-
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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E~F,t !5 (n50
N
An exp~ ivmt ! exp@ i ~cn1nkmL sinF!#,
~1!
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!
643© 2000 Society of Photo-Optical Instrumentation Engineers
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Tang et al.: Ultra-low-loss polymeric waveguide circuits for
optical . . .
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
n
t
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l
s
,
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.
-
Tang et al.: Ultra-low-loss polymeric waveguide circuits for
optical . . .
Fig. 2 Electrical diagram of a detector-switched optical
waveguide TD line for photonic phased-arrayantennas.
allya-
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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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Tang et al.: Ultra-low-loss polymeric waveguide circuits for
optical . . .
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.
646 Optical Engineering, Vol. 39 No. 3, March 2000
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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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Tang et al.: Ultra-low-loss polymeric waveguide circuits for
optical . . .
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.
sf
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f
r
f
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.
647Optical Engineering, Vol. 39 No. 3, March 2000
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Tang et al.: Ultra-low-loss polymeric waveguide circuits for
optical . . .
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.
susingandnergyindm--
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o-
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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
d
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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
i d~ t !5eh
hn@A1
21A2212F~Dv!A1A2 cos~Dv!t#, ~10!
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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Tang et al.: Ultra-low-loss polymeric waveguide circuits for
optical . . .
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.
,
-
-
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.
649Optical Engineering, Vol. 39 No. 3, March 2000
-
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lian,
Tang et al.: Ultra-low-loss polymeric waveguide circuits for
optical . . .
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.
650 Optical Engineering, Vol. 39 No. 3, March 2000
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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.
651Optical Engineering, Vol. 39 No. 3, March 2000