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

Gallium lanthanum sulfide (GLS) is a chalcogenide glass that offers unique optical properties, such as a wide transmission window (0.5–10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu m$</tex-math></inline-formula>) and a high non-linear refractive index. This makes it an ideal candidate for nonlinear integrated photonic devices, especially in the mid-infrared (mid-IR) wavelength region. This paper reports on the feasibility of utilizing a femtosecond-laser-inscribed waveguide array based on GLS glass as a novel saturable absorber for passively mode-locked mid-IR fiber laser systems. The waveguide array was fabricated via ultra-fast laser inscription using the multi-scan technique and then characterized using a mid-IR fiber laser to investigate the nonlinear propagation behavior, confirming its behavior as a saturable absorber that is based on the quasi-instantaneous Kerr nonlinearity. This waveguide array was then integrated into a linear fiber laser cavity with a highly reflective chirped fiber Bragg grating (CFBG) for wavelength-selective feedback. The observed mode-locked pulse train from this <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Ho^{3+}$</tex-math></inline-formula>/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Pr^{3+}$</tex-math></inline-formula> : ZBLAN fiber laser has a 26.2MHz repetition rate with a measured SNR of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 80dB, indicating a very stable mode-locked regime. An average output power of 850mW and a spectral width of 1.18nm were obtained. Our results show that a nonlinear waveguide array is a promising nonlinear device for all-integrated ultrafast fiber laser cavities in the important mid-infrared spectral region.