JPH11344434A - Optical absorption cell device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical absorption cell device for accurately measuring the amount of optical spectrum absorption of a sample gas by excluding an optical interference noise to be generated attended with change of a light path length. SOLUTION: When an absorption detection part 8A comprising an optical absorption cell device 20A samples a light intensity being detected by a light reception part 6 and detects the amount of optical spectrum absorption, at PZT(piezoelectric transducer) 5 is displaced through a PZT drive part 9A according to a sweep signal to change the multiple reflection light path length relating to mirrors 4A, 4B, and 4C and observes at least one period of the optical interference noise. Since the central value of the top and bottom of the observation waveform is detected as the amount of light spectrum absorption, the optical interference noise, that is, an amount of light spectrum absorption excluding what is called a fringe, can be measured accurately.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical absorption cell device suitable for application to a laser spectroscopic analyzer for irradiating a sample gas with a laser beam and observing its light absorption spectrum.

[0002]

2. Description of the Related Art First, the background of the present invention will be described.

[0003] The laser spectroscopic analysis technique mainly measures a trace molecular weight by optical spectrum analysis of a gas, and one of its promising applications is the measurement of isotopes.

Techniques for observing changes in isotopes can be used for diagnosing diseases in the medical field, researching photosynthesis and plant metabolism in the agricultural field, and tracing ecosystems in the earth science field.

[0005] Isotopes used in such applications include nitrogen, oxygen and the like. As is well known, carbon has a mass number of 12 (hereinafter simply abbreviated as ¹² C) and a mass number of ¹² . There are 13 stable isotopes (hereinafter simply abbreviated as ¹³ C). These stable isotopes, unlike radioisotopes, are not exposed to radiation and are easy to handle. Has been studied.

Conventionally, there has been an infrared spectrometer as a carbon isotope analyzer for such use. This apparatus used a lamp having a wide emission wavelength range in the infrared region as a light source, selected a light wavelength using a dispersion type spectroscope or the like, and observed the spectral absorption intensities of ¹² CO ₂ and ¹³ CO ₂ . However, in this method, the optical wavelength selection performance of the dispersion type spectrometer becomes a bottleneck,
Sufficient optical wavelength resolution was not obtained, and the measurement accuracy of carbon isotopes was not sufficient.

Further, as another apparatus for such a use,
There is a mass spectrometer. This device can measure carbon isotopes with high accuracy because it measures the mass of the molecule itself, but has the drawback that the device is large and difficult to handle, and the device price is very expensive.

Here, in the laser spectroscopic analysis method, a predetermined light wavelength is obtained by stabilizing the temperature and current of a semiconductor laser used as a light source, and gas molecules are changed by a change in light intensity of a laser beam after passing through a sample gas. The amount of light spectrum absorption due to is measured. However, in the laser spectroscopic analysis method, since very weak optical spectrum absorption is detected with high sensitivity, it is an important technical issue how to reduce laser optical interference noise, so-called fringe, in the measurement system. I have.

Next, a conventional example of an optical absorption cell device 20 for laser spectroscopic analysis will be described with reference to FIG.

An optical absorption cell device 20 shown in FIG. 2 has a sample cell 3 to which a gas port 2 is attached, and a light projecting unit 1 having a laser at one end of the sample cell 3 and a light receiving unit having a light receiving element. A mirror 4A to 4C (hereinafter, also typically referred to as mirror 4) and a piezoelectric transducer (hereinafter, also referred to as PZT) 5 are arranged inside the sample cell 3. ing. Further, the optical absorption cell device 20 includes a low-pass filter (LP
F) an absorption detection unit 8 to which the output of the light receiving unit 6 filtered through 10 is supplied; a laser control unit 7 to which light wavelength information is supplied from the absorption detection unit 8 to control the light projection unit 1;
Modulation circuit 11 for driving ZT5 through PZT driver 9
And

Hereinafter, the function of each component will be described.

The light projecting section 1 has a laser and generates an intensity stabilized laser beam controlled to a predetermined light wavelength by the laser control section 7 from the laser for the purpose of observing a light absorption spectrum.

The laser control unit 7 controls the current and temperature applied to the laser in accordance with the light wavelength information supplied from the absorption detection unit 8, and changes the predetermined light wavelength for observing the light absorption spectrum to a stable light intensity. It has a function to obtain.

The sample cell 3 generates optical spectrum absorption by a laser beam irradiated from the light projecting unit 1 for the introduced sample gas. The sample gas is introduced into and discharged from the sample cell 3 through the gas port 2.

In the sample cell 3, a multiple reflection optical path is formed by three mirrors 4A to 4C, and optically amplifies the weak optical spectrum absorption of the sample gas. FIG. 2 shows the structure of a white-type multiple reflection optical path combining three concave mirrors.

The PZT 5 is mounted on the sample cell 3 so that the mirrors 4A and 4C can be displaced in the directions of the arrows.
The multiple reflection optical path length is changed according to the voltage applied from the PZT drive unit 9. A continuous signal such as a sine wave or a triangular wave is applied to the PZT driving unit 9 at a low frequency of about 20 Hz to 1 kHz from the modulation circuit 11.

The light receiving section 6 detects the light intensity of the laser beam emitted from the sample cell 3. Since a fringe generated due to a change in the optical path length is superimposed on the detection signal as an AC signal, the fringe is smoothed by the LPF 10 to reduce the influence of the fringe. To detect.

Here, with reference to FIGS. 3A and 3B, the influence of the fringe in the detection process (observation process) of the light spectrum absorption amount will be described.

FIG. 3A is a schematic diagram for explaining the amount of absorption in the light intensity observation result. The horizontal axis is the light wavelength, the vertical axis is the observed light intensity, and the solid line is the light intensity as an example light spectrum absorption waveform. 5 shows a signal 22. In FIG. 3A, the difference between the light intensity A when the light wavelength is λ0 and the light intensity B at the light wavelength λ where there is no absorption is the light spectrum absorption amount. In practice, the measurement of the absorption amount does not need to observe the waveform itself, but can be realized by two fixed-point observations in which the light intensity A at the light wavelength λ0 and the light intensity B at the light wavelength λ without absorption are observed. Since it is a fixed point observation, a constant value should be obtained originally. However, since the fringe is superimposed on the actual observation light intensity, a change in the light intensity occurs due to a change in the optical path length.

FIG. 3B is a schematic diagram for explaining the influence of fringe on the light intensity observation. The horizontal axis represents the optical path length, the vertical axis represents the observed light intensity, and the solid line represents, for example, when the light wavelength is λ0. At the light intensity A, the light intensity signal 2 having an enlarged waveform when the optical path length is changed by displacing the PZT 5 in a single direction.
4 is shown.

As can be understood from FIG. 3B, inevitable fringes in the laser optical system having high coherence cause λ / 2 (where λ is emitted from the light projecting unit 1) with the displacement of the optical path length. The peak C and the trough D of the light intensity are generated on the light intensity signal 24 every time.

The optical path length in the multiple reflection optical path is determined by the mirror 4
When the distance between the mirrors 4 is fixed, a fixed fringe is superimposed on the light intensity (true value) A. As a result, the light intensity can be any one between the peak C and the valley D. Will be shown. However, the distance between the mirrors 4 always changes at the light wavelength level (about 1 μm) due to the thermal expansion of the structure constituting the sample cell 3. Always fluctuates depending on the

Therefore, when the optical path length is periodically changed by the PZT 5 as in the optical absorption cell device 20 according to the prior art in FIG. 2, an AC waveform (FIG. 3B) which turns the horizontal axis in accordance with the change amount of the optical path length. Light intensity signal 24 as observation signal)
Is replaced with a fringe component. More specifically, for example, the optical path length is linearly changed from the optical path length a to the longer optical path length b, and then linearly changed from the optical path length b to the optical path length a. When the optical path length is changed in a triangular wave so as to linearly change from the optical path length c to the shorter optical path length c and then return from the optical path length c to the optical path length a, the light intensity signal 24 shown in FIG. Along the solid line, the light intensity changes so as to turn back, and the fringe component (the amount of change in the light intensity) becomes apparent.

Therefore, as described above, by passing the light intensity signal 24 as the observation signal through the LPF 10, the fringe component is smoothed, and a value close to the true value A in which the influence of the fringe is reduced is obtained.

In the method shown in FIG. 2, the frequency of the fringe component replaced by the light intensity signal 24 as the AC waveform is increased by increasing the displacement of the PZT 5 or increasing the modulation frequency. , The attenuation rate can be increased.

[0026]

However, the larger the displacement of the PZT5, which is a capacitive load, the greater the capacitance. To drive the PZT5 at a high frequency, a large power (large power) is required. There is a problem that there is a limit.

When the change amount of the light wavelength is an integral multiple of the light wavelength λ0, the average value of the light intensity becomes the true value A. Otherwise, a DC residual component is generated, and the true value A is generated. There is a problem that the value A cannot be obtained.

In addition, since it is difficult to accurately obtain a displacement of an integral multiple of the optical wavelength λ0 at the optical wavelength level, when the weak optical spectrum absorption is to be detected, the fringe can be sufficiently increased by the conventional technique. There is a problem that it cannot be reduced.

The present invention has been made in consideration of such problems, and has as its object to provide an optical absorption cell device that can always obtain a constant fringe removal effect.

Another object of the present invention is to provide a small and simple optical absorption cell device in which the load on the displacement means, for example, the piezoelectric transducer can be reduced, and the environmental temperature change can be ignored.

[0031]

According to the present invention, for example, as shown in FIG. 1, a laser is used as a light source, and the light wavelength of the light source is controlled so that gas molecules introduced into the sample cell (3) can be reduced. In an optical absorption cell device for laser spectroscopy for observing weak optical spectrum absorption, a light projecting unit (1) using a laser as a light source, a gas port (2) for introducing and discharging a sample gas, and a sample gas sealed. And the mirrors (4A to 4A) constituting a multiple reflection optical path for optically amplifying a weak light absorption spectrum of the sample gas.
4C), a displacement unit (5) for displacing a part or all of the mirror, a light receiving unit (6) for detecting a light intensity of a laser emitted from the sample cell, and a light intensity of the light projecting unit. A laser control unit (7A) for controlling a light wavelength, an absorption detection unit (8A) for detecting an optical spectrum absorption amount from a detection signal of the light receiving unit, and a displacement unit drive unit (9A) for supplying a drive signal to the displacement unit The absorption detecting unit changes the multiple reflection optical path length by displacing the displacement unit through the displacement unit driving unit when detecting the optical spectrum absorption amount from the light intensity detected by the light receiving unit. Then, one or more periods of the optical interference noise are observed, and the median of the observed waveform is detected as an optical spectrum absorption amount.

According to this invention, when the absorption detecting section detects the light spectrum absorption amount from the light intensity detected by the light receiving section, the displacement means is displaced through the displacement means driving section to change the multiple reflection optical path length. Since one or more periods of optical interference noise are observed and the median of the observed waveform is detected as the optical spectrum absorption amount, it is necessary to accurately measure the optical spectrum absorption amount excluding the optical interference noise. Can be.

As the displacement means, a piezoelectric converter for converting an electric signal into a mechanical displacement signal can be used. Also, as other displacement means, an electromagnetic actuator,
One using non-contact displacement due to magnetic force, one using a diaphragm structure and pressure control, or the like can be used.

[0034]

An embodiment of the present invention will be described below with reference to the drawings. In the drawings referred to below, the same reference numerals are given to those corresponding to those shown in FIGS. In addition, in order to avoid complexity, description will be made with reference to FIGS. 2 and 3 as needed.

FIG. 1 shows a configuration of an optical absorption cell device 20A for laser spectroscopic analysis to which an embodiment of the present invention is applied.

The optical absorption cell device 20A shown in FIG. 1 has a sample cell 3 to which a gas port 2 is attached, and a light projecting unit 1 having a laser at one end of the sample cell 3 and a light receiving unit 6 having a light receiving element. Are mounted, and inside the sample cell 3, mirrors 4A to 4C (typically, also referred to as mirror 4) and a piezoelectric transducer (PZT) 5 as a displacement means are arranged. Further, the optical absorption cell device 20 includes an absorption detection section 8A to which the output of the light receiving section 6 is supplied, a laser control section 7A to which the output of the absorption detection section 8A is supplied to control the light projecting section 1, and an absorption detection section 8A. , A sawtooth sweep signal (also referred to as a repetitive ramp waveform) is supplied to the PZ
A PZT driving section 9A for driving T5.

The function of each component will be described below.

The light projecting section 1 generates an intensity-stabilized laser beam controlled to a predetermined light wavelength by the laser control section 7 for the purpose of observing a light absorption spectrum.

The laser control unit 7 controls the current and temperature applied to the laser constituting the light projecting unit 1 in accordance with the light wavelength information supplied from the absorption detecting unit 8, and controls the predetermined light for observing the light absorption spectrum. It has the function of obtaining a wavelength with a stable light intensity.

The sample cell 3 generates optical spectrum absorption by a laser beam emitted from the light projecting unit 1 in the introduced sample gas. The sample gas is introduced into and discharged from the sample cell 3 through the gas port 2.

In the sample cell 3, a multiple reflection optical path is formed by three mirrors 4 to optically amplify the weak optical spectrum absorption of the sample gas. FIG.
3 shows the structure of a white-type multiple reflection optical path combining a plurality of concave mirrors.

The PZT 5 is attached to the sample cell 3 so that the two mirrors 4A and 4C can be displaced in the direction of the arrow, and changes the multiple reflection optical path length in accordance with the voltage applied from the PZT drive unit 9. Note that the PZT 5 may be used to displace only one mirror, and the mirror 4
A, 4B, and 4C may all be displaced.

The light receiving section 6 detects (observes) the light intensity of the laser beam emitted from the sample cell 3.

The absorption detecting section 8A samples the light intensity signal supplied from the light receiving section 6 every 1 to 100 (ms), for example, and detects the amount of light spectrum absorption from the sampling result. The reason why the sampling time per point is set to 1 to 100 (ms) is that the total sampling time is, for example, about 1 to 60 seconds, and in order to observe one or more fringe periods during this period, about 100 -10
This is because it is determined that data of about 00 points is required.

A method of detecting the light spectrum absorption amount from the light intensity signal will be described with reference to FIG.

That is, FIG. 3A is a schematic diagram for explaining the amount of absorption in the light intensity observation result. The horizontal axis is the light wavelength, the vertical axis is the observed light intensity, and the solid line is the light spectrum absorption waveform as an example. 2 shows a light intensity signal 22. In FIG. 3A, the difference between the light intensity A when the light wavelength is λ0 and the light intensity B at a light wavelength where there is no absorption is the light spectrum absorption amount.

In this case, during the sampling of the light intensity A or the light intensity B, the absorption detection unit 8A sends the PZT 5 to the PZT 5 through the PZT driving unit 9A with the multiple reflection optical path length (light projection The optical path length from the unit 1 to the light receiving unit 6) is a constant-speed repetitive ramp waveform (the ramp period of the ramp waveform is equal to or more than one fringe period) of the optical interference noise of the laser. (Set for 1 to 60 seconds). Note that one cycle of the fringe corresponds to the target light wavelength λ0.
Thus, for example, when the light wavelength is λ0, it is determined to be λ0 / 2, and the corresponding displacement of the PZT 5 is determined by the configuration of the multiple reflection optical path and the mirrors 4A, 4B, and 4C that are displaced.

In this way, the light intensity signal (sampled signal) as an observation waveform output from the light receiving unit 6 and observed (detected) by the absorption detection unit 8A is shown in FIG. 3B.
The light intensity signal 24 shown in FIG. The sampled light intensity signal 24 is stored in a memory (not shown) in the absorption detection unit 8A. The light intensity signal 24 (as a result, the signal stored in the memory) as the observation waveform shown in FIG. 3B includes one or more fringe periods, and the light intensity of the peak C and the valley D (peak value C, peak value C). Value D), the peak values C and D are detected by the absorption detector 8A which also functions as a peak value detecting means and an average value calculating means, and the respective peak values C and D are detected. Is calculated, the true value A of the optical spectrum absorption amount (FIG. 3A,
3B) can be obtained with high accuracy by setting the average value A = (C + D) / 2.

Next, a specific example of each component in the example of FIG. 1 will be described.

The laser output means incorporated in the light projecting section 1 is a single mode DFB semiconductor laser (distribute
ted feedback semiconductor laser) can be used. In particular, in the carbon isotope measurement using breath as a sample gas, an infrared wavelength band of about 1.5 to 2.0 μm has high sensitivity and is effective.

As the gas port 2, a connection fitting for general piping can be used.

As the sample cell 3, a glass tube, a stainless steel tube or the like can be used.

As the mirrors 4A, 4B, and 4C, a reflection film suitable for the light wavelength of the light projecting unit 1 is suitable. In an infrared wavelength band that is advantageous for observation of carbon isotopes, gold deposition or dielectric multilayer film is used. Are used.

The optical absorption cell device 20A shown in FIG.
Now, three mirrors 4A, each of which is a concave mirror,
Although the white-type multiple reflection optical path composed of a combination of 4B and 4C has been exemplified, a Heriot type or the like composed of two concave mirrors can be used as another multiple reflection optical path.

In PZT5 as a piezoelectric element, low-pressure gas is often measured in laser spectroscopy.
At a low pressure of about 1 Torr, voltage leakage is likely to occur. In order to avoid this voltage leakage, it is effective to use, for example, a low-voltage PZT having a rated voltage of about 100 V or a vacuum PZT sealed in a metal case.

The displacement means is not limited to PZT5, but may employ an electromagnetic actuator, a means utilizing non-contact displacement by magnetic force, a means utilizing pressure control utilizing a diaphragm structure, or the like.

As the light receiving section 6, a photodiode suitable for the light wavelength of the light projecting section 1 is used. 1.50-2.0μ
Ge or InGaAs is suitable for an infrared wavelength of about m.

The laser control unit 7 is integrally formed with the light projecting unit 1 and includes an operational amplifier, a temperature sensor, a Peltier device, and the like.

The absorption detector 8 includes an operational amplifier, an A / D converter, a D / A converter, a memory as storage means,
And a CPU or the like functioning as each means of calculation, control, determination, processing, and the like.

In the example shown in FIG. 1, the most basic configuration (method) for capturing light intensity in a DC manner is shown. However, as a configuration (method) widely used for detecting weak light, an optical chopper is used for locking. It is needless to say that the present invention can be applied to a configuration in which the detection is performed by an in-amplifier or a configuration in which the light wavelength is modulated at a frequency f and the light intensity is detected by a 2f lock-in amplifier.

The PZT driving section 9A can use a DC high voltage generating circuit composed of a transformer and a transistor.

As described above, according to the above-described embodiment, the absorption detector 8A sets the PZ during the sampling of the light intensity signal.
A sweep signal having a multiple reflection optical path length corresponding to one or more fringe periods is applied to the PZT 5 through the T drive unit 9A,
Light intensity signal 24 which is an observation waveform of light intensity (see FIG. 3B)
, The peaks C and the valleys D of the fringe are clarified, and the true value A {A = (C + D) / 2} is calculated from the average value.

For this reason, each time the measurement is repeated, the distance between the mirrors 4A, 4B, and 4C changes at the optical wavelength level (about 1 μm) due to the thermal expansion of the structure, and the initial state of the fringe is different. Regardless of the light intensity between the valley D and the valley D, the peak C and the valley D of the fringe can be relatively captured, and by calculating the average value, the constant fringe removal effect can be obtained. Can be achieved.

Since the sampling can be sufficiently performed within a time period of about 1 to 60 seconds, the influence of the environmental temperature change can be neglected, and the required displacement for PZT5 is 1 μm or less and 1 Hz or less. As a result, the load on the PZT drive unit 9 is reduced, and the PZT drive unit 9 can be made a small and simple circuit. As a result, the size of the optical absorption cell device 20A can be reduced.

It should be noted that the present invention is not limited to the above-described embodiment, but can adopt various configurations without departing from the gist of the present invention.

[0066]

As described above, according to the present invention, when detecting the optical spectrum absorption amount from the light intensity, the multiple reflection optical path length is changed, and one or more periods of the optical interference noise are observed. Since the median value of the observed waveform is detected as the amount of optical spectrum absorption, optical interference noise,
The so-called fringe-eliminated light spectrum absorption amount can be accurately measured.

Therefore, according to the present invention, a constant fringe removing effect is always achieved.

Further, according to the present invention, it is possible to measure the amount of optical spectrum absorption in a relatively short time, and it is possible to disregard the environmental temperature change, and to reduce the load on the piezoelectric transducer. A simple optical absorption cell device can be manufactured.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional example.

FIG. 3A is a schematic diagram illustrating an optical spectrum absorption amount in a light intensity observation result, and FIG. 3B is a schematic diagram used for explaining optical interference noise in light intensity observation, a so-called fringe effect, and the like. FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light projection part 2 ... Gas port 3 ... Sample cell 4 (4A, 4
B, 4C) mirror 5 PZT (piezoelectric transducer) 6 light receiving unit 7 (7A) laser control unit 8 (8A) absorption detection unit 9 (9A) PZT drive unit 10 LPF 11 modulation circuit 20 (20A)
… Optical absorption cell device

Claims (2)

[Claims] An optical absorption cell apparatus for laser spectroscopy for observing weak optical spectrum absorption of gas molecules introduced into a sample cell by controlling a light wavelength of the light source using a laser as a light source. A light source, a gas port for introducing and discharging a sample gas, the sample cell filled with the sample gas, and a multiple reflection optical path for optically amplifying a weak light absorption spectrum of the sample gas. A constituent mirror; a displacement unit for displacing part or all of the mirror; a light receiving unit for detecting a light intensity of a laser emitted from the sample cell; and controlling a light intensity and a light wavelength of the light projecting unit. A laser control unit; an absorption detection unit that detects an optical spectrum absorption amount from a detection signal of the light receiving unit; and a displacement unit drive unit that supplies a drive signal to the displacement unit. The unit, when detecting an optical spectrum absorption amount from the light intensity detected by the light receiving unit, changes the multiple reflection optical path length by displacing the displacement unit through the displacement unit driving unit, and detects one of optical interference noise. An optical absorption cell device which observes a cycle or more and detects a median value of the observed waveform as an optical spectrum absorption amount. 2. The optical absorption cell device according to claim 1, wherein said displacement means is a piezoelectric transducer.