Complete crossing of Fano resonances in an optical microcavity via nonlinear tuning

We report on the modeling, simulation, and experimental demonstration of complete mode crossings of Fano resonances within chip-integrated microresonators. The continuous reshaping of resonant line shapes is achieved via nonlinear thermo-optical tuning when the cavity-coupled optical pump is partially absorbed by the material.The locally generated heat then produces a thermal field, which influences the spatially overlapping optical modes, allowing us to alter the relative spectral separation of resonances. Furthermore, we exploit such tunability to continuously probe the coupling between different families of quasi-degenerate modes that exhibit asymmetric Fano interactions. As a particular case, we demonstrate a complete disappearance of one of the modal features in the transmission spectrum as predicted by Fano [Phys. Rev. 124, 1866(1961)]. The phenomenon is modeled as a third-order nonlinearity with a spatial distribution that depends on the stored optical field and thermal diffusion within the resonator. The performed nonlinear numerical simulations are in excellent agreement with the experimental results, which confirm the validity of the developed theory.

Pole-placement self-tuning control of nonlinear Hammerstein system and its application to pH process control

By taking advantage of the separation characteristics of nonlinear gain and dynamic sector inside a Hammerstein model, a novel pole placement self tuning control scheme for nonlinear Hammerstein system was put forward based on the linear system pole placement self tuning control algorithm. And the nonlinear Hammerstein system pole placement self tuning control(NL-PP-STC) algorithm was presented in detail. The identi fication ability of its parameter estimation algorithm of NL-PP-STC was analyzed, which was always identi fiable in closed loop. Two particular problems including the selection of poles and the on-line estimation of model parameters, which may be met in applications of NL-PP-STC to real process control, were discussed. The control simulation of a strong nonlinear p H neutralization process was carried out and good control performance was achieved.

Observation of wave-breaking-free square pulses in a fiber ring laser

We experimentally and numerically demonstrate the generation of square pulses without any wave-breaking in a fiber ring laser. A segment of nonzero dispersion-shifted fiber is used to increase the laser cavity length and to optimize the parameters of the laser cavity. In the experiment, the pulse width can be tuned in a wide range from13.5 to 119.5 ns without wave-breaking while the peak power remains almost constant. The maximum singlepulse energy is up to 65.58 n J at a pump power of 508 m W. Numerical results are in good agreement with the experimental results. Numerical results also reveal the role of cavity length and nonlinearity in generating a square pulse without pulse breakup.