Optical Link Budget ECE 4006 Gb Ethernet Optoelectronic Links
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)
Sample VCSEL Specs Honeywell HFE4380-521 VCSEL specifications. - Honeywell HFE4384-522 VCSEL specifications. -
Sample Photodetector Specs Lasermate RSC M85A306 photodetector specifications. - Hammamatsu S5973 photodetector specifications.
HFE4380-521 HFE4384-522 Ith (mA) 6 6 DC bias of laser (mA) 7.2 7.2 Slope Efficiency (mW/mA) 0.04 0.15 Modulation Current of TX (mA) 30 30 Range of Power Output (mW) 0.288 - 1.2 1.08 - 4.5 Range of Power Output (w/ Loss) (mW) 0.144 - 0.6 0.54 - 2.25 Responsivity of Lasermate PD (A/W) 0.4 0.4 Responsivity of Hamamatsu PD (A/W) 0.47 0.47 Range of Current from Lasermate PD (uA) 57.6 - 240 216 - 900 Range of Current from Hamamatsu PD (uA) 67.7 - 282 253.8 - 1057.5 Sample Optical Power Link Budget
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).
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
Alignment Tolerant Fiber/OE Component Connection Fiber core Fiber core VCSEL/PD VCSEL/PD Longitudinal Misalignment Transverse Misalignment