  We study the properties of a soliton crystal, an bound state of several
optical pulses that propagate with a fixed temporal separation through the
optical fibres of the proposed approach for generation of optical frequency
combs (OFC) for astronomical spectrograph calibration. This approach - also
being suitable for subpicosecond pulse generation for other applications -
consists of a conventional single-mode fibre and a suitably pumped Erbium-doped
fibre. Two continuous-wave lasers are used as light source. The soliton crystal
arises out of the initial deeply modulated laser field at low input powers; for
higher input powers, it dissolves into free solitons. We study the soliton
crystal build-up in the first fibre stage with respect to different fibre
parameters (group-velocity dispersion, nonlinearity, and optical losses) and to
the light source characteristics (laser frequency separation and intensity
difference). We show that the soliton crystal can be described by two
quantities, its fundamental frequency and the laser power-threshold at which
the crystal dissolves into free solitons. The soliton crystal exhibits features
of a linear and nonlinear optical pattern at the same time and is insensitive
to the initial laser power fluctuations. We perform our studies using the
numerical technique called Soliton Radiation Beat Analysis.

Pattern Formation and Solitons;Optics