Composite substrate and method of producing composite substrate

A composite substrate, comprising in the following order:

• a surface emitting laser substrate;
• a wavelength conversion layer configured to convert incident light into light having a different wavelength;
• and
• a saturable absorber layer,
• wherein the composite substrate further comprises multilayer film arranged adjacent to the wavelength conversion layer, in at least one of a space between the surface emitting laser substrate and the wavelength conversion layer or a space between the saturable absorber layer and the wavelength conversion layer,
• wherein the multilayer film includes a plurality of refractive index layers,
• wherein the wavelength conversion layer has an end portion in a thickness direction on a side on which the multilayer film is arranged, and, in the end portion of the wavelength conversion layer in the thickness direction, a region in which an abundance of an inert gas atom is 0.5 at % or more is formed, and
• wherein, in the multilayer film, a first refractive index layer is positioned closest to the wavelength conversion layer among the plurality of refractive index layers, and an abundance of the inert gas atom in the first refractive index layer is less than 0.5 at %.