High-temperature stable fused taper fiber coupler and its fabrication method

Abstract

This invention discloses a high-temperature-stability fused taper fiber coupler and its fabrication method, relating to the field of fiber coupler technology. The high-temperature-stability fused taper fiber coupler consists of at least two single-mode fibers that are melted and stretched to form a taper region and a coupling region. The coupling region is coated with a negative thermal expansion material, which can be a modified epoxy resin with a negative thermal expansion coefficient. The negative thermal expansion material has opposite temperature characteristics to the fiber core and cladding, so the stress generated by temperature changes can partially cancel each other out, thereby reducing the change in the length of the coupling region and achieving high temperature stability of the splitting ratio of the fused taper fiber coupler.

Description

Technical Field

[0001] This invention relates to the field of fiber optic coupler technology, and in particular to a fused taper fiber optic coupler with high temperature stability and its fabrication method. Background Technology

[0002] Fused taper fiber couplers are formed by melting two or more optical fibers at high temperatures and stretching them to create a biconical waveguide structure. The overlapping of the closely spaced fiber core mode fields generates evanescent waves, enabling optical signal coupling and energy redistribution between the fibers. For a fixed wavelength and fiber structure, the splitting ratio of a fused taper fiber coupler is primarily determined by the length of the coupling region. This length is affected by the ambient temperature. When the ambient temperature changes, the fiber experiences stress due to thermal expansion and contraction, causing the length of the coupling region to expand or contract, resulting in fluctuations in the splitting ratio of the fused taper fiber coupler. Therefore, in practical applications such as fiber optic communication, photonics, fiber optic sensing, and precision measurement, the power of the split signal fluctuates with changes in ambient temperature, making it difficult to output high-power, stable signal light, thus significantly affecting measurement accuracy.

[0003] To address these issues, existing research has proposed methods such as PZT (piezoelectric ceramic) negative feedback control, ultraviolet laser photorefractive effect correction, and semiconductor Peltier effect temperature control to reduce the impact of ambient temperature changes on fused taper fiber couplers. However, these methods usually require high-precision temperature sensors, controllers, and drive circuits, making the system design, debugging, and maintenance quite complex. Furthermore, they may not be able to make timely and accurate compensations, resulting in poor control efficiency. Summary of the Invention

[0004] The high-temperature-stability fused taper fiber coupler provided by the present invention is formed by melting and stretching at least two single-mode optical fibers to form a taper region and a coupling region, wherein the coupling region is coated with a negative thermal expansion material.

[0005] Optionally, the negative thermal expansion material is a modified epoxy resin.

[0006] Optionally, the modified epoxy resin is formed by doping an epoxy resin matrix with inorganic particles having negative thermal expansion characteristics, or by doping an epoxy resin matrix with organic structural units having negative thermal expansion characteristics.

[0007] The epoxy resin matrix is ​​at least one of bisphenol A type epoxy resin, bisphenol F type epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, phenolic epoxy resin, and glycidylamine type epoxy resin.

[0008] Optionally, the coefficient of thermal expansion of the negative thermal expansion material is -3.5 × 10⁻⁶. -4 ~-7×10 -5 .

[0009] Optionally, the coupling region is coated with multiple layers of negative thermal expansion material.

[0010] Optionally, the multiple layers of the negative thermal expansion material have different coefficients of thermal expansion.

[0011] This invention proposes a method for fabricating a high-temperature-stability fused taper fiber coupler, comprising the following steps:

[0012] Step 1: Suspend the coupling region of the fused taper fiber coupler at the center of the U-shaped groove;

[0013] Step 2: Pour modified epoxy resin with a negative coefficient of thermal expansion into the U-shaped groove, and wait for the modified epoxy resin to cure to complete the preparation of the high-temperature stable fused taper fiber coupler.

[0014] Optionally, in step 2, the modified epoxy resin is cured with ultraviolet light.

[0015] Optionally, in step 2, the modified epoxy resin is simultaneously injected into the U-shaped groove through multiple injection points.

[0016] Optionally, the preparation method further includes the following steps:

[0017] Steps 1 and 2 are performed sequentially in multiple U-shaped grooves.

[0018] The present invention has the following beneficial effects:

[0019] A material with a negative thermal expansion coefficient is coated in the coupling region of the fused taper fiber coupler. The negative thermal expansion material has opposite temperature characteristics to the fiber core and cladding. Therefore, the stress generated by temperature changes can be partially canceled out, thereby reducing the change in the length of the coupling region and achieving high temperature stability of the splitting ratio of the fused taper fiber coupler. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the high-temperature stability fused taper fiber coupler proposed in an embodiment of the present invention;

[0022] Figure 2 This is a process flow diagram of the fabrication process of the high-temperature stability fused taper fiber coupler proposed in some embodiments of the present invention;

[0023] Figure 3 This is a schematic diagram of the planar structure of the U-shaped groove mold used in the embodiments of the present invention;

[0024] Figure 4 The figure shows the simulation data of the correlation between the splitting ratio temperature stability and the coating thickness of the high-temperature stability fused taper fiber coupler proposed in the embodiments of the present invention.

[0025] Figure 5 The figure shows the simulation data of the correlation between the splitting ratio temperature stability of the high-temperature stability fused taper fiber coupler proposed in the embodiments of the present invention and the thermal expansion coefficient of the coating material.

[0026] Figure 6 The figure shows the simulation data of the correlation between the splitting ratio temperature stability and the coating length of the high-temperature stability fused taper fiber coupler proposed in the embodiments of the present invention.

[0027] Figure 7 The figure shows the simulation data of the change in the splitting ratio with temperature before and after coating with a negative thermal expansion material for the high-temperature stability fused taper fiber coupler proposed in this embodiment of the invention.

[0028] Explanation of reference numerals in the attached diagram: 1. Fiber cladding; 2. Fiber core; 3. Negative thermal expansion material; 4. Input port; 5. First output port; 6. Second output port; 7. Uncoupled region; 8. Conical region; 9. Coupled region; 100. U-groove; 200. Glue injection port. Detailed Implementation

[0029] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0030] The basic principle of fused taper fiber couplers involves optical energy coupling between optical fibers. When transmitted in an optical fiber, most of the optical energy is concentrated in the fiber core, and only a small portion of the optical energy is transmitted outside the fiber core in the form of evanescent waves. Fiber couplers are mode coupling systems composed of two or more optical fibers, and their working mechanism is based on the evanescent wave mode coupling theory of optical fibers.

[0031] In single-mode fiber, the guided modes are two orthogonal fundamental modes (HE). 11When the transmitted mode enters the coupling region, as the fiber core becomes thinner, the normalized frequency (V value) of the fiber gradually decreases, and the optical power leaking into the cladding continuously increases. Simultaneously, the coupling coefficient also continuously increases, allowing for more efficient coupling of the optical signal between the two waveguides. At the output end, as the fiber core gradually thickens, the V value increases again. By controlling the length of the coupling region, the optical power is transmitted in a specific ratio between the two fiber cores. The structure of the coupling region determines the transmission efficiency of the optical signal from one fiber to another. The splitting ratio of a fused taper fiber coupler refers to the optical energy distribution between the two fibers, expressed by the formula:

[0032]

[0033] In the formula, λ is the wavelength of the light wave. The refractive index of the cladding is Let V be the diameter of the fiber in the coupling region, and V be a fiber parameter of the isolated fiber, expressed as:

[0034]

[0035] in , The refractive index is the core refractive index.

[0036] For a fixed wavelength and fiber structure, the splitting ratio of a fused taper fiber coupler is mainly determined by the length of the coupling region, which is affected by the external ambient temperature. When the ambient temperature changes, the fiber generates stress due to thermal expansion and contraction, causing the length of the coupling region to expand or contract, which in turn causes fluctuations in the splitting ratio of the fused taper fiber coupler.

[0037] See Figure 1 A typical fused taper fiber coupler consists of two optical fibers that are fused and stretched to form a mode coupling system. The optical fiber includes an optical cladding 1 and a fiber core 2, forming a non-coupling region 7, a taper region 8, and a coupling region 9. One end of one optical fiber is the input port 4, and the other end of the same optical fiber is the first output port 5. The output end of the optical fiber coupled to it is the second output port 6.

[0038] See Figure 1 The high-temperature-stability fused taper fiber coupler proposed in this application is based on the traditional fused taper fiber coupler, with the coupling region 9 covered by a negative thermal expansion material 3.

[0039] The negative thermal expansion material 3 has opposite temperature characteristics to the fiber core 2 and the fiber cladding 1. Therefore, the stress generated by temperature changes can partially cancel each other out, thereby reducing the change in the length of the coupling region and achieving high temperature stability of the splitting ratio of the fused taper fiber coupler.

[0040] In existing technologies, fused taper fiber couplers can be fabricated using the following method: After removing the coating from the central regions of two optical fibers, they are placed in a fiber fusion tapering machine, ensuring that the coated portions are parallel, seamless, and close together. A localized heat source is used to uniformly heat the coated fiber regions. Once the central portion of the fiber reaches a molten state, the stepper motor of the fusion tapering machine is controlled to taper the fiber. This stretching process gradually thins the fiber, eventually forming a tapered region in the middle section. The diameter of this region is typically reduced to a few micrometers, or even smaller. The stretching process not only changes the fiber's geometry but also alters its optical properties, especially the refractive index and mode structure of the core region. During stretching, two or more optical fibers will come into contact and couple with each other through the tapered portion of the fiber. When an optical signal enters the tapered region from one fiber, due to the structural change in the middle region, some of the optical signal is coupled to the core of the adjacent fiber via an electromagnetic field. This process is based on the optical mode coupling mechanism.

[0041] Because the diameter of the coupling region of the fused taper fiber is very small (a few micrometers or even smaller), and the optical field coupling between fibers requires extremely high precision, very high requirements are placed on the selection of negative thermal expansion materials and the coating process.

[0042] The inventors of this application explored the feasibility of coating various negative thermal expansion materials onto the coupling region of fused taper optical fibers during their research. For example, metal alloys, metal oxides, metal fluorides, and lead titanate-based anion-doped systems can be coated onto the surface of the coupling region using semiconductor processes such as physical vapor deposition and chemical vapor deposition, or by slurry coating or spraying. However, these processes are difficult to form a uniform coating on the outer periphery of the coupling region, often involve high-temperature annealing, and have high production costs and poor flexibility. Some organic materials with special molecular structures and doping states, such as polyaniline and polyimide, also have negative thermal expansion characteristics. However, the coefficient of thermal expansion and material strength of these materials are often difficult to control precisely. Similar to the preparation process of traditional optical fiber protective layers, they rely on coating cavities for coating. This process involves immersing the optical fiber in molten material, so that the material is uniformly wrapped around the surface of the optical fiber, and a protective layer is formed after cooling. Precise control of temperature and coating speed is required. At the same time, because the tolerance requirements of the mold are low, it is difficult to coat micron-sized fused taper optical fiber couplers.

[0043] In some embodiments, the negative thermal expansion material used in this application is modified epoxy resin.

[0044] The modified epoxy resin can be formed by doping an epoxy resin matrix with inorganic particles having negative thermal expansion characteristics, or by doping an epoxy resin matrix with organic structural units having negative thermal expansion characteristics.

[0045] Epoxy resin matrices include, but are not limited to, bisphenol A type epoxy resin, bisphenol F type epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, phenolic type epoxy resin, and glycidylamine type epoxy resin.

[0046] For example, by introducing an eight-membered carbon ring structure (polyamide) into the bisphenol A epoxy resin system, the coefficient of thermal expansion of the epoxy material can be changed from positive to negative. By controlling the proportion of polyamide, the coefficient of thermal expansion of the epoxy material can be precisely adjusted within a large range. There has been a lot of research on epoxy resin materials with negative coefficients of thermal expansion, which will not be elaborated in this application.

[0047] The preferred range for the coefficient of thermal expansion of the negative thermal expansion material used in this application embodiment is -3.5 × 10⁻⁶. -4 ~-7×10 -5 This is to make the coating thickness close to that of conventional optical fiber coatings.

[0048] In some embodiments, modified epoxy resin can be doped with nanoparticles such as titanium dioxide nanoparticles and silicon dioxide nanoparticles to form composite materials, thereby improving thermal stability and mechanical properties. The resulting composite material has a certain degree of flexibility as well as good hardness and stiffness. By adjusting the amount of nanoparticles, the elastic modulus and other performance parameters of the modified epoxy resin can also be controlled, making it similar to the adhesives commonly used for optical fiber coating, so as not to cause too much stress to the optical fiber and thus damage it.

[0049] In this embodiment of the invention, the modified epoxy resin coated on the fused taper fiber coupler can be cured by ultraviolet light, and can be cured at room temperature. It has a fast curing speed, flexible operation, and high curing quality, and is not prone to problems such as solvent residue and uneven curing.

[0050] See Figure 2 The high-temperature-stability fused taper fiber coupler proposed in this application embodiment can be fabricated by the following method:

[0051] Fabrication of fused taper optical fibers:

[0052] Remove the coating layer in the area between the two optical fibers. The area to be removed can be about 25mm. Clean the optical fiber with anhydrous ethanol.

[0053] The cleaned optical fiber is placed in a fused fiber taper machine, the light source is turned on, and the output end of the optical fiber is connected to an optical power meter.

[0054] Set the parameters of the melting tapering machine, including pre-drawing time, tapering speed, and heating temperature.

[0055] The area of ​​the optical fiber where the coating is removed is locally heated and then fused and tapered until the required splitting ratio is achieved, thus preparing a fused taper type optical fiber coupler.

[0056] Coating with modified epoxy resin:

[0057] Place the fused taper fiber coupler on a packaging platform with a U-shaped quartz groove, and adjust the platform so that the coupling area is suspended in the center of the U-shaped quartz groove.

[0058] A modified epoxy resin with a negative coefficient of thermal expansion is poured into a U-shaped quartz groove. After the modified epoxy resin cures, a fused taper fiber coupler with high temperature stability is prepared, followed by encapsulation and other subsequent processes.

[0059] In some embodiments, the coated modified epoxy resin can be cured by baking.

[0060] In a preferred embodiment, the coated modified epoxy resin is cured by ultraviolet light.

[0061] In some embodiments, modified epoxy resin can be injected into a U-shaped quartz groove using a multi-point injection method, allowing the material to be injected simultaneously from multiple locations. This reduces the flow distance and flow resistance of the material within the mold, which is beneficial for the uniform distribution and mixing of the material. The location and injection amount of the multi-point injection need to be optimized according to the structure and shape of the mold to ensure that the material can fill the mold uniformly.

[0062] In this embodiment of the invention, the mold for the U-shaped quartz channel needs to be designed according to specific materials and parameters. The structure of the U-shaped quartz channel mold can be found in [reference needed]. Figure 3 (A is the magnified area), including a U-shaped groove 100 provided on the substrate and a plurality of uniformly distributed injection ports 200. The length tolerance of the U-shaped groove 100 is ±1mm and the depth tolerance is ±1μm. The material of the U-shaped groove can be quartz.

[0063] For fiber couplers with different beam splitting ratios, the coating length and thickness of the negative thermal expansion material must be strictly controlled. The length error of the coating material must be controlled within ±0.2mm, and the thickness error must be controlled within ±2μm. After coating, the coating layer should be inspected. For areas with defects or that do not meet the accuracy requirements, manual trimming or laser repair methods can be used.

[0064] In some embodiments, the modified epoxy resin coating process can be carried out sequentially in multiple U-shaped grooves, with each layer of material being coated as a thin layer, then cured or dried, and then coated again. The desired coating thickness and precision can be achieved by stacking multiple layers.

[0065] Based on the multi-coating process, in some embodiments, modified epoxy resins with different elastic moduli can be sequentially coated in the coupling area to form a gradient, thereby optimizing the stress distribution in the coupling area, improving interfacial bonding, and enhancing the fatigue resistance of the coating layer.

[0066] Based on the multi-coating process, in some other embodiments, modified epoxy resins with different coefficients of thermal expansion can be sequentially coated in the coupling area to form a gradient, thereby optimizing the stress distribution in the coupling area under temperature changes. This avoids the coating layer from causing excessive stress to the optical fiber and damaging it, and also improves the interface bonding and enhances the fatigue resistance of the coating layer.

[0067] This application also investigated the effects of coating thickness, the coefficient of thermal expansion of the coating material, and coating length on the splitting ratio of the fused taper fiber coupler, and conducted simulation experiments. The simulation results are available in [reference needed]. Figures 4-7 .

[0068] in, Figure 4 This is a graph showing the correlation between beam splitting ratio temperature stability and coating material thickness. Figure 5 This is a graph showing the correlation between the temperature stability of the beam splitting ratio and the coefficient of thermal expansion of the coating material. Figure 6 This is a graph showing the correlation between the temperature stability of the beam splitting ratio and the coating length.

[0069] in accordance with Figures 4-6 The optimal parameters of the coating material can be optimized and obtained through simulation. Figure 7 The graph shown illustrates the change in the beam splitting ratio with temperature before and after coating with the negative thermal expansion material. Figure 7 The dashed and solid lines represent the changes in the splitting ratio with temperature before and after the coupling region of the fiber coupler is coated with a negative thermal expansion material. It can be seen that after coating with the negative thermal expansion material, the change in the splitting ratio of the fiber coupler decreases from the original 0.923% to 0.128%, and the maximum temperature sensitivity decreases from the original 0.076% / K to 0.004% / K, improving the temperature stability by an order of magnitude. Therefore, the technical solution of this application can greatly improve the temperature stability of the fused taper fiber coupler.

[0070] The high-temperature stability fused taper fiber coupler provided in this application embodiment is widely applicable to the fields of fiber optic communication, photonics, fiber optic sensing, and precision measurement.

[0071] In particular, lasers for space gravitational wave detection and high-precision fiber optic gyroscopes have extremely high requirements for the power stability of laser output. Fluctuations in beam splitting ratio and additional losses caused by ambient temperature fluctuations can bring significant intensity noise, greatly affecting their detection accuracy. The high-temperature stability fused taper fiber coupler provided in this application embodiment can effectively improve this problem.

[0072] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A high-temperature-stability fused taper fiber coupler, comprising at least two single-mode fibers formed by melting and stretching to create a taper region and a coupling region, characterized in that, The coupling region is covered with a negative thermal expansion material; The negative thermal expansion material is a modified epoxy resin; The modified epoxy resin is formed by doping an epoxy resin matrix with inorganic particles having negative thermal expansion characteristics, or by doping an epoxy resin matrix with organic structural units having negative thermal expansion characteristics. The epoxy resin matrix is ​​at least one of bisphenol A type epoxy resin, bisphenol F type epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, phenolic epoxy resin, and glycidylamine type epoxy resin. The coefficient of thermal expansion of the modified epoxy resin is -3.5 × 10⁻⁶. -4 ~-7×10 -5 .

2. The high-temperature-stability fused taper fiber coupler according to claim 1, characterized in that, The coupling region is covered with multiple layers of negative thermal expansion material.

3. The high-temperature-stability fused taper fiber coupler according to claim 2, characterized in that, The multilayered negative thermal expansion material has different coefficients of thermal expansion.

4. A method for fabricating a fused taper fiber coupler with high temperature stability, characterized in that, Includes the following steps: Step 1: Suspend the coupling region of the fused taper fiber coupler at the center of the U-shaped groove; Step 2: Pour modified epoxy resin with a negative coefficient of thermal expansion into the U-shaped groove, and wait for the modified epoxy resin to cure to complete the preparation of the high-temperature stable fused taper fiber coupler. The modified epoxy resin is formed by doping an epoxy resin matrix with inorganic particles having negative thermal expansion characteristics, or by doping an epoxy resin matrix with organic structural units having negative thermal expansion characteristics. The epoxy resin matrix is ​​at least one of bisphenol A type epoxy resin, bisphenol F type epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, phenolic epoxy resin, and glycidylamine type epoxy resin. The coefficient of thermal expansion of the modified epoxy resin is -3.5 × 10⁻⁶. -4 ~-7×10 -5 .

5. The method for fabricating a high-temperature-stability fused taper fiber coupler according to claim 4, characterized in that, In step 2, the modified epoxy resin is cured by ultraviolet light.

6. The method for fabricating a high-temperature-stability fused taper fiber coupler according to claim 4, characterized in that, In step 2, the modified epoxy resin is simultaneously injected into the U-shaped groove through multiple injection points.

7. The method for fabricating a high-temperature-stability fused taper fiber coupler according to claim 4, characterized in that, It also includes the following steps: Steps 1 and 2 are performed sequentially in multiple U-shaped grooves.

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