Passively Q-switched nanosecond pulse-train Nd:YAG laser system

Passively Q-switched nanosecond pulse-train Nd:YAG laser system
M. Szczurek, R. Jarocki, A. Bartnik, H. Fiedorowicz, J. Kostecki, R. Rakowski, A. Szczurek
Institute of Optoelectronics, Military University of Technology, 2 Kaliskiego Street,
00-908 Warsaw, Poland
ABSTRACT
A pulse-train Nd:YAG laser system consisting of repetitively Q-switched Nd:YAG oscillator with Cr4:YAG saturable
absorber, double pass Nd:YAG amplifier and stimulated Brillouin scattering (SBS) pulse compressor has been
demonstrated. Number and energy of the laser pulses were controlled by adjusting width and amplitude of the flash lamp
pumping pulses. Efficient SBS compression ofNd:YAG laser pulses was obtained using single-cell compressor.
Keywords: Nd:YAG laser, passive Q-switch, pulse-train, SBS compression
1. INTRODUCTION
In this paper we report our preliminary studies on the nanosecond laser pulse train generation, amplification and
compression. Passively Q-switched Nd:YAG laser system was specially designed for the laser-plasma EUV source with
a laser-irradiated gas puff target [1]. The gas puff target has several hundreds microseconds lifetime and can be created
with frequency not higher than 1 0 Hz due to the problems with gas evacuation from the vacuum chamber. However
because of the relatively long time duration of the gas puff target in accordance to a laser pulse it is possible to irradiate it
several times by so called laser pulse train. In this case an average power of the EUV source can be increased even an
order of magnitude. There are some conditions that have to be fulfilled. First of all the target after single excitation must
recover its initial parameters. It means that time separation between laser pulses should be at least several tens of
microsecond [2]. Secondly a power density ofa single laser pulse in the focus should be ofthe order of lOh1lO13 W/cm2
depending ofthe wavelength range ofradiation to be produced [3].
The major advantages of passive Q-switching compared to active Q-switching are the elimination of drive
electronics and simplified resonator configuration [4]. Conventional passively Q-switched laser emits single shots but
using long flash lamp pumping pulses, when operated well above threshold, repetitive Q-switching appears and laser
pulse train generation is possible. For reliable generation of the uniform laser pulse trains stable flat top flashlamp
pumping pulses are needed. Cr4:YAG crystal has been widely used as saturable absorber for Q-switching in Nd:YAG
lasers [5-7]. We obtain reliable nanosecond pulse train generation from passively Q-switched Nd:YAG laser using high
quality Cr4:YAG crystal. Passive Q-switching technique enables producing a single longitudinal mode laser pulses by
deliberately lengthening the built up time of the laser pulse [8]. This feature of the laser pulse is very important for
efficient operation of the SBS compressor. The basic concept of SBS is well understood and widely documented in
literature [9]. This phenomenon is an important tool for improvement of the performance of solid-state laser systems.
Particularly SBS can provide temporal compression of the laser pulses [10-14]. To obtain reliable SBS high microparticle
purity of material used for SBS generator is needed [14,15]. Two liquids were selected for SBS pulse
compressor: carbon tetrachioride (CC14) as well known reference medium and perfluorohexane (C6F14) as a safe, efficient
and exceptionally stable medium [14, 1 6]. We report one order compression of 1 3 ns Nd:YAG laser pulses in a single
SBS cell compressor.
348
2. EXPERIMENTAL SETUP
The experimental setup of the pulse train laser system shown in Fig. 1 has three major components: Nd:YAG laser
oscillator, double-pass Nd:YAG amplifier and SBS compressor. A laser head with 4 mm x 4" Nd:YAG laser rod pumped
by a linear Xe flashlamp was used at the oscillator. A conventional resonator consisting of a spherical HR mirror Ml
Solid State Lasers XIV: Technology and Devices, edited by Hanna J. Hoffman,
Ramesh K. Shori, Proceedings of SPIE Vol. 5707 (SPIE, Bellingham, WA, 2005)
0277-786X/05/$15 · doi: 10.1117/12.589125

i:i] T''""
:: ;UL_J "H— "'""""
!
1
" :: "I""'":: ::. :
..
"
__ -J
Fig. 1. Experimental setup ofthe pulse-train laser system: Ml, M2, resonator mirrors (spherical and flat, respectively); LH, Nd:YAG
laser head; DP, dielectric polarizers; SA, saturable absorber (AR coated Cr4:YAG crystal); A, aperture; T, telescope; AMP,
Nd:YAG amplifier; 2/4, retardation plates; HR, high-reflective mirrors; L, AR coated bi-convex lens; SBS, stimulated
Brillouin scattering cell.
and a plane output mirror M2 was passively Q-switched by a Cr4:YAG saturable absorber with an initial transmission of
25%. After oscillator a Galilean telescope was used. The telescope has two functions: changes the beam diameter by a
factor M=3 and allows for control ofthe laser beam divergence. Laser head with 5 mm x 3.5" Nd:YAG laser rod pumped
by a linear Xe flashlamp was used as a double-pass amplifier. A polarization switch consisting of 2/4 retardation plate
and dielectric polarizer was used to extract the laser pulse after second pass through the amplifier. After amplification a
laser pulses were directed to the SBS compressor through 2/2 retardation plate and polarization switch. The beam was
focused with a lens of 70 cm focal length into SBS cell. The SBS cell was a glass tube of 150 cm length containing
perfluorohexane (C6F14) or carbon tetrachioride (Cd4) as reference medium. Laser power supplies with high-current
insulated gate bipolar transistors were used for direct control of the flashlamp current in a microsecond time scale.
Rectangular pumping pulses with variable pulse width up to 2 ms were used. The laser system was operated with
repetition rate 1-10 Hz.
The laser pulse energy diagnostics were incorporated into the laser system using uncoated BK-7 beam splitters
and calibrated dual channel energy meter (Laser Precision RJ-7620) with the pyroelectric detectors (RJP-735). The
temporal profiles of the laser pulses were recorded using a large area vacuum photodiodes (ITL TF 1 850) coupled to the
digital oscilloscope (Tektronix TDS 620). The time integrated spatial profiles of the laser pulses were imaged onto a
CCD camera (Pulnix TM-745) and recorded using laser beam analizer (Spiricon LBA 100-A)
3. RESULTS
Temporally smooth, nearly Gaussian-shaped Q-switched pulses were achieved owing to the longitudinal-mode selection
properties of the saturable absorber. Typical temporal shape of the laser pulse is presented in Fig. 2. Very stable 13 ns
laser pulses were obtained using short cavity configuration and moderate pumping level to avoid depolarization and
thermal effects. The oscillator was operated at the fundamental transversal mode and near field spatial distribution of the
laser beam is presented in Fig. 3. The output laser beam was intracavity polarized by a dielectric polarizer for proper
operation of the polarization switches used to extract laser pulses at the amplifier and SBS pulse compressor systems. A
telescope of magnification M=3 was set up for normal adjustment at 632.8 nm with an autocollimator. A correction was
then made for 1064 nm to minimize of a laser beam divergence for selected pump and repetition rate using laser beam
analizer. The magnification of the telescope was chosen to match the beam profile to the double pass amplifier aperture
and avoid diffraction effects.
Proc. of SPIE Vol. 5707 349

Fig. 2. Temporal shape ofthe oscillator laser pulse.
Fig. 3. Spatial distribution ofthe oscillator laser beam.
Reliable repetitive Q-switching was achieved using stable rectangular flashlamp pumping pulses presented in Fig. 4 a.
Typical laser pulse train is presented in Fig. 4 b. It consists of ten laser pulses separated by about 100 ts. Generally the
number of pulses can be controlled by adjusting of the pump energy, the initial transmission of the absorber and the
output coupling of the resonator.
a,
0
) 508
::::
E
7
j
1
20 40 60
Time [nsj
13n FWHM
80 100 120
Fig. 4. a) Temporal shape of the flashlamp pumping pulse. b) Temporal structure of a laser pulse train.
350 Proc. of SPIE Vol. 5707
1000 1500
Time ['as]
a)
2000 2580
a,
So
OS
E
03t)
S
:: : :'
200 400 600 800 1000
Time ['as]
b)

In our experiment number of laser pulses in the laser pulse train was controlled by changing of the oscillator flashlamp
pumping energy for 1 ms duration of the flashlamp pulse to matched with lifetime of the gas puff target. The number of
laser pulses as a function of flashlamp pumping energy is shown in Fig. 5.
Q)Cl)
• E
0,4
0,2
0,0
200
400
600
800
1000
Fig. 5. Laser pulse trains as a function of oscillator pumping energy.
The laser pulses creating trains shown in Fig. 5 have equal energy and duration (FWHM). Laser pulse train energy vs.
number of laser pulses is shown in Fig. 6.
30
25
20
E 15
a)
w 10
2 4 6 8 10
15
Number of pulses
Fig. 6. Energy ofthe laser pulse train vs. number of pulses.
Low intensity laser pulses from the oscillator should be amplified to obtain intensity required for effective EUV and soft
X-ray production (101 1.4013 W/cm2). Assuming 100 tm diameter of the focus we should have 100 mJ of energy for 1 3 ns
laser pulse to reach power density of lOl 1W/cm2 after focusing of the laser beam. To obtain a flat envelope of the
amplified laser pulse train the energy of a single laser pulse should be chosen in such a way, that depletion of the
inversion caused by the passage of the one pulse will be equal to the production of the inversion between two pulses.
Then the inversion will be constant during a pulse train and each pulse will experience the same gain. For fixed oscillator
20
25
30
35
Proc. of SPIE Vol. 5707 351

output energy at the level of about 10 mJ (3 pulses, see Fig. 6), the performance of the double-pass Nd:YAG amplifier
and SBS compressor was investigated. Laser pulse train energy of about 240 mJ with 1 Hz repetition rate was obtained.
Full extraction of the energy stored at the amplifier was limited by lack of the sufficient optical isolation of the oscillator
from back-reflected laser pulses. Faraday rotators before optical switches should be applied. SBS reflectivity was
investigated as a function of the input energy delivered from the output of double pass Nd:YAG amplifier. Results of the
measurements are presented in Fig. 7.
0,8
0,7
0,6
0,5
> 0,4
0
a)
0,3 U CF
0,2 A CCI4
0,1
20 40 60 80 100 120 140
Input energy [mJ]
Fig. 7. SBS reflectivity as a function of input energy for two SBS active media: perfluorohexane (C6F14) and carbon tetrachioride
(Cd4). The bars represent the standard deviation obtained from an average of 10 shots for each value of reflectivity.
In general, the limit to the peak power that can be delivered to a SBS pulse compressor is determined by competing
processes such as stimulated Raman scattering, self focusing, and optical breakdown. In our experiment an optical
breakdown at the vicinity of the focus due to avalanche ionization of SBS medium initiated by spark on impurities for
input energy higher than 50 mJ was observed. Effective crossection for this process is proportional to the size of the
particle. Decreasing the size and amount of micro-particles gives us the possibility of reducing the generation of free
charge in the liquid necessary for electron avalanche ionization and avoid breakdown [15]. As a result, it is necessary to
remove these micro-particles using closed loop filtration system installed on the SBS cell. Such system is under
preparation and pressure filter funnel assemblies with removable fritted disc (Aldrich Z422924- 1 EA) and glass
microfibre filters (Whatman Z242543, Z258288) will be applied to retain micro-particles. Ultrafiltration below 0. 1 jtm is
needed [14, 16]. For the input energy less than 50 mJ reliable SBS reflection was observed. Temporal profiles of the
Stokes pulses measured at the output of the SBS compressor are presented in Fig. 8. For Cd4 10 times and for C6F14 8
times compression was obtained.
(I)
C
0V '
0,
E
::
...
352 Proc. of SPIE Vol. 5707
1.2 n&FWHM
.
5 10 15 20 25
Time [nd
a)
Fig. 8. Stokes pulses measured at the output of SBS compressor: a) for CC14 and b) for C6F14.
I
C
C
0,
'0
0.
E
0,10
I
1,6 n&FWHM
___
1
10
Time [no]
b)
15 20 25

3. CONCLUSIONS
In conclusion, we found repetitive passive Q-switching technology using Cr4:YAG saturable absorber to be an efficient
method for stable generation of the nanosecond pulse trains. Effective compression of a laser pulses was obtained.
Reliable operation of the SBS compressor for the laser pulse train energy higher than 50 mJ requires ultrafiltration of the
SBS active medium. For high efficiency of the EUV and soft X-ray production further amplification of the laser pulse
train is needed. It will be realized by applying second double pass Nd:YAG amplifier at the SBS phase conjugation
scheme.
ACKNOWLEDGEMENTS
The authors wish to thank Z. Mierczyk for providing Cr4:YAG saturable absorbers. We are also grateful P. Wachulak
for the technical assistance. This work was supported by the following grants of the Ministry of Scientific Research and
Information Technology ofPoland: No. 3 TO8C 002 27, No. 3 Ti lB 03 i 27 and No. 120/E-4i0/SPB/Eureka/O-13/DWN
727.
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