Optical fibres with sharp discontinuities as intracavity mode-locking devices for ultrashort-pulse fibre ring lasers

Abstract

1. In a Ken-lens mode-locking (KLM) resonator (Fig. la) for femtosecond pulse generation, the self-focusing effect within a Ken medium (KM) modifies the cavity mode profile. This is described as a self-amplitude modulation resulting in either decreasing loss through an internal aperture as the field intensity increases (hard aperturing) or increasing gain by adjusting the overlap between the cavity and the pump mode profiles (soft aperturing). Because the KLM uses bulk nonlinearities, it is well suited for mode-locking bulk solid-state lasers which have high intracavity intensities [l]. In fibre lasers, where nonlinear phase shifts are integrated over the fibre length, the additive pulse mode-locking (APM) is used. Polarisation APM (P-APM) relies on the intensity-dependent rotation of the elliptical polarisation state in a length of optical fibre. With proper settings of the initial polarization ellipse and phase bias, pulse shortening occurs. Optical fibre is well suited for nonlinear polarization rotation because the small mode diameter leads to high intensities and thus to a large nonlinear index change, and because long fibre length can be used. Unidirectional fibre ring cavity has low self-starting threshold. It can operate in the soliton regime, where the group velocity dispersion is negative, or in the stretched pulse regime (Fig. 1 b), where sections of both positiveand negative-dispersion fibre are used [ 2 ] . However polarization elements are sensitive to thermal and mechanical environmental changes. As an alternative to the polarization mode-locking devices, we suggest to use fibre discontinuities to achieve KLM operation in the fibre ring lasers. Step-like discontinuity in a fibre loop can be considered as an analog to a hard aperture inside linear resonator of the KLM laser. In our previous works, we have demonstrated applicability of the step-like discontinuities for pulse compression in optical fibres [3]. In this paper, mode-locking action of the both polarization rotator and step-like discontinuity is evaluated depending on parameters of the devices and compared. Ultra-short pulse propagation through nonlinear fibre segments is simulated numerically with account of material dispersion and gain in the fibre loop.