Yifei Shi

Dependence of leaky mode coupling on loss in photonic crystal fiber with hybrid cladding.

It is possible for core leaky mode to couple with cladding defect leaky mode when the cladding defect is close to fiber core. Dispersion properties and propagation loss of core mode will be affected in some extent once the coupling occurs. But a complete coupling between two leaky modes not always happens even the phase matching condition is satisfied. Leaky mode coupling in photonic crystal fiber with a hybrid cladding which includes low-index and high-index inclusions at the same time is numerically investigated based on a full vector finite element method. It is found that not only phase matching but also loss matching plays an important role in leaky mode coupling. The originally intersecting dispersion curves for the two leaky modes will split and become another two new curves due to the anti-crossing effect when both the real and imaginary parts of their mode effective refractive indices are equal. There is not splitting but some perturbation in dispersion curves for the two phase matching leaky modes when their losses are not equal. A theoretic explanation is also given to these phenomena.

Large Negative Dispersion in Dual-Core Photonic Crystal Fibers Based on Optional Mode Coupling

Chromatic dispersion of dual-concentric-core photonic crystal fibers (PCFs), which have an inner low-index core and an outer high-index core, is investigated based on a full vector finite-element method. Large negative dispersion can be obtained by introducing avoided crossing between modes of the two asymmetric cores. While increasing the negative dispersion, larger effective mode area is also possible by choosing a higher order mode in the outer core to couple with the fundamental inner core mode. During the analysis, a dual-core PCF with a dispersion peak value of -36 000 ps/(nmldrkm) and effective mode area of 34.8 mum2 is presented.

Finite element modeling of acoustic field induced by laser line source near surface defect

A numerical model of acoustic field induced by laser line source near the surface defect is established by finite element method (FEM), where a surface notch of rectangular shape has been introduced to represent the fatigue defect for the convenience of modeling. After calculating numerically the transient displacement distributions, which are generated by the laser irradiation, the ultrasonic wave modes on the surface and in the body of the plate material are presented in details. The longitudinal, transverse and surface acoustic waves (SAWs) excited by laser pulses near surface notch are compared under the situations that the notch depths are different. As the notch depth increases, the directivity of the bulk waves generation changes greatly. The amplitude of the reflected SAW rises observably at the same time, which is observed experimentally when the laser source is shifted near the surface notch in scanning laser line source (SLLS) measurement. Another effect induced by the surface notch is the time lag of the transmitted SAW pulse with respect to the original incident pulse. These phenomena can be explained from the results. The conclusions can be used to surface notch depth evaluation.

Resonance-Related Coupling of Symmetric Dual-Core Hybrid Photonic Crystal Fibers

Dual-core hybrid photonic crystal fibers in which high-index inclusions are introduced between the two cores are proposed and the directional coupling characteristics of them are investigated numerically. Alternative cutoffs of even and odd supermodes, decoupling effect, and maxima of coupling length are presented. It is demonstrated that the resonance and location of the high-index inclusions play an important role.

Study of Dual-Laser-Generated Surface Acoustic Waves Interacting with Multiangled Surface Breaking Cracks by Finite Element Method

In this paper we study the situation of a laser line source directly irradiating a crack modeled by a slot, which is an essential process in the scanning laser line source (SLLS) technique. The finite element method (FEM) was used to calculate the scattering of an ultrasonic wave. The propagation paths of ultrasonic waves were analyzed and corresponding waves were identified accordingly. The displacements for various slot orientations were then calculated and analyzed, and it was shown that the arrival times of some ultrasonic wave modes can be used for detecting the slot orientation. As a result, the relationship between the slot orientation and the ultrasound diffraction pattern was clarified.

A novel differential optical beam deflection detection system for measuring laser-generated surface acoustic waves

As the application of the laser ultrasonics developed rapidly, there is especial call for more sensitive and convenient optical installation to detect the ultrasonic waves induced by pulsed laser. The optical beam deflection (OBD) methods have abstracted the interest of people for many years for their merits over the interference method. In this paper a novel differential OBD detection system for measuring laser-generated surface acoustic waves (SAW) is presented. The detection principle of this optical system is discussed in detail according to the scheme. And we get the linear relation between the physical parameter of the SAW and the output of the detection system. For confirm the conclusion the Monte Carlo computation method is utilized to simulate the ray propagation in the system, adding the consideration of the light spot distribution of the detection laser. The numerical result agrees with the analytic method. The linear relation between the detection system output current and the deflection angle induced by SAW is validated. Furthermore, the sensitivity and the spatial resolution of the system proposed are also calculated for comparing with the other OBD methods. The results show that this differential optical beam deflection detection system is more sensitive to the small disturbance and has higher space resolution. It has considerable potential in ultrasonic measurement.

Numerical simulation of surface crack detection by laser-generated circumferential guided wave in elastic cylinders

Nondestructive and non-contact detection methods of surface defects of the elastic cylinders are urgently needed especially for the extensive application of such structure in the manufacture and daily life. In recent years there has been an increasing focus on laser ultrasonics, a novel technique in nondestructive evaluation, which can excite and receive high frequency ultrasonic waves from distance range in thermoelastic regime. This paper utilized the finite element method to investigate the transient mode conversion of circumferential guide waves by the surface crack of cylinder. The numerical model of the ultrasonic waves generated on aluminum cylinder whose diameter is 10mm by pulsed laser is established, considering the dependence of the material parameters on the temperature varying. The depth of the surface crack varies from 100 μm to 1 mm, which is much less comparing with the wavelength of the circumferential guide waves. The numerical results obtained from finite element model are presented in terms of vertical displacement, which is generated because of the localized temperature gradient on the surface. And the signals of the circumferential guide waves are analyzed by wavelet transformation. From the obtained results we can see that the amplitude of the scattered waves induced by the crack become more and more obvious as the depth of the crack increases and the time-frequency distributions of the scattered waves also change obviously. The results demonstrate the propagation of the circumferential guide waves on the surface of the cylinder can be applied in crack detection well.