Optical amplifier and an optical amplification method

Abstract

Two rare earth-doped optical fibers are connected in series and used to amplify input light. A splitter is installed between these two rare earth-doped optical fibers. The input light is monitored by having the portion of the input light that is branched off, by the splitter received by a photodiode. Excitation light output from a laser light source is guided by optical couplers and supplied to the above rare earth-doped optical fibers. A control circuit controls the output light level and, at the same time, stops the output from the laser light source when the input light level drops below a specified threshold value. The gain of the first stage rare earth-doped optical fiber while excitation light is being supplied is larger than the loss that occurs due to branching of the input light by the splitter.

What is claimed is: 
     
       1. An optical amplifier comprising:
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light; 
 an optical splitter splitting off a portion of the amplified input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and 
 an optical fiber amplifier, optically connected coupled to the optical splitter, amplifying the input light having said portion split off therefrom via by a second pumping light. 
 
     
     
       2. An optical amplifier according to  claim 1 , wherein the optical fiber amplifier is a rare earth-doped optical fiber amplifier. 
     
     
       3. An optical fiber amplifier according to  claim 1 , further comprising an optical isolator between the optical splitter and the optical fiber amplifier. 
     
     
       4. An optical fiber amplifier according to  claim 1 , further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power. 
 
     
     
       5. An optical fiber amplifier according to  claim 2 , further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power. 
 
     
     
       6. An optical fiber amplifier according to  claim 3 , further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power. 
 
     
     
       7. An optical amplifier comprising:
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light; 
 an optical splitter splitting off a portion of the amplified input light, the first pumping light being controllable in accordance with a monitored optical power of said split portion; and 
 an optical fiber amplifier, optically connected coupled to the optical splitter, amplifying the input light having said portion split off therefrom via by a second pumping light. 
 
     
     
       8. An optical amplifier comprising:
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light; 
 an optical splitter splitting off a portion of the amplified input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and 
 an optical fiber amplifier amplifying the input light having said portion split off therefrom via by a second pumping light. 
 
     
     
       9. An optical amplifier according to  claim 8 , wherein the optical fiber amplifier is a rare earth-doped optical fiber amplifier. 
     
     
       10. An optical fiber amplifier according to  claim 8 , further comprising an optical isolator between the optical splitter and the optical fiber amplifier. 
     
     
       11. An optical fiber amplifier according to  claim 8 , further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power. 
 
     
     
       12. An optical fiber amplifier according to  claim 9 , further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power. 
 
     
     
       13. An optical fiber amplifier according to  claim 10 , further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power. 
 
     
     
       14. An optical amplifier comprising:
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light; 
 an optical splitter splitting off a portion of the amplified input light, the first pumping light being controllable in accordance with a monitored optical power of said split portion; and 
 an optical fiber amplifier amplifying the input light having said portion split off therefrom via by a second pumping light. 
 
     
     
       15. An apparatus comprising:
 an optical amplifier including
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light, 
 an optical splitter splitting off a portion of the amplified input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and 
 an optical fiber amplifier amplifying the input light having said portion split off therefrom via by a second pumping light. 
 
 
     
     
       16. An apparatus comprising:
 an optical amplifier including
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light, 
 an optical splitter splitting off a portion of the amplified input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and 
 an erbium doped fiber amplifier (EDFA) including an erbium doped fiber (EDF) through which the input light having said portion split off therefrom travels and is amplified via by a second pumping light traveling through the EDF. 
 
 
     
     
       17. An apparatus comprising:
 an optical amplifier including
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light, 
 means for splitting off a portion of the amplified input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and 
 an optical fiber amplifier amplifying the input light having said portion split off therefrom via by a second pumping light. 
 
 
     
     
       18. An apparatus comprising:
 an optical amplifier including
 an optical fiber through which an input light travels, the input light being amplified by Raman scattering as the input light travels through the optical fiber via and a first pumping light traveling through the optical fiber in an opposite direction than the input light, 
 means for splitting off a portion of the amplified input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and 
 an erbium doped fiber amplifier (EDFA) including an erbium doped fiber (EDF) through which the input light having said portion split off therefrom travels and is amplified via by a second pumping light traveling through the EDF. 
 
 
     
     
       19. An apparatus comprising:
 an optical splitter splitting off a portion of an input light having been amplified by Raman scattering as the input light traveled through an optical fiber and a first pumping light traveling through the optical fiber in an opposite direction than the input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and   an optical fiber amplifier, optically coupled to the optical splitter, amplifying the input light having said portion split off therefrom by a second pumping light.   
     
     
       20. An apparatus according to claim 19, wherein the optical fiber amplifier is a rare earth-doped optical fiber amplifier. 
     
     
       21. An apparatus according to claim 19, further comprising an optical isolator between the optical splitter and the optical fiber amplifier. 
     
     
       22. An apparatus according to claim 19, further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power.   
     
     
       23. An apparatus according to claim 20, further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power.   
     
     
       24. An apparatus according to claim 21, further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power.   
     
     
       25. An apparatus according to claim 19, wherein the optical fiber is an erbium doped fiber. 
     
     
       26. An apparatus according to claim 19, wherein there are no optical components between the optical splitter and the optical fiber amplifier. 
     
     
       27. An apparatus comprising:
 an optical splitter splitting off a portion of an input light having been amplified by Raman scattering as the input light traveled through an optical fiber and a first pumping light traveling through the optical fiber in an opposite direction than the input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and   an optical fiber amplifier amplifying the input light having said portion split off therefrom by a second pumping light.   
     
     
       28. An apparatus according to claim 27, wherein the optical fiber amplifier is a rare earth-doped optical fiber amplifier. 
     
     
       29. An apparatus according to claim 27, further comprising an optical isolator between the optical splitter and the optical fiber amplifier. 
     
     
       30. An apparatus according to claim 28, further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power.   
     
     
       31. An apparatus according to claim 29, further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power.   
     
     
       32. An apparatus according to claim 27, wherein the optical fiber is an erbium doped fiber. 
     
     
       33. An apparatus according to claim 27, wherein there are no optical components between the optical splitter and the optical fiber amplifier. 
     
     
       34. An apparatus comprising:
 an optical splitter splitting off a portion of an input light having been Raman amplified by a first pumping light traveling in an opposite direction than, and along the same travel path as, the input light, the first pumping light being controlled in accordance with a monitored optical power of said split portion; and   an optical fiber amplifier, optically coupled to the optical splitter, amplifying the input light having said portion split off therefrom by a second pumping light.   
     
     
       35. An apparatus according to claim 34, further comprising an optical isolator between the optical splitter and the optical fiber amplifier. 
     
     
       36. An apparatus according to claim 34, further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power.   
     
     
       37. An apparatus according to claim 35, further comprising:
 a monitor monitoring the optical power of said split portion, to thereby provide said monitored optical power.   
     
     
       38. An apparatus according to claim 34, wherein there are no optical components between the optical splitter and the optical fiber amplifier. 
     
     
       39. An optical amplifier according to claim 1, wherein the optical fiber is doped with a rare earth element. 
     
     
       40. An optical amplifier according to claim 7, wherein the optical fiber is doped with a rare earth element. 
     
     
       41. An optical amplifier according to claim 8, wherein the optical fiber is doped with a rare earth element. 
     
     
       42. An optical amplifier according to claim 14, wherein the optical fiber is doped with a rare earth element. 
     
     
       43. An apparatus according to claim 15, wherein the optical fiber is doped with a rare earth element. 
     
     
       44. An apparatus according to claim 16, wherein the optical fiber is doped with a rare earth element. 
     
     
       45. An apparatus according to claim 17, wherein the optical fiber is doped with a rare earth element. 
     
     
       46. An apparatus according to claim 18, wherein the optical fiber is doped with a rare earth element. 
     
     
       47. An apparatus for receiving an optical signal transmitted through an optical fiber in a first direction, comprising:
 a pumping light source to output a pumping light to the optical fiber so that the pumping light travels through the optical fiber in a second direction opposite to the first direction for Raman scattering;   an optical coupler to receive the optical signal from the optical fiber and to output the received optical signal and a monitor signal of the received optical signal, the pumping light source being controlled in accordance with the monitor signal to thereby control the pumping light output by the pumping light source; and   an optical amplifier to amplify the received optical signal output from the optical coupler.   
     
     
       48. An apparatus according to claim 47, wherein the optical fiber is doped with a rare earth element. 
     
     
       49. An optical transmission system, comprising:
 an optical transmitting station to transmit an optical signal through an optical fiber in a first direction; and   an optical repeater, coupled to the optical fiber, including:
 a pumping light source to output a pumping light to the optical fiber so that the pumping light travels through the optical fiber in a second direction opposite to the first direction for Raman scattering, 
 an optical coupler to receive the optical signal from the optical fiber and to output the received optical signal and a monitor signal of the received optical signal, the pumping light source being controlled in accordance with the monitor signal to thereby control the pumping light output by the pumping light source and 
 an optical amplifier to amplify the received optical signal from the optical coupler and to output the amplified optical signal. 
   
     
     
       50. An optical transmission system, according to claim 49, wherein the optical fiber is doped with an rare earth element. 
     
     
       51. An optical transmission system, comprising:
 an optical repeater, coupled to an optical fiber through which an optical signal is transmitted in a first direction, including:
 a pumping light source to output a pumping light to the optical fiber so that the pumping light travels through the optical fiber in a second direction opposite to the first direction for Raman scattering, 
 an optical coupler to receive the optical signal from the optical fiber and to output the received optical signal and a monitor signal of the received optical signal, the pumping light source being controlled in accordance with the monitor signal to thereby control the pumping light output by the pumping light source, and 
 an optical amplifier to amplify the received optical signal output from the optical coupler; and 
   an optical receiver, operatively coupled to the optical repeater, to receive the amplified optical signal.