Transverse Coupling in Fiber Optics Part IV: Crosstalk

In multichannel communication systems, crosstalk between channels is a problem that must be considered. Typically, the crosstalk should be less than --20 dB. This means that, if an optical power of 1 mW is fed into one optical guide of a cable, no more than 10 /xW should be transferred into the other guides. Let us assume a typical link length of 10 km. The crosstalk measured over a 1-km-long fiber should be less than --40 dB if the power transfer is proportional to the square of the fiber length, less than --30 dB if the power transfer is proportional to the fiber length, and less than --20 dB if the power transfer is independent of the fiber length. As we shall see, the first power law is applicable to identical uniform fibers, the second to nominally identical irregular fibers, and the third to uniform dissimilar fibers. In optical fibers, the field decays exponentially in the cladding. Therefore, a modest increase in spacing between adjacent fibers is usually sufficient to reduce the optical coupling to tolerable values. 1431