May I ask what is the reference of the first equation showing up on the following page:



The equation essentially is the same same as the product of overlap integral for the calculation of mode coupling efficiency: https://www.rp-photonics.com/mode_matching.html. There are however, certain minor changes. For one, the ‘dxdy’ portion has been surmised to the vector differential form ‘dS’. Secondly, instead of the product of the integral one electric field and the the complex conjugate of the second field, it has that of the magnetic field.
I hope this was helpful!


Thanks a lot for your help.


Glad to have been of help!