When a grating is inscribed in a section of optical fiber through a coating of the optical fiber, using a light modulation mask to modulate the light beam that writes the grating, a fluid can be situated between the section of optical fiber and the back side of a mask component carrying the light modulation mask (e.g., on its front side) to reduce the refractive-index discontinuity encountered at the surface of the coating. In various embodiments, rather than running the fiber through a vessel containing the fluid, the fluid is run across the back side of the mask component or retained by capillary action between the fiber section and the mask component.