Broadband filterless tuneability in self-mode-locked Tm-doped fibre lasers

Over the last decade, the ultrafast short-wave infrared (SWIR: 1600 – 2500 nm) laser market has been promisingly growing with Thulium-doped fibre systems as key players. An advantageous low-loss atmospheric transmission, a deep biological tissue penetration as well as various absorption lines of gases and biomolecules drive the demand on efficient light sources operating at this wavelength band. To unleash the full potential for expanding applications the laser system has to present highly integrated, cost-effective, rugged, compact turn-key solutions. Broadband wavelength tuneability can ensure a one more level of versatility for laser systems and extend areas of their applications. Principle limitations of achieving wide tuning wavelength ranges are generally defined by the spectral bandwidth of the gain and traditional tuneability techniques, typically relying on implementation of bulk and expensive tuning elements. In this presentation, we will present nearly 90 nm tuneability in ultrafast Tm-doped fibre laser spanning from 1873 to 1962 nm by implementing variable feedback for efficient control of the excitation level of the active medium. To realise self-mode-locking we explore a heavily-doped active fibre enriched with Tm ion clusters to reinforce its saturable absorption mechanism with 23% modulation depth and 95 MW/cm2 saturation intensity. We observed both experimentally and numerically intriguing implications on the saturation level, gain, and glass matrix. The highest laser efficiency is observed with 20% feedback, generating 580-fs soliton pulses at 1877 nm central wavelength with 1.5 nJ output pulse energy. Numerical model combining the nonlinear Shrödinger and population inversion rate equations for the gain medium helps to unveil nonlinear pulse evolution under the influence of dynamically varying gain spectrum. The resulting laser system presents a compact and straightforward approach to achieve laser generation with a broad wavelength tuneability, high laser stability, and power performance, which can be translated to unexplored so far wavelength ranges.