High-power fiber lasers have been increasingly used for material processing, metal cutting, and welding applications due to high wall-plug efficiency, compactness, and superb long term reliability[ 1, 2]. Fused fiber components enable reliable, efficient, high-power fiber laser systems operating in continuous wave (high power) or pulsed regimes such as ultrafast (high peak power).
Common fused fiber components include the fiber mode-field adapter (MFA), fused fiber combiner (FFC), and fiber end cap (FEC) in addition to fusion splicing of dissimilar fibers[3, 4]. These devices allow power scaling of high-power fiber lasers. Key requirements for the components are low transmission loss, good long-term reliability, and high power handling capability. Fabrication of these parts requires both specific optical design and advanced fiber fusion process solutions.
A bi-directionally pumped single stage fiber laser is shown in Figure 1. The laser cavity consists of active gain fiber (e.g. Yb-doped double clad fiber), a high-reflection fiber Bragg grating (HR FBG) with reflectivity of typically >99%, and an output coupler (OC) FBG with reflectivity of typically around 5-10%. Low loss mode-field adapting splicing between gain fiber and HR/OC passive fiber is important for efficient laser operation and good output beam quality. The laser is pumped by combined multi-mode (MM) laser diodes (LD) from both directions to enable high-power laser output. Multiple LDs are combined with a fused fiber combiner to deliver highenergy power for pumping the laser cavity in an all-fiber configuration.
A fiber end cap, which allows expansion of the laser beam in the glass before exiting to the free space, mitigates detrimental end-face damage at high power by reducing the power density of the exit beam. Performance of these components is critical for maintaining efficient, reliable, long-term operation of the laser system.
To enable high output power of the fiber laser, efficient coupling of MM pump beams into the fiber and low loss transmission of the laser beam between different fibers are both critical. This imposes challenges for fabricating the necessary fused fiber components.
The fused-component optical design must be sound to ensure proper coupling of MM beams and laser beams between fibers. Brightness must be conserved for MM beam propagation and the fiber mode-field between dissimilar fibers needs to be properly matched. Fiber taper, geometric or diffusion taper, must be adiabatic.
A fusion process with a high degree of fusion temperature control, a high temperature uniformity and an accurate heating time control are necessary to ensure high-quality fiber fusion without introducing abnormality to the fibers. A good process results in low transmission loss and high splice strength. Macro and micro bending loss and excitation of high order beams must be minimized for single mode fiber laser operation.
This poses stringent requirements for the heat source used in the fusion process, including:
- uniform heating around the fiber due to its circular shape,
- wide temperature range to accommodate different fiber sizes,
- consistent heating to allow a precise fusion process repeatedly, and
- “clean” fusion process to ensure minimum temperature rise around the processed fiber area as the MM pump beam “sees” the surface of the fiber cladding. Minute surface contaminants could absorb laser energy and generate heat on the fiber surface.