NZ286845A

NZ286845A - Optical fibre amplifier: fluorescent particles distributed in core

Info

Publication number: NZ286845A
Application number: NZ28684596A
Authority: NZ
Inventors: Sergent Christian Le; Dieter Weber
Original assignee: Alcatel Australia
Priority date: 1995-06-29
Filing date: 1996-06-18
Publication date: 1997-04-24
Also published as: AU5617096A; AU712728B2

Description

<div class="application article clearfix" id="description"> Priority Dat©(s): Comply Specification Fii&ci: Cksc: '3. H.Q/.aa/.cfo;..6ca.e>^//^ Putftcntwn P.O. Jc*urn*l Mo: .. !M. HUE COPY NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "AMPLIFYING OPTICAL FIBRE" > WE, ALCATEL AUSTRALIA LIMITED, Cfrcn ooo C>^>£ 3^> A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 286845 This invention relates to an amplifying optical fibre. Such a fibre can be used to constitute an optical amplifier or an optical transmitter. It is conventionally constituted by a vitreous tran; mission material such as silica or a fluorinated glass. This medium is chosen so as to have a very low 5 absorption coefficient for a light to be amplified as it is guided through the fibre. The approximate wavelength of the light is typically 1,300 or 1,550 nm. An amplifying species is incorporated in the core so as to constitute therein active centers in which the amplification process takes place. It is conventionally constituted by erbium ions or praseodymium ions dissolved in the transmission 10 material. It has been proposed to associate with this amplifying species an auxiliary species capable of enhancing the performance of an amplifier implemented using such a fibre. The amplifier performance then depends upon both the nature of the transmission material and the chosen concentrations of the amplifying and auxiliary species. It includes in particular an efficiency spectral range, a pump light operating radiation efficiency, and a net amplifier gain which results not only from the intrinsic amplifying capabilities of the amplifying species but also from complex phenomena such as light absorption and deexcitation of the material through interaction with the surrounding materials constituted by the transmission material 20 and the amplifying species. Furthermore, the transmission material must allow the industrial manufacture 4 286845 of the fibre. More particularly, it has been proposed to enhance the performance of the amplifier using the following auxiliary species: Al203/ La203, Yb203 and P2Os. Dieter Weber proposed an auxiliary species constituted by aluminium fluoride AIF3 and forming a complex with an amplifying species constituted by neodymium fluoride NdF3 or praseodymium fluoride PrF3. For more information on this topic, the following documents may be consulted: - C.C Larsen -Al-La-Doped Amplifier fiber for extremely low Hydrogen Sensitivity- Proceedings of the 19th European Conference on Optical Communication - ECOC 93 - Montreux 1993. - Laser Focus - December 92 p.l 3 - German patent application no. 43 06 933.9 - M. Nakazawa et al. - Lanthanum codoped Erbium fiber amplifier -Electronics Letters 6th June 1991 V27 N12. Objects of the present invention are to enhance the performance of an amplifying fibre, to easily adapt this performance to various needs, and/or to facilitate the manufacture of the fibre. According to the invention there is provided an amplifying optical fibre comprising a core surrounded by an optical cladding, a main constituent of the core being a transmission material having low losses for a light to be amplified as it 28 6845 is guided through the fibre, other constituents of the core being: - an amplifying species forming active centers distributed in the transmission material and having fluorescent properties for amplifying said light; and - an auxiliary species distributed in the transmission material and being preferentially associated therein with said active centers so as to optimise their fluorescent characteristics, wherein the auxiliary species is associated with said active centers at least in part in the form of host particles containing said active centers and constituting a distinct phase dispersed in the transmission material. The invention is further described, by way of example only, with reference to the accompanying figure. This figure shows an amplifying optical fibre according to the invention. Referring to the figure, the fibre comprises a core 1 surrounded by an optical cladding 2. Said transmission material constitutes the core. A material having the same basic composition but using different dopants constitutes the cladding. The host particles are shown in 3. The fibre can be manufactured from known materials using known techniques. According to a first manufacturing possibility, the particles can be manufactured beforehand. The amplifying species and the auxiliary species are for this purpose dissolved together and the glass or solid compound thus obtained is 28 6845 ground so as to obtain particles of appropriate size. The particles are incorporated and dispersed in the transmission material during one of its manufacturing stages. According to a second manufacturing possibility, the amplifying species and the auxiliary species are incorporated in the transmission material at a temperature that is sufficient to allow a dissolution of the two species. The composition of the transmission material and/or the concentration of the auxiliary species are chosen so that a cooling stage, optionally associated with an intermediate-temperature heat treatment, produces a distinct phase constituted by submicronic particles comprising both the amplifying species and the auxiliary species. Common processes are then used to obtain a preform from the transmission material containing the particles and the material which is to constitute the cladding. Finally, the fibre is formed by drawing said preform. According to a first constitution possibility for the fibre according to the invention, said particles have dimensions lying in the range 3 to 500 nm and, for instance, diameters lying in the range 10 to 100 nm. The indicated dimensions are measured along transverse directions relative to the length of the fibre. These dimensions are chosen much lower than the wavelength of the light to be amplified so that the particles that are dispersed in the core of the fibre may not significantly diffuse said light. According to a second constitution possibility, said particles have dimensions lying in the approximate range 600 to 1,100 nm. The difference between the 28 684 5 refractive index of the host particles and that of the transmission material is then small enough not to produce scattering losses greater than 1 dB/m at the wavelength of said light. The difference in refractive indices typically lies in the vicinity of 0.04. These dimensions are chosen so that the light to be amplified and/or the pump light may be inwardly reflected several times in such a particle when the light penetrates therein. More precisely and in relation to the scattering losses, the main parameters are as follows: the radius a, the refractive index n,, the concentration in active species, the spatial frequency of the particle deemed to be spherical, the radius r and the refractive index n0 of the core, and the length of the fibre. These parameters are generally adjusted so that the light scattering losses do not exceed a level of around 1 dB for the length used. The losses are defined by a coefficient a adm/m= 10 Q.M.a2.r'2/l_(10) (1) where Q is the scattering coefficient Q = (8/3) x4 ((m2-l)/(m2 + 2))2 (2) L( 10) is the Naperian logarithm often, with M = number of particles per km of fibre x = 2 n a n0/A A, wavelength of the light used. m = n0/n, The formula (2) can only be strictly applied when x < < 1; it is given here as an approximation. Some examples of preparation modes for a preform, from which an optical fibre according to the invention can be obtained by hot drawing, will now be given. Example 1. Production of praseodymium-doped silica fibers for amplification at 1,300 nm. It is known that, in a silica medium, rnultiphonic deexcitation does not allow a sufficient lifetime for the active level 1 G4. This lifetime however reaches 258 /j.s in a LaF3 medium (lanthanum fluoride). An example of preparation is thus as follows: a) production of a lanthanum fluoride powder doped with 5% praseodymium fluoride using the common techniques of coprecipitation of an aqueous chloride solution by hydrofluoric acid. calcination in neutral gas atmosphere at around 950C. grinding by ultragrinder so as to obtain particles having a diameter in the vicinity of 50 nm. b) mixing 2% of said powder, measured in weight, with synthetic silica powder of 50 nm average particle size doped with 5 to 30% of germanium or 2 to 286845 7% of aluminium. A silica commercially known as "fumed silica" and prepared by oxidation or flame-hydroiysis of SiCI4 may for instance be used. c) precompaction in the form of a porous cylinder followed by drying and densification by progressive heating at temperatures lying in the range 1,000-1,400C under an atmosphere constituted by an oxygen-helium-chlorine mixture under commonly applied conditions for the production of porous preforms for optical fibers. d) the transparent rod obtained, having an approximate diameter of 8 mm and an approximate length of 1C cm is ground and polished, and then associated with tubes of commercial synthetic silica by successive sleeving-drawing-sleeving operations so as to achieve the desired core diameter and the desired outer diameter. A preform having an approximate diameter of 30 mm is thus obtained. During the final fibering operation, the preform is drawn under conventional conditions to produce a fibre having a diameter of 1 25 ns and a core radius of around 3 /j.s. A low fibering temperature is preferentially chosen. In this example 1, the transmission material is silica and the auxiliary species is lanthanum fluoride. The auxiliary species is doped with praseodymium as the amplifying species to constitute the material of the host particles. Example 2. Production of erbium-doped silica fibers for amplification at 1,550 nm. 28 6845 When implementing wavelength-division multiplexing, it is desirable to use amplifying fibers having amplifying properties that are little dependent upon the exact wavelength used in the window centered at 1,550 nm. An example of preparation is as follows: a) production of a submicronic amorphous lanthanum fluoride powder doped with about 3% erbium fluoride. b) production of a gel by addition of 2 g of said powder and vigorous agitation in a suspension at 40C comprising: 150 cm3 of water 25 cm3 of ethanol 2,5 g of aluminium chloride 20 g of submicronic synthetic silica commercially known as "fumed silica". The gel is poured into polymer tubes as it sets so as to prevent it from clinging to the walls. c) drying with all the usual precautions for this type of material, in particular in relation to the very progressive temperature rises (around 0,2C per minute) and the length of the plateaux (around ten hours). d) additional drying and densification in the temperature range of 900- 1,400C under an atmosphere constituted by a mixture of chlorine (30%), oxygen (30%) and helium (40%). e) grinding-polishing of the glass cylinder obtained in d 286845 f) production of a preform ready for fibering through successive sleeving and drawing operations using silica tubes having a 1 -2% fluorine concentration, sufficient to guarantee proper guiding of the light. In this example 2, the transmission material is silica and the host particle material is erbium-doped lanthanum fluoride. Example 3. Production of chalcogenide glass fibers for amplification at 1,300 nm. Glasses composed of La2S3-Ga2S3 mixtures can be doped with the chemical elements Pr3+ or Dy3+ which in this type of matrix have an extraordinary fluorescence quantum efficiency, close to ten times greater than values measured in fluorinated compositions of the ZBLAN type. Such glasses are unfortunately very sensitive to crystallisation and very difficult to fibre. A solution to this problem consists in: a) preparing a La2S3-Ga2S3 glass doped with 30,000 ppm of praseodymium or dysprosium. b) reducing the glass to submicronic particle size c) putting the glass in suspension with a concentration close to 3% in a core composition based on arsenic disulfide at a temperature of 800C in a sealed funnel. d) letting if cool so as to obtain a core rod having the same size as in the 10 280845 previous examples. e) performing sleeving and drawing operations as in the previous examples. In this example 3, the transmission material is arsenic disulfide and the host particle material is lanthanum-gallium sulfide doped with praseodymium or dysprosium. In view of the proportions and other indications given in relation to examples 1 to 3, two other examples can now be described more succinctly. Example 4. A host particle material constituted by lanthanum sulfide La2S3 and doped with praseodymium sulfide Pr2S3 and a transmission material constituted by gallium and lanthanum (Gax La2JS3 are prepared. These materials are reduced to particles having a diameter of around 50 nm, and then mixed. The mixture is sintered at a temperature that is approximately 10C greater than the glass transition temperature Tg of the transmission material. The core rod of the preform is thus formed. Example 5. A host particle material constituted by lanthanum fluoride LaF3 and doped with praseodymium fluoride PrF3 and a transmission material constituted by a fluorinated glass known as ZBLAN (zirconium, barium, lanthanum, aluminium and sodium fluoride) are prepared. These materials are reduced to particles having a 11 28 6845 diameter of around 50 nm, and then mixed. The mixture is sintered at a temperature that is approximately 10C greater than the glass transition temperature Tg of the transmission material. The core rod of the preform is thus formed. 12 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 286845 What we claim is:

1. An amplifying optical fibre comprising a core surrounded by an optical cladding for guiding light, a main constituent of the core being a transmission material, other constituents of the core being: - an amplifying species forming active centers distributed in the transmission material and having fluorescent properties for amplif/ing said light; and - an auxiliary species distributed in the transmission material and being preferentially associated therein with said active centres; wherein said auxiliary species is associated with said active centers at least in part in the form of host particles containing said active centers and constituting a distinct phase dispersed in said transmission material.

2. A fibre as claimed in claim 1, wherein said host particles have dimensions transverse to the fibre that lie in the range 3 nm to 500 nm.

3. A fibre as claimed in claim 1, wherein said host particles have dimensions transverse to the fibre that lie in the approximate range 600 nm to 1,100 nm.

4. A fibre as claimed in claim 3, wherein the refractive indices of said host particles and of said transmission material have a mutual difference that is small enough not to produce scattering losses greater than 1 dB/m at the wavelength of said light.

5. A fibre as claimed in claim 4, wherein said difference in refractive indices lies in the vicinity of 0.04. 10 286845

6. A fibre as claimed in claim 1, wherein said transmission material is silica and said auxiliary material is lanthanum fluoride, said amplifying species being praseodymium or erbium in fluoride form.

7. A fibre substantially as hereinbefore described with reference to the figure of the accompanying drawing. END OF CLAIMS ALCATEL AUSTRALIA LIMITED B. O'Connor Authorized Agent P5/1 /I 703 V zL K 1" JU»'*>6 Si;* 14 </div>