Hello Manoranjan,
If I remember correctly you are using OptiFDTD 14.0 which means you have access to the S-Parameters feature within the product. What you would do in this case is set one S-Param port across the single mode waveguide and set it as the input with that fundamental mode. Next you would set a second S-Param port across the multimode waveguide and configure it to the TE0 mode and run the S-Parameter script. This would give you S12 which would be the coupling coefficient. You would then repeat this process changing the mode of the second port to the TE1 and then the TE2 modes. Further documentation can be found in the technical background document that is included in the help menu in your product.
Scott