All-Optical Data Inverter Based on Free-Carrier Absorption Induced Cross-Gain Modulation in Si Quantum Dot Doped SiO$_{\bm x}$ Waveguide

The Si quantum dot (Si-QD) doped SiO x waveguide based all-optical cross-gain data inverter is demonstrated. The data inversion response of the probe at 1550 nm depends on the dispersed carrier lifetime in Si-QD, which is strongly correlated with the intensity of pulsed pump at wavelength of 405 nm. The free-carrier absorption cross-section of Si-QD is determined as 3.1 × 10-17cm 2 by fitting with four-level rate equations. Decreasing the pump power could maintain the band-filling effect in small Si-QDs and suppress the filling and scattering of excited free carriers to lower energy states in larger Si-QDs. This causes the carrier lifetime of Si-QD as well as the data inversion speed shortened from 35 to 19 μs by reducing pump power from 240 to 66 mW. Shrinking the Si-QD size and narrowing its size distribution is beneficial to the carrier lifetime shortening and dispersion by overlapping the electron-hole wave functions under strong quantum confinement. The modulation bandwidth can be enhanced by releasing the heating effect in Si-QDs, which further shortens the free carrier absorption induced data inversion response to 9 μs by shortening the pump duty-cycle from 10 to 0.5 μs. The all-optical SiOx:Si-QD waveguide cross-gain data inverter with a pulsed on-off keying data rate of 200 kbit/s is reported.

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