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Fourier-transform spectrographs (FTS) are among the most important tools for high-resolution spectroscopy over a broad spectral bandwidth. Usually, the frequency axis of an FTS is calibrated with relatively few atomic lines and an absolute wavelength reference, which is often a stabilized He–Ne laser. Normally, the phase-spectrum is measured using a continuous light source to enable phase correction. Laser frequency combs (LFC) provide a much higher stability. Their spectrum consists of closely spaced narrow lines, which are very well suited for the characterization and calibration of an FTS. Due to the pulsed nature of the LFC, however, the phase spectrum cannot be measured in the same way as for continuous light sources. We show how a proper phase spectrum from an FTS measurement of an LFC can be obtained and how the strongly varying phase spectrum noise can be filtered. We analyzed a narrow spectral band 10.200–12.500 cm−1 in which we detected ∼60.000 lines with sufficient intensity. Only with an accurate truncation of the interferogram and a proper shifting, the complex structure of the phase spectrum is revealed. For phase filtering, we adapted Mertz’s algorithm and show how the instrumental line shape is significantly improved.
Laser frequency combs have properties which make them promising spectrograph calibration light sources. One drawback for this application is the high dynamic range in the supercontinuum spectra of some frequency combs. We aim to flatten the spectrum of a Ti:sapphire laser frequency comb to improve the calibration performance for a Fourier transform spectrograph. For this, we develop a compact Fourier transform optical pulse shaping setup, which enables control of the spectral envelope via dispersion of the light onto a spatial light modulator. We demonstrate, that this setup allows us to flatten the comb spectrum from a dynamic range of 20 dB to less than 6 dB in the wavelength range 739-939 nm. For 86 % of the wavelength range, the dynamic range is below 1 dB.