The progression of the development of all-silicon based microring resonator modulators for high bitrates is reviewed. Trade-offs and challenges are explored.

We report a 224Gb/s per lane PAM4 DR4-Tx optical sub-system photonic integrated circuit and linear driver chip set and achieved OER 6.3 dB and 1.88 dB TDECQ. The PIC consists of 4-channels micro-ring modulators with integrated hybrid lasers and integrated SOA for 800G-DR4.

Explosive growth of datacenter traffic drives rapid scaling of optical interconnects architectures and technologies. We summarize the advancements in Intel’s heterogeneously integrated silicon photonics manufacturing platform enabling throughput scaling for IMDD and high-efficiency coherent links.

A fully integrated 800 Gbps PAM-4 2×FR4 and DR8 silicon photonics transmitter with eight heterogeneously integrated DFB lasers is demonstrated for data center applications over a temperature range of 0∼70°C and a reach of up to 2 km. © 2022 The Authors

Silicon photonics is a key element to datacenter connectivity. We review recent progress on fully integrated Silicon photonics transmitters with heterogeneously integrated DFB lasers at Intel Corporation, and discuss future areas of development.

We demonstrate a high-efficiency PAM4 silicon photonics transmitter optimized through end-to-end system modeling for applications up to 10km on four-channel CWDM4 grid. Our measurements show a close agreement with simulations meeting 400G-FR4 requirements with 1.7Vppd.

Photonic integrated circuits (PICs) suffer from birefringence due to high-index contrast. Polarisation handling devices improve the performance of the PICs by reducing the polarisation-dependent dispersion and loss. Furthermore, there is a growing interest in building polarisation division multiplexed transceivers using PICs, which require polarisation management. The authors provide an overview of the recent work on developing polarisation handling devices such as polarisation beam splitters and polarisers in indium phosphide and silicon-on-insulator platforms for optical communications and sensing applications. These devices expand the PICs library of polarisation handling devices and can be used to design more complex circuits with advanced or new functionalities.

A 400Gbps PAM-4 fully integrated DR4 silicon photonics transmitter with four heterogeneously integrated DFB lasers has been demonstrated for data center applications over a temperature range of 0∼70°C and a reach of up to 2km

Providing a low-cost, reliable and end-to-end quality of service (QoS) guaranteed connectivity is going to be a major challenge in optical transport due to the immense growth in number of connected devices and traffic volume triggered by fifth-generation mobile networks (5G) and cloud networks. First, we discuss the architectures of the 5G transport network and the data center network, to capture the full value of connectivity. The paper offers an overview of the network requirements and how the integrated photonic platforms, along with the advanced modulation formats, play a key role to support the unprecedented requirements of 5G and massive cloud deployment. Second, we provide an overview of the recent work on the development of high-speed silicon photonic (SiPh) modulators to generate and transport PAM-4 signals. Third, recent advancements of the photodetectors, optical hybrids, and integrated coherent receivers on the silicon-on-insulator (SOI) platform are also described. The results suggest that, in the first access and aggregation segments of the network, integrated silicon-based modulators aided by PAM-4 modulation formats can support the stringent requirements of new generation transport networks in terms of speed, footprint and power consumption.

In this paper, we present inphase-quadrature (IQ) modulation in the O-band using dual parallel Mach-Zehnder modulators on the silicon photonics platform. The detailed design of the IQ modulator (IQM) is discussed. We then report the DC and small signal characterization of the device and investigate the performance of the device in a coherent transmission system. A bit rate of 180 Gb/s with 16-QAM modulation is achieved over 20 km of single-mode fiber without any chromatic dispersion compensation. Furthermore, we demonstrate that 77 Gbaud QPSK transmission can be achieved with a low drive voltage of 3 Vpp.

We demonstrate 25 and 50 Gb/s four-level pulse amplitude modulation (PAM-4) transmission over 43- and 21-km standard single-mode fiber (SMF), respectively, using a silicon photonic 120 degrees hybrid-based integrated simplified coherent receiver in the C-band. The integrated receiver is composed of an edge coupler, a vertical grating coupler, a polarization splitter-rotator, a 3 x 3 multimode interference coupler-based 120 degrees hybrid, and three germanium p-i-n photodetectors. For 25 Gb/s PAM-4 transmission, we achieved the receiver sensitivities of -18, -17.5, -16.6, -14.5, and -9.8 dBm in back-to-back (B2B) and after 10.5, 21, 31, and 43 km, respectively, at a bit error rate (BER) below the hard-decision forward error correction (HD-FEC) threshold (i.e., 3.8 x 10(-3)) without using any receiver equalizer, while for the 50 Gb/s PAM-4 transmission, the receiver sensitivities of -13.6 dBm in B2B, and -13.5 and -11.6 dBm after 10.5 and 21 km are obtained using a 31-tap linear feed-forward equalizer (FFE), respectively. Furthermore, we achieved up to 44 Gb/s receiver equalizer-free PAM-4 transmission over 10.5 km below the HD-FEC. The characterization of the receiver at different wavelengths within the C-band is provided as well.

We present a four-lane silicon photonic transceiver for intra-datacenter optical interconnects. A net rate of 400 Gb/s is achieved at a bit error rate below the KP-4 forward error correction threshold of 2.4 × 10 −4 .

We present a silicon photonic dual parallel multielectrode Mach-Zehnder modulator (MEMZM) based transmitter targeting 200 Gb/s four-level pulse amplitude modulation (PAM4) short reach transceivers.The MEMZMs have an average V π and electro-optic (EO) bandwidth of 5 V and 38 GHz, respectively.The transmitter is characterized versus receiver equalizer taps, received signal power, driving voltage swing, crosstalk voltage swing, bitrate, and reach.Results reveal that using only a three-tap equalizer at the receiver, 100 Gb/s PAM4 net rate per lane can be achieved at a bit error rate (BER) below the KP4 forward error correction (KP4-FEC) threshold of 2.4 × 10 -4 .Moreover, up to 128 Gb/s can be received at a BER below the KP4-FEC threshold using only 2 Vpp and 1 Vpp driving the MEMZM segments.Then, both MEMZMs are driven simultaneously to assess the crosstalk impact on the BER performance at parallel operation.Driven by four binary signals, we demonstrate 200 Gb/s PAM4 transmission over up to 10 km of single mode fiber at a BER below the KP4-FEC threshold.

We present and experimentally demonstrate a silicon photonic (SiP)-based four-lane 400 Gb/s transmitter for fiber-rich intra-datacenter optical interconnects. Four parallel SiP series push-pull traveling wave Mach-Zehnder modulators (MZMs) operating in the O-band are used in the transmitter. The MZMs have an average electro-optic (EO) bandwidth of approximately 30 GHz at 3 V reverse bias voltage. To assess the parallel operation, we measure the EO crosstalk between the four MZMs, where the EO crosstalk between the closest MZMs is below -17 dB over 50 GHz bandwidth. Then, we use a four-channel digital-to-analog converter (DAC) to simultaneously drive the MZMs and characterize the performance of the transmitter versus various parameters. Results reveal that 53 Gbaud pulse amplitude modulation over 4-levels (PAM4), i.e., 100 Gb/s net rate, per lane can be received at a bit error rate (BER) below the KP4- forward error correction (KP4-FEC) threshold of 2.4×10-4 using only a 5-tap feed-forward equalizer (FFE) at the receiver. In addition, we show that 53 Gbaud and 64 Gbaud PAM4 per lane can be received at a BER below the KP4-FEC and 7% hard decision FEC (HD-FEC), respectively, using a driving voltage swing below 1.8 Vpp. To the best of our knowledge, these are the best results for 100 Gb/s PAM4 using a single electrode SiP TWMZM with a lateral PN junction in a multi-project wafer process. Finally, we show that the BER is still below the KP4-FEC at maximum crosstalk for all lanes, and an aggregate rate of 400 Gb/s can be achieved at an average BER of approximately 1×10-4.

A loop-mirror modulator (LMM) combines a Mach-Zehnder modulator with an integrated loop mirror. Differential phase shift keying modulation for the LMM with dual-drive traveling wave electrodes is investigated. We show analytically that when operated with the proper bias conditions, the LMM can achieve the arbitrary phase and amplitude modulation. We demonstrate experimentally an error-free transmission at 10 Gb/s.

We present a novel CSPR controllable silicon photonic transmitter circuit based on a tunable power splitter for VSB self-coherent transmissions. 112 Gb/s 16 QAM over 80 km at a BER below 3.8 × 10 -3 has been achieved.

We first optimize the design and compare the performance of thermo-optic phase-shifters based on TiN metal and N++ doped silicon, in the same SOI process. The designs don't require special material processing, show negligible loss, and have very stable power consumption. The optimum TiN design has a switching powerPπ=21.4 mW and a time constantτ=5.6 µs, whereasPπ=22.8 mW andτ=2.2 µs for the best N++ Si design, enabling 2.5x faster switching compared to the metal heater. Doped-Si-based heaters are therefore the most practical and efficient on standard SOI. In addition, to optimize the layout density of highly integrated dies, we experimentally characterize internal and external thermal crosstalk for tunable Mach-Zehnder interferometers (MZIs) based on both heater designs for various power, distances, and etching patterns. Deep trenches are the best structures not involving special fabrication techniques to mitigate heat leakage affecting phase-sensitive devices close to heaters. Given the numerous applications of thermal tuners, this work is relevant to almost all silicon photonics designers.

Four level pulse amplitude modulation (PAM-4) has become the modulation format of choice to replace ON-OFF keying (OOK) for the 400 Gb/s short reach optical communications systems. In this paper, we investigate the passible modifications to conventional Mach-Zehnder modulator structures to improve the system performance. We present three different silicon photonic Mach-Zehnder modulator architectures for generating PAM-4 in the optical domain using OOK electrical driving signals. We investigate the transfer function and linearity of each modulator and experimentally compare their PAM-4 generation and transmission performance with and without use of digital signal processing (DSP). We achieve the highest reported PAM-4 generation and transmission without the use of DSP. The power consumption of each modulator is presented,and we experimentally show that multielectrode Mach-Zehnder modulators provide a clear advantage at higher symbol rates compared to conventional Mach-Zehnder modulators.

We show that differential signaling with coupled electrodes for silicon photonic modulators increases the electro-optic bandwidth in low-resistive substrates. We also show that high DC resistance of transmission lines negates the benefit of longer modulators.

We characterize the impact of the modulator material on chirp, digital signal processing (DSP) algorithms and system-level performance in coherent digital optical links. We compare theoretically, in simulations and experimentally the lithium niobate (LiNbO3), indium phosphide (InP) and silicon (Si) integrated platforms. Distortions to vector diagrams are traced back to modulation physics, and are interpreted as quadrature crosstalk. In a back-to-back BPSK setup with an RF drive signal amplitude of 1.5Vπ, we measure chirp parameters α of ~0, 0.10 and 0.06 and error vector magnitude EVMRMS of 5.3%, 9.4% and 10.6% with the LiNbO3, InP and Si modulators respectively. Both α and EVMRMS are found to scale with the RF signal amplitude. In simulations, using a polynomial fit over a sinusoidal fit when pre-compensating the Si modulator transfer function slightly improves EVM (-0.6%). We also show that Si-related distortions can impact the efficiency of symbol timing recovery. In conclusion, phase and attenuation distortions in InP and Si modulators deteriorate the overall performance in coherent links, and cannot be neglected for large RF signal amplitudes. These results will benefit the optical communications community.