A dual-FPI integrated seawater salinity sensor based on PMPCF cursor sensitization

Abstract

The present invention provides a dual-FPI integrated seawater salinity sensor based on PMPCF cursor sensitization, which relates to the field of fiber optic sensing technology. The sensor etches two microfluidic channels on a single-mode fiber through femtosecond laser processing technology and connects them to the large holes of the PMPCF to form a seawater circulation channel. The fusion surface of the single-mode fiber and the polarization-maintaining photonic crystal fiber serves as the first reflection surface, and the gold-plated end face of the polarization-maintaining photonic crystal fiber serves as the second reflection surface, forming a dual-FPI structure. Due to the birefringence effect of the polarization-maintaining photonic crystal fiber, a cursor effect is generated in the superimposed spectrum of the dual-FPI in the reflection spectrum. By detecting the offset of the reflection spectrum envelope, the change of seawater salinity can be monitored in real time. The sensor of the present invention has the advantages of compact structure, high sensitivity, strong anti-interference ability, etc., and is suitable for long-term stable monitoring in complex marine environments, providing an efficient and reliable solution for the accurate measurement of seawater salinity.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic sensing, and in particular to a dual-FPI integrated seawater salinity sensor based on PMPCF cursor sensitization. Background Art

[0002] Seawater salinity is one of the core parameters for marine environmental monitoring, and its change directly affects ecological balance, ocean current dynamics, and marine resource development. Traditional salinity measurements mainly rely on electronic sensors (such as Conductivity Temperature Depth profiler CTD). Although they have high precision, they have defects such as large volume, high power consumption, and easy corrosion of metal probes, making it difficult to meet the long-term in-situ monitoring requirements of deep-sea high-pressure and high-salinity environments.

[0003] Fiber optic sensing technology has become a research hotspot for marine salinity monitoring due to its advantages such as small size, anti-interference, and corrosion resistance. Sensors based on the Fabry-Perot interferometer (FPI) principle achieve salinity sensing by detecting refractive index changes, but generally face challenges of insufficient sensitivity and poor anti-interference ability. For example, both salinity and temperature changes can cause refractive index shifts, requiring complex decoupling mechanisms or additional compensation structures, resulting in system complexity. Existing solutions such as FP interferometers can measure seawater salinity, but have problems such as low sensitivity, loose structure, high process difficulty, or poor long-term stability. Taking the Chinese patent "CN119147019A" as an example, it uses seven-core fiber to achieve FPI measurement of seawater salinity, but has a low salinity response and difficult signal demodulation, resulting in low practicality and insufficient reliability of the structure. Another Chinese patent "CN118190197A" amplifies the spectral shift by using the cursor effect through parallel-connected dual-FPIs. In theory, it can improve sensitivity. However, this technology requires strict matching of the free spectral range (FSR) of the dual interferometers, with an extremely low manufacturing tolerance; the discrete structure is vulnerable to vibration or temperature gradient interference in a dynamic environment, resulting in the failure of the cursor effect. Their common defect is that it is difficult to achieve both high sensitivity and compactness and environmental adaptability.

[0004] The fiber optic cursor effect provides a new idea for solving the sensitivity bottleneck. By superimposing two interference spectra with similar FSRs, a small wavelength shift can be significantly amplified. However, traditional cursor sensors mostly use discrete or cascaded FPI structures, which require precise alignment and are easily affected by environmental stress, resulting in difficulties in practical application. How to achieve a cursor effect sensor with a compact structure and high consistency has become the key to technological breakthrough.

[0005] In recent years, the unique advantages of polarization-maintaining photonic crystal fibers (PMPCFs) have provided new avenues for salinity sensing. PMPCFs achieve high birefringence and the photonic bandgap effect through a periodic air hole structure, enhancing the interaction between light and the external medium. Conventional photonic crystal fibers (PCFs) have air holes of only one diameter. Compared to conventional PCFs, the air hole structure of polarization-maintaining PCFs features two different diameters, with larger diameter holes (macropores) more conducive to the flow of seawater. The open cavity design increases the contact area with seawater, directly improving refractive index sensitivity. Furthermore, femtosecond laser micromachining technology, with its submicron precision and low thermal damage, enables precise fabrication of microstructures such as open cavities and reflective mirrors in PMPCFs, ensuring high consistency and mechanical strength of the sensing unit. For example, the open FPI etched by a femtosecond laser can be directly exposed to seawater, avoiding packaging stress interference. The integration of multimode fiber creates a compact interferometric optical path, enabling efficient separation of temperature and salinity signals. Summary of the Invention

[0006] In response to the shortcomings of the existing technology, the present invention provides a dual-FPI integrated seawater salinity sensor based on PMPCF vernier sensitivity enhancement. The high birefringence characteristics of PMPCF enable the optical fiber structure to produce dual FPIs, and the tiny FSR difference is used to form a built-in vernier effect. A femtosecond laser etches a microchannel in the single-mode optical fiber, allowing seawater to flow through the large pores of the PMPCF. The wavelength shift caused by salinity is amplified several times. At the same time, by taking the envelope of the dual FPI, the salinity response is demodulated. The integrated structure abandons the defects of traditional discrete designs and combines high sensitivity, compactness and environmental adaptability, providing a reliable solution for long-term and stable monitoring in complex marine environments.

[0007] A dual-FPI integrated seawater salinity sensor based on PMPCF vernier sensitization, specifically comprising: single-mode optical fiber and polarization-maintaining photonic crystal fiber;

[0008] The single-mode optical fiber is fused with one end of the polarization-maintaining photonic crystal fiber, and the fusion surface serves as the first reflection surface, i.e., reflection surface 1; the other end of the polarization-maintaining photonic crystal fiber is plated with a gold film, serving as the second reflection surface, i.e., reflection surface 2;

[0009] The single-mode optical fiber, the reflection surface 1 and the reflection surface 2 form a double reflection surface, forming a double FP interferometer.

[0010] The single-mode optical fiber is etched with two micro-flow channels by femtosecond laser, which are connected with the macropores of the polarization-maintaining photonic crystal fiber to form a seawater flow path.

[0011] Seawater salinity affects the birefringence of polarization-maintaining photonic crystal fiber, which in turn affects the reflection spectrum of the dual-FP interferometer, causing the dual-FP interferometer to form a vernier effect. The seawater salinity is detected by detecting the offset of the vernier envelope of the reflection spectrum.

[0012] The beneficial effects of adopting the above technical solutions are as follows:

[0013] The present invention provides a dual-FPI integrated seawater salinity sensor based on PMPCF cursor sensitization. Compared with the existing single FP interferometer, the present invention greatly improves the sensitivity of measuring seawater salinity and enhances the sensing characteristics of the sensor. In addition, the integrated design of the present invention can increase the error tolerance, make the dual FP interferometer more likely to generate the cursor effect, increase the practicability of the sensor, and provide a reliable solution for monitoring complex seawater environments. Brief Description of the Drawings

[0014] Figure 1 is a schematic structural diagram of an FP seawater salinity sensor sensitized by a photonic crystal fiber;

[0015] Figure 2 is a schematic diagram of the optical path propagation of a dual FP interferometer;

[0016] Figure 3 is a schematic diagram of the overall structure of the experimental system;

[0017] Figure 4 is a schematic diagram of the structure of a π-type tube package;

[0018] Figure 5 is the fundamental mode field distribution diagram of PMPCF in the x and y polarization directions and the relationship between the effective mode refractive index and wavelength;

[0019] Figure 6 is the upper envelope of the reflection superposition spectrum of a dual FP interferometer at different salinities;

[0020] Figure 7 is the salinity response curve diagram of an FP seawater salinity sensor sensitized by a photonic crystal fiber. Detailed Embodiments

[0021] The following combines the drawings and embodiments to further describe in detail the specific embodiments of the present invention. The following embodiments are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0022] A dual-FPI integrated seawater salinity sensor based on PMPCF cursor sensitization, as Figure 1 shown, specifically includes: a single-mode fiber and a polarization-maintaining photonic crystal fiber;

[0023] One end of the single-mode fiber is fusion-spliced with one end of the polarization-maintaining photonic crystal fiber, and the fusion-splicing surface serves as the first reflection surface, that is, reflection surface 1; the other end of the polarization-maintaining photonic crystal fiber is coated with a gold film, which serves as the second reflection surface, that is, reflection surface 2; the function of the gold film is to increase the reflectivity of the end face of the polarization-maintaining photonic crystal fiber.

[0024] Due to the birefringence effect of polarization-maintaining photonic crystal fiber, seawater flows through two microfluidic channels, which in turn affects the birefringence of the polarization-maintaining photonic crystal fiber. When a light beam passes through the polarization-maintaining photonic crystal fiber, the end-face reflected light of the polarization-maintaining photonic crystal fiber has different effective mode refractive indices along its path, thus forming FP interference spectra with different free spectral ranges. After the two FP interference spectra are superimposed, a Vernier effect is formed, and the real-time monitoring of seawater salinity can be achieved by detecting the movement of the envelope of the reflection spectrum.

[0025] The single-mode fiber and the reflecting surface 1 and the reflecting surface 2 form a double reflecting surface, forming a double FP interferometer.

[0026] The single-mode fiber is etched with two microfluidic channels by femtosecond laser, which is connected to the large holes of the polarization-maintaining photonic crystal fiber to form a seawater flow path.

[0027] Enhancement of the Vernier effect: The birefringence characteristics of the PMPCF result in a slight difference in the free spectral ranges of the two FP cavities. The superimposed interference spectra form the Vernier effect, and the spectral shift caused by the change in salinity is significantly amplified.

[0028] The salinity of seawater affects the birefringence of the polarization-maintaining photonic crystal fiber, and thus affects the superimposed spectrum. By detecting the shift of the Vernier envelope of the reflection spectrum, the detection of seawater salinity can be achieved.

[0029] This embodiment:

[0030] When light propagates in the structure, its optical path propagation diagram is as Figure 2 shown. When the light of the single-mode fiber ( I 0) reaches the reflecting surface 1 through the single-mode fiber, part of the light ( I 1) is reflected, and the transmitted light continues to propagate along the polarization-maintaining photonic crystal fiber. I 1 can be expressed as: I 1 = aI 0；

[0031] where a is the reflectivity of the reflecting surface 1.

[0032] When the light reaches the reflecting surface 2, due to the birefringence effect of the polarization-maintaining photonic crystal fiber, two reflected light beams are generated at the reflecting surface 2, I 2 and I 3 , I 2 and I 3 have perpendicular polarization directions. The birefringence B of the polarization-maintaining photonic crystal fiber can be expressed as: B = | nx - n y |;

[0033] wherein n x and n y are respectively x the effective modal refractive indices in the y direction and the

[0034] When the reflected light from the reflecting surface 2 reaches the reflecting surface 1, I 2 interferes with I 3 the transmitted light of I 1 respectively to form two FP interferometers. The two FP interferometers have similar free spectral ranges. Therefore, the coherent spectrum after the superposition of the two FP interferometers can form a Vernier effect. The free spectral range (FSR) of the envelope of the superposed spectrum can be expressed as: ;

[0035] wherein λ represents the wavelength of light in vacuum, L represents the length of the polarization-maintaining photonic crystal fiber.

[0036] When the seawater salinity changes, the refractive index of the large holes in the polarization-maintaining photonic crystal fiber changes accordingly, which will cause the envelope of the superposed spectrum to shift. By detecting the shift amount of the envelope, the change of seawater salinity can be detected.

[0037] The schematic diagram of the overall structure of the experimental system is as shown in Figure 3 . The system consists of a light source, a spectrometer, a circulator, and a constant temperature box. The light emitted by the light source passes through the circulator and interferes in the sensing structure, and then the reflected light is detected by the spectrometer. The function of the constant temperature box is to provide a constant temperature environment. The sensing structure is encapsulated by a π-shaped tube, and its encapsulation schematic diagram is as shown in Figure 4 .

[0038] Then, the salinity characteristics of the sensing structure were analyzed. The PMPCF has a lot of air hole structures, where the small hole diameter is 6μm, the large hole diameter is 15μm, and the cladding diameter is 125μm. In the region near the fiber core, the hole spacing between large holes is 17.2μm, the hole spacing between small holes is 8.6μm, and the fiber length is set to 1 cm. Based on the above information, the modal field of the PMPCF was analyzed using the finite element method to obtain x the effective modal refractive indices in the y direction and the Figure 5 direction. The distribution diagram of its fundamental mode field is as shown in the inset of Figure 5 . As the wavelength increases,x The effective mode refractive indices in the y direction and the Figure 6 direction both decrease, but the refractive index difference increases with the increase of wavelength. Salt solutions with different concentrations are used to simulate the change of seawater salinity, and the salinities are set to 26.29‰, 31.46‰, 36.64‰, 41.81‰, 46.99‰. Figure 7 is the upper envelope of the superimposed reflection spectra of the dual FP interferometer at different salinities. When the salinity increases, the troughs of the envelope shift to the red. Record the trough wavelengths at each salinity and then perform a linear fit. The linear fit graph of the salinity response is as shown in

[0039] The above description is only the preferred embodiments of the present disclosure and the description of the applied technical principles. 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 the specific combination of the above technical features, and should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the above inventive concept. For example, the technical solutions formed by mutually replacing the above features with the (but not limited to) technical features with similar functions disclosed in the embodiments of the present disclosure.

Claims

1. A dual-FPI integrated seawater salinity sensor based on PMPCF cursor sensitization, characterized in that, Specifically include: Single-mode optical fiber and polarization-maintaining photonic crystal fiber; One end of the single-mode optical fiber is fusion-spliced with one end of the polarization-maintaining photonic crystal fiber, and a gold film is plated on the other end of the polarization-maintaining photonic crystal fiber; The fusion-splicing surface of the single-mode optical fiber and the polarization-maintaining photonic crystal fiber serves as the first reflection surface, that is, reflection surface 1; the surface of the polarization-maintaining photonic crystal fiber plated with a gold film serves as the second reflection surface, that is, reflection surface 2; The single-mode optical fiber and reflection surface 1 and reflection surface 2 form a double reflection surface to form a double FP interferometer; Two microfluidic channels are etched on the single-mode optical fiber by femtosecond laser; The microfluidic channels are communicated with the large holes of the polarization-maintaining photonic crystal fiber; A seawater flow path is formed at the connection of the microfluidic channels and the polarization-maintaining photonic crystal fiber; In the seawater flow path, the seawater salinity affects the birefringence of the polarization-maintaining photonic crystal fiber, and further affects the reflection spectrum of the double FP interferometer, so that the double FP interferometer forms a vernier effect, and the detection of seawater salinity is realized by detecting the offset of the vernier envelope of the reflection spectrum.

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