Principles of Operation
The phenomenon of photoelasticity is the basis for operation of the PEM. If a sample of transparent solid material is stressed by compression or stretching, the material becomes birefringent, that is, different linear polarizations of light pass through the material at slightly different speeds.
If the optical element is relaxed, the light passes through with the polarization unchanged. If the optical element is stressed, the polarization components parallel or perpendicular to the modulator axis travel at slightly different speeds. The parallel component then either “leads” or “lags” the perpendicular component after passing through the modulator. The phase difference thus created between the two components oscillates as a function of time and is called the retardation or retardance.An important condition occurs when the maximum (peak) retardation reaches exactly one-fourth of the wavelength (/4) of light. When this happens, the PEM acts as an oscillating quarter-wave plate. At the peak, the polarization vector traces a right-handed spiral about the optical axis. Such light is called “right circularly polarized.” The polarization oscillates between right circular and left circular, with other polarization states between
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