CA1276824C

CA1276824C - Optical waveguides

Info

Publication number: CA1276824C
Application number: CA000465886A
Authority: CA
Inventors: Stephen Anthony Cassidy; Stephen Hornung; Raman Kashyap; Michael Harry Reeve
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1983-10-21
Filing date: 1984-10-19
Publication date: 1990-11-27
Anticipated expiration: 2007-11-27
Also published as: JPH07281064A; EP0162064B1; DE3479448T2; WO1985001802A1; JPS61500458A; GB8328204D0; JP2540293B2; JP2756529B2; DE3479448D1; EP0162064A1; US4923278A

Abstract

ABSTRACT OF THE DISCLOSURE

An optical fibre has a coating of, for example, liquid crystal polymer which causes temperature-dependent stress-induced changes in the optical fibre such as to counteract temperature-induced changes in the transmission delay of the fibre.

Description

OPTICAL ~AYEGUIDES

~hls lnvent10n relates to opt7c~1 w~vegu1des, ~nd 1n part1cul~r though not excluslvely to optlcal flbres and coatlngs for optlcal flbre ~aveguldes.
Opt1cal ~avegu~des generally compr1se an opt~cal gu~d~ng reg10n of a refract~ve lndex n1, embedded in materlal of A refract~ve 1ndex n2 where usually n2<nI. It should be observed that the guld1ng reglon ~s well as the hedd~ng materlal may themselves be structured by havlng two or more reglons of dlfferent refractlve 1nd~ces, as lllustrated by numerous known deslgns of opt~cal flbres.
It ts well known that optkal flbres of the k~nd used ~n opt k al commun1cat10ns and for optlcal f~bre sensors, for example, are usually coYered w~th ~
protect1ve coat~ng to protect the fibre surface from mechan~cal and chem~cal damage.
It 1s known also to reduce temperature dependence of transmiss~on losses of an optical fihre by appllcat~on to the flbre of an appropr~ate coat1ng. For example, publ1shed European patent appl~catlon EP-A-0076575 (nOptlcal f~bre 1nsens1tlYe to te~perature var1at~ons", Hughes A1rcraft Company) d~scloses an optical fibre su1table for operation at hlgh temperatures. Accordlng to the disclosure in EP-A-0076575 the temperature dependence of transm1ss{on losses, l.e. attenuatlon of optk al s~gnals passlng through the flbre, at hlgh operat1ng te~peratures ls reduced by apply1ng a metal coat1ng to the flbre and anneal~ng, ~he coating, whlch may be alumln1um or another metal or a met~l dlloy~ ls applled to the f1bre by pulllng the f1bre through a melt bath, for example, ~nd the flbre so coated ls then annealed at d temperature of , ~: :
~. . .
.: .

~.2~7~i~32~

several. hunclred decJreec; Ce~l.s:i.us. It ~.s ~t~ted in EP-A-0076575 that, ~rov:ided thc canneal:i.ncJ tempexclture :is suEEic:iently ll:Lgh ~or L:he transmission losses through the :E:ibre to be much the same as those at room temperature, the temperature dependence of -the transmission loss is substant.ially elimina-ted over a .range o:~ temperatures :Erom -200C to 560C.
'l'he eEfect of temperature var:ia-tions on the transm:iss.ion Loss oE opt:Lcal E:ibres :is cons:idered al.so .I.n "Op~.Lmum n~s:i.gll oF ~oak~-l Opt:ical. F:ibre~s Cons:icler;i.ncJ
~xcess at I.ow 'l'emperature", ~. Masuno ancl K. :r.sll:i.hara, .l. OE~t- Comm., 3(1982) ~, pp ;1.~2~ 5. 'rhe eC~Eect oE
temperature vaxiat:ions on transm:iss:i.on Loss :i.s considerecl with reference to optical fibres coated with a nyLon coating, and it is suggested that temperature dependence of transmission loss at low temperatures can be kept very low by employing nylon coatings with a linear t~ermal coefficient of expansion o~ the order of 10 5~C l It is an object of the present invention to reduce temperature dependence oE transmission delay in optical waveguides.
According to one aspect of the present invention a method oE r~ducing temperature dependence oE transmission delay in an optical waveguide comprises a-ttachiny the optical waveguide to a s-tressing means having a coe.E:E.icient of linear thermal expansion of opposite sign to that o~ the waveguide and arranged to substantially compensate :Eor temperature induced chanyes in optical path length :ln the wavegu;Lde by apulyiny a tempe.rature ciependent stress to the wave CJU lde.
Accordinc~ to another aspQc~t oE the pxesenk ~ venk:LQIl an opt:iGal wave~u:l.de assembl.y compr:lses an opti.cal wavegu.lde attached to stressin~ means hav:ing a coe:E:E:i.c.l.ent o:E thermal linear expans:i.on oE oppo~;:ite s:l.gn to that o~ the waveguide and arranged -to substant:Lally compensate Eor tempera-ture :inciucecl changes in optical path length i.n the waveguide by apply:ing a temperature dependent s-tress to the waveguide.
C~ , ' . ` . ' ' .
, , " ' - . ' , - . -:

- ~ ' ' ' . ' , 3~.Z'7~32~L

The opt~cal wavegu1de ls convenle~tly an optlcal f~bre. Alternat~vely, the optlcal wa~egu~de ~ay, for example, compr~se an opt~cal wavegulde structure In wh~ch the g~1d1ng reg10n 15 embedded ln a planar substrate, such os for example, a L~NbO3(Li-thiumniobate) th1n fllm ~avegu~de structure.
The stress1ng means may be attached to the ~avegu1de at dlscrete spaced pos~tlons, or ray be ~n lnt~mate contact w1th the wavegu~de or w1th an 1ntenmed~ate mater~al Itself attached to the wavegulde.
~he attachment between ~avegulde and stress1ng ~eans may be solely by ~ay of frlctlon, or ~ay be by means of, for example, an adhes1ve compound.
Preferably the stress~ng means comprises a sleeve about the wavegu~de.
Alternatlvely the stress~ng means nay, for examp~e, be a support ~ember such as, for example, a strength ~emher of an opt~cal f1bre cable.
ln a preferred embodlment of the pr~sent ~nvention the optlcal wavegu~de comprlses an optical fibre, and the stress1ng means compr1se a sleeve fonm1ng a ~acket tlghtly fltt~ng around the opt~cal f1bre or at least part of the length thereof.
Temperature-~nduced changes ~n trans~iss10n delay ~n for example, an optical flbre are caused by a comb~natlon of changes ~n the length of the fibre and ~n the refrdct~ve ~ndex of the f1bre. These changes ~n transm~ss10n delay are countera~ted ~n ~ccordance wlth the present ~nventlon by stra1n ~n, and/or changes 1n the refractlve lndex of, the flbre whlch result from the Appl1ed stress, The ~acket may be chosen such that the changes ln transm~sslon delays caused ~y the actlon of the ~acket counteract the thenmally 1nduced changes to such a degree as to substantlally compensate therefor.

~'' ' , - ' ' , .

,: '' : " ' ' 32~

In a ~urther preferred form of the present 1nvention the opttcfil flbre ~s of a ~ater~al,compos~t~on and structure such that the coefflc~ent of l~near thermal expanslon of the f~bre determ~nes the overall chanse ~n S the transm~ss10n delay, and the ~acket compr~ses mater~al hav~ng a coeff~clent of l~near thermal expans~on oppos~te to that of the f1bre.
In a yet further preferred form, the present lnvent~on compr1ses an optlcal flbre hav~ng a poslt~ve coefflclent of llnear thermal expanslon, and al ~acket of a mater~al havlng a negat1ve coeff~c~ent of l~near thermal expans~on.
The ~acket nay conveniently be formed of 3 liqu~d crystalltne polymer. The polymer may be extruded onto the optical ftbre.
The present ~nvention may be employed, for example, to provide a substant~ally temperature ~ndependent optical path length reference.
Thus, optical f~bres are known to be useful for example, as ~nterferometrlc sensor elements ow~ng to the~r ~nherent sens~t~v~ty to changes ln terperature, strain, pressure and electr~c current and magnetic fleld. Ho~ever most of these measurements 1n sens~ng ~equire the ab~l~ty to d~stingu~sh between the parameter being sensed and the other ~nfluences wh~ch may have a s~m~lar effect on the senslng propert~es of the f~bre. Th~s fs often achieved by us~ng a reference f~bre wh~ch ~s sub~ected to the sa~e lnfluences as the senslng flbre, except for the one parameter to be measured. Th~s requ~res careful layout 1n the des~gn of the sensor. Some control element ~s ~lso ~ncluded ~n the reference arm to keep track of the dr~ft ~nduced by d~fferent~a1 effects, espec~ally whcn temperature ~s a no~se source.

, - , ' '~ '' , . , .. . . .

- s -The present 1nventlon overcomes or ~t least m1tlgates some of these problems by provld~ng ~nter al~fi an optlc~l f~bre wh~ch ls coated w~th a mater~al ~h~ch has the effect of de-senslt~s~ng the optlcal delay ~n the f~bre w~th respect to changes ln temperlature.
~he present 1nvent~on w~ll now ble descr~bed further w~th reference to ~ theoretlcal ~odel, and by way of example w~th re~erenc~ to the accompany~ng draw~ngs of ~hlch:-F~gure l ~s a schemat~c cross-sect~on through a coated optlcal f1bre;
Flgure 2 ls A graphlcal representat~on of the relatlonshlp between temperature and transm~ss~on delay, as measured by an ~nterferometr~c method, of nn optlcal f~bre coated ln accordtnce w~th the present ~nvent~on; and Figures 3 to 5 111ustrate further embod~ments o~
the present ~nvention.
Referr~ng f~rst to Flgure 1, an optical ~bre l compr~ses a core 2 embedded 1n a cladding 3, and appl~ed to a primary coating 4 a coating ~ forming a tightly fi~ting ~acket to the optical f~bre 1. The coat~ng 5 wh~ch may be applled for example to a pr~mary coat~ng 4, as shown, or d~rectly to the surface of the cladd~ng 3, serves at least to counteract, or even to substant~ally compensate for, any temperature-~nduced changes ~n transmiss~on de1ay ~n the optical f~bre 1.
The propertles required o~ the coat~ng mater~a~, and the coatlng ~n gener~1, will become apparent from a br~ef out1lne of the theoret~cal b~ckground. Taklng the ex~mple of opt1c~1 flbre sens1ng dev~ces, sens~ng ln opt~cal flbres ls poss~ble as a result of the change ~n the optlcal path length, l,due to some ~nfluenc~ng cond~t~on such as ~ change ~n temperature. Thls has the ~, .~., ' - ' ~ ' ' ' .' ' ~, . ~ . .

31 ;~'7~

effect of alterin~ the group index, N, as well as the physical length of the fibre, L. The delay in a fibre can be represented by, t = NL....(1) c where c is the speed of light and 1 = NL.
When the temperature T changes, the delay is altered and the sensitivity to change is then given by, dt - l(NdL + LdN)....--(2) dT c dT dT
The change .in ~roup index and the length can be ~.ith~ add;Ltlvc or sllbtra~tive. The lenyth change is dependent on the thermal expans.ion coeEficient of ~he fibre which is large for borosilicate glasses (~10-5) and small ~or silica fibres (~10-6). The net value is generally positive with respect to temperature.
However, when a fibre is strained, it can be shown that the change in optical delay with respect to strain is given by the relation, dt = 1 (NdL + LdN)-----(3) d~ c d~ d~
where ~ is the stress induced in the fibre. In this equation, the two terms within the brackets have opposite signs and hence the overall effect is slightly reduced. However, the total effect is positive for an increase in stress.
The reader should now consider this fibre when coated with a material which has a coefficient of linear expansion opposite to that of the fibre. When the fibre is subjected to ~ t~mper~ture change, two e~fects will - ' - ~ ' . ' . :, .. .. .
.. . . . .
: . . . : -- : . . .
,, , ~ ~ , : . . . : .

- . ' . ' . . ~ :
: - . - ' ~ ~ ' ' . . - .

12~7~2~

occur. One will be the strain induced by length change as the net result of the competing coefficients of thermal expansions of the fibre and coating. The other effect will be the change in delay due to the chanye in the group index, N, as a result of the strain effect and because of the temperature dispersion of N. It can be seen that with the appropriate choice of coating material, the overall change in optical delay with respect to temp~rature can be reduced to zero. It can be shown by analysis o~ the composite structure oP Pibre ancl coating, that dt = t~K -~ lclN -~ EfdN(K -~ f)].. (4) dT NdT N da where, Ef is the Young's modulus of the fibre, a f its linear expansion coefficient and K = (ACEc~c + AfEfaf)/(ACEC + AfEf).. ( ) Here, the subscripts "c" and "f" refer to the coating and fibre respectively. A is the cross-sectional area, E is Young's modulus, and ~, the thermal expansion coefficient.
Usin~ equations 4 and 5, we arrive at the thermal expansion coefficient of the coating to be:
~C a (1 + K~/Kc)[ldt + EfafdN - ldN]/(1 + EfdN) tdT N d a NdT N d a - _f af.----(6) Kc where Kc + ACEc and Kf AfEf. For zero temperature sensitivity, dt = O.
dT

':' ~'-., - - ' ~ ' ' ' ~ 2~7~3 Uslng typlcnl values for s~ngle^mode sll1ca f~bre and us~ng parameters of the coat1ng ~s outl~ned ln table 1, we arr~ve at the requ~red thermal expans~on coeffk~ent of the coatlng to be ~pprox~mately -9*lQ ~.
A t~ght extrus~on coating package was made For sod~um boro-s111cate mult~mode and slllca monomode optkal fibres us~ng ~n or~ented thermotrop~c l~quld crystal polyester w~th a modulus of around 20GNm~2. These polymers are also called ~self re~nforc~ng" ~nd the polymer used ls a co-polyester contalnlng 73 mole /~
p-oxybenzoyl and 27 mole /- 6-oxy-2 naphthoyl Thts polymer possesses an ordered melt state wh~ch can be rearranged by shear and elongat~onal mælt flow dur~ng extruslon co~tlng. Us~ng th~s property the extrus~on 1~ cond~tlons can be tal10red to g~ve a range of polymer modul~ and thermal expans~on coeffic~ents. ~yp~ca1 condit~ons glve h~gh modul~ (20GNm 2), and low then~al expans~on coeff~clent (5 x 10-6) when c~mpared to conventional polymers.
The f~bres (two samples of graded ~ndex sodium boroslllcate mult~mode and two samples of s~llca monomode) were 125~m ln d~ameter w~th a s~l~cone rubber coat~ng br~ng~ng the total d~ameter to 250~m. They were coated using a conventional 19mm s~ngle-screw extruder. The key parameter ln determ~n~ng the degree of polymer orientat~on ~nduced by the extruslon process ls the draw ratio, given by the rat~o of the cross-sectlonal area of the dle to the cross-sect~onal ~rea of the coat~ng. FQr example, for sample I ~ lmm dle was used and the llne was run w~th the extruslon rate equal to the haul-off rate, Allow~ng the coatlng d~meter to rema~n ~t Imr. Th~s llmlts the or~entat~on process g~vlng low values of both ~ and E. In contrast~ for sample 2, a lsrger (2mm) die was used and the st~ll molten polymer was pulled down after :

: . , ... ~ , : - , ., - . ~ .

~7~324 extrusion. This increases the degree of orientation, and hence the resultant larger values of a and E. The extrusion conditions allow the alteration of theac from a small negative to a small positive value. The coefficients of linear expansion have been measured as follows:
Sample Strain Dia Line Die a ~10 6 E
No. Speed Dia.
Sodium Borosilicate multimode fibre.
1. 0.30~ l.Omm 17.5m/s lmm -~.313.0GNm 2 2~ 0.29~ l.Omm 22.Om/s 2mm -~;.326.5GNm 2 Silica Monomodc ~ibre 3. 0.05~ 0.9mm lO.Om/s 2mm -3.721.lGNm 2 4. 0.09% 2.Omm l.Om/s 2mm -9.7GNm 2 It is immediately apparent that there is a very high level of strain locked into the sodium borosilicate glass fibre samples, and a very low level in the silica fibre. This is caused by the difference in a-values of the borosilicate and the silica glasses (10-5 and 5*10-7 respectively) compared to the~ -value of the polymer. The polymer is deposited on fibre heated and expanded by it's passage through the extruder. As the polymer and glass cool the strong and now solidified polymer prevents the fibre from contracting and the fibre therefore remains in tension. This effect is greater for sodium borosilicate glass due to its large a . The expansion coefficient of the fibre material is therefore important in conjunction with this polymer.
For one sample, it was noted that the temperature sensitivity wa3 extremely low, being reduced from `,~

'. :

- ' : , . ' , -:', : ' ' :
.. . .
.

~L27~;~324 approximately 38 ps deg~lkm~l for the bare fibre to near zerofor the composite fibre structure, at a temperature around -20 deg C. In order to verify the large reduction in sensitivity to temperature and its application in sensors, a single-mode fibre Michelson interferometer was made with each arm approximately 30 metres of the coated fibre. (A single mode fibre Michelson interferometer is described, for example, in published UK patent application GB2136956A in the name of the present ~pplicants.) One arm was placed in a stable :~ tempera-ture cnvironment at room temperature while the other was temperature ramped around -25 clegree C. ~ ringe count was made at the output of the interferometer in order to compare it with the bare fibre subjected to a similar temperature ramping.
The sensitivity of the bare fibre has been measured to be approximately 8.33 fringes C ~ m ~ (16.66~ rads c 1 m 1).
A measurement of fringe count for the coated fibre revealed that an average of 0.92 fringes C-l m~1 (1.84~ rads C-1 m~l).
This represents a reduction in sensitivity to approximately 10%
of the bare fibre value. The measured fringe count data as a function of temperature is plotted in Fig 2, where the hysteresis is thought to be due to the temperature diPference between the thermocouple used to measure the temperature and the actual temperature distribution along the whole length of the coated fibre. As can be seen from the figure, the slope in both directions is similar approximately 2 degrees after the start of the measurement in each direction of the temperature ~xcursi~n.
Thq sensit~vity oP the coatecl ~lbre to temp~raturq ~h~nges may bq ~urthcr reduced by chan~ing ~he mell: and extrusion condi~ions which in turn alters the expansion coePPicient oP the mat.erial. With ~he correct expansivity ¦}i~?~l -, - : ', ,:

~LZ'7~j8Z4 of the coat~ng mater1al, 1t should be posslble to mln1m1se the sensltlvlty of transmlss10n delay of the f1bre to temperature fluctuat10ns over a larger useable temperature range. It should also ~e poss1ble to reduce acoust1c sens1t1vlty of the coated f1bre by extrudln~ lo~
compl1ance materlals. Th1s would ~llow the manufacture of sensors ~o be bu1le for spec1f1c appl1cat~ons.
Measurement has revealed the sens1t1vlty to have been reduced to approx1mately lO /~ of a bare s111ca f~bre.
By alter~ng the extruslon cond~tlons further reduct~ons ln sens~tlvlty to temperature changes are env1saged. Opt1cal flbres coated 1n accordance ~th the 1nventlon are expected to be useful 1n sensor appl1cat10ns. Also they ~ould, for example, for the f1rst t~me allow the construction of h~ghly stable dev1ces such as f~hre external cav~ty s~ngle-mode lasers.

TABLE I

FIBRE POLYMER

DIAMETER 125 ~m 900 ~m PRIMARY COATING 250 ~m COEFF. OF EXPN. ~ 5 * 10-7 3 7 * 10-6 (Room temp. data) YOUNG'S MODULUS 72 GN m~2 21 GN m~~

Referrlng now to F19ures 3 to 5, there are 111ustrated alternatlve embod~ments of the present ~nvent~on.

,` `'~:

:. ~ , ~ . . . . , ~

.
.
' : . ' ': ' ' : ' 1276i~Z4 In Flgure 3, the stress1ng means for the opt1c~1 f1bre I are prov1ded by a rtgtd polymer based substrate IO. The f1bre I 1s attached thereto by cl~mps I~ ~nd 15, at sp~ced pos~t~ons 12 find 13 wh1ch nay be of a releasable type such as screw clamps. Alternat1~ely 1nstead of reteasable clamps ~n adheslve compound may be employed to ~ttach the f1bre to the substrate, e~ther d~rectly or 1ndlrectly v1a mountlng blocks ~not shown). In the arrangement of F19ure 3, the f~bre I must ~e pre-stressed before attachment to the substrate 10 suff~c~ently to ensure that the tens10n 1n the f~bre ls not re1eased through thermal dlmens10n changes ln the substrate over the 1ntended range of operat1ng temperatures.
lt w~ll be apprec~ated that 1n the example of lS F~gure 3, the opt1cal f~bre cou~d readily be replaced by a~ opt~cal planar wavegu~de structure, e.g. a L~NbO3 th1n fllm wavegu~de structure, which then would preferably be attached to the substrate over its whole length.
However, ~n v~e~ of the r~gidity of such wavegu1de structures, prestress1ng would not normally be necessary.
In Flgure 4 there 1s sho~n an arrangement in whlch the f1bre 1 ~s wound about a polymer strength member 2I
hav1ng the des1red thermal expans~on properties as d~scussed above. The f~bre I ls aga1n pre-stressed. ~h1s ensures ~nt~mate contact between the f1bre and the strength memher 21 so that any var1at10ns, pr1mar11y 1n length but also 1n d1ameter, of the strength member 2I
cause a correspondlng change 1n stra1n 1n the f1bre I.
~n Flgure S the ~lbre I ls shown wound aroun~ A
drum 31 of polymer h~vlng the approprlate thermal propertl~s as prevlously d1scussed. lt wlll be read~ly understood that any change, pr~mJr11y ln d1ameter ~ut also axlally, of the drum wlll cause ~ change 1n the stra1n ln the f1bre I.

. .

,, . ~ , .. - . -. ~ . . . .

: . . .

Claims

1. A method of reducing temperature dependence of transmission delay in an optical waveguide, which comprises attaching the optical waveguide to a stressing means having a coefficient of linear thermal expansion of opposite sign to that of the waveguide and sufficient to substantially compensate for temperature induced changes in optical path length in the waveguide by applying a temperature dependent stress to the waveguide.

2. A method as claimed in claim 1 comprising attaching the waveguide to stressing means having a negative coefficient of thermal linear expansion.

3. A method as claimed in claim 1 including attaching stressing means comprising a tightly fitting jacket around the waveguide.

4. A method as claimed in claim 1 including attaching the waveguide to a stressing means formed of an oriented polymer.

5. A method as claimed in claim 4 including forming the stressing means by extrusion of a liquid crystal polymer.

6. A method as claimed in claim 3 including applying the jacket to the primary coating of the optical fibre.

7. A method as claimed in claim 6 in which the jacket material is extruded onto the fibre.

8. A method as claimed in claim 5, wherein at least one of the elastic modulus and the temperature coefficient of the extruded material are at least partly determined by the extrusion conditions.

9. A method as claimed in claim 1 or 2 including prestressing the waveguide and attaching the stressing means to the prestressed waveguide.

10. A method as claimed in claim 1 or 2 wherein the attachment is at least primarily through frictional contact.

11. An optical waveguide assembly comprising an optical waveguide attached to stressing means having a coefficient of thermal linear expansion of opposite sign to that of the waveguide and sufficient to substantially compensate for temperature induced changes in optical path length in the waveguide by applying a temperature dependent stress to the waveguide.

12. An assembly as claimed in claim 11 wherein the stressing means have a negative coefficient of linear thermal expansion.

13. An assembly as claimed in claim 11 or 12 wherein the waveguide is attached to the stressing means at discrete spaced positions.

14. An assembly as claimed in claim 11 or 12 wherein the waveguide is prestressed and attached to the stressing means in prestressed condition.

15. An assembly as claimed in claim 11 wherein the stressing means is formed of an oriented polymer.

16. An assembly as claimed in claim 15 wherein the stressing means is formed of a liquid crystal polymer.

17. An assembly as claimed in claim 11 wherein the waveguide comprises an optical fibre.

18. An assembly as claimed in claim 17 wherein the stressing means comprise a cylindrical member and the optical fibre is wound around the cylindrical member.

19. An assembly as claimed in claim 17 wherein the stressing means comprise a sleeve member around the fibre.

20. An optical fibre having at least part of its length enclosed in a tightly fitting jacket having a coefficient of thermal linear expansion opposite to the coefficient of thermal linear expansion of the optical fibre and capable of applying temperature dependent stress to the fibre such that changes in transmission delays induced by the applied stress counteract temperature induced transmission delay changes in the optical fibre.

21. An optical fibre as claimed in claim 20 wherein the jacket has a negative coefficient of thermal linear expansion.

22. An optical fibre as claimed in claim 20 or 21 wherein the jacket comprises a coating.

23. An optical fibre as claimed in claim 20 wherein the jacket comprises an oriented polymer.

24. An optical fibre as claimed in claim 23, wherein the polymer is a thermotropic liquid crystalline polymer.

25. a coated optical fibre as claimed in claim 22, wherein the jacket is a coating material applied to the primary coating of the fibre.

26. An optical fibre as claimed in claim 20 or 21, wherein the jacket material has a coefficient of thermal linear expansion of the order of -5 x 10-6.

27. An optical fibre as claimed in claim 20 or 21, wherein the optical fibre is a monomode silica optical fibre.

28. An optical fibre as claimed in claim 20 or 21, wherein the jacket material has an elastic modulus of the order of 20GNm-2.

29. An optical fibre assembly in which two or more optical fibres as claimed in claim 20 or 21 have a common coating.