1332 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 13, JULY 1, 2014

Optical and RF Characterization of a LithiumNiobate Photonic Crystal Modulator

Huihui Lu, Wentao Qiu, Clément Guyot, Gwenn Ulliac, Jean-Marc Merolla,Fadi Baida, and Maria-Pilar Bernal

Abstract— We optically characterize a micrometric radio fre-quency electro-optic modulator based on a lithium niobate pho-tonic crystal. With the vertically deposited sidewalls electrodes,the experimental results show a half-wave voltage length productVπ

∗L figure of merit of 0.0063 V-cm, a bandwidth of ∼1 GHzfor an active interaction area of ∼37 μm2. This is the smallestfigure of merit measured on an electro-optic modulator to date byseveral orders of magnitude with respect to bulk lithium niobate.

Index Terms— Modulator, electro-optic, photonic crystals,lithium niobate.

I. INTRODUCTION

THE success of future optical interconnects relies heavilyon the performance capabilities of compact electro-optic

modulators, designed to transfer an electronic bit stream ontoan optical carrier wave. Today’s state-of-the-art technologyfor electro-optical modulation in the telecom and datacomsector is based on an artificial crystal named lithium niobate(LiNbO3 - LN) [1]. In LN modulators the linear electro-opticeffect (Pockels effect) is utilized, which provides both high-speed and strict linear phase modulation. LN is a ferroelectriccrystal which possesses linear and nonlinear useful opticalproperties. As a consequence, it is a widely used material forguided-wave optical devices and high performance photonicintegrated functionalities being the material of choice notonly for modulators but also switches, frequency doublers,surface acoustic wave devices, Pockels cells, optical parameteroscillators and Q switching devices for lasers.

This material system only requires an electrical field anddoes not require a steady current flow, like it is the case insilicon modulators [2], [3]. This property exhibits tremendous

Manuscript received January 12, 2014; revised March 16, 2014; acceptedMay 6, 2014. Date of publication May 13, 2014; date of current versionJune 13, 2014. This work was supported in part by the ANR Project PhoXcryunder Contract ANR-09-NANO-004, in part by the Labex ACTION Programunder Contract ANR-11-LABX-01-01, and in part by the FundamentalResearch Funds for the Central Universities, China, under Grant 21613325.

H. Lu is with the Key Laboratory of Optoelectronic Information andSensing Technologies, Guangdong Higher Education Institute, Jinan Uni-versity, Guangzhou 510632, China, and also with the Department of Opto-electronic Engineering, Jinan University, Guangzhou 510632, China (e-mail:lucentlhh@gmail.com).

W. Qiu, C. Guyot, G. Ulliac, J.-M. Merolla, F. Baida, and M.-P. Bernal arewith the Institut FEMTO-ST, Université de Franche-Comté, Besançon CedexF-25030, France (e-mail: maria-pilar.bernal@univ-fcomte.fr).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2014.2323117

advantages in terms of power efficiency in the realization ofswitches and modulators.

In order to characterize the performance of a modulator,the half-wave voltage length product (Vπ

∗L) is a good figureof merit for comparing modulators of different lengths. Foroptical modulators that are not based in Mach-Zehnder (MZ)configuration, Vπ is defined as the driving voltage neededto go from the highest transmission to the lowest transmis-sion value [4]–[7]. L is defined, as in the conventional MZmodulators, as the interaction length with the electrodes. Twoparameters can lower this figure of merit. Since the half-wave voltage of the modulator scales with the separationbetween electrodes, reducing the electrode separation is criticalin achieving low Vπ in EO modulators. The use of resonant or“slow-light” structures can significantly further reduce Vπ

∗L.Low Vπ

∗L enables shorter electrodes and makes it easier toreduce the electrode capacity, and therefore to increase thebandwidth.

Photonic crystals (PhC) are capable of satisfying theserequirements [8]–If properly designed, they can exhibit slowlight propagation and their small size implies short electrodeseparation and short electrode lengths.

With all the arguments said before, it is natural to goa step further from the conventional, well-known LN mod-ulators and to propose PhC modulators fabricated on LNwaveguides. LN has hardly being considered as a substratefor the fabrication of photonic crystals due to numerousintegration challenges. It is well known the chemical resis-tivity of LN and its hardness to be mechanically etched.Since several years, attempts to get around this problemhave been done by several groups. Thus, techniques like FIBetching, ion-beam enhanced etching, and crystal ion slicing orfemto-second laser machining [9]–[15] have demonstrated thepossibility of micrometric size LN devices with nanometricsize features.

In this letter we report the optical and RF characterizationsof a micrometric electro-optic modulator based on LN PhC,which possesses a figure of merit half-wave product lengthproduct equal to 0.0063 V-cm, the smallest reported to thisdate in optical modulators, and an active interaction area ofonly 37 μm2 (with an active length of ∼5.3 μm). Bandwidthis approximately 1 GHz. It is important to note that this value,which is small for a performing modulator, but still far fromthe physical limit of LN [16] and it can be definitely increasedwith improved processing techniques to etch and thinned

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LU et al.: OPTICAL AND RF CHARACTERIZATION OF A MICROMETRIC MODULATOR 1333

Fig. 1. (a) Schematic of the LN PhC device. (b) Top view SEM (ScanningElectron Microscope) image of the fabricated modulator.

down LN at the nanoscale with a more suitable design ofelectrodes.

II. DESIGN AND FABRICATION

A schematic of the device is shown in Fig. 1(a). Lightmodulation is achieved by a Fabry-Perot cavity based ona 2D PhC geometry (a = 565 nm, r/a = 0.3, r is thehole radius and a is the period) that has been fabricatedon X-Cut LN with vertically deposited sidewalls electrodes.The fabrications processes are described in previous paper [17]and [18]. Due to the fact that LN crystal comes in wafersof several hundred microns of thickness, the PhC must beembedded in a previously fabricated waveguiding structurethat has been performed in several technological steps. Firstly,an annealed proton exchange (APE) with a depth of ∼2.2 μmhas been created on bulk LN in order to achieve verticallight confinement [17]. It is important to point out that thisprocess allows only TE (electric field is perpendicular to thePhC air holes) mode propagation. Optical grade dicing [19] isused to obtain a ridge waveguide allowing as a consequencein one hand, horizontal light confinement and on the otherhand, the possibility of sidewalls deposited lumped electrodeswith the purpose of increasing the overlap integral by matchingthe maximum of the RF external electric field and the opticalfield. Previous works have shown the effectiveness of this pro-cedure showing for instance a reduction of the applied externalfield of a 15% in comparison with electro-optic modulatorswithout this sidewalls extended electrode configuration [20].

For this particular structure the ridge width is of 17 μm(which corresponds to the separation between both electrodes).The PhC is based on a triangular lattice of air holes orientedalong the �M direction. The chosen geometry is a Fabry-Perot with a defect line perpendicular to the light propagationdirection. The Fabry-Perot is designed so that the cavity peakis located at λ = 1550 nm which implies r/a = 0.3 anda = 565 nm. Due to the well-known resistivity of LN tobe etched, air holes are drilled with a depth of ∼1.8 μmon APE waveguide with Focused Ion Beam (FIB) milling.Fig. 1 (b) shows a SEM image of a top view of the PhC.The active interaction length of the PhC Fabry-Perot structureis of ∼5.3 μm.

The theoretical transmission performed by FDTD is shownin Fig. 2. One can clearly see the cavity mode excited insidethe bandgap of the PhC, mode precisely at λ around 1550 nm.The spectral position of this cavity mode can be controlled viathe electro-optic effect. In fact, with an appropriate wafer cutdirection (optical axis of LN), the index of refraction of LN

Fig. 2. Theoretical transmission as a function of the wavelength for the PhCgeometry based on a Fabry Perot resonator (a). Experimental transmission ofthe resonator peak measured for three external voltages (b).

Fig. 3. Linear fitting of the dependence of the peak shift with the appliedexternal voltage.

can be modified thanks to the application of an external voltagethrough the vertically deposited electrodes. In addition, oneexpect enhancement of this modification of the refractive indexdue to the light confinement inside the cavity which leadsto the exaltation of non-linear effects [21], [22], the Pockelseffect for instance.

III. DEVICE CHARACTERIZATIONS

A. DC Electro-Optic Modulation

Figures 2 and 3 show the transmission spectra for variousDC drive voltages that were directly applied to the metal-lic electrodes through wire bonding. A tunable laser source(HP 8164A) was coupled through the waveguide using theend-fire technique. Single mode lensed fibers at a wavelengthof 1550 nm were used to couple light in and out fromthe device. Total overall losses (coupling in and out andpropagation losses) have been measured and correspond to∼21dB. The half-wave voltage Vπ can be roughly determinedas ∼11.8 V (this is the driving voltage needed to go frommaximum intensity to minimum intensity in the Fabry-Perotresonance peak at λ =1554.2 nm with a quality factor of ∼141when the applied voltage is of 0 V) from the Fig. 2 (b), and thecorresponding DC extinction ratio is measured to be ∼11.2 dB.Taken into account that the device active length is of ∼5.3 μmwe can immediately deduce that the modulator has a figureof merit VπL of ∼0.0063 V-cm. This value is 3 orders ofmagnitude smaller than the one measured with thin films of LNfabricated by the smart-cut technology [12], [13]. This fact isessentially due to the enhanced interaction between slow lightin photonic crystals and lithium niobate which possesses ahigh electro-optic coefficient. The value measured in this work

1334 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 13, JULY 1, 2014

Fig. 4. S11 parameter characteristics.

is even smaller than the ones measured in PhC modulatorsfabricated in polymer infiltrated on silicon slot waveguides,where VπL of 0.044 V-cm and 0.8 V-cm have been obtained[23], [24]. From the figure 3, the measured electro-optictunability is approximately 0.6 nm/V corresponding to aneffective electro-optic coefficient r33 = 563.6 pm/V whichis 18.3 times bigger than the one corresponding to bulk LN(r33 = 30.8 pm/V).

B. RF Modulation

Before characterizing the modulation performance of thedevice, we have measured the S11 parameter to have infor-mation about the impedance characteristics of the verticallydeposited sidewalls electrodes. This measurement is performedusing a vector network analyzer (ANRITSU 37369A). S11values as a function of applied frequencies are depicted inFig. 4. We can observe a deep cut-off frequency at ∼2.16 GHzthat was attributed to the electrodes configuration of thedevice which was not completely optimized for impedancematching.

The next step consists of measuring the RF modulationperformance of the PhC optical electro-optic modulator whichhas been performed by using the Intensity Modulation Fre-quency Response (IMFR) of the device. The output lensedfiber was coupled to an Optical Spectrum Analyzer OSA(Band C - AP2040A). In addition, a RF generator(HP E4422B) was connected to the electrodes. To ensure acorrect separation between the sidebands modulation signaland the input laser signal, we have started to sweep themodulation frequency at 50 MHz as the resolution of the OSAis 20MHz.

Fig. 5 illustrates the electro-optic modulation responsesshowing the output intensity of the laser at a wavelengthequal to 1546.9 nm, and the modulated sidebands signalsat 300 MHz, 600 MHz, 900 MHz, 1200 MHz, 1500 MHz,1800 MHz, 2100 MHz and 2400 MHz respectively. To do that,the driving electrical power of the RF generator was fixed to10 dBm which insures the modulation performance operatingin relative low signal regime. Note that a compromise wasfound among the low driving voltage, the modulation ratio andthe distortion of modulated signal. The modulation responsesshow the presence of the harmonic generations at relative low

Fig. 5. Sidebands modulation for different frequencies with a laser operatingat ∼1545.91 nm.

Fig. 6. Difference between the output powers of the sidebands modulationsignal and the laser light for electro-optical modulation response throughthe entire device. The inset figures illustrate the electro-optical modulationresponse showing the output powers of the pump laser and the modulatedsidebands signal at (a) 300 MHz, (b) 900 MHz, and (c) 1500 MHz,respectively.

frequency (300 MHz). However their intensities are signifi-cantly lower by 20 dB than the sidebands modulation signal.

In Fig. 6 we have plotted the output power differencebetween the sidebands modulation signals and the pump laser(λ = 1546 nm) through the entire device as the modulationfrequency increases from 50 MHz to 2.45 GHz. We canconclude from this data that the global frequency responsecorresponds to a relative low-pass filter. Since electro-optic

LU et al.: OPTICAL AND RF CHARACTERIZATION OF A MICROMETRIC MODULATOR 1335

crystals are also piezoelectric, the modulation at relative lowfrequencies (1-100MHz) can be accompanied by unwantedamplitude modulation and beam deflection. This effect ismost dramatic when the crystal is driven at a frequency thatcoincides with an acoustic resonance. These results are notsurprising to us since we had not completely optimized theelectrodes for 50 Ohm impedance matching creating thus acapacitive effect (with RC-constant of ∼160 ps).

IV. DISCUSSION

The experimental modulation results tell us that we are stillfar from the LN modulation capabilities that can be foundin classical bulk modulators [16]. Improvement in the LNnanofabrication will unambiguously lead to much better deviceoptical performance. It is crucial to have perfectly cylindricalholes which are particularly difficult to obtain in LN [25]. Theuse of LN thin films or air suspended membranes is clearly arequisite in order to hope for optimal performances but it isimportant to find easier and cheaper methods to obtain themthan the smart cut technology.

The electro-optic performance can be also improved bydecreasing the distance between the two electrodes. This canbe done with a dicing machine that provides better cuttingprecision. In addition, more suitable electrode design shouldeasily overcome these limits and reach ultra-high speed mod-ulation.

V. CONCLUSIONS

We have optically characterized a compact RF electro-optic modulator on a LN PhC cavity structure with anactive interaction area of ∼37 μm2 by combining Fabry-PerotPhC geometry and vertically deposited sidewalls electrodes.This novel configuration shows an electro-optical tunabilityof ∼0.6 nm/V, an r33 effective LN electro-optic coefficient18 times bigger than in bulk LN and a figure of merit VπL of0.0063 V-cm that represents the best figure of merit achievedfor a LN EO modulator. Such compact and highly efficientnanophotonic modulator is a competitive candidate for on-chipoptical interconnects.

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