Fiber optic Michelson sensor and arrays with passive elimination of polarization fading and source feedback isolation

• US 5,206,924 A
• Filed: 09/01/1992
• Issued: 04/27/1993
• Est. Priority Date: 01/31/1992
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Kersey, Alan D.
• Primary Examiner(s): Lee, John D.
• Assistant Examiner(s): NGO, HUNG NHAT
• Application Number:
  US07/937,782
• Time in Patent Office: 238 Days
• Field of Search: 250/227.27, 250/227.19, 356/349, 356/350, 356/345, 356/351, 385/11, 385/12, 385/24
• US Class Current: 385/24
• CPC Class Codes: G01B 2290/70 Using polarization in the interferometer
• CPC Class Codes: G01B 9/02056 Passive reduction of errors
• CPC Class Codes: G01B 9/02058 by particular optical compensation or alignment elements, e.g. dispersion compensation
• CPC Class Codes: G01D 5/344 using polarisation G01D5/35303 takes precedence
• CPC Class Codes: G02B 6/4209 Optical features

Abstract

A fiber optic Michelson interferometric sensor system with passive elimination of polarization fading and source feedback isolation is disclosed. In a preferred embodiment, an optical source provides light of a first polarization state to a polarization selective element. Output light from the polarization selective element passes through a length of optical fiber into a Michelson interferometric sensor which utilizes a Faraday rotator/mirror assembly, that is coupled to distal ends of sensing and reference arms of the Michelson interferometer, to provide for the passive elimination of polarization fading. The polarization selective element at the output of the optical source provides isolation of the optical source from the Michelson interferometer sensor.

Claims

1. A polarization-independent fiber optic interferometric sensor system comprising:

a polarization beamsplitter having first and second input ports and first and second output ports, said polarization beamsplitter being responsive to light applied to said first input port for coupling light of a first polarization state from said first input port to said first output port and light of a second polarization state orthogonal to said first polarization state from said first input port to said second output port and being further responsive to light applied to said first output port of said second polarization state for coupling said light to said second input port;

optical source means coupled to said polarization beamsplitter for providing light of said first polarization state to said first input port;

an interferometer including an optical coupler having third and fourth input ports and third and fourth output ports, sensing and reference arms having different associated birefringences and being respectively coupled to said third and fourth output ports, and first and second Faraday rotator mirror assemblies respectively coupled to distal ends of said sensing and reference arms;

means for conveying said light of said first polarization state from said first output port to said third input port of said optical coupler, said conveying means having a birefringence which changes the polarization of said light of said first polarization state to a first arbitrary polarization state, said output coupler allowing first and second portions of said light of said first arbitrary polarization state to respectively pass into said sensing and reference fiber arms to produce respective second and third arbitrary polarization states at said distal ends of said sensing and reference fibers, said first and second Faraday rotator mirror assemblies respectively converting said first and second portions of light of said second and third arbitrary polarization states into first and second portions of light of fourth and fifth arbitrary polarization states, said sensing and reference arms being responsive to light passing therethrough for developing an interference pattern output, said optical coupler combining said first and second portions of light of said fourth and fifth arbitrary polarization states into a beam of light and passing said beam of light through said conveying means to produce said second polarization state at said first output port of said polarization beam splitter, said beam of light of said second polarization state passing through said polarization beam splitter to substantially only said second input port of said polarization beam splitter; and

photodetection means responsive to said interference pattern output for developing an electrical signal that has a peak-to-peak amplitude proportional to the visibility of said interference pattern output.

2. The fiber optical interferometric sensor system of claim 1 wherein said optical source means includes:

a laser for generating light; and

a polarization controller coupled between said laser and said polarization beamsplitter and being responsive to said light from said laser for supplying light of said first polarization state to said first input port.

3. The fiber optic interferometric sensor system of claim 1 wherein:

said conveying means is a length of ordinary fiber coupled between said first output port of said polarization beamsplitter and said third input port of said optical coupler.

4. The fiber optic interferometric sensor system of claim 1 wherein:

said interferometer is a fiber optic Michelson interferometer.

5. The fiber optic interferometric sensor system of claim 1 wherein each of said first and second Faraday rotator mirror assemblies includes:

a Faraday rotator for rotating the state of polarization of light passing therethrough by 45°

; and

a mirror for reflecting the polarization-rotated light from said Faraday rotator back through said Faraday rotator to enable said Faraday rotator to rotate the polarization state of said polarization-rotated light by another 45°

on its second passage therethrough.

6. A polarization-independent fiber optic interferometric sensor system comprising:

optical source means for providing light of a first polarization state;

a polarization beamsplitter having first and second input ports and first and second output ports and being coupled to said optical source means, said polarization beamsplitter being responsive to the application of said light of said first polarization state to said first input port for coupling said light from said first input port to said first output port;

means coupled to said first output port for conveying said light of said first polarization state from said first output port to a distal end of said conveying means, said conveying means having a birefringence which changes the polarization state of said light of said first polarization state to a first arbitrary polarization state at said distal end of said conveying means;

a fiber Michelson interferometer including;

an optical coupler having third and fourth output ports and a third input port coupled to the distal end of said conveying means, sensing and reference fiber arms having different associated birefringences and being coupled at respective proximal ends thereof to said third and fourth output ports of said optical coupler for passing first and second portions of said light of said first arbitrary polarization state therethrough to produce respective second and third arbitrary polarization states at respective distal ends of said sensing and reference arms, and first and second Faraday rotator mirror assemblies respectively disposed at the distal ends of said sensing and reference fiber arms for converting said first and second portions of light of said second and third arbitrary polarization states into first and second portions of light of fourth and fifth arbitrary polarization states, said optical coupler combining said first and second portions of said fourth and fifth polarization states into a beam of light and allowing said beam of light to pass through said conveying means to produce said second polarization state at said first output port, said beam of light of said second polarization state passing through said polarization beam splitter to substantially only said second input port; and

output means coupled to said second input port of said polarization beamsplitter and being responsive to any interference pattern contained in said beam of light for developing an electrical signal that has a peak-to-peak amplitude proportional to the visibility of said interference pattern.

7. The fiber optic interferometric sensor system of claim 6 wherein:

said optical source means includes;

a laser for generating light; and

a polarization controller coupled between said laser and said polarization beamsplitter and being responsive to said light from said laser for supplying light of said first polarization state to said first input port;

said conveying means is a length of ordinary fiber coupled between said first output port of said polarization beamsplitter and said third input port of said optical coupler;

said interferometer is a fiber optic Michelson interferometer; and

each of said first and second Faraday rotator mirror assemblies includes;

a Faraday rotator for rotating the state of polarization of light passing therethrough by 45°

; and

a mirror for reflecting the polarization-rotated light from said Faraday rotator back through said Faraday rotator to enable said Faraday rotator to rotate the polarization state of said polarization-rotated light by another 45°

on its second passage therethrough.

8. A polarization-independent fiber optic interferometric sensor system comprising:

optical source means for providing a pulse of light of a first polarization state;

a polarization beamsplitter having first and second input ports and first and second output ports and being coupled to said optical source means, said polarization beamsplitter being responsive to the application of said pulse of light of said first polarization state to said first input port for coupling said pulse of light from said first input port to said first output port;

an input fiber having a birefringence and having proximal end coupled to said first output port;

a plurality of serially-coupled optical couplers serially disposed along said input fiber;

a plurality of delay elements, each of delay T, interleaved between adjacent ones of said plurality of optical couplers;

a plurality of fiber Michelson interferometers respectively coupled to said plurality of optical couplers, each of said fiber Michelson interferometers including;

a coupler having third and fourth output ports and a third input port coupled to an associated one of said plurality of serially-coupled optical couplers, said birefringence of said input fiber changing the polarization state of said pulse of light of said first polarization state to an associated first arbitrary polarization state at said coupler, sensing and reference arms having difference associated birefringences and being respectively coupled at proximal ends thereof to said third and fourth output ports of said coupler for passing first and second portions of said light of said first arbitrary polarization state therethrough to produce respective second and third arbitrary polarization states at respective distal ends of said sensing and reference arms, and first and second Faraday rotator mirror assemblies respectively coupled to distal ends of said sensing and reference arms for converting said first and second portions of light of said associated second and third arbitrary polarization states into first and second portions of associated fourth and fifth arbitrary polarization states, said optical coupler combining said first and second portions of said associated fourth and fifth polarization states into an associated pulse of light and allowing said associated pulse of light to pass through its associated one of said plurality of serially-coupled optical couplers into said input fiber, said associated pulse from each of said plurality of serially-coupled optical couplers combining into a sequence of pulses along said input fiber which pulses return through associated ones of said delay elements to said first output port of said polarization beamsplitter, each pulse in said sequence of pulses having said second polarization state upon its arrival at said first output port, said sequence of pulses of light sequentially passing through said polarization beamsplitter to said second input port; and

output means coupled to said second input port of said polarization beamsplitter and being responsive to any interference pattern contained in any one of said sequence of pulses of light for developing an associated electrical signal that has a peak-to-peak amplitude proportional to the visibility of said interference pattern.

9. The fiber optic interferometric sensor system of claim 8 wherein said optical source means includes:

a laser for generating light;

a pulse modulator responsive to said light from said laser for developing a sequence of light pulses; and

a polarization controller responsive to each said light pulse from said pulse modulator for supplying a light pulse of said first polarization state to said first input port.

10. The fiber optic interferometer sensor system of claim 8 wherein:

said input fiber is a length of ordinary input fiber.

11. The fiber optic interferometric sensor system of claim 8 wherein each of said first and second Faraday rotator mirror assemblies of each of said fiber Michelson interferometers includes:

a Faraday rotator for rotating the state of polarization of light passing therethrough by 45°

; and

a mirror for reflecting the polarization-rotated light from said Faraday rotator back through said Faraday rotator to enable said Faraday rotator to rotate the polarization state of said polarization-rotated light by another 45°

on its second passage therethrough.

12. The fiber optic interferometer sensor system of claim 8 wherein:

said output means is a photodetector.

13. The fiber optic interferometer sensor system of claim 8 wherein:

said optical source means includes;

a laser for generating light;

a pulse modulator responsive to said light from said laser for developing a sequence of light pulses; and

a polarization controller responsive to each said light pulse from said pulse modulator for supplying a light pulse of said first polarization state to said first input port;

said input fiber is a length of ordinary input fiber;

each of said first and second Faraday rotator mirror assemblies of each of said fiber Michelson interferometers includes;

a Faraday rotator for rotating the state of polarization of light passing therethrough by 45°

; and

a mirror for reflecting the polarization-rotated light from said Faraday rotator back through said Faraday rotator to enable said Faraday rotator the polarization state of said polarization-rotated light by another 45°

on its second passage therethrough; and

said output means is a photodetector.

14. A polarization-independent fiber optic interferometric sensor system comprising:

optical source means for providing consecutive first and second pulses of light of a first polarization state;

a polarization beamsplitter having first and second input ports and first and second output ports and being coupled to said optical source means, said polarization beamsplitter being responsive to the application of said consecutive first and second pulses of light of said first polarization state to said first input port for coupling said consecutive first and second pulses of light from said first input port to said first output port;

an input fiber having a birefringence and having a proximal end coupled to said first output port;

a plurality of optical couplers serially disposed along said input fiber for respectively tapping off portions of the light in each of said consecutive first and second pulses of light;

a plurality of delay sensing coils, each of delay T, interleaved between adjacent ones of said plurality of optical couplers;

a plurality of Faraday rotator mirror assemblies, each said Faraday rotator mirror assembly reflecting light applied thereto back with a change in polarization;

a plurality of fiber arms respectively coupled between associated ones of said plurality of optical couplers and associated ones of said plurality of Faraday rotator mirror assemblies, said pluralities of optical couplers, fiber arms, Faraday rotator mirror assemblies and delay sensing coils forming a plurality of fiber Michelson interferometers, each of said fiber Michelson interferometers including;

a first arm having an associated first optical coupler coupled by way of an associated first fiber arm to an associated first said Faraday rotator mirror assembly; and

a second arm having said associated first optical coupler serially coupled by way of an adjacent said delay sensing coil to an associated second optical coupler, and an associated second fiber arm coupled between said associated second optical coupler and an adjacent second Faraday rotator mirror assembly, said first and second Faraday rotator mirror assemblies in each of said plurality of fiber Michelson interferometers changing the polarization states of the portions of the light in each of said consecutive first and second pulses of light and returning such light by way of said input fiber to said first output port of said polarization beamsplitter, each of said consecutive first and second pulses of light from each Faraday rotator mirror assembly having said second polarization state upon its arrival at said first output port and passing through said polarization beamsplitter to said second input port; and

output means coupled to said second input port of said polarization beamsplitter and being responsive to any interference pattern contained in any one of said sequence of pulses of light for developing an associated electrical signal that has a peak-to-peak amplitude proportional to the visibility of said interference pattern;

wherein, in each one of said plurality of fiber Michelson interferometers, only the first pulse of the two pulses of light reflected off of said second Faraday rotator mirror assembly will overlap the second pulse of the two pulses of light reflected off of said first Faraday rotator mirror assembly to produce an interference pattern at said output means.

15. The fiber optic interferometer sensor system of claim 14 wherein said optical source means includes:

a laser for generating light;

a pulse modulator responsive to said light from said laser for developing consecutive first and second pulses of light of a first polarization state; and

a polarization controller responsive to each of said first and second light pulses from said pulse modulator for supplying a light pulse of said first polarization state to said first input port.

16. The fiber optic interferometer sensor system of claim 14 wherein each one of said plurality of Farady rotator mirror assemblies include:

a Faraday rotator for rotating the state of polarization of light passing therethrough by 45°

; and

a mirror for reflecting the polarization-rotated light back through said Faraday rotator to enable said Faraday rotator to rotate the polarization state of said polarization-rotated light by another 45°

on its second passage therethrough.

17. The fiber optic interferometric sensor system of claim 14 wherein:

said input fiber is a length of ordinary input fiber.

18. The fiber optic interferometric sensor system of claim 14 wherein:

said output means is a photodetector.