Polarization-Insensitive Tunable Silicon Photonic Optical Filters for High-Speed Data Centers

Introduction

The exponential growth of data traffic has driven the widespread adoption of high-speed Dense Wavelength-Division Multiplexing (DWDM) optical transceivers in data centers. However, filterless DWDM systems suffer from Optical Signal-to-Noise Ratio (OSNR) degradation due to the accumulation of Amplified Spontaneous Emission (ASE) noise from optical amplifiers. To mitigate this issue, tunable ASE optical filters have become indispensable components in advanced optical transceivers and transmission systems to meet the OSNR tolerance threshold. Silicon photonics integrated optical filters, particularly those based on Microring Resonators (MRRs), offer a compact, cost-effective, and spectrally efficient solution for such transceivers.

Device Design and Configuration

The demonstrated polarization-insensitive filter utilizes Polarization Splitter-Rotators (PSRs) and Polarization Combiner-Rotators (PCRs) in conjunction with two multistage second-order Vernier MRR-based filters. This configuration ensures polarization-independent performance and C-band tunability within feasible temperature constraints.

Fig. 1. (a) The fabricated device on SOI platform. (b) Control circuitry of the optical integrated filter.

As shown in Figure 1(a), the fabricated device is built on a silicon-on-insulator (SOI) platform. The input PSR routes the transverse electric (TE) component through the TE path to the upper MRR filter, while the transverse magnetic (TM) component is rotated into the TE polarization through the TM path and directed to the lower MRR filter. At the output, a PCR combines the filtered signals from each filter and routes them to the device output, reversing the TE and TM path connections to eliminate Polarization Dependent Loss (PDL).

Integrated heaters, connected to electrical pads and wire-bonded to a Printed Circuit Board (PCB), are used for thermal alignment and tuning of the resonance wavelength. The control circuitry, as depicted in Figure 1(b), consists of an Analog Front End (AFE) with high-precision voltage and current output Digital-to-Analog Converters (VDACs and IDACs), Transimpedance Amplifier (TIA) inputs, and a Microcontroller Unit (MCU) controlled via a Serial Peripheral Interface (SPI) bus. Photodiodes connected to the TIAs monitor the optical output power of each filter, providing feedback for controlling the heaters.

Characterization and Measurements

The device is configured using an automatic tuning technique that employs a customized hybrid optimization algorithm to generate look-up tables. An additional step is performed to enable temperature tracking and active regulation of the heaters by monitoring their resistance changes.

Fig. 2: (a) Reference resistance and TCR versus voltage for the first heater. (b) Resistance and TCR of the first heater by temperature at 0.2 V. (c) Normalized drop-port spectrum across the full FSR in the C-band at room and 80 °C temperatures. (d) Normalized peak transmission spectrum, accounting for fiber-chip coupling losses.

Figures 2(a) and 2(b) illustrate the relationship between the heater's reference resistance, temperature coefficient of resistance (TCR), and temperature, which are used to estimate and track the on-chip temperature for precise resonance wavelength tuning.

The filter resonance was aligned and tuned to the 1544.53 nm wavelength channel. Figure 2(c) shows the measured aligned and tuned spectrum at 24°C and 80°C, exhibiting a 40 nm Free Spectral Range (FSR), an average extinction ratio (ER) of 23 dB, and a minimum out-of-band suppression of 12.5 dB. The measured 3dB bandwidth at 1544.53 nm is around 90 GHz at 24°C and 92 GHz at 80°C, as shown in Figure 2(d). However, the bandwidth varies between 85-97 GHz across the C-band due to wavelength dependency of the couplers, which can be reduced by utilizing wavelength-independent filters.

The device's internal Insertion Loss (IL), excluding fiber-chip coupling loss, is less than 1.6 dB up to 80°C across the entire C-band, while the measured Polarization Dependent Loss (PDL) is negligible. The power required to tune each heater by 2π is around 70 mW, and the maximum power needed to tune the filter to any wavelength channel is approximately 560 mW using standard heaters, or 95 mW with thermal insulation trenches and undercut.

Conclusion

The presented polarization-insensitive integrated optical filter configuration demonstrates high performance and low temperature tuning requirements, making it an efficient alternative to bulky and expensive off-chip commercial filters for high-speed data center interconnects. With negligible PDL, up to 1.6 dB internal IL, 23 dB average ER, and a bandwidth of 85-97 GHz, this tunable filter offers a compact and cost-effective solution for mitigating OSNR degradation in advanced optical transceivers and transmission systems.

Reference

[1] S. Alnairat, B. Wohlfeil, S. S. Djordjevic, B. Schmauss, and J.-P. Elbers, "Polarization-insensitive Tunable Silicon Photonic Optical Filters for Data Center Transceivers," in Proc. of the IEEE Conference on Photonics, 2024, pp. 1-3, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.