For the reduction of the harmonics, you need a rugate filter design, or quasi-rugate (with refractive index steps). The Kaiser apodization window is used as an envelop function around the rugate design for reducing the sidelobes. The main reason being that the Fourier function of the Kaiser has almost no sidelobes.
I recommend the following paper to get an intuitive comprehension of the elimination of harmonics and its relation to the refractive index profile (I found the paper freely on the web):
J.A. Dobrowolski, and D. Lowe "Optical thin film synthesis program based on the use of Fourier transforms," Applied Optics, vol.17, no. 19, p.3039, 1978.
And for the application of the Kaiser window with rugate filters, look for papers from Bertrand Bovard in Applied Optics. It is possible to "discretize" a rugate index profile (to get a step-like index profile), and then optimize the thicknesses to reduce the sidelobes and harmonics, but that way you wont be able to eliminate all harmonics.
I hope that this response is useful!
Very very Thanks sir Daniel Poitras , for a clear explanation on how the Kasier window function will be useful for reducing the harmonics in multilayer filters. i will read the followed paper requested by you sir.
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