Challenges in high-intensity laser injection into multiple optical fibers

摘要

A growing number of applications involve the transmission of high-intensity laser pulses through optical fibers. Previously, our particular interests led to a series of studies on single-fiber transmission of Q-switched, 1064 nm pulses from multimode Nd:YAG lasers through step-index, multimode, fused silica fibers. The maximum pulse energy that could be transmitted through a given fiber was limited by the onset of laser-induced breakdown or damage. Breakdown at the fiber entrance face was often the first limiting process encountered, but other mechanisms were observed that could result in catastrophic damage at either fiber face, within the initial "entry" segment of the fiber, and at other internal sites along the fiber path. These studies examined system elements that can govern the relative importance of different damage mechanisms, including laser characteristics, the design and alignment of laser-to-fiber injection optics, fiber end-face preparation, and fiber routing. In particular, criteria were established for injection optics in order to maximize margins between transmission requirements and thresholds for laser-induced damage. Recent interests have led us to examine laser injection into multiple fibers. Effective methods for generating multiple beams are available, but the resulting beam geometry can lead to challenges in applying the criteria for optimum injection optics. To illustrate these issues, we have examined a three-fiber injection system consisting of a beam-shaping element, a primary injection lens, and a grating beamsplitter. Damage threshold characteristics were established by testing fibers using the injection geometry imposed by this system design.