  We propose a fibre-based approach for generation of optical frequency combs
(OFCs) with the aim of calibration of astronomical spectrographs in the low and
medium-resolution range. This approach includes two steps: in the first step,
an appropriate state of optical pulses is generated and subsequently moulded in
the second step delivering the desired OFC. More precisely, the first step is
realised by injection of two continuous-wave (CW) lasers into a conventional
single-mode fibre, whereas the second step generates a broad OFC by using the
optical solitons generated in step one as initial condition. We investigate the
conversion of a bichromatic input wave produced by two initial CW lasers into a
train of optical solitons, which happens in the fibre used as step one.
Especially, we are interested in the soliton content of the pulses created in
this fibre. For that, we study different initial conditions (a single
cosine-hump, an Akhmediev breather, and a deeply modulated bichromatic wave) by
means of soliton radiation beat analysis and compare the results to draw
conclusion about the soliton content of the state generated in the first step.
In case of a deeply modulated bichromatic wave, we observed the formation of a
collective soliton crystal for low input powers and the appearance of separated
solitons for high input powers. An intermediate state showing the features of
both, the soliton crystal and the separated solitons, turned out to be most
suitable for the generation of OFC for the purpose of calibration of
astronomical spectrographs.
