Hi Aruna,
Based on your description here, you need to model the effect of altered temperature on the optical properties and consequent phase shift. For the optical analysis you need MODE or FDTD. If you want to simulate the temperature as well (e.g. as a function of input power and vs different cooling conditions) you need a thermal analysis as well; for this HEAT can be used.
The closest application example I can suggest is the thermally tuned waveguide. Please study this article and let us know if you have any follow up questions.
Best regards
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