Nonlinear Fiber Optics - 4 ed. Agrawal

6.4. Vector Modulation Instability 205
Figure 6.12: Modulation-instability sidebands observed in a low-birefringence fiber. Pump is
polarized along the fast (top row) or slow axis (bottom row). Pump-probe detuning is 0.3 THz
for left and 1.2 THz for right columns. (After Ref. [61]; c○1998 OSA.)
used to seed the process. In a series of experiments, the pump beam was obtained from
a dye laser operating near 575 nm and consisted of 4-ns pulses that were wide enough
to realize quasi-CW operation. The pump-probe wavelength separation was tunable;
tuning allowed different regimes of modulation instability to be investigated. The fiber
was drawn using a rapidly rotating preform so that its intrinsic birefringence averaged
out to zero. A controlled amount of weak birefringence was introduced by winding the
fiber onto a 14.5-cm-diameter spool. The measured beat length of 5.8 m for the fiber
corresponded to a modal birefringence of only 10 −7 . The critical power P cr required
for the onset of polarization instability [see Eq. (6.3.10)] was estimated to be 70 W for
this fiber.
Figure 6.12 shows the pump spectra measured under several different experimental
conditions. In all cases, the pump power was 112 W (1.6P cr ) while the probe power
was kept low (∼1 W). Consider first the case of a pump polarized along the fast axis
(top row). For a pump-probe detuning of 0.3 THz, the probe frequency falls within the
gain spectrum of modulation instability (see Figure 6.9). As a result, the pump spectrum
develops a series of sidebands spaced apart by 0.3 THz. In contrast, the probe
frequency falls outside the gain spectrum for a detuning of 1.2 THz, and modulation
instability does not occur. When the pump is polarized along the slow axis (bottom
row), the situation is reversed. Now the 0.3-THz detuning falls outside the gain spectrum,
and modulation-instability sidebands form only when the detuning is 1.2 THz.
These experimental results are in agreement with the theory given earlier. In the time
domain, the pump pulse develops deep modulations that correspond to a train of dark
solitons with repetition rates in the terahertz regime [61]. A dark-soliton is also formed