Hi Wajahat,
Is there a particular platform you are targeting like Si or Si3N4 etc ? Are you planning on doing PIC fabrication ?
For circuit level modeling you can use OptiSPICE with splitter, combiner, phase shifter and waveguides to build MZM’s and MZI’s. If you would like to do physical design then you can use OptiMode (for the waveguide) and OptiFDTD for the splitter/combiner.
Best Regards,
Cem