Predistortion linearization of directly modulated DFB lasers and external modulators for AM video transmission

Abstract

Multichannel AM video transmission systems place stringent requirements on the linearity of optical sources. Nonlinearity in the transfer function of directly modulated DFB lasers and external optical modulators produces intermodulation distortion products which limit the modulation index M which can be used. Since the SNR is proportional to M2 for shot-noise-limited AM video transmission, increasing M greatly improves system performance. The DFB lasers are limited primarily by second-order distortion products, while Mach-Zehnder modulators are limited by third-order distortion products.1 This paper describes predistortion techniques which reduce the dominant distortion of a DFB laser and external modulator by 12 and 14 dB, respectively, allowing a large increase in modulation index.