Gigahertz optical tuning of an on-chipradio frequency photonic delay lineYANG LIU,1,2,* AMOL CHOUDHARY,1,2 DAVID MARPAUNG,1,2 AND BENJAMIN J. EGGLETON1,2

1Centre for Ultrahigh Bandwidth Devices for Optical System (CUDOS), Institute of Photonics and Optical Sciences (IPOS), School of Physics,University of Sydney, NSW 2006, Australia2Australian Institute for Nanoscale Science and Technology, University of Sydney, Sydney, NSW 2006, Australia*Corresponding author: yang.liu@sydney.edu.au

Received 1 February 2017; accepted 6 March 2017 (Doc. ID 286010); published 3 April 2017

Rapid and continuous tunability of time delay is a crucial functionality for radio frequency (RF) photonic signalprocessing systems. Recent developments in photonic integration have enabled realizations of integrated microwavephotonic (MWP) delay lines based on optical resonant devices such as ring resonators, typically tuned by slow thermo-optic effect. Here, we introduce an optical tuning approach to controlling and switching RF time delay from integratedoptical ring resonators with a fast tuning speed. We demonstrate seamless tuning between pulse delay and advance-ment, as well as gigahertz switch capability without modifying the properties of resonators. This scheme opens thepossibility for wideband advanced time-delay manipulation of RF signals for phase-arrayed antennas and radarapplications in a general and compatible approach. © 2017 Optical Society of America

OCIS codes: (060.5625) Radio frequency photonics; (130.0130) Integrated optics.

https://doi.org/10.1364/OPTICA.4.000418

1. INTRODUCTION

Microwave photonic (MWP) delay lines are key components insignal filtering, signal synchronization, and radar target simulatorsin advanced defense and radio frequency (RF) communicationsystems [1–4]. In these applications, fast delay tunability is nec-essary to provide a fast signal-pointing speed to microwave radarphased array systems [4] and the fast reconfigurability required indynamic cognitive RF systems [5]. To avoid suffering from largefootprints, MWP delay lines are preferably implemented in com-pact photonic devices based on optical resonances where thedispersive properties determine the signal group delay responses[6–8]—for instance, fiber Bragg gratings (FBGs) [9] and stimu-lated Brillouin scattering (SBS) [10] in fiber. However, theseschemes require long-length waveguides and high pump power.

Recent progress on photonic integrated circuits (PICs) has in-cluded an impressive demonstration of on-chip MWP delay lines[11], but they lack the required tuning speed. The fast tunabilitybased on the complementary phase-shifted spectra technique showsthe integration potential, but its on-chip implementation is con-strained by the difficulty of realizing a triangular-shaped spectralfilter for spectrum tailoring [12]. Compact tunable time-delay lineswere achieved by employing the optical resonances of integratedsilicon [13–15] and silicon nitride [16,17] ring resonators, butthe tuning speed is constrained by the slow thermal-optic effect.An on-chip photonic delay line was demonstrated using sub-wavelength grating waveguides in silicon-on-insulator (SOI), butit only offered discrete tuning of time delays [18]. A continuouslytunable time-delay realization was reported based on SBS in an

integrated photonic chip, but the intrinsic response time of theSBS process limits the tuning speed to ∼10 ns, governed by thelifetime of acoustic waves [19]. A controllable delay line basedon a highly dispersive photonic crystal waveguide was demon-strated to be a potential candidate for a fully integrated MWP sys-tem; however, the tuning speed of several microseconds was limitedby the frequency tunability of the optical source [20]. Althoughthese efforts towards miniaturization are crucial for fully integratedMWP processors, they bring complexity to the tuning mechanismof the dispersive properties, increasing the requirements for devicefabrication and power consumption. Hence, an entirely newapproach to creating dynamically tunable MWP delay linesfrom an otherwise passive optical device is desired to facilitatethe widespread implementation of this technology in real-worldapplications.

In this work, a principle for realizing gigahertz (GHz)tunable MWP delay lines in an integrated Si3N4 device is intro-duced and experimentally demonstrated. The technique relieson the interference between a data signal and a referencesignal [21,22] to synthesize larger phase shifts in the microwavedomain, which results in RF signal group delay enhancement.Through experiments, we demonstrate that this techniqueenables flexible switching between advancement and delay ofRF signals and allows for fast tunability up to a GHz tuning speedsolely by controlling the optical power. The results presented inthis work imply the feasibility of wideband operation and com-patibility with existing schemes based on dispersive photonicdevices.

2334-2536/17/040418-06 Journal © 2017 Optical Society of America

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2. PRINCIPLE OF PHASE AMPLIFICATION

The schematic configuration of a conventional tunable MWPdelay line is shown in Fig. 1(a). An RF signal is modulated ontoan optical carrier via electronic-to-optical (E-O) conversion, andthen processed by a photonic device which imparts dispersion-induced signal group delay, i.e., τg � ∂φOR�ω�∕∂ω, whereφOR�ω� is the imposed phase response over the signal frequencyω. In contrast to the conventional MWP delay line, where thegroup delay is consistently determined by the optical dispersionof photonic devices, in the tunable delay line based on phase sim-plification shown in Fig. 1(b), an optically controlled unit ofphase amplification is added to enhance the dispersion-inducedgroup delay. By changing the optical power, a larger phase slopeis synthesized, resulting in an enhanced group delay of the RFsignal through optical-to-electronic (O-E) conversion.

This method that allows for the phase-response amplification isbased on a vector-sum operation [22], as shown by a phasor dia-gram in Fig. 2(a). The vector in solid black represents a data signalA1 cos�ωt � Δφ� at a frequency of ω, with an initial phase shiftΔφ obtained from an optical resonance. Through a vector summa-tion with a reference signal A2 cos�ωt � π� illustrated by a vectorin solid blue, a synthesized signal depicted by a vector in solid redcan be obtained. It is clear to see that the phase shift φ 0 of thesynthesized signal is magnified with respect to the initial phase re-sponse Δφ of the data signal. The resultant phase shift φ 0 can becontrolled by varying the strength of the reference signal amplitudeA2. This amplified phase response imparted on the synthesizedRF signal results in an magnified signal group delay τ 0g, writtenas τ 0g � G · τg , where G is the amplification factor expressed byG � �1 − A2∕A1�−1 (for derivations, see Supplement 1).

In Fig. 2(b), the black line shows the phase response inducedby an optical ring resonator operated in the under-coupled (UC)regime where the coupling losses are less than the internal losses ofthe resonator, with an intrinsic quality factor (Q) of 1 × 106,which approximates the performance of the optical resonatorwe used in the experiments. In the UC region, the optical reso-nator has dispersive phase shifts over the resonance frequencyrange. The phase shift depicted by the black dot is enhancedby the phase-amplification method to a larger phase denotedby the red dot. By extending this principle across the frequencyrange around the resonance, the dispersion will be enhanced to

achieve a larger phase slope, as shown by the red curve, resultingin an improvement in the signal group delay accordingto τ 0g � G · τg .

3. DISPERSION CONTROL

With this scheme, we demonstrated the activation of tuning thesignal group delay by solely varying the strength of the referencesignal. We implemented the technique with a dual-sidebandmodulation where two signals were generated for vector summingby the mixing of sidebands and the optical carrier. As shown inFig. 3(a), an optical carrier is modulated by RF signals, generatingtwo intrinsically out-of-phase (π difference) optical sidebands forthe data signal and reference, respectively. The optical upper side-band acquires a phase shift Δφ imparted by the optical resonanceover the signal spectrum, while the lower sideband serves as the

Fig. 1. Basic block diagrams of (a) a conventional MWP delay line and(b) a MWP delay line with phase amplification. O-E, optical-to-electronic conversion; E-O, electronic-to-optical conversion.

Fig. 2. Schematic illustrations of the operational principle of(a) phase-amplification technique and (b) dispersion enhancement froman initial dispersion induced by an optical resonance.

Fig. 3. (a) Schematic implementation of dispersion control based onthe phase-response amplification, using dual-sideband phase modulation.(b) Simulation results of synthesized phase response with various ampli-tudes of the reference signal. The phase slope flips when the amplituderatio changes from A2∕A1 < 1 to A2∕A1 > 1.

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reference signal with a constant π-phase offset. The amplitude ofthe lower sideband is tuned by an optical filter so as to implementdifferent amplitude ratios A2∕A1. After signal mixing of the opticalcarrier and dual-sideband-based RF interferometry via photodetec-tion, a resultant RF signal is synthesized with an enhanceddispersion across the signal spectrum. The tuning of the amplituderatio A2∕A1 results in a tunable phase-amplification factor G.

Simulation results for investigating the feasibility of this tech-nique are shown in Fig. 3(b). In the simulations, a cascade of threeunder-coupled ring resonators, representing the experiment de-scribed later in this paper, is utilized to broaden the operation band-width for the phase amplification. The black trace shows the initialphase response on the upper sideband induced by the optical res-onance and the red and blue curves represent the enhancement ofthe synthesized dispersion, with operations under the condition ofthe amplitude ratioA2∕A1 < 1. Importantly, when operating in theregime of A2∕A1 > 1, the phase slope of the synthesized dispersionflips to the opposite sign, indicating the switching between negativeand positive signal group delay. Note that the RF signal delay ismanipulated by solely controlling the lower sideband power,without tuning the actual optical resonators.

Figure 4(a) depicts the experimental setup to demonstrate thephase-amplification technique for dispersion control. An opticalcarrier from a distributed feedback laser (Teraxion PureSpectrum, λ � 1550 nm) was modulated by RF signals drivenby a vector network analyzer (VNA, Agilent PNA 5224A) via aphase modulator (PM). The PM is used to generate the π phasedifference for dual sidebands, while the unbalanced sideband am-plitudes controlled by post processing will result in the intensitymodulation. The modulated light was then fed to three cascadedUC rings fabricated using low-loss TriPleX (Si3N4∕SiO2) technol-ogy [23]. Each ring has a free spectral range of 26 GHz and an

intrinsic Q of ∼1 × 106 (a 3 dB linewidth of ∼200 MHz ), andexhibited a low propagation loss of ∼0.1 dB∕cm. The chip wasequipped with on-chip tapers, leading to 7.5 dB fiber-to-fiber in-sertion loss. The amount of coupling, and hence the Q-factor andrejection of the ring resonator, can be tailored through thermo-optic tuning implemented using on-chip chromium heaters. Forthe demonstration, we set the rings to the UC state around10 GHz away from the optical carrier frequency and arrangedthe resonance frequencies with an equal interval of ∼100 MHzto broaden the 3 dB bandwidth up to ∼400 MHz with a rejectionof 5 dB. Subsequently, the upper sideband was processed by theseoptical resonances. A programmable optical filter (Finisar 4000s)was utilized to control the amplitude of the lower sideband.This can also be done by utilizing an IQ modulator that enablesindependent control of the amplitude and the phase. The processedoptical signal came out of the photonic chip, and was detected bya high-speed photodetector (u2t XPDV2120). Through mixingof the processed sidebands along with the optical carrier via photo-detection, the synthesized phase response in the RF spectral do-main was acquired by the VNA, as shown in Fig. 4(b). Anamplification factor G of 3 was obtained with A2∕A1 < 1, whilethe slope flipped to the opposite sign whenA2∕A1 > 1, as expectedfrom the simulation results shown in Fig. 3(b). These changes inmeasured phase slope indicate a flexible tunability of the signalgroup delay, which needs to be experimentally verified with realRF signal delay.

4. OPTICAL TUNING OF SIGNAL DELAY

Based on the dispersion control, here we performed a demonstra-tion of MWP signal delay with flexible tunability, using the setupshown in Fig. 5. In the experiment, we emulate a scenario wherea tunable MWP delay line works at 10 GHz within the x band(8–12 GHz), which is a frequency range for extensive microwaveapplications such as satellite communications, radars, and spacecommunications. RF pulse trains (a baseband RF signal with awidth of ∼3 ns and 20 MHz repetition rate) were generated froman RF signal source (Tabor, WS8352) and then were up-shiftedto 10 GHz by mixing with an RF carrier at 10 GHz from a localoscillator via a frequency mixer. The up-converted RF signals cen-tered at 10 GHz were modulated onto an optical carrier via a PM,generating two out-of-phase sidebands. The modulated opticalsignal was then processed by the optical filter and the photonicchip, as described previously in Fig. 4(a). After the photonic delayline, the detected RF signal was down-shifted from 10 GHz to thebaseband and monitored on an oscilloscope. One can note thatthe frequency bandwidth of the ∼3 ns signal pulse is ∼340 MHz,which can fit within the bandwidth of ∼400 MHz provided

Fig. 4. (a) Schematic of the experimental setup to confirm the workingprinciple of the phase-amplification technique. PM, phase modulator; PD,photodetector; VNA, network analyzer. Qualitative optical spectra denotedby red points are shown above the setup diagram. (b) Experimental resultsof synthesized phase response with various amplitude ratios. Notably, thephase slope flips when the amplitude ratio changes from A2∕A1 < 1 toA2∕A1 > 1.

Fig. 5. Schematic of the experimental setup to demonstrate tunableRF signal delay based on the phase-amplification technique. PM, phasemodulator; PD, photodetector; LO, local oscillator. Qualitative opticalspectra denoted by red points are shown above the setup diagram.

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by the cascade of three optical ring resonators. Potentially, thisscheme can also operate over a wide range of RF carrierfrequencies.

When operated in the region A2∕A1 < 1, as shown in Fig. 6(a),the RF pulse in purple trace using phase amplification shows an evi-dent enhancement in time delay, in contrast to the delay signal in redtrace with time-delay magnification. The dispersion enhancementallows for a group-delay magnification from −440 to −1110 ps withan amplification factor G of ∼3, as indicated by the correspondingmeasured phase responses. For the comparison of the group delay, anRF pulse without time delay is shown by the black trace.

We achieved flexible switching between negative group delay(signal advancement) and positive group delay by changing theamplitude ratio to the region A2∕A1 > 1. As shown in Fig. 6(b),the pulse indicated by the blue trace switches to positive groupdelay (�1570 ps) from signal advancement (−400 ps), which re-sults from the change in the sign of the synthesized phase slopeimparted on the RF signal spectrum. From the phase responsediagram in Fig. 6(b), the positive group delay can be tunablewhen the RF signal undergoes a different positive phase slopein yellow trace. The switching of delay can be explained bythe change in the sign of amplification factor G, when theamplitude ratio varies from A2∕A1 < 1 to A2∕A1 > 1. We notethat the symmetry of the phase responses originates from the dis-persive response imparted by optical resonances, and the phaseamplification is transparent to the chirp of modulated RF signalsas the relative phase offset of two sidebands is fixed.

5. GHZ TUNABILITY

We proceed to demonstrate that the phase-amplification tech-nique enables rapid tunability, which is a striking performancefor MWP systems, such as cognitive RF systems, where rapidlyadjustable MWP components are required.

We evaluated the tuning speed by measuring the change of thesynthesized phase shift at a single frequency, using the method basedon a phase detector [24] that can convert the phase change intovoltage signal (for details in methods and experiments, seeSupplement 1). A gate signal with a duty circle of 50% and rep-etition rate of 100 MHz was used to achieve the switching betweentwo discrete synthesized phase responses, leading to a time-variablevoltage signal that results from the phase difference. From Fig. 7, theconverted voltage signal denoted by the blue dotted line candynamically follow the variations of the gate signal depicted bythe solid red curve, which indicates that the tuning speed of thesynthesized phase response is dominated by the dynamic power con-trol via the high-speed IM. Since the rise and fall times of the gatesignal are below 1 ns, the dynamic tuning speed experimentallyachieved beyond 1 GHz. It is potentially expected to be as highas tens of GHz using a faster driven RF signal and high-speedIM. We compared the tuning speed of this work to other state-of-the-art tuning schemes for RF and MWP applications, as sum-marized in Table 1. The scheme we proposed shows a distinctadvantage over other works based on electric switches [25],micro-electro-mechanical system (MEMS) devices [26], laser fre-quency tuning [27], thermal tuning [28], and spatial light modu-lators (SLMs) [29], with a much faster speed by 4–5 orders ofmagnitude. The tuning speed can be further improved to be com-parable with the report work based on PM [30–32]. For this reason,there is a potential to develop rapidly tunable MWP delay lines.

6. DISCUSSION

Here we discuss the time-delay performance of this tunable MWPdelay line based on phase amplification. From the complex plane

Fig. 6. Experimental results of (a) pulse delay enhancement and(b) switching from negative delay to positive delay, along with corre-sponding spectra.

Fig. 7. Measured oscilloscope signals for demonstration of rapid tun-ing. The gate signal drives the phase switching that is reflected by theconverted voltage indicated by the synthesized signal.

Table 1. Tuning Speed Comparison of Reported MWPTuning Schemes

Tuning Schemes Response Time Functionality

IM (this work) <1 ns Delay linePM [30] <1 ns FilterPlasmonic modulator [31] <1 ns Phased array feederPM [32] ∼40 ns FilterElectric switches [25] ∼100 ns Delay lineMEMS RF devices [26] 1 ∼ 300 μs Phase shifter et al.Laser frequency tuning [27] ∼100 μs FilterThermal tuning [28] ∼170 μs FilterSLM [29] 5 ∼ 30 ms Delay line

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in Fig. 2(a), the maximum amplified phase shift is constrained to�π by the principle of vector-sum operation. This limit implies alarger group delay of half the signal pulse length according toτg � φmax∕B�ω�, where B�ω� is the bandwidth of a trans-form-limited signal, in comparison with the maximal time delayoffered by a maximum phase shift of � 1

2 π in Ref. [12].For the tunability, the performance can be flexibly tuned

through varying the amplitude ratio, achieving desirable amplifica-tion factorG. From the numerical analysis shown in Fig. 8(a), over-all, the output phase responses are obviously enhanced comparedwith the response when A2∕A1 � 0, which indicates no phaseamplification. Small phase shifts can be significantly amplified witha good response linearity. The increase in the amplitude ratio resultsin a magnified phase output, while the linearity of the phaseamplification begins to degrade at a ratio of 0.6. For amplitude ratioabove 0.6, although the phase response shows a saturation behaviorwhen the input phase shift increases, it keeps a good linearity withan input less than 10°. From Fig. 8(b), the smaller phase input canobtain a larger G when applying the same amplitude ratio.

It is worth noting that this scheme also introduces inevitablesignal loss for the synthesized RF signal, as partially destructive RF

interference always occurs. As shown in Fig. 9, the absolute groupdelay increases to peak values when the amplitude ratio ap-proaches 1, while the amplitude of the delayed RF pulse decreasesto a minimum. This can be explained by considering that strongerdestructive RF interference occurs when the amplitude ratio isclose to 1. However, this signal cancellation can be avoided byoperating in the region away from this critical point, and the re-duced signal is easily compensated by using a conventional RFamplifier. Considering an attenuation of 10 dB in the peak powerof the signal pulses for practical applications, the bounds of theoperation region are A2∕A1 � 0.75 for negative signal delay andA2∕A1 � 1.25 for positive signal delay, respectively.

Due to the radical functionality separation of power controlfrom the dispersion, this phase-amplification method providesa general method to manipulate MWP signal group delay, whichis compatible with other types of delay lines based on dispersiveoptical resonances such as integrated Bragg gratings [18] or on-chip SBS [33]. Furthermore, this scheme has the potential toachieve separate carrier tuning [34] for true time delay [35]through additional control of the phase shifts of the two opticalcarriers.

7. CONCLUSION

In conclusion, this simple and versatile scheme paves the way tothe realization of on-chip dynamically tunable photonic delaylines and the achievement of both MWP signal delay and ad-vancement in a passive platform. This technique is promisingfor enhancing conventional microwave photonic delay lines withflexibility and rapid tunability (GHz) and radically relaxing therequirements of complex designs and power consumption for in-tegrated MWP tunable delay lines. These features are beneficialfor applications in RF signal processors, multi-tap RF filters, andphased array antennas.

Funding. Air Force Office of Scientific Research (AFOSR)(FA2386-14-1-4030, FA2386-16-1-4036); Australian ResearchCouncil (ARC) (CE110001018, DE150101535, FL120100029,DE170100585).

See Supplement 1 for supporting content.

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