Active optical fibre doped with rare earth elements

Details Active optical fibre doped with rare earth elements Active optical fibre doped with rare earth elements

2001-03-30

2003-07-01

Healy, Brian (Department: 2874)

Optical waveguides

Having particular optical characteristic modifying chemical...

Of waveguide core

C385S123000, C385S126000, C385S127000, C065S385000, C065S428000

Reexamination Certificate

active

06587633

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active optical fibre doped with rare earth elements. Furthermore, the present invention relates to an optical amplifier comprising said active optical fibre, an optical communication system comprising said optical amplifier, and a method for producing said optical fibre.
2. Technical Background
As known, at present optical fibres are widely used in the field of telecommunications for transmitting signals. They essentially comprise an inner cylindrical region, called core, within which a signal is transmitted, and an outer annular region, called cladding. The refractive index of the cladding is lower than that of the core, so as to confine the transmitted signal within the latter.
Typically, both the core and the cladding are made of a silica-based glass material. The difference of refractive index between the core and the cladding is obtained by incorporating suitable additives (dopants) into the glass matrix of the core and/or of the cladding.
Typical examples of dopants used for modifying the refractive index of silica are germanium and phosphorous (which increase its refractive index) and fluorine (which decreases its refractive index).
An active optical fibre is an optical fibre whose core is further doped with particular substances capable of giving effects of optical amplification.
Typical examples of said substances are rare earth ions, whose spectroscopic properties are particularly suitable for the purpose. Among rare earths, erbium is the most frequently used component since its fluorescence spectrum has a band ranging between 1420 and 1650 nm, which corresponds to the third transmission window (centred at about 1550 nm) of a telecommunication signal.
Active optical fibres are used for producing optical amplifiers or, for example, super-fluorescent sources or lasers.
In general, an optical amplifier comprises an active optical fibre and a luminous source, called “pumping source”, suitable to provide a pumping signal having a wavelength (typically corresponding to a peak of the absorption spectrum of the dopant substance) capable of bringing the ions of the dopant substance to an excited energetic level. From said level, the ions spontaneously fall—in very short times—to a laser-emission level, or metastable level, where they remain for a relatively longer time (called mean lifetime of the metastable level).
When a luminous signal having a wavelength corresponding to said metastable level passes through an active optical fibre having a high number of excited ions on the metastable level, the excited ions decade to a lower level, thus causing a stimulated luminous emission having the same wavelength as the signal.
High concentrations of rare earth ions inside the core of an active optical fibre reduce the length of fibre needed for obtaining a high amplification of the signal.
Nevertheless, due to the extremely compact structure of silica, inside the core of an active optical fibre the rare earth ions tend to aggregate with one another as their concentration inside the core increases (this phenomenon is conventionally known as “clustering”).
Since—when aggregate—excited rare earth ions tend to fall from the metastable level in a non-radiative way, said aggregation causes a reduction of mean lifetime of the excited ions on the metastable level and thus, of the efficiency of fluorescence of the active optical fibre.
As the clustering prevents a corresponding increase of the fluorescence efficiency when the concentration of rare earth ions inside the core of an active fibre increases, it actually limits the value of maximum concentration of rare earth ions inside the core of an active optical fibre.
In the present invention and claims, the expression
“fluorescence efficiency” of a material indicates the ratio between the power of the amplified spontaneous emission (ASE) back-diffused by the material, in absence of transmission signal and during absorption of electromagnetic radiations by a pump source, and the power of the electromagnetic radiations received from said source (pumping power); and
“amplification efficiency” of an optical amplifier indicates the ratio between the optical power of a transmission signal in output from it and the optical pumping power provided to it.
The amplification efficiency of an optical amplifier increases as the fluorescence efficiency of the rare earth with which the active optical fibre is doped increases.
B. James Ainslie (“A review of the fabrication and properties of erbium-doped fibers for optical amplifiers”, Journal of Lightwave Technology, 1991, vol. 9, no. 2, pages 220-227) presents a review of methods of fabrication of active optical fibres doped with erbium and of the properties of said fibres. Among the other things, the Author states that the addition of P
2
O
5
and Al
2
O
3
to a silica-based glass enables the incorporation of several weight percents of rare earth ions without clustering effects. Furthermore, studies carried out on the concentration of erbium ions into Al
2
O
3
—SiO
2
and GeO
2
—SiO
2
have shown that the weight percents of erbium ions that can be incorporated without clustering effects into silica- and alumina-based glasses (Al
2
O
3
—Si
2
) is greater than the weight percents that can be incorporated into silica- and germanium-based glasses (GeO
2
—SiO
2
).
Nevertheless, the Applicant found that alumina has a limited solubility into the silica-based glass of conventional optical fibres (typically, the maximum quantity of alumina is less than or equal to, about 11%).
The Japanese patent application JP 3235923 describes an optical amplifier with an amplifying optical fibre suitable to amplify at the same time both signals in the band of 1300 nm and signals in the band of 1550 nm. Said amplifying optical fibre has both the core and the cladding made up of at least one glass material selected among a CaO—Al
2
O
3
—SiO
2
based glass, a phosphate-based glass and a fluorophosphate-based glass. Moreover, at least one between erbium and neodymium is included in at least one between the core and the cladding.
Nevertheless, the Applicant points out that said document does not give any further information on the composition of the CaO—Al
2
O
3
—SiO
2
based glass. In addition, as the active optical fibre it describes has both the core and the cladding made up of at least one glass material selected among a CaO—Al
2
O
3
—SiO
2
based glass, a phosphate-based glass and a fluorophosphate-based glass, it presents difficulties of junction with the optical fibres conventionally used for telecommunications, having a pure silica cladding.
For the production of an active optical fibre, the glasses of the core and cladding with different composition must be compatible with one another. For example, said glasses must be compatible in terms of temperature of glass transition.
This is an important requirement because during some steps of the production process of an optical fibre, the glass materials of core and cladding must both be at a plastic state (neither having a too high viscosity nor being at a melted state). This is only possible if the two materials have such temperatures of glass transition as to guarantee at least a partial superimposition of the temperature ranges in which they are at a plastic state.
SUMMARY OF THE INVENTION
The Applicant faced the technical problem of increasing the concentration of rare earth ions inside the core of an active optical fibre, limiting at the same time the ion-clustering phenomenon so as to obtain a corresponding increase in the fluorescence efficiency.
The Applicant has found that said problem can be solved with an active optical fibre having a silica glass cladding and a core comprising a quantity of silica (SiO
2
) of at least 50% in weight, and a quantity of an oxide XO not exceeding 40% in weight, wherein X is selected from the group comprising Ca, Sr, Ba and Zn.
The Applicant has proved that the core of the active optical fibre comprising, according to the invention, the above percen

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