Abstract:
An optical amplifying transmission system comprising a first optical amplifying transmission line including a first optical amplifier, a second optical amplifying transmission line including a second optical amplifier, a pumping light generator for generating pumping lights to be supplied to said first and second amplifiers, the powers of the pumping lights being variable, and first and second terminal stations which connect respectively to both ends of said first and second optical amplifying transmission lines.

Description:
FIELD OF THE INVENTION 
     This invention is related to an optical amplifying transmission system and an optical amplifying repeater, and more specifically, to an optical amplifying transmission system in which the transmission capacity of two optical transmission lines is alterable and an optical amplifying repeater therefor. 
     BACK GROUND OF THE INVENTION 
     A main method for international communication, until quite recently, has been a telephone and the volume of communication traffic between two countries has been mutually the same in both transmission directions. Therefore, up and down lines of a transmission system are designed symmetrically, namely as the capacity of both directions becomes equivalent. 
     However, due to the spread of an internet, a phenomenon has been occurring in which the volume of communication traffic of both transmission directions is greatly one-sided (asymmetrical traffic). Furthermore, the volume of communication traffic or the degree of asymmetry is subject to change with the passage of time. 
     In that circumstances, an extremely uneconomic means has been usually employed in which a transmission system capable of realizing the estimated maximum transmission capacity of both up and down transmission lines is constructed in the first place and only part of the system is used at the beginning. 
     Therefore, a transmission line has been greatly expected whose transmission capacity can be set up or altered more flexibly. 
     SUMMARY OF THE INVENTION 
     A purpose of this invention is to provide an optical amplifying transmission system in which the transmission capacity of two transmission lines is alterable even after completion of its construction and an optical amplifying. repeater used for the optical amplifying transmission system. 
     According to the invention, pumping powers can be varied flexibly, which are applied to a first amplifier disposed on a first optical amplifying transmission line and a second optical amplifier disposed on a second optical amplifying transmission line. Consequently, the transmission capacities of the first and second optical amplifying transmission lines can be set asymmetrically and, thus, there is no need to prepare an excessive transmission capacity in advance. 
     Preferably, the power distribution rate of each pumping power for being supplied to the first and second optical amplifiers is variable. Accordingly, the transmission capacities of the first and second optical amplifying transmission lines is easily controlled to be asymmetrical and its controlling structure is also simply realized. Furthermore, it is easy to insert the controlling structure as an optical amplifying repeater into a submarine optical cable. 
     By providing a divider for dividing pumping power control command light transmitted to the first optical amplifying transmission line from a first terminal and a controller for controlling the powers of pumping lights supplied to the respective first and second amplifiers by a pumping light generator according to the pumping power control command light divided by the divider, the pumping powers to be supplied to the first and second optical amplifiers or the power distribution rate can be remote-controlled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic block diagram of an embodiment of the invention; 
     FIG. 2 is a schematic block diagram of an embodiment of an optical repeater; 
     FIG. 3 is a plan view of an asymmetric X optical switch showing an illustration of a pumping light combiner/divider  50 ; 
     FIG. 4 is a schematic block diagram of another illustration of the pumping light combiner/divider  50 ; and 
     FIG. 5 is a schematic block diagram of another embodiment of the optical repeater. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention are explained below in detail with reference to the drawings. 
     FIG. 1 is a schematic block diagram showing an embodiment of an optical amplifying transmission system of the invention. An up optical transmission line  14  for transmitting signal light from a terminal station  10  to a terminal station  12  and a down optical transmission line  16  for transmitting signal light from the terminal station  12  to the terminal station  10  are connected between the terminal stations  10  and  12 . The terminal station  10  comprises an optical sender  10 S for sending the signal light to the up optical transmission line  14  and an optical receiver  10 R for receiving and processing the signal light input from the down optical transmission line  16 . The terminal station  12  comprises an optical sender  12 S for sending the signal light to the down optical transmission line  16  and an optical receiver  12 R for receiving and processing the signal light input from the up optical transmission line  14 . 
     The up optical transmission line  14  is an optical amplifying/repeating transmission line in which a plurality of transmission fibers  18  are connected through optical amplifiers  20 . The down optical transmission line  16  is also an optical amplifying/repeating transmission line in which a plurality of transmission fibers  22  are connected through optical amplifiers  24 . 
     Each of the optical amplifiers  20  and  24  comprises, for example, an erbium-doped optical amplifying fiber and a pumping light generator  26  generates pumping light for pumping the erbium-doped optical amplifying fibers. In a long distance optical transmission line system such as a submarine optical cable, the up optical transmission line  14  and the down optical transmission line  16  are kept in a single cable and the optical amplifiers  20  and  24  located within the same distance are stored in the same joint part. Each pumping light generator  26  supplies the pumping light to the optical amplifiers  20  and  24  stored in the same joint part. 
     The signal light output from the optical sender lOS of the terminal station  10  is optically amplified by the optical amplifier  20  while transmitting on the transmission optical fiber  18  of the up optical transmission line  14  and enters the optical receiver  12 R of the terminal station  12 . The signal light output from the optical sender  12 S of the terminal station  12  is optically amplified by the optical amplifier  24  while transmitting on the transmission optical fiber  22  of the down optical transmission line  16  and enters the optical receiver  10 R of the terminal station  10 . 
     In the conventional art, the signal light level of the up optical transmission line  14  and that of the down optical transmission line  16  are designed to be exactly the same. That is, the pumping light having basically the same power is supplied to both of the optical amplifier  20  of the up optical transmission line  14  and the optical amplifier  24  of the down optical transmission line  16  so that both amplifiers operate in the same state. As a result, optical signal levels of the up and down transmission lines become approximately equivalent and the transmission capacity of the up and down transmission lines is also the same. 
     However, according to the invention, the pumping light generator  26  comprises a distributor, the details are described later, which is externally controllable of the power rates to be distributed to the amplifiers  20  and  24 , and, therefore, the power of the pumping light supplied to the optical amplifier  20  and that of the pumping light supplied to the optical amplifier  24  can be varied freely. The pumping light generator  26 , therefore, supplies the pumping light of more power to an optical amplifier (for example, the optical amplifier  20  or  24 ) requiring a greater transmission capacity and the pumping light of less power to an optical amplifier (for example, the optical amplifier  24  or  20 ) requiring less transmission capacity. 
     In an optical transmission system designed satisfactorily, the transmission capacity is almost determined by an optical signal level of an optical transmission line. Further, the optical signal level is proportional to the pumping light power supplied to an optical amplifier. Therefore, by altering the distribution rate of the pumping light power, the desirable transmission capacity can be distributed to the up optical transmission line  14  and the down optical transmission line  16  without changing the total transmission capacity of the up and down optical transmission lines  14  and  16 . 
     If only the distribution rate is altered keeping the fixed total power of the pumping light generated from the pumping light generator  26 , the feeding power to the pumping light generator  26 , namely the feeding power to the repeating amplifier becomes regular and, as a result, there is no need to make an investment in equipment for an unnecessary transmission capacity. 
     As a whole transmission system, it is required that the optical senders  10 S,  12 S and the optical receivers  10 R,  12 R of the terminal stations  10  and  12  are capable of corresponding to the increase and decrease of the communication volume. However, in an international long distance communication system in which an optical submarine cable, which is difficult to be altered after construction, is employed as a part of transmission line, the utility value of the whole communication system remarkably increases since the form of utilization is extremely flexible as shown in the embodiment. 
     FIG. 2 is a schematic block diagram of an embodiment of an optical amplifying repeater comprising the optical amplifiers  20  and  24 , the pumping light generator  26  and their accompanying optical elements and circuits. 
     On the up optical transmission line  14 , an erbium-doped optical amplifying fiber  30  and an optical isolator  32  are connected in series between a transmission optical fiber  18 - 1  on the upper course and a transmission optical fiber  18 - 2  on the lower course. The optical isolator  32  prevents Rayleigh scattering light from entering the optical amplifying fiber  30  from the transmission optical fiber  18 - 2  on the lower course. An WDM coupler  34  for introducing pumping light to the optical amplifying fiber  30  in the opposite direction from signal light is arranged between the optical amplifying fiber  30  and the optical isolator  32 . 
     On the down optical transmission line  16 , the structure is basically the same with that on the up optical transmission line  14 . That is, an erbium-doped optical amplifying fiber  36  and an optical isolator  38  are connected in series between a transmission optical fiber  22 - 1  on the upper course and a transmission optical fiber  22 - 2  on the lower course. The optical isolator  38  prevents Rayleigh scattering light from entering the optical amplifying fiber  36  from the transmission optical fiber  22 - 2  on the lower course. A WDM coupler  40  for introducing pumping light to the optical amplifying fiber  36  in the opposite direction from signal light is arranged between the optical amplifying fiber  36  and the optical isolator  38 . 
     Furthermore, on the up optical transmission line  14 , a monitor coupler  42  for forwarding control command light sent from the terminal station  10  to a controller  44  is arranged between the optical isolator  32  and the transmission optical fiber  18 - 2 . The control command light is used to control the distribution rate of the pumping light toward the optical amplifying fiber  30  on the up optical transmission line  14  and the pumping light toward the optical amplifying fiber  36  on the down optical transmission line  16 . The signal light and the control command light transmitting on the up optical transmission line  14  enter the controller  44  through the monitor coupler  42 , and the controller  44  detects the control command light and generates a distribution control voltage signal. If it is also desired to control the distribution rate from the terminal station  12 , the same kind of monitor coupler is disposed between the optical isolator  38  and optical fiber  22 - 2  on the down optical transmission line  16  and output light of the monitor coupler is applied to the controller  44 . 
     Laser diodes  46  and  48  generate laser light having a wavelength for pumping the erbium to be doped to the optical amplifying fibers  30 ,  36  and apply to a pumping light combiner/divider  50 . The wavelengths of output light of the laser diodes  46  and  48  are slightly different each other, for example, approximately one nm or more, so as not to interfere mutually. The pumping light combiner/divider  50  combines the laser lights from the laser diodes  46 ,  48  and divides the combined light in two at a distribution rate according to the distribution control voltage signal from the controller  44 . Each divided light is supplied to the optical amplifying fibers  30  and  36  through the WDM couplers  34  and  40  respectively. 
     The operation for distributing pumping light in the optical amplifying repeater shown in FIG. 2 is explained below. The signal light (containing the control command light for controlling the pumping light distribution rate) outputted from the optical transmitter lOS of the terminal station  10  transmits on the up optical transmission line  14  and enters the optical amplifying fiber  30  from the transmission optical fiber  18 - 1 . As to be described later, pumping light from the WDM coupler  34  is introduced to the optical amplifying fiber  30  in the opposite direction from the signal light and the optical amplifying fiber  30  optically amplifies the signal light with the pumping light. The signal light optically amplified by the optical amplifying fiber  30  transmits the optical isolator  32  at low loss. The optical signal and the control command light output from the optical isolator  32  enter the controller  44  through the monitor coupler  42 . The controller  44  detects the control command light from the input light. The output light of the optical isolator  32  also enters the next transmission optical fiber  18 - 2  and transmits toward the optical receiver  12 R of the terminal station  12 . 
     The controller  44  generates the distribution control voltage signal by photoelectric-converting the control command light from the monitor coupler  42  and applies it to a control input of the pumping light combiner/divider  50 . The output laser lights of the laser diodes  46  and  48  enter to the two inputs of the pumping light combiner/divider  50 . The pumping light combiner/divider  50  combines the two laser inputs and then divides into two portions at the distribution rate according to the distribution control voltage signal from the controller  44 . The two portions of divided laser light are applied as pumping light to the optical amplifying fibers  30  and  36  through the WDM couplers  34  and  40  respectively. 
     As mentioned above, in the optical amplifying repeater shown in FIG. 2, the respective optical amplifying fibers  30  and  36  are pumped by the pumping light divided at the distribution rate determined by the distribution control voltage signal output from the controller  44 . The gains of the optical amplifiers  20  and  24  at each repeating position of the transmission system shown in FIG. 1 are respectively controlled similarly on the up and down optical transmission lines  14  and  16 . That is, signal levels on the up and down optical transmission lines  14  and  16  can be remote-controlled from the terminal station  10  (or  12 ). 
     The pumping light combiner/divider  50  comprises, for example, an asymmetric X optical switch as shown in FIG.  3 . FIG. 3 shows a plan view of the asymmetric X optical switch. The asymmetric X optical switch is formed, for example, as a waveguide on a crystal of lithium niobate. Two waveguides  60  and  62  on the input side of laser light and two waveguides  64  and  66  on the output side intersect at a point showing a shape of X. The output light of the laser diode  46  enters to the waveguide  60  and the output light of the laser diode  48  enters the waveguide  62 . For stabilizing the distributing operation of the optical switch, the waveguide structures of the waveguides  60  and  62  are made to be different each other. 
     The laser lights input to the waveguides  60  and  62  are transmitted to the junction with the waveguides  64  and  66 , combined there and distributed to the wavelengths  64  and  66 . At the upper parts of the waveguides  64  and  66 , electrodes  68  and  70  are respectively disposed. The electrode  68  is connected to the ground and, on the other hand, the control voltage from the controller  44  is applied to the electrode  70 . Electric fields of mutually opposite directions are applied to the waveguides  64  and  66  to be located beneath the electrodes  68  and  70 . With this operation, the applied voltage (the control voltage signal from the controller  44 ) of the electrode  70  can control the distribution rate for distributing the laser lights from the waveguides  60  and  62  to the waveguides  64  and  66 . The laser lights transmitted the waveguides  64  and  66  are respectively applied to the WDM couplers  34  and  40 . The laser lights from the laser diodes  46  and  48  is distributed equally to the waveguides  64  and  66  when no voltage is applied to the electrode  70 . 
     Although the asymmetric X optical switch shown in FIG. 3 can realize the two functions of combining and dividing with the single optical element, it is also applicable to realize the two functions separately using different elements. For instance, a Y branch optical switch can be used for dividing wavelength-multiplexed pumping light or polarization-multiplexed pumping light at a desired distribution rate. 
     FIG. 4 is a schematic block diagram showing another embodiment of the pumping light combiner/divider  50 . The output light of the laser diodes  46  and  48  is multiplexed in the same polarization state by a WDM coupler  72  and applied to a polarization rotator  74 . The polarization rotator  74  rotates a polarization plane of the input light according to the control voltage signal from the controller  44 . Output light of the polarization rotator  74  is applied to a polarization beam splitter  76  and split into two elements of mutually orthogonal polarization planes. The light of one polarization plane is applied to the WDM coupler  34  and the light of the other polarization plane is applied to the WDM coupler  40 . 
     As mentioned above, by using the polarization rotator  74  for rotating the polarization plane at an angle in accordance with the control voltage and the polarization beam splitter  76  together, the distribution rate, namely the rate of the pumping lights entering to the optical amplifying fibers  30  and  36  can be remote-controlled. 
     The simpler structure is that output lights of individual laser diodes pump the optical amplifiers  30 ,  36  and the terminal station  10  (or  12 ) controls the driving currents of the laser diodes. FIG. 5 shows a schematic block diagram of this embodiment. Identical elements are labelled with reference numerals common to those in FIG.  1 . 
     A controller  80  converts the control command light from the monitor coupler  42  into electric signal and applies the control signal showing the distribution of driving currents toward an LD driver  82 . The LD driver  82  supplies the driving currents to the respective laser diodes  84  and  86  at the distribution rate in accordance with the control signal from the controller  80 . That is, the LD driver  82  varies the distribution rate of driving currents to the laser diodes  84  and  86  according to the control signal from the controller  80  keeping a specific amount of feeding power. Generally, output of a laser diode is stabilized by an APC control and, therefore, its output power can be easily altered by changing a reference voltage of the APC circuit. The output laser lights of the laser diodes  84  and  86  are supplied to the optical amplifying fibers  30  and  36  through the WDM couplers  34 ,  40  and pump the optical amplifying fibers  30  and  36  respectively. 
     The structure shown in FIG. 5 also can vary the pumping light powers to be supplied to the up and down optical amplifying fibers  30  and  36  keeping the amount of feeding power for being supplied to the repeater and as a result the transmission capacity of the up and down lines can be made asymmetry. 
     Although the two laser diodes are employed in the abovementioned embodiment, this invention is naturally applicable to the structures of using more than two laser diodes. 
     As readily understandable from the above description, according to the invention, it is possible to easily provide an economical optical transmission system to meet the needs of asymmetric communication traffic. That is, it is no longer necessary to construct an optical transmission line having an excessive transmission capacity. 
     While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.