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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - WAVEFIL (WAvelength VErsatile Pulsed Raman FIbre Lasers)

Teaser

Summary

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 Î¼mâ€“2.0 Î¼m). RFLs play a particularly significant role for providing laser emissions in the practically important eye-safe wavelength range of 1.6 Î¼mâ€“1.8 Î¼m 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 Î¼mâ€“1.8 Î¼m 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 Î¼mâ€“1.8 Î¼m 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 Î¼m. 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 Î¼m and ~3.5 Î¼m. 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.

Work performed

The work performed throughout the project include: 1. Theoretical model development for TI-based ML RFL and manipulation of nonlinear pulse propagation. We constructed a TI-based ML RFL model based on the Ginzburg-Landau equation and the nonlinear absorption function of TI SA, which can be used to instruct the design of TI SA based ML RFL. Besides, we theoretically investigated the propagation and evolution characteristics of dark solitons. We found that the optical event horizon can provide an effective technique to actively control the propagation properties of a dark soliton with another weak probe wave. Careful power adjustment of the probe wave enables the black soliton converted into a gray one with varying grayness through the nonlinear interaction, corresponding to a nearly adiabatic variation of the solitonâ€™s speed. The probe wave manipulated collisional dynamics between both dark solitons are investigated as an analogue of the combined white-hole and black-hole horizons which provides some insights into exploring the transition between integrable and non-integrable systems. In addition, we proposed a manipulation approach to vary the wave speed, as well as the grayness, of dark solitons under the optical event horizon arising from the interaction between a dark soliton and a probe wave. The optical event horizon effect was demonstrated for the first time to be capable of inducing a reversible conversion between a black soliton and a gray one. This reversible soliton transformation and control process originates from the intrinsic competition between the probe-induced nonlinear phase shift and the internal phase of the dark soliton. 2. NPR and TI SA based Raman fibre lasers. We realised mode-locked operation of the 1.65 Î¼m Raman fibre laser based on NPR and TI SA, respectively. We compared the lasing characteristics using these two different techniques. 3. Development of 3.5 Î¼m high power mid-IR Er:ZBLAN fibre laser. We propose a simultaneous dual-end pumping scheme for the two different pump wavelengths (~976 nm and ~1973 nm) in the ~3.5 Î¼m Er3+-doped fluoride fibre laser to evenly distribute the large quantum defect heat and relieve the thermal loading accumulated in the pump injection fibre end facets that can ultimately lead to the fibre tip degradation and damage in high-power operation regime. A maximum output power of 1.72 W is achieved in the Er3+-doped fluoride fibre laser limited only by the available 1973 nm pump power with a corresponding slope efficiency of 26.2%.

Final results

The obtained results through the WAVEFIL project can lead to development of novel fibre-laser sources and related technologies and also enhance the EUâ€™s leading position in commercial fibre lasers and applications. For instance, high quality topological insulator Bi2Te3 was prepared. Through sandwiching Bi2Te3 layer with PMMA polymer which is self-assembled at the interface of the DI water and the air, a novel ultrathin PMMA-TI-PMMA saturable absorber has been successfully fabricated. This special structure can guard the sample against damage during transfer and oxidation. Our studies clearly show that TI could be a promising saturable absorber for mode-locking in Raman fibre lasers. We also experimentally found that the as-fabricated PMMA-TI-PMMA saturable absorber can successfully operate as an effective passive mode-locker without degrading the performance for several months, indicating the functionality of PMMA protection layer. This work suggests that the PMMA protection layer might not only just be suitable for TI, but also widely applicable for other 2-dimensional layer materials. The constructed mode-locked Raman fibre laser based on TI SA can have potential applications in air monitoring. Besides, we demonstrated a high power dual-end pumped Er3+-doped fluoride fibre laser at ~3.5 Âµm. We demonstrate that the dual-end pump scheme can be considered as an alternative approach to further scaling up the output power of ~3.5 Î¼m Er3+-doped fluoride fibre lasers. These specific laser radiations can have numerous practical and scientific applications in spectroscopy, environmental monitoring, medicine, defense countermeasures and many others, because this region corresponds to the transparency window of the atmosphere and strong absorption peaks of many molecules containing covalent bonds between carbon and hydrogen, or nitrogen or oxygen (such as CH4, HCN, CO2, CO). This projectâ€™s success can provide sustainable collaboration between hosts, co-hosts, and universities and institutions in China to improve their research excellence and broaden related industrial applications within Europe.