System Applications of Silicon Photonic Ring Resonators

1. 1 Shanghai Jiao Tong University 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 System applications of silicon photonic ring resonators

2. 2 Shanghai Jiao Tong University Motivation Electronic processing Optical processing in silicon photonics Complexity (# of units) High Low Line width 10s nm >100 nm Power mW - W mW - W Speed Gb/s Gb/s-Tb/s Optical processing may be desired in some high-speed applications

3. 3 Shanghai Jiao Tong University Parameters of digital differentiator Filter A/D DSP chip D/A Filter memory I/O Realization of digital differentiator using DSP TMS320C6455 DSP ADC:MAX109 Speed:2.2 G s/s Power dissipation:6.8 W Size:734.4 mm 2 DAC:MAX5881 Speed:4.3 G s/s Power dissipation:1160 mW Size:11 mmx11 mm DSP:TMS320C6455 Speed: 1.2 GHz clock rate; 9600MIPS (16bit) Size: 0.09-um/7-level Cu Metal Process (CMOS) BGA package: 24*24 mm 2 Power dissipation:1.76 W

4. 4 Shanghai Jiao Tong University Optical processing using ring resonator SEM photos of a silicon microring resonator 250-nm thickness 450-nm width Buffer layer: 3-m silica Mode area: ~ 0.1 m 2 Air gap : ~100 nm Signal processing functions: Slow light ( JSTQE 08) Fast light ( OE 09) Wavelength conversion ( APL 08) Format conversion ( OL 09) Optical differentiation ( OE 08)

5. 5 Shanghai Jiao Tong University Outline 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

6. 6 Shanghai Jiao Tong University Recent experiments on slow-light delay in silicon nano-waveguides Schemes Footprint (mm 2 ) 3dB Band width Duration/Delay Max storage capacity (bits) Publication SRS ~100GHz 3ps/ 4ps 1.3 Opt. Express 14(2006) cascaded microring resonator (APF / CROW) 0.09 0.045 54GHz -- 50ps/510ps 200ps/220ps 10 at 20bps 1 at 5bps Nature Photonics 1(2007) photonic crystal (PC) ~260MHz 1.9ns/1.45ns <1 Nature Photonics 1(2007) photonic crystal coupled waveguides (PCCW) 12nm 0.8ps/40ps LEOS 2007 Continuous tuning was not demonstrated Data format was limited to non return-to-zero (NRZ)

7. 7 Shanghai Jiao Tong University 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. 8 Shanghai Jiao Tong University Partial coupling Partial coupling Input Input DI DI More coupling More coupling Resonance Resonance 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 Ring resonator

9. 9 Shanghai Jiao Tong University Slow light Group delay Also see the animation

10. 10 Shanghai Jiao Tong University Tunable slow-light in silicon ring resonator Slow-light principle: / = group delay => Slow light

11. 11 Shanghai Jiao Tong University 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 The refractive index changes with the temperature - thermal dissipation time - density of the silicon C-thermal capacity V-volume of the microring K -thermo-optic coefficient Temperature tuning No need of additional procedure in the fabrication, very low threshold in tuning

12. 12 Shanghai Jiao Tong University Silicon microring used in the experiment SEM photos of the silicon microring resonator with a radius of 20 m 250-nm thickness 450-nm width Buffer layer: 3-m silica Mode area: ~ 0.1 m 2 Air gap : 120 nm ~8-dB notch depth ~0.1-nm 3-dB bandwidth

13. 13 Shanghai Jiao Tong University 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. 14 Shanghai Jiao Tong University 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. 15 Shanghai Jiao Tong University Continuous Tuning of 5-Gb/s Non- return-to-zero (NRZ) signal Delay versus the pump power Delayed waveforms (b) Maximum delay of ~100 ps

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

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

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

19. 19 Shanghai Jiao Tong University Delay comparisons Formats NRZ RZ CSRZ RZ-DB RZ-AMI Delays (ps) 100 80 95 110 65 Optical spectra the narrower, the larger delay Qiang Li et al., OSA Slow and Fast Light Topic Meeting , 2008

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

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

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

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

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

25. 25 Shanghai Jiao Tong University Signal conversions in mode-split ring The transmission function of the ring resonator is given by: Mode a is split into two resonance frequencies, 0 - 0 /(2 Q u ) and 0 + 0 /(2 Q u ). The resonance-splitting is determined by the mutual coupling factor Q u . 0 - the resonance frequency Q E - coupling quality factor Q L intrinsic quality factor Q u coupling quality factor Side wall roughness in E-beam results in two resonance modes :

26. 26 Shanghai Jiao Tong University 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. 27 Shanghai Jiao Tong University Experimental results dense wavelength conversion of 0.4nm nm 1. Signal light is originally set at the resonance -> 0 2. Resonance is shifted when pump is 1 3. Signal light off resonance -> 1 -> wavelength conversion 4. Inverted case can be realized pump signal Qiang Li et al., App. Phy. Lett ., 2008

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

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

30. 30 Shanghai Jiao Tong University Optical temporal differentiator : In the critical coupling region ( Q L = Q E ), the transfer function of the microring resonator is: A typical function for a first- order temporal differentiator

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

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

33. 33 Shanghai Jiao Tong University How to build an ultra-high-speed all- optical differentiator?

34. 34 Shanghai Jiao Tong University 80-G optical differentiator using a ring resonator with 2.5-nm bandwidth Radius: 20 m Bandwidth : 2.5 nm Resonance wavelength: 1551.73nm

35. 35 Shanghai Jiao Tong University Measurement setup

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

37. 37 Shanghai Jiao Tong University 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 B 3dB =5nm

38. 38 Shanghai Jiao Tong University Comparison of optical and electronic differentiators Species Speed Size Power dissipation Silicon ring 80 Gbps or higher 20 m (radius) < 1 mW Digital differentiator a few GHz mm 2 a few W All-optical differentiator : (1) ultra-high speed (2) compact structure DSP based : configurable; can fulfill more than one function

39. 39 Shanghai Jiao Tong University 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. 40 Shanghai Jiao Tong University Optical differential equation solver output port input port optical differentiator + - optical input signal x optical output signal y

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

42. 42 Shanghai Jiao Tong University 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. 43 Shanghai Jiao Tong University 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. 44 Shanghai Jiao Tong University Conclusions 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