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.
Commercial DFB Parameters Symbo l MinTypMaxUnit CW Output power(25C)Pf mW Threshold currentIt h--2560mA Operating currentIf mA Forward voltageVf V Center Wavelengthλc nm LinewidthΔ λ--2 MHz Monitor CurrentIm μA Monitor dark current(Vr=- 5V) Id-- 100nA Isolation(Optional)Iso-30-- dB TEC currentITEC A TEC voltageVTEC V Thermistor resistance(at 25 ℃ ) Rt h kΩ Operating Temperature Range To C Storage temperatureTst g C Components DFB diode Thermoelectric cooler Thermistor Photodiode Optical isolator Fiber-coupled lens
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
References  Verdeyen, J.T. - Laser Electronics, 3rd Ed., Prentice Hall, USA,  Yariv, A. - Optical Electronics in Modern Communications, 5th Ed., Oxford Un. Press, New York,  Ghafouri-Shiraz, H. and Lo, B.S.K. - Distributed Feedback Lasers- Principles and Physical Modeling, John Wiley & Sons, England,  Carrol, J., et. al. - Distributed Feedback Semiconductor Lasers, IEE, London,  Kinoshita, J.I. and Matsumoto, K. - “Transient chirping in distributed-feedback (DFB) lasers effect of spatial hole-burning along the laser axis”, IEEE J. Quantum Elec., Vol. 24, n.11, pp , November  Coldren. L.A. and Corzine, - Diode Lasers and Photonics Integrated Circuits, John Wiley & Sons, New York,  Kamioka, H., et. al. - “Reliability of an electro-absorption modulator integrated with a distributed feedback laser”, CLEO Pacific Rim 99: Procceedings, pp  Chu, S.N.G., et. al. - “Grating overgrowth and defect structures in distributed- feedback buried heterostructure laser diodes”, IEEE J. Sel. Top. in Quantum Elec., Vol. 3, n.3, pp , June 1997.
References  Aoki, M., et al. - “Novel structure MQW electroabsorption modulator/dfb-laser integrated device fabricated by selective area MOCVD growth”, Elec. Lett., Vol. 27, n.23, pp , November  Takigushi, T., et al. - “Selective area MOCVD growth for novel 1.3m DFB laser diodes with graded grating”, 10th Int. Conf. On InP and Related Materials: Proceedings, Tsukuba, Japan, May  Osowski, M.L., et al. - “An assymetric cladding gain-coupled DFB laser with oxide defined metal surface grating by MOCVD”, IEEE Phot. Tech. Lett., Vol. 9, n.11, pp , November  Luo, Y. et al. - “Fabrication and characteristics of gain-coupled DFB lasers with a corrugated active layer”, IEEE J. Quantum Elec., Vol. 27, n.6, pp , June  Koontz, E.M., et al. - “Overgrowth of submicron-patterned surfaces for buried index contrast devices”, J. of Semicond. Sci. Tech., 15, R1-12,  Iga, K. and Kinoshita, S. - Process technology for semiconductor lasers, Springer Series in Materials Science, New York, 1996.