Bright and dark pulsed fiber lasers based on multimode interference / Chen Yu

Chen , Yu (2025) Bright and dark pulsed fiber lasers based on multimode interference / Chen Yu. PhD thesis, Universiti Malaya.

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Abstract

Pulsed lasers, particularly ultrashort pulsed lasers, find widespread application across various fields including telecommunications, bio-imaging, spectroscopy, atmospheric measurements, surgery, and material processing. This study focuses on harnessing the potential of the single mode-multimode-single mode (SMS) structure as a robust avenue for developing simple, compact, and cost-effective bright and dark pulsed erbium-doped fiber lasers (EDFLs). The SMS configuration serves as an ingenious artificial saturable absorber (SA), facilitating the generation of ultrashort pulsed lasers through the Kerr Effect with nonlinear multimode interference. By precisely fusion-splicing single mode fibers to both ends of a designated length of multimode fiber, the SMS architecture is established. Subsequently, integrating this SMS structure into a ring EDFL and finetuning parameters via a polarization controller enables the successful demonstration of ultrashort pulsed lasers. Various experiments were conducted in both anomalous and normal dispersion regimes to explore the potential of different SMS structures for both bright and dark pulse generation. Within the net anomalous dispersion regime, utilizing step-index multimode fibers (SIMMF) with lengths of 96 mm, 156 mm, and 196 mm in the SMS structures, Q-switched fiber lasers were successfully observed. Remarkably, a notable milestone has been achieved with the observation of the shortest pulse width at 2.4 μs, coupled with a repetition rate of 155.8 kHz, attained at an input power of 343 mW and a center wavelength of 1530 nm. The artificial SA exhibited a significantly elevated damage threshold compared to conventional thin film types. The mode-locked EDFLs were also realized using the grade-index multimode fiber (GIMMF) based SMS structures, with lengths approximately at 45 mm, 100 mm, and 150 mm. These lasers produced both conventional soliton and bound soliton phenomena with pulse widths as low as 290 fs, accompanied by a repetition rate around 22 MHz and a center wavelength of around 1574 nm. Notably, the pulse width exhibited minimal variation across different lengths of GIMMF. Additionally, dark pulses were also realized with a minimum pulse width of 1.6 ns and a repetition rate of 21.5 MHz. Noteworthy spectral analysis revealed the dark pulse to be dual wavelength, with peak wavelengths centered at 1567.2 nm and 1569.4 nm, indicating the presence of a domainwall dark pulse phenomenon By integrating 230 meters of dispersion compensation fiber (DCF) into the cavity, the laser is ensured to operate within a significantly large net normal dispersion region. In this study, a 103 mm length GIMMF-based SMS structure is focused on within this domain. As the pump power increased, multiple pulsing emerged, with the pulse cluster ranging from 3 to 16 pulses. These multiple pulsing instances exhibited remarkably broad dissipative solitons, highlighting the potential of the SMS structure for direct supercontinuum generation. Additionally, harmonic pulses were also observed extending up to the 35th harmonic alongside the dissipative soliton.

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