CN106324749B

CN106324749B - Few-mode optical fiber for amplifier

Info

Publication number: CN106324749B
Application number: CN201610914554.1A
Authority: CN
Inventors: 陈苏; 杨玉诚; 曹蓓蓓; 朱继红; 汪洪海
Original assignee: Yangtze Optical Fibre and Cable Co Ltd
Current assignee: Yangtze Optical Fibre and Cable Co Ltd
Priority date: 2016-10-20
Filing date: 2016-10-20
Publication date: 2020-01-14
Anticipated expiration: 2036-10-20
Also published as: CN106324749A

Abstract

The invention relates to a few-mode optical fiber for an amplifier, which comprises a core layer and a cladding, and is characterized in that the core layer comprises an inner core layer and an outer core layer, wherein the relative refractive index difference delta 1 of the inner core layer is-0.1% -0.2%, the radius R1 is 2-5 mu m, the outer core layer tightly surrounds the inner core layer, the relative refractive index difference delta 2 of the outer core layer is 0.5% -1.5%, the radius R2 is 6-12 mu m, and the cladding tightly surrounds the outer core layer and is a pure quartz glass layer. The invention has simple structure and easy manufacture; by adopting a specific design of the concave annular core layer, the effective refractive index difference between the modes can be effectively improved, and the mode coupling is reduced, so that the coupling problem between the modes can be better solved, and the coupling between the transmission modes required by the system is reduced by inhibiting certain high-order modes which are easy to couple, thereby achieving the purpose of simultaneously transmitting a plurality of high-order modes with low crosstalk.

Description

Few-mode optical fiber for amplifier

Technical Field

The invention relates to an optical fiber used in an optical communication amplifier, in particular to a few-mode optical fiber used for an erbium-doped amplifier.

Background

The single-mode optical fiber is widely applied to optical communication networks due to the advantages of high transmission rate, large information carrying capacity, long transmission distance and the like. In recent years, with the increasing demand for capacity of communication and big data services, the network bandwidth is rapidly expanding, and the capacity of an optical transmission network is gradually approaching to the shannon limit of a single optical fiber: 100 Tb/s. The space division multiplexing and the module division multiplexing technology can break the traditional Shannon limit, realize the transmission with higher bandwidth, and is the best method for solving the problem of transmission capacity. The optical fibers supporting the multiplexing technology are multi-core optical fibers and few-mode optical fibers.

Experiments show that signals can be transmitted in more than one spatial propagation mode by using few-mode optical fibers in combination with the MIMO technology. However, as the number of modes in the fiber increases, in the conventional fiber, the MIMO process will rapidly become complex, which will result in a significant increase in the cost and difficulty of multiplexing for higher order modes.

On the other hand, in order to use mode division multiplexing in a long-distance optical transmission system, an erbium-doped fiber amplifier (EDFA) needs to be connected in the middle, and in the current system design for mode division multiplexing, the EDFA is designed for single-mode fiber transmission, when the single-mode EDFA is used for the mode division multiplexing system, the modes from the input few-mode transmission fiber need to be separated, then each mode is converted into a single mode and is amplified separately by the single-mode EDFA, and after amplification, the output single-mode signal from the amplifier is converted into the mode in the few-mode transmission fiber. This process is very complicated and costly.

US8848285 proposes an erbium doped few-mode fiber and fiber amplifier for simultaneous amplification of multiple modes, which can solve the problem of simultaneous amplification of modes, but the MIMO process is still complicated when the modes increase.

In fact, when the coupling between modes is reduced to a certain degree, MIMO can be avoided, thereby simplifying the system and increasing the reliability and scalability of transmission.

Disclosure of Invention

The present invention provides a few-mode optical fiber for an amplifier, which can effectively improve the effective refractive index difference between modes, achieve the purpose of reducing mode coupling and simultaneously transmitting a plurality of high-order modes with low crosstalk, and simplify the system.

For convenience in describing the summary of the invention, the following terms are defined:

performing: the glass rod or the combined body of the designed optical fiber can be directly drawn according to the design requirement of the optical fiber by the radial refractive index distribution consisting of the core layer and the cladding;

core rod: a solid glass preform comprising a core layer and a partial cladding layer;

radius: the distance between the outer boundary of the layer and the center point;

refractive index profile: the relationship between the refractive index of the glass of an optical fiber or an optical fiber preform (including a core rod) and the radius thereof;

relative refractive index difference:

n_iand n₀The refractive index of each part of each corresponding optical fiber and the refractive index of pure silica glass are respectively;

contribution amount of aluminum (Al): the relative refractive index difference (delta Al) of the aluminum (Al) doped quartz glass relative to the pure quartz glass is used as the quantity of the aluminum (Al) doped quartz glass;

contribution amount of phosphorus (P): the relative refractive index difference (delta P) of the phosphorus (P) -doped quartz glass relative to the pure quartz glass is used for expressing the amount of the phosphorus (P);

MCVD process: preparing quartz glass with required thickness by using improved vapor deposition and sintering processes;

solution method: rare earth elements are doped into the preform rod by a solution soaking method to form a soot-shaped glass layer, and a glassy structure is formed by sintering.

Gas phase method: rare earth elements are introduced into the inner wall of the liner tube by a gas phase doping method, and gas phase rare earth ions are directly deposited at high temperature to form a doping mode.

Bare fiber: which refers to a glass fiber without a coating layer in the optical fiber.

The technical scheme adopted by the invention for solving the problems is as follows: the composite material comprises a core layer and a cladding layer, and is characterized in that the core layer comprises an inner core layer and an outer core layer, the relative refractive index difference delta 1 of the inner core layer is-0.1% -0.2%, and the radius R1 is 2-5 mu m; the outer core layer tightly surrounds the inner core layer, the relative refractive index difference delta 2 is 0.5% -1.5%, the radius R2 is 6-12 μm, and the cladding layer tightly surrounds the outer core layer and is a pure quartz glass layer.

According to the scheme, the relative refractive index difference of the outer core layer decreases progressively from the center of the core layer to the inner side and the outer side, and the decreasing progressively is in a step type or a gradual change type.

According to the scheme, the inner core layer is mainly doped with aluminum (Al) and/or phosphorus (P) and is doped with a small amount of erbium (Er), the total contribution amount of the aluminum (Al) and/or the phosphorus (P) of the inner core layer is 0-0.2%, and the doping amount of the erbium (Er) is less than or equal to 200 ppm.

According to the scheme, the outer core layer is mainly doped with aluminum (Al) and phosphorus (P) and doped with erbium (Er), the total contribution amount of the aluminum (Al) and the phosphorus (P) of the outer core layer is 0.5-1.5%, and the doping amount of the Er (Er) is 800-4000 ppm.

According to the scheme, the inner core layer is doped with germanium (Ge) and/or fluorine (F) so as to adjust the refractive index.

According to the scheme, the optical fiber supports 2 or more than 2 stable transmission modes at the 1550nm wavelength.

In the above scheme, the optical fiber supports 6 stable transmission modes at 1550nm wavelength, LP01, LP11, LP21 and LP31, LP41 and LP51 respectively.

According to the scheme, the LP01 mode of the optical fiber can keep stable operation when the bending radius is 5 mm.

The invention has the beneficial effects that: 1. by adopting a specific design of the concave annular core layer, a part of high-order modes propagated in the common few-mode optical fiber can be inhibited, the inhibited high-order modes have similar effective refractive indexes with propagation modes existing in the optical fiber, crosstalk is easily formed between the inhibited high-order modes and the transmission modes, and normal transmission of signals is influenced. 2. Through a specific doping design, aluminum and phosphorus are simultaneously doped in the core layer, so that the solubility of rare earth erbium ions in the optical fiber is improved, and the doping of high-concentration rare earth ions is realized, so that higher gain performance can be realized in use, meanwhile, the photon darkening resistance of the optical fiber is improved, and the service life of the optical fiber is prolonged; 3. through the structural design and the doping design, the gain of the optical fiber between all modes of the C wave band is basically similar, and the optical fiber has good bending performance.

Drawings

FIG. 1 is a schematic view of a radial cross-section structure of an optical fiber according to an embodiment of the present invention.

FIG. 2 is a schematic representation of a cross-sectional view of the refractive index of an optical fiber according to one embodiment of the present invention.

FIG. 3 is a cross-sectional view of the refractive index of an optical fiber according to another embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to examples.

One embodiment of the present invention is shown in fig. 1 and 2 and includes inner and outer core layers and a cladding layer. The inner core layer 1 is composed of quartz glass doped with phosphorus (P) or quartz glass doped with fluorine and other dopant (erbium), has a relative refractive index difference Δ 1, and is prepared by an MCVD process. The outer core layer 2 closely surrounds the inner core layer, is composed of aluminum (Al) and phosphorus (P) co-doped quartz glass prepared by MCVD process, and may be doped with erbium. The cladding 3 closely surrounds the outer core layer and is composed of pure silica glass and is prepared by an MCVD or OVD process, and the radius R3 of the fiber cladding is 62.5 μm.

The coating layer of the optical fiber of the embodiment adopts a double-layer coating process, the drawing speed is 400-1000m/min, and the diameter of the optical fiber is 125 +/-2 mu m.

According to the technical scheme of the few-mode optical fiber, the parameters of the optical fiber are designed in the specified range, the core rod is manufactured according to the design requirements of the optical fiber through the known core rod manufacturing process such as an MCVD (micro-mechanical-deposition) process and the like, and the whole prefabricated rod is manufactured through the outer covering process such as a sleeve process, an OVD (optical vapor deposition) process and the like.

The refractive index profile of the drawn fiber was tested using an NR-9200 apparatus (EXFO), and the refractive index profile of the fiber and the main parameters of the doped material are shown in Table 1.

The optical fiber manufactured according to the technical scheme of the invention supports three to six stable transmission modes at the wavelength of 1550nm, namely LP01, LP11, LP21 and LP31, and LP41 and LP 51. As shown in table 2.

The refractive index profile of the optical fiber according to another embodiment of the present invention is shown in fig. 3, and is mainly characterized in that the relative refractive index difference of the outer core layer decreases gradually from the center of the core layer to the inner side and the outer side, and the decrease is gradual, the relative refractive index difference at the innermost side of the outer core layer is equal to or greater than the relative refractive index difference of the inner core layer, and the relative refractive index difference at the outermost side of the outer core layer is equal to or greater than the relative refractive index difference of the cladding layer. Table 1: example Structure and Material composition of few-mode optical fiber

Table 2: example major Performance parameters of few-mode fibers

Claims

1. A few-mode optical fiber for amplifier includes a core layer and a cladding layer, and is characterized in that the core layer includes an inner core layer and an outer core layer, the relative refractive index difference Delta 1 of the inner core layer is-0.1% -0.2%, and the radius R1 is 2 μm-5 μm; the outer core layer tightly surrounds the inner core layer, the relative refractive index difference delta 2 is 0.5% -1.5%, the radius R2 is 6-12 μm, and the cladding layer tightly surrounds the outer core layer and is a pure quartz glass layer; the optical fiber supports 2 or more than 2 stable transmission modes at a wavelength of 1550 nm; the inner core layer is doped with aluminum and/or phosphorus and is doped with a small amount of erbium, the contribution amount of the aluminum and/or phosphorus of the inner core layer is 0-0.05%, and the doping amount of the erbium is less than or equal to 200 ppm; the outer core layer is mainly doped with aluminum and phosphorus and doped with erbium, the total contribution of the aluminum and the phosphorus of the outer core layer is 0.5% -1.5%, and the doping amount of the erbium is 800ppm-4000 ppm.

2. The few-mode optical fiber for an amplifier according to claim 1, wherein the relative refractive index difference of said outer core decreases from the center of said core to the inner and outer sides thereof in a stepwise or gradual manner.

3. The few mode optical fiber for an amplifier according to claim 1 or 2, wherein said inner core layer is doped with germanium and/or fluorine.

4. The few-mode optical fiber for an amplifier according to claim 1 or 2, characterized in that said fiber supports 6 stable transmission modes at a wavelength of 1550nm, respectively LP01, LP11, LP21 and LP31, LP41 and LP 51.

5. The few-mode optical fiber for an amplifier according to claim 4, wherein the LP01 mode of said fiber is capable of maintaining stable operation at a bend radius of 5 mm.