Orbital Angular Momentum Erbium-Doped Fiber and Fiber Amplifier

Abstract

The present invention provides an orbital angular momentum erbium-doped optical fiber and an optical fiber amplifier. The orbital angular momentum erbium-doped optical fiber includes an inner cladding, an annular core and an outer cladding which are wrapped in sequence from the inside to the outside. The inner cladding is provided with a central air hole and a plurality of inner air holes, and the plurality of inner air holes are evenly arranged in an annular shape around the central air hole; erbium ions are evenly doped in the annular core; the outer cladding is provided with a plurality of outer air holes, and the plurality of outer air holes are evenly arranged in an annular shape around the annular core. The orbital angular momentum erbium-doped optical fiber of the present invention can well limit the distribution of the light field in the annular core region by setting the inner air holes and the outer air holes, so as to realize low differential mode gain amplification; there is no need to change the effective refractive index difference between the annular core and the cladding by doping or the like, and the annular core is evenly doped with a single layer of erbium ions. Compared with other multi-layer doping and regulated doping distribution, orbital angular momentum mode transmission can be realized by using a single material, which reduces the difficulty of drawing and has a simple manufacturing process.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber communication, and in particular to an orbital angular momentum erbium-doped optical fiber and an optical fiber amplifier. Background Art

[0002] Nowadays, the huge demand for data streaming has led to the rapid development of mobile Internet, cloud computing, big data, and the Internet of Things, which has also led to increasingly obvious limitations on network capacity. Communication based on a single single-mode optical fiber has approached its capacity limit of 100Tbit / s, corresponding to the C+L band in the communication of erbium-doped fiber amplifiers, and the demand for communication capacity is still increasing at a rate of 10 times every four years. Therefore, increasing the information capacity of communication systems is an urgent problem to be solved in the field of optical communications. Increasing the information capacity of communication systems is to use the different degrees of freedom and dimensions of light beams to improve the capacity of communications. Generally speaking, the physical dimensions of light waves include frequency (wavelength), complex amplitude (amplitude and phase), time, polarization, and space (cross-sectional spatial distribution). Wavelength division multiplexing, high-order orthogonal amplitude modulation, time division multiplexing, and polarization division multiplexing technologies are the application of light beam wavelength, amplitude and phase, time, and polarization, respectively. However, these existing technologies have basically reached their expansion limits.

[0003] Space division multiplexing uses the spatial dimension of light beams to increase communication capacity. It is one of the most promising technologies to break through the capacity bottleneck caused by nonlinear effects in current single-mode fiber communication systems. Mode division multiplexing is a type of space division multiplexing. It is based on mode division multiplexing of few-mode fiber. Since the transmission constants of each mode in the fiber are very close, the mode coupling is serious. The receiving end needs a complex multi-input multi-output digital signal processing algorithm to recover the signal, which increases the complexity and cost of the system. In order to achieve the receiving end without introducing or reducing the complex multi-input multi-output algorithm, the best choice is to use a large refractive index difference (greater than 10) between the modes. -4 ) orbital angular momentum fiber. The orbital angular momentum beam refers to a beam that carries a spatial phase factor Vortex beam, where l is the topological charge number, is the azimuth angle. Different orbital angular momentum modes have different spatial phase distributions, and the modes are orthogonal to each other, which provides a new spatial multiplexing dimension for multiplexing technology. The rich topological charge values ​​can greatly expand the total transmission capacity of communications. A 100-kilometer-long transmission system based on orbital angular momentum mode multiplexing has been verified.

[0004] Optical fiber amplifiers are key devices to ensure long-distance transmission in optical communication systems. For multi-channel orbital angular momentum mode amplification, it is hoped that the gain balance of all orbital angular momentum modes can be achieved. However, in the existing erbium-doped optical fibers for making optical fiber amplifiers with orbital angular momentum, the refractive index separation between different modes is small, and the overlap integrals of the field distributions of different modes with the erbium ion doping region cannot be matched simultaneously, resulting in a small number of amplified modes supported by the erbium-doped optical fiber with orbital angular momentum, low mode orders, complex processes and structures, low gain, and high noise figure, which cannot meet the requirements of orbital angular momentum mode multiplexing optical fiber communication. Summary of the Invention

[0005] The present invention provides an erbium-doped optical fiber with orbital angular momentum and an optical fiber amplifier to solve the defects in the prior art that the refractive index separation between different modes of the erbium-doped optical fiber with orbital angular momentum is small, the overlap integrals of the field distributions of different modes with the erbium ion doping region cannot be matched simultaneously, resulting in a small number of amplified modes supported by the erbium-doped optical fiber amplifier with orbital angular momentum, low mode orders, complex processes and structures, low gain, high noise figure, and inability to meet the requirements of orbital angular momentum mode multiplexing optical fiber communication, and to realize an erbium-doped optical fiber with orbital angular momentum and an optical fiber amplifier with low differential mode gain.

[0006] The present invention provides an erbium-doped optical fiber with orbital angular momentum, including: an inner cladding, a ring core, and an outer cladding wrapped in sequence from the inside to the outside. The inner cladding is provided with a central air hole and a plurality of inner layer air holes, and the plurality of inner layer air holes are arranged in a circular and uniform manner around the central air hole; erbium ions are uniformly doped in the ring core; the outer cladding is provided with a plurality of outer layer air holes, and the plurality of outer layer air holes are arranged in a circular and uniform manner around the ring core.

[0007] According to the erbium-doped optical fiber with orbital angular momentum provided by the present invention, the plurality of outer layer air holes are arranged in a circular pattern to form multiple outer air hole layers.

[0008] According to the erbium-doped optical fiber with orbital angular momentum provided by the present invention, the inner layer air holes are all tangent to the inner ring of the ring core, and the outer layer air holes close to the ring core are tangent to the outer ring of the ring core.

[0009] According to the erbium-doped optical fiber with orbital angular momentum provided by the present invention, the erbium-doped optical fiber with orbital angular momentum has a plurality of selectable structural parameters, including: the inner ring radius of the ring core, the outer ring radius of the ring core, the radius of the central air hole, the diameter and number of the inner layer air holes, the distance from the center of the inner layer air hole to the center of the central air hole, the diameter and number of the outer layer air holes, and the distance from the center of the outer layer air hole to the center of the central air hole.

[0010] According to the orbital angular momentum erbium-doped optical fiber provided by the present invention, the ratio of the inner ring radius to the outer ring radius of the annular core is in the range of 0.55-0.65.

[0011] According to the orbital angular momentum erbium-doped optical fiber provided by the present invention, the inner cladding and the outer cladding are made of silica, and the annular core is made of silica uniformly doped with erbium ions.

[0012] According to the orbital angular momentum erbium-doped optical fiber provided by the present invention, the orbital angular momentum erbium-doped optical fiber is pumped by an annular core pump and a fundamental mode pump source.

[0013] According to the orbital angular momentum erbium-doped optical fiber provided by the present invention, the shape and size of each of the inner-layer air holes are the same, and the cross-section of the inner-layer air holes is circular;

[0014] And / or, the outer layer air holes have the same shape and size, and the cross-section of the outer layer air holes is circular.

[0015] According to the orbital angular momentum erbium-doped optical fiber provided by the present invention, the amplified wavelength range of the orbital angular momentum erbium-doped optical fiber covers the C band.

[0016] The present invention also provides an optical fiber amplifier, comprising: an orbital angular momentum erbium-doped optical fiber and a fundamental mode pump source as described in any of the above items, wherein the orbital angular momentum erbium-doped optical fiber is used to amplify an input optical signal, and the fundamental mode pump source is used to perform ring core pumping on the orbital angular momentum erbium-doped optical fiber.

[0017] The orbital angular momentum erbium-doped optical fiber provided by the present invention can well limit the light field distribution in the annular core area by setting the inner layer air hole and the outer layer air hole, realize low differential mode gain amplification, and has the characteristics of high gain, small differential mode gain, low noise index, etc., and can be used as an online optical fiber amplifier in a mode division multiplexing optical fiber transmission system based on the orbital angular momentum mode; and there is no need to change the effective refractive index difference between the annular core and the cladding by doping or the like. The annular core is uniformly doped with a single layer of erbium ions. Compared with other multi-layer doping and regulation of doping distribution, the use of a single material The transmission and amplification of the orbital angular momentum mode can be realized by using only the raw materials, which reduces the difficulty of drawing, has a simple manufacturing process and a better effect, which is beneficial to improving the robustness of the optical fiber amplifier and solving the problems in the prior art that the refractive index separation between different modes of the orbital angular momentum erbium-doped optical fiber is small, the field distribution of different modes and the overlap integral of the erbium ion doping area cannot be matched at the same time, resulting in the orbital angular momentum erbium-doped optical fiber amplifier supporting a small number of amplification modes, low mode order, complex process and structure, low gain, high noise index, and cannot meet the needs of orbital angular momentum mode multiplexing optical fiber communication. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 is a schematic diagram of the cross-sectional structure of an orbital angular momentum erbium-doped optical fiber provided by an embodiment of the present invention;

[0020] Figure 2 is a graph showing the variation of the effective refractive index difference of adjacent eigenvector modes in an orbital angular momentum erbium-doped optical fiber with wavelength provided by an embodiment of the present invention;

[0021] Figure 3 1 is a schematic diagram of the normalized field intensity distribution of each eigenvector mode in the orbital angular momentum erbium-doped optical fiber provided by an embodiment of the present invention;

[0022] Figure 4 is a graph showing the variation of gains of different modes with wavelength in an orbital angular momentum erbium-doped optical fiber provided by an embodiment of the present invention;

[0023] Figure 5 is a graph showing the variation of the noise index of different modes in the orbital angular momentum erbium-doped optical fiber with the wavelength provided by an embodiment of the present invention;

[0024] Figure 6 The graph is a graph showing the variation of the maximum differential mode gain between different modes in the orbital angular momentum erbium-doped optical fiber with the wavelength provided by an embodiment of the present invention.

[0025] Reference numerals:

[0026] 1: inner cladding; 11: central air hole; 12: inner air hole;

[0027] 2: Ring core;

[0028] 3: outer layer; 31: outer air hole. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are 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] Orbital Angular Momentum (OAM), there are few erbium-doped gain fibers that support OAM mode, and there is little research in OAM mode multiplexing communication. To ensure the stable transmission of OAM mode, the refractive index line shape of the optical fiber should satisfy the distribution of the OAM annular light field intensity. The OAM mode is formed by the superposition of the odd and even modes of the same eigenvector mode with a phase difference of π / 2, and the eigenvector mode is obtained by solving the vector wave equation.

[0031] The superposition formula of different orbital angular momentum modes is as follows:

[0032]

[0033]

[0034] Among them, OAM represents orbital angular momentum mode; HE and EH, as well as TM and TE represent eigenvector modes; l represents topological charge; m represents radial order; i represents phase difference of π / 2; even represents even mode; odd represents odd mode.

[0035] The eigenvector modes in the same mode group will have strong coupling, which will affect the transmission stability of the orbital angular momentum mode. Therefore, it is necessary to ensure that the effective refractive index difference of the eigenvector modes in the mode group is greater than 10 -4 , which can be achieved by increasing the refractive index difference between the core and cladding or reducing the thickness of the core ring.

[0036] In order to achieve gain balance and low noise index of fiber amplifiers, low differential mode gain is currently achieved mainly by changing the pump structure and pump mode and double ring doping. However, these methods will increase the difficulty of actual manufacturing, especially double ring doping and non-uniform doping, it is difficult to avoid the diffusion of erbium ions during the doping process, making it difficult to achieve the designed erbium ion distribution line shape, resulting in difficulty in reducing differential mode gain.

[0037] Combine the following Figure 1 The orbital angular momentum erbium doped optical fiber of the present invention is described.

[0038] like Figure 1 As shown, the orbital angular momentum erbium-doped optical fiber provided by an embodiment of the present invention includes an inner cladding 1, an annular core 2 and an outer cladding 3 which are wrapped in sequence from the inside to the outside, the inner cladding 1 is provided with a central air hole 11 and a plurality of inner air holes 12, and the plurality of inner air holes 12 are uniformly arranged in a ring shape around the central air hole 11; erbium ions are uniformly doped in the annular core 2; the outer cladding 3 is provided with a plurality of outer air holes 31, and the plurality of outer air holes 31 are uniformly arranged in a ring shape around the annular core 2.

[0039] In this embodiment, a plurality of inner air holes 12 form an inner air hole layer distributed in an annular manner on the inner ring side of the annular core 2, and the effective refractive index of the inner air hole layer is different from the refractive index of the annular core 2, thereby realizing an effective refractive index difference between the annular core 2 and the inner cladding 1; a plurality of outer air holes 31 form an outer air hole layer distributed in an annular manner on the outer ring side of the annular core 2, and the effective refractive index of the outer air hole layer is different from the refractive index of the annular core 2, thereby realizing an effective refractive index difference between the annular core 2 and the outer cladding 3; annular air hole layers are respectively arranged on the inner and outer sides of the annular core 2 to form an annular inner and outer double cladding structure, thereby limiting the distribution shape of the orbital angular momentum mode light field intensity and ensuring the lateral direction of the orbital angular momentum erbium-doped optical fiber. The cross-sectional structure conforms to the characteristics of the orbital angular momentum mode annular light field intensity distribution, and the light field distribution of different orbital angular momentum modes can be limited to the same annular fiber core 2 area; and the single layer of erbium ions uniformly doped in the annular fiber core 2, that is, the light field distribution of different orbital angular momentum modes matches and overlaps with the erbium ion doped area of ​​the orbital angular momentum erbium-doped fiber, and the stimulated radiation performance of erbium ions can be used to achieve simultaneous amplification of multiple orbital angular momentum modes; the light fields of different orbital angular momentum modes are distributed in the same area, that is, the normalized field intensity distribution of different orbital angular momentum modes is less different, and the overlap between the areas of the erbium ion doped area is good, which can reduce the differential mode gain, achieve low differential mode gain amplification, and ensure the signal-to-noise ratio of the receiving end. In addition, the effective refractive index of the inner air hole layer is greater than the refractive index of air and less than the refractive index of silica, which can weaken the refractive index mutation between the annular fiber core 2 and the central air hole 11, and effectively reduce the loss of the orbital angular momentum erbium-doped fiber.

[0040] The orbital angular momentum erbium-doped optical fiber of the present invention can well limit the light field distribution in the annular core 2 region by setting the inner air hole 12 and the outer air hole 31, realize low differential mode gain amplification, and has the characteristics of high gain, small differential mode gain, low noise index, etc., and can be used as an online optical fiber amplifier in a mode division multiplexing optical fiber transmission system based on the orbital angular momentum mode; and there is no need to change the effective refractive index difference between the annular core 2 and the inner cladding 1 and the outer cladding 3 by doping or the like. The annular core 2 is uniformly doped with a single layer of erbium ions, and is compatible with other multi-layer doping and regulated doping. Compared with the conventional distribution, orbital angular momentum mode transmission can be achieved by using a single material, the difficulty of drawing is reduced, the manufacturing process is simpler, the effect is better, and it is beneficial to improve the robustness of the optical fiber amplifier, and solve the problems in the prior art that the refractive index separation between different modes of orbital angular momentum erbium-doped optical fiber is small, and the overlap integral of the field distribution of different modes and the erbium ion doping area cannot be matched at the same time, resulting in the orbital angular momentum erbium-doped optical fiber amplifier supporting a small number of amplification modes, low mode order, complex process and structure, low gain, high noise index, and cannot meet the needs of orbital angular momentum mode multiplexing optical fiber communication.

[0041] Specifically, the cross-sectional shape of the annular core 2 is adapted to the shape of the annular light field distribution of the supported orbital angular momentum mode.

[0042] Specifically, Figure 1 As shown, multiple outer air holes 31 are arranged in an annular manner to form multiple outer air hole layers. By providing multiple outer air hole layers, the effective refractive index difference between the annular core 2 and the outer cladding 3 can be increased, and a large refractive index difference between the annular core 2 and the outer cladding 3 can be achieved, which is conducive to better confining the light field distribution of each orbital angular momentum mode within the annular core 2 and improving the separation of the effective refractive index between adjacent orbital angular momentum modes, that is, ensuring that the effective refractive index difference between adjacent orbital angular momentum modes is greater than 10 -4 , thereby ensuring a stable mode and a small differential mode gain.

[0043] In a specific embodiment, the outer cladding 3 is provided with two circles of outer air hole layers, which can well confine the light field in the annular core 2, and the erbium ions are evenly distributed in the annular core 2 to match the annular refractive index line shape, thereby ensuring that the refractive index line shape of the orbital angular momentum erbium-doped optical fiber meets the distribution of the orbital angular momentum annular light field intensity, while meeting the size requirements of the orbital angular momentum erbium-doped optical fiber, and the manufacturing difficulty and cost are relatively low.

[0044] Specifically, the inner air holes 12 are all tangent to the inner ring of the annular core 2, and the outer air holes 31 close to the annular core 2 are tangent to the outer ring of the annular core 2. The inner air holes 12 are tangent to the inner ring of the annular core 2, that is, the inner air hole layer is tangent to the inner ring of the annular core 2; the outer air holes 31 are tangent to the outer ring of the annular core 2, that is, the outer air hole layer is tangent to the outer ring of the annular core 2; the inner air hole layer cooperates with the outer air hole layer to accurately confine the light field within the annular core 2, further reduce the overlap difference between the light field distribution of different orbital angular momentum modes and the erbium ion doping region, and further reduce the differential mode gain.

[0045] In one embodiment, Figure 1 As shown, the shapes and sizes of the inner air holes 12 are the same; the cross-section of the inner air holes 12 is circular, and the central axis of the inner air holes 12 is parallel to the central axis of the central air hole 11 .

[0046] In an embodiment not shown, the cross-section of the inner air hole 12 may also be elliptical, fan-shaped, waist-shaped, etc.

[0047] In one embodiment, Figure 1 As shown, the outer air holes 31 have the same shape and size; the cross section of the outer air holes 31 is circular, and the central axis of the outer air holes 31 is parallel to the central axis of the central air hole 11 .

[0048] In an embodiment not shown, the cross section of the outer air holes 31 may also be elliptical, fan-shaped, or waist-shaped, etc.; the shapes and sizes of the outer air holes 31 located in different circles of the outer air hole layers may be the same or different. For example, the cross section of the outer air holes 31 of the first circle of the outer air hole layer close to the annular fiber core 2 is elliptical, and the cross section of the outer air holes 31 of the second circle of the outer air hole layer located outside the first circle of the outer air hole layer is waist-shaped, and the size of the waist-shaped holes is larger than the size of the elliptical holes.

[0049] Specifically, the inner cladding 1 and the outer cladding 3 are made of silica, and the annular core 2 is made of silica uniformly doped with erbium ions. That is to say, the inner cladding 1, the annular core 2 and the outer cladding 3 are all made of silica, and a single layer of erbium ions is uniformly doped in the annular core 2, and air holes are opened in the inner cladding 1 and the outer cladding 3; the use of a single material of silica as a substrate reduces the difficulty of drawing the orbital angular momentum erbium-doped optical fiber, and the manufacturing process is simple.

[0050] In one specific embodiment, the inner cladding 1, the annular fiber core 2 and the outer cladding 3 are an integrally formed part. In another specific embodiment, the inner cladding 1, the annular fiber core 2 and the outer cladding 3 are nested and connected.

[0051] Specifically, the orbital angular momentum erbium-doped optical fiber is pumped by annular core 2 pumping and fundamental mode pumping source. By adopting the annular core 2 pumping method, the pumping method is simple, reducing the complexity of practical application; at the same time, the use of easily available fundamental mode pumping source pumping can save the cost of practical application.

[0052] Specifically, the amplified wavelength range of the orbital angular momentum erbium-doped optical fiber covers the C band, that is, the amplified wavelength range includes 1530 nanometers to 1565 nanometers.

[0053] In one embodiment, Figure 1 As shown, the orbital angular momentum erbium-doped optical fiber has a plurality of selectable structural parameters, including: the inner ring radius r of the annular core 2 3 , the outer ring radius r of the ring core 2 4 , the radius r of the central air hole 11 1 , the diameter D of the inner air hole 12 1 The number of holes and the distance r from the center of the inner air hole 12 to the center of the central air hole 11 2 , the diameter D of the outer air hole 31 2 and number, the distance r from the center of the outer air hole 31 to the center of the central air hole 11 5 and r 6By flexibly selecting the structural parameters of the orbital angular momentum erbium-doped fiber, it is possible to obtain a suitable ring core 2 area size, the size and arrangement of the inner air holes 12, and the size and arrangement of the outer air holes 31, so as to achieve a large refractive index difference between the ring core 2 and the outer cladding 3, ensuring that the effective refractive index separation of adjacent orbital angular momentum modes is greater than 10 -4 , that is, to ensure that the effective refractive index difference is greater than 10 -4 , ensuring the stable transmission of orbital angular momentum mode in orbital angular momentum erbium-doped optical fiber; it can also suppress the generation of high-order radial modes and ensure that the light field distribution of each orbital angular momentum mode is confined to the same area, supporting the transmission of as many orbital angular momentum modes as possible while suppressing high-order radial modes.

[0054] Specifically, based on the normalized frequency V of the orbital angular momentum erbium-doped fiber and the inner ring radius r of the annular core 2 3 With the outer ring radius r 4 The ratio ρ of orbital angular momentum determines the modes and quantity supported by the erbium-doped fiber. Among them, the normalized frequency V is an important parameter for the orbital angular momentum mode cutoff. The calculation formula of the normalized frequency V is:

[0055]

[0056] Where λ is the wavelength, n core is the refractive index of the ring core 2, n cladding is the effective refractive index of the outer cladding 3.

[0057] The calculation formula of the ratio ρ is:

[0058]

[0059] When the orbital angular momentum erbium-doped fiber is used, the wavelength λ range is determined; from the calculation formula of the normalized frequency V, it can be known that the outer ring radius r of the ring core 2 4 The larger the effective refractive index difference between the annular core 2 and the outer cladding 3, the larger the normalized frequency V value, and the more modes the orbital angular momentum erbium-doped fiber can transmit. At the same time, in order to ensure the stable transmission of the orbital angular momentum mode, it is also necessary to consider that the refractive index difference of the eigenvector mode in the same mode group is greater than 10 -4, which is related to the effective refractive index difference between the ring core 2 and the outer cladding 3 and the thickness of the ring core 2, so it can be achieved by adjusting the number and number of layers of the outer air holes 31 and the thickness of the ring core 2; when the normalized frequency V value is constant, the ratio of the inner ring radius to the outer ring radius of the ring core 2 is ρ. The smaller the ratio ρ, the more modes the orbital angular momentum erbium-doped fiber can support, but this will reduce the refractive index difference of the eigenvector mode in the same mode group, so a compromise is needed; finally, the manufacturing difficulty must be considered, and the inner cladding 1 and the outer cladding 3 must be filled with air holes as much as possible. Combined with the existing optical fiber situation, the size range of the orbital angular momentum erbium-doped fiber structure is determined to ensure that the expected effect is achieved.

[0060] Specifically, the ratio of the inner ring radius to the outer ring radius of the annular core 2 is in the range of 0.55-0.65. By limiting the ratio of the inner ring radius to the outer ring radius of the annular core 2, the area size and thickness of the annular core 2 can be adjusted to be within a suitable range, supporting as many orbital angular momentum mode transmissions as possible while suppressing the generation of high-order radial modes, avoiding the occurrence of high-order radial modes due to the thickness of the annular core 2 being too thick, and avoiding the transmission mode quality being particularly poor due to the thickness of the annular core 2 being too thin, thereby ensuring the stable transmission of the orbital angular momentum mode.

[0061] Specifically, by selecting the structural parameters of the orbital angular momentum erbium-doped optical fiber, the size of the annular core 2 region and the arrangement of the inner air holes 12 and the outer air holes 31 can be adjusted, thereby determining the light field distribution of each orbital angular momentum mode.

[0062] Optionally, the ratio of the radius of the central air hole 11 to the inner ring radius of the annular fiber core 2 is in the range of 0.55-0.65.

[0063] Specifically, by adjusting the size and arrangement of the inner air holes 12, the proportion of air in the inner air hole layer can be adjusted, that is, the duty cycle of the inner air hole layer can be adjusted; by adjusting the duty cycle of the inner air hole layer, the effective refractive index of the inner air hole layer can be adjusted, and then the effective refractive index of the inner cladding 1 can be adjusted. Similarly, by adjusting the size and arrangement of the outer air holes 31, the proportion of air in the outer air hole layer can be adjusted, that is, the duty cycle of the outer air hole layer can be adjusted; by adjusting the duty cycle of the outer air hole layer, the effective refractive index of the outer air hole layer can be adjusted, and then the effective refractive index of the outer cladding 3 can be adjusted.

[0064] In this embodiment, by adjusting the effective refractive index of the inner cladding 1 and the outer cladding 3, the effective refractive index difference between the annular core 2 and the inner cladding 1 and the outer cladding 3 can be increased, and the effective refractive index separation between each orbital angular momentum mode can be improved to ensure that the effective refractive index difference between adjacent orbital angular momentum modes is greater than 10 -4 , thereby ensuring the stable transmission of the mode.

[0065] In a specific embodiment, Figure 1 As shown, the structural parameters of the orbital angular momentum erbium-doped fiber are selected as follows: First, based on the core radius of an ordinary single-mode fiber being 4-5 microns, in order to match the optical field radius of the orbital angular momentum erbium-doped fiber of the present invention with that of an ordinary single-mode fiber, the inner ring radius r of the annular core 2 is set. 3 is 2.5 microns, and the outer ring radius r of the annular core 2 4 is 4.5 microns, and the annular core 2 is the erbium-doped region; then, based on 360 / 12 being an integer, an inner air hole layer is selected to arrange 12 inner air holes 12 in an annular manner, which is convenient for arrangement. Of course, 360 / 10 or 360 / 15 can also be selected; after the number of inner air holes 12 is determined, in order to ensure that the inner cladding 1 is filled with the inner air holes 12 as much as possible, the center spacing Λ of adjacent inner air holes 12 is selected 1 The diameter D of the inner air hole 12 is 1.035 microns. 1 Next, the radius r of the central air hole 11 can be determined 1 is 1.4 microns, and the distance r from the center of the inner air hole 12 to the center of the central air hole 11 2 Afterwards, two circles of outer air hole layers are selected. Similarly, based on 360 / 15 being an integer, the first circle of outer air hole layers has 15 outer air holes 31 arranged in a ring. In order to ensure that the first circle of outer air hole layers is filled with outer air holes 31 as much as possible, the center spacing Λ of adjacent outer air holes 31 is selected. 2 The diameter D of the outer air hole 31 is 2.287 microns. 2 The distance r from the center of the outer air hole 31 to the center of the central air hole 11 in the first outer air hole layer is 2 microns. 5 Based on 360 / 20 as an integer, the second outer air hole layer is arranged in a ring shape with 20 outer air holes 31. In order to ensure that the second outer air hole layer is filled with the outer air holes 31 as much as possible, the spacing between adjacent outer air holes 31 is Λ 3 The distance r from the center of the outer air hole 31 to the center of the central air hole 11 in the second outer air hole layer is 2.409 microns. 6 It is 7.7 microns.

[0066] In this embodiment, the simplified superposition formula of different orbital angular momentum modes is as follows:

[0067]

[0068]

[0069] Through simulation calculation, it is found that the eigenvector modes supported by the structure of the orbital angular momentum erbium-doped optical fiber in this embodiment are: HE11, TE01, TM01, HE21, HE31, EH11, HE41, EH21, HE51, EH31, HE61, EH41, and the effective refractive index difference of the eigenvector modes (i.e., HE and EH) in the same mode group is greater than 10 -4 , to ensure that the orbital angular momentum mode in the orbital angular momentum erbium-doped fiber can be stably transmitted. Due to the large difference in the propagation constants of the TE and TM modes, the orbital angular momentum mode formed by the two will split during the transmission process and cannot exist stably in the optical fiber, so the orbital angular momentum mode formed by the superposition of TE and TM is not counted. Then, according to the simplified superposition formula of the above different orbital angular momentum modes, 18 orbital angular momentum modes can be obtained by superposition, which are:

[0070] By adopting the optimized structural parameters of the orbital angular momentum erbium-doped fiber, the orbital angular momentum erbium-doped fiber of this embodiment is designed for the C band, can support the amplification of 18 orbital angular momentum modes, and ensure that the effective refractive index difference of the modes in the same mode group is greater than 10 -4 , to avoid the eigenvector mode from coupling into a linear polarization mode during transmission.

[0071] like Figure 2 As shown in FIG. 1 , it is a curve diagram of the difference in effective refractive index of adjacent eigenvector modes in the orbital angular momentum erbium-doped fiber of this embodiment as a function of wavelength; the horizontal axis in the figure is wavelength (nanometers), and the vertical axis is the effective refractive index difference. As can be seen from the figure, the eigenvector modes in the orbital angular momentum erbium-doped fiber of this embodiment are divided into 5 mode groups, corresponding to topological charges l=1-5, and all completely cover the C band. As the wavelength increases, the effective refractive index difference also increases, and the effective refractive index difference between the HE and EH modes in all mode groups is greater than 10 -4 , the orbital angular momentum mode can be stably transmitted.

[0072] like Figure 3 As shown in the figure, it is a schematic diagram of the normalized field intensity distribution of each intrinsic vector mode in the orbital angular momentum erbium-doped optical fiber of the present embodiment. The area between the two dotted lines in the figure is the erbium ion doped area, the abscissa represents the radial position (micrometer) of the orbital angular momentum erbium-doped optical fiber, and the ordinate represents the normalized light field intensity. Due to the different light field intensity distributions of different modes, the overlapping integrals of the erbium ion area and the light field intensity distribution are different, which is the source of the differential mode gain. As can be seen from the figure, the light field intensity distributions of different intrinsic vector modes in the orbital angular momentum erbium-doped optical fiber of the present embodiment are very close, and the spacing at the maximum values ​​of the light field intensity of different modes is about 0.2 micrometers, which overlaps very well with the erbium ion doped area, so the differential mode gain of the orbital angular momentum erbium-doped optical fiber of the present embodiment is small.

[0073] like Figure 4 As shown in the figure, it is a curve diagram of the gain of different modes in the orbital angular momentum erbium-doped optical fiber of this embodiment changing with wavelength; the horizontal axis in the figure is wavelength (nanometers) and the vertical axis is gain (decibels). It can be seen from the figure that when the pump power is 20dBm (100 milliwatts) and the signal power is -15dBm (0.0316 milliwatts), the minimum mode gain is about 22.5 decibels and the maximum can reach 24 decibels. The mode gain is large and can meet the needs of practical applications.

[0074] like Figure 5 The figure shows the curve of the noise figure (NF) of different modes in the orbital angular momentum erbium-doped fiber of this embodiment changing with wavelength; the horizontal axis in the figure is wavelength (nanometers) and the vertical axis is noise figure (decibel). It can be seen from the figure that the noise figure is lower than 4.3 decibels, with the lowest being 3.7 decibels, which is close to the theoretical limit.

[0075] like Figure 6 The graph shown is a graph of the maximum differential modal gain (DMG) between different modes in the orbital angular momentum erbium-doped optical fiber of this embodiment as a function of wavelength; the horizontal axis in the graph is wavelength (nanometers) and the vertical axis is differential modal gain (dB). As can be seen from the graph, the differential mode gain fluctuates with wavelength, but its value is relatively small, less than 0.10 dB.

[0076] An embodiment of the present invention also provides an optical fiber amplifier, comprising the orbital angular momentum erbium-doped optical fiber provided by any of the above embodiments, the orbital angular momentum erbium-doped optical fiber amplifies the input optical signal to improve the gain of the input optical signal; the optical fiber amplifier also includes a fundamental mode pump source, the fundamental mode pump source is used to pump the ring core 2 of the orbital angular momentum erbium-doped optical fiber.

[0077] The optical fiber amplifier of the present invention is used to compensate for the transmission line loss of ultra-long-distance orbital angular momentum mode multiplexing optical fiber communication; by adopting the orbital angular momentum erbium-doped optical fiber of the present invention and pumping the ring core 2 with a fundamental mode pump source, low differential mode gain amplification is achieved, the signal-to-noise ratio of the receiving end is guaranteed, and the gain balance and low noise index of the optical fiber amplifier are achieved, and the optical fiber amplifier has the characteristics of high gain, low noise, and low differential mode gain.

[0078] In a specific embodiment, the fundamental mode pump source includes a fundamental mode optical fiber, which includes a fundamental mode inner cladding, a fundamental mode annular core and a fundamental mode outer cladding which are wrapped sequentially from the inside to the outside, the fundamental mode inner cladding is provided with a fundamental mode central air hole and a plurality of fundamental mode inner layer air holes, and the plurality of fundamental mode inner layer air holes are evenly arranged in a ring shape around the fundamental mode central air hole; the fundamental mode outer cladding is provided with a plurality of fundamental mode outer layer air holes, and the plurality of fundamental mode outer layer air holes are evenly arranged in a ring shape around the fundamental mode annular core. In other words, the structure of the fundamental mode optical fiber is similar to that of the orbital angular momentum erbium-doped optical fiber of the present invention and has the same size, except that erbium ions are not doped in the fundamental mode annular core. The annular core 2 is pumped by the fundamental mode of the fundamental mode optical fiber, i.e., the HE11 mode.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. 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 invention.

Claims

1. An orbital angular momentum erbium-doped optical fiber, It is characterized in that include: An inner cladding, an annular core and an outer cladding are sequentially wrapped from the inside to the outside, the inner cladding is provided with a central air hole and a plurality of inner air holes, and the plurality of inner air holes are uniformly arranged in an annular shape around the central air hole; erbium ions are uniformly doped in the annular core; the outer cladding is provided with a plurality of outer air holes, and the plurality of outer air holes are uniformly arranged in an annular shape around the annular core; The inner air holes are all tangent to the inner ring of the annular core, and the outer air holes close to the annular core are tangent to the outer ring of the annular core; The inner air hole layer cooperates with the outer air hole layer to accurately confine the light field within the annular fiber core; The orbital angular momentum erbium-doped optical fiber is pumped by an annular core pump and a fundamental mode pump source.

2. The orbital angular momentum erbium-doped optical fiber according to claim 1, It is characterized in that The plurality of outer air holes are arranged in a ring shape to form a plurality of outer air hole layers.

3. The orbital angular momentum erbium-doped optical fiber according to claim 1, It is characterized in that The orbital angular momentum erbium-doped optical fiber has multiple selectable structural parameters, including: the inner ring radius of the annular core, the outer ring radius of the annular core, the radius of the central air hole, the diameter and number of the inner air holes, the distance from the center of the inner air hole to the center of the central air hole, the diameter and number of the outer air holes, and the distance from the center of the outer air hole to the center of the central air hole.

4. The orbital angular momentum erbium-doped optical fiber according to claim 1, It is characterized in that The ratio of the inner ring radius to the outer ring radius of the annular core is in the range of 0.55-0.

65.

5. The orbital angular momentum erbium-doped optical fiber according to claim 1, It is characterized in that The inner cladding and the outer cladding are made of silicon dioxide, and the annular fiber core is made of silicon dioxide uniformly doped with erbium ions.

6. The orbital angular momentum erbium-doped optical fiber according to claim 1, It is characterized in that The inner air holes have the same shape and size, and the cross-section of the inner air holes is circular; And / or, the outer layer air holes have the same shape and size, and the cross-section of the outer layer air holes is circular.

7. The orbital angular momentum erbium-doped optical fiber according to claim 1, It is characterized in that The amplified wavelength range of the orbital angular momentum erbium-doped optical fiber covers the C band.

8. An optical fiber amplifier, It is characterized in that include: The orbital angular momentum erbium-doped fiber and the fundamental mode pump source according to any one of claims 1 to 7, wherein the orbital angular momentum erbium-doped fiber is used to amplify an input optical signal, and the fundamental mode pump source is used to perform ring core pumping on the orbital angular momentum erbium-doped fiber.

Images