Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2002-11-05
2004-11-02
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341300
Reexamination Certificate
active
06813067
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable
1. Field of the Invention
The present invention relates generally to broadband amplifiers and, more specifically, to broadband Raman amplifiers for use in long-haul and ultralong-haul transmission systems.
2. Background of the Invention
Distributed Raman Amplification (DRA) is known by those of ordinary skill in the art. DRA is a powerful technique to improve the optical signal to noise ratio (OSNR) margin in a transmission optical fiber of long-haul Wavelength Division Multiplexing (WDM) systems, for example. The principle of the Raman amplifier is based on the stimulated emission process associated with Raman scattering in fiber for amplification of the signal. In quantum mechanics, Raman scattering is a process in which an incident photon excites an electron to a virtual state and then the stimulated/spontaneous emission occurs when the electron de-excites down to the upper photon energy level of the glass molecule of the optical fiber. In amorphous materials such as fused silica, molecular vibrational frequencies spread into bands that overlap and create a continuum. As a result, the Raman gain spectrum extends over a relatively large frequency range that is offset from the pump light frequency (up to 40 THz) with a broad peak located near 13.2 THz. Optical fibers can act as a broadband amplifier because of this feature.
A Raman pump is included as part of an amplifier and injects light into the fiber in the opposite direction of the source signal. The injected photons boost the optical signal where it is most needed—at the far end of the fiber where the signal is experiencing the most attenuation.
Referring now to
FIG. 1
, the energy levels and transitions associated with stimulated and spontaneous Raman emissions are shown. Generally, the available flat gain bandwidth for a single pump is about 15 nanometers (nm). To realize ultra-broadband (e.g., greater than about 75 nm, covering both C-band and L-band) amplification, pump lights (also referred to as pump lasers) with multiple wavelengths (typically greater than four) are necessary. In addition, to reduce the crosstalk caused by both pump power fluctuation and signal-induced pump depletion, it is advantageous to make the pump lights counter-propagating with the signals. In a multi-wavelength counter-pumped Raman fiber amplifier, it has been found that the noise performance in the shorter wavelength band is significantly worse than the noise performance in the longer wavelength band. This is due to temperature-dependent spontaneous Raman emission, the proximity of the signal to the pumps and rapid energy transfer of shorter-wavelength pumps to the longest-wavelength pump.
To flatten the noise performance in a multiple-wavelength pumped Raman fiber amplifier, a bidirectional-pumping scheme using specially designed pump lasers with very low relative intensity noise is used. A nearly 2 dB noise figure (NF) improvement was obtained in the shorter wavelength, band by use of this scheme. For such a scheme, however, the crosstalk originating from signal-induced co-propagating pump depletions is still serious and is difficult to overcome.
The origin of noise degradation in broadband signal transmission systems will now be described. A counter-pumped Raman fiber amplifier includes M pumps (P
1
, . . . , P
M
). The set of propagation equations governing forward signal light power evolution considering temperature-dependent spontaneous Raman emission is given by Equation 1 below:
ⅆ
S
n
(
z
)
ⅆ
z
=
B
n
(
z
)
S
n
(
z
)
+
C
n
(
z
)
-
α
n
S
n
(
z
)
B
n
(
z
)
=
∑
j
=
1
M
g
nj
2
A
eff
P
j
(
z
)
C
n
(
z
)
=
∑
j
=
1
M
g
nj
2
A
eff
[
hv
n
Δv
(
1
+
1
e
h
(
ς
l
-
v
n
)
/
κ
T
-
1
)
]
P
j
(
z
)
(
1
)
where &agr;
n
denotes the fiber loss at signal light frequency v
n
, &zgr;
j
denotes the frequency of the jth pump light, S
n
denotes the n
th
input signal and Z denotes fiber length. The subscript n denotes the n
th
signal, and g
nj
is the Raman gain coefficient. A
eff
is the fiber effective area. The term of
1
+
1
ⅇ
h
(
ς
l
-
v
n
)
/
κT
-
1
denotes the temperature-dependent spontaneous Raman emission factors, where h is the Plank's constant, &kgr; is Boltzman's constant, T is the temperature in Kelvin, and &Dgr;v is the noise bandwidth. In Equation 1, signal-signal Raman interaction and Rayleigh scattering have not been taken into account. The pump light power evolution has a similar equation as Equation 1. The signal gain G
n
(L) and noise power &THgr;
n
(L) at the fiber output end corresponding to Equation 1 are given by
G
n
(
L
)
=
exp
{
-
α
n
L
+
∫
0
L
B
n
(
z
)
ⅆ
z
Θ
n
(
L
)
=
∫
0
L
C
n
(
z
)
G
n
(
L
)
G
n
(
z
)
ⅆ
z
=
∫
0
L
C
n
(
z
)
G
n
(
z
,
L
)
ⅆ
z
(
2
)
where G
n
(z,L) means signal gain obtained from z to L. From the above, it can be seen that the noise power is dependent on both the noise generation factor C
n
(z) and the longitudinal gain spectrum profile G
n
(z). G
n
(z) is assumed to be identical for various signal light frequencies. When the signal light frequency is closer to the pump light frequencies, i.e., the value of &zgr;
1
−v
n
becomes smaller, the value of C
n
(z), and hence the noise power &THgr;
n
(L), increases accordingly. This is due to the fact that the temperature-dependent spontaneous Raman emission factor,
1
+
1
e
h
(
ς
l
-
v
n
)
/
κ
T
-
1
,
increases when &zgr;
j
−v
n
becomes smaller. For example, if T=300, while &zgr;
1
−v
n
=13.2 THz (corresponding to a peak Raman shift, a large frequency difference between the pump and the signal) and 1 THz (corresponding to a small frequency difference between the pump and the signal), the value of
1
+
1
e
h
(
ς
l
-
v
n
)
/
κ
T
-
1
becomes 1.125 and 5.55, respectively. This shows that the impact of temperature-dependent spontaneous Raman emission on signals in the shorter-wavelength side is much more serious than that in the longer-wavelength side.
The impact of the longitudinal gain spectrum profile G
n
(z) also should be considered. From Equation 2 it can be seen that, for the same value of G
n
(L), the value of &THgr;
n
(L) increases when the gain seen by the signal is closer to the output end of the fiber. Physically, this is due to different mechanisms for noise generation and signal amplification. Noise is generated along the fiber length. Moreover, from Equation 1 it can be seen that the noise generation factor C
n
(z) has a linear relationship with the pump light power. However, the longitudinal gain spectrum profile G
n
(z) has an exponential relationship with the pump light power. This implies that the noise generation is more distributed along the fiber length than is, the signal gain. As a result, when the signal gain is closer to the output end of the fiber, most of the noise components generated along the fiber length will experience a relatively large gain and, hence, result in a worse noise performance; however, when the signal gain is farther away from the output end of the fiber, there are relatively fewer noise components that experience large gain and, hence, the result is enhanced noise performance.
FIGS. 2
a-
2
c
give a simulated example of a conventional five-wavelength counter-pumped fiber Raman amplifier. The powers and wavelengths of the five pumps used are: 1421 nm (520 mw), 1435 nm (400 mW), 1450 nm (190 mW), 1472 nm (58 mW) and 1501 nm (98 mW). The input signal power is chosen to be −15 dBm/channel and 80 km of Stranded Single-Mode Fibers (SSMF) is used in the simula
Birk Martin
Zhou Xiang
AT&T Corp.
Hellner Mark
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