Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-06-02
2002-06-25
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S337130, C359S341410, C359S341420, C372S006000, C372S034000
Reexamination Certificate
active
06411430
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier mainly used in an optical communication system and suitable for amplifying a wavelength-division multiplexed optical signal having a band of 1.5 &mgr;m.
2. Related Background Art
In optical fiber communication systems, rare earth doped optical fiber amplifiers (referred to merely as “optical amplifiers” hereinafter) have remarkably been developed. Particularly, a D-WDM system in which a wide amplifying band of the optical amplifier is utilized and a communication capacity is increased by using wavelength-division multiplexed optical signals obtained by multiplexing a plurality of optical signals in the amplifying band has mainly been progressed. However, although the optical amplifier has the wide amplifying band, an amplifying property thereof has wavelength dependency, input intensity dependency and temperature dependency.
Thus, when the wavelength-division multiplexed optical signals are amplified collectively, there arises a problem regarding difference in gain between the respective different optical signal wavelengths (referred to as “channels” hereinafter). That is to say, in the D-WDM system, when the optical amplifiers are connected in a multi-stage fashion, the gain differences between the channels are accumulated, thereby causing great output signal intensity difference ultimately. Since the transmission property of the entire optical transmitting system is limited by a channel having minimum output signal intensity, even when there are channels having greater output signal intensity, the transmission property of the entire optical transmitting system will be reduced.
To solve this problem, various techniques have been developed. As one of these techniques, there is means for controlling a temperature of the entire rare earth doped optical fibers to keep the temperature constant in order to eliminate the temperature dependency. However, the means for controlling the temperature of the rare earth doped optical fibers increases power consumption and makes the entire system bulky, and, increase in the used temperature range results in additional increase in power consumption.
Further, there is gain constant control means for keeping the gain constant by adjusting output intensity in accordance with input intensity after gain spectrum is flattened by inserting a correction filter into an optical amplifier portion in order to eliminate the wavelength dependency from the amplifying property.
FIG. 44
shows an example of an optical amplifier utilizing such means. In the optical amplifier shown in
FIG. 44
, optical fiber amplifiers are connected in a two-stage fashion. The optical amplifier comprises an input optical connector
1
a
, an output optical connector
1
b
, optical couplers or beam splitters
2
a
,
2
b
,
2
c
,
2
d
, optical monitors PD
3
a
,
3
b
,
3
c
,
3
d
, optical isolators
4
a
,
4
b
,
4
c
,
4
d
, pumping light/optical signal wavelength-division multiplexers
5
a
,
5
b
,
5
c
, pumping light sources
6
a
,
6
b
,
6
c
, rare earth doped optical fibers (optical fiber amplifiers)
7
a
,
7
b
, an optical variable attenuator
8
, and optical signal gain constant pumping light source control circuits
9
a
,
9
b
. In this optical amplifier, a part of input optical signal outputted from the input optical connector
1
a
is picked up by the beam splitter
2
a
and light intensity thereof is measured by the optical monitor PD
3
a
. The optical signal passes through the optical isolator
4
a
and is incident on the optical fiber amplifier
7
a
which is now maintained in a pumping condition by the pumping light source
6
a
. In this optical fiber amplifier, the optical signal is subjected to optical amplification by stimulated emission. The optical-amplified optical signal passes through the optical isolator
4
b
, and a part of the light is picked up by the beam splitter
2
b
and light intensity thereof is measured by the optical monitor PD
3
b
. The pumping light source
6
a
is adjusted by the optical signal gain constant pumping light source control circuit (AGC)
9
a
so that a ratio between the input optical signal of the optical monitor PD
3
a
and the output optical signal of the optical monitor PD
3
b
becomes a constant value. The optical signal passed through the first stage passes through the optical variable attenuator
8
and is incident on the second stage. The second stage is operated in the similar manner to the first stage, so that the signals of the optical monitors PD
3
c
,
3
d
are compared by the optical signal gain constant pumping light source control circuit (AGC)
9
b
, and the pumping light sources
6
b
,
6
c
are controlled so that a ratio therebetween becomes a constant value. As a result, even if the light intensity of the input signal is changed, gain spectrums of the optical fiber amplifiers in the first and second stages are kept constant.
However, in the optical amplifier utilizing the gain constant control means as shown in
FIG. 44
, since the intensity of the pumping light is varied with the light intensity of the input signal, in a small input optical signal area within the operation input optical signal intensity range, the intensity of the pumping light becomes small, thereby deteriorating noise figure. Further, since the intensity of the pumping light is greatly changed, the first stage of the gain constant control requires forward pumping or bi-directional pumping.
In consideration of the above, an object of the present invention is to provide an optical amplifier of multi-stage type having a plurality of rare earth doped optical fibers and in which temperature dependency of gain spectrum can be compensated so as to be operated with constant gain spectrum regardless of used temperature. Another object of the present invention is to provide an optical amplifier which can be operated with constant gain spectrum regardless of intensity of input optical signal, insertion loss of parts between stages and light intensity of output optical signal. A further object of the present invention is to provide an optical amplifier in which noise figure is improved in a small optical signal area within an operation input optical signal range, and a gain configuration is kept constant regardless of intensity of input optical signal, and output variable control can be performed.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an optical amplifier having a plurality of rare earth doped optical fibers in a multi-stage and comprising one or more optical variable attenuator means, and an attenuation amount control means for changing an optical attenuation amount of the optical variable attenuator means on the basis of temperature of the rare earth doped optical fibers or an environmental temperature.
According to a second aspect of the present invention, there is provided an optical amplifier having a plurality of rare earth doped optical fibers in a multi-stage and comprising, a replaceable optical part between the rare earth doped optical fibers, one or more optical variable attenuator means, and an attenuation amount control means for changing an optical attenuation amount of the optical variable attenuator means on the basis of temperature of the rare earth doped optical fibers or an environmental temperature.
According to a third aspect of the present invention, in the optical amplifier according to the first or second aspect, the attenuation amount control means has an optical attenuation amount table associated with the temperature, and the optical attenuation amount of the optical variable attenuator means is changed on the basis of the optical attenuation amount table.
According to a fourth aspect of the present invention, in the optical amplifier according to the third aspect, when an equation obtained by applying the regression line based on the method of least squares to the optical attenuation amount table is represented by “optical attenuation amount=
Emori Yoshihiro
Fukushima Masaru
Ogino Atsushi
Ogoshi Haruki
Tashiro Yoshio
Moskowitz Nelson
The Furukawa Electric Co. Ltd.
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