Optical Carrier Research Articles

Experimental demonstration of coherent photonic neural computing based on a Fabry–Perot laser with a saturable absorber

As Moore’s law has reached its limits, it is becoming increasingly difficult for traditional computing architectures to meet the demands of continued growth in computing power. Photonic neural computing has become a promising approach to overcome the von Neuman bottleneck. However, while photonic neural networks are good at linear computing, it is difficult to achieve nonlinear computing. Here, we propose and experimentally demonstrate a coherent photonic spiking neural network consisting of Mach–Zehnder modulators (MZMs) as the synapse and an integrated quantum-well Fabry–Perot laser with a saturable absorber (FP-SA) as the photonic spiking neuron. Both linear computation and nonlinear computation are realized in the experiment. In such a coherent architecture, two presynaptic signals are modulated and weighted with two intensity modulation MZMs through the same optical carrier. The nonlinear neuron-like dynamics including temporal integration, threshold, and refractory period are successfully demonstrated. Besides, the effects of frequency detuning on the nonlinear neuron-like dynamics are also explored, and the frequency detuning condition is revealed. The proposed hardware architecture plays a foundational role in constructing a large-scale coherent photonic spiking neural network.

Analog stabilization of an electro-optic I/Q modulator with an auxiliary modulation tone.

Proper operation of electro-optic I/Q modulators relies on precise adjustment and control of the relative phase biases between the modulator's internal interferometer arms. We present an all-analog phase bias locking scheme where error signals are obtained from the beat between the optical carrier and optical tones generated by an auxiliary 2MHz R F tone to lock the phases of all three involved interferometers for operation up to 10GHz. With the developed method, we demonstrate an I/Q modulator in carrier-suppressed single-sideband mode, where the suppressed carrier and sideband are locked at optical power levels <-27d B relative to the transmitted sideband. We describe a simple analytical model for calculating the error signals and detail the implementation of the electronic circuitry for the implementation of the method.

Filter-Less Photonic RF Image-Reject Mixing With Simultaneous Self-Interference Cancellation and Fiber Transmission

A filter-free photonic method to simultaneously realize radio frequency (RF) image-reject mixing, self-interference cancellation (SIC), and fiber transmission for in-band full-duplex (IBFD) radio-over-fiber (RoF) link is reported. The key to achieving multi-functions is the joint manipulation of fiber dispersion-induced intermediate frequency (IF) amplitude and time delay changes, which is implemented by sideband phase-shifted optical carrier suppression (SPS-OCS) modulation and frequency tuning of the optical carrier. Owing to the control of IF amplitude and delay variation introduced by fiber dispersion, simultaneous deep image rejection and SIC over wide bandwidth can be realized after fiber transmission. Since no optical filter is required, the proposed system can work at a lower frequency, featuring a large frequency tuning range. For 25 km single-mode fiber (SMF) transmission, simultaneous 20 dB SIC within 1 GHz bandwidth and 29 dB image rejection ratio (IRR) at 6.6 GHz RF frequency in the C band, 21 dB SIC and 19 dB IRR within 1 GHz bandwidth in the X band, 24 dB SIC within 1 GHz bandwidth and 20 dB IRR at 16 GHz RF frequency in the Ku band are experimentally demonstrated. Furthermore, weak SOI with various modulation formats are well recovered after the cancellation of strong interference and fiber transmission.

Microwave Photonic Crystal Video Receiver for Radar Interception

This article introduces an approach of a crystal video receiver (CVR) based on microwave photonics (MWP) to detect pulsed radar signals. The pulsed radiofrequency (RF) envelope detection is achieved using the optical self-homodyne technique on the photodetection process. The proposed architecture uses a phase modulator (PM) to modulate the optical carrier and a fiber Bragg grating (FBG) to filter the spectrum to convert phase to intensity modulation. The self-homodyne technique eliminates the optical carrier at photodetector to reduce the shot noise and saturation on it. Moreover, the photodetection is performed at the radar pulse baseband, allowing to obtain transimpedance gain. The theoretical principle and an experimental demonstration are presented. A CVR sensitivity of 53 V/mW and a tangential signal sensitivity (TSS) equal to −40 dBm is achieved for a pulsed radar operating at 17 GHz.

Generating Loss Dependent Bifid Dips of Optical Carrier Based Microwave With Self-Feedback Anti-Symmetric Coupled Fiber Ring Resonator

A self-feedback anti-symmetric coupled fiber ring resonator (SACFRR) is proposed and demonstrated to generate the distinctive loss dependent bifid dips. The universal derivation of the ring resonators in both lightwave and optical carrier based microwave domains considering different conditions including attenuation, coupling and self-feedback is systematically elaborated, which is applicable to diverse ring resonator configurations. The lightwave and optical carrier based microwave experience different phase evolution during the cross-coupling procedure, thereby causing very different output. The anti-symmetric coupling strategy is creatively introduced into the ring resonator, which dominates the splitting of the original resonance dip for the optical carrier based microwave interferometry (OCMI). And OCMI shows the noticeable bifid dips within the lower envelope which receives less attention before. By performing the measurement of temperature and bend-induced loss, the bifid dips based demodulation of OCMI is employed to present the property of SACFRR and compared with the conventional upper envelope based demodulation, contributing to the utilization of lower envelope information. The temperature sensitivity is 521.4 kHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C at 3.05 GHz and the bend-induced loss sensitivity is 24.845 MHz/turn (diameter of 2.30 cm) at 10 turns where is adjacent to the critical point of bifid dips merging. The proposed scheme provides an extraordinary design avenue which can be extended to diverse ring resonators in both photonic integrated circuits and fiber links for exploring compelling functions.

Propagation of vortex symmetric Airy beam in the turbulent link

The vortex symmetric Airy beam (VSAB) which couples the autofocusing symmetric Airy beam (SAB) and topological charge can provide new possibilities for the optical carrier of free space optical communication (FSOC). This paper focuses on the performance of VSAB propagating through turbulent link. Based on the wave optics simulation, the model of VSAB propagating in atmospheric turbulence is constructed. By using the model, the evaluation parameters light intensity and aperture averaged scintillation of VSAB are calculated. The influence of different beam parameters on the characteristics of VSAB propagation in turbulent link is investigated. The results show that the VSAB with small decaying factor, small transverse length scales and less topological charge has more advantage in turbulence resistance.

Fermi surface anisotropy in plasmonic metals increases the potential for efficient hot carrier extraction

Realizing the potential of plasmonic hot carrier harvesting for energy conversion and photodetection requires new materials that resolve the bottleneck of extracting carriers prior to energy relaxation within the metal. Using first-principles calculations of optical response and carrier transport properties, we show that directional conductors with Fermi velocities restricted predominantly to one or two directions present significant advantages for efficient hot carrier harvesting. We show that the optical response of film-like conductors, PtCoO$_2$ and Cr$_2$AlC, resemble that of 2D metals, while that of wire-like conductors, CoSn and YCo$_3$B$_2$, resemble that of 1D metals, which can lead to high mode confinement and efficient light collection in small dimensions, while still working with 3D materials with high carrier densities. Carrier lifetimes and transport distances in these materials, especially in PtCoO$_2$ and CoSn, are competitive with noble metals. Most importantly, we predict that carrier injection efficiency from all of these materials into semiconductors can exceed 10% due to the small component of carrier velocity parallel to the metal surface, substantially improving upon the typical less than 0.1% injection efficiency from noble metals into semiconductors.

Stable optical and radio frequency joint transfer based on a passive phase compensation.

We propose a novel scheme that uses only a single passive phase compensation device to achieve stable optical and radio frequency joint transfer. The phase noises of optical and radio frequency can be simultaneously compensated by passively embedding their phase information on the two optical carrier sidebands generated by an electro-optical modulator without using the phase discrimination and active servo controller. As a result, this scheme has many advantages, such as high spectral purity, short settling time and infinite compensation accuracy. We experimentally demonstrate the joint transfer of optical and 1 GHz RF over 120 km fiber spools. The optical frequency stability achieves 6.9 × 10-17 at 1 s and 7.03 × 10-19 at 10000 s, while the 1 GHz RF is 6.47 × 10-13 at 1 s and 3.96 × 10-16 at 10000 s.

Stimulated Brillouin Scattering Based Image-Reject Microwave Signal Harmonic Down-Converter

We propose and demonstrate a photonic microwave signal harmonic down-converter with image rejection capability via stimulated Brillouin scattering (SBS). A dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM) is driven by a radio frequency/image (RF/IM) signal and a local oscillation (LO) signal to realize equivalent phase modulation and 4th-order double sideband modulation respectively. The optical carriers from the two arms of the DP-DPMZM can cancel out each other after polarization combining, thereby increasing the useful optical signal power in photodetection. Since the beating notes between the IM-modulated signals and LO-modulated 4th-order sidebands interfere destructively, the down-converted IM signals disappear naturally, which makes the down-converted image parts be suppressed in optical domain. By using an SBS-based dual-band microwave photonic filter to suppress the RF modulated -1st-order sideband and amplify the +1st-order sideband, destructive interference can be broken. Therefore, the RF signal over frequency range of 21-28 GHz can be down-converted to intermediate frequency (IF) band, and the image rejection ratio (IRR) can be also improved to more than 55 dB. Under -160 dBm/Hz noise floor, the SFDR is 105.1 dB·Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> .

Implementing a Single Gate Heterostructure Tunnel FET as a Low-Power Photosensor with Improved Sensitivity

This work investigates the effectiveness of a single gate SiGe/Si heterostructure tunnel field-effect transistor (SGHTFET) as a photosensor in the visible spectrum. A transparent zinc oxide (ZnO) layer is used as the optical region over the channel for sensing the incident light. When light impinges on the gate catalyst and creates optical charge carriers in the illumination region, the conductance of the device considerably rises and, consequently, the subthreshold current changes. For the suggested photosensor, the effect of varying drain-to-source voltage (VDS), germanium mole fraction (x), and silicon film thickness (tsi) on the sensor performance are investigated. The sensor offers enhanced sensitivity performance as compared to the traditional TFET in terms of several optical figures of merit (FOMs) such as available photocurrent, responsivity, quantum efficiency, sensitivity, and SNR, and can therefore be utilized as an efficient photosensor. The reported sensor has a peak responsivity (R) of 2.23 A W−1 and quantum efficiency (η) of 7.31 at a wavelength (λ) of 450 nm.

Photonic Integrated Circuit for Optical Phase Control of 1 × 4 Terahertz Phased Arrays

In this manuscript, we report on a 1 × 4 optical beam forming network (OBFN) chip using optical phase shifters (OPSs) based on thermo-optically controlled optical ring resonators (ORRs) for 1D beam steering at 0.3 THz. The 1 × 4 OBFN chip consists of four OPSs and is fabricated using TriPleX technology. Each of the four OPSs is realized by two cascaded identical ORRs, to reach a phase shift of 2π. To allow transfer of the optical phase shift to the THz domain by optical heterodyning in high-frequency 1.55 µm modified uni-travelling carrier photodiodes, the ORRs are designed such that one carrier of the optical heterodyne signal is at the ORR’s resonance frequency, whereas the second optical heterodyne signal is at its off-resonance. By adjusting the resonance frequencies of the two ORRs in each OPS synchronously, a relative phase variation between two optical heterodyne carriers of up to 2π with a tuning efficiency of 0.058 W/π, is experimentally demonstrated. Due to the dispersive power transmission loss of the ORRs, phase tuning leads to a power variation of the optical heterodyne-generated signals up to 3.8 dB, which is experimentally characterized at 0.295 THz. It is shown numerically that this power variation only has a minor impact on the steering performance of a 1 × 4 phased array. The determined beam direction deviation and maximum absolute radiation power change are smaller than 1° and 2 dB, respectively. By sweeping the phase difference between two adjacent THz antennas in the 1 × 4 phased array, from −120° to 120°, a beam steering range of ~62° is demonstrated numerically at 0.295 THz.

Stable secure key distribution scheme via orthogonal polarizations and a joint source-channel model.

Optical secure key distribution (SKD) based on reciprocity has been the subject of increasing discussion, for its inherent information-theoretic safety and because there is less occupation of fiber channels. The combination of reciprocal polarization and broadband entropy sources has proven effective in increasing the rate of SKD. However, the stabilization of such systems suffers from the limited span of polarization states and inconsistent polarization detection. The specific causes are analyzed in principle. To solve this issue, we propose a strategy for extracting secure keys from orthogonal polarizations. Optical carriers with orthogonal polarizations at interactive parties are modulated by external random signals using polarization division multiplexing dual-parallel Mach-Zehnder modulators. After bidirectional transmission through a 10-km fiber channel, error-free SKD with a rate of 2.07 Gbit/s is experimentally realized. The high correlation coefficient of the extracted analog vectors can be maintained for over 30 min. The proposed method is a step toward the development of secure communication with high speed and feasibility.

Resolving Intrinsic Modulation Spectral Lines from Electro-Optic Modulation Spectra Based on Boosted Iterative Deconvolution

Optical spectrum analysis is the most direct and effective method for characterizing electro-optic modulation spectra. According to the Rayleigh criterion, the best resolution of an optical spectrum analysis is limited by the point spread function of an optical spectrum analyzer (OSA) and the relative intensity of closely spaced spectral lines. In this paper, we propose a boosted iterative deconvolution (BID) method to resolve the intrinsic modulation of spectral lines from the measured optical carrier and modulation spectra. In our scheme, the electro-optic modulation spectrum is considered the convolution of the optical carrier spectrum and the intrinsic δ-function modulation spectrum, and the BID method enables fast and accurate extraction of the δ-function spectral lines from the measured modulation spectrum. The proof-of-concept experiment demonstrates that our method can improve the resolution of OSA by 10–30 p.m. at different relative intensities, with a best resolution of 10 p.m. in the iso-intensity case and wavelength errors of less than 2 p.m., which largely improves the measurement resolution and accuracy of the modulation spectrum.

Physically-Secured Ghost Diffraction and Transmission

A new approach to realizing physically-secured ghost diffraction and transmission is proposed in this letter. A series of random 2D arrays of numbers are used as optical information carriers to transmit original data, e.g., analog signals or images as ghosts. Computer-generated magnification factors are applied for optical data encoding, and physically-generated scaling factors are generated with absorptive filters in free-space optical data transmission. The series of computer-generated magnification factors and physically-generated scaling factors serves as security keys, and is explored to realize high-fidelity and high-security free-space optical data (ghost) transmission. It is experimentally demonstrated that the proposed method is feasible and effective in different environments, i.e., without or with scattering media. The proposed physically-secured ghost diffraction scheme offers a new research perspective on secured optical information (e.g., analog signal) transmission in free space.

Filter-free photonics-assisted frequency translator with a tunable phase shift and amplitude based on an integrated DP-QPSK modulator.

In this paper, we propose and demonstrate a novel, to the best of knowledge, filter-free photonics-assisted microwave frequency translator with a tunable phase shift and amplitude. The pivotal component of the proposed scheme is an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator, which is applied to generate a polarization orthogonal carrier-suppressed single sideband modulation signal and frequency shifted optical carrier signal. The polarization-multiplexed optical signal outputs from the DP-QPSK modulator is then sent to a photodetector (PD) via a polarization controller (PC) and a polarizer to implement photoelectric conversion. The electrical signal output from the PD is the desired frequency translated microwave signal, and the amount of frequency shift is determined by the frequency of the sawtooth wave applied to the DP-QPSK modulator. In addition, since the PC can be used to adjust the polarization angle and introduce a phase difference between the two orthogonally polarized optical signals, the phase shift and amplitude of the obtained translated signal can also be easily tuned. A theoretical analysis and simulation experiment are carried out to verify the feasibility of the proposed scheme. The simulation results show that the novel scheme can realize frequency translation with a 360° continuously tunable phase shift and adjustable amplitude for both a single-tone signal and linearly frequency modulated signal with a 50MHz bandwidth. The spurious suppression ratios of the single-tone signal and LFM signal after frequency translation are larger than 48 and 30dB, respectively.

Photonic scheme for broadband frequency response measurement of PDs.

We report a scheme to measure the broadband frequency response of photodetectors (PDs) with the capacity of self-calibration. The optical carrier at f 0 is modulated through one modulator, and this modulator is driven by a microwave signal with a fixed frequency f 1 to produce the carrier-suppressed double sideband optical signals f 0-f 1 and f 0+f 1. The frequency interval of the optical signals is 2f 1. Subsequently, the two optical signals are sent to another modulator driven by a swept microwave signal f m. Two pairs of carrier-suppressed double sideband signals with a high signal-to-noise ratio are generated. The frequencies of these signals are f 0-f 1-f m,f 0-f 1+f m, f 0+f 1-f m, and f 0+f 1+f m. After the PD under test, the frequency response can be extracted from the amplitude of the microwave signals at 2f 1, 2f 1-2f m, and 2f 1+2f m. Two PDs in our laboratory are experimentally characterized from 0.1 to 40GHz with a resolution of 100MHz. Compared with the traditional vector network analyzer swept method, the proposed method extends the measurement range from f m(max) to 2f 1+2f m(max) and has the capacity of self-calibration to eliminate the influence caused by the modulator frequency response on the measurement results.

Longwave IR lattice matched L-valley Ge/GeSiSn waveguide quantum cascade detector.

We propose a lattice-matched Ge/GeSiSn quantum cascade detector (QCD) capable of operating in the longwave infrared. The optical absorption and carrier transport based on intersubband transitions all occur within the L-valley of the conduction band of the group-IV material system using N-doped quantum wells (QWs). The waveguided lattice matched structure can be deposited strain free on top of a Ge buffer grown on Si substrate, and is end-coupled to low-loss on-chip Ge waveguides. We optimized the QCD structure through the analysis of the photoresponsivity and detectivity D*. The QCD operates in photovoltaic mode with narrow spectral response that is peaked anywhere in the 9 to 16 µm range, tunable by design. This work aims to push the optical response of the photodetectors made from the SiGeSn material system to longer wavelengths. The study suggests the QCD response can indeed significantly extend the spectral range beyond that of the photodiodes and photoconductors made from the same group-IV system for a wide variety of applications in imaging, sensing, lidar, and space-and-fiber communications.

Photonic Generation of Dual-Band Dual-Format Phase-Coded Microwave Signals

We propose and demonstrate a photonic scheme to generate dual-band dual-format phase-coded microwave signals based on a commercial dual-polarization dual-drive Mach-Zehnder modulator (DP-DDMZM), which consists of two sub-DDMZMs with orthogonal polarization states. Two microwave signals with different frequencies are fed to the sub-DDMZMs, separately. The power of microwave signals is optimized to well suppress the optical carrier at the corresponding arms to avoid the optical interference between two arms of the sub-DDMZMs. The key novelty of our scheme is that two different kinds of phase-modulated microwave signals can be generated simultaneously. One is a binary phase-coded microwave signal in pulse mode, the other is an arbitrary phase-modulated signal in continuous wave mode. The proposed scheme is theoretically analyzed and experimentally verified.

Programmable and tunable flat-top supercontinuum laser sources via electro-optic intensity and phase modulation scheme

In this study, we presented flat-topped coherent supercontinuum lasers with tunable repetition rates and programmable spectral bandwidths. Supercontinuum sources with ultra-broadband and high-repetition-rate coverage can be achieved by merging nonlinearly broadened electro-optic optical frequency combs with optical line-by-line spectrum shaping. Spectral bandwidth programming is implemented by iterative spectrum shaping and input power control of highly nonlinear stages, whereas repetition rate tuning is performed by modulation speed control in optical frequency combs. Herein, we implemented a programmable and tunable flat-topped supercontinuum with a maximum bandwidth and repetition rate of 55 nm at 10 dB and 50 GHz, respectively. To clarify the coherence of the supercontinuum during tuning and programming, we performed a phase-noise analysis. We proposed a remarkably modified self-heterodyne method to measure the phase noise of each mode precisely by filtering specific supercontinuum taps in a Mach–Zehnder interferometer. With this method, it has been proved that the single-sideband spectra in each mode are almost similar to that of the RF clock, indicating that our programmable and tunable supercontinuum generation process added minimal degradation to the phase noise properties. This study shows possibilities for generating hundreds of programmable and tunable flat-topped optical carriers with robustness and coherence.

Novel Li rich perovskites Li4NBI3 (B = Ge, Sn, or Pb) with high mobility based on super alkali cation Li4N

It is an important research goal to find new environment-friendly and stable materials for sustainable and green energy applications. This paper reports a new type of super alkali perovskite Li4NBI3 (B = Ge, Sn, or Pb) with high mobility that was designed and studied by using the first-principle calculation method based on density functional theory. The geometrical structures, thermodynamic stability, electronic properties, transport properties, mechanical properties, and optical absorption properties of Li4NBI3 perovskites were calculated. The effects of the super alkali cluster Li4N on the structure and electronic properties of these perovskites were investigated. The results show that ionic bonds are formed between Li4N and I, and that the local energy state provided by Li4N increases the density of states at the edge of the energy band, which affects the energy band structure and pseudo-bandgap, and further affects the optical absorption coefficient and carrier mobility. Mechanical parameters showed that these three perovskites have high ductility, so they are suitable for making thin films. With high conductivity, strong light absorption, these perovskites are likely to become potential candidates for new energy materials.