Abstract:
A system and a method for remotely sensing global motion of an ensemble of dynamically moving scattering sites. The system comprising a scattering medium under inspection, an optical transceiver and a detector in a double-pass geometry.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is relates to U.S. patent application Ser. No. 09/849,641 filed May 4, 2001, entitled “Vibrometer System Using a Phase Conjugate Mirror” this disclosure of which is hereby incorporated herein by reference.  
       TECHNOLOGICAL FIELD  
       [0002]     The technology disclosed herein relates to a system for and method of sensing net global motion of components in an ensemble of dynamically moving scattering sites. This technology is useful for remote sensing and reconnaissance data acquisition applications in various scattering systems, including ocean water, fog, clouds, suspensions, biological samples, as well as liquid and gaseous flow monitoring systems.  
       BACKGROUND INFORMATION  
       [0003]     The attempt to determine the net global motion of components in an ensemble of dynamically moving scattering sites has proved to be complex, given that many ensembles of scattering sites possess two modes of motion: a global velocity component and a differential velocity component. It is the presence of the random motion that adds noise to the system and inhibits the optimum performance of the sensor. By suppressing this noise in an optical manner, less of the system&#39;s dynamic range need be sacrificed. Therefore, optimal use of the dynamic range of the detection apparatus and/or post-processing can be realized, thus the sensor performance is highly optimized.  
         [0004]     In general, major concerns of such remote sensors include overall system efficiency, maintaining optical interrogation probe beams on the scattering sites under dynamic conditions, minimizing undesirable scattering, which can either corrupt the measurement or reveal the probing operation to an undesirable third part, and avoiding optical damage of the medium undergoing interrogation due to system inefficiency.  
         [0005]     Currently, the prior art that exists in remote sensors involves complex adaptive optical compensation systems and light detection and ranging (lidar) approaches. These approaches require intensive post processing which makes them unattractive in many applications.  
         [0006]     A conventional laser-ultrasonic non-destructive inspection system is taught Pepper et al. in U.S. Pat. No. 5,585,921 which issued on Dec. 17, 1996.  
         [0007]     There are two system bandwidth parameters that characterize the performance of a sensing system: the detection bandwidth, also known as the coherent bandwidth, defined as the maximum global motion, or Doppler shift, that can be detected by the system, and the noise reduction bandwidth, also known as the incoherent bandwidth, defined as the maximum differential Doppler shift that can be suppressed by the system.  
       BRIEF DESCRIPTION  
       [0008]     Briefly and in general terms, the presently disclosed technology relates to an optical system for sensing net global motion components in an ensemble of dynamically moving scattering sites which has a laser probe source; a scattering medium under inspection, which is illuminated by the laser probe source; a wavefront-reversal device for collecting light reflected from the scattering medium and returning a conjugated beam; and an optical detector for detecting and processing the conjugated beam reflected from the scattering medium.  
         [0009]     In another aspect, the presently disclosed technology relates to an optical system for suppressing noise components produced by an ensemble of dynamically moving scattering sites which has a laser probe source; a scattering medium under inspection, which is illuminated by the laser probe source; a wavefront-reversal device for collecting light reflected from the scattering medium and returning a conjugated beam; and an optical detector for detecting and processing the conjugated beam reflected from the scattering medium.  
         [0010]     In yet another aspect, the presently disclosed technology relates to a remote sensor comprising a laser probe source; a scattering medium under inspection, which is illuminated by the laser probe source; a wavefront-reversal device for collecting light reflected from the scattering medium and returning a conjugated beam; and an optical detector for detecting and processing the conjugated beam reflected from the scattering medium.  
         [0011]     In yet another aspect, the presently disclosed technology relates to method for sensing net global motion components in an ensemble of dynamically moving scattering sites. A laser probe source is provided that illuminates a scattering medium under inspection. A wavefront-reversal device is provided that collects light reflected from the scattering medium and returns a conjugated beam. A detector is provided that detects and processes the reflection of the conjugated beam from the scattering medium. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1   a  is a schematic of the basic system components of one embodiment: a laser probe, the scattering medium, wavefront-reversal module (a self-pumped phase-conjugate mirror), and a heterodyne detector/processor;  
         [0013]      FIG. 1   b  is a schematic of the basic system components of another embodiment: a laser probe, the scattering medium, wavefront-reversal module (a self-pumped phase-conjugate mirror), and a homodyne detector/processor;  
         [0014]      FIG. 2   a  is a schematic of the basic system components of another embodiment: a laser probe, the scattering medium, wavefront-reversal module (an externally-pumped phase-conjugate mirror), and a heterodyne detector/processor;  
         [0015]      FIG. 2   b  is a schematic of the basic system components of yet another embodiment: a laser probe, the scattering medium, wavefront-reversal module (an externally-pumped phase-conjugate mirror), and a homodyne detector/processor;  
         [0016]      FIG. 3   a  is a schematic of the basic system components of the another embodiment: a laser probe, the scattering medium, wavefront-reversal module (a spatial light modulator), and a heterodyne detector/processor;  
         [0017]      FIG. 3   b  is a schematic of the basic system components of the another embodiment: a laser probe, the scattering medium, wavefront-reversal module (a spatial light modulator), and a homodyne detector/processor;  
         [0018]      FIG. 4  is a block diagram of another embodiment where a spatial domain enhancement scheme of a mode homogenizer is added; and  
         [0019]      FIG. 4   a  is a drawing of a long, mutimode optical fiber with intentional imperfection on the outside wall thereof. 
     
    
     DETAILED DESCRIPTION  
       [0020]      FIG. 1   a  is a schematic diagram of the basic system architecture of one embodiment of the presently disclosed technology, utilizing a self-pumped Phase Conjugate Mirror (PCM) in a wavefront-reversal module  500  and a heterodyne detector  600 . A laser beam from a probe laser  100 , which laser may be implemented by a diode-pumped solid state laser, an argon ion laser, a laser diode or other lasing device, is preferably passed via an optical isolater  101  to an Acousto-Optic (AO) modulator  103 . The optical isolater  101  is preferably utilized to prevent reflected beams from re-entering the probe laser  100  which could otherwise cause instability in the probe laser  100 . The AO modulator  103  generates a local oscillator (LO) reference signal  26  that is offset in frequency from the signal beam for heterodyne detection of a received signal  105  by a heterodyne detector  600 . As will be seen, the received signal  105  reflects off the surface of beam splitter  200  in this embodiment. Those skilled in the art will appreciate that beam  26  and beam  105  would typically enter the heterodyne detector  600  superimposed upon each other so that they co-propagate and are co-polarized, but are shown separated in the figures for ease of illustration and understanding. Thus, additional beam splitters, mirrors and other optical devices would typically be used to superimpose those signal, but since the use of heterodyne detectors  600  in combination with AO modulators  103  is well known in the art, the particular techniques to superimpose the two beams  26 ,  105  need not be discussed here.  
         [0021]     The frequency shifted beam  26 , which may be shifted 10-80 MHz depending upon the desired bandwidth, that defracts from the AO modulator  103  is used as a Local Oscillator (LO) reference beam for the heterodyne detector  600 . The undifracted part, which is in general of greater amplitude, serves as the probe beam  25  that, after passing through a beam splitter  200 , propagates through the ensemble of dynamically moving scattering sites  300 , also referred to herein as a scattering medium. The scattering medium  300  may take the form of a collodial suspension of particles in a aqueous medium, e.g., biological species in ocean water, or levitated particles in an atmosphere. The propagation of beam  25  through the scattering medium may well result in the beam becoming diffused. Lens  400  collects the diffused beam  30  and directs it to illuminate a wavefront-reversal module  500 . The collected beam preferably propagates through an optional amplifier  510  that may be optionally used in both the self-pumped PCM as well as the externally-pumped PCM embodiments. The amplifier  510  may comprise a Er:Fiber, a two wave mixer, a raman amplifier, etc. The optionally amplified beam illuminates a phase conjugator  520 .  
         [0022]     The wavefront-reversal module  500  can be in the form of self-referencing device or an externally pumped device. Examples of self-referencing devices include self-pumped phase-conjugate mirrors  520  and adaptive optical elements such as spatial light modulators  720  shown in  FIGS. 3   a  and  3   b . Examples of externally pumped devices include externally phase-conjugate mirrors  520 ′ shown in  FIGS. 2   a  and  2   b  and other real-time holographic devices. A preferred embodiment is the self-pumped phase-conjugate mirror  520  shown in  FIG. 1   a . The self-pumped device can preferably accommodate the case where the frequency domains of the signal and noise overlap, where the externally pumped device is more constrained and may be used in cases where the noise is within the compensation bandwidth of the device, while the signal bandwidth is beyond the bandwidth of the device.  
         [0023]     The detection bandwidth is dictated by the Bragg condition of the phase-conjugate mirror  520 , as well as the wavelength-dependent spatial resolution of the system. These effects conservatively result in a bandwidth exceeding 10 GHz. This value corresponds to a maximum detectable globally induced Doppler speed by the scattering medium of 5 km/sec at an optical wavelength of 0.5 micrometers. On the other hand, the noise compensation bandwidth of this scheme is limited by the grating formation time within the phase-conjugate mirror  520 . In the case of a GaAs Multiple Quantum Well (MQW), the compensation bandwidth can approach 1 MHz, which corresponds to a maximum differential Doppler speed compensation capability of 0.5 m/sec within the scattering medium at an optical wavelength of 0.5 micrometers (on the other hand, a barium titanate device, which can approach a 1 kHz bandwidth, can compensate for Doppler speeds in the range of 0.5 mm/sec). By using a membrane spatial light modulator as a wavefront-reversal device, the real-time holographic response times can approach 1 microsecond, i.e., a 1 MHz bandwidth, again corresponding to a maximum differential speed compensation capability of 0.5 m/sec, at an optical wavelength of 0.5 micrometers.  
         [0024]     The optionally amplified beam that illuminates the phase conjugator mirror  520  contains two Doppler-shifted components: (i) a coherent signal component, which arises from any global motion of the scattering medium; and (ii) an incoherent noise component, resulting from differential motion within the scattering medium, which can be the result of Brownian motion, as an example. As a result of the propagated optionally amplified beam striking the self-pumped phase-conjugate mirror  520 , a wavefront reversed propagated beam is formed.  
         [0025]     The wavefront-reversed propagated beam passes through the lens  400  and the scattering medium  300 . The wavefront-reversed propagated beam is re-directed by the beam splitter  200  toward the heterodyne detector  600  and as beam  105 . The wavefront-reversed replica of the propagated beam  30  is not given a separate identifying number in  FIGS. 1   a ,  1   b ,  2   a ,  2   b ,  3   a ,  3   b , or  4 , but its presence is signified by the fact that the arrows representing the propagated beams  30  are shown as indicating the laser light of those beams is moving in two directions namely toward lens  400  and then back towards the scattering medium  300  after having been formed into a wavefront-reversed replica of the propagated probe beams traveling towards lens  400 . The wavefront reversed propagated beam will pick up global phase shift from the moving ensemble, thereby doubling the net detected phase shift, or global Doppler shift. On the other hand, the differential Doppler shift will be compensated during this return propagation path, due to the fact that the wavefront-reversed propagated beam retraces its incident path, undoing path distortions as wall as differential velocity motion(s).  
         [0026]     In another embodiment a homodyne receiver, as shown in  FIGS. 1   b  and  2   b , is utilized. Most of the elements of this embodiments are the same as in the embodiments of  FIG. 1   a  and  2   a  and therefore are not described in further detail here. Rather this description will focus on the elements which differ from the embodiment of  FIGS. 1   a  and/or  2   a . Detectors  600  and  600 ′ are both examples of coherent detectors. The homodyne detector  600 ′ requires two incident beams, as does the heterodyne detector: the signal beam  105  and the local oscillator beam  26 . The difference, however, is that the local oscillator of the heterodyne detector  600  is offset in frequency relative to the signal beam (in the example set forth in the embodiment of  FIG. 1   a , the offset is in the range of 10-80 MHz). By contrast, the LO of homodyne detector  600 ′ is not offset in frequency and therefore it possesses the same nominal frequency as does the signal beam. Therefore, the Acousto Optic modulator  103  in  FIG. 1   a , for example, is not required. The relative benefits and tradeoffs among these two coherent detector systems are well-known in the art. Simply stated, the homodyne system possesses a 3 dB advantage in signal-to-noise relative to the heterodyne system, but, requires a phase-tacking system to maintain quadrature operation, (i.e., a 90 degrees phase shift) between the signal and LO, which adds additional complexity to the homodyne detector  600 ′.  
         [0027]     Another embodiment of this technology involves the use of an externally pumped phase-conjugate mirror  520 ′ in  FIGS. 2   a  and  2   b . The externally pumped phase-conjugate mirror  520 ′ replaces the self-pumped phase-conjugate mirror  520  in the embodiments of  FIG. 1   a  and  1   b , therefore this description will focus on the elements that differ from the embodiments utilizing the externally pumped phase-conjugate mirror  520 . The externally pumped phase-conjugate mirror  520 ′, which is also known as a “kitty” conjugator, involves a photorefractive crystal with a pair of coherent input beams: a “pump” beam and a “probe” beam that are mutually coherent. The pump beam is the more powerful beam. In the presence of the pump beam, the weaker probe beam will “reflect” from the photorefractive crystal as a phase conjugate replica. The “kitty” conjugator allows the disclosed apparatus to function over a wide field of view (that is the diffused beam  30  can occur over a greater range of angles relative to the photorefractive crystal compared to the embodiment of  FIGS. 1   a  and  1   b ). The “kitty” conjugator also enables one to realize a faster responding PCM compared to the PCM of  FIGS. 1   a  and  1   b . Also, the “kitty” conjugator can have an amplifying effect so that more light occurs in the wavefront reversed light emanating from the photorefractive crystal than occurred in the probe light  30  which impinged upon the photorefractive crystal in the first place. The additional energy is derived from the pump beam  112 .  
         [0028]     In  FIGS. 2   a  and  2   b , the pump beam is labeled by reference number  112  while the probe beam is labeled with reference numeral  25 . The light reflecting from or passing through the scattering medium  300  is labeled by reference number  30 . A laser beam from the probe laser  100  passes through a beam splitter  102  creating the pump beam  112  and the probe beam  25 . The pump beam  112  is directed toward the externally pumped phase-conjugate mirror  520 ′. The probe beam  25  propagates through via AO modulator  103  (in the case of the embodiment of  FIG. 2   a ) to the scattering medium  300  and then is identified as the propagated probe beam  30 . The externally pumped phase-conjugate mirror  520 ′ operates in the same general manner as the self-pumped phase-conjugate mirror in that it reflects a wavefront-reversed replica of the propagated probe beam  30  so that the wavefront-reversed replica of the propagated probe beam  30  will converge on the scattering medium  300  in the same locations, or spots, drawn where the probe beam  25  impinges same, reflecting along the beam path taken by beam  25  until beam splitter  200  redirects the wavefront-reversed replica of the propagated beam towards the detector, shown as a heterodyne detector  600  in the embodiment of  FIG. 2   a  and homodyne detector  600 ′ in the embodiment of  FIG. 2   b . An externally pumped phase-conjugate mirror  520 ′ compensates for both the incoherent as well as the coherent components of the spectrum. In order for the eternally pumped device to yield the desired results, it is preferred that the incoherent component is within the compensation bandwidth of the device and that the coherent component is outside the bandwidth of the device.  
         [0029]     Another embodiment utilizes a Spatial Light Modulator (SLM)  700  in the wavefront-reversal system  500  as shown in  FIGS. 3   a  and  3   b . These two embodiments are very similar to the embodiments of  FIGS. 1   a  and  1   b , respectively, except that a SLM  700  and several supporting elements are used in the wavefront-reversal system  500  and in these embodiments the light emanating from optical element  400  has parallel beams in a direction towards the wavefront-reversal system  500 .  
         [0030]     In these two embodiments the wavefront-reversal system  500  includes an number of elements  710 - 730 . Beam splitter  710  directs the incident beam to a Wavefront-Error Sensor (WES) detector subsystem  725 . The WES detector subsystem  725  that may be implemented by a Shack-Hartman array of quad detectors. The electrical output of the WES detector subsystem  725  is processed by a processor  730 , whose output drives a phase-only Spatial Light Modualtor (SLM)  720 . Examples of SLMs  720  include deformable mirrors (driven by arrays of piezoelectric transducers), optical MEMS reflective piston arrays (a piston array is shown in a representative fashion in  FIGS. 3   a  and  3   b ), or liquid crystal SLMs that encode an incident optical beam with phase-only (wavefront) information. The incident beam from lens  400 , which is directed to the SLM  720  will, after convergence of the closed-loop processor  730 , emerge as a wavefront-reversed replica of the incident beam. This wavefront-reversed replica will retrace the path of the incident beam, pass back through optical element  400 , and retrace the path back to the scattering region  300 , and continue back to the element  200 , and into the coherent detector/processor  600 , similar to the evolution of the beam from the phase conjugate mirror in the embodiment of the wavefront-reversal system  500  of  FIGS. 1   a ,  1   b ,  2   a  and  2   b.    
         [0031]     A spatial domain enhancement system may be added to the disclosed embodiments. An example of an embodiment with a spatial domain enhancement system, such as a mode homogenizer  800 , is depicted by  FIG. 4 .  FIG. 4  is basically identical to  FIG. 1   b  except for (I) the addition of mode homogenizer  800  between optical element  400  and the wavefront-reversal system  500  and that the fact that the previously discussed optional amplifier is shown in dashed lines  350 .  
         [0032]     A mode-homogenizer  800  takes a highly structured incident beam (e.g. a complex spatial pattern of high-contrast bright and dark patches) with arbitrary “fill factor”, and, via passive techniques, provides an output beam with a more uniform spatial structure. One embodiment of a mode homogenizer  800  is a highly multimode optical fiber (see  FIG. 4   a ) with many mode-mixing perturbations  810  (such as microbends, tapers, internal defects, sidewall defects; a design consideration being the necessity to map an incident beam into as many modes as reasonably possible, while creating as little differential modal perturbation as reassonably possible). During operation, a highly structured intensity pattern at the input  820  to the homogenizer  800  (e.g. a few spatial modes) will emerge from the homogenizer (waveguide)  800  at output  830  with many spatial modes excited. Thus, the wavefront-reversal system  500  will “see” a beam with a smoother intensity profile over its field-of-view and result in a more homogenized wavefront reversed replica, thereby enhancing the system performance. Since the mode homogenization process is a reciprocal process, it is accomplished in a passive and reciprocal manner and, moreover, designed so that all photon transit-time differences through the device are much less than the inverse of the maximum bandwidth to be processed (e.g. the maximum transit-time difference is typically far less than a nsec, while the maximum bandwidth is expected to be less than a MHz), then, the mode homogenizer will not systematically affect or degrade the temporal performance of the system. Therefore, the ability of the system to extract the desired temporal features of the beam (the coherent component, while exorcising the incoherent component) will be optimized.  
         [0033]     The mode homogenizer  800  can be used with any of the embodiments of the wavefront-reversal system  500  described herein and with either a heterodyne detector  600  or homodyne detector  600 ′.  
         [0034]     In order to obtain highly optimized performance, no new gratings should be created during the measurement process, otherwise, new random noise terms can result in arbitrary phase shifts, thereby corrupting the sensor. There are two potential sources for such detrimental phase-noise affects to occur. The first is fundamental to stimulated scattering processes, while the second is related to the scattering medium under examination. Regarding the former, it should be noted that, in general, a global, yet fixed, phase shift is typically imposed onto wavefront-reversed replicas created by the phase-conjugate mirror, which is fundamental to, and generated by, the stimulated scattering process itself. Being a fixed phase factor in time, however, this overall constant phase value is of no consequence, so long as it remains fixed throughout the sensor measurement (which, in most cases is precisely what occurs).  
         [0035]     Regarding the latter phase-noise effect, it should be noted that it is possible for new gratings to be formed during the measurement if the input beams vary appreciable during this time. This can occur if, during the two-pass photon transit time to and from the wavefront-reversal system  500  back to the scattering ensemble, the scattering sites move appreciably (relative to the spatial resolving limit of the system, which is wavelength and particle size dependent). In many applications, however, this situation is not expected to occur, since the f/# (f-number) for most scenarios is rather large and the particle speeds are relatively small and the photon round trip time (for typical ranges) is relatively fast compared to the time scales in the system. Therefore, it is anticipated that, for most scenarios, the beams incident upon the wavefront-reversal system  500  do not vary appreciably, in a spatial sense. In this case, only the overall phase of the wavefront entering the wavefront-reversal system  500  changes during the measurement process, whose displacement is to be sensed, and not the shape of the wavefront.  
         [0036]     Other issues that affect the sensing system are the ensemble&#39;s spatial “fill factor”, which manifests itself in terms of residual phase noise, and the spatial resolution of the optical system.  
         [0037]     Having described this technology in connection with a number of embodiments, modification will now certainly suggest itself to those skilled in the art. As such, the appended claims are not to be limited to the disclosed embodiments except as specifically required by the appended claims.