JP7437022B2 - gas analyzer - Google Patents

Description

The present invention relates to a laser gas analyzer, and more particularly to a laser gas analyzer whose laser frequency can be adjusted.

In a laser-type gas analyzer, it is necessary to control the laser frequency used near the absorption line of the gas to be analyzed. When the laser oscillation element is a semiconductor, this control is performed by controlling the drive voltage (proportional to the drive current) applied to the laser oscillation element and the temperature of the laser oscillation element.

Therefore, the relationship between the laser frequency and the drive current and temperature of the laser oscillation element is investigated in advance, and the values of the drive current and temperature are set so that the laser oscillates at the desired frequency. However, even if the drive current and temperature values are set appropriately, the laser characteristics change over time, and this causes a phenomenon in which the oscillation frequency of the laser shifts. To perform highly accurate measurements, it is necessary to periodically correct this shift.

In order to correct this shift in the oscillation frequency over time, a gas analyzer incorporating a wavelength monitor (Patent Document 1) and a reference gas cell (Patent Document 2) has been proposed.

FIG. 5 is a conceptual diagram when the reference gas cell is used.

A sweep voltage of a predetermined width corresponding to the drive current is applied to the laser oscillation element 1 from the drive circuit 10, and a laser beam of a predetermined bandwidth is output from the laser oscillation element 1. Laser light from the laser oscillation element 1 is introduced into the optical fiber 11 via the optical fiber port 5, split by the splitter 15, guided via the optical fiber 12 to the measurement cell 20 by the collimator 14, and then via the optical fiber 13 to the collimator. 25 to the reference gas cell 300.

The laser beam that has passed through the measurement cell 20 is received by the light receiving element 21, and is input to the concentration calculating means 100, where it is used to calculate the concentration of the target substance.

On the other hand, a known substance is sealed in the reference gas cell 300 at a known concentration. The laser beam guided to the reference gas cell 300 by the collimator 25 as described above is received by the light receiving element 30 and input to the reference value detection means 200. The reference value detection means 200 obtains an absorption line at a frequency corresponding to the known substance and an absorption intensity corresponding to the known concentration. If the center position of the absorption line obtained here deviates from the set position, the sweep voltage is controlled so that the drive circuit 10 corrects it.

On the other hand, moisture in the atmosphere is considered to be a component that interferes with measurements, and in order to suppress its effects, the optical path is placed in a vacuum container or dry gas is introduced into the optical path. (Patent Document 3).

Japanese Patent Application Publication No. 2006-234810 Japanese Patent Application Publication No. 2008-232920 Japanese Patent Application Publication No. 2001-41877

As mentioned above, a gas analyzer that incorporates a wavelength monitor or a reference gas cell naturally has a large cost disadvantage, and if a reference gas cell is used, the device volume is large, making it unsuitable for portable use. be. Furthermore, there is a problem in that the reference gas cell itself deteriorates over time (gas leakage), which requires maintenance and expenses.

The present invention has been proposed in view of the above-mentioned conventional circumstances, and an object of the present invention is to present a simpler method and structure for correcting the laser frequency.

In the present invention, a driving voltage of a predetermined width is applied to a laser oscillation element installed in a laser device to obtain a laser beam, and the laser beam is transmitted within the laser device with a predetermined distance a between the laser oscillation element and the laser oscillation element. The concentration of the analyte is determined by receiving it at the oscillation-side optical fiber port arranged at a constant temperature, inputting it from the optical fiber port to a collimator via an optical fiber, and passing from the collimator through a measurement cell filled with a gas containing the analyte. A gas analyzer for measuring the following includes a branch circuit, a detector, a center voltage detection means, and a center voltage setting means.

The branch circuit branches the optical fiber into two branches, with the optical fiber to the collimator serving as one optical fiber, and the remaining optical fiber serving as the other optical fiber. The photodetector includes a light-receiving side optical fiber port arranged at the tip of the other optical fiber, and a light-receiving element arranged at a predetermined distance b from the light-receiving side optical fiber port and receiving laser light from the light-receiving side optical fiber port. Equipped with

The center voltage detection means detects a center voltage of an absorption line of moisture in the atmosphere, which is obtained by the light receiving element and is formed by a total distance of a predetermined interval a and a predetermined interval b . The center voltage setting means sets the center voltage in a drive circuit of the laser oscillation element .

In the present invention, when an optical resonator is used as the measurement cell, the reflected light from the mirror on the laser beam incident side of the optical resonator is returned to the one optical fiber via the collimator, and the optical fiber is connected to the optical fiber. and a return circuit leading from the optical fiber to the other optical fiber. At this time, the center voltage detection means detects the center voltage of the atmospheric moisture absorption line formed by the predetermined interval a, the predetermined interval b, and a total distance twice the distance c between the collimator and the mirror. do.

FIG. 1 is a diagram showing one embodiment of the present invention. FIG. 7 is a diagram showing another embodiment of the present invention. FIG. 3 is a diagram showing the relationship between absorption lines and driving voltage. A diagram showing the absorption wavenumber of water. FIG. 2 is a diagram showing a conventional example using a standard reference cell.

<Principle>
When a laser beam with a wavelength band that water absorbs (wavelengths of 1.4 μm, 1.8 μm, 2.6 μm, and 6.7 μm) is emitted into the atmosphere and received at a predetermined interval (for example, 2 cm), the received light intensity will be the same as that of the atmosphere. It shows an absorption line corresponding to the water concentration in the liquid (see Fig. 4, however, Fig. 4 is expressed in wave numbers).

Water absorption lines exist in the infrared to near-infrared range, and relatively strong absorption lines (absorption lines with a line intensity of 1.0 × 10 ^-20 cm or more in the HITRAN database) are 330 as shown in Figure 4. There are books. These absorption lines can be easily detected even in a low-humidity measurement environment at a temperature of 20 °C and a relative humidity of 30% RH, because if the optical path length is 2 cm, the amount of absorption is about 1% of the laser light. can be observed.

In a normal measurement environment, the humidity is higher than the above, so absorption lines can be observed more easily.

Therefore, by passing a laser beam through the atmosphere at a predetermined distance in one of the above wavelength bands in which water is absorbed, an absorption line of moisture present in the atmosphere is obtained.

The obtained absorption line signal is processed to find the center frequency. As an example, Fig. 3 shows the results obtained by fitting a signal using the Lorentz function (the shape of the absorption line under atmospheric pressure can be explained by the Lorentz function). If there is no change in the laser oscillation element over time, there will be no change in the relationship between the center frequency of the absorption line obtained as described above and the sweep voltage. However, if there is a change over time, the center frequency of the absorption line measured The frequency has a value different from the voltage (see S ₀ in FIG. 3(a) → S ₁ in FIG. 3(b)). In this case, by correcting the applied voltage corresponding to the center frequency after the change so that it is located at the center of the new sweep voltage, the shift in the laser oscillation frequency due to changes over time is corrected, and highly accurate measurement is possible. Become.

The center frequency of the absorption line can be obtained from a database such as HITRAN. Furthermore, although this center frequency slightly depends on the atmospheric pressure, even if the atmospheric pressure fluctuates by ±0.1 atm, the change in the central wave number due to this change is less than ±0.0013 cm ^-1 , which is necessary for quantitative gas analysis using the laser method. It is a negligible value at the measurement accuracy of the wave number of about 0.01 cm ^-1 .
<Device>
FIG. 1 is a functional block diagram of a device based on the above principle that corrects the frequency of laser light emitted from a laser oscillation element.

The drive circuit 10 drives the laser oscillation element 1 with a drive current proportional to the sweep voltage, which varies in a predetermined width. Thereby, the laser beam from the laser oscillation element 1 is set to vary in frequency within a predetermined range centered on the absorption frequency of the substance to be analyzed.

The laser beam from the laser oscillation element 1 is received by the optical fiber port 5 through the atmosphere at a predetermined distance a (for example, about 1 cm) inside the laser device 19, and is branched into two paths via the optical fiber 11 and the splitter 15. One of the branches is inputted into the measurement cell 20 via the optical fiber 12 and the collimator 14, and is subjected to gas analysis. The light emitted from the measurement cell 20 is photoelectrically converted by the light receiving element 21 and is passed to the concentration calculating means 100. Note that the distance a is not intentionally provided, but is a distance that inevitably occurs due to the design of the laser device 19 that incorporates the laser oscillation element 1. The same applies to distance b and distance c described below.

The other side branched by the splitter 15 is emitted into the atmosphere inside the photodetector 31 via the optical fiber 13 and the optical fiber port 6, and is sent to the light receiving element 30 arranged at a predetermined interval b (for example, 1 cm). Light is received. The output from the light receiving element 30 is input to the center voltage detection means 40. As mentioned above, the shape of the moisture absorption line under atmospheric pressure can be explained by the Lorentz function, so the center voltage detection means 40 obtains the absorption line from the output of the light receiving element 30 and fits it with the Lorentz function to determine the center The center voltage S ₀ corresponding to is detected (described later, FIG. 3(a)).

This result is input to the center voltage setting means 50, and the center voltage setting means 50 sets this value _S0 as the center value of the sweep voltage of the drive circuit 10. Thereafter, the drive circuit 10 drives the laser oscillation element 1 with a sweep voltage such that _S0 is the center value of the sweep voltage.

Then, after a predetermined period of time, the same process as above is performed again, and when the center voltage detection means 40 obtains the center voltage _S1 corresponding to the center of the absorption line, the center voltage setting means 50 drives that value. The driving circuit 10 drives the laser oscillation element 1 with a sweep voltage centered around the voltage _S1 (FIG. 3(b)).

Here, in FIG. 1, the temperature of the laser oscillation element 10 is fixed at 21 °C, the drive current is swept in the range of 90 mA to 155 mA, and the predetermined distance a is 1 cm, the predetermined distance b is 1 cm, and a total of 2 cm. The absorption lines of water existing in the atmospheric space were measured, and the absorption lines shown in Figure 3 were obtained.

Figure 3(a) shows the first measurement data. The vertical axis represents the absorbance, and the horizontal axis represents the sweep voltage applied to the drive circuit 10, which is proportional to the drive current. As described above, the fitting means 40 fitted the data obtained from the light receiving element 30 with a Lorentz function to obtain a center voltage of 0.17448 V corresponding to the center position of the absorption line. This value corresponds to the central wavenumber of this absorption line in air, 7181.14 cm ^-1 (referenced to the HITRAN database).

Figure 3(b) shows the results of a second run of the same experiment performed 12 weeks later. The center voltage in this figure is 0.17163 V, and it can be seen that the center of the absorption line shifts due to changes in laser characteristics over time. This amount of shift corresponds to 0.04 cm ^-1 in wave number. The center voltage setting means 50 detects the center voltage 0.17163 V, resets the 0.17163 V to correspond to a wave number of 7181.14 cm ^-1 , and passes it to the drive circuit 10 based on this new set value. Modifying the voltage can compensate for shifts in laser frequency that occur over a 12-week period.

The results also show that this laser oscillation device requires correction about once a week to ensure the wavenumber measurement accuracy of 0.01 cm ^-1 required for quantitative gas analysis. Therefore, in FIG. 1, the center voltage detecting means 40 and the center voltage setting means 40 are activated periodically (every time the power is turned on, or about once a week) to set the drive circuit 10 to a new center voltage. , the drive circuit 10 changes the median value of the sweep voltage accordingly. Alternatively, when an update is required, the update button may be turned on to activate the center voltage detection means 40 and the center voltage setting means 40 and pass a new center voltage to the drive means 10.

In the above, the laser beam is split by the splitter 15 and one of the laser beams is used to measure moisture in the atmosphere. When the reflected light from mirror 22 becomes strong enough, as shown in FIG. Then, it may be measured by the light receiving element 30.

In this case, the spatial distance a from the laser oscillation element 10 to the optical fiber port 5, the spatial distance b from the optical fiber port 6 to the light receiving element 30, and the spatial distance c from the collimator 14 to the high reflectance mirror 22 are used. However, since the spatial distance c is a round trip between the collimator 14 and the reflectance mirror 22, the distance is doubled, and since this distance is easy to adjust, the intensity of the absorption line can be easily increased with this method. can.

The collimator 14, splitter 15, circulator 16, laser device 19, and photodetector 31 are commercially available small-sized products that can be connected to the optical fibers 11, 12, and 13 via connectors, and these can be used. Therefore, compared to the case where a wavelength monitor or a reference gas cell is introduced, the installation space can be significantly reduced, so the present invention is effective in downsizing the gas analyzer.

In addition, here we have shown an example of detecting the center voltage by fitting with a Lorentz function, but other functions (Voigt function, quadratic function, etc.) may be used as the fitting function, or the intensity of the absorption line may be detected without fitting. The center voltage may be detected from the maximum value of .

Although only the analysis of moisture has been described above, the present invention is applicable to the analysis of gases containing substances that also have absorption frequencies near the wavelength bands where water absorbs (wavelength bands of 1.4 μm, 1.8 μm, 2.6 μm, and 6.7 μm). Of course, it can be used. Note that the center voltage detection means and the center voltage setting means can be realized not only as a circuit but also as a program linked to a computer.

As described above, the present invention is applicable not only to moisture analysis but also to gas analyzers that contain substances that have absorption frequencies near the wavelength bands where water absorbs (wavelength bands of 1.4 μm, 1.8 μm, 2.6 μm, and 6.7 μm). available and extremely effective.

1... Laser oscillation element 5, 6... Optical fiber port 10... Drive circuit 11, 12, 13... Optical fiber 14, 25... Collimator 15... Splitter 16... Circulator 19... Laser device 20... Measurement cell 21, 30... Light receiving element 22, 23... High reflectance mirror 31... Photodetector 40... Center voltage detection means 50... Center voltage setting means 100... Concentration calculation means 200... Reference value detection 300.・Reference gas cell 400...Optical resonators a, b, c...Distance

Claims (2)

A driving voltage of a predetermined width is applied to a laser oscillation element installed in a laser device to obtain laser light, and the laser light is arranged within the laser device with a predetermined distance a between it and the laser oscillation element. A gas that is received at an oscillation side optical fiber port, inputted from the optical fiber port to a collimator via an optical fiber, and passed from the collimator through a measurement cell filled with a gas containing the analyte, thereby measuring the concentration of the analyte. In the analyzer,
a branching circuit that branches the optical fiber into two branches, with the optical fiber to the collimator serving as one optical fiber and the remaining optical fiber serving as the other optical fiber;
a photodetector comprising a light-receiving optical fiber port disposed at the tip of the other optical fiber; and a light-receiving element arranged at a predetermined distance b from the light-receiving optical fiber port and receiving laser light from the light-receiving optical fiber port. and,
center voltage detection means for detecting a center voltage of an absorption line of atmospheric moisture obtained by the light receiving element and formed by the total distance of the predetermined interval a and the predetermined interval b ;
A gas analyzer comprising: center voltage setting means for setting the center voltage in a drive circuit of the laser oscillation element . An optical resonator is used as the measurement cell, and reflected light from a mirror on the laser beam incident side of the optical resonator is returned to the one optical fiber via the collimator, and guided from the one optical fiber to the other optical fiber. Equipped with a return circuit,
2. The center voltage detection means detects a center voltage of an atmospheric moisture absorption line formed by a total distance of the predetermined interval a, the predetermined interval b, and twice the distance c between the collimator and the mirror. 1. The gas analyzer according to 1.

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