train. The setup delivers ultrashort pulses
in the 10-ps range with adjustable PRR.
If there is a demand for finer granularity, operators can leave the mode-locking
technique and use gain-switching instead.
Gain-switching delivers arbitrary pulses
on demand, so a discrete base PRR is no
longer a concern. The accuracy, then, is
only limited by the temporal jitter between
the trigger and the emission time of the
corresponding pulse, which typically is
extremely low. Looking back to the resonant scanner application, the spot distance
∆s would be absolutely constant across the
whole working range (Figure 3).
The drawback of gain-switching diodes
is pulse length. The shortest pulses, directly obtained by gain-switching, are in
the range of approximately 40 ps. Many applications would be adversely impacted by
the longer pulse duration, in terms of ablation efficiency and thermal impact. Nevertheless, the authors showed that even processing of transparent media is possible
with these pulses5.
Figure 4 shows a schematic of an actual setup, based on gain-switching, that
was recently demonstrated6. The amplifier
chain consists of a fiber pre-amplifier, and
an InnoSlab power booster. An optional
SHG-stage can convert the 1030-nm radiation to the green spectral range, depending
on the application.
Varying pulse energies
A crucial issue for most applications is
maintaining a constant pulse energy while
changing the pulse repetition rate. Since
the PRR is altered in front of the amplifier chain, a closer look at the pulse energy
dynamics is required. In general, a lower
PRR would result in higher pulse energy
for a constant average output power. Varying pulse energies during large-scale processing, however, would have a strong impact on the quality of machined parts.
The very high processing speed helps in
addressing this problem. On the one hand,
typical laser active media, commonly used
in the amplifier chain, exhibit a long upper
state lifetime in the ms range. Conversely,
the laser’s PRR changes more than 10,000
times per second, which means that the
repetition rate changes so fast that the amplifier cannot adapt to the new situation.
An average, and nearly constant, pulse energy, independent of the PRR, is emitted.
In practice, the relaxation time of the
overall amplifier chain depends on a
number of parameters, like the number of
stages, the pump saturation of each indi-
vidual stage and the input power. Unfortu-
nately, all of these parameters increase the
relaxation rate of the amplifier, so entirely
constant pulse energies are typically not
achieved. However, an optimized design
of the amplifier chain allows for an effec-
tive damping of the pulse energy varia-
tion (Figure 5). Especially at high sweep-
frequencies, a quite constant value is pos-
sible. The remaining variation can easily
be actively compensated, if necessary.
Range of pulse repetition rates
A USP laser with an average power of
more than 200 W with pulse duration of 40
ps or less, depending on the oscillator, has
been realized. The PRR can be changed
more than 30,000 times per second between 5 MHz and 10 MHz with a pulse
energy variation of less than 10 percent
without an active compensation. An even
wider sweep range of the PRR between
a few 100 kHz and 40 MHz and average
output powers of more than 400 W are currently under development in a governmen-
Figure 4. Schematic setup of a highly dynamic laser system. The fiber pre-amplifier was optimized
to provide a large small-signal gain combined with minimized amplified spontaneous emission.
The output power was 200 W in the IR or 130 W after an optional SHG stage.
Figure 5. Temporal evolution of the pulse energy for PRRs between 5 MHz and 10 MHz at different
sweep-frequencies. For visibility reasons only every 10th pulse is plotted.