High-speed optical interconnects of data centers and high performance computers (HPC) have become the rapid development direction in the field of optical communication owing to the explosive growth of market demand. Currently, optical interconnect systems are moving towards higher capacity and integration. High-sensitivity receivers with avalanche photodiodes (APDs) are paid more attention due to the capability to enhance gain bandwidth. The impact ionization coefficient ratio is one crucial parameter for avalanche photodiode optimization, which significantly affects the excess noise and the gain bandwidth product (GBP). The development of silicon-germanium (Si-Ge) APDs are promising thanks to the low impact ionization coefficient ratio of silicon, the simple structure, and the CMOS compatible process. Separate absorption charge multiplication (SACM) structures are typically adopted in Si-Ge APDs to achieve high bandwidth and low noise. This paper reviews design and optimization in high-speed Si-Ge APDs, including advanced APD structures, APD modeling and APD receivers.

A CMOS-compatible avalanche photodiode (APD) with high speed and high sensitivity is a critical component of a low-cost, high-data-rate, and energy-efficient optical communication link. A novel waveguide-coupled silicon–germanium APD detector with three electric terminals was demonstrated with breakdown voltage of $-6\mathrm{V}$, bandwidth of 18.9 GHz, DC photocurrent gain of 15, open-eye diagram at a data rate of 35 Gb/s, and sensitivity of $-11.4\mathrm{dBm}$ at a data rate of 25 Gb/s. This three-terminal APD allows high-yield fabrication in the standard CMOS process and provides robust high-sensitivity operation under small voltage supply.

Silicon-germanium (Si-Ge) avalanche photodiodes (APDs) have large gain bandwidth product (GBP) and low excess noise due to the low impact ionization coefficient ratio of silicon. Optical receivers using APDs are able to achieve high-speed and energy efficient optical transceiver systems. We demonstrate a waveguide Si-Ge APD with low breakdown voltage of -10 V, achieving 60 Gb/s PAM4 successfully. A compact APD circuit model was constructed to allow photonic devices and transceiver circuitry co-design. The APD receiver has achieved -16 dBm sensitivity at 50 Gb/s PAM4 with a bit error rate (BER) of ${2.4\times 10^{-4}}$ . The sensitivity of APD receivers changes with the multiplication gain. In our analysis, compared to a PIN PD receiver the APD receiver operating at optimum gain can obtain ∼8 dB more sensitivity for NRZ signaling at <50 Gb/s and 3-4 dB more sensitivity for PAM4 signaling at 50-100 Gb/s. Also, the APD receiver operating at optimum gain can reduce power consumption by ∼10% at PAM4 data rates of 50 Gb/s and ∼15% at 100 Gb/s in a silicon carrier-depletion microring modulator based WDM photonic link.

We demonstrate low-voltage waveguide silicon-germanium avalanche photodiodes (APDs) integrated with distributed Bragg reflectors (DBRs). The internal quantum efficiency is improved from 60% to 90% at 1550 nm assisted with DBRs while still achieving a 25 GHz bandwidth. A low breakdown voltage of 10 V and a gain bandwidth product of near 500 GHz are obtained. APDs with DBRs at a data rate of 64 Gb/s pulse amplitude modulation with four levels (PAM4) show a 30%–40% increase in optical modulation amplitude (OMA) compared to APDs with no DBR. A sensitivity of around − 13 dBm at a data rate of 64 Gb/s PAM4 and a bit error rate of 2.4 × 10 − 4 is realized for APDs with DBRs, which improves the sensitivity by ∼ 2 dB compared to APDs with no DBR.

Reconfigurable photonic filters are promising candidates to meet the requirements of future microwave communication systems. However, sensitivity to process and temperature variations necessitates an automatic calibration solution to enable robust operation. This paper demonstrates fully automatic tuning of silicon photonic all-pass filter (APF)-based pole/zero filters using a monitor-based tuning method that calibrates the initial response by controlling each pole and zero individually via microheaters. The proposed tuning approach calibrates severely degraded initial responses to the designed elliptic filter shapes and allows us for automatic bandwidth and center-frequency reconfiguration of these filters. This algorithm is demonstrated on second- and fourth-order filters fabricated in a standard silicon photonics foundry process. After the initial calibration, only 300 ms is required to reconfigure a filter to a different center frequency. Thermal crosstalk between the microheaters is investigated, with substrate thinning demonstrated to suppress this effect and reduce filter calibration to less than half of the original thick substrate times. This fully automatic tuning approach opens the possibility of employing silicon photonic filters in real communication systems.

Utilizing four-level pulse-amplitude modulation (PAM-4) with vertical-cavity surface emitting lasers (VCSELs) is challenging due to device nonlinearity and bandwidth limitations. This paper presents a VCSEL-based PAM-4 transmitter that employs a 2.5-tap nonlinear equalizer to compensate for these issues. The transmitter utilizes lookup-table control of a current-mode digital-to-analog converter (DAC) output stage to enable independent equalization coefficients for the different PAM-4 symbol transitions. Utilizing half pre-cursor (MSB only), one main, and one post-cursor taps provides a good balance between circuit complexity and performance. Equalization resolution is increased by segmenting the output stage into a 2-bit symbol DAC and a fine 3-bit equalization DAC. Fabricated in GP 65nm CMOS, the serializing transmitter operates at 50 Gb/s with 3 dBm optical modulation amplitude (OMA) when driving an 11 GHz VCSEL and achieves 5.12 pJ/bit power efficiency.