Method and apparatus for reducing laser speckle using polarization averaging
First Claim
1. An apparatus for reducing laser speckle comprising:
- a. a polarizing beam splitter configured to divide a first polarized laser output into a second polarized laser output and a third polarized laser output, the first polarized laser output having a coherence length;
b. a light guide comprising a polarization preserving fiber optic, the light guide configured to create an optical path difference between the second polarized laser output and the third polarized laser output, the optical path difference being at least about the coherence length, the light guide being configured to direct the second polarized laser output to the polarizing beam splitter such that the polarizing beam splitter combines the second polarized laser output and the third polarized laser output into a fourth laser output; and
c. a depolarizing screen coupled to the fourth laser output, the fourth laser output illuminating the depolarizing screen.
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Abstract
A method and apparatus for reducing speckle uses polarization averaging. A polarizing beam splitter divides a first polarized laser output into a second polarized laser output and a third polarized laser output. A plurality of mirrors creates an optical path difference between the second and third polarized laser outputs. The optical path difference is at least about a coherence length for the first polarized laser output. The second and third polarized laser outputs are combined into a fourth laser output, which illuminates a depolarizing screen. If a human eye or an optical system having a intensity detector views the depolarizing screen, the eye or the intensity detector will detect reduced speckle, which results from uncorrelated speckle patterns created by the second polarized laser output and the third polarized laser output. A first alternative embodiment of the invention functions without the optical path difference being at least about the coherence length. In the first alternative embodiment, a piezoelectric transducer varies an optical path length by at least about a half wavelength of the first polarized laser output. By varying the optical path length by a sufficient frequency, the eye or the intensity detector will detect the reduced speckle. A second alternative embodiment combines two orthogonally polarized laser outputs, from two lasers, into a combined laser output. The combined laser output illuminates the depolarizing screen. A third alternative embodiment rotates the first laser output with a rotation frequency to form a rotating polarized laser output, which illuminates the depolarizing screen.
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Citations
33 Claims
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1. An apparatus for reducing laser speckle comprising:
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a. a polarizing beam splitter configured to divide a first polarized laser output into a second polarized laser output and a third polarized laser output, the first polarized laser output having a coherence length; b. a light guide comprising a polarization preserving fiber optic, the light guide configured to create an optical path difference between the second polarized laser output and the third polarized laser output, the optical path difference being at least about the coherence length, the light guide being configured to direct the second polarized laser output to the polarizing beam splitter such that the polarizing beam splitter combines the second polarized laser output and the third polarized laser output into a fourth laser output; and c. a depolarizing screen coupled to the fourth laser output, the fourth laser output illuminating the depolarizing screen.
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2. An apparatus for reducing laser speckle, comprising;
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a. a polarizing beam splitter configured to divide a first polarized laser output into a second polarized laser output and a third polarized laser output; b. a plurality of mirrors configured to create an optical path difference between the second polarized laser output and the third polarized laser output, the plurality of mirrors configured to direct the second polarized laser output to the polarizing beam splitter such that the polarizing beam splitter combines the second polarized laser output and the third polarized laser output into a fourth laser output; c. a piezoelectric transducer coupled to at least one of the mirrors, the piezoelectric transducer being driven by an electrical signal such that the optical path difference is varied by an amplitude, the amplitude being at least about a half wavelength of the first polarized laser output, the electrical signal having an electrical signal frequency; and d. a depolarizing screen coupled to the fourth laser output, the fourth laser output illuminating the depolarizing screen, the electrical signal frequency being at least a sufficient frequency such that laser speckle is reduced. - View Dependent Claims (3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
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17. An apparatus for reducing laser speckle:
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a. means for dividing a first polarized laser output into a second polarized laser output and a third polarized laser output, the first polarized laser output having a coherence length, the second polarized laser output and the third polarized laser output having orthogonal polarizations and having intensities that are about equal; b. means for oscillating an optical path length of the second polarized laser output by an amplitude and with an oscillation frequency, the amplitude being at least about a half wavelength of the first polarized laser output; c. means for combining the second polarized laser output and the third polarized laser output into a fourth laser output; and d. a depolarizing screen coupled to the fourth laser output, the fourth laser output illuminating the depolarizing screen, the oscillation frequency being at least a sufficient frequency such that laser speckle is reduced. - View Dependent Claims (18, 19, 20, 21, 22, 23)
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24. A method of reducing laser speckle comprising the steps of:
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a. dividing a first polarized laser output into a second polarized laser output and a third polarized laser output, the second polarized laser output and the third polarized laser output having orthogonal polarizations and having intensities that are about equal; b. oscillating an optical path length for the second polarized laser output by an amplitude and with an oscillation frequency, the amplitude being at least about a half wavelength of the first polarized laser output; c. combining the second polarized laser output and the third polarized laser output into a fourth laser output; and d. illuminating a depolarizing screen with the fourth laser output, the oscillation frequency being at least a sufficient frequency such that laser speckle is reduced. - View Dependent Claims (25, 26)
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27. An apparatus for reducing laser speckle comprising:
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a. means for dividing a first polarized laser output into a second polarized laser output and a third polarized laser output, the second polarized laser output and the third polarized laser output having orthogonal polarizations and having intensities that are about equal; b. means for switching between a first optical path length and a second optical path length for the second polarized laser output, a difference between the first optical path length and the second optical path length being about an odd multiple of a half wavelength of the first polarized laser output; c. means for combining the second polarized laser output and the third polarized laser output into a fourth laser output; d. means for diverging the fourth laser output in a first direction to create a fifth laser output; e. a scanning mirror coupled to the fifth laser output, the scanning mirror reflecting the fifth laser output to create a line illumination; and f. a depolarizing screen illuminated by the line illumination, the scanning mirror repeatedly scanning the line illumination across a portion of the depolarizing screen such that the means for switching maintains the first optical path length for a first scan, switches to the second optical path length for a second scan, and alternates between the first optical path length and the second optical path length for subsequent scans. - View Dependent Claims (28, 29)
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30. A method of reducing laser speckle comprising the steps of:
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a. dividing a first polarized laser output into a second polarized laser output and a third polarized laser output, the second polarized laser output and the third polarized laser output having orthogonal polarizations and having intensities that are about equal; b. switching between a first optical path length and a second optical path length for the second polarized laser output, a difference between the first optical path length and the second optical path length being about an odd multiple of a half wavelength of the first polarized laser output; c. combining the second polarized laser output and the third polarized laser output into a fourth laser output; d. diverging the fourth laser output in a first direction; and e. scanning the fourth laser output in a second direction across a portion of a depolarizing screen in a first scan with the first optical path length, in a second scan with the second optical path length, and in subsequent scans alternating between the first optical path length and the second optical path length, the second direction being orthogonal to the first direction. - View Dependent Claims (31, 32)
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33. An apparatus, for reducing laser speckle comprising:
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a. means for dividing a first polarized laser output into a second polarized laser output and a third polarized laser output, the means for dividing comprising a polarizing beam splitter, the first polarized laser output having a coherence length, the second polarized laser output and the third polarized laser output having orthogonal polarizations and having intensities that are about equal; b. a light guide comprising a polarization preserving fiber optic, the light guide coupled to the second polarized laser output, the light guide creating an optical path difference between the second polarized laser output and the third polarized laser output, the optical path difference being at least about the coherence length; c. means for combining the second polarized laser output and the third polarized laser output into a fourth laser output, the means for combining comprising the polarizing beam splitter; and d. a depolarizing screen coupled to the fourth laser output.
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Specification