Photonic bandgap fiber and fiber laser device using same

A photonic bandgap fiber comprising:

• a core region;
• and
• a cladding region having a large number of high refractive index portions disposed in a triangular lattice shape to surround the core region, the high refractive index portions having a refractive index higher than a refractive index of the core region, the cladding region including a medium having a refractive index equal to the refractive index of the core region and provided among the high refractive index portions, the photonic bandgap fiber being used in a state in which at least a part of the photonic bandgap fiber is bent with a predetermined bending radius, wherein
• the large number of the high refractive index portions are disposed in a nineteen-cell core type in three layers,
• a V value is 1.5 or greater and 1.63 or less, and
• Expression (1) and Expression (2) below are satisfied where a relative refractive index difference of the high refractive index portions to the medium is defined as Δ%, a lattice constant is defined as Λ μm, and the predetermined bending radius is defined as R cm,
• AHOM+BHOM+CHOM2+DHOM3≦Δ≦AFM+BFM+CFM2+DFM3 (1)
• 15≦R≦25 (2)
• where
• AHOM==171.37687+2.093345R−0.1275714R2
• BHOM=−43.29906+0.530802R+0.011094R2
• CHOM=3.48397−0.098659R+0.000365R2
• DHOM=−0.09034+0.003584R−0.0000348R2
• AFM=89.98658−2.94759R+0.04246R2
• BFM=−17.33858+0.743907R−0.011185R2
• CFM=1.0904−0.054614R+0.0008568R2 and
• DFM=−0.02229+0.001232R−0.00002R2,
• wherein the V value is obtained by:
• V
• =
• π
• λ
• ⁢
• d
• ⁢
• n
• high
• 2
• -
• n
• low
• 2
• where λ is a use wavelength, d is a diameter of the respective high refractive index portions, nhigh is the refractive index of the high refractive index portions, and nlow is the refractive index of the medium provided among the high refractive index portions.