waveguide high-speed circuits and systems laboratory b.m.yu high-speed circuits and systems...

High-Speed Circuits and Systems Laboratory

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Waveguide


B.M.Yu


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Content

1. Overview

2. Introduction

3. Design and fabrication

I. Simulation

II. measurement

4. Waveguide loss measurement

5. Coupling between shallow-ridge and narrow strip

6. Conclusion


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Overview

Optics Express (2010), Low Loss Shallow-Ridge Silicon Waveguide, Po Dong

Buried Oxide: 3 um, Cross section of waveguide: 0.25 um x 2 um

Target : Chip to Chip interconnect (a few tens of centimeter)

Average propagation loss: 0.2740.008 dB/cm in C-band (1530~1565 nm)

Double-level taper: to adiabatically couple from shallow ridge to strip waveguide

Cross section of WG

3 um

2 um

0.25 um


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Introduction

Silicon photonics: interest area for broad spectrum applications (opti-cal interconnect, sensing, RF photonics)

Submicron wide deeply etched waveguide structures- Efficient and high speed active photonic devices- SOI substrates (Top silicon thickness: 0.25 um)- Lowest propagation loss (in previous reports): 1~2 dB/cm @ 1550

nm - Chip to Chip interconnect & Narrow bandwidth filters in RF photon-

ics

Shallow ridge or thin silicon waveguide- Propagation loss: 0.3~1 dB/cm (selective oxidation fabrication

technique) - Difficult to control (device density, hard mask thickness, cross

section of WG)

In this paper- Low loss silicon ridge WG: 0.25 um silicon, average propagation

loss: 0.274 dB/cm


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Design and Fabrication

- Main reason of waveguide propagation loss: light scattering from etch

sidewalls

Minimizing optical field overlap with etched interface (increasing width

of WG,

decreasing etch depth)

- Cross section of wave guide: 2 um x 0.25 um (etch depth: 0.05 um)

- Power confinement: 84 %

Etch sidewall of WG Shallow-ridge WG

Simulation


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Group index: ~3.7, effective index: ~2.9

- Etch depth tolerance 0.01 um

- Group index variation: 0.0033 5ps delay time difference (50cm waveguide)

40Gbps with a reasonable fabrication tolerance

Simulation


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Bending loss

- 90 bending with various radii (50~120 um)

- @ 100um radii 10-4dB

Simulation

bending loss with various bending radii


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SEM image of WG cross section Top-view of 64 cm waveguide

- Soitec 6” wafers

- 0.25 um thick silicon 3um buried oxide

- Spiral waveguide (rmin= 300 um)

- 6 mm x 3 mm waveguide (length of waveguide: 64cm)

6 mm

3 mm

Fabrication


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Waveguide loss measurement

Test setup

ASE ( =1550 nm)

Optical spectrum analyzer

Optical fiber (TE mode Polarization)

Waveguide(horizontal taper)

Optical fiber


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Insertion loss measured for different WG length as a function of wavelength

Waveguide propagation loss using linear fitting @ =1550 nm

- Insertion loss is normalized to power measured from direct

to direct fiber coupling

- Insertion loss = waveguide propagation loss + coupling loss

- Waveguide loss: 0.281dB/cm (@ =1550 nm)

Loss measurement


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Loss measurement

Waveguide propagation loss as a function of wavelength in C-band

Waveguide loss with 9 chip on the same 6” SOI wafer

- Average loss ( variation): 0.274 dB/cm 0.008 dB/cm

- Same wafer but different average loss etch depth variation

- Average loss (same wafer): 0.299 dB/cm


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Coupling between shallow-ridge and narrow strip WG

Double level taper

- Narrow strip WG (450 nm x 250 nm): 1.5 um bending radius

- In Ring modulator, narrow strip WG is more efficient.

- Coupling would be need

Coupling between shallow-ridge and narrow strip waveguide

3-D simulation result


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Coupling between shallow-ridge and narrow strip WG

Simulation result

Coupling loss as a function of taper length

- 10um long taper is sufficient in order to achieve <0.25 dB coupling loss

- Highly index contrast between silicon and oxide

waveguide high-speed circuits and systems laboratory b.m.yu high-speed circuits and systems...

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