High power supercontinuum generation in a nonlinear ytterbium-doped fiber amplifier

High power supercontinuum generation with 70 W average output power in a nonlinear ytterbium-doped fiber amplifier is demonstrated using all-normal dispersion, all-fiber master oscillator power amplifier configuration. The supercontinuum covers from 1064 nm to beyond 1700 nm with spectral flatness better than 12 dB and 67.3% optical to optical conversion efficiency. The almost uniform spectral power density across the whole continuum is more than 70 mW/nm and the nanosecond bursts output have an effective peak power of 82.7 kW.     

Identification and separation of local and nonlocal optical nonlinear refraction effects: theory and experiment

Understanding the nonlinear optical effect of novel materials plays a crucial role in the fields of photonics and optoelectronics. Herein, we theoretically and experimentally investigate the simultaneous presence of third-order locally refractive nonlinearity and thermally induced nonlocal nonlinearity saturation. We present analytical expressions for the closed-aperture Z-scan trace and the number of spatial self-phase modulation(SSPM) rings,which allows one to unambiguously and conveniently separate the contributions of local and nonlocal nonlinear refraction in the case that both effects occur simultaneously. As a test, we study both the local and thermally induced nonlocal nonlinear refraction in fullerene/toluene solution by performing continuous-wave Z-scan and SSPM measurements at two different wavelengths. This work enriches the understanding of the physical mechanism of the optical nonlinear refraction effect in solution dispersions of nanomaterials, which can be exploited for nonlinear photonic devices.     

Thermoluminescence characterization of traps involved in the photodarkening of ytterbium-doped silica fibers

The photodarkening (PD) mechanisms of ytterbium-doped silica optical fibers have still not been elucidated, although hardening routes have been proposed. Most basic questions are still under debate about the assignment of the darkening excitation bands into the UV range, the nature of absorbing centers (photoionized centers or trapped carriers?), or of traps accepting photo-released carriers (electron or hole traps?). We used thermoluminescence measurements to characterize traps populated by different radiation types. It is notably demonstrated that photodarkening involves silica hole traps. The popular idea that color centers are formed upon carrier trapping is not consistent with our observations.     

Four-wave mixing in non-step-index optical fibers: a sensitive technique for measurement of refractive-index profiles

Investigation of four-wave mixing 6.4-mum core-diameter fiber with 520-610-nm pump light is used to develop a new technique for characterizing the refractive-index profiles of non-step-index fibers. Deviation from the step-index profile is measured. Small variations in the refractive index between different segments of the same commercial fiber is measured with a sensitivity of 0.0001.     

Permanent refractive-index modification in germanium-doped optical fibers by use of red light

We observed photoinduced birefringence in elliptical-core optical fibers by using a continuous-wave krypton-ion laser. We induced the birefringence by injecting 20 mW of 647-nm or 50 mW of 676-nm light into the fiber at 45 degrees to the slow axis. The rate of change of the refractive index was found to be proportional to the square of the average power. Polarization mode couplers written into the fibers have been stable for more than 2 years and can be erased by use of light polarized perpendicularly to the original writing beam.     

Pulse dynamics in dispersive nonlinear medium in presence of traveling refractive-index wave

The dynamics of wave packets propagating in nonlinear waveguides in presence of a traveling refractive-index wave is investigated. It is shown that the regime of self-steepening is possible in these waveguides not only at the trailing edge of the wave packet, but also at its leading edge. Nonreciprocal effects are typically observed when the optical pulse and the wave of optical inhomogeneity are either co- or counterpropagating.     

Chirped-pulse oscillators: theory and experiment

Theory of chirped-pulse oscillators operating in the positive dispersion regime is presented. It is found that the chirped pulses can be described analytically as solitary pulse solutions of the nonlinear cubic-quintic complex Ginzburg–Landau equation. Due to the closed form of the solution, basic characteristics of the regime under consideration are easily traceable. Numerical simulations validate the analytical technique and the chirped-pulse stability. Experiments with 10 MHz Ti:Sa oscillator providing up to 150 nJ chirped pulses, which are compressible down to 30 fs, are in agreement with the theory. PACS 42.65.Re; 42.65.Tg; 42.55.Rz     

Composition dependence of the photoinduced refractive-index change in lead silicate glasses

A large photoinduced refractive-index change (as great as Dn=0.21+/-0.04) is obtained in lead silicate glasses by irradiation with the frequency-quadrupled output of a Q -switched YAG laser (266 nm). An approximately exponential relationship is found between the photoinduced refractive-index change and the lead cation mole fraction over the composition range from 18.7% to 57%. The induced refractive-index change is permanent and shows no decay after heating to 360 degrees C during 1 h. Dispersion of the refractive-index change suggests that the photosensitivity is associated with changes in the intrinsic glass absorption edge.     

Steps on surfaces: experiment and theory

The properties of steps in thermal equilibrium are described in the context of prediction of the stability and evolution of nanostructures on surfaces. Experimental techniques for measuring the appropriate step parameters are described, and simple lattice models for interpreting the observations are reviewed. The concept of the step chemical potential and its application to the prediction of step motion (and therefore surface mass transport) is presented in depth. Examples of the application of this step-continuum approach to experimental observations of evolution of surface morphology are presented for morphological phase transitions, the decay of metastable structures, and the spontaneous evolution of metastable structure due to kinetic instabilities.     

Contribution of photoinduced densification to refractive-index modulation in Bragg gratings written in Ge-doped silica fibers

We have computed the contribution of UV-light-induced densification to the refractive-index modulation of fiber Bragg gratings. Our results confirm that, for strong gratings written in Ge-doped silica fibers with 248-nm UV light, density changes account for a major part of the photosensitivity effect.     

Range discrimination in ultrasonic vibrometry: theory and experiment

A technique has been developed to demodulate periodic broadband ultrasonic interrogation signals that are returned from multiple scattering sites to simultaneously determine the low-frequency displacement time histories of each individual site. The technique employs a broadband periodic transmit signal. The motions of scattering sites are separately determined from the echoed receive signal by an algorithm involving comb filtering and pulse synthesis. This algorithm permits spatial resolution comparable to pulse-echo techniques and displacement sensitivities comparable to pure-tone techniques. A system based on this technique was used to image transient audio-frequency displacements on the order of 1-10 μm peak (≥?50 nm/√Hz) that were produced by propagating shear waves in a tissue phantom. The system used concentric transmitting and receiving transducers and a carrier signal centered at 2.5 MHz with an 800 kHz bandwidth. The system was self-noise-limited and capable of detecting motions of strongly reflecting regions on the order of 1 nm/√Hz. System performance is limited by several factors including signal selection, component hardware, and ultrasonic propagation within the media of interest.     

Piezoelectric effect in GaAs nanowires: experiment and theory

The anomalous piezoelectric effect in GaAs nanowires was detected (the piezoelectric strain coefficient up to d₃₃ ≈ 26 pC/N) that is 12 times larger than the theoretically estimated value. This result is explained by the dominant content of the phase with the wurtzite‐type crystal structure in GaAs nanowires and an increased pressing force by the contact layer on the nanowire. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Bragg grating fabrication in germanosilicate fibers by use of near-UV light: a new pathway for refractive-index changes

Using 334-nm light, we demonstrate side writing of Bragg gratings with an index change of ~10(-4) in germanium-doped fibers. No hydrogen loading of the fibers was required. These gratings have the same temperature stability as gratings fabricated with 240-nm light. Our results suggest that photoionization is not needed for formation of gratings in Ge-doped glass. We observe an enhancement of the 334-nm photosensitivity in boron-codoped fibers and suggest that B facilitates a structural transformation of the glass.     

Kagome光纤超快非线性光学研究进展

Kagome光纤(简称KGF)是一种不依赖带隙导光的新型空芯微结构光纤,其结构设计灵活、损伤阈值高、损耗低(高透区损耗可低至~40 dB/km)、支持宽带传输(&;amp;lt;500 nm),并可通过纤芯改变所充气体及调节气压实现光纤色散、非线性效应的有效调制,在强场物理、超快激光技术等领域研究中优势突出。基于KGF在超快光学中的重要意义,该文对近年来国际上关于KGF在非线性光学变频及超短脉冲压缩等领域的研究成果进行介绍,并对关键性应用技术进行简要分析,最后对其发展前景进行展望。     

Oscillating neutrinos: theory vs experiment

Possible hints on neutrino masses are reviewed. They come from the deficits in the solar as well as atmospheric neutrinos and from need of a significant amount of hot component in the dark matter of the universe. The role of three generation mixing in simultaneously solving the solar and atmospheric neutrino problem is discussed. All the three hints can be reconciled if three neutrinos are almost degenerate. Models for neutrino masses and mixing implied by the above hints are briefly discussed.     

Sub-milliwatt sub-millisecond recording of population gratings in ytterbium-doped optical fibers at 976 nm

We report effective formation of population gratings in ytterbium-doped fibers by coherent light at 976 nm, i.e. at the wavelength that is typically utilized for optical pumping of these fibers. The dynamic gratings need sub-mW cw laser power to be recorded and have comparable amplitude (absorption) and phase (refractive index) components. Given the spontaneous relaxation time of a Yb³⁺ meta-stable level of ~ 0.8 ms, the grating formation time proved also to be in the sub-ms region with the phase grating component significantly slower than the amplitude one.     

Locally periodic Timoshenko rod: experiment and theory

The flexural vibrations of a locally periodic rod, which consists of N unit cells, are discussed both from the experimental and theoretical points of view. Timoshenko's beam theory and the transfer matrix method are used to calculate the normal-mode frequencies and amplitudes. The theoretical values are then compared with the experimental ones, which are obtained using an electromagnetic acoustic transducer (EMAT). Good agreement between the numerical results and the experimental measurements is obtained. It is shown that as N grows, a band spectrum emerges.     

K-shell photodetachment of He- : experiment and theory

High-resolution K-shell photodetachment measurements of He- giving rise to He+ ions have been performed using a merged synchrotron vacuum ultraviolet photon-ion beam technique. The measurements on this fundamental negative ion display dramatic structure differing substantially, qualitatively and quantitatively, from the corresponding process in neutral atoms and positive ions, owing to the dominance of correlation in both initial and final states of He-. In addition, this experimental investigation provides an unambiguous test of two independent theoretical calculations that report serious discrepancies and shows excellent agreement with one of them.