S1: a femtosecond laser direct-writing line-writing method for writing fiber Bragg gratings including:

focusing, through a fiber cladding, a femtosecond laser inside a fiber core from a positive upward direction of a non-polarization-maintaining gain fiber; positioning a focused laser spot of the femtosecond laser on a horizontal plane where a central axis of the fiber core is located; scanning in horizontal-movement, the focused laser spot, starting from one side of a boundary of the fiber core and the fiber cladding to another side of the boundary of the fiber core and the fiber cladding, across the central axis of the fiber core for line-writing; and forming a line-type grating plane inside the fiber core;

introducing a tiny angle between a projection of a laser line-writing direction on the horizontal plane where the central axis of the fiber core is located and a radial direction of the fiber core, thereby changing a distribution of refractive indices induced by a laser on the line-type grating plane into a trident-shaped refractive index distribution curve;

introducing a tiny angle between the laser line-writing direction and the horizontal plane where the central axis of the fiber core is located, thereby generating a tiny difference between refractive indices of small peaks on both sides of the trident-shaped refractive index distribution curve to exhibit a tiny asymmetry; that is, eventually forming an asymmetric trident-shaped refractive index distribution curve;

translating in horizontal-movement, after completing one line-type grating plane, the focused laser spot one Bragg grating period along an axis direction of the fiber core; and writing, along a direction that is opposite to a line-writing direction of a previous grating plane, a subsequent line-type grating plane; and

repeating the line-writing multiple times, and eventually writing the fiber Bragg grating with a preset length;

S2: writing, by means of femtosecond laser direct-writing line-writing, the fiber Bragg grating on both ends of the non-polarization-maintaining gain fiber to form a distributed fiber laser resonator;

launching, through a first optical fiber wavelength division multiplexer, pump light from a pumping light source into the distributed fiber laser resonator; and oscillating a laser signal whose laser wavelength is determined by a wavelength of the fiber Bragg grating back and forth within the fiber laser resonator for multiple round trips to implement a laser output;

forming, by the laser output from the fiber laser resonator, a forward output through a second optical fiber wavelength division multiplexer;

forming, by the laser output from the fiber laser resonator, a backward output through the first optical fiber wavelength division multiplexer; and

eventually, implementing, by the fiber laser resonator formed by the line-inscribed fiber Bragg grating prepared based on Step S1 according to Step S2, triple-wavelength laser output with high phase correlation, highly linear polarization and single-frequency narrow-linewidth through an all-fiber pumping scheme; and

S3: implementing, through a differential frequency technology that performs a beat frequency between any two of triple-wavelength laser output signals, multiple-frequency millimeter-wave or terahertz-wave output with high phase stability, since the triple-wavelength output implemented in Step S2 has highly linear polarization and single-frequency narrow-linewidth and a high phase correlation is between the laser signals of different wavelengths, wherein a fine structure with periodic frequency intervals, and close intensity between sidebands and a principal peak is in a frequency domain.