Distributed Feedback Lasers Overview

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Distributed Feedback Lasers Overview

• Mike Huang
• EE 290F
• February 17, 2004 Tuesday

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Semiconductor Lasers

• Add mirrors to provide optical feedback
• Add optical guiding to improve efficiency

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Optical Cavity (plane waves) 1
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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

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Threshold Condition

• Solve for gain ?

(a) gain profile 3.
(b) intensity spectrum 3.
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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

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Laser Spectra
3?
gt100?
Gain
?
?
Free Spectral Range
?
?
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DFB and DBR lasers 3
AR coating
HR coating
DFB DBR
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Cross section of DFB Lasers
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Laser output direction
Vertical Cavity Surface Emitting Lasers (VCSEL)

• Edge-Emitting Lasers
• Fabry-Perot (FP) Lasers
• DFB (distributed feedback) Lasers

Typical dimesion 2 um x 500 um 5 um x 5 um

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Periodic Structure with Gain
Incident and reflected intensities inside the
corrugated section with gain 2
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Solving for DFB Lasers
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Oscillation Condition
Reflection gain contour in the ??L - ?L plane 2
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Regular DFB Laser 2
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?/4-Shifted DFB Laser 2
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Gain-Coupled DFB Laser

• Complex ?

• plug-in coupled mode equations with

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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

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Fabrication (grating structure in DFB)

• Grating dimension l/4n 100nm (for l1.55mm)
• Electron-beam lithography (EUV, X-ray, ion-beam,
  )
• Interference of two UV lights.

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Dicing (edge-emitting-lasers)
To create reflection mirrors on two sides of the
cavity.
Substrate is thinned down (100mm) before
cleaving.

After cleaving, protective coating is deposited
on both facets to improve lifetime (mainly
degraded by COD).
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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).

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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.

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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.

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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.

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Commercial DFB

• Components
• DFB diode
• Thermoelectric cooler
• Thermistor
• Photodiode
• Optical isolator
• Fiber-coupled lens

  Parameters Symbol Min Typ Max Unit
  CW Output power(25C) Pf 10 --- 30 mW
  Threshold current It h -- 25 60 mA
  Operating current If -- 300 -- mA
  Forward voltage Vf -- 2.0 3.0 V
  Center Wavelength ?c 1540 1550 1570 nm
  Linewidth ? ? -- 2 -- MHz
  Monitor Current Im -- 200 -- µA
  Monitor dark current(Vr-5V) Id -- -- 100 nA
  Isolation(Optional) Iso -30 -- -- dB
  TEC current ITEC -- 1.2 -- A
  TEC voltage VTEC -- 3.2 -- V
  Thermistor resistance(at 25?) Rt h 9.5 10 10.5 kO
  Operating Temperature Range To -20 -- 65 C
  Storage temperature Tst g -40 -- 85 C
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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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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.

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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.