## 1 Introduction  
Microwave signal filtering plays an important role in microwave signal processing fields [1; 2], such as wireless local area network (WLAN) communication, mobile communication and radar systems. Once inferior to traditional electrical microwave filters [3], filters based on photonic links have now come to be considered an effective solution for low transmission-loss, high-frequency, large-bandwidth microwave signal processing. Such solutions find applications in next-generation wireless networks, radar systems and electronic warfare systems. Microwave photonic filters (MPFs) can typically be demonstrated using time-delay [4; 5] and non-delay structures [6; 7; 8]. However, the MPFs realized by using time-delay structures are limited by the intrinsic periodic spectral response due to the finite impulse response, hindering their periodic tunability. In addition, they usually need single-mode optical fibers (G. 652) several kilometersin length, or dispersion-compensating optical fibers of high dispersion coefficients, both of which make the structure of MPFs more highly multiplex. Numerous methods employing non-delay structures have been proposed and experimentally demonstrated in recent years, such as stimulated Brillouin scattering (SBS) [6], semiconductor optical amplifier [7] and phase-shifted fiber Bragg grating (PS-FBG) [8]. In [6], a high Q-factor MPF based on SBS was demonstrated, but the tuning range of the MPF was limited by the range of Brillouin frequency shift of low-tunability. In [7], an MPF based on a semiconductor optical amplifier with a frequency tuning range from 5 to 35 GHz was obtained, with a large 3-dB bandwidth around 4 GHz. In [8], the PS-FBG was used to implement conversion from phase-modulation to intensity-modulation (PM-IM), causing high insertion loss and low tunability of the passband bandwidth. Furthermore, dual-passband MPFs are indispensable and widely studied in different fields for a variety of practical applications, such as in multiband microwave systems like wireless and mobile communication, radar systems and satellite communication [9; 10; 11; 12]. In [9], a switchable, tunable, dual-passband MPF based on a reflective-fiber Mach-Zehnder interferometer and dispersive medium was proposed, but was limited by the poor tunability of the time-delay structure. In [10], PM-IM conversion in two channels was implemented in order to obtain an MPF with two independently tunable passbands using a phase modulator (PM) and an equivalent phase-shifted fiber Bragg grating (EPS-FBG). However, the wavelength spacing of the two wavelengths in the proposed MPF was small, making center frequencies of the two passbands close and the frequency spacing tunability of the two passbands poor. In [11], a tunable dual-passband MPF based on a polarization modulator and a PS-FBG was demonstrated, but due to the influence of the nonlinear phase response of the PS-FBG, the sidelobes of the passband were high. In [12], a tunable and reconfigurable MPF based on photonic waveguide platform and SBS was proposed, which can adjust the shape of the MPF by electrically tailoring the SBS pump using an arbitrary waveform generator (AWG). The use of an AWG in the proposed MPF also realized multiple filters with varying center frequencies and bandwidth. However, the filter response was not ideal with a high passband ripple induced by the influence of the nonlinear effect, so it was necessary to use a feedback loop controlled by a computer to reduce the passband ripple in the system, which increased the complexity of the MPF. In the present paper, we propose a new dual-passband MPF based on optical-injected distributed feedback (DFB) semiconductor lasers and a dual-output Mach-Zehnder modulator (DOMZM), with the merits of widely tunable range of center frequency, high flexibility and low insertion loss. In the proposed MPF, the microwave signals are applied to a DOMZM to achieve complementary intensity modulation.