Nonlinear Optics in Silicon Photonics

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2 Acknowledgement TU Berlin J. Bruns, G. Dziallas, A.Gajda, M. Jazayerifar, C. Stamatiadis, K. Voigt, B. Wohlfeil IHP R. Barth, J. Drews, M. Fraschke, O. Fursenko, T. Grabolla, B. Heinemann, D. Knoll, M. Kroh, A. Krüger, M. Lisker, S. Lischke, S. Marschmeyer, D. Micusik, P. Ostrovskyy, D. Petousi, H.H. Richter, K. Schulz, H.Tian, B.Tillack, A. Trusch, G.Winzer, Y. Yamamoto, L.Zimmermann HHI R. Elschner, T.Richter, C.Schubert DTU F. Da Ros, D. Vukovic, C. K. Dalgaard, M. Galili, C. Peucheret (now University Rennes) University of Virginia A.Beling, Q. Zhou TU Wien B. Goll, H. Zimmermann University of Southampton D. J. Thomson, F. Y. Gardes, Y. Hu, G. T. Reed PHOTLINE H. Porte 1

3 Photonic BiCMOS Introduction to Photonic BiCMOS Nonlinear Optical Signal Processing using Photonic BiCMOS Devices 2

4 BiCMOS Integrated Circuit Photonic Integrated Circuit IC PIC EPIC Electronic Photonic Integrated Circuit 3

5 Joint Lab Silicon Photonics Head: Dr. Lars Zimmermann IC technology Photonics Silicon Photonics Prof. Bernd Tillack Prof. 4

6 IHP location Frankfurt (Oder) 5

7 Pilot line 0.25µm/0.13µm SiGe BiCMOS SG25H1 npn HBTs up to f t /f max =180/220GHz SG25H3 npn HBTs up to f t /f max =110/180GHz V breakdown up to 7V SG13S npn HBTs up to f t /f max =250/300GHz 3.3V I/O CMOS 1.2V logic CMOS SG13G2 f t /f max =300/500GHz 6

8 Si Photonics Development Lines Passives PIC Photonic Integrated Circuits EPIC Electronic Photonic Integrated Circuits waveguide optics + modulators Ge-photodetector + full BiCMOS 7

9 IHP Si-Photonics technology Shallow etched Linear loss 0-10 signal [dbm] x10-6 wavelength [µm] 8

10 Coupling via Grating 10 single-mode fibre, adiabatic taper TE Grating coupler: No need for a polished facet Wafer scale testing Wafer-level packaging Compatible with SMF Flexible and cheap grating 10µm wide waveguide D. Taillaert et. al., IEEE J. Quantum Electron., vol. QE38 (2002), pp

11 Present optical i/o Standard linear 1550nm LD SM fiber Grating coupler PIC/EPIC TX & Rx Linear enhanced ~10µm spot 0 Coupling Lossl [db] Enhanced 1d-Grating Wavelength [µm] 10

12 Germanium photodetectors Ge waveguide photodiode Cross section S. Lischke IEEE Phot. Conf., San Francisco,

13 WG-coupled Ge lateral PIN PD Selectively grown Ge Ridge waveguide S 21 frequency response Nice dark current behavior 30(+) GHz (-3dB) bandwidth 0.6(+) A/W internal λ= 1.55µm 12

14 Photonic BiCMOS substrate issue Photonic components need SOI for low loss!! but optimum for photonics SOI dimensions are by far not optimal or even unfit for SOA-CMOS and, fortiori, SOA-BiCMOS!! Si on isolator: 220nm Buried SiO 2 layer Si substrate CMOS: both Si (220nm) and SiO 2 layer (2µm) much too thick 2µm HBT: Si layer much too thin for a low-r C collector fabrication; bad head dissipation due to higher R TH of SiO 2, compared to Si 13

15 Photonic BiCMOS substrate issue Photonic components need SOI for low loss!! but optimum for photonics SOI dimensions are by far not optimal or even unfit for SOA-CMOS and, fortiori, SOA-BiCMOS!! CMOS: both Si (220nm) and SiO 2 layer (2µm) much too thick HBT: Si layer much too thin for a low-r C collector fabrication; bad head dissipation due to higher R TH of SiO 2, compared to Si Solution: Local-SOI enables integration of best of Si-photonics into best Bi(CMOS)!! Si: 220nm Locally buried SiO 2 layer Si substrate Si on isolator: 220nm Buried SiO 2 layer Si substrate Reconstructed Si substrate 2µm 14

16 Local-SOI fabrication Locally removing SOI structure by RIE / wet etch sequence Selective Si epitaxy Planarization by Si-CMP Optimizing to keep usual device yield numbers of parent BiCMOS process!! epitaxy D. Knoll et al., ECS Transactions, 50, 9, pp (2012) 15

17 Receiver EPIC Photonic BiCMOS (SG25H1): Receiver (Ge-PD + SiGe-TIA) TIA D.Knoll et al, OFC

18 Receiver EPIC Photonic BiCMOS (SG25H1): Receiver (Ge-PD + SiGe-TIA) TIA D.Knoll et al, OFC

19 Receiver measurement results (I) Normalized receiver frequency response On wafer characterization Setup limitations Incoupling angle Incoupling power Reduced coupling efficiency GHz bandwidth fits to 20(+) Gbps functionality D.Knoll et al, OFC

20 Receiver measurement results (II) 10Gbps 120mV Eye diagrams for received data PRBS word length Nicely opened eye still at 25Gbps 20Gbps 25Gbps Sampling oscilloscope with 70 GHz sampling head Scale: vertical 20mV/division, horizontal 20ps/div D.Knoll et al, OFC

21 Coherent Receiver Coherent receiver with integrated TIAs, see G.Winzer et. al. Paper M3C.4, OFC 2015, 23rd March

22 Modulator with 10Gbit/sec driver High speed phase modulator Tuning section Input 2x1 MMI 2x2 MMI Outputs High speed phase modulator Low speed phase modulators Phase shift + tuning L. Zimmermann et al, ECOC

23 Photonic BiCMOS: Driver + Mach-Zehnder modulator L. Zimmermann et al, ECOC Ω termination DC tuning 10Gbit/sec MZI modulator Driver Operating point tuning Data input Power supply 22

24 Photonic BiCMOS Nonlinear Optical Signal Processing using Photonic BiCMOS Devices 23

25 Waveguides for Kerr-related nonlinear signal processing Material Nonlinear coefficient γ [W -1 m -1 ] HNLF 0.02 Si 3 N D.J. Moss et al.; Nature Photon.,vol.7, pp , August 2013 Chalcogenide glass 10 R. Neo et al.; Optics Express, vol. 21.,pp , April 2013 SOH (Silicon-organic-hybrid) 100 C. Koos et al.; Nature Photon. Vol. 3, pp , April 2009 c-si-nanowire (crystalline) 300 a-si-nanowire (amorphous) 1200 C. Grillet et. al.; Optics Express, vol. 20,pp

26 c-si for Kerr-related nonlinear optical signal processing positive High nonlinearity γ ~ 300 W -1 m -1 Low loss down to α ~ 0.3 db/cm 100m HNLF 1cm Si nanowire negative Two photon absorption β TPA ~ cmgw -1 Free carrier absorption due to TPA 25

27 Nonlinear loss mechanisms in silicon Operation at telecom wavelengths ~ 1.55 µm Two-photon absorption (TPA), λ< 2.12 µm Free carrier absorption (FCA) Free carrier induced index change (FCI) 26

28 Limitations due to Two-Photon-Absorption For telecom 1.55 µm Absorption α TPA = β TPA P/A eff due to β TPA negligible up to ~ 100 mw pump power Removal of free carriers essential (otherwise only pulsed operation) Maximum nonlinear phase 1.55 µm φ NL = γ P L eff < γ P / α TPA = φ NL,max φ NL,max = A eff γ /β TPA =2π FOM ~ 3 rad allowing for maximum parametric gain ~ 5 db 27

29 Several optical nonlinear effects in SOI waveguides were observed: Four Wave Mixing (FWM) 1,2,3,4,5 Self Phase Modulation (SPM) 6 Cross Phase Modulation (XPM) 6 Spontaneous and Stimulated Raman Scattering (SRS) 7,8 Applications utilizing nonlinear effects: Amplification of light Optical signal processing, e.g. wavelength conversion 1 Y. H. Kuo et al., Opt. Express 14, W. Mathlouthi et al., Opt. Express 16, M.A.Foster et al., Opt. Express 15, A. C. Turner-Foster et al., Opt. Express 18, J. R. Ong et al., IEEE PTL. 25, (2013) 6 Q. Lin et al., Opt. Express 15, R. Claps et al., Opt.Express 11, M. Krause et al., Opt. Express.12,

30 Silicon waveguides with reverse biased p-i-n junction p i n p-i-n junction across the nonlinear silicon waveguide Sweep the carriers generated by two-photon absorption Reduce effective carriers lifetime down to ps limits the impact of free-carrier absorption Y.H. Kuo et. al., Opt. Express 14 (2006) W. Mathlouthi et al., Opt. Express 18 (2008) J. R. Ong et al., IEEE Photon. Technol. Lett. 25 (2013)

31 Structure geometry 4 cm long waveguide 0.6 mm 2.5 mm 30

32 Waveguide technology cross section 31

33 Fabrication Used technology : BiCMOS (IHP Frankfurt (Oder)) 8 SOI wafers, 220 nm top Si layer and 2 μm buried oxide (BOX) Linear loss lower than 1 db/cm for waveguides with 50 nm slab and p-i-n diode Doping level in p and n regions of cm nm p i n 1,2 um Andrzej Gajda 32

34 Electric field distribution Shallow rib allows for higher field in the waveguide region A. Gajda et al., Opt. Express, vol. 19, pp , May

35 FCA vs Bias voltage for different slab heights Shallower etch Lower loss A. Gajda et al., Opt. Express, vol. 19, pp , May 2011 Intensity W/cm 2 w i = 1200 nm 34

36 Carriers screening effect simulation shallower etch depth higher carrier screening threshold A. Gajda et al., Opt. Express, vol. 19, pp , May

37 Photo current due to TPA with reversed pin junction H. Tian et al., JEOS:RP, Aug

38 Free carrier absorption threshold output power (dbm) w/o junction w/ junction 20 V 6.5 dbm 19.5 dbm coupled power (dbm) 37

39 Four-wave mixing in silicon Silicon Phase matching κ=0 required β 2 < 0 (anomalous dispersion) corresponds to dτ/dλ > 0 38

40 SiO 2 cladding W Chromatic Dispersion Si 3 N 4 cladding W s H Si SiO 2 SiO 2 s H Si Si 3 N 4 SiO 2 Anomalous dispersion, requirements: high H, low s, SiO 2 cladding 39

41 Silicon waveguides - optical properties Quantity Value Unit Length 4 cm Loss 1 db/cm TPA coeff. 0.5 cm/gw Nonlinear coeff. 280 W-1 m-1 Coupling loss * 4.5 db/facet Chromatic dispersion ps km -1 nm-1 Wrong sign *1D grating couplers with 35 nm bandwidth Waveguide parameters not yet optimum for optical signal processing 40

42 Four Wave Mixing measurement setup 41

43 Conversion efficiency Signal output to Idler output ratio η = P P idler signal ( L) ( ) (**) L used in experimental work (**) easy to measure (using Optical Spectrum Analyzer) (**) Y. Kuo et al. Opt. Express 14, (2006) 42

44 Waveguides without p-i-n Pump wavelength λ pump = nm Signal wavelength λ signal = nm Maximum efficiency -23 P pump = 26 dbm Waveguide lengths L = 1cm and L = 4 cm A. Gajda et al., Opt. Express, vol. 20, pp , June

45 FWM in p-i-n diode assisted waveguide Pump wavelength λ pump = nm Signal wavelength λ signal = nm Conversion efficiency η max ~-2 P pump = 26 dbm Waveguide length L = 4 cm A. Gajda et al., Opt. Express, vol. 20, pp , June

46 Wavelength conversion vs. detuning for different pump wavelengths -0.7 db our work -4.4 db Ong et.al., UC San Diego db Malouthi et. al., Intel db Kuo et. al., Intel 2006 Waveguide length L = 4 cm Bias voltage: U bias = 20 V Pump power P pump = 26 dbm Maximum efficiency η max (λ pump =1542nm, Δλ=3nm)=-0.7 db A. Gajda et al., Opt. Express, vol. 20, pp , June

47 Bit Error Rate (BER) Measurement setup 40 Gb/s NRZ OOK DC BIAS SIGNAL nm MZM PC DUT OBPF PRE AMPLIFIED RECEIVER PUMP nm EDFA OBPF PC Pump power in the waveguide : 20 dbm Signal power in the waveguide :0 dbm A. Gajda et al., Group IV Photonics

48 Wavelength converter output spectra relative power (db) no junction 0 V bias 20 V bias Bias (V) CE (db) no junction wavelength (nm) A. Gajda et al., Group IV Photonics

49 Measured bandwidth of FWM 3 db bandwidth of 10 nm Estimated dispersion of the waveguide D = ps/nm km η 0L contains incoupling loss of 4 db per coupler Conversion Efficiency [db] η 0L sim η 0L meas η LL sim η LL meas Signal wavelength [nm] A. Gajda et al., Group IV Photonics

50 Bit-Error Rate measurement Back to back Idler 0V bias Idler 20V bias w/o junction -log(ber) B2B Signal, 20V bias Idler, 20V bias Idler, 0V bias P rec [dbm] The power penalty of 0.2 db for idler in the 20V bias case A. Gajda et al., Group IV Photonics

51 Phase-sensitive Signal Processing 50

52 Four wave mixing (χ (3) ) Phase sensitive parametric processing (one mode PSA) non-degenerate 4-wave mixing pump 1 pump 2 signal idler pump 1 pump 2 signal + idler Processing of the signal becomes phase - sensitive 51

53 Phase sensitive amplification set up Power [dbm] CW Wavelength [nm] DC BIAS PM OPTICAL PROCESSOR EDFA PC Si WAVEGUIDE OSA F. Da Ros et al., ECOC

54 Evidence of phase-sensitivity in Si relative power (db) i/p i/p o/p max o/p min 28 dbm total power pump-to-signal power ratio: 30 db L= 4 cm Reverse bias: 25 V Signal phase shift: wavelength detuning (nm) 28 dbm total power correspond to 20.5 dbm(110 mw)/pump inside waveguide F. Da Ros et al., ECOC (2013) P

55 Phase sensitive extinction ratio optimisation pump power waveguide length normalised gain (db) L= 4 cm P p =24 dbm - 20 V P p =24 =26 dbm - 20 V P =24 p =26 =28 dbm - 20 V P p =24 =26 =28 dbm V phase (deg) gain (db) P p =24 dbm, -20V L= 4 cm w/o junction L= 4 cm w/ junction L= 2 cm w/ junction L= 1 cm w/ junction phase (deg) Up to 20 db phase-sensitive extinction ratio attainable F. Da Ros et al., ECOC (2013) P

56 Phase regeneration for 10 Gbit/s DPSK 40 GHz pumps D C optical processor CW PM signal EDF A OBPF Si waveguide PZT 10 Gbit/s phase noise emulation feedback loop APD DPSK Rx F. Da Ros et al., Opt. Express 22 (2014)

57 Phase-sensitive regenerator DPSK output spectrum 10 0 Input Output Power (dbm) , , , , ,40 Wavelength (nm) F. Da Ros et al., Opt. Express 22 (2014)

58 BER evaluation for phase-regeneration -log(ber) input - - w/o w/o noise output - w/o noise input - w/ noise output - w/ noise average received power (dbm) PRBS 4 GHz phase noise 0.57 phase modulation index input output w/o noise w/ noise Phase-noise penalty reduced from 10 db to less than 3.5 db F. Da Ros et al., Opt. Express 22 (2014)

59 Conclusions Successful electronic-photonic BiCMOS co-integrated circuits have been demonstrated FIRST DEMONSTRATIONS 10Gb/s modulator + driver 25Gb/s Germanium PD + TIA Si-nanowaveguides with pin-junctions allow foradvancedall-opticalsignalprocessing -Highly efficient wavelengh conversion -Phase sensitive parametric amplification 58