Apparatus include a photoelastic modulator (PEM) optical element, a controller having a frequency generator configured to produce a frequency signal at a selected frequency based on a clock signal of the controller wherein the controller is configured to produce a PEM driving signal based on the frequency signal, a PEM transducer coupled to the PEM optical element and the controller and configured to drive the PEM with the PEM driving signal, and a detector optically coupled to the PEM optical element and configured to receive a PEM modulated output and to produce a PEM detection signal that includes a PEM modulation signal, wherein the controller is configured to receive the PEM detection signal and to extract the PEM modulation signal from the PEM detection signal using the frequency signal and the clock signal.