is delivered to a photodiode (u2t BPD-02 with 50 GHz-bandwidth) to be converted into an electric signal. It is then amplified through a low noise electric amplifier (LNA) and directly modulates the DFB section of the AFL through a Bias-T to accomplish the optoelectronic loop. Another part (10%) of the optical power is feedback to the AFL to form an all-optical loop. The introduction of the all-optical fiber loop is to suppress the unwanted side modes in conjunction with the optoelectronic loop. One should be noted that the purpose of using a circulator instead of an optical coupler in the all-optical loop is to avoid the interference of the lights on the PD from the two loops, which are usually realized by using dual-electrical loop [17] or polarization orthogonal dual-optical loop methods [18]. Besides, the circulator also directs the light in the all-optical loop feedback to the AFL to realize a self injection locking, so as to narrow the linewidth of the laser as well as the beating frequency [19] and increase the Q value of this MPF. The total length of the all-optical and optoelectronic loops are about 5 km and 6 km (5 km þ 1 km), respectively. A 27-dB LNA is used in the optoelectronic loop to provide sufficient electrical gain. In addition, a variable optical attenuator (VOA) and a polarization controller (PC) are used before port 1 of the circulator to control the injection strength and polarization state of the optical feedback signal, respectively. The output signal is monitored through an RF spectrum analyzer (ESA) (Agilent PXA N9030A) and an optical spectrum analyzer (OSA) (Advantest Q8384). In the proposed OEO structure, the key device is the AFL. It is a monolithic integrated dualmode laser consisting of a DFB section and a short integrated feedback cavity, as shown in Fig. 2. The 220-m-long DFB section functions as a laser source, where a complex-coupled grating is applied to guarantee the single mode operation of this section. The short integrated feedback cavity consists of a 240-m phase section and a 320-m amplifier section, allowing the adjustment of the feedback phase and strength through injection currents. Under proper bias condition, the laser can work in dual-mode state due to the two external cavity modes getting comparable threshold gain. Detailed fabrication and performance of the device is presented in [20]. As shown in Fig. 1, the dual-mode AFL first servers as the laser source for the whole oscillator. Second, the beating of the two laser modes will greatly facilitate the initial oscillation of the OEO, which traditional relies on the transition from noise to oscillation in the normal OEO loop. The beating-frequency defines the central oscillating frequency, which functions as an active MPF. Tuning of this MPF can be easily realized through adjusting the DC currents of the laser sections. Third, the AFL can be direct modulated with high efficiency around the beating-frequencies due to the photon-photon resonance. The requirement for high electrical gain in the loop can thus be relaxed. In the experiment, the AFL was an unpackaged chip mounted on a Cu heat-sink. The temperature of the device was maintained at 25 °C by a thermoelectric cooler (TEC). The DFB section