Scattering field imaging along an optical waveguide in operando

The linear or nonlinear optical propagation behavior lengthwise of micro- and nano-waveguides in operation is difficult to evaluate experimentally, although this information is crucial for their optimal development. Previously, apart from the destructive cut-back method, far-field qualitative imaging of Rayleigh scattering (RS) or optical backscatter reflectometry (OBR) technique in infrared wavelength (the state-of-art technique in terms of spatial resolution) have been used for rapid analysis of the propagation of light in optical waveguides (optical nanofiber ONF in Refs. [1] , [2] ). In the present work, we demonstrate a novel method, nondestructive and non- invasive, with a high spatial (lower than 5 µm) and spectral resolution, to analyze the light propagation in an optical waveguide in operation in linear or nonlinear regime. By measuring with a confocal spectrometer the RS radiated out of an ONF in operation, we show that our method can spatially characterize the entire waveguide in the linear regime with a micrometric spatial resolution. This approach provides direct observation of the higher-order modes (HOMs) leaking from the ONF surface along the taper region and the transition from core–cladding to cladding– air guidance. This method can also be used to characterize the linear loss and provides the localization of defects along an optical waveguide.