Optimizing single mode robustness of the distributed modal filtering rod fiber amplifier

Area of Science:

• Optics and Photonics
• Materials Science

Background:

• High-power fiber amplifiers for pulsed applications necessitate large mode area (LMA) fibers with high pump absorption and near diffraction-limited output.
• Photonic crystal fibers offer shorter LMA fiber amplifiers with high pump absorption by decoupling the pump cladding from the outer fiber diameter.
• Achieving ultra-low numerical aperture (NA) for single mode (SM) guidance in LMA fibers presents a significant design challenge.

Purpose of the Study:

• To present a distributed modal filtering (DMF) design strategy for achieving SM guidance in ultra-low NA fibers with very large cores.
• To optimize the SM bandwidth of the DMF rod fiber design.
• To ensure stable SM bandwidths by targeting design parameters that tolerate fabrication uncertainties.

Main Methods:

• Utilizing photonic crystal fiber design principles to decouple pump cladding and outer fiber diameter.
• Implementing a distributed modal filtering (DMF) design to enable SM guidance in ultra-low NA, large-core fibers.
• Analyzing band gap properties to optimize SM bandwidth, specifically by leveraging the first band of cladding modes.

Main Results:

• A fourfold increase in SM bandwidth compared to previous designs was achieved.
• The optimized DMF design effectively utilizes the first band of cladding modes.
• The design covers a significant portion of the Ytterbium (Yb) emission band, including wavelengths at 1030 nm and 1064 nm.
• Design parameters were identified to tolerate refractive index fabrication uncertainties of ± 10⁻⁴, ensuring stable SM bandwidths.

Conclusions:

• The distributed modal filtering (DMF) design is a viable strategy for enabling single mode (SM) guidance in ultra-low NA, large-core fibers.
• The optimized DMF rod fiber design significantly enhances SM bandwidth, making it suitable for high-power pulsed fiber amplifiers.
• This approach provides robust performance by accommodating fabrication tolerances, crucial for practical implementation.