Multifrequency Rotatable Scanning Prisms

The subject matter of this application is related to the subject matter of commonly-owned U.S. patent application Ser. Nos. 800,937 entitled "Multimode, Multispectral Antenna", 800,938 entitled "Multimode, Multispectral Antenna", 913,890 entitled "Multimode, Multispectral Scanning and Detection", and 913,893 "MultiSpectral Radome" respectively, filed on even date herewith are expressly referenced to and incorporated herein by such reference.

This invention is directed toward the art of multimode frequency and wavelength scanning and detection systems, and more particularly, toward airborne multimode scanning and detection systems employing radar, visible and/or infrared scanning and detection techniques.

Many different kinds of multimode scanning and detection systems are currently known. Such systems may be active or passive in operation, being operationally effective in scanning or detecting multiple beams of radiation at multiple frequencies and wavelengths. The frequencies of operation include infrared radiation, in which heat is detected to identify a particular target or target region. Detection may be accomplished in the radar or radio frequency bands, either actively or passively or subject to a combination of active and passive modes.

The term multimode can further be taken to refer to detection first at one mode of energy operating at a given first frequency, and then detection at another selected mode or frequency. When several frequencies of the electromagnetic spectrum are thereby used, this approach is frequently referred to as multi-spectral. Multimode can further be taken to mean the use of both active and passive bands of radiation. It can additionally mean the use of one or more radar bands of radiation and one or more infrared bands. Multimode detection systems can moreover be ground based, ship based, airborne or set aloft in space.

In general, multimode detection systems enhance the detection flexibility and effectiveness of the system using the technique. For example, one beam may be designed to be wide in shape in order to conduct search operations for a target sought, and the other beam working in conjunction therewith is then narrow in order to accomplish tracking once the target has been identified. The different modes can relate to the distance or range of detection as well. For example, one mode can be used for short range target acquisition, while the other mode is employed at more extended ranges. For example, radar frequencies might be used at long ranges and infrared frequencies closer in.

The various modes of operating such detection systems can moreover be used in combination with each other in order to accomplish effective target classification and identification. For example, targets often appear different in different spectral regions, and the degree of difference can be used to distinguish one type of target from another.

As desirable as multimode systems may be, problems nonetheless arise in the development of multimode systems due to the relationships between the modes. For example, techniques and arrangements have been urgently needed to establish coordination between the modes of radiation selected, to permit effective handoff between the modes of operation to ensure a continuity of information and operation. Other problems faced in implementing multimode systems are caused by the limited nature of refractive materials available for use as protective domes, collimating lenses, and the scanning system itself, in order to permit unhampered egress and ingress of the selected beams of radiation to be scanned or detected.

The prior art often achieves beam scanning by mechanical pointing means, for example, by mounting entire antenna systems on gimbals. Such methods are more costly, cumbersome and prone to breakdown than the rotating refractive prism scanners according to the invention herein.

Other difficulties arise in designing an effective multimode scanning arrangement with rotating prisms when the beams scanned are at different frequencies, because beams of different frequencies typically are not deviated by the same amplitude. This not only causes such beams to point in different directions from time to time, but it also causes the difference in these directions to change by an amount which depends upon the pointing direction, thereby hampering transfer from one more of operation to the other. In other words, because the same scanning prisms are utilized for both beams, handoff from one mode to the other becomes more difficult to accomplish.

The invention herein is accordingly directed toward the establishment of a scanning arrangement for a multimode, multispectral detection system having beams of several frequencies which scan by the same amount. When the beams are optically superimposed, they are then pointed in the same direction and may be directed toward a selected target simultaneously, thereby enabling straightforward handoff between modes of operation.

In particular, the scanning arrangement includes a circumferentially rotatable pair of scanning prisms, each of the scanning prisms being constructed of cooperative subprisms of selected apex angle and materials, thereby ensuring that the parallel beams of radiation which enter the scanning prisms will also exit the prisms parallel to each other, and will thereby be directed toward the same target area or region in unison.

Another feature or aspect of the invention is directed toward construction of the subprisms and determining effective apex angles for complementary ones thereof. In particular, an arbitrary apex angle is selected and then complementary apex angles are evaluated for the different frequencies of operation selected.

Other features and advantages of the invention will be apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention.

FIG. 1 shows in axial cross section, a multimode detection system addressed herein.

FIGS. 2A and 2B show respective cross sections of a dual frequency scanning arrangement according to the invention herein, first with the arrangement set at maximum net angular deviation and then with no net angular deviation.

FIGS. 3A and 3B show a scanning arrangement according to the prior art.

FIG. 4 shows first and second beams of radiation having different wavelengths passing through a representative cross section of a scanning prism.

FIG. 5 is a flow chart indicating how to determine materials and apex angles according to the invention herein.

FIG. 1 shows generally a possible application for using a multimode detection system 11 including a scanning arrangement 13 having a cylindrical prisms 13' and 13". The detection system 11 particularly includes a radome 15 for passing beams of electromagnetic radiation operating in selected modes and/or frequencies including for example millimeter wave or Ku-band radar frequencies and infrared or visible frequencies. The detection system 11 further includes tubular walls 17 for containing electronic and optical equipment used for operating a detection system 11 and for acquiring and monitoring one or more selected external targets of interest and holding scanning prisms 13 and radome 15 in place. The detection system 11 further includes, according to a preferred embodiment of the invention, an infrared sensor element 27 and a pair of radar feeds 23 and 25 suitably mounted with respect to a support structure 33 of arrangement 11 which holds infrared sensor element 27 and feeds 23 and 25 in place within walls 17, as will be seen. Beams of radiation processing to and/or from respective sensors 23, 25, and 27 pass through collimating and shaping lens 29 and are scanned by first and second scanning prisms 13 and 13'.

As will be seen, scanning can be accomplished in an upward and downward direction, laterally back and forth, circularly, or in any one of a number of complex scan patterns, which can be programmed into a controller 41' suitably mounted in arrangement 11. The scanning prisms 13 eliminate the need for gimbals. Instead, they can be driven by a drive mechanism 41 acting under direction of controller 41', which operates mechanically for example with axially rotatable cylinder means 15' and 15" suitably rotatably seated within walls 17 and drivingly individually engaged to drive 41 either peripherally or flangedly along the surface of the circumference of the respective scanning prisms 13 and 13', or otherwise through an axially directed drive (not shown) extending to the center of the scanning prisms and then in turn through the collimating or shaping lens 29.

FIG. 1 further shows the collimating lens 29 held in place flangedly in a holding structure 29' which is in turn mounted on rotatable cylinder means 15" for example, according to one version of the invention. Further, the scanning prisms 13' and 13" are respectively secured and mounted in similar flanged structures 14' and 14 which as already noted are mounted on rotatable cylinder means 15' and 15" which are in turn suitably mechanically coupled to the drive mechanism 41.

FIG. 2A shows a cross-section of a preferred version of the scanning arrangement 13 according to one embodiment of the invention herein. Scanning prisms 13' and 13" are preferably cylindrical and rotatable about an axis parallel to input ray 19. In FIG. 2A, scanning prisms 13' and 13" are relatively rotated and disposed to reorient the direction of input beam 19 in the direction of output beam 19'. If an input beam 19 is at another selected frequency, it will nonetheless be deflected in the same fashion and to the same extent as beam 19 of a first selected frequency, because of the inventive feature of each of the prisms, namely that the subportions 13'a and 13'b and 13"a and 13"b of the respective prisms are cooperative. In particular, if what the first subportion does is greater for one frequency than for the other, this is undone by the cooperative subportion to precisely the same extent.

In FIG. 2B, a selected input beam 19 of electromagnetic radiation at a selected frequency passes directly through both scanning prisms 13' and 13" without any net angular deviation, since the second prism 13' reverses the deviation produced by the first prism 13" completely at the particular orientation to which it has been set.

The arrangement set forth in FIGS. 2A and 2B is an advance over the known prism systems of FIGS. 3A and 3B which display no subprisms.

FIG. 4 shows in detailed cross section one of the two scanning prisms 13' for example according to the invention herein, respectively depicting two subprisms 13'a and 13'b of respective first and second materials A and B. For convenience in analysis, first and second beams 19a and 19b of electromagnetic radiation of two selected frequencies and wavelengths are shown axially incident upon cylindrical prism 13'. The selected materials are respectively alumina and zinc sulfide for example.

In general, optical materials are characterized not only by different indices of refraction, but also by different degrees of variation of index with frequency and wavelength. Thus, for example, it is possible for two materials to each have the same refractive index at one wavelength, but different refractive indices at another.

A beam of electromagnetic radiation 19 is refracted at a surface through which it passes in proportion to the sine of its angle from the normal to that surface, and in proportion to the ratio of the refractive indices of the respective materials on opposite sides of the surface.

This concept establishes the operational basis for the cooperative multiprism assembly 13' shown in FIG. 4, in which the material of subprism A has apex angle "alpha.sub.a " and a refractive index "n.sub.a " as a function of wavelength lambda, while the material of subprism B has apex angle "alpha.sub.b " and a refractive index "n.sub.b " which again is a function of wavelength lambda. If the external medium is air or space, its refractive index is essentially unity for all wavelengths of interest.

Output deviation angles d.sub.1 and d.sub.2 correspond to wavelengths lambda.sub.1 and lambda.sub.2 respectively, and are equal to the net deviation after refraction by the three surfaces through which the radiation passes.

Without loss of generality, one design procedure for equalizing output angles d.sub.1 and d.sub.2 is possible by setting n.sub.a (lambda.sub.1)=n.sub.b (lambda.sub.1) and ensuring that n.sub.a (lambda.sub.2)<n.sub.b (lambda.sub.2)<n.sub.a (lambda.sub.1), while the initial directions of the input beams 19 are perpendicular to surface 63, and are therefore incident on surface 61 at the angle alpha.sub.b -alpha.sub.a from the normal to that surface. For lambda.sub.1, refraction occurs at the first surface such that the sine of the refracted angle is proportional to sine (alpha.sub.b -alpha.sub.a)/n.sub.a (lambda.sub.1), and the ray accordingly continues undeviated by surface 62, (because n.sub.a =n.sub.b for this wavelength), until it reaches surface 63, at which it is further refracted to the net deviation angle d.sub.1.

For lambda.sub.2, the first surface refraction is less than for lambda.sub.1, since n.sub.a (lambda.sub.2)<n.sub.a (lambda.sub.1). However, when this ray reaches surface 62, it is further refracted, because now n.sub.b (lambda.sub.2).noteq.n.sub.a (lambda.sub.2), and the amount of this refraction is controlled by both the ratio of these indices and by the magnitude of alpha.sub.b.

Since the values of n.sub.a (lambda.sub.2) and n.sub.b (lambda.sub.2) are known, the angle alpha.sub.b can be chosen so that the refracted lambda.sub.2 ray reaches surface 63 at the incident angle sin.sup.-1 [sin d.sub.1 ]/n.sub.b (lambda.sub.2)]. The exit angle d.sub.2 must then be equal to d.sub.1.

An example of a preferred version of the invention is to fashion subprism A, i.e., subprism 13'(a), out of an alumina-like material having refractive index of about 3 in the radar region of the electromagnetic spectrum, and a refractive index of about 1.7 for the IR region. Subprism B may be made of a material such as zinc selenide, which also has a refractive index approximately equal to 3 in the radar region, but which has an IR index of about 2.4. Then for the radar region lambda.sub.1, a choice of alpha.sub.b -alpha.sub.a =5 degrees would result in d.sub.1 =10.05 degrees. In order to make d.sub.2 =d.sub.1, this would require a beam angle for lambda.sub.2 within subprism B, i.e., subprism 13'(b) equal to 4.17 degrees from the normal to surface 63, while the angle of the same ray within subprism A would be 2.94 degrees from the normal to surface 61, or 2.06 degrees down from its original external direction. Since its original direction was perpendicular to surface 63, this means that the ray must be deviated an additional 2.11 degrees by surface 62. Ray tracing shows that since the ratio of refractive indices at surface 62 is 1.7:2.4, that surface must be tilted clockwise 9.27 degrees from the axis in order to produce this result. This example would therefore require alpha.sub.a =4.27 degrees and alpha.sub.b -9.27 degrees. By way of additional clarification, it should be noted that FIG. 4 depicts the circumstance in which "n.sub.b " is greater than or equal to "n.sub.a ". The concept, however, is equally valid for "n.sub.b " less than "n.sub.a ". Further, angles "alpha.sub.a " and/or "alpha.sub.b " could be negative angles as well under the inventive concept.

With respect to FIG. 4, the output angle of deviation "d" for a given wavelength lambda is: "d" =sin.sup.-1 [n.sub.b (lambda sin [alpha.sub.b -sin.sup.-1 [[n.sub.a (lambda)/n.sub.b (lambda)]sin[alpha.sub.a +sin.sup.-1 (sin(alpha.sub.b -alpha.sub.a)/n.sub.a (lambda))]]]]. This equation shows that "d" is imaginary (e.g. due to total internal reflection) unless [n.sub.a (lambda)/n.sub.b (lambda)]sin[alpha.sub.a +sin.sup.-1 [sin(alpha.sub.b -alpha.sub.a)/n.sub.a (lambda)]] is less than or equal to one. This condition can always be met when n.sub.b (lambda) is greater than or equal to n.sub.a (lambda), but it can be met only for a specific range of values when n.sub.a (lambda) is greater than n.sub.b (lambda); i.e. those for which sin[alapha.sub.a +sin.sup.-1 (sin(alpha.sub.b -alpha.sub.a)/n.sub.a (lambda))] is less than or equal to n.sub.b (lambda)/n.sub.a (lambda). Accordingly, materials A and B must be selected to conform with the indicated relationship.

For a desired value "d", either alpha.sub.a or alpha.sub.b may be independently chosen, but not both. For example, if a value is chosen for alpha.sub.b, then the following equation determines the required size of alpha.sub.a : alpha.sub.a +sin.sup.-1 [sin(alpha.sub.b -alpha.sub.a)/n.sub.a (lambda)]=sin.sup.-1 [(n.sub.b (lambda)/n.sub.a (lambda))(sin[alpha.sub.b -sin.sup.-1 (sin("d")/n.sub.b (lambda))])]. Since the right side of this equation consists of known values, it may be set equal to "gamma", a known constant angle. It follows that sin (alpha.sub.b -alpha.sub.a)=n.sub.a (lambda) sin(gamma-alpha.sub.a), which can be solved for alpha.sub.a : alpha.sub.a =tan.sup.-1 [(sin(alpha.sub.b)-n.sub.a (lambda)sin(gamma))/(cos(alpha.sub.b)-n.sub.a (lambda)cos(gamma))].

Further, for a single desired deviation or output angle "d" with two different wavelengths lambda.sub.a and lambda.sub.2, angles alpha.sub.a and alpha.sub.b are determined by the specified deviation angle "d" and the values n.sub.a (lambda.sub.1), n.sub.a (lambda.sub.2), n.sub.b (lambda.sub.1), n.sub.b (lambda.sub.2), as follows:

and

where

and

The simultaneous equations for alpha.sub.a and alpha.sub.b may be solved as desired. According to one technique, a numerical method can be implemented using either a computer or programmable calculator. In particular, a value is assumed for alpha.sub.b ; then gamma1 and gamma2 are evaluated; and the two equations for alpha.sub.a are finally independently evaluated and compared. Next, a new value is then chosen for alpha.sub.b which brings the two calculated values of alpha.sub.a closer together. This process is iterated until the difference between the calculated values for alpha.sub.2 is sufficiently small, and is produced by similarly small differences in successively assumed values of alpha.sub.b. For example, the criterion for these differences can be equal to or less than the tolerance to which such angles must be fabricated in order to produce sufficiently accurate deviation angles "d" for the required application.

Even more particularly, FIG. 6, shows a block diagram illustrating design process for choosing alpha.sub.a, alpha.sub.b, and material to achieve a desired deviation angle "d". This block diagram indicates the process involved in designing and making the inventive arrangement described herein.

Specifically, FIG. 6 calls for specification of a required deviation angle "d" in block 600 and making a choice of materials in block 610. Then a check is conducted at decision block 620 to see if it is possible to produce this deviation angle in a single prism of acceptable thickness with an averaged ((n.sub.a +n.sub.b)/2) index of refraction value. If not, consideration is given to evaluate whether a smaller deviation value is acceptable, as suggested at block 625.

If the desired deviation angle is deemed obtainable, a check is made at decision block 630 to determine whether n.sub.b is greater than n.sub.a for both desired wavelengths. If not, flag 635 is set and the operation continues.

Next, alpha.sub.b is chosen, its absolute value being less than 90 degrees, for an acceptably thin prism. Then, the gamma values indicated above are calculated. If one or both of the gamma values is imaginary and the absolute value of alpha.sub.b is not less than or equal to the arcsine of n.sub.a /n.sub.b, another value of alpha.sub.b is chosen, as per block 640. If the alpha.sub.b chosen causes one or both of the gamma values to be imaginary and the absolute value of alpha.sub.b is less than or equal to the indicated arcsine value, or is imaginary, a smaller deviation angle must be considered.

If both gamma values are real, first and second alpha.sub.a values are calculated, and if the flag has been set earlier at block 635, a check is conducted as set forth in block 666.

If the error between the calculated values of alpha.sub.a is acceptably small, the previous value of alpha.sub.b is updated as per block 675, if this has not already been accomplished. Then the error between successive alpha.sub.b values is checked to see if it is acceptably small. In this fashion, subprism angles alpha.sub.a and alpha.sub.b can be established.

It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit and scope of this novel concept as defined by the following claims.