A highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes of the refractive index of the surrounding liquid is presented. By using an open top ridge waveguide with a small core, the evanescent field interaction of the guided mode with the liquid analyte on the top of the waveguide is enhanced. The sensitivity measured via a shift in the resonance wavelength of the Bragg grating as high as 1 pm of wavelength shift for a change of 4 × 10−5 in the refractive index around 1.402 is realized. With a polarization insensitive Bragg grating, the polarization dependence of the sensor is improved. A theoretical analysis for the sensitivity of ridge waveguide sensors is given. The experimental results are in good agreement with the theoretical analysis.
A series of novel highly fluorinated aromatic−aliphatic copolyethers have been prepared by
copolycondensation reactions of decafluorodiphenylsulfone (DFPSO) with hexafluorobisphenol-A (6F-BPA) and
a highly fluorinated linear or branched diol (6CF-diol or 7bCF-diol) at different feed ratios of [diol]/[6F-BPA].
The reactions can be completed in 2−5 h in the presence of CsF in DMAc at room temperature to produce
polymers with number average molecular weight (M
n) up to 45 000 Da. 1H and 19F NMR studies show that the
composition of the resultant polymers agrees well with the feed ratio of the monomers in the reaction. This
feature leads to a feasible tune of glass transition temperatures (T
g) and refractive indexes of the produced polymers
by a simple control of the feed ratio. As the [diol]/[6F-BPA] ratio increases, the T
g of the copolymers decreases
gradually from 185 to 87 °C and 94 °C, and the refractive index decreases from 1.5120(n
TE)/1.5089(n
TM) to
1.4643(n
TE)/1.4629(n
TM) and 1.4487(n
TE)/1.4471(n
TM) for the copolymers from 6CF-diol and 7bCF-diol, respectively.
Meanwhile, the increase of the [diol]/[6F-BPA] ratio also effectively reduces the birefringence of the polymer
films from 3.1 × 10-3 to 1.4 × 10-3 for the polymers from 6CF-diol and to 1.6 × 10-3 for the polymers from
7bCF-diol. Well-developed waveguide structures with smooth ridge walls from these polymers have also been
demonstrated using a cost-effective soft lithography technique.
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