JM5A.13.pdf Advanced Photonics © 2014 OSA Spatial-coherence effect on damage occurrence in multimode optical fibers using nanosecond pulses Amir Herzog and Amiel A. Ishaaya Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105 hertzoga@post.bgu.ac.il Abstract: We investigate high peak power delivery in highly multimode optical fibers. By altering the spatial-coherence of the input beam, we demonstrate the transmission of 120mJ ns pulses at 355nm through a multimode fiber. OCIS codes: (140.3510) Lasers, fiber; (140.3330) Laser damage; (190.4370) Nonlinear optics, fibers. Multimode (MM) optical fibers are used for the delivery of pulsed laser radiation in a broad range of applications [1,2]; some of the applications require high peak-powers, and therefore optical damage may occur. Identifying the dominant damage mechanism and its root-causes, may potentially increase the amount of the delivered pulse energy. In the case of silica-based MM optical fibers, the damage threshold is expected to be clamped by the facet or bulk damage thresholds; however, in some cases optical damage occurs along the propagation path (inner damage), even below these thresholds. Such phenomenon might stem from various causes, including nonlinear phenomena, or due to the creation of hot-spots, caused by constructive interference along the propagation path. The ability of light to interfere depends on the spatial-coherence of the source, which is commonly quantified by the fringe visibility of two point sources in the beam, corresponding to the magnitude of the complex spatial-coherence degree [3]. Therefore, one can find a dependency between spatial-coherence and damage occurrence along propagation path in MM optical fibers. In our experiment, the spatial-coherence of the input beam was degraded using various diffusers and then a large multimode fiber for achieving further degradation of the spatial-coherence and larger uniformity of the beam intensity profile. The experiment included three parts: spatial-coherence measurements with various diffusers, spatial-coherence measurements at the output of a 1.5 mm core diameter fiber for 10cm and 120cm lengths (see experimental arrangement in Fig. 1), and damage threshold experiments for 0.6mm core fiber, positioned after the 1.5 mm core diameter fiber. The objective of the first part was to experimentally verify that degrading the input beam phase profile significantly affects the degree of spatial-coherence in the far-field. Results of the first and second parts of the experiment are described in Fig. 2. The experimental results, supported by numerical simulations, prove that there is a clear correlation between the degree of spatial-coherence in the beam and damage occurrence in MM fibers. In some cases, improvement ratio in the damage threshold after propagating through a short versus long MM fiber was 1:1.8. Due to the spatial- coherence degradation, we have delivered powers well exceeding the self-focusing limit, while avoiding inner damages. References [1] D. P. Hand, J. D. Entwistle, R. R. J. Maier, A. Kuhn, C. A. Greated and J. D. C. Jones,”Fibre optic beam delivery system for high peak power laser PIV illumination,” Meas. Sci. Technol. 10 239–45 (1999). [2] A.Vogel and V. Venugopalan, “Mechanisms of pulsed laser ablation of biological tissues,” Chem. Rev. 103(2), 577–644 (2003). [3] L. Mandel and E. Wolf, Optical Coherence and Quantum Optics (Cambridge University Press, 1995). Fig. 2. Visibility measurements (left to right): the raw beam, different diffusers, a short (10cm) and a long (120cm) MM fiber with 1.5mm core diameter. Inset: the raw-beam fringe visibility pattern image. Fig. 1. Experiment arrangement for the first and second parts of the experiment.