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.
Highlights
High power fiber lasers have revolutionized the laser world in the last decade [1]
Ming-Jun Li et al demonstrated that the maximum scalable mode area for effective single mode operation from a step index fiber (SIF) is 370μm2 for 1060nm operation taking into account the core numerical aperture (NA) to be 0.06 that could be achieved experimentally at the time of publishing the paper in 2009 [19]
We numerically showed the feasibility of achieving effective area around 700μm2 from a SIF with a NA of 0.038, while following the same criterion of single mode operation (FM loss lower than 0.1dB/m and higher order modes (HOMs) loss higher than 10dB/m) at 1060nm wavelength
Summary
High power fiber lasers have revolutionized the laser world in the last decade [1]. Fiber lasers offer great advantages such as better power handling, good beam quality, and enhanced flexibility over solid-state lasers. Non-linear effects often found to be the primary limiting factors for power scaling in fiber lasers and amplifiers [3] Several fiber designs such as Photonic Crystal Fiber (PCF) [4], Leakage Channel Fiber (LCF) [5], Resonantly-enhanced Leakage Channel Fiber (Re-LCF) [6], Bragg fiber [7], 2D-All Solid Photonic Bandgap Fiber (2D-PBGF) [8], Polygonal Chirally Coupled Core (P-CCC) [9], bend compensated Parabolic Fiber [10], Multi trench Fiber (MTF) [11,12,13,14], and Pixelated Trench Fiber (PTF) [15] have been proposed to scale the effective area of the fundamental mode (FM) in order to increase the threshold of non-linear effects. Experimental characterizations ensure a good laser efficiency of 81% with M2 less than 1.1
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