Double-Wall Carbon Nanotube Hybrid Mode-Locker in Tm-doped Fibre Laser: A Novel Mechanism for Robust Bound-State Solitons Generation

Maria Chernysheva,Alessio Gambetta,Gianluca Galzerano,Anastasia Bednyakova,Richard C T Howe,Mark Rümmeli,Guohua Hu,Aleksey Rozhin,Mohammed Al Araimi,Tawfique Hasan

doi:10.1038/srep44314

Abstract

The complex nonlinear dynamics of mode-locked fibre lasers, including a broad variety of dissipative structures and self-organization effects, have drawn significant research interest. Around the 2 μm band, conventional saturable absorbers (SAs) possess small modulation depth and slow relaxation time and, therefore, are incapable of ensuring complex inter-pulse dynamics and bound-state soliton generation. We present observation of multi-soliton complex generation in mode-locked thulium (Tm)-doped fibre laser, using double-wall carbon nanotubes (DWNT-SA) and nonlinear polarisation evolution (NPE). The rigid structure of DWNTs ensures high modulation depth (64%), fast relaxation (1.25 ps) and high thermal damage threshold. This enables formation of 560-fs soliton pulses; two-soliton bound-state with 560 fs pulse duration and 1.37 ps separation; and singlet+doublet soliton structures with 1.8 ps duration and 6 ps separation. Numerical simulations based on the vectorial nonlinear Schr¨odinger equation demonstrate a transition from single-pulse to two-soliton bound-states generation. The results imply that DWNTs are an excellent SA for the formation of steady single- and multi-soliton structures around 2 μm region, which could not be supported by single-wall carbon nanotubes (SWNTs). The combination of the potential bandwidth resource around 2 μm with the soliton molecule concept for encoding two bits of data per clock period opens exciting opportunities for data-carrying capacity enhancement.

Highlights

The unique double-wall structure gives DWNTs a larger modulation depth (64%) and faster carrier dynamics than single-wall carbon nanotubes (SWNTs), offering enhanced light-DWNT interaction
The present results suggest that DWNTs as saturable absorbers (SAs) are extremely effective for generating ultrashort conventional soliton pulses and stable bound-state multi-soliton complexes beyond 1.8 μm
This could be mainly attributed to their rigid structure, high modulation depth and faster carrier relaxation time, stabilising the phase-locking between multiple solitons

Summary

Introduction

The stable bound soliton pair is formed due to the interplay among phase modulation, anomalous group velocity dispersion (GVD), and the repulsive forces of adjacent split solitons during propagation along the laser cavity. The trailing soliton will experience an increase in the group velocity Such a soliton pair uses the dissipative nonlinear dynamics of the active cavity to remain stable for hours without the necessity for active stabilisation[18,19]. For a multi-wavelength generation, the cavity should possess an intensity or wavelength-dependent loss element This relieves the mode competition produced by homogeneous gain broadening, for example, through the nonlinear optical Kerr effect (NPE or nonlinear optical loop mirrors)[21]. Yan et al have presented a switchable and tuneable single-longitudinal-mode multi-wavelength Tm-doped fibre laser, based on NPE22,23, resulting in limited progress in the field

Methods

Results

Conclusion