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Cooling improvement for stacked SFP+ optical transceivers
Keywords: Optical Transceivers, Cooling Improvements, Test and Analysis
There is an ever increasing push in data transfer rate requirements in the communications industry. These higher data rates can be realized by either increasing the number of current generation optical transceivers with low data rate (and usually smaller form factor) or using higher data rate optical transceivers, which are usually more bulky. There is a tendency to maximize the number of current generation optics with a low data rate, before considering next generation optics which offer increased data rate. Besides physical limitations, the cooling of the optical modules limits the possibility to grow the number of ports. As an example, optical modules can be stacked in order to increase the port density. This is a common practice for SFP as well as SFP+. For SFP's, the cooling is usually less of an issue as the power dissipation per module is < 1 Watt and extended temperature range (up to 85C case temperature) optics are available. SFP+ optics SR (Short range) and LR (Long Rate) are, like SFP, dissipating less than 1 Watt and are available in extended temperature range. However the higher power SFP+ versions ER, ZR, DWDM are at the moment only available in commercial grade, supporting up to 70C case temperature. Furthermore the power dissipations of SFP+ ER, ZR and DWDM with fixed frequencies range from 1.25-1.5 Watt each. The latest power numbers for tunable DWDM (Dense Wavelength Division Multiplexing) SFP+ indicate 2 Watt each. Cooling these high power stacked SFP+s appears to be a challenge, especially for the row of SFP+'s that is closer to the PCB and is not directly exposed to the airflow. Analysis of temperature measurement results for stacked SFP+s in these types of configurations provided more insight in the cooling of the SFP+s. From the test results, it was concluded that leakage of airflow through the cage, from within the chassis to the ambient air or vice versa provides the dominant cooling mechanism. The thermal performance is further improved with the addition of vent holes to the cage and more importantly, the face plate. Prototypes have been developed in cooperation with cage vendors. These prototypes were evaluated both on thermal as well as on EMC performance. Significant temperature reduction (up to 30 percent) was achieved for the worst case SFP+ transceiver.
Arjan Kole, Staff Engineer
Electronic Cooling Solutions
Santa Clara, California

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