Optical isolatoris a necessary passive component that widely used in fiber optic communication networks. It is used to prevent unwanted light feedback into the sensitive devices or components. As we know, light can be reflected back and forth, which will also happen in the fiber optic networks.

But the fact is most of the refection will affect the stability of the system especially for lasers. Laser is essentially a resonant cavity between two semi-transparent mirrors. The lasing process happens between these two mirrors. The lasing process is very delicate and can be easily interfered. If back-reflected and scattered light enters into the laser, the lasing process will fluctuate and the output power of the laser will fluctuate. However to prevent the bad result caused by this unwanted refection, a fiber optic isolator is needed here. A fiber optic isolator is also called optical diode, which is issued to prevent any light from reflecting back down the fiber, stopping backscattering and feedback problems on these sensitive components in the networks.

Most of optical isolators in the current market are Faraday isolator that is based on Faraday rotators. According to the polarization sensitivity of the optical isolator, it can be divided into polarization sensitive and insensitive Faraday isolators:

Polarization-sensitive Faraday Isolators

Polarization-sensitive faraday isolator is the simplest type which only works when the input beam has a prescribed direction of linear polarization. This type of faraday isolator is composed of a pair of linear polarizers and a Faraday rotator.

The two linear polarizers are oriented so the planes in which they polarize light are 45° apart. The Faraday rotator sits between these two polarizers. The Faraday rotator rotates the plane of the polarization of light by 45° in a single direction no matter the light traveling direction, may it be from the first polarizer(left) or the second polarizer(right). So if the light goes from the first polarizer to the second polarizer (from left to right). The Faraday rotator will rotate the polarized light from the first polarizer by 45° which exactly matches the polarization plane of the second polarizer. So the light will go through with minimum loss.

But if the light goes from the second polarizer to the first polarizer (from right to left). The Faraday rotator will rotate the polarized light from the second polarizer also by 45°. But since it rotates the light as the same direction as from left to right, this time when the rotated light gets to the first polarizer, the polarization planes of the polarized light and the first polarizer are 90° cross. So all light are blocked and no light will go through.

Polarization-insensitive Faraday Isolators

In order to overcome this limitation of polarization sensitively, optical isolators have been developed which are polarization insensitive type. That is, regardless of the polarization state of the input beam, the beam will propagate through the isolator to the output fiber and the reflected beam will be isolated from the optical source.

Such devices are often required in the context of fiber optics, because many fibers are not polarization-maintaining. In particular, optical fiber communication systems are usually operated with undefined polarization state, and Faraday isolators as well as other components are then required to work with arbitrary polarization states.

The basic principle of a polarization-insensitive isolator is to first spatially separate the orthogonal polarization components of the input beam with some kind of polarizer, then send both through a Faraday rotator, and combine them again in the second polarizer. Since there is an arbitrary relative phase change between the two polarization components, polarization-insensitive isolator will in general not preserve the polarization state.

Insert loss and degree of isolation are essential features for the optic isolator. The insertion loss is mainly depend on the type and quality of the polarizers used. A high degree of isolation is generally more difficult to achieve for high-power devices, where the beam in the Faraday medium covers are larger area and is thus more sensitive to field inhomogeneities. The fiber optic isolators can be pre-installed with various kinds offiber connectors. Ingellen offers various type offiber optic isolatordevices which features high isolation and high return loss, minimal polarization dependent loss, and very low polarization mode dispersion.

Posted by: mikofy at 06:53 AM | No Comments | Add Comment
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