Patent GB-1421401-A (GB)
Title: Planar optical waveguides
IPC: G02B6/02
Filed: 1973-03-29
Abstract: 1421401 Planar optical waveguides CORNING GLASS WORKS 29 March 1973 [30 March 1972 (2)] 15263/73 Heading C1M [Also in Division G2] A method of forming a planar optical waveguide for light propagation comprises the steps of providing a substantially flat glass substrate; applying a first coating of glass having an index of refraction greater than that of said glass substrate to at least a portion of one flat surface of said substrate, said first coating of glass having an outer exposed surface substantially parallel to said one flat surface of said substrate, the width of said first coating being the dimension perpendicular to both said thickness and the light propagation axis of said first coating; and applying a second coating of glass having an index of refraction less than that of the first coating of glass over at least the outer exposed surface of the first coating of glass, the thickness of the first coating of glass for any desired finite width thereof being equal to or less than the thickness a determined in accordance with one of the following equations for TM om  modes (m even integer), TM om  modes (m odd integer), TE om  (m even integer) and TE om  modes (m odd integer) respectively where m # 10 and is the highest order mode to be propagated within a waveguide the first coating of which has thickness a and an infinite width, X is the wavelength of light to be propagated within said waveguide; n 2  is the index of refraction of said first coating, and one of n 1  and n 3  being the index of refraction of said substrate while the other of n 1  and n 3  being the index of refraction of the second coating of glass, the thickness of the first coating of glass being calculable as above where the propagation constant of the waveguide is equal to or less than K 1 , where K 1  is greater than K 3  and where K 1  = 2#n 1 /#, K 2  = 2#n 2 /# and K 3  = 2#n 3 /#, the thickness of said substrate and the second coating of glass each being at least twice the thickness of said first coating of glass. The first and second coatings of glass may be formed by the steps of depositing a layer of glass soot on at least a portion of one flat surface of said substrate by flame hydrolysis, the deposited soot either being sintered simultaneously with the deposition thereof or by a subsequent heating operation. Alternatively the first and second coatings of glass may be formed by depositing doped fused silica soot, the dopant being TiO 2 , Ta 2 O 5 , tin oxide, Nb 2 O 5 , ZrO 2 , Al 2 O 3 , La 2 O 3 , GeO 2  and B 2 O 3 . For example Al 2 O 3  may be present in preferably not more than 40% by weight and the TiO 2  should be maintained below 20% by weight. The first and second coatings may also be applied by a chemical vapour deposition technique or by applying a glass frit. In one embodiment the coatings are applied by using a burner which emits a flame in which a gasvapour mixture is hydrolysed to form a soot, the substrate being suitably mounted so that it can be translated so that a uniform thickness of soot is applied. In another embodiment (Fig. 5) the first coating 26 is applied in a narrow path, e.g having a width to thickness ratio of less than 10. A second coating 30 is applied over both the outer exposed surface 32 as well as the exposed side edges of coating 26. In another embodiment a laser beam is directed towards the first coating and a relative motion between the laser beam and the substrate is caused thereby producing a path of sintered glass. The unsintered portion of the first coating is removed from the substrate whereafter a layer of glass having an index of refraction less than that of the first coating may be applied over the path of sintered glass and at least a portion of the exposed surface of the substrate which is adjacent thereto. The specific example is concerned with the preparation of a planar optical waveguide the first glass layer consisting of 98% by weight SiO 2  and 2% by weight TiO 2  and the second layer of pure silica. After coating the structure is placed in an induction furnace having an oxygen atmosphere at 1500‹ C. to sinter the first and second soot layers.