GB2286390A - Infrared transmitting optical fibre materials - Google Patents

Abstract

An optical fibre amplifier is formed from glass doped with praseodymium. The glass may include one or more of cadmium mixed halide, hafnium halides, germaflium and silicon disulphide based vitreous materials or fluorozirconate glass fibres. It is possible to provide an optical fibre amplifier which operates at a 1300 nm window for passive optical networks.

Description

INFRARED TRANSMITTING OPTICAL FIBRE MATERIALS The present invention relates to infrared transmitting optical fibre materials, to a method of producing an optical fibre and to an optical fibre amplifier.

The development of silica glass single-mode optical fibres has led to the possibility of broad-band communication at second and third transmission windows situated at 1.3 ym and 1.5 ym respectively. In 1985, erbiumdoped silica optical fibre amplifier, known as EDFA, was developed for the third transmission window, with almost 97 quantum efficiency and a large signal gain of 50 dB.

The 1.5 ym optical fibre amplifiers are planned for use in the transoceanic submarine cable networks. EDFA will play a key role in the high-speed data transmission networks.

However globally, the terrestrial networks utilise 1.3 ym window and currently electronic repeaters are used at the signal wavelength. The electronic repeaters, used in the networks, are prohibitively high cost items.

Besides they inherently introduce incompatibility between optical and electronic components at high bit rate transmission ( > 2.5 Gbit/sec). Hence there is a need for providing distortion-free amplification without converting the optical signals into an array of electrical pulses and vice-a-versa. Optical fibre amplifiers operating at 1300 nm window for passive optical networks (PONs) are therefore required.

According to an aspect of the invention, there is provided an optical fibre comprising a glass doped with praseodymium. The rare-earth ions used as dopants are suitable for fluorescence in the 1300 nm wavelengthdomain.

According to another aspect of the invention, there is provided a method of producing a glass fibre doped with praseodymium, comprising the steps of providing starting compounds including cadmium fluoride and other halide ingredients; drying and fluorinating the cadmium fluoride material and drying and halogenating the halide ingredients; melting the dried material; adding a praseodymium dopant during the melting stage; and casting the melted material.

According to another aspect of the present invention, there is provided an optical fibre amplifier including an optical fibre as herein specified.

A critical factor in designing an efficient 1.3 im fibre amplifier based on Pr+3 as dopant ion is the selection of suitable glass hosts that would permit an efficient quantum yield of light at signal wavelength, i.e. hosts of adequate purity. We have developed a number of glass systems that are potentially suitable for fibre drawing trials and hence could be useful for fabricating fibre lasers and amplifiers. These glasses with Pr-ions as dopant are primarily cadmium mixed halide, hafnium halides, germanium and silicon disulphide based vitreous materials and their derivatives and modifications.

These are all potentially low phonon energy glasses which can be cast into 10-15 mm diameter rods of 10-15 cm in length for fibre drawing. All the above glasses have also excellent infrared transmission properties for other applications such as in sensors, detectors and medical devices.

As specified above, the land-based telecommunication network will utilise 1300 nm optical fibre amplifiers.

The first possible category of 1300 nm optical fibre amplifiers is based on the Pr-doped glasses which has been developed by HP & BT & . Currently fluorozirconate glass (ZBLAN) fibres are being developed for 1300nm Prdoped optical fibre amplifiers and the measured quantum efficiency is around 4 percent. The low quantum efficiency of ZBLAN fibres is limited due to large nonradiative decay process which determines the metastable lifetimes of 1G4 level to 3F4 level in Pr-doped glasses. The non-radiative lifetime is dependent on the phonon energy (580 cm 1) of the host glass, which is significantly lower than the silica (1200cm 1) glass, and permits the depletion of the pump energy (1010 nm) via multiphonon relaxation process.The larger phonon energy of the glass host increases the probability of the non-radiative decay process because the number of phonons required to provide relaxation of Pr-ions from metastable GA level is small, and hence the process becomes energetically more favourable than the glass hosts having phonon energies lower than 580 cm-l The number of phonon (p) involved in the non-radiative relaxation process can be determined by the energy gap (AE in cm-l) and the phonon energy (hw) relationship: p = /\E/hw. The relationship clearly explains the significance of hw of glass hosts for Pr-ions as do pants. Here ffE is the energy gap between the 1 and 4 3F4 levels.

Cadmium fluoride glasses have been made suitable for 1300 nm optical fibre amplifier application. In the bulk glass fabrication, the impurities must be controlled in order to eliminate the contribution of high phonon energy relaxation paths. The control of impurities and method of dopant addition has been systematically studied. The glass compositions melted are unsuitable for fibre fabrication, and the measured lifetimes are more than three times longer than the ZBLAN compositions. Table 1 summarises the effect of impurities on the measured fluorescence lifetimes from 1G4 level of Pr-ions in cadmium mixed halide glasses.

Table 1: Relationship between the fluorescence lifetime and impurities in mixed halide glasses.

Glass composition Lifetime Lifetime mole percent psec t,sec Impure glass Purified Clad glass CdF2-50,BaX2-1 0,NaX = 40, 180-210 290-330 X= F, Cl, Br, I.

Core glass CdF2-50,BaX2-1 0,NaX = 40-a, KBr = a, 180-210 290-330 X= F, Cl, Br, I.

The predicted fluorescence lifetimes in these glasses are expected to be of the order of 500 iisec. The shorter measured lifetimes in impure and relatively purified materials are due to the presence of high phonon energy impurities ( > 600 cm 1) in the glass which appear to cluster around Pr-ions and provide fast non-radiative relaxation paths. Preparatory bulk glass fabrication involves the following steps which are yet to be thoroughly optimised for achieving better results in terms of the fluorescence lifetimes, quantum efficiency and signal gain.

a) Extensive drying and fluorination of cadmium fluoride material which contains a significantly large quantities of oxides, nitrates (1018-1050 cm1), carbonates (1087 cm 1) and sulphates (1040-1210 cm~1).

b) Extensive drying and halogenation of other staring materials. These steps are described below.

c) CdF2 should be dried in a suitable fluorinating atmosphere of SF6 or HF. This drying process releases the moisture present in the fluoride powder. The other halide ingredients should similarly be dried in an atmosphere of HCl. After drying, the dried charge should be melted in an atmosphere of SF6/EF/HCl mixed with the nitrogen gas. The gas mixture should be maintained throughout the melting process.

In a successful preparation, the melting temperature was typically between 7500C-8000C. After melting the melted sample was cooled inside the furnace and after removing the crucible with frozen halide melt, the melt was remelted inside a dry glove box. The melting time was short which usually spanned over 30 minutes after which the core glass and cladding glass compositions were cast into the shape of a preform using rotational casting technique.

The prescribed melting step is radically different from the standard ZBLAN glass melting.

d) Control of glass melting procedure and dopant addition for preventing the clustering of impurities around Pr-ions. The addition of Pr-ions in particular for 1.3 ssm amplifier requires removal of oxide and related impurities from the glass prior to RE-ion addition. This is due to the higher affinity of RE-ions for oxygen related impurities.

e) Treatment of halide melts by vacuum technique prior to casting which involves elimination of gas bubbles that act as scattering centres. These bubbles however collapse during the fibre drawing stage.

f) Adopting the well-known rotational casting tech nique assists the removal of gas bubbles from the cast core-clad glass preforms.

g) Annealing of cast preforms in controlled atmosphere furnace for relieving thermal stresses and avoiding the surface contamination of bulk glass by moisture.

For this a particle vacuum furnace is the best option.

h) The use of 1000-2000 ppm mixture of HCl and HF with nitrogen gas is recommended for fibre drawing.

This has been established from thermodynamics calculations which was also verified by carrying out complementary experiments.

i) Drawing fibres with an external case around drawing furnace to prevent water attack on glass is also recommended for loss-less fibre fabrication.

j) Finally coating fibres with a suitable polymer and metal to prevent surface attack.

A good quality glass preform usually yields better quality fibres. The methods described above are recommended to achieve enhancement in the fluorescence lifetimes.

It is also possible to manipulate the local phonon energy of Pr-ions in the glass host by selecting the processing steps as well as chemistry of the dopants.

For suppressing the moisture attack on core glass, a new form of cladding glass with sodium phosphate (NaPO3) has been designed. In this glass sodium chloride from standard halide composition, listed in Table 1, is replaced by NaPO3. The range of substitution is between approximately 2 to 30 mole percent, with 10 mole percent being ideally suited for the high refractive index core (1.615). The measured refractive indices of a few phosphate compositions based on CdF2: (50), BaX2: (10) and NaX: (40-y) are listed below where y is the mole percent of NaPO3.

y, mol % refractive index Thermal Expansion coefficient, a/OC 5 1.5870 230 x 10-7 7 1.5861 10 1.5828 15 1.5804 225 x 10-7 The index and the coefficient of thermal expansion coeffient are matched to yield the value of NA greater than 0.3 which is a requirement for producing efficient Pr-doped fibre device for amplification.

The cladding glass of 7 and 10 mole percent containing phosphates have been drawn into fibres of 65 ym core diameter. The total loss measured in these fibres were 2 d B/m. The fibre does require polymer and metalcoatings for enhancing its environmental durability.

The addition of NaPO3 is carried out during the glovebox melting stage.

The disclosures in British patent application no.

94/02472.6, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

Claims (18)

1. An optical fibre comprising glass doped with praseodymium.

2. An optical fibre according to claim 1, wherein the glass is a low phonon energy glass.

3. An optical fibre according to claim 1 or 2, wherein the glass includes one or more of cadmium mixed halide, hafnium halides, germanium and silicon disulphide based vitreous materials or fluorozirconate glass fibres.

4. An optical fibre according to claims 1, 2 or 3, wherein the glass includes a core having a composition in mole percent substantially as follows: CdF2 (50%), BaX2 (10%), NaX (40-a) and KBr (a%), where X = F, C1, Br or I.

5. An optical fibre according to any preceding claim, wherein the glass includes a cladding having a composition in mole percent substantially as follows: CdF2 (50%), BaX2 (10%) and NaX (40%), where X = F, C1, Br or I.

6. An optical fibre according to any one of claims 1 to 4, wherein the glass includes a cladding having a composition in mole percent substantially as follows: CdF2 (50%), BaX2 (10%), NaX (40-j) and NaPO3 (j), where X = F, Cl, Br or I.

7. An optical fibre according to claim 6, wherein j is substantially between 2 and 30.

8. A method of producing a glass fibre doped with praseodymium, comprising the steps of providing starting compounds including cadmium fluoride and other halide ingredients; drying and fluorinating the cadmium fluoride material and drying and halogenating the halide ingredients; melting the dried material; adding a praseodymium dopant during the melting stage; and casting the melted material.

9. A method according to claim 8, wherein the dried material is dried in an atmosphere of SF6/HF/HC1 mixed with nitrogen gas.

10. A method according to claim 8 or 9, wherein the dried material is melted at a temperature of substantially 7500C to 8000C.

11. A method according to any one of claims 8 to 10, comprising the step of treating halide melts by a vacuum technique prior to casting.

12. A method according to any one of claims 8 to 11, comprising the step of annealing cast preforms in a controlled atmosphere furnace.

13. A method according to any one of claims 8 to 12, comprising the step of drawing a glass fibre from the cast in a 1000-2000 ppm mixture of HC1 and HF with nitrogen gas.

14. A method according to claim 13, wherein the fibre is drawn with a surrounding case.

15. A method according to any one of claims 8 to 14, comprising the step of coating the fibre with a polymer and metal.

16. An optical fibre amplifier including an optical fibre according to any one of claims 1 to 7 or an optical fibre produced by a method according to any one of claims 8 to 15.

17. An optical fibre substantially as hereinbefore described.

18. A method of producing an optical fibre substantially as hereinbefore described.