RARE EARTH-DOPED DOUBLE-CLAD OPTICAL FIBER AND PREPARATION METHOD THEREOF

Abstract

A rare earth-doped double-clad optical fiber includes a rare earth ion-doped fiber core, an inner cladding layer, and an outer cladding layer. A cross section of the inner cladding layer is a non-circular plane including at least two arcuate notches. According to the provided optical fiber, optical processing can be performed on a preform without changing a preform preparation process and a drawing process. The inner cladding is designed to have a non-circular planar structure having a cross section with at least two arcuate notches. While maintaining the same light absorption efficiency of pump light within the cladding layer, a preform polishing process is simplified, a risk of cracking the preform during polishing of multiple surfaces and a risk of contamination of the preform caused by impurities are reduced, wire drawing control precision is better, and comprehensive performance of the optical fiber is improved.

Claims

1. A rare earth-doped double-clad optical fiber, comprising a rare earth ion-doped fiber core, an inner cladding layer, and an outer cladding layer, wherein a cross section of the inner cladding layer is a non-circular plane comprising at least two arcuate notches.

2. The rare earth-doped double-clad optical fiber according to claim 1, wherein a relationship between a refractive index n.sub.1 of the fiber core and a refractive index n.sub.2 of the inner cladding layer is represented as (n.sub.1.sup.2n.sub.2.sup.2).sup.1/2, which is in a range from 0.01 to 0.25.

3. The rare earth-doped double-clad optical fiber according to claim 1, wherein a relationship between the refractive index n.sub.2 of the inner cladding layer and a refractive index n.sub.3 of the outer cladding layer is represented as (n.sub.2.sup.2n.sub.3.sup.2).sup.1/2, which is in a range from 0.2 to 0.5.

4. The rare earth-doped double-clad optical fiber according to claim 1, wherein rare earth ions doped in the fiber core comprise one or two of Tm, Yb, Ho, and Er.

5. The rare earth-doped double-clad optical fiber according to claim 1, wherein the inner cladding layer is a silica matrix with a refractive index of 1.4573 and a measurement wavelength of 632 nm.

6. The rare earth-doped double-clad optical fiber according to claim 1, wherein the cross section of the inner cladding layer is a non-circular plane comprising four identical arcuate notches, wherein the at least two arcuate notches are not connected with one another.

7. The rare earth-doped double-clad optical fiber according to claim 6, wherein the four identical arcuate notches are symmetrically distributed on the non-circular plane.

8. The rare earth-doped double-clad optical fiber according to claim 7, wherein a ratio between a height of the arcuate notches and a circle diameter of the inner cladding layer is in a range from 0.01 to 0.05.

9. A preparation method of the rare earth-doped double-clad optical fiber according to any of claim 1, the method comprising: preparing a circular quartz preform doped with a rare earth element by vapor deposition; adjusting a diameter of the circular quartz preform to meet a predetermined ratio between a diameter of a fiber core and a diameter of an inner cladding layer; performing optical processing and polishing on a side surface of the circular quartz preform to obtain a non-circular quartz preform having a cross section with at least two arcuate notches; and drawing the quartz preform and coating a coating outside the glass to form an outer cladding layer, thereby obtaining the rare earth-doped double-clad optical fiber.

10. The rare earth-doped double-clad optical fiber according to claim 9, wherein the cross section of the quartz preform is a non-circular plane comprising two or more identical and symmetrically distributed arcuate notches, and a ratio between a height of the arcuate notches and a circle diameter of the inner cladding layer is in a range from 0.01 to 0.05.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 schematically shows a refractive index profile of a double-clad optical fiber in an embodiment of the present disclosure;

[0025] FIG. 2 schematically shows a cross section of a 4D inner cladding layer in an embodiment of the present disclosure (a central circular part is a rare earth-doped fiber core, a2/a3=0.924, and a corresponding ratio between a height of an arcuate notch and a circle diameter of the inner cladding layer is 0.038);

[0026] FIG. 3 schematically shows a cross section of a 4D inner cladding layer in an embodiment of the present disclosure (a central circular part is a rare earth-doped fiber core, a2/a3=0.90, and a corresponding ratio between a height of an arcuate notch and a circle diameter of the inner cladding layer is 0.05);

[0027] FIG. 4 schematically shows a cross section of a 4D inner cladding layer in an embodiment of the present disclosure (a central circular part is a rare earth-doped fiber core, a2/a3=0.98, and a corresponding ratio between a height of an arcuate notch and a circle diameter of the inner cladding layer is 0.01);

[0028] FIG. 5 schematically shows a cross section of an octagonal inner cladding layer in a comparative embodiment (a central circular part is a rare earth-doped fiber core);

[0029] FIG. 6 shows normalized energy distribution percentages of 915 nm pump light in a fiber core of a 4D-type 20/130 Yb-doped double-clad optical fiber having arcuate notches of different heights in an embodiment of the present disclosure; and

[0030] FIG. 7 schematically shows a cross section of an inner cladding layer of a conventional D-type optical fiber.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] To illustrate the purposes, the technical solutions, and advantages of the present disclosure more clearly, the present disclosure will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely used for explaining the present disclosure, rather than restricting the present disclosure. Further, the technical features involved in the various embodiments of the present disclosure described below may be combined with one another as long as they do not constitute a conflict with one another.

[0032] To facilitate introduction of the technical solution of the present disclosure, the related terms in the present disclosure are first defined and explained as follows.

[0033] Core Layer: a portion having a relatively high refractive index in the middle of an optical fiber; having a circular shape; being a light guide layer of signal light in the optical fiber; and having a refractive index of n.sub.1.

[0034] Inner Cladding Layer: a portion next to a fiber core; having a refractive index of n.sub.2; generally being a quartz matrix; having a refractive index of 1.4573.

[0035] Outer Cladding Layer: a portion having a low refractive index next to an inner cladding layer in an optical fiber; being a coating of a low refractive index or a fluorine-doped coating;

[0036] having a refractive index of n.sub.3.

[0037] a.sub.1: a diameter of a core layer; a unit thereof being m.

[0038] a.sub.2: a distance between two parallel planes when a 4D-type inner cladding layer is taken as an example in an embodiment of the present disclosure; a unit thereof being m.

[0039] a.sub.3: a circle diameter of an inner cladding layer; a unit thereof being m.

[0040] FIG. 1 schematically shows a refractive index profile of a double-clad optical fiber in an embodiment of the present disclosure. As shown in FIG. 1, a refractive index relationship among a core layer, an inner cladding layer, and an outer cladding layer is n.sub.1>n.sub.2>n.sub.3.

[0041] As shown in FIG. 2, the present disclosure provides a rare earth-doped double-clad optical fiber, comprising a rare earth ion-doped fiber core, an inner cladding layer, and an outer cladding layer. A cross section of the inner cladding layer is a non-circular plane comprising at least two arcuate notches, and the at least two arcuate notches are not connected with one another.

[0042] The fiber core has a refractive index of n.sub.1. The inner cladding layer has a refractive index of n.sub.2, and is usually a quartz matrix with a refractive index of 1.4573 and a measurement wavelength of 632 nm. (n.sub.1.sup.2n.sub.2.sup.2).sup.1/2 is in a range from 0.01 to 0.25. The fiber core is doped with rare earth ions, including Tm, Yb, Ho, and Er, or is double-doped with Yb/Er.

[0043] The outer cladding layer of the optical fiber has a refractive index of n.sub.3, and is usually fluorine-doped quartz or a plastic coating. (n.sub.2.sup.2n.sub.3.sup.2).sup.1/2 is in a range from 0.2 to 0.5.

[0044] An outermost layer of the optical fiber is a protection layer with a refractive index which is usually about 1.5.

[0045] Pump light is mainly transmitted in the inner cladding layer. The D-type inner cladding layer in the prior art means that the inner cladding layer of the double-clad optical fiber has one and only one flat surface, and the rest surfaces are circular. As shown in FIG. 2, in an embodiment of the present disclosure, a cross section of the inner cladding layer is a non-circular plane comprising four identical arcuate notches (for simplicity, the optical fiber is referred to as a 4D double-clad optical fiber in the embodiments of the present disclosure), and the four identical arcuate notches are symmetrically distributed on the non-circular plane. It should be noted that all 4D double-clad optical fibers including different core doping ions as well as analogic 3D, 5D, 6D, 7D double-clad optical fibers etc., are all included in the protection scope of the present disclosure.

[0046] A ratio between a height of the arcuate notches on the cross section of the inner cladding layer and a circle diameter of the inner cladding layer is in a range from 0.01 to 0.05. The height of the arcuate notches herein is a chord height of an arch (a vertical line is drawn from a center of a circle to a chord, and a height of an arch refers to a length of a segment on the vertical line that is segmented by the chord and the arch).

[0047] Further, in order to prepare the above rare earth-doped double-clad optical fiber, improvement can be made to the existing preparation process. Specifically, following steps are included: a circular quartz preform doped with rare earth elements is prepared by vapor deposition or a solution method or a sol-gel method; a diameter of the circular quartz preform is adjusted to meet a predetermined ratio between a diameter of a fiber core and a diameter of an inner cladding layer; optical processing and polishing is performed on at least two random perpendicular planes of the circular quartz preform to obtain a non-circular quartz preform having a cross section with at least two arcuate notches; and the quartz preform is drawn and a coating is coated outside the glass to form an outer cladding layer, thereby obtaining the rare earth-doped double-clad optical fiber.

[0048] A specific process for preparing a 4D double-clad rare earth-doped optical fiber is as follows: a circular quartz preform doped with rare earth elements including Tm, Yb, Ho, Er or being double-doped with for example Yb/Er is prepared by a vapor deposition method and a solution immersion method; a diameter of the preform is adjusted by stretching a sleeving horizontally or vertically and by a corrosion process, so that the preform meets a predetermined ratio between a diameter of a fiber core and a diameter of an inner cladding layer; optical processing and polishing is performed on two arbitrary perpendicular surfaces of the circular preform to obtain four identical planes, and spacing between two planes parallel to each other is precisely controlled; and the preform is drawn at a high temperature by a wire drawing tower, and an inner layer coating having a refractive index of n.sub.3 is coated outside the glass to form a 4D type double-clad rare earth-doped optical fiber.

[0049] Taking a 20/130 double-clad optical fiber as an example, FIG. 6 shows test results of a normalized energy distribution of pump light with a wavelength of 915 nm in a 20 um fiber core at different ratios between arcuate notches and a circle diameter of an inner cladding layer. It can be seen from the results that, a ratio between a height of an arcuate notch and a circle diameter of an inner cladding layer corresponding to an octagonal cladding layer of FIG. 5 is 0.025, and a normalized energy distribution portion of the pump light with a wavelength of 915 nm in a 20 um fiber core is 10.374%. The results regarding the octagonal cladding layer are slightly different from those regarding the 4D cladding layer, which indicates that an effect of 4D cladding layer in destroying the helical pump light is equivalent to or even better than that of the octagon cladding layer, and. FIGS. 3, 2, and 4 are diagrams showing that ratios between the height of the arcuate notches and the circle diameter of the inner cladding layer are 0.05, 0.038, and 0.005, respectively.

[0050] Those skilled in the art will readily appreciate that the above description is only a preferred embodiment of the present disclosure and is not intended to limit the present disclosure. Any modifications, equivalent substitutions and improvements made within the spirit and the principle of the present disclosure are intended to be included within the protection scope of the present disclosure.