Advanced Ultrashort-Pulse Laser Diagnostics - Pro Physik
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Laser
all pictures: APE
A high contrast autocorrelator in
the lab can unlock hidden pulse
information.
Advanced Ultrashort-Pulse Laser Diagnostics
High-contrast autocorrelation provides deeper insights on pulsed fiber lasers.
Mateusz Ibek and Thomas Neicke
The simplest and most common me- on ytterbium-doped gain materials energy is timely distributed among
thod for the characterization of the such as Yb3+:YAG. Those fiber lasers the wings, pedestals and secondary
duration of an ultrashort pulse la- simplify the application of ultrashort pulses, the desired interaction bet-
ser is to measure an autocorrelation pulses on the one hand but also com- ween pulses and material cannot al-
function. However, especially with plicate it on the other hand. The reason ways be ensured anymore.
modern high-power fiber lasers, in is that the physics of the pulse genera High-contrast measurements are
some cases the sensitivity and dy- tion process includes various nonlinear essential for investigating high energy
namic range of standard autocorre- effects which may easily corrupt the laser pulses, as they allow the simulta-
lators are not sufficient and require pulse performance. neous display of the largest main peak
more sophisticated measurement Cost-efficient autocorrelators are and the smallest secondary peaks in
methods. With type II autocorrela- widely used when the knowledge of an autocorrelation measurement.
tors, a cost-efficient high-contrast the temporal duration of a pulse is The contrast, as it refers to a pulse
technique is available that can pro- required. But in some cases the sen- duration measurement, is the diffe-
vide more detailed insights into sitivity and the dynamic range of a rence between the highest and the lo-
a pulse topography than normal standard autocorrelator might not be west intensities, and often the lower
autocorrelators. sufficient and more sophisticated me- intensities are covered by background
thods are needed. signal. Autocorrelator measurements
T oday’s development of lasers, es-
pecially in material processing, is
moving in the direction of high en-
High contrast measurements are
essential in the field of ultrashort pulse
lasers delivering high pulse energies. It
with high contrast information re-
quire a modified autocorrelator, a
common approach is the so-called
ergy lasers with high repetition rates is important that the entire pulse en- type II (or type 2) layout.
and ultrashort pulse durations. Very ergy reaches the sample within a very
popular types of such lasers are based (ultra-)short time period. If the pulse
12 Physics’ Best, April 2020 © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Laser
Autocorrelation with pulse in the hundreds of femtosecond action. The autocorrelation function
high contrast regime. (ACF) is then measured by recording
There is an additional pulse at the interaction intensity dependence
A set of autocorrelation measure- about ±2 ps, which is revealed by the as a function of the distance (delay)
ments illustrates the difference bet- high contrast measurement. Since it variation.
ween a common autocorrelation is not moving with the compressor For the interaction of both pulses
and a high contrast autocorrelation setting, this is probably a duplicate of in the nonlinear crystal different sce-
(Fig. 1). A high-contrast autocorre- the main pulse. Such duplicates are narios can be described: the type I
lator (pulseCheck Type 2, APE) caused, for example, by coating de- case and the type II case.
and an additional reference autocor- fects of laser mirrors.
relator with a typical contrast range The measurement also shows a
(pulseCheck , APE) were used. All small secondary pulse, whose dis-
Type I autocorrelation
data sets have been normalized to tance to the main pulse increases Both pulses overlap with their iden-
the same peak intensity for a better with increasing stretching of the pulse. tical frequencies ω and identical pola-
comparison of the dynamic range of The pulse is located at approximate- rization e and pass through the non-
both autocorrelators. The measure- ly ±2 ps (Fig. 1a), ± ps (Fig. 1b) and linear type I crystal. The type I phase
ments were taken on a commercial ±2 ps (Fig. 1c). The analysis of this matching scheme (Fig. 2) results in the
high energy fiber laser with a repe- shows that the origin of this “mo- sum frequency 2ω with the polariza-
tition rate of MHz and a nominal ving” small feature is a spectrally and tion o. This part of the beam is used
pulse duration of fs. The output temporally separated part of the laser to record the ACF.
pulse duration of the laser is variably output. By an additional measurement In addition to the two overlapping
adjustable via an internal grating of the spectrum it would even be pos- pulses, each individual pulse generates
compressor. sible to clarify whether the laser out- an additional signal with the frequen-
The autocorrelation measurements put contains a red shifted post-pulse cies ω + 2ω. Here, 2ω is the result of
were performed at different pulse or a blue shifted pre-pulse. The pulse the nonlinear effect in the crystal and
compression levels. Fig. 1a shows the at ±2 ps does not change its temporal ω is the unconverted part of the in-
shortest possible pulse achieved with position with increasing stretching, cident beam.
the laser under the best compression only the pulse duration changes. Technically it is difficult to se-
conditions. Figs.1b and 1c were recor- parate the desired beam 2ω and the
ded at stretched pulses, the pulse stret- unwanted beams ω + 2ω by placing a
ching was created using the internal
High contrast layout vs. detector behind the crystal so that it
grating compressor.
common layout only measures the 2ω part that is ge-
The dynamic range of the type 2 The basic principle of an autocorrela- nerated by the two overlapping pulses.
autocorrelator is about 5 and there- tor for pulse duration measurements Instead, a portion from ω + 2ω is de-
fore up to three orders of magni- is to overlay a pulse with a variable tected and generates a certain level of
tude larger than that of the reference time- delayed copy of itself. Both background signal. The ACF trace will
autocorrelator. pulses interact in a nonlinear crys- then always contain some amount of
Pedestals become visible in the tal. The variation of the time-delay background signal even if the pulses
high contrast measurement, which are leads to a variable degree of temporal do not overlap.
clearly temporally separated from the overlapping of the two pulses at the
main peak. Please remember that the interaction area, and thus to different
duration of the main pulse is actually a intensities resulting from their inter-
a 100
b 100
c 100
10–1 10–1 10–1
intensity in a.u.
10–2
10–2 10–2
10–3
10–3
10–3
10–4
10–4
10–4
–20 –10 0 10 20 –20 –10 0 10 20 –20 –10 0 10 20
delay in ps delay in ps delay in ps
type II autocorrelation (pulseCheck type 2) type I autocorrelation (pulseCheck 50)
Fig. 1 A type II autocorrelator allows measurements with significantly higher contrast compared to a conventional autocorrelator (type I).
© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Physics’ Best, April 22 13
Laser
with a delayed fraction of the funda-
ω + 2ω
type I crystal mental pulse, leading to a measured
AC signal at the tripled frequency ω.
o 2ω This higher technical complexity is
ω e rewarded with a dynamic range of up
ω + 2ω to twelve orders of magnitude.
As third harmonic generation is
relatively weak due to lower conver-
ω e sion efficiencies, the method is less
sensitive and can only be used for ve-
ry high pulse energies in the µJ
Fig. 2 A common type I autocorrelation setup results in the sum frequency 2ω with the
regime. They are therefore – in most
polarization o.
cases – not suitable to work with high
energy fiber lasers with MHz repe-
ω tition rates, since significantly lower
type II crystal
pulse energies (i.e. in the range of µJ)
e
are achieved here.
2ω
In addition, the acquisition costs
ω o are up to four or five times higher than
an autocorrelator of type I or II.
ω
ω e Summary
With type II autocorrelators, like the
Fig. 3 A high-contrast type II autocorrelation setup results in the sum frequency 2ω with
the polarization e.
pulseCheck Type 2, a cost-efficient
high-contrast device is available that
can both measure the pulse duration
Type II autocorrelation unwanted background signal by non- and provide insights into a detailed
overlapping pulses occurs at all. pulse topography that is not visible
In order to achieve high contrast bet- This makes it possible to visualize with normal autocorrelators.
ween the peak of the autocorrelation the weak signals of satellite pulses and PulseCheck autocorrelators can
signal and the baseline (i.e. when no other types of secondary information be configured in type II layout to
pulse energy is present), the autocor- that would otherwise be covered by be easily operated like traditional
relator must be optically designed so background. autocorrelators.
that signal is only generated when the This makes type II autocorrelators
pulses overlap. This is achieved by an ideal tool for studying, understan-
the so-called type II phase matching
Another high contrast layout ding, and ultimately improving laser
scheme (Fig. 3). Both pulse trains enter For the sake of completeness, another pulses of high energy fiber lasers or
the type II crystal with a polarization layout should be mentioned. From solid-state lasers to a greater extent.
orthogonal to each other. Type II plasma physics research a different
phase matching results in the sum kind of autocorrelator is known which
frequency 2ω with the polarization e. is based on the third harmonic gene- Authors
Both incoming pulses must have a dif- ration, the so-called rd order auto-
ferent polarization state, i.e. e and o. correlator. Mateusz Ibek and Thomas Neicke,
APE Angewandte Physik & Elektronik GmbH,
Only the two overlapping beams with Compared to the type I and type II Plauener Str. 163-165, Haus N, 1353 Berlin,
different polarization states produce methods, two nonlinear crystals are Germany; phone: +49 3 986 11-3;
the desired sum frequency signal 2ω. used in series. The first crystal produ- e-mail: thomas_neicke@ape-berlin.de
An individual beam is not able to in- ces the doubled frequency 2ω. A frac- Web: www.ape-berlin.de
duce a nonlinear effect. Therefore, no tion of the 2ω pulse is then interacting
14 Physics’ Best, April 22 © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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