IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 21, NOVEMBER 1, 2010 1565

True Time Delay Photonic Circuit Based onPerfluorpolymer Waveguides

Aydin Yeniay and Renfeng Gao

Abstract—A high-performance 4-bit optical true time delay de-vice is realized on a perfluoropolymer integrated optic platform.The integrated waveguide circuit is based on athermal arrayedwavegide grating with monolithically integrated delay lines in awavelength-selective recirculating loop configuration. The deviceprovides precise delay length control (i.e., 1 m) over 16 channelswith 40-ps incremental time delays. The device offers athermal op-eration over the 0 C–50 C range with a maximum insertion lossof 6.8 dB and a polarization shift of 0.4 nm. This compact device

mm promises applications in code-division mul-tiple access and phased arrayed antenna systems, where the largechannel counts with high resolution delay length controls and smallfootprints are required.

Index Terms—Arrayed wavegide grating (AWG), optical truetime delay (TTD), perfluoropolymers, waveguides.

I. INTRODUCTION

P ERFLUOROPOLYMER waveguides offer a uniquehighly integrated optical signal processing platform with

benefits of ultralow loss, athermal, compact, flexible designof optical properties, and cost-effective features for high-den-sity integrated devices [1]–[3]. Unlike earlier generations ofpolymers, perfluoropolymers contain fully fluorinated systemswith little, or no, hydrocarbon content present, reducing the ab-sorption losses down to 0.001 dB/cm at infrared wavelengths.In such systems, thereby, process-induced losses become pre-dominant and can be minimized by optimizing each processingstep for defects and stress-free, symmetrical low loss channelwaveguide fabrications. In addition, polarization-dependentloss (PDL) is less of a concern for such waveguides whenemployed polymer substrates. PDL usually originates frommaterial birefringence, asymmetric waveguide cross sections,or induced stresses, for example. Stress usually arises duringwaveguide fabrication steps, especially when the materials ofthe waveguide structure and supporting substrate have dif-ferent coefficients of thermal expansion (CTEs). Moreover, theall-polymer system provides flexibility in arranging materialswith appropriate CTE and thermal optic (TO) coefficientswith optimized substrate or superstrate designs for realizingathermal operation. We previously presented such a platformfor passive device applications (Fig. 1) [3].

Manuscript received May 28, 2010; revised July 29, 2010; accepted August15, 2010. Date of publication August 23, 2010; date of current version October06, 2010. This work was supported by the Defense Advanced Research ProjectsAgency (DARPA) through Contract W31P4Q-05-C-R021.The authors are with Photon-X, Malvern, PA 19355 USA (e-mail:

ayeniay@photon-x.net).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2010.2069558

Fig. 1. Measured loss spectrum of the perfluoropolymer waveguides inbands.

In the present letter, we present a 16-channel optical truetime delay (TTD) module based on the similar platform withpropagation losses of 0.06 dB/cm over bands and aPDL of 0.4 dB for athermal operation over 0 C–50 C range.The optical TTD technique is an important element for both RFphotonics and optical telecommunication applications. In RFphotonics, besides many advantages of optical signal processingtechniques (e.g., low distribution loss, reduced size, higherfrequency operation, and increased immunity to electro-mag-netic interference), the major contribution of photonics is to beable to utilize innovative optical TTD techniques, especiallyfor phased array antenna (PAA) systems where electronictechniques are inadequate. In optical telecommunications,TTD is also a promising technique to provide dynamic opticalbuffering elements for optical routing in all-optical networks aswell as encoder/decoder elements for wavelength code-divisionmultiple access (CDMA) networks to increase the correlationproperties of codes and network security (i.e., encoding a databit in both wavelength and time domains) [4]–[6]. Both the op-tical telecommunication and RF photonics applications requiresimilar specifications for TTD modules [6]. These overlappingspecifications are mainly low insertion loss, low coupling lossto single-mode fiber (SMF), and low polarization dependenceas well as compact size with low energy consumption, all ofwhich can be met by the perfluoropolymer waveguide platform.Here, the proposed perfluoropolymer waveguide TTD module,based on the monolithic integration of arrayed wavegide grating(AWG) filtering with long delay lines, provides 4-bit athermaloperation with an overall loss of 6.5 dB. As employed with atunable laser (e.g., external cavity laser (ECL), sampled gratingdistributed Bragg reflector (SGBR), etc.), this configuration

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1566 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 21, NOVEMBER 1, 2010

Fig. 2. A 4 wafer of TTD chips with various designs with different delay lineincrements. The picture was taken after the RIE etching process.

may realize high resolution, squint-free, agile, wideband andfrequency-independent 4-bit TTD operations in PAA systems.

II. MODULE DESIGN AND FABRICATIONThe concept used in all-polymer AWG-based TTD is a sym-

metric feedback configuration, also known as recirculating pho-tonic filter (RPF) [6], where a modulated optical carrier of atunable laser is delayed and fed back into the symmetric inputport via selective optical fiber delay lines. Here, we demon-strate a monolithically integrated version of the RPF design,where the carrier signal can be steered by the AWG to the ap-propriate integrated delay line, depending on the carrier wave-length, then refocused into a single output port. This TTD canbe considered topologically equivalent to two AWGs in serieswith a delay element in between. Thus, three sequential oper-ations are performed in a single AWG: wavelength demulti-plexing, time delay, and wavelength multiplexing. It is impor-tant to note that the RPF has no loss; all the power at a givenwavelength is diffracted into the output port. Furthermore, dig-ital control of the TTD module can be processed remotely ata tunable laser module with 4-bit configurations, while mono-lithically integrated AWG and delay lines provide all passiveprocesses, resulting in more simplicity within the system.In fabrication of TTD circuits based on perfluoropolymer

channel waveguides, we utilized standard silicon very largescale integration (VLSI) fabrication steps, such as spin coatingof undercladding and core layers, photolithography, reactiveion etching (RIE), and a final spin coating of overcladding aswell as assembling superstrate for thermal management. Thewaveguide is m m in cross-section with a core andcladding refractive index difference of about 1.6%. Fig. 2 showsa 4 polymer wafer with fabricated several 4-bit TTD chipswith various delay increments. VLSI fabrication of monolithi-cally integrated AWG and delay lines enable superior controlof the resolution of the delay lines ( 1 m), thus providingsubpicosecond time delay control. Fig. 3 shows a mask designof a fabricated 4-bit TTD module with respective time delaysfor each channel. In this design, all wavelength channels willloop back into the AWG and focus into a single-channel wave-guide at the output end. Delay lines followed a raceway-like

Fig. 3. Fabricated 4-bit TTD design and calculated respective delay incrementswith a maximum of 600-ps relative time delay.

pattern with 2 mm in radius to ensure the bending losses arenot contributing (i.e., 1 mm). The delay length incrementsare selected to decrease from the channel#1 to 16 by 0.9 cm,corresponding to 40 ps in time delay as seen in the inset tableof Fig. 3. This particular design provides 4-bit operation withmaximum relative time delay of 600 ps with 40-ps incre-ments, where the maximum and minimum physical delaysare calculated 1073 and 473 ps, respectively. The AWG hasa diffraction order of 66, FSR of 23.5 nm, grating waveguidenumber of 126, and waveguide path difference of 77 m. Thefiber-to-fiber spectral transmission characteristics of the AWGwere measured in high resolution by synchronizing an ECL(i.e., ANDO 4321) with an optical spectrum analyzer (OSA).Adjacent crosstalk of such an AWG was previously measuredto be approximately 28 dB with a 2.8-dB insertion loss in asingle pass configuration [3].Fig. 4 shows the output spectra of all channels with delay

increments. All channels go through AWG twice and respec-tive delay lines in between. Insertion loss of the module variesfrom 6.8 to 5.6 dB due to additional loss acquired from delayline changes and AWG transmission characteristics, where theside channels suffer slightly higher loss compared to those ofcenter. We also measured polarization-dependent wavelengthshift of 0.4 nm and PDL of 0.4 dB, which is mainly due to thepolarization dependence of bending of delay lines. The fabrica-tion resolution of the delay lines is within 1 m owing to thefact that the features are directly transferred from the photomaskwith high fidelity. This TTD would operate down to the giga-hertz regime for beam forming applications in PAA systems.The overall chip size is 20 mm 32 mm. In case a longer delayis required for lower frequency PAA operation, the delay linesin the looped back region can be designed with spiral turns togenerate more delay time without a significant increase in theloss and chip size.

III. TTD ATHERMAL PROPERTIESDue to its phase-sensitive nature, AWG’s channel wave-

lengths usually exhibit highly sensitive temperature de-pendence. In AWGs, the wavelength shift as a func-tion of temperature can be represented by the relation:

YENIAY AND GAO: TTD PHOTONIC CIRCUIT BASED ON PERFLUORPOLYMER WAVEGUIDES 1567

Fig. 4. Fiber-to-fiber measured transmission spectrum of 4-bit TTD module.

Fig. 5. Calculated effective CTE variations with the superstrate thickness in asystem with substrate, thin film layers, and superstrate.

, where is the wavelength,the temperature, the effective index of the waveguide, andthe thermal expansion coefficient experienced by the wave-

guide array section within the AWG device [7]. The athermalcondition is, therefore, .In the usual case where the thin waveguide layers are placed

on top of a much thicker substrate, the substrate CTE dictatesthe value of . Previously, we presented a superstrate overlayapproach that sandwiches the polymer waveguides in betweena polymer superstrate and a polymer substrate [2]. EffectiveCTE at waveguide layers is determined by the thickness andthe Young’s modulus of the substrate and the superstrate. Fig. 5shows the thermal expansion simulation results for our systemfor effective CTE changes with respect to superstrate thickness.By tuning the superstrate thickness and, therefore, the effectivethermal expansion coefficient, we are able to tune the AWG tem-perature dependence to achieve athermal performance by can-celing the TO effect. Using this technique, the measured tem-perature shift of our perfluorinated polymer AWG is less than0.05 nm within 0 C–50 C range as seen in Fig. 6. This tech-nique has the immediate advantage of tunability after the deviceis fabricated. Further, the superstrate can be chosen from a large

Fig. 6. Measured temperature dependence of AWG channel wavelength shiftwith superstrate for the central channel at 1552.6 nm.

variety of materials since it need not meet the requirements ofthe waveguide substrate, such as surface smoothness and sta-bility under device fabrication conditions.

IV. CONCLUSION

We demonstrated a high-performance 4-bit TTD device ona perfluoropolymer integrated optic platform that is based onathermal AWG with monolithically integrated delay lines in awavelength-selective RPF configuration. The device providesprecise delay length control (i.e., 1 m) over 16 channels with40-ps incremental time delays. The device offers athermal op-eration over 0 C–50 C range with a maximum insertion lossof 6.8 dB and a polarization shift of 0.4 nm. This compact de-vice mm promises applications in CDMA andPAA systems, where the large channel counts with high resolu-tion delay length controls and small footprints are required.

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