An apparatus for use in tracking an eye (140, 440) that is illuminated by infrared light having an infrared wavelength, the apparatus comprising: a waveguide (112, 412) that is transparent and includes an input-coupler and an output-coupler; the input-coupler comprising a volume Bragg grating adapted to receive infrared light having the infrared wavelength that is reflected from an eye (140, 440) and couple the received infrared light into the waveguide (112, 412) so that at least a portion of the infrared light that is coupled into the waveguide (112, 412) travels by way of total internal reflection to the output-coupler of the waveguide (112, 412); a lens module that converts infrared light that exits the waveguide (112, 412) at the output-coupler from angularly encoded infrared light to spatially encoded infrared light; and a sensor that produces eye tracking data in dependence on the spatially encoded infrared light produced using the lens module; the volume Bragg grating including a lower boundary and an upper boundary, the upper boundary being closer to the output-coupler than the lower boundary; characterized in that the volume Bragg grating having a k-vector angle at the lower boundary that 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; the volume Bragg grating having a grating period at the lower boundary that is less than the grating period at the upper boundary, with the grating period of the volume Bragg grating between the lower and upper boundaries gradually increasing; and the volume Bragg grating having an angular bandwidth that is equal to or less than 5 degrees; wherein the k-vector angle is the angle of a k-vector relative to a surface normal of the volume Bragg grating. a waveguide (112, 412) that is transparent and includes an input-coupler and an output-coupler; the input-coupler comprising a volume Bragg grating adapted to receive infrared light having the infrared wavelength that is reflected from an eye (140, 440) and couple the received infrared light into the waveguide (112, 412) so that at least a portion of the infrared light that is coupled into the waveguide (112, 412) travels by way of total internal reflection to the output-coupler of the waveguide (112, 412); a lens module that converts infrared light that exits the waveguide (112, 412) at the output-coupler from angularly encoded infrared light to spatially encoded infrared light; and a sensor that produces eye tracking data in dependence on the spatially encoded infrared light produced using the lens module; the volume Bragg grating including a lower boundary and an upper boundary, the upper boundary being closer to the output-coupler than the lower boundary; characterized in that the volume Bragg grating having a k-vector angle at the lower boundary that 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; the volume Bragg grating having a grating period at the lower boundary that is less than the grating period at the upper boundary, with the grating period of the volume Bragg grating between the lower and upper boundaries gradually increasing; and the volume Bragg grating having an angular bandwidth that is equal to or less than 5 degrees; wherein the k-vector angle is the angle of a k-vector relative to a surface normal of the volume Bragg grating.