Photoelastic modulator
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An optical device used to modulate the polarization of a light source. The photoelastic effect is used to change the birefringence of the optical element in the photoelastic modulator.
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[edit] Description
The basic design of a photoelastic modulator consists of a piezoelectric transducer and a half wave resonant bar; the bar being a transparent material (now most commonly fused silica). The transducer is tuned to the natural frequency of the bar. This is determined by the length of the bar and the speed of sound in the optical element material. This resonance modulation results in highly sensitive polarization measurements. The fundamental vibration of the optic is along its longest dimension.
[edit] Fuctionality
The principle of operation for the photoelastic modulators is based on the photoelastic effect, in which a mechanically stressed sample exhibits birefringence proportional to the resulting strain. Photoelastic modulators are resonant devices where the precise oscillation frequency is determined by the properties of the optical element/transducer assembly. The transducer is tuned to the same frequency of the optical element as it vibrates along its long dimension. A current is then sent through the transducer to vibrate the optical element through stretching and compressing which changes the birefringence of the transparent material. Birefringence in a transparent material is caused by the stress of stretching and compressing that material. This is to say that different linear polarizations of light have slightly different speeds when passing through the material. The photoelastic modulator works by oscillating the birefringence of the optical element through a continuous cycle of compressing and stretching the optical element.
[edit] Uses
Photoelastic modulators are only able to do three things: modulate the polarization of a light source, detect the polarization of a light source, and “chop” light. All of this can also be done by rotating a wave plate while passing the light through it. The difference is that a photoelastic modulator has no moving parts therefore it can do all these things at a much faster frequency. Also, the resonate nature of the modulation means that the photoelastic modulator is capable of extremely sensitive measurements.
[edit] History
It was first invented by J. Badoz in the sixties and originally called a “birefringence modulator.” It was initially developed for physical measurements including optical rotary dispersion and Faraday rotation, polarimetry of astronomical objects, strain-induced birefringence, and Ellipsometry. The photoelastic modulator can work at one frequency; that being the natural frequency of the optical piece. Later developers of the photoelastic modulator include J.C Kemp, S.N Jasperson and S.E Schnatterly.
