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Operating the ultrafast intersubband transition- based In0.8Ga0.2As/AlAs0.56Sb0.44 coupled double-quantum-well all-optical cross-phase modulators at 40 Gb/s or higher is making it essential to know the pump-power ratings of these devices. Here, the pump-power rating of 10 samples taken from the same wafer was determined experimentally and used as the input to a detailed three dimensional thermal analysis to study the temperature distribution in these devices. Analysis shows the importance of phonon scatterings at the well-barrier interfaces and internal optical back-reflections from the tapers in order to produce meaningful results. As a result of the thorough thermal modeling approach, simulated results for all the samples are consistent and reveal that the highest temperature in these devices reaches the melting point of the In0.8Ga0.2As quantum-wells when operated at the pump-power rating. Simulated temperature distributions indicate that the hottest spot in the device occurs at a very close proximity to the input facet, hence explaining the destruction of the antireflection coatings and melted substance on the input facet seen on the photographs taken from scanning electron microscopy and clearly demonstrates the predictive power of the comprehensive thermal simulation tool developed herein.