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

Why Do We Need Fiber Optic Attenuators?

A fiber optic attenuator, also called an optical attenuator, simulates the loss the would be caused by a long length of fiber. Typically, this device performs receiver testing. While an optical attenuator can simulate the optical loss of a long length of fiber, it cannot accurately simulate the dispersion that would be caused by a long length of fiber.

Put it simply, for a fiber optic receiver, too much light can overload it and degrade the bit error ratio. In order to achieve the best bit error ratio (BER), the light power must be reduced. Fiber optic attenuators fit the requirement perfectly. This can happen when the transmitter delivers too much power such as when the transmitter is too close to the receiver.

What Is a Fiber Attenuator and How Does It Work?

Attenuators are like your sunglasses, which absorbs the extra light energy and protect your eyes from being dazzled. Attenuators typically have a working wavelength range in which they absorb the light energy equally.

An important characteristic of a good fiber attenuator is that they should not reflect the light, instead, they should absorb the extra light without being damaged. Since the light power used in fiber optic communications are fairly low, they usually can be absorbed without noticeable damage to the attenuator itself.

Types of Attenuators

Two types of fiber optic attenuators exist: fixed value attenuators and variable attenuators.

Fixed Value Attenuators

Fixed value attenuators have fixed values that are specified in decibels. Their applications include telecommunication networks, optical fiber test facility, Local Area Network(LAN) and CATV systems.

For example, a -3dB attenuator should reduce intensity of the output by 3 dB(50%).

Fixed value attenuator's attenuation value cannot be varied. The attenuation is expressed in dB. The operating wavelength for optical attenuators should be specified for the rated attenuation, because optical attenuation of a material varies with wavelength.

Fixed value attenuators are composed of two big groups: In-line type and connector type. In-line type looks like a plain fiber patch cable; it has a fiber cable terminated with two connectors which you can specify types.

Connector type attenuator looks like a bulk head fiber connector, it has a male end and a female end. It mates to regular connectors of the same type such as FC, ST, SC and LC.

Variable Attenuators

Variable attenuators come with many different designs. They are general used for testing and measurement, but they also have a wide usage in EDFAs for equalizing the light power among different channels.

One type of variable attenuator is built on a D-shaped fiber as a type of evanescent field device. If a bulk external material, whose refractive index is greater than the mode effective index, replaces a part of the evanescent field reachable cladding, the mode can become leaky and some of the optical power can be radiated. If the index of the external material can be changed with a controllable mean, through the effects such as thermo-optic, electro-optic, or acoustic-optic, a device with controllable attenuation is achievable.

Other types of variable attenuators include air gap, clip-on, 3-step and more.

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