Magnetic Quasi-Phase Matching All-Fiber Isolator

We have experimentally demonstrated an all-fiber optical isolator with 20？dB isolation. The result shows that the quasi-phase matching technique via a meter-long magnet array is highly feasible to generate more than 45 degrees of Faraday rotation in the fibers. The all-fiber isolator can also be temperature tuned to operate between 1048？nm and 1066？nm wavelength. 1. Introduction While ytterbium- (Yb- ) doped fiber laser amplifiers have reached multi-kW power level, the power limit of available fiber-coupled isolators is only around 50？W. Free-space bulk isolators have capability of handling higher power; however, it is inconvenient and highly inefficient to recouple light back into the fiber. To our best knowledge, there are no monolithic all-fiber isolators available for the Yb emission band. Without appropriate isolators to prevent back reflection and the build-up of backward-traveling-stimulated Brillouin scattering, backward amplification is capable of causing catastrophic failure of high-power laser systems. 2. All-Fber Isolator Design and Fiber PBL Measurements Our objective is to build an all-fiber isolator based on Faraday rotation in fused silica fiber [1]. The all-fiber isolator setup is shown in Figure 1. With the high magnetic field of Neodymium (NdFeB) magnets, we expect to obtain 45° Faraday rotation within the wavelengths of the Yb emission band using an array of magnets close to 1？m long. The magnets are mounted in two rows in a repelling configuration, such that the direction of the longitudinal field in the gap between the rows reverses periodically. A specialty fiber (Nufern low birefringence PM fiber) is inserted in the gap to produce the desired Faraday rotation, and its polarization beat length (PBL) is matched to the period of the magnet array. Due to this quasi-phase matching, laser emission entering the magnet array in one eigen-polarization of the fiber exits with equal amounts of the two eigen-polarizations [2, 3]. To demonstration an isolator, all-fiber polarizers are spliced to either ends of the birefringent fiber. The input polarizer is oriented in the direction of the eigen-polarizations; the polarization axis of the output polarizer is positioned at 45° with respect to the eigen-polarizations. The fiber is translated longitudinally to obtain linear polarization at 45° at the position of the latter polarizer. One major advantage of this setup is that more than one fiber can be inserted in the magnet gap to produce multiple isolators. Figure 1: All-fiber isolator setup. Phase matching condition for producing Faraday