Abstract
We propose an efficient method to determine the effective refractive index of step-index optical fibers from the visible to the mid-IR and thus allowing to infer their dispersive properties over a broad spectral range. The validity of the method, based on the writing of an array of fiber Bragg gratings (FBGs) with known periods using the fs scanning phase mask technique, is first confirmed with a standard silica fiber, then applied to various fluoride glass fibers to determine their effective refractive index and dispersion over more than three octaves, i.e. from 550 to 4800 nm.
Highlights
We propose an efficient method to determine the effective refractive index of step-index optical fibers from the visible to the mid-IR and allowing to infer their dispersive properties over a broad spectral range
The SMF-28, a standard silica fiber made by Corning, was chosen to validate the method, in part due to its well-known parameters, its batch-to-batch consistency and for the ease of writing fiber Bragg gratings (FBGs) in silica fibers compared to fluoride fibers
All made by Le Verre Fluoré, were chosen for this demonstration: a heavily-erbium-doped ZBLAN fiber commonly used in high power 2.8 μm fiber lasers, and three undoped fluoroindate fibers used for supercontinuum generation (SC) generation in the mid-IR
Summary
We propose an efficient method to determine the effective refractive index of step-index optical fibers from the visible to the mid-IR and allowing to infer their dispersive properties over a broad spectral range. In molecular spectroscopy and environmental sensing, for example, several atmospheric pollutants including carbon oxides (CO, CO2), hydrocarbons such as methane (CH4) and nitrogen oxides (NOX), possess absorption lines in the 3–5 μm spectral region that can be 10 to 100 times stronger than their respective harmonics lying in the near-infrared (near-IR)[1]. Other means exist for directly determining the dispersion of optical fibers, such as by inferring it from the Kelly side lobes associated with ultrafast pulses propagating inside a ring c avity[3] or by using complex or dedicated interferometric s etups[17,18] Another method that has been applied to highly sensitive silica-on-silicon waveguides is to write an array of weak FBGs to infer their dispersion from the measurement of their effective refractive index from 800 nm to 1600 nm[19]. This method has only been limited to a mere 50 nm spectral bandwidth[20], and its demonstration on a larger scale or in non-photosensitive fiber has yet to be made
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