Zonal analog holographic wavefront sensor

Abstract

Disclosed is a method of determining phase aberration in a given wavefront. The method includes recording a current array of holograms on a photosensitive recording medium, wherein the recording comprises recording a first pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium, rotating the photosensitive recording medium by 90°, recording a next pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, and form the current array of holograms, probing the current array of holograms with the given wavefront to generate a plurality of first order diffracted beams and a weak transmitted zero order beam. The method further includes capturing an image of the plurality of first order diffracted beams and the weak transmitted zero order beam by a detector, and zonally determining the phase aberration of the given wavefront based on comparison of intensity values of the plurality of first order diffracted beams.

Description

[0001] Title

[0002] Zonal analog holographic wavefront sensor

[0003] Field

[0004] The present invention relates to wavefront sensors, and more particularly to a zonal analog holographic wavefront sensor.

[0005] Background

[0006] Wavefront sensors (WFS) are devices that are used to determine the amount of phase aberrations present in a light wave. Phase aberrations can be defined as deviations from the ideal or reference path of the light wave. The aberrations are often mathematically represented by a set of modes (such as Zemike polynomials or Karhunen-Loeve functions) which describe the typical deviations, an optical wave can undergo. These modes range from simple angular deviations across the whole beam (tip and tilt), to familiar optical effects (such as de / focus and astigmatism) to more complex modes. An optical wave aberration can be composed of a certain magnitude of any number of modes. Wavefront sensors are a high value technology that have uses in advanced manufacturing, healthcare, metrology, diagnostic devices, scientific instrumentation, and high-resolution imaging systems. The wavefront sensors have applications in Adaptive Optics (AO), (including for astronomical imaging, free space optical communication, diffuse imaging and retinal imaging), and metrology (including for ophthalmology, optical and semiconductor device characterisation).

[0007] Nearly all wavefront sensors proposed and commercially available are zonal wavefront sensors. This means they compute the gradients of the incoming wavefronts phase at an array of discrete locations (local sub-apertures) and calculate / reconstruct the total wavefront phase from this information. This can be done in a number of ways, which typically fall into two categories: 1) Fitting the phase gradient data to a set of modes (often Zemike polynomials or specific response modes of the AO correction device) or 2) Through the use of other zonal and discontinuous algorithms (e.g. least squares, etc.). As WFS are used for a range of applications to solve a range of different problems, the requirements for the ideal WFS vary on the application. However typical features of which are of importance to end-users are: spatial sampling resolution, temporal sampling resolution (speed), wavefront measurement accuracy, wavefront measurement precision, dynamic range, wavelength dependence, vibrational insensitivity, cost, compact optical system, computational simplicity and efficiency.

[0008] FIGs. 1A and IB illustrate concept of a Shack-Hartmann wavefront sensor (SHWFS), wherein FIG.1A illustrates a set of reference spots obtained with a camera from a plane wave incident on SHWFS, and FIG. IB illustrates a distorted wavefront imaged through the SHWFS gives a set of displaced spots at each sub-aperture. The local tilts at each subaperture can be obtained geometrically. The SHWFS consists of an array of small lenses (lenslets) focused onto a CCD or CMOS sensor placed at the geometric focal plane of the lenslet array. When the lenslet array is illuminated with a plane reference wave, an array of replicated images fall in a grid pattern, each within their own sub-aperture. As the wavefront corresponds to a single field point, and because the parameters are typically selected such that the diffraction limit of the lenslets is satisfied, the circular lenslets create an array of Airy disc spot images and their locations are determined with a simple centroiding operation. When the lenslet array is illuminated with an aberrated nonscintillated beam, the centroids can be located and the displacement of each is proportional to the derivative of the wavefront across that specific lenslet area. Consequently, any phase aberration can be approximated by a set of discrete tilts in both the x and y dimensions. By sampling the wavefront with an array of lenslets, all local tilts can be measured and the whole wavefront reconstructed. However, the SHWFS dynamic range is conventionally restricted by the size of each subfield, that the spot can move within before it becomes confused with neighbouring spots.

[0009] FIG. 2 illustrates the schematic of the PWFS concept, with a beam from the exit pupil of a telescope (aberrated wavefront) modulated around the tip of a pyramidal optical element, placed at the focal plane of an aberrated optical system, and four replicated images of the exit pupil formed on the camera (one from each face of the pyramid). The analysis of the intensity patterns can establish the wavefront derivatives. These four intensity patterns, suitably combined, provide information on the local derivatives of the aberrated wavefront. In the absence of modulation, a given ray coming from point P on the exit pupil hits a face of the pyramid and only the pupil image corresponding to this face is illuminated at the point P conjugated to P'. When modulation is applied, instead, the ray spends a fraction of the time on every face of the pyramid, so all the four pupil images are illuminated in the point P conjugated to P', giving a useful signal to compute the wavefront derivatives.

[0010] An ITEK crossed grating wavefront sensor, also known as RTSI (real time shearing interferometer) uses crossed gratings (of a thin type) to create a real time shearing interferometer. The analysis of fringe patterns provides information on the aberrations in “unknown” wavefront. However, said sensor uses thin gratings, so the diffracted output can be confusing, and its dynamic range is limited by the diffraction efficiency.

[0011] A Modal Wavefront Sensor (MWFS) is a purely modal sensor that can rapidly and optically decompose an incoming wavefront into a set of aberration modes magnitudes. The input wavefront has a phase function representing the deviation of the phase of the aberrated wavefront from that of an unaberrated plane wave. This input beam is split into two identical beams that then pass through biasing phase plates placed in the back focal plane of a lens. These phase plates are designed such that the first adds a bias aberration to its input wavefront and the second subtracts the same aberration from its input wavefront. The beams are then focused by the lenses onto detector pinholes. A ratio of the intensities detected by photodiodes at the detector pinholes can be used to determine the degree of wavefront aberration present in the range of the phase bias bounds. However, said sensor can only measure chosen modes, it can’t measure discontinuous wavefronts, and suffers from intermodal crosstalk.

[0012] A modal Digital Holographic Wavefront Sensor (DHWFS) is a subset of the MWFS and in many implementations a digital device (such as a Spatial Light Modulator (SLM)) is used to produce the aberration biases. This is achieved through calculating the hologram needed to contain the aberrations biases as a Computer-Generated Hologram (GCH). The CGH is then encoded on the SLM and when the input wavefront is incident on the SLM it is diffracted into the various appropriate orders. The diffracted beams are then detected by an array of photodiodes or a camera. However, said sensor suffers from the disadvantages associated with thin gratings with limited diffraction efficiency and many diffracted orders. Further, spatial resolution is limited by pixel size of the SLM. As with the MWFS, the wavefront sensor is effectively blind to aberration modes that have not been encoded on the SLM. It also suffers from intermodal crosstalk and limited by speed and requirement of a SLM.

[0013] A modal Analog Holographic Wavefront Sensor (AHWFS) is also a subset of the MWFS but here physically recorded holograms are used to produce the aberration biases. The biases are recorded (and multiplexed) into a photosensitive recording medium using either digital recording techniques (such as an SLM) or refractive elements to generate the aberrations.

[0014] Summary

[0015] In an aspect of the present invention, there is provided a method of determining phase aberration in a given wavefront. The method comprises recording a current array of holograms on a photosensitive recording medium, wherein the recording comprises recording a first pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium; rotating the photosensitive recording medium by 90°; recording a next pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium when the photosensitive recording medium is rotated, wherein the first and next pairs are orthogonal to each other, and form the current array of holograms. The method further comprises probing the current array of holograms recorded in the photosensitive recording medium, with the given wavefront to generate a plurality of first order diffracted beams and a weak transmitted zero order beam, capturing an image of the plurality of first order diffracted beams and the weak transmitted zero order beam by a detector, wherein each first order diffracted beam in the image includes a plurality of zones, and zonally determining the phase aberration of the given wavefront based on comparison of intensity values of the plurality of first order diffracted beams.

[0016] In an embodiment of the present invention, the method further comprises recording a next array of holograms on the photosensitive recording medium after recording the current array, wherein the next array of holograms is at a predefined rotation angle with previous array, and wherein the next array comprises two pairs of orthogonal multiplexed volume holograms, each recorded at respective first and second spatial frequencies. In an embodiment of the present invention, a first volume hologram of the first or next pair is recorded by illuminating the photosensitive recording medium with a reference beam and another beam that forms a first angle with the reference beam, and wherein a second volume hologram of the first or next pair is recorded by illuminating the photosensitive recording medium, with the reference beam and another beam that forms a second angle with the reference beam.

[0017] In an embodiment of the present invention, the method further comprises generating four multiplexed volume holograms on the photosensitive recording medium, when the photosensitive recording medium is rotated once by 90 degrees.

[0018] In an embodiment of the present invention, the method further comprises generating the reference beam by a laser source, wherein the reference beam is perpendicular to an initial position of the photosensitive recording medium.

[0019] In an embodiment of the present invention, the method further comprises applying a tip / tilt aberration and / or other patterns and identifiers to the reference beam by a spatial light modulator (SLM) to generate the first and next pairs of multiplexed volume holograms with tip / tilt aberration, and / or identify the origin of specific beams among the plurality of diffracted beams.

[0020] In an embodiment of the present invention, the photosensitive recording medium is selected from one of: photopolymer, DCG, silver halide, or photopolymerizable sol-gel type materials.

[0021] In an embodiment of the present invention, the method further comprises comparing intensity values of one or more pixels in each zone, to measure slopes of the given wavefront in a plane similar to that of the photosensitive recording medium.

[0022] In an embodiment of the present invention, the method further comprises calculating diffraction efficiency of each diffracted beam in each zone, based on intensity values of the plurality of diffracted beams.

[0023] In an embodiment of the present invention, the method further comprises calculating tip and tilt locally in each zone based on intensity ratios of the plurality of diffracted beams, computing a local wavefront slope of a zone as a gradient of wavefront incident on said zone, and collecting a plurality of local gradients to compile the wavefront profile. In an embodiment of the present invention, the method further comprises illuminating the photosensitive recording medium at a wavelength different than a pre-defined design wavelength, and calculating wavelength shift by observing the angular movement of the plurality of diffracted beams.

[0024] In another aspect of the present invention, there is provided a zonal analog holographic wavefront sensor that comprises a photosensitive recording medium that is rotatable by 90°; an optical system configured to: record a current array of holograms on a photosensitive recording medium, wherein the recording comprises: recording a first pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium; recording a next pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium when the photosensitive recording medium is rotated by 90°, wherein the first and next pairs are orthogonal to each other, and form the current array of holograms; and probe the current array of holograms recorded on the photosensitive recording medium, with a given wavefront to generate a plurality of first order diffracted beams and a weak transmitted zero order beam. The sensor further includes a detector configured to capture an image of the plurality of first order diffracted beams and the weak transmitted zero order beam, wherein each first order diffracted beam in the image includes a plurality of zones; and zonally determine the phase aberration of the given wavefront based on comparison of intensity values of the plurality of first order diffracted beams.

[0025] Various embodiments of the present invention provide a zonal analog holographic wavefront sensor which is closest in concept to the PWFS and the AHWFs. In comparison to the PWFS, the proposed sensor has several advantages including that no modulation of the beam / pyramid is required. The size of the hologram allows for much easier alignment and greater flexibility in sensing beam diameter, and the sensor uses less expensive optical components. In comparison to the AHWFS, the proposed sensor has advantages in that it can measure a wider variety of wavefronts (e.g. discontinuous), and is not restricted to just those whose biases are included in the hologram. Furthermore, the sensor of the present invention has less complicated and stringent multiplexing requirements when recording the hologram, as only four standard gratings biased with some magnitude of tip / tilt are required, and it has a larger dynamic range.

[0026] Brief Description of the Drawings

[0027] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:

[0028] FIGs. 1 A and IB illustrate concept of SHWFS;

[0029] FIG. 2 illustrates the schematic of the PWFS concept;

[0030] FIG. 3A illustrates schematic of a first optical set-up to record volume Holographic Optical Element (vHOE) crossed gratings, in accordance with an embodiment of the present invention;

[0031] FIG. 3B illustrates schematic of a second optical set-up to record volume Holographic Optical Element (vHOE) crossed gratings, in accordance with another embodiment of the present invention;

[0032] FIG.4 is a flowchart illustrating a method of determining phase aberrations present in a given waveform, using first or second optical set-up, in accordance with an embodiment of the present invention;

[0033] FIG.5A illustrates an image including single pair of crossed gratings, in accordance with an embodiment of the present invention;

[0034] FIG.5B illustrates another image including two pairs of crossed gratings, in accordance with an embodiment of the present invention;

[0035] FIG.5C and 5D illustrate another image including two arrays of crossed gratings, in accordance with an embodiment of the present invention;

[0036] FIGs.6A and 6B illustrate measurement of phase aberrations of a wavefront using the recorded vHOE crossed gratings in the photosensitive recording medium, in accordance with an embodiment of the present invention.

[0037] Detailed Description of the Drawings

[0038] FIG. 3A illustrates schematic of a first optical set-up 300 to record volume Holographic Optical Element (vHOE) crossed gratings, in accordance with an embodiment of the present invention. The first optical set-up 300 includes a laser source 302, a spatial filter (SF) 304, variable apertures A1-A4, a collimating lens (CL), a first beam splitter (BS1) 308, a second beam splitter (BS2) 310, first and second planar mirrors Ml and M2 and a photosensitive recording medium 312. The laser source 302 generates a reference beam that is perpendicular to the photosensitive recording medium 312. Examples of the photosensitive recording medium 312 includes, but are not limited to, photopolymer, DCG, and silver halide. This optical-setup 300 records four multiplexed holographic gratings on the recording medium where a tip / tilt aberration is applied by rotating the photosensitive medium in the z-plane.

[0039] FIG. 3B illustrates schematic of a second optical set-up 320 to record volume Holographic Optical Element (vHOE) crossed gratings, in accordance with another embodiment of the present invention. The second optical set-up 320 includes a spatial light modulator (SLM) 322, a laser source, a spatial filter (SF), variable apertures (A1-A4), collimating lens (CL), partially reflective mirror (PRM), beamsplitter (BS1), planar mirrors (Ml -M3), and the recording medium 312. The second optical-setup 320 records four multiplexed holographic gratings on the recording medium in a manner same as that of FIG.3A, except for a tip / tilt aberration applied by the SLM.

[0040] It is to be noted that additional information can be encoded using an SLM during recording to provide more functionality. For example, the known static aberrations of an optical system (e.g. receiving telescope or microscope) or known beam divergence / convergence could be added into the reference beam. This would mean any aberrations measured by the sensor would be due to external factors (e.g. atmospheric turbulence, diffuse layers, etc.). Alternatively, digits or text could be included in the carrier beams as a visual aid for simpler iterations of the system that do not include a camera and / or for beam encoding / identification. This could be achieved using an array of photodiodes or similar for measurements, for increased speed with a trade-off in measurement resolution. Further, the visual aids can be encoded on the SLM. FIG.4 is a flowchart illustrating a method of determining phase aberrations present in a given wavefront using first or second optical set-up 300 or 320, in accordance with an embodiment of the present invention.

[0041] At step 402, a first pair of multiplexed volume hologram gratings are recorded with distinct carrier spatial frequencies (SF1 and SF2), on the photosensitive recording medium 312. In an embodiment of the present invention, the carrier spatial frequency is set based on the set-up and position of mirrors Ml and M2. Although, two mirrors are shown herein, it would be apparent to one of ordinary skill in the art, that more than two mirrors can be used to record the gratings on the medium 312. A first grating of the pair is recorded on the recording medium 312 with first spatial frequency SF1, by blocking mirror M2 and positioning mirror Ml so that the beam from the first mirror Ml forms a first inter-beam angle with the reference beam. The reference beam and the beam from the first mirror Ml are allowed to propagate, creating the interference pattern. The interference pattern is holographically recorded in the photosensitive recording medium 312. Then, a second grating of the pair is recorded on the recording medium 312 with second spatial frequency SF2, by blocking mirror Ml and positioning mirror M2 so that the beam from the second mirror M2 forms a second inter-beam angle with the reference beam.

[0042] In an embodiment of the present invention, the SLM 322 applies a tip / tilt aberration to the reference beam to produce gratings on the medium 312 with tip / tilt aberration.

[0043] The holographic recording step is similar to the recording of holographic crossed gratings. For practical holographic-recording purposes, this requires interference patterns created with recording beams set at different angles. The holograms with SF1 and SF2 may be recorded using object beams that illuminate the photosensitive recording medium 312 from opposite sides and for which the inter-beam recording angle may not be of the same magnitude. Thus, the first spatial frequency (SF1) is used to record first grating corresponding to first inter-beam angle, and the second spatial frequency (SF2) is used to record second grating corresponding to second inter-beam angle, thus creating a first pair of multiplexed recorded gratings with a spatially separated diffracted output. The first pair of multiplexed recorded gratings 500 with different spatial frequencies (SF1) and (SF2) is illustrated with reference to FIG.5A. At step 404, the photosensitive recording medium 312 is rotated at least once by 90°, in accordance with an embodiment of the present invention. In a preferred embodiment, the medium is rotated once by 90° in the x-y plane.

[0044] At step 406, a next pair of multiplexed volume hologram gratings are recorded in the medium 312 with separate carrier spatial frequencies (SF1 and SF2), whenever the photosensitive recording medium is rotated. Thus, upon one rotation of the photosensitive recording medium, the first spatial frequency (SF1) is used to record third grating, and the second spatial frequency is used to record fourth grating, thus creating a second pair / set of multiplexed recorded gratings. The second pair / set of multiplexed recorded gratings 502 with different spatial frequencies (SF1) and (SF2) is illustrated with reference to FIG.5B. The first and second pair of gratings may be referred to as a first array of gratings, in which first and second pair is orthogonal to each other.

[0045] Further pairs of multiplexed volume holograms may be recorded with further rotations of the photosensitive recording medium. It would be apparent to one of ordinary skill in the art, when the photosensitive recording medium is rotated by 90°, then four multiplexed holographic gratings are generated. An additional array of crossed gratings may be added at any given angle to the original array but again the rotation angle between the pairs of gratings within an array is to be 90 degrees. This allows for different dynamic range using different arrays.

[0046] Such multiplexing provides simultaneous sensing ability for two different wavelengths. Thus, in the preferred embodiment, the output from the recording stage is a multiplexed volume holographic optical element (vHOE) consisting of four holograms, with each grating pair set recorded in the recording medium perpendicular to each other. As these gratings are volume Bragg gratings they can have high diffraction efficiencies, with tuneable angular and wavelength selectivity (by changing material thickness and refractive index modulation, as well as the grating spatial frequency).

[0047] The multiplexed gratings refer to the hologram which is the physical structure recorded in the medium 312 that is used to diffract the light that is incident. An incoming beam of light is used to replay the hologram / holographic grating. The output from the hologram is referred to as diffracted beams of light. Referring back to FIG.4, at step 408, the volume holographic optical element (vHOE) crossed grating recorded in the medium 312 is probed with an unknown beam to determine the wavefront aberration of the unknown beam. The unknown beam may be made up of some unknown type and magnitude of aberration. The probing of the photosensitive recording medium 312 with the unknown beam is further illustrated with reference to FIG.6 A.

[0048] In an embodiment of the present invention, when the unknown beam is used to probe the vHOE, the multiplexed holograms illustrated in FIGs.5A and 5B may be replayed in the medium 312 and are detected by a CMOS / CCD detector or camera 502. FIG.5C illustrates a first image 504 generated by the CMOS / CCD detector 502 for the first pair of gratings 500, and a second image 506 generated for both the first and second pair of gratings 500 and 502. It can be seen in the first image 504 that there is not enough information to determine angle (tip / tilt) changes in both axes. In the second image 506, there is information about angle of wavefront (tip / tilt) incident on the hologram in both axes. The tip / tilt cannot be determined if the images are not orthogonal. Thus, the second image 506 illustrates an array of two pairs of gratings that are orthogonal to each other.

[0049] FIG.5D illustrates an image 508 generated for two arrays of gratings. The first array includes Ii, E, I3, and I4 gratings. The second array includes Ir, h’, I3’, and I4’ gratings. The second array may be recorded at a given rotation angle to the first array using spatial frequencies (SF1) and (SF2). However, within the second array, the pair of gratings are orthogonal to each other. In an example, the image 508 is obtained by recording second array @45 degree rotation to the first array using the same set of spatial frequencies. Similarly, the image 510 is obtained by recording second array @30 degree rotation to the first array using the same set of spatial frequencies. Referring back to FIGs.6A and 6B, the CMOS / CCD detector 602 generates an image 604 that includes four first order diffracted beams 606a-606b, and a weak transmitted zero order beam 608. The four diffracted beams create four visible spots 606a-606b in the image 604. When a beam probing the gratings is completely “off-Bragg” i.e. the hologram replay condition does not exactly match the hologram recording condition 610b, the beam will transmit through the grating and not be deflected by the gratings (instead forming the central 0thorder / transmitted beam). Referring back to FIG.4, at step 410, wavefront aberrations of the unknown beam are determined zonally using four first order diffracted beams 606a-606b and one zero order beam 608 by comparing the intensity values of the diffracted beams 606a-606b. The intensity values of the pixels in each of the beam images are compared and a calculation is used to produce a measure of the wavefront slopes in the x- and y-directions. This measurement is done over zones, whether pixel -by -pixel or over groups of pixels. A typical method of deriving the wavefront slopes is through a contrast calculation. In the x- and y-directions the wavefront slopes (Sxand Sy) are calculated by Sx= where I, is the intensity in the beam image and Io is the mean intensity in the four beam images. It should be noted many variants of this method can be used to derive the slopes. A wavefront slope is the calculated value of the tip / tilt across a zone, and is ideally, an average (of the actual) tip / tilt across multiple zones.

[0050] The diffraction efficiency across each zone of each diffracted beam depends on how close the replay beam wavefront is to being Bragg matched to a particular hologram at that zone. The diffraction efficiency is the intensity over a given region of the diffracted beam. The diffraction efficiency may be calculated as: (intensity of the first order beam) / (intensity of first order beam + intensity of any other diffracted beams + intensity in the zero order beam). This resulting intensity normalised differential in each corresponding zone in the paired sets of diffracted beams allow for the tip and tilt to be calculated locally at that zone using simple intensity ratios. These can then be utilised to determine the wavefront phase from the local wavefront slopes. The collection of local gradients may then be registered with respect to each other to compile the wavefront shape / profile. The local wavefront slope is the gradient of the wavefront incident on that particular zone of the sensor (part of the sensor). This is calculated in the x- and y-directions.

[0051] The use of the vHOE, during replay, to optically decompose the full aperture of an incoming optical beam into its tip / tilt components, through diffraction. The diffracted tip / tilt components are used to zonally determine the wavefront aberration through established methods of calculating wavefront slopes at each subaperture.

[0052] It is to be noted that the approximate position of the zero order could be used as a proximate global tip / tilt sensor to assist in aligning the beam entering the wavefront sensor (due to beam wander, jitter, etc.). Once it is within the “Bragg envelope” for the recorded holograms, the aberrations can be sensed with the zonal AHWFS sensing methodology proposed. In addition to this, it may be possible to include more channels in the zonal AHWFS to provide options regarding the dynamic range and the sensitivity of the sensor. This could be done for example by having one set of crossed gratings with a lower spatial frequency (e.g. 200-400 lines / mm) and another at a higher spatial frequency (e.g. 800-1000 lines / mm). This would provide the user with simultaneous information on the wavefront at different resolutions and increase the dynamic range of the system.

[0053] In an embodiment of the present invention, provided that the replay beam is within the Bragg envelope, the inclusion of an afocal imaging relay system with variable magnification positioned between the vHOE and the detector could provide capability to balance the desired application trade-off between system sensitivity and range. This adjustment could be dynamic and would allow user flexibility to control the ratio of pupil image size relative to detector pixel size.

[0054] In another embodiment of the present invention, there is provided a provision of multiwavelength analysis by illuminating the sensor at a wavelength different than the design wavelength (or in addition to the design wavelength sequentially) and calculating the wavelength shift by observing the angular movement of the diffracted beams. Typically, holograms are to use a specific 'design' wavelength. When they are replayed at that same wavelength (or similar wavelength) they behave as intended. If the Hologram is replayed with a wavelength other than the 'design' wavelength, their output departs from the model and may even have no output. This may have use in identifying chromatic aberrations or for metrology testing of optical el ements / sy stems at differing wavelengths. Another method of multi -wavelength analysis is through creating a number (2-4) of crossed gratings sensors, separated angularly (e.g. two sensors at the 0°, 90°, 180°, 270°; and 45°, 135°, 225°, 315°). Another method to allow for measurement of broadband spectral information would comprise stacking a corrective HOE grating between the photosensitive medium and the camera (602). The purpose of the corrective HOE grating would be to ensure that all wavelengths (within a given range), diffracted at a spectrum of angles, fall in the same spatial position on the camera. A corrective HOE grating would be required for each diffracted beam.

[0055] In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms include, includes, included and including" or any variation thereof are considered to be totally interchangeable, and they should all be afforded the widest possible interpretation and vice versa.

[0056] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.

Claims

Claims:

1. A method of determining phase aberration in a given wavefront, comprising: recording a current array of holograms on a photosensitive recording medium, wherein the recording comprises: recording a first pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium; rotating the photosensitive recording medium by 90°; recording a next pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium after the photosensitive recording medium is rotated, wherein the first and next pairs are orthogonal to each other, and form the current array of holograms; probing the current array of holograms recorded on the photosensitive recording medium, with the given wavefront to generate a plurality of first order diffracted beams and a weak transmitted zero order beam; capturing an image of the plurality of first order diffracted beams and the weak transmitted zero order beam by a detector, wherein each first order diffracted beam in the image includes a plurality of zones; and zonally determining the phase aberration of the given wavefront based on comparison of intensity values of the plurality of first order diffracted beams.

2. The method as claimed in claim 1 further comprising: recording a next array of holograms on the photosensitive recording medium after recording the current array, wherein the next array of holograms is at a predefined rotation angle with previous array, and wherein the next arraycomprises two pairs of orthogonal multiplexed volume holograms, each recorded at respective first and second spatial frequencies.

3. The method as claimed in any preceding claim, wherein a first volume hologram of the first or next pair is recorded by illuminating the photosensitive recording medium with a reference beam and another beam that forms a first angle with the reference beam, and wherein a second volume hologram of the first or next pair is recorded by illuminating the photosensitive recording medium, with the reference beam and another beam that forms a second angle with the reference beam.

4. The method as claimed in claim 1 further comprising generating four multiplexed volume holograms on the photosensitive recording medium, when the photosensitive recording medium is rotated once by 90 degrees.

5. The method as claimed in any preceding claim further comprising generating the reference beam by a laser source, wherein the reference beam is perpendicular to an initial position of the photosensitive recording medium.

6. The method as claimed in any preceding claim further comprising applying a tip / tilt aberration and / or other patterns and identifiers to the reference beam by a spatial light modulator (SLM) to generate the first and next pairs of multiplexed volume holograms with tip / tilt aberration, and / or identify the origin of specific beams among the plurality of diffracted beams.

7. The method as claimed in any preceding claim, wherein the photosensitive recording medium is selected from one of: photopolymer, DCG, silver halide, or photopolymerizable sol-gel type materials.

8. The method as claimed in any preceding claim further comprising comparing intensity values of one or more pixels in each zone, to measure slopes of the given wavefront in a plane similar to that of the photosensitive recording medium.

9. The method as claimed in any preceding claim further comprising calculating diffraction efficiency of each diffracted beam in each zone, based on intensity values of the plurality of diffracted beams.1710. The method as claimed in any preceding claim further comprising calculating tip and tilt locally in each zone based on intensity ratios of the plurality of diffracted beams, computing a local wavefront slope of a zone as a gradient of wavefront incident on said zone, and collecting a plurality of local gradients to compile the wavefront profile.

11. The method as claimed in claim 1 further comprising illuminating the photosensitive recording medium at a wavelength different than a pre-defined design wavelength, and calculating wavelength shift by observing the angular movement of the plurality of diffracted beams.

12. A zonal analog holographic wavefront sensor, comprising: a photosensitive recording medium that is rotatable by 90°; an optical system configured to: record a current array of holograms on a photosensitive recording medium, wherein the recording comprises: recording a first pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium; recording a next pair of multiplexed volume holograms with respective first and second carrier spatial frequencies, on the photosensitive recording medium when the photosensitive recording medium is rotated by 90°, wherein the first and next pairs are orthogonal to each other, and form the current array of holograms; and probe the current array of holograms recorded on the photosensitive recording medium, with a given wavefront to generate a plurality of first order diffracted beams and a weak transmitted zero order beam; and a detector configured to:18 capture an image of the plurality of first order diffracted beams and the weak transmitted zero order beam, wherein each first order diffracted beam in the image includes a plurality of zones; and zonally determine the phase aberration of the given wavefront based on comparison of intensity values of the plurality of first order diffracted beams.

Images