Page 510 - High Power Laser Handbook

478 Fi b er L a s er s Pulsed Fiber Lasers 479

chain. At low-pulse power levels, monolithic fiber-pigtailed modula-
tors are commercially available, which can be conveniently fusion
spliced in the chain and which avoid alignment-sensitive compo-
nents that might hinder ruggedness.
Finally, in cases of low-PRF operation, pulsed pumping can be
effectively implemented. This solution, often adopted for bulk DPSS
lasers, is especially viable due to the frequent use of single-emitter
diodes (which can be easily modulated at high rates) for fiber pumping.
This technique entails the judicious interruption of optical pumping
between pulses to prevent ASE buildup. However, maximum improve-
ment compared with CW pumping is anticipated for pulse periods
longer than the rare earth inversion (i.e., excited-state) lifetime and
for sufficiently long fibers, for which the ASE generated during one
pump pulse ends up propagating through a portion of fiber gain
medium that is no longer inverted and that corresponds to a sig-
nificant fraction of the overall fiber length. For silica-based Yb- and
Er-doped fibers, for example, this regime approximately corre-
sponds to pulse repetition rates less than 1 kHz and less than 100 Hz,
respectively.
16.3.3 MOPA versus Power Oscillators
In MOPA sources, a low-power laser acts as the seeder for a single- or
multistage amplifier. As such, MOPAs enable function separation
and independent optimization of the spectral and temporal aspects of
the pulse formation and the generation of high power. In many cases
documented in the literature, pulsed fiber-based MOPAs have fea-
tured a bulk solid-state laser as the master oscillator. A very common
choice, borrowed from optical telecommunications and highly com-
patible with end-to-end all-fusion-spliced-fiber MOPA designs, is to
use a fiber-coupled semiconductor laser. Pulsed operation from such
lasers is obtained through gain switching with a pulse-driving cur-
rent or by means of an external electronically controlled modulator.
These techniques support nearly one-to-one mapping of arbitrary
electrical-to-optical pulseforms and, therefore, can provide the high-
est degree of active control on the pulse format, including continuous
and independent adjustability of pulse duration (from a few picosec-
onds to CW), shape and repetition rate, as well as minimal-jitter syn-
chronization to a trigger signal. Moreover, high spectral purity with
support for single-frequency and near-Fourier-transform-limited
operation can be obtained by means of specially designed external-
cavity or distributed-feedback architectures. A disadvantage of
semiconductor lasers is that their limited energy storage results in
nanojoule-level pulse energies for few-nanosecond pulses, which
burdens the following fiber amplifiers with the task of supplying a
very large gain. This problem has reportedly been circumvented by
replacing the semiconductor lasers with microchip lasers. Such bulk