JP3229391B2 - Multipoint gas concentration measurement method and apparatus using optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas concentration measuring method and apparatus for optically measuring gas concentration, and more particularly to a gas concentration measuring method and apparatus capable of detecting gas even at a low concentration. The present invention relates to a multi-point gas concentration measuring apparatus 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 fiber.

[0002]

2. Description of the Related Art It is known that the presence or absence of a gas can be detected by utilizing the fact that laser light of a specific wavelength is easily absorbed by a certain kind of gas. Sensing technology using this principle is used in industrial measurement, pollution monitoring, etc. Widely used in Also,
If a laser beam is used as an optical fiber path as a transmission path, it is possible to detect gas in a remote place.

As an example, methane gas has a strong absorption spectrum line at 1.6 μm. Light containing this wavelength and another wavelength not absorbed by methane gas is guided to the measurement gas by 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 a light receiving section. In this case, a light source having a wide bandwidth such as an LED is used as the light source. In the light receiving unit, the light having a wavelength that is absorbed by methane gas and the light that is not absorbed by methane gas are spectrally separated using a band-pass filter or the like,
The ratio of the amount of attenuation of each light is obtained, and the concentration of methane gas is detected based on the ratio.

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

Accordingly, the present inventors have disclosed in Japanese Patent Application No. 2-78498.
A new remote methane gas detection device using an optical fiber as a transmission line has been developed by applying the technology. In a method using this principle, a driving power supply for a light source is modulated at a high frequency around a predetermined current, and a laser beam having a modulated wavelength and intensity is oscillated. At this time, a part of the laser light emitted from the semiconductor laser is used for monitoring, and the current and the temperature are controlled so that the center wavelength of the oscillation becomes the center of the methane absorption line. The laser light thus stably emitted is transmitted through an optical fiber to a measurement gas cell containing a gas of unknown concentration, and the transmitted light is guided to a light receiving unit by another optical fiber facing the laser light. The gas concentration is determined 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 remote methane gas detection device described above can measure only one gas concentration. Even if a plurality of gas cells are installed in series or in parallel along an optical fiber in order to measure gas concentrations at a plurality of locations, the sum of the absorption amounts at all measurement points can be obtained from the signal obtained at the light receiving unit. Is not enough to detect the gas concentration at each point. After all, the remote methane gas detection device can measure only one gas concentration, so if there are several places where concentration monitoring is necessary, a detection device including a light source, a light receiving unit, etc. is provided separately for each measurement point. And the cost increases.

On the other hand, as another method, a method of arranging a plurality of gas cells along an optical fiber and measuring a gas concentration at a plurality of places can be considered. 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 gas cell 6 via the optical transmission path 5. 6 is combined with another optical transmission line 5f and collected by the signal processing unit 4, and a plurality of light sources (one half of the measurement location) are transmitted from the light source unit 2.
Laser light modulated at the above modulation frequency. 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 of the problems is to prepare n / 2 or more different modulation frequencies in order to perform n multi-point density measurements. And 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 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 a low concentration, to measure gas concentrations at a plurality of locations by using a single set of light sources and signal processing means, and to simplify the control method. Another object of the present invention is to provide a method and an apparatus for measuring a multipoint gas concentration using an optical fiber which requires a small amount of optical fiber.

[0010]

[0011]

[0012]

[MEANS FOR SOLVING THE PROBLEMS] To achieve the above object
In addition, the present invention provides a wavelength and intensity according to the drive current and the temperature.
A laser that oscillates a laser beam of a certain degree
Modulate the drive current of the laser with a predetermined amplitude as
A driving circuit, a plurality of gas cells including a gas to be measured and arranged in series in an optical transmission line, and a transmission unit connected to each optical transmission line between the gas cells and transmitting light from the preceding gas cell to the succeeding gas cell. Optical branching means for branching into the measuring light and the transmitted light of the preceding gas cell; optical converging means for merging the transmitted light branched by each optical branching means with the transmitted light optical transmission path; and upstream of the optical transmission path. light intensity of the transmitted light and the optical coupler for coupling the downstream optical transmission line, and transmitted light branching means for branching the transmitted light from the measurement light and the transmitted light of the optical transmission path, branched transmitted light and
And the fundamental wave component and the second harmonic wave component of the modulation frequency are detected.
Signal processing means for phase-sensitive detection.

[0013]

[0014]

[0015]

[0016]

With the above arrangement, the wavelength and intensity of the laser light are modulated and the laser light is intermittently pulsed. Since a modulated wave of several cycles is included between the pulse widths, 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. The time required for that 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 lag. By performing phase-sensitive detection for each pulse, the concentration at each measurement point can be obtained even at a 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 fiber may be small.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below in detail 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 at the frequency ω, the oscillation frequency Ω and the oscillation intensity of the laser light are modulated at the frequency ω. Now, when the laser light whose frequency and intensity are modulated as described above is transmitted through an optical fiber into an atmosphere containing methane gas, the transmitted light P is expressed by the following equation (1).
Becomes

P = A [I ₀ + ΔI cos (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-order derivative dT / dΩ T ₀₂ ; second-order derivative d ² T / dΩ ² Also, φ ₁ is a laser beam emitted from a light source, transmitted through a fiber, and received. Φ ₁ = (2π / ω) · N · L / V (2) where N: refractive index of optical fiber core V: speed of light in vacuum L: light source Length of the optical transmission path from the 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 +
The frequency and phase components of φ ₁ )) are phase-sensitive detected and P (2
ω) yields Equation (3), where 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. In that case, since _T01 is 0 and _T02 is the maximum, the second term of the equation (3) can be deleted, 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 Expression (3), Expression (5) is obtained.

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

In addition, the minimum residual length Δl of the optical transmission path is determined so that the pulsed laser light modulated wave transmitted through each gas cell arrives at a time lag without overlapping the signal processing means. If the number of modulated waves included in one pulse is C _Y , then Δl ≧ (C _Y / ω) · (V / N) (7) In the embodiments described below, it is assumed that the length of the optical transmission path is configured to satisfy the above equation.

An embodiment of the present invention is constructed based on the above findings. Hereinafter, reference examples having portions common to the present invention.
And an embodiment of the present invention will be described.

As shown in FIG. 1, the multi-point gas concentration measuring apparatus 1 has a light source section 2 composed of a laser and a driving circuit, a pulse modulator 3, and a signal processing means 4. Is connected to a large number (n) of gas cells 6 provided at remote locations.

The laser included in the light source unit 2 oscillates a laser beam having a wavelength and intensity corresponding to the drive current and the temperature. Similarly, a 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 of an arbitrary width continuously or spontaneously, and the pulse
Can be intermittently pulsed.
The pulse modulator 3 is connected to the uppermost stream 5 a of the optical transmission line 5. In this embodiment, the pulse modulator 3 is composed of LiNbO
₃ guide and waveguide Mach-Zehnder modulator is used, by applying a pulse signal to the LiNbO ₃ waveguide type Mach-Zehnder modulator, it is possible to transmit the pulsed laser light 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 a modulation frequency. Can be subjected to phase sensitive detection to measure the concentration of the specific gas.

In the reference example shown in FIG. 1, the gas cell 6 has two transmission light paths, and one ends of these transmission light paths are connected to each other by a loop-shaped optical fiber 12. on the other hand,
The optical transmission path 5 is appropriately connected with a transmitted light input / output unit 7 for measurement, a light branching / joining unit 8, and a transmitted light branching unit 9. More specifically, the measuring light transmitting / receiving means 7 is provided for each gas cell 6 and mainly includes an optical branching coupler 10 and an optical isolator 11. Optical branching coupler 10
Can not only split light from the upstream side to the downstream side divided into two paths, but also merge light from both downstream sides to the upstream side and return it. The upstream side of the optical branching coupler 10 is connected to the optical transmission line 5 b allocated for each gas cell 6, and the other downstream side is connected to the optical isolator 11.
And the other is connected to one transmitted light path of the gas cell 6. The optical isolator 11 is capable of blocking light from one side and allowing light from the other side to pass unilaterally. The optical isolator 11 is connected to the other of the gas cell 6.
The light from the two transmitted light paths is connected so as to be able to unilaterally pass through the optical branching coupler 10. According to this configuration, the measuring light transmission light input / output unit 7 transmits the measuring light from the optical transmission line 5b to the optical branching coupler 10, one of the transmission light paths of the gas cell 6,
The looped optical fiber 12, the other transmitted light path of the gas cell 6, and the optical isolator 11 are guided in this order, and the transmitted light of the gas cell 6 that has passed through the optical isolator 11 is transmitted to the optical transmission path 5.
b.

The light branching / combining means 8 is capable of branching light from an upstream side to a downstream side divided into two paths, and also allowing light from both downstream sides to merge into the upstream side and return. is there. The light branching / combining means 8 is provided in the optical transmission path 5 in correspondence with the gas cells 6, and supplies the measuring light from the optical transmission path 5 to the measuring light transmitting / receiving means 7 of each gas cell 6. The transmitted light from the measuring light transmitting / receiving means 7 can be combined and taken into the optical transmission line 5.

The transmitted light branching means 9 is for passing light from the upstream side to the downstream side and branching light from the downstream side to the upstream branch path. The transmitted light branching means 9
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 of the transmitted light branching unit 9 is connected to the signal processing unit 4 via the optical transmission line 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 transmitted light to the signal processing unit 4.

Next, the operation of the reference example will be described.

In the light source unit 2, the driving wavelength and the temperature are adjusted with the oscillation wavelength of the semiconductor laser as the center frequency of the absorption line of the methane gas, and the frequency-modulated laser light is generated. This laser light is continuous light as shown in FIG. In the pulse modulator 3, this continuous light is
The signal is modulated by the pulse shown in FIG.
The measurement light whose wavelength and intensity are modulated and which are intermittently pulsed as shown in FIG. 7 is supplied to the optical transmission line 5a. The measurement light passes through the transmitted light branching means 9 and reaches the first light branching and joining means 8 as shown by the solid line arrow in FIG.

The light branching / combining means 8 branches the measuring light to the measuring light transmitting / receiving means 7 of each of the n gas cells 6 via each optical transmission line 5b. At this time, if the branching ratio of each optical branching and joining means 8 is set to, for example, 1: (n-1) on the upstream side and is closer to 1 toward the downstream side, an equal amount of measuring light is distributed to each gas cell 6. it can. Further, in the measuring light transmitting / receiving light input / output means 7, the light branching coupler 10 directly connects the gas cell 6
The light is distributed so that the light traveling toward is increased. Light branched from the optical branching coupler 10 to the optical isolator 11 is blocked there.

The measuring light distributed from the optical branching coupler 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 transmitted to the gas cell 6.
Through the transmitted light path. Here, the transmitted light that has reciprocated in the transmitted light path in the gas cell 6 is attenuated according to the gas concentration. The transmitted light (broken arrow in the figure) passes through the optical splitter / coupler 10 via the optical isolator 11, returns to the original optical transmission line 5 b, and reaches the optical splitter / merging unit 8. Further, the transmitted lights are sequentially merged by the light branching / joining means 8 and return to the original light transmission path 5 to reach the transmitted light branching means 9. Then, the light is branched by the transmitted light branching means 9 and enters the signal processing means 4.

Since the light in the optical fiber is transmitted at the speed of V / N, the pulse-like laser light generated by the pulse modulator 3 can be changed by changing the transmission path length of each gas cell. Arrive at the signal processing means 4 with a time delay. That is, as shown in FIG. 5D, transmitted light arrives in order from the gas cell having the shorter transmission path. The signal processing means 4 receives these pulsed transmitted lights, 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, it is displayed as a concentration measurement result of the gas cell having the shorter transmission path in the order of arrival. However, in this embodiment, since the measuring light is transmitted through the gas cell once back and forth, 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, gas can be detected even at a low concentration by phase sensitive detection. Moreover, by intermittently oscillating the laser light in a pulsed manner, a single light source and a signal processing means can measure gas concentrations at many points and simplify the control method. Further, since the measuring light and the transmitted light are reciprocated in the same optical transmission line, the amount of optical fiber may be small even if the measuring point is located at a remote place.

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

The directional coupler 41 shown in FIG. 4 can be used as the measuring light transmitting / receiving 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 side of the transmission line 42 to the optical transmission line 5b, the optical branching coupler 10 and the optical isolator 11 can be replaced.

Next, an embodiment of the present invention will be described.

In the embodiment shown in FIG. 2, the multipoint gas concentration measuring device 1 includes a light source unit 2 comprising a laser and a driving circuit.
It has a pulse modulator 3 and a signal processing means 4, which are the same as in the reference example . Also, the transmitted light branching means 9
Is the same.

Each gas cell 6 has one transmission light path, and each gas cell 6 is arranged in series on the light transmission path 5. Each of the optical transmission lines 5 between the gas cells 6 has an optical branching unit 12.
Is 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 unit 12 is connected to the transmitted light outlet of the gas cell 6 in the preceding stage, the transmitted light from the gas cell 6 in the preceding stage is branched at an appropriate branching ratio. It is split into light and the other into light transmitted through the gas cell 6 in the preceding stage. This branch is connected to the optical converging means 13.

The light converging means 13 has two paths upstream and one path downstream, and can converge the upstream light to the downstream. The transmitted lights of the gas cells 6 at each stage branched by the respective light branching means 12 are sequentially combined by the light combining means 13 into the transmitted light transmission line 5d. An optical isolator 11 is provided downstream of the transmitted light optical transmission line 5d, and an optical coupler 14 is connected further downstream.

The optical coupler 14 is capable of branching light from the upstream side to a downstream side divided into two paths, and also allows light from both downstream sides to be merged and returned to the upstream side. . The optical coupler 14 has an optical transmission line 5 on the upstream side.
One of the downstream sides is connected to the first stage gas cell 6, and the other is connected to the optical isolator 11. The optical coupler 14 couples the upstream of the optical transmission line 5 and the downstream 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 shown in FIG. 2, the light obtained by the signal processing means 4 is transmitted light only through the first gas cell 6, and transmitted light transmitted through the first gas cell 6 and then transmitted through the second gas cell 6. Thereafter, the transmitted light having the number of transmission stages successively arrives arrives in ascending order of the optical transmission path length. The signal processing means 4 calculates the gas concentration of the preceding gas cell 6 by performing phase sensitive detection, and sequentially calculates the gas concentration of the succeeding gas cell 6 using the result.

Next, another reference example will be described.

In the reference example shown in FIG. 3, the multipoint gas concentration measuring apparatus 1 includes a light source unit 2 composed of a laser and a driving circuit and a signal processing means 4 as in the above-described reference examples and embodiments.
, But 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 because the coupling configuration of the optical transmission line in the reference example, the embodiment, and the reference example of FIG. This shows that the method can be used for a modulation measurement method.

As in the second embodiment, gas cells 6 are arranged in series on the optical transmission line 5, and an optical branching unit 12 is connected between the gas cells 6, and an optical converging unit 13 is connected to the branch destination. Have been. The difference from the second embodiment is that
3 is that the merge direction is sequentially merged to the subsequent stage side. The last optical converging means 13 is connected to the signal processing means 4 via the transmitted light optical transmission line 5e.

[0050] In such a configuration, the light source unit 2 to the gas cell 6 transmission path length between l _0,0 gas cell 6 ₁ to the gas cell 6 ₂ transmission path length between ₁ and l _{0, 1} gas cell 6 ₂ - gas cell 6 ₃ The transmission path length between l _0,2 gas cell 6 _n-1 and the transmission path length between gas cell 6 _n is l _{0, n-1} The transmission path length between gas cell 6 _n and the last optical converging means 13 _n-1 l _{0, n} The transmission path length between the last optical combining means 13 _n-1 and the signal processing means 4 is l _{0, n + 1} The transmission path length between the optical branching means 12 ₁ and the optical combining means 13 ₁ is l.
_The length of the transmission path between the optical combining means 13 ₁ and the optical combining means 13 ₂ is 1
The transmission path length between the _1,2 optical converging means 13 _n-2 and the optical converging means 13 _n-1 is l _{1, n-1} and the transmission path length between the optical branching means 12 ₂ and the optical converging means 13 ₁ is l.
The transmission path length between the _2,1 optical branching means 12 ₃ and the optical combining means 13 ₂ is l
The transmission path length between the _2,2 optical branching means 12 _n-1 and the optical converging means 13 _n-2 is defined as l _{2, n-2} .

The signal detected by the signal processing means 4 is the sum of the light transmitted through each of the light combining means 13 and each of the gas cells 6. Ignore losses in the optical system for simplicity. 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 phase-sensitive detection signals PG _SUM is as follows: 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
Assuming that the transmitted light path length of each gas cell is constant and the gas concentration of the gas cell 6 _k is C _k for simplicity, the apparent gas concentration CK ( K = 1,2, ··, N) is, C1 = is represented by _{C 1 C2 = C 1 + C} 2 CN = C 1 + C 2 + ··· + C N, putting the PG _K = PG _K (CK) _{, PG K (CK) = a} K · I 0K · (ΔΩ) 2 · α (ω) · CK · L S / (2γ) 2 ... (9) However, a _K: signal transmission loss involved in PG _K I _0K : Central intensity of laser involved in 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 an amount dependent on the transmission path length in which the transmitted light is involved, and the optical path length LK of each transmitted light.
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 represented as _{0, n + 1} . If φ _k = φ _k (LK), φ _k (LK) = (2π / ω) · N · LK / V (10)

By the way, phase-sensitive detection of one modulation frequency means that two parameters of magnitude and phase have been obtained for equation (8). The transmission path is
Once installed, there is no possible to change, φ ₁ ~φ
_N is a known quantity, and eventually equation (8) is a binary simultaneous equation including n unknowns PG _{1 to} PG _n including the unknown concentration of the measured gas in each gas cell. Therefore, in order to solve this problem when n> 2, the laser beam is modulated at another frequency ω ′ (≠ ω), and the same measurement is performed as described above. Two parameters are obtained at one modulation frequency. Therefore, when n> 2, if phase-sensitive detection is performed at n / 2 different modulation frequencies (in the case of a small number, rounded up), PG ₁ , PG ₂ ,.
G _N is obtained.

The same applies to the fundamental wave phase sensitive signal. The magnitudes involved in gas concentration measurement are PW ₁ , PW
_2, ..., and PW _{n, 1} a phase difference phi, phi _2, ..., phi
Assuming that _n, the sum PW _SUM of the fundamental 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 involving the transmitted light, and if PW _K = PW _K (ΔI _K ), PW _K = A · ΔI _K (12)

In the measurement of the equation (11), two parameters of magnitude and phase are obtained as described above. PW _{1 to} PW _n
To solve the n unknowns of
The same measurement is performed by modulating the laser light with (≠ ω).

From the equations (9) and (12), the ratio is obtained.

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

Next, another reference example will be described.

As shown in FIG. 6, the series gas cells 6 and the optical system shown in the reference example of FIG. 3 form a gas cell row, and a plurality of gas cell rows are arranged. They are connected in parallel. Such a configuration,
It is suitable when the measurement location is divided into several locations and one location contains several measurement points.

Next, a case where the number of gases to be measured is two or more will be described. For example, a laser oscillating at a wavelength of 1.53 μm is prepared for acetylene detection. The driving current or the temperature of the laser is changed so that the oscillation frequency coincides with the center of the acetylene absorption line. An optical coupler is provided in order to transmit this laser light through an optical fiber and to join it with the optical fiber transmitting the methane detection laser light 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.
In the signal processing system of the light receiving unit, a signal processing device in which two signal processing devices for methane and an acetylene signal processing device are provided in parallel, or a switching device is provided in the processing device described in the embodiment, so that each If the same processing as described above is performed for each modulation frequency in the processing system, simultaneous detection of two or more types of gases becomes possible.

[0062]

The present invention exhibits the following excellent effects.

(1) The gas concentration can be measured at a plurality of locations with one set of light source and signal processing means, so that it is economical.

(2) Compared with the method using a plurality of modulation frequencies, the method of pulse-modulating the laser beam of the present invention has a simpler control method.

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

[Brief description of the drawings]

FIG. 1 is a block diagram showing a reference example of the present invention.

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

FIG. 3 is a block diagram showing a reference example of the present invention.

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

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

FIG. 6 is a block diagram showing a reference example of the present invention.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Multi-point gas concentration measuring device 2 Light source part 3 Pulse modulator 4 Signal processing means 5 Optical transmission line 6 Gas cell

Continued on the front page (72) Inventor Masahiko Uchida 5-1-1, Hidaka-cho, Hitachi City, Ibaraki Prefecture Nippon Cable Electric Co., Ltd. Opto-System Research Laboratory (72) Inventor Satoshi Yamamoto 5 Hidaka-cho, Hitachi City, Ibaraki Prefecture JP-A-3-4147 (JP, A) JP-A-3-248299 (JP, A) JP-A-3-237314 (Japanese) JP, A) JP-A-3-277945 (JP, A) JP-A-57-170738 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 21/00-21/61 G02B 6/00

Claims (1)

(57) [Claims] 1. A laser which oscillates a laser beam having a wavelength and intensity corresponding to a drive current and a temperature, a drive circuit for modulating a drive current of the laser with a predetermined amplitude around a predetermined current value, and a measurement circuit. A plurality of gas cells including the target gas and arranged in series in the optical transmission path, and the transmitted light from the preceding gas cell connected to each optical transmission path between the gas cells and the measurement light of the latter gas cell and the former Optical branching means for branching into the transmitted light of the gas cell, optical merging means for merging the transmitted light branched by each optical branching means with the optical transmission path for transmitted light, and the optical transmission path for the transmitted light upstream of the optical transmission path Optical coupler, a transmitting 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 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.