A C-type fiber vernier temperature sensor manufactured by femtosecond laser and a manufacturing method thereof

Abstract

The application provides a C-type fiber vernier temperature sensor manufactured by femtosecond laser and a manufacturing method, relates to the technical field of fiber sensing, and utilizes femtosecond laser processing technology to etch microgrooves on a hollow core fiber (HCF) to form a C-type fiber, so that PMDS can be fully filled into a microcavity to serve as a sensing medium to realize the temperature sensitization effect, and a FBG is directly written on a single-mode fiber to be used for positioning the interference wavelength of a high-sensitivity vernier spectrum. In view of the difficulty in simultaneously detecting high resolution and a large range of temperature, the application provides a cascaded vernier sensing structure based on a fiber grating and a C-type fiber manufactured by femtosecond laser, which is used for high-sensitivity and large-range measurement of seawater temperature. The sensor has the advantages of strong stability, flexible design, small structure, low cost and the like, and can be used as a new type of sensor to realize high-resolution and large-range detection of seawater temperature.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a C-type fiber optic vernier temperature sensor manufactured by femtosecond laser and its manufacturing method. Background Technology

[0002] 71% of the Earth's surface is covered by seawater. The ocean is the space upon which humanity depends for survival. Rich in marine biological and mineral resources, it is an asset supporting sustainable development. Many marine physical phenomena and environmental changes affect human production activities and economic development, among which temperature is a fundamental physical quantity characterizing the state of the ocean. Therefore, accurate and real-time monitoring of seawater temperature changes is crucial for ensuring the health of the marine ecological environment, promoting sustainable marine development, and understanding global climate change. Electrical sensors are widely used for detecting parameters such as seawater temperature due to their advantages of high precision, wide range, and mature technology. However, these sensors are susceptible to the effects of the marine environment, electromagnetic interference, and seawater corrosion, limiting their application in the field of marine monitoring.

[0003] In recent years, fiber optic sensors have been increasingly used for detecting marine environmental parameters due to their advantages such as resistance to electromagnetic interference, corrosion resistance, and small size. Many optical temperature sensors have been proposed, such as optical interferometers, surface plasmon resonance sensors, fiber optic gratings, and photonic crystal fibers. Fiber optic interferometry is a non-contact measurement method with advantages such as fast response and high accuracy; however, the interferometric structure is difficult to operate, has low measurement sensitivity, and is difficult to apply to high-resolution seawater parameter measurements in marine environments. Surface plasmon resonance sensors have high sensitivity and structural flexibility, but are easily affected by the seawater environment; the sensitive coating on the surface is prone to peeling and corrosion, making stability difficult to guarantee. Fiber optic grating sensors have advantages such as wide measurement range and stable performance, but are easily affected by parameter cross-sensitivity in practical measurements, limiting measurement accuracy. Photonic crystal fibers have structural diversity and high resolution, but their manufacturing process is complex and expensive. Therefore, there is an urgent need to design a temperature sensor with advantages such as high resolution, stable structure, and low cost to achieve rapid, real-time, high-resolution detection of seawater temperature.

[0004] Chinese patent CN116754093A proposes a method for enhancing the temperature measurement sensitivity of an intermodal interferometric fiber optic sensor. Its key feature is the use of a 1mm-5mm long coupling fiber as a coupler within the interferometric fiber optic sensor structure, serving as both a beam splitter and a beam combiner, ultimately achieving temperature enhancement. However, this method also introduces limitations to the measurement range. Chinese patent CN116698223A proposes a negative curvature hollow-core fiber optic temperature sensor based on the LSPR effect. This sensor incorporates localized plasma technology into the fiber, effectively improving the sensor's ability to control the evanescent field, stimulating the LSPR effect, and thus achieving a high-performance sensing response and high-sensitivity temperature detection. However, it also fails to solve the challenges of high resolution and simultaneous detection over a large range. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a C-type fiber optic vernier temperature sensor manufactured using femtosecond lasers and its fabrication method, for high-sensitivity and large-range measurement of seawater temperature. This sensor offers advantages such as high stability, flexible design, compact structure, and low cost, and can serve as a novel sensor for achieving high-resolution and wide-range detection of seawater temperature.

[0006] A C-type fiber optic vernier temperature sensor manufactured by femtosecond laser includes: single-mode fiber, hollow fiber, FBG, and C-type open cavity;

[0007] The hollow fiber is provided with a C-shaped open cavity, which is fabricated using femtosecond laser micro-nano processing technology and runs through the entire hollow fiber.

[0008] The single-mode fiber includes an input single-mode fiber and an output single-mode fiber. The input single-mode fiber, the C-type open cavity, and the output single-mode fiber are sequentially fused together to form a sensing cavity, with the fused end faces being M1 and M2, respectively. The input single-mode fiber, the hollow-core fiber, and the output single-mode fiber are sequentially fused together to form a reference cavity, with the fused end faces being M3 and M4, respectively.

[0009] The FBG is a fiber optic grating, which is built into a single-mode fiber at one end and connects the sensing cavity and the reference cavity.

[0010] A method for fabricating a C-type fiber optic vernier temperature sensor using femtosecond laser technology includes the following steps:

[0011] Step 1: Use a cleaver to cut single-mode fiber and hollow fiber to a set length. Use a fusion splicer to use the single-mode fiber as the input single-mode fiber and fuse it with the hollow fiber. Cut the hollow fiber to form an air cavity.

[0012] Step 2: Select another single-mode fiber as the output single-mode fiber and fusion splice it with the other end of the hollow fiber.

[0013] Step 3: Cut the output single-mode fiber to form an FP cavity, and use a femtosecond laser to perform cavity opening processing on the fused hollow fiber to form a C-shaped cavity structure; during the cavity opening process, the fiber is placed in water for laser processing;

[0014] Step 4: Using the processing software to control the micromachining platform at the set speed and according to the set parameters, perform laser etching to complete the fabrication of the temperature vernier sensor;

[0015] The beneficial effects of adopting the above technical solution are as follows:

[0016] This invention provides a C-type fiber optic vernier temperature sensor manufactured using femtosecond lasers and its fabrication method, for high-sensitivity and large-range measurement of seawater temperature. This sensor possesses advantages such as high stability, flexible design, small structure, and low cost. It can serve as a novel sensor for achieving high-resolution and wide-range detection of seawater temperature, and is expected to become a candidate sensor in the field of marine parameter detection. It has the following beneficial effects:

[0017] (1) This invention utilizes simple welding to construct the basic structure and uses femtosecond laser micro-nano processing technology to realize the fabrication of C-shaped open microcavity structure. The fabrication process is simple.

[0018] (2) By constructing a C-shaped open microcavity, this invention ensures full contact between the sensing beam and the sensitizing polymer PDMS in the FP interference structure, and utilizes the vernier effect to achieve high-resolution measurement of seawater temperature.

[0019] (4) This invention cleverly designs an integrated vernier parallel temperature sensing structure using a simple single-mode-hollow-single-mode structure, achieving high-resolution and large-range simultaneous detection of seawater temperature. This structure is a reflective sensing structure, which can achieve high-resolution and large-range measurement of seawater temperature by tracking the peak of the envelope spectrum in the vernier spectrum. Attached Figure Description

[0020] Figure 1 This is an example of a fiber optic vernier temperature sensing structure manufactured using femtosecond lasers in this invention embodiment;

[0021] Figure 2 This is the temperature experimental testing system in the embodiments of the present invention;

[0022] Figure 3 These are the vernier spectrum and FBG spectrum at 30.136℃ in the embodiments of the present invention;

[0023] Figure 4 These are the lower envelope spectra at different gradient temperatures in the embodiments of the present invention;

[0024] Figure 5This is the linear fitting result of the large-range vernier temperature sensing structure in the embodiments of the present invention. Detailed Implementation

[0025] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0026] A C-type fiber optic vernier temperature sensor manufactured by femtosecond laser includes: single-mode fiber, hollow fiber, FBG, and C-type open cavity;

[0027] The hollow fiber is provided with a C-shaped open cavity, which is fabricated using femtosecond laser micro-nano processing technology and runs through the entire hollow fiber.

[0028] The single-mode fiber includes an input single-mode fiber and an output single-mode fiber. The input single-mode fiber, the C-type open cavity, and the output single-mode fiber are sequentially fused together to form a sensing cavity, with the fused end faces being M1 and M2, respectively. The input single-mode fiber, the hollow-core fiber, and the output single-mode fiber are sequentially fused together to form a reference cavity, with the fused end faces being M3 and M4, respectively.

[0029] The FBG is a fiber optic grating, which is built into a single-mode fiber at one end and connects the sensing cavity and the reference cavity.

[0030] In this embodiment, the structure of the C-type fiber optic vernier temperature sensor is as follows: Figure 1 As shown, the sensing structure includes a photolithographic FBG, a photolithographic C-type fiber, and a parallel hollow fiber. The hollow fiber of the C-type structure L and the single-mode fiber form a sensing Fabry-Perot interferometer (SFPI), and the conventional hollow fiber L1 and the single-mode fiber form a reference Fabry-Perot interferometer (RFPI). Together, they form a parallel vernier sensing structure for achieving high-sensitivity temperature measurement.

[0031] The incident light beam enters the sensing cavity and the reference cavity respectively through a 2×2 coupler. In the sensing cavity, the incident light first passes through a photolithographic FBG of length L2. Light that satisfies the fiber Bragg wavelength will be reflected, and the remaining beam enters a C-shaped sensing structure of length L2.

[0032] Because the refractive index of the introduced single-mode fiber core differs from that of the polydimethylsiloxane (PDMS) filled in the C-type structure, emission occurs at M1. The remaining light passes through the PDMS and is reflected at the single-mode fiber core interface M2. For the reference cavity, the beam propagation path is similar to that of the sensing cavity, with two reflections occurring at M3 and M4.

[0033] A method for fabricating a C-type fiber optic vernier temperature sensor using femtosecond laser technology includes the following steps:

[0034] Step 1: Use a cleaver to cut single-mode fiber and hollow fiber to a set length. Use a fusion splicer to use the single-mode fiber as the input single-mode fiber and fuse it with the hollow fiber. Cut the hollow fiber to form an air cavity.

[0035] In this embodiment, single-mode fiber (8.2μm / 126μm) and hollow fiber (105μm / 128μm) of a certain length were selected and their ends were cut flat using a cleaver. Then, the cut single-mode fiber and hollow fiber were fused together using a Fujukawa fusion splicer with a discharge intensity of 85 units and a discharge time of 240ms. Under this discharge intensity and time, the hollow fiber did not collapse. The fused hollow fiber was then cut using a fixed-length cleaving system, with a hollow core length of approximately 310μm.

[0036] In this embodiment, the cleaver cuts single-mode optical fiber of 8.2μm / 125μm and hollow optical fiber of 105μm / 130μm to a set length, and the fusion splicer is a Fujikawa fusion splicer, model S179C;

[0037] Step 2: Select another single-mode fiber as the output single-mode fiber and fusion splice it with the other end of the hollow fiber to form a reference FPI;

[0038] Step 3: Cut the output single-mode fiber to form an FP cavity, and use a femtosecond laser to perform cavity opening processing on the fused hollow fiber to form a C-shaped cavity structure; during the cavity opening process, the fiber is placed in water for laser processing;

[0039] The initial fabrication process of the sensing FPI is the same as that of the reference FPI, but the hollow core length of the sensing FPI is approximately 235 μm. Next, a femtosecond laser is used to etch the hollow fiber of the sensing FPI, with an etching length, width, and depth of 235 μm, 64 μm, and 64 μm, respectively, forming a C-shaped fiber. During processing, the fabricated sensing FPI is placed in water, followed by femtosecond laser etching. This process eliminates imaging distortion and improves the smoothness of the C-shaped fiber fabrication. The frequency during femtosecond laser processing is set to 10 kHz, the objective lens is 20×, and the laser power is approximately 51 μW.

[0040] Step 4: Using the processing software to control the micromachining platform at the set speed and according to the set parameters, perform laser etching to complete the fabrication of the temperature vernier sensor;

[0041] In this embodiment, the processing software uses Newport μFab to perform laser etching at a speed of 65 μm / s according to the set parameters. Next, a certain length of single-mode fiber is selected to fabricate an FBG (Fiber-Factored Grating). The single-mode fiber is placed in an optical gel with a refractive index of 1.52, which facilitates the positioning of the fiber core and the etching of the grating. The etching length is 1500 μm. The frequency during the femtosecond laser processing is set to 50 Hz, and the objective lens is 40×. The host computer system software controls the micromachining platform to perform laser etching at a speed of 78.5 μm / s according to the set parameters of the grating.

[0042] Experimental testing system such as Figure 2 As shown, the system mainly includes a supercontinuum light source, a spectrometer, and a temperature control chamber. The supercontinuum light source is activated and adjusted using a mobile phone operating software. In the experiment, a 2×2 coupler connects the reference FPPI, the sensor FPPI, and the lithography FBG at one end, and the supercontinuum light source and spectrometer at the other end. The incident light is emitted via the supercontinuum light source, transmitted through the coupler to the reference FPPI and the sensor FPPI, and finally the converged beam returns to the spectrometer. The temperature control chamber is used to set different temperature gradients, and a platinum resistance thermometer is used to display the actual temperature in the temperature control chamber in real time. The platinum resistance thermometer has a resolution of 0.001℃.

[0043] Figure 3 The vernier spectrum and FBG spectrum at 30.136℃ in this embodiment of the invention clearly show that the center wavelength of the FBG is between 1552nm and 1553nm. In the experiment, we set the laser frequency to 50Hz and the laser etching rate to 78.5μm / s. The center wavelength of the FBG was around 1550nm, and the experimental results are consistent with the theory. The figure shows that three interference wavelengths of the lower envelope of the vernier spectrum appeared in the range of 1450 to 1620nm. The FSR of the lower envelope near 1550nm is 59.32nm. Figure 4 These are the lower envelope spectra at different temperature gradients in this embodiment of the invention, ranging from 30.136℃ to 40.502℃. As the temperature increases, the lower envelope spectrum undergoes a blue shift, shifting by a total of 67.64 nm.

[0044] Figure 5 This is the linear fitting result of the large-range vernier temperature sensing structure in this embodiment of the invention, divided into 5 temperature measurement intervals with different interference orders (m+2, m+1, m, m-1, m-2). The measurement results of the heating process of the vernier sensing structure are as follows: Figure 5As shown in the figure, the interference wavelength positions of different interference orders exhibit a blue shift with increasing temperature. The measurement sensitivities are -6.22 nm / ℃, -6.24 nm / ℃, -6.41 nm / ℃, -6.53 nm / ℃, and -6.65 nm / ℃. The measurement sensitivity increases with increasing temperature for different interference orders, which is related to the thermo-optic coefficient of PDMS.

[0045] The above description is merely a preferred embodiment of this disclosure and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in the embodiments of this disclosure is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the above-described inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features with similar functions disclosed in the embodiments of this disclosure.

Claims

1. A C-type fiber optic vernier temperature sensor manufactured by femtosecond laser, characterized in that, Including single-mode fiber, hollow fiber, FBG, and C-type open cavity; The hollow fiber is provided with a C-shaped open cavity, which is fabricated using femtosecond laser micro-nano processing technology and runs through the entire hollow fiber. The single-mode fiber includes an input single-mode fiber and an output single-mode fiber. The input single-mode fiber, the C-type open cavity and the output single-mode fiber are sequentially fused together to form a sensing cavity. The input single-mode fiber, the hollow fiber and the output single-mode fiber are sequentially fused together to form a reference cavity. The FBG is a fiber grating, which is built into a single-mode fiber at one end and connects the sensing cavity and the reference cavity. The fusion end faces of the sensing cavity are M1 and M2, and the fusion end faces of the reference cavity are M3 and M4. The C-type fiber optic vernier temperature sensor manufactured by femtosecond laser is produced by the following method, including the following steps: Step 1: Use a cleaver to cut single-mode fiber and hollow fiber to a set length. Use a fusion splicer to use the single-mode fiber as the input single-mode fiber and fuse it with the hollow fiber. Cut the hollow fiber to form an air cavity. Step 2: Select another single-mode fiber as the output single-mode fiber and fusion splice it with the other end of the hollow fiber. Step 3: Cut the output single-mode fiber to form an FP cavity, and use a femtosecond laser to open the cavity of the fused hollow fiber to form a C-shaped cavity structure. During the cavity opening process, the optical fiber is placed in water for laser processing. Step 4: Using the processing software, control the micromachining platform to perform laser etching according to the set parameters to complete the fabrication of the temperature vernier sensor.

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