Raman fibre lasers (RFLs) can emit at a broad wavelength range, due to the flexible Raman gain available at any wavelength across the transparency window of silica (0.3 μm–2.0 μm). RFLs play a particularly significant role for providing laser emissions in the practically important eye-safe wavelength range of 1.6 μm–1.8 μm that typically cannot be covered by traditional rare-earth-doped fibre lasers. This concept of wavelength versatility can also be applied in ultrashort pulsed laser sources, such as mode-locked (ML) lasers, thanks to the broad Raman gain bandwidth. These pulsed RFLs operating within the special wavelength region of 1.6 μm–1.8 μm can attract
considerable attention due to their potential applications in areas such as optical coherence tomography, communication, air monitoring, and medical surgery. Two dimensional (2-D) materials like graphene and graphene oxide, characterised by their ultra-broadband nonlinear saturable absorption effect, have been widely used in passively ML lasers as saturable absorbers recently. Topological insulators (TIs), another novel 2-D material, have opened the possibility of a more universal solution, since TI benefits from a virtually wavelength-independent saturable absorption effect and low cost. The WAVEFIL project relates to the novel application of ultra-broadband TI SA in the wavelength-versatile RFL for achieving the special wavelength range at 1.6 μm–1.8 μm that cannot be covered by traditional rare-earth doped lasers. Based on the TI SA, stable harmonically mode-locked operation of a Raman fibre laser was achieved at 1.658 μm. A maximum average output power of up to 130 mW was obtained at the repetition rate of 466.2 MHz, corresponding to the 1250th order harmonic mode-locking. The temporal width of the mode-locked pulse train is 350 ps. Besides, we have extended the lasing wavelength to the mid-IR wavelength region such as ~2.8 μm and ~3.5 μm. High power Er:ZBLAN fiber lasers operating at these two wavelengths have been achieved. Through the project the overall knowledge transfer, including the research methodology and skills from the host institution to the Fellow that are related to the Raman fibre-laser technology, nonlinear optics, and material sciences, has been implemented. The major deliverables achieve from this project can have potential applications in CO2 detection.
