A system (1000, 3000), comprising: a swept laser light source configured to generate laser light having a frequency swept across a frequency bandwidth as a function of time; a sample path configured to direct a first portion of the laser light to an eye as a probe beam and to receive a returned portion of the probe beam from the eye, returned by at least one of reflection and scattering, wherein the sample path includes a fiducial generator configured to introduce a fiducial signal into the returned portion of the probe beam from the eye; a reference path for passing therethrough a second portion of the laser light, the reference path having a defined optical path length; and a detector (1050, 3050) configured to receive the returned portion of the probe beam from the eye and the second portion of the laser light from the swept laser light source which passed through the reference path, and in response thereto to output an optical coherence tomography, OCT, output signal having a fiducial peak corresponding to the fiducial signal and having OCT peaks whose relative timing represents the depths of surfaces of structures of the eye, wherein the fiducial generator comprises: an optical path length; characterized in that the fiducial generator additionally comprises a reflective surface disposed at a first end of the optical path length; and a beam splitter configured to couple part of the first portion of the laser light to a second end of the optical path length, and to combine reflected light from the reflective surface with the returned portion of the probe beam from the eye to be provided to the detector, wherein the fiducial signal is produced in response to the reflected light from the reflective surface. a swept laser light source configured to generate laser light having a frequency swept across a frequency bandwidth as a function of time; a sample path configured to direct a first portion of the laser light to an eye as a probe beam and to receive a returned portion of the probe beam from the eye, returned by at least one of reflection and scattering, wherein the sample path includes a fiducial generator configured to introduce a fiducial signal into the returned portion of the probe beam from the eye; a reference path for passing therethrough a second portion of the laser light, the reference path having a defined optical path length; and a detector (1050, 3050) configured to receive the returned portion of the probe beam from the eye and the second portion of the laser light from the swept laser light source which passed through the reference path, and in response thereto to output an optical coherence tomography, OCT, output signal having a fiducial peak corresponding to the fiducial signal and having OCT peaks whose relative timing represents the depths of surfaces of structures of the eye, wherein the fiducial generator comprises: an optical path length; characterized in that the fiducial generator additionally comprises a reflective surface disposed at a first end of the optical path length; and a beam splitter configured to couple part of the first portion of the laser light to a second end of the optical path length, and to combine reflected light from the reflective surface with the returned portion of the probe beam from the eye to be provided to the detector, wherein the fiducial signal is produced in response to the reflected light from the reflective surface. an optical path length; characterized in that the fiducial generator additionally comprises a reflective surface disposed at a first end of the optical path length; and a beam splitter configured to couple part of the first portion of the laser light to a second end of the optical path length, and to combine reflected light from the reflective surface with the returned portion of the probe beam from the eye to be provided to the detector, wherein the fiducial signal is produced in response to the reflected light from the reflective surface. a reflective surface disposed at a first end of the optical path length; and a beam splitter configured to couple part of the first portion of the laser light to a second end of the optical path length, and to combine reflected light from the reflective surface with the returned portion of the probe beam from the eye to be provided to the detector, wherein the fiducial signal is produced in response to the reflected light from the reflective surface.