Gain-controlled Broadband Tuneability in Mode-locked Thulium-doped Fibre Laser through Variable Feedback

Broadband wavelength tuneability can ensure a new 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, relying on electrically controlled or bulk filters, reducing laser efficiency and generation stability. In this work, we present nearly 90 nm tuneability in ultrafast Tm-doped fibre laser within the span from 1873 to 1962 nm by implementing variable feedback for efficient control of the excitation level of the active medium and, hence, the gain spectrum. 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. By combining the nonlinear Shrodinger equation and population inversion rate equations for the gain medium, the developed numerical model 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 range tuneability of wavelength and operation regimes, which is not impaired by the limitations on laser stability or power performance and can be translated to other wavelength ranges.