Chalcogenide-glass Waveguide Arrays for Mid-Infrared Mode-Locked Fiber Lasers

<p dir="ltr">The mid-infrared (mid-IR) spectral region is of great importance for numerous applications, including spectroscopy, gas sensing, and material processing. Therefore, lasers emitting at mid-IR regions have received significant attention in ongoing research and industry. Fiber lasers are powerful tools to generate light at mid-IR wavelengths. However, the development of compact, high-performance mid-IR laser sources remains a challenge due to the lack of fiber-coupled optical components to create an all-fiber laser cavity, which severely limits the practical applications.</p><p dir="ltr">This thesis presents the feasibility of using femtosecond laser (fs) pulses to inscribe a nonlinear device, namely a waveguide array, which can be combined with an in-fiber component such as fiber Bragg grating to create a platform for the generation of ultrashort pulses of mid-IR light. The research begins with the fabrication and characterization of single-mode waveguides in highly nonlinear Gallium Lanthanum Sulphide (GLS) glass in the near- and mid-IR spectral regions. The refractive index profiles of the waveguides were mapped using quandriwave lateral shearing interferometry (QWLSI). Nonlinear optical properties were characterized via Z-scan measurement and self-phase modulation (SPM)-induced spectral broadening experiments, revealing that waveguides written in the athermal regime better preserve the high intrinsic nonlinearity of bulk GLS glass compared to those written in the thermal regime.</p><p dir="ltr">Based on these findings, 2D and 3D GLS waveguide arrays were fabricated and characterized as nonlinear photonic components. When excited with a mid-IR laser at 3400 nm, these arrays exhibited an intensity-dependent mode coupling effect, where low-power excitation resulted in linear diffraction, while high-power excitation led to nonlinear selftrapping of light. This behavior mimics a saturable absorber (SA), a key component for ultrafast pulse generation.</p><p dir="ltr">Finally, these waveguide arrays were integrated into a mode-locked fluoride fiber laser cavity, demonstrating their capability as mid-IR saturable absorbers. The last chapter provides a big picture overview of the topics covered in this thesis and takes a look at the future directions in which this work is headed. Our findings presented in this thesis demonstrate the feasibility of developing a hybrid chip-fiber platform for the mid-IR light thereby paving the way to the development of robust high-power, alignment-free, and all-fiber mid-IR laser systems.</p>