Determination of modal content in multimode fibers by simultaneous intensity and phase measurement

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. This paper proposes a simple method based on the simultaneous measurement of both intensity and phase profiles of the beam, performed by a high resolution (300times400 measurement points) shearing interferometer-based wavefront analyzer (SID4-HR, Phasics). A complete (amplitude and phase) and accurate analysis of the modal content, even for highly multimode beams is demonstrated. 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).