Photonic Crystals in Optical Communications

Agilent Technologies

Optical Interconnects

& Networks Department

Photonic Crystals in Optical Communications

Mihail M. SigalasAgilent Laboratories, Palo Alto, CA

[email protected]




Outline

-Trends in Optical Communications

-Photonic Crystals

-Numerical Methods

-Photonic Crystal Waveguides

-Resonators in Photonic Crystals

-Conclusions




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.




Computer Interconnects Hierarchy




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




Finite Difference Time Domain Method

4. Use absorbing boundary conditions (ABC) to close the space

ABC

PhotonicCrystal




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).




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.




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)




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)




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




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




2D Slab PBG Waveguide Bends: Circular Air Holes




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!




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.




3D Photonic Crystals

Ho et. al., Solid State Commun. 89, 413 (1994)

Layers of Si rods surrounded by air




3D PBG Waveguide Bend

Photonic Crystal Total Thickness: 20 layers

Projection of the 10th and 11th layers

Straight waveguide (Black)Bend (Red)




3D PBG Waveguide Splitter

Photonic Crystal Total Thickness: 20 layers

Projection of the 10th and 11th layers

Straight waveguide (Black)Splitter (Red)




3D PBG Waveguide Splitter

10th Layer 11th Layer

Guiding mostlythrough the air




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).




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)




Disk Resonators

Si on SiO2

Disk Radius: 2aDisk Thickness: 0.6a

Mode Volume: ~ (2a)^2 a=4a^3




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.




Future Work

- Loss mechanisms

- Theoretical models

- Coupling to photonic crystal waveguides

Photonic Crystals in Optical Communications

Documents

Transcript of Photonic Crystals in Optical Communications