1.  Field of the Invention
The present invention relates to an optical amplifier, an optical transmission equipment, an optical transmission system, and a method thereof and in particular, to an optical amplifier, an optical transmission equipment, an optical transmission system, and a method thereof, with which can be obtained a low noise figure (NIF) as well as dispersion compensation for an optical signal therein. 
2.  Description of Prior Art
In general, it is already known that an input loss of an optical signal at a stage in front of a doped fiber causes Deterioration in a SIN ratio thereof, in particular in an optical amplifier using such the doped fiber therein.  However, as is described in xe2x80x9cOptical Amplifier and Applications thereofxe2x80x9d (published by Ohm Co. Ltd., May, 1992) 5-3[1], it is indispensable to insert an optical isolator at the front stage of the doped fiber in the conventional optical transmission equipment, for the purpose of suppressing the reflection of stimulated or induced emission (i.e., ASE: amplified spontaneous emission) light.  In the optical transmission equipment with such the construction thereof, however, optical parts necessitated to be inserted at the front stage of the doped fiber are not only the optical isolator.  Namely, in general, they also includes optical parts, including a wavelength divider for wavelength of a supervising light, a coupler for monitoring strength of a transmission signal, a multiplexer for multiplexing an pumping or stimulation light, etc.,  and they have respective losses therein.  For instance, for obtaining a gain from 25 dB to 35 dB, it is necessary to combine a semiconductor laser of about 100 mW for excitation and a doped fiber of length from 20 m to 30 m, and in that case, noise figure (i.e., noise index; being abbreviated as NF hereinafter) of the doped fiber cannot be neglected. 
In the optical transmission equipment with such the construction, the optical signal which is once damaged or receive losses therein on a transmission path or line is amplified by using the doped fiber having high NF, after being further damaged or lost thereon, therefore it is difficult to keep the NF less than 6 dB, which can be defined by a ratio between the S/N ratio of an input side and that of an output side. 
Further, in a case where an optical signal of high velocity is applied onto an optical path of ordinal transmission fiber (NDSF: Non-Dispersion Shifted Fiber), there is a necessity of inserting a device for compensating the dispersion.  As the result of this, there is caused another necessity for compensating the loss due to that device for the dispersion compensating. 
An example of the structure of such the optical amplifier of conventional art is disclosed in a publication, xe2x80x9cTrial of 2xc3x972 Bi-directional Relay optical Fiber Amplifier (BDLA)xe2x80x9d (1997 Society Conference of Electronic Information Communication Society, B-10-184), with which the NF can be suppressed at 7.5 dB. Furthermore, a structural example is disclosed for example in U.S. Pat.  No.  5,831,754 (Japanese Patent Laying-Open No.  Hei 7-301831 (1995)), for compensating the loss due to the disperse compensating device. 
In case of transmitting the optical signal by multiple relays or repeaters with use of k optical amplifiers, the deterioration amount in the S/N ratio rises up in proportional to the number of the stages k. Therefore, in an actual optical transmission system where there is an upper limit in total amount of the S/N ratio deterioration, the number of the relay or repeater stages must be lessened following the increase in the S/N deterioration amount of the optical amplifier.  As a result of this, the distance of the optical transmission must be shortened. 
For instance, under regulation of total amount of the S/N deterioration ratio to be equal to or less than 12 dB, if an optical amplifier of S/N ratio deterioration at 4 dB and an optical amplifier of S/N ratio deterioration at 6 dB are positioned at a distance 80 km, respectively, then the total S/N ratio deterioration amount comes to be 12 dB for the transmission path relayed or amplified with three (3) stages of the optical amplifiers of 4 dB, while the same total SIN ratio deterioration amount of 12 dB is obtained by relaying with two (2) stages of the optical amplifiers of 6 dB. Namely, the optical signal can be transmitted at the distance 240 km with the optical amplifiers of 4 dB in S/N ratio deterioration since it can be relayed at three (3) stages therewith, while it can be transmitted only at the distance 160 km with the optical amplifiers of 6 dB since it can be relayed at only two (2) stages therewith. 
Though the S/N deterioration amount is not one being corresponding to the NF one by one, however, it becomes large when the optical amplifier inferior in the NF is applied to, therefore there is a problem that a distance for regenerative relaying or repeating comes to be short, in which the optical signal is returned once into an electric signal to be relayed or repeated. 
Furthermore, with the optical amplifier in which the doped fiber is divided into a plurality of stages, a plurality of exciting or pumping light sources are necessary, therefore bringing about a rise-up of cost of the optical amplifier, as well as the large-sizing and increase in electric power consumption thereof. 
Accordingly, a first object of the present invention is, for dissolving such the problems as mentioned in the above, to provide an optical amplifier including a function of compensating such the dispersion with a low NF, and being economical with a low electric power consumption. 
A second object of the present invention is to provide an optical transmission equipment including a function of compensating such the dispersion with a low NF, and being economical with a low electric power consumption therewith. 
A third object of the present invention is also to provide an optical transmission system including a function of compensating such the dispersion with a low NF, and being economical with a low electric power consumption therewith. 
A fourth object of the present invention is to provide a method for amplifying an optical signal, including a function of compensating such the dispersion with a low NF, and being economical with a low electric power consumption therewith. 
According to the present invention, for dissolving the problems and for achieving the objects mentioned in the above, there is provided an optical amplifier, for use in an optical transmission equipment, comprising:
a first optical doped fiber;
a second optical doped fiber;
a third optical doped fiber;
an optical part of bringing loss in the optical signal, and being provided between said first optical doped fiber and said second optical doped fiber;
a dispersion compensator being provided between said second optical doped fiber and said third optical doped fiber; and
a pumping light source for pumping being optically connected to so that at least two optical doped fibers are excited in common among said first, second and third optical doped fibers. 
With such the construction mentioned above, the feeble optical signal which is weakened on the transmission path(s) due to the propagation thereof is amplified once before being damaged with the loss of the optical part, such as the optical isolator, etc.,  without deterioration in the NF thereof, while the first, second and third optical doped fibers for amplifying thereof are pumped or excited in common, thereby achieving an optical amplifier being economical and small-sized with a low electric power consumption. 
Further, according to the present invention, for achieving the objects mentioned in the above, there is provided an optical transmission equipment for, transmitting optical signal from an equipment in upper stream to an equipment of down stream, comprising:
an optical amplifier portion for amplifying the transmitted optical signal for transmission thereof; and
a supervisor/controller portion for receiving information from the equipment in upper stream and for sending information including that of the optical transmission equipment itself to the equipment of down stream, wherein said optical amplifier portion comprises:
a first optical doped fiber;
a second optical doped fiber;
a third optical doped fiber;
an optical part of bringing loss in the optical signal, and being provided between said first optical doped fiber and said second optical doped fiber;
a dispersion compensator being provided between said second optical doped fiber and said third optical doped fiber; and
an pumping light source for pumping at least two optical doped fibers in common among said first, second and third optical doped fibers. 
With such the construction mentioned above, the feeble optical signal which is weakened on the transmission path(s) due to the propagation thereof is amplified once before being damaged with the loss of the optical part, such as the optical isolator, etc.,  without deterioration in the NF thereof, while the first, second and third optical doped fibers for amplifying thereof are pumped or excited in common, thereby achieving an optical transmission equipment being economical and small-sized with a low electric power consumption. 
Further, according to the present invention, also for achieving the above-mentioned object, there is provided an optical transmission system for transmitting an optical signal, comprising:
an optical sender for sending an optical signal converted from an electric signal onto a transmission path;
an optical transmission equipment for receiving the optical signal being attenuated on said transmission path, and for compensating with dispersion by amplification thereof; and
an optical receiver for the optical signal from said optical transmission equipment so as to convert it into the electric signal, wherein, said optical transmission equipment comprises an optical amplifier portion comprising:
a first optical doped fiber;
a second optical doped fiber;
a third optical doped fiber;
an optical part of bringing loss in the optical signal, and being provided between said first optical doped fiber and said second optical doped fiber;
a dispersion compensator being provided between said second optical doped fiber and said third optical doped fiber; and
an pumping light source for pumping at least two optical doped fibers in common among said first, second and third optical doped fibers. 
With such the construction mentioned above, also, the feeble optical signal which is weakened on the transmission path(s) due to the propagation thereof is amplified once before being damaged with the loss of the optical part, such as the optical isolator, etc.,  without deterioration in the NF thereof, while the first, second and third optical doped fibers for amplifying thereof are pumped or excited in common, thereby achieving an optical transmission system being economical and small-sized with a low electric power consumption. 
Furthermore, according to the present invention, for achieving the above-mentioned object, there is also provided an optical transmission system, comprising:
a plurality of optical senders, each for sending an optical signal of one wavelength converted from a plurality of electric signal onto a transmission path;
a first transponder for inputting said optical signal of one wavelength to convert into a plurality of optical signals being different to one another in the wavelength thereof;
a wavelength multiplexer for multiplexing said plurality of optical signals different to one another in the wavelength;
an optical amplifier for amplifying said multiplexed optical signals;
a wavelength divider for dividing said multiplexed optical signals amplified into the plurality of optical signals different to one another in the wavelength;
a second transponder for receiving said the plurality of optical signals different to one another in the wavelength to convert into an optical signal of one wavelength;
a plurality of an optical receivers, each for converting said optical signal of one wavelength into an electric signal, and further providing:
a supervisor/controller portion at sender side; and
a supervisor/controller portion at receiver side, wherein said optical amplifier has noise figure (NF) being equal or less than 4.5 dB.
With such the construction mentioned above, also, The feeble optical signal which is weakened on the transmission path(s) due to the propagation thereof is amplified once before being damaged with the loss of the optical part, such as the optical isolator, etc.,  without deterioration in the NF thereof, while the first, second and third optical doped fibers for amplifying thereof are pumped or excited in common, thereby achieving an optical transmission system being economical and small-sized with a low electric power consumption. 
And, also according to the present invention, there is also provided a method for amplifying an optical signal between two transmission paths, comprising following steps:
receiving the optical signal from one of said two transmission paths;
optically amplifying the optical signal with a first amplification factor, by means of a first optical doped fiber;
optically suppressing reflection of light due to ASE;
optically amplifying the optical signal with a second amplification factor being higher than the first amplification factor, by means of a second optical doped fiber; and
optically amplifying, the optical signal with a third amplification factor, by means of a third optical doped fiber, so as to be transmitted to the other one of said two transmission paths, wherein at least two are pumped in common among said first, second and third optical doped fibers. 
With such the method mentioned above, also, the feeble optical signal which is weakened on the transmission path(s) due to the propagation thereof is amplified once before being damaged with the loss of the optical part, such as the optical isolator, etc.,  without deterioration in the NF thereof, while the first, second and third optical doped fibers for amplifying thereof are pumped or excited in common, thereby achieving an optical transmission, economically, with a small-sized construction and a low electric power consumption.