Wavelength locker and optical system using the same

An optical system comprising:

• a tunable optical device;
• and
• apparatus for use in tuning said tunable optical device to a target wavelength, said apparatus comprising:
• a beam splitter for tapping a portion of the light emerging from said tunable optical device;
• a walk-off reflector for dividing said portion of the light emerging from said beam splitter into a plurality of beams;
• a multiple etalon for tailoring the light from at least two of said plurality of beams provided by said walk-off reflector;
• a multiple detector for detecting light from said multiple etalon and said walk-off reflector, said multiple detector providing a plurality of output signals;
• and
• a control unit for controlling said tunable optical device by providing a control signal to said tunable optical device according to said output signals provided by said multiple detector.