Parallel wavelength-division-multiplexed signal transmission and dispersion compensation enabled by soliton microcombs and microrings.

The proliferation of computation-intensive technologies has led to a significant rise in the number of datacenters, posing challenges for high-speed and power-efficient datacenter interconnects (DCIs). Although inter-DCIs based on intensity modulation and direct detection (IM-DD) along with wavelength-division multiplexing technologies exhibit power-efficient and large-capacity properties, the requirement of multiple laser sources leads to high costs and limited scalability, and the chromatic dispersion (CD) restricts the transmission length of optical signals. Here we propose a scalable on-chip parallel IM-DD data transmission system enabled by a single-soliton Kerr microcomb and a reconfigurable microring resonator-based CD compensator. We experimentally demonstrate an aggregate line rate of 1.68 Tbit/s over a 20-km-long SMF. The extrapolated energy consumption for CD compensation of 40-km-SMFs is ~0.3 pJ/bit, which is calculated as being around 6 times less than that of the commercial 400G-ZR coherent transceivers. Our approach holds significant promise for achieving data rates exceeding 10 terabits.

Continuously tunable silicon optical true-time delay lines with a large delay tuning range and a low delay fluctuation.

On-chip switchable optical true-time delay lines (OTTDLs) feature a large group delay tuning range but suffer from a discrete tuning step. OTTDLs with a large delay tuning range and a continuous tuning capability are highly desired. In this paper, we propose and experimentally demonstrate a silicon-based broadband continuously tunable OTTDL comprising a 7-bit delay line and a switch-based continuously tunable delay line. The group delay of the entire OTTDL can be continuously tuned from 0 to 1020.16 ps. A delay error within -1.27 ps to 1.75 ps, and a delay fluctuation of less than 2.69 ps in the frequency range of 2∼25 GHz are obtained. We analyze the causes of the delay fluctuation and its influence on beamforming. Moreover, we also propose a simplified non-invasive calibration method that can significantly reduce the complexity of the delay state calibration and can be easily extended to delay lines with more stages of optical switches. The high performance of our OTTDL chip and the calibration method drive practical applications of integrated OTTDLs.

Power-efficient polarization-insensitive tunable microring filter on a multi-layer Si3N4-on-SOI platform.

We develop and demonstrate a non-duplicate polarization-diversity tunable bandpass optical filter by leveraging the bi-directional transmission of add-drop dual-coupled microring resonators (MRRs) on a multi-layer Si3N4-on-silicon-on-insulator (SOI) platform. By using Euler-bends, we implement compact and low-loss Si3N4 MRRs with an equivalent bending radius of ∼38 µm. Fiber-to-fiber (on-chip) insertion loss of 4.3 dB (1.7 dB) with a low polarization-dependent loss of <0.5 dB and low differential group delay of <2.5 ps is achieved. The extinction ratio is more than 30 dB. The thermo-optic tuning efficiency of the Si3N4 MRRs is improved with a suspended micro-heater design. As a result, a wavelength tuning range of ∼2 nm and a 3-dB bandwidth tuning range of 20 GHz are experimentally demonstrated. The tuning efficiency is 33 pm/mW, which is ∼7.5 times higher than the previous design. This reconfigurable polarization-insensitive filter, with low loss, low cross talk, and high power efficiency, is highly promising for practical applications in optical communication and signal processing.

Polarization-insensitive 1D unidirectional compact grating coupler for the C-band using a 500 nm SOI.

Grating couplers are an important optical interconnect and have increasingly found their utility in sensing and LIDARs as well. Optical systems in general have been struggling to increase their bandwidths, making polarization insensitivity highly desirable. The standard 220 nm silicon-on-insulator (SOI) platform used for integrated photonics suffers from physical bottlenecks in the form of large modal differences in effective refractive index, propagation loss, and dispersion. In this paper, we present a grating coupler for polarization-insensitive coupling with polarization-dependent loss of less than 0.2 dB for more than 80% of the C-band on an alternative 500 nm SOI platform. We further show that the same design can be extended to polarization inflexible coupling and can reduce the polarization-dependent loss to less than 0.08 dB for the complete C-band. This platform is devoid of shortcomings, making it better suited for polarization-insensitive photonics, and the coupler is able to achieve these results through a simple and compact 1D design.

Silicon mode-insensitive modulator for TE0 mode and TE1 mode.

Mode-division multiplexing (MDM), which could further increase the capacity and flexibility of the communication systems, has attracted much attention. In this Letter, we demonstrate a proof-of-principle silicon mode-insensitive modulator based on the balanced Mach-Zehnder interferometer that could realize modulation of both TE0 and TE1 modes using a horizontal PN junction. The PN junction is offset from the center of the waveguide to the n-type doped region to modulate both TE0 and TE1 modes effectively. An adiabatic directional coupler is used as a mode-insensitive 3-dB power splitter for both modes. A mode-insensitive thermal phase shifter is used to change the operation point of the modulator. On-off keying modulation at 32 Gb/s is successfully demonstrated for both TE0 and TE1 modes. This modulator can be potentially used in MDM-assisted optical sampling systems.

High-resolution on-chip Fourier transform spectrometer based on cascaded optical switches.

Chip-level spectrometers provide a stable and cost-effective solution for spectral analysis in various applications. Here we present a silicon on-chip digital Fourier transform spectrometer consisting of eight cascaded optical switches connected by delay waveguides. By configuring the states of the optical switches, this chip can realize 127 Mach-Zehnder interferometers with linearly increased optical path differences. A machine-learning regularization method is utilized to reconstruct the spectrum. Experimental results show that our chip can retrieve both sparse and broadband optical spectra with negligible reconstruction errors. The spectral resolution can be further improved by cascading more stages of optical switches. Our method has the advantages of compact size, high scalability, and high signal-to-noise ratio, making it a promising candidate for realizing miniaturized spectrometers.

Hybrid WDM-MDM transmitter with an integrated Si modulator array and a micro-resonator comb source.

We demonstrate a multi-channel silicon photonic transmitter based on wavelength division multiplexing (WDM) and mode division multiplexing (MDM). The light source is realized by a silicon nitride (Si3N4) Kerr frequency comb and optical modulation is realized by silicon electro-optic modulators. Three wavelengths and two modes are employed to increase the optical transmission capacity. The accumulated data rate reaches 150 Gb/s. The dense integration of WDM and MDM components with a compact optical comb source opens new avenues for the future high-capacity multi-dimensional optical transmission.

Phase change material enabled 2 × 2 silicon nonvolatile optical switch.

Recently, large-scale photonic integrated circuits have seen rapid development. Optical switches are the elementary units used to realize optical routers and processors. However, the high static power and large footprint of silicon electro-optic and thermo-optic switches are becoming an obstacle for further scaling and high-density integration. In this Letter, we demonstrate a 2×2 nonvolatile silicon Mach-Zehnder optical switch enabled by low-loss phase change material Sb2S3. Changing the phase state of Sb2S3 can switch the optical transmission between the bar and cross paths. As no static power is required to maintain the phase state, it can find promising applications in optical switch matrices and reconfigurable optical circuits.

Hybrid dual-gain tunable integrated InP-Si3N4 external cavity laser.

We present a hybrid dual-gain integrated external cavity laser with full C-band wavelength tunability. Two parallel reflective semiconductor optical amplifier gain channels are combined by a Y-branch in the Si3N4 photonic circuit to increase the optical gain. A Vernier ring filter is integrated in the Si3N4 photonic circuit to select a single longitudinal mode and meanwhile reduce the laser linewidth. The side-mode suppression ratio is ∼67 dB with a pump current of 75 mA. The linewidth of the unpackaged laser is 6.6 kHz under on-chip output power of 23.5 mW. The dual-gain operation of the laser gives higher output power and narrower linewidth compared to the single gain operation. It is promising for applications in optical communications and light detection and ranging systems.

Integrated multi-beam optical phased array based on a 4 × 4 Butler matrix.

We design and demonstrate the first, to the best of our knowledge, silicon-based multi-beam optical phased array (MOPA), incorporating a 4×4 Butler matrix beamforming network. The one-dimensional end-fire array consists of 16 emitters at a uniform pitch together with their corresponding phase shifters and is shared among the beams to realize large-scale aliasing-free beam-steering at reduced complexity. Experimental results show that the device is capable of individual beam aliasing-free operation with a field of view up to 46°. The steering envelope shows a plateau where the peak intensities fluctuate within 0.5 dB. The beamforming and beam-steering performance are also evaluated for simultaneous multi-beam operation. Our work validates the feasibility of beamforming-network-based MOPAs, which are promising for applications including light detection and ranging and free-space optical communication.

Silicon reconfigurable mode-selective modulation for on-chip mode-multiplexed photonic systems.

Recently, optical mode-division multiplexing has drawn a lot of attention due to its ability to increase the optical communication capacity in one physical channel with a single wavelength carrier. In this Letter, we demonstrate reconfigurable mode-selective modulation which is potentially useful for on-chip mode-multiplexed photonic systems. The device consists of two mode exchangers and one TE1 mode modulator. The mode exchanger is based on a Mach-Zehnder interferometer that performs mode exchange between TE0 and TE1 modes. The TE1 mode modulator consists of a pair of 1×3/3×1 multimode interferometers acting as a mode (de)multiplexer. It only selectively modulates the TE1 mode while bypassing the TE0 mode. 32 Gb/s on-off keying (OOK) modulation is successfully demonstrated for both input TE0 and TE1 modes. This device can be used as a building block for on-chip multimode interconnect networks.

Broadband continuously tunable microwave photonic delay line based on cascaded silicon microrings.

We present a novel broadband continuously tunable microwave photonic delay line consisting of a modulator, a four-stage microring resonator delay line, a tunable optical bandpass filter, and a photodetector. Unlike the traditional microring delay lines working at the on-resonant wavelength, the microring resonators in our chip work at the anti-resonant wavelengths, leading to a large delay bandwidth and a small delay ripple. The experimental results show that relative group delay can be continuously tuned from 0 to 160 ps for microwave frequencies in the range of 0 to 16 GHz. The delay ripple is less than 6.2 ps. These results represent an important step towards the realization of integrated continuously tunable delay lines demanded in broadband microwave phased array antennas.

Comparison of the phase change process in a GST-loaded silicon waveguide and MMI.

In the past decades, silicon photonic integrated circuits (PICs) have been considered a promising approach to solve the bandwidth bottleneck in optical communications and interconnections. Despite the rapid advances, large-scale PICs still face a series of technical challenges, such as large footprint, high power consumption, and lack of optical memory, resulting from the active tuning methods used to control the optical waves. These challenges can be partially addressed by combining chalcogenide phase change materials (PCMs) such as Ge2Sb2Te-5 (GST) with silicon photonics, especially applicable in reconfigurable optical circuit applications due to the nonvolatile nature of the GST. We systematically investigate the phase change process induced by optical and electrical pulses in GST-loaded silicon waveguide and multimode interferometer. Using optical pulse excitation to amorphize GST has a clear advantage in terms of operation speed and energy efficiency, while electrical pulse excitation is more suitable for large-scale integration because it does not require complex optical routing. This study helps us better understand the phase change process and push forward the further development of the Si-GST hybrid photonic integration platform, bringing in new potential applications.

Optical generation of UWB pulses utilizing Fano resonance modulation.

In this paper, we reported an integrated method to generate ultra-wideband (UWB) pulses of different orders based on a reconfigurable silicon micro-ring resonator-coupled Mach-Zehnder interferometer. Under proper operating conditions, the device can produce Fano resonances with a peak-to-valley extinction ratio of above 20 dB. UWB monocycle and doublet signals with picosecond pulse widths are produced when the microring resonator is modulated by square and Gaussian electrical pulses, respectively. With our Fano resonance modulator on silicon photonics, it is promising to foresee versatile on-chip microwave signal generation.

Automatic calibration of silicon ring-based optical switch powered by machine learning.

Calibrating ring-based optical switches automatically is strongly demanded in large-scale ring-based optical switch fabrics. Supported by a machine-learning algorithm, we build an artificial neural network (ANN) model to retrieve the parameters of a 2×2 dual-ring assisted Mach-Zehnder interferometer (DR-MZI) switch from the measured spectra for the first time. The calibration algorithm is verified on several devices. The operating wavelength of the optical switch can be tuned to any wavelength in a free spectral range with an accuracy better than 90 pm. The extinction ratio exceeds 20 dB at the cross- and bar-states with no more than 7 calibration cycles. The voltage difference between the automatic calibration and manual tuning is less than 30 mV, showing the high accuracy of the calibration algorithm. Our scheme provides a new way to calibrate ring-based devices that work as optical switch fabrics and tunable optical filters.

Contra-directional switching enabled by Si-GST grating.

We present the design, simulation, and experimental demonstration of a Si-GST grating assisted contra-directional coupler for optical switching. The effective refractive index of the GST-loaded silicon waveguide changes significantly when the GST is switched from the amorphous state to the crystalline state, allowing for large tuning of the propagation constant. The two coupled waveguides are designed to satisfy the phase-match condition only at the amorphous state to achieve Bragg reflection at the drop-port. Experimental results show that the device insertion loss is less than 5 dB and the extinction ratio is more than 15 dB with an operation bandwidth of 2.2 nm around the 1576 nm operating wavelength. Due to the nonvolatile property of GST, there is no static power consumption to maintain the two states. It is the first demonstration of a GST-enabled grating coupler that can be switched by phase change material.

Programmable SCOW Mesh Silicon Photonic Processor for Linear Unitary Operator.

Universal unitary multiport interferometers (UMIs) can perform any arbitrary unitary transformation to a vector of input optical modes, which are essential for a wide range of applications. Most UMIs are realized by fixed photonic circuits with a triangular or a rectangular architecture. Here, we present the implementation of an N × N rectangular UMI with a programmable photonic processor based on two-dimensional meshes of self-coupled optical waveguide (SCOW) resonant structures. Our architecture shows a high tolerance to the unbalanced loss upon interference. This work enriches the functionality of the SCOW mesh photonic processors, which are promising for field-programmable photonic arrays.

Design of Ultra-Compact Optical Memristive Switches with GST as the Active Material.

In the following study, we propose optical memristive switches consisting of a silicon waveguide integrated with phase-change material Ge2Sb2Te5 (GST). Thanks to its high refractive index contrast between the crystalline and amorphous states, a miniature-size GST material can offer a high switching extinction ratio. We optimize the device design by using finite-difference-time-domain (FDTD) simulations. A device with a length of 4.7 µm including silicon waveguide tapers exhibits a high extinction ratio of 33.1 dB and a low insertion loss of 0.48 dB around the 1550 nm wavelength. The operation bandwidth of the device is around 60 nm.

Silicon optical filters reconfigured from a 16 × 16 Benes switch matrix.

Reconfigurable optical filters with tailorable performances are highly demanded in multi-purpose adaptive signal processing applications. We demonstrate infinite impulse response (IIR) silicon optical filters with a variable filter order by switching the optical path in a 16 × 16 Benes switch chip. The basic unit of the optical filter is a dual-ring assisted Mach-Zehnder interferometer. TiN microheaters are integrated in both ring resonators for resonance control, allowing for continuous tuning of the filter center wavelength and the bandwidth. Multiple high-order optical filters from the 2nd order up to the 14th order are obtained. The filter bandwidth tuning range is from 0.19 nm (23.75 GHz) to 1.06 nm (132.5 GHz) with a 1-dB in-band ripple. The out-of-band rejection ratio exceeds 30 dB for the 8th and 10th-order filters, limited by the inter-path optical crosstalk in the Benes architecture. The results point to new ways of reutilizing an existing switch matrix to flexibly construct wavelength-filtering devices.

Aliasing-free optical phased array beam-steering with a plateau envelope.

We investigate the feasibility of generating a plateau envelope for beam-steering with optical phased arrays (OPAs). The design guidelines are summarized from numerical simulations and verified with a fabricated chip, which incorporates both a coupling-suppressed curved waveguide array with a pitch of 0.8 µm for light emission and a 1-µm-long silica cavity for envelope tailoring. This silicon-on-insulator (SOI) based device demonstrates aliasing-free beam-steering over the entire field-of-view available (-32°~32°) with a far-field addressability of 6.71°. The steered beam exhibits a plateau envelope, with a peak intensity fluctuation of less than 0.45 dB, from -30° to 30°. These results represent a significant step towards realizing integrated OPA for optical beam-forming with a large aliasing-free steering range and a uniform beam intensity.