JP2007218783A - Optical fiber type gas concentration detection method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber type gas concentration detection method and a device suitable for detection of an object gas wherein the range of an absorption band is wider than a wavelength range where modulation and sweeping of laser light are possible. <P>SOLUTION: One sweeping range of a light source part 2 for modulating and sweeping laser light is set in a range including an object gas absorption band center wavelength, and two sweeping ranges are set on the shorter wavelength side and on the longer wavelength side. The laser light is transmitted through the atmosphere to be measured and received, and one-time and double detection signals are measured from a received signal, and each gas signal waveform in each sweeping range comprising the ratio between both detection signals is determined. Each gas signal waveform is synthesized together, and a peak value is determined from the synthesized gas signal waveform, and the concentration of the object gas is determined from the peak value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical fiber type gas concentration detection method and apparatus suitable for detecting a target gas whose absorption band is wider than a wavelength range in which laser light can be modulated and swept.

  Gas molecules have the property of absorbing laser light having a specific wavelength (referred to as absorption band), and the presence or absence of gas can be detected using this property. This gas detection method is used in fields such as industrial measurement and pollution monitoring. By transmitting this laser light through an optical fiber, it is possible to detect gas remotely.

  An optical fiber type gas concentration detection method and apparatus is described in Patent Document 1. In this technique, a laser beam having a modulated wavelength and intensity is oscillated by modulating a driving current of a semiconductor laser with a high-frequency sine wave around a predetermined current value. This laser light is incident on an optical fiber, guided to a gas cell, gas of unknown concentration in the gas cell is transmitted, the transmitted light is guided to a light receiver by an optical fiber, the received light signal is phase-sensitively detected, and a single detection signal The double detection signal is measured, a gas signal comprising the ratio of the single detection signal and the double detection signal is obtained, and the concentration of the target gas is obtained from the gas signal.

JP-A-5-256769

  On the other hand, since the center wavelength of the semiconductor laser is difficult to stabilize, the drive current is controlled to sweep the center wavelength near the center of the absorption band.

  However, the size of the wavelength range in which the laser beam can be modulated and swept by controlling the current of the semiconductor laser is about 0.2 nm to 1.4 nm. When the range of the absorption band of the target gas is wider than the wavelength range that can be modulated and swept, concentration detection becomes difficult.

  Therefore, an object of the present invention is to provide an optical fiber type gas concentration detection method and apparatus suitable for detecting a target gas in which the above-mentioned problem is solved and the absorption band range is wider than the wavelength range in which laser light can be modulated and swept. There is.

  In order to achieve the above object, the method of the present invention includes three or more light source units that linearly sweep the modulation center wavelength within a predetermined sweep range while modulating the wavelength and intensity of the laser light with a sine wave. The sweep range of one light source unit is set to a range including the center wavelength of the absorption band of the target gas, and the sweep range of the two light source units is shorter and longer than the center wavelength of the absorption band of the target gas. Set to the wavelength side, transmit these laser lights through the atmosphere to be measured and receive them, phase-detection of the received light signals for each sweep range, measure the 1x detection signal and 2x detection signal, Obtain a gas signal waveform for each sweep range consisting of the ratio of these 1-fold detection signal and 2-fold detection signal, synthesize these gas signal waveforms, find the peak value from the synthesized gas signal waveform, and calculate the peak value of the target gas from this peak value. The concentration is obtained.

  Further, the method of the present invention is provided with three or more light source units that modulate the wavelength and intensity of laser light with a sine wave and linearly sweep the modulation center wavelength within a predetermined sweep range, one of which is provided. The sweep range of the light source unit is set to a range including the center wavelength of the absorption band of the target gas, and the sweep range of the two light source units is set to the short wavelength side and the long wavelength side from the center wavelength of the absorption band of the target gas. These laser beams are transmitted through the atmosphere to be measured and received, and the received signals are phase-sensitively detected for each sweep range to measure a single detection signal. The magnitude of the inclination of the single detection signal A gas signal waveform for each sweep range is obtained, these gas signal waveforms are synthesized, a peak value is obtained from the synthesized gas signal waveform, and a concentration of the target gas is obtained from the peak value.

  A straight line connecting the gas signal waveform on the short wavelength side and the gas signal waveform on the long wavelength side may be used as the reference line of the peak value, and the highest value from the reference line among the gas signal waveforms in the central wavelength range may be used as the peak value. .

  The apparatus of the present invention is a light source unit that linearly sweeps the modulation center wavelength within a predetermined sweep range while modulating the wavelength and intensity of the laser beam with a sine wave, and the sweep range is within the absorption band of the target gas. Three or more light source units including one set in the range including the center wavelength and two set the sweep range on the shorter wavelength side and the longer wavelength side than the center wavelength of the absorption band of the target gas, and these laser beams The optical system that transmits light into the atmosphere to be measured and the transmitted light are received, the received signal is phase-sensitively detected for each sweep range, and the 1-fold detection signal and the 2-fold detection signal are measured. A gas signal waveform for each sweep range consisting of a ratio of the detection signal and the double detection signal is obtained, these gas signal waveforms are synthesized, a peak value is obtained from the synthesized gas signal waveform, and a concentration of the target gas is obtained from the peak value. And a signal processing unit.

  The apparatus of the present invention is a light source unit that linearly sweeps the modulation center wavelength within a predetermined sweep range while modulating the wavelength and intensity of laser light with a sine wave, and the sweep range is absorbed by the target gas. Three or more light source units including one set to a range including the center wavelength of the band and two set the sweep range to a shorter wavelength side and a longer wavelength side than the center wavelength of the absorption band of the target gas, An optical system that transmits laser light into the atmosphere to be measured, and the transmitted light are received. The received light signal is phase-sensitively detected for each sweep range to measure a 1 × detection signal. A signal processing unit that obtains a gas signal waveform for each sweep range having a magnitude of inclination, synthesizes these gas signal waveforms, obtains a peak value from the synthesized gas signal waveform, and obtains the concentration of the target gas from the peak value; It is provided.

  The optical system includes an optical multiplexer that combines the laser beams from the respective light source units, a light source side optical fiber that guides the combined laser beams to the measurement atmosphere, and a laser beam that has passed through the measurement atmosphere. You may provide the light reception side optical fiber guide | induced to a signal processing part.

  You may provide the selection clock generator which supplies the clock which selects a light source part one by one with respect to each light source part.

  The present invention exhibits the following excellent effects.

  (1) It is possible to detect a target gas whose absorption band is wider than the wavelength range in which laser light can be modulated and swept.

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

  As shown in FIG. 1, the optical fiber type gas concentration detection apparatus 1 according to the present invention linearly sweeps the modulation center wavelength within a predetermined sweep range while modulating the wavelength and intensity of laser light with a sine wave. One light source unit 2a, 2b, 2c that has a sweep range set to a range that includes the center wavelength of the absorption band of the target gas (light source unit 2b) and the sweep range is shorter than the center wavelength of the absorption band of the target gas Three or more light source units 2a, 2b, 2c including two (light source units 2a, 2c) set on the wavelength side and the long wavelength side, and an optical system 3 that transmits these laser beams into the measured atmosphere The transmitted light is received, and the received light signal is phase-sensitively detected for each sweep range to measure the 1 × detection signal K (1f) and the 2 × detection signal K (2f), and these 1 × detection signal K (1f) and double detection signal K (2f) Seek gas signal waveform synthesizing these gases signal waveforms, it obtains the peak value from the synthesis gas signal waveform, in which a signal processing unit 31 for determining the concentration of the target gas from the peak value.

  In this embodiment, the measurement atmosphere is provided by a gas cell 4 having a predetermined volume filled with a target gas having an unknown concentration at a predetermined temperature and pressure and having a predetermined transmission optical path length. In order to transmit the light from the light source units 2a, 2b, and 2c to the gas cell 4 and guide the light to the signal processing unit, the optical system 3 combines the laser beams from the light source units 2a, 2b, and 2c. And a light source side optical fiber 7 that guides the branched laser light to each gas cell 4 and a light receiving side optical fiber 8 that guides the laser light transmitted through the gas cell 4 to the signal processing unit.

  In this embodiment, the gas cells 4 are installed at a plurality of locations. Therefore, in order to transmit the light from the light source units 2 a, 2 b, 2 c to each gas cell 4 and collect it in the signal processing unit, the optical system 3 uses a plurality of laser beams combined in the subsequent stage of the optical multiplexer 5. A branching device 6 for branching is provided, and a plurality of light source unit side optical fibers 7 for guiding the branched laser light to each gas cell 4 and a plurality of laser beams for guiding the laser light transmitted through each gas cell 4 to the signal processing unit. A light receiving side optical fiber 8.

  Further, one of the light source unit side optical fibers 7 is directly connected to one of the light receiving side optical fibers 8 for the reference optical path without passing through the gas cell 4. Identification symbols z1, z2,... Zn-1, zn are attached to the optical paths of the outputs of the respective light receiving side optical fibers 8. The optical path z1 is a reference optical path, and the optical paths z2 to zn are sensor optical paths.

  As shown in FIG. 1, the light source unit 2 used in the light source units 2a, 2b, and 2c has a triangular wave sweeper 21 that sweeps a voltage so that a triangular wave is repeatedly generated, and a current proportional to the voltage of the triangular wave. A bias current source 22 that generates a DC bias current that increases or decreases, an inductor 23 that interrupts AC and passes the bias current, a transmitter 24 that generates a modulation AC current and voltage having a predetermined frequency f, From a frequency divider 25 that generates a voltage having a double frequency f from the voltage output of the transmitter 24, a capacitor 26 that cuts off a direct current and passes an alternating current for modulation, a bias current from the inductor 23, and a capacitor 26 A DFB-LD 27 that is a laser diode to which a modulation AC current is superimposed and applied, and a Peltier element 28 for heating and cooling the DFB-LD 27 And a Peltier element power supply 29 for controlling the temperature of the Peltier device 28. The output A from the transmitter 24 and the output from the frequency multiplier 25 are provided to a signal processing unit described later.

  The light source unit 2 becomes the light source units 2a, 2b, and 2c of the optical fiber type gas concentration detection device 1 by varying the sweep range for sweeping the modulation center wavelength. The light source units 2a, 2b, and 2c have the same modulation frequency f. The optical fiber type gas concentration detection device 1 is provided with a selection clock generator 9 that supplies a clock for sequentially selecting the light source units one by one for each of the light source units 2a, 2b, 2c. The clock from the selected clock generator 9 is also supplied to the signal processing unit, and the order of measurement described later is given by this clock.

  The signal processing unit 31 performs phase-sensitive detection on the frequencies f and 2f using a plurality of light receivers 32 connected to the respective light receiving side optical fibers 8 and the outputs A and B from the light source unit 2. Thus, a plurality of phase detection circuits 33 for obtaining the 1 × detection signal K (1f) and the 2 × detection signal K (2f), and the 1 × detection signal K (1f) and the 2 × detection signal K (2f) are stored. A signal storage unit 34 and a calculation unit 35 that calculates a signal read from the signal storage unit 34 are provided. In this embodiment, the calculating part 35 calculates | requires the gas signal waveform for every sweep range which consists of ratio of 1 time detection signal K (1f) and 2 time detection signal K (2f), synthesize | combines these gas signal waveforms, and synthesize | combines them. The peak value is obtained from the obtained gas signal waveform, and the concentration of the target gas is obtained from the peak value. Details of this calculation will be described in the following operation description.

  In the optical fiber type gas concentration detection apparatus 1 of FIG. 1, when the selected clock generator 9 sends a clock to each of the light source units 2a, 2b, 2c as shown in FIG. 4, each of the light source units 2a, 2b, 2c sequentially The following laser beam transmission operation is performed during the time from the rising edge to the falling edge of the clock.

  Here, in a certain light source unit, a triangular wave current flows for a time during which the clock is raised by the selected clock generator 9, and at that time, no clock is coming to the other light source units. For this reason, since the triangular wave current does not flow in the other light source units and the light source unit is not driven, the laser light transmission from the certain light source unit is not affected by the other light source units.

  In the laser beam transmission operation, the temperature of the DFB-LD 27 is fixed at a constant level by controlling the temperature of the Peltier element 28 by the Peltier element power source 29. The bias current source 22 generates a bias current in proportion to the triangular wave voltage output from the triangular wave sweeper 21. Thereby, the bias current is swept in one direction from a small value to a large value. At the same time, an alternating current output from the transmitter 24, that is, a sinusoidal modulation current is superimposed on the bias current and applied to the DFB-LD 27. FIG. 4 shows the applied current as a triangular wave + sine wave. Each light source unit 2a, 2b, 2c modulates with the modulation width d and the modulation center wavelength λ, and sweeps the modulation center wavelength λ linearly within the sweep range.

  In the optical system 3, the laser beams from the light source units 2 a, 2 b, and 2 c are multiplexed by the multiplexer 5 and branched to the light source unit side optical fibers 7 by the branching unit 6. The laser light passes through each gas cell 4 without passing through the gas cell 4, is guided to the light receiving side optical fiber 8 in the reference optical path z 1 and the sensor optical paths z 2 to zn, and enters each light receiver 32. Each light receiver 32 outputs a light reception signal. Since the laser light is absorbed along the absorption spectrum of the target gas shown in FIG. 2 when passing through the gas cell 4, the received light signal indicating the light intensity change of the laser light that has passed through the gas cell 4 is as shown in the figure. The wave is deformed based on the curve of the absorption spectrum.

  The signal processing unit 31 performs the following gas concentration calculation operation every time from the rising edge to the falling edge of the clock.

  In the gas concentration calculation operation, each phase detection circuit 33 obtains the 1 × detection signal K (1f) and the 2 × detection signal K (2f) from the received light signals of the reference optical path z1 and the sensor optical paths z2 to nz, respectively. All the obtained signals are temporarily stored in the signal storage unit 34. The computing unit 35 computes the signal read from the signal storage unit 34. That is, for the light reception signal in the reference optical path z1, the ratio z1K (2f) / z1K (1f) of the double detection signal z1K (2f) to the double detection signal z1K (1f) is calculated, and the reference gas shown in FIG. A signal is obtained, and a ratio ziK (2f) / ziK (1f) of the double detection signal ziK (2f) with respect to the single detection signal ziK (1f) (i is 2 to n) with respect to the received light signals in the sensor optical paths z2 to zn. To obtain a gas signal of the target gas. Next, a difference between the gas signal ziK (2f) / ziK (1f) of the target gas with respect to the reference gas signal z1K (2f) / z1K (1f) is calculated to obtain a peak value determination signal.

  Here, in the present embodiment, the sweep range of the light source unit 2b is set to a range including the center wavelength of the absorption band of the target gas, and the sweep range of the light source unit 2a is shorter than the center wavelength of the absorption band of the target gas. And the sweep range of the light source unit 2c was set longer than the center wavelength of the absorption band of the target gas. Therefore, the sweep ranges of the light source units 2a, 2b, and 2c are discretely arranged in a wide range that is the absorption band of the target gas. As a result, a discrete gas signal is obtained as shown by a solid line in FIG. These gas signals can be combined and regarded as one gas signal. Furthermore, as shown by a broken line in FIG. 3, an estimated gas signal can be interpolated between gas signals obtained by measurement.

  The peak value determination signal in FIG. 3 subtracts the reference gas signal from the gas signal of the target gas, thereby removing the wavelength dependency of the light source unit 2, the multiplexer 5, the branching unit 6, and the signal processing unit 31. This peak value determination signal is a gas signal of the target gas in an accurate sense.

  The calculator 35 obtains a peak value that is the height from the minimum value to the maximum value of the peak value determination signal. Specifically, the straight line connecting the gas signal waveform on the short wavelength side and the gas signal waveform on the long wavelength side is used as the reference line of the peak value, and the highest value from the reference line in the gas signal waveform in the central wavelength range is set. The peak value.

  Thereafter, the computing unit 35 obtains the gas concentration by applying the peak value to a relational expression or relation table between the peak value and the gas concentration obtained in advance using a reference gas having a known concentration.

  As described above, the present invention uses three light source units for modulating and sweeping laser light, transmits the laser light through the atmosphere to be measured, and receives the gas for each sweep range obtained from the received light signal. Since the signal waveform is synthesized and the peak value is obtained from the synthesized gas signal waveform, the peak value can be correctly obtained even when the absorption band range of the target gas is wider than the wavelength range possible with one light source unit. Can do. In the above embodiment, three light source units 2 are used, but it goes without saying that four or more light source units 2 are also effective.

  Next, another embodiment will be described.

  The apparatus configuration of FIG. 1 is the same as that of the above-described embodiment (however, 2f of the frequency multiplier 25 and the phase detection circuit 33 is not necessary), and the calculation unit 35 of the signal processing unit 31 has an inclination of the single detection signal (the horizontal axis The gas concentration is determined from the slope of the wavelength and the vertical axis representing the signal magnitude, ie, the ratio of wavelength to signal.

  Since the laser beam transmission operation in each of the light source units 2a, 2b, and 2c is the same as that in the above embodiment, only the gas concentration calculation operation performed by the signal processing unit will be described.

  Assume that a light reception signal as shown in FIG. Since the light of the absorption band is absorbed by the target gas, the light reception signal has a small peak in the sweep range. The peak wavelength is the center wavelength of the absorption band. In each phase detection circuit, the signal processing unit detects the 1 × detection signal K (1f) from the received light signals of the reference optical path z1 and the sensor optical paths z2 to zn, and stores them in the signal storage unit. The 1 × detection signal K (1f) is as shown in FIG.

  The computing unit 35 reads the 1 × detection signal, obtains the inclination of the 1 × detection signal z1K (1f) obtained from the light reception signal of the reference optical path z1, and uses this as the gas signal of the reference gas. In FIG. 4B, the inclination is shown. The calculation unit 35 similarly obtains the inclination of the 1 × detection signal ziK (1f) obtained from the received light signals of the sensor optical paths z2 to nz and sets it as the gas signal of the target gas.

  The double detection signal of FIG. 4C is not calculated in this embodiment, but is shown for reference of the embodiment.

  As shown in FIG. 5, one gas signal of the target gas and one gas signal of the reference gas obtained from the slope of each 1-fold detection signal are obtained in one sweep range.

  The computing unit 35 calculates a difference between the gas signal of the target gas and the gas signal of the reference gas to obtain a peak value determination signal. Since this is performed to remove the wavelength dependence of each part of the apparatus, the peak value determination signal is an accurate gas signal from which the wavelength dependence has been removed.

  The calculator 35 obtains a peak value that is the height from the minimum value to the maximum value of the peak value determination signal. Specifically, the straight line connecting the gas signal waveform on the short wavelength side and the gas signal waveform on the long wavelength side is used as the reference line of the peak value, and the highest value from the reference line in the gas signal waveform in the central wavelength range is set. The peak value.

1 is an overall configuration, details of a light source section, and a configuration diagram of a signal processing section in an optical fiber type gas concentration detection device showing an embodiment of the present invention. It is an operation | movement conceptual diagram of the light source part in this invention. It is a wave form diagram of the gas signal obtained by this invention, and the signal for peak value determination. (A) is a light reception signal, (b) is a 1 × detection signal, and (c) is a waveform diagram of a 2 × detection signal. It is a wave form diagram of the gas signal obtained by this invention, and the signal for peak value determination.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber type gas concentration detection apparatus 2, 2a, 2b, 2c Light source part 3 Optical system 4 Gas cell 5 Multiplexer 6 Branch device 31 Signal processing part

Claims (7)

  Three or more light source units that linearly sweep the modulation center wavelength within a predetermined sweep range while modulating the wavelength and intensity of the laser beam with a sine wave, of which the sweep range of one light source unit is the target Set to a range that includes the center wavelength of the absorption band of the gas, and set the sweep range of the two light sources to the short wavelength side and the long wavelength side from the center wavelength of the absorption band of the target gas, and measure these laser beams Light is transmitted through the atmosphere to receive light, and the received light signal is phase-sensitively detected for each sweep range to measure the 1 × and 2 × detection signals. From the ratio of these 1 × and 2 × detection signals, An optical fiber type gas concentration characterized by obtaining a gas signal waveform for each sweep range, synthesizing these gas signal waveforms, obtaining a crest value from the synthesized gas signal waveform, and obtaining a concentration of a target gas from the crest value Detection method.   Three or more light source units that linearly sweep the modulation center wavelength within a predetermined sweep range while modulating the wavelength and intensity of the laser beam with a sine wave, of which the sweep range of one light source unit is the target Set to a range that includes the center wavelength of the absorption band of the gas, and set the sweep range of the two light sources to the short wavelength side and the long wavelength side from the center wavelength of the absorption band of the target gas, and measure these laser beams Light is transmitted through the atmosphere to receive light, and the received light signal is phase-sensitively detected for each sweep range to measure a 1 × detection signal, and a gas signal for each sweep range consisting of the magnitude of the slope of this 1 × detection signal. An optical fiber type gas concentration detection method characterized by obtaining a waveform, synthesizing these gas signal waveforms, obtaining a peak value from the synthesized gas signal waveform, and obtaining a concentration of a target gas from the peak value.   The straight line connecting the gas signal waveform on the short wavelength side and the gas signal waveform on the long wavelength side is the reference line of the peak value, and the highest value from the above reference line is the peak value among the gas signal waveforms in the central wavelength range. The optical fiber type gas concentration detection method according to claim 1 or 2.   A light source unit that modulates the wavelength and intensity of a laser beam with a sine wave and linearly sweeps the modulation center wavelength within a predetermined sweep range, wherein the sweep range is within a range including the center wavelength of the absorption band of the target gas. Three or more light source sections including one set and two sweep ranges set shorter and longer than the center wavelength of the absorption band of the target gas, and these laser lights in the atmosphere to be measured The optical system to transmit and the transmitted light are received, and the received light signal is phase-sensitively detected for each sweep range to measure the 1 × detection signal and the 2 × detection signal, and these 1 × detection signal and 2 × detection signal are measured. A signal processing unit that obtains a gas signal waveform for each sweep range including a signal ratio, synthesizes these gas signal waveforms, obtains a peak value from the synthesized gas signal waveform, and obtains a concentration of the target gas from the peak value; Optical fiber type gas Concentration detection device.   A light source unit that modulates the wavelength and intensity of a laser beam with a sine wave and linearly sweeps the modulation center wavelength within a predetermined sweep range, wherein the sweep range is within a range including the center wavelength of the absorption band of the target gas. Three or more light source sections including one set and two sweep ranges set shorter and longer than the center wavelength of the absorption band of the target gas, and these laser lights in the atmosphere to be measured The optical system to transmit and the transmitted light are received, the received light signal is phase-sensitively detected for each sweep range, and a 1-fold detection signal is measured, and a sweep range consisting of the slope of the 1-fold detection signal And a signal processing unit that obtains a gas signal waveform for each, synthesizes these gas signal waveforms, obtains a crest value from the synthesized gas signal waveform, and obtains the concentration of the target gas from the crest value. Fiber type gas concentration detector.   The optical system includes an optical multiplexer that combines the laser beams from the respective light source units, a light source side optical fiber that guides the combined laser beams to the measurement atmosphere, and a laser beam that has passed through the measurement atmosphere. 6. The optical fiber type gas concentration detection device according to claim 4, further comprising a light receiving side optical fiber led to the signal processing unit.   The optical fiber type gas concentration detection device according to claim 6, further comprising a selection clock generator for supplying a clock for sequentially selecting the light source units one by one for each light source unit.

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