GLS Waveguide Array: A Novel Saturable Absorber for Mid-Infrared Mode-Locked Fiber Lasers

The mid-infrared (mid-IR) spectral region (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.5-25 \mu \mathrm{~m}$</tex>) plays a crucial role in numerous applications spanning spectroscopy, material processing, security, and industrial sectors [1]. A true all-fiber continuous wave or pulsed lasers are essential for developing compact and robust mid-IR laser systems suitable for field deployment. However, pulsed operation is limited by the lack of commercial mid-IR saturable absorbers (SAs). To address this, we propose integrating a novel waveguide array into the laser cavity to enable passive mode-locking in the mid-IR region. Gallium Lanthanum Sulfide (GLS), characterized by its wide transmission window (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.5-10 \mu \mathrm{~m}$</tex>) and high nonlinear refractive index [2], emerges as a promising material for fabricating these nonlinear integrated photonic devices. Ultrafast laser inscription (ULI) was employed to fabricate a 2D waveguide array within the GLS substrate. The 2D waveguide array was then characterized at 3400 nm wavelength to investigate the propagation behavior at low and high input peak powers, confirming its behavior as an SA [3]. At low input powers, evanescent wave coupling resulted in power redistribution among neighboring waveguides. However, as the input power increased, nonlinear self-focusing effects overcame linear diffraction, leading to power confinement within the central waveguide as shown in Fig. 1(a). This observed behavior closely resembles the operational characteristics of a SA. Finally, this device was integrated into a linear fiber laser cavity to achieve the stable, passive mode-locking in the mid-IR regime with laser emission at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.8 \mu \mathrm{~m}$</tex> (see Fig. 1(b)).