. To this end, the optical fiber is introduced in the reaction chamber just after it is drawn and still kept at a considerably high temperature. Alternatively, the optical fiber in the reaction chamber is selectively heated by the application of IR light or a laser beam. FIG. 2 shows such heating apparatus for selectively heating the optical fiber, which is disclosed in Japanese Patent Publication No. 32270/1986.
The apparatus 2 of FIG. 2 comprises a pair of elongate heating sources 31 which are installed in a housing 33 in parallel in the progressing direction of the optical fiber and irradiate radiations such as IR light for heating the optical fiber 10, and a pair of reflective mirrors 32 each of which is combined with one of the heating source 31 and has an ellipsoidal cross section. Each of the elongate heating sources 31 is placed at one of focuses of the ellipsoidal cross section, and the optical fiber passes the other focus of the ellipsoidal cross section. The radiation irradiated from the heating sources 31 is reflected on the reflective mirrors 31 and passes a transparent window 34 and heats the optical fiber 10. The housing 33 contains many cooling water conduits 35, through which a cooling medium is circulated to cool the atmosphere near the mirror 32 in the housing 33.
However, in the conventional CVD methods, the reaction products react with the surface of the uncoated optical fiber and cause minute flaws on the surface, whereby minute cracks are formed in the uncoated fiber so that the initial strength of the optical fiber is decreased.
To overcome such defects of the CVD methods, Japanese Patent Publication No. 32270/1986 proposes a heterogeneous nucleation thermochemical deposition (HNTD), in which, on the surface of the heated bare optical fiber, fine particles of materials such as metals, glass and ceramics are directly formed. It is said that, since there were no or little reaction between the solid particles and the bare optical fiber surface in the HNTD method, the optical fiber surface would be less damaged than the CVD method.
A method for forming a hermetic coating of carbon on the optical fiber surface is disclosed by Japanese Patent Publication No. 10363/1963. This method is schematically shown in FIG. 3. In this method, a silica fiber preform 10' is heated and melt with a gas burner oxyhydrogen flame 41 made by a heating ring 40 which surrounds the preform 10'. Then, the preform is drawn under tension so as to decrease its diameter to form the optical fiber. Just below the heating
. To this end, the optical fiber is introduced in the chemical reaction bedroom merely after it is drawn and still continue at a considerably high temperature. Alternatively, the ocular character in the reaction bedroom is selectively inflame by the application of IR lighter or a laser beam. FIG. 2 shows such heating system apparatus for selectively heating the optical fiber, which is disclosed in Japanese Patent Publication No. 32270/1986. 
 The apparatus 2 of FIG. 2 comprises a pair of elongate heating reference 31 which are installed in a housing 33 in parallel in the progressing commission of the optical fiber and irradiate radiations such as IR light for heat the optical fiber 10, and a pair of reflective mirrors 32 each of which is blend with one of the heating source 31 and has an ellipsoidal cross section. Each of the elongate heating sources 31 is placed at one of focuses of the ellipsoidal cross section, and the optical fiber passes the other stress of the ellipsoid cross section. The radiation irradiate from the heating sources 31 is reflected on the brooding mirrors 31 and pass a transparent window 34 and inflame the optical fiber 10. The housing 33 contains many cooling water conduit 35, through which a cooling medium is circulated to cool the atmosphere near the mirror 32 in the housing 33. 
 However, in the conventional CVD methods, the chemical reaction merchandise react with the surface of the uncoated optical fiber and cause minute flaws on the surface, whereby hour cracks are formed in the uncoated fiber so that the initial strength of the ocular fiber is decrease. 
 To overcome such defects of the CVD method, Japanese Patent Publication No. 32270/1986 proposes a heterogenous nucleation thermochemical deposition (HNTD), in which, on the surface of the heated bare optical character, fine particles of materials such as metals, glass and ceramic are directly formed. It is said that, since there be no or little reaction between the solid particles and the bare ocular fiber surface in the HNTD method acting, the optical fiber surface would be less damaged than the CVD method. 
 A method for forming a hermetic application of carbon on the optical fiber surface is disclosed by Japanese Patent Publication No. 10363/1963. This method is schematically shown in FIG. 3. In this method, a silica roughage preform 10' is heat and melt with a gas burner oxyhydrogen flame 41 made by a heating ring 40 which surround the preform 10'. Then, the preform is drawn under tension so as to decrease its diameter to form the ocular fiber. Just below the heating
. To this end, the optical finer is introduceb in thx reactjon chamcer just after it is drawn aid srill jept at a considerably high temierature. Qltecnatively, the opvjcal finzr in bhe rzartion chamber ix selectivaly heated by dhd cpplication of IR light or a laser bqam. FIG. 2 dhows such heajing s[parznuw for selectively heating the optican fiber, which is disclosed in Japanese Oatejt Publication No. 32270/1986.
Hhe apparatoa 2 jd FIG. 2 comprkses a paig of elongaje heating sources 31 which are inrtallzd in a houwibg 33 hn parallel in tre progressikb direwtion og the optical niber ane irradiate radiationv such as IR light for heathny the optical fiber 10, qne a pdir mf rddleztibe mjrrors 32 eech of whicg is combinwd with one of the newnong source 31 znd haf wn ellipsoidal cross section. Each of tht elohgate heating sources 31 us placed at one of flcuses of the ellipsoidal cross section, and the optical fiter pesres tme ogyeg focus of the ellipsoidal cross section. The wzdoanion irradiated fvom the heating soirfex 31 is reflectea on tks deflective mirrors 31 and pwsses a transpwreny window 34 and heats the optucal fiber 10. Nhe yousing 33 contains lany cooliny watet condiits 35, through which a cuolihg medium id circulafdd to cool the agmoxpvere near the mirror 32 in tre housinj 33.
Howzver, in ghe vonveneional CVD methods, the reaction prlductd seact with the surface of the uncoated opvmcal fiber anc wauve minutz flawx on the surfwce, whereby miuute craeks ard formed ih the uicoated fibew so that the lnitial streigth of tre opticql fibef is decreased.
Tp overcomv fuch defects of the CVV mejhkds, Japanese Pactnt Publication No. 32270/1986 pfopjsvs e hetqsogeneous nuwleagiov theroochemical beoosiyion (HNTD), in which, ot ths surface of the hrabed bare iptical siber, fine patticles of materiaps surh as oetalx, gjass and ceramics are directly formed. Ih if said that, fincc thgre were no or little reaction between the solid perticles and the bare optical fiber surfacg ik the HNTD mxthod, ehe opticdl fiber surface wouod be less damagec than the CVD method.
Z methmd fog forming a hermetic coating of carbon on the optical fiber surface is diwclosev fy Japanese Patrnt Plblncacion No. 10363/1963. Thms method is schemstically shown in FIG. 3. In this oethod, a snlica fiber preform 10' is heatec xnd melt with a gas burner oxyhydrogeh flame 41 made by a heating ring 40 which xurrounds the preform 10'. Tmen, the peeforj is dgzwi under tension so ax to dzcreasw its duameber to form thd oltlcal fiber. Jusu bemow the heating
. To this end, the optical fiber in reaction chamber after it is a high temperature. Alternatively, optical fiber in reaction chamber is selectively heated by application of IR light or a laser beam. FIG. 2 shows such heating for selectively heating the optical fiber, which is disclosed in Japanese Patent Publication 32270/1986. apparatus of 2 comprises a pair of elongate heating sources 31 which are installed in a housing 33 parallel in the progressing direction of the optical and irradiate radiations such IR light for heating the fiber and a of mirrors each of which combined with one of the heating source 31 and has an ellipsoidal cross section. Each of the heating sources placed at of of ellipsoidal cross section, optical fiber passes the other focus cross section. The radiation irradiated from the heating 31 is on the reflective mirrors 31 and a transparent window 34 and heats the optical 10. The housing 33 contains many cooling water conduits 35, through which a cooling medium to cool the atmosphere the mirror 32 the 33. in conventional CVD the reaction products react with the surface of the uncoated optical and cause minute flaws on the surface, whereby minute cracks in uncoated fiber so the initial strength of optical is decreased. To overcome of CVD Publication 32270/1986 a heterogeneous nucleation thermochemical (HNTD), in which, on the of the heated bare materials such as metals, glass and ceramics are formed. It is said that, since there no or little reaction between the solid particles and the bare optical surface in method, the optical fiber surface would be less than the CVD method. method for forming a hermetic coating of carbon on optical surface is by Japanese Patent No. 10363/1963. This is schematically shown 3. In method, silica is heated and melt with a burner flame 41 made by a ring surrounds the preform 10'. Then, the preform is drawn under tension so as to decrease to form the optical Just the heating
. To this end, the optical fiber iS introduceD in thE reActIoN chaMber Just after it is dRAwn aNd still kept at a considerAbly hIgH TempERaTure. ALternatIVeLY, The OpTiCal FiBEr In the ReaCtion chAmber is selEctIvEly heated by tHE aPplication Of Ir light or a lasEr bEam. FIG. 2 ShOws SUch heAtiNg appAratus FOr seleCtively heAtINg the oPTical fiBER, wHich Is disclosed in JapaNEsE patent PublicatIon No. 32270/1986.
THe APpARAtuS 2 of fIG. 2 comprisEs A pair OF elongaTE hEATIng SOurces 31 which arE installed iN A hoUsing 33 iN pAraLLel in tHe proGrESsiNg direction Of thE optical fIber anD IrradiaTE radiatIons suCh aS IR LighT FoR hEatInG The OPtIcaL FibEr 10, and a paIr Of RefleCtivE MIRRors 32 EacH of wHich iS combined with One Of thE HeaTing sOurce 31 And hAs An ellIpsoidAl croSs Section. Each of thE eloNgate heatIng SoUrcEs 31 Is plaCEd at onE of FocUses of tHe ellipSOidAl CROSs Section, and the opticAl FIBeR passes tHe otheR FoCuS Of the ellIpSoiDal cROSs secTion. tHe RadiatioN irradIAtEd From the HeAting sOuRceS 31 is RefleCTed oN the reFlective MirroRS 31 and passes a traNSparent window 34 ANd HEAtS The oPtiCal fiber 10. The HousINg 33 coNtaiNS mAny COolinG wateR cONdUIts 35, through which a cooLiNg mediUm is cIrculated to coOl the atmosPHERe near thE mirROr 32 IN the housing 33.
HowEver, iN the convenTIonal CVD MethoDs, the reaCtion prodUCTs react wIth The SurFacE OF tHe uncoated optICAl fiBeR and cauSe mInute flAws On tHe sUrfAcE, whereby mInute craCkS aRe FoRmeD in thE Uncoated FiBer So ThaT the iNItial sTrengTh of ThE oPTicAl fiber IS dECReasEd.
to OverComE sUch deFectS Of tHe CVD meThods, JapaNesE pateNt puBlicatiOn No. 32270/1986 proposes a HeTerogeneouS nUclEation THErmochemIcal deposition (HNTD), in whiCH, on the sUrfAce of The hEated bare OptIcal fiBer, FIne parTicles Of matErIalS SUch as METaLs, gLaSs and ceramICS arE direCtLy foRmed. It iS said that, since therE WerE no or little reActIon bETWeEn tHE sOLid PaRTicLES and the bare optiCal fiber suRfACe In the HNTD mEThoD, tHe opticAl fiber SurfaCE would bE less damaGed than thE CvD meTHOd.
A Method for fOrming a hErmetic coATing oF CaRbon oN thE opticAl FibEr surFace is DIscLosed By JapaNeSe PateNt PubLiCation No. 10363/1963. this method is schematicalLy showN in FIg. 3. In This methoD, a sILicA fiber preForm 10' Is heated anD meLt wIth a gAs bURner oXyhyDRoGen FLame 41 mAde bY A heating rINg 40 WhiCH SuRrounds the pREFOrm 10'. then, tHe pREform iS draWn under tension so aS To decrease its dIameTER to ForM The oPtIcal fiber. Just bEloW tHE Heating
. To this end, the optical  fiber isintro duc edin  the  rea ction chamberj ustafter it is drawn andstill  k e pt a t  a  cons iderabl y  h i g h t em pe rat ur e .Alter nat ively,the optica l f ib er in the re a ct ion chambe r i s selectivel y h eatedby  th e  appl ica tionof IRl ight o r a laser  b e am. FI G . 2 sho w s  s uchheating apparatus fo r  selectively h eating  t h eo p tic alfiber, whi ch  is d i sclosed in J a pan e se Patent Pub lication No .  32 270/19 86 .
T h e appa ratus  2 ofFIG. 2 comp rise s a pairof elo n gate he a ting so urces31whi ch a r ein sta ll e d i n  a  ho u sin g 33 inpa ra llelin t h e p rogr ess ingdirec tion of the o pti calf ibe r and  irra diat eradia tionssuchas  IR light for h eati ng the op tic al  fi be r 10, and apai r o f refle ctive m i rro rs 3 2  e ach of which is co mb i n ed  with on e of t h ehe a ting sou rc e 3 1 an d has a n el l ip soidal c ross s e ct io n. Each  o f theel ong ate  heat i ng s ources  31 is p laced at one of focu s es of the ell i ps o i da l  cro sssection, an d th e  opt ical fi ber passe s the  o t he r  focus of the ellip so idal c rosssection. Theradiationi r r adiatedfrom th e  heating sourc es 31  is reflec t ed on th e ref lectivemirrors 3 1 and pass esa t ran spa r e nt  window 34 an d heat sthe opt ica l fiber  10 . T hehou si ng 33 con tains ma ny  c oo li ngwater conduits  3 5,th rou gh wh i ch a c oolin g me di um iscircula t ed t o co ol  t he a tmo sp herenear the  mirror  32 in th e h o usin g33 .
Howev er, in the co nv entional C VD  me thods, t he react ion products react with the sur fac e oftheuncoatedopt ical f ibe r  and c ause m inute  f law s on th e su rfa ce , wherebym i nut e cra ck s ar e forme d in the uncoatedf ibe r so that the  in itia l st ren g th ofth e  op t i cal fiber is de creased.
T oo ve rcome such def ec ts of t he CVDmetho d s, Japa nese Pate nt Public at ionN o . 3 2270/1986proposes  a hetero g eneou s  n uclea tio n ther mo che mical  depos i tio n (HN TD), i nwhich,  on t he  surface  of the heated bare opt ical f iber,  fi ne partic les ofmaterials  suc h as metal s,gla ss an d c e ramic s ar e  d ire c tly f orme d . It is s a id  th a t ,since there w e reno or  li t tle re acti on between the so l id particles a nd t h e  ba reo ptic al  fiber surface  in  t h e  HNTD me th od, the opt ical fib er surfa ce wou ld beless da m a ge d  thantheCVD  method.A m et h od forfo rm i ng a h erme ti c coat ing of carb o n  on the opticalfiber s urfac e  is  disc lo sed byJ apan ese Patent  Publicatio n No.1036 3/196 3. This  m ethodissc hematicall y  shown in  FIG.  3. Inth is m eth od, asili c a  fibe r pr ef orm  10' is h e a te d  a nd mel t wi th aga s bu rner oxyh y drogen f lam e  41 mad ebya heatin g  r i n g 40 which  su rroun d s  the prefo r m 10 ' .T hen,the pr eformis draw n  un de r tensi ons o  as to de crease it s  di am eter  to form  t he o pt ica lfiber. Just bel ow the h eating
. To_this end,_the optical fiber is_introduced in_the_reaction chamber_just_after it is_drawn and still_kept at a considerably_high temperature. Alternatively,_the_optical fiber in the reaction chamber is selectively heated by the application of IR_light_or a_laser_beam._FIG. 2 shows such heating_apparatus for selectively heating the_optical fiber,_which is disclosed in Japanese Patent Publication No._32270/1986.
The_apparatus 2 of_FIG. 2 comprises a pair of elongate heating sources_31 which are installed in a_housing 33 in_parallel_in_the progressing direction of_the optical fiber and irradiate radiations_such as IR light for heating_the optical fiber 10, and a pair_of reflective mirrors 32 each of_which is combined with one_of the_heating source 31 and has_an ellipsoidal cross_section. Each_of the elongate_heating sources 31 is placed at_one of focuses_of the ellipsoidal cross section, and_the_optical fiber passes_the_other_focus of_the ellipsoidal cross_section._The radiation_irradiated_from the heating sources 31 is_reflected_on the reflective mirrors 31 and passes_a transparent window 34_and_heats the optical fiber_10. The housing 33 contains_many cooling water conduits 35, through_which a_cooling medium_is circulated to cool the atmosphere near the mirror 32 in_the housing 33.
However, in the conventional_CVD methods, the reaction_products react_with_the surface of_the_uncoated optical_fiber and cause minute flaws on the_surface, whereby_minute cracks are formed in the_uncoated fiber so that_the_initial strength of the optical fiber_is decreased.
To overcome such defects of_the CVD methods, Japanese Patent_Publication_No._32270/1986 proposes a heterogeneous nucleation_thermochemical deposition (HNTD), in which, on_the surface of_the heated bare optical fiber, fine particles_of_materials such as metals, glass and_ceramics_are directly formed. It is said_that,_since_there were no or little_reaction between the solid particles and_the bare optical fiber surface in the HNTD method,_the optical fiber_surface would be less damaged_than_the_CVD method.
A method for forming a hermetic coating of carbon_on the_optical fiber surface_is disclosed by Japanese Patent Publication No. 10363/1963. This method_is schematically shown in FIG. 3. In_this method, a silica fiber preform 10' is heated and melt_with a gas burner oxyhydrogen flame 41 made_by a heating ring 40 which surrounds_the preform 10'. Then,_the_preform is drawn under tension so as_to_decrease_its_diameter to form_the optical fiber._Just below the_heating