Abstract

The frequency chirp of the directly modulated lasers (DML) has long been regarded as the performance barrier preventing the DML from being employed in high-speed optical transmissions. In contrast, we regard the chirp as the combination of intensity modulation (IM) and frequency modulation in this paper. By utilizing coherent detection which provides a wavelength reference by the local oscillator, FM can be converted to phase modulation via time integral. Namely, we realize a 2 dimensional complex modulation (CM) using a single DML. The channel of a complex modulation of directly modulated lasers (CM-DML) is convolutional due to the integral operation. The maximum likelihood sequence estimation can be applied for demodulation. CM-DML offers more than 10-dB system OSNR sensitivity advantages over the conventional IM-DML using coherent detection, which shrinks the sensitivity penalty to <6 dB compared to the QPSK at 7% FEC threshold. Using CM-DML, we experimentally demonstrate the first dual polarization (DP) PAM-4 signal over >1000-km transmission of the standard single mode fiber. This reach record of the optical PAM reveals the great potentials of the CM-DML to replace the commercialized DP-QPSK products for medium reach applications.

© 2015 IEEE

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