1. 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

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

3. 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

4. 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

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

6. 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

7. 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)

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

9. 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

10. 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