Lenses Of Different Focal Lengths Research Articles

Trapping multiple absorbing particles in air using an optical fiber by photophoretic forces

We demonstrate photophoretic force-based optical trapping of multiple absorbing particles in air by loosely focusing a Gaussian beam emanating from a single mode fiber using convex lenses of different focal lengths, and investigate the dependence of the number of trapped particles and their sizes on the focal length. We observe the formation of particle chains at a particular focal length, and measure the axial dynamic range of optical trapping for each lens system. We then develop a numerical simulation to explain this observed dynamic range by estimating the temperature distribution across a particle surface, and determining the axial photophoretic force. Our simulation results are in reasonable agreement with experimental results. Interestingly, we also observe that the average size of trapped particles reduces as we increase the lens focal lengths. This is somewhat intriguing as each lens produces the same intensity profile, albeit at different axial distances. However, the axial intensity gradient reduces as the lens focal length is increased, which suggests that such gradients may somehow be involved in the mechanism of photophoretic confinement.

The Effects of Initial Laser Intensity on the Nonlinear Optical Properties of The Laser Dye DQOCI Doped Films Using Z-Scan Technique

This study is dedicated to investigate the effects of initial laser intensity on the nonlinear optical properties of the laser dye DQOCI dissolved in methanol with a concentration of 10 -5 M and doped with PMMA film. The properties were studied by using open and closed aperture Z-scan technique, with different levels of initial intensity (I0), excited by continuous diode solid-state laser at a wavelength of 532 nm. Three lenses of different focal lengths were employed to change the radius of the Gaussian laser beam and then change the initial intensity. For I0= 6.83 and 27.304 kWatt/cm2, the Z-scan curves show a saturation of absorption (SA) known as the negative type of nonlinearity, in which the absorption coefficient β 2 decreases and the transmittance increases with increasing the initial laser intensity. With I0 equal to 3.03 KWatt/cm2, the nonlinear absorption changes from SA to RSA, where the transmittances is reduced with the increase of intensity (z0) as analyzed by the theory of free carrier nonlinearities. The closed aperture z-scan shows a pre-focal transmittance minimum (valley) and a post focal transmittance maximum (peak) which reflects the z-scan signature of a positive nonlinearity (self-focusing) due to Kerr effect. Each of nonlinear refractive index (n2), nonlinear absorption coefficient (β 2), and third-order nonlinear optical susceptibility (χ3) are intensity-dependent.

Controlled generation of array beams of higher order orbital angular momentum and study of their frequency-doubling characteristics

We report on a simple and compact experimental scheme to generate high-power, ultrafast, higher-order vortex-array beams. Simply by using a dielectric microlens-array (MLA) and a plano-convex lens, we have generated array-beams carrying the spatial property of the input beam. Considering the MLA as a 2D sinusoidal phase-grating, we have numerically calculated the intensity pattern of the array-beams in close agreement with the experimental results. Using vortex beams of order as high as l = 6, we have generated vortex array-beam with individual vortices of orders up to l = 6. We have also theoretically derived the parameters controlling the intensity pattern, size, and the array-pitch and verified experimentally. The single-pass frequency-doubling of vortex-array at 1064 nm in a 1.2 mm long BiBO crystal produced green vortex-array of order, lsh = 12, twice the order of pump beam. Using lenses of different focal lengths, we have observed the vortex-arrays of all orders to follow a focusing dependent conversion similar to the Gaussian beam. The maximum power of the green vortex-array is measured to be 138 mW at a single-pass efficiency as high as ~3.65%. This generic experimental scheme can be used to generate the array beams of desired spatial intensity profile across a wide wavelength range by simply changing the spatial profile of the input beam.

A paraxial cloak with four lenses of different focal lengths

We demonstrate cloaking with four lenses of different focal lengths. To achieve this, we compute the separation distances between lenses such that the effective ABCD matrix is equal to just a propagation in free space. Previously, calculations using this method have been restricted to two pairs of lenses with equal focal lengths. Our computations, on the other hand, have no such restrictions: we generalize even with an asymmetric case. We derive expressions that show the dependence of the distances on the lenses and their focal lengths. Furthermore, we propose a way to minimize the length of this cloak. Our equations reduce to the Rochester cloak when its restriction is imposed. A general paraxial cloak could be useful in advanced undergraduate experiments in optics because (i) the limit imposed by the focal length has been lifted, and (ii) there is a pedagogical benefit of doing experiments with lenses beyond the usual imaging experiments, such as reinforcing concepts in Gaussian optics.

Generation of "perfect" vortex of variable size and its effect in angular spectrum of the down-converted photons.

The “perfect” vortex is a new class of optical vortex beam having ring radius independent of its topological charge (order). One of the simplest techniques to generate such beams is the Fourier transformation of the Bessel-Gauss beams. The variation in ring radius of such vortices require Fourier lenses of different focal lengths and or complicated imaging setup. Here we report a novel experimental scheme to generate perfect vortex of any ring radius using a convex lens and an axicon. As a proof of principle, using a lens of focal length f = 200 mm, we have varied the radius of the vortex beam across 0.3–1.18 mm simply by adjusting the separation between the lens and axicon. This is also a simple scheme to measure the apex angle of an axicon with ease. Using such vortices we have studied non-collinear interaction of photons having orbital angular momentum (OAM) in spontaneous parametric down-conversion (SPDC) process and observed that the angular spectrum of the SPDC photons are independent of OAM of the pump photons rather depends on spatial profile of the pump beam. In the presence of spatial walk-off effect in nonlinear crystals, the SPDC photons have asymmetric angular spectrum with reducing asymmetry at increasing vortex radius.

Camera Focal Length and the Perception of Pictures

Photographers, cinematographers, and computer-graphics engineers use certain techniques to create striking pictorial effects. By using lenses of different focal lengths, they can make a scene look compressed or expanded in depth, make a familiar object look natural or distorted, or make a person look smarter, more attractive, or more neurotic. Photographers have a rule of thumb that a 50 mm lens produces natural-looking pictures. We asked why pictures taken with a 50 mm lens look natural, while those taken with other focal lengths look distorted. We found that people's preferred viewing distance when looking at pictures leads them to view long-focal-length pictures from too near and short-focal-length pictures from too far. Perceptual distortions occur because people do not take their incorrect viewing distances into account. By following the rule of thumb of using a 50 mm lens, photographers greatly increase the odds of a viewer looking at a photograph from the correct distance, where the percept will be undistorted. Our theory leads to new guidelines for creating pictorial effects that are more effective than conventional guidelines.

A Retrospective Photometric Study of 82 Published Reports of Mastopexy and Breast Reduction

A Retrospective Photometric Study of 82 Published Reports of Mastopexy and Breast Reduction

The perceptual basis of common photographic practice

Photographers, cinematographers, and computer-graphics engineers use certain techniques to create striking pictorial effects. By using lenses of different focal lengths, they can make a scene look compressed or expanded in depth, make a familiar object look natural or distorted, or make a person look smarter, more attractive, or more neurotic. We asked why pictures taken with a certain focal length look natural, while those taken with other focal lengths look distorted. We found that people's preferred viewing distance when looking at pictures leads them to view long-focal-length pictures from too near and short-focal-length pictures from too far. Perceptual distortions occur because people do not take their incorrect viewing distances into account. By following the rule of thumb of using a 50-mm lens, photographers greatly increase the odds of a viewer looking at a photograph from the correct distance, where the percept will be undistorted. Our theory leads to new guidelines for creating pictorial effects that are more effective than conventional guidelines.

Optimization of laser hole drilling process on thick gold spherical hohlraums for intense X-ray generation

Hohlraums of high-Z materials are used as soft X-ray sources to study indirect drive fusion, equation of state of materials etc. Here, we describe a method to develop spherical gold hohlraums of large wall thickness (∼70–80 µm) on which laser entrance and diagnostics holes are drilled using a 10 Hz Nd:YLF laser. Holes of different diameters have been drilled with lenses of different focal lengths. The back wall of the hohlraum is protected from the damage by shutting off the laser at pre-determined hole drilling time.

Holographic lens recorded on photopolymers: fabrication and study of the image quality

In recent years, holographic optical elements have been introduced in different applications such as high-density data storage, interconnections, spatial and temporal filters and three-dimensional displays. Simultaneously, more sensitive, efficient and durable holographic materials have been developed. Thus it is necessary to analyze the characteristics of these elements in the holographic materials developed. In this paper a method to obtain holographic lenses in a photopolymer is presented. In order to obtain, reconstruct and analyze these lenses, an optical device was designed. Once the holographic lenses are obtained, the device allows us to capture the images provided by these lenses. The imaging quality of these lenses was evaluated by means of the modulation transfer function (MTF) and the contrast. Lenses of different focal lengths were recorded. The holographic lenses obtained had high diffraction efficiency and temporal stability. Moreover, the resolution was greater than that of other lenses with the same diaphragm number.

Moiré beating digital technique to collimation testing

A digital collimation technique based on the self-imaging phenomenon is proposed. Basically, a moiré pattern is observed by viewing the Fresnel image of an original self-imaged grating through a second grating. Decollimation of the laser beam causes a variation in the self-image period. In our method, the moiré beating is produced through a digital subtraction operation of a replica of the original grating and the self-images provided by a decollimated beam. The variation of the moiré period is related to the amount of decollimation of the illuminating beam. Lenses of different focal lengths were used in the collimation procedure, as well as gratings with different periods. The validity of the approach is experimentally demonstrated and setting sensitivities are compared.

Parametric study of a fiber-optic laser-induced breakdown spectroscopy probe for analysis of aluminum alloys

In the present work we demonstrate a fiber-optic laser-induced breakdown spectroscopy (FO LIBS) system for delivering laser energy to a sample surface to produce a spark as well as to collect the resulting radiation from the laser-induced spark. In order to improve the signal/background (S/B) ratio, various experimental parameters, such as laser energy, gate delay and width, detector gain, lenses of different focal lengths and sample surface, were tested. In order to provide high reliability and repeatability in the analysis, we also measured plasma parameters, such as electron density and plasma temperature, and determined their influence on the measurement results. The performance of FO LIBS was also compared with that of a LIBS system that does not use a fiber to transmit the laser beam. LIBS spectra with a good S/B were recorded at 2-μs gate delay and width. LIBS spectra of six different Al alloy samples were recorded to obtain calibration data. We were able to obtain linear calibration data for numerous elements (Cr, Zn, Fe, Ni, Mn, Mg and Cu). A linear calibration curve for LIBS intensity ratio vs. concentration ratio reduces the effect of physical variables (i.e. shot-to-shot power fluctuation, sample-to-surface distance, and physical properties of the samples). Our results reveal that this system may be useful in designing a high-temperature LIBS probe for measuring the elemental composition of Al melt.

Fabrication and characterization of diffractive optical elements in InP for monolithic integration with surface-emitting components

We report on the fabrication and subsequent characterization of binary diffractive optical elements (DOE's) in InP for operation at 1.3 microm. Fresnel lenses of different focal lengths and a DOE that splits and focuses an incident beam into a 1 x 4 array of spots (optical fan-out) were fabricated. We realized the surface reliefs by patterning resist, using electron-beam lithography and etching with a chemically assisted ion beam, which produced well-defined patterns with smooth sidewalls and little if no surface roughness. The measured efficiency for the lenses was 36%. For the fan-out element the efficiency and the uniformity error were 26% and 30%, respectively. Spot sizes as small as 16 microm were measured.

Nonsymmetric Fourier transforming with an anamorphic system

The idea of obtaining a nonsymmetric Fourier transform with crossed cylindrical lenses of different focal lengths is presented. The anamorphic rotation-variant system produces a scaled Fourier transform F(u,mv) of an object f(x,y), where m is a scaling constant. The system performs controlled angular magnification of an object spectrum. It is shown that the super resolution in one direction is gained by reducing the number of degrees of freedom of the optical message in the other. Experimental results are shown where the scaling constant m of up to 10 has been obtained.

VIEWERS' PERCEPTIONS OF VELOCITY AND DISTANCE IN TELEVISED EVENTS

Subjects from third-grade, seventh-grade, and college-age levels who differed with respect to photographic skills and knowledge were asked to judge the comparative velocity and distance traveled by two trains on videotape. In each of four comparisons, the velocity and distance traveled by both trains were equal, but photographing the trains with lenses of different focal lengths produced apparent differences between the velocity and distance traveled by the trains. Both children and adults frequently interpreted the effect of lens focal length as actual differences between the two trains in velocity and distance traveled. Hands-on experience with the tools of visual media only slightly mitigated against this tendency. The implications of this study are that many viewers do not understand how television production techniques manipulate perceptual information, and may thus be misled by their use.