Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-12-07
2003-09-30
Epps, Georgia (Department: 2873)
Optical waveguides
Optical fiber waveguide with cladding
C385S042000
Reexamination Certificate
active
06628870
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a block gain equalizer for adjusting gain in an optical transmission system, and more particularly to a block gain equalizer suitable for being inserted in a relaying section of an optical fiber transmission path to adjust gain of an optical signal.
DESCRIPTION OF THE PRIOR ART
Optical transmission systems often transmit a large quantity of signals by wavelength multiplexing an optical signal to transmit through an optical fiber transmission path.
FIG. 17
represents a principal part of an optical transmission system in which optical fiber transmission paths are connected together. A plurality of optical fiber transmission paths
101
1
,
101
2
, . . .
101
N
have optical repeaters
102
1
,
102
2
, . . .
102
N−1
disposed among those to amplify a reduced optical signal. In such an optical transmission system, the respective optical fiber transmission paths
101
1
,
101
2
, . . .
101
N
do not uniformly reduce the gain with distance for all frequencies used for optical signals, but a rate of the reduction differs depending upon the frequency. This also applies to a case of each optical repeater
102
1
,
102
2
, . . .
102
N−1
, and Raman amplification causes ununiformity in terms of frequency in an amplification factor. When it is approximately considered that attenuation or amplification of these frequencies occurs at a predetermined slope, it can be regarded that the respective optical fiber transmission paths
101
1
,
101
2
, . . .
101
N
have loss slopes, and the respective optical repeaters
102
1
,
102
2
, . . .
102
N−1
have gain slopes. In other words, one optical transmission system has a frequency characteristic determined by a sum of loss slopes due to the respective optical fiber transmission paths
101
1
,
101
2
, . . .
101
N
and gain slopes due to the respective optical repeaters
102
1
,
102
2
, . . .
102
N−1
.
The optical transmission system is designed so as to flatten the gain of the entire system in consideration of these loss slopes and gain slopes. Actually, however, the characteristics of the optical fiber transmission paths
101
1
,
101
2
, . . .
101
N
and the optical repeaters
102
1
,
102
2
, . . .
102
N−1
may be different from what has been expected due to variations in their manufacture and the like. When this “deviation” exceeds a certain tolerance, the system called “transmission path” does not hold. Thus, in order to prevent such a matter, the block gain equalizer is inserted in the transmission path for each predetermined relaying section in the optical transmission system. Thus, the gains for the respective frequencies are flattened by these block gain equalizers.
FIG. 18
illustrates the state in descriptively. As an example here, a block gain equalizer
103
is disposed behind an optical repeater
102
N−1
at the last stage shown in FIG.
17
. In this example, the frequency characteristic of the first optical fiber transmission path
101
1
is a flat characteristic with respective frequencies having equal signal levels as shown as the first characteristic measurement result
104
1
. If the frequency characteristic of the optical fiber transmission path
101
N
behind the optical repeater
102
N−1
at the last stage inclines by a large amount depending upon the frequency as shown as the N-th characteristic measurement result
104
N
, the block gain equalizer
103
is disposed behind it. This block gain equalizer
103
has such a frequency characteristic
105
as to offset the gain slope corresponding to a change in the frequency. Accordingly, the characteristic measurement result
106
of the optical signal after passage of this block gain equalizer
103
is flattened with respect to the frequency.
FIG. 19
represents a circuit configuration of a block gain equalizer for controlling such a gain. This block gain equalizer
103
has an optical coupler
112
which branches an optical signal
111
inputted from the optical fiber transmission path
101
N
. One optical signal
113
branched by the optical coupler
112
is inputted to a variable gain equalizer
114
, where the gain is flattened, and thereafter, it is transmitted to an optical fiber
116
at the following stage as the optical signal
115
.
On the other hand, the other optical signal
117
branched by the optical coupler
112
is inputted to a band pass filter (BPF)
118
, where only optical signals of a predetermined band are selected. Assuming the optical signals inputted into the block gain equalizer
103
are constituted by signals having respective wavelengths as shown at (a) in
FIG. 19
(differences in intensity of signal level are not indicated here), a specific wavelength &lgr;
SV
of those wavelengths is used as a supervisory signal. The band pass filter
118
selects the supervisory signal
119
having this wavelength &lgr;
SV
. (b) in
FIG. 19
shows a state in which only the supervisory signal
119
has passed through the band pass filter
118
.
The supervisory signal
119
thus selected is inputted into a photodiode (PD)
121
, and is photoelectric-converted into an electric signal
122
. This electric signal
122
representing the supervisory signal
119
is inputted into a gain control circuit
123
, which reads data on loss slope previously incorporated in the supervisory signal
119
to input, into the variable gain equalizer
114
, such a voltage signal
125
as to become a loss slope designated. As a result, the gain of the optical signal
115
(see FIG.
19
(
c
)) obtained is flattened with respect to the used frequency band. Of course, this data on the loss slope may be data transmitted to this block gain equalizer
103
through other paths.
FIG. 20
represents an example of a characteristic of the conventional variable gain equalizer. The variable gain equalizer
114
is adapted such that the loss slope relative to wavelength changes in response to the voltage level of the voltage signal
125
. In this figure, when the voltage signal
125
indicates 3V (volt), the frequency characteristic is flat as indicated by solid line, and no correction is performed. When the voltage signal
125
indicates 2V, the variable gain equalizer has such a characteristic that the more the wavelength &lgr; is increased, the less becomes the loss. When the voltage signal
125
indicates 4V, the variable gain equalizer has such a characteristic that the more the wavelength &lgr; is increased, the more becomes the loss. Therefore, the variable gain equalizer
114
is capable of adjusting the gain of the optical signal
113
in response to the voltage signal
125
to output an optical signal
115
, which has become flat with respect to the frequency.
SUMMARY OF THE INVENTION
The variable gain equalizer
114
shown in
FIG. 19
flattens the gain by changing the attenuation factor relative to the frequency of an optical signal inputted. Therefore, the optical signal inputted into the block gain equalizer
103
does not increase the signal level although the signal level may be reduced. For this reason, apart from a case where the block gain equalizer
103
is inserted at the last stage of the transmission path, when it is inserted in a section along the path, the length of the optical fibers in the section has to be made shorter than other sections. This leads to a problem that the optical transmission system becomes expensive.
It is an object of the present invention to provide a block gain equalizer which controls an amount of gain slope and does not cause any loss in optical signal.
According to the invention specified in claim
1
, there is provided a block gain equalizer having: (a) doped fibers added with rare earth elements for receiving an optical signal transmitted from a transmission path on the upstream side and relaying to a transmission path on the downstream side; (b) a pumping source for injecting pumping light into the doped fibers; and (c) power setting means for setting power of the pumping light to be outputted by this pumping source to power, which
Dinh Jack
Epps Georgia
McGinn & Gibb PLLC
NEC Corporation
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