EE 230: Optical Fiber Communication Lecture 10

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EE 230 Optical Fiber Communication Lecture 10
Light Sources and Transmitters
From the movie Warriors of the Net
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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

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Laser Diode Transmitter Block Diagram
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Source-Fiber Coupling Lambertian Sources
Lambertian Source radiance distribution
Generalized Coupled Power
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Step and Graded Index Fiber Coupling
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Graded Index Fiber Coupling Continued
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Source Fiber Coupling - II
Schematic of a typical assembly of coupling optics
Transmitters employing a) butt-coupling and b)
lens-coupling designs
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Turn-on delay
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Extinction Ratio Penalty
If the transmitter does not turn all the way
off during the transmission of a zero then
the extinction ratio r ( the ratio to a power
transmitted during a 0 to that during a
1) will cause a bit error rate penalty and a
reduction in sensitivity. For a PIN receiver the
peak power required for a given signal to noise
ratio will become
r0 if the optical signal is completely
extinguished during a logical 0 r1 if the
optical power during a 0 equals that during a
1 in this case the power required approaches
For APD detectors with gain the effect of the
multiplied noise during the 0 is more severe,
this case is shown in the graph to the left. k
is the ratio of the hole and electron ionization
coefficients and is a property of the material in
the avalanche multiplication region
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Traditional Laser Transmitter Approaches
Use a transmission line and impedance match
Keep it close and dont worry about the match
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Laser Driver Stabilization
Average Power and Mark Density Compensation
Average Power, Mark Density and Modulation
A variety of feedback approaches are available to
compensate for laser imperfections and the
consequences of temperature variation and aging
Average and Peak Power Stabilization
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Packaging
Drawing of Packaging Approach
Optical Module (a), Electrical module (b)

• 10 Channels
• 12.5 Gb/s aggregate bandwidth
• 1300 nm commercial laser array
• 50/125 Multimode fiber ribbon
• 130 mW/channel
• CMOS Driver Array
• BERlt10-14
• 1.2 km transmission with no
• BER degradation

Close-up of assembled module
Completed module integrated on test board
Bostica et. al., IEEE Transactions on Advanced
Packaging, Vol. 22, No 3, August 1999
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Example Commercial Transmitter Module
Palomar Technologies
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DFB-HEMT OEIC Laser Transmitter

• Transistor Technology
• InGaAs-InAlAs HEMT
• 1.5 mm gate length
• Laser
• Distributed Feedback Laser
• Self-Aligned Constricted Mesa (SACM)
• 7 MHz linewidth at 3 mW output power
• 19 GHz 3db frequency
• 8 mA average threshold
• Fabrication
• l/4 shifted cavity fabricated by e-beam
• 2-step MOCVD
• OEIC Performance
• Clean output eyes for all pattern lengths
• up to 5 Gb/s
• Operation at shorter patterns up to 10 Gb/s

Lo et. al. IEEE Photonics Technology Letters,
Vol. 2, No. 9, September 1990
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Polarization

• In molecules, PµaEßE2?E3
• In materials, PX(o)X(1)EX(2)E2X(3)E3
• If multiple electric fields are applied, every
  possible cross term is generated.
• At sufficiently high values of E, quadratic or
  higher terms become important and nonlinear
  effects are induced in the fiber.

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Electro-Optic Coefficient r (Pockels Effect)
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Electro-Optic Material Figures of Merit

• Phase shift efficiency n3r favors lithium
  niobate in most cases
• Bandwidth per unit power n7r2/e favors organic
  materials