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Precisely Wavelength-Controlled Corrugation for DFB Laser Diodes Delineated by Weighted-Dose Electron-Beam Lithography
Yoshiharu MUROYA Kenji SATO Tetsuro OKUDA Takahiro NAKAMURA Hirohito YAMADA Toshitaka TORIKAI
IEICE TRANSACTIONS on Electronics
Publication Date: 1998/08/25
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on High-Capacity WDM/TDM Networks)
electron-beam lithography, DFB laser diode, grating, wavelength control, WDM,
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Well-defined wavelength distributed feedback laser diodes (DFB-LDs) are required in WDM network systems. Since the EDFA gain bands have been expanded, even more wavelengths are needed for large-capacity dense-WDM transmission systems. A precisely pitch-controlled Bragg grating fabricated by electron beam (EB) lithography is very attractive for realizing these DFB-LDs. This paper describes this precise pitch- and phase-controlled grating delineated by a novel method called weighted-dose allocation variable-pitch EB-lithography (WAVE). In this method, an EB-dose profile for the grating is precisely controlled by a combination of the allocation and weighting of multiple exposures. This enables us to fabricate a precise fixed-pitch grating as well as a flexible grating with a continuously chirped structure. The stitching error at the exposure field boundary, the grating pitch, and the phase shift were evaluated by using a moire pattern generated by superimposing the microscope raster scan and the grating on a wafer. We also estimated amounts of the stitching errors from fabricated and calculated lasing characteristics, and clarified that the affect of the errors on the single-mode stability of LDs is negligible. Precise wavelength controlled λ/4 phase shifted DFB-LDs were successfully demonstrated as a result of both the WAVE method and the highly uniform MOVPE crystal growth.