Multimode fiber, optical amplifier, and fiber laser
An object is to improve the efficiency of amplification by rare earth ion while maintaining beam quality of output light in a multi-mode fiber doped with rare earth ion. A multi-mode fiber ( 11 ) that includes a rare-earth-ion-doped core and that has a normalized frequency of not less than 2.40 includes a filter portion ( 111 ) that is formed by bending a partial section of or entirety of the multi-mode fiber ( 11 ), the filter portion ( 111 ) having a smallest diameter (diameter R 1 ) that is set so that (1) only LP 01 , LP 11 , LP 21 , and LP 02 modes propagate or only LP 01 and LP 11 modes propagate and (2) a loss of a highest-order one of the modes that propagate is not more than 0.1 dB/m.
1. A multi-mode fiber that comprises a rare-earth-ion-doped core and that has a normalized frequency of not less than 2.40,
the multi-mode fiber comprising a filter portion that is formed by bending a partial section of the multi-mode fiber,
the filter portion having a smallest diameter of a bend of the filter portion that is set so that (1) only LP 01 , LP 11 , LP 21 , and LP 02 modes propagate or only LP 01 and LP 11 modes propagate and (2) a loss of a highest-order one of the modes that propagate is not more than 0.1 dB/m,
wherein the multi-mode fiber comprises a remainder portion that is formed by bending a partial section of the multi-mode fiber,
the filter portion is positioned on an upstream side of the remainder portion,
the smallest diameter of the bend of the filter portion is smaller than the smallest diameter of a bend of the remainder portion.
2. The multi-mode fiber as set forth in claim 1 , wherein:
the normalized frequency of the multi-mode fiber is not less than 3.83; and
the smallest diameter of the bend of the filter portion is set so that only the LP 01 , LP 11 , LP 21 , and LP 02 modes propagate and that a loss of the LP 02 mode is not more than 0.1 dB/m.
3. The multi-mode fiber as set forth in claim 1 , wherein the smallest diameter of the bend of the filter portion is set so that only the LP 01 and LP 11 modes propagate and that a loss of the LP 11 mode is not more than 0.1 dB/m.
4. The multi-mode fiber as set forth in claim 1 , wherein the filter portion is formed by winding at least a partial section of the multi-mode fiber into a loop form or a coil form.
5. An optical amplifier comprising:
a multi-mode fiber recited in claim 1 ; and
one or more excitation light sources configured to supply excitation light to the multi-mode fiber.
6. The optical amplifier as set forth in claim 5 , wherein:
the multi-mode fiber has opposite ends, one of the opposite ends being arranged to receive incoming seed light and serving as an entrance end; and
the filter portion is formed by winding a partial section, which includes a vicinity of the entrance end, into a loop form or a coil form.
7. The optical amplifier as set forth in claim 6 , wherein:
the other of the opposite ends, which is opposite from the entrance end, of the multi-mode fiber serves as an exit end; and
the filter portion is formed by winding a middle section into a coil form, the middle section being a section that extends from the vicinity of the entrance end of the multi-mode fiber to a vicinity of the exit end of the multi-mode fiber.
8. A fiber laser comprising:
an optical amplifier recited in claim 5 ; and
a seed light source configured to supply seed light to the multi-mode fiber.
9. The fiber laser as set forth in claim 8 , wherein:
the multi-mode fiber has opposite ends, one of the opposite ends being arranged to receive incoming seed light and the other of the opposite ends serving as an exit end; and
the one or more excitation light sources are coupled to the exit end.
10. The optical amplifier as set forth in claim 1 , wherein:
the multi-mode fiber has opposite ends, one of the opposite ends being arranged to receive incoming seed light and serving as an entrance end,
the filter portion is positioned on an entrance-end side of the remainder portion.