GB1569132A

GB1569132A - Optical fibre having low mode dispersion

Info

Publication number: GB1569132A
Application number: GB35404/76A
Authority: GB
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1975-09-22
Filing date: 1976-08-25
Publication date: 1980-06-11
Also published as: SE7609777L; NL7608107A; IT1067082B; ES451740A1; CA1038669A; DE2630208A1; FR2325059A1; JPS5240144A

Description

(54) OPTICAL FIBRE HAVING LOW MODE DISPERSION (71) We, NORTHERN TELECOM LIMITED, a Canadian company of 1600 Dorchester Blvd, West, Montreal, Quebec, Canada H3H 1R1, do 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: This invention relates to optical fibres.

An optical fibre conveys, or transmits, light from an input end to an output end by the phenomenon of internal reflection. A fibre generally comprises a core surrounded by a cladding and the light is retained within the core by the internal reflection.

One of the faults of previously proposed optical fibres is "mode dispersion", which causes pulse spreading. Differential mode delay can be eliminated if a fibre is allowed to propagate only a single fundamental mode. However this requires the fibre to have a small core diameter, making fibre splicing and connecting difficult. Also single-mode laser sources are required for efficient light insertion into such a single-mode fibre. The diameter of the core cannot be arbitrarily increased by reducing the numerical aperture as too small a numerical aperture requires a thick cladding for containment of the evanescent wave and also too large a radius of curvature for low bending losses.

According to the present invention there is provided a single mode optical fibre wsprising a core and at least three rings concentrically surrounding the core, alternate ones of the core and the rings being formed of light transmitting material having a refractive index within a first range and of cladding material having a refractive index within a second range lower than and exclusive of said first range, the V values as herein defined of the light transmitting paths at the wave length of light to be transmitted, being such as to satisfy the equation

where b and a are respectively the external diameter and the internal diameter (if any) of the element providing the optical path and NA is its numerical aperture, whereby said fibre provides at least two single-mode light transmission paths each surrounded by a respective cladding layer.

In this specification and in the appended claims the V value of the solid core of the optical fibre is equal to :rD/.NA, where is the iameter of the core, while the V value of a ring of the optical fibre is equal to 2zA ç b - a .NA where a is the inner radius and b the outer radius of the ring, No being in each case the numerical aperture of the core or ring and X being the wavelength of light to be transmitted.

In one form of the invention the core and at least one said ring are formed of said light transmitting material and constitute said optical paths, the ring adjacent the core and each alternate one of of any remaining rings being of a said cladding material.

In another form of the invention the ring immediately adjacent the core and at least one other said ring are formed of said light transmissive material and constitute said light transmitting paths, the core, the ring intermediate said light transmissive rings and each alternate one of any remaining rings being of a said cladding material.

A preferred embodiment provides an optical fibre having a cross-sectional area which enables efficient coupling and a fairly large numerical aperture and which substantially eliminates mode dispersion.

The invention will be more readily understood by the following description of certain embodiments, by way of example when taken in conjunction with the accompanying diagrammatic drawings, in which: Figure 1 is a cross-section through one known form of optical fibre having a steppedrefractive index; Figure 2 illustrates the form of the refractive index curve for the fibre of Figure 1; Figure 3 is a longitudinal cross-section of a fibre of the form of Figure 1, illustrating light ray propagation therein; Figure 4 is a curve illustrating the refractive index characteristic of another known form of optical fibre; Figure 5 is a longitudinal cross-section of the known form of fibre, the refractive index characteristic of which is illustrated in Figure 4; Figure 6 is a cross-section through an optical fibre in accordance with the present invention; Figure 7 is a curve illustrating the refractive index characteristic of the fibre of Figure 6; Figure 8 is a cross-section through yet a further form of optical fibre in accordance with the present invention; Figure 9 is a curve illustrating the refractive index characteristic of the fibre of Figure 8; and Figures 10 and 11 are curves illustrating the refractive index characteristics of two further forms of optical fibre in accordance with the present invention.

As previously stated, an optical fibre conveys or transmits light from an input end to an output end by the phenomenon of internal reflection. One form of optical fibre has a core 10 and a cladding 11, as illustrated in Figure 1, with the core 10 having one refractive index and the cladding 11 a lower refractive index - as seen in Figure 2. Waveguiding occurs via internal reflection for all light rays launched within the full cone angle as illustrated in Figure 3. The cone angle ;p is given by the numberical operture (NA):

where n is the refractive index of the core 10 and n(1-A) is the refractive index of the cladding 11.

Typically, for low loss guides, A = 1/2% to 4%, NA L 0.15 to 0.42 and 4) 170 to 50".

The core diameter D NA and cladding thickness T all determine the nature of modes propagating along the fibre. Thus, for example, a light ray 12 launched at a large angle to the fibre axis (near the critical angle /2) will experience a large number of reflections at the core/cladding interface 13, compared to a light ray 14 entering at a shallower angle. At the end of a length L of optical fibre the time delay difference between these highest and lowest order modes is: T s L A = 50A ns/km, c where c is the velocity of light in a vacuum, A being in %, and n being the refractive index of the core.

This differential mode delay is termed mode dispersion and causes pulse spreading even with mono-chromatic light. The frequency - length product band-width of the step-index fibre is thus limited at about 5 to 40 MHz-km.

Differential mode dispersion is eliminated if the fibre is allowed to propagate only the single fundamental (HE11) mode. At a wavelength k this occurs if the "V-value" of the guide satisfies V a JR i,D. NA 2.405.

In a single mode operation, the fibre's information capacity is limited essentially by chromatic material dispersion (about 0.8 to 1 ns/km per 100 A of source spectral width in the GaA1As range of 8000 to 8600 ). However, this requires D, the core diameter, to be approximately equal to 1-51lm. This is a small core cross-section and makes splicing and connecting difficult. As previously stated, single-mode laser sources are required for efficient light insertion into such a single-mode fibre. The diameter D cannot be arbitrarily increased by reducing the NA, since too small an NA requires too thick a cladding for containment of the evanescent wave and too large a radius of curvature for low bending losses.

An alternative form of optical fibre has a core with a non-uniform refractive index. This is illustrated in Figures 4 and 5, Figure 4 showing the core refractive index decreasing approximately parabolically, at 16. The cladding has a lower refractive index, as in the fibre of Figures 1 to 3. As seen in Figure 5 light rays 17 follow quasi-sinusiodal paths, rather than algg-zag one. Light travels a shorter distance in regions of high refractive index than in regions of lorv index in a given time and this tends to equalize the average velocities of the Various rays. The time delay between highest and lowest order modes 17 and 18 respectively is

where K is a number ranging from 1/8 to about 2 depending upon the accuracy with which the profile is maintained; A in %.

With fibres having a non-uniform, or graded, refractive index, mode delays of 1/2 to 2 ns/km have been obtained. Manufacturing tolerances must be extremely tight if the theoretical limits of 1/64 - 1 ns/km are to be achieved. A disadvantage of graded index fibres is that they accept only half as much light from an incoherent source as do step-index fibres, and also require twice the curvature radius in bends.

In addition to the above disadvantages, graded index fibres when produced from concentric double crucibles require a fast ion diffusion exchange in the fibre drawing step.

Further, soft glasses of attenuation higher than that of fused silica are used, small cores of 30 to less than 50 > m diameter are produced, and it is difficult to attain a closely parabolic profile of the refractive index.

Graded index fibres can also be produced by chemical vapor deposition (CVD) methods, but high precision in dopand concentrations is required.

The embodiments of the present invention to be described use fibres having a "stepped" gradient for the refractive index while obtaining advantages of reduced mode dispersion.

Figures 6 and 7 illustrate such a fibre. The fibre comprises a core 20 and a series of concentric rings or layers 21. The core 20 and each alternate ring, i.e. rings 22, 24, 26, 28 and 30 are of higher refractive index than intervening and outer rings 21, 23, 25, 27, 29 and 31, as will be seen from Figure 7. In the example of Figure 6 the light is conveyed through the core 20 and rings 22, 24, 26, 28 and 30. The thickness of each light conducting ring is reduced relative to the next inner ring and the innermost ring - 22 - is of a thickness slightly less than the radius of the core 20.

All the rings 22, 24, 26, 28 and 30 have "V" values, here denoted Vr values, defined by the equation

where a and b are the inner and outer radii of a light transmitting ring. To ensure that all model group velocities are approximately equal, the V and Vr values of core 20 and rings 22,24, 26,8and30 should be the same. Hence the diameter of the core 20 and the inner and outer radii of the light transmitting rings are such that all areas are equal. If the core/cladding index differences are held constant, that is index differences between core 20 and rings 22, 24, 26, 28 and 30 and the cladding rings 21, 23, 25, 27, 29 and 31, then the light transmitting ring thickness will decrease as radius increases. It is desirable that the thicknesses of the cladding rings 21, 23, 25, 27, 29 and 31 be large enough so that evanescent field coupling between the cores or light transmitting rings is minimized, as such coupling causes some spreading in modal velocities.

However, the efficiency of input light insertion is related to the fraction of cross-sectional area occupied by core and light transmitting rays, and therefore the cladding ring thicknesses should not be too large. As an indication, 30 to 50% of the total cross-section of a fibre is an optimum to be aimed at, for the light transmitting core and rings.

The number of light conducting rings can vary, and to some extent is controlled by the NA. A large NA, for example 0.2, reduces the number of rings, and a smaller NA - 0.1 permits a larger number of rings. A small NA permits more and larger rings and a larger light source but light source must be more collimated than a small one. A larger NA requires less input light collimation and permits tighter bends. the arrangement of Figure 6 gives a constant NA, with varying light transmitting ring thickness.

Figures 8 and 9 relate to an optical fibre having a light transmitting core and a plurality of light transmitting rings of constant ring thickness, the refractive index changing in a stepwise manner along the radius of the fibre. There is a light transmitting core 35 and light transmitting rings 37 and 39, and cladding rings 36, 38 and 40.

In both arrangements as in Figures 6 and 8, the core can be of "cladding" that is of the lower refractive index and succeeding alternate rings also of cladding, with intermediate rings of high refractive index. That is, in Figure 6, the refractive index as indicated in Figure 7 can be reversed, although an outer ring of cladding will then be required.

In a further alternative, not shown, ring thicknesses and index differences are both varied suitably.

It is also possible to vary the refractive index for each ring across the ring thickness.

Typical examples are shown in Figures 10 and 11, for an optical fibre arrangement as in Figure 8, for example.

Multi ring optical fibres are readily produced by chemical vapour deposition (CVD), plasma deposition or flame hydrolysis, all known methods for producing optical fibres.

There is a particular advantage in producing rings which have a constant refractive index across their thickness in that the doping can readily be obtained. The doping level is constant and it is a matter of doping or not doping - so far as each dopant is concerned - for either a light transmitting ring or a "cladding" ring.

For a graded refractive index it is more difficult as the doping level must be varied during the production of a ring. A typical example of a process for CVD production of an optical fibre is as follows: A tube of fused silica is mounted for rotation about its longitudinal axis - the axis vertical.

Oxygen is bubbled separately through reservoirs holding Si Cl4 and GeCl4 in liquid form, the oxygen picking up a vapour from the liquid. The oxygen and vapour from each reservoir are fed to a collecting chamber plus a direct flow of oxygen. The flows are combined and fed through the fused silica tube. The tube is rotated and a flame is traversed up and down the tube. At the heated position in the tube the gases and vapour dissociate and oxidation of the silicon and germanium occur with a resultant deposition on the wall of the tube. The deposition is in the form of a sooty deposit which is fused onto the wall of the tube in the form of a glassy layer. Several passes of the flame are made to form a particular ring. The doping level is adjusted by varying the rate of oxygen flow through the germanium chloride solution.

After the required number of rings have been formed, the tube is collapsed, again by passing the flame along the tube, but with a higher temperature so that the tube softens and collapses under surface tension forces. Thus the inner ring becomes the core. The collapsed tube is then pulled into a fibre in a conventional manner, for example by feeding into a furnace and pulling from the lower end and winding on a drum.

WHAT WE CLAIM IS: 1. A single mode optical fibre comprising a core and at least three rings concentrically surrounding the core. alternate ones of the core and the rings being formed of light transmitting material having a refractive index within a first range and of cladding material having a refractive index within a second range lower than and exclusive of said first range, the V values as herein defined of the light transmitting paths at the wavelength of light to be transmitted. being such as to satisfy the equation

where b and a are respectively the external diameter and the internal diameter (if any) of the element providing the optical path and NA is its numerical aperture, whereby said fibre provides at least two single mode light transmission paths each surrounded by a respective cladding layer.

2. An optical fibre in accordance with claim 1, wherein the core and at least one said ring are formed of said light transmitting material and constitute said optical paths, the ring adjacent the core and each alternate one of any remaining rings being of a said cladding material.

3. An optical fibre in accordance with claim 1, wherein the ring immediately adjacent the core and at least one other said ring are formed of said light transmissive material and constitute said light transmitting paths, the core, the ring intermediate said light transmissive rings and each alternate one of any remaining rings being of a said cladding material.

4. An optical fibre in accordance with claim 2 or claim 3, wherein the radial thickness of the or each said light-transmitting ring is less than that of any said ring which it surrounds,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. requires less input light collimation and permits tighter bends. the arrangement of Figure 6 gives a constant NA, with varying light transmitting ring thickness. Figures 8 and 9 relate to an optical fibre having a light transmitting core and a plurality of light transmitting rings of constant ring thickness, the refractive index changing in a stepwise manner along the radius of the fibre. There is a light transmitting core 35 and light transmitting rings 37 and 39, and cladding rings 36, 38 and 40. In both arrangements as in Figures 6 and 8, the core can be of "cladding" that is of the lower refractive index and succeeding alternate rings also of cladding, with intermediate rings of high refractive index. That is, in Figure 6, the refractive index as indicated in Figure 7 can be reversed, although an outer ring of cladding will then be required. In a further alternative, not shown, ring thicknesses and index differences are both varied suitably. It is also possible to vary the refractive index for each ring across the ring thickness. Typical examples are shown in Figures 10 and 11, for an optical fibre arrangement as in Figure 8, for example. Multi ring optical fibres are readily produced by chemical vapour deposition (CVD), plasma deposition or flame hydrolysis, all known methods for producing optical fibres. There is a particular advantage in producing rings which have a constant refractive index across their thickness in that the doping can readily be obtained. The doping level is constant and it is a matter of doping or not doping - so far as each dopant is concerned - for either a light transmitting ring or a "cladding" ring. For a graded refractive index it is more difficult as the doping level must be varied during the production of a ring. A typical example of a process for CVD production of an optical fibre is as follows: A tube of fused silica is mounted for rotation about its longitudinal axis - the axis vertical. Oxygen is bubbled separately through reservoirs holding Si Cl4 and GeCl4 in liquid form, the oxygen picking up a vapour from the liquid. The oxygen and vapour from each reservoir are fed to a collecting chamber plus a direct flow of oxygen. The flows are combined and fed through the fused silica tube. The tube is rotated and a flame is traversed up and down the tube. At the heated position in the tube the gases and vapour dissociate and oxidation of the silicon and germanium occur with a resultant deposition on the wall of the tube. The deposition is in the form of a sooty deposit which is fused onto the wall of the tube in the form of a glassy layer. Several passes of the flame are made to form a particular ring. The doping level is adjusted by varying the rate of oxygen flow through the germanium chloride solution. After the required number of rings have been formed, the tube is collapsed, again by passing the flame along the tube, but with a higher temperature so that the tube softens and collapses under surface tension forces. Thus the inner ring becomes the core. The collapsed tube is then pulled into a fibre in a conventional manner, for example by feeding into a furnace and pulling from the lower end and winding on a drum. WHAT WE CLAIM IS:

1. A single mode optical fibre comprising a core and at least three rings concentrically surrounding the core. alternate ones of the core and the rings being formed of light transmitting material having a refractive index within a first range and of cladding material having a refractive index within a second range lower than and exclusive of said first range, the V values as herein defined of the light transmitting paths at the wavelength of light to be transmitted. being such as to satisfy the equation

or of the core, respectively and wherein the refractive index of all said light-transmitting members is substantially the same.

5. An optical fibre in accordance with claim 2 or claim 3, wherein the refractive index of the or each said light-transmitting ring is less than that of any said ring which it surrounds, or of the core, respectively, and wherein the radial thicknesses of all of said lighttransmitting members are substantially the same.

6. An optical fibre substantially as described herein with reference to Figures 6 and 7 of the accompanying drawings.

7. An optical fibre substantially as described herein with reference to Figures 8 and 9 of the accompanying drawings.

8. An optical fibre substantially as herein described with reference to Figure 10 or Figure 11 of the accompanying drawings.