Mode Beating Research Articles

Mode-locking and self-mode-locking-like operation in a resonantly pumped gain-switched Tm-doped fiber laser

Mode-locking and mode-locking-like operation in a fast gain-switched Tm-doped fiber laser have been investigated experimentally. Applying a SESAM mirror, stable gain-switched and regular mode-locked pulses at a wavelength of ∼2μm are reported. The repetition rates of the gain-switched pulses and mode-locked sub-pulses are 50 kHz and 111 MHz, respectively. The duration of sub-pulses recorded within one gain-switched pulse envelope is ∼110 ps. Besides, in the same laser system configuration, it has been found that self-starting mode-locked resembling pulses, attributed to mode beating, can be generated. To the best of our knowledge, it is the first report on a gain-switched fiber laser where both modes of operation have been demonstrated.

Flexible x-ray source with tunable polarization and orbital angular momentum from Hermite-Gaussian laser modes driven plasma channel wakefield

A plasma channel undulator/wiggler may be created through the plasma wakefield excited by the beating of several Hermite-Gaussian laser modes propagating in a parabolic plasma channel. Control over both the betatron and undulator forces is conveniently achieved by tuning the amplitude ratios, colors, and order numbers of the modes. A special structure of the undulator/wiggler field without the focusing force near the propagation axis is generated inside the plasma wakefield by matching the strengths of the fundamental and first-order Hermite-Gaussian modes. The electron beam only experiences forced undulator oscillations in such a field, which significantly improves the quality of the emitted radiation. Since the value of the undulator strength parameter could be in a wide range, less or larger than unity, it is capable of generating narrow bandwidth x-ray, as well as the synchrotronlike high-energy $\mathrm{x}/\ensuremath{\gamma}$-ray, radiation by harmonics. Additionally, controlling the relative phases between the laser modes allows for polarization control of the plasma undulator. High-order harmonics produced from a circularly polarized plasma undulator clearly show the vortex nature and carry well-defined orbital angular momentum.

Experimental research on an L-band multi-wavelength erbium-doped fiber laser based on a cascaded Sagnac loop and M–Z filters

In this paper, we proposed and experimentally demonstrated an L-band multi-wavelength erbium-doped fiber (EDF) laser with a cascaded Sagnac loop, M–Z interferometer filters and a non-linear optical loop mirror (NOLM). The NOLM acted as the intensity-dependent transmissivity; it can effectively alleviate the mode competition and beating caused by homogeneous broadening. The transfer characteristic of the Sagnac loop was investigated firstly and got the optimized parameters. Then the laser output spectrum with different kinds of filters in the cavity was experimentally tested when the pump power was fixed at 450 mW. The experimental results showed that the EDF laser with the cascaded filters in the cavity had a better performance, such as a better flat lasing bandwidth within 3 dB, higher side-mode-suppression ratio and more comb lines generated. The peak power fluctuation was less than 0.35 dB within a 30 min monitor time.

Gravitational floating orbits around hairy black holes

We show that gravitational floating orbits may exist for black holes with rotating hairs. These black hole hairs could originate from the superradiant growth of a light axion field around the rotating black holes. If a test particle rotates around the black hole, its tidal field may resonantly trigger the dynamical transition between a co-rotating state and a dissipative state of the axion cloud. A tidal bulge is generated by the beating of modes, which feeds angular momentum back to the test particle. Following this mechanism, an extreme-mass-ratio-inspiral (EMRI) system, as a source for LISA, may face delayed merger as the EMRI orbit stalls by the tidal response of the cloud, until the cloud being almost fully dissipated. If the cloud depletes slower than the average time separation between EMRI mergers, it may lead to interesting interaction between multiple EMRI objects at comparable radii. Inclined EMRIs are also expected to migrate towards the black hole equatorial plane due to the tidal coupling and gravitational-wave dissipation. Floating stellar-mass back holes or stars around the nearby intermediate-mass black holes may generate strong gravitational-wave emission detectable by LISA.

基片式多纵模拍频光纤激光位移传感器

基片式多纵模拍频光纤激光位移传感器

Characterization of the coherent dynamics of bacteriochlorophyll a in solution

Disclosing the physical origin of quantum beatings in the early dynamics of biological light-harvesting pigment-protein complexes is one of the major challenges in the ultrafast spectroscopy community. 2D electronic spectroscopy (2DES) is a powerful tool for this purpose, but the complexity of the beating behavior in multichromophoric systems complicates the interpretation. For this reason, the availability of control datasets providing a full characterization of the response of isolated chromophores is highly desirable to untangle the features of intermolecular interactions from the properties of individual pigments. Here, a thorough 2DES characterization of the frequencies and dephasing times of intramolecular vibrational coherences of bacteriochlorophyll a in solution is provided. Several beating modes in the ground and excited state have been found and their dephasing times have been determined. The obtained results represent an essential interpretation key of the beating dynamics of pigment-protein complexes binding bacteriochlorophyll a.

Probing ultrafast vibrational dynamics of intramolecular hydrogen bonds with broadband infrared pump-probe spectroscopy

In this study we use ultrafast infrared (IR) pump-probe spectroscopy to study the vibrational dynamics of two intramolecular hydrogen bonded complexes, 10-hydroxybenzo[h]quinoline (HBQ) and 2-(2′-hydroxyphenyl)benzothiazole (HBT) dissolved in carbon tetrachloride. We pump the OH stretching (νOH) mode and probe across 1800 cm−1–3300 cm−1 using a broadband IR probe pulse. Both systems exhibit large anharmonicities (>250 cm−1), which are characteristic of medium strong hydrogen bonded systems. Additionally, we observe the coherent beating of low-frequency modes of 245 cm−1 and 118 cm−1 across the νOH stretch of HBQ and HBT, respectively. Anharmonic frequency calculations at the DFT level identify these low-frequency structural modes as in-plane bending modes, which modulate the intramolecular hydrogen bonding distance. These findings provide evidence that the anharmonic nature of the fundamental νOH stretch is primarily dictated by the coupling to low-frequency structural modes and these motions play an important role in the hydrogen bonding dynamics.

Intensity noise in high-frequency range of an external cavity diode laser and its reduction by second harmonic generation.

Characteristics of intensity noise of optically pumped vertical-cavity surface-emitting lasers and solid-state lasers, induced by the beating of the main lasing mode and non-lasing side modes and its reduction, have been reported in recent years. The mode beating noise of an external cavity diode laser composed of an electrically pumped edge-emitting laser diode chip is studied experimentally in this Letter. The noises due to the beating of the main mode with the first- to third-order side modes are observed, and multiple sub-peaks in the beating noise are measured. It is assumed that the new phenomena are coming from the enhanced four-wave mixing in the longer-active medium. Intensity noise reduction is also demonstrated by using the second harmonic generation of a beta barium borate crystal as a nonlinear absorber.

Emotion Painting: Lyric, Affect, and Musical Relationships in a Large Lead-Sheet Corpus

How are lyrical emotions expressed in music? This paper explores the correlation between affect-carrying lyrics and musical features such as beat strength, duration, pitch height, consonance, and mode. Using computer-aided musicology software music21 and the NRC emotion lexicon, we conduct a corpus study on 1,895 folk and popular song lead-sheets encoded as MusicXML. The study reveals that metrical strength and note lengths are highly correlated with affects, while correlations of pitch height, consonance, and mode are in general less significant, at times contradicting previous research. Measurements of minor vs. major chordal context and tonal certainty, however, reveal certain previously unknown differences among emotional states. The paper uses a larger dataset of observations and gives greater values of significance than has appeared in symbolic corpus analysis of emotions in the past, and includes general discussions and directions for future work.

Phase-matched harmonic generation in gas-filled waveguides in the vicinity of a multiphoton resonance

We investigate phase-matched harmonic generation of ultrashort (picosecond) laser pulses, tuned in the vicinity of a five-photon resonance in argon, confined in a hollow-core optical waveguide. This combines resonance enhancements and tight spatial confinement to increase the conversion efficiency toward vacuum-ultraviolet radiation pulses. We demonstrate that appropriate choice of the gas pressure maintains optimal phase-matching conditions also in the presence of inevitable dynamic level shifts at high intensities. Moreover, we reveal the considerable contribution of higher-order transversal waveguide modes to the total conversion efficiency and investigate the role of cascading frequency conversion processes. Finally, we study additional signal enhancements by buffer gas admixtures. The experimental data are compared with numerical simulations, taking higher transversal waveguide modes and cascade frequency conversion into account, identifying also the potential of quasi-phase matching by polarization mode beating. Our investigations show that proper choice of experimental parameters enables significant resonance enhancements in the conversion efficiency of harmonic generation.

Fiber Birefringence Measurement of Single-Mode Fiber by a Polarimetric Fiber Ring Laser

A novel and low-cost fiber birefringence measurement scheme is proposed and studied by a polarimetric fiber ring laser interrogated by the beat frequency technique. For the fiber ring laser, modes splitting occur between two orthogonal polarization modes because of intrinsic fiber birefringence. As a result, two types of beat frequency signals (i.e., longitudinal mode beat frequency and polarization mode beat frequency signals) are generated simultaneously. The principle of measurement of fiber birefringence by the fiber ring laser has been analyzed theoretically in detail. Two alternative expressions for fiber birefringence change are obtained, which is very convenient for users to choose different order longitudinal and polarization mode beat frequency signals. The variation of fiber birefringence caused by applied strain has been calculated in terms of beat frequency shift. To validate the proposed sensing system, the strain-induced fiber birefringence change in commercial Erbium-doped fiber is investigated and obtained by measuring frequency shift of two types of beat frequency signals. The experimental results show that the fiber birefringence experiences a linear decrease when the applied strain is increased. A strain response coefficient of –1.756 × 10–11/μe is obtained. The proposed sensing scheme has obvious advantages of simple structure, low cost, and high sensitivity and it will provide great convenience for measuring fiber birefringence for short-length single mode fiber.

Crystallinity-dependent capacity of a LiBC anode material in Li-ion batteries.

LiBC is a layered boron carbide material that was first studied as a superconducting material. Theoretical study showed that LiBC is a promising cathode material in Li-ion batteries, while our early computational and experimental studies demonstrated the feasibility of LiBC as a high-capacity anode material for Li-ion batteries. In this work, LiBC has been synthesized by a solid-state method using different carbon precursors of acetylene black and graphite, marked a-LiBC and g-LiBC, respectively, and their electrochemical properties and corresponding structure evolutions have been investigated in Li-ion batteries. As a result, a-LiBC delivers a higher specific capacity than g-LiBC, 500 mA h g-1vs. 200 mA h g-1, after 50 cycles at a rate of 0.1C. Actually, a significant structural evolution of a-LiBC was observed by in situ XRD during the charge/discharge processes, while a nearly constant structure was detected for g-LiBC. Meanwhile, in situ Raman results revealed the evolution of vibrational beating modes at different charge/discharge states. Therefore, the specific capacity of LiBC is closely related to its crystallinity, and LiBC with amorphous carbon precursors could be adopted as a promising anode material for Li-ion batteries.

Design of Compact but Fabrication-Tolerant Vertical Coupler for Active–Passive Integration

We propose a novel scheme of compact but fabric-ation-tolerant vertical coupler based on resonant coupling for active–passive integration. The transfer matrix method is used to carry out the design and optimization of various vertical coupler structures efficiently. Simulations show that transfer efficiency higher than 96% can be achieved for the proposed coupler with a length as short as 55 μm which is about half of a classic adiabatic coupler design. By employing a mode beating section with a length twice the beating length and a relatively steep linear taper, the proposed resonant coupler breaks the strongly periodic behavior of mode beating in conventional resonant couplers, thus dramatically improving the fabrication tolerance. The proposed resonant coupler is predicted with a fabrication tolerance of 380 nm which is about 2.5 times of the conventional resonant coupler, under the condition that the transfer efficiency is higher than 90%. The scheme of this paper can be widely used to design compact and fabrication-tolerant couplers.

Fabrication and Characterization of 2 × 2 Microfiber Coupler for Generating Two Output Stable Multiwavelength Fiber Lasers

A compact 2 × 2 microfiber coupler (MC) is proposed and experimentally demonstrated for the generation of stable multiwavelength fiber laser in the C-band region. The 2 × 2 MC is fabricated by twisting and fusing together two lateral dual single mode fibers using the flame-brushing technique. The 2 × 2 MC has a waist diameter of 8 μ m with a coupling ratio of 38:62 and 45:55 at ports 1 and 2, respectively. The 2 × 2 MC exploits super mode coupling to produce spatial mode beating and suppress mode competition of the homogeneous gain medium in a fiber ring laser. The channel spacing of the output wavelengths is immune to changes in the external environment and can be extracted from the peaks of the 2 × 2 MC's periodic oscillation. The proposed laser is capable of generating stable single, dual, and triple lasing wavelengths from its two output ports over a range of more than 29 nm, which is further enhanced by the polarization rotation mechanism. The generating lasing wavelengths have a 3 dB line-width of less than ∼30 pm and peak-to-floor ration of about 58 dB at a pump power of 38 mW.

Amplification of a turbulence driven seed magnetic island by bootstrap current

The amplification of a turbulence driven seed magnetic island by the bootstrap current is investigated numerically in a reduced magnetohydrodynamic model as a possible additional channel for the excitation and development of a neoclassical tearing mode (NTM) in a tokamak. The novel mechanism for such an excitation involves a two-step process, namely, the formation of a seed island due to a nonlinear beating of interchange turbulence modes followed by an amplification of the seed island by the bootstrap current leading to a large saturated island. Our study indicates that the level of turbulence controls both the critical seed island size and the saturated island size.

Temporal correlations of spectrally narrowband photon pair sources

We report on theoretical and experimental investigations of time-resolved cross- and auto-correlation measurements of spectrally narrowband photon pairs generated in sources based on parametric down conversion in resonant waveguide structures. We show that time-resolved measurements provide detailed and useful information about the spectral and modal structure of the bi-photon state. The shape of the cross-correlation function is asymmetric with exponential decays determined by the lifetimes of the signal and idler photons in the cavity. The time-resolved auto-correlation has Lorentzian shape. The measured value convoluted with the detector windows and mode beating can be used to characterise the spectral longitudinal mode behaviour. The temporal width of the auto-correlation function is more than two times longer that the cross-correlation time. This reveals that the spectral bandwidth of the single-photon component is much broader than the spectral width of the two-photon component.

Stable C-band fiber laser with switchable multi-wavelength output using coupled microfiber Mach-Zehnder interferometer

Abstract A compact coupled microfiber Mach-Zehnder interferometer (CM-MZI) is proposed and experimentally demonstrated for C-band region multi-wavelength tuning and switching in a fiber laser. The CM-MZI is fabricated using a 9 μm single tapered silica optical microfiber fabricated by flame-drawing technique and exploits multi-mode interference to produce spatial mode beating and suppress mode competition of the homogeneous gain medium. The output wavelength spacing is immune to changes in the external environment, but can be changed from 1.5 nm to 1.4 nm by slightly modifying the path-length difference of the CM-MZI. The proposed laser is capable of generating single, dual, triple, quintuple, and sextuple stabilize wavelengths outputs over a range of more than 32 nm using polarization rotation (PR) and macro-bending. The lasers having a 3 dB line-width of less than ∼30 pm and peak-to-floor of about 55 dB at a pump power of 38 mW.

Obtaining spontaneously beating cardiomyocyte-like cells from adipose-derived stromal vascular fractions cultured on enzyme-crosslinked gelatin hydrogels

Heart failure often develops after acute myocardial infarction because the injured myocardial tissue fails to recover or regenerate. Stem cell transplantation using adult cell sources, such as adipose-derived stromal vascular fraction (SVF), draws extensive attention. In this study, SVF cells were isolated from rat adipose tissue and cultivated on enzyme-crosslinked gelatin hydrogels. Morphological features of cell development and spontaneous beating behavior from these cells were observed and recorded. Cardiac phenotypes were characterized via immunofluorescence staining, and the expression of cardiac-specific genes was measured via RT-PCR. The functional assessment of SVF-derived cardiomyocyte-like cells (SVF-CMs) was performed by detecting cellular calcium transient activities and pharmacological responses. Results showed that most SVF-CMs exhibited elongated myotubule shapes and expressed cardiac troponin I strongly. SVF-CMs expressed cardiac-specific RNA (including transcription factors GATA binding protein 4) and myocyte enhancer factor 2c, as well as the structural proteins, namely, sarcomere actinin alpha 2, cardiac troponin I type 3, cardiac troponin T type 2, and cardiac gap junction protein alpha 1. Their beating mode, calcium activities, and pharmacological responses were similar to those of native CMs. Spontaneously beating SVF-CMs can be derived from adipose tissue-derived SVFs, and enzyme-crosslinked gelatin hydrogel promoted the cardiac differentiation of SVF cells.

New features of the MAX IV thermionic pre-injector

The MAX IV facility in Lund, Sweden consists of two storage rings for production of synchrotron radiation. The smaller 1.5GeV ring is presently under construction, while the larger 3GeV ring is being commissioned. Both rings will be operating with top-up injections from a full-energy injector. During injection, the electron beam is first delivered to the main injector from a thermionic pre-injector which consists of a thermionic RF gun, a chopper system, and an energy filter. In order to reduce losses of high-energy electrons along the injector and in the rings, the electron beam provided by the thermionic pre-injector should have the correct time structure and energy distribution. In this paper, the design of the MAX IV thermionic pre-injector with all its sub components is presented. The electron beam delivered by the pre-injector and its dependence on parameters such as optics, cathode temperature, and RF power are studied. Measurements are here compared with simulation results obtained by particle tracking and electromagnetic codes. The chopper system is described in detail, and different driving schemes that optimize the injection efficiency for the two storage rings are investigated. During operation, it was discovered that the structure of the beam delivered by the gun is affected by mode beating between the accelerating and a low-order mode. This mode beating is also studied in detail. Finally, initial measurements of the electron beam delivered to the 3GeV ring during commissioning are presented.

Theoretical study of gain-induced mode coupling and mode beating in few-mode optical fiber amplifiers.

We developed a generalized field-propagating model for active optical fibers that takes into account mode beating and mode coupling through the amplifying medium. We applied the model to the particular case of a few-mode erbium doped fiber amplifier. Results from the model predict that mode coupling mediated by the amplifying medium is very low. Furthermore, we applied the model to a typical amplifier configuration. In this particular configuration, the new model predicts much lower differential modal gain than that predicted by a classical intensity model.