1. Distributed Feedback Lasers Overview Mike Huang EE 290F February 17, 2004 [Tuesday]

2. Semiconductor Lasers I (pump current) Add mirrors to provide optical feedback Add optical guiding to improve efficiency

3. Optical Cavity (plane waves) [1]

4. Transmission of the optical cavity Transmission as function of the electrical length for different reflectivities (R) [1]. Maximum transmission for  = q  Cavity with gain: R  G( ).R

5. Threshold Condition Solve for gain  (a) gain profile [3]. (b) intensity spectrum [3].

6. Single Longitudinal Mode Oscillation Shorter cavity [VCSEL] increase mode spacing wider spectral width Injection of external light careful tuning External coupled cavity mechanical vibration, temperature and pressure changes Diffraction grating inside the laser structure [DFB]

7. Laser Spectra >100 ~3 Gain Free Spectral Range

8. DFB and DBR lasers [3] DFB DBR HR coating AR coating

9. Cross section of DFB Lasers

10. Laser output direction Edge-Emitting Lasers: Fabry-Perot (FP) Lasers DFB (distributed feedback) Lasers Vertical Cavity Surface Emitting Lasers (VCSEL) Typical dimesion: 2 um x 500 um5 um x 5 um

11. Periodic Structure with Gain Incident and reflected intensities inside the corrugated section with gain [2]

12. Solving for DFB Lasers

15. Oscillation Condition Reflection gain contour in the  L -  L plane [2]

16. Regular DFB Laser [2]

17. /4-Shifted DFB Laser [2]

18. Gain-Coupled DFB Laser plug-in coupled mode equations with: Complex  :

19. Index- x Gain(Loss)-Coupled DFBs [2] 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

20. 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

21. 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).

22. 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).

23. Grating Alignment [8] For growing into direction, grating must be aligned along the direction. Generally, the dominant growth inside a v-groove is along the [111] plane.

24. Surface Mass Transport (SMT) [8] Generation of [100] facets at the bottom of the grooves due to diffusion of surface atoms. This process may eliminate the [111] facet.

25. Wet-etched grating [8] 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.

26. Commercial DFB Parameters Symbo l MinTypMaxUnit CW Output power(25C)Pf10---30mW Threshold currentIt h--2560mA Operating currentIf--300--mA Forward voltageVf--2.03.0V Center Wavelengthλc154015501570nm LinewidthΔ λ--2 MHz Monitor CurrentIm--200--μA Monitor dark current(Vr=- 5V) Id-- 100nA Isolation(Optional)Iso-30-- dB TEC currentITEC--1.2--A TEC voltageVTEC--3.2--V Thermistor resistance(at 25 ℃ ) Rt h9.51010.5kΩ Operating Temperature Range To-20--65C Storage temperatureTst g-40--85C Components DFB diode Thermoelectric cooler Thermistor Photodiode Optical isolator Fiber-coupled lens

27. 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

28. References [1] Verdeyen, J.T. - Laser Electronics, 3rd Ed., Prentice Hall, USA, 1995. [2] Yariv, A. - Optical Electronics in Modern Communications, 5th Ed., Oxford Un. Press, New York, 1997. [3] Ghafouri-Shiraz, H. and Lo, B.S.K. - Distributed Feedback Lasers- Principles and Physical Modeling, John Wiley & Sons, England, 1996. [4] Carrol, J., et. al. - Distributed Feedback Semiconductor Lasers, IEE, London, 1998. [5] 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.2160-69, November 1988. [6] Coldren. L.A. and Corzine, - Diode Lasers and Photonics Integrated Circuits, John Wiley & Sons, New York, 1995. [7] Kamioka, H., et. al. - “Reliability of an electro-absorption modulator integrated with a distributed feedback laser”, CLEO Pacific Rim 99: Procceedings, pp.1202-3. [8] 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.862-873, June 1997.

29. References [9] 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.2138-40, November 1991. [10] 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 1998. [11] 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. 1460-62, November 1997. [12] 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.1724-31, June 1991. [13] Koontz, E.M., et al. - “Overgrowth of submicron-patterned surfaces for buried index contrast devices”, J. of Semicond. Sci. Tech., 15, R1-12, 2000. [14] Iga, K. and Kinoshita, S. - Process technology for semiconductor lasers, Springer Series in Materials Science, New York, 1996.