JP2796649B2 - 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 detector 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 by using a semiconductor laser, the oscillation wavelength of the laser beam needs to be stabilized in accordance with the center wavelength of the absorption line of the gas because the laser element greatly changes the oscillation wavelength depending on the temperature.

As an example of a gas detection device used for quantitative analysis of gas or the like, a device as shown in FIG. 5 has been considered. This gas detection device controls a semiconductor laser element 1 whose wavelength changes mainly in accordance with a temperature, a modulation system 2 for modulating a laser beam of the semiconductor laser element 1 and a temperature of the semiconductor laser element 1 to be constant. And a detection system 4 for detecting the concentration of gas.

The modulation system 2 comprises a constant current source 5, a function synthesizer 6 for generating an arbitrary function signal, a capacitor 7, resistors 8, 9 and a coil 10, and the control system 3 comprises a cell 11 , Photodiode 12, current-to-voltage converter 13, phase-sensitive detector 14, and subtracter 1.
5, the offset voltage generator 16, and the PID (Prop
orientational, Integral and Dif
and a Peltier element 18. The detection system 4 includes:
It comprises a cell 19, a photodiode 20, a current-to-voltage converter 21, phase-sensitive detectors 22 and 23, and a divider 24.

The semiconductor laser device 1 is excited by a constant DC current supplied from a constant current source 5 and generates a sinusoidal voltage V ₁ (Asinω) generated by a function synthesizer 6.
It is represented by t. Where A is the amplitude, ω is the angular frequency, and t is the time), and is modulated by the sinusoidal current I ₁ converted via the resistor 8.

[0006] In the control system 3, the laser beam L ₂ generated in the rear (left direction in the drawing) from the semiconductor laser element 1 which is modulated by the AC current I _1, cell 1 reference gas is sealed
1, the laser light passing through the cell 11 is received by the photodiode 12, and the output current is converted by the current-voltage converter 13 into a voltage as shown in the signal (a) of FIG. When this signal (a) is input to the phase-sensitive detector 14, a primary phase-sensitive detection signal shown in FIG. 2B is output.

FIG. 2 shows an example of an output characteristic of the phase sensitive detector. The horizontal axis represents the temperature (proportional to the wavelength) of the semiconductor laser device, and the vertical axis represents the output voltage of the current-voltage converter and the phase-sensitive detector. The dents appearing in FIG. 2A represent absorption lines, and FIG. 2B corresponds to the first derivative signal of the absorption lines. Here, it should be noted that an offset voltage d is generated in FIG. This offset voltage is caused not only by a change in wavelength but also by an increase in output when the input current of the semiconductor laser element is increased. FIG. 3 shows the relationship between the input current and the oscillation output of the semiconductor laser device. The horizontal axis represents the input current, and the vertical axis represents the oscillation output.

A first-order phase-sensitive detection signal (f signal) is input from the phase-sensitive detector 14 to an inverting input terminal of a subtractor 15. The voltage corresponding to d is applied to the non-inverting input terminal of the subtracter 15 by the offset voltage generator 16.

A signal obtained by removing the offset from the output signal of the phase sensitive detector 14 is sent from the subtracter 15 to the PID controller 17 as an error signal.
The surface temperature of both surfaces of the Peltier element 18 is adjusted to a predetermined temperature by the output voltage or current of the controller 17. The near-infrared semiconductor laser device 1 has a characteristic that the wavelength of the laser beam becomes longer when the temperature of the semiconductor laser device 1 itself becomes higher, and the wavelength becomes shorter when the temperature of the laser device itself becomes lower. The wavelength of the laser beam is controlled by controlling the temperature of the Peltier device 18 attached to the device 1. The PID controller 17 outputs a voltage or a current necessary for controlling the temperature of the Peltier element 18 as a control target by adding the input signal proportionally, integrated and differentiated at a predetermined ratio. The oscillation wavelength of the semiconductor laser device 1 is stabilized by controlling the temperature of the Peltier device 18.

On the other hand, the detection system 4 passes the laser beam L ₁ output in front of the semiconductor laser device 1 (to the right in the figure) through a cell 19 filled with gas as a detection atmosphere, and passes through the cell 19. The laser light is received by the photodiode 20, and the output current of the photodiode 20 is converted by the current-voltage converter 21 into a voltage as shown in the signal (a) of FIG. The voltage-converted signals are input to phase-sensitive detectors 22 and 23, respectively.
2 shows a first-order phase-sensitive detection signal (f signal) as shown in signal (b) of FIG. 2, and a second-order phase-sensitive detection signal as shown in signal (c) of FIG. Detection signals (2f signals) are output, and these signals are input to the divider 24. The divider 24 outputs a 2f signal proportional to the gas concentration and the amount of light passing through the cell 19 to f
By dividing by the signal, the concentration of the gas is obtained and the terminal 2
5 is output.

In general, a phase-sensitive detector extracts only a component having a specific frequency and a specific phase from a signal to be detected and demodulates it. The signal (c) in FIG. 2 is a second-order phase-sensitive detection signal (2f), and corresponds to the signal obtained by performing the second-order differentiation on the frequency of the signal (a). When the laser beam stabilized at the center of the absorption line of methane is received by the photodiode 20, the 2f signal is proportional to the methane concentration unless other conditions such as temperature and pressure change. Used to measure gas concentration.

[0012]

However, as shown in FIG. 3, the oscillation output of the semiconductor laser device does not actually increase linearly with respect to the input current as shown by the broken line, but as shown by a solid line, as shown in FIG. To increase. As shown in FIG. 2C, this curvature appears as an offset e of the second-order phase-sensitive detection signal of the phase-sensitive detector. Since the density of the detection atmosphere is obtained based on the secondary differential signal of the phase sensitive detector based on the primary differential signal, if the offset e remains, an error occurs in the detected density.

The present invention has been made in view of the above-mentioned points, and has as its object the purpose of the present invention based on primary and secondary phase-sensitive detection signals obtained by phase-sensitive detection of laser light after passing through a detection atmosphere. To remove the offset of the secondary phase-sensitive detection signal in the gas detection device for obtaining the concentration of the detection atmosphere, thereby eliminating the concentration measurement error.

[0014]

According to the present invention, there is provided a laser which oscillates a laser beam which is modulated by a current having a first AC component superimposed on a constant current and whose wavelength changes in accordance with temperature. A current-voltage converter for converting the laser light after passing through the detection atmosphere into a voltage, two phase-sensitive detectors for phase-sensitive detection of the output voltage of the current-voltage converter, and 1 obtained from one of the phase-sensitive detectors A gas detection device for detecting the concentration of a detection atmosphere based on a next phase-sensitive detection signal and a second phase-sensitive detection signal obtained from the other phase-sensitive detector, wherein the gas detection device is twice as large as the first AC component. AC current generating means for generating a second AC component varying with frequency;
A superimposed AC component on the first AC component to offset the offset of the secondary phase-sensitive detection signal due to the curved portion in the output characteristics of the laser. Achieved.

[0015]

According to the gas detecting device of the present invention, a laser which oscillates a laser beam whose wavelength changes according to the temperature is modulated by a first AC component, and the first AC component is added to the first AC component.
The second AC component that changes at twice the frequency is superimposed to offset the offset of the secondary phase-sensitive detection signal due to the curved portion in the output characteristics of the laser.

[0016]

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

FIG. 1 shows a schematic configuration of a gas detection device according to the present invention. The same components as those shown in FIG. 5 are denoted by the same reference numerals.

The gas detecting device includes a semiconductor laser device 1 for generating a laser beam having a wavelength that changes according to temperature, a modulation system 2 for modulating the semiconductor laser device 1, and a semiconductor laser device 1 having a constant temperature. Control system 3 for controlling temperature
And a detection system 4 for detecting the gas concentration.

The semiconductor laser element 1 emits the laser beams L ₁ and L ₂ in both the front and rear directions as described above, and outputs the forward output light L _1.
(Right direction in the figure) is passed through the cell 19 and the rear output light L ₂
Pass through the cell 11. The wavelengths of the laser beams L ₁ and L ₂ emitted by the semiconductor laser device 1 become longer as the temperature increases, and become shorter as the temperature decreases.

FIG. 3 shows the output characteristics of the semiconductor laser device 1. The horizontal axis represents the input current (mA), and the vertical axis represents the oscillation output (mW). The semiconductor laser device 1 generates a laser beam when the input current value exceeds a certain value as shown in FIG.
The output of the laser beam also increases as the current value increases. As described above, the oscillation output ideally desirably increases linearly as indicated by a broken line, but actually increases in a curved manner as indicated by a solid line. This curvature corresponds to the offset e of the secondary phase-sensitive detection signal described later.

Returning to FIG. 1, the modulation system 2 includes a constant current source 5, a function synthesizer 100, a capacitor 7, a coil 10, and resistors 8, 9, 101, 10
And 2.

The constant current source 5 supplies a constant current to the semiconductor laser device 1 via the resistor 9 and the coil 10 to cause the semiconductor laser device 1 to oscillate.

The function synthesizer 100 includes an AC voltage V ₁ represented by a sine wave Asinωt (amplitude A, angular frequency ω) and a sine wave Bsin (2ωt + φ).
(Amplitude B, the angular frequency is 2 [omega, phase phi) generates an AC voltage V ₂ represented by. These AC voltage V ₁ and AC voltage V ₂ are connected to I ₁ , I ₂ via resistors 101 and 102.
_It becomes ₂ and is mixed as a current of I ₁ + I ₂ , and is applied to the semiconductor laser device 1 via the capacitor 7 and the resistor 8, so that the laser light is modulated. The amplitude B and the phase φ of the sine wave Bsin (2ωt + φ) representing the AC voltage V ₂ generated by the function synthesizer 100
Is an offset e of a second-order phase-sensitive detection signal described later.
Has been adjusted to offset.

The control system 3 includes a cell 11, a current-voltage converter 13, a phase-sensitive detector 14, a subtracter 15, an offset voltage generator 16, a PID controller 17,
It comprises a Peltier element 18. A gas for reference (for example, 5% methane gas) is sealed in the cell 11, and the laser beam L ₂ output backward passes through the cell 11. The laser light that has passed through the cell 11 is received by the photodiode 12, converted into a current, and input to the current-voltage converter 13. The output current of the photodiode 12 is converted into a voltage by a current-voltage converter 13 composed of a resistor 103 and an operational amplifier 104, and the phase-sensitive detector 1
4 is output.

The phase sensitive detector 14 extracts only a component having a specific frequency and a specific phase from a signal to be measured as described above, demodulates and outputs the extracted signal. This makes it possible to detect a very small signal with an extremely high S / N ratio. The phase sensitive detector 14 receives the input signal (a) as shown in FIG.
Is input, the signal (b) (first-order phase-sensitive detection signal f)
Alternatively, the signal (c) (second-order phase-sensitive detection signal 2f) is output.

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 will include the modulation frequency and the DC component in addition to the DC component. It consists of a fundamental component having the same frequency and its harmonic components. When the fundamental wave 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. In the present invention, detection of a gas having a specific absorption wavelength and measurement of the gas concentration are performed using the second derivative signal.

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, f is the frequency) while keeping the temperature constant, Ω becomes Therefore, it is modulated at the angular frequency ω.

[0028]

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 angular 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.

[0030]

T = exp (−αcL) where c is the concentration of methane (partial pressure), L is the optical path length, T is the transmittance of the laser beam (proportional to the current of the photodiode 22), and α is the methane concentration. It is an absorption coefficient. In particular, α has a Lorentz-type frequency-dependent characteristic as shown in the following Expression 3 when focusing on one absorption line at atmospheric pressure.

[0031]

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

[0032]

[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, will vary according to the cosωt and cos2ωt Has components. 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.

[0033]

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

[0034]

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

[0035]

T ₀ ″ = −2 [3 (Ω ₀ −ω _m ) ² −γ ² ] γ ² α ₀ cL / [(Ω ₀ −ω _m ) ² + γ ² ] ³ Cosω of the transmittance T
If the t and cos2ωt components are subjected to phase-sensitive detection, signals proportional to T ₀ ′ and T ₀ ″, respectively, are obtained.
₀ 'corresponds to the signal (b) of FIG.
₀ "corresponds to the signal (c), that is, the 2f signal. The phase-sensitive detector 14 inputs the first-order phase-sensitive detection signal f to the inverting input terminal of the subtracter 15.

The resistors 105 and 106 and the operational amplifier 107
The non-inverting input terminal of the subtractor 15 comprises the voltage Vo of the slider of the variable resistor 16 pulled up by the power supply voltage.
s is input, and this voltage Vos is offset d
It corresponds to. The subtracter 15 calculates a signal obtained by subtracting the primary offset d from the primary phase-sensitive detection signal as a PID.
Output to the controller 17.

When the oscillation wavelength of the semiconductor laser device 1 is shorter than a predetermined wavelength, the PID controller 17 determines whether the Peltier device 18
Is output such that the temperature of the semiconductor laser element 1 is increased, and when the wavelength becomes longer than a predetermined wavelength, a voltage that decreases the temperature of the Peltier element 18 is output, so that the oscillation wavelength of the semiconductor laser element 1 is stabilized.

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 is applied, not only Joule heat is generated but also heat is generated at the junction, or 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 the figure, 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 18 or inverting the polarity, the Peltier element 18
By changing the temperature of the semiconductor laser device 1 attached to the device, the wavelength of the laser beam can be changed. The detection system 4 includes a cell 19, a photodiode 20, a current-to-voltage converter 21, two phase-sensitive detectors 22 and 23, and a divider 24. Cell 19 is filled with a gas for detection, forward output light L ₁ of the semiconductor laser element 1 passes.

As described above, the photodiode 20 converts the laser beam after passing through the cell 19 into a current, and
-To-voltage converter 2 composed of
Output to 1. The phase-sensitive detector 22 performs phase-sensitive detection on the output of the current-voltage converter 21 and outputs a primary phase-sensitive detection signal f, and the phase-sensitive detector 23 outputs a secondary phase-sensitive detection signal 2f. . The divider 24 divides the secondary phase-sensitive detection signal 2f by the primary phase-sensitive detection signal f. Since the signal f is proportional to the light amount and the signal 2f is proportional to the light amount and the gas concentration, a signal representing the gas concentration can be obtained from the terminal 25 of the divider 24.

Next, the operation of the gas detection device of the present invention for removing the offset from the secondary phase-sensitive detection signal will be described.

The semiconductor laser device 1 emits laser beams L ₁ and L ₂ by the DC current generated by the constant current power supply 5, and the function synthesizer 100 generates an AC voltage V ₁ and an AC voltage V _{2. 1} , L ₂ is the alternating current I
It is modulated by the ₁ + I _2. Of these modulated laser beams L ₁ and L ₂ , the laser beam L ₁ passes through the cell 19, is converted into a current by the photodiode 20, is converted into a voltage by the current-voltage converter 21, and is converted into a phase-sensitive detector. 22, 2
3 is input. The phase-sensitive detector 22 outputs the primary phase-sensitive detection signal f to the divider 23.

On the other hand, the offset of the output characteristic of the semiconductor laser device 1 shown in FIG. 3 is offset by mixing the AC current I ₂ with the AC current I ₁ , and the second-order phase-sensitive detection signal shown in FIG. The offset e is removed. When the secondary phase-sensitive detection signal from which the offset e has been removed is output to the divider 24, the divider 24 converts the secondary phase-sensitive detection signal 2f from which the offset e has been removed into the primary phase. A value divided by the sensitive detection signal f, that is, a signal representing the concentration of gas in the cell 19 is output to the terminal 25.

Thus, the function synthesizer 100 for generating the AC voltage V ₁ represented by the sine wave Asinωt and the AC voltage V ₂ represented by the sine wave Bsin (2ωt + φ) is modulated by the AC current I ₁ + I _2. a semiconductor laser element 1 that oscillates laser light of different wavelengths depending on the temperature, the current-voltage converter 21 for converting the laser light L ₁ after the cell 19 passes to a voltage, a current-voltage converter 2
Two phase-sensitive detectors 22 and 23 for phase-sensitive detection of the output voltage 1 and the secondary phase-sensitive detection signal 2f obtained from each phase-sensitive detector 22 and 23 are divided by the primary phase-sensitive detection signal f. by superimposing an alternating current I ₂ frequency alternating current I ₁ in the gas detection device for detecting the value as the concentration of the gas in the cell 19 is changed at twice the frequency of the alternating currents I _1, 2-order phase-sensitive Since the offset e of the detection signal 2f is canceled, the accurate concentration of the gas sealed in the cell 19 can be detected.

[0046]

As described above, in the present invention, a laser that oscillates a laser beam whose wavelength changes according to temperature is modulated by the first AC component. The second that changes at twice the frequency of the first AC component
Is superimposed to offset the offset of the secondary phase-sensitive detection signal due to the curved portion in the output characteristics of the laser, so that the gas concentration can be accurately detected.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of a gas detection device of the present invention.

FIG. 2 shows output characteristics of a phase sensitive detector.

FIG. 3 shows output characteristics of a semiconductor laser device.

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

FIG. 5 is a block diagram showing a schematic configuration of an example of a gas detection device by the present applicant.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser element 2 Modulation system 3 Control system 4 Detection system 5 Constant current source 12, 20 Photodiode 14, 22, 23 Phase sensitive detector 100 Function synthesizer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-57-29933 (JP, A) JP-A-63-9843 (JP, A) JP-A-64-9677 (JP, A) JP-A-60-1985 253953 (JP, A) JP-A-1-235835 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 21/00-21/01 G01N 21/17-21/61

Claims (1)

(57) [Claims] 1. A laser which oscillates a laser beam which is modulated by a current having a first AC component superimposed on a constant current and whose wavelength changes in accordance with a temperature, and a laser beam which has passed through a detection atmosphere and has a voltage Current-voltage converter, two phase-sensitive detectors for phase-sensitive detection of the output voltage of the current-voltage converter, a primary phase-sensitive detection signal obtained from one phase-sensitive detector, and the other phase-sensitive signal 2 obtained from a sensitive detector
What is claimed is: 1. A gas detection device for detecting a concentration of a detection atmosphere based on a next phase-sensitive detection signal, comprising: an AC current generating means for generating a second AC component which changes at a frequency twice as high as the first AC component. And offset canceling means for superimposing the second AC component on the first AC component to cancel the offset of the secondary phase-sensitive detection signal due to the curved portion in the output characteristic of the laser. A gas detection device, comprising: