Optical Fiber Communications 4th Edition by Gerd Keiser

Exercise 8

Let the electric field of an optical signal at a carrier frequency v i be
where P i is the signal power at the carrier frequency v i and i is the carrier phase. If N optical signals each at a different frequency v i are traveling along a fiber, show that the signal power is
where jk = 2( v j - v k ) represents the beat frequency at which the carrier population oscillates.