Isolators Protect Optical Amplifiers And Fiber-optic Systems

By Author: Ryan
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A fiber optic component that transmits light in only one direction is the high power polarization maintaining Isolator. From returning to the source, it prevents reflected light. It is capable of maintaining a well-defined state of polarization (SOP) of the light and is built with PM Panda fibers. Based on the required optical isolation degree of the propagating light, this high-power polarization-maintaining isolator is used. It can be of double or single stage. In many laser applications, it’s a good choice since on the operating conditions of optical devices; back-reflected light often has deleterious effects.


To perform various functions, many types of passive fiber-optic components are used in a modern optical transmission system. Into four categories, these components typically fall --connecting devices such as splices and connectors; branching devices such as splitters and couplers; filtering devices such as fiber Bragg gratings, wavelength-division multiplexers (WDMs)/demultiplexer, drop/add ...
... filters, and performance-improving devices such as isolators, polarizer’s, attenuators, and dispersion compensators.


In fiber-optic systems, Isolators are playing an increasingly important role. Isolators are normally nonreciprocal whereas most passive components are reciprocal, --that is, in the forward direction, they allow the passage of an optical beam with minimal losses while in the backward direction, the transmission with 40- to 70-dB losses are blocked.


Various types of optical isolators such as all-fiber isolators 2, all-fiber isolators1, waveguide-based isolators4, fiber Faraday rotator isolators3, and fiber-embedded isolators, have been numerous designs. A Faraday rotator with 45° rotation and a pair of birefringent crystals or a pair of polarizers is contained in the core of a typical commercially available isolator. The optical beam from one fiber to the other is usually coupled by a pair of micro lenses such as gradient-index lenses. A polarization-independent isolator is made by the birefringent crystal-based structure, and a polarization-dependent isolator is made by the polarizer-based module.


Refocused by a lens into the optical fiber, a polarization maintaining circulator and polarization-independent isolator splits the input beam into two parallel beams.


The incoming beam is split by the second wedge again splits into e and o beams in the backward direction.


Increasingly, in an optical system such as a fiber-optic gyro, designers are taking advantage of maintaining the polarization status and some critical components are polarization-dependent in some systems. So that their optical axes are 45° apart, the two polarizers are aligned. The first polarizer transmits only one polarization in the forward direction. So that it goes through the second polarizer without loss, The Faraday rotator rotates the polarization of the transmitted beam by 45°. By the Faraday rotation, the remaining component of the beam (after the second polarizer) is rotated in the same direction if the beam comes from the backward direction. Therefore, to the optical axis of the first polarizer, its polarization is perpendicular and absorbed. Usually, on both the output and the input ends, polarization-maintaining (PM) fibers are used. On the input or both ends, regular single-mode fibers are used in some cases. When with a polarization-independent isolator, PM fibers are combined correctly a single fiber can transmit two signals channels. Polarization maintaining Isolator offers isolation both channels with crossed polarization.

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