Multi-microjoule, MHz repetition rate
Ti:sapphire ultrafast regenerative amplifier
system
Xiaoshi Zhang,1,*
Eric Schneider,1 Greg Taft,
1 Henry Kaptyen,
1,3 Margaret Murnane,
1,3
and Sterling Backus1,2
1Kapteyn-Murnane Laboratories Inc., 1855 S 57th Ct, Boulder, Colorado 80301,USA
2Colorado State University, Electrical and Computer Engineering, 1373 Campus Delivery, Ft. Collins, Colorado
80523, USA 3University of Colorado at Boulder, Department of Physics, Boulder, Colorado 80301, USA
Abstract: We demonstrate a cryogenically cooled Ti:sapphire ultrafast
regenerative amplifier laser system producing >20 µJ energies at 50 kHz,
>12 µJ at 200 kHz and >3.5 µJ at 1MHz with repetition rates continuously
tunable from 50 kHz up to 1.7 MHz in a footprint of only 60x180 cm2. This
laser uses down-chirped pulse amplification employing a grism stretcher
and a glass-block compressor, achieving sub-60-fs pulse duration. This
laser represents a several-times improvement in repetition-rate and average
power over past Ti:sapphire-based ultrafast lasers in this class. We discuss
the unique challenges and solutions for this laser system. This laser system
has wide applications especially in ultrafast photoemission, nonlinear
imaging and spectroscopy, as well as for micro/nano-machining and
ultrafast laser therapy and surgery.
©2012 Optical Society of America
OCIS codes: (320.0320) Ultrafast optics; (140.0140) Lasers and laser optics.
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#158879 - $15.00 USD Received 28 Nov 2011; revised 3 Feb 2012; accepted 6 Feb 2012; published 13 Mar 2012(C) 2012 OSA 26 March 2012 / Vol. 20, No. 7 / OPTICS EXPRESS 7015
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1. Introduction
A large number of scientific and industrial applications benefit from the use of laser pulses
with moderate energy (~µJ) and ultrashort sub-100fs duration at the highest possible
repetition rate. In scientific applications such as pump-probe multi-dimensional spectroscopy
[1] for studying chemical/biological dynamics, or photoelectron spectroscopy, highly tunable
laser pulses and extensive signal averaging are necessary to extract small signals. Ultrafast
Ti:sapphire laser systems with multi-µJ pulse energy are ideal for pumping tunable optical
parametric amplifiers which are tunable over a very large wavelength range. Other
applications that benefit from moderate energy laser pulses at high repetition rates include
ultrafast electron microscopy and diffraction imaging [2, 3], which are powerful techniques
for investigating structural dynamics in condensed matter under unprecedented “atomic”-
scale spatial and femtosecond-scale temporal resolutions. In these techniques, ultrafast laser
pulses are used to generate < 100 fs electron pulses, which are severely limited in fluence to
only a few electrons per pulse due to the space charge effects. Thus, a laser system with a
MHz repetition-rate is essential for making this technique viable.
In the case of industrial applications, an excellent example is ultrafast laser micro/nano
machining/surgery [4–7]. A tightly focused laser pulse requires only modest ~µJ energy with
sub-100-fs pulse duration to ablate even hard-to-machine materials such as fused silica and
diamond. The result is very precise ablation with low debris and minimal collateral damage,
but also with a very small amount of material removed. The highest possible repetition rate is
essential for maximizing machining throughput. Considerable evidence suggests that the
shortest possible pulses with very “clean” pulse structure are necessary for machining the
most-fragile materials, making a Ti:sapphire laser a viable contender in this area—especially
if the repetition-rate of the laser can be increased to maximize throughput. Another example
is the use of high harmonic generation to achieve a table-top “laser” like EUV/Soft X-ray
source for applications such as extreme-UV (EUV) metrology, high-resolution EUV imaging,
and microscopy, that benefit from high average power and high repetition rate. All of these
applications can benefit from ultrashort pulses, high repetition rate, and high average power.
#158879 - $15.00 USD Received 28 Nov 2011; revised 3 Feb 2012; accepted 6 Feb 2012; published 13 Mar 2012(C) 2012 OSA 26 March 2012 / Vol. 20, No. 7 / OPTICS EXPRESS 7016
In the past, these needs have been addressed using a number of technologies. Ti:sapphire
regenerative amplifiers were developed in the early 1990’s [8], with 100-300 kHz repetition
rates, multiple microjoule pulse energies, and <100 fs pulse durations. Very high repetition-
rate multipass amplifiers also have been implemented using Ti:sapphire [9]. However, these
systems have not to-date succeeded in operating in the few MHz repetition-rate range and a
few microjoule energy level. Cavity-dumped [10, 11] and long-cavity [12] Ti:sapphire
oscillators can generate single-pulse energies significantly higher than a typical mode locked
oscillator, but the pulse energies are still in the sub-µJ range and marginal for many of the
above-mentioned applications. Although it has been shown previously that pulses from a
cavity-dumped oscillator can be amplified in a CW-pumped regenerative amplifier, the
maximum amplified pulse energies were less than a micro-joule [13]. Still other approaches
have employed Yb based gain media, in either cavity-dumped [14]. free-space regenerative
amplifier [15–17], fiber [18], or fiber-rod [19] amplifier configurations. However, these
systems are at fixed wavelength(~1-1.03 µm) and have fairly long pulse durations (>200 fs).
A satisfactory solution has not existed until now for applications that require ultrashort pulses
at MHz repetition rates.
In this work, we demonstrate, to our knowledge, the first Ti:sapphire regenerative
amplifier producing laser pulses with energies up to 3.7uJ, durations <60 fs and repetition
rates up to 1.7 MHz. We studied the amplifier at various seeding pulse energies up to 150 nJ.
Seed pulses below 6 nJ are produced by an ultrafast Ti:sapphire laser of standard design [20],
while seed pulses of >6 nJ were produced using a cavity-dumped Ti:sapphire laser operating
in the positive dispersion regime [11]. One challenge in implementing this system is the
requirement for higher seed pulse energies to avoid dynamic instabilities. This challenge and
the theoretical model developed to address this are discussed further.
2. Laser system
Fig. 1. Schematic of cryogenically-cooled MHz regenerative amplifier system.
Figure 1 shows a schematic of the laser amplifier system. The amplifier crystal is pumped by
two 11-W continuous wave green lasers at 532 nm and is cryogenically cooled to avoid
thermal lensing [21, 22]. The seed pulse is generated from a femtosecond oscillator and
stretched to ~10 ps using a grism pair [23, 24] that introduces negative group velocity
dispersion (GVD) as well as negative third order dispersion (TOD). The stretched pulse is
then sent through a Faraday isolator and injected from a thin film polarizer (TFP) into the
regenerative amplifier. The amplifier cavity consists of two flat mirrors, two 20cm radius-of-
curvature mirrors, a cryogenically-cooled Ti:sapphire rod, and an electrical optical (EO)
#158879 - $15.00 USD Received 28 Nov 2011; revised 3 Feb 2012; accepted 6 Feb 2012; published 13 Mar 2012(C) 2012 OSA 26 March 2012 / Vol. 20, No. 7 / OPTICS EXPRESS 7017
switch. The EO switch consists of a TFP, a quarter waveplate and a 2 cm long BBO Pockels
cell. The Pockels cell high voltage driver operates at quarter wave rotation voltage and
repetition rate up to 1.7 MHz. Once injected into the laser cavity, the pulse is trapped inside
the cavity by turning on the EO switch. After passing through the amplifier crystal for 30 to
40 times (15 to 20 round trips), the amplified laser pulse is switched from the amplifier at the
polarizer by turning off the EO switch. The pulse is then dumped by the Faraday isolator into
a SF6 glass compressor, which adds positive GVD and TOD. After passing through the
compressor glass for 1 m path length (5x20 cm), as shown in Fig. 2, the amplified laser pulse
is measured to be ~51 fs using a SHG FROG device. The pulse duration is currently
bandwidth limited by the optics inside the laser cavity. The compressed output of the
amplifier is 3.7 W; i.e. 3.7 uJ pulse energy at 1MHz repetition rate. In this case, the seed pulse
energy required was 200 nJ. We also measured an amplified spatial beam quality of 1.3 M2, a
long-term power stability of 0.71% RMS over 8 hours (Fig. 3b), and a pulse-to-pulse energy
stability of 0.5% at 1.5 MHz.
Fig. 2. Laser pulse measurement. a) Laser spectral intensity and phase; b) Laser temporal
intensity and phase; Inset: SHG FROG trace. The 51fs duration of the measured pulses is close
to the transform limit of the spectrum centered at 793 nm.
Cryogenic cooling of the Ti:sapphire crystal results in both higher overall efficiency of the
regenerative amplifier (for example, the Coherent RegA employs 12W pump energy to
generate 1.6W output [25]), as well as increasing the versatility of the laser system by
allowing for tunable repetition-rate. At lower repetition rate the pulse duration is typically
slightly shorter that 60 fs since the higher gain requires fewer passes in the amplifier
introducing less higher order dispersion from the Pockels cell and amplifier crystal. Figure 3
shows the pulse energy and average power of the laser as a function of repetition rate, and
also as a function of seed pulse energy. We found that higher seeding energy yields higher
amplifier output for fewer round trips in the cavity. Furthermore, we also found that the
higher seeding energy can saturate the gain of the amplification in fewer round trips, which
yields a more stable pulse train, as discussed later in this paper.
#158879 - $15.00 USD Received 28 Nov 2011; revised 3 Feb 2012; accepted 6 Feb 2012; published 13 Mar 2012(C) 2012 OSA 26 March 2012 / Vol. 20, No. 7 / OPTICS EXPRESS 7018
Fig. 3. a) Amplifier output energy versus repetition rate for different seed pulse energies; b)
laser power long-term stability for 8 hours at 1.5 MHz using 2 nJ seed energy.
3. Challenges
There are two main challenges to achieve this high repetition rate while using CW pump
lasers in the amplifier system. The first challenge is that the gain of this amplifier system is
limited by CW pumping, which requires a tightly focused laser beam (<100 µm diameter) in
the amplifier laser crystal to reach a reasonable gain. The pump lasers can be focused well
with M2 values less than 1.1 and are combined by positioning the two parallel pump beams
close together on the focusing lens. Given a 22 W pump and an 80µm focal spot on the
crystal, we estimated the focal length of the thermal lens in the crystal to be 3 mm if the
crystal were kept at room temperature. This strong lensing would make any type of laser
amplifier virtually impossible. This thermal lens focal length increases to 5.4 m with the
crystal at cryogenic temperature, which is very manageable and in fact does not require that
the regenerative amplifier cavity be specifically engineered for a particular focal length. This
laser uses a closed-loop, helium, cryogenic cooler with 90W capacity for cooling the crystal
below 50K. As a result, we did not observe any significant thermal lens in our amplifier
crystal and achieved 0.8% RMS power stability in long term (12 hr) tests. Likely the pump
power can be increased by several times as more powerful green pump lasers become
available. This is likely to further increase the extraction efficiency of the amplifier.
Fig. 4. Oscilloscope traces of amplified laser pulse train at 1 MHz with 2 nJ seed pulses for a)
20 passes, b) 35 passes, and c) 50 passes through the amplifier laser crystal.
#158879 - $15.00 USD Received 28 Nov 2011; revised 3 Feb 2012; accepted 6 Feb 2012; published 13 Mar 2012(C) 2012 OSA 26 March 2012 / Vol. 20, No. 7 / OPTICS EXPRESS 7019
The second challenge with this amplifier system is that adjacent amplified pulses compete
for gain when the repetition rate is higher than 312.5 kHz, where the inter-pulse spacing at
this repetition rate is shorter than 3.2 µs which is the spontaneous lifetime of the Ti:sapphire
laser level. The gain in the laser thus does not fully recover to a steady-state level between
cycles of regenerative amplifier, resulting in the potential for a pulse bifurcating instability. If
the amplifier gain is not sufficient to saturate the amplifier, a “runt pulse” will be generated
and excited state population will not be fully depleted. Continued pumping means that on the
next cycle of the regenerative amplifier, the stored energy will be higher. The result is
saturated amplification on every other cycle of the amplifier. The solution to this issue is to
ensure saturated amplification by using a high injection energy and operating at a gain that
allows for saturated amplification within a limited number of passes of the amplifier. Figure 4
shows experimental oscilloscope traces of the laser output as a function of the number of
passes through the amplification crystal between switching the pulse in and out of the cavity.
With less than 20 passes, the laser pulse train is normal and stable at 1 MHz. However, Fig.
4b shows that one pulse gains more energy than the other for 35 passes through the laser
crystal. Figure 4c shows that the previous pulse depletes the gain for the next pulse so there is
no significant amplification of the later pulse for 50 passes through the laser crystal.
To understand how to optimize the amplifier system, we developed a simple model based
on the recurrence relation developed in reference [26] for regenerative amplifiers:
1
,k s
J TJ+= (1)
1
1 10.5( ) / ,
k k k k sg g J T J J
−
+ += − − (2)
where Jk and gk are the laser fluence and low signal gain for the kth
pass through the laser
crystal; T and JS are the laser cavity transmission efficiency and the saturation fluence
respectively; and exp( )k k
G g= .
After the laser pulse is amplified, the gain is reduced. We can simplify the buildup of the
laser gain under the influence of a continuous pumping rate and spontaneous decay as a
function of time by
/
0 0( ) ( ) ,ft
fg t g g g e
τ−
= − − (3)
where gf is the gain immediately after the previous pulse amplification, and g0 is the
asymptotic gain, which is approached as t becomes large compared to the spontaneous decay
time τf.
Using the recurrence relation (1) and (2), we calculate the laser intensity and gain after
each time a pulse passes through the amplifier crystal in the regenerative amplifier cavity.
Then we can use Eq. (3) to estimate the gain recovery after the pulse is dumped out of the
laser cavity. In order to compare with experimental results, we measured our gain in our laser
as 0
1.2g = . As demonstrated in Fig. 5, we calculated the pulse energy ratio of the two
adjacent pulses, defined as ( ) /previous later previous
J J J− , as a function of number of passes through
the amplifier crystal at different repetition rates—ideally this parameter is zero for a stable
pulse train. The results in Fig. 5 are obtained using a pump power of 22W at 532 nm, an 80
µm pump mode at the crystal, and 2% loss in the amplifier cavity, which saturates the laser
pulse amplification at 50 passes. At 10 kHz (the black curve in Fig. 5), the pulse energy ratio
is constant at zero even with up to 200 passes. At 100 kHz, instability begins to appear as the
number of passes exceeds 25. Interestingly, as the number of passes keeps increasing, the
ratio eventually returns to zero at ~62 passes. This is due to the loss introduced in the laser
cavity that reduces the energy of the previous pulse to the level that the later pulse can reach
at lower gain. For repetition rates above 312.5 kHz, the ratio can be as high as 1 above 50
passes when the previous laser pulse amplification is completely saturated; i.e. as in the case
#158879 - $15.00 USD Received 28 Nov 2011; revised 3 Feb 2012; accepted 6 Feb 2012; published 13 Mar 2012(C) 2012 OSA 26 March 2012 / Vol. 20, No. 7 / OPTICS EXPRESS 7020
of Fig. 4c. From this we conclude that in order to achieve good pulse-to-pulse energy
stability, the pulse should be switched-out before its energy significantly saturates the laser
gain. This also results in a (modest) reduction in amplifier efficiency. For example, we obtain
an overall efficiency of 16.8% at 1 MHz, while the typical efficiency at a lower repetition rate
using a pulsed pump laser is >20%.
0 50 100 150 200
0.0
0.2
0.4
0.6
0.8
1.0
Pu
lse
en
erg
y R
atio
Number of Passes
10 kHz
100 kHz
300 kHz
500 kHz
1 MHz
3 MHz
Fig. 5. Laser pulse energy ratio ( ) /previous later previous
J J J− v.s. number of passes through the laser
crystal at different repetition rates. When the ratio is zero, the two adjacent pulses have the
same pulse energy. When the ratio is 1, the first pulse completely suppresses the gain of a later
pulse.
4. Conclusion and summary
We demonstrate the first ultrafast amplifier that can produce 52 fs, 3.7 uJ pulses at 1 MHz
and 800 nm. The repetition rate is continuously tunable from 50 kHz up to 1.7 MHz. We
discussed the technical challenges and how they were overcome. Although the output power
is currently limited by the available pump laser power at 22 W, the maximum pump power
this laser can accommodate is likely to be >100 W, limited by crystal cooling capacity.
Acknowledgments
This work was funded in part by Department of Energy SBIR grant DE-FG02-06ER84469.
#158879 - $15.00 USD Received 28 Nov 2011; revised 3 Feb 2012; accepted 6 Feb 2012; published 13 Mar 2012(C) 2012 OSA 26 March 2012 / Vol. 20, No. 7 / OPTICS EXPRESS 7021