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An optical isolator is a passive magneto-optic device that makes travel of light unidirectionally. The operation of the device is based on the Faraday effect. It is used in applications to avoid unwanted feedback to the system using the optical isolator.
Optical isolators are also known as optoisolators, optocouplers and photocouplers.
An optical isolator is comprised of a Faraday rotator with magnet, input polarizer and output polarizer. The input polarizer is arranged to act as a filter and only allows linearly polarized light into the rotator. The rotator then rotates the polarization of the input light by 45 degrees and is allowed to exit through the output polarizer. In this manner, energy is either absorbed or reflected depending on the polarizer type. Optical isolators are available with high power and low-power polarizers.
Optical isolators can also be categorized into polarization-dependent isolators and polarization-independent isolators. Polarization-dependent isolators use input and output polarizers along with a Faraday rotator, whereas polarization-independent isolators use input and output birefringent wedges along with a Faraday rotator.
Optical isolators are used in many applications, such as laser applications where they are used to prevent unwanted feedback into the laser source. This helps in not affecting the coherence of the laser affecting any damage to the diode itself as feedback is capable of frequency shift, noise, mode hopping or amplitude fluctuation. It is thus a key device in order to achieve stable laser diode operation. In high-speed optical fiber transmittance amplifiers and routes, they are considered indispensable devices due to their capability of eliminating the adverse effect of return beams.