Nonlinear Fiber Optics - 4 ed. Agrawal

494 Chapter 12. Novel Nonlinear Phenomena
Figure 12.34: Averaged spectra (bottom traces) and the degree of coherence (top traces) at fiber
output when input pulses at 850 nm are launched with 10 kW peak power and their width is
varied from 150 to 50 fs. (After Ref. [99]; c○2002 IEEE.)
Coherence degradation of a supercontinuum has been observed in several experiments
[137]–[141]. In one experiment in which 100-fs pulses were launched into a
6-cm-long tapered fiber, the GVD of fiber at the wavelength of input pulses was found
to play a crucial role [140]. As expected, coherence was high in the normal-GVD
regime, as soliton fission did not occur, but the bandwidth of supercontinuum was also
limited to 200 nm. Close to the ZDWL (around 820 nm), coherence was nonuniform
across the supercontinuum with spectral regions of low and high coherence. In the
anomalous-GVD regime, coherence degraded severely at 860 nm, but was relatively
high at a wavelength of 920 nm. In this case, the supercontinuum extended from 500
to 1300 nm with mean value of g 12 ≈ 0.7 across the entire bandwidth. These results
can be understood by noting that an increase in β 2 has the same effect as reducing the
pulse width since both the soliton order N and the dispersion length L D decrease for
larger values of β 2 .
The noisy nature of a supercontinuum has practical implications. For example, it
limits the stability of supercontinuum-based frequency combs useful for metrological
applications [142]. It also sets a fundamental limitation on the compression of ultrashort
pulses when their spectra are broadened using a highly nonlinear fiber [143].
Even when a CW laser is used for generating a supercontinuum, the coherence of
the CW laser affects the noise at the fiber output. In one experiment, output of a
low-coherence semiconductor laser operating at 1480 nm was amplified to 1.6 W before
launching it into a 5-km-long highly nonlinear fiber [144]. The relative intensity
noise (RIN) of the resulting supercontinuum was found to be enhanced by more than
15 dB/Hz across a bandwidth >1 GHz. Temporal traces of the fiber output exhibited
power fluctuations that often exceeded 50% of the average power level and had
a standard deviation of 35%. Numerical simulations based on Eq. (12.4.1), modified
to include spontaneous Raman noise, show that soliton fission plays an important role
and leads to enhanced intensity noise [147]. They also show that the smoothness of