Nonlinear Fiber Optics - 4 ed. Agrawal

11.1. Nonlinear Parameter 431
Figure 11.4: Experimental setup for measuring n 2 using a technique based on FWM-induced
sidebands. (After Ref. [12]; c○1993 IEEE.)
operating at 1.553 μm was modulated externally to produce 25-ns pulses at a repetition
rate of 4 MHz. Such pulses were amplified using two cascaded fiber amplifiers and then
launched into a 10.1 km of DSF. Spontaneous-emission noise added by the amplifiers
provided a seed for the modulation instability. As a result, the optical spectrum at the
fiber output exhibited two well-developed sidebands, similar to those seen in Figure
5.2. The amplitude of these sidebands changed with the peak power of pulses and was
used to deduce a value n 2 = 2.35 × 10 −20 m 2 /W for the test fiber.
11.1.5 Variations in n 2 Values
As seen in Table 11.1, the measured values of n 2 for silica fibers vary in a relatively
wide range extending from 2.23 to 3.95×10 −20 m 2 /W, depending on the fiber type and
the technique used. To understand the reason behind such a large range of variations in
n 2 values, one should first note that the core and the cladding of silica fibers are doped
with other materials (such as GeO 2 and fluorine) to ensure that their refractive indices
differ by a small amount (typically