New Design of a Thulium–Aluminum-Doped Fiber Amplifier Based on Macro-Bending Approach

A new method for gain enhancement in a S-band thulium-doped fiber amplifier (TDFA) co-doped with aluminum is demonstrated using a macro-bending approach. The macrobending of the doped fiber in a small radius suppresses both amplified spontaneous emissions (ASEs) at 800 and 1800 nm band and thus increases the population inversion in the S-band region. The numerical aperture and core radius of the doped fiber are optimized so that 800 nm ASE propagates with higher order modes to achieve a significant suppression while the loss is minimum in the S-band region. Meanwhile, the 1050 nm pump wavelength should propagate in the fundamental mode to maximize the overlap factor and thulium ion absorption so that the ASE loss is maximum at the 1800 nm region. Gain enhancements of about 5-8 dB are obtained with macrobending at the wavelength region between 1420 and 1470 nm.