Why would you need a bend-insensitive fiber?
Customers turn to specialty fiber manufacturers like OFS to provide fibers that have characteristics outside those of standard telecommunication fibers. Bend insensitivity is one of those characteristics since standard fibers are not designed for tight bends. In single-mode fibers the single or “fundamental” mode travels mostly in the fiber core but partly (30 to 50%) in the cladding. Because light in the cladding can easily be stripped away during macrobending, fibers that more closely confine the mode to the core are called for in many applications. Each of the following applications is made possible and more reliable with bend-insensitive fiber:
Towed Decoys - where fiber pre-wound on reels is further subjected to the stress of rapid payout into the airstream at jet speeds
Acoustic Sensor Arrays – where fiber containing multiple gratings is wrapped around small mandrels and deployed on the ocean floor or trailed behind submarines
Gyroscopes and other Interferometric Sensors – where fiber is wound into a coil of small diameter and used to measure rotation rate or a phase shift
Packaged Devices – the trend to smaller component size can require tighter bends for fiber packaged inside telecommunications devices and components.
How is bend insensitivity achieved in a single-mode fiber?
For light guidance to occur in conventional optical fibers, the index of refraction of the core is always higher than that of the surrounding cladding. Although the difference in index Δ is generally not large (<1%), the larger the value of Δ, the more bend insensitive the fiber will be. Since the base material of an optical fiber is fused silica, variations in the index of a core or cladding material is achieved by adding dopants to the glass. Dopants used to raise the index include germania (most commonly used) and phosphorous. Boron and fluorine can be used to reduce the index.
Using various dopants in different layers of the core and cladding makes a variety of designs possible. These designs are best shown in an index profile, which is a line graph of the index value across the diameter of the fiber. A matched clad design may use pure silica as the cladding material and germania dopant to raise the core index. Another common design called depressed clad fiber uses pure silica as the outer cladding, an inner cladding is created by down-doping, and the core area is updoped. More complex designs that create additional levels can be employed to achieve larger delta values.
