An optical transmission system includes an optical source such as a laser
having an optical output, this optical output being modulated such that
it has periods of operation having a first set of characteristics
interspersed with periods of operation having a second set of
characteristics. The output is then split into at least two signals, and
one of the signals delayed with respect to the other signal before the
two are mixed, such that a portion of the modulated optical output having
the first set of characteristics in one of the signals corresponds with a
portion of the modulated optical output having the second set of
characteristics of another of the signals. This allows a single optical
source to provide to a receiver a local oscillator signal simultaneously
with a data carrying signal. The system is particularly suitable for
lidar applications, including gas sensing, but also has utility in data
communications systems.