FIELD OF THE INVENTION

The present invention pertains generally to systems and methods for wavefront shaping. More particularly, the present invention pertains to systems and methods for shaping wavefronts that can include light having several different wavelengths. The present invention is particularly, but not exclusively, useful for creating a polychromatic wavefront having a pre¬ selected wavefront shape.

BACKGROUND OF THE INVENTION

The term "wavefront" can be defined as an imaginary surface joining points of constant phase in a wave propagating through a medium. For light waves, a wavefront can be thought of as a three-dimensional imaginary surface of constant optical path length, orthogonal to a family of rays that emanate from a source of radiation. In terms of shape, a wavefront can be spherical, planar or arbitrarily shaped. Indeed, for a monochromatic wave propagating from a point source through a medium of constant refractive index, a spherically shaped wavefront will be emitted from the source. At large distances from the source, however, the wavefront can be considered to be approximately planar. On the other hand, imperfect optical systems, natural phenomena (e.g. atmospheric turbulence) and many other factors can lead to non-uniform, irregularly shaped wavefronts. For example, a component of an optical system, such as an imperfectly ground lens, may create an aberration which distorts an otherwise uniform (e.g. planar) wavefront. Heretofore, insofar as monochromatic light is concerned, several types of apparatus for measuring wavefront shape have been developed. For example, methods for measuring phase deviations have been disclosed in conjunction with devices like the so-called "Hartmann-Shack sensor" and in publications such as U.S. Patent No. 5,062,702 which issued to BiIIe for an invention entitled "Device for Mapping Corneal Topography." An interferometer is another, common type of apparatus that can be used to measure the shape of a wavefront. In addition to wavefront measuring, devices and methods for wavefront reshaping have been reported. For example, U.S. Patent No. 6,220,707 (hereinafter the '707 patent) which issued to BiIIe for an invention entitled "Method for Programming an Active Mirror to Mimic a Wavefront" discloses the use of a faceted mirror to reshape a wavefront. U.S. Patent No. 6,220,707 is hereby incorporated by reference herein. Specific applications disclosed in the '707 patent include the reshaping of a distorted wavefront into a substantially planar wavefront, and vice versa. Moreover, this reshaping can be accomplished for distorted wavefronts in which the depth of the three dimensional wavefront, measured in the direction of light propagation, exceeds one wavelength. In greater detail, the '707 patent discloses a phase-wrapping technique in which the outputs from a Hartmann-Shack wavefront analyzer are processed to determine a total deviation in phase shift for each of a plurality of contiguous sub-beams in a wavefront. These phase shifts can be measured relative to the phase of corresponding sub-beams in a reference wavefront, such as a plane wavefront. For light having a wavelength, λ, each measured "total deviation" includes a modular "nλ" (also called modular "n2π") phase shift component and a modulo "λ" (also called modulo 2π) phase shift component. After measuring the total phase shift deviation, the particular modular phase shift for each sub-beam is compensated for by subtracting nλ, (n+1)λ, or (n-1)λ, etc. as appropriate, from the total phase shift of each sub- beam. Each element of the faceted mirror is then adjusted to minimize the modulo λ phase shift deviation of each respective sub-beam to effectively transform a light beam between a distorted wavefront and a plane wavefront. Although the above-described achievements have been successful in measuring and modifying the wavefronts of monochromatic light, many applications require the use of polychromatic light. For these applications, it may be desirable to control and modify wavefronts in a polychromatic light stream. One such application, by way of example, is the correction of aberrations created by an optical system during the imaging of a multi-colored object. Another exemplary application includes the creation of a polychromatic light stream having controlled wavefront shapes for use in testing the influence of optical aberrations on human vision. Accordingly, in light of the above, it is an object of the present invention to provide a system and method for reshaping the wavefronts of a light stream that contains light of several different wavelengths. Another object of the present invention is to provide systems and methods for reshaping wavefronts in polychromatic light having a three dimensional wavefront depth, measured in the direction of light propagation, that exceeds one wavelength. Still another object of the present invention is to provide systems and methods for shaping polychromatic wavefronts using phase shifting elements which are simple to use, relatively easy to manufacture and comparatively cost effective.

SUMMARY OF THE INVENTION

The present invention is directed to systems and methods for actively reshaping wavefronts of an input source of light having at least two different wavelengths (λ-i, λ2). For purposes of this disclosure, the input light can be described in terms of one or more light beams, with each beam being made up of a plurality of contiguous sub-beams. These sub-beams establish a wavefront for the λi light (i.e. a λi wavefront) and wavefront for the λ2 light (i.e. a Xz wavefront). The input light is received by an optical phase shifting device that can include one or more arrays, with each array having a plurality of elements. Functionally, within a particular array, each element is independently adjustable to selectively alter the optical pathlength of a corresponding sub- beam. Thus, the array can be programmed to selectively reshape a wavefront. More specifically, once programmed into a selected configuration, the array of elements operate to receive an incoming beam having a first, initial wavefront, and process the beam to create an outgoing beam having a second, modified wavefront. For the present invention, the array of elements can be, but is not necessarily limited to, a faceted active mirror, a liquid crystal array or a foil mirror having an array of actuators that are independently operable to selectively deform the foil mirror surface. In a typical embodiment, an active mirror having approximately forty-thousand individual facets is used, with each facet being independently moveable along a respective substantially parallel path. For source light having two wavelengths (X1, X2), a first array configuration is used to reshape the initial λi wavelength waveform and a second array configuration is used to reshape the initial X2 wavelength waveform. As described in more detail below, for the present invention, the first and second array configurations can be accomplished using either a single array of elements or two different arrays. In either case, once the wavefronts have been reshaped, both the λi wavelength light and the X2 wavelength light are directed onto a common beam path. Once on the common beam path, the light can be viewed, imaged or further processed. In one particular embodiment of the present invention, the input light includes alternating pulses of the λi wavelength light and the X2 wavelength light. For this embodiment, a single, common array of elements can be used to reshape the multi-wavelength light. Specifically, movements of the individual array elements can be synchronized with the alternating input light source to sequentially and selectively reshape the pulsed λ-i and X2 wavefronts. In another embodiment of the present invention, the input light includes the λi wavelength light and the X2 wavelength light simultaneously. For this embodiment, the input light is split (spatially) to direct the λi wavelength light onto a first beam path and direct the X2 wavelength light onto a second beam path. Once separated, a first array of elements is used to reshape the λi wavelength wavefront and a second array of elements is used to reshape the X2 wavelength wavefront. After wavefront reshaping, the exit beams from the arrays are recombined onto a common beam path. For some applications of the system, a wavefront sensor, such as a Hartmann-Shack sensor, can be provided to measure the λi wavefront, the X2 wavefront, or both. This measurement can be performed on light propagating toward an array, light propagating away from an array, or both. The output from the sensor is then used to program the array to effectuate a selected wavefront reshaping. In one implementation of the system, the sensor is used to measure a total deviation in phase shift for each of the sub-beams in the wavefront. These phase shifts are measured relative to the phase of corresponding individual sub-beams in a reference wavefront such as a plane wavefront. Each measured "total deviation" includes a modular "nλ" phase shift component and a modulo "λ" phase shift component, for light having a wavelength, λ. Once the total phase shift has been determined for each sub-beam in the measured wavefront, the array of elements is divided into regions. Specifically, one region is identified with an integer "n" wherein all of the sub- beams incident on elements in the "n" region have a same modular phase shift. Next, boundary facets are detected such that all of the boundary facets have an (n+1)λ modular phase shift, with a zero modulo λ phase shift deviation. An "n+1" region is then identified that is adjacent the boundary facets, but outside of the "n" region. Similarly, other boundary facets may be detected which have an (n-1)λ modular phase shift, with a zero modulo λ phase shift deviation. If so, an "n-1" region is identified. In a like manner, "n+2" and "n+3" regions etc., as well as "n-2" and "n-3 regions etc., can be identified. The particular modular phase shift for each region is compensated for by subtracting nλ, (n+1)λ, or (n-1)λ, etc. as appropriate, from the total phase shift of each sub-beam within the region. In this manner, the modulo λ phase shift deviations for each sub-beam in the wavefront are determined. Thus, if the application dictates, each element can be adjusted to minimize the modulo λ phase shift deviation of each respective sub-beam. Collectively, when this compensation is made for all elements, the active array is able to effectively transform a light beam between a distorted wavefront and a plane wavefront.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: Fig. 1 is a schematic view of the primary components of a system for reshaping wavefronts in polychromatic light; Fig. 2 is a detailed, schematic view of the system shown in Fig. 1 ; Fig. 3 is a front view of an array as seen in the direction of arrow 3-3 in Fig. 2; Fig. 4 is a schematic view showing the reshaping of a wavefront having a wavefront depth, measured in the direction of light propagation, that exceeds one wavelength; Fig. 5 is a schematic view of the primary components of another embodiment of a system for reshaping wavefronts in polychromatic light; Fig. 6 is a detailed, schematic view of the system shown in Fig. 5; Fig. 7 is a front view of a light source having a filter wheel for use in the embodiment shown in Figs. 5 and 6; and Fig. 8 is a cross sectional view of a filter wheel as seen along line 8-8 in Fig. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig. 1 , a wavefront reshaping system is shown and generally designated 20. As shown in overview in Fig. 1, the system 20 includes a source 22 for generating a light beam 24 having three different wavelengths (λ-i, X2, λ3). Although three wavelengths are shown and described, it is to be appreciated that light having more than three and as few as two wavelengths can be reshaped by the system 20. Moreover, the system 20 is not limited to light within the visible spectrum. Fig. 1 further illustrates that the beam 24 can be conveniently described as being made up of a plurality of contiguous sub-beams, of which exemplary sub-beams 26a-d have been shown and labeled. These sub-beams simultaneously establish initial λi, X2, λ3wavefronts 28 in the beam 24. From the source 22, the light beam 24 is made incident upon an optical phase shifting device 30. For the system 20, the optical phase shifting device 30 can be programmed to independently reshape the initial λi, X2, λ3 wavefronts 28 to produce modified λi, X2, λ3 wavefronts 32. Once modified, the λi, X2, Xz wavefronts 32 exit the optical phase shifting device 30 along a common beam path 34. Fig. 1 further shows that a detector 36 can be positioned on the beam path 34 to allow the modified wavefronts 32 exiting from the device 30 to be viewed, imaged or further processed. Fig. 2 shows the system 20 in greater detail, including the individual components of the optical phase shifting device 30. As shown, a source 22 generates a continuous light beam 24 that simultaneously includes three different wavelengths (λi, X2, λ3). Although different rays are used to illustrate the three different wavelengths leaving the source 22 in Fig. 2, it is to be appreciated that the entire continuous beam 24, as it leaves the source 22, includes the three different wavelengths (λ-ι, X2, λ3). By way of example, the source 22 can be a multicolor object that is illuminated by natural light to include white light, artificial light, or light that has been specially produced or filtered to include only the three desired wavelengths (λi, X2, λ3). In one implementation, the three desired wavelengths (λi, X2, λ3) correspond to a set of three primary colors. As used herein, the term 'primary colors' refers to any set of three or more colors which when added in appropriate combination will yield white. For example, λi can be blue light in the wavelength range 435 - 480nm, X2 can be red light in the wavelength range 605 - 750nm, and λ3 can be green light in the wavelength range 500 - 560nm. In some applications of the system 20, light in the "traditional" RGB space (i.e. Red at 700nm, Green at 546.1 nm and Blue at 435.8nm) is used. For other applications, it may be preferable to use eye sensitive wavelengths. Specifically, receptors (cones) of the human eye which are responsible for color vision are most sensitive in the following wavelength regions: 550-580nm (yellow-green), 520-540nm (green), and 415-450nm (blue). Fig. 2 shows that from the source 22, the beam 24 is first incident upon splitter 38a, which directs λi wavelength light onto beam path 40a and towards array 42a. As shown, the remainder of the light from splitter 38a, including light having X2 and λ3 wavelengths, is directed along path 44 towards splitter 38b. At splitter 38b, light having a wavelength X2 is directed onto beam path 40b and towards array 42b. The remainder of the light from splitter 38b, including light having wavelength, λ3, is directed along path 46 towards splitter 38c. At splitter 38c, light having a wavelength λ3 is directed onto beam path 40c and towards array 42c. It can be further seen in Fig. 2 that a portion of the X^ wavelength light traveling toward the array 42a on path 40a is directed to a wavefront sensor 48a. In a somewhat similar manner, a portion of the X2 wavelength light traveling toward the array 42b on path 40b is directed to a wavefront sensor 48b and a portion of the λ3 wavelength light traveling toward the array 42c on path 40c is directed to a wavefront sensor 48c. Each wavefront sensor 48a-c can be, for example, a Hartmann-Shack sensor or some other suitable device known in the pertinent art for measuring a wavefront. Fig. 2 further shows that the system 20 includes a processor 50 that is connected in electronic communication with a control unit 52 via wire 54. For the system 20, each wavefront sensor 48a-c is connected to the processor 50 via wire 56. It can further be seen that the control unit 52 is connected to each array 42a-c via wires 58a-c, respectively. With this cooperation of structure, the output from a sensor 48a-c can be used to program a respective array 42a-c to individually effectuate a selected wavefront reshaping for light on each of the respective beam paths 40a-c. In greater detail, and as best appreciated with cross-reference to Figs. 2 and 3, each array 42a-c includes a plurality of elements 60, of which exemplary elements 60a-c have been labeled. Functionally, within a particular array 42a-c, each element 60 is independently adjustable to selectively alter the optical pathlength of a corresponding sub-beam 26 (see Fig. 1). Thus, each array 42a-c can be programmed to selectively reshape a wavefront. For the system 20, each array 42 can be, but is not necessarily limited to, a faceted active mirror, a foil mirror having an array of actuator elements that are independently operable to selectively deform the foil mirror surface, or a liquid crystal array. Thus, each array 42, depending on its particular configuration, may operate via reflection or transmission to reshape a wavefront. In a typical embodiment, an active mirror having approximately forty- thousand individual facet elements 60 is used, with each facet element 60 being independently moveable along a respective, substantially parallel path. A more detailed description of an active, faceted mirror can be found in U.S. Patent No. 6,220,707 which was previously incorporated by reference herein. Functionally, as shown in Fig. 2, once programmed into a selected configuration, each array 42a-c operates to receive a respective incoming beam having a first, initial wavefront 62a-c and process the beam to create a respective outgoing beam having a second, modified wavefront 64a-c. Fig. 2 shows that after reshaping, the modified wavefronts 64a-c are directed onto a common beam path 34 by respective mirrors 66a-c for receipt by a detector 36, which in this case is a human eye. Applications of the particular embodiment shown in Fig. 2 can include, but are not limited to, enhancement of the optical quality and characteristics of binoculars, microscopes, endoscopes, and other imaging equipment. Alternatively, the system 20 shown in Fig. 2 can be used to produce polychromatic light having pre-selected wavefront characteristics. In this case, a source 22 which consists of one or more high-quality, light emitters (which can be monochromatic or polychromatic) are typically used (rather than an illuminated object). With regard to the embodiment shown in Fig. 2, it is to be appreciated that the wavefront sensors 48a-c can be selectively positioned to measure the initial wavefronts 62a-c, the modified wavefront 64a-c, or both. In all of these cases, the sensor outputs can be used to program the arrays 42a-c to obtain modified wavefronts 64a-c having pre¬ selected shapes. In some applications of the system 20, the shape of the source wavefronts may be known, or may be predicted or calculated. In these applications, it may not be necessary to use a wavefront measuring device (e.g. a Hartmann-Shack sensor) to modify and produce a pre-selected wavefront shape. For the system 20, the arrays 42a-c can be used to reshape initial monochromatic wavefronts 62a-c in which the depth of the three dimensional wavefront, measured in the direction of light propagation, exceeds one wavelength. This technique for use with monochromatic light was fully described and claimed in co-owned U.S. Patent No. 6,220,707, which was previously incorporated by reference herein. In particular, the 707 patent shows and describes a computer operation for processing the outputs of a Hartmann-Shack wavefront analyzer to determine a total deviation in phase shift for each of a plurality of contiguous sub-beams in a wavefront. In one instance, these total phase shifts can be measured relative to the phase of corresponding sub-beams in a reference wavefront, such as a plane wavefront. Fig. 4 illustrates a phase-wrapping technique for use in conjunction with an active mirror 68 having a plurality of facets 70, of which exemplary facets 70a-c have been labeled. Specifically, Fig. 4 shows a converging monochromatic wavefront 72 having wavelength, λ, that is incident upon the facets 70 of the active mirror 68. As shown, each facet 70 is independently moveable through a distance λ/2 along a respective substantially parallel path. It can be further seen that the converging wavefront 72 has a wavefront depth, measured in the direction of light propagation, that exceeds one wavelength, λ. It can also be seen that after interaction with the facets 70 of the mirror 68, a substantially planar wavefront 74 is produced and propagates away from the active mirror 68. To accomplish the reshaping shown in Fig. 4, a computer operates on the outputs from a Hartmann-Shack wavefront analyzer to determine a total deviation in phase shift for each of a plurality of contiguous sub-beams in a wavefront. For this purpose, the desired wavefront shape, after reshaping, can be used as a reference wavefront to measure the "total deviation." For light having a wavelength, λ, each measured "total deviation" includes a modular "nλ" phase shift component and a modulo "λ" phase shift component. After measuring the total phase shift deviation, the particular modular phase shift for each sub-beam is compensated for by subtracting nλ, (n+1 )λ, or (n- 1)λ, etc. as appropriate, from the total phase shift of each sub-beam. Each element of the active array can then be adjusted to minimize the modulo λ phase shift deviation of each respective sub-beam to effectively transform a light beam, such as the converging wavefront 72 to a plane wavefront, such as the reshaped wavefront 74. Referring now to Fig. 5, another embodiment of a wavefront reshaping system is shown and generally designated 20'. For the system 20', the source 22' is configured to sequentially emit pulses of light, which alternate in wavelength from pulse to pulse (e.g. λ-i, λ2, λ3, λi, λ2, λ3 ... λi, λ2, λ3 ...) as shown. Each pulse, for convenience, can be described as being made up of a plurality of contiguous sub-beams, of which exemplary sub-beams 26a' and 26b' have been shown and labeled. These sub-beams establish a repeating train of initial λi, λ2, λ3 wavefronts that are made incident upon an optical phase shifting device 30'. For the system 20', the optical phase shifting device 30' is synchronized with the source 22' and can be programmed to sequentially and independently reshape the initial λ-i, λ2, λ3 wavefronts. As further shown in Fig. 5, the pulses exit the optical phase shifting device 30' along a common beam path 34' and have modified X1, λ2, λ3 wavefronts. Fig. 5 also shows that a detector 36' can be positioned on the beam path 34' to allow the modified wavefronts exiting from the device 30' to be viewed, imaged or further processed. Fig. 6 shows the system 20' in greater detail, including the individual components of the source 22' and optical phase shifting device 30'. As shown, a source 22' includes an object 76 (which may or may not be multi¬ color) that is illuminated by three light emitters 78a-c (e.g. bulbs), with each emitter 78a-c generating a different wavelength (λ-i, λ2, λ3) of light. In one implementation, the three desired wavelengths (λi, X2, λ3) correspond to a set of three primary colors. For example, λi can be blue light in the wavelength range 435 - 480nm, X2 can be red light in the wavelength range 605 - 750nm, and λ3 can be green light in the wavelength range 500 - 560nm. In some applications of the system 20', light in the "traditional" RGB space (i.e. Red at 700nm, Green at 546.1 nm and Blue at 435.8nm) is used. For other applications, it may be preferable to use eye sensitive wavelengths. Specifically, receptors (cones) of the human eye which are responsible for color vision are most sensitive in the following wavelength regions: 550- 580nm (yellow-green), 520-540nm (green), and 415-450nm (blue). The intensities of the emitters 78a-c can be independently adjusted, if desired, to produce light having a pre-selected composite color. As shown, each emitter 78a-c is connected to the control unit 52' which is programmed to sequentially energize the three emitters 78a-c to produce the time varying light stream described above. Figs. 7 and 8 show an alternate arrangement of a source (designated source 22") for use in the system 20'. As shown, a polychromatic (e.g. white light) emitter 80 directs a beam of polychromatic light through a filter wheel 82 and onto beam path 34". The filter wheel 82 includes filters 84a-c that are azimuthally distributed as shown in Fig. 8. For the source 22", each filter 84a- c passes a respective wavelength of light, λi, X2, X3. A motor 86 is attached to the filter wheel 82 to rotate the filter wheel 82 in the direction of arrow 88. With this cooperation of structure, the concerted interaction of the emitter 80 and rotating filter wheel 82 produce a sequence of light pulses that alternate in wavelength (λi, X2, λ3 ... λ-i, X2, λ3 ...). Referring back to Fig. 6, it can be seen that a portion of each pulse propagating away from the source 22' (or alternatively source 22") is directed to a wavefront sensor 48', which can be, for example, a Hartmann-Shack sensor or some other suitable device known in the pertinent art for measuring a wavefront. Fig. 6 further shows that the system 20' includes a processor 50' that is connected in electronic communication with a control unit 52' and the wavefront sensor 48'. It can be further seen that the control unit 52' is connected to the array 42'. The processor 50' and control unit 52' are synchronized with the alternating emitters 78a-c (or filter wheel 82 when source 20" is used). With this cooperation of structure, the output from the sensor 48' can be used to program the array 42' to sequentially effectuate a selected wavefront reshaping, pulse by pulse, for a pattern of pulses generated by the source 22' or 22". After reshaping, the modified wavefronts are directed toward a detector 36', which in this case is a human eye. For some types of detectors 36', a minimum pulse repetition rate should be maintained. For example, for viewing by the human eye, each wavelength should be pulsed at greater than fifty hertz and preferably greater than sixty hertz. For three wavelengths, the state of the array 42' would then change with a wavelength of one-hundred eighty hertz, or greater. Applications of the particular embodiment shown in Fig. 2 can include, but are not limited to enhancement of the optical quality and characteristics of binoculars, microscopes, endoscopes, and other imaging equipment. Alternatively, the system 20 shown in Fig. 2 can be used to produce polychromatic light having pre-selected wavefront characteristics. With regard to the embodiment shown in Fig. 6, it is to be appreciated that the wavefront sensor 48' can be selectively positioned to measure a wavefront before the array 42', after the array 42', or both. In all of these cases, the sensor outputs can be used to program the arrays 42' to obtain modified wavefronts having pre-selected shapes. In some applications of the system 20', the shape of the source wavefronts may be known, or may be predicted or calculated. In these applications, it may not be necessary to use a wavefront measuring device (e.g. a Hartmann-Shack sensor) to modify and produce a pre-selected wavefront shape. Like the system 20 described above, the array 42' for the system 20' can be used to reshape initial wavefronts in which the depth of the three dimensional wavefront, measured in the direction of light propagation, exceeds one wavelength. While the particular systems and methods for shaping wavefronts in polychromatic light as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.