- #1
Vajhe
- 6
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Hi guys, I have been trying to solve the Helmholtz equation with no luck at all; I'm following the procedure found in "Engineering Optics with MATLAB" by Poon and Kim, it goes something like this:
Let's start with Helmholtz eq. for the complex amplitude ## \psi_p ##:
[tex] \nabla^2 \psi_p + k_0 ^2 \psi_p = 0 , k_0=\frac{w_0}{v} [/tex]
According to the authors, it should be possible to find a solution to that equation applying the two dimensional Fourier Transform to it; just one thing: apparently in engineering, the Fourier Transform is defined like this
[tex] \int_{- \infty}^{\infty} \int_{-\infty}^{\infty} f(x,y) e^{j k_x x + j k_y y} \, dx \, dy [/tex]
I don't really get why the kernel has positive signs, instead of negatives; the authors mention something to do with a convention of traveling waves, but I don't have the referenced book they mention (Principles of Applied Optics, Banerjee and Poon). If someone could explain me that I will be in a great debt .
Now, my problem is this: according to the authors, the result I should have after some manipulation should be
[tex] \frac{d^2 \Psi_p}{dz^2}+ k_0 ^2 (1 - \frac{k_x^2}{k_0^2} - \frac{k_y^2}{k_0^2}){\Psi_p} = 0 [/tex]
Where ##\Psi_p## is the Fourier Transform of ##\psi_p.##
When I apply the FT to the Helmholtz eq. I use the Laplace operator in both ##\Psi_p## and ##e^{j k_x x + j k_y y}##, that will give me several terms (actually a lot). I do some math and I continue working on it but I don't get the desired expression. I find particularly complicated the total derivative: the Laplace operator gives me partial derivatives, but the result should consist of a total one. Also, several terms are missing. I think a possible solution to the latter problem would be something akin to the relation
[tex] d = \frac{\partial}{\partial x}dx + \frac{\partial}{\partial y}dy + \frac{\partial}{\partial z}dz [/tex]
But, I don't find an answer about what is ##\frac{d^2}{dx^2}## in partial derivatives (as an operator), the answer I found is not that quite satisfactory, and even in that case, it presents me with some problems like the total derivates of x and y in z (which will be zero in any case).
So I'm pretty stuck at this point, and I would like to see some fresh ideas.
Thanks in advance!
P.D. Sorry if there are some confounding parts, I'm not sure what to do at this point .
Homework Statement
Homework Equations
Let's start with Helmholtz eq. for the complex amplitude ## \psi_p ##:
[tex] \nabla^2 \psi_p + k_0 ^2 \psi_p = 0 , k_0=\frac{w_0}{v} [/tex]
According to the authors, it should be possible to find a solution to that equation applying the two dimensional Fourier Transform to it; just one thing: apparently in engineering, the Fourier Transform is defined like this
[tex] \int_{- \infty}^{\infty} \int_{-\infty}^{\infty} f(x,y) e^{j k_x x + j k_y y} \, dx \, dy [/tex]
I don't really get why the kernel has positive signs, instead of negatives; the authors mention something to do with a convention of traveling waves, but I don't have the referenced book they mention (Principles of Applied Optics, Banerjee and Poon). If someone could explain me that I will be in a great debt .
Now, my problem is this: according to the authors, the result I should have after some manipulation should be
[tex] \frac{d^2 \Psi_p}{dz^2}+ k_0 ^2 (1 - \frac{k_x^2}{k_0^2} - \frac{k_y^2}{k_0^2}){\Psi_p} = 0 [/tex]
Where ##\Psi_p## is the Fourier Transform of ##\psi_p.##
The Attempt at a Solution
When I apply the FT to the Helmholtz eq. I use the Laplace operator in both ##\Psi_p## and ##e^{j k_x x + j k_y y}##, that will give me several terms (actually a lot). I do some math and I continue working on it but I don't get the desired expression. I find particularly complicated the total derivative: the Laplace operator gives me partial derivatives, but the result should consist of a total one. Also, several terms are missing. I think a possible solution to the latter problem would be something akin to the relation
[tex] d = \frac{\partial}{\partial x}dx + \frac{\partial}{\partial y}dy + \frac{\partial}{\partial z}dz [/tex]
But, I don't find an answer about what is ##\frac{d^2}{dx^2}## in partial derivatives (as an operator), the answer I found is not that quite satisfactory, and even in that case, it presents me with some problems like the total derivates of x and y in z (which will be zero in any case).
So I'm pretty stuck at this point, and I would like to see some fresh ideas.
Thanks in advance!
P.D. Sorry if there are some confounding parts, I'm not sure what to do at this point .