WDM-PON has been considered as the ultimate solution for access networks. To make it real, a cost effective optical source that supports color-free operation is essential. This paper will discuss recent progress of optical sources for WDM-PON. Recently we have experienced a change of paradigm in access networks. The voice and text oriented services were evolved to data and image based services with help of the internet and the world wide web. The evolution is continuing to video based service

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... th accelerated speed. In addition, the traffic pattern becomes more and more symmetric [1] . Definitely, this change of paradigm requires a new access network that supports high speed (> 100 Mb/s) symmetric guaranteed bandwidth for future video services with high definition TV quality. WDM-PON is an ideal solution to meet these requirements. The current access networks, B-PON, E-PON, and G-PON, that share a feeder fiber and a complicated TDMA (time division multiple access) protocol offer limited guaranteed-bandwidth and scalability. However, WDM-PON provides point-to-point connectivity with a point-to-multipoint physical infrastructure by sharing the feeder fiber with WDMA (wavelength division multiple access). It also provides bit rate and protocol transparencies. Fig. 1 shows a typical architecture of WDM-PON that has an OLT inside of the CO, a RN located at outside of the CO, and many ONTs/ONUs that are located at customer premises and share a feeder fiber from the CO to the RN. It may be noted that the RN can be realized with an AWG (arrayed waveguide grating) by using its periodic transmission characteristic [2] . It is clear that we have to specify each user (ONT/ONU) in a WDM-PON with a pair of different wavelengths to communicate with the CO. This can be achieved by using a wavelength specified optical source such as a DFB laser used commonly in WDM transmission systems. It brings about many issues including high cost of the laser itself. Most of all, each user should have a different ONT/ONU specified by a different wavelength. In addition, the wavelength should match the transmission peak wavelength of the two AWGs located at the CO and the RN during the entire lifetime of the ONT/ONU. Another main issue is installation of WDM-PON. The craftsman should make sure that he is dealing with the correct wavelength. There have been investigated many optical sources that support wavelength independent operation of ONT/ONU (color-free operation) to overcome these problems. A broadband light such as amplified spontaneous emission from optically pumped active medium can be used for the WDM source. In this case, we suffer from slicing loss, since only a small part of light is used for each communication channel. A widely tunable optical source can be used to generate light at a specified wavelength [3] . Then, the ONT/ONU can be tuned to a specified wavelength provided by prior information of the wavelength. We show many optical sources for the ONT in Table 1 and compare their major features. It may be noted that the OLT can also be realized by using the methods listed in Table 1 . An elegant method for the color-free operation of ONT/ONU is seeding a light from the CO to the ONT/ONU through the RN. We can use an array of wavelength specified lasers, e.g, DFB lasers, or a broadband light source to seed the ONT/ONU. For the latter case, the RN spectrally slices the broadband OTuA1.pdf