Abstract:
A sub-surface imaging system includes a broadband set of coherent beams with low divergence angle. It further includes an optical processing system which provides a probe and a reference beam, applies the probe beam to the target to be analyzed, recombines the beams interferometrically and varies the relative temporal relationship of the coherence phase of the two beams at a frequency compatible with video display rates. It further includes control and processing systems.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims priority from provisional application Ser. No. 60/479,811 filed on Jun. 18, 2003. 
     
    
     
       FIELD OF USE  
         [0002]    The invention relates to real-time imaging and analysis and in particular to sub-surface imaging and analysis at video rates.  
         BACKGROUND  
         [0003]    In a typical imaging and analysis systems a target is scanned in two dimensions by a probe signal. A corresponding detected signal provides information about the scanned points in the target which can then be displayed as an image and analyzed visually or by electronic processing.  
           [0004]    Real-time imaging consists of repeatedly scanning, detecting and displaying the resulting images at a sufficiently high rate to observe changes in the target scanned as the changes actually occur and to avoid motion related analysis issues.  
           [0005]    For real-time scanning and display, one dimension of the image must be scanned and displayed at high speed. For example, in a typical video system the high speed dimension is referred to as the horizontal scan rate, while the lower scan rate dimension is referred to as the vertical rate.  
           [0006]    Real-time non-invasive imaging is a powerful technique for non-destructive imaging or quantitative analysis of a variety of targets including, semiconductor wafers, materials, human tissue, etc. The analysis can include scanning for defects, discontinuities, or in the case of tissue, abnormalities such as malignant growths.  
           [0007]    Real-time imaging is particularly valuable in the case of in vivo analysis of human tissue, where non-destructive, non-invasive sub-surface scanning allows convenient analysis of potentially abnormal tissue without the need for a costly, time consuming and invasive biopsy.  
           [0008]    The two dimensional scanning of real-time imaging systems typically consists of a high speed scan along one axis and a low speed scan along an orthogonal direction. A typical video rate imaging system requires a high speed scanning rate of the order of 15 KHz and low speed scan of the order of 30 Hz, providing a progressive scan image rate of 30 images per second. These rates are typically sufficiently high, with respect to the motion speeds, to support in vivo display of tissue without blurring motion artifacts.  
           [0009]    There are existing scanning technologies suitable for low speed scanning. These include electromechanical based technologies, such as galvanometers or moving coils actuators. However, these technologies are not suitable for high speed scanning. Existing high speed scanning electromechanical technologies include rotating polygons which are expensive and physically large and in addition have significant alignment issues.  
           [0010]    High scanning rates can also be achieved by acousto-optic scanning where an optical beam is deviated by a chirped acoustic wave propagating through a crystal. The acoustic wave is generated by applying an RF signal to the crystal by means of a transducer. The RF signal has a repetitive and linearly varying frequency which provides a matching linearly varying frequency (chirp) to the acoustic wave. The acoustic wave intercepts the optical wave and deviates it by an angle proportional to the RF frequency. This technique allows high speed scanning, however, it is expensive, requires significant RF power because the angular deviation is small, and the system is physically large.  
           [0011]    One or more of these aspects of high speed moving parts, high cost components, high power consumption and large physical size make existing scanning systems unsuitable for cost effective, compact, robust, high speed imaging systems.  
           [0012]    A typical sub-surface imaging technology, such as confocal microscopy, can generate tomographic images for example of tissue, containing information similar to biopsy sections by scanning a one dimensional array, parallel to the surface of the tissue (x-scan), at varying depths (z-scan) in tissues. The series one dimensional scans at various depths can be displayed as a single tomographic image. Such imaging systems, however, have many of the undesirable aspects described above.  
           [0013]    Another sub-surface imaging technology, optical coherence tomography, can also generate tomographic, biopsy-like images. Such systems use a super-luminescence diode (SLD) as the optical source. The SLD output beam has a broad bandwidth and short coherence length. Optical coherence tomography involves splitting the output beam into a probe and reference beam. The probe beam is applied to the system to be analyzed (the target). Light scattered or reflected back from the target is combined with the reference beam to form the measurement signal.  
           [0014]    Because of the short coherence length only light that is scattered from a depth within the target such that the total optical path lengths of the probe and reference are equal combine interferometrically. Thus, the interferometric signal provides a measurement of the scattering value at a particular depth within the target. By varying the length of the reference path length, a measurement of the scattering values at various depths can be measured and in this manner, the z-axis can be scanned. The reference path length is typically varied by physically moving a reflecting mirror.  
           [0015]    In order to get the biopsy-like image, the second dimension scan (the x-scan) is obtained by translating the probe focusing mirror parallel to the target surface. However, at least some of the above mentioned limitations apply to this imaging method also and, in general, these limitations represent a barrier to applying current imaging technologies to compact, cost effective real-time applications.  
           [0016]    There is therefore an unmet need for a cost effective, compact, robust, high speed scanning technology that has no moving parts and that is compatible with imaging systems.  
         SUMMARY OF THE INVENTION  
         [0017]    The invention provides a method, apparatus and system for sub-surface imaging and analysis. The invention includes a broadband set of coherent beams with low divergence angle. It further includes a signal processing system which provides a probe and a reference beam, applies the probe beam to the target to be analyzed, recombines the beams interferometrically and varies the relative temporal relationship of the coherence phase of the two beams. It further includes control and processing systems. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is an illustration of the real-time imaging and analysis system according to the invention.  
         [0019]    [0019]FIG. 2 is an illustration of a preferred embodiment of the invention.  
         [0020]    [0020]FIG. 3 is a frequency domain illustration of an output of a mode-locked laser optical source.  
         [0021]    [0021]FIG. 4 is a time domain illustration of the outputs of mode-locked laser optical sources.  
         [0022]    [0022]FIG. 5 is a frequency domain illustration of outputs of two mode-locked laser optical sources.  
         [0023]    [0023]FIG. 6 is an alternate time domain illustration of the outputs of mode-locked laser sources.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    A novel imaging and analysis system is illustrated in and described with reference to FIG. 1, where a real-time imaging system is shown. It includes two generators  101  and  106 , each of which generates a set of repetitive discrete coherent signals. Because all signals generated are coherent and have a repetitive determined phase relationship with each other, it is possible to combine these signals interferometrically to produce a signal that is related to the relative phase of the two sets of signals.  
         [0025]    By controlling the relative repetitive frequencies of the two generators, the point in time at which the two sets interfere can be scanned sequentially with a scanning speed determined by the difference in the two frequencies. This can be used as the basis for a system that scans repeatedly through a section of a target. Furthermore, the difference between the repetitive frequencies of the discrete coherent sets can be such that high scanning rates can be achieved. By combining this electronic based high speed scanning with a conventional lower speed scan in an orthogonal direction, two dimensional high speed imaging can be achieved and therefore this system constitutes a real-time non-invasive tomographic system.  
         [0026]    As illustrated in FIG. 1, the first generator  101  outputs a set of discrete coherent signals  102  that is applied to a target  104  to be imaged or analyzed. At least part of the set of discrete coherent signals, applied to the target is reflected or scattered back and redirected by a signal steering element  103  to a signal combining element  108 .  
         [0027]    The part of the set of discrete coherent signals directed to the combining element  108  by the steering element  103  is referred to as the captured signal  105  and because the strength of the signal reflected or scattered back from any point in the target is dependent on the characteristics of the target at that point, this captured signal contains information contributing to an image of the target.  
         [0028]    The second generator  106  outputs a second set of discrete coherent signals, called the reference signal  107  that is also applied to the combining element  108  where it is combined interferometrically with the captured signal  105 . The resulting interferometric signal  109  is detected by a detector  110 .  
         [0029]    The detected signal is processed by an processing module  111  in conjunction with timing signals from the control module  112 , which also controls repetitive discrete coherent signal generators  101  and  106 . The resulting information constitutes a one dimensional scan of a segment of the target along the depth or horizontal axis  113  of the target. This scan can be achieved at a high rate, because it is based on electronic control signals.  
         [0030]    This process is repeated at different locations along an axis orthogonal to the horizontal axis, referred to as the vertical axis  114 . Scanning in this vertical axis can be at lower rates and accomplished by conventional electromechanical translational techniques. The sequence of scans thus acquired constitute a real-time image sequence which can be electronically analyzed in real-time.  
         [0031]    A preferred embodiment of the invention is illustrated in and described with reference to FIG. 2 where a real-time sub-surface imaging and analysis system is shown. The system includes a first electronically pumped and mode-locked laser diode  201 , whose output beam  202 , (which in this case is a set of discrete coherent optical signals), consists of a broad band set of wavelengths or modes that have a repetitive phase relationship with each other; the output beam  202 , is collimated by a first lens  203 . The output beam  202  is passed through a first beam splitter  204 , such as a polarization beam splitter, through a quarter wave plate  205  and a second lens  206 , with a relatively long Rayleigh range, e.g. 1 mm, and focused in a target  207 . At least part of the optical signal applied to the target is reflected or scattered back and captured by the second lens  206 . Reflection or scattering occurs because of discontinuities, such as changes of material properties, defects or changes of refractive index. The captured signal passes through the quarter waveplate  205 , back to the first beam-splitter  204 , where at least part of the captured signal  208  is directed to a second beam-splitter  209 .  
         [0032]    A second electronically mode-locked laser diode  210 , whose output  211  is collimated by a third lens  212  and is also applied to the second beam splitter  209 , where it is combined interferometrically with the captured signal  208 . The resulting interference signal is detected by first and second opto-electronic detectors  213  and  214  and processed by a processing module  215 . A control module  216  controls the mode-locked operation of the laser diodes  201  and  210  and also provides timing information to the processing module  215 . The processing module combines this timing information with the detected interference signals to compute intensity as a function of depth within the target, which constitutes a one dimensional scan of the target. A single opto-electronic detector may be used. An advantage of using a first and a second opto-electronic detector is that it provides a means to suppress noise by exploiting the complementary nature of the signals and having them detected differentially.  
         [0033]    The optical components,  201 ,  203 ,  204 ,  205 ,  206 ,  209 ,  210 ,  212 ,  213  and  214 , enclosed by the dashed box  217  in FIG. 2, do not involve any moving parts and can be assembled in a compact manner on an optical micro-bench. Use of a compact micro-bench facilitates vertical axis scanning with conventional electromechanical techniques.  
         [0034]    The control module  216 , along with the processing module  215 , combine successive one dimensional scans to generate a two dimensional scan. The control module  216  also stores the scans and control parameters in non-volatile memory for display, for further analysis and future operation. The electronic high speed scanning technique enables a real-time non-invasive imaging system. The resulting images can be analyzed visually using a real time display monitor or analyzed electronically, for example, by comparing a current image with previously acquired images.  
         [0035]    The output of a mode-locked laser diode is further illustrated in the frequency domain in FIG. 3 and consists of a set of modes, one of which is  301 , which are separated from each other by a constant frequency difference  302 . This frequency difference (delta_F) is related to the length of the laser diode according to the relationship delta_F=c/(2 nL) where c is the speed of light, n is the refractive index of the lasing material and L is the length of the laser diode cavity.  
         [0036]    Mode-locking is achieved by modulating the laser diode at a frequency equal to or harmonically related to the frequency delta_F. The output  202  of the laser diode  201 , of FIG. 2, referred to as the probe signal, is illustrated in the time domain in FIG. 4, where it is shown as a pulse train  401  with a repetition period  403 , (T1) which is the reciprocal of its repetition frequency delta_F 1 . The output of the second laser diode  210 , of FIG. 2, is shown as the pulse train  402  with a repetition period  404 , (T2) which is the reciprocal of its frequency delta_F 2 .  
         [0037]    The difference between the two periods  405  corresponds to the difference between the two frequencies delta_F 1  and delta_F 2  and is referred to as a frequency offset. Pulses from the two pulse trains go from being aligned in time, as shown at point  406 , to a systematic increase in misalignment until they come back into alignment. The frequency with which pulses come back into alignment is related to the frequency offset. The actual temporal relative alignment of the two pulse trains is referred to as their coherence phase offset.  
         [0038]    When the captured signal  208  is combined with the reference signal  211 , an interference signal will only exist when the captured signal is substantially aligned in time with the reference pulse. Since the reference and captured signals have different pulse frequencies, at any given time this alignment will correspond to only the optical signal reflected or scattered from a particular depth in the target.  
         [0039]    Thus, having a frequency offset between the reference and probe signals has the effect of selectively discriminating in favor of detecting a signal reflected or scattered from different depths in the target at different times. This effectively provides an electronic method of scanning in depth (or along the horizontal axis), with the advantage of having no moving parts. The range of the depth or horizontal axis scan corresponds to the optical path length of the laser cavity. A full scan occurs with a frequency corresponding to the frequency offset  405 .  
         [0040]    The optical system  217  in FIG. 2 can then be translated in a direction perpendicular to the horizontal axis by conventional electromechanical techniques, to provide a two dimensional scan of the target.  
         [0041]    The control module  216  in FIG. 2 generates the electronic signals to mode-lock both laser diodes  201  and  210  and provides a signal representing the frequency off set between them to the processing module  215 . This signal represents the coherence phase offset between the reference and probe signals. This allows the processing module  215  to determine from what depth in the target the detected interferomatric signal was reflected or scattered.  
         [0042]    A frequency offset between the two mode-locked lasers of the order of 15 KHz, allows for depth or horizontal scan rates that are compatible with video horizontal scan rates. This also allows corresponding wavelengths from the sets of wavelengths output by the two mode-locked lasers to have substantially the same wavelength values (Each pair is incrementally offset by 15 KHz). Frequencies other than 15 KHz are also usable.  
         [0043]    The two wavelength sets can optionally have a frequency offset, that is substantially the same for all corresponding wavelengths from the two sets. In FIG. 5, where the output of two mode-locked lasers are illustrated in the frequency domain and consists of a first of modes, one of which is  501 , and a second set of modes, one of which is  502 . (The second set of modes or wavelengths is illustrated by dashed lines.) These are offset by a frequency offset that is substantially the same for all corresponding modes (or wavelengths) of the two sets. This offset enables more sophisticated signal detection techniques, which includes but is not limited to coherent heterodyne techniques.  
         [0044]    In an alternative embodiment, depth scanning can be accomplished by varying the temporal relationship between the captured scattered signal and the reference signal by modifying the coherence phase offset between the first and second mode-locked lasers. This can be done by, for example, having both lasers mode-locked at substantially the same frequency and varying the phase relation between RF (radio frequency) signals electronically mode-locking the lasers.  
         [0045]    Typical outputs of the lasers in such an embodiment are illustrated in FIG. 6, where the first laser output  601  and the second laser output  602  have substantially the same repetition rate indicated by the periods  603  and  604 . The phase coherence between the two pulse trains  605  is aligned so the pulse trains are substantially 180 degrees out of phase when the signals are combined interferometrically.  
         [0046]    Scanning is accomplished by varying this phase relationship by an amount, for example, indicated by  606 . The scanning frequency is determined by the frequency with which the phase coherence is modulated. The scanning range is determined by the magnitude of the phase coherence variation  606 .  
         [0047]    It is understood that the above description is intended to be illustrative and not restrictive. Many of the features have functional equivalents that are intended to be included in the invention as being taught.  
         [0048]    For example, the mode-locked laser could be optically pumped, it could be a solid state laser, such as a Cr:LiSAF laser optically pumped by a diode laser and it could be passively mode locked by a Kerr lens or a semiconductor saturable absorber mirror. Gain switched optical sources, with optical feedback to lock modes may also be used. For purposes of this invention, mode-locked lasers will include gain switched sources.  
         [0049]    The scanned information may be analyzed for other than imaging purposes, for example, it may be used to map discontinuities. Such discontinuities include defects and refractive index changes. The discontinuity map may be analyzed to provide characteristic information about the target, such as concentration of particular constituent analytes. Depth scans at multiple entry points can be averaged to further analyze these characteristics. For purposes of this invention, imaging will include analysis in general.  
         [0050]    The technique is not restricted to discrete coherent optical signals. The invention could also be implemented using generators of discrete coherent acoustic signals or using discrete coherent RF signals.  
         [0051]    Other examples will be apparent to persons skilled in the art. The scope of this invention should therefore not be determined with reference to the above description, but instead should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.