JP5594041B2 - Gas concentration measuring device - Google Patents

Description

  The present invention relates to a gas concentration measuring apparatus for measuring a gas concentration in a sample cell using a wavelength tunable 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 sample cell 2, a light receiving element 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 sample cell 2. The sample cell 2 encloses a measurement gas and guides the laser light from the semiconductor laser 1 to the light receiving element 3 through the measurement gas. At this time, the laser light that has passed through the sample cell 2 in which the measurement gas is sealed is absorbed by the measurement gas according to the gas concentration.

  The light receiving element 5 detects the amount of transmitted light that is absorbed and reduced by the measurement gas in the sample cell 2. The arithmetic processing unit 5 is, for example, a lock-in amplifier, and measures the concentration of the measurement gas in the sample cell 2 by synchronously detecting twice the frequency of the modulation signal in the transmitted light amount detected by the light receiving element 3.

JP 2001-21493 A

  However, when the laser light emitted from the semiconductor laser 1 is reflected by the light receiving element 3 and enters the light emitting portion of the semiconductor laser 1 as return light, the stimulated emission is confused, the intensity of the light fluctuates, and the resonator The oscillation of the laser beam at becomes unstable.

In addition, the laser light emitted from the semiconductor laser 1 is reflected by the light receiving element 3 and re-reflected on the optical axis such as a window on the optical axis or the inner wall of the cell. When incident, the reciprocal part becomes an optical path difference, which causes fringe noise. Since the optical path difference changes depending on the temperature change of the gas to be measured, the brightness of interference moves. As a result, the signal intensity of the laser beam changed with time, and the gas concentration could not be measured accurately.
An object of the present invention is to provide a gas concentration measuring apparatus capable of significantly reducing the return light to the semiconductor laser and the fringe noise.

In order to solve the above problems, the gas concentration measuring apparatus according to the present invention has a wavelength tunable semiconductor laser that generates laser light and an inclination angle that is set in accordance with the direction of polarization of the laser light from the semiconductor laser. A first polarizing plate that transmits laser light having a specific polarization direction of laser light from the semiconductor laser, and a first polarizing plate that is disposed at an angle of 45 ° with respect to the first polarizing plate. A first quarter-wave plate that circularly polarizes laser light from the plate; a sample cell that encloses a gas and that receives laser light from the first quarter-wave plate; and a traveling direction of the laser light , The second quarter-wave plate that is set to the same tilt angle as that of the first quarter-wave plate and linearly polarizes the laser beam from the sample cell, and the progress of the laser beam The tilt of the first polarizing plate when viewed from the direction It is set to the same angle, and the second polarizing plate that transmits laser light having a specific polarization direction of the laser light from the second quarter wave plate, from the second polarizer And a light receiving element for receiving laser light.

According to the present invention, the first quarter-wave plate disposed at 45 ° with respect to the first polarizing plate and the first polarizing plate is disposed between the semiconductor laser and the sample cell. and when viewed from the traveling direction of the laser beam between the light receiving element, as viewed from the second traveling direction of the polarizing plate and the laser beam is set to the same angle as the inclination angle of the first polarizer, the first 1 Since the second quarter-wave plate set at the same tilt angle as the quarter-wave plate is provided as an isolator, the return light to the semiconductor laser and the fringe noise can be greatly reduced.

It is a block diagram of the gas concentration measuring apparatus of Example 1. It is a figure which shows the arrangement | positioning relationship of the deflection | deviation plate with respect to the optical axis of a laser beam, and the quarter wavelength plate in the gas concentration measuring apparatus of Example 1. FIG. It is a figure which shows the inclination angle of the quarter wavelength plate when a laser beam advances to a P direction in the gas concentration measuring apparatus of Example 1. FIG. It is a figure which shows the inclination angle of the quarter wavelength plate when a laser beam advances to a Q direction in the gas concentration measuring apparatus of Example 1. FIG. It is a block diagram 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. 1 further includes a polarizing plate (corresponding to the first polarizing plate) between the semiconductor laser 1 and the sample cell 2 compared to the gas concentration device shown in FIG. 6 and a quarter wave plate (corresponding to the first quarter wave plate) 8, and polarizing plates (corresponding to the second polarizing plate) 7 and 1 between the sample cell 2 and the light receiving element 3. A quarter wave plate (corresponding to the second quarter wave plate) 9 is provided as an isolator.

  1 is the same as the configuration shown in FIG. 5, the same parts are denoted by the same reference numerals, and the description thereof is omitted. Here, the polarizing plates 6 and 7 and the quarter wave plates 8 and 9 will be described.

  The polarizing plates 6 and 7 are optical elements that allow light having a specific polarization direction to pass therethrough. As shown in FIG. 2A, the polarizing plate 6 has an inclination angle set in accordance with the polarization direction of the laser light from the semiconductor laser 1, and is a straight line in accordance with the polarization direction of the laser light from the semiconductor laser 1. Pass polarized laser light.

  As shown in FIG. 2A, the quarter-wave plate 8 is disposed, for example, by 45 ° clockwise with respect to the inclination angle of the polarizing plate 6, and the linearly polarized laser light from the polarizing plate 6 is received. Circularly polarized light is guided to the sample cell 2.

  As shown in FIG. 2B, the polarizing plate 7 is set to the same tilt angle as the tilt angle of the polarizing plate 6 when viewed from the traveling direction of the laser beam. As shown in FIG. 2B, the quarter wavelength plate 9 is set to the same tilt angle as the tilt angle of the quarter wavelength plate 8 when viewed from the traveling direction of the laser beam.

  The quarter wave plate 9 linearly polarizes the circularly polarized laser beam from the sample cell 2. The polarizing plate 7 passes the linearly polarized laser light from the quarter wavelength plate 9 and guides it to the light receiving element 3. 2A and 2B, O represents the optical axis of the laser beam.

  Next, the operation of the gas concentration measuring apparatus according to the first embodiment configured as described above will be described in detail with reference to FIGS. FIG. 3 is a diagram illustrating the inclination angles of the quarter-wave plates 8 and 9 when the laser light travels in the P direction in the gas concentration measurement apparatus of the first embodiment.

  First, the laser light emitted from the semiconductor laser 1 having a short coherence distance is linearly polarized light, and the polarizing plate 6 is aligned with the polarization direction of the laser light. The linearly polarized light having the tilt angle of the polarizing plate 6 is obtained. The linearly polarized light signal having the tilt angle of the polarizing plate 6 at this time is indicated by V1 in FIG.

  Then, as shown in FIG. 3 (a), a linearly polarized light signal V1 (90 ° component) and a polarized light signal V2 ( The linearly polarized light is converted into circularly polarized light C1.

  Next, as shown in FIG. 3 (b), the circularly polarized light C1 is converted into linearly polarized light (signal V1) by the quarter wavelength plate 9 disposed at 45 ° clockwise with respect to the polarizing plate 7. The

  Further, since the polarizing plate 7 is arranged so as to match the polarization direction of the linearly polarized laser light, the laser light that has passed through the polarizing plate 7 becomes linearly polarized light having the inclination angle of the polarizing plate 7, and the light receiving element. 3 is received.

  On the other hand, when the laser light (returned light) reflected and returned by the light receiving surface of the light receiving element 3 passes through the polarizing plate 7 and passes through the quarter wavelength plate 9, the linearly polarized light returns to circularly polarized light. This return light has a circularly polarized light in the reverse direction.

  The operation at this time will be described with reference to FIG. FIG. 4 is a diagram showing the tilt angles of the quarter-wave plates 8 and 9 when the laser light travels in the Q direction, that is, when it is return light. As shown in FIGS. 4A and 4B, the quarter-wave plates 8 and 9 are counterclockwise with respect to the linearly polarized signal V3 (same as the linearly polarized signal V1 with a 90 ° component) when returning light. Tilted 45 °.

  The return light from the light receiving element 3 is linearly polarized by the polarizing plate 7 and is guided to the quarter-wave plate 9 as a linearly polarized signal V3 (90 ° component) as shown in FIG.

  As shown in FIG. 4 (a), a linearly polarized light signal V3 (90 ° component) and a polarized light signal V4 (180) are provided by a quarter wave plate 9 that is inclined 45 ° counterclockwise with respect to the polarizing plate 7. The linearly polarized light is converted into circularly polarized light.

  Next, the circularly polarized light is converted into linearly polarized light (signal V4) as shown in FIG. 4 (b) by the quarter wavelength plate 8 which is disposed 45 ° counterclockwise with respect to the polarizing plate 6. The The direction of the linearly polarized light (signal V4) is inclined by 90 ° with respect to the linearly polarized light (signal V1) of the laser light emitted from the semiconductor laser 1.

  For this reason, the return light is blocked by the polarizing plate 6 so that the return light is not incident on the semiconductor laser 1 and the laser oscillation can be stably performed.

  The laser light emitted from the semiconductor laser 1 is reflected by the light receiving element 3, re-reflected by the window on the optical axis, the inner wall of the sample cell 2, etc., and the re-reflected laser light is incident on the light receiving element 3 again. This round trip was an optical path difference and fringe noise.

  On the other hand, in the gas concentration measuring apparatus according to the first embodiment, when the laser light from the light receiving element 3 is incident on the quarter wavelength plate 9, as shown in FIG. 9 is inclined 45 ° counterclockwise with respect to the linearly polarized light signal V3, and when the laser light from the sample cell 2 is incident on the quarter-wave plate 9, as shown in FIG. The quarter-wave plate 9 is inclined 45 ° clockwise with respect to the linearly polarized light signal V1.

That is, when the laser light from the light receiving element 3 is incident on the quarter-wave plate 9 and when the laser light from the sample cell 2 is incident on the quarter-wave plate 9, the direction of circularly polarized light is reversed. It is facing. Therefore, the circularly polarized light of the laser light from the sample cell 2 is converted into linearly polarized light (signal V2) by the quarter wavelength plate 9. The direction of this linearly polarized wave (signal V2) is inclined by 90 ° with respect to the linearly polarized wave (signal V3) of the laser light from the light receiving element 3. For this reason, the laser light re-reflected by the window on the optical axis or the inner wall of the sample cell 2 is blocked by the polarizing plate 7 and is not incident on the light receiving element 3. For this reason, fringe noise can be significantly reduced.
In the gas concentration measuring apparatus of Example 1, a polarizing plate 6 and a quarter wavelength plate 8 are provided as an isolator between the semiconductor laser 1 and the sample cell 2, and the sample cell 2 and the light receiving element 3 are provided. A polarizing plate 7 and a quarter-wave plate 9 are provided between them. As an isolator, a Faraday rotator is provided between the semiconductor laser 1 and the sample cell 2 and between the sample cell 2 and the light receiving element 3. May be.

  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 Light receiving element 4 Laser drive part 5 Arithmetic processing part 6, 7 Deflection plate 8, 9 1/4 wavelength plate

Claims (1)

A wavelength tunable semiconductor laser that generates laser light;
A first polarizing plate having a tilt angle set in accordance with a direction of polarization of laser light from the semiconductor laser, and passing laser light having a specific polarization direction of laser light from the semiconductor laser;
A first quarter-wave plate disposed at an angle of 45 ° with respect to the first polarizing plate and circularly polarizing laser light from the first polarizing plate;
A sample cell enclosing gas and receiving laser light from the first quarter-wave plate;
A second quarter-wave plate that is set to the same tilt angle as that of the first quarter-wave plate as viewed from the traveling direction of the laser beam and linearly polarizes the laser beam from the sample cell; ,
Laser light having a specific polarization direction within the laser light from the second quarter-wave plate set to the same angle as the tilt angle of the first polarizing plate when viewed from the traveling direction of the laser light A second polarizing plate that passes through,
A light receiving element for receiving the laser light from the second polarizing plate;
A gas concentration measuring device comprising:

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