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Lucent optimizes thermal design for optical networks.

Courtesy of CAS Ltd.

CAS Ltd, Haifa, Israel successfully developed a thermal management design for a Lucent Technologies communication system, replacing a failed design provided by a Massachusetts-based consulting company. The consultant, who had used a competitor's thermal analysis software, had predicted that no heatsinks were required on the system's optical modules. CAS, using Coolit, determined that just the opposite was true, and prototype testing proved that the Coolit analysis was correct.

The original communications system shown in Figure 1 did not specify heat sinks for the optical modules. The single card/slot held two, 3 x 4 inch optical modules, three power supply modules, plus some additional electronic components. Total card power consumption was approximately 42 W, with the two optical modules drawing 11 W and 14 W, respectively.

Since optical modules are the most temperature-sensitive units in a telecom system, engineers must take particular care to ensure they have a safe thermal environment. With a maximum allowable case temperature of about 70oC, and inlet temperatures of 55oC there is little margin for thermal designers. Indeed, for the original design without a heat sink, Coolit predicted, and an experiment confirmed that the 14 W optical module would reach an average case temperature of over 79oC.

Unfortunately, the modules occupied all but 6 mm of space within the slot, so there was not enough room to mount a standard heat sink above the module. So, CAS proposed two cooling scenarios, one using a novel, low profile "winged" heat sink and a second that combined the "winged" heat sink with a base heat sink on the back-side of the modules.

Coolit analysis predicted that, in the first scenario, the average module case temperatures would be 68.0oC and 70.5oC for the 11 W and 14 W modules, respectively, Figure 2. Experiments verified this prediction by giving average case temperatures of 68.0oC and 70.7oC (see Table below). With the base heat sink added, Coolit predicted average case temperatures would decrease to 67oC and 69oC. Flow ribbons and temperatures are shown in Figure 3.


Coolit Analysis (oC)

Experiment (oC)

% error

11 W module




14 W module




After completing its final analyses, CAS successfully delivered the thermal design to its customer. The customer, impressed by how well the Coolit predictions matched test results, felt that, at last, it had a thermal design on which it could depend.

Prototype of the Lucent communication system by CAS Ltd.
2D section of Coolit model with temperatures for 11 W and 14 W modules.
Streamribbon visualization of air flow in Coolit model of the optical system.

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