Abstract:
A method for measuring the direction and velocity of a target in a measurement volume in a laser Doppler velocimetry system includes selecting a discrete number of grating positions for an electrically switchable grating, placing the electrically switchable grating in a light path of a laser source. The laser source outputs a laser beam along the light path. The method further includes switching the electrically switchable grating sequentially through the discrete number of grating positions at a grating frequency to generate diffracted light beams from the laser, directing the diffracted light beams through a lens, focusing the diffracted light beams on a measurement volume, passing a target through the measurement volume in a target direction at a target velocity and receiving scattered light in a detector and calculating the power spectrum of light intensity fluctuations. The power spectrum includes a maximum peak. The maximum peak occurs at a frequency shifted from the grating frequency an amount corresponding to the target velocity and in a direction corresponding to the target direction. A laser Doppler velocimetry system for measuring the direction and velocity of a target in a measurement volume is also disclosed.

Description:
This application claims priority from U.S. Provisional Patent Application No. 60/947,371 filed on Jun. 29, 2007 and entitled “SYSTEM METHOD AND APPARATUS FOR A PHASE-STEPPED GRATING FOR DOPPLER VELOCIMETRY,” which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates generally to systems and methods for measuring velocity, and more particularly, to methods and systems for Doppler velocimetry providing directional vectors. 
     Laser Doppler velocimetry (LDV) has been in development for several decades and provides one of the best methods for high-accuracy, single-point, fluid or surface velocity measurement. However, typical LDV systems and methods cannot easily determine a direction of the fluid flow. This has been addressed in various ways. By way of example, a Bragg cell can be used in a two-beam laser system to create a frequency shift between the laser beams. 
     One of the earliest methods of making the LDV system direction-sensitive used a mechanically rotating diffraction grating in the optical path.  FIG. 1  is a schematic of a typical LDV system  100  with a mechanically rotating diffraction grating  106 . Typically, the rotating diffraction grating  106  is produced by mounting the grating radially on a small wheel mounted coupled to a motor  108 . The motor  108  then rotates the grating continuously. 
     A laser source  102  emits a laser beam  104  along a light path  105  toward the mechanically rotating diffraction grating  106 . The mechanically rotating diffraction grating  106  diffracts the laser beam  104  into two laser beams  104 A and  104 B. It should be noted that for clarity purposes only the +/−first diffraction order laser beams  104 A and  104 B are shown in  FIG. 1  and that higher diffraction order laser beams (not shown) can also be produced. 
     The mechanically rotating diffraction grating  106  is typically moved by a motor  108 . The mechanically rotating diffraction grating  106  moves transverse to the laser beam  104  with a velocity v so that the first order diffracted laser beams  104 A and  104 B are shifted up and down in frequency, respectively. The diffracted laser beams  104 A and  104 B are then incident on an input surface  110 A of the lens  110 . The lens  110  focuses diffracted laser beams  104 A and  104 B. The lens  110  emits the focused diffracted laser beams  104 A′ and  104 B′ from an output surface  110 B toward a measurement volume  112 . Where the focused, diffracted laser beams  104 A′ and  104 B′ cross is the measurement volume  112 . Interference fringes are formed in the measurement volume  112 . Due to the motion of the diffraction grating  106 , the fringes  112 A- 112 D in the measurement volume move in the y direction with a frequency 2v/d. 
     The diffraction grating  106  performs two roles in the above system  100 . First, rotating the diffraction grating  106  splits the input laser beam  104  into two laser beams  104 A and  104 B. If a sinusoidal phase diffraction grating  106  is used, only two laser beams  104 A and  104 B may be generated. If a non-sinusoidal diffraction grating is used all but the +/−1 first diffraction order laser beams are blocked. 
     Secondly, mechanically moving the diffraction grating  106  induces a frequency shift onto the two diffracted laser beams  104 A and  104 B. One diffracted laser beam  104 A shifts up in frequency and the other diffraction order laser beam  104 B shifts down in frequency. This frequency shift causes the fringes to move. 
     The two, focused diffracted, laser beams  104 A′ and  104 B′ recombine in the measurement volume  112 . When a particle  120  is stationary in the measurement volume there is a fluctuation of the scattered light intensity as the interference fringes move past the particle. The frequency of these fluctuations is 2v/d. Light is reflected from the particle in all directions (i.e., scattered). 
     When the particle moves it will either move in a direction opposite of the moving fringes (i.e., forward) or in the same direction as the moving fringes (i.e., reverse). When the particle moves forward, the measured frequency of the intensity fluctuations is more than 2v/d. When the particle moves in reverse, the measured frequency of the intensity fluctuations will be less than 2v/d. This increase or decrease of the frequency allows the direction of motion to be determined. 
     The mechanical rotating grating system  100  of frequency shifting works and is commercially available. The mechanical rotating grating system  100  has several shortcomings. One shortcoming is a spatially varying fringe  112 A- 112 B period in the measurement volume. 
     In another shortcoming, the motor  108  induces vibration into the mechanically moving grating  106 . This vibration causes additional intensity fluctuations. 
     In view of the foregoing, there is a need for an improved LDV system capable of determining direction and yet without needing a rotating grating. 
     SUMMARY 
     Broadly speaking, the present invention fills these needs by providing an improved LDV system capable of determining direction using an electronically switchable grating. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, computer readable media, or a device. Several inventive embodiments of the present invention are described below. 
     One embodiment provides a method for measuring the direction and velocity of a target in a measurement volume in a laser Doppler velocimetry system includes selecting a discrete number of grating positions for an electrically switchable grating, placing the electrically switchable grating in a light path of a laser source. The laser source outputs a laser beam along the light path. The method further includes switching the electrically switchable grating sequentially through the discrete number of grating positions at a grating frequency to generate diffracted light beams from the laser, directing the diffracted light beams through a lens, focusing the diffracted light beams on a measurement volume, passing a target through the measurement volume in a target direction at a target velocity and receiving scattered light in a detector and calculating the power spectrum of light intensity fluctuations. The power spectrum includes a maximum peak. The maximum peak occurs at a frequency shifted from the grating frequency an amount corresponding to the target velocity and in a direction corresponding to the target direction. 
     The discrete number of grating positions can include three positions. The discrete number of grating positions can include more than three positions. The discrete number of grating positions can be equally spaced along a 2π radian cycle. 
     The discrete number of grating positions can be electronically displayed images. The electrically switchable grating can be an electronic image displayed on an electronic display and the discrete number of grating positions can include a discrete number of electronic grating images. The electronic display can be a liquid crystal display. The electronic display can be a digital micro-mirror display. 
     The maximum peak can occur at a frequency less than the grating frequency if the target is moving in a reverse direction. The maximum peak can occur at a frequency greater than the grating frequency if the target is moving in a forward direction. The grating frequency can be between less than about one Hertz and more than about 100 MHz. The target can include at least one of a fluid flow and a surface and the target is moving relative to the measurement volume. 
     Another embodiment provides a laser Doppler velocimetry system for measuring the direction and velocity of a target in a measurement volume. The system includes a lens, a laser source, electrically switchable grating, a moving target, a detector and a controller. The laser source outputs a laser beam along a light path and the light path is directed toward the lens. The electrically switchable grating includes a discrete number of grating positions and the electrically switchable grating is in the light path between the laser source and the lens. The moving target passes through a measurement volume in a target direction and at a target velocity. The lens focuses a pair of diffracted light beams from the electrically switchable grating into the measurement volume. The detector is directed along the light path. The detector is on an opposite side of the lens from the laser source. The controller is coupled to the electrically switchable grating and the detector. The controller includes circuitry for determining a maximum peak in the power spectrum of a signal received in the detector. The maximum peak occurs at a frequency shifted from the grating frequency an amount corresponding to the target velocity and in a direction corresponding to the target direction. 
     The discrete number of grating positions can include three positions. The discrete number of grating positions can include more than three positions. The discrete number of grating positions can be equally spaced along a 2π radian cycle. 
     The electrically switchable grating is an electronic image displayed on an electronic display and wherein the discrete number of grating positions include a discrete number of electronic grating images. The electronic display can be a liquid crystal display. The electronic display can be digital micro-mirror display. 
     The maximum peak can occur at a frequency less than the grating frequency if the target is moving in a reverse direction. The maximum peak can occur at a frequency greater than the grating frequency if the target is moving in a forward direction. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic of a typical LDV system with a mechanically moving diffraction grating. 
         FIG. 2  is a diagram of an electronically switchable grating, in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram of an LDV system including the electronically switchable grating, in accordance with an embodiment of the present invention. 
         FIG. 4  is a graph of the drive voltage for a piezoelectric translation stage, in accordance with an embodiment of the present invention. 
         FIG. 5  is a diagram of an LDV system including the electronically switchable grating and a movable screen, in accordance with an embodiment of the present invention. 
         FIGS. 6A through 6E  are graphical representations of power spectra of light intensity fluctuations of an actual test of the performance of the LDV system, in accordance with one or more embodiments of the present invention. 
         FIG. 7  is a diagram of an LDV system including a non-mechanical, electronically switchable grating, in accordance with an embodiment of the present invention. 
         FIG. 8  is a flowchart diagram that illustrates the method operations performed in measuring the direction and velocity of a target in a measurement volume in the LDV system, in accordance with one embodiment of the present invention. 
         FIG. 9  is a diagram of an LDV system including the electronically switchable grating, in accordance with an embodiment of the present invention. 
         FIG. 10  is a diagram of an LDV system including an electronically switchable reflective grating and a reflector, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Several exemplary embodiments for an improved LDV system capable of determining direction using an electronically switchable grating will now be described. It will be apparent to those skilled in the art that the present invention may be practiced without some or all of the specific details set forth herein. 
     One embodiment provides a system and method for electronically switching the diffraction grating. Determining direction does not require a continuous change in phase position of the diffraction grating but rather the phase need only shift or switch between predefined phase shifts within a single the 0 to 2π radians cycle. Further, only three or more discrete steps or shifts are needed within the 0 to 2π radian interval. 
     As described above, a moving diffraction grating imposing frequency shifts on the +/−first diffraction order laser beams in order to give moving fringes is the same as projecting a (spatially filtered) image of the moving grating into the flow of particles. 
     The grating can be shifted so that one period of the projected fringe pattern is covered in a single complete cycle of the image of the grating. The cycle can be repeated for as many repetitions as desired. One approach to switching the position of the diffraction grating is to physically move the grating into the desired discrete positions. In another approach the diffraction grating can be an image on a display (e.g., liquid crystal, MEMS device, etc.) and the image can be electronically switched into the desired number of different discrete positions in the display. The display position can be physically moved and the image position displayed therein can also be electronically switched in the display. The present invention can be used to detect movement and determine direction of movement of a target in the measurement volume. The target can be one or more particles in a gaseous or liquid (i.e., fluid) stream moving relative to the measurement volume. The target can be a surface that is moving relative to the measurement volume. By way of example, a surface moving under an optical computer mouse and the invention is included in the computer mouse to detect motion and direction of the mouse relative to the surface. 
       FIG. 2  is a diagram of an electronically switchable grating  200 , in accordance with an embodiment of the present invention. The electronically switchable grating  200  includes a mechanical diffraction grating  200  which can be discretely positioned or switched under electronic control of a piezoelectric stage  308 .  FIG. 3  is a diagram of an LDV system  300  including the electronically switchable grating  200 , in accordance with an embodiment of the present invention.  FIG. 4  is a graph  400  of the drive voltage for a piezoelectric translation stage  308 , in accordance with an embodiment of the present invention. 
     The electronically switchable grating  200  is shown in three phase steps in even intervals of 2π/3 radians. The electronically switchable grating  200  can be an amplitude grating mounted on a piezoelectric translation stage  308  that is driven by a drive voltage source  306 . The drive voltage can be stepped through the discrete steps as shown in the graph  400  of  FIG. 4 . Stepping the drive voltage through the discrete steps at 2π/3 radians intervals causes the piezoelectric translation stage to move the electronically switchable grating  200  in the corresponding discrete steps. The piezoelectric translation stage  308  can accurately step the switchable grating  200  to three discrete positions  202 ,  204  and  206  (i.e., 0π, ⅔π, and 4/3π, respectively). 
     While three discrete positions are described herein, it should be understood that more than three discrete positions could also be used. By way of example the, the electronically switchable grating  200  could be shifted between four positions (0π, ½π, 1π, and 3/2π, respectively) or five positions (0π, ⅖π, ⅘π, 6/5π, and 8/5π, respectively) or six positions (0π, ⅓π, ⅔π, 1π, 4/3π and 5/3π, respectively) or even more positions. 
     While a piezoelectric translation stage  308  is used, the frequency limitations of such devices (typically up to about 500 Hz for the fastest that are commercially available) can limit the use of the piezoelectric translation stage to only relatively slower particle flow rates. As will be described below, alternative embodiments allow the switchable grating  200  to be used in a selected number of discrete positions at much higher frequencies. 
       FIG. 5  is a diagram of an LDV system  500  including the electronically switchable grating  200  and a movable screen  352 , in accordance with an embodiment of the present invention. Referring to both  FIGS. 3 and 5 , the laser source  102  can be a low power HeNe laser (λ=633 nm) is incident on a 40 lp/mm Ronchi grating (25 μm pitch). The +/−first order laser beams  104 A and  104 B are selected. As shown in  FIG. 5 , the focused +/−first order laser beams  104 A′ and  104 B′ cross on a ground glass screen  352  with a grating magnification of approximately 1. The movable ground glass screen  352  simulates a moving particle in the measurement volume  112 . The ground glass screen  352  can be made by grinding the surface of a piece of glass with grit to make an optically rough surface. 
     Forward scattered light from the screen  352  is collected and imaged through an opening  330  in a screen  332  and detected with the detector  102 . The opening  330  can have a diameter of roughly the size of one coherence area to maximize signal contrast. The optical intensity fluctuations can be recorded and Fourier transformed using a digital oscilloscope. 
     In one embodiment, the grating  200  is mounted on a Polytec PI P733.2CL piezo stage and the stage driven with a Polytec PI E503 LVPZT amplifier (i.e., available from Polytec in Tustin, Calif. The frequency response of this system can be limited to several hundred hertz. 
     Recall that the fringes  112 A-D are formed using the +/−1 orders of the grating  200 , therefore a shift of half the period of the grating is needed to translate the fringes by one period, which is 12.5 μm in one embodiment. As described above, the translations of 0, ⅓ and ⅔ of the fringe period are used, so the positions of the diffraction grating should be 0 μm, 4.2 μm and 8.3 μm. 
     A function generator (e.g., Agilent 33120A available from Agilent in Palo Alto, Calif.) or similar drive voltage source can be used as the drive voltage source  306  to supply the correct drive voltages to the PZT amplifier which then drove the piezo stage  308 . 
     When driven at low frequencies (e.g., less than about 50 Hz) the required positions could be achieved to +/−0.1 μm. At higher frequencies (greater than about 50 Hz) the piezo stage  308  can experience ringing and overshoot. 
       FIGS. 6A through 6E  are graphical representations power spectra of light intensity fluctuations  610 ,  620 ,  630 ,  640  and  650  of an actual test of the performance of the LDV system  500 , in accordance with one or more embodiments of the present invention. 
     The power spectra  614 ,  624 ,  634 ,  644  and  654  are the raw test data. The power spectra  614 ,  624 ,  634 ,  644  and  654  are superimposed on respective summary curves  612 ,  622 ,  632 ,  642  and  652 . The summary assumes only one particle in the measurement volume  112  and specifically neglects the finite extent of the fringe pattern. The intensity of the scattered light as a function of time can be written as shown in Formula 1. 
     
       
         
           
             
               
                 
                   
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     where I 0  is a constant representing the maximum intensity of the light, v is the component of the particle velocity across the fringe pattern, A is the fringe period which is the separation between the bright or dark fringes and f 0  is the drive frequency of the grating  200 . The operator floor rounds its argument to the nearest integer in the direction of negative infinity. 
       FIG. 6A  shows the power spectrum  610  when the grating  200  is not moving and the target (i.e., the ground glass screen  352 ) is driven forward by piezo stage  308  at 5 Hz. A spectral peak  612 A occurs at 5 Hz to indicate the movement of the screen  352 . 
       FIG. 6B  shows the power spectrum  620  and the screen  352  is driven backwards by drive stage  308 . The spectral peak  622 A occurs at 7 Hz. The difference in location of peak  612 A and peak  622 A (i.e., 5 Hz v. 7 Hz) is due to variations in the rate at which the piezo stage  308  moves in forward and reverse directions. Specifically, the piezo stage  308  moves slightly faster in the reverse direction than in the forward direction, but there is no way to determine a priori the direction of the travel of the screen  352  in forward or reverse direction only that the screen is moving to produce peaks  612 A,  622 A at either 5 Hz or 7 Hz, respectively. 
       FIG. 6C  shows the power spectrum data  630   f  where the screen  352  is stationary and the grating  200  is oscillated at 20 Hz. As a result, a distinct peak  632 A at 20 Hz is detected. Due to the quantized nature of the grating position, a significant harmonic peak  632 B also occurs at 40 Hz. 
       FIG. 6D  shows the power spectrum data  640  where the screen  352  is driven forward at 5 Hz and the grating  200  is oscillated at 20 Hz. The predominant peak  642 A is at 25 Hz. 25 Hz is the sum of the frequency due to the motion of the screen  352  (5 Hz) and the frequency due to the oscillating grating  200  (20 Hz). The occurrence of the peak  642  at the 25 Hz sum of the two frequencies indicates that the screen  352  is driven forward at 5 Hz. 
     Peak  642 A stands about 10 dB above the next highest peak  642 B at 35 Hz (a difference frequency of 2*20 Hz minus 5 Hz). Lower intensity peaks  646 A,  646 B and  646 C are also harmonic peaks at 5 Hz (the frequency of the screen  352  motion), 15 Hz (a difference frequency 20 Hz-5 Hz) and 45 Hz (a multiple of the two harmonics of the two frequencies). 
       FIG. 6E  shows the power spectrum data  650  where the screen  352  is driven in reverse at 7 Hz and the grating  200  is oscillating at 20 Hz. The resulting largest peak  652 A occurs at 13 Hz which is 7 Hz less than 20 Hz grating frequency. The 13 Hz peak indicates that the screen  352  is driven in reverse at 7 Hz. 
     Peak  652 A stands about 10 dB above the next highest peak  655  at 47 Hz (a sum frequency of 2*20 Hz plus 7 Hz). Lower intensity peaks  656 A,  656 B and  656 C are also harmonic peaks at 7 Hz (the frequency of the screen  352  motion), 27 Hz (a sum frequency 20 Hz+7 Hz) and 33 Hz (a difference frequency of 2*20 Hz minus 7 Hz). 
     Due to the strong harmonics introduced by the discrete nature of the phase stepping, there are many additional spectral peaks  642 B,  646 A,  646 B,  646 C,  655 ,  656 A,  656 B and  656 C in the detected spectra  640  and  650 , respectively. As long as the additional spectral peaks  642 B,  646 A,  646 B,  646 C,  655 ,  656 A,  656 B and  656 C are well separated from the dominant signal peaks  642 A,  652 A, respectively, the dominant signal peaks  642 A,  652 A can be easily detected to determine valid velocity and direction measurements. 
     As shown in the above  FIGS. 2-6E  the electronically switchable grating  200  can be used to remove the velocity ambiguity in a laser Doppler velocimeter. The electronically switchable grating  200  can be stepped discretely in precise phase. 
       FIG. 7  is a diagram of an LDV system  700  including the electronically switchable grating  720 , in accordance with an embodiment of the present invention. As described in  FIGS. 2-6E  above, the mechanical electrically switchable grating  200  was mechanically switchable using a device such as a piezo stage  308  that can accurately move the grating  200  into several selected discrete positions. However, this mechanical switchable grating  200  is still limited in switching speed due to the mass of the grating itself. One embodiment of the present invention replaces the mechanical electrically switchable grating  200  a non-mechanical or much faster electronically switchable grating  720 . 
     The non-mechanical or much faster electronically switchable grating  720  is formed from any device that can accurately interfere with or otherwise modulate the passage of the laser light  104  with a known periodic or quasi-periodic pattern of the phase and/or amplitude modulation of the laser beam  104 . There are several high-speed technologies that suggest themselves for implementation of this LDV system  700 . By way of example, the non-mechanical electronically switchable grating  720  can be manufactured from a liquid crystal display (LCD). In another example, the non-mechanical electronically switchable grating  720  can be manufactured from micro-electrical mechanical systems (MEMS). An LCD can have sub-microsecond switching speeds. The MEMS (e.g., as used in the digital micro-mirror (DMD) display devices by Texas Instruments) have switching times in the tens of microseconds. 
     The non-mechanical, or much faster electronically switchable grating  720  can achieve frequency ranges of less than one Hertz to more than 100 MHz and frequency offsets on the order of about one Hertz to more than about 100 MHz. One benefit of implementing this technique with a electronically switchable grating  720  is that the frequency shift is only determined by the timing of the drive electronics which can be performed easily and accurately. Further, eliminating or minimizing the mechanical portion of the LDV allows the LDV system  700  to be much smaller than previous LDV systems. In the case of compact LDV probes the LDV system  700  would also be advantageous since an electric motor would not have to be accommodated within the compact LDV probe head. 
     The electronically switchable grating  720  can be an LCD or DMD that can display portions  720 A that interfere with the light  104  and portions  720 B that minimally interferes with the light similar to the grating  200  shown in  FIG. 2  above. As the electronically switchable grating  720  is an electronically controlled display or an image displayed on an electronic display, the controller  340  can cause the display to switch through the different discrete images. Each of the discrete images is analogous to the discrete positions of the grating  200  described in  FIGS. 2-6E  above. 
       FIG. 8  is a flowchart diagram that illustrates the method operations performed in measuring the direction and velocity of a target in a measurement volume  112  in the LDV systems  300 ,  500  and  700 , in accordance with one embodiment of the present invention. In an operation  805 , a number a discrete grating positions or images is selected. The electronically switchable grating  720  or image thereof can switch through three phase images in even intervals of 2π/3 radians. Alternatively, the electronically switchable grating  720  or image thereof can switch through more than three discrete phase images. By way of example the, the electronically switchable grating  720  could be displayed between four positions (0π, ½π, 1π, and 3/2π, respectively) or five positions (0π, ⅖π, ⅘π, 6/5π, and 8/5π, respectively) or six positions (0π, ⅓π, ⅔π, 1π, 4/3π and 5/3π, respectively) or even more phase images. 
     In an operation  810 , the switchable grating  720  is placed in the light path  104 . The grating  720  is switched or displayed sequentially through the selected number of phase images in an operation  815 . 
     In an operation  820 , the first order light beams  104 A,  104 B are directed through the lens  110  to a measurement volume  112  and to a detector  302 . In an operation  825 , the frequency spectrum of the light fluctuations at the detector are determined. Light scattered from the particle  120  in the measurement volume  112  is received in the detector  302 . 
     In an operation  830 , the controller  340  can determine a direction and a velocity of a target (e.g., the particle  120 ) in the measurement volume  112  by calculating the power spectrum of the scattered light fluctuations (e.g., as shown in  FIG. 6A-6E ). 
     In an alternate embodiment, the controller can determine the direction and velocity of the target by processing the signal received from the detector by other methods. By way of example, the signal received from the detector can be processed by calculating an autocorrelation or by use of other transforms such as wavelet transforms. 
       FIG. 9  is a diagram of an LDV system  900  including the electronically switchable grating  720 , in accordance with an embodiment of the present invention. The LDV system  900  is a more physically compact than the LDV system  700  described above because the detector  902  is located between the lens  110  and the non-mechanical, electronically switchable grating  720 . 
     In operation the LDV system  900  operates substantially similar to the LDV systems described above in other embodiments of the invention except that the signals  904 A and  904 B reflected from the particle  120  and back toward the lens  110 . The signals  904 A and  904 B pass through a secondary lens  910  that focuses the signals  904 A′ and  904 B′ to lens  110  which then focuses the signals  904 A″ and  904 B″ into the detector  902 . 
       FIG. 10  is a diagram of an LDV system  1000  including an electronically switchable reflective grating  1020  and a reflector  1012 , in accordance with an embodiment of the present invention. The reflector  1012  provides a reflective surface placed at an angle θ relative to light path  105 . Angle θ can be any suitable angle in a range from less than one degree to more than 179 degrees so as to allow the laser beam  104  to be offset to one side of the light path  105  at an angle α relative to the light path  105 . The angle α can be any suitable angle in a range from less than one degree to more than 179 degrees. 
     Offsetting the source  102  allows the LDV system  1000  to be more physically compact. The reflector  1012  can also be combined with relocating the detector  902  as shown in  FIG. 9  to further physically package the LDV systems  900  and  1000  as may be desired. 
     The reflective grating  1020  operates by diffracting and reflecting the light emitted from the reflector  1012 . The diffracted light beams  104 A,  104 B are then used as described above. 
     The laser source  102  emits a laser beam  104  toward the reflector  1012 . The reflector  1012  can be a solid reflective surface (i.e., a mirror) or alternatively, a reflective surface that reflects at least a portion of the laser beam  104  toward the grating  1020 . As described above, the grating  1020  reflects and diffracts the light  1004 A and the reflected and diffracted light beams  104 A,  104 B pass through openings  1030 A and  1030 B, respectively, to the lens  110 . The screen  1032 A,  1032 B,  1032 C substantially prevents undesired light beams  1004 B from impinging the lens  110 . 
     While the above embodiments have been described in terms of laser light, it should be understood that other types of light, visible and beyond the visible spectrum and other electromagnetic signals in other frequencies of the electromagnetic spectrum. 
     With the above embodiments in mind, it should be understood that the invention may employ various computer-implemented operations involving data stored in computer systems. These operations are those requiring physical manipulation of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. Further, the manipulations performed are often referred to in terms, such as producing, identifying, determining, or comparing. 
     Any of the operations described herein that form part of the invention are useful machine operations. The invention also relates to a device or an apparatus for performing these operations. The apparatus may be specially constructed for the required purposes, or it may be a general-purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general-purpose machines may be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations. 
     The invention can also be embodied as computer readable code and/or logic. The computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include hard drives, network attached storage (NAS), logic circuits, read-only memory, random-access memory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and other optical and non-optical data storage devices. The computer readable medium can also be distributed over a network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     It will be further appreciated that the instructions represented by the operations in the above figures are not required to be performed in the order illustrated, and that all the processing represented by the operations may not be necessary to practice the invention. Further, the processes described in any of the above figures can also be implemented in software stored in any one of or combinations of the RAM, the ROM, or the hard disk drive. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.