Sheng-Pin Su

Si-rich SiNx based Kerr switch enables optical data conversion up to 12 Gbit/s

Silicon photonic interconnection on chip is the emerging issue for next-generation integrated circuits. With the Si-rich SiNx micro-ring based optical Kerr switch, we demonstrate for the first time the wavelength and format conversion of optical on-off-keying data with a bit-rate of 12 Gbit/s. The field-resonant nonlinear Kerr effect enhances the transient refractive index change when coupling the optical data-stream into the micro-ring through the bus waveguide. This effectively red-shifts the notched dip wavelength to cause the format preserved or inversed conversion of data carried by the on-resonant or off-resonant probe, respectively. The Si quantum dots doped Si-rich SiNx strengthens its nonlinear Kerr coefficient by two-orders of magnitude higher than that of bulk Si or Si3N4. The wavelength-converted and cross-amplitude-modulated probe data-stream at up to 12-Gbit/s through the Si-rich SiNx micro-ring with penalty of −7 dB on transmission has shown very promising applicability to all-optical communication networks.

All-Optical Data Inverter Based on Free-Carrier Absorption Induced Cross-Gain Modulation in Si Quantum Dot Doped SiO

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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To meet the demand of all-optical data processing applications, the SiNx layer with enriching Si quantum dots (Si-QDs) for achieving strong optical nonlinearity is demonstrated with its χ(3) coefficient enhanced by three orders of magnitude larger than that of stoichiometric Si3N4. With excessive Si concentration enriched from 16.3% to 23.4%, the dense Si-QDs apparently self-assemble in the SiNx matrix with an average size of ∼0.95 nm and volume density of 5 × 1019 # cm−3. The Si-QD doped Si-rich SiNx not only enlarges its third-order nonlinear absorption coefficient from 0.01 to 1.8 m GW−1, but also increases its nonlinear refractive index from 5.7 × 10−13 to 9.2 × 10−12 cm2 W−1 at a wavelength of 800 nm, as attributed to the localized excitons with decreased effective Bohr radius in quantum confined Si-QDs. Such a SiNx:Si-QD material enables strong optical nonlinearity in compact nonlinear nanophotonic waveguide devices developed for future all-optical data processors.

Waveguide

The degenerate four-wave-mixing (DFWM) effect in Si quantum dot doped SiNx (SiN_x :Si-QD) channel waveguide has been preliminarily demonstrated. By enhancing the optical nonlinearity of SiN_x at 1550 nm with doped Si-QDs, the DFWM can be enhanced with its conversion efficiency of -46 dB in the SiN_x :Si-QD channel waveguide with length of 8 mm. The nonlinear refractive index of the SiN_x:Si-QD is estimated to be 3 × 10-14cm2/W. The chromatic dispersion analysis indicates that the SiN_x :Si-QD channel waveguide exhibits 3-dB conversion bandwidth of larger than 16 nm. Such a SiN_x :Si-QD waveguide is free of multi-photon absorption at 1550 nm even when injecting an intense laser pulse as large as 1.3 GW/cm². These results show great potential of Si-QD doped SiN_x waveguide for nonlinear optical processing.

Enriching Si quantum dots in a Si-rich SiNx matrix for strong χ(3) optical nonlinearity

The Si quantum dot-doped SiOx rib waveguide-based free-carrier absorption (FCA) modulator with enhanced all-optical modulation depth is demonstrated by integrating with a micro-ring waveguide resonator. The micro-ring waveguide resonator with a Q-factor of 6×103 induces a throughput transfer function in wavelength domain, such a transmittance notch can be blue-shifted by varying the excited free-carrier density of Si-QD. When injecting the continuous-wave probe at central wavelength of the transmittance notch, the probe signal can be inversely modulated by optically pumping the micro-ring waveguide resonator to blue-shift the notch away from its original wavelength. By optimizing the pump wavelengths, the largest FCA loss and highest free-carrier density can be enhanced to 2.9 cm-1 and 7.83 × 1016 cm-3, respectively. With the excited free-carrier density of ~1.3 × 1016 cm-3 inside the micro-ring waveguide, the maximal wavelength of the transmittance notch can be blue-shifted by 0.033 nm. The optical pumping also induces the broadened linewidth of transmittance notch from 0.25 to 0.27 nm. With the integrated micro-ring waveguide resonator, the all-optical modulation depth can be further enhanced from 52.5% to 63.5% by shifting the notched transmission spectrum of the micro-ring waveguide with the excited free-carrier density of Si-QD at probe wavelength of 1563.5 nm.

Degenerate Four-Wave Mixing in Si Quantum Dot Doped Si-Rich SiNx Channel Waveguide

The free-carrier cross-section of Si-QD is decreased from 5.5×10^-17 to 9×10^-18 cm² with shortened lifetime from 10 to 0.48 μs when shrinking Si-QD size from 4.3 to 1.7 nm due to the quantum confinement effect.