its purpose isn't to produce photons, neither is the lithium niobate. The purpose is act as a material to mediate the nonlinear interactions between photons to achieve fun things like supercontinuums, frequency comb generation, etc.

Think of it like the phosphor on your white LED - it produces no photons by itself, but it takes in the blue photons and remits them in a different spectrum. Obviously different physics, but still a similar concept.