Is optical attenuation used in your fiber network environment?

A fiber optic attenuator is a passive optical device that is mainly used to reduce optical power of an optical signal to prevent signal distortion when optical power reaching optical receiver exceeds its transmission range; in optical fiber In a channel system, optical signal strength of each channel should be approximately same, so an optical fiber attenuator should be used to balance optical power of each channel. This tutorial details types and operation of fiber optic attenuators.

Why use fiber optic attenuators?

Excessive optical signal power will overload optical receiver, resulting in signal distortion, so a fiber attenuator must be used to reduce power. In another case, in a fiber optic system with multiple wavelength channels, it is necessary to make optical signal power of each channel approximately same, so a fiber optic attenuator is used to balance power of each channel.

Fiber optic attenuators are commonly used in two cases: one is permanently installed in fiber optic link to equalize optical signal strength between optical transmitter and optical receiver, and other is used to test optical power. signal, that is, after power of optical signal is reduced by a part using a fiber optic attenuator, then power margin of optical signal in fiber optic communication line is checked.

Types of optical attenuators

According to different attenuation methods, fiber optic attenuators can be divided into fixed fiber optic attenuators, continuously adjustable fiber optic attenuators, and continuously adjustable fiber optic attenuators.

Fixed fiber optic attenuator

A fixed fiber attenuator has a fixed attenuation of optical power in fiber optic channel, and theoretically attenuation can be set to any value. This optical attenuator has a variety of connectors, such as FC, SC, ST, LC, etc. It is easy to install and has a simple structure.

Variable fiber attenuator

The attenuation of optical power in a fiber with a graduated variable fiber attenuator is not fixed, it can be 0.1dB, 0.5dB or 1dB and so on. This fiber optic attenuator is typically used in applications with multiple optical power sources. For example, an optical receiver must receive optical signals from three different light sources. In order to keep power of each optical signal within allowable range of optical receiver, a step-adjustable fiber optic attenuator must be used for each optical power to be attenuated to a different degree.

Infinitely adjustable optical attenuator

The optical power attenuation of stepless optical fiber attenuator can be adjusted according to specific needs. It is commonly used in applications where input optical power and output optical power are constantly changing, and is used to quickly and accurately adjust optical power. in a fiber optic line.

How a fiber optic attenuator works

Fiber optic attenuators can use various methods to attenuate optical power by a given amount. General operating principles: bias loss, sweateri for absorption and reflection loss.

Loss of bias principle

Offset loss refers to intentional misalignment of an optical fiber when it is connected, so that some of optical power is lost in order to achieve goal of attenuation control. This principle can also be divided into two types: side dislocation and longitudinal dislocation. Side dislocation means that two optical fibers are not in a straight line in horizontal direction, and cores of two fibers have a certain dislocation. a straight line in horizontal direction, but there is a certain distance in middle, as shown in picture below:

The optical fiber attenuator using bias loss principle is very sensitive to mode field distribution near optical transmitter. It has been proven that farther optical fiber attenuator is from optical transmitter, worse attenuation effect is, so we must use bias type. Fiber optic attenuators are used after optical transmitter to provide expected optical attenuation.

Absorption loss principle

The principle of absorption loss is to attenuate optical power by absorbing part of optical signal. A fiber optic attenuator using this principle takes advantage of light absorption characteristics of material, and puts a sheet of optically polished glass with neutral absorption into a sheet or strip of neutral dark glass in optical path (as shown in figure below), thereby reducing the light optical signal power. One piece of light-absorbing glass can be made into a fixed attenuator, several pieces of light-absorbing glass can be made into a graduated variable fiber optic attenuator with a tape measure, and strip-shaped light-absorbing glass can be made into a continuous variable optical fiber attenuator with continuous bias fiber attenuator.

Reflection loss principle

An optical fiber attenuator using this principle mainly uses light reflection characteristic. We know that reflection and scattering of light is one of main sources of loss in optical signal transmission process. The optical fiber attenuator, using principle of reflection loss, adds a layer of reflective film to inner optical fiber (as shown in figure below) to effectively attenuate optical power.

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