WO2018064822A1

WO2018064822A1 - Permeable optical fiber for gas sensing

Info

Publication number: WO2018064822A1
Application number: PCT/CN2016/101522
Authority: WO
Inventors: Bin Zhou; Sune Svanberg; Sailing He
Original assignee: South China Normal University
Priority date: 2016-10-09
Filing date: 2016-10-09
Publication date: 2018-04-12

Abstract

A permeable optical fiber (100, 202, 304, 403, 701) for distributed gas sensing is described. The fiber (100, 202, 304, 403, 701) has got core (101, 502, 602) and cladding (102, 501, 601) structure. It can be arranged in a plurality of ways, but could have a permeable polymer cladding (102, 501, 601) to let the gas go inside the fiber (100, 202, 304, 403, 701) and a silica core (101, 502, 602) to guide light. The polymer cladding (102, 501, 601) is also designed to protect the fiber (100, 202, 304, 403, 701) from physical damage. In the optical fibers (100, 202, 304, 403, 701) some small part of light is distributed in the region of cladding (102, 501, 601), and the permeable cladding (102, 501, 601) makes the interaction between surrounding gas the guided light possible, and then the sensing of the gas surrounding the fiber (100, 202, 304, 403, 701) is realized by the laser absorption spectroscopy. The proposed sensing fiber (100, 202, 304, 403, 701) is preferentially used for long distance gas sensing system using a variety of arrangements, including the use of sections of the gas permeable fiber (100, 202, 304, 403, 701) as sensors located at various sites, and optically accessed through normal fiber transmission or by using the differential absorption lidar technique, there spatial concentrations along the same long fiber are measured by time-of-flight or phase shift techniques. Numerous gases can be monitored in connection with industrial, agricultural, mining operations, etc.

Description

Permeable Optical Fiber for Gas Sensing

FIELD OF THE INVENTION

The invention describes a permeable optical fiber which can be applied as long distance gas sensors including single point gas sensor, multipoint gas sensor, distributed gas sensor and so on.

BACKGROUND OF THE INVENTION

Optical fiber gas monitoring is an important issue in many applications. Optical spectroscopy technology provides non-intrusive, sensitive and selective gas analysis in real-time (See, e.g. Reference [1-3] ) . Optical fiber itself is a dielectric material and environment robust. It is safety for flammable gas monitoring as the risk of electronic spark has been overcame. As a waveguide the optical fiber can guide the light wherever needed and then using the absorption spectroscopy technology to sense the target gas just as the well-known lidar system. Lidar is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. With the help of Mie and Rayleigh scattering, the lidar is also able to record a range-resolved signal from atmospheric particulates and molecules in a free atmospheric volume. (Reference [4-6] ) . Using the differential absorption lidar (dial) variety, where the transmitted wavelength is alternatingly switched between an absorption line of a gas of interest (the on-wavelength) and a neighboring non-absorbed reference wavelength (the off-wavelength) , it is possible to derive a range-resolved gas concentration map for the studied gas (Reference [7] ) . However, it is difficult for a lidar system to cover arbitrary gas volumes, since an atmospheric lidar system operates on the line-of-sight principle. Also, there might be eye safety considerations. Hence, in some important applications such as monitoring the gas concentration inside buildings, tunnels, factories and mines, the conventional lidar system becomes impractical.

A number of solutions have been proposed for multipoint gas monitoring. One of the most important solutions is using wireless technology to connect the gas sensors and create a wireless sensing network. The number of sensors in a sensing network is limited by the network protocol and the cost. To enlarge the number of sensors, the distributed sensing technology is required. There is no practical solution up to date for distributed gas monitoring. The present invention discloses the principles of such a distributed gas sensing system, where the optical evanescent field surrounding a light-carrying fiber is sensing the ambient gas surrounding the fiber. This is in important contrast to evanescent field gas sensing using CW lasers providing no range-resolution as discussed in (Reference [8] ) . Like-wise, range-resolved monitoring of liquids using fiber technology has been described in (Reference [9] ) . We here disclose how differential absorption lidar (dial) technology is utilized to probe the gas concentration along a fiber using a pulsed or modulated source, and a time-resolving detection system.

SUMMARY OF THE INVENTION

The permeable optical fiber has got core and cladding structure. It can be arranged in a plurality of ways, but could have a permeable polymer cladding to let the gas go inside the fiber and a silica core to guide light. The polymer cladding is also designed to protect the fiber from physical damage. In the optical fibers some small part of light is distributed in the region of cladding, and the environment gas can goes into the permeable cladding and interacts with the light in the cladding area. Then the sensing of the gas surrounding the permeable fiber is realized by the laser absorption spectroscopy.

The proposed sensing fiber is preferentially used for long distance gas sensing using a variety of arrangements, including the use of sections of the gas permeable fiber as sensors located at various sites, and optically accessed through normal fiber transmission or by using the differential absorption lidar technique, there spatial concentrations along the same long fiber are measured by time-of-flight or phase shift techniques. Numerous gases can be monitored in connection with industrial, agricultural, mining operations, etc.

According to a first aspect of the present invention a gas permeable optical fiber is provided comprising a solid core to guide light, and a permeable polymer cladding which can also increase the mechanical strength of the fiber and protect the fiber from physical damage. The refractive index of the cladding material is somewhat smaller than that of the core material (usually at the magnitude of 10^-2) at room temperature and small part of the light propagating along the fiber is distributed at the cladding area. The polymer cladding is permeable to surrounding gas and the interaction between the gas and the guided light of the fiber becomes possible.

According to a second aspect of the present invention an optical fiber gas sensing system is provided comprising a continuous wave (CW) tunable laser, a permeable optical fiber and a signal detecting module. The wavelength tunable range of the tunable laser should include the absorption wavelength of the tested gas. The wavelength of the laser is modulated using a triangular wave as a non-limiting example of the application, so the wavelength of the laser comes cross twice of the absorption line of the tested gas. The permeable optical fiber is surrounded by the tested gas and its guided light is absorbed twice per one modulation period by Beer-Lambert law. The signal detecting module includes an optical power meter and a spectrum analyzer as a non-limiting example. At the frequency domain, the gas concentration can be readout by mature technology to date. By changing the laser wavelength to other gases absorption band, the presented system is capable for variety gases monitoring.

According to a third aspect of the present invention an optical fiber multipoint sensing system is provided comparing a tunable laser, N pieces of gas permeable fibers, a signal detecting module, an optical circulator, an optical switch and optical fiber loop mirrors. The N pieces of gas permeable fibers are connected parallelly by an optical switch. They can be used as sensitive sensors at N different locations to locally measure the concentration of surrounding gas. The length of each piece of the permeable fibers, which might be coiled up in a compact arrangement is chosen to achieve the sensitivity required. The testing light can be reflected by the optical fiber loop mirrors as a non-limiting example, and returns back to the signal detecting module through the optical switch and the optical circulator. The sensor fibers are coupled to a central unit by conventional optical fibers which may be connected to a single laser source and a single detector by conventional telecom switch, to sequentially measure the gas concentration at the N locations.

According to a forth aspect of the present invention an optical fiber distributed sensing system is provided comparing a pulsed or modulated tunable laser, a long gas permeable fiber (usually hundreds meters to kilometers) , and the signal detecting module. The permeable fiber is long and its wiring scheme is decided by the sensing topology to measure the concentration of the variety gases at different position. The signal detecting module is provided comprising an optical detector, e.g. a photomultiplier tube (PMT) and a high speed data acquisition card which is similar with the optical fiber multipoint sensing system. The optical detector can convert weak back-reflected light from the fiber, generated in Rayleigh scattering, to an electrical signal. The pulse width of the sensing laser is at the magnitude of tens of nanoseconds to have a high spatial resolution. To maintain that spatial resolution, the response time of the detector should be matched accordingly. A high speed data acquisition card to record the amplitude of the reflected light detected is further provided. The acquiring speed should be fast enough to achieve the spatial resolution required.

According to a fifth aspect of the present invention an optical fiber mode stripper is provided comparing a short (at the magnitude of centimeters) permeable optical fiber with micro bending structure. The mentioned micro bending structure can be the fiber ring with small diameter (usually smaller than 5 mm) as a non-limiting example of the application of the present invention. According to the silica fiber drawing technology up to date, the diameter of the silica core of the presented fiber can not be very small, and it would be a multimode fiber. In the permeable multimode fibers, the optical power distribution at cladding area is different between different orders of modes, which means the interface area of the guided light and the tested gas is different between different orders of modes, thus the sensitivity to gas of the different modes are different. Once the mode coupling happens the sensitivity would change which is unacceptable for sensing. Fortunately the mode coupling coefficient between lower order modes is much smaller than that between the high order modes. The mode striper which is applied to remove the high order modes of the permeable fiber overcomes the problem of the sensitivity changing along the fiber.

According to a sixth aspect of the present invention a spectroscopic light source is provided comprising a pulsed or modulated tunable laser. Probing laser light is launched into the fiber. The operation wavelength of the laser should be tuned to match the spectral region of the absorption band of the tested gas and means to quickly change the output between alternating different wavelengths are provided, employing electronics for a flexible operation of the laser, related to laser output characteristics.

According to a seventh aspect of the present invention, a differential absorption algorithm based on by Beer-Lambert law is provided comprising the signal detecting module. Signal intensities are recorded separately for absorbed wavelengths and non-absorbed wavelengths of the laser transmitter. The two operation modes when the laser is turned to absorbed wavelengths and non-absorbed wavelengths are defined as “on” and “off” modes, respectively. By dividing signals from an on-wavelength by the signal from an off-wavelength, intensity variations due to e.g. different range intervals are eliminated and only the influence of gas absorption as a function of time/range is obtained. From this data the range-resolved information on gas concentration can be obtained after calibration for the specific gas (es) to be probed.

Preferably, the core material of the sensing fiber is silica, in order to lower down the optical losses. The permeable cladding material of the sensing fiber is a silicone material, since firstly the silicone is not fragile and can enhance the mechanical strength effectively； and secondly the gas permeability of silicone is much better than for most polymers； and thirdly the loss of the silicone at visible and infrared wavelengths is acceptable allowing the maximal length of the sensing fiber to be beyond 1 km； and fourthly, the silicone cladding offers protection to water and dust.

Preferably, the pulse width of the pulsed laser in distributed gas system is ten nanoseconds, the response time of the optical detector is less than ten nanoseconds and the data acquiring rate of the data acquisition card is more than 100 MHz, as to achieve the required spatial resolution of range.

Preferably, the micro bending diameter of the mode striper is 5 mm, to erase the high order modes efficiently.

It should be noted

that as an extension to the conventional two-wavelength differential absorption lidar concept, selected above as a non-limiting example of the application of the present invention, a multi-wavelength differential absorption lidar concept (Reference [10-12] ) can be implemented also for the fiber lidar case. Then the laser source is switched between multiple wavelengths, for which absorption of different gases occur, as well as wavelengths, where there is no gas absorption. Using extended dial evaluation routines, the range-resolved concentration of multiple gases can be evaluated, as illustrated for the case of free atmosphere propagation in Ref. (Reference [12] ) .

We also note, that the gas correlation detection principle, earlier demonstrated for the open atmosphere lidar case (Reference [13] ) and the CW non-fiber gas monitoring case employing multi-mode lasers (Reference [14, 15] ) as well as light emitting diodes (Reference [16] ) can be extended to the fiber laser case, as now being disclosed. The output of the light source, as filtered through a gas-correlation cell containing an optically thick concentration of the gas of interest, is compared with the light intensity which passed the gas of interest, either in a lidar fashion, or in a long-path absorption implementation, to reveal the presence of the gas in the probed gas volume.

We also note that the loss of the permeable polymer which made of the cladding is higher than the material of the core. The more light distributed at cladding area would lead to higher loss of the mode and also shorter transmission distance. While the interference of the light and the tested gas happens only in the cladding area, so the more light distributed in the cladding, the higher sensitivity it is. That is the tradeoff between the sensing range and the sensitivity. By controlling the mode stripper the loss and the sensitivity can be adjusted and are expected to be moderate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in detail, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

Figure 1 shows the cross section structure of the permeable optical fiber according to the first embodiment of the invention.

Figure 2 shows the invented permeable optical fiber used for gas sensor according to a second embodiment of the invention.

Figure 3 shows the invented permeable optical fiber used for multipoint gas sensor according to a third embodiment of the invention.

Figure 4 shows the invented permeable optical fiber used for distributed gas sensor according to a forth embodiment of the invention.

Figure 5 shows the optical power profile of the fundamental fiber mode over the permeable optical fiber cross section according to a fifth embodiment of the invention.

Figure 6 shows the optical power profile of a higher-order fiber mode over the permeable optical fiber cross section according to a fifth embodiment of the invention.

Figure 7 shows mode coupling coefficient of different modes in permeable optical fiber according to a fifth embodiment of the invention.

Figure 8 shows the optical fiber mode stripper which is used to erase the high order fiber modes in the permeable optical fiber according to a fifth embodiment of the invention.

Figure 9 shows the OTDR (optical time domain reflectometer) tested loss of a typical permeable optical fiber at 1550 nm according to a sixth embodiment of the invention.

Figure 10 shows the absorption spectrum of the tested gas and the operation wavelength of the tunable pulsed laser at the on-and off-wavelengths of the system according to a seventh embodiment of the invention.

Figure 11 shows the sequence of the controlling signals and received signals, and the differential absorption algorithm for the distributed optical fiber gas sensor according to a seventh embodiment of the invention.

DETAILED DESCRIPTION

Referring to Figure 1, a first embodiment of the invention provides a gas permeable optical fiber (100) comprising a solid core (101) to guide light, and a permeable polymer cladding (102) which can also increase the mechanical strength of the fiber and protect the fiber from physical damage. The refractive index the cladding material is somewhat smaller (usually at the magnitude of 10^-2) than that of the core material and small part of the light propagating along the fiber is distributed at the cladding area. The polymer cladding (102) is permeable to surrounding gas and the interaction between the gas and the guided light of the fiber becomes possible.

Referring to Figure 2, a second embodiment of the invention provides an optical fiber gas sensing system (200) is provided comparing a continuous wave (CW) tunable laser (201) , a permeable optical fiber (202) and a signal detecting module (204) . The wavelength tunable range of the tunable laser (201) should include the absorption wavelength of the tested gas (203) . The wavelength of the laser (201) is modulated using a triangular wave as a non-limiting example of the application, so the wavelength of the laser comes cross twice of the absorption line of the tested gas (203) . The permeable optical fiber (202) is surrounded by the tested gas (203) and its guided light can be absorbed twice per one modulation period by the Beer-Lambert law. The signal detecting module (204) includes an optical power meter and a spectrum analyzer as a non-limiting example. At the frequency domain, the gas concentration can be readout by mature technology. By changing the laser wavelength to other gas absorption band or by changing another laser, the presented system is capable for variety gases monitoring.

Referring to Figure 3, a third embodiment of the invention provides an optical fiber multipoint gas sensing system (300) is provided comparing a tunable laser (301) , N pieces of gas permeable fibers (304) , a signal detecting module (306) , an optical circulator (302) , an optical switch (303) and optical fiber loop mirrors (305) . The N pieces of gas permeable fibers (304) are connected parallelly by an optical switch (303) . They can be used as sensitive sensors at N different locations to locally measure the concentration of surrounding gas (307) . The length of each piece of the permeable fibers (304) , which might be coiled up in a compact arrangement is chosen to achieve the sensitivity required. The testing light can be reflected by the optical fiber loop mirrors (305) as a non-limiting example, and returns back to the signal detecting module (306) through the optical switch (303) and the optical circulator (302) . The sensor fibers (304) are coupled to a central unit by conventional optical fibers which may be connected to a single laser source (301) and a single detector (306) by conventional telecom switch (303) , to sequentially measure the gas concentration at the N locations.

Referring to Figure 4, a forth embodiment of the invention provides an optical fiber distributed gas sensing system (400) is provided comparing a pulsed or modulated tunable laser (401) , a long gas permeable fiber (403) , a signal detecting module (405) and a 3-dB optical coupler (402) . The permeable fiber (403) is long and its wiring scheme is decided by the sensing topology to measure the concentration of the variety gases (404) at different position. The signal detecting module (405) is provided comprising an optical detector, e.g. a photomultiplier tube (PMT) and a high speed data acquisition card which is similar with the optical fiber multipoint sensing system. The optical detector can convert weak back-reflected light from the fiber, generated in Rayleigh scattering, to an electrical signal. The pulse width of the sensing laser (401) is at the magnitude of tens of nanoseconds to have a high spatial resolution. To maintain that spatial resolution, the response time of the detector should be matched accordingly. A high speed data acquisition card to record the amplitude of the reflected light detected is further provided. The acquiring speed should be fast enough to achieve the spatial resolution required.

Figure 5 shows the optical power distribution of the fundamental mode over a fiber cross section (500) according to a fifth embodiment of the invention. Most of the guided light is distributed in the fiber core (502) , and part of the guided light is distributed in the cladding (501) through the evanescent field and interacts with the gas. Likewise, Figure 6 shows the optical power distribution of the higher-order modes over a fiber cross section (600) . Again, most of the guided light is distributed in the fiber core (602) and a little bit more part of the guided light is distributed in the cladding (601) compared with the light distribution of the fundamental mode (500) .

Figure 7 shows mode coupling coefficient of different modes in permeable optical fiber according to a fifth embodiment of the invention. The core diameter is difficult to be made very small, so the permeable optical fiber is usually a multimode fiber. The problem of the long transmission light in a multimode fiber is the mode coupling. Figure 7 shows the bigger the propagation constant difference is the smaller the mode coupling coefficient would be (Reference [17] ) . In the circularly symmetric multimode optical fiber the propagation constant differences between the first few modes are bigger than the differences between higher order modes.

Referring to Figure 8, a fifth embodiment of the invention provides an optical fiber mode stripper (700) is provided comparing a short permeable optical fiber (701) with micro bending structure (702) . The mentioned micro bending structure (702) can be the fiber ring with small diameter (usually less than 5 mm) as a non-limiting example of the application of the present invention. According to the silica fiber drawing technology up to date, the diameter of the silica core of the presented fiber can not be very small, and it would be a multimode fiber. In the permeable multimode fibers (100) , the optical power distribution at cladding area (102) is different between different orders of modes, which means the interface area of the guided light and the tested gas is different between different orders of modes, thus the sensitivity to gas of the different modes are different. Once the mode coupling happens the sensitivity would change which is unacceptable for sensing. Fortunately the mode coupling coefficient between lower order modes is much smaller than that between the high order modes (Reference [17] ) . The mode striper which is applied to remove the high order modes of the permeable fiber (701) overcomes the problem of the sensitivity changing along the fiber.

Figure 9 shows a tested results of a fiber designed to fit the present invention needs according to a sixth embodiment of the invention. A standard optical time domain reflectometer (OTDR) is employed, and a useful optical attenuation of 10 dB/km is demonstrated.

Figure 10 shows the absorption spectrum of the tested gas and the operational wavelength of the tunable laser at the on-and off wavelengths according to a seventh embodiment of the invention. At the on mode the operation wavelength of the laser should be tuned exactly at the peak wavelength of the absorption band of the tested gas, and at the off mode, the operation wavelength of the laser should be tuned off, but not too far away from the absorption band of the tested gas.

Figure 11 shows the full intended operation of the distributed gas sensor using the invented permeable optical fiber according to a seventh embodiment of the invention. The gas probing fiber is extending into the environment, where at some particular locations gas to be probed are present. The time-resolved fiber lidar return signals are shown for the affected on-resonance wavelength and the non-affected off-resonance wavelength. After division of the two curves the differential absorption curve is obtained, where the influence of non-spectroscopic signal attenuation is eliminated by forming a dimension-less quantity (Reference [7] ) . The curve is horizontal, except at locations where gas present as illustrated in the figure. From the slope, the concentration of the gas surrounding the fiber is calculated (Reference [7] ) , with higher concentrations the larger the slope of the curve is, basically reflecting the Beer-Lambert law (Reference [3] ) . Calibration is provided by surrounding the fiber with gas of known concentration, whereby all influences of spectroscopic absorption cross sections, as well as of the fraction of the total light which was able to interact with the gas through the evanescent field are included.

References

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Ed., Springer Handbook of Lasers and Optics, 2nd Edition (Springer, Heidelberg 2012) , pp 1146

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Claims

An gas permeable optical fiber for distributed gas sensing， characterized by comprising

a core being made of silica，

a cladding being made of silicone to achieve permeability for gas to reach the core area wherein interaction of the gas with sensing light occurs.
A fiber according to Claim 1， wherein optical radiation is transmitted through the fiber to interact with the surrounding gas at the cladding area.
An gas permeable optical fiber sensing system including a tunable laser， a gas permeable optical fiber for distributed gas sensing and a signal detection module； wavelength of the tunable laser is intermittently switched between absorbed and non-absorbed wavelengths of one or several types of gas of interest； the permeable optical fiber is surrounded by the tested gas， and by laser radiation conduction， guided light is absorbed so as to realize gas selective sensing； the signal detecting module includes an optical power meter and a spectrum analyzer.
An optical fiber multipoint sensing system including wavelength tunable lasers， N pieces of gas permeable fibers and a signal detection module， wherein N pieces of gas permeable fibers are utilized to measure gas concentrations at N different locations； and length of the fiber， which might be coiled up in a compact arrangement， is chosen to achieve required sensitivity； and the gas permeable fibers are coupled to a central unit by conventional optical fibers which may be connected to a single laser source and a single detector by conventional telecom switches， so as to sequentially measure gas concentration at the N locations； fiber or multiple tunable lasers and multiple pieces of gas permeable fibers might also be fiber-optically coupled for measuring one or several different types of gas.
An gas permeable optical fiber sensing system according to any of claims 3-4， wherein spatially resolved gas concentrations along the length of the fiber are acquired with laser radar method， which utilizes temporal analysis of the light returning to the signal detection module to gain spatial resolution of gas concentrations along the fiber length via evanescent optical field.
An gas permeable optical fiber sensing system according to Claim 5， wherein differential absorption lidar (dial) technique is being used to achieve gas specificity with range resolution.
An gas permeable optical fiber sensing system according to any of claims 3-5， wherein an optical fiber mode stripper is used to erase the high order modes in the permeable optical fiber， and the optical fiber mode stripper is inserted between the laser and the fiber， thus the mode crosstalk between the remaining modes is very small and acceptable for the gas sensing application.
An gas permeable optical fiber sensing system according to any of the claims 3-7， wherein spectral resolution and gas identification is achieved using gas correlation method， and transmission of a broader light distribution from an appropriate source， and absorption bands cover of the gas (es) of interest， is observed through optically thick absorption cells containing the gas(es) of interest， and acquired intensity is compared with the one observed from the fiber，

and/or

time resolution is achieved using phase-shift method， where continuous light sources are modulated at a plurality of frequencies and phase-shifts and demodulations are observed at the detector.
A method for gas monitoring using an optical fiber according to any of Claims 1-2.
A method according to Claim 9， where the gases to be monitored can be found among O₂， O₃， H₂O， H₂S， CO， CO₂， Hg， SO₂， NO， NO₂， CH₄， C₂H₆， and C₃H₈ as non-limiting examples of the application.
A method according to Claim 9 or 10， where the gas monitoring relates to mines， in particular coal mines； drilling holes， including those for petroleum and natural gas extraction， geophysical gas monitoring， including mineral exploration and earth quake prediction， industries， tunnels， buildings or urban areas.