Generation of Circularly Polarized Light
Photoelastic modulator (PEM) is an optical device used to modulate the polarization of light. It is composed of an optical element and a piezoelectric transducer. The transducer oscillates at the natural frequency of the optical element, which is 50 kHz.
This oscillation induces the oscillation of the birefringence of the optical element by the photoelastic effect. Thus, The retardation value of the PEM oscillates from +λ/4 to -λ/4. If we pass a linearly polarized laser pulse through the PEM when its retardation value becomes either +λ/4 or -λ/4, we can generate LCP and RCP pulses.
Synchronization scheme between the PEM oscillation and the firing of laser pulses. The 50- kHz reference pulses from the PEM are down-modulated to 10 Hz by a frequency divider (FD), then fed into two digital delay generators (DG1 and DG2) as trigger pulse. DG1 and DG2 further divide the 10- Hz pulses into 5- Hz pulses alternating with each other, which are combined into 10- Hz pulses for triggering the laser. δt1 and δt2 are adjusted such that the laser pulses fired by the 5- Hz pulses from DG1 and DG2 passed through the PEM when its retardation become -λ/4 and +λ/4, respectively.