What Is an Optical Isolator?

Andrew Kirmayer
Andrew Kirmayer

An optical isolator enables a beam of light to travel in one direction while preventing it from being reflected back the opposite way. The device includes an input polarizer as well as a faraday rotator, a crystalline piece that can rotate a light beam’s plane of polarization when a magnetic field is applied from outside the rotator. An output polarizer reflects the polarized light at a 45 degree angle before it leaves the isolator. The input polarizer, which passes the beam into the isolator, allows light to pass through at the same angle it enters before being altered by the faraday effect. Such a setup is common with lasers and optical communication systems.

Woman holding a disc
Woman holding a disc

The direction the plane of polarization rotates is controlled by how the faraday rotator affects the light beam. The change in the state of the light can be controlled precisely with a magnet. In an optical communication system, electric signals are first converted to light before entering the optical isolator, and then transmitted in one direction. Otherwise, a portion of the light reflected back would alter the frequency of a laser, possibly rendering it ineffective, or make the signals in a communication system unable to convey information. Data in a fiber network would be corrupted and completely unreadable once receivers reprocess it back to electrical signals.

The optical isolator is used along with other components such as optical amplifiers, optical combiners, and optical depolarizers to make full use of light in a communication or measurement system. While passing through the output polarizer and then through a magnetic field, each of which rotates the polarization plane 45 degrees, light is reflected at a total of 90 degrees. An optical diode is another term given to a device capable of directing light in this way. The effect prevents a light beam from reflecting back through a fiber after a laser beam, used as a light source, passes through a coupling lens and into the fiber optic wire.

In addition to use with lasers and optical communication systems, an optical isolator is sometimes used on a circuit for the purpose of electrical isolation. It offers protection against excessively high voltages. Noise levels are also reduced, making measurements more accurate and optimizing the quality of data. An optical isolator is typically very small and can be mounted, directly in the light path, to a circuit board or onto a semiconductor-based laser device.

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