Single longitudinal mode fiber laser with integration of gallium-erbium co-doped fiber

Single longitudinal mode (SLM) lasers have gained much attention because of its ability to generate narrow linewidth output with low phase noise necessary for applications in optical communication and sensing. The generation of SLM in fiber laser is highly desirable due to its excellent performance and coupling simplicity. The issue however, is the complexity imposed by the inherently long cavity length and doped fiber gain medium, which lead to small free spectral range (FSR) and broad gain bandwidth. The experimental work presented in this thesis demonstrates SLM operation using Ga- EDF as gain medium together with several mode filtration techniques in the laser setup. High Erbium ion concentration in the fiber leads to high gain coefficient allowing short gain medium length to be used hence reducing the cavity length and widening the FSR. Ring cavity configuration employing optimum Ga-EDF length of 2 m was employed as the base structure. In order to suppress and filter the multi longitudinal modes, tunable bandpass filter (TBF), sub-ring structure and cascaded dissimilar fiber taper were utilized in this laser system. Each of the filter mechanism was tested individually within the laser cavity to assess its performance. Once the performance of each filter was obtained, all of them were deployed into the laser system. Ultimately, the 1561.47 nm SLM output was able to achieve a narrow linewidth laser of 1.19 kHz with output power of 1.814 mW and optical signal-to-noise ratio of 55.37 dB. Additionally, the stability and tunability of the laser were investigated. The laser can be tuned from 1522 nm to 1563 nm by tuning the TBF and 0.16 dB of power fluctuation was observed in duration of one hour. Evidently, the use Ga-EDF is feasible and significant for SLM operation thanks to its high gain coefficient. Simple mode selectors such as TBF, sub-ring cavity and dissimilar cascaded tapered fiber are influential as longitudinal mode eliminator. The findings from this study highlighted the attractiveness of high dopant co-doped fiber as gain medium for various laser configuration. Among the works that had been proposed by researchers, to the best of our knowledge, this is the first SLM laser demonstration utilizing Ga-EDF as the gain medium.

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