Large Positive Dispersion Research Articles

Collective particle dynamics of molten NaCl by inelastic x-ray scattering

An inelastic x-ray scattering experiment has been performed on molten NaCl over wide wave vector and energy transfer ranges. Data of high statistical quality are analyzed using a memory function approach within a generalized Langevin equation. The approach with two relaxation times for the memory function provides a very good data description over the whole wave vector range beyond the hydrodynamic regime. A slow thermal and a fast structural relaxation process in the memory function completely define the density fluctuations in molten NaCl and evidences the thermal-viscoelastic model as the minimal description for collective particle dynamics in molten alkali halides. The obtained excitation frequencies demonstrate a large positive dispersion effect, which can be related to the viscoelastic reaction of the molten salt. A transition from the viscoelastic to a hydrodynamic response of the molten salt at small wave vectors is observed. In the hydrodynamic regime the resulting thermal diffusivity agrees well with values obtained through light scattering. The modeling indicates some deficiencies at small wave vectors and large energy transfers and the spectra of the current correlation function evidences additional intensity at high frequency. The frequency of these additional modes approach a non-zero value at zero wave vector and indicates a non-acoustic character of these excitations. The frequency center of this additional inelastic intensity coincides with optic-type modes in molten NaCl predicted by simulations.

Dispersion-managed, high-power, Tm-doped ultrashort pulse fiber laser using single-wall-carbon-nanotube polyimide film

We demonstrated a dispersion-managed, high-power, Tm-doped ultrashort pulse fiber laser using a single-wall-carbon-nanotube (SWNT) polyimide film. SWNTs with a diameter of 1.6 nm were synthesized with the enhanced direct injection pyrolytic synthesis (e-DIPs) method, and thin polyimide films in which SWNTs were dispersed were developed as saturable absorbers in the wavelength range λ = 1.8–2.0 µm. An all-fiber type, passively mode-locked, ultrashort-pulse Tm-doped fiber laser was demonstrated using the developed SWNT films. Wavelength tuning operation with gain fiber control and dispersion management of the developed fiber laser were investigated. Stable soliton and dissipative soliton mode locking operations were observed. High-power (102.6 mW) single-pulse mode-locking operation was achieved in a large positive dispersion regime. The repetition rate was 21.6 MHz, and the corresponding pulse energy was 4.75 nJ. To the best of our knowledge, this is the highest power operation of a Tm-doped fiber laser using carbon nanotubes and film-type devices with nano-carbon materials. The developed laser showed self-staring, stable performance and is useful for practical applications.

Spectral hole burning of surface plasmon polaritons via soliton waves at the interface of sodium and gold media

We investigate the possibility of creating spectral hole burning in surface plasmon polaritons (SPPs) at the interface of sodium vapor and gold media. The spectral holes are measured at specific parameters, phases and directions of the control fields. The direction and phase of the control field affect the creation of spectral hole burning in SPPs the most. The pulse shape in the spectral burning hole region is uniform and undistorted due to the cancellation of nonlinear Doppler broadening and large group velocity dispersion. The behavior of SPPs pulse in the spectral hole region is that of solitons, which arise due to cancellation of large positive dispersion for phase shift with nonlinear effect.. The results may have potential applications in nano-photonics, plasmonsters, photovoltaic devices, cloaking and sensing technology.

Effective Removal of Gordon–Haus Jitter in Mode-Locked Fiber Lasers

We demonstrate the effective removal of Gordon–Haus (GH) jitter in mode-locked fiber lasers based on the narrow bandpass filtering, verified from large positive dispersion to large negative dispersion. Simulations and experiments indicate that there exists an optimum filtering bandwidth for the near-zero GH jitter along with the constant directly coupled jitter. The corresponding measured root-mean-square timing jitter (10 kHz to 10 MHz) is restrained within 0.6 dB, even though the net dispersion is largely varied by 70% of the intracavity fiber dispersion. The demonstrated effective GH jitter elimination free of the dispersion management gives an easy way for the low-jitter laser design and the related high-precision applications outside the laboratory.

Transition from hydrodynamic to viscoelastic propagation of sound in molten RbBr.

Inelastic neutron scattering was applied to measure the acoustic-type excitations in the molten alkali halide rubidium bromide. For molten RbBr neutron scattering is mainly sensitive to the number density fluctuation spectrum and is not influenced by charge fluctuations. Utilizing a dedicated Brillouin scattering spectrometer, we focused on the small-wave-vector range. From inelastic excitations in the spectra a dispersion relation was obtained, which shows a large positive dispersion effect. This frequency enhancement is related to a viscoelastic response of the liquid at high frequencies. Towards small wave vectors we identify the transition to hydrodynamic behavior. This observation is supported by a transition of the sound velocity from a viscoelastic enhanced value to the adiabatic speed of sound for the acoustic-type excitations. Furthermore, the spectrum transforms into a line shape compatible with a prediction from hydrodynamics.

Nanosecond Pulse Generation in Lumped Normally Dispersive All-Fiber Mode-Locked Laser

We report an all-normal dispersion, all-fiber integrated ytterbium laser mode-locked by a single wall carbon nanotube (SWNT) saturable absorber, with a chirped fiber Bragg grating (CFBG) providing dispersion of 35.71 ps ·nm-1 allowing us to directly obtain nanosecond-scale duration pulses. In this large positive dispersion regime, mode-locking is initiated by the saturable action of the SWNT device, with pulse formation stabilized by the combined bandwidth limiting effect of the gain dispersion and the narrow reflection band of the CFBG.

Passively Q-switched mode-locking Erbium-doped fiber laser with net-normal dispersion using nonlinear polarization rotation technique

We experimentally demonstrate a passively Q-switched mode-locking (QML) operation in an Erbium-doped fiber ring laser with net normal dispersion by using nonlinear polarization rotation technique. A 2 m long section of dispersion compensating fiber (DCF) with extra large positive dispersion was inserted into the cavity to ensure the fiber laser working in the region of net positive dispersion. By carefully adjusting the polarization controller, both uniform dissipative mode-locking pulses with fundamental repetition rate and QML pulse trains with tunable repetition rate from 71.58 to 98.83 kHz are achieved. It is found that the QML operation is caused by the interaction between the polarization state of the pulse and the intracavity polarizer.

All-fiber mode-locked nanosecond laser employing intracavity chirped fiber gratings

We demonstrate that nanosecond pulses are generated directly from an all-fiber mode-locked ytterbium-doped fiber laser. A pair of Chirped Fiber Gratings (CFGs) with different sign of dispersion is employed for intracavity dispersion management. Self-starting stabilized mode-locking operation is achieved by nonlinear polarization evolution (NPE). The 1.27 ns pulses are obtained after one CFG with large positive dispersion. The pulse energy is up to 15 nJ at a repetition rate of 3.48 MHz.

Dynamical structure in liquid Te using inelastic x-ray scattering: from semiconductor-metal transition to metallic regime

We have carried out inelastic x-ray scattering measurements of liquid Te at SPring-8 in Japan and obtained the dynamic structure factors S(Q,W) at 500, 650 and 800°C in Q-range from 1.3 to 43 nm1 and W-range from -40 to 40 meV, respectively. Collective excitation is ascertained in each S(Q,W). Dynamical velocity vQ of the excitation mode decreases with increasing temperature, which is normal but contradicts to the behaviour of the ultrasonic sound velocity vs that anomalously increases against temperature. This contradiction means that the ultrasonic anomaly is not derived from the dynamics in this Q-region and larger-size (> a few nm) structural relaxation process exists. The value of "positive dispersion" (≡ (vQ - vs)/vs) decreases from 70% at 500°C to 15% at 800°C. Large positive dispersion is also observed at the nonmetal-metal transition regime in other liquids such as supercritical Hg, which imply that this is a common phenomenon concerning with nonmetal-metal transition.

Modal properties of solid-core photonic bandgap fibers

The modal behavior of a ∼500-μm 2 mode area photonic bandgap fiber was theoretically and experimentally investigated. First, a full-vector finite element algorithm was used to predict the modal properties. Then, a low-coherence reflectometry technique was employed to measure the chromatic dispersion and evaluate the number of guided modes. A short length of fiber was found to guide only two LP modes with different attenuation coefficients. The measured chromatic dispersion of the fundamental mode was found in close agreement with theoretical value. The large positive dispersion is theoretically explained by the positive waveguide dispersion induced by weak a spectral variation of radial electric field distribution in Bragg fibers.

Propagating particle density fluctuations in molten NaCl

In this paper we present the observation of acoustic modes in the spectra of molten NaCl measured over a large momentum transfer range using synchrotron radiation. A surprisingly large positive dispersion was deduced witha mode velocity exceeding the adiabatic value by nearly 70%. The large effect seems to be describable as a viscoelastic reaction of the liquid. Additionally, the derived dispersion resembles the Q-ω relation of the acoustic modes in liquid sodium. As an explanation for the large positive dispersion we propose that the density fluctuations in molten NaCI can be interpreted as a decoupled motion of the lighter and smaller cations on a nearly resting anionic background. These molten alkali halide measurements are the first experimental evidences for the so-called fast sound in a binary ionic liquid.