JP3114959B2 - Gas concentration detection method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for detecting a gas concentration in an atmosphere from the amount of transmitted laser light, and more particularly to a method for removing drifting background noise.
The present invention relates to a method and an apparatus for detecting a gas concentration capable of compensating a temporal change of a laser.

[0002]

2. Description of the Related Art Gas molecules have the property of absorbing laser light of a specific wavelength. It is known that the presence or absence of gas can be detected by utilizing this phenomenon, and a sensing technique using this principle is widely used in industrial measurement, pollution monitoring, and the like. Further, if this laser beam is transmitted through an optical fiber, remote monitoring becomes possible.

A remote gas detector using an optical fiber as a transmission line according to the present applicant modulates a driving current of a semiconductor laser at a high frequency around a predetermined current, and modulates a laser beam having a modulated wavelength and intensity. To oscillate. Further, the current and temperature are controlled so that the center wavelength of the oscillation becomes the center of the gas absorption line. In order to obtain stable oscillation, laser light emitted behind the semiconductor laser is used for monitoring. The laser beam which is stabilized and emitted forward is guided to a gas cell for measurement filled with gas of unknown concentration by an optical fiber. The transmitted light that has passed through the gas atmosphere is guided to the light receiver by another optical fiber, and the gas concentration is detected from the double detection signal or the basic detection signal of the laser light. Thereby, the gas concentration can be detected at a high S / N ratio.

However, focusing on one isolated absorption line of the gas, the shape of the absorption line changes depending on the pressure of the gas atmosphere, and the double detection signal used for quantitative measurement of the gas also has a value dependent on the pressure. Have. For this reason, if concentration measurement is performed using this gas detection device in a place such as a coal mine or a plant where the atmospheric pressure changes rapidly, accurate concentration measurement cannot be performed unless a pressure sensor is separately provided to monitor the pressure and pressure correction is performed.

Next, the present applicant uses a laser that oscillates a laser beam having a wavelength and intensity corresponding to the drive current and temperature.
After changing the drive current or temperature of this laser to oscillate laser light whose wavelength and intensity are modulated, sweep the center wavelength of the laser light, and pass the laser light through the gas atmosphere to be measured. A method was proposed in which the intensity of transmitted light was detected, a specific component of the detection signal was phase-sensitive detected, and the specific gas concentration under the above-mentioned atmospheric pressure was measured from the detection signal.

FIG. 4 shows the relationship between the wavelength of a laser beam and the ratio between a double phase-sensitive detection signal obtained by phase-sensitive detection and a fundamental-wave phase-sensitive detection (hereinafter, this ratio is referred to as a gas signal). Shown in In FIG. 4, the horizontal axis represents the center wavelength of the laser beam, and the vertical axis represents the gas signal. This is an output signal when the detection gas is acetylene gas and the absorption wavelength of the gas is 1.532 μm. The gas concentration is determined from this signal.
Further, the wavelength width of the two extreme values appearing on both sides near the gas absorption line depends on the spectrum width of the gas absorption line, and if the relationship between the spectrum width of the absorption line and the gas atmosphere pressure is grasped in advance, From this value, the pressure of the gas atmosphere can be obtained.

[0007]

However, the laser light from the light source has a small excess distortion component due to the wavelength dependence of the coupling between the laser and the optical fiber or the deterioration of the laser. Then it appears in a drifted form. If a low-concentration gas is detected under the same modulation condition, the gas signal component is buried in the drifting background noise signal, and an accurate gas concentration cannot be obtained. FIG. 5 is an example of this, and since the drift component increases from the short wavelength side to the long wavelength side, the waveform shown in FIG. 4 cannot be obtained.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a gas concentration detecting method and apparatus capable of solving the above-mentioned problems, removing drifting background noise, and compensating for a change with time of a laser.

[0009]

In order to achieve the above object, a method of the present invention uses a laser that oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature, and modulates the wavelength by driving the drive current. And oscillates laser light with modulated intensity, and sweeps a drive current to sweep the center wavelength of the laser light, passes the laser light through a gas atmosphere to be measured, receives transmitted light, and receives the received light. Phase-sensitive detection of a specific component of the signal, and from this detection signal, the gas concentration
In the gas concentration detecting method of detecting the gas concentration by correcting the gas concentration with the gas pressure and detecting the gas concentration, a part of the laser light is directly received without passing through the gas atmosphere to detect the transmitted light of the gas atmosphere. Drift, which is the detection component of the component and direct light
The difference from the noise component as the gas signal,
Part of is passed through a gas atmosphere of known concentration to receive transmitted light
Then, the first detection component is stored as an initial value,
After that, the detected component of transmitted light of gas of known concentration
Calculates the amount of correction for the change over time of the laser by comparing with
The concentration of the gas is detected by correcting the gas signal with the amount .

[0010]

The apparatus further includes a laser that oscillates laser light having a wavelength and intensity corresponding to the drive current and temperature, a modulation circuit that oscillates laser light whose wavelength and intensity are modulated by modulating the drive current, A sweep circuit that sweeps the center wavelength of the laser light by sweeping the drive current, an optical fiber that guides the laser light, a gas cell that is inserted into the optical fiber and fills the gas atmosphere to be measured, and a light transmitted through the gas cell. A receiver that receives the light and phase-sensitive detection of a specific component of the received light signal . From this detected signal, the gas concentration and pressure are detected.
In a gas concentration detection device comprising a detection circuit for detecting a force and correcting the gas concentration with a gas pressure to detect the gas concentration, a branching means for branching a part of the laser light, A receiver that directly receives light without passing through a gas atmosphere;
Is detected component of the detection component and the direct light gas atmosphere transmitted light
A noise elimination circuit that uses the difference from the drift noise component as a gas signal; a branching unit that branches a part of the laser light;
Standard for filling gas atmosphere of known concentration connected to diverter
Gas cell and receiver for receiving light transmitted through reference gas cell
And the initial detection component of the transmitted light of this reference gas cell
A storage circuit for storing the values as values, and
Compare the over-light detection component with the initial
A correction amount is obtained, and the correction amount of the gas signal is corrected with the correction amount.
And a positive circuit .

[0012]

[0013]

According to the above construction, a part of the laser beam is branched by the branching means, and the branched light is directly received by the light receiver without passing through the gas atmosphere. When the center wavelength of the laser light is swept, a drift component based on the wavelength dependence of the coupling between the laser and the optical fiber is detected. On the other hand, a drift component is superimposed on a gas signal in a light that is transmitted through a gas atmosphere to be measured and received. Therefore, only the gas signal is obtained from the difference between the detection component of the light transmitted through the gas atmosphere and the detection component of the direct light, and the gas concentration can be detected from the gas signal.

[0014] The distortion caused by the deterioration of the laser can be removed by comparing the detection results with gases of known concentrations. That is, a part of the laser light is branched by the branching means, and the branched light is guided to the reference gas cell. The initial detection component of the transmitted light of the reference gas cell is stored as an initial value, and the detected component of the transmitted light of the subsequent reference gas cell is compared with the initial value, whereby the change with time of the laser can be corrected.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

As shown in FIG. 1, the gas concentration detecting device according to the present invention mainly comprises a laser unit, an optical system, a laser driving circuit 1, and a signal processing unit 20.

The laser section includes a distributed feedback semiconductor laser (DFB-LD) 2 for oscillating laser light of a single wavelength,
A Peltier element 3 for mounting the FB-LD 2 and controlling its temperature. Optical components including a connector for coupling the laser beam from the DFB-LD 2 to the light branching unit 4 and a condenser lens, or an optical isolator for cutting the return light from the condenser lens are omitted in the drawings. Further, the end faces of these optical components are subjected to an anti-reflection coating process, and the return light to the DFB-LD 2 is extremely small.

The optical system includes an optical branching means 4, outgoing optical fibers 5a, 5b, 5c, a gas cell 6 to be detected, a reference gas cell 7, returning optical fibers 5d, 5e, light receivers 8a, 8b, 8c. The light splitting means 4
The laser light from the DFB-LD 2 is transmitted to the optical fiber 5 for the forward path.
a, 5b and 5c.

The gas cell 6 to be detected is a container filled with various gases (methane, acetylene, etc.) of unknown concentration to be measured, and can be easily installed at a position to be detected. I have. The outgoing optical fiber 5c is connected to one side of the gas cell 6 to be detected, and the returning optical fiber 5e is connected to the opposite side thereof.
Part of the laser light from the LD 2 passes through the atmosphere of the gas to be measured, and is received by the light receiver 8c at one end of the return optical fiber 5e.

The reference gas cell 7 is a container filled with an arbitrary gas serving as a reference. The gas can be easily injected and discharged, and the pressure can be arbitrarily set. One side of the reference gas cell 7 is connected to an outgoing optical fiber 5b,
On the opposite side, a return optical fiber 5d is connected, and a part of the laser light from the DFB-LD 2 passes through the reference gas atmosphere, and a light receiver 8 at one end of the return optical fiber 5d is connected.
b.

The outgoing optical fiber 5a is directly connected to the optical receiver 8a.
And a part of the laser light from the DFB-LD 2 is received by the light receiver 8a as it is. The end faces of the optical fibers 5a to 5e are treated by oblique cut anti-reflection coating or the like so that there is no interference system.

The laser drive circuit 1 includes an oscillator 10 for outputting a sine wave signal having a frequency ω, a frequency divider 11 for generating a second harmonic signal having a frequency 2ω from the sine wave signal having the frequency ω,
Bias current power supply 12 for adding bias current
And a sweeper 13 for determining how to sweep the bias current
And a power supply 9 for the Peltier element. The laser drive circuit 1 superimposes the sine wave signal from the oscillator 10 on the bias current from the bias current power supply 12
D2 is driven. Bias current power supply 1
2 is provided with an inductance L for preventing the influence of the output of the oscillator 10, and a capacitor C for cutting a DC component is provided on the output side of the oscillator 10.

The signal processing unit 20 includes a lock-in amplifier (ω) 14 for performing phase-sensitive detection of the outputs of the photodetectors 8a and 8b in synchronization with a sine wave signal of frequency ω from the oscillator 10, and a A lock-in amplifier for detection (ω) 16 for performing phase-sensitive detection of the output and a frequency 2 from the frequency multiplier 11
A lock-in amplifier (2ω) 15 that performs phase-sensitive detection of the outputs of the photodetectors 8a and 8b in synchronization with a second harmonic signal of ω, and a lock-in amplifier (2ω) that performs phase-sensitive detection of the output of the photodetector 8c ( 2ω) 17, an analyzing recorder 18 for recording and calculating the output ratio of these lock-in amplifiers, and an analysis processing device 19.

Next, the operation of the embodiment will be described.

An ordinary method for measuring gas concentration will be described.

First, the laser driving circuit 1 uses the following method to sweep the center wavelength of the laser light on the gas absorption line.

1) The temperature of the DFB-LD 2 is fixed at a constant value by the Peltier device 3 controlled by the Peltier device power supply 9.

2) The sweep current of the bias current of the DFB-LD 2 is linearly increased and decreased by the sweeper 13 so as to form a triangular wave, and the sweep is performed.

3) The modulation current of the DFB-LD 2 is a sine wave from the oscillator 10 and is superimposed on the bias current.

The laser light thus oscillated is guided to the light branching means 4 by an optical component (not shown) which has been subjected to an anti-reflection coating process, and is branched.

A part of the branched laser light enters the gas cell 6 to be detected via the outgoing optical fiber 5c, passes through a gas of unknown concentration, and is received by the optical receiver 8c via the returning optical fiber 5e. You. A part of the split laser light is directly passed through the optical receiver 8a through the outward optical fiber 5a.
Received.

In the signal processing section 20, the signal detected by the photodetector 8a is phase-sensitive detected by the lock-in amplifier (ω) 14 as a signal synchronized with the sine wave signal of the frequency ω from the oscillator 10, and the fundamental wave phase is detected. It is a sensitive detection signal. In addition, a lock-in amplifier (2ω) 15 performs phase-sensitive detection as a signal synchronized with a second-harmonic signal having a frequency of 2ω from the frequency multiplier 11 to obtain a second-harmonic phase-sensitive detection signal. These detected signals are recorded in the analyzing recorder 18.

On the other hand, the signal detected by the photodetector 8c is phase-sensitive detected by the lock-in amplifier (ω) 16 as a signal synchronized with the sine wave signal of the frequency ω from the oscillator 10, and the fundamental phase is detected. I do. In addition, a lock-in amplifier (2ω) 17 performs phase-sensitive detection as a signal synchronized with a second-harmonic signal having a frequency of 2ω from the frequency multiplier 11 to obtain a second-harmonic phase-sensitive detection signal. These detected signals are recorded in the analyzing recorder 18.

At this time, noise is superimposed on the second-harmonic phase-sensitive detection signal detected by the light receivers 8a and 8c and phase-sensitive detected by the lock-in amplifiers (2ω) 15 and 17. This is due to the wavelength dependence of the coupling between the laser and the optical fiber in the laser light of the light source, and there is a minute excess distortion component,
This is due to the fact that it appears as drifted noise when phase detected.

Since the laser beams split by the light splitting means 4 have different intensities, the received signal intensities also differ. Therefore, it is not possible to simply extract only the gas signal component from the difference between the two second-harmonic phase-sensitive detection signals. Accordingly, the fundamental wave phase sensitive detection signal recorded simultaneously is used.
That is, the signal processing unit 20 normalizes the signals by dividing both the second-harmonic phase-sensitive detection signals by the respective fundamental-wave phase-sensitive detection signals. The normalization removes the influence of the difference in received signal intensity.

A second-harmonic phase-sensitive detection signal detected by the light receiver 8a and phase-sensitive detected by the lock-in amplifier (2ω) 15, and a fundamental-wave phase-sensitive detection signal detected by the lock-in amplifier (ω) 14 Is the drift noise component, which is detected by the photodetector 8c and the lock-in amplifier (2
ω) 17 and the ratio of the second-harmonic phase-sensitive detection signal phase-sensitively detected by the lock-in amplifier (ω) 16 to the fundamental phase-sensitive detection signal is the gas signal component + drift noise component. Thus, a gas signal component can be obtained by taking these differences.

The gas concentration is determined from the peak value of the gas signal, and the pressure can be determined from the extreme value or half width value appearing on both sides of the peak value. At this time, drift noise has been removed, and accurate pressure and pressure-corrected accurate concentration can be obtained.

Next, a method of measuring and correcting the distortion of the gas signal due to the deterioration of the laser and the like will be described.

A part of the laser beam from the DFB-LD 2 is branched by the light branching means 4, enters the reference gas cell 7 via the outgoing optical fiber 5b, transmits the reference gas, and passes through the returning optical fiber 5d. The light is received by the light receiver 8b.

In the signal processing section 20, the signal detected by the photodetector 8b is phase-sensitive detected by the lock-in amplifier (ω) 14 as a signal synchronized with the sine wave signal of the frequency ω from the oscillator 10, and the fundamental wave phase is detected. It is a sensitive detection signal. In addition, a lock-in amplifier (2ω) 15 performs phase-sensitive detection as a signal synchronized with a second-harmonic signal having a frequency of 2ω from the frequency multiplier 11 to obtain a second-harmonic phase-sensitive detection signal. These detected signals are recorded in the analyzing recorder 18. Further, the ratio between the second-harmonic phase-sensitive detection signal and the fundamental-wave phase-sensitive detection signal is calculated and recorded in the signal processing unit 20.

In order to correct the time-dependent change of the laser, first, the reference gas cell 7 is filled with a gas of a specific known concentration, the above measurement is performed, and the initial value of the reference gas signal is recorded. Thereafter, the gas is filled with the above-mentioned known concentration every certain period, and the measurement is performed, and the result is recorded.

At the time of the normal gas concentration measurement, the correction value of the change with time of the laser is obtained by using the initial value and the latest measured value of the reference gas signal. The measured gas signal of the unknown concentration is corrected by this correction amount.

10 ppm by the gas concentration detector of FIG.
2 and 3 show the results of measuring methane at a concentration of. FIG. 2 shows the obtained drift noise component and gas signal component + drift noise component. FIG. 3 shows gas signal components extracted from these differences. The gas concentration can be determined from the peak value of the extracted gas signal, and the gas pressure can be determined from the extreme values on both sides of the peak value.

The gas concentration detecting device is a gas cell 6 to be detected.
Is attached to the connection point of the OF cable, the gas concentration at the connection point can be detected. This enables remote monitoring of the OF cable.

In the above embodiment, one-component gas was measured. However, multi-component gas can be measured as long as it is a gas molecule having a property of absorbing laser light of a specific wavelength. By switching or switching, the concentration of a plurality of gas components can be measured.

[0046]

The present invention exhibits the following excellent effects.

(1) Drift background noise alone can be detected by directly receiving a part of the laser light without passing through the gas atmosphere, so that the gas concentration can be detected without the background noise.

(2) By comparing the initial measured value and the measured value after a lapse of time with respect to a gas having a known concentration, a temporal change of a laser or the like can be detected. Becomes

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a gas concentration detection device showing one embodiment of the present invention.

FIG. 2 is a frequency distribution diagram of a gas signal (including a drift component) according to an embodiment of the present invention.

FIG. 3 is a frequency distribution diagram of only a gas signal according to an embodiment of the present invention.

FIG. 4 is a frequency distribution diagram of a gas signal used for measuring gas concentration and pressure.

FIG. 5 is a frequency distribution diagram of a gas signal buried in background noise.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser drive circuit 2 Distributed feedback semiconductor laser (DFB-LD) 3 Peltier element 4 Optical branching device 6 Gas cell to be detected 7 Reference gas cell 8a, 8b, 8c Light receiver

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akiyuki Nakamura 5-1-1, Hidaka-cho, Hitachi City, Ibaraki Prefecture Inside the Opto-System Research Laboratories, Inc. (72) Inventor Masahiko Uchida Hitachi City, Ibaraki Prefecture 5-1-1 Takamachi Inside Nippon Cable Co., Ltd. Optrosystem Research Laboratories (56) References JP-A-5-99845 (JP, A) JP-A-5-256768 (JP, A) JP-A-6 JP-A-2556769 (JP, A) JP-A-4-115142 (JP, A) JP-A-1-301149 (JP, A) JP-A-2-163603 (JP, A) JP-A-4-50639 (JP, A) (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01N 21/00-21/01 G01N 21/17-21/61 WPI / L (QUESTEL) JICST file (JOIS)

Claims (2)

(57) [Claims] 1. A laser that oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature, modulates the drive current, oscillates a laser beam whose wavelength and intensity are modulated, and sweeps the drive current. Sweeps the center wavelength of the laser light, passes the laser light through the gas atmosphere to be measured, receives the transmitted light, performs phase-sensitive detection of a specific component of the received light signal, and calculates the peak value of the detected signal. In a gas concentration detection method for detecting a gas concentration in which a gas concentration is detected by detecting a pressure from an extreme value or a half-width value, a part of the laser light is directly received without passing through the gas atmosphere, and the gas atmosphere is detected. detection of the detection signal of the transmitted light and direct light
The difference between the drift noise component is a signal to the gas signal, further the transmitted light received through a part of the laser light to a gas atmosphere of known concentration, is stored the first detection signal as an initial value, detection signal of the transmitted light of subsequent known concentration of gas
The initial value compared to the calculated correction amount of change over time of the laser, the gas concentration detection method characterized by detecting the concentration of a gas by correcting the gas signal at the correction amount No.. 2. A laser that oscillates laser light having a wavelength and intensity corresponding to a drive current and a temperature, a modulation circuit that oscillates laser light whose wavelength and intensity are modulated by modulating the drive current, A sweep circuit for sweeping the center wavelength of laser light by sweeping, an optical fiber for guiding the laser light, a gas cell inserted into the optical fiber to fill a gas atmosphere to be measured, and a light receiving device for receiving light transmitted through the gas cell And a detection circuit that detects a specific component of the received light signal in a phase-sensitive manner, detects a gas concentration based on a peak value of the detected signal, and detects a pressure based on an extreme value or a half-width value, thereby detecting a gas concentration corrected for pressure. A gas concentration detecting device comprising: a branching means for branching a part of the laser light; a light receiver for directly receiving the branched light without passing through the gas atmosphere; Transmitted light detection signal and direct light detection signal
A reference gas cell a difference between the drift noise component to be filled with a noise reduction circuit for a gas signal, and a branching means for branching a part of the laser light, the gas atmosphere of the connected known concentration to the branching unit is No., A receiver that receives the transmitted light of the reference gas cell and the first detection signal of the transmitted light of this reference gas cell
And a correction circuit for comparing the detection signal of the transmitted light of the reference gas cell with the initial value to obtain a correction amount of the change with time of the laser, and correcting the gas signal with the correction amount. A gas concentration detection device comprising: