A laser diode chip comprising: a plurality of semiconductor layers arranged on a substrate, the semiconductor layers collectively defining an active region layer of higher refractive index sandwiched between upper and lower cladding layers (12, 16) of lower refractive index; mutually opposed front and back end facets (22, 24) on which are formed front and back mirrors; a ridge extending laterally between the front and back end facets (22, 24) and having a pair of sides that extend part way to the active region layer to define a waveguide in the active region layer; a partially transmissive mirror arranged part way between the front end facet and the back end facet to subdivide the waveguide into a back waveguide section and a front waveguide section which form independent optical cavities, wherein the partially transmissive mirror is shared by the front and back waveguide sections (FS, BS), has a reflectivity R M greater than or equal to 70 percent and has a dual function of forming an output coupler for the back waveguide section and a back reflector for the front waveguide section; a plurality of electrical contacts (21, 23) configured to allow the front and back waveguide sections to be independently driven; and a controller configured to drive the electrical contacts (21, 23) in a first mode of operation to operate the laser diode chip with low slope efficiency for low output powers and in a second mode of operation to operate the laser diode chip with high slope efficiency for high output powers, wherein to produce the first, low slope efficiency mode of operation, the controller is configured to drive the electrical contacts (21, 23) to operate the back waveguide section as a laser diode and the front waveguide section as a bias-dependent tunable absorber, and wherein to produce the second, high slope efficiency mode of operation, the controller is configured to drive the electrical contacts (21, 23) to operate the front waveguide section as a laser diode and to deactivate the back waveguide section. a plurality of semiconductor layers arranged on a substrate, the semiconductor layers collectively defining an active region layer of higher refractive index sandwiched between upper and lower cladding layers (12, 16) of lower refractive index; mutually opposed front and back end facets (22, 24) on which are formed front and back mirrors; a ridge extending laterally between the front and back end facets (22, 24) and having a pair of sides that extend part way to the active region layer to define a waveguide in the active region layer; a partially transmissive mirror arranged part way between the front end facet and the back end facet to subdivide the waveguide into a back waveguide section and a front waveguide section which form independent optical cavities, wherein the partially transmissive mirror is shared by the front and back waveguide sections (FS, BS), has a reflectivity R M greater than or equal to 70 percent and has a dual function of forming an output coupler for the back waveguide section and a back reflector for the front waveguide section; a plurality of electrical contacts (21, 23) configured to allow the front and back waveguide sections to be independently driven; and a controller configured to drive the electrical contacts (21, 23) in a first mode of operation to operate the laser diode chip with low slope efficiency for low output powers and in a second mode of operation to operate the laser diode chip with high slope efficiency for high output powers, wherein to produce the first, low slope efficiency mode of operation, the controller is configured to drive the electrical contacts (21, 23) to operate the back waveguide section as a laser diode and the front waveguide section as a bias-dependent tunable absorber, and wherein to produce the second, high slope efficiency mode of operation, the controller is configured to drive the electrical contacts (21, 23) to operate the front waveguide section as a laser diode and to deactivate the back waveguide section.