WO1991013038A1

WO1991013038A1 - A rare-earth doped fibre

Info

Publication number: WO1991013038A1
Application number: PCT/AU1991/000068
Authority: WO
Inventors: Steven James Frisken
Original assignee: Otc Limited
Priority date: 1990-02-28
Filing date: 1991-02-28
Publication date: 1991-09-05

Abstract

A rare-earth metal doped fibre (1) of small radius for use in an optical amplifier or fibre laser. The fibre (1) has its inner core (3) doped with a rare-earth metal so as to have a high Numerical Aperture. The outer core (4) has a low Numerical Aperture and surrounds the inner core (3). A cladding (2) surrounds the outer core (4).

Description

A Rare-Earth Doped Fibre

TECHNICAL FIELD

The present invention relates to rare-earth doped fibres for optical amplifiers and fibre lasers.

BACKGROUND ART

Fused silica fibres form the basis of most single mode optical fibre applications. The effective operation of the fibre is dependant on the confinement of the light wave to the core of the fibre. This is achieved by different refractive Indexes, with a high refractive index being provided in the core as compared to that of the surrounding cladding.

When the core radius is large and the refractive index difference is high, the vast majority of the light is confined to the core of the fibre.

This difference in refractive indexes is referred to as the Numerical

Aperture (N.A.). The extent of confinement is dependant upon the number of modes that operate within the core, the wavelength of the light and the N.A.

It is desirable to have a high intensity light transmission to thereby provide a strong signal. This is particularly Important in the case of optical amplifiers. A number of conclusions have been reached by researchers recently regarding the design of fibres to optimise their use as amplifiers. Fibre design has concentrated on increasing the N.A. and reducing the core size in an attempt to increase the intensity of both the signal wavelength and pump in the core.

However there are constraints on the N.A. that can be achieved in manufacturing processes. Further the compatability with existing telecommunications fibres is also a problem, which problem is exacerbated by the increase in refractive index of the core. This incompatability results in losses in coupling light from a standard fibre to a gain fibre.

Preferably these losses must be kept minimal by ensuring that the overlap of the intensity profiles of both fibres remain large for the signal wavelength. The fibre should also remain single moded at a single wavelength.

OBJECT OF THE INVENTION

It is the object of the present invention to overcome or substantially ameliorate the above disadvantages.

DISCLOSURE OF THE INVENTION

There is disclosed herein a rare-earth metal doped fibre of small radius for use in an optical amplifier or fibre laser, said fibre having a rare-earth metal doped inner core of a high Numerical Aperture, and an outer core of larger radius than the inner core, said outer core having a

SUBSTITUTE SHEET low Numerical Aperture and surrounding the said inner core, and a cladding surrounding the said outer core.

Preferably the inner core is doped with Erbium, and the outer core is not doped.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing there is schematically depicted in transverse cross-section an optical fibre which may be used in an optical amplifier or fibre laser.

BEST MODE FOR CARRYING OUT THE INVENTION

In the accompanying drawing, there is schematically depicted, in cross-section, an optical fibre 1 which may be used in an optical amplifier or fibre laser.

The fibre 1 has an Inner core 3 which is doped with a rare-earth metal, preferably Erbium. The inner core has a high Numerical Aperture and is preferably doped in Its central region. Surrounding the inner core is an outer core 4 which is not doped with a rare-earth metal, or has a low dopant concentration. The outer core has a larger radius relative to the inner core and has a low Numerical Aperture relative to the inner core 3. Surrounding the outer core 4 is a cladding 2.

Where the fibre 1 is provided with a two-step Numerical Aperture fibre design, the inner core 3 allows a high intensity light to be obtained for both the pump and the signal wavelengths. This is done without affecting the coupling ability with a standard telecommunications fibre (the signal coupling efficiency). As the Erbium dopant is confined to a small area within the core, greater increases can be obtained in the gain in the amplifier.

It should be appeciated that the definition of Numerical Aperture (N.A) means the local Numerical Aperture relative to the cladding, which could, for example, be pure silica, with a refractive index of 1.46.

2 2 N.A = (n(r) -nc_,la_d. ) where n is the local refractive index and ⁿclad ^^{s tne re}^^racti^ve i^nde^χ of the cladding.

This equation gives the local N.A at the radial distance r.

The main advantage of this preferred fibre design comes when the pump wavelength is considerably shorter than the signal. For example, for Erbium doping the signal wavelength is at about 1.536 μm, and two of the possible pumping bands are at 980 nm and 820 nm. In this case as well as the advantages already listed of high Intensity for pump and signal and good signal coupling to standard fibre, there is an additional advantage of low bending loss for the signal wavelength compared with standard fibres which are single-moded fibres at the pump wavelengths.

SUBSTITUTE SHE

Claims

CLAIMS :

1. A rare-earth metal doped fibre of small radius for use in an optical amplifier or fibre laser, said fibre having a rare-earth metal doped inner core of a high Numerical Aperture, and an outer core of larger radius than the inner core, said outer core having a low Numerical Aperture and surrounding the said inner core, and a cladding surrounding the said outer core.

2. The fibre of claim 1, wherein said inner core is doped with Erbium.

3. The fibre of claim 1 or 2, wherein said inner core is doped in its central region.

4. The fibre of claim 1, 2 or 3, wherein said outer core is not doped with a rare-earth metal.

5. The fibre of claim 1, 2 or 3, wherein said outer core has a low dopant concentration.

6. An optical fibre for use 1n an optical amplifier or fibre laser, substantially as hereinbefore described with reference to the accompanying drawings.

SUBSTITUTESHEET