Interferometric fiber optic sensor configuration with pump-induced phase carrier

Abstract

An interferometric fiber optic sensor and method are provided for controlling the optical phase of a fiber interferometer by an optically induced change in the refractive index for one arm of the fiber interferometer and providing a passive all-optical phase shift interrogation in response to this dependency on the optically induced change in the refractive index. The interferometric fiber optic sensor includes a laser source for generating light at a first predetermined wavelength, a fiber interferometer coupled to the laser source and having first and second fiber arms with a predetermined optical path difference between the fiber arms, a predetermined one of the first and second fiber arms being doped with an element for introducing an optically adjustable absorption spectrum, and a pump laser coupled to the predetermined fiber arm for generating light at a second predetermined wavelength so that an effective index for a guided mode in the predetermined fiber arm and a phase delay of the light passing through the fiber interferometer are changed. As a result, the interferometric fiber optic sensor and method allows the phase interrogation of the fiber interferometer by using a passive all-optical approach based on a pump induced refractive index change in the doped fiber arm so that balanced all fiber interferometer elements are used as sensors for eliminating laser induced phase noise.

What is claimed and desired to be secured by Letters Patent of the United States is: 
     
       1. An interferometric fiber optic sensor comprising: a laser source for generating light at a first predetermined wavelength;   a balanced fiber interferometric for sensing a time-varying field to be measured, said balanced fiber interferometer coupled to said laser source and having first and second fiber arms, a predetermined one of said first and second fiber arms being doped with an element for introducing an optically adjustable absorption spectrum into said predetermined fiber arm when said predetermined fiber arm is pumped by light at a second predetermined wavelength;   means coupled to said predetermined fiber arm for selectively providing light at the second predetermined wavelength to produce a change in a refractive index of said predetermined fiber arm and a change in phase delay of the light passing through said balanced fiber interferometer to provide a passive all-optical phase shift interrogation of said balanced fiber interferometer; and   means coupled to said balanced fiber interferometer being responsive to light passing through said balanced fiber interferometer with a changed phase delay for producing a demodulated output phase difference signal corresponding to the time-varying field being sensed.   
     
     
       2. The interferometric fiber optic sensor of claim 1 wherein: said balanced fiber interferometer comprises a balanced Mach-Zehnder interferometer.   
     
     
       3. The interferometric fiber optic sensor of claim 2 wherein: the output of said balanced fiber interferometer is (1/2) I 0  [1+kcos(φ s  -φ r )] where I 0  is a source input intensity, k is a fringe visibility value and φ s  and φ r  are respective optical phase shifts in said first and second fiber arms which are given by φ s  =(2π/λ 0 )n s  L s  and φ r  =(2π/λ 0 )n r  L r  with n s  and n r  being respective effective indices of said first and second fiber arms and L s  and L r  being the respective lengths of said first and second fiber arms.   
     
     
       4. The interferometric fiber optic sensor of claim 1 wherein: said predetermined fiber arm is doped with a rare-earth ion.   
     
     
       5. The interferometric fiber optic sensor of claim 1 wherein: said predetermined fiber arm is doped with Erbium (Er 3+ ).   
     
     
       6. The interferometric fiber optic sensor of claim 1 further including: means responsive to the light passing through said balanced fiber interferometer with the change in phase delay for producing a demodulated output phase difference signal corresponding to the time varying field being measured.   
     
     
       7. The interferometric fiber optic sensor of claim 6 wherein said producing means includes: a detector for converting the light passing through said balanced fiber interferometer to an electrical signal corresponding to the phase difference between the light in said first and second fiber arms; and   processor means for demodulating the electrical signal to produce an output signal corresponding to the time-varying field being sensed.   
     
     
       8. The interferometric fiber optic sensor of claim 1 wherein said selectively providing light means includes: a pump laser coupled to said predetermined fiber arm for providing light at the second predetermined wavelength in an on mode of operation to provide a passive all-optical phase interrogation of said balanced fiber interferometer during said on mode of operation; and   a modulator having on and off modes of operation for modulating said pump laser only during the on mode of operation to enable said pump laser to produce the second predetermined wavelength to provide the passive all-optical phase interrogation of said balanced fiber interferometer during the on mode of operation.   
     
     
       9. An interferometric fiber optic sensor comprising: a laser source for generating light at a first predetermined wavelength;   a balanced fiber interferometer for sensing a time-varying field to be measured, said balanced fiber interferometer coupled to said laser source and having first and second fiber arms, a predetermined one of said first and second fiber arms being doped with an element for introducing an optically adjustable absorption spectrum into said predetermined fiber arm when said predetermined fiber arm is pumped by light at a second predetermined wavelength;   a pump laser for generating light at the second predetermined wavelength so that an effective index for a guided mode in said predetermined fiber arm and a phase delay of output light passing through said balanced fiber interferometer are changed;   a first wavelength division multiplexing coupler for applying light from said pump laser to said balanced fiber interferometer in the opposite direction to the light from said laser source;   a modulator for causing said pump laser to selectively generate and apply light at the second predetermined wavelength to said predetermined fiber arm to provide a passive all-optical phase shift interrogation of said balanced fiber interferometer; and   means coupled to said balanced fiber interferometer being responsive to light passing through said balanced fiber interferometer with a changed phase delay for producing a demodulated output phase difference signal corresponding to the time-varying field being sensed.   
     
     
       10. The interferometric fiber optic sensor of claim 9 wherein: said (balanced) fiber interferometer comprises a balanced Mach-Zehnder interferometer.   
     
     
       11. The interferometric fiber optic sensor of claim 10 wherein: said balanced fiber interferometer comprises a second wavelength division multiplexing coupler for selectively coupling said pump laser to said predetermined fiber arm as a 50:50% splitter at said first predetermined wavelength and as a unidirectional coupler at said second predetermined wavelength.   
     
     
       12. The interferometric fiber optic sensor of claim 9 wherein: said predetermined fiber arm is doped with a rare-earth ion.   
     
     
       13. The interferometric fiber optic sensor of claim 9 wherein: said predetermined fiber arm is doped with Erbium (Er 3+ ).   
     
     
       14. An interferometric fiber optic sensor comprising: a laser source for generating light at a first predetermined wavelength;   a pump laser for generating light at a second predetermined wavelength in the same direction as the light from said laser source;   a wavelength division multiplexing coupler for coupling the outputs from said laser source and said pump laser;   a balanced fiber interferometer for sensing a time-varying field to be measured, said balanced fiber interferometer being coupled to said wavelength division multiplexing coupler, said balanced fiber interferometer having first and second fiber arms, a predetermined one of said first and second arms being doped with an element for introducing an optically adjustable absorption spectrum into said predetermined fiber arm when said predetermined fiber arm is pumped by light at a second predetermined wavelength;   a modulator for modulating said pump laser during an on mode of operation to cause said pump laser to emit light at the second predetermined wavelength during the on mode of operation so that an effective index for a guided mode in said predetermined fiber arm and a phase delay of the light passing through said balanced fiber interferometer are changed in order to provide a passive all-optical phase shift interrogation of said fiber interferometer and to cause said pump laser to not emit light at the second predetermined wavelength during the off mode of operation; and   a pump filter coupled to said balanced fiber interferometer for removing the light from said pump laser.   
     
     
       15. The interferometric fiber optic sensor of claim 14 wherein: said balanced fiber interferometer comprises a balanced Mach-Zehnder interferometer.   
     
     
       16. The interferometric fiber optic sensor of claim 14 wherein: said predetermined fiber arm is doped with a rear-earth ion.   
     
     
       17. The interferometric fiber optic sensor of claim 14 wherein: said predetermined fiber arm is doped with Erbium (Er 3+ ).   
     
     
       18. A method for optically controlling the interrogation of a balanced fiber interferometer in an interferometric fiber optic sensor, said method comprising the steps of: sensing in a sensing fiber arm of the balanced fiber interferometer a time-varying field to be measured;   optically inducing a selective change in a refractive index for one of the fiber arms of the fiber interferometer;   providing a passive all-optical phase shift interrogation of the balanced fiber interferometer in response to said step of optically inducing the selective change in said refractive index; and   detecting and demodulating the output of the interrogated balanced fiber interferometer to produce an output phase difference signal corresponding to the time-varying field being sensed.   
     
     
       19. A method for optically controlling the interrogation of a balanced fiber interferometer having sensing and reference arms, said method comprising the steps of: generating light at a first predetermined wavelength from a laser source;   coupling the balanced fiber interferometer to the laser source;   doping a predetermined one of the sensing and reference fiber arms with an element for introducing an optically adjustable absorption spectrum to the balanced fiber interferometer when the predetermined fiber arm is pumped by light at a second predetermined wavelength;   sensing in the sensing fiber arm of the balanced fiber interferometer a time-varying field to be measured;   selectively generating light at the second predetermined wavelength by a pump laser enabled by a modulator and coupled to the predetermined fiber arm to produce a phase delay in output light passing through the balanced fiber interferometer to provide a passive all-optical phase shift interrogation of the balanced fiber interferometer; and   detecting and demodulating the output of the interrogated balanced fiber interferometer to produce an output phase difference signal corresponding to the time-varying field being sensed.