Silicon photonics is poised to meet the increasing demand for high-bandwidth, low-power, and densely integrated optical communications in CMOS-compatible environments. Microring resonators in particular have become ubiquitous photonic building blocks that have already been utilized to demonstrate modulators, filters, and switches. However, the large frequency dependence with geometry (~100GHz/nm) and thermo-optic coefficient (~10GHz/°C) innate to silicon microrings threatens to preclude their use in dense wavelength division multiplexed (DWDM) applications, where the channel spacings are tight and temperatures may vary by as much as 15°C. Several promising solutions to address this challenge have come in the form of low-power (4.4µW/GHz) and high-speed thermal tuning, sensor-based thermal compensation, and athermal devices. However, while temperature sensor and athermal solutions address the thermal stability issue, they do not address fabrication based frequency variations. A recent study has leveraged scattering of the microring filters for wavelength locking; however, scattered light based techniques are insufficiently reliable to enable large-scale implementations.

In our work ^[1] , we experimentally demonstrated the first high-speed and scalable on-chip optical wavelength recovery capable of compensating both fabrication and thermal induced frequency variations on a silicon photonic chip. Using a thermally tunable adiabatic resonant microring (ARM) resonator, shown in Figure 1a, combined with a unique wavelength recovery algorithm implemented using a field-programmable gate array (FPGA), we demonstrate low-power (less than 1mW for ±10°C) and high-speed (as low as 200 µs) wavelength recovery. Furthermore, this approach is capable of being implemented using advanced CMOS electronics, hybrid or monolithically integrated with silicon photonics. The implemented algorithm and the experimental results are depicted in Figure 1. Using the algorithm, the recovery time is experimentally reduced from 4.3ms to 200µs.

Figure 1: A) Diagram of the experimental setup for wavelength recovery using the ARM resonator and top-view scanning electron microscope (SEM) image of the device. B) Measured frequency shift and calibrated temperature shift as a function of heater power. C) Coarse and fine loop flow charts for wavelength recovery decision-making for thru port where P_H(t) is real-time power dissipated in the heater; ΔP_H(t) is real-time power variation in the heater; P_MAX/MIN is maximum/minimum heater power; ΔP_C/F is coarse/fine minimum power variation in the heater; I(t) is real time output intensity; I₀ is the threshold intensity; e(t) is the error signal; and I_T(t) is the target intensity, which is constantly updating for global minima locking. Drop port decision-making algorithm can be implemented by changing comparison statements and using the rest of the algorithm. D) Microring wavelength recovery results as a function of increasing minimum power variation in the heater and stability of the recovered signal is investigated as a function of loop speed (inset).

1. E. Timurdogan, 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.

High-performance devices and systems based on CMOS-compatible silicon photonics are increasingly gaining momentum in the optical interconnects community as essential building blocks for high-speed, energy-efficient communication systems. In silicon photonic interconnects, a critical role is played by the integrated modulator, which represents one of the major components still being refined. Silicon modulators operate by free-carrier injection, by modulating the width of the depletion region, or based on charging metal-oxide-semiconductor (MOS) capacitors. Already, compact 3.5-μm microdisk modulators have been demonstrated with a power consumption of only 3 fJ/bit and 1-V operation ^[1] . Microdisks were chosen for the ease of implementing interior contacts and for their hard outer waveguide walls to enable the smallest footprints (~10 μm²) and bend radii (~2 μm). However, microdisks support higher-order modes that corrupt the otherwise extensive free spectral range (FSR) by introducing unwanted resonances. Maintaining a large, uncorrupted FSR is important for wavelength-division multiplexing (WDM) multiple resonant modulators on a single communication line. Microrings eliminate the undesired modes, but directly contacting a microring leads to scattering and loss. Further, the use of external ridge waveguides to enable electrical contact increases the diameter to 10 μm, thereby increasing the area by nearly an order of magnitude.

In our work ^[2] , we demonstrated a new class of modulators, adiabatic resonant microring (ARM) modulators, which enable the integration of vertical p-n junctions and interior contacts while maintaining a high quality factor and compact size, while preserving single radial mode propagation and thereby achieving an uncorrupted FSR. Adiabatic resonant microrings (ARMs) have been shown both numerically ^[3] and experimentally to enable interior contacts and a compact size while maintaining high quality factors. ARMs operate on the principle of mode-evolution. In the coupling region, the ring waveguide is made narrow to ensure single-mode operation, and the waveguide is then slowly widened to enable contact to the microring where there is no optical field. Here, we demonstrated a vertical p-n junction based on the ARM modulator operating in depletion mode, with data rates up to 12 Gb/s, while occupying less than 12.5 µm² of chip area and maintaining an uncorrupted 6.9 THz FSR. Figure 1 depicts the device and summarizes the experimental results.

Figure 1: A) 3D diagram of the ARM modulator that we used in these experiments; B) scanning electron micrograph (SEM) of the silicon layer of the ARM modulator; C) measured spectral response showing the uncorrupted FSR (6.9 THz) of the resonator; and D) high-speed output optical eye diagrams for data rates: 5Gb/s (left), 12Gb/s (middle), and 15Gb/s (right).

1. M. R. Watts, W. A. Zortman, D. C. Trotter, R. W. Young, and A. L. Lentine, “Vertical junction silicon microdisk modulators and switches,” Opt. Express vol. 19, pp. 21989–22003, 2011.
2. E. Timurdogan, M. Moresco, A. Biberman, J. Sun, W. A. Zortman, D. C. Trotter, and M. R. Watts, “Adiabatic resonant microring (ARM) modulator,” in Proc. Optical Interconnects Conference, TuC6, 2012, pp. 48–49.
3. M. R. Watts, “Adiabatic microring resonators,” Opt. Lett. Vol. 35, pp. 3231–3233, 2010.