System method and apparatus for optical directional determination

1. A method for measuring the direction and velocity of a target in a measurement volume in a laser Doppler velocimetry system comprising:

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, wherein the laser source outputs a laser beam along the light path;

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, wherein the power spectrum includes a maximum peak, wherein 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.

2. The method of claim 1 , wherein the discrete number of grating positions includes three positions.

3. The method of claim 1 , wherein the discrete number of grating positions includes more than three positions.

4. The method of claim 1 , wherein the discrete number of grating positions are equally spaced along a 2π radian cycle.

5. The method of claim 1 , wherein the discrete number of grating positions are electronically displayed images.

6. The method of claim 5 , wherein 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.

7. The method of claim 6 , wherein the electronic display is a liquid crystal display.

8. The method of claim 6 , wherein the electronic display is a digital micro-mirror display.

9. The method of claim 1 , wherein the maximum peak in the power spectrum occurs at a frequency less than the grating frequency if the target is moving in a reverse direction.

10. The method of claim 1 , wherein the maximum peak in the power spectrum occurs at a frequency greater than the grating frequency if the target is moving in a forward direction.

11. The method of claim 1 , wherein the grating frequency is between less than one Hertz and more than about 100 MHz.

12. The method of claim 1 , wherein the target includes at least one of a fluid flow and a surface and wherein the target is moving relative to the measurement volume.

13. A laser Doppler velocimetry system for measuring the direction and velocity of a target in a measurement volume comprising:

a lens;

a laser source, wherein the laser source outputs a laser beam along a light path, wherein the light path is directed toward the lens;

an electrically switchable grating having a discrete number of grating positions wherein the electrically switchable grating is in the light path between the laser source and the lens;

a moving target passing through a measurement volume in a target direction and at a target velocity, wherein the lens focuses a pair of diffracted light beams from the electrically switchable grating into the measurement volume;

a detector directed along the light path, wherein the detector is on an opposite side of the lens from the laser source; and

a controller coupled to the electrically switchable grating and the detector, wherein the controller includes circuitry for determining a maximum peak in a power spectrum of a signal received in the detector, wherein 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.

14. The system of claim 13 , wherein the discrete number of grating positions includes three positions.

15. The system of claim 13 , wherein the discrete number of grating positions includes more than three positions.

16. The system of claim 13 , wherein the discrete number of grating positions are equally spaced along a 2π radian cycle.

17. The system of claim 13 , wherein 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.

18. The system of claim 17 , wherein the electronic display is a liquid crystal display.

19. The system of claim 17 , wherein the electronic display is a digital micro-mirror display.

20. The system of claim 13 , wherein the maximum peak occurs at a frequency less than the grating frequency if the target is moving in a reverse direction.

21. The system of claim 13 , wherein the maximum peak occurs at a frequency greater than the grating frequency if the target is moving in a forward direction.