ments, possible approaches and a working
setup for next-generation USP lasers, capable of adapting the pulse repetition rate in
the MHz range with a very fine resolution.
The requirements will be specified for the
resonant scanner, since these are the highest among the possible applications.
Dynamically variable pulse
The laser deflection speed in the focal
plane of a resonant scanner-based micromachining setup can be described by a
sinusoidal function. Since the spot distance is proportional to the scanning speed
(∆s = v/PRR), a repetition rate, which
changes proportionally to the deflection
speed, leads to a fixed spot distance across
the whole scanning range. Moreover, since
a resonant scanner typically oscillates in
the 10-kHz range, the laser’s PRR has to
be changed more than 10,000 times per
second. Due to the very high velocity of
the deflected beam of about 1000 m/s, a
high PRR in the MHz range is required to
reach an adequate pulse overlap. In order to
maintain high accuracy and a precise positioning on the workpiece, the PRR ideally
has to be tuned continuously in this range.
In a mode-locked-based USP laser system, the PRR is given by the cavity round-trip time of the laser oscillator. Therefore,
the variation of the PRR of these USP lasers is done by pulse picking with the help
of fast electro-optical or acousto-optical
modulators, suitable for high-power operation. The base PRR can be divided by integers and is reduced to an effective PRR.
This is shown in Figure 2 for a base PRR
of 10 MHz, 100 MHz, 1000 MHz and a
true arbitrary pulse-on-demand operation.
A typical USP laser, operating at a fundamental PRR of 100 MHz, offers very
few effective PRRs in the MHz range. If
the application requires a PRR variation
between 10 MHz and 5 MHz, only 11 individual repetition rates are accessible. This
is not enough to ensure high positioning
accuracy after the resonant scanner. Only
a much higher base PRR — or even an
arbitrary pulse-on-demand technology —
can fulfill this requirement (Figure 3).
The accuracy of the spots for a base PRR
of 100 MHz is only ∆s = ± 4.1 µm, which
is too poor for high precision surface structuring with focus diameters in the 10-µm
range. A base PRR of 1 GHz allows for a
much higher accuracy of ∆s = ± 0.4 µm.
This accuracy is competitive with current
technologies and sufficient for USP laser
This calculation also holds true for
speeding up the processing of narrow
curves or homogenizing the ablation with
laser turning machine applications. Pro-
cessing narrow curves with variable PRRs
in the 100-kHz range already ensures
high accuracy, since the granularity is
fine enough (compare to Figure 2). A true
scaling to the MHz range, however, is only
achieved if the base PRR is strongly in-
creased or an arbitrary pulse-on-demand
setup is used. Increasing the throughput
of these applications by an order of mag-
nitude leads to a reduction of a 10-second
production process to just one second.
Evaluating semiconductor lasers,
ultrafast pulse picking
Using a high-base PRR with ultrafast
pulse picking or an arbitrary pulse on-demand technique are the best approaches.
A pulse repetition rate in the GHz range
requires a short cavity length of a few
mm, making semiconductor lasers a feasible option. However, a key challenge is
Figure 1. Schematic picture of a pulse train with (a) a constant pulse repetition rate (PRR), (b) a linear sweep
of the PRR, (c) a periodic modulation of the PRR and (d) a picture of the lab setup of the prototype.
Figure 2. Achievable repetition rates with different seed sources by pulse picking. A higher base PRR
enables more possible effective PRRs in the upper MHz range.