A system in which the controller (24) of a multi section diode laser such
as a SG-DBR (10) is configured so that the laser can be swept rapidly in
a pre-determined frequency direction through a series of frequency points
by asserting a pre-calibrated series of sets of control input values to
the sections of the diode laser, wherein the frequency points are
obtained from cavity modes in a plurality of different supermodes, and
the sets of control input values are pre-determined to take account of
thermal transients that are known to arise from jumps in the output modes
that occur when sweeping through the pre-calibrated series of sets of
control input values in the pre-determined frequency direction.