Overview of laser communication technology at NASA Goddard Space Flight Center

Overview of laser communication technology at NASA Goddard Space Flight Center

William L. Hayden, Michael A.Krainak, Donald M. Cornwell, Jr. Anthony W. Yu Xiaoli Sun

NASA Goddard Space Flight Center Hughes SiX Corp. Johns Hopkins UniversityMC 721 .2 Lanham, Maryland Electrical Engineering Department

Greenbelt, Maryland 20771 20706 Baltimore, Maryland 21218

ABSTRACT

A 650 Mbps geosynchronous free space optical communications crosslink system is describedwhich is based on the proposed TDRSS II crosslink. System analysis and technology development for adirect detection digital optical crosslink is presented for the A1GaAs and InGaAs semiconductor lasers andfor the Nd:YAG/Nd:YLF solid state lasers. Laser power requirements for each of these technologieswere calculated based on Si avalanche photodiode detectors, 30 cm transmit and receive apertures, and ahigh-performance pointing system. These average power requirements are 0.56 watts for the AlGaAswavelengths, 1.49 watts for the InGaAs wavelengths, and 1.64 watts for the Nd:YAG/Nd:YLFwavelengths which are well within the current state of the art presented in the paper. Aperture versuslaser power parametrics are also presented and compared to recent work in Nd:YAG homodyne PSK.The A1GaAs approach is shown to need only 3 dB more laser power than the P5K approach.

1. INTRODUCHON

The present NASA satellite communications system is called the Tracking and Data Relay SateffiteSystem (TDRSS)1. TDRSS uses microwave technology for space to ground (White Sands, NM) andgeosynchronous (GEO) satellite to low earth orbiting (LEO) vehicles or platforms. NASA isinvestigating an upgrade to the TDRSS network called TDRSS ll. The TDRSS II transponder requiressufficient bandwidth to support 650 Mbps data transmission. An intersatellite GEO to GEOcommunications crosslink has been proposed for the TDRSS IP. A microwave communications system(Ka/Ku bands) is currently planned as the solution for the TDRSS II intersatellite link requirement. Inthis paper, we present research on the system analysis and the technology development required forproviding a baseband digital optical crosslink using the TDRSS II crosslink requirements. This conceptis shown in Figure 1.

2. SYSTEM DESCRIPTION AND ANALYSIS

The systems analysis presented in this paper is focused on the pointing acquisition and trackingsubsystem performance and the communications subsystem performance, both of which are the salientfundamental differences between the microwave system and the laser system. The performancerequirements for these two subsystems were derived from the mission requirements specified in section 1in combination with what we considered state-of-the-art for lasers (described in section 3), receivers(described in section 4), and precision pointing devices. Link budgets for AlGaAs laser technology, forInGaAs laser technology, and for Nd:YAGINd:YLF laser technology are given in Tables 1-3. Thebaseband digital format was chosen for (1) its ability to regenerate the original digital data and thusimprove the end-to-end TDRSS performance; and (2) its simplicity in both modulating the lasers(especially semiconductor lasers) and in direct detection demodulation.

The TDRSS II crosslink requirement3 is for 650 Megabits per second (Mbps) with additional low-rate data in the return direction (user to ground); and 100 Mbps also with low-rate data in the forwarddirection (ground to user). The TDRSS II constellation baseline is for an East satellite service, a West

0819440934/93/$6.OO SPIE Vol. 1866/45

satellite service and a backside (out-of-view:OOV) satellite service. This baseline has these three relayservices separated by approximately 1200 in longitude. One of the in-view satellites (East or West) willserve as the relay for the OOV and is designated as the in-view relay (IVR). This allows for an evendistribution of services and closes the zone of exclusion (ZOE) currently over the Indian Ocean.

The optical crosslink, because of its very narrow transmitter beam divergence, must deal withproblems unique to this technology. The transit time delay from the transmitter to the receiver creates anangular difference (in this TDRSS II scenario) of 36 microradians (irad) between the receiver line-of-sight (LOS) and the transmiuer beam. This is several times larger than the main lobe of the transmitterbeam (typically 5 to 10 p.rad) at the divergence angle where the optical intensity is reduced by e2. This'tpoint-ahead" angle must be accommodated in the terminal hardware. Similarly, the spacecraft attitudeuncertainty (typically lO) is magnitudes larger than this beam divergence and is handled by the open-loop acquisition process. The position uncertainty adds to the open-loop pointing error during acquisitionbut is insignificant compared to the attitude uncertainty. The attitude error is the dominant error in theacquisition uncertainty field of regard.

A general design for this type of optical terminal is illustrated in Figure 2. It is a two-channel(each at 325 Mbps), full-duplex design using wavelength and/or polarization discrimination for each ofthe transmitter channels and each of the receiver channels. The pointing, acquisition and trackingsubsystem (PATS) is composed of three functions: (1) the gimbaled telescope providing coarse trackingand a wide-field of regard; (2) the fine steering mechanism providing wide-band, precision line-of-sight(LOS) tracking; and (3) the point-ahead mechanism (here shown as a Risley prism pair) which providesthe correct alignment between the receiver LOS and the transmitter beam. A wide field-of-view (FOV)acquisition detector, a narrow field-of-view (FOV) track detector, and an alignment monitor for point-ahead control are associated with the PATS mechanisms. A beam diverger (for spoiling the beam) and anacquisition/communication mechanism are used for acquisition as shown in Figure 2. The track laser isan independent track beacon that is partially retro-reflected (via the cube corner) back into the receiver forcalibrating the alignment monitor to the track detector boresight.

Once the on-orbit terminals have acquired each other by way of the acquisition sequence, theymust precision track to better than the transmitter beam divergence. This is implemented by closed-loopboresight tracking of each terminal by the other terminal. The criteria for precision fine tracking is that thecombined tracking detector noise-equivalent-angle4 (NEA) and the uncompensated base motiondisturbance5 are smaller than the transmitter beam divergence. Thus, the root of the sum of the squares(RSS) of these two values must be � 0. 1 of the beam divergence to avoid deep fades in the receivedsignal power. State of the art fme tracking systems6 have performances of —O.5 to 1 .0 prad rrns. Thus, a5 irad transmitter beam is considered the narrowest practical beam divergence. Another trackingrequirement is that the point-ahead angle also be properly implemented to � 0. 1 of the beam divergence.The sources of point-ahead error are: (1) point-ahead command generation, resolution, linearity and drift;(2) internal alignment resolution, linearity and drift; and (3) gimbal and point-ahead mechanism frictionand non-linearities. The tracking error and the point-ahead error represent the rms jitter and bias,respectively, for the tracking statistics.

Besides the known receiver advantages7, the pulse-position modulation (PPM) format providesadvantages for the laser transmitter in direct detection systems. Historically, laser diodes have been peakpower limited from one to two times their average power making high-order PPM inefficient. Recently,the high-power laser diodes and the master-oscillator/power amplifier (MOPA) technology have changedthis. These devices can be operated at a constant average power with duty cycles of �25%. With thiscapability, 4-slot or quaternary PPM (Q-PPM) can be used to reduce the transmitter average powerrequirements. This format places a pulse within one of four slots within a word frame regardless of thedata. It is desirable to ac couple the receiver to block strong dark and background signals as well as avoidthe 1/f noise spectrum. A long string of zero's or one's will not change the laser duty cycle or shift the

46/SPIE Vol. 1866

data spectrum. Thus, for direct detection systems using intensity modulation, these PPM properties arewell matched to the laser device operating parameters for extending laser reliability and achievingoptimum communications performance. Given that the laser is average power limited (at least up to a25% duty cycle), Q-.PPM requires -3 dB fewer photons per bit and thus requires -3 dB less averagetransmitter power for a given BER over binary PPM (Manchester). The BPPM and Q-PPM each have thesame pulse width for a given data rate thus the receiver bandwidth is the same and consequently thereceiver noise is the same, so there is no change to the receiver noise characteristics8.

The analyses in this paper are based upon Q-PPM and direct detection at three wavelengths:AlGaAs at 860 nm, InGaAs at 980 nm, and Nd:YAG at 1064 nm. These transmitter technologies willbe discussed in section three. The transmitter aperture gain was calculated using the method in Reference9. To minimize the transmitter power and to maximize the receiver gain, a 30 cm (12") aperture waschosen as a reasonable upper bound for all wavelengths. However, since the fine tracking systemperformance is limited, as discussed above, the transmitter feed beam diameter was selected to keep thedivergence above —5 p.rad. The system wavefront error was chosen to be X/1O and the wavefront losswas calculated by the Strehl ratio loss. The required signal power for acquisition was determined from aprobability of detection = 0.98 and a probability of false alarm = 0.001. The required signal power fortracking was determined from a detector NEA of 5 prad for coarse tracking and 0.35 prad for finetracking. The required signal power for a i06 bit error rate (BER) communication was determined usingthe method in Reference 10. The links have a 3 dB margin that includes a 3 dB end-of-life systemdegradation. The detector parameters for acquisition and tracking are given in Table 4 and the detectorparameters for communications are given in Table 5. Note, in Table 4, the 10 Hz bandwidth shown foracquisition and coarse tracking, and 1000 Hz bandwidth for fine tracking. Note, in Table 5, that thetotal 650 Mbps is divided into the two separate 325 Mbps channels. This was chosen to be consistentwith the integrated receiver and transmitter Q-PPM electronics11'12.

To assess the effects of pointing bias and jitter and to select an aperture size and laser powerconsistent with current technology, some trade analyses were performed. Figure 3 gives this parametricanalysis for the A1GaAs laser wavelength (the InGaAs and Nd:YAG are not shown). It is clear from thisgraph that in order to stay below 1 watt per channel, the aperture must be > 20 cm and the bias and jittererrors must be � 0.5 irad, as stated above. The transmitter beam divergence was kept smaller than 10times the jitter which results in the 1 trad bias/jitter case having a break point at 1 5 cm. This break pointon the 0.5 .trad case is near 30 cm.

As shown in Table 1 , the A1GaAs technology resulted in the least required transmitter power ofthe three technologies. The average power for the sum of both of the 325 MHz channels is 0.56 watts.The higher performance of the A1GaAs technology is due to the shorter wavelength and the high detectorquantum efficiency which provides a calculated receiver sensitivity of 70 photons/bit. Note that thevalues shown in the budgets are peak power per channel. The average power is derived by doubling fortwo channels and then dividing by a factor of four for the 25% Q-PPM duty cycle.

The InGaAs link budget is given in Table 2. The longer wavelength compared to AlGaAsreduces both the aperture efficiency and the silicon avalanche detector quantum efficiency which results inmore required transmitter power for the link and a calculated receiver sensitivity of 210 photons/bit.Other detector materials were explored but they also have poor quantum efficiencies and low avalanchegain at this wavelength thus making silicon the best choice.

The Nd:YAG link budget is given in Table 3. The transmitter technologies discussed below arethe Nd:YAG (1064 nm) and the Nd:YLF (1047 nm) needed for duplex operation. The link budget wascalculated for Nd:YAG because it has the longer of the two wavelengths and thus has slightly poorerperformance resulting in a calculated receiver sensitivity of 252 photons/bit.

SPIE Vol. 1866/47

Recent excellent work in optical coherent PSK13 reports a 13 photon/bit sensitivity at i06 BER atthe 1064 nm wavelength. Figure 4 is a comparison of average power requirements for the PSK systemand the A1GaAs direct detection system discussed in this paper. Three losses were included that were notrelevant to the direct detection system: (1) The receiver must also be near diffraction-limited so an X/1OStrehi ratio loss (-1 .7 1 dB) is added; (2) the reported case used a fiber optic receiver so a free-space tofiber coupling loss (-1 dB) is added; and (3) since E-field spatial matching is critical, a mode matchingefficiency loss (-1 dB) is also added.14'15 The effects of transmitter pointing are the same as for the 0.86micron Q-PPM case. From the curve, only ''3 dB more average power is required for the Q-PPM directdetection design given the assumed parameters.

3. LASER TRANSMJTI'ER TECHNOLOGY

The laser transmitter for the free space optical communication system described in the first sectionhas several requirements. These include having near diffraction limited optical beam quality, operating at>1.0 W average power, being mechanically robust enough to withstand the launch environment (>15 Gon each axis), having high electrical to optical conversion efficiency (>5%), being capable of 325 Mbpsrate Q-PPM modulation, operating at a wavelength that is compatible with high sensitivity high bandwidthdetectors, having a long operating lifetime (multiple years). The rapid advancement of high powersemiconductor laser technology leads to a system trade analysis for various laser technologies. We areinvestigating three different master oscillator power amplifier (MOPA) laser technologies (1)semiconductor aluminum gallium arsenide (A1GaAs) (2) semiconductor indium gallium arsenide(lnGaAs) and, (3) Nd doped crystals with semiconductor laser pumps and semiconductor laser masteroscillator injection.

Commercial fundamental mode (TEMiyj) AlGaAs semiconductor lasers are available at 150 mWaverage power16. Individual device wavelengths can be selected in the 8 10-860 nm region. Anengineering model of a 1W MOPA using discrete components (a 150 mW master oscillator and asemiconductor broad area amplifier) is being developed following previous success with the laboratoryprototype17. The prototype was modulated at 325 Mbps using the Q-PPM format and has produced>0.8W of diffraction limited power using first generation semiconductor broad area amplifiers.Improved broad area amplifiers18 indicate that 10 W average powers are achievable. The best mechanicalstability would come from a fully monolithic MOPA on a chip. In conjunction with other USGovernment agencies, NASA-GSFC has developed a 1W 860 nm monolithic MOPA under contract.This device was delivered shortly before the time of this writing and no test results are yet available. Dueto the progress of strained laser quantum well material growth, InGaAs commercial devices are nowavailable at the 1 W average power level19. These monolithic MOPA devices operate in the 910-1000 nmwavelength regime. No communications modulation results have been reported for these commercialdevices. Alexander et al. 20 reported excellent results on an InGaAs discrete MOPA laser transmitter in acoherent communications experiment.

A third approach to the laser transmitter uses a diode pumped Nd amplifier. Baer21 reportedresults of a MOPA system using a Nd laser modulated by a commercially available electroopticmodulator as the master oscillator. We have constructed a similar all solid state MOPA system using acommercially available 1047 nm semiconductor laser diode22 master oscillator and a modified commerciallaser diode pumped Nd:YLF optical power amplifier. We have achieved over 1 Gbps modulation rateswith > 0.6 W of average power in a near diffraction limited beam23. The greatest advantage of thissystem is that very high optical powers (10-100 W or larger) can be attained by cascading several diodepumped Nd doped host amplifier stages.

48 / SPIE Vol. 1866

4. OPTICAL DETECTOR AND RECEWERTECHNOLOGY

The receiver sensitivity required by free space communications systems is typically much higherthan receiver sensitivities in fiber optic systems. In addition, most fiber optic systems have beenoptimized at the 1.3 or 1.5 micron wavelengths where fiber attenuation is low. Shorter wavelengths arepreferred for free space communications because the required antenna gains can be met with smallertelescopes and high sensitivity silicon avalanche photodetectors can be used at wavelengths less than 900nm. Unfortunately, high bandwidth, high sensitivity, avalanche photodiodes and preamplifiercombinations are not readily available. NASA-GSFC developed a low noise avalanche photodiode(APD) and 900 MHz bandwidth preamplifier hybrid integrated circuit using the EG&G super lowionization k-factor (SLIK) APD and an Anadigic preamplifier. Using this optical receiver, we achieved adirect detection receiver sensitivity of less than 84 photons/bit at iø6 bit error rate at the 325 Mbps datarate at the 820 nm wavelength using the Q-PPM modulation format.

Conventional silicon APD detectors have low quantum efficiency at the near 1 micron Ndcompatible wavelengths. A commercial silicon APD, enhanced for operation at 900-1 100 nmwavelengths, was used for communications experiments we conducted at the 1 .047 micron wavelength.At 50 Mbps, a sensitivity of 200 photons/bit was achieved at i06 bit error rate using a 1047 nm laserdiode transmitter.24 At data rates less than 50 Mbps, the 1.047 mm systems are very competitive with the800-860 nm systems. It is easy to see the utility of the laser diode master oscillator diode pumped Nddoped optical power amplifier laser transmitter system described earlier. However, using a 1 micronwavelength direct detection system at data rates higher than 50 Mbps is difficult due to the lack ofwavelength compatible high sensitivity detectors with bandwidths >1 00 MHz. In contrast,Wondernoth13 achieved impressive results using the phase shift keying (PSK) modulation format andhomodyne receiver techniques.

5. OTHER TECHNOLOGY

A complete set of 650 Mbps Q-PPM transceiver electronics is being developed by NASA - LewisResearch Center12.

6. CONCLUSION

Technology development and system analysis for a geosynchronous free space opticalcommunications crosslink system is being conducted at the NASA Goddard Space Flight Center. Thechosen optical approach is a direct detection baseband digital optical crosslink at 650 Mbps which usedthe proposed TDRSS II system to set design goals. It is our hope that this type of system would bedeployed in the NASA network early in the twenty first century.

7. ACKNOWLEDGMENTS

Many of the ideas presented in this paper were the result of long term efforts by Dr. MichaelFitzmaurice. This work was supported by Dr. Ramon DePaula of NASA/HQ Code RS.

SPIEVo!. 1866/49

7. REFERENCES

1 . D. Elwell, et a!, "The tracking and data relay satellite system: An historical perspective",AIAA 14th International Communication Satellite Systems Conference, March 1992.

2. A. Comberiate, "User services in the TDRSS II era", AIAA 14th InternationalCommunication Satellite Systems Conference, March 1992, Paper AIAA-92-1885

3. P. Heffernan, et al, "Implementation of future service growth as TDRS II intersatellitelink (ISL) and UHF capabilities", NASA/GSFC white paper #405-WP-TDRS 11-001, 1992.

4. M. Ross, et al, "Space optical communications with the Nd:YAG laser", Proceedings ofthe IEEE Transactions, Vol. 66 (3), pp. 319-344, March 1978.

5. W. Hayden, et al, "Wide-band precision two-axis beam steerer tracking servo designand test results", SPIE Free-Space Laser Communications V, Vol. 1866, January 1993.

6. E. A. Swanson, J. K. Roberge, "Design considerations and experimental results for directdetection spatial tracking systems", Optical Engineering 28 (6) 659, June 1989.

7. R. Gagliardi, S. Karp, Optical Communications., R. E. Krieger Publisher, 1988.

8. F.M. Davidson, X. Sun, M.A. Krainak, "Bandwidth requirements for direct detectionoptical communication receivers with PPM signaling", Free-Space Laser CommunicationTechnologies III, SPIE Vol. 1417, Los Angeles, January 1991.

9. B. Klein, J. Degnan, "Optical antenna gain. 1 : Transmitting antennas", Applied Optics,13 (9) Sept. 1974, pp. 2134-2141.

10. C. Chen, C. Gardner, "Impact of random pointing and tracking errors and the design ofcoherent and incoherent optical intersatellite communication links", IEEE Transactionson Communications, 37 (3) pp. 252-260, March 1989.

11. W.L. Hayden, et a!, "NASA's flight-technology development program: A 650-Mbit/slaser communications testbed", SPIE Free-Space Laser Communication Technologies III,Vol. 1417, pp. 182-199, January 1991.

12. J.M Budinger, et a!, "Quaternary pulse position modulation electronics for free spacelaser communications", AIAA Conference on Advanced SEI Technologies. Paper AIAA-91-3471, September 4-6, 1991, Cleveland, Ohio.

13. B. Wandernoth, "20 Photon/bit 565 Mbit/s PSK homodyne receiver usingsynchronization bits", Electronics Letters, 28 (4) Feb. 13, 1992, pp. 387-388.

14. K. Winick, P. Kumar, "Spatial mode matching efficiencies for heterodyned GaA1Assemiconductor lasers", Journal of Lightwave Technology 6 (4) April 1988, pp. 513-520.

50/SPIEVo!. 1866

15. R. Gross, P. Meissner, E. Patzak, 'Theoretical investigation of local oscillator intensitynoise in optical homodyne systems", Journal of Lightwave Technology, 6 (4) April 1988,pp. 521-530.

16. Spectra Diode Laboratory model 5420 data sheet

17. D. M. Cornwell, "AM and FM response of an AlGaAs high power semiconductormaster oscillator broad area power amplifier (MOPA) " CLEO '92 Paper CTHI12.

18. L. Goldberg, D. Mehuys, "High power broad area diode amplifiers and applications".IEEE LEOS Proceedings 1992. Paper DLTA11.1

19. SDL Model 5760 monolithic MOPA laser diode data sheet

20. S. B. Alexander, et al, "1 Gbit/s coherent optical communication system using a 1 Woptical power amplifier". Electronics Letters 29 (1) 114. 1993.

21. T.M. Baer, et a!, "Performance of diode-pumped Nd:YAG and Nd:YLF lasers in atightly folded resonator configuration," IEEE burn. Quant. Elect.. 28, pp. 1131-1138. 1992.

22. EG&G Optoelectronics Model C86125E 1060 nm Single Quantum Well CW Lasers datasheet.

23. A.W. Yu, M.A. Krainak, G.L. Unger, "1047 nm lasr diode master oscillator Nd:YLFpower amplifier laser system". Submitted for publication to Electronics Letters.

24. K.R. Baker, A.W. Yu, M.A. Krainak, "Direct detection free space opticalcommunications link using a 1 micron wavelength laser diode transmitter", IEEEPhotonics Technology Letters. February 1993.

SP1EVo!. 1866/51

LINK PARAMETERS VALUE ACO JE TRACK COMM

TRANSMITTER PARAMETERSTRANSMIT POWER (dBW)TRANSMISSION EFFICIENCY

TRANSMIT ANTENNA GAIN (dBi)

WAVEFRONT ERROR LOSS

FINE TRACK BEACON, PER CHANNEL FOR COMM 2

5O/ OPTICS; 70% EOL DEGRADATiON30 cm TELESCOPE, 20% SECONDARY

18cm FEEDBEAM.5 irad BIAS 8 JITTER

JiO RMS

-2.47-4.5676.90

-1 .71

-2.47-4.5&76.90

-1.71

-25.084.56

1 1 6.7

1 .71

.534.56

116.7

-1.71CHANNEL PARAMETERS

RANGE LOSSBACKGROUND

75,000 kmSTARS FOR ACO & COARSE TRACK;MOON FOR FINE TRACK & COMM

-300.80 -300.80 -300.80 -300.80

RECEIVER PARAMETERSRECEIVER ANTENNA GAIN (dBi)TRANSMISSION EFFICIENCY

30 cm TELESCOPE, 20% OBSCURATION50% OPTICS; 70% EOL DEGRADATiON

120.62-4.56

120.62-4.56

120.62-4.56

120.62-4.56

RECEIVED POWER (dBW) PER CHANNEL FOR COMM -116.59 -116.59 -99.39 -73.78

REQUIRED RECEIVER POWER (dBW) PER CHANNEL FOR COMM; 16' BER 1 9.59 -1 22.55 -102.39 -76.78

MARGIN (END-OF-LIFE) +3.00 +5.96 +3.00 +3.00

1 . Average of all transmitter lasers for acqUisition and coarse track peak power, and only the track laser for fine track peak power.2. Two 325 Mbps QPPM encoded channels; Peak power listed.3. All values in dBW or dB, as relevant.

Table 1 - A1GaAs TDRSS II link budgets

LINK PARAMETERS VALUE ACQFINE

COMM

TRANSMITTER PARAMETERSTRANSMIT POWERTRANSMISSION EFFICIENCY

TRANSMIT ANTENNA GAIN (dBi)

WAVEFRONTERRORLOSS

FINE TRACK BEACON, PER CHANNEL FOR COMM2

5% OPTiCS; 70% EOL DEGRADATION

3o cm TELESCOPE, 20% SECONDARY22cm FEEDBEAM.5 uad BIAS & JITTER

.J1oRMS

-4.5676.39

-1.71

i-4.5676.39

-1.71

-21 .754.56

1 16.7

-1.71

4.73-4.561 16.7

-1.71

CHANNEL PARAMETERSRANGE LOSSBACKGROUND

75,000 kmSTARS FOR ACO & COARSE TRACK:MOON FOR FINE TRACK & COMM

-299 66. -299 66. 29 6. 299 66.

RECEIVER PARAMETERSRECEIVER ANTENNA GAIN (dBi)TRANSMISSION EFFICIENCY

30 cm TELESCOPE, 20% OBSCURATiON50% OPTICS; 70% EOL DEGRADATION

1 1 9.48-4.56

119.48-4.56

1 1 9.48-4.56

119.48-4.56

RECEIVED POWER (dBW) PER CHANNEL FOR COMM -1 1289 -1 12.89 -96.30 -69.58

REQUIRED RECEIVER POWER (dBW) PER CHANNEL FOR COMM; iO' BER . 15.89 -1 18.85 -99.30 -72.58

MARGIN (END-OF-LIFE)+3.00 +5.96 +3.00 +3.00

1 . Average of all transmitter lasers for acqUisition and coarse track peak power, and only the track laser for fIne track peak power.

2. Two 325 Mbps QPPM encoded channels; Peak power listed.3. All values in dBW or dB, as relevant.

Table 2. InGaAs TDRSS II link budgets

52/SPIEVo!. 1866

LINK PARAMETERS VALUE ACQ COARSETRACK COMM

TRANSMITTER PARAMETERS

TRANSMITPOWER (dBW)1TRANSMISSION EFFICIENCYTRANSMIT ANTENNA GAIN (dBi)

WAVEFRONTERRORLOSS

FINETRACKBEACON, PERCHANNELFORCOMM2

50% OPTiCS 70% EOL DEGRADATION3o cm TELESCOPE, 20% SECONDARY

27.9 cm FEED BEAM.5 irad BIAS & JITTER

?1ORM5

2.17-4.5676.39

-1.71

2.17-45676.39

-1.71

-21.40-4.561 16.7

-1.71

517-4.561 16.7

-1.71CHANNEL PARAMETERS

RANGE LOSSBACKGROUND

75,000 kmSTARS FOR ACO & COARSE TRACK;MOON FOR FiNE TRACK & COMM

-298 95. -298.95 2 . 2 .

RECEIVER PARAMETERSRECEIVER ANTENNA GAIN (dBi)TRANSMISSION EFFICIENCY

30 cm TELESCOPE, 20% OBSCURATION50% OPTICS; 70% EOL DEGRADATION

1 1 8.77-4.56

1 1 8.77

-4.561 18.77-4.56

118.77-4.56

RECEIVED POWER (dBW) PER CHANNEL FOR COMM -1 12.45 -112.45 -95.69 -69.14

REOUIREDRECEIVERPOWER(dBW) PERCHANNELFORCOMM; 1O BER -11545 -118.42 -98.71 -72.14

MARGIN (END-OF.LIFE) +3.00 +5.97 +3.00 +3.00

1 . Average of all transmitter lasers for aoquisition and coarse track peak power, and only the track laser for fine track peak power.2. Two 325 Mbps QPPM encoded channels; Peak power listed.3. All values in dBW or dB, as relevant.

Table 3. Nd:YAG TDRSS II link budgets

TRACK DETECTOR PARAMETERS AIGaAs InGaAs Nd:YAGDETECFOR TYPE QAPD QAPD QAPDNOISE FACTOR, db 165 3.65 3.65QUANTUMEFFICIENCY,% 80 30 25NOISE EQUIVALENTPOWER, watts/'IHz 0.36 x 10-14 2.2 x 10-14 2.43 x i014PREAMP NOISE C

pAt'IHzURRENT DENSITY, 0.05 0.05 0.05

NOISE EQUIVALENTBANDWIDTH, Hz 10/1000 10/1000 10/1000PROCESSING LOSS, db -1 -1 -1

Table 4. Acquisiton and track detector parameters

COMM DETECTOR PARAMETERS A1GaAs InGaAs Nd:YAGDETECTOR TYPE APD APD APDNOISE FACTOR, db 4.45 4.45 4.45QUANTUM EFFICIENCY, % 90 30 25NOISE EQUIVALENTPOWER, wattsNHz 0.48 io-15 1.26 x iO'5 1.39 x iO15PREAMP NOISE C

DAPIHzURRENT DENSITY, 5.0 5.0 5.0

NOISEEQUIVALENT BANDWIDTH, MHz 325 325 325PROCESSING LOSS, db -1 -1 -1

Table 5. Communicationsdetector parameters

SPIEVoI. 1866/53

54/SPIE Vol. 1866

In View GEO

Out of View GEOGEO-LEOOptical Link LEO spacecraft

Figure 1. NASA TDRSS II optical communications links being researched at GSFC

Figure 2. TDRSS II optical terminal concept design

APERTURE SIZE, METERS

Figure 3. TDRSS II average laser power vs telescope diameter

APERTURE SIZE, METERS

Figure 4. Average laser power comparison of coherent PSK and Q-PPM

SPIEVo!. 1866/55

LINK PARAMETERS:70 photons/bIt @ 10-6 BER0-PPM 325 Mbps75,000 km 3 dB margin3 dB optics loss 3 dB EOL link

degradationI .71 dB wavefront error loss

10

P... —Blas=Jltter=O— —— — Bias =Jltter =0.5 mlcrorad

- - -Bias =Jer =1 mlcrorad

i ; i I

0.10 0.15 0.20 0.25 0.30

10

1:1

T:T%;T,T:..s::— 8:l=

r MICRORAD BIAS & JifTER ERROR

0.10.10 0.15 0.20 0.25 0.30