A method and apparatus for wavelength switching a laser source. An external cavity (2) having a Bragg grating (5) is provided in an optical fiber (4). Space division switching is performed in the external cavity between a plurality of optical fibers (4) each having a different Bragg grating (5). A space division switch (3) is provided in the external cavity for switching between a plurality of optical fibers (4) each carrying a different Bragg grating (5).