High speed silicon

Mach-Zehnder modulator

High speed silicon

Mach-Zehnder modulator

Hyun-Yong Jung

High-Speed Circuits and Systems Labo-ratory

Bandwidth of 10 GHz & Data transmission from 6 Gbps to 10

Gbps

Outline

Introduction

Device design

Phase-shifter performance

High speed data transmission

Conclusion

Introduction

• Improvements in material quality, device design & driver circuitry• 10 Gbps data transmission, 3.8 dB ER & ~10 dB of on-chip loss

• Transmitting data : 4 GHz by customizing drive circuitry

• Small signal BW : 2.5 GHz • Transmitting data : 1 GHz - limited by driver

MOS Capacitor

Device design

1 um n type-doped

0.55 um p 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 groundedP-type ELO-Si VD applied

A thin charge layer onboth 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 transmis-

sion

Results

•Intrinsic bandwidth (as measured by RC cutoff) of 10 GHz•Driver-limited data transmission at 10 Gbps with 3.8 dB ER