All PEMs are capable of generating left and right circularly polarized light. In the case of the Hinds I/FS50 PEM, you would be able modulate light in a continuously varying fashion between left-circular and right-circular at a frequency of 50 kHz.
The PEM modulates polarization as the function:
f(t) = A*cos(wt), where w = 2π *50kHz.
The polarization output of the PEM is not a step function but varies sinusoidally in time. The operating frequency of the PEM is nonadjustable, being equal to the resonant frequency of the optical element.