Use of bend loss to obtain single-transverse-mode operation of a multimode fiber amplifier

Summary form only given.Use of single-mode (SM) fiber lasers and amplifiers for applications requiring optical pulses with high energy and/or high peak power has been limited by the relatively low energy storage of SM fibers and by the onset of nonlinear processes in the fiber. Similarly, for cw applications requiring narrow linewidth, the maximum power is limited by nonlinear processes. Two approaches to overcoming these limitations have been reported: (1) use of large-mode-area SM fibers; and (2) use of multimode (MM) gain fibers, in which suppression of high-order modes is achieved by adjusting the fiber index and dopant distributions, cavity configuration, and/or launch conditions of the seed beam. We report a new approach to obtain SM operation of a laser or amplifier employing MM fiber. By wrapping the gain fiber around a cylindrical mandrel whose radius is chosen to provide low loss for the fundamental mode and high loss for LP/sub 11/ and the other high-order modes, bend loss can be used as a form of distributed spatial filtering, thereby allowing the core diameter to be increased significantly beyond the SM limit.