CA1168938A

CA1168938A - Radiation resistance optical fibers and a process for the production of the same

Info

Publication number: CA1168938A
Application number: CA000371387A
Authority: CA
Inventors: Motohiro Nakahara; Shiro Kurosaki; Shigeru Tanaka
Original assignee: Nippon Telegraph and Telephone Corp; Sumitomo Electric Industries Ltd
Current assignee: Nippon Telegraph and Telephone Corp; Sumitomo Electric Industries Ltd
Priority date: 1980-02-22
Filing date: 1981-02-20
Publication date: 1984-06-12
Also published as: DE3106389A1; DE3106389C2; GB2071644A; GB2071644B

Abstract

TITLE: Radiation resistance optical fibers and a process for the production of the same ABSTRACT OF THE DISCLOSURE

An improved radiation resistance optical transmi-ssion fiber of this invention comprises a higher refractive index part and a lower refractive index part, the higher refractive index part consisting predominantly of a silica glass synthesized by oxidation reaction of a hydride such as SiH4 or an organic compound such as Si(OC2H5)4 at a rela-tively low temperature, and is prepared by feeding a hydride such as SiH4 or an organic compound such as Si(OC2H5)4 into an oxyhydrogen flame consisting of H2 and/or D2 and °2 to synthesize glass particles, depositing the glass particles on a starting member to form a higher refractive index part of silica glass and combining the higher refractive index part with a lower refractive index part to form one body as a fiber.

Description

1 16893~

1 BACKGROUND OF ~HE INVENTION

1. FIE~D OF TH~ INVENTION
This invention relates to an optical transmission glass fiber and a process for the production of the same and more particularl~, it is concerned with a radiation resistance optical transmission glass fiber which is used in optical communication systems under radiations such as X-ra~s and ~-rays, image transmission s~stems or illumina-tion systems, and a process for the production of the same.
~he range of use of optical transmission glass fibers is increasing in the fields of the control lines of air planes, the control lines of ships, the wirings of com-puters and the control or communication lines of works or buildings in addition to the fields of telegraphs and tele-phones in the prior art, since they have various advantages that signals of large capacity can be made with a small space and they are non-inductive due to electrical insula-tion and show a light weight as well as an excellent flexi-bility because of narrow glass fibers. Under the situation, much endeavor has been made to improve the optical transmi-ssion glass fibers and their properties are thus elevated in transmission loss, broad band transmission and practical strength. Of late, therefore, the optical transmission glass fibers have been put to practical use.

2. DESCRIPTION OF TH~ PRIOR ART
Up to the present time, the following glass fibers have been known as an optical transmission glass fiber:
(1) Fiber consisting of a core of SiO2 glass and a cladding of silicone resin, (2) Quartz glass fiber consisting of a core of P2O5-GeO2-SiO2 or P2O5-B2O~-GeO2 and a cladding of B2O3-SiO2 --1-- ,!,~lr 1 ~8938 l or B2o3-p2o5-sio2~

(3) Quartz glass fiber consisting of a core of SiO2 glass and a cladding of B203 and/or ~-doped SiO2 glass,

(4) Multicomponent glass fiber in which both of a core and cladding consist of a borosilicate glass or a soda lime glass and

(5) Eigh silicate glass fiber consisting of a core of Cs2-B203-SiO2 glass and a cladding of B203-SiO2 glass.
~ atel~, it has strongl~ been required to use these optical glass fibers, in particular, under radiation of X-rays in medical or indu~trial fields or r-ra~s in the fields of handling nuclear reactors or radioactive element so as to make the best use of the electrical insulation and flexibility of the optical glass fiber. In the presence of such a radiation, howe~er, glass meets with its structural defects to result in a very large transmission loss. In particular, the abo~e described glass fibers of the types (2), (4) and (5) or plastics fibers meet with a largel~
increased transmission loss and those of the types (1~ and (3) also meet with a considerably increased transmission 109s. Increase of the transmission loss in these fibers is discussed b~, for example, E. J. Friebele et al in "~aser Focus" Sept., 1978, page 50-56.
Therefore, fibers which have hitherto been proposed or developed cannot be used in the presence of radiations because of their largely increased transmission loss. In the s~nthesis of the prior art glass fibers, halogen co~-pounds such as SiCl4 with a high cohesive strength are used as a raw material and OH groups are intended to be decreased for the purpose o~ lowering the transmission loss, but the hydrolysis or thermal oxidation is carried out at a hightempe-rat~re such as higher than 1650 ~, which tends to cause structural ~ 168938 l defects such as oxygen defect. Thus, the quantity of defects produced by radiation i8 large while the diminishing speed of the defects is tos low considering for the quantit~ of the defects produced by radiation, resulting in increase of the transmission loss.
On the other hand, as a window glass under radia-tion, there is used a glass doped with CeO2 or Sb203 whereb~
electrons or positive holes produced b~ radiation damage are trapped b~ the electron capture centers of Ce4+ and the posi-tive hole capture centers of Ce3 and not trapped by lattice defects$ thus the structural defects being held as it is and suppressing coloration of the glass. However, the composi-tion of such a window glass is not suitable for an optical transmission fiber since hardly purified compounds are used as a starting material and the length of transmission of light is inferior b~ the order of 102 _ 105sO that the loss due to radiation be too large (~ 10 dB/km at 10 R-y ).
SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical transmission glass fiber and a method of produ-cing the same, which overcome the heretofore noted disadvan-tages.
It is another object of the present invention to provide an optical transmission glass fiber in which light advances concentrically in a part consisting of a high purity silica glass free from structural defects so as to minimize the influence of the structural defects.
It is a further object of the present invention to provide an optical transmission glass fiber in which OH
groups or OD groups are somewhat incorporated to promote diminishing of defects produced and excited by radiation and - - , 1 to decrease further the transmission loss.
It is a still further object of the present inven-tion to produce an optical wave guide with a decreased radi-ation damage by the use of a glass with deoreased structural defects, which is synthesized at a relatively low tempera-ture, without using as a raw material compounds decomposable at a high temperature auch as chlorides.
It is a still further obJect of the present inven-tion to provide a process for the production of an optical fiber having an SiO2-Sb203 glass b~ lowering the synthesis temperature of the glass to suppress formation of structural defects and to decrease radiation damages.
It is a still further object of the present inven-tion to provide a process for the production of an optical transmission glass fiber by using as a core a fiber consis-ting of an SiO2 glass containing Ce2O3 + CeO2.
These objects can be attained by a radiation resi-stant optical glass fiber comprising a higher refractive index part and a lower refractive index part, the higher .
refractive index part consisting predominantly of a silica glass synthe~ized by oxidation reaction of a hydride such as SiH4 or an organic compound such as 9i(0C ~ 5)4 and a process for the production of a radiation resistant optical trans-mission glass fiber which comprises feeding an organic compound such as Si(OC2H5)4 or a hydride such as SiH4 into an oxyhydrogen flame consisting of H2 and/or D2 and 2 to synthesize glass particles, depositing the glass particles on a starting member to form a high refractive index part of silica glass and combining the higher refractive index part with a lower refractive index part to form one body as a fiber, or which comprises oxidizing SbH3 gas and SiH4 gas or Si(OC2H5)4 to form an Sb2O3_SiO2 glass, depositing this glass .

1 16893~

1 and an SiO2 glass obtained b~ oxidizing SiH4 gas or Si(OC ~ 5)4 and subjecting to melt spining.

BRI~F D~SCRIP~ION OF THE DRAWING

~ he accompanying drawings are to illustrate the present invention in more detail.
Fig. 1 (A), (B) and (C) are cross-sectional views of embodiments of the optical transmission fiber accoraing to the present invention;
Fig. 2 is a cross-sectional view of the optical glass fiber of th0 present invention to illustrate the pri-mar~ coating and secondary coating;
Fig. 3 (a) is a graph showing the relationship between the increased quantity of transmission loss and the ~-rays irradiation time for comparison of the optical fiber of the present invention with the prior art;
Fig. 3 (b) is a graph showing the same relationship as that of Fig. 3(a) but in the case of an intermittent irradiation;
Fig. 4 (a) is a graph showing the relationship between the increased quantity of transmission 105s and the ~-ra~s irradiation time for comparision of the optical fiber of the present invention with the prior art in a case where the cladding and jacket consist of glass;
Fig. 4 (b) is a graph showing the same relationship as that of Fig. 4(a) but in the case of using other optical fibers;
Fig. 5 and ~ig. 6 are cross-sectional views of o~her embodiments of the optical transmission glass fibers accor-ding to the present invention;
Fig. 7 is a schematic view of an apparatus suita-ble for the production of the optical fiber according to the ~ ~8938 l present invention b~ VAD method;

DE~AILED D~SCRIP~ION OF THE INV~N~ION

In accordance with the present invention, there is provided a radiation resistant optical glass fiber compri-sing a higher refractive index part and a lower refractive index part, the higher refractive index part consisting pre-dominantl~ of a silica glass synthesized by oxidation reac-tion of a hydride such as SiH4 or an organic compound such as Si(OC2H5)4 at a relatively low temperature such as less than 1550 C, preferably less than 1400 a and a process for the production of a radiation resistant optical transmission glass flber, which comprises feeding an organic compound such as Si(OC2H5)4 or a hydride such as SiH4 into an ox~hydrogen flame consisting of H2 and/or D2 and 2 to syn-thesize glass particles, dipositing or accumulating the glass particles on a starting member to form a higher refractive index part of silica glass and combining the higher refrac-tive index part with a lower refractive index part to form one body as a fiber.
In the present invention, a glass with a decreased structural defect by lowering the synthesis temperature thereof is used without using a compound decomposable at a high temperature such as chlorides thus obtaining a glass fiber with a decreased radiation damage and, if necessary, OH groups or OD groups are incorporated in the glass to some extent so that the diminishing speed of defects produced and excited by radiation is promoted and the transmission loss is further decreased.
Furthermore, the present invention provides a process of producing a radiation resistant optical fiber which com-prise oxidizing SbH3 gas with SiH4 gas or/and Si(OC2H5)4 1 ~893~
1 gas to form an Sb2O3-SiO2 glass, depositing the resulting glass and another SiO2 glass obtained by oxidizing SiH4 gas or/and Si~oC2H5)4 gas, and then subjecting the accumulate : to melt spinning.
In the present invention, the synthesis temperature of the glass is lowered for example, to lower than 1600C, preferably lower than 1400C, in an optical fiber having an SiO2-Sb2O3 glass and the structural defects are thereby suppressed to decrease the radiation damage. In this glass ?O composition, O~ groups or OD groups are optionally incor-porated to increase the diminishing speed of defects produced and excited by radiation and to suppress increase of the transmission loss. The transmission loss due to structural defects may be minimized by doping a silica glass with Sb2O3 only as a dopant so that light advances concentrically in a part consisting of a high purity silica glass free from other impurities than OH group or OD group.
In addition, the present invention provides a radia-tion resistant optical transmission fiber comprising a higher ~0 refractive index part and lower refractive index part, the higher refractive index part consisting predominantly of a Ce-doped SiO2 glass synthesized by oxidizing or decomposing a hydride such as SiH4 or an organic compound such as Si~O2 ~5)4 with a Ce-containing compound at a relatively low temperature such as lower than 1600C, preferably 1100 to 1550C. That is to say, a SiO2 glass containing preferably 0.001 to 1~ of Ce2O3 and CeO2 is prepared whereby electrons and positive holes are trapped by the electron capture center of Ce and the positive hole capture center of Ce3 with holding the effect of suppressing increase of the transmi-ssion loss. Using this glass as a core, an optical trans-mission fiber is produced wherein increase of the transmi-1 16893~
.

l ssion loss is suppresed even under radiation.
Examples of the other hydrides which can be used inthe present invention are SiH2Cl2, SiHC13, Si2H6, Si~H8 and the like~ Examples of the other organic compounds which can be used in the present invention are CH~C13Si, CH3Cl2SiH, CH3ClSiH2, aH3SiH~ C2H5Cl3Si. C2H5~l2Si~ 2 5 2 C2H5~iH3' C3H7C13~i~ C3~7Cl2SiH, C3H7clSiH2, 3 7 3~ 4 9 l2SiH~ C4HgSiH3~ C5H11Cl2SiH, C5H11SiH
C6H13SiH3~ C7~15SiH3, CgH17SiH3~ (CH30)4Si, and the like.
Fig. 1 ~A) shows one embodiment of the optical fiber of the present invention~ in which core 11A is of a high purity silica glass containing some OH groups or OD
groups and synthesized at a low temperature and cladding 12 is of a silicone resin or fluorine resin. Fig. 1 (B) shows another embodiment of the optical fiber of the present inven-tion in which core 11~ is of a high purity silica glass con-taining some OH groups or OD groups and synthesized at a low temperature and cladding 12B is of a high purity silica glass containing any one of ~23~ F~ B203-F and P205-~ and synthesized at a low temperature, which may contain some OH
groups or OD groups~ Fig~ 1 (C) shows a further embodiment of the optical fiber of the present invention, in which 11C
is the same as 11B, 12C is the same as 12B and jacket 13~ is of, for example, a silica glass. ~hese glass fibers are radiation resistant, but in order to prevent from deteriora-tion of the mechanical strength thereof, it is desirable to provide the outside of glass fiber 21 as shown in ~ig. 2 with a primary coating 22 of a thermosetting resin such as polyi-mide resin or epoxy resin (which should be coated directlyafter forming the glass fiber) and further with a thermo-plastic resin 23 such as ethylene propylene rubber or brid-ged polyethylene by extrusion.

1 Methods of making a glass rod or preporm as a raw material of glass fiber, and a glass fiber will now be illu-strated. In one example, H2 and/or D2 as a combustion gas and 2 as a combustion aid are fed to a burner consisting of a quartz glass tube to form a flame, into which an orga-nic compound such as ~i(OC2H5)4 or hydride such as SiH4 that is decomposable at a relatively low temperature is introduced by the aid of a suitable carrier gas to effect a flame oxida-tion reaction and to form a fine powder of silica glass con-taining some OH groups and/or OD groups, and the resulting powder is blown against and deposited in the state of fused glass on a revolving target by O-CVD (Outside Chemical Vapor Deposition) method or VAD (Vapor-phase Axial Deposition) method. Depending on the deposition method, any form of round bars or cylinders can be obtained as well known in the art.
In another example, a gaseous Si-containing compound such as Si(OC2~5)4 or SiH4 and 2 gas are introduced into the above described oxyhydrogen flame and a glass fine powder mass is prepared by OLCVD method of VAD method, which is then subjected to sintering and clear vitrification in an atmos-phere containing some H2O and/or D2O, or some H2 and/or D2, thus obtaining a high purit~ silica glass bar containing large amounts of OH groups and/or OD groups. ~hen, the sur-face of this glass bar is subjected to cylinder grinding or polishing and further to HF polishing, CO2 laser polishing or flame polishing to obtain a clean and smooth glass bar.
~his glass bar is subjected -to melt spinning in a furnace at a high temperature and coated with a silicone resin or fluo-rine resin before taking up the fiber on a reel, followed by baking, to thus obtain a glass fiber of the present invention.
In a further exmple, a gaseous raw material containing at least one of F, B and P and a gaseous raw material containing ~ 168938 l silicon such as Si(OC2~5)4 or SiH4 with 2 a ced into a plasma flame of high freguency plasma torch by the aid of a suitable carrier gas and a silica glass doped with any one of F, B203, B203-F and P205-F is deposited on the outer surface of a rotating glass rod synthesized as described above. During the same time, if necessary, a com-pound containing H or D such as H20 or D20 can be added to synthesize a glass containing OH groups or OD groups, from which a glass fiber of the present invention can be obtained through melt spinning. In this case, compounds such as SiH4, SiF4, BF3, B2H6, PF3 and the like are introduced into an oxyhydrogen flame made up of H2 and/or D2 and 2 to synthesize silica glass fine particles doped with any one of F, B20~, B203-F and P205-F and deposited in fused state on a glass rod as a core described above. Of course, the above described glass fine particles can be deposited as a powder soot, follo-wed by sintering in an atmosphere containing H20 or D20, thus obtaining a transparent glass. Subsequently, on the outer surface of this synthesized glass rod is deposited a silica glass by introduc:ing a gaseous raw material containing Si such as SiCl4 with 2 by the aid of a carrier gas into a flame or plasma flame. During the same time, if necessary, ~iCl4, AlCl3 or ZrCl4 can be added to deposit a glass doped with ~iO2, Al203 or ZrO2. In spite of depositing a synthesized glass on the outside thereof, the above described synthesized glass rod with the cladding can be inserted in a suitable vycor -glass or quartz glass tube, collapsed to form a rod and then sub3ected to spinning, or the glass tube can be spun with collapsing as it is. In a still further example ? a silica glass doped with any one of B203, F, B203 F or P20s-F is 0 synthesized at a low temperature and deposited on the inside M-CVD
of a quartz glass or vycor glass tube by the prior art/(Modi-fied Chemical Vapor Deposition) method or P-CVD (Plasma-activated 1 16893~

l CVD) method using a hydride such as SiH4, an organic compo-und such as Si(OC2H5)4 and/or other compounds such as Si~4, ~F3, B2~6 and PF3 as a gaseous raw material. A silica glass rod synthesized at a low temperature and containing some OH
groups or OD groups is inserted in the inside of this tube and subjected to melt spinning after collapsed tG be a rod or with collapsing, thus obtaining a glass fiber of the pre-sent invention. In this M-CVD method, a glass fiber can further be prepared by synthesizing and depositing at a low temperature a silica glass doped with B2O3 and/or F, synthe-sizing and depositing at a low temperature using a hydride such as SiH4 or an organic compound suCh as Si(OC2H5)4 as a gaseous raw material and then subaecting the resulting com-posite tube to melt spinning directly or after collapsed to be a rod.
Fig. 5 shows a cross-sectional view of an optical fiber consisting of core 51 of Sb2O3-SiO2 glass, cladding 52 of SiO2~ B23-si2' F-SiO2 or F-~2O3-SiO2 glass and aacket 53 of a quartz or vycor glass. These glass fibers are radiation resistant, but in order to prevent from dete-rioration of the mechanical strength thereof, it is desira-ble to provide the outside of glass fiber 61 as shown in Fig. 6 with a primary coating 62 of a thermoplastic resin such as polyimide resin, epoxy resin or silicone resin (which should be coated directly after forming the glass fiber) and further with a thermoplastic resin 63 such as ethylene propylene rubber or bridged polyethylene by ext-rusion.
The Sb2O3-SiO2 glass used herein is excellent in 30 radiation resistance as well known in the art and is ordi-narily synthesized by ~-CVD method or by flame hydrolysis as follows:

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1 SiC14 + 2 = Si2 + 2Cl 2SbC15 + 3/2O2 = Sb23 + 5Cl2 SiC14 + 2H2 + 2 = Si2 +
2SbC15 + 5~2 + 3/202 = Sb23 +
In M-CVD method, however, the oxidation decomposition of chlorides is carried out at a high temperature and Cl remains in a large amount, thus resulting in many oxygen defects and increasing the loss in the presence of radiation. In the method of flame hydrolysis, the reaction temperature is so bigh due to use of chlorides that there remain a number of structural defects such as oxygen defect and the transmission loss increases in the presence of radiation.
~ he above described problem can be solved by selec-ting, as a raw material, hydrides such that can be oxidized at a lower temperature and have a high radiation resistance as well as a relatively low vitrification temperature as the oxides thereof, according to the present invention.
Up to the present time, the use of such hydrides as a raw material has not taken into consideration since they cause to increase the transmission loss in a longer wavelength zone.
Methods of making really the above described optical fiber will now be illustrated. In one example, H2 and/or D2 as a combustion gas and 2 as a combustion aid are fed to a burner consisting of a quarts glass tube to form a flame, into ~hich gaseous hydrides of SiH4 and SbH3 as raw material gases are introduced by the aid of a suitable carrier gas to effect a flame oxidation reaction to form a fine powder of silica glass doped with Sb2O3 containing some OH groups and/or OD groups, and the resulting powder is blown against and deposited in the state of fused glass on a rotating ~L 16~938 l target by O-CVD method or VAD method. Depending on the deposition method, any form of round rods or cylinders can be obtained. In another example, a fine powder of the above described Sb2O3-SiO2 glass can be prepared in the form of a powder mass by O-CVD method or VAD method and sintered to be a transparent glass. Then, the surface of this glass rod is subjected to cylinder grinding or polishing and ~urther to HF polishing, CO2 laser polishing or flame polishing to obtain a cleaned and smooth glass rod. ~2 and/or D2 as a combustion gas and 2 gas as a combustion aid are fed to a burner consisting of a quart~ glass tube to form a flame, into which a gaseous hydride of SiH4 as a raw material is introduced by the aid of a suitable carrier gas to effect a flame oxidation reaction and to form a fine powder of SiO2 containing some OH groups and/or OD groups, and the resul-ting powder is blown against and deposited in the state of fused glass on the above described glass rod.- This glass rod is subjected to melt spinning in a furnace at a high temperature and coated with a thermoplastic resin before taking up the fiber on a reel, followed by baking7 to thus obtain a glass fiber of the present invention. In a further example, a gaseous raw material containing F or B and a gaseous raw material containing Si such as SiCl4 with 2 are introduced into a plasma flame of high frequency plasma torch by the aid of a suitable carrier gas and a silica glass doped with F and/or B2O3 is deposited on the outer surface of a rotating glass rod synthesized as described above. During the same time, if necessary, a compound con-taining H or D such as H2O or D2O can be added to synthesize a glass containing OH groups or OD groups, from which a glass fiber of the present invention can be obtained through melt spinning. In this case, compounds such as SiH4, SiF4, BF3 and the like are introduced into an oxyhydrogen flame 1 1~893~
.

1 made up of H2 and/or D2 and 2 to synthesize silica glass fine particles and subse~uently a silica glass doped with B2O3 and/or F is deposited in fused state. At this time, in particular, glass fine particles are synthesized at a low temperature, deposited as powder and subsequently sintered in an atmosphere containing H2O or D2O, thus obtaining a transparent glass. Subsequently, on the outer surface of this synthesized glass rod is deposited a silica glass by introducing a gaseous raw material containing Si such as SiC14 with 2 by the aid of a carrier gas into a flame or plasma flame. During the same time, if necessary, TiC14, AlC13 or ZrC14 can be added to deposit a glass doped with ~iO2, A12O3 or ZrO2. Melt spinning of these glass rods result in glass fibers of the present invention.
In spite of depositing a synthesized glass on the outside thereof, the above described synthesized glass rod with the cladding can be inserted in a suitable vycor glass or quartz glass tube, collapsed to form a rod and then sub-jected to spinning, or the glass tube can be spun with collapsing as it is.
In a further example, a silica glass consisting of pure SiO2 or doped with B2O3 and/or F is synthesized at a low temperature and deposited on the inside of a quartz glass or vycor glass tube by the prior art M-CVD method or P-CVD method using a hydride such as SiH4, an organic com-pound such as Si(OC2H5)49 SiF4 and/or BF3 as a gaseous raw material. A silica glass rod synthesized at a low tempera-ture and containing some OH groups and/or OD groups is inter-ted in the inside of this tube and subjected to melt spinning 30 after collapsed to be a rod or with collapsing, thus obtain-ing a glass fiber of the present invention.
Furthermore, a method by P-CVD or M-CVD is available. SiH4 1 16893~

l gas optionally diluted with N2 and an oxidation gas such as C2 or 2 are fed in a quartz glass tube or vycor tube the outside of which is heated, and SiO2 glass synthesized and deposited on the inner wall of the tube. Then, if necessary, SbH3 and SiE4 gas diluted with Ar and an oxidation gas such as C02 or 2 are fed therein, an S~203 SiO2 glass is synthe-siæed and deposited on the inner wall of the tube. ~here-after, the tube is heated at a high temperature to laminate the synthesized glass layer, collapsed to form a rod and subjected to melt spinning to form a fiber. In some cases, another fiber can be prepared by synthesizing and depositing at a low temperature a silica glass doped with B203 and/or F by M-CVD method, further synthesizing and depositing at a low temperature a silica glass using SiH4 as a raw material and then sub~ecting this composite tube to melt spinning directly or after collapsed to a rod.
In the present invention, an SiO2 glas~ containing 0.001 to 1 % of Ce203 + CeO2 is used as a glass through which light advances concentrically, since if the amount of Ce203 + CeO2 is less than 0.001 %, the centers of Ce 4~ and Ce 3+
are so dilute that electrons or positive holes are trapped by the structural defects to increase the transmission loss, while if more than 1 %9 the loss increases due to ultraviolet absorption by Ce3 and Ce4+. When using a wavelength of 10 ~m or more, however, the sum total can be allowed to at most 2 % because the effect of ultraviolet absorption is decreased.
5ince such a core glass has a refractive index similar to, although somewhat higher than, that of quartz glass, the cladding is to be of plastics such as silicone resin and fluorine resin, or of a glass having a refractive index lower than that of quartz glass, such as ~203-F-SiO2, 2 5 F Si2~ F-si2 or ~2o~-P2os-F-sio2 glass. In the latter l case, SiO2 or an SiO2 glass doped with TiO2, ZrO2, Al2O3 or ~f2 can further be provided on the outside thereof in order to increase the water resisting property ~nd strength~
Onto the glass fiber is applied, as a primary coa-ting, a thermosetting resin such as polyimide or epoxy resins that are resistant to radiations directly after melt spinning the preform. In the case of preparing a plastic cladding fiber, of course, a silicone resin or fluorine resin is coated and baked directl~ after spinning. Onto this primary coating is further applied, as a secondary coa-ting, a thermoplastic resin such as ethylene propylene rubber or bridged polyethylene for the purpose of reinforcing.
When the transmission loss is increased after irradiation of electron ray, the fiber can be subjected to a heat treatment at a suitable temperature.
The present invention will now be illustrated by the use of SiH4 as a starting material without limiting the same~ Of course, other organic compounds such as Si(OC2H5)4 or dopants such as B ~ 6, P~I3, SbH3, etc. which do not so increase the transmission loss under radiation can be used.
Referring to Fig. 7~ ~2 gas and 2 gas, as a com-bustion gas, are fed to oxyhydrogen burner 71 to make flame 72, into which SiH4 gas diluted with, for example, He gas is introduced via burner 71, and SiO2 glass fine particles are obtained through the reaction of ~iH4 + 202 = SiO2 + 2H20.
During the same time, an aqueous solution of Ce compound is spouted in the form of a spray 75 from exhaust nozzle 74 and glass fine particles having Ce2O3 and CeQ2 in or on the glass fine particles are deposited on target 77' to form a glass soot or transparent glass body 77. If necessar~, carrier gas 78 is fed to exhaust nozzle 74 and compressed gas 79 is added to vessel 79' to forward the aqueous solution 79" via Jl 168938 l pipe 79"' and to form a flow of the aqueous solution 75 with the form of a spray. As the aqueous solution, there are pre-ferably used aqueous solutions of cerium nitrate Ce(NO3)3 -xH2O and cerium ammonium nitrate Ce(NO3)3 . NH4NO3 . ~H2O or (NH4)3 Ce(NO3)6 zH2O. Of course, other Ce salts can also be used. When the temperatures of target 77', the surface of glass body 77 and glass fine particles73, 76 are sufficiently high, a transparent glass body 77 is obtained, but when the temperatures are low, glass body 77 is deposited as powder.
In the latter case, the powder can thereafter be sintered in a furnace at a high temperature to give a transparent glass bod~.
The above described embodiment is carried out by VAD method, but is not always limited thereto. Modifications of the embodiment shown in Fig. 7 are of course possible and O-CVD method can also be adapted thereto.
The transparent glass rod obtained in this wa~ is stretched to give a glass rod with a suitable diameter on which a cladding glass of B2O3-SiO2, P2O5-F-SiO2, ~-SiO2 or B203-F-SiO2 is deposited by O-CVD method. If necessary, for the protective purpose, SiO2 or ZrO2-, ~iO2-, Al2O3_or HfO2-doped SiO2 can be deposited on the outside thereof b~
O-CVD method. In some cases, a cladding glass of B203-SiO2, P2Os-F-SiO2,F_SiO2 or B2O3-~-SiO2 can be provided inside a quartz tube, from which a preform can be obtained as "rod-in-tube".
A glass fiber with a number of OH groups meets with a great loss under radiation at wavelengths correspon-ding to the vidration absorption of OH group or the vibra-tion absorption of OH group and SiO4 , i.e. 2.7~U~n, 1.3JU~ , O.95 ~, etc., which has some influences on the other wave-length rane. Even in the presence of O~ groups, however, 1 1 ~893~

l increase of the loss caused thereby is not so large at the loss of light source wavelengths of ~ED (Light Emitting Diode) or ~D (Laser Diode) in the range of ~ = 0.82-0.87 ~m.
In a glass with OD groups rather than OH groups, the wave-lengths at which the absorption loss is great are shifted to wavelength ~ times9 resulting in decreased effect. OD group has the similar effect of promoting to diminish the defects to OH group.
The content of OE groups and/or OD groups in the glass fiber of the present invention will hereinafter be illustrated. In the prior art optical transmission fiber, the loss due to OH groups is 1.25 dB/km per 1 ppm wt of OH
groups at A = 0.945 ~m, i.e. 50 dB/km in the presence of 40 ppm of OH groups, and is increased by 2 to 20 d3/km in the range of ~ = 0.80-0.90 ~m. ~hus, such a fiber i5 not con-sidered to be used for optical communication systems. In the presence of radiations such as X-rays and ~-rays, however, there can be used a fiber whose transmission loss is high but is not so increased in the presence of radiations, since the distance of optical transmission is short, i.e. 100 m or less. For example, an optical fiber consisting of a silica core and a silicone resin clad, containing 40 ppm of OH groups, meets with only a loss of 0.2 to 2 dB in 100 m. If less than 40 ppm, recovery of the loss due to structural defects is too late to be put to practical use, while if more than 40 ppm, the more, the better.
Methods of making a glass rod containing OH groups and/or OD groups and a glass fiber will now be illustrated without limiting the present invention. In particular, it is important herein to prepare a silica glass containing OH groups or OD groups. In one example, H2 and/or D2 as a combustion gas and 2 gas as a combustion aid are fed to a burner consisting of a quartz glass tube to form a flame, : - 1 168~38 into which an organic compound such as Si(OC2H5)4 or a hydride such as SiH4, as a raw material, is introduced by the aid of a suitable carrier gas to effect a flame hydro--~ lysis or falme oxidation reaction and to form a fine powder of silica glass containing OH groups and/or OD groups, and the resulting powder is blown against and deposited in the state Or fused glass on a rotating target by O-CVD method or VAD method. Depending upon the deposition method, any form of round rods or cylinders can be obtained as well known in tbe art. In another example, a gaseous Si-containing compound and 2 gas are introduced into the above described oxyhydrogen flame or plasma flame and a glass fine powder mass is prepared by O-CVD method VAD method, which is then sub~ected to sintering and clear vitrification in an atmos-phere containing H2O and/or D2O, or H2 and/or D2, thus ,~
obtaining a high purity silica glass containing large amounts of OH groups and/or OD group~. Then, the surface of this ., ~
glass rod is subaected to cylinder grinding or polishing and further to HF polishing, C02 laser polishihg or flame polishing to obtain a clean and smooth glass rod. ~his glass rod is subJected to melt spinning in a furnace at a high temperature and coated with a silicone resin or fluo-rine resin before taking up the fiber on a reel, followed by ; baking, to thus obtain a glass fiber of the present inven-- tion. In a further example, a gaseous raw material contai-ning at least one of F and B and gaseous raw material con-taining Si such as SiC14 with 2 are introduced into a plasma flame of high freguency plasma torch by the aid of a suitable carrier gas and a silica glass doped with F and/or B2O3 is deposited on the outer surface of a rotating glass rod synthe-sized as above. During the same time, if necessary, a com-pound containing H or D such as H2O or D2O can be added to 1 ~893~

1 synthesize a glass containing OH groups or OD groups, from which a glass fiber of the present invention can be obtained through melt spinning. Subsequently, on the outer surface of this synthesized glass rod is further deposited a silica glass by introducing a gaseous raw material containing Si such as SiC14 with 2 by the aid of a carrier gas into a flame or plasma flame. During the same time, if necessary, ~iC14, AlC13 or ZrC14 can be added to deposit a glass doped with TiO2, A12O3 or ZrO2. In spite of depositing a synthe-sized glass rod on the outside thereof, the above describedsynthesized glass rod with the cladding can be inserted in a suitable vycor glass or quartz glass tube, collapsed toa-rod and then subjected to spinning, or the glass tube can be spun with collapsing as it is. In a still further example, a silica glass doped with B2O3 and/or F is deposited on the inside of a quartz glass or vycor glass tube by the prior art M-CVD method or P-CVD method. A silica glass rod con-taining OH groups and/or OD groups1 as described above, is inserted in the inside of this tube and subjected to melt spinning after or during collapsing. If a compound of H
or D is added to the gaseous raw material when this silica glass doped with ~23 and/or F is deposited, a clad glass containing a number of OH groups or OD groups can be prepared.
In some cases, a silica glass core containing a number of OH groups or OD groups can simultaneously be prepared by M-CVD method.
According to the present invention, there is provi-ded an optical transmission glass fiber whose increase of the loss is small even in the presence of radiations. ~hat is to say, the use of fibers consisting of a silica glass synthe-sized at a low temperature and containing some OH groups or OD groups, a silica glass synthesized at a low temperature and containing some OH groups or OD groups in addition to --~0--`-` 116893~

l Sb2O3 or a silica glass containing a number of OH groups or OD groups according to the present invention makes possi-ble optical communications, illuminations and image trans-missions in high radiation ranges, because increases of the transmission loss is thereby suppressed in the presence of radiations such as X-rays and ~-rays.
~ he follosing examples are given in order to illu-strate the present invention in more detail without limiting the same.
Example Various glass fibers were prepared by the conditions as shown in Table 1 and subjected to examination of the change of the transmission loss under irradiation of ~-rays.

Table Sample Core No. Material Making Method 1CeO2-P2O5_SiOM-CVD, high temperature 2 SiO2P-CVD, high temperature 3 P205-SiO2P-CVD, lower than Sample No. 2 A SiO2Flame Oxidation (SiH~ SiO2) B SiO2Plasma Synthesis (SiCl4~SiO2) C SiO2 Bernoulli Method Cladding Jacket P2o5-B2o3-sio2M-CVD, high temperature Quartz Glass F-doped SiO2P-CVD, high temperature Quartz Glass F-doped SiO2 P-CVD, hiæh temperature Quartz Glass Silicone Resin Coating and Baking Silicone Resin Coating and Baking Silicone Resin Coating a~d Baking l These glass fibers were reinforced by a primary coating of silicone resin and a secondary coating of polyethylene or nylon, wound round an aluminum reel of 280 mm in diameter with a length of 5 to 30 m and subjected to irradiation of ~-rays at a dose rate of 1.2 x 106 R/H from 60Co placed at the center thereof. Change of the transmission loss during irradiation is examined by monitoring the intensity of the transmission light of IED ( ~ = 0.83JUm) and change of the transmission loss after irradiation is examined in the range of 0.8 to 1.6J~m.
When the various fibers were subjected to irradi-ation of ~-rays, the output of the transmission light inten-sity of ~ED was varied as shown in Fig. 3-(a) and Fig. 4-(a).
~ig. 3 (a) is a graph showing the relationship between the increased quantity of transmission loss and the ~-rays irradiation time when the cladding is of a plastics, in which Curve A shows that of an optical fiber according to the present invention consisting of a core glass of SiO2 synthesized at a low temperature from SiH4 while Curves B
and C show respectively that of the prior art optical fibers, the former consisting of a core glass of SiO2 synthesized by the plasma synthesis of SiGl4 and the latter consis-tng of a core glass of SiO2 obtained by heating and melting natural quartz crystal at a high temperature by the Bernoulli method (H2 + 2 flame). In the case of C, the transmission loss is larger because of the higher synthesis temperature and larger amounts of impurities. In comparison of the cases A and B, on the other hand, there is a large difference in the quantity of loss in spite of that there is not such a large difference in purity between them. This is possible due to the diffe-rence of synthetic temperatures of glass. That is, in the fiber A of the present invention, the glass is synthesized l at a relativel~ low temperature because of using ~iH4 as a raw material, thus resulting in decreased structural defects as well as high radiation resistance. ~his tendenc~ can also be seen in a case where the cladding and jacket consist of glass, as shown in Fig. 4 (a). ~he optical fiber of Sample No. 3 acccrding to the present invention, containing additio-nally P205, can be synthesized at a lower temperature than the optical fiber of Sample No. 2, containing SiO2 only.
Therefore, even if core glasses are prepared by the same P-CVD method, there are few str~ctural defects resulting in decrease of the transmission loss in a case where the core glass is synthesized at a lower temperature. In addition, even in the case of doping P205, increase of the transmission loss cannot be so suppressed as Sample No. 3 of the present invention according to circumstances, i.e. when the core glass is synthesized by M-CVD method with codoping GeO2.
When the fiber of Sample No. A is subjected to intermittent irradiation of r -rays, the increased quantity of transmission loss is changed as shown in Figo 3 (b) in which the hatched portions under the axis of abscissa mean irradiation time (hour).
Moreover, the similar data were measured as to the following samples to obtain results shown in Fig. 4 (b).
Sample No. 4 GeO2-SiO2 core/SiO2 clad fiber No. 5 Ge02-B203-siO2 cre/B23-si2 clad fiber No. 6 Ge02-P2os-sio2 Cre/P205 Si2 clad fiber No. 7 Geo2-p2o5-B2o~-sio2 core/B2o~-p2o5-sio2 clad fiber No. 8 P205-SiO2 core/F-SiO2 clad fiber No. 9 SiO2 core/F-SiO2 clad fiber (Core was s~nthesized by plasma oxidation and decomposition of SiCl4) No.10 Cs20-B203-SiO2 cre/B23-si2 clad fiber -2~-1 ~893~

l In this figure, T on the axis of abscissa means "20 minutespast after removal of 60Co".
Example 2 A guartz tube of 8 mm~ x 10 mm~ was placed in an electric furnace moved reciprocatedly at a rate of 50 cm/hr and heated at 1400 C~ Into this tube were flowed 5 % Si~4 - 95 N2 gas at a rate of 100 cm3/min and 40 % 2 - 60 % ~e gas at a rate of 2000 cm3/min for 10 hours to synthesize and deposit an SiO2 glass on the inner wall thereof. ~hen, 5 %
SbH3 - 95 % N2 gas at a rate of 50 cm3/min, 5 % SiH4 - 95 N2 at a rate of 100 cm3/min and 40 % 2 ~ 60 ~ He gas at a rate of 3000 cm3/min were flowed therein for 5 hours to syn-thesize and deposit an Sb203-SiO2 glass on the deposited inner wall. The thus resulting tube was subjected to glass lathe , heated at 1800 C in an ~2/2 flame and collapsed to give a glass rod (D) with an outer diametex of 8 mm~, a clad diameter of 5 mm~ and a core diameter of 3 mm~.
In the similar manner, another glass rod (E) having an outer diameter of 8 mm~, a clad diameter of 5 mm~ and a core diameter of 3 mm~ was prepared using a quartz tube of the same dimension, i.e. 8 mm~ x 10 mm~ and using SiC14 and 2 as a raw material for cladding and SbC15,SiCl4 and 2 as a raw material for core which were subjected to M-CVD method.
These glass rods were charged in an electric furnace, heated at 1800 C to form a fiber of 150 ~m~ and then coated with a primary coating of epoxy resin and further with a secondary coating of ethylene propylene rubber thus obtaining optical fibers. When the thus obtained optical fibers were subjected to irradiation of ~-rays at a rate of 1 x 106 R/h for 5 hours, the transmission losses measured were increased by 150 dB/Km in the case of Fiber D and by 1500 dB/Km in the case of Fiber E

1 l6ss3a 1 Example 3 ~sing a system as shown in Fig. 7, 10 wt ~ aqueous solution of Ce(N03)~ was fed at a rate of 3 x 10 5 mol/min as Ce(N03)3 to form a spray-like aqueous solution 75 while H2 at a rate of 2000 ml/min, 2 at a rate of 4QOO ml/min and SiH4 at a rate of 112 ml/min were simultaneously fed to burner 71, thus obtaining a glass rod 77 with a diame-ter of 15 mm~. This glass rod was stretched to a diameter of 10 mm~, inserted into a quartz pipe of 12 mm~ x 14 mm~
on the inner wall of which B203-F-SiO2 glass had been pre-viously deposited by M-CVD method to give a thickness of 1 mm~, and collapsed to form a preform of 14 mm~ in diameter.
The thus resulting preform was charged in a resistance fur-nace of carbon, melt spun in a fiber of 140 ~m~ and coated with an epoxy resin. When this fiber was subjected to irra-diation of ~-rays at a dose rate of 1 x 106 R/h for 1 hour, increase of the transmission loss was small, i.e. 50 dB/km at ~ m.
Example 4 SiH4 gas with a carrier gas of D2 were introduced into an oxyhydrogen flame by a quartz tube burner, subjected to flame oxidation to form glass fine particles and deposited or accumulated in fused state by VAD method to form a glass r d of 12 mm~ x 200 mml containing 900 ppm of OH groups and 100 ppm of OD groups. ~his glass rod was then subjected to grinding and polishing to a diameter of 10 mm~.
The thus resulting glass rod and another glass rod containing less than 10 ppm of OH groups, synthesized by high frequency plasma flame, were melt spun to form glass fibers of 150 ~m~ and immediately coated two times with a silicone resin to give a thickness of 50 ~m + 50 ~m. ~hese fibers with a length of 100 m was subjected to irradiation of X-rays 1 16893~

1 of 2000 R. ~he fiber of the present invention showed no increase of loss while the comparative fiber containing less than 10 ppm of OH groups showed a considerable increase of loss.
On the other hand, in a Ge-doped fiber prepared by the prior art M-CVD method, the increase of loss was too large to allow light to be passed therethrough.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A radiation resistant optical glass fiber compris-ing at least two portions having different refractive indexes the portion having the higher refractive index consisting pre-dominantly of a silica glass prepared by the oxidation of a silicon compound selected from the group consisting of SiH4, SiHCl 3, SiH2C12, Si2H6, Si3H8, (C2H5O)4Si, (CH3O)4Si, CH3Cl3Si, CH3Cl2SiH, CH3ClSiH2, CH3SiH3, C2H5C13si, C2H5Cl2SiH, C2H5ClSiH2, C2H5SiH3, C3H7C13Si, C2H7C12SiH2, C3H7ClSiH, C3H7SiH3, C4H9C12Si C4H9SiH, C5H11Cl2SiH, C5HllSiH, C8H13SiH3, C7H15SiH3 and C8H17SiH3 at a temperature of at most 1600 °C.

2. The radiation resistant optical fiber of claim 1, wherein the higher refractive index part consists of an Sb2O3-Sio2 glass.

3. The radiation resistant optical fiber of claim 2, wherein the Sb2O3-SiO2 glass is prepared by oxidation of SiH4 or an organic compound of Si and SbH3 or an organic compound of Sb.

4. The radiation resistant optical fiber of claim 1, wherein the higher refractive index part consists of a Ce-doped SiO2 glass.

5. The radiation resistant optical fiber of claim 4, wherein the Ce-doped SiO2 glass is synthesized by oxidizing or decomposing at a relatively low temperature a hydride of Si or an organic compound of Si with a Ce-containing compound.

6. The radiation resistant optical fiber of claim 5, wherein the Ce-containing compound is selected from the group consisting of cerium nitrate and cerium ammonium nitrate.

7. The radiation resistant optical fiber of claim 1, wherein the lower refractive index part consists of at least one member selected from the group consisting of silicone resins and fluorine resins.

8. The radiation resistant optical fiber of claim 1, wherein the lower refractive index part consists of silica glass.

9. The radiation resistant optical fiber of claim 8, wherein the silica glass is synthesized by oxidizing a relative-ly low temperature a hydride of Si or an organic compound of si .

10. The radiation resistant optical fiber of claim 1, wherein the lower refractive index part consists of at least one member selected from the group consisting of F-SiO2, B2O3-SiO2, P2O5-F-SiO2, B2O3-F-SiO2 and B2O3-P2O5-F-SiO2.

11. The radiation resistant optical fiber of claim 10, wherein the lower refractive index part is synthesized by oxidizing at a temperature of 1100 to 1500 °C SiH4, SiF4, BF3, B2H6, PF3 and/or organic compounds of Si, B, P and F.

12. The radiation resistant optical fiber of claim 1, wherein the outside of the lower refractive index part is further coated with at least one member selected from the group consisting of quartz glass and SiO2 glass doped with at least one of TiO2, ZrO2, A12O3 and HfO2.

13. The radiation resistant optical fiber of claim 1, where the fiber has a primary coating consisting of a thermo-setting resin selected from the group consisting of polyimides and epoxy resins, and a secondary coating consisting of a thermoplastic resin selected from the group consisting of ethylene propylene rubbers and bridged polyethylenes.

14. A process for the production of a radiation resistant optical fiber, which comprises introducing a silicon compound selected from the group consisting of SiH4, SiHC13, SiH2C12, SiH6, Si3H8, (C2H5O)4Si, (CH3O)4Si, CH3C13Si, CH3C12SiH
CH3ClSiH2, CH3SiH3, C2H5C13Si, C2H5C12SiH, C2H5ClSiH2, C2H5SiH3, C3H7Cl 3Si, C2H7C12SiH2, C3H7ClSiH, C3H7SiH3, C4H9ClSiH

C8H17SiH3 at a temperature of at most 1600 °C to synthesize glass fine particles, depositing the glass fine particles on a starting member to thus form a glass rod as a core, and combin-ing the core with a cladding to form one body.

15. A process for the production of a radiation resistant optical fiber, which comprises introducing a hydride of Si or an organic compound of Si into an oxhydrogen flame made of at least one of H2 and D2 and O2 to synthesize glass fine particles, depositing the glass fine particles in fused state on a starting member to thus form a glass rod as a core, and combining the core with a cladding to form one body.

16. A process for the production of a radiation resistant optical fiber, which comprises introducing a hydride of Si or an organic compound of Si into an oxyhydrogen flame to synthesize glass fine particles, depositing the glass fine particles as a soot on a starting member, subjecting to sin-tering and clear vitrification in He gas or in an atmosphere containing at least one of H2O and D2O, thereby forming a silica glass rod as a core, and combining the core with a clad-ding to form one body.

17. The process of claim 15 wherein a hydride of Sb or an organic compound of Sb is further added to the oxyhydrogen flame to synthesize Sb2O3-SiO2 glass fine particles.

18. The process of claim 16 wherein a hydride of Sb or an organic compound of Sb is further added to the oxyhydrogen flame to synthesize Sb2O3-SiO2 glass fine particles.

19. The process of claim 17 or 18 wherein the hydride of Sb is SbH3.

20. The process of claim 15 wherein at least one compound selected from the group consisting of hydrides, halides and organic compounds of Si, B, P and Sb is further added to the oxyhydrogen flame.

21. The process of claim 16 wherein at least one compound selected from the group consisting-of hydrides, halides and organic compounds of Si, B, P and Sb is further added to the oxyhydrogen flame.

22. The process of claim 15 or 16 wherein at least one Ce-compound selected from the group consisting of cerium nitrate and cerium ammoniam nitrates is further added to the oxyhydrogen flame.

23. The process of claim 20 or 21 wherein at least one Ce-compound selected from the group consisting of cerium nitrate and cerium ammonium nitrates is further added to the oxyhydrogen flame.

24. The process of claim 14 comprising introducing SiH4 gas and oxygen gas or a gas capable of yielding oxygen at a high temperature into a quartz glass tube, heating and reacting the mixed gases to form an SiO2 glass, depositing the glass on the inner wall of the tube, then introducing further thereinto SbH3, SiH4 and oxygen gas or a gas capable of yielding oxygen at a high temperature, heating and reacting the mixed gas to form an Sb2O3-SiO2 glass, depositing the glass on the deposited inner wall of the tube, heating and collapsing the tube at a higher temperature and then subjecting to melt spinning.

25. The process of claim 24, wherein the gas capable of yielding oxygen at a high temperature is CO2.

26. The process of claim 14 comprising depositing SiO2 glass or an SiO2 glass doped with at least one member selected from the group consisting of F, B2O3-F, B2O3 and P2O5-F in the hollow of a silica pipe, inserting a silica glass rod to be a higher refractive index part in the pipe and subjecting the composite body of the pipe and glass rod to melt spinning directly or after collapsing to form one body by a glass lathe.

27. The process of claim 14 comprising introducing SiH4 or SiF4 gas optionally with at least one selected from the group consisting of BF3, B2H6 and PF3 into a plasma flame or oxyhydrogen flame consisting of at least one of H2 and D2, and O2 to synthesize fine particles of SiO2 glass or an SiO2 glass doped with at least one member selected from the group consiting of B203, F, B203-F and P2O5, corresponding to a lower refractive index part, depositing the glass fine parti-cles on the outside of a silica glass rod corresponding to a higher refractive index part, optionally depositing further a silica glass thereon or covering with a silica pipe and then subjecting to melt spinning.

28. The process of claim 26 or 27, wherein the silica glass rod is prepared by introducing an Si hydride or an Si organic compound into an oxyhydrogen flame made up of at least one of H2 and D2, and O2 to synthesize glass fine particles and depositing the glass fine particles in fused state on a starting member.

29. The process of claim 26 or 27, wherein the silica glass rod is prepared by introducing an Si hydride or an organic Si compound into an oxyhydrogen flame to synthesize glass fine particles, depositing the glass fine particles as a soot on a starting member, and subjecting to sintering and clear vitrification in He gas or in ah atmosphere containing at least one of H2O and D2O.

30. The process of claim 27, wherein the SiO2 glass of the lower refractive index part is deposited in fused state.

31. The process of claim 27, wherein the SiO2 glass of the lower refractive index part is deposited in the state of a soot and sintered.