Ellipticized Acoustical Liquid Filled Lens Providing Balanced Astigmatism

This application is a continuation of application Ser. No. 138,013, filed Apr. 7, 1980 for Ellipticized Acoustical Lens Providing Balanced Astigmatism, which was a continuation-in-part of application Ser. No. 949,711 filed Oct. 10, 1978 now U.S. Pat. No. 4,224,626 for Ellipticized Lens Providing Balanced Astigmatism.

In the design of singlet acoustic lenses for underwater sound or sonar system applications in which wide angle scanning or fixed off-axis operation is required, it is frequently desired to achieve greater off-axis scan in one plane than in the other, wide angle azimuth performance usually being more necessary than elevation scan. Due to requirements for weight minimization and compact packaging for submersible vehicle installations where both system volume and weight are at a premium, it is further generally desirable to extremize the lens in the sense of maximizing the lens aperture or minimizing the lens volume and sometimes simultaneously to achieve a minimum, or at any rate a near minimum, F-number.

On the other hand, in such acoustic applications for the most part, excluding only those areas of medical acoustic radiography and related fields in which frequencies higher than 10 megahertz are commonly used, the wavelengths encountered are of the order of a millimeter, a centimeter, or a decimeter so that there is no need to restrict the acoustic lens design to spherical surfaces grindable only by self-correcting motions as in optical lens manufacture, but rather, nonspherical acoustic lenses of quite complex shape may be readily used. With this relaxation of the design requirements, consideration in sonar and underwater acoustics was first given to nonspherical but rotationally invariant acoustic lenses, i.e., nonspherical acoustic lenses whose surfaces are surfaces of revolution about the lens axis. Such acoustic lenses, however, like any ordinary singlet acoustic lens, spherical or nonspherical, invariably suffer when extremized from astigmatism at off-axis points with the magnitude of that aberration increasing with increasing scan angle and decreasing F-number.

In order to circumvent the astigmatic defect of all single element rotationally invariant extremized acoustic lenses at off-axis angles for the purpose of achieving increased scan in the azimuth plane at the anticipated sacrifice of a reduction in elevation scanning, the possibility of designing a geometrically perfect nonspherical bifocal acoustic lens in three-space, symmetric with respect to two orthogonal planes through the lens axis, one of which contains its design foci, and complete with minimal lens volume and a very low, or in some instances an even minimal, F-number is considered. Such a nonspherical acoustic lens, has axial symmetry about the z-axis, and also plane symmetry about both the x,z and y,z-planes, but no surfaces of revolution and has balanced astigmatism; i.e., negative astigmatism on axis, zero astigmatism at its off-axis design points or primary foci F and F' and positive astigmatism at still further off-axis points and, consequently, offers the opportunity for an optimized trade off between azimuth and elevation scanning capability by suitable optimum choice of the basic lens parameters.

FIG. 1 illustrates an ellipticized acoustic lens with surfaces S and S' and their refractive affect on the ray R and R.sub.o ' from the focus F to the wavefront W;

FIG. 2 illustrates the meridional plane cross-section of the ellipticized acoustic lens of FIG. 1 and the rays R.sub.N.sbsb.1 to R.sub.N.sbsb.N and R'.sub.N.sbsb.1 to R'.sub.N.sbsb.N used in calculation of the eigenvalue like angles .phi..sub.1 and .phi.'.sub.1 ;

FIG. 3 illustrates meridional plane cross-section of the ellipticized acoustic lens of FIG. 1 showing the particular case of FIG. 2 where N=1; and

FIG. 4 illustrates the ray tracing results for the ellipticized acoustic lens of FIG. 1.

The geometrical shape of the ellipticized azimuth versus elevation optimized and aperture extremized nonspherical singlet acoustic lens comprising the present invention is characterized by the following boundary value problem and is an extension of the inventor's prior lens antenna invention as described in U.S. Pat. No. 3,146,451.

Referring now to FIG. 1 there is shown an acoustic lens with a graphical representation of a general ray structure. A dual ray structure is also described but is not shown in FIG. 1 as it would excessively complicate the drawing and can be readily understood from the description. The acoustic lens can be made of Sylgard rubber RTV3120 or as a liquid filled thin metal walled shell lens using as the liquid filling any suitable organic or inorganic liquid such as carbon tetrachloride, fluorolube or certain liquid metal mixtures.

Given the focal distance .vertline.z.sub.o .vertline., the acoustic lens diameter 2b.sub.o, the index of refraction of the lens material n.sub.o >1, the off-axis design angle .psi..sub.o and the wavelength .lambda..sub.o of the incident underwater sound or acoustic wave to be focused by the lens, i.e., given the parameters ##EQU1## let the lens surfaces S and S' in FIG. 1 have equations of the form ##EQU2## and let a general ray R from the off-axis design focal point F(x.sub.o,y.sub.o,z.sub.o) pass through the lens surface S at the point x, y,z and through the surface S' at the point x',y',z'. At each of the interfaces the ray is refracted in accordance with Snell's Law. The ray emerges from the acoustic lens in the direction .psi..sub.o, that is to say parallel to the y,z-plane and at the angle .psi..sub.o with respect to the z-axis. Let W denote the corresponding orthogonal sonic wavefront. In order to achieve a dual ray structure from a symmetrically placed dual focal point at F' (x.sub.o,-y.sub.o,z.sub.o) so that a general ray R' from the latter point emerges from the acoustic lens in the corresponding dual direction -.psi..sub.o, we require the lens surfaces S and S' to be symmetrical about the x,z-plane and, taking x.sub.o =0, we require further and without loss of generality, that the surfaces S and S' also be symmetric about the y,z-plane. Finally, extremizing the acoustic lens design so as to obtain a maximal lens aperture, or what is the same thing, a minimal lens volume, by requiring that the lens surfaces S and S' intersect in a sharp boundary curve, or lens periphery, .GAMMA., our problem then is to so determine the shape of the lens and the form of the functions z=z(x,y) and z'=z'(x',y') specifying the lens surfaces S and S' in (2) as to insure that all rays R from the focal point F(x.sub.o,y.sub.o,z.sub.o) incident on the lens within the lens periphery .GAMMA. are perfectly collimated by the lens into a parallel ray plane wave sonic beam perfectly focused at infinity in the direction .psi..sub.o and, by symmetry, that all rays from the dual focal point F'(x.sub.o,-y.sub.o,z.sub.o) similarly incident on the acoustic lens within the periphery .GAMMA. are similarly perfectly focused by the lens at infinity in the dual direction -.psi..sub.o.

Taking appropriate vector cross products between the incident ray R and the normals to the acoustic lens surfaces S and S' at the points x,y,z, and x',y',z', respectively, in the illustration, and applying Snell's Law at each interface, and using the Jacobian notation for appropriate two by two determinants of partial derivatives, we readily obtain for the lens surfaces S and S' the system of partial differential equations ##EQU3## where A denotes the arguments A=(x,y,z,x',y',z') and the coefficient functions F(A) to G'(A) are of the forms ##EQU4## and where p and p' denote the path length elements defined by the expressions ##EQU5## and are thus the geometrical lengths of the segments of the ray R indicated in the figures.

In writing the partial differential equations (3) for the acoustic lens surfaces S and S' we have taken as independent variables the variables x and y which are the x,y coordinates of the point of intersection of the ray R with the lens surface S and we have considered the unknown coordinate z of this point and the unknown coordinates x',y',z' at which the ray R intersects the lens surface S', each to be functions of these variables.

That is to say we suppose the equations (2) for S and S' can also be written in the form ##EQU6## the latter of these being a parameterization of the equation for surface S' in the terms of the variables x and y.

Essentially equivalent to either pair of the partial differential equations in (3) and capable of being substituted for one of those pairs of equations by the acoustical analogue of Malus's Theorem or, as may be shown by direct substitution, is the acoustical path length condition that ##EQU7## for all rays R from the focal point F(x.sub.o,y.sub.o,z.sub.o) to the sonic wave front W propagating in the direction .psi..sub.o with instantaneous intercept a on the z-axis as illustrated and, applying this relation to the extremal rays R.sub.o and R'.sub.o which pass through the lens extremities E(0,b.sub.o 0) and E'(0,-b.sub.0 0), we find for the focal point F(x.sub.o,y.sub.o,z.sub.o) the coordinates ##EQU8## and for the constant P.sub.o in (7) the expression ##EQU9## similarly as in U.S. Pat. No. 3,146,451. In terms of (8) the dual focal point is now F'(x.sub.o,-y.sub.o,z.sub.o).

The natural symmetry of the acoustic lens surfaces S and S' about the x,z-plane imposed by the requirements of providing for a dual ray structure from the dual focus F'(x.sub.o,-y.sub.o,z.sub.o) and the symmetry about the x,z-plane corresponding to the assumption that x.sub.o =0 results in the four symmetry conditions ##EQU10## as additional requirements on the functions defining the lens surfaces S and S' in (2) in terms of the variables x,y and x',y' which in terms of the variables x and y as independent variables become of course much more complicated relations. The symmetry conditions will subsequently be seen to play a very strong role in the mathematics of the problem.

Applying the extremizing condition that the acoustic lens has a maximal aperture or, equivalently, has a minimal volume in the sense described previously so that the lens surfaces S and S intersect in a sharp periphery .GAMMA. passing through the lens extremities E(0,b.sub.o,0) and E'(0,-b.sub.o,0) as in the figures, we readily derive as the boundary conditions of the problem the requirement that that the functions z(x,y) and z'(x',y') in (2) satisfy the relations ##EQU11## on the ellipse ##EQU12## To derive (11) and (12) we use relations (7) to (10) all together and equate z(x,y) to z'(x,y).

Finally, similarly as in U.S. Pat. No. 3,146,151 we have as the conditions for F-number minimization that the exiting segment of the ray R.sub.o from the focal point F(x.sub.o,y.sub.o,z.sub.o) be tangent to the acoustic lens surface S' at the lens extremity E(0,b.sub.o,0) and similarly for the corresponding ray from the dual focal point F'(x.sub.o,-y.sub.o,z.sub.o). Note that the acoustic lens in the illustration has an almost, but not quite, minimal F-number where F=.vertline.z.sub.o .vertline./2b.sub.o.

Our present invention is now characterized as a solution for the lens surfaces S and S' in the boundary value problem consisting of the system of partial differential equations (3) and (4) solved subject to the conditions (7) to (12) with or without the optional additional condition of F-number minimization appended.

A variety of analytic, approximate, and numerical approaches and solution algorithms are viable for application to the solution of the acoustic lens boundary value problem with nearly minimal F-numbers and, to the problem with the F-number minimized. Such solutions were given by the inventor in U.S. Pat. No. 3,146,451 for the corresponding two-dimensional lens problem. These solutions pertained to microwave lenses but are equally applicable to acoustic lenses. Here we extended these solutions to the present partial differential equations problem in three dimensions.

To the foregoing end, ellipticizing the two-dimensional successive approximation solution of the lens problem presented in U.S. Pat. No. 3,146,451 and appending appropriate Taylor series terms, we solve the boundary value problem (1) to (12) in the mixed forms ##EQU13## assuming the F-number is nonminimal. Here the .beta..sub.n (N) and .beta.'.sub.n (N) are the successive approximation coefficients obtained in U.S. Pat. No. 3,146,451 for the two-dimensional problem and .beta..sub.mn and .beta.'.sub.mn are correction, or completion, terms corresponding to the slight but finite deviations of the lens surfaces S and S' in the three-dimensional problem from a simple elliptic transformation of the corresponding acoustic lens surfaces S and S' in the essentially two-dimensional problem discussed in U.S. Pat. No. 3,146,451.

To obtain the coefficients .beta..sub.n (N) and .beta.'.sub.n (N) in the polynominal approximations in (13) and (14) with the F-numbers nonminimal we begin exactly as in the two-dimensional problem of U.S. Pat. No. 3,146,451 by fitting 2N+2 rays R.sub.o,R'.sub.o,R.sub.N.sbsb.1,R'.sub.N.sbsb.1 . . . , R.sub.N.sbsb.N and R'.sub.N.sbsb.N from the focal point F(x.sub.o,y.sub.o,z.sub.o) in the y,z-plane cross section of the acoustic lens as in FIG. 2 such that these rays are perfectly controlled in both direction and path length by the polynominal approximates to the acoustic lens surfaces S and S' of degree 4N+2. As in U.S. Pat. No. 3,146,451 the technique hinges on solutions for approximate values of the eigenvalue like angles .phi..sub.1 and .phi.'.sub.1 in the illustration associated with the rays R.sub.N.sbsb.1 and R'.sub.N.sbsb.1 which pass through the acoustic lens vertices, or axial intercepts of the acoustic lens surfaces S and S', as shown. This solution in turn depends on solution of simultaneous determinantal equations of the form ##EQU14## for the quantities .phi..sub.1 and .phi.'.sub.1. In (15) and (16) the elements of the determinants .DELTA.(.phi..sub.1,.phi.'.sub.1) and .DELTA.'(.phi..sub.1,.phi.'.sub.1) and, hence, the determinants themselves, depend only on .phi..sub.1 and .phi.'.sub.1 ; i.e., each element of each determinant is either a constant or a function of .phi..sub.1 or .phi.'.sub.1 or both. Moreover, the functions defining the elements are simple elementary algebraic and trigonometric expressions as in U.S. Pat. No. 3,146,451 so no great difficulties attend the solution of the system (15) and (16) for .phi..sub.1 and .phi.'.sub.1 for given N.

Letting the polynominal approximations in the solutions for the lens surfaces S and S' in (13) and (14) converge to analytic limits and noting that the infinite series (13) and (14) are suitably convergent it then follows that the coefficients .beta..sub.mn and .beta.'.sub.mn in the latter parts of (13) and (14) can be obtained by a Cauchy-Kovalevsky type process, i.e., by the method of limits using known expressions for the derivatives (d.sup.k /dy.sup.k)z(0,y) and (d.sup.k /dy'.sup.k)z'(0,y') developed for the two-dimensional problem in U.S. Pat. No. 3,146,451 together with the chain rule, to compute the quantities ##EQU15## required in the process. For practical purposes one can sometimes even determine a few of the coefficients by trial and error using ray tracing methods to check the results 18.

Computational experience with the mixed forms of solution (13) and (14) indicates that the convergence of both the limit expressions and the infinite series in (13) and (14) is exceedingly rapid even when the F-numbers are nearly minimal. Thus, for example, for the case of a Sylgard rubber acoustic lens made of RTV 3120 with F-number one-half at 20 degrees off-axis having as basic parameter values the numbers ##EQU16## and .lambda..sub.o arbitrary, and taking the simplest case N=1 in (13) and (14), as in FIG. 3 we obtain for the coefficients .beta..sub.n (N) and .beta.'.sub.n (N) the values ##EQU17## which, with all .beta..sub.mn and .beta.'.sub.mn values zero, yields the ray tracing results shown in FIG. 4. Here the deviations .DELTA.P in the sonic path length or the sonic phase for the wave front is seen to be flat to within 0.01 units over a circular region of diameter 7 cos.psi..sub.o units, or to be flat to within one part in 658.

By the acoustic analogue to the quarter wavelength Rayleigh criterion for diffraction limited beam forming, this acoustic lens with F=1/2 at 20 degrees is capable of forming a collimated sonic beam of half-power width of approximately four-tenths of a degree, or 24 minutes of arc, or twice this with a one-eighth wavelength Rayleigh criterion, a very sharply focused sonic beam in either case for a bifocal acoustic lens with so very low an F-number.

Finally, if the F-number is actually to be a minimum, the foregoing treatment needs modification along the lines of the related well known treatment of the generalized aplanatic lens design problem as will be easily recognized by those skilled in the art. In particular if such is to be the case, it will normally be best to give up the analytic expressions for the lens surface S' in (13) and (14) and determine S' from S by satisfaction of the path length condition after previously determining S as before.

After solving the acoustic lens boundary value problem we then seek to adjust the basic lens parameters .vertline.z.sub.o .vertline.,n.sub.o,.psi..sub.o in (1) particularly the index of refraction n.sub.o and the off-axis design angle .psi..sub.o, so as to balance astigmatism and optimize the acoustic lens for best balance of azimuth versus elevation scanning performance. Generally we seek a maximum azimuth scanning capability for given acceptable elevation performance. In carrying out this azimuth versus elevation scanning trade off, any of several varied but more or less equivalent optimization criteria may be applied.

It has therefore been shown that an ellipticized azimuth versus elevation optimized and aperture extremized nonspherical acoustic lens having perfect primary off-axis focal points F and F' and perfect conjugate off-axis focal points F.sub..infin. and F'.sub..infin. at infinity is attainable. It has further been shown that such a lens balances the astigmatism in the sense of having negative astigmatism on axis, zero astigmatism at its off-axis or primary design foci F and F' and positive astigmatism at still further off-axis points and can be optimized for best trade off between azimuth and elevation scanning capability by suitable optimal choice at the basic lens parameters.

It will be understood that various changes in details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.