Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode fiber ring laser

A novel approach for the generation of high-frequency microwave signals using a dual-wavelength single-longitudinal-mode fiber ring laser is proposed and demonstrated. In the proposed configuration, a dual-wavelength fiber Bragg grating (FBG) with two ultranarrow transmission bands in combination with a regular FBG is used to ensure single-longitudinal-mode operation of the fiber ring laser. A semiconductor optical amplifier is employed as the gain medium in the ring cavity. Since the two lasing wavelengths share the same gain cavity, the relative phase fluctuations between the two wavelengths are low and can be used to generate a low-phase-noise microwave signal without need of a microwave reference source. Three dual-wavelength ultranarrow transmission-band FBGs with wavelength spacing of 0.148, 0.33, and 0.053 nm are respectively incorporated into the laser. Microwave signals at 18.68, 40.95, and 6.95 GHz are obtained by beating the dual wavelengths at a photodetector. The spectral width of the generated microwave signals as small as 80 kHz with a frequency stability better than 1 MHz in the free-running mode at room temperature is obtained.