Spectral Shaping of an Astrocomb with Single-Comb-Mode Specificity

Astrocombs-broadband laser frequency combs comprising a sequence of ultra-narrow, drift-free, regularly spaced optical frequencies on a selectable multi-GHz grid-are being adopted as calibration sources for high-resolution astronomical spectrographs to enable precision radial-velocity measurements [1]. Broadband astrocombs feature significant spectral structure introduced by supercontinuum generation that must be mitigated by a spectral flattening process to avoid pixel saturation / under-illumination reducing the calibration precision. Typical spectral flattening devices employ a diffraction grating to disperse the astrocomb output across a liquid crystal on silicon (LCOS) spatial light modulator (SLM), whose programmable birefringence is leveraged to modify the spectral envelope [2]–[4]. Previously, we showed how an ancillary high resolution, cross-dispersion spectrograph with a lithographically-defined pinhole mirror in the Fourier plane could be used to spatially isolate a single comb mode for generating an absolute wavelength solution of an astronomical spectrograph [5]. Here, we augment this approach by placing an LCOS SLM at the Fourier plane (Fig. 1(a)), now enabling the individual intensities of thousands of astrocomb modes to be adjusted dynamically and independently.