When I am thinking about the interference of two laser beams, a curious question often comes to my head: Take a Mach-Zehnder interferometer as example and suppose the frequency of the leaser beams is the same, namely they are all monochromatic plane wave.

As we know, if we measure the optical power in different positions in the propagation direction, an interference fringe will appear.

What puzzled me is that in the destructive interference position, the optical power is zero, however, if the power meter keep fixedly in this position, the power is zero all the time, but the laser is still working, and where has the energy gone?


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