Alkeskjold, Thomas Tanggaard

Hansen, Kristian Rymann

Jørgensen, Mette Marie

Laurila, Marko

Lægsgaard, Jesper

Noordegraaf, Danny

Optical fiber amplifiers have gained increased scientific and commercial interest due to high output powers with easy operation and maintenance. The main advantages of fiber amplifiers are good thermal properties, due to a large surface/volume ratio, excellent beam quality and stability [1]. Very large core fiber areas are highly desirable to mitigate various nonlinear processes, such as Kerr, four-wave mixing, Raman, and Brillouin effects. It is difficult to scale up the core area without going into a multi-moded regime. Microstructured fiber technology has allowed core diameters of 60-100 microns, by reducing all index contrasts to very low values, and possibly utilizing advanced photonic-band gap cladding designs to filter out higher-order modes [2], see Fig. 1. The guided modes of such cores are very sensitive to perturbations. An unavoidable perturbation at large average power is the formation of thermal gradients across the core, which induces a power dependent transverse index modification due to the thermo-optic effect. A phase shift between the thermally induced index grating and the signal intensity causes power transfer from the fundamental mode to a higher order mode leading to modal instability [3-5], represented in Fig. 2 as the growth of the higher order mode content. This is a fundamental obstacle for power scaling in fiber amplifiers that significantly reduces beam quality and stability.We combine analytic considerations for thermally induced mode coupling with the finite element method to allow complex micro structured fibers to be considered. Thereby the modal instability threshold is estimated for very large mode area fiber amplifiers of various photonic crystal fiber designs. Experimentally the modal instability threshold for very large mode area fiber amplifiers is determined repeatedly and a decrease in the modal instability threshold level is observed as the threshold is reached multiple times

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 20, 2017Modal instabilities in very large mode area rod fiber amplifiersJohansen, Mette Marie; Hansen, Kristian Rymann; Laurila, Marko; Noordegraaf, Danny; Alkeskjold,Thomas Tanggaard; Lægsgaard, JesperPublication date:2012Link back to DTU OrbitCitation (APA):Jørgensen, M. M., Hansen, K. R., Laurila, M., Noordegraaf, D., Alkeskjold, T. T., & Lægsgaard, J. (2012). Modalinstabilities in very large mode area rod fiber amplifiers. Abstract from Northern Optics 2012, Snekkersten,Denmark.Modal instabilities in very large mode area rod fiber amplifiers Mette M. Jørgensen1, Kristian R. Hansen1, Marko Laurila1, D. Noordegraaf2, Thomas T. Alkeskjold2, Jesper Lægsgaard1 1DTU Fotonik, Ørsteds Plads, building 343, 2800 Kgs. Lyngby, Denmark, mmajo@fotonik.dtu.dk 2NKT Photonics A/S, Blokken 84, 3460 Birkerød, Denmark Optical fiber amplifiers have gained increased scientific and commercial interest due to high output powers with easy operation and maintenance. The main advantages of fiber amplifiers are good thermal properties, due to a large surface/volume ratio, excellent beam quality and stability [1]. Very large core fiber areas are highly desirable to mitigate various nonlinear processes, such as Kerr, four-wave mixing, Raman, and Brillouin effects. It is difficult to scale up the core area without going into a multi-moded regime. Microstructured fiber technology has allowed core diameters of 60-100 microns, by reducing all index contrasts to very low values, and possibly utilizing advanced photonic-band gap cladding designs to filter out higher-order modes [2], see Fig. 1. The guided modes of such cores are very sensitive to perturbations. An unavoidable perturbation at large average power is the formation of thermal gradients across the core, which induces a power dependent transverse index modification due to the thermo-optic effect. A phase shift between the thermally induced index grating and the signal intensity causes power transfer from the fundamental mode to a higher order mode leading to modal instability [3-5], represented in Fig. 2 as the growth of the higher order mode content. This is a fundamental obstacle for power scaling in fiber amplifiers that significantly reduces beam quality and stability. We combine analytic considerations for thermally induced mode coupling with the finite element method to allow complex micro structured fibers to be considered. Thereby the modal instability threshold is estimated for very large mode area fiber amplifiers of various photonic crystal fiber designs. Experimentally the modal instability threshold for very large mode area fiber amplifiers is determined repeatedly and a decrease in the modal instability threshold level is observed as the threshold is reached multiple times. [1] J. Nilsson and D. N. Payne, “High-power fiber lasers,” Science 332, 921-2 (2011). [2] M. M. Jørgensen, S. R. Petersen, M. Laurila, J. Lægsgaard, and T. T. Alkeskjold, “Optimizing single mode robustness of the distributed modal filtering rod fiber amplifier,” Opt. Express 20, 7263-7273 (2012). [3] A. V. Smith and J. J. Smith, "Mode instability in high power fiber amplifiers," Opt. Express 19, 10180-10192 (2011). [4] T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H.-J. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, "Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers," Opt. Express 19, 13218-13224 (2011). [5] K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Lægsgaard, "Thermally induced mode coupling in rare-earth doped fiber amplifiers," Opt. Lett. 37, 2382-2384 (2012).  Fig. 1: Microscope picture of a complex micro-structured photonic crystal rod fiber [2].  Fig. 2: Threshold like increase of higher order mode (HOM) content as a function of output power yielding modal instability. 

Modal instabilities in very large mode area rod fiber amplifiers

http://orbit.dtu.dk/ws/files/51205842/Modal_instabilities_fiber_amplifiers.pdf

Modal instabilities in very large mode area rod fiber amplifiers

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology