A method of preparing a planar optical waveguide assembly, the method comprising: (i) applying a silicone composition to a surface of a substrate to form a first silicone film, wherein the silicone composition comprises: an organopolysiloxane resin having the formula (R 1 R 2 2 SiO 1/2) v (R 2 2 SiO 2/2) w (R 2 SiO 3/2) x (R 3 SiO 3/2) y (SiO 4/2) z, wherein R 1 is C 1 to C 10 hydrocarbyl, R 2 is independently selected from R 1 or an epoxy-substituted organic group,, v is from 0 to 0.3, w is from 0 to 0.5, x is from 0 to 0.9, y is from 0.1 to 0.8, z is from 0 to 0.5, and v + w + x + y + z = 1, provided the organopolysiloxane resin has an average of at least two silicon-bonded epoxy-substituted organic groups per molecule; an organic solvent; and a photoinitiator; (ii) exposing at least one selected region of the first silicone film to radiation having a wavelength of from 150 nm to 800 nm to produce a partially exposed film having at least one exposed region and at least one non-exposed region; (iii) removing the non-exposed region of the partially exposed film with a developing solvent to form a patterned film; and (iv) heating the patterned film for an amount of time sufficient to form at least one silicone core having a refractive index from about 1.45 to about 1.60 at 23 °C for light having a wavelength of 589 nm, such that the refractive index of the silicone core is greater than the refractive index of the substrate, characterised in that R 3 is C 4 to C 8 alkyl. (i) applying a silicone composition to a surface of a substrate to form a first silicone film, wherein the silicone composition comprises: an organopolysiloxane resin having the formula (R 1 R 2 2 SiO 1/2) v (R 2 2 SiO 2/2) w (R 2 SiO 3/2) x (R 3 SiO 3/2) y (SiO 4/2) z, wherein R 1 is C 1 to C 10 hydrocarbyl, R 2 is independently selected from R 1 or an epoxy-substituted organic group,, v is from 0 to 0.3, w is from 0 to 0.5, x is from 0 to 0.9, y is from 0.1 to 0.8, z is from 0 to 0.5, and v + w + x + y + z = 1, provided the organopolysiloxane resin has an average of at least two silicon-bonded epoxy-substituted organic groups per molecule; an organic solvent; and a photoinitiator; an organopolysiloxane resin having the formula (R 1 R 2 2 SiO 1/2) v (R 2 2 SiO 2/2) w (R 2 SiO 3/2) x (R 3 SiO 3/2) y (SiO 4/2) z, wherein R 1 is C 1 to C 10 hydrocarbyl, R 2 is independently selected from R 1 or an epoxy-substituted organic group,, v is from 0 to 0.3, w is from 0 to 0.5, x is from 0 to 0.9, y is from 0.1 to 0.8, z is from 0 to 0.5, and v + w + x + y + z = 1, provided the organopolysiloxane resin has an average of at least two silicon-bonded epoxy-substituted organic groups per molecule; an organic solvent; and a photoinitiator; (ii) exposing at least one selected region of the first silicone film to radiation having a wavelength of from 150 nm to 800 nm to produce a partially exposed film having at least one exposed region and at least one non-exposed region; (iii) removing the non-exposed region of the partially exposed film with a developing solvent to form a patterned film; and (iv) heating the patterned film for an amount of time sufficient to form at least one silicone core having a refractive index from about 1.45 to about 1.60 at 23 °C for light having a wavelength of 589 nm, such that the refractive index of the silicone core is greater than the refractive index of the substrate,