Abstract:
An impact resistant waveguide includes a clad surrounding a core. A reinforcing filler is incorporated in a curable silicone composition used to prepare the core, thereby imparting impact resistance to the core. A method of manufacture of the waveguide includes injecting the curable silicone composition into a clad made of silicone elastomeric tubing and thereafter curing the curable silicone composition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage filing under 35 U.S.C. §371 of PCT Application No. PCT/US06/048010 filed on 15 Dec. 2006, currently pending, which claims the benefit of U.S. Provisional Patent Application No. 60/764,196 filed 1 Feb. 2006 under 35 U.S.C. §119 (e). PCT Application No. PCT/US06/048010 and U.S. Provisional Patent Application No. 60/764,196 are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to impact resistant optical waveguides and efficient methods for the manufacture thereof. 
     BACKGROUND 
     Sensors comprising a waveguide composed of a core and a clad each made from an elastomeric material are known. Such a sensor may include light emitting means and light receiving means each connected to ends of the optical waveguide. The core or clad, or both, may be transparent, and the core has a refractive index somewhat larger than that of the clad. Both core and clad may be made from a synthetic rubber or gel. 
     Compositions used in preparing waveguides have been disclosed, for example in U.S. Pat. No. 4,529,789. However, this composition cures to form a gel having inadequate refractive index and insufficient impact resistance for some applications. The resin present in this composition may also cause cloudiness, which is undesirable for optical waveguide applications. 
     Existing waveguides are frequently prepared by first fabricating a core and thereafter coating the core with a clad material. Such methods include extruding a high viscosity transparent composition through a nozzle, curing the composition to form the core, applying a liquid clad material to the outer periphery of the core, and thereafter curing the clad material. Sensors made from such waveguides suffer from the drawback that additional connectors are required to attach parts of the sensor, such as radiation sources and detectors at the ends of the waveguide. This adds process steps and cost to fabricating the sensor. 
     Problem to be Solved 
     It is an object of this invention to provide a sensor comprising an impact resistant waveguide by an efficient method of manufacture. 
     SUMMARY 
     This invention solves the above problem by providing an impact resistant waveguide comprising a clad surrounding a core. A reinforcing filler is incorporated in a curable silicone composition used to prepare the core, thereby imparting impact resistance to the core. A method of manufacture of the waveguide comprises injecting the curable silicone composition into a clad comprising tubing and thereafter curing the curable silicone composition. 
     DETAILED DESCRIPTION 
     All amounts, ratios, and percentages are by weight unless otherwise indicated. The articles “a”, “an”, and “the” each mean one or more. 
     Waveguide 
     The waveguide comprises a core surrounded by a clad. The core is a silicone elastomer having a refractive index higher than that of the clad. The difference between the core refractive index and the clad refractive index may be selected based on the end use of the waveguide. For example, in some applications, the difference may range from 0.005 to 0.55, alternatively 0.1 to 0.3. A difference in this range may be desirable, for example, when a relatively small bend radius is needed, e.g., when the waveguide will have to be bent to angles of 90° or higher in small areas. Alternatively, a smaller refractive index difference, such as 0.005 to 0.04 may be desirable when the waveguide will be used for digital data transmission applications over large distances. 
     Core 
     The core may be prepared by injecting a curable silicone composition into the clad and thereafter curing the composition. The core has a refractive index, which may range from 1.43 to 1.59, alternatively 1.46 to 1.54. A suitable composition comprises: 
     (I) a polyorganosiloxane with an average of at least two reactive, unsaturated organic groups per molecule, 
     (II) a curing agent having an average of at least two silicon-bonded hydrogen atoms per molecule, 
     (III) a hydrosilylation catalyst, and 
     (IV) a reinforcing filler. 
     Component (I) Polyorganosiloxane 
     Component (I) in the composition is a polyorganosiloxane with an average of at least two unsaturated organic groups per molecule capable of undergoing a hydrosilylation reaction. Component (I) can be a homopolymer or a copolymer. Component (I) can have a linear, branched, or cyclic structure. The reactive, unsaturated organic groups per molecule in the component (I) can be located at terminal, pendant, or at both terminal and pendant positions. Component (I) may have a refractive index of 1.38 to 1.59. The amount of component (I) in the composition is 100 parts by weight. Component (I) can comprise siloxane units including, but not limited to, R 2 R 1   2 SiO 1/2 , R 1   3 SiO 1/2 , R 2 R 1 SiO 2/2 , R 1   2 SiO 2/2 , and R 1 SiO 3/2  units. In the preceding formulae, each R 1  is independently a monovalent organic group. Suitable monovalent organic groups include, but are not limited to, alkyl groups such as methyl, ethyl, propyl, and butyl; alkenyl groups such as vinyl, allyl, and butenyl; alkynyl groups such as ethynyl and propynyl; aromatic groups such as phenyl, styryl, tolyl, xylyl, and benzyl. However, at least 5%, alternatively at least 50% of R 1  are aromatic groups. The amount of R 1  groups being aromatic groups may range from 10% to 90%. The exact amount of R 1  groups that are aromatic groups will depend on various factors including the filler selected. In general, when more aromatic groups are present, refractive index increases. Each R 2  is independently an unsaturated monovalent organic group. R 2  is exemplified by alkenyl groups such as vinyl, allyl, and butenyl and alkynyl groups such as ethynyl and propynyl. Component (I) may comprise a polydiorganosiloxane of the formula
     (A) R 1   3 SiO(R 1   2 SiO) α (R 1 R 2 SiO) β SiR 1   3 ,   (B) R 1   2 R 2 SiO(R 1   2 SiO) α (R 1 R 2 SiO) β SiR 1   2 R 2 , or   (C) a combination thereof.
 
In these formulae, R 1  and R 2  are as described above, α has an average value of 0 to 20,000, and β has an average value of 2 to 20,000.
   

     Component (I) may comprise polydiorganosiloxanes such as dimethylvinylsiloxy-terminated poly(dimethylsiloxane/methylphenylsiloxane); dimethylvinylsiloxy-terminated poly(dimethylsiloxane/diphenylsiloxane); phenyl,methyl,vinyl-siloxy-terminated polydimethylsiloxane; and combinations thereof. 
     Component (I) can be a single polydiorganosiloxane or a combination comprising two or more polydiorganosiloxanes that differ in at least one of the following properties: structure, viscosity, average molecular weight, siloxane units, and sequence. Methods of preparing polydiorganosiloxanes suitable for use as component (I), such as hydrolysis and condensation of the corresponding organohalosilanes or equilibration of cyclic polydiorganosiloxanes, are well known in the art. 
     Component (II) Curing Agent 
     Component (II) may be a polyorganohydrogensiloxane having an average of at least two silicon-bonded hydrogen atoms per molecule. Component (II) can be a homopolymer or a copolymer. Component (II) can have a linear, branched, or cyclic structure. The silicon-bonded hydrogen atoms in the component (II) can be located at terminal, pendant, or at both terminal and pendant positions. The amount of component (II) in the composition is sufficient to cure the composition. The amount of component (II) is sufficient to provide 0.5 to 2 silicon bonded hydrogen atoms per reactive unsaturated organic group in, component (I) (SiH/unsaturated group ratio). Alternatively, SiH/unsaturated group ratio may range from 1.0 to 1.8. 
     Component (II) can comprise siloxane units including, but not limited to, HR 3   2 SiO 1/2 , R 3   3 SiO 1/2 , HR 3 SiO 2/2 , R 3   2 SiO 2/2 , and HSiO 3/2  units. In the preceding formulae, each R 3  is independently selected from monovalent organic groups free of aliphatic unsaturation. 
     Component (II) may comprise a compound of the formula
     (a) R 3   3 SiO(R 3   2 SiO) γ (R 3 HSiO) δ SiR 3   3 , or   (b) R 3   2 HSiO(R 3   2 SiO) γ (R 3 HSiO) δ SiR 3   2 H,   (c) a combination thereof.   

     In these formulae, γ has an average value of 0 to 20,000, and δ has an average value of 2 to 20,000. Each R 3  is independently a monovalent organic group free of aliphatic unsaturation. Suitable monovalent organic groups free of aliphatic unsaturation include alkyl groups such as methyl, ethyl, propyl, and butyl; aromatic groups such as phenyl, styryl, tolyl, xylyl, and benzyl. At least 5%, alternatively at least 50% of R 3  may be aromatic groups. The amount of R 3  groups being aromatic groups may range from 10% to 90%. The exact amount of R 3  groups that are aromatic groups will depend on the various factors including the filler selected. 
     Component (II) is exemplified by dimethylhydrogensiloxy-terminated poly(dimethylsiloxane/methylphenylsiloxane); dimethylhydrogensiloxy-terminated poly(dimethylsiloxane/diphenylsiloxane); dimethylhydrogensiloxy-terminated poly(methylhydrogensiloxane/methylphenylsiloxane)siloxane; trimethylsiloxy-terminated poly(dimethylsiloxane/methylhydrogensiloxane/methylphenylsiloxane); phenyl,methyl,vinyl-siloxy-terminated poly(dimethylsiloxane/methylhydrogensiloxane); and combinations thereof. 
     Component (II) can be a combination of two or more organohydrogenpolysiloxanes that differ in at least one of the following properties: structure, average molecular weight, viscosity, siloxane units, and sequence. Methods of preparing linear, branched, and cyclic organohydrogenpolysiloxanes suitable for use as component (II), such as hydrolysis and condensation of organohalosilanes, are well known in the art. 
     Component (III) Hydrosilylation Catalyst 
     Suitable hydrosilylation catalysts are known in the art and commercially available. The hydrosilylation is added in an amount sufficient to catalyze the curing reaction of the composition. The amount of component (III) may range from 0.1 to 1000 ppm of a platinum group metal based on the weight of the composition, alternatively 10 to 100 ppm of a platinum group metal. The hydrosilylation catalyst may comprise a platinum group metal selected from platinum, rhodium, ruthenium, palladium, osmium or iridium metal or organometallic compound thereof, or a combination thereof. The hydrosilylation catalyst is exemplified by compounds such as chloroplatinic acid, chloroplatinic acid hexahydrate, platinum dichloride, and complexes of said compounds with low molecular weight organopolysiloxanes or platinum compounds microencapsulated in a matrix or coreshell type structure. Complexes of platinum with low molecular weight organopolysiloxanes include 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane complexes with platinum. These complexes may be microencapsulated in a resin matrix. 
     Suitable hydrosilylation catalysts are described in, for example, U.S. Pat. Nos. 3,159,601; 3,220,972; 3,296,291; 3,419,593; 3,516,946; 3,814,730; 3,989,668; 4,784,879; 5,036,117; and 5,175,325 and EP 0 347 895 B. Microencapsulated hydrosilylation catalysts and methods of preparing them are known in the art, as exemplified in U.S. Pat. No. 4,766,176 and the references cited therein; and U.S. Pat. No. 5,017,654. 
     Component (I) Reinforcing Filler 
     Component (IV) is a reinforcing filler. The amount of component (IV) added to the composition depends on the type of filler selected, however, component (IV) may be added to the composition in an amount of 0.01% to 65% based on the weight of the composition. Suitable reinforcing fillers include silica such as fumed silica or colloidal silica; titania, silicon metal, aluminum, or a combination thereof. Suitable reinforcing fillers are known in the art and commercially available, such as fumed silica sold under the name CAB-O-SIL by Cabot Corporation of Massachusetts. The filler selected depends on various factors including the wavelength of the radiation to be transmitted by the waveguide and the properties of the filler. For example, when the particle size of the filler is greater than 10% of the wavelength of radiation to be transmitted, then the filler and polydiorganosiloxane are selected such that their refractive indexes match. “Match” means that refractive index of the filler and refractive index of the polydiorganosiloxane have values sufficiently close to one another such that the optical loss of the resulting waveguide is sufficiently low for a detector at the outlet of the waveguide to detect the radiation. For example, with a refractive index match within 1% between components (I) and (IV), a waveguide with a length of 2 m to 10 m can be prepared having a loss of 0.5 decibel per centimeter (dB/cm) or less. When the particle size of the filler is sufficiently less than approximately 10% of the wavelength of radiation to be transmitted, matching the refractive indexes of components (I) and (IV) is not necessary to reduce optical loss. Alternatively, the filler may have a particle size less than or equal to 10% of the wavelength of radiation to be transmitted through the wave guide, and the filler may have a filler refractive index, the polydiorganosiloxane may have a polydiorganosiloxane refractive index, and the filler refractive index may match the polydiorganosiloxane refractive index. One skilled in the art would be able to determine when refractive indexes of components (I) and (IV) should be matched without undue experimentation. 
     The filler may optionally be surface treated with a treating agent. Treating agents and treating methods are known in the art, see for example, U.S. Pat. No. 6,169,142 (col. 4, line 42 to col. 5, line 2). The filler may be treated with the treating agent prior to combining the filler with the other components of the composition, or the filler may be treated in situ. 
     The treating agent can be an alkoxysilane having the formula: R 4   ε Si(OR 5 ) (4-ε) , where ε is 1, 2, or 3; alternatively ε is 3. R 4  is a monovalent organic group exemplified by monovalent hydrocarbon groups of 1 to 50 carbon atoms, alternatively 8 to 30 carbon atoms, alternatively 8 to 18 carbon atoms. R 4  is exemplified by alkyl groups such as hexyl, octyl, dodecyl, tetradecyl, hexadecyl, and octadecyl; and aromatic groups such as benzyl, phenyl and phenylethyl. R 4  can be saturated or unsaturated, branched or unbranched, and unsubstituted. R 4  can be saturated, unbranched, and unsubstituted. 
     R 5  is an unsubstituted, saturated hydrocarbon group of 1 to 4 carbon atoms. The treating agent is exemplified by hexyltrimethoxysilane, octyltriethoxysilane, decyltrimethoxysilane, dodecyltrimethyoxysilane, tetradecyltrimethoxysilane, phenyltrimethoxysilane, phenylethyltrimethoxysilane, octadecyltrimethoxysilane, octadecyltriethoxysilane, and a combination thereof. 
     Alkoxy-functional oligosiloxanes can also be used as treating agents. Alkoxy-functional oligosiloxanes and methods for their preparation are known in the art, see for example, EP 1 101 167 A2. For example, suitable alkoxy-functional oligosiloxanes include those of the formula (R 6 O) υ Si(OSiR 7   2 R 8 ) 4-υ . In this formula, υ is 1, 2, or 3, alternatively υ is 3. Each R 6  can be an alkyl group. Each R 7  can be independently selected from saturated and unsaturated monovalent hydrocarbon groups of 1 to 10 carbon atoms. Each R 8  can be a saturated or unsaturated monovalent hydrocarbon group having at least 11 carbon atoms. 
     Silazanes may also be used as treating agents. For example, hexamethyldisilazane may be used. 
     Optional Component (V) Cure Modifier 
     Component (V) is an optional cure modifier that may be added to the composition. Component (V) can be added to extend the shelf life or working time, or both, of the composition. Component (V) can be added to raise the curing temperature of the composition. Suitable cure modifiers are known in the art and are commercially available. 
     Component (V) is exemplified by acetylenic alcohols such as methyl butynol, ethynyl cyclohexanol, dimethyl hexynol, and combinations thereof; cycloalkenylsiloxanes such as methylvinylcyclosiloxanes exemplified by 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane, 1,3,5,7-tetramethyl-1,3,5,7-tetrahexenylcyclotetrasiloxane, and combinations thereof; ene-yne compounds such as 3-methyl-3-penten-1-yne, 3,5-dimethyl-3-hexen-1-yne; triazoles such as benzotriazole; phosphines; mercaptans; hydrazines; amines such as tetramethyl ethylenediamine, dialkyl fumarates, dialkenyl fumarates, dialkoxyalkyl fumarates, maleates, and combinations thereof. Suitable cure modifiers are disclosed by, for example, U.S. Pat. Nos. 3,445,420; 3,989,667; 4,584,361; and 5,036,117. 
     The amount of component (V) added to the composition will depend on the particular cure modifier used, the nature and amount of component (III), and the composition of component (II). However, the amount of component (V) may be 0.001% to 10% based on the weight of the composition. 
     Clad 
     Suitable clads are known in the art and are commercially available. For example, SILASTIC® brand tubing, which is commercially available from Dow Corning Corporation of Midland, Mich., U.S.A. is suitable for use as a clad. When SILASTIC® tubing is used, the clad refractive index is less than 1.45, alternatively 1.40 to 1.42. Suitable coating compositions that may be applied inside the clad are known in the art and are commercially available. For example, SYLGARD® 184, which is commercially available from Dow Corning Corporation, is suitable to use as a coating composition. Without wishing to be bound by theory, it is thought that the coating improves loss characteristic of the waveguide. 
     Sensors 
     The waveguide described above has an inlet and an outlet. The sensor of this invention comprises the waveguide and a radiation source at the inlet of the waveguide or a detector at the outlet of the waveguide, or both. Alternatively, the sensor may comprise a radiation emitting and detecting device at the inlet of the waveguide and a reflector, such as a mirror, at the outlet of the waveguide. The radiation source may be for example, a LED device. The detector may be, for example, a photodiode or phototransistor. 
     Method of Manufacture 
     This invention further relates to a method for preparing the waveguide and sensor described above. The method comprises:
     (1) injecting a curable silicone composition into a clad having an inlet and an outlet,   (2) curing the composition to form a core after step (1), thereby forming a waveguide having an inlet and an outlet. The method may optionally further comprise inserting a radiation source into the composition at the inlet of the waveguide before step (2). The method may optionally further comprise inserting a detector into the composition at the outlet of the waveguide before step (2).   

     The method may optionally further comprise applying a coating composition to the inside of the clad and curing the coating composition to from a cured coating before step (1). Step (2) and the optional step of applying the coating composition may be performed, for example, by heating at a temperature of 100 to 200° C., alternatively 150 to 200° C. for 30 minutes to 2 hours. This may provide the benefit of reducing interface roughness, thereby reducing optical loss. 
     The method of this invention may provide the benefit that insertion of the radiation source and detector eliminates the need to couple the light into the waveguide through complex optical lens arrangements. 
     Methods of Use 
     The sensor of this invention may be used as pressure sensor or light guide for automotive or medical applications. When the emitter is a LED, the sensor may be used for collimation of light. Automotive applications include fender crash sensors, vehicle safety zone sensors, anti-pinch sensors for sunroofs, electric sliding doors, and windows, pedestrian impact severity reduction mechanisms, and passenger detection in a seat. Without wishing to be bound by theory, it is thought that the optical sensor of this invention has greater accuracy, faster response time, and higher reliability than electro-mechanical sensors (e.g., piezo electric). 
     EXAMPLES 
     These examples are intended to illustrate the invention to one of ordinary skill in the art and should not be interpreted as limiting the scope of the invention set forth in the claims. 
     Example 1 
     A core composition was prepared by mixing 60 grams (g) Base, 115 g dimethylvinyl siloxy-terminated, poly(dimethylsiloxane/phenylmethylsiloxane), 3.9 g Crosslinker, 2.4 g Chain Extender, 0.013 g Catalyst, and 0.03 g Surfynol 61. The Base contained 32% hexamethyldisilazane treated fumed silica and 68% phenyl,methyl,vinyl siloxy-terminated poly(dimethylsiloxane/phenylmethylsiloxane) having a refractive index ranging from 1.465 to 1.475, viscosity ranging from 1425 to 2600 cSt at 25° C., and vinyl content ranging from 0.67 to 0.78%. The Crosslinker was trimethylsiloxy-terminated, poly(dimethylsiloxane/methylhydrogensiloxane) having Si—H content ranging from 0.71 to 0.85% and viscosity 5 cSt (mm 2 /s). The Chain Extender was a dimethylhydrogen siloxy-terminated polydimethylsiloxane from Gelest, Inc. The Catalyst was a mixture containing 0.8% platinum and tetramethyldivinyldisiloxane. Surfynol 61 is 3,5-dimethyl-1-hexyn-3-ol from Nissan Chemical Industry Co., Ltd. 
     The mixture was first de-aired under vacuum (&lt;20 Torr) at room temperature. A syringe was then filled with the resulting de-aired mixture. The syringe was used to inject the de-aired mixture into a 0.5 meter (m) long SILASTIC® Laboratory Tubing manufactured by Dow Corning Corporation. A 3 millimeter (mm) diameter LED emitter from Fiber Optic Products, Inc. was pushed into the inlet. The resulting article was placed into an oven and heated for 1 hour (h) at 150° C. The resulting waveguide has a loss of 0.375 decibels per centimeter (dB/cm). No crack was observed when the waveguide was repeatedly hit with a hammer. 
     A waveguide according to example 1 is shown in  FIG. 1 . The waveguide comprises the SILASTIC® clad  101  and a silicone elastomer core  102  prepared by curing the composition as described above. 
     Example 2 
     A core composition was prepared by mixing 60 g Base, 115 g dimethylvinyl siloxy-terminated, poly(dimethylsiloxane/phenylmethylsiloxane), 3.9 g Crosslinker, 2.4 g Chain Extender, 0.013 g Catalyst, and 0.03 g Surfynol 61. The Base, Crosslinker, Chain Extender, Catalyst, and Surfynol were the same as in Example 1. 
     The mixture was first de-aired under vacuum (&lt;20 Torr) at room temperature. A syringe was then filled with the resulting de-aired mixture. The syringe was used to inject the de-aired mixture into a 0.5 meter (m) long SILASTIC® Laboratory Tubing manufactured by Dow Corning Corporation. The resulting article was placed into an oven and heated for 1 hour (h) at 150° C. to prepare a waveguide. The waveguide was taken out of the oven and cooled to room temperature. The core to 0.5 centimeter (cm) from the end was removed with a spatula. The tubing was filled with several drops of uncured mixture prepared as above. The article was de-aired again under vacuum. The end of the tubing was filled with uncured mixture again. A 3 mm LED light source from Fiber Optic Products, Inc. was pushed into the tube. The resulting article was placed into an oven and heated for 30 min. This immobilized the LED in the resulting sensor. 
     The optical loss was measured using the cut-back method with the tube end re-aligned close to a Si photodiode sensor after each cut. The data are compiled in Table 1 below. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Loss measurement (cut-back method) of filled tube 
               
             
          
           
               
                 Initial tube 
                 Cut-off piece length 
                 Tube length l 
                 Light power 
                   
               
               
                 length [cm] 
                 [cm] 
                 [cm] 
                 P [mW] 
                 ln P 
               
               
                   
               
             
          
           
               
                 73.4 
                 0 
                 73.4 
                 1.17 
                 0.1570 
               
               
                   
                 12.1 
                 61.3 
                 3.66 
                 1.2975 
               
               
                   
                 10.5 
                 50.8 
                 8.79 
                 2.1736 
               
               
                   
                 9.1 
                 41.7 
                 16.28 
                 2.7899 
               
               
                   
                 9.0 
                 32.7 
                 38.3 
                 3.6454 
               
               
                   
                 6.3 
                 26.4 
                 68.1 
                 4.2210 
               
               
                   
                 6.6 
                 19.8 
                 129 
                 4.8598 
               
               
                   
                 5.8 
                 14.0 
                 213 
                 5.3613 
               
               
                   
               
               
                 Note: 
               
               
                 The LED current was kept constant at 16.8 mA 
               
             
          
         
       
     
       FIG. 2  is a semi logarithmic plot of output power (measured at the tube end) versus tube length showing the expected exponential dependence. The loss for example 2 is L=0.38 dB/cm. 
     Example 3 
     A 36-cm long SILASTIC® Laboratory Tubing (¼″ ID; ⅜″ OD) manufactured by Dow Corning Corporation was filled with a core composition. The core composition was prepared by mixing 60 grams (g) Base, 115 g dimethylvinyl siloxy-terminated, poly(dimethylsiloxane/phenylmethylsiloxane), 3.9 g Crosslinker, 2.4 g Chain Extender, 0.013 g Catalyst, and 0.03 g Surfynol 61. The Base, Crosslinker, Chain Extender, Catalyst, and Surfynol were the same as in Example 1. The mixture was first de-aired under vacuum (&lt;20 Torr) at room temperature. A syringe was then filled with the material and used to push it into the tubing. A 3 mm LED light emitter and detector (from Radio Shack) were pushed into each end of the tubing. The tubing was placed into an oven and heated for 1 h at 150° C. 
     The response of the tube sensor was evaluated under conditions of variable compression applied to the tube. A ⅜″ diameter rod was placed at 90° angle across the sensor tube. Pressure was applied to the metal rod, and the output power of the detector was measured as a function of the compression distance relative to the flat surface on which the tube sensor was resting. The result is shown in Table 2 and plotted in  FIG. 3 . 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Out power as a function of compression 
               
             
          
           
               
                   
                 Compression distance (mil) 
                 Output Power (mW) 
               
               
                   
                   
               
             
          
           
               
                   
                 0 
                 538 
               
               
                   
                 20 
                 466 
               
               
                   
                 40 
                 394 
               
               
                   
                 60 
                 328 
               
               
                   
                 80 
                 265.8 
               
               
                   
                 100 
                 207.2 
               
               
                   
                 120 
                 158.8 
               
               
                   
                 140 
                 122.6 
               
               
                   
                 160 
                 95.3 
               
               
                   
                 180 
                 76.3 
               
               
                   
                 200 
                 60.3 
               
               
                   
                 164 
                 89 
               
               
                   
                 9 
                 476 
               
               
                   
                 0 
                 509 
               
               
                   
                   
               
             
          
         
       
     
     The last three data points in the table show that there remains a small temporary deformation of the tube after the compression is removed (not plotted). 
       FIG. 4  shows a sensor according to example 3. The sensor includes a waveguide  404  including a core  402  and a clad  407 . The sensor further includes a radiation source (LED emitter)  401  inserted in the core  402  at the inlet  403  of a waveguide  404  and a radiation detector  405  at the outlet  406  of the waveguide  404 . 
     Example 4 
     In this example, the internal surface of the SILASTIC® Laboratory Tubing manufactured by Dow Corning Corporation was coated with SYLGARD® to decrease interface roughness and thereby improve the loss characteristic. To a 3 m long tubing was injected 12 g of SYLGARD® 184. The tubing was hung at one end to the ceiling and SYLGARD® 184 ran down the inside surface of the tubing. The resulting coating was cured at room temperature for 24 hours (hr). The tube was cut to a length of 2.5 m and was then filled with a mixture that was prepared by combining 60 g Base, 115 g dimethylvinyl siloxy-terminated, poly(dimethylsiloxane/phenylmethylsiloxane), 3.9 g Crosslinker, 2.4 g Chain Extender, 0.013 g Catalyst, and 0.03 g Surfynol 61. The Base, Crosslinker, Chain Extender, Catalyst, and Surfynol were the same as in Example 1. The resulting filled tubing was placed into a 150° C. oven and heated for 1 h. The optical loss of the cured light guide as measured using cutback method is 0.211 dB/cm. No crack was observed when the waveguide was repeatedly hit with a hammer. 
     A cross section of a waveguide according to example 4 is shown in  FIG. 5 . The waveguide comprises the SILASTIC® clad  501 , a cured SYLGARD® 184 coating  502  inside the clad, and a silicone elastomer core  503  prepared by curing the composition as described above. 
     Comparative Example 1 
     A curable composition without reinforcing filler was prepared containing 11 parts tris(dimethylvinylsiloxy)methylsilane, 68 parts phenyl,methyl,vinylsiloxy-terminated poly(dimethylsiloxane/phenylmethylsiloxane), 21 parts dimethylhydrogensiloxy-terminated, phenyl silsesquioxane resin, less than 1 part 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane complexes with platinum, 0.06 part tetramethyldivinyldisiloxane, and 0.30 part dimethyl-1-hexyn-3-ol. This composition was first de-aired under vacuum (&lt;20 Torr) at room temperature. A 0.5 m long SILASTIC® Laboratory Tubing manufactured by Dow Corning Corporation was then filled with the de-aired composition. The filled tubing was placed into a 150° C. oven and heated for 1 h to form a waveguide. The optical loss measured using cutback method was 0.314 dB/cm. When the waveguide was hit with a hammer, the core broke and shattered. This comparative example shows the importance of filler to make the waveguide impact resistant and the inability of a resin to impart a desired degree of impact resistance in this composition. 
     INDUSTRIAL APPLICABILITY 
     The waveguide and sensor of this invention may be advantageous in that no primer or adhesive is needed between the core and the clad. This renders the methods for their preparation more efficient than if a primer or adhesive has to be applied between the core and clad. Furthermore, optical waveguides 2 to 10 meters long may be prepared according to this invention. Low loss in the optical waveguides of this invention allow optical waveguides of these lengths to be prepared. Waveguide loss may be less than 0.5 dB/cm, alternatively 0.2 to 0.5 dB/cm. Matching refractive index of the filler with the polydiorganosiloxane (when the particle size of the filler exceeds 10% of the wavelength of radiation to be transmitted) and decreasing interface roughness may reduce optical loss. 
    
    
     DRAWINGS 
       FIG. 1  is a cross section of a waveguide according to example 1. 
       FIG. 2  is a graph showing light power transmitted through a waveguide as function of waveguide length. 
       FIG. 3  is a graph showing response to waveguide deformation under compression. 
       FIG. 4  is a cross section of a sensor according to example 3. 
       FIG. 5  is a cross section of a waveguide according to example 4. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Reference Numerals 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 101 
                 clad 
               
               
                 102 
                 core 
               
               
                 401 
                 radiation source 
               
               
                 402 
                 core 
               
               
                 403 
                 inlet to waveguide 
               
               
                 404 
                 waveguide 
               
               
                 405 
                 radiation detector 
               
               
                 406 
                 outlet of waveguide 
               
               
                 407 
                 clad 
               
               
                 501 
                 clad 
               
               
                 502 
                 coating 
               
               
                 503 
                 core