Selecting the Right Attenuator 1

Fiber optic attenuators are devices that reduce signal power in fiber optic links by inducing a fixed or variable loss. They are used to control the power level of optical signals at the outputs of light sources and electrical-to-optical (E/O) converters. They are also used to test the linearity and dynamic range of photo sensors and photo detectors.
Fiber optic attenuators use several methods of attenuation. Examples include air gaps, microbends, acousto-optic modulators, and electro-optic modulators. Air gaps between optical fibers cause light to be reflected because of the change in refractive index. Microbends are sharp curvatures with local axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Microbending can cause significant radiative loss and mode coupling. Acousto-optic modulators use sound waves to modify the amplitude, frequency, or phase of light passing through an acousto-optic material.
Similarly, electro-optic modulators use an electric field to alter the characteristics of light passing through an electro-optic material.
Fiber optic attenuators can use single-mode and/or multi-mode optical cable. Single-mode fiber optic cable allows only one mode to propagate and features very small core diameters of approximately 8 μm. Single-mode cable permits signal transmissions at extremely high bandwidths and allows very long transmission distances. By contrast, multi-mode cable supports the propagation of multiple modes and features core diameters ranging from 50 to 100 μm. Multi-mode cable has a graded or stepped refractive index and allows the use of inexpensive light emitting diode (LED) light sources. With multi-mode cables, connector alignment and coupling is less critical than with single-mode fibers. Because of dispersion, however, multi-mode cable provides reduced transmission distances and transmission bandwidths.
Many types of connectors are used with fiber optic attenuators. Biconic connectors have precision-tapered ends for low insertion loss. D4 and FC Fiber Optic Attenuator are durable, zirconia- ceramic ferrules with a keyed body for repeatability. FC Fiber Optic Connector is used primarily with single-mode fibers, but are also used in telephone systems, instruments, and high- speed communication links. Designed for use in FDDI networks, FDDI connectors are 2.5 mm ferrules that include a fixed shroud. ESCON connectors have the same measurements, but use an adjustable shroud. LC connectors are high-precision, zirconia- ceramic ferrules that feature an RJ-45 push-pull housing and latching. MT-RJ connectors hold two fibers with a ferrule that is smaller than the one used in MTP connectors, devices that are threaded and well-suited for high-density applications. ST connectors are easy-to-assemble devices that feature a bayonet mounting system. They are used with both single-mode and multi-mode fibers in communications applications. SMA connectors include a low-cost, multi-mode coupling that is suitable for military applications. Loop back connectors are used to test transceiver systems.
Important specifications for fiber optic attenuators include wavelength range, attenuation range, resolution, polarization dependent loss (PDL), and return loss. Attenuation range measures the signal loss produced by fiber optic attenuators. For fixed attenuators, this is a single value. For variable attenuators, this is a range of values. Resolution measures attenuation sharpness. As a rule of thumb, higher resolutions indicate sharper distinctions between attenuation levels. PDL is the polarization dependent signal loss produced by fiber optic attenuators. Return loss is the ratio of reflected power to incident power. Expressed in decibels (dB), return loss also measures the amount of reflected power on a transmission line that is terminated or connected to a passive or active device. Some fiber optic attenuators are rack-mounted or include a pigtail. Others maintain the polarization of the incoming signal.