A broadband laser source comprising: a resonant cavity (1) containing a
medium (2) which emits optical energy (3) in a continuum of wavelengths
in response to application of pump energy (4) to the medium (2);
a source of pump energy (5) for producing said optical energy (3);
a frequency shifter (6) within the resonant cavity (1); and a tuneable
spectral filter (7) within the resonant cavity (1), and the apparatus
being such that in use the tuneable spectral filter (7) has a peak wavelength
which is repeatedly swept in a substantially resonant fashion over a wavelength
range equal to, or a fraction of, a linewidth of the broadband source such that
the rate of change of the peak wavelength of the tuneable spectral filter (7)
is substantially equal to the rate of change of the frequency of the optical energy
(3) as it is frequency shifted within the resonant cavity (1) by
the frequency shifter (6).
