Page 502 - High Power Laser Handbook

470 Fi b er L a s er s Pulsed Fiber Lasers 471

over short distances, comparable to the FWM coherence length
defined earlier. This occurrence breaks down the oscillatory nature of
the power exchange between the pump and scattered beams and may
lead to substantial FWM buildup even in non-phase-matched condi-
tions in the normal dispersion wavelength region (~1 μm). 10,11
Similar to the behavior of SBS and nonlinear phase modulation dis-
cussed earlier, the impact of FWM also becomes enhanced when a frac-
tion of the main beam power is distributed over secondary, closely
spaced spectral components, such as multiple longitudinal modes. In
such cases, FWM side bands can become effectively “seeded” at these
satellite frequencies (rather than building up from diminutive parametric
noise) and, thus, set in at much lower peak powers, especially if the ini-
tial spectral features are separated by few tens of gigahertz or less.
This scenario, known in optical telecommunication applications
as quasi-phase-matched cross talk, can lead to substantial spectral broad-
ening, as shown in Fig. 16.1, which pertains to the Yb-doped fiber
amplification of ~1-ns pulses emitted by a single-frequency Nd:YAG
having side-longitudinal mode suppression of less than 10 dB. This
important effect must be considered in the PFL design optimization,
particularly in terms of the choice of seeding sources for amplifiers,
as discussed in Sec. 16.3.

∆ν

0
Peak power
Normalized spectrum (dB) −20 ~50 kW
Seed only (~5 kW)
~100 kW

−40

−60

1063 1064 1065
Wavelength (nm)
Figure 16.1 Peak-normalized logarithmic scale spectrum of the fiber-amplified output
of an actively Q-switched Nd:YAG seed laser (pulse duration ~1 ns, pulse repetition
frequency [PRF] = 10 kHz, output peak power ~5–10 kW). Black trace: Seed
throughput only (unpumped fiber); Red trace: Amplified pulse peak power ~50 kW
(~10 dB optical gain); Blue trace: Peak power ~100 kW. The gain fiber was a ~3-m
long, 25-μm core, Yb-doped fiber. The frequency spacing of the cross-talk comb (∆ν) is
approximately equal to the longitudinal mode spacing in the seed laser.