An apparatus, comprising: a first optical waveguide including a bulk-substrate, an input-coupler and an output-coupler; the bulk-substrate of the first optical waveguide including a first major surface and a second major surface opposite the first major surface; the input-coupler of the first optical waveguide configured to couple, into the bulk substrate of the first optical waveguide, light that is within an input angular range and within a first wavelength range; the output-coupler of the first optical waveguide configured to couple, out of the bulk-substrate (1060 of the optical waveguide, light within an output angular range and within the first wavelength range that travels through the first optical waveguide from the input-coupler to the output-coupler at least in part by way of total internal reflection, wherein the input-coupler and the output-coupler are both located in or on the same one of the major surfaces; a first coating covering a given one of the first and second major surfaces of the first optical waveguide, wherein the given one of the major surfaces is the opposing major surface to the one of the major surfaces in or on which the input-coupler and the output-coupler are located; the first coating configured to have a low reflectance of no more than 2 percent for light within the first wavelength range that is traveling within the bulk-substrate of the first optical waveguide and is incident on the given one of the first and second major surfaces of the first optical waveguide on which the first coating is located at each angle below a low threshold angle of no more than 25 degrees relative to a normal of the one of the first and second major surfaces of the first optical waveguide on which the first coating is located; and the first coating configured to have a high reflectance of at least 50 percent for light within the first wavelength range that is traveling within the bulk-substrate of the first optical waveguide and is incident on the given one of the first and second major surfaces of the first optical waveguide on which the first coating is located at each angle above a high threshold angle of 35 degrees relative to the normal of the given one of the first and second major surfaces of the first optical waveguide on which the first coating is located. a first optical waveguide including a bulk-substrate, an input-coupler and an output-coupler; the bulk-substrate of the first optical waveguide including a first major surface and a second major surface opposite the first major surface; the input-coupler of the first optical waveguide configured to couple, into the bulk substrate of the first optical waveguide, light that is within an input angular range and within a first wavelength range; the output-coupler of the first optical waveguide configured to couple, out of the bulk-substrate (1060 of the optical waveguide, light within an output angular range and within the first wavelength range that travels through the first optical waveguide from the input-coupler to the output-coupler at least in part by way of total internal reflection, wherein the input-coupler and the output-coupler are both located in or on the same one of the major surfaces; a first coating covering a given one of the first and second major surfaces of the first optical waveguide, wherein the given one of the major surfaces is the opposing major surface to the one of the major surfaces in or on which the input-coupler and the output-coupler are located; the first coating configured to have a low reflectance of no more than 2 percent for light within the first wavelength range that is traveling within the bulk-substrate of the first optical waveguide and is incident on the given one of the first and second major surfaces of the first optical waveguide on which the first coating is located at each angle below a low threshold angle of no more than 25 degrees relative to a normal of the one of the first and second major surfaces of the first optical waveguide on which the first coating is located; and the first coating configured to have a high reflectance of at least 50 percent for light within the first wavelength range that is traveling within the bulk-substrate of the first optical waveguide and is incident on the given one of the first and second major surfaces of the first optical waveguide on which the first coating is located at each angle above a high threshold angle of 35 degrees relative to the normal of the given one of the first and second major surfaces of the first optical waveguide on which the first coating is located.