Nonlinear Fiber Optics - 4 ed. Agrawal

3.2. Dispersion-Induced Pulse Broadening 61
10 2 Distance, z/L D
m = 4
Broadening Factor
10 1
10 0
2
1
C = 5
10 −1
0 0.5 1 1.5 2 2.5 3 3.5 4
Figure 3.5: Broadening factor σ/σ 0 as a function of z/L D in the anomalous-dispersion regime
for chirped super-Gaussian pulses with C = 5. The case m = 1 corresponds to a Gaussian pulse.
Pulse edges become steeper with increasing values of m.
drawn for the case of initially chirped pulses with C = 5. When the pulses are initially
chirped, the magnitude of pulse broadening depends on the sign of the product β 2 C.
The qualitative behavior is similar to that shown in Figure 3.2 for the case of a Gaussian
pulse (m = 1), although pulse compression is reduced considerably for super-Gaussian
pulses.
3.2.5 Experimental Results
The initial compression of chirped pulses has been observed experimentally using
pulses emitted from a directly modulated semiconductor laser. In one experiment [10],
the incident pulse at a wavelength of 1.54 μm was positively chirped (C > 0). It compressed
by about a factor of 5 after propagating 104 km in the anomalous-GVD regime
of a fiber with β 2 ≈−20 ps 2 /km. In another experiment, the semiconductor laser emitted
a negatively chirped pulse (C < 0) at a wavelength of 1.21 μm [11]. After propagating
a distance of 1.5 km in the normal-dispersion regime (β 2 = 15 ps 2 /km), the
pulse width decreased from 190 to 150 ps. When the fiber length was increased to
6 km, the pulse width increased to 300 ps, in agreement with the qualitative behavior
shown in Figure 3.2. In a different experiment much shorter optical pulses (initial
FWHM ≈ 26 ps) at 1.3 μm were obtained from a distributed-feedback (DFB) semiconductor
laser by using the gain-switching technique [15]. As the pulses were negatively
chirped (C < 0), a dispersion-shifted fiber was employed with a positive GVD at
1.3 μm(β 2 ≈ 12 ps 2 /km). The pulse compressed by a factor of three after propagating