Depolarizers

Depolarizers obliterate the polarization of a polarized beam by reflecting the beam in all directions at right angles to its axis. Also known as polarizations scramblers, depolarizers change plane polarization into a mix of polarization states, which resembles unpolarized light. This is especially useful when dealing with polarization sensitive instruments such as reflecting spectrometers. Most depolarizers, have excellent transmittance,

92% over the usable 190 to 2500 nm range. There are two common forms of depolarizers (although custom and specialized varieties are available): wedge depolarizers and Lyot depolarizers.

Wedge depolarizers use a crystalline quartz wedge together with a compensating fused silica wedge to correct the angular deviation. The optic axis of the quartz wedge lies in the plane of the wedge and at 45° to the input polarization. The overall effect is like that of a retardation plate. Variable retardation occurs across the aperture, which produces depolarization for all wavelengths.

Lyot depolarizers consist of two crystalline quartz plates assembled with their optic axes lying in the plane of the plates, aligned at 45°. One plate is exactly twice the thickness of the other. Depolarization occurs due to the production of varying amounts of circularly and elliptically polarized radiation of different wavelengths. This type of depolarizer is not should not be used in situations where monochromatic light must be depolarized. Lyot depolarizers are generally fashioned from quartz crystals, calcite, and magnesium fluoride (MgF<SUB>2</SUB>). Quartz crystals are the most commonly used type, and are generally the cheapest. Calcite depolarizers may be as much as twice as expensive as quartz crystal types, but they are far more effective when used in narrow spectral ranges, due to a much higher birefringence. Magnesium fluoride depolarizers are also useful in very narrow spectral ranges.

Depolarizers are widely used in conjunction with Raman amplification, as the Raman scattering effect is highly dependant upon polarization, especially when used with pump lasers. Depolarizers are also used in instrumentation, although this is generally a niche field.

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