Multi-microjoule, MHz repetition rate Ti:sapphire ......Multi-microjoule, MHz repetition rate...

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

*[email protected]

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.


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)


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.


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.


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


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.


Multi-microjoule, MHz repetition rate Ti:sapphire ......Multi-microjoule, MHz repetition rate...

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