An ultra-fine diameter heterogeneous fiber optic hydrophone and its manufacturing process

Abstract

The present invention discloses an ultra-fine diameter heterogeneous fiber optic hydrophone and its manufacturing process. The hydrophone includes a core shaft, a sensitizing cylindrical shell, and a heterogeneous fiber optic interferometer. A cavity is disposed through the core shaft. Sensitizing cylindrical shells are symmetrically arranged on both sides of the core shaft. A heterogeneous fiber optic interferometer is installed in the cavity of the core shaft. The heterogeneous fiber optic interferometer includes a heterogeneous fiber optic coupler, a reference arm ultra-short fine diameter fiber, and a sensing arm ultra-fine diameter fiber. The heterogeneous fiber optic coupler, the reference arm ultra-short fine diameter fiber, and the reference arm mirror are fixedly arranged flatly in the cavity. The sensing arm ultra-fine diameter fiber is evenly wound around the sensitizing cylindrical shell and its end is connected to a sensing arm mirror. The present invention uses ultra-fine diameter ultra-small mode field fibers and fine diameter small mode field fibers to form a heterogeneous fiber optic interference structure, and the fiber is coiled and packaged in an ultra-fine high-sensitivity structure, realizing the fabrication of an ultra-fine diameter, low optical insertion loss, and high-sensitivity hydrophone, which can be applied to fields such as underwater acoustic target detection, environmental noise measurement, and undersea mineral resource exploration.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic acoustic sensing, and particularly relates to an ultra-fine diameter heterogeneous fiber optic hydrophone and its manufacturing process. Background Art

[0002] The fiber optic hydrophone array module is an effective device for realizing applications such as underwater acoustic target detection, ocean noise environment measurement, and seabed mineral resource exploration. It has the advantages of being passive underwater, anti-electromagnetic interference, corrosion-resistant, capable of long-distance transmission, and easy to expand, and has been widely applied in multiple fields. With the popularization and application of small and medium-sized underwater platforms, as a platform equipment load, the fiber optic hydrophone array has become a new development trend towards miniaturization, lightweight, low power consumption, expandability, and improved capabilities. Therefore, new requirements such as small size, low weight, and high reliability have been put forward for fiber optic hydrophones.

[0003] Currently, internationally and domestically, there are solutions for fiber optic hydrophones to obtain underwater acoustic signals by using changes in parameters such as light intensity, light phase, and light polarization to achieve sensing. Among them, the light phase change transducer method is the most extensive and stable way to obtain and restore underwater acoustic signals. It usually uses specific fiber optic optical path structures and devices such as fiber optic interferometer types, fiber optic F-P interferometer types, and active fiber optic grating types to achieve this function.

[0004] The point-type fiber optic hydrophone unit uses optical fiber wound around a sensitized organic housing structure with a specific diameter size, and is connected with optical structures such as fiber optic grating pairs or fiber optic device groups to construct a fiber optic interferometer. High-sensitivity acquisition of underwater acoustic and vibration signals is achieved through the wound sensitive optical fiber, and the interference signal converted into a light phase change is output, and is detected and demodulated by a photoelectric detector and restored. The fiber optic hydrophone array module is the combined application of fiber optic hydrophones, existing in the forms of linear arrays, planar arrays, and volume arrays. The diameter, thickness, bending radius, transmission distance, element scale of the array module, as well as the array channel noise, array gain, characteristic size and specifications, etc. are all limited by the specification size and optical characteristics of the fiber optic hydrophone, that is, there are the following implicit relationships.

[0005] Table 1 Factors Limiting the Size of Fiber Optic Hydrophones and Response Factors

[0006]

[0007] Through the analysis of the above table, it can be seen that the small-size design of fiber optic hydrophones is mainly limited by the fiber winding radius, which should aim at the specific technical requirements of the long-term reliability, insertion loss, and sensitivity of fiber optic hydrophones, and thus determines the performance of the array such as the operating frequency, noise level, acoustic gain, and its volume and weight.

[0008] As the requirements for the underwater platform's underwater acoustic payload capabilities continue to increase to meet the needs of longer distances, higher accuracy, and greater depth in environmental and target measurements, the payload is required to have a smaller unit weight, a smaller volume, and comparable or even better performance (such as sensitivity, noise, etc.). Fundamentally, this needs to be achieved by designing sensors with smaller sizes and better parameters.

[0009] In the optical path of fiber optic hydrophones, the fiber optic interferometer structure is mostly adopted. Either fiber Bragg gratings or a coupler + two mirrors can form the interference of optical signals to extract optical phase information. At the same time, in terms of structure, fiber optic hydrophones need to encapsulate gratings, couplers, mirrors, etc. into them to achieve the purpose of the most refined and lowest interference. Therefore, the sizes of fiber optic devices and the installation layout within the hydrophone structure will also limit the minimum size of their encapsulation.

[0010] The specific problems encountered in the ultra-fine diameter of fiber optic hydrophones include the following four aspects:

[0011] (1) The reliability of ultra-fine winding decreases: According to the fiber bending microcrack growth model, as the winding diameter decreases, the microcracks will accelerate and expand, resulting in a significant shortening of the bending reliable life. The mathematical relationship can be simply described as t 寿命 ∝(1 / 10) r , which is significantly contradictory to the long-term reliability requirements of its encapsulation application. Reducing the fiber diameter, increasing the fiber screening stress, and shortening the fiber winding length can all delay the crack growth. However, for the requirements of sensing sensitivity, multiplexing delay, etc., the fiber length needs to be maintained at a certain value; the fiber screening stress is limited by the existing process and raw materials and is difficult to be significantly increased in the short term. Therefore, reducing the fiber diameter is a direct and effective method to improve fiber winding.

[0012] (2) The sensitivity after ultra-fine winding decreases: Based on the theory of axial static pressure-strain of a cylindrical shell, the strain is proportional to the cube of the diameter ( ), and inversely proportional to the square of the thickness ( ). The ultra-fine diameter design will surely cause a decrease in the diameter of the sensitizing structure. It can be compensated in the reverse direction by the thickness. And the fiber layer in the thin sensitizing shell also becomes an important factor in the thickness. Therefore, using finer fibers can not only reduce the thickness of a single layer but also increase the number of winding turns in a single layer and reduce the number of winding layers, thus thinning the overall thickness of the fiber layer;

[0013] (3) The insertion loss of ultra-fine winding increases: After the fiber is wound with an ultra-small diameter, the incident angle of the optical signal transmitted in the single-mode fiber at the refractive index transition interface becomes smaller, making it easier for the optical signal to leak. Therefore, a smaller mode field diameter is required to constrain the optical signal;

[0014] (4)Insufficient applicability of existing processes: The ultra-fine diameter fiber optic hydrophone uses ultra-fine diameter and ultra-small mode field single-mode fibers. The existing fusion splicing processes and equipment for fine diameter fibers are not applicable, which limits applications such as testing and arraying of pure ultra-fine diameter fiber optic hydrophones. Summary of the Invention

[0015] In view of the problems of reliability, sensitivity, insertion loss, and process applicability that need to be solved for realizing ultra-fine diameter of fiber optic hydrophones as discussed above, the present invention proposes a fiber optic hydrophone structure with a small diameter and high sensitivity enhancement structure based on an integrated interferometer of heterogeneous fine diameter small mode field fiber - ultra-fine diameter ultra-small mode field fiber. The ultra-fine diameter fiber is wound around a small sensitivity enhancement shell to achieve acoustic signal sensing and sensitivity enhancement, while the fine diameter fiber realizes input docking, thereby realizing an ultra-fine diameter fiber optic hydrophone with good integration of high sensitivity, high reliability, low insertion loss, and process convenience, providing device module-level support for sensing application scenarios with small space, low load, and high performance.

[0016] I. Scheme Design:

[0017] 1. Fiber heterogeneous integrated interferometer scheme:

[0018] This scheme uses a long-arm difference interferometer as the basic structure of the sensing optical path, as shown in Figure 1 . The conventional structure includes 1 coupler and 2 mirrors. The coupler and the mirrors are connected by the same type of fiber. The length difference between the two arms of the fiber is controlled within a value of more than ten meters to dozens of meters to ensure that the two arms of the fiber respectively obtain the acoustic signal and the reference signal.

[0019] Considering that the fiber needs to be wound around a specific sensitivity enhancement structure to enhance the acquisition of acoustic signals, the present invention uses a cylindrical shell as the sensitivity enhancement structure of the hydrophone. After different diameters and lengths of fibers are wound into coils, they correspond to different reliability lifetimes, as shown in Figure 4 (a). For example, for the bending at a diameter of φ8mm, taking a fiber with a length of 21m as an example, fibers with diameters of 160μm and 100μm will have lifetimes of less than 1 year and more than 10 years respectively; while in Figure 4 (b), for 160μm fibers with lengths of 21m and 10m wound into φ8mm diameter coils, the expected lifetimes will be less than 1 year and more than 10 years respectively. That is, using a fiber with a smaller diameter can make the coil have a long bending lifetime, or using a shorter fiber length can also achieve long-term reliability, but the latter conflicts with the high sensitivity requirements of the hydrophone, so it is discarded. The sensing ratio of the interferometer uses an ultra-fine diameter long fiber to obtain both high sensitivity and high reliability after being wound into a hydrophone.

[0020] At the same time, to ensure that the optical signal is confined to the core under small fiber bending, that is, to ensure a smaller bending transmission optical loss, a design with an ultra-small fiber mode field diameter, such as 5μm, is adopted. Therefore, the sensing arm of the interferometer is an ultra-fine diameter and ultra-small mode field fiber.

[0021] When the input and output optical fibers of the interferometer, as well as the sensing and reference optical fibers, all adopt ultra-fine-diameter and ultra-small-mode-field optical fibers, although it ensures the advantages of high sensitivity, high reliability, and low optical loss of the fiber optic hydrophone, the existing plugging, fusion splicing, and testing technologies are not well adapted to ultra-fine optical fibers (with a diameter of 100 μm and a mode field diameter of 5.5 μm), making it impossible to popularize and apply the fiber optic hydrophone in engineering. Based on this, one arm of the interferometer retains a fine-diameter and small-mode-field optical fiber (with a diameter of 160 μm and a mode field diameter of 7 μm), and uses its mature process supporting such as plugging and fusion splicing.

[0022] Thus, the interferometer forms a coupler by melting and tapering a fine-diameter and small-mode-field optical fiber and an ultra-fine-diameter and ultra-small-mode-field optical fiber. The input and output optical fibers respectively contain 1 pair of fine-diameter and small-mode-field optical fibers and ultra-fine-diameter and ultra-small-mode-field optical fibers. At the output end of the optical fiber, the lengths of two arms are retained according to the designed arm length difference, and a mirror is fabricated at the tail end to return the optical signal. Since the overly long reference arm optical fiber needs to be accommodated in the hydrophone structure or coiled in the insensitive area of the sensitization housing, it will cause the reference arm optical fiber to form a small bend, which will in turn increase the insertion loss of the fine-diameter optical fiber in the reference arm, thus affecting the extinction ratio of the interferometer and the signal quality. Therefore, the reference arm is designed to have an ultra-short optical fiber length (within a few millimeters) or be integrated and encapsulated with the coupler to ensure that it can be arranged straight inside the hydrophone structure, avoiding microbend loss and failure risks. The optical path structure is shown in Figure 3 .

[0023] 2. Hydrophone sensor structure scheme:

[0024] The hydrophone structure design fully considers the requirements for effects such as hydrophone miniaturization and acceleration noise suppression, and also takes into account the requirements for the fixed installation of optical fiber devices. All optical fiber devices are designed with a fine and short encapsulation. The designed hydrophone sensitization structure consists of two completely identical organic columnar shells, whose diameters are determined according to the miniaturization target, such as Φ8 mm or even smaller; the shell thickness is designed to balance the hydrophone sensitivity target and the hydrostatic pressure resistance design, that is , a balance between the relationships of

[0025] The sensitization columnar shells are fixed symmetrically in mirror image on the metal core shaft, and the metal core shaft is also designed with mirror symmetry. The formed fiber optic hydrophone assembly structure is a mirror-symmetrical structure with respect to the plane passing through the axis and the central vertical plane. Its overall encapsulation structure is: sensitization columnar shell - core shaft - sensitization columnar shell (from left to right along the axis), sensitization housing - core shaft (from inside to outside radially), as shown in Figure 4 . A sealed air cavity is formed between the columnar shell and the core shaft. When working in a liquid environment, the acoustic impedance mismatch at the interface forms a sensitization effect, that is , where (ρc) water > (ρc) air , so P air < P water(where P water , P air are the sound pressure amplitudes in water and air respectively, and (ρc) water , (ρc) air are the products of the sound speed and the medium density in water and air respectively).

[0026] When the hydrophone structure moves axially with acceleration, the deformation generated on the sensitizing cylindrical shell is exactly such that the deformations of the two cylindrical shells are the same in magnitude and opposite in phase, thus canceling each other out; while when moving radially with acceleration, the expansion and contraction amounts of the points symmetric about the plane passing through the axis perpendicular to the acceleration direction on each sensitizing cylindrical shell are exactly the same in magnitude and opposite in phase. Assuming a sinusoidal signal is discussed, the acceleration signal of the left sensitizing cylindrical shell is described as: ɑ ι = A﹒sin(wt + φ), and the acceleration signal of the right sensitizing cylindrical shell is described as: ɑ r = -A ﹒ sin(wt + φ), (where ɑ ι , ɑ r represent the acceleration signals obtained by the left and right cylindrical shells respectively, A is the amplitude of the noise signal, w is the signal frequency, t is the time, and φ is the initial phase), so they cancel each other out, thus having a good acceleration noise cancellation effect.

[0027] The axial center position of the mandrel is designed to be hollow for realizing the input and output of optical fibers; the symmetric middle section is designed as a cavity structure, mainly for the accommodation of the optical fiber coupler and the optical fiber mirror. To reduce the overall length and diameter of the hydrophone, the middle cavity is designed to accommodate the length of one coupler and the width of the side-by-side arrangement of the coupler and the mirror.

[0028] 3. Optical fiber hydrophone assembly scheme:

[0029] The coupler in the optical fiber heterogeneous interferometer is encapsulated on the installation structure of the middle cavity of the metal mandrel, and the input pigtail is led out through the hollow shaft at one end of the mandrel; the thin-diameter optical fiber of the ultra-short reference arm is laid straight, and the connected mirror is fixed in the hollow shaft at the other end of the mandrel; the ultra-thin-diameter optical fiber of the sensing arm is bisected and wound around the two sensitizing shell structures respectively. The number of optical fiber layers and the relative positions of the winding turns of the two shells are completely symmetric and consistent. The connected mirror is fixed in the middle cavity, arranged side by side with the coupler, and the outer cover is sealed with glue to achieve device protection; the fiber output end and the mirror laying end of the hollow mandrel core are sealed by structural or glue-sealing methods to achieve the sealing of the inner cavity of the mandrel. Considering the needs of isolation protection such as shock resistance and corrosion resistance, after the hydrophone structure and the optical fiber interferometer are encapsulated, a low-permeability colloid is used for overall potting and other methods to achieve isolation from the external environment.

[0030] Therefore, the design of the present invention primarily comprises three components: a fiber-optic heterogeneous integrated interferometer solution, a hydrophone sensing structure solution, and a fiber-optic hydrophone assembly solution. The interferometer, the optical structural foundation of the fiber-optic hydrophone, utilizes a unique heterogeneous design to enable winding into smaller diameter coils and smaller packaging. The sensing structure underpins the fiber-optic hydrophone's enhanced underwater acoustic signal acquisition and serves as a protective packaging mechanism for the fiber-optic device, combining ultra-fine diameter and noise suppression. The fiber-optic hydrophone assembly solution primarily utilizes the unique packaging design of the heterogeneous interferometer and the fine-diameter sensitivity enhancement structure to ensure highly sensitive, low-noise sensing and reliable, stable packaging of the fiber-optic device.

[0031] Specifically, the present invention provides the following technical solutions:

[0032] An ultra-fine diameter heterogeneous fiber optic hydrophone comprises a core shaft, a sensitivity-enhancing column shell, and a heterogeneous fiber optic interferometer. A cavity is provided through the core shaft, and sensitivity-enhancing column shells are symmetrically provided on both sides of the core shaft. A heterogeneous fiber optic interferometer is installed in the cavity of the core shaft. The heterogeneous fiber optic interferometer comprises a heterogeneous fiber optic coupler. The input end of the heterogeneous fiber optic coupler is respectively connected to a thin-diameter input fiber and an ultra-fine-diameter input fiber. The output end of the heterogeneous fiber optic coupler is respectively connected to an ultra-short thin-diameter fiber of a reference arm and an ultra-fine-diameter fiber of a sensing arm. The reference arm ultra-short diameter optical fiber is connected to the reference arm reflector, the sensing arm ultra-fine diameter optical fiber is connected to the sensing arm reflector, and the heterogeneous optical fiber coupler, the reference arm ultra-short diameter optical fiber and the reference arm reflector are fixed flatly in the cavity. The sensing arm ultra-fine diameter optical fiber extends from the cavity and is evenly wound on the sensitivity-enhancing column shells on both sides. The end of the sensing arm ultra-fine diameter optical fiber is connected to the sensing arm reflector fixed on the middle section of the cavity. A packaging box cover is provided on the middle section of the cavity, and the core shaft, the packaging box cover and the sensitivity-enhancing column shell are all provided with an external potting layer.

[0033] Furthermore, the diameter of the thin-diameter input optical fiber is greater than 120 μm, the diameter of the ultra-fine-diameter input optical fiber is less than 120 μm, the diameter of the ultra-short thin-diameter optical fiber of the reference arm is greater than 120 μm, the length of the ultra-short thin-diameter optical fiber of the reference arm is within 5 mm, the diameter of the ultra-fine-diameter optical fiber of the sensing arm is less than 120 μm, the length of the ultra-fine-diameter optical fiber of the sensing arm is 10m-30m, and the mode field of the thin-diameter input optical fiber and the ultra-short thin-diameter optical fiber of the reference arm is greater than 6.0 μm, and the mode field of the ultra-fine-diameter input optical fiber and the ultra-fine-diameter optical fiber of the sensing arm is less than 6.0 μm.

[0034] Furthermore, a middle cavity is provided in the middle section of the core shaft, both sides of the core shaft are hollow shafts, and the middle cavity is larger than the cavity of the hollow shaft and is interconnected.

[0035] Furthermore, the outer layer of the core shaft is a metal tube, and the inner layer of the core shaft is a quartz tube or a glass tube.

[0036] Furthermore, the sensitivity-enhancing column shell is composed of two sensitivity-enhancing column shell bodies symmetrically installed above and below, and a sensitivity-enhancing cavity is formed between the sensitivity-enhancing column shell bodies and the core shaft.

[0037] Furthermore, the core shaft and the sensitivity-enhancing column shell are provided with a guide channel for the extension and winding of the ultra-fine optical fiber of the sensing arm.

[0038] Furthermore, the heterogeneous fiber coupler, the reference arm ultra-short thin-diameter optical fiber and the reference arm reflector are an integrated packaging structure.

[0039] The present invention also provides a process for manufacturing an ultra-fine diameter heterogeneous optical fiber hydrophone, comprising the following steps:

[0040] S1. Prepare a core shaft, a sensitivity-enhancing column shell, and a heterogeneous fiber interferometer respectively, wherein a cavity is horizontally provided in the core shaft, each sensitivity-enhancing column shell includes two sensitivity-enhancing column shell bodies, and the heterogeneous fiber interferometer includes a heterogeneous fiber coupler, connecting the input end of the heterogeneous fiber coupler to the thin-diameter input fiber and the ultra-fine-diameter input fiber, and connecting the output end of the heterogeneous fiber coupler to the ultra-short-diameter fiber of the reference arm and the ultra-fine-diameter fiber of the sensing arm respectively, connecting the ultra-short-diameter fiber of the reference arm to the reference arm reflector, and connecting the ultra-fine-diameter fiber of the sensing arm to the reflector of the sensing arm;

[0041] S2. Fix the sensitivity-enhancing column shell on both sides of the core shaft by means of structure or gluing, so that a closed sensitivity-enhancing cavity is formed between the sensitivity-enhancing column shell and the core shaft;

[0042] S3. Fix the heterogeneous fiber coupler, the reference arm ultra-short thin fiber, and the reference arm reflector flatly in the cavity of the core shaft, wind the sensing arm ultra-fine fiber evenly around the sensitivity-enhancing cylinder shells on both sides, fix the sensing arm reflector at the end of the sensing arm ultra-fine fiber to the middle section of the cavity by means of structure or adhesive, and cover the packaging box lid;

[0043] S4. The two ends of the cavity of the core shaft are sealed, and a colloid is used to encapsulate the outer surface of the optical fiber hydrophone structure to form an external potting layer to obtain a complete optical fiber hydrophone.

[0044] Furthermore, in step S1, the heterogeneous fiber coupler, the reference arm ultra-short thin-diameter optical fiber, and the reference arm reflector are packaged into an integrated structure through colloid.

[0045] Furthermore, in step S3, the ultra-fine optical fiber of the sensing arm is marked with an equal-division point, and is laid out on one of the sensitivity-enhancing column shells along the guiding mechanism of the core axis. The fiber is wound using zero tension, and when it reaches the equal-division point, the other half of the optical fiber is guided to the other sensitivity-enhancing column shell and wound in the same manner as the symmetrical side sensitivity-enhancing column shell to form an ultra-fine optical fiber sensing coil layer.

[0046] Based on the above scheme, the beneficial effects of the present invention are:

[0047] 1. High reliability and long life effect:

[0048] (1) The reliability of the fiber optic hydrophone is mainly determined by the reliability of the fiber optic coil, the reliability of the device, and the overall environmental adaptability. For the requirement of ultra-fine diameter, according to the fiber coil failure model, the prediction curve of life - fiber diameter - length - coil diameter is obtained. The diameter and length of the fiber are designed and selected. For example, for a fiber optic coil with a diameter of φ8mm and a length of 20m, a fiber with a diameter of 100μm is selected to ensure a life of more than 10 years; for a fiber optic coil with a diameter of φ8mm and a length of 10mm, a fiber with a diameter of 160μm is selected, which also ensures a life of more than 10 years. At the same time, by strengthening the screening intensity control of the fiber, its reliability is further enhanced. Combining the above, the matching of the ultra-fine diameter and long life requirements of the hydrophone can be achieved;

[0049] (2) The fiber optic device adopts a thin and short type of sealed packaging, which is convenient for the device to be packaged in a small-size structure and can also ensure the long-term reliability of the device. By using mature processes and combining strongly screened integrated interferometer devices, the failure rate can be controlled at a level of 200 FITs or even smaller, and it has a long intrinsic life;

[0050] (3) In terms of packaging, the design of hollow core shaft end sealing and overall glue pouring of the hydrophone is adopted, so that the fiber optic coil and fiber optic device inside can be isolated from the external environment such as water and corrosive liquid, and at the same time, the anti-physical damage capabilities such as shock resistance and knock resistance are improved, so that its intrinsic reliability can be fully obtained.

[0051] 2. Low loss, low noise, and high sensitivity effect:

[0052] (1) The ultra-fine diameter fiber adopts a design with an ultra-small mode field diameter. For example, the mode field diameter is below 5.5μm. Based on the waveguide transmission theory, it can be predicted that there is almost no light transmission mode leakage under small bending. After the process of winding the loop, the increase in the measured loss is within the error range of 0.5dB, achieving the low loss effect of the optical signal;

[0053] (2) The structural components of the hydrophone, such as the core shaft and the sensitizing column shell, are all designed and packaged in a mirror image. The length of the sensitizing coil and the relative winding area are strictly the same. When the overall layout scheme of the hydrophone with central clamping or both-end clamping is adopted, when non-acoustic signals such as vibration are introduced, they can be completely cancelled out, and the acceleration sensitivity within the frequency band is generally lower than -20dB, achieving the noise suppression effect;

[0054] (3) The sensitivity of the hydrophone column shell is related to its diameter, thickness, material, etc. It has a cubic relationship with the diameter, that is and an inverse square relationship with the thickness, that is While reducing the diameter, the thickness is thinned, and the material is preferably matched. Still, the ultra-fine diameter hydrophone can have a sensitivity similar to that of the thick hydrophone, achieving the high sensitivity effect. Description of the Drawings

[0055] Figure 1 is a structural diagram of a conventional fiber optic interferometer;

[0056] Figure 1 Label description: 1 - equal - splitting ratio coupler; 2 - mirror; 3 - thin - diameter input optical fiber; 4 - thin - diameter reference - arm optical fiber; 5 - thin - diameter sensing - arm optical fiber;

[0057] Figure 2 is a structural diagram of a heterogeneous fiber optic interferometer, where: Figure 2 in (a) is a structural diagram of a heterogeneous fiber optic interferometer with an ultrashort reference arm, Figure 2 in (b) is a structural diagram of a heterogeneous fiber optic reference - arm integrated interferometer;

[0058] Figure 2 Label description: 6 - heterogeneous fiber optic equal - splitting ratio coupler; 7 - ultra - thin - diameter input optical fiber; 8 - ultrashort - thin - diameter reference - arm optical fiber; 9 - ultra - thin - diameter sensing - arm optical fiber; 10 - heterogeneous fiber optic reference integrated coupler;

[0059] Figure 3 is a structural diagram of an ultra - thin - diameter heterogeneous fiber optic hydrophone;

[0060] Figure 3 Label description: 12 - mandrel; 13 - sensitizing cylinder shell; 14 - sensitizing cavity; 15 - middle cavity; 16 - hollow shaft; 17 - left - right symmetry plane; 18 - up - down symmetry plane;

[0061] Figure 4 is a prediction curve graph of the fiber optic coil life and related factors, where: Figure 4 in (a) is a graph of fiber optic coil life - fiber diameter - winding radius relationship, Figure 4 in (b) is a graph of fiber optic coil life - fiber length - winding radius relationship;

[0062] Figure 5 is an assembly process diagram of an ultra - thin - diameter heterogeneous fiber optic hydrophone, where: Figure 5 in (a) is a front - view schematic diagram of the coupler fixation and ultra - thin - diameter optical fiber winding, Figure 5 in (b) is an upper - view schematic diagram of the hydrophone mirror installation, Figure 5 in (c) is an upper - view schematic diagram of the hydrophone middle - cavity encapsulation, Figure 5 in (d) is a schematic diagram of the hydrophone potting finished product;

[0063] Figure 5 Label description: 19 - ultra - thin - diameter optical fiber sensing coil layer; 20 - middle - cavity encapsulation box cover; 21 - mandrel tail - end hollow seal; 22 - mandrel fiber - inlet - end hollow seal; 23 - external potting layer. Detailed Implementation Modes

[0064] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0065] As Figure 1 shown, the structure of a conventional fiber optic interferometer includes an equal-splitting ratio coupler 1. The input end of the equal-splitting ratio coupler 1 is connected to two thin-diameter input fibers 3. The output ends of the equal-splitting ratio coupler 1 are respectively connected to a reference arm thin-diameter fiber 4 and a sensing arm thin-diameter fiber 5. The reference arm thin-diameter fiber 4 is connected to a mirror 2 of the reference arm, and the sensing arm thin-diameter fiber 5 is connected to a mirror 2 of the sensing arm.

[0066] As Figure 2 shown, the heterogeneous fiber optic interferometer provided by the present invention adopts a heterogeneous fiber optic interferometer with an ultrashort reference arm as shown in (a) of Figure 2 , or a heterogeneous fiber optic reference arm integrated interferometer as shown in (b) of Figure 2 . As Figure 2 shown in (a) of , for the heterogeneous fiber optic interferometer with an ultrashort reference arm, the heterogeneous fiber optic coupler adopts a heterogeneous fiber optic equal-splitting ratio coupler 6. Its input end is connected to the thin-diameter input fiber 3, and an ultra-thin-diameter input fiber 7 (as an application backup) is connected. The output ends of the heterogeneous fiber optic equal-splitting ratio coupler 6 are respectively connected to a reference arm ultra-short thin-diameter fiber 8 (a thin-diameter fiber with an ultra-short length) and a sensing arm ultra-thin-diameter fiber 9. The reference arm ultra-short thin-diameter fiber 8 is connected to the reference arm mirror 2, and the sensing arm ultra-thin-diameter fiber 9 is connected to the sensing arm mirror 2.

[0067] As Figure 2 shown in (b) of , the main structure of the heterogeneous fiber optic reference arm integrated interferometer is the same as that of the heterogeneous fiber optic interferometer with an ultrashort reference arm. The difference is that the heterogeneous fiber optic coupler adopts a heterogeneous fiber optic reference integrated coupler 10. The heterogeneous fiber optic reference integrated coupler 10 is obtained by slightly lengthening and thickening the original heterogeneous fiber optic coupler, and the heterogeneous fiber optic coupler, the reference arm ultra-short thin-diameter fiber 8 and the reference arm mirror 2 are fixed flatly and encapsulated integrally by using a structure and / or colloid, so as to improve the performance and reliability of the reference arm.

[0068] As Figure 3As shown in the figure, a superfine-diameter heterogeneous fiber optic hydrophone provided by the present invention includes a core shaft 12, a sensitivity-enhancing cylindrical shell 13, and a heterogeneous fiber optic interferometer. The core shaft 12 is mainly made of a metal material, with a metal tube on the outer layer and a quartz tube or a glass tube on the inner layer. A middle cavity 15 is provided in the middle section of the core shaft 12. Both sides of the core shaft 12 are hollow shafts 16. The middle cavity 15 is larger than the cavities of the hollow shafts 16 and they are interconnected. Two sensitivity-enhancing cylindrical shells 13 are symmetrically installed on the left and right sides of the core shaft 12. Each sensitivity-enhancing cylindrical shell 13 is composed of two sensitivity-enhancing cylindrical shell bodies symmetrically installed up and down. A sensitivity-enhancing cavity 14 is formed between the sensitivity-enhancing cylindrical shell body and the core shaft 12. The sensitivity-enhancing cylindrical shells 13 are completely mirror-symmetrical with respect to the left-right symmetry plane 17 and the up-down symmetry plane 18 respectively.

[0069] A heterogeneous fiber optic interferometer is installed in the cavity of the core shaft 12. The heterogeneous fiber optic coupler, the reference arm ultra-short fine-diameter fiber 8, and the reference arm mirror 2 are fixed flatly in sequence on the channels from the cavity of one side hollow shaft 16, through the middle cavity 15 to the cavity of the other side hollow shaft 16. The sensing arm ultra-fine-diameter fiber 9 extends out from one side hollow shaft 16 and is evenly wound around the sensitivity-enhancing cylindrical shells 13 on both sides of the core shaft 12 to form a superfine-diameter fiber optic sensing coil layer 19. The wound sensing coil layer 19 is mirror-symmetrical with respect to the left-right symmetry plane 17 and the up-down symmetry plane 18 respectively. The end of the sensing arm ultra-fine-diameter fiber 9 is connected to the sensing arm mirror 2 fixed on the upper part of the middle cavity 15. On the hollow shafts 16 on both sides of the core shaft 12, there are respectively a core shaft fiber inlet end hollow seal 22 and a core shaft tail end hollow seal 21. On the middle cavity 15, there is a middle cavity encapsulation box cover 20. The core shaft 12, the middle cavity encapsulation box cover 20, and the sensitivity-enhancing cylindrical shells 13 on both sides are provided with an external potting layer 23.

[0070] Further, the diameter of the fine-diameter input fiber 3 is 160 μm, the diameter of the ultra-fine-diameter input fiber 7 is 100 μm, the diameter of the reference arm ultra-short fine-diameter fiber 4 is 160 μm, the length of the reference arm ultra-short fine-diameter fiber 4 is 3 mm, the diameter of the sensing arm ultra-fine-diameter fiber 9 is 100 μm, the length of the sensing arm ultra-fine-diameter fiber 9 is 20 m, and the mode fields of the fine-diameter input fiber 3 and the reference arm ultra-short fine-diameter fiber 4 are M7 μm, forming a fine-diameter small mode field fiber, and the mode fields of the ultra-fine-diameter input fiber 7 and the sensing arm ultra-fine-diameter fiber 9 are M5.5 μm, forming an ultra-fine-diameter ultra-small mode field fiber.

[0071] As Figure 5 shown, a manufacturing process of a superfine-diameter heterogeneous fiber optic hydrophone provided by the present invention includes the following steps:

[0072] S1. Manufacture a mandrel 12, a sensitization column shell 13 and a heterogeneous fiber optic interferometer respectively. Among them, a cavity is arranged inside the mandrel 12, a middle cavity 15 is arranged in the middle section of the mandrel 12, both sides of the mandrel 12 are hollow shafts 16, the middle cavity 15 is larger than the cavities of the hollow shafts 16 and they are interconnected. Each sensitization column shell 13 includes two sensitization column shell bodies;

[0073] The heterogeneous fiber optic interferometer includes a heterogeneous fiber optic coupler. Connect the input end of the heterogeneous fiber optic coupler to the thin-diameter input fiber 3 and the ultra-thin-diameter input fiber 7. The output ends of the heterogeneous fiber optic coupler are respectively connected to the reference arm ultra-short thin-diameter fiber 8 and the sensing arm ultra-thin-diameter fiber 9. The reference arm ultra-short thin-diameter fiber 8 is connected to the reference arm mirror 2, and the sensing arm ultra-thin-diameter fiber 9 is connected to the sensing arm mirror 2;

[0074] S2. Symmetrically install the two sensitization column shell bodies of the sensitization column shell 13 onto the mandrel 12, and achieve the fastening and sealing between the contact surfaces of the two by means of structure or gluing, etc., to form a sealed sensitization cavity 14. At this time, the two sensitization column shells are completely mirror-symmetrical with respect to the left-right symmetry plane 17 and the up-down symmetry plane 18 respectively;

[0075] S3. Fix the heterogeneous fiber optic coupler, the reference arm ultra-short thin-diameter fiber 8 and the reference arm mirror 2 flatly in the channel from the hollow shaft 16 to the middle cavity 15 of the hydrophone mandrel 12 to ensure the reliability of the reference arm. See Figure 5 of (a);

[0076] Mark the equally divided points on the sensing arm ultra-thin-diameter fiber 9, lay it along the guiding mechanism of the mandrel 12 to one of the sensitization column shells 13, and wind the fiber with zero tension. When reaching the equally divided points, guide the other half of the fiber to the other sensitization column shell 13 and wind the wire in the same way as the symmetrically arranged sensitization column shell 13 to form an ultra-thin-diameter fiber sensing coil layer 19. The wound ultra-thin-diameter fiber sensing coil layer 19 is mirror-stacked with respect to the left-right symmetry plane 17 and the up-down symmetry plane 18 respectively. Fix the sensing arm mirror 2 in the middle cavity 15 of the mandrel by means of structure or gluing, etc. See Figure 5 of (b), and cover the cover of the middle cavity encapsulation box 20. See Figure 5 of (c);

[0077] S4. Conduct sealing protection on the hollow shafts 16 at both ends of the mandrel 12. By setting the hollow seal 22 at the fiber inlet end of the mandrel and the hollow seal 21 at the tail end of the mandrel, ensure the isolation between the inner and outer cavities of the mandrel 12. See Figure 5 of (c). Conduct encapsulation protection outside the fiber optic hydrophone structure, and use a low-water-permeability and impact-resistant colloid to form an external encapsulation layer 23 to obtain a complete fiber optic hydrophone. See Figure 5 of (d).

[0078] Furthermore, in the step S1, for manufacturingFigure 2 In the way of integrating the heterogeneous fiber reference arm interferometer shown in (b), the heterogeneous fiber coupler, the reference arm ultra-short and ultra-thin fiber 8 and the reference arm mirror 2 are flatly fixed and integrally encapsulated by using a structure and / or colloid, so as to improve the performance and reliability of the reference arm. At the same time, in step S3, directly installing this encapsulated structure can improve the manufacturing efficiency.

[0079] Herein, the product process implementation, performance testing, etc. of the fiber optic hydrophone of the present invention are introduced in more detail as follows:

[0080] 1. Construction of ultra-thin fiber heterogeneous interferometer:

[0081] The ultra-thin fiber heterogeneous interferometer is constructed by improving on the basis of the conventional interferometer structure shown in Figure 1 Parallel fusion tapering is performed on a thin-diameter small-mode field fiber such as a φ160μm / M7μm fiber and an ultra-thin-diameter ultra-small-mode field fiber such as a φ100μm / M5.5μm fiber to realize an optical fiber coupling structure with an equal splitting ratio by taking the splitting ratio as a monitoring parameter. Miniaturized reliability packaging protection with functions such as sealing, vibration isolation, dehumidification, watertightness, and pressure resistance is carried out on it, which at least includes two layers of quartz / glass tubes and metal tubes. An encapsulated heterogeneous fiber coupler 6 is formed. Its input optical fiber includes a thin-diameter input optical fiber 3 and an ultra-thin-diameter input optical fiber 7. The output end also includes an ultra-thin fiber of the sensing arm 9 for signal sensing, and the other thin-diameter optical fiber is designed as the reference arm ultra-short and ultra-thin fiber 8, usually with a length of several mm, for the phase reference of optical interference. Small mirrors 2 are made at the ends of the two output optical fibers to reflect the two optical signals to the heterogeneous fiber coupler 6 to form interference, and the mirrors are also subjected to miniaturized packaging protection with the same functions as the coupler.

[0082] When the heterogeneous fiber coupler 6 and the reference arm mirror 2 are integrally packaged, one independent packaging device can be reduced, and only the size of the original coupler needs to be slightly lengthened and thickened to form a heterogeneous fiber reference integrated coupler 10.

[0083] According to the above packaging process, the Figure 3 heterogeneous fiber interferometer structure is constructed, in which the input optical fiber shown includes two paths of a thin-diameter input optical fiber 3 and an ultra-thin-diameter input optical fiber 7, the heterogeneous fiber coupler 6 is a thin-ultra-thin fiber heterogeneous configuration, the ultra-thin fiber of the sensing arm 9 is an ultra-thin fiber dozens of meters long, the reference arm ultra-short and ultra-thin fiber 8 is a thin-diameter fiber several mm long, and a mirror 2 is integrated at the tail end.

[0084] Its optical path works as follows: Light enters through the thin - diameter input optical fiber 3, is equally split by the heterogeneous fiber coupler 6. One path goes to the reference - arm ultra - short thin - diameter optical fiber 8, is reflected back to the heterogeneous fiber coupler 6 at the reference - arm mirror 2, and the other path goes to the sensing - arm ultra - fine - diameter optical fiber 9, also returns to the heterogeneous fiber coupler 6 through the sensing - arm mirror 2. Through time - delay matching, an optical interference signal is formed and output through the thin - diameter input optical fiber 3. The ultra - fine - diameter input optical fiber 7 is mainly used for application backup.

[0085] 2. Process implementation of the ultra - fine - diameter fiber hydrophone:

[0086] Install the two sensitizing cylinder shells 13 in the fiber - optic hydrophone structure in pairs onto the mandrel 12, and achieve the fastening and sealing between their contact surfaces through gluing and other methods, forming a sealed sensitizing cavity 14. At this time, the two sensitizing cylinder shells 13 are completely mirror - symmetric with respect to the left - right symmetry plane 17 and the up - down symmetry plane 18 respectively.

[0087] Fix the heterogeneous fiber coupler 6, the reference - arm ultra - short thin - diameter optical fiber 8, and the reference - arm mirror 2 flatly in the channel from the hollow shaft 16 to the middle cavity 15 of the fiber - optic hydrophone mandrel 12 to ensure the reliability of the reference arm 8. Mark the equal - division points on the sensing - arm ultra - fine - diameter optical fiber 9, lay it along the guiding mechanism of the mandrel 12 onto one of the sensitizing cylinder shells 13, and use zero - tension fiber winding. When reaching the equal - division point, guide the other half of the optical fiber to the other sensitizing cylinder shell 13 and wind the wire in the same way as the symmetric - side sensitizing cylinder shell 13, forming the ultra - fine - diameter fiber sensing coil layer 19. The wound ultra - fine - diameter fiber sensing coil layer 19 is mirror - stacked with respect to the left - right symmetry plane 17 and the up - down symmetry plane 18 respectively. Fix the sensing - arm mirror 2 in the middle cavity 15 of the mandrel and fix it by gluing and other methods, then cover the cover 20 of the middle - cavity encapsulation box.

[0088] Seal and protect the hollow shafts 16 at both ends of the mandrel 12, including the hollow seal 22 at the fiber - entry end of the mandrel and the hollow seal 21 at the tail end of the mandrel, to ensure the isolation between the inner and outer cavities of the mandrel 12. Conduct encapsulation protection outside the above - mentioned fiber - optic hydrophone structure, and use a low - water - permeability and impact - resistant colloid to form an external potting layer 23, thus obtaining Figure 5 the complete fiber - optic hydrophone shown in (d).

[0089] 3. Performance testing of the ultra - fine - diameter fiber hydrophone:

[0090] The performance testing of ultra-fine fiber hydrophones primarily focuses on the overall optical insertion loss after the ring packaging, the insertion loss balance between the two arms, the optical path difference between the arms, as well as the acoustic pressure sensitivity and acceleration sensitivity. By using various test devices and systems, spliced or connected to the thin input fiber 3, the corresponding optical signals (such as DC, pulsed, narrow-linewidth, broadband, externally loaded acoustic and vibration signals) are input. The return optical signals are then received, and various physical parameters are directly or indirectly obtained to evaluate the hydrophone's performance.

[0091] In summary, the present invention utilizes a device-based fiber interferometer structure. Taking into account factors such as winding reliability, low light leakage, and the applicability of splicing processes and tools for fine-diameter optical fibers, a design employs parallel fusion tapering of two optical fibers to form a heterogeneous coupler. This allows the fine-diameter fiber at the coupler's input to be spliced using established processes, while the ultra-fine-diameter fiber in the sensor arm at the output can meet the requirements for reliable, low-loss ultra-fine-diameter winding. Furthermore, focusing on the thinning and noise-resistant packaging structure of the hydrophone, the interferometer components are designed to be packaged in a short and slender package, while the reference arm's fine-diameter fiber adopts an ultra-short design. This reduces overlap and juxtaposition between packaged components, allowing the interferometer components to be installed in a smaller hydrophone structure, thus enabling the fabrication of an ultra-fine hydrophone.

[0092] This hydrophone utilizes ultra-fine fiber, allowing for more fiber turns per unit length of the sensitivity-enhancing column. This thinner layer also reduces the binding force on the housing, resulting in a lower housing stiffness, thus ensuring the high sensitivity of the thin-diameter fiber-optic hydrophone. Furthermore, the hydrophone utilizes a mirrored design, with the sensing fiber equally wound around two identical sensitivity-enhancing structures, which are mounted symmetrically on the core shaft. This provides excellent resistance to acceleration noise.

[0093] Therefore, the ultra-fine hydrophone designed in the present invention has similar performance effects as the fine-diameter hydrophone in terms of sensitivity, reliability, noise, insertion loss, etc. while greatly reducing the diameter size. It can well support the application in light, thin and slender sensor arrays. It is expected to greatly reduce the radial size of the linear array and break through the limitations of the aperture due to volume and weight, and enhance the application capabilities of the fiber-optic hydrophone system in underwater acoustic detection, underwater environmental research, seabed mineral exploration and other fields.

[0094] It should be noted that the above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Equivalent modifications made based on the above embodiments fall within the scope of protection of the present invention.

Claims

1. An ultra-fine diameter heterogeneous optical fiber hydrophone, characterized by: The invention comprises a core shaft, a sensitivity-enhancing column shell and a heterogeneous fiber interferometer, wherein a cavity is provided through the core shaft, sensitivity-enhancing column shells are symmetrically provided on both sides of the core shaft, a heterogeneous fiber interferometer is installed in the cavity of the core shaft, and the heterogeneous fiber interferometer comprises a heterogeneous fiber coupler, the input end of the heterogeneous fiber coupler is respectively connected to the thin-diameter input fiber and the ultra-fine-diameter input fiber, the output end of the heterogeneous fiber coupler is respectively connected to the ultra-short-diameter fiber of the reference arm and the ultra-fine-diameter fiber of the sensing arm, the ultra-short-diameter fiber of the reference arm is connected to the reference arm reflector, the ultra-fine-diameter fiber of the sensing arm is connected to the sensing arm reflector, and the heterogeneous fiber coupler, the ultra-short-diameter fiber of the reference arm and the reference arm reflector are fixed flatly in the cavity, the ultra-fine-diameter fiber of the sensing arm extends from the cavity and is connected to the sensitivity-enhancing column shells on both sides. The optical fiber is evenly wound on the cylindrical shell, the end of the ultra-fine optical fiber of the sensing arm is connected to the sensing arm reflector fixed on the middle section of the cavity, the middle section of the cavity is provided with a packaging box cover, and the core shaft, the packaging box cover and the sensitivity-enhancing cylindrical shell are all provided with an external potting layer; the diameter of the thin-diameter input optical fiber is greater than 120μm, the diameter of the ultra-fine-diameter input optical fiber is less than 120μm, the diameter of the ultra-short-diameter optical fiber of the reference arm is greater than 120μm, the length of the ultra-short-diameter optical fiber of the reference arm is within 5mm, the diameter of the ultra-fine-diameter optical fiber of the sensing arm is less than 120μm, the length of the ultra-fine-diameter optical fiber of the sensing arm is 10m-30m, and the mode field of the thin-diameter input optical fiber and the ultra-short-diameter optical fiber of the reference arm is greater than 6.0μm, and the mode field of the ultra-fine-diameter input optical fiber and the ultra-fine-diameter optical fiber of the sensing arm is less than 6.0μm.

2. The ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 1, characterized in that: The middle section of the core shaft is provided with a middle section cavity, both sides of the core shaft are hollow shafts, the middle section cavity is larger than the cavity of the hollow shaft and is interconnected.

3. The ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 1, characterized in that: The outer layer of the core shaft is a metal tube, and the inner layer of the core shaft is a quartz tube or a glass tube.

4. The ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 1, characterized in that: The sensitivity-enhancing column shell is composed of two sensitivity-enhancing column shell bodies that are symmetrically installed up and down, and a sensitivity-enhancing cavity is formed between the sensitivity-enhancing column shell bodies and the core shaft.

5. The ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 1, characterized in that: The core shaft and the sensitivity-enhancing column shell are provided with a guide channel for the ultra-fine optical fiber of the sensing arm to extend and be wound.

6. The ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 1, characterized in that: The heterogeneous optical fiber coupler, the reference arm ultra-short thin-diameter optical fiber and the reference arm reflector are an integrated packaging structure.

7. A process for manufacturing the ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 1, characterized in that: The steps include: S1. Prepare a core shaft, a sensitivity-enhancing column shell, and a heterogeneous fiber interferometer respectively, wherein a cavity is horizontally provided in the core shaft, each sensitivity-enhancing column shell includes two sensitivity-enhancing column shell bodies, and the heterogeneous fiber interferometer includes a heterogeneous fiber coupler, connecting the input end of the heterogeneous fiber coupler to the thin-diameter input fiber and the ultra-fine-diameter input fiber, and connecting the output end of the heterogeneous fiber coupler to the ultra-short-diameter fiber of the reference arm and the ultra-fine-diameter fiber of the sensing arm respectively, connecting the ultra-short-diameter fiber of the reference arm to the reference arm reflector, and connecting the ultra-fine-diameter fiber of the sensing arm to the reflector of the sensing arm; S2. Fix the sensitivity-enhancing column shell on both sides of the core shaft by means of structure or gluing, so that a closed sensitivity-enhancing cavity is formed between the sensitivity-enhancing column shell and the core shaft; S3. Fix the heterogeneous fiber coupler, the reference arm ultra-short thin fiber, and the reference arm reflector flatly in the cavity of the core shaft, wind the sensing arm ultra-fine fiber evenly around the sensitivity-enhancing cylinder shells on both sides, fix the sensing arm reflector at the end of the sensing arm ultra-fine fiber to the middle section of the cavity by means of structure or adhesive, and cover the packaging box lid; S4. The two ends of the cavity of the core shaft are sealed, and a colloid is used to encapsulate the outer surface of the optical fiber hydrophone structure to form an external potting layer to obtain a complete optical fiber hydrophone.

8. The process for manufacturing an ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 7, characterized in that: In the step S1, the heterogeneous fiber coupler, the reference arm ultra-short thin-diameter fiber, and the reference arm reflector are packaged into an integrated structure through colloid.

9. The process for manufacturing an ultra-fine diameter heterogeneous optical fiber hydrophone according to claim 7, characterized in that: In step S3, the ultra-fine optical fiber of the sensing arm is marked with an equal-division point, and is laid out on one of the sensitivity-enhancing column shells along the guiding mechanism of the core axis. The fiber is wound using zero tension, and when it reaches the equal-division point, the other half of the optical fiber is guided to the other sensitivity-enhancing column shell and wound in the same manner as the sensitivity-enhancing column shell on the symmetrical side to form an ultra-fine optical fiber sensing coil layer.

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