Abstract: A photoelastic modulator (PEM) has two parts (11, 13) of the same or dissimilar cross sections, joined together to form an abrupt junction (16). A transducer, for which the natural oscillation is at the half acoustic wavelength mode, excites a longitudinal standing wave of high purity in the PEM. A feedback system is used to achieve stable phase synchronisation of multiple PEMs working at the same oscillation frequency, where the outputs of an amplitude and phase sensor for one PEM is used to correct phase variations of other PEMs and to compensate optical retardation errors.

Abstract: A photoelastic modulator, comprising a driving block and a driver transducer for perturbing said block in a predefined direction to establish a standing wave extending longitudinally in said block and hence perpendicularly to said predefined direction. The transducer is affixed to said block at two regions of the transducer mutually displaced in said predefined direction, to minimize the coupling of lateral perturbation perpendicular to the predefined direction. A recess or gap may be provided under the transducer.