In-cavity pulse shaping by spectral sculpturing in mode-locked fibre lasers

Abstract

We review our recent progress on the realisation of pulse shaping in passively-mode-locked fibre lasers by inclusion of an amplitude and/or phase spectral filter into the laser cavity. We numerically show that depending on the amplitude transfer function of the in-cavity filter, various advanced temporal waveforms can be generated, including parabolic, flattop and triangular pulses. An application of this approach using a flattop spectral filter is shown to achieve the direct generation of high-quality sinc-shaped optical Nyquist pulses with a widely tunable bandwidth from the laser oscillator. We also present the operation of an ultrafast fibre laser in which conventional, dispersion-managed and dissipative soliton mode-locking regimes can be selectively and reliably targeted by adaptively changing the dispersion profile and bandwidth programmed on an in-cavity programmable filter. Keywords: mode-locked fibre lasers, nonlinear pulse shaping, frequency filtering, nonlinear fibre optics. INTRODUCTIONAt the level of fundamental research, mode-locked lasers constitute an ideal platform for the investigation of original and complex nonlinear dynamics of ultrashort pulses, while at the applied research level, pulses with different and optimised features -e.g., in terms of pulse duration, temporal and/or spectral shape, width, energy, repetition rate and emission bandwidth -are sought with the general constraint of developing efficient cavity architectures. In these two aspects, fibre laser cavities, which offer compact and versatile design, represent an area of intense development. Recently, the inclusion of a spectral pulse shaper into the cavity of a mode-locked fibre laser has emerged as a method to achieve a potentially high degree of control over the dynamics and the output of the laser [1-3], while obviously entailing a more power efficient technique than pulse shaping implemented through direct filtering of a laser output. The use of a spectral filter based on a Fourier-domain programmable optical processor [4], when placed inside a laser cavity, has the potential to allow the operation of lasers that exhibit pulse characteristics that can be controlled purely through software control. In this paper, we review our recent results and advances in the area, by numerically describing a novel approach to achieving different regimes of pulse generation in a passively-mode-locked fibre laser, which relies on nonlinear in-cavity pulse dynamics and pulse shaping by an in-cavity amplitude and phase spectral filter [5]. By changing the amplitude profile applied on the filter, we are able to create various output temporal waveforms of fundamental and practical interest, ranging from bright and dark parabolic profiles to a flat-top profile and triangular and sawtooth (asymmetric triangular) profiles. As an application of this general approach, we show that the use of an incavity flat-top spectral filter makes it possible to directly generate sinc-shaped Nyquist pulses of high quality and of a widely-tunable bandwid...