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Problems with far-field calculation in diffraction grating simulation
Posted 19 lug 2012, 18:05 GMT-4 RF & Microwave Engineering, Wave Optics Version 4.3, Version 4.4 5 Replies
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Hello! Thank you for your time and patience to have a look at my question!
I am trying to simulate MEMS diffraction grating with COMSOL RF module and I have a problem with far-field domain calculation. The sensing scheme is attached here for your reference. The laser light source and photo detectors are placed 500um down under diffraction grating fingers(made of poly-silicon). Part of the light will be reflected by fingers and part will go through the slits between fingers and hit the gold mirror. The light reflected by mirrors and polisilicon fingers will have interference pattern in the far field. When the distance "g" between the mirror and fingers is changing, the diffraction pattern should also change.
My purpose is to calculate the light diffraction angle in far field. Based on this paper: www.comsol.com/papers/11452/ (also attached here), I choose to use far-field domain to calculate the diffraction angle. When I was simulating one cell of the structure with periodic boundary condition, the result was fine. But when I try to simulate the real design our group fabricates, there are several problems.
1. The real structure has 51 fingers. I started with 11 fingers to see if far-field domain could work. But the far field pattern is almost indiscernible.
2. In my case with 51 fingers(with symmetry cut), I use background calculation and solve for scatter field. Based on Frauhofer diffraction theory, the light pattern will go to far field when d>> 2*D^2/lamda (D:gating period lamda: wavelength d: distance between grating and observer In my case d>>37.5um). Here I start far field integration at d=35um and do a parametric sweep to the distance "g" between mirror and fingers. However the case is too big for the server to solve.
My questions are:
1. Is there a more efficient way to calculate diffraction pattern? Far field calculation is too time consuming!!
2. In far field integration, does it matter if I start to integrate in a smaller domain(inside the near field)? How to choose the size of near field?
I have contacted COMSOL technical support twice and they suggested me to look at Plasmonic Wire Grating www.comsol.com/showroom/gallery/10032/ . But in this example, the wavenumber k of high order diffraction patterns are acquired analytically, it's more like "drawing" these patterns with COMSOL rather then solving them. In my case the wavenumber is still unknown so I did not use port boundary condition.
Does anyone have any suggestions?? I really appreciate your help!
Shuang
I am trying to simulate MEMS diffraction grating with COMSOL RF module and I have a problem with far-field domain calculation. The sensing scheme is attached here for your reference. The laser light source and photo detectors are placed 500um down under diffraction grating fingers(made of poly-silicon). Part of the light will be reflected by fingers and part will go through the slits between fingers and hit the gold mirror. The light reflected by mirrors and polisilicon fingers will have interference pattern in the far field. When the distance "g" between the mirror and fingers is changing, the diffraction pattern should also change.
My purpose is to calculate the light diffraction angle in far field. Based on this paper: www.comsol.com/papers/11452/ (also attached here), I choose to use far-field domain to calculate the diffraction angle. When I was simulating one cell of the structure with periodic boundary condition, the result was fine. But when I try to simulate the real design our group fabricates, there are several problems.
1. The real structure has 51 fingers. I started with 11 fingers to see if far-field domain could work. But the far field pattern is almost indiscernible.
2. In my case with 51 fingers(with symmetry cut), I use background calculation and solve for scatter field. Based on Frauhofer diffraction theory, the light pattern will go to far field when d>> 2*D^2/lamda (D:gating period lamda: wavelength d: distance between grating and observer In my case d>>37.5um). Here I start far field integration at d=35um and do a parametric sweep to the distance "g" between mirror and fingers. However the case is too big for the server to solve.
My questions are:
1. Is there a more efficient way to calculate diffraction pattern? Far field calculation is too time consuming!!
2. In far field integration, does it matter if I start to integrate in a smaller domain(inside the near field)? How to choose the size of near field?
I have contacted COMSOL technical support twice and they suggested me to look at Plasmonic Wire Grating www.comsol.com/showroom/gallery/10032/ . But in this example, the wavenumber k of high order diffraction patterns are acquired analytically, it's more like "drawing" these patterns with COMSOL rather then solving them. In my case the wavenumber is still unknown so I did not use port boundary condition.
Does anyone have any suggestions?? I really appreciate your help!
Shuang
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5 Replies Last Post 23 gen 2015, 09:21 GMT-5
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Posted:
1 decade ago
I'm not sure I understood you problem... but for calculating far-field your domain doesn't need to extend to "far". There are plenty of example that show how far field is calculated for antennas. I think you can use same approach.
Another way is calculating near-field in COMSOL and then doing far-field propagation yourself using Fourier-transform. That's the way I'm doing my far-field calculations.
Another way is calculating near-field in COMSOL and then doing far-field propagation yourself using Fourier-transform. That's the way I'm doing my far-field calculations.
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Posted:
1 decade ago
I'm not sure I understood you problem... but for calculating far-field your domain doesn't need to extend to "far". There are plenty of example that show how far field is calculated for antennas. I think you can use same approach.
Another way is calculating near-field in COMSOL and then doing far-field propagation yourself using Fourier-transform. That's the way I'm doing my far-field calculations.
Thank you Alexander!!
I will try to use small domain. Could you please explain more about how to do the far field integration with Fourier-transformation? It seems like faster than solving for the far-field by COMSOL.
--
helena
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Posted:
1 decade ago
far-field calculations are tricky to setup if you are not familiar with Fourier optics. There are many places where things can potentially go wrong. And usually it's hard to tell if something is not calculated correctly. I strongly advise using COMSOL's far-field calculations unless you are comfortable doing Fourier.
But basically, here is how I do it:
1. Use COMSOL to calculate the near-field;
2. Extract field along the horizontal line (or plane for 3D problem) above the grating;
3. Normalize the field to incident excitation;
4. Fourier-transform near-field E(x) into far-field E(k) (I use fft() function of Matlab). Simplifying, E(k)=sum(E(x)*exp(i*k*x))
But basically, here is how I do it:
1. Use COMSOL to calculate the near-field;
2. Extract field along the horizontal line (or plane for 3D problem) above the grating;
3. Normalize the field to incident excitation;
4. Fourier-transform near-field E(x) into far-field E(k) (I use fft() function of Matlab). Simplifying, E(k)=sum(E(x)*exp(i*k*x))
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Posted:
10 years ago
Dear Alexander,
can you please specify more precisely unique steps? Mainly after the fft()? The size of amplitudes depends on the amount of periods you take... so this is not the right "far-field apmlitude".
More detailed description would be extremely helpful.
Thank you very much!
Katerina
can you please specify more precisely unique steps? Mainly after the fft()? The size of amplitudes depends on the amount of periods you take... so this is not the right "far-field apmlitude".
More detailed description would be extremely helpful.
Thank you very much!
Katerina
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Posted:
9 years ago
Hello
I'm new in Comsol and I'm modeling acoustic pressure distribution around a loudspeaker. My model is 2D axisymmetryic. I couldn't figure out what exactly happen when I use far field calculation to obtain total pressure! i want t calculate pressure in a point inside my domain. but when i use far field calculation, the results are completely different with the results which obtained by usual approach and i don't know which one is more reliable!!!!
another thing is i couldn't understand the difference between these two expression for calculating far field pressure in my desire point(0,1). pfar(0,1) and ffc1.Lp_pfar .
Thank you for your tips.
Sadaf shariati
I'm new in Comsol and I'm modeling acoustic pressure distribution around a loudspeaker. My model is 2D axisymmetryic. I couldn't figure out what exactly happen when I use far field calculation to obtain total pressure! i want t calculate pressure in a point inside my domain. but when i use far field calculation, the results are completely different with the results which obtained by usual approach and i don't know which one is more reliable!!!!
another thing is i couldn't understand the difference between these two expression for calculating far field pressure in my desire point(0,1). pfar(0,1) and ffc1.Lp_pfar .
Thank you for your tips.
Sadaf shariati