Integrated Microring Tuning in Deep-trench Bulk CMOS
- Category: Circuits & Systems, Optics & Photonics
- Tags: chen sun, vladimir stojanovic
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Download as PDFWavelength-division multiplexed (WDM) silicon-photonic links form a promising alternative to traditional electrical interconnects. However, the essential component of these WDM networks, the microring resonator, suffers from resonance drifts due to either static process variations or dynamic temperature fluctuations, necessitating methods for active resonance tuning[1],[2]. In this work, we demonstrate a heater driver system for microring resonators consisting of a fully-digital Δ∑-based heater driver circuit and an ARM filter (Figure 1, left), both monolithically integrated in a commercial 0.25-µm-equivalent bulk CMOS process with deep-trench isolation capability. Integration of the heater driver alongside the integrated heater minimizes the parasitic resistance from the driver to the heater, provides a digital heater control interface through which multiple rings can be controlled on-chip, and eliminates the dedicated pads for external heater control used in the majority of previous work.
The driver circuit consists of a synthesized pipelined accumulator and a custom one-transistor driver head (Figure 1, right). The circuit utilizes pulse density modulation (PDM) to output a heater drive waveform consisting of a digital pulse train, with a duty-cycle corresponding to an 8-bit digital input setting. The clock frequency of the driver can be used to trade-off the backend power overhead with the size of the digital ripples. The 6-μm diameter ARM filter[3] is made using Poly-Si waveguides above deep oxide trenches (Figure 1, left) and achieves an uncorrupted free spectral range (FSR) of 3.7THz (Figure 2, left). Using a process-compatible supply voltage of 2.5V, the system is able to tune the resonance by 350GHz, with an efficiency of 10-15µW/GHz (Figure 2, right).
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- Figure 1: Layout of the integrated heater driver connected to the ARM filter, both integrated in a deep trench CMOS process (left). Block diagram of the Δ∑-based heater driver with SEM of the connected ARM filter (right).
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- Figure 2: Optical response of the ring filter at minimum and maximum heater power settings (left) and tuning cost per gigahertz, with driver cost included, to reach a specific tuning offset (right).
- A. Biberman, D. C. Trotter, C. Sun, M. Moresco, V. Stojanović, and M. R. Watts, “Automated wavelength recovery for microring resonators,” in Proc. Conference on Lasers and Electro-Optics, CM2M.1 2012. [↩]
- M. Georgas, J. Leu, B. Moss, and C. Sun, “Addressing link-level design tradeoffs for integrated photonic interconnects,” Custom Integrated Circuits Conference (CICC), Sept. 2011. [↩]
- M. R. Watts, “Adiabatic Microring-Resonators,” Optics Letters, vol. 35, no. 19, 2010. [↩]