Linearized external modulator for analog applications

Abstract

A relatively simple electronic predistortion unit to linearize a Mach Zehnder intensity modulator was demonstrated. Limitation of the linearity improvement due to the effect of multi-carriers induced third order distortion was explained. A 13 to 16 dB third order distortion reduction was achieved in a 20 channels system. introduction For analog lightwave distribution external modulation is very attractive because high power diode pump laser can be used to achieve good power budget and also the intermodulation distortion (IM) is independent of frequency. The primary disadvantage of external modulation is the nonlinear distortion associated with the standard external modulator. Linearization schemes such as electronic predistortion [1] feed-forward compensation [2] and polarization mixing [3] have been proposed to reduce the nonlinear distortion. Those techniques achieved relatively the same level of linearity improvement. For the present CATV application the primary frequency range of interest is in the 50 to 600 MHz. While the frequency is relatively low the broadband nature of the system and the high dynamic range requirement make the design of broadband compensation difficult. In this paper we demonstrated a unique predistortion circuit which offered fairly good linearization with simpler electronic circuit. In the next section an experimental set-up to evaluate the linearized modulator was described. Several limitations due to the distribution of composite distortion product arising from multi-carriers were explained. The two-tones third order IM distortion measured was below the noise of the spectrum analyzer. The degradation for wideband multi-channels system was discussed. Experiment In this section the linearized modulator and the experimental evaluation techniques were described. This work was intended for multi-carriers CATV system operating in the frequencies range of 50 to 600 MHz. However the approach could be valuable for other applications as well. The goal of this work was to reduce the third order nonlinearity generated by the modulator. With this higher optical modulation index could be employed which would result in a higher link power budget for a given optical source power. The predistortion unit presented here offered separate independent phase and amplitude control. The functional block diagram of the predistortion unit was shown in Figure 1. The modulating RF input was split by using a 10 dB RF directional coupler. The coupled output RF signal was fed into the third order distortion generator. A matched pair diode biased in the opposite direction was used to generate the third order distortion. The distortion generator output was followed by a variable gain amplifier. At the output end a 3 dB RF power combiner was used to sum both the main modulating signal and the generated distortion signal. The output was 8 / SPIE Vol. 1371 High-Frequency Analog Fiber Optic Systems(1990)