the present invention which solves the problems described above in the prior art is to provide silicone resin , epoxy resin , and acrylic resin with photocurability without deteriorating transparency , heat stability and optical isotropy characteristic of such resins which are capable of directly forming a core - ridge . in the present invention , a photosensitive substance is formed in layer on a substrate or a clad layer . this photosensitive substance contains a reactive oligomer and a photopolymerization initiator and may further contain a crosslinking agent . the reactive oligomer used in the present invention is an epoxy - type oligomer , silicone - type oligomer or an acrylic - type oligomer . specifically , the oligomer is a compound represented by one of above formulae ( 1 ) to ( 6 ), and these oligomers may be mixed . advantages that the photosensitive substance used in the present invention is a reactive oligomer are : 1 ) because the state of the polymer material before curing can be made very homogeneous , it has superior optical transmission characteristic in the ultraviolet region or visible region , and has a sufficient resolution even when the film formed by curing is thick , 2 ) because the state of the polymer material before curing is an oligomer , leveling is possible even when the substrate or the like has irregular parts , and since the oligomer penetrates into every part , film formation suitable for various shapes is possible , 3 ) because the oligomer is randomly connected and cured , the cured material is small in birefringence . polymerization of the epoxy oligomer used in the present invention is performed by bonding and crosslinking by uv - irradiation between epoxy or alkoxy group and hydroxy group contained in the epoxy oligomer . in order to effect the crosslinking reaction efficiently and sufficiently , it is desirable to add a photopolymerization initiator . when the photosensitive substance contains a crosslinking agent , polymerization may be performed by bonding and crosslinking by uv - irradiation between epoxy or alkoxy group and hydroxy group contained in the crosslinking agent and epoxy oligomer material . a silicone oligomer inherently has a property to proceed crosslinking in the presence of a cationic photopolymerization initiator which is so - called a acid generator . therefore , optical crosslinking is possible in principle under a specific condition even with a combination of only a silicone oligomer and a cationic photopolymerization initiator . however , according to intensive studies conducted by the inventors , it is very rare that sufficient crosslinking is actually achieved by a combination of only a silicone oligomer and a cationic photopolymerization initiator . in addition , the higher the molecular weight of the oligomer , the more crosslinking tends to proceed , however repeatability is poor and the effect is insufficient . furthermore , this method can not be a general - purpose technique since an increase in molecular weight of the material itself is difficult when composition of substituent is rich in aromatic groups . next , the inventors have attempted a method in which a cationic photopolymerization initiator is added to silicone oligomer to form a film on a substrate , then the film is heat - treated to increase the molecular weight , and then the film is irradiated with light to cure . sufficient crosslinking is rarely achieved depending on the type of silicone oligomer and heating condition , however , in most cases , crosslinking is insufficient and there are problems in repeatability as in the above - described cases . finally , the inventors have found that desired photocurability can be achieved by a method in which a crosslinking agent with a remarkable function as a polymerization coinitiator is added as a third , component in addition to silicone oligomer and a cationic photopolymerization initiator . addition of a silane coupling agent ( crosslinking agent ) to the photosensitive composition may be used , for example , for the purpose of increasing adhesiveness when the photosensitive composition is cured . this is to achieve close adhesion between the cured material and a material contacting the cured material by containing a crosslinking agent in the photo - cured composition to produce siloxane between both the materials . therefore , such purpose is substantially different from the use as the polymerization coinitiator in the present invention . in the present invention , attention is given to the fact that the photosensitive composition for optical waveguides includes mainly a silicone oligomer . the crosslinking agent is added for the purpose that the agent selectively reacts with the silicone oligomer by irradiation with light in the presence of an acid generator to effectively progress an increase in molecular weight in the initial stage of photopolymerization . further , addition of a polymerization coinitiator in the present invention has an effect for enhancing plasticity of the entire composition and promoting progress of polymerization reaction under irradiation . this method is found to provide sufficient crosslinking as compared with the above - described molecular weight control method or pre - baking method without being affected by the structure of silicone oligomer , high in repeatability , and can be widely applied to photo - crosslinking of silicone oligomer . crosslinking agents particularly effective as polymerization coinitiators can include the compounds represented by a general formula ( 7 ). more specifically , γ - glycidoxypropyl - triethoxysilane represented by structural formula ( 8 ), 1 , 2 - bis ( triethoxysilyl ) ethane , 1 , 4 - bis ( triethoxysilyl ) benzene , 1 , 6 - bis ( triethoxysilyl ) ethanehexane , and bi - functional epoxy compounds represented by formulae ( 9 ), ( 10 ), ( 11 ) and ( 12 ) and the like can be exemplified . in the above formula , a and a ′ are independently selected by one of the following three types of structure . in addition to the above , typical crosslinking agents effective as polymerization coinitiators for silicone oligomer are those which undergo ring opening or dehydration condensation by proton supply from the cationic photopolymerization initiator such as epoxy compounds , silane coupling agent , silanol compounds , and alkoxy compounds , including alicyclic epoxy of bifunctional or more other than the above - described ones , ( 3 - glycidoxypropyl ) trimethoxysilane , ( 3 - glycidoxypropyl ) methyldiethoxysilane , and diphenylsilanediol . examples of other silane coupling agents can include aminopropyltriethoxysilane , tridecafluoro - 1 , 1 , 2 , 2 ,- tetrahydro - octyltriethoxysilane , n - β -( aminoethyl )- γ - aminopropyltrimethoxysilane , n - β -( aminoethyl )- γ - aminopropyltriethoxysilane , n - bis [ β -( aminoethyl )]- γ - aminopropylmethyldimethoxysilane , γ - mercaptopropyltrimethoxysilane , γ - mercaptopropyltriethoxysilane , γ - methacryloxypropyltrimethoxysilane , n - β -( n - vinylbenzylaminoethyl )- γ - aminopropyl - trimethoxysilane hydrochloride , methyltrimethoxysilane , methyltriethoxysilane , vinyltriacetoxysilane , γ - chloropropyltrimethoxysilane , hexamethyldisilazane , γ - anilinopropyltrimethoxysilane , vinyltrimethoxysilane , octadecyldimethyl [ 3 -( trimethoxysilyl ) propyl ] ammonium chloride , γ - chloropropylmethyldimethoxysilane , γ - mercaptopropylmethyldimethoxysilane , methyltrichlorosilane , dimethyldichlorosilane , trimethylchlorosilane , vinyltriethoxysilane , benzyltrimethylsilane , vinyltris ( 2 - methoxyethoxy ) silane , γ - methacryloxypropyltris ( 2 - methoxyethoxy ) silane , β -( 3 , 4 - epoxycyclohexyl ) ethyltrimethoxysilane , γ - ureidopropyltriethoxysilane , γ - isocyanurpropyltriethoxysilane , n - octyltriethoxysilane and the like . in the present invention , polymerization of the silicone oligomer material includes polymerization by a reaction of a crosslinking agent with the silicone oligomer . typical crosslinking agents used here can include azide compounds such as azidepyrene , bisazide compounds such as 4 , 4 ′- diazidebenzalacetone , 2 , 6 - di -( 4 ′- azidebenzal ) cyclohexanone , 2 , 6 - di -( 4 ′- azidebenzal )- 4 - methylcyclohexanone , diazo compounds and the like . polymerization of acrylic oligomer is performed by a reaction of the crosslinking agent with the acrylicoligomer . as the crosslinking agent used here , carbonyl compounds such as diphenyltriketonebenzoin , benzoinmethylether , benzophenone , acetophenone , and diacetyl ; peroxides such as benzoyl peroxide ; azo compounds such as asobis - isobutyronitrile ; azide compounds such as azidepyrene ; bisazide compounds such as 4 , 4 ′- diazidebenzalacetone , 2 , 6 - di -( 4 ′- azidebenzal ) cyclohexanone , 2 , 6 - di -( 4 ′- azidebenzal )- 4 - methylcyclohexanone , and diazo compounds are exemplified as typical ones . the photopolymerization initiator used in the present invention is not specifically limited if it is generally used as a photopolymerization initiator , and typical cationic photopolymerization initiators include sulfonium salt , osmium salt , antimonium salt and the like . specifically , preferred cationic photopolymerization initiators include but are not limited to n - benzyl - 4 - benzoylpyridinium hexafluoroantimonate , n -( 3 - metyl - 2 - butenyl )- 2 - cyanopyridinium hexafluorophosphate , p - chlorobenzenediazonium hexafluorophosphate , diphenyliodonium hexafluorophosphate , and tris ( ethylacetoacetato ) aluminum . the amount of the crosslinking agent to be added is as small as 1 to 2 % and not more than several %. the optical waveguides produced using the photosensitive composition according to the present invention can be released from the near infrared absorption effect inherent to the crosslinking agent , and maintains the characteristic of optical propagation loss , as is , in the communication wavelength band intrinsic of silicone resin . normally , to provide silicone oligomer with photocurability , epoxy or vinyl ether or acrylic group is introduced into the silicone oligomer itself by covalent bond . however , the amount of crosslinking agent used in this case is converted to the amount more than about 50 times as large as the present invention . thus , the bond between side chains by crosslinking is dominant rather than the strong siloxane bond , which generates not only problems such as heat stability but also an inevitable increase in optical propagation loss due to increase in proportion of hydroxy group in the case of epoxy and increase in proportion of hydrocarbon in the case of vinyl ether or acrylic . that is , formation of a core - ridge directly by light without deteriorating the heat stability and low - loss characteristic in communication - wavelength band specific to silicone resin is possible for the first time using the photosensitive composition for optical waveguides according to the present invention . a method for producing an optical waveguide using a reactive oligomer material according to the present invention will be described with reference to fig1 a to 1 d . fig1 a to 1 d are schematic sectional views illustrating processes of forming an optical waveguide according to the present invention . in the present invention , the reactive oligomer is coated on a substrate or a clad , after position registration , the coating is irradiated with uv light through a mask or directly , and an unirradiated portion is dissolved away with a solvent to form a waveguide ridge pattern . specifically , as shown in fig1 a , a lower clad forming resin layer 2 is formed on a substrate 1 , and a core part forming photosensitive substance layer 3 is formed thereon . next , as shown in fig1 b , a mask 4 having a pattern of core part shape is placed on the photosensitive substance layer 3 , and irradiated with uv light 5 through the mask 4 . this cures only the core part 6 of the photosensitive substance layer 3 . after that , a uv light unirradiated portion of the photosensitive substance layer 3 , is dissolved out to form a ridge pattern of the core part 6 as shown in fig1 c . the same photosensitive resin as the clad part forming resin layer 2 is coated so that the core part 6 is buried thereby forming a clad part 7 as shown in fig1 d . the thus produced optical waveguide has superior solvent resistance , is small in polarization dependence because the material used is small in birefringence , low in propagation loss , and has superior heat stability and moisture resistance . the present invention will be described in further detail with reference to examples , however , the present invention is not limited to these examples . phenyltrichlorosilane ( 211 . 5 g ) and methyltrichlorosilane ( 36 . 3 g ) were dissolved in 1 liter of anhydrous tetrahydrofuran , and 3 equivalents of water ( 67 . 5 g ) was slowly added dropwise to the solution so that the liquid temperature does not increase . after that , 315 g of sodium hydrogencarbonate was added while agitating the reaction liquid . after the completion of carbon dioxide gas evolution , agitation was continued for further 1 hour . next , the reaction liquid was filtered , then tetrahydrofuran was distilled out by a rotary evaporator to give a colorless , transparent viscous liquid . further , this liquid was vacuum dried to give an oligomer a . the molecular weight of resulting oligomer a was measured by gpc and the result was mw = 3300 , mn = 1500 . next , a photosensitive substance a was prepared which comprises 50 g of oligomer a , 25 g of uv resin , 2 weight % of n - benzyl - 4 - benzoylpyridinium hexafluoroantimonate as a photopolymerization initiator and 25 g of methylisobutylketone as a solvent . an optical waveguide using the photosensitive substance a in the core , and an ultraviolet curable resin ( uv resin ) in the clad was produced as shown in fig1 a to 1 d . first , the uv resin was applied to a silicon substrate by a spin coating method to form a layer . at this moment , the rotational speed of the spin coater was adjusted so that the film thickness is 15 μm . the formed layer was irradiated with uv light of 7000 mj / cm 2 , and then heated at 90 ° c . for 30 minutes to form a lower clad layer 2 . next , the photosensitive substance a was applied by the spin coating method to form a core part formation layer . in this case , the rotational speed of the spin coater was adjusted so that the film thickness is 8 μm . at this moment , no intermixing was noted between the lower clad layer and the core part formation layer . the formed core part formation layer was heated at 120 ° c . for 10 minutes to remove the solvent . a mask 4 having a core part pattern to provide a core width of 8 μm was placed on the core part formation layer , and uv light of 7000 mj / cm 2 was irradiated through the mask . after that , the core part 6 was patterned by heating at 120 ° c . for 2 minutes . next , development was performed using a 1 : 1 mixed solvent of methylisobutylketone and isoproylalcohol to remove a uv light unirradiated portion . the developed layer was heated at 120 ° c . for 30 minutes to form a core ridge . the same uv resin used for the lower clad was applied onto it and then photocured as the lower clad to produce a buried type channel optical waveguide as shown in fig1 which has a structure with the core part 6 buried in the clad part 7 . in this case , the thickness of the upper clad is 8 μm from the upper surface of the core . the resulting optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for propagation loss which showed 1 . 0 db / cm at wavelength 1 . 3 μm , and 1 . 2 db / cm or less at wavelength 0 . 633 μm . further , this optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . a photosensitive substance b was prepared which comprises 50 g of the oligomer a produced in example 1 , 1 g of ( 3 - glycidoxypropyl ) trimethoxysilane as a crosslinking agent , 2 weight % of n - benzyl - 4 - benzoylpyridinium hexafluoroantimonate as a photopolymerization initiator , and 25 g of methylisobutylketone as a solvent . using the same procedure as in example 1 except that the photosensitive substance b was used in place of the photosensitive substance a as a core part forming photosensitive substance , a buried type channel optical waveguide comprising a core - clad layer structure as shown in fig1 d was produced . in this case , the thickness of the upper clad is 8 μm from the upper surface of the core . further , no intermixing was noted between the lower clad layer and the core part formation layer . as in example 1 , the resulting optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for optical propagation loss which showed 0 . 5 db / cm at wavelength 1 . 3 μm , and 0 . 8 db / cm or less at wavelength 1 . 55 μm . further , this optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . oligomer b was produced using the same procedure as in example 1 except that deuterated phenyltrichlorosilane was used in place of phenyltrichlorosilane in the production of oligomer a in example 1 . and a photosensitive substance c was produced using the same procedure as in example 2 except that oligomer b was used in place of oligomer a . next , a buried type channel optical waveguide of core diameter 8 μm × 8 μm was produced by the same process as in example 2 . the resulting optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for optical propagation loss which showed 0 . 1 db / cm at wavelength 1 . 3μm , and 0 . 5 db / cm or less at wavelength 1 . 55μm . further , this optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . using quite the same procedure as in example 3 , eight types of optical waveguide differing in compositional ratio of silicone oligomer side chain of the core , crosslinking agent and initiator were produced . the respective optical waveguides were measured for optical propagation loss , and the results of measurement are shown in table 1 comparing with the results of example 1 to 3 . a photosensitive substance d was prepared from 100 weight % of liquid oligomer having the following structural formula ( 15 ) ( where z = 0 to 2 ) and 2 weight % of n - benzyl - 4 - benzoylpyridinium hexafluoroantimonate as a photopolymerization initiator . epoxy resin having a thickness of 100 μm was formed on a substrate . this epoxy resin had a refractive index of 1 . 52 at wavelength 1 . 55 μm . next , the photosensitive substance d was applied to the epoxy resin layer by spin coating method to form a photosensitive substance layer . after that , the layer was irradiated with uv light through a mask having a waveguide pattern of core part shape . in this case , irradiation amount of uv light was 2000 mj / cm 2 . when the layer was developed by an isopropanol solution , a uv light unirradiated portion of the photosensitive substance layer was dissolved , and only a uv light irradiated portion of the cured liquid epoxy oligomer was remained to form a ridge pattern of core part shape . the refractive index of the core part after curing was 1 . 525 at wavelength 1 . 55 μm . after that , this ridge pattern was coated with an epoxy resin adjusted to provide a refractive index at photocuring of 1 . 52 at wavelength 1 . 55 μm , and cured to produce an optical waveguide where the core is buried in the clad as shown in fig1 d . thus , a single mode channel waveguide ( core diameter 8 μm × 8 μm , δn = 0 . 3 %) having a clad comprising epoxy resin of refractive index 1 . 52 and a core comprising uv - cured epoxy resin of refractive index 1 . 525 was produced . the resulting optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for optical propagation loss which was 1 . 5 db or less at wavelength 1 . 3 μm , and 3 . 0 db or less at wavelength 1 . 55 μm . polarization dependence of the optical propagation loss was 0 . 1 db or less at both wavelength 1 . 3 μm and 1 . 55 μm . further , the resulting optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . example 13 a single mode channel waveguide ( core diameter 8 μm × 8 μm , δn = 0 . 3 %) was produced using the same procedure as in example 12 except that a photosensitive substance e prepared from 100 weight % of liquid silicone epoxy oligomer ( desirably having a molecular weight of 1000 to 10000 ) represented by the following structural formula ( 16 ) and 2 weight % of n - benzyl - 4 - benzoylpyridinium hexafluoroantimonate as a photopolymerization initiator was used in place of the photosensitive substance d . the resulting optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for optical propagation loss which showed 1 . 0 db or less at wavelength 1 . 3 μm , and 1 . 5 db or less at wavelength 1 . 55 μm . polarization dependence of the optical propagation loss was 0 . 1 db or less at wavelength both 1 . 3 μm and 1 . 55 μm . further , the optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . a single mode channel waveguide ( core diameter 8 μm × 8 μm , δn = 0 . 3 %) was produced using the same procedure as in example 12 except that a photosensitive substance f prepared from 100 weight % of liquid silicone oligomer represented by the following structural formula ( 17 ) and 2 weight % of n - benzyl - 4 - benzoylpyridinium hexafluoroantimonate as a photopolymerization initiator was used in place of the photosensitive substance d . the resulting optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for optical propagation loss which was 1 . 5 db or less at wavelength 1 . 3 μm , and 3 . 0 db or less at wavelength 1 . 55 μm . polarization dependence of the optical propagation loss was 0 . 1 db or less at wavelength both 1 . 3 μm and 1 . 55 μm . further , the optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . next , a multi - mode optical waveguide ( depth 40 μm , width 40 μm , δn = 1 %) was produced using the same procedure as in example 1 except that the above photosensitive substance f was used in place of the photosensitive substance a . this optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for optical propagation loss which was 1 . 0 db or less at wavelength 0 . 85 μm , 0 . 5 db or less at wavelength 1 . 3 μm and 1 . 0 db or less at wavelength 1 . 55 μm . polarization dependence of the optical propagation loss was less 0 . 1 db or less . further , the optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . using quite the same procedure as in example 14 , nine types of optical waveguide differing in compositional ratio of silicone oligomer side chain of the core , crosslinking agent and initiator were produced . the respective optical waveguides were measured for optical propagation loss , and the results of measurement are shown in table 2 . a single mode channel waveguide ( core diameter 8 μm × 8 μm , δn = 0 . 3 %) was produced using the same procedure as in example 12 except that a photosensitive substance g prepared from 100 weight % of liquid silicone vinylether oligomer represented by the following structural formula ( 18 ) and 2 weight % of 2 , 6 - di ( 4 ′- azidebenzal )- 4 - methylcyclohexanone as a photopolymerization initiator was used in place of the photosensitive substance d . the resulting optical waveguide was cut to a length of 5 cm by a dicing saw , and measured for optical propagation loss which was 1 . 5 db or less at wavelength 1 . 3 μm , and 3 . 0 db or less at wavelength 1 . 55 μm . polarization dependence of the optical propagation loss was 0 . 1 db or less . further , the optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . a single mode channel waveguide ( core diameter 8 μm × 8 μm , δn = 0 . 3 %) was produced using the same procedure as in example 12 except that a photosensitive substance h prepared from 100 weight % of liquid acrylic oligomer represented by the following structural formula ( 19 ) and 2 weight % of diphenyltriketonebenzoin as a photopolymerization initiator was used in place of the photosensitive substance d . the resulting optical waveguide was cut to a length of 5 cm by a dicing saw and measured for optical propagation loss which was 0 . 5 db or less at wavelength 1 . 3 μm , and 5 . 0 db or less at wavelength 1 . 55 μm . polarization dependence of the optical propagation loss was 0 . 1 db or less at wavelength both 1 . 3 μm and 1 . 55 μm . further , the optical propagation loss of the waveguide was unchanged for more than 1 month under the condition of 75 ° c ./ 90 % rh . 240 g of phenyltriethoxysilane , 20 g of methyltriethoxysilane , 10 g of water , 100 g of isopropylalcohol , and 0 . 1 g of hydrochloric acid were mixed , heated under ref lux for 4 hours . then solvents and the like were removed under vacuum using a rotary evaporator to give 112 g of colorless , transparent oligomer c . 50 g of the oligomer c , 1 g of 3 - glycidoxypropyltriethoxysilane , 7 g of water , 20 g of isopropylalcohol , 2 g of ion - exchange resin ( amberlite ira900 from aldrich co .) were charged into a reaction vessel , heated at 80 ° c . for 12 hours while agitating . the ion exchange resin was removed by filtration , and then isopropylalcohol was removed under vacuum using a rotary evaporator to give 44 g of a colorless , transparent oily substance d . next , using the same procedure as above except that 253 g of phenyltriethoxysilane and 6 g of methyltriethoxysilane were used , 110 g of colorless , transparent oligomer e was obtained , which gave 40 g of a colorless , transparent oily substance f . using these oily substances d and f , an optical waveguide was produced according to the conventional photolithographic technique using the following procedure . first , the oily substance d was mixed with a cationic photopolymeization initiator to give a photosensitive composition g for optical waveguides , which was applied onto a silicon wafer by a spin coating method . in this case , the rotational speed of the spin coater was adjusted so that the film thickness is about 15 μm . the formed film was photocured by irradiation of uv light for 10 minutes , and thoroughly cured at 150 c . to form a lower clad layer . next , the oily substance f was mixed with a cationic photopolymerization initiator to give a photosensitive composition h for optical waveguides , which was applied to the above - mentioned lower clad layer , and irradiated with uv light for 10 minutes through an 8 μm wide linear mask . an uncured portion was washed out by a solvent , and thoroughly cured at 150 ° c . to form a rectangular core of 8 μm in width and 8 μm in height . an upper clad was formed on it using the same procedure as the lower clad to give a buried optical waveguide . the waveguide was measured for optical propagation loss at 1 . 55 μm band which was confirmed to be less than 0 . 5 db / cm . the oligomers shown in table 3 were used in place of oligomer e , respectively treated with ion - exchange resin as in example 26 to prepare photosensitive substances mixed with photopolymerization initiator . further , a buried channel optical waveguide having core - clad layer structure as shown in fig1 d was produced using the same procedure as in example 1 except that the above treated photosensitive substance was used in place of the photosensitive substance a as the core part forming photosensitive substance . in this case , the thickness of the upper clad was controlled to 8 μm . no intermixing was noted between the lower clad layer and the core part formation layer . the results of optical propagation loss measurement are shown in table 3 . the present invention has been described in detail with respect to preferred embodiments , and it will now be apparent that changes and modifications may be made without departing from the invention in its broader aspects , and it is the intention , therefore , in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention .