We demonstrate a phase shifter based on a silicon slot waveguide infiltrated with liquid crystal. We achieve a phase shift of 73 pi for a 5V drive voltage, with a voltage-length product of 0.022V.mm around 1V. We can drive the phase shifter directly with a 1V, duobinary pulse-width-modulated signal, allowing direct digital CMOS control of an analog optical phase shifter

Absil, P

Ako, Thomas

Beeckman, Jeroen

Bogaerts, Wim

Koos, C

Korn, D

Lepage, G

Leuthold, J

Pantouvaki, M

Puthenparampil George, John

Verheyen, P

Xing, Yufei

Yu, Hui

English

Ghent University Academic Bibliography

Direct Digital Control of an EfficientSilicon+Liquid Crystal Phase ShifterYufei Xing1257, Thomas Ako15, John P. George125, Dietmar Korn 3, Hui Yu 156,Peter Verheyen 4, Marianna Pantouvaki 4, Guy Lepage 4, Philippe Absil 4,Christian Koos 3, Juerg Leuthold 38, Jeroen Beeckman 15, Wim Bogaerts2591Liquid Crystal and Photonics Group, Ghent University,Department of Electronics and Information Systems, Gent, Belgium2Photonics Research Group, Ghent University - imec,Department of Information Technology, Gent, Belgium3Karlsruhe Institute of Technology (KIT), Institutes IPQ and IMT, Engesserstr. 5, 76131 Karlsruhe, Germany4IMEC vzw, Leuven, Belgium5Center of Nano- and Biophotonics (NB-photonics), Gent, Belgium6now with: Zhejiang University, Department of Information Science and Electronic Engineering, Hangzhou 310027, China7now with: Dept. of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore8now with: Electromagnetic Fields Laboratory, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland9now also with: Luceda Photonics, Dendermonde, BelgiumAbstract—We demonstrate a phase shifter based on a siliconslot waveguide infiltrated with liquid crystal. We achieve a phaseshift of 73 pi for a 5V drive voltage, with a voltage-length productof 0.022V·mm around 1V. We can drive the phase shifter directlywith a 1V, duobinary pulse-width-modulated signal, allowingdirect digital CMOS control of an analog optical phase shifter.I. INTRODUCTIONOptical phase shifters are essential building blocks for tun-ing, switching or modulation. For modulation the first criteriumto satisfy is operation speed, but for tuning and switching thepower consumption is a greater concern. Thermal tuning isefficient, but power hungry. Carrier-based effects are very fastbut weaker. Silicon-organic hybrids based on slot waveguidescan achieve much higher electro-optic efficiencies that canbe tailored with a proper choice of organic material [1]. Wepresent a phase shifter based on a slot waveguide infiltratedwith liquid crystal, which have a huge anisotropy, and a verystrong (but rather slow) electro-optic effect. In combinationwith silicon photonics they can be used for very efficient tuners[2]. A liquid crystal in a slot waveguide enhances this effectby increasing the confinement of light in the LC while at thesame time increasing the electric field strength between the LCelectrodes, dramatically reducing the modulation voltage [3].A second aspect is the control of the phase shifter. Whenelectrically driven, the accuracy of the phase shifter is closelyrelated to the accuracy of the digital-to-analog converter (DAC)that provides the drive voltage or current. This can actuallybe the limiting factor unless high-quality DACs are used,especially in combination with a low driving voltages. DACsalso consume power and chip real estate. We demonstrate adirect digital control of a liquid crystal phase shifter usingfast 1V pulse-width-modulation control. The relatively slowresponse of the liquid crystal will average this digital signalin an analog phase shift.II. CONCEPT AND DESIGNThe phase shifter is part of a Mach-Zehnder modulatordesigned by Karlsruhe Institute of Technology and fabricatedin IMEC. The device consists of an MMI splitter and combinerand 1mm long slot phase shifters in both arms. The slot iscontacted through a thin doped silicon slab, and connected toelectrodes using tungsten vias and copper wiring. The backendstack is etched back down to the waveguide. The resultingcross section is drawn in Fig. 1.The liquid crystal is applied on the waveguide and allowed tospread across the modulator. It is difficult to verify the fillingof the slot, but from the experiment we can assume than theinfiltration is quite good. Applying a voltage over the rails ofthe slot waveguide will realign the LC molecules, changing theeffective index of the TE mode. The phase shift will translatein an intensity change at the output of the MZI. We only driveone arm of the MZI modulator, grounding the central electrodeand one of the outer electrodes. The phase shifter is operatedin alternating current (AC): otherwise ion impurities in theLC will accumulate, negating the electric field. The operationfrequency should be between 30kHz and 1MHz. The upperlimit is dictated by the choice of liquid crystal (E7) whichgradually loses its anisotropy beyond beyond this frequency.Fig. 1. Conceptual drawing the device under test.III. DEVICE CHARACTERIZATIONA. Phase responseTo characterize the static response, we swept the amplitudeof the AC signal from 0V to 5V. We measured the intensitymodulation of the MZI, recording the peaks and zeros corre-sponding to a 0 and pi phase difference. Fig. 2 shows the phasemodulation (in multiples of pi): we achieved a modulationof 73pi for 5V amplitude, and the effect saturates, indicatinga complete realignment of the LC. The highest modulationefficiency lies between 0.2V and 2.5V, with a differential phaseshift of dφ/dU = 44.53pi/V around 1.0V. This correspondswith the very low voltage-length product of 0.022 V·mm.0 0.5 1 1.5 2 2.5 3 3.5 4 4.501020304050607080Applied Voltage (V)Phase Shift (pi)Phase Shift in Relation with Applied Voltage MF 1 MHzPhase Shift (pi)Fig. 2. Phase shift as a function of control voltage. A phase shift of 73 pi isachieved at drive voltage of 5 V.B. Switching timeWe measured the switching time by switching the devicefrom an off-state near 1.0V to a nearby on-state, and back. Forlarger voltages, we go through multiple minima and maximabefore the signal settles. The switch-on and switch-off timeare between 1.5 ms and 2.5 ms, independent of the drivevoltage. We can speed up the switching time by applying anoverdrive or pre-emphasis. When we apply a 4V overdrive, wecan reduce the switching time to 100µs. Such an overdrive isdifficult to apply for a reduction in voltage, where switchingis limited by the relaxation time of the LC.IV. DIGITAL DRIVING WITH PWMInstead of an analog drive voltage, we now directly applya digital drive voltage to the phase shifter. We use fast pulse-width modulation (PWM), and the LC will respond only to theRMS of the voltage. The precision can be controlled by theword length, as long as it is much shorter than the switchingtime. We used a word rate of 25.6kHz. Because the liquidcrystal requires an AC signal (net zero voltage), we use aduobinary encoding where pulses are alternatingly positive ornegatively. This is shown in Fig. 3.When we sweep the duty cycle of the PWM in steps of 0.5%,we can again record the output of the MZI. The observed10%50%90%VoltageTimeFig. 3. Duobinary PWM drive signal.0 20 40 60 80 10005101520253035Pulse Duration Cycle (%)Phase Shift (pi)Fig. 4. Phase shift with increasing duty cycle of the PWM drive signal at1.0V. We observe a phase shift of 32 pi.response (phase shift obtained for a given duty cycle) fora 1.0V drive is shown in Fig. 4, with a pulse frequency of25.6KHz. We observe a 32pi phase shift.The power consumption of the device is extremely low anddifficult to measure directly. We could establish an upperlimit of 2nW. As the power consumption is related to the(dis)charging of the slot capacitor, it should be constant duringthe PWM modulation scheme.V. CONCLUSIONWe demonstrated a liquid crystal on silicon phase shifterwith a very high modulation efficiency of 0.0211V·mm and adirect digital control using duobinary pulse-width modulation.The device is overdimensioned and achieves a 32pi phase shiftwith a 1.0V drive. This makes it possible to reduce the devicelength from 1mm to 70µm.ACKNOWLEDGEMENTPart of this work was supported by the European Unionthrough the FP7-SOFI project.REFERENCES[1] J. Leuthold, C. Koos, W. Freude et al. J. Sel. Top. Quantum Electron.,vol. 19, no. 6, pp. 114–126, Nov 2013.[2] W. De Cort, J. Beeckman, T. Claes et al. Opt. Lett., vol. 36, no. 19, pp.3876–3878, Oct 2011.[3] J. Pfeifle, L. Alloatti, W. Freude et al. Opt. Express, vol. 20, no. 14, pp.15 359–15 376, 2012.

Direct digital control of an efficient silicon+lequid crystal phase shifter

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Direct digital control of an efficient silicon+lequid crystal phase shifter

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