Increase Phase Velocity by Losing Power ?

[Jeffrey Yang]

I just read some words from the book Optical Waveguide Theory by Snyder, Allan W., p226. It says that

"However, although it is possible for the phase velocity of a mode to exceed the maximum speed of light in the cladding (of waveguide), c/n_cl, this cannot occur without losing power to radiation"

My question is how to understand this? Why losing power can increase the phase velocity of the wave?

 

[Simon Bridge]

The loss of power is the result of setting up such a high phase velocity wave, it is not the cause of the high velocity.

 

[Jeffrey Yang]

The loss of power is the result of setting up such a high phase velocity wave, it is not the cause of the high velocity.

Thanks for your reply. I still cannot understand the internal relationship between "losing energy" and "change of phase velocity", could you please explain more about this?

 

[Simon Bridge]

If you make the phase velocity too fast, some of the light inevitably escapes.
It's like if you try to drive too fast around a bend you lose some power to skidding... going fast has more energy but you are also losing energy.