Composite cladding large-mode-field-area single-mode fiber based on parabolic refractive index fiber core

Abstract

The invention relates to the technical field of optical fibers, and particularly discloses a large-mode-field single-mode transmission optical fiber which sequentially comprises a fiber core layer, a trapezoidal refractive index gradient ring, a multi-gully groove and a wrapping layer from inside to outside. The fiber core layer comprises two layers of fiber cores, and the refractive index of the outer fiber core is lower than that of the inner fiber core; the refractive index change of the trapezoidal refractive index ring structure is trapezoidal, the refractive index of the inner side is increased from a cladding value to an inner-layer fiber core value, the refractive index value of the middle part is equal to the inner-layer fiber core value, and the refractive index of the outer side is reduced from the inner-layer fiber core value to the cladding value; the multi-gully groove structure comprises two gullies, and the refractive index value of the gullies is lower than the refractive index value of the cladding. The mode field area of the optical fiber can be increased, the bending loss of a fundamental mode can be reduced, the transmission quality of the optical fiber can be improved, and the manufacturability can be improved under the condition of medium-size radius bending.

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. Summary of the Invention

[0003] The purpose of this invention is to overcome the bending loss of the optical fiber FM (mode field area) which is greater than 0.1 dB / m when the bending radius is 13.5 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 consists of a parabolic refractive index core layer, a tilted refractive index coupling ring, an eight-lobed recessed trench structure, and a cladding structure. The refractive index of the parabolic refractive index core layer gradually decreases from the inside out. The recessed trench structure is configured as an eight-lobed, petal-shaped recessed trench structure, and the refractive index of the eight-lobed recessed trench structure is less than that of the cladding.

[0005] The core radius is 55 μm.

[0006] The distance between the fiber core layer and the tilted refractive index coupling ring is 0 μm.

[0007] The width of the tilted refractive index coupling ring is 10 μm.

[0008] The distance between the tilted refractive index coupling ring and the eight-lobed lobed recessed groove structure is 0 μm.

[0009] The width of the eight-lobed, petal-shaped sunken groove structure is 57.5 μm.

[0010] The angular width of the eight-petal groove depression layer is 29.25°, and the angular width between two adjacent grooves of the eight-petal groove depression layer is 15.75°.

[0011] The radius of the cladding is 182.5 μm.

[0012] The parabolic fiber core has a maximum refractive index of 1.4448 and a minimum refractive index of 1.4445.

[0013] The maximum refractive index of the tilted refractive index cladding is 1.4445, and the minimum refractive index is 1.444.

[0014] The refractive index of the eight-lobed grooved depression layer is 1.4434.

[0015] The cladding has a refractive index of 1.444.

[0016] This invention provides a bend-resistant, large-mode-area single-mode transmission optical fiber based on refractive index variation. The core layer is composed of a parabolic core layer, with the refractive index gradually decreasing from the inside to the outside. By adjusting the refractive index difference between the core layer and the composite cladding, the mode area of ​​the optical fiber is increased while still satisfying single-mode transmission requirements. The tilted refractive index coupling ring is a graded-ratio ring, which enhances the resonant coupling effect between higher-order modes and the coupling ring, increases the bending loss of higher-order modes, and better meets the requirements for single-mode transmission. The recessed trench structure adopts an eight-lobed, petal-shaped recessed trench structure, which indirectly increases the refractive index difference between the cladding structure and the core structure, increases the numerical aperture, increases the light-gathering ability of the optical fiber, and increases the mode area of ​​the optical fiber. 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 satisfying the single-mode transmission capability over a long wavelength range, demonstrating good versatility. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 merely some embodiments of the present invention and are not intended to define the present invention.

[0018] Figure 1 This is a schematic diagram of a single-mode optical fiber with a parabolic refractive index core structure, low bending loss, and large mode field, provided in Example 1 of the present invention.

[0019] Figure 2 Example 2 of this invention provides a low-bending-loss, large-mode-field single-mode fiber with a parabolic refractive index core and composite cladding structure, and a fundamental mode (LP) of the fiber. 01 ) and higher-order modes (LP) 11v and LP 11h The curves of bending loss, effective modal area of ​​FM, and loss ratio as a function of core radius.

[0020] Figure 3 Example 2 of this invention provides a low-bending-loss, large-mode-field single-mode fiber with a parabolic refractive index core and composite cladding structure, and a fundamental mode (LP) of the fiber. 01 ) and higher-order modes (LP) 11v and LP 11h The curves of bending loss, effective mode area of ​​FM, and loss ratio as a function of tilted refractive index cladding thickness t1 are shown.

[0021] Figure 4 Example 2 of this invention provides a low-bending-loss, large-mode-field single-mode fiber with a parabolic refractive index core and composite cladding structure, and a fundamental mode (LP) of the fiber. 01 ) and higher-order modes (LP) 11v and LP 11h The curves of bending loss, effective mode area of ​​FM, and loss ratio as a function of the thickness t2 of the depression trench structure.

[0022] Figure 5 Example 2 of this invention provides a low-bending-loss, large-mode-field single-mode fiber with a parabolic refractive index core and composite cladding structure, and a fundamental mode (LP) of the fiber. 01 ) and higher-order modes (LP) 11v and LP 11h The curves of bending loss, effective mode area of ​​FM, and loss ratio as a function of the number of sunken layers N are shown.

[0023] Figure 6 The electric field mode distribution diagram of the low bending loss, large mode field single-mode fiber with a parabolic refractive index core and composite cladding structure provided in Example 2 of the present invention is shown in the FM mode distribution diagram under a bending radius of 13.5 cm.

[0024] Figure 7 The electric field mode distribution diagram of the low bending loss, large mode field single-mode fiber with parabolic refractive index core and composite cladding structure provided in Example 2 of the present invention is shown in the figure of HOMs under the condition of bending radius of 13.5 cm.

[0025] Figure 8 The curves provided in Example 2 of this invention, showing the bending loss and effective mode area (FM) of the fundamental mode (LP01) and higher-order modes (LP11v and LP11h) of a low-bending-loss, large-mode-field single-mode fiber with a parabolic refractive index core and composite cladding structure, and the loss ratio as a function of the negative refractive index cladding thickness, are shown. Figure 9 Example 2 of this invention provides curves showing the effective mode area and loss ratio of a low-bending-loss, large-mode-field single-mode fiber with a parabolic refractive index core and composite cladding structure, comprising the fundamental mode (LP01) and higher-order modes (LP11v and LP11h), as a function of bending angle. Figure 10The electric field diagram of the fundamental mode (LP01) of a low-bending-loss, large-mode-field single-mode fiber with a parabolic refractive index core and composite cladding structure provided in Example 2 of the present invention is shown under the condition of a bending radius of 13.5 cm. Figure 11 The electric field diagram of a low-bending-loss, large-mode-field single-mode fiber HOM mode (LP11) with a parabolic refractive index core and composite cladding structure provided in Example 2 of the present invention is shown under the condition of a bending radius of 13.5 cm. Detailed Implementation To make the technical solutions and advantages of the present invention clearer, the present invention 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 the present invention and are only used to explain the present invention, and are not intended to limit the present invention.

[0026] Please see Figures 1 to 7 This invention provides a composite cladding large mode area single-mode fiber based on a parabolic refractive index core. From the inside out, the fiber consists of a core layer, a tilted refractive index coupling ring, a recessed trench structure, and a cladding. The refractive index of the core layer gradually decreases from the inside out, which meets the requirements of a parabolic function, and the refractive index of the recessed trench structure is lower than that of the cladding.

[0027] The core radius is 55 μm.

[0028] The distance between the fiber core layer and the tilted refractive index coupling ring is 0 μm.

[0029] The width of the tilted refractive index coupling ring is 10 μm.

[0030] The distance between the tilted refractive index coupling ring and the eight-lobed sunken groove structure is 0 μm.

[0031] The width of the eight-lobed sunken groove structure is 57.5 μm.

[0032] The radius of the cladding is 182.5 μm.

[0033] The fiber core has a maximum refractive index of 1.4448 and a minimum refractive index of 1.4445, and its refractive index follows a parabolic distribution.

[0034] The highest refractive index of the tilted refractive index coupling ring is 1.4445, and the lowest refractive index is 1.444.

[0035] The refractive index of all the sunken groove structures is 1.4434.

[0036] The refractive index of the cladding is 1.444.

[0037] 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.

[0038] The mode field area measures the power density per unit area of ​​an optical fiber. The formula for calculating the mode field area is:

[0039]

[0040] In equation (1), E(x, y) represents the transverse electric field of the fiber cross section.

[0041] 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:

[0042]

[0043] In equation (2), β is the propagation constant of each mode and λ is the incident wavelength.

[0044] Example 1

[0045] This example provides a composite cladding large-mode-field bending-resistant, low-loss single-mode transmission fiber with tilted refractive index refractive ring and depressed trench based on refractive index variation. From the inside out, it consists of a core layer, a tilted refractive index coupling ring, an eight-lobed depressed trench structure, and a cladding structure.

[0046] The core layer consists of a fiber core with a parabolic refractive index, which gradually decreases from the center outwards. The core layer structure can alter the refractive index difference between the core and cladding layers by adjusting the refractive index, thus satisfying stable single-mode transmission conditions and ensuring universality in maintaining single-mode transmission over long wavelengths.

[0047] When the tilted refractive index coupling loop bends with the fiber, mode coupling occurs, causing the mode field to leak outward. This helps to increase the mode field area and reduce the bending loss of the FM.

[0048] The recessed groove structure consists of recessed petal-shaped grooves. Its refractive index is lower than that of the cladding, which can reduce the refractive index of the cladding and thus increase the refractive index difference between the core and the cladding, thereby increasing the numerical aperture and enhancing the light-gathering ability of the optical fiber when bent.

[0049] The refractive indices of the aforementioned core layer and tilted refractive index coupling ring are greater than those of the cladding, while the refractive index of the sunken petal-shaped groove structure is less than that of the cladding.

[0050] 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.

[0051] Example 2

[0052] This embodiment is based on embodiment 1:

[0053] In the core layer, the core radius a = 30 μm, the highest refractive index is 1.4448, the lowest refractive index is 1.4445, and its distribution satisfies the distribution pattern of a parabolic core.

[0054] In the tilted refractive index coupling ring, the width of the refractive ring is 10 μm, and the maximum refractive index is 1.4445; the minimum refractive index of the refractive ring is 1.444, which gradually decreases from the position near the fiber core outwards.

[0055] The width of the sunken groove structure is 62.5 μm, and the refractive index is 1.4434.

[0056] The distance between the fiber core layer and the tilted refractive index coupling ring is 0 μm, and the distance between the tilted refractive index coupling ring and the sunken groove structure is 0 μm.

[0057] The cladding radius is d = 182.5 μm and the refractive index is 1.444.

[0058] The cladding is surrounded by a perfectly matched layer with a width of 20 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.

[0059] The optical fiber structure was simulated and analyzed using COMSOL software based on the finite element method. The simulation results show that a reasonable core refractive index can achieve single-mode transmission with a large mode area. As the radius a of the fiber core increases, the bending loss of FM and HOMs increases, and the mode area of ​​FM also increases with the increase of a. The increase of the width t2 of the groove structure has no significant effect on the refractive index difference between the cladding and the core, so the mode area does not change significantly.

[0060] Figure 3 and Figure 4The graphs show the bending loss of FM and HOMs, and the effective mode area of ​​FM as a function of core radius *a*. When the width of *a* is within the range of 55-67 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 the increase of core radius *a*, rising from 6.9093 dB / m to 16.3667 dB / m; the bending loss of FM increases from 0.0232 dB / m to 0.0415 dB / m. The increase of *a* indirectly increases the refractive index difference between the cladding and core layers, which is beneficial for mode field leakage during bending. The mode area increases with the increase of *a*, rising from 1909.4237 μm. 2 Rising to 2335.28089μm 2 .

[0061] Figure 5 and Figure 6 The graphs show the bending loss of FM and HOMs, and the effective mode area of ​​FM as a function of the duty cycle γ of the eight-lobed recessed trench layer. The bending loss of both FM and HOMs decreases with increasing γ. When γ is between 0.6 and 0.7, the fiber maintains single-mode transmission, with bending losses of 0.046219 dB / m and 0.002884 dB / m for FM and 5.295915 dB / m for HOMs. As γ increases, the eight-lobed recessed trench layer gradually increases in size, reducing the refractive index difference between the core and cladding. Consequently, the mode area decreases with increasing γ, reaching 1884.1210 μm. 2 1874.1462μm 2 .

[0062] Figure 7 and Figure 8The graphs show the bending loss of FM and HOMs, and the effective mode area of ​​FM as a function of the tilted cladding width t1. When t1 is in the range of 8-12 μm, the fiber meets the single-mode transmission operating condition. As t1 increases, the refractive index of the core layer decreases indirectly, and the refractive index difference between the core and cladding increases, causing the bending loss of FM to gradually increase to 0.003829 dB / m, 0.005589 dB / m, 0.011737 dB / m, 0.028056 dB / m, and 0.078817 dB / m; the bending loss of HOMs increases significantly to 1.24235 dB / m, 1.98577 dB / m, 3.00123 dB / m, 5.40619 dB / m, and 6.35917 dB / m. 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 increases with the increase of t1, and is 1837.032dB / m, 1854.759dB / m, 1878.734dB / m, 1913.316dB / m, and 1967.845dB / m respectively.

[0063] Figure 9 The graphs show the loss ratio of FM and HOMs and the effective mode area of ​​FM as a function of bending angle from 0 to 45°. Both FM and HOMs satisfy the single-mode condition within the symmetrical range of 0-13° and the fiber single-mode transmission condition within the effective bending range. The mode area initially increases and then decreases with increasing bending angle, reaching a symmetry axis at a bending angle of 22.5°, with the minimum and maximum values ​​being 1878.7343 μm at 0°. 2 and 1884.575 μm at 22.5° 2 .

[0064] Figure 10 and Figure 11 This diagram shows the electric field distribution of FM and HOMs in the optical fiber of this embodiment when the bending radius is 13.5 cm. This invention can adjust the refractive index difference between the core and cladding layers by adjusting the refractive index and width of the core layer, which is beneficial for achieving large mode area single-mode transmission performance with a small bending radius. The tilted refractive index coupling ring resonates with the mode, causing 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 eight-lobed grooved recessed 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.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core, characterized in that, The fiber core layer is a single-layer core, and the refractive index value meets the requirements of a parabolic core. The refractive index value gradually decreases from the center to the outside. The recessed segmented groove adopts an eight-lobed groove recessed layer, and the refractive indices of the eight lobes are equal and less than the refractive index of the cladding.

2. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The radius of the fiber core layer is 55 μm.

3. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The distance between the fiber core layer and the tilted refractive index cladding is 0 μm.

4. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The width of the tilted refractive index cladding is 10 μm.

5. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The distance between the eight-lobed grooved depression layer and the tilted refractive index cladding is 0 μm.

6. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The width of the eight-lobed groove depression layer is 57.5 μm.

7. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The angular width of the eight-lobed groove depression layer is 29.25°, and the angular width between two adjacent grooves of the eight-lobed groove depression layer is 15.75°.

8. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The radius of the cladding is 182.5 μm.

9. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The parabolic fiber core has a maximum refractive index of 1.4448 and a minimum refractive index of 1.4445.

10. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The maximum refractive index of the tilted refractive index cladding is 1.4445, and the minimum refractive index is 1.

444.

11. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The refractive index of the eight-lobed grooved depression layer is 1.4434.

12. The composite cladding large-mode-area single-mode optical fiber based on a parabolic refractive index core as described in claim 1, characterized in that, The cladding has a refractive index of 1.444.

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