Apparatus and method for determining the optical power passing through an optical fiber

Abstract

An apparatus and method for determining the optical power transmitted through an optical fiber. The invention is based on measuring the intensity of the fluorescence produced by a doped segment of an optical fiber. The dopant is selected so that it emits light at a different wavelength than that responsible for producing the fluorescence. The doped segment is of sufficient length and dopant concentration to provide a detectable signal, but short enough to prevent the doped segment from serving as a gain medium, resulting in amplified spontaneous emission and excess fluorescence traveling along the optical fiber. The dopant material is excited by the optical signal carried by the fiber, causing a fluorescence. In the preferred embodiment the intensity of the fluorescence is proportional to the intensity of the propagating light. The signal power is then determined from the intensity of the fluorescence. The intensity of the fluorescent signal is measured by a photodetector placed so as to detect the light emitted through the side of the doped segment. The detector may wrap around the circumference of the fiber, or be placed to one side and used in conjunction with a reflector placed on the opposing side of the fiber. Filters may be used to shield the detector from other light sources and assist with accurately determining the optical power of the signal propagating within the fiber.

I claim: 
     
       1. An apparatus for determining the power carried by an optical signal propagating through an optical fiber, the apparatus comprising: a doped optical fiber segment formed in the optical fiber, the segment having a core doped with a dopant, the dopant being such that it emits a fluorescence when excited by the optical signal;   a detector for detecting the fluorescence emitted by the doped segment; and   means responsive to the detector for determining the power of the optical signal based on the fluorescence detected by the detector.   
     
     
       2. The power determining apparatus of claim 1, wherein the dopant is a rare earth metal. 
     
     
       3. The power determining apparatus of claim 2, wherein the rare earth metal is erbium. 
     
     
       4. The power determining apparatus of claim 2, wherein the rare earth metal is neodymium. 
     
     
       5. The power determining apparatus of claim 1, wherein the dopant is an organic fluorescent dye. 
     
     
       6. The power measuring apparatus of claim 1, wherein the dopant is uniformly distributed across the cross sectional area of the doped segment. 
     
     
       7. The power determining apparatus of claim 1, wherein the intensity of the fluorescence emitted by the doped segment is proportional to the intensity of the propagating signal. 
     
     
       8. The power determining apparatus of claim 1, wherein the detector is disposed adjacent to and on one side of the doped segment. 
     
     
       9. The power determining apparatus of claim 8 further comprising: a reflector disposed adjacent to the doped segment and opposite the side of the doped segment on which the detector is disposed for reflecting a portion of the emitted fluorescence back to the detector.   
     
     
       10. The power determining method of claim 1, wherein the detector wraps around the circumference of the doped segment. 
     
     
       11. The power determining apparatus of claim 1, wherein the doped segment is of sufficient length and dopant concentration to provide a detectable signal, but short enough to prevent the doped segment from serving as a gain medium. 
     
     
       12. A method for measuring the power carried by an optical signal propagating through an optical fiber, the method comprising: adding a dopant to a segment of an optical fiber, the dopant emitting a fluorescence when excited by the optical signal;   detecting the fluorescence emitted by the doped segment; and   determining the power carried by the optical signal based on the detected fluorescence.   
     
     
       13. The power measuring method of claim 12, wherein the dopant is a rare earth metal. 
     
     
       14. The power measuring method of claim 13, wherein the rare earth metal is erbium. 
     
     
       15. The power measuring method of claim 13, wherein the rare earth metal is neodymium. 
     
     
       16. The power measuring method of claim 12, wherein the dopant is an organic fluorescent dye. 
     
     
       17. The power measuring method of claim 12, wherein the dopant is uniformly distributed across the cross sectional area of the doped segment. 
     
     
       18. The power measuring method of claim 12, wherein the intensity of the fluorescence emitted by the doped segment is proportional to the intensity of the propagating signal. 
     
     
       19. The power measuring method of claim 12, wherein the fluorescence is detected by means of a detector disposed adjacent to and on one side of the doped segment. 
     
     
       20. The power measuring method of claim 19 further comprising: disposing a reflector adjacent to the doped segment and opposite the side of the doped segment on which the detector is disposed for reflecting a portion of the emitted fluorescence back to the detector.   
     
     
       21. The power measuring method of claim 12, wherein the fluorescence is detected by a detector which wraps around the circumference of the doped segment. 
     
     
       22. The power measuring method of claim 12, wherein the doped segment is of sufficient length and dopant concentration to provide a detectable signal, but short enough to prevent the doped segment from serving as a gain medium.