Free space optical communication using orbital angular momentum multiplexing

Emma Yann Zhang

Hong Kong University of Science and Technology May 11, 2013

History of Orbital Angular Momentum

1932 – C. Darwin identified another part of angular momentum in photons besides spin.

• Angular momentum of photons has external part due to motion of the center of gravity and intrinsic part due to spin.

During high order atomic transitions, photon emits at a distance r from atom. The product of linear momentum of photon and radial distance contributes to an angular momentum independent of spin.

C.G. Darwin. (1932). “Notes on the Theory of Radiation,” Proceedings of the Royal Society of London.

History of Orbital Angular Momentum

1992 – L. Allen found all helically phased beams have OAM, and can be generated using cylindrical lenses

• OAM is a natural property in helically phased beams • LG modes (helical beams) can be easily produced using cylindrical lenses

mode converters from HG modes.

L. Allen et al. (1992). “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,” Physical Review A

Helical light beams with OAM = +ħ Cylindrical lenses mode converter

History of Orbital Angular Momentum

1992 – Helical beams with optical vortex generated using forked diffraction grating.

• More lines at the bottom, light diffracts at different angle • Changing the number of split lines changes the mode produced.

V. Yu. Bazhenov et al. 1992."Screw dislocations in light wavefronts." Journal of Modern Optics 39.5

Forked diffraction grating produces screwed wavefronts with optical vortex at the center

History of Orbital Angular Momentum

1995 – OAM can be transferred and rotate microscopic particles

• Direction of rotation

determined by the sign of the OAM state.

• A laser beam of a few mW gives a rotation speed of 1-10 Hz.

He, H., et al. 1995. "Direct observation of transfer of angular momentum to absorptive particles from a laser beam with a phase singularity." Physical Review Letters, 75.5

The rotation of a trapped red blood cell under (i)15ħ, (ii)−15ħ. Dasgupta et al. 2011.Optics Express, Vol. 19, Issue 8

History of Orbital Angular Momentum

2004 – Gibson et al. demonstrated free-space optical communication with OAM-carrying light beams

• Information encoded in OAM states, transmitted in air over 15m.

• Computer generated hologram on spatial light modulators (SLM)

Gibson, Graham, et al. 2004. "Free-space information transfer using light beams carrying orbital angular momentum." Opt. Express 12.22

History of Orbital Angular Momentum

2012 – Terabit data transfer rate using OAM multiplexing.

• Data rate of 2.56Tbit/s over 1 meter in free space

• Combining OAM multiplexing and polarization multiplexing to increase capacity and spectral efficiency

Wang, Jian, et al. 2012."Terabit free-space data transmission employing orbital angular momentum multiplexing." Nature Photonics.

Helically phased beams

Azimuthal phase dependence Optical vortex Carry OAM of per photon

l = 0

l = -1

l = +1

l = +2

l = +3

http://www.gla.ac.uk/schools/physics/research/groups/optics/research/orbitalangularmomentum/

Visualize helical waves…

Laguerre-Gaussian Mode

LGpl p – number of radial nodes l – azimuthal index

Orbital and spin angular momentum

Angular momentum Orbital Spin

Wave fronts: Helically phased waves Circularly polarized waves

AM per photon l ħ σħ

No. of states ±1, 2, 3, 4….. ±1

Each photon has unlimited OAM states that can carry information without interference!

OAM Multiplexing in FSO Communication System

Laser resonator Generating LG beams

Multiplexing LG beams

Propagating in free space

Demultiplexing LG beams Recover data

Holograms using SLM

Methods of Generating Light Carrying OAM Asymmetric laser resonators (direct generation) Cylindrical lenses Forked diffraction grating Spiral phase plate Computer generated holograms

iPad app: iHologram Spatial Light Modulator (SLM)

Advantages of OAM Multiplexing

Advantages Limitations

High data capacity Higher when combined

with other multiplexing methods

Enhanced security

Capacity limited by atmospheric turbulence

Unable to use in long haul fiber communication systems

Mode coupling in waveguide Wong, KLGG, et al. 2012. "Quantization of

orbital angular momentum in helically twisted photonic crystal fiber." Frontiers in Optics.

Applications Deep space and near-Earth

optical communications

LAN to LAN connections in campus / city area network

Djordjevic, I. B. (2011). Deep-space and near-Earth optical communications by coded orbital angular momentum (OAM) modulation. Opt. Express, 19

Interesting videos on OAM Talk by Miles Padgett at SPIE Photonic West: Miles J.

Padgett Photonics West 2013 plenary talk: Light in a Twist: Optical Angular Momentum http://www.youtube.com/watch?v=5yyTEUYY8o8

Short introductory on OAM and optical tweezers: Miles Padgett: Optical tweezers and twisted beams of light http://www.youtube.com/watch?v=2hdKXMRKSY8