GB2071644A

GB2071644A - Radiation Resistant Optical Fibers and a Process for the Production Thereof

Info

Publication number: GB2071644A
Application number: GB8105047A
Authority: GB
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: 1981-02-18
Publication date: 1981-09-23
Also published as: CA1168938A; GB2071644B; DE3106389C2; DE3106389A1

Abstract

An improved radiation resistance optical transmission fiber of this invention comprises a portion having a higher refractive index and a portion having a lower refractive index the higher refractive index portion consisting predominantly of a silica glass synthesized by oxidation of a hydride such as SiH4 or an organic compound such as Si(OC2H5)4 at a relatively 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 O2 to synthesize glass particles, depositing the glass particles on a starting member to form a higher refractive index portion of silica glass and combining the higher refractive index portion with a lower refractive index portion to form one body as a fiber.

Description

SPECIFICATION Radiation Resistant Optical Fibres and a Process for the Preparation Thereof This invention relates to an optical transmission glass fiber and a process for the production thereof. More particularly, it is concerned with a radiation resistant optical transmission glass fiber which is used in optical communication system, which may be affected by radiation such as X-rays and y-rays, image transmission systems or illumination systems, and a process for the production thereof.

The range of use of optical transmission glass fibers is increasing in the fields of the control lines of aeroplanes, the control lines of ships, the wirings of computers and the control or communication lines of works or buildings in addition to the telecommunications field. These fibers have various advantages in that signals of large capacity can be made using a small space, they are non-inductive due to electrical insulation and they are light as well as possessing an excellent flexibility. Thus much effort has been made to improve the optical transmission glass fibers and their properties have thus been improved in that transmission loss, broad band transmission and practical strength. Of late, optical transmission glass fibers have been put to practical use.

Up to the present time, the following giassfibers have been known as optical transmission glass fibers: (1) Fibers consisting of a core of SiO2 glass and a cladding of a silicone resin, (2) Quartz glass fibers consisting of a core of P205-GeO2-Si02 or P205-B203-GeO2 and a cladding of B203-SiO2 or B203-F2O5-SiO2.

(3) Quartz glass fibers consisting of a core of SiO2 glass and a cladding of B203 and/or F-doped SiO2 glass, (4) Multicomponent glass fibers which both of the core and cladding consist of a borosilicate or a soda lime glass and (5) High silicate glass fibers consisting of a core of Cs2-B203-Si02 glass and a cladding of B203-SiO2 glass.

Recently, it has been desired to use these optical glass fibers, in particular, under the influence of the radiation of X-rays in the medical or industrial fields or y-rays in the field of handling nuclear reactors or radioactive elements, so as to make the best use of the electrical insulation and flexibility of the optical glass fibers. In the presence of such radiation, however, glass undergoes structural defects to result in a very large transmission loss. In particular, the above described glass fibers of types (2), (4) and (5) or plastics fibers undergo a large transmission loss and those of types (1) and (3) also have a considerably increased transmission loss. Increase in the transmission loss of these fibers is discussed by, for example E. J. Friebele et al in "Laser Focus" Sept., 1978, page 50--56.

Therefore, fibers which have hitherto been proposed or developed cannot be used in the presence of radiation because of their increased transmission loss. In the preparation of the prior art glass fibers, halogen compounds such as SiCi4 with a high cohesive strength are used as a raw material with a number of OH groups is decreased in order to lower the transmission loss. However, the hydrolysis or thermal oxidation is carried out at a high temperature such as above 1 6500C, which tends to cause structural defects such as defects due to oxygen. Thus, the number of defects produced by radiation is large while the diminishing speed of the defects is too low considering for the quantity of the defects produced by radiation, resulting in an increse in the transmission loss.

On the other hand, a glass doped with CeO2 or Sb203 is used as a window glass subjected to radiation. The electrons or positive holes produced by radiation damage are trapped by the electron capture centers of Ce4+ and the positive hole capture centers of Ce3+ and are not trapped by lattice defects. Thus the structural defects are not altered and coloration of the glass is suppressed. However, the composition of such a window glass is not suitable for use as an optical transmission fiber since scarcely purified compounds are used as the starting materials and the length of transmission of light is inferior by the order of 1 02--105, SO that the loss due to radiation would be too large ( 102 dB/km at 1 06 R-y).

We have now developed an optical transmission glass fiber and a method for the producing thereof, which overcome the hertofore noted disadvantages.

Accordingly, the present invention provides a radiation resistant optical glass fiber comprising at least two portions having different refractive indexes, the portion having the high refractive index consisting predominantly of a silica glass prepared by the oxidation of a silicon hydride or a siliconcontained organic compound at a relatively low temperature.

In the optical transmission glass fibers of the invention light advances concentrically in the part consisting of a high purity silica glass substantially free from structural defects, so as to minimize the infiuence of the structural defects. The optical transmission glass fibers may incorporate some OH groups or OB groups to decrease the defects produced by radiation and to decrease further the transmission loss.

The optical glass fibers of the invention comprise a higher refractive index portion and a lower refractive index portion, the higher refractive index portion consisting predominantly of a silica glass synthesized by the oxidation of a silicon hydride, such as SiH4, or a silicon-containing organic compound, such as Si(OC2H5)4. The radiation resistant optical transmission glass fibers of the invention may be produced by 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 02 to synthesize glass particles, depositing the glass particles onto a starting member to form a high refractive index portion of silica glass and combining the higher refractive index portion with a lower refractive index portion to form one body as a fiber, by oxidising SbH3 gas and SiH4 gas or Si(OC2H5)4 to form an Sb203-Si02 glass, depositing this glass and an SiO2 glass obtained by oxidizing SiH4 gas or Si(OC2H5)4 and subjecting the composite to melt spinning.

In accordance with the present invention, a glass having decreased structural defects produced by lowering the synthesis temperature thereof without using a compound decomposable at high temperatures such as chlorides is used, thus providing a glass fiber with a decreased radiation susceptibility. If necessary, OH groups or OD groups are also incorporated in the glass to some extent so that the transmission loss is further decreased.

The present invention also provides a process for producing radiation resistant optical fibers which process comprises oxidizing SbH3 gas with SiH4 gas and/or Si(OC2H5)4 gas to form an Sb203- 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 composite to melt spinning.

In the present invention, the synthesis temperature of the glass is lowered for example, to below 1 5500C, preferably below 1 4000 C, for an optical fibers comprising an SiO2-Sb203 glass and the structural defects are thereby suppressed. In this glass composition, OH groups or OD groups are optionally incorporated to decrease the transmission loss. The transmission loss due to structural defects is kept as low as possible by doping a silica glass with Sb203 as a dopant so that light advances concentrically in the part consisting of a high purity silica glass free from impurities other than OH groups or OD groups.

In addition, the present invention provides a radiation resistant optical transmission fiber comprising a higher refractive index portion and lower refractive index portion the higher refractive index portion consisting predominantly of a Ce-doped SiO2 glass synthesized by oxidising or decomposing a hydride such as SiH4 or an organic compound such as Si(O2H5)4 with a Ce-containing compound at a relatively low temperature such as below 1 6000 C, preferably 1100 to 1 5500 C. That is to say, a SiO2 glass containing preferably 0.001 to 1% of Ce203 and CeO2 is prepared whereby electrons and positive holes are trapped by the electron capture center of Ce4+ and the positive hole capture center of Ce3+ with the effect of suppressing increase of the transmission loss.Using this glass as a core, an optical transmission fiber is produced in which increase of the transmission loss is suppressed even under radiation.

Examples of the other hydrides which can be used in the present invention are SiH2CI2, SiHCI3, Si2H6 and Si3HB. Examples of the other organic compounds which can be used in the present invention are CH3CI3Si, CH3CI2SiH, CH3CISiH2, CH3SiH3, C2H5Cl3Si, C2H5Cl2SiH, C2H5CISiH2, C2H 5SiH3, C3H7C13Si, C3H,CI2SiH, C3H7CISiH2, C3H7SiH3, C4HgCI2SiH, C4H9SiH3, C5H,1CI2SiH, C5H"SiH3, 0eH13SiH3, C,H,5SiH3, C8H"SiH3 and (CH30)4Si.

The present invention will be further described with reference to the accompanying drawings, Figure 1(A), (B) and (C) are cross-sectional views of embodiments of optical transmission fibers according to the present invention; Figure 2 ;s a cross-sectional view of an optical glass fiber of the present invention to illustrate the primary coating and secondary coating; Figure 3(a) is a graph showing the relationship between the increase in transmission loss and the y-rays irradiation time for comparison of an optical fiber of the present invention with the prior art; Figure 3(b) is a graph showing the same relationship as that of Figure 3(a) but for an intermittent irradiation;; Figure 4(a) is a graph showing the relationship between the increase in transmission loss and the y-rays irradiation time for comparison of an optical fiber of the present invention with the prior art for a case where the cladding and jacket consist of glass; Figure 4(b) is a graph showing the same relationship as that of Figure 4(a) but using other optical fibers; Figure 5 and Figure 6 are cross-sectional views of other embodiments of the optical transmission glass fibers according to the present invention; Figure 7 is a schematic view of an apparatus suitable for the production of the optical fiber according to the present invention by the VAD method;; Figure 1 (A) shows one embodiment of the optical fiber of the present invention, in which core 1 A is of a high purity silica glass containing some OH groups or OD groups and synthesized at a low temperature and cladding 1 2A is of a silicone resin or fluorine resin. Figure 1 (B) shows another embodiment of the optical fiber of the present invention in which core 11 B is of a high purity silica glass containing 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 B203, F, B203-F and P205-F and syntheiszed at a low temperature, which may contain some OH groups or OD groups. Figure 1(C) shows a further embodiment of the optical fiber of the present invention, in which 1 1C is the same as 11 B, 1 2C is the same as 12B and jacket 1 3C is of, for example, a silica glass. These glass fibers are radiation resistant, but in order to prevent deterioration of the mechanical strength thereof, it is desirable to provide the outside of glass fiber 21 as shown in Figure 2 with a primary coating 22 of a thermosetting resin such as polyimide resin or epoxy resin (which should be coated directly after forming the glass fiber) and further with a thermoplastic resin 23 such as ethylene propylene rubber or bridged polyethylene by extrusion.

Methods of making a glass rod or preform as a raw material of glass fiber, and a glass fiber will now be illustrated. In one example, H2 and/or D2 as a combustion gas and 02 as a combustion aid are fed to a burner consisting of a quartz glass tube to form a flame, into which an organic compound such as Si(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 oxidation reaction and to form a fine powder of silica glass 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 revolving target by the O-CVD (Outside Chemical I Vapour Deposition) method or VAD (Vapour-phase Axial Deposition) method.Depending on the deposition method, any form of round bars or cylinders can be obtained as is well known in the art. In another example, a gaseous Si-containing compound such as Si(OC2H5)4 or SiH4 and 02 gas are introduced into the above described oxyhydrogen flame and a glass fine powder mass is prepared by the O-CVD method or VAD method, which is then subjected to sintering and clear vitrification in an atmosphere containing some H2O and/or D2O, or some H2 and/or D2, thus obtaining a high purity silica glass bar containing large amounts of OH groups and/or OD groups. The surface of this glass bar is then 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.This glass bar is subjected to melt spinning in a furnace at a high temperature and coated with a silicone resin or fluorine resin before takiing taking up the fiber on a reel, followed by baking, to thus obtain a glass fiber of the present invention. In a further example, a gaseous raw material containing at least one of F, B and P and a gaseous raw material containing silicon such as Si(OC2H5)4 or SiH4 with 02 are introduced into the plasma flame of a high frequency plasma torch by the aid of a suitable carrier gas and a silica glass doped with any one of F, B203, 82O3-F and P205-F is deposited on the outer surface of a rotating glass rod synthesized as described above.At the same time, if necessary, a compound containing H or D such as H2O 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 by melt spinning. In this case, compounds such as SiH4, SiF4, SF3, B2H6 or PF3 are introduced into an oxyhydrogen flame made up of H2 and/or D2 or 2 to synthesize silica glass fine particles doped with any one of F, B203, 8203-F and P205-F and deposited in the fused state on a glass rod as a core as described above.Of course, the above described glass fine particles can be deposited as a powder soot, followed by sintering in an atmosphere containing H2O or D2O, thus can 2' providing a transparent glass. Subsequently, on the outer surface of this synthesized glass rod a silica glass is deposited by introducing a gaseous raw material containing Si such as SiCI4 with 02 by the aid of a carrier gas into a flame or plasma flame. At the same time, if necessary, TiCI4, AICI3 or ZrCI4 can be added to form a glass dopoed with TiO2,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 subjected to spinning, or the glass tube can be spun as it is. In a still further example, a silica glass dopoed with any one of B203, F, 8203F or P205-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 (Modified Chemical Vapour Deposition) method or P-CVD (Plasma-activated CVD) method using a hydride such as SiH4, an organic compound such as Si(OC2H5)4 and/or other compounds such as SiF4, SF3, B2H6 and PF3 as gaseous raw materials.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 to form a glass fiber of the present 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 B203 and/or F, synthesizing 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 subjecting the resulting composite tube to melt spinning directly or after collapsing to form a rod.

Figure 5 shows a cross-sectional view of an optical fiber consisting of core 51 of Sb2O3-Si02 glass, cladding 52 of SiO2, 820 3-SiO2, F-S lO2 or F-8203-SiO2 glass and jacket 53 of a quartz or vycor glass. These glass fibers are radiation resistant, but in order to prevent deterioration in the mechanical strength thereof, it is desirable to provide the outside of glass fiber 61 as shown in Figure 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 extrusion.

The Sb203-SiO2 glass used herein is excellent in radiation resistance as well known in the art and is ordinarily synthesized by M-CVD method or by flame hydrolysis as follows: SiCI4+02=SiO2+2CI2 2SbCI5+3/202=Sb203+5Cl2 SiCl4+2H2+O2=SiO2+4HCI 2SbCl6+5H2+3/202=S8203+ 1 OHCI In the 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 transmission loss in the presence of radiation. In the method of flame hydrolysis, the reaction temperature is so high due to the use of chlorides that there remain a number of structural defects such as oxygen defects and the transmission loss increases in the presence of radiation.

The above described problem can be solved according to the present invention by selecting as the raw materials, 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.

Previously, the use of such hydrides as raw materials has not been taken into consideration since they cause an increase in the transmission loss in a longer wavelength zone.

Methods of making the above described optical fibers will now be illustrated. In one example, H2 and/or D2 as a combustion gas and 02 as a combustion aid are fed to a burner consisting of a quartz glass tube to form a flame, into which 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 gas 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 target by O-CVD method or VAD method. Depending on the deposition method, any types of round rods or cylinders can be obtained.In other example, a fine powder of the above described Sb203-Si02 glass can be prepared in the form of a powder mass by the O-CVD method or VAD method and sintered to form a transparent glass. Then, the surface of this glass rod is subjected to cylinder grinding or polishing and further to HF polishing, CO2 laser polishing or flame polishing to obtain a cleaned and smooth glass rod.H2 and/or D2 as a combustion gas and O2 gas as a combustion aid are fed to a burner consisting of a quartz 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 resulting powder is blown against and deposited in the state of a 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 baking, to thus ontain 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 SiCI4 with 02 are introduced into the plasma flame of a high frequency plasma torch by the aid of a suitable carrier gas a silica glass doped with F and/or B203 is deposited on the outer surface of a rotating glass rod synthesized as described above. At the same time, if necessary, a compound containing H or D such as H20 or D20 can be added to produce a glass containing OH groups or OD groups, from which a glass fiber of the present invention can be obtained by melt spinning.In this case, compounds such as SiH4, SiF4, or BF3 are introduced into an oxhydrogen flame made up of H2 and/or D2 and 02 to synthesize silica glass fine particles and subsequently a silica glass doped with B203 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 H20 or D20, thus providing a transparent glass. Subsequently, on the outer surface of this synthesized glass rod a silica glass is deposited by introducing a gaseous raw material containing Si such as SiCI4 with 02 by the aid of a carrier gas into a flame or plasma flame.At the same time, if necessary, TiCI4, AICI3 or ZrCI4 can be added to form a glass doped with TiO2, Al203 or Zero2. Melt spinning of these glass rods result in the 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 subjected to spinning, or the glass tube can be spun as it is.

In a further example, a silica glass consisting of pure SiO2 or doped with B203 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 compound such as Si(OC2H5)4, SiF4 and/or BF3 as a gaseous raw material. A silica glass rod synthesized at a low temperature and containing some OH groups and/or OD groups is placed in the inside of this tube and subjected to melt spinning thus producing a glass fiber of the present invention.

Furthermore, formation by the P-CVD or M-CVD is available. SiH4 gas optionally diluted with N2 and an oxidation gas such as CO2 or 02 are fed in a quartz 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 SiH4 gas diluted with Ar and an oxidation gas such as CO2 or O2 are fed therrein, and an Sb203-SiO2 glass is synthesized and deposited on the inner wall of the tube. Thereafter, the tube is heated at a high temperature to iaminate 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 the M-CVD method, further synthesizing and depositing at a low temperature a silica glass using SiH4 as a raw material and then subjecting this composite tube to melt spinning directly or after collapsing to form a rod.

In the present invention, an siO2 glass containing 0.001 to 1% q/o of Ce203+CeO2 is used as a glass through which light advances concentrically, since if the amount of Ce2O3+CeO2 is less than 0.001%, the centers of Ce4+ and Ce3+ are so dilute that electrons or positive holes are trapped by the structural defects to increase the transmission loss, while if more than 1%, the loss increases due to ultraviolet absorption by Ce3+ and Ce4+. When using wavelength of 1 0 ,um or more, however, the sum total can be allowed to at most 2% because the effect of ultraviolet absorption decreases.

Since such a core glass has a refractive index similar to, although somewhat higher than, that of quartz glass, the cladding should 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 8203-F-SiO2, P205-F-SiO2 F SiO2 or 82O3-P205-F-SiO2 glass. In the latter case, SiO2 or an SiO2 glass doped with TiO2, ZrO2, Al203 or HfO2 can further be provided on the outside thereof in order to increase the water resisting property and strength.

Onto the glass fiber is applied, as a primary coating, a thermosetting resin such as polyimide or epoxy resin that is resistant to radiation, directly after melt spinning the preform. When preparing a plastic clad fiber a silicone resin or fluorine resin is coated and baked directly after spinning. Onto this primary coating is further applied, as a secondary coating, a thermoplastic resin such as ethylene propylene rubber or bridged polyethylene for the purpose of reinforcement. When the transmission losI increases after electron irradiation the fiber can be subjected to heat treatment at a suitable temperature.

The present invention will now be illustrated by the use of SiH4 as a starting material without limitation thereof. Of course, other organic compounds such as Si(OC2H6)4 or dopants such as B2H6, PH3, SbH3, etc. which do not so increase the transmission loss under radiation can be used.

Referring to Figure 7, H2 gas and O2 gas, as a combustion 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 SiH4+202=SiO2+2H2O. At the same time, an aqueous solution of a Ce compound is spouted in the form of a spray 75 from exhaust nozzle 74 and glass fine particles having Ce203 and CeO2 in or on the glass fine particles are deposited or target 77' to form a glass soot or transparent glass body 77. If necessary, 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 pipe 79"' and to form a flow of the aqueous solution 75 with the form of a spray.As the aqueous solution, there are preferably used aqueous solutions of cerium nitrate Ce(NO3)3 xH2O and cerium ammonium nitrate Ce(NO3)3 NH4NO3 yH2O or (NH4)3Ce(NO3)6 . zH2O. Of course, other Ce salts can also be used. When the temperature of target 77', the surface of glass body 77 and glass fine particles 73, 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 body.

The above described embodiment is carried out by the VAD method, but is not always limited thereto. Modifications of the embodiment shown in Figure 7 are of course possible and the O-CVD method can also be adapted thereto.

The transparent glass rod obtained in this way is stretched to give a glass rod of a suitable diameter on which a cladding glass of 8203-SiO2, P205-F-SiO2, F-SiO2 or 8203-F-SiO2 is deposited by the O-CVD method. If necessary, for protecting purposes, Six, or ZrO2-, TiO2-, At 203 or HfO2-doped SiO2 can be deposited on the outside thereof by the O-CVD method. In some cases, a cladding glass of 8203-SiO2, P20-F-SiO2, FlSiO2 or 8203-FlSiO2 can be provided inside a quartz tube, from which a preform can be obtained as a "rod-in-tube".

A glass fiber with a number of OH groups meets with a great loss under radiation at wavelengths corresponding to the vibration absorption of OH group or the vibration absorption of OH groups and SiO4, i.e. 2.7 ym, 1.3 form, 0.95 ym, etc., which has some influences on the other wavelength ranges.

Even in the presence of OH groups, however, increase of the loss caused thereby is not so large as the loss of light source wavelengths of LED (Light Emitting Diode) or LD (Laser Diode) in the range of A=0.82--0.87 ,um. In a glass with OD groups rather than OH groups, the wavelengths at which the absorption loss is great are shifted to wavelengths atimes, resulting in a decreased effect. OD groups have the effect of diminishing the defects of OH groups.

The content of the OH groups and/or OD groups in the glass fiber of the present invention will hereinafter be illustrated. In the priort art optical transmission fibers, the loss due to OH groups is 1.25 dB/km per 1 ppm wt of OH groups at A=0.945 ym, i.e. 50 dB/km in the presence of 40 ppm of OH groups, and is increased by 2 to 20 d8/km in the range of A=0.80--0.90 ssm. Thus, such a fiber is not considered useful for optical communication systems. In the presence of radiation such as X-rays, and y-rays, however, a fiber can be used whose transmission loss is high but does not increase in the presence of radiation, 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 silicone resin clad, containing 40 ppm of OH groups, meets with only a loss of 0.2 to 2 dB in 100 m.

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, into which an organic compound such as Si(OC2Hs)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 hydrolysis or flame 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 of fused glass on a rotating target by the O-CVD method or VAD method. Depending upon the deposition method, any type of round rods or cylinders can be obtained as is well known in the art. In another example, a gaseous Si-containing compound and 02 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 subjected to sintering and clear vitrification in an atmosphere containing H20 and/or D20, or H2 and/or D2, thus producing a high purity silica glass containing large amounts of OH groups and/or OD groups.Then, the surface of this glass rod 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 rod. This glass rod is subjected to melt spinning in a furnace at a high temperature and coated with a silicon resin or fluorine resin before taking up the fiber on a reel, followed by baking, to thus obtain a glass fiber of the present invention. In a furthor example, a gaseous raw material containing at least one of F and B and a gaseous raw material containing Si such as SiCI4 with 02 are introduced into a plasma flame and a high frequency plasma torch by the aid of a suitable carrier gas and a silica glass doped with F and/or B203 is deposited on the outer surface of a rotating glass rod synthetized above.At the same time, if necessary, a compound 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 by 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 SiCI4 with 02 by the aid of a carrier gas into a flame or plasma flame. At the same time, if necessary, TiCI4, AICI3 or ZrCI4 can be added to deposit a glass doped with TiO2, Al203 or ZrO2. In spite of depositing a synthesized glass rod 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 a rod and then subjected to spinning, or the glass tube can be spun as it is. In a still further example, a silica glass doped with B203 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 containing OH groups and/or OD groups, 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 B203 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 the M-CVD method.

According to the present invention, there is provided an optical transmission glass fiber whose increase in transmission loss is small even in the presence of radiation. Thus, the use of fibers consisting of a silica glass synthesized 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 Sb203 or a silica glass containing a number of OH groups or OD groups according to the present invention makes possible optical communications, illuminations and image transmissions in high radiation ranges, because the increase in the transmission loss is thereby suppressed in the presence of radiation such as X-rays and y-rays.

The following examples are given in order to illustrate the present invention in more detail.

Example 1 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 1 Sample Core No. Material Making Method 1 CeO2-P205-SiO M-CVD, high temperature 2 SiO2 P-CVD, high temperature 3 P20s-sio2 P-CVD, lower than Sample No. 2 A SiO2 Flame Oxidation (SiH4~SiO2) B SiO2 Plasma Synthesis(SiC14eSiO2) C SiO2 Bernoulli Method Cladding Material Making Method Jacket P205-8203-SiO2 M-CVD, high temperature Quartz Glass F-doped SiO2 P-CVD, high temperature Quartz Glass F-doped SiO2 P-CVD, high temperature Quartz Glass Silicone Resin Coating and Baking Silicone Resin Coating and Baking Silicone Resin Coating and Baking These glass fibers were reinforced by a primary coating of silicone resin and a secondary coating of polyethylene or nylon, wound around an aluminium reel of 280 mm in diameter with a length of 5 to 30 m and subjected to irradiation of y-rays at a dose rate of 1.2 x 1 OG 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 LED (;1=0.83 ym) and change of the transmission loss after irradiation is examined in the range of 0.8 to 1.6 ym.

When the various fibers were subjected to irradiation of p-rays, the output of the transmission light intensity of LED was varied as shown in Figure 3(a) and Figure 4(a). Figure 3(a) is a graph showing the relationship between the increased quantity of transmission loss and the y-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 SiCI4 and the latter consisting of a core glass of SiO2 obtained by heating and melting natural quartz crystal at a high temperature by the Bernoulli method (H2+O2 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 difference of synthetic temperatures of glass. That is, in the fiber A of the present invention, the glass is synthesized at a relatively low temperature because of using SiH4 as a raw material, thus resulting in decreased structural defects as well as high radiation resistance. This tendency can also be seen in a case where the cladding and jacket consist of glass, as shown in Figure 4(a).The optical fiber of Sample No. 3 according to the present invention, containing additionally P2O5, 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 structural 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 P2O5, 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 p-rays, the increased quantity of transmission loss is changed as shown in Figure 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 Figure 4(b).

Sample No. 4 GeO2SiO2 core/SiO2 clad fiber No. 5 GeO2-B2O3-SiO2 core/B2O3-SiO2 clad fiber No. 6 GeO2-P205-SiO2 core/P205SiO2 clad fiber No. 7 GeO2-P205-B2O3-SiO2 core/B203-P205-SiO2 clad fiber No. 8 P2O5-5i02 core/F-SiO2 clad fiber No. 9 SiO2 core/F-SiO2 clad fiber (Core was synthesized by plasma oxidation and decomposition of SiC14) No. 10 Cs2O-B2O3-SiO2 core/B2O3-SiO2 clad fiber.

In this figure, T on the axis of the abscissa means "20 minutes past after removal of ##Co".

Example 2 A quartz tube of 8 mmf x 10 mmq was placed in an electric furnace moved reciprocatedly at a rate of 50 cm/hr and heated at 1 4000 C. Into this tube were flowed 5% SiH495% N2 gas at a rate of 100 cm3/min and 40% 0260% He gas at a rate of 2000 cm3/min for 10 hours to synthesize and deposit an SiO2 glass on the inner wall thereof. Then, 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% 0260% He gas at a rate of 3000 cm3/min were flowed therein for 5 hours to synthesize and deposit an Sb2O3-SiO2 glass on the deposited inner wall.

The thus resulting tube was subjected to glass lathe, heated at 1 8000C in an HO2 flame and collapsed to give a glass rod (D) with an outer diameter of 8 mum0, a clad dimaterdiameter of 5 mm and a core diameter of 3 mm .

In the similar manner, another glass rod (E) having an outer diameter of 8 mass, a clad dimater diameter of 5 mm) and a core diameter of 3 mm) was prepared using a quartz tube of the same dimension, i.e. 8 mmXx o mm) and using SiCI4 and 02 as a raw material for cladding and SbCl5, SiCI4 and 02 as a raw material for core which were subjected to M-CVD method.

These glass rods were charged in an electric furnace, heated at 18000C to form a fiber of 1 50 Mm) 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 y-rays at a rate of 1 x 106 R/h for 5 hours, the transmission lossess measured were increased by 150 dB/Km in the case of Fiber D and by 1 500 dB/Km in the case of Fiber E.

Example 3 Using a system as shown in Figure 7, 10 wt% aqueous solution of Ce(NO3)3 was fed at a rate of 3x 10-5 mol/min as Ce(NO3)3 was fed at a rate of 3x 10-5 mol/min as Ce(NO3)3 to form a spray-like aqueous solution 75 while H2 at a rate of 2000 ml/min, 02 at a rate of 4000 ml/min and SiH4 at a rate of 112 mI/min were simultaneously fed to burner 71, thus obtaining a glass rod 77 with a diameter of 1 5 mm) this glass rod was stretched to a diameter of 10 mum0, inserted into a quartz pipe of 12 mmçSx 14 mm) on the inner wall of which B2O3-F-SiO2 glass had been previously deposited by M CVD method to give a thickness of 1 mmd, and collapsed to form a preform of 14 mm in diameter.

The thus resulting preform was charged in a resistance furnace of carbon, melt upon spun in a fiber of 140 ,um) and coated with an epoxy resin. When this fiber was subjected to irradiation of y-rays at a dose rate of 1 x l 05 R/h for 1 hour, increase of the transmission loss was small, i.e. 50 dB/km at A=1.1 um.

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 fo to form a glass rod of 12 mmbx200 mml containing 900 ppm of OH groups and 100 ppm of OD groups. This glass rod was then subjected to grinding and polishing to a diameter of 10 mums5.

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 1 50 ymsI and immediately coated two times with a silicone resin to give a thickness of 50 ,um+50 ym. These fibers with a length of 100 m was subjected to irradiation of X-rays of 2000 R. The 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

1. A radiation resistance optical glass fiber comprising at least two portions having different refractive indexes the portion having the higher refractive index consisting predominantly of a silica glass prepared by the oxidation of a silicon hydride or a silicon-containing organic compound at a relatively low temperature.

2. A radiation resistant optical fiber as claimed in claim 1, wherein the hydride is SiH4, SiHCI3, SiH2Cl2, Si2H6 orSi3H8.

3. A radiation resistant optical fiber as claimed in claim 1 wherein the organic compound is (C2H50)4Si, (CH30)4Si, CH3CI3Si, CH3Cl2SiH, CH3CISiH2, CH3SiH3, C2H5Cl3Si, C2H5C12SiH, C2H5CISiH2, C2H5SiH3, C3H,CI3Si, C2H,CI2SiH2, C3H,CISiH, C3H,SiH3, C4H9CI2SiH, C4H 9SiH3, C5H"CI2SiH, C5H11SiH3, C ,H,3SiH3, C,H,5SiH3 C8H17SiH3.

4. A radiation resistant optical fiber as claimed in any one of the preceding claims wherein the portion having the higher refractive index is an Sb203-SiO2 glass.

5. A radiation resistant optical fiber as claimed in claim 4 wherein the Sb203-SiO2 glass is prepared by the oxidation of SiH4 or an organic compound of Si and SbH3 or an organic compound of Sb.

6. A radiation resistant optical fiber as claimed in any one of claims 1 to 3, wherein the portion having the higher refractive index is a Ce-doped SiO2 glass.

7. A radiation resistant optical fiber as claimed in claim 6 wherein the Ce-doped SiO2 glass is prepared by oxidising or decomposing at a relatively low temperature a hydride of Si or an organic compound of Si with a Ce-containing compound.

8. A radiation resistant optical fiber as claimed in claim 7 wherein the Ce-containing compound is cerium nitrate or cerium ammonium nitrate.

9. A radiation resistant optical fiber as claimed in any one of the preceding claims wherein the lower refractive index portion consists of a silicone resin or a fluorine resin.

10. A radiation resistant optical fiber as claimed in any one of claims 1 to 8 wherein the lower refractive index portion consists of silica glass.

1 A radiation resistant optical fiber as claimed in claim 10 wherein the silica glass is synthesized by oxidizing at a relatively low temperature a hydride of Si or an organic compound of Si.

12. A radiation resistant optical fiber as claimed in any one of claims 1 to 8 wherein the lower refractive index portion of F-5i02, B2O3-5i02, P 205-F-SiO2, B2O3-F-SiO2 or 8293 P205-F-SiO2.

1 3. A radiation resistant optical fiber as claimed in claim 1 2 wherein the lower refractive index portion is synthesized by oxidizing SiH4, SiF4, BF3, B2He, PF3 and/or organic compounds of Si, B, P and F at a temperature of from 1100 to 1 5O00C.

14. A radiation resistant optical fiber as claimed in any one of the preceding claims wherein the outside of the lower refractive index portion is further coated with quartz glass or SiO2 glass doped with at least one of TiO2, ZrO2, Al203 orHfO2.

1 5. A radiation resistant optical fiber as claimed in any one of claims 1 to 13 wherein the fiber has a primary coating consisting of a polyimide or epoxy resin, and a secondary coating consisting of ethylene propylene rubber or a bridged polyethylene.

1 6. 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 consisting of at least one of H2 and D2 and O2 to produce fine glass particles, deposting the fine glass particles in the fused stated onto a starting member to form a glass rod as a core, and combining the core with a cladding to form one body.

1 7. 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 produce fine glass particles, depositing the fine glass particles as a soot onto a starting member, subjecting the soot to sintering and clear vitrification in He gas or in an atmosphere containing at least one of H20 and D20, thereby forming a silica glass rod as a core, and combining the core with a cladding to form one body.

1 8. A process as claimed in claim 1 6 or claim 1 7 wherein the hydride of Si is Six4.

19. A process as claimed in claim 16 or claim 17 wherein the organic compound of Si is si(oC2H5)4

20. A process as claimed in claim 16 or claim 1 7 wherein a hydride of Sb or an organic compound of Sb is further added to the oxyhydrogen flame to synthesize Sb203-SiO2 fine glass particles.

21. A process as claimed in claim 20 wherein the hydride of Sb is SbH3.

22. A process as claimed in claim 1 6 or claim 1 7 wherein at least one hydride, halide or organic compound of Si, B, P or Sb is further added to the oxyhydrogen flame.

23. A process as claimed in any one of claims 1 6, 17 or 22 wherein cerium nitrate of cerium ammonium nitrate is further added to the oxyhydrogen flame.

24. A process for the production of a radiation resistant optical fiber, which comprises 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 gas 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 Sb203-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 it to melt spinning.

25. A process as claimed in claim 24 wherein the gas capable of yielding oxygen at a high temperature is CO2.

26. A process for the production of a radiation resistant optical transmission fiber, which comprises depositing SiO2 glass or an SiO2 glass doped with F, 82O3-F, B203 or P,O,--F in the hollow of a silica pipe, inserting a silica glass rod of a higher refractive index 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.

27. A process for the production of a radiation resistant optical fiber, which comprises introducing SiH4 or SiF4 gas optionally with 8F3, 82H6or P F3 into a plasma flame or oxyhydrogen flame consisting of at least one of H2 and D2,and 02 to synthesize fine particles of SiO2 glass or an SiO2 glass doped with 8203, F, B2O3-F or P205, corresponding to a lower refractive index portion, depositing the glass fine particles on the outside of a silica glass rod corresponding to a higher refractive index portion, optionally depositing further a silica glass thereon or covering with a silica tube and then subjecting to melt spinning.

28. A process as claimed in claim 26 or claim 27 wherein the silica glass rod is prepared by introducing an Si hydride or an Si organic compound into an oxyhydrogen flame consisting of at least one of H2 and D2, and 02 to synthesize fine glass particles and depositing the fine glass particles in the fused state on a starting member.

29. A process as claimed in claim 26 or claim 27 wherein the silica glass rod is prepared by introducing an Si hydride or an organic Si compound into an oxyhydrogen flame to synthesize fine glass particles, depositing the fine glass particles as a soot on a starting member, and subjecting the soot to sintering and clear vitrification in He gas or in an atmosphere containing at least one of H20 and D2O.

30. A process as claimed in claim 27 wherein the SiO2 glass of the lower refractive index portion is deposited in fused state.

31. A process as claimed in claim 27 wherein the SiO2 glass of the lower refractive index portion is deposited in the state of a soot and sintered.

32. A process for producing a radiation resistant optical transmission fiber substantially as hereinbefore described with reference to any one of the Examples.

33. A radiation resistant optical fiber whenever produced by a process as claimed in any one of claims 1 6 to 32.

34. A radiation resistant optical fiber substantially as hereinbefore described with reference to any one of the Examples.

35. An optical transmission system comprising at least one radiation resistant optical fiber as claimed in any one of claims 1 to 1 5, 33 or 34.