We report on experimental observations of formation and evolution of transient plasma produced in fused silica glass with spatiotemporally focused (STF) femtosecond laser pulses using a pump-probe shadow imaging technique. Surprisingly, the observation shows that the track of the plasma is significantly curved, which is attributed to an asymmetric density distribution of the transient plasma produced in the focal volume caused by the pulse front tilt of the STF laser field.

We report on fabrication of tubular optical waveguides buried in ZBLAN glass based on transverse femtosecond laser direct writing. Irradiation in ZBLAN with focused femtosecond laser pulses leads to decrease of refractive index in the modified region. Tubular optical waveguides of variable mode areas are fabricated by forming the four sides of the cladding with slit-shaped femtosecond laser pulses, ensuring single mode waveguiding with a mode field dimension as small as ~4 μm.

In this paper, we consider the stabilization of combustion oscillations in the Rijke tube, which is modeled by a linearized wave equation with heat release fluctuation ODE acting as a point source term. A boundary PI feedback controller is designed to suppress the combustion oscillations. The Nyquist criterion is applied to prove that eigenvalues of the operator of the close-loop system are all located on left-hand side of the complex plane. The closed-loop system is proved to be exponentially stable by using the Riesz basis approach. Numerical simulations are presented to verify the effectiveness of the control design. Moreover, the experimental results are obtained in an experimental Rijke tube, which validates the effectiveness of the method.

Because of the need for low pollutant emissions, industrial gas turbines typically use partially premixed gases for combustion. However, the nonlinear dynamic characteristics of partially premixed flames have not been studied sufficiently. Therefore, this study focuses on the dynamics of a partially premixed flame generated by a swirler with fuel holes on its surface and designs a flame describing function (FDF) identification method based on the parallel subsystem model. This method can separate the flame dynamic characteristics into a parallel connection of the nonlinear and linear models. The nonlinear model is related to the disturbance frequency and velocity perturbation amplitude, whereas the linear model depends only on the disturbance frequency. This method is verified using a simulation. Finally, experimental research on partially premixed flames is conducted. Based on the experimental data, the identification method successfully separates the FDF into a nonlinear model with saturation characteristics and a linear model with Gaussian distribution characteristics. The flame model obtained by the identification method is the foundation for the analysis of combustion thermoacoustic stability and active/passive control strategy.