Optical Feedback-Tolerant Gain-Coupled DFB Lasers for Isolator-Free Modules in the Access Networks

Passive optical network topology has been widely adopted in access networks due to its low-cost and yet flexible network structure. To further promote the passive optical networks, the cost reduction of optical modules is critical. Relatively expensive combination of a conventional index-coupled distributed feedback laser diode (IC-DFB-LD) and an optical isolator is commonly used for passive optical networks with transmission distance more than 30 km. Although gain-coupled DFB-LDs (GCDFB-LD) have been widely investigated in the hope of eliminating the isolator in optical modules, their limited output power keeps them from practical use in passive optical networks. In this paper, we describe the development of 1.31 μm and 1.49 μm GC-DFB-LDs with high output power and optical feed back tolerance for isolator-free optical modules in access networks. The relative intensity noise (RIN) degradation was well suppressed below −120 dB/Hz at −8 dB optical feedback in the temperatures range from 0◦C to 85◦C from both 1.31 μm and 1.49 μm GC-DFBLDs. Optical feedback tolerance of 1.31 μm and 1.49 μm GC-DFB-LDs were improved by more than 6 dB and 4 dB as compared with conventional IC-DFB-LDs. Dispersion power penalty after over 30 km transmission at 1.25 Gbps were achieved less than 0.3 dB and 0.7 dB under −15 dB optical feedback conditions. The proposed 1.31 μm GC-DFB-LD prototypes experimentally demonstrated 14mW output power with over 5,000-hour operation at 85◦C. Our devices are found to fully complying IEEE 802.3ah standard and seem to be promising for the low-cost optical modules in longreach access network applications. The details of the device structure as well as transmission experiments are also reported. key words: gain coupled distributed feedback lasers, relative intensity noise, optical isolators, optical feedback semiconductor lasers