Fiber Optic Forum

n a market where lower back reflection is becoming a requirement, selecting the correct style of attenuator becomes more important. Attenuators, some more than others, do effect back reflection. There are a variety of methods used to achieve attenuation in an optical system:

- Air-gap style
- Film or translucent material
- Lens technology
- Intentional misalignment of fiber cores
- Micro/macro bending of the fiber

Perhaps the most common method being an air-gap style attenuator. This concept is used in spacer washers, bulkheads and some variable style attenuators. All of these styles by definition create back reflection due to the elimination of physical contact between the two mated fibers. A glass-air-glass environment is created, guaranteeing reflection.

Another approach used to create attenuation is to place a small piece of film or translucent material in a bulkhead mating sleeve. The films filtering ability reduces through light and causes back reflection that is detectable when used with SPC, UPC and APC polished connectors. Also, please note that angled connectors are not allowed to mate when inserted into a filtered attenuator.

Lens technology is also used to create attenuation. Proper lens selection and placement allows a lens created attenuation when terminating with low reflection connectors.

The intentional misalignment of fiber cores is another approach to achieve a low back reflection attenuation. The inline fixed attenuator patchcord commonly uses this technology and is commonly used in low back reflection requirements. For example, a l0dB singlemode in-line attenuator patchcord with UPC connectors has a typical back reflection reading of -53.2dB. The same patchcord, absent the attenuator, has a reading of -55dB. The 1.8dB increase in back reflection is a direct result of the in-line attenuator, but is insignificant when compared to a -24dB air gap reflection.

Micro/macro bending of the fiber is the attenuation technique which produces the lowest back reflection. Keep in mind, environmental stresses placed on fiber can have a physical or mechanical effect causing varying dB losses.

Selecting an attenuator is an important factor in obtaining low back reflection in fiber optic systems. In applications where low back reflection is not required, air-gap attenuators are the most economical method for attenuation. However when low back reflection is critical, micro/macro bending of fibers, lens type or in-line attenuators provide desired conditions.