Bi-directional Use Of Phase And Polarization Modulators Incorporated In A Sagnac Loop For Fiber-wireless Links

As an effective solution for transmission of the wideband and high dynamic range microwave or millimeter wave, fiber-wireless links is required for a variety of commercial and military applications, such as high-capacity wireless network, and RF sensor network. Due to the increasingly high requirements on performance of the fiber-wire links, if we focus on the devices, it will be difficult to find a link solution. The thesis is aimed to find solutions of high performance links from the available devices and the photonics structure, focusing on bi-directional use of phase and polarization modulators which are incorporated in a Sagnac loop. Four novel schemes, including photonic frequency up-conversion, photonic vector signal generation, high dynamic range photonic link and wavelength reuse, have been proposed and experimentally demonstrated, these schemes not only provide the idea and results for improving the performance of the fiber-wireless links, but also provides the reference for the design of the novel photonics devices.At first, a novel technique to generate a frequency-doubled millimeter-wave (mm-W) vector signal that is immune to fiber chromatic-dispersion-induced power fading and free from interband beating interferences (IBBIs) is proposed and experimentally demonstrated. The fundamental concept of the proposed approach is to use a polarization modulator (PolM) that is incorporated in a Sagnac loop at which a light wave is modulated by an intermediate frequency vector signal along one direction and with no modulation along the opposite direction due to the traveling-wave nature of the PolM. The combination of the modulated and un-modulated signals at a photodetector will generate an mm-W signal that is immune to the fiber chromatic-dispersion-induced power fading and free from the IBBIs. The generation of a 100 and a 500 MSym/s 16-QAM signal at 30.75 GHz and the transmission of the signals over a 25-km single-mode fiber are evaluated. An error-free transmission of the 100 MSym/s signal and the transmission of the 500 MSym/s signal with a bit-error-rate below the forward error correction threshold of 3.8 x 10-3 are achieved.In addition, a novel technique to photonic generate a microwave vector signal based on a bi-directional use of a PolM and a phase modulator (PM) in a Sagnac loop is proposed and experimentally demonstrated. The in-phase (I) and quadrature-phase (Q) components of a baseband signal are mixed with a microwave carrier and then applied, respectively, to the PolM and the PM. Beating between the optical carrier and the +1st order sideband of a phase-modulated signal and beating between the optical carrier and the +1st order sideband of an intensity-modulated signal generate two microwave signals and the combination of the two microwave signals generate a microwave vector signal. A quadrature phase is introduced due to the inherent π/2 phase shift between the optical carrier and the +1st order sideband of the phase-modulated signal. The generation of a 4.5 GHz microwave signal at 625 MSym/s with QPSK modulation and the transmission of the signal over a 25-km single-mode fiber (SMF) are evaluated. An error-free transmission is achieved at a received optical power of 6 dBm. By incorporating I/Q imbalance compensation, an error-free transmission is achieved at a lower received power of 2 dBm.Furthermore, the technique of the bi-directional use of phase modulator incorporated in a Sagnac loop is used to extend the transmission distance of the microwave photonic link. A novel technique to transport a microwave signal over an optical fiber based on phase-modulation and coherent I and Q demodulation. In the transmitter, a Sagnac loop incorporating a PM is used to generate two orthogonally polarized optical signals with one being phase modulated and the other with no modulation that acts as a remote optical reference signal. The orthogonally polarized optical signals are transmitted over a SMF to a polarization and phase diversity coherent receiver (PPDCR), and are coherently detected with a free-running optical local oscillator (OLO) at the receiver. Since the phase-modulated and the reference signals are transmitted over the same SMF, the optical phases are correlated, and the original signal can be recovered based on digital signal processing (DSP) algorithm. The proposed technique is experimentally evaluated. Compared with a phase-modulated coherent I/Q demodulated link without using an optical phase correlated reference signal, the transmission distance is extended to 10 km, while providing a link gain and a shot-noise limited spurious-free dynamic range (SFDR) of-9.5 dB and 115.8 dB·Hz2/3, respectively.Finally, A novel wavelength reuse scheme to directly phase encode the upstream data on downstream light wave is demonstrated. In the base station, the downstream signal is fed to a Sagnac loop through a polarization beam splitter (PBS), and used as the upstream carrier. A PM that is incorporated in the Sagnac loop is employed to encode the upstream data to the light wave travelling along one direction of the loop, leaving light wave travelling along another direction unmodulated, due to the traveling-wave nature of the PM. At the output of the Sagnac loop, the oppositely transmitted light waves are polarization multiplexed, and are sent to a polarization and phase diversity coherent receiver over an SMF. Since the orthogonally polarized optical signal are travelled over the same optical path, the optical phase of them are correlated, thus, in the receiver, the upstream signal can be linearly recovered from the polarization multiplexed optical signal. The performance of phase re-modulation of a 2.5 GHz,625 MSym/s, QPSK signal on a downstream light wave which is also phase encoded by another 2.5 GHz,625 MSym/s, QPSK signal is evaluated, and a bit error rate of 7.6x10-11 over 25 km SMF at an optical power of-21 dBm is achieved.