# US 11,309,677 B2 · Multi-clad optical fiber with delocalization of pedestal modes

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- Number: US 11,309,677 B2
- Document ID: US11309677B2
- Status: Granted
- Kind: B2
- Application number: 17/162,852
- Filed: 2021-01-29
- Published: 2021-08-05
- Granted: 2022-04-19
- Earliest claimed: 2020-01-31
- Assignees: Institut National D'Optique
- Applicants: Institut National D'Optique
- Inventors: Antoine Proulx; Vincent Roy; Mathieu Boivin; Yves Taillon
- Web page: https://patentgrep.com/patent/US11309677B2

## CPC

- H01S 3/06733: {Fibre having more than one cladding}
- G02B 6/03627: {arranged - +}
- G02B 6/03661: {having 4 layers only}
- H01S 3/06716: {Fibre compositions (per se C03C13/04)or doping with active elements (lasing materials in general H01S3/14)}
- H01S 3/06712: {Polarising fibre; Polariser}
- H01S 3/06754: {Fibre amplifiers (H01S3/06708 takes precedence)}
- H01S 3/08045: Single-mode emission
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

A multi-clad optical fiber is provided. The fiber includes, concentrically and radially outwards from the center of the optical fiber, a core doped with at least one rare-earth dopant material, a pedestal cladding structure, an inner cladding and an outer cladding. The pedestal cladding structure includes a pedestal layer having a refractive index smaller than a refractive index of the core, and a raised index layer having a refractive index larger than the refractive index of the pedestal layer. The raised index layer has a thickness and a refractive index which preserve the confinement of the core mode in the core and minimize the overlap of one or more pedestal modes with the core.

## Claims

### Claim 1

A cladding-pumped multi-clad optical fiber comprising, concentrically and radially outwards from a center axis of the optical fiber:
a core configured to guide a light signal in a core mode, said core having a core diameter dcore smaller than about 30 μm, the core being doped with at least one rare-earth dopant material providing amplification of the light signal, the core having a refractive index ncore;
a pedestal cladding structure supporting light propagation in a plurality of pedestal modes, the pedestal cladding structure having a pedestal cladding structure diameter dpcs, the pedestal cladding structure comprising:
a pedestal layer contiguously surrounding the core and configured to confine the light signal in said core mode, the pedestal layer having a refractive index nped smaller than the refractive index of the core ncore; and
a raised index layer contiguously surrounding the pedestal layer and having a thickness Δw, the raised index layer having a refractive index nril larger than the refractive index of the pedestal layer nped and smaller than the refractive index of the core ncore, the thickness Δw and the refractive index nril of the raised index layer preserving a confinement of the core mode in the core;
an inner cladding having a refractive index nicl smaller than the refractive index nril of the raised index layer; and
an outer cladding having a refractive index nocl smaller than the refractive index nicl of the inner cladding;
wherein the pedestal cladding structure is a thin pedestal cladding structure characterized by a ratio of the pedestal cladding structure diameter dpcs to the core diameter dcore smaller than about 3, and the thickness Δw and the refractive index nril of the raised index layer minimize an overlap of one or more modes of the plurality of pedestal modes with the core to reduce a probability of amplification of the pedestal modes without requiring extraction of said pedestal modes.

### Claim 2 (depends on 1)

The multi-clad optical fiber according to claim 1, wherein the diameter of the core is between about 10 μm and about 25 μm.

### Claim 3 (depends on 1)

The multi-clad optical fiber according to claim 1, wherein the diameter of the core is smaller than about 10 μm.

### Claim 4 (depends on 1)

The multi-clad optical fiber according to claim 1, wherein the core is made of silica and the at least one dopant material comprises ytterbium, erbium, thulium or a combination thereof.

### Claim 5 (depends on 4)

The multi-clad optical fiber according to claim 4, wherein the core is co-doped with at least one of aluminum oxide, phosphorus oxide, cerium oxide and fluorine.

### Claim 6 (depends on 1)

The multi-clad optical fiber according to claim 1, wherein the pedestal layer is made of silica doped with at least one index-raising dopant.

### Claim 7 (depends on 6)

The multi-clad optical fiber of claim 6, wherein the at least one index-raising dopant of the pedestal layer comprises germanium oxide, phosphorus oxide, aluminum oxide, titanium oxide or a combination thereof.

### Claim 8 (depends on 1)

The multi-clad optical fiber according claim 1, wherein the raised index layer is made of silica doped with at least one index-raising dopant.

### Claim 9 (depends on 8)

The multi-clad optical fiber of claim 8, wherein the at least one index-raising dopant of the raised index layer comprises germanium oxide, phosphorus oxide, aluminum oxide, titanium oxide or a combination thereof.

### Claim 10 (depends on 1)

The multi-clad optical fiber according to claim 1, wherein at least one of the core, the pedestal layer, the raised index layer, the inner cladding and the outer cladding has a non-uniform refractive index profile.

### Claim 11 (depends on 1)

The multi-clad optical fiber according to claim 1, having a V parameter of less than about 4.

### Claim 12 (depends on 1)

The multi-clad optical fiber according to claim 1, comprising a pair of stress-applying parts extending parallel to the core.

### Claim 13 (depends on 12)

The multi-clad optical fiber according to claim 12, wherein the stress-applying parts are enclosed within the inner cladding.

### Claim 14 (depends on 12)

The multi-clad optical fiber according to claim 12, wherein the stress-applying parts extend within the inner cladding and partially into the pedestal cladding structure.

### Claim 15 (depends on 1)

The multi-clad optical fiber according to claim 1, wherein the one or more modes of the plurality of pedestal modes having a minimized overlap with the core is an LP11-like mode.