JPS63275935A - Gas sensor - Google Patents

Abstract

PURPOSE:To suppress noises due to optical standing waves, by making the amplitude of wavelength modulation to be X times the interval of the optical standing wave, which is generated in an optical system. CONSTITUTION:A sawtooth wave signal having a specified period is generated 21 and supplied to a laser 1 as a bias current. The oscillating wavelength of the signal is determined. The oscillating output undergoes A/D conversion 22 and the result is inputted into a ROM 25. The data of the modulating amplitude from the ROM 25 are read. D/A conversion 24 of the data is performed. The result is multiplied 29 by the amplitude of a sine wave from an oscillator 23. The amplitude of the sine wave is changed at every oscillating wavelength. Thus condition, under which X times of the period of the optical standing wave that is generated in an optical system are obtained, is satisfied in all wavelength [X satisfies J2(piXX)=0 and J is a quadratic Bessel function]. The multiplied output and the sawtooth signal are added 26, and the current of the laser 1 is set. As a result, noises due to the optical standing waves, which are generated in a long optical path cell 28 for measuring gas to be measured 2, can be suppressed.

Description

[Detailed Description of the Invention] [Summary] The present invention is an infrared semiconductor laser type gas sensor using a differential measurement method, and the present invention solves fluctuations in concentration measurement values caused by optical standing waves generated in the optical system due to changes in environmental temperature. In order to suppress this, the amplitude of the wavelength modulation must be X times the interval of the optical standing waves generated in the optical system (X satisfies J(π×X)=O, J is a quadratic Bessel function). The structure is designed to suppress noise caused by optical standing waves.

[Industrial application field]

The present invention relates to an infrared semiconductor laser type gas sensor, particularly 1! The present invention relates to an infrared semiconductor laser type gas sensor that suppresses fluctuations in concentration measurement values caused by optical standing waves generated in an optical system due to changes in loading a, thereby making it possible to perform highly accurate concentration measurements.

Gas sensors are required to be small, highly sensitive, short-time measurement, and highly accurate. In this highly accurate measurement,
In particular, it is necessary to eliminate fluctuations in concentration measurement values caused by optical standing waves generated in the optical system due to temperature changes, and a method for this purpose is required.

[Conventional technology]

FIG. 5 is a block diagram of a conventional infrared laser type gas sensor, and FIGS. 6 and 7 are diagrams for explaining its operation.

In FIG. 5, the conventional gas sensor is divided into a laser 1 that outputs a variable wavelength laser beam, a lens 6 that converts the laser beam into parallel light, a half mirror 1O that divides the parallel light, and a half mirror 10. On the other hand, a long optical path cell 28 through which the laser beam passes through the gas to be measured 2, a lens 7 which condenses the transmitted light, and a detector 8 which detects the light condensed by the lens 7 and converts it into an electrical signal. It is equipped with

Further, the other laser beam divided by the half mirror 10 is passed through the reference cell 12 which passes through a known gas housed through the mirror 1), and the laser beam which has passed through the reference cell 12 is passed through the lens 13. A detector 14 that detects and converts it into an electrical signal, signal processing circuits 3 and 15 that process the output signals of the detector 8 and the detector 14, respectively, and calculates the output signal of each signal processing circuit and displays the concentration. The configuration includes a divider 16 and a display 9.

Its operation will be explained with reference to the drawings.

In FIG. 5, the light emitted from a semiconductor laser 1 is made into parallel light by a lens 6, and passes through a minute gas to be measured (for example, a pollution gas such as NO2) 2 in the atmosphere. The transmitted light is focused by a lens 7 onto an infrared detector 8 and converted into an electrical signal.

As shown in FIG. 6, the semiconductor laser 1 can continuously sweep the wavelength by changing the current I.

FIG. 7B shows the absorption spectrum of 1) constant gas 2. In the differential measurement method, a minute alternating current is superimposed on the bias current flowing through the laser. The temporal change of the laser oscillation wavelength is as shown in FIG. 7A. Therefore, the power of the laser transmitted through the gas to be measured 2 changes over time as shown in FIG. 7C, and is detected by the detector 8. h in the figure is approximately proportional to the gas concentration.

The signal processing circuit 3 detects the alternating current shown in FIG. 7C and measures h. In this way, in the differential measurement method, the oscillation wavelength of the laser is modulated.

On the other hand, the light that is split by the half mirror 10 immediately after passing through the lens 6 and travels upward in the figure passes through the reference cell 12 and is focused by the lens 13 onto the detector 14 . The reference cell 12 stores a gas with a known concentration, and the detector 14 and signal processing circuit 15 operate in the same manner as the detector 8 and signal processing circuit 3 described above to generate a signal h for the gas with a known concentration. ′ is measured.

The divider 16 calculates and outputs the ratio of the signal h' of the measured gas of unknown concentration to the signal h' of known concentration. The display device 9 converts the output of the divider 16 into a gas concentration and displays it.

As shown in FIGS. 7A and 7B, the amplitude a of wavelength modulation for differential measurement has conventionally been used at about twice the half-width at half maximum of the absorption line. This is because the magnitude of the detection signal (h) in FIG. 7C is maximized at this time.

[Problem that the invention seeks to solve]

In the conventional infrared laser gas sensor described above, multiple reflections occur on surfaces such as the lens and the window of the laser container. Therefore, optical standing waves are generated. Changes in environmental temperature change the distance between optical components, or the refractive index of optical components such as lenses changes, causing the phase of the optical standing wave to change. This limits measurement sensitivity.

FIG. 8 is a diagram showing noise due to optical standing waves, which is in the form of a series of sinusoidal waves with constant intervals. The phase of this wave changes from a solid line to a dotted line as the temperature changes.

FIG. 9 shows the absorption spectrum of the gas, and during measurement, the magnitude of the absorption spectrum of the gas is measured from the superimposed spectrum of FIGS. 8 and 9. At this time, since the width of the absorption spectrum waveform and the period of the waveform of the optical standing wave that becomes noise are close, the absorption spectra cannot be separated. For this reason, noise due to optical standing waves is added to the measured concentration value, resulting in the disadvantage that the measured concentration value fluctuates due to temperature changes.

The present invention was created in view of these points, and it is an object of the present invention to provide a gas sensor that can prevent the measured concentration value from fluctuating due to noise caused by optical standing waves generated in the optical system due to temperature changes. The purpose is

[Means for solving problems]

FIG. 1 shows a block diagram of the gas sensor of the present invention, which includes a triangular wave generation circuit 21, an A/D converter 22, an oscillator 23, a ROM 25, a D/A converter 24, an adder 26, Laser current setting circuit 2 consisting of a multiplier 27
9 is attached to a conventional gas sensor, and the laser current setting circuit 27 adjusts the amplitude of the wavelength modulation @ (the peak t of the signal in Fig. 3).
o peak value) is the period of the optical standing wave generated in the optical system
(X satisfies J2(π×X)=0, J is a quadratic Bessel function).

[Effect]

The triangular wave generation circuit 21 supplies a bias current to the laser 1;
In other words, it determines the oscillation wavelength. The tuning rate of the laser 1 differs depending on the oscillation wavelength. By changing to
The condition is such that (π×X)=0, J is a quadratic Bessel function).

〔Example〕

FIG. 2 is a block diagram of a gas sensor according to an embodiment of the present invention.
FIG. 3 is a diagram showing the signal amount for an optical standing wave in one embodiment.

As shown in FIG. 2, the gas sensor of one embodiment includes a triangular wave generation circuit 21, an A/D converter 22, and an oscillator 23.
A laser current setting circuit 27 consisting of a ROM 25, a D/A converter 24, an adder 26, and a multiplier 29 is attached to a conventional gas sensor.

The triangular wave generating circuit 21 generates a fast wave signal having a predetermined period. This fast wave signal is a bias current flowing through the laser 1,
That is, the oscillation wavelength of the laser 1 is determined, which is converted into a digital signal by the A/D converter 22 and input to the ROM 25.

The modulation amplitude corresponding to the oscillation wavelength is stored in the ROM 25 in advance, and the data of the modulation amplitude is read out using the input output oscillation wavelength of the A/D converter 22 as an address.
/A converter 24.

The D/A converter 24 converts the modulation amplitude data read from the ROM 25 into an analog quantity. A multiplier 29 changes the amplitude (peak to peak) of the sine wave generated by the oscillator 23 for each oscillation wavelength. As a result, the period of the optical standing wave generated in the optical system at all wavelengths is multiplied by X (X is J2 (
, π×X)−〇, where J is a quadratic Bessel function).

The adder 26 adds the output signal of the triangular wave generating circuit 21 and the output signal of the multiplier 27 to set the current of the laser 1.

Figure 3 (a), (b), (cl is a diagram showing the signal amount for an optical standing wave, Figure 3 (1)) shows the optical standing wave generated in the optical system, Figure 3 (a) shows wavelength modulation for differential measurement, and FIG. 3(C) shows the signal of the detector output.

As shown in FIG. 3(a), Cb+, fc), since second-order differential measurement is performed, the signal amount is proportional to the Co52t coefficient of the Fourier series expansion of fD). In other words, XCo
5nt ・・・・・・・・・(1) The coefficient of Cos2t is: B / πCo5 (ASint + φ) Cos2tdt=
2 BJ,! (A) ・・・・・・・・・・・・
- (2) Since the formula (2) is obtained, the signal amount for the optical standing wave becomes O at A where J2(A) - 0, and noise can be suppressed.

Here, since A-π×(amplitude of wavelength variable ÷ interval of optical standing waves), for example, as shown in Fig. 4, the ratio of the amplitude of wavelength modulation to the interval of optical standing waves is 1.63. .2.68.3.7
If it is 0°..., noise can be suppressed.

By applying the above conditions to the drive current of the laser 1, the long optical path cell 28 for measuring the gas 2 to be measured, for example.
It is possible to suppress noise caused by optical standing waves generated in

〔Effect of the invention〕

As described above, according to the present invention, noise caused by standing waves generated in the optical system can be suppressed, so that stable gas concentration measurement can be performed against changes in environmental temperature.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of main parts of a gas sensor of the present invention, Fig. 2 is a block diagram of a gas sensor of an embodiment of the invention, Fig. 3 is a diagram showing the signal amount for an optical standing wave of an embodiment, FIG. 4 shows the ratio between the amplitude of wavelength modulation and the interval of optical standing waves in one embodiment. Figure 5 shows the signal amount for optical standing waves. Figure 5 is a schematic diagram of a conventional infrared laser gas sensor. Figure 6 is a diagram to explain the wavelength sweep of the laser. Figure 7 is an explanation of the absorption spectrum. FIG. 8 is a diagram showing noise due to optical standing waves, and FIG. 9 is an absorption spectrum due to gas. In the figure, 1 is a laser, 2 is a gas to be measured, 3.15 is a signal processing circuit, 6, 7.13 is a lens, 8, 14 is a detector, 9 is a display, 10 is a half mirror. ,
12 is a reference cell, 16 is a divider, 21 is a triangular wave generation circuit, 22 is an A/D converter, 23 is an oscillator, and 24 is a D/D converter.
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Claims (2)

[Claims] (1) By using an infrared semiconductor laser (1) as a light source and measuring the absorption spectrum of the gas using the differential measurement method,
In a gas sensor that measures the concentration of a trace amount of gas, the amplitude (peak to
A gas sensor characterized in that the peak value) is set to be X times the period of an optical standing wave generated in an optical system (X satisfies J_2(π×X)=0, J is a quadratic Bessel function). (2) ROM (25) with the wavelength of the laser as an address and the modulation current value for wavelength modulation of the laser as data
2. The gas sensor according to claim 1, wherein the relationship set forth in claim 1 is always maintained in a wavelength range necessary for measuring the absorption spectrum.