PRECISELY WAVELENGTH-CONTROLLED CORRUGATION FOR DFB LASER-DIODES DELINEATED BY WEIGHTED-DOSE ELECTRON-BEAM LITHOGRAPHY

Well-defined wavelength distributed feedback laser diodes (DFB-LDs) ar e required in WDM network systems. Since the EDFA gain bands have been expanded, even more wavelengths are needed for large-capacity dense-W DM transmission systems. A precisely pitch-controlled Bragg grating fa bricated by electron beam (EB) lithography is very attractive for real izing these DFB-LDs. This paper describes this precise pitch- and phas e-controlled grating delineated by a novel method called weighted-dose allocation variable-pitch EB-lithography (WAVE). In this method, an E B-dose profile for the grating is precisely controlled by a combinatio n of the allocation and weighting of multiple exposures. This enables us to Fabricate a precise fixed-pitch grating as well as a flexible gr ating with a continuously chirped structure. The stitching error at th e exposure field boundary, the grating pitch, and the phase shift were evaluated by using a moire pattern generated by superimposing the mic roscope raster scan and the grating on a wafer. We also estimated amou nts of the stitching errors from fabricated and calculated lasing char acteristics, and clarified that the affect of the errors on the single -mode stability of LDs is negligible. Precise wavelength controlled la mbda/4 phase shifted DFB-LDs were successfully demonstrated as a resul t of both the WAVE method and the highly uniform MOVPE crystal growth.