  In this paper we discuss the bandwidth enhancement that can be achieved in
multi-Joule optical parametric chirped pulse amplification (OPCPA) systems
exploiting the tunability of parametric amplification. In particular, we
consider a pair of single pass amplifiers based on potassium dideuterium
phosphate (DKDP), pumped by the second harmonic of Nd:glass and tuned to
amplify adjacent regions of the signal spectrum. We demonstrate that a
bandwidth enhancement up to 50% is possible in two configurations; in the first
case, one of the two amplifiers is operated near its non-collinear broadband
limit; to allow for effective recombination and recompression of the outgoing
signals this configuration requires filtering and phase manipulation of the
spectral tail of the amplified pulses. In the second case, effective
recombination can be achieved simply by spectral filtering: in this
configuration, the optimization of the parameters of the amplifiers (pulse,
crystal orientation and crystal length) does not follow the recipes of
non-collinear OPCPA.
