A new method for side pumping of double-clad fiber sources

We report a new method for pumping of double-clad, rare-earth-doped fiber sources using diode lasers, diode bars, or fiber-coupled pump sources. In this technique, the pump beam is launched by reflection from a mirror that is embedded in a channel cut into the inner cladding. The mirror may be curved to reduce the divergence of the pump beam, thereby allowing the use of highly divergent pump sources and low-numerical-aperture inner claddings. Additional advantages include high coupling efficiency, little loss of brightness in coupling the pump beam, relatively low sensitivity to misalignment, no obstruction of the fiber ends, no loss for light propagating in the core, simplicity (low parts count), compact and rugged packaging, scalability to high power, and low cost. Because of the large alignment tolerances, the technique is uniquely well suited to the direct coupling of the output of a diode bar into one or more double-clad fibers. We present a detailed description and characterization of the technique. We also describe the performance of Yb-doped and Er-Yb-doped fiber amplifiers constructed with this method, including amplifiers with saturated output powers of 5.2 W (1064-nm wavelength) and 2.6 W (1550 nm) when pumped with two laser diodes.