A laser and method for generating an electromagnetic wave signal having high phase coherence. A distributed reflective optical fiber laser resonant cavity is formed by a non-polarization-maintaining gain optical fiber and a pair of fiber Bragg gratings of which both ends are written by a femtosecond laser direct-writing and line-etching method. During optical fiber grating writing, a tiny included angle is introduced between the projection of a laser line-etching direction on a horizontal plane where a central axis of an optical fiber core is located and a radial direction of the optical fiber core, and a tiny included angle is introduced between the laser line-etching direction and the horizontal plane where the central axis of the optical fiber core is located. The laser resonant cavity generates a three-wavelength laser output signal and eventually a millimeter wave or terahertz wave is generated between the output signals for outputting.