Wave Division Multiplexing (WDM) technology has been widely used in optical fiber communications in order to increase the transmission capacity for point to point connection through a single optical fiber. Various conventional WDM laser technologies have been developed and widely deployed over the years. Solutions suitable for Dense WDM (DWDM) applications and with high speed modulation performance at 10 G or above are generally desired for efficient use of optical fibers to deliver the overwhelmingly increasing data transmission demands, especially in long haul and metro markets. Fixed wavelength externally modulated distributed feedback (DFB) lasers, e.g., electro-absorption modulated lasers (EML), and wavelength tunable lasers are currently dominating the market deployment. Tunable lasers have gradually increased its share in the market with the advantage of being colorless for better inventory management. The sophisticated and costly fabrication process tunable lasers may potentially pose a limit to its continuing increase of market share.
WDM technology is not only desired for point to point transmission in long haul and metro markets but also desirable for multipoint to multipoint connections through a single fiber in aggregation, sometimes referred to as a WDM Passive optical network (WDM-PON) system. Both EML and tunable lasers are typically too expensive for WDM-PON systems. Cost effective colorless laser solutions are desired for WDM-PON applications and there has been substantial research and development efforts in the past pursuing such solutions. Among the earlier efforts, an Optical Network Unit with injection locked Fabry-Perot (FP) laser and reflective optical semiconductor amplifier (RSOA) based on a seed light coming from a high power broadband light source sliced by the Arrayed Waveguide Grating multiplexer/demultiplexer (AWG MUX/DEMUX) or from a continuous wave (CW) WDM light source have been proposed. Other options have been proposed including a self-seeding architecture that uses the light emission from the RSOA in the ONU as its own seed light by placing a partial reflective mirror at the output of the AWG MUX to feedback some of the light emission from the RSOA itself. This approach virtually forms an external cavity laser that has a cavity length as long as the length of the fiber that connects the ONU to the passive WDM MUX node of the WDM-PON system. Concern of stability for such a long external cavity laser has been studied using a Faraday rotator mirror (FRM) to stabilize the polarization through the long optical fiber. The self-seeding architecture greatly simplified the WDM-PON system since it limits the light seeding configuration to the ONU service area between the ONUs and passive node, which is beneficial for the seamless deployment of the WDM-PON service and facilitates the potential convergence between future WDM-PON and the existing time division multiplexing-PON (TDM-PON) architectures. Nonetheless, the relevant solutions proposed so far are not able to provide satisfactory transmission performance in speed and distance.
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