I. Introduction
Binarily modulated microwave signals are always used in electronic warfare, radar and communication systems. Amplitude shift keying (ASK), phase shift keying (PSK) and frequency-shift keying (FSK) microwave signals are basic binarily modulated signals and can be used to synthesize high-order modulated signals to increase the system performance [1]–[4]. Limited by the electronic bottleneck, the ASK, PSK, and FSK signals generated in the electronic domain suffer from low carrier frequency and coding bit rate, which cannot cope with the increasing communication capability and millimeter-wave radar. Microwave photonics has the characteristics of high frequency, large bandwidth, anti-electromagnetic interference, and reconfiguration [5], [6]. Therefore, photonic generation of binarily modulated microwave signals has become a promising signal generation technique. For the generation of ASK signal, frequency up-conversion is a commonly used method [7]. However, it is necessary to further filter out the severe leaked local oscillator (LO) signal to decrease the interference to the receiver detection. Based on direct photonic conversion technique [8] and frequency-to-time mapping [9], the LO signal can be suppressed. Whereas, some baseband components, which are called “background noise” are involved in the generated ASK signal. The background noise may result in crosstalk with other signals. By taking use of the phase modulation and intensity modulation switching technique [10], [11], LO-free and background-free ASK signal can be generated. For the PSK signal, it can be generated by coding the relative phase of two optical signals [12], [13]. Based on the vector sum method, photonic generation of background-free PSK signal is achieved [14]. By switching the modulation state of the modulator at the positive and negative quadrature bias points of its transmission curve, PSK signal can also be successfully generated [15]. Through the combination of double-sideband modulated signal and phase modulated optical carrier, binary PSK signal is produced [16]. For the FSK signal generation, a polarization modulator (PolM) is always used to switch the orthogonal polarization state of the output optical signal. By cascading a polarization-dependent modulator which is driven by two microwave signals of different frequencies, an FSK signal can be obtained [17], [18]. Photonic generation of FSK signal can be realized via equivalently changing the bias of a dual-drive Mach-Zehnder modulator (DDMZM), driven by a coding signal and a microwave carrier, at quadrature and the minimum transmission points [19]. In order to eliminate the background noise, a unipolar ASK signal is mixed with an RF clock, and a DDMZM is driven to generate a background-free FSK signal [20]. Nevertheless, the reconfigurability of the all above schemes is insufficient, since they can only generate binarily modulated signals in one format.