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.
Methods of Attenuation
Fiber optic attenuators use several methods of attenuation including 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 radiant 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.
Electro-optic modulators use an electric field to alter the characteristics of light passing through an electro-optic material.
Fixed and Variable Attenuators
Fiber optic attenuators are available with either a fixed or variable attenuation range.
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Fixed fiber optic attenuators are used to reduce the optical power signal in communication links. They work analogous to a step-down transformer. As the signal approaches a device or node in a communication link the power is reduced to a level that is suitable for its application. Communication links such as LAN, CATV and telecommunication networks use fixed fiber optic attenuators.
Variable fiber optic attenuators offer a range of attenuation values. They are used for testing and measurement, or when you need to equalize the power between different signals.
Fiber optic attenuators are characterized by a wavelength range and attenuation range.
The electromagnetic spectrum is the full bandwidth of all wavelengths. What we refer to as the visible spectrum is a range of light waves that can be detected by the human eye. Fiber optic systems transmit light waves from the far visible region, red (650nm), to the near infrared region (1700nm). This range is used due to characteristics of the transport medium, the optical fiber. Shorter wavelengths are attenuated due to scattering of the light source and absorption bands at certain frequencies further attenuate the signal. The most common wavelengths used for fiber optic transmissions are illustrated below.
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Attenuation, measured in decibels (dB), is a value used to describe the ratio of input power to output power. The value is equal to ten times the logarithm of input power over output power.
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Fiber Optical Cable
Fiber optic attenuators are available in a two cable types, single mode and multimode, which will allow either single or multiple paths for light to travel through the fiber respectively. Multimode cables limit the distance that the signal can travel as multiple paths of transmission cause modes of light to disperse, limiting the transmission bandwidth.
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Single mode fiber optic cable will allow only one mode to propagate. The fiber has a very small core diameter of approximately 9 µm. It permits signal transmission at extremely high bandwidth and allows very long transmission distances. These fibers are often coupled with laser diode light sources and can cost two to four times as much as an equivalent system using multimode fiber.
Multimode fiber optic cable supports the propagation of multiple modes. They typically have a core diameter of 50 to 100 µm with a refractive index that is graded or stepped. They allow the use of inexpensive light emitting diode (LED) light sources and connector alignment and coupling is less critical than single mode fiber. Distances of transmission and transmission bandwidth are less than with single mode fiber due to dispersion.
Graded index multimode fiber contains a central core of the fiber that has a high index of refraction that gradually reduces towards the peripheral of the optical fiber. Modes of light that propagate along the central core of the fiber move slower while other modes of light are refracted toward the core of the fiber shortening the distance they travel and allowing for individual pulses to travel in unison.
Step index multimode fiber is the least expensive fiber and the dispersion rate is greater than all other optical fibers limiting the bandwidth and transmission distance. This is due to the speed and distance variation between each mode of light traveling in a given pulses. The reflected modes travel a greater distance causing dispersion and reflection of the light source.
Fiber Optic Connectors
Many types of connectors are used with fiber optic attenuators.
Biconic connectors have precision-tapered ends for low insertion loss.
D4 and FC connectors are durable, zirconia-ceramic ferrules with a keyed body for repeatability. FC connectors are used primarily with single-mode fibers, but are also used in telephone systems, instruments, and high-speed communication links.
FDDI connectors, designed for use in FDDI networks, 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.