High-Speed Board-Level Polymer Optical Sub- Systems

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2 High-Speed Board-Level Polymer Optical Sub- Systems I. H. White, N. Bamiedakis, J. Chen, and R. V. Penty Department of Engineering, University of Cambridge, UK

3 Motivation 3

4 Motivation - exponential growth in amount of information exchanged globally - data servers, storage systems, supercomputers Increasing size for a large data centre: : ~ ft : ~ ft : ~ ft : ~ ft 2 Optical interconnects within high-performance electronic systems 4

5 Why use Optics in Data Server Units from OFC 2011 IBM, A. Taunblatt 5

6 Opto-electronic PCBs use: optics for high-speed links electronics for low-speed/control signals and power increase: interconnect density ( x18 at 10 Gb/s) reduce board area ( ~60 %) Intensive research in industry-academia various: R.C.A Pitw on et al. OI w orkshop, 2014 optical material and fabrication methods/ OE board design /OE packaging and assembly IBM Xyratex Mitsui Dellmann L. et al, ECTC, , 2007 Nakagaw a S. et al. ECTC, , 2008 Papakonstantinou I. et al, ECTC, ,2008 Teck Guan Lim et al, IEEE TAP, pp ,

7 Translation for optical interconnects Therefore requirements for optical interconnects: from J. Kash, Photonics Society Ann. Meeting 2010 imposes big challenge for next generation short-reach optical links -- cost & power : < $1 Gb/s, < 25 pj/bit 7

8 Board-Level Optical Interconnects Various approaches proposed: free space interconnects fibres embedded in substrates waveguide-based technologies Jarczynski J. et al., Appl. Opt, 2006 our work Tyco FlexPlane Basic waveguide & component studies Interconnection architectures Board-level OE integration PCB-integrated Optical units 8

Siloxane Polymer Materials Siloxane materials engineered to exhibit suitable mechanical, thermal and optical properties: are flexible exhibit high processability coating, adhesion to substrates,

9 Siloxane Polymer Materials Siloxane materials engineered to exhibit suitable mechanical, thermal and optical properties: are flexible exhibit high processability coating, adhesion to substrates, dicing exhibit high thermal and environmental stability withstands ~ 350 C (solder reflow) low intrinsic loss at datacommunications wavelengths: nm low birefringence offer refractive index tunability suitable for integration on PCBs offer high manufacturability are cost effective 9 9

Multimode Waveguides Cost-efficiency targets relaxed alignment tolerances multimode waveguides top cladding n ~ 1.5 core n ~ 1.

10 Multimode Waveguides Cost-efficiency targets relaxed alignment tolerances multimode waveguides top cladding n ~ 1.5 core n ~ um - typical cross section used: µm 2 1 db alignment tolerances > ± 10 µm assembly possible with pick-and-place machines - pitch of 250 µm to match ribbon fibre and VCSEL/PD arrays - facets exposed with dicing saw (low-cost process) 20um bottom cladding n ~ 1.5 substrate propagation losses: nm crosstalk (up to 150 µm spacing) < -25 db large number of parallel on-board waveguides 10 10

RF amplifier 50 µm Oscilloscope 32 µm record error-free (BER<10-12 ) 40 Gb/s data transmission N.

11 40 Gb/s NRZ data transmission - 1 m long spiral waveguide Voltage source Pattern generator A Cleaved 50 μm MMF 50 μm MMF patchcord Bias 16x 16x Tee 850 nm VCSEL 1 m spiral waveguide MM VOA 30 GHz PD B 40 GHz RF amplifier 50 µm Oscilloscope 32 µm record error-free (BER<10-12 ) 40 Gb/s data transmission N. Bamiedakis et al., IEEE PTL, vol. 26, pp , 2014 N. Bamiedakis, et al., IEEE JLT, vol. 33, pp. 1-7,

$Bandwidth (GHz) Bandwidth (GHz) Bandwidth studies Demonstration of waveguide bandwidth-length product of at > 40 GHz m Two 1 m long spiral samples tested with different refractive index profiles$ 5 1 0.5 0 input B2B - x= pulse +0.0 m t in -0.5 0 0.25 0.5 0.75 1 1.25 Time (ps) input time domain measurements Data FWHM = 0.25 ps Gaus FWHM = 0.18 ps Sech FWHM = 0.16 ps Loren FWHM = 0.

5 1 0.5 0 input B2B - x= pulse +0.0 m t in -0.5 0 0.25 0.5 0.75 1 1.25 Time (ps) input time domain measurements Data FWHM = 0.25 ps Gaus FWHM = 0.18 ps Sech FWHM = 0.16 ps Loren FWHM = 0.

12 Bandwidth (GHz) Bandwidth (GHz) Bandwidth studies Demonstration of waveguide bandwidth-length product of at > 40 GHz m Two 1 m long spiral samples tested with different refractive index profiles graded -index (GI) step -index (SI) different profiles generated by adjusting fabrication parameters potential for dispersion engineering SI GI Autocorrelation Trace Amplitude SI GI input B2B - x= pulse +0.0 m t in Time (ps) input time domain measurements Data FWHM = 0.25 ps Gaus FWHM = 0.18 ps Sech FWHM = 0.16 ps Loren FWHM = 0.12 ps Data Gauss fit Sech fit Lore fit R 2 Gaus = R 2 Sech = R 2 Loren = output Autocorrelation Trace Amplitude Sp2 output SI WG#3 In:x10, Out: x16- pulse x= +0.0 m t out Time (ps) Data FWHM = ps Gaus FWHM = ps Sech FWHM = ps Loren Data FWHM = ps Gauss fit Sech fit Lore fit R 2 Gaus = R 2 Sech = R 2 Loren = no mode mixer Offset ( m) with mode mixer Offset ( m) SI 32 µm 35 µm GI 32 µm estimated bandwidth: SI: GHz GI: GHz potential to achieve 100 Gb/s over a single multimode polymer waveguide 35 µm J. Chen, et al., in ECOC, paper Mo.3.2.3, pp. 1-3, 2015 J. Chen et al., IEEE JLT, pre-publication available online,

Multimode Waveguide Components OEM OEM Optical board S-bend S-bend Optical board OEM 90 90 crossing Use of passive

13 Multimode Waveguide Components OEM OEM Optical board S-bend S-bend Optical board OEM crossing Use of passive multimode waveguide components: on-board routing flexibility & advanced topologies However, limited power budget (e.g. 10 GbE has 8 db power budget) for high-speed on-board links low-loss components required Components designed and fabricated: - Waveguide crossings - Bent waveguides: 90 o bends and S-bends) - Y-splitters/combiners - Waveguide couplers - Waveguide Tapers 90 bend 90 bend OEM Performance characterisation under varying launch conditions and input offsets restricted launches (SMF, lens) and partially overfilled launches (MMF) 90 crossings S-bends 90 bends Y-splitters FR4 board 13 13

Optical coupling schemes Optical coupling achieved either by: - out-of-plane coupling using beam-turning elements + simplifies assembly and electrical connection of active devices - requires

14 Optical coupling schemes Optical coupling achieved either by: - out-of-plane coupling using beam-turning elements + simplifies assembly and electrical connection of active devices - requires additional fabrication steps typically, 45 mirrors in optical layer & micro-lenses - end-fired coupling + eliminates the need for additional optical structures - requires embedding the OE devices in the board and efficiently routing the electrical signal from the board surface to the devices 14 14

15 Optical Coupling Examples End fire IBM optical rod integrated 45 o mirror Neyer A. et al, ECTC, 2005 Takagi Y. et al, IEEE JLT, vol 28 (20), 2010 Dellmann L. et al, ECTC, , 2007 fibre-based 90 o connections microlens assisted coupling S. H. Hwang et al, IEEE PTL, vol 19 (6), 2007 Ishii Y. et al, IEEE TAP, vol. 26 (2),

16 Cambridge Approach to Optical Coupling Using simple tools, low-cost materials and minimal technical know-how... FR4 substrate PCB prototyping machine, Mask aligner, EVG 620 LPKF Protomat C60 Clad layer Core layer VCSEL Photodiode TIA Solder reflow machine, PACE Thermoflo 2000 SMA FR4 SMA Via Rx Via Copper Solder mask 16

PCB-integrated 10 Gb/s optical units Proof-of-principle demonstrators

transceiver built on low-cost FR4 10 Gb/s transmit - 10 Gb/s

Data SMA inputs Data SMA outputs FR4 OE PCB polymer layers LD module

PCB waveguide facet PD LD N. Bamiedakis et al., IEEE TCPMT, vol.

17 PCB-integrated 10 Gb/s optical units Proof-of-principle demonstrators integrating optics and high-speed electronics - 10 Gb/s optical transceiver built on low-cost FR4 10 Gb/s transmit - 10 Gb/s chip-to-chip on-board communication link 10 Gb/s receive power input Data SMA inputs Data SMA outputs FR4 OE PCB polymer layers LD module Rx module PD Y-splitter embedded in optical layer waveguide facet OE PCB waveguide facet PD LD N. Bamiedakis et al., IEEE TCPMT, vol. 3, pp , 2013 A.Hahim et al., IET Optoelectronics, vol. 6, pp ,

Blade servers typically have 14 blades and another 2 external network connections, making a total of 16 backplane

18 On-board interconnection architectures Blade servers are a popular method of increasing packing density in IT environments. Network connectivity is currently provided by an electrical backplane capable of providing several Gb/s total throughput. Blade servers typically have 14 blades and another 2 external network connections, making a total of 16 backplane connections. There is a need for a low cost backplane which will enable one blade to talk to any other in the chassis at >1 Gb/s.

28 (2005) Fujitsu Labs optical backplane

19 Optical Backplanes: Widespread Industry Interest Numerous demonstrations of simple point-to-point on-board polymer links Intel optical chip-to-chip link Mohammed et al, Intel Tech. J. 8 (2004) Asperation Perlos Co/Vtt Electronics Immonen et al, IEEE Trans. Elect. Pack. Manuf. 28 (2005) Fujitsu Labs optical backplane Glebov et al, Opt. Eng. 46 (2007) Fraunhofer/Siemens et al Schroder et al, Opt Int. Circ. VIII, Proc.SPIE 6124 (2006) IBM Terabus Optocard Schares et al, IEEE J. Sel. Top. Q. Elect. 12 (2007)

passively optically interconnect different electrical cards/modules Shuffle router Optical bus one dedicated

20 Advanced on-board interconnection architectures Backplanes are next level of integration of optics into highperformance electronic systems, e.g. blade servers cost-effective systems with reduced power consumption Ways to passively optically interconnect different electrical cards/modules Shuffle router Optical bus one dedicated waveguide one common communication channel for each on-board link on-board waveguide links 1 Rx 1 2 Rx 2 3 Rx 3 4 optical backplane Rx 4 20

Shuffle router design details Backplane design - exploits all four substrate edges - uses low loss waveguide components 90 crossings and 90 bends:

01 db/crossing with MMF bend loss ~ 1 db for RoC > 8 mm non-blocking architecture scalable waveguide design ribbon fibre connection to Rx ribbon fibre

21 Shuffle router design details Backplane design - exploits all four substrate edges - uses low loss waveguide components 90 crossings and 90 bends: simultaneous fabrication of all waveguides in single plane crossing loss ~0.01 db/crossing with MMF bend loss ~ 1 db for RoC > 8 mm non-blocking architecture scalable waveguide design ribbon fibre connection to Rx ribbon fibre connection from - opposite edges populated with like-connection types ( or Rx) & spatial offset minimises crosstalk reaching I/O connections requires only one 90 bend per waveguide - scalable with increasing card number max # crossings per wg link = N 2 -N 21 21

10-Card Optical Backplane Card interfaces (10 waveguides x 10 4 Rx each) Rx Rx Rx Rx 10 8 10 FR4 : 100 90 -bends ~ 1800 90 -crossings 2.

22 10-Card Optical Backplane Card interfaces (10 waveguides x 10 4 Rx each) Rx Rx Rx Rx FR4 : bends ~ crossings 2.25 U 6 (10 cm) Rx Rx Rx Rx Rx x 10 4 Schematic of 10-card backplane layout 100 waveguides single 90 bend per waveguide 90 crossings or less per waveguide Input Type Insertion Loss Worst-case Crosstalk 50 μm MMF 2 to 8 db < -35 db SMF 1 to 4 db < -45 db Terabit capacity enabled by 100 waveguides, 10 Gb/s in multicast mode J. Beals, et al., Applied Physics A, vol. 95, pp , 2009, 22 22

23 Regenerative optical bus architecture Proposed optical bus architecture: - polymeric optical bus modules and multiple optical channel - two optical transmission directions to allow full card connectivity - signal drop and signal add functions at each card interface 1 M optical bus segments 1 2 N cards N N+1 2 3R 3R N+2 2 N optical signal direction 3R M polymeric waveguide bus structures regenerator units - number of cards per segment limited by available optical power budget - 3R regenerator units to allow bus extension with multiple segments arbitrary number of cards can be connected onto the bus implementation costs that linearly scale up with the number of cards M N 23 23

24 next bus segment next bus segment Optical Bus Architecture Waveguide Design Schematic of a single bus segment: - two optical transmission directions - signal drop at each Rx port and signal add at each port Rx N optical channels 3R Rx 1 2 N 1 2 N 3R Regenerator Rx Rx Card 1 Rx Rx Card 2 signal drop signal add Rx Rx Card M transmission direction Rx 3R Rx 3R Regenerator bus repeating unit 24

25 50 mm 50 mm 3R 3R Proof-of-principle bus module Rx Rx Rx Rx Rx Card Card Card Card Card Rx Rx Rx Rx 3R Rx Card Rx 3R Rx Card Rx Repeating unit Power budget studies: using realistic component losses and a 15 db power budget for 10 Gb/s links 3 cards possible before regeneration required Design of a proof-of-principle 4-channel 3-card bus module compatible with 1x4 VCSEL/PD arrays and transceivers and 3R chips size: 90x50 mm 2, fits 4 wafer Rx 1 Rx 2 Rx 3 a b c d e f g h i j k l 50 µm WGs 50 µm WGs 50 µm WGs 25 µm 40 µm 50 µm 50 µm 50 µm 100 µm 60 µm 100 µm 60 µm 1 mm 3.35 mm 3.35 mm 3R out ' 2' 3' 4' 3R in 50 µm, w=0 µm 50 µm, w=10 µm main bus geometry 50 µm, w=40 µm bus repeating unit mm Optical layer: Y-splitters/combiners, 90 bends, 90 crossings, raised-cosine S-bends, tapers N. Bamiedakis et al., in Opt. Expr., vol. 20, iss. 11, pp ,

50 mm 50 mm 4-channel 3-card Bus Module Sample polymeric bus

materials using standard photolithography size: 90 x 50 mm 2 -

drop I III I optical signal II signal add II Fabrication

Rx 3 b e f i j bus outputs 3R 1 2 3 4 90 mm 1' 2' 3' 4' Rx 3R

26 50 mm 50 mm 4-channel 3-card Bus Module Sample polymeric bus modules fabricated on low-cost FR4 substrates from siloxane materials using standard photolithography size: 90 x 50 mm 2 - facets exposed with dicing saw (no polishing steps) signal drop I III I optical signal II signal add II Fabrication Details III Rx 1 Rx 2 Rx 3 a b c d e f g h i j k l Rx 1 Rx 2 Rx 3 b e f i j bus outputs 3R mm 1' 2' 3' 4' Rx 3R 3R bus inputs Rx 3R 1' 2 2' optical bus module

3R signal regeneration 50 mm Data Transmission 1-

27 3R signal regeneration 50 mm Data Transmission 1- Rx4 ch2 1- Rx6 ch2 Rx 1 Rx 3 Rx 4 Rx OBUS1_S5 3R Regenerator 4 6 OBUS2_S6 10 Gb/s data transmission experiments for all channels through 3R regeneration e.g. error-free (BER<10-12 )10 Gb/s transmission from 1 to Rx 4 and Rx 6 (channel 2) 27

28 1a 1e 1i 11' 2b 2f 2j 22' 3c 3g 3k 33' 4d 4h 4l 44' 5e 5i 51' 6f 6j 62' 7g 7k 73' 8h 8l 84' 9i 91' 10j 102' 11k 113' 12l 124' 131' 142' 153' 164' Path Loss (db) Insertion loss characterisation Insertion losses of all 40 optical paths measured with butt-coupled 50 µm MMF and a 1 4 VCSEL array target: loss < 15 db 3R out mm 1 Rx 1 Rx 2 Rx 3 a b c d e f g h i j k l mm 1' 2' 3' 4' 3R in - ALL within the 15 db target db power budget x4 VCSEL input 50 µm MMF input 2 0 Path N. Bamiedakis et. al, Opt. Expr. vol. 20, 2012 N. Bamiedakis et. al, JLT, vol. 32, pp ,

Data Transmission All possible data transmission paths for all channels when: - only specific channel ON - all channels ON (DC biased - not data) 1 Rx 1 Rx 3 3R signal regeneration 50 mm OBUS1_S5 3

29 Data Transmission All possible data transmission paths for all channels when: - only specific channel ON - all channels ON (DC biased - not data) 1 Rx 1 Rx 3 3R signal regeneration 50 mm OBUS1_S5 3 3R Regenerator Rx 4 Rx OBUS2_S6 IN 1 Rx 3 Rx 4 3 Rx 6 4 INPUT OUTPUT OUT Rx 3 Rx 4 Rx Ch. 1ONLY Ch. 2 ALL ON Ch. 3 ONLY Ch. 4 ALL Ch. ON 1 Ch. ONLY 2 Ch. ALL 3 ON Ch. 1 EF EF EF EF EF EF ONLY Ch. 2 EF EF EF EF EF EF Ch. 3 ALL ON EF EF EF EF EF EF Ch. 4 EF EF EF EF EF EF ONLYCh. 1 EF EF EF EF Ch. 2 EF EF EF EF ALL ONCh. 3 EF EF EF EF Ch. 4 EF EF EF EF ONLYCh. 1 EF EF Ch. 2 EF EF ALL ON Ch. 3 EF EF Ch. 4 EF EF Ch. 4 Ch. 1 Ch. 2 4 Ch. 3 Ch. 4 Error-free (EF: BER<10-12 ) transmission achieved for all on-board links N. Bamiedakis et. al, JLT, vol. 32, pp

mode mixer broad area detector bent 16x R

30 180 deg excess loss (db) Toward high-density low-cost interconnects - polymer waveguide arrays - large arrays feasible high aggregate capacity - suitable connectors under development - flexible substrates - bending radius < 5 mm - twisted waveguides fibre patchcord cleaved input fibre R. Dangel,et al., JLT, vol. 31, pp , µm WGs 850 nm VCSEL mode mixer broad area detector bent 16x R twisted SMF 50 µm MMF 100 µm MMF Radius (mm) 30

High-density ultra low-cost interconnects - interface polymer waveguide arrays with micro-pixelated LED arrays

micro-pixelated LEDs (µleds): small active area (20-100 µm) large bandwidth ( > 100 MHz) relatively large

J.J.D. McKendry et al., JLT, vol. 30, pp. 61-67, 2012.

demonstrated using a single µled and a multimode WG - potential for coarse WDM multiplexing : e.g. 4-λ CDWM 62.

31 High-density ultra low-cost interconnects - interface polymer waveguide arrays with micro-pixelated LED arrays potential to achieve relatively larger aggregate capacity with ultra-low cost optical interconnects - micro-pixelated LEDs (µleds): small active area ( µm) large bandwidth ( > 100 MHz) relatively large output power ( >1 mw) can be formed in array configurations high bandwidth per pixel larger total output power J.J.D. McKendry et al., JLT, vol. 30, pp , µled array 30 µm wide WGs - µled-based PWG links: - small area (<100 µm) matches waveguide size - 5 Gb/s demonstrated using a single µled and a multimode WG - potential for coarse WDM multiplexing : e.g. 4-λ CDWM 62.5 µm aggregate data transmission > 1 Tb/s/mm 2 using ultra low-cost optical components N. Bamiedakis et al., in ICTON, pp. 1-4, 2015 N. Bamiedakis et al., to be presented in ICTON,

32 Other Routes to Higher Bandwidth: Wavelength Division Multiplexing TX Single Wavelength RX TX TX RX RX TX n n Multiple Wavelengths RX c.f. Australian Photonics Animation 32

33 Integrated De/multiplexers for Guided-wave WDM Links Grating collimated field diffracted field Parabolic mirror λ 2 λ 1 λ 0 Waveguide connection with slab Component 1 : Slab waveguide one dimensional confinement (vertical only) output waveguides expanded field input field 1 input waveguides Component 2 : Deep etched waveguides - two dimensional confinement Triangular elements for transmission diffraction grating Component 3 : Reflecting surfaces top view Demultiplexer design Parabolic collimating mirror 33

34 Integrated De/multiplexers for Guided-wave WDM Links Grating collimated field diffracted field Parabolic mirror λ 2 λ 1 λ 0 Waveguide connection with slab Component 1 : Slab waveguide one dimensional confinement (vertical only) output waveguides expanded field input field 1 input waveguides Component 2 : Deep etched waveguides - two dimensional confinement Triangular elements for transmission diffraction grating Component 3 : Reflecting surfaces top view Parabolic collimating mirror Demultiplexer design 34

35 Integrated De/multiplexers for Guided-wave WDM Links Grating collimated field diffracted field λ 1 Parabolic mirror 2 λ 2 λ 0 expanded field output waveguides input field 1 input waveguides top view 35

36 Integrated De/multiplexers for Guided-wave WDM Links output waveguides expanded field Grating collimated field Waveguide Parabolic connection mirror with slab 2 3 diffracted field λ 2 λ 1 λ 0 Component 1 : Slab waveguide one dimensional confinement (vertical only) input field 1 input waveguides λ 0, λ 1, λ 2, Triangular elements for transmission diffraction grating λ 1 λ 2 λ 0 top view Component 2 : Deep etched waveguides - Component 3 : Reflecting surfaces 36

37 Integrated De/multiplexers for Guided-wave WDM Links Grating collimated field 3 diffracted field λ 1 Parabolic mirror 2 λ 2 λ 0 expanded field 4 output waveguides 5 input field 1 input waveguides top view 37

38 Integrated De/multiplexers for Guided-wave WDM Links Predicted Integrated Multiplexer/Demultiplexer Performance - Gaussian mode power distribution at input restricted launch condition 0 =0.45 m, =10 nm, r 0 =5.0, w 0 =8.0 m,sep =125 m,tm Exp (0.25,0.25) uniform mode power distribution at input worst-case scenario 0 =0.45 m, =10 nm, r 0 =5.0, w 0 =8.0 m,sep =125 m,tm Uni Power 0-5 Spectral Response (db) db WG- 1 WG- 2 WG- 3 WG- 4 Spectral Response (db) db WG- 1 WG- 2 WG- 3 WG Wavelength ( m) Wavelength ( m) on-going fabrication work N. Bamiedakis et al., to be presented in ICTON,

Conclusions Multimode polymer waveguides: a cost-effective optical technology

optical links direct integration onto PCBs, low-cost assembly various

39 Conclusions Multimode polymer waveguides: a cost-effective optical technology for board-level optical interconnects low loss, low-crosstalk on-board optical links direct integration onto PCBs, low-cost assembly various interconnection architectures for passive backplanes potential to achieve even higher data rates > 100 Gb/s! Siloxane waveguides Basic waveguide components Interconnection architectures Board-level OE integration PCB-integrated optical units 39 39

Polymer Interconnects for Datacom and Sensing. Department of Engineering, University of Cambridge

Polymer Interconnects for Datacom and Sensing Richard Penty, Ian White, Nikos Bamiedakis, Ying Hao, Fendi Hashim Department of Engineering, University of Cambridge Outline Introduction and Motivation Material