Optical Communication Slides

8/8/2019 Optical Communication Slides

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Optics/Optical Communication

theories and techniques

Mark Kenneth E. Alonzo


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Free-space opticalcommunications systems for

satellite-to-ground and deep space communications

have been proposed, studied, and even implemented

in laboratory demonstration systems for more than 30

years. Nevertheless, few of these systems have actually

been deployed aboard spacecraft. Even though most of

the technical problems associated with opticalcommu-

nications systems have been solved, advances in micro-

wave sources and high-speed electronics have main-

tained traditional RF communications systems as the

technology of choice for space-based communications

system designers.


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This situation is now changing as a

result of several factors: ever-increasing requirementsfor high data rate (hundreds to thousands of Mbps)

communications, significant investments by NASA

and DoD, and significant advances in the telecom-

munications technology for fiber-optic communicationcomponents, including fiber amplifiers, fiber lasers, and

sensitive receivers. These components may be appli-

cable, in some form, to free-space optical communica-

tions systems for use in space.1 Compact beamsteeringtechnology; very fine pointing, tracking, and stabiliza-

tion control; and ultra-lightweight antennas are also

critical technologies.


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The motivation for transitioning to optical com-

munications for deep space applications, apart from

pragmatic considerations such as the increasing need

to acquire more scientific data and real-time imagery,

is fundamentally dependent on the wavelength of

light relative to RF bands currently used. As will be

described in the section on the link equation, which

governs all communications links (both RF and opti-

cal), there are three terms that explicitly depend on

wavelength: the transmitter antenna gain, the space

loss, and the receiver antenna gain. When these fac-

tors are combined, for equal antenna sizes, the advan-

tages of shorter (optical) wavelengths become obvious:

the received signal goes inversely as the square of the

wavelength.


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This is mitigated by the fact that optical

antennas are not always as big as RF antennas, and they

require greater precision to make and point because of

the shortness of the wavelength and the narrowness of

the opticalbeam.

On the other hand, having smaller antennas (as

well as other components) can be a weight advan-tage for optics. Optical modulation bandwidths are

also wider because for similar modulation electron-

ics, which are roughly equally limited in their rela-

tive (normalized) bandwidths between RF and optical

modulators, higher (optical) frequencies directly implywider bandwidths. This becomes compromised by the

fact that, for wider bandwidths (as in optics), in-band

noise energy is necessarily greater, thus potentially

reducing the signal-to-noise ratio (SNR) foroptical

receivers.


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. The key requirement foroptical communi-

cations development for space applications is to sup-

port science mission data retrieval at higher rates than

heretofore possible with RF systems for space

missions

as far out as interstellar space and all the way in to

near-Earth orbit (NEO) or geosynchronous Earth orbit(GEO) distances.


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Forms of optical communicationEarly optical communication

There are many forms of non-technological optical

communication, including body language and sign language.

Techniques such as semaphore lines, ship flags, smoke signals,

and beacon fires were the earliest form of technologicaloptical communication.

The heliograph uses a mirror to reflect sunlight to a distant

observer. By moving the mirror the distant observer sees

flashes of light that can be used to send a prearrangedsignaling code. Navy ships often use a signal lamp to signal in

Morse code in a similar way.


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Distress flares are used by mariners in emergencies, while

lighthouses and navigation lights are used to communicate

navigation hazards.

Aircraft use the landing lights at airports to land safely, especially at

night. Aircraft landing on an aircraft carrier use a similar system toland correctly on the carrier deck. The light systems communicate

the correct position of the aircraft relative to the best landing

glideslope. Also, many control towers still have an Aldis lamp to

communicate with planes whose radio failed.

Optical fiber is the most common medium for modern digital optical

communication.

Free-space optical communication is also used today in a variety of

applications.

Contd of Forms of optical communication


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Optics: Ultra-lightweight Large-

aperture CassegrainTelescope


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The Cassegrain is the most common system for the modernobservatory. It is available in several optical variations. The flatNewtonian diagonal is replaced with a secondary mirror with aconvex surface. Light is reflected back through a hole in theprimary mirror.

A Cassegrain telescope has thefollowing advantages:a) The tube length is compact.b) The focal plane and hence

instrumentation is readilyaccessible.There are several basicoptical variations.


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Classical CassegrainThe original design invented by French sculptor SieurCassegrain employed a parabolic primary and ahyperbolic secondary mirror. The field of view of theclassical Cassegrain is rather small. The diameter ofthe secondary mirror is also small. Typical focal ratiosare f/12 to f/15. For systems faster than about f/10 thecoma in produced by a classical Cassegrain will beseveral times worse than a Newtonian of the samefocal ratio.


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The goal of our strategic vision is to develop all the

critical components of an optical communications ter-

minal for spacecraft use in which its mass is kept to a

minimum and its data rate falls within the range requiredto give a significant gain over an RF system, as summarized

in Fig. 1. The Mars Laser Communication Demonstration,

as currently envisioned, has a goal of 30 Mbps from Mars,

but under frequent daylight receiver background conditionsit will be much lower than this and just barely competitive

with the RF downlink data rate, which is at best a few Mbps.


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The pointing accuracy required to support a given

mission is driven primarily by diffraction consistent

with the desired link margin. For a meter-class aper-

ture, this is a little less than 400 nrad as assumed inTable 2. These two factors, receiver sensitivity at the

Earth terminal and pointing accuracy at the space-

craft, present the greatest technical challenge to

future development and successful implementationof optical communications for deep space.

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