Dynamic spectral tailoring of a 10 GHz laser frequency comb for enhanced calibration of astronomical spectrographs

Abstract: Laser frequency combs (LFCs) are an important component of Doppler radial velocity (RV) spectroscopy that pushes fractional precision to the $10^{-10}$ level, as required to identify and characterize Earth-like exoplanets. However, large intensity variations across the LFC spectrum that arise in nonlinear broadening limit the range of comb lines that can be used for optimal wavelength calibration with sufficient signal-to-noise ratio. Furthermore, temporal spectral-intensity fluctuations of the LFC, that are coupled to flux-dependent detector defects, alter the instrumental point spread function (PSF) and result in spurious RV shifts. To address these issues and improve calibration precision, spectral flattening is crucial for LFCs to maintain a constant photon flux per comb mode. In this work, we demonstrate a dynamic spectral shaping setup using a spatial light modulator (SLM) over the wavelength range of 800nm to 1300nm. The custom shaping compensates for amplitude fluctuations in real time and can also correct for wavelength-dependent spectrograph transmission, achieving a spectral profile that delivers the constant readout necessary for maximizing precision. Importantly, we characterize the out-of-loop properties of the spectral flattener to verify a twofold improvement in spectral stability. This technique, combined with our approach of pumping the waveguide spectral broadener out-of-band at 1550 nm, reduces the required dynamic range. While this spectral region is tailored for the LFC employed at the Habitable-zone Planet Finder (HPF) spectrograph, the method is broadly applicable to any LFC used for astronomical spectrograph calibration.