What Are Fiber Optic Isolators? Fiber Optic Communication Tutorial Series By Colin Yao At Isnare.com Ezine Articles

Why do we need fiber optic isolator?

Light can be reflected back and forth. This is also true in fiber optic communication networks. But in fiber optic networks, most of the reflections are harmful to the stability of the system which is especially true 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.

So that is where fiber optic isolator comes to play. Optical isolators are devices that transmit light only in one direction. They play a vital role in fiber optic systems by stopping back-reflection and scattered light from reaching sensitive components, particularly lasers.

How do optical isolators work?

The inside workings of optical isolators depend on polarization. An 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.

From above mentioned principles, you see that fiber optic isolators transmit light only in one direction and they work like a one way street.

Polarization Dependent Isolator and Polarization Independent Isolator

In reality, the aforementioned isolator working principle is called polarization dependent isolator. Why? Since the first linear polarizer cuts the input light in half with its polarizing effect. So in order to achieve minimum loss, you would have to align the polarization of your input light to the same polarization as the first polarizer.

In order to overcome this limitation of polarization dependence, optical isolators have been developed which are polarization independent. 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.

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