Fabrication of fibers with high rare-earth concentrations for Faraday isolator applications

The Faraday effect provides a mechanism for achieving unidirectional light propagation in optical isolators; however, miniaturization requires large Verdet constants. High rare-earth content glasses produce suitably large Verdet values, but intrinsic fabrication problems remain. The novel powder-intube method, or a single-draw rod-in-tube method, obviates these difficulties. The powder-in-tube method was used to make silica-clad optical fibers with a high terbium oxide content aluminosilicate core. Core diameters of 2.4 µm were achieved in 125-µm-diameter fibers, with a numerical aperture of 0.35 and a Verdet constant of -20.0 rad/(T m) at 1.06 µm. This value is greater than 50% for crystals found in current isolator systems. This development could lead to all-fiber isolators of dramatically lower cost and ease of fabrication compared with their crystalline competitors.