Standalone, CMOS-based Faraday rotation in a silicon photonic waveguide.

Nonreciprocity is a fundamental requirement of signal isolation in optical communication systems. However, on chip isolator designs require either post-processing steps or external magnetic biasing, which are impractical for commercial applications. This raises the need for standalone devices which support nonreciprocal functionality using standardized fabrication techniques. Here, we report the first design of an electromagnetic coil surrounding a waveguide which exclusively employed the complementary metal-oxide-semiconductor (CMOS) process flow. The coil supported an electric current up to 14 mA. In simulations, it generated an alternating magnetic flux density up to 1.16 mT inside a strip waveguide and thereby induced a rotation of 50.71 picodegrees for the fundamental transverse-magnetic mode at a wavelength of 1352 nm. Our analysis further revealed methods to increase the rotation by orders of magnitude. It demonstrated the scope of manufacturing processes and serves as a building block for the development of a commercially viable, on-chip optical isolator.

Design, analysis, and characterization of a compact silicon photonic modulator with meandered phase shifters.

We present a C-band Mach-Zehnder modulator with meandered phase shifters and a compact footprint of 432 × 260 µm2 on the silicon-on-insulator platform. Electrode, p-n junction, and optical transit time are considered when performing the electro-optic bandwidth (EO BW) simulations. The simulation results prove that the dominant bandwidth limiting factor for this type of modulator is optical transit time. The insertion loss of the modulator without bias is 2.1 dB. The measured half-wave voltage (V π ) and 3-dB EO BW at -0.5 V bias are 6.4 V and 7.7 GHz, respectively. 53 Gbaud PAM-4 transmission over 2 km of standard single-mode fiber is achieved at a bit error rate (BER) below the 6.7% overhead hard-decision forward error correction BER threshold of 3.8×10 -3.

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.

180 Gb/s single carrier single polarization 16-QAM transmission using an O-band silicon photonic IQM.

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.

400 Gb/s O-band silicon photonic transmitter for intra-datacenter optical interconnects.

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.

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.

CMOS-compatible multi-band plasmonic TE-pass polarizer.

A CMOS-compatible plasmonic TE-pass polarizer capable of working in the O, E, S, C, L, and U bands is numerically analyzed. The device is based on an integrated hybrid plasmonic waveguide (HPW) with a segmented metal design. The segmented metal will avoid the propagation of the TM mode, confined in the slot of the HPW, while the TE fundamental mode will pass. The TE mode is not affected by the metal segmentation since it is confined in the core of the HPW. The concept of the segmented metal can be exploited in a plasmonic circuit with HPWs as the connecting waveguides between parts of the circuit and in a silicon photonics circuit with strip or slab waveguides connecting the different parts of the circuit. Using 3D FDTD simulations, it is shown that for a length of 5.5 µm the polarization extinction ratios are better than 20 dB and the insertion losses are less than 1.7 dB over all the optical communication bands.

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.

Modulator material impact on chirp, DSP, and performance in coherent digital links: comparison of the lithium niobate, indium phosphide, and silicon platforms.

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.

First demonstration of a 400 Gb/s 4λ CWDM TOSA for datacenter optical interconnects.

We present the first demonstration of a 4λ transmitter optical sub-assembly (TOSA) on the coarse wavelength division multiplexing (CWDM) grid, i.e., 20 nm spacing, targeting 400G-FR4 requirements over 2 km. The TOSA is based on uncooled InP external modulated laser (EML) technology and it utilizes four EMLs followed by a CWDM multiplexer. We characterize the performance of the TOSA versus received optical modulation amplitude (OMA), number of equalizer taps, reach, modulation format, TOSA case temperature, and bit rate. Four 53 Gbaud 4-level pulse amplitude modulation (PAM4) RF signals are used to drive the TOSA achieving a net rate of 400 Gb/s. Results reveal that 400 Gb/s can be transmitted over 2 km of single mode fiber (SMF) at a bit error rate (BER) below the KP4- forward error correction (KP4-FEC) threshold (i.e., 2.4 × 10-4) using only a 5 tap feed forward equalizer at the receiver. To the best of our knowledge, this is the first demonstration of 400 Gb/s using a 4λ CWDM TOSA over 2 km of SMF. Moreover, we achieve 400 Gb/s and 600 Gb/s over 20 km and 10 km below KP4-FEC and the 7% hard-decision FEC (HD-FEC) (i.e., 3.8 × 10-3) thresholds, respectively, without optical amplification. Furthermore, we show the performance of the TOSA against temperature, where it shows no significant change in the BER performance from 20 °C to 60 °C. Finally, we compare the performance of PAM2, PAM4, and PAM8 modulation formats where we show the possibility of achieving 400 Gb/s aggregate bit rate using 42 Gbaud PAM8 modulation format at the expense of utilizing a stronger FEC.

DSP-free 'coherent-lite' transceiver for next generation single wavelength optical intra-datacenter interconnects.

We propose a DSP-free coherent-lite system that requires neither high-speed DSP nor high-resolution signal converters for deployment inside datacenters over single mode fiber links with reaches of 10 km and less. The removal of converters and DSP, in which some subsystems are fundamental for successful coherent detection, is enabled by either replacing DSP subsystems with optics having equivalent functions or by re-engineering the system. We validate in a proof-of-concept experiment the proposed DSP-free system using 50 Gbaud DP-16QAM delivering 400 Gb/s over 10 km of single mode fiber (SMF) below the KP4 forward error correction (FEC) threshold of 2.2 × 10-4. In addition, we perform a detailed experimental parametric study of the coherent-lite system in which various system parameters are swept such as baud rate, reach, laser power and laser linewidth. Our results verify that the coherent-lite system can be realized using low-cost DFB lasers with linewidths of a few hundred kHz. Moreover, we compare the performance of the coherent-lite system with that of a conventional coherent transceiver leveraging the full DSP stack. Then, we evaluate the power consumption savings achieved by the coherent-lite scheme relative to a classic DSP-based coherent system. Assuming a CMOS node ranging from 28 to 7 nm for DSP implementation, our estimate shows that the coherent-lite scheme can save 95 to 78% of the power consumed by the following subsystems: analog-to-digital converters, chromatic dispersion compensation, 2 × 2 MIMO polarization demultiplexing and carrier recovery. Finally, we compare the power consumption of the coherent-lite scheme with more standard 400G IM-DD systems utilizing either eight or four parallel WDM lanes (8 × 50G and 4 × 100G). The coherent-lite system is found to have similar module power consumption requirements as a corresponding 4 × 100G IM-DD system while bringing the benefits of coherent detection including improved sensitivity and higher spectral efficiency leading to fewer light sources per transceiver module. To the best of our knowledge, this work represents the first experimental demonstration of a DSP-free coherent-lite system for single channel 400G datacenter 10 km interconnects, a potential attractive solution due to its scalability to future 800G and 1.6T intra-datacenter optical interconnects.

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.

200 Gb/s transmission using a dual-polarization O-Band silicon photonic intensity modulator for Stokes vector direct detection applications.

We present a dual-polarization O-band silicon photonic (SiP) transmitter for intra-datacenter optical interconnects. The transmitter is built using two identical O-band traveling wave Mach-Zehnder modulators with an average VπL and a bandwidth at 1.5 V bias voltage of 2.88 V.cm and 24.5 GHz, respectively. We experimentally demonstrate the transmitter in a Stokes vector direct-detection (SV-DD) system for dual-polarization intensity modulated signals with 2-level and 4-level pulse amplitude modulation (DP-PAM2 and DP-PAM4) formats. The direct-detection Stokes vector receiver (DD-SVR) followed by offline digital signal processing (DSP) is implemented for SOP de-rotation. We characterize the performance of the SV-DD system versus number of taps, received signal power, state of polarization (SOP), reach, and bit rate. Results reveal that 112 Gb/s DP-PAM2 can be transmitted over 10 km of single mode fiber (SMF) at a bit error rate (BER) below 10-5 at -1 dBm received signal power irrespective of the SOP. Moreover, a 168 Gb/s (42 Gbaud) DP-PAM4 signal can be transmitted over 2 km and 10 km at a BER below the 7% hard-decision forward error correction (HD-FEC) threshold (i.e., 3.8 × 10-3) at 0 dBm and 2 dBm, respectively. Furthermore, 224 Gb/s and 200 Gb/s DP-PAM4 are successfully received at a BER below the HD-FEC in the back-to-back and 2 km cases, respectively. Finally, we compare the performance of the 6 × 2 multiple-input multiple-output (MIMO) equalization to a simpler 4 × 2 MIMO equalization and explain the superior performance of the 6 × 2 in the presence of SVR imperfections.

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.

Self-homodyne system for next generation intra-datacenter optical interconnects.

We propose a self-homodyne system for next generation intra-datacenter networking. The proposed system has a higher spectral efficiency for the modulated signal compared to the intensity-modulation/direct-detection (IM/DD) systems and uses digital signal processing of reduced complexity compared to a conventional coherent system. The concept of the proposed system is to send the modulated signal and a tone originating from the same laser over the full-duplex fiber with the aid of circulators to be used remotely at the receiver for coherent detection. The overall system physical complexity approaches the equivalent IM/DD system giving the same target data rate for 400G systems and beyond. We experimentally demonstrate emulation of the proposed system and report data rates of 530 Gb/s, 448 Gb/s and 320 Gb/s on a single wavelength below the KP4 forward error correcting threshold over 500 m, 2 km and 10 km of single mode fiber, respectively.

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.