Efficient second harmonic generation in a high-Q Fabry-Perot microresonator on x-cut thin film lithium niobate.

Microresonators facilitate enhanced light-matter interactions within a limited space, showing great promise for nonlinear optics. Here, we demonstrate a high-quality (Q) factor Fabry-Perot microresonator (FPR) for second harmonic generation (SHG) on an x-cut thin film lithium niobate (TFLN) platform. The FPR exhibits Q factors of Qpump = 1.09 × 105 and QSH = 1.15 × 104 at the 1560 nm pump wavelength and 780 nm second harmonic wavelength, respectively. Under low pump power, a normalized SHG efficiency of 158.5 ± 18.5%/W is attained. We experimentally verify that increased temperatures mitigate photorefractive effects that degrade SHG performance. This work highlights the immense capabilities of one-dimensional planar optical waveguide resonators for efficient on-chip nonlinear wavelength conversion.

Ultra-efficient second harmonic generation via mode phase matching in integrated lithium niobate racetrack resonators.

High-efficiency second harmonic generation (SHG) relying solely on intermodal dispersion engineering remains a challenge. Here, we realize highly efficient SHG using a double-waveguide coupled racetrack microring resonator on X-cut lithium niobate on insulator (LNOI), where both pump and second harmonic (SH) approach critical coupling. Through precise temperature tuning, simultaneous pump and SH resonance is attained in the resonator, dramatically enhancing SHG efficiency. With low pump power, a normalized conversion efficiency of 9972%/W is achieved. Moreover, the resonator provides a 25.73 dB enhancement in SHG efficiency compared to a 4 mm straight waveguide with identical phase matching in our experiment. This work enables efficient wavelength conversion and quantum state generation on integrated X-cut LNOI platforms.

Hybrid-integrated wideband tunable optoelectronic oscillator.

As a photonic-based microwave signal generation method, the optoelectronic oscillator (OEO) has the potential of meeting the increasing demand of practical applications for high frequency, broadband tunability and ultra-low phase noise. However, conventional OEO systems implemented with discrete optoelectronic devices have a bulky size and low reliability, which extremely limits their practical applications. In this paper, a hybrid-integrated wideband tunable OEO with low phase noise is proposed and experimentally demonstrated. The proposed hybrid integrated OEO achieves a high integration level by first integrating a laser chip with a silicon photonic chip, and then connecting the silicon photonic chip with electronic chips through wire-bonding to microstrip lines. A compact fiber ring and an yttrium iron garnet filter are also adopted for high-Q factor and frequency tuning, respectively. The integrated OEO exhibits a low phase noise of -128.04 dBc/Hz @ 10 kHz for an oscillation frequency of 10 GHz. A wideband tuning range from 3 GHz to 18 GHz is also obtained, covering the entire C, X, and Ku bands. Our work demonstrates an effective way to achieve compact high-performance OEO based on hybrid integration, and has great potential in a wide range of applications such as modern radar, wireless communication, and electronic warfare systems.

Compact and fast-response optical switch based on complex refractive index engineering.

The optical switch is a crucial device in integrated photonic circuits. Among the various types of optical switches available, the on-off Mach-Zehnder interferometer is one of the most widely used structures. However, compared with other structures, such as a microring, the large footprint of a Mach-Zehnder interferometer significantly restricts the integration density. In this paper, we propose a compact Mach-Zehnder interferometer based on complex refractive index engineering. By manipulating the complex index of the material in the structure, the lateral size of the device can be compressed down to only 3.25 µm. Moreover, the reducing of the space between heaters and waveguides leads to a fast response of only 1.9 µs. Our work offers a new, to the best of our knowledge, approach of a compact integrated optical switch, and opens a new avenue for application of absorbing materials.

Complex-valued matrix-vector multiplication system for a large-scale optical FFT.

Recent advancements in optical convolutional neural networks (CNNs) and radar signal processing systems have brought an increasing need for the adoption of optical fast Fourier transform (OFFT). Presently, the fast Fourier transform (FFT) is executed using electronic means within prevailing architectures. However, this electronic approach faces limitations in terms of both speed and power consumption. Concurrently, existing OFFT systems struggle to balance the demands of large-scale processing and high precision simultaneously. In response, we introduce a novel, to the best of our knowledge, solution: a complex-valued matrix-vector system harnessed through wavelength selective switches (WSSs) for the realization of a 24-input optical FFT, achieving a high-accuracy level of 5.4 bits. This study capitalizes on the abundant wavelength resources available to present a feasible solution for an optical FFT system with a large N.

Compact and high Q-factor multimode racetrack ring resonator based on transformation optics.

The ring resonator is a versatile and functional component in the silicon-based integrated optical circuit. Most of the previously reported ring resonators work in the single-mode case. With the rapid development of mode division multiplexing technology, a multimode ring resonator (MMRR) has been proposed and the usage beyond the limit of a conventional single mode ring resonator has been explored. However, the reported MMRRs are either large in size or low in quality factor. In this paper, we designed a compact silicon MMRR with a small bending radius of 15µm, in which the three lowest TE modes all have high Q-factors. For suppressing the mode loss and inter-mode crosstalk in MMRR, a multimode waveguide bend (MWB) with mode adiabatic evolution was designed based on transformation optics and waveguide shape optimization. The independent excitation of each order mode of the MMRR is realized by using bending directional coupler and asymmetric directional coupler. We successfully fabricated the device on a silicon-on-insulator (SOI) platform using simple one-step lithography. The measured loaded Q-factors of the three lowest TE modes are 5.9 × 104, 4.5 × 104, and 4.7 × 104, respectively.

Simultaneous full set of three-input canonical logic units in a single nonlinear device for an all-optical programmable logic array.

All-optical canonical logic units-based programmable logic array (CLUs-PLA) is an important combinational logic device owing to its flexibility and user-defined feature. However, the limited number of three-input CLUs generated in a single nonlinear device hinders their practical application. In this study, we overcome this limitation and experimentally demonstrate the simultaneous generation of a full set of three-input CLUs in only one nonlinear device. By performing bidirectional four-wave mixing (FWM) and wavelength spacing optimization, the all-optical three-input PLA with a full set of CLUs enables arbitrary functions. We experimentally demonstrate the implementation of a series of combinational logic functions including, user-defined logic functions, full adder, and full subtractor, exhibiting error-free performances for all logic operations at 40 Gb/s. The scheme can reduce the number of nonlinear devices in CLUs-PLA, which simplifies the computing system and reduces power consumption. Therefore, the scheme has great potential for future high-speed optical computing systems.

Seven-channel all-optical reconfigurable canonical logic units multicasting at 40 Gb/s based on a nonlinearity-enhanced silicon waveguide.

All-optical canonical logic unit (CLU) is the basic building block of high-speed optical logic operation and complex optical computing. By utilizing the parallelism of optical signals, multichannel multicasting of all-optical CLUs can expand the capacity of the computing system effectively. Here, we propose and experimentally demonstrate the 40 Gb/s all-optical reconfigurable two-input CLUs generated in seven wavelength channels via four-wave mixing (FWM) in the nonlinearity-enhanced silicon waveguide. By introducing reverse-biased PIN junctions to reduce nonlinear loss, the output power of converted light can be increased over 10 dB. Moreover, pumped by two optical signals and a continuous wave beam, a full set of reconfigurable CLUs is multicasted in seven parallel wavelength channels. All logic signals with error-free performance are realized. Attributing to the rate transparency of FWM and parallel multicasting of logic functions, the proposed scheme offers more flexibility and expandability in future high-speed optical logic processing and complex optical computing.

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.

Compact high-contrast silicon optical filter using all-passive and CROW Fano nanobeam resonators.

We propose and experimentally demonstrate a high-order coupled-resonator optical waveguide (CROW) nanobeam filter with semi-symmetrical Fano resonance enhancement. Thanks to the tight arrangement of multiple nanobeams and assistance of the partial transmission element, the designed filter has a high-contrast transmission and low insertion loss. Finally, the fabricated filter has a compact size of 20µm×10µm, a high extinction ratio as much as 70 dB, and an insertion loss as low as 1 dB. This filter shows a passive structure without thermal control configuration for calibration on each resonator. This compact filter can be a basic building block for various applications requiring high extinction ratio filtering, such as single-photon source filtering of integrated photon chips.

Suppression and revival of single-cavity lasing induced by polarization-dependent loss.

For most photonics devices and systems, loss is desperately averted, since it will increase the power consumption and degrade the performance. However, in some non-Hermitian systems, loss can induce a modal gain when the parity-time symmetry is broken, which offers a new way to manipulate the lasing of active cavities. Here we experimentally observe the counterintuitive phenomenon in a single laser cavity assisted by the polarization-dependent loss. A parity-time symmetric system is constituted by the two orthogonally polarized photonic loops in a single laser cavity, which can guarantee the consistency of two coupling loops. The measured output power of the cavity depends on the cross-polarization loss, which reveals virtually opposite relationships before and after the critical point. It provides a novel, to the best of our knowledge, understanding of polarization loss and shows great potential for lasing manipulation in a single cavity with polarization control.

Deterministic design of focusing apodized subwavelength grating coupler based on weak form and transformation optics.

The focusing apodized subwavelength grating coupler (F-ASGC) has advantages of high coupling efficiency, small footprint and simple fabrication process, which make it a popular component for chip-scale coupling and testing of integrated optical circuit. However, the design of F-ASGC based on effective medium theory lacks accuracy, causing the drawbacks of peak wavelength deviation and performance degradation. In this work, we propose a deterministic design method of F-ASGC. Our grating coupler is formed by assembling various subwavelength grating units according to their complex effective indexes. The complex effective indexes of these grating units are accurately obtained by the weak form calculation. Then combining with transformation optics, we strictly analyze the F-ASGC for the first time. The simulation results show that the deterministically designed F-ASGC has high coupling efficiency of -2.51 dB, 3 dB bandwidth of 51 nm, and accurate central wavelength of 1553.1 nm. And we also fabricated it on the commercial SOI wafer. The measured maximum efficiency is -3.10 dB, the 3 dB bandwidth is 55 nm, and the central wavelength is 1551.5 nm.

Ultracompact optical switch using a single semisymmetric Fano nanobeam cavity.

We propose and theoretically analyze a generalized four-port waveguide-cavity system with a novel, to the best of our knowledge, semisymmetric Fano structure. By applying the system to a silicon-based photonic crystal nanobeam cavity (PCNC), we experimentally demonstrate an ultracompact crossbar optical switch with high drop port transmission. A low insertion loss of 1.5 dB in the drop port at the cross state is achieved comparing to the 8.5 dB insertion loss in a traditional non-Fano structure. The device is also benefited by the high quality factor and small mode volume for efficient switching and the ultracompact footprint of $ 14\;\unicode{x00B5} {{\rm m}^2} $14µm2 in the core structure. The proposed PCNC switch shows great potential in various optical systems on chip to increase the integration and reduce energy consumption.

Hamiltonian Hopping for Efficient Chiral Mode Switching in Encircling Exceptional Points.

Dynamically encircling exceptional points (EPs) can lead to chiral mode switching as the system parameters are varied along a path that encircles EP. However, conventional encircling protocols result in low transmittance due to path-dependent losses. Here, we present a paradigm to encircle EPs that includes fast Hamiltonian variations on the parameter boundaries, termed Hamiltonian hopping, enabling ultrahigh-efficiency chiral mode switching. This protocol avoids path-dependent loss and allows us to experimentally demonstrate nearly 90% efficiency at 1550 nm in the clockwise direction, overcoming a long-standing challenge of non-Hermitian optical systems and powering up new opportunities for EP physics.

Two-dimensional silicon photonic grating coupler with low polarization-dependent loss and high tolerance.

We design and demonstrate a two-dimensional grating coupler with an elliptical-like etching pattern on the silicon-on-insulator platform. The polarization-dependent loss of the fabricated device is measured to be 0.2 dB at the C-band, lower than that of most conventional 2D GCs. The etching unit can be easily fabricated with a large tolerance of ± 10 nm and a large feature size of 310nm, which is beneficial to its manufacturing process and wide application.

Theoretical study of polarization dependence of carrier-induced refractive index change of quantum dot.

The influences of dot material component, barrier material component, aspect ratio and carrier density on the refractive index changes of TE mode and TM mode of columnar quantum dot are analyzed, and a multiparameter adjustment method is proposed to realize low polarization dependence of refractive index change. Then the quantum dots with low polarization dependence of refractive index change (<1.5%) within C-band (1530 nm - 1565 nm) are designed, and it shows that quantum dots with different material parameters are anticipated to have similar characteristics of low polarization dependence.

Silicon-on-insulator-based microwave photonic filter with narrowband and ultrahigh peak rejection.

We propose and demonstrate a silicon-on-insulator (SOI)-based widely tunable microwave photonic filter (MPF), which is implemented by using an under-coupled microring resonator (MRR) assisted by two cascaded tunable Mach-Zehnder interferometers. In the experiment, the MPF achieves an ultrahigh peak rejection exceeding 60 dB, a full width at half-maximum bandwidth of 780 MHz, and a frequency tuning range of 0-40 GHz, even when the propagation loss of the MRR is 1.65 dB/cm. To the best of our knowledge, this MPF demonstrates ultrahigh peak rejection and narrow bandwidth simultaneously in SOI for the first time with MRR of such propagation loss and avoids using external electrical devices to improve the rejection.

Mode measurement of few-mode fibers by mode-frequency mapping.

Since few-mode fibers (FMFs) have great potential as the new transmission media for optical communications, the ability to distinguish different fiber modes is essential. Most of the traditional schemes do not yield phase information, or are limited by beam size and mechanical requirements. Here, a method is presented to analyze the mode distribution of FMFs. The fiber modes are mapped to different frequencies by using dynamic spatial phase masks. The complex amplitudes at these frequencies indicate the amplitudes and phases of the fiber modes. The method can extract not only the amplitude distribution, but also the phase distribution of the fiber modes, and no other assisted light is needed.

Spectrum Control through Discrete Frequency Diffraction in the Presence of Photonic Gauge Potentials.

By using optical phase modulators in a fiber-optical circuit, we theoretically and experimentally demonstrate large control over the spectrum of an impinging signal, which may evolve analogously to discrete diffraction in spatial waveguide arrays. The modulation phase acts as a photonic gauge potential in the frequency dimension, realizing efficient control of the central frequency and bandwidth of frequency combs. We experimentally achieve a 50 GHz frequency shift and threefold bandwidth expansion of an impinging comb, as well as the frequency analogue of various refraction phenomena, including negative refraction and perfect focusing in the frequency domain, both for discrete and continuous incident spectra. Our study paves a promising way towards versatile frequency management for optical communications and signal processing using time modulation schemes.

Tomographic polarization analyzer by polarization-mode-frequency mapping.

A tomographic polarization analyzer is presented by polarization-mode-frequency mapping. Two orthogonal circularly polarized components of the unknown polarization state of light are converted to two orbital angular momentum (OAM) modes by a q-plate, and then the OAM modes are mapped to two frequencies by using time-varying spatial modulation. The polarization state of light can be retrieved by tomographic reconstruction of the temporal intensity signal collected by a photodetector. The time-varying spatial modulation can be achieved by either a programmable spatial device or a spinning object. Our method can directly measure the Jones matrix of light with high accuracy due to the high-volume time sampling.