![]() ![]() In this case it is possible to choose a coupling length such that 100% of the signal light and 100% of the pump light leaves on the same fiber. Typically the signal light will be around 1550 nm and the pump will be 980 nm. At the input of an EDFA you want to mix the (low level) incoming signal light with (high level) light from the pump. On the left-hand side of the figure we see an example of coupling two different wavelengths into the same output fiber. That is a given device may allow input over a range of wavelengths in the 1310 nm band up to 50 nm wide and a range of wavelengths in the 1550 nm band also up to 50 nm wide. “Wavelength flattened” couplers or splitters of this kind operate over quite a wide band of wavelengths. The quoted insertion loss is usually between 1.2 and 1.5 dB and the channel separation is quoted as better than 40 dB. Note that each coupler or splitter must be designed for the particular wavelengths to be used.Ĭommercial devices of this kind are commonly available and are very efficient. Thus the coupler on the left can operate in the opposite direction and become a splitter and the splitter on the right can operate in the opposite direction and become a coupler (combiner). In fact both the processes described above are performed in the same coupler-the process is Bi-Directional (BiDi). At a particular point down the coupler the wavelengths will be in different waveguides so if we make this the coupling length then we have separated the wavelengths exactly. On the right-hand side of the diagram the reverse process is shown where two different wavelengths arrive on the same input fiber. If we make the coupler exactly this length then the signals have been combined. Thus in the left-hand section of the diagram (combining wavelengths) there will be a place down the coupler where all of the light is in only one waveguide. ![]() The period of the shift is different for the two different wavelengths. The graph of power transfer shows how power input on one of the fibers shifts back and forth between the two waveguides. These functions are shown in the figure below. All we need to do is choose the coupling length carefully and we can arrange for loss free wavelength combining or splitting. The process can be performed with very little loss.Īs the coupling length is wavelength dependent, the shifting of power between the two parallel waveguides will take place at different places along the coupler for different wavelengths. In the reverse direction light of two different wavelengths on the same fiber can be split so that one wavelength goes to one output fiber and the other wavelength is output onto the other output fiber. Light of two different wavelengths on different input fibers can be merged (combined) onto the same output fiber. Wavelength Selective Couplers (or Splitters) are used to either combine or split light of different wavelengths with minimal loss. ![]()
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