Modified SiP single-polarization CADD receiver.

In cost-sensitive application scenarios, increasing the data rate per channel under a limited receiver bandwidth is critical, and thus, the transceivers with low costs and high electrical spectral efficiencies (ESEs) are highly desirable. In this Letter, we demonstrate a modified silicon photonic (SiP) carrier-assisted differential detection (CADD) receiver with a record ESE for single polarization. The ESE of the conventional CADD is mainly limited by the transfer function that originated from the optical delay and hybrid. We modify the transfer function of the CADD by placing an additional delay in parallel to the original delay path. Consequently, the modified transfer function exhibits a sharper slope around the zero frequency, leading to a higher ESE. Here we employ complementary metal-oxide-semiconductor-compatible SiP integration to further reduce the cost and footprint of the modified CADD receiver. In the experiment, 280-Gb/s raw rate (net 226-Gb/s) 16-QAM OFDM signal after 80-km SMF transmission was detected using a 36.5-GHz SiP modified CADD receiver, with a bit error ratio below the 24% SD-FEC threshold. To our best knowledge, we achieve a record net 6.2-b/s/Hz ESE for an integrated single-polarization DD receiver with a 16-QAM format.

Ultrabroadband and compact 2 × 2 3-dB coupler based on trapezoidal subwavelength gratings.

We propose and experimentally demonstrate an ultrabroadband and compact 2 × 2 3-dB coupler based on the trapezoidal subwavelength gratings (SWGs). The adiabatic coupling is achieved between a trapezoidal SWG waveguide and a reversely tapered strip waveguide, which contributes to the ultrabroad operation bandwidth and the compact footprint of the coupler. Numerical results prove that our device has a power splitting imbalance of < ± 0.5 dB and an excess loss of < 0.2 dB in the ultrabroad bandwidth of 300 nm from 1400 nm to 1700nm, with a coupling length of 4.4 µm and a total length of 24.4 µm. The fabricated device is characterized in a 270-nm bandwidth from 1400 nm to 1670 nm, showing a measured power splitting imbalance of < ± 0.7 dB and an excess loss of < 0.5 dB.

GPU-accelerated iterative method for FD-OCT image reconstruction with an image-level cross-domain regularizer.

The image reconstruction for Fourier-domain optical coherence tomography (FD-OCT) could be achieved by iterative methods, which offer a more accurate estimation than the traditional inverse discrete Fourier transform (IDFT) reconstruction. However, the existing iterative methods are mostly A-line-based and are developed on CPU, which causes slow reconstruction. Besides, A-line-based reconstruction makes the iterative methods incompatible with most existing image-level image processing techniques. In this paper, we proposed an iterative method that enables B-scan-based OCT image reconstruction, which has three major advantages: (1) Large-scale parallelism of the OCT dataset is achieved by using GPU acceleration. (2) A novel image-level cross-domain regularizer was developed, such that the image processing could be performed simultaneously during the image reconstruction; an enhanced image could be directly generated from the OCT interferogram. (3) The scalability of the proposed method was demonstrated for 3D OCT image reconstruction. Compared with the state-of-the-art (SOTA) iterative approaches, the proposed method achieves higher image quality with reduced computational time by orders of magnitude. To further show the image enhancement ability, a comparison was conducted between the proposed method and the conventional workflow, in which an IDFT reconstructed OCT image is later processed by a total variation-regularized denoising algorithm. The proposed method can achieve a better performance evaluated by metrics such as signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), while the speed is improved by more than 30 times. Real-time image reconstruction at more than 20 B-scans per second was realized with a frame size of 4096 (axial) × 1000 (lateral), which showcases the great potential of the proposed method in real-world applications.

Fourier-inspired neural module for real-time and high-fidelity computer-generated holography.

Learning-based computer-generated holography (CGH) algorithms appear as novel alternatives to generate phase-only holograms. However, most existing learning-based approaches underperform their iterative peers regarding display quality. Here, we recognize that current convolutional neural networks have difficulty learning cross-domain tasks due to the limited receptive field. In order to overcome this limitation, we propose a Fourier-inspired neural module, which can be easily integrated into various CGH frameworks and significantly enhance the quality of reconstructed images. By explicitly leveraging Fourier transforms within the neural network architecture, the mesoscopic information within the phase-only hologram can be more handily extracted. Both simulation and experiment were performed to showcase its capability. By incorporating it into U-Net and HoloNet, the peak signal-to-noise ratio of reconstructed images is measured at 29.16 dB and 33.50 dB during the simulation, which is 4.97 dB and 1.52 dB higher than those by the baseline U-Net and HoloNet, respectively. Similar trends are observed in the experimental results. We also experimentally demonstrated that U-Net and HoloNet with the proposed module can generate a monochromatic 1080p hologram in 0.015 s and 0.020 s, respectively.

Hybrid lithium tantalite-silicon integrated photonics platform for electro-optic modulation.

Integrated electro-optic modulators are key components in photonic integrated circuits. Silicon photonic technology is considered to be promising for large-scale and low-cost integration. However, silicon does not exhibit any Pockels effect, and the electro-optic modulator based on free-carrier dispersion suffers from challenges such as high-power consumption, limited bandwidth, and large optical propagation loss. Here, a new, to the best of our knowledge, hybrid lithium tantalite-silicon platform is proposed for electro-optic modulators based on the Pockels effect. Benefiting from the strong Pockels coefficients of a thin-film lithium tantalite, a hybrid microring-based modulator is demonstrated. The quality factor and the extinction ratio of the hybrid microring are 1.7 × 104 and 10 dB, respectively. The linear bidirectional wavelength tuning efficiency is measured as 12.8 pm/V. The measured 3-dB bandwidth is > 20 GHz. High-quality eye diagrams of 20 Gbps non-return-to-zero signal and 20 Gbps four-level pulse amplitude modulation signals are generated experimentally. The proposed platform extends the toolbox of silicon photonics technology, which paves the way for high-speed modulators and phase shifters in optical communication and optical phased array.

Deep learning empowered highly compressive SS-OCT via learnable spectral-spatial sub-sampling.

With the rapid advances of light source technology, the A-line imaging rate of swept-source optical coherence tomography (SS-OCT) has experienced a great increase in the past three decades. The bandwidths of data acquisition, data transfer, and data storage, which can easily reach several hundred megabytes per second, have now been considered major bottlenecks for modern SS-OCT system design. To address these issues, various compression schemes have been previously proposed. However, most of the current methods focus on enhancing the capability of the reconstruction algorithm and can only provide a data compression ratio (DCR) up to 4 without impairing the image quality. In this Letter, we proposed a novel design paradigm, in which the sub-sampling pattern for interferogram acquisition is jointly optimized with the reconstruction algorithm in an end-to-end manner. To validate the idea, we retrospectively apply the proposed method on an ex vivo human coronary optical coherence tomography (OCT) dataset. The proposed method could reach a maximum DCR of ∼62.5 with peak signal-to-noise ratio (PSNR) of 24.2 dB, while a DCR of ∼27.78 could yield a visually pleasant image with a PSNR of ∼24.6 dB. We believe the proposed system could be a viable remedy for the ever-growing data issue in SS-OCT.

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.

Deep-learning-enabled high-performance full-field direct detection with dispersion diversity.

Data center interconnects require cost-effective photonic integrated optical transceivers to meet the ever-increasing capacity demands. Compared with a coherent transmission system, a complex-valued double-sideband (CV-DSB) direct detection (DD) system can minimize the cost of the photonic circuit, since it replaces two stable narrow-linewidth lasers with only a low-cost un-cooled laser in the transmitter while maintaining a similar spectral efficiency. In the carrier-assisted DD system, the carrier power accounts for a large proportion of the total optical signal power. Reducing the carrier to signal power ratio (CSPR) can improve the information-bearing signal power and thus the achievable system performance. To date, the minimum required CSPR is ∼7 dB for all the reported CV-DSB DD systems having electrical bandwidths of approximately half of baud rates. In this paper, we propose a deep-learning-enabled DD (DLEDD) scheme to recover the full optical field of the transmitted signal at a low CSPR of 2 dB in experiment. Our proposal is based on a dispersion-diversity receiver with an electrical bandwidth of ∼61.0% baud rate and a high tolerance to laser wavelength drift. A deep convolutional neural network enables accurate signal recovery in the presence of a strong signal-signal beat interference. Compared with the conventional method, the proposed DLEDD scheme can reduce the optimum CSPR by ∼8 dB, leading to a significant signal-to-noise ratio improvement of ∼5.8 dB according to simulation results. We experimentally demonstrate the optical field reconstruction for a 28-GBaud 16-ary quadrature amplitude modulation signal after 80-km single-mode fiber transmission based on the proposed DLEDD scheme with a 2-dB optimum CSPR. The results show that the proposed DLEDD scheme could offer a high-performance solution for cost-sensitive applications such as data center interconnects, metro networks, and mobile fronthaul systems.

Planar Alvarez tunable lens based on polymetric liquid crystal Pancharatnam-Berry optical elements.

Virtual reality (VR) and augmented reality (AR) have widespread applications. The vergence-accommodation conflict (VAC), which causes 3D visual fatigue, has become an urgent challenge for VR and AR displays. Alvarez lenses, with precise and continuously tunable focal length based on the lateral shift of its two sub-elements, are a promising candidate as the key electro-optical component in vari-focal AR display systems to solve the VAC problem. In this paper, we propose and fabricate a compact Alvarez lens based on planar polymetric liquid crystal Pancharatnam-Berry optical elements. It can provide continuous diopter change from -1.4 D to 1.4 D at the wavelength of 532 nm with the lateral shift ranging from -5 mm to 5 mm. We also demonstrate an AR display system using this proposed Alvarez lens, where virtual images are augmented on the real world at different depths.

On-chip mode division (de)multiplexer for multi-band operation.

We propose an on-chip mode division (de)multiplexer based on asymmetric directional couplers (ADCs) for multi-band operation. In mode-coupling process, the minimum coupling length is wavelength-dependent. The longer the wavelength is, the shorter the minimum coupling length is. A light of longer wavelength can be coupled back and forth multiple times with a total coupling length which equals to the minimum coupling length of a shorter wavelength light, thus realizing multi-band transmission at different wavelengths. As a proof-of-concept experiment, a four-mode (de)multiplexer for joint operation in the C- and O-Bands is designed and experimentally demonstrated. For the four modes (TE0, TE1, TE2 and TE3), the measured insertion losses (ILs) and crosstalk (CT) of the (de)multiplexer are < 4.7 dB and < -10.1 dB respectively from 1290 nm to 1360 nm, and they are < 3.5 dB and < -11.8 dB respectively from 1510 nm to 1580 nm.

Hybrid aluminum nitride and silicon devices for integrated photonics.

Aluminum nitride has advantages ranging from a large transparency window to its high thermal and chemical resistance, piezoelectric effect, electro-optic property, and compatibility with the complementary metal-oxide-semiconductor fabrication process. We propose a hybrid aluminum nitride and silicon platform for integrated photonics. Hybrid aluminum nitride-silicon basic photonic devices, including the multimode interferometer, Mach-Zehnder interferometer, and micro-ring resonator, are designed and fabricated. The measured extinction ratio is > 22 dB and the insertion loss is < 1 dB in a wavelength range of 40 nm for the Mach-Zehnder interferometer. The extinction ratio and intrinsic quality factor of the fabricated micro-ring resonator are > 16 dB and 43,300, respectively. The demonstrated hybrid integrated photonic platform is promising for realizing ultralow-power optical switching and electro-optic modulation based on the piezoelectric and electro-optic effects of aluminum nitride thin films.

Arbitrary access to optical carriers in silicon photonic mode/wavelength hybrid division multiplexing circuits.

The manipulation of optical modes directly in a multimode waveguide without affecting the transmission of undesired signal carriers is of significance to realize a flexible and simple structured optical network-on-chip. In this Letter, an arbitrary optical mode and wavelength carrier access scheme is proposed based on a series of multimode microring resonators and one multimode bus waveguide with constant width. As a proof-of-concept, a three-mode (de)multiplexing device is designed, fabricated, and experimentally demonstrated. A new, to the best of our knowledge, phase-matching idea is employed to keep the bus waveguide width constant. The mode coupling regions and transmission regions of the microring resonators are designed carefully to selectively couple and transmit different optical modes. The extinction ratio of the microring resonators is larger than 21.0 dB. The mode and wavelength cross-talk for directly (de)multiplexing are less than -12.8 dB and -19.0 dB, respectively. It would be a good candidate for future large-scale multidimensional optical networks.

Compact silicon three-mode multiplexer by refractive-index manipulation on a multi-mode interferometer.

Mode-division multiplexing can scale the capacity of optical communications and optical interconnects. We demonstrate an ultra-compact and fabrication-error tolerant silicon three-mode multiplexer by shallowly etching rectangular trenches on a multi-mode interferometer. Depending on the selected input port, the TE0 mode is converted to the eigenmodes of the bus waveguide. These modes are coupled to each other owing to the refractive-index perturbation induced by the shallow trenches and finally converted to a selected spatial mode at the output. A three-mode multiplexing device is experimentally demonstrated with a footprint of 2.00 × 17.05 µm2. The bandwidths for the three channels are >70 nm with crosstalk values below -10 dB.

Carrier-assisted differential detection with reduced guard band and high electrical spectral efficiency.

For high-capacity and short-reach applications, carrier-assisted differential detection (CADD) has been proposed, in which the optical field of a complex-valued double sideband (DSB) signal is reconstructed without using a sharp-edge optical bandpass filter or local oscillator laser. The CADD receiver features a transfer function with periodical nulls in the frequency domain, while the signal-signal beat interference (SSBI) is severely amplified around the frequency nulls of the transfer function. Since the null magnitude at the zero frequency is inevitable, a guard band is required between the carrier and the signal, leading to a higher receiver bandwidth and implementation cost. To reduce the needed guard band, we propose a parallel dual delay-based CADD (PDD-CADD), in which an additional delay is placed parallel to the original delay in the conventional CADD. By this means, the modified transfer function has a sharper roll-off edge around the zero frequency. Consequently, the requirement on the guard band can be relaxed, which maximizes the bandwidth utilization of the system. The parallel delay is first optimized through numerical simulation. We then perform a proof-of-concept experiment to transmit a 100-Gb/s orthogonal frequency division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal over an 80-km single-mode fiber (SMF). After the fiber transmission, the proposed PDD-CADD can reduce the required guard band from 3 to about 1.2 GHz compared with the single delay-based conventional CADD. To our best knowledge, for the direct detection of a single polarization complex-valued DSB signal without using a sharp-roll-off optical filter, we achieve a record electrical spectral efficiency of 5.9 b/s/Hz.

Ultra-compact and broadband silicon polarizer employing a nanohole array structure.

An ultra-compact and broadband transverse magnetic (TM)-pass polarizer is experimentally demonstrated using a photonic crystal nanohole structure. By engineering the period of the circular holes, the fundamental transverse electric mode is suppressed due to the bandgap of the nanohole array, while the T M 0 mode propagates with a negligible insertion loss. Simulation results predict that the bandwidth of the device can reach 245 nm with a 33 dB polarization extinction ratio (PER). In the experiment, an insertion loss <1.2d B for the T M 0 mode and a PER over 20.8 dB are demonstrated in a 70 nm wavelength range from 1520 to 1590 nm, mainly limited by the grating couplers used for fiber-to-chip coupling. The fabricated device is compact with a total length of 7.21 µm.

Ultra-narrow passband-tunable filter based on a high-Q silicon racetrack resonator.

An ultra-narrow narrow passband-tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on a SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multimode waveguide racetrack, and the Q factor is measured as high as 8.1×105. The structure of the device is based on a thermally tunable Mach-Zehnder interferometer coupled racetrack. The tunability of the bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range is about 0.28 nm (35 GHz), with the footprint of 0.21mm2.

Demonstration of terabit coherent on-chip optical interconnects employing mode-division multiplexing.

We experimentally demonstrate a net capacity per wavelength of 1.23 Tb/s with 30 GBaud 16-ary quadrature amplitude modulation (16-QAM) mode-division multiplexing (MDM) signals over a single silicon-on-insulator (SOI) multimode waveguide for optical interconnects employing $11 \times 11$ multiple-in-multiple-out (MIMO) digital signal processing. In order to simplify the receiver architecture for coherent optical interconnects, we further propose and evaluate an on-chip self-homodyne coherent detection (SHCD) scheme. In the experiment, 30 Gbaud quadrature phase shift keying (QPSK) signals carried by 10 waveguide modes are successfully recovered with bit error rates (BERs) below 7% forward error correction (FEC) threshold using the pilot tone delivered by ${{\rm TE}_0}$ mode as a local oscillator. Around 10% penalty on error vector magnitude (EVM) is observed due to modal cross talk compared to homodyne detection.

High-speed electro-optic modulator based on silicon nitride loaded lithium niobate on an insulator platform.

Electro-optic (EO) modulators, which convert signals from the electrical to optical domain plays a key role in modern optical communication systems. Lithium niobate on insulator (LNOI) technology has emerged as a competitive solution to realize high-performance integrated EO modulators. In this Letter, we design and experimentally demonstrate a Mach-Zehnder interferometer-based modulator on a silicon nitride loaded LNOI platform, which not only takes full advantage of the excellent EO effect of LiNbO3, but also avoids the direct etching of LiNbO3 thin film. The measured half-wave voltage length product of the fabricated modulator is 2.24 V·cm, and the extinction ratio is ∼20dB. Moreover, the 3 dB EO bandwidth is ∼30GHz, while the modulated data rate for on-off key signals can reach up to 80 Gbps.

Zero-guard band dual-SSB PAM4 signal transmission with joint equalization scheme.

We propose and experimentally demonstrate a joint equalization scheme to recover a zero-guard band dual-single sideband (dual-SSB) four-level pulse amplitude modulation (PAM4) signal in a system with moderate bandwidth limitation. In the joint equalization scheme, a multiple-input multiple-output feedforward equalizer (MIMO-FFE) is first used to mitigate the residual crosstalk resulting from non-ideal optical filtering. Then, a modified post filter (PF) is placed after the MIMO-FFE to suppress the MIMO-FFE-enhanced low-frequency noise, whereas the known inter-symbol interference introduced by the modified PF is further eliminated with the maximum likelihood sequence estimation algorithm. Based on the proposed scheme, we experimentally demonstrate a 112-Gb/s zero-guard band dual-SSB PAM4 signal transmission over an 80-km single mode fiber with the averaged bit error ratio of two sidebands below 3.8×10-3. We also achieve, to the best of our knowledge, a record electrical spectral efficiency of 7.1 b/s/Hz for single polarization direct detection systems using a dual-SSB PAM4 or dual-SSB 16-quadrature amplitude modulation format.

Design and experimental demonstration of a silicon multi-dimensional (de)multiplexer for wavelength-, mode- and polarization-division (de)multiplexing.

Leveraging the physical dimensions of an optical carrier (e.g., wavelength, mode, or polarization) allows significant scaling of the transmission capacity for optical communications. Here we propose a scheme for implementing on-chip silicon (de)multiplexers with simultaneous wavelength-, mode-, and polarization-division (de)multiplexing capability. The device is constructed by using cascaded subwavelength grating (SWG)-based contra-directional couplers. To verify the feasibility of the proposed structure, we perform a proof-of-concept experiment of an 8-channel (de)multiplexer with two wavelengths, two modes, and two polarizations. The insertion losses are lower than 6.6 dB and the crosstalk values are below -18.7dB at around 1540 nm and 1550 nm for all the eight channels.