Determining Maximum Power Ratings for Isolators

We describe the use of thermal models to determine maximum safe power ratings for Faraday rotation isolators. Models are constructed for two distinct isolator topologies. The first model is based on the topology used for legacy isolators. These are the isolators that have been in common usage for more than 40 years and are marketed by other vendors. The second model is based on the topology of the isolators produced by Micro Harmonics Corporation (MHC). The primary difference is that the MHC isolators employ a CVD diamond disc for increased power handling while the legacy isolators do not.

AutoDesk NASTRAN is used for the thermal simulations. NASTRAN uses a finite element analysis (FEA) to determine the temperature distribution in arbitrary 3-D objects. The simulator yields a detailed 3-D map of the temperature inside the isolator. Legacy isolators have well-established maximum power ratings. In the NASTRAN model for the legacy isolator, the thermal source is set to a value equivalent to the established maximum power rating. The simulation yields the maximum temperature in the legacy isolator when driven at the maximum rated power level. This temperature provides a baseline to determine the maximum power for the MHC isolator.