A weak fiber Bragg grating temperature-sensitive optical cable

Abstract

A weak fiber grating temperature-sensitive optical cable comprises a weak fiber grating array and a guide wire fixed to the weak fiber grating array in sections, wherein the weak fiber grating array and the guide wire are sleeved in a plastic sleeve, and the plastic sleeve is coated with a braided layer. The weak fiber grating array is engraved with a plurality of identical or wavelength division multiplexed fiber grating arrays, and the coating layer of the fiber grating array is a modified polyacrylate layer or a UV-transparent coating layer. The present invention improves the linearity of the sensor array by matching the coating layer of the fiber grating array, and at the same time generates axial length redundancy by controlling the tension of the guide wire, thereby eliminating the cross-effect of stress during cabling, and the temperature sensing accuracy reaches 0.01°C, and the linearity is good within the range of 0°C to 80°C, and the optical cable has a simple structure and is easy to prepare.

Description

Technical Field

[0001] The invention relates to a fiber grating temperature-sensitive optical cable, in particular to a weak fiber grating temperature-sensitive optical cable. Background Art

[0002] With the popularity of fiber Bragg grating strings and weak grating arrays, fiber Bragg grating sensors are increasingly used in distributed temperature measurement applications. In temperature monitoring such as dam temperature field, water environment, soil moisture content, etc., the sensor is required to have high accuracy and no self-heating. Fiber Bragg grating temperature sensor is a passive sensing device with poor thermal conductivity. It has significant advantages such as anti-electromagnetic interference, high sensitivity, light weight and flexibility, multi-parameter composite sensing, and large-space distributed measurement. It has broad application prospects in the above-mentioned sensing fields. However, fiber Bragg grating sensors are very sensitive to temperature and strain. Under the action of strain, their temperature characteristics will show obvious nonlinearity. When the grating is cabled or bent, local micro-strain will be generated, which seriously affects the absolute accuracy of the sensor. In addition, the optical fiber itself is a multi-layer structure, with a core of lightly doped SiO2 and a cladding of SiO2, and the thermal expansion coefficients of the two are similar. However, in order to improve the mechanical properties of optical fiber, traditional optical fiber is usually prepared by coating polyacrylate twice on the 125um diameter cladding to form a 250um diameter optical fiber. Since the thermal expansion coefficient of the polyacrylate coated twice is different from the expansion coefficient of SiO2, and polyacrylate will undergo an obvious glass transition when the temperature rises, the thermal expansion coefficient before and after the glass transition will fluctuate, affecting the linearity of the temperature sensing of the fiber Bragg grating sensor, and thus affecting the measurement accuracy. A large number of existing studies often only consider the intersection of temperature and strain, and rarely pay attention to the influence of the coating layer. There are also few studies on the packaging of fiber Bragg grating arrays composed of a large number of grating strings. The Chinese patent "A packaging structure and method for a quasi-distributed fiber Bragg grating temperature sensor" (patent number: ZL: 201010185984.7) proposes a packaging structure with three metal coatings. Not only is the manufacturing process complicated, but the thermal expansion coefficients are inconsistent after multiple layers of coating, which can easily lead to nonlinear temperature sensing and is not suitable for high-precision temperature sensing. Limited by the traditional grating cabling structure and method, the fiber grating temperature sensor generally has a measurement accuracy of only 0.5°C, poor linearity over a large temperature range, and a very limited scope of application. Summary of the invention

[0003] In view of the problems of low temperature accuracy and poor linearity of existing fiber grating temperature sensing cables, the present invention proposes a weak fiber grating temperature sensing cable. The invention improves the linearity of the sensor array by matching the design of the coating layer of the fiber grating array, and at the same time generates axial length redundancy by controlling the tension of the guide wire, thereby eliminating the cross-effect of stress during cabling. The temperature sensing accuracy reaches 0.01℃, and the linearity is good in the range of 0℃ to 80℃. The optical cable has a simple structure and is easy to prepare. It has important application value in the field of weak fiber grating temperature sensing.

[0004] The technical solution adopted by the present invention is:

[0005] A weak fiber grating temperature sensing optical cable comprises a weak fiber grating array and a guide wire fixed to the weak fiber grating array in sections, wherein the weak fiber grating array and the guide wire are sleeved in a plastic sleeve, and the plastic sleeve is coated with a braided layer. The weak fiber grating array is engraved with a plurality of identical or wavelength division multiplexed fiber grating arrays, and the coating layer of the fiber grating array is a modified polyacrylate layer or an ultraviolet transparent coating layer.

[0006] The guide wire has a diameter greater than 0.5 mm to 1 mm. After the pre-tension is applied, the guide wire is fixedly spaced apart from the weak fiber grating array 2, and the interval between the fixed points is 30 cm.

[0007] The fixing point is bonded with UV glue and cured by ultraviolet light, and the diameter of the fixing point is less than 1 mm.

[0008] The plastic sleeve is hollowly sleeved around the weak fiber grating array and the guide wire, and the inner diameter of the plastic sleeve is 2.0 mm to 3.0 mm, the outer diameter is 4.0 mm to 5.0 mm, and the thickness is 2 mm.

[0009] The pretension applied by the guide wire is related to the length of the optical cable. The longer the optical cable, the greater the pretension applied. The pretension applied by the guide wire is related to the inner diameter of the plastic sleeve. The smaller the inner diameter, the greater the pretension applied.

[0010] The braided layer is fiber braided, and the outer diameter of the plastic sleeve is coated twice.

[0011] The invention discloses a weak fiber Bragg grating temperature-sensitive optical cable, which has the following beneficial effects:

[0012] 1) High temperature accuracy:

[0013] The linearity of the sensor array is improved by matching the design of the coating layer of the fiber grating array. At the same time, the axial length redundancy is generated by controlling the pre-tension of the guide wire, eliminating the cross-effect of stress during cabling. The temperature sensing accuracy reaches 0.01℃, and the linearity is good in the range of 0℃ to 80℃.

[0014] 2) The optical cable has a simple structure, is easy to prepare, and has a high cost-effectiveness:

[0015] The fiber grating array is fixed on the guide wire with the guide wire as the carrier, eliminating the influence of stress on the fiber grating array during cabling, and the structure is simple. After the fiber grating array and the guide wire are fixed, they can be inserted into the plastic sleeve or extruded, which has simple process, low cost and high cost performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the structure of the present invention.

[0017] Figure 2 It is a schematic diagram of the structure of the weak fiber grating array of the present invention. DETAILED DESCRIPTION

[0018] like Figure 1 , Figure 2 As shown, a weak fiber Bragg grating temperature sensing optical cable comprises a weak fiber Bragg grating array 2 and a guide wire 5 segmentally fixed to the weak fiber Bragg grating array 2. The weak fiber Bragg grating array 2 and the guide wire 5 are sleeved in a plastic sleeve 4, and the plastic sleeve 4 is coated with a braided layer 1 to form a fiber Bragg grating temperature sensing optical cable.

[0019] The weak fiber grating array 2 is engraved with multiple identical gratings. For example, the wavelength of all weak grating arrays is 1550nm. Relevant theories and experiments have proved that when the reflection of the grating drops to about 0.01%, 1000 gratings of the same wavelength are allowed on one array. Of course, if the number of grating arrays is greater than 1000, or the spatial resolution is less than 3 meters, different wavelengths can be inserted between gratings of the same wavelength, which is referred to as wavelength division multiplexing. The specific combination form can be time division / wavelength division multiplexing, or wavelength division / time division multiplexing; the coating layer 6 of the fiber grating array is a modified polyacrylate layer or a UV-transmitting coating layer, and the thermal expansion coefficient is 5.5·10 -7 / ℃ is 20~30um.

[0020] The guide wire 5 has a diameter greater than 0.5 mm to 1 mm. After applying pre-tension, the guide wire 5 is fixed to the weak fiber grating array 2 at intervals, and the interval between the fixed points 3 is 30 cm. The fixed points 3 are bonded with UV glue and cured by ultraviolet light, and the diameter of the fixed points 3 is less than 1 mm. Since the spacing between every two grating sensing points ranges from tens of centimeters to several meters, there is not a grating sensing point between every two fixed points 3, and the distance between the grating sensing point and the fixed point 3 should not be less than 5 cm.

[0021] The plastic sleeve 4 is hollowly sleeved on the periphery of the weak fiber grating array 2 and the guide wire 5, and the plastic sleeve 4 has an inner diameter of 2.0 mm to 3.0 mm, an outer diameter of 4.0 mm to 5.0 mm, and a thickness of 2 mm.

[0022] The magnitude of the pre-tension applied by the guide wire 5 is related to the length of the optical cable. The longer the optical cable, the greater the pre-tension applied. When the length of the optical cable is large, the greater the friction force on the optical fiber in the casing, the more serious the local stretching, which requires sufficient margin to ensure that the grating area is not affected by stress. Therefore, by increasing the pre-tension, it can be ensured that the grating array has sufficient margin after the guide wire is contracted under normal circumstances.

[0023] The magnitude of the pre-tension applied by the guide wire 5 is related to the inner diameter of the plastic sleeve 4. The smaller the inner diameter, the greater the pre-tension applied. The smaller the inner diameter of the plastic sleeve 4, the greater the friction force on the optical fiber in the sleeve, and the more serious the local stretching, which requires sufficient margin to ensure that the grating area is not affected by stress. Therefore, by increasing the pre-tension, it can be ensured that the grating array has sufficient margin after the guide wire is contracted under normal circumstances.

[0024] The braided layer 1 is fiber braided, and the outer diameter of the plastic sleeve 4 is coated twice to improve the tensile strength of the plastic sleeve and facilitate direct placement.

[0025] The schematic diagram of the weak fiber grating array 2 of the present invention is as follows: Figure 2 As shown, it is composed of an optical fiber core 9, an optical fiber grating array 8, a core cladding 7, and a coating 6. The optical fiber core 9 is coated with a cladding made of SIO2 to form a bare fiber with a diameter of 125um; the grating preparation can be to engrave a number of identical or wavelength-division multiplexed optical fiber grating arrays 8 on the bare fiber through a mask template online writing technology, or to first coat a UV-transparent coating layer and then write a grating array according to demand. The length of a single grating is about 10um, and the spacing between two gratings ranges from tens of centimeters to several meters. The grating array is coated with an ultra-thin polyacrylate coating layer or a UV-transparent coating layer, and the thermal expansion coefficient is 5.5×10 -7 / ℃, thickness is 20~30um.

[0026] The fiber Bragg grating temperature sensing optical cable provided by the present invention uses nylon thread as a carrier, and fixes the fiber Bragg grating on the guide wire with redundant segments, thereby eliminating the stress cross-effect during cabling. The fixed fiber Bragg grating and the guide wire are hollowly sleeved in the plastic sleeve 4 to further eliminate the strain effect and protect the fiber Bragg grating from damage. The entire optical cable is simple to manufacture, low in cost, accurate in temperature measurement, and easy to mass produce.

Claims

1. A weak fiber Bragg grating temperature sensing optical cable, comprising a weak fiber Bragg grating array (2), and a guide wire (5) fixed to the weak fiber Bragg grating array (2) in sections, characterized in that: The weak fiber grating array (2) and the guide wire (5) are sleeved in a plastic sleeve (4), and the plastic sleeve (4) is coated with a braided layer (1); The guide wire (5) is fixedly spaced apart from the weak fiber grating array (2) after a pre-tension is applied; The magnitude of the pre-tension applied by the guide wire (5) is related to the length of the optical cable; the longer the optical cable, the greater the pre-tension applied; The magnitude of the pre-tension applied by the guide wire (5) is related to the inner diameter of the plastic sleeve (4); the smaller the inner diameter, the greater the pre-tension applied; By controlling the tension of the guide wire (5) to generate axial length redundancy, the redundant segments of the optical fiber Bragg grating are fixed on the guide wire (5), thereby eliminating the influence of stress crossover during cabling; the fixed optical fiber Bragg grating and the guide wire (5) are hollowly sleeved in the plastic sleeve (4), thereby further eliminating the influence of strain; The weak fiber grating array (2) is engraved with a plurality of identical or wavelength division multiplexed fiber grating arrays, the coating layer (6) of the fiber grating array is a modified polyacrylate layer or an ultraviolet transparent coating layer, and the linearity of the sensor array is improved by matching and designing the coating layer (6) of the fiber grating array.

2. A weak fiber Bragg grating temperature sensitive optical cable according to claim 1, characterized in that: The guide wire (5) has a diameter greater than 0.5 mm to 1 mm.

3. The weak fiber Bragg grating temperature sensitive optical cable according to claim 1, characterized in that: The interval between the fixing points (3) is 30 cm.

4. A weak fiber Bragg grating temperature sensitive optical cable according to claim 3, characterized in that: The fixing points (3) are bonded with UV glue and cured by ultraviolet rays.

5. A weak fiber Bragg grating temperature sensitive optical cable according to claim 3 or 4, characterized in that: The diameter of the fixing point (3) is less than 1 mm.

6. The weak fiber Bragg grating temperature sensitive optical cable according to claim 1, characterized in that: The plastic sleeve (4) is hollowly sleeved on the periphery of the weak fiber grating array (2) and the guide wire (5).

7. A weak fiber Bragg grating temperature sensitive optical cable according to claim 6, characterized in that: The plastic sleeve (4) has an inner diameter of 2.0 mm to 3.0 mm, an outer diameter of 4.0 mm to 5.0 mm, and a thickness of 2 mm.

8. The weak fiber Bragg grating temperature sensitive optical cable according to claim 1, characterized in that: The braided layer (1) is a fiber braid, which performs secondary coating on the outer diameter of the plastic sleeve (4).

9. The method for preparing the weak fiber Bragg grating temperature sensitive optical cable according to any one of claims 1 to 8, characterized in that: The invention is composed of an optical fiber core (9), an optical fiber grating array (8), a core cladding (7), and a coating layer (6); the optical fiber core (9) is coated with a cladding made of SIO2 to form a bare fiber; the grating is prepared by engraving a plurality of identical or wavelength division multiplexed optical fiber grating arrays (8) on the bare fiber through a mask template online writing technology; or first coating a UV-transmitting coating layer, and then engraving a grating array according to demand, wherein the length of a single grating is 10 um, and the spacing between two gratings ranges from tens of centimeters to several meters; the grating array is coated with an ultra-thin polyacrylate coating layer or a UV-transmitting coating layer.

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