Polarization-Insensitive High-Speed All-Silicon Double-Microring Avalanche Photodiodes

Introduction

Silicon photonics (SiPh) has emerged as a promising technology for low-cost and CMOS-compatible photonic integrated circuits, enabling large-scale manufacturing, electronic co-integration, and high-volume production. However, a common challenge for all-silicon avalanche photodiodes (APDs) is their inability to detect light in both transverse electric (TE) and transverse magnetic (TM) polarizations, which necessitates additional polarization handling components, increasing complexity and footprint.

This article discusses a novel polarization-insensitive high-performance all-silicon double-microring resonator (MRR) APD integrated with a two-dimensional (2D) grating coupler, as demonstrated by researchers at Hewlett Packard Labs.

Device Design

The key components of the device are the 2D grating coupler and the double-MRR APD structure.

1. 2D Grating Coupler

The 2D focusing grating coupler is designed to split any polarized light into the TE modes at two orthogonally oriented waveguides, as shown in Fig. 1(a).

Fig. 1. (a) Microscope image of the double-MRR APD with 2D GC. (b) 3-D schematic of the double-MRR APD. (c) Schematic cross section and (d) simulated electric field of the all-Si PN junction.

2. Double-MRR APD

The double-MRR APD consists of two 12-μm radius resonators with a waveguide height of 220 nm and a width of 500 nm. The first MRR is undoped with low propagation loss, while the second MRR includes a "Z" shaped PN junction that acts as a photodetector (PD). The new doping concentration allows for a better overlap (by ~40%) between the high electric field and the optical mode, improving the absorption probability.

Fig. 1(d) shows the simulated electric field distribution at -8.7 V in the MRR, where the photon-assisted tunneling (PAT) effect can induce non-negligible photocurrent with the narrower junction barrier, and the highest electric field is sufficient for triggering impact ionization to boost the photocurrent.

Measured Results

1. 2D Grating Coupler Performance

At a wavelength of 1310 nm, the input 2D grating coupler has an insertion loss (IL) of ~8 dB, and the polarization dependence of the IL is ~1 dB, measured by exciting the structure with two orthogonal polarization states (-45 and 45 degrees).

2. Double-MRR APD Performance

The double-MRR APD exhibits a free spectral range (FSR) of ∼5.6 nm and an extinction ratio (ER) of ∼10 dB, as shown in Fig. 2(a).

Fig. 2. (a) Normalized transmission spectrum. (b) Measured total and dark current at -10 dBm input. (c) Measured responsivity at different optical power and (d) measured frequency response at two orthogonal polarization states.

The I-V characteristics (Fig. 2(b)) demonstrate a very low dark current of ~1.5 nA when the responsivity is 0.68 A/W and 0.53 A/W at -8.7 V for x- and y-polarized input light, respectively, with -10 dBm optical input. The highest responsivity is ~1 A/W at -8.9 V, and the maximum polarization dependence of responsivity is ~1 dB.

The frequency response (Fig. 2(d)) is polarization-insensitive, with a high 3dB bandwidth >50 GHz at -7.8 V and ~40 GHz at -8.7 V, where the avalanche built-up time starts to dominate the bandwidth limitation.

High-Speed Data Transmission

To investigate the dynamic properties, eye diagrams of the double-MRR APD were measured at -8.7 V without an amplifier. As shown in Fig. 3, the APD demonstrates open and symmetrical 92 Gb/s non-return-to-zero (NRZ) eye diagrams with signal-to-noise ratios (SNRs) of 5 and 4.8 for x- and y-polarized light, respectively.

Fig. 3. Measured 92 Gb/s NRZ eye diagrams for (a) x- and (b) y-polarized light. Measured 144 Gb/s PAM-4 eye diagrams for (c) x- and (d) y-polarized light.

Furthermore, the double-MRR APD can support 144 Gbit/s PAM4 signals, with symbol error rate (SER) penalties of 4.8 dB and 4.5 dB for x- and y-polarized light, respectively, at the Soft Decision FEC (SD-FEC) threshold of SER 4.8×10^-4 according to the IEEE 802.3cd standard.

Conclusion

This work demonstrates a polarization-insensitive high-speed all-silicon double-MRR APD with a 2D grating coupler. The APD exhibits a responsivity of ~1 A/W, a low dark current of ~1.5 nA, a high bandwidth of >50 GHz at -7.8 V and ~40 GHz at -8.7 V, and the ability to detect 92 Gbps NRZ and 144 Gbps PAM4 eye diagrams.

The polarization insensitivity, suppressed channel crosstalk, high-speed characteristics, and manufacturability make this device suitable as a next-generation DWDM all-silicon receiver solution with reduced cost, complexity, and footprint.

Reference

[2] Y. Peng, S. Hooten, Y. Yuan, Z. Huang, S. Cheung, W. V. Sorin, D. Liang, M. Fiorentino, R. G. Beausoleil, "Polarization-Insensitive High-Speed All-Silicon Double-Microring Avalanche Photodiodes," Hewlett Packard Labs, Hewlett Packard Enterprise, Milpitas, CA, USA, 2024.