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A total internal reflection (TIR) coupler for side coupling pump light into a fiber for use in an amplifier or laser is mounted on a flat surface of the fiber's inner cladding. The TIR coupler has a reflecting surface that forms an angle of taper a with the inner cladding, which is effective to refl

A total internal reflection (TIR) coupler for side coupling pump light into a fiber for use in an amplifier or laser is mounted on a flat surface of the fiber's inner cladding. The TIR coupler has a reflecting surface that forms an angle of taper a with the inner cladding, which is effective to reflect pump light at a preselected angle of incidence θincand satisfy a TIR condition at its reflecting surface for folding pump light into the fiber. The pump light is launched into the fiber at an angle that also satisfies a TIR condition for guiding pump light inside the inner cladding.

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A total internal reflection (TIR) coupler for side coupling pump light into a fiber for use in an amplifier or laser is mounted on a flat surface of the fiber's inner cladding. The TIR coupler has a reflecting surface that forms an angle of taper a with the inner cladding, which is effective to refl

A total internal reflection (TIR) coupler for side coupling pump light into a fiber for use in an amplifier or laser is mounted on a flat surface of the fiber's inner cladding. The TIR coupler has a reflecting surface that forms an angle of taper a with the inner cladding, which is effective to reflect pump light at a preselected angle of incidence θincand satisfy a TIR condition at its reflecting surface for folding pump light into the fiber. The pump light is launched into the fiber at an angle that also satisfies a TIR condition for guiding pump light inside the inner cladding. er light, together with the optical signal, causes four wave mixing (FWM) to occur in the fiber, the FWM causing a converted optical signal to be produced in the fiber and having a wavelength different from the optical signal. 14. An apparatus as in claim 13, wherein: the fiber has a first end and a second end, the optical signal travels through the fiber from the first end to the second end, the converted optical signal is output from the second end of the fiber, and the pump source pumps the fiber with pump light travelling from at least one of the group consisting of the first end of the fiber to the second end, and the second end of the fiber to the first end. 15. An apparatus as in claim 14, wherein said at least one other light includes first and second lights, the apparatus further comprising: an optical multiplexer multiplexing the optical signal, the first light and the second light together into a multiplexed light, and then providing the multiplexed light to the first end of the fiber to travel through the fiber from the first end to the second end. 16. An apparatus as in claim 14, wherein said at least one other light includes first and second lights, the apparatus further comprising: an optical multiplexer multiplexing the optical signal and the first light together into a multiplexed light, and then providing the multiplexed light to the fiber to travel through the fiber; and a fiber grating formed in the fiber, the second light generated by oscillation in the fiber grating. 17. An apparatus as in claim 13, wherein said at least one other light includes first and second lights, the apparatus further comprising: a first grating formed in the fiber, the first light being generated by oscillation in the first grating; and a second grating formed in the fiber, the second light being generated by oscillation in the second grating. 18. An apparatus as in claim 14, wherein said at least one other light includes first and second lights, the apparatus further comprising: an optical multiplexer multiplexing the optical signal and the first light together into a multiplexed light, and then providing the multiplexed light to the first or second end of the fiber to travel through the fiber; and first and second fiber gratings optically connected to the first and second ends of the fiber, respectively, the second light generated by oscillation between the first and second fiber gratings. 19. An apparatus as in claim 13, further comprising: an optical fiber optically connected to the doped fiber for extracting the converted optical signal. 20. An apparatus as in claim 13, further comprising: an optical isolator optically connected to the doped fiber. 21. An apparatus as in claim 13, wherein the doped fiber is an erbium doped fiber provided with pump light having a wavelength included in a 0.98 μm band or a 1.48 μm band so that the erbium doped fiber has a gain band which includes 1.55 μm. 22. An apparatus as in claim 13, further comprising: a ferrule holding the doped fiber. 23. An apparatus as in claim 22, wherein the ferrule has opposite end faces inclined with respect to a plane perpendicular to an axis of the doped fiber. 24. An apparatus as in claim 22, further comprising an antireflection coating formed on each of opposite end faces of the ferrule. 25. An apparatus as in claim 13, wherein the fiber is a fluoride fiber. 26. An apparatus as in claim 13, wherein a concentration of the rare earth element doped in the fiber is 50,000 ppm or higher. 27. An apparatus as in claim 13, wherein the fiber is a polarization maintaining fiber causing a polarization plane of the optical signal to be substantially parallel to a polarization plane of a light of said at least one other light. 28. An apparatus as in claim 13, wherein the fiber is a polarization maintaining fiber. 29. An apparatus as in claim 13, further comprising a nonlinear optical medium optically connected to the dop ed fiber for extracting the converted optical signal. 30. An apparatus as in claim 29, wherein the nonlinear optical medium is an optical fiber. 31. An apparatus as in claim 29, wherein the nonlinear optical medium provides chromatic dispersion in the range of ±5 ps/nm/km to the converted optical signal. 32. An apparatus as in claim 29, wherein the nonlinear optical medium is a polarization maintaining fiber, and the doped fiber is a polarization maintaining fiber. 33. An apparatus as in claim 13, further comprising: a fiber grating, said at least one other light including a light generated by oscillation in a fiber grating; and a grating controller which changes a pitch of the fiber grating. 34. An apparatus as in claim 13, wherein the fiber has first and second ends, the apparatus further comprising: first and second reflectors optically connected to the first and second ends of the fiber, respectively, said at least one other light including a light generated by oscillation between the first and second reflectors. 35. An apparatus as in claim 34, wherein each of the first and second reflectors has variable wavelength selectivity. 36. An apparatus as in claim 14, further comprising: a filter optically connected to the second end of the fiber and having a passband which passes the converted optical signal. 37. An apparatus as in claim 14, further comprising: a filter optically connected to the second end of the fiber and having a passband which passes the converted optical signal and rejects light having a wavelength different from that of the converted optical signal. 38. An apparatus as in claim 13, wherein a frequency of a light of said at least one light is changeable to cause the wavelength of the converted optical signal to be at a target wavelength. 39. An apparatus comprising: a rare earth doped optical fiber having first and second lights travelling therethrough; a pump source providing pump light to the fiber so that the first light is amplified as the first light travels through the fiber, wherein the first and second lights together cause four wave mixing (FWM) to occur in the fiber, the FWM causing a third light to be produced in the fiber and having a wavelength different from the first light. 40. An apparatus as in claim 39, wherein: the fiber has a first end and a second end, the first light travels through the fiber from the first end to the second end, the third light is output from the second end of the fiber, and the pump light travels from at least one of the group consisting of the first end of the fiber to the second end, and the second end of the fiber to the first end. 41. An apparatus as in claim 40, further comprising: an optical multiplexer multiplexing the first and second lights together into a multiplexed light, and then providing the multiplexed light to the first end of the fiber to travel through the fiber from the first end to the second end. 42. An apparatus as in claim 39, further comprising: a fiber grating formed in the fiber, the second light generated by oscillation in the fiber grating. 43. An apparatus as in claim 39, wherein the doped fiber is an erbium doped fiber provided with pump light having a wavelength included in a 0.98 μm band or a 1.48 μm band so that the erbium doped fiber has a gain band which includes 1.55 μm. 44. An apparatus as in claim 39, wherein the fiber is a polarization maintaining fiber. 45. An apparatus as in claim 39, further comprising: a fiber grating, the second light generated by oscillation in a fiber grating; and a grating controller which changes a pitch of the fiber grating. 46. An apparatus as in claim 39, wherein the fiber has first and second ends, the apparatus further comprising: first and second reflectors optically connected to the first and second ends of the fiber, respectively, the second light generated by oscillation between the first and second reflectors. 47. An appara tus as in claim 46, wherein each of the first and second reflectors has variable wavelength selectivity. 48. An apparatus as in claim 40, further comprising: a filter optically connected to the second end of the fiber and having a passband which passes the third light. 49. An apparatus as in claim 40, further comprising: a filter optically connected to the second end of the fiber and having a passband which passes the third light and rejects light having a wavelength different from that of the third light. 50. An apparatus as in claim 39, wherein a wavelength of the second light is changeable to cause the wavelength of the third light to be at a target wavelength. 51. An apparatus as in claim 39, further comprising: a controller controlling a wavelength of the second light to cause the third light to be at a target wavelength. 52. An apparatus comprising: a rare earth doped optical fiber having first, second and third lights travelling therethrough; a pump source providing pump light to the fiber so that the first light is amplified as the first light travels through the fiber, wherein the first, second and third lights together cause four wave mixing (FWM) to occur in the fiber, the FWM causing a fourth light to be produced in the fiber and having a wavelength different from the first light. 53. An apparatus as in claim 52, wherein: the fiber has a first end and a second end, the first light travels through the fiber from the first end to the second end, the fourth light is output from the second end of the fiber, and the pump light travels from at least one of the group consisting of the first end of the fiber to the second end, and the second end of the fiber to the first end. 54. An apparatus as in claim 52, further comprising: an optical multiplexer multiplexing the second and third lights together into a multiplexed light, and then providing the multiplexed light to the fiber to travel through the fiber. 55. An apparatus as in claim 52, further comprising: a first grating formed in the fiber, the second light being generated by oscillation in the first grating; and a second grating formed in the fiber, the third light being generated by oscillation in the second grating. 56. An apparatus as in claim 53, further comprising: an optical multiplexer multiplexing the first and second lights together into a multiplexed light, and then providing the multiplexed light to the first or second end of the fiber to travel through the fiber; and first and second fiber gratings optically connected to the first and second ends of the fiber, respectively, the third light generated by oscillation between the first and second fiber gratings. 57. An apparatus as in claim 52, wherein the doped fiber is an erbium doped fiber provided with pump light having a wavelength included in a 0.98 μm band or a 1.48 μm band so that the erbium doped fiber has a gain band which includes 1.55 μm. 58. An apparatus as in claim 52, wherein the fiber is a polarization maintaining fiber. 59. An apparatus as in claim 52, wherein the fiber has first and second ends, the apparatus further comprising: first and second reflectors optically connected to the first and second ends of the fiber, respectively, at least one of the second and third lights generated by oscillation between the first and second reflectors. 60. An apparatus as in claim 59, wherein each of the first and second reflectors has variable wavelength selectivity. 61. An apparatus as in claim 53, further comprising: a filter optically connected to the second end of the fiber and having a passband which passes the converted optical signal. 62. An apparatus as in claim 53, further comprising: a filter optically connected to the second end of the fiber and having a passband which passes the converted optical signal and rejects light having a wavelength different from that of the converted optical signal. 63. An apparatus as in claim 52, wherein a wavelength of at least one of the second and third lights is changeable to cause the wavelength of the fourth light to be at a target wavelength. 64. An apparatus as in claim 52, further comprising: a controller controlling a wavelength of at least one of the second and third lights to cause the fourth light to be at a target wavelength. 65. An apparatus comprising: a rare earth doped optical fiber having first and second lights travelling therethrough; a pump source pumping the fiber so that the first light is amplified as the first light travels through the fiber, wherein the first and second lights together cause four wave mixing (FWM) to occur in the fiber, the FWM causing a third light to be produced in, and output from, the fiber, the third light having a wavelength different from the first and second lights; and a filter optically connected to the fiber to filter light output from the fiber, the filter having a passband which passes the third light and rejects light having a wavelength different from that of the third light. 66. An apparatus as in claim 65, further comprising: a fiber grating formed in the fiber, the second light generated by oscillation in the fiber grating. 67. An apparatus as in claim 65, further comprising: a light generating device generating the second light and having characteristics which are variable to set a wavelength of the second light. 68. An apparatus as in claim 65, wherein the fiber has first and second ends, the apparatus further comprising: first and second reflectors optically connected to the first and second ends of the fiber, respectively, the second light generated by oscillation between the first and second reflectors. 69. An apparatus as in claim 68, wherein each of the first and second reflectors has variable wavelength selectivity. 70. An apparatus as in claim 65, further comprising: a controller controlling a wavelength of the second light to cause the third light to be at a target wavelength. 71. An apparatus as in claim 65, wherein the first light is modulated by a transmission signal, so that the third light is also modulated by the transmission signal. 72. A method comprising: pumping a rare earth doped optical fiber so that a first light travelling through the fiber is amplified; providing a second light which travels through the fiber and, together with the first light, causes four wave mixing (FWM) to occur in the fiber, the FWM causing a third light to be produced in the fiber and having a wavelength different from the first light. 73. A method as in claim 72, wherein: the fiber has a first end and a second end, the first light travels through the fiber from the first end to the second end, the third light is output from the second end of the fiber, and said pumping pumps the fiber with pump light that travels from at least one of the group consisting of the first end of the fiber to the second end, and the second end of the fiber to the first end. 74. A method as in claim 73, further comprising: multiplexing the first and second lights together into a multiplexed light; and providing the multiplexed light to the first end of the fiber to travel through the fiber from the first end to the second end. 75. A method as in claim 72, further comprising: generating the second light by oscillation in a fiber grating formed in the fiber. 76. A method as in claim 72, wherein the fiber is a polarization maintaining fiber. 77. A method as in claim 72, further comprising: generating the second light by oscillation in a fiber grating formed in the fiber; and changing a pitch of the fiber grating to change a wavelength of the second light. 78. A method as in claim 72, wherein the fiber has first and second ends, the process further comprising: generating the second light by oscillation between first and second reflectors optically connected to the first and second ends of the fiber, respectively. 79. A method as in claim 78, wherein each of the first and second reflectors has variable wavelength selectivity. 80. A method as in claim 73, further comprising: filtering light output from the second end of the fiber with a passband which passes the third light. 81. A method as in claim 73, further comprising: filtering light output from the second end of the fiber with a passband which passes the third light and rejects light having a wavelength different from that of the third light. 82. A method as in claim 72, further comprising: A controlling a wavelength of the second light to cause the third light to be at a target wavelength. 83. A method as in claim 72, further comprising: modulating the first light by a transmission signal, so that the third light is also modulated by the transmission signal. 84. A method as in claim 82, further comprising: modulating the first light by a transmission signal, so that the third light is also modulated by the transmission signal. 85. An optical communication system comprising: a transmitter transmitting an optical signal; a rare earth doped optical fiber, the optical signal travelling through the fiber; a pump source providing pump light to the fiber so that the optical signal is amplified as the optical signal travels through the fiber; a light source providing a light to the fiber so that the light, together with the optical signal, causes four wave mixing (FWM) to occur in the fiber, the FWM causing a converted optical signal to be produced in the fiber and having a wavelength different from the optical signal transmitted by the transmitter; and a receiver receiving the converted optical signal from the fiber. 86. An optical communication method comprising: transmitting an optical signal through a rare earth doped optical fiber; pumping the fiber so that the optical signal is amplified as the optical signal travels through the fiber; providing a light to the fiber which, together with the optical signal, causes four wave mixing (FWM) to occur in the fiber, the FWM causing a converted optical signal to be produced in the fiber and having a wavelength different from the transmitted optical signal; and receiving the converted optical signal from the fiber.