This document presents a retroreflecting modulator for underwater optical communications. It begins with an overview of the current challenges with underwater communication and how a retroreflecting modulator could provide improvements. It then discusses retroreflectors, how they work, and their applications. The document introduces a Fabry Perot etalon and how it relates to the retroreflecting modulator. It provides details on the construction and testing of the modulator, showing its transmission response with and without a voltage bias. In under 3 sentences, the document summarizes a new technology for underwater optical communication using a retroreflecting modulator that could provide benefits over existing methods.

1. MEMS Blue/Green
Retroreflecting Modulators for
Underwater Optical
Communications
The technology, its
applications and future
prospects
Presented By Niranjan T

2. The structure of this presentation
o The concept in brief
o Underwater Communications- the
scenario
o Introduction to retroreflectors
o Retroflectivity
o A Fabry Perot etalon - overview
o The retroflecting modulator
o Modulator construction
o Acknowledgements
o Bibliography

4. Underwater Communication
• Dominated by RF waves, which have short propagation
Now: 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
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.

5. Underwater Communication
Interrogating
vehicle
Retroreflecting
modulator
Remote Remote
Dev1 Remote Dev3
Dev2

6. Retroreflectors
A retroreflector (sometimes called a retroflector or cataphote)
is a device or surface that reflects light back to its source with a
What?
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.
Many types of retroreflectors exist, and more are being invented
How? as we discuss.
The ray diagram for a simple corner reflector is shown below.

7. Retroreflectors
Prism type
Sphere/ Bead
type

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

9. Retroreflecting Modulator
A modulating retro-reflector (MRR) couples or combines an optical
What? 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.
How?
Why? • Larger bandwidth
• Low probability of intercept
• Immunity from interference or jamming
• Frequency spectrum allocation issue relief
• Smaller, lighter, lower power

10. A Fabry Perot etalon
In optics, a Fabry–Pérot interferometer or etalon is typically made of a
What? 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.
Phase difference between successive reflections:
How?
Maximum transmission (Te = 1) occurs when the optical path length difference
(2nlcos θ) between each transmitted beam is an integer multiple of the
wavelength.

11. 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.

12. Modulator Transmission Response
Transmission spectrum using a tungsten lamp to illuminate the
modulator with and without the voltage bias. The vertical line represents the
532 nm laser line.

13. Overview of the Experiment

14. 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