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The evolution of the light intensity along an optical waveguide is evaluated by analyzing far-field right-angle Rayleigh light scattering. The method is based on point-by-point-spectral mapping distributed along the waveguide with a micrometric spatial resolution given by a confocal microscope, a high spectral resolution given by a spectrometer, and a high signal-to-noise ratio given by a highly cooled detector. This non-destructive and non-invasive experimental method allows the observation, in linear or nonlinear regime of propagation, the general Rayleigh scattering profile of the optical waveguide in a nominal operation, i.e., whatever the power or the wavelength of the light source (CW or pulsed laser), and can be applied to micrometer-scale waveguides of several centimeters in length, for which the longitudinal characterization is challenging.
Applied to a tapered optical fiber with submicrometer final diameter and several centimeters long (called nanofiber), this method has proved its capacity in linear regime to collect different optical characteristics such as optical losses, mode beatings, transition from core-cladding to cladding–air guidance for different modes, localization of punctual defects, leaking of high order modes no longer guided by the fiber. Furthermore, the experimental results are successfully compared to measurements provided by the state-of-the-art Optical Backscatter Reflectometer system, and to numerical simulations. Moreover, coupled to the spectral resolution of the spectrometer, the method have allowed in the nonlinear regime the distributed measurements of a Raman cascading process along the nanofiber, for the first time to our knowledge.
Our experimental technique is complementary to other characterization methods, generally focused on the optical parameters of the waveguide input or output. This technique can also be extended to others waveguides whatever its geometry which represents a strong interest for deepen optical characterization of photonics waveguides, or for other optical regimes characterized by spectral evolution of the field propagating along the waveguide.
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