a1839_1.pdf JThD137.pdf
High-dynamic-range, 200-ps window, single-shot crosscorrelator for ultrahigh intensity laser characterization I. Jovanovic, C. Brown, C. Haefner, M. Shverdin, M. Taranowski, and C. P. J. Barty Photon Science and Applications Program, National Ignition Facility Programs Directorate, Lawrence Livermore National Laboratory, Mail Code L-470. 7000 East Avenue, Livermore, CA 94550, USA Author e-mail address:
[email protected]
Abstract: A novel high-dynamic-range cross-correlator is presented that enables single-shot characterization of pulse contrast for ultrahigh intensity lasers in the temporal region up to 200 ps. ©2007 Optical Society of America
OCIS codes: (120.3940) Metrology; (140.7090) Ultrafast lasers
Many applications of ultrahigh intensity lasers require high prepulse contrast. While the detailed requirements for pulse contrast depend on the experiment, it is generally desirable to measure the pulse shape with a record length up to tens of ns prior to the arrival of the main pulse with at least ~200-ps resolution, with better (~ps) resolution in the immediate vicinity (200-ps) of the main pulse. The pulse contrast in the ~200-ps time window preceding the arrival of the main pulse is the most challenging single-shot pulse contrast measurement: its timescale and resolution requirements make it incompatible with photodiodes and conventional cross/auto-correlation measurements, while its required dynamic range is beyond streak camera capabilities. The long record length of 200 ps or, equivalently, ~6 cm of distance, is difficult to satisfy in cross-correlation measurements since they require both very large nonlinear crystal apertures and associated pulse energies, along with large beam crossing angles. Approaches considered in the past to overcome those limitations include the splitpulse recording in multiple temporal windows and the use of tilted pulse fronts [1,2]. The use of tilted pulse front of a short pulse is a convenient method for extending the record length: it does not require “stitching” of multiple traces and can be produced by any angular dispersion-inducing optical system. The use of a tilted pulse front is effectively equivalent to the use of a large beam or a large crossing angle. As a disadvantage of this approach we can quote the somewhat reduced temporal resolution. The requirement for high dynamic range of the measurement presents another challenge, which can be addressed by using background-free measurements. The use of the third harmonic of the fundamental pulse allows distinction of the cross-correlation signal from the scatter and frequency doubling of the fundamental pulse, normally present in second-harmonic measurements. Furthermore, the full dynamic range cannot be easily captured in a single shot on a single CCD or CMOS detector due to typical dynamic range limitations of 105, in a time window of >200 ps, thus bridging the gap between conventional autocorrelation and photodiode measurements. A combination of photodiode and conventional pulse shape measurements together with this technique provides a more complete knowledge of pulse evolution used in ultrahigh intensity laser experiments. x
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Fig. 1. Extended temporal range in TOCC is achieved by using pulse front tilt of the frequency-doubled probe pulse. Object and image relay planes are denoted by “o” and “i1-3”, respectively. Dynamic range is extended by using a step-variable filter in the image plane i2 of the crosscorrelation UV trace. Top: cross-correlation of a tilted frequency doubled probe pulse with the fundamental pulse is illustrated in x-t space.
a1839_1.pdf JThD137.pdf
The design of our third-order cross-correlator (TOCC) with tilted-pulse front is shown in Fig. 1. A short probe pulse is used to produce the second harmonic, which is subsequently incident on a 3600 mm-1 grating with a large off-Littrow incidence angle of 84°. The diffracted pulse exhibits a pulse front tilt of ~200 ps/cm, and it is imaged to the 3ω crystal image plane i1 in the plane of diffraction using a cylindrical lens. It is combined with the input pulse on a cylindrical lens in an arrangement orthogonal to the plane of diffraction, producing parallel line foci crossing in the 3ω crystal. The resulting ultraviolet cross-correlation trace is first imaged onto a step variable filter (i2). The use of the step-variable filter allows selective obscuration of the trace and increases the effective dynamic range. The shaded trace is then imaged onto a CCD sensor. The recorded trace is corrected for dynamic range based on filter transmission and contains the information on pulse contrast in up to 200-ps window around the main pulse. While the pulse duration of the input pulse is arbitrary within the temporal window of the device, it is advantageous to use as short probe pulse as possible to improve the temporal resolution. In ultrahigh intensity lasers with variable pulse duration, such pulse could be conveniently derived from the main laser pulse. In our experiments we split a 380-fs, 1053-nm pulse into two replicas which were used as input and probe pulses. The resulting calculated temporal resolution is determined by the duration of the probe pulse and the projection of the tilted pulse duration onto the direction of pulse propagation. In our experiments, the duration of the frequency doubled pulse is estimated to be 270 fs. The 80° pulse front tilt of the probe pulse results in the temporal resolution limit of ~1.5 ps. The total required energy for the measurement was ~10 mJ, set by the conversion efficiency in the 3ω crystal with limited overlap of the incident and tilted probe pulses, and by the sensitivity of the CCD to UV light. For instrument validation we generated a prepulse 200 ps prior to the arrival of the main pulse using an uncoated glass wedge in front of a retroreflector. In Fig. 2 we show the acquired cross-correlation trace along with the measurement performed using a fast photodiode (New Focus 1434, 25 GHz) and oscilloscope (Tektronix TDS6154C, 15 GHz). Excellent agreement is observed between two measurements, indicating the validity of the previously performed temporal calibration by delaying the input pulse with respect to the probe pulse. For this initial measurement of the dynamic range we substituted a knife-edge for step-variable filter and selectively blocked the regions of the cross-correlation trace that would otherwise saturate the CCD, while varying the filtering used in front of the CCD. The recovered pulse shape obtained by combining three single-shot measurements is shown in Fig. 3. The achieved signal-to-noise ratio is >105, which can be further improved by more effective scatter control. 106
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Fig. 2. An example of prepulse measurement performed using the TOCC and the combination of a fast photodiode and oscilloscope, Excellent agreement exists between the two measurements.
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Fig. 3. In a three-shot measurement in which an insertable knife-edge is substituted for step-variable filter, a >105 dynamic range is achieved based on the ratio of the peak intensity of the UV trace and the corresponding background intensity.
In conclusion, we have demonstrated a technique that bridges the temporal range and resolution capability gaps present among photodiodes, conventional cross-correlators, and streak cameras. Using the tilted pulse-front technique extends the temporal range of the single-shot measurement to ~200 ps. Application of third harmonic frequency mixing and step-variable filtering increases the single-shot dynamic range. In this measurement a tradeoff exists between the temporal resolution and the temporal window, which can be improved by employing a shorter probe pulse. It is expected that this method, when used together with conventional photodetectors, will enable contiguous measurement of pulse contrast. This work was performed under the auspices of the U.S Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
References [1] [2]
K. Oba, P.-C. Sun, Y. T. Mazurenko, and Y. Fainman, “Femtosecond single-shot correlation system: a time-domain approach,” Appl. Opt. 38, 3810-3817 (1999). G. Figueira, L. Cardoso, N. Lopes, and J. Wemans, “Mirrorless single-shot tilted-pulse-front autocorrelator,” J. Opt. Soc. Am. B 22, 2709-2714 (2005).
