Apparatus for detecting coherent radiation and unequal path interferometers

• US 4,595,292 A
• Filed: 07/29/1983
• Issued: 06/17/1986
• Est. Priority Date: 07/29/1983
• Status: Expired due to Term

• Current Assignee: Goodrich Corporation (Rtx Corporation)
• Original Assignee: Perkinelmer Incorporated
• Inventors: Amodeo, Ralph J., Krohn, William
• Primary Examiner(s): Willis, Davis L.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US06/518,727
• Time in Patent Office: 1,054 Days
• Field of Search: 356/346, 356/352, 350/347 V
• US Class Current: 356/519
• CPC Class Codes: G01J 9/02 by interferometric methods using interferometers for measuring optically the linear dimensions of objects G01B9/02
• CPC Class Codes: G02F 1/216 using liquid crystals, e.g. liquid crystal Fabry-Perot filters

Abstract

The present invention is directed to an improved unequal path interferometer which is a particularly adapted, among many other possible uses, for detecting coherent radiation in the presence of non-coherent ambient radiation, which comprises an unequal optical path length interferometer of the type in which portions of the radiation impinging on the interferometric component are caused to be recombined after travelling two different optical paths; the optical paths differing in length by an amount substantially greater than the coherence length of the non-coherent radiation but substantially less than the coherence length of the coherent radiation; the unequal optical path length interferometric component comprising a liquid crystal cell having dielectric anisotropic properties; elements for applying a time varying electric field vector to the liquid crystal cell to vary the effective index of refraction of the liquid crystal cell in a preselected systematic manner; and elements for detecting the intensity of the recombined portions and producing a signal representative thereof, said signal having a variable component caused by the varying constructive and destructive interference of the recombined coherent radiation components, while the recombined non-coherent radiation components produce only a substantially steady background signal.

Claims

1. Apparatus for detecting the presence of coherent radiation in the presence of non-coherent ambient radiation comprising;

a Fabry-Perot etalon interferometer of the type in which portions of the radiation impinging on the interferometric component are caused to be recombined after travelling two different optical paths;

said optical paths differing in length by an amount substantially greater than the coherence length of the non-coherent radiation but substantially less than the coherence length of the coherent radiation;

said unequal optical path length interferometric component comprising a liquid crystal cell having dielectric anisotropic properties;

means for applying a time varying electric field vector to said liquid crystal cell to vary the effective index of refraction of said liquid crystal cell in a preselected systematic manner comprising a first electric field applied to said liquid crystal cell in the direction of the optical axis and a second electric field applied to said liquid crystal cell in an orthogonal direction, the electrical potential applied to said electrical fields being varied in a systematic manner so that a vector sum is applied to the cell which rotates with time;

means for detecting the intensity of said recombined portions and producing a signal representative thereof, said signal having a variable component caused by the varying constructive and destructive interference of the recombined coherent radiation components, while the recombined non-coherent radiation components produce only a substantially steady background signal.

2. Apparatus according to claim 1 wherein said liquid crystal cell is fabricated from cyanobiphenyl.

3. Apparatus according to claim 1 wherein said liquid crystal cell is fabricated from phenylcyclohexane.

4. In an unequal optical path length interferometer, a Fabry-Perot etalon comprising a liquid crystal cell having dielectric anisotropic properties, means for applying a time varying electric field vector to said liquid crystal cell to vary the effective index of refraction of said liquid crystal cell in a preselected systematic manner, said means for applying a time varying electric field vector comprises a first electric field applied to said liquid crystal cell in the direction of the optical axis and a second electric field applied to said liquid crystal cell in an orthogonal direction, the electrical potential applied to said electrical fields being varied in a systematic manner so that a vector sum is applied to the cell which rotates with time.

5. Apparatus according to claim 4 wherein said liquid crystal cell is fabricated from cyanobiphenyl.

6. Apparatus according to claim 4 wherein said liquid crystal cell is fabricated from phenylcyclohexane.

7. Apparatus for detecting the presence of coherent radiation in the presence of non-coherent ambient radiation comprising:

a Fabry-Perot etalon interferometer of the type in which portions of the radiation impinging on the interferometric component are caused to be recombined after travelling two different optical paths;

said optical paths differing in length by an amount substantially greater than the coherence length of the non-coherent radiation but substantially less than the coherence length of the coherent radiation;

said unequal optical path length interferometric component comprising a liquid crystal cell having dielectric anisotropic properties;

means for applying a time varying electric field vector to said liquid crystal cell to vary the effective index of refraction of said liquid crystal cell in a preselected systematic manner, said liquid crystal cell has two partially reflective sides which define the optical path, and wherein said means for applying a time varying electric field vector comprises a first set of electrodes applied to the first partially reflective side, a second set of electrodes alternating with a third set of electrodes applied to the second partially reflective side of the cell and means for controlling the electrical potential applied to the three sets of electrodes to systematically modulate the electrical potential so that gradually the major component thereof extends from a path between the first and second electrodes to a path between the first and third electrodes and then returns to a path between the first and second electrodes over a period of time in a systematic manner, whereby the liquid crystals in the cell will align themselves to and track the electric field to change the angle of the liquid crystal axes with respect to the optical axis and thereby change the index of refraction of the cell;

means for detecting the intensity of said recombined portions and producing a signal representative thereof, said signal having a variable component caused by the varying constructive and destructive interference of the recombined coherent radiation components, while the recombined non-coherent radiation components produce only a substantially steady background signal.

8. In an unequal optical path length interferometer, a Fabry-Perot etalon comprising a liquid crystal cell having dielectric anisotropic properties, means for applying a time varying electric field vector to said liquid crystal cell to vary the effective index of refraction of said liquid crystal cell in a preselected systematic manner, said liquid crystal cell has two partially reflective sides which define the optical path, and wherein said means for applying a time varying electric field vector comprises a first set of electrodes applied to the first partially reflective side, a second set of electrodes alternating with a third set of electrodes applied to the second partially reflective side of the cell and means for controlling the electrical potential applied to the three sets of electrodes to systematically modulate the electrical potential so that gradually the major component thereof extends from a path between the first and second electrodes to a path between the first and third electrodes and then returns to a path between the first and second electrodes over a period of time in a systematic manner, whereby the liquid crystals in the cell will align themselves to and track the electric field to change the angle of the liquid crystal axes with respect to the optical axis and thereby change the index of refraction of the cell.