The current state of the art of optical communications is briefly reviewed. This review covers NASA programs, DOD and other government agency programs, commercial aerospace programs, and foreign programs. Included is a brief summary of a recent NASA workshop on optical communications. The basic conclusions from all the program reviews is that optical communications is a technology ready to be accepted but needed to be demonstrated. Probably the most advanced and sophisticated optical communications system is the Laser Intersatellite Transmission Experiment (LITE) system developed for flight on the Advanced Communications Technology Satellite (ACTS). Optical communications technology is available for the applications of data communications at data rates in the under 300 MBits/sec for nearly all applications under 2 times GEO distances. Applications for low-earth orbiter (LEO) to ground will allow data rates in the multi-GBits/sec range. Higher data rates are limited by currently available laser power. Phased array lasers offer technology which should eliminate this problem. The major problem of cloud coverage can probably be eliminated by look ahead pointing, multiple ground stations, and knowledge of weather conditions to control the pointing. Most certainly, optical communications offer a new spectral region to relieve the RF bands and very high data communications rates that will be required in less than 10 years to solve the communications problems on Earth

Hendricks, Herbert D.

English

NASA Technical Reports Server

N90-27153
BRIEF STATE-OF-THE-ART REVIEW ON OPTICAL COMMUNICATIONS FOR
THE NASA ISES WORKSHOP
Herbert D. Hendricks
NASA Langley Research Center
Hampton, Virginia
Abstract
This is a brief review of the current state of the art of optical communications.
This review covers NASA programs, DOD and other government agency programs, commer-
cial aerospace programs, and foreign programs. Included is a brief summary of a
recent NASA workshop on optical communications. The basic conclusions from all the
program reviews is that optical communications is a technology ready to be accepted
but needed to be demonstrated. Probably the most advanced and sophisticated optical
communications system is the Laser Intersatellite Transmission Experiment (LITE) sys-
tem developed for flight on the Advanced Communications Technology Satellite (ACTS).
Optical communications technology is available for the applications of data communi-
cations at data rates in the under 300 MBits/sec for nearly all applications under 2
times GEO distances. Applications for low-Earth orbiter (LEO) to ground will allow
data rates in the multi-GBits/sec range. Higher data rates are limited by currently
available laser power. Phased array lasers offer technology which should eliminate
this problem. The major problem of cloud coverage can probably be eliminated by look
ahead pointing, multiple ground stations, and knowledge of weather conditions to
control the pointing. Most certainly, optical communications offer a new spectral
region to relieve the RF bands and very high data communications rates that will be
required in less than i0 years to solve the communications problems on Earth.
Introduction
NASA interest in optical communications has centered around a number of advantages
that optical communications offers. These advantages range from a factor of i0 re-
duction in antenna size to reduced antenna weight (which also helps improve space-
craft stability and pointing problems), reduced power requirements, a new spectral
region to help elevate RF band crowding, higher data transmission rates, and more
secure data transmission. Shown in figure I is an LEO to telecommunications data
relay satellite (TDRS) link comparison of laser communications vs. Ku band communica-
tion. From this data it is obvious that optical communications offer some very major
advantages over RF communications.
Even though NASA, DOD, Europeans, and the Japanese have planned experiments,
very little has been flown in space to demonstrate the technology. Anumber of
aircraft-to-ground demonstrations have been performed. Probably the most sophisti-
cated experimental system to date was planned as the LITE project to fly on the
ACTS. However the LITE project has been scratched from ACTS. The good news is that
the technology development is continuing. The remainder of this paper will be
directed toward the identification of some of the ongoing programs and what organiza-
tions are involved as well as a summary of where optical communications stand today
and what needs to be developed for the future.
131
https://ntrs.nasa.gov/search.jsp?R=19900017837 2020-03-19T21:15:13+00:00Z
NASA Programs
The major efforts in the development, fabrication, and demonstration of optical
communications components and systems have been focused at the NASA Goddard Space
Flight Center, Langley Research Center, and the Jet Propulsion Laboratory.
Jet Propulsion Laboratory
Research and development in this area has been directed toward the Deep Space
Missions. Current work has been aimed toward the development of coherent communica-
tions systems based on diode pumping of Nd:YAG and the development of external
modulation techniques. JPL is looking to the future for a 10-year lifetime device
with a 2-Watt laser requirement with an external modulator for a 20 MBits/s data
rate.
Goddard Space Flight Center
Research and development at this center has been directed toward Direct Detection
Light Technology (DDLT) component and system development for the LITE experiment
aboard ACTS. The DDLT system would share the Air ForCe/MaSSachusetts Institute' of
Technoiogy/eincoln eaborat0ry (MIT/eL)_coherent experimen_t_pt_s_n d would_fun_ti0 n
when the coherent experiment was in the_off_mode. Optical communications experiments
would be performed between ACTS and the Goldstone, AZ site and the CSFC Greenbelt, MD
site as well as to an experimental module called Light Technology Experimental
Facility (LTEF) aboard the Shuttle. This DDLT system will utilize the 20-cm optics
of LITE, a multiple laser power summer with 200-300 milliwatts power output, and pro-
vide direct modulation of the lasers at a 220 MBits/s data rate. GSFC has performed
a number of pointing, tracking, and acquisitions systems studies and development pro-
grams. GSFC has also addressed the problem of developing long life and stable
semiconductor lasers. _
Future missions requirements at GSFC are seen to require laser power in the 1-5 watts
range and communication data rates to 4 GBits/s to provide communications for all of
the upcoming NASA missions. These laser power level and modulation rates will re-
quire further development of high power semiconductor lasers.
Langley Research_ Center _
Research _an_ deve[6pment programs at eangiey_havecentered on the development of
laser technology, optical transceivers, and lately the possible use of optical
communications with ISES.
Langley has developed and demonstrated a number of semiconductor lasers for applica-
tions to optical communications. This tec_o_logy has mainly denteredon AiGaAssemi-
conductor lasers with emission in the 50-100 mW range and direct modulation demon°
strati0ns to _ GB_6s/s. Recent _deveiopments ha_e:_e_h-aimed _toward_the utiiizat_on
of phased _array _ [as_rs_t0 demons@_-a_e°=_uqtiple w_t devices _capabie0f m0dUlation £0
5 GBits/s 0ther recent efforts have centered on t_e development and demons:tr_ti0n
of a nar row lingwidtb semiconductor laser in InGaAsP for Coherent communication
applications.
i32
A joint program with the Air Force has been completed in the area of optical
transceivers. This development provided a military family of transceivers up to
i GBits/s. A second phase of the program currently under way is addressing a high
speed transceiver which will operate up to 4 GBits/s.
Langley Research Center has proposed an ISES which utilizes optical communications to
downlink data to interactive users of Eos or Space Station data. This system design
could utilize existing 20-cm optics, tracking, pointing, and acquisition systems.
Implementation of a simple 30-mW laser would easily provide communications rates in
the I00 to I000 MBits/s communications rates. Look ahead and smart pointing systems
to alleviate cloud coverage problems and a network of recieving stations interlinked
with high speed fiber optic systems would make it a very useful system.
Other Government Agency Programs
The Air Force and Navy support most technology developments in the way of components,
subsystems, and systems. Only the coherent optical communications research and
development, by MIT/Lincoln Laboratory, part of the LITE experimental system for
flight on the ACTS, will be discussed. This project provides the most information
available for general use and does not impact DOD restrictions or provide proprietary
contractor information.
The original approach for the coherent communications experiment was to downlink to
Goldstone, AZ and GSFC at Greenbelt, MD. By utilizing coherent communications, which
should have 15 dB better sensitivity than Direct Detection Light Wave Technology
(DDLT) there would be a direct comparison of the two optical communications tech-
niques. The coherent design consisted of a 30-mW laser which could be modulated to
220 MBits/s. The laser system was quadruple redundant with an analysis and tuning
system. The overall weight was 200 pounds and needed 200 electrical watts. The
parts were to be radiation hardened and flight qualified. Perkin Elmer supplied part
of the optics and Ball Aerospace part of the point system. No current plans exist
for flight. However, a flight version of the system will be available in 1992. This
particular system has the best design information available for comparison to other
system designs. In addition to the flight system, there was also a ground system
design which used 30-cm optics.
Commerical Aerospace Programs
The major commercial companies involved in optical communications component, sub-
system, and system development are Martin Marietta, TRW, McDonnell Douglas, Ball
Aerospace, Perkin Elmer, Hughes Aircraft Company, GE Aerospace, Spectra Diode
Laboratory, David Sarnoff Research Center, and Stanford Telecommunications, Inc.
None of these company projects will be discussed because of the many and varied
proprietary restrictions. There are a number of government-funded (DOD) projects
connected to each company. They range from crosslinks, uplinks, and downlinks
systems to a variety of subsystems and component developments. The main conclusion
is that there are few major program efforts of such size as the LITE experiment for
ACTS.
133
Foreign Programs
Research and development by foreign countries of which knowledge is available is
based on the major efforts of a European project and a Japanese project.
European Program
The European project, called the Semiconductor Laser Intersatellite Link Experiment
(SILEX), consists of a contractor team made up of Matra(prime) of France, Dornier of
West Germany, Bertin of France, ANT of West Germany, Telespazio of Italy, Selenia of
Italy, and Spazio of Italy. The SILEX project consists of two GEO laser communica-
tions packages for a '92-'93 launch. This system uses wavelength multiplexed AIGaAs
semiconductors with a data relay capability of 480 MBits/s.
Japanese Program
The Japanese project is being put together by a team from the National Space Develop-
ment Agency of Japan, the Radio Research Laboratory, and the Ministry of Posts and
Telecommunications. The project is a GEO-Ground experiment with a 1992 launch and a
GEO-GEO experiment for 1996 launch. The project is built for the ETS-6 satellite and
utilizes AIGaAs Semiconductor lasers.
NASA Optical Communications Workshop
A NASA optical communications workshop, sponsored by Code RC (Dr. M. M. Sokoloski),
was held at Annapolis, Md. on March 28-29, 1989. The major purpose of the worshop
was to review optical communications technology with a view toward satisfying future
communications needs of the Global Change Technology Initiative (GCTI). A review of
the "Mission to Earth" communications requirements was presented along with a view of
potential science applications of optical communications technology. A government
and industry briefing on optical communications was presented before the workshop
sessions.
The workshopswere split into three Sessions" Laser Technology, Systems Technology,
and Science Benefits. The laser technology working group concluded that semi-
conductor lasers with good lifetime existed with power outputs up to 35 mW.
Semiconductor lasers with power outputs up to i00 mW are available but with
questionable limits on their lifetime. By power multiplexing a system of lasers with
reasonable life, 200 mW power output was achievable with modulation capability to
GBit/sec data rates. Within the next 3-5 years phased array lasers, Nd:YAG la_ers,
and Master Oscillator PowEr Amplifier lasers should offer in excess of l-watt power
ouput at high modulation rates. _ _
The Systems Tec_n01ogy Working Group f0resaw that optical communications systems at _
I MBits/s at 2 times GEO and that 220 MBit/s systems at I times GEO were currently
available. Technology needs were high power lasers/amplifiers, modular highly
integrated packaging, better acquisition and tracking Systems, and novel approaches
to reducing the weight of the optical systems. Reasonable systems are available by
1991 whereas higher data rate systems will be available for deployment in the 1996198
timeframe with technology fixed by 1991-93.
The Science Benefits workshop concluaed that global wind sensing, water vapor mea-
surements, gravity field mapping, planetary sciences and fundamental theories such as
134
gravitational wave detection would be possible through the use of optical communica-
tions systems and associated technology.
ISES Optical Communications
The ISES optical communications system could be easily achievable on an LEO mission
such as Eos or Space Station Freedom. The optical communications system could imple-
ment a direct detection systems approach and utilize a 20-cm optic system with a
30-mW AIGaAs laser which could be modulated up to a multi-Gibits/sec data rate while
maintaining a signal margin. Pointing, acquisition, and tracking systems of the type
developed by Ball Aerospace would satisfy the system's needs. Electronics of the
LITE type developed by MIT/LL would be a model for the systems. For GEO-GEO and GEO-
Ground communications, either a coherent communications system such as that built by
MIT/LL or a DDLT type design such as that built by GSFC would provide a 200-300
MBits/sec data rate. Any higher data rate requirements would require a higher power
laser which requires greater lifetime than is currently available. These types of
laser devices should be available in the 1991-92 timeframe. A ground station design
could utilize 20- to 30-cm optics depending upon data rate for communications and be
of the type being developed by MIT/LL.
The item that will mean most to the success of optical communications is the ability
to communicate to a variety of ground stations located throughout the United States.
Therefore a point ahead capability to direct the optical beam to a clear cloud free
location is imperative. This will require a computer with knowledge of the weather
pattern and a selection alogrithm to assure a clear communications path. With the
advent of expanding high data rate fiber optics communications routes by a number of
communications companies, the problem of directing data to the user is no longer a
problem.
Summary
Optical communications systems can do the job at higher data rates than achievable in
the RF domain. Near-Earth systems requirments are ready. What needs to be done is
to qualify the optical communication system for use in space. A satellite for proof
of demonstration of optical communications needs to be available.
Optical communications systems require longer ranges such as those from GEO orbits,
and data rates higher than 200 MBits/s require higher laser power. This technology
is advancing more rapidly in the past four years than previously thanks to DOD
funding. Multi-GBits/s data rates for communications are a reality for optical
communications systems. The electronics to drive and sense the signals are
available.
What needs to be done is to utilize more systems development such as in the ACTS
LITE optical communications to drive out and develop optimized systems. Lighter
weight hardware with further reduction in system power would be the major goals.
System packaging and space qualification of the hardware are also major requirements.
Adequate funding has not been directed toward solving needed flight hardware systems
demonstrations.
Optical communications is a super data rate channel for the transfer of data and
informations. It is past ready for experiments and demonstrations. What needs to
135
be done is to properly implement the available technolgy into an acceptable
demonstration to prove the viability of optical communications.
Acknowledgments
The author would like to acknowledge many helpful discussions, literature and copies
of talks supplied by the following who have made this brief Summary of optical
communications technology possible:
Dr. M. M. Sokoloski, NASA Headquaters, Code RC
Dr. M. Fitzmaurice, GSFC
Mr. L. caudi_ NASA-Headquaters I COdg_EC
Dr. Vincent thanl _4iT-Lincoln Lab dra_-£ory
Dr. Jim Lesh, Jet Propuis-ion lJaboratory
Dr. Don Carlin, David Sarnoff Research_ Center
Dr/ Don Chanin(Dav_d'Sarnoff Research Center
Dr. Mike Ettenbefg_Da_id Sarnoff Research Center
Dr. William Streiffer, Spectra Diode Laboratory
Dr. David Welch, Spectra Diode Laboratory
Dr. Joe Weller, Naval Research Laboratory
Dr. Michael Gordinfer, McD0nneii-Douglas Astronautics, Co.
Dr. Nick Pchelkin, Kirtland AFB _ _
Dr. Vernon Heinen, NASA Headquaters, Code RC
Mr. Robert Marshaiek, Ball Aerospace Systems Group
Dr. Dan Botez, TRW
Mr. Steve Mecherle, TRW
Mr. Dale Mikelson, Martin Marietta Astronautics Group
Dr. Chandrasekhar Roychoudhuri, Perkin Elmer
Dr. James Sparkman, NOAA
Mr. Alan Wissinger, Perkin Elmer Corp.
Mr. Brian Hendrickson, Griffis AFB
Dr. Tom Giallorenzi, Naval Research Laboratory
Dr. L. Goldberg, Naval Research Laboratory
Dr. steVe Katzberg, Langley Research center
Dr. Harry F. Benz, Langley Research Center
Dr. Roger Breckenridge, Langley Research Center
136
LINK COMPARISON
LASER VS. Ku BAND
LINK CHARACTERISTICS
O 300 MBPS DATA RATE
o 10E-5 BIT ERROR PROBABILITY (UNCODED)
LASER Ku BAND *
LEO TRANSMITTER POWER
LEO ANTENNA DIAMETER
TDRS ANTENNA DIAMETER
LINK MARGIN
0.25 WATTS
7 INCHES
10 INCHES
6 DB
10 WATTS
12 FEET
16 FEET
6 DB
*FROM TDRSS USER'S GUIDE, REVISION 4, JANUARY 1980
Figure I. Low Earth orbiter to TDRS link comparison of
laser communication versus Ku band communication.
137
l_ !!ii_!i_!! ¸_


Brief state-of-the-art review on optical communications for the NASA ISES workshop

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server