Contains an introduction and reports on four research projects.Maryland Procurement Office Contract MDA 904-90-C5070Maryland Procurement Office Contract MDA 904-93-C4169U.S. Air Force - Office of Scientific Research Grant F49620-93-1-0604Charles S. Draper Laboratories Contract DL-H-441698MIT Lincoln Laboratory Contract CX-16335National Institute of Standards and Technology Grant 60-NANBOD-1052U.S. Army Research Office Grant DAAL03-90-G-0128U.S. Army Research Office Grant DAAH04-93-G-0399U.S. Army Research Office Grant DAAH04-93-G-0187U.S. Air Force - Office of Scientific Research Contract F49620-90-C-003

Brothers, L. Reginald

Fung, Irene

Golubovic, Boris

Joneckis, Lance G.

Lai, Bobby Y.

Lee, Dicky

Rediker, Robert H.

Shapiro, Jeffrey H.

Shepard, Scott R.

Sun, Ke-Xun

Wong, Ngai C.

DSpace@MIT

Chapter 2. Optical Propagation and CommunicationChapter 2. Optical Propagation and CommunicationAcademic and Research StaffProfessor Jeffrey H. Shapiro, Dr. Robert H. Rediker, Dr. Ngai C. WongVisiting Scientists and Research AffiliatesDr. Lance G. Joneckis'Graduate StudentsL. Reginald Brothers, Irene Fung, Boris Golubovic, Bobby Y. Lai, Dicky Lee, Scott R. Shepard, Ke-Xun SunTechnical and Support StaffBarbara A. King2.1 IntroductionThe central theme of our research programs hasbeen to advance the understanding of optical andquasi-optical communication, radar, and sensingsystems. Broadly speaking, this has entailed: (1)developing system-analytic models for importantoptical propagation, detection, and communicationscenarios; (2) using these models to derive the fun-damental limits on system performance; and (3)identifying and establishing through experimentationthe feasibility of techniques and devices which canbe used to approach these performance limits.2.2 Squeezed States of LightSponsorMaryland Procurement OfficeContract MDA 904-90-C5070Contract MDA 904-93-C4169Project StaffProfessor Jeffrey H. Shapiro, Dr. Ngai C. Wong, Dr.Lance G. Joneckis, Scott R. Shepard, Ke-Xun SunThe central focus of our research has been gain-saturated operation of an optical parametric ampli-fier (OPA). Under gain saturation, the amplifiedoutput intensity of an injected coherent-state signalbecomes amplitude squeezed and the large-signalsignal-to-noise ratio improves, an operating regimeknown as squeezed amplification.2 We have per-formed a detailed study of the mean-field charac-teristics, gain saturation, and quantum-noisespectra of an injection-seeded OPA including arbi-trary cavity detunings of the three interactingwaves.3 Comparison with our mean-field exper-imental results using a type-I phase-matchedLiNbO3 OPA has shown excellent qualitative agree-ment. We have also successfully analyzed athermal-hystersis effect encountered in our exper-iment that can severely impact the tuning capabilityof the OPA.4As part of our squeezed-amplifier theory, we haveestablished the quantum form of the Gaussianmoment-factoring theorem and used this theorem todeduce the direct-detection photocurrent noisespectrum from a squeezed amplifier in theGaussian-state approximation. We have also usedthis Gaussian-state formalism to explicitly unify theanalyses of the three principal nonclassical man-ifestations of signal-idler state entanglement, i.e.,nonclassical signal/idler photon correlation, nonclas-sical quadrature-noise squeezing, and nonclassical1 Laboratory for Physical Sciences, College Park, Maryland.2 N.C. Wong, "Squeezed Amplification in a Nondegenerate Parametric Amplifier," Opt. Lett. 16(21): 1698-1700 (1991).3 K.X. Sun, Classical and Quantized Fields in Optical Parametric Interactions, Ph.D. diss., Dept. of Physics, MIT, 1993.4 K.X. Sun, Classical and Quantized Fields in Optical Parametric Interactions, Ph.D. diss., Dept. of Physics, MIT, 1993; K.X. Sun, N.C.Wong, and J.H. Shapiro, "Thermal Hysteretic Effects in a Triply-Resonant Optical Parametric Oscillator," paper presented at theConference on Lasers and Electro-Optics, Baltimore, Maryland, May 2-7, 1993.139Chapter 2. Optical Propagation and Communicationsignal/idler fourth-order interference.5 This workclarifies previously ill-understood limits on the semi-classical theory of fourth-order interference. In par-ticular, we show that a quantum signature isdemonstrable, in the low-photon-flux regime, atfringe visibilities substantially below 50 percent.In another theoretical study, we have continued ourdevelopment of quantum propagation theory forsingle-mode fiber. Using a coarse-grained timeapproach based on a phenomenological Kerr-effecttime constant, we have been working to evaluatethe homodyne-measurement noise spectra for avariety of input and fiber conditions.6 We haveobtained results for continuous-wave (cw) inputsapplied to disperionless fiber in the absence andpresence of loss. The latter case was handledusing the terminated-cumulant expansion (TCE); weare now using this same approach to find the noisespectrum for dispersive propagation with a cw input.We have also begun exploring the value of local-oscillator optimization in pulsed-input homodynemeasurements.2.2.1 PublicationsJoneckis, L.G., and J.H. Shapiro. "Quantum Propa-gation in a Kerr Medium: Lossless, Dispersion-less Fiber." J. Opt. Soc. Am. B 10(6): 1102-1120(1993).Joneckis, L.G., and J.H. Shapiro. "Quantum Propa-gation in Single-Mode Fiber." Paper presentedat the Quantum Electronics and Laser ScienceConference, Baltimore, Maryland, May 2-7,1993.Joneckis, L.G., and J.H. Shapiro. "Quantum Propa-gation in Single-Mode Fiber." Proceedings of theThird Workshop on Squeezed States and Uncer-tainty Relations, Baltimore, Maryland, August10-13, 1993. Forthcoming.Shapiro, J.H., "Phase Conjugate Quantum Com-munication with Zero Error Probability at FiniteAverage Photon Number," Phys. Script. T48:105-112 (1993).Shapiro, J.H., and K.X. Sun. "Semiclassical vs.Quantum Behavior in Fourth-Order Interfer-ence." J. Opt. Soc. Am. B. Forthcoming.Sun, K.X. Classical and Quantized Fields in OpticalParametric Interactions. Ph.D. diss., Dept. ofPhysics, MIT, 1993.Sun, K.X., N.C. Wong, and J.H. Shapiro. "ThermalHysteretic Effects in a Triply-Resonant OpticalParametric Oscillator." Paper presented at theConference on Lasers and Electro-Optics,Baltimore, Maryland, May 2-7, 1993.2.3 Multiresolution Laser Radar RangeProfilingSponsorU.S. Air Force - Office of Scientific ResearchGrant F49620-93-1-0604Project StaffProfessor Jeffrey H. Shapiro, Irene FungThis effort is part of a collaboration on automatictarget detection and recognition, with ProfessorsAlan S. Willsky (MIT Laboratory for Information andDecision Systems) and W. Eric L. Grimson (MITArtificial Intelligence Laboratory) and their students.The unifying theme of the collaboration is the use ofmultiresolution (wavelet) methods at everystage-from sensor front-end processing, throughfeature extraction, to the object recognitionmodule-in an overall system.Our group's activity centers on the application ofmultiresolution techniques to laser radars. Previ-ously, we have studied maximum-likelihood (ML)estimation for planar range-profiling with a laserradar.7 The importance of the ML approach lies inits ability to suppress the range anomalies causedby laser speckle. The practicality of the MLapproach derives from the utility of the expectation-maximization (EM) algorithm for this problem. Ininitial work on the multiresolution range-profilingproblem, we have shown that the same EMapproach can be applied, at any desired wavelet5 J.H. Shapiro and K.X. Sun, "Semiclassical vs. Quantum Behavior in Fourth-Order Interference," J. Opt. Soc. Am. B, forthcoming.6 L.G. Joneckis and J.H. Shapiro, "Quantum Propagation in a Kerr Medium: Lossless, Dispersionless Fiber," J. Opt. Soc. Am. B 10(6):1102-1120 (1993); L.G. Joneckis and J.H. Shapiro, "Quantum Propagation in Single-Mode Fiber," paper presented at the QuantumElectronics and Laser Science Conference, Baltimore, Maryland, May 2-7, 1993.7 T.J. Green, Jr. and J.H. Shapiro, "Maximum-Likelihood Laser Radar Range Profiling with the Expectation-Maximization Algorithm,"Opt. Eng. 31(11): 2343-2354 (1992).140 RLE Progress Report Number 136scale, to perform ML profiling. We are nowning performance studies on this schemeHaar wavelets to fit simulated laser radardata.begin-usingrange2.4 Optical Frequency Division andSynthesisSponsorsCharles S. Draper LaboratoriesContract DL-H-441698MIT Lincoln LaboratoryContract CX-16335National Institute of Standards and TechnologyGrant 60-NANBOD-1052U.S. Army Research OfficeGrant DAAL03-90-G-0128Grant DAAH04-93-G-0399Grant DAAH04-93-G-0187Project StaffDr. Ngai C. Wong, L. Reginald Brothers, Bobby Y.Lai, Dicky LeeFrequency division and synthesis in the opticaldomain play an important role in modern opticalprecision measurements, optical frequency stan-dards, and coherent optical communication. Wehave demonstrated tunable optical frequency divi-sion using an optical parametric oscillator (OPO)approach based on an efficient, one-stepparametric downconversion process.8 An OPO con-verts an input pump into two intense, coherent sub-harmonic outputs whose frequencies are tunableand whose sum frequency equals the pump fre-quency. By phase locking the output frequency dif-ference to a microwave source, the outputfrequencies are precisely determined, and the OPOfunctions as an optical frequency divider. OPO fre-quency dividers can be operated in series or in par-allel to measure, compare, and synthesizefrequencies from optical to microwave, with highprecision and resolution.Chapter 2. Optical Propagation and Communication2.4.1 Terahertz Optical Frequency CombGenerationIn order to facilitate difference frequency measure-ments in the THz range, we have developed anoptical frequency comb generator based on an effi-cient electro-optic phase modulator design. Byincorporating a microwave waveguide resonatorstructure in a LiNbO3 electro-optic modulator, thephase velocities of the microwave and optical fieldscan be matched to maximize the electro-optic mod-ulation at a user-specified microwave frequency.The modulation is further enhanced by placing themodulator inside an optical cavity that is resonantfor the input optical beam and generatedsidebands. For 1 W of microwave power at 17GHz, we have obtained an optical frequency combwith a 3-THz span, as shown in figure 1.9 We haveemployed the terahertz comb to measure a differ-ence frequency of 1.45 THz between a YAG laserand the signal beam of an OPO with a measure-ment uncertainty of 1 MHz, limited only by the sta-bility of the two optical sources. In addition tooptical frequency metrology, terahertz optical fre-quency comb generation is potentially useful for fre-quency identification in a wideband opticalcommunication network.2.4.2 Tunable Optical Parametric OscillatorIn our first demonstration of a tunable optical fre-quency divider, the signal-idler difference frequencyof a type-II phase-matched KTP OPO was phaselocked to a microwave source at 12.3 GHz withexcellent signal-to-noise ratio.10 The tuning rangewas limited by the 25-GHz bandwidth of the high-speed photodetector. For many applications, atuning range that is much larger than the detectorbandwidth is desirable. We have made use of theterahertz optical frequency comb generator tophase lock the KTP OPO at a difference frequencyof 665 GHz. The idler beam was used to drive theterahertz optical frequency comb generator; theresultant beat frequency between the signal and the39th upper sideband of the comb was detected bya 1-GHz photodetector. We have measured a res-idual phase noise spectral density of the phase-8 N.C. Wong, "Optical Frequency Division using an Optical Parametric Oscillator," Opt. Lett. 15(20): 1129-1131 (1990); D. Lee andN.C. Wong, "Tunable Optical Frequency Division Using a Phase-Locked Optical Parametric Oscillator," Opt. Lett. 17(1): 13-15(1992).9 L.R. Brothers, D. Lee, and N.C. Wong, "Terahertz Optical Frequency Comb Generation and Phase Locking of Optical ParametricOscillator at 665 GHz," Opt. Lett., forthcoming.10 D. Lee and N.C. Wong, "Tunable Optical Frequency Division Using a Phase-Locked Optical Parametric Oscillator," Opt. Lett. 17(1):13-15 (1992).Chapter 2. Optical Propagation and CommunicationFigure 1. Optical spectrum analyzer trace of the optical frequency comb, with a mode spacing of 17.05 Ghz. The insetshows an amplified portion of the spectrum.locked OPO of 0.2 mrad/Hz .1 The high spectralpurity obtained in the experiment shows that phaselocking at an even higher difference frequency ispossible.We have also developed a cw dual-cavity doublyresonant OPO (DRO) that provides a wider tuningrange than the usual single-cavity OPO. Becauseof the stringent requirement of simultaneous cavityresonance of the signal and idler waves in a single-cavity DRO, mode hops usually occur if the cavitylength is tuned by more than 1-2 nm, or - 10 MHzin output frequencies in our DRO system. To over-come this lack of tuning capability, signal and idlerwaves are separated internally by a polarizingbeam splitter and resonated in separate cavities. Inthis way, the threshold remains low while contin-uous frequency tuning is achieved by varying thetwo cavities separately. Our preliminary resultsshow that the cavity length could be tuned over 60nm, or 600 MHz, in the output frequencies. Thethreshold for the dual-cavity DRO was 55 mW com-pared with 47 mW for a single-cavity DRO. Thetuning range of the dual-cavity DRO is expected tobe limited by the angle phase matching bandwidthof the KTP crystal of - 50-100 GHz. This develop-ment may lead to a wider use of cw OPOs as con-venient and tunable radiation sources for manyapplications.2.4.3 PublicationsBrothers, L.R., and N.C. Wong. "Terahertz Optical-Frequency Comb Generation." Paper presentedat the Optical Society of America AnnualMeeting, Toronto, Canada, October 3-8, 1993.Brothers, L.R., D. Lee, and N.C. Wong. "TerahertzOptical Frequency Comb Generation and PhaseLocking of Optical Parametric Oscillator at 665GHz." Opt. Lett. Forthcoming.Lee, D., and N.C. Wong. "Stabilization and Tuningof a Doubly Resonant Optical Parametric Oscil-lator." J. Opt. Soc. Am. B 10(9): 1659-1667(1993).Lee, D., N.C. Wong, and L.R. Brothers. "FrequencyTuning and Stabilization of a KTP Optical11 L.R. Brothers, D. Lee, and N.C. Wong, "Terahertz Optical Frequency Comb Generation and Phase Locking of Optical ParametricOscillator at 665 GHz," Opt. Lett., forthcoming.142 RLE Progress Report Number 136dB 60 70 80 90403020 1 Amplified2 0 \20 d B100 I I I 1-60 -40 -20 0 20 40 60Sideband OrderParametric Oscillator." Paper presented at theOptical Society of America Annual Meeting,Toronto, Canada, October 3-8, 1993.Wong, N.C., and D. Lee. "Frequency Tuning andPhase Locking of an Ultrastable Doubly Reso-nant Optical Parametric Oscillator." Paper pre-sented at the 1994 Advanced Solid-StateLasers, Salt Lake City, Utah, February 7-10,1994.Wong, N.C., D. Lee, and L.R. Brothers. "CWPhase-Locked Optical Parametric Oscillator as aTunable Source for Terahertz Radiation." Pro-ceedings of the SPIE Nonlinear Optics for High-Speed Electronics, Los Angeles, California,January 25, 1994.Wong, N.C., D. Lee, and L.R. Brothers. "OpticalFrequency Counting Based on ParametricDownconversion." Paper presented at the 1993International Symposium on Atomic FrequencyStandards and Coherent Quantum Electronics,Nara, Japan, August 18-20, 1993.2.5 Analog Processing of OpticalWavefronts Using IntegratedGuided-Wave OpticsSponsorU.S. Air Force - Office of Scientific ResearchContract F49620-90-C-0036Project StaffDr. Robert H. Rediker, Boris GolubovicIn wavefront sensing and correction, it is envisionedthat 103 - 104 basic modules would be used. Inintegrated optics, as in integrated circuits, it isimportant to relax the requirements for individualcomponents while requiring that operation of theintegrated optics (circuits) is independent of signif-icant component variations. The wavefront issensed by interferometers between the multiplicityof through waveguides with the arms of the inter-ferometers evanescently coupled to the adjacentwaveguides. The input powers to the interferom-eter arms will not be equal as a result of (1) theinput power to the waveguide array being non-Chapter 2. Optical Propagation and Communicationuniform and (2) unequal coupling by the evanescentcouplers.In RLE Progress Report No. 135, we stated that theexperimental results on the phase measurementand phase correction were independent of thepower difference between the interferometer armsup to a ratio of greater than 50:1. We also wrote inlast year's progress report that it was decided in1992 to embark on building a complete opticalmodule. The on-chip detector at the output of theinterferometer was to also be incorporated in thisplanned module. In the previous work, the detectorhas been off chip. In this past year, a semicon-ductor processing technique was developed andused so that fabrication of all components (e.g.,waveguides, couplers and detectors) of the modulewas compatible. The complete optical moduleswere fabricated; their performance was measured,and compared favorably with theoretical predictions.We can now report in more detail on the compo-nents of the complete module. The phase on theoutput of the through waveguides was varied usinga reverse biased p+-n--n+ structure incorporated intoa portion of the waveguide. A small dither was alsoimpressed on the phase. A Y-junction interferom-eter was used to obtain the relative phase differ-ence between the two waveguides. IntegratedMSM photodetectors, 17x69 mm in size with totalinternal reflection mirror coupling, were used forsignal detection. Both the interferometer anddetector were fabricated and operated between thetwo 30 pm-separated through waveguides. Thephase dither detection system made it possible toadjust appropriately the relative phase between theoutputs of the two through waveguides, largelyindependent of the power ration between the indi-vidual guides.2.5.1 PublicationsGolubovic, Boris. Basic Module for an IntegratedOptical Phase Difference Measurement and Cor-rection System. S.M. thesis, Dept. of Electr.Eng. and Comput. Sci., MIT, 1993.Lau, S.D., J.P. Donnelly, C.A. Wang and R.H.Rediker. "Integrated AIGaAs Waveguides forOptical Phase Difference Measurement and Cor-rection." IEEE J. Quantum Electron. Forth-coming.143Professor Qing Hu144 RLE Progress Report Number 136

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