Reversible UV degradation of PMMA plastic optical fibers

Laser-induced fluorescence, Raman and absorption spectroscopy are used to investigate reversible degradation of transmission in PMMA optical fibers. When exposed to 254 nm UV light, optical transmission of PMMA plastic optical fiber in 400–800 nm range shows a significant increase in attenuation for shorter wavelengths. Over a period of 10 days following UV exposure, the transmittance of the plastic fiber recovers to a significant fraction of its pre-exposure value. UV-exposed fiber exhibits strong laser-induced fluorescence with 488 nm argon-ion laser. This fluorescence spans a spectral region between 450 nm and 750 nm with a peak around 580 nm. The fluorescence intensity decreases over several days following UV exposure. Likewise, Raman is also used to investigate degradation process. Freshly UV-exposed fiber shows total absence of Raman spectrum of PMMA. Following UV exposure, recovery of Raman signal over several days is correlated to the recovery of fiber transmittance as well as the decay of laser-induced fluorescence. A widely believed plausible explanation for UV-induced increase of attenuation involves formation of different macro radicals which recombine progressively after UV is stopped. Laser-induced fluorescence over several days is reported here providing direct evidence for molecular-level deterioration and recovery of PMMA.