JPS6058810B2 - Infrared spectroscopy method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an infrared spectroscopic analysis method, particularly an analysis method suitable for detecting trace components in a gas.

Harmful gases that cause air pollution, such as carbon monoxide (
It is already well known that spectroscopic analysis using infrared absorption is convenient for detecting substances such as Co) and sulfur dioxide gas (SO0).

Therefore, the present inventors previously proposed a spectroscopic analyzer using a semiconductor infrared laser as a light source in Japanese Patent Application No. 52-128465. This device is a semiconductor tunable laser (TU).
The presence or absence of a specific component, such as Co, can be determined from the absorption curve obtained by sweeping the emission wavelength by gradually changing the current supplied to the laser and measuring the absorption of the sample gas at each wavelength. The device is configured to perform discrimination and concentration measurement. However, since the device according to this prior application directly measures the absorption of the sample gas to infrared light in a predetermined wavelength range, it is useful when detecting trace amounts of Co, etc. contained in the atmosphere, such as 1 pμm or less. has the disadvantage that measured values are disturbed by irregular temporal fluctuations in absorption due to atmospheric temperature changes, air currents, etc. In order to reduce the disturbance caused by the above-mentioned atmospheric conditions, the current flowing through the semiconductor laser element is modulated at a frequency of about 10KH2, and the spectral absorption curve is obtained by synchronously detecting the AC signal obtained by laser photoelectric conversion after passing through the atmosphere. A measurement method has been proposed in which a signal corresponding to the first derivative of is obtained and this derivative is further normalized by a primitive function.

According to this method, the above-mentioned atmospheric disturbance can be removed and a measured value directly proportional to the concentration of toxic gas can be obtained. However, this method has the disadvantage that the signal level becomes low and measurement becomes difficult for pollutant gases such as 500, which exist at low concentrations in the atmosphere. In addition, since the output power of a wavelength-tunable semiconductor laser generally depends on the wavelength of the output light, the value of the derivative is not zero regardless of the presence or absence of absorption, so the reference value (zero point) may be unclear. There is a disadvantage. The present invention solves the above-mentioned problems, uses a wavelength tunable laser as a light source, and calculates the derivative by taking the difference between the power of laser light before it is absorbed by the atmosphere or pollutant gas, and that of the laser light after absorption. Set the reference value, that is, the zero point of the measuring instrument,
Furthermore, it is an object of the present invention to provide a novel infrared spectroscopic analysis method that also makes it possible to obtain the second derivative of a spectral absorption curve.

Embodiments of the analysis method according to the present invention will be described in detail below with reference to the drawings.

In explaining the principle of construction of the present invention, a general background explanation about this type of measurement will be provided.

When infrared rays pass through a gas, they are absorbed and their intensity decreases. However, not all infrared rays are absorbed; only the infrared rays that resonate with the vibrational energy of gas molecules are absorbed. Therefore, the wavelength of infrared rays absorbed by each molecule is different.

For example, CO. (5N0 is C (the mass of 5N is different - so,
The vibration energy is different, CO is 4.7 μM, NO is 5
．． It is absorbed in resonance with infrared rays at 7 pm. Before explaining the embodiments of the analysis method according to the present invention, the measurement principle of the present invention will be explained. The concentration of the gas to be detected, such as carbon monoxide, in the atmosphere is C1, the optical path length in the atmosphere is L1, the frequency of the laser emission used as the measurement light source is υ, and the output power of the laser is determined by the light receiving element after absorbing PO1. Let Pr be the optical power collected on the light receiving surface.

However, α(υ) represents the CO absorption coefficient in the form of a function of υ, and f(t) represents the influence of atmospheric disturbance as a function of time. Further, K is a proportional constant.
, by differentiating both sides of the above equation (1) with respect to υ.

Here, PO (laser output power) needs to be treated as a function of the frequency υ when a wavelength tunable semiconductor laser is used as the light source and the supply current is variably controlled. This is because it changes depending on the current. Now, dividing both sides of equation (2) by equation (1) yields, and the term f(t) representing the influence of atmospheric disturbance is eliminated.

In Equation (3), a dash ('') is added to the derivative. From the result of Equation (3) above, the second term on the right side is proportional to the concentration of contaminant gas. Therefore, if the laser output power If PO is constant regardless of the frequency υ, the derivative P″O will be zero, so the left side of equation (3) is proportional to the concentration C of a pollutant gas, for example carbon monoxide.However, as mentioned above, the emission power PO is Since it depends on v, the derivative PO generally does not become zero, making it difficult to determine the zero point (the point where C=0). Therefore, if we set L=0 in equation (3), we get the following.

As is immediately clear by comparing both equations (3) and (4), if the value of P″o/P obtained under the condition of L=0 or C=0 is subtracted from equation (3), the amount of pollutant gas A measurement value that is directly proportional to the concentration C of the gas and returns to zero when the concentration is zero can be immediately obtained.This makes it easy to see the measurement results and makes it easy to evaluate the concentration of the pollutant gas.Here, the configuration of the present invention We will explain α and C used in equations (1) to (7) to explain the principle. αo: Absorption coefficient at absorption center r: Absorption half-width υo: Observed frequency (laser frequency) C: Gas Concentration L: Optical path length PO (v): Laser power P: (υ): Received power, where: The following measurement methods are available.

When a sawtooth current is applied to the laser, since the wavelength tunable laser is used in the present invention, the wavelength is swept and an absorption characteristic is obtained.

This is α. By superimposing a minute modulated current on the sawtooth current,
If a signal synchronized with this modulation frequency among the detector numbers is detected through a lock noise amplifier, α'' can be obtained.

α'' can be obtained by detecting a signal synchronized to a frequency twice the modulation frequency.

C indicates the gas concentration, and as mentioned above, the absorption wavelength differs depending on the laser, and the gas to be measured is determined by the oscillation wavelength of the laser.

Therefore, for example, a beam that outputs infrared rays that match the absorption of 4.7 μm (7) CO indicates the concentration of a specific substance called CO.

In this way, certain positions in the absorption spectrum have already been theoretically analyzed from the molecular vibration model, and the wavelength of the laser is set to the absorption wavelength of the substance to be measured.

Based on the principle explained above, the value of P″o/PO before absorption was determined in the vicinity of the light source, that is, the semiconductor laser, and its derivative was normalized by this value and the laser light power after passing through the atmosphere containing pollutant gas. If the difference from the value P″r/Pr is determined using a differential amplifier or the like, this becomes a desired measurement value that is proportional to the concentration of the pollutant gas.

Normally, an optical path length of about 100 to several hundred meters is required to observe detectable absorption by carbon monoxide in the atmosphere, so if you observe the laser light power at a distance of about 1 meter from the semiconductor laser, you can This value can be said to be a value in which there is virtually no absorption of contaminant gas, and it can also be regarded as a value when the optical path length L = 0. In any case, the second term on the right side of equation (3) is This applies if it is equal to zero. In addition, in order to obtain the derivative, or differential, of optical power, especially when a wavelength tunable semiconductor laser is used as a light source, it is necessary to use the Journal of Applied Physics.
fAppjedPhysics) No. 1, No. 4, No. 8
A convenient method is to superimpose some alternating current on the laser input current, as described in the paper published on pages 54-861. In other words, since the emission wavelength of a semiconductor wavelength tunable laser changes depending on the supplied current, if the supplied current is not just a direct current but a direct current with a constant amplitude and frequency alternating current superimposed, the output light of the laser will change. The wavelength (and therefore the frequency) changes depending on the instantaneous amplitude of the alternating current component.

When the amplitude of the alternating current component is small, naturally the fluctuation range of the frequency υ of the output light is also small. Therefore, if this variation range is expressed as Δυ, and the minute change in the power of the laser output light corresponding to Δυ is expressed as ΔPO, then the power variation rate ΔPO/Δυ is essentially the derivative of PO(υ), that is, the derivative of PO(υ). Function P″0(υ)
is equal to Therefore, the desired derivative P can be obtained by dividing the change in laser output power by the change in frequency υ.
″o(υ) will be obtained.

Next, the second derivative of the equation (1) given earlier is as follows.

10' Rearrange the above equation and write it using a dash as the differential sign.

At the maximum point (or minimum point) of absorption, "α"=0, so the third and fourth terms on the right side of equation (6) disappear. In this case, dividing both sides of equation (6) by equation (1) yields a form that is very similar to equation (3).

Therefore, as described above for equations (3) and (4), P″″/P when L=0 (near the laser)
By determining the value of O and taking the difference from equation (7), a measured value proportional to the concentration C of the contaminant gas can be obtained. Since the value of the second derivative P″r or P″″0 appears as a frequency 2fm which is twice the frequency Fm (of the alternating current that modulates the supply current to the laser), measuring the amplitude of the component of 2fm yields the second derivative P ″r
and P″O can be known.

Generally, the measured output often has a straight line slope, and a straight line slope means that the curvature is zero. Therefore, when the curvature is determined by second-order differentiation, only the changing part is extracted, resulting in improved sensitivity. An example of the apparatus used in the method of the present invention is shown in the accompanying drawings as a system diagram.

In the drawing, laser light emitted from a wavelength tunable semiconductor laser 1 is focused by a convex lens 2, further reflected by a concave mirror 3 to become parallel light, passes through an atmosphere 4 containing pollutant gas, and is further reflected by a bending reflector 5 to form an optical path. The length is set to a length suitable for absorption spectrum observation, is focused again by the Cassegrain mirror 6, and passes through the hole of the concave mirror 6a in the Cassegrain mirror to an infrared photoelectric conversion system (in this example, a photon type infrared detector).
The light is collected on the light-receiving surface of 7. Therefore, in this embodiment, a chopper 8 and a light branching device 9 are provided in the optical path between the convex lens 2 and the concave mirror 3.

Chopper 8 is commonly used in infrared measurement systems,
The optical branching device 9 is often not present in conventional devices. In this embodiment, using the light branched by the optical branching device 9, the laser output light power PO before absorption and its derivative P
O or P″2 is measured.For this purpose, a photon-type infrared detector 10 that receives the branched light and converts it into an electrical signal is provided.Hereafter, for the purpose of differentiation, an infrared detector 7 that receives the absorbed light will be used as the third infrared detector. The infrared detector 10 receiving the branched light is called the second detector.The output of the first detector 7 is input to a pair of phase-locked amplifiers 11 and 12.
The output of 2 is added to the divider 13. The output of the divider 13 is P″r/Pr in equation (3) or P″″r/ in equation (7).
This corresponds to Pr. On the other hand, the output of the second detector 10 is generated by another phase constraint increase. They are both input to width amplifiers 14 and 15, and their outputs are applied to a divider circuit 16, and the divider circuit 16 receives (3)
P″o/P on the right side of equation (7) or P″o/PO on the right side of equation (7)
produces the corresponding output. Hereinafter, the division circuit 13 will be referred to as a first division circuit, and the division circuit 16 will be referred to as a second division circuit. the above. The outputs of both divider circuits 13 and 16 are applied to a differential amplifier 17, and the output corresponding to the difference between the outputs of both divider circuits 13 and 16 is proportional to the concentration C of the contaminant gas, and no contaminant gas is present at all. produces an output that becomes zero when This output is given to the automatic recorder 18 to record the analysis results. During the above operation, the current control circuit 19 controls the current supplied to the semiconductor laser 1.

The current control circuit 19 supplies a direct current with a small amplitude alternating current component superimposed thereon to the semiconductor laser 1, and gradually changes the direct current component in accordance with a required measurement range. ``By programming the currents in this manner, P''r/Pr and P''o from the first divider circuit 13 and the second divider circuit 16, respectively.
An output signal proportional to /PO is obtained. According to the spectroscopic analysis method according to the present invention described above, even if a semiconductor laser whose output light power depends on the frequency is used as a light source, the zero point of the measured value is stabilized, and the concentration of the pollutant gas that is the target of direct analysis is stabilized. There is an advantage in that it is possible to obtain a measurement value that returns to zero when the concentration is zero in proportion to.

In particular, near the most important peak of the absorption curve, the use of the second derivative allows for more accurate measurements than when the first derivative is used. Therefore, the present invention is extremely advantageous when applied not only to the observation of air pollution but also to the spectroscopic analysis of all substances having absorption in the infrared region.

[Brief explanation of drawings]

The accompanying drawing is a schematic system diagram showing the configuration of an apparatus used in an embodiment of the spectroscopic analysis method according to the present invention. 1: wavelength tunable semiconductor laser, 2: convex lens, 3: concave mirror, 4: atmosphere, 5: bending reflector, 6: Cassegrain mirror,
6a: Concave mirror with hole, 7: First detector, 8: Chiyotsupa,
9: Optical branching device, 10: Second detector, 11 and 12:
Phase-locked amplifier, 13: first divider, 14 and 15:
Phase-locked amplifier, 16: second divider, 17: differential amplifier, 18: automatic recorder, 19: current control circuit.

Claims (1)

[Claims] 1. A wavelength tunable infrared laser is used as a light source, the laser light output from the light source is transmitted through an object to be analyzed, and the current supplied to the wavelength tunable laser is divided into alternating current components with constant amplitude and frequency. The differential coefficient of the power of the transmitted light with respect to the frequency of the laser beam is calculated from the change in the laser output light corresponding to the certain frequency, and the quotient is calculated by dividing the differential coefficient by the atomic function. Separately, the quotient of the differential coefficient of the spectrum of the output light when it is not absorbed by the object is divided by its atomic function is calculated, and from the difference between the two quotients, the predetermined absorbing substance present in the object to be analyzed is determined. An infrared spectroscopic analysis method characterized by determining the concentration of. 2. The infrared spectroscopic analysis method according to claim 1, wherein the differential coefficient is a second-order differential coefficient.