Optical wavefront sensor employing polarization-modifying elements

Abstract

An optical wavefront sensor has an array of microlenses and an associated focal plane array sensor. The optical wavefront sensor further includes a plurality of polarization- modifying elements, each associated with a microlens, that modify the polarization of light passing through the microlens. The polarization-modifying elements are arranged in first and second groups, and are deployed such that closest-neighbor microlenses of the array of microlenses are associated with polarization-modifying elements from different groups. The focal plane array sensor has multiple pixels, each having first and second subpixels configured to sense a different polarization state. The optical wavefront sensor further includes a processing system that processes outputs from the focal plane array sensor to analyze the optical wavefront of light entering the array of microlenses, employing relative intensities of the subpixels to associate focal plane spots with corresponding microlenses.

Description

[0001] Optical Wavefront Sensor Employing Polarization-Modifying Elements

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present invention relates to optical sensors and, in particular, it concerns optical wavefront sensors employing polarization-modifying elements.

[0004] Characterizing and analyzing the wavefront of an optical beam is crucial for a wide range of applications and research fields, including optical communications, lasers, astronomy, and many others. Since the 1960s, efforts have been made to characterize and correct wavefronts to enhance image quality in astronomical observations (adaptive optics). These efforts have expanded into various other domains, driven by the unique requirements and challenges of each field.

[0005] The Shack-Hartmann (SH) wavefront sensor is the most widely used wavefront detector. It operates based on a microlens array (MLA) positioned at a focal length from a camera plane (focal plane array sensor). The lenses focus the light onto the camera, and the deviation of the focused light from each lens is measured relative to the lens’ optical axis. From these deviations, the angular distribution of the light can be obtained, and consequently, the phase distribution or wavefront of the light beam can be determined. The optical arrangement for such an SH wavefront sensor is illustrated schematically in FIG. 1A, which depicts a simplified schematic of the Shack-Hartmann (SH) wavefront sensor 10 in two dimensions, having an MLA 12 of microlenses and a focal plane array sensor 14. An expanded representation of two lenses 12a and 12b from the microlens array 12 is used in FIGS. IB and 1C to illustrate operation of the sensor. In FIG. IB, the passage of light through two of the lenses onto the focal plane array sensor with a flat incident wavefront while in FIG. 1C the case of a distorted wavefront. Each lens 12a and 12b has a corresponding effective area 14a and 14b on the sensor within which the corresponding spot is typically expected to fall. Within each effective area, the center of mass of the light is calculated independently. These center of mass positions are then used to reconstruct the overall wavefront shape.

[0006] The spatial resolution of the SH sensor is determined by the ratio of the beam size to the microlens size, typically ranging from tens to hundreds of microns. The more lenses that spread the light beam, the more spatial information is obtained.

[0007] The angular resolution, i.e., the minimum measurable angle ($?), is simply determined by the ratio of the pixel size (d) of the sensor (camera) to the focal length (f) of the microlenses, assuming that the light passing through a single lens has a uniform angle (uniform phase gradient). With slightly more sophisticated algorithms, it is possible to achieve better resolution, for example, by calculating the focused light position using center of mass calculations based on the intensity ratio between the pixels on which the light is focused. In this case, the angular resolution depends on the dynamic range of the camera, the signal-to-noise ratio, and the number of pixels involved in the center of mass calculation.

[0008] SUMMARY OF THE INVENTION

[0009] The present invention is an optical wavefront sensor employing polarization-modifying elements.

[0010] According to the teachings of an embodiment of the present invention there is provided, an optical wavefront sensor comprising: (a) an array of microlenses; (b) a plurality of polarization-modifying elements, each polarization-modifying element associated with one of the microlenses so as to modify a polarization property of light passing through the microlens; (c) a focal plane array sensor deployed in a focal plane of the array of microlenses, the focal plane array sensor having a plurality of effective pixels, each effective pixel including at least a first subpixel and a second subpixel configured for selectively sensing light of a first polarization state and a second polarization state, respectively; and (d) a processing system associated with the focal plane array sensor, wherein the plurality of polarization-modifying elements include a first group of elements deployed to modify polarization of light in a first manner and a second group of elements deployed to modify polarization of light in a second manner, the first and second groups of elements being deployed such that closest-neighbor microlenses of the plurality of microlenses are associated with polarization-modifying elements from different groups, and wherein the processing system is configured to process outputs from the focal plane array sensor to analyze the optical wavefront of light entering the array of microlenses, the processing system employing relative intensities of the subpixels to associate focal plane spots with corresponding microlenses.

[0011] According to a further feature of an embodiment of the present invention, the first and second groups of polarization-modifying elements are arranged in a checkerboard pattern.

[0012] According to a further feature of an embodiment of the present invention, the first and second groups of polarization-modifying elements are linear polarizers with a polarization axis of the second group orthogonal to a polarization axis of the first group. According to a further feature of an embodiment of the present invention, the first and second groups of polarization-modifying elements are half-wave retarder plates with a fast axis of the second group at 45 degrees to a fast axis of the first group.

[0013] According to a further feature of an embodiment of the present invention, there is also provided a full-aperture halfwave retarder element mounted in front of the array of microlenses so as to be rotatable about a central optical axis.

[0014] According to a further feature of an embodiment of the present invention, the first and second groups of polarization-modifying elements are quarter-wave retarder plates with a fast axis of the second group at 90 degrees to a fast axis of the first group.

[0015] According to a further feature of an embodiment of the present invention, there is also provided a full-aperture quarter-wave retarder element mounted in front of the array of microlenses so as to be rotatable about a central optical axis.

[0016] According to a further feature of an embodiment of the present invention, the first and second subpixels are configured for selectively sensing linear-polarized light angularly spaced at 0 and 90 degrees, respectively.

[0017] According to a further feature of an embodiment of the present invention, each effective pixel includes four subpixels configured for selectively sensing linear-polarized light angularly spaced at 0, 45, 90 and 135 degrees, respectively.

[0018] There is also provided according to an embodiment of the present invention, an optical wavefront sensor for polarized input light, the optical wavefront sensor comprising: (a) an array of microlenses; (b) a plurality of optical elements each associated with one of the microlenses, at least a first group of the optical elements being configured to rotate a plane of polarization of polarized light passing through the microlens; (c) a focal plane array sensor deployed in a focal plane of the array of microlenses, the focal plane array sensor having a plurality of effective pixels, each effective pixel including at least a first subpixel and a second subpixel configured for selectively sensing light of a first polarization state and a second polarization state, respectively; and (d) a processing system associated with the focal plane array sensor, wherein the first group of the optical elements is deployed to rotate the plane of polarization of the polarized input light in a first manner and a second group of the optical elements is deployed to rotate the plane of polarization of the polarized light in a second manner or to leave the plane of polarization unchanged, the first and second groups of optical elements being deployed such that closest-neighbor microlenses of the plurality of microlenses are associated with optical elements from different groups, and wherein the processing system is configured to process outputs from the focal plane array sensor to analyze the optical wavefront of light entering the array of microlenses, the processing system employing relative intensities of the subpixels to associate focal plane spots with corresponding microlenses.

[0019] According to a further feature of an embodiment of the present invention, at least the first group of optical elements comprises half-wave retarder plates.

[0020] According to a further feature of an embodiment of the present invention, there is also provided a full-aperture halfwave retarder element mounted in front of the array of microlenses so as to be rotatable about a central optical axis.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0023] FIG. 1A, described above, is a schematic representation of a conventional Shack- Hartmann optical wavefront sensor;

[0024] FIGS. IB and 1C are enlarged schematic views of the region of FIG. 1A designated I, illustrating the operation of the sensor for a flat incident wavefront and a distorted wavefront, respectively;

[0025] FIG. 2 illustrates schematically four scenarios, labeled (a)-(d), which may give rise to ambiguity as to the micro lens from which each detected spot originates;

[0026] FIG. 3 is a block diagram of an optical wavefront sensor, constructed and operative according to the teachings of an embodiment of the present invention;

[0027] FIG. 4A is a schematic representation of the polarizer components associated with four subpixels of an effective pixel of a polarization- sensitive focal plane array sensor for use in the optical wavefront sensor of FIG. 3;

[0028] FIG. 4B is a schematic side view of the structure of each subpixel from the effective pixel of FIG. 4A;

[0029] FIG. 5A is a schematic front view of a polarization-modifying element implemented as a checkerboard pattern of orthogonal polarizers for use in the optical wavefront sensor of FIG. 3;

[0030] FIG. 5B is a schematic representation of the optical elements of an implementation of the optical wavefront sensor of FIG. 3;

[0031] FIG. 5C is an enlarged schematic view of the region of FIG. 5B designated V, and showing schematically the level of illumination sensed by each subpixel for effective pixels within the respective illumination spots from adjacent microlenses; FIG. 5D illustrates schematically an alternative arrangement of polarizer components for subpixels of two effective pixels, where each effective pixel senses only two orthogonal polarization components;

[0032] FIG. 6 illustrates schematically four scenarios, labeled (a)-(d), equivalent to those of FIG. 2, illustrating how the ambiguity described there is eliminated according to the teachings of the present invention; and

[0033] FIGS. 7A and 7B are schematic partial illustrations similar to FIG. 5C illustrating alternative implementations of the present invention based on retarder plates as the polarizationmodifying elements associated with the microlenses.

[0034] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The present invention is an optical wavefront sensor employing polarization-modifying elements.

[0036] The principles and operation of an optical wavefront sensor according to the present invention may be better understood with reference to the drawings and the accompanying description.

[0037] By way of introduction, it follows from the discussion of the SH sensor in the Background above that enhanced resolution can be achieved by using a longer focal length. At large angles, possible spill-over of the spots from one microlens into the effective area of an adjacent microlens lead to possible ambiguity in the wavefront sensing. Various scenarios of such spill-over are illustrated in FIG. 2 as scenarios (a)-(d). These include:

[0038] 1. Effective Area Overlap: In scenario (a), light from lens 12a spills over into the effective area 14b, causing confusion as the light in area 14b could originate from either lens 12a or lens 12b.

[0039] 2. Pixel Overlap: Scenario (b) depicts light spillover from lens 12a causing a single pixel to receive light from both lens 12a and lens 12b, making it difficult to determine the true source of the light.

[0040] 3. Inter-Lens Light Spillover: Scenario (c) illustrates light spillover from lens 12a into the effective area 14b and from lens 12b into the effective area 14a, further complicating the interpretation of the captured light patterns.

[0041] 4. Multiple Angles in Single Lens: Scenario (d) shows a wavefront passing through a single lens 12b with two different angles, resulting in two "light spots" within a single effective area 14b. This creates an ambiguity making it challenging to determine whether these spots originate from the same lens or represent light spillover from neighboring lenses.

[0042] To address these and other issues, as shown in FIGS. 3-7, an optical wavefront sensor 100 according to an embodiment of the present invention includes a microlens array (MLA) 12 providing a plurality of microlenses (individually 12a, 12b) and a corresponding array 16 of a plurality of polarization-modifying elements (individually 16a, 16b), each polarizationmodifying element associated with one of the microlenses so as to modify a polarization property of light passing through the microlens. A focal plane array sensor 14, deployed in a focal plane of MLA 12 of microlenses, has a plurality of effective pixels 18, each effective pixel including at least a first subpixel and a second subpixel configured for selectively sensing light of a first polarization state and a second polarization state, respectively. In the example illustrated in FIG. 4A, each effective pixel 18 is formed from a group of four pixel detectors having polarizing filters 20a, 20b, 20c, 20d at rotated polarization states of 0°, 45°, 90° and 135°, respectively. A typical structure for each subpixel 22 is illustrated in FIG. 4B and includes a microlens 24, the appropriate polarizer element 20a for each subpixel, and a photodiode 26 sensitive to the wavelength(s) of light to be sensed (e.g., visible, near IR or thermal radiation), preferably all integrated onto a suitable sensor chip (not shown), such as a CMOS chip. A processing system 28 (FIG. 3) is associated with the focal plane array sensor 14 to receive and process the output data.

[0043] Polarization-modifying elements 16 include a first group of elements 16a deployed to modify polarization of light in a first manner and a second group of elements 16b deployed to modify polarization of light in a second manner. These first and second groups of elements are deployed such that closest-neighbor microlenses of the plurality of microlenses are associated with polarization-modifying elements from different groups, for example, as a checkerboard pattern (FIG. 5A). Processing system 28 is configured to process outputs from the focal plane array sensor 14 to analyze the optical wavefront of light entering the array of microlenses 12, employing the relative intensities of the subpixels to associate focal plane spots with corresponding microlenses.

[0044] In other words, as will be further appreciated from the detailed examples below, the presence of polarization-modifying elements 16 provides characteristic polarization encoding of incoming light passing through adjacent microlenses, and thereby generates a distinctive polarization signature when sensed by FPA sensor 14. This provides additional information which is used by processing system 28 to decipher cases of potential confusion, such as those illustrated in FIG. 2 above, to determine which measured spot at the sensor derives from which microlens 12. This allows an increase in the angular range of the sensor without compromising angular sensitivity. Alternatively, for a given sensor resolution, a longer focal length can be used to achieve higher angular resolution than would otherwise be possible while maintaining reliable differentiation between the spots derived from adjacent microlenses.

[0045] Polarizer Implementations

[0046] In a first set of implementations, the first and second groups of polarization-modifying elements 16a, 16b are linear polarizers with a polarization axis of the second group orthogonal to a polarization axis of the first group, as represented schematically in FIG. 5A. In the enlarged depiction of two adjacent microlenses in FIG. 5C, the light passing through polarizationmodifying element 16a is marked with arrows showing an electric vector in the plane of the drawing (corresponding to the 90° sensor orientation of FIG. 4A) while the light passing through polarization-modifying element 16b is marked with circled dots indicating an electric vector into the page (corresponding to the 0° sensor orientation of FIG. 4A). As a result, effective pixels illuminated by the spot from microlens 14a generate a full-brightness response from subpixel 20c a 50% response from pixels 20b and 20d and zero response from subpixel 20a. In contrast, effective pixels illuminated by the spot from micro lens 14b generate a full-brightness response from subpixel 20a a 50% response from pixels 20b and 20d and zero response from subpixel 20c. As a result, the various scenarios described as resulting in ambiguity as illustrated above in FIG. 2 can now readily be distinguished by the ratios between the subpixel intensities within the illumination spot, as illustrated in FIG. 6. An area of overlap of the two spots can be identified unambiguously by a reduction in contrast between the orthogonal polarizations within the spot.

[0047] A sheet of polarization-modifying elements 16 is shown here schematically as a separate optical component deployed behind MLA 12 but may alternatively be positioned in front of MLA 12 and, in either position, may be combined into a single optical component with MLA 12. In certain particularly preferred but non-limiting implementations, MLA 12 is formed with a planar rear surface and the sheet of polarization-modifying elements 16 is formed as a film which is applied to the planar rear surface of MLA 12. Various technologies are known for producing a sheet of elements 16 including, but not limited to, successive selective photopolymerization of regions of a sheet containing liquid crystal molecules while exposed to different directions of electric fields. Micro-manufacture of conductive grid polarizer regions with differing orientations may also be used. One example of a product which could be used for polarizationmodifying element 16 is commercially available under the tradename COLORPOL® S Patterned Polarizer from Laser Components GmbH (DE). Depending on the technology and production methods used, in some cases, the polarization-modifying elements 16 may be integrally formed with the microlenses during manufacture of MLA 12.

[0048] The polarization- sensitive focal plane array sensor described above with reference to FIGS. 4 A and 4B corresponds to a detector commercially available under the tradename POLARSENS™ from Sony Semiconductor Solutions Corporation, where each effective pixel is formed from a group of four subpixels with respective polarizing filters at 0, 45, 90 and 135 degrees. In the commercially available sensors, the effective pixels are 7 pm x 7 pm, with each subpixel 3.5 pm square. The provision of four rotated polarizers is helpful for analyzing the polarization state of incident light in the absence of a priori information about the expected polarization. Such a sensor is also suitable for implementation of the present invention. However, given the well-defined orthogonal polarizations which are generated by the sub-regions of polarization-modifying elements 16, it may be possible to employ a sensor which has only two perpendicular orientations of polarizers 20a and 20c, as illustrated schematically in FIG. 5D. This reduces the number of subpixels per effective pixel to two and may double the effective resolution of the sensor in one dimension.

[0049] FIG. 6 illustrates how the present invention provides a solution for each of the “leakage effect” situations presented in FIG. 2, as follows:

[0050] Case (a) shows overlap in the effective area, with light leakage from lens A to effective area B so that in effective area B, light is received from both lenses A and B. The light from lens A is linearly polarized according to the polarization axis at 90 degrees, so only subpixel 20c with a polarization axis at 90 degrees receive maximum power, subpixels 20b and 20d with polarization axes of 45, 135 show power lower by a factor of 2, and subpixel 20a with a polarization axis of 0 degrees will not register any light. The light spot from lens B can be identified by pixels in which subpixel 20c does not register any light and subpixel 20a registers maximum power.

[0051] Case (b) shows overlap due to light leakage from lens A so that one pixel receives light from two microlenses A and B. This is identified by sensing of similar light intensities at subpixels 20a and 20c (as well as at subpixels 20b and 20d).

[0052] Case (c) shows light leakage from lens A to effective area B, and from lens B to effective area A. The polarization encoding of the light as sensed by the subpixels of the sensor areas provides additional information to facilitation resolution of such ambiguities.

[0053] Case (d) shows a wavefront passing through one lens with two angles so that in one effective area, two "light spots" are received, so that the number of spots does not match the number of microlenses. Here too, the polarization encoding and sensing facilitates determination of from which lens each sensed spot derives.

[0054] The invention is exemplified herein with reference to polarization-modifying elements 16 which include two groups generating orthogonally-polarized output beams from two subsets of microlenses. Although this option is typically considered to be highly advantageous, the same principles could be used to generate a larger number of subgroups. For example, polarizationmodifying elements 16 could include four groups of polarizing elements at 45 degree steps so that each group generates maximum intensity in a different one of the subpixels of each effective pixel. This would reduce the polarization-state contrast between the different groups but may allow differentiation between diagonally neighboring microlenses. For most applications, however, the option of orthogonally encoded polarization from two sets of polarizationmodifying elements is preferred.

[0055] The use of linear polarizers for polarization-modifying elements 16 works well for nonpolarized input light and can in principle also be used for polarized input light, so long as the orientation of the device or the input light polarization direction is not perpendicular to either set of polarizers. To optimize such a scenario, a rotatable halfwave retarder plate can be fitted at the sensor input aperture (equivalent to element 30 described below with reference to FIG. 7B) in order to optimize the angle of the plane of polarization reaching the MLA 12, for example, so as to be at 45 degrees to the polarization vectors of the polarization-modifying elements. However, even in this case, the use of linear polarizers reduces the intensity of light entering the system by 50 percent.

[0056] For this reason, when working with polarized input light, and particularly where working with relatively low intensity signals where high sensitivity is desired, the use of waveplates (retarders) may be preferred, as will now be exemplified with reference to FIGS. 7A and 7B. Retarder Implementations

[0057] For polarized light, it is possible to work with retarder plates for the polarizationmodifying elements to avoid loss of intensity of light passing through the MLA. In order to accommodate different angles of polarization relative to the device, a rotatable retarder 30 (FIGS. 3 and 7B) is preferably provided as an input aperture conditioner.

[0058] One such implementation is illustrated in FIGS. 7A and 7B, where two groups of microlenses 12c, 12d are provided with respective first and second groups of half-wave retarder plates 16c and 16d with a fast axis of the second group at 45 degrees to a fast axis of the first group. As a result, when plane polarized light with an electric vector parallel or perpendicular to the fast axis of one group is incident on the device, the plane polarized light passes through that group of microlenses unchanged while the light passing through the second group of microlenses is rotated by 90 degrees.

[0059] If the input polarized light is not aligned orthogonally to the fast axis of one of the sets of retarder plates, the light from adjacent microlenses will be orthogonally polarized, but will not be correctly aligned with the axes of sensitivity of polarization- sensitive FPA sensor 14, resulting in a reduction in the contrast detected by the polarization-sensitive FPA sensor 14. In certain cases, as illustrated in FIG. 7A, the orientation of the input polarization may be predetermined, or the entire sensor assembly may be rotatably mounted in order to ensure optimal contrast. However, for enhanced usability, wavefront sensor 100 preferably employs a rotatable half- wave retarder 30 as illustrated in FIG. 7B, so as to be rotatable about a central optical axis 32 of the entire sensor, as an input aperture conditioner (shown schematically in the partial view of FIG. 7B but covering the input aperture in front of the entire MLA 12). The input aperture conditioner is preferably mounted in a rotatable collar attached to the main aperture of the wavefront sensor. Since a half-wave plate has the effect of rotating the plane of plane- polarized light through twice the angle between the fast axis of the retarder plate and the plane of polarization, rotation through a range of 90 degrees is sufficient to achieve any desired plane of polarization entering the MLA 12. The input aperture conditioner may be turned until the FPA sensor generates maximum contrast between two orthogonal polarization components.

[0060] Optionally, linear polarizers may be associated with the polarization modifying elements, also defining a checkerboard of orthogonal polarizers similar to that of FIG. 5A, above. This reduces the number of valid positions for input aperture conditioner 30, since the polarization will need to be specifically parallel (rather than orthogonal, or vice versa) to the fast axis of one specific group of retarder plates, while the other group of retarder plates will have their fast axis at 45 degrees to the first group followed by a polarizer at 90 degrees to the polarizer of the first group. This combination may have certain advantages, simplifying the alignment of element 30 (which goes between an extinction position at one angle to a maximum-intensity position at 90 degrees thereto), and enhancing the signal-to-noise ratio due to non-polarized background radiation by excluding 50% of the background while allowing substantially all of the polarized light intensity through.

[0061] In the above example, since one set of halfwave retarder elements has its fast axis aligned with the plane of polarization so that the input polarization is unchanged, this set of polarizationmodifying elements may optionally be replaced by clear glass (or other clear optical material suitable to the manufacturing techniques) that does not change the polarization and / or by a linear polarizer alone, aligned with the intended direction of polarization of the input beam. Although the above example relates to halfwave retarder elements as the polarizationmodifying elements, a similar implementation would be possible with a checkerboard of quarterwave retarder elements with their fast axes mutually-rotated by 90 degrees at the microlenses and a quarter-wave plate as a rotatable as an input aperture conditioner in front of the entrance aperture. The input aperture conditioner can be rotated until it generates pure circular polarization from the plane-polarized input, and then this circular-polarized light is converted to planar polarized light at two orthogonal orientations at the microlens array polarizationmodifying elements. Here too, corresponding polarizers may optionally be added behind the quarter-wave retarder elements at the MLA in order to enhance the signal-to-noise ratio against unpolarized background radiation.

[0062] Processing

[0063] The processing performed by processing system 28 is essentially similar to a standard Shack-Hartmann sensor with resolution corresponding to the effective pixel size (e.g., 4 subpixels), but with additional polarization data available from the FPA sensor to ensure correct correlation of detected spots with the corresponding microlenses and to resolve potential ambiguities. Various interpolation techniques are known for use in achieving subpixel resolution, and these techniques may be modified to derive additional data from the subpixel data in the case that a spot falls on only certain subpixels within an effective pixel. Details of such techniques are discussed in Yang Y, Huang L, Xiao Y and Gu N (2023), Polarized Shack-Hartmann wavefront sensor. Front. Phys. 11: 1091848. doi: 10.3389 / fphy.2023.1091848.

[0064] By way of one non-limiting practical example based on the currently available commercial polarization-sensitive sensors, the sensor has a full sensor chip resolution of 2448 x 2048 in which the size of one subpixel is 3.45pm x 3.45pm, so the size of an effective pixel with 4 polarizations is 7pm x 7pm. This may be suitable for use with off-the-shelf MLAs, for example, with microlens pitch in the range of 100-400 pm. The size of the sensor as well as the size, microlens pitch and focal length of the MLA array may be selected as needed for various different applications, and various conventional optical arrangement may be used to scale up or down the aperture size to match a given sensor to a particular application.

[0065] It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. An optical wavefront sensor comprising:(a) an array of microlenses;(b) a plurality of polarization-modifying elements, each polarization-modifying element associated with one of said microlenses so as to modify a polarization property of light passing through said microlens;(c) a focal plane array sensor deployed in a focal plane of said array of microlenses, said focal plane array sensor having a plurality of effective pixels, each effective pixel including at least a first subpixel and a second subpixel configured for selectively sensing light of a first polarization state and a second polarization state, respectively; and(d) a processing system associated with said focal plane array sensor, wherein said plurality of polarization-modifying elements include a first group of elements deployed to modify polarization of light in a first manner and a second group of elements deployed to modify polarization of light in a second manner, said first and second groups of elements being deployed such that closest-neighbor microlenses of said plurality of microlenses are associated with polarization-modifying elements from different groups, and wherein said processing system is configured to process outputs from said focal plane array sensor to analyze the optical wavefront of light entering said array of microlenses, said processing system employing relative intensities of said subpixels to associate focal plane spots with corresponding microlenses.

2. The optical wavefront sensor of claim 1, wherein said first and second groups of polarization-modifying elements are arranged in a checkerboard pattern.

3. The optical wavefront sensor of claim 1, wherein said first and second groups of polarization-modifying elements are linear polarizers with a polarization axis of said second group orthogonal to a polarization axis of said first group.

4. The optical wavefront sensor of claim 1, wherein said first and second groups of polarization-modifying elements are half-wave retarder plates with a fast axis of said second group at 45 degrees to a fast axis of said first group.

5. The optical wavefront sensor of claim 4, further comprising a full-aperture halfwave retarder element mounted in front of said array of microlenses so as to be rotatable about a central optical axis.

6. The optical wavefront sensor of claim 1, wherein said first and second groups of polarization-modifying elements are quarter-wave retarder plates with a fast axis of said second group at 90 degrees to a fast axis of said first group.

7. The optical wavefront sensor of claim 6, further comprising a full-aperture quarterwave retarder element mounted in front of said array of microlenses so as to be rotatable about a central optical axis.

8. The optical wavefront sensor of claim 1, wherein said first and second subpixels are configured for selectively sensing linear-polarized light angularly spaced at 0 and 90 degrees, respectively.

9. The optical wavefront sensor of claim 1, wherein each effective pixel includes four subpixels configured for selectively sensing linear-polarized light angularly spaced at 0, 45, 90 and 135 degrees, respectively.

10. An optical wavefront sensor for polarized input light, the optical wavefront sensor comprising:(a) an array of microlenses;(b) a plurality of optical elements each associated with one of said microlenses, at least a first group of said optical elements being configured to rotate a plane of polarization of polarized light passing through said microlens;(c) a focal plane array sensor deployed in a focal plane of said array of microlenses, said focal plane array sensor having a plurality of effective pixels, each effective pixel including at least a first subpixel and a second subpixel configured for selectively sensing light of a first polarization state and a second polarization state, respectively; and(d) a processing system associated with said focal plane array sensor, wherein said first group of said optical elements is deployed to rotate the plane of polarization of the polarized input light in a first manner and a second group of said optical elements is deployed to rotate the plane of polarization of the polarized light in a second manner or to leave the plane of polarization unchanged, said first and second groups of optical elements beingdeployed such that closest-neighbor microlenses of said plurality of microlenses are associated with optical elements from different groups, and wherein said processing system is configured to process outputs from said focal plane array sensor to analyze the optical wavefront of light entering said array of microlenses, said processing system employing relative intensities of said subpixels to associate focal plane spots with corresponding microlenses.

11. The optical wavefront sensor of claim 10, wherein at least said first group of optical elements comprises half-wave retarder plates.

12. The optical wavefront sensor of claim 11, further comprising a full-aperture halfwave retarder element mounted in front of said array of microlenses so as to be rotatable about a central optical axis.

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