Fiber Optics Attenuators - The Ultime Guide on How they work?

An optical attenuator is a passive device used to reduce the power level of an optical signal, either in free space or in an optical fiber. There are various types of them from the fixed ones, step-wise variable, and continuously variable.

Attenuators are usually used when the signal arriving at the receiver is too strong and hence may overpower the receiving elements. This may occur because of a mismatch between the transmitters/receivers, or because the media converters are designed for a much longer distance than for which they are being used.

Sometimes attenuators are also used for stress testing a network link by incrementally reducing the signal strength until the optical link fails, determining the signal’s existing safety margin.

Although fiber optic attenuators are normally used in SM (Single Mode) circuits, because this is where the stronger lasers are used for distance transmission, there are also multi mode attenuators available.

The most common version of attenuators are male to female units, often called plug-style or buildout style. These plug-style attenuators simply mount on one end of a fiber optic cable, allowing that cable to be plugged into the receiving equipment or panel.

beyondtech attenuators available

There are also female to female (bulkhead) attenuators, often used to mount in patch panels or for connecting two fiber optic cables together. More expensive, but useful for testing, are variable attenuators which are adjustable between 1dB and 30dB.

Bear in mind that the dB ratings are a measure of signal strength and can sometimes be confusing. The chart below will give you an idea of the percent of attenuation of your signal for specific dB values.

Fiber optic attenuators are usually used in two scenarios.

The first case is in power level testing. Optical attenuators are used to temporarily add a calibrated amount of signal loss in order to test the power level margins in a fiber optic communication system. In the second case, optical attenuators are permanently installed in a fiber optic communication link to properly match transmitter and receiver optical signal levels.

How many types of Optical Attenuators (OA) can you find?

There are four different types of OA and they can take a number of different forms and are typically classified as fixed or variable attenuators. What's more, they can be classified as LC, SC, ST, FC, MU, E2000 etc. according to the different types of connectors.

1. Fixed Attenuators: Fixed optical attenuators used in fiber optic systems may use a variety of principles for their functioning. Preferred attenuators use either doped fibers, or misaligned splices, or total power since both of these are reliable and inexpensive.


Inline style attenuators are incorporated into patch cables. The alternative build out style attenuator is a small male-female adapter that can be added onto other cables.

Non-preferred attenuators often use gap loss or reflective principles. Such devices can be sensitive to modal distribution, wavelength, contamination, vibration, temperature, damage due to power bursts, may cause back reflections, may cause signal dispersion etc.

2. Loopback Attenuators: Loopback fiber optic attenuator is designed for testing, engineering and the burn-in stage of boards or other equipment. Available in SC/UPC, SC/APC, LC/UPC, LC/APC, MTRJ, MPO for single mode application. 

3. Built-in Variable Attenuators: Built-in variable optical attenuators may be either manually or electrically controlled. A manual device is useful for one-time set up of a system, and is a near-equivalent to a fixed attenuator, and may be referred to as an "adjustable attenuator". In contrast, an electrically controlled attenuator can provide adaptive power optimization.

Attributes of merit for electrically controlled devices, include speed of response and avoiding degradation of the transmitted signal. Dynamic range is usually quite restricted, and power feedback may mean that long-term stability is a relatively minor issue.

The speed of response is a particularly major issue in dynamically reconfigurable systems, where a delay of one millionth of a second can result in the loss of large amounts of transmitted data.

Typical technologies employed for high-speed response include liquid crystal variable attenuator (LCVA), or lithium niobate devices.

There is a class of built-in attenuators that is technically indistinguishable from test attenuators, except they are packaged for rack mounting, and have no test display.

4. Variable Optical Test Attenuators: this type generally uses a variable neutral density filter. Despite the relatively high cost, this arrangement has the advantages of being stable, wavelength insensitive, mode insensitive, and offering a large dynamic range.

Other schemes such as LCD, variable air gap etc. have been tried over the years, but with limited success.

They may be either manually or motor control. Motor control gives regular users a distinct productivity advantage since commonly used test sequences can be run automatically.

attenuation calibration

Attenuation calibration could be a real issue for the fiber infrastructure. The user typically would like an absolute port to port calibration. Also, calibration should usually be at a number of wavelengths and power levels, since the devices is not always linear. However, a number of instruments do not, in fact, offer these basic features, presumably in an attempt to reduce cost. The most accurate variable attenuator instruments have thousands of calibration points, resulting in excellent overall accuracy in use.


Calibration should usually be at a number of wavelengths and power levels since the device is not always linear. 

In recent years, there appeared many technologies on the manufacture of the variable optical attenuator, including mechanical VOA, magneto-optical VOA, LCD VOA, MEMS VOA, thermo-optic VOA and acoustic-optic VOA.

attenuation automation

Test Automation: Test sequences that use variable attenuators, can be very time-consuming. Therefore, automation is likely to achieve useful benefits. Both bench and handheld style devices are available.

How do Optical Attenuators work?

The power reduction is done by such means as absorption, reflection, diffusion, scattering, reflection, diffraction, and dispersion, etc.

Optical attenuators usually work by absorbing the light, like sunglasses absorb extra light energy.

They typically have a working wavelength range in which they absorb all light energy equally.

They should not reflect the light or scatter the light in an air gap since that could cause unwanted back reflection in the fiber system. Another type of attenuator utilizes a length of the high-loss optical fiber, that operates upon its input optical signal power level in such a way that its output signal power level is less than the input level.

Optical Attenuator Performance:

  • Amount of attenuation and insertion loss: insertion loss and the attenuation amount of the optical attenuator is an important indicator of the amount of attenuation of the optical attenuator indicator to actually insertion loss, and attenuation amount of the variable attenuator addition, there are separate indicators insertion loss, high quality can be variable attenuator insertion loss 1.0dB or less, in general, common variable attenuator of the index is less than 2.5dB can be used. When the actual selection adjustable attenuator insertion loss as low as possible. 
  • Optical attenuator accuracy: attenuation accuracy is an important indicator of the optical attenuator.
Typically mechanical type variable optical attenuator for attenuation accuracy of ± 0.1 times that amount. Its size depends on the degree of processing of precision mechanical components. High attenuation accuracy fixed optical attenuator. Typically the higher the attenuation accuracy, the higher the price.
  • Return loss: an important indicator of the impact of system performance in optical device parameters return loss.

The retroreflective optical network system effects are well known. Optical attenuator Return loss is the light energy incident on the optical attenuator and the attenuator light energy incident along the road reflecting ratio.

For now, you can understand how fiber optics attenuators work, and you also are aware of the importance of them for your fiber infrastructure. That's why Beyondtech has them available at our several distribution locations for 24 hours shipping and they were carefully tested each one of them for your reliability and for a complete solution-oriented approach.  

July 31, 2017 by
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