LASER INDUCED CRYSTALLIZATION OF AMORPHOUS Se80Te20 ALLOY THIN FILMS

The present study is concerned with the laser induced crystallization of amorphous thin films of Se₈₀Te₂₀ alloy. The films were prepared on a glass substrate by vacuum evaporation from bulk Se₈₀Te₂₀ alloy. The as-grown films were amorphous. The crystallization induced by an argon ion laser irradiation was studied at different beam intensities ranging from 50 mW to 600 mW and different time durations. The 486 nm (blue green) line was chosen for irradiation. The crystallization and growth processes in the laser irradiated samples were studied in the electron microscope at low temperature (173 K). it was seen that the crystallization was quicker at higher laser beam intensities as expected. The conditions for the onset of nucleation and the progress of crystallization in these films are compared with those observed in the films irradiated by electron beam.     

Adaptive clutter rejection filtering in ultrasonic strain-flow imaging

This paper introduces strain-flow imaging as a potential new technique for investigating vascular dynamics and tumor biology. The deformation of tissues surrounding pulsatile vessels and the velocity of fluid in the vessel are estimated from the same data set. The success of the approach depends on the performance of a digital filter that must separate echo signal components caused by flow from tissue motion components that vary spatially and temporally. Eigenfilters, which are an important tool for naturally separating signal components adaptively throughout the image, perform very well for this task. The method is examined using two tissue-mimicking flow phantoms that provide stationary and moving clutter associated with pulsatile flow.     

Extracting mode components in laser intensity distribution by independent component analysis

With increasingly sophisticated laser applications in industry and science, a reliable method to characterize the intensity distribution of the laser beam has become a more and more important task. However, traditional optic and electronic methods can offer only a laser beam intensity profile but, cannot separate the main mode components in the laser beam intensity distribution. Recently, independent component analysis has been a surging and developing method in which the goal is to find a linear representation of a non-Gaussian data set. Such a linear representation seems to be able to capture the essential structure of a laser beam profile. After assembling image data of a laser spot, we propose a new analytical approach to extract laser beam mode components based on the independent component analysis technique. For noise reduction and laser spot area location, wavelet thresholding, Canny edge detection, and the Hough transform are also used in this method before extracting mode components. Finally, the experimental results show that our approach can separate the principal mode components in a real laser beam efficiently.     

Bunched electron beam properties measurement by means of single-shot multibeam cross-correlation technique

A new single-shot multibeam cross-correlation technique is proposed for the measurement of a bunched electron beam temporal structure. As an optical laser pulse modulated by the Coulomb field of the electron bunch in an electro-optical crystal, it is proposed to use a non-uniform pulse modulated both in time and in space. In the proposed technique, measurement of the time profile is realized during a single laser shot. In this case, as a result of electro-optical modulation of a non-uniform femtosecond pulse, the information on the time profile of the Coulomb field is contained in only the spatially shifted spectral components of the modulated pulse.     

Optical breakdown in fused silica and argon gas: application to Nd:YAG laser limiter

A gas cell filed with argon gas under pressure is placed in a tightly focused laser beam to provide a limiter for laser pulses above a certain peak power, corresponding to the optical breakdown threshold for the creation of a laser-induced plasma. Measurements of the threshold intensity as a function of argon gas pressure are given for a laser wavelength of 1.064 microm (Nd:YAG) and a pulse length of 6.4 ns. Threshold intensities for optical breakdown in fused silica were measured with the same optical system, enabling a relative comparison of breakdown thresholds, of interest for protecting fused-silica optical components in fiber-optic delivery systems for laser material processing applications. The threshold intensity was measured to 220 GW/cm2 in Ar at 1.0 x 10(5) N/m2 (1 atm), 80 GW/cm2 in Ar at 8.0 x 10(5) N/m2 (7.9 atm), and 55 GW/cm2 in fused silica. Even though the threshold in argon is higher than that in fused silica, the limiter will protect the optical components if the laser beam is focused to a tighter spot in the gas cell than at the input end of the fiber.     

Resolution improvement in two-photon fluorescence microscopy with a single-mode fiber

The dependence of spectral broadening of an ultrashort-pulsed laser beam on the fiber length and the illumination power is experimentally characterized in order to deliver the laser for two-photon fluorescence microscopy. It is found that not only the spectral width but also the spectral blue shift increases with the fiber length and illumination power, owing to the nonlinear response in the fiber. For an illumination power of 400 mW in a 3-m-long single-mode fiber, the spectral blue shift is as large as 15 nm. Such a spectral blue shift enhances the contribution from the short-wavelength components within the pulsed beam and leads to an improvement in resolution under two-photon excitation, whereas the efficiency of two-photon excitation is slightly reduced because of the temporal broadening of the pulsed beam. The experimental measurement of the axial response to a two-photon fluorescence polymer block confirms this feature.     

Measurement of aerosol-particle trajectories using a structured laser beam

What is believed to be a new concept for the measurement of micrometer-sized particle trajectories in an inlet air stream is introduced. The technique uses a light source and a mask to generate a spatial pattern of light within a volume in space. Particles traverse the illumination volume and elastically scatter light to a photodetector where the signal is recorded in time. The detected scattering waveform is decoded to find the particle trajectory. A design is presented for the structured laser beam, and the accuracy of the technique in determining particle position is demonstrated. It is also demonstrated that the structured laser beam can be used to measure and then correct for the spatially dependent instrument-response function of an optical-scattering-based particle-sizing system for aerosols.     

Hybrid acousto-optic Fourier processor for imaging spatially inhomogeneous acoustic fields

A new method is proposed for imaging spatially inhomogeneous acoustic fields. The approach is based on the Fourier transform of a coherent light field formed as a result of the Bragg diffraction of a probing laser beam on an ensemble of quasi-plane acoustic waves in a lithium niobate crystal. These waves appear as a result of the transformation of an imaged acoustic field by a spherical acoustic lens. The proposed method has been experimentally verified using model multielement acoustic sources.     

Rapidly convergent phase-retrieval strategy for use with reflected laser light

An approach for wave-front sensing using reflected laserlight from a rough object is proposed. Light from a single laser beam is split into two beams, and the beams are launched from spatially separated apertures to illuminate an object. The reflected laser light is measured in the pupil plane of a receive telescope and in a plane conjugate to the object. By modulation of one of the two illuminator beams, the intensity pattern associated with each beam, as well as the field cross product of the two beams, is measured in each plane. A phase-retrieval algorithm is formulated by using projections onto constraint sets to recover the complex field associated with each illuminator. The algorithm is found to converge rapidly to the correct solution, particularly when compared with the convergence rates of more conventional phase-retrieval approaches. The new algorithm exhibits excellent performance in strong scintillation and is very tolerant to noise, exhibiting only a very small noise gain.     

Three-color differential interferometry

It is shown that differential interferometry using a Wollaston prism and a three-color laser source is an optical technique that has all the advantages of differential interferometry in polarized white light and of classical monochromatic interferometry. The interference fringe pattern obtained is very large and colored and presents a central white fringe that enables easy identification of the zero order of the interferogram. The three-color source is obtained by filtering the unwanted lines of the ionized laser (mixed argon and krypton) and balancing the three red, green, and blue lines by a technique that involves placing birefringent plates between the polarizer and the analyzer, the thickness of which has been calculated to create a natural filter. The unsteady aerodynamic flow downstream of a diamond shape airfoil has been visualized with this technique, which shows that the power of the light source is sufficient to record the interferograms at a high rate.     

Spatial filter pinhole for high-energy pulsed lasers

Spatial filters are essential components for maintaining high beam quality in high-energy pulsed laser systems. The long-duration (21 ns) high-energy pulses envisioned for future inertial-confinement fusion drive systems, such as the U.S. National Ignition Facility (NIF), are likely to lead to increased plasma generation and closure effects within the pinholes in the spatial filters. The design goal for the pinhole spatial filter for the NIF design is to remove small-angle scatter in the beam to as little as a ?100-murad divergence. It is uncertain whether this design requirement can be met with a conventional pinhole design. We propose a new pinhole architecture that addresses these issues by incorporating features intended to reduce the rate of plasma generation. Initial experiments with this design have verified its performance improvement relative to a conventional pinhole design.     

Design concept for diffractive elements shaping partially coherent laser beams.

A new two-step design algorithm for the calculation of a diffractive phase element (DPE) for use with partially coherent laser beams is presented. The optical reconstruction of the DPE is modeled by the convolution of a coherent diffraction pattern and the far-field intensity distribution of a partially coherent laser beam. Numerical deconvolution is applied to derive a suitable amplitude pattern as signal input to a standard iterative Fourier transform algorithm (IFTA). Theory and numerical results are presented. Compared with a single-step IFTA design, this new approach yields nearly equal diffraction efficiencies and a relative improvement of 15% in signal reconstruction error.     

Investigation of the influence of spatial coherence of a broad-area laser diode on the interference fringe system of a Mach-Zehnder interferometer for highly spatially resolved velocity measurements

Laser Doppler anemometry is a method for absolute velocity measurements that is based on a Mach-Zehnder interferometer arrangement and usually employs transverse fundamental-mode lasers. We employed inexpensive and powerful broad-area laser diodes and investigated ways in which an interference fringe system is influenced by the spatial coherence properties of a multimode beam. It was demonstrated that, owing to poor spatial coherence of the beam, interference is suppressed in the marginal regions of the intersection volume. Based on these results, a sensor for highly spatially resolved velocity measurements can be built. The inherent astigmatism of the broad-area diode is corrected by an arrangement of two crossed cylindrical lenses. An interference fringe system of length 200 microm and a relative variation in fringe-spacing of only 0.22% were demonstrated with light emitted from a broad-area laser diode with a 100 microm x 1 microm emitter size. Based on this principle a powerful, simple, and robust laser Doppler sensor has been achieved. Highly spatially resolved measurements of a boundary layer flow are presented.     

Self-interferometric technique for visualization of phase patterns encoded onto a liquid-crystal display

We report a new self-interferometric technique for visualizing phase patterns that are encoded onto a phase-only liquid-crystal display (LCD). In our approach, the LCD generates both the desired object beam as well as the reference beam. Normally the phase patterns are encoded with a phase depth of 2pi radians, and all of the incident energy is diffracted into the first-order beam. However, by reducing this phase depth, we can generate an additional zero-order diffracted beam, which acts as the reference beam. We work at distances such that these two patterns spatially interfere, producing an interference pattern that displays the encoded phase pattern. This approach was used recently to display the phase vortices of helical Ince-Gaussian beams. Here we show additional experimental results and analyze the process.     

Raman signal enhancement in deep spectroscopy of turbid media

A new, passive method for enhancing spontaneous Raman signals for the spectroscopic investigation of turbid media is presented. The main areas to benefit are transmission Raman and spatially offset Raman spectroscopy approaches for deep probing of turbid media. The enhancement, which is typically several fold, is achieved using a multilayer dielectric optical element, such as a bandpass filter, placed within the laser beam over the sample. This element prevents loss of the photons that re-emerge from the medium at the critical point where the laser beam enters the sample, the point where major photon loss occurs. This leads to a substantial increase of the coupling of laser radiation into the sample and consequently an enhanced laser photon-medium interaction process. The method utilizes the angular dependence of dielectric optical elements on impacting photon direction with its transmission spectral profile shifting to the blue with increase in the deviation of photons away from normal incidence. This feature enables it to act as a unidirectional mirror passing a semi-collimated laser beam through unhindered from one side, and at the other side, reflecting photons emerging from the sample at random directions back into it with no restrictions to the detected Raman signal. With substantial restrictions to the spectral range, the concept can also be applied to conventional backscattering Raman spectroscopy. The use of additional reflective elements around the sample to enhance the Raman signal further is also discussed. The increased signal strength yields higher signal quality, a feature important in many applications. Potential uses include sensitive noninvasive disease diagnosis in vivo, security screening, and quality control of pharmaceutical products. The concept is also applicable in an analogous manner to other types of analytical methods such as fluorescence or near-infrared (NIR) absorption spectroscopy of turbid media or it can be used to enhance the effectiveness of the coupling of laser radiation into tissue in applications such as photodynamic therapy for cancer treatment.     

Color fringe-projected technique for measuring dynamic objects based on bidimensional empirical mode decomposition

A triple-frequency color fringe-projected technique is presented to measure dynamic objects. Three fringe patterns with a carrier frequency ratio of 1:3:9 are encoded in red, green, and blue channels of a color fringe pattern and projected onto an object's surface. Bidimensional empirical mode decomposition is used for decoupling the cross talk among color channels and for extracting the fundamental frequency components of the three fringe patterns. The unwrapped phase distribution of the high-frequency fringe is retrieved by a three-step phase unwrapping strategy to recover the object's height distribution. Owing to its use of only a single snapshot, the technique is suitable for measuring dynamically changing objects with large discontinuity or spatially isolated surfaces.     

Application of a Genetic Algorithm on the Assessment of the Capabilities of Cctuator Systems for Laser Mircro Adjustment

Laser forming with actuator systems is a very attractive process to meet the rising demand for accuracy in micro system technology. However, the actuator systems have to be designed specifically for the given adjustment task. A computer assisted design system for actuator systems for laser based micro adjustment has been developed which supports the designer by creating a number of optimized design choices from a starting design. A multiobjective genetic algorithm has been successfully applied to this task. The algorithm for evaluation of adjustment capability of the actuator system uses another single objective genetic algorithm to determine the forming capability of the actuator system. The approach is similar to the inverse kinematics algorithm used in robotics but also considers elastic forming. The irradiated components of the actuator system are treated as robotic links for which the amount of laser based forming are the link parameter. An elastic structural mechanic simulation of the actuator system with beam elements calculates the resulting adjustment movement of the actuator system. The research performed to develop a configuration of this algorithm specifically to the needs of actuator system design evaluation is presented in this paper.     

Optical superresolution of focused partially spatially coherent laser beams

We report design theories of a diffractive superresolution element (DSE) to implement optical superresolution of focused partially spatially coherent laser beams. The design problem of the DSE can be transformed into a problem of linear programming to obtain a globally optimal solution. By using the design theories, some fundamental limits of optical superresolution of focused partially spatially coherent laser beams are proposed, and several design examples are provided. As expected, both the fundamental limits and the design examples show that worse spatial coherence will cause worse superresolution performance. The design theories provide a design approach with partially coherent beams and may be useful for other design problems under partially coherent illumination.     

Characterization of a low-speckle laser line generator

The goal of our investigation is to design a low-speckle laser line generator based on partial spatially coherent laser light. Low speckle is achieved by exploiting a regime of strongly reduced spatial coherence of a broad-area vertical-cavity surface-emitting laser, which is used as the line generator's light source. A comparative experimental study of different optical configurations is conducted, leading to the design of an optimal optical system. The results of our study are also valid for other sources of partial spatially coherent emission.