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.
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