Light amplification and lasing in a stilbenoid compound-doped glass-clad polymer optical fiber

We report on a large optical gain, over the wide spectral range, and lasing in a glass-clad polymer optical fiber that uses a novel highly fluorescent stilbenoid compound. The compound 1,4-bis(4-diphenylamino-styryl)-benzene is designed for the blue region of the spectrum and has a high quantum yield of 0.85 in polystyrene and a relatively large Stokes shift of ∼50 nm. A fiber doped with 0.2-wt.% of the compound is photoexcited with a thin striped-shape area at 355 nm with nanosecond optical pulses, and the emission from one end is monitored as a function of the excitation length to deduce the net gain coefficient. The gain spectroscopy has revealed a broad optical gain exceeding 25 cm^-1 and up to 36 cm^-1 at 494 nm that covers a wide spectral range of about 70 nm when the fiber is transversely photoexcited at 12 mJ/cm² at room temperature. An analysis shows that the saturation effect expected for homogeneously broadened gain accounts for the amplified spontaneous emission output behavior at longer excitation lengths. Waveguide loss is measured to be 0.7 cm^-1 at 494 nm. The large gain and low loss have been utilized to demonstrate blue laser emission at 489 nm from the fiber (which is only 1.4 cm in length) in a low finesse cavity defined by the Fresnel reflections at the fiber-air interfaces. The threshold for lasing is found to be 1.7 mJ/cm².