Index- x Gain(Loss)-Coupled DFBs  Index-coupled DFB lasers have two degenerate (longitudinal) modes Mode selection is based on facet phase very tricky and unreproducible Gain- or loss- coupled DFB Single wavelength More difficult to fabricate
Fabrication (grating structure in DFB) Grating dimension ~ /4n ~ 100nm (for ~1.55 m) Electron-beam lithography (EUV, X-ray, ion-beam, …) Interference of two UV lights. UV sensitive PR
Dicing (edge-emitting-lasers) Substrate is thinned down (~100 m) before cleaving. To create reflection mirrors on two sides of the cavity. After cleaving, protective coating is deposited on both facets to improve lifetime (mainly degraded by COD).
Notes on Fabrication Smoothness of the gratings depends strongly on crystal orientation. Holographic photolithography or e-beam lithography are used to define the grating mask. Wet etch is used to etch the gratings. Dry etch may cause defects on the structure that propagate during the overgrowth. V-groove preferable to rectangular (grating quality). Growth rate depends strongly on the crystallographic orientation. Orientation of the growth depends on temperature. Epitaxial overgrowth is more complicated on the GaAs material system than in InP (oxidation).
Grating Alignment  For growing into direction, grating must be aligned along the direction. Generally, the dominant growth inside a v-groove is along the  plane.
Surface Mass Transport (SMT)  Generation of  facets at the bottom of the grooves due to diffusion of surface atoms. This process may eliminate the  facet.
Wet-etched grating  Wavy grating lines, nonflat side-walls and linkages between grooves can be caused by undefined mask boarder or misalignment with respect to the crystal orientation.
Conclusion Overview of basic laser and DFB principles. Fabrication process depends on the growing method. Most critical step: grating. Transmitter used in most (all) long-haul WDM/DWDM systems. Tunable DFBs Forrest
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