Abstract

The new generation of optical communication networks requires new enabling sub-system elements and optical components – from laser sources to receivers, adjusted to novel coding, modulation and transmission techniques. In communications a special role is played by sinc-shaped (in the time domain) Nyquist pulses possessing a rectangular spectrum, which enable data encoding in a minimum spectral bandwidth and intrinsically satisfy the Nyquist criterion of zero intersymbol interference. These properties enable spectrally efficient data transmission (see e.g. [1–3] and references therein). Various approaches for the generation of optical Nyquist pulses have been suggested, including spectral reshaping of mode-locked laser, fibre optical parametric amplification and phase modulation [4], phase-locked flat-comb generation [5], and intensity modulation and four-wave mixing [6]. We numerically study here the Nyquist pulse generation scheme shown in the left panel of Fig. 1: the laser ring cavity consists of an erbium-doped fibre (EDF) gain segment followed by a saturable absorber (SA) element, and a spectral pulse shaping filter. The filter spectral response is H(f) = R(f)exp [iβ_2,acc(2πf)²/2], where R(f) is the amplitude transfer function corresponding to the impulse response r(t) = sin(2πt/τ_p)cos(2βπt/τ_p)/{(2πt/τ_p) [1−(4βt/τ_p)², where τ_p is the pulse duration between zero crossings, and β(0 ≤ b ≤ 1) is known as a roll-off factor. The spectral phase adds a specific amount of group-velocity dispersion β_2,acc to the cavity which permits to control the net cavity dispersion [7]. Note that the spectral filter can be realised in a variety of ways, such as a fibre Bragg grating or a programmable liquid crystal on silicon optical processor, which offers the additional advantage of being easily reconfigurable, or a Nyquist filter in series with a dispersive delay line for dispersion control.

© 2015 IEEE