COL 10(9), 090602(2012) CHINESE OPTICS LETTERS September 10, 2012
Experimental and theoretical analyses of RSOA-based
colorless-ONUs with 4PAM signal in the up-link
Yueying Zhan ()
∗
, Min Zhang ( ), Lei Liu ( ),
Mingtao Liu (), Zhuo Liu ( ), and Xue Chen ( )
State Key Laboratory of Information Photonics and Optical Communications,
Beijing University of Posts and Telecommunications, Beijing 100876, China,
∗
Corresponding author: zhanyueying@gmail.com
Received March 5, 2012; accepted May 17, 2012; posted online August 16, 2012
A scheme that utilizes the 4-level pulse-amplitude-modulated (4PAM) signal as the re-modulated signal
of colorless optical network units (ONUs) based on reflection semiconductor optical amplifiers (RSOAs) is
proposed. The system with 10-Gb/s non-return-to-zero (NRZ) downstream and 5-Gb/s 4PAM upstream
signals is theoretically analyzed and experimentally verified. Simulation and experimental results reveal
that the 4PAM re-modulated signal yields better performance than the NRZ signal in the upstream. A
receiver pow er penalty of 1.6 dB is also improved by the 4PAM signal at back-to-back (BtB) transmission,
whereas another receiver power penalty of 1.5 dB improved after 30-km single mode fiber transmission,
where 4PAM signals are used as upstream signal.
OCIS codes: 060.2330, 230.4480, 060.1155.
doi: 10.3788/COL201210.090602.
The demand for high-bandwidth services has fueled the
deployment of fiber access networks around the world.
Likewise, wavelength-division-multiplexing passive opti-
cal network (WDM-PON) with colorless optical network
units (ONUs) has attracted considerable attention
[1−7]
.
Recently, various re-modulation schemes for colorless-
ONUs based on reflection semiconductor optical am-
plifiers (RSOAs) have been reported
[8−20]
. However,
these approaches
[9−13]
have several disadvantages, such
as limited RSOA bandwidth and the severe impact of
fiber dispersion. Multilevel modulation with higher spec-
tral efficiency is one way of extending the reach of
multimode fiber at a high bit rate, although complex-
ity is critical in data communication links. Meanwhile,
the 4-level pulse-amplitude-modulated (4PAM) signal
offers the lowest implementation complexity in the mul-
tilevel modulation format with spectral efficiency of 2
bit/s/Hz
[14]
. The multilevel signal has been used in the
re-modulated signal, which has higher dispersion toler-
ance than the non-return-to-zero (NRZ) signal; however,
the light of the injected signal in RSOA is continuous
wave (CW) light
[15]
.
In this letter, we propose a novel wavelength re-
modulation scheme for the WDM-PON systems using 10-
Gb/s NRZ and 5-Gb/s 4PAM signals in the downstream
and upstream, respectively. The feasibility of the pro-
posal is evaluated by experiments and simulations. Sim-
ulation and experimental results show that the proposed
system with the 4PAM signal has better performance,
because it supports higher bit rate and longer distance
transmission.
The optical spectra of multilevel signals are typically
compressed against the NRZ signal; thus, they have a
lower dispersion tolerance than multilevel signals. Figure
1 shows the comparison of the optical spectra between the
4PAM and NRZ signals at 5 Gb/s, wherein the optical
spectra of the former comprise half of the optical spectra
of the latter. The modulated bandwidth of RSOA is an
important limiting factor in the high-rate transmission
system.
The rate equations for carriers (N) and total photon
density (S) of the RSOA are respectively given by
[21,22]
dN
dt
=
J
eV
− [(A
rad
+ A
nrad
)N +(B
rad
+ B
nrad
)N
2
+ C
aug
N
3
] − Γ ν
g
g(N, S)S, (1)
dS
dt
=
P
in
η
hvV
+βΓ (B
rad
+ B
nrad
)N
2
+ μΓ ν
g
g(N, S)S − α
int
ν
g
S, (2)
where S = S
+ASE
+ S
−ASE
+ S
+
i
+ S
−
i
, S
±ASE
,andS
±
i
are the spontaneously amplified emitted photon density
and input photon density of the right and left facets
of RSOA, respectively; J is the injection current of the
RSOA; A
rad
, A
nrad
, B
rad
, B
nrad
,andC
aug
are the recom-
bination coefficients of the RSOA, which are constant;
Γ is the optical confinement factor; ν
g
is the group ve-
locity; β is the spontaneous emission factor; α
int
is the
internal waveguide loss; e is the elementary charge of
the electron; V is the active region volume. Expressing
the carrier density N as the sum of a steady state term
N
Fig. 1. (Color online) Optical spectra of the NRZ and 4PAM
signals.
1671-7694/2012/090602(5) 090602-1
c
2012 Chinese Optics Letters