Stability peculiarities in the stretch pulse hybrid mode-locked erbium-doped all-fiber ring laser

Ultrashort pulse (USP) fiber lasers have become one of the most important instruments in various fields of science and industry during two past decades [1]. A number of applications requires stability of pulse duration, repetition rate and low intensity noise of an USP source. For example, stabilization of this parameters is in great demand in frequency metrology field, dual comb spectroscopy, THz pulse spectroscopy and etc. [2]. It should be noted, that hybrid Mode-locking (ML) for USP generation have attracted a great attention due to their capabilities of enhancing pulse quality and providing reliable ML startup by taking advantage of a co-action of two saturable absorbers (SA) — a slow SA such as SESAM, carbon nanostructures or graphene, and a fast absorber based on nonlinear polarization evolution (NPE) effect [3]. Recently Carbon:Boron Nitride Single-Walled Nanotubes(C:BNNTs) SA is proved to be reliable absorber with excellent saturation parameters for erbium-doped fiber laser ML [4]. In this paper we have obtained and measured stability of stretch pulse generation in the hybrid ML erbium-doped all-fiber ring laser by using C:BNNTs SA in the co-action with NPE effect. We have managed to realize two different types of stable single-pulse, self-starting stretch pulse generation at a repetition frequency of 42.2MHz with S/N ratio ∼ 63 dB. Note, that the total intracavity group velocity dispersion β
2
was −0.021 ps
²
at 1550 nm. Obtained sech
²
-type spectrum has a FWHM of ≈ 4nm at the output average power of 16.7mW and the Gauss-type spectra has a FWHM of ≈ 56 nm at the same output power. Pulse width measured by intensity autocorrelator is 650 fs for both USP generation regimes. Note, that the time-bandwidth product for the sech
²
pulse is TBP=Δ × τ min ≈ 0.322 and for the Gauss pulse TBP ≈ 4.45 (typical TBP for a Gauss pulse ≈ 0.44). Thus, the obtained sech
²
pulse is close to the bandwidth-limit (TBP ≈ 0.315 for a classical soliton). It should be noted, that minimal Gauss pulse width (up to sub-100 fs duration) can be achieved simply by proper fiber length control at the laser output.