Asymmetric self-coherent detection.

The bandwidth upgrade required in short-reach optical communications has prompted the need for detection schemes that combine field reconstruction with a cost-effective subsystem architecture. Here we propose an asymmetric self-coherent detection (ASCD) scheme for the field reconstruction of self-coherent (SC) complex double-sideband (DSB) signals based on a direct-detection (DD) receiver with two reception paths. Each reception path consists of a photodiode (PD) and an analog-to-digital converter for the detection of a part of the received optical signal that experiences a different optical transfer function via the configuration of an optical filter. We derive an analytical solution to reconstructing the signal field and show the optimal filter response in optimizing the signal SNR. Further, we numerically characterize the theoretical performance of a specific ASCD scheme based on a chromatic dispersion filter and validate the principle of the ASCD scheme in a proof-of-concept experiment. The ASCD scheme approaches the electrical spectral efficiency of coherent detection with a cost-effective DD receiver, which shows the potential for high-speed short-reach links required by edge cloud communications and mobile X-haul systems.

Cost-minimizing distribution matching supporting net 800 Gbit/s PS-PAM transmission over 2 km using a 4-λ EML TOSA.

Probabilistic shaping (PS) allows tunable spectral efficiency that is suitable for realizing high throughput intra-data center transceivers. In this Letter, we integrate the cost-minimizing distribution matching (CMDM) in the probability amplitude shaping scheme to generate PS-PAM signals with ultra-short symbol block lengths for reduced serial processing delay. We detail the principle of CMDM and present two different methods of implementation. We demonstrate that CMDM enables the transmission of single wavelength net 200 Gbit/s PS-PAM-8 over 2 km of single-mode fiber (SMF). We show that similar performance is achievable using a constant composition distribution matcher, yet requiring 10 times longer symbol block lengths. We also report, to the best of our knowledge, the first demonstration of net 800 Gbit/s transmission over 2 km of SMF using a packaged 4-λ electro-absorption modulated laser transmitter optical sub-assembly (TOSA).

23-dB average isolation using a silicon photonic Mach-Zehnder modulator.

We demonstrate an optical time-gate isolator entirely fabricated on the silicon-on-insulator (SOI) platform based on a conventional traveling-wave Mach-Zehnder modulator (TW-MZM) design. The device achieves 18.2 dB (22.7 dB) time-averaged isolation when driven with 2.0-Vpp (7.1-Vpp) differential clock signals at 6.8 GHz and biased at null. Under these conditions, the isolator blocks backward light at all time regardless of driver amplitude, but produces periodic modulation in the forward direction. Moreover, we embed our isolator in a digital communication link and measure a signal-to-noise ratio (SNR) penalty of only 0.5 dB due to the isolator at 13.6 Gbaud PAM-4 data rate. Our device can be integrated in larger circuits to protect laser sources or mitigate interference.

Asymmetric direct detection of orthogonal offset carriers assisted polarization multiplexed single-sideband signals.

We propose and demonstrate the asymmetric direct detection (ADD) of polarization division multiplexed single-sideband (PDM-SSB) signals with orthogonal offset carriers. ADD exploits the photocurrent difference to eliminate the Y-Pol interference in the X-Pol, and the X-Pol signal intensity to eliminate the X-Pol interference in the Y-Pol without resorting to iterative algorithms. This enables not only low-complexity signal linearization but also a simplified receiver front-end composed of a single optical filter, two single-ended photodiodes and two analog-to-digital converters (ADC). In the experiment, we first perform a parametric study of the proposed scheme at 40 Gbaud in the back-to-back configuration (B2B) to evaluate the performance impact of different system parameters including the carrier to signal power ratio (CSPR), the matched filter roll-off, and the filter guard band. Next, we demonstrate the transmission of 416 Gbit/s PDM 16-QAM signal over 80 km single-mode fiber (SMF) below the soft-decision forward error correction (SD-FEC) threshold of 2×10-2. We also numerically study the effectiveness of a 2×2 multiple-input-multiple-output MIMO equalizer in alleviating the inter-polarization linear crosstalk resulting from the non-orthogonal PDM-SSB signals due to polarization-dependent loss (PDL), which is not negligible for potential on-chip implementation of ADD.

Asymmetric direct detection of twin-SSB signals.

We propose the asymmetric direct detection (ADD) of twin-single sideband (SSB) signals based on a simple receiver front-end composed of one optical filter and two photodiodes. ADD exploits the photocurrent difference between a filtered and unfiltered signal pair to reconstruct and linearize the received twin-SSB signal with a high electrical spectral efficiency (ESE). We evaluate the performance impact of the critical system parameters on ADD and demonstrate 231 Gb/s net rate 16-QAM twin-SSB transmission with 6.03 b/s/Hz ESE over an 80 km standard single-mode fiber below the ${1} \times {{10}^{ - 2}}$1×10-2 hard-decision forward error correction threshold. We also found that the bit error rate performance of ADD is robust against the relative center wavelength drifting of the optical filter.

Optimization of thermo-optic phase-shifter design and mitigation of thermal crosstalk on the SOI platform.

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.

Broadband 1310/1550 nm wavelength demultiplexer based on a multimode interference coupler with tapered internal photonic crystal for the silicon-on-insulator platform.

We design and experimentally demonstrate a broadband 1310/1550 nm wavelength demultiplexer based on a multimode interference coupler with a tapered internal photonic crystal (PC) structure for the silicon-on-insulator platform. The tapered internal PC structure is engineered to reflect the C-band light while transmitting the O-band light. Novel PC nanotapers are introduced for the internal PC structure that effectively suppress the sidelobe of the photonic bandgap and enable our device to be operable over the O-band. The device was fabricated using electron beam lithography, and its performance has been experimentally characterized. The measured extinction ratios are higher than 15 dB over a 74 nm bandwidth from 1286 to 1360 nm at the O-band, and over a 103 nm bandwidth from 1527 to 1630 nm that covers the C-band and the L-band.

Design and analysis of high-speed optical access networks in the O-band with DSP-free ONUs and low-bandwidth optics.

In this paper, we demonstrate and investigate high-speed passive optical networks (PON) in the O-band without using any digital signal processing (DSP) at the ONU side, meanwhile still adopting cost-effective low-bandwidth optics. We show that with commercial 10G-class optics including DML and PIN-TIA, 50Gb/s PAM4 PON with a power budget up to 29dB is achieved under the HD-FEC BER limit. We present the detailed design procedure of the DSP including the downlink Nyquist pulse shaping and pre-equalization and uplink post-equalization, and conduct a comprehensive study on this system including DSP complexity, DAC/ADC sampling rate requirement and tolerance to the response diversity of ONU receivers. We conclude that, 1) 55 and 35 taps are sufficient for downlink and uplink digital filters to approach the BER floor; 2) the minimum required DAC and ADC sampling rate are respectively 33.75GS/s and 32GS/s to achieve the optimal performance; and 3) with 1dB sensitivity penalty at the HD-FEC BER limit, the allowable response diversity of ONU receivers can be from -2.2dB/10GHz to 1.8dB/10GHz. Furthermore, we explore and demonstrate the suitable data rate based on on-off keying and duobinary modulation formats in this system and present corresponding results.

Compact high-performance adiabatic 3-dB coupler enabled by subwavelength grating slot in the silicon-on-insulator platform.

We demonstrate a compact high-performance adiabatic 3-dB coupler for the silicon-on-insulator platform. The refractive index of the gap region between two coupling waveguides is effectively increased using subwavelength grating, which leads to high-performance operation and a compact design footprint, with a mode-evolution length of only 25 µm and an entire device length of 65 µm. The designed adiabatic 3-dB coupler has been fabricated using electron beam lithography and the feature size used in our design is CMOS compatible. The fabricated device is characterized in the wavelength range from 1500 nm to 1600 nm, with a measured power splitting ratio better than 3 ± 0.27 dB and an average insertion loss of 0.20 dB.

100 Gb/s PAM4 transmission system for datacenter interconnects using a SiP ME-MZM based DAC-less transmitter and a VSB self-coherent receiver.

We experimentally demonstrate a digital-to-analog-converter-less (DAC-less) vestigial sideband (VSB) 4-level pulse amplitude modulation (PAM4) transmission system for data center interconnects (DCIs) using a silicon photonic (SiP) multi-electrode Mach-Zehnder modulator (ME-MZM) based DAC-less transmitter and a VSB self-coherent receiver. The impacts of linear and nonlinear impairments on the proposed system and their mitigation methods are comprehensively studied. By using Kramer-Kronig (KK) detection, frequency domain chromatic dispersion compensation, and short-memory time domain Volterra equalization at the receiver, we report a 112 Gb/s PAM4 transmission over 40 km standard single mode fiber (SSMF) with a bit error rate (BER) below the 7% overhead (OH) hard-decision forward error correction threshold of 3.8 × 10-3, and a 120 Gb/s PAM4 transmission over 80 km SSMF with a BER below the 20% OH soft-decision forward error correction threshold of 2 × 10-2, without any transmitter side digital signal processing such as pre-emphasis and pulse shaping.

Silicon photonic dual-drive MIM based 56 Gb/s DAC-less and DSP-free PAM-4 transmission.

We report two designs of silicon photonic dual-drive Michelson interferometric modulators (MIMs) suitable for four-level pulse amplitude modulation (PAM-4) that do not require digital-to-analog converters or digital signal processing. The PN junctions in MIM-1 have an asymmetric geometry and 4 doping concentrations, while those in MIM-2 have a symmetric geometry and 6 doping concentrations. We simulate and experimentally demonstrate that MIM-2 has a larger modulation efficiency and a better electro-optic (EO) bandwidth than MIM-1. The measured VπLπ of MIM-2 at -2 V bias is 0.8 V-cm, and the measured 3-dB EO bandwidth at 0 V bias is 9.3 GHz. By carefully choosing the bias conditions of the device and the driving binary radio-frequency signals applied on each phase shifter, PAM-4 signals with even spacings are generated. Successful 56 Gb/s PAM-4 transmission over 2 km of standard single mode fiber is presented, with an estimated bit error rate below the hard-decision forward error correction threshold of 3.8 × 10-3.

Transversely coupled Fabry-Perot resonators with Bragg grating reflectors.

We design and demonstrate Fabry-Perot resonators with transverse coupling using Bragg gratings as reflectors on the silicon-on-insulator (SOI) platform. The effects of tailoring the cavity length and the coupling coefficient of the directional coupler on the spectral characteristics of the device are studied. The fabricated resonators achieved an extinction ratio (ER) of 37.28 dB and a Q-factor of 3356 with an effective cavity length of 110 µm, and an ER of 8.69 dB and a Q-factor of 23642 with a 943 µm effective cavity length. The resonator structure presented here has the highest reported ER on SOI and provides additional degrees of freedom compared to an all-pass ring resonator to tune the spectral characteristics.

Experimental study of performance enhanced IM/DD transmissions combining 4D Trellis coded modulation with precoding.

We propose to combine 4D trellis-coded modulation (TCM) with transmitter-side Tomlinson-Harashima precoding (THP) in IM/DD transmissions, and experimentally investigate the achieved performance improvement. Theoretically, THP can approximately produce an end-to-end additive white Gaussian noise (AWGN) channel even with severe bandwidth limitation, allowing TCM to maintain its coding gain in the presence of inter-symbol interference. In our experiments with off-the-shelf commercial components, which limit the 3 dB bandwidth of the system to be ~3.5 GHz, the combination of TCM and THP shows a better receiver sensitivity for various system bit rates from 56 Gbit/s to 112 Gbit/s, considering the KP4 threshold of BER = 2 × 10-4. In the 112 Gbit/s back-to-back (B2B) transmission, with the help of THP the receiver sensitivity is improved by 3.3 dB using 4D-PAM4 TCM at the KP4 FEC threshold compared with using conventional PAM4. In addition, combining TCM and THP also helps to lower the BER floor.

Compact single-etched sub-wavelength grating couplers for O-band application.

We demonstrate two single-etched sub-wavelength grating coupler (SWGC) designs for O-band application, one targeting at high coupling efficiency and the other targeting at broad operating bandwidth. The high-efficiency SWGC has a measured peak coupling efficiency of -3.8 dB and a 3-dB bandwidth of 40 nm, and the broadband SWGC has a measured peak coupling efficiency of -4.3 dB and a 3-dB bandwidth of 71 nm. Focusing gratings have been used in our SWGCs to reduce the design footprints and the dimensions of our SWGCs are smaller than 45 µm × 24 µm. The back reflections of our SWGCs are suppressed to be below -15 dB over the wavelength range from 1260 nm to 1360 nm.

High-speed low-chirp PAM-4 transmission based on push-pull silicon photonic microring modulators.

We report a silicon photonic modulator based on the use of dual parallel microring modulators (MRMs) inserted in a Mach-Zehnder interferometer (MZI). It is operated in a push-pull configuration for low-chirp transmission at approximately 1550 nm. The chirp parameters of the device are measured using 10 Gb/s on-off keying (OOK) transmission over 20 km of standard single mode fiber (SSMF), and they are less than 0.01, showing the low-chirp characteristic of the modulator. We further demonstrate four-level pulse amplitude modulation (PAM-4) transmission at 92 Gb/s over 1 km of SSMF and at 40 Gb/s over 20 km of SSMF. The measured bit error rates (BERs) are below the hard-decision (HD) forward error correction (FEC) threshold of 3.8 × 10-3.