Passive Optical Networks

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.

3.4.3.1 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)

MFL

10 km

1  waveguide

grating router

(WGR)

ONU

REC

LED

SCM or TDM

Remote node (RN)

REC

1.3/1.5

WDM

1.3/1.5

WDM

DEMUX

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