Nonlinear Fiber Optics - 4 ed. Agrawal

11.5. Non-Silica Fibers 447
Figure 11.19: (a) SPM-broadened spectrum of 3.2-ps pulses at peak power levels of 0.1, 1.5,
and 2.7 W. (b) Measured Raman-gain spectrum of the same fiber when pumped near 1550 nm.
(After Ref. [97]; c○2004 OSA.)
Because of their highly nonlinear nature, chalcogenide fibers have attracted considerable
attention for applications related to nonlinear fiber optics, in spite of their
relatively high losses. Their use for making fiber gratings and nonlinear switches can
reduce the required power considerably [92]. The values of γ can be further enhanced
by reducing the core diameter and adopting a microstructure for the cladding with air
holes. A chalcogenide holey fiber was made in 2000 using gallium and lanthanum
sulphide for the glass composition [79].
Tellurite glasses offer a value of n 2 ≈ 2.5 × 10 −19 m 2 /W, about 10 times larger
than that of silica [89]. At the same time, the peak Raman gain for them is up to 30
times larger when compared with silica glass [104]. In a 2003 experiment, tellurite
glass with the composition of 5% Na 2 CO 3 , 20% ZnO, and 75% TeO 2 was used to
fabricate a microstructured fiber [105]. Figure 11.20 shows the cross section of this
fiber together with the transmission view observed with a microscope. The fiber had
a core diameter of 7 μm. As the core was suspended by 100-nm-thick strands, it was
surrounded mostly by air, resulting in a strong confinement of the optical mode and an
effective mode area of only 21.2 μm 2 . The estimated value of γ was48W −1 /km for
this fiber. In another tellurite fiber with A eff = 2.6 μm 2 , the value of γ was measured to
be 580 W −1 /km, while ensuring that the fiber supported a single-mode at a wavelength
near 1050 nm [101].
Fibers based on bismuth oxide (Bi 2 O 3 ) have attracted considerable attention in
recent years [106]–[109]. Such a fiber provided in 2002 a value of 64.2 W −1 /km for
the nonlinear parameter γ [99]. By 2004, a bismuth-oxide fiber exhibited a value of
γ = 1360 W −1 /km at a wavelength of 1.55 μm when its core diameter was reduced
to 1.7 μm [106]. The refractive indices of the core and cladding were 2.22 and 2.13,
respectively, resulting in a numerical aperture of 0.6. The effective mode area of this
fiber was estimated to be only 3.3 μm 2 . The resulting value of n 2 ≈ 1.1 × 10 −18 m 2 /W
for Bi 2 O 3 is larger by a factor of about 50 compared with that of silica. Only 1-m
length of such a highly nonlinear fiber was needed to make a FWM-based wavelength
converter capable of operating at 80 Gb/s [107]. Because of their highly nonlinear