Laser Frequency Combs for High Resolution Spectrograph Calibration

Abstract

While laser frequency combs (LFCs) are in principle ideal calibration light sources for high resolution spectrographs, they are still not regularly used. This is not only due to high cost and maintenance, but also persistent technological challenges. One of these challenges is the high variability and instability of the spectral envelope of the LFC. This issue is caused by underlying physical effects and the sensitivity of the nonlinear broadening process used to generate the supercontinuum spectrum. In the context of ELT-HIRES and the Göttingen solar observatory, we aim to improve the calibration of high resolution spectrographs using an LFC. To mitigate the adverse effects of the variability and instability of the spectral envelope we explore three different approaches. The first is to design a tapered nonlinear fiber which yields a more stable and flat spectral envelope. This is done by creating a grid of simulations for various fiber taper parameters and then judging the quality of the simulated spectra. Results from the simulation are promising and the designed fiber was manufactured. First measurement results of a spectral broadening are still pending. The second approach is specific to the Fourier transform spectrograph . We develop a method to calculate the comb frequencies necessary for calibration directly from the interferogram. Thereby, we aim to simplify and improve the calibration process. This method is faster since we do not have to perform a phase correction and subsequently fit the individual comb lines in the spectrum. We show that the method of retrieving the comb frequencies from the interferogram and a calibration is possible. However, the precision is lower than for the conventional method. By calculating simulated interferograms and introducing different types of measurement errors into the simulation, we find that the main reason for reduced precision in this method is the change of the spectral envelope during the measurement. Additionally, phase errors and a dispersion depending on the optical path length difference are causing systematic errors. Last, we develop, construct and characterize a spectrum control setup to stabilize and manipulate the spectral envelope of the LFC. This is the first demonstration of such a setup in the near infrared. Using this setup, the stability of the resulting spectral envelope is increased and the dynamic range is decreased. In high resolution spectrograph measurements, more comb lines are thus detectable. However, the spectrum control setup causes systematic noise in the Fourier transform spectrograph measurements. Thus, this approach does not lead to an improvement in calibration precision and further work is necessary to remove this noise from the measurements. For echelle spectrographs this problem does not apply and this setup could therefore be suitable to improve LFC calibration of an astronomical spectrograph in the near infrared.