CA1122079A

CA1122079A - Manufacture of monomode fibers

Info

Publication number: CA1122079A
Application number: CA000334464A
Authority: CA
Inventors: Koichi Abe
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1979-08-27
Filing date: 1979-08-27
Publication date: 1982-04-20
Also published as: GB2071351A; JPS5635108A; IT1130698B; IT8022414A0; SE8005973L; NL8003105A

Abstract

MANUFACTURE OF MONOMODE FIBERS

Abstract of the Disclosure A monomode fiber has a core, cladding layer and jacketing layer. The cladding layer has a refractive index which is a predetermined value lower than that of the core and a predetermined value higher than that of the jacketing layer. The jacket can be formed by a fused silica tube substrate, the cladding layer of phosphorus doped fused silica and the core of doped fused silica, being doped with either phosphorus or germanium,or both.

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Description

This invention relates to monomode optical fibers and is concerned with a particular form of such a fiber.
The bandwidth of a multimode optical fiber is limited by intermodal dispersion and by chromatic dispersion. The intermodal dispersion can be minimized by closely equalizing optical path lengths of various modes.
To achieve this, the index profile across the fiber core has to be controlled very precisely. In the currently used fiber fabrication technologies, it is difficult to achieve the optimum index profile consistently. Alternatively, intermodal dispersion can be eliminated entirely by restricting the guided mode to the lowest order mode (monomode fiber).
The chromatic dispersion is determined mainly by the intrinsic property of the materials used to construct the fiber and the spectral bandwidthof the light source. In silica-based glasses, the chromatic effect is minimal at the wavelength of approximately 1.3 ~m(l).
In monomode fibers, a larger portion of the guided light is carried through the cladding layer compared to the multimode fibers. ThereFore,both core and cladding materials have to be chemically deposited to minimize theattenuation.
Boron-doped silica has been commonly used for the cladding because of its lower refractive index relative to the pure silica used for the core. A drawback of this fiber design is the high attenuation at the wavelengthrange of 1.3 ~m due to the intrinsic optical absorption of boron-doped silica.
Pure silica is the only cladding material used to date to make a monomode fiber with a low attenuation at the wavelength of 1.3 ~m. The core is made of either germanium or phosphorus doped silica. In this design, the deposition of the cladding requires a sustained high temperature which tends to give rise to serious distortion of the substrate tube with subsequent deformation of the coreand the cladding. This technical difficulty can be overcome by utilizing sophisticated tube diameter monitoring and pressure-controlling systems.

A theoretical requirement to design monomode fiber is given by V-value defined by:
V = 2~.a.NA

For the operating wavelength (~), the core radius (a) and the numerical aperature (NA) have to be chosen to make the V-value smaller than 2.405 for step-index profiles. For a parabolic profile, V <3.518, other profiles will have their unique maximal V-values. To minimize losses due to input coupling9 jointing, and bending, it is desirable to choose the core size and NA as large as possiblewithout violating the monomode requirement. However, even if the V value is slightly larger than the cut-off value, higher modes are usually lossy.
Practically such a quasi-monomode fiber can operate as a monomode fiber for a sufficiently long fiber length.
By utilizing an appropriate choice of dopants and a new fiber design, monomode fibers with low loss at the 1.3 ~m wavelength range can be fabricated. The modified chemical vapour deposition technique, well established for multimode fiber fabrication can be used withou~ any special modifications.
Phosphorus is incorporated into the silica cladding layer as a flux, the amount being chosen to satisfy the following two requirements: the deposition temperature being sufficiently low so as to avoid any distortion of the substrate tube, and the refractive lndex increase sufficiently small so as to minimize the amount of light sarried through the cladding layer, which has a refractive index slightly higher than that of the substrate silica tube. In a particular example, a germanium doped silica core is deposited following the phosphorus doped silica cladding deposition.
~ In its broadest aspect, the present invention provides a ;~ monomode fiber comprising core, cladding layer and jacketing layer. The cladding layer has a refractive index which is a predetermined value lower than that of the core and a predetermined value higher than that of the jacketing layer.

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Z~79 The invention will be readily understood by the following description of certain embodiments, by way of example in conjunction with the accompanying drawings, in which:-Figures 1 and 2 are cross-sections through conventional forms of multimode and monomode fibers, Figures 3 and 4 are diagrammatic representations of the refractive index of the two known forms of monomode fiber, being representative of across a fiber parallel to the longitudinal axis of the Fiber;
Figure 5 is a diagrammatic representation of the refractive index across a fiber in accordance with $he present invention, Figure 6 is a typical spectral attenuation curve of a fiber, in accordance with the present invention;
Figure 7 is a d;agrammatic illustration of one form of apparatus for making a preform for drawing into a fiber.
Figures 1 and 2 illustrate a multimode fiber 10 and a mono-mode fiber 11 respectively. The multimode fiber lO has a core 12 and a cladding layer 13. Typically the cladding can be fused silica and the core doped fused silica having a refractive index slightly higher than that of the cladding.
Alternatively the core can be fused silica and the cladding of doped fused siiica, . ~ 20 doped so as to have a lower refractive index than the core. Typical dimenslons ;
are 50 ~m for the core diameter and 125 ~m for the outside diameter of the cladding, although these dimensions can vary.
The monomode fiber 11 has a core 14, a cladding layer 15 and a jacket 16. Typical d1mensions for fiber 11 are approx;mately lO ~m for the diameter of the core~ 50 to 70 ~m for the outside diameter of the cladding and 125-150 ~m for the outside diameter of the iacket. These dimensions may vary slightly. It will be seen that the total quantity of deposited mater1al is approximately the same in both monomode and multimode fibers, that is, the core 12 in the multimode fiber and the core 14 and cladding 15 in the monomode fiber.

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~ -For the monomode fiber, 11, as stated previously, boron-doped silica has been used for the cladding with pure silica ~or the core~
deposited in a pure silica tube, for exatnple. The index of refraction profile for such a fiber is illustrated in Figure 3. Such a fiber has high attenuation at the wavelength range of 1.3 ~m. For low attenuation at 1.3 ~m~ pure fused silica has been used for the cladding layer. The core is of either germanium or phosphorus doped silica. The index of refraction profile for this latter form of fiber is illustrated in Figure ~.
The present invention provides a form of fiber which avoids the high attenuation characteristics of boron-doped silica cladding (Figures 2 & 3) and also avoids the manufacturing problems of pure fused silica cladding (Figures 2 & 4). The jacket, 16 in Figure 2, is of pure fused silica, for example from an original fused silica tube substrate, the cladding is of doped silica, phosphorus being used as a dopant and as a fusion temperature reducing flux or additive, and the core is also of doped fused silica, the core material being doped with either phosphorus or germanium or both. With phosphorus doping, the doping level will be higher, in the core, than in the cladding.
The resultant fiber has an index of refraction profile as in Figure 5. The core/clad light constitutes the monomode, but the cladding may also work as an effective multimode core utilizing the silica jacket as a cladding. However,for long fiber lengths (over lkm), almost all of the lossy modes reflecting at the cladding/jacket interface are attenuated and only monomode light remains.
Although the multimode cladding light is carried if the fiber is too short or if the amount of phosphorus in the cladding is too large, such light can be eliminated by locally removing the silica jacket and applying a cladding mode stripper. This technique is used to measure the attenuation of this type of monomode fiber.
A typical spectral attenuation curve of a fiber in accordance with the present invention is illustrated in Figure 6.

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- : . - . -Z~79 , Figure 7 illustrates one form oF apparatus for manufacturing preforms for drawing with fibers. Silicon tetrachloride is held, in liquid form in reservoir 20 and phosphorus oxychloride in liquid form in reservoir 21.
Oxygen is fed, via pipe 22 and pipes 23 and 24 into reservoirs 20 and 21 respectively, the oxygen bubbling through the liquids in the reservoirs, and in so doing carrying vapour from each liquid. The oxygen and vapour from each reservoir passes through pipes 25 and 26 to a collecting chamber 27. Oxygen is also fed direct from pipe 22 directly to the collecting chamber 27 via pipe 28. A control valve 29 is provided in each pipe 23, 24 and 28. In pipes 25 and 26 a monitoring device 30 is provided to monitor the oxygen/vapour composi-tion the monitoring devices controlling the valves 29 to maintain a preset composition by controlling the flow. Flow indicators 31 can also be provided, and a control valve 32 is provided in the oxygen pipe 28.
From the collecting chamber 27 the mixed vapour and oxygen pass via pipe 33 to flow through a glass tube 34, of fused silica. The tube 34 is rotated and a flame from a torch or burner 35 is traversed up and down the tube, the burner fed oxygen and hydrogen via pipes 36 and 37 respectively.
The gases and vapours dissociate as the burner is traversed to form a sooty ; or particulate material, which then gives a resultant deposition, in the example of silica and phosphorus, on the inner wall of the tube, in the form of a sooty deposit, which is fused onto the inner wall in the form of a glassy film. This is a conventional so-called modified chemical vapour deposition method.
If phosphorus is to be added to both cladding and core material, after the necessary number of passes of the burner to deposit and form the required thickness of cladding material, the supply of phosphorus is increased and a further pass of the burner made to deposit the core material.
If the core material is to have a different additive, for .. . . .
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example germanium, then a further reservoir with associated piping control valve, and monitor can be provided as indicated in dotted outline, and with references 40, 41, 42, 43 and 44 respectively. Then for forming the core material, control valve 29 from the phosphorus oxychloride reservoir is shut off and the control valve for the germanium tetrachloride reservoir is opened.
It is also possible to supply vapour containing both - phosphorus and germanium if desired. After deposition of the core material, the tube 3~ is collapsed to a solid preform~ by increasing the temperature of the flame of the burner 35 to collapse the tube by surface tension. The solid preform can then be used for pulling into a fiber.
A typical example of making a fiber is as follows:-- Flow rate of silicon tetrachloride approximately 100 cc/min;
flow rate of phosphorus oxychloride approximately 2 cc/min (total including carrier gas, about 600 cc/minute); number of passes, about 25; temperature about 1500O. This forms a cladding layer. Then a single pass to deposit the core material is made as follows; silicon tetrachloride about 75 cc/min;
germanium tetrachloride about 23 cc/min; again at about 1500C. The tube is then collapsed by heating to about 2100C - movlng the burner slowly along the tube.
The particular flow rates, relative values of the consti-` ~ tuents, and the temperatures are not critical in so far as the present invention is concerned and can vary in the manner as in other chemical vapour depositions inside a tube. Other doping materials can be used, as in other processes.
After the tube is collapsed to a rod, it may be placed in afurther tube and the combined tube and rod drawn to produce a fiber with desired core and cladding diameters. This is indicated in Figure 5 where the dotted lines 50 represent the refractive index of the tube. Alternatively, the preform may be drawn down into a fiber wlthout lnsertion in a further tube.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A monomode optical fiber comprising;
a core of pure fused silica or doped fused silica, a cladding layer on said core, the cladding layer of doped fused silica or pure fused silica, the relative doping of the core and the cladding layer being such that the refractive index of the cladding layer is a predetermined value lower than the refractive index of the core;
and a jacketing layer of fused silica which has a refractive index a predetermined value lower than the cladding layer.

2. A fiber as claimed in claim 1, including a further layer on said jacketing layer.

3. A fiber as claimed in claim 1, including a further layer on said jacketing layer, said further layer having the same refractive index as said jacketing layer.

4. A fiber as claimed in claim 1, said jacketing layer of pure fused silica, said cladding layer of doped fused silica and the core of doped fused silica.

5. A fiber as claimed in claim 4, said cladding layer being doped with phosphorus.

6. A fiber as claimed in claim 4, said core being doped with phosphorus.

7. A fiber as claimed in claim 4, said core being doped with germanium.

8. A fiber as claimed in claim 4, said core having an outside diameter of approximately 10 µm and said cladding layer having an outside diameter of between about 50 µm and about 70 µm.

9. A fiber as claimed in claim 8, said jacketing layer having an outside layer of between about 125 µm and about 150 µm.

10. A method of manufacturing a monomode fiber, comprising;
(i) mounting a fused silica tube for rotation about its axis;
(ii) passing a mixture of oxygen, silicon tetrachloride vapour and a vapour containing phosphorus through said tube;
(iii) rotating said tube and traversing a heating means relative to said tube to form a localized heated area in said tube to dissociate said oxygen and said vapours, silicon being oxidized by said oxygen and phosphorus combining with said oxidized silicon in said heated area to form a particulate material said material then depositing on an inner wall of said tube, the remainder of the oxygen and vapours flowing from said tube;
(iv) fusing the deposit on said inner wall to form a glassy film of fused silica with phosphorus as a dopant;
(v) continuing steps (iii) and (iv) a predetermined number of times;
(vi) passing a mixture of oxygen, silicon tetrachloride vapour and a vapour containing at least one of the group consisting of phosphorus and germanium through said tube;

(vii) traversing said heating means relative to said tube while still rotating to form a localized heated area in said tube to dissociate said oxygen and said vapours, silicon being oxidized by said oxygen and said at least one of said phosphorus and germanium combining with said oxidized silicon in said heated area to form a particulate material, said material then being deposited on said inner wall on said fused deposit, the remainder of the oxygen and vapours flowing from said tube;
(viii) fusing the deposit to form a further glassy film of fused silica doped with at least one of said phosphorus and germanium.

11. A method as claimed in claim 10 further comprising:-(ix) increasing the heating of said tube to collapse the tube to a solid rod.

12. A method as claimed in claim 11 further including drawing said solid rod to a fiber.

13. A method as claimed in claim 11 further including pushing said solid rod into a closely fitting glass tube.

14. A method as claimed in claim 13, further including drawing said tube and rod to a fiber.

15. A method as claimed in claim 10, wherein steps (vii) and (viii) are carried out once only.

16. A method as claimed in claim 10, wherein steps (iii) and (iv) are continued for about twenty-five passes.