MEMS Blue/Green Retroreflecting Modulators for Underwater Optical Communications

The technology, its applications and future

prospects

Presented By Niranjan T

The structure of this presentation

o The concept in briefo Underwater Communications- the scenarioo Introduction to retroreflectorso Retroflectivityo A Fabry Perot etalon - overviewo The retroflecting modulatoro Modulator constructiono Acknowledgementso Bibliography

Underwater Communication

• Dominated by RF waves, which have short propagation lengths.• Limited to tethered cables, acoustic waves or optical communication using visible wavelengths.• Low data rates and high latency.• Tethered communication only works well for stationary objects and P2P systems• For small autonomous vehicles, power and weight limitations impose severe constraints

Now:

This solution: • Up to 1Mbps data rates.• Up to 8 meters of underwater transmission.• No power source required by a power constrained system.• Power source only required at the interrogating vehicle.• No sources or associated pointing hardware at the system.• Quiet operation.

Underwater Communication

Interrogating vehicle

Retroreflecting modulator

Remote Dev1 Remote

Dev2

Remote Dev3

Retroreflectors

A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light back to its source with a minimum scattering of light.An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source.

What?

How?Many types of retroreflectors exist, and more are being invented as we discuss.The ray diagram for a simple corner reflector is shown below.

Retroreflectors

Prism type

Sphere/ Bead type

Retroreflectivity

Retroreflectors find a place in many day to day applications and are found in nature too.Where?

Like?

Retroreflecting Modulator

A modulating retro-reflector (MRR) couples or combines an optical retroreflector with a modulator to reflect modulated optical signals directly back to an optical receiver or transceiver, allowing the MRR to function as an optical communications device without emitting its own optical power.

What?

How?

• Larger bandwidth• Low probability of intercept• Immunity from interference or jamming• Frequency spectrum allocation issue relief• Smaller, lighter, lower power

Why?

A Fabry Perot etalon

In optics, a Fabry–Pérot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly reflecting mirrors. Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon.

What?

How? Phase difference between successive reflections:

Maximum transmission (Te = 1) occurs when the optical path length difference (2nlcos θ) between each transmitted beam is an integer multiple of the wavelength.

Modulator Construction

• Technology used: Bulk micromachining• Wafer 1- 17x17 mm Si sample with etched Silicon Nitride membranes• Wafer 2- 1x1 inch glass substrate with patterned mirrors.• The two wafers are bump bonded and Indium is used to provide electrical connection to Silicon Nitride layer. • 250x 250 um Aluminum layers to act as mirrors.

Modulator Transmission Response

Transmission spectrum using a tungsten lamp to illuminate themodulator with and without the voltage bias. The vertical line represents the

532 nm laser line.

Overview of the Experiment

References

William C. Cox, Kory F. Gray, Jim A. Simpson, Brandon Cochenour, Brian L. Hughes and John F. Muth, “A MEMS Blue/Green Retroreflecting Modulator for Underwater Optical Communications ”-Department of Electrical and Computer Engineering,North Carolina State University Mohd. Rizal Arshad, “Recent Advancements in Sensor Technology for underwater Applications” – Indian Journal of Marine Sciences, Sept 2009 Freespace Optics Resources at www.nrl.navy.mil Wikicommons reference on Retroreflectors.

Thank You for your attention