JP2796648B2 - Gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detecting device using a semiconductor laser device.

[0002]

2. Description of the Related Art It is known that the presence or absence of gas can be detected by utilizing the fact that laser light of a specific wavelength is easily absorbed by a certain kind of gas. Sensing technology using this principle is used for industrial measurement, pollution monitoring, etc. Widely used for As an example, the presence or absence of methane can be detected with high sensitivity by utilizing the fact that the 3.3922 μm oscillation line of the laser light generated by the He—Ne laser is strongly absorbed by methane. Since methane is the main component of city gas, leakage of city gas can be detected by detecting methane gas. However, when gas detection is performed using a semiconductor laser, the oscillation wavelength of the laser beam must be stabilized in accordance with the center wavelength of the gas absorption line because the laser element greatly changes the oscillation wavelength depending on the temperature.

[0003] Accordingly, the applicant of the present application filed an application on June 28, 1991 for a gas detection device in which the oscillation wavelength of a semiconductor laser device was accurately controlled, and FIG. 2 is a block diagram of a temperature control unit. is there.

In FIG. 2, a part L ₂ of a laser beam generated from a semiconductor laser device 1 modulated by an alternating current of a predetermined frequency passes through a cell 2 filled with gas, and the laser beam passing through the cell 2 Is detected by the photodiode 3, and the output current of the photodiode 3 is converted by the current-to-voltage converter 6 including the resistor 4 and the operational amplifier 5 into the signal A of FIG.
Is converted to a voltage as indicated by. When the converted voltage is input to the phase-sensitive detector 7, a primary phase-sensitive detection signal (f signal) (not shown) is output to the output terminal 8.

FIG. 3 shows an example of output characteristics of the current-voltage converter 6 and the phase-sensitive detector 7 with respect to the temperature of the semiconductor laser device. The horizontal axis is the temperature of the semiconductor laser device, and the vertical axis is the output voltage. The output signal of the phase sensitive detector 7 is a resistor 9, 1
0, a signal which is sent to an offset adjuster 13 composed of a variable resistor 11 and an operational amplifier 12 and from which an offset is removed from an output signal of the phase sensitive detector 7 is used as an error signal as a PID (Proportional, Integra) signal.
(Land Differential proportional, integral and differential) controller 14 and the output voltage or current of the PID controller 14 adjusts the surface temperatures of both surfaces of the Peltier element 15 to predetermined temperatures. Since the f-signal is the first-order derivative of the output signal of the current-voltage converter 6, the offset is equal to that of the semiconductor laser device 1.
In the input / output characteristics of the above, it occurs due to a gradient when the oscillation output increases with an increase in the input current.

The near-infrared semiconductor laser device 1 has a characteristic that the wavelength of the laser beam becomes longer when the temperature of the laser device itself becomes higher, and the wavelength becomes shorter when the temperature of the laser device itself becomes lower. The wavelength of the laser light is controlled by controlling the temperature of the Peltier element 15 attached to the laser element 1. PID controller 1
Reference numeral 4 denotes a signal required to control the temperature of the Peltier element 15 as a control target by adding the input signal proportionally, integrated, and differentiated at a predetermined ratio. For detection of a gas having a specific absorption wavelength, a secondary phase-sensitive detection signal (2f signal) as shown by a signal B in FIG. 3 is used. Wherein the horizontal axis of the peaks P ₁ and P ₂ is the temperature, but the temperature is proportional to the wavelength, these peaks because it corresponds to the gas absorption wavelength in the cell 2 P ₁ and P ₂ The gas component can be detected by the position.

The phase-sensitive detector 7 extracts only a component having a specific frequency and a specific phase from a signal to be detected and demodulates it. The signal B shown in FIG. 3 is a 2f signal from the phase-sensitive detector, and corresponds to the signal A that is obtained by performing second-order differentiation on the wavelength. The reason why the 2f signal is used is that when a laser beam stabilized at the center of the methane absorption line is received by the photodiode 3, the intensity of the 2f signal changes to the methane concentration unless other conditions such as temperature and pressure change. This is because it is proportional.

As another method, a thermocouple is attached to the side surface of the heat conducting plate 16 between the semiconductor laser device 1 and the Peltier device 15, and the voltage obtained from the thermocouple and the detection gas By controlling the applied voltage or current of the Peltier device 15 based on the difference from the output voltage of a voltage generator that generates a voltage corresponding to the wavelength, the semiconductor laser device 1
There has been proposed a method of controlling the oscillation wavelength.

[0009]

However, in the method of controlling the oscillation wavelength using the first-order phase-sensitive detection signal as shown in FIG. 2, the stability of the oscillation wavelength is good, but the external temperature changes rapidly. Then, the control cannot catch up and the oscillation wavelength shifts. Further, there is another problem that the stability of the oscillation wavelength is not so high only by controlling the oscillation wavelength by detecting the temperature of the semiconductor laser element.

The present invention has been made in view of the above points, and an object of the present invention is to provide a gas detection device which has high stability of oscillation wavelength and is not affected by a sudden external temperature change. It is in.

[0011]

According to the present invention, there is provided a laser which oscillates a laser beam whose wavelength changes in accordance with a temperature, a temperature detector which detects a temperature of the laser, and an oscillating laser beam. A first voltage generator for generating a voltage corresponding to the temperature of the laser whose wavelength is set to a predetermined wavelength; and a difference between the output voltage of the first voltage generator and the output voltage of the temperature detector. First temperature control means for controlling the temperature of the laser to a predetermined temperature based on the temperature, a photodetector for detecting the intensity of the laser beam after passing through the reference atmosphere, and phase-sensitive detection of the output of the photodetector. A phase-sensitive detector that outputs a primary phase-sensitive detection signal, a second voltage generator that generates a voltage corresponding to the offset of the primary phase-sensitive detection signal, and a temperature control unit that controls the temperature by a first temperature control unit. After a predetermined time after being controlled, Second temperature control means for controlling the laser temperature to a predetermined temperature based on the difference between the voltage of the primary phase sensitive detection signal obtained by the detector and the output voltage of the second voltage generator; This can be achieved by a gas detection device provided with:

[0012]

The first temperature control means detects the temperature of a laser that oscillates a laser beam whose wavelength changes according to the temperature with a temperature detector, and the first voltage generator sets the oscillation wavelength of the laser beam to a predetermined value. A voltage corresponding to the temperature of the laser set to be the wavelength is generated, and the temperature of the laser is made constant based on the difference between the output voltage of the first voltage generator and the output voltage of the temperature detector. The second temperature control means detects the intensity of the laser beam after passing through the reference atmosphere with a photodetector, outputs a primary phase-sensitive detection signal, and outputs the primary phase-sensitive detection signal using a second voltage generator. Generates a voltage corresponding to the offset of the sensitive detection signal,
A predetermined time after the temperature is controlled by the first temperature control means, the laser temperature becomes constant based on the difference between the voltage of the primary phase-sensitive detection signal and the output voltage of the second voltage generator. Control.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an embodiment of a temperature control unit of a gas detection device according to the present invention. The temperature control unit of the gas detection device includes a semiconductor laser device 20, a cell 21, a photodiode 22, a first temperature control unit 23, and a second
And the temperature control unit 24 of FIG. The first temperature controller 23 includes a Peltier element 25, a window comparator 26, a PID controller 27, resistors 28a and 28b as a first voltage generator 28, and a delay buffer 2
9, the second temperature control unit 24 includes a Peltier device 30, a current-voltage converter 31, a phase-sensitive detector 32, and a resistor 33 serving as a second voltage generator 33.
a, 33b, a subtractor 34, and a PID controller 3
5 is comprised.

The semiconductor laser device 20 is mounted on a Peltier device 30 via a heat conductive plate 36. The Peltier device 30 is bonded to a heat conductive plate 38 having a thermocouple 37 mounted on a side surface. The Peltier element 25 is mounted on the opposite side (lower side in the figure) of the heat conduction plate 38.

The semiconductor laser element 20 is a phase sensitive detector 3
The laser beam is modulated at a frequency equal to the detection frequency of 2, and its oscillation wavelength becomes longer as the temperature of the laser element itself becomes higher, and becomes shorter as the temperature of the laser element itself becomes lower, as described above. A part L _{2 of the} laser light emitted from the semiconductor laser element 20 (the laser light emitted in the left direction in the figure) passes through a cell 21 in which gas as a reference atmosphere is sealed.

In general, a Peltier element is an element utilizing the Peltier effect. When two kinds of metals or semiconductors are joined and an electric current flows, not only Joule heat is generated but also heat is generated at the junction. Heat absorption occurs. In this phenomenon, when the direction in which the current flows is reversed, heat generation and heat absorption are reversed.

FIG. 4 is an explanatory diagram for explaining the Peltier effect.

As shown in FIG. 4, when a P-type semiconductor and an N-type semiconductor are connected by a conductor a and a battery E is connected between a P-type semiconductor electrode b and an N-type semiconductor electrode c, the electrode a generates heat. At the same time, the electrodes b and c absorb heat. By combining a plurality of such elements, the amount of heat generated and the amount of heat absorbed can be increased. By changing the applied voltage of the Peltier element or inverting the polarity, the Peltier element 25,
The wavelength of the laser light can be changed by changing the temperature of the semiconductor laser element 20 provided in the vicinity of 30.
The surfaces of the Peltier elements 25 and 30 that are joined to the heat conducting plate 38 simultaneously generate or absorb heat.

In this embodiment, the thermocouple 37 uses the Seebeck effect (a phenomenon in which a current flows through a metal in accordance with the temperature when the temperature of the connection point of two kinds of metals is raised). It is provided for the purpose of measuring the temperature of the connection point, one end of which is grounded, and the voltage V _{37 at} the other end is input to the window comparator 26.

The window comparator 26 is composed of a subtractor composed of an operational amplifier 41, resistors 39 and 40, and a comparator 42.
The output voltage V 37 of the thermocouple ₃₇ , the resistor 28a and the resistor 2
A signal is output based on the difference from the voltage V _{28 at} the connection point with 8b. The voltage V ₂₈ is set to a voltage corresponding to the temperature of the semiconductor laser device 20 set so that the oscillation wavelength of the semiconductor laser device 20 becomes a predetermined wavelength. Subtractor outputs a voltage difference obtained from the output voltage V ₃₇ of the thermocouple 37 by subtracting the voltage V ₂₈ to the PID controller 27. On the other hand, when the output voltage V _{37 of} the thermocouple ₃₇ , that is, the temperature of the junction of the heat conduction plate 38 is out of the predetermined range, the window comparator 26 outputs a signal of, for example, “0” logic level to the delay buffer 29. When the temperature of the junction of the heat conduction plate 38 is within a predetermined range, a signal of a logic level “1” is output to the delay buffer 29. Peltier device 2
Since it takes time to stabilize the temperature of the semiconductor laser element 20 by the temperature control by the step 5, the output signal from the window comparator 26 is delayed by the delay buffer 29 until the output voltage V37 of the thermocouple ₃₇ is stabilized. Then, it is input to the relay 47.

The current-to-voltage converter 31 comprises a resistor 48 and an operational amplifier 49.
Is converted to a voltage.

The phase-sensitive detector 32 modulates the signal to be detected at a specific frequency as described above, extracts only a component having a specific frequency and a specific phase from the modulated signal, and demodulates it. Output. This results in an extremely high S / N
A small signal can be detected by the ratio.

In general, when a laser beam modulated at a specific frequency passes through a gas that absorbs light of a specific wavelength and is received by a sensor, the output signal of the sensor has a DC component and the same as the modulation frequency. It consists of a fundamental wave component having a frequency and its harmonic components. When the fundamental component and the second harmonic component are phase-sensitive detected by the phase-sensitive detector, signals corresponding to the first derivative and the second derivative with respect to the respective wavelengths as described later can be obtained. The secondary differential signal is used for detecting a gas having a specific absorption wavelength and measuring the concentration of the gas, and the present invention uses the primary differential signal to stabilize the oscillation frequency of the laser light at a specific frequency. Things.

In general, the oscillation angular frequency Ω of a semiconductor laser is a function of the temperature and the drive current. When the drive current is modulated at an angular frequency ω (2πf) while keeping the temperature constant, Ω becomes
At the angular frequency ω.

[0026]

Ω = Ω ₀ + ΔΩ cosωt where Ω ₀ is the central oscillation angular frequency of the semiconductor laser, ΔΩ
Is the oscillation angular frequency modulation amplitude, and ω is the modulation frequency.

The oscillation angular frequency modulation amplitude [Delta] [omega is Taylor expand the transmittance T of the laser beam is represented by the number 2 in the Omega = Omega ₀ is smaller, the number 4 is calculated up to the second order term of [Delta] [omega.

[0028]

T = exp (-αcL) where c is the concentration of methane (partial pressure), L is the optical path length, T is the transmittance of laser light (proportional to the current of the photodiode 22), and α Is the absorption coefficient of methane. Especially, in the atmosphere, when attention is paid to one absorption line, α has a Lorentz-type frequency-dependent characteristic as shown in Expression 3.

[0029]

Α = γ ² α ₀ / ((Ω−ω _m ) ² + γ ² ) where ω _m is the center frequency of the absorption wavelength, and α ₀ and γ are constants.

[0030]

[Number 4] _{T = T 0 + (ΔΩ)} 2 T 0 "/ 4 + ΔΩT 0 'cosωt + ((ΔΩ) 2 T 0" cos2ωt) / 4 + ... , and the addition of the DC component, the component that varies according to cosωt and cos2ωt With. Where T ₀ , T
₀ ′ and T ₀ ″ are respectively the transmittance T at Ω = Ω ₀ , its first derivative dT / dΩ, and its second derivative d ² T / dΩ ²
, And are given as Equations 5, 6, and 7 below.

[0031]

T ₀ = 1−γ ² α ₀ cL / ((Ω ₀ −ω _m ) ² + γ ² )

[0032]

T ₀ ′ = 2 (Ω ₀ −ω _m ) γ ² α ₀ cL / [(Ω ₀ −ω _m ) ² + γ ² ] ²

[0033]

T ₀ ″ = −2 [3 (Ω ₀ −ω _m ) ² −γ ² ] γ ² α ₀ cL / [(Ω ₀ −ω _m ) ² + γ ² ] ³ Equation 6 is the primary phase It corresponds to the sensitive detection signal f and
Corresponds to the second-order phase-sensitive detection signal 2f.

The subtractor 34 includes resistors 51 and 52.
And the operational amplifier 53, and the phase sensitive detector 3
A voltage obtained by subtracting the offset voltage of the second voltage generator 33 from the output voltage V ₃₂ of the second device is sent to the PID controller 35. As described above, the offset is caused by the inclination when the output increases with an increase in the input current in the input / output characteristics of the semiconductor laser device 20.

The voltage V _{33 at} the connection point between the resistors 33 a and ₃₃ b is set to an offset voltage corresponding to the offset of the voltage value V ₃₂ of the primary phase sensitive detection signal of the phase sensitive detector ₃₂ . . Since the offset voltage varies depending on the type of the semiconductor laser device 20, it needs to be removed. The offset voltage is removed by the subtractor 34.

The relay 47 applies the output voltage or current of the PID controller 35 to the Peltier element 30 only when the output of the delay buffer 29 is "1".

Next, the operation of controlling the oscillation wavelength in the gas detection device according to the present invention will be described.

In FIG. 1, the temperature of the semiconductor laser device 20 is converted into a voltage V ₃₇ by a thermocouple 37 and input to a window comparator 26. The window comparator 26 compares the output voltage V 37 of the thermocouple ₃₇ with the voltage V _28, and when the voltage V ₃₇ of the thermocouple 37 exceeds the voltage V ₂₈ (V ₂₈
−V ₃₇ ), a negative voltage is sent to the PID controller 27, and the PID controller 27 applies a voltage or a current to the Peltier element 25 so as to lower the temperature at the connection point of the thermocouple 37, so that the temperature of the semiconductor laser element 20 is reduced. Goes down.
When the voltage V ₃₇ conversely becomes lower than the voltage V ₂₈ from this sends a positive voltage to the PID controller 27, PI
Since the D controller 27 applies a voltage of the opposite polarity to the Peltier element 25 so as to increase the temperature at the connection point of the thermocouple 37, the temperature of the semiconductor laser element 20 increases. The window comparator 26 sends a signal of a logical level “1” or “0” to the delay buffer 29. Delay buffer 29
When a "1" signal is input from the window comparator 23, a "1" logical level signal is sent to the relay 47 after a predetermined time.

On the other hand, part L ₂ of the laser light from the semiconductor laser element 20 passes through the cell 21 and passes through the photodiode 2.
2 is incident. The output current of the laser light detected by the photodiode 22 is converted into a voltage by the current-voltage converter 31. When the converted voltage is input to the phase-sensitive detector 32, a first-order phase-sensitive detection signal (f signal) is output to an output terminal 50 and input to a subtractor 34, where the offset is applied to the offset. The signal from which the voltage has been removed is sent to the PID controller 35 as an error signal, and the output voltage of the PID controller 35 is
7 is sent. This relay 47 from the delay buffer 29
The voltage or the current of the PID controller 35 is applied to the Peltier element 30 only when the signal sent to the P1 is at the logic level “1”.

As a result, the first temperature controller 23 controls the temperature of the semiconductor laser device 20 roughly, and the second temperature controller 24 controls the temperature precisely. That is, 2
Since the temperature control is performed heavily, the oscillation wavelength of the semiconductor laser element 20 is stabilized without shifting even if the external temperature changes suddenly.

[0041] Thus output as a voltage V ₃₇ detects the temperature of the semiconductor laser device 20 with a thermocouple 37, the semiconductor laser device 20 based on the difference between the voltage V ₃₇ and the voltage V ₂₈
The temperature of the Peltier device 25 attached to the semiconductor laser device 20 is controlled, and a part L _{2 of the} laser light emitted from the semiconductor laser device 20 is converted into the photodiode 2 via the gas in the cell 21.
Detected at 2, converted to voltage, and 1 via phase sensitive detector
The oscillation wavelength of the semiconductor laser element 20 is stabilized by controlling the temperature of the Peltier device 30 attached to the semiconductor laser device 20 based on the difference between the voltage V ₃₂ and the voltage V ₃₃ of the following differential signal.

In this embodiment, the values of the resistors 28a, 28b, 33a, 33b for removing the offset voltage are fixed, but if they are variable, it is possible to detect different gases.

[0043]

As described above, in the present invention, in the present invention, the temperature of the laser is detected and converted into a voltage, and the laser is controlled based on the difference between the converted voltage and the voltage of the first voltage generator. , A laser beam emitted from the laser after a predetermined time has passed through the reference atmosphere, and a voltage of the primary differential signal obtained by phase-sensitive detection and a second voltage generator. Is controlled so that the laser temperature becomes a predetermined temperature on the basis of the difference between the voltage and the voltage, the oscillation wavelength of the laser can be stabilized without being affected by a sudden external temperature change.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of a temperature control unit of a gas detection device according to the present invention.

FIG. 2 is a block diagram of an example of a gas detection device according to the present applicant.

FIG. 3 is a graph showing an example of an output characteristic of a current-voltage converter and a phase-sensitive detector with respect to a temperature of a semiconductor laser device used in the present invention.

FIG. 4 is an explanatory diagram for explaining a Peltier effect.

[Explanation of symbols]

 Reference Signs List 20 semiconductor laser device 21 cell 22 photodiode 23 first temperature control unit 24 second temperature control unit 25, 30 Peltier device 37 thermocouple

Claims (4)

(57) [Claims] 1. A laser that oscillates a laser beam whose wavelength changes in accordance with a temperature, a temperature detector that detects the temperature of the laser, and an oscillation wavelength of the laser beam is set to be a predetermined wavelength. First to generate a voltage corresponding to the temperature of the laser
And a first temperature control means for controlling the temperature of the laser to be a predetermined temperature based on a difference between an output voltage of the first voltage generator and an output voltage of the temperature detector. A photodetector for detecting the intensity of the laser beam after passing through the reference atmosphere, a phase-sensitive detector for phase-sensitively detecting the output of the photodetector, and outputting a primary phase-sensitive detection signal; A second voltage generator for generating a voltage corresponding to the offset of the phase-sensitive detection signal, and a primary voltage obtained by the phase-sensitive detector a predetermined time after the temperature is controlled by the first temperature control means. Controlling the laser temperature to a predetermined temperature based on the difference between the voltage of the phase-sensitive detection signal and the output voltage of the second voltage generator.
And a temperature control means. 2. The method according to claim 1, wherein said laser is a semiconductor laser.
The gas detection device according to item 1. 3. The gas detection device according to claim 1, wherein each of the first and second temperature control means has a Peltier element. 4. The gas detecting device according to claim 1, wherein said second temperature control means includes a delay buffer.