Abstract
Microring modulators offer attractive solutions for high-speed optical interconnects due to their low loss and small footprint. However, these devices are sensitive to temperature and process variations, which can significantly impact their performance, particularly the optical modulation amplitude (OMA). Maximizing the OMA is crucial for achieving reliable and efficient data transmission. In this tutorial, we will explore a novel calibration technique proposed by Zabihpour et al. that addresses the challenges posed by process variations and ensures maximum OMA without relying on high-speed circuits or simulations.
Introduction
Traditional methods for maximizing OMA often rely on simulations and models to determine the optimal locking point for the thermal control unit (TCU). However, these approaches are susceptible to process variations, leading to reduced yield and performance. Figure 1(b) illustrates the impact of process variations on the expected yield for different target OMAs under two different TCU configurations: the drop-port and through-port methods.
As shown in Figure 1(b), for a target OMA of 0.5 mW (-3 dBm), the expected yield is only 36% for the drop-port method and 61% for the through-port method. Furthermore, in 35% and 16% of cases, respectively, the TCU fails to lock, resulting in zero OMA.
The proposed calibration technique aims to overcome these limitations by locking the TCU to the maximum OMA, irrespective of process variations, leading to the attainment of the red curves in Figure 1(b), where failures to lock are eliminated, and the expected yield for a 0.5 mW target OMA reaches 83%.
Proposed Design
The proposed design, shown in Figure 2(a), taps off a small portion (0.5%) of the laser power to monitor fluctuations and a portion (5%) of the modulated light inside the ring to the drop-port of the microring modulator (MRM). This modulated light is directed to a photodiode (PD), and the resulting current is converted to voltage by a transimpedance amplifier (TIA).
The ratio of the drop-port to input average laser power, regulated by an R2R-ladder digital-to-analog converter (RDAC), is employed within the TCU to establish a feedback loop for MRM thermal stabilization. The RDAC, together with a comparator and successive approximation register (SAR) logic, forms a SAR analog-to-digital converter (ADC) that allows the drop-port voltage to be measured during the calibration phase.
The calibration procedure is based on the Lorentzian property of the MRM filter, as illustrated in Figure 2(b). The drop-port voltages for the 0 and 1 driver levels (v0 and v1) follow the Lorentzian curve, and their average voltage (vave) and absolute value of their difference voltage (Δv) can be calculated.
Calibration Procedure
The calibration procedure consists of three steps:
Sweep the heater gate voltage while transmitting level 0 to measure the maximum drop-port voltage for level 0 (ADP0).
Sweep the heater gate voltage while transmitting level 1 to measure the maximum drop-port voltage for level 1 (ADP1).
Sweep the heater gate voltage while transmitting a clock data pattern (in lieu of dc-balanced data) to calculate dl/Δλ from max{vave(λn)}.
After obtaining these measurements, the average voltage associated with the maximum OMA can be determined by setting the derivative of Δv(λn) to zero, solving for the optimal value of λnopt, and inserting this value into the equation for vave.
If the peak voltages of v0 and v1 are not identical, both levels are sent separately, and ADP values corresponding to v0 and v1 are measured. Then, dl/Δλ is derived from max{vave} using a numerical method. The subsequent steps remain the same.
Measurement Results
The proposed calibration technique was implemented in an 8-channel wavelength-division multiplexing (WDM) transmitter utilizing MRM technology, fabricated in monolithic GF45SPCLO technology. Figure 3(a) illustrates the measurement setup, and Figure 3(b) shows the transmitted transient signal from the TIA's output before averaging.
Figure 3(c) outlines the three-step calibration procedure, involving sweeping the heater gate voltage while transmitting level 0, level 1, and a clock data pattern. The resulting peak drop-port average voltages were measured at 0.544 V, 0.56 V, and 0.535 V, respectively. These measurements were then used to calculate dl/Δλ as 0.178 V and the average drop-port voltage corresponding to maximum OMA as 0.40 V.
To verify the accuracy of the calibration, the heater gate voltage was swept again with higher resolution, as shown in Figure 3(d). The results confirm that the maximum OMA aligns with the calculated average voltage of 0.40 V, and Figure 3(b) exhibits the transient signal successfully locked at the peak OMA.
Conclusion
The proposed calibration technique for microring modulator thermal controllers offers a reliable and efficient method for maximizing OMA without relying on high-speed circuits or simulations. By leveraging the Lorentzian property of the MRM filter and a simple calibration procedure, the TCU can derive the average power corresponding to the maximum OMA, effectively addressing the challenges posed by process variations.
Through this approach, the expected yield for a target OMA of 0.5 mW is significantly improved, reaching 83% compared to traditional methods. This calibration technique has the potential to enhance the performance and reliability of microring modulator-based optical interconnects, enabling more efficient and robust data transmission in high-speed communication systems.
Reference
[2] M. Zabihpour, K. Hunter, E. Zailer, H. Shakiba, and A. Sheikholeslami, "A Calibration Technique for Microring Modulator Thermal Controller," in IEEE Journal of Solid-State Circuits, vol. 59, no. 2, pp. 484-492, Feb. 2024, doi: 10.1109/JSSC.2023.3373002
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