I am reading the beam propagation method (BPM) in optical imaging paper. I find a paper solve the Helmholtz equation in the inhomogeneous media. The paper is:
Light propagation in graded-index optical fibers, M. D. Feit and J. A. Fleck, Jr
They use the operator form to solve the equation, I never see this kind of operator calculation to solve the classical eletromagnetic proble before. I can intuitively understand how it is going on. But I want to learn more about this and acquire such method. However, i am not a mathmatical stduent. I don't want to learn a full book of the function analysis or some group theory to just acquire this. I mean is there some good,simple and not very long pages reading material to learn this kind of technique.
Besides, there are some places that i don't understand in the paper.
- Question 1:
Like the equation (3)
How to obtain this? Now the the expansion for $(1+x)^{1/2}$ is not hold for the Laplacian operator?
- The second place i don't understand is:
Why it is equivalent to the Helmholtz equation? Intuitively, I know that using the angular spectrum to solve the Helmholtz equation, we can get maybe the top solution. However, is there or mathmatical derivation of this? From top to expression to the equation (10).