Multi-kilohertz repetition rate Ti:sapphire
amplifier based on down-chirped pulse
amplification
David M. Gaudiosi1, Etienne Gagnon1, Amy L. Lytle1, Julie L. Fiore2, Emily A. Gibson1,
Steve Kane3, Jeff Squier4, Margaret M. Murnane1, Henry C. Kapteyn1, Ralph Jimenez2
and Sterling Backus1
1
Department of Physics and JILA, University of Colorado and National Institute of Standards and Technology,
Boulder, Colorado, 80309-0440
2
Departments of Chemistry and Biochemistry and JILA, University of Colorado and National Institute of Standards
and Technology,
Boulder, Colorado, 80309-0440
3
Horiba Jobin-Yvon, Edison, New Jersey, 08820
4
Colorado School of Mines, Golden, Colorado, 80401
[email protected]
Abstract: We present a novel ultrafast multipass laser amplifier design
optimized for sub-millijoule output energy and capable of being operated at
repetition rates exceeding 40 kHz. This ti:sapphire based system makes use
of a grism based stretcher, a cryogenically cooled ti:sapphire crystal and an
astigmatically compensated multipass amplifier design that allows for
pumping with significantly lower pump pulse energies than has been
demonstrated to date. We also make use of the downchirped pulse
amplification scheme to minimize loss in the pulse compression process.
Preliminary experiments demonstrate an output pulse energy of 290 µJ at 10
kHz and 270 µJ at 15 kHz with a pulse duration of 36 fs.
© 2006 Optical Society of America
OCIS codes: (140.3280) Laser amplifiers
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Received 13 June 2006; revised 2 August 2006; accepted 4 August 2006
2 October 2006 / Vol. 14, No. 20 / OPTICS EXPRESS 9277
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1. Introduction
Ultrafast laser amplifier systems have found widespread application in areas of science and
technology, such as high-harmonic generation [1], studies of protein dynamics [2] and
chemical reactions [3], laser machining [4] and laser particle acceleration [5]. Most highpower ultrafast laser amplifiers make use of the chirped-pulse amplification (CPA) technique
[6] in which a femtosecond pulse is positively chirped to increase its duration to 0.1-1 ns,
amplified and then recompressed back to femtosecond duration. The most-successful
architecture uses a ti:sapphire crystal, pumped by a green laser, as a gain medium. Ti:sapphire
laser systems have been implemented in two basic configurations: very high repetition rate
systems (>100 kHz) with low pulse energy (<10 uJ), pumped by CW lasers [7] and low to
medium repetition rate systems (10 Hz – 10 kHz) with high pulse energy (1 mJ – a few
Joules) pumped by pulsed lasers [8,9].
A very large fraction of ultrafast laser applications require pulses with moderate energy, in
the tens of microjoules to a millijoule range, and with peak powers of gigawatts— for
example, as an energy source for broadly-tunable optical parametric amplifiers (OPA’s), or
simply to generate peak focused intensity of ~1014-1015 W cm-2 for material ablation or
VUV/EUV harmonic generation. Although these applications often require pulse energies in
excess of that available from CW-pumped amplifier systems, they benefit from higher
repetition rates than those pumped by pulsed lasers. As ultrafast science progresses, many
experiments require extensive signal averaging to obtain the signal-to-noise ratio necessary
for observing subtle effects. Average powers available from ultrafast laser systems have been
increased from ~1 watt in the 1990’s, to ~10 W today through the use of cryogenic cooling of
the ti:sapphire crystal [9-11]. This has allowed millijoule-level ultrafast laser systems to be
implemented at repetitions rates as high as 10 kHz.
Nevertheless, a gap has remained in ultrafast technology at pulse repetition-rates of 20100 kHz and pulse energies of ~0.1 millijoule. CW laser-pumped systems have output pulse
energies of ~5 µJ, which is marginal or insufficient for many applications, such as pumping
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multiple OPA’s, or for efficient high-order harmonic generation. On the other hand, systems
using pulsed pump lasers are generally engineered to generate millijoule pulses. Many
experiments are compelled to use these kilohertz systems but only use a small fraction of the
total energy output. Recently, a 100 kHz regenerative amplifier based on DPA was
demonstrated, producing 28μJ, 39 fs pulses with horizontal and vertical M2 values of 3.6 and
5.3, respectively [12]. In addition, Yb-fiber laser amplifiers have been demonstrated that
operate at > 10 kHz and have a pulse energy of ~ 140 uJ [13]. In other work, fiber amplifiers
have produced very short (~43 fs) pulses [14], but fiber systems have not been demonstrated
that encompasses all three properties: >10 kHz repetition rate, sub-40 fs pulses, and >100 µJ
pulse energy.
A few papers report fractional millijoule CPA systems running at ~10 kHz repetitions
rates [15,16]. However, use of these systems has been limited, despite the fact that highpower frequency-doubled Nd:YAG lasers operating at 20-50 kHz and with average powers of
~100W are readily available. The reason is that thermal lensing is a serious limitation on
performance. When the pump laser is focused onto a ti:sapphire crystal, local heating causes a
lensing effect. This lensing gets progressively worse at higher repetition rates as the heat load
per unit area increases. The thermal lens focal length inside the amplifier crystal [17] is given
by
f =
2 k π r2
2k
=
(dn dT ) E ν (dn dT) F ν
(1)
where k is the thermal conductivity of the crystal, r is the pump radius, dn/dT is the change in
the index of refraction with temperature, E is the pump pulse energy that is converted to heat,
ν is the repetition frequency, and F= E/(πr2) is the pump fluence. For Ti:sapphire at 77 K, k =
9.8 W/(cm K) [18] and dn/dT = 1.8 x 10-6 /K [19] and at 50 K, k ≈ 30 W/(cm K) [18] and
dn/dT = 1.8 x 10-6 /K [19]. Figure 1 shows the thermal lens focal length, at 50 and 77 K, as a
function of repetition-rate, when the peak focused fluence is kept constant at 3 J cm-2 for a
gain of ~ 5 per pass. A number of laser amplifier systems have made use of cryogenic cooling
for higher performance [9-11,20], but this is, to our knowledge, the first presentation of a
system using ti:sapphire cooled to 50 K. For example, a 100 kHz system running at 50 K
would have a thermal lens of ~ 110 cm, while the same system at 77 K would have a thermal
lens of ~36 cm, or at room temperature, ~ 2 mm.
Fig. 1. Focal length of the thermal lens induced in a Ti:sapphire crystal at 77 K (solid) and 50
K (dashed). The fluence is kept constant at 3 J cm-2.
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In this work, we implement a cryogenically-cooled laser amplifier designed to be scaleable to
very high repetition rates. We use a modified multipass amplifier design that can use pump
pulse energies significantly less than the 8-10 mJ required in past designs. Since commercial
green pump lasers are limited to ~100W average power, the large pump pulse energy has
limited the repetition rate of such a single stage, single pump laser amplifier to 10-15 kHz. To
overcome this limitation, the new design makes use of tighter focusing in the amplifier ring,
and incorporates a new optical design that greatly reduces accumulated astigmatism. Finally,
we combine these new optical designs with downchirped pulse amplification (DPA), rather
than conventional CPA, to reduce compressor losses [11]. The result is a compact, single
stage amplifier design that is scalable to tens of kHz and produces moderate pulse energy with
excellent beam quality. The use of a cavity-dumped seed laser eliminates the need for a
pockels cell pulse selection, which has been another significant limitation on repetition rate.
2. Experimental setup
In DPA, the seed pulse is stretched using negative dispersion and then compressed using
positive dispersion by passing the beam through a block of glass. Figure 2 shows a schematic
of the amplifier system setup. The front end of the amplifier system is a cavity-dumped
oscillator with a repetition rate that is variable up to 2 MHz. At repetition rates of 10-15 kHz,
the cavity-dumped energy is ~15 nJ per pulse with a spectrum that can support a 12 fs pulse.
Two different stretcher designs were used in this project. For the comparison between old
and new ring designs, the stretcher consisted of a grating pair followed by a prism pair. The
grating pair provided the majority of the pulse stretching, while the prism pair precompensated for the large amount of third-order dispersion (TOD) in the material compressor.
Subsequently, we obtained a pair of grisms [20]. A grism is a single optical element
containing both a prism and a grating, and which is designed for an optimum ratio of second
to third-order dispersion, allowing for efficient recompression. All subsequent measurements
were made using the grisms as the stretcher (Fig. 2). The pulse from the grism stretcher is
adjustable from 5 – 30 ps, has a pulse energy of ~ 1 nJ. The beam is near Brewster’s angle on
all of the entrance and exit faces. These grisms were fabricated using off-the-shelf aluminum
coated gratings, which limited the double pass stretcher efficiency to ~10%; however, the use
of optimized gold coated gratings will increase this to > 75%. Nevertheless, the high input
energy of the cavity-dumped seed laser allowed us to demonstrate saturated gain of the
amplifier.
Fig. 2. Schematic diagram of the DPA amplifier system.
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After the stretcher, the beam is then sent into an amplifier consisting of a seven-mirror, ten
pass multipass ring and a cryogenically cooled Ti:sapphire amplifier crystal. To obtain a
smaller focal spot (~ 150 μm diameter) in the amplifier crystal, mirrors with a smaller (75 cm)
radius of curvature (ROC) are used in this ring, compared with the standard 1.0 m ROC
mirrors used previously[21]. However, use of shorter ROC mirrors in this multipass design
[22] causes a degradation of the output mode (Fig. 3(a)) due to increased astigmatism. A new
amplifier ring design was therefore implemented to minimize the incident angle of the beam
on the curved mirrors (Fig. 4) and resulted in virtually no astigmatism in the output beam
(Fig. 3(b)).
Fig. 3. M2 measurement of the amplified beam with (a) old ring design and (b) new ring
design. The output of the new design has an M2 of 1.18 and 1.26 in the x- and y-directions,
respectively.
Fig. 4. Schematic diagram of astigmatism-minimized amplifier ring. Only three passes are
shown, but up to 10 passes can be employed with minimal astigmatism.
The 6 mm amplifier crystal is housed in a ultra-high vacuum cell and cooled to ~ 50 K using a
closed loop helium cryo-cooler. The system was pumped by a commercially available,
doubled Nd:YAG laser (532 nm) capable of running at 5-20 kHz with up to 50 W average
power. The pump pulse energy was limited to 2.3 mJ to allow the repetition rate to be
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increased to > 40 kHz . The uncompressed, amplified output power was measured as a
function of seed pulse duration by adjusting the grism separation (Fig. 5). The output power
varies greatly with input seed pulse duration, reaching a maximum around 22 ± 3 ps. At
shorter pulse lengths, the finite lower state lifetime reduces the amplifier output. A simulation
based on the Frantz-Nodvik equations [23] was developed to extract the lower state lifetime;
the result is shown in Fig. 5(a). The simulation and the data are in very good agreement when
a lower state lifetime in the range of 0.25-5 ps is assumed. The variation in the lifetime is due
to the unknown ratio of the upper to lower level degeneracy in ti:sapphire. The product of
this degeneracy ratio with the lower state lifetime is 25 for a best fit with the data. The
simulation (Fig. 5(a) inset) shows that the output energy starts to saturate at a seed pulse width
of ~ 75 ps. The drop in efficiency at longer pulse lengths is a result of the experimental setup.
The gratings of the grisms are only 1” square, which limited the amount of stretch that could
be applied before serious spectral clipping occurred. The amplified power dropped when the
spectral clipping became too large—a pair of grisms with dimension 50-75 mm will allow for
sufficient stretch to obtain near-ideal energy extraction, while still maintaining a very compact
geometry. The maximum output pulse energy of the amplifier is 290 µJ at 10 kHz and 270 µJ
at 15 kHz, with a pulse-to-pulse intensity stability of 1.7% and 3.0% rms at 10 and 15 kHz,
respectively. At 15 kHz, the grism separation was adjusted and the relative timing of the
pump and seed pulses was reoptimized (Fig. 5(b), circles). This resulted in slightly higher
energies at the shorter seed pulse lengths.
Fig. 5. Power before compression as a function of seed pulse length for (a) 10 kHz repetition
rate and (b) 15 kHz repetition rate. Data was taken while only adjusting grism separation
(squares) and while adjusting both grism separation and timing between pump and seed pulses
(circles). Simulation results are shown as solid line in (a) and in (a) inset, which extrapolates
the model to estimate the pulse duration required for efficient energy extraction from
ti:sapphire.
After the amplifier, the beam is sent into a telescope to provide a beam that slowly
diverges from ~ 1 cm to ~ 2 cm diameter as the beam passes through the compressor material.
This divergence minimizes the B-integral in the compressor while also allowing for a smaller
block of glass for the compressor. Calculations and measurements indicate that the variation
of the pulse width across the beam profile is negligible. This telescope also serves as a spatial
filter, which ensures a spatial mode close to TEM00 with a throughput of 90%. The
compressor consists of 120 cm of uncoated SF18 glass. An AR coated slab, that is on order,
should provide 95% throughput for the compressor.
The pulse duration from the amplifier was measured using second-harmonic generation
frequency resolved optical gating (SHG FROG). Figure 6 shows the reconstructed pulse and
phase, with a FWHM of 35.5 ± 1 fs. The transform limit of the amplified spectrum is ~ 28 fs.
FROG measurements taken at different radial positions of the beam profile reconstruct to the
same value. This shows that the center and edges of the divergent beam do not traverse
significantly different amounts of material. The output beam quality is good, as can be seen
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by the M2 of the beam (Fig. 3(b)), which were measured to be 1.18 and 1.26 in the x- and ydirections, respectively. Nonlinear effects, such as B-integral, would distort the beam profile,
leading to a large M2 value. Since the M2 is close to one in both directions, we conclude that
nonlinear effects in the compressor are negligible. This is consistent with our calculations that
the B-integral should be less than 0.5 in this configuration.
Fig. 6. Temporal profile and phase of amplified pulse as measured with frequency-resolved
optical gating (FROG).
3. Conclusion
In conclusion, we have demonstrated a laser amplifier system based on downchirped pulse
amplification that can be operated at medium energies and very high repetition rates. The
modified design makes use of a tighter focus amplifier ring capable of achieving saturated
gain at lower pump pulse energies. This modified amplifier design greatly reduces
accumulated astigmatism in the multipass ring. The result is a compact single stage amplifier
that is scaleable to 40 kHz or greater repetition rate with excellent beam quality and no
degradation in amplifier performance. Such a system will provide a pulse energy and peak
power sufficient for many applications, at higher pulse repetition frequencies than are
currently available.
Acknowledgments
This work was supported by NSF PHY-0096822, by the Department of Energy DE-FG0399ER14982, and by NSF ECS-0216205. R.J. is a staff member in the Quantum Physics
Division of NIST. J.F. is supported by an NSF-IGERT grant. E.G. is supported by the NRC
post-doctoral fellowship program.
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Received 13 June 2006; revised 2 August 2006; accepted 4 August 2006
2 October 2006 / Vol. 14, No. 20 / OPTICS EXPRESS 9283