Patent Number: 048878855
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to novel arrangements, including both systems and methods, for generating narrow beams of traveling wave fields in space, and more particularly pertains to several embodiments for integrated radiation cavities (either LASER or MASER cavities) designed to generate in their own medium a Bessel mode diffraction free beam. Much of the disclosure herein is applicable to all types of waves as described by the basic Helmholtz wave equation, including electromagnetic waves such as radio frequency, microwave, infra-red, optical and x-ray waves, relativistic and nonrelativistic quantum waves associated with particle waves, such as electron, neutron, proton, atom and other quantum particle waves, and further including physical elastic waves such as material deformation waves and longitudinal waves including acoustical waves. 2. Discussion of the Prior Art Current state of the art techniques to concentrate a wave or form a parallel beam are generally successful only over a very limited range of beam propagation. This range is conventionally related inversely to the degree of concentration. This inverse relationship arises primarily because all wave fields are subject to diffraction (i.e., beam spreading). The arrangements of the subject invention have several advantages over all prior art techniques currently in use, with a principle advantage thereof being greatly improved resistance to diffraction. Two methods exist in the current state of the art for generating narrow beams, focusing and collimation. Due to the ever present effects of diffraction, a focus is never perfect. Instead, a focus is characterized as a finite region over which a beam has a minimum radius. The distance along the lens axis, on one side or the other of the focus, where the beam exhibits significant convergence is called the depth of field of the focus. The depth of field of a focus is generally limited by the sharpness of the focus. That is, a very small focal spot can be achieved only at the expense of depth of field. All light waves, such as those radiated by the sun, lamps and lasers, can be collimated as well as focused. Collimated (parallel) beams are generally preferred because they have much greater depth of field than focused beams, although they are less bright. Collimation is normally accomplished by a series of aligned apertures, which are basically just holes in opaque screens, which allow the light through along just one direction. A sequence of aligned holes along a collimation axis of a beam provides the normal manner of creating a well-defined parallel or collimated beam. Unfortunately, diffraction affects collimation adversely just as it does focusing. The effects of diffraction on collimation can be described with the explanation that a wave field bends outwardly from the edges of a hole as it proceeds therethrough, and thus the resulting beam is not as well collimated. FIG. 1 illustrates the characteristic behavior of waves traveling through holes. The diffractive bending of water waves that are entering a narrow harbor or passing by a jetty can be shown easily in aerial photographs thereof because of the large scales involved, but the bending of light waves is very difficult to notice under ordinary circumstances because the angle of bending is so small. The bending angle is approximately equal to the ratio of the wavelength of the light to the size of the hole, an angle that is usually less than 10.sup.-3 (one one-thousandth) of a degree. A standard criterion called the "Rayleigh range" identifies the distance over which a collimated beam remains well defined after passing through a hole with a given cross sectional area. The Rayleigh range is the ratio of the area of the hole to the wavelength of the light. The Rayleigh range (here denoted Z) is mathematically characterized by the formula Z=A/.lambda., where A denotes the hole's area and .lambda. denotes the light's wavelength. For visible light .lambda. is very small, in the range 15-30 millionths of an inch. A circular hole with a radius equal to one inch has a Rayleigh range of about Z=2 miles. For this reason the diffraction illustrated in FIG. 1 will ordinarily be simply undetectable. However, if an attempt is made to define the beam extremely well (to be able to illuminate a very small spot quite precisely) then the situation is very different. A spot radius of 50 microns (about two-thousandths of an inch) or smaller is conceivable in applications of modern optical technology. The Rayleigh range for a beam formed by passage through a 50 micron sized hole is only one inch or less. This is much greater than the depth of field of a normal sized lens focal spot, but is still very small on a practical working scale. These estimates indicate that current techniques for creating narrow collimated beams are simply unable to generate beams that have any significant range at all, particularly with respect to commercial operations such as drilling, embossing, scribing, testing, and other manufacturing or laboratory activities that might advantageously use very narrow beams. The present invention appears to have applicability and utilization in the semiconductor industry in areas of high precision instruments for optical surface treatments such as etching and marking operations. In these applications, the ability of ordinary light beams to achieve near-wavelength resolution without concern about depth of field or beam divergence could be applied to high-volume integrated circuit manufacturing operations. Tolerances unknown in wafer processing without electron beam or x-ray techniques could be met with ordinary light, perhaps to great advantage in reducing capital costs, magnetic field sensitivity, and worker protection requirements, while increasing instrument reconfiguration flexibility and reducing deadtime between job-runs. Additionally, in the area of high precision process diagnostics, a major change is evolving in process-flow diagnostic instruments. A new generation of instruments uses laser probes to tag (by excitation of fluorescence, for example) molecules participating in a flowing or mixing process at very precisely located highly sensitive regions of the process. The input probe and the signal received back from the light-sensitized molecule are optical and do not disturb the flow or mix in any way. This is in contrast to all of the previous methods that use mechanical sensors inserted into the process, or macroscopic markers or floats injected to accompany the process. These prior art approaches have the disadvantage that their presence necessarily disturbs the environment being measured. The purpose of localized observations is to provide early warnings of turbulent flow, to monitor the degree of completion of a reaction, etc. The present invention has the advantage of allowing highly precise positioning of its beam center and immunity against beam divergence over relatively great depth of field, compared with all other prior art laser devices. SUMMARY OF THE INVENTION The present invention overcomes the prior art limitations on the range of extremely well defined beams, with the term beam herein being utilized generally to refer to the central bright spot, not the full intensity pattern, and is based on the premise that wave fields are subjects to the laws of diffraction. The subject invention can be explained as an arrangement for causing diffractive influences on a beam to cancel each other, thereby allowing the preparation of narrow beams with extreme range or depth of field. To be specific, reconsider the last example hereinabove of a 50 micron beam. If a diffraction free aperture as described herein, with a radius of one inch, instead of 50 microns, is used to create a 50 micron size beam, the Rayleigh range becomes 500 times greater, about 33 feet. If narrow beams are important for truly distant wave transport, as in reconnaissance and laser range-finding, a somewhat larger diffraction free aperture would suffice. For example, if a diffraction free aperture with a one-foot radius is used to create a one-inch wide beam, the Rayleigh range grows to 30 miles. Accordingly, a principal object of the present invention is to provide an arrangement for transforming travelling wave fields into well-defined beams that are not affected by diffractive spreading. The arrangement depends upon a properly designed aperture, and can be applied to any wave field whose wave amplitude .PSI. satisfied these mathematical relations: EQU .PSI.(x,y,z,t)=.PSI.(x,y,z)e.sup.i.omega.t EQU [.gradient..sup.2 +(.omega./v).sup.2 ] .PSI.(x,y,z,t)=0 The letter v designates the velocity of the wave incident on the transmission plate. It is well known that an extremely wide variety of wave fields satisfy these conditions, including radio, microwave, infra-red, optical, x-ray, and all other electromagnetic waves, many types of sound, water, and elastic waves, and both relativistic and non-relativistic quantum waves associated with electrons, neutrons, protons, atoms and all other quantum particles. Considering, for illustration, only light waves, the beams generated pursuant to the teachings herein can find immediate application to laser printing, laser surgery, high precision instruments for optical treatment of surfaces such as laser etching, laser marking, high precision process diagnostic instruments, and other laser applications where depth of field and control of beam definition are more crucial than the irradiance thereof. Ranging and signalling and targeting with well defined, high power coherent electromagnetic and other waves over long distances may also be possible in nonabsorbing media and atmospheres. Pursuant to the teachings herein, nondiffracting apertures can be constructed by following precise criteria which are based upon mathematical principles of waves. The basic criterion of a nondiffracting aperture is to convert a wavefront of an input plane wave beam, obtained in a standard manner, from a laser beam for example, into a wavefront with a very specific form, so that the height and spacing of the modulations of the output electric field strength of the output beam are related to each other in such a way that the beam travels without any change in the modulations. This means that any very sharp maximum, such as the central beam spot, will maintain its small size and will not spread out. Nondiffracting apertures can be built to satisfy these criteria by using commercially available components such as lenses, screens, wave guides, masks, absorption filters, phase shifters, etc. The term nondiffracting as used herein is meant to apply to a well defined traveling wave beam not subject to beam spreading in the sense that the intensity pattern of the traveling wave beam in a transverse plane is substantially unaltered by propagation over a range which is substantially larger than the Rayleigh range of a Gaussian beam with equal central spot width. Pursuant to the teachings of the present invention, such a wave beam is formed by generating a traveling wave beam the amplitude of which has its transverse dependence substantially identical to J.sub.m (.alpha..rho.), the m.sup.th Bessel function of the first kind, wherein .alpha. is a geometrical constant and .rho. designates the transverse radial coordinate of the wave beam, and further wherein the Bessel function argument is independent of the distance z of propagation, which results in a well defined traveling wave beam not subject to beam spreading. Pursuant to the teachings of the present invention, a well defined traveling wave beam substantially unaffected by diffractive spreading can be generated from a recognition that certain exact, non-singular solutions exist for the free space Helmholtz wave equation which represent a class of fields that are nondiffracting in the sense that the intensity pattern in a transverse plane is substantially unaltered by propagation in free space. More specifically, the present invention recognizes that the only axially symmetric nondiffracting field other than a plane wave is the zero-order Bessel function of the first kind and this beam can have an effective spatial width as small as several wavelengths. In accordance with the teachings herein, the present invention provides arrangements, encompassing both systems and methods, for generating a well defined traveling wave beam substantially unaffected by diffractive spreading, comprising generating a beam having a transverse dependence of a Bessel function, and a longitudinal dependence which is entirely in phaser form, which results in a beam having a substantial depth of field which is substantially unaffected by diffractive spreading. In one disclosed embodiment, the beam is generated by placing a circular annular source of the beam in the focal plane of a focusing means, which results in the generation of a well defined beam thereby because the far field intensity pattern of an object is the Fourier transform thereof, and the two-dimensional Fourier transform of a Bessel function is a circular function. In a second disclosed embodiment, the beam is generated by transmitting a coherent beam sequentially through a phase modulator, having a periodic step function pattern, and a spatial filter, whose transmittance is the modulus of the Bessel function, to generate a beam having a transverse Bessel function profile. In different embodiments, the beam can be an electromagnetic wave, a particle beam, a transverse beam, a longitudinal beam such as an acoustic beam, or any type of beam to which the Helmholtz generalized wave equation is applicable. Moreover, the beam can be generated with a transverse dependence of a zero order Bessel function, or a higher order Bessel function, or any combination of different Bessel functions such as a zero order Bessel function and one or more higher order Bessel functions, as illustrated in FIG. 17. The present invention offers a significant advantage over prior art methods by permitting a bright central core of a beam to remain concentrated and available for use over much greater ranges of propagation than is currently possible with prior art methods of beam formation. The subject invention is generally applicable to processes that are activated by bright spots (of light, for example), but for which the distance at which the activity occurs is not easily controlled extremely well. These processes can vary from normal manufacturing and laboratory processes such as drilling, embossing, scribing, welding or testing, where the distance is in the few-inch range and beam spot sizes may be extremely small (10-100 microns), to open field processes such as ranging and aligning where the distances and beam spot sizes may both be many thousands of times greater, but relative tolerances about the same. Pursuant to the teachings of the present continuation-in-part application, several embodiments are described and disclosed of an integrated radiation cavity, as incorporated in a laser or maser, for increasing the efficiency of production of the radiation beams. More particularly, designs are disclosed for integrated optical or microwave cavities for lasers or masers which will generate directly from their own gain medium a Bessel-mode diffraction-free beam. The different disclosed embodiments for such integrated optical or microwave cavities have several common characteristics: (a) a close relation to a known stable laser or maser cavity design, (b) a large mode volume to permit exploiting the relatively high gain of the laser or maser systems, and (3) little departure in principle from the design that has already led to successful observation of non-diffracting beams. The several disclosed embodiments of FIGS. 12-16 are generally generic to either Light Amplification by Stimulated Emission of Radiation (LASER's) or Microwave Amplification by Stimulated Emission of Radiation (MASER's). Several of these embodiments are diffraction-free mode generators, and have the common characteristic of integrating the radiation source into the diffraction-free mode generator, as opposed to directing an externally generated beam through a diffraction-free aperture. One embodiment is somewhat of a hybrid specy in this regard as a diffraction-free aperture is incorporated into one end of the resonant cavity. All of these embodiments are generally expected to produce much higher output power and increased efficiency of operation. Moreover they can be used to produce intense high beams of very small diameter (60 microns or much smaller) having applications, for example, to precision pointing, micro-welding, and ultra-small scale data deposition and scanning.