Optical fibre amplifier

Abstract: An Ar⁺ion laser pumped optical fibre amplifier is provided with an optical fibre having a core which has been solution doped using a solution of an aluminium and an erbium salt.

Description: This invention relates to optical fibre amplifiers, and more particularly to a pumped optical fibre amplifier having an optical fibre comprising an erbium doped core. Such an amplifier is known, for example, from the first paragraph of an Article in Applied Physics Letters, vol. 51, no. 22, 30th November 1987. This article is primarily concerned with fibre lasers rather than amplifiers and discloses a fibre laser comprising a neodymium doped fibre fusion spliced with an erbium doped fibre an pumped to an Ar⁺ion laser at 514nm, mirrors being used at the free ends of the fibres for reflecting multiple wavelength oscillation at 910, 1080 and 1530 nm. An object of the invention is to provide an efficient optical fibre amplifier for amplifying signals at wavelengths of approximately 1532 nm. Erbium doped fibres are suitable for such amplifiers when pumped with an Ar⁺ion laser at 514.5 nm. To operate as an amplifier, the fibre is pumped with an Ar⁺ion laser operating at 514.5 nm, so as to invert the erbium ions to a metastable energy level. Provided that the pump power is sufficient to invert greater than 50% of the erbium ions a signal beam of wavelength approximately 1532 nm propagated through the same fibre will achieve gain. Ideally all the pump power should be converted into useful signal gain, however excited state absorption (ESA) of the pump light depletes the pump power by absorption of pump photons by ions resident in the metastable level and provides an extra loss mechanism which does not contribute to signal gain, thus significantly reducing the amplifier pumping efficiency. A measure of the amplifier pumping efficiency is the ratio of the excited-state absorption cross-section to ground-state absorption (GSA) cross-section, ESA/ GSA. The smaller this ratio, the more efficient the fibre at a particular pump wavelength. Silica cores of optical fibres are commonly doped with germania (GeO₂) to modify the refractive index of the core material. Fibres with such cores when codoped with erbium exhibit an unacceptably high ratio of ESA to GSA when pumped at wavelengths around 500 nm. We have found that using alumina (Al₂O₃) as the refractive index modifying dopant in a silica core doped with erbium provides a considerably lower ratio of ESA to GSA for the fibre at pump wavelengths around 500 nm. The use of alumina as the refractive index modifying dopant has the additional advantage that the fabrication of the fibre may be facilitated by solution doping the core with a solution of an aluminium salt and an erbium salt, thus introducing both dopants in the same step. The invention provides a pumped optical fibre amplifier having an optical fibre comprising an erbium doped silica core characterised in that the core has been solution doped using a solution of an aluminium salt and an erbium salt, and in that the optical fibre is arranged to be pumped by an Ar⁺ion laser. In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 diagrammatically illustrates an Ar⁺ion pumped optical fibre amplifier; Figure 2 illustrates the refractive index profile of an alumina doped fibre; Figure 3 illustrates the loss spectra of alumina and germania doped fibres co-doped with erbium; and Figure 4 illustrates the fluorescence spectra of alumina and germania doped fibres co-doped with erbium. Referring first to Figure 1, there is shown an optical fibre amplifier 10 (which may be used as a power amplifier, an in-line amplifier or a pre-amplifier) for amplifying signals of approximately 1532nm. The amplifier 10 comprises an alumina-erbium co-doped fibre 12 arranged to be pumped at 514.5 nm by an Ar⁺ion laser 14 via a dichroic fibre coupler 16. The fibre 12 is formed using a solution doping technique as an extension of the well-known MCVD technique. One or more conventional cladding layers (typically a SiO₂/P₂O₅/F glass) are first deposited on the inside surface of the substrate tube in a lathe, following which silica core layers are deposited at a reduced temperature to form a partially-sintered, porous soot. The alumina and erbium dopants are then introduced by removing the thus formed tube from the lathe and soaking the core layers in an aqueous or alcoholic solution of an aluminum salt and an erbium salt to ensure saturation of the porous soot. The tube is then replaced in the lathe, the core layers are dried and fused and the tube collapsed. Fibre drawing from the thus formed preform is conventional. An aluminium salt is required which is obtainable in highly pure form and is easily soluble in water or alcohol. Aluminium trichloride hexahydrate is a suitable material, being readily available in 99.9995% pure form and being highly soluble. Erbium trichloride hexahydrate is the preferred erbium salt. Figure 2 shows the refractive index profile of a typical preform made using the above technique. The profile is a good approximation to a step profile and indicates the consistency of the incorporation of the alumina across the core. A typical loss spectrum of an Al₂O₃/SiO₂ core Er³⁺-doped fibre (125 µm O.D. cut-off = 1250nm) containing about 300 ppm Er³⁺ is shown by the solid line in the graph of Figure 3 in which graph loss in dB/km is plotted along the Y axis and wavelength in nm is plotted along the X axis. Superimposed on the same graph for comparison in dotted line is the loss spectrum of a GeO₂/SiO₂ core Er³⁺ doped fibre containing 200 ppm Er³⁺ which shows the shifts in the relative intensities of the erbium absorption spectra caused by the presence of the alumina. The associated fluorescence spectra are shown in Figure 4 in which power in arbitrary units is plotted along the Y axis and wavelength in nm is plotted along the X axis with the fluorescence spectra for the alumina doped core being shown in solid line and that for the germania doped core being shown in dotted line. It will be noted that there is very little shift in the peak fluorescence wavelength, compared to the large shift observed in, for instance, the 4-level ⁴F 3/2 - ⁴I 11/2 transition in Nd³⁺. As mentioned hereinbefore, it is important for the ratio of excited-state absorption cross-section to ground-state absorption cross-section, ESA/ GSA to be small in the fibre of an optical fibre amplifier when pumped. This ratio was measured for the two fibre types at 514.5nm and found to be approximately 1 and approximately 0.5 for the Er³⁺/GeO₂/SiO₂ and Er³⁺/Al₂O₃/SiO₂ fibres respectively. Thus, the alumina co-doped fibre provides a significantly more efficient amplifier when pumped at 514.5 nm. Signal gains of 23dB at 1.536 um have been obtained for an Er³⁺/Al₂O₃/SiO₂ fibre with a numerical aperture of 0.15, pumped with 300 mW of pump light at 514.5nm. In an Er³⁺/GeO₂/SiO₂ fibre, 300mW of pump power at 514.5nm was insufficient to obtain any notable signal gain. It will be noted from the above that the core need only contain a few hundred ppm Er³⁺ in order to form an effective amplifier. It is to be understood that other dopants may be introduced into the erbium alumina co-doped core.

Claims: A pumped optical fibre amplifier (10) having an optical fibre (12) comprising an erbium doped silica core, characterised in that the core has been solution doped using a solution of an aluminium salt and an erbium salt, and in that the optical fibre (12) is arranged to be pumped by an Ar⁺ion laser (14). An amplifier as claimed in claim 1, wherein the aluminum salt is aluminium trichloride hexahydrate. An amplifier as claimed in claim 1, wherein the erbium salt is erbium trichloride hexahydrate. An amplifier as claimed in claim 1, 2 or 3, wherein the core contains a few hundred ppm Er³⁺. An amplifier as claimed in claim 4, wherein the core contains about 300 ppm Er³⁺.

Inventors: LAMING RICHARD IAN, POOLE SIMON BLANCHETTE, LAMING, RICHARD IAN, POOLE, SIMON BLANCHETTE
Applicants: PIRELLI GENERAL PLC
Application year: 1989
Main classification: H01S3
IPC codes: H01S3, G02F1, C03B37