Patent ID: 8641274

Claim:
A polarization-maintaining fiber comprising: a core made of germanium doped silica glass; a sensing portion comprising a fiber Bragg grating that is formed in the core; a pair of stress-applying part made of boron doped silica glass that are separated from the core, and are arranged so as to face each other symmetrically for the core; a cladding made of pure silica glass that encloses the core and the stress-applying parts; and a polyimide coating layer with a thickness of 10 μm or less that surrounds the outer periphery of the cladding, wherein: a modal birefringence at a wavelength of 1550 nm and at a temperature of +22° C. is larger than or equal to 5×10 −4 ; and a polarization crosstalk is −25 dB/100 m or less.