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Agilent Technologies
Optical Interconnects
& Networks Department
Photonic Crystals in Optical Communications
Mihail M. SigalasAgilent Laboratories, Palo Alto, CA
mihail_sigalas@agilent.com
Agilent Technologies
Optical Interconnects
& Networks Department
Outline
-Trends in Optical Communications
-Photonic Crystals
-Numerical Methods
-Photonic Crystal Waveguides
-Resonators in Photonic Crystals
-Conclusions
Agilent Technologies
Optical Interconnects
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Trends in Optical Communications
-Optical interconnects have been replacing electrical interconnects at shorter and shorter distances over time.
-Optical interconnects for chip to chip or even within one chip will be needed in the near future.
-Very short (microns scale) optical components (waveguides, bends, splitters, resonators) needed to achieve that.
-There are two ways to make micron scale optical components: Photonic crystals and high index contrast materials.
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Computer Interconnects Hierarchy
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Finite Difference Time Domain Method
1. Approximate the space and time derivatives in Maxwell’s equations with finite differences.2. The ``leap-frog’’ scheme for the E and H fields in time:
3. E and H fields in space (Yee grid):
E EEH H
nn-1/2 n+1/2Time
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Finite Difference Time Domain Method
4. Use absorbing boundary conditions (ABC) to close the space
ABC
PhotonicCrystal
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Requirements for photonic crystals interconnects
-Should be easily fabricated (2D slab PC are easier to be made than 3D PC)
-Should have low propagation and coupling losses.
-Most of the current 2D slab PC waveguides have narrow guiding bands with small group velocities. Small group velocities create higher internal and propagation losses. New structures are needed.
-Should be easily integrated with active devices (lasers, LEDs).
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2D Slab PBG Waveguides
Si slab on a SiO2 substrate
Triangular Lattice; Lattice constant: a; R/a=0.29; h/a=0.6
Band Gap for even modes(TE-like): 0.242-0.307 c/a
High index contrast confinementperpendicular to the slab.
Photonic band gap effect within the slab.
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2D Slab PBG Waveguides: Circular Air HolesGuiding along a line of circular Air holes with Rd=0.45a
3D FDTD Calculations
Guided Band is narrow with Small group velocity
Aslo see: Loncar, et.al., J. Opt. Soc. Am. B 18, 1362 (2001)
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2D Slab PBG Waveguides: Elliptical Air Holes
LeakyModes
Guiding along a line of elliptical Air holes withShort axis 0.66a and long axis 1.48a.
Guided band covers most of the band gap.
Plane Wave Expansion MethodJohnson, et.al., Opt. Express 8, 173 (2001)
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2D Slab PBG Waveguides: Elliptical Air Holes
Guiding along a line of elliptical Air holes withRatio of short to long axis 0.45
Short axis: 0.66a, 0.7a, 0.74a
Good coupling andlow propagation losses
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Fabrication of PC waveguides
substrate
SiO2 (0.15um) Si (0.26um)SiO2 (1um)
Alignment Marks Define PC Waveguide
Define Ridge Waveguide Silicon Etch SEM of PBG waveguide
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2D Slab PBG Waveguide Bends: Circular Air Holes
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Conventional Waveguide Bends
Si waveguide on SiO2120o Bend: ~70% Transmission60o Bend: ~90% Transmission
Also see: Manolatou et. al., J. Lightwave Techn. 17, 1682 (1999)
Good Transmission!
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PBG vs. Conventional Waveguides
-Both types of waveguides could give 100% efficiency along tight bends.
-There is ONLY one difference between the two types: PBG waveguides can guide light mostly through the air. However, ONLY 3D photonic crystals can do that.
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3D Photonic Crystals
Ho et. al., Solid State Commun. 89, 413 (1994)
Layers of Si rods surrounded by air
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3D PBG Waveguide Bend
Photonic Crystal Total Thickness: 20 layers
Projection of the 10th and 11th layers
Straight waveguide (Black)Bend (Red)
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3D PBG Waveguide Splitter
Photonic Crystal Total Thickness: 20 layers
Projection of the 10th and 11th layers
Straight waveguide (Black)Splitter (Red)
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3D PBG Waveguide Splitter
10th Layer 11th Layer
Guiding mostlythrough the air
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Micro-resonators
-Micron size resonators needed for sources and detectors.
-Micro-resonators also needed for add-drop filters in Wavelength Division Multiplexing.
-There are two ways to create micron-size resonators: Photonic crystals and High index contrast materials (micro-disk, micro-ring).
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2D Slab PBG Resonators
Rd/a=0.21, 0.17, 0.11
Air Holes in Si slab with SiO2 substrateLattice constant: aAir holes Radius: 0.29a
Mode Volume: ~a^3
See also: Vuckovic, et.al., Phys. Rev. E 65, 016608 (2001)
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Disk Resonators
Si on SiO2
Disk Radius: 2aDisk Thickness: 0.6a
Mode Volume: ~ (2a)^2 a=4a^3
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Conclusions
-There is a need for micron scale components (waveguides and resonators) for optical communications.
-There are two possible candidate materials for building the optical
components: Photonic crystals and high index contrast materials.
-For waveguides, both types of materials are expected to perform equally well.
-However, 3D photonic crystals can guide light mostly through the air.
-Photonic crystal resonators are expected to be 5-10 times smaller in size
than micro-disk resonators.
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Future Work
- Loss mechanisms
- Theoretical models
- Coupling to photonic crystal waveguides