Sharing and Placement of On-chip Laser Sources in Silicon-Photonic NoCs

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1 Sharing and Placement of On-chip Laser Sources in Silicon-Photonic NoCs Chao Chen, Tiansheng Zhang, Pietro Contu, Jonathan Klamkin, Ayse Coskun, Ajay Joshi {chen98, tszhang, contu, klamkin, acoskun, Department of ECE, Boston University, Boston MA This research has been partially funded by the NSF grants CCF and CNS

2 Silicon-Photonic NoC Research [Kirman 06] [Shacham 08] [Vantrease 08] [Gu 09] [Joshi 09] [Batten 09] [Pan 09] [Cianchetti 09] [Koka ] [Pan ] [Beamer ] [Xue ] [Hendary ] [Kao ] [Li 3] [Zulfiqar 3] 2

3 Silicon-Photonic NoC Challenges q Bandwidth utilization Current applications/architectures do not need Tbps on-chip bandwidth q Power consumption Large laser power and thermal tuning power could negate bandwidth benefits q Packaging Coupling tens of off-chip laser sources to photonic NoC is challenging 3

4 Silicon-Photonic NoC Challenges q Bandwidth utilization Current applications/architectures do not need Tbps on-chip bandwidth q Power consumption Large laser power and thermal tuning power could negate bandwidth benefits q Packaging Coupling tens of off-chip laser sources to photonic NoC is challenging 4

5 Contributions of this paper q Packaging solution We use on-chip laser sources to address the packaging problem q Laser power solution We operate the laser sources at their maximum efficiency We strategically place and share the laser sources 5 Waveguide loss (db/cm)

Related Work q On-chip laser sources AlGaInAs-Si [Fang 2006] Ge [Camacho-Aguilera 202] III-V [Liu 20] InAs/GaAs [Wang 20] q Silicon-photonic NoC with on-chip laser source ATAC [Kurian 202] Clos and

6 Related Work q On-chip laser sources AlGaInAs-Si [Fang 2006] Ge [Camacho-Aguilera 202] III-V [Liu 20] InAs/GaAs [Wang 20] q Silicon-photonic NoC with on-chip laser source ATAC [Kurian 202] Clos and crossbar [Heck 204] q Laser power management Channel sharing [Pan 20] [Li 203] NoC bandwidth scaling [Zhou 203] [Chen 203] q Silicon-photonic NoC design automation Optical layer routing for laser power minimization [Ding 2009] and waveguide crossings minimization [Condrat 203] Joint exploration of link-level and system-level design for performance evaluation [Chan 20] Automatic placement and routing of photonic devices [Hendry 20] [Boos 203] 6

7 Outline q Background and Motivation q Laser source basics q Laser source sharing and placement strategies q Evaluation q Summary 7

Laser Source Basics q Laser efficiency = P O /P IN q Laser output power (P O ) = η i η d (hc/λq) (I-I th ) q Laser input power (P IN ) = I 2 R S +IV d q Main take away - I th e T, η d e -T, V d T

8 Laser Source Basics q Laser efficiency = P O /P IN q Laser output power (P O ) = η i η d (hc/λq) (I-I th ) q Laser input power (P IN ) = I 2 R S +IV d q Main take away - I th e T, η d e -T, V d T JDSU From energy efficiency perspective it is critical to operate diode laser sources at minimum possible temperature T η i : laser internal efficiency η d : differential quantum efficiency h: Planck s constant c: speed of light λ: laser operating wavelength q: electric charge I: drive current I th : threshold current R s : laser series resistance V d : diode voltage 8

9 Laser Source Efficiency q Threshold current to switch ON laser increases with temperature q Laser efficiency decreases with increase in temperature q The current at which laser source efficiency is maximum changes with temperature 9

10 Optical Power per λ per Laser Source 0.7 W per core

11 Outline q Background and Motivation q Laser source basics q Laser source sharing and placement strategies q Evaluation q Summary

12 Sharing of Laser Sources q We need to share the laser sources to operate them at maximum efficiency q The degree of sharing depends on the optical power demand per wavelength per waveguide 2

13 Sharing of Laser Sources q We need to share the laser sources to operate them at maximum efficiency q The degree of sharing depends on the optical power demand per wavelength per waveguide 3

14 Laser sources Sharing strategies q Sharing using ring filters at the waveguide crossing 3.2 db loss if one laser source is shared by 64 waveguides (at 0.05 db/crossing) q Sharing using merging and splitting of waveguides.2 db loss if one laser source is shared by 64 waveguides (at 0.2 db/split) 4

15 Laser sources Placement strategies q Laser sources placed along the edge Waveguide loss (db/cm) is small Core temperature will have a smaller impact on laser source efficiency q Laser sources placed next to starting location of the photonic link Waveguide loss (db/cm) is large Core temperature will have a larger impact on laser source efficiency 5

16 Methodology to decide Laser Source Sharing and Placement 6

17 Outline q Background and Motivation q Laser source basics q Laser source sharing and placement strategies q Evaluation q Summary 7

18 Target System q 256 cores, Private L2, 6 MCs, 52 GB/s NoC bisection BW q Silicon-photonic NoC connects L2s and MCs q 22 nm technology node, V q P core (avg) = 0.46 W, P L2-bank (avg) = 0.0 W 8

19 Evaluation Cases q Logic topologies 8-ary 3-stage Clos, 6-ary 3-stage Clos, 6 x 6 Crossbar q Physical layouts U-shape W-shape q Sharing and Placement strategies Edge w/ sharing Local w/ & w/o sharing 9

20 Evaluation Laser Power (W) 00 0 local non- share local share edge share 8- router Clos (U- shaped) router Clos (U- shaped) 00 Crossbar (U- shaped) ary Clos (W- shaped) Laser Power (W) 0 Waveguide Loss (db/cm) 0 Waveguide Loss (db/cm) 20

21 Evaluation Laser Power (W) 00 0 local non- share local share edge share 8- router Clos (U- shaped) router Clos (U- shaped) 00 Crossbar (U- shaped) ary Clos (W- shaped) Laser Power (W) 0 Waveguide Loss (db/cm) 0 Waveguide Loss (db/cm) 2

22 Evaluation Laser Power (W) 00 0 local non- share local share edge share 8- router Clos (U- shaped) router Clos (U- shaped) 00 Crossbar (U- shaped) ary Clos (W- shaped) Laser Power (W) 0 Waveguide Loss (db/cm) 0 Waveguide Loss (db/cm) 22

23 Evaluation Laser Power (W) 00 0 local non- share local share edge share 8- router Clos (U- shaped) router Clos (U- shaped) 00 Crossbar (U- shaped) ary Clos (W- shaped) Laser Power (W) 0 Waveguide Loss (db/cm) 0 Waveguide Loss (db/cm) 23

24 Evaluation Laser Power (W) 00 0 local non- share local share edge share 8- router Clos (U- shaped) router Clos (U- shaped) 00 Crossbar (U- shaped) ary Clos (W- shaped) Laser Power (W) 0 Waveguide Loss (db/cm) 0 Waveguide Loss (db/cm) 24

25 Outline q Background and Motivation q Laser source basics q Laser source sharing and placement strategies q Evaluation q Summary 25

Summary q On-chip laser sources will simplify packaging q On-chip laser sources should be shared and strategically placed to minimize laser power q A cross-layer methodology is

26 Summary q On-chip laser sources will simplify packaging q On-chip laser sources should be shared and strategically placed to minimize laser power q A cross-layer methodology is required to determine the best sharing and placement of laser sources Sharing and placement choices change with logical topology, physical layout, photonic device designs, etc. 26

Sharing and Placement of On-chip Laser Sources in Silicon-Photonic NoCs

Sharing and Placement of On-chip Laser Sources in Silicon-Photonic NoCs Chao Chen, Tiansheng Zhang, Pietro Contu, Jonathan Klamkin, Ayse K. Coskun, Ajay Joshi Electrical and Computer Engineering Department,