FABRICATION OF NONCONVENTIONAL DISTRIBUTED FEEDBACK LASERS WITH VARIABLE GRATING PERIODS AND PHASE-SHIFTS BY ELECTRON-BEAM LITHOGRAPHY

Distributed feedback (DFB) lasers for a wide range in emission frequency have been fabricated in the AlGaAs system. Gratings with periods around 110 nm have been defined by electron beam lithography and dry etching. Two approaches are discussed to reach extremely fine tuning of the emission frequency, which becomes more and more essential the higher the band gap of the material and with that, the emission frequency of the laser. To reach single mode emission, phase shifts in the grating structure have been realized. A flat photon density distribution to reduce spatial hole burning could be achieved by distributing the pi/2 phase shift by using four lambda/16 phase shifts. In addition, resonators have been fabricated where the phase shift is spread over a wide region in the center of the resonator which corresponds to the introduction of a phase arranging zone with a slightly larger effective grating constant. The laser structures have been tested by optical pumping. DFB emission could be observed over a wide range in frequency at room temperature. The resonators with the distributed phase shifts show stable single mode operation.