Polarization-Diverse CWDM Silicon Photonic Receiver: A Tutorial

Introduction

The exponential growth of data traffic in data centers, fueled by emerging data-hungry applications like machine learning and large language models such as ChatGPT, has led to an alarming increase in data center power consumption. Optical communication systems offer a promising solution to address this challenge, as they are more energy-efficient than their electrical counterparts. However, the pluggable transceivers currently used in data centers are too large to cope with the anticipated traffic explosion. Co-packaged optics (CPO) has recently been proposed as a more compact and scalable solution.

For 3.2-Tbps CPO modules, the module width must be less than 22 mm, necessitating the use of coarse wavelength-division multiplexing (CWDM) and polarization-diverse operation. Silicon photonics (SiPh) is a key enabling technology for realizing high shoreline density and functionality simultaneously, but SiPh components typically exhibit significant polarization dependence.

To address these challenges, researchers at KYOCERA Corporation have developed a polarization-diverse CWDM silicon photonic receiver. In this tutorial, we will explore the design, components, and performance of this innovative receiver.

Polarization-Diverse Photonic Integrated Circuit

The polarization-diverse CWDM receiver is implemented as a photonic integrated circuit (PIC), as shown schematically in Figure 1. The input light from the fibers first passes through polarization splitter and rotator (PSR) components placed as close as possible to the input to minimize jitter caused by the difference in group index between TE (transverse electric) and TM (transverse magnetic) light.

Figure 1. Schematic of our developed 1.6-Tbps SiPh chip.

Next, demultiplexers (DeMUXs) split the multiplexed signals by wavelength. Although the goal is to eliminate polarization-dependent losses (PDLs), some residual PDL remains due to factors such as spot-size converters (SSCs), fabrication errors, and others. To compensate for these PDLs, variable optical attenuators (VOAs) are employed. Finally, each photodiode (PD) has two input ports to combine the TE and TM optical signals.

Optical Components

The PIC integrates several key optical components, each designed and optimized for polarization-diverse CWDM operation.

1. PSR: The PSR is based on a directional coupler design. Figure 2(b) shows the insertion loss spectra of the PSR for TE and TM input polarizations, respectively, with the largest PDL around 3 dB at the shortest wavelength.

Fig. 2. Overview of developed photonic integrated circuit (PIC): (a) photo of PIC, (b) insertion loss spectra for PSR with TE and TM inputs, (c) DeMUX characteristics, (d) VOA spectra under varying currents, and (e) responsivity spectrum of MMI-PD at -3.3-V bias showing actual measurements (solid line) and theoretical unit quantum efficiency (dotted line).

2. DeMUX: The DeMUX is designed to split CWDM light with low insertion loss and crosstalk, as shown in Figure 2(c). It includes a heater for shifting the center of the passband.

3. VOA: Two types of VOAs are employed: one for PDL compensation (inserted only in the TE lanes due to higher PSR insertion loss for TE input) and another for adjusting the input light amplitude. Figure 2(d) demonstrates the linear attenuation characteristics of the VOAs, with a measured efficiency of around 0.2 dB/mA.

4. Two-port PD: To combine the TE and TM optical signals, the PDs feature two input ports. A multi-mode interferometer (MMI) structure is used to focus the light onto a small area, enabling high responsivity and low capacitance. Figure 2(e) shows the responsivity spectrum of the MMI-PDs, with responsivities over 0.85 A/W and 3-dB bandwidths exceeding 35 GHz.

All-in-One Polarization-Diverse CWDM Receiver

By integrating the aforementioned components, KYOCERA researchers fabricated a prototype of the polarization-diverse CWDM receiver chip.

Figure 3(a) shows the spectral response of the photocurrent, confirming that the input light with TE and TM polarizations is correctly split and directed to the appropriate PDs. Without using the VOAs for PDL compensation, residual PDLs of 4–7 dB are observed.

Fig. 3. Photocurrent spectra of PD: (a) without VOA for TE and TM inputs, and (b) with adjusted VOA current across several polarization axes, at a PD bias of -3.3 V and input optical power of 10 dBm.

When the VOAs are employed for PDL compensation, as shown in Figure 3(b), the measured photocurrent spectra exhibit minimal PDL for input light with all polarizations, thanks to the integrated VOAs.

The prototype PIC achieves an impressive shoreline bandwidth density of 266 Gbps/mm, calculated from its 6-mm width and 1.6-Tbps capacity. This compact and high-performance design makes it well-suited for CPO modules and high-density pluggable transceivers.

Conclusion

The polarization-diverse CWDM silicon photonic receiver developed by KYOCERA Corporation represents a significant advancement in addressing the growing demand for high-bandwidth optical communication systems in data centers. By integrating key components such as PSRs, DeMUXs, VOAs, and two-port PDs onto a single PIC, the researchers have demonstrated a compact and high-performance solution that mitigates polarization-dependent losses and enables polarization-diverse CWDM operation.

With its high shoreline bandwidth density and ability to compensate for residual PDLs, this innovative receiver paves the way for more efficient and scalable optical communication systems in CPO modules and pluggable transceivers, ultimately contributing to reduced power consumption in data centers.

Reference

[1] N. Matsui, H. Uemura, R. Motoji, D. Maeda, and T. Sugita, "A Polarization-Diverse Coarse Wavelength-Division Multiplexing Silicon Photonic Receiver," in Proceedings of the IEEE International Conference on Photonics, Yokohama, Japan, 2024, pp. 1-2. doi: 979-8-3503-9404-7/24/$31.00.