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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

In Vitro Antiviral Activity and Resistance Profile of the Next-Generation Hepatitis C Virus NS3/4A Protease Inhibitor Glecaprevir.

Glecaprevir (formerly ABT-493) is a novel hepatitis C virus (HCV) NS3/4A protease inhibitor (PI) with pangenotypic activity. It inhibited the enzymatic activity of purified NS3/4A proteases from HCV genotypes 1 to 6 in vitro (half-maximal [50%] inhibitory concentration = 3.5 to 11.3 nM) and the replication of stable HCV subgenomic replicons containing proteases from genotypes 1 to 6 (50% effective concentration [EC50] = 0.21 to 4.6 nM). Glecaprevir had a median EC50 of 0.30 nM (range, 0.05 to 3.8 nM) for HCV replicons containing proteases from 40 samples from patients infected with HCV genotypes 1 to 5. Importantly, glecaprevir was active against the protease from genotype 3, the most-difficult-to-treat HCV genotype, in both enzymatic and replicon assays demonstrating comparable activity against the other HCV genotypes. In drug-resistant colony selection studies, glecaprevir generally selected substitutions at NS3 amino acid position A156 in replicons containing proteases from genotypes 1a, 1b, 2a, 2b, 3a, and 4a and substitutions at position D/Q168 in replicons containing proteases from genotypes 3a, 5a, and 6a. Although the substitutions A156T and A156V in NS3 of genotype 1 reduced susceptibility to glecaprevir, replicons with these substitutions demonstrated a low replication efficiency in vitro Glecaprevir is active against HCV with most of the common NS3 amino acid substitutions that are associated with reduced susceptibility to other currently approved HCV PIs, including those at positions 155 and 168. Combination of glecaprevir with HCV inhibitors with other mechanisms of action resulted in additive or synergistic antiviral activity. In summary, glecaprevir is a next-generation HCV PI with potent pangenotypic activity and a high barrier to the development of resistance.

All-optical non-volatile tuning of an AMZI-coupled ring resonator with GST phase-change material.

We present a Ge2Sb2Te5 (GST)-integrated ring resonator with the tuning enabled by an all-optical phase change of GST using a sequence of optical pulses. The tuning is non-volatile and repeatable, with no static power consumption due to the "self-holding" feature of the GST phase-change material. The 2 µm long GST can be partially crystallized by controlling the number of pulses, increasing the tuning freedom. The coupling between the ring resonator and the bus waveguide is based on an asymmetric Mach-Zehnder interferometer. The coupling strength is wavelength-dependent, so that an optimal wavelength can be selected for the probe light to get more than 20 dB transmission contrast between the amorphous and crystalline GST states.

In Vitro Antiviral Activity and Resistance Profile of the Next-Generation Hepatitis C Virus NS5A Inhibitor Pibrentasvir.

Pibrentasvir (ABT-530) is a novel and pan-genotypic hepatitis C virus (HCV) NS5A inhibitor with 50% effective concentration (EC50) values ranging from 1.4 to 5.0 pM against HCV replicons containing NS5A from genotypes 1 to 6. Pibrentasvir demonstrated similar activity against a panel of chimeric replicons containing HCV NS5A of genotypes 1 to 6 from clinical samples. Resistance selection studies were conducted using HCV replicon cells with NS5A from genotype 1a, 1b, 2a, 2b, 3a, 4a, 5a, or 6a at a concentration of pibrentasvir that was 10- or 100-fold over its EC50 for the respective replicon. With pibrentasvir at 10-fold over the respective EC50, only a small number of colonies (0.00015 to 0.0065% of input cells) with resistance-associated amino acid substitutions were selected in replicons containing genotype 1a, 2a, or 3a NS5A, and no viable colonies were selected in replicons containing NS5A from other genotypes. With pibrentasvir at 100-fold over the respective EC50, very few colonies (0.0002% of input cells) were selected by pibrentasvir in genotype 1a replicon cells while no colonies were selected in other replicons. Pibrentasvir is active against common resistance-conferring substitutions in HCV genotypes 1 to 6 that were identified for other NS5A inhibitors, including those at key amino acid positions 28, 30, 31, or 93. The combination of pibrentasvir with HCV inhibitors of other classes produced synergistic inhibition of HCV replication. In summary, pibrentasvir is a next-generation HCV NS5A inhibitor with potent and pan-genotypic activity, and it maintains activity against common amino acid substitutions of HCV genotypes 1 to 6 that are known to confer resistance to currently approved NS5A inhibitors.

16 × 16 non-blocking silicon optical switch based on electro-optic Mach-Zehnder interferometers.

We experimentally demonstrate a 16 × 16 non-blocking optical switch fabric with a footprint of 10.7 × 4.4 mm2. The switch fabric is composed of 56 2 × 2 silicon Mach-Zehnder interferometers (MZIs), with each integrated with a pair of TiN resistive micro-heaters and a p-i-n diode. The average on-chip insertion loss at 1560 nm wavelength is ~6.7 dB and ~14 dB for the "all-cross" and "all-bar" states, respectively, with a loss variation of ± 1 dB over all routing paths. The measured rise/fall time of the switch upon electrical tuning is 3.2/2.5 ns. The switching functionality is verified by transmission of 20 Gb/s on-off keying (OOK) and 50 Gb/s quadrature phase-shift keying (QPSK) optical signals.