The fundamental limits on ultrafast optics and strong-field physics are often governed by amplifier noise within the signal band. Such in-band noise is inaccessible by traditional out-of-band filtering devices. Now, scientists at Shanghai Jiao Tong University (Shanghai, China) have devised an in-band noise-filtering scheme based on spatiospectral coupling that enables high-gain, but low-noise amplification of ultrashort laser pulses.
The in-band filter consists of a pair of parallel diffraction gratings with a slit placed at the midplane between the gratings. In contrast to a monochromator, the near-field slit of the in-band filter activates the effect of spatiospectral coupling: the finite slit-width results in the selective transmission of narrowband components at each diffraction angle, and the transmitted light with angular dispersion is subsequently converted into a spatially chirped beam by the output grating. As a result, the filter can be characterized by a 2D transmission function that exhibits a narrow oblique line in the spatiospectral domain. While the local passband at a given spatial position is as narrow as 0.01 to 1 nm, its overall spectral transmission can be made very broadband (>10 nm). Thus, a broadband signal with matched spatiospectral coupling can pass the filter unimpeded, while broadband noise that is randomly distributed in the spatiospectral domain will be mostly blocked. Near-noiseless amplification of laser pulses with contrast as high as 1011 is demonstrated in a high-gain optical parametric chirped-pulse amplifier (OPCPA), resulting in approximately 40X enhancement in output contrast. Reference: J. Wang et al., Laser Photonics Rev., 12, 8, 1700316 (Jun. 19, 2018).
