GB2096351A

GB2096351A - Monomode optical fibre

Info

Publication number: GB2096351A
Application number: GB8208680A
Authority: GB
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1981-04-08
Filing date: 1982-03-24
Publication date: 1982-10-13
Also published as: GB2096351B

Abstract

The present invention provides an optical fibre comprising (a) a core having a diameter in the range of from 4 to 10 mu m and comprising silica and a dopant such that the refractive index of the core exceeds that of pure silica by at least 0.001, and (b) a cladding having a diameter of at least 20 mu m and comprising silica and a dopant such that the refractive index of the cladding is less than that of pure silica by at least 0.001, said fibre being capable of monomode transmission in the 1.3 mu m window and/or the 1.55 mu m window. The fibre may be used for optical telecommunications.

Description

SPECIFICATION Optical fibres This invention relates to monomode optical fibres, i.e. fibres capable of transmitting light by propagation as a single mode. Monomode fibres which comprise a silica-based core and cladding are physically characterised by their small dimensions and have typically a core diameter from 4 to 10 Mm and a cladding diameter of at least 20 ,um, conveniently from 20 to 50 ,um. The cladding is usually further surrounded.

In order to function there must be a difference between the refractive index of the core and that of the cladding and this difference will hereinafter be referred to, as is conventional, as An. The refractive index of the core is normally greater than the refractive index of the cladding. This may be achieved, in a silica-based fibre, by the use of a core comprising silica and germanium dioxide (the latter component serving to raise the refractive index above that of pure silica) and of a cladding having a refractive index which has hitherto been similar to that of pure silica.

Monomode optical fibres find application in telecommunications, e.g. the transmission of telephone messages. Their advantages over multimode optical fibres are now being appreciated. Single mode fibres can exhibit lower signal loss levels and may be used with higher data transmission rates than multimode fibres.

The prior art and background to the present invention will now be described with reference to the first two of the accompanying drawings of which Figure 1 shows the theoretical relationships between fibre core diameter, An, fibre dispersion zero, fibre cut-off wavelength, jointing losses and bending losses in a particular class of monomode fibres; and Figure 2 shows the theoretical intrinsic loss in a particular monomode optical fibre.

These Figures and the corresponding discussion hereinafter are intended to facilitate understanding of the present invention, but relate directly to the performance of present invention only in such manner as will be apparent.

The factors affecting fibre performance are indicated in Figure 1 for monomode fibres having a core comprising silica and germanium dioxide and a cladding having a refractive index close to that of pure silica. Among the detailed comments that can be made are: (a) The lower the An value of a particular fibre the greater its susceptibility to micro-bending loss becomes; therefore, fibres having low An values, in the order of 0.003, can suffer from a great deal of signal loss if they are allowed to bend.

(b) The smaller the core diameter of a fibre becomes, the more difficult it becomes to join it effectively and the average signal loss per joint in a fibre rises with a reduction in its core diameter.

(c) Curves 8 and 9 on Figure 1 run through values of core diamger and An for fibres which have second order mode cut off wavelengths, hereinafter referred to as AcO wavelengths, of 1.2 ,um and 1.0 Mm respectively. It is undesirable for Aco to be very close to or at a fibre operating wavelength because there is total signal loss of the second order mode from a fibre at its AcO and a considerable signal loss close to its Aco Fibres having a AcO of approximately 1.2 ym or less have the advantage that they transmit by monomode propagation any wavelengths in excess of about 1.3 Mm. However, if use only at somewhat higher wavelengths is desired, then a rather higher ,lCo will be permissible corresponding to a curve rather further out from the origin than curve 8. There is little advantage unless shorter wavelengths than approximately 1.3 ,um are to be used in fibres having AcO of less than 1.0 Mm, in view of the increased microbending and jointing losses towards the origin of Figure 1.

(d) Dispersion in a monomode optical fibre is the spreading out of a pulse propagated down the fibre thus limiting the number of discrete pulses which may be sent along a fibre, per given time period, without consecutive pulses becoming confused. The cause of dispersion in such a fibre is that signals of different wavelengths propagating along the fibre travel at different velocities. This problem can be significant even when a light source having a narrow output waveband is employed. Therefore in order to maximise the rate at which data may be sent along a fibre at a particular transmission wavelength the dispersion at that wavelength should be minimised, other things being equal.By balancing the dispersion caused by the refractive index profile of the fibre with that caused by the materials from which the fibre is constructed it is possible to make a fibre having zero dispersion at, or about, a particular chosen wavelength. This wavelength is calied the dispersion zero wavelength of a fibre and hereinafter will be referred to, as is conventional, as A0. Ideally A0 in a fibre should coincide with the operating wavelength of any communications system employing the fibre if large bandwidths are required.The dependence of AQ an An and core diameter is illustrated in Figure 1 where curves 7 run through values of core diameter and An which give the same For the class of fibres shown in Figure 1, it follows that the values of An and the core diameter of the fibre should preferably lie in the region between curves 8 and 9 on Figure 1 at a point on the line of the desired refractive index.

While the detailed values, etc., in Figure 1 are not universally applicable, being based on various assumptions, a general conclusion for present purposes is that practically useful fibres having high A0 (e.g. of approximately 1.55 ,um) normally have higher An and smaller core diameters than fibres having a relatively low A0 (e.g. of approximately 1.3 Mum).

The significance of the figures 1.3 um and 1.55 ym just mentioned is apparent from Figure 2. This displays the various factors affecting theoretical intrinsic loss as a function of wavelength for a monomode fibre having a core comprising silica and germanium dioxide and a cladding having a refractive index approximately equal to that of silica. Curve 2 is the "tail" of a uv absorption band; curve 3 is part of an infrared absorption band; curve 4 represents Rayleigh scattering.The sum of curves 2 to 4, represented by curve 1, has a single minimum at rather more than 1.5 ym. However, small quantities of water incorporated in the fibre during production normally result in a substantial absorption band centred at approximately 1.4 ,um. In principle, therefore, fibres of this type should have an intrinsic loss spectrum qualitatively similar to curve 5, with two regions of low loss centred at approximately 1.3 ,um and 1.55 ,am respectively.

These regions are termed "windows". Both the 1.3 ym window and the 1.55 ym window have been considered for the carrier signal in optical telecommunications. The 1.3 ,um window is attractive above about 1.2 ,um and as far towards 1.4 ,um as the water content permits ,and the 1.55 ,um window is attractive up to about 1.7 ,um and as far down towards 1.4 ym as the water content permits.

While the detailed values in Figure 2 are not universally applicable, being based on various assumptions (including a core germanium dioxide concentration), an important general conclusion for present purposes is that if loss similar to that indicated by the theoretical curve 5 could be realised in practice, then the 1.55 Hm window would be the better one, with the lower minimum loss. The 1.55 window is also effectively larger than the 1.3 ym window in spectral width for a given acceptable loss (which may be important if there are constraints on source wavelength or if wavelength multipiexing is to be used). Moreover, the 1.3 ,um window is, on balance, the more vulnerable to water-related absorption.If the water content of a fibre is ailowed to increase, as may happen in practice, the water absorption peak 6 on curve 5 would not only become taller, or more intense, but it would also become wider.

The initially narrower window at about 1.3 ,um would be encroached upon and transmission losses in a system using this window would be increased more than those in a system using the window centred at about 1.55 ym.

Still referring to fibres in which the core comprising silica and germanium dioxide and a cladding having a refractive index similar to that of pure silica, we shali now briefly review the results that have been obtained in practice. Fibres have Tor 1.3 ym have been fabricated (for this, An~0.003 and the core germanium dioxide content 2 mole percent), and these may be used at high data rates for a transmission wavelength of 1.3 ,um; these, however, have appreciable dispersion at 1.55 ,um, which wavelength would otherwise be a preferable wavelength because of the lower loss.Fibres having At~1.55 ym so as to achieve maximum bandwidth at 1.55 ,um have also been fabricated (for this, An~0.012 and the core germanium dioxide content 10 mole percent), but these may suffer from a high level of signal loss in both the 1.3 ,um window and the 1.55 ,am window. This high level of signal loss is an absorption phenomenon which we believe is associated with the core germanium content.

An object of the present invention is the reduction of non-intrinsic loss associated with the core dopant content for a given fibre An. This is achieved according to the present invention by reducing the core contribution to the An by depressing the refractive index of the cladding by doping with, for example, fluorine. It will be appreciated that the non-intrinsic loss associated with the core dopant content is not the only loss component and that therefore the present invention is to be seen as adding to the range of possibilities for fibre production from which selection for low total loss will in practice be made having regard also to other requirements and circumstances.

It is to be noted that it is known to use cladding of slightly reduced refractive index (for example see B. J. Ainslie, C. R. Day, P. W. France, K. J.

Beales, and G. R. Newns, Electronics Letters, volume 15, pages 411-413 (1979)), the objective being, however, to prevent the cladding from acting as a secondary waveguide (the outermost pure silica layer of the waveguide in that case acting as the optical cladding). It is also known to use depressed claddings about a pure silica core, the objective of which is to permit the use of a chemically simple core.

The present invention provides an optical fibre comprising (a) a core having a diameter in the range of from 4 to 10 ym and comprising silica and a dopant such that the refractive index of the core exceeds that of pure silica by at least 0.001, and (b) a cladding having a diameter of at least 20 ,um, conveniently in the range from 20 to 50 ym, and comprising silica and a dopant such that the refractive index of the cladding is less than that of pure silica by at least 0.001, said fibre being capable of monomode transmission in the 1.3 ym window and/or the 1.55 ,um window.

For the avoidance of any possible doubt, it is now stated that the words "capable of monomode transmission" refer only to the guidance properties of the fibre, and not to the value of;l,.

Preferably, the core dopant is germanium dioxide.

Preferably, the dopant in the cladding is fluorine.

Advantageously, the cladding also contains phosphorus pentoxide. This expedient permits low deposition temperatures in MCVD (see below).

Since phosphorus pentoxide raises the refractive index of silica, the amount of fluorine in the cladding should in this case be more than sufficient to compensate for this effect.

A convenient method of producing the optical fibres in accordance with the present invention comprises drawing an appropriate preform which has been prepared by the MCVD (modified chemical vapour deposition) process. In this, layers of cladding and then of core material are deposited from an appropriate vapour mixture onto the inside of a silica tube which is then collapsed to yield the preform, which may be sleeved with another silica tube before drawing so as to achieve a particular desired aspect ratio. The vapour mixtures that may be used as appropriate are, for example, mixtures of pure oxygen with one or more of SiC14, GeCI4, POCK3, and CCl2F2 (these latter compounds providing Si, Ge, P and F respectively). Advantageously, chlorine is present as a drying agent during collapsing of the tube.

Among other methods which may be used for producing a preform for drawing into a fibre in accordance with the present invention are outside vapour phase oxidation, vapour axial deposition and plasma modified chemical vapour deposition.

Generally, suitable drawing temperatures are in the range from 1 9000C to 22000C, especially in the lower part thereof (below 20000C). Reference is hereby made to our European patent application of even date (our ref 22715/1) relating to drawing of preforms.

It will be appreciated that the present invention may be particularly advantageous in the production of fibres that, fabricated with a conventional cladding, would have high core germanium concentrations for a desired (high) An such as 0.012 for operation at 1.55,us.

The fibre according to the present invention will now be illustrated by means of the following examples. In each Example a preform was produced by MCVD, layers of cladding material and then of core material being successively deposited on the inside of a silica tube which was then collapsed in the presence of chlorine to yield a preform. The core dopant was germanium dioxide and the cladding dopants were fluorine and phosphorus pentoxide. The preforms were drawn in a carbon resistance furnace, the temperature of whose hot zone was measured by optical pyrometry. The fibres, immediately after drawing, were coated with silicone resin which was then cured at approximately 3000 C.

Example 1 A fibre was produced (the preform being drawn at 1 5 metre/min and 20500C) which had an outer diameter of 100 ym, a core diameter of 8.5 ym, a core refractive index exceeding that of pure silica by 0.0025, a cladding diameter of 50 ym, and a cladding refractive index less than that of pure silica by 0.0025. (An, it will be readily seen, was 0.005.) The fibre had a At of 1.305 ssm, loss at 1.3 ym of 0.37 4m, and loss at 1.55 ym of 0.21 dB/km.

Example 2 A fibre was produced (the preform being drawn at 1 5 metre/min and 20500C) which had an outer diameter of 100,us, a core diameter of 4.8 Mm, a core refractive index exceeding that of pure silica by 0.009, a cladding diameter of 31 ym, and a cladding refractive index less than that of pure silica by 0.004. (An, it will be readily seen, was 0.013.) The fibre had a loss at 1.3 ym of 1.3 dB/km.

Considerably improved properties would have been expected had a larger cladding diameter been used.

Example 3 Example 2 was repeated with the indentical preform, at the same drawing speed but at a drawing temperature of 1 9500C. The loss at both 1.30 Mm and 1.50 Mm was 1.1 dB/km.

Claims

1. An optical fibre comprising (a) a core having a diameter in the range of from 4 to 10 m and comprising silica and a dopant such that the refractive index of the core exceeds that of pure silica by at least 0.001, and (b) a cladding having a diameter of at least 20 ym and comprising silica and a dopant such that the refractive index of the cladding is less than that of pure silica by at least 0.001, said fibre being capable of monomode transmission in the 1.3 Mm window and/or the 1.55 Mm window.

2. An optical fibre as claimed in Claim 1, wherein the dopant in the core is germanium dioxide.

3. An optical fibre as claimed in Claim 1 or 2, wherein the dopant in the cladding is fluorine.

4. An optical fibre as claimed in any one of Claims 1 to 3, wherein the cladding diameter is in the range from 20 um to 50 Mm.

5. A method of producing an optical fibre as claimed in any one of Claims 1 to 4, which comprises producing an appropriate preform by modified chemical vapour deposition and drawing the preform.