A transparent waveguide, which is for use in tracking an eye that is illuminated by infrared light having an infrared wavelength, includes a volume Bragg grating type of input-coupler adapted to receive infrared light having the infrared wavelength and couple the received infrared light into the waveguide. The volume Bragg grating includes a lower boundary and an upper boundary that is closer to the output-coupler than the lower boundary. A k-vector angle of the volume Bragg grating at the lower boundary is greater than a k-vector angle at the upper boundary, with k-vector angles of the volume Bragg grating between the lower and upper boundaries gradually decreasing as distances decrease between grating planes of the volume Bragg grating and the upper boundary. Additionally, the volume Bragg grating preferably has an angular bandwidth that is no greater than 5 degrees.