Out-Of-Band Bending Loss Measurements

Bending loss in singlemode fiber is wavelength dependent, so a result taken at one wavelength does not correlate directly to another. A common question is whether an existing 1550nm reflectometer, such as an LWA 7601-CL or OBR 4600, can be used to measure bending loss for applications that operate at 1310nm.  Conversely, can a LWA 7601-O or OBR 4613 be used to measure bending loss for applications that operate at 1550nm?  The short answer is that the measurement should be made at the operational wavelength.

Bend loss is very sensitive to wavelength

Because the mode field diameter is larger at longer wavelengths, 1550nm light strips out of the core at a given bend radius much more readily than 1310nm light does. A mandrel wrap that produces a clearly measurable loss at 1550nm can produce substantially less loss at 1310nm in the same fiber. A 1550nm measurement therefore overstates the bend loss the fiber will exhibit in a 1310nm application, and the offset is not a simple correction.

The reverse case is worth keeping in mind as well. Using an O-band instrument to characterize bends in a network that will operate at 1550nm understates the loss those bends will actually produce, so a set of measurements that looks well within budget at 1310nm can correspond to a link that fails its loss budget in service at 1550nm.

There is no fixed scaling factor between wavelengths

Published work on modeling bend loss in singlemode fiber indicates that the core and cladding structure, whether step index, graded index, depressed cladding or another profile, has a large effect on how bend loss varies with bend radius and wavelength. Therefore, the relationship is fiber-dependent, and not universal. It even differs between standard singlemode fiber and bend-insensitive singlemode fiber, which is designed specifically to suppress the effect. A single ratio applied to a 1550nm result will not reliably predict 1310nm loss across different fiber types.

What to do if only an out-of-band instrument is available?

Start by asking the fiber manufacturer for waveguide and bend performance data on the specific fiber, keeping in mind that some manufacturers publish considerably more of this data than others. With a comprehensive waveguide model or experimental data for that exact fiber, a 1550nm measurement can be used to predict 1310nm behavior, and vice-versa.  However, that requires modeling capability specific to the fiber in question rather than a generalized conversion formula. Without it, testing out-of-band will only give you a rough idea of what is going on.  Testing at the operating wavelength gives the most reliable data.