Low birefringence fibers
- Very low intrinsic birefringence
Low birefringence definition
Low birefringence fibers are optical fibers that can propagate optical radiation, preserving the state of polarization for long distances. Ideally this means that the fiber must have perfect geometry and be completely symmetrical along the optical axis. It must also be homogeneous along the axis. Any linear polarization can be represented by two linear polarizing modes orthogonal to each other. An ideal low birefringence fiber will propagate these two modes with identical velocity. In a real fiber there are a number of imperfections such as ellipticity, eccentricity, microbending, bending etc. Such imperfections lead to a difference in velocity between the two polarization modes of the fiber and hence a phase difference (retardance) between them.Standard telecomm type fibers typically have a large retardance that accumulates with length. The LB series fibers have a very low retardance that is not a linear function of length.
Application
These fibers have many uses, one of the most popular is fiber sensors. In a Faraday sensor, the fiber is wound in a coil and a conductor is passed through the coil. Polarized light in the fiber will have its plane of polarization rotated in proportion to the current passing through the conductor.Faraday effect
The strength of the Faraday effect is determined by the Verdett constant which is about V = 2.8 x 10-4 deg/ Oe cm for pure fused silica. It means that an axial magnetic field of 100 Oe acting on a straight 10 m length of fiber will rotate the polarization axis about 30 degrees. Note that such rotation is almost insensitive to temperature changes for pure silica, but if attempts are made to increase the verdett constant by doping silica with paramagnetic ions, there can be a big temperature dependence of retardance. We believe that pure silica low birefringence fiber is the optimal type for magnetic field sensors.
Other applications
In some applications where polarized light must be transmitted, it is not always convenient to align the fiber with the plane of polarization of the input signal that is essential when using HiBi fiber. In these situations LoBi fiber can more conveniently be used.Coating
The type coating used on low birefringence fiber is important if the properties of the fiber are to be preserved. We use a special low stress coating that maintains the low birefringence properties of the fiber.LB Series
These fibers are available in 80 µm and 125 µm diameters and for operating wavelengths of 633 nm, 850 nm, 1300 nm and 1550 nm.| LB600 | LB800 | LB1300 | LB1500 | |||||
| Operating wavelength (nm) | 633 | 850 | 1300 | 1550 | ||||
| Attenuation (dB/km) | <12 | <5 | <2 | <2 | ||||
| Cut-off wavelength | <600 | <800 | <1250 | <1400 | ||||
| Mode field diameter | 3.5 | 5 | 9 | 10 | ||||
| Numerical aperture (N.A) | 0.1 | 0.1 | 0.1 | 0.1 |
