Ultra-low-loss polymeric waveguide circuits for optical true-time … · 2018. 6. 22. ·...

Ultra-low-loss polymeric waveguide circuitsfor optical true-time delays in widebandphased-array antennas

Suning Tang, MEMBER SPIEBulang Li, MEMBER SPIENianhua JiangRadiant Research, Inc.3006 Longhorn Blvd., Suite 105Austin, Texas 78758E-mail: [email protected]

Dechang AnZhenhai FuLinghui WuRay T. Chen, FELLOW SPIEUniversity of Texas at AustinMicroelectronics Research CenterDepartment of Electric Computer

EngineeringAustin, Texas 78758E-mail: [email protected]

Abstract. The optical true-time-delay line is a key building block formodern broadband phased-array antennas, which have become one ofthe most critical technologies for both military and civilian wireless com-munications. We present our research results in developing an opticalpolymer-based waveguide true-time-delay module for multilink phased-array antennas by incorporating wavelength-division multiplexing (WDM)technology. The demonstrated optical polymeric waveguide circuits canprovide a large number of optical true-time delays with a dynamic rangeof 50 ns and a time resolution of 0.1 ps. Various fabrication techniquesare investigated for producing ultralong low-loss (0.02 dB/cm) polymericchannel waveguides with tilted waveguide grating output couplers. Fastphotodiode arrays are fabricated and rf signals with frequencies of 10 to50 GHz are generated through the optical heterodyne technique. A de-tailed study of waveguide amplification to achieve loss-less polymericwaveguide is conducted. The optical amplification of 3.8 dB/cm isachieved at a wavelength of 1064 nm in a Nd31-doped polymeric wave-guide. WDM techniques are also employed for potential multilink appli-cations. The presented methodologies enable hybrid integration with areduced cost in optoelectronic packaging and an increased reliability anddecreased payload for the next generation of phased-array antennas.© 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)00603-6]

Subject terms: optical true-time delay; phased-array antenna; polymericwaveguides; waveguide grating; waveguide amplifier; optical waveguide circuit.

Paper IO-06 received June 30, 1999; revised manuscript received Sep. 8, 1999;accepted for publication Sep. 12, 1999.

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1 Introduction

The increasing demand on bandwidth and the reliabilityairborne communications networks have stimulated theplacement of mechanically scanned antennas by phaarray antennas, which enable independent electronictrol of each antenna element, thus increasing the flexiband the speed of beam forming. In phased-array antenthe phase and amplitude of each radiating element areditionally controlled through switching the length of eletrical delays feeding the antenna elements. Howeverprovide broadband capability, future generations of phasarray antennas must be built by invoking the recentlyveloped optical true-time-delay~TTD! technology. OpticalTTD lines provide phase shifts to each phased-arraytenna element through optical delays via the optical fibethe waveguide that serves as a carrier for rf signals.

The mechanism of phased-array antennas emploelectronically driven antenna elements with individuacontrollable phase shifts can be described as follows.wavefront direction of the total radiated carrier wavecontrolled through a continuously and progressively varphase shift at each radiating element, achieving a contous steering of the antenna. For a linear array radiaelements with individual phase control, the far-field pattealong the direction ofF can be expressed as1

Opt. Eng. 39(3) 643–651 (March 2000) 0091-3286/2000/$15.00

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where An is pattern of the individual element,vm is themicrowave frequency,km5vm /c is the wave vector,cn isthe phase shift,L is the distance between radiating elments, andF is the direction angle of the array beam reltive to the array normal. The dependence of the array faon the relative phase shows that the orientation of the mmum radiation can be controlled by the phase excitatbetween the array elements. Therefore, by varying the pgressive phase excitation, the beam can be oriented indirection. For continuous scanning, phase shifters are uto continuously vary the progressive phase. For examplepoint the beam at an angleF0 ,cn is set to the followingvalue:

cn52nkmL sinF0 . ~2!

Differentiating Eq.~2!, we have

DF52tanF0S Dvmvm D ~rad!. ~3!

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It is clear that for a fixed set ofcn’s, if the microwavefrequency is changed by an amountDvm , the radiatedbeam will drift by an amountDF0 . This effect increasesdramatically asF0 increases. This phenomenon is the scalled ‘‘beam squint,’’ which leads to an undesirable drof the antenna gain in theF0 direction.

For wideband operation, it is necessary to implemoptical TTD steering technique such that the far-field ptern is independent of the microwave frequency.2 In theapproach of optical TTD, the path difference between tradiators is compensated by lengthening the microwfeed to the radiating element with a shorter path tomicrowave phase front. Specifically, the microwave excing the (n11)’th antenna element is made to propagthrough an additional delay line of lengthDn5nL(F0).The length of this delay line is designed to provide a timdelay of

tn~F0!5~nL sinF0!/c ~4!

for the (n11)’th delay element. For all frequenciesvm ,cnis given by

cn52vmtn~F0!. ~5!

With such a delay setup, when the phase termnkmL sinFin Eq. ~1! is changed due to frequency ‘‘hopping,’’ thphase termcn will change accordingly to compensate fthe change such that the sum of the two remainschanged. Thus, constructive interference can be obtainethe directionF0 at all frequencies. In other words, the eemental vector summation in the receiving mode or intransmit mode is independent of frequency, which is crucfor ultrawide and operation for future phased-array antnas~PAAs!.

The existing PAA technologies include microstrip rflecting array antennas with mechanical phasing,3 fibergrating prisms,4 and thermo-optically switched silica-basewaveguides circuits.5 These attempts have demonstratthe low-weight potential and some good performance chacteristic. Mechanical phased microstrip antennas dorequire expensive beam-forming transmission-line nworks and/or phase-shifting circuits. The beam steerinprovided by the mechanical rotation of each antennaments. In fiber Bragg grating prism technology, higperformance reflection gratings can be easily fabricateultra-low-loss optical fibers, but they require very expesive fast wavelength tunable laser diodes. The thermoptically switched silica-based waveguide circuit offers ecellent delay time control in a compact structure wherelength of waveguide is defined by photolithography.

There are several severe problems of these existingproaches. Existing approaches fail to provide high-spbeam steering due to the speed limitation of mechandriving motors, wavelength-tunable laser diodes, and/o3 2 thermo-optic switches. The existing approaches arequire a large number of expensive components sucminiaturized motors, wavelength-tunable laser diodes,2 3 2 thermo-optic switches, which makes the systempractical for commercial applications. The techniquesimproving these existing approaches demonstrated so

644 Optical Engineering, Vol. 39 No. 3, March 2000

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in general, add to system complexity, employ very expsive devices, and/or require extremely difficult fabricatiprocesses.

2 Polymeric Waveguide Circuits for OpticalTrue-Time Delay

Optical polymers have recently shown great potentialfabricating practical photonic waveguide devices. Becapolymeric waveguide technology is conceptionally hybrit opens up the possibility for a large-scale optoelectrointegration on any substrate in a cost-effective mannerthis paper, we present a new approach for developing ocal TTD lines for wideband PAAs using polymeric wavguide technologies.6 In this approach, optical TTD lines arcomposed of photonic polymeric waveguide circuits aelectrically switched high-speed photodetectors, as shoin Fig. 1. This PAA system uses an ultralong photonpolymeric channel waveguide circuit on a semiconducsubstrate, where a high-speed photodetector array is prericated. The photonic polymeric waveguide circuits consof ~1! polymeric channel waveguides,~2! waveguide grat-ing couplers, and~3! waveguide amplifiers. Such a polymeric waveguide circuit is capable of providing opticTTDs from 1 ps to 50 ns for wideband multiple communcation links in a compact miniaturized scheme. Note tthe bandwidth of this approach is currently limited by tbandwidth of photodetectors at 60 GHz. The optical ampfication along the waveguide is important to compensthe optical loss due to the waveguide propagation and ging fanout. The optical heterodyne technique is usedgenerating an optical rf carrier by employing two coherelaser diodes with slightly different wavelength. A largnumber of TTD combinations can be provided for the PAsimultaneously by electronic switching the photodetecarray fabricated under the polymeric waveguide circuThis system eliminates the need for fast wavelength tunalaser diodes, long bulky bundles of fibers, and/or expensoptical 23 2 waveguide switches. Unlike any convetional approach where one TTD line can provide only odelay signal at a time, this TTD module is capable of geerating all required optical TTD signals simultaneouslyall antenna elements.

Compared to expensive electro-optic switches awavelength-tunable laser diodes, high-performance phdetectors are inexpensive and can be cost-effectively fa

Fig. 1 Schematic diagram of the compact multilink optical TTD linebased on a polymer-based photonic waveguide circuit.


Fig. 2 Electrical diagram of a detector-switched optical waveguide TD line for photonic phased-arrayantennas.

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cated into a large array based on the technologies origindeveloped for optical imaging and fiber-optic communictions. High-speed Newport MSM photodetectors havebandwidth up to 60 GHz or a rise time of 7 ps. The eletrical diagram of the detector-switched optical TTD moduis shown in Fig. 2. Such a hybrid integration of detectorsthe optical waveguides eliminates the most difficoptoelectronic-packaging problem associated with the dcate fiber-detector interface and/or fiber-switch interfacenot only reduces the cost associated with optoelectropackaging, but also reduces the system payload withimproved reliability for airborne applications. The TTDfor multiple communication links can be simply provideby employing multiple optical rf modulated beams at dferent wavelengths over the same delay line basedwavelength-division multiplexing technique.

The unique optical amplification feature of photonpolymers enables us to fabricate an ultralong optical chnel waveguide with a large number of fanout gratings.7 Theoptical propagation loss and fanout loss are compensby the optical gain throughout the waveguide delay line.a result, a large number of time delays can be obtainedusing a single laser diode for advanced photonic radartems that often have 103 to 105 antenna elements. The optical gain is provided within the photonic polymeric wavguide doped with rare-earth ions such as Nd31 and pumpedby a third laser (l3) from another end of the waveguidcircuit. To obtain uniform fanout, the optical gain in thwaveguide section between two fanout gratings can begineered to exactly compensate the sum of the wavegpropagation loss and optical fanout loss. The delay at edetector is equal to the time of flight along the wavegucircuit to the selected waveguide grating coupler. Becathe length of waveguides is defined by photolithograpthe optical polymeric waveguide delay lines can provid0.1-ps TTD resolution over a 50-ns dynamic range. Tthin-film nature of polymers enables us to fabricateTTD module ~made of waveguide circuits and waveguigratings! on any substrate of interest, using standard vlarge scale integration~VLSI! technologies originally de-veloped for microelectronics industries.

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3 Ultralong Polymeric Channel Waveguide

A high-performance PAA with dynamic range of 50 nrequires the optical polymeric waveguide to be over 10to provide sufficient optical TTD. To fabricate such ultrlong polymeric waveguide circuits, we have developthree waveguide fabrication technologies:8–10 ~1! thecompression-molding technique,~2! the VLSI lithographytechnique, and the~3! laser-writing technique. Our experimental results indicate that high-performance polymewaveguide circuits with a waveguide propagation loss lthan 0.02 dB/cm can be produced by using these thpolymeric waveguide technologies. The compressimolding technique has demonstrated its uniqueness inducing three-dimensional~3-D! tapered waveguide circuitswhich are crucial for obtaining efficient optical couplinbetween the input laser diode and the waveguide circMass-producible waveguides with excellent repeatabihave been obtained by using the VLSI lithography tecnique, originally developed for fabricating very large scaintegrated circuits on silicon wafer. The laser writing wavguide technology has shown its flexibility in fabricatinhigh-performance large-scale polymeric waveguide ccuits.

Due to the excellent repeatable results, standard Vlithography techniques was selected for fabrication of10-m-long polymeric waveguide circuits. Since the lengof waveguides is defined by photolithography, the wavguide length can be precisely controlled and circledmore than 10 m with accuracy in the submicrometer ranAs a result, the polymeric waveguide delay circuits canfabricated with a 0.1-ps TTD resolution over a 50-ns dnamic range. We successfully fabricated a 10-m-long pomeric waveguide circuit using the VLSI lithography tecniques. Figure 3 shows the 10-m-long polymeric chanwaveguide circuit with a waveguide dimension of35 mm. The waveguide propagation loss is about 0dB/cm measured atl51064 nm. Ultra-low-loss opticapolyimides were employed for the waveguide fabricatioThese polyimides have shown excellent optical transmsion characteristics with good thermal and chemical sta

645Optical Engineering, Vol. 39 No. 3, March 2000

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ity over time and temperature. They have been proven tosilicon complementary metal-oxide semiconduc~CMOS! process compatible.

For a PAA with element-to-element spacing ofd5l/2,wherel is the wavelength of the rf radiation, the maximupossible delay time is11

Ti max5 il sinfm/2c i51,2,3,. . . ,K, ~6!

where um is the maximum scan angle,c is the speed oflight, and K is the number of elements of a PAA. Thminimum delay corresponding to the antenna angular relution uR is

Ti min5 il sinuR/2c. ~7!

Equations~6! and ~7! determine theTi max and Ti min andthe total numberR of different delays required for steerinthe antenna overum with resolutionuR .

For example, for the designed antenna operating af511 GHz ~or l527.3 mm!, with um545 deg, uR50.7 deg, a 6-bit delay line (R526564) is required withTmax52.06 ns andTmin535.6 ps. These correspond tomaximum delay line ofLmax5Ti maxc/n542 cm, and a mini-mum delay step ofLmin5Ti minc/n57.1 mm, respectively.Here n51.5 is the optical refractive index of polymeriwaveguide. The antenna element separation isd5l/2513.65 mm. The required dimension of the 2-D PAAS5(dR)25(13.653 64)25873.63 873.6 mm2. As manyas 643 6454096 antenna elements may be required. Sa 2-D PAA can electronically scan in two dimensions acan cover at least nine satellites at all times in all locatio

4 Tilted Waveguide Grating Couplers for OpticalFanout

To obtain optical TTD, output couplers must be fabricatalong the polymeric waveguide at an interval determinby the minimum delay step sized as already described.optical waveguide grating coupler is an ideal candidatecoupling out the rf modulated optical waves into photodtectors, which propagate through the polymeric wavegucircuit. The unique nonblocking feature of gratings enabus to have a large number of optical fanouts alongwaveguide propagation, where each fanout correspondsTTD. Since the proposed photonic polymer-based waguide delay lines are fabricated in a planarized geomewhile the photodetector array employed receives optsignal perpendicular to the substrate surface, surfa

Fig. 3 Photographs of (a) the 10-m-long polymeric waveguide cir-cuit and (b) the waveguide cross section.


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normal optical grating couplers are required. To provieffective surface-normal coupling, the microstructuregrating coupler should be tilted for creating the requirBragg phase-matching condition just for one output dirtion. Such surface-normal waveguide grating couplersachieved by using tilted-surface relief microstructure.9,10

The tilted waveguide grating coupler is fabricatedusing the reactive-ion etching~RIE! technique. In this pro-cess, the optical channel waveguide is first fabricated usphotolithography. The fabricated channel waveguide hathickness of 10mm and a width of 50mm. For simplicity, aglass substrate is selected where waveguide cladding isrequired due to the low refractive index of glass. A thaluminum metal mask is further required on top of tchannel waveguide. Then a 500-Å aluminum layercoated on top of the waveguide using electron-beam evaration, followed by a layer of 5206E photoresist with spspeed of 3000 rpm. The grating pattern on photoresistpatterned by a photomask, which was then transferredthe aluminum layer by wet etching, to open a grating-liwindows on top of the waveguide. We used an RIE procwith a low oxygen pressure of 10 mtorr to transfer tgrating pattern on the aluminum layer to the polyimilayer. A Faraday cage12 was used in the RIE process. Tform the tilted grating pattern on the polyimide waveguidthe sample is placed at a tilted angle of 40 deg with respto the incoming oxygen ions inside the cage. The final swas to remove the aluminum mask by another RIE proceThe waveguide tilted grating couplers were successfufabricated. Figure 4 shows the scanning electron micscope picture of the tilted waveguide grating fabricated

The gratings are designed to surface-normally couplelaser beam out of the waveguide at an operating walength of 1060 nm. A large number of gratings canfabricated on top of the waveguide simultaneously. Toutput coupling efficiency is measured at 5% when a YAlaser with output wavelength of 1060 nm is employeCoupling efficiency can be well controlled by adjusting tgrating depth from 1 to 8%. The nonblocking nature of twaveguide grating enables a large number of fanouts althe waveguide propagation. In other words, a large numof optical TTDs can be generated along the wavegupropagation with the delay time equal to the time of fligalong the waveguide circuit.

5 Polymeric Waveguide Amplification forLossless Operation

Optical waveguide amplification provides a convenient wto amplify optical signals without the need for optoeletronic conversion. Due to the large number of opticfanouts in a very long waveguide delay line, an optic

Fig. 4 Scanning electron microscope picture of tilted waveguidegratings.

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waveguide amplifier is highly desired for fabricatingdetector-switched optical waveguide TTD module. Thesulting signal amplification is crucial to compensate thetical fanout loss and propagation loss for creating a ‘‘loless’’ optical waveguide delay line.8,13–18 Realization oflossless optical waveguides based on photonic polymrepresents a new technology that may create a new claphotonic devices with superior performance at a reducost. The application of the lossless photonic polymerthe optical TTD module would eliminate the necessity uing multiple input laser diodes that must be operated cohently not only in frequency but also in phase. It also enabuniform optical outputs to each photodetector by adjustthe optical gain of the waveguide equal to the sum oftical propagation loss and fanout loss. Such a losslesstonic polymer is obtained by doping rare-earth ions suchNd31 in a host polyimide.

To develop a photonic polymeric amplifier, the rarearth ions must be doped uniformly in the host polymSince organic solvents are often used to prepare the pimide solution, while rare-earth ions such as NdCl3 arehighly soluble in water, it is reasonable to use a mixturewater and an organic material as the solvent. Figurshows the developed preparation procedure for photopolymers. The host polyimide is first dissolved in an oganic solvent and kept in hot bath for 4 h at 40°C. TheNdCl3•6H2O is dissolved in pure water solvent, and kepthot bath for 4 h at 40°C. Then, the two solutions wermixed together and put in hot water bath at 40°C for aother 4 h. A uniform solution containing Nd31 ions is thusformed. The quality of the solution is pivotal to make higperformance optical waveguide amplifiers. Polymeric thfilms are obtained by spin-coating the polymer on silicsubstrate, and dried in vacuum at 80°C. The thicknesthe film can be well controlled within 1 to 10mm by ad-justing the spin speed and/or polymer concentration.

To optimize the optical amplification efficiency, thfluorescence lifetime of the metastable states of doNd31 ions must be kept long. It is well known that the moserious quenchers are the admixed O–H groups from wmolecules for glass waveguides.19,20 The underlyingmechanism is due to the vibronic coupling betweeneffective phonons and the metastable electronic stateNd31 though overtone vibration. If the energy gap betwe

Fig. 5 Fabrication procedures for preparing a lossless photonicpolymeric waveguide film.

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the excited state and the ground state of Nd31 is less thanfour times of the phonon frequency, the fluorescence ofmetastable state will be fully quenched.20,21 Therefore wedeveloped an effective dehydration process to eliminatewater molecules within the polymer.8,13,14The transmissionspectra of two samples, a pure polyimide film andNd31-doped polyimide film, are shown in Fig. 6, measurby a Lambda spectrometer. Within the range of 500 to 12nm, three main absorption bands of Nd31 were observed,centered at 578, 745, and 796 nm. The absorption specdue to Nd31 is very similar to that of Nd31-doped silicafibers.21

We experimentally demonstrated the optical amplifiction in the photonic polymeric waveguides fabricated. Fure 7 shows the setup for optical gain measurement.waveguide under test was mounted on a prism coupstage. The pumping beam at wavelength of 796 nm, frotunable Ti:sapphire laser, was coupled into the waveguusing prismP1 . The 1064-nm signal beam was provideby a Nd:YAG laser and coupled into the waveguide usprism P2 . Note thatP1 also functions as the output prismfor the signal beam. The pumping beam and the sigbeam were carefully aligned to ensure the overlap weach other to achieve the optimum amplification. A lasbeam analyzer and an IR CCD camera were employedthe alignment. The 1064-nm amplified signal was detecafter passing through a wavelength-filtering system ctaining rejection filterF1 and a laser bandpass filterF2 ,both working at 1064 nm.

The relationship among the optical gain, pumpipower, Nd31 doping concentration, and the interactiolength of the signal and pump beams was experimentinvestigated. Figure 8 shows the variation of optical gaversus the pumping power with a Nd31 doping concentra-tion of 6.731019/cm3. The interaction length of the signaand pumping beams in the waveguide was fixed to 1.8A saturated gain of 3.8 dB was observed, correspondina pump power of 4.9 mW. The relationship betweengain and the concentration of Nd31 is further illustrated inFig. 9. The optimized concentration of Nd31 for amplifica-tion was;6.73 1019/cm3. Gain quenching occurred serously when the Nd31 doping concentration was determineat ;7.83 1019/cm3.

Nd31 has two broad absorption bands centered atand 796 nm, as indicated in Fig. 6. These absorption ba

Fig. 6 Transmission spectra of polymeric waveguide films for purepolyimide film and polyimide film doped with 2.1% (by weight)NdCl3.



648 Optical Engi

Fig. 7 (a) Schematic of the test setup for demonstrating optical amplification in polymeric waveguidesand (b) photograph of the experiment setup and test parameters.

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were further confirmed by the measuring the gain verpumping wavelength, as shown in Fig. 10. The pumpefficiency reached maximum around 745 and 796 nmdecreased slowly when the pump wavelength was detuaway from the peaks. This result confirms that the enelevels of Nd31 in amorphous polymer are similar to theseamorphous glasses. In short, the rare-earth ions of N31

were successfully doped into the host polymer. Optical aplification of a photonic polymeric waveguide were demonstrated with 3.8 dB net gain atl51064 nm in a 1.8-cm-long planar waveguide.

6 Generation of Wideband rf Signals Using theOptical Heterodyne Technique

To provide the ultrawideband operation from 11 to 40 GHseveral rf techniques can be employed with differebandwidth-tunable capabilities. These include harmogeneration in a Mach-Zehnder modulator,22 heterodynemixing of two lasers,23 resonance enhanced modulation olaser diode24 ~LD!, and a dual-mode distributed feedba~DFB! laser in mode-locked operation.25 Direct modulationof the LD seems straightforward to generate a millimewave. However, the high insertion loss, high drive voltanonlinear response, and small modulation depth limitusefulness of this technique.26

Compared with direct modulation of an LD or usinexternal modulators, the optical heterodyne technique is

Fig. 8 Measured optical gain in Nd31-doped polymeric film at l51064 nm as a function of optical pumping power at l5796 nm.

neering, Vol. 39 No. 3, March 2000

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pable of providing hundreds of gigahertz base bandwidwhile maintaining a high modulation depth. We have sucessfully generated up to 50-GHz rf signals using two tuable LDs oscillating at single longitudinal mode basedoptical heterodyne technique. Figure 11 shows the scmatic diagram of the experimental setup. The outputs frthese two lasers with slightly different wavelengths acombined by a two-to-one polarization maintaining fibbeam combiner and then sent to wideband photodetect

Suppose that the outputs of these two lasers are give

E1~ t !5A1 exp~ j v1t !, ~8!

E2~ t !5A2 exp~ j v2t !5A2 exp@ j ~v11Dv!t#, ~9!

whereDv is the beat frequency. The output of the photdetector is given by23

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wheree is the electron charge,h is the quantum efficiencyof the detector,hn is the photon energy, andF(Dv) is thefrequency response function of photodetector.

Due to the limitation of the bandwidths of microwavamplifier and the spectrum analyzer, this 50-GHz sigcannot be detected directly. To solve this problem, a th

Fig. 9 Measured optical gain at l51064 nm as a function of Nd31

concentration in an optical polyimide waveguide.

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tunable diode laser with wavelength between the precedtwo lasers is used to down-convert this 50-GHz signatwo signals at about 25 GHz. This 50-GHz signal was sdirectly through the optical waveguide delay line fabcated. The optical fanout from the waveguide TTD linecombined with the output of the third laser and is then sto an ultrafast photodetector with a 25-GHz microwave aplifier, which is connected to an rf spectrum analyzer. Tmeasured signals ofDv5v12v25(v12v3)1(v32v2)524.85125.90550.75 GHz is shown in Fig. 12.

7 Detector-Switched Optical True-Time-DelayLines

Figure 13 shows a photo of a polymeric waveguide TTline fabricated on an 8-cm-long glass substrate with waguide thickness of 10mm and width of 50mm. Surface-normal waveguide grating couplers are fabricated with50-mm coupling length and a 10-mm separation. The opcal rf signals, propagating through the channel waveguare coupled surface-normally into a high-speed twphotodetector array, placed right under the waveguidelay line. The electrical output of two high-speed photodtectors are electrically combined with a single output. Tbandwidth of these detectors is;60 GHz with a 5-V biasvoltage. The output of the electrical response from thetectors is first amplified through a 20-GHz microwave aplifier and then connected to a sampling scope for meaing the optical true delay times. The schematic diagrammeasuring the optical TTDs is also illustrated in Fig. 13.the experiment, the delay time interval of the optical wavguide TTD line is measured by employing a Ti:sapph

Fig. 10 Variation of optical gain at 1064 nm versus pumping wave-length. The optical pumping power is fixed at 5 mW in the measure-ment.

Fig. 11 Generation of rf signals using the optical heterodyne tech-nique.

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femtosecond laser system. Sequential equivalent time spling technique is employed for measuring the small timdelay~;50 ps!. Since the delay signal is repetitive, samplcan be acquired over many repetitions of the signal, wone sample taken on each repetition. When a synchrontrigger is detected, a sample is taken after a very short,well-defined delay. When the next trigger occurs, a smtime increment is added to this delay and the scope taanother sample. This process is repeated many timesthe time window is filled. This enables the oscilloscopeaccurately capture signals whose frequency componentsmuch higher than the scope’s sample rate. A 50-ps deinterval, corresponding to a 10-mm fanout separation ofpolymeric waveguide delay line, is obtained using thsetup and the result is also illustrated in Fig. 13. Thecertainty due to jittering is estimated to be less than 5 psthis experiment.

The detector bias switching is successfully obtainedlunching a short electrical pulse into the photodetector bcircuit while monitoring the photodetector output responunder cw optical rf illumination. Figure 14 shows the eletrical diagram of the experiment. A 500-ps electrical puis coupled into the detector bias circuit. A high-speelectro-optic response is obtained at the photodetectorput end. The output pulse is measured with a linewidth ons, which implies a nanosecond switching speed forphotodetector-switched optical polymeric waveguide TTline.

Fig. 12 Indication of a 50.75-GHz optical rf signal generated byoptical heterodyne technique.

Fig. 13 Schematic diagram for measuring the optical TTDs using afemtosecond Ti:sapphire laser system.


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8 Wavelength-Division Multiplexing in PolymericChannel Waveguides

To provide the multilink optical TTD functionality, thewavelength-division multiplexing~WDM! technique can beemployed in conjunction with polymeric waveguide graticouplers. Waveguide grating couplers are ideal for proding a large number of optical rf modulated TTD signalsphotodetectors when the WDM technique is employedmultilink communications. The unique nonblocking featuenables us to have a large number of optical fanouts fmultiple laser beams along the waveguide propagation.cause of the strong wavelength selectivity of optical grings, waveguide gratings can be designed and fabricatediffract light at a desired wavelength by adjusting gratiperiod. In other words, it can function as a wavelengdivision demultiplexer in the waveguide delay line circuwhen multiple laser beams are used for multiple commucation links.

To demonstrate the concept of a simple multilink aproach, a set of waveguide surface-normal grating coupwith operating wavelengths ofl351550 nm were fabri-cated over a polymeric waveguide delay line. In the expment, three laser beams with output wavelength atl15950 nm, l251300 nm, andl351550 nm, respectivelywere employed and coupled into the testing waveguidelay line, as shown in Fig. 15. To determine the opticcrosstalk among the multiple channels, three input laswere further amplitude modulated at three different fquencies~0.9, 0.7, and 1.1 MHz!, respectively. This en-abled us to separate the measured crosstalk and signthe display screen simultaneously. The input power of emodulated laser beam was adjusted at the same l~;500mW!. The optical output from a grating coupler wadetected by a fiber pig-tailed photodetector through a figraded-index~GRIN! lens. In the experiment, the detectwas positioned at the waveguide grating coupler desigfor surface-normal coupling atl51550 nm. The fabricatedwaveguide grating has a 30-mm interaction length with acoupling efficiency of 5%. The optical crosstalk was me

Fig. 14 Electric diagram for measuring the switching speed of bi-ased photodetectors.

Fig. 15 Schematic of a multilink waveguide delay line using theWDM technique.


-

o

-

n

l

sured by an rf spectrum analyzer~Model hp 8566B!. Thechannel crosstalk was successfully determined at a sigto-noise ratio~SNR! of 32 dB, as shown in Fig. 16. Atunable laser with a wavelength tuning range from 14701650 nm was further used to determine the coupling wdow of waveguide gratings. The measured transmissspectrum had a 40-nm, 3-dB linewidth with a 100-nwavelength separation between the first two minima.

9 Conclusions

We successfully demonstrated a photonic waveguide-baTTD line using polymeric waveguides, waveguide ampliers, and wavelength-selective grating couplers in conjution with bias-switched photodetectors. Polymeric wavguide technology, including ultra-low-loss polymerwaveguides, optical waveguide amplifiers, and wavelengselective grating couplers in conjunction with bias-switchphotodetectors, offers a unique hybrid integration in reaing advanced photonic PAAs based on optical TTD linSuch a hybrid integration of photonic devices eliminathe most difficult optoelectronic packaging problem in dveloping advanced photonic PAAs. This integrated aproach not only reduces the cost associated with optoetronic packaging, but also reduces the system payload wan improved reliability for airborne applications. Currentlall of the building blocks essential for the fabricationwideband PAAs are becoming available, while the eleccally switched optical polymeric waveguide delay lines ctainly present a very promising technology in this field.

Acknowledgments

This research is supported by the Ballistic Missile DefenOrganization~BMDO!, the Air Force Office of ScientificResearch~AFOSR!, the Office of Naval Research~ONR!,the 3M Foundation, and Raytheon Systems Co.

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Suning Tang is chief scientist with Radiant Research, Inc., Austin,Texas. He received his BS degree in electrical engineering in laserdevices from Nanjing Institute of Technology, China, his MS degreein optics from the Weizmann Institute of Science, Israel, and hisPhD degree in electrical engineering in optoelectronic interconnectsfrom the University of Texas at Austin. He was with Cirrus Logic andAdvanced Photonics Technologies for 4 years before he joined Ra-diant Research, Inc. His work in the past 15 years has includedoptical interconnects, polymer-based waveguide devices, holo-graphic devices, fiber optic devices, optical modulators/switches,optical control of microwave signals, and semiconductor photonicdevices. He has been the principal investigator for many awardedSBIR research programs sponsored by the Department of Defenseand by private industries. Dr. Tang has chaired several internationalconferences organized by SPIE, he has published more than 50papers in IEEE, OSA, AIP, and SPIE journals and holds severalpatents, and he is a member of SPIE and OSA.

Ray T. Chen is the Temple Foundation Endowed Professor at theUniversity of Texas, Austin. His research includes GaAs all-optical2-D cross bar switch arrays, 2-D and 3-D optical interconnections,polymer-based integrated optics for true-time-delay lines, polymerwaveguide amplifiers, graded index polymer waveguide lenses, ac-tive optical back planes, traveling wave polymer waveguide switch-ing devices, and holographic optical elements. Dr. Chen has chairedand was a program committee member for more than 10 domesticand international conferences organized by SPIE, IEEE, and PSC.He is also the invited lecturer for the short course on optical inter-connects for the international technical meetings organized by SPIE.Dr. Chen has published more than 130 papers and has deliverednumerous invited talks for professional societies. He is a fellow ofSPIE and a member of IEEE, OSA, and PSC.

Biographies of the other authors not available.


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