state was measured by a vector network analyzer (VNA) (HP 8510c), as shown in Fig. 3(c). As can be seen, the modulation response exhibits strong resonance at 33.5 GHz due to the photonphoton oscillation, which results from the dual-mode emission with 33.5-GHz mode-spacing. Zoom-in frequency response in a scanning range of 100 MHz with linear scale is plotted Fig. 3(d), which shows a −3-dB linewidth of 5.8 MHz through Lorentz fitting. This frequency response peak defines the oscillation frequency, as well as providing strong modulation response, which dramatically decreases the RF threshold gain of the whole loop. By adjusting the bias current of IA from 30 to 130 mA with IDFB fixed at 70 mA andIP biased at 0 mA, respectively, the beating-frequency of the device can be continuously tuned from 28 to 41 GHz, as shown in Fig. 4(a). The dual-mode state is chosen with the criteria that the intensity difference between the two modes is smaller than 10 dB. The overlapped frequency responses under different beating frequencies are shown in Fig. 4(b). The frequency response becomes weaker with the increasing of the beating frequency, which is due to the damping of the deliverable driving RF signal to the DFB section, since the contact pad of DFB section was not specially designed for high-frequency operation. Further increase of the frequency response can be expected if high frequency design of the DFB section is adopted. When the bias current of amplifier section exceeded 130 mA, three or more modes appeared simultaneously, which limited the further tuning of the beating-frequency. By closing the OEO loop with an optical injection power of −16 dBm measured at port 1 of the circulator, the OEO starts to oscillate at the beating-frequency of the AFL. The two lasing modes will be automatically locked to the FSR of the feedback loop due to the frequency pulling effect happed in the AFL. The dual-loop configuration performs a fine mode selection. Fig. 5(a) shows the RF spectrum of the generated 33.5-GHz signal in a 1-GHz span. The zoom-in view of this signal in 1-MHz span and 9.1-kHz RBW is also plotted in Fig. 5(b), showing a > 60 dB sidemode suppression ratio. The single-sideband (SSB) phase noise spectrum of the obtained signal is measured using the build-in phase noise module of the RF spectrum analyzer. As