there are many parts of the optical communication,except the WDM which is just a part of the fiber optic network u mention above.there are fiber optic transceiver,fiber optic transmission and the other fields in the fiber optical communication u could try to realize or learn.
recently,there is a hot topic in this industry:the news"Google is backing a $300mil fiber network pipe between US and Japan, planned for 2016 ",it cause many people of this field interested in it so that many topics about it appears in many forums or experts communities .
u can also join some experts communities to pick your inspiration about your phd topic.