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Distributed Feedback Lasers Overview

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Title: Distributed Feedback Lasers Overview


1
Distributed Feedback Lasers Overview
  • Mike Huang
  • EE 290F
  • February 17, 2004 Tuesday

2
Semiconductor Lasers
  • 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
3?
gt100?
Gain
?
?
Free Spectral Range
?
?
8
DFB and DBR lasers 3
AR coating
HR coating
DFB DBR
9
Cross section of DFB Lasers
10
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

11
Periodic Structure with Gain
Incident and reflected intensities inside the
corrugated section with gain 2
12
Solving for DFB Lasers
13
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14
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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
  • Complex ?
  • plug-in coupled mode equations with

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

21
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).
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
  • 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
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.
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