What are Fiber Optic Attenuators?
Fiber optic attenuators are used in applications where the optical signal is too strong and needs to be reduced. There are many reasons why this need may arise. You may need to equalize channel strength in a multi-wavelength system or reduce the optical level to meet the input specifications of an optical receiver. In both of these cases, a point reduction in optical signal strength is required.
Types of Fiber Optic Attenuators
Optical attenuators can take a number of different forms and are typically classified as fixed or variable. Fixed attenuators can be broken down into either build out style or incorporated into a patch cord. The build out variety is a small (~ 1.25 inch long) attenuator with a male connector interface on one end and a female interface connector on the opposite end. The build out style is typically fabricated with either air gap attenuation or doped fiber attenuation.
Air Gap Attenuators
In air gap attenuators, the specified loss of optical power is accomplished by two fibers that are separated by air to yield the correct loss. Air gap attenuators are susceptible to dust contamination and can be sensitive to moisture and temperature variations. And if air gap attenuators are applied in multi-channel analog systems such as those used by CATV companies, it can be a huge mistake as the attenuator itself can create second order distortions that hamper the performance of the system.
Doped Fiber Attenuators & Benefits
Doped fiber attenuators are exactly as the name implies. A short piece of fiber with metal ion doping that provides the specified attenuation and the interface between the male and female connections on the attenuator. The doped fiber attenuator is the preferred component for a number of reasons. Doped fiber is not susceptible to dirt build up, moisture, or temperature variations. And they are made to perform with stability over wide wavelength variations and band passes. Attenuation incorporated into a patch cord can be fabricated with either a fusion splice or the leg of a fused coupler. These types can sometimes be wavelength sensitive because of how they are fabricated.
Doped fiber attenuators are the preferred attenuators for all of the reasons provided above in the appropriate point reduction of an optical signal. However, they can also be mistakenly applied when testing the performance of optical equipment and using attenuators (in place of real fiber lengths) to simulate a fiber length/distance or network. Only real fiber lengths simulate the various chromatic and polarization dispersions with which real systems must operate. Therefore, real fiber should always be used whenever possible to test equipment that will utilize optical fiber in the field.
