regime have been demonstrated [61]. Since the slave VCSEL emission wavelength
is typically dependent on its bias current or heat sink temperature, a ‘‘training’’
session may be required to find the lockable wavelength range. This may be done
with the assistance of a look-up table, or by forming a feedback loop with
measurements of the slave VCSEL reflected power through Port 3 of the optical
circulator in Fig. 3.42 or its junction voltage while sweeping the slave VCSEL
wavelength. The training session needs to be done only once when the ONU is
started up or infrequently as situation becomes necessary, similar to the rebooting
of a personal computer.
3.4.3 Colorless ONUs
The emission wavelength of this category of ONUs is nonspecific though it
can be selectively determined by external factors such as the filtering properties
of the AWG in the remote node or the wavelength of an injection/seeding light
into the ONU. This flexibility thereby enables the exact same colorless ONU
configuration to be deployed across the network, facilitating mass production. In
literature, there exists many schemes that facilitate the colorless feature in
ONUs, and these schemes can also be categorized into three main groups
described below. Colorless ON Us Based on Spectral-Slicing Techniques
The colorless ONUs in this category benefits from the spectrum-slicing
technique to generate unique upstream wavelength channels for transmission.
In this technique, the combination of a broadband source and optical filters are
exploited to spectrally slice the spectrum of a broadband optical source into
narrowband and unique channels for upstream transmission. The earliest pro-
posals were based on implementing a broadband optical source at each ONU
with spectral-slicing occurring at the AWG in the remote node [62–64]. Due to
spectral slicing, only a narrowband of sliced-spectrum is received at the CO
from each ONU, even though upstream data is modulated on the entire
broadband spectrum. Figure 3.45 shows the schematic diagram of a WDM-
PON with an LED implemented as the broadband optical source in each ONU
[62]. Other proposals for broadband optical sources at the ONU include the
superluminescent LED (SLED) [63, 64], and erbium doped fiber amplifier
(EDFA) as amplified spontaneous emission (ASE) light sources [65], and
multimode FP-LDs [66].
In some other proposals, a single centralized broadband source is implemen-
ted in the CO instead of distributing broadband optical sources across the
Transmitter Sources at Subscriber Premises 129
network, thus lowering component cost at each ONU [67]. By the same token,
the spectrum of the centralized broadband optical source is sliced into multiple
narrowband and unique channels at the remote node, with each one distributed
toward a separate ONU. At the ONU, the spectrally sliced narrowband wave-
length channel is externally modulated with the upstream data to be sent back to
the CO. With all the spectral-slicing techniques mentioned above, only a narrow
part of the broadband source is effectively utilized for data transport. As a result,
the transmission bit-rate and distances are limited by the high spectral-slicing
loss, which is further compounded by the low launch powers of LEDs and
SLEDs. One can increase the passband of the AWG to reduce spectral slicing
losses, but will in turn encounter dispersion issues and will limit the number of
users supported in the network. A recent proposal shown in Fig. 3.46 aims at
overcoming the power issue by using a centralized supercontinuum broadband
light source to achieve 10 Gb/s bidirectional transmission over 40-km distance
[67]. The centralized broadband supercontinuum broadband light source con-
sists of an actively mode-locked laser, an EDFA, a polarization controller, a
nonlinear photonic crystal fiber, and a C-band band-pass filter to limit the
spectral width of the supercontinuum to the C-band for upstream carrier use.
Consequently, a C-band supercontinuum is distributed downstream and spec-
trally sliced at the remote node into multiple upstream carriers which are then
modulated with upstream data by the ONUs.
Headend (HE) or
Central office (CO)
10 km
1 waveguide
grating router
Remote node (RN)
Figure 3.45 Schematic diagram of WDM-PON with broadband optical source (LED) in ONU.
(From Ref [62])
130 Optical Technologies in Passive Optical Access Networks

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