Temperature and salinity optical fiber sensor based on femtosecond laser writing and its use method

Abstract

The present invention provides a temperature and salinity optical fiber sensor based on femtosecond laser writing and a method for using the same, and relates to the field of optical fiber sensing technology. The sensor of the present invention is composed of an input single-mode optical fiber, a hollow-core optical fiber, and an output single-mode optical fiber spliced ​​together, referred to as an SHS structure. Based on this structure, femtosecond laser micromachining technology is introduced to write waveguides and open cavities, thereby realizing the integration of Fabry-Perot interferometer (FPI) and Mach-Zehnder interferometer (MZI). The writing of the cladding waveguide in the present invention provides a basis for the realization of the MZI, and the semi-open cavity realizes direct contact between the sensing arm and the external environment, providing a basis for high-sensitivity measurement. The present invention has good stability, repeatability and reproducibility, and shows the prospect of measuring ocean temperature and salinity.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to a temperature and salinity optical fiber sensor based on femtosecond laser writing and a method for using the same. Background Art

[0002] There are many oceanographic parameters, among which seawater salinity and temperature are fundamental for ocean observation. Their distribution and changes directly or indirectly impact marine ecology, marine aquaculture, and military activities. A change of 0.15‰ in seawater salinity can trigger internal waves, known as "underwater demons," which seriously endanger the safety of coastal facilities and underwater vehicles, and even cause submarines to "sink." A sustained, six-month-long abnormal seawater temperature fluctuation of 0.5°C can trigger the El Niño phenomenon, leading to dramatic global climate shifts, such as this year's global heatwave and other extreme weather events. Therefore, it is essential to accurately monitor changes in ocean temperature and salinity.

[0003] Currently, instruments used to monitor ocean temperature and salinity parameters are primarily electronic sensors, offering accurate measurements. However, foreign products, such as the US Seabird, Sweden's SAIV, and Canada's RBR, dominate the domestic market for ocean monitoring products. Furthermore, these sensors suffer from large size, high power consumption, corrosion susceptibility, and short lifespan. Compared to electronic sensors, fiber optic sensing technology offers unique advantages in ocean exploration. It is sensitive to multiple parameters, compact, fast, low-cost, long-lasting, and has low transmission loss, enabling long-distance transmission. Consequently, it has garnered widespread attention from researchers.

[0004] Fiber Bragg grating (FBG) sensors are the most mature fiber devices, offering excellent stability, but their low sensitivity limits their applications. In addition, fiber interferometers and surface plasmon resonance (SPR) sensors based on various interference principles offer various possibilities for measuring ocean parameters. In 2011, an open-cavity fiber sensor milled by focused ion beam (FIB) was proposed. This sensor achieved a salinity sensitivity of 48 pm / ppt and a temperature sensitivity of 29 pm / °C. Tong et al. experimentally demonstrated a simple sensor based on mode interferometry, which exploits the interference of different cladding and core modes to simultaneously measure temperature and refractive index. Later, Su et al. proposed a similar spherical structure. However, their sensitivity was unsatisfactory. Based on theoretical analysis, Wang et al. proposed a fiber-microfiber coupler (OMC). Experimental results showed salinity and temperature sensitivities as high as nm / ‰ and nm / °C, respectively. Subsequently, Yu et al. tested the depth response characteristics of this device. Furthermore, Zhao et al. combined a polarization-maintaining fiber (PMF) with an OMC to simultaneously measure temperature and salinity by measuring changes in the intensity and wavelength of the interference spectrum. This sensor has high sensitivity but is bulky and difficult to package. Zhang et al. constructed a dual-channel SPR sensor using different metal films. The results showed that its temperature sensitivity and salinity sensitivity can reach 0.3769 nm / ‰ and -0.956 nm / ℃, but the life of this type of sensor depends on the performance of the metal film. In recent years, with the development of femtosecond laser micromachining technology, more and more researchers have used it to manufacture sensors. Tian et al. constructed a micro multi-cavity sensor using femtosecond laser. The air open cavity and the quartz cavity were constructed into a multi-cavity Fabry-Perot (FP) interference structure, and fast Fourier transform (FFT) analysis was used to obtain different responses of different cavities. In addition, many open cavity schemes have been proposed, and the spectral quality has been continuously improved. In addition, femtosecond direct writing of waveguides is also a common method for constructing Mach-Zehnder (MZ) structures in recent years. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention provides a temperature and salinity fiber optic sensor based on femtosecond laser inscription and its use method. This sensing structure offers high sensitivity, excellent stability, repeatability, and reproducibility, and has the potential to simultaneously measure seawater temperature and salinity.

[0006] A temperature and salinity optical fiber sensor based on femtosecond laser writing, specifically comprising: an input single-mode optical fiber, a hollow-core optical fiber, an output single-mode optical fiber, and a waveguide;

[0007] The input single-mode optical fiber, hollow-core optical fiber and output single-mode optical fiber are aligned and fused in sequence by a fusion splicer;

[0008] The waveguide is divided into three sections, wherein the starting point of the first section of the waveguide is set on the core of the input single-mode optical fiber, 1000 microns away from the fusion interface of the input single-mode optical fiber and the hollow-core optical fiber, the angle between the first section of the waveguide and the core is 2.29 degrees, and the end point stops at the fusion interface of the input single-mode optical fiber and the hollow-core optical fiber; the end point of the first section of the waveguide is used as the starting point of the second section of the waveguide, the length of the second section of the waveguide is 100 microns, and the end point of the second section of the waveguide is used as the starting point of the third section of the waveguide. The end point of the third section of the waveguide is on the core of the output single-mode optical fiber, and the end point is 1000 microns away from the fusion interface of the output single-mode optical fiber and the hollow-core optical fiber.

[0009] A rectangular half-open cavity is provided in the center of the hollow-core optical fiber. The rectangular half-open cavity is prepared by using femtosecond laser micromachining technology, which opens half of the hollow-core optical fiber to ensure the connection between the air hole of the hollow-core optical fiber and the outside world.

[0010] The cladding direct-writing waveguide of the single-mode optical fiber and the hollow-core optical fiber serves as the reference arm of the MZI, the semi-open cavity path serves as the sensing arm of the MZI, and the two fusion interfaces of the hollow-core optical fiber and the single-mode optical fibers on both sides serve as the two reflective end faces of the FPI, forming a Mach-Zehnder interferometer (MZI) and a Fabry-Perot interferometer (FPI).

[0011] On the other hand, a method for using a temperature and salinity optical fiber sensor based on femtosecond laser inscription is implemented based on the aforementioned temperature and salinity optical fiber sensor based on femtosecond laser inscription, and specifically includes the following steps:

[0012] Step 1: Encapsulate the optical fiber sensor with a π-shaped tube;

[0013] The π-shaped tube includes ports 1 to 4. The optical fiber sensor enters the π-shaped tube from port 2 and exits from port 1. External fluid flows in and out through ports 3 and 4.

[0014] Step 2: Connect the input single-mode fiber end of the fiber optic sensor to the supercontinuum light source and spectrometer 1 through a circulator, and directly connect the output single-mode fiber end to spectrometer 2.

[0015] Step 3: Based on the interference principles of Mach-Zehnder interferometer and Fabry-Perot interferometer, the relationship between their interference trough, the effective refractive index in the open cavity, and the effective refractive index of the waveguide is obtained, as shown in Formula 1.

[0016]

[0017] where n e , n w are the effective refractive indices of light transmitted in the cavity and waveguide, respectively. MZ and λ FPare the trough wavelengths of the interference patterns of MZI and FPI, respectively. m is a constant. L1 is the distance from the waveguide starting point to the interface between the input single-mode fiber and the hollow-core fiber. L2 is the length of the hollow-core fiber. θ is the eccentricity angle of the waveguide. n co It is the effective refractive index of light transmitted in the fiber core.

[0018] When the refractive index within the cavity changes, the interference spectra of the Mach-Zehnder and Fabry-Perot interferometers shift, enabling salinity and temperature monitoring. The packaged structure is placed in a constant temperature chamber, and salinity is measured by injecting seawater of varying salinity into the π-shaped tube. Temperature is measured by varying the temperature of the constant temperature chamber. The reflection and transmission spectra of the two spectrometers are simultaneously monitored, and a sensitivity matrix is ​​constructed to enable simultaneous measurement of both parameters.

[0019] The beneficial effects of adopting the above technical solution are:

[0020] The present invention provides a temperature and salinity optical fiber sensor based on femtosecond laser inscription and a method for using the same. The present invention realizes the construction of a MZI by inscribing a waveguide, and ensures direct contact between the sensing beam and the measured substance in the MZ interference structure by constructing a semi-open cavity, thereby achieving high-sensitivity measurement of the measured substance.

[0021] This method integrates FP and MZ interferometer structures to simultaneously obtain dual-spectral information, enabling dual-parameter measurement of temperature and salinity. Leveraging the sensitivity of the FP and MZ interferometers to both temperature and salinity, a dual spectrum containing information on temperature and salinity variations is acquired. Wavelength demodulation is used to determine their different sensitivity characteristics, and by constructing a sensitivity matrix, simultaneous temperature and salinity measurement is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of an optical fiber sensor in an embodiment of the present invention;

[0023] Figure 2 A diagram showing the steps for making an optical fiber sensor according to an embodiment of the present invention;

[0024] Figure (a) shows welding and cutting; Figure (b) shows welding; Figure (c) shows femtosecond waveguide writing; Figure (d) shows femtosecond cavity opening.

[0025] Figure 3 This is a diagram of the optical propagation path in an embodiment of the present invention;

[0026] Figure 4 This is a diagram of an experimental system in an embodiment of the present invention;

[0027] Figure 5 Graph showing salinity response characteristics of the MZI and FPI in the optical fiber sensor according to an embodiment of the present invention;

[0028] Figure (a) - salinity response characteristic diagram of FPI; Figure (b) - salinity response characteristic diagram of MZI;

[0029] Figure 6 Graph showing the temperature response characteristics of the MZI and FPI in the optical fiber sensor according to an embodiment of the present invention;

[0030] Figure (a) shows the temperature response characteristics of FPI; Figure (b) shows the temperature response characteristics of MZI. DETAILED DESCRIPTION

[0031] The following embodiments of the present invention are described in further detail with reference to 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.

[0032] A temperature and salinity optical fiber sensor based on femtosecond laser writing, specifically comprising: an input single-mode optical fiber, a hollow-core optical fiber, an output single-mode optical fiber, and a waveguide;

[0033] The input single-mode optical fiber, hollow-core optical fiber and output single-mode optical fiber are aligned and fused in sequence by a fusion splicer;

[0034] The waveguide is divided into three sections, wherein the starting point of the first section of the waveguide is set on the core of the input single-mode optical fiber, 1000 microns away from the fusion interface of the input single-mode optical fiber and the hollow-core optical fiber, the angle between the first section of the waveguide and the core is 2.29 degrees, and the end point stops at the fusion interface of the input single-mode optical fiber and the hollow-core optical fiber; the end point of the first section of the waveguide is used as the starting point of the second section of the waveguide, the length of the second section of the waveguide is 100 microns, and the end point of the second section of the waveguide is used as the starting point of the third section of the waveguide. The end point of the third section of the waveguide is on the core of the output single-mode optical fiber, and the end point is 1000 microns away from the fusion interface of the output single-mode optical fiber and the hollow-core optical fiber;

[0035] A rectangular half-open cavity is provided in the center of the hollow-core optical fiber. The rectangular half-open cavity is prepared by using femtosecond laser micromachining technology, which opens half of the hollow-core optical fiber to ensure the flow of the external measured substance.

[0036] The waveguides on the cladding of a single-mode fiber and a hollow-core fiber, as well as the semi-open cavity in the hollow-core fiber, are fabricated using femtosecond laser micromachining. The direct-write waveguides in the cladding of the single-mode fiber and hollow-core fiber serve as the reference arm of the MZI, while the semi-open cavity serves as the sensing arm of the MZI, ensuring the flow of the external measured material. The two fusion-spliced ​​interfaces between the hollow-core fiber and the single-mode fibers on either side serve as the two reflective end faces of the FPI, forming a Mach-Zehnder interferometer (MZI) and a Fabry-Perot interferometer (FPI).

[0037] Using femtosecond laser micromachining, the first waveguide segment was created on the core of the input single-mode fiber, 1000 microns from the interface between the input single-mode fiber and the hollow-core fiber. The waveguide angle was 2.29 degrees, stopping at the interface. A second straight waveguide segment, 100 microns in length, was then added from the end of the first waveguide segment. The third waveguide segment was then added, starting from the end of the second waveguide segment and ending at the core of the output single-mode fiber, 1000 microns from the interface between the output single-mode fiber and the hollow-core fiber.

[0038] The use of hollow-core optical fiber provides the foundation for the construction of FPI, and femtosecond laser micromachining technology provides technical support for the construction of MZI, realizing the integration of FPI and MZI. The interference spectra of MZ and FP can be obtained simultaneously. By detecting the spectral shift caused by seawater salinity and temperature, the simultaneous measurement of temperature and salinity can be achieved.

[0039] The waveguides on the cladding of single-mode and hollow-core fibers and the semi-open cavity on the hollow-core fibers were fabricated using femtosecond laser micromachining technology. The direct-write waveguides on the cladding of the single-mode and hollow-core fibers served as the reference arm of the MZI, and the semi-open cavity served as the sensing arm of the MZI, while ensuring the flow of the external measured substance.

[0040] The two fusion interfaces of the hollow-core fiber and the single-mode fibers on both sides serve as the two reflective end faces of the FP interferometer.

[0041] The MZI and FPI are integrated in a compact and simple fiber optic sensing structure.

[0042] Changes in seawater temperature and salinity will cause shifts in the MZ and FP interference spectra. By simultaneously monitoring the dual spectra, simultaneous detection of seawater temperature and salinity can be achieved.

[0043] On the other hand, a method for using a temperature and salinity optical fiber sensor based on femtosecond laser inscription is implemented based on the aforementioned temperature and salinity optical fiber sensor based on femtosecond laser inscription, and specifically includes the following steps:

[0044] Step 1: Encapsulate the optical fiber sensor with a π-shaped tube;

[0045] The π-shaped tube includes ports 1 to 4. The optical fiber sensor enters from port 2 of the π-shaped tube and exits from port 1. The external fluid flows in and out through ports 3 and 4.

[0046] Step 2: Connect the input single-mode fiber end of the fiber optic sensor to the supercontinuum light source and spectrometer 1 through a circulator, and directly connect the output single-mode fiber end to spectrometer 2.

[0047] Step 3: Based on the interference principles of Mach-Zehnder interferometer and Fabry-Perot interferometer, the relationship between their interference trough, the effective refractive index in the open cavity, and the effective refractive index of the waveguide is obtained, as shown in Formula 1.

[0048]

[0049] where n e , n w are the effective refractive indices of light transmitted in the cavity and waveguide, respectively. MZ and λ FP are the trough wavelengths of the interference patterns of MZI and FPI, respectively. m is a constant. L1 is the distance from the waveguide starting point to the interface between the input single-mode fiber and the hollow-core fiber. L2 is the length of the hollow-core fiber. θ is the eccentricity angle of the waveguide. n co It is the effective refractive index of light transmitted in the fiber core.

[0050] When the refractive index within the cavity changes, the interference spectra of the Mach-Zehnder and Fabry-Perot interferometers shift, enabling salinity and temperature monitoring. The packaged structure is placed in a constant temperature chamber, and salinity is measured by injecting seawater of varying salinity into the π-shaped tube. Temperature is measured by varying the temperature of the constant temperature chamber. The reflection and transmission spectra of the two spectrometers are simultaneously monitored, and a sensitivity matrix is ​​constructed to enable simultaneous measurement of both parameters.

[0051] The optical fiber sensor designed in the embodiment of the present invention is as follows Figure 1 As shown, it includes an input single-mode fiber, a hollow-core fiber, an output single-mode fiber, and a waveguide; the inner and outer diameters of the hollow-core fiber are 30 microns and 125 microns, and the length of the hollow-core fiber is 100 microns. The starting point (point A) or the end point (point B) of the waveguide is 1000 μm away from the fusion interface of the SMF and the HCF, and the angle (θ) is 2.29 degrees. The length, width, and height of the semi-open cavity are 80 microns, 36 microns, and 62.5 microns, respectively. The specific preparation process is as follows Figure 2 shown.

[0052] In the present invention, we use Rsoft to simulate and analyze it, and the specific light path propagation path is as follows: Figure 3 As shown in Figure 2, the input light at point A will be split into two beams and propagate separately, and then re-coupled back into the output SMF at point B.

[0053] In the present invention, the overall view of the experimental equipment is as follows Figure 4As shown. A supercontinuum light source serves as the light source. Spectrometer 1 (OSA1: AQ6370D) collects the reflectance spectrum via a circulator, and spectrometer 2 (OSA2: AQ6370D) collects the transmission spectrum. A thermostat is used to provide different temperature environments. The sensor probe is sealed in a custom π-tube. Ports 1 and 2 of the π-tube are sealed with UV glue, port 3 serves as the liquid input port, and port 4 serves as the liquid output port. For salinity testing, the incubator temperature is set to 25°C, and saline solutions of varying salinity are injected into the π-tube via a syringe to measure salinity characteristics under constant temperature. For temperature experiments, the salinity of the liquid under test in the π-tube remains constant, while the incubator temperature is varied between 25°C and 55°C.

[0054] First, the salinity response of the sensing structure was tested in a constant temperature environment of 25°C. The measurement solutions with salinities of 0‰, 9.335‰, 20.004‰, 30.007‰, 34.999‰, and 40.001‰ (RI = 1.3329, 1.3345, 1.3356, 1.3377, 1.3384, and 1.3399) were injected into the π-shaped tube respectively. The salinity characteristic response is shown in the figure below. Figure 5 As shown in Figure 2 , the phase difference of the FPI interferometer increases with increasing salinity, causing the spectrum to redshift. Conversely, as the salinity of the measured solution increases, the phase difference of the MZI interferometer decreases, causing the spectrum to blueshift. After linear fitting, the salinity sensitivities of the FPI and MZI are 0.14 nm / ‰ (896 nm / RIU) and -0.7 nm / ‰, respectively. Furthermore, both the FPI and MZI exhibit excellent spectral quality, with fringe visibility greater than 10 dB.

[0055] Then, the temperature response of the sensing structure was tested at a constant salinity of 0‰. The test environment temperature changed from 25℃ to 55℃ with an interval of 5℃. The temperature characteristic response is as follows Figure 6 As shown in Figure 2 , it is well known that the refractive index of salt water decreases as the temperature increases. Therefore, as the temperature rises, the phase difference of the FPI decreases and that of the MZI increases, resulting in a blue shift in the FPI spectrum and a red shift in the MZI spectrum. Fitting analysis shows that the sensitivities of the FPI and MZI at 25°C are -0.08 nm / °C and 0.6 nm / °C, respectively. At 55°C, the sensitivities of the FPI and MZI are -0.26 nm / °C and 1.381 nm / °C, respectively.

[0056] The temperature and salinity optical fiber sensor based on femtosecond laser inscription mentioned in the present invention can simultaneously achieve high-sensitivity sensing of seawater temperature and salinity. It has the advantages of in-fiber integration, compact structure, and simple preparation. It is suitable for measuring seawater temperature and salinity in complex environments.

[0057] The above description is merely a preferred embodiment of the present 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 the present disclosure is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also encompass other technical solutions formed by any combination of the above-mentioned technical features or their equivalents without departing from the above-mentioned inventive concept. For example, a technical solution formed by mutually replacing the above-mentioned features with (but not limited to) technical features with similar functions disclosed in the embodiments of the present disclosure.

Claims

1. A temperature and salinity optical fiber sensor based on femtosecond laser writing, characterized in that: include: Input single-mode fiber, hollow-core fiber, output single-mode fiber, waveguide; The input single-mode optical fiber, hollow-core optical fiber and output single-mode optical fiber are aligned and fused in sequence by a fusion splicer; The waveguide is divided into three sections, wherein the starting point of the first section of the waveguide is set on the core of the input single-mode optical fiber, 1000 microns away from the fusion interface of the input single-mode optical fiber and the hollow-core optical fiber, the angle between the first section of the waveguide and the core is 2.29 degrees, and the end point stops at the fusion interface of the input single-mode optical fiber and the hollow-core optical fiber; the end point of the first section of the waveguide is used as the starting point of the second section of the waveguide, the length of the second section of the waveguide is 100 microns, and the end point of the second section of the waveguide is used as the starting point of the third section of the waveguide. The end point of the third section of the waveguide is on the core of the output single-mode optical fiber, and the end point is 1000 microns away from the fusion interface of the output single-mode optical fiber and the hollow-core optical fiber; A rectangular half-open cavity is provided in the center of the hollow-core optical fiber. The rectangular half-open cavity is prepared by using femtosecond laser micromachining technology, which opens half of the hollow-core optical fiber to ensure the connection between the air hole of the hollow-core optical fiber and the outside world.

2. The temperature and salinity optical fiber sensor based on femtosecond laser writing according to claim 1, characterized in that: The cladding direct-writing waveguide of the single-mode optical fiber and the hollow-core optical fiber serves as the reference arm of the MZI, the semi-open cavity path serves as the sensing arm of the MZI, and the two fusion interfaces of the hollow-core optical fiber and the single-mode optical fibers on both sides serve as the two reflective end faces of the FPI, forming a Mach-Zehnder interferometer (MZI) and a Fabry-Perot interferometer (FPI).

3. A method for using a temperature and salinity optical fiber sensor based on femtosecond laser inscription, which is implemented based on the temperature and salinity optical fiber sensor based on femtosecond laser inscription according to claim 1, characterized in that: The following steps are involved: Step 1: Encapsulate the optical fiber sensor with a π-shaped tube; The π-shaped tube includes ports 1 to 4. The optical fiber sensor enters the π-shaped tube from port 2 and exits from port 1. External fluid flows in and out through ports 3 and 4. Step 2: Connect the input single-mode fiber end of the fiber optic sensor to the supercontinuum light source and spectrometer 1 through a circulator, and connect the output single-mode fiber end directly to spectrometer 2; Step 3: Based on the interference principles of Mach-Zehnder interferometer and Fabry-Perot interferometer, the relationship between their interference trough, the effective refractive index in the open cavity, and the effective refractive index of the waveguide is obtained, as shown in Formula 1. （1） in , are the effective refractive indices of light transmitted in the open cavity and waveguide, respectively; and are the trough wavelengths of the interference patterns of MZI and FPI, respectively, m is a constant, It is the distance from the waveguide starting point to the fusion interface between the input single-mode fiber and the hollow-core fiber. is the hollow-core fiber length, is the waveguide's eccentricity angle, It is the effective refractive index of the light transmitted in the fiber core; When the refractive index in the open cavity changes, the interference spectra of the Mach-Zehnder interferometer and the Fabry-Perot interferometer will shift, thereby enabling the monitoring of salinity and temperature; the packaged structure is placed in a constant temperature box, and seawater of different salinities is injected into the π-shaped tube to achieve salinity measurement, and the temperature is measured by changing the temperature of the constant temperature box; the reflection spectrum and transmission spectrum on the two spectrometers are monitored simultaneously, and their sensitive matrix is ​​constructed to achieve simultaneous measurement of dual parameters.

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