Fiber Optic Attenuators

What is an attenuator?

A fiber attenuator is a passive device that is used to reduce the power of an optical signal as it passes through the optical fiber. Although strong signal power is needed to achieve long-haul data transmission, too much can cause an overload in the fiber optic receiver and degrade the bit error ratio (BER).

The role of an attenuator is to reduce the signal power of light by absorbing it to prevent overload and degradation. Think of it as sunglasses that absorb extra light energy or an air gap that scatters light.

Optic attenuators can be installed permanently in a fiber optic link or temporarily added for the purpose of fiber optic power level testing. Permanent installation ensures that signal levels at the transmitter and receiver are properly matched, while temporary use adds a calibrated amount of signal loss to test a fiber optic system’s power lever margins.

Two Common Fiber Optic Attenuators

1. Fixed optical attenuator

As the name suggests, a fixed fiber attenuator has unchanging attenuation levels expressed in decibels (dB). It is often used in single-mode systems that has too much power that can saturate receivers. These include telecommunication networks, local area networks (LANs), CATV systems, and optical fiber test facilities.

It consists of an in-line type and connector type. The former resembles a plain fiber patch cable that has a fiber cable terminated with two connectors, the type of which you can specify. The latter, on the other and, is like a bulkhead fiber connector with two connector interface on opposite ends--one male and the other female.

Connector type attenuators mate with regular connectors (FC, LC, ST, and SC) of a similar type.

2. Optical variable attenuator

Also known as a variable optical attenuator, this type of attenuator uses a variable neutral density fiber. It is generally used for measurement and testing, and equalizes light power among different channels in Erbium-Doped Fiber Amplifier (EDFA).

Advantages of an Optical Variable Attenuator

• Stable and offers a large dynamic range
• Wavelength insensitive
• Mode insensitive

Optical variable attenuators are commonly available as a stepwise variable attenuator or a continuously variable attenuator. But they can also be made as a plug-in card.

Fiber Optic Attenuators Specifications

In choosing optic attenuators, several important specifications are considered.

• Wavelength range is the spectrum of light in the infrared region. Normal wavelengths are 850, 1300 and 1550 nm.
• Attenuation range is the measure of the signal loss produced by the attenuator. It is a single value for fixed attenuators and a range of values for variable attenuators. It is measured in decibels over a set distance (dB/km). The loss of optical power usually happens when the fibers are joined or during fusion splicing and mechanical splicing.
• Resolution is the measure of the sharpness of attenuation.
• Polarization dependent loss (PDL) is the loss of signal produced by a fiber attenuator.
• Return loss is also known as back reflection or reflectance and is the ratio of reflected power to incident power.