CA1340929C - Polymer claddings for optical fibre waveguides - Google Patents

Abstract

An optical fiber comprises a core coated with a cladding having a lower refractive index than the core, the cladding composition comprising a fluorinated monoacrylate, a polyfunctional cross-linking acrylate being difunctional or higher, a photoinitiator, and optionally at least one of a thermal stabilizer and antioxidant, the composition comprising lees than 0.3% by weight of a mono- or polyfunctional thiol and being cured or cross-linked.
Optical fibers are used as a means of transmitting data.

Description

_ 1 _ 1340929 POLYMER CLADDINGS FOR OPTICAL FIBRE WAVEGUIDES
This invention relates to opt=ical fibres and in particular to optical fibres comprising a glass core coated with a polymer cladding.
The use of optical fibres as a means of transmitting data has received widespread interest. Information transfer using a modulated light beam guided by a glass or plastics fibre has been utilized in many applic<~tions including telecommunications and computer link-up and data base use.
Advantages of the use of fibre optic linkages are very high information carrying capacity compared to metal wires carrying electrical signals and freedom from ext=ernal interference.
Optical fibres comprise a core, generally an inorganic glass such as fused silica or a synthetic resin, and a cladding of a material having a lower refractive index than the core, which cladding confines the .Light energy to propagate in the core by total internal reflection. The refractive index of fused silica is 1.458 at room temperature and there is a limited range of materials which have refractive indices below this value. The efficiency of propagation increases as the difference in refractive index between the core increases. The refractive index of the cladding should be at least 0.03 units less than that of the core, preferably at least 0.05 units less.
Prior art cladding materials include thermoplastic polymers which are coated on the optical fibre by melt extrusion through a die. This method of cladding suffers from the disadvantages that it is difficult to obtain thin coatings and the coatings tend to be loosely bonded to the silica core.
Other polymers have been applied by so7_vent coating. However, solvent coating has the disadvantages that it may be necessary to coat a fibre several times until the desired thickness is - la -obtained and the n.ecessi.ty of handling high solids solutions with attendant prcblems of bubbling of the coating.
Furthermore, there is the additional problem of pollution of the environment during evaporation of the solvent.
Cross-linkable polymeric coating compositions have been employed which are rapidly cured after coating by heating or exposure to ultra violet light. Examples of such compositions are disclosed in United States Patent Nos. 4099837, 4125644 and 4511209.
British Patent No. 1262526 discloses an optical element, e.g., a lens, view aperture, etc., comprising a solid transparent base, e.g., a thermoplastics material, on which is coated a fluorine-containing transparent, thermoset organic polymer having an index of refraction within 0.02 units of the base. The polymer is formed from a fluorine-free acrylic monomer and a fluorine-containing acrylic monomer, at least one of which monomers being polyfunctional. The specific coating compositions disclosed utilise at least 70% by weight of the fluorine free acrylic monomer.
United States Patent No. 4511209 discloses a cladding composition for plastic clad silica optical fibres comprising:
a highly fluarinated monofunctional acrylate with a refractive index below 1.38 and constituting more than 50%
by weight of the composition, a polyfunctional acrylate being trifunctional or higher serving as a cross-linking agent, a mono- or polyfunctional thiol that functions as a synergist, preferably a thiol containing silane, e.g., gamma-mercaptopropyl trimethoxy silane, and a photoinitiator.
The cladding compositions are dip or spray coated onto the fibre and exposed to ultra violet radiation to cure the coating. The hard clad optical fibres produced have attenuation aften below 10 dB/km and exhibit superior temperature behaviour than silicone clad optical fibres.
The present invention provides alternative cladding formulations for optical fibres.

'340929 _3.
Therefore, according to the present invention is provided an optical fibre comprising a core coated with a cladding having a refractive index,: less than that of the core. The cladding composition comprises at least one fluorinated mono-acrylate, a polyfunctional cross-linking acrylate which is difunctional or higher, and a photoinitiator. The cladding composition also comprises less than 0.3~~ (by weight) of a mono- or polyfunctional thiol and is either cured or crosslinked. The mono-acrylates of t:he cladding composition are selected from either or both of the following groups:
(i) thos;e fluorinated mono-acrylates containing a fluorinated cycloaliphatic radical which has a minimum of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and (ii) compounds of the general formula R O X
RfSOzN--~; CHz~OC-C=CHZ
in which X is a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, x is 1. or 2, R is an alkyl group of 1 to 5 carbon atoms, and Rf is a fluoroaliphatic radical which has a minimum of three C-F bonds or in which at least 25%
of the C-H bonds have been replaced by C-F, whichever degree of fluorination is higher. Also according to the present invention there is provided a cladding composition for optical fibres comprising from 50% to 95%
(by weight) of a fluorinated mono-acrylate selected from either group (i) or group (ii) above, from 2% to 35% (by weight) of a p~~lyfunctional crossl:inking acrylate which is difunctional or higher, and from 0.5% to 20% (by weight) of a plzotoinitiator. The r_ladding composition comprises less than 0.3% (by weight.) of a mono- or polyfunctional thiol. The cladding composition of the invention may oeadily be applied by dip coating and can be immediately photopolymerised to cause curing or cross-linking, by e:~cposure to ultraviolet light to provide optical fibre; having equivalent and often superior properties to those of the prior art. In particular the adhesion to g:Lass of the cladding composition of the invention is ;superior to that of the composition of the United States Patent No. 4,511,209. If the formulations of the invent:LOn additionally comprise other vinyl functionalize<i components, e.g., I;meth)acrylic silanes and (meth)acr~tlic acid, adhesion to glass is further increased.
The cladding composition of the invention differ from the compositions of United States Patent No.
4,511,209 in 1=hat they do not require the presence of a thiol synergi;~t. The function of the synergist is not defined although suitable compounds are identified as being of the gamma-mercaptopropyl trimethoxy silane type.
The most likely function of these compounds is to chain transfer the polymer chain to the mercapto function and then via the :~ilane, bond the polymer to the core surface. This results in the polymer matrix having silane groups appended only at the termini of the polymer chains. In the composition of the present invention when adhesion enhancers, e.g., acrylic silanes, are employed it results in the incorporation of the adhesion enhancer through the matrix thereby significantly promoting adhesion.
The fluorinated mono-acrylates used in the compositions of the invention possess one polymerisable vinyl group. The term acrylate is used in the generic sense and inc:Ludes not only derivatives of acrylic acid, but also methacrylic and other modified acrylic acids.
The fluorinated mono-acrylates possess a fluoro aliphatic group in which either a minimum of three C-F
bonds are present or 25~ of the C--H bonds have been replaced by C--F bonds, whichever degree of fluorination is greater.
The fluoroaliphatic radical i_s generally a fluorinated, preferably saturated, monovalent, non _5_ 1340929 aromatic, aliphatic radical of at least two carbon atoms.
the chain may be straight, branched, or, if sufficiently large, cyclic, and may be interrupted by oxygen atoms or nitrogen atoms bonded only to carbon atoms. A fully fluorinated group is preferred, but hydrogen or chlorine atoms may be present as substituents in the fluorinated aliphatic radical; generally not more than one atom of either is present in the radical for every two carbon atoms. Preferably the radical contains a terminal perfluoromethyl group. Preferably, the fluorinated aliphatic .radical cortai.ns not more than 20 carbon atoms.
More preferably the fluorinated aliphatic radical is cycloaliphatic, such as perfluorohexyl or perfluoropentyl.
The presence of a fluarinated cycloa:Liphatic radical provides the unc~bvious advantage of a tougher cured coating composition in which thermal and oxidative degradation process are retarded compared to acyclic compositions. It is then possible to extend the use of the cladded fibre to a broader temperature operating range without sacrificing the optical clarity and colorlessness of the coating.
Typical fluorinated mono-acrylates used in the invention are selected from the group consisting of compounds of the general formulae O X R O X
Y-C"Zy--~CHZ-~~ -OC-C---CHZ and RfSO2N-fCH2-~-OC-C=CHZ
wherein Y and Z are, independently, members selected from the group consisting of H, F, and C1, n is an integer from 3 to 12, q is an integer from 4 to 24, m is 0, 1, o~~ 2, and X, Rf, R, and x are as previously defined, with the proviso that, with respect to the cyclic group C"Zq, not more than one atom of hydrogen or chlorine is present for every two carbon atoms. Specific examples of compounds represented by the first formula include 1,1-dihydroperfluorocyclohexane carbinol acrylate, 1,1-dihydroperfluorocyclohexane carbinol methacry late , 1,1-dihydroperfluorocyclopentane carbinol acrylate, 1,1-dihydroperfluorocyclopentane carbinol metlzacrylate, Compound: represented by the second formula contain heteroatoms, ca.g., sulphur and nitrogen, outside the fluoroaliphat:ic radical. Example:a of such compounds include 2-(N-eth~~l perfluorooctane sulphonamido)ethyl acr~Tlate , 2-(N-ethyl perfluorooctane sulphonamido)ethyl methacrylate, 2-(N-but~Tl perfluorooctane sulphonamido)ethyl acr~~late .
Mixtures of t~ao or more fluorinated mono-acrylates may also be employed.
The poly-functional cross-linking acrylates used in the invention are at least difunct;ional, preferably trifunctional or higher. The compounds generally have a molecular weight of less than 600..
Typical i;.ri- and tetrafunctional acrylates have the general formu:La:
O X

3 0 R' CHIC ( CHI-O-C-C=CHZ ) 3 in which:
X is as defined above, and R' represents an alkyl group, generally of 1 to 5 carbon atoms (e.g. methyl), hydroxy, or -O.COC(X)CHZ in which X is as defined above.
Suitable difunctional acrylates are of the general f ormu 1 a 1340g2g t it CHZ=C-C-a--~CHZ~C-C=CHZ
in which:
each X i:a as defined above and p is an :integer from 3 to 8.
Examples of cross-linking acrylates include:
Trimethylol propane tri(meth)acrylate, 1,4-butanedio:l di(meth)acrylate, 1,3-butanedio:L di(meth)acrylate, 1,6-hexanedio:L di(meth)acrylate, Pentaerythritol tetra(meth)acrylate, Pentaerythritol tri(meth)acrylate, Dipentaerythr:~tol penta(meth)acrylate, and Hydantoin hex<iacrylate.
Mixtures of cross-linking acrylates may be employed.
The photoinitiator may comprise any of the photoinitiato~.~s known in the art, e.g., hydroxyacetophenone type photoinitiators. Examples of photoinitiators are those commercially available from Ciba Geigy under the Trade Marks Irgacure 651, Irgacure 500, Irgacure 184, and those commercially available from Merck under the Trade Marks Darocur 1173 (2-hydroxy-2-methyl-1-phen'tl-1-propanone) and I)arocur 1116 (2-hydroxy-2-methyl-1-(4--isopropylphenyl)-1-propanone).
In general suitable cladding compositions will have the following formulation:
fluorinai=ed mono-acrylate 50 to 95% by weight cross-linking acrylate 2 to 35% by weight photoinii~iator 0.5 to 20% by weight.
Preferab:Ly the components are selected within the following ranges:
fluorinai~ed mono-acrylate 75 to 95% by weight cross-linking acrylate 2 to 10% by weight photoinii:iator 0.5 to 10% by weight.
In addition to the three components, the compositions preferably include an adhesion enhancer, e.g., a compound having a vinyl functionality and being _8_ different from the fluorinated mono-acrylate and the cross-linking acrylate.
The adhe:aion enhancers possess a single polymerisable vinyl group, and may be methacrylate, preferably acrylate silanes such as 3-tri(m)ethoxy-silylpropyl(mcah)acrylate. Alternatively acrylic or methacrylic ac: id may be used as an adhesion enhancer.
The adhe:~ion enhancers generally increase the bonding strength of the cladding composition to the silica by at :least 10%, generally at least 20% compared with cladding compositions in which the adhesion enhancers are absent. Generally, the adhesion improves as the conceni=ration of adhesion enhancer is increased.
However, the refractive index of the cladding material also rises as the proportion of non-fluorinated material is increased, which is undesirable. Thus a compromise must be made, and best results have been obtained with the adhesion sanhancer present in the range 1 to 25%, preferably 2 i:o 15% by weight of t:he composition.
The cladding compositions may also comprise a thermal stabi:Lizer/antioxidant. how loss optical fibres must pass a si:ringent cold/hot temperature cycling test.
the optical f:Lbres are kept at -6°_i°C for four hours and monitored as <~ function of loss in dB/Km. The fibres are then brought hack to room temperature for two hours and thereafter he:Ld at +125°C for four hours. Again the temperature i:~ brought to ambient for two hours and the loss of the optical fibre is recorded. It has been found that the preseance of up to 5%, generally 0.001% to 5%, preferably 0.01 to 1% by weight of: one or more thermal stabilizers/antioxidants compatible with the cladding composition provides improved stability to the cold/hot cycling test. Suitable stabilizers/antioxidants include low melting hindered phenols and t:hioesters. Specific examples include 2,6-di-tert-butyl-4-methyl phenol commercially <~vailable under the ~~rade ~'iark Ultranox 226, octadecyl-3,5--di-tert-butyl-4-hydroxyhydrocinnamate -8a-commercially available under the grade ~elarl~ Irganox 1076, 2,6-di-tert-butyl-4-sec-butyl phenol commercially available under the Trade Marks Isonox 132 or Vanox 1320, and dilauryl ~thiodipropionate commercially available under the Trade Mark Cyanox LTDP. A combination of thioester and hindered phenol has proved to be particularly effective.
The core of the optical fibres of the invention is preferably formed of an inorganic glass, more preferably fused silica.
The cladding composition is selected to provide a refractive index lower than that of the core, preferably at least 0.03 units less, more preferably at least 0.05 units less then the refractive index of the core.
The optical fibres of the present invention may be prepared by conventional techniques employing the cladding compositions of the invention.

- ~~40929 The optical fibres of the invention may also posses a protective layer ~~uch as those known in the art. For example, a protective coating of a fluoropolymer, e.g., copoly)ethylene-tetrafluoroethylene), may be coated as an extrusion by passing the clad fibre thz~ough a melt of the fluoropolymer. A suitable fluoropolymer is commercially available from Dupont under the trade name Tefzel* 210.
The accompanying drawings illustrates suitable apparatus for preparing a coated optical fibre in accordance with the invention.
A glass fibre 2 is drawn from a pure glass preform 4 held in a furnace 6 on a standard glass drawing tower. The tower is provided with a coating station comprising a coating cup 8 containing t:he cladding composition and the metering die 10. The coated fibre is passed immediately through an ultra violet curing station 12 where it is photopolymerised and cooled on a take-up spool 14. A typical fibre will have an overall diameter of 200 microns and a cladding thickness of 10 microns.
2C The invention will now be illustrated by the following Example: .
In the F;xamples the following components were employed:
Mono-acrylate A: 2-(N-ethyl perfluoro-octane 2~ sulphonamido)ethyl acrylate commercially available from Minnesota Mining and Manufacturing Co., purified by extraction with ethanol.
*Trade-mark - 9a - ~ 3 4 ~ 9 2 9 Mono-acrylate B: 1,1--dihydroperfluoro-octylacrylate (commercially available from Minnesota Mining & Manufacturing Co.) TMPTMA: trirnethylolpropane trimethacrylate HHA: hydantoin hexa-acrylate (commercially available from Minnesota Mining &
Manufacturing Co. ) 134~92~
HDDA: hexanediol diacrylate acrylate silane: 3-trimethoxysilylpropyl methacrylate IG651: Irgacure 651 commercially available from Ciba Geigy (benzildimethylketal) IG500: Irgacure 500 commercially available from Ciba Geigy IG184: Irgacure 184 commercially available from Ciba Geigy thiol synerg~ist: gamma-mercaptopropyl trimethoxy-silane ULTRANOX: Commercially available from Borg Warner Chemicals CYNOX LTDP Commercially available from A.M.

Cynanoid DAROCUR 1116 E.M. Industries IRGANOX 1076 Ciba Geigy wa~rvr ~ 1 A cladding composition was prepared by mixing the following components:
Mono-acrylate A 92 parts by weight TMPTMA 4 parts by weight IG651 4 parts by weight Coated fibres were prepared by the technique described with reference to Figure 1 to produce coated fused silica optical fibres having an overall diameter of 200 micrometres and a cladding thickness of 10 micrometres.
The coated fibre was tested using a Photon Tektronix TM 506 spectrophotometer commercially available from Laser Precision Corporation and an attenuation figure of 4.8 dB/Km at 812 nm was recorded. Commercially available optical fibres in accordance with United States Patent No.
4511209 were tested under identical conditions and recorded an attenuation figure of 6.2 dB/Km.

_11- ~0 929 , 1 'S

Comparison with fibres of US Patent No. 4511209 The formulations reported in Table 1 were prepared.
Table 1 (parts by weiqht) Example Mono- Mono- TMPTMA IG651 thiol acrylic acry-acry- acry- silane acid late late A ate B silane 2 949: - 44 54 - - -3 89C~ - 44 54 60 - -The refractive index of the uncured formulations were measured. z~he adhesion of the cured formulations to glass was measured. by a drag test.
The drag test consisted of coating the formulation to be tested. on to a large glass slide. Lines were scored diagonally across the coating and a stylus to which a weight was attached. was applied to the coating. The stylus was pulled across the coating. The weight applied to the stylus when a half of the lines are stripped away was recorded, in grams, as a measure of adhesion.
The refractive indices of the uncured, uncross-linked formulation at sodium D line wavelength and adhesion values are reported in the following Table 2.

Table 2 Example Adhesion Refractive Index 2 240 1.393 3 <160 1.397 4 >800 1.397 5 720 1.396 Examples 2 to 5 show the refractive index of the 3 component system was lower than the comparison formulation.
Further that. the glass adhesion of the 3 component system and adhesion enhanced systems was greater than that of the thiol silane containing formulation.
Coating using mono-acrylate B (Examples 6 to 9) were subjectively much more brittle than those using mono-acrylate A.
E~pLES 10 TO 13 Comparison of the effect of fluorinated acrvlate on adhesion The formulations reported in Table 3 were prepared and refractive index and adhesion measurements were conducted as in Example 2.
Table 3 Example Mono- Mono- TMPTMA IG651 Adhesion Refrac-acrylate acrylate tive B A Index 10 928 - 57 40 240 1.356 11 - 927 54 44 640 1.392 12 922 - 49 41 <160 not measured 13 - 946 51 44 720 not measured ~ 3~+Og~g , These results indicate that. the fluoro-octyl sulphonamide acrylate monomers exhibit advantageous adhesion properties.

Comparison of the effect of photoinitiator choice on adhesion The formulations reported in Table 4 were used to coat glass slide:.. The adhesion properties of the formulations were measured as in Example 2.

Table 4 Example Mor~o- TMPTMA acrylic IG651 IG500 IG184 Ad-acryl- silane hesion ate A

All of these comme rcial photoinitiators effected acceptable re.
levels of cu Comparison of the effect of cross-linking agent selection on The formulation reported in Table 5 were used to coat glass slides. The adhesion of the formulations was measured as in Example 2.
Table 5 Example Mono- IG651acry- TMPTMA HHA HDDA Adhes-acrylate late ion silane 20 715 60 105 - 114 - (a) ~ 1200 ,.

23 702 61 100 - - 5y;~'-' 720 (a) Very poor alitycoatings led to inconsistent results qu 134~929 Effect of acry lic silane concentration on adhesion To a formulation Mono-acrylate A, IG651 comprising and TMPTMA in the weight ratio 92:4:4was added the following proportion of acrylate silane.
The adhesion to glass was measured as in Example 2.

weight % acrylate silane Adhesion 2.4 640 22.5 1040 It can be clearly seen that any amount of the acrylate silane over 1% was beneficial to core cladding adhesion.

Comparison with fibres of U.S. Patent No. 4511209 Optical fibres were prepared as in Example 1 using the following cladding compositions:
Example Mono- IG651 TMPTMA acrylic acrylate thiol acrylate acid silane synergist A

25 88 4 4 - 4 , 28t1~ 72.1 0.9 23.3 - - 3_~

____________.____________________._____________________________ Table IIi, Example 14 of U.S. Patent 4511209 1340~2~
Comparitive Attenuation at 820 nm Example Attenuation dB/Km 26 11.9 28 48.4 The presence of the thiol synergist of the prior art does not lead to lower attenuation than the cladding compositions of the invention.
Comparison of adhesion by lap shear test 0.5 inch (12.8 mm) overlap.
Example Immediately After % Change after cure 24 hrs 25 100.3 108.4 8.0 26 123.7 178.2 44.0 28 66.6 71.1 6.7 Adhesion of cladding to silica of Examples 25 and 26 containing acrylic acid or acrylate silane is considerably improved over the cladding of Example 28 containing thiol synergist. The improvement is particularly pronounced upon ageing.

Addition of thermal stabilizer Optical fibres were prepared as in Example 1 using a cladding composition of the following formulation:
% by weight mono-acrylate A 88.5 acrylate silane 2 Ultranox 226 0.5 1 3 ~O g2 ~
After i:hermal cycling at +1.25°C the fibre exhibited a permanent loss damage of 7 dB/Km compared to 10 to 14 dB/Km of fibres having the same cladding formulation without Ultranox 22fi thermal stabilizer.

Combination of two thermal stabilizers An optical fibre was prepared as in Example 1 using the following cladding formulation.
by weight mono-acrylate A 82.25 acrylai:e silane 5 Darocur 1116 2 Cyanox LTDP 0.5 Irganox 1076 0.25 After curing the clad fibre was extrusion coated with Tefzel 210 :in a conventional mariner. The permanent loss damage of the resulting buffered optical fibre was 1.1 dB/Km after being annealed at +125°C for four hours.

A cladcjing composition was prepared by mixing the following components:
1,1-dihydroperfluorocyclohexane carbinol acrylate 88 parts by weight 2-ethyl-2-()zyroxymethyl)-1,3-propanediol triacrylate 5 parts by weight 3-(trimetho:Kysilyl)propyl methacrylate 5 parts by weight Darocur 117:3 2 parts by weight Coated fibres were prepared and evaluated using the procedure according to Example 1. An attenuation figure of 5.73 dB/Km ~~t 820 nm was recorded. After curing the cladded '~3~092~
fibre was e~:trusion coated with Tefzel 210 in a conventional manner to provide a buffered optical fibre with an attenuation loss of 6.23 dB/Km a.t 820 nm. The thermal stability of the Tefzel buffered fiber was evaluated after maintaining it at 125°C for four hours and was recorded as 10.58 dB/Km.
The 1,1.-dihydroperfluorocyclohexane carbinol acrylate used in this, example was prepared according to the procedure disclosed by D. W. Codding et al., "Journal of Polymer Science", _15., 518 (1955) except that the charge was 210 g trifluoroacetic anh Bride 79.8 y , g acrylic acid, 250 g perfluorocyclohexylmethylol, and 0.1 g phenathiazine as inhibitor; the reaction mixture was stirred for 2 hours after the reaction. had subsided, and the product was purified by vacuum distillation.
A comparison was made of the Tefzel coated cladded fibre prepared in this example with a commercially available hard clad silica fibre sold by Ensign-Bickford (Simsbury, CT).
Both fibres were maintained at 125°C for four hours. The instant fibre was colorless and transparent and showed no increase in attenuation at 600 nm whereas the Ensign-Bickford fibre yellowed and showed an increase in attenuation of 120 dB/Km at 600 nm. "Transparent" means that the cladded fiber when viewed under an optical microscope (e. g., at 100X), have the property of transmitting rays of visible light so that bodies beneath the fiber, for example, such as bodies having essentially the same nature as the fiber, can be clearly seen through the fiber.
Improved properties of the cladded fiber can be realized by including antioxidant/thermal stabilizer in the formulation as has been shown in Example 30.

Claims (14)

1. An optical fiber comprising a core coated with a cladding having a lower refractive index than the core, said cladding composition comprising (a) one or more fluorinated mono-acrylates selected from the group consisting of (i) fluorinated mono-acrylates having a fluorinated cycloaliphatic radical having either a minimum of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and (ii) compounds of the general formula in which X represents H or an alkyl group of 1 to 5 carbon atoms, x is 1 or 2, R represents an alkyl group of 1 to 5 carbon atoms, and R f represents a fluoroaliphatic radical having either a minimum of three C-F bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, (b) a polyfunctional cross-linking acrylate being difunctional or higher, and (c) a photoinitiator, said composition comprising less than 0.3% (by weight) of a mono- or polyfunctional thiol and being cured or cross-linked.

2. An optical fiber as claimed in Claim 1 in which the cladding composition is free from mono- or polyfunctional thiols.

3. An optical fiber as claimed in Claim 1 in which the fluorinated mono-acrylate is selected from the group consisting of compounds of the general formula in which Y and Z are, independently, members selected from the group consisting of H, F, and Cl, X represents a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, n is an integer from 3 to 12, q is an integer from 4 to 24, and m is 0, 1, or 2, with the proviso that, with respect to the cyclic group C n Z q, not more than one atom of hydrogen or chlorine is present for every two carbon atoms, and in which X represents a hydrogen atom or an alkyl group of 1 to 5 carbon atoms;
R represents an alkyl group of 1 to 5 carbon atoms;
R f represents a fluoroaliphatic radical having either a minimum of three C-F bonds or in which at least 25% of the C-H
bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and x is 1 or 2.

4. An optical fiber as claimed in Claim 3 characterized in that the fluorinated acrylate is selected from 1,1-dihydroperfluorocyclohexane carbinol acrylate, 1,1-dihydroperfluorocyclohexane carbinol methacrylate, 1,1-dihydroperfluorocyclopentane carbinol acrylate, 1,1-dihydroperfluorocyclopentane carbinol methacrylate, 2-(N-ethyl perfluorooctane sulphonamido)ethyl acrylate, 2-(N-ethyl perfluorooctane sulphonamido)ethyl methacrylate, 2-(N-butyl perfluorooctane sulphonamido)ethyl acrylate, and mixtures thereof.

5. An optical fiber as claimed in Claim 1 characterized in that the cross-linking acrylate is selected from compounds of the general formulae in which X represents a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, R1 represents an alkyl group of 1 to 5 carbon atoms or in which X is as defined above, and p is an integer from 3 to 8.

6. An optical fiber as claimed in Claim 1 characterized in that the cross-linking acrylate is selected from trimethylol propane tri(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, di(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, and hydantoin hexaacrylate, and mixtures thereof.

7. An optical fiber as claimed in Claim 1 in which the cladding composition comprises fluorinated mono-acrylate 50% to 95% (by weight) cross-linking acrylate 2% to 35% (by weight) photoinitiator 0.5% to 20% (by weight)

8. An optical fiber as claimed in Claim 7 in which the cladding composition comprises fluorinated mono-acrylate 75% to 95% (by weight) cross-linking acrylate 2% to 10% (by weight) photoinitiator 0.5% to 10% (by weight)

9. An optical fiber as claimed in Claim 1 in which the cladding composition additionally comprises from 1% to 50% (by weight) of the total cladding composition of an adhesion enhancer having a vinyl functionality different from the fluorinated mono-acrylate and the cross-linkable acrylate.

10. An optical fiber as claimed in Claim 9 in which the adhesion enhancer is a (meth)acrylic silane.

11. An optical fiber as claimed in Claim 1 in which the cladding composition additionally comprises from 0.001% to 5%
(by weight) of the total cladding composition of a thermal stabilizer/antioxidant.

12. An optical fiber as claimed in Claim 11 in which the thermal stabilizer/antioxidant is selected from 2,6-di-tert-butyl-4-methyl-phenol, octadecyl-3,5-du-tert-butyl-4-hydroxyhydrocinnamate, 2,6-di-tert-butyl-4-sec-butyl phenol, dilauryl thiodipropionate, and combinations thereof.

13. An optical fiber as claimed in Claim 1 in which the core is fused silica and refractive index of the cladding in at least 0.03 units less than that of the core.

14. A cladding composition for optical fibers comprising (a) from 50% to 95% (by weight) of a fluorinated mono-acrylate selected from the group consisting of (i) fluorinated mono-acrylates having either a minimum of three C-F bonds or in which at least 25% of the C-H
bonds have been replaced by C-F bonds, whichever degree of fluorination is higher, and (ii) compounds of the general formula in which X represents a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, x is 1 or 2, R represents an alkyl group of 1 to 5 carbon atoms, and R f represents an fluoroaliphatic radical or a fluorinated cycloaliphatic radical having either a minimum of three C-F
bonds or in which at least 25% of the C-H bonds have been replaced by C-F bonds, whichever degree of fluorination is higher;

(b) from 2% to 35% (by weight) of a polyfunctional cross-linking acrylate being difunctional or higher; and (c) from 0.5% to 20% (by weight) of a photoinitiator, the composition comprising less than 0.3% (by weight) of a mono- or polyfunctional thoil.