In an uncoated, pure Bragg grating structure, two strongest reflection peaks present at each side of forbidden band . When the Bragg grating structure is integrated to a laser diode to form a distributed feedback (DFB) laser, these two reflection peaks will be the two dominant cavity modes . In order to achieve single frequency operation, such symmetry must be broken.
Different causes that can break this symmetry are studied and simulated using transfer matrix method (TMM). Once the symmetry is broken, the stronger mode will stay dominant throughout laser operation. The switching of dominant modes can be realized by adding a phase shifter next to the gain region . In our experiment, optical injection of carrier can induce mode switching at very low drive current and relatively high speed . This phenomenon can be used to generate pure frequency modulation. However, the criteria for laser itself are too restrictive, and mode hopping range cannot be modified for it is determined by the physical dimension of device. As a result, mode hopping is not ideal for our purpose.