Optical isolators are passive optical devices that allow light to be transmitted in only one direction. They are most often used to prevent any light from reflecting back down the fiber, as this light would enter the source and cause backscattering and feedback problems. Optical feedback degrades signal-to-noise ratio and consequently bit-error rate. Ideally an isolator would pass all light in one direction and block all light in the reverse direction.
There are two major classifications of optical isolators: inline isolators (fiber optic isolators) and free space isolators. Inline fiber optical isolators are designed in pigtail fashion. That is to say that they come with built-in fiber optic cable and connectors so that they may be integrated directly into a fiber optic system. Free space isolators, by contrast, do not have an integral connection system. They must be directly mounted to the object that needs isolation.
Important specifications for optical isolators include center wavelength, isolation, insertion loss, and polarization dependant loss. Center wavelength is the center of the wavelength range in which the isolator is designed to function optimally. This characteristic is usually measured in nm. Isolation, generally measure in decibels (db), is a measure of how effectively back reflections are prevented and the degree to which the isolator can transmit. Insertion loss is the attenuation caused by the insertion of an optical component. Polarization dependant loss is the attenuation caused by polarization.
Optical isolators are used in many optical applications in corporate, industrial, and laboratory settings. They are reliable devices when used in conjunction with fiber optic amplifiers, fiber optic ring lasers, fiber optic links in CATV applications, and high-speed and coherent fiber optic communication systems. Single polarization fiber optic isolators are also used with laser diodes, gyroscopic systems, optical modular interfaces, and a variety of other mechanical control and testing applications.