each colorless ONU is nonspecific and is determined by the filtering properties of
the AWG in the remote node or the wavelength of an injection/seeding light into
the ONU. Directly modulated broadband optical sources such as LEDs, which
are spectrally sliced at the AWG in the remote node, have been proposed as one
form of colorless ONUs. Due to inherently low modulation speeds of these
earlier solutions, alternative solutions based on injection-locked FP-LDs and
wavelength-seeded RSOAs have been proposed. The emission wavelength of an
injection-locked FP-LD or a wavelength-seeded RSOA follows that of the seed-
ing light, which originates from a narrowband slice of light from a broadband
optical source located at the CO. More recently, schemes that remove the need for
the centralized broadband source have been proposed using self-injection-locking
and self-seeding configurations which typically employ an optical feedback elem-
ent in the remote node. On the other hand, source-free ONU configurations have
been proposed to reuse the optical downstream carrier sent from the CO for
upstream modulation. To minimize the influence of the downstream modulation
on the upstream data, various schemes including the adoption of different modu-
lation formats for downstream and upstream transmissions, as well as the use of
subcarriers have been proposed.
It is envisioned that the rapid advancement of optical technologies for passive
optical networks will continue to surge ahead to ensure cost-competitiveness
with current access technologies. Already, the advancements in the above-men-
tioned areas have contributed significantly to the success and widespread uptake
of power-splitting PONs, especially in Asian countries, with the deployment of
the future-proof WDM-PON to follow suit in the near future.
 K. Okamoto, ‘‘Planar Lightwave Circuits for FTTH and GMPLS,’’ in Proc. of APOC
2006, paper 6351–28, Kwangju, Korea. Sept. 3–7, 2006.
 I.-B. Sohn, M.-S. Lee, J.-Y. Chung, ‘‘Fabricatiom of optical splitter and passive alignment
technique with a femtosecond laser,’’ IEEE PTL, vol.17, no.11, pp2349–2351, Nov., 2005.
 E. Edmon, K.G. McCammon, R. Estes, J. Lorentzen, ‘‘Chapter 2: Today’s broadband
fiber access technologies and deployment considerations at SBC,’’ Broadband optical access
and FTTH, ed. Chin-Lon Lin, Chap. 3, p17, (John Wiley & Sons, 2006).
 Alliance Fiber Optic Products, Inc., PLC Splitter data sheet, available from http://
 R. Gritters, H. Taguchi, and A. Geraci, ‘‘Low Insertion Loss PLC Splitters for Improved
PON Performance,’’ White paper from Seikoh Gaiken USA, Fiber optic product news http://
www.fpnmag.com/scripts/ShowPR.asp?PUBCODE ¼ 025&ACCT ¼ 0004717&ISSUE ¼
0312&RELTYPE ¼ PR&PRODCODE ¼ 0000&PRODLETT ¼ A
 K.A. McGreer, H. Zu, C. Ho, N. Kheraj, Q. Zhu, M. Stiller, and J. Lam, ‘‘Planar lightwave
circuits for PON applications’’ Proc. of OFC, paper NWD4, Mar., 2006.