Multi-point refractive index measurement optical fiber sensor based on periodic series side polishing sub-crystal optical fiber structure

Abstract

The invention provides a multi-point refractive index measurement optical fiber sensor based on a periodic series side polishing sub-crystal optical fiber structure. The sensor sequentially comprises a first single-mode optical fiber, a first section of coreless optical fiber, a plurality of periodic series structure units, a second section of coreless optical fiber and a second multimode optical fiber, wherein each periodic structure unit consists of a section of single-mode optical fiber and a section of photonic crystal optical fiber. The photonic crystal fiber part is subjected to side polishing treatment, and the side polishing depths of all the sections are sequentially different. By adjusting the polishing depth, the mode coupling strength between each structural unit and an external medium can be controlled, so that resonant wavelength responses at different positions are generated in a transmission spectrum. And each formant corresponds to an independent space sensing area, so that the change of refractive indexes at a plurality of positions can be sensed at the same time. The structure has the advantages of flexible assembly, compact structure, high response sensitivity and the like, and is particularly suitable for real-time monitoring of multi-region refractive indexes in a micro-fluidic chip system. Compared with a traditional single-point sensor, the number of sensing channels is increased through the modular series connection design, the information acquisition capacity and the detection efficiency of a microfluidic platform are improved, and good realizability is achieved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a multi-point refractive index measurement fiber optic sensor based on a periodically serially polished sub-crystal fiber structure. Specifically, it is applied to the synchronous detection of multi-point refractive index and belongs to the category of high-resolution refractive index measurement devices in fiber optic sensors, fiber optic interferometers, and microfluidic detection systems. Background Technology

[0002] With the widespread application of fiber optic sensing technology in fields such as biological detection, environmental monitoring, chemical analysis, and microfluidic systems, higher demands are being placed on multi-point, high-sensitivity refractive index measurement methods. Traditional fiber optic sensors, such as fiber Bragg gratings (FBGs) or long-period fiber gratings (LPFGs), while possessing advantages such as compact structure and strong resistance to electromagnetic interference, generally only achieve single-point or few-point refractive index monitoring, making it difficult to meet the simultaneous sensing requirements of multiple detection sites in microfluidic systems. On the other hand, side-projected fiber structures are widely used for high-sensitivity refractive index detection due to their enhanced evanescent field effect. However, existing side-projected fiber sensors are mostly single-segment in form, with fixed structural parameters, making them unable to flexibly adapt to different detection environments. Meanwhile, photonic crystal fibers (PCFs), due to their controllable guiding mode properties and special microstructures, are also used for refractive index sensing, but their integration capabilities in multi-segment assembly and tunable response are limited. For example, patent CN202411618096 proposes an interferometer sensing array based on photonic crystal fibers and its fabrication method. The photonic crystal fiber-based interferometer sensor array is formed by cascading multiple photonic crystal fiber interferometers with different collapse lengths, each of the photonic crystal fiber interferometers.

[0003] Existing multi-point refractive index sensing methods often rely on complex beam splitting systems, fiber arrays, or multi-channel demodulators, which not only increase system cost and size but may also lead to problems such as optical path mismatch and signal crosstalk, reducing system stability and reliability.

[0004] Therefore, there is a need for a new fiber optic sensing structure that is compact, simple to manufacture, highly sensitive, and has multi-point response capability, which can achieve simultaneous measurement of refractive index at multiple spatial locations without introducing complex optical paths and external auxiliary systems. It is particularly suitable for online monitoring scenarios in highly integrated platforms such as microfluidic chips. Summary of the Invention

[0005] The purpose of this invention is to provide a multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure, to solve the problems of limited detection points, difficulty in structural expansion, difficulty in adjusting sensitivity, and inability to adapt to different refractive index detection environments in existing fiber optic sensing technologies. This sensor has a compact structure, is easy to assemble, possesses multi-point resonant response capability, and can flexibly adjust the side-polishing depth of each sensing area according to the characteristics of the liquid being measured, thereby achieving simultaneous refractive index measurement at multiple spatial locations. It is particularly suitable for real-time monitoring and analysis of the refractive index of liquids in multiple regions within a microfluidic chip platform.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: the sensor sequentially includes a first single-mode fiber, a first coreless fiber segment, multiple periodic tandem structural units, a second coreless fiber segment, and a second multimode fiber; wherein, each periodic structural unit is composed of a multimode fiber segment and a grapefruit-shaped photonic crystal fiber segment, the grapefruit-shaped photonic crystal fiber being side-polished to extract the evanescent field of the optical guiding mode to enhance the sensing capability of the refractive index of the external medium.

[0007] The fiber optic structure is connected by fusion splicing. After the overall fusion splicing is completed, multiple structural units undergo multi-segment directional polishing at a preset depth on a high-precision side-polishing platform. This method effectively ensures the positional accuracy and structural continuity of the side-polishing area, simplifies the process, and enhances the repeatability and mechanical stability of the overall sensor.

[0008] The grapefruit-shaped photonic crystal fibers in the multiple structural units have different side-throwing depths, and the side-throwing positions of each segment are independent and non-overlapping along the fiber axis. By adjusting the side-throwing depths of different segments, the response sensitivity to different test liquids can be optimized, enabling each sensing area to form a resonant wavelength with distinct characteristics in the transmission spectrum, thus achieving multi-point synchronous monitoring.

[0009] When light travels from a single-mode fiber to a coreless fiber, it excites multiple high-order and low-order modes. The input optical field of a coreless fiber can be represented as:

[0010] In its formula, The number of modes excited in the fiber core. The coordinates are the radial coordinates on the cross-section of the optical fiber. and These represent the magnetic field profile and the excitation coefficient, respectively.

[0011] The evanescent field near the coreless fiber interface will induce a non-negligible attenuation of the optical field as it passes through the NCF. Therefore, over the propagation distance... The total light field distribution at that location is expressed as:

[0012] In the formula, and These represent the transport constant and the evanescent absorption coefficient, respectively. Based on the absorption theory of the cladding evanescent field, gamma... It can be represented as:

[0013] In the formula, Vacuum wavelength, The attenuation coefficient of the external liquid under test at a specific wavelength. and These refer to the effective refractive index of the fiber cladding and the external liquid, respectively. It is the critical angle. The output light intensity 𝐼(𝑟,𝐿) can be expressed as:

[0014] In the formula, It is the total length of the sensor structure.

[0015] Because this invention employs multiple cascaded structural units, and each PCF side-throw depth is different, the sensitivity factor of each segment is... They are all different. Through reasonable design of each section... (e.g., 2μm, 4μm, 6μm, etc.) can make each resonant wavelength... The spectra show clear distinctions, enabling multi-point decoupled detection. In practical applications, by calibrating the spectral drift curves corresponding to each polishing depth, the refractive index changes of the external liquid at multiple locations can be inverted. Attached Figure Description

[0016] Figure 1 This is a 2D and 3D schematic diagram of a multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished photonic crystal fiber structure. The structure consists of a single-mode fiber 1, a coreless fiber 2, a grapefruit-shaped photonic crystal fiber 3, and a multimode fiber 4, wherein the side polishing depths of the grapefruit-shaped photonic crystal fiber with the periodic structure are 2μm, 4μm, 6μm, and 8μm, respectively.

[0017] Figure 2 This is a schematic diagram of the cross-section of a grapefruit-shaped photonic crystal fiber.

[0018] Figure 3 This is a schematic diagram of a multi-point refractive index measurement fiber optic sensor system based on a periodically tandemly polished sub-crystal fiber structure applied to droplet microfluidics. Detailed Implementation

[0019] In this embodiment, the sensor is embedded in a typical "X-shaped reaction channel" structure to detect refractive index changes in different confluence regions. It is particularly suitable for online monitoring of mixing, reaction, or diffusion processes of multi-component liquids in microchannels. The following is illustrated in the appendix... Figure 2 The technical solutions of the present invention will be further explained in the examples and embodiments.

[0020] Before use, the user should prepare a stable light source. The broadband light source (BBS) emits a stable light signal, and its wavelength and power should be set to the values ​​required by the system. The light source output should be stably coupled into the single-mode fiber input of the sensor via an optical fiber coupler. Simultaneously, the sensor output should be connected to an optical detection device such as a spectrometer to collect the output spectrum of the transmitted signal. After the connection is complete and the system stability is checked, actual measurements can begin.

[0021] During the measurement process, an optical signal emitted from a stable light source is injected into one end of the fiber optic sensor via an optical fiber coupler. The optical signal from the light source passes sequentially through multiple periodic structural units inside the sensor. Each unit contains a section of side-polished photonic crystal fiber, and the position of each structural section precisely corresponds to a reaction point in the X-shaped channel. The optical signal passes sequentially through four sections of photonic crystal fiber with different side-polishing depths inside the sensor. The side-polished region of each G-PCF section constitutes a sensing unit, corresponding to regions A, B, C, and D in the figure, respectively. Because the side-polished structure enhances the evanescent field coupling between the guided mode and the external medium, even a small change in the liquid refractive index can cause mode coupling of the local optical signal, thereby forming a resonant attenuation peak with a fixed position and variable intensity in the output spectrum.

[0022] The four side-projection zones are located in different fluid environments. For example, regions A and B are close to the main inlet and may be pure liquid 1 or liquid 2; while regions C and D are located in the middle of the mixing zone, where liquid mixing, concentration gradients, or the formation of reaction products occur. Therefore, the local refractive index of the four regions has spatial differences, and the sensor can obtain complete spatial information of the reaction process by synchronously acquiring the corresponding resonant wavelength changes.

[0023] The output is connected to the spectrometer via optical fiber to achieve real-time acquisition of multi-point resonance spectra. During measurement, the output spectrum can be acquired at different time points, and the positions of the spectral attenuation peaks corresponding to the four sensing points A, B, C, and D can be extracted to analyze their shift over time.

[0024] If the refractive index of the liquid at a certain measuring point increases, the corresponding attenuation peak in the sensor output spectrum will shift towards a longer wavelength, i.e., a red shift occurs; conversely, a blue shift occurs. If a peak shows a red shift (longer wavelength), it indicates that the refractive index of the liquid at that location has increased, possibly due to increased solute concentration, reactant formation, or enhanced liquid mixing; if a blue shift occurs (shorter wavelength), it indicates dilution or fluid switching. By tracking the output wavelength positions of multiple side-projection regions, the refractive index change trend at different spatial locations in the reaction region can be obtained. If multiple measuring points simultaneously show wavelength shifts in similar directions, it indicates that the overall liquid concentration in the entire channel has increased or decreased; if the wavelength shift trends differ significantly between measuring points, it reflects different local liquid mixing states, indicating the existence of concentration gradients or reaction rate differences.

[0025] By analyzing the spectral drift behavior of the four regions, it is possible not only to determine whether a liquid reaction has occurred in the channel, but also to further infer whether the mixing process is symmetrical and whether the reaction diffusion is uniform. For example, if the spectral drift in regions B and C is greater than that in regions A and D, it indicates that there is a more significant concentration gradient or reactivity in the middle region; if the spectral drift directions of the four sensing points are consistent but the amplitudes are different, it indicates that the reaction occurs throughout the channel, but there are differences in the intensity of diffusion.

[0026] Furthermore, by comparing the response amplitudes of different side-projection zones, the reaction intensity or the degree of inter-liquid diffusion can be preliminarily determined. For example, if there is a significant wavelength shift at the central measuring point while the changes at the two side measuring points are smaller, it may indicate that the main reaction occurs in the central region; if the response at the edge measuring points is enhanced, there is liquid flow shift or increased diffusion at the reaction interface.

[0027] After the test, the spectral drift value of each measurement point can be converted into a quantitative change in refractive index by combining the established calibration relationship curve between refractive index and spectral drift. This will output a set of real-time updated "spatial refractive index distribution data", providing a data basis for subsequent concentration calculation, reaction kinetic analysis, fluid control optimization, etc.

[0028] The multi-point refractive index measurement fiber sensor based on a periodically tandemly polished photonic crystal fiber structure of the present invention can provide good measurement results. This structure achieves synchronous measurement of refractive index changes in multiple key regions in a microfluidic reaction channel without increasing the complexity of the external structure by tandemly connecting multiple photonic crystal fiber modules with adjustable side polishing depths. It has the advantages of high sensitivity, simple structure, and strong adaptability, and is suitable for efficient integration and online monitoring in complex micro-reaction systems.

[0029] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0030] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms are not limited to referring to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0031] The above description is merely an embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure, characterized in that: It consists of the following main components:

1. Single-mode fiber, 2. Coreless fiber, 3. Grapefruit-type Photonic Crystal Fiber (G-PCF), and 4. Multimode fiber. In the sensor structure: incident light is coupled through the single-mode fiber into the first segment of the coreless fiber, and further into the periodically arranged G-PCF structure. The beam spreading effect entering the coreless fiber causes mode mismatch and excites higher-order modes. Then, a portion of the light is coupled into the cladding of the grapefruit-type photonic crystal fiber. Each side-thrown G-PCF segment, by disrupting the integrity of its cladding, allows some mode fields to leak to the outside of the fiber, thereby enhancing the coupling between the fiber's guided mode and the external medium. The cladding modes and core modes propagating within the grapefruit-type photonic crystal fiber are then coupled into the rightmost coreless fiber, forming a resonant attenuation band (i.e., resonant wavelength) in the transmission spectrum when the refractive index of the external environment changes. Furthermore, due to the different lateral projection depths of each G-PCF segment, its effective refractive index and mode coupling strength also differ, resulting in certain differences in the resonance wavelength of each structural unit. This ultimately manifests as multiple resolvable resonance peaks in the spectrum. Each resonance peak corresponds to a sensing region at a spatial location, and parallel monitoring of refractive index changes at multiple spatial points can be achieved by utilizing spectral variations.

2. The multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claim 1, characterized in that: This multi-point measurement fiber optic sensor is assembled using a modular method. Its core lies in achieving optical axial continuity and stable structural repeatability by splicing and precisely aligning multiple grapefruit-shaped photonic crystal fiber segments.

3. The multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claim 2, characterized in that: The fusion-spliced ​​multi-period fiber structure is fixed on a high-precision side-polishing platform, which allows for segment-by-segment adjustment of polishing position and depth. A segmented side-polishing strategy is employed, with each G-PCF segment polished to different depths (2μm, 4μm, 6μm, 8μm) at predetermined positions according to design requirements. The polishing process utilizes precision-controlled fiber clamps and an automatic displacement control system to ensure that each polished area corresponds to the correct position after fusion splicing, preventing structural misalignment or damage to adjacent areas.

4. The multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claim 2, characterized in that: The single-mode fiber is 400µm long, the coreless fiber is 500µm long, the multimode fiber in the periodic fiber structure is 400µm long, and the grapefruit-shaped photonic crystal fiber is 100µm long. The grapefruit-shaped photonic crystal fiber has a cladding refractive index of 1.457, a cladding diameter of 125µm, a SiO2 core diameter of 11.2µm, a core refractive index of 1.457, a wall thickness of 1.82µm, and a hole spacing of 21.7µm.

5. A multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claim 1, characterized in that: After side-spraying, a transparent low-refractive-index protective adhesive or an integrated microfluidic chip channel can be encapsulated on the outside of each side-sprayed area. After encapsulation, connection loss testing and spectral scanning are performed to verify the resonant response characteristics and wavelength decoupling of each side-sprayed area, confirming the independence and sensitivity of each sensing area.

6. A multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claim 2, characterized in that: The side-throw depth of the G-PCF in multiple structural units can be set according to the refractive index characteristics of the liquid to be tested, thereby adjusting the coupling strength between the guided mode of each sensing area and the external medium, and not relying solely on the side-throw depth described in the patent. This allows each structural unit to form a distinguishable resonant wavelength in the transmission spectrum, and by monitoring the changes in the resonant wavelength, the refractive index of the liquid at multiple spatial locations can be sensed simultaneously.

7. A multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claim 4, characterized in that: The assembly lengths of the periodic grapefruit-shaped photonic crystal fiber and single-mode fiber in the sensor can be set according to the refractive index characteristics of the liquid to be tested, and are not solely dependent on the assembly length described in the patent, which facilitates the adjustment of the transmission modes of different light fields in the fiber.

8. A multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claim 1, characterized in that: The light source is connected to the input of the sensor, and the receiver (such as a spectrometer) is connected to the output of the optical fiber. After the light source passes through the sensing area, when the environment of the target object changes, the output spectral pattern is fed back to the receiver in real time, and the corresponding changes in the resonant wavelength are observed.

9. A multi-point refractive index measurement fiber optic sensor based on a periodically tandemly polished sub-crystal fiber structure according to claims 1-8, characterized in that, The method is as follows: S1. Before using a multi-point refractive index measurement fiber optic sensor with a periodic tandem side-polished sub-crystal fiber structure, a suitable light source must first be prepared. Laser sources with stable wavelengths and intensities, such as semiconductor lasers or helium-neon lasers, are typically used. It is essential to ensure that the light source provides sufficient optical power and stability so that the optical signal can be transmitted through the optical fiber and effectively interfered with. S2. Connect the light source output to the sensor system input via a high-stability fiber optic coupler to ensure coupling efficiency and optical axis alignment. The input optical signal enters from the first single-mode fiber (SMF), expands through the coreless fiber, and enters the periodic structure section, sequentially transmitting through multiple side-polished sub-crystal fiber units. During this process, keep the fiber connections secure to avoid micro-bending losses and unnecessary scattering. S3. When the optical signal propagates through a multi-segment side-polished G-PCF, the evanescent field of its guided mode interacts with the external liquid being measured. G-PCF segments with different polishing depths respond differently to changes in the external refractive index, leading to coupling resonance at multiple spatial locations. This results in multiple distinguishable resonant wavelengths in the transmission spectrum, each corresponding to a sensing region. This process does not rely on an external reference optical path and exhibits good optical stability. S4. The output optical signal enters the spectral detection system, typically using a high-resolution spectrometer or an array-type photodetector to record the output spectrum in real time. The detector converts the captured light intensity variation with wavelength into an electrical signal and sends it to the signal processing module. This module analyzes the multi-peak resonance spectrum and extracts the resonance wavelength drift in each sensing region caused by external refractive index changes through information such as spectral shifts and intensity variations. S5. The signal processing unit calculates the drift of each resonant wavelength and, combined with the structural parameters of each side-projectile G-PCF segment and a pre-established sensitivity model, performs inversion calculation of the refractive index at the corresponding spatial point. The final output is the real-time refractive index value at each measurement location, which can be further used for monitoring and analysis of liquid concentration changes, biochemical reaction processes, or multi-regional environmental changes within the microfluidic channel.

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