A method of forming a singulation line in an optical waveguide sheet, the optical waveguide sheet comprising a body that defines opposite first and second surfaces (12, 14) and that supports at least one sheet optical waveguide at a waveguide location, the method comprising: irradiating the optical waveguide sheet with a focused laser beam at discrete locations along an irradiation path by passing the focused laser beam through the first surface at the discrete locations to form spaced apart modified regions (138, 300) that extend from at least one of the first surface and the second surface and into the body of the optical waveguide sheet, wherein the spaced apart modified regions (138, 300) are separated by unmodified regions of the optical waveguide sheet to define at least one modified section of the optical waveguide sheet; leaving at least one section of the optical waveguide sheet along the irradiation path at the waveguide location unmodified to define at least one unmodified section wherein the singulation line is formed by the at least one modified section and the at least one unmodified section of the optical waveguide sheet; separating the optical waveguide sheet along the singulation line to form an optical waveguide substrate comprising: an end face residing substantially at the location of the singulation line and having at least one first section with a first surface roughness at the at least one unmodified section and a second section with a second surface roughness at the at least one modified section, wherein the second surface roughness is greater than the first surface roughness; at least one substrate optical waveguide constituted by a section of the at least one sheet optical waveguide, the at least one substrate optical waveguide comprising opposite first and second surfaces and a waveguide end surface at the at least one first section of the end face. irradiating the optical waveguide sheet with a focused laser beam at discrete locations along an irradiation path by passing the focused laser beam through the first surface at the discrete locations to form spaced apart modified regions (138, 300) that extend from at least one of the first surface and the second surface and into the body of the optical waveguide sheet, wherein the spaced apart modified regions (138, 300) are separated by unmodified regions of the optical waveguide sheet to define at least one modified section of the optical waveguide sheet; leaving at least one section of the optical waveguide sheet along the irradiation path at the waveguide location unmodified to define at least one unmodified section wherein the singulation line is formed by the at least one modified section and the at least one unmodified section of the optical waveguide sheet; separating the optical waveguide sheet along the singulation line to form an optical waveguide substrate comprising: an end face residing substantially at the location of the singulation line and having at least one first section with a first surface roughness at the at least one unmodified section and a second section with a second surface roughness at the at least one modified section, wherein the second surface roughness is greater than the first surface roughness; at least one substrate optical waveguide constituted by a section of the at least one sheet optical waveguide, the at least one substrate optical waveguide comprising opposite first and second surfaces and a waveguide end surface at the at least one first section of the end face. an end face residing substantially at the location of the singulation line and having at least one first section with a first surface roughness at the at least one unmodified section and a second section with a second surface roughness at the at least one modified section, wherein the second surface roughness is greater than the first surface roughness; at least one substrate optical waveguide constituted by a section of the at least one sheet optical waveguide, the at least one substrate optical waveguide comprising opposite first and second surfaces and a waveguide end surface at the at least one first section of the end face.