IRAQI JOURNAL OF APPLIED PHYSICS

All Rights Reserved ISSN (printed) 1813-2065, (online) 2309-1673 Printed in IRAQ 9

Tayyab Imran1 Mukhtar Hussain2

Using Frequency Resolved

Optical Gating for

Optimization of Thermal

Lensing Compensated

Ti:Sapphire Femtosecond

Laser System

1 Department of Physics and

Astronomy,

College of Science, King Saud University,

P.O. Box 2454, Riyadh 11541,

SAUDI ARABIA 2 Department of Physics,

Govt. College University Faislabad,

Sahiwal, PAKISTAN

We report the characterization and optimization of thermal lensing compensated

high peak power Ti:Sapphire femtosecond laser system, 4.0mJ pulse energy

operating at 1-kHz repetition rate. Thermal lensing is compensated by employing

convex folding mirrors and Peltier coolers while thermal eigenmode post-

amplifier has introduced to retain the amplified laser pulse beam on the

amplifier crystal. Single-shot second harmonic generation (SHG) frequency-

resolved optical gating (FROG) diagnostic technique is employed to characterize

the output compressed laser pulses. FROG image is monitored by charged-

couple device (CCD) attached to the personal computer and optimization of the

laser system is observed by FROG image in real time. Grating detuning is carried

out in the compressor to optimize the minimum possible pulse duration and pulse

of 30 fs duration is measured at the zero detuning scale.

Keywords: Ultrafast lasers; Ti:Sapphire laser; Chirped pulse amplification; Optical gating

1. Introduction

A swift advancement in the development of high

average peak power laser systems have been

observed in recent years [1-5]. The high peak power

laser systems have become quite important in

various experimental applications such as high

harmonics generations (HHG), white-light

continuum (WLC), plasma and optical field

ionization [6-8]. High peak power laser system

needs high power pump laser beam to pump the

crystal in the amplifier. This high pump power in

amplifiers induced the thermal effect in amplifying

crystal which leads to the distortion in the amplified

pulses that ultimately reduce the efficiency of the

amplifier. To perform experiments, it is essentially

required to characterize and optimize the spectral

and temporal evolution of thermal lensing

compensated laser systems.

Different diagnostics techniques have been

employed to characterize the high power

femtosecond laser systems such as auto-correlation

[9,10], spectral phase interferometry for direct

electric-field reconstruction (SPIDER) [11], and

frequency resolved optical gating (FROG) [12-16].

The autocorrelation technique fails to provide

information about the phase of the pulse therefore

the shape of temporal profile is guessed before to

make experimental measurement, on the other hand

SPIDER technique can provide spectral and

temporal information but the experimental setup is

quite complicated and difficult to align. The FROG

technique which can be described as a spectrally

resolved auto-correlation measurement, simple in

setup and efficient to characterize the spectral and

temporal evolution of the femtosecond pulses. There

are different versions of FROG diagnostic

techniques [14], the most sensitive version of FROG

is second harmonic generation (SHG) FROG.

Further it can be a categorized into multi-shot

FROG and single-shot FROG. In this article, we

explain and investigate the characterization and

optimization of thermal lensing compensated high

power Ti:Sapphire femtosecond laser system

operating at 1-kHz repetition rate by employing

SHG-FROG technique.

2. Femtosecond laser system

A Ti:Sapphire femtosecond laser system

operating at 1-kHz repetition rate consist of an

oscillator, a grating stretcher, multi pass pre-

amplifier, post-amplifier, and a grating compressor.

The block diagram of femtosecond laser system is

shown in Fig. (1), femtosecond pulses which are

generated from a mode locked femtosecond

Ti:Sapphire laser oscillator in the long cavity

arrangement running at 27 MHz repetition rate [17].

The laser pulses from the oscillator are stretched to

220 ps in 1400 grooves/mm ruling grating stretcher

[18]. Pulses are made to pass through Faraday

rotator to block the back reflection and backward

amplified spontaneous emission (ASE) from the

amplifier. The pulses are then sent to 8-pass pre-

IJAP, vol. (11), no. (3), July-September 2015, pp. 9-12

10 © Iraqi Society for Alternative and Renewable Energy Sources and Techniques (I.S.A.R.E.S.T.)

amplifier where the pulse train is extracted from the

amplifier after four passes which are selected by a

Pockel’s cell at 1-kHz repetition rate. The extracted

pulse is directed again to pre-amplifier to complete

the remaining 4-passes where pulse energy raises up

to 1.2 mJ. The output pulses from the pre-amplifier

are further directed to post-amplifier after passing

through the second Pockel’s cell, which improve the

contrast ratio and minimize the ASE emerges out

from the pre-amplifier. Finally the output pulse

energy reaches up to 7.0 mJ at the output of the

second amplifier [19].

When a high power laser is used as a pump

source, thermal lensing arises which is induced by

heating of Ti:Sapphire crystal of the amplifier which

results distortion of wave fronts of laser pulses that

reduces the focusing stability of laser spot.

Therefore, for adequate amplification efficiency and

good beam quality, one has to compensate the

thermal lensing [5, 20]. At the constant value of

input pumping power, the simple convex mirror or

lens can be used in order to compensate the thermal

lensing [21,22]. To compensate the thermal lensing,

we employed convex folding mirrors and the Peltier

cooler that is installed to lower the temperature of

the crystal up to -40°C. The Peltier cooler is

attached with the copper block on which the crystal

is mounted. By cooling the Ti:Sapphire crystal,

thermal conductivity increases and change in

refractive index per unit temperature decreases

which results the decrease of thermal lensing [23].

Further, to compensate the thermal lensing effects,

thermal eigenmode type 4-pass post-amplifier [12,

20] introduced to keep the beam size of the

amplified laser pulses on the crystal. The output

pulses of the post-amplifier are made to double pass

through a pair of parallel 1480 grooves/mm ruling

gratings compressor, and compressed pulse of 4.0

mJ obtained at the output.

Fig. (1) Schematic setup of 1-kHz repetition rate Ti:Sapphire femtosecond laser system: FR (Faraday Rotator), PC (Pockel’s Cell)

3. Results and Discussion

The frequency resolved optical gating (FROG)

technique [14] has been used for the characterization

of femtosecond laser pulses in time and frequency

domains simultaneously [14,24]. We employed the

most sensitive single-shot second harmonic

generation (SHG) FROG diagnostic technique to

align, characterize and optimize the high power

laser system. The experimental scheme of the

FROG setup is shown in Fig. (2). Compressed

pulses after the compressor is split up into two

identical pulses by using 50% beam splitter, these

pulses are line-focused by using cylindrical mirror,

the focused pulses are then recombined in a 100 µm

thick BBO (Beta Barium Borate) type nonlinear

crystal. The splitted pulses are line focused with a

small angle of 2° to accomplish single-shot FROG

configuration [25,26]. The overlapping beam with

small angle geometry in the nonlinear crystal

reduces the phase mismatching, but shortened the

measurable temporal range.

The output pulse from the nonlinear crystal is

spectrally resolved by a 150 grooves/mm ruling

grating and captured by a CCD camera. This

captured image is called the FROG trace or image.

To retrieve the information from FROG trace,

commercial FROG software (Femtosecond, Inc.) is

used. The FROG software algorithm uses the input

experimental data, starting with an initial guess

value, a better and closer guess is generated through

iterative Fourier transform algorithm, which

approaches the correct electric field. After running

number of iterations the FROG error decides the

reliability of the retrieval process [24].

IRAQI JOURNAL OF APPLIED PHYSICS

All Rights Reserved ISSN (printed) 1813-2065, (online) 2309-1673 Printed in IRAQ 11

Fig. (2) Schematic of FROG diagnostic to characterize the

laser pulses

The FROG trace or image is a two-dimensional

spectrogram, which has a delay time axis and a

wavelength axis. By using the FROG algorithm, the

intensity and phase profiles of the test pulse can be

retrieved from the experimental FROG trace, which

may provide complete information about the test

pulse in terms of intensity and phase profile. As in

our experimental setup, SHG-FROG trace can be

expressed as [12-16,25,26].

𝐼𝐹𝑅𝑂𝐺(𝜔, 𝜏) = |∫ 𝐸(𝑡)𝐸(𝑡 − 𝜏)𝑒𝑥𝑝(𝑖𝜔𝑡)𝑑𝑡∞

−∞|2 (1)

FROG error G(k) [24,25] is a root mean square

average across the trace which is difference of

experimental FROG trace and the retrieved FROG

trace.

𝐺(𝑘) = √1

𝑁2∑ |𝐼𝐹𝑅𝑂𝐺(𝜔𝑖 , 𝜏𝑗) − 𝐼𝐹𝑅𝑂𝐺

(𝑘)(𝜔𝑖 , 𝜏𝑗)|

2𝑁𝑖,𝑗=1 (2)

Where IFROG(i,j) and I(k)

FROG(i,j) are

representing the experimental and retrieved FROG

traces respectively, which are always normalized to

a peak of unity. The information of the femtosecond

pulse retrieved from the FROG trace is considered

to be reliable if FROG error is below than the noise

level of the experimental trace [23-25]. A two

dimensional FROG trace is retrieved from the

measured trace (Fig. 3) by running a number of

iterations using FROG software (Femtosecond,

Inc.). From the two dimensional FROG trace,

retrieved temporal and spectral evolution of the

compressed pulse was plotted, which reveals

temporal and spectral phase variations. From the

retrieved plots we observe, the FWHM of retrieved

temporal profile is 30 fs with relatively flat temporal

phase variations, which changes about 1 radians

peak to peak as shown in Fig. (3a).

Similarly retrieved spectral profile shows that

the phase distortion is less than 1 radian over the

bandwidth of 70 nm, as shown in Fig. (3b). The

FROG error for 256 X 256 trace was G=0.0028,

which is considerably low.

(a)

(b)

Fig. (3) (a) Retrieved temporal profiles of amplified laser

pulses, inset FROG trace, (b). Retrieved spectral profiles of

amplified laser pulses

At the compressor end of the laser system, the

optimized output spectrum is shown in Fig. (4a), the

efficiency of a laser pulse was optimized by

adjusting the incident angle into the grating and the

separation between gratings. We have adjusted the

grating separation in the compressor in order to

compensate the second and third group delay

dispersion and to optimize the minimum possible

pulse duration [23]. As it can be seen in Fig. (4b),

by detuning the grating, pulse duration changes

correspondingly, it is observed that the change in

pulse duration is very small when grating detuned

between -50 to 50 µm and the pulse duration has a

minimum (~30fs) at the zero detuning scale. These

results conclude the characterization and

optimization of the thermal lensing compensated

high peak power 1-kHz repetition rate femtosecond

laser system.

IJAP, vol. (11), no. (3), July-September 2015, pp. 9-12

12 © Iraqi Society for Alternative and Renewable Energy Sources and Techniques (I.S.A.R.E.S.T.)

(a)

(b)

Fig. (4) (a) Optimized output spectrum of the femtosecond

laser system, (b). Grating detuning to optimize the

compressor for minimum possible pulse duration

4. Conclusion

A thermally compensated Ti:Sapphire based

high power femtosecond laser system, 4.0mJ energy

per pulse, 30fs pulse duration operating at 1-kHz

repetition rate has been described, characterized and

optimized. Long cavity oscillator used as front end

of femtosecond laser system because of broadband

spectrum, low ASE and long interval between

pulses. To compensate the thermal lensing, convex

folding mirrors and Peltier cooler was used to cool

down the Ti:Sapphire crystal of the amplifier and

thermal eigenmode post-amplifier was introduced to

overcome the thermal lensing effects by keeping the

beam size of the amplified laser pulses on the

amplified crystal. Sensitive SHG-FROG single-shot

technique was employed to characterize the

thermally compensated femtosecond laser system.

The detuning of grating compressor vs pulse

duration was studied to optimize the compressor for

minimum possible pulse duration.

Acknowledgment

The authors acknowledge this research was

carried out at Korea Advanced Institute of Science

and Technology (KAIST), South Korea. One of the

author would also like to greatly acknowledge the

support of Prof. Dr. Nam Chang Hee and senior

fellow Dr. Jae Hee Sung.

References

[1] T. J. Yu et al., Opt. Express 20, 10807 (2012).

[2] S. Sawai et al., Appl. Phys. Express 7, 022702

(2014).

[3] J. H. Sung et al., Opt. Lett., 35, 3021 (2010).

[4] Y. Chu et al., Opt. Express 21, 29231 (2013).

[5] C. L. Blanc et al., IEEE J. Quantum Electron.,

4, 407 (1998).

[6] A. Flettner et al., Appl. Phys. B 73, 192

(2001).

[7] V. P. Kandidov et al., Appl. Phys. B 77, 149

(2003).

[8] H. G. Roskos et al., Laser & Photon. Rev., 1,

349 (2007).

[9] R. Trebino, and D. J. Kane, J. Opt. Soc. Am. A

10, 1101-1111 (1993).

[10] Ming-Sung Chao et al., Opt. Lett. 40, 902-905

(2015).

[11] C. Iaconis and L. A. Walmsley, Opt. Lett.

23,792-794 (1998).

[12] D. J. Kane, and R. Trebino, IEEE J. Quant.

Electronics. 29, 571-579 (1993).

[13] S. Linden, J. Kuhl, and H. Giessed, Opt. Lett.

24, 569-571 (1999).

[14] R. Trebino et al., Rev. Sci. Instrum., 68, 3277

(1997).

[15] J. Gagnon, E. Goulielmakis, and V. S.

Yakovlev, Appl. Phys. B 92, 25–32 (2008).

[16] R. Itakura et al., Opt. Express, 23, 10914

(2015).

[17] J. H. Sung et al., Jpn. J. Appl. Phys., 41, L931

(2002).

[18] G. Cheriaux et al., Opt. Lett., 21, 414 (1996).

[19] J. H. Sung, K, -H, Hong, and C. H. Nam, J.

Korean Phys. Soc. 7, 135 (2003). [19]

[20] C. L. Blanc et al., IEEE J. Quantum Electron.

4, 407 (1998)

[21] S. Ito et al., Appl. Phys. B 76, 497 (2003).

[22] S. Fourmaux et al., Opt. Express, 17, 178

(2010).

[23] R. Salin et al., Opt. Lett. 23, 718 (1998)

[24] K. H. Hong, Y. H. Cha, and C. H. Nam, J.

Korean Phys. Soc. 33, 315 (1998).

[25] K. W. Delong, and R. Trebino, J. Opt. Soc.

Am. A 11, 2429 (1994).

[26] K. H. Hong et al., Opt. Comm. 213, 193

(2002).