Broadband polarization/mode insensitive 3-dB optical coupler for silicon photonic switches.

In this work, we experimentally demonstrate a four-mode polarization/mode insensitive 3-dB coupler based on an adiabatic coupler. The proposed design works for the first two transverse electric (TE) modes and the first two transverse magnetic (TM) modes. Over an optical bandwidth of 70 nm (1500 nm to 1570 nm), the coupler exhibits at most 0.7 dB insertion loss with a maximum crosstalk of -15.7 dB and a power imbalance not worse than 0.9 dB. A multimode photonic switch matrix using this optical coupler is proposed simultaneously exploiting wavelength division multiplexing (WDM), polarization division multiplexing (PDM), and mode division multiplexing (MDM). Based on the coupler experimental measurements, the switching system loss is estimated to be 10.6 dB with crosstalk limited by the MDM (de)multiplexing circuit.

Addressing the programming challenges of practical interferometric mesh based optical processors.

We demonstrate a novel mesh of Mach-Zehnder interferometers (MZIs) for programmable optical processors. We thoroughly analyze the benefits and drawbacks of previously known meshes and compare our newly proposed mesh with these prior architectures, highlighting its unique features and advantages. The proposed mesh, referred to as Bokun mesh, is an architecture that merges the attributes of the prior topologies Diamond and Clements. Similar to Diamond, Bokun provides diagonal paths passing through every individual MZI enabling direct phase monitoring. However, unlike Diamond and similar to Clements, Bokun maintains a minimum optical depth leading to better scalability. Providing the monitoring option, Bokun's programming is faster improving the total energy efficiency of the processor. The performance of Bokun mesh enabled by an optimal optical depth is also more resilient to the loss and fabrication imperfections compared to architectures with longer depth such as Reck and Diamond. Employing an efficient programming scheme, the proposed architecture improves energy efficiency by 83% maintaining the same computation accuracy for weight matrix changes at 2 kHz.

Fabrication error tolerant broadband mode converters and their working principles.

Computational inverse design techniques have shown potential to become reliable means for designing compact nanophotonic devices without compromising the performance. Much effort has been made to reduce the computation cost involved in the optimization process and obtain final designs that are robust to fabrication imperfections. In this work, we experimentally demonstrate TE0-TE1 and TE1-TE3 mode converters (MCs) on the silicon-on-insulator platform designed using the computationally efficient shape optimization method. These MCs have mode conversion efficiencies above 95%, and the insertion loss ranges from 0.3 dB to 1 dB over a wavelength span of 80 nm ranging from 1.5 µm to 1.58 µm. Maximum modal crosstalk found experimentally in the C-band is -19 dB. The conversion efficiency drops at most by 2.2% at 1.55 µm for 10 nm over/under etch, implying good robustness to dimensional variations. We present the mode conversion mechanism of these MCs by studying the simulated electromagnetic field patterns and validate with supportive data. We also demonstrate their performance in the time domain with a 28 Gbps OOK and a 20 GBaud PAM-4 payload transmissions, which supports their utility for high throughput data communications. The open eye diagrams exhibit Q-factors of 8 dB.

Efficient mode exchanger-based silicon photonic switch enabled by inverse design.

A novel and energy efficient mode insensitive switch building block is proposed and experimentally demonstrated on a silicon-on-insulator platform. Based on a Mach-Zehnder interferometer, the switch uses a relatively compact mode insensitive phase shifter which includes a mode exchanger. The novel structure realizes the exact same phase shift for all modes by exchanging the modes midway within the phase shifter. The design approach leads to reduced power consumption otherwise not possible. Switching the first two quasi transverse electric (TE) modes simultaneously consumes 25.6 mW of power, an approximately 30% reduction from previous reported demonstrations. The measured insertion loss is 3.1 dB on average with a worst-case crosstalk of -14.9 dB over a 40 nm optical bandwidth from 1530 nm to 1570 nm. The design methodology enables scalability up to four optical modes.

Dual-mode broadband compact 2 × 2 optical power splitter using sub-wavelength metamaterial structures.

The multimode power splitter is a fundamental component in mode-division multiplexed systems. In this paper, we design and characterize a broadband compact dual-mode multimode interferometer (MMI) optical power splitter based on subwavelength grating (SWG) structures. The optimized dual-mode MMI is three times more compact than its conventional mode insensitive MMIs and shows low loss and low crosstalk flat response over 100 nm bandwidth. Characterizations of the fabricated dual-mode splitter show that the total excess loss in the experiment is less than 0.1 dB and 0.65 dB for TE0 and TE1, respectively, and the modal crosstalk is less than -17 dB for both input modes.

Spectral-dependent electronic-photonic modeling of high-speed VCSEL-MMF links for optimized launch conditions.

We present spectral-dependent electronic-photonic modeling of vertical-cavity surface-emitting laser (VCSEL)-multimode fiber (MMF) links for next-generation high-speed interconnects. The beam coupling processes, between the VCSEL and the MMF and between the MMF and the photodetector (PD), are discussed, with spectral-dependent three-dimensional launch conditions analyzed. The model accounts for fiber effects on the transmission performance, specifically modal attenuation, dispersion, mode mixing, and mode partition noise. An advanced split-step small-segment (4-S) method simulates the signal evolution over the MMF with high accuracy and high efficiency. Experimental validation at 25 Gbps confirms the high accuracy of the VCSEL-MMF link model. The model reveals that larger radial offsets can further excite lower-order mode groups reducing the power distributed to higher-order groups when a tilted beam couples to the input fiber facet. With an optimized misalignment launch, the modal bandwidth is greatly improved by 3.8-fold compared to the conventional center launch. The model helps determine the optimum launch condition to improve link performance metrics such as transmission reach.

Topological inverse design of nanophotonic devices with energy constraint.

In this paper, we introduce an energy constraint to improve topology-based inverse design. Current methods typically place the constraints solely on the device geometry and require many optimization iterations to converge to a manufacturable solution. In our approach the energy constraint directs the optimization process to solutions that best contain the optical field inside the waveguide core medium, leading to more robust designs with relatively larger minimum feature size. To validate our method, we optimize two components: a mode converter (MC) and a wavelength demultiplexer. In the MC, the energy constraint leads to nearly binarized structures without applying independent binarization stage. In the demultiplexer, it also reduces the appearance of small features. Furthermore, the proposed constraint improves the robustness to fabrication imperfections as shown in demultiplexer design. With energy constraint optimization, the corresponding spectrum shifts under ±10 nm dimensional variations are reduced by 17% to 30%. The proposed constraint is unique in simultaneously taking both geometry and electric field into account, opening the door to new ideas and insights to further improve the computationally intensive topology-based optimization process of nanophotonic devices.

The diamond mesh, a phase-error- and loss-tolerant field-programmable MZI-based optical processor for optical neural networks.

This paper presents the performance analysis of a phase error- and loss-tolerant multiport field-programmable MZI-based structure for optical neural networks (ONNs). Compared to the triangular (Reck) mesh, our proposed diamond mesh makes use of a larger number of MZIs, leading to a symmetric topology and adding additional degrees of freedom for the weight matrix optimization in the backpropagation process. Furthermore, the additional MZIs enable the diamond mesh to optimally eliminate the excess light intensity that degrades the performance of the ONNs through the tapered out waveguides. Our results show that the diamond topology is more robust to the inevitable imperfections in practice, i.e., insertion loss of the constituent MZIs and the phase errors. This robustness allows for better classification accuracy in the presence of experimental imperfections. The practical performance and the scalability of the two structures implementing different sizes of optical neural networks are analytically compared. The obtained results confirm that the diamond mesh is more error- and loss-tolerant in classifying the data samples in different sizes of ONNs.

Mode insensitive switch for on-chip interconnect mode division multiplexing systems.

A mode insensitive switch is proposed and experimentally demonstrated on a silicon-on-insulator platform using a balanced Mach-Zehnder interferometer structure with a mode insensitive phase shifter for on-chip mode division multiplexing interconnects. Switching the first three quasi-transverse electric (TE) modes, consuming less than 40 mW power is demonstrated. The whole system exhibits approximately $ - {2},\;{ - 3.7}$-2,-3.7, and $ - {5.2}\;{\rm dB}$-5.2dB insertion loss for the TE0, TE1, and TE2 modes at 1550 nm, respectively. The corresponding crosstalk is less than $ - {8.6}\;({\rm - 9}), {- 8} ({ - 10.3})$-8.6(-9),-8(-10.3), and $ - {10}\;{\rm dB}$-10dB ($ - {10.3}\;{\rm dB}$-10.3dB) within the wavelength range of 40 nm (1535-1575 nm) for the cross (bar) states, respectively. The extinction ratios (ERs) for the cross (bar) states are 20.1 (19.5), 22.8 (33.7), and 15.4 dB (18.1 dB) for the TE0, TE1, and TE2 modes at 1550 nm, respectively. The payload transmission is also conducted using non-return-to-zero pseudorandom binary sequence (PRBS)-31 data signals at 10 Gb/s for single-mode transmission and simultaneous three-mode transmissions. For all the scenarios, open eyes are observed.

8 × 8 SOA-based optical switch with zero fiber-to-fiber insertion loss.

The design, fabrication, and characterization of an 8×8 lossless optical switch, based on semiconductor optical amplifier (SOA) gates, is reported. It comprises three stages of 2×2 switches into an 8×8 Banyan switch, for a total of 48 SOAs. Three SOAs on each optical path provide gain to compensate for on-chip and fiber coupling loss, thereby making the optical switch lossless. All 64 optical paths demonstrate error-free 10 Gbps NRZ PRBS-31 transmission with at least 30 dB signal-to-noise ratio and less than 0.9 dB power penalty.

Modal crosstalk in Silicon photonic multimode interconnects.

We investigate modal crosstalk in silicon photonic MDM-based interconnects using tapered multiplexers. Crosstalk from coherent optical interference originates from variation in the physical structure and alters the transmission link performance. Through simulations and experimental work, optical crosstalk as a function of wavelength is analyzed to understand its impact in MDM and MDM-WDM dual-multiplexing applications. The detrimental effects are validated in the frequency and time domains through fabricated MDM interconnects of various lengths. Results indicate modal crosstalk must be < -22 dB to maintain a BER of 10-12. The experimental methodology assesses the optical modal crosstalk's impact on the data, towards a mitigation approach to improve the payload signal integrity and enable system-level optimization such as channel wavelength allocation.

3×10 Gb/s silicon three-mode switch with 120° hybrid based unbalanced Mach-Zehnder interferometer.

We propose and experimentally demonstrate a reconfigurable mode division multiplexing (MDM) silicon photonics three-mode switch (3MS) in C-band using a 120° optical hybrid based unbalanced Mach-Zehnder interferometer (UMZI) and Ti/W metal heater phase-shifter. The novel 3MS enables reconfigurable switching of the first three transverse electric (TE) modes by exploiting the relative phase difference of the 120° hybrid. A proof-of-concept realization of this 3MS demonstrates <12.0 µs switching time and >12.3 dB switching extinction ratio at 1560 nm wavelength with 94.8 mW average heater power consumption. Simultaneous (de)multiplexing and switching of 10 Gb/s non-return-to-zero (NRZ) PRBS31 optical payload over three spatial channels experimentally demonstrates 3 ×10 Gb/s aggregated bandwidth. Open eye diagrams in all output channels with >9.6 electrical signal-to-noise ratio (SNR) exhibits reliable data transmission. The 3MS has potential applications in MDM silicon photonics interconnects for the implementation of high throughput switch matrix.

Dynamic switching of a packaged photonic integrated network-on-chip using an FPGA controller.

A packaged photonic integrated network-on-chip (NoC) based on multi-microrings with a controller and scheduler implemented in FPGA is demonstrated under dynamic packet-switched traffic. Multiple transmission scenarios have been investigated, comprising up to three interfering signals at the same wavelength. The dynamic switching exhibits a power penalty of approximately 0.5 dB at a BER of 10-9. The presence of up to three interferers induces a power penalty below 1 dB.

High-speed grating-assisted all-silicon photodetectors for 850 nm applications.

This paper presents the design, fabrication, and measurement results of a novel lateral p-i-n silicon photodetector (Si-PD) for 850 nm in a silicon-on-insulator (SOI) platform. In the proposed photodetector, the incident light is directed horizontally using a grating coupler, significantly increasing optical absorption in the depletion area thereby increasing the PD's responsivity. The measurement results show that the grating coupler increases the responsivity by 40 times compared with the Si-PD without a grating coupler. The grating-assisted Si-PD with 5µm intrinsic width has a responsivity of 0.32 A/W and a dark current of 1 nA at 20 V reverse-bias voltage. Further, it shows an open eye diagram for 10 Gb/s PRBS­31 non-return-to-zero on-off keying (NRZ-OOK) data and has a 3-dB bandwidth of 4.7 GHz at this bias voltage. In addition, the design parameters of three variations of the grating-assisted Si-PD for high-speed applications (>25 Gb/s) are presented. The optimized grating-assisted Si-PD uses a focusing grating coupler and its p-i-n diode has a 0.3 µm intrinsic width. It has a responsivity of 0.3 A/W, an avalanche gain of 6, a dark current of 2 µA, and a 3-dB bandwidth of 16.4 GHz at 14 V reverse-bias voltage that, to the best of our knowledge, is the largest 3-dB bandwidth reported for a Si-PD. As a result, it has a figure of merit (FoM) of 4920 GHz×mA/W. Further, it shows an open eye diagram of 35 Gb/s PRBS­31 NRZ-OOK data.

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.

Mode selecting switch using multimode interference for on-chip optical interconnects.

A novel mode selecting switch (MSS) is experimentally demonstrated for on-chip mode-division multiplexing (MDM) optical interconnects. The MSS consists of a Mach-Zehnder interferometer with tapered multi-mode interference couplers and TiN thermo-optic phase shifters for conversion and switching between the optical data encoded on the fundamental and first-order quasi-transverse electric (TE) modes. The C-band MSS exhibits a >25 dB switching extinction ratio and < -12 dB crosstalk. We validate the dynamic switching with a 25.8 kHz gating signal measuring switching times for both TE0 and TE1 modes of <10.9 µs. All channels exhibit less than 1.7 dB power penalty at a 10-12 bit error rate, while switching the non-return-to-zero PRBS-31 data signals at 10 Gb/s.

Responsivity optimization of a high-speed germanium-on-silicon photodetector.

This paper experimentally demonstrates a design optimization of an evanescently-coupled waveguide germanium-on-silicon photodetector (PD) towards high-speed (> 30 Gb/s) applications. The resulting PD provides a responsivity of 1.09 A/W at 1550 nm, a dark current of 3.5 µA and bandwidth of 42.5 GHz at 2 V reverse-bias voltage. To optimize the PD, the impact of various design parameters on performance is investigated. A novel optimization methodology for the PD's responsivity based on the required bandwidth is developed. The responsivity of the PD is enhanced by enlarging its geometry and using off-centered contacts on top of the germanium, while an integrated peaking inductor mitigates the inherent bandwidth reduction from the responsivity optimization. The performance of the optimized PD and the conventional, smaller size non-optimized PD is compared to validate the optimization methodology. The sensitivity of the optimized PD improves by 3.2 dB over a smaller size non-optimized PD. The paper further discusses the impact of top metal contacts on the photodetector's performance.

100G and 200G single carrier transmission over 2880 and 320 km using an InP IQ modulator and Stokes vector receiver.

We demonstrate experimentally the transmission of single carrier 56 Gbaud 16-QAM, 8-QAM and QPSK optically modulated signals over 320, 960 and 2,880 km, respectively, using a fully packaged InP IQ modulator and a Stokes vector direct detection (SV-DD) receiver realized using discrete optics. Results show that by optimizing the carrier-to-signal-power ratio, the total throughput-times-distance product for 16 QAM and QPSK are 71,680 Gbps.km and 322,560 Gbps.km, respectively, at bit error rate (BER) below the hard decision forward error correcting threshold (HD-FEC) of 4.5 × 10-3.

Integrated optical deserialiser time sampling based SiGe photoreceiver.

A novel photoreceiver architecture enabling parallel processing in the electronic domain of a high-speed optical signal is demonstrated. This allows the electronics to operate at significantly lower frequency than the optical signal and hence reduce power consumption and the impact of parasitics. The photoreceiver performs optical time sampling with four integrated SiGe photodetectors connected in series by waveguide delay lines. Four variations of the optical time sampling receiver are designed and demonstrated which differ by the data rate (10 Gb/s and 20 Gb/s) and silicon delay waveguide loss (2.5 dB/cm and 0.2 dB/cm). The bit error rate performance of the photodetectors in the receiver was measured individually and reached a performance below 1 × 10⁻¹° at an input optical power between 4.8 and 6.3 dBm through an off-chip 50 Ω load at the output. After O/E conversion, the electrical signal (one segment of 2¹5 - 1 PRBS data) from each of the photodetector is processed without errors at a quarter of the bit rate, leading to an overall more power efficient receiver front-end.

Simultaneous high-capacity optical and microwave data transmission over metal waveguides.

The implementation of power efficient and high throughput chip-to-chip interconnects is necessary to keep pace with the bandwidth demands in high-performance computing platforms. In recent years, considerable effort has been made to optimize inter-chip communications using traditional copper waveguides. Also, optical links are extensively investigated as an alternative technology for fast and efficient data routing. For the first time, we experimentally demonstrate simultaneous microwave and optical high-speed data transmission over metallic waveguides embedded in polymer. The demonstration is significant as it merges two layers of communications onto the same structure towards increased aggregated bandwidth, and energy-efficient data movement.