Glass substrate and optical integrated device

A glass substrate comprising:

• a core portion to be an optical waveguide;
• and
• a cladding portion,
• wherein the core portion and the cladding portion both comprise a glass,
• the core portion has a higher Ag concentration than the cladding portion,
• a Ag concentration gradient is present from a boundary between the core portion and the cladding portion toward a region in the core portion where the Ag concentration is maximum,
• the core portion is a region where a refractive index is equal to or greater than a value represented by {N+(Δn/2)}, where Δn is a refractive index difference represented by (Nmax−N), Nmax is a maximum value of a refractive index in the core portion, and N is a refractive index of the cladding portion,
• the refractive index difference Δn is 0.005 or more, and
• a core thickness Δd of the core portion in a thickness direction of the glass substrate is 2.5 μm to 10 μm.