Enabling Complex Modulation of Directly Modulated Signals Using Laser Frequency Chirp

• Published in: IEEE photonics technology letters Vol. 27; no. 22; pp. 2407 - 2410
• Main Authors: Di Che, Qian Hu, Feng Yuan, Qi Yang, Shieh, William
• Format: Journal Article
• Language: English
• Published: IEEE 15.11.2015
• Subjects: Chirp; Coherence; Coherent detection; Frequency chirp; Maximum likelihood decoding; Modulation; Optical fiber communication; Optical receivers; Optical sensors; Optical transmitters; Phase modulation; Viterbi algorithm; coherent detection; Optical fiber communication; Viterbi algorithm; maximum likelihood decoding; frequency chirp; phase modulation
• ISSN: 1041-1135; 1941-0174
• DOI: 10.1109/LPT.2015.2467171

Directly modulated laser (DML) with direct detection (DD) dominates the commercial optical short reach applications. When upgrading the DML-DD to higher data rate, the frequency chirp is commonly regarded as a performance barrier. On the contrary, in this letter, we take advantage of the chirp to generate phase modulation along with the intensity, namely, realize complex modulation at transmitter using a single DML. Coherent detection (CD) is applied to recover this 2 dimensional (2-D) signal. A modified Viterbi algorithm (VA) conducts maximum likelihood decoding utilizing the signal intensity and differential phase in receiver digital signal processing (DSP). By making decision using 2-D information, VA provides more than 9-dB optical signal to noise ratio (OSNR) sensitivity advantage over the conventional DML-CD with intensity only decision, in a 10-GBaud pulse-amplitude modulation (PAM)-4 system. We demonstrate the first PAM-8 experiment over 160-km standard single mode fiber using the phase diversity DML.