Optimized shift-pattern overlay for high coupling efficiency waveguide grating couplers: erratum.

In this erratum, we correct the reference numbers in Table 1 of our Letter [Opt. Lett.47, 3968 (2022)10.1364/OL.464652]. This does not change the scientific results and conclusions of the original Letter.

High-visibility energy-time entanglement system enabled by a low-loss silicon-integrated platform.

Energy-time (E-T) entanglement is widely employed in long-distance quantum entanglement distribution due to its strong robustness against transmission fluctuations. In this Letter, we report what we believe to be the first silicon monolithically integrated E-T entanglement system, which integrates the photon sources, wavelength demultiplexers, and Franson interferometers on a single chip. Also, by utilizing low-loss multimode waveguides in Franson interferometers, we measured an on-chip quantum interference visibility of 99.66% (±0.47%), to our knowledge one of the highest values for integrated E-T entanglement systems reported to date. The quantum interference after 1- and 5-km fiber propagation shows visibilities of 96.72% (±0.78%) and 97.46% (±1.23%), respectively. These results demonstrate the potential of using silicon monolithic integration for advance E-T entanglement-based quantum communication networks.

Compact supermode switch for photonic matrix processing.

A 2 × 2 switch based on differential effective thermo-optic (TO) coefficients of waveguide supermodes is proposed and experimentally demonstrated as a more compact alternative to Mach-Zehnder interferometer (MZI)-based switches used in coherent photonic matrix processing networks. The total waveguide width of the device is 1.335 µm. Using a novel, to the best of our knowledge, supermode coupler with a wideband 3-dB coupling ratio, the switch was engineered to have on-off extinction ratios (ERs) ranging from 24.1 to 38.9 dB for the two output ports over a 135 nm bandwidth. Insertion losses (ILs) of less than 0.3 and 0.4 dB over the 100 nm bandwidth were measured for bar and cross transmission, respectively. The waveguide width error tolerance is +/-30 nm. The proposed device has the potential to improve the scalability of a programmable coherent mesh for matrix processing by increasing the integration density without sacrificing the overall accuracy or limiting the operational wavelength range of the mesh.

Combined polysilicon and silicon gratings for dual-wavelength-band waveguide grating couplers.

We proposed a novel, to the best of our knowledge, design for a dual-wavelength-band waveguide grating coupler. The proposed structure works in both the C band and O band. The proposed device is optimized from an initial design of two independent gratings formed on the silicon and polysilicon overlay layers, respectively. We designed the up layer (polysilicon) for the C band and the down layer (silicon) for the O band as the initial optimization seed. After numerical optimization of this structure using a genetic algorithm, the grating coupler has a coupling efficiency of -3.86 dB at the C band and -4.46 dB at the O band. We validate the approach in a commercial foundry using 193-nm photolithography in a multi-project wafer, and the experimental result has coupling efficiencies of -4.37 dB in the C band and -5.8 dB in the O band.

Compact integrated mode-size converter using a broadband ultralow-loss parabolic-mirror collimator.

In this Letter, we propose and demonstrate an integrated mode-size converter (MSC) with a compact footprint, low losses, and a broad bandwidth. By exploiting a parabolic mirror, the divergent light from a narrow waveguide (450 nm) is collimated to match the mode size of a wide waveguide (10 µm). The measured insertion loss (IL) is ≈ 0.15 dB over a 100-nm bandwidth. The mode-size conversion is achieved with a footprint as small as ≈ 20 × 32 µm2, which is much shorter than the linear taper length required to attain the same level of losses.

Efficient 330-Gb/s PAM-8 modulation using silicon microring modulators.

We propose and demonstrate a high-efficiency silicon microring modulator for next-generation optical transmitters operating at line rates above 300 Gb/s. The modulator supports high-order PAM-8 modulation up to 110 Gbaud (330 Gb/s), with a driving voltage of 1.8 Vpp. The small driving voltage and device capacitance yields a dynamic energy consumption of 3.1 fJ/bit. Using the modulator, we compare PAM-8 with ultrahigh baud rate PAM-4 of up to 130 Gbaud (260 Gb/s) and show PAM-8 is better suited for 300-Gb/s lane rate operation in bandwidth-constrained short-reach systems.

High coupling efficiency waveguide grating couplers on lithium niobate.

We propose and validate a new, to the best of our knowledge, approach for high coupling efficiency (CE) grating couplers (GCs) in the lithium niobate on insulator photonic integration platform. Enhanced CE is achieved by increasing the grating strength using a high refractive index polysilicon layer on the GC. Due to the high refractive index of the polysilicon layer, the light in the lithium niobate waveguide is pulled up to the grating region. The optical cavity formed in the vertical direction enhances the CE of the waveguide GC. With this novel structure, simulations predicted the CE to be -1.40 dB, while the experimentally measured CE was -2.20 dB with a 3-dB bandwidth of 81 nm from 1592 nm to 1673 nm. The high CE GC is achieved without using bottom metal reflectors or requiring the etching of the lithium niobate material.

Optimized shift-pattern overlay for high coupling efficiency waveguide grating couplers.

We propose and validate a new, to the best of our knowledge, approach for increasing the coupling efficiency of waveguide grating couplers by introducing an optimized shift-patterned polysilicon overlay above the silicon grating structure. After optimizing the shifts in position and duty cycles of each period in the polysilicon overlay and silicon grating, the silicon grating and polysilicon overlay can form composite subwavelength structures which improve both the mode matching and the directionality of the grating coupler, and enable the design of a high-efficiency perfectly vertical grating coupler (PVGC) with -0.91 dB simulated coupling efficiency. The devices are fabricated using photolithography in a standard commercial multi-project wafer fabrication service by IMEC and have a measured coupling loss of approximately 1.45 dB.

Polarization-independent waveguide grating coupler using an optimized polysilicon overlay.

We propose and validate a new, to the best of our knowledge, approach to designing a polarization-independent waveguide grating coupler, using an optimized polysilicon overlay on a silicon grating structure. Simulations predicted coupling efficiencies of about -3.6 dB and -3.5 dB for TE and TM polarizations, respectively. The devices were fabricated using photolithography in a multi-project wafer fabrication service by a commercial foundry and have measured coupling losses of -3.96 dB for TE polarization and -3.93 dB for TM polarization.

C-band 67†GHz silicon photonic microring modulator for dispersion-uncompensated 100 Gbaud PAM-4.

A very-high-bandwidth integrated silicon microring modulator (MRM) designed on a commercial silicon photonics (SiP) platform for C-band operation is presented. The MRM has a 3 dB electro-optic (EO) bandwidth of over 67 GHz and features a small footprint of 24 µm × 70 µm. Using the MRM, we demonstrate intensity modulation-direct detection (IM-DD) transmission with 4-level pulse amplitude modulation (PAM-4)  signaling of over 100 Gbaud. By utilizing the optical peaking effect and negative chirp in the MRM, we extend the transmission distance, which is limited by the fiber-dispersion-induced frequency fading. Using a standard single-mode fiber (SSMF) for transmission across distances of up to 2 km, we measured the data transmission of 100 Gbaud PAM-4 signals with a bit error rate (BER) under the general 7% hard-decision forward-error correction (HD-FEC) threshold. The MRM enables an extended transmission distance for 100 Gbaud signaling in the C-band without dispersion compensation.

High-extinction CROW filters for scalable quantum photonics.

We report an integrated tunable-bandwidth optical filter with a passband to stop-band ratio of over 96 dB using a single silicon chip with an ultra-compact footprint. The integrated filter is used in filtering out the pump photons in non-degenerate spontaneous four-wave mixing (SFWM), which is used for producing correlated photon pairs at different wavelengths. SFWM occurs in a long silicon waveguide, and two cascaded second-order coupled-resonator optical waveguide (CROW) filters were used to spectrally remove the pump photons. The tunable bandwidth of the filter is useful to adjust the coherence time of the quantum correlated photons and may find applications in large-scale integrated quantum photonic circuits.

Optimal Bezier curve transition for low-loss ultra-compact S-bends.

S-bends with a widening of the width at the mid-bend and a Bezier curve transition are proposed and demonstrated for low-loss S-bends. The increased optical confinement and reduced transition loss enable low insertion loss (IL) and compact S-bends with longitudinal offsets as small as 2.5 µm and a wide operating bandwidth (∼100nm) on a 220 nm thick silicon-on-insulator platform. The simulation results show ILs less than (0.22, 0.20, 0.20, 0.20) dB in the wavelength range of (1.5-1.6) µm, while the minimum ILs are (0.13, 0.13, 0.15, 0.16) dB for lateral offsets of (3, 6, 9, 12) µm. The experimental results show that ILs remain less than (0.41, 0.38, 0.36, 0.39) dB for the mid-bend widening Bezier (MWB) S-bends.

Inverse design of multi-band and wideband waveguide crossings.

Photonic integrated circuits for wideband and multi-band optical communications will need waveguide crossings that operate at all the wavelengths required by the system. In this Letter, we use the modified gradient decedent method to optimize the dual-wavelength band (DWB) crossings on both single- and double-level platforms. On the single-level platform, the simulation results show insertion losses (ILs) less than 0.07 and 0.11 dB for a crossing working at a DWB of 1.5-1.6 and 1.95-2.05 µm. ILs are less than 0.1 and 0.2 dB for a crossing operating in the DWB of 1.5-1.6 and 2.2-2.3 µm. On the double-layer platform, the simulated results show IL less than 0.08 dB across the wavelength range of 1.25-2.25 µm. We experimentally demonstrate the DWB crossing operating at 1.5-1.6 and 2.2-2.3 µm to have IL less than 0.3 and 0.4 dB and crosstalk of -28 and -26dB in the two bands, respectively.

Compact high-extinction tunable CROW filters for integrated quantum photonic circuits.

We describe the use of cascaded second-order coupled-resonator optical waveguide (CROW) tunable filters to achieve one of the highest reported measured extinction ratios of $ {\gt} {110}\;{\rm dB}$>110dB. The CROW filters were used to remove the pump photons in spontaneous four-wave mixing (SFWM) in a silicon waveguide. The SFWM generated quantum-correlated photons that could be measured after the cascaded CROW filters. The CROW filters offer a compact footprint for use in monolithic quantum photonic circuits.

Experimental demonstration of 111.1-Gb/s net information rate using IM/DD probabilistically shaped orthogonal chirp-division multiplexing with a 10-GHz-class modulator.

We propose probabilistically shaped quadrature amplitude modulation (PS-QAM) formats to maximize the capacity in fiber transmission systems using orthogonal chirp-division multiplexing (OCDM). OCDM possesses the property of chirp spread spectrum (CSS), leading to improved resilience to system impairments. We further investigate the recently proposed robust channel estimator based on pulse compression and noise rejection and experimentally demonstrate its feasibility in an intensity-modulated/direction-detection (IM/DD) OCDM system. By applying the proposed PS-QAM based OCDM to an IM/DD optical system, a net information rate of 111.1 Gb/s has been successfully achieved using a 10-GHz class Mach-Zehnder modulator (MZM) and has also shown improved performance compared to the conventional PS-QAM based orthogonal frequency-division multiplexing (OFDM) systems. Moreover, due to the superior characteristics of OCDM, there is no need for additional feedback to obtain the prior knowledge of channel state information in the proposed system, leading to reduced complexity and cost.

Dual-wavelength-band subwavelength grating coupler operating in the near infrared and extended shortwave infrared.

We show that dual-wavelength-band (DWB) subwavelength grating couplers (SWGCs) can be designed for simultaneous coupling of the near-infrared and extended shortwave-infrared (SWIR) fundamental transverse electric polarized light at the same diffraction angle. Numerical simulations predict coupling efficiencies (CEs) larger than 34% for the DWB SWGCs operating in the S/C band (1.48/1.55 µm) and extended SWIR band (1.8-2.8 µm) with a widely and continuously tailorable peak wavelength separation between 250 and 1250 nm. The fabricated DWB SWGCs with peak wavelengths of (1.56, 2.255) µm and (1.487, 2.331) µm respectively obtain CEs of (20.2, 25.8)% and (20.6, 26.9)%, 1-dB bandwidths of 38 nm and 54 nm at a diffraction angle of 2°.

Efficient perfectly vertical grating coupler for multi-core fibers fabricated with 193 nm DUV lithography.

We propose a novel high-efficiency, low-reflection, and fabrication-tolerant perfectly vertical grating coupler (PVGC) with a minimum feature size >200 nm to allow for fabrication using 193 nm deep-ultraviolet lithography. The structural parameters of PVGC were optimized by a genetic optimization algorithm. Simulations predicted the coupling efficiency to be -2.0 dB (63.0%) and the back reflections to be less than -20 dB in the wavelength range of 1532-1576 nm. The design was fabricated in a multi-project wafer run for silicon photonics, and a coupling efficiency of -2.7 dB (53.7%) with a 1 dB bandwidth of 33 nm is experimentally demonstrated. The measured back reflection is less than -16 dB over the C-band. The PVGC occupies a compact footprint of 30 µm×24 µm and can be interfaced with the multi-core fibers for future space-division-multiplexing networks.

High-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors.

Waveguide photodetectors integrated with graphene have demonstrated potential for ultrafast response and broadband operation. Here, we demonstrate high-performance chemical vapor deposited graphene-on-silicon nitride waveguide photodetectors by enhancing the absorption of light propagating in the transverse-magnetic mode through a metal-graphene junction. A doubling in responsivity is experimentally observed. In our zero-biased metal-graphene junction, a 15 mA W-1 intrinsic responsivity and a 30 GHz bandwidth are achieved at ∼1550 nm. The results are comparable to those obtained from the best pristine graphene-based photodetectors. Our work enables new architectures for high-performance optoelectronic devices based on the graphene-on-silicon nitride platform.

Tailorable dual-wavelength-band coupling in a transverse-electric-mode focusing subwavelength grating coupler.

A transverse-electric-mode focusing subwavelength grating coupler (FSWGC) is proposed and demonstrated for dual-wavelength-band (DWB) coupling from a single-mode fiber into a suspended-membrane waveguide for the first time, to the best of our knowledge. Location and separation of the two coupling peaks can be flexibly tailored based on a proposed design methodology. As a proof of concept, two DWB FSWGCs working at (1486.0, 1594.5) nm and (1481.5, 1661.5) nm are experimentally demonstrated with coupling efficiencies of (18.3%, 20.1%) and (14.5%, 17.5%), 3-dB bandwidths of (55.0, 30.5) nm and (44.0, >39.5) nm, respectively. A DWB FSWGC working at (1480, 1830) nm with a wavelength separation of 350 nm and coupling efficiencies of (38.0%, 33.2%) is also numerically predicted.

Integrated germanium-on-silicon Franz-Keldysh vector modulator used with a Kramers-Kronig receiver.

We propose an integrated vector modulator based on two compact and high-speed germanium-on-silicon Franz-Keldysh electro-absorption modulators. The proposed vector modulator is extremely compact with a total footprint of only 1800 µm×200 µm. We further experimentally demonstrate a 4-quadrature-amplitude-modulation (4-QAM) at 40 Gb/s over a 20-km standard single-mode fiber transmission. The complex signal is successfully re-constructed with a single-ended photodiode in a recently proposed Kramers-Kronig receiver for future low-cost, low-power, and low-footprint datacenter interconnect applications. The preliminary performance of the vector modulator with a 16-QAM is also investigated.