September 15, 2009 / Vol. 34, No. 18 / OPTICS LETTERS 2757

Optically fed microwave true-time delay basedon a compact liquid-crystal

photonic-bandgap-fiber device

Lei Wei,1,* Weiqi Xue,1 Yaohui Chen,1 Thomas Tanggaard Alkeskjold,2 and Anders Bjarklev1

1DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark2Crystal Fibre A/S, Blokken 84, DK-3460 Birkerød, Denmark

*Corresponding author: lewe@fotonik.dtu.dk

Received May 20, 2009; revised August 11, 2009; accepted August 13, 2009;posted August 19, 2009 (Doc. ID 111644); published September 9, 2009

An electrically tunable liquid-crystal, photonic-bandgap-fiber-device-based, optically fed microwave, true-time delay is demonstrated with the response time in the millisecond range. A maximum electrically con-trolled phase shift of around 70° at 15 GHz and an averaged 12.9 ps true-time delay over the entire modu-lation frequency range of 1–15 GHz are obtained. © 2009 Optical Society of America

OCIS codes: 060.5295, 350.4010, 230.3720.

Microwave photonics has attracted increasing atten-tion for processing microwave and millimeter-wavesignals directly in the optical domain [1]. One impor-tant application is the true-time delay based on pho-tonic technologies, which is an effective way to real-ize broadband phased-array systems [2]. Thereported techniques can be classified into two catego-ries: active optical devices, such as active semicon-ductor waveguides using coherent population oscilla-tion [3] or optical fiber using stimulated Brillouinscattering [4], and passive optical devices [5–10].

In general, the true-time delay based on passiveoptical devices is realized mostly by exploring wave-guide dispersion through the time-domain phase-shift method, which detects the phase of light sinu-soidally modulated at gigahertz frequencies [2].Among the proposed approaches, fiber grating de-vices, as a well-developed one-dimensional periodicstructure, still play an important role in designingcompact and reliable true-time delay units [5]. Aminimum broadband true-time delay of 1 ps at up to30 GHz based on a linearly chirped fiber Bragg grat-ing has been analyzed by changing either the tem-perature or strain along the grating region [6].

Meanwhile, progress in two-dimensional photoniccrystal waveguides (PCWs) reveals a promising solu-tion for buffering and time-domain processing of op-tical signal by tailoring the dispersion properties. Arapid increase in group index up to several tens is ob-served near the bandgap edge [7]. At present, solu-tions to control the group velocity are based mainlyon local heating [8] or microfluid infiltration [9].However, the transmission loss in PCWs (typically20 dB transmission drop near bandgap edge for a200-�m-long device [7,8]) is also a challenge andmust to be conquered in practical applications. More-over, photonic bandgap (PBG) fibers, which confinelight using bandgap effects by two-dimensional peri-odic cladding, have also been employed in the true-time delay modules. A continuously tunable time de-lay from 0 to 500 ps over 1 GHz bandwidth at the Xband was demonstrated with a broadband light

source through a 20.13-m-long PBG fiber [10]. PBG

0146-9592/09/182757-3/$15.00 ©

fibers improve the stability and integratability oftrue-time delay modules. Nevertheless, external pre-cisely broadband optical tunable filters are required.Recently, electrically driven liquid-crystal (LC) infil-trated photonic-bandgap (LCPBG) fiber devices havebeen developed [11–13], which indicates that LC asan infiltration candidate provides the attractive basisfor externally tuning the group index of waveguide.

In this Letter, we demonstrate for the first time (toour knowledge) the realization of true-time delay ofmicrowave signals on a fixed wavelength optical car-rier using a compact electrically tunable LCPBG fi-ber device. Compared with previously reported re-sults, we explore an alternative mechanism to tailorthe dispersion by electrically changing the effectiveindex of the core mode of the LCPBG fiber near thebandgap edge. A maximum phase shift of around 70°at 15 GHz is observed with the driving voltage of260 V rms, which corresponds to an averaged 12.9 pstrue-time delay over the whole modulation frequencyrange of 1–15 GHz. The response time of the deviceis also experimentally demonstrated in the millisec-ond range. The compact design enables this all-in-fiber device easily integrated into microwave photo-nic systems.

The fiber used in the experiments is a large-mode-area photonic crystal fiber (PCF) (LMA-13, CrystalFibre A/S, Denmark), with a solid core surrounded byfive rings of air holes arranged in a triangular lattice.The liquid crystal is MDA-00-3969 (Merck, Ger-many), which has a wavelength-dependent ordinaryand extraordinary refractive index of no=1.4978 andne=1.7192 at 589.3 nm.

The cross section of the LCPBG device is illus-trated in the inset of Fig. 1(a). The LC MDA-00-3969is infiltrated for 20 mm of the length of the fiber byusing capillary forces and then mounted between twov-grooves fabricated on a silicon substrate. Twosingle-mode-fiber pigtails are fixed in the grooves ateach end of the LCPBG fiber for coupling in and outof the device. The Au electrodes are deposited on theside walls of the grooves, forming two sets of elec-

trodes. To ensure a high-fiber-coupling efficiency,

2009 Optical Society of America

2758 OPTICS LETTERS / Vol. 34, No. 18 / September 15, 2009

SU-8 fiber fixing structures are built up on the elec-trodes [13]. The device is driven in bipolar mode byusing two 1 kHz sine wave signals +V, −V, as shownin the inset of Fig. 1(a). Figure 1(b) shows the trans-mission bandgap in the wavelength range1400–1600 nm with different driving voltages at23°C. The total insertion loss is 4.8 dB. It is evidentthat the short-wavelength edge is shifted towardlonger wavelengths by increasing the driving voltage.

Figure 1(a) describes the experimental setup tomeasure the rf phase shift or time delay in theLCPBG fiber device. The electrical microwave signalis generated by the network analyzer and modulatedby a Mach–Zehnder modulator (MZM) onto a laserbeam with a wavelength of 1510 nm, shown as theblack dotted line in Fig. 1(b). The modulated opticalsignal is coupled into the LCPBG fiber device andthen converted back to electrical microwave signal bya high-speed photodetector inside the network ana-lyzer. The network analyzer compares the receivedsignal with the internal reference and infers the mi-crowave phase shift and corresponding powerchange. An erbium-doped fiber amplifier (EDFA) isused to control the input optical power to the device,and the polarization state of the input beam is se-lected by polarization controllers (PCs).

Figure 2(a) shows the measured rf phase shift andthe relative rf power change as a function of the driv-ing voltage for different modulation frequencies. The

Fig. 1. (Color online) Experimental setup for investigatingthe rf phase shift or time delay in the LCPBG fiber device.The inset is the cross section of this compact LCPBG fiberdevice with the electric connection in bipolar mode.(b) Transmission spectrum of this LCPBG fiber device fordifferent voltages. The black dotted line is the optical car-rier at 1510 nm used in experiments.

results demonstrate a continuously tunable rf phase

shifter by changing the driving voltage. A maximumrf phase shift of around 70° at a modulation fre-quency of 15 GHz is achieved. An obvious phase shiftfor different modulation frequencies is observed onlywhen the driving voltage is higher than 220 V rms.Since the short-wavelength bandgap edge starts tocross 1510 nm above 220 V rms, a distinct change ofgroup index at this wavelength is expected. For thesame voltage control, the rf power at different modu-lation frequencies has a small variation, whichagrees well with the optical power change shown inFig. 1(b).

Figure 2(b) shows the rf phase shift �� and corre-sponding time delay �t against modulation frequencyf. A simple relation with group index change �ng isgiven by ��=2�f�t=�ng��2�fL� /c�, where c is thespeed of light in vacuum and L is the length of thedevice. The values of group index change 0.071 at240 V rms, 0.113 at 250 V rms, and 0.194 at260 V rms are estimated through linear regressionfrom the experimental results shown in Fig. 2(b). Thegroup index change �ng over the optical frequency �,as given by �ng=�neff+�����neff� /��� [7], is origi-nated from the change in the effective index �neff ofthe core mode of infiltrated PCF near the bandgapedge when the driving voltage is applied. The first-order derivative term addresses the extra contribu-tion from the shifting of steep bandgap edge. Consid-ering different PCF structures and mode confinement[14], the typical value of �neff due to biased LC is onthe order of 10−3. By conducting a simulation basedon the model proposed in [15], we estimated a maxi-mum value of group index change 0.079 at 260 V rmsfor optical carrier at 1510 nm, which is within thesame order of our experimental observation. The

Fig. 2. (Color online) (a) Radio-frequency phase shift andrelative rf power change versus the driving voltage for dif-ferent modulation frequencies; (b) rf phase shifts and cor-responding time delay as a function of the modulation fre-quency for different driving voltages.

quantity mismatch might be further minimized,

September 15, 2009 / Vol. 34, No. 18 / OPTICS LETTERS 2759

since the simulated fiber structure and extrapolatedrefractive indices, as well as the filling ratio of LC(which are critical for the bandgap calculation), varyslightly from reality. At 260 V rms, an averaged12.9 ps time delay over the entire measurementbandwidth is achieved, which indicates a broadbandmicrowave true-time delay. If the driving voltage in-creases further, a longer time delay will be expected.However, a larger power decrease has to be consid-ered. One possible solution is to combine optical am-plifiers as power boosters. The maximum modulationfrequency here is limited only by the capacity of thenetwork analyzer. The large rf phase variations athigh frequencies around 15 GHz are caused mainlyby the electrical modulation signal out of the operat-ing range of the network analyzer. Furthermore, thesignal quality could be largely improved by diminish-ing the mismatch between the bandgap position ofthe device and the optimal working wavelengthrange of EDFA and the modulator. No memory effectof this device is found, i.e., when the voltage is tunedoff, the phase returns to the initial state.

The response time of this device is also investi-gated. When the driving voltage is on, there is a lowtransmission at the working wavelength 1510 nm,where microwave signals are carried and the corre-sponding phase shift is obtained, whereas no phaseshift is found when the electric field is off. Thereforeswitching is performed by amplitude modulating the1 kHz sine driving voltage by a 1 Hz square enve-lope, as shown in Fig. 3. The dots show experimentalphase shift results with a solid line by exponentialfitting. Here, the rise and decay time ton and toff aredefined as the time required for phase shift changefrom 10% to 90% at the corresponding switchingedge. Although a longer decay tail is expected for LCmolecules to relax back to equilibrium, a faster decay

Fig. 3. (Color online) Time-resolved rf phase shift at15 GHz when a 1 kHz sine driving voltage is amplitudemodulated by a 1 Hz square envelope. Dots are experimen-

tal results with a solid line by exponential fitting.

time toff is found, since the corresponding phase shiftis nearly turned off at the driving voltage around220 V rms, as shown in Fig. 2(a), and thus speeds up.

Here, we demonstrate the true-time delay of micro-wave signals on an optical carrier using a compactelectrically tunable LCPBG fiber device with the re-sponse time in the millisecond range. The time delayis continuously tuned by changing the driving volt-age. At 260 V rms, an averaged around 12.9 ps true-time delay in the entire modulation frequency rangeof 1–15 GHz is obtained. The reported results dem-onstrate that the compact LCPBG fiber device notonly is easily integrated into microwave photonic sys-tems but also has the potential to achieve a tunabletrue-time delay at different microwave or millimeter-wave frequency bands. To improve the signal quality,the bandgap position shaping of the LCPBG fiber de-vice is demanded.

References

1. J. Seeds and K. J. Williams, J. Lightwave Technol. 24,4628 (2006).

2. J. Yao, J. Lightwave Technol. 27, 314 (2009).3. F. Öhman, K. Yvind, and J. Mørk, IEEE Photonics

Technol. Lett. 19, 1145 (2007).4. Y. Okawachi, M. S. Bigelow, J. E. Sharping, Z. Zhu, A.

Schweinsberg, D. J. Gauthier, R. W. Boyd, and A. L.Gaeta, Phys. Rev. Lett. 94, 153902 (2005).

5. A. Molony, C. Edge, and I. Bennion, Electron. Lett. 31,1485 (1995).

6. V. Italia, M. Pisco, S. Campopiano, A. Cusano, and A.Cutolo, IEEE J. Sel. Top. Quantum Electron. 11, 408(2005).

7. T. Baba, Nat. Photonics 2, 465 (2008).8. Y. A. Vlasov, M. O’Boyle, H. F. Hamann, and S. J.

McNab, Nature 438, 65 (2005).9. M. Ebnali-Heidari, C. Grillet, C. Monat, and B. J.

Eggleton, Opt. Express 17, 1628 (2009).10. Z. Liu, X. Zheng, H. Zhang, Y. Guo, and B. Zhou, Opt.

Lett. 31, 2789 (2006).11. M. W. Haakestad, T. T. Alkeskjold, M. D. Nielsen, L.

Scolari, J. Riishede, H. E. Engan, and A. Bjarklev,IEEE Photonics Technol. Lett. 17, 819 (2005).

12. C. Zografopoulos, E. E. Kriezis, and T. D. Tsiboukis, J.Lightwave Technol. 24, 3427 (2006).

13. L. Wei, T. T. Alkeskjold, and A. Bjarklev, “Compactdesign of an electrically tunable and rotatable polarizerbased on a liquid crystal photonic bandgap filter”(submitted to IEEE Photonics Technol. Lett.).

14. J. Lægsgaard, A. Bjarklev, and S. E. B. Libori, J. Opt.Soc. Am. B 20, 443 (2003).

15. J. Weirich, J. Laegsgaard, L. Scolari, L. Wei, T. T.Alkeskjold, and A. Bjarklev, Opt. Express 17, 4442

(2009).