PASSIVE TOPOLOGICALLY BIASED SAGNAC INTERFEROMETER AS A ROTATIONAL SENSOR CAPABLE OF SENSING MAGNITUDE AND DIRECTION OF ROTATION

Abstract

Many optical gyroscopes are based on an optical Sagnac interferometer configuration including various interferometric fiber-optic gyroscopes (IFOG) to measure magnitude and direction of rotation. IFOGs require active phase modulation in their fiber coil to decipher direction of rotation. This patent document discloses a new type of IFOGs that utilizes a passive topological (also known as geometric) phase shift to sense magnitude and direction of rotation without requiring active phase modulation.

Claims

1. A fiber optic Sagnac interferometer passively biased through topological phase.

2. A passively biased Sagnac interferometer comprising: a light source; a photo detector; a linear polarizer a depolarizer a topological phase bias element a fiber coil; a first non-polarizing beam splitter/circulator; a second non polarizing beam splitter; output of said light source optically connected to input of a non-polarizing first beam splitter/circulator; reflection output of first said beam splitter optically connected to said photo detector; transmission output of the said first beam splitter/circulator optically connected to input of said linear polarizer; output of said linear polarizer optically connected to input of a second non-polarizing beam splitter; transmission output of said second beam splitter optically connected to input of said depolarizer; output of said depolarizer optically connected to said topological phase bias element; output of said topological phase bias element optically connected to first input of said fiber coil; reflection output of said second beam splitter optically connected to second input of said fiber coil.

3. Apparatus of claim 2, wherein light source is depolarized.

4. Apparatus of claim 2, wherein light source is linearly polarized and aligned to axis of said linear polarizer.

5. A tethered passively biased Sagnac interferometer comprising: output of said light source optically connected to input of a non-polarizing beam splitter/circulator residing in said first box; reflection output of said beam splitter optically connected to said photo detector; transmission output of the said beam splitter optically connected to first input end of a fiber cable tethered outside of said first box; the second optics box optically connected to second end input of said tethered fiber cable; residing in said second optics box, a linear polarizer, a non-polarizing beam splitter, a depolarizer, a topological phase bias element, a non PM single mode fiber coil; output of said linear polarizer optically connected to input of non-polarizing beam splitter; transmission output of said beam splitter optically connected to input of said depolarizer; output of said depolarizer optically connected to said topological phase bias element; output of said topological phase bias element optically connected to first input of said fiber coil; reflection output of said beam splitter optically connected to second input of said fiber coil.

6. A tethered passively biased Sagnac interferometer of claim 5, wherein light source is depolarized.

7. A tethered passively biased Sagnac interferometer of claim 6, wherein optical cable is a single mode fiber.

8. A tethered passively biased Sagnac interferometer of claim 5, wherein light source is polarized.

9. A tethered passively biased Sagnac interferometer of claim 8, wherein the optical fiber cable residing outside of first said optoelectronic box is a PM single mode fiber with its axis aligned to polarization axis of said polarized light source.

10. A tethered passively biased Sagnac interferometer of claim 9, wherein polarization axis of said PM single mode fiber cable is aligned to said polarizer in second optics box.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a schematic diagram of a conventional Sagnac interferometric fiber optic gyroscope (IFOG).

[0020] FIG. 2 is a schematic diagram of the present invention fiber-optic gyroscope with topological phase bias element and depolarized light source.

[0021] FIG. 3 is a schematic diagram depicting one possible topological phase bias element.

[0022] FIG. 4 is a schematic diagram of a passively biased fiber-optic gyroscope based upon present invention with a depolarized light source and tethered single mode optical fiber.

[0023] FIG. 5 is a schematic diagram of a passively biased fiber-optic gyroscope based upon present invention with a polarized light source and tethered PM single mode optical fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] FIG. 2 depicts first preferred embodiment of the present invention. Optical source (1) emanates preferably low coherence depolarized light through optical path (4) that goes through a non-polarizing beam splitter (3), polarizer (5), non-polarizing beam splitter (6), depolarizer (7), a topological phase bias element (8), and fiber coupling element (9). In case path (4) is a collimated free optical space, element (9) is a collimator pigtailed with single mode non PM fiber (11). Light from fiber (11) couples to a non PM single mode optical fiber coil (10) clockwise. Non-polarizing beam splitter (6) also redirects part of light path (4) into light path (14). Light path (14) goes through fiber coupling element (13) which is coupled to single mode non PM fiber (12). Light from fiber (12) reaches coil (10) counterclockwise. Clockwise light from fiber coil (10) goes through optical fiber (12), collimator (13), and through optical path (14) gets redirected back into polarizer (5) by non-polarizing beam splitter (6). Counterclockwise light from coil (10) goes through fiber (11) into collimator (9), depolarizer (7), non-polarizing beam splitter (6) and interfere with clockwise light at polarizer (5). The result of this interference reaches photo detector (2) by way of optical path (4), non-polarizing beam splitter (3) and optical path (19). Since clockwise and counterclockwise paths have crossed the topological phase bias element (8), they each acquire a phase shift topologically. The bias phase shift of the Sagnac interferometer is then the difference between clockwise and counterclockwise topological phase shifts. It should be noted that optical paths (4), (14) and (19) can be free space, optical fibers or integrated optical waveguides. Non-polarizing beam splitter (3) can also be a circulator or 2?1 or 2?2 single mode fiber coupler or an integrated optical waveguide. Non-polarizing beam splitter (6) can be a 1?2 or 2?2 single mode fiber coupler or of integrated optics. Further we can choose non-polarizing beam splitter (6) as a polarizing beam splitter with polarizer (5) rotated by 45 degree respect to it. Depolarizer (7) preferably is of a Lyot type. Depolarizer (7) can also be placed on optical path (14) instead of path (4).

[0025] FIG. 3 represents one possible embodiment of topological phase bias element (8). Elements (8a) and (8c) are achromatic quarter wave plates with their optical axis rotated at 45 degrees respect to optical axis of (8b) which is an achromatic half wave plate. The Sagnac interferometer bias phase shift preferably is tuned approximately to 90 degrees by rotating axis of polarizer (5) respect to (8) which set the interferometer in its quadrature point of operation.

[0026] FIG. 4 represents a second preferred and tethered embodiment of the present invention. Here box (23) has a fiber optic bulk head (21), such as FC or LC connector, and is connected to fiber optic bulk (22) through a non PM single mode optical fiber to box (24). Box (23) comprises of depolarized light source (1), non-polarizing beam splitter or circulator (3), and photo detector (2) with same functionality as explained in first embodiment of present invention. Box (24) comprises of polarizer (5), non-polarizing beam splitter (6), depolarizers (7), topological phase bias element (8) and fiber coupling elements (9) and (13) and fiber coil (10) with same functionality as the first embodiment. Optical single mode non PM fiber cable (20) connects light path (4A) on box (23) to (4B) on box (24). If light paths (4A) and (4B) chosen to be optical fibers then connections (21) and (22) can also represent fusion spliced connections.

[0027] FIG. 5 represents a third preferred and tethered embodiment of the present invention. Here the optical fiber cable (27) represents a single mode PM fiber connecting box (26) to box (24). Box (24) has the same functionality as explained in second embodiment of present invention. Source (1a) in box (26) is linearly polarized and its polarization axis aligned to slow or fast axis of PM fiber cable (27). The PM axis of fiber (27) is also aligned to linear polarization axis of polarizer (5) of box (24). Accordingly, polarization axis of reflected light from box (24) is also aligned to slow or fast axis of PM fiber cable (27).

REFERENCES

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