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With the growing demand for driving safety and convenience, Head-Up Displays (HUDs) have gained more and more interest in recent years. In this paper, we propose a HUD system with the ability to adjust the relative brightness of ambient light and virtual information light. The key components of the system include a cholesteric liquid crystal (CLC) film, a geometric phase (GP) liquid crystal lens, and a circular polarizer. By controlling the voltage applied to the GP lens, the contrast ratio of the virtual information light to ambient light could be continuously tuned, so that good visibility could always be obtained under different driving conditions.

The Gerchberg–Saxton (GS) iteration algorithm is widely used for phase-only computer-generated holograms. However, as the GS algorithm was originally developed for single-plane holographic reconstruction, its reconstruction capability for multiple planes is limited. The additive cross-talk between different image planes results in image-quality degradation. In this study, we propose a dynamic compensatory GS algorithm, where the weighting factors of the amplitude-constraining function are dynamically adjusted during each iteration. Both simulation and experimental results prove significant improvement on average image quality of multiple planes.

We propose and experimentally demonstrate a multiple input multiple output - artificial neural network (MIMO-ANN) nonlinear equalizer (NLE) to process the complex quadrature amplitude modulation (QAM) signal in a single-sideband (SSB) self-coherent detection (SCD) system. In the proposed scheme, a 2-by-2 MIMO structure with two ANNs is employed to effectively mitigate the signal distortions induced by in-phase and quadrature (IQ) imbalance and fiber nonlinear effects. By using the proposed MIMO-ANN NLE, we successfully transmit a 112-Gb/s SSB 16-QAM signal over a single-span 120-km single mode fiber (SMF) in a direct detection (DD) system with a bit error rate (BER) lower than 3.8 × 10−3. We also conduct a comparative study between the proposed MIMO-ANN NLE, a feedforward equalizer (FFE), a NLE consisting of two independent real-valued Volterra filters, and a MIMO-Volterra filter. The proposed MIMO-ANN NLE outperforms other equalizers with the longer fiber length and thus stronger nonlinearities, since it can easily approximate a complicated nonlinear function. To the best of our knowledge, this is the first experimental demonstration of an ANN-based equalizer in an SSB SCD system.

Dielectrically loaded thin-film lithium niobate (LiNbO3) on insulator (LNOI) platforms have enabled a range of photonic integrated circuit components, such as high-speed optical modulators, switches, and nonlinear devices, while avoiding the direct etching of the LiNbO3 thin film. Silicon nitride (Si3N4) is one of the most attractive dielectric loading materials as it has a similar refractive index and transparency window to LiNbO3 and can be deposited and patterned by mature fabrication processes. The patterning of Si3N4 opens the opportunity to fabricate grating couplers in the same fabrication step, providing efficient optical interfaces for wafer-scale testing. In this paper, we investigate and demonstrate single-step etched grating couplers on a Si3N4-LNOI (X-cut) platform. The grating couplers (straight and curved) are designed and fabricated for TE-polarized modes along the Y and Z crystallographic directions, considering the LiNbO3 crystal’s birefringence. The experimentally demonstrated coupling losses are as low as 4.02 and 4.24 dB along the crystallographic Y and Z directions, respectively. The corresponding peak wavelengths are 1609 and 1615 nm, respectively. The measured 3-dB bandwidths are wider than 70 nm for both crystallographic directions. We also numerically investigated the influence of fabrication variations and the fiber angle on the transmission. To the best of our knowledge, this work is the first demonstration of grating couplers with different light propagation directions on the Si3N4 loaded LNOI platform.

We review the current status of mode-division multiplexing (MDM) techniques in fibers and on chips. Three system applications are introduced, including quasi-single mode transmission, multicore few-mode amplifier, and fiber sensing. We also discuss the technology development trend in terms of multiple-input-multiple-output-free MDM, economics of MDM, and quantum information processing. Finally, we provide perspectives on emerging applications beyond communications by leveraging the optical properties of high order modes, e.g., nonlinear optics in the visible regime, broadband frequency comb generation, and super resolution endoscopy.

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 × 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 T E 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.

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.

In this article, polymer network liquid crystal (PNLC) grating/Fresnel lens is fabricated by holography. The exposure light pattern for the grating is obtained by interfering two planar wave fronts, while the Fresnel pattern is achieved by interfering a planar wave front and a spherical wave front. Owing to the alignment effect and anchoring power of polymer network, the holographic PNLC grating achieves improved diffraction efficiency, and remarkably reduced operation voltage (reduced by 80%) compared with holographic polymer-dispersed-liquid-crystal and holographic polymer-stabilised blue-phase liquid-crystal gratings, while maintaining submillisecond response. Moreover, it achieves high spatial frequency with a 2-μm grating period, thanks to the holographic fabrication. The holographic PNLC Fresnel lens also exhibits attractive electro-optical properties.

In this paper, we use a dynamic mask lithography system to fabricate a polymer-stabilized blue-phase Fresnel lens. Fresnel-zone patterned light is directly generated by a programmable spatial light modulator and then projected onto the blue phase precursor. As blue phase is cured, a Fresnel-zone refractive index distribution is achieved simultaneously. The fabricated Fresnel lens features polarization-insensitivity, a broad temperature range, submillisecond response and high spatial frequency. With easy fabrication and programmable flexibility, this method has great potential for various optical applications.

We propose and experimentally demonstrate a photonic generation scheme for multiband ultra-wideband (MB-UWB) signal by employing a dual-drive Mach–Zehnder modulator (DDMZM). In this scheme, four electrical binary pulse streams with different bit rates are combined to drive one radio frequency (RF) port of the DDMZM, while the other RF port is modulated by the sum of their 1-b delayed duplicates. By properly adjusting the bit rate and the amplitude of each pulse stream, as well as the bias voltage of the DDMZM, an MB-UWB signal with four sub-bands can be obtained and the transmission capacity can be quadrupled. The feasibility of the proposed scheme is verified by a system demonstration, where the signal modulated by amplitude-phase-shift-key format reaches a high bit rate of 2 Gb/s.

We present our recent progress on microwave signal processing (MSP) using on-chip passive silicon photonic devices, including tunable microwave notch filtering/millimeter-wave (MMW) signal generation based on self-coupled micro-resonators (SCMRs), and tunable radio-frequency (RF) phase shifting implemented by a micro-disk resonator (MDR). These schemes can provide improved flexibility and performances of MSP. The experimental results are in good agreement with theoretical predictions, which validate the effectiveness of the proposed schemes.

Liquid crystals (LCs) are widely used in nonlinear optics because of their sensitive responses to optical stimulation. Combined with other optoelectronic materials such as azo dyes, quantum dots (QDs), etc., LCs show very large optical nonlinearity which makes them suitable for applications in optical information processing, including holographic display and holographic storage. In this review, we present an overview of recent efforts on holographic display and storage based on liquid crystalline materials. Emphasis is placed on the dynamic holographic display which uses fast-response LCs doped with azo dye and QD. In this application, we discuss how to improve the response speed, resolution, and diffraction efficiency of liquid crystalline materials to realize high frame rate, large-size and high-brightness holographic displays. Some LC materials for holographic storage are also introduced, including polymer-dispersed liquid crystals, blue-phase liquid crystals, and azobenzene polymer liquid crys...

Augmented reality (AR) display technology greatly enhances users' perception of and interaction with the real world by superimposing a computer-generated virtual scene on the real physical world. The main problem of the state-of-the-art 3D AR head-mounted displays (HMDs) is the accommodation-vergence conflict because the 2D images displayed by flat panel devices are at a fixed distance from the eyes. In this paper, we present a design for an optical see-through HMD utilizing multi-plane display technology for AR applications. This approach manages to provide correct depth information and solves the accommodation-vergence conflict problem. In our system, a projector projects slices of a 3D scene onto a stack of polymer-stabilized liquid crystal scattering shutters in time sequence to reconstruct the 3D scene. The polymer-stabilized liquid crystal shutters have sub-millisecond switching time that enables sufficient number of shutters to achieve high depth resolution. A proof-of-concept two-plane optical see-through HMD prototype is demonstrated. Our design can be made lightweight, compact, with high resolution, and large depth range from near the eye to infinity and thus holds great potential for fatigue-free AR HMDs.

We demonstrate a real-time holographic display using quantum-dot doped liquid crystal with a build-up time of several to tens of milliseconds, and the maximum diffraction efficiency is measured up to 30%. Holographic videos of red, green, and blue colors at a refresh rate of 60 Hz are realized. The dependences of response time on the recording intensity, applied voltage, and grating period are investigated.

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 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 for the 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.

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 × 1 0 − 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.

A GFDM-OFDM hybrid modulation scheme is experimentally demonstrated in a 60-Gb/s IM-DD system. 1.9-dB and 3.1- dB receiver sensitivity improvements are achieved compared to the conventional GFDM and OFDM signals in the back-to-back case, respectively.

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 − 18.7 d B at around 1540 nm and 1550 nm for all the eight channels.

Mode blocking may be required in mode-division multiplexing (MDM) systems. We demonstrate a silicon mode blocking filter using a subwavelength grating-based contra-directional coupler. The device is capable of blocking the undesired mode channel without affecting the propagation of the other modes. As a proof-of-concept experiment, two mode blocking filters are experimentally demonstrated, which can block the TE0 mode and the TE1 mode, respectively. Low crosstalk (≤21.0 dB) and reasonable insertion losses (≤2.3 dB) are achieved for both the TE0-mode- and the TE1-mode blocking filters.

We present our recent work on thermal-optical switching on silicon chips, including two nanobeam switches with wavelength tuning, and a mode and polarization selective switch having a 748-Gb/s capacity on a single wavelength.

A modified KK receiver based on an exponential operation is proposed to accurately reconstruct the field at a low CSPR in a DD system. We experimentally demonstrate a 2-dB sensitivity improvement after 40-km SMF transmission.

A silicon polarization splitter and rotator is experimentally demonstrated using a nonlinearly-tapered directional coupler. Adiabatic polarization splitting and rotating are achieved over a length of 129 μm. Fabrication tolerance to waveguide widths is also verified.

We propose and experimentally demonstrate a neural-network-based wavelength locking algorithm for a 1 × 2 silicon microring switch. The wavelength locking is performed at a 20-nm/ms locking speed over a full free spectral range.

Due to the inherent in-direct bandgap nature of Silicon, heterogeneous integration of semiconductor lasers on Silicon on Insulator (SOI) is crucial for next-generation on-chip optical interconnects. Compact, high-efficient and high-tolerant couplers between III-V light source and silicon chips have been the challenge for photonic integrated circuit (PIC). Here, we redesign the taper adiabatic coupler with the total coupling length of only 4 {\mu}m, and propose another two novel slot coupler and bridge-SWG coupler with both coupling length of 7 {\mu}m, to heterogeneously integrate III-V lasers and silicon chips. We study theoretically the optical mode coupling process through the redesigned taper coupler, the final coupling results match well with the simulation in 3D-FDTD. The three compact couplers represent fundamental TE mode coupling efficiencies all over 90%, even 95.7% for bridge-SWG coupler, to the best of our knowledge, are also the shortest coupling structures (7 um). Moreover, these coupling structures also possess excellent fabrication tolerance., Comment: 7 pages, 7 figures

Neuromorphic photonics that aims to process and store information simultaneously like human brains has emerged as a promising alternative for the next generation intelligent computing systems. The implementation of hardware emulating the basic functionality of neurons and synapses is the fundamental work in this field. However, previously proposed optical neurons implemented with SOA-MZIs, modulators, lasers or phase change materials are all dependent on active devices and quite difficult for integration. Meanwhile, although the nonlinearity in nanocavities has long been of interest, the previous theories are intended for specific situations, e.g., self-pulsation in microrings, and there is still a lack of systematic studies in the excitability behavior of the nanocavities including the silicon photonic crystal cavities. Here, we report for the first time a universal coupled mode theory model for all side-coupled passive microresonators. Attributed to the nonlinear excitability, the passive microresonator can function as a new type of all-optical spiking neuron. We demonstrate the microresonator-based neuron can exhibit the three most important characteristics of spiking neurons: excitability threshold, refractory period and cascadability behavior, paving the way to realize all-optical spiking neural networks., Comment: 8 pages, 7 figures

A DSP-free phase alignment scheme is proposed and experimentally demonstrated in a 56-Gb/s SSB-PAM4 transmission system. BERs are reduced significantly with the new phase alignment scheme in both the back-to-back case and after 40-km transmission.

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.

We propose and demonstrate a novel modulation scheme termed as generalized frequency division multiplexing - orthogonal frequency division multiplexing (GFDM-OFDM) for intensity-modulated direct-detection (IM-DD) optical communication systems. In our scheme, the low-frequency band is allocated to a GFDM signal to achieve a low peak-to-average power ratio (PAPR), while the high-frequency band is filled with an OFDM signal that is robust to frequency-domain variations. By this means, the PAPR of the whole signal can be reduced and the high-frequency fading effects induced by the electronic components can be mitigated, leading to an improved system performance. Simulation results show that our proposed GFDM-OFDM format outperforms OFDM and GFDM formats in terms of optical receiver sensitivity for both the back-to-back (BTB) and 10-km transmissions. The GFDM-OFDM signal achieves receiver sensitivity improvements of approximately 3.0 dB and 1.6 dB, compared to the OFDM and GFDM signals, respectively.

Photonic integrated circuits (PICs) technologies in China have made rapid progress in the past decade. In this paper, recent advances and future prospective of PICs in China are introduced and discussed, respectively. More than nine major projects in the field of PICs have been issued. Various material platforms and technologies have been developed to realize photonic integrated devices for many applications focusing on broadband communication, such as optical and wireless network, optical interconnects, and optical coherent communication. Many research achievements and breakthrough have been made under the support of these projects, such as silicon photonics transceiver, InP-based transceiver, and optical and microwave signal processing chips. It can be expected that the PICs technology will be continuously supported in the next five years, since it has been considered as one of the key enabling technologies for supporting several national major assignments and strategies, such as broadband communication, supercomputing and datacenter.

We propose and experimentally demonstrate an on-chip all-optical differential-equation solver capable of solving second-order ordinary differential equations (ODEs) characterizing continuous-time linear time-invariant systems. The photonic device is implemented by a self-coupled microresonator on a silicon-on-insulator platform with mutual coupling between the cavity modes. Owing to the mutual mode coupling within the same resonant cavity, the resonance wavelengths induced by different cavity modes are self-aligned, thus avoiding precise wavelength alignment and unequal thermal wavelength drifts as in the case of cascaded resonators. By changing the mutual mode coupling strength, the proposed device can be used to solve second-order ODEs with tunable coefficients. System demonstration using the fabricated device is carried out for 10-Gb/s optical Gaussian and super-Gaussian input pulses. The experimental results are in good agreement with theoretical predictions of the solutions, which verify the feasibility of the fabricated device as a tunable second-order photonic ODE solver.

We present a transmissive-type interferometric display featuring a single layer of Fabry-Perot (FP) filter using blue phase liquid crystal (BPLC) in tandem with a quantum-dot backlight. With this design, no polarizers and color filters are needed. The intensity modulation is implemented by electrically controlling BPLC’s Kerr effect, while the color modulation employs a sequential color scheme. Based on numerical simulations, device performance has been studied in depth.

We report a polymer-stabilized blue-phase liquid crystal (PSBPLC) cured with a visible laser in an in-plane-switching (IPS) cell. Its electro-optic properties are compared with those of a conventional PS-BPLC cured by ultraviolet (UV) light. Besides, a PS-BPLC grating is fabricated using visible-laser holography.

Coherent backlight is an important component in holographic 3D displays. In this paper, a novel coherent backlight featuring a compact design is presented. The feasibility of this structure is experimentally verified by reconstructing 3D images with a spatial light modulator. The diffraction efficiency of the system is ~4.3%.

A modified three-dimensional (3D) Gerchberg-Saxton (GS) algorithm for phase-only holograms in holographic displays is proposed. Numerical and experimental results indicate that compared with the traditional 3D GS algorithm, image-quality difference in the proposed method is reduced by four orders of magnitude, while the average image-quality is increased by 28.7%.

We design an optical see-through head mounted display (HMD) employing multi-plane volumetric technique for augmented reality applications. With a stack of fast-switching polymerstabilized liquid crystal scattering shutters, the HMD provides correct depth information and solves the accommodationconvergence conflict.

A novel fabrication method for polymer-stabilized blue-phase liquid-crystal Fresnel lens is proposed. Blue phase is stabilized by patterned light illumination directly obtained from a spatial light modulator. The lens diffraction efficiency can be continuously tuned. With full programmable capability, this method holds great potential in optical applications.

We report an updatable holographic 3D display based on photo-refractive azo-dye-doped liquid crystals (ADDLCs). The display is capable of recording and displaying 3D images with all depth cues, including accommodation. A holographic 3D image of 4K resolution is demonstrated in this paper.

In this paper, we report a polymer network liquid crystal grating fabricated using interfered visible light. Exposed to the periodic light pattern, polymer concentration gradient is achieved as the precursor is cured. The grating is simple to fabricate and has low voltage, sub-millisecond response and high spatial resolution.

We discuss microwave signal processing (MSP) using on-chip passive silicon photonic devices. By using the through port of a self-coupled micro-resonator (SCMR), a microwave photonic notch filter is achieved with a tuning range over 20 GHz and a maximum rejection ratio of 25 dB. Through frequency extracting from an optical frequency comb using the drop ports of SCMRs, photonic generation of 39-GHz and 29-GHz MMW signals are achieved. A photonic RF phase shifter based on a single micro-disk resonator can provide a phase-shift approaching 2π.

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.

Augmented reality (AR) technology, which integrates virtual computer-generated information into the real world scene, is believed to be the next-generation human-machine interface. However, most AR products adopt stereoscopic 3D display technique, which causes the accommodation-vergence conflict. To solve this problem, we have proposed two approaches. The first is a multi-planar volumetric display using fast switching polymer-stabilized liquid crystal (PSLC) films. By rapidly switching the films between scattering and transparent states while synchronizing with a high-speed projector, the 2D slices of a 3D volume could be displayed in time sequence. We delved into the research on developing high-performance PSLC films in both normal mode and reverse mode; moreover, we also realized the demonstration of four-depth AR images with correct accommodation cues. For the second approach, we realized a holographic AR display using digital blazed gratings and a 4f system to eliminate zero-order and higher-order noise. With a 4k liquid crystal on silicon device, we achieved a field of view (FOV) of 32 deg. Moreover, we designed a compact waveguidebased holographic 3D display. In the design, there are two holographic optical elements (HOEs), each of which functions as a diffractive grating and a Fresnel lens. Because of the grating effect, holographic 3D image light is coupled into and decoupled out of the waveguide by modifying incident angles. Because of the lens effect, the collimated zero order light is focused at a point, and got filtered out. The optical power of the second HOE also helps enlarge FOV.

In this paper we propose an optical see-through multi-plane display with reverse-mode polymer-stabilized liquid crystal (PSLC). Our design solves the problem of accommodation-vergence conflict with correct focus cues. In the reverse mode PSLC system, power consumption could be reduced to ~1/(N-1) of that in a normal mode system if N planes are displayed. The PSLC films fabricated in our experiment exhibit a low saturation voltage ~20 Vrms, a high transparent-state transmittance (92%), and a fast switching time within 2 ms and polarization insensitivity. A proof-of-concept two-plane color display prototype and a four-plane monocolor display prototype were implemented.

We experimentally demonstrate a 102-Gb/s DMT signal transmission over 105-km single-span SMF in an IM-DD system. This work reports the highest single-wavelength and single-polarization net data rate×single span length product in IM-DD systems.