Microwave Photonic Filter Based On Polarization Modulator

With the rapid development of modern communication, wireless transmission carries higher frequency due to the ever-growing requirements of system capacity. However, there are several bottlenecks that limit the feasibility of high-frequency microwave systems. Firstly, the limited bandwidth of microwave signals seriously restricts the further development of microwave communication. Secondly, RF signals with high frequency suffer from serious loss after long-distance transmission. Because of the disadvantages of traditional electrical filters in limited bandwidth and sampling frequency, such filters generally work in low-frequencies. Moreover, electromagnetic interference and additional loss are inevitable. In fact, the biggest disadvantage of electrical filter is that when the frequency of microwave signal is changed, it is necessary to re-design the whole filter circuit.Microwave photonic filter could achieve the processing of high-frequency microwave signals in optical domain by utilizing both of microwave and photonic devices. Such filters offter high bandwidth, high sampling rate, low loss, anti-electromagnetic interference and other advantages of fiber communication for overcoming the drawbacks of electrical approaches. In addition, the tunability of microwave photonic filters makes it more flexible in practical applications, which can achieve the filtering functionality for microwave signals in different frequencies without changing the filter structure.In this thesis, first we introduce the principle of microwave photonic filters, and analyze their main characteristics. Then the principle of phase modulation used in microwave photonic filter design is presented. Moreover, we describe the conversion principle and implementation process of phase modulation to intensity modulation with two examples by using either the single-mode fiber or the phase-shift fiber Bragg grating. The related frequency responses of such microwave photonic filters are given accordingly.Microwave photonic filters could be achieved by using the polarization properties of light. Utilizing a polarization modulator and a single laser source, microwave photonic filters with variable coefficients and complex coefficients are demonstrated. The tunability is achieved through changing the filter tap coefficients by adjusting the polarization controller. Microwave photonic filter based on polarization mode interference is also achieved through changing the filter tap coefficients by controlling the bias voltage applied on the polarization modulator. The frequency responses of microwave photonic filters are analyzed through experiments or simulation, which coincide with theoretical calculation and confirm the effectiveness of proposed schemes.