Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2001-05-11
2002-07-23
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337400
Reexamination Certificate
active
06424456
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical fiber amplifier for clamping and equalizing gain in optical communication systems, and particularly to an optical fiber amplifier for clamping and equalizing gain, capable of clamping gain automatically by maintaining the same population inversion, and capable of maintaining the same gain equalization for the wide signal gain band to achieve the high transmission capacity, in the case that the number of channel varies in the input signal.
BACKGROUND OF THE INVENTION
Generally, in optical communication systems, an optical amplifier is used to compensate loss in a signal. Such an optical amplifier is a device for compensating loss of attenuation of the optical signal due to a long distance transmission.
Optical fiber amplifiers are widely used in optical communication systems. Especially, an erbium doped fiber amplifier (EDFA) is used to amplify a signal periodically in certain distance so as to compensate the attenuation of optical signal caused by the long distance transmission, when a great number of data are transmitted over long distance, without being regenerated through a strand of optical fiber.
If the wavelength division multiplexing (WDM) optical transmission system is used, the whole transmission capacity increases rapidly in proportion to the number of wavelengths used, since a number of data can be transmitted on many carriers of different wavelength each other.
In the WDM network, changes in the number of channels frequently occur on account of changes in network configuration, malfunctions in constituent elements, and frequent add/drops of channels. Change of the total power in the input signal associated with the above change can induce a gain change and a transient to the output of the remaining optical channels. As a result, transmission errors can increase due to the above mentioned gain variations. Accordingly, in WDM networks, the gain variation on account of a change of the number of input signal must be minimized.
There are two typical methods in automatic gain clamping for an optical fiber amplifier. One method is by adjusting the pumping light power and the other is by applying a compensating signal. Recent studies on all-optical automatic gain clamping methods show the relatively low output power and the equalized gain band of approximately
16
nm.
However, recent trends are to employ more than 32 channels and the total output power should be considerably high in proportion to the number of channels, since the signal power at each channel is almost the same. In this case, the sufficient output power cannot be produced with existing configurations of gain-clamped optical fiber amplifier. In addition, since the equalized gain band must be sufficiently wide and the equalizing degree must be quite good for long distance transmissions, a gain-equalizing filter is inevitably required. Since the amplifier can be used in WDM networks, only after solving such problems simultaneously, the development of such an amplifier is very important. The present invention provides an optical fiber amplifier with a simple and reliable configuration capable of solving such problems.
FIG. 1
is a block diagram showing an optical fiber amplifier for clamping and equalizing gain of a prior art.
As shown in
FIG. 1
, the amplifier comprises: a first isolator
21
for passing the input signal light only in one direction; a first optical coupler
11
for coupling the outputs of the first isolator
21
and a filter
31
; a pump
71
for producing the population inversion in an amplifying medium (EDF)
41
by increasing the pump power; a second optical coupler
12
for coupling the outputs the first optical coupler and the pump
71
; an amplifying medium
41
for amplifying the input signal from the second optical coupler
12
; a second isolator
22
for blocking the reflected light and passing the transmitted light in the output from the amplifying medium
41
; a third optical coupler
13
for allotting the output of the second isolator
22
; a third isolator
23
for isolating the allotted signals at the third optical coupler
13
; a filter
31
for passing only a specified wavelength among the output from the third isolator
23
and transmitting it to the first optical coupler
11
; and a fourth isolator
24
for isolating the allotted signals at the third optical coupler
13
after inputting it, and outputting the amplified optical signal, in conjunction with a compensating signal.
As mentioned above, the construction of the optical fiber amplifier of
FIG. 1
is for applying a compensating signal.
That is, an amplifying medium for amplifying the input signal from a second optical coupler
12
, a pump to provide the population inversion in the amplifying medium
41
, two optical couplers
11
,
13
of the ring shaped construction for applying a compensating signal, and a filter
31
to transmit only a specified wavelength are included in the optical fiber amplifier for clamping gain of the prior art.
The gain of the optical fiber amplifier for clamping and equalizing gain of the prior art is determined by the losses of two ring shaped couplers
11
,
13
. When the power of the pump
71
is increased to increase the output signal power, only the laser signal used as a compensating signal is amplified without changing the power of the amplified signal output. Also, the output power will be decreased due to the loss of the third optical coupler
13
.
Eventually, the optical fiber amplifier for clamping gain of the prior art shown in
FIG. 1
cannot be applied to the WDM system because the equalization of gain is not considered.
FIG. 2
is a block diagram showing an optical fiber amplifier of another example of a prior art that adds a gain equalizing filter to the output part of the optical fiber amplifier in FIG.
1
.
As shown in
FIG. 2
, the amplifier comprises: a first isolator
21
for passing the input signal light only in one direction; a first optical coupler
11
for coupling the outputs of the first isolator
21
and a filter
31
; a pump
71
for producing the population inversion in an amplifying medium (EDF)
41
by increasing the pump power; a second optical coupler
12
for coupling the outputs the first optical coupler and the pump
71
; an amplifying medium
41
for amplifying the input signal from the second optical coupler
12
; a second isolator
22
for blocking the reflected light and passing the transmitted light in the output from the amplifying medium
41
; a third optical coupler
13
for allotting the output of the second isolator
22
; a third isolator
23
for isolating the allotted signals at the third optical coupler
13
; a filter
31
for passing only a specified wavelength among the output from the third isolator
23
and transmitting it to the first optical coupler
11
; a gain equalizing filter
51
for equalizing the gain of the output of the allotted signal from the third optical coupler
13
; and a fourth isolator
24
for isolating the output of the gain equalizing isolator
51
, and outputting the optical signal with an additional compensating signal.
The optical fiber amplifier including a gain equalizing filter of a prior art as shown in
FIG. 2
simply equalizes and clamps the gain by providing the gain equalizing filter at the output terminals of the optical fiber amplifier.
According to the construction of
FIG. 2
, an equalized gain band of approximately 30 nm can be obtained from 1530-1560 nm.
However, in the case of the optical fiber amplifier including a gain-equalizing filter, there is a problem of the relatively large loss for the output of the signal since the filter is located at the output terminals.
FIG. 3
is a block diagram showing an optical fiber amplifier of still another example of a prior art.
As shown if
FIG. 3
, the amplifier comprises: a first isolator
21
for passing the input signal light and isolating the light coming from the opposite direction; a first pump
71
for producing the population inversion in an amplifying medium
41
by increasing th
Choi Hyunbeom
Lee Dong-han
Greenblum & Bernstein P.L.C.
Hellner Mark
Neotek Research Co., Ltd.
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