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We here discuss recent progress on astronomical optical frequency comb generation at innoFSPEC-Potsdam. Two
different platforms (and approaches) are numerically and experimentally investigated targeting medium and low
resolution spectrographs at astronomical facilities in which innoFSPEC is currently involved. In the first approach, a
frequency comb is generated by propagating two lasers through three nonlinear stages – the first two stages serve for the
generation of low-noise ultra-short pulses, while the final stage is a low-dispersion highly-nonlinear fibre where the
pulses undergo strong spectral broadening. In our approach, the wavelength of one of the lasers can be tuned allowing
the comb line spacing being continuously varied during the calibration procedure – this tuning capability is expected to
improve the calibration accuracy since the CCD detector response can be fully scanned. The input power, the dispersion,
the nonlinear coefficient, and fibre lengths in the nonlinear stages are defined and optimized by solving the Generalized
Nonlinear Schrodinger Equation. Experimentally, we generate the 250GHz line-spacing frequency comb using two
narrow linewidth lasers that are adiabatically compressed in a standard fibre first and then in a double-clad Er/Yb doped
fibre. The spectral broadening finally takes place in a highly nonlinear fibre resulting in an astro-comb with 250
calibration lines (covering a bandwidth of 500 nm) with good spectral equalization.
In the second approach, we aim to generate optical frequency combs in dispersion-optimized silicon nitride ring
resonators. A technique for lowering and flattening the chromatic dispersion in silicon nitride waveguides with silica
cladding is proposed and demonstrated. By minimizing the waveguide dispersion in the resonator two goals are targeted:
enhancing the phase matching for non-linear interactions and producing equally spaced resonances. For this purpose,
instead of one cladding layer our design incorporates two layers with appropriate thicknesses. We demonstrate a nearly
zero dispersion (with +/- 4 ps/nm-km variation) over the spectral region from 1.4 to 2.3 microns.
The techniques reported here should open new avenues for the generation of compact astronomical frequency comb
sources on a chip or in nonlinear fibres.
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