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We report the first complete 10G silicon photonic ring modulator with integrated ultra-efficient CMOS driver and closed-loop wavelength control. A selective substrate removal technique was used to improve the ring tuning efficiency. Limited by the thermal tuner driver output power, a maximum open-loop tuning range of about 4.5nm was measured with about 14mW of total tuning power including the heater driver circuit power consumption. Stable wavelength locking was achieved with a low-power mixed-signal closed-loop wavelength controller. An active wavelength tracking range of > 500GHz was demonstrated with controller energy cost of only 20fJ/bit.
© 2014 Optical Society of America
1. Introduction
Microring based devices constitute a significant group in the silicon photonics components’ tool box. Utilizing the resonance characteristics of microrings, various photonic components including modulators, wavelength division multiplexing (WDM) filters, and switches can be made with very attractive performance metrics in both small physical size and low power consumption. In particular, microring based silicon modulators have been demonstrated with ultra-compact sizes, ultra-high modulation speed with ultra-low modulation power and total power penalty [1–3]. In addition, simple low voltage-swing CMOS drivers can be implemented to drive ring modulators with high energy efficiencies [4]. These unique advantages are ideal for building highly efficient and large-bandwidth WDM photonic interconnects for future communication and computing systems.
However, practical application of microring modulators remains a big challenge because they are especially susceptible to both manufacturing tolerances and thermal fluctuations due to the relatively high thermo-optic coefficient of silicon and the resonant nature of microrings. To register and lock a ring with its corresponding laser wavelength, resonance adjustment is needed not only to compensate for the static offset from the manufacturing tolerances, but also to combat the dynamic drift in a thermally volatile environment. The most promising approaches to solving this problem have involved the use of closed-loop control systems to thermally stabilize the ring modulator with different feedback mechanisms [5–9]. Results reported so far include: wavelength tracking and control by monitoring the optical scattering from the ring resonator [5]; monitoring the mean power of the modulated signal [6]; measuring the bit-error-rate [7]; using a local thermal dithering [8]; or balanced homodyne phase detection of an optical carrier to mitigate the input laser power fluctuation [9]. However, in all these experiments, external closed-loop controllers, e.g. a personal computer, a PID controller, or a FPGA system, were used. To date, there has been no demonstration of a fully integrated closed-loop controller with low power.
In this work, we report, to our knowledge, the first complete 10G silicon ring modulator with fully integrated CMOS driver and active wavelength control. A reverse biased depletion silicon ring modulator with integrated Ge waveguide photodetector (PD) for mean power monitoring and metal heater for ring resonance tuning, fabricated using a 130nm SOI CMOS process, was hybrid integrated with a 40nm bulk CMOS chip containing a 10G driver and a mixed-signal low-power bang-bang closed-loop controller. An active wavelength tracking range of more than 500GHz was achieved with only 20fJ/bit controller energy cost at a data rate of 10Gbps.
2. A complete 10G Si ring modulator with integrated active wavelength control
The active microring device in this demonstration is a reverse-biased depletion ring modulator capable of 10Gbps modulation, fabricated using a commercial 130nm SOI CMOS process with 300nm silicon film thickness. Shown in Fig. 1, the device consists of a 7.5-μm-radius ring with one side coupled to a signal bus waveguide for laser input and modulated signal output, and the other side coupled to a second bus waveguide which is connected to a Ge waveguide PD for power monitoring. The ring waveguide has a width of 380nm and a slab height of 80nm and is doped (2 × 1018cm−3) 100% to form a symmetric lateral PN junction for high-speed modulation. It is designed to be wider than the bus waveguide (300nm) for smaller bending loss and better coupling phase matching. The gap is 285nm between the ring and the signal bus waveguide, and 525nm between the ring and the monitor bus waveguide to achieve a coupling coefficient of about 0.2%. The P contacts of the ring modulator between and outside of the two bus waveguides are connected using M2 layer metal. A metal resistor is implemented with daisy chained metal vias right above the ring waveguide between metal layers from M2 to M5 for thermal adjustment of the ring resonance wavelength. An array of 8 such ring modulators with slightly different radii are arrayed along a shared signal waveguide to form a synthetic resonant comb with a channel spacing of 1.6nm, and a physical spacing of 125μm.
We monitor the mean power of the modulated signal using the integrated monitor PD to align and lock the ring modulator with its corresponding laser wavelength using a bang-bang controller. In contrast to an analog PI controller that produces a continuous error signal proportional to the difference between the measured photocurrent and the ideal threshold, a bang-bang control system provides a discrete 1 or −1 as the error signal to the digital loop accumulator, indicating whether the monitored photocurrent is above or below the ideal threshold. Figure 1 outlines the schematics of the feedback control system. The circuit compares the average output of the monitor photodiode to the ideal threshold which can be obtained via calibration at bring-up for given constant input laser optical power. An average value higher (lower) than the threshold indicates a change in ring resonance to a shorter (longer) wavelength and, therefore, a need to heat (cool) the ring to counteract this shift. The bang-bang control loop will eventually lock, and the comparator output will reach a limit cycle, dithering between 0 and 1. The overall transfer function of the system is a high-pass filter: thermal disturbances slower than the corner frequency (τ = 1 ms) can be corrected, but those due to faster thermal fluctuations pass through un-attenuated. The fine adjustment of the ring resonance to a desired wavelength is achieved by driving a carefully controlled current through the integrated metal resistor. As we reported earlier [4], we used a 15-bit DAC to drive the resistor with a maximum output power of 8 mW, giving us 32768 steps of 0.24 μW each. The DAC uses a delta-sigma approach to dither between two currents separated by 125 μA, giving us the precise resolution required. The details of the controller circuit design can be found in [10].
The closed-loop control circuit, co-integrated with a pulsed-cascode ring modulator driver circuit as described in [11], was fabricated using a TSMC 40nm bulk CMOS process. Each modulator driver cell was designed with three pairs of 17 μm bonding pads to match the pads on the ring modulator chip for high speed modulation, ring tuning and ring power monitoring, respectively. These bonding pads are arranged in a 2x3 array with a separation of 50 μm. As pictured in Fig. 2(a), the fabricated VLSI chip was bonded with the SOI ring modulator chip using a micro solder bump based hybrid integration technique. The silicon handler substrate underneath the ring modulator array was then removed using a back side etch-pit process [4] for improved tuning efficiency. Finally, the hybrid chip assembly was die-attached to a test PCB for test and characterization. A picture of the test setup is shown in Fig. 2(b). A fiber array was used for laser light input and modulated optical signal output via the grating couplers on chip. The open-loop tuning efficiency of the metal resistor based thermal tuner was measured first with results shown in Fig. 2(c). A maximum tuning range of about 4.5nm can be achieved with about 14mW of total tuning power including the heater driver circuit power consumption.
Fig. 2 (a) A picture of the hybrid integrated chip assembly die-attached and wire bonded on a test PCB. (b) A picture of the test setup for a closed-loop controlled ring modulator. (c) Measured open-loop ring tuning efficiency including the heater driver power.
We first drove one of the ring modulators open loop using an on-chip PRBS generator (231-1) at a data rate of 10Gbps with no current to the heater. A clean open “eye” was obtained with an extinction ratio of better than 6dB after aligning the laser wavelength to the ring resonance at 1548.06nm with about 1mW input power to the ring. The measured modulation power is about 0.8 mW, similar to what we measured in other demonstrations [4]. We then turned on the ring’s closed-loop controller with a calibrated ideal threshold and observed stable locking of the ring modulator to the laser wavelength. As we continued tuning the input laser wavelength, the controlled ring modulator was able to track the laser wavelength change and remained locked over a 4.25nm range (limited only by the tuning range of the thermal tuner), corresponding to an equivalent thermal fluctuation range of more than 50K. Figure 3 shows the 10G eye diagrams recorded at different wavelengths, indicating a largely un-changed modulation quality over the entire tracking range.
Fig. 3 10G eye diagrams at a different wavelength showing the wavelength tracking of a closed-loop controlled ring modulator.
The controller performance under thermal crosstalk from the neighboring channels was characterized by setting two nearest neighbor rings tuners to their maximum output, and then turning them off suddenly. With the control-loop turned off, we observed a clear ring resonance shift from the accumulated eye (left picture in Fig. 4(a)) recorded through the process while no change to the modulator eye was observed with the control loop turned on (right picture in Fig. 4(a)).
Fig. 4 Measured ring modulator performance with thermal crosstalk from neighboring channels (a) and long-term (>24 hours) ambient temperature change (b).
The closed-loop controller further proved its effectiveness in a long-term stability experiment. The modulation eye of the ring modulator would not normally remain open for overnight operation, due to ambient temperature change as the left picture in Fig. 4(b) indicates. However, it remained open for more than 24 hours with the control-loop turned on as shown in the right picture in Fig. 4(b).
The measured total power consumption of the closed-loop controller excluding the power consumed by the metal heater and its driver was only 0.2 mW, corresponding to an energy cost of 20 fJ/bit for 10Gbps operation. Table 1 summarizes the power consumption of the different components of the closed-loop controlled ring modulator, along with the corresponding circuit area. The total circuit area is about 2600 μm2. The digital circuit component area will scale directly with technology while the analog component area will scale slowly, if at all. However, if the area of the scaled technologies increases in an adverse manner to analog blocks, we may choose to add more calibration and trimming circuitry to compensate, resulting in a slower improvement in areal efficiency.
Table 1. Summary of the Circuit Power Consumption and Area of Different Components.
3. Summary
We demonstrated, for the first time, a complete hybrid-integrated silicon photonic ring modulator with fully integrated low power 10G driver and active closed-loop wavelength control. The SOI photonic chip integrates the Si ring modulator with a mean power monitoring PD and a metal resistor thermal tuner while the bulk CMOS VLSI chip integrates the low power 10G modulator driver with closed-loop wavelength control including a power monitoring TIA, a bang-bang controller and a high resolution heater driver. The closed-loop controlled ring modulator achieved wavelength tracking and stable locking over a range of 4.25nm (>500GHz), equivalent to a temperature change over 50K, with an energy cost of only 20fJ/bit excluding the thermal tuner power consumption. This successful demonstration paves the way for the application of ring modulators in practical low power WDM silicon photonics links.
4. Acknowledgments
This material is based upon work supported, in part, by DARPA under Agreements HR0011-08-9-0001. The views expressed are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government. The authors thank Dr. Jag Shah of DARPA MTO for his inspiration and support of this program. Approved for Public Release. Distribution Unlimited.
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