CN101140220A - A multi-point optical fiber gas sensor system based on optical fiber buffer - Google Patents

Abstract

A multi-point gas sensing system based on an optical fiber buffer includes an address encoder, a dual-wavelength light source, a multi-point optical fiber sensing network, and a demodulator. The address encoder generates address codes and read-write control signals for different monitoring points; each detection unit of the multi-point optical fiber sensor network adopts a buffer structure in which a gas absorption chamber is introduced into the optical fiber loop; a double-ring coupled all-optical buffer can be used With the polarization type all-optical buffer; under the control of the write signal, the address code optical signal is "written" into the optical fiber loop of the detection unit, which is absorbed by the gas for multiple cycles; the absorbed optical signal is "read" and sent to The demodulator performs photoelectric conversion, comparison with reference light, address decoding and data processing to obtain gas concentrations at different points and realize multi-point gas monitoring. The system obtains beneficial effects by improving the structure of the absorption chamber, adding an optical amplifier, polarization control and the improvement of the 1665nm wavelength device, and is used for monitoring other gases with absorption peaks near the 1665nm wavelength.

Description

A multi-point optical fiber gas sensor system based on optical fiber buffer

technical field

The invention relates to a multi-point gas sensing system based on an optical fiber buffer, and belongs to the technical field of measuring gas concentration by optical fiber sensing systems.

Background technique

Energy shortage has become an important factor restricting the sustainable development of my country's current national economy. Among the primary energy sources (coal, nuclear energy, oil, natural gas, wind power, tide, solar energy), coal is the most important primary energy source in my country and plays a pivotal role. However, the current safety situation in coal mines is very worrying. Among various mine disasters, gas explosion ranks first, so gas monitoring technology has always been an important technology for coal mine safety. After long-term research and development, a variety of series of gas detection instruments have been developed: such as the use of flame height changes to detect gas test lamps; gas testers based on the principle of light interference; A gas detection tube that changes color (shade or displacement) during the reaction to determine the gas concentration; a thermal conductivity gas detector that uses the difference in thermal conductivity between the measured gas and air to achieve detection; and uses the surface of a gas-sensitive material to adsorb some Gas-sensitive gas detectors, etc., whose conductivity changes with the concentration of gases, have not been applied because of their many shortcomings.

At present, the methane detector based on the principle of electrochemical thermal catalysis is widely used. The catalyst is used to continuously burn (oxidize) low-concentration methane at a lower temperature. The heat released by different concentrations of methane during catalytic combustion is different, resulting in temperature changes, so methane concentrations can be measured. The advantage of this detector is that the output signal is large, the linearity is good, the signal processing and display are more convenient, the structure of the circuit and the instrument is simplified; the measurement is less affected by the environmental conditions, and various absorbents can be omitted; Other combustible gases (butane, pentane, etc.) can also react; it is easy to realize automatic continuous monitoring. Its disadvantages are poor anti-poisoning (hydrogen sulfide, organic silicon vapor) performance; frequent calibration (one week); measurement range is generally below 5%, and when the concentration is high, not only will there be activation phenomenon, but also when it is above 10%, "Double value" (same reading at different concentrations) also occurs.

There are already several gas safety monitoring systems in China, such as KJ1 and KJ2 systems of Changzhou Automation Research Institute, JK4 system of Great Wall Technology Company, A1 system, KJ2000 system, etc., which have played an important role in coal mine safety production. However, these systems mainly use advanced computer technology and communication technology to transform the monitoring system, and work hard on information transmission and collection. There is no substantial improvement on the front-end gas sensor. For many years, the gas monitoring system has consistently used the gas sensor based on the electrochemical principle. It must be powered by a long distance, which has potential safety hazards, poisoning, and short life. And its reaction speed is slow, can't reflect in time when there is gas outburst. Alternatives and new sensing principles have been sought for many years. About ten years ago, it was proposed to use optical fiber to make gas sensor. However, progress has been slow in the past ten years, and so far no optical fiber gas sensor has reached a practical level to work in coal mines.

The basic principle of optical fiber gas sensing is based on Beer’s law that gas absorbs special spectral lines in the spectrum, that is, I=I ₀ exp(-αCL), where I ₀ and I are the light intensity before and after the light passes through the gas, and α is the absorption coefficient , C is the gas concentration and L is the absorption length. It can be seen from the above formula that if higher detection sensitivity is required, the absorption coefficient or absorption length must be increased. If an optical fiber is used, the absorption wavelength must also fall within the optical transmission wavelength range of the optical fiber.

Gas has four absorption wavelengths for light: 3.43 μm, 6.522 μm, 3.3 12 μm and 7.658 μm, none of which can pass through the silica fiber, so only their harmonics can be measured. In the past research on fiber optic gas sensors, the wavelength of 1.31μm was used, and the combined harmonic of v ₂ +3v ₃ appeared at this wavelength, and the absorption coefficient was very low. When the length of the gas chamber reaches 0.5m, even if the concentration of the gas chamber reaches 25% of the upper critical limit of the explosion, the absorption loss for the wavelength of 1.3312μm is only 0.03dB. This level of loss is already below the unstable range of the light source, and it is very difficult to require accurate measurement.

In order to obtain higher detection sensitivity, a longer absorption length is necessary. At present, the length of the gas chamber used by people is about 50 cm. This means that it is of course very difficult to send light out of the fiber, pass through a 50cm absorption chamber and then converge into the fiber. Although feasible in principle, it is not feasible technically. Moreover, the absorption length of 50cm is too bulky for a sensor head, which is greatly affected by the environment. In a mine full of dust and moisture, practical application is impossible. Moreover, when the gas concentration in the environment changes, the concentration in the gas chamber of the sensor should also change accordingly, but an excessively large gas chamber will inevitably lead to a large difference between the gas concentration in the gas chamber and the gas concentration in the environment, or lag in time, reducing the Sensitivity and response speed of the sensor. Therefore, although optical fiber gas sensors have been proposed for many years, and many research institutions have reported their research results in the laboratory, they have not been practical so far, and are difficult to use in the narrow space of the harsh coal mining face.

There are two ways to improve detection sensitivity and response speed: (1) reduce the volume of the absorption chamber; (2) increase the absorption per unit length. In order to reduce the volume, but ensure sufficient absorption length, the only way is to increase the number of absorption. In other words, the same light pulse can be used to pass through the gas cell multiple times to increase the absorption length equivalently.

The purpose of the present invention is to propose a new structure aimed at the deficiencies of the prior art, that is, to use light pulses to absorb the gas absorbing gas chamber for multiple cycles in the optical fiber buffer, greatly reducing the length of the gas chamber and increasing the equivalent absorption. length; and use coding and modulation technology to control "writing" and "reading" for each buffer, so as to realize multiplexing of multiple detection points with one light source and demodulation system.

In order to improve the absorption rate, the present invention transfers the absorption wavelength from the combined harmonic of v ₂ +3v ₃ to the second harmonic of v ₃ (1.665 μm), and its absorption coefficient increases by an order of magnitude, which can shorten the length of the air chamber from 50cm to 5cm. Therefore, the flexibility of detection is greatly improved, and it is convenient for practical application.

In order to further improve the detection accuracy, the present invention adopts a dual-wavelength mode, one is the absorption wavelength, and the other is the reference wavelength. The two pass through the same optical path, except for the absorption loss, the other optical path losses are the same, and a high detection sensitivity can be obtained through differential comparison.

Contents of the invention

The purpose of the present invention is to realize a kind of multi-point optical fiber gas sensor system based on optical fiber buffer by the following technical scheme, it consists of an address encoder, a dual-wavelength light source, a multi-point optical fiber sensor network, and a demodulator Composed of devices, it is used for high-precision, high-sensitivity, real-time monitoring of gas concentration at multiple points in underground coal mines.

The address encoder generates serial address codes for different monitoring points, modulates the dual-wavelength light source, and generates control signals for "writing" and "reading" different monitoring points.

The light emitted by the dual-wavelength light source is modulated by the address encoder and sent to the multi-point optical fiber sensor network.

The multi-point optical fiber sensor network includes a plurality of detection units, each unit adopts the structure of the optical fiber type all-optical buffer, and a gas absorption chamber is added in the optical fiber loop of the optical buffer. The optical buffer includes an optical fiber loop for buffering optical signals, amplifiers in the loop, polarization controllers, wavelength division multiplexing couplers and other accessories, a nonlinear element for "writing" and "reading", and a gas absorption chamber. Under the control of the address encoder, each detection unit "writes" the optical signal carrying the address information of the detection point into the optical fiber loop of the unit, and circulates it multiple times; each cycle passes through the absorption chamber once, Thus being absorbed by gas many times. Then, under the control of the address encoder, the optical signal absorbed by the gas is "read out" and sent to the demodulator.

The demodulator performs photoelectric conversion on the received optical signal carrying different addresses and gas absorption information, and then compares it with the optical signal of the reference wavelength, performs address decoding and data processing, and obtains the gas concentration parameters of different absorption gas chambers. Realize real-time monitoring of multi-point gas concentration. Different patterns correspond to different monitoring points, and it is easy to distinguish them when they arrive at the same demodulator.

The technical scheme of the invention is also applicable to the sensing and monitoring of other gases with light absorption peaks near the wavelength of 1665nm.

The beneficial effects of the present invention are:

(1) The multi-point optical fiber gas sensor system based on the optical fiber buffer fully combines the high-tech in the field of modern optical communication and the high-tech in the field of sensing, including optical coding technology, all-optical buffer technology, optical fiber transmission technology, modulation Demodulation technology and multiplexing technology are used for distributed detection of methane gas in coal mines. It can monitor the concentration of methane gas in multiple locations with high sensitivity, high precision and real-time, providing an important basis for ensuring the safety of coal mines.

(2) The technical effect of the multi-point optical fiber gas sensing system based on the optical fiber buffer can also be improved by the following means.

First, improving the composition and structure of the gas absorption chamber, improving the performance of the absorption chamber, and reducing the transmission loss of the optical path can improve the light absorption efficiency of the gas and reduce noise, thereby improving the performance of the sensing system.

Second, improve the structure of the fiber ring cavity, including installing an optical amplifier in the fiber ring to compensate for the loss of the optical path. Due to the harsh environment of some optical fiber lines in underground coal mines, it may cause unstable factors such as additional loss and self-excited noise, which have a great impact on optical fiber transmission characteristics and ring cavity performance. Therefore, improving the structure and control mode of the optical fiber loop can improve the performance of the sensing system.

Third, improving or selecting a more optimized code pattern, modulation, and multiplexing method can improve the control signal and time synchronization performance of the optical switch in the all-optical buffer, and improve the flexibility and accuracy of distributed optical fiber sensing technology, thereby Improve the performance of sensing systems.

Fourth, when selecting a dual-wavelength light source, its two working wavelengths must be able to distinguish between the absorption wavelength and the reference wavelength, and must be as close as possible to ensure that they can serve as a reference, and their wavelength and optical power should be stable. Therefore, selecting a high-quality dual-wavelength light source with stable power can improve the performance of the sensor.

Fifth, adopting fast polarization stabilization technology can improve the interference effect when multiple sensing heads (absorbing chambers) work together, thereby improving the performance of the sensing system.

Sixth, the improvement of the function of the signal processing part of the sensor system is also conducive to improving the technical effect of the sensor.

Seventh, the functions of other parts of the sensor system are improved, and the relevant optical devices such as dual-wavelength continuous lasers, couplers, circulators, filters, isolators, etc. matching with the second harmonic absorption peak of gas gas (1665nm) are selected and improved. performance, are conducive to improving the technical effect of the sensor system.

Description of drawings

Fig. 1 is a schematic structural diagram of a multi-point optical fiber gas sensing system based on an optical fiber buffer;

Fig. 2 is the schematic structural diagram of the multi-point optical fiber gas sensing system based on the double-ring coupled all-optical buffer;

Fig. 3 is a schematic structural diagram of a multi-point optical fiber gas sensing system based on a polarization-type all-optical buffer.

Detailed ways

The present invention will be further described below in combination with examples of implementation and accompanying drawings.

Embodiment 1: A multi-point optical fiber gas sensor system using a dual-ring coupled all-optical buffer;

As shown in Figure 1, the multi-point optical fiber gas sensor system based on dual-ring coupled all-optical buffers consists of an address encoder 1, a dual-wavelength light source 2, and a multi-point optical fiber transmission system composed of multiple dual-ring coupled all-optical buffers. Sense network 3, and a demodulator 4. The multi-point optical fiber sensor network 3 is shown in Figure 2, and it is made up of multiple double-ring coupled all-optical buffers, and each double-ring coupled all-optical buffer includes a circulator 5, a 3X3 coupler 6 arranged in parallel, left and right two An optical fiber loop 7 and 8, an absorption chamber 9 and a nonlinear element 10.

The address encoder 1 sets different patterns for different monitoring points, modulates the 1665nm dual-wavelength light source 2 in Figure 2, and generates control signals P _ctl for "writing" and "reading" different monitoring points 1, P _ctl 2 and P _ctl 3. After the light emitted by the dual-wavelength light source 2 is modulated by the address encoder 1, it is sent to the circulator 5 of the double-ring coupling all-optical buffer used by the first monitoring point A of the multi-point optical fiber sensor network 3, and then enters the parallel arrangement Middle port of 3×3 fiber coupler 6. The 3×3 coupler divides this signal into two paths, which are averagely output from the upper and lower ports on the right side of the coupler and enter the right loop 7 of the optical fiber. The nonlinear element 10 receives the “write” control signal P _ctl 1 from the address encoder 1 , and “writes” the address code corresponding to the monitoring point A into the annular cavity, and enters the left loop 8 . A gas absorption chamber 9 is arranged in the left loop 8, and this address signal is just absorbed partly by the methane gas, and then returns to the right loop 7. In this way, the pulse of this code pattern repeatedly circles in the annular cavity formed by these two loops, and is absorbed by the gas gas many times until another control signal "read" signal reads it out from the absorption loop. The read signal is output from the middle port on the left side of the 3×3 coupler 6 arranged in parallel, and leaves the optical buffer of the first monitoring point A through the circulator 5 again, and passes through the circulators of the following monitoring points B and C and finally reaches demodulator 4.

The optical signals carrying different addresses and gas absorption information received by the demodulator 4 are first subjected to photoelectric conversion, and then address decoding and data processing are performed to obtain gas concentration parameters of different absorption chambers, thereby realizing real-time monitoring of gas concentrations at multiple points. monitor.

The function of the nonlinear element 10 here is to generate a nonlinear phase shift, so any device capable of generating a nonlinear phase shift at a wavelength of 1665nm can realize this function. Specifically, ordinary optical fibers, highly nonlinear optical fibers, photonic crystal fibers, and semiconductor optical fibers can be used. Amplifiers and electroabsorption modulators, etc.

Embodiment 2: A multi-point optical fiber gas sensor system using a polarization-type all-optical buffer;

As shown in Figure 1, the multi-point optical fiber gas sensor system based on dual-ring coupled all-optical buffers consists of an address encoder 1, a dual-wavelength light source 2, and a multi-point optical fiber transmission system composed of multiple polarization-type all-optical buffers. Sense network 3, and a demodulator 4. The multi-point optical fiber sensing network 3 is shown in Figure 3, each monitoring point uses a polarization type all-optical buffer, and a gas absorption chamber 6 is placed in the optical fiber loop of the buffer. Therefore, each polarization-type all-optical buffer includes a nonlinear element 5 , an absorption chamber 6 , and a section of optical fiber loop 7 . The optical fiber loop 7 referred to here includes a section of optical fiber and accessories to improve loop performance, such as polarization controllers, amplifiers, and the like.

The address encoder 1 sets different patterns for different monitoring points, modulates the 1665nm dual-wavelength light source 2 in Figure 2, and generates control signals for "writing" and "reading" different monitoring points. The light emitted by the dual-wavelength light source 2 is sent to the multi-point optical fiber sensor network 3 after being modulated by the address encoder 1 . It first reaches the nonlinear element 5 of the polarization-type all-optical buffer used in the first monitoring point A. The nonlinear element 5 receives the "write" signal from the address encoder 1, generates polarization rotation, and "writes" the address code corresponding to the monitoring point A into the optical fiber ring 7, and passes through the gas absorption chamber 6, and the address signal is Part of it is absorbed by the gas, and then returns to the nonlinear element 5. Since no new control signal arrives at this time, the pulse of the address signal circles repeatedly in the optical fiber loop and is absorbed by the gas many times. When another control signal "read" arrives, the nonlinear element 5 will again rotate the polarization of the optical signal so that it can be read out of the absorbing loop. When the read signal passes through the non-linear elements 5 of the monitoring points B and C, since there is no corresponding "write" signal, it will directly pass through them. By analogy, this signal will finally reach the demodulator 4 . The signal process of other monitoring points is similar to this. After the demodulator 4 receives the optical signal carrying different addresses and gas absorption information, it first performs photoelectric conversion and compares it with the signal of the reference light, and then performs address decoding and data processing to obtain the gas concentration parameters of different absorption chambers. Realize real-time monitoring of multi-point gas concentration.

The function of nonlinear element 5 here is to generate nonlinear polarization rotation, so any device that can generate nonlinear rotation at a wavelength of 1665nm can realize this function, specifically ordinary optical fiber, highly nonlinear optical fiber, photonic crystal optical fiber, semiconductor optical amplifier can be used and electroabsorption modulators.

Claims (7)

1. A multi-point optical fiber gas sensing system based on optical fiber buffer is characterized in that: the system is mainly composed of an address encoder, a dual-wavelength light source, a multi-point optical fiber sensing network, and a demodulator. Composition; of which: The address encoder generates serial address codes for different monitoring points, modulates the dual-wavelength light source, and generates control signals for "writing" and "reading" different monitoring points; The light emitted by the dual-wavelength light source is modulated by the address encoder and sent to the multi-point optical fiber sensor network; The multi-point optical fiber sensor network includes multiple detection units, each unit adopts the structure of a fiber-optic all-optical buffer, and a gas absorption chamber is placed in the optical fiber loop of the buffer, and each detection unit is located at the address encoder Under control, the optical signal carrying the address information of the detection point is "written" into the optical fiber loop of the unit, and circulated multiple times; each cycle passes through an absorption chamber, and is absorbed by the gas multiple times; then, Under the control of the address encoder, the optical signal absorbed by the gas is "read out" and sent to the demodulator; After receiving the optical signals carrying different addresses and gas absorption information, the demodulator performs photoelectric conversion, address decoding and data processing to obtain gas concentration parameters of different absorption chambers, thereby realizing real-time monitoring of multi-point gas concentration. 2. A kind of multi-point optical fiber gas sensing system based on optical fiber buffer according to claim 1, it is characterized in that: adopt 1665nm dual-wavelength mode, one wavelength is the absorption wavelength, and one wavelength is the reference wavelength; An optical path has the same optical path loss except the absorption loss. Through differential comparison, a high detection sensitivity can be obtained. 3. a kind of multi-point gas sensing system based on optical fiber buffer according to claim 1, is characterized in that: each monitoring point of multi-point optical fiber sensing network uses a double-ring coupled all-optical buffer, and utilizes 1550nm The control light realizes the nonlinear phase shift for the signal light with a wavelength of 1665nm. The nonlinear element can be ordinary optical fiber, high nonlinear optical fiber and photonic crystal optical fiber, so as to realize the "reading and writing" operation. 4. The multi-point gas sensing system based on optical fiber buffer according to claim 1, characterized in that: each monitoring point of the multi-point optical fiber sensing network uses a polarization type all-optical buffer, and utilizes 1550nm control light Realize nonlinear polarization rotation for 1665nm wavelength signal light; the nonlinear element can be ordinary optical fiber, highly nonlinear optical fiber and photonic crystal optical fiber, so as to realize "reading and writing" operation. 5. A kind of multi-point optical fiber gas sensing system based on optical fiber buffer according to claim 1, characterized in that: the optical signals passing through all the gas absorption chambers arrive at the same demodulator, and are compared with the optical signals of the reference wavelength After comparison, after demodulation and subsequent signal processing, the absorption loss of the absorption chambers at different monitoring points is obtained, so as to obtain the gas concentration of each monitoring point, and realize multi-point gas sensing. 6. A kind of multi-point gas sensing system based on optical fiber buffer according to claim 1, is characterized in that: select the double-wavelength continuous laser that matches with gas gas second harmonic absorption peak corresponding wavelength (1665nm), coupling devices, circulators, filters, isolators. 7. A multi-point gas sensing system based on an optical fiber buffer according to claim 1, characterized in that: the system is used for sensing and monitoring other gases that have light absorption peaks near the wavelength of 1665nm.

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