A Millimeter Wave Electro-Optical Transmitter for Radio over Fiber Systems

Complies with the regulations of the University and meets the accepted standards with respect to originality and quality. Date_____________________________________ iii ABSTRACT Millimeter wave radio over fiber (RoF) system has emerged with competitive advantages of high bandwidth and low distribution cost to meet the increasing demanding for high data rate wireless access. In RoF system, optical transmitter is a key device that is utilized to transmit radio frequency (RF) signal over optical fiber. An optical transmitter design involves two important components: Mach Zehnder Modulators (MZM) and Disturbed Feedback (DFB) Lasers. Unfortunately, the DFB laser faces aging problem that its slope efficiency and threshold currents change over time and temperature; the MZMs suffer from bias voltage drift effect, which leads to an unsteady output power. Power fluctuation reduces gain stability, which is indeed a problem of RoF system. The objective of this thesis is to develop an optical transmitter with low cost circuit design to overcome the above-mentioned issues, by utilizing print circuit board (PCB) integrated design. The proposed transmitter consists of an MZM, a DFB laser, a current control circuit, a Thermo Electric Cooler (TEC) control circuit, and a bias controller, with which the DFB laser produces constant optical power and the MZM bias controller provides an automatic bias control to obtain a iv steady system gain. The designed optical transmitter consists of a stable optical source and electro-optical modulator (MZM). Experiments are conducted to assess the performance of the designed optical transmitter: (1) The DFB laser with control circuit produces a stable optical output power (10mW) at wavelength 1550nm. Temperature of the DFB laser has been investigated which fluctuates no more than 0.2° C. (2) Flexible bias controller reduces the power penalty for the MZM. It is found that the MZM's output power (proportional to system's gain) biased at quadrature drifts more than 0.89dB in 30 minutes. On contrary, the output power, with bias controller, reduces its fluctuation less than 0.03dB. Experiments prove that the microcontroller is a more advanced technique for MZM's bias control design. (3) The RoF system, consisting of designed optical transmitter and a commercial optical receiver, has been investigated, which provides a 30GHz bandwidth for a millimeter wave application. Moreover, by maintaining the bias voltage, the bias controller improves the RoF system's spurious free dynamic range (SFDR) by 5.35 dB. v (4) It is found that the RoF system provides a linear range about 10 dB …