Pump Coupler Co-propagating vs Power Combiner
I am designing a Radio-over-Fiber (RoF) system using external modulation. My requirements are to separate the optical components (say f1, f2, f3 and f4) at the output of the the modulator and then recombining specific components (f1 with f4 and f2 with f3). In order to separate the components I am using FGBs centered at the frequency of interest. Using Optical Power Meters I noted the value of power at the output of the modulator and individual FBGs and found that the power value of the FBGs approximately add up to the value noted by a power meter at the modulator.
Now, when I combined the components f1 and f4 using a power combiner (loss set to 0 dB) I expected the powers to be added up but the power meter showed a different result. If I replace the power combiner with a pump coupler co-propagating, the power seems to adds up and is more than the input powers (with signal and pump attenuation both set to 0 dB). Also, if I use an x-coupler for a coupling coefficient of 0.5 the power levels at the two o/p arms approx. add up to the input power as displayed in the power meters.
My question is if the power combiner is equivalent to a single arm of the x-coupler? How different is a pump coupler from an x-coupler and power combiner?
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