specialty optical fibers overview

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Specialty Optical Fibers - Overview

Specialty Optical Fibers - Overview

Specialty optical fibers have become an enabling technology for a variety of industries ranging from communications to oil and gas exploration, laser surgery, high power lasers, automotive and aerospace. Specialty fibers in these various systems can play the role of a sensing element, a laser cavity, an amplifying medium, or a filtering device to name a few. Verrillon is the leading supplier of optical fibers for harsh environments, used in temperature and pressure monitoring systems, gyroscopes, fiber lasers and communication components and modules.Examples of Specialty fibers and their Apps:

Bend-Insensitive Fiber:Optical fiber is sensitive to stress, particularly bending. When stressed by bending, light in the outer part of the core is no longer guided in the core of the fiber so some is lost, coupled from the core into the cladding, creating a higher loss in the stressed section of the fiber. If you put a visible laser in a fiber and stress it, you can see the light lost by the stress as in this multimode fiber

1. Fundamentals of Macro-bending in Multimode Fiber:Multimode fiber has many modes of light travelling through the core. As each of these modes moves closer to the edge of the core, it is more likely to escape, especially if the fiber is bent. In a traditional multimode fiber, as the bend radius is decreased, the amount of light that leaks out of the core increases.

In 2007, a new type of "bend-insensitive" singlemode fiber was introduced, followed by multimode fiber in 2009. Manufacturers liked to demonstrate this fiber by bending it around impossibly small bends or stapling it to a piece of wood - demonstrations that made veterans of the business cringe at seeing fiber treatedso badly! But the demonstrations showed that these fibers could be bent in what seemed like impossibly small radii without significant light loss.

2. How Does Bend Insensitive Fiber Work?Bend-insensitive fiber cables are designed for improved bend performance in reduced-radius applications. The fiber cables employ a moderately higher NA than standard single mode telecommunication fiber cables, and offer improved bend performance for applications in the 1310- and 1550-nm range.Optical fiber manufacturers used a refractive index trench, which means a ring of lower refractive index material, to basically reflect the lost light back into the core of the fiber. This trench configuration is shown below.

Adv:Assurance against problems caused by careless installation.Small incremental cost.

Comparison of StandardOFC vs BI fiber:Up to 10x better bend performance than standard 50um fiberHigh bandwidth OM3 and OM4 capabilityImproved optical performanceFully standards compliant; compatible with installed baseMay be connected with commercially available equipmentTermination Fiber:Optical devices - multi-ports - often have one or more open branches-Back reflections from these ports causes instabilities -needs to be suppressed.Sol: Use Termination Fiber! Eg: coreless silica construction.

Polarization-maintaining optical fiber:

PMForPM fiber isoptical fiberin which thepolarizationoflinearly polarized light waveslaunched into the fiber is maintained duringpropagation, with little or no cross-coupling of opticalpowerbetween the polarizationmodes.Apps: fiber optic sensing, interferometry,quantum key distribution.in telecommunications for the connection between a sourcelaserand amodulator, since the modulator requires polarized light as input.

Disadv:rarely used for long-distance transmission, because PM fiber is expensive and has higherattenuationthansinglemode fiber.

PM fiber maintains the existing polarization of linearly polarized light that is launched into the fiber with the correct orientation. If the polarization of the input light is not aligned with the stress direction in the fiber, the output will vary between linear andcircular polarization. The exact polarization --sensitive to variations in temperature and stress in the fiber.The output of a PM fiber is typically characterized by itspolarization extinction ratio(PER)the ratio of correctly to incorrectly polarized light, expressed indecibels.

The quality of PM patchcords and pigtails can be characterized with aPER meter.

Most work by inducingstressin thecorevia a non-circularcladdingcross-section, or via rods of another material included within the cladding. Several different shapes of rod are used, and the resulting fiber is sold under brand names such as "Panda" and "Bow-tie".

Light circles: cladding.Dark areas: core config.Goal -> use stress-applying parts to create slow and fast axes in the core.Each of these will guide light at diff velocity.Crosstalk b/n 2 axes is suppressed to maintain the state of polarized light, launched into either of the axes, as it travels along fiber.

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