Microscopic imaging along tapered optical fibers by right-angle Rayleigh light scattering in linear and nonlinear regime

Abstract

The evolution of the light intensity along an optical waveguide is evaluated by analysing far-field right-angle Rayleigh light scattering. The method is based on point by point spectral mapping distributed along the optical 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 of 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, 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, called nanofiber, with submicrometer final diameter and several centimeters long, the method has proved its capacity 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 the distributed measurements of the Raman cascading process along the nanofiber, for the first time to our knowledge. The experimental technique developed in this work 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.

Distributed spectral measurement of nonlinear Raman cascade process along an optical nanofiber

“…The scattering profiles are similar with a general point of view, which proves that our system is relevant. These scattering profiles are indeed due to the characteristics of the different scattering sources, corresponding to the fiber core at the beginning of the transition where light is still guided into the core of the fiber, the silica cladding when the core is no longer guiding in the tapered region, and the ONF surface for the small diameter when the surface scattering takes over the cladding scattering [13,14]. As the ONF has a homogenous diameter along about 20 mm length, the scattering level remains constant as observed in the 1D-X trace.…”

“…In addition, the large width of the entrance slit of the spectrometer (1 mm) used for this measurements increases the number of the detected Rayleigh photons. Moreover, the 1D-scan-X has been performed only for the ONF part (between X=50-70 mm), i.e., with the homogeneous diameter, where the strength of the scattering process is maximal and constant due to the large difference in the refractive index of air and dielectric [13]. This ensures a scattered recorded signal with a high intensity and a stable SNR in order to observe the real dynamics of the Raman amplifications of the all generated spectral components along the ONF region.…”

Distributed Spectral Measurement of Supercontinuum Generation Along an Optical Nanofiber

“…The experimental setup is based on the detection of far-field right-angle Rayleigh light scattering radiated out the waveguide using a micro-spectrometer [3] allowing a high spatial resolution given by the confocal microscope, and a high spectral resolution given by the spectrometer. Notch filters are used to remove the pump spectral components and to allow simultaneous Stokes and anti-Stokes measurements.…”

“…Therefore, there is no experimental investigation of the SC generation along an optical nanofiber. For that purpose, we have recently developed a non-destructive and non-invasive experimental method in order to characterize spatially and spectrally the guided light along an optical waveguide [3]. By using this method, we present in this paper preliminary results of a distributed longitudinal measurement of SC generation along a nanofiber of millimeter length and sub-micrometric size.…”

Optical microfiber or nanofiber: a miniature fiber-optic platform for nanophotonics

“…By scanning a partly stripped 125-μm-diameter fiber along an MNF and detecting the optical transmission of the MNF, sub-nanometer measurement accuracy of the MNF diameter was realized. Besides, many other techniques based on external gratings [161,162] , near-field probing [52,163–165] , stress-strain analysis [62] , Rayleigh scattered light imaging [166,167] , optical backscattering reflectometry technique [168] , short-time Fourier transform analysis on the modal evolution [169] , second- and third-harmonic generation [170] , and forward BS [171] , with a resolution from 15 nm to 40 pm, have been reported in recent years.…”