High speed silicon Mach-Zehnder modulator Bandwidth of 10 GHz & Data transmission from 6 Gbps to 10 Gbps Hyun-Yong Jung High-Speed Circuits and Systems Laboratory

Outline Introduction Device design Phase-shifter performance High speed data transmission Conclusion

Introduction MOS Capacitor Small signal BW : 2.5 GHz Transmitting data : 1 GHz - limited by driver Transmitting data : 4 GHz by customizing drive circuitry Improvements in material quality, device design & driver circuitry 10 Gbps data transmission, 3.8 dB ER & ~10 dB of on-chip loss

Device design 0.55 um p type-doped 1 um n type-doped Epitaxial lateral overgrowth(ELO) is used to grow the crystalline Si - ELO reduces the density of dislocations Poly-Si  ELO Si(crystalline-Si) - Poly-Si is more lossy due to defects To target high BW performance, the doping concentrations of Si are higher than those of previous poly-Si To minimize the metal contact loss, design 2~3 um wide poly-Si pieces overlap the top corners of the ELO-Si rib

Device design All wave guide dimensions are smaller than the first version  The optical mode is more tightly confined

Device design < Schematic of MZI, wire-bonds, and driver IC > Overall length of the MZI modulator : 15 mm - Each arm : 3.45 mm long high-doping - High speed RF MOS capacitor phase shifter : 2~4.75 mm long lightly doped - Low-speed phase shifters are driven with DC voltages to electrically Driver  Using 70 GHz-FT SiGe HBT process & employs a push-pull emitter-coupled logic output stage  Improved driver design & phase-shifter efficiency lead to reduced power dissipation

Phase-shifter performance N-type Si  grounded P-type ELO-Si  VD applied A thin charge layer on both side of the gate dielectric index(n) & absorption(α) of Si are changed neff changes Optical phase shift depends on  neff changes, device length, and the optical wave length Figure of merit  (VπLπ) – voltage swing & device length for π-radian phase shift (0.15 π radian phase shift in this each MZI arm) 7.8 V-cm  3.3 V-cm (minimizing  shorten device length)

Phase-shifter performance The intrinsic bandwidth  (2πRC)-1 (2πRC)-1 = 10.2 GHz

High speed data transmission Optical eye diagram of modulator at λ = 1.55 um Total insertion loss of 19 dB (10 dB - on chip, 9 dB - coupling) Both eye diagrams have the same vertical and horizontal scales Still slower than modulators based on LiNbO3 or III-V  40 GHz This modulation can be more optimized - Phase efficiency (MZI arm < 0.2 cm, on-chip loss < 2 dB) - Higher BW can be obtained by increasing doping concentration W/ higher loss

Conclusion Efforts Materials improvement Optimization of dopant distribution MZI splitter design to reduce on-chip loss Incorporation of optical tapers to reduce coupling loss Reduction of waveguide dimensions to scale phase modulation efficiency Improvement of drive circuitry to realize higher data transmission Results Intrinsic bandwidth (as measured by RC cutoff) of 10 GHz Driver-limited data transmission at 10 Gbps with 3.8 dB ER