Patent ID: 7440665

Claim:
An optical fiber comprising: (a) an optical core region that includes the center axis; (b) an optical cladding region that has a refractive index lower than that of the optical core region and that surrounds the optical core region; and (c) a ring-shaped acoustic core region as a part of the optical core region, each of the optical core region, the acoustic core region, and the optical cladding region being placed coaxially around the center axis and stretching along the center axis; a longitudinal-mode acoustic velocity in the acoustic core region being lower than that in the region positioned directly inside the acoustic core region and being lower than that in the region positioned directly outside the acoustic core region; the longitudinal-mode acoustic velocity in the acoustic core region and the radial thickness of the acoustic core region being predetermined so that a longitudinal mode acoustic wave that is localized in the acoustic core region and that propagates along the center axis can exist; and the optical fiber having a Brillouin gain spectrum whose 3-dB width is at least 60 MHz, and the optical fiber satisfying the relationship of t λ a ⁢ { 1 - ( V 1 V 0 ) 2 } 1 2 > 0.35 , ( 1 ) where: V 0 is a longitudinal-mode acoustic velocity that is the slower one in the group consisting of a longitudinal-mode acoustic velocity in the region directly inside the acoustic core region and a longitudinal-mode acoustic velocity in the region directly outside the acoustic core region; V 1 is a longitudinal-mode acoustic velocity in the acoustic core region; t is a radial thickness of the acoustic core region; and λ a is a wavelength of an acoustic wave and has a value of 0.537 μm.