Frequency Chirp Supported Complex Modulation of Directly Modulated Lasers

• Published in: Journal of lightwave technology Vol. 34; no. 8; pp. 1831 - 1836
• Main Authors: Di Che, Feng Yuan, Qian Hu, Shieh, William
• Format: Journal Article
• Language: English
• Published: New York IEEE 15.04.2016; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Chirp; coherent detection; frequency chirp; intensity modulation; maximum likelihood decoding; Maximum likelihood estimation; Optical fiber communication; Optical transmitters; phase modulation; Phase shift keying; Sensitivity; Signal to noise ratio; Viterbi decoding; frequency chirp; intensity modulation; optical fiber communication; maximum likelihood decoding; Viterbi decoding; Coherent detection; phase modulation
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/JLT.2015.2512298

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.