resonator length and the refractive index of the optical path, but strictly speaking, the refractive index changes depending on the type and concentration of gas in the optical path, so the oscillation wavelength changes slightly. However, since this method is intended for gas detection in everyday life, it is not expected to have a significant impact. When detecting high-concentration gases, gas concentrations can be detected accurately by combining it with a peak search using wavelength scanning.

[0022] Embodiment 2 FIG. 4 is a diagram showing a gas sensor according to a second embodiment. Optical switches 13a, 13b, and 13c switch whether or not to pass incident light. Laser light of different wavelengths emitted from three resonators 4a, 4b, and 4c is passed through optical switches 13a, 13b, and 13c in turn, and is incident on one light-receiving element 9. In other words, gas type identification is time-shared. This makes it possible to reduce the number of light-receiving elements 9. The other configurations and effects are the same as those of the first embodiment.

[0023] Embodiment 3 5 is a diagram showing a gas sensor according to a third embodiment. The distances between the mirror 1 and each of the plurality of semi-transparent mirrors 2a, 2b, and 2c are the same. A plurality of delay elements 14a, 14b, and 14c are provided in each of the plurality of resonators 4a, 4b, and 4c, respectively. The refractive indices of the delay elements 14a, 14b, and 14c are different from one another.

[0024] The delay devices 14a, 14b, and 14c are elements used for signal modulation in optical communication, and are made of an insulator, LiNbO 3、 Or it is made of semiconductors such as InP or Si. A retarder made of LiNbO3 adjusts the refractive index by the Pockels effect caused by applying voltage. A retarder made of InP or Si adjusts the refractive index by the thermo-optic effect or carrier plasma effect caused by current.

[0025] When the refractive index differs, the light travels at a different speed. Therefore, when the refractive index is changed, the effective cavity length for the laser light changes, and the oscillation wavelength changes. Therefore, since the multiple cavities 4a, 4b, and 4c have different cavity lengths, multiple laser lights having different wavelengths can be simultaneously generated from the output light of the gain medium 3. The other configurations are the same as in embodiment 1, and the same effects as in embodiment 1 can be obtained.

[0026] FIG. 6 is a diagram showing a modified example of the gas sensor according to the third embodiment. In the configuration shown in FIG. 5, the positions of multiple semi-transparent mirrors 2a, 2b, and 2c are shifted to roughly tune to the target oscillation wavelength, and then the delay devices 14a, 14b, and 14c are used to fine-tune the wavelength and increase the resolution. On the other hand, in the configuration shown in FIG. 6, the delay devices 14a, 14b, and 14c have sufficient dynamic range and resolution. In this case, the oscillation wavelength can be controlled by the delay devices 14a, 14b, and 14c, eliminating the need to control the oscillation wavelength by adjusting the position of the semi-transparent mirrors. Therefore, a single semi-transparent mirror 2 can be used regardless of the number of laser light wavelengths being handled. In other words, multiple resonators 4a, 4b, and 4c share a single mirror 1 and a single semi-transparent mirror 2 facing each other.

[0027] Embodiment 4 FIG. 7 is a diagram showing a gas sensor according to a fourth embodiment. The temperature adjustment unit 15 adjusts the temperature of the gain medium 3 to adjust the refractive index of the gain medium 3. When the gain medium 3 is a semiconductor amplifier, the temperature adjustment unit 15 may be, for example, a Peltier element or a current source for temperature adjustment using the self-heating of the semiconductor amplifier. The position adjustment unit 16 is a piezoelectric element or MEMS that physically moves the semi-transparent mirror 2, and adjusts the position of the semi-transparent mirror 2. The position adjustment unit 16 may also be configured to individually adjust the positions of the semi-transparent mirrors 2a, 2b, and 2c. The other configurations are the same as those of the first to third embodiments.

[0028] Figure 8 shows the light intensity detected by the light-receiving element. The difference between the gas detection peak, which occurs when the measured gas 10 absorbs the laser light, and the background level of the sensor, including contamination and aging, is detected. The temperature of the gain medium 3 is then adjusted to determine the required oscillation wavelength range. The positions of the semi-transparent mirrors 2, 2a, 2b, and 2c are adjusted to manipulate the laser light within the absorption spectrum range of the measured gas 10. The delay devices 14a, 14b, and 14c are adjusted to individually set the oscillation wavelengths of the resonators 4a, 4b, and 4c. By adjusting the temperature of the gain medium 3, the positions of the semi-transparent mirrors 2, 2a, 2b, and 2c, or the refractive index of the delay devices 14a, 14b, and 14c to scan the oscillation wavelengths of the resonators 4a, 4b, and 4c, the accuracy of gas detection can be improved. [Explanation of symbols]

[0029] 1 mirror, 2 semi-transparent mirror, 3 gain medium, 4a, 4b, 4c resonator, 5 optical waveguide, 6a, 6b, 6c lenses, 9, 9a, 9b, 9c light receiving element, 11 detection unit, 12 memory unit, 13a, 13b, 13c optical switch, 14a, 14b, 14c delay unit, 15 temperature adjustment unit, 16 position adjustment unit

Claims

1. a gain medium; a plurality of resonators having different resonator lengths and configured to simultaneously generate a plurality of laser beams having different wavelengths from the output light of the gain medium; a light receiving element for detecting the plurality of laser beams; A gas sensor characterized in that the plurality of laser beams are made to interfere with the gas to be measured inside the plurality of resonators.

2. 2. The gas sensor according to claim 1, further comprising a detection unit for determining the type of the measurement gas by detecting absorption of the plurality of laser beams by the measurement gas from the output of the light receiving element.

3. Further comprising a storage unit that stores reference data; 3. The gas sensor according to claim 2, wherein the detection section compares the detection result of the light receiving element with the reference data to determine the type or concentration of the measurement gas.

4. an optical waveguide that splits the output light of the gain medium into a plurality of beams; 4. The gas sensor according to claim 1, further comprising a lens that converts each of the plurality of light beams into collimated light and provides the collimated light to each of the plurality of resonators.

5. 4. The gas sensor according to claim 1, further comprising an optical switch that causes the laser beams emitted from the plurality of resonators to be incident on the light-receiving element in sequence.

6. 4. The gas sensor according to claim 1, further comprising a plurality of delay elements each having a refractive index different from one another and provided in each of the plurality of resonators.

7. 7. The gas sensor according to claim 6, wherein the plurality of resonators share one mirror and one semi-transparent mirror that face each other.

8. a temperature adjusting unit for adjusting the temperature of the gain medium; 4. The gas sensor according to claim 1, further comprising a position adjustment unit that adjusts the positions of the mirrors of the plurality of resonators.

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