On-chip silicon photonic nanohole metamaterials enabled high-density waveguide arrays.

High-density silicon waveguide arrays manufactured on a complementary metal-oxide-semiconductor (CMOS)-foundry platform hold great promise for optical information processing and photonic integration. However, evanescent waves arising from nanoscale confinement would cause significant optical crosstalk in waveguide arrays, which remains a vital issue in various applications. Here, by utilizing silicon photonic nanohole metamaterials, we propose a scheme to greatly suppress the crosstalk in the devices and then demonstrate ultra-compact low-crosstalk waveguide arrays. For a 100-µm-long waveguide array at a half-wavelength pitch, low crosstalk of -19 dB can be obtained in a wide range of wavelengths (1500 nm-1580 nm). In the experimental demonstrations, our approach exhibits the ability to suppress the crosstalk over a broad bandwidth without substantially increasing the propagation loss as well as the promising design flexibility, which shall pave the way for metamaterials enabled high-density waveguide arrays.

Graphene plasmonic spatial light modulator for reconfigurable diffractive optical neural networks.

Terahertz (THz) diffractive optical neural networks (DONNs) highlight a new route toward intelligent THz imaging, where the image capture and classification happen simultaneously. However, the state-of-the-art implementation mostly relies on passive components and thus the functionalities are limited. The reconfigurability can be achieved through spatial light modulators (SLMs), while it is not clear what device specifications are required and how challenging the associated device implementation is. Here, we show that a complex-valued modulation with a π/2 phase modulation in an active reflective graphene-plasmonics-based SLM can be employed for realizing the reconfigurability in THz DONNs. By coupling the plasmonic resonance in graphene nanoribbons with the reflected Fabry-Pérot (F-P) mode from a back reflector, we achieve a minor amplitude modulation of large reflection and a substantial π/2 phase modulation. Furthermore, the constructed reconfigurable reflective THz DONNs consisting of designed SLMs demonstrate >94.0% validation accuracy of the MNIST dataset. The results suggest that the relaxation of requirements on the specifications of SLMs should significantly simplify and enable varieties of SLM designs for versatile DONN functionalities.

Resolving the scalability challenge of wavelength locking for multiple micro-rings via pipelined time-division-multiplexing control.

Micro-ring resonator (MRR) is a key photonic device that has a wide range of applications but suffers from wavelength uncertainties. For almost all practical applications, a wavelength controller is required for each MRR. The wavelength controller is usually much larger than the MRR. With more complicated control algorithms, the controller size becomes even larger. Equipping each MRR with a wavelength controller will not be scalable. We propose a pipelined time-division-multiplexing (PTDM) control scheme that achieves high scalability while maintaining good loop bandwidth by exploiting the speed mismatch between the heater and the controller. To verify this proposed scheme, a hybrid integrated controller supporting four MRRs is designed. Measurement results show that it achieves a sine tracking speed of about 15 nm/s while achieving a locking accuracy of 7 pm and a tuning range of 9 nm.

Demonstration of an ultra-compact 8-channel sinusoidal silicon waveguide array for optical phased array.

Here we demonstrate an ultra-compact 8-channel sinusoidal silicon waveguide array for an optical phased array. In our device, based on sinusoidal bending, the cross talk (CT) between waveguides can be efficiently reduced with a waveguide pitch of only 695 nm. For the transverse electric (TE) mode, the simulation results show that the insertion loss (IL) of the 100-µm-long device is 0.1 dB and the CT between all waveguides is lower than -25 dB at 1550 nm. In the measurements, an IL of less than 1 dB and CT lower than -18 dB are obtained. Since the pitch is related to the beam-steering range and power consumption of the optical phased array, such an ultra-compact device could potentially be a good candidate to build the emitter for an energy-efficient optical phased array with a large field of view.

Ultra-compact multimode waveguide bend with shallowly etched grooves.

In this work, an ultra-sharp multimode waveguide bend (MWB) based on gradient shallowly etched grooves is proposed and demonstrated. With a bending radius of only 5.6 µm, our shallowly-etched-groove multimode waveguide bend (SMWB) can enable low excess loss and low-crosstalk propagation with the four lowest-order TE mode-channels, simultaneously. In the simulation, the excess losses of the proposed 90°- SMWB for TE0-TE3 are all below 0.46 dB and the inter-mode crosstalks are lower than -18 dB in 1500 nm-1600 nm. Furthermore, the measured results of the fabricated 90°- SMWB show that the excess losses for TE0-TE3 are less than 1 dB and the inter-mode crosstalks are all below -14 dB in 1510 nm-1580 nm. Such a proposed device thus provides a promising solution for ultra-compact MWBs in multimode silicon photonics.

Design of an ultra-compact low-crosstalk sinusoidal silicon waveguide array for optical phased array.

In this work, an ultra-compact low-crosstalk sinusoidal silicon waveguide array is proposed and analyzed. We first design a pair of low-crosstalk sinusoidal silicon waveguides with a pitch of 695 nm, where the sinusoidal bends are the key to reduce the crosstalk between waveguides. Then, based on this idea, we propose a low-crosstalk sinusoidal silicon waveguide array with a 695 nm pitch. The simulation results show that for an array length of 100 µm, the insertion loss is as low as 0.08 dB, and the crosstalk is lower than -26 dB at 1550 nm. The 695 nm pitch waveguide array also exhibits a favorable fabrication error tolerance when taking into account the waveguide width variations in practice. Moreover, within the acceptable range of crosstalk, the center-to-center distance between adjacent waveguides of this array can be further reduced to 615 nm. Since the pitch is related to the power consumption and beam-steering range of the optical phased array, our design provides an effective method to build the emitter for an energy-efficient optical phased array with a large field of view.

Self-homodyne wavelength locking of a silicon microring resonator.

We propose and experimentally demonstrate a self-homodyne locking method for a silicon microring resonator (MRR). The device employs a self-homodyne detection structure and consists of a tunable MRR with two directional couplers along the ring for monitoring, two phase shifters to calibrate the phase difference between the two monitored optical signals, and a Y-branch to combine the two signals. A single photodetector is used to detect the output power of the Y-branch. If the MRR is on resonance, a destructive interference occurs in the Y-branch, therefore the monitored photocurrent is minimized. By using such a device structure and the homodyne detection scheme, the MRR with a Q factor of 1.9 × 104 can be accurately locked to the signal wavelength, and the locking process is insensitive to input power variation. The wavelength locking range is larger than one free spectral range (FSR) of 6 nm, and the locking errors are ≤0.015 nm.

Compact CWDM interleaver based on an interfering loop containing a one-dimensional Fabry-Perot cavity.

We propose and experimentally demonstrate a compact silicon photonic interleaver based on an interfering loop containing a 1D Fabry-Perot (FP) cavity for coarse wavelength division multiplexing (CWDM) applications. The interleaver consists of a directional coupler and a FP cavity designed to minimize the channel crosstalk. Instead of using an off-chip optical circulator, the reflection light of the interleaver can be separated from the input by placing two identical interleavers in a Mach-Zehnder interferometer (MZI) structure for practical applications. We also study the impacts of fabrication errors of the MZI structure. The fabricated device has a footprint of 64 µm×70 µm and a channel spacing of ∼19 nm. The maximum crosstalk is -16 dB in a wavelength range from 1508 nm to 1590 nm.

Single-resonance silicon nanobeam filter with an ultra-high thermo-optic tuning efficiency over a wide continuous tuning range.

Energy-efficient tunability is highly desired for silicon photonic devices. We demonstrate a thermo-optic tunable filter with an ultra-high tuning efficiency based on a suspended photonic crystal nanobeam cavity. Attributed to the ultra-small mode volume and free-standing waveguide structure, a tuning efficiency of 21 nm/mW is achieved over a wide single-resonance tuning range of ∼43.9 nm. The 10%-90% switching times are 67.0 µs and 68.8 µs for the rising edge and the falling edge, respectively. The demonstrated energy-efficient tunable device can find applications in reconfigurable photonic integrated circuits.

Silicon photonic bandpass filter based on apodized subwavelength grating with high suppression ratio and short coupling length.

A compact silicon bandpass filter with high sidelobe suppression is proposed and experimentally demonstrated using an apodized subwavelength grating (SWG) coupler. The device is implemented by placing a SWG waveguide next to a strip waveguide, and apodization is employed with a Gaussian profile to taper the gap between the two waveguides. A high sidelobe suppression ratio of 27 dB can be obtained with a 3-dB bandwidth of 8.8 nm and an insertion loss of 2.5 dB. Owing to the large optical phase mismatch between the two waveguides and the presence of the SWG waveguide, the coupling length of the device is reduced to 100.3 µm. The experimental results validate our proposed apodized-SWG-based contradirectional coupler (contra-DC) as a promising device in suppressing out-of-band components in coarse wavelength division multiplexed (CWDM) optical communication systems.

Ultra-compact tunable silicon nanobeam cavity with an energy-efficient graphene micro-heater.

We propose and experimentally demonstrate an ultra-compact silicon photonic crystal nanobeam (PCN) cavity with an energy-efficient graphene micro-heater. Owing to the PCN cavity with an ultra-small optical mode volume of 0.145 µm3, the light-matter interaction is greatly enhanced and the thermo-optic (TO) tuning efficiency is increased. The TO tuning efficiency is measured to be as high as 1.5 nm/mW, which can be further increased to 3.75 nm/mW based on numerical simulations with an optimized structure. The time constants with a rise time constant of τrise = 1.11 µs and a fall time constant of τfall = 1.47 µs are obtained in the experiment.

Wavelength and bandwidth-tunable silicon comb filter based on Sagnac loop mirrors with Mach-Zehnder interferometer couplers.

We propose and experimentally demonstrate a wavelength and bandwidth-tunable comb filter based on silicon Sagnac loop mirrors (SLMs) with Mach-Zehnder interferometer (MZI) couplers. By thermally tuning the MZI couplers in common and differential modes, the phase shift and reflectivity of the SLMs can be changed, respectively, leading to tunable wavelength and bandwidth of the comb filter. The fabricated comb filter has 93 comb lines in the wavelength range from 1535 nm to 1565 nm spaced by ~0.322 nm. The central wavelength can be red-shifted by ~0.462 nm with a tuning efficiency of ~0.019 nm/mW. A continuously tunable bandwidth from 5.88 GHz to 24.89 GHz is also achieved with a differential heating power ranging from 0.00 mW to 0.53 mW.

High-extinction-ratio silicon polarization beam splitter with tolerance to waveguide width and coupling length variations.

We demonstrate a compact silicon polarization beam splitter (PBS) based on grating-assisted contradirectional couplers (GACCs). Over 30-dB extinction ratios and less than 1-dB insertion losses are achieved for both polarizations. The proposed PBS exhibits tolerance in width variation, and the polarization extinction ratios remain higher than 20 dB for both polarizations when the width variation is adjusted from + 10 to -10 nm. Benefiting from the enhanced coupling by the GACCs, the polarization extinction ratio can be kept higher than 15 dB and the insertion loss is lower than 2 dB for both polarizations when the coupling length varies from 30.96 to 13.76 µm.

Analysis of an electro-optic modulator based on a graphene-silicon hybrid 1D photonic crystal nanobeam cavity.

We propose and numerically study an on-chip graphene-silicon hybrid electro-optic (EO) modulator operating at the telecommunication band, which is implemented by a compact 1D photonic crystal nanobeam (PCN) cavity coupled to a bus waveguide with a graphene sheet on top. Through electrically tuning the Fermi level of the graphene, both the quality factor and the resonance wavelength can be significantly changed, thus the in-plane lightwave can be efficiently modulated. Based on finite-difference time-domain (FDTD) simulation results, the proposed modulator can provide a large free spectral range (FSR) of 125.6 nm, a high modulation speed of 133 GHz, and a large modulation depth of ~12.5 dB in a small modal volume, promising a high performance EO modulator for wavelength-division multiplexed (WDM) optical communication systems.

High-capacity and low-cost long-reach OFDMA PON based on distance-adaptive bandwidth allocation.

We propose and experimentally demonstrate a distance-adaptive bandwidth allocation scheme to realize high-capacity long-reach orthogonal frequency division multiple access passive optical network (OFDMA PON) with cost-effective electro-absorption modulator (EAM). In our scheme, the subcarriers in downstream OFDM signal are properly allocated to the optical network units (ONUs) with different fiber transmission lengths. By this means, the detrimental influence of power fading induced by dispersion and chirp can be avoided, thus all OFDM subcarriers can be modulated with high-order quadrature amplitude modulation (QAM) format, leading to a high transmission capacity. A proof-of-concept experiment is performed, in which three ONUs with transmission distances of 25, 50, and 100 km are assigned with different subcarriers, respectively. By using distance-adaptive bandwidth allocation technique, an OFDM signal of 34.5 Gb/s is successfully delivered to the ONUs with a bit error ratio (BER) lower than 2 × 10(-3).

Broadband photodetection in a microfiber-graphene device.

We propose and experimentally demonstrate a microfiber-graphene device. Owing to the interaction between the graphene film and the evanescent field leaked from the microfiber, the hybrid photoconductive device exhibits a high photoresponse. A maximum photocurrent responsivity of ~2.81 mA/W is achieved in the telecommunication band. A nearly flat photoresponse spectrum within broad operational band ranging from 1500 nm to 1600 nm is also obtained as a consequence of the dispersionless and flat absorption of graphene. These results show that the proposed photocurrent generation device could provide an effective solution for broadband photodetection.

Proposed high-speed micron-scale spatial light valve based on a silicon-graphene hybrid structure.

We propose a new ultracompact CMOS-compatible variable-transmission spatial light valve based on a silicon-graphene hybrid structure. Normally incident ∼1560 nm light can be coupled to a silicon-graphene-based 1D photonic crystal cavity through a perturbation-based diffractive coupling scheme. The lightwave modulation is achieved by tuning the Fermi level of the graphene, which can change both the loss and the resonant wavelength of the cavity. Based on finite-difference time-domain simulation, the modulation depth is larger than 10 dB with driving voltage of about 4.8 V(pp) while the modulation speed is estimated to be higher than 45 GHz.

Efficient modulation of 1.55 µm radiation with gated graphene on a silicon microring resonator.

The gate-controllability of the Fermi-edge onset of interband absorption in graphene can be utilized to modulate near-infrared radiation in the telecommunication band. However, a high modulation efficiency has not been demonstrated to date, because of the small amount of light absorption in graphene. Here, we demonstrate a ∼ 40% amplitude modulation of 1.55 µm radiation with gated single-layer graphene that is coupled with a silicon microring resonator. Both the quality factor and resonance wavelength of the silicon microring resonator were strongly modulated through gate tuning of the Fermi level in graphene. These results promise an efficient electro-optic modulator, ideal for applications in large-scale on-chip optical interconnects that are compatible with complementary metal-oxide-semiconductor technology.

Compact tunable silicon photonic differential-equation solver for general linear time-invariant systems.

We propose and experimentally demonstrate an all-optical temporal differential-equation solver that can be used to solve ordinary differential equations (ODEs) characterizing general linear time-invariant (LTI) systems. The photonic device implemented by an add-drop microring resonator (MRR) with two tunable interferometric couplers is monolithically integrated on a silicon-on-insulator (SOI) wafer with a compact footprint of ~60 µm × 120 µm. By thermally tuning the phase shifts along the bus arms of the two interferometric couplers, the proposed device is capable of solving first-order ODEs with two variable coefficients. The operation principle is theoretically analyzed, and system testing of solving ODE with tunable coefficients is carried out for 10-Gb/s optical Gaussian-like pulses. The experimental results verify the effectiveness of the fabricated device as a tunable photonic ODE solver.

Reconfigurable electro-optical directed-logic circuit using carrier-depletion micro-ring resonators.

Here we demonstrate a reconfigurable electro-optical directed-logic circuit based on a regular array of integrated optical switches. Each 1×1 optical switch consists of a micro-ring resonator with an embedded lateral p-n junction and a micro-heater. We achieve high-speed on-off switching by applying electrical logic signals to the p-n junction. We can configure the operation mode of each switch by thermal tuning the resonance wavelength. The result is an integrated optical circuit that can be reconfigured to perform any combinational logic operation. As a proof-of-principle, we fabricated a multi-spectral directed-logic circuit based on a fourfold array of switches and showed that this circuit can be reconfigured to perform arbitrary two-input logic functions with speeds up to 3 GB/s.