High-performance optical fiber temperature-salt sensor based on femtosecond laser inscribing and manufacturing method thereof

Abstract

The invention provides a high-performance optical fiber temperature-salt sensor based on femtosecond laser inscribing and a manufacturing method, and relates to the technical field of optical fiber sensing technologies, and the high-performance optical fiber temperature-salt sensor specifically comprises a Fabry-Perot interferometer (FPI), a Mach-Zehnder interferometer (MZI), a single-mode fiber (SMF), a hollow-core fiber (HCF), a temperature-sensitive material, a double-C-type microchannel and a single-clad fiber (SCF). The FPI is formed by welding a double-C-shaped micro-channel filled with a temperature-sensitive material, a first SMF and a second SMF, and the double-C-shaped micro-channel is formed by inscribing on the HCF by femtosecond laser; the MZI is formed by welding the SCF, the second SMF and the third SMF through axis offset and dislocation; the FPI is a temperature sensing area, the MZI is a salinity sensing area, and the FPI provides temperature compensation for the MZI; and by adopting a binary quadratic polynomial curved surface fitting decoupling algorithm, the cross sensitivity between the temperature and the salt is reduced, the sensitivity and the resolution are relatively high, and the method has important significance for constructing a global transparent ocean strategy.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing and a manufacturing method thereof. Background Art

[0002] The ocean accounts for 97% of the Earth's water resources, but so far, humans have only explored and developed less than 10% of the ocean area. As the impact of global climate change becomes increasingly prominent, the ocean has become the focus of global attention. Accurately obtaining information on seawater parameters, such as seawater salinity and temperature, has important theoretical significance for oceanographic research, marine ecological environment monitoring, marine resource development and utilization, marine culture, and military security, thus enabling people to observe the ocean more accurately.

[0003] At present, although the traditional method of measuring the refractive index and temperature of the solution is widely used, it has many limitations. For example, the detection technology based on electronic sensors is susceptible to electromagnetic interference and has poor stability in complex biological environments. However, some chemical detection methods are not only cumbersome and time-consuming, but may also cause damage to the liquid sample, affecting the accuracy of the test results and the subsequent analysis of the sample. These problems limit its application in real-time monitoring, and a more efficient, accurate and reliable detection technology is urgently needed.

[0004] As a new type of sensing technology, fiber optic sensors have shown great potential in the field of seawater parameter measurement in recent years. Based on the changes in the transmission characteristics of light in optical fibers, they can sense changes in external physical quantities and have significant advantages such as anti-electromagnetic interference, small size, high sensitivity, and distributed measurement. Especially in applications involving the detection of human physiological parameters, fiber optic sensors can directly interact with seawater samples to achieve on-site real-time monitoring of temperature and salinity, effectively avoiding the drawbacks of traditional detection methods. Therefore, fiber optic sensors have attracted the attention of many researchers. According to different structural characteristics, they can be divided into fiber grating type, fiber interferometer type, and fiber surface plasmon resonance type.

[0005] In 2021, Yu et al. proposed a structure based on double-hole fiber (DSHF), in which DSHF was fused between two sections of coreless optical fiber by offset welding, and its temperature sensitivity can be further improved. In 2022, Zhang et al. proposed a salinity measurement structure based on dual C-type fiber (DCTF), in which a large-sized exposed DCTF microfluidic channel was formed by etching the double-hole fiber with hydrofluoric acid, but the resolution was low. With the continuous advancement of fiber optic sensing technology, researchers have begun to explore cascading different principles in the same sensor to achieve the measurement of more parameters and obtain better performance. In 2022, Zhang et al. proposed a few-mode fiber offset cascade sensor for measuring ocean salinity and temperature, and the temperature had a large crosstalk on salinity. In 2023, Wang et al. proposed a salinity and temperature hybrid interferometer based on SPR and MZI, which has low resolution and cannot work stably in marine environments.

[0006] The Chinese invention patent "CN112781633A" proposed a fiber optic seawater temperature and salinity sensor based on the vernier effect, but the parallel vernier design is adopted, and the overall structure is not compact enough. The Chinese invention patent "CN215984925U" proposed a salinity / temperature synchronous measurement sensor and its system based on optical fiber staggered fusion, and the salinity sensitivity still needs to be further improved.

[0007] Therefore, by comparing the above results, it can be found that both fiber Bragg grating sensors and hybrid interferometers have low sensitivity and cross-sensitivity to temperature and salinity. The SPR sensor has low resolution, and its metal film is not conducive to its further development in underwater environments. Therefore, in the field of seawater multi-parameter measurement, although fiber optic sensors have many advantages, there are still problems of low sensitivity and resolution that need to be solved urgently. Summary of the invention

[0008] In view of the shortcomings of the prior art, the present invention provides a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing and a manufacturing method; the present invention can realize the simultaneous measurement of seawater temperature and salinity, and effectively improve the sensitivity and resolution of dual-parameter measurement, and has certain potential in the fields of marine scientific research, seawater aquaculture, marine navigation, etc.

[0009] On the one hand, a high-performance fiber-optic temperature-salinity sensor based on femtosecond laser inscription, including Fabry-Perot interferometer FPI and Mach-Zehnder interferometer MZI; The Fabry-Perot interferometer FPI comprises a double C-type microchannel, a first single-mode optical fiber and a second single-mode optical fiber, wherein two ends of the double C-type microchannel are fused with the first single-mode optical fiber and the second single-mode optical fiber respectively; The double C-shaped microchannel is formed by femtosecond laser writing on a hollow core optical fiber HCF and is filled with a temperature-sensitive material; The Mach-Zehnder interferometer MZI comprises a single-clad optical fiber SCF and a third single-mode optical fiber, wherein two ends of the single-clad optical fiber SCF are respectively fused with the second single-mode optical fiber and the third single-mode optical fiber through axis offset dislocation; The Fabry-Perot interferometer FPI is a temperature sensing area, the Mach-Zehnder interferometer MZI is a salinity sensing area, and the Fabry-Perot interferometer FPI provides temperature compensation for the Mach-Zehnder interferometer MZI.

[0010] On the other hand, the above-mentioned method for manufacturing a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing comprises the following steps: Step 1: Fabrication of Fabry-Perot interferometer (FPI) structure; First, a fusion splicer is used to splice the first single-mode optical fiber and one end of the hollow-core optical fiber, and then the hollow-core optical fiber is cut using a fixed-length cutting system; then the other end of the hollow-core optical fiber is spliced ​​with the second single-mode optical fiber to form a single-mode-hollow-core-single-mode SHS structure; Step 2: Fabrication of Mach-Zehnder interferometer (MZI) structure; After the Fabry-Perot interferometer FPI structure is made, the single-clad optical fiber is staggered and fused with the second single-mode optical fiber in the horizontal X direction by a fusion splicer, and the stagger amount is 65μm; at this time, in the vertical Y direction, the single-clad optical fiber and the second single-mode optical fiber are manually turned to the same plane by the optical fiber angle rotator and fixed; then the other end of the single-clad optical fiber is staggered and fused with the third single-mode optical fiber to form the Mach-Zehnder interferometer MZI structure; Step 3: Preparation of temperature-sensitive materials; First, the original material of the temperature-sensitive material PDMS and the curing agent are mixed in a set ratio, stirred fully, placed in a vacuum reactor to extract air, and wait for the bubbles to disappear to obtain a liquid temperature-sensitive material; Step 4: Femtosecond laser processing; The hybrid structure composed of FPI and MZI was processed by femtosecond laser, placed on a three-dimensional displacement platform, and scanned at a speed of 100μm / s using a 10x objective lens to prepare two C-shaped microchannels with a length of 15μm, a depth of 40μm, and a width of 125μm. The same processing parameters were used to write rectangular microgrooves of the same size on the first and second single-mode optical fibers, with a length of 30μm. Step 5: Filling and curing of temperature-sensitive materials; The prepared liquid temperature-sensitive material is filled into the air cavity of the hollow-core optical fiber through the principle of capillary action, and then placed in an electric blast drying oven and dried at 80°C for 2 hours, and the temperature-sensitive material is completely solidified to obtain a prepared sensor; Step 6: Sensor packaging; The prepared sensor is encapsulated using a glass tube and UV glue.

[0011] The beneficial effects of adopting the above technical solution are: The present invention provides a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing and a manufacturing method thereof, which has the following beneficial effects:

[0012] (1) A high-performance fiber temperature-salinity sensor based on femtosecond laser inscription is proposed, and temperature measurement is performed using FPI. The disclosed FPI is made by inscribing a double C-shaped microcavity on a hollow-core optical fiber using a femtosecond laser to form a microchannel, and then filling the microchannel with PDMS. It has the advantages of high sensitivity and compact structure.

[0013] (2) A high-performance fiber temperature-salinity sensor based on femtosecond laser inscription is proposed, which uses MZI for salinity measurement and FPI for temperature compensation. The disclosed MZI is to stagger the SCF between the transmission SMF and the lead-out SMF, causing part of the optical signal to leak out of the optical fiber and come into direct contact with seawater. Changes in seawater salinity will change its refractive index, thereby significantly affecting the mode coupling efficiency and transmission loss of the leaked light, making the sensor extremely sensitive to salinity changes.

[0014] (3) Experimental results show that the sensor has a sensitivity of -32.76nm / ℃ and -2.80nm / ‰ for seawater temperature and salinity respectively, and a resolution of 0.0015℃ and 0.018‰, which indicates that it has the potential to measure seawater temperature and salinity with high performance.

[0015] (4) The quadratic bivariate polynomial surface fitting decoupling algorithm is used to effectively reduce the crosstalk between temperature and seawater salinity (refractive index). The indication error of seawater refractive index is only 0.016%, which provides a new method for the high-precision measurement of seawater temperature and salinity. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic diagram of a cascade fiber optic sensor based on femtosecond laser processing; Figure 2 This is a schematic diagram of the manufacturing process of cascade optical fiber sensors based on femtosecond laser processing; Figure 3 This is a schematic diagram of a cascade optical fiber sensor test system based on femtosecond laser processing; Figure 4 It is the temperature reflection spectrum and temperature response curve under the wavelength demodulation method; Among them (a) - temperature reflection spectrum, (b) - three-dimensional temperature response diagram; Figure 5 It is the temperature reflection spectrum and temperature response curve under the cavity length demodulation method; Among them (a) - temperature reflection spectrum, (b) - three-dimensional temperature response diagram; Figure 6 It is the salinity transmission spectrum and salinity response curve under the wavelength demodulation method; Among them (a) - temperature reflection spectrum, (b) - three-dimensional temperature response diagram; Figure 7 It is the three-dimensional coordinate diagram of the refractive index, temperature, and wavelength of the interference valley of FPI and MZI under the quadratic polynomial fitting algorithm; Among them, (a) - three-dimensional coordinate diagram of FPI interference valley, (b) - three-dimensional coordinate diagram of MZI interference valley. DETAILED DESCRIPTION

[0017] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0018] On the one hand, a high-performance fiber-optic temperature-salinity sensor based on femtosecond laser inscription, including Fabry-Perot interferometer FPI and Mach-Zehnder interferometer MZI; The Fabry-Perot interferometer FPI comprises a double C-type microchannel, a first single-mode optical fiber and a second single-mode optical fiber, wherein two ends of the double C-type microchannel are fused with the first single-mode optical fiber and the second single-mode optical fiber respectively; The double C-shaped microchannel is formed by femtosecond laser writing on a hollow core optical fiber HCF and is filled with a temperature-sensitive material; The Mach-Zehnder interferometer MZI comprises a single-clad optical fiber SCF and a third single-mode optical fiber, wherein two ends of the single-clad optical fiber SCF are respectively fused with the second single-mode optical fiber and the third single-mode optical fiber through axis offset dislocation; The Fabry-Perot interferometer FPI is the temperature sensing area, the Mach-Zehnder interferometer MZI is the salinity sensing area, and the Fabry-Perot interferometer FPI provides temperature compensation for the Mach-Zehnder interferometer MZI. The binary quadratic polynomial surface fitting decoupling algorithm is used to reduce the cross-sensitivity between temperature and salt, and has high sensitivity and resolution, which is of great significance for building a global "transparent ocean" strategy.

[0019] On the other hand, the above-mentioned method for manufacturing a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing comprises the following steps: Step 1: Fabrication of Fabry-Perot interferometer (FPI) structure; First, a fusion splicer is used to splice the first single-mode optical fiber and one end of the hollow-core optical fiber, and then the fixed-length cutting system is used to cut the hollow-core optical fiber; in this embodiment, the theoretical cutting length is 100 μm, and the actual measured cutting length is also 100 μm. Then, the other end of the hollow-core optical fiber is spliced ​​with the second single-mode optical fiber to form a single-mode-hollow-core-single-mode SHS structure.

[0020] Step 2: Fabrication of Mach-Zehnder interferometer (MZI) structure; After the Fabry-Perot interferometer (FPI) structure is fabricated, the single-clad optical fiber is staggered-fused with the second single-mode optical fiber in the horizontal X direction by a fusion splicer, with a stagger of 65 μm. At this time, the single-clad optical fiber and the second single-mode optical fiber are manually turned to the same plane in the vertical Y direction by a fiber angle rotator and fixed. Then, the other end of the single-clad optical fiber is staggered-fused with the third single-mode optical fiber to form a Mach-Zehnder interferometer (MZI) structure.

[0021] Step 3: Preparation of temperature-sensitive materials; First, the original material of the temperature-sensitive material PDMS and the curing agent are mixed in a set ratio, stirred fully, placed in a vacuum reactor to extract the air, and wait for the bubbles to disappear to obtain a liquid temperature-sensitive material.

[0022] Step 4: Femtosecond laser processing; The hybrid structure composed of FPI and MZI was processed by femtosecond laser and placed on a three-dimensional displacement platform. A 10x objective lens was used to scan at a speed of 100μm / s to prepare two C-type microchannels with a length of 15μm, a depth of 40μm, and a width of 125μm. In order to speed up the filling speed and avoid PDMS clogging, the same processing parameters were used to engrave rectangular microgrooves of the same size on the first and second single-mode optical fibers, and the length of the microgrooves was 30μm.

[0023] Step 5: Filling and curing of temperature-sensitive materials; The prepared liquid temperature-sensitive material is filled into the air cavity of the hollow-core optical fiber through the principle of capillary action, and then placed in an electric blast drying oven and dried at 80°C for 2 hours. The temperature-sensitive material is completely solidified to obtain a prepared sensor.

[0024] Step 6: Sensor packaging: In order to provide a stable solution exchange environment, the prepared sensor is packaged using a glass tube and UV glue.

[0025] The test system of the present invention is as follows Figure 3As shown, it includes a circulator, a broadband light source, two spectrometers, a high-precision constant temperature water bath and a high-precision thermometer. The incident light enters the circulator from the broadband light source, and reaches the hollow-core optical fiber filled with PDMS through ports 1 and 2 of the circulator for reflection and transmission. The reflected light reaches the first spectrometer through port 3. The remaining transmitted light passes through the offset region and finally reaches the second spectrometer. The packaged sensor is placed in a high-precision water bath, and the salinity of seawater is measured by injecting seawater with different refractive indices into the packaging structure. The temperature is measured by adjusting the temperature in the water bath. The spectral data on the two spectrometers are read at the same time, and the salinity data is processed by the cavity length demodulation method, and the temperature data is processed by the wavelength demodulation method. Finally, the quadratic surface fitting algorithm is used for decoupling to achieve simultaneous measurement of dual parameters.

[0026] The present invention is a cascade optical fiber sensor for simultaneously measuring temperature and refractive index based on femtosecond laser writing technology. The schematic diagram of the cascade sensor is shown in FIG. Figure 1 As shown, the sensor is composed of a cascade of FPI and MZI. The hollow core fiber (HCF) is fused between the introduction single-mode fiber and the transmission single-mode fiber to form two reflection surfaces M1 and M2, and the single-clad fiber is staggered fused between the transmission single-mode fiber and the lead-out single-mode fiber to form two staggered fusion surfaces M3 and M4. In the FPI, the incident light Iin enters the core of the introduction single-mode fiber and first reaches the first reflection surface M1, where reflection and transmission occur to form the first beam of reflected light I1. The remaining transmitted light passes through the PDMS and reaches the second reflection surface M2, where it is reflected again to generate the second beam of reflected light I2. The two beams of reflected light are finally coupled into reflected light IR. In the MZI, the transmitted light I3 passing through the FPI reaches the first staggered surface M3 along the core of the single-mode fiber and is divided into two beams of light I4 and I5 at M3. I4 is in direct contact with the external solution, and I5 is transmitted in the cladding of SCF. There is a phase difference between the two, and finally both reach the second misalignment plane M4 and are coupled into a beam of transmitted light IT in the core of the single-mode optical fiber.

[0027] Since the sensor is composed of FPI cascade MZI, the sensing principle of the sensor is analyzed separately. For M1 and M2, the refractive index of the two dielectric cores of PDMS and SMF is different, which will produce Fresnel reflection, and its end face reflectivity is shown in formula (1): (1); n 1 and n 2 are the refractive indices of the fiber core and PDMS, respectively. Since the reflectivity of M1 and M2 is not high, FPI can be regarded as double-beam interference. The output intensity of the reflected light can be expressed as formula (2): (2); In the formula, I 1 is the reflected light intensity of M1, I 2 is the reflected light intensity of M2, L 1 is the length of HCF, λ is the wavelength of the incident light, is the phase difference between the two beams. It is worth noting that the PDMS completely enters the cavity of the HCF and solidifies under the capillary action, so it can be considered that the length of the PDMS is approximately equal to the length of the HCF.

[0028] It can be calculated from formula (2) that when satisfy hour, m The order interference valley can be expressed by formula (3): (3); For λ m Derivative, we get λ m The wavelength drift can be expressed as formula (4): (4); When the temperature changes to When the thermal expansion coefficient is set to α , the thermo-optic coefficient is set to β , then the change of length and RI can be expressed as formula (5): (5); In formula (5), A is the multiple of outward expansion or contraction when the temperature changes. Combining equation (4) with equation (5), It can be further written as formula (6): (6); Therefore, the temperature sensitivity of FPI is as shown in formula (7): (7); The free spectral range (FSR) of FPI can be expressed as formula (8): (8); In MZI, the output intensity and phase difference of the two transmitted light beams can be expressed by formula (9): (9); I 4 is the output intensity of light in seawater, I 5 is the output intensity of the light in the cladding, n 3 , n 4are the RI of the cladding and seawater, L 2 is the length of the SCF fiber in the misaligned area.

[0029] when (m is an integer), the output light intensity IMZI reaches the maximum value, and the m-order trough can be expressed as formula (10): (10); The wavelength shift is obtained by differentiating λ, which can be described by equation (11): (11); and Respectively express n 3 and n 4 The derivative value is that when the RI of seawater changes, due to L 2 The wavelength shift can be expressed as formula (12): (12); The relationship between wavelength offset and RI sensitivity can be obtained from equation (11) and expressed as equation (13): (13); The FSR of MZI can be expressed by formula (14): (14); Using the FSR of the FPI and MZI, combined with the spectral range, the required number of troughs can be calculated, and the theoretical length required for the HCF in the experiment is obtained to be 100 μm, and the SCF length is 450 μm.

[0030] The manufacturing system of the sensing structure usually consists of a fiber optic three-dimensional displacement platform, a fiber optic cutter, a fusion splicer, a fixed pulley, a weight, a fiber optic angle rotator, a femtosecond processing platform, and a fiber optic micro-nano filling system. The detailed manufacturing process of the fiber optic sensor includes six steps: Figure 2 shown.

[0031] (1) FPI structure production. First, use a fiber cleaver to cut the end faces of the hollow-core fiber and the introduced single-mode fiber flat, put it into the fusion splicer to splice the introduced single-mode fiber and the hollow-core fiber, and then use the fixed-length cutting system to cut the hollow-core fiber to a theoretical length of 100 μm. The actual measured cutting length after cutting is also 100 μm. Then, splice the hollow-core fiber with another section of transmission single-mode fiber to form a single-mode-hollow-core-single-mode (SHS) structure. Put the SHS structure into the fixed-length cutting system again, and cut the transmission single-mode fiber to 450 μm. (2) MZI structure production. After the FPI structure is made, the single-clad optical fiber is placed in the fusion splicer and fused with the transmission single-mode optical fiber in the X direction. The misalignment amount is set to 65μm, the welding parameters are 57 units, and the welding time is 510ms. The fused misaligned structure is placed in the fixed-length cutting system, and the single-clad optical fiber is cut to a theoretical value of 450μm. The actual measured value after cutting is 460μm. At this time, the cut misaligned structure and the lead-out single-mode optical fiber are placed in the fusion splicer. In the Y direction, the single-clad optical fiber and the single-mode optical fiber are manually turned to the same plane through the optical fiber angle rotator and fixed. This ensures that the three optical fibers after fusion are consistent in the X and Y planes. Afterwards, the other end of the single-clad optical fiber is fused with the lead-out single-mode optical fiber with the same parameters to produce the MZI structure.

[0032] (3) Preparation of temperature-sensitive materials. First, mix the liquid PDMS raw material and the curing agent in a ratio of 3:1, stir thoroughly, place in a vacuum reactor to extract air for 30 minutes, and wait for the bubbles to disappear.

[0033] (4) Femtosecond laser processing. The sensor was processed by femtosecond laser and placed on a three-dimensional displacement platform. A 10x objective lens was used to scan at a speed of 100 μm / s. The laser frequency was 1 kHz and the working laser intensity was 24.3 μW. Two C-type microchannels with a length of 15 μm, a depth of 40 μm and a width of 125 μm were prepared on the two reflection surfaces of the hollow-core optical fiber. In order to speed up the filling speed and avoid PDMS clogging, the same processing parameters were used to write rectangular microgrooves of the same size on the single-mode optical fibers at both ends. The length of the microgrooves was 30 μm, and the depth and width were consistent with the C-type microchannels.

[0034] (5) PDMS filling and curing. First, place the sensor under a microscope, then taper the single-mode optical fiber under a flame lamp to prepare a microprobe. Dip the microprobe in a small amount of liquid PDMS and fix it on the fixture of the three-dimensional displacement platform. Continuously adjust the position of the microprobe under the microscope. When the microprobe is infinitely close to one of the microchannels, the liquid PDMS will slowly enter the air cavity due to the principle of capillary action. It should be noted that in order to squeeze out as much residual air as possible and avoid a long air cavity between the PDMS and the reflective surface, more PDMS should be filled near the microchannels and microgrooves on both sides to form PDMS microspheres. Finally. Place the filled sensor in an electric blast drying oven, dry it at 80°C for 2 hours, and wait for the PDMS to completely cure.

[0035] (6) Sensor packaging: In order to provide a stable solution exchange environment, the prepared sensor is packaged using a glass tube and UV glue.

[0036] Based on the above preparation process, a cascade sensor was successfully prepared and constructed as follows Figure 3 The test system for simultaneous measurement of refractive index and temperature shown in FIG. The system consists of a circulator, a broadband light source with a spectral range of 1520 nm to 1620 nm, two spectrometers with a resolution of 0.05 nm, a high-precision constant temperature water bath, and a high-precision thermometer. First, before the experiment begins, the package structure filled with solution is placed in a constant temperature water bath, and then the high-precision thermometer is fixed in the water bath to ensure that the temperature probe can record the temperature in real time. Finally, when the temperature reaches 5°C, data is recorded. The incident light enters the circulator from the broadband light source, passes through ports 1 and 2 of the circulator to the hollow-core optical fiber filled with PDMS, and is reflected and transmitted. The reflected light reaches the first spectrometer through port 3, which is displayed as a reflection spectrum. The remaining transmitted light passes through the dislocation area and finally reaches the second spectrometer, which is displayed as a transmission spectrum.

[0037] In the present invention, the temperature characteristics of the sensing structure are first tested. The initial temperature of the water bath is set to 5°C, and the refractive index of the seawater in the package structure is 1.3326. Then, starting from 5°C, the stabilized spectral data is recorded every 5°C until 30°C. Figure 4 (a) shows the temperature reflection spectrum of the sensor when the temperature rises from 5℃ to 30℃. It can be seen that as the temperature rises, the interference valley dip shows a regular blue shift phenomenon, and based on this, the temperature response curve is drawn, as shown in Figure 2. Figure 4 (b) As shown. The temperature sensitivity of the sensor is -0.39nm / ℃, and the linearity is 0.99929. In addition, the temperature reproducibility of the sensor is also explored to exclude accidental situations. Based on the first experiment, temperature ascending and descending changes were performed, and the spectral changes and temperature responses when the temperature was increased from 5℃ to 30℃ and decreased from 30℃ to 5℃ were recorded, as shown in Figure 2. Figure 4 As shown in (b), the temperature sensitivities are -0.39nm / ℃ and -0.38nm / ℃, and the linearities are 0.98962 and 0.99241, respectively. Through these three temperature tests, it can be proved that the proposed sensor has good temperature sensitivity and reproducibility.

[0038] At the same time, when the temperature measurement range is 5℃~40℃, the movement of the interference valley has exceeded one FSR, and a cross-cycle phenomenon has occurred, which may cause the phase information to be unclear. It is impossible to accurately determine whether the phase change is caused by the real change of the measured parameter, which will bring great difficulties to the subsequent signal processing and parameter solution, and also reduce the resolution of the sensor. Therefore, we adopted a three-parameter cosine fitting (TPCF) demodulation algorithm based on the FPI spectral optical cavity length (OCL) extraction to calculate the phase difference between the filter spectrum and the constructed spectrum. The phase difference can be used to compensate for the OCL and improve the calculation accuracy of the OCL. Compared with the traditional wavelength demodulation method, the TPCF method has a wider demodulation range, and the demodulation accuracy and demodulation resolution are also improved. In the wavelength demodulation method, the interference valley needs to be tracked, and the fitting linearity is different due to the different selected inclination angles. The demodulation of the cavity length is based on the OPD of the signal, which comprehensively utilizes the entire spectrum. There is no need to select a specific peak, and the wavelength movement is not restricted, so real-time and fast demodulation can be achieved. The three-dimensional temperature reflection spectrum processed by the cavity length demodulation method is shown in Figure 2. Figure 5 (a) shows the corresponding temperature response curve. Figure 5 (b) The sensitivities of the three temperature tests were -31.89nm / ℃, -32.28nm / ℃, and -32.76nm / ℃, and the corresponding linearities were 0.99996, 0.99993, and 0.99956, respectively.

[0039] The temperature of the water bath was adjusted to 15.00°C without changing the temperature value. Salt water with salinities of 0.1‰, 3.1‰, 8.7‰, 10.3‰ and 12.3‰ was selected. The RI values ​​of seawater were calibrated with an Abbe refractometer, which were 1.3326, 1.3332, 1.3343, 1.3346 and 1.3350, respectively. Figure 6 (a) recorded the salinity transmission spectrum of the sensor, Figure 6 (b) is the corresponding salinity response curve of the sensor. It can be seen that as the salinity of seawater increases, the dip angle of the transmission spectrum blue shifts. The sensitivity of the three salinity experiments is -2.70nm / ‰, -2.80nm / ‰ and -2.72nm / ‰, and the corresponding linearity is 0.99583, 0.99896 and 0.99632, respectively.

[0040] Usually, the sensitivity matrix decoupling method is used in the measurement of temperature and seawater salinity. However, it cannot be ignored that in the process of salinity measurement, the change of temperature will also greatly affect the salinity value, causing very significant crosstalk. Therefore, we use the quadratic polynomial surface decoupling algorithm to reduce crosstalk. The measurement of seawater salinity is often achieved indirectly by measuring the refractive index. The empirical formula of seawater refractive index is shown in formula (15): (15); where a0 -a 9 is a constant coefficient; Figure 7 (a) and Figure 7 As shown in (b), the relationship between the characteristic wavelength of the interference valley in the FPI reflection spectrum and the MZI transmission spectrum and different temperatures and seawater refractive index is depicted using 3D coordinates. According to the quadratic polynomial surface decoupling algorithm, the indication error of the seawater refractive index is only 0.016%, which effectively reduces the crosstalk of temperature on seawater salinity (refractive index).

[0041] In summary, the cascade sensor in the present invention can measure seawater temperature and salinity simultaneously, and has the advantages of high sensitivity, low cost, compact structure, and can effectively reduce the interference of temperature on salinity, etc., providing a novel and effective idea for the field of seawater multi-parameter detection.

Claims

1. A high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing, characterized in that: Including Fabry-Perot interferometer FPI, Mach-Zehnder interferometer MZI; The Fabry-Perot interferometer FPI comprises a double C-type microchannel, a first single-mode optical fiber and a second single-mode optical fiber, wherein two ends of the double C-type microchannel are fused with the first single-mode optical fiber and the second single-mode optical fiber respectively; The Mach-Zehnder interferometer MZI comprises a single-clad optical fiber SCF and a third single-mode optical fiber, wherein two ends of the single-clad optical fiber SCF are respectively fused with the second single-mode optical fiber and the third single-mode optical fiber through axis-center offset misalignment.

2. According to claim 1, a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing is characterized in that: The double C-type microchannel is formed by writing on a hollow core optical fiber HCF using a femtosecond laser and is filled with a temperature-sensitive material.

3. According to claim 1, a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing is characterized in that: The Fabry-Perot interferometer FPI is a temperature sensing area, the Mach-Zehnder interferometer MZI is a salinity sensing area, and the Fabry-Perot interferometer FPI provides temperature compensation for the Mach-Zehnder interferometer MZI.

4. A method for manufacturing a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing, used to manufacture a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing as claimed in claim 1, characterized in that: The following steps are involved: Step 1: Fabrication of Fabry-Perot interferometer (FPI) structure; Step 2: Fabrication of Mach-Zehnder interferometer (MZI) structure; Step 3: Preparation of temperature-sensitive materials; First, the original material of the temperature-sensitive material PDMS and the curing agent are mixed in a set ratio, stirred fully, placed in a vacuum reactor to extract air, and wait for the bubbles to disappear to obtain a liquid temperature-sensitive material; Step 4: Femtosecond laser processing; Step 5: Filling and curing of temperature-sensitive materials; The prepared liquid temperature-sensitive material is filled into the air cavity of the hollow-core optical fiber through the principle of capillary action, and then placed in an electric blast drying oven and dried at 80°C for 2 hours, and the temperature-sensitive material is completely solidified to obtain a prepared sensor; Step 6: Sensor packaging: The prepared sensor is packaged using a glass tube and UV glue.

5. The method for manufacturing a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing according to claim 4 is characterized in that: The step 1 is specifically as follows: firstly, a fusion splicer is used to splice the first single-mode optical fiber and one end of the hollow-core optical fiber, and then the hollow-core optical fiber is cut using a fixed-length cutting system; and then the other end of the hollow-core optical fiber is spliced ​​with the second single-mode optical fiber to form a single-mode-hollow-core-single-mode SHS structure.

6. The method for manufacturing a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing according to claim 4 is characterized in that: The step 2 is specifically as follows: after the Fabry-Perot interferometer FPI structure is manufactured, the single-clad optical fiber is staggered and fused with the second single-mode optical fiber in the horizontal X direction by a fusion splicer, and the stagger amount is 65 μm; at this time, in the vertical Y direction, the single-clad optical fiber and the second single-mode optical fiber are manually turned to the same plane by a fiber angle rotator and fixed; then the other end of the single-clad optical fiber is staggered and fused with the third single-mode optical fiber to form a Mach-Zehnder interferometer MZI structure.

7. The method for manufacturing a high-performance optical fiber temperature-salinity sensor based on femtosecond laser writing according to claim 4 is characterized in that: The step 4 is specifically as follows: using a femtosecond laser to process the hybrid structure composed of FPI and MZI, placing it on a three-dimensional displacement platform, using a 10x objective lens, scanning at a speed of 100 μm / s, and preparing two C-type microchannels with a length of 15 μm, a depth of 40 μm, and a width of 125 μm; using the same processing parameters to write rectangular microgrooves of the same size on the first and second single-mode optical fibers, and the length of the microgrooves is 30 μm.

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