AbstractAttenuation loss of plastic optical fibers (POFs) gradually increases with long use at high temperatures. We separated attenuation loss of the POFs before and after heating at 150°C in air into four loss factors, and identified the main factor for attenuation loss increase as electronic transition absorption loss (αe). The increase of αewas caused by a thermal oxidation reaction of the core polymer in the POF. But, according to NMR and FT‐IR spectra, elementary analysis, etc., hardly any reaction products were detected. We assume that the reaction products were only a small amount of the conjugated carbonyl groups. So we fabricated POFs containing model compounds with conjugated carbonyl groups, and measured their attenuation loss. The attenuation loss spectra of these POFs were similar to those of thermally degraded POFs. Consequently, a very small amount of conjugated carbonyl groups were formed by the thermal oxidation reaction of core polymer so that αeincreased to an unacceptable
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