## 1 Introduction  
In recent years, the microwave signal generation method using microwave photonic technology has become a research hotspot. Photonics-assisted methods are considered as a promising replacement for electronic technology, with the advantages of having a wide bandwidth, low loss, and immunity to electromagnetic interference [1, 2, 3, 4, 5]. Various photonic arbitrary waveform generation schemes have been proposed in recent years. Among all those methods, the external optical modulation method is an efficient and promising solution that generates microwave waveforms with a full duty cycle [6, 7, 8, 9]. Typically, the external optical modulation method is based on the external modulation of a continuous wave (CW) light using electro-optical modulators. The basic principle of this method is to manipulate the modulated optical sidebands such that the generated harmonics correspond to the Fourier series expansion components of the desired pulse train in the frequency domain. A method of generating multi-functional signals, including short pulses, trapezoidal, triangular, and sawtooth waves, and double pulses based on a dual-driven Mach-Zehnder modulator (DDMZM) was proposed in [10], but additional dispersive fiber was required to filter the signal sideband. [PERSON] proposed a method to generate microwave waveform by using a sinusoidal signal to modulate the sideband generated by the polarization modulator (PolM) in Sagnac loop, but the system was vulnerable to nonlinear effects [11]. A photonic approach to generate triangular and rectangular waveforms based on a dual-parallel Mach-Zehnder modulator (DPMZM) and a tunable bandpass filter (TBPF) was proposed in [12]; however, the TBPF increased the complexity of the system. Recently, a polarization-division multiplexing Mach-Zehnder modulator (PDM-MZM) and a balanced photodetector (BPD) to generate a microwave waveform was proposed in [13]. Ref. [14] proposed a novel photonic approach for generating an arbitrary waveform, which was based on the property of real-time Fourier transform in the temporal Talbot effect. In addition, the optoelectronic oscillator (OEO) is a well-known high-frequency oscillator for generating low-phase microwave and millimeter-wave signals without employing any external microwave sources, which typically consists of many components such as a laser diode, optical fiber, modulator, photodiode, and a feedback loop complete with an electrical bandpass filter and electrical amplifier. A possible implementation of a centralized micro- and millimeter-wave signal generated by OEO distribution as a local oscillator for a 5G radio access network (RAN) was demonstrated in [15]. In paper [16], a dual-polarization modulator was used to realize a polarization multiplexed optoelectronic oscillator, by properly setting the amplitudes of the two signals that had been generated by the proposed OEO, a triangular pulse train could be generated. In [17], a self-starting OEO was built to offer stable microwave signals to generate microwave waveforms, but the scheme needed a filter both in the optical and the electrical domain. Most of the microwave waveform generators based on OEO require stabilization for short-term, long-term, and multimode operation [15]. In this paper, we propose and experimentally demonstrate a simple photonic method for the generation of full-duty-cycle sawtooth, square, and triangular waveforms by DpolDMZM. Hence, the filtering or dispersive optical devices which have been involved in previous approaches [10; 11; 12; 15; 17] obviate the need. This scheme only requires a sinusoidal radio frequency (RF) source, a laser diode (LD), a 3 dB divider, a phase shifter (PS), a dual-polarization dual-drive Mach-Zehnder modulator (Dpol-DDMZM), and a photodetector (PD). According to the Fourier series of the desired waveform, the working point and modulation depth of Dpol-DDMZM are properly set to make the first-order, second-order, and third-order harmonic signals of the photocurrent from the spectrum of the desired pulse sequence.