Devices and methods for determining relative motion

1. An interferometer, comprising:

a body;

a light source located within the body and configured to project a coherent light to an opposing surface exterior to the body;

a plurality of light detectors located within the body and configured to convert reflections of the coherent light into photocurrents, wherein the plurality of light detectors includes at least two pairs of light detectors;

at least one processor configured to:

receive data input indicative of the photocurrents from the at least two pairs of light detectors;

apply a plurality of quadrature pair filters to the data input, wherein each quadrature pair filter is associated with a differing pair of light detectors;

refine the data input by removing non-quadrature signals from the data input, wherein the refined data input has higher signal-to-noise ratio than the data input;

identify in the refined data input a motion signal and a noise signal, wherein the motion signal is associated with a first change in the photocurrents that occurs in response to a relative motion between the body and the opposing surface, and the noise signal is associated with a second change in the photocurrents that does not occur in response to a relative motion between the body and the opposing surface; and

determine relative motion between the body and the opposing surface based on the motion signal identified in the refined data input.

2. The interferometer of claim 1 , wherein the second change in the photocurrents is caused by unwanted reflections of light.

3. The interferometer of claim 1 , wherein the second change in the photocurrents is caused by fluctuations in light source illumination intensity.

4. The interferometer of claim 1 , wherein a power of the light source is lower than 0.9 mW and a velocity of the determined relative motion ranges between 0.5 to 1000 mm/sec.

5. The interferometer of claim 1 , wherein the at least one processor is further configured to determine relative motion between the body and the opposing surface when the body is at a distance from the opposing surface and a ratio between a power of the motion signal and a power of the noise signal is below 1 and when the body is in contact with opposing surface and the ratio is below 5.

6. The interferometer of claim 1 , wherein the at least one processor is further configured to determine relative motion between the body and the opposing surface when the body is at a distance from the opposing surface and a ratio between a power of the motion signal and a power of the noise signal is below 0.1 and when the body is in contact with opposing surface and the ratio is below 1.

7. The interferometer of claim 1 , wherein, when the body is moved a distance of 100 mm at a constant distance from the opposing surface, the at least one processor is further configured to determine the relative motion with a measurement error below 5%.

8. The interferometer of claim 1 , wherein, when the body is moved a distance of 100 mm at a constant distance from the opposing surface, the at least one processor is further configured to use the refined data input for determining the relative motion with a measurement error below 0.25%.

9. The interferometer of claim 1 , wherein, when the body is moved a distance of 100 mm at a constant distance from the opposing surface, the at least one processor is further configured to use the refined data input for determining the relative motion with a measurement error below 0.1%.

10. The interferometer of claim 1 , wherein the least one processor is configured to detect changes while the body moves out of contact with the opposing surface and substantially parallel to the opposing surface.

11. The interferometer of claim 10 , wherein substantially parallel to the opposing surface includes movement of under 5 mm in a direction orthogonal to the plane parallel to the opposing surface.

12. The interferometer of claim 1 , wherein the plurality of light detectors includes at least three pairs of light detectors and the at least one processor is further configured to determine the relative motion in three dimensions based on refined data input from the at least three pairs of light detectors.

13. The interferometer of claim 1 , wherein the at least one processor is further configured to determine the relative motion substantially in real time.

14. The interferometer of claim 13 , wherein determining the relative motion substantially in real time includes a delay smaller than 0.1 sec from time-of-movement until time-of-determination.

15. The interferometer of claim 13 , wherein determining the relative motion substantially in real time includes a delay smaller than 0.01 sec from time-of-movement until time-of-determination.

16. The interferometer of claim 1 , wherein the data input includes interfering signals caused by at least one of: unwanted reflections of light, electrical noise, and fluctuations in light source illumination intensity, and wherein at least some of the interfering signals are non-quadrature.

17. The interferometer of claim 1 , wherein the data input includes motion signals and interfering signals, a typical amplitude of the interfering signals is larger than a typical amplitude of the motion signals and frequencies of the interfering signals differ from frequencies of the motion signals.

18. The interferometer of claim 1 , further comprising at least one analog-to-digital converter for generating from data input from at least three pairs of light detectors a data stream including information regarding a movement of the body in at least two non-collinear directions, and wherein the at least one processor is further configured to:

identify a range of frequencies of interest in the data stream based on the identified motion signal;

remove from the data stream information associated with frequencies out of the range of frequencies of interest to generate a second data stream; and

determine the relative motion between the body and the opposing surface from the second data stream.

19. A method for determining relative motion between an interferometer and an opposing surface, the method comprising:

projecting a coherent light to an opposing surface exterior to a body of the interferometer;

using a plurality of light detectors located within the body to convert reflections of the coherent light into photocurrents, wherein the plurality of light detectors includes at least two pairs of light detectors;

receiving data input indicative of the photocurrents from the at least two pairs of light detectors;

applying a plurality of quadrature pair filters to the data input, wherein each quadrature pair filter is associated with a differing pair of light detectors;

refining the data input by removing non-quadrature signals from the data input, wherein the refined data input has higher signal-to-noise ratio than the data input;

identifying in the refined data input a motion signal and a noise signal, wherein the motion signal is associated with a first change in the photocurrents that occurs in response to a relative motion between the body and the opposing surface, and the noise signal is associated with a second change in the photocurrents that does not occur in response to a relative motion between the body and the opposing surface;

and

determining the relative motion between the body and the opposing surface based on the motion signal identified in the refined data input.