Coupled Waveguides
Most recent answer: 03/03/2013
Q:
I studied in mode coupling theory that in case of two optical waveguides having same refractive index when placed near to each other causes the optical power to transfer from one waveguide to another completely after a certain distance. what makes the light wave move from its own medium to an identical medium (the second waveguide). Also what confuses me more is after some distance when both the waveguide has equal power, even then the power continues to be transferred from the first waveguide to the second until the power at the first waveguide is completely transferred to the second waveguide. what is the reason behind that?
- Dipal Ghosh (age 25)
Hamburg,Germany
- Dipal Ghosh (age 25)
Hamburg,Germany
A:
Very nice question.
Let me answer by describing a slightly simpler system with essentially the same physics. Take two identical mass-on-spring oscillators and couple them with a very weak spring. There are still two normal modes of the system, but they aren't quite the separate ones of the two oscillators any more. One normal mode has the oscillators swinging together, so it has the same frequency as they had before (the extra spring doesn't stretch) and the other has them swinging opposite, so the extra spring contributes some more restoring force and just slightly raises the frequency.
Now start with just one oscillator going. That's a combination of those two normal modes, so that they add up for that oscillator but cancel for the other one. Since the two modes are slightly different frequencies, their relative phase gradually changes. After a while, they're adding where they had cancelled and cancelling where they had added. The other oscillator now is swinging. Then the energy gradually goes back to the first one, and so on.
First try turned up a nice video:
The same thing is going on with the waveguides, except that here the energy is also traveling along as it goes back and forth between them.
Mike W.
Let me answer by describing a slightly simpler system with essentially the same physics. Take two identical mass-on-spring oscillators and couple them with a very weak spring. There are still two normal modes of the system, but they aren't quite the separate ones of the two oscillators any more. One normal mode has the oscillators swinging together, so it has the same frequency as they had before (the extra spring doesn't stretch) and the other has them swinging opposite, so the extra spring contributes some more restoring force and just slightly raises the frequency.
Now start with just one oscillator going. That's a combination of those two normal modes, so that they add up for that oscillator but cancel for the other one. Since the two modes are slightly different frequencies, their relative phase gradually changes. After a while, they're adding where they had cancelled and cancelling where they had added. The other oscillator now is swinging. Then the energy gradually goes back to the first one, and so on.
First try turned up a nice video:
The same thing is going on with the waveguides, except that here the energy is also traveling along as it goes back and forth between them.
Mike W.
(published on 03/03/2013)