SILICON PHOTONICS FOR DATA COMMUNICATIONS · Caveats on Silicon Photonics • Not cheap just because it’s silicon – Expensive mask set, process, don’t have volume • Performance

ADOPT Winter School 2014 1

SILICON PHOTONICS FOR DATA COMMUNICATIONS

Gideon YoffeKaiam Corporation, California

Visitor at ICT, KTH Kista

• Introduction– Kaiam packaging technology– Data communication, Datacenters

• Silicon Photonics• Two possible commercial applications

– Multi-wavelength transmitters– Low-cost tunable lasers

• Conclusions

KAIAM: Use Si MEMS to build complex optical assemblies

4. Standard packaging and testing follows

2

1. Build a “PCB” using a silicon MEMS breadboard• simple low‐cost process, can be done

at many foundries

2. Depending on the PIC, bond components on the “PCB”• standard die‐bonding tools used for electronics

3. Micro‐machine optically connects the components • micro‐lenses move to maximize coupling,

micro‐heaters lock with solder• quick process, cheap tools, tolerant of

mechanical positioning errors and shifts

ADOPT Winter School 2014

Implementation of MEMS alignment

3

Shunt driver

Microlens

PLC upside‐down on spacer

Laser diodeMEMS bench


4

MEMS alignment40Gb/s (4 x 10Gb/s) optical subassembly

All parts assembled using conventional tools,then aligned with MEMS and locked

PLC (upside down on spacer)

Shunt driver

laser

lens

Note: MEMS details not shown for simplicity


5

Solder lock of MEMS structure1) On chip heater melts solder ball2) MEMS moves the lens into optimal

position. Tab is somewhere in solder ball

3) Heater is turned off, locking part in position

tab moves into solder ball

AuSn solder

buried Ni/Crheater

Air gap for thermal isolation


6

Tolerance to die bonding error

• Lens adjustment compensates for initial non-optimal component placement

• 20um placement error ~< 0.6dB penalty

• MEMS design also demagnifiespost-solder shift


7

Advantages of Kaiam approach

• Leverages generally available single-function components– No need to build complex monolithically integrated chips

• Much higher performance– Discrete chips can be optimized for high performance. Better than

monolithically integration, where material compromises must be made

• Very low development time and resources– For each Photonic Integrated Circuit, only a new “PCB” is needed

• Very high yield– Questionable parts can be tested / burned-in before assembly– Don’t have to reject the assembly because one part is bad

7Kaiam Corporation, ECOC 2012 WS09

Source: Independent Analyst Research and Cisco Analysis; Cisco Visual Networking IndexFrom Ori Gerstel, Cisco

Data use growing fast, but not revenue!

Telecom Revenue growth is limited (GDP based)

Internet traffic growth is high (30-100% CAGR)

0

30

60

2008 2009 2010 2011 2012 2013

Exa

byt

es

pe

r mo

nth

MobilityBusiness InternetBusiness IP WANConsumer InternetConsumer IPTV/CATV


Datacenters


• Vast amount of data to/from datacenters

• 30000-50000 servers per datacenter

• Need power, cooling –

• Facebook set up in Luleå

Datacenter Interconnects


Need layers of switches between servers

• Far more data travels within a datacenter than to/from a datacenter• A “search” might be sent to 1000+ servers• Many layers of switches required• Server-to-switch links now moving from 1Gb/s to 10Gb/s• Links between switches moving to 40Gb/s now, some to 100Gb/s


Dreams for Integrated Silicon Photonics• Si electronic circuits perform switching (logic) of signals• Photonics is very appealing for, transport, routing of signals

– Fiber optics used first for long haul, now for shorter and shorter links

• Main cited application for silicon photonics is optical interconnect, chip-to-chip or on-chip

DARPAIBM

Near-term uses for Silicon Photonics• As electronics moves to 25Gb/s I/O, optical transceivers on “faceplate” will suffer.• Optics embedded on or very close to Si IC will be needed• Power dissipation from III-V’s would be a concern• Clear opportunity here for Si photonics chip, maybe with remote light source• For now, Si photonics chip likely to be separate from electronics



Silicon Photonics• Use CMOS line to make optical components, in silicon on insulator, 220nm thick• Foundries like imec, IME, have processes well controlled

– Offer multi-project wafers;circuits generally perform as expected!

Univ Delaware / Opsis /IME

Silicon Waveguide

• Very small optical mode, <0.5um• Very high refractive index contrast

– Silicon n=3.46– SiO2 n=1.46

• Very tight bends, tiny circuits possible• Output beam at facet very divergent, hard

to couple


“High delta” Silica waveguide4um mode diameter

Single-mode fiber9um mode diameter

Waveguide Couplers, Splitters


Y-branch

Multi-mode interference (MMI) device

Directional coupler, tap

1X2 2X2

Ring Resonators• Resonant coupling of light into a ring• Can resonantly couple out into a second waveguide


B. Little MIT1997

Imec

through

drop


AWG Arrayed Waveguide Gratings• Integrated optics device in silica or

other waveguides

• Used as mux or demux, channel spacing as low as 50GHz (0.4nm)

input guide

free-space regions

waveguide array, different lengths

output guides


Silica, Silicon AWGSilica AWGTypically 20 X 30mm

Silicon AWG0.2 X 0.35mm

• Bend radius for silica ~few mm• Bend radius for silicon ~10um• Problem for Silicon – wavelength accuracy

– Thickness tolerance gives 10nm uncertainty

NTT

Optical Coupling - Edge• Direct attachment of single-mode fiber would give 20dB loss• Need to expand optical mode• Inverse taper, coupling to waveguide with effective index ~ 1.6, often polymer• Obtain 2-3um spot size, <1dB loss to lensed fiber or via a lens to SMF

– similar to coupling a laser diode to fiber


U Gent

IBM

“CMOS compatible” edge coupler

• Some labs insist on only using CMOS processes – no polymer

• Can get good results with inverse taper alone, etched facet to control position of tip.

• But what does “CMOS compatible” really mean?


Optical Coupling - Grating

• Grating couplers couple light out of a waveguide, into a fiber• Generally 10 degrees off vertical to break backward-forward symmetry and to

minimize back-reflections• Waveguide tapers out to 10um width to match single-mode fiber• Basic grating coupler gives about 25-30% coupling efficiency to fiber


Advanced Grating Couplers

• With added complexity, still CMOS compatible, can achieve up to 70% coupling at peak, fairly broad spectrum

• Add poly-silicon overlay to break up-down symmetry

• “Apodize”, vary grating duty-cycle, to try to match output beam profile to fiber mode


Luxtera

U Gent / Imec

Choice, Grating vs Edge Coupler?


Parameter Grating Edge ChoiceCoupling efficiency

1.5dB loss to SMF 1dB to lensed fiber Edge

Optical Bandwidth

Typically 60nm 3dB, higher to smaller spot.

>100nm Edge

Back Reflections

~2%, very hard to eliminate. May require isolator.

<0.1% with good design Edge

Convenience Place coupler anywhere on chip. Full-wafer testing.

At edge only. Dicing and extra steps required.

Grating

CMOS compatibility

Standard process throughout

Non-standard steps required. Breaks metal guard ring.

Grating

Package cost /complexity

Low-cost custom component to turn beam.Large/selected spot size for easy alignment.

Standard geometry.Smaller spotsize requires more precise alignment.

Tie


Active Devices: Refractive index Change

• Index change through free-carriers, “plasma effect”, known since 1987• ∆n can be 0.001 for doping 1E18/cm^3, but depletion region width small compared

to waveguide so effect on mode is small– very weak effect for micron-scale waveguides

• Holes give bigger effect than electrons, with lower loss• Some accompanying free-carrier absorption

Carrier density change:reverse biased pn diode

• Change depletion region size in pn diode• For given reverse bias V, doping density N,

very roughly:• Depletion region width ~ 1/√N• No. of carriers moved, modal index change ~ √N• Length for pi phase shift ~ 1/√N• Capacitance/unit length ~ √N• Absorption/unit length ~N

• Higher N gives:• Shorter modulator for pi phase shift• Little change in capacitance• Higher absorption loss

• Tradeoff length for loss through doping, little effect on speed ~0.4um

Depletion region at pn junction, due to drift/diffusion



Silicon Modulators - Ring• Shift transmission resonance by applied signal

• Very compact, fast

• Very temperature-sensitive, 0.07nm/deg C

• Need active tuning

• Not suitable for low-cost uncooled applications

Sun/ Kotura


Silicon Mach-Zehnder Modulators• Amplitude modulation through refractive

index change in one path of an interferometer

• Operates over wide wavelength range

• Not too sensitive to temperature

• Doesn’t need active tuning

• Better suited to communications, but bigger than ring resonator From IMEC


Depletion width, Capacitance vs Voltage

• Index change ~N (doping level), depletion width ~1/sqrt(N)• Higher doping gives bigger modal index change, phase shift, but higher capacitance• Note depletion region, where action takes place, ~0.1um wide

Capacitance, Depletion Width vs Voltage3mm long, doping level 1E18/cm^3

0.00

0.25

0.50

0.75

1.00

1.25

1.50

0 1 2 3 4 5 6

Reverse bias (Volts)

Junc

tion

capa

cita

nce,

pF

0

0.025

0.05

0.075

0.1

0.125

0.15

Dep

letio

n re

gion

wid

th,

umcapacitance

IMEC data

depletion w idth

From S. Sze, “Semiconductor Devices”


Phase shift vs Voltage

• Calculate change of mode effective index with voltage, through overlap of changing depletion region width

• Calculate phase shift

• Vpi = 9.7V

• Vpi.L = 14.5V.mm

• Capacitance at 0V = 0.7pF

• Simulations, data agree, despite simple 1-D model

Phase Shift, Capacitance vs Voltage1.5mm long, doping 1E18/cm^3

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10 12 14Reverse bias (Volts)

Phas

e sh

ift (r

adia

ns)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Cap

acita

nce

(pF)

Phase shift

Capacitance


Mach-Zehnder Modulator Operation• The available refractive index change in silicon is fairly small, so modulation is not

very efficient.

• Vπ.L product around 26V.mm for good high-speed devices

• With 3mm long device, push-pull, can get decent extinction ratio with 2-3V swing

MZM output, Vpi.L=26Vmm, L=3mm, push-pull

0

0.020.04

0.06

0.080.1

0.12

0.140.16

0.18

0 1 2 3 4 5 6V

Out

put i

nten

sity

2Vp-p


Modulator Performance• With “lumped” electrodes, speed limited to about 10Gb/s for good extinction ratio

– direct tradeoff of phase shift and capacitance with doping

• Traveling-wave electrodes get past RC limitations, used for all 25Gb/s applications

• But electrode characteristic impedance typically ~30 ohms, due to capacitance

• On-chip modulator insertion loss typically ~5dB– Mostly due to P/N doping in phase-shifters

• All published data has been at 1550nm. IMEC, Opsis/IME, now starting 1310nm

From IMEC modulator multi-project wafer run announcement.


Integrated Light Source for Silicon• Silicon diodes do not emit

light, unlike GaAs, InP

• No easy integrated light source

• Some hero experiments showing light emission without III-V:– “Porous silicon”, 1990’s

– Strained Ge, GeSn, on Si

– Thulium Silicates

optically-pumped lasing of strained Ge on SiMIT, Gp IV Photonics Meeting 2012


Hybrid-Integrated Light Source• Wafer bonding

– UCSB, Intel– Inefficient laser, poor confinement in

gain region– Thermal problems – SiO2

• 40 deg/W for 800um laser– Yield questions

• Epitaxial InP on Silicon– S. Lourdudoss, KTH– Very appealing– Looks very difficult

passive

active


Caveats on Silicon Photonics

• Not cheap just because it’s silicon– Expensive mask set, process, don’t have volume

• Performance of devices is mediocre– Losses higher than SiO2, InP– Electro-optic effects weaker than in InP, simpler physics– Detector (Ge) efficiency 0.5 to 0.7A/W, while InGaAs is close to 1A/W

• No easy light source• Optical coupling is difficult• Real benefits expected when integrated with electronic circuitry

– But generally photonics not made on same CMOS line as top-end electronics– Beware power consumption/heating

Application 1: 4 wavelength transmitter design exercise• 10Gb/s datacenter links moving to 40Gb/s, QSFP package, 4 lanes at 10Gb/s each• Short reach using 850nm VCSEL’s, 4 parallel multimode fibers, up to 100~300m• For longer reach use 4 wavelengths multiplexed onto single-mode fiber• Use directly-modulated semiconductor lasers, uncooled to save power• Standard is 20nm channel spacing: 1270, 1290, 1310, 1330nm.


PLC (upside down on spacer)

Shunt driver

laser

lens

Note: MEMS details not shown for simplicity

Existing Kaiam 4X10Gb/s optical sub-assembly for QSFP transceiver for 10km link


4 channel eyes from QSFP TOSA

Luxtera Silicon short-reach version


• Single laser diode as light source, split between 4 modulators• Silicon photonics integrated with drivers – nice for distributed travelling wave drive• Sold in “Active Optical Cable”, short links, 4 single-mode optical fibers, 10Gb/s each.• Maybe cheap in volume for short distance, but ribbon fiber, termination expensive• Customers often prefer connectorized transceivers.

Silicon for Next-Gen 100Gb/s• Now we need to plan for 4X25Gb/s, for 100Gb/s link.• Strong preference for uncooled operation to save electric power• Not clear that directly modulated semiconductor lasers can give 25Gb/s at high T• Interest in using Silicon Photonics to generate the 25Gb/s signals

– Modulators + Multiplexer, tap waveguides to monitor laser power• How good a chip can we make in a multi-project wafer run, e.g. at IMEC?


Si Chip

Mux

Mod

Mod

Mod

Mod

CW Lasers

Output fiber

det

det

det

det


Modulators with MMI splittersLayout in “Fimmprop”

apply index modulation

zero modulation: quadrature

pi/2 modulation: output high

-pi/2 modulation: output low

MZM output, Vpi.L=26Vmm, L=3mm, push-pull

0

0.020.04

0.06

0.080.1

0.12

0.140.16

0.18

0 1 2 3 4 5 6V

Out

put i

nten

sity

quadrature

Estimated Loss Budget• Estimate losses from foundry guidance, to see how much laser power we need• We want about 0dBm, or 1mW average power per wavelength in the output fiber• For the current design, we need laser power 27dBm,=500mW!!!• Totally impractical. Need to be able to run off 30mW lasers, maximum• Output grating coupler is a big contributor because of 60+nm wavelength range

– Good edge couplers will save up to 8dB, but we still need more savings elsewhere.


Item Loss dB Comments

Input coupler 4

Grating coupler specified 2.5dB loss to SMF;additional loss transforming laser mode to SMF. Use edge coupler when available

MZM insertion loss 5 Mostly due to doping of phase shifters

MZM modulation loss 4For low voltage operation will need to bias with some loss at "1" level

Passive waveguide loss 1 Loss is 1.5 to 2.5dB/cm in undoped waveguideMux loss 4 AWGTaps 1 Taps on input guides to monitor optical powerOutput coupler 8 Limited bandwidth of grating. Want edge coupler!Total losses 27

Feasibility of Si Photonics

• Well-characterized building blocks through most of the design• Modulators should give about 15GHz bandwidth, able to achieve 6dB extinction

ratio• Uncertainty of precise silicon thickness leads to wavelength uncertainty

– Multiplexer, grating wavelengths can easily be wrong by up to 10nm

• Losses will be quite high, over 20dB from laser chip to output fiber– Would need an optical amplifier in order to measure “eye diagrams”

• Chip would not be good enough to make a product• Could be used for lab demonstrations and investigations of silicon photonics• Performance is always improving as the foundries tune their processes and designs• Maybe the concept can be practical in 2-3 years.



Example 2: Tunable laser for WDM fiber-to-the-home

• Bandwidth demand in the “last mile” is pushing interest in WDM fiber-to-the-home

• Many architectures use tunable transceiver at end user

• Requires very low-cost tunable laser

TX/RX 32

TX/RX 5

TX/RX 4

TX/RX 3

TX/RX 2

TX/RX 1

Central Office

Homes,

Labs,

Companies

Commercial Tunable Lasers

• Integrated devices dominate in compact tunable transmitter market

• Complex, large InP chips – too expensive


Syntune

JDSU


Kaiam Tunable Laser• Exploit silicon photonics: integrate tunable filter function into silicon

• Couple to external InP gain chip

• Package can be very compact, cheap

• Kaiam performed proof-of-concept demonstration, reported at OFC 2012.Silicon PLC prism lens InP gain chip

Proposed TO-style package


PLC tunable reflectors

• Vernier tuning of two sets of reflection peaks

• Silicon tuning ~ 0.07nm per deg C

• Thermally tuned micro-ring resonators, diameter ~50um

Vernier tuning with ring resonators

0

0.2

0.4

0.6

0.8

1

1530 1535 1540 1545 1550

wavelength

inte

nsity Ring 1

Ring 2

gain chip

PLC reflector with micro‐ring resonators

grating coupler

heaters


Custom PLC’s in Sub-Micron SOI• PLC’s, 900X300um, were fabricated to our design on 193nm 8-inch CMOS line

– SOI 0.25um thick, waveguide width ~0.5um

– Near-normal incidence grating couplers for 70% coupling

Bragg grating

grating coupler heater electrodes

1. Rings in loop configuration

2. Rings in series, Bragg mirror for return path

Kaiam, OFC 2012


Tunable Reflection spectra• Measured using broadband SLD source and a fiber-optic circulator

• Envelope of spectrum corresponds to grating coupler and SLD, each 40-50nm FWHM

Thermally tuned reflection spectra Heat applied to one ring only

0

1

2

3

4

5

6

7

1510 1520 1530 1540 1550

Wavelength (nm)

Ref

lect

ed p

ower

(a.u

.)

0mw5mw11mw18mw


Lasing Results 1. Spectra

• Lab bench external-cavity laser using ring-resonator PLC coupled via lens to AR/cleaved gain chip (Alphion)

-60

-50

-40

-30

-20

1520 1530 1540 1550 1560 1570 1580

Wavelength (nm)

dB


Lasing Results 2. Fine tuning

• Align two rings by heating one, then apply heat to both to tune the whole spectrum

• Lasing mode stays on aligned peaks

Fine tuning

00.20.40.60.8

11.21.4

0 25 50 75 100 125

Thermal tuning power (mW)

Wav

elen

gth

shift

(nm

)


Lasing Results 3. L-I

• Rings tuned for efficient lasing on one peak

• Achieve desired 5mW facet power

Output power

01234567

0 25 50 75 100 125 150 175 200

SOA current (mA)

Out

put p

ower

(mW

)


Lasing Results 4. Modulation

• Directly modulate gain chip with square-wave

• Rise/fall times 200-250ps, adequate for 1.25Gb/s

• Speed limited by gain chip design – intended for DC drive


Path Forward• Efficiency can be improved by optimization of gain chip for application:

– Threshold - MQW BH vs wide ridge with bulk active

– Slope efficiency - Low-reflectance front facet vs as-cleaved

• Improvements also from optimization of PLC, coupling

Simulated Efficiency Improvements

02

468

10

121416

1820

0 25 50 75 100 125 150 175 200

Current (mA)

Pow

er (m

W)

presentOptimize gain chip onlyImprove PLC also


Conclusions

• Silicon photonics offer possible path to low-cost optical data links

• Many functions available – modulators, detectors, filters, multiplexers …

• Chips are made in well-characterized CMOS fabs, so they generally behave as expected

• Light source needs to be in another material

• Challenges – optical coupling, high loss

• Commercial possibilities for datacenter interconnect and fiber-to-the-home

• Not easy, even if it is Silicon!

Documents

SILICON PHOTONICS FOR DATA COMMUNICATIONS · Caveats on Silicon Photonics • Not cheap just because it’s silicon – Expensive mask set, process, don’t have volume • Performance