Explainations on free space optical propagator

I am looking for a freelancer who can provide an in-depth technical explanation of a MATLAB free space optical propagator. The focus of the explanation should be on the theory and principles of the technology. I got part of the code from a book that I can provide, but I have to really grasp the physics and mathematics behind it. The intended audience for this explanation is experts in the field.

Only individual writers. Who can accept low budgets. Thank!