  Lasers that generate ultra-intense light pulses are under development for
experiments in high-field and high-energy-density physics, as well as for
applications such as particle acceleration. Extensions to even higher powers
are being considered for future investigations that can only be imagined today,
such as the quantum electrodynamics of plasmas and isolated attosecond-pulse
generation with solid targets. For all of these areas, it is vital to produce
high-contrast pulses, so that no pre-plasma is created in the target before the
arrival of the main pulse. However, noise is unavoidable in high-gain
amplification, and is manifested in the form of background light that
accompanies pulses generated by chirped-pulse amplification (CPA). Here, we
introduce a linear filtering technique based on spatio-spectral coupling, which
allows in-band filtering of amplified pulses for the first time. Experiments
demonstrate approximately 40 times contrast enhancement in optical parametric
chirped-pulse amplification (OPCPA) and provide a foundation for scaling to
much higher performance. The simplicity, efficiency, and direct compatibility
with existing techniques for short-pulse generation will make spatio-spectral
filtering attractive to a wide range of applications in ultrafast optics and
time-resolved spectroscopy, and may open new directions in noise reduction.
