Three-parameter sensor based on double-clad optical fiber

Abstract

According to the surface plasmon resonance (SPR)-based three-parameter optical fiber sensor and the preparation method thereof provided by the invention, the Bragg grating (FBG) and the long-period grating (LPFG) are utilized, so that synchronous measurement of temperature, stress and salinity can be realized in the same sensor structure, and the sensor has the advantages of high sensitivity, quick response, compact structure and the like. According to the structural design of the sensor, the problem of cross sensitivity between temperature and stress can be effectively solved, the performance of the sensor is remarkably improved, and the application prospect of the sensor in the fields of marine environment monitoring, biomedical detection and the like is widened.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, specifically a surface plasmon resonance (SPR) three-parameter sensor based on a double-clad optical fiber structure, which is used to simultaneously measure temperature, stress, and salinity in a liquid environment. This sensor is suitable for use in fields such as ocean monitoring, environmental testing, and industrial process control. Background Art

[0002] With the development of modern sensing technology, traditional single-parameter sensors are no longer able to meet the demand for simultaneous sensing of multiple physical or chemical parameters in complex environments. Real-time, high-precision measurement of key parameters such as temperature, stress, and salinity is particularly important in fields such as marine engineering, resource development, and ecological monitoring.

[0003] Currently, multi-parameter measurement has become a research hotspot. Traditional sensors can only measure a single parameter, but practical applications often require the simultaneous detection of multiple physical or chemical parameters to improve system integration and practicality. In marine engineering, changes in temperature, stress, and salinity are particularly important for monitoring the health of seafloor structures, resource development, and natural disaster warnings.

[0004] Surface plasmon resonance (SPR) technology, due to its high sensitivity, fast response, and label-free nature, has been widely used in biosensing, gas detection, and liquid refractive index measurement. Integrating SPR technology into fiber-optic sensing systems enables the construction of compact, electromagnetically resistant, and easily remotely deployed sensor devices.

[0005] SPR technology utilizes a metal film (such as gold) coated on the surface of an optical fiber. When the refractive index of the external environment changes, the SPR resonance wavelength shifts, enabling the measurement of external parameters. However, most current SPR sensors can only measure a single parameter, making it difficult to meet the requirements for simultaneous multi-parameter detection in complex environments.

[0006] In addition, Fiber Bragg Grating (FBG) and Long Period Grating (LFPG) are widely used in fiber optic sensing because they are sensitive to temperature and strain.

[0007] FBGs are primarily affected by thermal expansion and thermo-optic effects, while LPFGs are more sensitive to changes in the refractive index of the surrounding medium. Combining SPR with FBGs / LFPGs is expected to enable simultaneous measurement of multiple parameters.

[0008] However, the existing technology still has the following shortcomings:

[0009] Most sensors adopt discrete structures, which makes the system complex and costly.

[0010] Most existing SPR sensors can only measure salinity or temperature and cannot achieve simultaneous detection of multiple parameters.

[0011] The sensing signals between different parameters are likely to affect each other, reducing the measurement accuracy.

[0012] Therefore, it is urgent to propose a new type of optical fiber sensor that organically combines the three sensing mechanisms of SPR, FBG and LFPG to achieve synchronous measurement of temperature, stress and salinity, and has good stability, anti-interference ability and repeatability. Summary of the Invention

[0013] The purpose of the present invention is to provide an SPR three-parameter sensor based on double-clad optical fiber and its application method, aiming to overcome the limitations of existing optical fiber sensors in multi-parameter measurement, realize the synchronous measurement of temperature, stress and salinity, and have high sensitivity, good stability and anti-interference ability.

[0014] The SPR sensing unit achieves highly sensitive detection of changes in the refractive index of the external environment based on the surface plasmon resonance effect. The basic principle is as follows: when light propagates through a single-mode optical fiber, it undergoes total reflection at the interface between the optical fiber and the metal film, generating an evanescent wave. This evanescent wave can couple with the surface plasmon wave excited by the collective oscillation of free electrons on the metal film surface. When the wave vectors of the two match, resonance occurs, resulting in a significant absorption of light energy at a specific wavelength, forming a distinct resonance valley in the output spectrum.

[0015] The resonance condition satisfies the following formula:

[0016]

[0017] Among them, among them, is the wave vector of the surface plasmon wave, is the wave vector in vacuum, is the refractive index of the optical fiber core, θ is the incident angle, is the refractive index of the surrounding medium.

[0018] When the refractive index of the external environment changes (such as salinity changes), it will cause The change causes the resonance condition to shift, which is manifested as a drift in the resonance wavelength.

[0019] In practical applications, changes in the salinity of the external liquid environment directly cause changes in the refractive index of the cover layer, which in turn changes the refractive index, causing a shift in the resonance condition, manifested as a shift in the SPR resonance wavelength. By monitoring this shift in resonance wavelength in real time, quantitative measurement of salinity changes in the liquid environment can be achieved.

[0020] Preferably, in the present invention, a 40–50 nm thick gold film is deposited on the surface of a single-mode optical fiber. This metal film not only exhibits excellent electrical conductivity and chemical stability, but also exhibits strong surface plasmon excitation capabilities in the visible to near-infrared range. By monitoring changes in the SPR resonance wavelength, high-precision measurements of salinity parameters in liquid environments can be achieved.

[0021] Fiber Bragg grating (FBG) is a new type of photonic device developed in recent years. It has important application prospects in the fields of fiber-optic communications and sensing. The advantages of this device are small size, light weight, resistance to electromagnetic interference, and ability to operate normally in complex chemical environments.

[0022] According to the coupled mode theory, the central reflection wavelength of the fiber Bragg grating (FBG) is:

[0023]

[0024] In the formula is the effective refractive index of the guided mode, Λ is the grating period, and when the wavelength meets the Bragg condition, the incident light will be reflected back to the original path by the fiber Bragg grating.

[0025] From the above formula, we can know that the central reflection wavelength λ of the fiber Bragg grating varies with The stress effect λ is caused by the elastic-optic effect and the change of the fiber grating period Λ, while the temperature effect λ is caused by the thermo-optic effect and the thermal expansion effect.

[0026] Long period grating (LFPG) is a new type of fiber grating with a grating period generally greater than 100 The basic principle of light transmission is to couple the forward-propagating fundamental mode to the forward-propagating p-order cladding mode, which is then attenuated after a certain distance. The phase matching condition of the long-period fiber grating can be expressed as:

[0027]

[0028] in 、 are the refractive indices of the core and cladding, respectively, is the transmission wavelength of the p-th order cladding mode. The incident light wavelength can only be transmitted through the long-period grating when it satisfies the relationship.

[0029] The SPR three-parameter optical fiber sensor provided by the present invention consists of the following parts:

[0030] The overall structure is DCF-SMF-DCF (Dual Cladding - Single Mode Fiber - Dual Cladding).

[0031] Among them, the front section and the rear section are double-clad optical fiber (DCF), and the rear section is used to write the grating.

[0032] The middle part is single-mode fiber (SMF), which serves as the SPR sensing area.

[0033] Among them, a gold film with a thickness of 40-50 nm is plated on the surface of the SMF segment to form the SPR sensing area.

[0034] Among them, FBG is written into the core of the rear double-clad optical fiber, and its sensitivity to temperature and stress is used to measure stress and temperature through the reflection wavelength drift caused by thermal expansion effect, thermo-optic effect and elastic-optic effect respectively.

[0035] Among them, LPFG is written into the double-clad optical fiber core at the back end to enhance the sensor's response sensitivity to temperature and improve the temperature measurement accuracy of the overall system.

[0036] LPFG and FBG have different sensitivities to strain and temperature (the LPFG used in the experiment has a much greater temperature sensitivity than FBG and a smaller stress sensitivity than FBG). A hybrid sensor consisting of two FBGs and one LPFG can achieve simultaneous measurement of strain and temperature.

[0037] Define the function F(R1, R2) as the normalized intensity difference of the two FBG reflection signals:

[0038]

[0039] Among them, R1 and R2 are the reflectivities of the two FBGs respectively.

[0040] Combined with the transmission spectrum information of LFPG, the following matrix equation is constructed:

[0041]

[0042] in is the temperature change, is the strain change, is the experimental calibration coefficient.

[0043] By solving the above matrix, the true change values ​​of temperature and strain can be separated. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a schematic diagram of a three-parameter optical fiber with a DCF-SMF-DCF structure.

[0045] Figure 2 It is a schematic diagram of the cross section of a double-clad optical fiber.

[0046] Figure 3 This is a schematic diagram of DCF and SMF welding.

[0047] Figure 4 It is a schematic diagram of directional cutting of optical fiber.

[0048] Figure 5 This is a schematic diagram of the overall system of the three-parameter fiber optic sensor, which includes: a spectrometer 1, a transmission fiber 2, a syringe 3, a silicone tube 4, a microfluidic control area 5, a waste liquid tank 6, a spectrometer 7, and a host computer 8. DETAILED DESCRIPTION

[0049] In order to enable those skilled in the art to better understand the technical solutions and beneficial effects of the present invention, the present invention will be further described in detail below in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0050] In the specific implementation process of the present invention, the experimental equipment, instruments and materials used can be obtained through conventional commercial channels unless otherwise specified. The preparation and testing methods of the double-clad fiber-based SPR three-parameter sensor described in the present invention, including but not limited to the steps of fiber fusion splicing, metal film coating, grating writing, multi-parameter sensing experiment, etc., are all performed according to the common technical means in the field and can be appropriately adjusted without departing from the spirit of the present invention.

[0051] In addition, the parameter measurement, signal demodulation and data processing methods involved in the present invention, such as SPR resonance wavelength detection, FBG and LPFG spectrum analysis, matrix decoupling algorithm, etc., are all technical contents well known to technicians in this technical field and can also be implemented using alternative methods in the existing technology.

[0052] In summary, the present invention integrates SPR, FBG and LPFG sensing units to construct a three-parameter optical fiber sensor with a compact structure, sensitive response, and suitable for liquid environments. It can realize the simultaneous measurement of temperature, stress and salinity, and has good application prospects and promotion value.

[0053] To make the argument clear, Figure 1 A diagram illustrating the DCF-SMF-DCF structure is given, in which a grating is engraved in the fiber core to enable temperature and strain measurement, and a gold film is coated on the surface of the single-mode fiber area to produce the SPR effect.

[0054] Figure 2 This is a cross-sectional view of a double-clad optical fiber, where the core diameter is 10 , the inner cladding diameter is 64 , the outer cladding diameter is 125um.

[0055] In a preferred embodiment of the present invention, a phase mask method is used to write a Bragg grating in a DCF. The phase mask method comprises the following steps:

[0056] The excimer laser used as the light source has an output wavelength of 248 nm, making it suitable for writing gratings in the ultraviolet band. The maximum single-pulse output energy of this excimer laser is 300 mJ.

[0057] Preferably, in this embodiment, the laser pulse energy actually used is about 100 mJ.

[0058] Furthermore, a section of photosensitivity-rich single-mode optical fiber was selected as the matrix material for writing the Bragg grating. In the writing area, where the Bragg grating is to be formed, the fiber's coating must be completely stripped to prevent the coating from evaporating during laser irradiation and depositing on the phase template surface, which would affect the efficiency and quality of the grating writing.

[0059] Furthermore, the ultraviolet laser beam output by the excimer laser is focused by a cylindrical mirror, concentrating the laser energy on the phase mask. The focused laser beam is then incident perpendicularly on the surface of the phase mask, which has a periodic structure and is used to produce interference fringes.

[0060] Furthermore, a DCF with its coating removed is placed closely beneath the phase mask. After the laser beam passes through the phase mask, it creates a spatially periodic intensity distribution in the fiber core, inducing a change in the refractive index and forming a periodically modulated refractive index structure, namely a fiber Bragg grating (FBG).

[0061] Furthermore, in this embodiment, the laser pulse repetition frequency is set to 10 Hz, ensuring stable pulse intervals and good exposure uniformity during the writing process. Furthermore, after being focused by the cylindrical mirror, the incident power density on the DCF is approximately 400 mJ / cm² / pulse, sufficient to stimulate the photoinduced refractive index change effect in the optical fiber, thereby effectively writing high-quality Bragg gratings.

[0062] Furthermore, after writing is completed, the performance of the formed Bragg grating is tested, including parameters such as central wavelength, reflectivity, and bandwidth.

[0063] Depending on application requirements, the writing area can be re-coated with a protective layer or encapsulated to enhance the mechanical strength and environmental stability of the device.

[0064] To make the argument clear, Figure 3and Figure 4 The fabrication method of DCF-SMF is given.

[0065] Specifically, a commercial fiber fusion splicer (such as the Fujikura 87s) is used to perform high-precision splicing of double-clad fiber and single-mode fiber. This splicer features automatic alignment, discharge intensity adjustment, and image recognition, enabling low-loss, high-strength connections between different types of optical fibers.

[0066] Furthermore, a section of double-clad optical fiber and a section of single-mode optical fiber are cut respectively, and a special optical fiber cutter is used to cut the end faces of the two optical fibers to ensure that the end faces are flat and burr-free, and the cutting angle is controlled within ±0.5° to ensure the quality of the fusion splicing.

[0067] Furthermore, the double-clad optical fiber and single-mode optical fiber to be fused are respectively fixed in the left and right clamps of the fusion splicer, and the automatic alignment program is started. The system collects images of the two optical fiber end faces through the built-in camera and automatically adjusts the optical fiber position according to the image information so that the central axes of the two optical fiber cores are aligned.

[0068] Furthermore, before the formal fusion splicing, a short-time, low-power discharge pre-fusion is performed to remove possible tiny contaminants or moisture on the optical fiber end face, thereby improving the fusion splicing success rate.

[0069] Furthermore, appropriate splicing parameters are set, including discharge current, discharge time, and thrust. These parameters are optimized based on the fiber type (e.g., DCF and SMF material, diameter, and number of cladding layers). The discharge splicing process is then initiated, generating a high-temperature arc between the two electrodes, causing the fiber ends to partially melt and fuse together.

[0070] Furthermore, after the welding is completed, the system automatically estimates the welding loss value based on the welding image and displays it on the operation interface. At the same time, the welding point can be tested for insertion loss using an external light source and an optical power meter to ensure that the welding loss is less than 0.1dB.

[0071] Preferably, the metal film is a gold film with a thickness controlled between 40–50 nm. This thickness range effectively supports the excitation of surface plasmon waves while balancing the penetration depth of the optical signal and the stability of the sensor. To this end, magnetron sputtering is used to deposit a gold film on the surface of a single-mode optical fiber. This method offers advantages such as excellent film uniformity, high density, strong adhesion, and good process controllability, making it suitable for the preparation of functional thin films on the surfaces of micron-sized optical fibers.

[0072] Specifically, a section of single-mode optical fiber that has been fused and cut to a fixed length is selected, and ultrasonic cleaning is performed using deionized water and anhydrous ethanol in sequence to remove organic pollutants and particulate impurities that may exist on the surface, and then it is blown dry with high-purity nitrogen for use.

[0073] Furthermore, the cleaned single-mode optical fiber is placed in a dedicated coating fixture, and the optical fiber surface is ensured to be in a horizontal or rotated state to ensure uniformity during the coating process.

[0074] Furthermore, the fixture and the optical fiber sample were placed in the vacuum chamber of the magnetron sputtering coating equipment. After closing the door, the vacuum system was started to make the basic vacuum degree inside the chamber ≤5×10 -3 Pa.

[0075] Furthermore, high-purity argon gas is introduced as sputtering gas, the gas pressure is adjusted to the range of 0.5-1.0 Pa, the sputtering power supply is started, and the target surface is pre-sputtered without turning on the substrate bias to remove oxides and impurities on the target surface.

[0076] Furthermore, the substrate bias mode was switched to, the sputtering power was set to 80-150 W, the sputtering time was 60-120 s, and the sputtering rate was controlled at approximately 0.3-0.5 nm / s, ultimately obtaining a gold film with a thickness of 40-50 nm. During this process, the film thickness can be precisely controlled by adjusting the sputtering time.

[0077] Furthermore, after the sputtering is completed, the power is turned off and the vacuum state is maintained until the cavity temperature returns to room temperature, and then dry air is slowly introduced and the coated optical fiber sample is taken out.

[0078] Furthermore, a scanning electron microscope (SEM) is used to characterize and analyze the surface morphology and thickness of the gold-plated film to confirm that the film layer is continuous, dense, and has no obvious defects. At the same time, a step meter or ellipsometer can be used to further verify whether the film thickness meets the design requirements.

[0079] To fabricate the proposed fiber helical long-period grating, an arc discharge-based heating system was employed. The core components of this system include a high-precision rotation device, a holder for fixing the optical fiber, and a pair of electrodes for generating the arc.

[0080] To ensure stability and uniformity during the grating modulation process, the rotator and electrodes of the entire system are automatically controlled by a computer program, thereby achieving precise adjustment of process parameters.

[0081] In the actual fabrication process, one end of a fiber optic cable is firmly fixed to a fiber holder; the other end is rotated at a constant speed V1 by a rotator. Simultaneously, an electrode moves along the fiber's axial direction at another constant speed V2. This synchronized motion imparts a helical structure to the fiber as it softens due to heat. The resulting grating period, Λ, can be expressed as:

[0082]

[0083] Throughout the entire process, the fiber remains aligned between the two electrodes. Furthermore, the electrode alignment is rigorously calibrated to ensure arc discharge stability and uniform heating distribution, thereby enhancing the structural consistency and optical performance of the resulting fiber.

[0084] To make the argument clear, Figure 5 The overall sensing system diagram is shown. The light source 1 transmits the optical signal into the transmission optical fiber 2. The syringe 3 injects the liquid to be measured through the silicone tube 4 into the microfluidic controller 5. The waste liquid flows into the treatment tank 6 for treatment. The salinity, temperature and other variables in the solution in the microfluidic controller cause the optical signal to change, which is transmitted to the spectrometer 7. Finally, the host computer 8 processes the signal to achieve parameter measurement.

[0085] The above embodiments are merely typical examples of the present invention and are not intended to limit the present invention. Those skilled in the art may, without departing from the spirit and substance of the present invention, make various equivalent transformations or substitutions to the technical solutions of the present invention based on different application scenarios, material selection, structural design, and changes in process parameters. Such transformations shall fall within the scope of protection defined by the claims of the present invention.

Claims

1. A multi-parameter optical fiber sensor based on surface plasmon resonance, characterized in that: The system comprises a first double-clad fiber segment, a single-mode fiber sensing segment and a second double-clad fiber segment connected in sequence; the surface of the single-mode fiber segment is coated with a gold film for stimulating the SPR effect; a Bragg grating (FBG) and a long-period grating (LPFG) are written into the double-clad fiber segment for temperature and stress detection.

2. The sensor according to claim 1, characterized in that The length of the single-mode optical fiber segment is 10 cm, and the thickness of the gold film is 40–50 nm.

3. The sensor according to claim 1, wherein The central wavelength of the FBG is within the range of 1290–1320 nm, the central wavelength of the LPFG is 1306 nm, and the 50% transmission point of the LPG transmission spectrum is close to the FBG reflection spectrum.

4. The sensor according to claim 1, characterized in that The LPFG is a spiral long period grating produced by an arc discharge heating system.

5. The sensor according to claim 1, wherein The matrix equation is constructed by normalizing the reflected light intensity difference function F(R1, R2) and the wavelength change Δλ to achieve decoupled measurement of temperature and stress.

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