Techniques for ultra-sharp edge distributed spectral filtering in optical fiber

We demonstrate, to the best of our knowledge, novel distributed spectral filtering techniques in optical fibers, enabling ultra-sharp wavelength discrimination below 0.1 nm. Our first method uses highly dispersive resonator rods with engineered high NA and small diameters for steep dispersion slopes, enabling sharp core-to-rod mode coupling. The second exploits localized HOMs ( L P l 1 family, l >1) resonantly coupled with the FM, producing narrow stopbands with ∼10pm–level wavelength selectivity. This method produces precise, distributed in-line filtering without additional components, and this makes them highly attractive for various applications, including fiberized notch filters, high-power narrow linewidth fiber lasers, and fiber lasers employing tandem pumping schemes.