Fused silica optical fibers with arbitrary gradient index nanostructured core

Gradient index (GRIN) fibers have recently been identified as the potential next generation components for today's classic fiber optic applications, ranging from telecommunications to laser technologies [1]. In particular the nonlinear beam self-cleanup leading in multimode (MM) GRIN fibers to spatial organization into a fundamental mode on a weak background of higher order modes, is hoped to disrupt telecommunications due to its potential to harness the MM and its promise of superb information density [2]. For lasers, this approach means a new quality of high power delivery with bending loss or speckle formation strongly mitigated against existing solutions. Yet, arbitrary transverse profiles of refractive index in GRIN fibers would even add to these advantages e.g. the possibility of transmitting nonstandard states of polarization or beam shaping without the need for bulk optics. Current CVD technologies however limit freedom of development of GRIN fibers. Only radially symmetric or, to some extent, elliptical refractive index distributions in are feasible. Recently we have presented a new approach to fibers with arbitrary GRIN profiles [3]. A continuous refractive index profile at the fiber cross-section is obtained by varying distribution of two types of subwavelength rods (Fig.1a). Since every single rod in the final nanostructured core fiber is sub-wavelength, the propagating light beam experiences a certain effective refractive index described with the Maxwell-Garnett effective medium model. This method allow to obtain arbitrary gradient distribution in the core. Here we report for the first time the development of fiber with effective parabolic GRIN profile in the core based on fused silica (FS) glass. The core is composed of 2107 rods of 200 nm diameters, made of two types of glass: low refractive index FS (nD= 1.4588) and high index Ge doped FS (nD= 1.4637). with 3.18% mol concertation (Fig.1b). The fiber core has an effective parabolic refractive index distribution with gradient g=0.077 m
⁻¹
. The proposed fiber has a flat anomalous dispersion with ZDW varying between 1.26 μm and 1.37 μm for the fibers with different core diameters between 8 and 11 μm (Fig. 1d). The measured dispersion characteristics perfectly match simulation results obtained with continuous parabolic profiles. Such profile is used in our discussion to compare properties of the developed fiber with standard GRIN fibers, although arbitrary index profiles are possible using the same technological platform. Our results also prove, that the Ge dopant distribution is well controlled during fiber fabrication, as the effective refractive index profile of the nanostructured core perfectly matches the design specification.