Iron-oxide-nanoparticle-enabled broadband all-optical microfiber phase shifter.

A broadband, all-optical phase shifter based on an iron-oxide-nanoparticle (IONP)-coated microfiber Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The device exhibits broadband phase shift capability from the visible to the near-infrared region through absorption-induced photothermal control. The microfiber diameter is ~2.8 μm, and the interferometer has an arm-length difference of ~1.27 mm, corresponding to a free spectral range of 1.28 nm, while the IONP-coated MZI introduces an insertion loss of 4.2 dB. The measured phase shift efficiencies are 0.035 π/mW, 0.03 π/mW, and 0.025 π/mW at the control wavelengths of 690, 980, and 1557 nm, respectively. A maximum phase shift of 2.16π is achieved at 980 nm with a control power of 73.68 mW. The rise and fall times of the photothermal response are measured to be 649 and 520 μs, respectively. Owing to its compact footprint, broadband operation, and efficient phase tunability, the device provides a promising platform for all-optical phase shifting, tunable filtering, sensing, and other reconfigurable fiber-optic devices.