The basic reason behind the harmonics generation is due to the mismatch of the refractive index variation of the coated film and substrate material.
but what is physics behind for the generation of harmonics in optical filter and its significance.
Due to index variation of the material RI and coated material. The harmonics will be generated
One way to look at it is from the distribution of the light intensity (or irradiance) inside an optical filter, linked to the phase of the light.
For example, in the case of a quarterwave stack (passband reflector), at harmonic wavelengths you will see that the irradiance at the interfaces inside the filter is similar to that of the 'fundamental' wavelentgth, but the number of extremums of irradiance inside the layers is different, so that the phases of the beams reflected at the interfaces align in the same way for the harmonic and the fundamental mode, even if the distribution of irradiance inside the layers is different. That is a way to explain the harmonics.
The explanation of the sidelobes is different, but can also be understood when considering the phase values of the light waves reflected at each interfaces. For the harmonics, the phases are aligned in the same way as the fundamental wavelength (for example, leading to constructive interference); in the case of sidelobes, the phases are not aligned, but their 'addition' leads to a maximum value, similar to any other beating phenomena when you mix several waves together.
This is one way to visualize these phenomena. If you are familiar with Fourier analysis of wave signals, you may find their another way to visualize it.
I hope it helps...
Thanks to daniel poitras sir. but why at near to the stop band wavelength the side lobes are generated high as compared to the far away from the stop band?
Hello Vemuri,
For a better understanding of the sidelobes, I suggest that you read about the Gibbs phenomenon in Fourier analysis. Start with the wikipedia entry:
https://en.wikipedia.org/wiki/Gibbs_phenomenon
I am sure that you remember that there is a Fourier transform relationship between the refractive index profile of a filter and its optical spectrum. The Gibbs phenomenon occurs at discontinuities in the refractive index profile (in particular, the discontinuities at the top and bottom of the filter). It could be eliminated if one could use an infinitely thick filter; it can be reduced by reducing the discontinuities in the index profile.
Very Very thanks sir (Dr.Daniel poitras). i will read and discuss with you sir if any doubts means. please guide sir for my Ph.D
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