Field-Modulated Generators for Wind Energy

It's probably that the output frequency and phase corresponds to the local mains, at whatever speed the rotor is turning.

How is this coupled? Why MOSfet?

MOSFETs typically drop less voltage than say transistors.
The On resistance of MOSFETs has become very low over the years.
Transistors and especially Darlingtons drop quite a voltage even when hard on, say a volt, at a high currents this represents quite a power loss. 10amps at 1volt is 10watts whereas 10amps at 10mΩ is 1 watt (I²R) if my catly arithmetic is right...
Del

I think what is intended here is that an HF or polyphase ac is modulated with the local freq (50Hz in your case) which then can be rectified in alternat directions to give a sine wave output at the modulation frequency.

Chas