FIELD OF THE INVENTION
The present invention relates generally to the field of optical communications
and in particular to an improved Raman amplified, erbium-doped fiber amplifier.
BACKGROUND OF THE INVENTION
The demand for higher capacity transmission systems generated by
the evolution of voice and data networks has led to the development of multiwavelength,
wavelength-division-multiplexed (WDM) optical communication systems and networks
having a large number of individual channels. (See, e.g., Y. Sun, J.B. Judkins,
A.K. Srivastava, L. Garrett, J.L. Zyskind, J.W. Sulhoff, C. Wolf, R.M. Derosier,
A.H. Gnauck, R.W. Tkach, J. Zhou, R.P. Espindola, A.M. Vengsarkar, and A.R. Chraplyvy,
"Transmission of 32-WDM 10-Gb/s Channels Over 640 Km using Broad Band, Gain-Flattened
Erbium-Doped Silica Fiber Amplifiers, " IEEE Photon. Tech. Lett., Vol. 9, No. 12,
pp. 1652-1654, December 1997; and A.K. Srivastava, Y.Sun, J.W. Sulhoff, C. Wolf,
M. Zirngibl, R. Monnard, A.R. Chraplyvy, A.A. Abramov, R.P. Espindola, T.A. Strasser,
J.R. Pedrazzini, A. M. Vengarkar, J.L. Zyskind, J. Zhou, D.A. Ferrand, P.F. Wysocki,
J.B. Judkins and Y.P. Li, "1 Tb/s Transmission of 100 WDM 10-Gb/s Channels Over
400 Km of TRUEWAVE Fiber", OFC Technical Digest, Postdeadline Papers, PD 10-1-10-4,
San Jose, CA, February 22-27, 1998; P. B. Hansen and L. Eskildsen, Optical Fiber
Technology, Vol. 3, pp.221-237, 1997; and A. K. Srivastava, L.Zhang, Y. Sun and
J. W. Sulhoff, Proc. OFC'99, Paper FC2, pp. 53-55, San Diego, CA 1999.
Appropriately, rare earth-doped optical fiber amplifiers are used
to amplify optical signals used in such communications systems and networks. These
rare earth-doped optical fiber amplifiers are found to be cost effective, exhibit
low-noise, provide relatively large bandwidth which is not polarization dependent,
display substantially reduced crosstalk, and present low insertion losses at relevant
operating wavelengths. As a result of their favorable characteristics, rare earth-doped
optical fiber amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs), are replacing
current optoelectronic regenerators in many optical lightwave communications systems
Accordingly, due to their broad application and importance, a continuing
need exists for improved rare earth-doped fiber amplifiers and in particular, improved
erbium-doped fiber amplifiers.
SUMMARY OF THE INVENTION
We have developed a Raman amplified optical amplifier that advantageously
improves the noise figure of the amplifier as a result of a Raman pump which is
provided within the amplifier itself. Advantageously, such amplifiers have wide
applicability in dense wavelength division multiplexed transmission systems.
Further features and advantages of the present invention, as well
as the structure and operation of various embodiments of the present invention
are described in detail below with reference to the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is schematic of a multi-stage optical amplifier which may serve
as a foundation for the present invention; and
Fig. 2 is a schematic of a Raman amplified optical amplifier according
to the present invention; and
Fig. 3 is a schematic of an exemplary WDM transmission system including
Raman amplified optical amplifiers according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 illustrates the basic principles of a multi-stage optical amplifier
which serves as the foundation for our invention. Specifically, and with reference
now to that figure, optical amplifier 100 includes two separate and distinct stages
of amplification which are separated by an optical element such as an optical isolator.
The first stage of amplification 120 is coupled to a second stage of amplification
140 through a passive element 160 such as an optical isolator, an optical filter,
or the like. The first stage 120 may comprise a doped amplifying fiber 180 coupled
via a coupler 181 to both an input port 182 and to a pump port 183 for receiving
energy from a pump 185 such as a laser diode pump. Coupler 181 may be a multiplex/demultiplex
type of filter or an optical interference filter manufactured by, for example JDS
Optics of Ottowa, Canada.
The second stage 140 of the multi-stage amplifier may comprise a
second doped amplifying fiber 141 coupled via coupler 128 to both an output port
130 for providing a signal which has been amplified and to a pump port 132 for
receiving energy from a laser diode pump 134. Similar to the first stage, coupler
128 can be a multiplex (demultiplex) type of filter or an optical interference
filter manufactured by, for example, JDS Optics.
The output port 136 of the fiber 180 is coupled to the input port
138 of fiber 141 via a passive optical element 160. The optical element 160 can
be an optical isolator of an optical filter or both. The optical isolator can take
the form of an optical diode which permits optical energy to travel from the first
stage 120 to the second stage 140, but restricts the travel of optical energy in
the opposite direction. The optical filter can take the form of a multiplexer
which can be made from a diffraction grating, a thin film, or a filter which can
restrict the amplified spontaneous emission (ASE) and/or has filtering characteristics
for modifying the gain characteristics of the multi-stage amplifier or other type
of filter which reduces energy in the form of spontaneous emission from flowing
between the two stages and causing saturation of either stage. The optical isolator
and or the optical filter can be coupled to the optical fibers of the first stage
120 and the second stage 140 either optically, by fusion, by splicing or other
Light from laser diode pump 185 can be launched into the first stage
120 via a lens located at the pump port 183 . A filter such as a holographic grating
139 can be formed in the fiber between the end 183 and the coupler 181 to reject
an undesired mode from the laser diode pump by providing backscattering.
As noted above, the coupler 181 is constructed to couple both the
input signal received by input port 182 and the pump signal received by the pump
port 183 to amplifying fiber 180. In a similar manner, coupler 128 is constructed
to pass the amplified signal from the multistage amplifier to the output port 130
and to couple the pump signal received by pump port 132 to amplifying fiber 141.
In operation, an optical signal which is to be amplified is coupled
by some convenient means such as, for example, optically or the like to input
port 182 and a pump signal is coupled to pump port 139 and pump port 132. The received
optical signal amplified by the Erbium-doped fiber amplifier 180 of the first stage
120 of the optical fiber amplifier is couple to the input port of the second stage
140 of the optical amplifier 100 via passive optical element.
It is to be understood that the Erbium-doped fiber amplifier 180 of
the first stage 120 can be pumped from either or both ends and that the Erbium-doped
fiber amplifier 141 of the second stage 140 can also be pumped from either or
both ends. This being so, the laser diode and a multiplexer for pumping the first
and second stages 120, 140 of the optical amplifier 100 can be fabricated on a
single substrate can include the passive optical element or elements such as an
optical isolator and/or an optical fiber.
Fig. 2 is a schematic which illustrates the basic principle of our
Raman amplified, optical amplifier which is the subject of the present invention.
The optical amplifier shown there 200, is divided primarily into a number of stages
202 ... 202[N]. Briefly stated, optical signals enter an input of the optical
amplifier 204 and exit an output 206 after traversing the multiple stages 202
... 202[N]. As is shown in this exemplary structure 200, the stages include a
section of Rare Earth, i.e, Erbium-doped fiber 210 ... 210[N].
Shown further in Fig. 2, the first stage 202 of the amplifier 200
is pumped by an optical pump at an appropriate wavelength such as 980nm. As can
be appreciated, there is some design freedom in the choice of the pump. The pump
shown 212, is shown exemplary as 980nm, which will maintain a high inversion level
resulting in a low noise figure. The pump is coupled to a stage of the optical
amplifier 200 via wavelength selective coupler 216.
Shown further in Fig. 2, and the subject of the present invention,
is a Raman pump 214, coupled to a stage of the amplifier 200 via wavelength selective
coupler 218. The Raman pump may advantageously be a laser device of a number of
wavelengths, although for the purposes of our exemplary embodiment, 1400 to 1550nm.
While not explicitly shown in Fig. 2, the Raman pump 214 may be coupled
to the optical amplifier 200 in any of the stages 202 ... 202[N], although a
mid-stage is perhaps the most convenient. As such, the 980nm pump wavelength selective
coupler 216 should be transparent at Raman wavelength with very small insertion
loss. Amplifiers constructed according to our inventive teachings the input signal
power will be larger (due to the Raman gain) and the presence of a Raman pump at
a wavelength longer than the 980nm pump 212 will further increase the inversion
level of the first stage of the exemplary amplifier shown in Fig. 2. As can be
further appreciated, the inversion level in such a first stage is kept high to
improve the noise figure of the amplifier.
Finally, and with reference now to Fig. 3., there is shown in schematic
form an exemplary wavelength-division multiplexed (WDM) transmission system incorporating
our inventive Raman amplified optical amplifiers. Specifically, transmission system
300 which interconnects transmitter 302 with receiver 304 includes a number of
fiber spans 306, 308 and 310 each interconnected by Raman amplified optical amplifiers
307, 309, and 311, which are shown illustratively as Erbium-doped fiber amplifiers,
and demultiplexer 315. In operation, wavelength division multiplexed optical signals
(not shown) emanate from transmitter 302 traverse the multiple fiber spans 306,
308 and 310 as they are amplified by our inventive Raman amplified optical amplifiers
307, 309 and 311. The signals are subsequently demultiplexed through the action
of demultiplexer 315 and then received by receiver 304. As can be readily appreciated,
optical fiber spans and component materials, transmitters 302, demultiplexers 315
and receivers 304 are all well known and their actual embodiment in a particular
application is largely a matter of design choice.
With continued reference to Fig. 3, at each of the amplifiers 307,
309 and 311, and according to our inventive teachings and consistent with our
discussion of Fig. 2, high power Raman pump is sent through a transmission fiber
which acts as a gain medium for the Raman amplification. Raman pump and optical
signals will propagate through the transmission system in opposite directions.
As should now be apparent and a result of our inventive teachings, the use of Raman
amplification within the optical amplifier improves a noise-figure of the amplifier
thereby improving the overall system margin.