Grating device, light source unit, and optical system

摘要

The present invention relates to a grating device and the like which can lower the degree of polarization of light, while having a structure which can be obtained inexpensively in a small size. The grating device comprises a polarization-maintaining optical fiber, and a long-period grating formed in at least a core region of the polarization-maintaining optical fiber. The grating period of the long-period grating, its effective refractive index with respect to a first mode light component, its effective refractive index with respect to a second mode light component, and respective wavelengths of the first and second mode light components satisfy a phase matching conditional expression.