Nonlinear Fiber Optics - 4 ed. Agrawal

446 Chapter 11. Highly Nonlinear Fibers
Figure 11.17: (a) SEM image of the cross section of a holey fiber fabricated with the extrusion
technique. The insets show details of the core and cladding regions. The dashed circle represents
the core with a 950-nm diameter. (b) Measured mode profiles together with the constant-intensity
contours in the transverse plane. (After Ref. [100]; c○2006 IEEE.)
the fiber composition [91]–[98]. Enhancement in n 2 by a factor of 500 was measured
in a 2000 experiment for the As 2 Se 3 glass [95]. By 2004, the same material with a
higher purity exhibited an enhancement in n 2 by a factor of about 1000 [97].
The 85-cm-long fiber used in the 2004 experiment had a 7-μm core diameter with
an effective mode area of about 40 μm 2 . Figure 11.19 shows the SPM-broadened spectrum
of 3.2-ps pulses at three peak power levels together with the measured Ramangain
spectrum for the same fiber when pumped at 1550 nm. The observed spectral
broadening is consistent with a value of γ = 2450 W −1 /km. Using A eff = 40 μm 2 , n 2 is
deduced to be 2.4×10 −17 m 2 /W, a value about 1000 times larger than that of silica. At
the same time, the Raman gain g R for this material is about 5.1 × 10 −11 m/W at pump
wavelengths near 1.55 μm, a value nearly 800 times larger than that of silica fibers.
Figure 11.18: Dispersion parameter D as a function of wavelength for lead-silicate fibers with
core diameters ranging from 0.5 to 1.4 μm. The dispersion curve for SF57 bulk glass is provided
for comparison. (After Ref. [100]; c○2006 IEEE.)