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Fiber Isolators & Circulators
Frequently Asked Questions
An optical isolator contains a Faraday rotator, a component characterized by rotating input linear polarization by 45 degrees. It is important to note that the direction of polarization rotation is dependent on the orientation of the Faraday rotator but not on the light travel direction. In other words, if, for instance, the polarization is rotating clockwise relative to the propagation direction, in the reverse-propagation direction, the rotation will be counter-clockwise. At the input of an optical isolator there is a linear polarizer ensuring that only linearly polarized line enters the system. There is a second linear polarizer present at the exist that let’s through the light that is now rotated by 45 degrees relative to the input. If there is a back reflection from any subsequent optical components in the system, the light might be able to back enter from the exit polarizer and further rotate in the same direction by 45 degrees resulting in a 90 degree rotation relative the orientation of the entrance polarizer, which will block the light from passing through the system in the reverse direction.
Yes, the rotation degree of the polarization is dependent on the optical wavelength of the input light. Additionally, the polarizers and the other elements of an optical isolator may have special optical coatings which are also wavelength dependent. Some optical isolators are only suitable for a specific wavelength, while others can operate over a range of wavelengths.
Yes, this is true for optical isolators containing polarizers. However, polarization-independent isolators exist.
Yes, optical circulators and isolators that are suitable for free-space coupling have been designed and are used in free-space communication systems. They are often used to reduce or eliminate back-reflection of light from the optical system that otherwise might enter the laser and deteriorate its performance and even damage it.
Yes, in the case of polarization-dependent circulators, the polarization of the input beam should be linear and match the input polarizer orientation. However, polarization-independent optical circulators exist and are used in fiber optical communications as the polarization state of light is likely to vary as it propagates through the fiber.
Both components are widely used in optical communications systems to send signals along different channels. They are also used in dispersion compensation devices or as means to protect laser systems from back reflection. Optical isolators are also used in multistage amplified laser systems to prevent back-reflection from entering back into the laser which can deteriorate or damage the laser system.
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