Optical Link Budget

1
Optical Link Budget

• ECE 4006
• Gb Ethernet Optoelectronic Links

2
Logical Flow for Optical Link Power Budget
Output current from laser driver (mA)
Vertical Cavity Surface Emitting Laser Slope
Efficiency (mW/mA)
Optical signal on optical fiber
Input optical signal to photodetector Responsivity
(A/W)
Input current to receiver (mA)
3
Sample VCSEL Specs


Honeywell HFE4380-521 VCSEL specifications.
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Honeywell HFE4384-522 VCSEL specifications.
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4
Sample Photodetector Specs

Lasermate RSC M85A306 photodetector
specifications.
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Hammamatsu S5973 photodetector specifications.



5
Sample Optical Power Link Budget

6
Optical Link Budget Issues

• Choose only components that will achieve Gb
  Ethernet compliant data rates.
• What is the range of output currents from the
  laser driver?
• What is the slope efficiency and min/max output
  from the VCSEL?
• What are the fiber and connector losses?
• What is the total input power to the PD?
• What is the responsivity of the PD?
• What is the dark current of the PD?
• What is the max PD input power?
• What is the min/max PD output current such that
  the receiver operates?
• What is the capacitance of the PD? (this needs to
  be matched to the receiver to achieve Gb Ethernet
  compliant data rates).

7
Optical Packaging Issues

• Alignment tolerance and power dissipation
  directly affect the cost of the module. Consider
  these issues when specifying the VCSEL, PD, and
  fiber.
• Alignment tolerance maximize the alignment
  tolerance (how much the fiber/VCSEL and fiber/PD
  can be misaligned, yet still get a Gb Ethernet
  compliant eye)
• Evaluate the effect of the VCSEL aperture and
  divergence angle
• Evaluate the effect of the MMF (multimode fiber)
  core diameter and numerical aperture (NA)
• Evaluate the effect of the PD responsivity and
  aperture
• Power dissipation minimize the power dissipation
  (run the optical link at the lowest power
  possible, and with the least power dissipation,
  yet still get a Gb Ethernet compliant eye)
• Evaluate the effect of the VCSEL slope efficiency
• Evaluate the effect of the PD responsivity
• Note the bandwidth/distance product for 50 mm
  diameter core MMF is larger than for 62.5 mm
  diameter core fiber. We will use 62.5 mm MMF in
  this class

8
Alignment Tolerant Fiber/OE Component Connection
Fiber core
Fiber core
VCSEL/PD
VCSEL/PD
Longitudinal Misalignment
Transverse Misalignment