Bend-Insensitive Fiber
Another important consideration in designing bend insensitive fibers is the ratio of the operating wavelength to the cutoff wavelength of the fiber. The cutoff wavelength is the lower limit of singlemode guidance and below that wavelength multiple modes are guided. Above the cutoff, the fiber operates single-mode and the size of the MFD increases with wavelength. Since the core remains the same size, a larger percentage of the mode travels in the cladding as you operate at wavelengths farther away from the cutoff and this light is in danger of being attenuated when the fiber is bent. Therefore the best bend performance is achieved by operating just above the cutoff wavelength of the fiber. As an example, operating at 1550nm with a 1500nm cutoff would offer good bend performance.
What is the minimum bend radius of a fiber?
How much can you safely bend an optical fiber? The answer to that is fiber specific and dependent upon whether you are concerned with the optical or the mechanical integrity of the fiber. Minimum optical bend radius pertains to the smallest radius the fiber can be bent to and still guide light without a rise in attenuation. Minimum mechanical bend radius pertains to the mechanical reliability of the fiber and how such bending affects the fiber lifetime. In fibers with low NA such as 0.11 and 0.12, the optical bend radius is the limiting factor. As the NA of fibers is increased (up to 0.21), the fiber will continue to guide light at smaller bend radii with low attenuation so that the minimum mechanical radius becomes the limiting factor.
Minimum optical bend radius - When a fiber is bent, the mode continues to be guided but the index of the glass can be considered to be distorted in the bent region. More light is radiated into the cladding where it is highly attenuated. The minimum radius of curvature (Rc) can be modeled for fibers with different Δ and operating-wavelength-to-cutoff-wavelength ratios. In general, the higher the NA the better the optical bend performance. As an example: a 0.21 NA fiber with cutoff at 1260 nm has an (Rc) of 2.77 mm at 1310 nm and 4.65 mm at 1550 nm. The attenuation rises very sharply as the Rc is approached.
Minimum mechanical bend radius – This is unrelated to fiber NA but is related to fiber diameter. When a fiber is bent, the outside of the bend is under tensile stress and the inside under compressive stress. The bending stress can be calculated:
σbending = E (r / R)
where: E = Young’s modulus = 10440 kpsi (72GPa)
r = Fiber radius
R = Bend radius
Time to failure can then be calculated using the Power Law Model:
Tf= (σp / σa)n
where: tf = Time to failure (seconds)
σp = Prooftest stress
σa = Application stress = bending stress or σ bending
n = Fatigue factor (typically 20 for non-carbon coated fibers)
There are 2 ways to achieve lower bend radius. One is to increase the fatigue factor (n) and the other is to increase the proof test level. Increasing the n-value or fatigue factor of the fiber can be done through the application of carbon as a primary coating. Carbon is a very thin coating of 0.03 to 0.04 µm, applied under acrylate, silicone or polyimide coatings, that hermetically seals the glass surface inhibiting the growth of microscopic surface cracks in the glass. It raises the fatigue factor from 20 to 100 or more.
Proof testing is a procedure that OFS performs on 100% of fibers manufactured. It puts the fiber under tensile load to a specific level (commonly 100kpsi or 200kpsi). If flaws exist, they will propagate and break under the load. The remaining lengths have a very high probability of being free from cracks that will compromise fiber lifetime. The recommended operation stress level is 20% of the proof test level for non-carbon coated fibers. Carbon-coated fibers can operate at 80% of the proof test level or can operate at a lower percentage of the proof test level for an extended lifetime.
For a 125 µm diameter fiber with a required 20 year lifetime:
At 100 kpsi proof test minimum bend radius is 18 mm
At 200 kpsi, 9 mm
For the same fiber with carbon coating:
At 100 kpsi proof test the minimum bend radius is 8 mm
At 200 kpsi, 4 mm
For an 80 µm diameter fiber with a required 20 year lifetime:
At 100 kpsi proof test minimum bend radius is 12 mm
At 200 kpsi, 6 mm
For the same fiber with carbon coating:
At 100 kpsi proof test the minimum bend radius is 5 mm
At 200 kpsi, 3 mm
Reduced Fiber Size 
Many sensing applications use 80µm fiber as the standard cladding size rather than the standard cladding size of 125µm. These reduced diameter or Micro fibers can provide improved mechanical bend performance for a specific bend diameter.
Future Bend Insensitive Designs
Looking beyond the typical step-index design, other solutions become possible such as microstructure fiber, where the material index of both core and clad are identical – the index of pure silica, which is 1.46. Rather than employing dopants to raise or lower the index of the glass, the cladding index is lowered by the presence of air holes. Because the index of air is 1, the effective index of the cladding area is less than the core, providing the correct condition for singlemode guidance. The greater the air to glass ratio, the greater the index Δ will be. With this method, NAs approaching 1 are possible.
One of the problems of increasing bend insensitivity by raising the index Δ is that the higher the Δ gets, the smaller the MFD becomes, holding the cutoff wavelength constant. This can increase the attenuation of interconnections. Hole Assisted Fiber (HAF) is a novel design being developed by Furakawa (parent company of OFS) and OFS Laboratories. This 80µm clad diameter fiber design combines the characteristics of standard fibers and microstructure fibers. The core is doped just as in a conventional fiber, but holes are added to the cladding region to create a larger delta. Designed to be several moded, all but the fundamental mode are stripped away when bent. The MFD is large at 9µm @ 1550nm and it also has excellent bend performance with <.001dB/km of loss at 7.5mm bend diameter.
Choosing a bend resistant fiber
OFS makes a range of single-mode fibers under the brand name Clearlite Photonic Fibers. The minimum mechanical bend radius is listed for each fiber in the specifications. Fibers with with higher NAs of 0.16, 0.17, 0.20 and 0.21 offer increasingly good bend performance. If you need assistance in selecting a fiber for your application, we can help. We will need the following information:
What is your application?
What is the operating wavelength?
What is the minimum bend radius your fiber will experience?
What is the expected fiber lifetime?
What is the smallest MFD you can tolerate?
Can carbon coating be used?
Can you use an 80µm Micro fiber?
Other requirements such as temperature range and environmental conditions.
