Group Delay Research Articles

Enhancing phase modulation and group delay in reflective Huygens metasurfaces via a Fabry-Perot cavity

Huygens metasurfaces exhibit excellent optical properties such as 2π phase modulation and slow light effects. However, they face challenges including wide bandwidth and low group delay due to their high radiation losses. Here, we propose a reflective Huygens metasurface coupled with an F-P cavity. We demonstrate that F-P resonance modes can couple with magnetic-quasi-bound-state (M-QBIC) and electric-quasi-bound-state (E-QBIC) in the Huygens metasurface through constructive interference, significantly enhancing the quality factors of both QBICs. Through structural parameter optimization, our reflective Huygens metasurface achieves 4π phase modulation and a high group delay of up to 166 ps. Compared to the non-coupled Huygens metasurface with the same structural asymmetry, the group delay of the F-P coupled reflective Huygens metasurface is enhanced by up to 30 times. Our design reduces the fabrication precision requirements for Huygens metasurfaces, enabling similar group delays to be achieved in low-symmetry coupling structures as in highly symmetric non-coupling structures. Additionally, the performance of this metasurface shows robustness to changes in incident light polarization. This design highlights the potential for achieving high-quality factors, large phase modulation, and large group delay, offering new avenues for the design of highly sensitive tunable devices, efficient nonlinear optical devices, and narrowband slow light devices.

An epsilon‐near‐zero circuit based on half‐mode SIW with enhanced integrability for applications of narrowband large group delay

AbstractThe tunneling effect of an epsilon‐near‐zero (ENZ) is exploited to fulfill a narrowband large group delay by using a compact half‐mode substrate integrated waveguide structure built on thin substrate. The ENZ channel possesses a finite input impedance at the tunneling frequency point, and the finite input impedance can be matched to the final I/O port simply by a quarter wavelength impedance transformer. Thus, the enhanced integrability is realized in a planar form in the proposed circuit comparing to the conventional waveguide ENZ channel. Meanwhile, in the real ENZ channel, the effective impedance is observed to be close to 50 Ω, so that the ENZ can be connected with 50 Ω microstrip directly, which has been experimentally validated. The final measured net group delay is 1.36 ns at 3.08 GHz with 3.6 dB insertion loss, and the proposed method provides an alternative for the group delay engineering.

Log‐periodic antenna effect on modulated signals in regions of negative group delay

Log‐periodic antenna effect on modulated signals in regions of negative group delay

A Method of Ship Target Modulation Feature Enhancement Based on Phase Information

Abstract Ship modulation features are commonly used in underwater acoustic target recognition, which contains propeller parameter information of the target. The basis processing of modulation spectrum is demodulation. Traditional envelope demodulation methods only consider amplitude information and make little use of phase information, which can fully characterize the dynamic characteristics of radiated noise. In this paper, a modulation feature enhancement method based on phase information is proposed, which takes full account of the phase information of the target signal. It uses the modified group delay function to demodulate the target signal, and utilizes the phase stability characteristics of the modulated line spectrum to enhance the modulation features of the ship target by weighted phase features. Simulation and sea trial data verification show that the proposed method can effectively enhance the energy of the line spectrum, suppress the background noise, and improve the signal-to-noise ratio of the modulation line spectrum, which lays a good foundation for further using the modulation line spectrum to carry out the estimation of modulation parameters, and has a good application prospect in the field of underwater acoustic target recognition.

A novel non-invasive method for measuring the spatial kinematic behavior of cardiomyocytes regulated by mechanical cues

The intact heart undergoes complex and multiscale mechanical remodeling processes. Measuring rhythmic spatial contraction of the myocardium is crucial for assessing mechanical durability and the ability to mount coordinated responses to pressure, electrical, and hemodynamic signals. However, current cardiomyocyte measurement platforms typically focus on action potentials and XY-plane contractions. Therefore, effective evaluation methods for studying the influence of mechanical cues on the spatial dynamic contraction of cardiomyocytes are still lacking. In this study, we developed a topographic guiding combined with an optical spatial motion tracking method to provide controllable mechanical stimulation for inducing directed contraction of cardiomyocytes and obtaining spatial motion information in vitro. We first performed a detailed investigation of cell connections and cytoskeleton orientations by combining the proposed method with immunofluorescence. Next, spatial constrictive modes, features, and key parameters of microgroove-guided cardiomyocytes were studied. Finally, the three-dimensional (3D) motions of the cardiomyocytes at different positions on the structure were compared. We found that the XY-plane contraction of cardiomyocytes typically has only one direction and shows a significant phase delay compared to the axial motion. In addition, cardiomyocytes located near the edges of the microgrooves were restricted by stronger mechanical forces, resulting in a significant height change reduction. These results provide new perspectives for structural and functional research on cardiomyocytes under long-term mechanical regulation. Overall, this study provides a highly precise and convenient method for evaluating the 3D cardiomyocyte motion under mechanical induction. This method is expected to enhance understanding of cardiomyocyte development and be useful for research on cardiac mechanics and functions.

Apodized chirped fiber Bragg grating for measuring the uniform and non-uniform characteristics of physical parameters

An apodized Chirped Fiber Bragg Grating (CFBG) is presented to compute and depict the sensing response for various uniform and non-uniform profiles of the temperature and the strain for different chirp parameters. The effectiveness of the CFBG can be reduced owing to the appearance of wide-scale side lobes as well as increasing group delay ripples (GDR). The introduction of apodization profile is highly impactful to enhance the sensing efficiency by minimizing the side lobes level along with the quantity of GDR. The sensing response of the apodized CFBG with different chirp parameters are evaluated for various ranges of uniform temperature (30 °C–80 °C) and strain (300 µ∊–550 µ∊). Furthermore, different non-uniform Gaussian profiles are also introduced to illustrate the overview for detecting and locating the spot size sources of measurands. This approach is highly desirable for estimating the structural integrity by ensuring the safety, efficiency, and reliability of the CFBG-based sensing outcomes.

Generation of second-order sidebands and slow–fast light in cavity magnomechanics with a parametric amplifier

In this work, we theoretically investigate the optical second-order sideband (OSS) efficiency and group delay of an output probe field in a two-coupled cavity magnomechanics (CMM) system. The setup comprises a gain (active) cavity and a passive (loss) cavity, which simultaneously includes an optical parametric amplifier (OPA) and a yttrium iron garnet sphere to facilitate magnon–photon coupling. Employing experimentally attainable parameters, we show that the presence of an OPA can significantly enhance the optical transmission rate and OSS efficiency. We show that photon–magnon strong coupling can enhance OSS efficiency substantially when compared to weak photon–magnon coupling. Our results, in particular, show that the OSS’s efficiency may be significantly improved for active–passive-coupled cavities as compared to the passive–passive cavities system. Furthermore, we demonstrate that modifying the system configurations may change the group delay, affecting the transition from rapid to slow light propagation, and vice versa. Our results provide a novel and easy approach to designing CMM devices for altering light propagation, with possible applications including optical switching, information storage, and accurate measurement of weak signals.

Sleep disorder syndromes of post-acute sequelae of SARS-CoV-2 (PASC) / Long Covid

IntroductionCOVID-19 infection has resulted in a high prevalence of a post-infectious syndrome, known as post-acute sequelae of SARS-CoV-2 (PASC) or “Long COVID”. PASC is a heterogeneous disease with a high prevalence of sleep disturbances, varying from an insomnia disorder to excessive daytime sleepiness. MethodsPatients seen in the Covid Survivorship Program at the Beth Israel Deaconess Medical Center Boston, USA, were screened for sleep disorders as part of a comprehensive multi-system evaluation. Those who screened positive were referred for a comprehensive sleep evaluation in a dedicated COVID-19-Sleep clinic, followed by diagnostic sleep testing and treatment. This report summarizes patients who completed an American Academy of Sleep Medicine (AASM) accredited facility-based diagnostic evaluation. International Classification of Sleep Disorders 3rd Edition-Revised criteria were met for all diagnoses. ResultsIn 42 patients with PASC, five categories of sleep disorder syndromes were observed following a sleep clinic evaluation, including obstructive sleep apnea, chronic insomnia disorder, primary hypersomnia, REM behavior disorder (RBD), and new onset circadian phase delay. Seven patients met criteria for idiopathic hypersomnia, and two had narcolepsy type 2. RBD patients were infected in three different waves; circadian disturbance patients were all infected in the winter wave of 2020/21, and the primary hypersomnolence group occurred during all waves, predominantly the initial wave of 2020. A peculiar form of insomnia was a persistent loss of sleep regularity. ConclusionsSpecific sleep symptoms/syndromes are reported in this select group of patients with PASC/Long Covid. As new onset sleep complaints are prevalent in PASC, we recommend a complete clinical and investigative sleep evaluation for persistent severe sleep symptoms following COVID-19 infection.

Real-time in situ phase sensitivity calibration of interferometric fiber-optic ultrasonic sensors.

Output from a fiber-optic interferometric ultrasonic sensor can be affected by the operating point changes and/or the optical power fluctuations. We report a method for real-time and in situ calibration of the responses to ultrasound-induced phase delays for a low-finesse Fabry-Perot (FP) ultrasonic sensor demodulated using a phase-generated carrier. The carrier is produced from an electro-optic phase modulator, and its zero- and fundamental-carrier frequency components yield two quadrature signals. The same phase modulator is employed to generate controllable laser frequency modulations and calibration phase delay modulations. The response to the known calibration phase delays is used to gauge the sensor output in real time and in situ. The experiment has verified the effectiveness of the calibration method against the signal variations from both operating point changes and optical power variations.

Research on High-Frequency PGC-EKF Demodulation Technology Based on EOM for Nonlinear Distortion Suppression

In this study, a phase-generated carrier (PGC) demodulation algorithm combined with the extended Kalman filter (EKF) algorithm based on an electro-optic modulator (EOM) is proposed, which can achieve nonlinear distortion (such as modulation depth drift and carrier phase delay) suppression for high-frequency phase carrier modulation. The improved algorithm is implemented on a field-programmable gate array (FPGA) hardware platform. The experimental results by the PGC-EKF method show that total harmonic distortion (THD) decreases from −32.61 to −54.51 dB, and SINAD increases from 32.59 to 47.86 dB, compared to the traditional PGC-Arctan method. This indicates that the PGC-EKF demodulation algorithm proposed in this paper can be widely used in many important fields such as hydrophone, transformer, and ultrasound signal detection.

Generation of a tripartite photonic state via a double-Λ configuration in a four-level system

Triphotons have a more abundant energy structure compared to biphotons. Furthermore, as the number of photons increases, excellent properties such as entangled multi-qubit states, high security, flexibility, and information capacity are observed. This leads to a growing demand for multi-body quantum information processing. Here, a method is proposed to generate a three-photon entangled state using a single six-wave mixing process in an atomic ensemble. The research examines the temporal correlation characteristics of the triphoton produced in photon coincidence counting measurements, with a focus on the linear and nonlinear susceptibilities of the six-wave mixing process. These properties primarily depend on the fifth-order nonlinear coupling coefficients responsible for the damping Rabi oscillations and the group delay determined by the longitudinal detuning function. To enhance the nonlinear interaction between the optical field and the atomic ensemble, placing the atomic ensemble in a high-quality cavity and utilizing laser cooling techniques to eliminate the internal Doppler broadening effect in the atomic gas hold promise.

Enhanced transmission and group delay in optomechanics with an optical parametric amplifier

In this paper, we investigate the phenomenon of optomechanically induced transparency (OMIT) in a cavity that has a moving end mirror and is subjected to an external force. Furthermore, we place an optical parametric amplifier (OPA) inside the cavity. We show that the transmission intensity of the probe field and the group delay is enhanced by the parametric gain and phase of the OPA. We also show that this enhancement is influenced by external forces. We believe that these findings could be valuable in the area of quantum information processing.

Investigation on the randomly-coupled unit grouping multi-core fibers with different unit arrangement

Randomly-coupled unit grouping multi-core fiber (RC-UG-MCF) is a new fiber structure that combines weakly and strongly coupled multi-core fiber technologies. Compared to the weakly-coupled MCF with a large cladding size, the RC-UG-MCF can shrink the cladding size by grouping the cores. On the other hand, the RC-UG-MCF only maintains a few cores in the strongly-coupled condition, which can reduce the multiple-input multiple- output digital signal processing (MIMO-DSP) complexity compared to the coupled MCFs with full MIMO-DSP for all the cores. In this paper, we intro- duce three types of randomly-coupled unit grouping 12-core fiber, which are randomly-coupled 2-core unit grouping MCF (RC-2CUG-MCF), RC-3CUG- MCF and RC-4CUG-MCF. Subsequently, we compare these three RC-UG- MCFs in terms of group delay spread, inter-unit crosstalk (XT), and cladding size. To achieve an inter-unit XT of −40 dB/100 km, RC-3CUG-MCF can provide a solution to maintain MIMO-DSP scale at the level of 6 × 6 with 180-µm cladding radius. However, the RC-2CUG-MCF has worse inter-unit XT, and the RC-4CUG-MCF has larger MIMO complexity. Based on the analysis, there is a trade-off relationship between the MIMO-DSP scale and the fiber cladding size, assuming that the target inter-unit XT is fixed.

Spray swirling flame dynamic under combined modulation of air and fuel

Combustion instability is still a main challenge in developing advanced gas turbine combustors. It is essential to conduct instability control once it occurs. In this paper, liquid fuel modulation by a high-speed on/off valve is used to control flame oscillation generated by loudspeakers. Spray swirling flame dynamic under the combined modulation of air and fuel is experimentally studied. Flame dynamical characteristics and coherent structures at different excitation phase delays are discussed with nonlinear time-series analysis and proper orthogonal decomposition method. Experimental results show flame heat release rate fluctuation is decreased by 86% at 40° while flame instability is intensively enhanced at 220°. Phase portraits and recurrence plots show the flame is in a chaotic oscillation with low amplitude aperiodic oscillations at 40° while it suffers from the largest fluctuation at 220°. Difference in flame dynamic is caused by the destructive (out of phase) and constructive (in phase) interference of heat release rate fluctuations induced by air excitation and fuel modulation. Flame returns back to its nature dynamical behavior under out-of-phase modulation. Longitudinal modes of the spray flame are enhanced while the spiral modes are inhibited under in-phase modulation.

Bolt looseness detection in lap joint based on phase change of Lamb waves

Bolts are widely applied to lap joints in many industrial fields. Due to harsh environment and low quality of operation, bolt looseness may appear during service. For lack of an understanding of the Lamb wave propagation process across bolted joints, most of the existing methods exploit some basic characteristics such as the transmitted energy as the looseness index, which is insensitive to bolt looseness at the early stage. To take full advantage of the information in the Lamb wave propagation and improve the detection performance, this study proposes a method for bolt looseness detection based on the phase change of Lamb waves during the propagation process. Firstly, the contact status is investigated based on the distribution of contact press, and the change of contact area along with bolt torque is analyzed. Subsequently, the propagation process of Lamb waves across bolted joints is revealed, based on which the monotonic relationship between the phase delay of Lamb waves and the extent of bolt looseness is derived. Thus, a looseness index based on phase change can be established to detect bolt looseness. Finally, the experiment is carried out in a bolted joint composed of two aluminum plates. The phase change in received signals conforms to the analyzed propagation mechanism of Lamb waves in bolted joints and verifies the effectiveness of the established bolt looseness index. Compared to the traditional methods based on the transmitted energy of Lamb waves, the proposed method is more sensitive to the change of bolt torque, especially at the early stage of bolt looseness.

Tunable multifunctional terahertz metamaterial device based on metal-dielectric-vanadium dioxide

The Electromagnetically Induced Transparency (EIT) effect in terahertz (THz) metamaterial devices has garnered extensive attention owing to its distinctive advantages in the slow light devices and quantum communication. However, the application of EIT has been constrained by the limitation to a single transparent window and the fixed intensity of the electromagnetic response at the transmission peak after fabrication. To achieve a dual-frequency EIT effect and enable active tunability, this paper presents a tunable multifunction THz metamaterial device based on vanadium dioxide (VO2). The transition of VO2 from the insulator to the metal allows for the active control of EIT and multi-band absorption switching in the THz range. When VO2 is in its dielectric state, two EIT-like windows could be observed with a maximum value of 86.9 % and 71.7 % of the transmission amplitude, respectively. By adjusting the temperature of the VO2, a modulation depth of up to 99 % can be achieved, enabling active control of the EIT-like effect. In addition, active adjustment of group delay of up to 5.47 ps was demonstrated. When VO2 is in the metallic state, the device serves as a tunable multiband absorber, capable of achieving perfect absorption and modulation amplitude of up to 92 %. The results indicated that the presented devices hold significant value for research in the THz range, particularly for applications related to switching and slow light devices. Therefore, the tunable multifunctional THz metamaterial device is of great significance for THz technology.

Horizontal rearrangement frequency domain chirplet transform: algorithm and applications

Abstract Based on the idea of optimal time–frequency analysis (TFA) theory, a time–frequency post-processing method called horizontal rearrangement frequency domain chirplet transform (HRFCT) is proposed. The proposed method is designed to analyze non-stationary transient signals with nonlinear group delay (GD), aiming at obtaining energy-concentrated and accurate time–frequency representation (TFR). First, based on the image characteristics of the constructed amplitude function, the selection criteria of GD are defined, that is, the local maximum of amplitude function and the local minimum of absolute value of derivative of amplitude function are calculated. Next, according to the calculated GD, the coefficients on the GD trajectory are adaptively extracted, and the coefficients that cause the TFR to be blurred are discarded. Finally, the analysis result of a two-component simulation signal demonstrates the satisfactory performance of the HRFCT, and compared with four advanced TFA methods, the competitiveness of the HRFCT is verified. Additionally, the application of the HRFCT in the processing of constant and variable speed bearing signals highlights its prospects in transient signal analysis and rotating machinery condition monitoring.

Bistatic radar cross section estimation by using DDM for better spatial resolution of ocean surface wind speed

The delay-Doppler map (DDM) is the signal power distribution of the Coarse/Acquisition code (C/A code) of the received Global Navigation Satellite System (GNSS) signal in different code phase delay and Doppler frequency. When the received signal is reflected from ocean surface, the DDM can be used to retrieve the ocean surface wind speed, which is the GNSS-reflectometry (GNSS-R) technique. Due to the signal power distribution in DDM is the correlation results of received and artificial C/A code from receiver in different code phase delay and Doppler frequency, the Woodward ambiguity function (WAF) occurs in the DDM. In the case of DDM, the WAF is the correlation results of two square waves in different code phase delay and Doppler frequency, and is approximated a triangular function and a sinc function in code phase delay and Doppler frequency axes, respectively. It means the correlation results not only show the code phase delay and Doppler frequency of the received signal but also influence the surrounding code phase delay and Doppler frequency values and cause the structure of DDM more complex. Using more bins in DDM in the wind speed retrieval process can reduce the influence of WAF but cause the spatial resolution to worsen. In order to use as less bins as possible in the retrieval process and not reduce the retrieval efficiency too much, a simple method to estimate bistatic radar cross section (BRCS) from DDM is developed in this study. Furthermore, a retrieval process is also developed for ocean surface wind speed retrieving by using less bins in the DDM from Cyclone Global Navigation Satellite System (CYGNSS) mission.

Generation and evolution dynamics of gain-guided dissipative solitons in a Tm-doped fiber laser

The generation of dissipative solitons from a mode-locked fiber laser can enhance emission energy, while the insertion of a bandwidth filter exposes the system to complexity. This study presents the first generation and evolution dynamics of gain-guided dissipative solitons (GGDSs) in Tm-doped fiber lasers exploiting the finite gain bandwidth. The results highlight the intricate interplay of finite gain bandwidth with dispersion and nonlinearity in governing the dynamics of GGDSs. An increase in pump power significantly broadens the emission spectrum, which in turn introduces the finite gain bandwidth to the generation of GGDSs. Consequently, linear chirp accumulates, promoting high-energy ultrashort pulse generation after external compression. However, an increase in round-trip group delay dispersion leads to a narrower spectral bandwidth, which can weaken the influence of the finite gain bandwidth, resulting in decreased soliton energy and broader pulse duration. The investigation of GGDS evolution dynamics within the cavity also provides us with a deeper insight into the characteristics of GGDS. Overall, this research offers important guidance for designing high-performance Tm-doped fiber lasers with simple structures that fully exploit the finite gain bandwidth.

A Metamaterial Bandpass Filter with End-Fire Coaxial Coupling

A miniaturized metamaterial (MTM) bandpass filter (BPF) based on end-fire coaxial coupling is proposed. End-fire coaxial coupling is achieved by using the coaxial cavity to connect with the SubMiniature version A connector. The subwavelength characteristics of the MTM lead to the miniaturization advantages of the filter in transverse dimensions. Moreover, the longitudinal length of the coaxial cavity can be sharply reduced by introducing matched blocks. As a result, the proposed filter has miniaturization merit both in transverse and longitudinal dimensions. The full-wave simulation results further reveal that the MTM BPF exhibits the advantages of low loss, low reflection, and low group delay. Additionally, the fractional bandwidth is approximately 13% when |S11| is less than −15 dB. The MTM BPF might have potential applications to array antennas for easily being expanded to two dimensional arrays.