REVIEW OF SCIENTIFIC INSTRUMENTS 85, 126103 (2014)

Note: Pre-pulse characterization of femtosecond laser pulse by filamentation in transparent media X. L. Liu (),1,a) X. Lu (),2 J. L. Ma (),2 Z. G. Du (),1 Y. He (),1 Y. T. Li (),2 L. M. Chen (),2 and J. Zhang ()2,3 1

Academy of Opto-electronics, Chinese Academy of Sciences, Beijing 100094, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3 Key Laboratory for Laser Plasmas (MoE) and Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China 2

(Received 3 September 2014; accepted 27 November 2014; published online 12 December 2014) A new method and associating system has been presented to characterize pre-pulses of femtosecond laser using laser filamentation in transparent media. Pre-pluses of the laser system has been measured experimentally and it is in good agreement with the results obtained by third order cross-correlator. This method can be used for fast detection of temporal laser intensity relatively in order to avoid formation of pre-plasmas before laser matter interaction experiments. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4903853] Development of chirped-pulse amplification (CPA) technique1 together with titanium-doped sapphire media made it possible to produce table-top extreme laser system in laboratories. These systems produce laser pulses with picosecond (ps) to femtosecond (fs) in duration and up to petawatt (PW) peak powers,2–6 which reached a focused intensity of more than 1020 W/cm2 on target. Consequently, experimental study of laser-matter interaction in ultrahigh electromagnetic fields, such as nonlinear optics,7 bright xray sources,8 electron or ion acceleration,9, 10 and nuclear activation,11 can now be done using ultraintense and ultrashort laser pulses. However, typical CPA laser system generates not only a clean ultrashort pulse temporally but also satellite pulses as well as amplified spontaneous emission (ASE) background. These pre-pulses would generate plasmas before arrival of the main pulse, changing the target and eventually laser-matter interaction dynamics. So it is crucial to characteristic temporal intensity of the laser pulse relatively before ultraintense laser matter interaction studies. Conventionally, temporal intensity of the ultrashort laser pulse is characterized by third-order intensity cross-correlator (such as Sequoia, Amplitude Technologies) in a temporal range of a few nanoseconds with intensity dynamic of more than 109 . However, there are some shortcomings in the third order cross-correlation. First, pulse broadening would induce measuring error due to group velocity dispersion in transparent optical elements of the cross-correlator,12 which is even more critical for ultraintense laser with tens of fs pulse duration. Besides, some ps “ghost pre-pulses” are observed in the measuring results. They are generated by replica of post pulses from internal reflections in crystals and other optical elements.13 Eventually, the energy of laser pulse should be carefully attenuated below damage threshold of optical elements in cross-correlator. So the contrast ratio measured is not fully amplified laser pulse directly used in experiments a) Electronic mail: [email protected]

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though it is believed that there are little differences with the results.14 In the meantime, detailed and accurate temporal contrast ratio measurement is somewhat not much necessary in all cases. Particularly in ultraintense laser matter interaction experiments, the main focus is whether laser intensity before the main pulse is high enough to generate pre-plasmas. As a result, fast detection of relative laser intensity before the arrival of main pulse is of the most importance. In this letter, we demonstrated new approach to measure pre-pulse of femtosecond laser, based on laser filamentation15–17 in transparent media. Pre-pulse of Xtreme Light II (XL-II) laser system18 has been characterized experimentally and it is in good agreement with that by the commercial third-order cross-correlator. Pre-pulse characterization is performed using pumpprobe method. Schematic view of the system is shown in Fig. 1. Femtosecond laser pulse is first divided by a beam splitter. The reflected pump beam, energy of which is monitored by an energy meter, is focused by a 130 mm lens to induce filamentation in air. Uniformly distributed part of the transmitted beam is selected by an aperture with a diameter of 10 mm and is frequency doubled by a Potassium dihydrogen phosphate (KDP) crystal to 400 nm as the probe beam. The probe beam, which is large enough compared to the filamentation size, goes through the plasma area with a direction perpendicularly to that of the pump pulse to get the properties of filamentation. Filamentation induced by focused fs laser pulses is scanned using time-resolved shadowgraphs in pump-probe measurement. Area of the geometric focus is imaged on a 16 bit charge-coupled device (CCD) camera by a × 10 microscope. Appropriate neutral density filters are put in front of the CCD camera to avoid saturation. Time delay between the pump and probe pulse is varied by an optical delay line. Pulse duration of 400 nm probe pulse is estimated to be 170 fs by group velocity mismatch between the two pulses in the KDP crystal, while moving step of the translation motorized stage

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FIG. 1. Experimental setup for pre-pulse characterization. BS is a beam splitter, ODL is an optical delay line, ND is adjustable neutral density filters, OL is an objective lens of a microscope, L is a focusing lens, and EM is an energy meter.

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(SGSP26-100) is about 10 μm in the optical delay line. So temporal resolution of the pump-probe system is about 170 fs, which is the large one between pulse duration of probe pulse and that of moving accuracy. Pre-pulses of the laser system are measured by the following routines. First, define the temporal zero point. Set output energy of the pump pulse to be two or three times of the threshold power15 to induce filamentation in air. Temporal zero point is the moment when filamentation is just formed in the geometric focus of shadowgraphs by changing time delay between the pump and probe pulse. Second, increase laser energy to nominal energy of the laser facility and then monitor geometric focus when scanning the temporal range before the main pulse. If filamentation is formed before arrival of the main pulse in shadowgraphs, pre-pulse exists at this time and time delay between pre-pulse and main pulse can be confirmed. At last, keep optical delay line unchanged and decrease the output energy of pump laser continuously from the nominal energy. Record the minimum energy of the pump pulse that pre-pulse filamentation exists. So intensity ratio between main pulse and pre-pulse can be calculated using the above minimum energy and theoretical threshold power of filamentation in air. Using the above method and associating system, we have performed pre-pulse measurement on XL-II laser facility. It delivers laser pulses with up to 640 mJ and 60 fs in pulse duration. Fluctuation of the laser facility is less than 5%, so shadowgraphs taken in different time showing dynamics of the filamentation formation. Fig. 2(a) shows shadowgraphs of filamentation by scanning the temporal range before the main pulse with laser energy of 16.5 mJ. Attention should be paid on the area around geometric focus, which is indicated by a rectangular line in one of the shadowgraphs for example, to find out whether there is plasma channel before the arrival of the main pulse. If plasma channel formed in this area, it can be judged by the obvious changes around geometric focus in the subtracting image between the present and its previous one. As shown in Fig. 2(a), there would be prefilamentation in the area of interest around 1.4 ps. Then subtraction of shadowgraphs before this time is done which is

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FIG. 2. Shadowgraphs of filamentation generated by laser energy of (a) 16.5 mJ and (b) 37 mJ; subtracting image between shadowgraphs in different temporal delay with laser energy of 16.5 mJ: (c) between shadowgraphs at −1.6 ps and −1.8 ps and (d) between shadowgraphs at −1.4 ps and −1.6 ps. The interest area of geometric focus is indicated with rectangular dotted lines.

shown in Figs. 2(c) and 2(d). By comparison of these two figures, obvious changes of light intensity have taken place in the area around geometric focus which indicates that prefilamentation formed at 1.4 ps before arrival of the main pulse. This pre-filamentation is also demonstrated at this time when the energy of pump laser is increased, as shown in Fig. 2(b), with laser energy of 38 mJ, for example. So it must be induced by the pre-pulse, and temporal delay between the prepulse and main pulse can be characterized as 1.4 ps. When energy of the pump laser is decreased below 16.5 mJ, this pre-pulse filamentation disappears, demonstrating that energy of the pre-pulse is not high enough to induce filamentation

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which is still ensured by the translation motorized stage with high-precision. In conclusion, we have presented a new method and associating system to characterize pre-pulse of ultrashort laser pulse using laser filamentation in air. We demonstrate that the system can measure pre-pulse properties of fs laser pulse and the result is in good agreement with that measured by the third order cross-correlator. This method is especially suitable for fast demonstration of whether the contrast ratio of the laser system meets the requirements of specific physical experiment.

FIG. 3. Contrast ratio measurement by third order cross-correlator.

in air. So energy ratio of the pre-pulse and main pulse can be calculated by the ratio of threshold power of filamentation and that of the minimum energy mentioned above. It is 1.2 × 10−2 in this experiment (Pcr t/Emin ). Fig. 3 shows contrast ratio obtained by third-order cross-correlator Sequoia. There is a pre-pulse in about 1.4 ps before the peak intensity with energy ratio of 1.6 × 10−2 and it is in good agreement with the results got by the above new method. The method presented here is efficient to characterize pre-pulses of femtosecond laser, especially for detection of pre-plasmas formation before laser matter interaction experiments. For example, if a contrast ratio of 10−6 is the basic requirement for laser matter interaction experiment, the energy of pump laser should be set as 106 times of the critical power for filamentation in air in this measurement. Scan the interesting temporal range before arrival of the main pulse using the above system to detect whether pre-pulse filamentation exists. If no pre-pulse filamentation formed, contrast ratio of the laser facility meets requirements of the experiment. In this process, energy of pump laser may exceed the nominal energy of the laser facility. Then, condensed transparent media, such as water, can be used instead of air as the propagation media. In that case, pulse energy for filamentation is greatly reduced. Further improvements of the system can be done. On the one hand, energy ratio of the pre-pulse and main pulse can be calculated by the ratio between the total pump pulse energy for the main pulse to induce filamentation and that for the prepulse in the experiment. It may provide more accurate measuring results compared to that using the theoretical threshold power for filamentation. On the other hand, temporal range of the measuring system can be extended to nanosecond using meters long optical line, while the temporal resolution of

This work is supported by the National Natural Science Foundation of China (Grants Nos. 11404335 and 71402176) and the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201310). 1 D.

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