intersatellite laser crosslinks

Intersatellite Laser Crosslinks

JOHN E. MULHOLLAND, Senior Member, IEEEVillanova University

SEAN ANTHONY CADOGANMartin Marietta Corp.

Intersatellite laser crosslinks (ISL) provide a method

of communication that has significantly increased the data

throughput that can be managed over typical RF communication

systems, and has significant growth potential. Optical

communications offer very wide bandwidths which can be

effectively utilized with wavelength division multiplexing

techniques. The data rate growth potential is well beyond the few

gigabit per second range of RF technology. The use of lasers in

transmitting optical data takes advantage of its small wavelength

and low beam divergence to send highly directed signals over

significant distances with controlled losses in intensity. The

high directivity of the laser aids in resistance to jamming

communications between satellites, or between satellites and

ground stations.

Various intersatellite laser optical crosslink systems are

discussed including the Massachusetts Institute of Technology’s

Laser Intersatellite Transmission Experiment (LITE), the

McDonnell Douglas Electronic Systems Company Laser

Crosslink System, and The Ball Aerospace Optical Intersatellite

Link, in order to display the various subsystems and their

implementations. Link budget calculations are performed on the

most commonly used modulation formats to determine system

parameters necessary to close the crosslink.

Background is provided on primal system architectures and

methods of laser communication, as well as presently implemented

systems. The authors provide some insights on where ISL systems

have opportunity to increase their data throughput and reduce

acquisition time.

Manuscript received June 18, 1994; revised March 27, 1995.

IEEE Log No. T-AES/32/3/05872.

Authors’ addresses: J. E. Mulholland, Dept. of Electrical andComputer Engineering, Villanova University, Villanova, PA19085-1681; S. A. Cadogan, Martin Marietta Corp., Managementand Data Systems, King of Prussia, PA.

0018-9251/96/$10.00 c° 1996 IEEE

I. INTRODUCTION

Intersatellite laser crosslinks (ISLs) provide amethod of communication that has significantlyincreased the data throughput that can be managedover typical RF communication systems. The data rategrowth potential is well beyond the few gigabit persecond range of RF technology. The use of lasers intransmitting optical data takes advantage of its smallwavelength and low beam divergence.The ISL is subdivided into five major subsystems.

The transmitter is typically a semiconductor laser, orlaser diode. The receiver has a design very dependenton the method of communication, and transmitterconstruction. The acquisition subsystem is responsiblefor aligning the transmitter and receiver to preparefor communication. The tracking subsystem mustmaintain the link with the stability necessary to allowfor reliable data transmission. The communicationsubsystem is responsible for encoding and decodingthe data to be sent between satellites.The RF atmospheric coefficient of attenuation is

very low, which results in RF signals slowly losingstrength in the atmosphere and can therefore travellong distances, including over the horizon. On thecontrary, laser signals are highly directional, permitlarge bandwidths, and are attenuated to a significantextent by the atmosphere. This, in addition to the factthat they are line-of-sight [1], causes some importantdesign problems that must be addressed.Various ISL systems are discussed in order to

display the various subsystems which comprise a lasercrosslink, and their implementations. Discussion onthe strengths of laser communications is provided, andrelated to RF technology.Background is provided on earlier system

architectures and methods of laser communication,as well as presently implemented systems. Optical linkbudget calculations are performed for various methodsof communications. The author provides some insightson where intersatellite laser optical crosslink systemshave opportunity to increase their data throughput andreduce acquisition time.

II. INTERSATELLITE LASER CROSSLINKS

A. Why Satellite?

McDonnell Douglas Electronic Systems Company(MDC) was chosen by the U.S. government in 1981to bring laser communications into production bydeveloping a satellite-to-satellite crosslink. The systemwas to be installed on an already existing satellite.Therefore to minimize any impact to the satellite, thelaser crosslink needed to be a stand-alone, bolt-onpackage, which provided terminal control, a despunline of sight, and could operate from raw spacecraft

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996 1011

power [2]. The profitability of communications bysatellite becomes evident when reviewing the keyfeatures of the MDC laser crosslink subsystem:1) reduction of the reliance on foreign ground stations,2) survivability, 3) jam resistance, 4) low probability ofdata intercept, and 5) field of view (FOV) limited onlyby gimbal location relative to the sensor.With one centralized U.S. ground station which

takes inputs from multiple satellites, the dependenceon multiple foreign ground stations is greatly reduced.This alleviates the time delays and increased factors oferror associated with the distributive nature of multipleforeign ground stations. Survivability is especiallyimportant in times of natural disaster, war, or otherevents which can be detrimental to low altitudecommunication devices (i.e., air craft systems) andground-station-to-ground-station communications. Jamresistance and low probability of error, features of theMDC laser crosslink system, are results of the narrowbeamwidth used. The high altitude of the satellitesleads to a much more expansive FOV which is onlylimited by the gimbal location relative to the sensor.

B. Why Optical?

The Optical Communications Group at M.I.T.Lincoln Laboratory has been investigating anddeveloping the technologies required to make highto very high data rate optical intersatellite crosslinka reality for over ten years. According to Boroson[3], optical communications allows the use ofcomparatively small antenna (telescope) packagesbecause of its very short wavelength. RF technology,even in the upper EHF region over 60 GHz, requiresantenna apertures on the order of several feet indiameter to support links with capabilities of morethan a few tens of megabits per second. Fig. 1compares the package apertures for 40,000 km linkswhich quantifies the difference in aperture size for RFversus optical communications at various data rates.With the utilization of satellites, special attention mustbe taken to payload constraints on size and weightadded by the communications subsystem.Optical communications also offers very

wide bandwidths, especially when utilizingwavelength-division multiplexing techniques. RFtechnology, on the contrary, does not have data rategrowth potential beyond a few gigabits per second,especially in a network where frequency reuse may notbe possible.

C. Why Laser?

The development of laser communication beganat MDC in the late 1960s under both U.S. Air Forceand company sponsorship. Laser communicationat short wavelengths theoretically holds a great

Fig. 1. Synchronous range crosslink aperture.

advantage over the best achievable communicationin the RF spectrum. The extremely low beamdivergence minimizes signal loss and a narrow receiverFOV makes it extremely difficult to jam. The shortwavelength of lasers offers the opportunity to modulateat very high data rates. Laser communication offers1) low probability of data intercept, 2) jam resistance,and 3) high bandwidth capabilities.The highly directional nature of lasercom makes

it difficult to intercept and jam communication. Thehigh directivity arises from the short wavelengths ofvisible and nearly infrared energy. Lasercom sidelobesare also generally much lower than RF or millimeterwave sidelobes, resulting in an inherent resistanceto interception or jamming [1]. There are manyconstraints which must be taken into account whenchoosing a laser subsystem. Some of these constraintsare identified in a later section, which discusses thetransmitter of a laser crosslink system.

III. SYSTEMS APPROACH

A. General Parameters

There are many parameters which the systemdesigner must consider in the development of an ISL.For instance, in order to get maximum use of therelatively low power of the laser diodes, the designermust pay particular attention to beam pointingand tracking, wavefront quality, package rigidity,point-ahead accuracy, and maintenance of theseproperties through the temperature and vibrationalextremes of the lifetime of a satellite. In order toarrive at a successful lasercom design, all of theseconstraints must be fulfilled simultaneously in a fullsystem context. The lasercom should be compact,lightweight, and have a relatively simple packagedesign as a result of the solution with these constraints.

1012 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996

Fig. 2. LITE engineering model block diagram.

It must be noted that there are many different waysof configuring an ISL, and along with this, differentsystem parameters which must be considered.In general, the total weight of the transmitter,

receiver, acquisition, tracking and communicationssubsystems should be within the range of 200—300 lb.It was also noted that state of the art systems transmitat approximately 300 Mbit/s. Using Fig. 1, for a datarate on the order of 100 Mbit/s, a 0.1 W laser requiresan aperture diameter of about 0.4 ft, and a 1 W laserof only about 0.2 ft.

B. Transmitter

There are many considerations in designing atransmitter. The laser used must not only be powerfulenough to transmit the necessary beam over aspecified distance, but it must pass a screening testdesigned to select lasers with acceptable operatingtemperature, narrow linewidths, acceptable opticalproperties, reasonable FM responses, and prospectsfor long life [4]. A laser must qualify for spaceusage in a satellite crosslink system. For examplegas lasers (i.e., He-Ne) are not practical in spacedue to their relatively low efficiency and large size.Inability to maintain uniformity of the vapor in thedischarge region has ruled out metal vapors suchas Zn, Hg, Sn, and Pb which have displayed lasertransition in the visible spectrum [11]. Therefore asolid state or semiconductor laser is the device ofchoice. Semiconductor lasers, particularly the GaAlAsfamily, are good candidates for the laser sourcein a heterodyne system. Semiconductor lasers arecompact, have high power conversion efficiency (withprime-to-optical output power conversion efficiencybetween 20% and 50%), architectural simplicity,and utilize single-frequency operation. As a part ofthe Laser Intersatellite Transmission Experiment(LITE) project [3], M.I.T. has built many laboratorycommunications links based on commercially available30 mW GaAlAs lasers with wavelengths between 0.83and 0.86 ¹m. These lasers were adequate for crosslinksin the 100 Mbit/s class (see Fig. 2).

Mass, prime power, and volume estimatesfor reliable ISL payloads were performed in atelecommunications system that provides a full-duplexinterconnection of three wideband transpondersbetween two spacecraft separated by 60 deg alongthe geostationary arc by R. Marshalek of the BallAerospace Systems Group and D. Paul of COMSATLaboratories [5]. The following conclusions related totransmitter laser choice were made.

1) The CO2 system demands excessive laserredundancy and large payload mass to support a 10-yrhigh reliability (0.9) mission.2) Redundancy increases payload weight by about

20 to 30 kg int he Nd:YAG, InGaAsP, and GaAlAssystems.3) GaAlAs systems entail lower payload mass

and prime power, and are recommended for atelecommunications ISL.

C. Receiver

Many different types of receivers can be utilizedin the lasercom system. Some of these receiversare introduced in a general nature. In general, thereceiver or detector must be able to transform lightinto electrical signals. Many, but not all, have someamount of built-in gain to better detect the incomingsignal. Depending on the operating wavelength, therewill be different materials used. The receiver is alsoapplication dependent. If direct modulation is used asa communications method, a detector and amplifier isneeded. If a synchronous detection method (such asRF links) is employed, a local oscillator (laser) mustbe used. This is the heterodyne case. The detectorof choice for Nd:YAG and GaAs wavelengths is theavalanche photodiode (APD). An APD can be usedin the tracking and communication phase, which isdiscussed in a later section.The system impact of resonant laser receivers for

free-space laser communications has been studied,and the major advantage of the resonant receiverdesign approach is that it enables laser communicationlink closure for many applications, by using anavailable 14 cm aperture and existing compact diodelaser sources (for acquisition and high-data-ratecommunication). The problem of having to close thecommunication link with a reasonably sized aperturehas now been circumvented. This alleviates theproblem that previously existed with the traditionaldirect-detection approach to laser communicationswith a reliable, high-power high-beam-quality (Strehlratio) transmitter that controls system mass andbeam-pointing requirements [6]. The resonant receiverdesign approach also has immunity to large opticalbackground interference, while not overloadingrequirements on transmitter frequency, thermalstability, or receiver frequency tracking that increase

MULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1013

the complexity of the alternative heterodyne-detectionapproach. For most parameters, the resonant receiverrequirements lie between those for direct detectionand heterodyne detection. The resonant receiverapproach attacks those key areas that have been majordrivers on system reliability, performance, and cost byoffering a balanced design approach to long-distancehigh-data-rate laser communications.For digital traffic, the full bandwidth, direct,

and heterodyne-detection GaAlAs systems entailcomparable mass, power, and volume. However,for analog traffic, the GaAlAs heterodyne-detectionsystem is superior because it uses far less massand volume. The major reason that the GaAlAsheterodyne-detection system is so successful for analogtraffic is that it efficiently accommodates the threemultiplexed communication transponders with adirect-optical-carrier-frequency modulation technique[7]. The LITE engineering model at M.I.T. utilizes asemiconductor coherent (heterodyne) detection whichallows for nearly quantum-limited performance withsensitivity better than that of direct detection at allbut the very lowest data rates. Heterodyne detectionalso allows operation with a bright object, such as thesun, in the FOV; whereas, direct detection systems aresignificantly degraded.

D. Acquisition and Tracking

Acquisition refers to the process in which thereceiving satellite determines where the incomingbeam sent by the transmitting satellite is located.Bridging a 42,000 km link with the very narrowbeamwidth of a laser poses a serious design problem,however, multiple sequential methods of acquisitionare discussed.One goal for laser communication is the reduction

in acquisition time and the improvement of acquisitiontechniques. The relation between the beamwidth ofthe transmitted beam, the receiver’s FOV, and themaximum time it takes for acquisition is well displayedin Fig. 3. This figure plots curves of maximumacquisition time against azimuth uncertainty angle (forconstant elevation uncertainty) for a number of beamsize and FOV combinations. The curve on the far leftindicates that the acquisition time may be more thanfive min for a 0.5 deg initiator beam and a one degresponder FOV. An examination of the curves towardsthe right of Fig. 6 indicates that short search timescan be implemented over much larger volumes ofuncertainty if the FOV of the detector and the beamdivergence of the initiator are large enough. It shouldbe noted that for wide initiator beam divergences, highpower lasers must be employed in order to close thelink.For mutual acquisition to occur, each satellite must

reduce its initial knowledge of the opposing satellite’slocation to values compatible with fine tracking

Fig. 3. Acquisition time.

and communications. Initially, there is a large ratiobetween the initial angular uncertainty and the narrowbeam divergences in the tracking and communicationlinks to conserve the limited laser power.The Laser Crosslink Subsystem (LCS) of

McDonnell Douglas Electronic Systems Companyuses the direct pulse detection technique, andtherefore their acquisition algorithm is differentfrom one using coherent (heterodyne) detection.A 100¹ rad acquisition beam is initially scannedover the region of uncertainty. The pulse rate of thelaser is reduced during acquisition to provide higherpeak pulse power required to compensate for theexpanded beam divergence. When each LCS terminaldetects illumination from the opposite terminal, thepointing converges and scan fields are reduced inorder to increase scan frequency. This continuesuntil pointing accuracies are sufficient to supportcommunications. The scans are then suspended andeach LCS transitions to 10¹ rad communications beampointing and data transmission [2].

E. Tracking and Maintaining Links

Tracking refers to the process in which thesatellites maintain their communication links. In theLITE system the high bandwidth steering mirror(HBSM) also correctly points the transmitted beamto the other terminal as it keeps the incomingbeacon signal centered on the tracking detector.This allows the compensation of pointing variationscaused by spacecraft motion and vibration. Oncethe laser transmitter is set up and stabilized, and thebeam-steering system has completed the bore-sightingprocedure (alignment of transmit and receive beams),LITE is ready to acquire and track the incoming signal.Once the signal is acquired the beam is narrowedwhich increases its power. When the other terminal


senses this, it sends the signal over to its trackingreceiver, narrows its beacon and points it towardLITE. LITE senses this increased power and increasesthe tracking bandwidth to 1 KHz to improve trackingperformance. After all acquisition is complete, thecommunications session can begin [4].Fine tracking allows the use of narrow

communication beams for high-data-rate transferbetween the two satellites, simultaneously maintainingpoint-induced burst communication errors atacceptably low levels. A burst error rate of 10¡5 to10¡6 is acceptable and is achievable with a 1 ¾ trackingaccuracy of about one-twentieth of the null-to-nulltransmit beamwidth. The spectral content of thesatellite platform disturbance errors determines thetrack detector update rate (bandwidth).

F. Communications

There are different methods of modulation of thelaser beam which can be used to send informationin the beam. In the beginning of lasercom, directmodulation was the only option available. Informationwas sent via the duration of pulses of laser power.Now the lasers can be modulated like RF carriers (i.e.,frequency or even phase modulation, see [5]).The communications subsystem is composed of

five major parts in the Ball Aerospace design; thelaser/modulator, detector/local oscillator, LO laser(heterodyne system), communication electronics,processing electronics, and passive optics.The data format has to be constructed according to

the nature of the data. For pulse-position modulation(PPM), digital modulators are required [7], whileon—off keying (OOK) systems are usually implementedusing scramblers, or forward error correction coding inorder to improve the correlation properties of the datasignal [8]. In order to recover the signal and regeneratethe information sent, PPM maximum likelihoodreceivers require a symbol clock recovery circuit.For OOK systems the amplitudes are regenerated bythreshold decision. The synchronization requires aphase-locked loop triggered by data transitions [11].A quadrant detector composed of four APDs splitat a focal plane by a pyramid, light pipes, or fibersmay also be used as a communications detector (3 nsrise and fall) in addition to a track detector, if thequadrant outputs are summed. Duchmann and Plancheindicate that in their communications system, thereceive function consists of a low noise APD-baseddirect detection of the incoming signal followed bya non-return-to-zero (NRZ) regeneration of thebaseband electrical signal [9].In fiber-based state of the art heterodyne receivers,

continuous phase frequency-shift keying (FSK),or differentially encoded phase-shift keying (PSK)modulation is used. The detection principle consistsof the active mixing of a local oscillator signal and the

receive signal in conjunction with envelope detectionas in RF systems [8]. The key components are transmitand local oscillator lasers, optical input couplers, andhigh bandwidth electronics.

IV. LINK BUDGET

A. SNR and Data Errors

The final performance of the system dependson the signal-to-noise ratio (SNR). Noise in an RFsystem is usually thermal noise, or device noise. Inlink budget calculations for an RF communicationlink the carrier is an electromagnetic wave. Noise in anoptical system consists of thermal, as well as quantumnoise generated by asynchronous photon plinking insignals, because for an optical communication system,the carrier is a photon.In free-space laser optic communications, the link

budget is defined by an allowable bit error rate (BER).The acceptable BER commonly used in analysisof optical links is 10¡9. From this BER, a SNR isdetermined. These two factors, in conjunction withthe range of transmission, are utilized in choosingthe transmitter and receiver. If the digital trafficis received with a 10¡9 BER it corresponds to aSNR of 16.2 dB for quadrature PSK (QPSK) trafficand 17 dB for baseband digital traffic, including a1.4 dB modem implementation margin in both cases[10]. The analog traffic requires a 17 dB SNR at thereceiver output. For the receiver, when determiningthe receiver specifications, the modulation format, aswell as the detection scheme must be considered. Thenecessary SNR must also be taken into account. Thesensitivity of the receiver is measured as number ofdetected photons per bit (at peak power) necessary toachieve a BER of 10¡9. Once the receiver sensitivityis known, the amount of power needed from thetransmitter must be determined. There are variousother noise-inducing factors, such as differences intemperature throughout the atmosphere significant tocause a perceptible change in the index of refractionpresented to a laser beam as it passes through. Thiscan result in beam broadening, tearing and steeringof portions of the beam, causing fades and surges inthe optical beam as a result of variations in powerdensity. The probability of bit error is thereforedramatically increased. Atmospheric turbulence, andpointing inaccuracies are other factors which canintroduce bit errors and degrade the performance ofa communications link.

B. Link Budget Calculations

The link budget is a numerical calculation thatproves link closure. It is used to determine whetherthe SNR is high enough for data to be successfullytransferred. Link budget calculations determine system


TABLE IOptical Intersatellite Link System Parameters

performance for communication configurations bytrading off such parameters as aperture, transmitterpower, and data rate. This section shows the designand definition of the communications link. Opticalpower budget calculations are performed in six systemsto determine antenna diameter requirements as afunction of average transmitter power. The systemsare: 1) carbon dioxide laser system with heterodynedetection, 2) neodymium-doped laser system withdirect detection, as in the LCS laser of McDonnellDouglas, which utilizes solid state GaAs diodes topump a Nd :YAG rod, 3) In GaAsP laser systemwith direct detection, 4) GaAlAs laser system withdirect detection, 5) GaAlAsP laser system withwavelength division multiplexing and direct detection,and 6) GaAlAs laser system with heterodyne detection.These systems have been previously introduced inearlier sections which discuss transmitter and receiveroptions.Typical parameter values are used throughout

this discussion in order to determine the antennarequirement for each of the six systems as a functionof optical transmitter power. Table I gives the systemparameters used in the link budget calculationsfor the different optical intersatellite link systems.These calculations were performed for each of themodulation formats discussed, and a link margin of5 dB was assumed in all cases. The results for the CO2and AgAlAs systems are displayed in Figs. 4 and 5.

Fig. 4. Antenna diameter requirements for CO2 system.

The Nd-doped system was evaluated uner mode-lockedconditions. Results for the other systems are similarto the GaAlAs System. It must also be noted thatthe actual average transmitter power for the analog


TABLE IIAntenna Diameter Requirements for Baseband Digital Transmission (360-Mbit/s Total Throughput With 10-9 BER) Over 42,000 km

Range

Note: *Optical power of each transmitter.

Fig. 5. Antenna diameter requirements for GaAlAs system.

formats is 0.75 times the value read from the graph[11].Anticipated transmitter power levels in an ISL

were estimated to compare the six systems, and thecorresponding antenna diameters were then obtainedfor transmission of three baseband digital transpondersignals. Table II gives the modulation formats, powerlevels, and calculated diameters of these basebandsignals. Table III provides a similar comparison for thetransmission of three QPSK transponder signals. TheCO2 and Nd systems require the smallest diametersfor transmission of baseband digital signals but usemore complicated modulation techniques and lessefficient transmitters than systems based on GaAlAsheterodyne detection system, although difficult to

engineer, requires a substantial reduction in averagetransmitter power than the nonheterodyne case.The CO2 heterodyne, GaAlAs WDM, and GaAlAsheterodyne systems require the smallest antennas foranalog transmission. Since the WDM system is veryreliable and simpler to implement, it is preferred inshort-term applications. With technological advancesin GaAlAs heterodyne systems it will become thepreferred choice for analog transmission of threeseparate transponder signals [11].

V. CONCLUSIONS

Different methods of laser beam modulation havebeen used over the years to send information. In theearly days of lasercom, direct modulation was usedwhere the laser was turned on and off just as Morsecode signals were used. The speed of modulationhad to be checked as well. Now the lasers can bemodulated like RF carriers (i.e., frequency or evenphase modulation).The range in data rate from tens of kilobits to

tens of megabits was previously exclusively coveredby Nd :YAG lasers modulated with an M-ary PPMformat. Now, new pulsed diode arrays are capable ofoperating with PPM modulation with peak powersof tens of watts at megabit data rates. State of the artsystems now transmit at approximately 300 Mbit/s.It is the authors’ opinion that there are several

methods which can be incorporated into theacquisition and tracking phase, as well as thecommunications phase to improve system performance.Sections VA and VB discuss these proposed systemenhancements.

A. Acquisition and Tracking Enhancements

The narrow beamwidth of the transmitted opticalbeam presents design difficulties in the actualacquisition of the signal. To broaden the beam andsend it out from the laser calls for much more power


TABLE IIIAntenna Diameter Requirements for Transmission of Three 72 MHz QPSK Transponders Over 42,000 km Range

Note: *Optical power of each transmitter.**In CW operations.

than the laser may be able to provide if the sameamount of intensity is to be sent over 42,000 km links.The authors propose using the fact that in the farfield (Fraunhouffer) light sent through an aperturewill be captured in the focal plane, at the receiver asthe Fourier transform of the signal. This cannot onlybe done in time and frequency but also in space andspatial frequency. A rectangular slit will result in asinx=x, and a sinx=x will result in a square pulse. Theresulting far field pattern should be chosen so that itssymmetry will facilitate finding the “center” where theactual beam will be present. It should have an areaof increased spatial area, or of spatial area significantenough to make it useful to place the initial fieldthrough an aperture. If the signal is broader, it willbe easier to find. It has been discussed that a 2-phaseacquisition phase can be used to save energy while thereceiver satellite is trying to locate the transmittingsatellite. Another suggestion by the author is to usea three-phase approach. Trades should be made todetermine if a very large, very powerful pulse orpulse sequence as an initial phase will cut down thereceivers initial field of uncertainty or FOV enough tosignificantly decrease acquisition time.

B. Communications Enhancements

It has been noted in certain systems (such as theLITE system), that redundant laser diodes are presentbut are used solely as backup when other diodes fail.They may also be used to provide the necessary powerin the case of weaker lasers. The authors suggestthat data throughput be increased by simultaneousoperation of multiple lasers in the transmitter section,to be received by an array of receivers at the receiving

terminal. Either multiple processors can be used toprocess the different incoming beams, which wouldlinearly increase payload size and weight, or a singleprocessor can take multiple inputs and process themseparately, and multiplex the results accordingly.This approach would allow certain options suchas multiple users transmitting data simultaneously,utilizing only one transmitting and one receivingsatellite without concern for their data becomingavailable to other users (particularly important inpersonal communications and proprietary or securecommunications). Due to the short wavelength ofoptical systems, it has been noted that there is ahigh degree of directivity. Careful attention must bepaid so that once the acquisition signal is receivedand the system switches to communication beamsthat the beam divergence is not wide enough toallow interference between the various incomingcommunication signals.The Optical Communications Technology group

at Lincoln Laboratory believes that the technologyis available for deployment of operational lasercommunication systems in the several hundredmegabits per second range, with near term technologyto be able to support multipke gigabits per secondlinks in small and reliable packages [3].

C. Receiver Enhancements

In recently developed low-effective k silicon APDs(k = 0:002 to 0.005, depending on wavelength), asensitivity of 68 photons per bit at a BER of 10¡9

has been measured on a direct-detection receiverdeveloped using a lser diode (¸= 810 nm) with anextinction ratio of 0.02 [13].


REFERENCES

[1] Casey,W. L., Doughty, G. R., Marston, R. K., andMuhonen, J.(1990)Design considerations for air-to-air laser communications.In SPIE Proceedings, 1417, Los Angeles, CA, 21-2, 1990.

[2] Deadrick, R. B., and Deckelman, W. F. (1992)Laser crosslink subsystem–An overview.SPIE, Vol. 1635, Los Angeles, CA, Jan. 23—24, 1992.

[3] Boroson, D. M.An overview of Lincoln Laboratory development oflasercom technologies for space.MIT Lincoln Laboratory.

[4] Marshalek, R. G., and Paul, D. K. (1990)Mass, prime power, and volume estimates for reliableoptical intersatellite link payloads.In SPIE Proceedings, 1218, Los Angeles, CA, Jan. 15—17,1990.

[5] Marshalek, R. G., Smith, R. J., and Begley, D. L. (1992)System impact of the resonant laser receiver for free-spacelaser communications.SPIE Proceedings, 1635, Los Angeles, CA, Jan. 23—24,1992.

[6] Borner, S., and Heicher, J. (1989)4-PPM modulator/demodulator with fully digital signalregeneration.In SPIE Proceedings, 1131 (1989), 195.

[7] Noldeke, C. (1992)Survey of optical communication system technology forfree-space transmission.In SPIE Proceedings, 1635, Los Angeles, CA, Jan. 23—24,1992.

[8] Duchmann, O., and Planche, G. (1991)How to meet intersatellite links mission requirements byan adequate optical terminal design.In SPIE Proceedings, 1417, Los Angeles, CA, Jan. 21—22,1991.

[9] Feher, K. (1983)Baseband transmission systems and power efficientmodulation techniques for linear and nonlinear satellitechannels.Digital Communications: Satellite/Earth StationEngineering.Englewood Cliffs, NJ: Prentice-Hall, 1983.

[10] Marshalek, R. G., and Koepf, G. A. (1988)Comparison of optical technologies for intersatellite linksin a global telecommunications network.Optical Engineering, 27, 1 (Aug. 8, 1988).

[11] McIntyre, R. J. (1991)Comments on performance of coherent optical receivers.Proceedings of the IEEE, 79, 7 (July 1991), 1080—1082.

[12] Boroson, D. M. (1993)LITE engineering model–I: Operation and performanceof the communications and bean-control subsystem.In SPIE Proceedings, 1866, Los Angeles, CA, Jan. 1993.

[13] Hect, E. (1987)Optics (2nd ed.).Reading, MA: Addison-Wesley, 1987.

[14] Pillsbury, A. D., Taylor, J. A. (1990)Optomechanical design of a space-based diode lasertransmitter assembly.In SPIE Proceedings, 1218, Los Angeles, CA, Jan. 15—17,1990.

[15] Verdeyen, J. T. (1989)Laser Electronics (2nd ed.).Englewood Cliffs, NJ: Prentice-Hall, 1989.

[16] Ross, M. (1975)Direct photodetection space laser communications.In Convention Record: Electronics and Aerospace SystemsConb., 1975, 174-I—174-H.

[17] Chan, V. W. (1983)Heterodyne lasercom systems using GaAs lasers for ISLapplications.In Conference Record: International Conference onCommunications, 1983, E1.5.1—1.5.7.


Sean A. Cadogan was born in Brooklyn, NY in 1968. He received his B.S. in electrical engineering from theMassachusetts Institute of Technology, Cambridge, in 1990, and an M.S. in electrical engineering from VillanovaUniversity, Villanova, PA, in 1993.From 1990 to 1992, he worked at General Electric Aerospace as an Edison Engineering Program member

holding positions in the Systems Integration, Systems Analysis, and Verification and Test Engineering groupsin Management and Data Systems. While in the Sensor Systems Engineering groups in Management and DataSystems. While in the Sensor Systems Engineering group he was the project leader on a study that quantifiedthe impacts of bit errors on digital processing, and the implementation of the Bose—Chaudhuri—Hocquenghem(BCH) coding algorithm to detect and correct bit errors. He is presently a Hardware Systems Engineer at MartinMarietta Aerospace, formerly GE, in Valley Forge, PA and resides in Norristown, PA.

John E. Mulholland (S’57–M’61–SM’87) received the B.E.E. degree from Villanova University, Villanova,PA, in 1960, the M.S.E.E. degree from Drexel Institute of Technology, Philadelphia, PA, in 1965, and the Ph.D.degree in electrical engineering from the University of Pennsylvania, Philadelphia, in 1969.In 1985, he joined the faculty of the Department of Electrical and Computer Engineering at Villanova

University to develop the microwave engineering technology area for both education and research. Before joiningVillanova University, he was employed at the General Electric Space Division as Manager of the CommunicationEquipment and Antenna Engineering Laboratories. His assignments have included the development ofmicrowave filter analytical techniques and the design of waveguide and directional filters and the Ku and Xfrequency bands and the development of automated RF measurement techniques for components and systems.More recently he has led the development of the interface definition of the command and control segment withthe microwave transmission segment of a major military satellite data communications system. Prior to joiningGeneral Electric, he provided consultation in radar clutter, multipath, propagation effects and radiation hazardsat the RCA Missile and Surface Radar Division. He also provided analytical support for the AN SPY-1 radar inthe areas of antenna matching, random materials, monopulse tracking collimation and alignment, and sidelobeblanking.Dr. Mulholland is a registered Professional Engineer in Pennsylvania, past Chairman of the Antenna

Propagation/Microwave Theory and Techniques (AP/MTT) Society, Philadelphia Section of IEEE.


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