Fiber Optic Attenuators


When looking at fiber optic attenuators, people always have several questions as to why would you purposely put attenuation on a fiber optic network. When testing fiber, attenuation is one of the key points that you are looking for. You want your attenuation to be as low as possible, so that you can fit the link budget that you are trying to meet or beat. For those that don’t know, attenuation is the amount of light or signal lost over a span or link. When building a network, customers are always looking to cover distances as far as possible and sometimes they add a fiber amplifier to boost signals to make sure that they reach to the end of where they are running fiber so that a signal will reach every customer or place that it needs to. Sometimes adding in an amplifier causes a higher power laser to be used, which in turn, if the signal is only going a short distance to your first location, can cause the light signal to be too strong when it hits the optical receiver. It can either distort the signal or it can burn up the optical receiver. This is where an attenuator that purposely adds in loss is useful and necessary. Attenuators come in several different types, and have many different levels of attenuation to assist in creating the perfect balance for a network. Let’s dig into the different types of fiber optic attenuators to give you a better idea of the different ways you can help to create loss and potentially save you money in equipment costs.



Male to Female Optical Attenuator


The common optical attenuator that I usually sell the most of is the male to female style attenuators. These attenuators are used at the end of a patch cord and typically get plugged in at the receiving side of your transmission. Some refer to these as in-line attenuators, because they are put directly into a switch and a patch cable is plugged in. These come in 1 db increments and go from 1dB to 25dB in attenuation levels. They come in the four most common fiber connector types in both UPC and APC polishes. Often the level of attenuation that is perfect for the application is unknown. Most customers will buy a hand full of each dB level from 1 to 10. When on a job the amount of attenuation needed varies so they have a variety and it becomes a “try it till it works” process. Meaning, if they start with 5dB and it is too much and shuts the signal off, you would know that you need less attenuation and your next level to try would be 4dB. Repeat the process until the desired attenuation is achieved.



Female to Female Optical Attenuator


There are attenuators that allow you to plug two male connectors to each other. These are the bulkhead style female to female attenuators. They tend to be wavelength specific. This means for the desired attenuation it is only guaranteed at the specified wavelengths - 1310nm or 1550nm. This style uses a wavelength sensitive neutral density filter that assists with achieving the proper level of attenuation. When using this type, they only come in four different dB levels. In increments of 5 - so 5, 10, 15 or 20dB is what you can get these in. Use this style when you know exactly the dB level of attenuation needed and what wavelength you need the attenuation to be on.



Variable Optical Attenuator


Rather than getting several different dB levels of attenuators, there are some other types of attenuators that allow you to change the amount of loss. One of these styles is called a variable optical attenuator. It uses a device in the middle of a patch cord that allows you to turn a screw to change the amount of attenuation. It ranges from the 1dB to 20 dB and works on both 1310nm and 1550nm wavelengths. This one will have a specific connector on each end. By turning a nut on the device it helps separate the connectors or helps to pull them back together. This will change the amount of loss in a system because connectors are meant to have the end faces touching to eliminate that loss.

Another type that we have is an air gap attenuator. These are only available for the ST and FC style connectors. This one uses its name and puts different levels of space (air) between a mated pair of fiber optic connectors. Air attenuators involve the use of “washers” to cause attenuation by creating distance between the end faces. The washers have different thicknesses to help change the amount of loss. This kind of washer style attenuator does not have specific dB levels as there are other factors that can affect the amount of loss at that point. They do however have different colors to help you to know which one you have tried through your test process. Unlike the male to female attenuators, the washers are placed at the light source or transmitter and not the receiving side.



Passive Optical Attenuators


Another form of attenuator is known as passive style attenuators. This is where you use a device to help attenuate a patch cord that is already in place. We have two different options that we would consider this type. One is a Clip On attenuator that can be used on a 3 millimeter jacketed fiber and has the ability to go from zero dB to 47dB for the wavelength of 1550nm. For a wavelength of 1310nm the levels will be zero to 21 dB. When the Clip On attenuator is put on a fiber, the way it works is by bending the fiber to cause the desired loss that is required. This device can be reused and does not affect the performance of the fiber once removed. The other one that would fall in this category would be the 3 step attenuator. This one only works on wavelength of 1310nm. There are three different ranges, 2-7dB, 5-15dB and 5-20dB. This device works by exceeding the recommended bend radius of the fiber without introducing any back reflection.

As you can see, attenuators can have a very important role in certain networks. They may sound counterproductive in the sense that when running fiber you are looking for the smallest amount of loss. Attenuators not only create loss but can assist in a network that has too much light going through which can affect the signal and cause data loss on a link. It is funny to think that a part that costs around 12 dollars can have a big impact on a network that cost a lot more to put together.