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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
High-Speed Circuits and Systems Lab. 3
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)
High-Speed Circuits and Systems Lab. 4
Input
CL Cross(P2)
Parallel(P1)
(coupling length)
1. Theorem of Directional Couplers
High-Speed Circuits and Systems Lab. 5
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
1. Theorem of Directional Couplers
High-Speed Circuits and Systems Lab. 6
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
1. Theorem of Directional Couplers
High-Speed Circuits and Systems Lab. 7
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. Theorem of Directional Couplers
High-Speed Circuits and Systems Lab. 8
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, …
High-Speed Circuits and Systems Lab. 9
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
High-Speed Circuits and Systems Lab. 10
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
High-Speed Circuits and Systems Lab. 11
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
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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)
High-Speed Circuits and Systems Lab. 13
Output powers for DCs with coupled waveguide lengths
3. Measurement Results
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
High-Speed Circuits and Systems Lab. 14
3. Measurement Results
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
High-Speed Circuits and Systems Lab. 15
ER
IL
3. Measurement Results
Observation with an infrared microscope at these wavelength (CCD) Light scattered from the top surface of the device
High-Speed Circuits and Systems Lab. 16
Photomicrographs
3. Measurement Results
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
High-Speed Circuits and Systems Lab. 17
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.
High-Speed Circuits and Systems Lab. 18
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.
High-Speed Circuits and Systems Lab. 19
Thank you for listening!
Q & AJeong-Min Lee
High-Speed Circuits and Systems Lab. 20