Abstract
Laser frequency combs have properties that 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 (SLM). We demonstrate that this setup allows us to flatten the comb spectrum in the wavelength range 780 to 980 nm. For 90% of the wavelength range, the dynamic range is below 3 dB. In the flattened spectra 3% more comb lines can be detected. The line center fit precision, however, is reduced due to noise caused by the SLM.
Highlights
Laser frequency combs (LFCs) provide evenly spaced narrow emission lines that cover a broad spectral range
The spectrum of the LFC we use has a strong variability in power of more than 20 dB, resulting in low signal-to-noise-ratio (SNR) for some wavelength ranges in our Fourier transform spectrograph (FTS) measurements
We are able to detect about 3% more comb lines in flattened spectra compared to spectra where the control loop was disabled
Summary
Laser frequency combs (LFCs) provide evenly spaced narrow emission lines that cover a broad spectral range. Their spacing and position can be controlled to a very high accuracy and precision. These properties make LFCs suitable wavelength calibration light sources for astronomical spectrographs.[1,2]. There are a number of challenges involved in creating such an astro-comb.[3] To cover a large spectral range, a broadening of the LFC spectrum in a nonlinear fiber is often required.[4,5] The resulting broadened spectrum exhibits strong intensity variations. This can lead to over- or under-exposures in calibration measurements of a spectrograph as well as wavelength errors in the calibration process
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