JPH0273138A - Laser type gas sensor - Google Patents

Abstract

PURPOSE:To continue measurement of the gas concentration even at the time of the occurrence of discontinuity of a spectrum due to mode hop by providing a mode hop detecting means, a temperature control means, and a laser driving current control means in a signal processing device. CONSTITUTION:A mode hop detecting means 10, a temperature control means 11, a laser driving current control means 12 are provided in the signal processing device 9 to constitute a laser type gas sensor. The means 10 detects the mode hop based on the level of a difference signal generated from a second standardized secondary differential signal from a signal processing circuit 8 with a certain difference width. The means 11 controls the element temperature of a temperature variable semiconductor laser 1 so that the mode hop position detected between two minimum points in the second standardized secondary differential signal is shifted out of the range between two minimum points. The means 12 controls the laser driving current in accordance with the second standardized secondary differential signal shifted by the control of the element temperature of the laser 1 so that the laser oscillation wavelength is within two minimum points.

Description

[Detailed Description of the Invention] [Summary] Regarding a laser-type gas sensor that measures the concentration of a gas to be measured using laser light emitted from a wavelength tunable semiconductor laser, it is affected by discontinuity in the absorption spectrum due to mode hops. The purpose is to measure the gas concentration without any interference, and the laser beam emitted from the wavelength tunable semiconductor laser is split into two optical paths by optical means, one of which is transmitted through the gas to be measured in the measurement cell and sent to the first photodetecting element. The other one is transmitted through the reference gas in the reference cell and is incident on the second photodetecting element, and the ゛M electric signal 1st is obtained by photoelectric conversion by the first and second photodetecting elements, respectively. and a second signal processing circuit to generate first and second normalized second-order differential signals, respectively, and the signal processing device calculates the gas concentration of the gas to be measured based on these normalized second-order differential signals. In the laser type gas sensor that calculates mode hop detection means for detecting a mode hop based on the level of a difference signal generated with a constant difference width from the second normalized second-order differential signal; 2 in the normalized second-order differential signal of
The mode pop position detected between the two minimum points is
temperature control means for controlling the element temperature of the semiconductor laser so as to shift it outside the two small points; , the laser drive f! controls the sea 1F drive current so that the laser oscillation wavelength falls between the above two minimum points. I current control means is configured to provide for the signal processing operation.

[Industrial application field]

The present invention relates to a laser gas sensor, and more particularly to a laser gas sensor that measures the concentration of limb measurement gas using laser light emitted from a wavelength tunable semiconductor laser.

The laser type gas sensor has a configuration as schematically shown in FIG. In the figure, reference numeral 1 denotes a wavelength tunable semiconductor laser, and a laser beam in the infrared region emitted from the laser beam is split into two optical paths by a beam splitter 2. The optical path of the laser beam is split into two by a beam splitter 2. The other one passes through the reference cell 5 and enters the photodetector element 6 . The measurement cell 3 is filled with a gas to be measured whose gas concentration is unknown, and the reference cell 5 is filled with a reference gas whose type and concentration are known.

The photodetecting elements 4 and 6 photoelectrically convert the attenuated laser light in the wavelength band specific to the above-mentioned gas to be measured and the reference gas, and the resulting electric signal is sent to a microcomputer through a signal processing circuit 7.8. It is supplied to the configured signal processing device @9. The signal processing device @9 calculates the gas temperature of the gas to be measured based on this input electrical signal.

Further, the signal processing device 9 supplies a drive current (laser drive current) to the semiconductor laser 1 to oscillate and output a laser beam with a wavelength corresponding to the current value, and gradually increases the laser drive current in stages at regular intervals. By increasing the value, the wavelength sweep of the laser beam is performed near a specific absorption line. Furthermore, this wave movement is performed periodically. The signal processing device 9 also outputs temperature control means for maintaining the semiconductor laser 1 at a constant low temperature.

In a laser type gas sensor having such a configuration, it is necessary to accurately measure the gas concentration even if the oscillation wavelength characteristics of the semiconductor laser change.

[Conventional technology]

In the conventional laser type gas sensor, each of the signal processing circuits 7.8 measures the received power having the characteristics as shown in FIG. The changes in the input electrical signal were synchronously detected to generate second-order differential signals as shown in FIG. 8(B), and the signal processing device 9 divided these second-order differential signals by the received power to generate the The absorption amount is determined from a normalized signal having two minimum points a1 and a2 and one maximum point b1 as shown in FIG. 8(C).

Here, the above absorption amount is the normalized signal (
That is, from one maximum point b1 of the normalized absorption spectrum) to two minimum points 1. This is the signal m to point d on the line segment C connecting point a2 with the same wavelength as the maximum point b1 (however, the signal processing device 9 estimates the wavelength from the laser current).

Conventionally, the signal processing device 9 adjusts the scanning width of the cold current of the semiconductor laser 4f1 so as to cover one absorption line in the absorption band of a specific gas and oscillate in this manner. The gas concentration was calculated by multiplying the determined absorption amount of the measured gas by the absorption amount of the reference gas and the proportional value of the gas concentration.

[Problem to be solved by the invention]

However, the oscillation wavelength characteristics of the semiconductor laser 1 may change over time, and in this case, a discontinuous point may occur in the spectrum (this is called a mode hop).

That is, as shown by the broken line in FIG. 9, a discontinuous portion where no normalized signal is obtained occurs as the laser current changes.

This is because the longitudinal oscillation mode of the laser changes at this position, and the oscillation wavelength of the semiconductor laser 1 changes by more than 200 Å with this discontinuity point as a boundary. In other words, the oscillation wavelength of the semiconductor laser 1 changes depending on the laser drive current, but in the discontinuous portion, the oscillation wavelength significantly changes nonlinearly with respect to a slight change in the laser drive current.

As a result, of the two minimum points of the normalized signal, the minimum point where the discontinuous portion exists becomes a minimum value different from the original minimum value, as shown by a3 in Fig. 9, and the gas concentration at this time Measurement data is invalid.

However, conventionally, measurement data was generated by estimating changes in the oscillation wavelength of the laser light from changes in the laser current.
As long as the laser drive vJ current is changing normally, it cannot be detected whether or not the mode hop has occurred, and when the mode hop occurs, the gas concentration is measured from invalid measurement data, resulting in low measurement reliability.

The present invention has been made in view of the above points, and an object of the present invention is to provide a laser gas sensor that measures gas concentration without being affected by discontinuities in the absorption spectrum due to mode pops.

[Means to solve the problem]

FIG. 1 shows a principle configuration diagram of the main part of the present invention. The laser type gas sensor according to the present invention has basically the same configuration as shown in FIG. 11 and laser drive 8 current control means 12.

The mode hop detection means 10 detects a mode hop based on the level of the difference (g) generated from the second normalized quadratic differential signal from the signal processing circuit 8 with a constant difference width.

The temperature control means 11 controls the element temperature of the semiconductor laser 1 so as to shift the mode hop position detected between the two minimum points in the second normalized second-order differential signal to the outside between the two minimum points.

Further, the laser drive current control means 12 drives the laser so that the laser oscillation wavelength falls between two minimum points in response to the second normalized second-order differential signal shifted by the element temperature υ1111 of the semiconductor laser 1. Control the current.

[Effect]

The oscillation wavelength of the wavelength tunable semiconductor radar 1 is uniquely determined by the cooling temperature and laser drive current. However, if there is a change over time, the oscillation wavelength may change even under the same operating conditions, and the position of the mode hop (in terms of wavelength) may change. If this mode hop occurs between two minimum points in the normalized second-order differential signal, the turning point will be different from the original position, making accurate a1 degree measurement impossible.

Therefore, in the present invention, the mode hop detection means 10 detects the mode hop position, and the temperature control means 11 controls the cooling temperature until the mode hop position is detected between the two minimum points of the absorption line. This corresponds to changes over time.

The mode hop detection means 10 generates a difference signal with a constant difference width when detecting a mode hop, but in the spectrum of the normalized second-order differential signal shown in FIG. Assuming that the second-order differential signal m is one point, for example, when there are 1024 points and the width of the difference is 10 points, a differential signal spectrum as shown in FIG. 2 is obtained.

In FIG. 2, the maximum point MAX corresponds to the maximum point b1 in FIG. 9, and the minimum point ML on the left side and the minimum point MR on the right side correspond to minimum points a and a3 in FIG. 9, respectively. That is, the minimum points ML and MR are the points at which the difference signal changes from negative to positive, and the maximum point MAX is the point at which the difference signal changes from positive to negative.

On the other hand, the difference signal at the mode hop occurrence point indicated by e in FIG. 9 reaches an extremely high level as indicated by E in FIG. 2.

Therefore, the mode hop detection means 10 detects a laser drive current value of such a large level that this difference signal exceeds the predetermined range between the positive threshold value T+ and the negative threshold value T- as the mode hop position. do.

The temperature control means 11 utilizes the fact that the position of the mode hop shifts depending on the cooling temperature of the semiconductor laser. In other words, if the mode hop wavelength shifts to the shorter wavelength side when the element cooling temperature is increased (cooled), the normalized second-order differential signal spectrum will change to the element cooling temperature T1, which is not shown in FIG. 3(A). When obtained, the element cooling temperature is T2.
(<T1), the normalized second-order differential signal spectrum becomes as shown in FIG. 3(B).

As can be seen from Fig. 3 (A) and (B), the absorption center laser driving current ■. By lowering the cooling temperature from T1 to T2, the position of the mode hop, which was on the longer wavelength side by ΔIo, changes to the position of Δ11 as measured from the absorption center laser drive current of the same absorption line. Δ
11<△■.

This shows that the position of the mode hop has shifted to the shorter wavelength side. The temperature control means 11 shifts the mode hop position outside the two minimum points by utilizing the property that the mode hop position shifts by controlling the element temperature of the semiconductor laser 1.

On the other hand, the laser drive current control means 12 controls the element temperature by the temperature control means 11 as shown in FIG.
As shown in B), the absorption center laser driving current of the normalized second-order differential signal spectrum is [.

Since there is a shift from ■1 to ■1, in order to compensate for this shift,
The center laser drive current is I. Control is performed to increase the number from T1 to T1.

If a mode hop appears between the left minimum point and maximum point as shown in Figure 3 (△), by shifting the mode hop to the longer wavelength side, the mode hop will be moved to the left of the left minimum point. The position (wavelength) can be shifted. Further, since the relationship between the element temperature and the mode hop level (wavelength) differs depending on the type of semiconductor laser 1, if the above assumption is reversed, the operation described above may be performed in reverse.

By doing so, a stable laser gas sensor that can cope with changes in the semiconductor laser 1 over time can be obtained.

(Example) FIG. 4 shows a block diagram of an embodiment of the main part of the present invention.

In the figure, the same components as those in FIG. 7 are denoted by the same reference numerals, and their explanations will be omitted. FIG. 4 shows the signal processing circuit 8 and signal processing device 9 in FIG. 7, and the configuration of the signal processing device 9 in this embodiment is different from the conventional one.

In FIG. 4, an electrical signal from a photodetector 8 inputted through an input terminal 14 is supplied to a Zumble hold circuit (S/H circuit) 15 to detect the received power, while being supplied to a lock-in amplifier 16. and a second-order differential signal is detected.

The above received power and second-order differential signal are sent to the multiplexer 17
The signals are alternately synthesized in time series and inputted to the A/D converter 18, where they are converted into digital signals and then supplied to the microcomputer 19.

As in the conventional case, the microcomputer 19 receives the received power of the gas to be measured and the second-order differential signal (however, these are digital signals) from the signal processing circuit 7 (not shown), and A
/r) The received power of the reference gas from the converter 18 and a digital signal related to the second-order differential signal are supplied, and the gas concentration is calculated in the same manner as in the conventional method, and the digital signal from the A/D converter 18 is Based on this, the setting temperature is instructed to the cooling temperature control circuit 20, and the eight values of the scan start point and scan end point of the laser drive current are instructed to the laser drive circuit 21.

That is, the microcomputer 19 controls the cooling temperature control 2.
11 circuit 20 constitutes the temperature control means 11, and together with the laser drive circuit 21 constitutes the laser drive current control means 12. The output signal of the cooling temperature control circuit 20 is sent to the heater 2 shown in FIG. 5 via the output terminal 22.
9 as a control current to variably control its temperature. Further, the output signal of the radar drive circuit 21 is transmitted to the output terminal 23.
The current is supplied to the semiconductor radar 1 (laser diode 25 in FIG. 5) as a driving current.

In the semiconductor laser 1, as shown in FIG. A temperature sensor 28 is attached to the heat sink 27, and a heater 29 made of, for example, a manganin wire wound thereon is provided on the other end of the heat sink 27, and one surface of the heater 29 is heated with liquid nitrogen or the like. refrigerant 30 is in contact with the refrigerant 30.

Therefore, the radar diode 25 is cooled by the coolant 30, and the element temperature is variably controlled by the heater 29. The relationship between the element temperature of the radar diode 25 and the wavelength at which mode hops occur has been investigated in advance.
This performs the temperature control.

Next, the mode hop shift operation according to this embodiment will be explained in more detail. Δ by the microcomputer 19
The normalized second-order differential signal spectrum during normal operation obtained based on the output digital signal (i) of the /D converter 18 is as shown in FIG. 6(A), and at this time, the element cooling temperature is T1, the absorption center laser drive current at the maximum point is ■.The laser drive JJM flow is assumed to be scanned within the range of IO±Δ1, which covers the two minimum points with the center at 1°. .

Thereafter, as the semiconductor laser 1 (laser diode 25) changes over time, the normalized second-order differential signal spectrum changes from the maximum point P to the minimum point P on the right side, as shown in FIG. 6(B).
If a mode hop AX PHo occurs between HR and HR, the original minimum point PHR will not appear, resulting in a measurement error.

Therefore, in this embodiment, assuming that when the laser diode 25 lowers the element temperature, the wavelength position of the mode hop shifts to the shorter wavelength side, the microcomputer 1
9 changes the set temperature for the cooling temperature control circuit 20;
An instruction is given to further cool the wavelength position of the mode hop PHo in the spectrum so that it moves to the right of the minimum point PHR.

As a result, the entire spectrum shifts to the shorter wavelength side, and when the element temperature (cooling temperature) is T2 (<T1), a minimum point PHR' appears as shown in FIG. 6(C), and
Mode hop PHo' is located on the right side thereof.

However, if this continues as it is, as shown in Figure 6 (C),
Same laser drive 1st class I. If the oscillation wavelength is observed at ±ΔI, it will be shifted to the long wavelength side, and the minimum point P on the right side
It cannot cover the oscillation wavelength of H11'. Therefore, the microcomputer 19 instructs the laser drive circuit 21 to change the starting point and end point values of the scanning radar drive current, as shown in FIG. 6(D).
As shown in FIG.
11±ΔI that can obtain oscillation wavelengths that cover each of
The laser drive current is changed within a range (scan width).

In this way, according to this embodiment, even if a mode hop occurs, it is possible to obtain two minimum points and one maximum point in the normalized second-order differential signal spectrum, and from this, the gas concentration can be calculated. be able to.

〔Effect of the invention〕

As described above, according to the present invention, even if mode hops occur in the normalized second-order differential signal spectrum due to changes in the semiconductor laser over time and spectral discontinuity occurs, gas concentration can be measured without being affected by the discontinuity. It has the advantage that it can be used continuously, improve the reliability of gas concentration measurement by the laser type gas sensor, and increase the apparent lifespan of the semiconductor laser.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the principle configuration of the main parts of the present invention, Figure 2 is a diagram showing a difference spectrum, Figure 3 is a diagram explaining mode hop shifts due to changes in element cooling temperature, and Figure 4 is a diagram showing the principle of the present invention. A block diagram of an embodiment of the main part, Fig. 5 is a diagram showing an example of a semiconductor laser and its temperature control structure, Fig. 6 is a normalized second-order differential signal spectrum diagram for explaining the operation of Fig. 4, Fig. 7 8 is a schematic block diagram of a laser type gas sensor, FIG. 8 is a signal spectrum diagram for explaining the operation of FIG. 7, and FIG. 9 is a diagram showing an example of the spectrum of a normalized signal when the characteristics of a semiconductor laser change. In the figure, 1 is a wavelength tunable semiconductor laser, 2 is a beam splitter, 3 is a measurement cell, 4.6 is a photodetector, 5 is a reference cell, 7.8 is a signal processing circuit, 9 is a signal processing [1°10 is A mode hop detection means, 11 a temperature control means, 12 a laser drive current control means, 19 a microcomputer, 20 a cooling temperature control circuit, and 21 a sea 11 drive circuit are not shown. Fig. 1 is a diagram illustrating the principle structure of the main parts of the present invention. Fig. 2 is a diagram showing the laser drive current differential vector. 9 Signal processing circuit, 8 Block diagram of an embodiment of the main part of the present invention. A diagram showing an example of a control structure. FIG. 5 Laser drive current Laser drive current FIG. Immersion torque diagram Figure 6 (part 2) Laser drive current Laser drive current Current chart

Claims (1)

[Claims] The laser beam emitted from the wavelength tunable semiconductor laser (1) is split into two optical paths by the optical means (2), and one optical path is transmitted through the gas to be measured in the measurement cell (3) to the first photodetector element (4). ), and the other is transmitted through the reference gas in the reference cell (5) and is incident on the second photodetecting element (6), and the first and second photodetecting elements (4, 6) respectively detect The electrical signal obtained by photoelectric conversion is sent to the first and second signal processing circuits (7,
8) to generate first and second normalized second-order differential signals, respectively, and the signal processing device (9) calculates the gas concentration of the gas to be measured based on these normalized second-order differential signals. In the laser type gas sensor, mode hop detection means (1
0), and shifting the mode hop position detected between two minimum points in the second normalized quadratic differential signal by the mode hop detection means (10) to outside between the two minimum points. temperature control means (11) for controlling the element temperature of the semiconductor laser (1), and corresponding to the second normalized second-order differential signal shifted by controlling the element temperature of the semiconductor laser (1), A laser-type gas sensor characterized in that the signal processing device (9) is equipped with a laser drive current control means (12) for controlling the laser drive current so that the laser oscillation wavelength falls between the two minimum points.