EE 230: Optical Fiber Communication Lecture 9

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EE 230 Optical Fiber Communication Lecture 9
Light Sources
From the movie Warriors of the Net
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Conditions for gain (lasing)

• E2-E1ltFc-Fv (population inversion)
• g?(1/L)ln(1/R)? (net gain)
• ?2nL/p, p an integer (phase coherence)

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Reflectivity
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Longitudinal mode spacing
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Laser Diode Structure and Optical modes
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Conditions for continuous lasing (steady state)

• Net rate of change of density of conduction band
  electrons is zero (injection minus recombination
  and depletion)
• Net rate of change of density of photons created
  is zero (stimulated emission minus leakage and
  spontaneous emission)

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Laser Electrical Models
Simple large signal model
Package Lead Inductance
Bond wire Inductance
Laser contact resistance
Use a large signal diode model for the
laser junction, this neglects the optical
resonance
Package Lead Capacitance
Laser Junction
Laser Pad Capacitance
More exactly the laser rate equations can be
implemented in SPICE to give the correct
transient behavior under large signal modulation
Assume that the light output is proportional to
the current through the laser junction
Small signal model
(Hitachi)
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Steady-state lasing conditions
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Turn-on delay
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Turn-on Delay
To reduce the turn on delay Use a low
threshold laser and make Ip large Bias the
laser at or above threshold
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Relaxation oscillation

• Decays as e-?t/2, where
• and with a freqency ?, where

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Modulation frequency

• Difference between optical output at
  modulation frequency ?m and steady-state output
  is proportional to

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Resonance Frequency
Semiconductor lasers exhibit an inherent second
order response due to energy sloshing
back-and-forth between excited electrons and
photons
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Large Signal Transient Response
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Effects of current and temperature

• Applying a bias current has the same effect as
  applying a pump laser electrons are promoted to
  conduction band. Fc and Fv get farther apart as
  well
• Increasing the temperature creates a population
  distribution rather than a sharp cutoff near the
  Fermi levels

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Fabry Perot Laser Characteristics
(Hitachi Opto Data Book)
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Quantum efficiency

• Internal quantum efficiency ?i, photons emitted
  per recombination event, determined empirically
  to be 0.65?0.05 for diode lasers
• External quantum efficiency ?e given by

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Total quantum efficiency

• Equal to emitted optical power divided by applied
  electrical power, or h??e/qV
• For GaAs lasers, TQE ? 50
• For InGaAsP lasers, TQE ? 20

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Chirping
Current modulation causes both intensity and
frequency modulation(chirp) As the electron
density changes the gain (imaginary part of
refractive index ni) and the real part of the
refractive index (nr) both change. The
susceptability of a laser to chirping is
characterized by the alpha parameter.
??1-3 is expected for only the very best
lasers Chirping gets worse at high
frequencies Relaxation oscillations will produce
large dp/dt which leads to large chirping Damping
of relaxation oscillations will reduce
chirp Correctly adjusting the material
composition and laser mode volume can reduce ?
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Reflection Sensitivity
Problem
Solution
R. G. F. Baets, University of Ghent, Belgium
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Example

• A GaInAs diode laser has the following
  properties
• Peak wavelength 1.5337 ?m
• Spacing between peaks 1.787x10-3 ?m
• J/Jth1.2
• What are the turn-on delay time, the cavity
  length, the threshold electron density, and the
  threshold current?

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Turn-on delay time

• 3.7 ln(1.2/1.2-1) 6.63 ns

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Cavity length

• L (1.5337)2/(2)(3.56)(1.787x10-3)
• 184.9 ?m

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Threshold electron density

• R 0.3152
• g?(1/L)ln(1/R)?
• gth1/.01849 ln(1/.3152)100162.4 cm-1
• From figure, N1.8x1018 cm-3

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Threshold current

• J/2de I/2deLW
• Ith(0.5x10-4)(1.6x10-19)(1.8x1018)(.01849)(4x10-4
  )/(3.7x10-9)
• Ith29 mA

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Laser Diode Structures
Most require multiple growth steps Thermal
cycling is problematic for electronic devices
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Laser Reliability and Aging
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Power degradation over time

• Lifetime decreases with current density and
  junction temperature

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Problems with Average Power Feedback control of
Bias
Problem L-I curves shift with Temperature and
aging
Turn on delay increased Frequency response
decreased
Average Power
Average Power
Light
Light
Current
Current
L-I Characteristic with temperature dependent
threshold
Ideal L-I Characteristic
Output power, frequency response decreased
Light
Average Power
Current
Average number of 1s and Os (the Mark Density)
is linearly related to the average power. If
this duty cycle changes then the bias point
will shift
L-I Characteristic with temperature dependent
threshold and decreased quantum efficiency
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Light Emitting Diodes
An Introduction to Fiber Optic Systems-John Powers
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LED Output Characteristics

• Typical Powers
• 1-10 mW
• Typical beam divergence
• 120 degrees FWHM Surface emitting LEDs
• 30 degrees FWHM Edge emitting LEDs
• Typical wavelength spread
• 50-60 nm

An Introduction to Fiber Optic Systems-John Powers
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Distributed Feedback (DFB) Laser Structure

• Laser of choice for optical
• fiber communication
• Narrow linewidth, low chirp for direct modulation
• Narrow linewidth good stability for external
  modulation
• Integrated with Electro-absorption modulators

Distributed FeedBack (DFB) Laser
Distributed Bragg Reflector(DBR) Laser
As with Avalanche photo-diodes these structures
are challenging enough to fabricate by themselves
without requiring yield on an electronic
technology as well Hidden advantage the facet
is not as critical as the reflection is due to
the integrated grating structure
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Bragg wavelength for DFB lasers
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Thermal Properties of DFB Lasers
Light output and slope efficiency decrease at
high temperature

• Wavelength shifts with temperature
• The good Lasers can be temperature tuned for WDM
  systems
• The bad lasers must be temperature controlled,
• a problem for integration

Agrawal Dutta 1986
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VCSELs

• Much shorter cavity length (20x)
• Spacing between longitudinal modes therefore
  larger by that factor, only one is active over
  gain bandwidth of medium
• Mirror reflectivity must be higher
• Much easier to fabricate
• Drive current is higher
• Ideal for laser arrays

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Choosing between light sources

• Diode laser high optical output, sharp
  spectrum, can be modulated up to tens of GHz, but
  turn-on delay, T instability, and sensitivity to
  back-reflection
• LED longer lifetime and less T sensitive, but
  broad spectrum and lower modulation limit
• DFB laser even sharper spectrum but more
  complicated to make
• MQW laser less T dependence, low current, low
  required bias, even more complicated
• VCSEL single mode and easy fabrication, best
  for arrays, but higher current required