Effects of Solvent Etching on PMMA Microstructured Optical Fiber Bragg Grating

Polymer optical fiber Bragg gratings (POFBGs) can be solvent etched to achieve higher sensitivity sensors. To analyze the influence of etch on the mechanical and thermal performance of PMMA-based microstructured POFBGs (mPOFBGs), we etched annealed and unannealed samples in pure acetone, while at the same time monitoring the induced Bragg wavelength shifts. Optical-thermodynamical interpretation of these shifts together with thermal and mechanical testing of etched fibers, reveal that solvent etching can lead to trapping of residual plasticizing solvent in the polymer matrix and consequently to change of Young's modulus and thermal instability of the etched mPOFBG. Removal of these solvent residuals by thermal treatment results in a stable linear response with a thermal sensitivity indistinguishable of that of the unetched mPOFBG. The maximum found change of Young's modulus due to acetone plasticization during etch is ~5% for both unannealed and annealed mPOFBGs. The mPOFBG force sensitivity increases greatly with reduced diameter and the relatively small changes in strain sensitivity and Young's modulus introduced by the etching process in this case do not have a big effect.