How to Select Fiber Optic Polarizers
Fiber optic polarizers (FOP) are placed inline to improve the extinction characteristics of fiber optic cable. They allow the transmission of only one polarization, blocking light in unwanted polarization states. Fiber optic polarizers are used because degradation in polarization extinction can cause noise interference and limit the performance of the entire fiber optic system.
Polarization of Light Waves
Light waves are highly susceptible to noise and interference. In order to avoid unwanted interference patterns and to improve signal performance in fiber optic systems, polarizers are used. The ideal optical fiber polarizer will transmit linearly polarized light with a high extinction ratio and low insertion loss.
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Linear polarization is the confinement of the electric or magnetic field to a plane along the direction of wave propagation.
Extinction ratio is the ratio of the power of a plane-polarized beam that is transmitted through a polarizer as compared with the transmitted power when the polarizer's axis is perpendicular to the beam's plane.
Insertion loss is the attenuation caused by the insertion of an optical component.
Fiber optic systems transmit light waves which are characterized by the length of the wave, referred to as the wavelength. The carrier signal is further distinguished by the optical power of the signal measured in dBm or mW.
Wavelengths detected by the human eye measure approximately from 400 to 700 nanometers. This is referred to as the visible region. Fiber optic systems typically transmit longer light waves from red (650nm) to the infrared region. This is 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 as illustrated bellow.
Image Credit: The Fiber Optic Association
The three prime wavelengths used in fiber optics systems are 850, 1300 and 1550 nm. Devices that operate at these wavelengths are supported and calibrated by NIST (the US National Institute of Standards and Technology). The 850nm wavelengths are primarily used in plastic optical fiber (POF) and multimode optical fiber. Multimode fiber also is used to transmit 1300nm carrier signals, while single mode fiber uses even longer wavelengths, 1310nm, and 1490nm to 1625nm.
The power of the optical signal is a measure of wavelength and photon density. Fiber optic communications use very low power signals. The most common units used to represent optical power are dBm and milliwatts (mW). dBm is a measurements where "m" refers to a reference power of 1 milliwatt. It is a logarithmic relationship, and a power level of 0 dBm is equivalent to 1 milliwatt. Furthermore, -10 dBm is 0.1 milliwatt and +10 dBm is 10 milliwatts.
Image Credit: The Fiber Optic Association, Inc.
Fiber optic polarizers are available in a two cable types, single mode and multimode, which will allow for 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 as much as an equivalent system using multimode fiber.
The following illustration outlines the most common construction 9/125. 9/125 is used to describe the core to cladding diameter ratio, 9 microns to 125 microns.
Image Credit: Multicom, Inc.
Multimode fiber optic cable supports the propagation of multiple modes. They typical have a core diameter of 50 to 62.5 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.
Image Credit: Multicom, Inc.
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.
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Fiber Optic Connections
Fiber optic polarizers mate with fiber optic cables by the use of optical fiber connectors. The connector is used to secure the polarizer to the cabling system and may come with or without pigtails. Connectors are selected by application and performance requirements. GlobalSpec's help file describes the most common connector types used with fiber optic polarizers.
Some fiber optic polarizers can be pigtailed into a cabling system. Pigtails are short optical fibers that are stripped and spliced into cabling systems.