Showing papers by "Xinliang Zhang published in 2023"

All-Optical Nonlinear Activation Function Based on Germanium Silicon Hybrid Asymmetric Coupler

Abstract: Nonlinear activation functions are crucial for optical neural networks (ONNs) to achieve more various functions. However, the current nonlinear functions suffer from some dilemma, including high power consumption, high loss, and limited bandwidth. Here, we propose and demonstrate an all-optical implementation of a nonlinear activation function based on germanium silicon hybrid integration. The principle lies in the intrinsic absorption and the carrier-induced refractive index change of germanium in C -band. It has a large operating bandwidth and a response frequency of 70 MHz, with a loss of 4.28 dB and a threshold power of 5.1 mW. Adopting it to the MNIST handwriting data set classification, it shows an improvement in accuracy from 91.6% to 96.8%. This proves that our scheme has great potential for advanced ONN applications.

Engineering Nonlinear Effects of Quantum-Well Semiconductor Optical Amplifiers for Applications in Optical Signal Processing

Abstract: Quantum-well semiconductor optical amplifiers (QW-SOAs) have been widely used in all-optical signal processing functions because of their various nonlinear effects. For different optical signal processing functions, the requirements for performance of the QW-SOAs are different, and nonlinear effects of the QW-SOAs should be controlled selectively. Engineering nonlinear effects of QW-SOA is very important for different applications in optical signal processing. Here, a comprehensive QW-SOA model by combining the QW band structure calculation with the dynamic model is established to connect the structure parameters with the nonlinear effects of QW-SOA. Those characterized parameters of the QW-SOAs, such as gain coefficient, differential gain coefficient, refractive index change, differential refractive index change, linewidth enhancement factor, third-order susceptibility and polarization dependent gain, are used to characterize the nonlinear effects, and are calculated with different structure parameters based on this developed model. Take the wavelength conversion based on cross gain modulation effect and four wave mixing effect as examples, the structure parameters of the QW-SOAs are optimized for engineering the nonlinear effects and achieving the best output performance. This theoretical work presents a guide for choosing the optimized structure parameters while fabricating the QW-SOAs for different optical signal processing functions.

Parity–Time-Symmetric Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

Abstract: Parity–time (PT) symmetry has been intensively exploited in optoelectronic oscillators (OEOs), enabling mode selection in mutually coupled feedback loops. However, two mutually coupled feedback loops are usually established to achieve a spatial PT-symmetric architecture, which increases system complexity and instability. Here, we propose and experimentally prove a PT-symmetric OEO based on stimulated Brillouin scattering (SBS). The PT symmetry is implemented in an overlapping spatial loop in which the gain and loss are realized based on SBS gain and SBS loss, respectively. By adjusting the pump power and the polarization states of the probe and pump lights, the gain and loss can be balanced, thus achieving stable single-mode oscillation. In experiment, a 9.66 GHz microwave signal with a side-mode suppression ratio of 41 dB is generated. At 10 kHz offset frequency, the single-sideband phase noise reaches as low as −120.1 dBc/Hz. This demonstration opens new avenues to enhance mode selectivity in an OEO and can potentially be integrated on a chip.

High-performance waveguide coupled Germanium-on-silicon single-photon avalanche diode with independently controllable absorption and multiplication

Abstract: Abstract Germanium-on-silicon (Ge-on-Si) single photon avalanche diodes (SPADs) have received wide attention in recent years due to their potential to be integrated with Si photonics. In this work, we propose and demonstrate a high-performance waveguide coupled Ge-on-Si separate-absorption-charge-multiplication SPAD with three electric terminals. By providing two separate voltage drops on the light absorption and multiplication regions, the drift and multiplication of carriers can be optimized separately. This indeed improves the freedom of voltage regulation for both areas. Moreover, thanks to the separate controlling, doping profile of the charge layer is greatly released compared to that of the conventional device because of the flexible carrier injection. In this scenario, the dark counts of the detector can be largely reduced through decreasing the electric field on the sidewalls of the Ge absorption region without affecting the detection efficiency. The proposed SPAD exhibits a high on-chip single photon detection efficiency of 34.62% and low dark count rates of 279 kHz at 1310 nm with the temperature of 78 K. The noise equivalent power is as low as 3.27 × 10−16 WHz−1/2, which is, to the best of our knowledge, the lowest of that of the reported waveguide coupled Ge-on-Si SPADs. This three-terminal SPAD enables high-yield fabrication and provides robust performance in operation, showing a wide application prospect in applications such as on-chip quantum communication and lidar.

Laser‐Printed Terahertz Plasmonic Phase‐Change Metasurfaces

Abstract: Chalcogenide phase‐change materials (PCMs) have offered an appealing material solution by acting as a switchable dielectric layer to tune the electromagnetic properties of terahertz metamaterials and metasurfaces. Here, this work demonstrates large‐scale and lithography‐free manufacturing of all‐PCM terahertz metasurfaces based on direct laser switching of crystalline micro‐domains in a thin film with high switching ratio of the emerging plasmonic PCM, In3SbTe2 (IST). The fabricated high‐quality IST metasurfaces achieve efficient plasmonic resonances and a large modulation depth with ultrafast response (full width at half maxima of the modulation time ≈1.6 ps) in a deep‐subwavelength switching volume. For the dynamic evolution of terahertz resonance modes, theoretical modeling reveals a delicate interplay between amorphous and crystalline IST due to the bonding‐structure‐induced different carrier lifetimes and spatially localized electric fields. These studies open new avenues for realizing all‐PCM terahertz ultrafast nanophotonics.

Dual-band achromatic metalens-assisted grating couplers for wavelength demultiplexing.

Abstract: The design of grating couplers (GCs) that can (de)multiplex and couple arbitrarily defined spatial light into photonic devices is crucial for miniaturized integrated chips. However, traditional GCs have a limited optical bandwidth due to their wavelength's dependency on the coupling angle. In this paper, we propose a device that addresses this limitation by combining a dual-broadband achromatic metalens (ML) with two focusing GCs. By controlling the frequency dispersion, the waveguide-mode-based ML achieves excellent dual-broadband achromatic convergence and separates broadband spatial light into opposing directions at normal incidence. The focused and separated light field matches the grating diffractive mode field and is then coupled into two waveguides by the GCs. This ML-assisted GCs device exhibits a good broadband property with -3 dB bandwidths of 80 nm at 1.31 µm (CE ∼ -6 dB) and 85 nm at 1.51 µm (CE ∼ -5 dB), which almost covers the entire designed working bands, representing an improvement over traditional spatial light-GC coupling. This device can be integrated into optical transceivers and dual-band photodetectors to enhance the bandwidth of wavelength (de)multiplexing.

Real-time Fourier-domain optical vector oscilloscope

Abstract: To meet the constant demands of high-capacity telecommunications infrastructure, data rates beyond 1 terabit per second per wavelength channel and optical multiplexing are widely applied. However, these features pose challenges for existing data acquisition and optical performance monitoring techniques because of bandwidth limitation and signal synchronization. We designed an approach that would address these limitations by optically converting the frequency limit to an unlimited time axis and combining this with a chirped coherent detection to innovatively obtain the full-field spectrum. With this approach, we demonstrated a real-time Fourier-domain optical vector oscilloscope, with a 3.4-terahertz bandwidth and a 280-femtosecond temporal resolution over a 520-picosecond record length. In addition to on-off keying and binary phase-shift keying signals (128 gigabits per second), quadrature phase-shift keying wavelength division–multiplexed signals (4 × 160 gigabits per second) are simultaneously observed. Moreover, we successfully demonstrate some high-precision measurements, which indicate them as a promising scientific and industrial tool in high-speed optical communication and ultrafast optical measurement.

On-chip tunable parity‐time symmetric optoelectronic oscillator

Abstract: Abstract. Parity‐time (PT) symmetry breaking offers mode selection capability for facilitating single‐mode oscillation in the optoelectronic oscillator (OEO) loop. However, most OEO implementations depend on discrete devices, which impedes proliferation due to size, weight, power consumption, and cost. In this work, we propose and experimentally demonstrate an on-chip tunable PT‐symmetric OEO. A tunable microwave photonic filter, a PT‐symmetric mode‐selective architecture, and two photodetectors are integrated on a silicon‐on‐insulator chip. By exploiting an on‐chip Mach–Zehnder interferometer to match the gain and loss of two mutually coupled optoelectronic loops, single‐mode oscillation can be obtained. In the experiment, the oscillation frequency of the on-chip tunable PT‐symmetric OEO can be tuned from 0 to 20 GHz. To emulate the integrated case, the OEO loop length is minimized, and no extra-long fiber is used in the experiment. When the oscillation frequency is 13.67 GHz, the single‐sideband phase noise at 10-kHz offset frequency is −80.96 dBc / Hz and the side mode suppression ratio is 46 dB. The proposed on-chip tunable PT‐symmetric OEO significantly reduces the footprint of the system and enhances mode selection.

High Performance Broadband Full Linear Polarimeter based on ReS 2 Nanobelts

Abstract: Polarization-sensitive photodetectors can simultaneously sense the polarization states and intensity of light. Traditional polarimeters integrate photodetectors with complex and bulky optical systems such as polarizers and waveplates, which can not meet the current demand of on-chip polarization photonic devices. This work has demonstrated a full linear polarization polarimeter based on two independent ReS2 nanobelt devices vertically stacked with a designed twist angle. The device achieves a high responsivity (959 A W−1) without applying an external gate voltage. In addition, the ReS2 nanobelt photodetector displays a strong polarization-sensitive photoresponse with a linear dichroic ratio of 1.72 at 665 nm. By utilizing those two vertically stacked nanobelt devices, the linear polarization states of the incident light can be distinguished in a wide range of wavelengths from 590 to 800 nm. This study provides a simple route for future polarization-sensitive optoelectronic devices.

ANN-Assisted High-Resolution and Large Dynamic Range Temperature Sensor Based on Simultaneous Microwave Photonic and Optical Measurements

Abstract: We proposed and demonstrated a temperature sensor with both high resolution and large dynamic range based on simultaneous microwave photonic and optical measurements. To simultaneously increase the sensitivity and dynamic range of optical measurement, two cascaded ring resonators (CRRs) with different temperature sensitivities and free spectral ranges (FSRs) are designed as the sensing probe and fabricated based on silicon-on-insulator (SOI) wafer. Based on the ultrahigh-Q-factor microring in the CRR, a microwave photonic notch filter (MPNF) is obtained and used to achieve a high temperature resolution. Meanwhile, the optical transmission spectrum of the CRR is used to achieve a large dynamic range. Combining the two measurements, a temperature sensor with both high resolution and large dynamic range is realized. To improve the response speed and robustness of the sensing system, the artificial neural network (ANN) is used to directly extract the temperature from the CRR spectrum, which is particularly helpful at low signal-to-noise ratio (SNR). The sensing resolution and dynamic range achieved by our scheme are $7.02 \times 10^{-3}$ °C and 106.94 °C, respectively.

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

Abstract: 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.

Source-configured symmetry-broken hyperbolic polaritons

Abstract: Abstract Polaritons are quasi-particles that combine light with matter, enabling precise control of light at deep subwavelength scales. The excitation and propagation of polaritons are closely linked to the structural symmetries of the host materials, resulting in symmetrical polariton propagation in high-symmetry materials. However, in low-symmetry crystals, symmetry-broken polaritons exist, exhibiting enhanced directionality of polariton propagation for nanoscale light manipulation and steering. Here, we theoretically propose and experimentally demonstrate the existence of symmetry-broken polaritons, with hyperbolic dispersion, in a high-symmetry crystal. We show that an optical disk-antenna positioned on the crystal surface can act as an in-plane polarized excitation source, enabling dynamic tailoring of the asymmetry of hyperbolic polariton propagation in the high-symmetry crystal over a broad frequency range. Additionally, we provide an intuitive analysis model that predicts the condition under which the asymmetric polaritonic behavior is maximized, which is corroborated by our simulations and experiments. Our results demonstrate that the directionality of polariton propagation can be conveniently configured, independent of the structure symmetry of crystals, providing a tuning knob for the polaritonic response and in-plane anisotropy in nanophotonic applications.

Photonic generation of phase-coded linearly frequency modulation signals with increased TBWP

Abstract: As a new pulse compression signal, the phase-coded linearly frequency modulated (LFM) signal is proposed to faciliate multi-purpose radars. Here, we propose and demonstrate a phase-coded LFM signal generator, whose center frequency and bandwidth can be both adjusted. The phase-coded LFM signal generator is realized by feeding a phase-modulated optical signal into an optoelectronic oscillator (OEO). When the Fourier domain mode-locking is achieved, a phasecoded LFM signal is successfully generated. The center frequency and bandwidth of the generated microwave signal can be adjusted by changing the wavelength of the tunable laser source (TLS) and the peak-to-peak voltage of the driving signal, respectively. Experiment results show that a phase-coded LFM microwave signal with a center frequency of 9.5 GHz and a bandwidth of 5 GHz is successfully generated, whose TBWP is 5.6×105. To demonstrate the flexibility of the proposed scheme, the center frequency and bandwidth of the generated phase-coded LFM signal are adjusted to 12.4 and 2.6 GHz, respectively. The signal generator can be used in pulse compression radar in the future.

Large group delay and low loss optical delay line based on chirped waveguide Bragg gratings.

Abstract: On-chip optical delay lines (ODLs) based on chirped waveguide Bragg gratings (CWBG) have attracted much attention in recent years. Although CWBGs are well developed, the CWBG which have large group delay (GD), large delay-bandwidth product and low loss while is circulator-free have little been investigated so far. In this work, we propose and experimentally demonstrate such a CWBG-based ODL. This device is fabricated on a low-loss 800-nm-height silicon nitride platform, combining 20.11-cm long index-chirped multi-mode spiral waveguide antisymmetric Bragg gratings with a directional coupler. The bandwidth of this circulator-free ODL is 23 nm. The total GD is 2864 ps and the delay-bandwidth product is 65.87 ns·nm, which both are the largest values achieved by on-chip CWBG reported to our knowledge. Its loss is 1.57 dB/ns and the total insertion loss of the device is 6 dB at the central wavelength near 1550 nm. This integrated CWBG can be explored in practical applications including microwave photonics, temporal optics, and optical communication.

Multi-port real-time observation for ultrafast intracavitary evolution dynamics

Abstract: Abstract Recent advances in real-time spectral measurements of a mode-locked fiber laser have found many intriguing phenomena and which have verified the soliton theory. However, most current results are based on laser single-port observation, and are rarely involved in the cavity evolution, which also has rich nonlinear dynamics according to the soliton theory. Here we present an approach for the intracavitary soliton evolution processes, where spectra from multi-ports are collected in time-division multiplexed sequence to realize synchronous real-time observation. The sinusoidal evolution of the spectral beating is observed clearly, agreeing with the reported prediction. Furthermore, the intracavitary spectral dynamics of the period-doubling bifurcation are also revealed. Our scheme observed the spectral expanding and shrinking alternately and periodically over two round trips, matching well with simulations. This work may open up possibilities for real-time observation of various intracavitary nonlinear dynamics in photonic systems.

Generation of all-fiber third-order orbital angular momentum modes based on femtosecond laser irradiation

Abstract: In this article, a long period fiber grating (LPFG) based on femtosecond laser irradiation is proposed to generate the third order Orbital Angular Momentum (OAM) mode. With the advantage of lower heat absorption efficiency, lower power and small focused spot size，this method is more reliable ,repeatable and promising compared with the traditional method based on carbon dioxide laser. The experiments results demonstrated that the fundamental mode can be converted to the linear polarization mode LP31 mode even OAM±3 modes with 98% mode conversion efficiency near 1550nm. The proposed method has the potential to generate higher order mode.

BP neural network enables 2bits precision improvement for microring weight bank

Abstract: We experimentally demonstrate a method for predicting and compensating the system error of a 3×3 on-chip microring resonator (MRR) weight bank using a pre-designed back propagation (BP) neural network. The system error can be quickly predicted and well compensated to improve the computation precision of optical matrix-vector multiplication (MVM), without bulky experimental devices. The results show that the computation precision can be increased from 5- bit to 7-bit. This work provides a weight accuracy improvement method for MRR-based photonic integrated chips for applications such as optical computing and optical communication.

An Efficient, Fast‐Responding, Low‐Loss Thermo‐Optic Phase Shifter Based on a Hydrogen‐Doped Indium Oxide Microheater

Abstract: Thermo-optic (TO) phase shifters are very fundamental units in large-scale active silicon photonic integrated circuits (PICs). However, due to the limitation of microheater materials with a trade-off between heating efficiency and absorption loss, designs reported so far typically suffer from slow response time, high power consumption, low yields, and so on. Here, an energy-efficient, fast-responding, low-loss TO phase shifter is demonstrated by introducing hydrogen-doped indium oxide (IHO) films as the microheater, and the optimized electron concentration with enhanced mobility endows the IHO with high conductivity as well as high near-infrared (NIR) transparency, which allow it to directly contact the silicon waveguide without any insulating layer for efficient tuning and fast response. The TO phase shifter achieves a sub-microsecond response time (970 ns/980 ns) with a π phase shift power consumption of 9.6 mW, and the insertion loss introduced by the IHO microheater is ≈0.5 dB. The proposed IHO-based microheaters with compatible processing technology illustrate the great potential of such material in the application of large-scale silicon PICs.

Large-Scale Reconfigurable Integrated Circuits for Wideband Analog Photonic Computing

Abstract: Photonic integrated circuits (PICs) have been a research hotspot in recent years. Programmable PICs that have the advantages of versatility and reconfigurability that can realize multiple functions through a common structure have been especially popular. Leveraging on-chip couplers and phase shifters, general-purpose waveguide meshes connected in different topologies can be manipulated at run-time and support a variety of applications. However, current waveguide meshes suffer from relatively a low cell amount and limited bandwidth. Here, we demonstrate a reconfigurable photonic integrated computing chip based on a quadrilateral topology network, where typical analog computing functions, including temporal differentiation, integration, and Hilbert transformation, are implemented with a processing bandwidth of up to 40 GHz. By configuring an optical path and changing the splitting ratio of the optical switches in the network, the functions can be switched and the operation order can be tuned. This approach enables wideband analog computing of large-scale PICs in a cost-effective, ultra-compact architecture.

Compact microring resonator based on ultralow-loss multimode silicon nitride waveguide

Abstract: Silicon nitride (Si3N4) waveguides with high confinement and low loss have been widely used in integrated nonlinear photonics. Indeed, state-of-the-art ultralow-loss Si3N4 waveguides are all fabricated using complex fabrication processes, and all of those reported that high Q microring resonators (MRRs) are fabricated in laboratories. We propose and demonstrate an ultralow-loss Si3N4 racetrack MRR by shaping the mode using a uniform multimode structure to reduce its overlap with the waveguide. The MRR is fabricated by the standard multi project wafer (MPW) foundry process. It consists of two multimode straight waveguides (MSWs) connected by two multimode waveguide bends (MWBs). In particular, the MWBs are based on modified Euler bends, and an MSW directional coupler is used to avoid higher-order mode excitation. In this way, although a multimode waveguide is used in the MRR, only the fundamental mode is excited and transmitted with ultralow loss. Meanwhile, thanks to the 180 deg Euler bend, a compact chip footprint of 2.226 mm perimeter with an effective radius as small as 195 μm and a waveguide width of 3 μm is achieved. Results show that based on the widely used MPW process, a propagation loss of only 3.3 dB / m and a mean intrinsic Q of around 10.8 million are achieved for the first time.

Chip-to-chip optical multimode communication with universal mode processors

Abstract: The increasing amount of data exchange requires higher-capacity optical communication links. Mode division multiplexing (MDM) is considered as a promising technology to support the higher data throughput. In an MDM system, the mode generator and sorter are the backbone. However, most of the current schemes lack the programmability and universality, which makes the MDM link susceptible to the mode crosstalk and environmental disturbances. In this paper, we propose an intelligent multimode optical communication link using universal mode processing (generation and sorting) chips. The mode processor consists of a programmable 4*4 Mach Zehnder interferometer (MZI) network and can be intelligently configured to generate or sort both quasi linearly polarized (LP) modes and orbital angular momentum (OAM) modes in any desired routing state. We experimentally establish a chip-to-chip MDM communication system. The mode basis can be freely switched between four LP modes and four OAM modes. We also demonstrate the multimode optical communication capability at a data rate of 25 Gbit/s. The proposed scheme shows significant advantages in terms of universality, intelligence, programmability and resistance to mode crosstalk, environmental disturbances and fabrication errors, demonstrating that the MZI-based reconfigurable mode processor chip has great potential in long-distance chip-to-chip multimode optical communication systems.

Performing photonic nonlinear computations by linear operations in a high-dimensional space

Abstract: Abstract As photonic linear computations are diverse and easy to realize while photonic nonlinear computations are relatively limited and difficult, we propose a novel way to perform photonic nonlinear computations by linear operations in a high-dimensional space, which can achieve many nonlinear functions different from existing optical methods. As a practical application, the arbitrary binary nonlinear computations between two Boolean signals are demonstrated to implement a programmable logic array. In the experiment, by programming the high-dimensional photonic matrix multiplier, we execute fourteen different logic operations with only one fixed nonlinear operation. Then the combined logic functions of half-adder and comparator are demonstrated at 10 Gbit/s. Compared with current methods, the proposed scheme simplifies the devices and the nonlinear operations for programmable logic computing. More importantly, nonlinear realization assisted by space transformation offers a new solution for optical digital computing and enriches the diversity of photonic nonlinear computing.

Optoelectronic oscillator based on SBS-assisted parity-time symmetry

Abstract: A parity-time symmetric optoelectronic oscillator is constructed based on stimulated Brillouin scattering. A stable microwave signal at 9.66 GHz is generated with a phase noise of −103.9 dBc/Hz at an offset frequency of 10 kHz.

Broadband hyperbolic thermal metasurfaces based on the plasmonic phase-change material In3SbTe2.

Abstract: Thermal radiation modulation facilitated by phase change materials (PCMs) needs a large thermal radiation contrast in broadband as well as in a non-volatile phase transition, which are only partially satisfied by conventional PCMs. In contrast, the emerging plasmonic PCM In3SbTe2 (IST) that undergoes a non-volatile dielectric-to-metal phase transition during crystallization offers a fitting solution. Here, we have prepared IST-based hyperbolic thermal metasurfaces and demonstrated their capabilities to modulate thermal radiation. By laser-printing crystalline IST gratings with different fill factors on amorphous IST films, we have achieved multilevel, large-range, and polarization-dependent control of the emissivity modulation (0.07 for the crystalline phase and 0.73 for the amorphous phase) over a broad bandwidth (8-14 μm). With the convenient direct laser writing technique that supports large-scale surface patterning, we have also demonstrated promising applications of thermal anti-counterfeiting with hyperbolic thermal metasurfaces.

Thermo-optic phase shifter based on hydrogen-doped indium oxide microheater

Abstract: Thermo-optic (TO) phase shifters are very fundamental units in large-scale active silicon photonic integrated circuits (PICs). However, due to the limitation of microheater materials with a trade-off between heating efficiency and absorption loss, designs reported so far typically suffer from slow response time, high power consumption, low yields, and so on. Here, we demonstrate an energy-efficient, fast-response, and low-loss TO phase shifter by introducing hydrogen-doped indium oxide (IHO) films as microheater, and the optimized electron concentration with enhanced mobility endows the IHO high conductivity as well as high nearinfrared (NIR) transparency, which allow it to directly contact the silicon waveguide without any insulating layer for efficient tuning and fast response. The TO phase shifter achieves a submicrosecond response time (970 ns/980 ns) with a π phase shift power consumption of 9.6 mW. And the insertion loss introduced by the IHO microheater is ~ 0.5 dB. The proposed IHO-based microheaters with compatible processing technology illustrate the great potential of such material in the application of large-scale silicon PICs.

Analytic theory for parametric amplification in high-Q micro-ring resonators

Abstract: Optical parametric amplifiers (OPAs) are optical amplifiers based on four-wave mixing processes, showing great potential for applications in communications, optical signal processing, quantum optics, etc. In recent years, significant progress has been made to integrated OPAs based on highly nonlinear micro-ring resonators (MRRs), benefiting from the greatly enhanced optical-matter interactions. Notable parametric gain becomes available at unprecedented low power levels, allowing for example the source-integrated coherent optical frequency combs. Therefore, an analytical formula for OPAs in high-quality (Q) MRRs is of great importance in the design of optical parametric devices. Analytical theory for OPAs in high-Q MRRs in the high pump power scenario remains elusive, where intensity-dependent nonlinear phase that brings significant parametric gains cannot be ignored. In this work, analytical formulas for the parametric gain and conversion efficiency (CE) of a high-Q MRR with high-pump excitation are derived. We show the interplay between parametric gain and field enhancement: the field enhancement of the signal and idler wave can be greatly boosted due to the compensation of the round-trip loss by the parametric gain, which in turn leads to increased field enhancement as well as greater parametric gain and CE. Our theory agrees well with numerical and experimental results.

Generation of phase-coded LFM signals based on Fourier domain mode-locked optoelectronic oscillator

Abstract: Phase-coded linearly frequency modulated (LFM) signals play essential roles in enhancing the performance of radar as well as exploring wireless communication. To generate phase-coded LFM signals, we propose to launch a phase-coded signal into an optoelectronic oscillator (OEO). When the OEO is Fourier domain mode-locked (FDML), a phase-coded LFM signal is generated. The center frequency, bandwidth, coding pattern, and bit rate of the generated signals can be flexibly adjusted. In the experiment, a phase-coded LFM microwave signal with a bandwidth of 5.0 GHz is generated, corresponding to a time bandwidth product (TBWP) of 2.8×10 ⁵ . The center frequency and bandwidth of the generated signals are adjusted from 9.7 to 10.7 GHz and from 5.0 to 3.0 GHz, respectively. Meanwhile, different coding patterns and rates are also implemented, indicating that the phase-coded LFM signal can be used in wireless communications. The proposed approach can potentially be applied to multifunctional integration in radar.