Nonlinear Fiber Optics - 4 ed. Agrawal

12.4. Temporal and Spectral Evolution 487
Figure 12.28: Predicted spectra when 27-ps pulses with 1.7 kW peak power propagate inside
a microstructured fiber together with second-harmonic pulses at 450 nm with only 5.6 W peak
power: (a) spectra for different initial delay of 200-fs pulses; (b) spectra for different pulse widths
launched with no initial delay. (After Ref. [123]; c○2005 OSA.)
interaction between the two leads to considerable spectral broadening in the visible
region of the generated supercontinuum. The use of XPM for blue-light generation
was proposed in a numerical study in which 27-fs pulses at 900 nm with 1.7-kW peak
power were propagated inside a microstructured fiber with 200-fs pulses at 450 nm with
only 5.6-W peak power [123]. The ZDWL of the fiber was set at 650 nm so that the
two pulses propagate with nearly the same group velocity (resulting in enhanced XPM
interaction). The width of the second pulse as well as its relative delay was found to
affect the visible spectrum. Figure 12.28 shows the predicted spectra in the blue region
at the output of a 80-cm-long fiber as a function of (a) initial delay of 200-fs pulses and
(b) width of the second-harmonic pulse with zero delay.
The XPM phenomenon can be used with success to enhance the bandwidth of supercontinuum
generated with the pump pulse alone. In a 2005 study, 30-fs pulses at a
wavelength of 1028 nm were launched into a 5-m-long microstructured fiber with or
without weak second-harmonic pulses [125]. Their relative delay was adjusted using
a delay line. The fiber had two ZDWLs located at 770 and 1600 nm. Figure 12.29
compares the output spectra as a function of launched power when pump pulses were
propagated (a) alone and (b) with the second-harmonic pulses. When only pump pulses
are launched, the supercontinuum is generated through the formation of Raman solitons
that shift toward longer wavelengths until their RIFS is suppressed near the second
ZDWL at 1600 nm. The NSR emitted by them in the spectral region near 1700 nm is
clearly visible. However, the supercontinuum does not extend in the spectral region
below 900 nm. When second-harmonic pulses at 514 nm are also launched, the preceding
scenario does not change because these pulses are too weak to affect the main
pump pulse. However, the XPM-induced phase shift produced by pump pulses chirps
the green pulses such that the supercontinuum at the end of fiber extends in the visible
region and covers a wide range from 350 to 1700 nm. In another experiment, when
the power ratio between the pump and second-harmonic pulses was adjusted, it was
possible to produce a smooth and nearly flat supercontinuum that covered the entire