JP2008298635A - Optical gas detection method and optical gas detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical gas detection method which enables the suppression of the variation of a gas signal due to the variation of a detection signal, and to provide an optical gas detector. <P>SOLUTION: In the optical gas detector equipped with a light source part 2 for sweeping a modulation center wavelength within a predetermined sweep range while modulating the wavelength of a laser beam, an optical system 3 for transmitting the laser beam through an atmosphere to be measured, a light detection part 4 for detecting a detection signal from the light detection signal of transmitted light by phase sensitive detection and a signal processing part 5 for calculating a gas signal from the detection signal, calculating the wave height value of the waveform of the gas signal and detecting the concentration of a predetermined gas in the atmosphere to be measured from the wave height value, the signal processing part 5 divides the gas signal by the divisor value corresponding to the level of the gas signal to calculate the wave height value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical gas detection method and an optical gas detection device that can suppress fluctuations in a gas signal due to fluctuations in detection signals.

  Gas molecules have the property of absorbing light in a specific wavelength band called an absorption band, and the amount of absorption depends on the gas concentration. Therefore, the gas concentration can be detected using this property. This optical gas detection method is used in fields such as industrial measurement and pollution monitoring. By transmitting this light through an optical fiber, it is possible to detect gas remotely.

  In the conventional technology such as Patent Document 1, the light source unit oscillates a laser beam having a modulated wavelength and intensity by modulating the driving current of the semiconductor laser with a high-frequency sine wave around a predetermined current value. . By sweeping the temperature of the semiconductor laser, the oscillation wavelength is swept within a predetermined wavelength range around the absorption band of the target gas at a predetermined sweep cycle.

  The laser light is incident on an optical fiber and guided to the optical system, and the target gas having an unknown concentration in the optical system is transmitted. The transmitted light is guided to the light receiving detection unit by the optical fiber, and the 1st detection signal and the 2nd detection signal are detected from the received light signal that has received the transmitted light by the phase sensitive detection.

  The ratio of the 1 × detection signal and the 2 × detection signal is obtained for the sweep period of the modulation center wavelength to obtain a gas signal. Is detected.

JP-A-5-256769

  Since the light source unit and the light receiving detection unit are composed of semiconductor elements, they have temperature characteristics and drift characteristics. An optical system such as an optical fiber also has temperature characteristics. The main factor of drift characteristics is due to temperature characteristics of the laser diode and changes in the characteristics of the optical fiber.

  Due to such temperature characteristics and drift characteristics, subtle fluctuations in the magnification of the circuit within the light receiving detection section occur, resulting in subtle fluctuations in the 1 × detection signal and 2 × detection signal, which causes the gas signal to fluctuate. As a result, the gas detection accuracy decreases.

  Accordingly, an object of the present invention is to provide an optical gas detection method and an optical gas detection device that can solve the above-described problems and can suppress a change in a gas signal due to a change in a detection signal.

  In order to achieve the above object, the optical gas detection method according to the present invention modulates the wavelength of the laser light, sweeps the modulation center wavelength within a predetermined sweep range, and transmits the laser light into the measured atmosphere. Detecting a detection signal by phase sensitive detection from the received light signal that has received the transmitted light, obtaining a gas signal from the detection signal, obtaining a peak value of the waveform of the gas signal, and obtaining a predetermined value in a measured atmosphere from the peak value In the optical gas detection method for detecting the gas concentration, the peak value is obtained after dividing the gas signal by a divisor value corresponding to the level of the gas signal.

  The gas signal may be obtained from a ratio of a DC (direct current) component signal detected from the detected signal or a detected signal from the received light signal and a detected signal twice.

  The gas signal may be obtained from a double detection signal among the detection signals.

  The divisor value may be the value of the gas signal at the beginning of the sweep cycle of the modulation center wavelength.

  The divisor value may be calculated from the value of the gas signal at the beginning and the value of the gas signal at the end of the sweep cycle of the modulation center wavelength.

  In order to achieve the above object, an optical gas detector of the present invention includes a light source unit that modulates the wavelength of a laser beam and sweeps the modulation center wavelength within a predetermined sweep range, and the laser beam in a measured atmosphere. An optical system that transmits light, a light receiving detection unit that detects a detection signal by phase sensitive detection from a light reception signal that has received the transmitted light, a gas signal is obtained from the detection signal, and a peak value of the waveform of the gas signal is obtained, And a signal processing unit that detects a concentration of a predetermined gas in the atmosphere to be measured from the peak value, wherein the signal processing unit converts the gas signal to a divisor value corresponding to the level of the gas signal. The peak value is obtained after dividing by.

  The signal processing unit may determine the gas signal from a ratio of a DC (direct current) component signal detected from the detected signal or a detected signal from the received light signal and a detected signal twice.

  The signal processing unit may obtain the gas signal from a double detection signal of the detection signals.

  The signal processing unit may use the divisor value as the value of the gas signal at the beginning of the sweep cycle of the modulation center wavelength.

  The signal processing unit may calculate the divisor value from a gas signal value at the beginning and a gas signal value at the end of the sweep cycle of the modulation center wavelength.

  The present invention exhibits the following excellent effects.

  (1) The fluctuation of the gas signal due to the fluctuation of the detection signal can be suppressed.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, an optical gas detection device 1 according to the present invention includes a light source unit 2 that modulates the wavelength and intensity of laser light and sweeps the modulation center wavelength within a predetermined sweep range, and the laser. An optical system 3 that transmits light into the atmosphere to be measured; a light receiving detector 4 that detects a detection signal from the received light signal that has received the transmitted light by phase sensitive detection; and a sweep period of the modulation center wavelength from the detection signal. In the optical gas detection device comprising the signal processing unit 5 for obtaining a gas signal of the gas signal, obtaining a peak value of a waveform of the gas signal, and detecting a concentration of a predetermined gas in the measured atmosphere from the peak value. The processing unit 5 obtains the peak value after dividing the gas signal by a divisor value corresponding to the level of the gas signal.

  The light source unit 2 includes a semiconductor laser 6, a Peltier element 7 that heats and cools the semiconductor laser 6, a temperature control constant current source 8 that controls the current of the Peltier element 7, a bias constant current source 9 that controls a bias current, and a bias An application inductor 10, an oscillator 11 that oscillates at a modulation frequency ω, an AC application capacitor 12, and a frequency multiplier 13 that generates a double frequency 2ω are provided.

  The optical system 3 is provided with a collimating lens 14 and a condensing lens 15 at both ends of a cylindrical body (gas cell) 21 having an atmosphere to be measured so that light propagates a predetermined distance in the atmosphere to be measured including the target gas. Is. The optical fiber 16 connects the light source unit 2 to the optical system 3 and the optical system 3 to the light receiving detection unit 4.

  The optical gas detector 1 includes, for example, a light source unit 2, a light receiving detector 4, and a signal processor 5 in a device main body (not shown), and a gas cell 21 is disposed at a desired site in the optical system 3. By laying the optical fiber 16 from the main body to the gas cell 21, remote monitoring becomes possible.

  The light receiving detection unit 4 includes a light receiving element 17 connected to the optical fiber 16, a lock-in amplifier 18 that detects a 1 × detection signal using the oscillation signal ω from the oscillator 11, and a frequency multiplied signal from the frequency multiplier 13. A lock-in amplifier 19 that detects a double detection signal using 2ω and a divider 20 that calculates a ratio of both detection signals are provided.

  Although the signal processing unit 5 is shown as a single block, the signal processing unit 5 is configured by a computer including a CPU, a memory, a clock, an A / D converter, and the like, and performs signal processing in a procedure to be described later.

  An outline of the operation of the optical gas detector 1 will be described.

  Light controlled to detect the target gas is emitted from the semiconductor laser 6 of the light source unit 2. Control by the light source unit 2 is to sweep the center wavelength of vibration at a relatively slow time interval (0.5 to 10 sec) while vibrating the wavelength of light at a relatively fast frequency ω (100 to 100 KHz). .

  In the optical system 3, light is incident on the gas cell 21 through the optical fiber 16. In the gas cell 21, the light collimated by the collimating lens 14 propagates a predetermined distance in the atmosphere to be measured, is condensed by the condenser lens 15, and is emitted from the gas cell 21. As light passes through the atmosphere to be measured, light absorption by the target gas occurs.

  The light that has passed through the optical system 3 is received by the light receiving element 17 of the light receiving detector 4. In the received light detection unit 4, a 1 × detection signal and a 2 × detection signal, which are 1 × components of the frequency ω, of the received light signal are detected, and the divider 20 calculates the ratio of the 1 × detection signal and the 2 × detection signal. To get a gas signal.

  The signal processor 5 performs necessary calculations (details will be described later) to detect the gas concentration.

  FIG. 2 shows the gas when the division of the double detection signal by the single detection signal is continuously performed for a predetermined time interval (sweep cycle) with the central wavelength sweep cycle in the light source unit 2 as a partition. 3 is a conceptual diagram of a signal 31. FIG. Multiple sweep cycles are displayed in an overlapping manner.

  The gas concentration is obtained by performing a known calculation on a vertical axis value (referred to as a peak value) at the peak 32 shown in the figure. However, in actual measurement, the measurement becomes unstable due to temperature characteristics and drift characteristics, and the waveform of the gas signal gradually shifts in the height direction at each sweep (the vertical axis value decreases overall or (Increased) phenomenon. Since the gas concentration is obtained by performing a predetermined calculation on the peak value of the gas signal 31, a measurement error gradually occurs in the gas concentration in accordance with the deviation.

FIG. 3 shows a graph of gas signals for a plurality of sweep periods when actual measurement is performed.
The numerical value on the horizontal axis represents the number of sampling timings. That is, sampling is performed at predetermined time intervals during the period of sweeping the wavelength over a predetermined range. Since the gas signal of each time is slightly different, the waveform spreads in a band shape. The fluctuation of the peak value is about 0.15 on the vertical scale.

  The present inventor has found that the gas signals for a plurality of sweep periods in FIGS. 2 and 3 can be transformed into waveforms that are very similar to each other by applying appropriate geometric deformation. That is, the gas signal for each sweep cycle of the modulation center wavelength is divided by a divisor value (for example, an instantaneous value at a specific time) obtained from the gas signal in the sweep cycle as a value corresponding to the level of the gas signal. To normalize the waveform. If the level of the gas signal is large over the sweep period, the divisor value is naturally a large value, so that a level reduction effect by division can be expected. On the contrary, if the level of the gas signal is small over the sweep period, the divisor value is naturally small, so that the level expansion effect by division can be expected. As a result, the difference in magnitude between the normalized gas signals for a plurality of sweep cycles is reduced as compared with the original gas signal. Thereby, the fluctuation | variation of a waveform can be restrained small.

  FIG. 4 shows the gas signal 31 that is the same as that in FIG. 2, in which the divisor value is the value of the gas signal 31 at the head 51 (33) of the modulation center wavelength sweep period, 3 is a conceptual diagram of a signal 52. FIG. As a result of the normalization process, the standardized gas signals 52 of a plurality of sweep periods are equal to each other at the head 51, and the respective waveforms are similar to each other. Therefore, the fluctuation of the peak value is reduced. Thereby, the error of gas concentration can be reduced.

  That is, the signal processing unit 5 extracts the value of the gas signal at the head from the gas signal for one sweep cycle and stores it as a divisor value, and divides the gas signal of the sweep cycle by the divisor value to obtain the normalized gas signal. Obtain the peak value from the normalized gas signal and use it as the gas concentration.

  FIG. 5 shows the gas signal 61 (31) for one sweep period of FIG. 2 with the divisor value as the value of the gas signal 31 at the head 62 (33) of the sweep period of the modulation center wavelength and the gas signal at the tail 63 (34). It is the conceptual diagram which computed from the value of 31 and showed the divisor value with the gas signal 61. FIG. Specifically, the value of the gas signal 61 at the head 62 and the value of the gas signal 61 at the tail 63 are connected by a straight line (referred to as a reference line) 64, and the vertical axis value at each time point of the reference line 64 is taken as a divisor value. The instantaneous value of the gas signal 61 is divided by the divisor value at each time point.

Here, the gas signal for one sweep cycle is represented by an integer n data strings. If the k-th value from the beginning is Vk, the leading value is represented by V ₀ and the trailing value is represented by V _n−1 . At this time, the kth value Lk of the reference line is calculated by the equation (1).

  FIG. 6 is a conceptual diagram of the gas signal 71 normalized by dividing the instantaneous value Vk of the gas signal 31 by the divisor value Lk. Overlays multiple sweep cycles. As a result of the normalization process, the standardized gas signal 71 having a plurality of sweep periods has the same magnitude at the head 72 and the tail 73 and is equal to each other, so that the fluctuation of the peak value is smaller than in the case of FIG. Thereby, the error of the gas concentration can be further reduced.

  In FIG. 7, the data actually measured in FIG. 3 is normalized by the divisor calculated from the first value and the last value of the gas signal sweep cycle for a plurality of sweep cycles. The graph of a result is shown. Since the normalized gas signals at each time are almost the same, the waveform appears to be one. The fluctuation of the peak value is about 0.01 on the vertical scale.

  As described above, in the present invention, even if the gas signal fluctuates due to fluctuations in the detection signal, fluctuations in the gas signal can be suppressed by correcting the gas signals by normalization. Therefore, the gas concentration error can be reduced.

  In the optical gas detection device 1 of FIG. 1, the light detection unit 4 detects a 1 × detection signal and a 2 × detection signal by phase sensitive detection from a received light signal that has received the transmitted light of the optical system 3, and the 1 × detection signal. The gas signal for the sweep period of the modulation center wavelength was obtained from the double detection signal. However, since the double detection signal reflects the gas concentration, the light reception detection unit 4 may detect only the double detection signal and use the double detection signal as it is as a gas signal. Further, the double detection signal is detected from the received light signal by phase sensitive detection, and the received light signal is passed through a low-pass filter (not shown) to detect the DC (direct current) component signal of the received light signal, and the double detected signal is converted to the DC ( The gas signal may be obtained by dividing by a (DC) component signal. In the signal processing unit 5, the gas signal is normalized as in the case of FIG. 1, and the gas concentration is detected from the peak value of the waveform of the normalized gas signal.

It is an optical and electric circuit block diagram of the optical gas detection apparatus which shows one Embodiment of this invention. It is a wave form diagram of a gas signal (a horizontal axis is a wavelength and a vertical axis is an unknown number). It is a waveform diagram of the gas signal by actual measurement (the horizontal axis is the wavelength, and the vertical axis is the anonymous number). It is a wave form diagram (a horizontal axis is a wavelength and a vertical axis is an unknown number) of a standardized gas signal obtained by the present invention. FIG. 4 is a waveform diagram of a gas signal that defines the reference line of the present invention (the horizontal axis is the wavelength, and the vertical axis is the anonymous number). It is a wave form diagram (a horizontal axis is a wavelength and a vertical axis is an unknown number) of a standardized gas signal obtained by the present invention. It is a wave form diagram (a horizontal axis is a wavelength and a vertical axis is an unnamed number) of a standardized gas signal obtained by the present invention based on a gas signal by actual measurement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical gas detector 2 Light source part 3 Optical system 4 Light reception detection part 5 Signal processing part

Claims (10)

  While modulating the wavelength of the laser beam, the modulation center wavelength is swept within a predetermined sweep range, the laser beam is transmitted through the measured atmosphere, and the detection signal is detected by the phase-sensitive detection from the received light signal. In the optical gas detection method of detecting, obtaining a gas signal from the detection signal, and detecting the concentration of a predetermined gas in the atmosphere to be measured from the peak value of the waveform of the gas signal, the gas signal is set to the level of the gas signal. An optical gas detection method characterized in that the peak value is obtained after dividing by a corresponding divisor value.   2. The optical gas detection according to claim 1, wherein the gas signal is obtained from a ratio of a DC (direct current) component signal detected from the detected signal or a detected signal from the received light signal and a detected signal twice. Method.   2. The optical gas detection method according to claim 1, wherein the gas signal is obtained from a double detection signal among the detection signals.   The optical gas detection method according to claim 1, wherein the divisor value is a value of a gas signal at the head of the sweep cycle of the modulation center wavelength.   The optical gas detection method according to claim 1, wherein the divisor value is calculated from a gas signal value at the beginning and a gas signal value at the end of the sweep cycle of the modulation center wavelength.   A light source unit that modulates the wavelength of the laser light and sweeps the modulation center wavelength within a predetermined sweep range, an optical system that transmits the laser light into the measurement atmosphere, and a phase from a received light signal that receives the transmitted light A light receiving detector for detecting a detection signal by sensitive detection, a signal for obtaining a gas signal from the detection signal, obtaining a peak value of the waveform of the gas signal, and detecting a concentration of a predetermined gas in the measured atmosphere from the peak value An optical gas detection device comprising a processing unit, wherein the signal processing unit divides the gas signal by a divisor value corresponding to the level of the gas signal, and obtains the peak value. Gas detector.   7. The signal processing unit obtains the gas signal from a ratio of a DC (direct current) component signal detected from the detected signal or a detected signal from the received light signal to a detected signal twice. Optical gas detector.   7. The optical gas detection device according to claim 6, wherein the signal processing unit obtains the gas signal from a double detection signal of the detection signals.   9. The optical gas detection device according to claim 6, wherein the signal processing unit uses the divisor value as a value of a gas signal at the head of the sweep cycle of the modulation center wavelength.   9. The optical system according to claim 6, wherein the signal processing unit calculates the divisor value from the value of the gas signal at the beginning and the value of the gas signal at the end of the sweep cycle of the modulation center wavelength. Gas detector.

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