External Modulation Optical Transmitter Key Technologies

This thesis mainly discusses the external modulation optical fiber transmission system for radar signal. the features of external modulation optical fiber transmission and the development of external modulation technique are involved. The structure of the External Modulation Transmitter and the applications of the external modulation optical fiber transmission system is also mentioned. The external modulator which is the key device of the optical fiber transmission system is discussed in detail. Because of the nolinear transmission function, the external modulator's output would introduce the nolinear distortion. To compensate the nonlinear distortion ,this thesis apply bias control and predistortion technique.The bias controller can control the bias point automatically and compensate the second order distortion effectively. It is based on the principle of feedback to keep the bias point. Photodiode monitoring convert optical signal to electrical signal,then the error signal can be obtained by bandpass filter and the error signal would be converted to DC signal by small signal detector.At last,we can use ADuC824 to collect the error signals.The bias voltage is adjusted automatically by the MCU's programs keep the bias point.Controlling the bias voltage can eliminate the external modulator's second order distortion,but the external modulator will produce odd order distortion. In this thesis,modulator's third order distortion is theoretically analysed. Considering the relation between modulator's distortion and RF drive's distortion,we seek a kind of method which can compensate the whole External Modulation Transmitter's distortion. Predistortion technique is discussed in detail. The results with and without predistortion circuit are given by simulation in ADS. It show that distortionreduction could be achieved.