Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-02
2003-09-02
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S049010, C372S099000
Reexamination Certificate
active
06614822
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to optical communications systems, and particularly to dense wavelength division multiplexing laser amplification systems and laser modules for use in such systems.
BACKGROUND OF THE INVENTION
With the explosive growth of the Internet and other communications needs, there has developed a commensurate need for transmission systems to handle the ever increasing demand for capacity to transmit signals. Fiber optic systems have become the technology of choice for meeting this demand. Significant attention has been directed to systems which use dense wavelength division multiplexing (DWDM) to increase the number of signal channels that can be transmitted through a single optical fiber. DWDM systems rely on erbium doped fiber amplifiers (EDFA) which are pumped by semiconductor lasers. Generally, EDFAs are pumped by lasers having a wavelength of about 1480 nm or about 980 nm. There are relative advantages and disadvantages of using lasers in each of these wavelengths, which are known to those skilled in the art. EDFA's are capable of simultaneously amplifying multiple signal channels.
A schematic overview of a known generic EDFA in a DWDM system is depicted in FIG.
1
. Signals enter the system through optical fiber
100
and pass through an optical isolator
120
where they are amplified in erbium doped fiber (EDF)
130
which is pumped by a plurality of semiconductor lasers
150
a,
150
b,
150
c
and
150
d,
each of which is fixed, or set, to a specific unique wavelength. Pumping lasers
150
a
-
150
d
are coupled to EDF
130
using a conventional multiplex couplers
160
and
170
. The amplified signals pass through a second optical isolator
180
and out of the system through optical fiber
190
. Pumping lasers
150
a
-
150
d
may be fixed at different wavelengths. For example, pumping laser
150
a
-
150
d
may have output wavelengths of 1460 nm, 1470 nm, 1480 nm and 1490 nm, respectively. It will be appreciated that more than four pumping lasers may be employed in the system of
FIG. 1
to further increase the number of signal channels that can be simultaneously transmitted by the system. However, the number of pumping lasers that can be effectively employed is subject to various constraints. The lasers are required to emit high power, or they are required to produce output spectrums that result in as low loss as possible when being wavelength-multiplexed with the wavelength-multiplex coupler
160
. The 3 dB wavelength bandwidth of output light is therefore required to be less than 3 nm, or still more preferably less than 2 nm in order to eliminate the coupling loss at the wavelength-multiplex coupler.
FIG. 2
shows a basic exemplary structure of a Raman amplifier. A WDM optical input signal is coupled into a Raman amplifying fiber loop
2
(a single mode fiber) by way of an input optical fiber
12
and polarization independent isolator
25
. A high power beam of light, or pumping light, is generated by pumping source
1
and coupled into fiber loop
2
in the opposite direction by a WDM coupler
13
at an end of fiber
2
which is opposite to the input signal. When the high power pumping light of over 300 mW is coupled to Raman fiber loop
2
, a stimulated scattering phenomenon occurs in the molecules of the fiber which causes power from the high-power pumping light to be coupled the input signal light, which acts to amplify the input signal light. The transfer of power occurs only if the frequency of the pumping light is about 13 THz greater than the frequency of the input signal light (which corresponds to the wavelength of the pumping light being about 100 nm shorter than the wavelength of a 1.55 &mgr;m input signal). This Raman gain has a −1 dB bandwidth of about 20 nm. In order to generate flat and wide gain band over a 80 nm band like an EDFA amplifier, a wavelength multiplexed pumping light source
1
is needed. The Raman amplifier requires a greater pump power in order to obtain the same gain as that of the EDFA amplifier. Thus, the coupling loss at the wavelength-multiplex coupler is also a important issue in this application. Therefore, in order to realize a stable Raman gain spectrum efficiently, it is important that the power level of pumping source 1 be well controlled and at the same time, the output power bandwidth of each pumping laser should be less than 3 nm, or more desirably be less than 2 nm, and the fluctuation of the center wavelength must be controlled to be less than +−1 nm. Because of the predictable nature of stimulated Raman Scattering phenomenon, a Raman amplifier can be constructed to amplify any desired wavelength so long as a pumping light source can be prepared, which is an advantage over the EDFA amplifier.
The present invention is directed towards semiconductor lasers that can be employed by pumping source
1
. As a brief background, pumping source
1
comprises semiconductor lasers
3
1
,
3
2
,
3
3
, and
3
4
of Fabry-Perot type, wavelength stabilizing fiber gratings
5
1
,
5
2
,
5
3
, and
5
4
, polarization couplers (polarization beam combiner)
6
1
and
6
1
, and a WDM coupler
11
. The fiber gratings
5
1
,
5
2
,
5
3
, and
5
4
are wavelength-selective reflectors which set the center wavelengths of lasers
3
1
,
3
2
,
3
3
, and
3
4
, respectively. Gratings
5
1
,
5
2
set the center wavelengths of lasers
3
1
,
3
2
to a first wavelength &lgr;
1
, and gratings
5
3
,
5
4
set the center wavelengths of lasers
3
3
,
3
4
to a second wavelength &lgr;
2
. The difference between &lgr;
1
and &lgr;
2
is between 6 nm and 35 nm, and additional sets of lasers and gratings at different wavelengths may also be added to pumping source
1
. Light outputs from lasers
3
1
,
3
2
,
3
3
,
3
4
are polarization-multiplexed by the polarization coupler
6
for each wavelength &lgr;
1
, &lgr;
2
, and output lights from the polarization coupler
6
are combined by the WDM coupler
11
to obtain the output light of pumping source
1
. Polarization maintaining fibers
17
are connected between the semiconductor lasers
3
and the polarization coupler
6
to obtain two pumping lights having different polarization planes.
A portion of the output light is coupled by a branching coupler
14
and analyzed by a monitoring and control circuit
15
, which determines the amount of amplification that is occurring and generates a feedback control signal to pumping source
1
which ensures consistent amplification (gain).
While
FIG. 1
shows the basic construction of an EDFA amplifier and
FIG. 2
shows the basic construction of a Raman amplifier, there are many challenges remaining for improving the performance and efficiency of the system, and it is believed that nearly every component of the system can be improved. Among the challenges addressed by the present invention is the need for a higher power pumping laser which has a center wavelength and a level of power that are well-controlled, and which has a very narrow bandwidth output that can be wavelength multiplexed with other such lasers, each typically fixed at different center wavelengths.
Greater power from the pumping lasers enables a repeater in a DWDM system, which typically comprises an EDFA or Raman Amplifier, to amplify the incoming signals to a greater degree, which enables the distance between repeaters to be increased. The latter enables one to reduce the number of repeaters in the system, thereby lowing the cost of the system and increasing the reliability of the system. Greater power also enables the EDFA to amplify more signal channels, and thereby enables the DWDM system to carry more signal channels.
Stable and narrowly confined power from the pumping lasers enables a low loss multiplexing of the individual pumping light with wavelength multiplexing couplers, thereby enabling a multiplexed power to be greater. A stable center wavelength and a well-controlled level of power also enables a Raman gain produced by the pumping light to be stable, thereby preventing associated noise from being modulated on
Aikiyo Takeshi
Koyanagi Satoshi
Tsukiji Naoki
Yoshida Junji
Leung Quyen
Sheppard Mullin Richter & Hampton LLP
The Furukawa Electric Co. Ltd.
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