WO1995033971A1

WO1995033971A1 - Method and apparatus for acquiring images

Info

Publication number: WO1995033971A1
Application number: PCT/US1995/006075
Authority: WO
Inventors: Eric A. Swanson
Original assignee: Massachusetts Institute Of Technology
Priority date: 1994-06-02
Filing date: 1995-05-16
Publication date: 1995-12-14
Also published as: US5465147A

Abstract

An apparatus and method for acquiring an at least one dimensional digital image of a region of an object (28) using an optical source (12) which outputs a first optical beam having a short coherence length. A splitter (22) splits the first optical beam into a reference beam (30) and an object beam (40). The reference beam travels to a reference scatterer (32) and the object beam (26) is directed toward the region of the object. An array detector (54) such as a charge coupled device receives a portion of the object beam (18a') and a portion of the reference beam (18b') and detects the resulting incident intensity over the at least one dimension and outputs a signal (823). Since the coherence length of the source is short, the signal output from the detector array corresponds to one or more dimensional slice of the object which represents the above region.

Description

METHOD AND APPARATUS FOR ACQUIRING IMAGES

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to an apparatus and method for acquiring an at least one dimensional image of an object and in particular to an apparatus and method for acquiring a series of at least one dimensional images of an object at high rates using a charge coupled device (CCD) array, whereby each at least one dimensional images are captured in a parallel fashion without requiring any transverse scanning.

2. Description of Related Art

There are many industrial, medical, and other applications where one or two dimensional images of an object are required. In addition, these applications often require both high spatial resolution and high longitudinal resolution (less than 10 micrometers) , measurements of distances, thicknesses, and optical properties of the object. The applications can include measurements of biological tissue layers, semiconductors and other applications involving multiple thin layers of material, as well as in the non¬ destructive testing of small structures such as integrated optical circuits, optical connectors, optical couplers, semiconductor lasers and semiconductor optical amplifiers. Such applications also include various medical applications including laser microsurgery, microscopy and diagnostic instrumentation.

Existing techniques for acquiring one or two dimensional images include scanning laser or confocal microscopes and scanning laser ophthalmoscopes (SLO) , provide highly spatially resolved images, for example being able to generate real time video images of the eye with a lateral resolution of a few micrometers. However, the depth resolution of SLOs quickly degrade with decreasing numerical aperture. For example, SLO measurements of the retina through the pupil aperture restrict the depth resolution to roughly 200 microns. SLOs are also expensive, costing in the range of a quarter million dollars.

Optical triangulation offers fairly high resolution, but requires parallel boundaries. Such devices also have relatively poor signal-to-noise ratios and have degraded resolution at greater depths, where numerical aperture is restricted.

Existing techniques for performing such measurements include optical coherence domain reflectometers (OCDR) , optical time domain reflectometry (OTDR) , ultrasound, scanning laser microscopes, scanning confocal microscopes, scanning laser ophthalmoscopes and optical triangulation. Existing OCDR systems do not normally have the rapid data acquisition rate required for the measurement of biological or other samples having the potential for dynamic movement; while OTDR systems are very expensive and have only limited resolution and dynamic range.

Ultrasound, which is perhaps the most commonly used technique, is disadvantageous for applications such as taking measurements on the eye in that, in order to achieve the required acoustic impedance matches, and to thus avoid beam losses and distortion, contact is generally required between the ultrasonic head or probe and the product or patient being scanned. While such contact is not a problem when scans are being performed on, for example, a patient's chest, such probes can cause severe discomfort to a patient when used for taking eye measurements such as those used for measuring intraocular distances for computing the power of lens implants. The relatively long wavelengths employed in ultrasound also limit spatial resolution. Further, ultrasound depends on varying ultrasound reflection and absorption characteristics to differentiate and permit recording or display of tissue, or other boundaries of interest. Therefore, when the acoustic characteristics of adjacent layers to be measured are not significantly different, ultrasound may have difficulty in recognizing suc boundaries.

A need, therefore, exists for an improved method an apparatus for performing high resolution measurements and i particular for optically performing such measurements, whic improved technique does not require contact with the bod being measured, which maintains substantially constant hig resolution over a scanning depth of interest, regardless o available apertures size and which is relatively compact an inexpensive to manufacture. Such a system should also b capable of providing differentiation between sample layer and be able to provide identification of layer material o of selected properties thereof. Such a system should als be able to provide one, two and three-dimensional images o a scanned body and should be rapid enough for use i biological and other applications where the sample bein measured changes over relatively short time intervals Finally, it would be desirable if such techniques could als provide information concerning the birefringence property an spectral properties of the sample.

U.S. Patent Application Ser. No. 08/033,194 relates t optical measuring systems which can perform high resolutio measurements and provide the above advantages. I particular, these systems can perform such measurement without contacting the object or body being measured. Th systems maintain substantially constant high resolution an are relatively compact and inexpensive to manufacture. Suc systems are also capable of providing differentiation betwee sample layers, identification of layer material or o selected properties thereof. The systems also provid measurements at rapid enough rates for use in biological an other applications where the sample being measured change over relatively short time intervals. In fact, they can eve provide information concerning the birefringence property an spectral properties of the sample. Figure 1 shows an object 28 being scanned by an optical beam. In particular, a first beam 101 is incident on a scanning mirror 71 which reflects beam 101 as beam 103 which is incident on object 28 at region A. Mirror 71 rotates about pivot P in a controlled manner by some type of rotating driver (not shown) which causes mirror 71 to move to a new position represented by reference numeral 71'. As mirror 71 moves to its new position, beam 103 scans across object 28 in the transverse direction to region B. As beam 103 passes across object 28, object 28 scatters the radiation back towards some detection unit (not shown) which detects that scattered radiation coherently using radiation from a reference arm (not shown) .

The detection unit for capturing a one or two dimensional image of object 28 involves capturing a series of values corresponding to a series of intensity measurements. This series of intensity measurements in turn correspond to a series of regions of object 28 transversely scanned by beam 103 across object 28. Hence, this approach to acquiring measurements across a transverse direction of an object is essentially a serial image capturing process. Consequently, the rate of image capture by such a serial system may be limited by the rate at which individual measurements are acquired and hence at the rate beam 103 is scanned across object 28. This holds regardless of whether or not beam 103 is scanned using a mechanical scanner such as mirror 71 or some other non-mechanical scanning mechanism such as acousto-optic devices. In addition, if a second scanning means such as a second mirror is added to enable scanning in a second transverse direction, the rate of acquisition of resulting two dimensional images is even further reduced.

In some applications, the power of radiation incident on a unit area is fixed, e.g., optical measurements of the human eye, illuminating a human retina. In such cases, simultaneous or parallel illumination and acquisition of information represents a more efficient approach t performing optical measurements than any transverse scannin approach.

Hence it is further desirable to perform a longitudina scan through an object while acquiring one or two dimensiona slices in a completely parallel fashion as the longitudina scan proceeds. It is desirable to acquire these one or tw dimensional images without the use of transverse scannin mirrors which can slow down the rate of image capture an introduce noise which reduce system sensitivity an resolution.

SUMMARY OF THE INVENTION An object of the invention, therefore, is to provide a apparatus and method of acquiring images of an object in a parallel manner without any transverse scanning across the object.

Another object of the invention is to provide an apparatus and method which can acquire one or two dimensional images at a high rate. Another object of the invention is to provide an apparatus and method of acquiring a series of two dimensional images along a third direction.

Another object of the invention is to provide an apparatus and method for acquiring one or two dimensional transverse images of an object without having to scan a beam across the object.

Another object of the invention is to provide a relatively inexpensive apparatus and method which can acquire multi-dimensional images. One advantage of the invention is that it detects a one or two dimensional slice or region of an object without scanning the target radiation over the object.

Another advantage of the invention is that it can output two dimensional images of the object at high rates. Another advantage of the invention is that it can scan and output a series of two dimensional images of the object along a third direction of the object.

Another advantage of the invention is that it is not limited to capturing regions of the object in the form of a straight line or a flat plane but instead can capture curved lines or curved two dimensional regions of the object.

One feature of the invention is that it uses a source for outputting radiation having a short coherence length. Another feature of the invention is that it employs heterodyne detection over two dimensions.

Another feature of the invention is that it uses a two dimensional array detector such as a charge coupled device.

Another feature of the invention is that it employs heterodyne detection over the two dimensions of the charge coupled device.

Another feature of the invention is that the charge couple device outputs signals corresponding to two dimensional slices of the object. These and other objects, advantages and features are accomplished by an apparatus for acquiring an at least one dimensional image of a region of an object without any transverse scanning, including: a source for outputting radiation having a short coherence length; means for splitting the radiation into reference radiation and object radiation; means for receiving the object radiation and directing the object radiation toward the region of the object; means for receiving the reference radiation and directing the reference radiation through a reference path; and array detecting means for receiving a portion of the object radiation scattered off of the region and a portion of the reference radiation, and for detecting incident intensity in a parallel fashion over the at least one dimension resulting from the portion of the reference radiation coherently interfering with the portion of the object radiation scattered from the region and for outputting a signal corresponding to the incident intensity, wherein t portion of the reference radiation requires a reference del time to travel from the means for splitting to the arr detecting means and the portion of the object radiati requires an object delay time to travel from the means f splitting to the region and then to the array detecti means.

The above objects, advantages and features are furth accomplished by the provision of the above apparatus, where the array detecting means comprises a two dimensional arr detecting means for receiving the portion of the obje radiation and the portion of the reference radiation, f detecting the incident intensity over two dimensio resulting from the portion of the reference radiati coherently interfering with the portion of the obje radiation scattered from the region and for outputting t signal corresponding to the incident intensity.

The above objects, advantages and features are al accomplished by an apparatus for acquiring a two dimension image of a region of an object, comprising: an optic source for outputting a first optical beam having a sho coherence length; means for splitting the first optical be into a reference beam and an object beam; means for receivi the object beam and directing the object beam toward t region of the object and for collecting a portion of t object beam scattered off of the region; means for receivi the reference beam and directing the reference beam throu a reference path; and a two dimensional array detecting mea for receiving a portion of the object beam and a portion the reference beam, for detecting incident intensity over t dimensions resulting from the portion of the reference be coherently interfering with the portion of the object be scattered from the region and for outputting a signa corresponding to the incident intensity, wherein the porti of the reference beam requires a reference delay time travel from the means for splitting to the two dimensiona array detecting means and the portion of the object beam requires an object delay time to travel from the means for splitting to the region to the two dimensional array detecting means. The above and other objects, advantages and features are alternatively accomplished by the provision of a method for acquiring a two dimensional image of a region of an object, comprising the steps of: outputting radiation having a short coherence length; splitting the radiation using a splitter into reference radiation and object radiation; receiving, using an optical guide, the object radiation and directing the object radiation toward the region of the object and for collecting a portion of the object radiation scattered off of the region; receiving the reference radiation and directing the reference radiation through a reference path; receiving a portion of the object radiation and a portion of the reference radiation and detecting incident intensity over two dimensions using a two dimensional array detector resulting from the portion of the reference radiation coherently interfering with the portion of the object radiation scattered from the region; and outputting a signal corresponding to the incident intensity, wherein the portion of the reference radiation requires a reference delay time to travel from the splitter to the two dimensional array detector and the portion of object radiation requires an object delay time to travel from the splitter to the region and back to the two dimensional array detector.

The above and other objects, advantages and features of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an object being transversely scanned by a beam according to previous methods. Figure 2 shows an imaging system according to on embodiment of the invention.

Figure 3 is a simplified version of the optical path within the system of Figure 2. Figure 4A shows one example of an intermediate frequenc signal resulting from coherently detecting an interferenc pattern produced by overlapping radiation scattered from th reference path and the object path and Figure 4B shows thi same output portion after demodulation. Figure 5 corresponds to Figure 3 with spatially expande radiation 17a, 17a', 17b, 17b', 18a, 18b' as shown in Figur 2.

Figures 6A, 6B and 6C show a front view of the referenc scatterer, a region in the object, and the detecting surfac of the detector array, respectively.

Figure 7 shows an embodiment in which the reference uni of Figure 2 has been replaced by an alternate reference uni and, in particular, the scatterer of Figure 2 has bee replaced with a curved scatterer. Figure 8 shows a more detailed schematic representatio of image detection and processing unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 2 shows an imaging system 10 according to on embodiment of the invention. In particular, a source 1 outputs radiation with a short coherence length CL output radiation 14 which is collimated by collimating lens 15 int collimated radiation 16 which travels toward a splitter 22. A short coherence length CL is a coherence length which i less than the desired longitudinal resolution (Δz' in Figur 2) . Collimated radiation 16 can be made elliptical using a anamorphic beam expander or cylindrical lens (i.e., lens 1 can be an anamorphic beam expander or a cylindrical lens) o it can be made more circular (i.e., lens 15 can be a len with approximately equal power in all transverse directions) . A first portion of radiation 16 referred to here as reference radiation 17a is directed into a reference arm 30 by splitter 22 and a second portion of radiation 16 referred to here as image radiation 17b passes into an image arm 26. Reference radiation 17a travels to a reference unit 31 which includes a reference scatterer 32 and a scanning mechanism 39. A portion of reference radiation 17a is scattered back from reference scatterer 32 toward splitter 22 as radiation 17a'. Image radiation 17b travels to object or sample 28 through optics 81 which can be one or more lenses, a beam expander, a beam contractor or .even possibly free space depending on the desired size of beam 17b incident on object 28. A portion of image radiation 17b is scattered back toward splitter 22 as radiation 17b' . A portion of radiation 17a' from reference arm 30 passes through splitter 22 toward a charge coupled device (CCD) array detector 54 as radiation 18a'. The shape of CCD array detector 54 can be made to match that of the collimated radiation. For example, CCD array detector 54 can be a one dimensional CCD array if radiation 16 is elliptical and a two dimensional detector array if radiation 16 is more cylindrical . Likewise, a portion of radiation 17b' from image arm 26 is redirected by splitter 22 toward detector 54 as radiation 18b' .

The path length from splitter 22 to reference scatterer 32 back to splitter 22 and then through to detector 54 will be referred to as the reference path length L_R and the time for radiation 16 to travel the reference path length will be referred to as the reference delay time T_R. Similarly, the path length from splitter 22 to object 28 back to splitter 22 and then to detector 54 is the object path L₀ and the time for radiation 16 to travel the object path length is the object delay time T₀.

The scattered radiation 18b' and 18a' received from object 28 and scatterer 32, respectively, are combined at splitter 22, resulting in interference fringes for length matched reflections (i.e., reflections for which the difference in reflection path lengths is less than the sourc coherence length) and the resulting combined output i transmitted to optics 50 which can be used to expand, compress or otherwise fill CCD detector 54. Hence, optic 50 can include a beam compressor, beam expander or othe optical elements known in the art. Optics 50 then compresse or expands radiation 18a' and 18b' and outputs that radiatio to an image detection and processing unit 52. Imag detection and processing unit 52 includes a charge couple device (CCD) 54 which detects the intensity pattern fro radiation 18a' and radiation 18b'. This intensity patter will include coherent interference signals, i.e., homodyne or heterodyne detection can be achieved, provided the absolute value of the difference between path length _R an LQ is approximately within the coherence length (CL) of source 12. Image detection and processing unit will be described in more detail with reference to Figure 8 below. A computer 57 allows a user to control the process of image acquisition performed by image detection and processing unit 52 and displays of resulting images on monitor 59 or stores them on a hard disk (not shown) or some other digital storage device.

Figure 3 is a simplified version of the optical paths within system 10 using a single ray of radiation 17a, 17a', 17b, 17b', 18a', 18b' of Figure 2. In particular, the path length from splitter 22 to reference scatterer 32 is defined as dl, the path length from splitter 22 to a region 202 to be imaged is defined as d2 and the path length from splitter 22 to CCD detector 54 is defined as d3. Hence, the reference path length L_R discussed above is 2dl + d3 and the object path length L_Q is 2d2 + d3. Hence, since the absolute value of the difference between L_R and LQ = |2(d2-dl)|, if this value is less than or equal to CL, coherent detection occurs.

Coherent or Heterodyne Detection for a single Ray When scatterer 32 moves at a velocity V, a Doppler shift frequency having a frequency f_D = 2V/λ occurs, where λ is the nominal wavelength of the radiation output by source 12. When reference radiation 17a' is shifted by this frequency f_D (due to the motion of scatterer 32) and resulting radiation 18a' spatially overlaps object radiation 18b' which did not undergo such a Doppler shift, then the intensity will vary at a "beat" or intermediate frequency equal to f_D. This intermediate frequency signal will only occur, however, if radiation 18a' and 18b' coherently interfere with each other which in turn only occurs if the absolute value of the path difference |L_R - L₀| is less than the coherence length CL of source 12 as discussed above. Hence, the above conditions are satisfied, detector 54 can detect and output this intermediate frequency signal. Moreover, when the velocity V is large enough, the resulting Doppler shift f_D will be higher than the predominant low frequency (1/f type) noise spectrum. For an 830 nm wavelength output from source 12, which might be a typical source wavelength, this occurs for a velocity V above approximately 1mm/sec.

Longitudinal Scanning for a Single Ray Longitudinal scanning, i.e., scanning in the z direction of object 28, is also achieved as a result of the motion of scatterer 32. Referring again to Figure 3, path lengths L_R and LQ are initially equal with the beam at a desired initial scan depth in object 28. As scatterer moves away from splitter 22, the point in object 28 at which the path lengths are equal is scanned to successively greater depths within object 28. At each point in the scan, radiation scattering occurs and is a function of optical characteristics of object 28 such as refractive index variations, as well as absorption, scattering and spectral characteristics of object 28 through which radiation 17b' is passing.

Figure 4A shows one example of an intermediate frequency signal output by detector 54 for a single scatterer at given depth point (see Figure 3) in object 28, where the absolute value of the difference between the object path length L₀ and the reference path length L_R is less than the coheren length (CL) of the light source (i.e., |L_Q - L_R| < CL) . T intermediate frequency signal appears for a time T which the absolute value of the difference between the referen delay time T_R and the object delay time T₀, i.e., |T_R - T₀| Note that the intermediate frequency signal shown in 4A al has an envelope signal superimposed thereon as discussed the parent to this application. This envelope signal occu because as the reference length L_R varies due to the moti of scatterer 32, |L_R - Lo| is scanned through the coheren length CL of source. Consequently, the intermedia frequency signal disappears while |L_R - L_o| sweeps by mo than the coherence length CL beyond the depth point. Figu 4B shows this same output portion after demodulation. variety of demodulation approaches are discussed in U. Patent Application 07/875,670 which is one of the pare applications to this application.

The above discussion showed that the coherence leng CL of source 12 determines available system resolution (Δz' in the z direction and the intermediate frequency output fr detector 54 is indicative of scattering obtained at particular depth z within object 28. Consequentl successive interferometric outputs obtained during a sc form profiles of the optical characteristics of object 2 The profiles can be index, scattering (even microstructu scattering) , absorption or refractive index profiles with the samples where scattering is normally maximum and may ha some lesser peaks in a predetermined pattern, depending the scattering characteristics of the medium at the sc depth.

In view of the above, it is apparent that is not ju the peak wavelength of source 12 which determines the syst longitudinal resolution Δz', but rather the coherence leng of source 12 as successive depth points are longitudinal scanned as discussed in the parent to this applicatio Hence, the higher the desired longitudinal resolution t lower the coherence length CL of source 12. Moreover, the lower the coherence length CL of source 12 the wider the spectral width of radiation output by that source. In addition, the spectral width of many sources with longer coherence lengths such as lasers and light emitting diodes can also be modified using mechanical, electrical modulation and other known techniques.

Parallel Image Capture Figure 5 is a simplified depiction of a parallel image capture system which corresponds to Figure 3 with spatially expanded radiation 17a, 17a',17b, 17b', 18a', 18b' as shown in Figure 2. Here, however, four rays, ray 1, ray 2, ray 3 and ray 4 of radiation 16 are shown. Figure 5 is referred to as simplified, because it depicts rays 1-4 with no crossing or overlapping which in practice occurs to some degree. Again, these rays can be in one or two transverse directions. The path lengths from splitter 22 to reference scatterer 32 are defined as dl(x_;, y,, z,) , where i=l, 2, 3 and 4 for rays ray 1, ray 2, ray 3 and ray 4, respectively. Similarly, the path lengths from splitter 22 to a region 202 to be imaged are defined as d2(x , y , z,') and the path lengths from splitter 22 to CCD detector 54 are defined as d3(X_j ^M, y_j", z,") , where i=l, 2, 3 and 4 for ray 1, ray 2, ray 3 and ray 4, respectively. Hence, the reference path length L_R discussed above is 2dl(Xj, y_t, z,) + d3(x_;, y_t, z,) and the object path length L_Q is 2d2(X;', y,', z,') + d3(x_if y,, z,) where i=l, 2, 3 and 4 for ray 1, ray 2, ray 3 and ray 4, respectively. For example, coherent detection (and consequently intermediate frequency f_D) for a particular ray- -say ray 1—only occurs when the absolute value of the optical path difference Lκ(xi,yj,zk) - I__o(xi', yj', zk') for that ray is within the coherence length CL of source 12. Moreover, this must be true for each ray of radiation 18a' and 18b'. Figures 6A, 6B and 6C show a front view of referenc scatterer 32, a region 202 in object 28 and detector surfac 302 of detector array 54, respectively. The front views ar shown in two transverse directions, it being understood tha when radiation 16 is elliptical, only one dimension need b shown. In the discussion that follows, radiation in tw transverse directions is discussed, however, all aspects o image acquisition in one dimension is analogous to the belo discussion of two directions. In addition, the followin discussion represents a simplified discussion of the primar processes which take place in parallel image capture, i being understood that there will be crosstalk between ray in the reference or object paths.

Figure 6A shows ray 1, ray 2, ray 3 and ray 4 which ar incident on scatterer 32 at locations (xl, yl, zl), (x2, yl, zl) , (x3, yl, zl) and (x4, yl, zl) , respectively. Th longitudinal position of scatterer 32 is determined by th coordinate z and hence zl, z2 and z3 represent thre different longitudinal positions of reference scatterer 32. As scatterer 32 moves toward and away from splitter 22, th longitudinal position z of scatterer 32 changes but th transverse coordinates (xi, yj) of rays 1-4 does no necessarily have to vary.

Figure 6B shows rays 1, 2, 3, and 4 which are inciden on region 202 of object 28 at locations (xl', yl', zl'), (x2', yl', zl'), (x3', yl', zl') and (x4', yl', zl'), respectively. The longitudinal position of region 202 i determined by the coordinate z' and hence zl'. z2' and z3' represent three different longitudinal positions of regio 202. Moreover, the longitudinal positions z' of region 20 being imaged, changes from zl', z2' and z3', as scatterer 3 changes from zl, z2, and z3, respectively.

Figure 6C shows rays 1, 2, 3 and 4 which are inciden on scatterer 32 on detection elements 302 at location (jel",yl"), (x2",yl"), (x3,yl") and (x4",yl") respectively. The number of subregions 302 can range from a few to severa thousand depending on the desired spatial resolution Δx' and Δy' of regions 1, 2, 3 in object 28. I(x",y") is the intensity of radiation incident on element 302 at location (x",y"). Moreover, if no ray is scattered from a particular subregion of object 28 no intermediate frequency signal can appear at the element 302 corresponding to that ray. For example, suppose there is no radiation 17b' from ray 1 scattered back from location (xl',yl',zl') of object 28, then there will be no intermediate frequency signal I(xl",yl") detected at the element 302 at location (xl",yl"). In addition, if an intermediate frequency signal does appear at element 302 at location (xl",yl"), then the amplitude of the demodulated signal (see Figure 4B) provides an indication of the magnitude of the amount of radiation 17b' scattered from location (xl',yl',zl') as a result of ray 1 incident on that subregion. In this manner, a one or two dimensional image of region 1 (a longitudinal slice of object 28) can be acquired in a parallel manner.

Simultaneous capture of an image as discussed above requires that system 10 be properly aligned. This means that a significant portion of radiation 18a' from reference scatterer 32 physically overlaps with radiation 18b' from object 28 to insure that radiation 18a' can interfere with 18b' at CCD detector 54. Note that the cross-sectional view of radiation incident on array surface 302 need not necessarily be the same size as the transverse dimensions of radiation 17a or 17b, because optics 50 can be a beam compressor or beam expander depending whether the transverse dimensions of radiation 18a' and 18b' are to be compressed or expanded before reaching array surface 302.

Longitudinal two dimensional slices are captured in a parallel manner as follows. Referring again to Figures 6A- 6C, the longitudinal position of scatterer 32 is determined by the coordinate z and hence zl, z2 and z3 represent three different planar longitudinal positions of reference scatterer 32. Similarly, object radiation 17b is incident on object 28 at planar regions 1, 2 and 3 which is a plane determined by longitudinal coordinates zl', z2' and z3', respectively. Now interference fringes occur for dept points in the sample where the absolute value of the difference between the object path length from splitter 22 for a point (xi',yj') in object 28, L₀(xi',yj'), and the reference path length for a corresponding point (xi^,,yj') on reference scatterer 32, L_R(xi,yj) , is less than the coherence length (CL) of the light source (i.e., |L_o(xi',yj') L_R( i,yj) I < CL) . Therefore, the coherence length of the light source determines available system resolution in the z direction.

Suppose path lengths L_R(xi,yj,zk) and Lo(xi',yj',zk') for all rays are initially equal at a desired initial scan depth z' in object 28. As scatterer 32 moves away from splitter 22, an entire plane (or line if radiation 16 is elliptical) in object 28 at which the path lengths are equal is scanned to successively greater depths within object 28. At each plane in the scan, radiation scattering occurs and is a function of the refractive index variation for the material through which the radiation is passing and of such index boundaries at each subregion (xi',yj'). The longitudinal scan involved scanning a plane (or line if only one transverse is being used) through object 28. This longitudinal scan is not, however, limited to a plane, but instead can be any two dimensional shape (or one dimensional curve) and it is the shape of scatterer 32 that determines the shape of regions longitudinally scanned through object 28 as will be discussed below.

Coherent or Heterodyne Detection

The heterodyne detection process will now be discussed when subregions (x,y) all move at a single velocity V. When scatterer 32 and its subregions move at a velocity V, a Doppler shift frequency having a frequency of f_D = 2V/λ occurs, where V is the velocity at which scatterer 32 is moved and λ is the wavelength of source 12. The Doppler shift frequency f_D, is superimposed on the envelope signal as shown for a small portion of an intensity output in Figure 4A. The envelope signal shown in Figure 4A will be present at every detecting surface 302 (see Figure 6C) which receives object scattered radiation 18b' from a corresponding subregion (x,y) on object 28. Figure 4B shows this same output portion after demodulation. Again, if the velocity V is large enough, the resulting Doppler shift frequency will be above the 1/f type noise spectrum of the system.

Uniform Longitudinal Scanning Reference scatterer 32 is secured to mechanism 39 (Figure 2) which moves the scatterer 32 toward and away from splitter 22 in a particular pattern. Referring to the embodiment of Figure 2, mechanism 39 moves scatterer 32 away from splitter 22 at a uniform, relatively high velocity V. The desired value of velocity V depends on the wavelength of radiation output by source 12. Regardless of the value of velocity V, it is desirable that all subregions (xi,yj) on scatterer 32 move at a single velocity V to ensure that each ray scattered by each subregion (xi',yj') is Doppler shifted by the same frequency f_D. In that case, each pixel will have the same intermediate frequency signal (provided object radiation is received from object 28) indicative of the optical characteristics of subregion (xi',yj'). Moreover, each of the subregions (xi^,,yj') that does have such an intermediate frequency signal can be demodulated in a manner analogous to that discussed above with respect to a single ray system. However, in order to achieve a single intermediate frequency Doppler shift, all subregions (xi,yj) of scatterer 32 must move at a single velocity. This can be achieved in several ways.

In one embodiment, when scatterer 32 reaches the far end of its travel path, scatterer 32 is rapidly returned to the initial position, the scan having a generally ramp or sawtooth profile, with measurements being taken on the ramp. Mechanism 39 may also return scatterer 32 to its initia position at substantially the same rate V, movements of th mirror thus being in a triangular pattern. With a triangula scan, readings or measurements can be taken with scattere 32 moving in either one of the two directions, or can b taken with scatterer 32 moving in both directions. Mechanis 39 may be any one of a variety of devices adapted fo performing the mirror translation function. For example, mechanism 39 could be a stepper motor, the motion of whic is applied to scatterer 32 through"an averaging mechanism t provide uniform velocity. A DC servo motor might also b utilized to obtain the desired motion. Variou electromagnetic actuators, for example, a speaker coil, ma also be utilized for moving scatterer 32. With suc electromagnetic actuators, detection of scatterer positio and servo-control thereof are also required in order t achieve the desired uniform motion. More specifically, i such a system, a signal indicative of desired scattere position at each point in the scatterer travel path would b compared against a signal from a detector of actual scattere position and any resulting error signals utilized to control the actuator to maintain scatterer 32 moving at the desire constant velocity. It would also be possible to use a servo- controlled galvanometer driven linear translator fo mechanism 39. One potential problem is that when scattere 32 is being translated at high speed by mechanism 39, it is nearly impossible to completely eliminate all wobbling of scatterer 32 which may adversely affect the accuracy of distance determinations. Various mechanisms may be utilize to correct for such wobble so that more of radiation 17a is scattered back as radiation 17a' toward splitter 22.

In the above discussion relating to system 10 in Figur 2, reference unit 31 and in particular mechanism 39 wit scatterer 32, can be modified to provide a series of tw dimensional images in the transverse direction (or one transverse dimensional images if radiation 16 is elliptical at a single depth point z. One way to accomplish this is i mechanism 39 vibrates or dithers scatterer 32 back and fort about a single point zl at a frequency f . It should b noted that the vibrating frequency f_m should be selected t be great enough to ensure that the resulting intermediat frequency signal is above the 1/f type noise floor of syste 10 and sufficient to prevent aliasing. In addition t dithering, one or more acousto-optic modulators can be use to impose the desired optical frequency offset between th reference and object beam.

As previously discussed, an intermediate frequenc signal output from a particular element 302 will be presen only if radiation 17b' is scattered from a particula subregion (x',y')« Hence, a two dimensional image can b created by determining which elements 302 at (xi",yj") outpu an intermediate frequency signal and hence which subregion (xi',yj') scatter radiation 17b'. Moreover, if region zl physically varies with time so that subregions (xi',yj') ma scatter radiation 17b' back towards splitter 22 during on data acquisition time and not scatter radiation 17b' at later data acquisition time, the resulting two dimensiona image will itself change with time. Hence, system 10 use in this mode provides a two dimensional real time monitorin system.

In the above discussion, an additional frequency shif of either radiation 17a, 17a' and/or radiation 18a, 18a' ca be introduced by adding modulating units in either path 2 or path 30. In addition, suppose that radiation in path 2 or 30 is shifted by a modulation frequency F_s where F_s » f_m The resulting intermediate frequency signal output at eac element 302 will be a signal having a carrier frequency F and a frequency modulated by a signal proportional t cos(f_mt). If F_s is greater than the noise floor of system scatterer 32 need not be scanned at high frequencies f_m an still be well above the noise floor. Figure 7 shows an embodiment in which reference unit 3 of Figure 2 has been replaced by reference unit 830 and, i particular, scatterer 32 has been replaced with a curve scatterer 832. Here, curved scatterer 832 is moved towar and away from splitter 22 as before. However, this motio translates into longitudinally scanning curved regions 1, 2 3 through object 28. Any other shaped scatterer can be use as scatterer 32 and the shape of that scatter translates int correspondingly shaped regions in object. Consequently, thi control over the shape of regions scanned in object 28 ca be tailored to the specific application. For example, if 2 represents an human eye, it may be desirable to form scannin regions in the eye with approximately the same curvature a the cornea or lenses in the eye. Also, reflector 832 can b a deformable mirror such as a metal plated piece of rubbe the exact shape being either mechanically or electricall controllable, the latter making it possible to modify th shape of the region longitudinally scanned in real time.

Figure 8 shows a more detailed schematic representatio of image detection and processing unit 52. In particular, image detection and processing unit 52 includes CCD array 5 followed by front end electronics 801. In the preferre embodiment, CCD detector 54 itself includes an imaging are 804, a frame store area 809 and CCD interface unit 815. Front end electronics 801 includes analog-to-digita converters 821, logic unit 832', first-in-first-out (FIFO) unit 841 and digital signal processing unit 851. A syste clock 861 provides a synchronizing clock signal t synchronize CCD array 54 with front end electronics 801. Commercially available CCDs can be used as CCD array 5 and preferably CCD 54 should have a fast frame rate, hig quantum efficiency, frame transfer and multiple readouts. A/D converters can also be standard, commercially availabl A/D converters, preferably having a several MHz conversio rates and at least 12 bit conversion. DSP 851 can be fro Texas Instrument's Model TMS 3200 family. Image detection and processing unit 52 operates as follows. CCD array 54 integrates an intensity pattern on imaging area 804 made of elements 803. Once the image is captured, it is mapped to frame store area 809 to be transferred out of CCD array 54 via interface unit 815. Frame store area 809 is used in order that imaging area 804 can be freed up to capture a new image while frame store area 809 transmits the previous image via interface unit 815 to front end electronics 801. In a preferred embodiment, frame store area 809 has multiple output lines 811 (four such lines are shown but more of fewer are possible) to interface unit 815 which includes buffer amplifiers, double correlated sampling and filtering electronics none of which are shown in Figure 8 but are known in the art. The resulting signals are then transmitted via lines 823 to analog-to-digital converter 821 which includes parallel analog-to-digital converters 821. Since the rate at which images are transmitted from CCD array 54 largely determines the rate at which images can be acquired, multiple lines 811 and 823 are used to transmit signals from frame store area 809 to front end electronics 801. Consequently, the more output lines frame store area 809 has, the higher the rate at which frames of signals are captured by CCD array 54.

The signals output on lines 823 are respectively analog- to-digitally converted into digital signals which in turn are output to logic unit 832' via respective digital lines 833. Logic unit 832' which rearranges the digital signals into a serial form outputs the data to FIFO 841 via line 843. The digital signals received by FIFO 841 are then output via line 853 to digital signal processor (DSP) 851 in the order they were output from frame store area 809.

Clock 861 synchronizes the rate at which CCD 54 captures and outputs images as well as the rates A/D converters 821 digitize the signals received from CCD 54 as well as the rate at which logic unit 832' filters and outputs data to FIFO 841. The data is then output in the order it was receive to DSP 851 which processes the data as follows.

In a most basic implementation, detection unit 5 performs digital signal processing on the received images i a manner analogous to that discussed in the paren applications to this application. In particular, fram capture time is chosen so that there are more than tw samples captured per intermediate frequency cycle, which a discussed above, can result from Doppler induced frequenc shift, acousto-optic frequency shift, etc, ... . By samplin more than twice per intermediate frequency cycle, aliasin can be avoided. In the case of uniform velocity scanning the scanner sweeps the measurement window longitudinall through sample region 202. During this time, a series o samples are recorded for each pixel in Figure 6C. Thus, three dimensional data set is captured indicating the optica profiles of the object.

There are numerous real time and non-real time method to extract information about the optical properties of th sample. In one method, DSP 851 processes sequential sample from the same pixel by digitally band pass filtering aroun the center frequency followed by digitally rectifying and lo pass filtering to remove double frequency terms. Thus, DS 851 performs a parallel envelope detection operation on th CCD output. Additional filtering can be used to extrac and/or compare information (such as cross pixel scattering) from nearby pixels to further increase the utility of th measurements. The processing can involve multipl measurements and averaging to further increase sensitivity. Since the processing is performed digitally, additiona information such as phase information can be extracted and/o utilized to improve sensitivity (e.g., improving resolutio of the system) as well as provide other useful information. All of the above processing can be done in real time or th digital samples can be stored and then processed later i computer 57.

Claims

What is claimed is:

1. An apparatus for acquiring an at least on dimensional image of a region of an object without an transverse scanning, comprising: a source for outputting radiation having a shor coherence length; means for splitting said radiation into referenc radiation and object radiation; means for receiving said object radiation and directin said object radiation toward the region of the object; means for receiving said reference radiation an directing said reference radiation through a reference path; and array detecting means for receiving a portion of sai object radiation scattered off of said region and a portion of said reference radiation, and for detecting incident intensity in a parallel fashion over said at least one dimension resulting from said portion of said reference radiation coherently interfering with said portion of the object radiation scattered from said region and for outputting a signal corresponding to said incident intensity, wherein said portion of the reference radiation requires a reference delay time to travel from said means for splitting to said array detecting means and said portion of the object radiation requires an object delay time to travel from said means for splitting to said region and then to said array detecting means.

2. The apparatus as claimed in claim 1, wherein said array detecting means comprises a two dimensional array detecting means for receiving said portion of the object radiation and said portion of the reference radiation, for detecting said incident intensity over two dimensions resulting from said portion of said reference radiation coherently interfering with said portion of the object radiation scattered from said region and for outputting sai signal corresponding to said incident intensity.

3. The apparatus as claimed in claim 2, wherein sai two dimensional array detecting means comprises a charg coupled device.

4. The apparatus as claimed in claim 2, furthe comprising image processing means for receiving said signa and outputting a two dimensional image corresponding to sai region.

5. The apparatus as claimed in claim 2, wherein sai two dimensional array detecting means comprises a charg coupled device including a plurality of active detectin areas arranged in rows and columns and a plurality of storag areas also arranged in rows and columns, wherein sai plurality of active detecting areas detect said inciden intensity and output a first signal to said plurality o storage areas and said plurality of storage areas output sai first signal.

6. The apparatus as claimed in claim 5, furthe comprising a series of analog to digital converter respectively coupled to said rows of the storage areas fo receiving and converting said first signal in parallel an outputting corresponding parallel groups of digital data.

7. The apparatus as claimed in claim 6, furthe comprising a logic unit coupled to said series of analog t digital converters for receiving, filtering and the outputting said groups of digital data as filtered digita data.

8. The apparatus as claimed in claim 7, furthe comprising a first-in-first-out unit having an input couple to said logic unit for receiving and storing said filtered digital data and periodically outputting said filtered digital data in block units.

9. The apparatus as claimed in claim 8, further comprising a digital signal processor coupled to said first- in-first-out unit, for receiving said block units and arranging said unit to be displayed as an image.

10. The apparatus as claimed in claim 1, wherein said means for receiving said reference radiation comprises reference path length varying means for varying said reference path, thereby causing said portion of reference radiation to undergo a plurality of reference delay times such that said array detecting means detects a plurality of incident intensities over said at least one dimension and outputs a plurality of signals corresponding to a plurality of regions of said object having a respective plurality of object delay times corresponding to said plurality of reference delay times, respectively.

11. The apparatus as claimed in claim 10, further comprising image processing means for receiving said plurality of signals and outputting a respective plurality of two dimensional images corresponding to said plurality of signals.

12. The apparatus as claimed in claim 11, further comprising three dimensional image processing means for receiving said plurality of signals and outputting a three dimensional image corresponding to said plurality of signals.

13. The apparatus as claimed in claim 12, further comprising a digital signal processing means for receiving said plurality of signals and outputting two dimensional digital images corresponding to said region.

14. The apparatus as claimed in claim 1, wherein sai source outputs said radiation having a coherence length o less than a desired longitudinal resolution.

15. The apparatus as claimed in claim 1, wherein sai source outputs said radiation having a peak wavelength in th near infrared spectrum.

16. The apparatus as claimed in claim 1, wherein sai source outputs said radiation having a peak wavelength in th near infrared spectrum and a coherence length of less tha approximately 10 micrometers.

17. An apparatus for acquiring a two dimensional imag of a region of an object, comprising: an optical source for outputting a first optical bea having a short coherence length; means for splitting said first optical beam into reference beam and an object beam; means for receiving said object beam and directing sai object beam toward the region of the object and fo collecting a portion of said object beam scattered off o said region; means for receiving said reference beam and directin said reference beam through a reference path; and a two dimensional array detecting means for receivin a portion of said object beam and a portion of said referenc beam, for detecting incident intensity over two dimension resulting from said portion of said reference beam coherentl interfering with said portion of the object beam scattere from said region and for outputting a signal correspondin to said incident intensity, wherein said portion of th reference beam requires a reference delay time to travel fro said means for splitting to said two dimensional arra detecting means and said portion of the object beam require an object delay time to travel from said means for splitting to said region to said two dimensional array detecting means.

18. The apparatus as claimed in claim 17, further comprising digital image processing means for receiving said signal and outputting a two dimensional digital image corresponding to said region.

19. The apparatus as claimed in claim 17, wherein said two dimensional array detecting means comprises a charge coupled device.

20. The apparatus as claimed in claim 1, further comprising digital image processing means for receiving said signal and outputting a two dimensional digital image corresponding to said region.

21. The apparatus as claimed in claim 1, wherein said two dimensional array detecting means comprises a charge coupled device including a plurality of active detecting areas arranged in rows and columns and a plurality of storage areas also arranged in rows and columns, wherein said plurality of active detecting areas detect said incident intensity and output a first signal to said plurality of storage areas and said plurality of storage areas output said first signal.

22. The apparatus as claimed in claim 17, wherein said means for receiving said object beam comprises reference path length varying means for varying said reference path thereby causing said portion of reference radiation to undergo a plurality of reference delay times such that said array detecting means detects a plurality of incident intensities over said at least one dimension and outputs a plurality of signals corresponding to a plurality of at least one dimensional regions of said object having a respective plurality of object delay times corresponding to sa plurality of reference delay times, respectively.

23. The apparatus as claimed in claim 22, furth comprising image processing means for receiving sa plurality of signals and outputting a respective plurali of two dimensional images corresponding to said plurality signals.

24. The apparatus as claimed in claim 23, furth comprising digital image processing means for receiving sa plurality of signals and outputting a plurality of t dimensional digital images corresponding to said plurali of signals, respectively.

25. The apparatus as claimed in claim 1, wherein sa means for receiving said reference radiation and directi said reference radiation through a reference path compris a scatterer which receives said reference radiation a scatters said reference radiation through the reference pat

26. The apparatus as claimed in claim 25, wherein sai scatterer comprises a mirror.

27. The apparatus as claimed in claim 25, wherein sai scatterer is curved, thereby causing said region in th object to be similarly curved.

28. A method for acquiring a two dimensional image o a region of an object, comprising the steps of: outputting radiation having a short coherence length; splitting said radiation using a splitter into referenc radiation and object radiation; receiving, using an optical guide, said object radiatio and directing said object radiation toward the region of th object and for collecting a portion of said object radiation scattered off of said region; receiving said reference radiation and directing said reference radiation through a reference path; receiving a portion of said object radiation and a portion of said reference radiation and detecting incident intensity over two dimensions using a two dimensional array detector resulting from said portion of said reference radiation coherently interfering with said portion of the object radiation scattered from said region; and outputting a signal corresponding to said incident intensity, wherein said portion of the reference radiation requires a reference delay time to travel from said splitter to said two dimensional array detector and said portion of object radiation requires an object delay time to travel from said splitter to said region and back to said two dimensional array detector.

29. An apparatus for acquiring an at least one dimensional image of a region of an object without any transverse scanning, comprising: a source for outputting radiation having a short coherence length; a splitter which splits said radiation into reference radiation and object radiation and allowing said object radiation to pass toward the region of the object and for collecting a portion of said object radiation scattered off of said region; a reference scatterer which receives said reference radiation along a reference path and scatters said reference radiation back toward said splitter; and an array detector which receives a portion of said object radiation and a portion of said reference radiation and detects incident intensity in a parallel fashion over said at least one dimension resulting from said portion of said reference radiation coherently interfering with said portion of the object radiation scattered from said region and for outputting a signal corresponding to said incident intensity, wherein said portion of the reference radiation requires a reference delay time to travel from said splitter to said array detector and said portion of object radiation requires an object delay time to travel from said splitter to said region and back to said array detecting means.

30. The apparatus as claimed in claim 29, wherein said array detector comprises a two dimensional array detector which receives a portion of said object radiation and a portion of said reference radiation, detects incident intensity over two dimensions resulting from said portion of said reference radiation coherently interfering with said portion of the object radiation scattered from said region and outputs a signal corresponding to said incident intensity.

31. The apparatus as claimed in claim 29, wherein said scatterer comprises a mirror.

32. The apparatus as claimed in claim 29, wherein said scatterer is curved, thereby causing said region in the object to be similarly curved.

33. The apparatus as claimed in claim 10, further comprising processing means for receiving said plurality of signals and digitally utilizing phase information contained in said plurality of signals.