1999_tmtt packaged semiconductor laser optical phase-locked loop (opll) for photonic generation,...

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 7, JULY 1999 1257

Packaged Semiconductor Laser OpticalPhase-Locked Loop (OPLL) for Photonic

Generation, Processing andTransmission of Microwave

SignalsL. N. Langley, M. D. Elkin, C. Edge,Member, IEEE,M. J. Wale,Member, IEEE,

U. Gliese, X. Huang, and A. J. Seeds,Fellow, IEEE

Abstract—In this paper, we present the first fully packagedsemiconductor laser optical phase-locked loop (OPLL) microwavephotonic transmitter. The transmitter is based on semiconductorlasers that are directly phase locked without the use of anyother phase noise-reduction mechanisms. In this transmitter, thelasers have a free-running summed linewidth of 6 MHz and theOPLL has a feedback bandwidth of 70 MHz. A state-of-the-artperformance is obtained, with a total phase-error variance of 0.05rad2 (1-GHz bandwidth) and a carrier phase-error variance of7��10

�4 rad2 in a 15-MHz bandwidth. Carriers are generatedin the range of 7–14 GHz. The OPLL transmitter has been fullypackaged for practical use in field trials. This is the first time thistype of transmitter has been fabricated in a packaged state whichis a significant advance on the route to practical application.

Index Terms—Optical phase-locked loops, optical transmitters,packaging, phased arrays, semiconductor lasers.

I. INTRODUCTION

M ICROWAVE photonic systems incorporating photonicsignal processing and fiber optic transmission of mi-

crowave and millimeter-wave signals have been the subjectof increasing interest over the last decade [1]–[10]. Pro-posed and investigated applications are manifold, rangingfrom optical signal processing and optically fed phased-arrayantenna beamformers to optical backbone networks for cellularphone systems. Common to all types of microwave photonicsystems is the fundamental need for photonic transmittersand receivers. Most often, the receiver is implemented witha wide-bandwidth photodetector and succeeding amplifiers,

Manuscript received October 19, 1998; revised March 13, 1999. This workwas supported by the US Army CECOM under Contract DAA-B07-95-C-D155.

L. N. Langley, M. D. Elkin, C. Edge, and M. J. Wale are with the PhotonicsDivision, Marconi Materials Technology (MMT), Caswell, Towcester NN128EQ, U.K.

U. Gliese is with Research Center COM, Technical University of Denmark,DK-2800 Lyngby, Denmark.

X. Huang and A. J. Seeds are with the Department of Electronic andElectrical Engineering, University College London, Torrington Place, LondonWC1E 7JE, U.K.

Publisher Item Identifier S 0018-9480(99)05195-9.

whereas the transmitters are more complex and challengingto implement.

Microwave photonic transmitters can vary significantly incomplexity depending on the chosen signal generation anddetection techniques. In systems relying on intensity modu-lation and direct detection (IM-DD), the transmitter is mostoften implemented with a single laser (direct modulation) ora laser in conjunction with an intensity modulator (externalmodulation). On the other hand, dual-frequency laser trans-mitters for coherent remote heterodyne detection (RHD) canrange from the fairly simple to the more complex. The opticalphase-locked loop (OPLL) microwave photonic transmitter isan example of the latter.

Since the successful construction of the first OPLL [11],there has been significant interest in this type of device[12]–[48], including its use in microwave photonic systems[17], [22], [36]–[38], [42], [44]–[48]. Solid-state laser OPLL’sare relatively easy to implement due to their very narrowlaser linewidth, which means that they do not require wide-band feedback electronics or a short-loop-propagation delay.However, solid-state lasers tend to be too bulky and powerconsuming for practical use. Semiconductor laser OPLL’s tendto be difficult to design and implement due to their requirementfor wide-band feedback electronics and small loop delays,consequent from the wider laser linewidth.

At the moment, the field of microwave photonics is at avery important turning point. Many different technologies,techniques, and systems have been investigated in laboratoriesall over the world. The challenge of the next decade is tobring microwave photonics from the laboratory into practicalapplication in the field. To make this feasible, it is of great im-portance to investigate and fabricate integrated and packageddevices and subsystems.

In this paper, we present the first fully packaged semicon-ductor laser OPLL microwave photonic transmitter. The entireOPLL transmitter has been successfully designed [49] andconstructed [50], [51] in one fabrication run. It is implementedwith two custom-designed three-electrode distributed feedback(DFB) lasers, bulk microoptics, and a combination of discrete

0018–9480/99$10.00 1999 IEEE

1258 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 7, JULY 1999

Fig. 1. Schematic of microwave photonic system based on dual-frequencytransmission and RHD.

and integrated-loop electronics. The laser diodes are designedfor narrow linewidth, wide bandwidth, and uniform and sensi-tive FM response, while the main challenge in the constructionof the OPLL is the packaging and the realization of a widefeedback bandwidth. A state-of-the-art performance is ob-tained and the work and results obtained clearly demonstratethat semiconductor OPLL’s can be designed for productionand packaged for practical use. This is an important stepforward within the field of microwave photonic systems and,in particular, OPLL microwave photonic transmitters.

The paper is organized as follows. The system contextand the basics of OPLL microwave photonic transmitters aredescribed in Section II. This is followed by a description inSection III of the design and construction of the packagedOPLL module. After this, the obtained performance results arepresented and discussed in Section IV. Finally, conclusionsare drawn in Section V.

II. OPLL MICROWAVE PHOTONIC TRANSMITTER

A. Dual-Frequency Signal Generation and RHD

Microwave photonic systems may be based on either co-herent or noncoherent techniques. The principles of coherentRHD are quite different from those of noncoherent DD [47],[48].

A simplified schematic of the RHD principle is shownin Fig. 1. At the transmitter end, two phase-correlated lasersignals with a frequency offset corresponding to the desiredmicrowave frequency are generated by a dual-frequency lasertransmitter. Both signals are transmitted through the fiber linkto the receiver, where heterodyning takes place, resulting ina microwave signal proportional to the photodetector current,given by

(1)

where is the responsivity of the photodetector,is thetime, is the instantaneous power, is the instantaneousfrequency, and is the instantaneous phase of the lasersignals. Equation (1) shows that it is necessary to maintaina well-defined relationship between the frequency and phaseof the two signals. This makes RHD systems more complexthan DD systems. Despite this, RHD systems are attractivefor many applications as they offer a number of advantagescompared to DD systems: higher link gain [52], comparableor higher carrier-to-noise ratio (CNR) [52], lower sensitivityto chromatic dispersion [53], and a variety of photonic signal-processing capabilities, such as amplitude control, frequencycontrol, phase control, time-delay control, filtering, modula-tion, frequency conversion, and signal recovery [47], [48],[54], [55].

Fig. 2. Schematic of an OPLL microwave photonic transmitter.

The major challenge in bringing RHD systems into practi-cal application is the realization of suitably packaged dual-frequency laser transmitter modules. These can be imple-mented in different ways, and a significant number of con-cepts such as dual-mode lasers, pulsed lasers, optical fre-quency shifters, single-side band modulators, suppressed car-rier double-side band modulators, optical injection phase-locked loops, OPLL’s, and optical feedforward modulatorshave been proposed and experimented with in laboratoriesaround the world. An overview of these is given in [47],[48], and [54]. The work described in this paper takes thesemiconductor laser OPLL from the laboratory table-top stageinto a fully packaged module for practical use in field trialswhile maintaining state-of-the-art performance.

B. Basics of the OPLL Solution

A schematic for the OPLL microwave photonic transmitteris shown in Fig. 2. It consists of a free-running master laser,an optoelectronic front end (PIN photodiode and microwaveamplifier), a phase detector (microwave mixer), a loop filter,a slave laser, and a microwave reference oscillator. Themicrowave signal generated by heterodyning the two lasersignals is compared to the signal from the microwave referenceoscillator in the phase detector. The resulting phase-errorsignal is fed back to the slave laser which is forced to trackthe master laser at a frequency offset corresponding to thefrequency of the microwave reference oscillator. This causesa significant reduction of the phase noise on the opticallygenerated microwave signal.

The required feedback bandwidth is determined by thesummed laser linewidth, the requirements for loop stability andthe phase noise requirements placed on the optical microwavesignal by the system in which the OPLL is to be applied[56]–[58]. However, the feedback bandwidth is independentof the desired microwave frequency. The available frequencyrange is, therefore, solely determined by the operational fre-quency of the photodiode and microwave components. Withcurrent technology, OPLL’s can be constructed for operationfrom RF frequencies to well within the millimeter-wave re-gion. The tuning range of any of these frequency bands islargely determined by the bandwidth of the photodiode andmicrowave components.

C. Implementation Complexity

The major challenge in implementing OPLL’s that operatewith semiconductor lasers is the realization of a wide feedback

LANGLEY et al.: PACKAGED SEMICONDUCTOR LASER OPPL 1259

bandwidth. This is necessary, since semiconductor lasers, with-out external noise-reduction circuits such as external cavitiesor negative electrical feedback, have a large amount of phasenoise that [from (1)] appears directly on the generated opticalmicrowave signal and has to be removed by the OPLL. Therequirement for a wide feedback bandwidth leads to a numberof other demanding requirements. In order to achieve thewide feedback bandwidth, the loop-propagation delay mustbe small, and the response bandwidths of the microwavecomponents, together with the slave-laser FM response, mustbe wide and uniform in both magnitude and phase. Theseare not requirements that are easy to fulfill, and they makeit challenging to design and construct both components andthe entire OPLL.

Previously, only eight laboratory table-top OPLL’s, basedon direct phase locking of semiconductor lasers, have beenimplemented. Of these, three are homodyne [16], [34], [41],five are heterodyne [17], [37], [38], [42], [45], and onlyone [38] has been applied in microwave photonic systemsexperiments [47], [48]. This clearly reflects the design andimplementation complexity involved in semiconductor laserOPLL’s. Furthermore, packaging is a complex task, due to thelarge number of components and the coherent optics. However,as is shown in this paper, it is possible to design and implementpackaged semiconductor laser OPLL’s for production.

III. D ESIGN AND CONSTRUCTION OFPACKAGED OPLL

A packaged semiconductor laser OPLL has been designedand constructed for evaluation in a proof-of-concept coher-ent optical beamforming system [59]. The design involvedoptimization of the loop configuration and development of cus-tom single-frequency laser diodes with the necessary narrowlinewidth, frequency modulation, and tuning characteristics.An important design parameter for the OPLL is the feedbackbandwidth, which must be wide to give the necessary phase-noise improvements. Consequently the loop-propagation delaymust be minimized [56]–[58] and the physical dimensionsof the OPLL must be reduced as much as possible. Toachieve the desired performance, the optical parts of the OPLLare based on microoptics, and the loop-feedback electronicshave been implemented using high-performance discrete andintegrated microwave components and a custom-designed loopfilter. Each laser has a miniature collimating lens and opticalisolator. Other miniature lenses are used to focus the opticalsignals into the output fiber and photodiode of the feedbackelectronics. Detailed design of the OPLL module required awell-founded OPLL theory [56]–[58], [60]–[62] and the useof expertise in several microwave and photonic areas, includ-ing the custom lasers, loop-feedback electronics, microoptics,and packaging and assembly. The OPLL performance wasmodeled continuously throughout package development. Noperformance sacrifices were made in developing the packagedOPLL module, compared to previous bench-top experimentalsystems [37], [38]. Also, the design and construction tech-niques employed in its fabrication are those regularly usedby Marconi Materials Technology (MMT) in a number ofstandard products and modules.

Fig. 3. Typical static-tuning characteristic of the three-section DFB laser. (I1,I2, and I3 are the rear-, middle-, and front-section bias currents, respectively.)

A. Lasers

Semiconductor lasers for use in an OPLL must meet a num-ber of requirements. First, for a given loop-propagation delay,the laser linewidth sets the minimum phase-error variancethat can be obtained [57], [58]. For the packaged OPLL, thedelay properties of the microoptic and microwave componentsresulted in a minimum delay of approximately 0.4 ns, requiringa beat linewidth of less than 8 MHz to meet the phase-error variance specification. To achieve a delay of0.4 ns,only an integrated or hybrid/microoptic solution is feasible.A fiber-based OPLL cannot give acceptable performance forsemiconductor lasers due to their megahertz linewidths. Sec-ond, the FM response of the laser directly affects the looptransfer function. Standard current-tuned lasers have an FMresponse dominated by thermal effects at low frequencies,giving a red-shifting response with increasing current, whileat high frequencies, the response is dominated by band-filling and plasma effects, giving a blue-shifting response[63], [64]. Such a characteristic cannot be easily equalizedin the loop filter. Third, laser output power above 5 mWis required to maintain a high loop gain. Finally, intensitynoise and IM should be low, as any fluctuation in outputpower will be converted to phase noise by the loop. To meetthese requirements, custom 1.3-m semiconductor lasers havebeen designed and fabricated for use in the OPLL module.The lasers are of a three-section InGaAsP multiple-quantum-well first-order uniform-grating DFB design, and are operatedwith asymmetric bias currents to the multiple sections togive a carrier-injection effect dominated, blue-shifting FMperformance [65]. In this bias configuration, IM is suppressed.

The laser output power is 15–20 mW (dependent on device)under uniform bias conditions suitable for the master laser.When biased asymmetrically to generate the required FMproperties, the output power can still be as large as 13 mW.The individual laser linewidths are in the range 1–4 MHz,depending on the bias conditions. Fig. 3 shows a typical static-tuning characteristic. An FM sensitivity of 1–3 GHz/mA has


Fig. 4. Typical dynamic FM response of the three-section DFB laser.

Fig. 5. Schematic circuit diagram for the loop-feedback electronics (biascircuits not shown, for clarity).

been achieved. All lasers have regions of blue- and red-shifting FM characteristics. In the OPLL module, the laseris biased at a linear blue-shifting operation point of the static-tuning response. The FM characteristics have been measuredusing a hi-bi fiber Mach–Zehnder interferometer operating asa frequency discriminator. A typical dynamic FM responseis shown in Fig. 4, where the response is flat and withoutany phase changes due to transition between thermal andcarrier tuning [66]. A 3-dB FM bandwidth of 700 MHz withside-mode suppression of 30–40 dB is obtained.

B. Loop-Feedback Electronics

A circuit diagram of the loop-feedback electronics is shownin Fig. 5. Bias circuits are not shown, for clarity. The currentapplied to the slave laser from the loop-feedback circuitensures that the slave-laser frequency dynamically tracks themaster-laser frequency offset by the local oscillator (LO)frequency. The system was designed for operation with lasershaving the previously measured FM bandwidth and linewidth.In addition to this, the phase noise needed to be minimizedfor the 15-MHz noise bandwidth of the system in which thisOPLL is to be used. Critical to reducing the phase noiseare narrow laser linewidths and a wide feedback bandwidth.If the phase noise is too large, not only will this degradethe quality of the output signal, but also cause the loop tolose lock. The photodiode is a 20-GHz MMT device andthe microwave pre-amplifier is a MMT 2–20-GHz GaAs highelectron-mobility transistor (HEMT) traveling wave amplifiermonolithic-microwave integrated circuit (MMIC) with inte-grated photodiode bias. A Magnum 4-14-GHz double-balanceddiode ring mixer is used as a phase detector. The phase-

Fig. 6. Schematic representation of the module.

error signal from the phase detector is applied to the tuningsection of the slave laser via the loop filter. A phase-leadcompensated low-passRC design was considered to give thebest performance for the OPLL. The main advantage overother loop filters is that it significantly reduces the carrier-phase noise over the noise bandwidth, while maintaining goodloop stability with the wide feedback bandwidth requiredfor semiconductor lasers [62]. The filter was implementedusing discrete surface-mount components. For the microwavecomponents of the loop (amplifier and phase detector), a fullMMIC integration would be a good solution [67]. However,due to time constraints, this has not been carried out in thisproject.

C. Microoptics, Assembly, and Packaging

The OPLL module was designed for use with a coherentoptical-beamforming demonstrator. In order to achieve theoptical phase-shift processing in the subsequent beamformer,the two optical signals emitted by the OPLL must be arrangedwith orthogonal polarization states [55]. Semiconductor lasersemit light in the TE polarization only, so miniature/2plates are used to rotate the polarization of the laser beams.A polarizing beamsplitter is then utilized to divert a smallproportion of the light onto the photodiode of the loop-feedback electronics. The larger proportion of the two beams(TM for the master laser and TE for the slave laser) is coupledinto a polarization-maintaining (PM) output fiber. Efficientwavefront overlap of the two beams is required to give a strongmicrowave signal from optical mixing in the photodiode.

The complete OPLL unit is assembled into a custom opto-electronic package, as illustrated in Fig. 6. Various construc-tion techniques were used, such as laser welding for criticalalignment components, plus solder and epoxy for less criticalalignments.

The laser subassemblies are constructed using a laser mount,to which the laser and associated components are soldered. Mi-crooptic lenses and isolators are laser welded in place in frontof the laser to give a collimated beam of 1-mm diameter. Eachlaser has a thermistor placed adjacent to it. This is used by ex-


Fig. 7. Photograph of the OPLL module.

ternal temperature-control circuitry to control the laser wave-lengths via a thermocooler, upon which the laser mount sits.

The polarizing beamsplitter subassembly is constructed us-ing standard commercially available 3.2-mm polarizing cubebeamsplitters. Two half-waveplates and a polarizer are fixedto its faces using optical cement. The half-waveplates are setat angles to give the required split ratio for each laser tothe photodiode. The feedback electronics is assembled usingstandard microwave-construction techniques. A prism is fixedabove the photodiode using epoxy to deflect the optical signalsonto it.

The module is assembled by first placing the loop electron-ics within the package and then adding the optical components.The beamsplitter subassembly and slave laser are alignedtogether such that a signal is seen on the photodiode. Thebeamsplitter is then fixed using epoxy, and the laser is solderedinto place using the thermocooler as the heating element. Somebackground heat is also applied to the module. This processis then repeated for the master laser. A focusing lens is thenpositioned in front of the photodiode. The signal from theslave laser is maximized and the lens is then laser welded inplace. The master laser is then also turned on and the beatsignal is then maximized using a beamsteering lens. This lensis laser welded in front of the master laser. Finally, a furtherlens and the output fiber are then laser welded into place toprovide the module output. Prior to the addition of the outputfiber, the OPLL performance is monitored by focusing the twolaser beams (through the output fiber tube) onto an externalphotoreceiver.

This resulted in a rigid and robust module suitable foroperation in military specification environments. A photographof the OPLL module is shown in Fig. 7. Overall moduledimensions are 6555 17 mm.

D. Estimated Performance

From the design and layout of the OPLL, the loop-propagation delay can be estimated. The optical and electrical

contributions to the delay can be estimated from physical pathlengths using knowledge of refractive indices and dielectricconstants, respectively. The resultant estimated value for theoptical delay is 90 ps. Circuit modeling and measurementswere also used to estimate the electrical delay, the resultbeing 290 ps. Therefore, the total estimated loop-propagationdelay is 380 ps. A loop-feedback bandwidth (implemented bythe filter) of 70 MHz has been chosen, taking into accountthe laser and loop-feedback electronics bandwidths, to give asufficient phase margin for stable operation. Here, the feedbackbandwidth is given as the frequency of unity open-loop gain.This is considered the best measure for feedback bandwidth, asit gives a fixed comparable reference point, independent on thetype of loop (loop order and type of filtering). For the feedbackloop, the coupling efficiency of the laser light to the photodiodeis estimated as 3 dB for the microoptic assembly methodchosen. A further 3 dB is lost due to the 45polarizer, and also5 dB due to the fraction of light diverted to the photodiodeby the polarizing beamsplitter. The remainder is coupled intoa PM output fiber. The photodiode responsivity is 0.6 A/Wat 1300 nm, and the MMIC amplifier has a gain of10 dB.The mixer has a conversion loss of approximately 6 dB. Theestimated wavefront overlap loss of the two beams in the pho-todiode is 2 dB. A slave-laser FM efficiency of 1.3 GHz/mAresulted in a modeled loop gain of 0.55 GHz. Further detailedmodeling [56]–[58], [60]–[62] during the design processpredicted a loop phase-error variance (infinite bandwidth)of 0.02 rad with a carrier phase-error variance (in 15-MHznoise bandwidth) of rad . The acquisition range atan LO frequency of 9 GHz was calculated as280 MHz.

IV. M EASURED PERFORMANCE

From the OPLL module, the output power into the PMoutput fiber is 2.8 and 0.6 mW for the slave and masterlasers, respectively. The measured locked OPLL output signalis shown in Fig. 8 for an LO frequency of 9 GHz. Close tothe carrier, its spectral shape corresponds exactly to that ofthe reference LO signal. The phase-noise level is95 dBc/Hzat a 50-MHz offset, corresponding to the loop resonancefrequency, and decreases to less than105 dBc/Hz at loweroffsets. Of the total signal power, 96% is phase locked inthe carrier, and the total phase-error variance is 0.05 rad(1 GHz bandwidth), of which the part within the 15-MHznoise bandwidth is 710 rad . The variation of phase-errorvariance with the noise bandwidth is shown in Fig. 9. Fornarrow noise bandwidths below a few megahertz, the phase-error variance is constant and equal to that of the reference LOsignal. As the noise bandwidth approaches the loop-resonancefrequency, the phase-error variance increases significantly andthen approaches a constant value corresponding to the totalphase-error variance (infinite-noise bandwidth). Fig. 10 givesthe acquisition range for various LO frequencies, showingthat the loop operates from 7 to 14 GHz. The acquisitionrange is 370 MHz at 9 GHz, and as high as700 MHzat 12 GHz. The lower acquisition range at 9 GHz is dueto a minimum in the loop gain at that frequency, resultingfrom ripple in the loop-electronics transfer response. Key


Fig. 8. Measured power spectrum for a 9-GHz locked output signal.

Fig. 9. Variation of phase-error variance with noise bandwidth for the signalof Fig. 8.

Fig. 10. Measured acquisition range against LO frequency, showing that theOPLL module operates from 7–14 GHz.

performance parameters of the loop have been summarizedin Table I, which show that the packaged OPLL has achieveda performance very close to that of the best bench-top exper-imental OPLL [38], [47].

TABLE IKEY OPLL PERFORMANCE PARAMETERS

V. CONCLUSION

A packaged semiconductor laser OPLL microwave photonictransmitter has been constructed for the first time. It is suitablefor field trial operation and is intended for evaluation ina proof-of-concept coherent optical beamforming system fora phased array antenna communications system. The OPLLhas been implemented with semiconductor laser diodes, mi-crooptics, and discrete and integrated electronics to give astate-of-the-art performance. The laser diodes are designedfor narrow linewidth, wide FM bandwidth, and high FMefficiency, while the main challenge in the construction ofthe OPLL is the packaging and the realization of a widefeedback bandwidth. A total phase-error variance of 0.05 radhas been achieved (1-GHz bandwidth), and that within a 15-MHz noise bandwidth is 7 10 rad . The OPLL can operatewith LO frequencies from 7–14 GHz. At LO frequenciesfrom 8–13 GHz, the acquisition range is 370 MHz. Theperformance of the packaged OPLL module is equivalent toprevious bench-top experimental demonstrations. The resultsof this work clearly demonstrate that semiconductor OPLL’scan be designed for production and packaged for practicaluse, while maintaining a state-of-the-art performance that isadequate for system application. This is an important step


forward within the field of microwave photonics. We expectloops of this type to find application in a range of opticalbeamforming and broad-band fiber-radio applications.

ACKNOWLEDGMENT

The authors wish to thank L. Coryell and J. Wright forsupport and useful discussions.

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L. N. Langley, photograph and biography not available at the time ofpublication.

M. D. Elkin , photograph and biography not available at the time of publi-cation.

C. Edge (M’98), photograph and biography not available at the time ofpublication.

M. J. Wale (M’92), photograph and biography not available at the time ofpublication.

U. Gliese, for photograph and biography, see this issue, p. 1150.

X. Huang, photograph and biography not available at the time of publication.

Alwyn J. Seeds(M’81–SM’92–F’97) received thePh.D. degree from the University of London, U.K.,in 1980, for work on the optical control of IMPATToscillators.

From 1980 to 1983, he was a Staff Memberat Lincoln Laboratory, Massachusetts Institute ofTechnology, where he worked on GaAs mono-lithic millimeter-wave integrated circuits for use inphased-array radar. In 1983 he was with QueenMary College, University of London, where helectured in telecommunications until moving to Uni-

versity College, Torrington Place, London in 1986, where he is now Professorof opto-electronics and Head of the Opto-electronics and Optical NetworksGroup. He has published over 150 papers on microwave and opto-electronicdevices and their systems applications is the presenter of the video “Mi-crowave Opto-electronics” in theIEEE Emerging Technologiesseries. Hiscurrent research interests include microwave bandwidth tunable lasers, opticalcontrol of microwave devices, mode-locked lasers, optical phase-lock loops,optical frequency synthesis, dense WDM networks and nonlinear opticaltransmission.

Dr. Seeds is Vice Chairman of Commission D (Electronics and Photonics)of the International Union for Radio Science (URSI) and Chairman of theTechnical Committee on Lightwave Technology of the IEEE MicrowaveTheory and Techniques Society. He has served on the program committees formany international conferences and is General Chair for the IEEE MTT/LEOSInternational Topical Meeting on Microwave Photonics (MWP 2000) to beheld in Oxford, U.K.

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