Patent Publication Number: US-8121486-B2

Title: Optical transmission apparatus

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority from Japanese application JP 2007-064338 filed on Mar. 14, 2007, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to an optical transmission apparatus and a control method thereof. More specifically, the present invention is directed to an optical transmission apparatus and a control method thereof, which are provided for an optical transmission system by which a plurality of signal light is wavelength-multiplexed so as to transmit the wavelength-multiplexed signal light. 
     In optical communication systems, generally speaking, while communication capacities are enlarged, in order to reduce costs of the optical communication systems, wavelength-multiplexed light transmission techniques have been applied. That is, in the wavelength-multiplexed light transmission techniques, a plurality of signal light having wavelengths different from each other are bundled within a single optical fiber so as to communicate the bundled signal light via the signal optical fiber. Also, in actual optical communication systems, in order to compensate losses as to power levels of optical signals occurred in an optical fiber which constitutes a transmission path between two points separated from each other over a distance, optical fiber amplifies are installed on the transmission path, and while the optical signals are not converted into electric signals during transmissions, the optical fiber amplifiers amplify the plurality of signal light having the different wavelengths from each other in a batch mode. 
     Optical fiber amplifiers have wavelength dependent characteristics in amplification factors (gains) with respect to signal light. For instance, in such a case of an optical fiber amplifier which amplifies optical signals having a wavelength range defined from 1530 nm up to 1560 nm, an amplification gain with respect to signal light having a wavelength near 1530 nm becomes higher than an amplification gain with respect to signal light having a wavelength near 1560 nm. This wavelength dependent characteristic of the gain changes in response to a change in gains of the optical fiber amplifier. In order to adjust the wavelength dependent characteristic of the gain of the optical fiber amplifier, it is desirable to realize that the wavelength dependent characteristic is not varied by keeping the gain constant. To this end, in optical fiber amplifiers, in order to achieve flat gain characteristics of signal light having a plurality of multiplexed wavelengths, a constant gain control capable of keeping a gain constant has been carried out. A constant gain control in an optical fiber amplifier may be realized by that for instance, while intensity of wavelength-multiplexed signal light on the input side of the optical fiber amplifier and intensity of wavelength-multiplexed signal light on the output side thereof are measured, pumping light of the optical fiber amplifier is controlled in such a manner that an intensity ratio (gain) of the input signal light to the output signal light always becomes constant. 
     Also, if a loss occurred in a transmission path changes only under the constant gain control, then intensity of an input optical signal of a signal repeater optical amplifier changes, and intensity of an output optical signal of the signal repeater optical amplifier changes in response to the first-mentioned intensity change. As a result, intensity of an input optical signal of a reception-sided optical transmission apparatus may eventually change, and thus, there are some possibilities that an input signal level of a receiver may be deviated from a dynamic range. In an optical transmission system, in order to solve the above-explained restriction in the input dynamic range of the receiver and also non-linear effects of optical fibers, a constant output intensity control (will be simply referred to as “constant output control” hereinafter) capable of keeping intensity of output signal light constant every wavelength is carried out in addition to the above-described constant gain control. 
     In a constant output control, for instance, total output light intensity of an optical amplifying unit containing an optical fiber amplifier is calculated based upon a total number of previously designated multiplexed signal light (namely, total multiplexed wavelength number) and output light intensity every wavelength; and an attenuating amount of an optical attenuating unit such as an optical attenuator provided at a post stage of the optical fiber amplifier may be controlled in such a manner that the total output light intensity of the optical amplifying unit becomes desirable light intensity. In accordance with this constant output control, the light signal outputted from the optical amplifying unit is controlled to have the desirable constant light intensity in such a manner that a loss change is canceled with respect to a variation in light intensity in combination with the loss change of the optical fiber. 
     As previously described, there are two control modes in the optical amplifying unit; namely, the constant gain control and the constant output intensity control capable of compensating the change in the signal intensity, which is caused by the loss variation of the transmission path. In this case, when a wavelength-multiplexed optical signal is optically amplified based upon the constant output intensity control, as previously described, since the total number of the multiplexed wavelengths is employed, a problem may occur in such a case that a difference is produced between the total multiplexed wavelength value, and a total wavelength number of multiplexed signal light which has been actually inputted to the optical fiber amplifier. In an optical transmission system, for example, in the case that a portion of plural sets of transmitters stored in optical fibers is brought into malfunction, and/or an optical fiber for coupling a transmitter to a wavelength multiplexing unit is extracted, a total number of signal light (total wavelength number) may change which is wavelength-multiplexed on the optical fiber. In this case, at a time instant when a failure happens to occur, the respective optical fiber amplifiers instantaneously cannot grasp a status of the actually multiplexed wavelength number. As a result, such a matching condition can be no longer established between a total wavelength number which constitutes an initial condition for the constant output intensity control, and a total number of wavelengths which have been physically multiplexed on the optical fiber. 
     As a consequence, when an optical signal having a certain wavelength drops due to a maintenance work, or a failure, while such a total output light intensity is employed as the target value, which is calculated based upon a larger signal light number than a total number of actually multiplexed signal light, a constant output intensity control of an optical fiber amplifier is performed. In this case, output light intensity per 1 signal light becomes higher than the predetermined intensity value. As a result, there is such a problem that each of the signal light reaches a receiver in an excessively high input signal level. As a consequence, when there is a change in a total number of multiplexed wavelengths, the execution of not the constant output intensity control, but the constant gain control is required. If the constant gain control is carried out, even when the total wavelength number changes, amplification gains of optical signals are constant, so that the optical signals having the respective wavelengths are amplified in the constant gain, and thus, excessively higher amplifications can be avoided. 
     As previously explained, the constant gain control must be selected with respect to the change in the total wavelength numbers, whereas the constant output intensity control must be selected with respect to the loss variation of the transmission path. As methods for switching these two control modes, for example, one technical idea “OUTPUT LEVEL CONTROL SYSTEM FOR WDM-PURPOSE OPTICAL AMPLIFIER” has been proposed in “Communication Society Meeting held by Electronic Information Telecommunication Institute in 1996”, Lecture No. B1096 lectured by YOSHIDA et. al. That is, in this output level control system, the intensity of the output light per 1 signal light is controlled to become the desirable intensity by detecting the total intensity of the signal light outputted from the optical amplifier, and the total number of the wavelengths stored in the optical transmission system. In this output level control system, the below-mentioned initial condition has been conducted. That is, under this initial condition, the speed as to the loss variation of the transmission path is sufficiently slow, as compared with the control speed of the optical amplifier, and on the other hand, the transient response characteristic of the signal intensity change in combination with the change in the total number of the wavelengths to be multiplexed is sufficiently fast, as compared with the control speed of the optical amplifier. Based upon the above-described initial condition, in the above-described output level control system, two sorts of the above-described variation factors may be discriminated from each other in accordance with a change in the changing speeds of the optical signal intensity measured in the optical transmission apparatus. The change in the wavelength numbers corresponds to, for example, such an event occurred in a changing operation of a communication path for connecting a transmission point to a reception point. Then, it is so supposed that a changing speed of optical signal intensity due to the above-described change in the wavelength numbers is lower than, or equal to several hundreds microseconds. The loss variation of the transmission path corresponds to, for example, such an abnormal event occurs when a maintenance worker of an optical transmission system pulls an optical fiber, or hooks an optical fiber. Then, it is so assumed that a changing speed of optical signal intensity due to the above-explained loss variation of the transmission path is higher than, or equal to several milliseconds. 
     When an attention is paid to the above-explained difference in the changing speeds, while a frequency threshold value has been previously set with respect to a total signal intensity change detected from multiplexed input light of the optical amplifier, the above-described control system can judge that the occurrence factor of the signal intensity variation is caused by either the change in the wavelength numbers or the loss change of the transmission path by checking whether or not a changing speed of total signal intensity in combination with an occurrence of a certain event exceeds the preset frequency threshold value. Also, in response to the judged occurrence factor of the total signal intensity change, such a control mode which should be performed in the optical amplifying unit can be determined. In this case, as the control mode of the optical amplifying unit, as previously explained, when the occurrence of the loss variation is detected, the constant output intensity control is employed, whereas when the change in the wavelength numbers is detected, the constant gain control is employed. 
     Also, as another method capable of solving the problems caused by the change in the wavelength numbers and the loss variation of the transmission path, JP-A-2001-257646 has proposed the below-mentioned control method: That is, the monitoring control-purpose monitoring light called as pilot light (probe light) is extracted by the branching element provided on the output side of the optical amplifier, and then, the optical amplifier is controlled in such a manner that the optical intensity of the probe light becomes constant. In the control method of JP-A-2001-257646, both the constant output intensity control and the constant gain control of the optical amplifier are carried out by paying an attention only to the intensity change of the probe light. As a result, in this control method, the factors as to the intensity changes of the signal light need not be discriminated from each other, whereas these factors are discriminated from each other in “Communication Society Meeting held by Electronic Information Telecommunication Institute in 1996”, Lecture NO. B1096 lectured by YOSHIDA et. al. Also, since there is also no specific restriction as to the responding time constant of the optical amplifier, the above-described optical amplifier control method can be applied with respect to such a high-speed loss variation (lower than, or equal to several milliseconds) of the transmission path, and further, such a low-speed change (higher than, or equal to several milliseconds) in the wavelength numbers. 
     SUMMARY OF THE INVENTION 
     However, in the above-described method for judging the occurrence of the event based upon the frequency threshold value, for instance, in such a case that a change in wavelength numbers occurs at a low speed higher than, or equal to 1 millisecond, it is not so recognized that a change in total signal intensity is caused by the change in the wavelength numbers, but it is so recognized that the change in the total signal intensity is caused in the loss change occurred in the transmission path. In this case, as the control mode of the optical amplifier, the constant output intensity control for holding the output light intensity as the target value is selected, and thus, such a control operation is carried out by which the signal light intensity of the respective wavelengths is increased at a level higher than the necessary level. As previously described, when the discriminating operation as to the variation factors can be hardly carried out, there are some possibilities that although any one of these variation factors actually occurs, such a control method which does not correspond to this actually occurring variation is performed, so that an adverse influence may be eventually given to the signal qualities. In other words, the method for controlling the optical amplifier by detecting the multiplexed-wavelength number can be adapted only to the loss variation in the transmission path, whose changing speed is relatively slow, or only to the variation of the total wavelength numbers, whose changing speed is relatively fast. 
     Also, in the method with employment of the probe light, the control operation of the optical amplifier depends upon the probe light corresponding to the specific light. As a result, when an abnormal event happens to occur in the probe light due to some reasons such as a failure of a light source, the control operation of the optical amplifier is carried out by employing intensity of such a probe light where the abnormal event occurs. As a result, there is such a risk that the control operation of the optical amplifier is erroneously carried, and thus, a problem may occur in the transmission of the signal light. 
     An object of the present invention is to provide an optical transmission apparatus and a control method thereof, which are operable as follows: That is, even under a steady-state condition that a loss variation of a transmission path and a change in total multiplexed-wavelength numbers are not present, and also, even under an abnormal condition that the loss variation of the transmission path and the change in the total multiplexed-wavelength numbers happen to occur, since the constant output intensity control and the constant gain control corresponding to the control systems are properly determined based upon a changing reason of light intensity, the optical transmission apparatus and the control method thereof can properly control intensity of signal light so as to secure a communication quality. 
     In order to achieve the above-described object, the optical transmission apparatus, according to the present invention, is featured by that while an optical attenuator is arranged at a prestage of an optical amplifier, a constant output intensity control by the optical attenuator is carried out by employing an optical signal having a certain wavelength such as monitoring light; intensity of wavelength-multiplexed signal light is monitored at a front stage and a rear stage of the optical amplifier respectively; and a constant gain control is carried out in such a manner that a ratio of the monitored intensity of the wavelength-multiplexed signal light is approximated to a constant value. 
     In accordance with the optical transmission apparatus related to the present invention, the constant output intensity control and the constant gain control can be properly determined in response to the changing reason of the light intensity, and thus, the intensity of the signal light can be controlled in a proper manner. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for illustratively showing an embodiment of a network to which an optical transmission apparatus according to the present invention is applied. 
         FIG. 2  is a diagram for indicating an embodiment of a schematic structure of a ring type optical wavelength multiplexing transmission system, to which the present invention is applied. 
         FIG. 3  is a diagram for showing an internal arrangement of an optical amplifying unit  30  which is mounted on an optical transmission apparatus  1  ( 1 A to  1 B) of  FIG. 1 . 
         FIG. 4  is an explanatory diagram for explaining a relationship between a constant gain control and a constant output intensity control of the optical amplifying unit  30 . 
         FIG. 5  is an explanatory diagram for explaining the constant output intensity control with employment of optical intensity of monitoring control light of the optical amplifying unit  30 . 
         FIG. 6  is a flow chart of process operations for updating a monitoring light target value “Em_target (n).” 
         FIG. 7  is a flow chart for describing a constant output intensity control of a monitoring control unit  20 . 
         FIG. 8  is a flow chart for describing process operations for updating the target value “Em_target (n)” in the case that a total wavelength number changes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to drawings, a description is made of an optical wavelength multiplexing transmission system and a control method thereof, according to an embodiment of the present invention. 
       FIG. 1  illustratively shows a general network structure with employment of the optical wavelength multiplexing transmission system to which the present invention is applied. The general network structure is arranged by an access  102 , an edge  104 , a metropolis  105 , and a core  106 . The access  102  provides the FTTH (Fiber To the Home) services with respect to subscribers in the unit of a region by employing an OLT  100  (Optical Line Terminal) apparatus, or an ONU  101  (Optical Network Unit) apparatus. The edge  104  intensively collects communications from the subscribers in the unit of the region in a region group by employing a plurality of L2 (Layer-2) switches  103 . The metropolis  105  extensively collects the communications which have been extensively collected by the L2 switches  103  in the unit of a city. The core  106  is mainly constituted by OXCs  107  in order to transmit a large capacity of communications in a higher efficiency among large cities over long distances, while the large capacity of communications have been exclusively collected in the unit of the city. The respective hierarchical layers are properly connected to each other by employing routers  108 , and the like. In the above-described network, an OADM (Optical Add Drop Multiplexer) apparatus  1  is such an optical transmission system which is provided in order that communications which have been dispersed in a relatively wide range are intensively collected to one place. 
       FIG. 2  is diagram for representing a schematic structure of a ring type optical wavelength multiplexing system corresponding to one embodiment of an optical transmission system to which the present invention is applied. The optical transmission system is arranged by a plurality of optical transmission apparatuses  1  (namely,  1 A,  1 B,  1 C) such as OADMs, which have been connected to each other via optical fibers  2  (namely,  2 - 1 ,  2 - 2 ). The optical fibers  2  constitute transmission paths. Signal light outputted from a transmitter  10  mounted on the optical transmission apparatus  1 A is wavelength-multiplexed by an optical multiplexing/demultiplexing unit  40 - 1 , the wavelength-multiplexed signal light is amplified by a transmission light amplifying unit  50 - 1 , and thereafter, the amplified signal light is propagated via an optical fiber  2 - 1 , the optical transmission apparatus  1 B, and an optical fiber  2 - 2  to reach the optical transmission apparatus  1 C provided on the opposite side, and then, the reached signal light is wavelength-demultiplexed by a multiplexing/demultiplexing unit  40 - 3 . Thereafter, the wavelength-demultiplexed signal light is received by a receiver  11  of the optical transmission apparatus  1 C. While the signal light transmitted from the optical transmission apparatus  1 A passes through the optical fibers  2 , optical intensity thereof is lowered due to a propagation loss. In order to compensate this lowered optical intensity, a reception light amplifying unit  30 - 2 , a multiplexing/demultiplexing unit  40 - 2 , and a transmission light amplifying unit  50 - 2  have been mounted on the optical transmission apparatus  1 B, whereas a reception light amplifying unit  30 - 3  has been mounted on the optical transmission apparatus  1 C. 
     The respective optical transmission apparatuses  1 A to  1 C have been equipped with monitoring control units  20 - 1  to  20 - 3 , respectively. A monitoring control unit  20  produces monitoring information, for example, alarm information, status information of an apparatus, and a total number of multiplexed signal light, which are required in the relevant optical transmission apparatus, and then, transmits/receives the produced monitoring information as monitoring light with respect to other optical transmission apparatuses. As represented as broken lines in  FIG. 2 , the monitoring light transmitted from the monitoring control unit  20 - 1  of the optical transmission apparatus  1 A has been reception-processed by the monitoring control unit  20 - 2  of the relay-purpose optical transmission apparatus  1 B which is located adjacent to the above-described optical transmission apparatus  1 A, whereas the monitoring light transmitted from the monitoring control unit  20 - 2  of the optical transmission apparatus  1 B has been reception-processed by the monitoring control unit  20 - 3  of the optical transmission apparatus  1 C. 
       FIG. 3  is a structural diagram for showing an embodiment of one optical amplifying unit  30  which is mounted on the optical transmission apparatus  1 . The optical amplifying unit  30  has been equipped with an optical attenuating unit  35 , an optical demultiplexer  31 , a monitoring light receiver  41 , an optical branching unit  32 , an optical amplifier  33 , another optical branching unit  36 , a monitoring control unit  20 , a monitoring light transmitter  47 , and an optical multiplexer  37 . The optical attenuating unit  35  is capable of freely adjusting a passing loss of incident light thereof with respect to the optical amplifier  33 . The optical demultiplexer  31  optically demultiplexes monitoring light from output light derived from the optical attenuating unit  35 . The monitoring light receiver  41  converts the monitoring light into monitoring information which corresponds to an electric signal. The optical branching unit  32  such as a optical coupler optically branches a portion of the signal light which has passed the monitoring information demultiplexer  31 . The optical amplifier  33  is constructed of an optical fiber amplifier, or the like, and optically amplifies signal light within such a wavelength range of the light which has passed the optical branching unit  32 . The optical branching unit  36  such as an optical coupler optically branches a portion of the signal light outputted from the optical amplifier  33 . The monitoring control unit  20  receives monitoring information from another optical transmission apparatus and processes the received monitoring information so as to produce monitoring information which is transmitted to a further optical transmission apparatus. The monitoring light transmitter  47  outputs the monitoring information from the monitoring control unit  20  as monitoring light. The optical multiplexer  37  optically multiplexes the monitoring light outputted from the monitoring light transmitter  47  with the signal light passed through the optical coupler  36  so as to transmit the optically multiplexed signal light to an optical fiber provided on the output side. 
     The monitoring light which has been optically demultiplexed from the signal light by the optical demultiplexer  31  is entered to the monitoring light receiver  41 , and then, both monitoring light intensity “Em” and control information “D 1 ” extracted from the monitoring light are outputted from the monitoring light receiver  41  to the monitoring control unit  20 . These optical demultiplexer  31  and the monitoring light receiver  41  constitute such a means for monitoring intensity of monitoring light. The signal light branched by the branching unit  32  is inputted to a detector  42  for detecting intensity of the signal light, and then, an output of the detector  42  is entered as input intensity “Es” of the signal light which has been wavelength-multiplexed to the monitoring control unit  20 . These optical branching unit  32  and the detector  42  constitute such a means for monitoring intensity of the wavelength-multiplexed signal light before being inputted to the optical amplifier  33 . The signal light branched by the branching unit  36  is entered to another detector  46 , and then, an output of the detector  46  is entered to the monitoring control unit  20  as intensity “Eo” of the signal light after being amplified. These branching unit  36  and detector  46  constitute such a means for monitoring intensity of wavelength-multiplexed signal light after being outputted from the optical amplifier  33 . 
     The monitoring control unit  20  monitors intensity of light outputted from the monitoring light receiver  41 , the detector  42 , and the receiver  46 ; performs the above-described constant gain control, constant output intensity control, and a control of suppressing the constant output intensity control; and also, relays the control information “D 1 ” to the monitoring light transmitter  47  as monitoring information “D 2 .” Under the constant gain control, energizing light of the optical fiber amplifier  33  is adjusted in such a manner that a ratio of the signal light input intensity “Es” to the amplified signal light intensity “Eo” is kept constant. 
     In the present embodiment, as represented by broken lines in  FIG. 2 , the monitoring light transmitted from the monitoring control unit  20 - 1  of the optical transmission apparatus  1 A passes through the optical attenuating unit  35  mounted on the optical amplifying unit  30 - 2  of the relay-purpose optical transmission apparatus  1 B, is optically demultiplexed from the signal light before being inputted to the optical amplifier  33 , and then, the optically demultiplexed monitoring light is entered to the monitoring control unit  20 - 2 . Also, the monitoring light outputted from the monitoring control unit  20 - 2  is optically multiplexed with the signal light on the output side of the optical amplifying unit  30 - 2 , the optically multiplexed monitoring light passes through the optical attenuating unit  35  mounted on the optical amplifying unit  30 - 3  of the optical transmission apparatus  1 C, is optically demultiplexed from the signal light before being inputted to the optical amplifier  33 , and then, the optically demultiplexed monitoring light is entered to the monitoring control unit  20 - 3 . 
     In other words, both the signal light and the monitoring light are entered to the optical attenuating unit  35  mounted on the optical amplifying unit  30 , and the monitoring light is not entered to the optical amplifier  33 , whereas only the signal light is entered to the optical amplifier  33 . As a consequence, for instance, in such a case that a loss variation happens to occur within the section of the optical fiber  2 - 1 , not only intensity of the signal light, but also intensity of the monitoring light entered to the monitoring control unit  20 - 2  change at the same time in the relay-purpose optical transmission apparatus  1 B subsequent to the optical fiber  2 - 1 . Also, when a total number (namely, total number of wavelengths) of signal light transmitted from the optical transmission apparatus  1 A to the optical fiber  2 - 1  is increased and/or decreased, while intensity of the monitoring light entered to the monitoring control unit  20 - 2  does not change, only intensity of the signal light entered to the optical amplifier  33  changes. 
     As a consequence, the monitoring control unit  20 - 2  performs the constant output intensity control in such a manner that power of the detected monitoring light becomes predetermined light intensity with respect to the optical attenuating unit  35  while the power of the monitoring light is set as a target value. Also, in the case that a total number (total number of wavelengths) of the signal light changes, the monitoring control unit  20 - 2  performs the constant gain control of the optical amplifier  33  based upon intensity of wavelength-multiplexed signal light inputted to the optical amplifier  33  and intensity of wavelength-multiplexed signal light outputted from this optical amplifier  33 . As previously explained, the monitoring control unit  20 - 2  of the present embodiment monitors the intensity of the signal light inputted/outputted with respect to the optical amplifier  33  so as to perform the constant gain control with respect to the intensity of the signal light. Moreover, the monitoring control unit  20 - 2  monitors the intensity of the monitoring light outputted from the optical attenuating unit  35  so as to perform the constant output intensity control with respect to the intensity of the monitoring light. The optical attenuating unit  35  is arranged at the prestage of the optical amplifier  33 , and this optical attenuating unit  35  is controlled based upon the monitoring light. As a result, while fluctuations contained in the intensity of the original light are compensated which are caused by the loss variation of the transmission path and the like, the constant output intensity control can be carried out which cannot be seriously and adversely influenced based upon the change in the total wavelength number. Also, since the constant gain control by the optical amplifier  33  provided at the post stage of the optical attenuater  35  is carried out based upon not the monitoring light, but the intensity of the input/output signal light, even when an abnormal event happens to occur in the monitoring light, the adverse influence is not given to the constant gain control. 
     Next, a description is made of a method for controlling an output of the optical attenuating unit  35  by employing intensity of monitoring light. Basically, the monitoring control unit  20  controls the optical attenuating unit  35  in such a manner that intensity “Em” of actual monitoring light observed by the monitoring light receiver  41  is approximated to a target value “Em_target” of intensity of monitoring light, which has been previously determined. In this case, alternatively, the target value “Em_target” may be set as a value which is fixedly determined based upon a characteristic of an optical transmission system which is to be constructed. 
     However, a control subject for the constant output intensity control is such a signal light which has been wavelength-multiplexed, whereas signal light which is employed in the constant output intensity control for the optical attenuater  35  is monitoring light. As a result, there are some cases that there is an error between a target value of signal light intensity which is wanted to be controlled as constant light intensity, and intensity of actual signal light. If this error is reflected to the control operation with employment of the monitoring light, then reliability of the optical transmission system may be improved. As a consequence, in the present embodiment, the target value “Em_target” of the monitoring light intensity is updated by employing an actually measured intensity value “Es” of signal light detected by the detector  42 , and another target value “Es_target” of signal light intensity per 1 wavelength, which has been previously determined. 
     It should be noted that the target value of the intensity of the monitoring light will be expressed as “Em_target (n)” in the below-mentioned description. Symbol “Em_target (n−1)” indicates a target value which was used in a preceding control operation; symbol “Em_arget (n)” represents a target value which is presently used in a control operation; and symbol “Em_target (n+1)” shows a target value which is newly updated.  FIG. 6  indicates a process flow chart for updating the above-explained target value “Em_target (n).” 
     First of all, the monitoring control unit  20  calculates intensity “Ese” of signal light per 1 wavelength based upon the actually measured value “Es” of the intensity of the signal light inputted to the optical amplifier  33 , and also, a multiplexing wavelength number “W” of the signal light (step  601 ):
 
 Ese=Es/W   (formula 1)
 
     Next, the monitoring control unit  20  calculates an error “ΔP” between the signal light intensity “Ese” and the control target value “Es_target” of the signal light intensity per 1 wavelength (step  602 ):
 
Δ P=Ese−Es _target  (formula 2)
 
     Then, the monitoring control unit  20  subtracts the previously calculated error “ΔP” from the target value “Em_target (n)” of the intensity of the monitoring light, which is presently set, so that the monitoring control unit  20  acquires a new target value “Em_target (n+1)” of the intensity of the monitoring light by performing a calculation (step  603 ):
 
 Em _target( n+ 1)= Em _target( n )−Δ P   (formula 3)
 
     The monitoring control unit  20  makes the correction with respect to the monitoring light target value “Em_target (n)” used to perform the constant output intensity control based upon the above-described (formula 1) to (formula 3), and then, performs the constant output intensity control by employing the new monitoring light target value “Em_target (n+1)” after the correction. As a result, the monitoring control unit  20  can reflect the change in the signal light intensity onto the target value of the monitoring light, and equivalently controls the intensity “Ese” of the signal light per 1 wavelength to the desirable target value “Es_target” of the signal light intensity in accordance with the constant output intensity control. 
     The target value “Es_target” of the signal light intensity corresponds to such a numeral value which is exclusively determined based upon the optical characteristic of the optical amplifier  33  in order to achieve a uniform gain over the entire amplification range, and may be determined as, for instance, −20 dBm. Also, an initial value of the monitoring light target value “Em_target” may be properly determined in correspondence with a characteristic of an optical transmission system which is to be constructed. It should also be understood that, as described above, since the target value “Em_target (n)” of the monitoring light in the present embodiment is such a value which is properly updated from the observed value “Es” of the signal light intensity and the target value “Es_target” of the signal light intensity, there is no need to strictly determine this target value “Em_target (n)” of the monitoring light intensity. As a result, for example, this target value “Em_target (n)” of the monitoring light intensity may be made equal to the target value “Es_target” of the signal light intensity, for instance, may also be set as −20 dBm. 
     It is so assumed that, for instance, the target value “Es_target” of the signal light intensity and the target value “Em_target (n)” of the monitoring light were −20 dBm, whereas the value of the calculated signal light intensity “Ese” was −23 dBm. In this case, the error “ΔP” is calculated as follows:
 
Δ P=Ese−Es _target=(−23)−(−20)=−3 dBm
 
At this time, the intensity of the signal light has not yet been controlled as −20 dBm corresponding to the original control target value “Ese” but has been controlled as −23 dBm, so that this signal light intensity is such a value lower than the original control target value “Ese” by 3 dB. As a consequence, if a new target value “Em_target (n+1)” of the monitoring light is calculated by employing the immediately-before calculated target value “Em_target (n)” of the monitoring light, and the error “ΔP”, then the below-mentioned calculation is performed:
 
 Em _target( n+ 1)= Em _target( n )−Δ P =(−20)−(−3)=−17 dBm
 
If the monitoring control unit  20  controls the optical attenuating unit  35  by employing the target value “Em_target (n+1)” of the monitoring light intensity corresponding to this new control target value, then the controlled monitoring light is controlled to be attenuated by the optical attenuating unit  35  to become −17 dBm, and at this time, the signal light is controlled to be attenuated to become −20 dBm, so that the target may be achieved. As previously explained, the signal light can be controlled to become the control target value by employing the monitoring light.
 
     Next, referring now to a signal waveform diagram shown in  FIG. 4 , a description is made of both a constant gain control and a constant output intensity control, which are executed by the monitoring control unit  20  of the optical amplifying unit  30 . An abscissa of  FIG. 4  indicates an elapse of time. In  FIG. 4 , intensity (E 1 ) of monitoring light indicates monitoring light intensity “Em” detected by the monitoring light receiver  41 ; intensity (C 1 ) of input signal light indicates signal light intensity “Es” observed as the output of the detector  42  provided at the prestage of the optical amplifier  33 ; and intensity (D) of output signal light represents a change in signal light output intensity “Eo” observed as the output of the detector  46  provided at the post stage of the optical amplifier  33 . 
     Symbol “Slos” in the input signal light intensity (C 1 ) indicates a threshold value which is set in order to judge whether or not the input signal light is present. When the signal light intensity “Es” exceeds the threshold value “Slos,” the monitoring control unit  20  judges that the signal light is inputted, and thus, the monitoring control unit  20  commences the constant gain control of the optical amplifier  33  as represented in a constant gain control (A). The threshold value “Slos” may be properly selected in correspondence with an optical transmission system which is to be constructed, it is conceivable that, for example, such a value in the vicinity of −30 dBm may be applied to this threshold value “Slos.” 
     Symbols “Sth(H)” and “Sth(L)” shown in the input signal light intensity (C 1 ) represent threshold values of signal light intensity which are set in order to judge whether or not there is a change in a total wavelength number of the signal light. In this embodiment, when the monitoring light intensity “Es” is present between the upper limit threshold value “Sth(H)” and the lower limit threshold value “Sth (L)”, the monitoring control unit  20  judges that a variation of light intensity is limited within an allowable range, and does not update the threshold values with respect to the signal light. In such a case that the monitoring light intensity “Es” is deviated from these upper/lower limit threshold values, the monitoring control unit  20  sets both the upper limit threshold value “Sth (H)” and the lower limit threshold value “Sth (L)”, while a value (indicated by black point) of monitoring light intensity “Sm” per 1 wavelength observed at the time when the threshold values was updated is employed as a reference. When intensity “Es” of signal light detected by the detector  42  exceeds the upper limit threshold value “Sth (H)”, the monitoring control unit  20  judges that a total wavelength number of the signal light is increased. When intensity “Es” of signal light detected by the detector  42  becomes lower than the lower limit threshold value “Sth (L)”, the monitoring control unit  20  judges that a total wavelength number of the signal light is decreased. Since these upper/lower limit threshold values “Sth (H)” and “Sth (L)” must be updated in response to the increase/decrease of the total wavelength number, the monitoring control unit  20  updates these threshold values every time a change of the signal light intensity occurs. 
     Trigger pulses for updating both the threshold values “Sth (H)” and “Sth (L)” in order to detect this change of the signal light intensity correspond to updating (C 2 ) of the signal light threshold values, and are generated after a variation of the input signal light intensity (C 1 ) with respect to the signal light intensity “Es” had been confirmed, and thereafter, a predetermined time expressed by either “T 4 ” or T 2  has elapsed. The reason why the trigger pulses are delayed by the predetermined time is given as follows: That is, such a time duration is considered that variations occurred after the variation with respect to the signal light intensity “Es” had occurred are converged, and then, the intensity of the signal light becomes stable. For example, in the embodiment shown in  FIG. 3 , in such a case that in the input signal light intensity (C 1 ), the monitoring light intensity “Es” exceeds the threshold value “Slos” for judging whether or not the signal light intensity is present, the monitoring control unit  20  updates the threshold values “Sth (L)” and “Sth (H)” for detecting the variation in the signal light intensity after the time “T 4 ” passes since the monitoring control unit  20  had judged that the monitoring light intensity Es exceeded the threshold value “Slos.” Also, the monitoring control unit  20  updates the threshold value for detecting the variation after the monitoring control unit  20  had judged that the signal light intensity exceeded the variation threshold value “Sth (H)” and the change in the total wavelength number occurred, and thereafter, the time “T 2 ” has elapsed. Similarly, the values of these threshold values “Sth (L)” and “Sth (H)” may be selected to be various values in correspondence with an optical transmission system which is to be constructed. For instance, it is conceivable that such values of the signal light intensity “Sm”±3 dB may be applied to these threshold values “Sth (L)” and “Sth (H).” 
     Also, symbol “Mlos” in the monitoring light intensity (E 1 ) indicates a threshold value of monitoring light intensity which is set in order to judge whether or not the monitoring light is present. When intensity “Em” of monitoring light detected by the detector  46  exceeds the threshold value “Mlos”, the monitoring control unit  20  judges that the monitoring light has been entered, and then, commences the constant output intensity control by the optical attenuating unit  35  as indicated in the constant output intensity control (B). 
     Furthermore, in such a case that the input signal light intensity (C 1 ) has such a change which exceeds the range of the threshold value of either “Sth (H)” or “Sth (L)”, the monitoring control unit  20  may alternatively update the monitoring light target value “Em_target (n)” for performing the constant output intensity control with employment of the intensity “Em” of the monitoring light. A trigger pulse for this updating operation is indicated in updating (E 2 ) of the monitor light target value, and the monitoring control unit  20  generates the trigger pulse after a predetermined time expressed by symbol “T 3 ” has elapsed. The reason why the trigger pulse is delayed by the predetermined time is given as follows: That is, such a time duration is considered that variations occurred after the variation with respect to the monitoring light intensity “Em” had occurred are converged, and then, the intensity of the monitoring light becomes stable. Also, in such a case that in the monitoring light intensity (E 1 ) of  FIG. 3 , the monitoring light intensity “Em” exceeds the threshold value “Mlos” for judging whether or not the signal light intensity is present, the monitoring control unit  20  sets the target value “Em_target (n)” for performing the constant output intensity control after the time “T 3 ” passes since the monitoring control unit  20  had judged that the monitoring light intensity “Em” exceeded the threshold value “Mlos.” At this time, the target value “Em_target (n)” may be set by a value of “Ese” if signal light is inputted, whereas the target value “Em_target (n)” may be alternatively set by the initial value of the target value “Em_target (n)” which has been previously set if the signal light is not entered. 
     Referring now to the signal waveform diagram shown in  FIG. 4 , a description is made of operations of the optical amplifying unit  30  in the case that a fiber loss changes. Even when a loss  61  of a transmission path shown in a transmission pass loss (E 3 ) occurs, since the constant output intensity control has been carried out with employment of the monitoring light by the optical attenuating unit  35  at the input portion of the optical amplifying unit  30 , an intensity of an optical signal passed through the optical attenuating unit  35  does not largely change, so that the intensity variation of the optical signal caused by the fiber loss can be suppressed. Also, since the constant output control has been carried out in the optical attenuating unit  35 , as indicated by numeral  62  of the input signal light intensity (C 1 ), the signal light intensity “Es” is converged within the allowable range between the upper limit threshold value “Sth (H)” and the lower limit threshold value “Sth (L)”, and the threshold values with respect to the signal light are not updated. 
     Referring now to the signal waveform diagram shown in  FIG. 4 , a description is made of operations of the optical amplifying unit  30  in the case that a total number of multiplexed wavelengths changes. As shown in numeral  63  in a wavelength number change (C 3 ), when a total number of wavelengths is increased from “N” waves to “M” waves (M=N 1 , N 2 , N 3  in  FIG. 5 ), intensity of monitoring light does not change, but only intensity of signal light is increased. As a result, as indicated in the input signal light intensity (C 1 ), the signal light intensity “Es” is varied to exceed the upper limit threshold value “Sth (H)”, and thus, the monitoring control unit  20  judges that a change of wavelength numbers has occurred. Then, during a time period from the detection of the wavelength number change up to a time “T 2 ”, as represented in the monitoring light target value update (E 2 ), the monitoring control unit  20  suppresses updating of the target value “Em_target (n)” corresponding to the control target value of the constant output intensity control. After the time T 2  has elapsed, the monitoring control unit  20  again updates the control target value “Em_target (n)” with respect to the constant output intensity control so as to obtain such a target value “Em_target (n+1).” It should be understood that only updating of the control target value with respect to the constant output intensity control is suppressed, and the constant output intensity control itself is being continuously operated by employing the control target value “Em_target (n).” 
     In this case, the reason why the control target value of the constant output intensity control is suppressed is given as follows: That is, under such a condition that a total number of multiplexed wavelength has changed, a loss itself as to a transmission path such as an optical fiber does not change, so that a strength of an optical signal outputted from the optical attenuating unit  35  should not be changed. There is such a temporal delay that after the total number of multiplexed wavelengths inputted to the optical amplifying unit  30  has physically changed, the monitoring control unit  20  detects this number change and then updates setting of the wavelength number. While this temporal delay is neglected, if the monitoring control unit  20  updates the control target value “Em_target (n)” by employing the previous wavelength number when a total number of multiplexed signal light changes, since a total number of multiplexed wavelengths grasped by the optical amplifier  33  is different from a total number of multiplexed wavelengths physically entered to the optical amplifier  33 , then optical intensity per 1 signal light is controlled to become an erroneous intensity value. As a consequence, at numeral  64  shown in the input signal light intensity (C 1 ), at a time instant when the monitoring control unit  20  senses that a total number (total wavelength number) of multiplexed signal light has changed, the monitoring control unit  20  suppresses updating for the control target value used to perform the constant output intensity control only for the time “T 2 ”, as represented in the monitoring light target value update (E 2 ). 
     Under such a time duration defined by the time “T 2 ”, the total number of multiplexed wavelengths physically inputted to the optical amplifying unit  30  is specified; setting operation of new light intensity “Ese” per 1 wavelength which constitutes the target value of the constant output intensity control with respect to the signal light is accomplished; and after the setting operation is accomplished, the control target value “Em_target (n)” of the constant output intensity control with respect to the monitoring light is updated. It should also be understood that as methods for specifying a multiplexed wavelength number by the monitoring control unit  20 , the below-mentioned methods may be conceived: Namely, a method that an optical transmission apparatus provided on the upper stream side of an optical transmission system employs a total number of multiplexed signal light contained in monitoring information which is transmitted by monitoring light; a method that the monitoring control unit  20  itself discriminates signal light inputted to the optical amplifier  33  from each other based upon wavelengths thereof so as to detect a present total number of multiplexed signal light; and other methods may be conceived. 
     Also, in the present embodiment, even when the monitoring control unit  20  judges that there is no intensity of signal light entered to the optical amplifier  33 , in such a case that the monitoring control unit  20  judges that there is intensity of monitoring light outputted from the optical attenuating unit  35 , the monitoring control unit  20  can continue the constant output intensity control of the optical attenuating unit  35  by employing the target value Em_target (n)″ which is presently used. Alternatively, when a new path for an optical signal is opened, if only monitoring light is transmitted through the transmission path prior to signal light and a target value of arbitrary monitoring light intensity such as the initial value of the control target value “Em_target (n)” is set, then a constant output intensity control may be commenced earlier than the conduction of the signal light. Generally speaking, it takes a certain time in order that the optical attenuating unit  35  is brought into such a condition that a constant output intensity control thereof can be carried out. As a result, if only the optical attenuating unit  35  can be brought into the operable condition in advance within the optical transmission apparatus  1 , then opening times of optical signal paths can be shortened which are provided in an entire optical transmission system in which plural sets of the above-described optical transmission apparatuses  1  are coupled to each other. As previously described, even when there is no signal light intensity, if the monitoring light intensity is present, then the monitoring control unit  20  of the present embodiment can control the attenuating amount of the optical attenuating unit  35  so as to be brought into such a condition that the constant output intensity control can be carried out. 
     As previously described, when an optical transmission system is arranged by employing the optical transmission apparatuses according to the present embodiment, even if a loss variation of a transmission path happens to occur, and also, even if a total number of multiplexed wavelengths changes, which are stored in a wavelength multiplexing system, both a constant gain control and a constant output intensity control can be controlled at the same time. As a result, since stability with respect to disturbances and the like can be improved, high quality communication services can be provided. 
     Next, a description is made of a method for determining the monitoring light target value “Em_target (n)” in such a case that a status where signal light intensity and monitoring light intensity are not present is changed into another status where the signal light intensity and the monitoring light intensity are present, and also, in the case that a total number of wavelengths changes with reference to a signal waveform diagram indicated in  FIG. 5 . Under such a condition before a time instant “T 5 ”, both the monitoring light and the signal light have intensity lower than, or equal to the threshold values “Slos” and “Mlos” for determining whether or not these monitoring light and signal light are present, so that the intensity thereof is under non-intensity present condition. At this time, the constant output intensity control is also stopped. 
     Next, under such a condition that the monitoring control unit  20  judges that the monitoring light intensity is present, but judges that the signal light intensity is not present in a status of a time instant “T 5 ”, since the signal light intensity is not present, the monitoring light target value “Em_target (n)” is set to an arbitrary initial value, and then, the monitoring control unit  20  performs the constant output intensity control by employing this set monitoring light initial value. 
     Next, in a status of a time instant “T 6 ”, in the case that signal light having a wavelength number “N 1 ” is inputted to the optical amplifying unit  30  and the monitoring control unit  20  detects both monitoring light intensity and signal light intensity, the monitoring control unit  20  calculates a monitoring light target value “Em_target (n)” by employing the above-described (formula 1) to (formula 3), and then, controls the optical attenuating unit  35  while the calculated monitoring light target value is defined as a target value of a constant output intensity control. At this time, although the constant output intensity control has been actually carried out with respect to the monitoring light, since the constant output intensity control with respect to the signal light intensity has been equivalently established, such a target has been achieved in which the intensity of the signal light to the optical amplifying unit  33  is controlled to become a predetermined value. 
     Next, as indicated at time instants T 7  and T 8 , when a total number of wavelengths changes, the signal light intensity “Es” to be observed is increased/decreased in response to the total number of wavelengths. After the time “T 2 ” elapses since the change in the total wavelength numbers has been detected, the monitoring control unit  20  updates the monitoring light intensity “Em_target (n)” to become such a monitoring light intensity “Em_target (n+1)” by employing the above-explained (formula 1) to (formula 3). 
     Furthermore, at a time instant “T 9 ”, when the signal light intensity becomes lower than the threshold value “Slos”, the signal light intensity is not completely present, and the total wavelength number becomes zero wave, the monitoring control unit  20  is operated in such a manner that the monitoring light target value “Em_target (n)” is maintained as such a target value obtained before the signal light intensity is not present. As a consequence, even when the signal light intensity is not present, the monitoring control unit  20  can continue the operation of the constant output intensity control. 
     Moreover, at a time instant T 10 , in such a case that the signal light intensity exceeds the threshold value “Slos” and the monitoring control unit  20  judges that the signal light intensity is present, after the time “T 4 ” has elapsed, the monitoring control unit  20  again calculates a monitoring light target value “Em_target (n)” based upon the above-explained (formula 1) to (formula 3), and continuously performs the constant output intensity control. 
     As indicated in the time instant “T 9 ”, when the monitoring light target value “Em_target (n)” is calculated in the above-described manner, since the monitoring control unit  20  can store thereinto the preceding value, the monitoring control unit  20  can continuously perform the constant output intensity control by employing the optical attenuating unit  35 , and thus, can shorten such a time duration until the signal light intensity is again recovered. 
     As an initiating method of the optical attenuating unit  35  in the present embodiment, the below-mentioned method is provided. In an actual operation of a wavelength multiplexing system, in order to confirm a connection validity of optical fibers connected to the wavelength multiplexing system, the following work is performed. That is, signal light is conducted to the optical fibers in a test mode so as to confirm intensity of the signal light. In other words, while this confirmation work is carried out, the monitoring control unit  20  can store the monitoring light target value “Em_target (n)” corresponding to the control target value with respect to the constant output intensity control. As a consequence, even in such a case that this connection confirmation work was accomplished and the signal light conducted in the test mode was deleted, the monitoring control unit  20  can continuously perform the constant output intensity control by employing the monitoring light target value “Em_target (n)” which was stored during the test work. Also, when signal light is actually conducted, similar to a time instant “T 10 ”, since the constant output intensity control of the optical attenuating unit  35  with employment of the monitoring light target value “Em_target (n)” has already been carried out, so that the initiation time for the optical amplifying unit  30  can be shortened. 
       FIG. 7  is a flow chart for describing a constant output intensity control performed by the monitoring control unit  20 . Firstly, the monitoring control unit  20  sets an initial value of the monitoring light target value “Em_target (n)” (step  701 ). Subsequently, the monitoring control unit  20  checks whether or not signal light has been entered, and if the signal light has been inputted (“YES” of step  702 ), then the monitoring control unit  20  updates the monitoring light target value “Em_target (n)”, if required (step  703 ). Such a case that this updating operation is required is given as follows: That is, for instance, as shown in a flow chart of  FIG. 8 , the monitoring control unit  20  judges whether or not signal light intensity “Es” exceeds the upper, or lower limit threshold value “Sth (H)”, or “Sth (L)” (step  801 ); when the signal light intensity “Es” exceeds the threshold value “Sth (H)”, or “Sth (L)”, the monitoring control unit  20  is brought into a waiting status only for the time “T 2 ” (step  802 ); and then, the monitoring control unit  20  performs an updating process operation indicated in  FIG. 6  (step  803 ). The updating process operation shown in  FIG. 6  includes the process operation (step  601 ) of the (formula 1), the process operation (step  602 ) of the (formula 2), and the process operation (step  603 ) of the (formula 3). 
     Even when the monitoring control unit  20  judges that the signal light has not been inputted (“NO” of step  702 ), in such a case that the monitoring control unit  20  judges that the monitoring light has been inputted (“YES” of step  704 ), the monitoring control unit  20  performs a constant output intensity control with respect to the optical attenuating unit  35  by employing the value of the previously set monitoring light target value “Em_target (n)” (step  705 ). Also, in the case that the signal light has been entered, when the monitoring control unit  20  judges that the monitoring light has been entered (“YES” of step  704 ), the monitoring control unit  20  similarly executes the constant output intensity control (step  705 ). Otherwise, in such a case that the monitoring control unit  20  judges that the monitoring light has disappeared (“NO” of step  704 ), since the monitoring control unit  20  cannot perform the constant output intensity control, the monitoring control unit  20  ceases the process operation. 
     When a loss variation happens to occur on an optical transmission path through which both monitoring light and signal light are wavelength-multiplexed and the wavelength-multiplexed signal light is transmitted, optical intensity of the monitoring light changes in addition to optical intensity of the signal light. Also, in such a case that a total number (total wavelength number) of the multiplexed signal light changes on the upstream side, only the intensity of the signal light changes. While the optical transmission apparatus of the present invention utilizes the above-described property of the optical transmission system, since the optical transmission apparatus of the present invention performs the constant output intensity control with respect to the intensity of the light as the control subject, the optical transmission apparatus can maintain the intensity of the light outputted from the optical amplifier as the constant light intensity. Also, since the optical transmission apparatus of the present invention performs the constant gain control with respect to the intensity of the signal light as the control subject, the optical transmission apparatus can maintain the intensity of the signal light per a wavelength as the constant light intensity. 
     As a consequence, in accordance with the present embodiment, even in such a case that the total number of the signal light changes, which has been stored in the optical transmission system, the constant gain control can be realized, so that the communication qualities can be guaranteed with respect to the signal light which has already been stored and to which the services have already been commenced. 
     Also, in accordance with the present embodiment, even in such a case that the losses of the optical fibers and the like change, which couple the optical transmission systems arranged in remote places to each other, the constant output intensity control can be realized, so that the communication qualities can be guaranteed with respect to the signal light which has already been stored and to which the services have already been commenced. 
     Also, in accordance with the present embodiment, since the control subjects with respect to the above-described constant output intensity control and the above-explained gain constant control are different from each other, the respective control subjects can be independently controlled. As a consequence, even in such a case that both the change for the total number of the signal light, and the loss change of the optical fiber occur at the same time, both the above-described controls can be driven at the same time. 
     Also, in accordance with the present embodiment, even when there is no signal light inputted to the optical transmission system, the optical attenuating unit mounted on the optical amplifying unit can be previously adjusted. Thus, when the signal light is firstly transmitted in the optical transmission system, the initiation time of the optical amplifying unit can be shortened. 
     Further, in accordance with the present embodiment, even when there is no signal light inputted to the optical transmission system, since the signal light intensity once passes through the optical transmission system, the correct attenuation amount with respect to the optical attenuating unit can be held, and when the signal light is transmitted at a second time, the initiation time of the optical amplifying unit can be furthermore shortened. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.