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What Is a Faraday Rotator?

Paul Reed
Paul Reed

A faraday rotator is a device with no moving parts that changes the polarization, or waveform angle, of light passing through it. Light passes through air or other materials as a series of waves, called electromagnetic radiation, with the characteristics of both electric and magnetic fields. The device works on the principle that light passing through a crystal or solid transparent material will change polarization if a magnetic field is present.

Discovered by scientist Michael Faraday in 1845, the effect of magnetic fields on light waves was the first evidence that light was an electromagnetic wave. He found that changing the magnetic field strength affected the polarization angle of the light. Named The Faraday Effect, this is the basis for the rotator, which uses the experimental effect in a practical device.

Woman holding a disc
Woman holding a disc

Light passing through many materials, including glass and water, can have the polarization angle affected without the use of magnetic fields. This effect is called optical polarization, and manufacturers of sunglasses take advantage of this by producing lenses that block polarized angles other than normal light. The effect of glare is reduced, because reflected light from water or buildings will have a different polarization angle.

To build a faraday rotator, a magnet surrounds a transparent material. As light passes through, the magnetic field causes the light wave to be turned by a specific amount. The amount of rotation can be determined by an equation that uses the magnetic field strength, the length of the crystal, and the verdet constant of the material. This constant is different for all materials and changes with temperature; tables of constants are published for materials at different temperatures.

Laser equipment often uses a faraday rotator as a protection device, to prevent reflected laser energy into the unit. When a laser creates a beam of light, it is very coherent, which means it contains light of one specific waveform. As the light leaves the laser, it often is reflected or passes through other equipment and potentially some of the light could be reflected back to the laser. Adding a faraday rotator prevents this, because the light passing through the rotator is polarized typically 45° from the original beam and cannot reflect back. The angle can be varied, but more polarization requires additional magnetic field strength.

An additional benefit of a faraday rotator is that light passing through it and then returning in the opposite direction is not rotated back. If light is polarized 45° by the rotator, and then strikes a mirror and returns, the rotator will polarize it another 45°. Optical polarizing filters, or devices that create specific degrees of polarization for laboratory use, can take advantage of this effect. This works by reflecting some of the light back through the rotator, creating two light beams that are polarized at different angles.

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