Lambda transition and Bose-Einstein condensation in liquid

We present the theory describing Bose-Einstein condensation (BEC) and superfluidity in a liquid ^{4}He based on the concept that for some temperature interval, there exist metastable diatomic clusters or diatomic quasiparticles which are the bound states of two atoms of ^{4}He. It is shown that in liquid ^{4}He for the temperature region 1K≤T≤T_{λ} diatomic quasiparticles macroscopically populate the ground state which leads to BEC in liquid ^{4}He. The approach yields the lambda temperature as T_{λ}=2.16K, which is in excellent agreement with the experimental lambda transition temperature T_{λ}=2.17K. The concept of diatomic quasiparticles also leads to superfluid and BEC fractions which are in good agreement with experimental data and Monte Carlo simulations for liquid ^{4}He. It is also shown that the condensate fraction for low temperature (T≤0.5K) at saturated vapor pressure is ρ_{0}/ρ=7.22%, which is very close to the value 7.25±0.75% obtained in recent measurements.

Periodic and localized waves in parabolic-law media with third- and fourth-order dispersions.

Considering the higher-order nonlinearity is essential in a broad range of real physical media as it significantly influences the wave dynamics in these systems. We study the propagation of femtosecond light pulses inside an optical fiber medium exhibiting higher-order dispersion and cubic-quintic nonlinearities. Pulse evolution in such a system is governed by a higher-order nonlinear Schrödinger equation incorporating second-, third-, and fourth-order dispersions as well as cubic and quintic nonlinearities. The periodic and solitary wave solutions are identified using the equation method. Results presented indicated the potentially rich set of periodic waves in the system under the combined influence of higher-order dispersive effects and cubic-quintic nonlinearity. The velocity of these structures is uniquely dependent on all orders of dispersion. Conditions on the optical fiber parameters for the existence of these exact stable solutions are found by analytical stability analysis.

Propagation of dipole solitons in inhomogeneous highly dispersive optical-fiber media.

We consider ultrashort light pulse propagation through an inhomogeneous monomodal optical fiber exhibiting higher-order dispersive effects. Wave propagation is governed by a generalized nonlinear Schrödinger equation with varying second-, third-, and fourth-order dispersions, cubic nonlinearity, and linear gain or loss. We construct a type of exact self-similar soliton solution that takes the structure of a dipole via a similarity transformation connected to the related constant-coefficients one. The conditions on the optical-fiber parameters for the existence of these self-similar structures are also given. The results show that the contribution of all orders of dispersion is an important feature to form this kind of self-similar dipole pulse shape. The dynamic behaviors of the self-similar dipole solitons in a periodic distributed amplification system are analyzed. The significance of the obtained self-similar pulses is also discussed. By performing numerical simulations, the self-similar soliton solutions are found to be stable under slight disturbance of the constraint conditions and the initial perturbation of white noise.

Mode-locked femtosecond all-normal all-PM Yb-doped fiber laser using a nonlinear amplifying loop mirror.

We report on a new design for a passively mode locked fibre laser employing all normal dispersion polarisation maintaining fibres operating at 1 µm. The laser produces linearly polarized, linearly chirped pulses that can be recompressed down to 344 fs. Compared to previous laser designs the cavity is mode-locked using a nonlinear amplifying fibre loop mirror that provides an additional degree of freedom allowing easy control over the pulse parameters. This is a robust laser design with excellent reliability and lifetime.

Parabolic and hyper-Gaussian similaritons in fiber amplifiers and lasers with gain saturation.

We present a new asymptotically exact analytical similariton solution of the generalized nonlinear Schrdinger equation for pulses propagating in fiber amplifiers and lasers with normal dispersion including the effect of gain saturation. Numerical simulations are in excellent agreement with this analytical solution describing self-similar linearly chirped parabolic pulses. We have also found that for small enough values of the dimensionless saturation energy parameter the fiber amplifiers and lasers can generate a new type of linearly chirped self-similar pulses, which we call Hyper-Gaussian similaritons. The analytical Hyper-Gaussian similariton solution of the generalized nonlinear Schrdinger equation is also in a good agreement with numerical simulations.

Quasi-parabolic pulse propagation and breakup in fiber amplifiers with third-order dispersion.

We present an analytical solution for propagating pulses in normal dispersion fiber amplifiers, including the effect of third-order dispersion. The solution of the generalized nonlinear Schrödinger equation is based on asymptotic methods, first-order perturbation theory, and a renormalization procedure and leads to determination of the critical length corresponding to pulse breakup. We have also found and confirmed numerically the condition on the parameters that govern the propagation, as is necessary to ensure a highly accurate analytical description of the pulses and critical lengths in fiber amplifiers with third-order dispersion.

Experimental realization of a mode-locked parabolic Raman fiber oscillator.

We report here the first demonstration of a mode-locked fiber laser delivering parabolic pulses (similaritons) at 1534 nm. The use of a Raman-based gain medium potentially allows its implementation at any wavelength. The 22nJ output similariton pulses have a true parabolic shape both in the time and spectral domains and a linear chirp. Linear recompression close to Fourier limit is demonstrated allowing us to obtain 6 ps compressed pulses with a compression factor of 75.

Experimental demonstration of similariton pulse compression in a comblike dispersion-decreasing fiber amplifier.

Self-similar propagation of linearly chirped hyperbolic-secant pulses in a comblike decreasing-dispersion fiber amplifier has been observed experimentally for the first time to our knowledge. The scheme takes advantage of an exact solution of the generalized nonlinear Schrödinger equation with distributed coefficients.