Increasing the dimensions of a waveguide provides the simplest means of reducing detrimental nonlinear effects, but such systems are inherently multi-mode, reducing the brightness of the system. Furthermore, using rectangular dimensions allows for improved heat extraction, as well as uniform temperature profile within the core. We propose a method of using the angular acceptance of a transmitting Bragg grating (TBG) to filter the fundamental mode of a fiber laser resonator, and as a means to increase the brightness of multi-mode fiber laser. Numerical modeling is used to calculate the diffraction losses needed to suppress the higher order modes in a laser system with saturable gain. The model is tested by constructing an external cavity resonator using an ytterbium doped ribbon fiber with core dimensions of 107.8μm by 8.3μm as the active medium. We show that the TBG increases the beam quality of the system from M² = 11.3 to M² = 1.45, while reducing the slope efficiency from 76% to 53%, overall increasing the brightness by 5.1 times.

Date Published: 4 March 2015
PDF: 11 pages
Proc. SPIE 9344, Fiber Lasers XII: Technology, Systems, and Applications, 93441W (4 March 2015); doi: 10.1117/12.2079358

Published in SPIE Proceedings Vol. 9344:
Fiber Lasers XII: Technology, Systems, and Applications
L. Brandon Shaw, Editor(s)