Yikai Su

1. System applications of silicon photonic ring resonators Yikai Su State Key Lab of Advanced Optical Communication Systems and Networks , Department of Electronic Engineering, Shanghai Jiao Tong University, China yikaisu@sjtu.edu.cn

2. Motivation Optical processing may be desired in some high-speed applications

3. Filter A/D DSP chip D/A Filter memory I/O Parameters of digital differentiator Realization of digital differentiator using DSP ADC:MAX109 Speed:2.2Gs/s Power dissipation:6.8W Size:734.4mm2 DAC:MAX5881 Speed:4.3Gs/s Power dissipation:1160mW Size:11mmx11mm TMS320C6455 DSP DSP:TMS320C6455 Speed: 1.2GHz clock rate;9600MIPS(16bit) Size: 0.09-um/7-level Cu Metal Process (CMOS) BGA package: 24*24 mm2 Power dissipation:1.76 W

4. Optical processing using ring resonator 250-nm thickness 450-nm width Buffer layer: 3-µm silica Mode area: ~ 0.1µm2 Air gap : ~100 nm SEM photos of a silicon microring resonator • Signal processing functions: • Slow light (JSTQE 08) • Fast light (OE 09) • Wavelength conversion (APL 08) • Format conversion (OL 09) • Optical differentiation (OE 08)

5. Tunable delay in silicon ring resonators Optically tunable buffer for different modulation formats at 5-Gb/s rate Optically tunable phase shifter for 40-GHz microwave photonic signal Signal Conversions Dense wavelength conversion and multicasting in a resonance-split silicon microring Format conversions (NRZ to FSK, NRZ to AMI) Optical temporal differentiator Concentric rings for bio-sensing Conclusions Outline

6. Recent experiments on slow-light delay in silicon nano-waveguides • Continuous tuning was not demonstrated • Data format was limited to non return-to-zero (NRZ)

7. Tuning signal delay in resonator-based slow-light structure • Tunable group delay is important for implementing a practical buffer • Single microring-resonator is a basic building block of the resonator-based slow-light structure Tuning methods: • Electro-optic effect by forming a p-i-n structure • Thermo-optic effect by implanting a micro-heater • MEMS actuated structure

8. More coupling Resonance Input DI Partial coupling Ring resonator • Incoming light is partially coupled into the ring • The signal in the ring interferes with the input light after one round-trip time • Only the signal of resonance can be coupled into the ring

9. Slow light Group delay Also see the animation

10. Tunable slow-light in silicon ring resonator Slow-light principle: Δθ/Δω = group delay => Slow light

11. When a pump light is injected into the microring resonator, the absorbed energy is eventually converted to the thermal energy and leads to a temperature shift Temperature tuning τ- thermal dissipation time ρ-density of the silicon C-thermal capacity V-volume of the microring Kθ-thermo-optic coefficient The refractive index changes with the temperature No need of additional procedure in the fabrication, very low threshold in tuning

12. Silicon microring used in the experiment ~8-dB notch depth ~0.1-nm 3-dB bandwidth SEM photos of the silicon microring resonator with a radius of 20 μm 250-nm thickness 450-nm width Buffer layer: 3-µm silica Mode area: ~ 0.1µm2 Air gap : 120 nm

13. Vertical coupling • Gold grating coupler to couple light between the single mode fiber (SMF) and the silicon waveguide • The gold grating coupler is designed to support TE mode only Measured fiber-to-fiber coupling loss: ~20dB The technique was invented by Ghent SEM photo of the gold grating coupler

14. Experimental setup A dual-drive MZM is used when generating RZ-DB and RZ-AMI Fangfei Liu et al., IEEE JSTQE May/June 2008

15. Continuous Tuning of 5-Gb/s Non-return-to-zero (NRZ) signal (b) Delay versus the pump power Delayed waveforms Maximum delay of ~100 ps

16. Return-to-zero (RZ) signal 5G RZ eye diagram Delay versus the pump power 5Gb/s Maximum delay of 80 ps for 5-Gb/s RZ signal Qiang Li et al., IEEE/OSA J. Lightw. Technol., Vol 26, No. 23, 2008

17. 5-Gb/s carrier-suppressed RZ (CSRZ) signal Maximum delay of 95 ps CSRZ is used in long haul 0  0 Eye diagrams and waveforms for the 5-Gb/s CSRZ signal

18. RZ-DB 5-Gb/s RZ-Duobianry (DB) and RZ-Alternating-Mark-Inversion (AMI) signals Maximum delay of 110 ps RZ-DB is good for dispersion uncompensated system in metro RZ-AMI Maximum delay of 65 ps RZ-AMI is tolerant to nonlinear impairments

19. Delay comparisons Optical spectra the narrower, the larger delay Qiang Li et al., OSA Slow and Fast Light Topic Meeting, 2008

20. Larger delay with cascaded rings Resonator-based slow-light structures : • Single channel side-coupled integrated spaced sequences of resonators (SCISSOR) • Double channel SCISSOR • Coupled resonator optical waveguides (CROW)

21. Operation principle Optically tunable microwave photonic phase shifter The two tones of the microwave optical signal experience different phase shifts, resulting in group delay change

22. Experimental setup 20-GHz microwave photonic signal Silicon microring Temperature tuning Q. Li et al., ECOC 2008, paper P2.12

23. Maximum phase shift: -4.6 rad 40GHz result – phase shift Qingjiang Chang et al., IEEE Photon. Technol. Lett，vol. 21, no. 1, Jan. 2009

24. Continuous tuning based on thermal nonlinear effect by changing the control light power Phase shift vs. pump power

25. Signal conversions in mode-split ring Side wall roughness in E-beam results in two resonance modes: ω0 - the resonance frequency QE - coupling quality factor QL – intrinsic quality factor Qu –coupling quality factor The transmission function of the ring resonator is given by: Mode a is split into two resonance frequencies, ω0-ω0/(2Qu) andω0+ω0/(2Qu). The resonance-splittingis determined by the mutual coupling factorQu.

26. Observation of mode splitting Resonance-splitting Motivation: shift the resonance to convert signals by using free carrier dispersion (FCD) effect Ziyang Zhang et al., CLEO/QELS 2008 Tao Wang et al., JLT 2009

27. Experimental results – dense wavelength conversion of 0.4nm nm signal  pump • Signal light is originally set at the resonance -> ‘0’ • Resonance is shifted when pump is ‘1’ • Signal light off resonance -> ‘1’ -> wavelength conversion • Inverted case can be realized Qiang Li et al., App. Phy. Lett., 2008

28. Wavelength multicasting FSR p s1 s2 • Conversions of 2 wavelengths -> wavelength multicasting • By setting the signal wavelengths properly, non-inverted and inverted multicasting can be implemented Qiang Li et al., App. Phy. Lett., 2008

29. Format conversion- NRZ to FSK FSK Spectrum 5dB/div 0.5nm/div p s1 s2 FSK Eye diagram Input NRZ signal 500μW/div 2.5ns/div 500μW/div 500ps/div demodulated signal: upper sideband Fangfei Liu et al., APOC 2008 demodulated signal: upper sideband

30. Optical temporal differentiator In the critical coupling region (QL = QE), the transfer function of the microring resonator is: : A typical function for a first-order temporal differentiator

31. Experimental results 10G 5G Input Output Input Output Gaussian Sine Square Fangfei Liu, et al., Opt. Express 2008

32. Format conversion- NRZ to AMI A microring is a high pass filter NRZ + high pass filtering => AMI 10G NRZ 10G AMI Qiang Li et al., Chin. Opt. Lett., Vol 7, No. 2, 2009

33. How to build an ultra-high-speed all-optical differentiator?

34. 80-G optical differentiator using a ring resonator with 2.5-nm bandwidth Radius: 20 μm Bandwidth : 2.5 nm Resonance wavelength: 1551.73nm

35. Measurement setup

36. 80-Gb/s differentiation result G. Zhou et al., Electron. Lett. 2011

37. Future work: 160-G differentiation • Design of new ring resonator: critical coupling, large 3-dB bandwidth • One possible design: • Large bandwidth: small diameter and high loss • Critical coupling: long coupling length B3dB=5nm

38. Comparison of optical and electronic differentiators All-optical differentiator:(1) ultra-high speed (2) compact structure DSP based: configurable; can fulfill more than one function

39. Differential equation solver • Differential equations are widely employed in virtually any field of science and technology: • Physics • Biology • Chemistry • Economics • Engineering • All constant-coefficient linear differential equations can be modeled with finite number of: • Differentiators • Couplers/Subtractors • Splitters • Feedback branches

40. - optical input signal x + optical differentiator optical output signal y output port input port Optical differential equation solver

41. Silicon microring for bio-sensing DNA hybridization DNA probe After hybridization: The effective index changes around the waveguide results in resonance shift DNA probe is attached to the ring • Problems with the single ring: • limited sensing area • not easy to control the notch depth (air gap between the ring and the straight waveguide)

42. Proposal: concentric rings Single ring concentric ring Two samples Field distribution The field is evenly distributed among the two concentric rings, thus increasing the sensing area

43. Enhanced notch depth Blue: single ring Red: double rings Enhanced notch depth, easier detection of resonance shift More rings? Xiaohui Li, et al., Applied Optics 2009

44. Silicon ring resonators with nano-scale SOI waveguides can perform many functions: Tunable delay Digital: different modulation formats at 5 Gb/s Analog: 40-GHz microwave photonic signal Signal conversions Dense wavelength conversion and multicasting Format conversions Optical temporal differentiator Concentric rings for sensitive bio-sensing Conclusions