An external cavity laser has an external cavity that is formed between a
target surface and a laser diode facet located farthest from the target
surface. A chromatic dispersive element is disposed in the external
cavity, and focuses light from the laser diode such that one wavelength
is focused at a focal spot on the target surface. Light reflected from
the focal spot on the target surface is the predominant wavelength
coupled back into the laser waveguide, which provides a feedback signal
that determines the oscillation wavelength of the laser diode. The laser
diode therefore outputs a well-defined wavelength of light which
corresponds to the distance between the chromatic dispersive element and
the target surface.