Semiconductor Diode Lasers Overview

1
Semiconductor Diode Lasers Overview

• EECS 233
• Fall, 2002

2
Module 1.1 Lasers

• What is a laser? What makes a laser?
• What is a semiconductor diode laser? What does
  it look like?
• Topics of concern for an end user.
• Single-wavelength laser
• Wavelength selection in diode laser
• DFB, VCSEL

3
What is a LASER

• Laser has
• Spectral coherence
• Spatial coherence
• Laser is made with
• Optical gain medium
• Optical resonator to provide feedback
• Pump source

4
Semiconductor Diode Lasers
Vertical Cavity Surface Emitting Laser
Edge Emitting Laser

• Two major types of diode lasers
• Diode laser
• Emission from p-I-n junction
• Current modulation
• EEL Chip size 500 x 500 x 100 um3

5
Equivalent Model of VCSEL
VCSEL
Equivalent Model
p-DBR
composite mirror 1
gain medium
n-DBR
composite mirror 2

• (DBR distributed Bragg reflector)
• VCSEL gain mirrors for feedback

6
Optical Cavity (plane waves) 1
7
Topics of Concern

• Modulation response
• Power
• Beam profile light coupling into an optical
  fiber
• Linewidth
• Wavelength
• Current
• Voltage
• DFB (distributed feedback) laser and VCSEL

8
Example of Typical Link Experiment
Eye diagram
9
Transmission Performance

• Bit Error Rate (BER) vs. Min. Ave. Received Power

Optical SNR
Optimal decision level
Q6 ? BER1e-9 Q7 ? BER1e-12
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Performance Impairment

• Power penalty is typically due to
• Mode partition noise
• Dispersion
• Chirp
• Noise
• Jitter
• Extinction ratio
• Etc.

Power Penalty
Example of a closed eye
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Topics of Concern

• Mode partition noise
• Dispersion
• Chirp
• Noise
• Jitter
• Extinction ratio

• Modulation response
• Power (photon density)
• Beam profile light coupling into an optical
  fiber
• Linewidth
• Wavelength
• Current
• Voltage

12
Topics of Concern

• Mode partition noise
• Dispersion
• Chirp
• Noise
• Jitter
• Extinction ratio

• Modulation response
• Power (photon density)
• Beam profile light coupling into an optical
  fiber
• Linewidth
• Wavelength
• Current
• Voltage

13
Optical Emission
I (pump current)
E
SCH
holes
QW
p-doped
n-doped

electrons
x
14
Wavelength Selection

• Semiconductor gain spectrum is very broad,
  typically 20-40 nm, depending on pumping
• Longitudinal mode (standing wave mode) spacing
  1/L
• Typically gain is not clamped at threshold

15
Threshold Condition

• Solve for gain ?

(a) gain profile 3.
(b) intensity spectrum 3.
16
Fabry-Perot EEL vs. VCSEL
Edge Emitting Lasers
VCSELs
Free Spectral Range
Gain
?
?
?
?
17
DFB and DBR lasers 3
Distributed Feedback Distributed Bragg
Reflector DFB DBR
Grating order multiples of ?o/2nr of the
grating 1st order InP grating at 1.55mm
wavelength 0.25mm period
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Coupled Mode Theory
Incident and reflected intensities inside the
corrugated section 2
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Periodic Structure with Gain
Incident and reflected intensities inside the
corrugated section with gain 2
20
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

21
?/4-Shifted DFB Laser 2
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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.

24
Strain free growth 8
Strain free grating overgrowth showing horizontal
composition fluctuation.
Strain free DFB laser.
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Topics of Concern

• Mode partition noise
• Dispersion
• Chirp
• Noise
• Jitter
• Extinction ratio

• Modulation response
• Power (photon density)
• Beam profile light coupling into an optical
  fiber
• Linewidth
• Wavelength
• Current
• Voltage

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Linewidth
Schawlow-Townes Limit
27
Topics of Concern

• Mode partition noise
• Dispersion
• Chirp
• Noise
• Jitter
• Extinction ratio

• Modulation response
• Power (photon density)
• Beam profile light coupling into an optical
  fiber
• Linewidth
• Wavelength
• Current
• Voltage

28
Modulation Response

• Rate equations

• Resonance frequency depends on gain, differential
  gain, photon density and photon lifetime.

29
Frequency Chirp

• Transient variation of the lasing wavelength
  during direct modulation of the injection
  current.

Optical Power
Optical power vs. time
Wavelength
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Chirp
Linewidth enhancement factor

• Nonlinear gain ?
• Small ? reduces adiabatic chirp but enhances the
  transient chirp
• Large ? reduces oscillation but degrades laser
  bandwidth

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Chirp in DFB Lasers 5
Current pulse waveform.
Temporal variation of wavelength shift (top) for
shifted DFB lasers ((a)?/8, (b)?/4), just
detuning effect of the Bragg wavelength (c) and
just spectral hole-burning (d).
32
Integrated Electro-Absorption Modulator

• Typical data length of 300?m, voltage of 10V and
  3 to 4 growth steps.
• Reliability limited by the laser lifetime 7.

?1.58?m
33
Topics of Concern

• Mode partition noise
• Dispersion
• Chirp
• Noise
• Jitter
• Extinction ratio

• Modulation response
• Power (photon density)
• Beam profile light coupling into an optical
  fiber
• Linewidth
• Wavelength
• Current
• Voltage

34
Extinction Ratio

• Extinction Ratio

35
Trade-off Extinction Ratio and Chirp
36
Topics of Concern

• Mode partition noise
• Dispersion
• Chirp
• Noise
• Jitter
• Extinction ratio

• Modulation response
• Power (photon density)
• Beam profile light coupling into an optical
  fiber
• Linewidth
• Wavelength
• Current
• Voltage

37
Relative Intensity Noise
38
Power Penalty

• Ex 10 Gbps system Q7 and BER1e-12
• RIN -117 dB/Hz corresponds to 1 dB power penalty
• RIN -127 dB/Hz ? 0.1 dB power penalty
• Ex 8-bit analog ? SNR48 dB
• rRIN-51dB
• 1GHz ? -141dB/Hz

39
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.

40
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.