Optical device

An optical device comprising:

• a plurality of cores each including an inner core and an outer core surrounding an outer circumferential surface of the inner core, the outer core having a refractive index lower than a refractive index of the inner core;
• and
• a cladding surrounding the outer circumferential surface of the core and having a refractive index lower than the refractive index of the outer core, wherein:
• the optical device includes a tapered portion in which the each core is tapered in diameter from a first end to a second end of the core in a longitudinal direction and a pitch between the cores adjacent to each other is decreased;
• when light propagates through the core tapered in diameter, a bending loss of an LP02 mode light beam and a bending loss of an LP21 light beam at a wavelength of 1.53 μm are 1.0 dB/m or greater at a radius of 140 mm, and a bending loss of an LP11 mode light beam at a wavelength of 1.625 μm is 0.5 dB/100 turns or less at a radius of 30 mm;
• and
• 0.5377×r2−7.7≦V2/V1≦0.5377×r2−5.7
• 3≦r2/r1≦5
• are satisfied,
• where a radius of the inner core before tapered in diameter is defined as r1, a radius of the outer core before tapered in diameter is defined as r2, a refractive index volume formed of a product of a cross sectional area of the inner core and a relative refractive index difference of the inner core to the cladding before tapered in diameter is defined as V1, and a refractive index volume formed of a product of a cross sectional area of the outer core and a relative refractive index difference of the outer core to the cladding before tapered in diameter is defined as V2.