Optical phase singularities and superluminal motion in unbounded space

In this paper, we summarize a remarkable result obtained by Soskin et al. in Phys Rev A 56, 4064 (1997). We show that for an on-axis superposition of two different-waist Laguerre-Gauss beams with numbers (0, n) and (0, m), the topological charge equals TC=m up to a plane where the waist radii become the same, given that the beam (0, m) has a greater waist radius, changing to TC=n after this plane. This occurs because in the initial plane the superposition has an on-axis op-tical vortex with TC=m and on different axis-centered circles there are (n – m) vortices with TC= +1 and (n – m) vortices with TC= –1. On approaching the above-specified plane, the vortices with TC= -1 "depart" to infinity with a higher-than-light speed, with the TC of the total beam becoming equal to TC=n. If, on the contrary, the beam (0, m) has a smaller waist, then the total TC equals n on a path from the initial plane up to a plane where the waist radii become the same, changing to TC=m after the said plane. This occurs because after the said plane, n–m vortices with TC= –1 "arrive" from infinity with a higher-than-light speed.

Magnetic Field Sensing Characteristics Based on Optical Microfiber Coupler Interferometer and Magnetic Fluid

In this paper, a novel and compact magnetic field sensor based on the combination of an optical microfiber coupler interferometer (OMCI) and magnetic fluid (MF) is proposed. The sensor is made up of an OMCI cover with polydimethylsiloxane (PDMS) and MF, and it uses MF as a material for adjusting the magnetic refractive index and magnetic field response. The sensing characteristics of the sensor are analyzed, and the experimental test is carried out. Under the condition of the same OMC waist length, the sensor sensitivity increases with the decrease of the OMC waist radius. The sensitivity of 54.71 and 48.21 pm/Oe was obtained when the OMC waist radius was set at 3.5 and 4 μm, respectively. In addition, we also tested the sensing response time and vector response characteristics of the sensor. At the same time, we discuss the demodulation idea about the cross-sensitivity of the magnetic field and temperature. The sensor has the advantages of high sensitivity, low cost, small size, optimized performance, and convenient integration. It has huge application potential in the fields of navigation and industrial intelligent manufacturing.

Simulation of thermal lensing in an end-pumped Nd:YAG laser rod with Gaussian and super-Gaussian pump beam profile

A numerical simulation is made on the thermal lensing effect in an laser diode end-pumped Nd:YAG laser rod. Based on finite element method (FEM), the laser rod temperature distribution is calculated and the focal length is deduced for a Gaussian and super-Gaussian pump beam profiles. At the pump power of 20W, the highest temperature located at the center of end-pumped face was 345K, and the thermal lens focal length was 81.4mm along the x-z axis. The results indicate that the thermal lensing effect sensitively depend on the pump power, waist radius of the pump beam and the pump distribution in a laser rod geometry.

Effect of Excitation Beam Divergenceon the Goos–HänchenShift Enhanced byBloch Surface Waves

The Goos–Hänchen (GH) shift is simulated and experimentally studied when the Bloch surface wave is excited in the forbidden band of a one-dimensional photonic band-gap structure for an excitation beam with different divergence. By changing the beam waist radius, a correspondingly changed Goos–Hänchen shift curve can be observed, where with a larger waist radius and the corresponding less divergence of the excitation beam, a greater GH shift that is closer to the Artmann estimation is obtained. The experimental demonstration of this phenomenon agrees well with the theoretical simulation.

Effect of Bump Shape on Current Density and Temperature Distributions in Solder Bump Joints under Electromigration

Three dimensional thermo-electrical finite element analysis was employed to simulate the current density and temperature distributions for solder bump joints with different bump shapes. Mean-time-to-failure (MTTF) of electromigration was discussed. It was found that as the bump volume increased from hourglass bump to barrel bump, the maximum current density increased but the maximum temperature decreased. Hourglass bump with waist radius of 240 μm has the longest MTTF.

Research on the Micro-Cutting of 0Cr18Ni9 Stainless Steel Films by Femtosecond Laser

In order to research the ablation of 0Cr18Ni9 stainless steel films whose thickness is 20μm, stainless steel films were ablated and micro-cut by fs laser in the paper. Firstly, the ablation threshold of 0Cr18Ni9 stainless steel films and the beam waist radius of fs laser were obtained based on the functional relation between ablation area and the laser energy. And, experimental results indicate that the waist radius of fs laser is 10.416μm and the ablation threshold of stainless steel films whose thickness is 20μm is 0.455J/cm2. Secondly, in order to observe the heat-affected zone, metallographic phase and electrical resistivity of stainless steel films ablated by fs laser were researched in detail. And through the experimental results, we can deduce that metallographic phase of specimens which were cut by fs laser was in recovery process and the metallographic was not change. Finally, in order to research the composition of stainless steel films phase affected by fs laser ablation, specimens were tested by XRD. And through the XRD test, it can be deduced the composition of stainless steel films phase was not affected by fs laser cut and the content of phase was affected by it.