All-optical modulator based on tapered optical fiber configuration deposited with silver nanoparticles

Abstract This study experimentally demonstrates a compact all-optical modulator based on a tapered single-mode fiber coated with silver nanoparticles (Ag NPs) dispersed in a polyvinyl alcohol (PVA) solution. The tapered fiber was fabricated from standard SMF-28 by using a fusion splicing technique, and the waist diameter was precisely reduced to 70.824 µm to ensure strong evanescent field interaction. The Ag NPs had a particle size of 20 nm and were doped into a PVA solution with a concentration of 5 mg. This solution satisfies the two critical conditions (|W| ≫ 1 and |T| ≪ 1), which enable the Ag NPs to act effectively as all-optical devices. The nanoparticles were subsequently deposited onto the waist region of the tapered fiber that enhances the local light–matter interaction. Under optical pumping with a 980 nm laser diode, the Ag NPs induce a strong nonlinear refractive index change through the Kerr effect, leading to intensity-dependent modulation of a 1,550 nm probe signal. A clear inverted square waveform was observed in response to square-wave pump modulation, with stable operation achieved in the frequency range from 1 Hz to 1 kHz. These findings confirm that Ag NPs–coated tapered fibers provide a simple, low-cost, and efficient platform for all-optical modulation, with strong potential for ultrafast photonic and telecommunication applications.