Few-mode rare earth doped optical fiber, optical fiber laser and optical fiber amplifier

Abstract

The invention belongs to the technical field of optical fiber amplification, and discloses a few-mode rare earth doped optical fiber, an optical fiber laser and an optical fiber amplifier. The few-mode rare earth doped optical fiber comprises a fiber core, an inner cladding and an outer cladding which are sequentially arranged from inside to outside, the fiber core is of a multi-layer annular structure, counting is conducted on the fiber core from inside to outside, and the molar content ratio of rare earth ions doped in every two adjacent layers meets the following relation: # imgabs0 #, nX is the molar content ratio of rare earth ions doped in the outer even number of core layers in the two adjacent layers, NY is the molar content ratio of rare earth ions doped in the inner odd number of core layers in the two adjacent layers, # imgabs 1 # is a first constant, and # imgabs 2 # and # imgabs 3 # are provided. The optical gain medium adopts the few-mode rare earth doped optical fiber. The pumping efficiency of the few-mode rare-earth-doped optical fiber can be improved, and the optical fiber laser and the optical fiber amplifier have the advantage of being high in pumping efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber amplification, and more specifically, relates to a few-mode rare-earth-doped optical fiber, an optical fiber laser, and an optical fiber amplifier. Background Art

[0002] In recent years, space-division multiplexing technology has been proposed as an effective way to expand communication capacity in the future. One of the space-division multiplexing technologies is based on few-mode fiber (FMF). The few-mode fiber has only one core and can support the transmission of multiple modes. Compared with the traditional single-core single-mode fiber, the transmission capacity of the few-mode fiber can be increased by several times or dozens of times. However, the current development of space-division multiplexing active devices is not particularly mature. In long-distance communication systems, optical fiber amplifiers are needed to compensate for signal losses during transmission. Few-mode rare-earth-doped fiber amplifiers have received attention because they can support the amplification of multi-mode signals, especially in applications that require high power and broadbandwidth, such as lidar, laser cooling, and gravitational wave detection.

[0003] The prior art, such as Chinese invention patent (CN202322662247.7), discloses a four-mode erbium-doped optical fiber and an amplifier for L-band amplification. The four-mode erbium-doped optical fiber uses layered doping to achieve mode gain equalization in the L-band. The US patent (US20140063592AL) proposes a few-mode erbium-doped fiber amplifier for transmission in a mode-division multiplexing system. Since a spatial optical structure is adopted, there are problems such as mode impurity and large insertion loss in the converted modes when using high-order pump modes. Therefore, a method of using annular doping of erbium ions is proposed to achieve gain equalization of two signal modes. Chinese patent (CN112180499A) proposes a three-layer core multi-layer erbium-ion-doped four-mode optical fiber. The refractive index distribution assisted by the three-layer core grooves enables the optical fiber to have a high mode refractive index difference, which can weaken inter-mode crosstalk and reduce bending loss during application. Moreover, erbium ions are doped in layers, and mode gain equalization is achieved by optimizing the doping region and concentration ratio. However, the above three studies have not well solved the problems of low pump power and uneven gain of few-mode optical fibers.

[0004] Chinese invention patent (CN117434643B) discloses a radiation-resistant multi-clad doped optical fiber. The outer cladding consists of N claddings and can achieve high-concentration and high-uniformity doping of fluorine, aluminum, phosphorus, and cerium ions. This article aims to increase the ion doping concentration by increasing the outer cladding and fails to solve the mode equalization problem.

[0005] Chinese Invention Patent (CN117008241A) discloses a gain-equalized erbium-doped fiber based on hierarchical doping and its application. The inner core layer is doped with erbium ions, and the outer core layer is co-doped with erbium and ytterbium ions. The inner core layer and the outer core layer have different components and different concentrations of doped ion layers. By adjusting the types and ratios of doped ions in the inner core layer and the outer core layer, the interaction effect between ytterbium ions and erbium ions is caused, and the gain difference is reduced. This article aims to co-dope erbium and ytterbium in the outer layer of the fiber core to solve the mode equalization problem, but it will cause the narrowing of the mode amplification band.

[0006] In the design of fiber amplifiers, the cladding pumping technology is an efficient pumping method. It allows light of multiple modes to be transmitted in the inner cladding of the fiber and effectively couples the pump light to the core of the rare-earth-doped fiber. Theoretically, by continuously optimizing the design of few-mode rare-earth-doped fibers, the performance of fiber amplifiers can be improved, thereby meeting the requirements of future communication systems. However, at present, the few-mode rare-earth-doped fibers still have the problem of insufficiently high pumping efficiency. How to optimize the design to improve the relevant performance of few-mode rare-earth-doped fibers and obtain few-mode rare-earth-doped fibers with better amplification effects is an issue of concern and needs to be solved in this field. Summary of the Invention

[0007] The present invention provides a few-mode rare-earth-doped fiber, a fiber laser, and a fiber amplifier to solve the problem of low pumping efficiency of few-mode rare-earth-doped fibers in the prior art.

[0008] In a first aspect, the present invention provides a few-mode rare-earth-doped fiber, including a core, an inner cladding, and an outer cladding arranged in sequence from the inside to the outside; the core is a multi-layer annular structure, and when counting from the inside to the outside of the core, the molar content ratios of rare-earth ions doped in adjacent two layers satisfy the following relationship: Where N X is the molar content ratio of rare-earth ions doped in the even core layer closer to the outside in adjacent two layers, and N Y is the molar content ratio of rare-earth ions doped in the odd core layer closer to the inside in adjacent two layers. is a first constant.

[0009] Preferably, the following is also satisfied among multiple odd core layers in the core: |N i -N i+2 | / N i <20%, where N i is the molar content ratio of rare-earth ions doped in the i-th annular structure from the inside to the outside of the core, and N i+2 is the molar content ratio of rare-earth ions doped in the (i + 2)-th annular structure from the inside to the outside of the core.

[0010] Preferably, the core contains rare earth ion oxides, aluminum oxide, germanium oxide and phosphorus oxide; the rare earth ions are erbium, ytterbium, thulium or holmium ions; the molar content ratio of the rare earth ions ranges from 0.1% to 1%, the molar content ratio of germanium oxide ranges from 3% to 5%, the molar content ratio of aluminum oxide ranges from 5% to 9%, and the molar content ratio of phosphorus oxide ranges from 2% to 3%.

[0011] Preferably, the molar content ratio N of aluminum element in the core Al and the molar content ratio N of germanium element Ge satisfy the following relationship: wherein, is the second constant,

[0012] Preferably, the outer contour of the cross-section of the inner cladding is a regular polygon.

[0013] Preferably, the few-mode rare earth-doped optical fiber further includes: a coating layer;

[0014] The diameter of the core is 15 μm to 30 μm, and the relative refractive index difference Δ of the inner core layer in two adjacent layers Y is in the range of 0.6% to 0.8%, and the relative refractive index difference Δ of the outer core layer in two adjacent layers X is in the range of 0.7% to 0.9%;

[0015] The diameter of the circumscribed circle of the inner cladding is 110 ± 5 μm, and the relative refractive index difference Δ of the inner cladding 2 is in the range of -0.5% to -0.1%;

[0016] The diameter of the outer cladding is 125 ± 10 μm, and the relative refractive index difference Δ of the outer cladding 3 is 0;

[0017] The diameter of the coating layer is 250 ± 15 μm, and the relative refractive index difference of the coating layer is in the range of -5% to -3%.

[0018] Preferably, the relative refractive index difference Δ of the inner core layer in two adjacent layers Y and the relative refractive index difference Δ of the outer core layer in two adjacent layers X satisfy the following relationship: Δ X = Δ Y + γ / u, where u is the total number of layers of the core, γ is the third constant, and 0.2 ≤ γ ≤ 1.

[0019] Preferably, the core is a four-layer annular structure. The core is sequentially divided into four layers A, B, C, and D from the inside to the outside, and the molar content ratios of the rare earth ions doped in each layer are respectively denoted as N A 、NB , N C , N D , and satisfy: |N A -N C | / N A < 20%.

[0020] In a second aspect, the present invention provides an optical fiber laser, the optical fiber laser is a cladding-pumped structure, and the optical gain medium of the optical fiber laser adopts the above-mentioned few-mode rare-earth-doped optical fiber.

[0021] In a third aspect, the present invention provides an optical fiber amplifier, the optical fiber amplifier is a cladding-pumped structure, and the optical gain medium of the optical fiber amplifier adopts the above-mentioned few-mode rare-earth-doped optical fiber.

[0022] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0023] (1) The few-mode rare-earth-doped optical fiber provided by the present invention includes a core, an inner cladding, and an outer cladding arranged in sequence from the inside to the outside. The core is a multi-layer annular structure. When counting from the inside to the outside of the core, the molar content ratios of the rare-earth ions doped in adjacent two layers satisfy the following relationship: Wherein, N X is the molar content ratio of the rare-earth ions doped in the even core layer closer to the outside in adjacent two layers, and N Y is the molar content ratio of the rare-earth ions doped in the odd core layer closer to the inside in adjacent two layers, is a first constant, This concentration relationship between adjacent layers can increase the interaction length between the pump light and the doped ions, thereby improving the coupling efficiency and energy conversion efficiency of the pump light. That is, the present invention can improve the pump efficiency and amplification gain of the few-mode rare-earth-doped optical fiber, and at the same time can also adjust the amplification effects between different modes and reduce the inter-mode gain difference.

[0024] (2) The core in the present invention contains rare-earth ion oxides, aluminum oxide, germanium oxide, and phosphorus oxide, and the molar content ratios of each component are set. The molar content ratio N Al of aluminum element in the core and the molar content ratio N Ge of germanium element satisfy the following relationship: Wherein, is a second constant, This design has ultra-low noise.

[0025] (3) The relative refractive index difference Δ Y of the core layer closer to the inside in adjacent two layers and the relative refractive index difference Δ X of the core layer closer to the outside in adjacent two layers satisfy the following relationship: Δ X=Δ Y +γ / u, where u is the total number of core layers, γ is the third constant, and 0.2 ≤ γ ≤ 1. This design can further adjust the optical field distribution of each mode to achieve high gain and low mode gain difference.

[0026] (4) In the present invention, the outer contour of the cross-section of the inner cladding is a regular polygon, which can further improve the pumping efficiency.

[0027] (5) The few-mode rare-earth-doped fiber provided by the present invention can be used as the optical gain medium of a fiber laser or a fiber amplifier, and the corresponding device has the advantages of high pumping efficiency and low inter-mode gain difference. Description of the Drawings

[0028] Figure 1 is a schematic cross-sectional structure diagram of a few-mode rare-earth-doped fiber provided by an embodiment of the present invention;

[0029] Figure 2 is a schematic diagram of the layering of the core in a few-mode rare-earth-doped fiber provided by an embodiment of the present invention;

[0030] Figure 3 is a refractive index profile diagram of a few-mode rare-earth-doped fiber provided by an embodiment of the present invention. Detailed Embodiments

[0031] To better understand the above technical solutions, the above technical solutions will be described in detail below in conjunction with the accompanying drawings of the specification and specific embodiments.

[0032] In a first aspect, the present invention provides a few-mode rare-earth-doped fiber. Refer to Figure 1 , which includes a core 1, an inner cladding 2, and an outer cladding 3 arranged in sequence from the inside to the outside; the core 1 is a multi-layer annular structure. When counting from the inside to the outside of the core 1, the molar content ratios of the rare-earth ions doped in adjacent two layers satisfy the following relationship: where N X is the molar content ratio of the rare-earth ions doped in the even core layer closer to the outside in adjacent two layers, and N Y is the molar content ratio of the rare-earth ions doped in the odd core layer closer to the inside in adjacent two layers, is the first constant,

[0033] In the present invention, there is a correlation between the molar content ratios of the rare-earth ions doped in adjacent two layers. Through this spaced doping method, the interaction length between the pump light and the doped ions can be increased, thereby improving the coupling efficiency and energy conversion efficiency of the pump light, and reducing the inter-mode gain difference.

[0034] In a preferred solution, where when, the inter-mode gain difference is less than 3 dB; When the mode - to - mode gain difference is less than 2 dB.

[0035] In addition, among multiple odd - numbered core layers in the core 1, the following is also satisfied: |N i - N i+2 | / N i <20%, where N i is the molar content ratio of rare - earth ions doped in the i - th annular structure from the inside to the outside of the core, and N i+2 is the molar content ratio of rare - earth ions doped in the (i + 2) - th annular structure from the inside to the outside of the core. In a preferred embodiment, |N i - N i+2 | / N i <10%.

[0036] The core 1 contains rare - earth ion oxides, alumina, germanium oxide, and phosphorus oxide; the rare - earth ions are erbium, ytterbium, thulium, or holmium ions; the molar content ratio of rare - earth ions is in the range of 0.1% to 1%, the molar content ratio of germanium oxide is in the range of 3% to 5%, the molar content ratio of alumina is in the range of 5% to 9%, and the molar content ratio of phosphorus oxide is in the range of 2% to 3%. The refractive index of the core is regulated by multiple elements such as rare - earth ions, aluminum, germanium, and phosphorus, enabling it to support the amplification of multiple modes (e.g., ten - mode) simultaneously and further improving the gain effect in the C - band to more than 25 dB.

[0037] In order to further improve the amplification effect of the few - mode rare - earth - doped fiber in the C - band, the present invention further regulates the molar content ratios of aluminum and germanium elements. The molar content ratio N Al of aluminum element in the core and the molar content ratio N Ge of germanium element satisfy the following relationship: When the amplification effect is the best, and the amplification band can be extended to the L - band, where is the second constant. In a preferred embodiment, where the noise is less than 6 dB; when

[0038] The outer contour of the cross - section of the inner cladding 2 is a regular polygon, which can further improve the pumping efficiency.

[0039] In addition, the few - mode rare - earth - doped fiber may further include a coating layer. The diameter of the core 1 is 15 μm to 30 μm, and the relative refractive index difference Δ Y of the inner - layer core in adjacent two layers is in the range of 0.6% to 0.8%, and the relative refractive index difference Δ XIn the range of 0.7% to 0.9%. The diameter of the circumscribed circle of the inner cladding 2 is 110 ± 5 μm, and the relative refractive index difference Δ 2 is in the range of -0.5% to -0.1%. The inner cladding 2 is preferably made of fluorine-doped quartz material. The low-refractive-index quartz cladding can increase the bending resistance of the optical fiber. The diameter of the outer cladding 3 is 125 ± 10 μm, and the relative refractive index difference Δ 3 of the outer cladding 3 is 0. The outer cladding 3 is preferably made of pure quartz material. The diameter of the coating layer is 250 ± 15 μm, and the relative refractive index difference of the coating layer is in the range of -5% to -3%.

[0040] In order to further adjust the optical field distribution of each mode and achieve high gain and low mode gain difference, the refractive indices between adjacent layers need to follow certain rules. Specifically, the relative refractive index difference Δ Y of the inner core layer in adjacent two layers and the relative refractive index difference Δ X of the outer core layer in adjacent two layers satisfy the following relationship: Δ X = Δ Y + γ / u, where u is the total number of core layers, and γ is the third constant, 0.2 ≤ γ ≤ 1.

[0041] It can be found that as the number of layers increases, the difference in the refractive indices of each annular layer becomes smaller and smaller. The reason for following this principle is that when the refractive index difference of the annular structure is smaller, the coupling between modes in the core will decrease, which helps to reduce the crosstalk between modes, thereby improving the gain and reducing the noise. This is very important for ensuring the independent transmission of modes between cores.

[0042] The few-mode rare-earth-doped optical fiber can support multiple modes according to regulation. For example, the few-mode rare-earth-doped optical fiber can support LP 01 , LP 11 , LP 21 , LP 02 , LP 31 , LP 12 , LP 41 , LP 22 , LP 03 , LP 51 and other multiple modes.

[0043] Next, taking the core containing a four-layer annular structure as an example, several embodiments are given in combination with data to further illustrate the present invention.

[0044] See Figure 2 and Figure 3 , the core is a four-layer annular structure. The core is divided into four layers A, B, C, and D from the inside to the outside. The molar content ratios of the rare-earth ions doped in each layer are respectively denoted as N A , N B , NC , N D , and satisfy: In addition, |N A -N C | / N A < 20%, that is, the core N A and N C have a difference ratio of less than 20%.

[0045] Example 1:

[0046] The few-mode rare-earth-doped optical fiber provided in Example 1 has a core diameter of 15 μm, and the relative refractive index differences Δ AC of the A layer and the C layer are 0.8%, and the relative refractive index differences Δ BD of the B layer and the D layer are 0.9%; the diameter of the circumscribed circle of the inner cladding is 110 μm, and the relative refractive index difference Δ 2 is -0.2%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0047]

[0048]

[0049] The core in Example 1 contains rare-earth ion oxides, alumina, germanium oxide, and phosphorus oxide; among them, the molar content ratio of rare-earth ions in the A layer is 0.49%, and the molar content ratio of rare-earth ions in the C layer is 0.45%. In the A layer and the C layer, the molar content of germanium oxide is 3%, the molar content of alumina is 5.4%, and the molar content of phosphorus oxide is 2%; the molar content ratio of rare-earth ions in the B layer is 0.12%, and the molar content ratio of rare-earth ions in the D layer is 0.1%. In the B layer and the D layer, the molar content of germanium oxide is 5%, the molar content of alumina is 7%, and the molar content of phosphorus oxide is 3%.

[0050] The coupling efficiency of the pump light of the few-mode rare-earth-doped optical fiber provided in Example 1 is 88%, the energy conversion efficiency is 4%, under the C-band test, the gain is 25 dB, the inter-mode gain difference is 2.5 dB, and the noise is 6 dB.

[0051] Example 2:

[0052] The few-mode rare-earth-doped optical fiber provided in Example 2 has a core diameter of 18 μm, and the relative refractive index differences Δ AC of the A layer and the C layer are 0.6%, and the relative refractive index differences Δ BD of the B layer and the D layer are 0.7%; the diameter of the circumscribed circle of the inner cladding is 115 μm, and the relative refractive index difference Δ 2is -0.2%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0053]

[0054]

[0055] The core in Example 2 contains rare earth ion oxides, alumina, germanium oxide, and phosphorus oxide; among them, the molar content of rare earth ions in layers A and C is 0.75%, in layers A and C, the molar content of germanium oxide is 4%, the molar content of alumina is 8.2%, and the molar content of phosphorus oxide is 2%; the molar content of rare earth ions in layers B and D is 0.3%, in layers B and D, the molar content of germanium oxide is 4%, the molar content of alumina is 8.2%, and the molar content of phosphorus oxide is 3%.

[0056] The coupling efficiency of the pump light of the few-mode rare earth-doped fiber provided in Example 2 is 90%, the energy conversion efficiency is 4%, under the C-band test, the gain is 24 dB, the inter-mode gain difference is 3 dB, and the noise is 5 dB.

[0057] Example 3:

[0058] For the few-mode rare earth-doped fiber provided in Example 3, the diameter of the core is 20 μm, and the relative refractive index difference Δ AC between layers A and C is 0.8%, and the relative refractive index difference Δ BD between layers B and D is 0.9%; the diameter of the circumscribed circle of the inner cladding is 110 μm, and the relative refractive index difference Δ 2 is -0.2%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0059]

[0060]

[0061] The core in Example 3 contains rare earth ion oxides, alumina, germanium oxide, and phosphorus oxide; among them, the molar content of rare earth ions in layer A is 0.32%, the molar content of rare earth ions in layer C is 0.3%, in layers A and C, the molar content of germanium oxide is 3%, the molar content of alumina is 7.4%, and the molar content of phosphorus oxide is 3%; the molar content of rare earth ions in layer B is 0.11%, the molar content of rare earth ions in layer D is 0.1%, in layers B and D, the molar content of germanium oxide is 5%, the molar content of alumina is 9%, and the molar content of phosphorus oxide is 2%.

[0062] For the few-mode rare-earth-doped fiber provided in Example 3, the coupling efficiency of the pump light is 92%, the energy conversion efficiency is 4%, the gain is 24 dB, the inter-mode gain difference is 2.2 dB, and the noise is 5.5 dB under the C-band test.

[0063] Example 4:

[0064] For the few-mode rare-earth-doped fiber provided in Example 4, the diameter of the core is 25 μm, the relative refractive index difference Δ between the A layer and the C layer AC is 0.7%, and the relative refractive index difference Δ between the B layer and the D layer BD is 0.8%; the diameter of the circumscribed circle of the inner cladding is 112 μm, and the relative refractive index difference Δ 2 is -0.2%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0065]

[0066]

[0067] The core in Example 4 contains rare-earth ion oxides, alumina, germanium oxide, and phosphorus oxide; among them, the molar content of rare-earth ions in the A layer and the C layer is 0.9%, in the A layer and the C layer, the molar content of germanium oxide is 3%, the molar content of alumina is 5.4%, and the molar content of phosphorus oxide is 3%; the molar content of rare-earth ions in the B layer and the D layer is 0.4%, in the B layer and the D layer, the molar content of germanium oxide is 5%, the molar content of alumina is 7%, and the molar content of phosphorus oxide is 3%.

[0068] For the few-mode rare-earth-doped fiber provided in Example 4, the coupling efficiency of the pump light is 95%, the energy conversion efficiency is 5%, the gain is 26 dB, the inter-mode gain difference is 2.1 dB, and the noise is 5.8 dB under the C-band test.

[0069] Example 5:

[0070] For the few-mode rare-earth-doped fiber provided in Example 5, the diameter of the core is 30 μm, the relative refractive index difference Δ between the A layer and the C layer AC is 0.6%, and the relative refractive index difference Δ between the B layer and the D layer BD is 0.9%; the diameter of the circumscribed circle of the inner cladding is 110 μm, and the relative refractive index difference Δ 2 is -0.2%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0071]

[0072]

[0073] The core in Example 5 contains rare earth ion oxide, alumina, germanium oxide, and phosphorus oxide; among them, the molar content of rare earth ions in the A layer and the C layer is 0.85%, in the A layer and the C layer, the molar content of germanium oxide is 3%, the molar content of alumina is 5.4%, and the molar content of phosphorus oxide is 3%; the molar content of rare earth ions in the B layer and the D layer is 0.3%, in the B layer and the D layer, the molar content of germanium oxide is 5%, the molar content of alumina is 7%, and the molar content of phosphorus oxide is 3%.

[0074] For the few-mode rare earth-doped optical fiber provided in Example 5, the coupling efficiency of the pump light is 95%, the energy conversion efficiency is 5%, under the C-band test, the gain is 23 dB, the intermodal gain difference is 2.3 dB, and the noise is 6 dB.

[0075] Example 6:

[0076] For the few-mode rare earth-doped optical fiber provided in Example 6, the diameter of the core is 30 μm, and the relative refractive index difference Δ AC of the A layer and the C layer is 0.6%, and the relative refractive index difference Δ BD of the B layer and the D layer is 0.7%; the diameter of the circumscribed circle of the inner cladding is 110 μm, and the relative refractive index difference Δ 2 is -0.1%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0077]

[0078]

[0079] The core in Example 6 contains rare earth ion oxide, alumina, germanium oxide, and phosphorus oxide; among them, the molar content of rare earth ions in the A layer is 0.95%, the molar content of rare earth ions in the C layer is 0.93%, in the A layer and the C layer, the molar content of germanium oxide is 3%, the molar content of alumina is 5.4%, and the molar content of phosphorus oxide is 3%; the molar content of rare earth ions in the B layer is 0.4%, the molar content of rare earth ions in the D layer is 0.39%, in the B layer and the D layer, the molar content of germanium oxide is 5%, the molar content of alumina is 7%, and the molar content of phosphorus oxide is 3%.

[0080] For the few-mode rare earth-doped optical fiber provided in Example 6, the coupling efficiency of the pump light is 95%, the energy conversion efficiency is 5%, under the C-band test, the gain is 25 dB, the intermodal gain difference is 1.5 dB, and the noise is 6 dB.

[0081] Example 7:

[0082] The few-mode rare-earth-doped optical fiber provided in Example 7 has a core diameter of 30 μm, and the relative refractive index differences Δ AC between the A layer and the C layer are 0.6%, and the relative refractive index differences Δ BD between the B layer and the D layer are 0.8%; the diameter of the circumscribed circle of the inner cladding is 110 μm, and the relative refractive index difference Δ 2 is -0.1%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0083]

[0084]

[0085] The core in Example 7 contains rare-earth ion oxides, alumina, germanium oxide, and phosphorus oxide; among them, the molar content of rare-earth ions in the A layer and the C layer is 0.65%, and in the A layer and the C layer, the molar content of germanium oxide is 3%, the molar content of alumina is 7.4%, and the molar content of phosphorus oxide is 3%; the molar content of rare-earth ions in the B layer and the D layer is 0.2%, and in the B layer and the D layer, the molar content of germanium oxide is 3%, the molar content of alumina is 7.4%, and the molar content of phosphorus oxide is 3%.

[0086] The coupling efficiency of the pump light of the few-mode rare-earth-doped optical fiber provided in Example 7 is 95%, the energy conversion efficiency is 5%, under the C-band test, the gain is 24 dB, the inter-mode gain difference is 2 dB, and the noise is 5 dB.

[0087] Example 8:

[0088] The few-mode rare-earth-doped optical fiber provided in Example 8 has a core diameter of 20 μm, and the relative refractive index differences Δ AC between the A layer and the C layer are 0.7%, and the relative refractive index differences Δ BD between the B layer and the D layer are 0.9%; the diameter of the circumscribed circle of the inner cladding is 110 μm, and the relative refractive index difference Δ 2 is -0.1%; the diameter of the outer cladding is 125 μm, and the relative refractive index difference Δ 3 is 0; the diameter of the coating layer is 250 μm, and the relative refractive index difference is -5%.

[0089]

[0090]

[0091] The core in Example 8 contains rare earth ion oxides, aluminum oxide, germanium oxide, and phosphorus oxide; among them, the molar content of rare earth ions in layers A and C is 0.95%, in layers A and C, the molar content of germanium oxide is 3%, the molar content of aluminum oxide is 5.4%, and the molar content of phosphorus oxide is 3%; the molar content of rare earth ions in layers B and D is 0.4%, in layers B and D, the molar content of germanium oxide is 4%, the molar content of aluminum oxide is 6.2%, and the molar content of phosphorus oxide is 3%.

[0092] The coupling efficiency of the pump light of the few-mode rare earth-doped optical fiber provided in Example 8 is 95%, the energy conversion efficiency is 5%, under the C-band test, the gain is 25 dB, the inter-mode gain difference is 1 dB, and the noise is 5.8 dB.

[0093] In the second and third aspects, the present invention also provides an optical fiber laser and an optical fiber amplifier, which will be described below with Examples 9 and 10 respectively.

[0094] Example 9:

[0095] Example 9 provides an optical fiber laser, the optical fiber laser is of a cladding-pumped structure, and the optical gain medium of the optical fiber laser uses the above-mentioned few-mode rare earth-doped optical fiber (for example, the few-mode rare earth-doped optical fiber described in Examples 1 to 8 can be used).

[0096] Example 10:

[0097] Example 10 provides an optical fiber amplifier, the optical fiber amplifier is of a cladding-pumped structure, and the optical gain medium of the optical fiber amplifier uses the above-mentioned few-mode rare earth-doped optical fiber (for example, the few-mode rare earth-doped optical fiber described in Examples 1 to 8 can be used).

[0098] In summary, through the optimized design of the core refractive index profile, doping region, doping content ratio, and doping components, the few-mode rare earth-doped optical fiber provided by the present invention has a higher pump efficiency compared with the traditional few-mode rare earth-doped optical fiber, and in addition, it also has a lower inter-mode gain difference. The few-mode rare earth-doped optical fiber provided by the present invention is used as the optical gain medium of an optical fiber amplifier and an optical fiber laser, and has the advantages of high pump efficiency and low inter-mode gain difference. On the premise of improving the transmission system capacity, the present invention provides a new idea for the cladding-pumped scheme of a space-division multiplexing amplifier.

[0099] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the examples, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A few-mode rare-earth-doped optical fiber, characterized in that: The fiber core comprises a fiber core, an inner cladding and an outer cladding arranged in sequence from the inside to the outside; the fiber core is a multi-layer ring structure, and the molar content ratio of rare earth ions doped in two adjacent layers of the fiber core from the inside to the outside satisfies the following relationship: Among them, N X is the molar content ratio of rare earth ions doped in the outer even-numbered core layer of the two adjacent layers, N Y is the molar content ratio of rare earth ions doped in the inner odd-numbered core layer of the two adjacent layers, is the first constant, 2. The few-mode rare-earth-doped optical fiber according to claim 1, characterized in that: The plurality of odd-numbered core layers in the core also satisfy: |N i -N i+2 | / N i <20%, where N i is the molar content ratio of rare earth ions doped into the i-th ring structure from the inside to the outside of the core, N i+2 It is the molar content ratio of rare earth ions doped into the i+2th ring structure from the inside to the outside of the fiber core.

3. The few-mode rare-earth-doped optical fiber according to claim 1, characterized in that: The fiber core contains rare earth ion oxide, aluminum oxide, germanium oxide and phosphorus oxide; the rare earth ions are erbium, ytterbium, thulium or holmium ions; the molar content of rare earth ions is in the range of 0.1% to 1%, the molar content of germanium oxide is in the range of 3% to 5%, the molar content of aluminum oxide is in the range of 5% to 9%, and the molar content of phosphorus oxide is in the range of 2% to 3%.

4. The few-mode rare-earth-doped optical fiber according to claim 3, characterized in that: The molar content of aluminum in the fiber core is N Al The molar content of germanium is N Ge The following relations are satisfied: in, is the second constant, 5. The few-mode rare-earth-doped optical fiber according to claim 1, characterized in that: The outer contour of the cross section of the inner cladding is a regular polygon.

6. The minority-mode rare-earth-doped optical fiber according to claim 1, characterized in that: Also includes: coating layer; The diameter of the fiber core is 15 μm to 30 μm, and the relative refractive index difference Δ Y In the range of 0.6% to 0.8%, the relative refractive index difference Δ X In the range of 0.7% to 0.9%; The diameter of the circumscribed circle of the inner cladding is 110±5 μm, and the relative refractive index difference Δ2 of the inner cladding is in the range of -0.5% to -0.1%; The diameter of the outer cladding is 125±10 μm, and the relative refractive index difference Δ3 of the outer cladding is 0; The diameter of the coating layer is 250±15 μm, and the relative refractive index difference of the coating layer is in the range of -5% to -3%.

7. The few-mode rare-earth-doped optical fiber according to claim 1, characterized in that: The relative refractive index difference Δ between the inner core layers of the two adjacent layers Y The relative refractive index difference Δ between the outer core layer and the adjacent two layers X Satisfies the following relationship: Δ X =Δ Y +γ / u, u is the total number of layers of the fiber core, γ is the third constant, 0.2≤γ≤1.

8. The minority-mode rare-earth-doped optical fiber according to claim 2, characterized in that: The fiber core is a four-layer ring structure, and the fiber core is divided into four layers A, B, C, and D from the inside to the outside. The molar content of rare earth ions doped in each layer is recorded as N A 、N B 、N C 、N D , and satisfy: |N A -N C | / N A <20%.

9. A fiber laser, characterized in that: The fiber laser is a cladding pumped structure, and the optical gain medium of the fiber laser is the few-mode rare-earth-doped fiber according to any one of claims 1 to 8.

10. An optical fiber amplifier, characterized in that: The optical fiber amplifier is a cladding pumped structure, and the optical gain medium of the optical fiber amplifier adopts the few-mode rare-earth-doped optical fiber according to any one of claims 1 to 8.

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