We present a thermo-optic silicon phase shifter with integrated diode, which allows multiplexed access to heaters and reduces the number of pads to drive complex silicon photonics circuits. We also demonstrate that such heaters can be driven using digitally modulated signals, which increases the linearity of the phase shift response of the device

Alves Júnior, Antônio Ribeiro

Bogaerts, Wim

Ghent University Academic Bibliography

Thermo-optical Phase Shifter with IntegratedDiodes for Multiplexed ControlAntonio Ribeiro and Wim BogaertsGhent University - IMEC, Photonics Research Group, Department of Information Technology, Ghent, BelgiumCenter for Nano and Biophotonics (NB-Photonics), Ghent, BelgiumAntonio.Ribeiro@UGent.beAbstract: We present a thermo-optic silicon phase shifter with integrated diode, whichallows multiplexed access to heaters and reduces the number of pads to drive complexsilicon photonics circuits. We also demonstrate that such heaters can be driven using dig-itally modulated signals, which increases the linearity of the phase shift response of the device.OCIS codes: (130.3120) Integrated optics devices, (130.4815) Optical switching devices, (130.4110) Modulators.1. Introduction and ConceptHeaters are an attractive solution for phase shifting in silicon photonics due their simplicity of implementation andtheir compactness, compared to alternatives such as free-carrier modulators [1–3]. Although compact, heaters requirebulky pads to connect them to external driver electronics. One way to save space is multiplexing the use of the pads,driving multiple heaters with the same set of pads.A typical side heater is obtained by placing a doped silicon strip close to the target waveguide, and using it as anelectric resistor. The resistance of the heater is determined by its electrical properties (both inherent from the siliconand obtained from its dopants) and its geometry. Using both P-type and N-type dopants to create a PN junction createsa heater that is, at the same time, a diode (Fig.1).2. Design and FabricationThe heater consist of a silicon strip doped with N-type dopants, while one of its edges was doped with P-type, to createthe PN junction (Fig.1). We implemented the device in imec’s ISIPP25G+ silicon photonics platform. The heater wasplaced 0.7um away from the target waveguide. The implemented heaters are 50um long and 1um wide, fabricated ona 220nm thick silicon layer. The I-V response of the heater is shown in Fig.1(b). The breakthrough voltage stayed at−7V and it yielded 9.6mW when forward-biased at 7V . To achieve higher power output without compromising thethe operational range of ±7V we placed multiple diodes in parallel. For the fabricated phase shifter we report a pi radphase shift at 20.9mW , which is a result compatible with the reported values for this technology [4].Fig. 1: The conceptual design (a) and an IV curve (b) of the diode heater .Fig. 2: MZI with diode heaters in each arm. The conceptual design (a) and the fabricated chip (b). The curve (c) showsthe spectrum shift in function of a PWM and DC driving signal.3. Applications and Obtained ResultsAs a demonstration. we placed the diode heaters in two circuits: a pair of ring resonators with distinct resonances(Fig.3) and a Mach-Zehnder Interferometer (MZI) operating in push-pull (Fig.2) .3.1. Red-Blue shifting of MZIs filtersIn the MZI circuit (Fig.2) the heaters are placed on both arms of the interferometer, allowing it to operate in push-pull.By routing the electrical connection of the two heaters in parallel, but with opposite polarity, we can use one pair ofbond pads to drive either the top heater or the bottom one, depending on the polarity of the driving signal. It allows toeither red-shift or blue-shift the spectrum response of the device with only one pair of contact pads.We operated the circuit driving the pair of heater with a variable voltage source (from −4V to +4V ) and also witha pulse width modulated (PWM) signal (fixed voltage, variable duty cycle. A negative duty cycle indicates a signalwith negative polarity). From that we recorded the induced shift in the spectrum of the MZI filter, shown in Fig.2(c).It is clear from the plot that changing the bias will result in either a positive (red) or a negative (blue) shift. Also, it isnoticeable that the response of the circuit to a digital PWM driving signal is much more linear than for DC driving.This is true because the induced phase shift is proportional to the temperature, which is proportional to the dissipatedpower. For a DC driving scheme, the power is quadratic to the applied voltage, while in the PWM scheme, the powergrows linearly with the duty cycle. PWM driving is possible due to the high time constant of the heater (77.5us,measured). When the digital driving signal changes much faster than the thermal response time, the induced phaseshift is averaged over time. For driving our circuit we used a PWM signal with fixed amplitude of +5V (or −5V fornegative bias) and frequency f = 2MHz.3.2. Simultaneous control using PWM signal with two polarityIn Fig.3(b) we show the spectrum of the two rings responding independently, based on the bias of the driving signal.This technique allows to address either one heater or the other, but if we time-multiplex the driving signal we canaddress both heaters at the same time. To achieve that we used a duobinary PWM signal that has both a positive anda negative cycle that that can be independently defined (Fig.3(c), bottom), allowing us to address, independently, thetwo heaters. Fig.3(c) shows the correlation between the duty cycle values (rows 1 and 3) and the measured phase shift(rows 2 and 4), as well as the PWM signal (bottom). This demonstrates that we can use one pair of pads to control twoheaters at the same time, in an independent fashion.4. Extending the Concept for Matrix AddressingThe multiplexing can be extended by placing the phase shifters in a matrix topology, connecting the heaters as M rowsand N columns arrangement (Fig.4). We can now use the asymmetric behaviour of the diode to address one row attime and individually drive each heater in that row. If we time-multiplex the addressing of the rows, and rely on thehigh time constant of the heaters to delivery a steady phase shift, we can sequentially address all the heaters of thecircuit. This way we reduce the number of pads from MxN +1 (ground) to M +N.Fig. 3: The circuit with two rings (a), the spectrum shift of both rings for different DC driving voltages (b). The plot(c) shows the correspondence between the duty-cycle values of the duobinary PWM signal (rows 1 and 3), measuredspectrum shift for rings (rows 2 and 4) and the respective duobinary PWM signal (row 5).Fig. 4: The heaters can be disposed in a M×N arrangement for the multiplexing driving (a). Microscope image of afabricated demonstration switch with 15 MZIs with diode heaters, placed in a 3×5 matrix configuration (b).Our experimental results indicate that we are able to deliver pi rad phase shift using a PWM signal with amplitudeof 7V and a duty cycle of 28%, so with these heaters we can multiplex up to 3 rows of N heaters using this matrixaddressing technique.5. ConclusionWe demonstrated the use of silicon photonics heaters with integrated diodes to allow bias-dependent operation. That,combined with digital driving, can provide multiplexed access to the heaters, reducing the number of necessary pads.We also demonstrated that PWM driving improves the linearity response of the heaters.References1. M. R. Watts, J. Sun, C. DeRose, D. C. Trotter, R. W. Young, and G. N. Nielson, ”Adiabatic thermo-optic Mach-Zehnder switch”, Opt. Lett. 38, 733-735 (2013)2. Y. Maegami, G. Cong, M. Ohno, M. Okano, and K. Yamada, ”Strip-loaded waveguide-based optical phaseshifter for high-efficiency silicon optical modulators,” Photon. Res. 4, 222-226 (2016)3. Y. Yang, Q. Fang, M. Yu, X. Tu, R. Rusli, and G. Lo, ”High-efficiency Si optical modulator using Cu travelling-wave electrode,” Opt. Express 22, 29978-29985 (2014).4. A. Masood, M. Pantouvaki, G. Lepage, P. Verheyen, J. Van Campenhout, P. Absil, D. Van Thourhout, W.Bogaerts, ”Comparison of heater architectures for thermal control of silicon photonic circuits”, Group IV Pho-tonics, South Korea, p.ThC2 (2013)

Thermo-optical phase shifter with integrated diodes for multiplexed control

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Thermo-optical phase shifter with integrated diodes for multiplexed control

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