Showing papers on "Bessel beam published in 1992"

Experimental investigation of Bessel beam characteristics.

Abstract: We report on an experimental characterization of Bessel beams with finite apertures. We show that real Bessel beams can be generated with intensity profiles that closely resemble the ideal Jo2 transverse-intensity distribution of Bessel beams. We also show interferometrically that these beams have planar phase fronts with π-phase shifts from one Bessel ring to the next. We report tolerance conditions for Bessel beam generation and give an example of this generation that uses an unstable resonator as the light source.

Review of non-diffracting Bessel beam experiments

Abstract: The theory of non-diffracting Bessel beam propagation and experimental evidence for nearly-non-diffracting Bessel beam propagation are reviewed. The experimental results are analysed to show that the observed propagation distances are characteristic of the optical systems used to transport the beams, and are not unique to the Bessel beam intensity profiles. The results indicate that, despite the innovative techniques employed to generate finite radius Bessel beam distributions, nearly-non-diffracting Bessel beam propagation has not yet been conclusively demonstrated.

Optimization of real phase-mask performance

Abstract: Classical phase mask lithography designs can be understood by the application of Fourier optics to the phase mask pattern. For maximum resolution, the mask design for a circularly symmetric contact hole will have a Fourier pattern with most of the energy near the edge of the system aperture. The inverse Fourier transform of an ideal annular Fourier plane pattern is the Bessel function, Jo. This function consists of a central lobe and an infinite number of rings, with each ring having equal energy and alternating phase shifts. This function is a solution to the wave equation in cylindrical coordinates. A characteristic of this solution is that the field has the same transverse profile, independent of the position along the axis. Therefore, the diameter of the central lobe of an ideal Bessel beam will have infinite depth of focus. Although approximations to such diffraction free beams have been reported for coherent light, the depth of focus depends on the number of outer lobes of the Bessel function that can be produced. Practical mask design and imperfect coherence limit the number of lobes that are actually useful. However, a coarse approximation to the Bessel beam can be created with only one or two phase shifted lobes. This is what is attempted in the optimized "outrigger" patterns. The ideal design, however, is one in which one phase- edge is placed at the zero of the Jo function, and a second is placed so that the power in the outer lobe approximates that of the Bessel function. Masks were fabricated with such designs, and the wafer exposures show that the image is significantly brighter than with other techniques, and the depth of focus is also significantly increased.

Transporting and focusing radially polarized laser beams

Abstract: This paper reports that a radially polarized laser beam is useful for applications such as laser particle acceleration. The issues of transporting and focusing (with an axicon) a radially polarized beam that was generated in the laboratory are examined. Problems of preserving the polarization while directing the beam are solved by using a compound 90-deg-fold out-of-plane pair of mirrors. When focused by an axicon, the radially polarized beam produces a diffraction-free Bessel beam. The transverse intensity distribution agrees with theory.

Generation of low-divergence laser beams

Abstract: Apparatus for transforming a conventional beam of coherent light, having a Gaussian energy distribution and relatively high divergence, into a beam in which the energy distribution approximates a single, non-zero-order Bessel function and which therefore has much lower divergence. The apparatus comprises a zone plate having transmitting and reflecting zones defined by the pattern of light interference produced by the combination of a beam of coherent light with a Gaussian energy distribution and one having such a Bessel distribution. The interference pattern between the two beams is a concentric array of multiple annuli, and is preferably recorded as a hologram. The hologram is then used to form the transmitting and reflecting zones by photo-etching portions of a reflecting layer deposited on a plate made of a transmitting material. A Bessel beam, containing approximately 50% of the energy of the incident beam, is produced by passing a Gaussian beam through such a Bessel zone plate. The reflected beam, also containing approximately 50% of the incident beam energy and having a Bessel energy distribution, can be redirected in the same direction and parallel to the transmitted beam. Alternatively, a filter similar to the Bessel zone plate can be placed within the resonator cavity of a conventional laser system having a front mirror and a rear mirror, preferably axially aligned with the mirrors and just inside the front mirror to generate Bessel energy distribution light beams at the laser source.

Bessel beam ultrasonic transducer: fabrication method and experimental results

Abstract: We report experimental results from a first‐of‐a‐kind ultrasonic transducer that generates a beam with a Bessel function profile. Using a technique of nonuniform poling, an axially symmetric Bessel function pattern is ‘‘polarized into’’ a piezoelectric ceramic element. The resulting circular‐disk transducer has the usual full‐plating electrode configuration, but produces an ultrasonic beam with a radial displacement profile approximating that of the Bessel function J0 (r), both in amplitude and in phase. The radiation field of a 1‐in.‐diam, 2.25 MHz Bessel transducer mapped out with a point probe shows good agreement with calculated results using a Gauss‐Hermite model. Bessel transducers are of particular interest in attempts to achieve ‘‘diffractionless’’ beams.

Method and arrangement of generating a non-diffractive beam at a location which is remote from optical element and application thereof

Abstract: In order to generate a Bessel beam at a location remote from an optical arrangement, the optical arrangement includes, a first beam converging element which is illuminated by a collimated beam, a second beam converging element which is illuminated by a beam which has passed through the first beam converging element. The first and second beam converging elements are arranged in a manner to radiate a ring beam which is parallel with an optical axis. A third beam converging element is further provided which is illuminated by the ring beam. The third beam converging element is arranged to generate a non-diffractive beam at a location which is remote therefrom. Further, an improvement of a bar code reader is disclosed, which includes a laser source for producing a laser beam. A laser beam deflecting member is provided to deflect the laser beam so as to form a scan pattern comprised of two scan lines. The two scan lines are oriented at a predetermined angle with respect to one another. A combination of photo sensor and electric circuitry is provided to detect the beginning of the scan pattern and to discriminate between the two scan lines.

Weighted conical transducer for generation of Bessel beam ultrasound

Abstract: A conical transducer with a weighted velocity distribution is proposed as a simple and practical way of generating a quasi-non-diffracting beam. It is demonstrated theoretically that the weighted conical transducer can create a sound field almost the same as that of the Bessel beam transducer in continuous-wave excitation. In the case of short-pulse excitation, sidelobes are averaged and the pulse duration time is shorter than that of the Bessel beam transducer. It is also shown through the preliminary experiments that the weighted conical transducer has the capability of generating a narrow beam ultrasound over a long axial range. >

Generation of nondiffracting X waves using annular‐array transducers

Abstract: Nondiffracting beams have been suggested for ultrasonic imaging and tissue characterization applications because of their long depth of field. Recently, a new class of nondiffracting beams, termed the X waves, has been discovered. An advantage of these beams over the J0 Bessel beam is that the X waves are nondispersive. An ideal zero‐order X wave has infinite time duration and requires infinitely large aperture area, excited by a spatially nonuniform, circular‐symmetric, input, h(r,t), where r and t are the radial distance and time, respectively. In practice, X waves can be generated employing a finite size annular‐array transducer. In this paper, annular‐array implementation of the X waves is investigated and the errors introduced in the field due to such implementation are analyzed. Each ring in the array is excited by a different time‐limited waveform. Quantitative comparisons between the field of the array and the ideal X wave is made possible by computer simulations. Results indicate that, in the ran...

Production of a Diffractionless Ultrasonic Beam

Abstract: The propagation of ultrasonic beams is a phenomenon of widespread interest to a variety of technologies including sonar, medical ultrasound, and nondestructive evaluation. One goal in most applications is the production of a narrow, highly collimated beam of sound. Rigid piston radiators have often been employed and have been thoroughly analyzed. This type of source has the generally undesirable attributes of a complicated near field interference structure as well as far field side lobes. Sources which produce a Gaussian amplitude distribution have been studied since, for this case, the previous disadvantages are eliminated. Unfortunately, Gaussian radiators are more difficult to manufacture [1,2]. Various types of focusing probes have also been analyzed for concentrating the sound in a narrow band over a short depth of field. Conically focussed, or axicon, probes have been examined for the purpose of extending the focal region for resolution over a greater depth of field. One disadvantage common to all of the above sources, and indeed to any physically realizable source, is the phenomenon of beam spread due to diffraction.