Showing papers on "Bessel beam published in 1998"

Bessel and conical beams and approximation with annular arrays

Abstract: The Bessel beam is one of the relatively new limited-diffraction beams that have been discovered. It is compared with the conical transducer, which also gives an approximate limited-diffraction solution to the wave equation. The conical transducer's field deviates from the predicted field in the nearfield, where it is wider. Therefore, the Bessel beam is better for use in a hybrid system where a limited-diffraction beam is used for transmission and a dynamically focused beam for reception. The limited-diffraction Bessel beam of order zero can be excited on an annular transducer with equal-area division of elements and with a fixed prefocus, i.e., conventional transducers used in commercial medical imaging equipment. The element division implies that the scaling parameter must be chosen to contain the first lobe of the Bessel function in the first element. In addition, the prefocus must be such that the array is steerable to infinite depth with minor loss. Even when the Bessel beam yields a larger depth of field than that of an unfocused transducer, it has the advantage of a narrower beam. Simulated examples are shown where the approximate Bessel beam compares favorably with a spherically focused beam with a fixed focus, an unfocused beam, and a conical transducer.

Generation of nondiffracting beams by diffractive phase elements

Abstract: We present a design of the diffractive phase elements (DPE’s) that produce nondiffracting beams according to the beam-shaping scheme, in which the incident Gaussian-profile beam is converted into a Bessel-function J0 beam. An optimization method is applied to solving this special beam-shaping problem. Numerical investigation of the generating J0 Bessel beam shows that the designed DPE can satisfactorily produce the J0 Bessel beam.

Evanescent Bessel beams

Abstract: Bessel beams of the evanescent kind are presented and analyzed. They rapidly decay with propagation but retain their original transversal shape. A physical method of generation is proposed in the form of propagation of an ordinary (nonevanescent) Bessel beam across an interface between two different dielectric media. Transmission and reflection coefficients are calculated for this type of beam. The analysis is vectorial and is fully consistent with Maxwell’s equations. Apodized beams of Gauss–Bessel and Circ–Bessel types are propagated by numerical simulation and are shown also to retain a narrow central lobe. Beams of these types have evident advantages in near-field applications, microscopy, and high-density data storage with subwavelength resolution.

Beam-preshaping axicon focusing

Abstract: An approach to efficient axially symmetric focusing of a light beam to an extended line segment is studied wherein the on-axis intensity and the scale of transverse intensity distribution can be controlled precisely and simultaneously at any point of the segment. Schematic realization is based on employing two phase-only optical components in tandem, the first of which, with a disk form, performs prior central light beam shaping and marginal correction to smooth the edge slopes, while the second one, with an annular form, is used for the appropriate axicon-type focusing of a light beam to the segment. An instance is given of converting a collimated Gaussian laser beam into an oscillation-free nondiffracting zero-order Bessel beam reproduced on a finite interval.

Two-element refracting system for annular Gaussian-to-Bessel beam transformation

Abstract: A refracting system consisting of two lenses is designed to transform an annular Gaussian laser beam into a circular Bessel beam. The slopes of the input and output surfaces fit well with a sixth-order polynomial. A smooth variation of the radii of curvature of the resulting aspheric surfaces is very attractive for easy machining of the surfaces. The diffraction-free length for the designed system is 59.4735 m at 633 nm.

Generation of the second harmonics of bessel light beams in a KTP crystal

Abstract: Theoretical and experimental investigations were made of the characteristic features of doubling the frequency of Bessel light beams by the oe — e interaction in a KTP crystal. All possible and scalar interactions of plane—wave components of Bessel beams were observed experimentally. The spatial structure of the second harmonic in the far zone consisted of a central spot and a concentric ring. The diffraction divergence of the central beam was weak ( ~ 1.7 mrad). The energy conversion efficiency was ~ 21% for a Bessel beam, which was three times higher than the efficiency of conversion of the initial laser beam. A theoretical model was based on representation of the field of the second harmonic as a superposition of Bessel beams, which provided a correct description of the observed spatial and energy characteristics of the second harmonic.

Refracting System for Annular Gaussian-to-Bessel Beam Transformation.

Abstract: A refracting system is designed for transforming an annular Gaussian laser beam into a circular Bessel beam. The slope of the input and output surfaces fitted well with a sixth-order polynomial. The radii of curvature of the resulting aspheric surfaces are found to vary smoothly over the surface. The diffraction-free length for this system is found to be 59.4735 m at 633 nm.

Non-linear propagation of limited diffraction beams [in medical US imaging]

Abstract: Recently, harmonic imaging has been shown to increase image quality in medical imaging. This is believed to be caused by decreased sidelobes. It is also due to a decreased sensitivity to near-field aberrations owing to the lower transmission frequency, and the lower relative amplitude of the second harmonic in the near-field. One of the problems of limited diffraction beams has been the sidelobe level. We performed a simulation of non-linear propagation for Bessel beams and X waves to see what performance gains that could be achieved. We used the angular spectrum method of P.T. Christopher and K.J. Parker (1991) which includes a non-linear substep to predict higher harmonic generation. Simulations of continuous wave Bessel beam excitation on a 15 mm transducer at 2.275 MHz showed about 9 dB decrease in sidelobe level for the second harmonic. The beamwidth was approximately the same as for the first harmonic. Near the end of the depth of field, the sidelobes increased and gave a poorer beam profile. For pulsed X wave excitation on the same transducer, high sidelobes were present throughout the depth of field. A large 50 mm Bessel transducer at 1.65 MHz gave a second harmonic with half the beamwidth and the same sidelobe level as the first harmonic. By deleting the central lobe of the J/sub 0/ Bessel beam for the 15 mm transducer, improved second harmonic properties were achieved. The second harmonic beam was characterized by low amplitude in the near-field, a narrow beam and reduced sidelobe level even for large depths. However, the modified Bessel beam loses some of its limited diffraction properties. Comparable results were achieved for the modified X wave, where the second harmonic also showed less energy in the X branches.

Fabrication and properties of light spiral filters

Abstract: A simple technique of obtaining light spiral (helium) filters in UV exposed photoresist layers consisting in a combined use of binary masks and revolution of the photosensitive layers during exposition is presented. The filters investigated experimentally are inspected by observing intensity distribution in a laser beam reflected from the sample. The filter giving the angular phase shift variation which is close to that needed for the generation of the first order Bessel beam is realized.

Optical head for multi-layered recording

Abstract: PROBLEM TO BE SOLVED: To enable multi-layered by making use of a Bessel beam SOLUTION: When collimating a laser light from a laser diode 1 into parallel light by a collimator lens 3 and converting it into an annular beam through a liquid crystal shutter panel 10, this optical head selectively turns ON and OFF respective annular parts of the liquid crystal shutter panel 10 to vary the diameter of the annular light beam, thereby enabling the depth-direction multi-layered recording of a recording medium 7 COPYRIGHT: (C)1999,JPO

Characteristics of parametric frequency conversion making use of Bessel light beams

Abstract: It is proposed that Bessel light beams be used to pump optical parametric oscillators. The advantages of such pumping are demonstrated by a comparison with Gaussian light beams. In particular, the maximum mode coupling coefficient is achieved in a parametric oscillator pumped by a Bessel beam, which enhances parametric amplification. A nonlinear conversion regime that does not destroy the spatial structure of the intracavity fields is found. This opens up the possibility of increasing the efficiency of conversion of the energy of an input Bessel beam, compared with a Gaussian beam.

Optical breakdown of helium in Bessel laser radiation beams

Abstract: Numerical simulation is used to investigate the dynamics of formation of a helium plasma in Bessel beams, shaped by an axicon and a phase converter from a laser radiation pulse with Gaussian temporal and radial intensity profiles. The beam intensities at the breakdown threshold are determined as a function of the pulse duration for various radial field distributions in a beam characterised by Bessel functions of order m (m = 0 - 5). It is shown that in the investigated range of parameters the threshold intensity is independent of m. The temporal and spatial evolution of the resultant plasma, and the dependence of the plasma characteristics on the pulse parameters are considered. Conditions are found for the formation of tubular plasma channels in beams of orders m{>=}1. The adopted model of the optical breakdown of helium is shown to be satisfactory because of a good agreement between the results of calculations of the moment of breakdown in a zeroth-order Bessel beam and experimental results. (interaction of laser radiation with matter. laser plasma)

Single-pass picosecond optical parametric generator pumped by Bessel beam

Abstract: A novel approach to forming the high brightness output of the travelling-wave optical parametric generator (TOPG) in single pass is demonstrated. TOPG based on KTP crystal was pumped by the Bessel beam formed by an axicone. This is a new approach to diffraction-limited parametric superfluorescence generation in nonlinear crystal pumped by a Bessel beam.

Bessel-Beam Pumped Optical Parametric Generator

Abstract: A novel method of exciting the diffraction-limitcd output of the single pass travelling ware optical parametric generator (TOPG) io demonmtcd. TOPG bared on KIT crystal was pumped by the Bessel beam formed by an Micone. In [I-Z] possibility to obtain the diffraction limited beam h m TOPG using twoand threebeam pump has been shown. In the case ofBessel beam all plane waves compiling pump bcam are lying on the cone surface. This represm a limiting w e of multibeam pump. The Bessel beam was formed in thc glass axicone d i c h was illuminated by the second hamionic of 10 Hz pasrive modelocked NdYAG laser with active negstive feedback and amplification system (pulse duration I5 ps). The spatial profile ofthe Bessel beam and its angular spectrum are presented in Fig.a, b , accordingly. The quality of the Bersel beam WBS 63 (the ratio ofthe lins radius to the ring width in m&ar s p e c m ) . The K I F crystal (8 m . 6 mm.9 m) was cut at 9 = 90' and 0 = 24' for tpe-ll phase-matching what conespmds to nearly noncritical phaw-matching. The crystal was orientated for the conical pump noncollinear phase-matching correspondingly to micone apex angle. ,

Asymmetric diffractionless beams in nonlinear optics

Abstract: We have shown that there exists a generalization of the Bessel beam that can be more efficient for some nonlinear optical interactions. The intensity-length product maybe larger for asymmetric beams but still maintains the advantageous characteristics of the Bessel beam, such as an extended longitudinal focus and a well-defined angle with respect to the z-axis.

Stimulated scattering with a diffraction-free pump beam

Abstract: Summary form only given. We investigated the conditions under which conical or on-axis emission is observed in forward stimulated Raman scattering in H/sub 2/ gas and backward stimulated Brillouin scattering in liquid acetone. The conical emission of stimulated Raman Stokes light was observed for cone angles of the Bessel beam in the range from 0.7 to 3.5 mrad.

Superluminal Localized Waves in Femtosecond Optics

Abstract: Theoretical and experimental study of optical implementations of various new nonspreading wave-packet solutions to linear wave equations is overviewed. Startling superluminal group velocity of pulsed Bessel beams and so-called X-type waves is discussed and mathematically described in terms of electromagnetic field of a tachyonic source.