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 Coskun, Ajay Joshi {chen98, tszhang, contu, klamkin, acoskun, joshi}@bu.edu Department of ECE, Boston University, Boston MA This research has been partially funded by the NSF grants CCF-49549 and CNS-49703.

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

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

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 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

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 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

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

Optical Power per λ per Laser Source 0.7 W per core

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

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

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

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

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

Methodology to decide Laser Source Sharing and Placement 6

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

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, 800 MHz @ 0.65 V q P core (avg) = 0.46 W, P L2-bank (avg) = 0.0 W 8

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

Evaluation Laser Power (W) 00 0 local non- share local share edge share 8- router Clos (U- shaped) 00 0 6- router Clos (U- shaped) 00 Crossbar (U- shaped) 00 6- ary Clos (W- shaped) Laser Power (W) 0 Waveguide Loss (db/cm) 0 Waveguide Loss (db/cm) 20

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 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