Enhancement of temporal contrast in a femtosecond petawatt Ti:sapphire laser

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2013 Laser Phys. Lett. 10 095302

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IOP PUBLISHING LASER PHYSICS LETTERS

Laser Phys. Lett. 10 (2013) 095302 (4pp) doi:10.1088/1612-2011/10/9/095302

LETTER

Enhancement of temporal contrast in afemtosecond petawatt Ti:sapphire laser

Y Xu, X Y Guo, Y S Huang, Y Y Li, X M Lu, C Wang, Y Q Liu,W P Wang, H Zhang, Y X Leng, X Y Liang, B F Shen, R X Li and Z Z Xu

State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics,Shanghai 201800, People’s Republic of China

E-mail: lengyuxin@mail.siom.ac.cn, ruxinli@mail.shcnc.ac.cn and zzxu@mail.shcnc.ac.cn

Received 7 April 2013, in final form 20 May 2013Accepted for publication 20 May 2013Published 2 August 2013Online at stacks.iop.org/LPL/10/095302

AbstractSignificant enhancement of temporal contrast is achieved in a femtosecond petawattTi:sapphire laser. A pulse cleaning technique, combining double-CPA, non-collinear opticalparametrical amplification and second harmonics generation, is applied to improve theamplified spontaneous emission (ASE) contrast. Meanwhile, pre-pulse leaking from theregenerative amplifier is suppressed by double Pockels cells and a saturable absorber.Measurement results show that pre-pulse on nanosecond scale can be effectively suppressedand the ASE contrast can be improved from the original ∼10−8 to ∼2× 10−12. Protonacceleration experimental results with a foil target also demonstrate a significant enhancementof temporal contrast.

(Some figures may appear in colour only in the online journal)

1. Introduction

With the remarkable development of petawatt (PW)-classchirped pulse amplification (CPA) laser systems [1–4], manyexciting opportunities are created for laser–matter interactionsin the relativistic regime [5, 6]. In the particular case ofsolid–target experiments, laser pulses with high temporalcontrast are required to prevent unwanted intense pre-pulses.Intense pre-pulses can generate pre-plasma before the mainpulse arrives at the target and destroy the experiment.Generally, the temporal contrast should be better than 10−10 torestrict destructive pre-plasma dynamics at the 1020 W cm−2

incident intensity level [7]. Furthermore, a pulse with highertemporal contrast is required for some specific targets, such asfoil targets for proton acceleration.

Typical petawatt Ti:sapphire laser systems based onthe CPA scheme generally suffer two kinds of pre-pulses:amplified spontaneous emission (ASE) and pre-pulses leakingfrom the amplifiers. ASE can usually be suppressedby injecting a high energy, high temporal contrast seed

pulse into the CPA amplifier stages [8]. Based on thedouble-CPA scheme [9], various pulse cleaning techniqueshave been applied to suppress ASE, such as rotation ofthe polarization ellipse in gases [10], cross-polarized wave(XPW) generation [11, 12], saturable absorber [3, 13] andoptical parametrical amplification (OPA) [4]. Pre-pulsesleaking from the regenerative amplifier can usually besuppressed by the utilization of an external Pockels cell (PC).However, considering the limited extinction of the single PC,additional methods, such as double PC and saturable absorber,are still required to achieve a better pre-pulse suppression.

In this work, we present the recent technical progresson temporal contrast enhancement in our femtosecondpetawatt Ti:sapphire laser. A newly developed pulse cleaningtechnique [14, 15], which combines double-CPA, non-collinear optical parametrical amplification (NOPA) andsecond harmonics generation (SHG), is applied to improvethe ASE contrast. Meanwhile, the pre-pulse leaking fromthe regenerative amplifier is suppressed by double PC anda saturable absorber. In addition, the output energy of the

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Laser Phys. Lett. 10 (2013) 095302 Y Xu et al

Figure 1. Layout of high temporal contrast petawatt Ti:sapphire laser.

final amplifier is boosted to 50.8 J under the pump energyof 105 J. After compression, with a transmission efficiencyof 72%, a laser pulse with peak power of 0.8 PW andpulse duration of 45 fs can be achieved. Temporal contrastmeasurements show that pre-pulse on nanosecond scale canbe effectively suppressed, and the ASE contrast is improvedfrom the original ∼10−8 to ∼2 × 10−12 (100 ps before themain pulse). Experimental results of proton acceleration witha foil target also demonstrate that the temporal contrast can beenhanced significantly.

2. Setup of high temporal contrast femtosecondpetawatt Ti:sapphire laser system

The schematic diagram of our high temporal contrast petawattTi:sapphire CPA laser system is shown in figure 1.

The high temporal contrast front-end consists of acommercial 1 kHz/3.8 mJ/35 fs Ti:sapphire CPA laser anda novel pulse cleaning system [14, 15]. This pulse cleaningsystem is based on NOPA and SHG processes. It consistsof four parts: (1) a white light continuum (WLC) generationstage for generating broadband seed pulses; (2) WLC-seeded infrared NOPA stage for generating broadband idlerpulses near 1.6 µm with stabilized carrier–envelop phase;(3) infrared NOPA stage seeded by the idler pulses forgenerating 1.6 µm pulse with 0.5 mJ energy; (4) SHGprocess for generating cleaned femtosecond pulses by usingthe infrared pulse frequency doubling with a nonlinear crystal(0.1 mm thick BBO plate). After the SHG process, 0.1 mJcleaned pulses with ∼35 fs pulse duration can be generatednear 800 nm.

In the following second CPA stage, the high temporalcontrast seed pulse is stretched to ∼600 ps (FWHM) durationin an all-reflective Offner type stretcher, which is designedto match the compressor in our previous petawatt laser [2].The transmission efficiency of the stretcher is 34%, whichresults in seed pulses with energy 34 µJ being injected

into the regenerative amplifier (Ti:s Reg Amp). The 4.5 mJoutput pulse from the regenerative amplifier is passed throughdouble PC and a saturable absorber (45% transmission) forsuppression of pre-pulse on nanosecond scale.

After pre-pulse suppression, the pulse is expanded to4 mm in diameter and introduced into a six-pass amplifier(Ti:s Amp. I). The output pulse energy is 50 mJ under thepump energy of 200 mJ. Then the pulse is expanded to10 mm in diameter and injected into a four-pass amplifier (Ti:sAmp. II). Two antireflection-coated DOE with diffractionefficiency of 81% are used for pump beam homogenization inthis amplifier. With total pump energy of 2 J on the Ti:sapphirecrystal, output pulse with energy of 840 mJ can be obtained atthe repetition of 10 Hz.

To increase the laser peak power to the PW-level, wefurther boost the pulse energy in a four-pass power amplifier(Ti:s Amp. III) and a three-pass final amplifier (Ti:s Amp. IV),which operates on a single shot basis limited by the pump laser(four shots per hour). Based on previous work [2], we enhancethe output energy of the final amplifier. The output energyof the Nd:glass pump laser is upgraded with added Nd:glassamplifier stages, and the beam sizes of the pump and signalpulses are enlarged to 68 mm and 65 mm in the final amplifierrespectively. Transverse parasitic lasing is suppressed by thecladding method using refractive index-matched liquid dopedwith ink powder, and with careful adjustment of time delayand spatial overlap between the signal pulse and the pumppulse [16]. Then output energy of 50.8 J is achieved withpump energy of 105 J in the final amplifier, corresponding tothe conversion efficiency of 47.5%.

The amplified laser pulse from the final amplifier isexpanded to 150 mm in diameter and introduced into thepulse compressor using four 1480-groove mm−1 gratings.Due to the pulse duration of the seed and the gain narrowingeffect mainly in the regenerative amplifier, a compressed pulsewith duration of only 45 fs is achieved now. Therefore, thepulse peak power of 0.8 PW can be obtained now with 72%transmission efficiency of the compressor.

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Laser Phys. Lett. 10 (2013) 095302 Y Xu et al

Figure 2. Comparable contrast curves. Pre-pulses are identified asthe artifacts generated by multiple reflections at the opticalcomponents in the cross-correlator and the amplifier.

3. Experimental results and discussion

The ASE contrast is measured with a scanning third-order cross-correlator (Amplitude Technology, Sequoia). Forcomparison, the ASE contrast ratio of the high contrast front-end, the high contrast petawatt Ti:sapphire laser (without thesaturable absorber) and our original petawatt Ti:sapphire laserare all measured. Figure 2 presents the comparable temporalcontrast curves under different conditions.

Our original petawatt Ti:sapphire laser is not a double-CPA system, a Ti:sapphire oscillator rather than a hightemporal contrast front-end is used as the seed source. Onlya single Pockels cell is used after the regenerative amplifierfor pre-pulse suppression. The measured ASE contrast of ouroriginal petawatt Ti:sapphire laser is about ∼10−8 at 100 psbefore the main pulse (shown as the red curve in figure 2),which is the typical temporal contrast ratio for the traditionalCPA laser.

In our high temporal contrast petawatt Ti:sapphire lasersystem, the high temporal contrast front-end can deliver0.1 mJ seed pulses with contrast of ∼2 × 10−12 (shown asthe gray curve in figure 2). The measurement has reached themaximal dynamic range of the third-order cross-correlator.

Thanks to the cascaded second-order nonlinear processes(NOPA with SHG), the front-end in our petawatt lasersystem can generate a seed pulse with very high temporalcontrast [15]. Meanwhile, the energy (0.1 mJ) of the front-endis high enough for ASE suppression in the second CPA stage.The high temporal contrast front-end can serve as an excellentseed source for the petawatt laser system.

Based on a double-CPA scheme and the introduction ofthe high temporal contrast front-end, ASE contrast can beimproved by more than three orders of magnitude in ourpetawatt Ti:sapphire laser. ASE contrast of ∼2 × 10−12 at100 ps before the main pulse can be achieved in the hightemporal contrast petawatt Ti:sapphire laser (shown as theblack curve in figure 2). This measurement result is obtainedwhile the saturable absorber is removed from the amplifier. Itdemonstrates that effective ASE contrast enhancement can beaccomplished just by introducing the high temporal contrastfront-end into the petawatt laser. Limited by the dynamicrange of the measurement, further improvement of the ASEcontrast by the saturable absorber cannot be observed withthe third-order cross-correlator, and the measured temporalcontrast curves are similar whether there is a saturableabsorber or not.

Besides the ASE noise, pre-pulse leaking from theregenerative amplifier is another cause of contrast problem.The pre-pulse is suppressed by double PC and a saturableabsorber in our high temporal contrast petawatt Ti:sapphirelaser. The suppression of pre-pulse on nanosecond scale canbe observed by a photodiode (shown in figure 3). All themeasurement results are obtained after amplification in the10 Hz four-pass amplifier (Ti:s Amp. II).

Figure 3(a) shows the pre-pulses leaking from theregenerative amplifier. The measured intensity ratio betweenthe first pre-pulse (∼11 ns before the main pulse) and the mainpulse is about 2.2×10−3. This measurement result is obtainedwithout the application of double PC and saturable absorber.

Figure 3(b) shows the suppression of pre-pulse bydouble PC and saturable absorber. This measurement resultis obtained after the application of double PC and saturableabsorber. No pre-pulses can be observed even when the mainpulse causes severe saturation on the photodiode. Limited by

Figure 3. (a) Shows the pre-pulses leaking from the regenerative amplifier. (b) Shows the suppression of pre-pulse by double PC andsaturable absorber.

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Laser Phys. Lett. 10 (2013) 095302 Y Xu et al

the dynamic range of the measurement, the exact contrast ratiocannot be directly measured by the photodiode. However, anestimation of pre-pulse contrast can be given here.

Switching off the two PC and using half wave plates tohelp pulse passing through the PC assembly, the measuredintensity ratio between the pre-pulse and the main pulseis about 2.5 × 10−5 after inserting the saturable absorber.It means that pre-pulse contrast can be improved about 90times by the saturable absorber. The saturable absorber is aneconomical and effective method for pre-pulse suppression,and it can serve as an assistant of general Pockels cell.

Removing the saturable absorber and using only one PCassembly after the regenerative amplifier, the measured inten-sity ratio between the pre-pulse and the main pulse is about2×10−6. It means that pre-pulse contrast can be improved byabout three orders of magnitude by one PC assembly. Thus,pre-pulse contrast on nanosecond scale should be improved byabout six orders of magnitude by the utilization of double PC.

On ignoring the different amplification gain between thepre-pulse and the main pulse in the following single shotamplifiers, and ignoring the different compressed pulse du-ration between the pre-pulse and the main pulse, the pre-pulsecontrast on nanosecond scale can be estimated to be ∼10−11.

To test the enhancement of temporal contrast, apreliminary proton acceleration experiment with a foil targetis carried out. Using an f /4 off-axis parabola mirror, theefficient focused intensity on the target is∼1×1019 W cm−2.Under the same focused intensity, different laser interactionresults can be obtained with different temporal contrastconditions.

Based on our original petawatt Ti:sapphire laser system(ASE contrast∼10−8, pre-pulse contrast on nanosecond scale∼10−6), no obvious energetic protons can be detected with a30 nm thick C target and a 2.5 µm thick Al target.

Based on the high temporal contrast petawatt Ti:sapphirelaser (ASE contrast ∼2 × 10−12, pre-pulse contrast onnanosecond scale ∼10−11), the maximum proton energy of2.2 MeV can be obtained with a 30 nm thick C target and themaximum proton energy of 7 MeV can be obtained with a2.5 µm thick Al target.

The proton acceleration experimental results alsodemonstrate temporal contrast enhancement in our petawattTi:sapphire laser. With further improvement of laser focusedintensity by a deformable mirror and an f /2 off-axis parabolamirror, a detailed investigation of proton acceleration undera higher focused intensity (above 1020 W cm−2) will bepresented in our future work.

4. Conclusion

In conclusion, temporal contrast enhancement in a fem-tosecond petawatt Ti:sapphire laser system is achieved bycombining different pulse cleaning techniques. A pulsecleaning technique, based on the combination of double-CPA,NOPA and SHG, is applied to improve the ASE contrast.Meanwhile, double PC and a saturable absorber are applied tosuppress the pre-pulse on nanosecond scale. The measurementresults show that pre-pulse on nanosecond scale can be

effectively suppressed and the ASE contrast can be improvedfrom the original ∼10−8 to ∼2 × 10−12 (100 ps before themain pulse). Preliminary proton acceleration experimentalresults with a foil target also demonstrate that temporalcontrast can be enhanced significantly.

Acknowledgments

This work was supported by the National Basic ResearchProgram of China (No. 2011CB808101), the ChineseAcademy of Science, and the National Natural ScienceFoundation of China (NSFC) (Nos 11127901, 10734080,60921004, 60908008, 61078037).

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