Narrow linewidth high frequency microwave signal generator based on composite filter structure

In order to realize the output of single longitudinal mode dual-wavelength laser signal at lower pump power and obtain the high frequency microwave signal with narrow line width, the narrow linewidth high frequency microwave signal generator based on multiple filter compound structure was proposed and demonstrated. The dual-wavelength Stokes optical signal with four times Brillouin frequency shift interval was realized through the eight shaped Brillouin cavity structure and the wavelength selective filter composed by reflective fiber grating. The 200 m length single mode fiber was used as gain medium, and it forms a cascaded fiber ring structure with 50 m long single-mode fiber. A three-port coupler and 2 m long unpumped polarization-maintaining erbium-doped fiber was used to form a Sagnac ring structure. The cascaded fiber ring configuration and Sagnac ring configuration were designed to select mode for single longitudinal mode Stokes optical signal. The experiment proves that the high-frequency microwave signal of 42.85 GHz can be generated by the beat frequency detection of the output single-longitudinal-mode dual-wavelength Stokes optical signal, and the line width is 38 kHz; Changing the output wavelength of the tunable pump laser, frequency tuning in the range of 42.25-43.51 GHz can be achieved; Through the stability test, the frequency change of the 42.85 GHz high-frequency microwave signal is in the range of 0.83 MHz, and the peak power change is in the range of ±0.8 dB. It has good stability and meets the actual application requirements.