Delay Dispersion Research Articles

Network Massive MIMO Transmission Over Millimeter-Wave and Terahertz Bands: Mobility Enhancement and Blockage Mitigation

Mobility and blockage are two critical challenges in wireless transmission over millimeter-wave (mmWave) and Terahertz (THz) bands. In this paper, we investigate network massive multiple-input multiple-output (MIMO) transmission for mmWave/THz downlink in the presence of mobility and blockage. Considering the mmWave/THz propagation characteristics, we first propose to apply per-beam synchronization for network massive MIMO to mitigate the channel Doppler and delay dispersion effects. Accordingly, we establish a transmission model. We then investigate network massive MIMO downlink transmission strategies with only the statistical channel state information (CSI) available at the base stations (BSs), formulating the strategy design problem as an optimization problem to maximize the network sum-rate. We show that the beam domain is favorable to perform transmission, and demonstrate that BSs can work individually when sending signals to user terminals. Based on these insights, the network massive MIMO precoding design is reduced to a network sum-rate maximization problem with respect to beam domain power allocation. By exploiting the sequential optimization method and random matrix theory, an iterative algorithm with guaranteed convergence performance is further proposed for beam domain power allocation. Numerical results reveal that the proposed network massive MIMO transmission approach with the statistical CSI can effectively alleviate the blockage effects and provide mobility enhancement over mmWave and THz bands.

Simultaneous microwave frequency conversion and idler filtering based on polarization manipulating of an amplified spontaneous emission source

We propose a technique to simultaneously realize microwave frequency conversion and idler filtering by polarization manipulating of an amplified spontaneous emission (ASE) source. By simply adjusting the tunable differential group delay (DGD) introduced by a polarization division multiplexed (PDM) emulator, switchable and idler-free frequency conversion can be implemented. Thanks to the intrinsic frequency selection of the dispersive delay structure, no electrical filter is involved to remove the mixing idlers, which ensures a broad operation bandwidth and flexible tunability. In addition, the dispersion-induced mixing power fading and the environmental vibration sensitivity that exist in previous approaches are both avoided owing to the PDM optical paths. The proposed method is theoretically and experimentally verified.

Blind and low-complexity modulation format identification scheme using principal component analysis of Stokes parameters for elastic optical networks.

We propose a blind and low-complexity modulation format identification (MFI) scheme for elastic optical networks (EONs). Since the square operation reduces half the number of the clusters in Stokes space, the scheme directly performs principal component analysis (PCA) on Stokes parameters after square operation. This greatly reduces the dimensionality of received signals from 3 × N to 3 × 3. Subsequently, three obtained principal components (PCs) are employed synthetically to identify the modulation formats. The effectiveness is first verified through 28 GBaud polarization division multiplexing (PDM)-BPSK/-QPSK/-8QAM/-16QAM/-32QAM/-64QAM simulation systems. Only using 2048 symbols, the required minimum optical signal-to-noise ratio (OSNR) values to achieve 100% MFI success rate are all equal to or lower than their corresponding 7% forward error correction (FEC) thresholds. Besides that, the scheme also obtains significant tolerances to residual chromatic dispersion (CD) and differential group delay (DGD). Finally, the proposed scheme is further verified by 20 GBaud PDM-QPSK/-16QAM/-32QAM long-haul transmission experiments. The results demonstrate that the scheme exhibits good resilience towards fiber nonlinear impairments. More importantly, compared with other four kinds of MFI schemes, the used symbol number to achieve 100% MFI success rate notably equals to at most 2/5 as that of other schemes, and its time complexity can be reduced to O(N).

Obtaining more energetic modelocked pulses from a SESAM-based fiber laser.

A major design goal for femtosecond fiber lasers is to increase the output power but not at the cost of increasing the noise level or narrowing the bandwidth. Here, we perform a computational study to optimize the cavity design of a femtosecond fiber laser that is passively modelocked with a semiconductor saturable absorbing mirror (SESAM). We use dynamical methods that are more than a thousand times faster than standard evolutionary methods. We show that we can obtain higher pulse energies and hence higher output powers by simultaneously increasing the output coupling ratio, the gain, and the anomalous group delay dispersion. We can obtain output pulses that are from 5 to 15 times the energy of the pulse in the current experimental design with no penalty in the noise level or bandwidth.

Subclinical cardiac structural and electrical abnormalities in fibromyalgia syndrome

Background/aim In the literature, there is a paucity of data about the effects of fibromyalgia syndrome (FMS) on myocardial function and electrophysiological properties of atrium and ventricles. In this study, we investigated cardiac functions and noninvasive predictors of arrhythmias in patients with FMS.Materials and methods The study included 43 female patients diagnosed with FMS and 30 age- and sex-matched healthy subjects. The presence of fragmented QRS (fQRS) morphology, P dispersion, QT dispersion, inter- and intraatrial electromechanical delay was evaluated in the groups with 12-lead ECG and standard and tissue Doppler echocardiography.Results Among electrocardiographic parameters, P dispersion, QT dispersion, and the ratio of presence of fQRS morphology were found to be significantly higher in the study group as compared to the control group. In lateral and septal, the ratio of the early transmitral flow velocity to the early diastolic tissue velocity (E/Em) was significantly higher in the study group. Additionally, intra- and interatrial electromechanical delay was found significantly prolonged in the study group. Conclusion FMS is found to be associated with significant cardiac electrical alterations that may indicate the increased risk of atrial and ventricular arrhythmias in this group of patients.

Flexible all-PM NALM Yb:fiber laser design for frequency comb applications: operation regimes and their noise properties

We present a flexible all-polarization-maintaining (PM) mode-locked ytterbium (Yb):fiber laser based on a nonlinear amplifying loop mirror (NALM). In addition to providing detailed design considerations, we discuss the different operation regimes accessible by this versatile laser architecture and experimentally analyze five representative mode-locking states. These five states were obtained in a 78-MHz configuration at different intracavity group delay dispersion (GDD) values ranging from anomalous (-0.035 ps2) to normal (+0.015 ps2). We put a particular focus on the characterization of the intensity noise as well as the free-running linewidth of the carrier-envelope-offset (CEO) frequency as a function of the different operation regimes. We observe that operation points far from the spontaneous emission peak of Yb (∼1030 nm) and close to zero intracavity dispersion can be found, where the influence of pump noise is strongly suppressed. For such an operation point, we show that a CEO linewidth of less than 10-kHz at 1s integration can be obtained without any active stabilization.

Self-stabilization mechanism in ultra-stable Fourier domain mode-locked (FDML) lasers

Understanding the dynamics of Fourier domain mode-locked (FDML) lasers is crucial for determining physical coherence limits, and for finding new superior methods for experimental realization. In addition, the rich interplay of linear and nonlinear effects in a laser ring system is of great theoretical interest. Here we investigate the dynamics of a highly dispersion-compensated setup, where over a bandwidth of more than 100 nm, a highly coherent output with nearly shot-noise-limited intensity fluctuations was experimentally demonstrated. This output is called the sweet-spot. We show by numerical simulation that a finite amount of residual dispersion in the fiber delay cavity of FDML lasers can be compensated by the group delay dispersion in the swept bandpass filter, such that the intensity trace exhibits no dips or high-frequency distortions, which are the main source of noise in the laser. In the same way, a small detuning from the ideal sweep filter frequency can be tolerated. Furthermore, we find that the filter’s group delay dispersion improves the coherence properties of the laser, and acts as a self-stabilizing element in the cavity. Our theoretical model is validated against experimental data, showing that all relevant physical effects for the sweet-spot operating regime are included.

Stability switches in a ring-structured predator\u2013prey metapopulation model with dispersal delay

In this paper, we consider a predator–prey metapopulation model with a ring-structured configuration of an arbitrary and finite number of patches. The prey are assumed to disperse between the connected patches with a constant dispersal delay. We show that the dispersal delay can induce stability switches exhibiting both stabilizing and destabilizing roles in the stability of the symmetric coexistence equilibrium. Numerical simulations are presented to further illustrate the effects of the dispersal delay, the dispersal rate, the fraction of dispersal due to predation avoidance and the network topology on the number of stability switches.

Passband-Switchable and Frequency-Tunable Dual-Passband Microwave Photonic Filter

A passband-switchable and frequency-tunable dual passband microwave photonic filter (MPF) implemented using a broadband optical source (BOS) in a delay-line configuration is proposed and experimentally demonstrated. In the proposed MPF, the BOS is sliced by a Mach-Zehnder interferometer (MZI), in which a phase modulator (PM) is incorporated in the upper arm. The phase-modulated light signal combined with an unmodulated light at the output of the MZI is sent through a dispersive delay line and detected at a photodetector (PD). The entire operation is equivalent to a microwave bandpass filter with its central frequency determined by the time delay difference between the two arms of the MZI. To implement a dual passband MPF, a second lower arm with a different arm length corresponding to a different time delay is added. The independent switchability of the passband is realized by controlling the polarization state of the light wave in each of the lower arms to be parallel or orthogonal with that of the upper arm. The independent frequency tunability is achieved by tuning the time delay of each of the lower arms. The proposed MPF is experimentally evaluated. A dual-passband MPF with independent passband switchability and independent frequency tunability is demonstrated. The results show that each of the two passbands can be switched on and off with an extinction ratio of about 20 dB. The passband frequency can also be continuously tuned with a tunable range from 1 to 18 GHz.

Multimodal spectral focusing CARS and SFG microscopy with a tailored coherent continuum from a microstructured fiber

We report a technologically novel microscopy system for bioimaging based on a 100 fs titanium:sapphire (Ti:Sa) laser pumped coherent continuum from a tailored, 9-cm long, all normal dispersion (ANDi) fiber, enabling concurrent image contrast with (a) spectral focusing coherent anti-Stokes Raman scattering (SF-CARS) (spanning 900–3200 cm−1) and (b) sum frequency generation (SFG). Both modalities were efficiently excited with power levels at the microscope focus compatible with biological samples. Moreover, using the continuum, images were recorded in the back-scattering (epi-detection) geometry, without the necessity for an expensive, computer-controlled, spatial light modulator (SLM), clearly demonstrating the strong signal levels achieved. Image contrast from the multiple modalities provided greater chemical and structural insights than imaging with any single technique in isolation. Numerical simulations supported these developments in regard to both the optimum fiber length for SC generation and the achievement of high spectral resolution in SF-CARS via careful group delay dispersion matching across the pump and Stokes pulses using just an inexpensive sequence of short glass blocks inserted into the Stokes beam. We show bio-images of mouse tissue recorded concurrently via label/stain-free contrast from multiple modalities: CARS, two-photon auto-fluorescence (TPaF) and second harmonic/sum frequency generation (SHG/SFG). Overall, our approach delivers optimum performance in back-scattered (epi-) detection configuration, suited for thick samples, at reduced complexity and cost. The addition of this simple fiber add-on to lasers already widely used for TPF microscopy can thus extend the capabilities of a significant number of existing microscopy laboratories.

Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution: erratum.

We correct the values of the group delay dispersion of the cavity mirrors and N2, as well as the concentration of CO2, obtained from the measurement of the center frequencies of cavity modes using a comb-based Fourier transform spectrometer. The corrected values of group delay dispersion are a factor of 3 higher, which implies that the precision and accuracy of the dispersion measurements is 0.3 fs2 and 3 fs2, respectively.

Comparison of magnetron sputtering and ion beam sputtering on dispersive mirrors

We have compared two kinds of dispersive mirrors (DMs) produced by magnetron sputtering and ion beam sputtering. One of them is a broadband DM which is known as double-angle DM, providing a group delay dispersion (GDD) of -40 mathrm{{fs}}^2 in the range of 550–1050 nm. The other one is a robust highly dispersive mirror, which provides a GDD of about -275 mathrm{{fs}}^2 at 800 nm and covers the wavelength range from 690 to 890 nm. For the first time, a comparison between magnetron-sputtering-produced and ion-beam-sputtering-produced dispersive mirrors is performed.

Optical steering of electron beam in laser plasma accelerators.

Using a Dazzler system and tilting a compressor grating, we provide an effective way of using the laser group delay dispersion to continuously steer the electron beam accelerated by an asymmetric laser wakefield. The deviation angle of the electron beam was the same as that of the angularly chirped laser pulse from its initial optical axis, which is determined by the laser pulse-front-tilt (PFT). This method can be utilized to continuously control over the pointing direction of electron bunches to the requisite trajectories, especially for practical applications in highly sensitive alignment devices such as electron-positron colliders or undulators. Additionally, we investigate the effect of PFT on the properties of the electron beam.

Nonlinear Spectral‐Imaging Study of Second‐ and Third‐Harmonic Enhancements by Surface‐Lattice Resonances

AbstractAn ultrafast spectral imaging optical system featuring adaptive precompensation for group delay dispersion, automated tuning and collimation, rapid power stabilization, and rapid spectral imaging switching is developed. In this system, real‐time visualization ensures that the excited Gaussian beam remains focused on the nanoarray of the electron‐beam lithographed metasurface, as well as maximum gain of the nonlinear effect information. Based on the system, two surface‐lattice resonance metasurfaces under broadly tunable excitation (800–1300 nm) are investigated. The results demonstrated a 16‐fold enhancement in the third harmonic generation intensity and a 168‐fold enhancement in the second harmonic generation intensity at a 1200 nm excitation compared to the unpatterned films, with a narrow resonant bandwidth of 8 nm. This can be attributed to the frequency‐matching lattice‐enhanced localized surface plasmon resonance, which results in effective and vast excitation of plasmons that produce numerous energetic carriers. The method offers the possibility to reveal various ultrafast dynamics in nonlinear plasmonic metasurfaces.

Cascaded nonlinearities in high-power femtosecond optical parametric oscillator

A femtosecond optical parametric oscillator (OPO) pumped by the second harmonic of a Yb:KGW solid-state oscillator was investigated. A comparison between lithium triborate (LBO) and barium borate (BBO) crystals was carried out, with particular attention to the impact of phase mismatch on pulse characteristics. The phase mismatch induces cascaded nonlinearity, which acts as an effective nonlinear refractive index. Negative group delay dispersion of the resonator and the effective nonlinear refractive index allows soliton-like pulse formation with varying pulse durations and a bandwidth-limited time-bandwidth product. Deviation from the soliton equation was observed when intracavity power was varied. The measured pulse durations were lower than the classical soliton duration, indicating that the pulses were not classical solitons. A Z-scan experiment was used to determine nonlinear refractive indices under conditions identical to those in the oscillator cavity. Unexpectedly, the signs of nonlinear refraction as measured by the Z scan and indirectly observed in the OPO proved to be the opposite. This sign reversal could be interpreted in terms of the competing cascaded nonlinearities corresponding to self-phase modulation and cross-phase modulation between the signal and the pump pulses in the nonlinear OPO crystal.

Simultaneous measurement of DC two-photon absorption signal offset and amplitude of the intensity autocorrelation in the focusing of femtosecond pulses.

In this work, the DC two-photon absorption signal offset (${{\rm DC}_{\rm TPA}}$DCTPA) and the amplitude of the autocorrelation (${{\rm A}_{\rm AC}}$AAC) are measured simultaneously around the focal point of an apochromatic microscope objective using the z-scan autocorrelation technique. The ${{\rm A}_{\rm AC}}$AAC is obtained from the nonlinear sensor response given by the two-photon-absorption, generated in a GaAsP photodiode, for femtosecond laser pulses. We verify that the change in the ${{\rm DC}_{\rm TPA}}$DCTPA signal along $z$z is coincident with the amplitude of the intensity autocorrelation, and that the highest amplitude of the AC is reached at the same position as the highest amplitude of the ${{\rm DC}_{\rm TPA}}$DCTPA signal. The ${{\rm DC}_{\rm TPA}}$DCTPA signal is typically used as a reference for the alignment in a collinear intensity autocorrelator, and we show that it can also be used as a practical procedure to estimate the depth of focus. The ${{\rm DC}_{\rm TPA}}$DCTPA signal measurement allows us to locate the optimum spatial-temporal coupling given by the highest amplitude of the intensity autocorrelation. Additionally, we find a variation in the pulse duration within the same region due to the radially varying group delay dispersion.

The Effects of Plasma Lensing on the Inferred Dispersion Measures of Fast Radiobursts

Abstract Radio signals are delayed when propagating through plasma. This type of delay is frequency dependent and is usually used for estimating the projected number density of electrons along the line of sight, called the dispersion measure (DM). The dense and clumpy distribution of plasma can cause refractive deflections of radio signals, analogous to lensing effects. Similar to gravitational lensing, there are two contributions to the time delay effect in plasma lensing: a geometric delay, due to increased path length of the signal, and a dispersive delay due to the change of speed of light in a plasma medium. We show the delay time for two models of the plasma distribution, and point out that the estimated DM can be biased. Since the contribution of the geometric effect can be comparable to that of the dispersive delay, the bias in the measured DM can be dramatically large if plasma lensing effects are not taken into account when signals propagate through a high-density gradient clump of plasma.

Highly-Dispersive Mirrors With Advanced Group Delay Dispersion

We report on a novel starting design structure of highly-dispersive mirror (HDM) which combines quarter-wavelength structure and multi-cavities, introducing more wavelength-dependent storage times of incident radiation and group delay (GD). Based on such structure, a HDM with new advanced group delay dispersion (GDD) of −20 000 fs2 in the wavelength range of 1035–1045 nm is designed, fabricated and characterized. Our proposed initial multilayer structure is instructive in designing HDM aiming at introducing large GDD. Our HDM further pushes the maximum negative GDD allowed by the design’s total physical thickness to its limitation. It offers an ideal trade-off between GDD and total mirror thickness. The HDM has been applied in a fiber-chirped system, providing a considerable compensation of GDD reaching −200 000 fs2 with only 10 reflections between two mirrors, and laser pulse compression from 2.8 ps to 213 fs is realized.

Hormone-mediated dispersal and sexual maturation in males of the social paper wasp Polistes lanio.

Sex-biased dispersal is common in social species, but the dispersing sex may delay emigration if associated benefits are not immediately attainable. In the social Hymenoptera (ants, some bees and wasps), newly emerged males typically disperse from the natal nest whilst most females remain as philopatric helpers. However, little information exists on the mechanisms regulating male dispersal. Furthermore, the conservation of such mechanisms across the Hymenoptera and any role of sexual maturation are also relatively unknown. Through field observations and mark-recapture, we observed that males of the social paper wasp Polistes lanio emerge from pupation sexually immature, and delay dispersal from their natal nest for up to 7 days whilst undergoing sexual maturation. Delayed dispersal may benefit males by allowing them to mature in the safety of the nest and thus be more competitive in mating. We also demonstrate that both male dispersal and maturation are associated with juvenile hormone (JH), a key regulator of insect reproductive physiology and behaviour, which also has derived functions regulating social organisation in female Hymenoptera. Males treated with methoprene (a JH analogue) dispersed earlier and possessed significantly larger accessory glands than their age-matched controls. These results highlight the wide role of JH in social hymenopteran behaviour, with parallel ancestral functions in males and females, and raise new questions on the nature of selection for sex-biased dispersal.

A Novel Hyperbolic Tangent Profile for Optical Fiber for Next Generation OAM-MDM Systems

In this work, we propose and theoretically/numerically investigate a novel parametric refractive index profile for few mode fibers, referred to as a hyperbolic tangent few mode fiber (HTAN-FMF). To the best of our knowledge, the HTAN-FMF is adopted for the first time, to simultaneously maximize the inter-channels separation and OAM purity. The proposed FMF achieves a wide separation between cylindrical vector modes ( $\geq 3\times 10\,\,^{\mathrm{ -4}}$ between TE0,1, HE2,1, and TM0,1). This enables low-level crosstalk and clearly overcomes recently reported levels in literature. On the other hand, simulations show that the generated and evaluated OAM modes are mostly high pure (≥ 99.9%) leading to intrinsic crosstalk as minimum as -30 dB during propagation through HTAN-FMF. Other propagation performance metrics of the proposed fiber including chromatic dispersion and differential mode delay are also evaluated over the C ITU-T band showing that HTAN-FMF has the potential to achieve stable OAM modes transmission. Bending insensitivity is numerically proved showing that the proposed FMF has the potential to enhance the transmission capacity and the spectral efficiency in next generation OAM mode division Multiplexing (OAM-MDM) systems.