with the starting material is not too reactive has a reasonable gel time and yields crosslinked polymer with low optical loss and good optical quality generally the crosslinking procedure is carried out as follows the catalyst is mixed with the vinyl terminated dimethyldiphenylsiloxane and degassed this mixture is added to the crosslinking agent mixed and again degassed this mixture is applied to the device and curing is effected either by allowing the mixture to stand at room temperature for 1 10 hours or heating typically to about 100 u00b0 150 u00b0 c for a shorter time higher temperatures may be used but generally no advantage are obtained and very high temperatures above about 250 u00b0 to 300 u00b0 c are avoided because of decomposition of the polymer composition of the mixture is an important variable in determining the mechanical properties of the crosslinked polymer and the gel time of the mixture composition of the mixture of crosslinking agent and vinyl terminated siloxane is given in terms of the ratio of number of hydride groups on the crosslinking agent si h groups to number of vinyl groups on the vinyl terminated siloxane starting material hydride vinyl ratio this ratio may vary over.