Tunable laser diode having a distributed feedback structure

A method for controlling the frequency of a coherent optical beam produced by a distributed feedback laser diode, said distributed feedback laser diode comprising:

• a substrate, a waveguide layer provided on the substrate for guiding an optical radiation therethrough, an active layer provided on the waveguide layer, said active layer being injected with carriers and producing an optical radiation as a result of recombination of the carriers, diffraction grating means provided at an interface between the substrate and the waveguide layer for reflecting the optical radiation propagating in the waveguide layer back and forth, said optical radiation having an inhomogeneous distribution of intensity along the active layer such that there is a maximum of the intensity and a minimum of the intensity, a clad layer provided on the active layer for confirming the optical radiation within the active layer, a plurality of electrodes provided on the top surface of the clad layer for injecting the carriers into the active layer along the clad layer, the plurality of electrodes being disposed on the top surface of the clad layer so as to align generally in the longitudinal direction, and backside electrode provided at the bottom surface of the substrate for injecting the carriers into the active layer through the substrate, said method comprising the steps of:
• supplying driving currents to the plurality of electrodes so that the distributed feedback laser diode produces a coherent optical beam;
• supplying a modulation signal, in addition to the driving current, to one of the plurality of electrodes provided in correspondence to the maximum of the intensity of the optical radiation formed in the active layer;
• monitoring an output power of the coherent optical beam produced in response to the drive currents and the modulation signal;
• and
• controlling the driving currents to the supplied to the plurality of electrodes which is provided in correspondence to the minimum of the intensity of the optical radiation in the active layer.

A tunable laser diode for producing a coherent optical beam with a frequency that is changed in response to a modulation signal, comprising;

• a substrate elongated in a longitudinal direction between a front end of said substrate and rear end of said substrate facing the front end, said substrate having a top surface and a bottom surface;
• a waveguide layer elongated in the longitudinal direction between a front end and a rear end of said waveguide layer respectively corresponding to the front end and the rear end of said substrate, said waveguide layer further having a top surface and bottom surface, said waveguide layer being provided on the top surface of said substrate for guiding an optical radiation therethrough such that the optical radiation is propagated through said waveguide layer in the longitudinal direction;
• an active layer elongated in the longitudinal direction between a front end and a rear end of said active layer respectively corresponding to the front end and the rear end of said waveguide layer, said active layer having a top surface and being provided on the top surface of said waveguide layer, said active layer connected to receive carriers and producing an optical radiation as a result of recombination of the carriers, said active layer being provided on said waveguide layer such that the optical radiation produced in said active layer is transferred to said waveguide layer and guided therealong;
• said waveguide layer and said active layer forming a mesa structure projecting upwards from said substrate, said mesa structure extending in the longitudinal direction with a substantially same cross section when viewed in the longitudinal direction:
• diffraction grating means provided at an interface between the top surface of said substrate and the bottom surface of said waveguide layer, for reflecting the optical radiation propagating in said waveguide layer back and forth in the longitudinal direction;
• a clad layer elongated in the longitudinal direction between a front end and a rear end of said clad layer respectively corresponding to the front end and the rear end of said active layer, said clad layer having a top surface and being provided on the top surface of said active layer so that the optical radiation within said active layer such that the optical radiation produced in said active layer is prevented from leaking into said clad layer;
• a plurality of grooves provided on said clad layer at both sides of said mesa structure so as to reach a depth corresponding to said mesa structure, each of said plurality of grooves extending in the longitudinal direction, said plurality of grooves being aligned, at respective sides of the mesa structure, in the longitudinal direction and separated from each other;
• a plurality of electrodes provided on the top surface of said clad layer in correspondence to said plurality of grooves for injecting the carriers into said active layer along said clad layer, said plurality of electrodes being disposed on the top surface of said clad layer so as to be aligned generally in the longitudinal direction, wherein a first electrode of said plurality of electrodes is positioned in correspondence to a part of said active layer in which the optical radiation formed in said active layer has a maximum intensity level, said first electrode being connectable to receive the modulation signal while other electrodes being connectable to receive a drive current, said first electrode being electrically isolated from said other electrodes;
• and
• a backside electrode provided at the bottom surface of said substrate for injecting the carriers into said active layer through said substrate.