Two-photon absorption allows fast modulation in silicon-core fiber

Scientists from the Optoelectronics Research Centre, University of Southampton (Southampton, England) and Penn State University (University Park, PA) have created an optical fiber with a hydrogenated amorphous silicon (a-Si:H) core, allowing all-optical modulation based on two-photon absorption (TPA). Modulation with an extinction ratio of more than 4 dB at sub-picosecond speeds shows the fiber’s potential for use in high-speed signal processing.

The fiber was fabricated using high-pressure microfluidic chemical deposition. The researchers note that the a-Si:H in the fiber is different in hydrogen content from that used in on-chip a-Si:H waveguides, and thus has its own nonlinear properties. In cross-absorption modulation (XAM) based on TPA, a strong pump pulse co-propagates with a weak probe pulse down the fiber. Both pulses are absorbed, resulting in modulation that is the inverse of the weak probe pulse. Pulses at 1540 nm with a 650 fs duration and a 40 MHz repetition rate were spilt into pump (250 W peak power) and probe (10 mW peak power) pulses. The probe was also modulated with a chopper to distinguish it from the pump. The absorption within the probe pulse occurred within 800 fs with a 4.5 dB extinction ratio, but a slower recovery. In addition, a weak (0.5 mW) continuous-wave signal was modulated with 1570 nm light from a laser diode, showing a response time on the order of the 650 fs pump width.

Contact Anna Peacock at [email protected].