CN111470769A

CN111470769A - Preparation method of rare earth doped few-mode optical fiber

Info

Publication number: CN111470769A
Application number: CN202010330513.4A
Authority: CN
Inventors: 黄宏琪
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-04-24
Filing date: 2020-04-24
Publication date: 2020-07-31

Abstract

The invention provides a preparation method of a rare earth doped few-mode optical fiber. A liner tube or a prefabricated rod is formed when a cladding layer or an inner core layer formed by silicon dioxide, germanium dioxide and fluorine is deposited by a PCVD (plasma chemical vapor deposition) process; preparing a rare earth-doped core layer by MCVD gas-phase doping process, and drawing the rare earth-doped core layer into the required rare earth-doped few-mode optical fiber by a pipe-in-pipe rod process. The advantages of precise control of the refractive index, good roundness, good bow and small loss of the PCVD process are fully utilized; the advantages of high rare earth doping concentration, uniform rare earth distribution, low loss and enough thickness of a doped core layer in an MCVD (metal-oxide-semiconductor deposition) gas-phase doping process are utilized; the drawn rare earth doped few-mode optical fiber has the characteristics of more accurate refractive index profile, uniform rare earth doping, less loss, good roundness and good concentricity by drawing through a pipe sleeve rod process. The finally prepared optical fiber accords with the initial structure design and parameter design, namely the doped few-mode optical fiber has better quality in the using process.

Description

Preparation method of rare earth doped few-mode optical fiber

Technical Field

The invention belongs to the field of preparation of optical fiber rods, and particularly relates to a preparation method of a rare earth doped few-mode optical fiber.

Background

In recent years, with the rapid development of services such as mobile internet, cloud computing, intelligent manufacturing and the like, people have increasingly strong demands for communication bandwidth, and standard single-mode optical fiber communication capacity based on time division multiplexing, wavelength division multiplexing, polarization multiplexing and quadrature amplitude modulation is already very close to the nonlinear shannon limit (100 Tbit/s) of the standard single-mode optical fiber, and the upper limit cannot meet the continuously increasing demands for the bandwidth of the next generation internet. The spatial dimension is the only physical dimension that has not been fully exploited. Mode-multiplexed optical transmission based on few-mode optical fibers is of great interest to scientists.

The most critical technology of the mode division multiplexing optical transmission is to realize the accurate control of the conduction mode in the optical fiber, and in order to meet the requirement of the large-capacity mode division multiplexing optical transmission, the few-mode optical fiber needs to meet the following requirements:

(1) the multiple modes of conduction are low in loss and the difference in loss between the different modes (i.e., differential mode loss DMA) is low. For multiple input-multiple output (MIMO) based signal processing systems, large DMA limits the transmission capacity of the system.

(2) The differential mode group delay DMGD, i.e. the difference in the transmission speed of the different modes in the fibre, is sufficiently small. When the modes carrying the independent mode division multiplexing data channels have different group velocities, different mode pulses simultaneously injected into the few-mode optical fibers cannot simultaneously reach the receiver, so that the MIMO digital signal processing of the mode division multiplexing receiver becomes very complicated.

On the other hand, in order for the mode division multiplexing optical transmission system to be used over a long distance, signal light of different propagation modes needs to be amplified. In current system designs for mode division multiplexing, the modes from the input few-mode transmission fiber are first separated, then each mode is converted to a single mode and amplified separately by a single-mode EDFA, and after amplification, the output single-mode signal from the amplifier is converted to the mode in the few-mode transmission fiber. This process is very complicated and costly. In view of the above, a ring rare earth doped few-mode fiber for amplification is proposed, in which the few-mode fiber amplifier can simultaneously amplify multiple modes of few-mode conduction, and the coupling between the modes is very low, thereby greatly reducing the difficulty of MIMO digital signal processing and well achieving the equalization of gains between the modes.

However, the structural design and parameter design of the rare earth doped few-mode optical fiber make the preparation of the optical fiber extremely difficult to control. The cladding NA is required; the NA, radius, doping concentration, doping distribution and refractive index profile of the annular rare earth doped core; the NA, the radius and the like of the inner layer core are controlled in an extremely precise range, so that the DMA, the DMGD and the intermode coupling of the few-mode optical fiber are as small as possible, and the intermode gain is as uniform as possible. The MCVD vapor phase doping process for preparing the rare earth doped optical fiber (the process for preparing the rare earth doped optical fiber which is the best at present) has the defects of poor refractive index accurate control, difficult preparation of a complex section structure, easy crystallization of rare earth high-concentration doping, easy roundness difference and poor bow curvature of large-core-diameter preform doping and the like. The few-mode optical fiber requires high doping concentration, large doping core diameter and complex section structure, when the optical fiber preform is prepared by using MCVD (metal chemical vapor deposition) doping process, a cladding layer is deposited firstly, then a rare earth doped annular core layer is deposited, and then the inner core layer is deposited, the requirement on the thickness of the annular doping layer for preparing the required structure and parameters is large, the rare earth doping concentration is high, in general, co-doped Al is adopted to improve the concentration and uniformity of rare earth doping, but the general viscosity is large when Al and rare earth are co-doped, the subsequent liner tube is easy to deform when the inner core layer is deposited due to more deposited layers, the subsequent liner tube is shriveled and bent due to subsequent fusion, and the prepared preform can not meet the design requirement.

Disclosure of Invention

In view of the above-mentioned drawbacks or needs for improvement in the prior art, the present invention provides a method for preparing a rare-earth doped few-mode optical fiber. The method is suitable for preparing the annular rare earth-doped optical fiber with a complex refractive index profile.

For clarity of the disclosure, the following explanations of the terms used in this patent are made:

few-mode optical fiber: refers to an optical fiber that supports more than one spatial mode but less than the currently used multimode optical fiber as a transmission medium for a transmission system or communication link, typically supporting about 2-50 modes, and that does not have the same non-linearity problems as single mode optical fibers, and that can be configured without the modal dispersion problems common to multimode optical fibers.

Optical fiber preform: it refers to a glass rod or its combination body composed of core layer and cladding layer, which can be directly drawn into the designed optical fiber.

Rare earth doping: the rare earth ions are doped in the core layer of the optical fiber perform.

Core layer: refers to the portion of the optical waveguide transmission that acts as an optical fiber.

Cladding: refers to a portion that forms a total reflection interface with the core layer and can affect the light transmission quality of the core layer.

Radius: refers to the distance between the outer boundary of the layer and the center point;

refractive index profile: the relationship between the refraction of the glass of the optical fiber or optical fiber preform (including the core rod) and its radius.

Relative refractive index difference:

wherein

The absolute refractive index corresponding to the ith cladding layer or the core layer,

refractive index of pure silica glass.

MCVD process: is an improved chemical vapor deposition preparation process, and belongs to a tube-in-tube method. The method is a process that silicon tetrachloride, germanium tetrachloride, oxygen, helium and the like to be reacted are introduced into a liner tube, then a chemical reaction is carried out under the heating of oxyhydrogen flame, and the root of the reaction is deposited on the inner wall of the liner tube under a thermophoresis mechanism. The process is suitable for preparing the prefabricated rod with a complicated section, but the prefabricated rod with rough refractive index control and unfavorable large core diameter (the diameter of the deposited part is more than 10 mm) is the main process for preparing the rare earth-doped optical fiber prefabricated rod at present.

The PCVD process comprises the following steps: the method is a plasma chemical vapor deposition process, and belongs to a tube-in-tube method. Silicon tetrachloride, germanium tetrachloride, oxygen and the like to be reacted are introduced into the liner tube, and the reactants are ionized under the action of high frequency to generate chemical reaction to generate silicon dioxide, germanium dioxide and the like which are directly deposited on the inner wall of the liner tube in a glass form. The process has high deposition efficiency, fine control on refractive index and extremely low loss, and is very suitable for preparing optical fiber preforms with complex sections and doped different components.

MCVD (metal-chemical vapor deposition) doping process: the method is a process of forming corresponding saturated vapor pressure by performing certain high-temperature treatment on rare earth chelate (or rare earth chloride) and a co-doping agent, and then introducing a liner tube through a pipeline to deposit the liner tube and other doping components on the inner wall of the liner tube together, so that the rare earth-doped optical fiber preform can be directly formed.

In order to achieve the above object, the present invention employs the following steps:

(1) depositing a required cladding in the liner tube, depositing a rare earth doped annular core layer, depositing a pure silicon dioxide layer with the thickness of 0.2mm, and taking down the deposited liner tube for later use;

(2) another liner tube is taken to deposit a core layer and is fused into a solid prefabricated rod, and the radius of the core layer of the prefabricated rod, the doping concentration of each component and the refractive index profile need to meet the structural design and the parameter design of the inner core layer of the few-mode optical fiber;

(3) cleaning and corroding the liner tube in the step (1) to corrode the deposited pure silicon dioxide layer, rounding the preform in the step (2) to polish off the outer pure silicon dioxide part to 0.2mm, and polishing;

(4) and (3) drawing the liner tube treated in the step (3) and the preform rod in the step (2) in a combined mode to obtain the few-mode optical fiber with the required size.

According to the step (1), the cladding is mainly used for adjusting the optical waveguide performance of the few-mode optical fiber, the doping of the cladding is realized through the codoping of silicon dioxide, germanium dioxide and fluorine according to the cladding structure and parameter design of the few-mode optical fiber, and the thickness of the cladding is calculated through calculating the thickness of the corresponding cladding of the few-mode optical fiber. It should be understood that the cladding layer herein may be where the refractive index profile is complex (i.e., the cladding profile may be any shape desired). The cladding can be prepared by a PCVD process or a MCVD process, preferably by the PCVD process (the advantages of high control precision, low loss, small non-roundness of the liner and small bow of the liner in the refractive index profile of the PCVD are exerted).

According to the step (1), the rare earth doped annular core layer is a liner tube of the cladding prepared by introducing rare earth chelate steam, aluminum trichloride steam, silicon tetrachloride steam and oxygen into a PCVD or MCVD process, and then the liner tube is heated by oxyhydrogen flame to carry out chemical reaction to directly form a doped core layer containing rare earth ions, wherein the doping concentration of rare earth and a co-doping agent thereof and the thickness of the core layer are determined by the parameters of the few-mode optical fiber. The rare earth doping process is completed by MCVD gas phase doping process (the advantages of high rare earth doping concentration, uniform doping and small loss when the process is used for preparing the rare earth doped prefabricated rod are exerted).

And (2) depositing a pure silicon dioxide layer with a certain thickness (about 0.2mm thick) on the doped core layer according to the step (1). The aim is to protect the deposited rare earth doping layer; and secondly, when the subsequent liner tube is cleaned, the control is convenient by corroding the deposited silicon dioxide layer but not corroding the rare earth doped layer.

According to the step (2), the inner core layer is realized by depositing silicon dioxide, germanium dioxide, fluorine and the like in another liner tube, and the doping concentration and thickness are calculated by the structural design and the parameter design of the inner core layer of the few-mode optical fiber. The core layer can be prepared by PCVD or MCVD process, preferably PCVD is selected

The process preparation, namely, the required prefabricated rod is formed by deposition in PCVD and then fusion shrinkage on HEC, and the process can ensure that the prefabricated rod has small loss, good roundness, good bow and accurate refractive index control.

And (4) according to the step (3), rounding the preform, namely, grinding off the pure silica part of the preform (leaving a layer of about 0.2mm thick for corrosion), and polishing for later use.

And (4) according to the step (3), the corrosion of the liner tube and the preform rod refers to removing impurities on the inner surface and the outer surface of the liner tube and the preform rod through corrosion and drying.

And (4) inserting the prefabricated rod into the liner tube, and drawing the prefabricated rod into the required optical fiber by a tube-in-tube rod process.

The invention has the following beneficial effects:

(1) when a cladding or an inner core layer formed by silicon dioxide, germanium dioxide and fluorine is deposited by utilizing a PCVD (plasma chemical vapor deposition) process, the refractive index of a liner tube or a prefabricated rod is accurately controlled, and the advantages of good roundness, good bow and small loss are achieved;

(2) the MCVD gas phase doping process is utilized to prepare the rare earth doped core layer, and has the advantages of high rare earth doping concentration, uniform rare earth distribution, small loss and enough thickness of the doped core layer;

(3) through the combination of the doping liner tube and the prefabricated rod, the defects of poor roundness and large camber of the prefabricated rod caused by sequential deposition of the cladding, the doping core layer and the inner core layer by the MCVD process when the rare earth doping core layer has large viscosity and a large number of deposition layers can be well avoided. Particularly, when the roundness is poor, the roundness of the core layer and the cladding layer of the preform is also poor; when the bow is large, it causes a problem that the drawn optical fiber may be eccentric.

Drawings

FIG. 1 is a cross-sectional view of a rare earth doped few-mode fiber;

FIG. 2 is a cross-sectional view of the refractive index of a rare-earth doped few-mode fiber;

FIG. 3 is a cross-sectional and side view of a doped liner;

fig. 4 is a sectional view and a side view of the inner core preform.

The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:

the radius of the inner core layer of the 1-rare earth doped few-mode optical fiber is R1, and the relative refractive index difference is

The radius of the annular rare earth doped layer of the 2-rare earth doped few-mode optical fiber is R2, and the relative refractive index difference is

The doped cladding of the 3-rare earth-doped few-mode fiber has a radius of R3 and a relative refractive index difference of

4-pure silica cladding of rare earth-doped few-mode fiber, inner tube with relative refractive index difference of 05-doped liner tube, pure silica layer with radius R56-doped liner tube, rare earth annular doping layer with radius R6 (R6 = R5+0.2 mm) 7-doped liner tube, and relative refractive index difference of R7

The doped cladding of the 8-doped liner has a radius R8 relative refractive index difference of

9-pure silica glass cladding of doped liner with radius R9 relative refractive index difference 010-side view of doped liner 11-core of doped preform, and radius R11 relative refractive index difference of

Pure silica cladding (rounded) from a 12-doped preform with a radius R12 relative refractive index difference of

13-rounding the treated doped preform.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

As shown in fig. 3, this example provides a process for preparing a doped liner cladding, that is, a pure silica layer is deposited by a PCVD process to a certain thickness, so that a pure silica cladding of the deposited liner is formed 9 (i.e., with a radius of R9); then depositing a doped cladding layer 8 (i.e. with radius R8 and relative refractive index difference of R8) by the co-doping of silicon dioxide, germanium dioxide and fluorine

）。

As shown in fig. 3, a process for preparing a rare earth doped layer is provided in this example. Taking off the liner tube deposited with 8 and 9, installing the liner tube on MCVD lathe, and introducing rare earth chelate, aluminum trichloride and tetrachlor into the liner tubeDepositing a certain thickness of rare earth doped layer 7 (i.e. the relative refractive index difference of R7 is as follows) by heating silicon chloride, oxygen, helium and the like in oxyhydrogen flame

) (ii) a A thickness of pure silicon dioxide is then deposited on the rare earth doped layer to form 6 (i.e., radius R6). The doped liner 10 is formed.

As shown in fig. 4, in this example, a preform for preparing an inner core layer is provided. Another liner tube is taken, a core layer required by deposition of silicon tetrachloride, germanium tetrachloride, oxygen, freon and the like is introduced into the liner tube through a PCVD (plasma chemical vapor deposition) process, then the liner tube is fused and contracted into a solid preform on an HEC (high-temperature chemical vapor deposition) with the core diameter of 11 (namely the radius is R11, and the relative refractive index difference is R11

). The preform's pure silica cladding is formed 12 by a rounding process. The preform formed was 13.

The rare earth doped low-modulus optical fiber is prepared by etching 6 of 10 and 12 of 13, inserting 13 into 10 (i.e., the rod-in-tube process described above, which generally requires R6-R11 to be between 0.5mm and 1mm to ensure concentricity of the optical fiber), and drawing.

It should be noted that R11, R7, R8 and R9 in this example are all derived from the calculation and process preparation requirements of R1, R2, R3 and R4 of the few-mode optical fiber to be prepared.

According to the invention, the cladding and the inner core are prepared by using the PCVD process, and the rare earth doped core is prepared by using the MCVD process, so that the prepared rare earth doped few-mode optical fiber has the advantages of more accurate refractive index profile, uniform rare earth doping, less loss, good roundness and good concentricity. So that the prepared optical fiber conforms to the initial structural design and parameter design.

It will be understood by those skilled in the art that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. The preparation method of the rare earth doped few-mode optical fiber is characterized by comprising the following steps of:

2. The method for preparing rare earth doped few-mode optical fiber according to claim 1, wherein the doping of the cladding in step (1) is realized by co-doping of silica, germanium dioxide and fluorine.

3. Method for producing a rare-earth doped few-mode optical fiber according to claim 2, characterized in that the cladding is produced by a PCVD or MCVD process, preferably by a PCVD process.

4. The method for preparing the rare earth-doped few-mode optical fiber as claimed in claim 3, wherein the rare earth-doped annular core layer in the step (1) is formed by introducing rare earth chelate vapor, aluminum trichloride vapor, silicon tetrachloride vapor and oxygen into the liner tube of the cladding prepared by PCVD or MCVD process, and then performing chemical reaction under oxyhydrogen flame heating to directly form the doped core layer containing rare earth ions.

5. The method for preparing rare earth doped few-mode optical fiber according to claim 1, wherein the inner core layer in step (2) is prepared by depositing silica, germanium dioxide and fluorine in another selected liner tube, and the core layer can be prepared by PCVD or MCVD process.

6. The method of claim 5 wherein the core layer is formed by a PCVD process, deposited in PCVD and then fused on a HEC to form the desired preform.