Microwave Signal Generation From a Dual-Wavelength Single-Frequency Highly $\hbox{Er}^{3+}/\hbox{Yb}^{3+}$ Co-Doped Phosphate Fiber Laser

Narrow linewidth microwave signals generated from a heterodyne detection configuration of a dual-wavelength (DW) single-frequency highly Er
³⁺
/Yb
³⁺
co-doped phosphate fiber laser is presented. The oscillating cavity of the fiber laser consists of a dual-channel narrow-band fiber-Bragg-grating (DC-NB-FBG), a 0.7-cm-long Er
³⁺
/ Yb
³⁺
co-doped phosphate fiber and a wideband FBG (WB-FBG). The wavelength selecting gratings are spatially separated to create partially separated resonant cavities. Highly Er
³⁺
/Yb
³⁺
co-doped phosphate fiber ensures that the mode competition is relatively weak under low pump power. DW single-frequency lasing with laser linewidths of 3 kHz is achieved. A 12.014-GHz microwave signal with a 3-dB linewidth of 3 kHz is obtained from the heterodyne detection of the DW fiber laser.