Ultra wide band achromatic Risley prism scanner

A system and method for scanning a wide band beam is presented. An apparatus includes a pair of prism triplets. Each prism triplet includes a first wedge prism, a second wedge prism and a third wedge prism all formed with different optical materials. In operation, a beam passing through the wide band team scanning apparatus first passes through the first, second and third wedge prisms of the first prism triplet. The beam then passes through the wedge prisms of the second prism triplet in a mirrored order (the third, then second, then first wedge prisms) than that of the first prism triplet. This apparatus with two prism triplets allows wide band light transmitted through it to emerge with its plurality of different wavelengths of light travelling in the same direction to equalize net dispersive effects each of different wavelengths.

BACKGROUND OF THE INVENTION

1. Field of Invention

The current invention relates generally to apparatus, systems and methods for controlling waves. More particularly, the apparatus, systems and methods relate to steering (e.g., scanning) achromatic waves using one or more prisms. Specifically, the apparatus, systems and methods provide for steering achromatic beams with a triplet pairs of prisms so that the beams behave as though they are monochromatic.

2. Description of Related Art

In modern optical sensors and imaging systems it is often desirable to steer or deflect incoming beams of light in order to capture data in multiple fields of the beam. This beam steering, also known as beam scanning, is conventionally performed using a Risley prism scanner.FIG. 1illustrates a prior art Risley prism pair. A Risley prism scanner is an optical device comprised of two identical prisms placed into an optical beam such that the prisms can be rotated about an axis parallel to the optical beam. This action results in the steering of the beam as it leaves the prism pair, and for example, can act as a beam steering device to direct a laser beam into the far-field through a large steerable angle, characteristic of each prism's makeup of optical material and wedge angle.

However, because the index of refraction of an optical material is a strong function of the wavelength (color), the amount of deflection interposed on the beam varies strongly with wavelength. Although this is a useful phenomenon in some circumstances, it is highly undesirable in many others because it limits the spectral bandwidth of beams that are steerable using a conventional Risley prism scanner. For this reason, conventional Risley prism scanners are only useful for monochromatic systems, such as single-wavelength laser systems or optical imaging systems having very narrow bandpass filters. A need therefore exists for a device, system and/or apparatus that is capable of steering an achromatic beam so that the beam behaves as if it were a monochromatic beam.

SUMMARY

The preferred embodiment of the invention includes a wide band achromatic beam scanning device that uses two multi-prisms wherein each prism is comprised of multiple components and each component is constructed of a different type of optical material. The relative position, wedge angle orientation, and optical material of each component is optimized such that wide band light transmitted through all of the components emerges with all wavelengths travelling in the same direction thereby equalizing net dispersive effects for all wavelengths. Therefore, in general, most embodiments of the present invention provides for beam scanning devices that steer an achromatic beam so that the beam behaves as if it were a monochromatic beam. Particular applications require different positions, orientations, and optical materials to achieve these desired effects and therefore it is contemplated that the present invention has a plurality of exemplary embodiments.

In one configuration of the preferred embodiment, an apparatus includes a pair of prism triplets. Each prism triplet includes a first wedge prism, a second wedge prism and a third wedge prism all formed with different optical materials. In the preferred embodiment, the first wedge prism is formed out of zinc sulfide (ZnS), the second wedge prism is formed out of zinc selenide (ZnSe) and the third wedge prism is formed out of gallium arsenide (GaAs).

In operation, a beam passing through the wide band beam scanning apparatus first passes through the first, second and third wedge prisms of the first prism triplet. The beam then passes through the wedge prisms of second prism triplet in a mirrored order (the third, then second, then first wedge prisms) than that of the first prism triplet. This apparatus with two prism triplets allows wide band light transmitted through it to emerge with its plurality of different wavelengths of light travelling in the same direction to equalize net dispersive effects each of different wavelengths.

In more detail, the first wedge prisms is formed with a first planar surface and a second planar surface tilted at a first angle (e.g., wedge angle) with respect to the first planar surface. The second wedge prism further includes third and fourth planar surfaces at second wedge angle and the third wedge prism further includes fifth and sixth planar surfaces formed with a third wedge angle. In the preferred embodiment, the first, second and third wedge angles are all different. In some embodiments, the third angle is less that the first angle and the second angle and additionally, the second angle is greater than the second angle. The third angle can be about 10 degrees, the second angle can be about 40.3 degrees, and the third angle can be about 13.95 degrees.

DETAILED DESCRIPTION

FIGS. 2 and 3illustrate exemplary embodiments of wide band beam scanning devices200,300that use two multi-prisms202,302each. One of the multi-prisms202of the wide band beam scanning device200ofFIG. 2has been rotated 180 degrees in the direction of arrow R about center point CP. Orienting the two multi-prisms202in this way provides for a wide band beam scanning device200that passes an input beam240through it so that its output beam242has zero degrees of scan. In another configuration shown inFIG. 3, one of the multi-prisms302of the wide band beam scanning device300has been flipped about a centerline CL of the wide band beam scanning device300so that its two multi-prisms302mirror each other. This configuration provides for a wide band beam scanning device that does not pass a beam through it with zero degrees of scan. In the example ofFIG. 3, the mirrored orientation of the multi-prisms302receive an input beam340and provides for about 45 degrees of scan at the output beam342. Those of ordinary skill in the art will appreciate that multi-prisms can be designed so that the scan could be other angles.

The wide band beam scanning device200ofFIG. 2is now explained in greater detail and this explanation also applies to the scanning device300ofFIG. 3. Both multi-prisms202, are comprised of three separate prisms204,206,208. In the preferred embodiment, each prism204,206,208is constructed of a different type of optical material and are generally wedge-shaped. The three prisms204,206,208are stacked together and then the two multi-prisms are combined to form a wide band beam scanning device200with a left side210, a right side212, a top side214and a bottom side216. Each wedge-shaped prism204,206,208has left and right surfaces that form angles with respect to each other. In the preferred embodiment, for example, prism204has a left surface220and a right surface222that form a wedge angle (θ) at point P1of about 13.95 degrees; prism206has a left surface224and a right surface226that form a wedge angle (φ) at point P2of about 40.3 degrees; and prism208has a left surface228and a right surface230that form a wedge angle (ρ) at point P3of about 10.0 degrees.

Similar to the scanning device200ofFIG. 2, the scanning device300inFIG. 3is formed with two multi-prisms302that are also comprised of three separate prisms304,306,308. The three prisms304,306,308are stacked together and then the two multi-prisms302are combined to form a wide band beam scanning device300with a left side310, a right side312, a top side314and a bottom side316. Also, similar to the scanning device200ofFIG. 2, prism301has a left surface320and a right surface322; prism306has a left surface324and a right surface326; and prism308has a left surface328and a right surface330.

Returning toFIG. 2, the relative position, wedge angle orientation, and optical material type of each prism204,206,208(e.g, components) is optimized such that wide band light transmitted through all of the components204,206,208emerges with all wavelengths travelling in the same direction, thereby equalizing net dispersive effects for all wavelengths. Particular applications require differing positions, orientations, and optical materials, therefore it is contemplated that the present invention has a plurality of exemplary embodiments. For the application shown inFIG. 2, each prism triplet202is comprised of three different material types, ZnS, ZnSe, and GaAs. Even in this application, there are other material types that could perform equally well as these materials so this example is not meant to limit the usage of other glass types in other embodiments of this invention.

In order to find the best geometrical configuration for each component prism within a multi-prism202, the wedge angle orientation of one of the component prisms should be varied systematically and input into a conventional ray-trace program such as Zemax, for example. The spectrum of transmitted rays can then be computed for several wavelength values and the relative angles between them. In an exemplary embodiment, the optical design program's optimization capability should be used to determine the values of the other component prism wedge angles so as to minimize the angular separation between the emerging wavelengths. The results of the optimization leading to the prism pair illustrated inFIG. 2are shown in Table 1. The last column, labeled “S DEV,” is the standard deviation of the differing transmitted ray angles computed over a wavelength. It shows that the prism geometry which gave the best performance was obtained when the wedge angle (ρ) of the GaAs prism was about 10 degrees. In this analysis, the values of Θ, χ, and ρ represent the wedge angles of the ZnS, ZnSe, and GaAs prisms, respectively. Therefore, an embodiment of the present invention provides a beam scanning device200,300that is achromatic over a large wavelength band.

TABLE 1(Optimization Results of varying the value of the wedgeof the GaAs prism from 8 to 12 degrees. The last columnrepresents the variation in scan angle (in units of micro-radians)for the spectral band ranging from 3 to 10 microns)Achromatic Prism: Doublet Trioρ (deg)Θ (deg)Φ (deg)S DEV(GaAs)(ZnS)(ZnSe)(micro-radians)8.00013.47136.5752339.00013.60738.18411010.00013.74239.79647.511.00013.87841.41216512.00014.00943.031300

FIG. 4illustrates a method400of scanning a wide band beam. The method400begins by passing the wide band beam through a first prism triplet, at402, and generating a first prism triplet output. The first prism triplet can be formed with a first wedge prism, a second wedge prism and a third wedge prism. The method400can pass the beam through a first wedge prism formed of zinc sulfide (ZnS) and having a wedge angle of about 13.95 degrees, a second wedge prism formed of zinc selenide (ZnSe) and having a wedge angle of about 40.3 degrees, and a third wedge prism formed of gallium arsenide (GaAr) and having a wedge angle of about 10.0 degrees.

The method400passes the first prism triplet output through a second prism triplet, at404, to generate a second prism triplet output. Passing light through two multi-prisms in this way ensures all wavelengths of the output travel in the same direction to equalize any net dispersive effects for wavelengths of the original light beam.

In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. Therefore, the invention is not limited to the specific details, the representative embodiments, and illustrative examples shown and described. Thus, this application is intended to embrace alterations, modifications, and variations that fall within the scope of the appended claims.

Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described. References to “the preferred embodiment”, “an embodiment”, “one example”, “an example”, and so on, indicate that the embodiment(s) or example(s) so described may include a particular feature, structure, characteristic, property, element, or limitation, but that not every embodiment or example necessarily includes that particular feature, structure, characteristic, property, element or limitation. Furthermore, repeated use of the phrase “in the preferred embodiment” does not necessarily refer to the same embodiment, though it may.