Does anyone know how we can excite a certain mode, e.g. the second order mode, in a waveguide? I'm a little confused. According to the ray-optics theory, modes are related with incident angles, which means by ensuring a certain range of incident angle we can excite a desired mode. However, based on the wave theory where each mode has different spatial power distribution. Thinking in this way, it seems that what we need is to make sure the shape of the power profile of the incident light matches the profile of the desired mode and also that the light is projected on the right position on the end face of the waveguide. And in this case, the incident angle doesn't matter.
It seems strange that these two theories give different results which don't agree with each other. Can anyone see where the problem is? Thanks!