Composite-core-layer large-mode-field single-mode optical fiber

Abstract

The utility model relates to the field of fiber lasers, and particularly discloses a composite core layer large-mode-field single-mode fiber. The structure of the optical fiber sequentially comprises a composite fiber core layer, a trapezoidal gradient refractive index coupling ring, a sunken groove structure and a wrapping layer from inside to outside. The fiber core layer is formed by compounding three layers of fiber cores with different refractive indexes, and the refractive indexes of the fiber cores are sequentially reduced from inside to outside; the refractive index change condition of the trapezoidal gradient refractive index coupling ring is an isosceles trapezoid, the refractive index of the top of the trapezoidal gradient refractive index coupling ring is consistent with the refractive index of the innermost fiber core, and the refractive index values of the waist parts at the two sides are uniformly and progressively decreased from the top fiber core value to the two sides to the refractive index value of the cladding; the refractive index of the sunken groove structure is lower than that of the cladding, and the sunken groove structure is of an annular structure. The optical fiber can improve the transmission quality of the optical fiber under the bending condition, increase the mode field area of the optical fiber and reduce the bending loss generated by a fundamental mode under the condition of small-size radius bending, has the capability of meeting the single-mode transmission condition under the condition of long-wave-band wavelength, and has good use potential.

Description

Technical Field

[0001] This invention relates to the field of fiber lasers, and in particular to a large-mode-field single-mode transmission fiber based on refractive index variation. Background Technology

[0002] Increasing the mode field area of ​​optical fibers can not only reduce internal power density and thus improve nonlinear effects, but also enhance the capacity of optical fiber communication. Increasing the fiber core diameter is the most direct and effective method to increase the mode field area; however, as the core diameter increases, the number of internal modes increases, leading to mode competition and impairing the quality of the propagating beam. Compared to multimode fibers, single-mode fibers have no intermode dispersion and low total dispersion, making them suitable for long-distance, high-capacity optical fiber communication systems, optical fiber local area networks, and various optical fiber sensors. In practical applications, bending can make it difficult for ordinary optical fibers to maintain a stable single-mode transmission state. Therefore, designing large-mode-field, bend-resistant single-mode transmission fibers is a feasible way to promote the continued development of high-power fiber lasers. Utility Model Content

[0003] The purpose of this invention is to overcome the bending loss of the optical fiber FM (mode field) which is greater than 0.1 dB / m when the bending radius is 10 cm, so as to increase the mode field area of ​​the FM, reduce the bending loss of the FM, and improve the transmission quality of the beam in the optical fiber while meeting the single-mode transmission conditions.

[0004] To achieve the above objectives, this invention employs an optical fiber structure based on a large-mode-field single-mode transmission structure with varying refractive index. From the inside out, it comprises a composite core layer, a trapezoidal graded-index coupling ring, a recessed trench structure, and a cladding structure. The composite core layer is configured as an inner core, a middle core, and an outer core, with the refractive index decreasing sequentially from the inside out. The recessed trench structure is configured as a single-groove recessed ring structure, and the refractive index of the single-groove recessed ring structure is less than that of the cladding.

[0005] The inner fiber core has a radius of 30 μm, the middle fiber core has a width of 18 μm, and the outer fiber core has a width of 13.2 μm.

[0006] The distance between the composite fiber core layer and the trapezoidal gradient refractive index coupling ring is 20 μm.

[0007] The trapezoidal gradient refractive index coupling ring has a waist width of 5 μm on both sides and a top width of 4 μm.

[0008] The distance between the trapezoidal gradient refractive index coupling ring and the sunken groove structure is 39 μm.

[0009] The width of the sunken groove structure is 8 μm.

[0010] The inner fiber core has a refractive index of 1.439, the middle fiber core has a refractive index of 1.4385, and the outer fiber core has a refractive index of 1.4384.

[0011] The refractive index of the sunken groove structure is 1.436.

[0012] The refractive index of the cladding is 1.43815.

[0013] This invention provides a bend-resistant, large-mode-area single-mode transmission optical fiber based on refractive index variation. The composite core layer consists of three core layers with different refractive indices, decreasing from the inside to the outside. By adjusting the refractive index difference between the composite core layer and the cladding, the mode area of ​​the optical fiber is increased while still meeting the requirements for single-mode transmission. The trapezoidal graded-index coupling ring enhances the resonant coupling effect between higher-order modes and the coupling ring, increasing the bending loss of higher-order modes and better meeting the requirements for single-mode transmission. The recessed groove structure adopts a single-groove ring structure, which indirectly increases the refractive index difference between the cladding and core layers, improves the numerical aperture, increases the light-gathering ability of the optical fiber, and increases the mode area. Through the above structural design, the bending loss of the fundamental mode can be reduced and the mode area increased under a small radius of bending, improving the transmission quality of the optical fiber and maintaining single-mode transmission capability over a long wavelength range, demonstrating good versatility. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the following is a brief introduction to the drawings required in the embodiments or the prior art. The following drawings are only some embodiments of this utility model and are not intended to define this utility model.

[0015] Figure 1 This is a schematic diagram of a two-dimensional cross-sectional structure of a single-mode fiber with a composite core and a large mode field, provided as Example 1 of this utility model.

[0016] Figure 2 This is a schematic diagram of the refractive index distribution of a single-mode fiber with a large mode field and a composite core, provided as Example 1 of this utility model.

[0017] Figure 3 The curves of bending loss and effective mode field area of ​​FM and HOMs with large mode field of composite core layer provided in Example 2 of this utility model are given under the condition that the width of the outermost delt2 layer varies.

[0018] Figure 4The curves for bending loss and effective mode area of ​​FM and HOMs of single-mode fiber with composite core and large mode field provided in Example 2 of this utility model are given under the condition that the distance t from the composite fiber core layer to the trapezoidal graded refractive index coupling ring varies.

[0019] Figure 5 The curves for bending loss and effective mode area of ​​FM and HOMs with large mode field of composite core single-mode fiber provided in Example 2 of this utility model are given under the condition that the distance t2 from the sunken trench structure to the trapezoidal graded refractive index coupling ring varies.

[0020] Figure 6 The curves showing the bending loss and effective mode area of ​​FM in single-mode optical fibers with a composite core and large mode field provided in Example 2 of this utility model vary with wavelength from 1600 to 2000 nm.

[0021] Figure 7 The electric field mode distribution diagram of the FM of the single-mode optical fiber with a large mode field and a composite core layer provided in Example 2 of this utility model under the condition of a bending radius of 10cm.

[0022] Figure 8 The electric field mode distribution diagram of the large mode field single-mode optical fiber with composite core layer provided in Example 2 of this utility model is shown in the HOMs under the condition of a bending radius of 10cm. Detailed Implementation

[0023] To make the technical solution and advantages of this utility model clearer, the present utility model will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the drawings described herein are merely some embodiments of this utility model and are only used to explain this utility model, and are not intended to limit this utility model.

[0024] Please see Figures 1 to 8 This utility model provides a composite core layer large mode field single-mode fiber, which consists of a composite core layer, a trapezoidal graded refractive index coupling ring, a recessed trench structure and a cladding from the inside to the outside. The refractive index of the core layer gradually decreases from the inside to the outside, and it has an inner, middle and outer three-layer structure. The refractive index of the recessed trench structure is lower than that of the cladding.

[0025] The inner core has a radius of 30 μm, the middle core has a width of 18 μm, and the outer core has a width of 13.2 μm.

[0026] The distance between the composite fiber core layer and the trapezoidal gradient refractive index coupling ring is 20 μm.

[0027] The trapezoidal gradient refractive index coupling ring has a waist width of 5μm on both sides and a top width of 4μm.

[0028] The distance between the trapezoidal gradient refractive index coupling ring and the sunken groove structure is 39 μm.

[0029] The width of the sunken groove structure is 8 μm.

[0030] The radius of the cladding is 200 μm.

[0031] The inner fiber core has a refractive index of 1.439, the middle fiber core has a refractive index of 1.4385, and the outer fiber core has a refractive index of 1.4384.

[0032] The refractive index increments on both sides of the waist of the trapezoidal tapered ring are 1.7 × 10⁻⁶. 4 / μm, -1.7×10 4 / μm, the top refractive index of the trapezoidal refractive index gradient ring is 1.439.

[0033] The refractive index of the sunken groove structure is 1.436.

[0034] The refractive index of the cladding is 1.43815.

[0035] In this embodiment, the fundamental mode field strength distribution across the fiber cross-section is approximately Gaussian, concentrated at the fiber core. The beam forms a critical propagation angle with the fiber axis in the straight or flat portion of the fiber. When the fiber bends, the propagation angle formed at the boundary of the bend exceeds this critical value. This results in total internal reflection not being satisfied in the bent fiber, causing the beam that was originally propagating in the core to leak into the cladding, leading to mode loss. Therefore, as the bending radius decreases, more beams leak into the cladding, resulting in greater bending loss.

[0036] The mode field area measures the power density per unit area of ​​an optical fiber. The formula for calculating the mode field area is:

[0037]

[0038] In equation (1), E(x, y) represents the transverse electric field of the fiber cross section.

[0039] The leakage loss of each mode within an optical fiber can be obtained by solving for the imaginary part of its propagation constant. The bending loss formula is as follows:

[0040]

[0041] In equation (2), β is the propagation constant of each mode and λ is the incident wavelength.

[0042] Example 1

[0043] This example provides a single-mode transmission fiber with a large mode field, bend resistance, and low loss based on a composite core layer with a trapezoidal refractive ring and a recessed trench, which consists of a composite core layer, a trapezoidal graded refractive index coupling ring, a recessed trench structure, and a cladding structure from the inside out.

[0044] The composite core layer consists of three layers of fiber with different refractive indices: inner, middle, and outer. The refractive index decreases from the inner to the outer layer. This three-layer composite core structure increases the core radius and mode field area. Furthermore, by adjusting the refractive index, the refractive index difference between the core and cladding layers can be changed, thus satisfying stable single-mode transmission conditions and ensuring the universality of maintaining single-mode transmission in the long wavelength range.

[0045] When the trapezoidal graded refractive index coupling ring is bent with the optical fiber, mode coupling is generated, causing the mode field to leak outward. This is beneficial for increasing the mode field area and reducing the bending loss of the FM.

[0046] The recessed groove structure consists of a single-groove recessed ring structure. Its refractive index is lower than that of the cladding, which can reduce the refractive index of the cladding part, thereby increasing the refractive index difference between the fiber core and the cladding, thus increasing the numerical aperture and enhancing the light-gathering ability of the optical fiber when bent.

[0047] The refractive indices of the composite fiber core layer and the trapezoidal gradient refractive index coupling ring are greater than the refractive index of the cladding, while the refractive index of the sunken groove structure is less than that of the cladding.

[0048] Based on the above structure, the optical fiber in this embodiment can increase the mode field area of ​​the optical fiber and reduce the bending loss of the fundamental mode while having a small bending radius, thus meeting the stable single-mode transmission conditions.

[0049] Example 2

[0050] This embodiment is based on embodiment 1:

[0051] In the composite fiber core layer, the inner fiber core has a radius r0 = 30 μm and a refractive index of 1.439, the middle fiber core has a width delt1 = 18 μm and a refractive index of 1.4385, and the outer fiber core has a width delt2 = 13.2 μm and a refractive index of 1.4384.

[0052] In the trapezoidal graded refractive index coupling ring, the width of the inner waist of the refractive ring is 5 μm, and the refractive index increase is 1.7 × 10⁻⁶. 4 / μm; top width is 4μm, refractive index is 1.439; outer waist width of the refractive ring is 5μm, refractive index increment is -1.7×10 4 / μm.

[0053] The width of the sunken groove structure is 8 μm, and the refractive index is 1.436.

[0054] The distance between the composite fiber core layer and the trapezoidal graded refractive index coupling ring is 20 μm, and the distance between the trapezoidal graded refractive index coupling ring and the sunken groove structure is 39 μm.

[0055] The cladding radius r1 = 200 μm and the refractive index is 1.43815.

[0056] The cladding is surrounded by a perfectly matched layer with a width of 10 μm for absorbing boundary conditions. The purpose of the perfectly matched layer is to gradually attenuate or even completely absorb the incident light to achieve zero reflection. When the width is greater than 5 μm, the loss of each mode remains basically unchanged during the simulation.

[0057] The optical fiber structure was simulated and analyzed using COMSOL software based on the finite element method. The simulation results show that a reasonable composite core refractive index can achieve single-mode transmission with a large mode area. As the distance t from the composite core to the trapezoidal graded refractive index coupling ring increases, the bending loss of FM and HOMs increases, and the mode area of ​​FM also increases with the increase of t. The increase of the distance t from the depression trench structure to the trapezoidal graded refractive index coupling ring has no significant effect on the refractive index difference between the cladding and the core, so the mode area does not change significantly.

[0058] Figure 3 The graphs show the bending loss of FM and HOMs, and the effective mode field area of ​​FM as a function of the outermost delt2 width. When the delt2 width is within the range of 13-15 μm, the bending loss of FM is less than 0.1 and the bending loss of HOMs is greater than 1, indicating that the optical fibers satisfy a stable single-mode transmission state. The bending loss of HOMs increases with increasing delt2, specifically: 1.2266 dB / m, 1.2974 dB / m, 1.6323 dB / m, and 2.4524 dB / m; the bending loss of FM is: 0.08757 dB / m, 0.07808 dB / m, 0.07361 dB / m, and 0.08575 dB / m. Increasing delt2 indirectly increases the refractive index difference between the cladding and core layers, which is beneficial for mode field leakage during bending. The mode field area increases with increasing delt2, reaching 2074.40146 μm. 2 2075.876μm 2 2091.984μm 2 2120.162μm 2 .

[0059] Figure 4The graphs show the bending loss of FM and HOMs, and the effective mode area of ​​FM as a function of the distance *t* from the composite core to the trapezoidal graded-index coupling ring. When *t* is within the range of 20-24 μm, the optical fiber satisfies a stable single-mode transmission state. The bending loss of both FM and HOMs increases with increasing *t*: 0.0656 dB / m, 0.06761 dB / m, 0.07022 dB / m, 0.07074 dB / m, and 0.08647 dB / m for FM; and 1.5369 dB / m, 3.284 dB / m, 3.4031 dB / m, 2.755 dB / m, and 3.7727 dB / m for HOMs. With increasing *t*, the refractive index difference between the core and cladding decreases, but the core diameter increases, and the mode area overcomes the effect of the refractive index, reaching 2076.09911 μm. 2 2080.62589μm 2 2085.56433μm 2 2091.02352μm 2 2097.75566μm 2 .

[0060] Figure 5 The graphs show the bending loss of FM and HOMs, and the effective mode area of ​​FM as a function of the distance t2 from the grooved structure to the trapezoidal graded-index coupling ring. When t is in the range of 38-42 μm, the fiber meets the single-mode transmission operation requirements. As t2 increases, the refractive index of the core layer decreases indirectly, and the refractive index difference between the core and cladding increases. The bending loss of FM does not change significantly, and is 0.09527 dB / m, 0.07808 dB / m, 0.07402 dB / m, 0.07697 dB / m, and 0.08631 dB / m, respectively. The bending loss of HOMs increases significantly, and is 1.2296 dB / m, 1.2974 dB / m, 1.6541 dB / m, 2.4954 dB / m, and 6.5043 dB / m, respectively. At this point, the increased high basis ratio is beneficial to improving the bending loss of the fiber. Although the area of ​​the model field does not vary much, it decreases as t2 increases, and is 2077.42dB / m, 2074.968dB / m, 2074.224dB / m, 2074.331dB / m, and 2074.762dB / m respectively.

[0061] Figure 6 The graphs show the bending loss of FM and HOMs and the effective mode area of ​​FM as a function of wavelength from 1600 to 2000 nm. The bending loss of FM is less than 0.1 dB / m, and that of HOMs is greater than 1 dB / m, satisfying the single-mode transmission conditions in the long-wavelength range. Their mode areas increase with increasing wavelength, reaching 1474 μm.2 1612.705μm 2 1758μm 2 1911.328μm 2 2075.258μm 2 .

[0062] Figure 7 and Figure 8 This diagram shows the electric field distribution of FM and HOMs in the optical fiber of this embodiment when the bending radius is 10 cm. This invention adjusts the refractive index difference between the core and cladding layers by regulating the refractive index and width of each layer in the composite core, which is beneficial for achieving large mode area single-mode transmission performance with a small bending radius. The trapezoidal graded refractive index coupling ring resonates with the modes, allowing the mode field to leak outward to obtain a larger mode area while reducing the bending loss of FM and HOMs. The refractive index of the single-groove recessed ring structure is lower than that of the cladding, increasing the refractive index difference between the core and cladding layers, increasing the focusing ability of the optical fiber, and improving the beam transmission quality.

[0063] The above description is only a preferred embodiment of the present utility model and is not intended to limit the present utility model. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present utility model should be included within the protection scope of the present utility model.

Claims

1. A composite core large mode area single mode optical fiber, characterized in that, the optical fiber comprises: a composite core layer, a trapezoidal graded index coupling ring, a sunken trench structure, a cladding layer; the core layer comprises an inner core, a middle core and an outer core arranged from inside to outside, the refractive index value decreases layer by layer from inside to outside, the sunken trench structure adopts a single trench, and the refractive index of the single trench is less than the refractive index of the cladding layer.

2. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the inner core radius is 30 μm, the middle core width is 18 μm, and the outer core width is 13.2 μm.

3. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the distance between the composite core layer and the trapezoidal graded index coupling ring is 20 μm.

4. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the waist width of the trapezoidal graded index coupling ring on both sides is 5 μm, and the top width is 4 μm.

5. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the distance between the trapezoidal graded index coupling ring and the sunken trench structure is 39 μm.

6. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the width of the sunken trench structure is 8 μm.

7. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the radius of the cladding layer is 200 μm.

8. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the refractive index of the inner core is 1.439, the refractive index of the middle core is 1.4385, and the refractive index of the outer core is 1.4384.

9. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the refractive index of the sunken trench structure is 1.

436.

10. The composite core large mode area single mode optical fiber of claim 1, characterized in that, the refractive index of the cladding layer is 1.43815.

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