High-temperature-stability fused taper type optical fiber coupler and preparation method thereof

Abstract

The invention discloses a high-temperature-stability fused taper type optical fiber coupler and a preparation method thereof, and relates to the technical field of optical fiber couplers. According to the high-temperature-stability fused taper type optical fiber coupler, at least two single-mode optical fibers are fused and stretched to form a taper area and a coupling area, the coupling area is coated with a negative thermal expansion material, and the negative thermal expansion material can adopt modified epoxy resin with a negative thermal expansion coefficient; the negative thermal expansion material, the fiber core of the optical fiber and the cladding have opposite temperature characteristics, so that the stress generated along with the temperature change can be partially counteracted, the change of the length of a coupling area is reduced, and the high temperature stability of the beam splitting ratio of the fused taper type optical fiber coupler is realized.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber couplers, and in particular to a fused-taper optical fiber coupler with high temperature stability and a preparation method thereof. Background Art

[0002] The fused-taper fiber coupler is formed by melting two or more optical fibers at high temperature and stretching them to both sides to form a double-taper waveguide structure, and uses the overlapping of the adjacent core mode fields to generate evanescent waves to achieve optical signal coupling and energy redistribution between different optical fibers; for a fixed wavelength and optical fiber structure, the splitting ratio of the fused-taper fiber coupler is mainly determined by the length of the coupling zone, which is affected by the external ambient temperature. When the ambient temperature changes, the optical fiber generates stress due to the thermal expansion and contraction effect, causing the length of the coupling zone to expand and contract, which in turn causes the splitting ratio of the fused-taper fiber coupler to fluctuate. Therefore, in actual use, when applied to optical fiber communications, photonics, optical fiber sensing and precision measurement, the signal light power after splitting will fluctuate with the change of ambient temperature, making it difficult to output high-power stable signal light, which greatly affects the measurement accuracy.

[0003] In response to these problems, existing research work has proposed using PZT (piezoelectric ceramic) negative feedback control, ultraviolet laser photorefractive effect correction, semiconductor Peltier effect temperature control and other methods to reduce the impact of ambient temperature changes on fused-taper fiber couplers. However, these methods usually require supporting high-precision temperature sensors, controllers, drive circuits, etc. The design, debugging and maintenance of the system are relatively complicated, and accurate compensation may not be made in a timely manner, resulting in poor control efficiency. Summary of the invention

[0004] The fused-taper type optical fiber coupler with high temperature stability 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 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 is formed 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 epoxy resin, bisphenol F epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, phenolic epoxy resin, and glycidylamine epoxy resin.

[0008] Optionally, the thermal expansion coefficient of the negative thermal expansion material is .

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

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

[0011] The present invention provides a method for preparing a fused-taper optical fiber coupler with high temperature stability, comprising the following steps:

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

[0013] Step 2, pouring a modified epoxy resin having a negative thermal expansion coefficient into the U-shaped groove, and waiting for the modified epoxy resin to solidify, thereby completing the preparation of the fused-taper type optical fiber coupler with high temperature stability.

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

[0015] Optionally, in step 2, the modified epoxy resin is poured into the U-shaped groove through multiple injection points at the same time.

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

[0017] The processes of step 1 and step 2 are performed in sequence in a plurality of U-shaped grooves.

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

[0019] A layer of material with a negative thermal expansion coefficient is coated on the coupling area of ​​the fused-taper fiber coupler. The negative thermal expansion material has opposite temperature characteristics to the core and cladding of the optical fiber. Therefore, the stress generated by temperature changes can partially offset each other, thereby reducing the change in the length of the coupling area and achieving high temperature stability of the splitting ratio of the fused-taper fiber coupler. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

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

[0022] Figure 2 A flow chart of the preparation process of a high temperature stability fused-taper optical fiber coupler proposed in some embodiments of the present invention;

[0023] Figure 3 A schematic diagram of the planar structure of a U-shaped groove mold used in an embodiment of the present invention;

[0024] Figure 4 A simulation data diagram showing the correlation between the temperature stability of the splitting ratio and the coating thickness of the high temperature stability fused-taper optical fiber coupler proposed in an embodiment of the present invention;

[0025] Figure 5 A simulation data diagram showing the correlation between the temperature stability of the splitting ratio of the high temperature stability fused-taper optical fiber coupler and the thermal expansion coefficient of the coating material proposed in an embodiment of the present invention;

[0026] Figure 6 A simulation data diagram showing the correlation between the temperature stability of the splitting ratio and the coating length of the high temperature stability fused-taper optical fiber coupler proposed in an embodiment of the present invention;

[0027] Figure 7 This is a simulation data diagram of the change in splitting ratio with temperature of the high temperature stability fused-taper optical fiber coupler proposed in an embodiment of the present invention before and after negative thermal expansion material coating.

[0028] Explanation of the reference numerals: 1. optical fiber cladding; 2. optical fiber core; 3. negative thermal expansion material; 4. input port; 5. first output port; 6. second output port; 7. non-coupling region; 8. cone region; 9. coupling region; 100. U-shaped groove; 200. glue injection port. DETAILED DESCRIPTION

[0029] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] The basic principle of the fused-taper fiber coupler involves the coupling of light energy between optical fibers. When transmitting in the optical fiber, most of the light energy is concentrated in the core of the optical fiber, and only a small part of the light energy is transmitted outside the core in the form of evanescent waves. The fiber coupler is a mode coupling system composed of two or more optical fibers, and its working mechanism is based on the evanescent wave mode coupling theory of optical fibers.

[0031] In single-mode optical fiber, the transmission modes are two orthogonal fundamental modes (HE 11) signal, when the conduction mode enters the coupling zone, as the core becomes thinner, the normalized frequency (V value) of the optical fiber gradually decreases, and the optical power leaked to the cladding increases; at the same time, the coupling coefficient will continue to increase, and the optical signal will be more effectively coupled between the two waveguides. At the output end, as the core becomes thicker, the V value increases again. By controlling the length of the coupling zone, the optical power is transmitted in a specific ratio in the two cores. The structure of the coupling zone determines the transmission efficiency of the optical signal from one optical fiber to another. The splitting ratio of the fused-taper fiber coupler refers to the distribution of light energy between the two optical fibers, and its expression formula is:

[0032]

[0033] In the formula, λ is the wavelength of the light wave, is the refractive index of the cladding, is the fiber diameter in the coupling region, V is the fiber parameter of the isolated fiber, and its expression is:

[0034]

[0035] in , is the core refractive index.

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

[0037] See also Figure 1 A typical fused-taper fiber coupler is a mode coupling system formed by melting and stretching two optical fibers. The optical fiber includes an optical fiber cladding 1 and a core 2, forming a non-coupling region 7, a tapered region 8, and a coupling region 9. One end of one of the optical fibers is an input port 4, and the other end of the corresponding optical fiber is a first output port 5, and the output end of the optical fiber coupled thereto is a second output port 6.

[0038] See also Figure 1 The fused-taper type optical fiber coupler with high temperature stability proposed in the present application is based on the traditional fused-taper type optical fiber coupler, and the coupling area 9 is coated with a negative thermal expansion material 3.

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

[0040] In the prior art, a fused-taper type optical fiber coupler can be prepared by the following method: after removing the coating from the central area of ​​two optical fibers, the optical fibers are placed in an optical fiber fused-taper machine to ensure that the portions where the coating is removed are parallel and close together without any gaps; a local heat source is used to uniformly heat the optical fiber area where the coating is removed, and when the central portion of the optical fiber reaches a molten state, the stepper motor of the fused-taper machine is controlled to perform taper drawing. This stretching process causes the optical fiber to gradually become thinner, and eventually a tapered area is formed in the middle portion of the optical fiber. The diameter of this area is usually reduced to a few microns or even smaller; the stretching process not only changes the geometric shape of the optical fiber, but also changes its optical properties, especially the refractive index and mode structure of the core region. During the stretching process, two or more optical fibers will come into contact with each other and couple with each other through the tapered portion of the optical fiber; when an optical signal enters the tapered area from one optical fiber, due to the structural change in the middle region of the optical fiber, part of the optical signal is coupled to the core of the adjacent optical fiber through the electromagnetic field. This process is based on the mode coupling mechanism of light.

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

[0042] The inventor of the present application explored the feasibility of coating a variety of negative thermal expansion materials on the coupling area of ​​the fused cone type optical fiber during the research process. For example, metal alloys, metal oxides, metal fluorides, lead titanate-based anion doping systems, etc. can be coated on the surface of the coupling area by physical vapor deposition, chemical vapor deposition and other semiconductor process technologies or by slurry coating and spraying. However, these processes are difficult to form a uniform coating on the periphery of the coupling area, and often involve a high-temperature annealing process, and have high production costs and poor flexibility; some organic substances with special molecular structures and doping states, such as polyanilines and polyimides, also have negative thermal expansion characteristics, but the thermal expansion coefficients and material strengths of these materials are often difficult to accurately control, and similar to the preparation process of traditional optical fiber protective layers, they rely on coating mold cavities for coating. This process immerses the optical fiber in the molten material so that the material is evenly wrapped on the surface of the optical fiber, and a protective layer is formed after cooling. It is necessary to accurately control the temperature and coating speed. At the same time, it is difficult to coat the micron-level fused cone type optical fiber coupler because the tolerance requirements of the mold are relatively low.

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

[0044] The modified epoxy resin may 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] The epoxy resin matrix includes, but is not limited to, bisphenol A epoxy resin, bisphenol F epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, novolac epoxy resin, and glycidylamine epoxy resin.

[0046] For example, by introducing an eight-membered carbon ring structure (polyamide) into the bisphenol A epoxy resin system, the thermal expansion coefficient of the epoxy material is changed from positive to negative, and by controlling the proportion of polyamide, the thermal expansion coefficient of the epoxy material can be accurately adjusted within a large range; there have been many related studies on epoxy resin materials with negative thermal expansion coefficients, which will not be described in detail in this application.

[0047] Among them, the preferred range of thermal expansion coefficient of the negative thermal expansion material used in the embodiment of the present application is , so that the coating thickness is close to that of conventional optical fiber coating.

[0048] In some embodiments, some nanoparticles such as titanium dioxide nanoparticles, silicon dioxide nanoparticles, etc. can be doped into the modified epoxy resin to form a composite material to improve thermal stability and mechanical properties. The resulting composite material has a certain flexibility and good hardness and stiffness. By adjusting the doping amount of nanoparticles, the performance parameters such as the elastic modulus of the modified epoxy resin can also be controlled to make it close to the glue commonly used for optical fiber coating, so as not to bring too much stress to the optical fiber and damage the optical fiber.

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

[0050] See also Figure 2 The fused-taper type optical fiber coupler with high temperature stability proposed in the embodiment of the present application can be prepared by the following method:

[0051] Preparation of fused-taper optical fiber:

[0052] Remove the coating in the middle area of ​​the two optical fibers. The removal area can be about 25 mm. Use anhydrous ethanol to clean the optical fiber.

[0053] Place the cleaned optical fiber in a fused optical fiber taper machine, turn on the light source, and connect the output end of the optical fiber to an optical power meter.

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

[0055] The area where the coating of the optical fiber is removed is locally heated to perform melt taper drawing until the splitting ratio requirement is met, thereby preparing a melt taper type optical fiber coupler.

[0056] Coating of modified epoxy resin:

[0057] The fused-cone optical fiber coupler is placed on a packaging platform with a U-shaped quartz groove, and the platform is adjusted so that the coupling area is suspended at the center of the U-shaped quartz groove.

[0058] A modified epoxy resin with a negative thermal expansion coefficient is poured into the U-shaped quartz groove, and after the modified epoxy resin is solidified, the preparation of a fused-cone optical fiber coupler with high temperature stability is completed, and then subsequent processes such as packaging are carried out.

[0059] In some embodiments, the applied 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, the modified epoxy resin can be injected into the U-shaped quartz trough by multi-point injection, so that the material can be injected from multiple positions simultaneously, reducing the flow distance and flow resistance of the material in the mold, which is beneficial to the uniform distribution and mixing of the material; the position 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 evenly fill the mold.

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

[0063] For optical fiber couplers with different splitting ratios, the coating length and thickness of the negative thermal expansion material need to be strictly controlled. The length error of the coating material needs to be controlled within ±0.2mm, and the thickness error of the coating material needs to be controlled within ±2μm. After coating, the coating layer is inspected. For parts with defects or that do not meet the accuracy requirements, manual trimming, laser repair and other methods can be used to handle them.

[0064] In some embodiments, the modified epoxy resin coating process can be carried out in multiple U-shaped grooves in sequence, with a thinner layer of material applied each time, followed by curing or drying, and then the next coating, to achieve the desired coating thickness and accuracy through multiple stacking.

[0065] Based on the multiple coating process, in some embodiments, modified epoxy resins with different elastic moduli can be sequentially coated on the coupling area to form a gradient, so as to optimize the stress distribution in the coupling area, improve the interface bonding, and enhance the fatigue resistance of the coating layer.

[0066] Based on the process of multiple coatings, in some other embodiments, modified epoxy resins with different thermal expansion coefficients can be coated in sequence in the coupling area to form a gradient, so as to optimize the stress distribution in the coupling area under temperature changes, avoid the coating layer causing too much stress on the optical fiber and damaging the optical fiber when the temperature changes, and also improve the interface bonding and enhance the fatigue resistance of the coating layer.

[0067] This application also studies the influence of coating thickness, thermal expansion coefficient of coating material and coating length on the splitting ratio of fused-taper fiber coupler, and conducts simulation experiments. The simulation results are shown in Figure 4-Figure 7 .

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

[0069] in accordance with Figure 4-Figure 6 The optimal parameters of the coating material can be optimized and simulated. Figure 7 The curve diagram of the change of the beam splitting ratio with temperature before and after the negative thermal expansion material is coated is shown. Figure 7 The dashed line and the solid line respectively represent the change of the splitting ratio with temperature before and after the negative thermal expansion material is wrapped in the coupling area of ​​the optical fiber coupler; it can be seen that after coating with the negative thermal expansion material, the change range of the splitting ratio of the optical fiber coupler is reduced from the original 0.923% to 0.128%, and the maximum temperature sensitivity is reduced from the original 0.076% / K to 0.004% / K, and the temperature stability is improved by an order of magnitude; therefore, the technical solution of the present application can greatly improve the temperature stability of the fused-cone optical fiber coupler.

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

[0071] In particular, lasers for space gravitational wave detection and high-precision fiber gyroscopes have extremely high requirements for the power stability of laser output. Fluctuations in the splitting ratio and additional loss caused by ambient temperature fluctuations will bring large intensity noise, greatly affecting their detection accuracy. The high-temperature stability fused-cone fiber coupler provided in the embodiments of the present application can better improve this problem.

[0072] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A fused-taper type optical fiber coupler with high temperature stability, wherein at least two single-mode optical fibers are melted and stretched to form a taper region and a coupling region, characterized in that: The coupling area is coated with negative thermal expansion material.

2. The fused-taper type optical fiber coupler with high temperature stability according to claim 1, characterized in that: The negative thermal expansion material is modified epoxy resin.

3. The fused-taper type optical fiber coupler with high temperature stability according to claim 2, characterized in that: 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 epoxy resin, bisphenol F epoxy resin, alicyclic epoxy resin, aliphatic epoxy resin, phenolic epoxy resin, and glycidylamine epoxy resin.

4. The fused-taper type optical fiber coupler with high temperature stability according to claim 1, characterized in that: The thermal expansion coefficient of the negative thermal expansion material is .

5. The fused-taper type optical fiber coupler with high temperature stability according to claim 1, characterized in that: The coupling area is coated with multiple layers of negative thermal expansion material.

6. The fused-taper type optical fiber coupler with high temperature stability according to claim 5, characterized in that: The multiple layers of negative thermal expansion material have different thermal expansion coefficients.

7. A method for preparing a fused-taper optical fiber coupler with high temperature stability, characterized in that: The following steps are involved: Step 1, suspending the coupling region of the fused-taper optical fiber coupler at the center of the U-shaped groove; Step 2, pouring a modified epoxy resin having a negative thermal expansion coefficient into the U-shaped groove, and waiting for the modified epoxy resin to solidify, thereby completing the preparation of the fused-taper type optical fiber coupler with high temperature stability.

8. The method for preparing a fused-taper optical fiber coupler with high temperature stability according to claim 7, characterized in that: In the step 2, the modified epoxy resin is cured by ultraviolet light.

9. The method for preparing a fused-taper optical fiber coupler with high temperature stability according to claim 7, characterized in that: In the step 2, the modified epoxy resin is poured into the U-shaped groove through multiple injection points at the same time.

10. The method for preparing a fused-taper optical fiber coupler with high temperature stability according to claim 7, characterized in that: The following steps are also included: The processes of step 1 and step 2 are performed in sequence in a plurality of U-shaped grooves.

Images