Abstract

Large-mode-area (LMA) fibers have enabled great advances in high-power fiber lasers and amplifiers by allowing single-mode operation with an effective area (A_eff) much larger than in standard single-mode fibers. However, the size of LMA fibers is limited due to the presence of higher-order-modes (HOMs) which can degrade the output beam quality. In these systems, it is essential to characterize the spatial properties of the output beam accurately, for instance to determine the beam pointing stability. Different techniques have been proposed to evaluate the modal content of a beam exiting a LMA fiber and its amount of HOMs [1–3]. Some of them are based on intensity measurements and either need time consuming phase retrieval algorithm [1], or can only lead to the relative powers contained in the modes, without any information on the relative phase [2]. Another is based on a careful scanning of the intensity profile of a LMA fiber with a single-mode fiber [3], which is time consuming as well. As an alternative, we propose a simple method based on the simultaneous measurement of both intensity and phase profiles of the beam, performed by a high resolution (300×400 measurement points) shearing interferometer-based wavefront analyzer (SID4-HR, Phasics). We demonstrate a complete (amplitude and phase) and accurate analysis of the modal content, even for highly multimode beams. Since the measurement can be performed at video rates, this method opens the path for active wavefront correction techniques in multimode guiding structure (multimode or multicore fibers).

© 2009 IEEE