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Ultra-compact mode-order converters with dielectric slots are demonstrated on a silicon-on-insulator platform. We propose a mode converter that converts the TE0 mode into the TE1 mode with an ultra-small footprint of only 0.8×1.2µ m 2 . The measured insertion loss is less than 1.2 dB from 1520 nm to 1570 nm. To reduce the insertion loss, we further optimize the structure and design two mode converters that convert the TE0 mode into the TE1 mode and the TE2 mode with footprints of 0.88×2.3µ m 2 and 1.4×2.4µ m 2 , respectively. Their measured insertion losses are both less than 0.5 dB. Additionally, the proposed devices are cascadable and scalable for high-order mode conversion.

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.

We propose and experimentally demonstrate a self-homodyne locking method for a silicon microring resonator (MRR). The device employs a self-homodyne detection structure and consists of a tunable MRR with two directional couplers along the ring for monitoring, two phase shifters to calibrate the phase difference between the two monitored optical signals, and a Y-branch to combine the two signals. A single photodetector is used to detect the output power of the Y-branch. If the MRR is on resonance, a destructive interference occurs in the Y-branch, therefore the monitored photocurrent is minimized. By using such a device structure and the homodyne detection scheme, the MRR with a Q factor of 1.9 × 10 4 can be accurately locked to the signal wavelength, and the locking process is insensitive to input power variation. The wavelength locking range is larger than one free spectral range (FSR) of 6 nm, and the locking errors are ≤0.015 nm.

In this Letter, a novel humidity sensor based on a chip-scale silicon nanobeam cavity with polymethyl methacrylate (PMMA) cladding is demonstrated. This sensor is easy to fabricate and compatible with CMOS technology. It shows a humidity sensing with a linear wavelength dependence of 22.9 pm/% relative humidity (RH) in a wide range of RH from 10% to 85%, an ultra-fast response time of 540 ms, and high stability. After annealing, the sensor exhibits high reversibility, repeatability, and temperature insensitivity at the range of 25°C-40°C. To the best of our knowledge, this is the first application of integrated photonics in high performance humidity detection. It provides a new way for the chip-scale sensor to integrate with photonic devices and optical systems.

In this paper we demonstrate fast-response Pancharatnam-Berry (PB) phase optical elements (PBOEs) based on polymer-stabilized liquid crystal (PSLC). First, a non-interferometric photo-alignment technique is employed to generate PB patterns in a dye-doped liquid crystal by green laser light. Then the samples are exposed to UV light to form polymer networks. Due to the greatly increased elastic constant in PSLC, all PBOEs can achieve submillisecond response time, while maintaining high diffraction efficiency (>90%). Furthermore, a varifocus PB lens (PBL) is implemented based on two identical PB lens elements and its application in fatigue free augmented-reality (AR) displays is verified. The fast response PBOEs based on PSLC hold great potential for various display and photonics applications.

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.

To address the accommodation-convergence conflict problem in conventional augmented reality (AR) head-mounted displays, we propose a compact multi-plane display design based on cholesteric liquid crystal (CLC) reflective films and a polarization switch. Because of the polarization selectivity of CLC films, circularly-polarized light with different handedness is reflected by different CLC films, resulting in different optical path lengths and different image depths by the lens. A flicker-free dual-plane prototype with correct focus cues and relatively low operating voltage has been implemented. Moreover, a multi-plane AR display scheme with more than 2 depth planes is proposed by stacking multiple CLC films and polarization switches together.

We report a non-interferometric single-exposure technique for fabricating Pancharatnam-Berry (PB) devices with arbitrary wavefronts, via photo-patterning an azo-dye doped LC with a two-dimensional linear polarization field, whose local polarization direction can be controlled by a spatial light modulator (SLM) on the pixel level. Upon one exposure, different local LC orientations are generated simultaneously. The non-interferometric approach is insensitive to environmental disturbance, and moreover, the dynamic phase mask on the SLM can be conveniently reconfigured by a computer. Our fabricated PB gratings, q-plates and hologram exhibit good optical performances. Such a simple yet reconfigurable fabrication method enables new PB devices to be developed, and it would open a new gateway towards widespread applications.

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.

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 TE 0 mode and the TE 1 mode, respectively. Low crosstalk (≤21.0 dB) and reasonable insertion losses (≤2.3 dB) are achieved for both the TE 0 -mode- and the TE 1 -mode blocking filters.

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.

Holographic AR display is a promising technology for head-mounted display devices. However, it usually has a complicated optical system and a large form factor, preventing it from widespread applications. In this work, we propose a flat-panel design to produce a compact holographic AR display, where traditional optical elements are replaced by two holographic optical elements (HOEs). Here, these two thin HOEs together perform the optical functions of a beam expander, an ocular lens, and an optical combiner. Without any bulky traditional optics, our design could achieve a compact form factor that is similar to a pair of glasses. We also implemented a proof-of-concept prototype to verify its feasibility. Being compact, lightweight and free from accommodation-convergence discrepancy, our design is promising for fatigue-free AR displays.

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.

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 V rms , 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 propose and experimentally demonstrate an ultra-compact silicon photonic crystal nanobeam (PCN) cavity with an energy-efficient graphene micro-heater. Owing to the PCN cavity with an ultra-small optical mode volume of 0.145 µm 3 , the light-matter interaction is greatly enhanced and the thermo-optic (TO) tuning efficiency is increased. The TO tuning efficiency is measured to be as high as 1.5 nm/mW, which can be further increased to 3.75 nm/mW based on numerical simulations with an optimized structure. The time constants with a rise time constant of τ rise = 1.11 μs and a fall time constant of τ fall = 1.47 μs are obtained in the experiment.

A compact silicon bandpass filter with high sidelobe suppression is proposed and experimentally demonstrated using an apodized subwavelength grating (SWG) coupler. The device is implemented by placing a SWG waveguide next to a strip waveguide, and apodization is employed with a Gaussian profile to taper the gap between the two waveguides. A high sidelobe suppression ratio of 27 dB can be obtained with a 3-dB bandwidth of 8.8 nm and an insertion loss of 2.5 dB. Owing to the large optical phase mismatch between the two waveguides and the presence of the SWG waveguide, the coupling length of the device is reduced to 100.3 μm. The experimental results validate our proposed apodized-SWG-based contradirectional coupler (contra-DC) as a promising device in suppressing out-of-band components in coarse wavelength division multiplexed (CWDM) optical communication systems.

In this paper, we demonstrate a dynamic holographic display in a quantum dot (ZnS/InP) doped liquid crystal device, where one of the interior cell surfaces is covered by a ZnSe layer. Such a hybrid device shows substantially improved photorefractive sensitivity of 2.2 cm 3 /J, which is almost 300 times larger than that in ZnS/InP doped liquid crystal device without the ZnSe layer. The holographic grating can form at intensities as low as ~0.8 mW/cm 2 , and exhibit a fast optical response of several to tens of milliseconds. Exploiting the superior performances of photosensitivity and fast response of this device, we obtain dynamic holographic videos of red, green, and blue colors, as well as a reconstructed image of high gray-scale fidelity.

We demonstrate a compact silicon polarization beam splitter (PBS) based on grating-assisted contradirectional couplers (GACCs). Over 30-dB extinction ratios and less than 1-dB insertion losses are achieved for both polarizations. The proposed PBS exhibits tolerance in width variation, and the polarization extinction ratios remain higher than 20 dB for both polarizations when the width variation is adjusted from + 10 to -10 nm. Benefiting from the enhanced coupling by the GACCs, the polarization extinction ratio can be kept higher than 15 dB and the insertion loss is lower than 2 dB for both polarizations when the coupling length varies from 30.96 to 13.76 μm.

We propose and experimentally demonstrate a wavelength and bandwidth-tunable comb filter based on silicon Sagnac loop mirrors (SLMs) with Mach-Zehnder interferometer (MZI) couplers. By thermally tuning the MZI couplers in common and differential modes, the phase shift and reflectivity of the SLMs can be changed, respectively, leading to tunable wavelength and bandwidth of the comb filter. The fabricated comb filter has 93 comb lines in the wavelength range from 1535 nm to 1565 nm spaced by ~0.322 nm. The central wavelength can be red-shifted by ~0.462 nm with a tuning efficiency of ~0.019 nm/mW. A continuously tunable bandwidth from 5.88 GHz to 24.89 GHz is also achieved with a differential heating power ranging from 0.00 mW to 0.53 mW.

We propose and experimentally demonstrate a microfiber-graphene device. Owing to the interaction between the graphene film and the evanescent field leaked from the microfiber, the hybrid photoconductive device exhibits a high photoresponse. A maximum photocurrent responsivity of ~2.81 mA/W is achieved in the telecommunication band. A nearly flat photoresponse spectrum within broad operational band ranging from 1500 nm to 1600 nm is also obtained as a consequence of the dispersionless and flat absorption of graphene. These results show that the proposed photocurrent generation device could provide an effective solution for broadband photodetection.

We propose and numerically study an on-chip graphene-silicon hybrid electro-optic (EO) modulator operating at the telecommunication band, which is implemented by a compact 1D photonic crystal nanobeam (PCN) cavity coupled to a bus waveguide with a graphene sheet on top. Through electrically tuning the Fermi level of the graphene, both the quality factor and the resonance wavelength can be significantly changed, thus the in-plane lightwave can be efficiently modulated. Based on finite-difference time-domain (FDTD) simulation results, the proposed modulator can provide a large free spectral range (FSR) of 125.6 nm, a high modulation speed of 133 GHz, and a large modulation depth of ~12.5 dB in a small modal volume, promising a high performance EO modulator for wavelength-division multiplexed (WDM) optical communication systems.

In this paper, we demonstrate a holographic polymer-stabilized blue-phase liquid crystal grating fabricated using a visible laser. As blue phase is stabilized by the interfered light, polymer-concentration gradient is achieved simultaneously. With the application of a uniform vertical electric field, periodic index distribution is obtained due to polymer-concentration gradient. The grating exhibits several attractive features such as polarization-independency, a broad temperature range, sub-millisecond response, simple fabrication, and low cost, thus holding great potential for photonics applications.

We demonstrate ultrafast optical modulation using a single 1-μm-diameter graphene-decorated microfiber, which is fabricated with a convenient and controllable evanescent-field-induced deposition method. Benefitting from the significantly enhanced light-graphene interaction of the subwavelength transvers dimension of the microfiber and accumulation of the saturable absorption of the piled graphene flakes, the microfiber shows nonlinear saturable absorption with a peak power threshold down to 1.75 W (60 MW/cm(2)), with a measured response time of about 3.5 ps.

Coherent backlight is an essential component for holographic displays. In this paper, a compact design of edge-lit coherent backlight featuring two holographic optical elements for two-dimensional beam expansion is presented. Its diffraction efficiency is numerically studied using the coupled-wave theory. In experiments, the diffraction efficiency is measured as 4.3% and the feasibility of this design is verified by reconstructing 3D images with a spatial light modulator.

We propose and experimentally demonstrate a distance-adaptive bandwidth allocation scheme to realize high-capacity long-reach orthogonal frequency division multiple access passive optical network (OFDMA PON) with cost-effective electro-absorption modulator (EAM). In our scheme, the subcarriers in downstream OFDM signal are properly allocated to the optical network units (ONUs) with different fiber transmission lengths. By this means, the detrimental influence of power fading induced by dispersion and chirp can be avoided, thus all OFDM subcarriers can be modulated with high-order quadrature amplitude modulation (QAM) format, leading to a high transmission capacity. A proof-of-concept experiment is performed, in which three ONUs with transmission distances of 25, 50, and 100 km are assigned with different subcarriers, respectively. By using distance-adaptive bandwidth allocation technique, an OFDM signal of 34.5 Gb/s is successfully delivered to the ONUs with a bit error ratio (BER) lower than 2 × 10(-3).