| In radio over fiber systems, a linear optical transmitter is desirable in order to achieve an extended signal dynamic range. This thesis introduces and discusses a predistortion-compensation linearization technique to improve the optical transmitter's linearity. In order to compensate for the second and the third order laser distortions, the predistorter's quadratic and cubic law circuits are required to work at up to 2 or 3 times the carrier frequency. In this thesis, an alternative design approach is proposed to relieve the bandwidth requirement of the predistortion circuits. This approach uses multiple tank circuits to approximate the frequency profile shaping filters in the frequency band of interest. At the same time, tunability is provided to account for the changes of the laser's distortion behavior due to component variations, thermal effects and aging. In order to tune the predistortion circuit adaptively, a calibration algorithm was proposed and implemented for minimizing the laser distortion. This algorithm is based on multiple variable feedback, which directly processes the RF signal and does not require a DSP unit or an optimization routine. This suggests a potential complete analog implementation with less circuit complexity. A prototype predistorter IC has been designed using 0.18 mum CMOS technology. It operates near 2GHz for demonstration purpose and has more than 300MHz linearized bandwidth. 5 to 15 dB reduction of the second harmonic distortion and the third order intermodulation distortion can be obtained. |
