High-performance optical fiber temperature and salinity sensor fabricated by femtosecond laser writing and fabrication method thereof

Abstract

The present invention provides a high-performance fiber optic temperature and salinity sensor based on femtosecond laser writing and a manufacturing method thereof, which relates to the technical field of fiber optic sensing technology. Specifically, it includes 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-shaped microchannel, and a single-clad fiber (SCF). The FPI is formed by fusing a double C-shaped microchannel filled with a temperature-sensitive material, a first SMF, and a second SMF. The double C-shaped microchannel is formed by femtosecond laser writing on the HCF. The MZI is formed by axially offset misalignment fusion of the SCF, the second SMF, and the third SMF. The FPI is the temperature sensing region, the MZI is the salinity sensing region, and the FPI provides temperature compensation for the MZI. The use of a binary quadratic polynomial surface fitting decoupling algorithm reduces the cross-sensitivity between temperature and salinity, and has high sensitivity and resolution, which is of great significance for constructing the global "transparent ocean" strategy.

Description

Technical Field

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

[0002] The ocean occupies 97% of the Earth's water resources. However, to date, humans have only explored and developed less than 10% of the ocean area. With the increasing impact of global climate change, the ocean has become the focus of global attention. Accurately obtaining seawater parameter information, such as seawater salinity and temperature, has important theoretical significance for fields such as oceanography research, marine ecological environment monitoring, marine resource development and utilization, marine culture, and military security, enabling people to observe the ocean more precisely.

[0003] Currently, although traditional methods for measuring the refractive index and temperature of solutions are widely used, there are many limitations. For example, detection techniques based on electronic sensors are easily affected by electromagnetic interference and have poor stability in complex biological environments. However, some chemical detection methods are not only cumbersome and time-consuming but may also damage liquid samples, affecting the accuracy of detection results and subsequent analysis of the samples. These problems limit their application in real-time monitoring, and there is an urgent need for a more efficient, accurate, and reliable detection technology.

[0004] Fiber optic sensors, as a new type of sensing technology, have shown great potential in the field of seawater parameter measurement in recent years. Based on the change in the transmission characteristics of light in optical fibers, they can sense the change of external physical quantities and have significant advantages such as anti-electromagnetic interference, small volume, 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 further divided into fiber grating type, fiber interference type, and fiber surface plasmon resonance type.

[0005] In 2021, Yu et al. proposed a structure based on double - hole single - mode fiber (DSHF), which was fusion - spliced between two coreless fibers by misalignment welding, but its temperature sensitivity could still be further improved. In 2022, Zhang et al. proposed a salinity - measurement structure based on double - C - type fiber (DCTF), in which a large - size exposed DCTF micro - flow channel was formed by etching the double - hole fiber with hydrofluoric acid, but the resolution was low. With the continuous progress of fiber - optic sensing technology, researchers began 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 misalignment cascaded sensor for measuring ocean salinity and temperature, and the crosstalk of temperature to salinity was large. In 2023, Wang et al. proposed a salinity and temperature hybrid interferometer based on SPR and MZI, with low resolution and unable to work stably in the marine environment.

[0006] The Chinese invention patent "CN112781633A" proposed an optical - fiber seawater temperature and salinity sensor based on the Vernier effect, but with a parallel Vernier design method, the overall structure was not compact enough. The Chinese utility model patent "CN215984925U" proposed a salinity / temperature synchronous measurement sensor and its system based on fiber - optic misalignment fusion splicing, and the salinity sensitivity still needed to be further improved.

[0007] Therefore, by comparing the above achievements, it can be found that both fiber - Bragg grating sensors and hybrid interferometers have the problems of low sensitivity and cross - sensitivity between temperature and salinity. The SPR sensor has low resolution, and its metal film is not conducive to its further development in the underwater environment. 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] Aiming at the deficiencies of the prior art, the present invention provides a high - performance fiber - optic temperature and salinity sensor based on femtosecond - laser writing and its 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 and salinity sensor based on femtosecond - laser writing includes a Fabry - Perot interferometer FPI and a Mach - Zehnder interferometer MZI;

[0010] The Fabry - Perot interferometer FPI includes a double - C - type micro - channel, a first single - mode fiber, and a second single - mode fiber, where the two ends of the double - C - type micro - channel are respectively fusion - spliced with the first single - mode fiber and the second single - mode fiber;

[0011] The double C-shaped microchannel is formed by femtosecond laser inscription on a hollow-core fiber (HCF) and filled with a temperature-sensitive material;

[0012] The Mach-Zehnder interferometer (MZI) includes a single-clad fiber (SCF) and a third single-mode fiber. The two ends of the single-clad fiber (SCF) are respectively fusion-spliced with the second single-mode fiber and the third single-mode fiber with an axial offset misalignment;

[0013] The Fabry-Perot interferometer (FPI) is the temperature sensing region, the Mach-Zehnder interferometer (MZI) is the salinity sensing region, and the Fabry-Perot interferometer (FPI) provides temperature compensation for the Mach-Zehnder interferometer (MZI).

[0014] On the other hand, the manufacturing method of a high-performance fiber optic temperature and salinity sensor based on femtosecond laser inscription described above includes the following steps:

[0015] Step 1: Fabrication of the Fabry-Perot interferometer (FPI) structure;

[0016] First, use a fusion splicer to splice one end of the first single-mode fiber and the hollow-core fiber, and then use a fixed-length cutting system to cut the hollow-core fiber; then splice the other end of the hollow-core fiber with the second single-mode fiber together to form a single-mode-hollow-core-single-mode (SHS) structure;

[0017] Step 2: Fabrication of the Mach-Zehnder interferometer (MZI) structure;

[0018] After fabricating the Fabry-Perot interferometer (FPI) structure, the single-clad fiber is fusion-spliced with the second single-mode fiber with a misalignment of 65 μm in the horizontal X direction by a fusion splicer; at this time, in the vertical Y direction, manually rotate the single-clad fiber and the second single-mode fiber to the same plane by a fiber angle rotator and fix them; then fuse the other end of the single-clad fiber with the third single-mode fiber with a misalignment to form the Mach-Zehnder interferometer (MZI) structure;

[0019] Step 3: Preparation of the temperature-sensitive material;

[0020] First, mix the original temperature-sensitive material PDMS with the curing agent in a set ratio, stir well, place it in a vacuum reactor to extract air, and wait for the bubbles to disappear to obtain a liquid temperature-sensitive material;

[0021] Step 4: Femtosecond laser processing;

[0022] The hybrid structure composed of FPI and MZI is processed using femtosecond laser, placed on a three-dimensional displacement platform, and scanned at a speed of 100 μm / s using a 10× objective lens to fabricate two C-shaped microchannels with a length of 15 μm, a depth of 40 μm, and a width of 125 μm; rectangular microgrooves of the same size are inscribed on the first and second single-mode optical fibers using the same processing parameters, and the length of the microgrooves is 30 μm.

[0023] Step 5: Filling and curing the temperature-sensitive material;

[0024] The prepared liquid temperature-sensitive material is filled into the air cavity of the hollow-core optical fiber by the principle of capillary action, and then it is placed in an electric drum drying oven and dried at 80 °C for 2 hours, waiting for the temperature-sensitive material to be completely cured to obtain the prepared sensor.

[0025] Step 6: Sensor encapsulation;

[0026] The prepared sensor is encapsulated using a glass tube and ultraviolet glue.

[0027] The beneficial effects produced by adopting the above technical solutions are as follows:

[0028] The present invention provides a high-performance fiber optic temperature and salinity sensor based on femtosecond laser writing and a manufacturing method, having the following beneficial effects:

[0029] (1) A high-performance fiber optic temperature and salinity sensor based on femtosecond laser writing is proposed, and FPI is used for temperature measurement. The disclosed FPI is formed by inscribing double C-shaped microcavities on a hollow-core optical fiber using femtosecond laser to form microchannels and filling PDMS in the microchannels, having advantages such as high sensitivity and compact structure.

[0030] (2) A high-performance fiber optic temperature and salinity sensor based on femtosecond laser writing is proposed, MZI is used for salinity measurement, and FPI is used for temperature compensation. The disclosed MZI is formed by misaligned fusion splicing the SCF between the transmission SMF and the lead-out SMF, resulting in partial optical signal leakage to the outside of the optical fiber and direct contact with seawater. The change in seawater salinity will change its refractive index, thus significantly affecting the mode coupling efficiency and transmission loss of the leakage light, making the sensor extremely sensitive to salinity changes.

[0031] (3) Experimental results show that the seawater temperature and salinity sensitivities of this sensor are -32.76 nm / °C and -2.80 nm / ‰ respectively, and the resolution can reach 0.0015 °C and 0.018 ‰, having the potential for high-performance measurement of seawater temperature and salinity.

[0032] (4) The decoupling algorithm of quadratic binary polynomial surface fitting is adopted to effectively reduce the crosstalk of temperature to seawater salinity (refractive index). The indication error of seawater refractive index is only 0.016%, providing a new method for the high-precision measurement field of seawater temperature and salinity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a schematic diagram of a cascaded fiber optic sensor based on femtosecond laser processing;

[0034] Figure 2 is a schematic diagram of the manufacturing process of a cascaded fiber optic sensor based on femtosecond laser processing;

[0035] Figure 3 is a schematic diagram of a test system for a cascaded fiber optic sensor based on femtosecond laser processing;

[0036] Figure 4 is a graph of temperature reflection spectrum and temperature response curve under the wavelength demodulation method;

[0037] where (a) - temperature reflection spectrum diagram, (b) - three - time temperature response diagram;

[0038] Figure 5 is a graph of temperature reflection spectrum and temperature response curve under the cavity length demodulation method;

[0039] where (a) - temperature reflection spectrum diagram, (b) - three - time temperature response diagram;

[0040] Figure 6 is a graph of salinity transmission spectrum and salinity response curve under the wavelength demodulation method;

[0041] where (a) - temperature reflection spectrum diagram, (b) - three - time temperature response diagram;

[0042] Figure 7 is a three - dimensional coordinate diagram of refractive index, temperature, and wavelength of the interference valleys of FPI and MZI under the quadratic polynomial fitting algorithm;

[0043] where (a) - three - dimensional coordinate diagram of the FPI interference valley, (b) - three - dimensional coordinate diagram of the MZI interference valley. DETAILED DESCRIPTION OF THE INVENTION

[0044] Hereinafter, with reference to the drawings and embodiments, the specific embodiments of the present invention will be further described in detail. The following embodiments are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0045] On the one hand, a high - performance fiber optic temperature - salinity sensor based on femtosecond laser writing includes a Fabry - Perot interferometer FPI and a Mach - Zehnder interferometer MZI;

[0046] The Fabry - Perot interferometer (FPI) includes a double - C - shaped microchannel, a first single - mode fiber, and a second single - mode fiber, where the two ends of the double - C - shaped microchannel are respectively fusion - spliced to the first single - mode fiber and the second single - mode fiber;

[0047] The double - C - shaped microchannel is formed by femtosecond laser writing on a hollow - core fiber (HCF) and is filled with a temperature - sensitive material;

[0048] The Mach - Zehnder interferometer (MZI) includes a single - clad fiber (SCF) and a third single - mode fiber, where the two ends of the single - clad fiber SCF are respectively offset - misaligned fusion - spliced with the second single - mode fiber and the third single - mode fiber through axial offset;

[0049] The Fabry - Perot interferometer FPI is a temperature - sensing region, and the Mach - Zehnder interferometer MZI is a salinity - sensing region. 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 salinity, having high sensitivity and resolution, which is of great significance for constructing the global "transparent ocean" strategy.

[0050] On the other hand, the manufacturing method of a high - performance fiber temperature - salinity sensor based on femtosecond laser writing described above includes the following steps:

[0051] Step 1: Fabrication of the Fabry - Perot interferometer FPI structure;

[0052] First, use a fusion splicer to splice one end of the first single - mode fiber and the hollow - core fiber, and then use a fixed - length cutting system to cut the hollow - core fiber; In this embodiment, the theoretical cutting length is 100 μm, and the actually measured cutting length is also 100 μm. Then, splice the other end of the hollow - core fiber with the second single - mode fiber together to form a single - mode - hollow - core - single - mode (SHS) structure.

[0053] Step 2: Fabrication of the Mach - Zehnder interferometer MZI structure;

[0054] After fabricating the Fabry - Perot interferometer FPI structure, the single - clad fiber is misaligned fusion - spliced with the second single - mode fiber in the horizontal X direction by a fusion splicer, and the misalignment amount is 65 μm; At this time, in the vertical Y direction, manually rotate the single - clad fiber and the second single - mode fiber to the same plane through a fiber angle rotator and fix them; Then, misalign and fusion - splice the other end of the single - clad fiber with the third single - mode fiber to form a Mach - Zehnder interferometer MZI structure.

[0055] Step 3: Preparation of the temperature - sensitive material;

[0056] First, mix the original temperature - sensitive material PDMS with a curing agent in a set ratio, stir well, place it in a vacuum reactor to extract air, and wait for the bubbles to disappear to obtain a liquid temperature - sensitive material.

[0057] Step 4: Femtosecond laser processing;

[0058] The hybrid structure composed of FPI and MZI is processed by femtosecond laser. It is placed on a three-dimensional displacement platform, and a 10× objective lens is used to scan at a speed of 100 μm / s to fabricate two C-shaped microchannels with a length of 15 μm, a depth of 40 μm, and a width of 125 μm. To accelerate the filling speed and avoid PDMS blockage, rectangular micro-grooves with the same size are inscribed on the first and second single-mode optical fibers using the same processing parameters, and the length of the micro-grooves is 30 μm.

[0059] Step 5: Filling and curing of temperature-sensitive material;

[0060] The prepared liquid temperature-sensitive material is filled into the air cavity of the hollow optical fiber by the principle of capillary action, and then it is placed in an electric blast drying oven and dried at 80 °C for 2 hours. Wait for the temperature-sensitive material to be completely cured to obtain the prepared sensor.

[0061] Step 6: Sensor packaging: To provide a stable solution exchange environment, the prepared sensor is packaged using a glass tube and ultraviolet glue.

[0062] The test system of this invention patent is as Figure 3 shown, and it consists of a circulator, one 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 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. The salinity of seawater is measured by injecting seawater with different refractive indices into the packaging structure, and the temperature is measured by adjusting the temperature in the water bath. At the same time, the spectral data on the two spectrometers are read. The cavity length demodulation method is used to process the salinity data, the wavelength demodulation method is used to process the temperature data, and finally the quadratic surface fitting algorithm is used for decoupling to achieve the simultaneous measurement of dual parameters.

[0063] This invention is a cascaded fiber optic sensor for simultaneous measurement of temperature and refractive index based on femtosecond laser writing technology. The schematic diagram of the cascaded sensor is as Figure 1As shown in the figure, the overall sensor is composed of a cascaded FPI and MZI. An empty-core fiber (HCF) is fusion-spliced between the input single-mode fiber and the transmission single-mode fiber to form two reflecting surfaces M1 and M2. The single-clad fiber is misaligned and fusion-spliced between the transmission single-mode fiber and the output single-mode fiber to form two misaligned splicing surfaces M3 and M4. In the FPI, the incident light Iin enters the core of the input single-mode fiber and first reaches the first reflecting surface M1, where reflection and transmission occur, forming the first reflected light I1. The remaining transmitted light passes through the PDMS and reaches the second reflecting surface M2, where reflection occurs again, generating the second reflected light I2. The two reflected lights are finally coupled into the reflected light IR. In the MZI, the transmitted light I3 passing through the FPI travels along the core of the single-mode fiber to the first misalignment surface M3, where it is divided into two beams of light I4 and I5. I4 is directly in contact with the external solution, and I5 travels in the cladding of the SCF. There is a phase difference between them. Finally, both reach the second misalignment surface M4 and are coupled into a transmitted light IT in the core of the output single-mode fiber.

[0064] Since the sensor is composed of a cascaded FPI and MZI, the sensing principle of the sensor is analyzed separately. For M1 and M2, due to the different refractive indices of the two media cores of PDMS and SMF, Fresnel reflection will occur, and its end-face reflectivity is shown in formula (1): (1);

[0065] n 1 and n 2 are the refractive indices of the core and PDMS respectively. Since the reflectivities of M1 and M2 are not high, the FPI can be regarded as a two-beam interference. The output light intensity of the obtained reflected light can be expressed as formula (2): (2);

[0066] In the formula, I 1 is the intensity of the reflected light of M1, I 2 is the intensity of the reflected light of M2, L 1 is the length of the HCF, λ is the wavelength of the incident light, is the phase difference between the two beams of light. It should be noted that under capillary action, the PDMS completely enters the cavity of the HCF and solidifies. Therefore, it can be considered that the length of the PDMS is approximately equal to the length of the HCF.

[0067] It can be calculated from formula (2) that when satisfies , m the m-th order interference valley can be expressed by formula (3): (3);

[0068] Derive with respect to λ m to obtain the wavelength drift of λ m which can be expressed as Equation (4): (4);

[0069] When the temperature change is the length of PDMS and the RI of the external solution will change under the action of the thermo-optic effect and the thermal expansion effect. Let the thermal expansion coefficient be α and the thermo-optic coefficient be β then the changes in length and RI can be expressed as Equation (5): (5);

[0070] In Equation (5), A is the multiple of expansion or contraction outward when the temperature changes. Combining Equation (4) and Equation (5), can be further written as Equation (6):

[0071] (6);

[0072] Therefore, the temperature sensitivity of the FPI is as shown in Equation (7): (7);

[0073] The free spectral range (FSR) of the FPI can be expressed as Equation (8): (8);

[0074] In the MZI, the output light intensity and phase difference of the two transmitted light beams can be expressed by Equation (9): (9);

[0075] I 4 is the output light intensity of light in seawater, I 5 is the output light intensity of light in the cladding, n 3, n 4 are the RIs of the cladding and seawater respectively, L 2 is the length of the SCF fiber in the dislocation region.

[0076] When (m is an integer), the output light intensity IMZI reaches the maximum value, and the m-th wave valley can be expressed as Equation (10): (10);

[0077] Deriving with respect to λ to obtain the wavelength displacement, which can be described by Equation (11): (11);

[0078] and respectively represent the values after taking the derivatives of n 3 and n 4. When the RI of seawater changes, since L 2 remains unchanged, the wavelength shift can be expressed as Equation (12):

[0079] (12);

[0080] The relationship between the wavelength offset and the RI sensitivity can be obtained from Equation (11) and can be expressed as Equation (13): (13);

[0081] The FSR of the MZI can be expressed by Equation (14): (14);

[0082] Using the FSRs of the FPI and MZI and combining with the spectral range, the required number of valleys can be calculated, and the theoretical length required for the HCF in the experiment is 100 μm, and the length of the SCF is 450 μm.

[0083] The fabrication system of the sensing structure usually consists of a three-dimensional fiber optic displacement platform, a fiber optic cutter, a fusion splicer, a fixed pulley, weights, a fiber optic angle rotator, a femtosecond processing platform, a fiber optic micro-nano filling system, etc. The detailed fabrication process of the fiber optic sensor includes six steps as Figure 2 shown.

[0084] (1) Fabrication of the FPI structure. First, use a fiber optic cutter to cut the end faces of the hollow core fiber and the input single mode fiber flat, place them in a fusion splicer to splice the input single mode fiber and the hollow core fiber, then use a fixed-length cutting system to cut the hollow core fiber to a theoretical length of 100 μm. After cutting, the actual measured cutting length is also 100 μm. Then splice the hollow core fiber with another section of the transmission single mode fiber to form a single mode - hollow core - single mode (SHS) structure. Again, place the SHS structure in the fixed-length cutting system and cut the transmission single mode fiber to 450 μm;

[0085] (2) Fabrication of MZI structure. After the FPI structure is fabricated, the single-clad optical fiber is placed in a fusion splicer and fused with the transmission single-mode optical fiber in the X direction. The offset is set to 65 μm, the welding parameters are 57 units, and the welding time is 510 ms. The fused offset structure is placed in a fixed-length cutting system, and the single-clad optical fiber is cut to a theoretical value of 450 μm. The actual measured value after cutting is 460 μm. At this time, the cut offset 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 using 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 using the same parameters to fabricate the MZI structure.

[0086] (3) Preparation of thermosensitive 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.

[0087] (4) Femtosecond laser processing. The sensor was processed using a 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, a laser frequency of 1 kHz, and a working laser intensity of 24.3 μW. Two C-shaped microchannels with a length of 15 μm, a depth of 40 μm, and a width of 125 μm were prepared on the two reflective surfaces of the hollow-core fiber. 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 those of the C-shaped microchannels.

[0088] (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 and one of the microchannels are infinitely close, 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 and dry it at 80°C for 2 hours, waiting for the PDMS to completely cure.

[0089] (6) Sensor packaging. In order to provide a stable solution exchange environment, the prepared sensor is packaged using a glass tube and UV glue.

[0090] Based on the above preparation process, a cascaded sensor was successfully fabricated, and a test system for simultaneously measuring refractive index and temperature as shown in Figure 3 was built. 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, the encapsulated structure filled with the solution was placed in the constant temperature water bath, and then the high-precision thermometer was fixed in the water bath to ensure that the temperature probe could record the temperature in real time. Finally, when the temperature reached 5 °C, data recording was carried out. The incident light enters the circulator from the broadband light source and reaches the hollow 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 and is shown as the reflection spectrum. The remaining transmitted light passes through the misalignment region and finally reaches the second spectrometer, which is shown as the transmission spectrum.

[0091] In the present invention, the temperature characteristics of the sensing structure were first tested. The initial temperature of the water bath was set at 5 °C, and the refractive index of the seawater in the encapsulated structure was 1.3326. Immediately afterwards, starting from 5 °C, the spectral data after stabilization were recorded every 5 °C until 30 °C. Figure 4 (a) shows the temperature reflection spectrum of the sensor when the temperature rises from 5 °C to 30 °C. It can be seen that as the temperature increases, the interference dip shows a regular blue shift phenomenon. Based on this, the temperature response curve was plotted as shown in Figure 4 (b). The temperature sensitivity of the sensor is -0.39 nm / °C, and the linearity is 0.99929. In addition, the temperature reproducibility of the sensor was also investigated to rule out accidental situations. Based on the first experiment, temperature ascending and descending change experiments were carried out, and the spectral changes and temperature responses when the temperature was increased from 5 °C to 30 °C and decreased from 30 °C to 5 °C were recorded respectively, as shown in Figure 4 (b). The temperature sensitivities are -0.39 nm / °C and -0.38 nm / °C respectively, and the linearities are 0.98962 and 0.99241 respectively. Through these three temperature experiments, it can be proved that the proposed sensor has good temperature sensitivity and reproducibility.

[0092] Meanwhile, when the temperature measurement range is 5°C to 40°C, the movement of the interference valley has exceeded one FSR, resulting in a cross-cycle phenomenon, which may cause the phase information to be blurred. It is impossible to accurately determine whether the change in phase is caused by the real change of the measured parameter, which will bring great difficulties to subsequent signal processing and parameter calculation, and also reduce the resolution of the sensor. Therefore, we adopted a three-parameter cosine fitting (TPCF) demodulation algorithm based on the extraction of the optical cavity length (OCL) of the FPI spectrum to calculate the phase difference between the filtered spectrum and the constructed spectrum. The phase difference can be used to compensate 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, it is necessary to track the interference valley, and due to different selected inclination angles, the fitting linearity is also different. The demodulation of the cavity length is based on the OPD of the signal, comprehensively using the entire spectrum, without the need to select specific peaks, and the wavelength shift is not restricted, enabling real-time and fast demodulation. The three temperature reflection spectra processed by the cavity length demodulation method are as shown in Figure 5 (a), and the corresponding temperature response curve is as shown in Figure 5 (b). The sensitivities of the three temperature tests are -31.89 nm / °C, -32.28 nm / °C, and -32.76 nm / °C respectively, and the corresponding linearities are 0.99996, 0.99993, and 0.99956 respectively.

[0093] Adjust the temperature of the water bath to 15.00°C, keep the temperature value unchanged, select brines with salinities of 0.1‰, 3.1‰, 8.7‰, 10.3‰, and 12.3‰, and calibrate the RI values of the seawater with an Abbe refractometer, which are 1.3326, 1.3332, 1.3343, 1.3346, and 1.3350 respectively. Figure 6 (a) records 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 the seawater increases, the inclination angle of the transmission spectrum blue-shifts. The sensitivities of the three salinity experiments are -2.70 nm / ‰, -2.80 nm / ‰, and -2.72 nm / ‰ respectively, and the corresponding linearities are 0.99583, 0.99896, and 0.99632 respectively.

[0094] Usually, the sensitivity matrix decoupling method is used in the measurement of temperature and seawater salinity. However, it cannot be ignored that during the salinity measurement process, the change in temperature will also greatly affect the salinity value, causing very significant crosstalk. Therefore, we adopted a quadratic polynomial surface decoupling algorithm to reduce crosstalk. The measurement of seawater salinity is often indirectly realized by measuring the refractive index. The empirical formula of the refractive index of seawater is as shown in formula (15):

[0095] (15); wherein, a0 - a9 are constant coefficients;

[0096] Figure 7 (a) and Figure 7 (b) as shown, the relationship between the characteristic wavelengths of the interference valleys in the FPI reflection spectrum and the MZI transmission spectrum and different temperatures and seawater refractive indices is respectively depicted using 3D coordinates. According to the quadratic polynomial surface decoupling algorithm, the indication error of the seawater refractive index is only 0.016%, effectively reducing the crosstalk of temperature on seawater salinity (refractive index).

[0097] In summary, the cascaded sensor in the present invention can simultaneously measure seawater temperature and salinity, and has the advantages of high sensitivity, low cost, compact structure, and can effectively reduce the interference of temperature on salinity, providing a novel and effective idea for the field of seawater multi-parameter detection.

Claims

1. A high-performance fiber optic temperature and salinity sensor based on femtosecond laser inscription, characterized in that, It includes a Fabry - Perot interferometer (FPI) and a Mach - Zehnder interferometer (MZI); The Fabry - Perot interferometer (FPI) includes a double - C - type microchannel, a first single - mode fiber, and a second single - mode fiber, where the two ends of the double - C - type microchannel are respectively fusion - spliced to the first single - mode fiber and the second single - mode fiber; The Mach - Zehnder interferometer (MZI) includes a single - clad fiber (SCF) and a third single - mode fiber, where the two ends of the single - clad fiber (SCF) are respectively fusion - spliced to the second single - mode fiber and the third single - mode fiber with an axial offset misalignment; The double - C - type microchannel is formed by writing with femtosecond laser on a hollow - core fiber (HCF) and is filled with a temperature - sensitive material; The Fabry - Perot interferometer (FPI) is a temperature - sensing region, and the Mach - Zehnder interferometer (MZI) is a salinity - sensing region. The Fabry - Perot interferometer (FPI) provides temperature compensation for the Mach - Zehnder interferometer (MZI); A high - performance fiber temperature - salinity sensor based on femtosecond - laser writing is as follows, and the manufacturing method includes the following steps: Step 1: Fabrication of the Fabry - Perot interferometer (FPI) structure; Step 2: Fabrication of the Mach - Zehnder interferometer (MZI) structure; Step 3: Preparation of the temperature - sensitive material; First, mix the original temperature - sensitive material PDMS with the curing agent in a set ratio, stir well, place it 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 the temperature - sensitive material; Fill the air cavity of the hollow - core fiber with the prepared liquid temperature - sensitive material through the principle of capillary action, then put it into an electric drum - drying oven and dry it at 80 °C for 2 hours, and wait for the temperature - sensitive material to be completely cured to obtain the prepared sensor; Step 6: Sensor packaging: Package the prepared sensor using a glass tube and ultraviolet glue; The specific content of Step 1 is: First, use a fusion splicer to splice one end of the first single - mode fiber and the hollow - core fiber, and then use a fixed - length cutting system to cut the hollow - core fiber; then splice the other end of the hollow - core fiber to the second single - mode fiber to form a single - mode - hollow - core - single - mode (SHS) structure; The specific content of Step 2 is: After fabricating the Fabry - Perot interferometer (FPI) structure, splice the single - clad fiber to the second single - mode fiber with a misalignment in the horizontal X direction by a fusion splicer, and the misalignment amount is 65 μm; at this time, in the vertical Y direction, manually turn the single - clad fiber and the second single - mode fiber into the same plane through a fiber angle rotator and fix them; then splice the other end of the single - clad fiber to the third single - mode fiber with a misalignment to form a Mach - Zehnder interferometer (MZI) structure; The specific content of Step 4 is: Use femtosecond laser to process the hybrid structure composed of FPI and MZI. Place it on a three - dimensional displacement platform, use a 10 - times objective lens, and scan at a speed of 100 μm / s to prepare two C - type microchannels with a length of 15 μm, a depth of 40 μm, and a width of 125 μm; use the same processing parameters to write rectangular micro - grooves with the same size on the first and second single - mode fibers, and the length of the micro - grooves is 30 μm.

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