Computation efficient sparse DNN nonlinear equalization for IM/DD 112 Gbps PAM4 inter-data center optical interconnects.

In this Letter, we propose and experimentally demonstrate a novel, to the best of our knowledge, sparse deep neural network-based nonlinear equalizer (SDNN-NLE). By identifying only the significant weight coefficients, our approach remarkably reduces the computational complexity, while still upholding the desired transmission accuracy. The insignificant weights are pruned in two phases: identifying the significance of each weight by pre-training the fully connected DNN-NLE with an adaptive L2-regularization and then pruning those insignificant ones away with a pre-defined sparsity. An experimental demonstration is conducted on a 112 Gbps PAM4 link over 40 km standard single-mode fiber with a 25 GHz externally modulated laser in O-band. Our experimental results illustrate that, for the 112 Gbps PAM4 signal at a received optical power of -5dBm over 40 km, the proposed SDNN-NLE exhibits promising solutions to effectively mitigate nonlinear distortions and outperforms a conventional fully connected Volterra equalizer (VE), conventional fully connected DNN-NLE, and sparse VE by providing 71%, 63%, and 41% complexity reduction, respectively, without degrading the system performance.

Reducing computation complexity by using elastic net regularization based pruned Volterra equalization in a 80 Gbps PAM-4 signal for inter-data center interconnects.

Volterra equalization (VE) presents substantial performance enhancement for high-speed optical signals but suffers from high computation complexity which limits its physical implementations. To address these limitations, we propose and experimentally demonstrate an elastic net regularization-based pruned Volterra equalization (ENPVE) to reduce the computation complexity while still maintain system performance. Our proposed scheme prunes redundant weight coefficients with a three-phase configuration. Firstly, we pre-train the VE with an adaptive EN-regularizer to identify significant weights. Next, we prune the insignificant weights away. Finally, we retrain the equalizer by fine-tuning the remaining weight coefficients. Our proposed ENPVE achieves superior performance with reduced computation complexity. Compared with conventional VE and L1 regularization-based Volterra equalizer (L1VE), our approach show a complexity reduction of 97.4% and 20.2%, respectively, for an O-band 80-Gbps PAM4 signal at a received optical power of -4 dBm after 40 km SMF transmission.

Inter-polarization mixers for coherent detection of optical signals.

Electro-magnetic (EM) mixers are fundamental building blocks in communication systems. They are used in frequency/wavelength filters, interferometric modulators, amplitude-phase receivers, to name a few. Traditional EM mixers have two or more input ports and work only for co-polarized signal and local-oscillator (LO) incident on its inputs. Here we report on novel designs, in silicon, of inter-polarization EM mixers operating at 1550 nm wavelength. The 180-degree optical mixer comprising a single input port is demonstrated to coherently mix orthogonally polarized signal and LO. Using the proposed 180-degree mixer, we report on a novel design for a 90-degree optical mixer on silicon with small footprint, broadband response, low loss and good fabrication tolerance. It exploits birefringence of a waveguide to achieve broadband and fabrication-tolerant 90° phase difference between the signal/LO relative phase in the in-phase and quadrature components. A monolithic silicon photonics coherent receiver is demonstrated using the reported 90-degree mixer, and its operation at 22 Gbaud and 44 Gbaud is shown. These mixers pave the way for novel coherent receiver architectures in long-haul, metro, passive optical networks and data-center interconnect applications.

Simultaneous four-channel thermal adaptation of polarization insensitive silicon photonics WDM receiver.

We propose a novel approach to demonstrate simultaneous multi-wavelength locking during temperature changes in a silicon photonic polarization insensitive microring-based wavelength division multiplexing (WDM) receiver. The DC component of a single monitoring photodetector at the through port of the microring filter array is exploited as a feedback signal with no additional power consumption. This feedback signal is used in control circuitry to properly tune the microring filters using ohmic heating, thus creating a feedback loop for thermal adaptation. We describe the necessary information, specifically each microring filter's room temperature resonant wavelength and tunability, which can be used to calibrate and achieve proper wavelength configurability and locking. In addition, we describe a simple control algorithm based on an adaptive gradient method often used in machine learning, allowing the receiver to endlessly demultiplex at different temperatures. We successfully achieve thermal adaptation over a temperature range >37°C and demultiplex a 4 × 25 Gb/s on-off-keying signal of 150 GHz channel spacing, all while the polarization is scrambling.

Simultaneous wavelength locking of microring modulator array with a single monitoring signal.

A microring modulator array coupled to a common bus waveguide can be used to construct low power, compact and flexible wavelength-division-multiplexing (WDM) transmitters. However, due to extremely small working bandwidths of the rings, it is challenging to find the right resonant wavelength setting and locking the resonance to an external laser. In the paper, we propose a novel technique enabling simultaneous wavelength locking of a microring modulator array with a single monitor, together with automatically optimizing the wavelength setting. We experimentally demonstrate locking three rings over a temperature range >40 °C at 3x20 Gb/s on-off-keying (OOK) modulation and ~3x75 Gb/s discrete multi-tone (DMT) modulation.

Optical filter requirements in an EML-based single-sideband PAM4 intensity-modulation and direct-detection transmission system.

The feasibility of a single sideband (SSB) PAM4 intensity-modulation and direct-detection (IM/DD) transmission based on a CMOS ADC and DAC is experimentally demonstrated in this work. To cost effectively build a >50 Gb/s system as well as to extend the transmission distance, a low cost EML and a passive optical filter are utilized to generate the SSB signal. However, the EML-induced chirp and dispersion-induced power fading limit the requirements of the SSB filter. To separate the effect of signal-signal beating interference, filters with different roll-off factors are employed to demonstrate the performance tolerance at different transmission distance. Moreover, a high resolution spectrum analysis is proposed to depict the system limitation. Experimental results show that a minimum roll-off factor of 7 dB/10GHz is required to achieve a 51.84Gb/s 40-km transmission with only linear feed-forward equalization.

128-Gb/s 100-km transmission with direct detection using silicon photonic Stokes vector receiver and I/Q modulator.

Recently, there is increasing interest in utilizing Stokes vector receiver, which is a direct-detection technique with the capability to digitally track the polarization changes in fibers and decode information in multiple dimensions. Here, we report a monolithically integrated silicon photonic Stokes vector receiver, which consists of one polarization beam splitter, two polarization rotators, one 90-degree optical hybrid, and six germanium photodetectors. Paired with a silicon in-phase/quadrature modulator incorporating a power-tunable carrier in the orthogonal polarization, transmission at 128-Gb/s over 100-km fiber is achieved with direct detection.

High-power dual-fed traveling wave photodetector circuits in silicon photonics.

We introduce the concept of dual-illuminated photodetectors for high-power applications. Illuminating the photodetector on both sides doubles the number of optical channels, boosting DC and RF power handling capability. This concept is demonstrated utilizing multiple-stage dual-illuminated traveling wave photodetector circuits in silicon photonics, showing a maximum DC photocurrent of 112 mA and a 3-dB bandwidth of 40 GHz at 0.3 mA. Peak continuous-wave RF power is generated up to 12.3 dBm at 2 GHz and 5.3 dBm at 40 GHz, at a DC photocurrent of 55 mA. High speed broadband data signals are detected with eye amplitudes of 2.2 V and 1.3 V at 10 Gb/s and 40 Gb/s, respectively. A theoretical analysis is presented illustrating design tradeoffs for the multiple-stage photodetector circuits based on the bandwidth and power requirements.

Novel integration technique for silicon/III-V hybrid laser.

Integrated semiconductor lasers on silicon are one of the most crucial devices to enable low-cost silicon photonic integrated circuits for high-bandwidth optic communications and interconnects. While optical amplifiers and lasers are typically realized in III-V waveguide structures, it is beneficial to have an integration approach which allows flexible and efficient coupling of light between III-V gain media and silicon waveguides. In this paper, we propose and demonstrate a novel fabrication technique and associated transition structure to realize integrated lasers without the constraints of other critical processing parameters such as the starting silicon layer thicknesses. This technique employs epitaxial growth of silicon in a pre-defined trench with taper structures. We fabricate and demonstrate a long-cavity hybrid laser with a narrow linewidth of 130 kHz and an output power of 1.5 mW using the proposed technique.

Silicon optical modulator with shield coplanar waveguide electrodes.

A silicon Mach-Zehnder Interferometer (MZI) optical modulator with a shield coplanar waveguide (CPW) transmission line electrode design was demonstrated. This shield-CPW electrode suppresses the signal distortion caused by the parasitic slot-line (SL) mode and improves the electrical bandwidth and the electro-optical (EO) bandwidth. With the shield-CPW electrodes and 5.5 mm-long phase shifters, the silicon MZI optical modulator delivered an EO bandwidth of above 24 GHz and a V (π) = 3.0 V was achieved at λ = 1310 nm. When modulated at 28-Gb/s data rate, it achieved an extinction ratio of 5.66 dB under a driving voltage of V (pp) = 1.3 V, corresponding to a power consumption of 0.8 pJ/bit.

Cascaded uncoupled dual-ring modulator.

We demonstrate, by coherent driving two uncoupled rings in same direction, that the effective photon circulating time in the dual-ring modulator is reduced, with increased modulation quality. The inter-ring detuning-dependent photon dynamics, Q factor, extinction ratio, and optical modulation amplitude of two cascaded silicon ring resonators are studied and compared with that of a single-ring modulator. Experimentally measured eye diagrams, together with coupled mode theory simulations, demonstrate the enhancement of the dual-ring configuration at 20 Gbps with a Q∼20,000.

112-Gb/s monolithic PDM-QPSK modulator in silicon.

We present a monolithic dual-polarization quadrature phase-shift keying (QPSK) modulator based on a silicon photonic integrated circuit (PIC). This PIC consists of four high-speed silicon modulators, a polarization rotator, and a polarization beam combiner. A 112-Gb/s polarization-division-multiplexed (PDM) QPSK modulation is successfully demonstrated.

50-Gb/s silicon quadrature phase-shift keying modulator.

We report the first successful demonstration of quadrature phase-shift keying (QPSK) modulation using two nested silicon Mach-Zehnder modulators. 50-Gb/s QPSK signal is generated with only 2.7-dB optical signal-to-noise ratio penalties from the theoretical limit at a bit-error ratio of 10(-3). This result validates that silicon photonics could be a viable and powerful platform of photonic integrated circuits in coherent optical communications.

Compact, low-loss and low-power 8×8 broadband silicon optical switch.

We demonstrated a 8×8 broadband optical switch on silicon for transverse-electrical polarization using a switch-and-selector architecture. The switch has a footprint of only 8 mm × 8 mm, minimum on-chip loss of about 4 dB, and a port-to-port insertion loss variation of only 0.8 dB near some spectral regions. The port-to-port isolation is above 30 dB over the entire 80-nm-wide spectral range or above 45 dB near the central 30 nm. We also demonstrated a switching power of less than 1.5 mW per element and a speed of 2 kHz, and estimated the upper bound of total power consumption to be less than 70 mW even without optimization of the default state of the individual switch elements.

Experimental demonstration of microring quadrature phase-shift keying modulators.

Advanced optical modulation formats are a key technology to increase the capacity of optical communication networks. Mach-Zehnder modulators are typically used to generate various modulation formats. Here, we report the first experimental demonstration of quadrature phase-shift keying (QPSK) modulation using compact microring modulators. Generation of 20 Gb/s QPSK signals is demonstrated with 30 µm radius silicon ring modulators with drive voltages of ~6 V. These compact QPSK modulators may be used in miniature optical transponders for high-capacity optical data links.

High-speed low-voltage single-drive push-pull silicon Mach-Zehnder modulators.

We demonstrate a single-drive push-pull silicon Mach-Zehnder modulator (MZM) with a π-phase-shift voltage of 3.1 V and speed up to 30 Gb/s. The on-chip insertion loss is 9 dB due to the use of a 6 mm-long phase shifter. Higher switching speed up to 40-50 Gb/s is also demonstrated in devices with shorter phase shifters which require higher drive voltages but have lower insertion losses.

Compact polarization rotator on silicon for polarization-diversified circuits.

We demonstrate a polarization rotator based on adiabatic mode evolution on silicon for polarization-diversified circuits. The rotator has a device length of 420 µm, a polarization-conversion efficiency of more than 90%, and an insertion loss less than 1 dB for a wavelength range of 80 nm. Combining the rotator with a compact, broadband polarization beam splitter based on cascaded directional couplers enhances the polarization conversion extinction ratio to over 30 dB with less than 1.5 dB total insertion loss over a 60 nm spectral range.

Monolithic silicon chip with 10 modulator channels at 25 Gbps and 100-GHz spacing.

We demonstrate a chip containing ten low-chirp silicon modulators, each operating at 25 Gbps, multiplexed by a SiN arrayed-waveguide grating with 100-GHz spacing, showing the potential for 250 Gbps aggregated capacity on a 5×8 mm(2) footprint.