Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2000-08-04
2004-01-13
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S337000, C359S337210, C359S341100, C359S341200, C359S341300, C372S006000
Reexamination Certificate
active
06678087
ABSTRACT:
This application is based on Japanese Patent Application Nos. 11-224534 (1999) filed Aug. 6, 1999, and 11-264493 (1999) filed Sep. 17, 1999, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier applicable to an optical fiber communication system and the like, and utilizing an active optical fiber as its gain medium, and to a Raman amplifier capable of improving its gain and pumping efficiency and to an optical fiber communication system using the Raman amplifier.
2. Description of the Related Art
As conventional examples of such optical amplifiers, there are a first configuration as shown in
FIG. 21
(see, P. F. Wysocki, et al., IEEE Photon. Technol. Lett., vol.9, pp.1343-1345, 1997), and a second configuration as shown in
FIG. 22
(see, E. Desurvire, Erbium-Doped Fiber Amplifiers, John Wiley & Sons, Inc., Section 6.2, 1994). In the optical amplifier of the first conventional configuration as shown in
FIG. 21
, signal light incident on a first optical isolator
41
is combined by a first optical combiner
42
with pump light fed from a first pumping light source
43
, and is incident on a first erbium-doped fiber (called EDF from now on)
44
. The EDF
44
amplifies the signal light using the pump light, and the amplified signal light passes through a second optical isolator
45
to be incident on a partially reflective gain equalizer
46
. The signal light emerging from the partially reflective gain equalizer
46
passes through a third optical isolator
47
, and is combined by a second optical combiner
48
with pump light fed from a second pumping light source
49
to be incident on a second EDF
51
. The second EDF
51
amplifies it using the pump light, and the amplified light signal is emitted through a fourth optical isolator
52
. In
FIG. 21
, the symbol “a” designates a fusion splice.
In the second conventional optical amplifier as shown in
FIG. 22
, signal light is incident on a first port of an optical circulator
61
and emitted from a second port so that it is combined by an optical combiner
63
with pump light fed from a pumping light source
64
, and the coupled signal light and pump light are incident on an erbium-doped fiber (EDF)
65
, in which the signal light is pumped and amplified by the pump light. The amplified signal light is incident on a wavelength independent reflector
66
which reflects off the signal light to be incident on the EDF
65
from the opposite direction to the first incident direction. Then, the signal light passes through the EDF
65
and is incident on the second port of the optical circulator
61
via the optical combiner
63
. The signal light is supplied from the third port of the optical circulator
61
to a gain equalizer
62
to be output therefrom.
The optical amplifiers of the first and second conventional configurations as shown in
FIGS. 21 and 22
use rare-earth doped fibers such as the erbium-doped fibers (EDF)
44
,
51
and
65
as a gain medium. The rare-earth doped fibers include besides the erbium-doped fiber a praseodymium-doped fiber, thulium-doped fiber depending on the type of the rare-earth material.
The partially reflective gain equalizer
46
is placed in an intermediate position to carry out gain equalization with maintaining the noise figure and optical output of the optical amplifier at good values. The partially reflective gain equalizer
46
consists of a Bragg fiber grading that equalizes the gain spectrum by reflecting part of the signal light in the direction opposite to its propagation direction to give a transmission loss, thereby providing the transmission loss with wavelength dependence.
The partially reflective gain equalizer
46
is interposed between the optical isolators
45
and
47
to prevent the signal light or spontaneously emitted light, which is reflected or amplified by the partially reflective gain equalizer
46
, from returning to the first or second EDF
44
or
51
, thereby preventing noise because of their return. The two optical isolators
41
and
52
at the two ends are installed to prevent the optical amplifier from becoming instable because of residual reflection light from outside.
FIGS. 23A and 23B
illustrate gain spectra of the optical amplifier of
FIG. 21
, and a loss spectrum of the gain equalizer
46
, respectively.
FIG. 23A
comparatively illustrates two spectra when the gain equalizer is used and not used, which illustrates that the gain equalization is carried out between the wavelengths &lgr;
1
and &lgr;
2
.
In the second configuration of the optical amplifier as shown in
FIG. 22
, the pumping efficiency is improved by using the optical circulator
61
and wavelength independent reflector
66
. Since the signal light reflected by the wavelength independent reflector
66
passes through the erbium-doped fiber (EDF)
65
twice, the gain (in terms of dB) is twice that of one passage. In addition, since the pump light is also reflected by the wavelength independent reflector
66
, the pump light in the EDF is enhanced.
FIGS. 24A and 24B
illustrate a reflectivity spectrum and a loss spectrum of the wavelength independent reflector
66
, respectively. The reflectivity is set high (close to 100%), and the loss is set low. The signal light passing through the EDF
65
twice is guided to the signal light output port by the optical circulator
61
, and is output through the gain equalizer
62
. The gain spectrum of the amplifier and the loss spectrum of the gain equalizer
62
are the same as those of
FIGS. 23A and 23B
. The gain equalizer
62
used by the second configuration can be either partially reflective or antireflective. The antireflective gain equalizer includes a Mach-Zehnder optical filter or long period fiber grating.
FIGS. 25A and 25B
each show part of the erbium-doped fiber
65
and wavelength independent reflector
66
used in the second configuration:
FIG. 25A
shows a silicate erbium-doped fiber
65
a
, whereas
FIG. 25B
shows a non-silicate erbium-doped fiber
65
b
. As for the silicate erbium-doped fiber (EDF)
65
a
, it usually undergoes fusion splicing (denoted by the symbol “a”) with adjacent pigtail fibers
67
. Generally, the core diameter of the EDF is considerably smaller than that of the pigtail fibers
67
, and hence the fusion splicing of the two requires much expense and time. On the other hand, as for the non-silicate erbium-doped fiber (EDF)
65
b
, since the fusion splicing between the EDF and the pigtail fibers
67
cannot be achieved, high NA (high numerical aperture) silicate fibers
68
and the EDF are butted and bonded at angled polished surfaces (denoted by the symbol “c”), followed by the fusion splicing between the high NA silicate fibers
68
and the pigtail fiber
67
(denoted by the symbol “a”)
The first configuration of the conventional optical amplifier as shown in
FIG. 21
includes many components such as the EDFs
44
and
51
, the pumping light sources
43
and
49
, and the optical combiners
42
and
48
, which presents a problem of increasing the size and cost of the optical amplifier.
As for the second configuration of the conventional optical amplifier as shown in
FIG. 22
, since the gain equalizer
62
is installed outside the gain medium EDF, that is, on the output side of the signal light, a problem arises of reducing the optical output power by an amount of the loss of the gain equalizer
62
.
Furthermore, since the configuration comprising the EDF
65
a
or
65
b
in connection with the wavelength independent reflector
66
as shown in
FIG. 25A
or
25
B requires the fusion splicing made by butting the angled polished surfaces followed by bonding, it has a problem of increasing the number of optical components, and hence increasing the cost of components and assembly.
FIG. 46
is a block diagram showing a first configuration of a conventional Raman amplifier. The Raman amplifier comprises an optical fiber
161
, a gain medium of the Raman amplification; a pumping light source
162
;
Masuda Hiroji
Takachio Noboru
Black Thomas G.
Cunningham Stephen
Nippon Telegraph and Telephone Corporation
Ostrolenk Faber Gerb & Soffen
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