title: Demonstration of ultra-low NA rare-earth doped step index fiber for applications in high power fiber lasers.
abstract: In this paper, we report the mode area scaling of a rare-earth doped step index fiber by using low numerical aperture. Numerical simulations show the possibility of achieving an effective area of ~700 um² (including bend induced effective area reduction) at a bend diameter of 32 cm from a 35 μm core fiber with a numerical aperture of 0.038. An effective single mode operation is ensured following the criterion of the fundamental mode loss to be lower than 0.1 dB/m while ensuring the higher order modes loss to be higher than 10 dB/m at a wavelength of 1060 nm. Our optimized modified chemical vapor deposition process in conjunction with solution doping process allows fabrication of an Yb-doped step index fiber having an ultra-low numerical aperture of ~0.038. Experimental results confirm a Gaussian output beam from a 35 μm core fiber validating our simulation results. Fiber shows an excellent laser efficiency of ~81%and aM² less than 1.1.
authors.forename: Deepak
authors.initials: D
authors.lastname: Jain
authors.forename: Yongmin
authors.initials: Y
authors.lastname: Jung
authors.forename: Pranabesh
authors.initials: P
authors.lastname: Barua
authors.forename: Shaiful
authors.initials: S
authors.lastname: Alam
authors.forename: Jayanta K
authors.initials: JK
authors.lastname: Sahu