Variable Optical Attenuators

A simple approach would be to use fixed attenuators which can easily be exchanged and possibly combined. However, it is more convenient to use a filter wheel, e.g. of neutral density filter type, where a variable degree of attenuation can be adjusted simply by rotating the wheel to another position. The rotation may be done manually or with a motorized drive, possibly with computer control. Some variable reflectivity mirrors are also suitable for such applications.

The attenuation may vary in steps or continuously; in the latter case, one will of course have some variation of attenuation across the beam profile.

Absorbing filters are quite limited in terms of power handling capability.

For high-power applications, one will usually use a device where a variable fraction of the incident power can be sent into a robust beam dump. One such possibility is to use rotated dielectric mirrors.

For an input with constant wavelength, one may use an edge filter with a variable tilt angle, so that the filter edge can be varied. While the reflected beam will have a substantially varying propagation direction, the transmitted beam exhibits only a variable parallel offset, which may either be small enough to be tolerated (if a thin substrate is used) or can somehow be compensated, e.g. by using two such devices in a sequence, which are rotated in synchronism.

The attenuation of such devices will typically be polarization-dependent, which however is often not a problem when using such devices with linearly polarized laser beams.

If the input beam is linearly polarized, one may first apply a variable rotation of the polarization direction with a rotatable half-waveplate and then use a polarizer. This type of device may also be able to handle very high optical powers; the rejected beam can be sent into a beam dump.

Liquid crystal modulators can be used for obtaining electrically adjustable attenuation. The liquid crystal device actually only modifies the polarization state, which causes variable attenuation in a subsequent polarizer. Usually, linear polarization at the input is expected, although polarization-independent designs are possible. The power handling capability is modest.

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