Tunable 360° photonic radio frequency phase shifterbased on optical quadrature

double-sideband modulation and differential detectionXiaoxiao Xue,1,* Xiaoping Zheng,1,2 Hanyi Zhang,1 and Bingkun Zhou1

1State Key Laboratory on Integrated Optoelectronics/Tsinghua National Laboratory for Information Science and Technology,Department of Electronic Engineering, Tsinghua University, Beijing 100084, China

2email: xpzheng@tsinghua.edu.cn*Corresponding author: xxx03@mails.tsinghua.edu.cn

Received September 2, 2011; revised October 20, 2011; accepted October 25, 2011;posted October 26, 2011 (Doc. ID 153999); published November 28, 2011

We propose a novel structure of a photonic RF phase shifter based on the vector-sum principle. The optical signalwith quadrature double-sideband modulation passes through a dual-output Mach–Zehnder interferometer (MZI),and the two outputs are differentially detected. Two phase-quadrature RF terms are generated, and their amplitudescan be controlled in a triangularly complementary way by changing the phase of the MZI. A full tuning range of0°–360° at 14GHz is demonstrated experimentally accompanied by nearly constant RF amplitude. The validity ofusing our scheme in all-optical RF phase modulation is also verified. © 2011 Optical Society of AmericaOCIS codes: 060.5625, 350.4010.

The photonic RF phase shifter is a device that can manip-ulate the microwave phase in optical means, and it hasgreat potential in the optically controlled phase array an-tenna [1], analog signal processing [2], and long-distancemicrowave distribution [3]. Compared to its electricalcounterparts, the photonic RF phase shifter has the ad-vantages of high frequency, light weight, and immunity tothe electromagnetic interferences.One well-known method for RF phase shifting is the

vector-sum technique. In the vector-sum scheme, theRF phase is controlled by adjusting the amplitudes oftwo RF components with quadrature phases. The mostdirect way to implement a photonic RF vector-sum phaseshifter is by independently modulating and jointly detect-ing two optical signals, each with one RF component [4].The amplitude of each component is adjusted by control-ling each optical power and modulation slope. Such asystem is complicated in practice, because two RF portsare needed for modulation and multiple controls areneeded for phase tuning. One simpler method is usingsingle-drive modulation. An optical differential time de-lay element is inserted after the modulation stage to in-troduce a quadrature phase difference between the twoRF components [5–7], and the RF amplitudes are con-trolled in a complementary way via a directional coupleror a polarization controller. Though the complexity is re-duced in these schemes, one problem is that the outputRF amplitude also changes as the phase is tuned.In this Letter, we propose a novel photonic RF vector-

sum phase shifter based on optical quadrature double-sideband modulation and differential detection. Onlyone RF port is needed for modulation and one controlvoltage for RF phase tuning. Experimentally, the RF out-put phase can be tuned continuously between 0°–360°while the RF amplitude keeps nearly constant. The ad-vantages of our scheme include simple structure, easytunability, and large-scale integratability.The experimental setup is shown in Fig. 1. A dual-

parallel LiNbO3 Mach–Zehnder modulator (DPMZM) isused for quadrature double-sideband modulation. There

are three MZ structures in the DPMZM: MZ1 is driven bythe RF input and biased at the carrier-suppression point,MZ2 is RF short connected and biased at the maximumtransmission point, and the bias voltage of MZ3 is ad-justed such that the phase shift between MZ1 and MZ2is π=4. Under small-signal modulation, only the twofirst-order sidebands are considered. The optical fieldafter the DPMZM can be expressed by

eðtÞ ¼ a exp½jðωtþ π=4Þ� þX

m exp½jðωt� ΩtÞ�; ð1Þ

where a and m are the optical amplitude of the centralcarrier and the sidebands, respectively; ω and Ω are theoptical and the RF frequency, respectively; and

Pstands

for summation of the two terms with different signs.The modulated light then goes through a phase-tunable

MZ interferometer (PTMZI), which is composed of aLiNbO3 z-cut phase modulator (PM) cascaded with apolarization beam splitter (PBS). The illustration ofthe PTMZI is shown in the insets of Fig. 1. Polarization

Fig. 1. (Color online) Experimental setup of the vector-sumRF phase shifter: PC, polarization controller; DPMZM, dual-parallel Mach–Zehnder modulator; PM, LiNbO3 phasemodulator; PBS, polarization beam splitter; BPD, balancedphotodetector; LNA, RF low-noise amplifier; PTMZI, phase-tunable Mach–Zehnder interferometer.

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controllers PC2 and PC3 are adjusted so the polarizationsof both the input lightwave and the PBS are 45° to theprincipal axis of the PM. The differential time delay be-tween the TE and TM modes in the PM is represented byτ. The amplitude transfer function of the upper and loweroutputs of the PTMZI is

TuðjωÞ ¼ cos½ðωτ þ ϕÞ=2�; ð2Þ

TlðjωÞ ¼ cos½ðωτ þ ϕÞ=2þ π=2�; ð3Þ

respectively. Here ϕ stands for the phase difference be-tween the two polarizations due to the control voltageapplied to the PM, and is given by

ϕ ¼ πVc=Vπ;TE − πVc=Vπ;TM; ð4Þ

where Vc is the control voltage and Vπ;TE and Vπ;TM re-present the half-wave voltage for the TE and TM modes,respectively. The modulation sensitivity is quite differentfor the two polarizations in the PM (Vπ;TE > Vπ;TM) [8];thus, the phase of the PTMZI can be tuned very quicklyby changing Vc. The optical fields of the upper and loweroutputs of the PTMZI are given by

euðtÞ ¼ a exp½jðωtþ π=4Þ� cos½ðωτ þ ϕÞ=2�þm

Xexp½jðωt� ΩtÞ� cos½ðωτ � Ωτ þ ϕÞ=2�;

ð5Þ

elðtÞ ¼ a exp½jðωtþ π=4Þ� cos½ðωτ þ ϕÞ=2þ π=2�þm

Xexp½jðωt� ΩtÞ� cos½ðωτ � Ωτ þ ϕÞ=2

þ π=2�: ð6Þ

At the balanced photodetector (BPD), the first-orderRF current is then

iðtÞ ¼ euðtÞ × e�uðtÞ − elðtÞ × e�l ðtÞ¼ 2am

XcosðΩt∓π=4Þ cosðωτ þ ϕ� Ωτ=2Þ: ð7Þ

When Ωτ ¼ π=2, the RF current is then

iðtÞ ¼ 2am cosðΩtþ ωτ þ ϕÞ: ð8Þ

Here we omit the DC and the small second-order harmo-nic components for simplicity. We can see that the opti-cal phase ϕ is introduced to the RF output. The output RFphase can then be tuned simply by adjusting the controlvoltage applied to the PM. The physical principle isclearly shown by the intermediate result of Eq. (7). Theoutput current consists of two terms that result from thecentral carrier mixing with the upper and lower side-bands, respectively. Because there is an additional π=4phase shift between the carrier and the sidebands, theresulting phases of the two RF terms are quadrature.The amplitudes of this two phase-quadrature RF termscan be controlled in a triangularly complementary wayby changing the phase of the PTMZI, resulting in a

summed RF current with a tunable phase and a constantamplitude.

In our experiments, the differential time delay of thePM is 18:2 ps and the RF central frequency is calculatedto be 13:7GHz. The RF half-wave voltage of the DPMZMis 3V. The responsivity of the BPD is 0:7A=W. The RFlow-noise amplifier has a gain of 35 dB over 10–20GHz.We applied an RF test signal of 14GHz and 10 dBm to theDPMZM. The measured optical carrier-to-sideband ratioafter the DPMZM is about 13 dB. The optical power at theinput of the PBS is 3:5dBm. The measured RF outputwaveforms are shown in Fig. 2. The RF phase is tunedfrom 0° to 360° with a step of 45° by changing Vc from0V to 5:8V. The RF amplitude variation in the tuning pro-cess is about 0:4 dB, which is mainly due to the deviationof the RF frequency from the central value (13:7GHz).Theoretically, when Ωτ ≠ π=2, the RF amplitude willnot keep constant as the phase is tuned, and the relation-ship between the RF phase and the control voltage is nolonger linear. Detailed plots of the RF amplitude andphase versus the PTMZI phase and the RF frequencyare shown in Figs. 3 and 4, respectively. If we definethe 1 dB bandwidth B1dB as the range in which the am-plitude variation is less than 1dB and the 5° bandwidth

Fig. 2. (Color online) Measured output waveforms of thephotonic RF phase shifter (averaged by 5; RF frequency,14GHz).

Fig. 3. (Color online) (a) Theoretical 3D plot of RF amplitudeversus optical MZI phase and RF frequency and (b) overlapped2D plots of RF amplitude versus RF frequency at differentoptical MZI phases (the 1 dB bandwidth is 2GHz).

4642 OPTICS LETTERS / Vol. 36, No. 23 / December 1, 2011

B5° in which the phase variation is less than 5°, then B1dBand B5° will be 2GHz and 1:5GHz, respectively.To further verify the validity of the new photonic RF

phase shifter, we did an experiment of all-optical RFphase modulation. The experimental setup is shown inFig. 5. The RF carrier of 14GHz is phase modulated by1:4Gbps non-return-to-zero (NRZ) baseband data witha fixed pattern of “10110100.” The normalized basebandwaveform is shown in Fig. 6(a). The actual peak-to-peakvoltage is 3:1V; thus, the RF phase shift between “1” and“0” is approximately π. The normalized RF waveformafter modulation is shown in Fig. 6(b). A phase hop isclearly seen between “1” and “0.” The RF signal is thendemodulated, and the output baseband waveform isshown in Fig. 6(c). The local oscillator power is 10 dBm,and the RF data power is 3 dBm. The cut-off frequency ofthe low-pass filter is 6GHz. The actual peak-to-peak vol-tage of the output baseband waveform is about 150mV.The baseband waveform after demodulation follows thedata source quite well, which demonstrates the validityof using our scheme in all-optical RF phase modulation.Our proof-of-concept system was stable in experi-

ments although no temperature control was usedbecause of the high-quality laser we used and the

short-term stability of the differential time delay. How-ever, temperature control is still needed for long-termstability. In practice, the PTMZI can be implemented ina two-arm form and integrated together with the DPMZMto build a system more compact and stable.

In summary, we have proposed and experimentally de-monstrated a novel photonic RF vector-sum phase shifterbased on optical quadrature double-sideband modulationand differential detection. A full tuning range of 0°–360°accompanied by nearly constant RF amplitude (variation0:4 dB) is demonstrated at 14GHz. The validity of usingour scheme in all-optical RF phase modulation is alsoverified experimentally.

We acknowledge support from the 973 Project undergrant 2012CB315603/04 and the National Natural ScienceFoundation of China (NSFC) under grants 60736003,61025004, and 61032005.

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Fig. 4. (Color online) (a) Theoretical 3D plot of RF phase ver-sus optical MZI phase and RF frequency and (b) overlapped 2Dplots of RF phase versus RF frequency at different optical MZIphases (the 5° bandwidth is 1:5GHz).

Fig. 5. Experimental setup of RF phase modulation using thefast photonic phase shifter: LPF, low-pass filter.

Fig. 6. (Color online) Waveforms of the (a) baseband datasource, (b) phase-modulated RF signal, and (c) baseband dataafter demodulation (RF frequency, 14GHz; baseband bitrate,1:4Gbps).

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