Graded-Index Core Polymer Optical Waveguide for High-bandwidth-density On-Board Interconnect

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1 European Cluster for Optical Interconnects (ECO) Workshop Sep. 25, 2013 Graded-Index Core Polymer Optical Waveguide for High-bandwidth-density On-Board Interconnect Takaaki Ishigure Faculty of Science and Technology Keio University

2 Performance Development in Super Computer Ref.

3 Super Computers from IBM 2012? IBM Power 775

4 Polymer waveguides for on-board application Optical Wiring Board (PCB) Chip to chip link: < 0.3 m Board to board link: 0.3 m ~ 1 m Connect to other devices GI multimode fiber ribbon MT connector

$Existing Waveguide n Step-Index (SI) Rectangular cores 250 mm 50 mm 矩形 Refractive index profile$ core-cladding boundary The scattering loss caused by the structural irregularity at the

core-cladding boundary The scattering loss caused by the structural irregularity at the

5 Existing Waveguide n Step-Index (SI) Rectangular cores 250 mm 50 mm 矩形 Refractive index profile Concerned issues Large propagation loss Crosstalk at highly-integration Scattering loss at the core-cladding boundary The scattering loss caused by the structural irregularity at the core-cladding boundary would easily increase, because all the modes propagate by total internal reflection at the boundary.

6 Existing Waveguide n Step-Index (SI) Rectangular cores 250 mm 50 mm 矩形 Refractive index profile Concerned issues Large propagation loss Crosstalk at highly-integration Inter-channel crosstalk crosstalk Since all the modes are reflected at the core-cladding boundary, inter-channel crosstalk is concerned in the case of densely-aligned waveguide.

7 Graded-Index Core Waveguide Graded-Index (GI) Circular cores n Refractive index profile Advantages of GI circular core Low propagation loss Low inter-channel crosstalk High coupling efficiency with a fiber Reduction of inter-channel crosstalk Modes in GI profile propagate around the core center, which can reduce inter-channel crosstalk even at highly-integration.

8 Graded-Index Core Waveguide Graded-Index (GI) Circular cores n Refractive index profile Minimum loss of GI profile Advantages of GI circular core Low propagation loss Low inter-channel crosstalk High coupling efficiency with a fiber It is expected that the propagation loss could be as low as possible, because GI profile confines mode fields around the core center.

9 Polymer Waveguide Based Link This figure shows design of waveguide-based optical link O-PCB LSI VCSEL or SMF Polymer waveguide Fiber ribbon GI-MMF Polymer waveguide PD

Connection Loss Simulation Link Model (Solver:FIMM-WAVE,

th 10 cm All modes WG A 1 st 2 nd N th B All modes 1 m 50

MMF 60 mm ncl=1.44 SI Square core WG 60 mm ncl=1.

45 50 mm 60 mm nco=1.55 10~ 50 mm 60 mm nco=1.

10 Connection Loss Simulation Link Model (Solver:FIMM-WAVE, FIMM-PROP) Connection Loss Connection Point1 Connection Point 2 Connection Point 3 VCSEL (NA=0.1) l = 850 nm w 0 = 9 mm Gaussian spherical wave 1 st 2 nd N th 10 cm All modes WG A 1 st 2 nd N th B All modes 1 m 50 GI-MMF 1 st 2 nd N th All modes 10 cm WG C GI Circular core MMF 60 mm ncl=1.44 SI Square core WG 60 mm ncl=1.54 GI Circular core MMF 60 mm ncl= mm nco= mm 60 mm nco= ~ 50 mm 60 mm nco= ~ 50 mm 50 mm 10 ~ 50 mm 10 ~ 50 mm NA = 0.17 NA = 0.17 NA = 0.17 Core size dependency of the connection loss is simulated.

Coupling loss [db] Coupling loss [db] Connection Loss Simulation Mode Profiles 50x50 mm SI waveguide 50 mm-f GI waveguide 50x50 mm GI waveguide Calculated Results Connection 2 3.

11 Coupling loss [db] Coupling loss [db] Connection Loss Simulation Mode Profiles 50x50 mm SI waveguide 50 mm-f GI waveguide 50x50 mm GI waveguide Calculated Results Connection WG GI-MMF WG GI-MMF WG GI-MMF Connection GI SI GI Core size [db] SI GI GI Core size [db] GI core shows low loss in wide rage of core size at connection 2

12 Coupling loss [db] Coupling loss [db] Connection Loss Simulation Mode Profiles 50x50 mm SI waveguide 50 mm-f GI waveguide 50x50 mm GI waveguide Calculated Results Connection WG GI-MMF WG GI-MMF WG GI-MMF GI GI SI Core size [db] Connection GI SI GI Core size [db] 矩形 GI 型コアは接続点 3 において低損失を示した

13 The Mosquito Method Utilizing A Micro Dispenser For Fabricating Circular GI Core

Coating cladding layer *Before UV cure(viscous) Inserting a needle into cladding layer Core 2. Forming core by Mosquito method 3.

14 New Fabrication Method: Mosquito Method Cladding Silicone frame (0.5 mm-thickness) Core Dispenser Parameter of Dispenser 1 Pressure 2 Velocity 3 Viscosity 4 Needle size Substrate 1. Coating cladding layer *Before UV cure(viscous) Inserting a needle into cladding layer Core 2. Forming core by Mosquito method 3. UV exposure Substrate High viscosity material is used for maintaining circular core Formation of GI distribution using monomer diffusion Fabrication of circular core less than 50 mm diameter is investigated for high density wiring.

Forming a smaller circular core needs to dispense with

Dispensing Conditions and Obtained Waveguides Pressure

250, 350 kpa 10, 12, 14, 16, 18, 20 mm/s FX-W712(ADEKA

) 190 mm Core-diameter(mm) 110 90 70 50 250 kpa 350 kpa

core-diameter Scan velocity (mm/s) of fabricated

15 Forming a smaller circular core needs to dispense with lower pressure and to scan more quickly. Dispensing Conditions and Obtained Waveguides Pressure Scan velocity Core Cladding Needle Dispensing condition 250, 350 kpa 10, 12, 14, 16, 18, 20 mm/s FX-W712(ADEKA Corp.) FX-W713(ADEKA Corp.) 190 mm Core-diameter(mm) kpa 350 kpa The repeatability of core-diameter Scan velocity (mm/s) of fabricated waveguides is confirmed. Core-diameter dependence on dispensing pressure and scan velocity Variance:± 5 % LOW Scan velocity HIGH

$Refractive-Index Fabricated$ 12-Ch. PPOW with GI-core Ch.

$Waveguide-width[mm] Refractive-index$

16 Refractive-Index Fabricated Waveguides and Their Index Profiles 12-Ch. PPOW with GI-core Ch Cross-section Core-diameter 40 mm; Pitch 250 mm 15-, 8- and 5-cm straight waveguide Index profile measured using an interference microscope D & 3D NFP Image Waveguide-width[mm] Refractive-index profile Interference pattern PPOWs with GI-circular-core are fabricated successfully.

Comparison between SI and GI Waveguides SI and GI Core

Photolithography(SI) Core/Cladding FX-W712/FX-W713

17 Comparison between SI and GI Waveguides SI and GI Core waveguides using the same polymer materials Mosquito(GI) Photolithography(SI) Core/Cladding FX-W712/FX-W713 FX-W712/FX-W713 Core-diameter 40 mm 40x40 mm Pitch 250 mm x 12 ch. 250 mm x 12 ch. Waveguide-length 5 cm 5 cm Cross-section NFP Image Insertion loss and crosstalk of each waveguide are measured.

Loss(dB/cm) Propagation loss Propagation loss of

) fabricated by Mosquito method is measured by

033 db/cm @ 850 nm 15 cm Fabricated waveguide 0.

850 900 950 1000 Wavelength(mm) 2D NFP image

18 Loss(dB/cm) Propagation loss Propagation loss of the waveguide (ADEKA Corp.) fabricated by Mosquito method is measured by cut-back method ( cm) nm 15 cm Fabricated waveguide FX-W712 Cross-section Wavelength(mm) 2D NFP image FX-W712 (ADEKA Corp.) is expected as one of the low loss waveguide materials.

19 GI-core waveguides exhibit lower insertion loss than SI-type. Insertion Loss Experimental setup VCSEL (850 nm) MMF or SMF PPOWs 5 cm probe 1 m GI(Mosquito method) SI(Photolithography) GI Result Launching probe Power meter 50 mm GIMMF probe 1 m SMF 25GI 50GI Loss[dB] SI Launching probe SMF 25GI 50GI Loss[dB] * Insertion loss average of 12-channels

Inter-Channel Crosstalk Experimental setup Scan VCSEL (850

SI(Photolithography) GI core 125-mm pitch GI core 250-mm

For high-density wiring, the waveguide with a 125-mm pitch is

20 Inter-Channel Crosstalk Experimental setup Scan VCSEL (850 nm) 25 mm GIMMF PPOWs 5 cm probe 1 m GI(Mosquito method) SI(Photolithography) GI core 125-mm pitch GI core 250-mm pitch Power meter 50 mm GIMMF probe 1 m SI core 250-mm pitch For high-density wiring, the waveguide with a 125-mm pitch is fabricated and compared crosstalk value to waveguides with a 250-mm pitch.

Crosstalk(dB) Inter-Channel Crosstalk Experimental setup Scan 0-10 -20-30 -40-50 -60 VCSEL (850 nm) 25 mm GIMMF PPOWs 5 cm probe 1

SI-250 um-pitch Ch. GIWG 125 mm[db] Power meter 50 mm GIMMF probe 1 m GIWG 250 mm[db] SIWG 250 mm[db] 2-36.4-43.6-34.8 3-41.6-44.

21 Crosstalk(dB) Inter-Channel Crosstalk Experimental setup Scan VCSEL (850 nm) 25 mm GIMMF PPOWs 5 cm probe 1 m GI(Mosquito method) SI(Photolithography) Result 25 mm GIMMF WG 50 mm GIMMF 125 um-pitch 250 um-pitch SI-250 um-pitch Ch. GIWG 125 mm[db] Power meter 50 mm GIMMF probe 1 m GIWG 250 mm[db] SIWG 250 mm[db] Waveguide-width(mm) Crosstalk of waveguide with narrower pitch than 250 mm is lower than SI-WG.

For Satisfying the Single-Mode Condition Fabrication

47) (From TOK) Clad Silicone Resin TPIR-224(n=1.

22 For Satisfying the Single-Mode Condition Fabrication Conditions Fabricated Waveguide Core Silicone Resin PO-46(n=1.47) (From TOK) Clad Silicone Resin TPIR-224(n=1.43) (From TOK) 500 mm Loss Evaluation Setup LED 850nm SMF Waveguide 5.0 cm SMF Power meter Insertion Losses 1 2 Cross-section 10 mm 10 mm Low insertion loss achieved Average [db] Minimum [db]

23 3-Dimensional Narrow-Pitch Fan-Out Structure Cross-sections of narrower pitch waveguides 80 mm 60 mm 40 mm Slight deformation and pitch deviation 40 mm Horizontal position of the neighbor core(s) varied 60~80-mm pitch could be the minimum (from a 230-mm O. D. needle) For further narrower pitch, height variation is a promising way. Core 2 Core 1 Core 3 Core 3 Core 2 Core 1 40 mm 250 mm Fan-out structure is realized.

24 The Photo-Addressing Method For Fabricating Square GI Core

25 63 rd IEEE ECTC Las Vegas, NV: May 28 31, 2013 Low-Loss Design and Fabrication of Multimode Polymer Optical Waveguide Circuit with Crossings for High-Density Optical PCB T. Ishigure, K. Shitanda, T. Kudo Keio Univ. / Yokohama, JP S. Takayama, K. Moriya, T. Mori, K. Choki Sumitomo Bakelite / Utsunomiya, JP May 28 31, 2013 Takaaki Ishigure, et al. -25-

$52) 50mm Photo-mask Core layer Substrate Heat Refractive index$ $Cladding layer Some kinds of refractive index modifiers were$

26 How GI-core Waveguide Fabricated? Photo-Addressing Technique UV UV Cladding layer (R.I. 1.52) 50mm Photo-mask Core layer Substrate Heat Refractive index profile n Laminate and Heat n SI-GI 50mm Cladding layer Core layer Cladding layer Some kinds of refractive index modifiers were incorporated in cladding layer. Heat GI-GI profile Refractive index profile May 28 31, 2013 Takaaki Ishigure, et al. -26-

27 Concentration Distribution leads to Index Profile Photo-mask UV Core layer Polymer matrix with high refractive index Monomer with low refractive r.t Substrate No pattern visible Heat (>45deg.C) Co-polymer Crosslinked Monomer diffusion into exposed area Concentration Distribution Core Cladding Core May 28 31, 2013 Takaaki Ishigure, et al. -27-

28 Optical PCB with Crossed Waveguides J. Beals IV, et al., Appl. Phys. A, 95, 983 (2009) Polymer Waveguides May 28 31, 2013 Takaaki Ishigure, et al. -28-

Loss Estimation using Ray-Trace Method Power-Law Form n co = 1.553 n cl = 1.

29 Loss Estimation using Ray-Trace Method Power-Law Form n co = n cl = / 2 n( x, y) nco{1 2 [ f ( x) g( y)]} f ( x) x p x, g( y) a y a y q Launch Condition for Ray-Trace Simulation 50 mm 250 mm l = 850 nm Beam spot size (2e = 30 mm) 50 mm Input NA is set to be 0.2. Intensity distribution is Gaussian. May 28 31, 2013 Takaaki Ishigure, et al. -29-

30 Results of Simulation Very low loss in GI-GI despite SI-SI intersection May 28 31, 2013 Takaaki Ishigure, et al. -30-

31 GI-Core Crossed Waveguide by Photo Addressing 100mm Crossed channels Transmitted channels 90 degree May 28 31, 2013 Takaaki Ishigure, et al. -31-

32 Comparison: Simulated & Measured 0.12 db/cross 0.02 db/cross May 28 31, 2013 Takaaki Ishigure, et al. -32-

33 Comparison: Simulated & Measured 0.12 db/cross 0.02 db/cross 0.01 db/cross The loss of GI-GI is one-order lower than the lowest results of SI-SI db/cross May 28 31, 2013 Takaaki Ishigure, et al. -33-

34 Crossing Angle Dependence SI-SI: Excess loss is observed SI-GI: Loss abruptly increases with decreasing the angle. GI-GI: Good agreement with simulated results. Remarkably low loss even at 20-deg. angle. May 28 31, 2013 Takaaki Ishigure, et al. -34-

35 Conclusions We introduce the advantages of GI-Core waveguide as follows: Low propagation loss because of the smaller effect of the core-cladding boundary roughness. Low inter-channel crosstalk even under smaller pitch due to the optical confinement High modal bandwidth GI-core polymer waveguides will play an important role for high-speed and high-density on-board optical interconnects.

36 Thank you very much for your attention

Low-loss light coupling with graded-index core polymer optical waveguides via 45-degree mirrors

Low-loss light coupling with graded-index core polymer optical waveguides via 45-degree mirrors Yoshie Morimoto 1,* and Takaaki Ishigure 2 1 Graduate School of Science and Technology, Keio University,