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Q Factor - Optical Cavity

Posted 2010年5月12日 GMT-4 02:05 Version 4.3b 5 Replies

Perumal Dharanipathy
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I am trying to find the Q factor for an optical cavity in 3D geometry in RF Module.

I made the 3D geometry and I ran the eigen value solver to find the eigen modes and I am able to find the eigen mode which looks similar to the one I am looking for. At this moment, If I use the "Imag(lambda)/2*real(lambda)" expression and plot in Global Variables plot, I am seeing a Q factor which is very small and not the real Q. I don't know how to get get to the real Q by finding the correct damping factor. If anyone has done this before, can you brief me the way to compute this damping factor and the real Q of the system.

Thank you,

5 Replies Last Post 2014年2月14日 GMT-5 01:15
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2010年5月12日 GMT-4 16:29
Hi

I'm not pleased with my reply here, but I must admit that I give up mostly to run after the Q factors, as the Q factor is mainly the inverse of the damping factor, and in most cases one have very little knowledge of the true damping factor.

This holds for structural eigenmodes, as well as for RF or optics, but perhaps in ACDC electric behaviour one have better and more precise data of the losses in the materials.

For PZT and magnets the hysteresis is also very case to case dependant, so there too Q factor extraction is delicate.

The positive thing is that if you have some good measurements, then you can use COMSOL to optimise its model and get some interesting results, also for the Q factor.

Hopefully your case ;)

Good luck
Ivar
Hi I'm not pleased with my reply here, but I must admit that I give up mostly to run after the Q factors, as the Q factor is mainly the inverse of the damping factor, and in most cases one have very little knowledge of the true damping factor. This holds for structural eigenmodes, as well as for RF or optics, but perhaps in ACDC electric behaviour one have better and more precise data of the losses in the materials. For PZT and magnets the hysteresis is also very case to case dependant, so there too Q factor extraction is delicate. The positive thing is that if you have some good measurements, then you can use COMSOL to optimise its model and get some interesting results, also for the Q factor. Hopefully your case ;) Good luck Ivar
Perumal Dharanipathy
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Posted: 1 decade ago 2010年5月14日 GMT-4 03:35
Hello Ivar,

You are definitely right. It is good to know that I am not alone in this struggle.

But, I found this piece of documentation from COMSOL which is what I am following at the moment.
www.comsol.com/showroom/gallery/2186/


Though, it has no relevance in terms of its physics, the model flow seems to be the same. So I made the 3D structure first, ran the eigen value solver, found the values and then for the specific eigen value which is my desired mode, I run a harmonic propagation paratmetric sweep around this frequency and in my case, I am plotting the Norm of the E.Field in a point where the field is maximum in the device. I did this only yesterday night and I seem to get a Q very close to the experimental Q of my device. (already measured).

So I guess this works, but don't know yet if it can be generalised so. I will have to confirm with some other examples.

Do let me know if you find any other way.

Thanks

Hello Ivar, You are definitely right. It is good to know that I am not alone in this struggle. But, I found this piece of documentation from COMSOL which is what I am following at the moment. http://www.comsol.com/showroom/gallery/2186/ Though, it has no relevance in terms of its physics, the model flow seems to be the same. So I made the 3D structure first, ran the eigen value solver, found the values and then for the specific eigen value which is my desired mode, I run a harmonic propagation paratmetric sweep around this frequency and in my case, I am plotting the Norm of the E.Field in a point where the field is maximum in the device. I did this only yesterday night and I seem to get a Q very close to the experimental Q of my device. (already measured). So I guess this works, but don't know yet if it can be generalised so. I will have to confirm with some other examples. Do let me know if you find any other way. Thanks
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2010年5月14日 GMT-4 04:05
Hi

Indeed runing first a stationary to set up the bulk conditions/loads/stress/fields and then run an eigenvalue linearised through the stationary case, and then possibly a harmonic scan is the "book" approach, and I beleive is going tobe easier to set up in V4.

So the methodology is correct in my opinion, remains to implement it correctly and to verify all that. It's good to hear you get good match with your expected values

Have fun Comsoling
Ivar
Hi Indeed runing first a stationary to set up the bulk conditions/loads/stress/fields and then run an eigenvalue linearised through the stationary case, and then possibly a harmonic scan is the "book" approach, and I beleive is going tobe easier to set up in V4. So the methodology is correct in my opinion, remains to implement it correctly and to verify all that. It's good to hear you get good match with your expected values Have fun Comsoling Ivar
Manas Ranjan Gartia
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Posted: 1 decade ago 2010年8月6日 GMT-4 19:21
Hi,
Can you please upload your model...I am also trying to calculate the Q-factor for a silica bead optical mode..

Thanks

Manas
Hi, Can you please upload your model...I am also trying to calculate the Q-factor for a silica bead optical mode.. Thanks Manas
Liaquat Ali
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Posted: 1 decade ago 2014年2月14日 GMT-5 01:15
Dear ,

I am trying to model the Silicon Toroid with Terfenol D in it, so it would be of great help that you can upload your model here

Regards
Ali
Dear , I am trying to model the Silicon Toroid with Terfenol D in it, so it would be of great help that you can upload your model here Regards Ali

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