Design of WDM PON With Tunable Lasers:
The Upstream Scenario
Jingjing Zhang, Student Member, IEEE, and Nirwan Ansari, Fellow, IEEE
Abstract—Tunable lasers are potential upstream optical light
generators for wavelength-division-multiplexing (WDM) passive
optical network (PON), which is a promising solution for next-gen-
eration broad-band optical access. The wavelength provisioning
flexibility of tunable lasers can increase the admissible traffic in
the network as compared to wavelength-specific lasers. Generally,
the broader the lasers’ tuning ranges, the more the traffic can be
admitted to the network. However, broad tuning range requires
sophisticated technology, and probably high cost. To achieve the
optimal tradeoff between the admissible traffic and the cost, we
investigate the relationship between lasers’ tuning ranges and
the network’s admissible traffic and then design WDM PON by
selecting lasers with proper tuning ranges for the upstream data
transmission. Specifically, we focus on addressing two issues under
three scenarios. The two issues are: how to admit the largest traffic
by properly selecting lasers, and how to admit given upstream
traffic using lasers with tuning ranges as narrow as possible. The
three scenarios are: full-range tunable and wavelength-specific
lasers are available, limited-range tunable lasers are available, and
the exact number of lasers with specific tuning ranges are given.
Index Terms—Admissible traffic, tunable laser, tuning range,
wavelength-division-multiplexing (WDM) passive optical network
(PON).
I. INTRODUCTION
WAVELENGTH-DIVISION-MULTIPLEXING (WDM)
passive optical network (PON), which efficiently
exploits the large capacity of optical fibers, is becoming one
promising next-generation broad-band optical access solution
[1]–[3]. As compared to time-division-multiplexing (TDM)
PON such as Ethernet PON [4] and gigabit PON (GPON) [5],
WDM PON increases its capacity by utilizing optical devices
with multiwavelength provisioning capability. Many WDM
PON architectures with different optical devices have been
proposed to provision multiple wavelengths [6]–[8].
To provision multiple wavelengths for upstream transmis-
sion, WDM PON can be realized in two major architectures,
depending on the placement of optical light generators [9]. The
first scheme is to equip optical network units (ONUs)with lasers
for their own upstreamtraffic transmission. The lasers are placed
at the ONU side. An alternative scheme is to utilize lasers at
Manuscript received April 22, 2009; revised July 12, 2009, October 14, 2009.
First publishedDecember 22, 2009; current phiên bản publishedJanuary 15, 2010.
This work was supported in part by the National Science Foundation (NSF)
under Grant 0726549.
The authors are with the Advanced Networking Laboratory, Department
of Electrical and Computer Engineering, New Jersey Institute of Technology,
Newark, NJ 07032 USA (e-mail: [email protected]; [email protected]).
Color versions of one or more of the figures in this paper are available online
at
Digital Object Identifier 10.1109/JLT.2009.2039020
the optical line terminal (OLT) side to supply seed light for up-
stream transmission. The unmodulated light supplied by OLT
is first transmitted down to ONUs and then modulated and re-
flected back by ONUs. Instead of lasers, reflective receivers and
modulators are equipped at ONUs to realize colorless ONUs
[10]. The reflective modulator can be based on reflective semi-
conductor optical amplifier combined with an electroabsorp-
tion modulator [11]. Since the signal and seed lights are trans-
mitted in opposite directions on the same wavelength, this kind
of network may need to consider the effect of optical reflection,
including Rayleigh backscattering, which limits the maximum
network reach and largest channel bit rate [12]. In this paper,
we focus on the former architecture, which is simpler, more re-
liable, and is potentially able to achieve a higher loss budget and
larger bit rate [9], [13].
There are three major classes of optical source generators
depending on the wavelengths generation capability, namely,
wavelength-specific sources, wavelength-tunable sources, and
multiwavelength sources [14]. A wavelength-specific source
emits only one specific wavelength, e.g., the common DFB/dis-
tributed Bragg reflector (DBR) laser diode (LD), or the ver-
tical-cavity surface-emitting LD. A multiple-wavelength source
is able to generate multipleWDM wavelengths simultaneously,
including multifrequency laser, gain-coupled DFB LD array,
and chirped-pulse WDM. Besides multiwavelength sources, a
wavelength-tunable source can generate multiple wavelengths
as well [15]. However, it can only generate one wavelength at
a time. Tunable lasers can employ many technologies such as
DFB array, sampled grating DBR, external cavity diode laser
etalon, etc. Different technologies may yield different tuning
ranges. Among these three kinds of optical source generators,
wavelength-specific lasers or wavelength-tunable lasers are
usually adopted. Multi-frequency lasers are currently not fa-
vored owing to their high cost.
As compared to wavelength-specific lasers, wavelength-tun-
able lasers have two main benefits [16]. First, from the mul-
tiple access (MAC) layer’s point of view, in the case of wave-
length-specific lasers, one wavelength channel is utilized by a
fixed set of lasers, and thus the statistical multiplexing gain
cannot be exploited for traffic from lasers using different wave-
length channels. In the case of wavelength-tunable lasers, the
wavelength tunability of tunable lasers facilitates statisticalmul-
tiplexing of traffic from a larger set of lasers, thus potentially
yielding better system performance. Second, for network oper-
ators, tunable lasers offer advantages such as simple inventory
management and reduced sparing cost.
In this paper, we equip each ONU with one tunable laser for
its own upstream data transmission. We try to exploit the tun-
able lasers’ merit of statistical gain in the MAC layer. Each
Link download cho anh em:
The Upstream Scenario
Jingjing Zhang, Student Member, IEEE, and Nirwan Ansari, Fellow, IEEE
Abstract—Tunable lasers are potential upstream optical light
generators for wavelength-division-multiplexing (WDM) passive
optical network (PON), which is a promising solution for next-gen-
eration broad-band optical access. The wavelength provisioning
flexibility of tunable lasers can increase the admissible traffic in
the network as compared to wavelength-specific lasers. Generally,
the broader the lasers’ tuning ranges, the more the traffic can be
admitted to the network. However, broad tuning range requires
sophisticated technology, and probably high cost. To achieve the
optimal tradeoff between the admissible traffic and the cost, we
investigate the relationship between lasers’ tuning ranges and
the network’s admissible traffic and then design WDM PON by
selecting lasers with proper tuning ranges for the upstream data
transmission. Specifically, we focus on addressing two issues under
three scenarios. The two issues are: how to admit the largest traffic
by properly selecting lasers, and how to admit given upstream
traffic using lasers with tuning ranges as narrow as possible. The
three scenarios are: full-range tunable and wavelength-specific
lasers are available, limited-range tunable lasers are available, and
the exact number of lasers with specific tuning ranges are given.
Index Terms—Admissible traffic, tunable laser, tuning range,
wavelength-division-multiplexing (WDM) passive optical network
(PON).
I. INTRODUCTION
WAVELENGTH-DIVISION-MULTIPLEXING (WDM)
passive optical network (PON), which efficiently
exploits the large capacity of optical fibers, is becoming one
promising next-generation broad-band optical access solution
[1]–[3]. As compared to time-division-multiplexing (TDM)
PON such as Ethernet PON [4] and gigabit PON (GPON) [5],
WDM PON increases its capacity by utilizing optical devices
with multiwavelength provisioning capability. Many WDM
PON architectures with different optical devices have been
proposed to provision multiple wavelengths [6]–[8].
To provision multiple wavelengths for upstream transmis-
sion, WDM PON can be realized in two major architectures,
depending on the placement of optical light generators [9]. The
first scheme is to equip optical network units (ONUs)with lasers
for their own upstreamtraffic transmission. The lasers are placed
at the ONU side. An alternative scheme is to utilize lasers at
Manuscript received April 22, 2009; revised July 12, 2009, October 14, 2009.
First publishedDecember 22, 2009; current phiên bản publishedJanuary 15, 2010.
This work was supported in part by the National Science Foundation (NSF)
under Grant 0726549.
The authors are with the Advanced Networking Laboratory, Department
of Electrical and Computer Engineering, New Jersey Institute of Technology,
Newark, NJ 07032 USA (e-mail: [email protected]; [email protected]).
Color versions of one or more of the figures in this paper are available online
at
You must be registered for see links
.Digital Object Identifier 10.1109/JLT.2009.2039020
the optical line terminal (OLT) side to supply seed light for up-
stream transmission. The unmodulated light supplied by OLT
is first transmitted down to ONUs and then modulated and re-
flected back by ONUs. Instead of lasers, reflective receivers and
modulators are equipped at ONUs to realize colorless ONUs
[10]. The reflective modulator can be based on reflective semi-
conductor optical amplifier combined with an electroabsorp-
tion modulator [11]. Since the signal and seed lights are trans-
mitted in opposite directions on the same wavelength, this kind
of network may need to consider the effect of optical reflection,
including Rayleigh backscattering, which limits the maximum
network reach and largest channel bit rate [12]. In this paper,
we focus on the former architecture, which is simpler, more re-
liable, and is potentially able to achieve a higher loss budget and
larger bit rate [9], [13].
There are three major classes of optical source generators
depending on the wavelengths generation capability, namely,
wavelength-specific sources, wavelength-tunable sources, and
multiwavelength sources [14]. A wavelength-specific source
emits only one specific wavelength, e.g., the common DFB/dis-
tributed Bragg reflector (DBR) laser diode (LD), or the ver-
tical-cavity surface-emitting LD. A multiple-wavelength source
is able to generate multipleWDM wavelengths simultaneously,
including multifrequency laser, gain-coupled DFB LD array,
and chirped-pulse WDM. Besides multiwavelength sources, a
wavelength-tunable source can generate multiple wavelengths
as well [15]. However, it can only generate one wavelength at
a time. Tunable lasers can employ many technologies such as
DFB array, sampled grating DBR, external cavity diode laser
etalon, etc. Different technologies may yield different tuning
ranges. Among these three kinds of optical source generators,
wavelength-specific lasers or wavelength-tunable lasers are
usually adopted. Multi-frequency lasers are currently not fa-
vored owing to their high cost.
As compared to wavelength-specific lasers, wavelength-tun-
able lasers have two main benefits [16]. First, from the mul-
tiple access (MAC) layer’s point of view, in the case of wave-
length-specific lasers, one wavelength channel is utilized by a
fixed set of lasers, and thus the statistical multiplexing gain
cannot be exploited for traffic from lasers using different wave-
length channels. In the case of wavelength-tunable lasers, the
wavelength tunability of tunable lasers facilitates statisticalmul-
tiplexing of traffic from a larger set of lasers, thus potentially
yielding better system performance. Second, for network oper-
ators, tunable lasers offer advantages such as simple inventory
management and reduced sparing cost.
In this paper, we equip each ONU with one tunable laser for
its own upstream data transmission. We try to exploit the tun-
able lasers’ merit of statistical gain in the MAC layer. Each
Link download cho anh em:
You must be registered for see links