Hello burhan num mina llah,
Cross Coupler for combining or splitting optical signals. the process is cores of two identical parallel fibers are so close to one another that the one wave can “leak” from one fiber core into the other core. This allows an exchange of energy, The amount of energy exchange is dependent upon the proximity of the two cores, d, and the length over which this exchange takes place, L. It is easy to see that if the coupling length is long enough, a complete transfer of energy can take place from one core into the other. If the length is longer still, the process will continue, shifting the energy back into the original core. By selecting the proper length, any given power transfer ratio can be realized. This is how a 50/50 or a 10/90 coupler is made.
In reality, energy transfers back and forth between the two fibers many times over the coupling region (also called the Interaction Length). The transfer rate is a function of wavelength, so if a wavelength is used that is different from the design wavelength, the energy transfer (or coupling ratio) will be different. As an example, assume that two different wavelengths are launched into the two input ports of a coupler.
In case of fork no such mechanism is involved. Regenerate the signal at the two output point.
Regerds,
Dhiman