Sensitivity-adjustable fiber-optic spr curvature sensor and method of making use thereof

Abstract

The application belongs to the field of optical fiber sensing, and mainly relates to a sensitivity-adjustable optical fiber SPR curvature sensor and a manufacturing method thereof. The optical fiber SPR curvature sensor comprises sequentially arranged light injection areas, modulation areas, sensing areas and light receiving areas. The light injection areas are longitudinal and horizontal core optical fibers, the modulation areas and the light receiving areas are gradient refractive index optical fibers, and the sensing areas are gradient refractive index optical fibers with flat-bottomed grooves. The modulation areas, the sensing areas and the light receiving areas are located on the same gradient refractive index optical fiber. The light signal is injected into the fiber core of the modulation area from the light injection area. After modulation, the light beam transmission track in the fiber core is approximately a cosine function, effectively making the transmission light mode in the fiber core single. The light beam reaches the groove and contacts the metal film covering the surface of the groove to generate the SPR effect. Meanwhile, the depth of the groove and the light injection fiber core are adjusted to change the SPR incident angle and adjust the curvature sensing sensitivity and linearity. The application solves the problems of low sensitivity, difficulty in adjusting sensitivity, linearity, multi-channel and direction recognition of the current optical fiber SPR curvature sensor.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensors, specifically relating to a fiber optic SPR curvature sensor with adjustable sensitivity and its manufacturing and usage methods. Background Technology

[0002] The proposed fiber optic SPR curvature sensors are either step-index multimode fiber core-type or fiber cladding-type. A sensing metal film is deposited on the surface of the step-index multimode fiber core or cladding. When light enters the fiber core or cladding, multiple higher-order modes of the transmitted light contact the metal film, resulting in SPR (Strain Resonance) and generating an SPR resonance valley in the received spectrum. When the fiber bends, the angles between each mode of the transmitted light and the metal film change (but the initial angles between each mode and the metal film are different, and the magnitude of the change is also different). The wavelength of the SPR resonance valley shifts (due to the combined superposition of the shifts of each mode), and the curvature value can be demodulated from the shift in the resonance wavelength. Some researchers have constructed fiber optic SPR curvature sensors by etching step-index multimode fiber and depositing a sensing film on the fiber core as a bending sensing region, achieving a sensitivity of 1.50 nm / m. -1 (SU Y, WEI Y, ZHANG Y, et al. Surface-Plasmon-Resonance-Based Optical Fiber Curvature Sensor with Temperature Compensation by Means of Dual Modulation Method[J]. Sensors(Basel), 2018, 18(8).). Furthermore, a sensing semi-film was deposited on the upper surface of the core of a step-index multimode fiber to achieve the detection of curvature and bending direction (WEI Y, HU J, LIU C, et al. Fiber semi-film SPR curvature sensor with the function of directional recognition[J]. Results in Optics, 2020, 1.), with a sensitivity of 0.71 nm / m. -1Researchers have also used asymmetrical D-type multimode fiber to achieve fiber optic SPR multichannel curvature sensing and bending direction recognition (WEI Y, LIU C, LIU C, et al. Multichannel Directional Recognition SPR Curvature Sensor Based on D-Type Double-Clad Multimode Fiber[J].IEEE Sensors Journal,2022,22(22):21719-26.) with a sensitivity of 0.61 nm / m. -1 Researchers have constructed a fiber optic SPR curvature sensor with a heterogeneous core structure (LIU C, HU J, WEI Y, et al. Fiber cladding SPRbending sensor characterized by two parameters[J]. Chinese Physics B, 2020, 29(12).), with a maximum wavelength sensitivity of 0.76 nm / m. -1 Researchers have proposed depositing a sensing film on the cladding surface of a single-mode fiber and using bending to couple light from the fiber core into the fiber cladding, thus realizing fiber cladding-type SPR curvature sensing (WEI Y, LIU C, LIU C, et al. Single-mode fiber curvature sensor based on SPR[J]. Appl Opt, 2022, 61(15): 4620-6.), with a maximum sensitivity of 0.26 nm / m. -1 .

[0003] The proposed fiber optic SPR curvature sensors involve full-light injection into the core or cladding of a step-index multimode fiber. The transmitted light propagates within the core or cladding, exciting different modes of light. Numerous SPR modes occur, each with a different SPR incident angle and corresponding SPR resonance valley wavelength. The overall SPR resonance valley is ultimately formed by the superposition of resonance valleys generated by each mode, resulting in a relatively wide valley. When the fiber bends, the SPR incident angle at which each mode of light contacts the sensing membrane changes, thus altering the wavelength corresponding to the resonance valley. However, the SPR incident angle differs for different modes. During bending, for the same angle change, the amount of SPR resonance valley movement varies for different modes. The overall movement of the SPR resonance valley with bending is a combined result of the superposition of the movements of SPR resonance valleys from each mode, leading to the relatively low sensitivity of current fiber optic SPR curvature sensors. Meanwhile, since different SPR incident angles have different sensitivities, but only knowing the changing trend of the total reflection angle between each transmission mode and the sensing film is insufficient to single out the mode or control the total reflection angle between the transmitted light and the sensing film, thus preventing sensitivity adjustment. Therefore, it is necessary to develop a novel fiber optic SPR curvature sensor with high curvature sensitivity and adjustable curvature sensitivity. Summary of the Invention

[0004] Therefore, the purpose of this invention is to provide an optical fiber SPR curvature sensor with a single transmission optical mode and adjustable sensitivity, as well as a method for its fabrication and use.

[0005] The fiber optic SPR curvature sensor with adjustable sensitivity includes, from left to right, a light-injection area, a modulation area, a sensing area, and a light-receiving area. The modulation area, sensing area, and light-receiving area are located on the same graded-index fiber, and the right end of the light-injection area and the left end of the modulation area are fused together opposite each other.

[0006] The light-focusing region is a cross-core fiber with seven quartz cores, each 5 μm in diameter. In this context, a cross-core fiber refers to multiple quartz cores arranged side-by-side in an L-shaped cross-section. One core is positioned along the fiber axis, three cores are arranged vertically upwards, and three cores are arranged horizontally to the right. The distance between two adjacent quartz cores on the same axis is 10 μm, and the quartz cladding diameter is 125 μm. The modulation region is a graded-index fiber with a core diameter of 105 μm, a quartz cladding diameter of 125 μm, a numerical aperture of 0.3, and a length of 1000 μm, which is the length of two / three self-focusing effect cycles of light propagation in the graded-index fiber. The sensing area is a graded-index multimode optical fiber with a first groove. The first groove is a flat-bottomed groove, and its base is a quartz core of the graded-index optical fiber. The length of the first groove is 1000 μm, its depth is 56 μm, and its opening direction is vertically upward. The surface of the first groove is coated with a first sensing metal film with a thickness of 50 nm. The surface of the first sensing metal film is coated with a first UV-curable adhesive with a refractive index of 1.33 using an optical fiber coating machine, so that the diameter of the coated optical fiber reaches 250 μm, which is consistent with the diameter of the coating layer. The light-receiving area is a graded-index optical fiber with a plastic coating layer diameter of 250 μm and a length greater than 0.5 m.

[0007] The sensing principle of a fiber optic SPR curvature sensor with adjustable sensitivity: A single-mode optical signal is emitted from the injection zone into the fiber core of the modulation zone. After modulation, the beam's propagation trajectory within the core approximates a cosine function, effectively ensuring a single transmission mode. The beam reaches the first groove sensing area and contacts the metal film covering the groove surface, resulting in surface plasmon resonance (SPR). It then continues to propagate forward, converging in the receiving area before being transmitted to a spectrometer for processing. When the groove sensing area bends, the SPR incident angle changes, causing the SPR resonance valley on the sensing spectrum to shift. This establishes a relationship between the amount of bending and the SPR resonance wavelength, achieving curvature sensing. Conversely, when the groove bends inward, the SPR incident angle increases, and the SPR resonance valley shifts towards shorter wavelengths; when it bends outward, the SPR incident angle decreases, and the SPR resonance valley shifts towards longer wavelengths. The different directions of SPR resonance valley movement are used to identify the bending direction.

[0008] The sensitivity adjustment principle of the fiber optic SPR curvature sensor is as follows: There are two sensitivity adjustment methods. In the first method, the depth of the first groove is adjustable, ranging from 10 to 62.5 μm. When the light-emitting position of the illumination area is fixed (i.e., the transmission trajectory of the cosine beam remains unchanged), as the groove depth gradually increases, the SPR incident angle between the modulated cosine beam and the sensing film on the groove surface increases. At this point, bending the groove sensing area reduces the curvature sensitivity. Therefore, the SPR incident angle of the groove sensing area increases with increasing groove depth, and the curvature sensitivity decreases accordingly; conversely, the SPR incident angle decreases with decreasing groove depth, and the curvature sensitivity increases accordingly. Simultaneously, the linearity of the sensor's curvature line decreases with increasing groove depth. Therefore, by setting different groove depths, the sensor's sensitivity and linearity can be adjusted.

[0009] The second sensitivity adjustment method involves changing the light-emitting position of the illumination area when the groove depth remains constant. This involves emitting light to different cores of the cross-core fiber in the illumination area, thus altering the cosine path of the beam transmission. When light is transmitted to a core with a larger eccentricity in the illumination area, the amplitude of the cosine beam is larger, resulting in a smaller SPR incident angle when it contacts the sensing gold film on the groove surface. This leads to a greater curvature sensitivity when the groove is bent. Therefore, controlling the distance between the injection fiber core and the central axis of the fiber controls the SPR incident angle, thereby adjusting the curvature sensitivity of the sensor. A smaller SPR incident angle in the sensing area corresponds to a higher curvature sensing sensitivity but lower linearity. The linearity of the sensor can be adjusted by changing the position of the injection fiber core. Alternatively, both the position of the injection fiber core and the groove depth can be changed to jointly adjust the curvature sensitivity.

[0010] The method for fabricating the aforementioned sensitivity-adjustable fiber optic SPR curvature sensor includes the following steps:

[0011] S1 Take a 1m long section of graded refractive index multimode fiber, strip the coating layer with a length of 5cm in the middle of the fiber, wipe it with alcohol, clamp the fiber on the three-dimensional micro-motion stage fixture, keep the fiber horizontal, adjust the micro-motion stage to place the bare fiber directly below the CO2 laser beam, set the parameters of the CO2 laser, and after processing the fiber, form a flat-bottomed groove with a length of 1mm and a depth of 56μm.

[0012] S2 places the grooved graded refractive index fiber into the fiber optic dicing system and rotates the helical differential head to make the dicing blade cut 1000μm in front of the planar groove structure.

[0013] S3 cuts the graded-index fiber with a groove structure to a fixed length, places it into the fiber clamp of the polarization-maintaining fusion splicer, adjusts the orientation of the groove under a microscope so that the groove opening faces vertically upward and fixes it, and then places it into the right end of the polarization-maintaining fusion splicer.

[0014] S4 places the flattened cross-core fiber into the fiber end face micro-positioning device of the polarization-maintaining fusion splicer to observe the fiber end face. Rotate the fiber clamp so that the fiber cores on the two axial surfaces of the cross-core fiber are vertically upward and horizontally to the left, respectively, and lock the fiber clamp. Then place the fixed cross-core fiber and fiber clamp into the left end of the polarization-maintaining fusion splicer, and finally fusion splice it directly opposite the left end of the modulation zone of the graded refractive index fiber.

[0015] S5 deposits a 50nm gold film on the groove area using a plasma sputtering machine, and then coats it with a first UV-curable adhesive with a refractive index of 1.33 using an optical fiber coating machine, so that the diameter of the coated optical fiber reaches 250μm, which is consistent with the diameter of the coating layer.

[0016] The specific usage method of the aforementioned adjustable-sensitivity fiber optic SPR curvature sensor is as follows:

[0017] S1 First, use the fiber end face micro positioning device of the polarization maintaining fusion splicer to observe the left end face of the cross-sectional cored fiber. Rotate the fiber clamp so that the fiber cores on the two axial surfaces of the cross-sectional cored fiber are vertically upward and horizontally to the right, respectively. Then lock the fiber clamp and place the fiber clamp with the cross-sectional cored fiber fixed into the right end of the three-dimensional micro-motion stage.

[0018] S2 places a single-mode fiber with a flat end face and a diameter of 4μm into the left end of the three-dimensional micro-motion stage, and modulates the three-dimensional micro-motion stage so that the core of the single-mode fiber is aligned with the middle core of the cross-shaped fiber.

[0019] S3 Then, the groove sensing area is clamped directly below the cylindrical microscope. The clamp is rotated so that the groove opening is vertically upward. The fiber optic bending sensing area is clamped by the fiber optic clamping rod fixed on the lifting platform. When the lifting platform is moved up, the groove sensing area is bent outward. When the lifting platform is moved up, the groove sensing area is bent inward. This achieves bending in different directions.

[0020] S4 controls the single-mode fiber on the left side of the three-dimensional micro-motion stage to move vertically or horizontally to the right, with each movement being 10μm. Light is injected into different cores of the cross-shaped fiber. After light passes through different cores, the SPR incident angle generated with the groove plane is different, so the bending groove sensing area has different curvature sensitivity. The closer the core is to the outside, the smaller the SPR incident angle generated with the groove plane. Therefore, the curvature sensitivity increases with the distance of the off-core.

[0021] As a preferred embodiment, this application also provides a sensitivity-adjustable fiber optic SPR dual-channel curvature sensor. Its overall structure is basically the same as the aforementioned sensitivity-adjustable fiber optic SPR curvature sensor, with the main difference being that the sensing area includes a first sensing area and a second sensing area. Specifically, the light-focusing area is a cross-core fiber with seven quartz cores, each with a diameter of 5 μm, arranged in an L-shape. One core is positioned along the fiber axis, three cores are arranged upwards along the longitudinal axis, and three cores are arranged to the right along the transverse axis. The distance between two adjacent quartz cores on the same axis is 10 μm, and the quartz cladding diameter is 125 μm. The modulation area is a graded-index fiber with a core diameter of 105 μm, a quartz cladding diameter of 125 μm, a numerical aperture of 0.3, and a length of 1000 μm, which is the length of two / three self-focusing effect cycles of light propagation in the graded-index fiber. The first sensing area is a graded-index multimode optical fiber with a first groove. The first groove is a flat-bottomed groove, and the substrate of the groove is the quartz core of the graded-index optical fiber. The length of the first groove is 1000 μm, the depth is 56 μm, and the opening direction of the first groove is vertically upward. A first sensing metal film with a thickness of 50 nm is deposited on the surface of the first groove. The surface of the first sensing metal film is coated with a first UV-curable adhesive with a refractive index of 1.33 using an optical fiber coating machine, so that the diameter of the coated optical fiber reaches 250 μm, which is the same as the diameter of the coating layer. The second sensing area is... A graded-index multimode optical fiber with a second groove, the distance between the second groove and the first groove being greater than 10 cm, the second groove being a flat-bottomed groove, the base of the groove being a quartz core of the graded-index optical fiber, the length of the second groove being 1000 μm and the depth being 56 μm, the opening direction of the second groove being vertically upward, the surface of the second groove being coated with a second sensing metal film with a thickness of 50 nm, the surface of the second sensing metal film being coated with a second UV-curable adhesive with a refractive index of 1.39 using an optical fiber coating machine, so that the diameter of the coated optical fiber reaches 250 μm, consistent with the diameter of the coating layer. The light receiving area is a graded-index optical fiber, its plastic coating layer having a diameter of 250 μm and a length greater than 0.5 m.

[0022] A sensitivity-tunable fiber optic SPR dual-channel curvature sensor is disclosed. Its dual-channel curvature sensing principle is as follows: A single-mode beam from the longitudinal plane of a cross-core fiber is incident into the fiber core of the modulation zone. After modulation, the beam's propagation trajectory within the core approximates a cosine function. The beam contacts the first groove sensing area at a certain angle, and then reacts with a metal film covering the surface of the first groove, resulting in surface plasmon resonance (SPR). The beam then continues propagating forward to the second groove, where it reacts again with a metal film covering the surface of the second groove, also resulting in SPR. Finally, the beam converges in the receiving area and is transmitted to a spectrometer for processing. By using UV-curable adhesives with different refractive indices (1.33 for the first groove and 1.39 for the second groove) to coat the surfaces of the first and second grooves, two SPR resonance valleys are generated in the sensing spectrum, located in different wavelength bands. The SPR resonance valley generated by the first groove is closer to shorter wavelengths, while the SPR resonance valley generated by the second groove is closer to longer wavelengths. When the two grooves bend, the corresponding SPR resonance valleys shift. The curvature values ​​of the corresponding sensing areas are demodulated based on the amount of movement of different resonance valleys, thus realizing dual-channel curvature measurement.

[0023] As a preferred embodiment, this application also provides a sensitivity-tunable fiber optic SPR two-dimensional curvature sensor, whose overall structure is basically the same as the aforementioned sensitivity-tunable fiber optic SPR curvature sensor. The main difference lies in that the sensing area includes a first sensing area and a second sensing area. Specifically, the light-focusing area is a cross-core fiber with a total of 7 quartz cores, each with a diameter of 5 μm, arranged in an L-shape. One core is located along the fiber axis, three cores are arranged upwards along the longitudinal axis, and three cores are arranged to the right along the transverse axis. The distance between two adjacent quartz cores on the same axis is 10 μm, and the quartz cladding diameter is 125 μm. The modulation area is a graded-index fiber with a core diameter of 105 μm, a quartz cladding diameter of 125 μm, a numerical aperture of 0.3, and a length of 1000 μm, which is the length of 2 / 3 of the self-focusing effect cycle of light propagation in the graded-index fiber. The first sensing area is a graded-index multimode optical fiber with a first groove. The first groove is a flat-bottomed groove, and the substrate of the groove is the quartz core of the graded-index optical fiber. The length of the first groove is 1000 μm, the depth is 56 μm, and the opening direction of the first groove is vertically upward. A first sensing metal film with a thickness of 50 nm is deposited on the surface of the first groove. A first UV-curable adhesive with a refractive index of 1.33 is coated on the surface of the first sensing metal film using an optical fiber coating machine, so that the diameter of the coated optical fiber reaches 250 μm, which is the same as the diameter of the coating layer. The second sensing area is a multimode optical fiber with a graded-index multimode fiber. The second groove is a graded-index multimode fiber. The horizontal distance between the second groove and the first groove is 1000 μm. The second groove is a flat-bottomed groove, and its substrate is a quartz core of the graded-index fiber. The length of the second groove is 1000 μm, and its depth is 56 μm. The opening direction of the second groove is horizontal and forward. A second sensing metal film with a thickness of 50 nm is deposited on the surface of the second sensing metal film. A second UV-curable adhesive with a refractive index of 1.39 is coated on the surface of the second sensing metal film using an optical fiber coating machine, so that the diameter of the coated fiber reaches 250 μm, which is consistent with the diameter of the coating layer. The light-receiving area is a graded-index fiber with a plastic coating layer diameter of 250 μm and a length greater than 0.5 m.

[0024] The sensitivity-adjustable fiber optic SPR two-dimensional curvature sensor operates on the following principle: Light is simultaneously transmitted to one single-mode fiber core on both the longitudinal and transverse planes of the fiber, allowing two single-mode beams from the injection region to simultaneously enter the fiber core in the modulation region. After modulation, the beam propagation trajectory within the fiber core approximates a cosine function, and the two cosine beams propagate perpendicularly to each other. The beam on the longitudinal plane reaches the first groove sensing region, where it contacts the metal film covering the surface of the first groove, resulting in surface plasmon resonance (SPR). The beam on the transverse plane reaches the second groove sensing region, where it contacts the metal film covering the surface of the second groove, also resulting in surface plasmon resonance (SPR). The beams then continue propagating forward, converging in the receiving region within the fiber core, and finally being transmitted to a spectrometer for processing. Because the refractive index environments surrounding the two groove sensing areas are different, they correspond to two different SPR resonance valleys in the sensing spectrum (where the groove is covered with a UV-cured adhesive with a low refractive index, and the SPR resonance wavelength is closer to the short wavelength; and the groove is covered with a UV-cured adhesive with a high refractive index, and the SPR resonance wavelength is closer to the long wavelength). When the optical fiber bends longitudinally, the first groove bulges outward and inward; when the optical fiber bends laterally, the second groove bulges outward and inward. The two SPR resonance valleys in the sensing spectrum move respectively. By detecting the amount and direction of movement of the two SPR resonance valleys, the curvature in different dimensions can be detected, and the bending direction can be determined, thus realizing two-dimensional curvature sensing.

[0025] The beneficial effects of this invention are as follows:

[0026] 1. By utilizing the self-focusing effect of graded-index fiber, single-mode light is injected at the non-central axis of the incident end face of the graded-index fiber, so that the beam in the core of the sensing graded-index fiber is transmitted along a cosine path. The transmission mode is singular, and the resulting resonance valley has a small half-width at half-maximum. Therefore, when the fiber is bent, the SPR incident angle of the single beam transmission mode is consistent, making the overall resonance valley move significantly and improving the sensitivity of the fiber SPR curvature sensor.

[0027] 2. By using single-mode fiber to transmit light through the cores of cross-sectional fiber with different eccentric distances, the amplitude of the cosine transmission beam in the core of the sensing graded refractive index fiber can be changed, thereby changing the SPR incident angle at the groove sensing area. This results in different curvature sensitivities when the fiber is bent, thus adjusting the curvature sensitivity and linearity of the sensor.

[0028] 3. By changing the depth of the groove sensing area on the graded refractive index fiber, the SPR incident angle at the groove sensing area can be changed, resulting in different curvature sensitivities when the fiber is bent, thus achieving adjustment of curvature sensitivity and detection range.

[0029] 4. By etching two groove sensing areas at different positions along the axis of the graded refractive index fiber in the same direction, and by controlling the SPR incident angle and applying different external refractive index environments to the groove sensing areas, the operating range of the resonant wavelength can be adjusted, thus realizing a wavelength division multiplexing dual-channel fiber SPR curvature sensor.

[0030] 5. On two mutually perpendicular axial surfaces of a graded refractive index fiber, groove sensing areas are etched in close proximity. By controlling the SPR incident angle of the groove sensing area and applying different external refractive index environments to the groove sensing area, the working range of the resonant wavelength is adjusted to realize a fiber SPR curvature sensor for wavelength division multiplexing two-dimensional curvature sensing. Attached Figure Description

[0031] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the following figures are provided for illustration:

[0032] Figure 1 A schematic diagram of a fiber optic SPR curvature sensor with adjustable sensitivity.

[0033] Figure 2 Figure 1 shows cross-sectional views of different parts of a fiber optic SPR curvature sensor with adjustable sensitivity. Figure 2 shows a cross-sectional view of a four-core fiber optic cable corresponding to the AA' plane; Figure 3 shows a cross-sectional view of a graded multimode fiber corresponding to the BB' plane; and Figure 4 shows a cross-sectional view of the coating on the groove surface of the graded multimode fiber corresponding to the CC' plane.

[0034] Figure 3 A schematic diagram of a fiber optic SPR curvature sensor device with adjustable sensitivity for testing.

[0035] Figure 4 This is a schematic diagram of the light-transmitting SPR curvature sensor probe of a four-core optical fiber with different eccentric cores according to an embodiment of the present invention.

[0036] Figure 5 This is a schematic diagram of a sensor probe for SPR curvature sensor with different groove depths in a graded multimode fiber according to an embodiment of the present invention.

[0037] Figure 6 Data graph for testing a fiber optic SPR curvature sensor with adjustable sensitivity;

[0038] Figure 7 This is a schematic diagram of a fiber optic SPR dual-channel curvature sensor with adjustable sensitivity.

[0039] Figure 8 A schematic diagram of a fiber optic SPR two-dimensional curvature sensor probe with adjustable sensitivity. Detailed Implementation

[0040] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0041] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0042] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0043] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0044] like Figure 1 and Figure 2As shown, this embodiment relates to a fiber optic SPR curvature sensor with adjustable sensitivity. The overall structure includes: a light-injecting region 1, a modulation region 2, a sensing region 3, and a light-receiving region 4. The light-injecting region 1 consists of longitudinally and transversely arranged cored optical fibers, with a total of seven quartz fiber cores 1-1, each with a diameter of 5 μm, arranged in an L-shape. One quartz fiber core 1-1 is located along the fiber axis, three cores are arranged upwards along the longitudinal axis, and three cores are arranged to the right along the transverse axis. The spacing between two adjacent quartz fiber cores on the same axis is 10 μm. The quartz cladding 1-4 has a diameter of 125 μm, and the plastic coating layer 1-5 has a diameter of 250 μm. The modulation region is a graded-index optical fiber, with a quartz fiber core 2-1 having a diameter of 105 μm and a numerical aperture of 0.3, and a quartz cladding 2-2 having a diameter of 125 μm and a length of 1000 μm. The sensing area is a graded-index multimode optical fiber with a first groove 3-1. The first groove is a flat-bottomed groove, and the substrate of the groove is the quartz core of the graded-index optical fiber. The length of the first groove is 1000 μm and the depth is 56 μm. The opening direction of the first groove is vertically upward. The surface of the first groove is coated with a first sensing metal film 3-2 with a thickness of 50 nm. The surface of the first sensing metal film is coated with a first UV-curable adhesive 3-3 with a refractive index of 1.33 using an optical fiber coating machine. The light receiving area is a graded-index optical fiber, and its plastic coating layer 4-2 has a diameter of 250 μm and a length greater than 0.5 m.

[0045] like Figure 3 As shown, the test setup for the adjustable-sensitivity fiber optic SPR curvature sensor comprises: a broadband light source 9, a three-dimensional micro-motion stage 10, a lifting stage 11, a cylindrical microscope 12, and a spectrometer 13. A single-mode fiber with a flat end face and a diameter of 4 μm is placed at the left end of the three-dimensional micro-motion stage. The stage is modulated so that the core of the single-mode fiber is aligned with the middle core of the cross-core fiber. The grooved sensing area is then clamped directly below the cylindrical microscope. The clamp is rotated so that the groove opening is vertically upward, and the fiber bending sensing area is held in place by a fiber clamping rod fixed to the lifting stage. The left end of the single-mode fiber is connected to the broadband light source, and the right end of the graded-index fiber receiving area is connected to the spectrometer. The lifting stage is then controlled to achieve both outward and inward bending of the grooved sensing area.

[0046] The specific fabrication steps for a sensitivity-adjustable fiber optic SPR curvature sensor are as follows:

[0047] S1 takes a 1m long graded-index multimode optical fiber, strips off the coating layer in the middle of the fiber for 5cm, wipes it with alcohol, clamps the fiber on the three-dimensional micro-motion stage fixture, keeps the fiber horizontal, adjusts the micro-motion stage to place the bare fiber directly below the CO2 laser beam, sets the parameters of the CO2 laser (processing parameters are set to processing speed 800 mm / s, power 50%, frequency 5KHz), and after processing the fiber, a flat-bottomed groove with a length of 1mm and a depth of 56μm is formed.

[0048] S2 places the grooved graded refractive index fiber into the fiber optic dicing system and rotates the helical differential head to make the dicing blade cut 1000μm in front of the planar groove structure.

[0049] S3 cuts the graded-index fiber with a groove structure to a fixed length, places it into the fiber clamp of the polarization-maintaining fusion splicer, adjusts the orientation of the groove under a microscope so that the groove opening faces vertically upward and fixes it, and then places it into the right end of the polarization-maintaining fusion splicer.

[0050] S4 places the flattened cross-core fiber into the fiber end face micro-positioning device of the polarization-maintaining fusion splicer to observe the fiber end face. Rotate the fiber clamp so that the fiber cores on the two axial surfaces of the cross-core fiber are vertically upward and horizontally to the left, respectively, and lock the fiber clamp. Then place the fixed cross-core fiber and fiber clamp into the left end of the polarization-maintaining fusion splicer, and finally fusion splice it directly opposite the left end of the modulation zone of the graded refractive index fiber.

[0051] S5 deposits a 50nm gold film on the groove area using a plasma sputtering machine, and then coats it with a first UV-curable adhesive with a refractive index of 1.33 using an optical fiber coating machine, so that the diameter of the coated optical fiber reaches 250μm, which is consistent with the diameter of the coating layer.

[0052] The specific steps for using the adjustable-sensitivity fiber optic SPR curvature sensor are as follows:

[0053] S1 uses the fiber end face micro positioning device of the polarization maintaining fusion splicer to observe the left end face of the cross-core fiber. Rotate the fiber clamp so that the fiber cores on the two axial surfaces of the cross-core fiber are vertically upward and horizontally to the right, respectively. Lock the fiber clamp and then place the fiber clamp with the cross-core fiber fixed into the right end of the three-dimensional micro-motion stage 10.

[0054] S2 places a single-mode optical fiber with a flat end face and a diameter of 4μm into the left end of the three-dimensional micro-motion stage 10, and modulates the three-dimensional micro-motion stage so that the core of the single-mode optical fiber is aligned with the middle core of the cross-core optical fiber.

[0055] S3 Then, the groove sensing area is clamped directly below the cylindrical microscope 12, the clamp is rotated so that the groove opening is vertically upward, and the fiber bending sensing area is clamped by the fiber clamping rod fixed on the lifting platform 11.

[0056] The left end of the S4 single-mode fiber is connected to the broadband light source 9, and the right end of the graded refractive index fiber receiving area is connected to the spectrometer 13. Then, the lifting platform is controlled to realize the outward convex bending and inward concave bending of the groove sensing area, which corresponds to the SPR resonance valley on the sensing spectrum moving to a shorter wavelength and moving to a longer wavelength. The curvature value is demodulated according to the relationship between the wavelength shift and the curvature.

[0057] Method for adjusting the sensitivity and linearity of the S5 fiber optic SPR curvature sensor: Control the single-mode fiber on the left side of the three-dimensional micro-stage to move vertically or horizontally to the right, with each movement being 10 μm. Light is injected into different cores of the cross-core fiber, such as... Figure 4 As shown, different fiber cores, after being exposed to light, produce different SPR incident angles with the groove plane. Therefore, the curved groove sensing area has different curvature sensitivities. The closer the fiber core is to the outer edge, the smaller the SPR incident angle with the groove plane. Thus, the curvature sensitivity increases with the distance between the eccentric cores. Simultaneously, the linearity of the sensor decreases with increasing eccentric distance. Curvature sensitivity can be improved by using an eccentric fiber with a larger eccentric distance to fabricate the sensing probe, while the linearity of the curvature sensitivity can be improved by using an eccentric fiber with a smaller eccentric distance. Figure 5 As shown, the SPR incident angle generated by the groove plane of different depths is different, so the curved groove sensing area has different curvature sensitivity. The deeper the groove, the smaller the SPR incident angle and the greater the curvature sensitivity.

[0058] like Figure 6 The figure shows the actual test curves of the fiber optic SPR curvature sensor with adjustable sensitivity proposed in this patent. Light was transmitted through the fiber core with an eccentricity of 29 μm on the longitudinal plane of the cross-core fiber. Figure (a) shows the data when the groove sensing area bends in the -X direction (i.e., the groove bends inward). From the data, it can be concluded that the SPR resonant wavelength shifts to shorter wavelengths as the curvature increases, within a curvature range of 0-14.06 μm. -1 The wavelength shift range is 736.5-663.7 nm, and the average curvature sensitivity is 5.18 nm / m. -1 Figure (b) shows the data when the groove sensing area bends in the X direction (i.e., the groove bends outward). The data shows that the SPR resonant wavelength shifts to longer wavelengths as the curvature increases, within a curvature range of 0-14.06 m. -1 The wavelength shift range is 740.1-830.2 nm, and the average curvature sensitivity is 6.41 nm / m. -1 .

[0059] like Figure 7 As shown, this embodiment relates to a fiber optic SPR dual-channel curvature sensor with adjustable sensitivity, the overall of which is... Figure 1 The right side of the light-receiving area 4 of the central sensing structure is etched with the same second groove 5-1. The second groove opens vertically upward, has a length of 1000μm and a depth of 56μm. The distance between the second groove and the first groove is at least 10cm. The groove depth is 56μm. The surface of the groove is coated with a second sensing metal film 5-2 with a thickness of 50nm. The surface of the second sensing metal film is coated with a second UV-curable adhesive with a refractive index of 1.39.

[0060] The specific fabrication steps of a sensitivity-adjustable fiber optic SPR dual-channel curvature sensor are as follows:

[0061] S1 Take a 1m long graded-index multimode optical fiber, strip the 5cm coating layer in the middle of the fiber, wipe it with alcohol, and clamp the fiber on the three-dimensional micro-motion stage fixture to keep the fiber horizontal. Adjust the micro-motion stage to place the bare fiber directly below the CO2 laser beam. Set the parameters of the CO2 laser (processing parameters are set to processing speed 800 mm / s, power 50%, frequency 5KHz). Process the first groove 3-1 with a length of 1mm and a depth of 56μm on the fiber. Then move the three-dimensional micro-motion stage 20cm and place it directly below the CO2 laser beam. Process the second groove 5-1 with a length of 1mm and a depth of 56μm on the fiber.

[0062] S2 places the grooved graded refractive index fiber into the fiber optic slitting system and rotates the helical differential head to make the slitting blade cut 1000μm in front of the first groove structure.

[0063] S3. After the graded-index fiber with grooved structure has been cut to a fixed length, it is placed into the fiber clamp of the polarization-maintaining fusion splicer. Under a microscope, the orientation of the grooves is adjusted so that the openings of the first and second grooves face vertically upwards and then fixed. It is then placed into the right end of the polarization-maintaining fusion splicer. S4. Repeat step S4 above for fabricating the adjustable-sensitivity fiber SPR curvature sensor.

[0064] S5 deposits 50nm gold films on two groove regions using a plasma sputtering instrument. Then, it coats the first groove with a first UV-curable adhesive with a refractive index of 1.33 and the second groove with a second UV-curable adhesive with a refractive index of 1.39, so that the diameter of the coated optical fiber reaches 250μm, which is consistent with the diameter of the coating layer.

[0065] like Figure 8 As shown, this embodiment relates to a fiber optic SPR two-dimensional curvature sensor with adjustable sensitivity, the overall of which is... Figure 1 A second groove 7-1 with a different opening orientation is etched 1000 μm behind the light-receiving area 4 of the central sensing structure. The opening direction of the second groove is horizontal and forward, which is 90 degrees different from the opening direction of the first groove on the axial plane. The length of the second groove is 1000 μm and the depth is 56 μm. A second sensing metal film 7-2 with a thickness of 50 nm is deposited on the second groove. The second sensing metal film is covered with a second UV-curable adhesive with a refractive index of 1.39 using an optical fiber coating machine, so that the size of the sensing area is consistent with the original optical fiber size.

[0066] The specific fabrication steps for a sensitivity-adjustable fiber optic SPR two-dimensional curvature sensor are as follows:

[0067] S1. Take a 1m long graded-index multimode optical fiber, strip the 5cm coating layer in the middle of the fiber, wipe it with alcohol, and clamp the fiber on the three-dimensional micro-motion stage fixture to keep the fiber horizontal. Adjust the micro-motion stage to place the bare fiber directly under the CO2 laser beam. Set the parameters of the CO2 laser (processing parameters are set to processing speed 800 mm / s, power 50%, frequency 5KHz). Process the first groove 3-1 with a length of 1mm and a depth of 56μm on the fiber. Then move the three-dimensional micro-motion stage 1000μm. Rotate the fiber fixture on the three-dimensional micro-motion stage 90 degrees and place it directly under the CO2 laser beam. Process the second groove 7-1 with a length of 1mm and a depth of 56μm on the fiber.

[0068] S2 repeats the specific fabrication steps S2, S3, S4, and S5 of the above-described sensitive adjustable fiber optic SPR dual-channel curvature sensor.

[0069] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made to it in form and detail without departing from the scope defined by the claims of the present invention.

Claims

1. A fiber-optic SPR curvature sensor with adjustable sensitivity, characterized in that, The light injection area, the modulation area, the sensing area and the light receiving area are sequentially arranged, and the modulation area, the sensing area and the light receiving area are arranged on the same graded-index fiber, and the light injection area is opposite to the modulation area and is fused; The light injection area is a longitudinal and horizontal core fiber, and there are 7 L-shaped quartz cores, wherein 1 core is arranged on the axis of the fiber, 3 cores are arranged in the longitudinal direction, and 3 cores are arranged in the horizontal direction. The modulation area is a graded-index fiber, the core diameter is 105 μm, the quartz cladding diameter is 125 μm, the numerical aperture is 0.3, the plastic coating layer diameter is 250 μm, and the length is 1000 μm, which is the length of 2 / 3 self-focusing effect period of light transmission in the graded-index fiber. The sensing area is a graded-index multimode fiber with a first groove, the first groove is a flat-bottomed groove, the base of the groove is the quartz core of the graded-index fiber, the length of the first groove is 1000 μm, the depth is 56 μm, the opening direction of the first groove is vertically upward, the surface of the first groove is coated with a first sensing metal film with a thickness of 50 nm, and the surface of the first sensing metal film is coated with a first ultraviolet curing glue with a refractive index of 1.33 by a fiber coating machine, and the diameter of the coated fiber reaches 250 μm consistent with the coating layer diameter. The light receiving area is a graded-index fiber with a length greater than 0.5 m.

2. The sensitivity-adjustable fiber-optic SPR curvature sensor according to claim 1, characterized in that: The curvature sensitivity decreases with the increase of the groove depth, and the curvature linearity increases with the increase of the groove depth; different core light injection in the light injection area is used to adjust the sensitivity and linearity of the sensor, the greater the distance between the light injection core and the central axis of the fiber, the smaller the SPR incident angle of the sensing area, the greater the corresponding curvature sensing sensitivity, and the lower the linearity.

3. The sensitivity-adjustable fiber-optic SPR curvature sensor according to claim 1, characterized in that: The right side of the light receiving area is also provided with a second groove, the second groove is vertically upward, the length is 1000 μm, the depth is 56 μm, the distance between the second groove and the first groove is at least 10 cm, the groove depth is 56 μm, and the surface of the groove is coated with a second sensing metal film with a thickness of 50 nm, and the surface of the second sensing metal film is coated with a second ultraviolet curing glue with a refractive index of 1.

39.

4. The sensitivity-adjustable fiber-optic SPR curvature sensor according to claim 1, characterized in that: The second groove is etched 1000 μm behind the sensing area of the graded-index fiber, the opening direction axis of the second groove is different from that of the first groove by 90 degrees, the length of the second groove is 1000 μm, the depth is 56 μm, the second groove is coated with a second sensing metal film with a thickness of 50 nm, and the second sensing metal film is coated with a second ultraviolet curing glue with a refractive index of 1.39 by a fiber coating machine.

5. A manufacturing method of the sensitivity-adjustable fiber SPR curvature sensor according to claim 1, characterized in that: S1. A section of graded-index multimode fiber is taken, and the middle coating layer of the fiber is removed, then the fiber is wiped and clamped on a three-dimensional micro-motion stage clamp to keep the fiber horizontal, the bare fiber is adjusted to be below the CO2 laser beam, and a flat-bottomed groove with a length of 1 mm and a depth of 56 μm is etched on the fiber by a CO2 laser. S2 puts the graded-index fiber with grooves into the fiber length cutting system, rotates the screw differential head to make the cutting knife cut 1000 μm in front of the planar groove structure; S3 puts the graded-index fiber with grooves after length cutting into the fiber clamp of the polarization maintaining fusion splicer, adjusts the orientation of the grooves under the microscope to make the groove opening face vertically upward and fix, and then puts it into the right end of the polarization maintaining fusion splicer; S4 puts the cut longitudinal and horizontal core array fiber into the fiber end face microscopic positioning device of the polarization maintaining fusion splicer to observe the fiber end face, rotates the fiber clamp to make the cores on the two axial surfaces of the longitudinal and horizontal core array fiber vertically upward and horizontally left respectively, locks the fiber clamp, puts the fixed longitudinal and horizontal core array fiber and the fiber clamp into the left end of the polarization maintaining fusion splicer, and finally fuses the left end of the graded-index fiber modulation area.

6. A use method of the sensitivity-adjustable fiber SPR curvature sensor of claim 1, characterized in that: S1 observes the left end face of the longitudinal and horizontal core array fiber with the fiber end face microscopic positioning device of the polarization maintaining fusion splicer, rotates the fiber clamp to make the cores on the two axial surfaces of the longitudinal and horizontal core array fiber vertically upward and horizontally right respectively, locks the fiber clamp, and then puts the fixed longitudinal and horizontal core array fiber and the fiber clamp into the right end of the three-dimensional micro stage; S2 puts the single-mode fiber with a 4 μm diameter and a flat end face into the left end of the three-dimensional micro stage, adjusts the three-dimensional micro stage to make the core of the single-mode fiber face the middle core of the longitudinal and horizontal core array fiber; S3 clamps the groove sensing area of the graded-index fiber right below the barrel microscope, rotates the clamp to make the planar groove opening vertically upward, and clamps the fiber bending sensing area with the fiber clamping rod fixed on the lifting table; S4 connects the left end of the single-mode fiber with the wide-spectrum light source, connects the right end of the light receiving area of the graded-index fiber with the spectrometer, then controls the lifting table to realize the convex bending and concave bending of the groove sensing area, and the corresponding SPR resonance valley on the sensing spectrum moves to the short wavelength and the long wavelength, and the curvature value is demodulated according to the relationship between the wavelength movement and the curvature; S5 adjusts the sensitivity and linearity of the fiber SPR curvature sensor: controls the single-mode fiber on the left side of the three-dimensional micro stage to move vertically or horizontally to the right, and the movement amount is 10 μm each time, the light is injected into different cores of the longitudinal and horizontal core array fiber, after the light passes through different cores, the SPR incidence angle generated by the groove plane is different, the farther the core is from the center, the smaller the SPR incidence angle is, the curvature sensitivity increases with the increase of the distance from the core, and the linearity of the sensor decreases with the increase of the distance from the core, a larger eccentric distance of the eccentric fiber is selected to make the sensing probe to improve the curvature sensitivity, and a smaller eccentric distance of the eccentric fiber is selected to make the sensing probe to improve the linearity of the curvature sensitivity. ​ 7. A method of using a sensitivity-adjustable fiber-optic SPR curvature sensor according to claim 3, characterized by: The first groove sensing area on the graded refractive index fiber and the second groove sensing area on the graded refractive index fiber are respectively placed in two fiber rotating clamps and placed directly below the barrel microscope, the two fiber clamps are rotated to make the two groove sensing area notches vertically upward, finally the two fiber groove sensing areas are clamped by the fiber clamping rod fixed on the lifting platform, the two lifting platforms are controlled to move upward, the two groove sensing areas are bent in the outward convex or inward concave direction, the dual-channel curvature sensing is realized, and the curvature is demodulated by using the moving amount of the corresponding two SPR resonance valleys on the sensing spectrum.

8. A method of using a sensitivity-adjustable fiber-optic SPR curvature sensor according to claim 4, characterized by: The first groove and the second groove of the graded refractive index fiber are placed in the same fiber rotating clamp and placed under the barrel microscope, the fiber clamp is rotated to make the first groove opening direction vertically upward and the second groove opening direction horizontally forward, then the fiber clamp is fixed, the fiber clamping rod on the lifting platform is clamped in the middle position of the two fiber groove sensing areas, the lifting platform is controlled to move upward and downward, the first groove is bent in the outward convex and inward concave direction, then the direction of the fiber clamping rod is rotated to keep vertical and clamped in the middle position of the two fiber groove sensing areas, the lifting platform is controlled to move left and right, the second groove is bent in the outward convex and inward concave direction, the wavelength moving conditions of different sensing areas corresponding to different SPR resonance valleys are recorded, the curvature value is demodulated by using the wavelength, and the multi-dimensional bending direction recognition is realized.

Images