Distributed fiber optic sensing device

Abstract

The present invention provides a distributed optical fiber sensing device, which comprises a looped optical fiber, an optical fiber circulator and a detector; a pulsed laser emitted by a first light source enters the looped optical fiber through the optical fiber circulator, and backscattered light of the pulsed laser is received by the detector; a continuous laser emitted by a second light source enters the looped optical fiber, and the continuous laser is received by the detector through the optical fiber circulator; the pulsed laser transmitted forward in the looped optical fiber is blocked at a blocker, and the continuous laser transmitted backward in the looped optical fiber passes through the blocker. The present invention has the advantages of long effective detection distance, short detection time, etc.

Description

Technical Field

[0001] The present invention relates to temperature and strain detection, and in particular to a device for sensing external temperature and / or strain using optical fiber. Background Art

[0002] Distributed fiber optic sensing technology has the advantages of continuous distributed detection, long detection distance, accurate positioning, rich measurement information, inherent safety, and low cost. It has been widely used in the fields of electricity, petroleum, bridges, tunnels, slopes, etc.

[0003] Among various fiber optic sensing technologies, the distributed fiber optic sensing device based on the Brillouin scattering effect is a new type of sensing device. It directly uses the optical fiber as a sensing element, combining "sensing" and "sensing" into one, and can sense the temperature and / or strain along the optical fiber. The distributed fiber optic sensing device includes a light source, an optical fiber circulator, an optical fiber, and a detector. The laser emitted by the light source is coupled into the optical fiber after passing through the optical fiber circulator. Various scattering effects will be generated during the transmission of the laser in the optical fiber. The frequency shift of the back Brillouin scattering is related to the temperature and / or strain along the optical fiber. The backscattered light in the optical fiber is detected by the detector after passing through the optical fiber circulator, thereby obtaining the temperature, strain distribution and position information along the optical fiber.

[0004] The effective detection distance is one of the core indicators of distributed fiber optic sensing technology. As the effective detection distance increases, due to the transmission loss of the optical fiber, the scattered signal at the end of the optical fiber becomes smaller and even fails to meet the detection requirements, which limits the improvement of the effective detection distance. In order to increase the effective detection distance, the potential solutions are:

[0005] 1. Increase the power entering the optical fiber; increasing the power entering the optical fiber will lead to the emergence of nonlinear effects, which will affect the measurement.

[0006] In order to prevent the occurrence of nonlinear effects, the power entering the optical fiber cannot be too large; and the longer the distance, the lower the power allowed to enter the optical fiber, which will further shorten the effective detection distance.

[0007] 2. Improve the performance of the detector to achieve a higher signal-to-noise ratio; Improving the performance of the detector is difficult and costly, so this solution is generally not considered.

[0008] 3. Increase the pulse width of the pulse laser; increasing the pulse width of the pulse laser will sacrifice the spatial resolution, that is, the width of the pulse laser cannot be increased blindly.

[0009] 4. Increase the number of cumulative averages. Obviously, increasing the number of cumulative averages will increase the measurement time, which means that the number of cumulative averages cannot be increased blindly due to the measurement time limit.

[0010] At present, the longest effective detection distance of distributed fiber optic sensing devices based on the Brillouin scattering effect is only 75 kilometers, which cannot fully meet the application needs in the fields of electricity, petroleum, etc. Summary of the invention

[0011] In order to solve the deficiencies in the above-mentioned prior art solutions, the present invention provides a distributed optical fiber sensing device with a long effective detection distance and short detection time.

[0012] The objective of the present invention is achieved through the following technical solutions:

[0013] A distributed optical fiber sensing device, comprising a looped optical fiber, an optical fiber circulator and a detector, wherein the distributed optical fiber sensing device further comprises:

[0014] A first light source, wherein the pulsed laser emitted by the first light source enters the looped optical fiber after passing through the optical fiber circulator;

[0015] A second light source, wherein a continuous laser beam emitted by the second light source enters the looped optical fiber;

[0016] A blocker is provided at which the pulsed laser transmitted forward in the looped optical fiber is blocked, and the continuous laser transmitted backward in the looped optical fiber passes through the blocker and is received by the detector via the optical fiber circulator.

[0017] In this solution, the pulse laser emitted by the first light source is transmitted forward in the loop optical fiber and is blocked at the blocker. At this time, the transmission distance of the pulse laser emitted by the first light source is less than the total length of the loop optical fiber. The smaller the transmission distance of the pulse laser, the higher the power allowed to enter the loop optical fiber. At this time, the scattered signal of the loop optical fiber will be stronger, which can improve the effective detection distance; on the other hand, the smaller the transmission distance of the pulse laser, the greater the repetition frequency of the pulse laser. At this time, more times of accumulation and averaging can be accumulated within the same measurement time, further improving the effective detection distance.

[0018] Furthermore, the first light source, the second light source, the optical fiber circulator and the detector are all located at the detection end, the looped optical fiber is located in the sensing area, and the blocker is located on the optical path of the looped optical fiber.

[0019] Preferably, the distributed optical fiber sensing device further comprises:

[0020] An optical amplifier is located on the optical path upstream or downstream of the blocker, and the power of the continuous laser light transmitted in the loop optical fiber in the reverse direction is increased by the optical amplifier.

[0021] The optical amplifier is an erbium-doped fiber amplifier EDFA or a semiconductor optical amplifier SOA.

[0022] Preferably, the blocker is built into the optical amplifier to facilitate on-site deployment.

[0023] The blocker is a unidirectional double-stage optical fiber isolator or an optical fiber circulator.

[0024] The optical frequency difference between the first light source and the second light source is 8-14 GHz, and the optical frequency of one light source is fixed, while the optical frequency of the other light source changes periodically and gradually, thereby covering the Brillouin spectrum of the looped optical fiber.

[0025] Furthermore, the present solution also provides a low-cost multi-channel distributed optical fiber sensing device, which includes a looped optical fiber, an optical fiber circulator, a detector, a first light source, a second light source, a blocker and an optical amplifier. The distributed optical fiber sensing device also includes: a first optical switch, a second optical switch, a first optical fiber coupler, and a second optical fiber coupler; the first light source enters the plurality of looped optical fibers after passing through the optical fiber circulator and the first optical switch in sequence; the second light source enters the plurality of looped optical fibers after passing through the second optical switch; the plurality of looped optical fibers are respectively combined by the first optical fiber coupler and enter the blocker and the optical amplifier, and are connected to the plurality of looped optical fibers after being split by the second optical fiber coupler.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. Long effective detection distance;

[0028] A creative proposal was made to add a blocker in the middle of the loop optical fiber to block the pulse laser emitted by the first light source. The blocker prevents the further transmission of the pulse laser in the loop optical fiber, that is, shortens the transmission distance of the pulse laser, which means that the power of the pulse laser can be increased and the repetition frequency of the pulse laser can be increased. The former makes the scattering signal of the loop optical fiber stronger, and the latter makes the number of accumulated averages more within the same measurement time. Both can effectively improve the effective detection distance.

[0029] Experiments show that with the same light source, optical fiber and detector, the effective detection distance can be increased to more than 120 kilometers by setting up a blocker, which significantly improves the effective detection distance.

[0030] 2. Shorter measurement time;

[0031] The transmission distance of the pulse laser is shortened, the repetition frequency of the pulse laser is higher, and the measurement time required for the same number of cumulative averages is shorter.

[0032] 3. Lower cost;

[0033] The cost of the multi-channel distributed optical fiber sensing device is reduced by combining / splitting the first optical fiber coupler and the second optical fiber coupler, multiplexing the blocker and the optical amplifier. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The disclosure of the present invention will become easier to understand with reference to the accompanying drawings. It is easy for those skilled in the art to understand that these drawings are only used to illustrate the technical solution of the present invention and are not intended to limit the scope of protection of the present invention. In the drawings:

[0035] Figure 1 is a simplified structural diagram of a distributed optical fiber sensing device according to Embodiment 1 of the present invention;

[0036] Figure 2 is a simplified structural diagram of a distributed optical fiber sensing device according to Embodiment 2 of the present invention;

[0037] Figure 3 4 is a simplified structural diagram of a distributed optical fiber sensing device according to Example 4 of the present invention. DETAILED DESCRIPTION

[0038] Figure 1-3 The following description describes the optional embodiments of the present invention to teach those skilled in the art how to implement and reproduce the present invention. In order to teach the technical solution of the present invention, some conventional aspects have been simplified or omitted. It should be understood by those skilled in the art that variations or substitutions derived from these embodiments will be within the scope of the present invention. It should be understood by those skilled in the art that the following features can be combined in various ways to form multiple variations of the present invention. Thus, the present invention is not limited to the following optional embodiments, but is only limited by the claims and their equivalents.

[0039] Embodiment 1:

[0040] Figure 1 A schematic diagram of the structure of a distributed optical fiber sensing device according to an embodiment of the present invention is shown as follows: Figure 1 As shown, the distributed optical fiber sensing device comprises:

[0041] The loop optical fiber 41 is, for example, an optical cable not less than a double-core optical fiber, the length of the optical cable is 100 kilometers, and the double-core optical fibers are fused together at the far end of the optical cable to form the loop optical fiber 41. At this time, the total length of the loop optical fiber 41 is 200 kilometers;

[0042] The optical fiber circulator 21 is a three-port optical fiber circulator, wherein the incident laser enters from the first port and reaches the second port, and the backscattered light enters from the second port and reaches the third port;

[0043] The detector 31, such as an InGaAs photodetector, is used to detect the backscattered light of the looped optical fiber 41 and convert the backscattered light into an electrical signal for collection and analysis;

[0044] The above-mentioned looped optical fiber, optical fiber circulator and detector are all prior art in this field, and the specific structure and working method are not described in detail here;

[0045] The first light source 11 is, for example, an externally modulated narrow-linewidth semiconductor laser with a central wavelength of 1550.12 nm. The pulsed laser emitted by the first light source 11 enters the looped optical fiber 41 after passing through the optical fiber circulator 21.

[0046] A second light source 12, such as a narrow linewidth semiconductor laser, with a central wavelength of 1550.04 nm, the continuous laser emitted by the second light source 12 enters the loop optical fiber 41;

[0047] The optical frequency of the first light source 11 is fixed, and the optical frequency of the second light source 12 is periodically changed by periodically changing the driving current, so that the optical frequency difference between the first light source 11 and the second light source 12 is 8 to 14 GHz, covering the Brillouin spectrum of the looped optical fiber;

[0048] The backscattered light of the pulsed laser interacts with the continuous laser and is received by the detector after passing through the optical fiber circulator;

[0049] The blocker 51, such as a unidirectional double-stage optical fiber isolator, has an operating wavelength of 1550±20nm, unidirectional transmission, and an isolation greater than 35dB. The pulsed laser transmitted forward in the loop-back optical fiber 41 is blocked at the blocker 51, and the continuous laser transmitted backward in the loop-back optical fiber 41 passes through the blocker 51 almost without loss.

[0050] To facilitate installation and maintenance, further, the first light source 11, the second light source 12 and the detector 31 are all at the detection end (the near end of the optical cable), the loop optical fiber 41 (optical cable) is laid in the sensing area, and the blocker 51 is in the optical path of the loop optical fiber 41 (the far end of the optical cable).

[0051] In this embodiment, the pulse laser emitted by the first light source is transmitted forward in the loop optical fiber and is blocked at the blocker. At this time, the transmission distance of the pulse laser emitted by the first light source is reduced to 100 kilometers, which is less than the total length of the loop optical fiber of 200 kilometers. To prevent the occurrence of the stimulated effect, the power of the pulse laser incident on the loop optical fiber shall not exceed the stimulated power threshold. Since the stimulated power threshold gradually decreases with the transmission distance, a blocker is added in the middle of the loop optical fiber, and the transmission distance of the pulse laser is reduced from the previous 200 kilometers to 100 kilometers. At this time, the power allowed to enter the loop optical fiber can be increased by about 3dB, so the scattering signal of the loop optical fiber will be stronger, and the effective detection distance can be increased by about 15 kilometers accordingly; on the other hand, the smaller the transmission distance of the pulse laser, the more the repetition frequency of the pulse laser can be doubled. At this time, under the same cumulative average number of conditions, the measurement time can be shortened by half; or the number of cumulative averages can be doubled in the same measurement time, further improving the effective detection distance (about 7 kilometers).

[0052] Embodiment 2:

[0053] The distributed optical fiber sensing device according to the embodiment of the present invention is different from the embodiment 1 in that:

[0054] like Figure 2 As shown, the distributed optical fiber sensing device further includes an optical amplifier 61 , which is located on the optical path upstream or downstream of the blocker 51 . The power of the continuous laser transmitted in the reverse direction in the loop optical fiber 41 is increased by the optical amplifier 61 .

[0055] Since the optical amplifier is in the middle of the loop optical fiber and its gain is relatively large, the power of the continuous laser emitted by the second light source and entering the loop optical fiber can be greatly reduced. When the continuous laser transmitted in the loop optical fiber reaches the optical amplifier, the continuous laser is relayed and amplified at the optical amplifier, compensating for the transmission loss of the continuous laser in the loop optical fiber.

[0056] Compared with Example 1, due to the addition of an optical amplifier, the power of the continuous laser emitted by the second light source is greatly reduced (far below the stimulated power threshold), and it only needs to meet the condition that the power after amplification by the optical amplifier does not exceed the stimulated power threshold. This means that the power of the continuous laser interacting with the pulsed laser is greatly increased, thereby achieving a significant increase in the intensity of the backscattered signal.

[0057] The optical amplifier can be an erbium-doped fiber amplifier EDFA or a semiconductor optical amplifier SOA, and the gain of the optical amplifier for small signals is not less than 10dB. In this embodiment, the optical amplifier uses a semiconductor optical amplifier SOA, the operating wavelength is 1550±20nm, and the gain for small signals is 20dB. The power of the continuous laser emitted by the second light source is 1mW. After being transmitted through an optical cable with a length of 100 kilometers, the power of the continuous laser is attenuated to about 10μW. The continuous laser is restored to about 1mW after being amplified by the semiconductor optical amplifier SOA. At this time, the intensity of the backscattered signal is greatly increased, and the temperature and strain measurement of the entire optical cable of not less than 100 kilometers can be achieved.

[0058] Embodiment 3:

[0059] The distributed optical fiber sensing device according to the embodiment of the present invention is different from that in Embodiment 2 in that:

[0060] The blocker is built in the optical amplifier and is located behind the output end of the optical amplifier, that is, the optical amplifier is a unidirectional optical amplifier, which is convenient for on-site deployment.

[0061] In this embodiment, the optical amplifier uses an erbium-doped fiber amplifier EDFA, with an operating wavelength of 1550±20nm and a gain of 15dB for small signals. The blocker uses a unidirectional two-stage fiber isolator, with an operating wavelength of 1550±20nm, unidirectional transmission, and an isolation greater than 35dB. The unidirectional two-stage fiber isolator is directly behind the output end of the erbium-doped fiber amplifier EDFA, realizing unidirectional small signal amplification, an integrated structure, high integration, and convenient on-site deployment.

[0062] Embodiment 4:

[0063] The distributed optical fiber sensing device according to the embodiment of the present invention is different from that in Embodiment 2 in that:

[0064] like Figure 3 As shown, the distributed optical fiber sensing device also includes:

[0065] The first optical switch 71, the second optical switch 72, the first fiber coupler 81 and the second fiber coupler 82, the first light source 11 enters the plurality of loop optical fibers 41 after passing through the fiber circulator 21 and the first optical switch 71 in sequence; the plurality of loop optical fibers 41 are respectively combined by the first fiber coupler 81 and enter the blocker and the optical amplifier, and are connected to the plurality of loop optical fibers after being split by the second fiber coupler 82; the second light source 12 enters the plurality of loop optical fibers 41 after passing through the second optical switch 72.

[0066] Embodiment 5:

[0067] An example of application of the distributed optical fiber sensing device according to embodiment 4 of the present invention in submarine cable monitoring.

[0068] In this application example, the submarine cable has a distance of 70 km and has three phases, A, B, and C. Each phase of the submarine cable has a single-mode optical cable built into it;

[0069] The first light source uses an externally modulated narrow-linewidth semiconductor laser with a central wavelength of 1550.12nm, a pulse width of 10ns, and a peak power of 300mW;

[0070] The second light source uses a narrow linewidth semiconductor laser with a central wavelength of 1550.04nm and a continuous power of 4mW;

[0071] The blocker uses a unidirectional double-stage optical fiber isolator with an operating wavelength of 1550±20nm, unidirectional transmission, and an isolation greater than 35dB;

[0072] The optical amplifier uses erbium-doped fiber amplifier EDFA with a gain of 26dB;

[0073] The first optical switch and the second optical switch are both 1×4 single-mode micromechanical optical switches;

[0074] The first optical fiber coupler and the second optical fiber coupler are both 1×4 fused-taper optical fiber couplers with a splitting ratio of 25:25:25:25.

[0075] The pulse laser emitted by the first light source passes through the three-port optical fiber circulator and the first optical switch in turn and then enters the loop optical fiber of the A, B, and C phase submarine cables (there is an optical switch channel as a backup); the continuous laser emitted by the second light source passes through the second optical switch and then enters the loop optical fiber of the A, B, and C phase submarine cables. The loop optical fibers of the A, B, and C phase submarine cables are respectively combined by the first optical fiber coupler and then enter the blocker and optical amplifier. They are then separated by the second optical fiber coupler and then connected to the loop optical fibers of the A, B, and C phase submarine cables in turn.

[0076] Due to the high cost of optical amplifiers and blockers, and the high gain of optical amplifiers for small signals, the continuous laser beam is combined through the second optical fiber coupler and enters the input end of the optical amplifier. The output end of the optical amplifier outputs the amplified continuous laser beam, which is then split through the first optical fiber coupler after passing through the blocker and enters the loop optical fiber of the A, B, and C phase submarine cables.

[0077] The cost of the multi-channel distributed optical fiber sensing device is reduced by combining / splitting the first optical fiber coupler and the second optical fiber coupler, multiplexing the blocker and the optical amplifier;

[0078] In this embodiment, after the optical amplifier and the blocker are set at the end of the submarine cable, the repetition frequency of the pulse laser of the first light source is increased from 0.7kHz to 1.4kHz, and the power of the continuous laser of the second light source after reaching the end of the submarine cable is increased from 16μW to 400uW. Finally, under the premise of an effective detection distance of 70 kilometers, the measurement time is reduced from 120s / channel to 60s / channel, and the spatial resolution is increased from 5 meters to less than 2 meters. And by multiplexing the blocker and the optical amplifier, the cost of the four-channel distributed optical fiber sensing device is not much increased, and the beneficial effect is significant.

Claims

1. Distributed optical fiber sensing device, including looped optical fiber, optical fiber circulator and detector; Features: The distributed optical fiber sensing device also includes: A first light source, wherein the pulsed laser emitted by the first light source enters the looped optical fiber after passing through the optical fiber circulator; A second light source, wherein a continuous laser beam emitted by the second light source enters the looped optical fiber; A blocker is arranged in the middle of the loop optical fiber. The pulse laser transmitted forward in the loop optical fiber is blocked at the blocker, and the continuous laser transmitted backward in the loop optical fiber passes through the blocker and is received by the detector via the optical fiber circulator.

2. The distributed optical fiber sensing device according to claim 1, Features: The first light source, the second light source, the optical fiber circulator and the detector are all located at the detection end, the looped optical fiber is located in the sensing area, and the blocker is located on the optical path of the looped optical fiber.

3. The distributed optical fiber sensing device according to claim 1, Features: The distributed optical fiber sensing device further comprises: an optical amplifier, which is located on the optical path upstream or downstream of the blocker.

4. The distributed optical fiber sensing device according to claim 3, Features: The optical amplifier is an erbium-doped fiber amplifier EDFA or a semiconductor optical amplifier SOA.

5. The distributed optical fiber sensing device according to claim 3, Features: The blocker is built in the optical amplifier.

6. The distributed optical fiber sensing device according to claim 1, Features: The blocker is a unidirectional double-stage optical fiber isolator or an optical fiber circulator.

7. The distributed optical fiber sensing device according to claim 1, Features: The optical frequency difference between the first light source and the second light source is 8-14 GHz.

8. The distributed optical fiber sensing device according to claim 3, Features: The distributed optical fiber sensing device also includes: A first optical switch, a second optical switch, a first fiber coupler and a second fiber coupler, the first light source enters the plurality of looped optical fibers after passing through the fiber circulator and the first optical switch in sequence; the second light source enters the plurality of looped optical fibers after passing through the second optical switch; the plurality of looped optical fibers are respectively combined by the first fiber coupler and enter the blocker and the optical amplifier in sequence, and are connected to the plurality of looped optical fibers after being split by the second fiber coupler.

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