Optical densitometer and optical waveguide

An optical densitometer for measuring a density of a gas or liquid of interest, the optical densitometer comprising:

• a light source capable of introducing light into a core layer;
• a detector capable of receiving the light that has propagated through the core layer;
• and
• an optical waveguide, the optical waveguide comprising:
• a substrate;
• and
• the core layer comprising a light propagation portion capable of propagating the light in an extending direction of the light propagation portion, and a diffraction grating portion,
• the diffraction grating portion comprising a diffraction grating region and an extension region connected to the diffraction grating region, and
• a first optical coupling region included in the extension region and a second optical coupling region included in the light propagation portion being optically coupled with respect to the light propagating through the core layer, wherein
• at least a portion of the diffraction grating portion is spatially separated from the light propagation portion via a material having a relatively lower refractive index than that of the core layer in a thickness direction, and
• in a region where the light propagation portion and the diffraction grating portion overlap in plan view, a first distance modification region is provided in which a distance in a thickness direction between the diffraction grating portion and the light propagation portion reduces in a direction approaching from the diffraction grating region toward the first optical coupling region.