Abstract:
An optical system provides information about tangential vibration components of a surface at remote location. The optical system includes a light source assembly that emits first and second beams, each having one or more wavelengths and one or two polarizations. The first and second beams are directed to the interrogated surface. A detector system is positioned to detect a third beam formed by at least a portion of the first and second beams being reflected from the interrogated surface. The first, second and third beams having incident and reflection angles relative to the interrogated surface that do not lay in a same plane. The detector system positioned remotely from the interrogated surface, and providing information on a phase change in the third beam relative to the first and second beam. The phase change is indicative of at least one surface vibration vector component of the interrogated surface. The detector system is a 90 degree optical hybrid balanced detector with four photodiodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Ser. No. 60/543,065, filed Feb. 9, 2004. This application is also a continuation-in-part of U.S. Ser. No. 10/669,130, filed Sep. 22, 2003. Both of the above applications are fully incorporated herein by reference. 
     
    
     FIELD OF INVENTION  
       [0002]     This invention relates generally to the systems and methods for measuring vibration frequency of an interrogated surface, and more particularly to systems and methods for measuring vibration frequency in three dimensions in response to optical signal detection.  
       BACKGROUND OF THE INVENTION  
       [0003]     The principles of Doppler Laser vibrometry are well known to the person skilled in the art, the general description can be found for example at the web site of Polytec company, CA: http://www.polytec.com/int/158 — 1004.asp?highlightSubMenu=Vibrometer%20University &amp;highlightPopupMenu=Signal%20Processing, the entire content of which is incorporated by reference herein. Vibrometers based on this principle can remotely measure surface velocities, or vibrations, with high spatial resolution and over a broad frequency and amplitude range.  
         [0004]     A coherent laser beam is projected on to the surface under investigation. Light scattered back from the surface is shifted in frequency by an amount proportional to the velocity of the surface (the Doppler effect). The instrument measures this frequency shift to produce an instantaneous velocity signal, which can subsequently be analyzed.  
         [0005]     By adding coordinate control using scanning mirrors, a single point vibrometer sensor can be used to scan across a surface, gathering multi-point data from vibrating objects. See for example, vibrometers produced by company Metrolaser, CA, Laser Doppler vibrometers at: http://www.metrolaserinc.com/vibrometer.htm [2].  
         [0006]     The most basic laser Doppler vibrometer is the one with a reference beam, see for example models LV-1100 and LV-1300 produced by Ono Sokki, Japan [3] http://www.onosokki.co.jp/English/hp_e/whats_new/SV_rpt/SV — 2/svtec.html. Laser light released from the light source is split into two beams, one of which serves as an incident beam directed at the object under measurement, while the other serves as a reference beam fed back within the vibrometer. The beam reflected from the object experiences Doppler shift in proportion to the vibratory velocity of the object. This beam is then made to interfere with the reference beam, which is given a frequency shift beforehand by the acoustooptic modulator, so that a beat frequency can be obtained. From this beat frequency signal, only the Doppler-shift component is singled out at the detector circuit and sent to the FM demodulator to be output as a voltage signal that is proportional to the vibratory velocity.  
         [0007]     The disadvantage of the described above system is in rather low sensitivity of the signal detection, and further improvement of the detection scheme is required to provide the measurement on longer distances and to improve the reliability of the system.  
         [0008]     An object does not always vibrate in one direction only. It may in fact vibrate in a complex manher in three-dimensional directions. The LV-3300 [3] http://www.onosokki.co.jp/English/hp_e/whats_new/SV_rpt/SV — 2/svtec.html is a vibrometric system comprising three reference-beam laser Doppler vibrometer units. The system performs vector calculations when signals are received from these three vibrometers to simultaneously measure the X-, Y-, Z-axis vibratory velocity and the direction in which the object under measurement vibrates. One of the three optical heads is arranged so that the direction of its incident light coincides with the direction (Z-axis) in which the object moves; while the other two are arranged so that the directions of their incident light (ZX and ZY) are specific angles away from the Z-axis. Signals from these angled optical heads represent the vibratory velocities and vibrations in the ZX and ZY directions. Consequently, beams reflected from all three optical heads contain signal components for the Z, ZX and ZY directions in which the object vibrates. Thus, we can measure the vibratory characteristics of the object for the X-, Y- and Z-axis directions simultaneously, by inputting these three signals into a vector calculator.  
         [0009]     The sensitivity of described above system is not very high, that&#39;s why the measuring is carried out on short distances, usually less than one meter to avoid signal fading due to air turbulence effects as well as random vibrations of various elements in the system. In multiple applications a remote vibration measurement is required. One of the examples is a target recognition based on measured vibration frequencies. A target can be hundreds or thousands of meters away from the detector system, and its vibration frequencies must be measured in order to distinguish friend or foe.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, an object of the present invention is to provide systems, and their methods of use, for measurement of a vibration vector of a surface.  
         [0011]     Another object of the present invention is to provide systems, and their methods of use, for measurement of a vibration vector of tangential and three-dimensional components.  
         [0012]     Yet another object of the present invention is to provide systems, and their methods of use, that provide high sensitivity detection of vibration components of a surface at a location remote from the surface.  
         [0013]     A further object of the present invention is to provide systems, and their methods of use, that provide high sensitivity detection of vibration components of a surface at a location remote from the surface with the use of a coherent receiver of an optical signal.  
         [0014]     These and other objects of the present invention are achieved in an optical system that provides information about tangential vibration components of a surface at remote location. The optical system includes a light source assembly that emits first and second beams, each having one or more wavelengths and one or two polarizations. The first and second beams are directed to the interrogated surface. A detector system is positioned to detect a third beam formed by at least a portion of the first and second beams being reflected from the interrogated surface. The first, second and third beams having incident and reflection angles relative to the interrogated surface that do not lay in a same plane. The detector system positioned remotely from the interrogated surface, and providing information on a phase change in the third beam relative to the first and second beam. The phase change is indicative of at least one surface vibration vector component of the interrogated surface. The detector system is a 90 degree optical hybrid balanced detector with four photodiodes.  
         [0015]     In another embodiment of the present invention, a method is provided for determining information about a surface vibration at a remote location. An optical system is provided that produces at least first and second beams. The first and second beams are directed to an interrogated surface located remotely from the optical system. A third beam is formed and has a phase change relative to the first and second beams that corresponds with a surface vibration of the interrogated surface. A phase of the first and the second beams is changed by Doppler effect when the first and second beams are reflected from the interrogated surface. The third beam is a redirection of the first and the second beams following their reflections from the interrogated surface. At least a portion of the third beam is received at a detector system positioned remote from the interrogated surface. A phase shift of the third beam is measured. The phase shift is induced by the interrogated surface by Doppler effect and is indicative of at magnitude of at least one of a vibration vector component of the interrogated surface.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a block diagram of one embodiment of a system of the present invention for measurement of tangential components of surface vibration vector.  
         [0017]      FIG. 2  is a block diagram of one embodiment of a system of the present invention for three-dimensional surface vibration vector measurement.  
         [0018]      FIG. 3  is a schematic diagram of one embodiment of a balanced detector of the present invention with a 90-degrees optical hybrid.  
         [0019]      FIG. 4  illustrates a tangential Doppler shift measurement that can be utilized with an embodiment of the present invention.  
         [0020]      FIG. 5  illustrates one embodiment of a system configuration of the present invention with four beams used in a 2-by-2 formation. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]     In one embodiment of the present invention, an optical device is provided, the block diagram of which is shown in  FIG. 1 , where  1  is a unit that combines elements for interrogated surface illumination, while  2  is the unit for a signal detection and information on vibration recovery block. The system operates as follows: the light source  3  outputs the beam  4  splitted by a splitter  5  into two beams  6  and  7 . The light source  3 , preferably a laser, generates beam at least one wavelength from in the UV or visible or near infrared or far infrared ranges (from 0.2 nm to 20 μm). The beams  6  and  7  impinge the beam directing devices  8  and  9 . These beam directing devices are selected from, a mechanical devices, a mirror on gimbals and a MEMS devices. Beam directing devices enable to perform two functions: (1) target the beams on the particular spot on the interrogated surface; (2) scan the beams over the interrogated surface.  
         [0022]     Output beams  10  and  11  are directed on the same spot  12  on the vibrating interrogated surface  13 . Light beam  14  reflected from the vibrating spot  12  forms the third beam. The third beam experiences the frequency shift caused by Doppler effect. The phase of the third beam carries information on the vibration frequency of the interrogated surface.  
         [0023]     Beam  14  enters the detection unit  2  where it impinges the coherent balanced detector system  15  followed by digital signal processing block  16 . In one embodiment of the present invention, the coherent detector includes 90-degrees optical hybrid.  
         [0024]     The third beam is formed by redirected light from the first and the second beams.  
         [0025]     In one embodiment of the present invention, the light of the first and the second light beams are either time or frequency or polarization multiplexed in order to simplify their separation at the detection stage. This time/wavelength/polarization multiplexing can be done in the following way.  
         [0026]     1) Polarization multiplexing: the first and the second beam must have orthogonal polarization. After splitting by the splitter  5  in illuminating unit  1 , a polarization-rotating element is introduced in one of the beams, for example, beam  6  that turns its polarization by 90 degrees. Before entering the detection unit  2 , the beam  14  passes polarization beam splitter that separates the beams with orthogonal polarizations.  
         [0027]     2) Frequency multiplexing: the first and the second beam must have different wavelengths. These two beams can be produced from the same light source followed by DEMUX. Another DEMUX must be installed at the entrance of the detection unit  2  to separate the beams with different wavelengths.  
         [0028]     3) Time multiplexing: the first and second beams are emitted at different time frame. At the entrance of the detector unit  2  a splitter divides the third beams into two equal beams, and the delay introduced in one of the arms provides time compensation in order to equalize the arrival time of the first and the second beam.  
         [0029]     Using balanced detection scheme improves sensitivity of the vibration measurement by canceling the relative intensity noise (RIN) of the laser. In principle quantum limit of measurement can be attained.  
         [0030]     In order to achieve full three-dimensional vector components set one should use more than two illuminating beams, namely three, four or more.  
         [0031]      FIG. 2  shows an embodiment of the present invention in which four laser beams illuminate the spot of the interrogated surface. Illumination unit  20  consists of a light source  3 , a beam splitter  21  and four beam directing devices  22 ,  23 ,  24 , and  25 . Output beam  26  from the light source  2  is splitted by the splitter  21  into four beams  27 ,  28 ,  29 , and  30 . These four beams  27 - 30  are redirected by beam directing devices  22 - 25  in such a way that all beams  31 , 32 , 33 , 34  target the same spot  12  on the interrogated surface  13 . At least a portion of the light from these four beams is reflected by the surface forming the beam  14 .  
         [0032]     Similar to described above scheme with two illuminating beams, the beam  14  enters the detection unit  2  where it impinges the coherent balanced detector system  15  followed by digital signal processing block  16 . In one embodiment of the present invention, the coherent detector system includes four balanced photodetectors with 90-degrees optical hybrid. A schematic diagram of one embodiment of the coherent balanced detector system is shown in  FIG. 3 . It consists of a 90° optical hybrid  40  and four balanced photodetectors  41 - 44 . Two incoming optical signals  45  and  46 , called, respectively, the signal S and the local oscillator L, impinge two inputs  47  and  48  of the optical hybrid. Both signal beam S and local oscillator L beam are divided by the first set of 3 dB couplers  49  and  50  as shown in  FIG. 3 . The beam  51  passes through the phase shifter  52  and gains the additional phase shift of 90°. The beams  53  and  54  are combined together at the directional coupler  55 . Respectively, the beams  56  and  57  are combined together at the directional coupler  58 . The resulting four output signals A, B, C, D coming, respectively, from the outputs  59 ,  60 ,  61  and  62 , all having 90° relative phase difference of the form: A=S+L, B=S−L, C=S+jL and D=S−jL.  
         [0033]     In one embodiment of the present invention, the balanced detector is used as described in the U.S. patent application Ser. No. 10/669,130 “Optical coherent detector and optical communications system and method” by I. Shpantzer et al. incorporated herein by reference.  
         [0034]     In various embodiment, the system of the present invention can either measure one vibration frequency at the time or multiple frequencies at the time. In order to measure multiple frequencies at the same time, the third beam  14  is splitted into multiple beams by splitter  70  as shown in  FIG. 4 . Each of output beams from the splitter  70  is processed by its own detector unit  2 ,  2 ′,  2 ″,  2 ′″, etc. that is adjusted for the particular frequency to be measured.  
         [0035]     A method for measuring the tangential vibration components by using two incident beams is depicted in  FIG. 5 . The two illuminating beams  10  and  11  (the first and the second beams) have angles φ 1  and φ 2  in respect the normal, while the detector is placed at a viewing angle φ s  (third beam  14  angle). The resulting Doppler frequency shifts are  
           Δ   ⁢           ⁢     f   1       =         (         d   ^     1     -       d   ^     s       )     ⁢     ρ   v       λ       ,       Δ   ⁢           ⁢     f   2       =         (         d   ^     2     -       d   ^     s       )     ⁢     ρ   v       λ           
 
 where {circumflex over (d)} 1 , {circumflex over (d)} 2 , {circumflex over (d)} s  are unit vectors in the directions of the incident and reflected beams,           is the relative velocity between the source and target, and λ is the light wavelength. The frequency difference between the first and the second beams that form the third beam will be  
           Δ   ⁢           ⁢   f     =         Δ   ⁢           ⁢     f   1       -     Δ   ⁢           ⁢     f   2         =           (         d   ^     1     -       d   ^     2       )     ⁢     ρ   v       λ     =         (       cos   ⁢           ⁢     ϕ   1       +     cos   ⁢           ⁢     ϕ   2         )     ⁢     v   p       λ           ,       
 
 where v p  is the component of the velocity between the source and target in the direction {circumflex over (d)} 1 −{circumflex over (d)} 2 . The frequency difference does not depend on the direction of the detector ({circumflex over (d)} s ). Notice that since  
         D   R     ⁢     &lt;&lt;   1         
 
 the Doppler shift is diminished by a factor of  
           cos   ⁢           ⁢     ϕ   1       +     cos   ⁢           ⁢     ϕ   2         =     D   R         
 
 where D is the distance between the beam sources and R is the distance to the target. This means that in order to have good velocity resolution we will need long observation times. 
 
         [0036]     In order to measure the frequency difference we can beat the reflected beams (the first and the second ones) together in one of the following ways:  
         [0037]     1. “Direct detection”—detecting the returned signal with a diode: |A 1 e j(2πΔf     1     t+φ     1     ) +A 2 e j(2πΔf     2     t+φ     2     ) | 2 =|A 1 | 2 +A 2 | 2 +2A 1 A 2  cos(2πΔft+Δφ), which gives a large DC and no information about the sign of the Doppler frequency shift. By giving one of the beams an extra frequency shift f 0 , we can distance the wanted signal from the DC (practically it is close to heterodyne detection). 
 
 2. “Balanced detection”—the two beams will be sent using, for example, two orthogonal polarizations (other multiplexing schemes, namely time and wavelength multiplexing operate in the similar manner), the incoming beam will be split using a polarization beam splitter oriented at 45° to the principal axes, and the two outputs detected with a pair of balanced diodes:  
                        1     2       ⁢     A   1     ⁢     ⅇ     j   ⁡     (       2   ⁢   π   ⁢           ⁢   Δ   ⁢           ⁢     f   1     ⁢   t     +     φ   1       )           +       1     2       ⁢     A   2     ⁢     ⅇ     j   ⁡     (       2   ⁢   π   ⁢           ⁢   Δ   ⁢           ⁢     f   2     ⁢   t     +     φ   2       )                  2     -                1     2       ⁢     A   1     ⁢     ⅇ     j   ⁡     (       2   ⁢   πΔ   ⁢           ⁢     f   1     ⁢   t     +     φ   1       )           -       1     2       ⁢     A   2     ⁢     ⅇ     j   ⁡     (       2   ⁢   πΔ   ⁢           ⁢     f   2     ⁢   t     +     φ   2       )                  2       =     2   ⁢     A   1     ⁢     A   2     ⁢     cos   ⁡     (       2   ⁢   πΔ   ⁢           ⁢   f   ⁢           ⁢   t     +   Δφ     )           ,       
 
 thus getting rid of the DC level. 
 
 3. “Direct Mixing”—the two beams will be sent using, for example, two orthogonal polarizations (other multiplexing schemes, namely time and wavelength multiplexing operate in the similar manner), the incoming beam will be split using a polarization beam splitter oriented to the principal axes, and the two outputs mixed in the hybrid: 2A 1 A 2 e j(2πΔft+Δφ) . 
 
 4. “Indirect Mixing”: As in direct mixing except that each polarization will be mixed with a local oscillator, the outputs can then be mixed electrically (analogically or digitally): (A 1 e j(2πΔf     1     t+φ     1     ) A 1 e −j(2πΔf     1     t+φ     1       (1)     ) )(A 2 e j(2πΔf     2     t+φ     2     ) A 1 e −j(2πΔf     1     t+φ     1       (2)     ) )*=P 1 A 1 A 2 e (j(2πΔft+Δφ+Δφ     1     ). 
 
 (Using “Indirect Mixing” further improves the sensitivity by using powerful local oscillator.) 
 
         [0038]     The measurement of the radial velocity, can be done by beating each beam with a local oscillator, such as is done in the “Indirect Mixing” method in the first step. Here we will have two measurements, one of the velocity components in the direction of {circumflex over (d)} 1 −{circumflex over (d)} s  and the other in the direction of {circumflex over (d)} 2 −{circumflex over (d)} s , which are almost orthogonal to {circumflex over (d)} 1 −{circumflex over (d)} 2 . Alternatively, by mixing the two measurements we would get the velocity component in the direction of {circumflex over (d)} i +{circumflex over (d)} 2 − 2 {circumflex over (d)} s , which is orthogonal to {circumflex over (d)} 1 −{circumflex over (d)} 2  if {circumflex over (d)} s  is orthogonal to {circumflex over (d)} 1 −{circumflex over (d)} 2 .  
         [0039]     Two more beams are utilized in order to determine the velocity in the third axis, this measurement will have to be time mixed with the previous one, since we can&#39;t distinguish more beams based on polarization. Alternatively, we could use a different wavelength, or use a slight frequency shift (bigger than the possible Doppler values), for the two additional beams.  
         [0040]      FIG. 6  shows a possible system configuration, four beams are used in a 2-by-2 formation ( 22 , 23  and  24 , 25 ), such that their returns can be distinguished in the receiver  2  located in the middle (using polarization/wavelength/time). Each is mixed with a local oscillator and sampled to get S i =A i e j(2πΔf     1+φ       1     ) , i=1 . . . 4. The radial Doppler is measured by looking at S 1 S 2 S 3 S 4 , and the tangential Doppler components by looking at S 1 S 2 S 3 *S 4*  and S 1 S 3 S 2 *S 4 *.  
         [0041]     The sensitivity of the disclosed technique for vibration components measurement is e superior to other systems and methods as a result of the methodology as well as the use of balanced detection at the receiver followed digital signal processing (e.g., noise compensation) for further sensitivity improvement. Balanced detection using matched pair of photodiodes provides approximately 3-dB improvement of SNR compared with detection using a single photodiode as suggested by prior art. The usually high DC component associated with single photodiode detection is removed using balanced detection. Furthermore, balanced detection allows suppression of laser RIN from the local laser, which is not possible with single photodiode detection.  
         [0042]     The procedure described above discloses the measurement of the vibration vector at the particular point of the interrogated surface. In order to receive a whole map of the surface vibration, the scanning over the surface is applied. In the disclosed system the scanning is performed by beam directing devices  8  and  9  ( FIG. 1 ) and  22 - 25  ( FIG. 2 ).  
         [0043]     The vibrometer disclosed in the present invention comprises three features mentioned above, namely reference beam, scanning ability, and three-dimensional measurement of the vibration vector combined with improved sensitivity and reliability compared with the standard approach.  
         [0044]     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.