A dielectric-grating waveguide free-electron laser device generating coherent or laser-like radiation is provided. An electron beam propagates next to a dielectric waveguide with a built-in grating structure to generate highly confined coherent or laser-like radiation in the waveguide through the Bragg resonance, the backward-wave resonance, or the Fabry-Perot resonance provided by the grating-waveguide structure. The dielectric-grating waveguide can be made of linear optical materials or nonlinear optical materials or combination of linear and nonlinear optical materials to enable versatile functionalities, such as laser generation, laser-wavelength conversion, and laser signal processing. Owing to the build-up of the laser modes inside the dielectric waveguide, coherent or laser-like Smith-Purcell radiation is also generated above the grating via coupling and bunching of the electrons with the surface mode fields.