optical directional coupler based on si-wire waveguides · device might be used as a wavelength...

Optical Directional Coupler Basedon Si-Wire Waveguides

Jeong-Min Lee([email protected])

High-Speed Circuits and Systems Lab.

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Title of the Paper

Optical Directional Coupler Based on Si-Wire Waveguides

Publication Journal IEEE Photonics Technology Letters (PTL, 2005)

Summary of the Paper

Author: Hirohito Yamada, Tao Chu, Satomi Ishida, and Yasuhiko

Contents: 3 pages, 5 figures, 5 references

High-Speed Circuits and Systems Lab. 2

Contents

1. Introduction1) Concept of Directional Couplers (DCs)2) Theorem of Directional Couplers (DCs)

2. Structure and Fabrication

3. Characteristics1) Simulation Results2) Measurement Results

4. Conclusion

5. References


1. Concept of Directional Couplers

Directional coupler– Structure: two optical waveguides are brought in close.

• S-shaped waveguides: need for the input and output ports of the devices• Silicon-wire waveguides: large difference between the refractive indexes of

Si core (n = 3.5) and silica cladding (n = 1.5) reduce bending radius (~ um)


Input

CL Cross(P2)

Parallel(P1)

(coupling length)

1. Theorem of Directional Couplers


Input

Cross(P2)

Parallel(P1)

Ce o

L

Analysis: using interference with odd and even mode– One single mode waveguide has even (symmetric) & odd mode (anti-symmetric)– Energy exchange in two parallel waveguide interference of symmetric and

anti-symmetric shape– At z=0 odd and even mode exist interference light exists at top

waveguide

Odd

Even

z=0 z=Lc



Input

Cross(P2)

Parallel(P1)

Ce o

L

Odd

Even

z=0 z=Lc

Analysis: using interference with odd and even mode– Multi-mode waveguide has even (symmetric) & odd mode (anti-symmetric)– Energy exchange in two parallel waveguide interference of symmetric and

anti-symmetric shape– At z=0 Odd and even mode exist interference light exists at top

waveguide



Input

Cross(P2)

Parallel(P1)

Ce o

L

Analysis: using interference with odd and even mode– At z=Lc the phase of odd mode is changed (π shift) interference most of light exists at bottom waveguide

For specific L LC (coupling length) Input power transfer to cross port

– Extinction ratio (ER) =

Odd

Even

1

1 2

10 log( )PP P

z=0 z=Lc

1. Introduction

Usage of directional couplers: filters, polarizers, modulators, switches, …


Directional Couplers

Types of WG Conventional waveguide Si-wire waveguide

Materials optical fiber & waveguide (LiNbO3, Si, SiO2-based)

SOI (Silicon-on-Insulator)

Sizemore than several mm

(large separation between 2 parallel waveguides)

hundreds of um to mm

Characteristics -dense integration,

compatible with CMOS technology

1. Introduction

Detailed studies on directional couplers with Si-wire waveguides have not yet been reported

In this paper,– Present optical DCs based on Si-wire waveguides– Discuss their characteristics– Describe structure, the fabrication process, calculated and measured results


2. Structure and Fabrication

Structure– SOI: 0.3 um (Si), 1 um (buried oxide)

Process:– Pattering: use electronic beam lithography to form the waveguide core pattern– Etching: etch down to the SiO2 layer (inductively coupled plasma dry etcher)– Cutting: cut for measurement from the wafer


3. Simulation Results

Electric field profiles calculated for coupled waveguides with different waveguide core spacing (0.3 and 0.2 um)

Coupling length: – 10 um (for 0.3 um core spacing), 4.9 um (for 0.2 um core spacing)

Coupling length: parallel > perpendicular


3. Measurement Results

Output powers for both parallel and perpendicular polarizations Both output ports were complementary and they changed sinusoidaly

according to the coupled waveguide Coupling length: 10 um (perpendicular), 11 um (parallel)


Output powers for DCs with coupled waveguide lengths


Measurement environment– Directional coupler with a 100-um-long

coupled waveguide

Strong dependence on wavelength

Transmittance results– At 1549 nm: most of the light power was

output from the parallel port– At 1569 nm: most of the light power was

output from the cross port

Ripple: caused by Fabry-Pérot reflection because both facets of the DC were uncoated



Extinction ratio (ER): greater than 20 dB

Insertion loss (IL): 15 dB at 1550 nm

Coupling loss: 6.1 dB x 2 facet

Calculation vs. Measurement results – Core width is 10% smaller than designed

size (0.3 um)– Similar to that observed in the light output

power measured fabrication process


ER

IL


Observation with an infrared microscope at these wavelength (CCD) Light scattered from the top surface of the device


Photomicrographs


Optical output was reciprocally changed with 2.5-nm wavelength spacing between the parallel port and the cross port

ER: greater than 10 dB IL: not greater than 100-um-long coupled waveguide


Characteristics for a DC with long (800 um) coupled waveguide

4. Conclusion

Fabricated optical directional couplers with Si-wire waveguide and evaluated their fundamental characteristics.

Found extremely short coupling lengths (10 um), which means that they can be used as compact power dividers or combiners.

Demonstrated wavelength-selective characteristics of light output from long directional couplers.

Optical outputs from the parallel and cross ports of an 800-um-long device changed reciprocally with a 2.5-nm wavelength periodicity. indicates this device might be used as a wavelength multiplexer-demultiplexer in wavelength-division-multiplexing (WDM) systems.


5. References

[1] N. Ofusa, “An optical add-drop multiplexer with a grating-loaded directional coupler in silica waveguides”, IEICE Trans. Commun., vol. E82-B, pp. 1248-1251, 1999

[2] H. Kogelnik, “Switched directional couplers with alternating ∆β”, IEEE J. Quantum Electron, vol. QE-12, no. 7, pp. 396-401, 1976

[3] B. E. Little, “Ultra-compact Si-SiO microring resonator optical channel dropping filters”, IEEE Photon. Technol. Lett., vol. 10, no. 4, pp. 549–551, 1998

[4] A. Sakai, “Propagation characteristics of ultrahigh-∆ optical waveguide on silicon-on-insulator substrate”, Jpn. J. Appl. Phys., vol. 40, pp. L383–L385, 2001.

[5] K. Yamada, “Silicon-wire based ultra small lattice filters with wide free spectral ranges”, Opt. Lett., vol. 28, pp. 1663–1664, 2003.


Thank you for listening!

Q & AJeong-Min Lee

([email protected])


optical directional coupler based on si-wire waveguides · device might be used as a wavelength...

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