CMOS Circuitry for the Wavelength Locking of a Ring-based Silicon Photonics Transmitter
The aggregate bandwidth of the inter-chip and intra-chip interconnects needs to be increased to multi-terabyte/s at an energy efficiency of < 1 pJ/bit. To do so, SiPh-based optical interconnects have been proposed to replace the existing electrical interconnects. An E/O modulator is a key component of such an optical interconnect. It executes amplitude modulation of the incoming laser wavelength (λlas) with the to-be-transmitted electrical signal. OMA is considered as the figure of merit for this modulation since it determines the amplitude of the transmitted optical eye. The wavelength corresponding to the maximum OMA, therefore, is denoted as the optimum wavelength (λopt).
A ring modulator is an attractive E/O modulator for realizing high-bandwidth-density energy-efficient SiPh optical interconnects. It exhibits distinct advantages such as high bandwidth, compact footprint, low capacitance, low drive voltage and WDM compatibility. A ring modulator, however, also poses an important challenge due to its resonant behavior. Its OMA is significantly reduced when λopt is not aligned to λlas. Typically, even a slight misalignment of +/- 30 pm inhibits modulation by reducing the OMA by ~50%. Thus, process, thermal (80 pm/°C) and λlas variations need to be compensated within few picometers, which can otherwise cause misalignments up to several nanometers. Such a compensation will lock λopt to λlas, and hence stabilize the modulation at the maximum OMA. This will, in turn, pave the way for the commercial realization of ring-based SiPh optical interconnects.
This thesis presents wavelength locking wherein λopt is appropriately tuned to be aligned with the incoming λlas. This is achieved by a feedback control of the ring modulator temperature. Thus, a heater is driven by a control circuitry according to the modulated optical power obtained at the drop port of the modulator. This drop-port optical power is fed to the control circuitry in the form of photocurrent generated by a photodiode. The resulting drop-port based wavelength-locking feedback loop has the advantage of not interfering with the actual through-port transmission. As a result, it is suitable for the WDM transmitter architectures. However, this feedback loop is more challenging since the drop-port power is designed to be much smaller than the through-port power so as not to induce excessive optical loss in the modulator.
To address the above wavelength-locking challenges, a control circuitry based on a novel concept of direct monitoring of drop-port OMA is proposed. It also includes a CDS-based quantizer to determine the slope of the monitored OMA. Two versions of such a slope quantizer, called dither-based and dither-less, are presented. The implementation of the control circuitry, meanwhile, is ensured to be low noise, low offset, low power and scalable for the high transmission rates.
Owing to the control circuitry, the performance of wavelength locking is significantly improved compared to the prior state of the art. This is experimentally verified by demonstrating three wavelength-locked ring-based optical transmitters. Each transmitter consists of a SiPh chip and a CMOS chip, which are either wire-bonded or flip-chip bonded. The SiPh chip, designed in 130 nm SOI technology, contains a ring modulator, heater and photodiode. On the other hand, the CMOS chip, designed in either 40 nm or 28 nm technology, contains a ring modulator driver and a control circuitry.
The first and second transmitters operate at 2 Gb/s. They constitute a dither-based and dither-less wavelength-locking feedback loops, respectively. The third transmitter also constitutes a dither-based feedback loop, but operates at 4 Gb/s and employs a highly-integrated control circuitry. It achieves OMA stabilization with an accuracy of 0.3% (1.7 pm ~ 0.02 °C), a speed of 500 pm/s (~6.3 °C/s), a tuning range of 5 nm (~62.5 °C), and a penalty of only ~2%. The stabilization is also immune to the input laser-power drifts and to the self-heating of the modulator. Moreover, the control circuitry is potentially scalable and low-power of 234 µW (59 fJ/bit). Consequently, it can enable a stabilized WDM ring transmitter for Nx20 Gb/s at < 761 fJ/bit.