A continuous process

To help you understand the continuity of the conversion into circularly polarised light, different points in time are being highlighted now and the PEM modulation is being displayed.

The PEM modulation effectively slows down the two linear, orthogonal components relative to each other that the incoming linearly polarised light consists of. This introduces a phase shift between the two components, which gradually changes between plus/minus a quarter of the wavelength, λ, i.e., between -0.25 * λ and +0.25 * λ. That is, at the end points of its oscillation, the PEM acts as a quarter waveplate.

At a modulation of -0.25 * λ, the resulting light is right-handed circularly polarised
At a modulation of +0.25 * λ, the resulting light is left-handed circularly polarised
At a modulation of 0 * λ, there is no phase shift, and the light passing through the PEM's optical element retains its linear horizontal polarisation
At any other modulation, the resulting light is elliptically polarised, with right-handedness for negative phase shifts and left-handedness for positive phase shifts