Polarization response of planarized optical waveguides to determine the anisotropic complex refractive index of graphene oxide thin films.

The polarization response of graphene oxide (GO)-coated planarized optical waveguides is used to determine the complex refractive index of GO film. GO films with thicknesses between 0.10 and 0.71 µm were coated on planarized optical waveguides. GO-coated waveguides exhibit large polarization dependent losses-and the polarization response depends strongly on the GO coating thickness. The response was used, together with finite element analysis, to determine the complex refractive index of the GO film. The complex refractive indices of GO films for both TE- and TM-polarized light at a wavelength of 1550 nm were found to be 1.71+0.09i and 1.58+0.05i, respectively. The uncertainties of nGO and kGO for TE-polarized light are ±0.02 and ±0.03, respectively, whereas the uncertainties of nGO and kGO for TM-polarized light are ±0.05 and ±0.02, respectively.

Thermal stress modification in regenerated fiber Bragg grating via manipulation of glass transition temperature based on CO2-laser annealing.

In this work, we have demonstrated thermal stress relaxation in regenerated fiber Bragg gratings (RFBGs) by using direct CO2-laser annealing technique. After the isothermal annealing and slow cooling process, the Bragg wavelength of the RFBG has been red-shifted. This modification is reversible by re-annealing and rapid cooling. It is repeatable with different cooling process in the subsequent annealing treatments. This phenomenon can be attributed to the thermal stress modification in the fiber core by means of manipulation of glass transition temperature with different cooling rates. This finding in this investigation is important for accurate temperature measurement of RFBG in dynamic environment.

Reflection spectra of etched FBGs under the influence of axial contraction and stress-induced index change.

We present a new theoretical model for the broadband reflection spectra of etched FBGs which includes the effects of axial contraction and stress-induced index change. The reflection spectra of the etched FBGs with several different taper profiles are simulated based on the proposed model. In our observation, decaying exponential profile produces a broadband reflection spectrum with good uniformity over the range of 1540-1560 nm. An etched FBG with similar taper profile is fabricated and the experimental result shows good agreement with the theoretical model.

Axial contraction in etched optical fiber due to internal stress reduction.

When an optical fiber is dipped in an etching solution, the internal stress profile in the fiber varies with the fiber diameter. We observed a physical contraction as much as 0.2% in the fiber axial dimension when the fiber was reduced from its original diameter to ~6 µm through analysis using high resolution microscope images of the grating period of an etched FBG at different fiber diameters. This axial contraction is related to the varying axial stress profile in the fiber when the fiber diameter is reduced. On top of that, the refractive index of fiber core increases with reducing fiber diameter due to stress-optic effect. The calculated index increment is as much as 1.8 × 10(-3) at the center of fiber core after the diameter is reduced down to ~6 µm. In comparison with the conventional model that assumes constant grating period and neglects the variation in stress-induced index change in fiber core, our proposed model indicates a discrepancy as much as 3nm in Bragg wavelength at a fiber diameter of ~6 µm.