Nonlinear Fiber Optics - 4 ed. Agrawal

196 Chapter 6. Polarization Effects
Figure 6.8: Transmittivity of a birefringent fiber of length L = L B as a function of input power
for different input angles. (After Ref. [34]; c○1986 OSA.)
intensity of these pulses exhibited 76-MHz modulation because of longitudinal-mode
beating inside the laser. These small-amplitude modulations were unaffected when the
signal was polarized near the slow axis of the fiber, but became amplified by as much
as a factor of 6 when input pulses were polarized near the fast axis. The experimental
results were in good qualitative agreement with theory, especially when the theory was
generalized to include twisting of the fiber that resulted in elliptical birefringence [45].
The power-dependent transmittivity seen in Figure 6.8 can be useful for optical
switching. Self switching of linearly polarized beams induced by polarization instability
has been demonstrated in silica fibers [46]. It can also be used to switch the
polarization state of an intense beam through a weak pulse. Polarization switching can
also occur for solitons [48]. In all cases, the input power required for switching is quite
large unless fibers with low modal birefringence are used. A fiber with a beat length
L B = 1 m requires P 0 ∼ 1kWifweuseγ = 10 W −1 /km in Eq. (6.3.10). This value
becomes larger by a factor of 100 or more when high-birefringence fibers are used. For
this reason, polarization instability is not of concern when highly birefringent fibers are
used because P 0 remains