Can I use the amplitude modulator as a rotating fixed waveplate?
No, the Models 4103 and 4104 amplitude modulators can be used as variable waveplates only. They will not work as a substitute for a rotating fixed wave plate. The Model 410X modulators function as a waveplate whose retardance is varied by changing the voltage that is applied to the electro-optic crystals. So, you can simulate a waveplate with a fixed orientation and an electronically variable retardance. A rotating fixed waveplate would require a fixed retardance with a variable orientation, which is not possible with the 410X.