As one of fiber laser devices which are used for a processing machine or medical equipment, an MO-PA (master oscillator-power amplifier) type fiber laser device in which light generated by a seed light source, such as a laser oscillator (MO: Master oscillator), is amplified by an amplifier (PA: power amplifier) to be emitted has been known. As one of amplifiers which are used as the above amplifier, an optical fiber amplifier which amplifies the light using an amplification optical fiber is known.
In such an amplification optical fiber, a double clad fiber in which an active element such as a rare-earth element is added in a core is generally used. The double clad fiber includes a double clad fiber in which the core propagates only single mode. light and a double clad fiber in which the core propagates multi-mode light. Therefore, in the double clad fiber in which the core propagates only the single mode light, a cross-sectional area of the core is small. Accordingly, in order to obtain a high power laser output, a density of light which may propagate in the core may be increased. In this case, an optical energy is transited to an undesired wavelength by a non-linear optical effect and thus an expected laser output cannot be obtained. Therefore, in accordance with demands for increasing a power of the optical fiber amplifier in recent years, an optical fiber amplifier using a double clad fiber in which a core propagates multi-mode light attracts attentions.
When the double clad fiber in which the light is propagated in a multi-mode is used as the amplification optical fiber, in the propagated light, an LP01 mode (basic mode) or a higher order mode such as an LP02 mode is pumped. Patent Document 1 discloses an amplification optical fiber in which an active element is added so as to be distributed in accordance with a higher order mode intensity profile.    [Patent Document 1] JP 2010-516633 W