Displacement Of Focal Point Research Articles

Effect of varied beam diameter of picosecond laser on Foturan glass volume microprocessing

Foturan glass is a photosensitive transparent material which has attracted much interest for microfluidic applications due to possibility of volume processing by ultrafast lasers. In this work, we have investigated the effect of picosecond laser on volume processing in Foturan glass when varying the beam diameter incident on a lens. To this end, specific laser focusing configurations have been designed using raytracing models and an analysis protocol has been developed in the lens focusing region in order to describe the focal point displacement occurring at the variation of the incident laser beam diameter. The numerically simulated results were explained in association with Rayleigh length and found to be in good agreement with the experimental data obtained at well-defined conditions. Specifically, it was found that the hollow microstructures developed by thermal treatment and chemical etching after laser irradiation were significantly displaced along the propagation direction when the incident beam diameter varied in the range of 1-3.5 times. This approach aims to bring an essential contribution to the field of ultrashort pulse lasers micro- and nanoprocessing in transparent materials proving that the laser beam focus position and its size can be precisely controlled with high precision by automated optics for the variation of incident laser beam diameter in predefined conditions. This approach has the potential for laser multi-beam processing at various volume depths using the same optics setup and may even be applicable to two-photon excitation microscopy. On the other hand, the processing protocol in Foturan glass may allow understanding transparent material modification by tailoring laser beam characteristics.

Systematic investigation of terahertz wave generation from liquid water lines.

Understanding the process of terahertz (THz) wave generation from liquid water is crucial for further developing liquid THz sources. We present a systematic investigation of THz wave generated from laser-irradiated water lines. We show that water line in the diameter range of 0.1-0.2 mm generates the strongest THz wave, and THz frequency red shift is observed when diameter of the water line increases. The pump pulse energy dependence is decoupled from self-focusing effect by compensating the focal point displacement. As the pump pulse energy increases, saturation effect in THz peak electric field is observed, which can be mainly attributed to the intensity clamping effect inside the plasma and have never been reported previously, using water line or water film as the THz source. The proposed mechanism for saturation is supported by an independent measurement of laser pulse spectrum broadening. This work may help to further understand the laser-liquid interaction in THz generation process.

The Correction of Focal Point Displacement Caused by the Refraction of the Beams in High-Intensity Focused Ultrasound

Nowadays, high-intensity focused ultrasound (HIFU) as nonionizing radiation is used for cancer treatment. Basically, the function of HIFU is similar to conventional ultrasound. Ultrasound beams are perverted when crossing the border of different environments. This decreases the beam's focus within the tumor and may induce damage to the normal tissues. In this study, we aim to develop appropriate algorithms for correcting the focal point displacement duced by the beam's refraction. First, the level of displacement due to difference in two specific tissues was calculated for one element of the transducer and, then, it extended to all of the elements. Finally, a new focal point was calculated, which is considered as a desired focal point of the transducer in which the maximum temperature occurs. Designed algorithms were implemented in MATLAB software. A HIFU simulator (by the Food and Drug Administration of US) was used to simulate HIFU therapy. The proposed algorithm was tested on four models with two layers of tissue. Results illustrated the use of proposed algorithm results for 78% correction in the focal point displacement. In addition, it was noted that a part of this displacement was caused by the absorption of the beam in the tissues. The proposed algorithm can significantly correct the focal point displacement in HIFU therapy and consequently prevent damage to the normal tissues.

Focal Shift of Nano-Optical Lens Affected by Periodic Resonance With Substrate

A method of focal-shift resonance with substrate based on two commonly used structures of nano-optical lens is presented with numerical simulations. This resonance exhibits periodicity with the variation of substrate refractive index. Both types of lenses are based on gradual changed nanoslits and analyzed by the extraordinary optical transmission theory. This displacement of focal point can reach the order of microns. A periodic intensity change of focal light is observed between two absorption peaks in metal nanolens. The origin of this focal shift is also discussed with the analysis of the Fresnel number in diffraction. The finite element method is utilized in this paper.

Flexible All-Optical Wavelength Shifters Using Strong Focusing in a Wideband Engineered PPLN

Wavelength shift of modified second-harmonic bandwidth in a 20-mm-long engineered chirped periodically poled lithium niobate is experimentally demonstrated by focal-point displacement of a strongly focused pump beam of a 30- $\mu \text{m}$ beam waist. Utilizing this non-confocal arrangement in a cascaded second harmonic generation and difference frequency generation process, a CW probe as a signal wavelength is variably shifted within an 8-nm span by pump tuning using a $\sim 7$ -nm second-harmonic bandwidth.

An Ultrasonic-Adaptive Beamforming Method and Its Application for Trans-skull Imaging of Certain Types of Head Injuries; Part I: Transmission Mode

A new adaptive beamforming algorithm for imaging via small-aperture 1-D ultrasonic-phased arrays through composite layered structures is reported. Such structures cause acoustic phase aberration and wave refraction at undulating interfaces and can lead to significant distortion of an ultrasonic field pattern produced by conventional beamforming techniques. This distortion takes the form of defocusing the ultrasonic field transmitted through the barrier and causes loss of resolution and overall degradation of image quality. To compensate for the phase aberration and the refractional effects, we developed and examined an adaptive beamforming algorithm for small-aperture linear-phased arrays. After accurately assessing the barrier's local geometry and sound speed, the method calculates a new timing scheme to refocus the distorted beam at its original location. As a tentative application, implementation of this method for trans-skull imaging of certain types of head injuries through human skull is discussed. Simulation and laboratory results of applying the method on skull-mimicking phantoms are presented. Correction of up to 2.5 cm focal point displacement at up to 10 cm depth under our skull phantom is demonstrated. Quantitative assessment of the method in a variety of temporal focusing scenarios is also reported. Overall temporal deviation on the order of a few nanoseconds was observed between the simulated and experimental results. The single-point adaptive focusing results demonstrate strong potential of our approach for diagnostic imaging through intact human skull. The algorithms were implemented on an ultrasound advanced open-platform controlling 64 active elements on a 128-element phased array.

Thermal Effects in High-Power Continuous-Wave Single-Pass Second Harmonic Generation

We present a theoretical model which describes the effects of thermal load distribution on single-pass second harmonic generation (SP-SHG) of high-power continuous-wave (cw) radiation in MgO:sPPLT nonlinear crystal to provide green output at 532 nm. Numerical simulations are performed based on real practical values and actual operating conditions associated with a recent SP-SHG experiment, generating 10 W of cw green radiation using a Yb-fiber laser. The model is used for four oven configurations to simulate the implications of thermal effects on SH power. The observed asymmetric feature of the phase-matching curves, particularly at higher fundamental powers up to 50 W, are characterized and explained by considering the generation of heat due to crystal absorption. The concept of optical path difference (OPD) is introduced to study the formation of thermal lens and its effects on the displacement of focal point inside the thermally loaded crystal. We further study the dependence of the SH power on the different oven schemes by increasing the input fundamental power up to 50 W. It is found that a top-sinked oven design is the optimum configuration for achieving maximum SHG efficiency without saturation. Comparison of the simulation results with experimental data confirms the validity of the theoretical model.

Numerical simulation of the tissue ablation in high‐intensity focused ultrasound therapy with array transducer

AbstractThe development of high‐intensity focused ultrasound (HIFU) therapy for deeply situated cancer has been desired. One problem is focal point displacement due to the inhomogeneity of the human body. The objectives are the realization of appropriate phase control of an array transducer and support for preoperative planning of HIFU therapy by the computational prediction of ablation regions. To these ends, in this study we have developed an HIFU simulator that employs a voxel phantom constructed from CT/MRI data of a living human body. To reproduce the pressure propagation through an inhomogeneous medium, the mass and momentum equations for a mixture with the equation of state of the medium are solved. The ablation of tissue is modeled as a phase transition by using the phase field model. Then, the heat equation with viscous dissipation as a heat source and the Allen–Cahn equation with a free energy model are solved to predict the development of the ablation region. The basic equations are discretized by the finite difference method. HIFU therapy with an array transducer for liver cancer is reproduced numerically. Although the results without phase control show displacement and diffusion of the focal point due to the inhomogeneity of the human body, a clear focal point is obtained by using the array transducer with an appropriate phase delay obtained from pre‐computation. The HIFU simulator predicts that the ablation region will develop close to the target, owing to the phase control of the array transducer. Copyright © 2010 John Wiley & Sons, Ltd.

Compensation for displacement of the focal point in cone beam single photon emission computed tomography reconstruction

This study examined the effects of focal point displacement on image quality in cone beam single photon emission computed tomography (SPECT). A new image reconstruction algorithm that accounts for the focal point shift was derived and three shift geometries were investigated. The geometries included a lateral shift with a fixed focal length but off-center focusing, a linear axial shift with a variable focal length that depends linearly on the distance between a bin of the detector and the center of the detector, and a random axial shift with a randomly varying focal length. Computer simulation was conducted to evaluate the shift effects with a phantom that was composed of 118 small spherical sources. The results demonstrated that the lateral shift of the focal point was more critical to image quality than was the axial shift. With a 0.64 cm (1 pixel) lateral shift, noticeable artifacts was observed, while an axial shift resulted in minimal changes in image quality until it reached 8 cm (12.5 pixels). The derived reconstruction algorithm eliminated most of the artifacts caused by a fixed lateral shift or a linear axial shift of the focal point, but failed to do so for a random axial shift since the linear distribution assumed in image reconstruction did not match the random shift occurred in acquisition of the data.