Study Of RF Photonic Front-end Technology Based On DSP Coherent Receiver

Transport of high dynamic range, broadband RF signals from a remotely located antenna over several tens of meters or several thousands of meters is required for a variety of commercial and military applications. In this so-called RF photonic front-end an RF signal is converted into an optical signal, distributed via an optical fiber and subsequently restored to the electrical format at the recipient’s end using a photodetector. On the other hand, in RF photonic front-end systems, RF signals cab be down-convert to lower intermediate frequency (IF) in optical manner where it can be more easily digitized and processed using a signal processor. The major advantage of all optical frequency down-conversion is that the electrooptic modulator response bandwidth can extend from near dc to millimeter-wave frequencies in a single device. Compared to conventional electronic links operating over coaxial cable, RF photonic front-end can provide significant advantages in the areas of bandwidth, propagation loss, and immunity against electromagnetic interference. These unique advantages of RF photonic front-end have resulted in diverse applications, such as telecommunications, military or defense systems, and aeronautics&astronautics. The major challenge of high performance RF photonic front-end is find ways to relizing high stability noise reduction and nonlinear distortion suppression. In this dissertation, several techniques to optimize the performance of DSP based RF photonics coherent receiver are intensively investigated. The main works of this dissertation are summarized as follows: Firstly, we proposed an improved DSP based linear RF photonic front-end based on a single phase modulator (PM). The fundamental principle of our approach lies in a single device that can simultaneously play the role of two phase modulators connected in parallel, which is realized using the anisotropic electrooptic coefficient of lithium niobate. The use of both orthogonal TE and TM modes of PM allows the homodyne interferometer link to be implemented with a single modulator, which reducing the matching requirements of dual modulator scheme. Experimental results show that the DSP-based linearized system increases the output third-order intermodulation intercept point (OIP3) from7.75dBm to25.25dBm and improves the Spurious Free Dynamic Range (SFDR) from112dB·Hz2/3to123.8dB·Hz2/3. Moreover, a simple and stable system can be achieved.Secondly, Although The above scheme uses a single PM to minimize the matching requirements of dual modulator scheme, it requires digital phase-locked loop to remove the remains optical phase noise. we proposed a high linear DSP-based RF photonic link based on polarization modulator (PolM) and dual-parallel polarization beam splitter (PBS). The fundamental principle of our approach is that when a PBS is placed at the output of the PolM, the polarization modulation is converted to differential mode intensity demodulation. By choosing via tuning the polarization controller (PC) in each channel, each is optically biased to produce the desired in-phase/quadrature (I/Q) demodulated signal. Thanks to the polarization-modulation to intensity-modulation conversion, the fluctuations of optical phase noise have no impact on the coherent I/Q demodulated signals. Experimental results show that the DSP-based linearization leads to suppression of the third-order intermodulation (IMD3) by more than40dB, and improves the third-order limited spurious free dynamic range of the link to124dB in a1Hz bandwidth. Moreover, there is no need peripheral equipment to minimize laser phase noise effects. Thirdly, we progress the proposed architecture to demonstrate a linear down-conversion from microwave frequency to intermediate frequency. Here we show that by using two cascaded polarization modulators, one driven by a microwave input signal and the other by a strong microwave local oscillator tone, a down-converted signal can be produced at the difference frequency. Two parallel PC and PBS is connected to the output of receiver PolM to realize I/Q intensity demodulation. We further show that by simply merging the coherent demodulation with DSP, the dynamic range of the system is significantly improved from102.7dB·Hz2/3to112dB·Hz2/3within Ku band. Most importantly, the performance of linear demodulation is flat across the entire test operational bandwidth.