Preparation method of optical fiber end face temperature-salt sensor based on structured AFM probe processing

Abstract

The invention discloses a preparation method of an optical fiber end face temperature-salt sensor based on structured AFM probe processing, and the method comprises the steps: replacing a traditional probe with a structured diamond probe which is subjected to FIB transformation and is provided with a chip groove for nanometer milling, and packaging a waveguide layer on a processed metal structure; on the basis, the multi-parameter optical fiber sensor for simultaneously measuring the temperature and the salinity of the seawater is constructed. By means of the innovative structured diamond AFM probe, the problems that when a traditional AFM probe is used for etching a gold film, cuttings are accumulated, and the probe is seriously abraded are solved, only one AFM probe needs to be modified at a time through the FIB technology, the situation that in a traditional method, each optical fiber needs to be directly etched through expensive FIB equipment is avoided, and the cost is reduced. The preparation cost of a single optical fiber sensor is reduced, the ultraviolet resin waveguide layer with the thickness of 1.65 microns is packaged above the gold grating, and the structure can excite a plurality of guided wave surface plasma resonance modes with high Q values.

Description

Technical Field

[0001] This invention belongs to the field of marine environmental monitoring and relates to a method for preparing plasmonic nanostructures on the end face of optical fibers. Specifically, it relates to a method for precisely fabricating plasmonic nanostructures on the end face of single-mode optical fibers based on structured AFM probes. Background Technology

[0002] In the field of marine environmental monitoring, temperature and salinity are crucial physical parameters. Traditional fiber optic multi-parameter sensors typically rely on stacking multiple sensing elements (such as interferometers and fiber gratings) on optical fibers, leading to increased sensor size and structural complexity. In recent years, a micro-sensor based on fiber end-face guided waveguide surface plasmon resonance (GWSPR) has attracted significant attention due to its ultra-compactness, multifunctionality, and high sensitivity. This type of sensor requires the fabrication of micro / nano-scale metallic grating structures on the end face of a single-mode fiber, covered by a polymer waveguide of a specific thickness.

[0003] Currently, the fabrication of nanostructures on the end face of optical fibers mainly relies on focused ion beam (FIB) etching technology. However, these existing fabrication methods have significant limitations: 1. High cost and extremely low efficiency: FIB equipment is expensive, and point-by-point processing of a single optical fiber is extremely time-consuming, making it difficult to achieve low-cost mass production; 2. Defects of traditional AFM scribing: If a conventional atomic force microscope (AFM) probe (usually a standard conical tip) is used to directly scribble (nanomill) on the metal film, the lack of a chip removal structure at the probe tip makes it easy for chips to accumulate during the cutting of the gold film, resulting in rapid probe wear, rough edges of the fabricated nanostructures, and extremely poor yield and consistency.

[0004] Therefore, there is an urgent need for a method to fabricate plasmonic nanostructures at the fiber end face that combines low cost, high efficiency, and high processing precision in order to meet the high-quality manufacturing requirements of fiber optic temperature and salinity sensors. Summary of the Invention

[0005] This invention provides a method for fabricating an optical fiber end-face temperature and salinity sensor based on structured AFM probe processing. The method uses a structured diamond probe with chip removal grooves modified by FIB to replace the traditional probe for nanomilling, and encapsulates a waveguide layer on the processed metal structure. Based on this, a multi-parameter optical fiber sensor for simultaneous measurement of seawater temperature and salinity is constructed.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for fabricating an optical fiber end-face temperature-salt sensor based on structured AFM probe fabrication includes the following steps:

[0008] Step 1: Preparation of structured AFM probes:

[0009] A high-hardness diamond AFM probe (originally a conical tip) was selected, and nanoscale grooves were precisely machined on both sides of its conical tip using focused ion beam (FIB). These grooves serve as "chip removal grooves," enabling the probe to have cutting and chip removal capabilities similar to macro-milling cutters. The specific preparation method is as follows: Starting from 35nm away from the bottom surface of the cone, an outer arc with a radius of 150nm and an angle of 100° is formed with the center of the bottom surface as the center. The chord of the outer arc is used as a common chord, and an inner arc with a radius of 165nm is drawn on the other side of the common chord. The two arcs are connected to form a closed shape, and a groove is made from the bottom surface to the tip in this shape. Another groove is made symmetrically with the first groove.

[0010] Step 2, fiber end face coating:

[0011] A single-mode fiber is cut with an extremely flat end face using a fiber cleaver, and then placed into a high-vacuum magnetron sputtering thin film deposition system to uniformly deposit a gold film on the fiber end face with a thickness of 40~80nm.

[0012] Step 3: Efficient fabrication of plasmonic structures:

[0013] The coated optical fiber is fixed in the AFM device with the fiber clamp facing upward. The structured AFM probe prepared in step 1 is used to perform nanomilling on the gold film. Thanks to the chip removal grooves on both sides of the probe, gold material chips can be efficiently scraped and discharged, and a plasmonic metal structure with smooth edges and precise period is prepared.

[0014] Step 4, Waveguide Packaging:

[0015] On the fiber end face of the fabricated plasmonic metal structure, a layer of ultraviolet-cured liquid polymer resin (OrmoComp, Micro resist technology) is coated, and the resin is encapsulated using ultraviolet curing technology. The resin thickness is preferably controlled at 1.65μm with an error of 0.2μm to form a dielectric waveguide layer covering the plasmonic metal structure, thus completing the fabrication of the GWSPR fiber end face sensor.

[0016] Step 5: Construction of the two-parameter measurement system:

[0017] The fiber optic end-face sensor prepared in step 4 is connected to the demodulation system to build a dual-parameter measurement system. The dual-parameter measurement system is used to obtain the wavelength shift of the high-Q resonance trough (Dip) at different tilt angles in the reflection spectrum, realizing the simultaneous measurement of two parameters of seawater temperature and salinity. The demodulation system includes a tunable laser source, a polarization controller, a 2×1 fiber coupler, a photodetector, and a data acquisition card (DAQ). The optical signal of the tunable laser source is input to the input end of the 2×1 fiber coupler; the common end of the 2×1 fiber coupler is output to the polarization controller to excite light with a specific polarization state (TM polarization is dominant); the polarization-adjusted light is connected to the fiber optic end-face sensor probe through a fiber flange and fiber jumper; during measurement, the fiber optic end-face sensor probe is immersed in the seawater to be measured at different temperatures and salinities. The optical signal excites the plasma resonance on the waveguide surface of the fiber end face. The reflected light carrying the resonance absorption information returns along the original path, enters the photodetector through the other output end of the 2×1 fiber coupler, and is converted into an electrical signal. The reflection spectrum is recorded by the data acquisition card (DAQ) and the computer (PC).

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. Significantly improves the quality and efficiency of nanofabrication: The innovative "structured diamond AFM probe" (with FIB chip removal grooves on both sides) solves the problems of chip accumulation and severe probe wear when traditional AFM probes scribing on gold films. The cutting is sharper and the chip removal is smoother. The prepared nanograting structure has high edge smoothness, which improves processing efficiency and yield.

[0020] 2. Significantly reduced manufacturing costs: Only one AFM probe needs to be modified using FIB technology. This probe can then be reused for mechanical scribing of a large number of fiber end faces. This avoids the need for expensive FIB equipment to directly etch each fiber in the traditional method, thus reducing the manufacturing cost of a single fiber sensor.

[0021] 3. Achieving high-sensitivity measurement of both temperature and salinity parameters: A 1.65μm thick ultraviolet resin waveguide layer is encapsulated above a gold grating. This structure can excite multiple high-Q guided wave surface plasmon resonance (GWSPR) modes. Since different resonance tilt angles have different sensitivities to the environmental refractive index (salinity) and the thermal expansion effect of the waveguide layer (temperature), combined with a sensitivity matrix algorithm, high-precision, crosstalk-resistant synchronous measurement of seawater temperature and salinity can be achieved using a single optical fiber. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the fabrication of structured AFM probes;

[0023] Figure 2 A structured AFM probe three-view diagram;

[0024] Figure 3 This is a flowchart of the sensor fabrication process.

[0025] Figure 4 This is a schematic diagram of a fiber optic temperature and salinity sensor testing system. Detailed Implementation

[0026] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0027] This invention provides a method for fabricating a dual-parameter temperature and salinity sensor for seawater integrated on the end face of a single-mode optical fiber. The specific implementation steps are as follows:

[0028] (1) Fabrication of structured AFM processing probes:

[0029] A diamond AFM probe with a high cantilever beam elastic constant and a conical tip was selected. This probe was placed in an ion beam processing electron microscope, and specific geometric grooves were etched downwards on both symmetrical sides of the diamond conical tip using the ion beam. The specific method is as follows: the radius of the cone's base is approximately 183 nm, the height is 175 nm, the radius of the spherical cap at the tip is 20 nm, and the cone's half-angle is 45°. Starting 35 nm from the base, an outer arc with a radius of 150 nm and an angle of 100° is drawn, centered on the base. Using the chord of the outer arc as a common chord, an inner arc with a radius of 165 nm is drawn on the other side of the common chord. The two arcs are connected to form a closed shape, and a groove is created from the base to the tip using this shape. Another groove is then created symmetrically. After machining, the probe tip forms a structure similar to a micro-milling cutter, and the grooves on both sides provide chip containment and removal space for subsequent machining.

[0030] (2) Fiber end face preparation and metal film deposition:

[0031] A section of standard single-mode fiber (SMF-28) is cut, the end coating is stripped, and then cut using a high-precision fiber cleaver to obtain a smooth, mirror-like fiber end face (surface roughness <1nm). The cut fiber is then vertically fixed on a fixture and placed in the vacuum chamber of a magnetron sputtering instrument. Sputtering parameters are set, and a pure gold film with a thickness of 40~80nm (e.g., 40nm) is uniformly deposited on the fiber end face.

[0032] (3) High-speed milling of plasmonic nanostructures at the fiber end face:

[0033] The gold-plated optical fiber is fixed to the sample stage of the AFM (Atomic Force Microscope) using an optical fiber clamp, with the fiber end face facing upwards. The structured diamond probe prepared in step (1) is then applied. The required processing structure parameters are set in the operating software: the diameter of the single-mode optical fiber is 125 μm, and a two-dimensional grating is processed at a position of 10 × 10 μm at the center of the optical fiber (called the plasma crystal cavity), wherein: the period along the X direction is 1025 nm, the period along the Y direction is 1250 nm, the width of the processed groove is 60 nm, and a distributed Bragg reflector (DBR) grating is processed around the plasma crystal cavity. The period of the DBR grating in the X direction is 530 nm, the gap between the DBR grating and the plasma crystal cavity in the X direction (i.e., the distance between the first DBR grating and the plasma crystal cavity) is 900 nm, the period in the Y direction is 600 nm, and the gap between the DBR grating and the plasma crystal cavity in the Y direction is 550 nm. The probe scans and cuts the gold film in contact mode. Due to the chip removal grooves on both sides, the cut gold nanochips are smoothly discharged to the outside of the groove, preventing the "built-up edge" phenomenon at the probe tip. A flat and regular nano-metal grating structure is produced in one operation.

[0034] (4) Precision packaging of the fiber end face waveguide layer:

[0035] On the end face of the optical fiber with the gold grating structure, a small amount of refractive index-matched UV-curable polymer resin (OrmoComp) is drop-coated. The resin thickness is precisely controlled to 1.65 μm using an alignment method or an imprinting method (using a PDMS mold for positioning). Subsequently, UV light is used to irradiate and completely cure the resin, forming a dielectric waveguide layer covering the plasmon structure, thus completing the fabrication of the sensor probe.

[0036] (5) Construction and measurement of temperature and salinity dual-parameter sensing and detection system:

[0037] The measurement system is constructed according to the optical path: the optical signal of the tunable laser source is input to the input end of the 2×1 fiber coupler, and the common end of the 2×1 fiber coupler is output to the polarization controller to excite light with a specific polarization state (TM polarization is the main polarization); the light after polarization adjustment is connected to the fiber end face sensor probe through the fiber flange and fiber jumper.

[0038] During measurement, the sensor probe is immersed in seawater of different temperatures and salinities. The optical signal excites surface plasmon resonance (GWSPR) at the end face of the optical fiber. The reflected light carrying the resonance absorption information returns along the original path, enters the photodetector through the other output end of the coupler, and is converted into an electrical signal. The reflection spectrum is recorded by the data acquisition card (DAQ) and the computer (PC).

[0039] As the salinity (due to changes in the environmental refractive index) and temperature (due to thermal expansion and thermo-optic effects of polymer waveguides) in seawater change, the wavelength shifts of the resonance troughs (Dip) corresponding to two different tilt angles in the spectrum will vary by different proportions. By using a pre-calibrated sensitivity matrix for calculation, the temperature and salinity values ​​of seawater can be obtained simultaneously with high precision.

Claims

1. A method for fabricating an optical fiber end-face temperature-salt sensor based on structured AFM probe fabrication, characterized in that... The method includes the following steps: Step 1: Preparation of structured AFM probes: A diamond AFM probe was selected, and nanoscale grooves were precisely machined on both symmetrical sides of its conical head using a focused ion beam. Step 2, fiber end face coating: A single-mode fiber is cut with an extremely flat end face using a fiber cleaver, and then placed into a high-vacuum magnetron sputtering thin film deposition system to uniformly deposit a gold film on the fiber end face. Step 3: Efficient fabrication of plasmonic structures: The coated optical fiber is fixed in the AFM device with the fiber clamp facing upward. The structured AFM probe prepared in step 1 is used to perform nanomilling on the gold film to prepare a plasmonic metal structure with smooth edges and precise period. Step 4, Waveguide Packaging: A layer of ultraviolet-cured liquid polymer resin is coated on the fiber end face after the plasmonic metal structure has been fabricated, and the resin is encapsulated using ultraviolet curing technology to form a dielectric waveguide layer covering the plasmonic metal structure, thus completing the fabrication of the GWSPR fiber end face sensor. Step 5: Construction of the two-parameter measurement system: The fiber end face sensor prepared in step 4 is connected to the demodulation system to build a dual-parameter measurement system. The wavelength shift of the high Q value resonance valley at different tilt angles in the reflection spectrum is obtained using the dual-parameter measurement system, so as to realize the simultaneous measurement of seawater temperature and salinity.

2. The method for fabricating an optical fiber end-face temperature and salinity sensor based on structured AFM probe fabrication according to claim 1, characterized in that... In step 1, the specific preparation method of the nanoscale groove is as follows: Starting from 35nm from the bottom surface of the cone, the outer arc is centered on the bottom center, with a radius of 150nm and an angle of 100°. The chord of the outer arc is used as the common chord. On the other side of the common chord, an inner arc with a radius of 165nm is made. The two arcs are connected to form a closed shape. A groove is made from the bottom surface to the tip in this shape, and another groove is made symmetrically with the first groove.

3. The method for fabricating an optical fiber end-face temperature and salinity sensor based on structured AFM probe fabrication according to claim 1, characterized in that... In step 2, the thickness of the gold film is controlled at 40~80nm.

4. The method for fabricating an optical fiber end-face temperature and salinity sensor based on structured AFM probe fabrication according to claim 1, characterized in that... In step 3, the structural parameters of the nanomilling process are as follows: the diameter of the single-mode fiber is 125 μm, and a two-dimensional grating is processed at the center of the fiber at a position of 10 × 10 μm, i.e., the plasma crystal cavity. The period along the X direction is 1025 nm, the period along the Y direction is 1250 nm, and the width of the processed slot is 60 nm. A distributed Bragg reflector grating is processed on the periphery of the plasma crystal cavity. The period of this grating in the X direction is 530 nm, and the gap between it and the plasma crystal cavity in the X direction is 900 nm. The period in the Y direction is 600 nm, and the gap between it and the plasma crystal cavity in the Y direction is 550 nm.

5. The method for fabricating an optical fiber end-face temperature and salinity sensor based on structured AFM probe fabrication according to claim 1, characterized in that... In step 4, the resin thickness is controlled at 1.65 μm, and the error is controlled at 0.2 μm.

6. The method for fabricating an optical fiber end-face temperature and salinity sensor based on structured AFM probe fabrication according to claim 1, characterized in that... In step 5, the demodulation system includes a tunable laser source, a polarization controller, a 2×1 fiber coupler, a photodetector, and a data acquisition card. The optical signal from the tunable laser source is input to the input end of the 2×1 fiber coupler, and the common end of the 2×1 fiber coupler is output to the polarization controller to excite light in a specific polarization state. The polarization-adjusted light is connected to the fiber end face sensor probe through a fiber flange and a fiber jumper. During measurement, the fiber end face sensor probe is immersed in seawater of different temperatures and salinities. The optical signal excites plasma resonance on the waveguide surface of the fiber end face. The reflected light carrying resonance absorption information returns along the original path, enters the photodetector through the other output end of the 2×1 fiber coupler, and is converted into an electrical signal. The reflection spectrum is recorded by the data acquisition card and the computer.

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