Investigation Of All-Optical Microwave Signal Processing Techniques

As the increasing demand of wireless communication bandwidth, the wireless communications will be extended to high frequency microwave band. However, limited by the processing speed, it is hard for conventional electrical techniques to process high speed microwave signals. At the same time, as the increasing of the microwave frequency, the transmission loss of the microwave signal in the air is also increased, thus microwave signals cannot be transmitted in the air over a long distance. Photonics exhibites the nature advantage of broad bandwidth, therefore using photonic techniques to process high speed microwave signals can overcome the bottle neck existing in the electrical techniques. Meanwhile, as the optical fiber exhibits the character of low-loss transmission, thus using the optical fiber communication technique can transmit microwave signal over a long distance, such as radio over fiber (RoF) and ultra-wideband (UWB) over fiber. Besides, microwave signal processing with photonic approach has strong immunity to electromagnetic interference (EMI). Thus, the technique of microwave photonics has important applications in military, civil communication, etc.In the thesis, microwave photonic techniques are studied based on electrooptic modulators, optical filters and semiconductor optical amplifiers (SOAs). The main study area includes microwave photonic filter, photonic generation and modulation of UWB, and photonic generation of phase-coded microwave signal. The contributions of the thesis are list as follows:(1) Novel microwave photonic filters realized by using detuned optical bandpass filters to filt the phase modulated optical signal. It is proposed to achieve a microwave photonic filter based on an electrooptic phase modulator (EOPM) and two tunable optical bandpass filters (TOBFs). By controlling the detuning direction of the tunable optical bandpass filters (TOBFs), the microwave addition and the microwave subtraction in the optical domain are achieved respectively. Correspondingly, a microwave photonic filter switchable between bandpass filtering and bandstop filtering is achieved. An improved scheme based on arrayed waveguide grating (AWG) is also proposed. Two channels of AWG are oppositely detuned from the phase modulated signal and a microwave bandstop filter with much flatter passband and higher frequency selectivity is achieved. It also proposed to achieve a microwave photonic filter realizing bandpass and bandstop filtering shape simultaneously. By using the opposite transfer functions at the two output ports of the delay interferometer (DI), a microwave bandpass and microwave bandstop filtering shapes are obtained at the two output ports of DI respectively. Thus, microwave routing can be realized.(2) A microwave photonic filter by using a Fabry-Perot semiconductor optical amplifier (FP-SOA) to filt the phase modulated signal is proposed. By simply adjusting the bias current of FP-SOA to shift the gain spectrum of FP-SOA, a single-passband microwave photonic filter with a Q factor of184, and a flat-top microwave photonic filter with transition edges and a shape factor of1.26are obtained respectively. The tunability of the microwave photonic filter is also demonstrated.(3) Photonic generation of UWB impulses by using cascaded Mach-Zehnder modulators (MZMs) is proposed. Two Mach-Zehnder modulators (MZM) are cascaded to generate a pair of polarity-reversed UWB monocycle pulses. When the two MZMs are operating under the push-pull condition, the generated UWB impulses are chirp free. The generation of higher order UWB pulses by cascading more MZMs is also demonstrated by simulation.(4) Photonic generation of UWB impulses by using the MZM and the SOA. By cascading an MZM and an SOA to exploit the gain saturation effect of SOA to ununiformly amplify the modulated optical pulse, a pair of polarity-reversed monocycle impulses is generated. To electrically alternate the bias of MZM to realize binary phase modulation of monocycle impulse is also proposed and simulated. It is also proposed to generate a pair of polarity-reversed doublet impulses by cascading an MZM and two SOAs to exploit the gain saturation effect twice. It is also proposed to generate UWB impulses by cascading an MZM and an FP-SOA. By exploiting the feedback of FP cavity and the gain saturation effect of SOA, a pair of polarity-reversed UWB impulses is generated. The UWB transmission in the optical fiber and the space is also experimentally studied.(5) Millimeter wave band UWB impulses generation by using frequency up conversion of centimeter wave band UWB impulses. In the experiment, the DI is detuned from the phase modulated optical signal to achieve centimeter wave band UWB. By using carrier suppressed modulation in the MZM, the centimeter wave band UWB is frequency up converted and millimeter wave band UWB impulses are achieved.(6) A quadrature phase shift keying (QPSK) modulator is used to achieve phase-coded microwave signals. By adjusting the biases of the two MZMs included in the QPSK modulator seperately, carrier suppressed modulation and binary phase shift keying (BPSK) modulation are achieved respectively. Correspondingly, a phase-coded microwave signal is obtained at the output of the QPSK modulator, and the phase shift is precise π.