Specialty Optical Fibers - Overview

Specialty optical fibers have become an enablingtechnology for a variety of industries ranging fromcommunications to oil and gas exploration, lasersurgery, high power lasers, automotive andaerospace. Specialty fibers in these varioussystems can play the role of a sensing element, alaser cavity, an amplifying medium, or a filteringdevice to name a few. Verrillon is the leadingsupplier of optical fibers for harsh environments,used in temperature and pressure monitoringsystems, gyroscopes, fiber lasers andcommunication components and modules.

Examples of Specialty fibers and their Apps:

Bend-Insensitive Fiber:

• Optical fiber is sensitive tostress, particularly bending.When stressed by bending,light in the outer part of thecore is no longer guided in thecore of the fiber so some islost, coupled from the coreinto the cladding, creating ahigher loss in the stressedsection of the fiber. If you puta visible laser in a fiber andstress it, you can see the lightlost by the stress as in thismultimode 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 thisfiber by bending it around impossibly small bends or stapling it to a piece ofwood - demonstrations that made veterans of the business cringe at seeingfiber treated so badly! But the demonstrations showed that these fibers couldbe 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 Standard OFC vs BI

fiber:

•Up to 10x better bend performance than standard 50um fiber

•High bandwidth OM3 and OM4 capability

•Improved optical performance

•Fully standards compliant; compatible with installed base

•May be connected with commercially

available equipment

Termination 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:PMF or PM fiber is optical fiber in which

the polarization of linearly polarized light waves launched into the fiber is maintained during propagation, with little or no cross-coupling of optical power between the polarization modes.

Apps: • fiber optic sensing, interferometry , quantum key

distribution.

• in telecommunications for the connection between a source laser and a modulator, since the modulator requires polarized light as input.

• Disadv:

rarely used for long-distance transmission, because PM fiber is expensive and has higher attenuation than singlemode 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 and circular polarization.

• The exact polarization --sensitive to variations in temperature and stress in the fiber.

•The output of a PM fiber is typically characterized by its polarizationextinction ratio (PER)—the ratio of correctly to incorrectly polarizedlight, expressed in decibels.

•The quality of PM patchcords and pigtails can be characterized witha PER meter.

•Most work by inducing stress in the core via a non-circular cladding cross-section, or via rods of another materialincluded 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.