Patent Publication Number: US-6335823-B2

Title: Optical amplifier for use in optical communications equipment

Description:
This application is a divisional of application Ser. No. 09/677,759, filed Oct. 3, 2000, now pending, which is a divisional of application Ser. No. 09/042,790, filed Mar. 17, 1998, now pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical amplifier for use in optical communications equipment. 
     2. Description of the Related Art 
     In recent years, wavelength division multiplexing (WDM) transmission systems have been introduced. In transmitting stations, increasing the number of signal channels results in an increase in the number of post amplifiers used. An optical amplifier takes up a lot of space in optical transmitting equipment. It is therefore desired that the optical amplifier be decreased in size and produced in integrated form. 
     In the optical amplifier, a laser diode (LD) is used as a pumping light source and a component that gives off heat generated by the LD is associated with the light source. Of components that construct the optical amplifier, the light source and its associated radiator are very large, preventing the optical amplifier from becoming reduced in size. Thus, separating the pumping light source from the optical amplifier and grouping components necessary for use as a pumping light source, such as a light source and its associated driving circuit, as a pumping light source unit were thought of. 
     FIGS. 1A,  1 B and  1 C show arrangements of conventional pumping light source units. 
     As shown in FIGS. 1A,  1 B and  1 C, three types of pumping light source units may be considered (note that the example of FIG. 1A uses four pumping LDs). The example of FIG. 1C is used in the WDM system developed by CIENA (see catalog “CIENA Multiwave Line Amplifier Block Diagram”). 
     In FIGS. 1A to  1 C,  1  denotes a circuit for driving a pumping light source,  2 ,  3 ,  4 ,  5  and  8 ; pumping light sources,  6 ; a polarization beam splitter (PBS),  7 ; a wavelength division multiplexing (WDM) coupler,  9 ; an optical splitter for separating pumping light, and  10 ; an optical coupler/splitter for coupling and separating pumping light. 
     FIG. 1A, each of the four pumping light sources  2 ,  3 ,  4  and  5  is driven (for example, supplied with current) by a respective one of the four pumping light driving circuits  1  to produce light. The light from each of the pumping light sources  2  to  5  is output as linearly polarized light. The beams of light output from the pumping light sources  2  and  3  can be set substantially equal to each other in wavelength but will be differently polarized, The beams of light which are substantially equal to each other in wavelength but differently polarized are polarization-coupled by the polarization beam splitter  6 . The pumping light sources  4  and  5  are of the same type as the pumping light sources  2  and  3  but will differ from the pumping light sources  2  and  3  in output wavelength. This indicates that, even if the pumping light sources  4  and  5  are of the same type as the pumping light sources  2  and  3 , their wavelengths do not necessarily match because of variations in manufacturing process. The polarization-coupled pumping light from the pumping light sources  2  and  3  and the polarization-coupled pumping light from the pumping light sources  4  and  5  are coupled by the WDM coupler  7 . The light output from the WDM coupler is sent to an optical amplification medium, for example, an erbium doped fiber (EDF), for use as pumping light for amplifying light signals. The WDM coupler is so named because it couples wavelength-multiplexed signal light and pumping light of a specific wavelength, but in practice it can be an ordinary optical coupler. 
     This arrangement is used when only one pumping light source cannot provide a sufficient amplification action to the optical amplification medium and intended to obtain pumping light with a larger power through the use of two or more pumping light sources. 
     The pumping light source unit of FIG. 1B comprises one pumping light source driver  1 , one pumping light source  8 , and an optical splitter  9  for separating pumping light from the pumping light source  8 . This arrangement is used when the pumping light source has a power large enough to supply two or more optical amplification media (not shown). This arrangement allows the two or more optical amplification media to be operated equally with one pumping light source having a single wavelength and a single polarized wave. 
     The pumping light source unit of FIG. 1C comprises two or more pumping light source drivers  1 , an equal number of pumping light sources  8 , and an optical coupler/splitter  10  that couples and separates pumping beams of light from the pumping light sources. This arrangement is intended to supply two or more optical amplification media with pumping light from a single pumping light source unit, but it has two or more pumping light sources  8  to provide pumping light with higher power because a single pumping light source alone cannot give all of the optical amplifiers a sufficient amplifying action. As described above, however, there are variations in wavelength between two or more pumping light sources  8 . Thus, the use of each of the pumping light sources  8  for a respective one of the optical amplification media will cause their respective amplification actions to vary. For this reason, this pumping light source unit is arranged such that beams of pumping light from the pumping light sources  8  are first coupled to produce a single beam of light and then the single beam of light is separated to thereby supply each of the optical amplification media with pumping light of the same property. 
     Hereinafter, what includes a pumping light source unit, an optical amplification medium or media, and other circuits including a control circuit shall be called an optical amplifier. 
     In an optical amplifier, automatic gain control (AGC) or automatic level control (ALC) is sometimes performed to control the supply amount of pumping light to an optical amplification medium. In conventional optical amplifiers, the amount of output light of a pumping light source is varied to vary the supply amount of pumping light to the optical amplification medium by changing a drive current to the pumping light source. 
     In the WDM transmission system, signal light beams of different wavelengths are collectively amplified by an optical amplifier. After the start of system implementation, the WDM transmission system is sometimes modified to increase the signal multiplexing degree (i.e., the system is upgraded). When the multiplexing degree is increased, the optical amplifier requires more pumping power in order to increase the supply amount of pumping light to the optical amplification media. 
     In the pumping light source units shown in FIGS. 1B and 1C, the ratio of the output power of each pumping light is fixed to the ratio of the splitting by the splitter  9  or the optical coupler/splitter  10 , and the amount of pumping light at each output port cannot be varied arbitrarily. 
     The optical amplifier contains components that are associated with a pumping light source to radiate heat generated by it. These components are relatively large among components composing the optical amplifier, preventing the optical amplifier from becoming reduced in size. When LDs as pumping light sources that generate heat and driver circuits therefor are placed close together within the optical amplifier, it will create an excessive rise in temperature, reducing the performance and reliability of the optical amplifier. 
     Heretofore, even if the WDM transmission system is upgraded, pumping light can only be output up to the allowable maximum output of a pumping light source installed in an optical amplifier at the beginning of system implementation. As an example, assume that, in a 16-channel WDM transmission system, only four channels are employed at the beginning of system implementation. In this case, the optical amplifier used is naturally equipped with pumping light sources necessary to accommodate 16 channels, which increases the initial investment at the time of system installation. 
     In an optical communications device equipped with an optical amplifier of the built-in pumping light source type, heat generated in the device is difficult to radiate and it is therefore necessary to cool the device with a fan, dissipating extra power. 
     When pumping light of a narrow spectrum width emitted by an LD as a pumping light source reflects from optical parts composing an optical amplifier or a fiber junction back to the pumping LD, the operation of the LD becomes unstable, which makes the operation of the optical amplifier unstable. To avoid this problem, conventionally the optical amplifier has an optoisolater built in on the output side of the pumping LD. This arrangement requires more optical parts. 
     In the most used type of an optical amplifier, the amplification characteristic of an amplification medium is wavelength-dependent, and pumping LDs have variations in wavelength due to variations in the manufacturing process. For this reason, the optical amplifier has variations in amplification characteristic due to variations in wavelength. 
     In an optical amplifier in which a pumping light source unit and an optical amplification medium are coupled together by means of a connector, it is necessary to sound an alarm in the case that the connector has come off, because pumping light leaking out through the connector is very dangerous for persons at work. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical amplifier which has a function of changing the supply amount of pumping light to an amplification medium, and is small in size and little affected by heat generated by a pumping light source. 
     According to a first aspect of the present invention, there is provided an optical amplifier for amplifying incoming signal light in response to pumping light applied thereto, characterized by including a variable attenuator for varying the input level of the pumping light applied to the optical amplifier to thereby tune the amplification characteristic of the optical amplifier. 
     In the optical amplifier of the present invention, two or more optical amplification units that contain amplification media are assembled into one package. 
     The optical communications device of the present invention comprises a pumping light source unit having at least one pumping light source for generating pumping light and a unit for separating or coupling pumping light from the at least one pumping light source, and an optical amplification unit for amplifying incoming signal light in response to application thereto of pumping light from the pumping light source unit and is characterized in that the pumping light source unit is placed in a location in the optical communications device where heat radiation conditions are good. 
     According to a second aspect of the present invention, there is provided an optical amplifier for amplifying incoming signal light in response to application thereto of pumping light from a pumping light source unit having a pumping light source for generating a pumping light beam and an optical coupler unit for coupling multiple pumping light beams, characterized in that the pumping light source unit includes a polarization plane rotating unit for rotating the plane of polarization of output pumping light from the optical coupler unit through a first angle of rotation for transmission and rotating the plane of polarization of return light, resulting from the output pumping light being reflected from a connector connecting the pumping light source unit and other components of the optical amplifier back to the pumping light source unit, through a second angle of rotation, thereby inputting to the pumping light source return light, different in wavelength from the pumping light source generated by the pumping light source. 
     According to a third aspect of the present invention, there is provided an optical amplifier for amplifying incoming signal light in response to application thereto of pumping light from a pumping light source unit having multiple pumping light sources each generating a pumping light beam and an optical coupler/splitter unit for coupling multiple pumping light beams and splitting into individual light beams, characterized in that the pumping light source unit includes a polarization plane rotating unit for rotating the plane of polarization of output pumping light from the optical coupler unit through a first angle of rotation for transmission and rotating the plane of polarization of return light, resulting from the output pumping light being reflected from a connector connecting the pumping light source unit and other components of the optical amplifier together back to the pumping light source unit, through a second angle of rotation, thereby inputting to the pumping light source return light different in wavelength from the pumping light source generated by the pumping light source. 
     According to a fourth aspect of the present invention, there is provided an optical amplifier in which a pumping light source unit having a pumping light source for generating pumping light and an optical amplification unit having an amplification medium for amplifying incoming signal light in response to application of the pumping light thereto are connected together by means of a connector that allows the pumping light to be transmitted to the optical amplification unit, characterized in that the optical amplification unit includes a unit for determining whether or not the connection between the pumping light source unit and the optical amplification unit is established by means of the connector on the basis of the output level of the pumping light from the pumping light source unit. 
     According to a fifth aspect of the present invention, there is provided an optical amplifier in which a pumping light source unit having a pumping light source for generating pumping light and an optical amplification unit having an amplification medium for amplifying incoming signal light in response to application of the pumping light thereto are connected together by means of a connector that allows the pumping light to be transmitted to the optical amplification unit, characterized in that the pumping light source unit includes a unit for determining whether or not the connection between the pumping light source unit and the optical amplification unit is established by means of the connector on the basis of the level of return light reflected from the connector. 
     According to a sixth aspect of the present invention, there is provided an optical amplifier in which a pumping light source unit having a pumping light source for generating pumping light and an optical amplification unit having an amplification medium for amplifying incoming signal light in response to application of the pumping light thereto are connected together by means of a connector that allows the pumping light to be transmitted to the optical amplification unit, characterized by comprising a unit for determining whether or not the connection between the pumping light source unit and the optical amplification unit is established by means of the connector. 
     An optical amplification unit of the present invention has an amplification medium for amplifying incoming signal light in response to application thereto of pumping light from a separate pumping light source unit, the optical amplification unit and the pumping light source unit being connected by a connector to form an optical amplifier, and is characterized by the provision of a variable attenuator for adjusting the level of pumping light input to the amplification medium. 
     A pumping light source unit of the present invention has a pumping light source for generating pumping light to be output to a separate optical amplification unit, the pumping light source unit and the optical amplification unit being connected by a connector to form an optical amplifier, and is characterized by the provision of a variable attenuator for adjusting the level of pumping light to be output to the optical amplification unit. 
     According to the present invention, the pumping light, which, in the conventional system, has its output level maintained constant or adjusted by controlling the pumping light source itself, can be level-adjusted easily by the use of the variable attenuator, which allows pumping light with a suitable intensity to be supplied to the amplification medium. 
     In addition, since the pumping light source is separated from the optical amplification unit, two or more optical amplification units can be grouped into one package, ensuring compactness of the optical amplifier. 
     Being separated from the optical amplification unit, the pumping light source can be placed in a location where heat radiating conditions are good and can suppress the effect of heat on the light amplification unit. 
     By the provision of the unit for rotating the plane of polarization of pumping light output from the pumping light source through a predetermined angle in the pumping light source unit, the operational instability of the light source due to return light can be eliminated. 
     According to the present invention, the optical amplifier is separated into the pumping light source unit containing the pumping light source and the optical amplification unit containing the amplification medium and a connector is therefore required to couple these units together. The connector may come off while a person is working. Not only has the pumping light high power, but it is converged by an optical fiber. In the event that the connector has come off, therefore, the person at work may be exposed to danger. To avoid such danger, the optical amplifier of the present invention is provided with means for detecting whether the connector is off or not. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B and  1 C show arrangements of conventional pumping light source units; 
     FIG. 2 illustrates the principle of the present invention; 
     FIG. 3A shows an arrangement of a variable attenuator; 
     FIG. 3B shows an arrangement of the Faraday rotator of FIG. 3A; 
     FIG. 4 shows a first embodiment of an optical amplifier of the present invention; 
     FIG. 5 shows a second embodiment of the optical amplifier of the present invention; 
     FIG. 6 shows a third embodiment of the optical amplifier of the present invention; 
     FIG. 7 shows a first arrangement of the pumping light source unit; 
     FIG. 8 shows a second arrangement of the pumping light source unit; 
     FIG. 9 shows a third arrangement of the pumping light source unit; 
     FIG. 10 shows a fourth arrangement of the pumping light source unit; 
     FIG. 11 shows a fifth arrangement of the pumping light source unit; 
     FIG. 12 shows a sixth arrangement of the pumping light source unit; 
     FIG. 13 shows a seventh arrangement of the pumping light source unit; 
     FIG. 14 is a schematic representation of the optical amplifier; 
     FIG. 15 shows a first arrangement of the optical amplification unit; 
     FIG. 16 shows a second arrangement of the optical amplification unit; 
     FIG. 17 shows a third arrangement of the optical amplification unit; 
     FIG. 18 shows a fourth arrangement of the optical amplification unit; 
     .FIG. 19 shows a fifth arrangement of the optical amplification unit; 
     FIG. 20 shows an arrangement of the pumping light source unit adapted to operate the pumping light source stably; 
     FIG. 21 shows a fourth embodiment of the optical amplifier of the present invention; 
     FIG. 22 shows an arrangement adapted to monitor connection between the optical amplification unit and the pumping light source unit; and 
     FIGS. 23A and 23B are diagrams for use in explanation of how to connect a pumping light source or sources in the pumping light source unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 2, there is illustrated the principle of the present invention. 
     An arrangement of an optical amplifier, with a pumping light source excluded, is illustrated here. Hereinafter, the other portion of an optical amplifier than a pumping light source is referred to as an optical amplification unit. 
     Reference numeral  11  denotes an amplification medium that amplifies signal light by being injected with pumping light,  12 ; a coupler that couples the signal light and the pumping light,  13 ; a variable attenuator that varies the supply amount of the pumping light to the amplification medium, and  14 ; a driver that drives the variable attenuator. 
     In the present invention, signal light and pumping light having its output level adjusted by the variable attenuator  13  driven by the driver  14  are input to the coupler  12 , then coupled and transmitted to the amplification medium  11 . The amplification medium consists of, for example, an erbium doped fiber. By the pumping light, the amplification medium (fiber amplifier)  11  is excited to initiate stimulated emission, thereby amplifying the signal light. 
     With the arrangement of FIG. 2, even if the light multiplexing degree increases, sufficient light amplification can be achieved by merely adjusting the output level of the pumping light with the variable attenuator  13 . 
     In addition, control of the output power of the pumping light through the use of the variable attenuator  13  eliminates the need of controlling a pumping light source itself and moreover allows the pumping light source and the optical amplification unit to be arranged separately. Separating the heat emitting pumping light source from the optical amplification unit susceptible to heat ensures that the optical amplifier operates stably. 
     FIGS. 3A and 3B illustrate an example of the variable attenuator. 
     FIG. 3A illustrates the entire arrangement of the variable attenuator. 
     The variable attenuator comprises a lens  20  which collimates incoming light from an optical fiber  25 , a birefringent wedge  21  which splits an incoming light beam into two components, depending on a difference in polarization, a variable Faraday rotator  22  which can vary the angle of Faraday rotation, a birefringent wedge  23  which splits an incoming light beam into two components, and a lens  24  which focuses light emerging from the birefringent wedge  23 . The light transmitted through the lens  24  is directed to an optical fiber  26 , in which case, depending on the Faraday rotation angle in the Faraday rotator  22 , there are two cases; the case where 100% of light is directed to the optical fiber, and the case where only a part of light is directed. When only a part of light is directed to the optical fiber, the intensity of light directed to the fiber is attenuated. On the other hand, when 100% of light is directed to the fiber, the intensity of light reaches the maximum level. 
     Thus, by controlling the Faraday rotation angle of the Faraday rotator, the degree to which a light path is bent in the birefringent wedge  23  is varied, allowing the intensity of light directed to the optical fiber  26  to be controlled. 
     FIG. 3B illustrates an example of the variable Faraday rotator. 
     This Faraday rotator comprises a permanent magnet  27 , an electromagnet  28 , a magneto-optical crystal  29 , and a variable current source for supplying current to the electromagnet. A light beam  31  incident on the magneto-optical crystal  29  has its plane of polarization rotated by a magnetic field produced within the magneto-optical crystal by the permanent magnet  27  and the electromagnet  28 . The direction of the magnetic field produced within the magneto-optical crystal changes with the magnitude of a magnetic field produced by the electromagnet. The angle of rotation of the plane of polarization of the light beam  31  is determined by that component of the magnetic field which is parallel to the direction in which the light beam travels. Thus, the angle of rotation of the plane of polarization of the light beam  31  can be changed by changing the direction of the magnetic field within the magneto-optical crystal  29  with its magnitude unchanged. Here, the permanent magnet  27  is used to saturate the magnetic field within the magneto-optical crystal  29 . By saturating the magnetic field within the magneto-optical crystal, the magnitude of the internal magnetic field can be kept unchanged even if any magnetic field is produced by the electromagnet  28 . By changing the direction of the internal magnetic field, the magnitude of the magnetic field component parallel to the direction in which the light beam  31  travels is increased or decreased. Thereby, the angle of rotation of the plane of polarization of the light beam  31  is controlled. 
     FIG. 4 illustrates a first embodiment of the optical amplifier of the present invention. 
     This optical amplifier comprises a pumping light source unit  40  and an optical amplification unit  41 . The pumping light source unit couples pumping light emitted from two or more pumping light sources  43  in a coupler  42  for transmission to the optical amplification unit  41 . The provision of two or more light sources allows a pumping light beam of large power to be transmitted to the optical amplification unit  41 . If the pumping light source unit  40  is arranged such that a pumping light source or sources can be added, the optical amplifier requirements of having higher-power pumping light at a later time will be accommodated. 
     The optical amplification unit  41  is equipped with a WDM coupler  44  that couples incoming signal light and pumping light for transmission to an amplification medium  46  via an optical isolator  45 . The amplification medium  46  is excited by the pumping light to amplify the light signal, which in turn is directed to an optical isolator  47 . The amplified signal is then input to a wavelength selective filter  48  where only the signal is derived and then input to an optical splitter  49 . When the signal light is amplified in the amplification medium  46  by the use of the pumping light, which emit signal light in the opposite direction to the transmission direction. The optical isolators  45  and  47  are provided to prevent the opposite direction signal light. 
     In the optical splitter  49 , most of the signal light propagates straight, but part of the signal light branches off and its total output is detected by a photodiode  50 . The output of the photodiode is input to a monitor circuit  51  to make a decision of whether or not the amplified signal light has reached a predetermined output level. If the decision is that the amplified signal light has not reached the predetermined output level, then the monitor circuit sends a signal to an attenuator driver circuit  52 . In response to this signal, the driver circuit controls the variable attenuator  53  to thereby adjust the pumping light output to the fiber amplifier  46 . By using such a feedback loop, the output level of the amplified signal light can be maintained constant. The monitor circuit  51  may be formed of a differential amplifier. 
     In this arrangement, being separated from the pumping light sources  43  grouped as a pumping light source unit, the optical amplification unit  41  will not suffer from heat emitted by the pumping light sources. In addition, unlike the conventional system, the pumping light output level is adjusted by the variable attenuator  53 . Thus, even if the signal multiplexing degree in signal light is changed, a required output level of pumping light can be obtained readily by controlling the variable attenuator  53 . 
     FIG. 5 illustrates a second embodiment of the optical amplifier of the present invention. 
     In this figure, the same reference numerals are used to denote parts corresponding to those in FIG.  4  and their descriptions are omitted. 
     The second embodiment of FIG. 5 is arranged such that a single light source  62  can provide sufficient pumping light to each of two or more amplification media  70  and  46 . A pumping light source unit  60  comprises the pumping light source  62  and an optical splitter  63  that splits pumping light from the pumping light source for transmission to the amplification media  70  and  46 . An optical amplification unit  61  comprises two or more amplifiers (first and second amplification units  74  and  75 ). The arrangement of the second optical amplification unit  75  is the same as that described in conjunction with FIG.  4  and hence its description is omitted here. 
     The first optical amplification unit  74  takes a configuration called bidirectional excitation in which pumping light is input to the amplification medium  70  from its both ends. Two pumping light beams from the pumping light source unit  60  are directed to the first optical amplification unit  74  where their output levels are respectively adjusted by variable attenuators  64  and  67  driven by driver circuits  65  and  66  and then coupled with signal light by WDM couplers  68  and  71 . The two pumping light beams are input to the amplification medium  70  from its both ends to thereby amplifying the signal light. Optical isolators  69  and  72  are provided, as described previously, so as to remove spurious light that propagates in the opposite direction to the direction in which the signal light propagates. The light transmitted through the optical isolator  72  is input to a wavelength selective optical filter  73  where only the main signal light is extracted for transmission over a transmission path (not shown). 
     In the first optical amplification unit  74 , unlike the second optical amplification unit  75 , no feedback is used to maintain the output level of amplified signal light constant. Of course, a feedback arrangement may be provided in the first optical amplification unit  74  as well. In this case as well, the feedback arrangement is such that the output level of a fraction of main signal light emerging from the wavelength selective optical filter  73  is detected through the use of a photodiode and a monitor circuit and the variable attenuators  64  and  67  are controlled by the driver circuits  65  and  66  supplied with an output signal of the monitor circuit. 
     The first optical amplification unit  74  uses bidirectional excitation, which is effective in using a very long erbium-doped fiber for the amplification medium  70 . That is, in such a case, pumping light input from the WDM coupler  68  to an end of the amplification medium  70  will be dissipated before it reaches the other end of the medium. By inputting pumping light from the WDM coupler  71  to the other end of the amplification medium as well, therefore, the pumping light is allowed to spread over the entire medium. According to this scheme, even if the amplification medium  70  consists of a very long erbium-doped fiber, the entire medium can be used to amplify signal light. 
     FIG. 6 shows a third embodiment of the optical amplifier of the present invention. 
     In FIG. 6, as in FIG. 5, the optical amplification unit  61  comprises the first and second optical amplification units  74  and  75  and like reference numerals are used to denote corresponding parts to those in FIG.  5 . 
     Using bidirectional excitation, the first optical amplification unit  74  in the optical amplification unit  61  is arranged such that pumping light is directed to both ends of the amplification medium  70 . The advantage of this scheme has been described previously. The second optical amplification unit  75 , arranged identically to that of FIG. 4, uses only one pumping light beam. As with the second optical amplification unit  75 , in the first optical amplification unit  74  feedback control may be performed on the variable attenuators  64  and  65 . 
     In FIG. 6, a pumping light source unit  80  containing pumping light sources and the optical amplification unit  61  are provided separately and, in the optical amplification unit  61 , pumping light from the pumping light source  80  to the amplification media  70  and  46  is controlled by the variable attenuators  64 ,  67  and  53 . Such an arrangement prevents heat emitted by the pumping light sources  81  from adversely affecting the stable operation of the optical amplification unit  61  and allows the required pumping light to be obtained only by controlling the variable attenuators  64 ,  67  and  53 . In addition, by arranging the pumping light source unit so as to allow a pumping light source or sources to be added to its optical coupler/splitter  82 , a requirement of providing such high-power pumping light as is not obtainable from the previously installed pumping light sources  81  can be accommodated simply by adding a pumping light source or sources to the pumping light source unit  80 . 
     The pumping light source unit  80  of FIG. 6 is equipped with two or more pumping light sources  81  to thereby provide an intense pumping light that is not obtainable from a single pumping light source. That is, lights output from the pumping light sources  81  are coupled and then split by an optical coupler/splitter  82  for transmission to the amplification media  70  and  46 . As described previously, by coupling lights from two or more pumping light sources and then splitting them for transmission to two or more amplification media, there is provided an advantage that all the amplification media can operate with substantially the same characteristics. That is, variations in operating characteristics between amplification media due to variations in a manufacturing process among pumping light sources, which would occur if one pumping light source were allocated for one amplification medium, can be eliminated. 
     FIG. 7 illustrates a first arrangement of the excitation light source unit. 
     The pumping light source unit of FIG. 7 is provided with two or more pumping light sources  90  to supply one amplification medium with a high-power pumping light which is not obtainable from a single pumping light source. Unlike the previous arrangement, in this arrangement a variable attenuator  93  and an attenuator driver circuit  92  are installed in the pumping light source unit as opposed to the optical amplification unit. In this case, in order to adjust the output level of the pumping light in response to the output level of signal light amplified by the amplification medium, it is required to input a feedback control signal to the driver circuit  92  from the optical amplification unit (not shown). As described previously, since the pumping light source unit and the optical amplification unit are assembled as separate units, electrical wiring is needed to input the feedback control signal to the attenuator driver circuit  92 . Although the feedback control arrangement will not be particularly mentioned below, it will be apparent to those skilled in the art. 
     FIG. 8 shows a second arrangement of the pumping light source unit. 
     In this arrangement as well, the pumping light source is equipped with variable attenuators  97  and an attenuator driver circuit  96 . This arrangement is suitable for a case where a single pumping light source  94  has an output high enough to provide pumping light to each of two or more amplification media (not shown). An optical splitter  95  splits light from the pumping light source  94 . The variable attenuators  97  each adjust the output level of corresponding pumping light from the optical splitter. This is intended to tune each of the amplification media individually. Although, in FIG. 8, the variable attenuators  97  are controlled in common by the one driver circuit  96 , one driver circuit may be provided for each variable attenuator. For pumping light output control feedback, a monitor circuit is provided to detect the output level of signal light amplified by an amplification medium to produce a control signal and the control signal is then applied to the attenuator driver circuit  96  connected to the monitor circuit. 
     FIG. 9 shows a third arrangement of the pumping light source unit. 
     In this arrangement, lights from two or more pumping light sources  98  are first coupled and then split by an optical coupler/splitter  99 . This is intended to obtain pumping light of a required output level by coupling the two or more pumping light sources  98  and to prevent variations in wavelength among lights to the amplification media by first coupling all lights from the pumping light sources into one light beam and then dividing it. The division of one light eliminates variations in wavelength among light beams sent to the amplification media, allowing each amplification medium to operate uniformly. In the arrangement of FIG. 9 as well, variable attenuators  101  and an attenuator driver circuit  100  are provided in the pumping light source unit. The variable attenuators  101  are each provided for a light output of the optical splitter and controlled by the driver circuit  100 . As described previously in conjunction with FIG. 8, an attenuator driver circuit may be provided for each of the variable attenuators. For pumping light output feedback control, a control signal is applied from a monitor circuit not shown to the attenuator driver circuit  100 . 
     In the arrangements of FIGS. 8 and 9, pumping light divided by the optical splitter  95  or optical coupler/splitter  99  is power-controlled by each variable attenuator  97  or  101 . This is equivalent to varying the dividing ratio in the splitter or coupler/splitter. 
     FIG. 10 shows a fourth arrangement of the pumping light source unit. 
     This pumping light source unit is arranged such that lights from two or more pumping light sources  110  are coupled by an optical coupler  111  into one for application to a single amplification medium and moreover the number of light sources connectable to the coupler can be increased or decreased. For example, the optical coupler  111  has light source connectors and optically coupling components built in which are larger in number than pumping light sources required at optical transmission system startup, and a minimum required number of pumping light sources is connected at the time of system installation. When there arises a need to increase the optical multiplexing degree in the optical transmission system (for system upgrade), an additional pumping light source or sources (including a separate pumping light source unit or units) are coupled to the connectors of the optical coupler  111  which have been provided in advance. That is, when the optical multiplexing degree of the optical transmission system increases, the power of signal light increases accordingly. In this case, when the power of pumping light remains unchanged, the gain of the fiber amplifier drops. It is thus required to elevate the output level of the pumping light. For this reason, the pumping light source unit is arranged to allow a pumping light source or sources to be added. This avoids the need to install more pumping light sources than is necessary at the stage of initial investment. In addition, higher-power pumping light requirements at the time of system upgrade can be accommodated readily. 
     FIG. 11 shows a fifth arrangement of the pumping light source unit. 
     In the arrangement of FIG. 11, lights from two or more pumping light sources  110  are first coupled and then separated for transmission to each amplification medium. In this case as well, an optical coupler/splitter  113  is provided beforehand with connectors and optical elements which allow an additional pumping light source or sources to be added. When higher-power pumping light becomes necessary for system upgrade, an upgrading pumping light source or a separate pumping light source unit can be connected to the optical coupler/splitter  113 . 
     As shown in FIG. 10 or FIG. 11, since the optical coupler  111  or optical coupler/splitter  113  is formed to allow for connection of additional pumping light source or sources, a minimum required number of pumping light sources suffices at system startup. For system upgrade, a pumping light source or sources have only to be added to obtain pumping light at a required level. Thus, the initial investment in the optical transmission system can be controlled to a minimum and the system can be upgraded readily. 
     FIG. 12 shows a sixth arrangement of the pumping light source. 
     This arrangement contains a variable attenuator  115  and an attenuator driver circuit  114  in addition to the arrangement of FIG.  10 . Although, in the arrangement of FIG. 10, the variable attenuator  115  and the attenuator driver circuit  114  are provided in the optical amplification unit not shown, they may be provided in the pumping light source unit as shown in FIG.  12 . This arrangement corresponds to the arrangement of FIG. 7 but differs in that the optical coupler  111  is formed to allow for addition of a pumping light source or sources. As described in conjunction with FIG. 7, to feedback control the pumping light output from the optical coupler  111 , wiring is needed to input a control signal to the attenuator driver circuit  114 . 
     FIG. 13 shows a seventh arrangement of the pumping light source unit. This arrangement, corresponding to the arrangement of FIG. 11, has variable attenuators  116  each corresponding to a respective one of pumping light outputs of an optical coupler/splitter  113  and an attenuator driver circuit  117  for driving the attenuators. This arrangement allows pumping light output to each amplification medium to be controlled individually. This is substantially equivalent to varying the light dividing ratio in the optical coupler/splitter  113 . 
     The arrangement of FIG. 13, which is adapted to supply pumping light to multiple amplification media, can meet situations where the optical multiplexing degree is increased only for some of transmission paths and the pumping light output level need not be elevated for all the amplification media that the pumping light source accommodates. That is, with system upgrade, pumping light from a pumping light source  112  or separate pumping light source unit is input to the optical coupler/splitter  113  and the light attenuation level in each variable attenuator  116  is set individually. More specifically, pumping light of the same output level as prior to the upgrade is applied to amplification media for which the pumping light output level does not need to be elevated, while pumping light of a higher output level is applied to amplification media for which the pumping light output level needs to be elevated. Thus, the arrangement of FIG. 13 can accommodate various system upgrades. 
     As described previously, in the arrangement of FIG. 13, one attenuator driver circuit may be provided for each variable attenuator. For feedback control of the output level of each pumping light, a control signal is applied from a monitor circuit, not shown, to the attenuator driver circuit. 
     FIG. 14 is a schematic of an optical amplifier. 
     The optical amplifier of FIG. 14 is arranged such that a pumping light source unit  120  is independent of an optical amplification unit and many optical amplification units are incorporated into a package  121 . By incorporating optical amplification units  122 - 1  to  122 -n into one package in this manner, optical amplifiers on multiple light transmission paths can be integrated, which will not take up too much space in the optical communications device. The pumping light source unit  120  is installed independently of the optical amplifier unit integrating package  121 , which provides greater freedom in placement of the pumping light source unit in the optical communications device. Thus, the pumping light source unit  120 , which emits a large amount of heat and may adversely affect the amplification media in the optical amplification units, can be placed in a cooling-efficient location within the optical communication device e.g., in the upper portion of the device or near to a fan. 
     To provide an optical amplifier with uniform characteristics, optical amplification units of the same type are connected to the pumping light source unit, For this reason, pumping lights having the same wavelength or the same wavelength component are input to the amplification media in the optical amplification units, which allows the optical amplifier to have uniform characteristics regardless of the wavelength dependence of the amplification media. 
     FIGS. 15 to  18  show various arrangements of the optical amplification unit described in conjunction with FIGS. 4 and 5. 
     FIG. 15 shows a first arrangement of the optical amplification unit. 
     In FIG. 15,  131  denotes an amplification medium (fiber amplifier) consisting of an erbium-doped fiber,  132 ; a WDM coupler for coupling pumping light and signal light,  133 ; an optical isolator for preventing oscillation of light propagating in the reverse direction,  134 ; a wavelength selective optical filter which allows signal light wavelength components to pass through,  135 ; a variable attenuator which varies the supply amount of pumping light to the amplification medium,  136 ; an attenuator driver circuit for supplying a drive current to the optical attenuator,  137 ; an optical splitter for branching part of signal light emerging from the filter for the purpose of controlling the output level of the pumping light,  138 ; a photodiode for converting signal light branched by the optical splitter into an electrical signal, and  139 ; a control circuit which receives the electrical signal and performs necessary calculations to send a control signal to the attenuator driver circuit. 
     The optical amplification unit of FIG. 15 performs automatic level control (ALC) to keep the level of output light constant and has the same arrangement as that described in conjunction with FIG.  4 . Part of signal light is branched by the optical splitter  137  and then converted into an electrical signal, which, in turn, is compared in magnitude with a preset value in the control circuit  139 . The ALC is performed by controlling the drive current to the variable attenuator  135  via the driver circuit  136  so that a difference between the electrical signal and the preset value will reach zero. 
     Separating the optical amplifier into the optical amplification unit containing the amplification medium susceptible to heat and the pumping light source unit containing a pumping light source or sources allows the amplification medium to operate stably and the pumping light source unit to have facilities for system upgrades as described previously. Further, unlike the conventional amplifier in which the output level of pumping light is controlled by varying current to a pumping light source, the present invention performs the pumping light level control by varying the power of pumping light to the amplification medium using the variable attenuator while keeping the output level of the pumping light source constant. This makes it possible to form an optical amplifier from separate units: an optical amplification unit and a pumping light source unit. 
     FIG. 16 shows a second arrangement of the optical amplification unit. 
     In this figure, like reference numerals are used to denote corresponding parts to those in FIG.  15  and their descriptions are omitted. 
     In FIG. 16,  140  denotes an optical splitter which branches part of incoming signal light to control the output level of pumping light,  141 ; a photodiode which converts the signal light branched by the optical splitter into an electrical signal, and  142 ; a control circuit which receives the electrical signals produced by the photodiodes  138  and  141  and performs necessary calculations to produce a control signal for the attenuator driver circuit  136 . 
     This amplification unit is arranged to perform automatic gain control (AGC) to keep the signal light gain constant. Part of output signal light branched by the splitter  137  is converted into an electrical signal by the photodiode  138 , while part of incoming signal light branched by the optical splitter  140  is converted into an electrical signal by the photodiode  141 . These electrical signals are applied to the control circuit  142  where the ratio of output signal light to input signal light, i.e., the gain, is obtained. This gain is compared with a preset value to produce a difference therebetween. The AGC control is performed by controlling current to the variable attenuator  135  via the driver circuit  136  so that the difference will reach zero. 
     The WDM coupler  132 , the optical isolator  133 , the wavelength selective filter  134  and the amplification medium  131  operate as described previously. 
     FIG. 17 shows a third arrangement of the optical amplification unit. 
     In this figure, like reference numerals are used to denote corresponding parts to those in FIGS. 15 and 16 and their descriptions are omitted. 
     In FIG. 17,  143  denotes an optical splitter which branches part of pumping light for control,  144 ; a photodiode which converts the pumping light branched by the optical splitter into an electrical signal, and  145 ; a control circuit which receives the electrical signal produced by the photodiode and performs necessary calculations to produce a control signal for the attenuator driver circuit  136 . 
     This amplification unit is arranged to perform automatic power control (APC) to keep the amount of pumping light to the amplification medium constant. Part of pumping light branched by the optical splitter  143  is converted into an electrical signal by the photodiode  144 . The electrical signal is applied to the control circuit  145  where it is compared with a preset value to produce a difference therebetween. The APC control is performed by controlling current to the variable attenuator  135  via the driver circuit  136  so that the difference will reach zero. 
     The pumping light having a constant level is coupled with incoming signal light in the WDM coupler  132  to amplify the signal light in the amplification medium  131 . The amplified signal light is applied to the wavelength selective filter  134  from which only the main signal light is derived for transmission over a light transmission path (not shown). As described previously, the optical isolator  133  is provided to avoid oscillation of light propagating in the reverse direction. 
     In this arrangement in which the power of pumping light is constant, when higher-power pumping light becomes necessary for system upgrade, the preset value in the control circuit  145  must be changed to a suitable value. However, if there is no need of increasing the power of pumping light, the arrangement will function identically to the ALC control and the AGC control. 
     FIG. 18 shows a fourth arrangement of the optical amplification unit. This arrangement is adapted for bidirectional pumping light amplification. By varying current to each of variable attenuators  135  through a corresponding one of driver circuits  136 , the supply amount of pumping light to the amplification medium from each of its input and output ends can be controlled individually. In the arrangement of FIG. 18, two pumping light sources are used: one for pumping light supply to the amplification medium from its input end, and one for pumping light supply from the output end. 
     It is desirable that a common pumping light source be used to supply pumping light to the amplification medium from its both ends. An arrangement that uses such a common pumping light source is shown in FIG.  19 . 
     The operation of each component for bidirectional excitation has been described in conjunction with FIG.  5  and the description is thus omitted here. 
     FIG. 19 shows a fifth arrangement of the optical amplification unit. 
     In FIG. 19, like reference numerals are used to denote corresponding parts to those in FIGS. 15 to  18  and their descriptions are omitted. In this figure,  146  denotes an optical splitter which splits pumping light. 
     The optical amplification unit of FIG. 19 has the same bidirectional excitation arrangement as the unit of FIG. 18 but differs in that pumping light from a pumping light source is split by the optical splitter  146  and then input to the variable attenuators  135 . Instead of placing the variable attenuators  135  as shown in FIG. 19, a single variable attenuator may be placed ahead of the optical splitter  146  to vary the amount of pumping light before it is split by the splitter. In this case, the ratio between the supply amount of pumping light to the amplification medium from its input end and the supply amount of pumping light from the output end depends on the dividing ratio in the optical splitter. 
     FIG. 20 shows an arrangement for operating pumping light sources stably in the pumping light source unit of the optical amplifier. 
     In general, the connectors adapted for optical components, though having varying degrees of reflectiveness, reflect no little incoming light to produce return light. When the return light falls on a pumping light source, its operation becomes unstable, causing the wavelength of oscillating light to drift. This is because, when a pumping light source consists of a laser which uses a resonator structure, the return light upon incidence disturbs the resonant state of the resonator. 
     To solve such a problem, there are provided two pumping light sources  157  and  158  of different wavelengths. The pumping light sources  157  and  158  are respectively set to output light beams of λ1 and λ2 in wavelength which are linearly polarized perpendicular to each other. The output beams of the light sources  157  and  158  are polarization-coupled by a polarization beam splitter (PBS)  159 . A Faraday rotator  160  is set to rotate the plane of polarization of incoming light through 22.5 degrees. After having been polarization-coupled, the pumping light has its plane of polarization rotated through 22.5 degrees by the Faraday rotator  160  and then output from the pumping light source unit. Return light, resulting from the output pumping light being reflected from optical components within the optical amplifier, has its plane of polarization rotated through 22.5 degrees again in the Faraday rotator. It thus follows that the return light has its plane of polarization rotated through 45 degrees with respect to the original light (the output light of the PBS). Thus, the return light will have wavelength components of λ1 and λ2. 
     Thus, if the return light has a component different in wavelength from the output light of an pumping light source, its action of disturbing the resonant state within the resonator of a laser as the pumping light source will be suppressed. The operational instability of the pumping light source due to return light can therefore be eliminated. 
     That is, output light of pumping light sources (LDs) having different output wavelengths is used as pumping light so that part of the pumping light that is reflected by optical components and falls on the pumping light source unit will not be uniform in wavelength. This allows return light to an LD to contain a wavelength component other than the output wavelength of the LD, thus eliminating the operational instability of the optical amplifier. 
     FIG. 21 shows a fourth arrangement of the optical amplifier. 
     In this arrangement, optical amplification unit  170  is provided with a pumping light input monitor  177  which comprises an optical splitter  174  for branching part of pumping light, a photodiode  175  for receiving the part of pumping light branched by the optical splitter, and a monitor circuit  176  for monitoring the output level of the received pumping light. The other components of the optical amplification unit remain unchanged from those of FIG.  4  and their descriptions are thus omitted. In this arrangement, the output level of pumping light is monitored by the pumping light input monitor  177  to activate an alarm, such as a buzzer or lamp, when the output level of pumping light has dropped abnormally. 
     In addition, when the input amount of pumping light to the optical amplification unit  170  has dropped, the monitor circuit sends an electrical signal to the pumping light source unit  171  to shut off the pumping light source unit. Alternatively, when the pumping light source unit has an output adjusting optical attenuator (not provided in the arrangement of FIG.  21 ), the attenuator is controlled to increase its light attenuation amount to lower the output amount of pumping light to zero or a safety level. 
     Such control is performed by sending a control signal from the monitor circuit  176  to a control circuit  173  in the pumping light source unit  171 , thereby causing light source drivers  172  to shut off pumping light sources  43  or causing a variable attenuator, if provided, to increase the light attenuation amount. 
     Such control is intended to detect a state where a connector  178  that connects the pumping light source unit  171  and the optical amplification unit  170  together has become disconnected. Not only has the pumping light output from the pumping light source unit  171  very high power, but it is also converged by an optical fiber. If, when the connector has come off, a person at work should be exposed to pumping light, this would entail great dangers to his or her skin and eyes. For this reason, the pumping light input monitor  177  is provided in the optical amplification unit to detect the output level of the pumping light, thereby monitoring the state of the connector  178 . When the output level of the pumping light has dropped below a predetermined level, the connector is considered to have become disconnected, whereupon the alarm is sounded or the pumping light source is shut down to keep the person at work from dangers. 
     FIG. 22 shows another arrangement for monitoring the connection between the optical amplification unit and the pumping light source unit. 
     In this arrangement, light beams from multiple pumping light sources  183  are first coupled and then split by an optical coupler/splitter  184 . The pumping light source unit is also connected with the optical amplification unit by means of connectors  180 . In this case as well, the state of each connector is monitored. 
     That is, there is provided a reflection monitor  182 , in which reflected light from the connector  180  is branched by an optical splitter  185  and then detected by a photodiode  190  which sends the detected output to a monitor circuit  186 . When the connector is off, the power of the reflected light from the connector becomes higher than when the connector is connected. When the power of the reflected light has become higher than a preset level, the monitor circuit  186  produces an alarm signal. Otherwise, when a variable attenuator is provided in the pumping light source unit, the monitor circuit controls the attenuator to reduce the output amount of pumping light to zero or a safety level. Alternatively, the monitor circuit  186  sends a control signal to a control circuit  187  to cause driver circuits  188  to shut off the pumping light sources  183 . 
     FIGS. 23A and 23B illustrate how pumping light sources are connected in the pumping light source unit. 
     In FIG. 23A, pumping light sources (laser diodes; LDs) are connected in series with a transistor  200 . This arrangement requires less transistors and less power dissipation than when each of the LDs is driven individually. 
     As an example, consider a case where, as shown in FIG. 23B, one LD is attached to a transistor  201 . Assuming that the voltage across the LD is V and the current flowing through the transistor is I, the power dissipation in the transistor becomes P=(Vcc−V) I. On the other hand, when multiple LDs are connected in series as shown in FIG. 23A, the power dissipation in the transistor  200  becomes P′=(V′cc−4V) I where Vcc=(1+α)V, V′cc=(4+α′) V, and α and α′ are assumed to be nearly equal to each other. 
     A comparison in transistor power dissipation between the arrangements of FIGS. 23A and 23B indicates that 4P−P′=(4α−α′) and hence 4P is larger than P′. Thus, a combination of series-connected LDs and one transistor requires less power dissipation than with multiple combinations of each of an LD and a transistor. Further, less transistors are required. 
     However, the pumping LDs need not be connected in series. For example, if pumping LDs are connected in parallel within the light source driver circuits, a failing LD will be replaced with a good one more easily than with the series connection, providing redundancy. 
     The embodiments have been described in terms of an optical fiber amplifier. An optical semiconductor amplifier can also be used which uses a semiconductor as the amplification medium. 
     The output power of pumping light to the amplification medium is adjusted by a variable attenuator, making it easy to adjust the output power of pumping light. 
     The optical amplifier is separated into the pumping light source unit containing a pumping light source or sources and the optical amplification unit susceptible to heat, which allows the amplification medium in the optical amplification unit to be free from the influence of heat emitted by the pumping light source and operate stably. 
     System upgrades, such as are intended to increase the signal multiplexing degree, can be accommodated by adjusting the attenuation amount of the variable attenuator. By arranging the pumping light source unit so that an additional pumping light source or sources can be attached thereto, it becomes unnecessary to install redundant pumping light sources at the time of system installation, allowing the initial investment to be cut. 
     When pumping light is supplied to multiple amplification media from the pumping light source unit, the power of pumping light can be adjusted for each amplification medium.