Nickel metal and nickel oxide for saturable absorber femtoseconds pulse laser performance

Ultrashort pulses generated from fibre lasers can be achieved through passive mode- locking using a saturable absorber. This study investigated the use of nickel metal and nickel oxide as saturable absorbers integrated with tapered fibres. The conventional method for creating saturable absorbers with these materials predominantly utilised a fibre ferrule configuration. While this approach offers compatibility and flexibility, it suffers from a limited interaction length between light and the saturable absorber material. To address this limitation, this research focused on the fabrication of nickel- and nickel oxide-based saturable absorbers embedded in polydimethylsiloxane as a host polymer. By exploiting the evanescent field along the tapered fibre, efficient light-matter interaction was achieved, enabling effective modulation of transmitted light. The saturable absorbers were characterised for their morphological, optical absorption, and crystallinity properties. The fabrication process involved liquid-phase exfoliation, ultrasonication, and spin coating onto the tapered fibre. The performance of nickel-based saturable absorbers was first evaluated by integrating them into an erbium-doped fibre laser ring cavity for dispersion management studies. The nickel-based absorber demonstrated nonlinear transmission characteristics, including a modulation depth of 4.77%, a saturation intensity of 265 MW/cm2, and nonsaturable absorption of 61.6%. Adjusting the single-mode fibre length within the cavity significantly influenced the pulse properties. Longer single-mode fibre lengths resulted in narrower spectral bandwidth and slower pulse width but caused increased attenuation losses. An optimised single-mode fibre length of 24.2 metres provided the best results, achieving stable pulses with a peak extension ratio of 57.55 dB at a repetition rate of 7.99 MHz. Subsequent experiments focused on nickel oxide-based saturable absorbers to assess the influence of various material concentrations on laser performance. Nickel oxide concentrations ranging from 0.1 to 3.0 mg/mL were tested. Concentrations below 0.5 mg/mL failed to support mode-locking due to insufficient material, while concentrations exceeding 2.1 mg/mL led to particle re-aggregation and performance degradation. The saturable absorber with a nickel oxide concentration of 0.9 to 1.5 mg/mL exhibited the optimal balance, offering a modulation depth of 4.1 to 4.9 %, reduced transmission losses, and stable mode-locking performance. In conclusion, this research advanced ultrashort pulse lasers by integrating nickel- based saturable absorbers with tapered fibres, while optimising fibre length and material concentration. This study aligns with the united nations sustainable development goal (SDG 9), which emphasises industry, innovation, and infrastructure, by providing high-precision, energy-efficient solutions for advanced manufacturing processes such as cutting, drilling, and welding. Furthermore, these innovations hold significant potential for applications in various fields, including telecommunications, biomedical imaging, and precision measurement.

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