JPH06148071A - Method and instrument for measuring multi-point gas concentration using optical fiber - Google Patents

Abstract

PURPOSE:To provide a multi-point gas concentration measuring method by which a gas can be detected even when the concentration of the gas is low and the concentration of the gas can be measured at multiple points by means of one set of a light source and signal processing means and, at the same time, optical fibers can be controlled easily and no large amount of optical fibers is required. CONSTITUTION:By using a laser which oscillates laser light having the wavelength and intensity corresponding to the driving current and temperature of the laser, the wavelength and intensity of the laser light are modulated 2 and, at the same time, the laser light is intermittently discontinued 3 like a pulse by changing the driving current and temperature of the laser. The pulse-like laser light is passed through a plurality of gas atmospheres to be measured through an optical transmission line 5 and the laser light transmitted through the atmospheres is joined together. The intensity of the joined transmitted laser light is detected and the concentration of a specific gas is measured by phase-sensitively detecting 4 a fundamental and second harmonic components from the intensity detecting signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration measuring method and device for optically measuring a gas concentration, and in particular, it is capable of detecting a gas even at a low concentration and has a large number of light sources and a signal processing means. The present invention relates to a multipoint gas concentration measuring device using an optical fiber which can measure a gas concentration at a location, has a simple control method, and requires a small amount of optical fibers.

[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 a laser beam of a specific wavelength is easily absorbed by a certain type of gas. Sensing technology applying this principle is used in industrial measurement, pollution monitoring, etc. Widely used in. Also,
If laser light is used as an optical fiber transmission line, it is possible to detect gas at a remote place.

As an example, methane gas has a strong absorption spectrum line at 1.6 μm. Light including this wavelength and another wavelength that is not absorbed by methane gas is guided to the measurement gas with an optical fiber, and the light is transmitted through the measurement gas atmosphere,
The transmitted light is received by another opposing optical fiber and guided to the light receiving portion. In this case, a light source having a wavelength with a wide bandwidth such as an LED is used as the light source. In the light receiving part, using a bandpass filter or the like, the light having a wavelength absorbed by methane gas and the light having a wavelength not absorbed by methane gas are separated into
The ratio of the attenuation of each light is obtained, and the concentration of methane gas is detected from this.

However, such a methane gas detection system requires a complicated optical system such as using a large number of half mirrors. Further, in terms of SN ratio, it is disadvantageous for detecting low concentration gas. For example, in a gas cell with a spatial optical path length of 10 cm, λ = 1.66 as the methane absorption wavelength.
The attenuation when light of 5 μm (absorption coefficient α = 16 m ^-1 · atm ^-1 ) is transmitted is 0.65 at 50% concentration, 0.02 at 1% concentration, and 0.002 at 1000 ppm concentration. , The attenuation becomes extremely small as the concentration becomes lower. If the amount of attenuation is extremely small, the measurement becomes difficult in terms of SN ratio.

Therefore, the present inventors have filed Japanese Patent Application No. 2-78498.
We have developed a new remote methane gas detector using an optical fiber as a transmission line. In the method applying this principle, the driving power source of the light source is modulated at a high frequency centering on a predetermined current to oscillate a laser beam whose wavelength and intensity are modulated. At this time, a part of the laser light emitted from the semiconductor laser is used for monitoring, and the current and temperature are controlled so that the center wavelength of oscillation becomes the center of the methane absorption line. Then, the stably emitted laser light is transmitted through the optical fiber to the measurement gas cell containing the gas of unknown concentration, and the transmitted light is guided to the light receiving portion by another optical fiber facing the measurement gas. The gas concentration is obtained from the double detection signal and the fundamental detection signal. With such a remote methane gas detection device, the gas concentration can be detected at a high SN ratio.

However, the above-mentioned remote methane gas detector can only measure the gas concentration at one point. Even if a plurality of gas cells are installed in series or in parallel along the optical fiber in an attempt to measure gas concentrations at a plurality of points, the signal obtained at the light receiving section gives the sum of absorption amounts at all measurement points. However, it is not enough to detect the gas concentration at each point. After all, the remote methane gas detection device can only measure the gas concentration at one point. Therefore, when there are several places where concentration monitoring is required, a detection device including a light source, a light receiving part, etc. is separately provided for each measurement point. However, there is a problem that the cost is increased.

On the other hand, as another method, a method of arranging a plurality of gas cells along the optical fiber and measuring the gas concentration at a plurality of points is also conceivable. That is, as shown in FIG. 7, a plurality of gas cells 6 are arranged in parallel, and measurement light is branched and supplied from the light source unit 2 to each of the gas cells 6 via the optical transmission line 5, The transmitted light that has passed through 6 is merged with another optical transmission path 5f and collected in the signal processing unit 4, and a plurality of light sources (half of the measurement points) are provided from the light source unit 2.
The laser light modulated by the above modulation frequency is given. By doing so, the gas concentration of each gas cell 6 can be obtained by solving the relational expression obtained for each modulation frequency.

[0008]

However, this method still has a problem. One is to prepare n / 2 or more different modulation frequencies in order to carry out n multi-point density measurement. Therefore, a control circuit for controlling the plurality of modulation frequencies is required. In addition, an optical transmission line for guiding the measuring light to the gas cell and an optical transmission line for guiding the transmitted light from the gas cell are separately required, and many optical fibers are required for the remote measurement. There's a problem.

Therefore, an object of the present invention is to solve the above-mentioned problems, to detect gas even at low concentration, and to measure gas concentration at multiple points with one set of light source and signal processing means, and to simplify the control method. Another object of the present invention is to provide a multipoint gas concentration measuring method and apparatus using an optical fiber that requires a small amount of optical fiber.

[0010]

In order to achieve the above object, the present invention uses a laser that oscillates a laser beam having a wavelength and intensity according to the drive current and temperature, and changes the drive current or temperature of the laser. The wavelength and intensity of the laser light are modulated, and this laser light is intermittently pulsed, and this pulsed laser light is transmitted through the optical transmission line to multiple gas atmospheres to be measured. Are combined, the light intensity of the combined transmitted light is detected, and the fundamental wave component and the second harmonic component of the modulation frequency are phase-sensitively detected from the detection signal to measure the concentration of the specific gas.

Further, the multipoint gas concentration measuring apparatus includes a laser that oscillates a laser beam having a wavelength and intensity according to a driving current and temperature, and a driving current of the laser with a predetermined amplitude centered on a predetermined current value. A drive circuit for modulation, a plurality of gas cells containing the gas to be measured, an optical transmission line for transmitting laser light, a measurement light is guided from the optical transmission line to the gas cell, and the transmitted light of the gas cell is also transmitted optically. Measuring light transmission light input and output means for each gas cell returned to the path, and the measurement light is branched from the optical transmission path to each measurement light transmission light input and output means, and from each measurement light transmission light input and output means. Optical branching / combining means for combining the transmitted light to be taken into the optical transmission path, transmitted light branching means for branching the transmitted light from the measuring light and the transmitted light in the optical transmission path, and detecting the light intensity of the branched transmitted light And modulation frequency A fundamental wave component and second harmonic component number can be configured and a signal processing means for phase sensitive detection.

Alternatively, the laser and the drive circuit,
A plurality of gas cells arranged in series in the optical transmission line, and the transmitted light from the gas cells in the previous stage connected to each optical transmission line between each gas cell is used as the measuring light for the gas cells in the subsequent stage and the transmitted light for the gas cells in the previous stage. Optical branching means for branching, optical joining means for joining the transmitted light branched by each optical branching means to the optical transmission path for transmitted light, and the upstream of the optical transmission path and the downstream of the optical transmission path for transmitted light are coupled. It may be configured to include an optical coupler, a transmitted light branching unit that branches the transmitted light from the measuring light and the transmitted light in the optical transmission line, and the signal processing unit.

Alternatively, the laser and the drive circuit,
A plurality of gas cells, a light branching means, a light combining means,
An optical transmission line for transmitted light from the merging unit and a signal processing unit connected to the optical transmission line for transmitted light may be provided.

Further, a pulse modulator for interrupting the measuring light from the laser in a pulse shape may be provided.

Further, a plurality of gas cell rows each including the plurality of gas cells, the light branching means, and the light merging means may be arranged in parallel.

[0016]

With the above construction, the laser light is modulated in wavelength and intensity and is intermittently pulsed. Since the modulated wave of several cycles is included in this pulse width, phase sensitive detection is possible. On the other hand, the pulsed laser light passes through each gas cell and returns to the signal processing means, but the required time for this depends on the optical transmission path length of each gas cell. Therefore, the pulsed transmitted light from each gas cell arrives at the signal processing means with a time shift. By performing the phase sensitive detection for each pulse, the concentration at each measurement point can be obtained even at low concentration.

Further, by combining the transmitted light and taking it into the optical transmission line, the measuring light and the transmitted light can be transmitted by one optical fiber. That is, the amount of optical fibers may be small.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. Here, an example of measuring methane gas using a semiconductor laser as a light source will be described.

When the driving current of the semiconductor laser is modulated with the frequency ω, the oscillation frequency Ω and the oscillation intensity of the laser light are modulated with the frequency ω. Now, when the laser light whose frequency and intensity are thus modulated is transmitted through an optical fiber into an atmosphere containing methane gas, the transmitted light P is expressed by the formula (1).
Becomes

P = A [I ₀ + ΔIcos (ωt + φ ₁ + φ)] × [C ₀ + ΔΩ · T ₀₁ cos (ωt + φ ₁ ) + T ₀₂ cos {2 (ωt + φ ₁ ) + φ}] (1) where C _{0 = T 01 + T 02 (ΔΩ} ) 2/4 here, a; modulation frequency φ of the drive current; intensity modulation amplitude omega; central intensity ΔI of the laser output; constant I ₀ which depends on the transmission loss of the optical-system intensity modulation and Phase difference from frequency modulation T; Transmittance T ₀₁ ; First derivative dT / dΩ T ₀₂ ; Second derivative d ² T / dΩ ^{2 In} addition, φ ₁ is the laser light emitted from the light source and transmitted through the fiber to receive light. Phase difference caused by the time delay until reaching the chamber, φ ₁ = (2π / ω) N L / V (2) where N: optical fiber core refractive index V; speed of light in vacuum L; light source To the detector (here, the optical fiber length is several kilometers, the propagation length in the gas cell Several tens centimeters from a few centimeters, the latter in is ignored.) Thus, the transmitted light P, in addition to the DC component, cos (.omega.t
+ Φ ₁ ) component and cos (2 (ωt + φ ₁ )) component.

In the above equation (1), cos (2 (ωt +
φ ₁ )) frequency and phase components are detected with phase sensitivity and P (2
When ω) is obtained, equation (3) is obtained, and P (2ω) is T
It can be seen that it changes based on ₀₁ and T ₀₂ .

[0022] P (2ω) = A {I 0 · T 02 (ΔΩ) 2/4 + ΔI · ΔΩ · cosφ · T 01} ... (3) the detection signal P (2 [omega) is changed by a change in the optical frequency Ω The maximum sensitivity is obtained when the center frequency Ω _{0 of the} laser coincides with the center frequency W of the absorption line of methane gas. At that time, since T ₀₁ is 0 and T ₀₂ is maximum, the second term of the equation (3) can be erased and only the first term remains. In other words, T ₀₂ at the time of Ω ₀ = W is a _{T 02 = 2 · α (W} ) · C · l / γ 2 ... (4). Therefore, when this is substituted into the equation (3), the equation (5) is obtained.

P (2ω) = A · I ₀ · (ΔΩ) ² · α (W) · C · l / (2γ) ² (5) where α (W); Ω ₀ = W Absorption coefficient of methane gas 2γ; full width at half maximum of gas absorption line C · l; product of gas concentration and spatial optical path length in gas cell On the other hand, the frequency and phase components of cos (ωt) are phase-sensitively detected to obtain P (ω) When obtained, P (ω) = A · ΔI (6)

Further, in order that the pulsed laser light modulated waves which have passed through each gas cell arrive at the signal processing means with a time shift without overlapping, the minimum extra length Δl of the optical transmission line is determined. When the number of modulated waves included in one pulse is C _Y , Δl ≧ (C _Y / ω) · (V / N) (7) In the embodiments described below, the length of the optical transmission line is assumed to satisfy the above equation.

An embodiment of the present invention is constructed based on the above findings.

As shown in FIG. 1, the multipoint gas concentration measuring apparatus 1 has a light source section 2 including a laser and a driving circuit, a pulse modulator 3, and a signal processing means 4, which are optical transmission lines 5. Is connected to a large number (n number) of gas cells 6 provided at a remote location.

The laser included in the light source section 2 oscillates laser light having a wavelength and intensity according to the drive current and temperature. Similarly, the drive circuit included in the light source unit 2 changes the drive current or temperature of the laser to modulate the wavelength and intensity of the laser light. Light source 2
Are connected to the pulse modulator 3.

The pulse modulator 3 generates a pulse having an arbitrary width continuously or singly, and the light source section 2 is generated by this pulse.
It is possible to intermittently pulse the laser light from.
The pulse modulator 3 is connected to the uppermost stream 5a of the optical transmission line 5. In this embodiment, the pulse modulator 3 is LiNbO.
_{A 3-} waveguide type Mach-Zehnder modulator is used, and by applying a pulse signal to this LiNbO ₃ waveguide-type Mach-Zehnder modulator, pulsed laser light can be transmitted to the optical transmission line 5.

The signal processing means 4 has a light receiving element for receiving the transmitted light from the gas cell 6, converts the transmitted light into an electric signal, and converts the electric signal into a fundamental wave component and a second harmonic wave component of the modulation frequency. The phase sensitive detection can be performed to measure the concentration of the specific gas.

In the first embodiment shown in FIG. 1, the gas cell 6 has two transmission optical paths, and one ends of these transmission optical paths are connected to each other by a loop-shaped optical fiber 12.
On the other hand, to the optical transmission line 5, a measuring light transmitted light input / output means 7, an optical branching / merging means 8 and a transmitted light branching means 9 are appropriately connected. More specifically, the measuring light transmitted / received light input / output means 7
Is provided for each gas cell 6 and mainly includes an optical branching / coupling device 10 and an optical isolator 11. The optical branching / coupling device 10 can branch the light from the upstream side to the downstream side, which is divided into two paths, and can combine and return the light from both downstream sides to the upstream side. Optical splitter / coupler 10
The upstream side of is connected to the optical transmission line 5b allocated for each gas cell 6, and the opposite downstream side is connected to the optical isolator 11 on the one side and the transmission optical path on the other side of the gas cell 6 on the other side. The optical isolator 11 is capable of blocking light from one side and passing light from the other side unilaterally. The optical isolator 11 unidirectionally splits light from the other transmission optical path of the gas cell 6 into an optical branching / coupling device 10.
Is connected so that it can pass through. With this configuration,
The measuring light transmitted light entering / exiting means 7 sends the measuring light from the optical transmission line 5b to the optical branching / coupling device 10, one transmission optical path of the gas cell 6, the loop-shaped optical fiber 12, the other transmission optical path of the gas cell 6, The transmitted light of the gas cell 6 that has passed through the optical isolator 11 can be returned to the optical transmission line 5b while being guided in the order of the optical isolator 11.

The light branching / combining means 8 is capable of branching light from its upstream side into two downstream paths, and at the same time returning light from both downstream sides by combining it into the upstream side. is there. The optical branching / combining means 8 is provided in the optical transmission path 5 so as to correspond to the gas cells 6, and the measuring light is branched from the optical transmission path 5 to the measuring light transmitting light entering / exiting means 7 of each gas cell 6 and supplied. The transmitted light from the respective measuring light transmitted light input / output means 7 can be merged and taken into the optical transmission path 5.

The transmitted light branching means 9 passes the light from the upstream side to the downstream side and branches the light from the downstream side into the upstream branch path. The transmitted light splitting means 9 is
The upstream side is the uppermost stream 5a of the optical transmission line 5 from the pulse modulator 3.
, And the downstream side is connected to the optical transmission line 5. The branch path of the transmitted light branching means 9 is connected to the signal processing means 4 via the optical transmission path 5c. With such a configuration, the transmitted light branching unit 9 can branch the transmitted light from the measurement light and the transmitted light in the optical transmission path 5 and guide the branched light to the signal processing unit 4.

Next, the operation of the embodiment will be described.

In the light source section 2, the oscillation wavelength of the semiconductor laser is centered on the absorption line of methane gas, and the driving current and temperature are adjusted to generate frequency-modulated laser light. This laser light is continuous light as shown in FIG. In the pulse modulator 3, this continuous light is generated as shown in FIG.
It is modulated with a pulse as shown in FIG.
The measurement light whose wavelength and intensity are modulated and which is intermittently pulsed as shown in (3) is supplied to the optical transmission line 5a. The measuring light passes through the transmitted light branching means 9 and reaches the first light branching / merging means 8 as shown by the solid arrow in FIG.

The light branching / combining means 8 branches the measurement light into the measurement light transmitted light input / output means 7 of each of the n gas cells 6 through each light transmission path 5b. At this time, if the branching ratio of each optical branching / combining means 8 is set to, for example, 1: (n-1) on the upstream side and a branching ratio closer to 1 is used toward the downstream side, an equal amount of measuring light is distributed to each gas cell 6. it can. Further, in the measuring light transmitted / received light input / output means 7, the light branching coupler 10 is used to directly connect the gas cell 6
Allocated so that more light goes to. The light branched from the optical branch coupler 10 to the optical isolator 11 is blocked there.

The measuring light distributed from the optical branching / coupling device 10 to the gas cell 6 passes through the transmission optical path in the gas cell 6, is transmitted through the loop-shaped optical fiber 12, and is again in the gas cell 6.
Through the transmitted light path inside. Here, the transmitted light that reciprocates in the transmitted optical path in the gas cell 6 is attenuated according to the gas concentration. The transmitted light (indicated by a broken line arrow in the figure) passes through the optical isolator 11 and the optical branching / coupling device 10, returns to the original optical transmission line 5b, and reaches the optical branching / combining means 8. Further, the transmitted lights are sequentially combined by the optical branching / combining means 8 and return to the original optical transmission path 5 to reach the transmitted light branching means 9. Then, the light is split by the transmitted light splitting means 9 and enters the signal processing means 4.

By the way, since the light in the optical fiber is transmitted at the speed of V / N, the pulsed laser light generated by the pulse modulator 3 is generated by making the transmission path length of each gas cell different. , Arrives at the signal processing means 4 with a time lag. That is, as shown in FIG. 5D, the transmitted light sequentially arrives from the gas cell having the shortest transmission path. The signal processing means 4 receives the pulsed transmitted light, performs phase sensitive detection on the fundamental wave component and the second harmonic component of the modulation frequency, and calculates the gas concentration. Then, based on the optical transmission path length of each gas cell stored in advance, the results are displayed as the concentration measurement results of the gas cells having the shortest transmission paths in the order of arrival. However, in the present embodiment, since the measuring light is transmitted back and forth through the gas cell once, the spatial light path length in the gas cell is determined in consideration of this in the concentration calculation using the equation (5).

As described above, according to the present invention, the gas can be detected even at a low concentration by the phase sensitive detection. Moreover, by intermittently pulsing the laser light, the gas concentration can be measured at multiple points with one set of light source and signal processing means, and the control method is simplified. Further, since the measuring light and the transmitted light are reciprocated in the same optical transmission line, the amount of optical fibers may be small even if the measurement point is at a remote place.

In this embodiment, the gas cell 6 has two transmission optical paths, and one ends of these transmission optical paths are connected to each other by the loop-shaped optical fiber 12.
The transmitted light path is defined as one path, and the transmitted light is directly reflected by the optical isolator 11.
May be returned to.

Further, the directional coupler 41 shown in FIG. 4 can be used as the measuring light transmission / transmission / reception means 7.
The directional coupler 41 allows the light incident from the transmission path 42 to pass through the transmission path 43 and allows the light incident from the transmission path 44 to pass through the transmission path 42. By connecting the transmission line 42 side to the optical transmission line 5b, the optical branching coupler 10 and the optical isolator 11 can be replaced.

Next, a second embodiment of the present invention will be described.

In the second embodiment shown in FIG. 2, the multipoint gas concentration measuring device 1 has a light source section 2 consisting of a laser and a driving circuit, a pulse modulator 3 and a signal processing means 4, which are provided. , The same as in the first embodiment. Further, the transmitted light splitting means 9 is also the same.

The gas cell 6 has one transmission optical path, and each gas cell 6 is arranged in series on the optical transmission path 5. An optical branching means 12 is provided in the optical transmission line 5 between the gas cells 6, respectively.
Are connected. The light branching means 12 is capable of branching light from the upstream side to the downstream side divided into two paths. Since the upstream side of the light branching means 12 is connected to the transmitted light outlet of the gas cell 6 in the front stage, the transmitted light from the gas cell 6 in the front stage is branched at an appropriate branching ratio, and one is used for measuring the gas cell 6 in the rear stage. The light is split into the light and the other is split into the light transmitted through the gas cell 6 in the preceding stage. This branch is connected to the optical combining means 13.

The light merging means 13 has an upstream side of two paths and a downstream side of one path, and can combine the light on the upstream side with the downstream side. The transmitted lights of the gas cells 6 at the respective stages branched by the respective optical branching means 12 are sequentially combined by the optical combining means 13 into the transmitted light optical transmission path 5d. An optical isolator 11 is provided downstream of the transmitted light transmission path 5d, and an optical coupler 14 is connected further downstream thereof.

The optical coupler 14 can branch the light from the upstream side to the downstream side, which is divided into two paths, and can combine and return the light from both downstream sides to the upstream side. . The upstream side of the optical coupler 14 is the optical transmission line 5
One of the downstream sides is connected to the first-stage gas cell 6 and the other side is connected to the optical isolator 11. The optical coupler 14 couples the upstream side of the optical transmission line 5 and the downstream side of the transmitted light optical transmission line 5d, and the optical isolator 11 can block the measurement light from the transmitted light optical transmission line 5d.

In the configuration of FIG. 2, the light obtained by the signal processing means 4 is the transmitted light of only the first-stage gas cell 6 or the transmitted light of the first-stage gas cell 6 and then the second-stage gas cell 6. The transmitted light in which the number of transmission stages is sequentially stacked will arrive in the order of the shorter optical transmission path length. The signal processing means 4 performs phase sensitive detection to calculate the gas concentration of the gas cell 6 at the front stage, and utilizes this result to sequentially calculate the gas concentration of the gas cell 6 at the rear stage.

Next, a third embodiment will be described.

In the third embodiment shown in FIG. 3, the multipoint gas concentration measuring device 1 has a light source section 2 composed of a laser and a driving circuit and a signal processing means 4 as in the above-mentioned embodiments. However, the pulse modulator 3 is not included. Then, the light source unit 2 modulates the laser light with a plurality of modulation frequencies. This is applicable not only to the measuring method in which the coupling configuration of the optical transmission lines in the first, second and third embodiments described above is pulse-modulated for laser light, but also for the measuring method for modulating laser light at a plurality of modulation frequencies. Is also available.

Similar to the second embodiment, the gas cells 6 are arranged in series in the optical transmission line 5, the optical branching means 12 is connected between the gas cells 6, and the optical joining means 13 is connected to the branching destination. Has been done. The difference from the second embodiment is that the light merging means 1
The merging direction of 3 is that they are sequentially merged to the rear side. Then, the last optical merging unit 13 is connected to the signal processing unit 4 via the transmitted light optical transmission line 5e.

In such a configuration, the transmission line length between the light source section 2 and the gas cell 6 ₁ is l _0,0 the transmission line length between the gas cell 6 ₁ and the gas cell 6 ₂ is l _0,1 gas cell 6 ₂ -gas cell 6 ₃ the transmission path length between l _{0, n-1} gas cell 6 _n ~ last optical combining means 13 _n-1 transmission path length between the transmission path length l _{0, 2} gas cell 6 _n-1 ~ gas cell 6 _n between l _{0, n} The transmission path length between the last optical combining means 13 _n-1 to the signal processing means 4 is l _{0, n + 1} The transmission path length between the optical branching means 12 ₁ to the optical combining means 13 ₁ is l
The transmission path length between the _1,1 optical merging unit 13 ₁ and the optical merging unit 13 _{2 is set} to 1
_1,2 The transmission path length between the optical merging means 13 _n-2 and the optical merging means 13 _n-1 is l _{1, n-1,} and the transmission path length between the optical branching means 12 ₂ and the optical merging means 13 ₁ is l.
_2, the transmission path length between the optical branching means 12 ₃ and the optical combining means 13 _{2 is set} to l
The transmission path length between the _2,2 optical branching means 12 _n-1 and the optical combining means 13 _n-2 is set as l _{2, n-2} .

The signal detected by the signal processing means 4 is the total sum of the transmitted light of each light combining means 13 and each gas cell 6. For simplicity, neglect the loss in the optical system. Of each of the transmitted light, PG ₁ the magnitude of the second harmonic phase-sensitive detection signal involved in the gas concentration, PG _2, · ·, and PG _N, the phase differences _{φ 1, φ 2, ··,} φ N Then, the signal processing means 4
The sum of the second harmonic wave phase-sensitive detection signal at the PG _{SUM is, PG SUM = PG 1 cos {} 2 (ωt + φ 1)} + PG 2 cos {2 (ωt + φ 2)} + ·· + PG N cos {2 (ωt + φ N) } (8) Here, each second harmonic phase sensitive detection signal PG _k is
For the sake of simplicity, the transmission optical path length of each gas cell is constant, and the gas concentration of the gas cell 6 _k is C _k. K = 1, 2, ..., N) is represented by C1 = C ₁ C2 = C ₁ + C ₂ CN = C ₁ + C ₂ + ... + C _N , where PG _K = PG _K (CK) , PG _K (CK) = A _K · I _{0 K} · (ΔΩ) ² · α (ω) · CK · L _S / (2γ) ² (9) where A _K : signal transmission loss I related to PG _{K 0K} : central intensity of laser related to PG _K ΔΩ: frequency modulation amplitude L _S : optical path length in gas cell (constant) 2γ: full width at half maximum of gas absorption line

Further, φ _k is a quantity depending on the transmission path length involved in the transmitted light, and the optical path length LK of each transmitted light.
Is L1 = l _0,0 + l _1,1 + ... + l _{1, n-1} + l _{0, n + 1} L2 = l _0,0 + l _0,1 + l _2,1 + l _1,2 + ... + l
_{1, n-1} + l _{0, n + 1} LN = l _0,0 + l _0,1 + ... + l _{0, n-1} + l _{0, n} + l
_It is expressed as _{0, n + 1} . When φ _k = φ _k (LK) is set, φ _k (LK) = (2π / ω) · N · LK / V (10)

By the way, the phase sensitive detection for one modulation frequency means that the two parameters of the magnitude and the phase are obtained in the equation (8). The transmission line is
Once installed, it will not fluctuate, so φ ₁ ~ φ
_N is a known amount, and after all, the equation (8) is a binary simultaneous equation including n unknown numbers of PG _{1 to} PG _n , which includes the unknown concentration of the measurement gas of each gas cell. Therefore, in order to solve this when n> 2, the laser light is modulated at another frequency ω '(≠ ω) and the same measurement as above is performed. Two parameters are obtained at one modulation frequency. Therefore, in the case of n> 2, PG ₁ , PG ₂ , ..., P can be obtained by phase-sensitive detection with n / 2 (rounded up when the number is small) different modulation frequencies.
G _N is found.

The same applies to the fundamental wave phase sensitive signal. The size involved in gas concentration measurement is PW ₁ , PW
_2, ..., and PW _{n, 1} a phase difference phi, phi _2, ..., phi
_{If n} is set, the total sum PW _SUM of the fundamental wave phase sensitive detection signals in the signal processing means 4 is PW _SUM = PW ₁ cos (ωt + φ ₁ + φ) + PW ₂ cos (ωt + φ ₂ + φ) + ···· + PW _n cos ( ωt + φ _n + φ) (11)

Here, each fundamental wave phase sensitive detection signal PW
_k is an amount that depends on the amplitude intensity ΔI _{K related} to the transmitted light, and if PW _K = PW _K (ΔI _K ) is set, then PW _K = A · ΔI _K (12)

The measurement of the equation (11) means that the two parameters of the magnitude and the phase are obtained as described above. PW _{1 to} PW _n
To solve for the n unknowns of
The laser beam is modulated with (≠ ω) and the same measurement as above is performed.

The ratio is obtained from the equations (9) and (12).

R _k = PG _k / PW _k = I _0k / ΔI _k · (C ₁ + ··· + C _K ) The previous two coefficients are constants determined from the characteristics of the laser, and the concentration at each measurement point can be obtained.

Next, a fourth embodiment will be described.

As shown in FIG. 6, the series gas cells 6 and the optical system shown in the third embodiment form a gas cell array, and a plurality of gas cell arrays are arranged. Each gas cell array is formed by an optical system. It is connected in parallel. Such a configuration
It is suitable when the measurement points are divided into several points and several measurement points are included in one point.

Next, the case where there are two or more kinds of gases to be measured will be described. For example, a laser that oscillates at a wavelength of 1.53 μm is prepared for acetylene detection. The laser drive current or temperature is changed to adjust so that the oscillation frequency coincides with the center of the absorption line of acetylene. An optical coupler is provided to transmit this laser light through an optical fiber and join the optical fiber transmitting the laser light for methane detection shown in the embodiment. However, the modulation frequency ω _{A at} that time is the modulation frequency ω of the methane detection laser.
Set a different value from _M. It is expected that the greater the least common multiple of the two values, the higher the measurement accuracy.
Then, in the signal processing system of the light receiving section, by providing a signal processing device in which two methane signal processing devices and an acetylene signal processing device are provided in parallel, or by providing a switching mechanism in the processing device shown in the embodiment, If the processing system performs the same processing as described above for each modulation frequency, it is possible to detect two or more kinds of gases simultaneously.

[0062]

The present invention exhibits the following excellent effects.

(1) It is economical because it is possible to measure the gas concentration at multiple points with one set of light source and signal processing means.

(2) Compared to the method using a plurality of modulation frequencies, the method of pulse-modulating the laser light of the present invention is simpler in control method.

(3) Since the measuring light and the transmitted light are reciprocated in the same optical transmission line, the amount of optical fibers can be small even if the measuring point is at a remote place.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a block diagram showing a second embodiment of the present invention.

FIG. 3 is a block diagram showing a third embodiment of the present invention.

FIG. 4 is a directional coupler used in the present invention.

FIG. 5 is a waveform diagram for explaining the method of the present invention.

FIG. 6 is a block diagram showing a fourth embodiment of the present invention.

FIG. 7 is a block diagram showing a conventional example.

[Explanation of symbols]

 1 multipoint gas concentration measuring device 2 light source 3 pulse modulator 4 signal processing means 5 optical transmission line 6 gas cell

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiko Uchida 5-1-1 Hidaka-cho, Hitachi City, Ibaraki Prefecture Hitachi Cable Ltd. Optro System Research Laboratories (72) Inventor Satoshi Yamamoto Hidaka-cho, Hitachi City, Ibaraki Prefecture 5-1-1, Hitachi Cable Ltd., Optoro System Laboratories

Claims (6)

[Claims] 1. A laser that oscillates a laser beam having a wavelength and intensity according to a drive current and temperature is used, and the drive current or temperature of the laser is changed to modulate the wavelength and intensity of the laser beam and the laser beam is emitted. The pulsed laser light is intermittently transmitted, and this pulsed laser light is transmitted through the optical transmission line to multiple gas atmospheres to be measured, the transmitted light of these gas atmospheres is combined, and the light intensity of the combined transmitted light is detected. Then, a multipoint gas concentration measuring method using an optical fiber is characterized in that the fundamental wave component and the second harmonic component of the modulation frequency are phase-sensitively detected from the detection signal to measure the concentration of the specific gas. . 2. A laser that oscillates laser light having a wavelength and intensity according to a drive current and temperature, a drive circuit for modulating the drive current of the laser with a predetermined amplitude around a predetermined current value, and measurement. A plurality of gas cells containing the target gas, an optical transmission line that transmits laser light, and a measurement light that guides the measurement light from the optical transmission line to the gas cell and returns the transmitted light of the gas cell to the optical transmission line. The transmission light input / output means and the measurement light transmitted light input / output means from the optical transmission path are branched and given, and the transmission light from the respective measurement light transmission light input / output means are combined to form an optical transmission path. An optical branching / combining means to be taken into, a transmitted light branching means for branching the transmitted light from the measuring light and the transmitted light in the optical transmission line, a light intensity of the branched transmitted light, and a fundamental wave component of a modulation frequency. With the second harmonic component A multipoint gas concentration measuring device using an optical fiber, comprising a signal processing means for phase sensitive detection. 3. A laser that oscillates laser light having a wavelength and intensity according to a drive current and temperature, a drive circuit for modulating the drive current of the laser with a predetermined amplitude centered on a predetermined current value, and measurement. A plurality of gas cells that contain the target gas and are arranged in series in the optical transmission line, and the transmitted light from the gas cells in the previous stage that is connected to each optical transmission line between each gas cell Optical branching means for branching to the transmitted light of the gas cell, optical joining means for joining the transmitted light branched by each optical branching means to the transmitted light optical transmission path, and upstream of the optical transmission path and the transmitted light optical transmission path An optical coupler for coupling the downstream side of the optical transmission line, a transmitted light splitting unit for splitting the transmitted light from the measurement light and the transmitted light in the optical transmission line, a light intensity of the split transmitted light, and a basic modulation frequency. Phase sensitive detection of wave component and second harmonic component A multipoint gas concentration measuring device using an optical fiber, comprising: a signal processing means. 4. A laser that oscillates a laser beam having a wavelength and intensity according to a drive current and temperature, a drive circuit for modulating the drive current of the laser with a predetermined amplitude around a predetermined current value, and measurement. A plurality of gas cells that contain the target gas and are arranged in series in the optical transmission line, and the transmitted light from the gas cells in the previous stage that is connected to each optical transmission line between each gas cell Optical branching means for branching to the transmitted light of the gas cell, optical joining means for joining the transmitted lights branched by the respective optical branching means, and an optical transmission path for transmitted light for transmitting the joined transmitted light from the joining means. And a signal processing means for detecting the light intensity of the combined transmitted light and performing phase sensitive detection of the fundamental wave component and the second harmonic component of the modulation frequency. Gas concentration measuring device. 5. A multipoint gas concentration measuring device using an optical fiber according to claim 2, further comprising a pulse modulator for intermittently measuring the measuring light from the laser in a pulsed manner. 6. A multipoint using an optical fiber according to any one of claims 2 to 4, wherein a plurality of gas cell rows each consisting of the plurality of gas cells, the light branching means, and the light merging means are arranged in parallel. Gas concentration measuring device.