Challenges in Silicon Photonics WDM Technology

Challenges in Silicon Photonics WDM Technology

Yung-Jr Hung, Professor

Department of Photonics, National Sun Yat-sen University

Outline

Motivations

Fabrication-tolerant MZI design

Cascaded MZIs for WDM (de)multiplexers

Conclusions

WDM in data center

A hyperscale data center spent several billions of dollars a year just on opticaltransceivers.

Hyperscale company like Facebook invests in single mode fiber to ensure the cablinginfrastructure is ready for future upgrades.

Hyperscale data center

Conventional optical multiplexing technologies

PLC based WDM (de)multiplexers suffer from high optical transmission loss while theassembly of thin-film based WDM (de)multiplexers requires high precisionaxial/angular positioning.

On-chip WDM (de)multiplexing using photonic integration technology is promising.

T. Saeki et al., SEI Technical Review, 2016.

Advantages of photonic integration

Integrating the same photonic functions with easier assembly!

MACOM CDRGe/Si PD

TIA

PLC DeMUX

FiberArray

Optical isolatorSi Lens arraySi

MUXSi MZI modMod Driver

InP/Si Hybrid Laser

Inside Intel’s 100G CWDM4 optical transceiver

InP/Si Hybrid Laser

Si MZI mod 1

Si MZI mod 2

Si MZI mod 3

Si MZI mod 4

RF Pads DC Pads

Echelle grating MUX

Spot size converter

Optical transceiver based on CWDM8

Intel released CWDM8 400 Gbps optical transceivers based on 8-channel CWDM technology.

Wavelength (nm)

12711411 nm

J. B. Driscoll et al., IEEE GFP, 2018.

Roadblock for on-chip WDM (de)multiplexers

(A Blessing and A Curse) Silicon photonics waveguide• Sub-micrometer dimensions• Small bend radius• High-density photonics• Every nm3 matters

Significant filter performance variation across the wafer evenwith advanced CMOS fabrication technology

W. Bogaerts et al., IEEE JSTQE, 2019. (IMEC)

Impacts on interferometric filter devices


Cascaded MZIs Arrayed Waveguide GratingEchelle Grating

Linewidth uniformity/WG lossExtreme thickness uniformity

Small featuresLinewidth uniformityWG loss/Small gaps

Impacts on interferometric filter devices


Cascaded MZIs Arrayed Waveguide Grating

Linewidth uniformity/WG lossLinewidth uniformityWG loss/Small gaps

Extreme thickness uniformitySmall features

Echelle Grating

Poor device yield in cascaded MZIs

Linewidth variation leads to serious peak wavelength drift of MZIs. Only ~10% device yield !

S.-H. Jeong et al., JOSAB, 2020. (PETRA) W. Bogaerts et al., IEEE JSTQE, 2019. (IMEC)

Outline

Motivations



Conclusions

The impact of WG dimensional variation

Local (intra-chip) and global (inter-chip) linewidth variation

300-mm SOI wafer demonstrated a thickness variation (3s) of 2.5 nm. Resultant SOI WGsexhibit a linewidth variation (3s) of 7.65 nm.

A wider WG width could effectively reduce the impact of linewidth variation, but this issue cannot be completely eliminated.

Print bias in lithographically defined pattern

WG width on Layout Fabricated WG width

650 nm 690 nm

500 nm 530 nm

Print bias issue determines the change of mean WG width !!

Print bias issue deviates the actual WG dimension from the one defined on the mask layout. Such linewidth deviation varies with the waveguide width and is also dependent of the local

pattern density.

Spectral shift in a regular MZI

Regular MZIs with identical waveguide widths on both arm exhibit a spectral shift of 0.75 nmper 1 nm waveguide width change.

Largest spectral shift happens when the waveguide widths for two arms change in oppositedirections.

∆휆 ∆푤⁄ = 휆 휕n 휕w⁄ 푛

T.-H. Yen et al., JLT , 2020.

The proposed fabrication-tolerant MZI design

This approach allows the cancellation of the effective index change to the waveguide widthvariation in a MZI by combining narrow and wide waveguides on two different arms.

∆휆 ∆푤⁄ = 휆휕푛휕푤 퐿 −

휕푛휕푤 퐿 푛 퐿 − 푛 퐿

Waveguide width dependent spectral shift in a MZI

푛 , = 푛 , − , 휆

∆휆 ∆푤⁄ = 휆 휕n 휕w⁄ 푛

퐿 − 퐿 =0

δλ = 휆 푛 , 퐿 − 푛 , 퐿

Fabrication tolerant MZI design:


Spectral shift in a fabrication-tolerant MZI

Spectral shift in a fabrication-tolerant MZI can be relaxed when the waveguide width of twoarms are changed simultaneously with the print bias value.

퐿 − 퐿 =0

δλ = 휆 푛 , 퐿 − 푛 , 퐿


Outline

Motivations



Conclusions

Four-channel CWDM (de)multiplexer – design


Four-stage cascaded MZIs 21 fabrication-tolerant MZIs DCs with 5 different coupling ratios Designed for 1510, 1530, 1550, and 1570 nm of wavelengths

Four-channel CWDM (de)multiplexer – experiments

193-nm DUV lithography Edge coupler pair for optical I/Os Footprint: 1.68x0.87 mm (further reduced by smaller bends) W1/W2 = 650/450 nm; L1/L2 = 23.25/8.014 mm

Regular MZIs Fabrication-tolerant MZIs


Four-channel CWDM (de)multiplexer – device yield

Regular MZIs

Fabrication-tolerant MZIs


Four-channel CWDM (de)multiplexer – performance


Precise alignment to the defined wavelength grids leads to lower insertion loss and channelcrosstalk in fabrication-tolerant MZIs.

Degraded channel crosstalk in CH1 is attributed to the narrowband spectral response ofdirectional couplers. The coupling coefficient at CH1 wavelength deviates from the one atdesigned wavelength.

Roadblocks for dense WDM (de)multiplexer

Large device footprint for dense WDM (de)multiplexer Implementing multiple cascaded MZI stages is not practical.

Channel spacing = 4.5 nm


1-1 Regular MZIs

1-1 Compensated MZIs


1-1 Regular MZIs


1-1 Regular MZIs


3-2 Regular MZIs


T.-H. Yen et al., Proc. OECC 2020.

Measured spectral responses (fabrication-tolerant MZIs)

1-1 cascaded MZIs (FSR = 4.5 nm) 1-1 cascaded MZIs (FSR = 0.8 nm) 1-1 cascaded MZIs (FSR = 0.08 nm)

Wavelength (nm)

Opt

ical

tran

smis

sion

(dB)

Wavelength (nm)

Opt

ical

tran

smis

sion

(dB)

Wavelength (nm)

Opt

ical

tran

smis

sion

(dB)

1-1 cascaded MZIs does not provide box-like filter response. No active phase tuning is employed. So process variation induced spectral shift affects the

ultimate optical crosstalk.


Spectral shift, loss, and crosstalk


Regular MZIs Fabrication-tolerant MZIsDl

(nm) sl (nm) Loss (dB)

Xtalk(dB)

Dl(nm) sl (nm) Loss

(dB)Xtalk(dB)

CH1 9.14 4.79 -0.15 -16.44 0.34 0.43 -0.12 -18.79CH2 8.49 4.14 -0.14 -16.97 0.39 0.52 -0.12 -18.49CH3 8.78 4.39 -0.13 -16.26 0.63 0.39 -0.24 -14.71CH4 9.42 4.25 -0.13 -16.2 0.57 0.46 -0.21 -14.16

Regular MZIs Fabrication-tolerant MZIsDl

(nm) sl (nm) Loss (dB)

Xtalk(dB)

Dl(nm) sl (nm) Loss

(dB)Xtalk(dB)

CH1 1.32 1.03 -0.83 -7.84 0.43 0.2 -2.18 -8.9CH2 1.36 1.03 -0.81 -7.73 0.41 0.19 -1.97 -10.2CH3 0.85 0.88 -0.43 -21 0.38 0.21 -2.15 -8.3CH4 0.81 0.9 -0.43 -19.23 0.37 0.22 -2.35 -7.67

Channel spacing = 4.5 nm Channel spacing = 0.8 nm

Actively-tuned cascaded MZIs for dense WDM

Fabrication-tolerant MZI design can reduce the spectral shift to the order of sub-nm. This approach is promising for coarse WDM applications. However, for dense WDM (de)multiplexing we still need active phase tuning to precisely align

each MZI for optimal optical crosstalk.

FSR = 0.083 nm

10.5 dB

1549.7 1549.8 1549.9 1550.0 1550.1 1550.2 1550.3-45

-40

-35

-30

-25

-20

-15

-10

-5

0 1549.921549.96 1550.04

1550

Opt

ical

tran

smis

sion

(dB)

Wavelength (nm)

Ch. 1 (1V) Ch. 3 (2.1V) Ch. 2 (1V) Ch. 3 (2.1V)

FSR = 0.038 nm

11.1 dB

1549.8 1550.0 1550.2-45

-40

-35

-30

-25

-20

-15

-10

-5

0T = 25.3℃

1549.841549.92 1550.08

1550O

ptic

al tr

ansm

issi

on (d

B)

Ch. 1 (1.05V) Ch. 2 (1.05V) Ch. 3 (1.78V) Ch. 4 (1.78V)

1549.7 1549.8 1549.9 1550.0 1550.1 1550.2 1550.3-45

Wavelength (nm)

Outline

Motivations



Conclusions

Conclusions


Due to the manufacturing linewidth variability, serious spectral drift happens in as-fabricated MZIs on silicon-on-insulator.

A fabrication-tolerant MZI design methodology is demonstrated to solve this issue byoptimizing the waveguide widths/lengths of two arms.

Four-channel CWDM (de)multiplexer based on fabrication-tolerant cascaded MZIsexhibit lower insertion loss (< -3dB) and channel crosstalk (< -20 dB) with its spectralresponses well aligned to the defined wavelength grids.

The same design concept can be applied for dense WDM, but cascaded MZIs exhibithigher channel crosstalk and larger device footprint.

Actively optical phase tuning may be required for dense WDM (de)multiplexing.

Acknowledgements

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Thank you for your attention!

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Challenges in Silicon Photonics WDM Technology