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Undercladding and uppercladding layers that form part of an optical waveguide are deposited with a thermal CVD technique. The optical waveguide has a structure in which optical cores are formed over the undercladding layer and in which the uppercladding layer is formed over and between the optical c

Undercladding and uppercladding layers that form part of an optical waveguide are deposited with a thermal CVD technique. The optical waveguide has a structure in which optical cores are formed over the undercladding layer and in which the uppercladding layer is formed over and between the optical cores. Generally, the optical cores have a refractive index greater than a refractive index of the cladding layers. A borophosphosilicate-glass layer may be used as one or both of the cladding layers. Thick cladding layers may be formed by cyclically depositing and annealing portions of the layer.

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What is claimed is: 1. A method for forming an optical waveguide, the method comprising: providing a gaseous mixture to a chamber at a pressure substantially between 100 torr and 800 torr; depositing an undercladding layer over a substrate with the gaseous mixture using thermal chemical-vapor depos

What is claimed is: 1. A method for forming an optical waveguide, the method comprising: providing a gaseous mixture to a chamber at a pressure substantially between 100 torr and 800 torr; depositing an undercladding layer over a substrate with the gaseous mixture using thermal chemical-vapor deposition, wherein depositing the undercladding layer comprises: depositing a first portion of the undercladding layer using thermal chemical-vapor deposition; thermally annealing the first portion; and thereafter, depositing a second portion of the undercladding layer using thermal chemical-vapor deposition; and forming a plurality of optical cores over the deposited undercladding layer, wherein the optical cores have a refractive index greater than a refractive index of the undercladding layer. 2. The method recited in claim 1 wherein the gaseous mixture comprises a boron-containing gas, a phosphorus-containing gas, a silicon-containing gas, and an oxygen-containing gas, and the undercladding layer comprises a borophosphosilicate-glass layer. 3. The method recited in claim 2 wherein a boron concentration of the undercladding layer is substantially between 3 wt. % and 8 wt. %. 4. The method recited in claim 2 wherein a boron concentration of the undercladding layer is substantially between 4.5 wt. % and 6.5 wt. %. 5. The method recited in claim 2 wherein a phosphorus concentration of the undercladding layer is substantially between 1 wt. % and 3 wt. %. 6. The method recited in claim 1 wherein the pressure is substantially between 100 torr and 300 torr. 7. The method recited in claim 1 wherein thermally annealing the first portion is performed in a nitrogen atmosphere. 8. The method recited in claim 1 wherein thermally annealing the first portion is performed in an oxygen atmosphere. 9. The method recited in claim 1 wherein thermally annealing the first portion is performed without removing the substrate from the chamber. 10. The method recited in claim 1 wherein thermally annealing the first portion is performed at a temperature substantially between 900째 C. and 1050째 C. 11. The method recited in claim 1 further comprising depositing an uppercladding layer over the optical cores substrate using thermal chemical-vapor deposition. 12. The method recited in claim 1 wherein the refractive index of the undercladding layer is between 1.4500 and 1.4650. 13. The method recited in claim 1 wherein the refractive index of the undercladding layer is between 1.4550 and 1.4600. 14. The method recited in claim 1 wherein the refractive index of the undercladding layer varies by less than 2% of 1.4575. 15. A method for forming an optical waveguide, the method comprising: providing a gaseous mixture to a chamber at a pressure substantially between 100 torr and 800 torr; and depositing an uppercladding layer over a substrate having a plurality of optical cores with the gaseous mixture using thermal chemical-vapor deposition, wherein the optical cores have a refractive index greater than a refractive index of the uppercladding layer, wherein depositing the uppercladding layer comprises: depositing a first portion of the uppercladding layer using thermal chemical-vapor deposition; thermally annealing the first portion; and thereafter, depositing a second portion of the uppercladding layer using thermal chemical vapor deposition. 16. The method recited in claim 15 wherein the gaseous mixture comprises a boron-containing gas, a phosphorus-containing gas, a silicon-containing gas, and an oxygen-containing gas, and the uppercladding layer comprises a borophosphosilicate-glass layer. 17. The method recited in claim 16 wherein a boron concentration of the uppercladding layer is substantially between 3 wt. % and 8 wt. %. 18. The method recited in claim 16 wherein a boron concentration of the uppercladding layer is substantially between 4.5 wt. % and 6.5 wt. %. 19. The method recited in claim 16 wherein a phosphorus concentration of the uppercladding layer is substantially between 1 wt. % and 3 wt. %. 20. The method recited in claim 15 wherein the pressure is substantially between 100 torr and 300 torr. 21. The method recited in claim 15 wherein thermally annealing the first portion is performed in a nitrogen atmosphere. 22. The method recited in claim 15 wherein thermally annealing the first portion is performed in an oxygen atmosphere. 23. The method recited in claim 15 wherein thermally annealing the first portion is performed without removing the substrate from the chamber. 24. The method recited in claim 15 wherein thermally annealing the first portion is performed at a temperature substantially between 900째 C. and 1050째 C. 25. The method recited in claim 15 wherein the refractive index of the uppercladding layer is between 1.4500 and 1.4650. 26. The method recited in claim 15 wherein the refractive index of the uppercladding layer is between 1.4550 and 1.4600. 27. The method recited in claim 15 wherein the refractive index of the undercladding layer varies by less than 2% of 1.4575.