Nonlinear Fiber Optics - 4 ed. Agrawal

204 Chapter 6. Polarization Effects
Figure 6.11: Spectral sidebands observed at the output of a 51-m-long high-birefringence fiber
when the pump is polarized at 45 ◦ from the principal axes. Black and gray traces correspond to
light polarized along the fast and slow axes, respectively. The central multipeak structure results
from the scalar modulation instability. (After Ref. [69]; c○2005 OSA.)
modulation instability was seen when the input field was polarized at a 45 ◦ angle from
the principal axes [69]. Figure 6.11 shows the observed spectrum at the output of a 51-
m-long fiber, with a birefringence-induced differential group delay of 286 fs/m, when
3.55-ns pulses at a 2.5-kHz repetition rate (with an average power of about 1 mW) were
launched into it. The central multipeak structure is due to scalar modulation instability,
but the two extreme peaks result from vector modulation instability. These two peaks
are orthogonally polarized along the fast and slow axes, a feature unique to the vector
modulation instability.
Modulation instability in low-birefringence fibers was observed in 1995 using 60-
ps pulses (with peak powers >1 kW) obtained from a krypton-ion laser operating at
647 nm [58]. Fibers used in the experiment were a few meters long, and their birefringence
was controlled through stress induced by winding the fiber on a spool with a relatively
small diameter. When input pulses were polarized along the slow axis, the two
sidebands indicative of modulation instability had the same polarization and were polarized
along the fast axis. Their spacing could be varied over a range of 20 nm or so by
simply changing the spool size (a smaller spool diameter produces more stress-induced
birefringence), resulting in larger sideband spacing. In a variation of this idea, fibers
with periodically varying birefringence along their length were produced by wrapping
the fiber around two spools [59]. Such a periodic variation can create new sidebands
through quasi-phase matching, similar to the periodic variation of dispersion and nonlinearity
discussed in Section 5.1.
A systematic study of induced modulation instability in low-birefringence fibers
was performed in 1998 using a pump-probe configuration [61]. The probe beam was