Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots

The use of glasses doped with PbS nanocrystals as intracavity saturable absorbers for passive Q-switching and mode locking of c-cut Nd:Gd_0.7Y_0.3VO₄, Nd:YVO₄, and Nd:GdVO₄ lasers is investigated. Q-switching yields pulses as short as 35 ns with an average output power of 435 mW at a repetition rate of 6–12 kHz at a pump power of 5–6 W. Mode locking through a combination of PbS nanocrystals and a Kerr lens results in 1.4 ps long pulses with an average output power of 255 mW at a repetition rate of 100 MHz.     

Widely Tunable cw Diode-Pumped 1.9-μm Tm:GdVO4 Laser at Room Temperature

A widely tunable cw diode-pumped room-temperature Tm：GdVO4 laser is built. Output power of 2.8 W and a slope effic/ency of 22% pumped by a 18 W Fibre-coupled diode laser at 795nm have been obtained. Continuous tunability from 1820nm to 1946nm is achieved. In addition, the factors that contribute to the efficiency of oscillation are studied.     

Photon echo chirp transform using a stabilized frequency agile laser

We report on the demonstration of the chirp transform algorithm for the instantaneous optical frequency analysis of wide band radiofrequency signals. The process exploits photon echoes in an Er³⁺:YSO crystal at 1536 nm. The analysis covers a 1.6 GHz instantaneous bandwidth together with more than 24,000 independent channels. These performances require a frequency agile laser source that is phase-locked in order to take advantage of the tens of μs of coherence time offered by the crystal optical transition.     

Crystal field and microscopic spin Hamiltonians approach including spin-spin and spin-other-orbit interactions for d and d ions at low symmetry C3 symmetry sites: V in Al2O3

A new module has been developed within the CFA/MSH computer package, which is applicable for d² and d⁸ ions at sites of trigonal symmetry type I (C_3v,D₃,D_3d) and type II (C₃,C_3i), including the ‘imaginary’ CF term. For the first time the spin-spin (SS) and spin-other-orbit (SOO) interactions have also been included in the Hamiltonian. This module enables to study the contributions to the energy levels and the spin Hamiltonian parameters, i.e. zero-field splitting D and g-factors: g_∥ and g_⊥. The contributions arising from the spin-orbit (SO), SS, and SOO interaction as well as those due to the low symmetry CF effects induced by the distortion angle ?, which describes the difference between C₃ and C_3v symmetry, can be studied. As an application of the new module, calculations have been carried out for V³⁺(3d²) ions in α-Al₂O₃ crystal, taking into account for the first time the SS and SOO interactions, and the low symmetry CF effects. The results show that (i) the contributions from the SS and SOO interactions to the energy levels are larger for free V³⁺ ions than those for V³⁺ ions in α-Al₂O₃ crystal, (ii) both the contributions to the SH parameters and the energy levels arising from the SOO interaction are larger than those arising from the SS interaction, (iii) the contributions due to the low symmetry CF effects induced by the distortion angle ? are in general significant, (iv) D and g_⊥ are sensitive to the distortion angle ?, whereas g_∥ is insensitive to ?, and (v) the influence of the lattice distortions on the spectroscopic properties of V³⁺ ion in α-Al₂O₃ is pronounced. It appears important for similar ion-crystal cases to consider the lattice distortions in detailed calculations, which take into account the relevant contributions from the SO, SS and SOO interactions. A good agreement between the theoretical and experimental results has been obtained.     

SOA-based actively mode-locked fiber ring laser by forward injecting an external pulse train

An actively mode-locked fiber ring laser based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) is demonstrated to operate stably with a simple configuration. By forward injecting an easily-generated external pulse train, the mode-locked fiber laser can generate an optical-pulse sequence with pulsewidth about 6 ps and average output power about 7.9 mW. The output pulses show an ultra-low RMS jitter about 70.7 fs measured by a RF spectrum analyzer. The use of the proposed forward-injection configuration can realize the repetition-rate tunability from 1 to 15 GHz for the generated optical-pulse sequences. By employing a wavelength-tunable optical band-pass filter in the laser cavity, the operation wavelength of the designed SOA-based actively mode-locked fiber laser can be tuned continuously in a wide span between 1528 and 1565 nm. The parameters of external-injection optical pulses are studied experimentally to optimize the mode-locked fiber laser.     

Misidentification or Identification Rates for Four Kinds of Particles on BES-II

Using the data samples collected at and around the center-of-mass energies of 3.097 GeV and 3. 773 GeV with the BES-Ⅱ detector at the BEPC collider, we select the pure particle samples of muon, electron, kaon and pion, and their mutual （mis）identification rates are also obtained.     

A continuous-mode model of the mode locking in a pump-modulated laser

The active mode-locking process of the multimode laser with an external pump modulation is theoretically investigated in the frequency domain within the framework of the continuous-mode approximation. Intermode interaction and mode-coupling effects, including both AM and FM modulations, are naturally considered in a hierarchical equation of the mode components derived from the multimode Maxwell-Bloch equations. It is reduced to a continuous-mode equation that can be solved analytically in a stationary case, and used to discuss the spectral line shape and the phase dynamics of mode-components as a function of modulation amplitude and detuning of the modulation frequency. We predict a novel oscillation existing below the threshold of the ordinary complete mode-locking: The intensity of the total electric field yields a stable pulse train but its phase varies irregularly in time. This semi-locked state is characterized by a nonlinear chirping, an asymmetric spectrum, and drifting phases of the field mode-components.     

Linear hydrodynamics and viscoelasticity of nematic elastomers

We develop a continuum theory of linear viscoelastic response in oriented monodomain nematic elastomers. The expression for the dissipation function is analogous to the Leslie-Ericksen version of anisotropic nematic viscosity; we propose the relations between the anisotropic rubber moduli and new viscous coefficients. A new dimensionless number is introduced, which describes the relative magnitude of viscous and rubber-elastic torques. In an elastic medium with an independently mobile internal degree of freedom, the nematic director with its own relaxation dynamics, the model shows a dramatic decrease in the dynamic modulus in certain deformation geometries. The degree to which the storage modulus does not altogether drop to zero is shown to be both dependent on frequency and to be proportional to the semi-softness, the non-ideality of a nematic network. We consider the most interesting geometry for the implementation of the theory, calculating the dynamic response to an imposed simple shear and making predictions for effective moduli and (exceptionally high) loss factors. Received 16 October 2000 and Received in final form 10 December 2000     

Magnetic resonance on LiCuVO 4

EPR and ⁷Li NMR measurements were performed in the distorted inverse spinel V(LiCu)O₄ down to 1.5 K. Anisotropy effects on magnetic resonance spectra due to the Jahn-Teller distortion of the oxygen octahedra surrounding the copper ions are discussed. The estimation of the spin-spin interactions deduced from the EPR-relaxation rate Δ H reveals a situation comparable to the prototypical one-dimensional S = 1/2 Heisenberg antiferromagnet CuGeO₃. Approaching three-dimensional antiferromagnetic order ( T _N ≈ 2 K) from above, both magnetic relaxation rates, Δ H _EPR and ⁷ (1/ T ₁ ), respectively, exhibit nearly the same critical divergence reminding to the onset of three-dimensional order in two-dimensional layered systems. Received 22 January 2001 and Received in final form 6 May 2001     

Active synchronization of two mode-locked lasers with optical cross correlation

Two mode-locked Ti:sapphire lasers of different wavelengths were precisely synchronized by a simple feedback system employing sum-frequency generation (cross correlation). When the timing error exceeded the pulse duration, the periodic bunch of the sum-frequency pulse was used for rough timing adjustment. Using cross correlation with a stretched pulse, we struck a balance between wide locking range and sensitive timing detection. When the two lasers were well-synchronized, we obtained a continuous cross-correlation pulse train for 3 min. The holding time of the laser synchronization was extended to over one hour by adding a motorized stage to the PZT-mounted cavity mirror. We estimated the rms timing jitter between the two lasers by a scanning cross-correlation measurement. We confirmed that the rms timing jitter of the two lasers during 1.8 s was 28 fs. Received: 30 January 2002 / Revised version: 14 June 2002 / Published online: 8 August 2002