Double-FPI integrated seawater salinity sensor based on PMPCF vernier sensitization

Abstract

The invention provides a dual-FPI integrated seawater salinity sensor based on PMPCF vernier sensitization, and relates to the technical field of optical fiber sensing, the sensor is characterized in that two micro-flow channels are etched on a single-mode optical fiber through a femtosecond laser processing technology, and the micro-flow channels are connected with a large hole of a PMPCF to form a seawater flow channel. The welding surface of the single-mode fiber and the polarization-maintaining photonic crystal fiber is used as a first reflecting surface, and the gold-plated end surface of the polarization-maintaining photonic crystal fiber is used as a second reflecting surface, so that a double-FPI structure is formed. Due to the birefringence effect of the polarization-maintaining photonic crystal fiber, the superposition spectrum of the double FPIs in the reflection spectrum generates the vernier effect. And the change of the seawater salinity can be monitored in real time by detecting the deviation of a reflection spectrum envelope line. The sensor provided by the invention has the advantages of compact structure, high sensitivity, strong anti-interference capability and the like, is suitable for long-term stable monitoring in a complex marine environment, and provides an efficient and reliable solution for accurate measurement of seawater salinity.

Description

Technical Field

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

[0002] Seawater salinity is one of the core parameters for marine environmental monitoring, and its changes directly affect ecological balance, ocean current dynamics, and marine resource development. Traditional salinity measurement mainly relies on electronic sensors (such as temperature-salinity-depth meters (CTDs), which have high accuracy but have defects such as large size, high power consumption, and easy corrosion of metal probes. They are difficult to adapt to the long-term in-situ monitoring needs of deep-sea high-pressure and high-salinity environments.

[0003] Optical fiber 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 principle of Fabry-Perot interferometer (FPI) achieve salinity perception by detecting changes in refractive index, but generally face challenges of insufficient sensitivity and poor anti-interference ability. For example, changes in salinity and temperature will cause refractive index shifts, requiring complex decoupling mechanisms or additional compensation structures, leading to system complexity. Existing solutions such as FP interferometers can measure seawater salinity, but they 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 optical fiber to achieve FPI measurement of seawater salinity, but the salinity response is low and the signal demodulation is difficult, resulting in low practicality and insufficient reliability of the structure. Another Chinese patent "CN118190197A" connects two FPIs in parallel and uses the vernier effect to amplify the spectral shift, which can theoretically improve sensitivity. However, this technology requires strict matching of the free spectral range (FSR) of the dual interferometers, and the manufacturing error tolerance is extremely low; the discrete structure is easily disturbed by vibration or temperature gradient in a dynamic environment, resulting in the failure of the vernier effect. The common defect of both is that it is difficult to achieve high sensitivity, compactness, and environmental adaptability at the same time.

[0004] The fiber optic vernier effect provides a new idea for solving the sensitivity bottleneck. By superimposing two interference spectra with similar FSRs, small wavelength shifts can be significantly amplified. However, traditional vernier sensors mostly use discrete or cascaded FPI structures, which require precise alignment and are easily affected by environmental stress, making them difficult to apply in practice. How to achieve a compact and highly consistent vernier effect sensor has become the key to technological breakthroughs.

[0005] In recent years, the unique advantages of polarization-maintaining photonic crystal fiber (PMPCF) have provided a new direction for salinity sensing. PMPCF achieves high birefringence and photonic bandgap effect through periodic air hole structure, which can enhance the interaction between light and external media. Ordinary photonic crystal fiber has only one diameter of air hole. Compared with ordinary photonic crystal fiber, the air hole structure of polarization-maintaining photonic crystal fiber has two air hole structures with different diameters, and the hole with larger diameter (large hole) is more conducive to the circulation of seawater. The open cavity design can expand the contact area with seawater and directly improve the refractive index sensitivity. In addition, femtosecond laser micromachining technology can accurately prepare microstructures such as open cavity and reflective mirror in PMPCF with submicron precision and low thermal damage characteristics, ensuring high consistency and mechanical strength of the sensing unit. For example, the open FPI etched by femtosecond laser can be directly exposed to seawater to avoid packaging stress interference, and at the same time integrate multimode optical fiber to build a compact interference optical path to achieve efficient separation of temperature and salt signals. Summary of the invention

[0006] In view of the deficiencies of the prior art, the present invention provides a dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization. The high birefringence characteristics of PMPCF enable the optical fiber structure to produce dual FPI, and the tiny FSR difference is used to form a built-in vernier effect. The femtosecond laser etches a micro-channel 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 demodulation of the salinity response is achieved. The integrated structure abandons the defects of the traditional discrete design, and has 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: a single-mode optical fiber and a polarization-maintaining photonic crystal fiber; The single-mode optical fiber is fused with one end of the polarization-maintaining photonic crystal fiber, and the fusion surface is used as the first reflection surface, namely, reflection surface 1; the other end of the polarization-maintaining photonic crystal fiber is plated with a gold film, which is used as the second reflection surface, namely, reflection surface 2; The single-mode optical fiber and the reflection surface 1 and the reflection surface 2 form a double reflection surface, forming a double FP interferometer.

[0008] The single-mode optical fiber is provided with two micro-flow channels etched by femtosecond laser, which are connected with the macro-pores of the polarization-maintaining photonic crystal optical fiber to form a seawater flow path.

[0009] 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.

[0010] The beneficial effects of adopting the above technical solution are: The present invention provides a dual FPI integrated seawater salinity sensor based on PMPCF vernier 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 fault tolerance rate, make the dual FP interferometer more likely to produce a vernier effect, increase the practicality of the sensor, and provide a reliable solution for monitoring complex seawater environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is a schematic diagram of the structure of a photonic crystal fiber-sensitized FP seawater salinity sensor; Figure 2 It is a schematic diagram of the optical path propagation of the dual FP interferometer; Figure 3 It is the overall structural diagram of the experimental system; Figure 4 It is a schematic diagram of the structure of the π-type tube package; Figure 5 is the relationship between the fundamental mode field distribution diagram of the PMPCF in the x and y polarization directions and the effective mode refractive index and wavelength; Figure 6 is the upper envelope of the reflection superposition spectrum of the dual FP interferometer under different salinities; Figure 7 This is the salinity response curve of the FP seawater salinity sensor sensitized by photonic crystal fiber. DETAILED DESCRIPTION

[0012] 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.

[0013] A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization, such as Figure 1 As shown, specifically including: single-mode optical fiber and polarization-maintaining photonic crystal fiber; 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, namely, reflection surface 1; the other end of the polarization-maintaining photonic crystal fiber is coated with a gold film, serving as the second reflection surface, namely, reflection surface 2; wherein the function of the gold film is to increase the reflectivity of the end face of the polarization-maintaining photonic crystal fiber.

[0014] Since polarization-maintaining photonic crystal fiber has a birefringence effect, seawater flows through two micro-flow channels, which in turn affects the birefringence of the polarization-maintaining photonic crystal fiber. When the 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 through the path, thereby forming FP interference spectra with different free spectral ranges. The two FP interference spectra are superimposed to form a cursor effect, and the real-time monitoring of seawater salinity can be achieved by detecting the movement of the envelope of the reflection spectrum.

[0015] The single-mode optical fiber and the reflection surface 1 and the reflection surface 2 form a double reflection surface, forming a double FP interferometer.

[0016] The single-mode optical fiber is provided with two micro-flow channels etched by femtosecond laser, which are connected with the macro-pores of the polarization-maintaining photonic crystal optical fiber to form a seawater flow path.

[0017] Enhanced vernier effect: The birefringence characteristics of PMPCF cause slight differences in the free spectral ranges of the two FP cavities, and the superposition of interference spectra forms a vernier effect, which significantly amplifies the spectral shift caused by salinity changes.

[0018] The salinity of seawater affects the birefringence of polarization-maintaining photonic crystal fiber, and further affects the superimposed spectrum. The salinity of seawater can be detected by detecting the offset of the cursor envelope of the reflection spectrum.

[0019] This embodiment: When light is transmitted in the structure, its optical path propagation diagram is as follows Figure 2 As shown. When the light of single-mode fiber ( I 0) When it reaches the reflecting surface 1 through the single-mode optical fiber, part of the light ( I 1) Reflection occurs, and the transmitted light continues to transmit along the polarization-maintaining photonic crystal fiber. I 1 can be expressed as: I 1= I 0； in a is the reflectivity of reflective surface 1.

[0020] When the light reaches the reflection surface 2, due to the birefringence effect of the polarization-maintaining photonic crystal fiber, the light beam generates two reflected lights at the reflection surface 2. I 2 and I 3 , I 2 and I 3 The polarization directions of the polarization-maintaining photonic crystal fibers are perpendicular to each other. B It can be expressed as: B =| n x - ny |; in n x and n y They are x Direction and y The effective mode refractive index in the direction.

[0021] When the reflected light from reflective surface 2 reaches reflective surface 1, I 2 and I 3 The transmitted light is respectively I 1 Interference occurs, forming two FP interferometers. The two FP interferometers have similar free spectral ranges, so the coherent spectrum after the two FP interferometers are superimposed can form a vernier effect. The free spectral range (FSR) of the envelope of the superimposed spectrum can be expressed as: ; in λ is the wavelength of light in a vacuum, L Represents the length of the polarization-maintaining photonic crystal fiber.

[0022] When the salinity of seawater changes, the refractive index of the large hole of the polarization-maintaining photonic crystal fiber changes accordingly, which will cause the superimposed paving lines to shift. By detecting the offset of the envelope line, the change in seawater salinity can be detected.

[0023] The overall structure diagram of the experimental system is shown in Figure 3 As shown in the figure. 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 packaged by a π-shaped tube, and its packaging schematic diagram is shown in the figure. Figure 4 shown.

[0024] Then the salinity characteristics of the sensing structure were analyzed. PMPCF has a lot of air pores, where the small pore diameter is 6μm, the large pore diameter is 15μm, and the cladding diameter is 125μm. In the area near the core, the pore spacing between the large pores is 17.2μm, the pore spacing between the small pores is 8.6μm, and the fiber length is set to 1cm. Based on the above information, the mode field of PMPCF is analyzed using the finite element method, and the result is x Direction and y The effective mode refractive index in the direction. The fundamental mode field distribution diagram is as follows Figure 5 As the wavelength increases, x Direction and yThe effective mode refractive index in all directions decreases, but the refractive index difference increases with the increase of wavelength. Salt solutions of different concentrations were used to simulate the change of seawater salinity, and the salinity was set to 26.29‰, 31.46‰, 36.64‰, 41.81‰, and 46.99‰. Figure 6 is the upper envelope of the superimposed reflection spectrum of the dual FP interferometer at different salinities. When the salinity increases, the trough of the envelope redshifts. The trough wavelength at each salinity is recorded, and then a linear fit is performed. The linear fit diagram of the salinity response is shown in Figure 7 As shown, according to the fitting results, the salinity sensitivity of the superimposed spectrum of the dual FP interferometer is 3.952nm / ‰, and the linearity is 0.98485. According to the results of related patents, the salinity sensitivity of the present invention is 19.76 times and 21.41 times that of the Chinese patents "CN116380278A" and "CN116559117A", respectively.

[0025] The above description is only a preferred embodiment of the present disclosure and an explanation of the technical principles used. 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 technical features, but 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 above features are replaced with (but not limited to) technical features with similar functions disclosed in the embodiments of the present disclosure to form a technical solution.

Claims

1. A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization, characterized in that: Specifically include: Single-mode optical fiber, and polarization-maintaining photonic crystal fiber; The single-mode optical fiber is fused with one end of the polarization-maintaining photonic crystal optical fiber, and the other end of the polarization-maintaining photonic crystal optical fiber is plated with a gold film.

2. A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization according to claim 1, characterized in that: The fusion surface of the single-mode optical fiber and the polarization-maintaining photonic crystal fiber is used as the first reflection surface, namely, reflection surface 1; the surface of the polarization-maintaining photonic crystal fiber coated with a gold film is used as the second reflection surface, namely, reflection surface 2.

3. A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization according to claim 2, characterized in that: The single-mode optical fiber and the reflection surface 1 and the reflection surface 2 form a double reflection surface, forming a double FP interferometer.

4. A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization according to claim 1, characterized in that: The single-mode optical fiber is etched with two microfluidic channels by femtosecond laser.

5. A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization according to claim 4, characterized in that: The microfluidic channel is communicated with the macropore of the polarization-maintaining photonic crystal optical fiber.

6. A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization according to claim 5, characterized in that: The connection point between the micro-flow channel and the polarization-maintaining photonic crystal optical fiber forms a seawater flow path.

7. A dual FPI integrated seawater salinity sensor based on PMPCF vernier sensitization according to claim 6, characterized in that: 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 dual FP interferometer, so that the dual FP interferometer forms a vernier effect, and the seawater salinity is detected by detecting the offset of the vernier envelope of the reflection spectrum.

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