Contains research objectives and reports on three research projects.National Science Foundation (Grant ENG78-21603)U.S. Army Research Office - Durham (Contract DAAG29-90-C-0010)U.S. Navy - Office of Naval Research (Contract N00014-80-C-0941)U.S. Army Research Office - Durham (Contract DAAG29-80-K-0022

Kennedy, Robert S.

Papurt, David M.

Rediker, Robert H.

Schloss, Robert P.

Shapiro, Jeffrey H.

Warde, Cardinal

DSpace@MIT

COMMUNICATION SCIENCES AND ENGINEERINGXXI. OPTICAL PROPAGATION AND COMMUNICATIONAcademic and Research StaffProf. R.S. Kennedy Prof. J.H. Shapiro Prof. C. WardeProf. R.H. Rediker Dr. H.P.H. YuenGraduate StudentsP.L. Bogler W.P. Jaeger D.M. PapurtR.S. Bondurant J. Nakai W.S. RossJ.J. Fratamico T.T. Nguyen R.P. SchlossThe broad objectives of our programs are to determine the fundamental limitson the detection and communication performance that can be realized with importantoptical channels and to identify, and establish the feasibility of, techniquesand devices which can be used to achieve these limits.1. IMPROVED LOW-VISIBILITY COMMUNICATIONNational Science Foundation (Grant ENG78-21603)U.S. Army Research Office - Durham (Contract DAAG29-90-C-0010)Robert S. Kennedy, Jeffrey H. Shapiro, Cardinal WardeThis program, part of which is carried out jointly with MIT's Center forMaterials Science and Engineering (CMSE), concerns the problem of line-of-sightatmospheric optical communications under low-visibility weather conditions. Duringthe past year substantial progress has been made in our studies of optical propa-gation in turbid atmospheres,1-4 communication through optical scattering chan-nels,556 and devices and techniques for space-distributed optical-phase compensa-tion. 6-8 The sections that follow provide an overview of our recent researchresults in the first two of the preceding areas; the work on phase compensationresides in CMSE.Our principal objectives for next year are (a) to begin communication-typeexperiments over our 7.4-km propagation range, and (b) continue to refine andextend our ability to do space-distributed phase compensation. The communication-PR No. 123 139(XXI. OPTICAL PROPAGATION AND COMMUNICATION)type experiments will initially employ a frequency-doubled Nd:YAG laser but willevolve towards more realistic systems that might be used in practice. The secondaspect of next year's program will entail further experimentation with interfer-ence phase loop systems that employ the microchannel spatial light modulator.8' 9This work will also include a more thorough examination of the limit set by atmo-spheric coherence time on the utility of phase compensation systems.6a. Propagation StudiesDuring the past year we have made the transition, in our atmospheric propa-gation studies, from exploratory propagation measurements to the development andverification of approximate theories to predict the behavior of the low-visibilityatmospheric channel. In particular, we have focused our effort on two theories,viz., the multiple-forward scatter (MFS) and the strong multiple-scatter (SMS)approaches. Both theories originate in experimentally observed channel behavior.The MFS approach (first suggested by Mooradian et al.10) is an ad hoc small-angleapproximation based on a truncated single-scatter phase function. We have devel-oped this model to the point where it can provide '5 simple yet realistic predic-tions for the attenuation, beam spread, angular spread and multipath spread en-countered in a line-of-sight low-visibility communication link; a coherence timeprediction has also been obtained from this model, but no experimental validationis available as yet. We have also explored the incorporation of a diffusionlikepropagation mode into the MFS model so as to have a natural decay of MFS behaviorat very large optical thickness.?The second theory we have pursued, the SMS approach, relies for its founda-tion on the experimentally observed insensitivity of off-axis received power to3inhomogeneities in the distribution of scatterers. At present, the SMS model islimited to point sources. It provides, in its present form, an analytical assess-ment of the transition to diffusion-mode propagation.For the next year we do not anticipate a major thrust in the propagation area.We will be working on the coherence time question, as it bears on the utility ofphase compensation; we may also continuell an effort begun earlierl2 to reformu-late the multiple-scattering problem in a manner that leads to more explicit re-sults with relatively weak assumptions.PR No. 123 140(XXI. OPTICAL PROPAGATION AND COMMUNICATION)b. Communication StudiesThe essential communication-theory features of the low-visibility channelwere established some time ago. 13 ,14 These studies predate the development ofrealistic propagation models so they are based on ad hoc received-field statisti-cal models. Nevertheless, the early work established two vital principles: thatbackground-light suppression is the key to reliable low-visibility channel opticalcommunication, and that adaptive phase compensation is an important option forachieving this suppression. Lately, we have begun reassessing the communicationtheory work based on the predictions of the MFS channel model. 5'6 We have foundthat background-light suppression is indeed vital to maintaining daytime linkoperability in bad weather, and have considered the merits of the three genericapproaches (development of narrow-band wide-angle filters, operation at mid-ultra-violet wavelengths, and the use of adaptive phase compensation) for achieving sig-nificant background suppression.For the next year, our communication studies will be aimed at establishinga demonstration link to explore the experimental viability of some of the receiverstructures we have analyzed. Theoretical work on the use of error-correctingcodes for the low-visibility channel is also anticipated.References1. J. Nakai, "A Spatio-Temporal Channel Model of Low-Visibility Optical WavePropagation," S.M. Thesis, Department of Electrical Engineering and ComputerScience, M.I.T., January 1980.2. T.T. Nguyen, "A Combined Forward-Scatter and Diffusion Analysis of Low-Visibility Optical Propagation," S,M. Thesis, Department of Electrical Engi-neering and Computer Science, M.I.T., August 1980.3. W.S. Ross, "Point-Source Optical Propagation in a Multiple Scattering Medium,"Ph.D. Thesis, Department of Electrical Engineering and Computer Science,M.I.T., August 1980.4. W.S. Ross, W.P. Jaeger, J. Nakai, T.T. Nguyen, and J.H. Shapiro, "AtmosphericOptical Propagation: An Integrated Approach," to appear in Opt. Engr.5. J.H. Shapiro, "Optical Communications through Low-Visibility Weather Condi-tions," 1980 National Telecommun. Conf. Record, IEEE, New York (1980),pp. 27.6.1-27.6.6.PR No. 123 141(XXI. OPTICAL PROPAGATION AND COMMUNICATION)6. J.H. Shapiro and C. Warde, "Optical Communication through Low-VisibilityWeather," Opt. Engr. 20, 76-83 (1981).7. C. Warde, A.M. Weiss, and A.D. Fisher, "LiNb0 3 and LiTaO3 Michrochannel Spa-tial Light Modulators," Proc. SPIE 218, 59-66 (1980).8. A.D. Fisher, "High-Resolution Adaptive Phase Compensation," Ph.D. Thesis,Department of Electrical Engineering and Computer Science, M.I.T., November1980.9. C. Warde and A.D. Fisher, "Adaptive Phase Compensation for Low-VisibilityOptical Communication," Proc. SPIE 209, 144-151 (1979).10. G.C. Mooradian, M. Geller, L.B. Stotts, D.H. Stephens, and R.A. Krautwald,"Blue-Green Pulsed Propagation through Fog," Appl. Opt. 18, 429-441 (1979).11. P.L. Bogler, unpublished Ph.D. thesis research, Department of ElectricalEngineering and Computer Science, M.I.T. (1980).12. D.C. Simmons, "An Investigation of the Reciprocity of a Scattering Atmo-sphere," S.M. Thesis, Department of Electrical Engineering and ComputerScience, M.I.T., September 1978.13. R.S. Kennedy, "Communications through Optical Scattering Channels: An Intro-duction," Proc. IEEE 58, 1651-1665 (1970).14. S.R. Robinson, "Phase Compensation Receivers for Optical Communication,"IEEE Trans. Commun., COM-25, 900-909 (1977).15. C. Warde, A.M. Weiss, A.D. Fisher, and J.I. Thackara, "Optical InformationProcessing Characteristics of the Microchannel Spatial Light Modulator,"submitted to Appl. Opt.2. FIBER-COUPLED EXTERNAL-CAVITY SEMICONDUCTOR HIGH-POWER LASERU.S. Navy - Office of Naval Research (Contract N00014-80-C-0941)Robert H. Rediker, Robert P. SchlossThis program has as its goal the demonstration of the feasibility of cou-pling low-power semiconductor lasers in parallel to produce a high-average-powercoherent laser beam. Such a laser system would combine the reliability of diodelasers with the higher power and monochromaticity required for several communi-cations applications. In the proposed system, optical fibers from the individuallow-power semiconductor lasers are gathered together to act as a source for theexternal cavity with an appropriate spatial filter to assure that coherence isestablished over the entire optical bundle emitter.PR No. 123 142(XXI. OPTICAL PROPAGATION AND COMMUNICATION)In the first phase of this program, the coupling of diode laser radiationinto, its propagation through, and its outcoupling from fibers are being studied.Experiments are under way to combine a diode laser and a fiber inside an externalcavity.3. ATMOSPHERIC PROPAGATION EFFECTS ON INFRARED RADARSU.S. Army Research Office - Durham (Contract DAAG29-80-K-0022)Jeffrey H. Shapiro, David M. PapurtThis program is aimed at obtaining a quantitative understanding of the effectsof atmospheric propagation and target reflection characteristics on the perfor-mance of compact CO2 laser heterodyne-reception radars. Under a collaborationarrangement with the Optics Division of the M.I.T. Lincoln Laboratory, the experi-mental portions of the research are to be carried out on the compact infrared radarunder development there.Since the inception of this program, progress has been made in the followingareas:(i) Initial clear-weather infrared radar measurements have been collectedwith simultaneous scintillation and seeing data over a 1-km path;(ii) An analysis of the relative merits of linear and logarithmic frameintegration of speckle and scintillation-limited imagery has been carried out;2(iii) A doctoral thesis aimed at extending the models to include the3multiple-scattering effects present in bad weather conditions has been initiated.The research plan for the next year involves: (1) reduction and evaluationof initial clear-weather (turbulence) data; (2) development of computer programsto simulate radar imagery; (3) additional data collection in a variety of tur-bulence conditions.References1. J.H. Shapiro, "Linear vs Logarithmic Frame-Integration for Coherent LaserRadars," to be presented at the 1981 IEEE International Symposium on Informa-tion Theory, Santa Monica, CA, 1981.PR No. 123 143(XXI. OPTICAL PROPAGATION AND COMMUNICATION)2. J.H. Shapiro, "Imaging and Target Detection with a Heterodyne-Reception Opti-cal Radar," Project Report TST-24, M.I.T. Lincoln Laboratory (Oct. 13, 1978).3. D.M. Papurt, Ph.D. thesis research, Department of Electrical Engineering andComputer Science, M.I.T., Fall 1980.PR No. 123 144

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