We demonstrate a novel polarization beam splitter (PBS) based on a subwavelength grating (SWG) multimode interference (MMI) coupler for the silicon-on-insulator platform. The birefringence of the MMI coupler is engineered using SWG, which leads to a compact design footprint, with a device length of less than 100 μm. Our PBS has simulated extinction ratios (ERs) better than 20 dB for both polarizations over the wavelength range from 1530 to 1625 nm that covers the entire Cand L-bands. The fabricated device achieves the measured ERs larger than 20 dB at the wavelength of 1550 nm for both polarizations, and the insertion losses of 1.9 and 2.5 dB for the transverse electric (TE) and transverse magnetic (TM) polarizations, respectively, at 1550 nm. In addition, the measured ERs are larger than 14.5 dB for the TE polarization and 11.7 dB for the TM polarization over an 84-nm spectral range covering the entire C-band.

We present the design and characterization of O-band and C-band silicon photonic (SiP) traveling wave Mach-Zehnder modulators (TW-MZM) allowing 220 Gbps/ net rate operation. The designed modulators show over 45 GHz 3-dB E-O bandwidth with a single-segment design. In the O-band, with simple linear feed forward equalization, we transmit net 203 (200) Gbps signal over 2 km (10 km) of single-mode fiber (SMF) below the hard-decision forward error correction (HD-FEC) BER threshold of 3.810-3. With the aid of nonlinear Volterra equalizer and one 2.3Vpp driving signal, we transmit net 225 (216) Gbps PAM8 signals assuming 20% overhead soft-decision FEC with a normalized general mutual information (NGMI) threshold of 0.8798 over 2 km (10 km) of SMF. The C-band design enables net 220 Gbps in B2B and net 215 Gbps over 500 m of SMF above the specified NGMI threshold. These results are the highest reported net rate for SiP MZM in an intensity modulation direct-detection (IM/DD) system, fabricated entirely in a commercial foundry.

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 manuscript, we investigate the possible modifications to conventional Mach-Zehnder modulator structures to improve the system performance. We present 3 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 multi-electrode Mach-Zehnder modulators provide a clear advantage at higher symbol rates compared to conventional Mach-Zehnder modulators.

The performance of faster-than-Nyquist intensity modulation and direct detection (IM/DD) systems can be severely degraded by the inter-symbol interference (ISI) due to the component bandwidth limitation and the faster-than-Nyquist filtering. In addition, nonlinearities caused mainly by optical modulation and square-law detection can further deteriorate signal quality. Here, we propose to incorporate transmitter-side Tomlinson-Harashima precoding to mitigate the ISI and a Volterra nonlinear equalizer (VNLE) to mitigate the nonlinear distortions for PAM4 faster-than-Nyquist systems. We experimentally demonstrate a single wavelength 145 Gb/s PAM4 IM/DD transmission with bandwidth limited commercial components and sub-Nyquist sampling at 70 GSa/s. The optimal VNLE filter length is also investigated to balance the implementation complexity and system performance. In addition, compared with the scheme using transmitter-side duobinary precoding (DuoBP), receiver-side VNLE and maximum likelihood sequence estimation (MLSE), both performance improvement and implementation complexity reduction are achieved.