Abstract:
A device for detecting a wavefront that is defined by a plurality of contiguous light beams includes an array of lenslets for isolating the individual light beams and focusing each individual light beam to a focal point in an x-y plane. The device also has a plurality of clusters which are positioned in the x-y plane, and each cluster includes a plurality of pixels that are arranged in rows aligned in an x-direction, and columns aligned in a y-direction. Additionally, each pixel of a cluster includes both a first photocell for generating an x-signal and a second photocell for generating a y-signal, respectively, in response to an illumination of the pixel by a light beam. Further, the device includes circuitry for converting the x and y signals to digital signals and then using the digital signals to determine an x-y position for the focal point of the particular light beam that is incident on the cluster. A computer then compares the respective x-y positions of the various focal points to detect the wavefront. Depending on the particular application of the device, either photodiodes or phototransistors may be selected for use as the photocells in the pixels.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to light wavefront detection devices and their methods of use. More particularly, the present invention pertains to wavefront detection devices that measure deviations between the actual focal points of individual light beams in the wavefront and the idealized focal point of corresponding individual light beams in a plane wavefront. The present invention is particularly, but not exclusively useful for using light beam focal point deviations for the purposes of restructuring, mimicking or otherwise detecting a light wavefront. 
     BACKGROUND OF THE INVENTION 
     Wavefront detectors are typically used as measurement systems for detecting the shape of the wavefront that is characterized by a plurality of aberrated light beams. To do this, the phase information of the aberrated light beams, i.e. wavefront, can be described by comparing this information to the phase information of an ideal wavefront which is sometimes called the reference wavefront. For this purpose, the reference wavefront is assumed to have its vertex parallel to the optical axis for each point in a plane. The optical path difference (OPD) between the phase information of the aberrated wavefront and the phase information of the ideal reference wavefront is thus quantified at each point of the reference wavefront. The result is that the wavefront of the aberrated light beams will correspond to the sum of all optical aberrations introduced into this beam while travelling through different optical elements within the optical path. Due to the precise nature of their measurements, wavefront detectors have found numerous applications in areas of optical information processing, especially astronomical research, quality control for optical elements, and ophthalmologic diagnostics. 
     In most wavefront detection schemes interferometers are used for precise and reliable measurements of optical path differences. An accuracy of several fractions of the used radiation wavelength is achievable using interferometer techniques and this accuracy allows high resolutions in quantification of the wavefront&#39;s shape. Different types of interferometers have been invented for this purpose, with the most important being commonly known as the Michelson interferometer. Special redesigns of the Michelson interferometer, however, have been adapted to special measuring purposes. For example, the Mach-Zehnder interferometer or the Saganc interferometer have been widely used. It is common to all types of interferometers that they consist of a system of two optical paths. One path guides a reference wavefront, and the other path contains an element with the optical aberrations that are to be measured. The superposition of the reference wavefront and the aberrated wavefront in an imaging plane then allows the quantification of the amount of aberrations with high accuracy. The disadvantage of this method, however, is the need for two optical paths. 
     In addition to interferometers, two other type wavefront detection schemes are now offering the possibility of measuring the wavefront&#39;s shape by examining images of a single aberrated light beam. One such method uses a back-projection of the point-spread-function (PSF) of the focused light beam to calculate the light beams wavefront before being focused. In this measuring scheme, the intensity distribution of a focal point generated by a lens is examined by use of a CCD camera and a digital signal processing system. By reconstructing the incident light beams wavefront in front of the lens and comparing it to the focal points PSF, the wavefronts shape can be derived. 
     The second commonly applied detection scheme with image examination is referred to as the Hartmann-Shack detection scheme. For this scheme, the Hartmann-Shack-wavefront sensors use an array of micro-lenses to divide the incident light beam into a matrix of sub-apertures. Each lens then focuses its partial incident light into a focal point. In case of a local tilt of the incident light beams wavefront within the margins of this sub-aperture, the focal point emerges at a deviation perpendicular to the optical axis. The amount of this deviation, in first order, is proportional to the amount of the local tilt of the wavefront. Thus, this tilt can be quantified. A measurement of all focal point deviations in a wavefront allows the reconstruction of the global wavefront&#39;s shape by use of a least-square-fit method for calculation. This results in a mathematical standard description of the OPD with respect to a reference wavefront by use of high order polynomials. 
     The basic Hartmann test has been commonly used to measure the surface quality of primary mirrors in astronomical telescopes as they are polished. To do this, a pinhole is placed in the entrance pupil of a lens of high quality. The pinhole is then movable perpendicular to the optical axis to cover every point within the apertures area. Consequently, the wavefront of the incident light beam is divided into a number of sampling points. For each point, the image of the pinhole will result in a focal point on the image plane of the lens and a local tilt of the wavefront within the pinhole will cause a deviation of the focal point perpendicular to the optical axis. As indicated above, this deviation is measurable. 
     To overcome the critical time limitations of the former described Hartmann-test, a parallel use was invented by Dr. Roland Shack of the University of Arizona&#39;s Optical Sciences Center which is now widely referred to as a Hartmann-Shack wavefront detection scheme. Instead of only one pinhole, a number of equidistant micro lenses is used to generate a matrix of focal points on an image plane. 
     A Hartmann-Shack-Sensor (HSS) commonly includes an optical system for imaging the aberrated light beam onto a lens array, and an image detector for measuring deviations of the resultant focal points. When using micro-lens array having sufficient quality, HSSs can be used in many applications of wavefront measurement. Further, apart from monitoring for quality control of optical elements where only measurements of wavefront shapes are performed, active optics which use HSS are also realizable. For instance, astronomical telescopes usually compensate for atmospherical distortions by using a closed loop active optics which include a Hartmann-Shack-Sensor. 
     The critical function of an HSS as part of an image system is to measure the amount of the focal points deviations with a sufficient repetition rate and acceptable accuracy. In operation, the image data is transmitted to an image processing system with digital data acquisition possibilities that are able to perform pattern recognition. The matrix of focal points that needs to be examined for intensity distribution and for the center of each focal point, in order to measure the amount of focal point deviation due to a local wavefront tilt, is often extensive. In this regard, standard CCD (charge coupled devices)-cameras are restricted to a frame repetition rate of about 50 Hz Some high performance CCD-systems, however, with the capability of reading out randomly accessible imaging detectors, are capable of repetition rates of several hundred Hz. Nevertheless, even more responsive and more accurate focal point detections are desirable. 
     The crucial part of focal point deviation detection is the classification of the focal points intensity distribution for the center of each spot. It is commonly known, that the center of the spots are best derived by use of a center of gravity algorithm. This algorithm achieves a high accuracy in position detection despite the cost of the duration of calculation. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     A device for detecting a wavefront in accordance with the present invention includes an optical means, such as a lenslet array, for separating the wavefront into a plurality of contiguous light beams. The individual light beams are then individually focused by the optical means to a plurality of respective focal points in an x-y plane. As contemplated by the present invention, the optical means that is used for separating the wavefront into the plurality of contiguous light beams, and for then focusing these light beams, can be of a type commonly referred to as a Hartmann-Shack-Sensor (HSS). 
     For the present invention, an array of clusters are positioned to lie in the x-y plane so that the focal point of each individual light beam will be incident on a respective cluster. More specifically, the clusters are contiguously arranged in rows and columns in the x-y plane. Further, each cluster includes a plurality of pixels which are also aligned in rows and columns. In particular, each row in the cluster has a plurality of pixels that are aligned in the x-direction, and each column has a plurality of pixels that are aligned in the y-direction. Still further, each pixel includes a first photodetector (photocell) that will generate an x-signal in response to an illumination of the pixel by the focal point of a light beam, and each pixel includes a second photodetector (photocell) that will generate a y-signal in response to an illumination of the pixel. 
     Depending on the size of a light beam&#39;s focal point, and the number and size of the pixels that are in a cluster, the actual location of a focal point on the cluster can be determined with good accuracy. For most applications, it is necessary that only about five percent of the pixels in a cluster be illuminated by the focal point of a light beam at any point in time. This is generally sufficient for determining the x-y position of the focal point on the cluster and further, depending upon the location of the particular cluster in the array, the x-y position of a particular focal point can be determined relative to the x-y positions of all other focal points of light beams in the wavefront. 
     As contemplated by the present invention, the two photodetectors (photocells) which make up each pixel can be either a photodiode type photocell or a phototransistor type photocell. As is well known, various structural configurations are possible for the manufacture of the photodiodes, or the phototransistors, and the selection of a particular type photocell will, for the most part, depend on the particular application that is to be accomplished by the device, and on the wavelengths of the light beams that are to be detected by the device. 
     In the operation of the device of the present invention, each of the plurality of contiguous light beams that make up the wavefront to be detected is focused onto a respective cluster in the array. The respective “x” and “y” signals thereby generated are first converted into compressed digital signals. These digital signals are then sent to a computer where they are used to determine the actual x-y position of the particular light beam&#39;s focal point on the cluster. Similarly, the actual x-y positions of the focal points for all of the contiguous light beams in the wavefront are determined by the computer. Next, these actual x-y positions are compared with the corresponding idealized x-y positions of contiguous light beams in a plane wavefront. These comparisons are then used to determine any deviation there may be between the actual x-y position of a light beam&#39;s focal point and a corresponding idealized x-y position for the focal point. Using the deviations for each of the light beam focal points, the actual wavefront that is characteristic of the detected wavefront can be analyzed, reconstructed or otherwise used as desired. 
    
    
     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 top plan view of an application specified integrated circuit (ASIC) for use in wavefront detection in accordance with the present invention; 
     FIG. 2 is a top plan view of a cluster used for the present invention and an enlarged view of several pixels used in the cluster; 
     FIG. 3A is a perspective view of a lenslet array shown focusing a representative light beam of a wavefront onto a focal point in an x-y plane; 
     FIG. 3B is a schematic view of a focal point deviation between a light beam of a plane wavefront and a light beam of a distorted wavefront; 
     FIG. 3C is a schematic view of a plurality of focal point deviations between a plane wavefront and a distorted wavefront; 
     FIG. 4 is a top plan view of a cluster with a light beam focal point incident thereon, together with related graphs depicting signal processing techniques used for determining the x-y position of the focal point on the cluster; 
     FIGS. 5A-E are a sequence of cross sectional views showing respective structural configurations for various photocells as would be seen along the line  4 — 4  in FIG. 2; 
     FIG. 6 is a graph showing plots of wavelength vs. quantum efficiency for each of the photocells shown in FIGS. 4A-E; and 
     FIG. 7 is a schematic view of a control configuration for the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1 a device in accordance with the present invention is shown and is generally designated  10 . For the purposes of the present invention, the device  10  functions as an Application Specific Integrated Circuit (ASIC) which is formed as an array  12  of clusters  14 . While the particular device  10  shown in FIG. 1 is shown as a three by three array  12  which includes nine clusters  14 , this is only exemplary. Preferably, the device  10  may be as much as a sixteen by sixteen array  12  having two hundred and fifty six clusters  14 . In any case, regardless the number of clusters  14 , the operation of the device  10  and the function of the individual clusters  14  will remain essentially the same. 
     As shown in FIG. 1, the array  12  is mounted on a substrate in an x-y plane  16  with the clusters  14  aligned in a plurality of rows  18  and a plurality of columns  20 . For purposes of this disclosure the rows  18  from top to bottom are appropriately primed and the columns are identified with lower case letters. Thus, the top row of the array  12  includes the clusters  14   a ,  14   b  and  14   c , while the left hand column of the array  12  includes the clusters  14   a ,  14   a ′ and  14   a ″. As also shown in FIG. 1 the various clusters  14  are electronically interconnected. For example, the cluster  14   a  is electronically interconnected with the cluster  14   b  by x-data link  22   a , and with the cluster  14   a ′ by the y-data link  24   a . Also by way of example, it is to be appreciated that the x-data link  22   c  from cluster  14   c  is cumulative with x-data link  22   b  and x-data link  22   a . The x-data from all of the rows  18  are likewise cumulative. Therefore, the x-data  26  includes the x-data links  22  of all clusters  14  in the array  12 . Similarly, y-data link  24   a ″ includes y-data link  24   a ′ and y-data link  24   a , and the y-data  28  includes all of the y-data links  24  in array  12 . 
     The contribution of each cluster  14  to the x-data  26  and y-data  28  from the array  12  will be best appreciated by cross referencing FIGS. 2,  3 A-C and  4 . First, in FIG. 2, using the cluster  14   b ′ as an example, it can be seen that each cluster  14  includes a plurality of pixels  30  which are arranged as a plurality of rows  32  and a plurality of columns  34 . Specifically, the rows  32  extend in the x-direction, and the plurality of columns  34  extend in the y-direction. As shown, all of the rows  32  terminate at a current detector  36  and all of the columns  34  terminate at a current detector  38 . Further, still referring to FIG.  2  and using the pixel  30   a  as an example, it will be appreciated that each pixel  30  includes a pair of photodetectors (photocells)  40  and  42 . More specifically, the photodetector  40  is electronically connected to a circuit  44  which will conduct any current that is generated by the photodetector  40  to the current detector  38 . Similarly, the current detector  38  receives currents from corresponding photodetectors  40  in all of the pixels  30  in the cluster  14  via respective circuits  44 . The information from these respective circuits  44  is then passed by the current detector  38  to a data shift  48  where it is included with the x-data link  22   a ′ from cluster  14   a ′ to create the x-data link  22   b ′. In a similar manner, any currents that are generated by the photodetectors  42  will be passed via respective circuits  46  to a data shift  50  where they are included with the y-data link  24   b  from cluster  14   b  to create the y-data link  24   b′.    
     In a manner well known in the pertinent art, the currents from photodetectors  40 , 42  which pass through the respective circuits  44 , 46  (referred to above) are generated whenever a focal point  52  is incident on a particular photodetector  40  or  42 . The utility of this is, perhaps, best appreciated with reference to FIGS. 3A-C. Beginning with FIG. 3A, it is appropriate to consider a wavefront as comprising a plurality of individual light beams  54 . For this analysis, consider the plurality of individual light beams  54  (i.e. the entire wavefront) is incident on an array  56  of individual lenslets  58  with each light beam  54  passing through a respective lenslet  58 . Specifically, consider the light beam  54   a . As Shown in FIG. 3A, the light beam  54   a  will be focused by the lenslet  58   a  to a focal point  52  in the x-y plane  16 . For continuity of discussion, consider the focal point  52  of light beam  54   a  is incident on the cluster  14   b ′ in the x-y plane  16 . If the wavefront, of which light beam  54   a  is a part, is a plane wavefront, it can be shown that the focal point  52  will be as substantially shown in FIG.  3 B. On the other hand, if the wavefront is not planar and, instead, is somehow aberrated or distorted, the light beam  54   a  will be altered to react as a light beam  54   a ′ which approaches the lenslet  58   a  at an angle. The consequence of this is that the focal point  52  (light beam  54   a ) is shifted to the focal point  52 ′ (light beam  54   a ′). Such a shift is shown, as an example, in FIGS. 3B and 3C. A similar consequence, of course, results for all of the other individual light beams  54  in the wavefront. As intended for the present invention, shifts in the locations of the focal points  52  in the x-y plane  16  for the respective individual light beams  54  can be used for reconstructing the distorted wavefront. Specifically, the amount of shift of focal points  52  in the x and y directions from their predetermined locations in the x-y plane  16  for a plane wavefront provides information with which the incident wavefront can be reconstructed. For the present invention, this detection of shifts in focal points  52  is done on each of the individual clusters  14 . 
     In FIG. 4, the cluster  14   b ′ is again used as an example. As such, in FIG. 4 it will be seen that the focal point  52  is incident on several pixels  30  in the in the x-y plane  16  of the array  12 . Actually, the intensity (J ph ) of focal point  52  has a gaussian distribution which, for the y direction is shown by the intensity curve  60  in FIG.  4 . Further, although the focal point  52  is two-dimensional in the x-y plane, as shown, it will be substantially circular. Thus, for purposes of discussion, it is sufficient to consider only the y dimension of focal point  52  and recognize that the x dimension will have similar consequences. Accordingly, consider the photodetector  42  of a pixel  30  in cluster  14   b ′ that is illuminated by the focal point  52 . If this photodetector  42  is illuminated by the focal point  52 , it will generate a current which, together with all other photodetectors  42  in the same row  32  will generate a responsive current spike  62  (e.g. I y3 ) which will flow through a circuit  46  to the current detector  36 . Similarly, for each row  32 , a current spike  62  will be generated by all photodetectors  42  in the same row  32  (e.g. I y1 , I y2  - - - I y8 ). As can be easily appreciated, the magnitude of the current spikes  62 , as shown in FIG. 4, will vary according to how many photodetectors  42  in the row  32  are illuminated by the focal point  52 . In any event, the current detector  36  compares the current spikes  62  to generate an output signal  64  which is representative of the y coordinate of the focal point  52  on cluster  14   b ′. As indicated in FIG. 4, the output signal  64  is compressed in the data shift  50  to a binary code. For example, as shown, the current associated with I y3  is converted to a null binary digit  66   (0)  while the current associated with I y4  is converted to a unit binary digit  66   (1) . Similarly, the currents associated with the various rows  32  are converted into binary signals which are compressed and transmitted from the cluster  14   b ′ as the y-data link  24   b ′. A similar discussion for the photodetectors  40  in the columns  34  will result in the generation of the x-data link  22   b′.    
     In accordance with the present invention the photodetectors  40 ,  42  can be any of several types well known in the pertinent art. Specifically, the array  12  and all of the photodetectors  40 ,  42  and associated circuitry can be fabricated by processing silicon wafers with standard industrial processes. The result is a monolithic integration of photodetectors  40 , 42  and signal processing circuitry. More specifically, insofar as the photodetectors  40 , 42  are concerned, several embodiments of photodiodes or phototransistors, such as those shown in FIGS. 5A-E can be employed. 
     FIG. 5A shows an embodiment for an nplus-photodiode  68  which can be used as a photodetector  40 , 42 . As shown, the nplus-photodiode  68  includes a highly doped n-active cathode  70  which is embedded into a slightly p-doped substrate  72 . Also embedded into the substrate  72  is a highly doped p-active anode  74 . Photoconversion for the nplus-photodiode  68  occurs due to a reverse polarized voltage within the depletion region  71  between the cathode  70  and the substrate  72 . For a slightly different embodiment of the photodetectors  40 , 42 , FIG. 5B shows a pplus-photodiode  76 . As shown, the pplus-photodiode  76  has a highly doped p-active anode  78  which is embedded into a slightly doped n-well  80 . The n-well  80  is embedded into a slightly p-doped substrate  82 , and a highly doped n-active cathode  84 , like the anode  78 , is embedded into the n-well  80 . For the pplus-photodiode  76 , photoconversion will take place in the depletion region  79  between the anode  78  and the n-well  80 . A third type embodiment for a photodiode which can be used as the photodetectors  40 , 42  is the nwell-photodiode  86  shown in FIG.  5 C. For the nwell-photodiode  86 , a slightly doped nwell  88  is provided and a highly doped n-active cathode  90  is embedded into the nwell  88 . A highly doped p-active anode  92  is embedded into a slightly p-doped substrate  94  and a depletion region  96  is provide between the nwell  88  and the substrate  94 . For the nwell-photodiode, photoconversion takes place in the depletion region  96 . 
     As indicated above, phototransistors may also be used for the photodetectors  40 , 42 . One embodiment for such a phototransistor is a pnp-phototransistor  98  as shown in FIG.  5 D. As shown, the pnp-phototransistor  98  includes a highly doped p-active emitter  100  which is embedded into a slightly n-doped nwell  102 . A highly doped n-active base  104  can also be embedded into the nwell  102 . A highly doped p-active collector  106  is embedded into a substrate  108  and a depletion region  110  wherein photoconversion takes place separates the nwell  102  from the substrate  108 . In yet another embodiment of a phototransistor type photodetector  40 , 42 , an npn-phototransistor  112  is provided as shown in FIG.  5 E. Included in the npn-phototransistor  112  are a highly doped n-active emitter  114  which is embedded into a slightly p-doped base  116 . The base  116 , in turn, is embedded into a slightly n-doped nwell  118 . The npn-phototransistor  112  also includes a highly doped n-active collector (sinker)  120  which is connected to a buried layer  122 . If desired, the base  116  can have a highly doped p-active base lead  124 . A substrate  126  is provided to support this structure and a depletion region  128  is provided between the nwell  118  and the base  116  where photoconversion will take place. 
     In FIG. 6 the effective quantum efficiencies for the various photodetectors  40 , 42  ( 68 ,  76 ,  86 ,  98  and  112 ) are shown in FIGS. 5A-E with respect to the incident photon wavelengths in focal point  52 . As shown in the FIG. 6, the peak value of each curve corresponds to the maximum of effective quantum efficiency of the respective photodetector ( 68 ,  76 ,  86 ,  98  and  112 ) and, therefore, gives a selection specification for the photodetectors  40 , 42  according the particular application of the device  10 . For example, a near ultraviolet application will motivate the implementation of the npn-phototransistor  112 , whereas a near infrared application will motivate the implementation of the nplus-photodiode  68  or the nwell-photodiode  88 . Both the pnp-phototransistor  98  and the npn-phototransistor  112  offer a gained photocurrent (compared to the photodiodes) but they suffer from long charge carrier base transit times and, accordingly have a slow transient response. Thus, for different applications with different specification concerning photosensitivity and repetition rates, different photodetectors  40 , 42  may be used. 
     Referring now to FIG. 7, it can be seen that an architecture for the device  10  of the present invention provides for the device  10  to be mounted on a printed circuit board  130  and controlled by a computer  132 . Specifically, the computer  132  is connected via an interface  134  with a field programmable gate array (FPGA)  136  which is mounted on the printed circuit board  130 . With this connection digital control signals from the computer  132  and FPGA  136  can be sent directly to the device  10  via a line  138 . Also, digital control signals can be sent from the FPGA  136  via a line  140  to a digital/analog converter  142 , and analog control signals can then be sent to the device  10  via a line  144 . For the present invention, wavefront data collected by the ASIC device  10  is sent via a line  146  to a random access memory (RAM)  148  for subsequent transfer via a line  150  to the FPGA  136 . The data is then sent from the FPGA  136  to the computer  132  where a least square fit for calculation of the mathematical description of the wavefront phase information is performed. 
     While the particular Application Specified Integrated Circuit For Use in Wavefront Detection 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.