Extreme prepulse contrast utilizing cascaded-optical parametric amplification Page: 3 of 4
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Extreme prepulse contrast utilizing cascaded-optical
I. JovanovicI, C. Haefner2, B. Wattellier3, and C. P. J. Bartyl
Lawrence Livermore National Laboratory, 7000 East Ave., L-470, Livermore, CA 94550, USA
Phone: (925) 424-9773, FAX: (925)423-6195, email: firstname.lastname@example.org
2Nevada Terawatt Facility, University of Nevada, Reno, Nevada 89506
3Phasics, Campus de l'Acole Polytechnique, Bat. 404, 91128 Palaiseau Cedex, France
It has been shown recently that an optical parametric chirped-pulse amplifier can be easily reconfigured
into a cascaded-optical parametric amplifier (COPA), enabling complete prepulse removal and optical
switching with a window defined by the pump pulse duration. We have demonstrated instrument-limited
measurement of the COPA prepulse contrast >1.4x1011 using 30-mJ pulses. The COPA technique is
applicable to all energy ranges and pulse durations. A convenient millijoule-scale implementation of this
technique is presented using a single, large-aspect-ratio quasi-phase-matched nonlinear crystal.
It has been shown previously that the use of OPCPA amplifies only a single pulse out of
the oscillator pulse train resulting in a prepulse contrast enhancement equal to the total saturated
gain, provided that the pump pulse duration is sufficiently short and there is no overlap in time
with preceding or subsequent pulses. Notwithstanding the significant prepulse contrast
enhancement, OPCPA does not remove prepulses; it merely reduces their relative peak intensity,
typically on the order of 108 in a large laser system. Here we report the measurement of the
extreme contrast enhancement of a modified OPCPA technique, termed cascaded optical
parametric amplification (COPA).  The technique is implemented by reconfiguration of a two-
stage OPCPA into a phase-conjugated COPA system. Its feasibility has been described and
recently demonstrated . The method removes any pre- and post-pulses in the neighborhood of
the main, amplified pulse. Instrument-limited experimental measurement of the prepulse contrast
of 1.4x101:1 is presented at a 30-mJ level using cascaded electro-optical modulators to protect
the photodiode from damage by the main pulse.
Demonstration of a large prepulse contrast requires a high dynamic range prepulse
measurement technique. We have chosen a relatively simple setup using a sensitive photodiode
and a cascade of fast Pockels cells to protect the photodiode from damage by the main pulse.
Pockels cells were operated such that the main pulse was rejected, while the prepulses were
transmitted to the photodiode. The attenuation of the main pulse was 1.1x105 and was sufficient
to protect the diode from damage by the main pulse. We first determined the contrast ratio for the
conventional OPCPA system. The prepulse contrast was 1.4x108, which is approximately
equivalent to the gain of the system. We reconfigured the OPCPA setup to the COPA
configuration and we could not detect any prepulses within the noise level of our instrumentation.
The baseline was measured with and without COPA turned on to correct for the noise level. The
noise level was 6.2 0.05x10-3 on the normalized scale of Fig. 2. The main source of noise is
digitizer and crosstalk effects in the oscilloscope. We determined the contrast ratio by comparing
the pulse amplitude with the leading baseline ahead after subtracting the noise level, and obtained
an instrument-limited contrast ratio of 1.4x101. Fig. 1 shows the comparison between OPCPA
and COPA contrast measurement. The measurement dynamics were limited by the maximum
available energy determining the signal to noise ratio.
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Jovanovic, I; Haefner, C; Wattellier, B & Barty, C J. Extreme prepulse contrast utilizing cascaded-optical parametric amplification, article, June 15, 2006; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc886906/m1/3/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.