A hierarchically doped few-mode gain-flattened optical fiber

Abstract

The application belongs to the field of optical fiber communication, and particularly relates to a layered doped few-mode gain equalization optical fiber. The optical fiber comprises coaxially arranged core and cladding, the refractive index of the core is greater than that of the cladding; the core comprises a first core and a second core, the refractive index of the second core is higher than that of the first core, and the doping concentration ratio of rare earth ions in the first core and the second core is 0.6-0.9:1.0; the cladding comprises a first cladding and a second cladding, the refractive index of the second cladding is greater than that of the first cladding. The few-mode gain equalization optical fiber has a high modal refractive index difference and small inter-mode crosstalk, which is beneficial to realize gain equalization between modes; has important application value for relay amplification of a mode large-capacity communication system, and can be applied to a multi-mode transmission system and a space division multiplexing large-capacity optical fiber communication system.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical fiber communication, and particularly relates to a layered-doped few-mode gain-equalized optical fiber. BACKGROUND

[0002] With the globalization of information, the demand for network data traffic and communication capacity is increasing, which makes the transmission rate of single-mode optical fiber close to the Shannon limit, resulting in the shortage of communication capacity. The space division multiplexing (SDM) technology proposed in 2009 has become an effective method to improve the transmission capacity of optical fiber and break through the capacity limit, and is considered as a key technology to solve the communication demand. The commonly used space division multiplexing technology is mode division multiplexing technology based on few-mode optical fiber. Mode division multiplexing (MDM) is an effective method to solve the capacity limit of single-mode optical fiber. When the optical fiber is used for long-distance information transmission, fiber loss will occur, so fiber amplifiers are needed to compensate for the loss during transmission. In order to realize long-distance and high-efficiency transmission, gain equalization of the fiber amplifier needs to be realized. Therefore, when designing the fiber amplifier, the gain of each mode needs to be improved while the gain difference between modes is reduced, so that the gain of each mode is equalized. In practical application, the fiber amplifier has the advantages of high gain, low loss, small mode crosstalk, etc., and has good application prospect.

[0003] In various types of multimode optical fiber communication systems, gain equalization between different modes can be achieved by adjusting the doping distribution of erbium-doped optical fiber, changing the pump intensity, and changing the refractive index distribution of the optical fiber. Although changing the pump intensity can reduce the differential mode gain to some extent, as the number of multiplexed modes increases, it leads to great difficulty in manufacturing process and complex reconfiguration of pump modes.

[0004] Chinese patent CN112099129A proposes an air hole few-mode gain-equalized optical fiber based on layered doping. Due to the design of the central air hole, the fiber core cannot be effectively utilized, resulting in a low maximum gain of each mode in the optical fiber.

[0005] US patent WO2020181586A1 proposes a low-differential-mode-gain few-mode erbium-doped fiber amplifier. The invention uses a few-mode erbium-doped fiber, which can obtain a smaller differential mode gain by changing the pump mode, but the pump mode needs to be properly configured, thereby increasing the difficulty of system construction.

[0006] Chinese patent CN111123427A proposes a layered-doped step-index few-mode optical fiber for mode gain equalization. The optical fiber mainly relies on the optimization of the distribution of doping ions, which has great optimization difficulty and increases the complexity of the fiber manufacturing process.

[0007] US patent WO2021143579A1 proposes a fiber filter and a fiber amplifier, in which the fiber waveguide structure adopted is basically similar to patent CN111123427A, and can realize gain equalization between different wavelength optical signals, but cannot realize gain equalization of two or more modal optical signals.

[0008] In summary, the prior art cannot well realize the lower gain difference between the modes.

[0009] For ease of understanding, the professional terms involved in the present application are as follows:

[0010] DMG: differential modal gain, differential modal gain represents the amplification effect on all signals, and the formula is: DMG(λ)=max{|G(m,λ)-G(n,λ)|}, where m≠n, G is the gain, m and n are the mode numbers, and λ is the wavelength.

[0011] Fiber refractive index distribution function: Wherein n2 is the second core layer refractive index, n1 is the lowest refractive index of the first core, Δ is the relative refractive index, r is the radial distance from the core center, a is the core radius, and α is the fiber refractive index profile distribution parameter.

[0012] C band: C band (Conventional Band) ranges from 1530nm to 1565nm. SUMMARY

[0013] The present application provides a layered doped few-mode gain equalization optical fiber, which realizes a lower gain difference between modes and solves the problem of large-capacity transmission in the next generation of modal space division multiplexing optical fiber communication system.

[0014] According to the technical scheme of the present application, the layered doped few-mode gain equalization optical fiber comprises a core layer and a cladding layer arranged coaxially, the refractive index of the core layer is greater than the refractive index of the cladding layer;

[0015] The core layer comprises a first core layer (trench) and a second core layer, the first core layer is located at the center of the optical fiber, and the refractive index of the first core layer increases with the increase of the radius, the second core layer is wrapped outside the first core layer, and the refractive index n2 of the second core layer is higher than the refractive index n1 of the first core layer; the first core layer and the second core layer are both rare earth ion doped layers, and the doping concentration ratio of the rare earth ions in the first core layer and the second core layer is 0.6-0.9:1.0;

[0016] The cladding comprises a first cladding (groove) and a second cladding, the first cladding is wrapped outside the second core layer, the second cladding is wrapped outside the first cladding, the refractive index n4 of the second cladding is greater than the refractive index n3 of the first cladding.

[0017] The present application adopts layered doping and special refractive index design, can regulate the mode field distribution and gain amplification characteristics of the optical fiber, not only can regulate the gain balance between modes, but also can reduce the crosstalk between modes.

[0018] The layered doping few-mode gain equalization optical fiber of the present application can regulate four LP modes (LP 01 , LP 11 , LP 21 , LP 02 ) in the C band.

[0019] Further, the refractive index of the first core layer satisfies the optical fiber refractive index distribution function Wherein, the value range of the refractive index profile parameter α is -2.15-5.29.

[0020] Further, the difference between the refractive index n1 of the first core layer and the refractive index n2 of the second core layer is -0.001-0.003.

[0021] Further, the rare earth ion is erbium, and the doping concentration of the rare earth ion in the second core layer is 2.0×10 25 -5.0×10 25 / m 3 .

[0022] Further, the radius b of the second core layer is 1.4-3.0 times the radius a of the first core layer.

[0023] Further, the radius a of the first core layer is 2.0μm-3.5μm, and the radius (outer radius) b of the second core layer is 3.0μm-6.0μm.

[0024] Further, the thickness of the first cladding (i.e. the refractive index groove width thickness, equal to the difference between the radius c of the first cladding and the radius b of the second core layer) is 1.5μm-3.9μm.

[0025] Further, the difference between the refractive index n2 of the second core layer and the refractive index n3 of the first cladding is 0.005-0.011; the difference between the refractive index n3 of the first cladding and the refractive index n4 of the second cladding is -0.001- -0.002.

[0026] Further, the core layer is doped with aluminum element, germanium element and fluorine element, the aluminum element doping content is 0.7-2.2 mol%, the germanium element doping content is 3.6-13.6 mol%, and the fluorine element doping content is 5.8-12.9 mol%.

[0027] Further, in the first core layer and the second core layer, the doping concentration ratio of the aluminum element is 0.4-0.6:1.0, the doping concentration ratio of the germanium element is 0.7-0.9:1.0, and the doping concentration ratio of the fluorine element is 3.1-3.9:1.0.

[0028] The technical scheme of the present application has the following advantages compared with the prior art:

[0029] (1) The present application adopts a design scheme combining layered doping and special refractive index, and realizes effective amplification of each mode by adjusting the mode field distribution of different modes, and reduces the gain difference between modes;

[0030] (2) The present application adopts a layered doping design, which effectively adjusts the doping of erbium ions and facilitates control and process preparation;

[0031] (3) The present application adopts a special refractive index design, which reduces intermodal crosstalk, so that the light field of signal light and pump light in the optical fiber reaches the best overlapping state, and realizes balanced gain of the signal;

[0032] (4) The present application introduces grooves, so that the optical fiber maintains sufficient intermodal spacing, thereby avoiding mode coupling caused by bending, greatly reducing the bending loss in actual application, and improving the application value of the optical fiber. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 It is a schematic diagram of the end face structure of the present application.

[0034] Figure 2 It is a schematic diagram of the refractive index profile of the optical fiber of the present application.

[0035] Figure 3 It is a schematic diagram of the rare earth ion doping concentration profile of the optical fiber of the present application.

[0036] BRIEF DESCRIPTION OF DRAWINGS: 1-1, first core layer; 1-2, second core layer; 2-1, first cladding layer; 2-2, second cladding layer; a is the radius of the first core layer, b is the outer radius of the second core layer, c is the outer radius of the first cladding layer, and d is the outer radius of the second cladding layer. DETAILED DESCRIPTION

[0037] The present application will be further described below in conjunction with the drawings and specific embodiments, so that those skilled in the art can better understand the present application and implement it, but the embodiments are not limiting to the present application.

[0038] Example 1: A multimode optical fiber with layered doping and gain equalization

[0039] like Figure 1 As shown, the optical fiber consists of two coaxial core layers and two cladding layers. The first core layer 1-1 is a trench located at the center of the optical fiber. The second core layer 1-2 is located in a concentric circle around the first core layer 1-1. The first cladding layer 2-1 is a trench located outside the second core layer 1-2. The second cladding layer 2-2 is located outside the first cladding layer 2-1.

[0040] The radius *a* of the first core layer 1-1 is 2.1 μm, the radius *b* of the second core layer 1-2 is 5.0 μm, the ratio *b* / *a* of the radius *b* of the second core layer 1-2 and the radius *a* of the first core layer 1-1 is 2.3, the refractive index difference between the first core layer refractive index *n1* and the second core layer refractive index *n2* is -0.001, and the refractive index of the first core layer satisfies the fiber refractive index distribution function. The refractive index profile parameter α is -2.15; the width of the refractive index groove in the first cladding 2-1 is 1.5 μm; the difference between the refractive index n2 of the second core layer and the refractive index n3 of the first cladding layer is 0.005, and the difference between the refractive index n3 of the first cladding layer and the refractive index n4 of the second cladding layer is -0.001; rare earth ion (erbium) doping is mainly concentrated in the core, including the first core layer 1-1 and the second core layer 1-2. The erbium ion doping concentration ratio in the first core layer 1-1 and the second core layer 1-2 is 0.6:1.0, and the erbium ion concentration in the second core layer 1-2 is 2.0 × 10⁻⁶. 25 / m 3 This optical fiber can achieve LP 01 LP 11 LP 21 LP 02 Modal amplification: the gains of the four modes in the C-band are 21.9dB, 21.4dB, 21.5dB, and 21.6dB, respectively, while the DMG gain is 0.5dB.

[0041] Example 2: A multimode optical fiber with layered doping and gain equalization

[0042] like Figure 1 As shown, the optical fiber consists of two coaxial core layers and two cladding layers. The first core layer 1-1 is a trench located at the center of the optical fiber. The second core layer 1-2 is located in a concentric circle around the first core layer 1-1. The first cladding layer 2-1 is a trench located outside the second core layer 1-2. The second cladding layer 2-2 is located outside the first cladding layer 2-1.

[0043] The radius *a* of the first core layer 1-1 is 3.0 μm, and the radius *b* of the second core layer 1-2 is 7.0 μm. The ratio of the radius *b* of the second core layer 1-2 to the radius *a* of the first core layer 1-1, *b / a*, is 2.33. The refractive index difference between the first core layer refractive index *n1* and the second core layer refractive index *n2* is -0.002. The refractive index of the first core layer satisfies the fiber refractive index distribution function. The refractive index profile parameter α is -5.29; the width of the refractive index groove in the first cladding 2-1 is 3.9 μm; the difference between the refractive index n2 of the second core layer and the refractive index n3 of the first cladding layer is 0.010; the difference between the refractive index n3 of the first cladding layer and the refractive index n4 of the second cladding layer is -0.00125; rare earth ion (erbium) doping is mainly concentrated in the core, including the first core layer 1-1 and the second core layer 1-2. The erbium ion doping concentration ratio in the first core layer 1-1 and the second core layer 1-2 is 0.81:1.0; the erbium ion concentration in the second core layer 1-2 is 3.0 × 10⁻⁶. 25 / m 3 This optical fiber can achieve LP 01 LP 11 LP 21 LP 02 Modal amplification: the gains of the four modes in the C-band are 23.8dB, 23.4dB, 23.5dB, and 23.6dB, respectively, while the DMG gain is 0.4dB.

[0044] Example 3: A Layered Doping Gain-Equalized Multimode Optical Fiber

[0045] like Figure 1 As shown, the optical fiber consists of two coaxial core layers and two cladding layers. The first core layer 1-1 is a trench located at the center of the optical fiber. The second core layer 1-2 is located in a concentric circle around the first core layer 1-1. The first cladding layer 2-1 is a trench located outside the second core layer 1-2. The second cladding layer 2-2 is located outside the first cladding layer 2-1.

[0046] The radius *a* of the first core layer 1-1 is 3.5 μm, and the radius *b* of the second core layer 1-2 is 8.0 μm. The ratio *b* / *a* of the radius of the second core layer 1-2 to the radius *a* of the first core layer 1-1 is 2.3. The refractive index difference between the first core layer refractive index *n1* and the second core layer refractive index *n2* is -0.003. The refractive index of the first core layer satisfies the fiber refractive index distribution function. The refractive index profile parameter alpha is -3.56; the refractive index groove width in the first cladding 2-1 is 2.5 μm, the difference between the second core layer refractive index n2 and the first cladding refractive index n3 is 0.011, and the difference between the first cladding refractive index n3 and the second cladding refractive index n4 is -0.002; the rare earth ion (erbium) doping is mainly concentrated in the core, including the first core layer 1-1 and the second core layer 1-2, and the erbium ion doping concentration ratio in the first core layer 1-1 and the second core layer 1-2 is 0.9:1.0, and the erbium ion concentration in the second core layer 1-2 is 5.0×10 25 / m 3 ; the optical fiber can realize amplification of LP 01 , LP 11 , LP 21 , LP 02 modes, and the gain of the four modes in the C waveband is 22.3 dB, 21.8 dB, 21.9 dB and 22.1 dB respectively, and the DMG is 0.5 dB.

[0047] In summary, the few-mode gain equalization optical fiber has a high modal refractive index difference and a small intermodal crosstalk, which is conducive to realizing gain equalization between modes. The new optical fiber has important application value for the relay amplification of a modal large-capacity communication system, and can be applied to a multi-modal transmission system and a space division multiplexing large-capacity communication system.

[0048] Obviously, the above embodiments are only examples for clearly illustrating, and are not limited to the embodiments. For those skilled in the art, other different forms of changes or variations can be made on the basis of the above description. Here, all the embodiments do not need to be exhausted, and the obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A layered doped few-mode gain equalized optical fiber, characterized in that, It includes a core layer and a cladding layer arranged coaxially, wherein the refractive index of the core layer is greater than the refractive index of the cladding layer; The core layer includes a first core layer (1-1) and a second core layer (1-2). The first core layer (1-1) is located at the center of the optical fiber, and the refractive index of the first core layer (1-1) increases with the increase of the radius. The second core layer (1-2) is wrapped around the first core layer (1-1), and the refractive index n2 of the second core layer (1-2) is higher than the refractive index n1 of the first core layer (1-1). Both the first core layer (1-1) and the second core layer (1-2) are rare earth ion doped layers, and the doping concentration ratio of rare earth ions in the first core layer (1-1) and the second core layer (1-2) is 0.6 to 0.9:1.

0. The cladding includes a first cladding (2-1) and a second cladding (2-2), the first cladding (2-1) wraps around the second core layer (1-2), the second cladding (2-2) wraps around the first cladding (2-1), and the refractive index n4 of the second cladding (2-2) is greater than the refractive index n3 of the first cladding (2-1). The refractive index of the first core layer (1-1) satisfies the fiber refractive index distribution function. The refractive index profile parameter α ranges from -2.15 to -5.29, and a is the radius of the first core layer (1-1).

2. The layered doped few-mode gain equalized fiber as described in claim 1, characterized in that, The difference between the refractive index n1 of the first core layer (1-1) and the refractive index n2 of the second core layer (1-2) is -0.001 to -0.

003.

3. The layered doped few-mode gain equalized fiber as described in claim 1, characterized in that, The rare earth ion doping concentration in the second core layer (1-2) is 2.0 × 10⁻⁶. 25 ~5.0×10 25 / m 3 .

4. The layered doped few-mode gain equalized fiber as described in claim 1, characterized in that, The radius b of the second core layer (1-2) is 1.4 to 3.0 times the radius a of the first core layer (1-1).

5. The layered doped few-mode gain equalized fiber as described in claim 1 or 4, characterized in that, The radius a of the first core layer (1-1) is 2.0 μm to 3.5 μm, and the radius b of the second core layer (1-2) is 3.0 μm to 6.0 μm.

6. The layered doped few-mode gain equalized fiber as described in claim 1, characterized in that, The thickness of the first cladding layer (2-1) is 1.5 μm to 3.9 μm.

7. The layered doped few-mode gain equalized fiber as described in claim 1, characterized in that, The difference between the refractive index n2 of the second core layer (1-2) and the refractive index n3 of the first cladding layer (2-1) is 0.005 to 0.011; the difference between the refractive index n3 of the first cladding layer (2-1) and the refractive index n4 of the second cladding layer (2-2) is -0.001 to -0.

002.

8. The layered doped few-mode gain equalized fiber as described in claim 1, characterized in that, The core layer is also doped with aluminum, germanium, and fluorine, with aluminum doping content of 0.7–2.2 mol%, germanium doping content of 3.6–13.6 mol%, and fluorine doping content of 5.8–12.9 mol%.

9. The layered doped few-mode gain equalized fiber as described in claim 8, characterized in that, In the first core layer (1-1) and the second core layer (1-2), the doping concentration ratio of aluminum is 0.4-0.6:1.0, the doping concentration ratio of germanium is 0.7-0.9:1.0, and the doping concentration ratio of fluorine is 3.1-3.9:1.0.

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