Optical signal level control apparatus

In an optical signal level control apparatus used in a WDM system, the amount of circuitry per wavelength is reduced. Optical power level is detected on the output side of a variable optical attenuator, and the amount of attenuation in the variable optical attenuator is adjusted so that the output level becomes equal to a constant value L1. At this time, if the detected level is lower than a threshold value Th0 or Th-d, it is determined that a signal off condition has occurred, and the amount of attenuation is set to a constant value A1. The amount of attenuation, A1, is chosen to be sufficiently larger than the amount of attenuation used in the output level constant control but small enough to be able to detect the restoration of the signal. When the amount of attenuation is being held at the constant value A1, if the output level is restored to a level higher than the threshold value Th0 or Th1 (Th1<Th-d), the output constant control is resumed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

With growing numbers of Internet, mobile telephone, and other network users, network traffic has been increasing, and this, coupled with ever diversifying services from electronic commerce and electronic mail to delivery of moving images, has been increasing capacity needs. To meet such needs, large-capacity networks are indispensable, and the introduction of optical communication networks has been increasing; in particular, wavelength division multiplexing communication networks using wavelength division multiplexing (WDM) technology have been deployed rapidly. In WDM, transmission quality degrades if there occurs a variation in optical power level between wavelength multiplexed optical signals due to variation in characteristics among optical components such as optical fibers, optical amplifiers, etc. In particular, when an optical node (OADM, OXC, etc.) constructed by combining various optical components is used, the variation increases, and it becomes necessary to provide an optical level adjusting function. The present invention relates to an optical level control method, and an apparatus, that have a function to suppress such variation, and that autonomously perform control so as to prevent the occurrence of an optical surge and like phenomenon in the event of a failure or during protection.

2. Description of the Related Art

FIG. 1shows one example of prior art optical signal level control in a WDM system. A wavelength multiplexed input signal (λ1to λn) is separated into signals of different wavelengths by an optical splitter10, and the separated signals are each passed through an optical branching device12, a variable optical attenuator14, and an optical branching device16, and are again wavelength multiplexed by an optical combiner18and amplified by an optical amplifier20.

A photodetector22detects the power of light separated by the optical branching device16provided on the output side of the variable optical attenuator14, and feedback-controls the variable optical attenuator14through a control circuit24to maintain the power level of the light of the corresponding wavelength at a constant level. On the other hand, a photodetector26is provided to detect whether the optical signal of the corresponding wavelength has been input normally and also to detect a signal off condition.

As described above, as the prior art optical signal level control requires the provision of two photodetectors for each wavelength, one for feedback control and the other for the detection of a signal off condition, the prior art has had the problem that the amount of circuitry, and the overall cost of the apparatus increase as the number of wavelengths increases.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to achieve both the optical power level constant control and the signal off detection control while reducing the amount of circuitry and the overall cost.

According to the present invention, there is provided an optical signal level control apparatus comprising: a variable optical attenuator; a photodetector for detecting optical power level at an output of the variable optical attenuator; and a control circuit for controlling the amount of attenuation in the variable optical attenuator in accordance with the detected optical power level, and wherein the control circuit includes: means for controlling the amount of attenuation in the variable optical attenuator so that the detected optical power level becomes equal to a target value; and means for maintaining the amount of attenuation in the variable optical attenuator at a predetermined value when the detected optical power level has dropped below a first threshold value, the predetermined value being chosen to be small enough to be able to detect restoration of the optical power level.

According to the present invention, there is also provided an optical signal level control apparatus comprising; a variable optical attenuator; a photodetector for detecting optical power level at an input of the variable optical attenuator; and a control circuit for controlling the amount of attenuation in the variable optical attenuator in accordance with the detected optical power level, and wherein the control circuit includes: means for storing the relationship between the optical power level at the input of the variable optical attenuator and a setting for the variable optical attenuator for bringing the optical power level at an output of the variable optical attenuator to a target value; means for controlling the optical power level at the output of the variable optical attenuator to the target value by setting the amount of attenuation in the variable optical attenuator based on the detected optical power level and the stored relationship; and means for maintaining the amount of attenuation in the variable optical attenuator at a predetermined value when the detected optical power level has dropped below a first threshold value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2is a diagram showing one embodiment of the present invention. The same constituent elements as those inFIG. 1are designated by the same reference numerals, and the description of those elements will be omitted here.

In the embodiment ofFIG. 2, the control circuit30provided for each wavelength achieves both the optical power level constant control and the detection and control of a signal off condition and restoration from the signal off condition, based only on the result of the detection done on the output of the variable optical attenuator14by the optical branching device16and the photodetector22provided on the output side of the variable optical attenuator14. As shown in the flowchart ofFIG. 3, first the optical power level detected on the output side of the variable optical attenuator14by the photodetector22is compared with a threshold value Th0 (step1000), and the output constant control is performed if the optical power level (output power) on the output side is higher than the threshold value Th0. In the output constant control, if the output power is substantially equal to the target level L1 (step1002), the amount of attenuation in the variable optical attenuator14is not adjusted (step1004), but if the output power is not equal to the target level L1, the amount of attenuation is adjusted so that the output power becomes equal to the target level (step1006).

If the output power is lower than the threshold value Th0, control is performed to maintain the attenuation amount constant (step1008). In the attenuation amount constant control, the amount of attenuation in the variable optical attenuator14is controlled to a constant level A1. The amount of attenuation, A1, is chosen to be small enough that the signal restored to the normal state can be detected with the threshold value Th0, but preferably, it is sufficiently larger than the amount of attenuation used in the output constant control. With this setting, by detecting only the output of the variable optical attenuator, the output power can be controlled at a constant level during normal operation; on the other hand, when a signal off condition is detected, the amount of attenuation is maintained at the predetermined value, so that an abnormal value can be prevented from being output when the signal is restored from the off condition to the normal condition. The control circuit30for performing such operation can be implemented by a CPU and a storage device for storing a program for the same, but it can also be implemented by logic circuits only.

FIGS. 4,5, and6are diagrams showing the transitions of the optical power level (input power level) on the input side of the variable optical attenuator, the amount of attenuation in the variable optical attenuator, and the optical power level (output power level) on the output side, respectively, for the case where a signal off condition occurs and the signal is restored thereafter. When a signal off condition occurs, and the input power level drops from the normal level to the signal off input power level as shown inFIG. 4, the output power level drops from the output constant control level L1 to the no-signal level as shown inFIG. 6; when this level drop is detected with the threshold value Th0, the amount of attenuation is changed from the amount of attenuation used in the output constant control to a larger attenuation amount, i.e., A1, as shown inFIG. 5. Thereupon, the output power level further drops by A1-(attenuation in constant control), achieving a substantially shut-off condition (FIG. 6). When the signal is restored from the signal off condition (FIG. 4), as the threshold value Th0 is set lower than a level lower than the signal level by A1 (signal level-A1), the signal restoration is detected with the threshold value Th0 and the output constant control is thus resumed.

FIG. 7is a flowchart showing a second example of the control performed in the control circuit30. InFIG. 7, if the output level drops below threshold value Th-d during the output constant control (step1100), the control is switched to the attenuation amount constant control using the attenuation amount A1 (step1104). If the output level is restored to a level higher than threshold value Th1 during the attenuation amount constant control (step1106), the control is switched back to the output constant control. The transitions of the output level and the attenuation amount in the above process are shown inFIGS. 8 and 9, respectively. As shown inFIG. 8, the threshold value Th-d is set to a value different from the threshold value Th1, and preferably larger than Th1. By so setting, the signal restoration can be detected with Th1 even when the amount of attenuation, A1, to be used in the attenuation amount constant control is increased.

FIGS. 10 and 11show the transitions of the output power level and the attenuation amount, respectively, in a third example of the control performed in the control circuit30.

Generally, the response of a variable optical attenuator is relatively slow; therefore, when the amount of attenuation is changed from A1 used in the signal off condition to the amount of attenuation used in the output constant control, the actual amount of attenuation does not change instantly, but changes relatively slowly, as shown inFIG. 11. In view of this, in the third example, a plurality of threshold values Th1, Th2, . . . , Thm are set for the detection of signal restoration (FIG. 10), and attenuation amounts A1, A2, . . . , Am are prestored in association with the respective threshold values.FIG. 12is a flowchart showing the third example of the control performed in the control circuit30. InFIG. 12, steps1200,1202,1204, and1206are the same as the corresponding steps1100,1102,1104, and1106inFIG. 7. When it is detected in step1206that the output power is larger than Th1, the attenuation amount setting in the variable optical attenuator is changed to the attenuation amount setting for the output constant control (step1208). The amount of attenuation in the variable optical attenuator then decreases and, when the actual amount of attenuation drops to A2 in step1210, it is determined whether the output power level exceeds the threshold value Th2 prestored in association with A2 (step1212). If the output power level is not higher than Th2, the process returns to the attenuation amount constant control in step1204. If the output power level is higher than Th2, then when the amount of attenuation drops to the next attenuation amount Am (m=3, 4, . . . ) (step1214) a determination is made against the threshold value Thm (step1216); if the output power level is not higher than Thm, the process returns to the attenuation amount constant control in step1204. In this way, the output power level is checked against the threshold values Th2, Th3, . . . , Thm in sequence, and if it is determined that the output power is higher than any threshold value, the process finally returns to the output constant control in step1200. In this example, instead of storing A2, A3, . . . , differences ΔB1, ΔB2, . . . , relative to A1 may be stored.

As the response of the variable optical attenuator is relatively slow, as earlier noted, when the signal is restored from the signal off condition, the output power rises relatively slowly after the control is switched to the output constant control, as shown inFIGS. 8and10. Accordingly, by checking the output power a plurality of times using the threshold values Th1, Th2, . . . , Thm set to match the slow response, erroneous detection can be prevented even though low threshold values are used.

FIG. 13is a flowchart showing a fourth example of the control performed in the control circuit30. InFIG. 13, when the output constant control is being performed (step1300), if the output level drops below the first threshold value Th-d (step1302), the output constant control continues to be performed until a predetermined hold-off time elapses (step1304); if the output level is restored to a level higher than Th-d within that predetermined time, the process returns to the output constant control. If the output level continues to stay below Th-d until the hold-off time has elapsed, the process proceeds to the attenuation amount constant control (step1306). When the attenuation amount constant control is being performed, if it is detected that the output level is higher than the second threshold value Th1 (step1308), the process returns to the output constant control (step1300) only when the output level continues to remain higher than Th1 until the hold-off time has elapsed.

In the fourth example, when an output level lower than the threshold value Th-d is detected during the target value control, or when an output level higher than the threshold value Th1 is detected during the attenuation amount constant control, switching to the other control is not done immediately, but the control is switched to the attenuation amount constant control or the target value control, respectively, only when the detected condition continues to last until the hold-off time has elapsed. This serves to prevent an erroneous operation due to an instantaneous change in the output level.

FIGS. 14 and 15show the transitions of the output power level and the attenuation amount, respectively, in the fourth example of the control. As can be seen fromFIGS. 14 and 15, as the output constant control continues to be performed during the hold-off period before switching from the output constant control to the attenuation amount constant control due to a signal off condition, the amount of attenuation is brought to zero, so that the output power level temporarily rises. At this time, if the optical power level happens to be restored, then if the power level is restored to the same level as the level before the instantaneous off condition, the output level becomes higher than the level before the instantaneous off condition. To prevent this, when the output level drops below Th-d, and switching is made from the output constant control to the attenuation amount constant control, the value of the attenuation amount immediately before the output level drops below Th-d should be retained, and the attenuation amount should be set to the predetermined attenuation amount A1 when the output level is lower than Th-d even after the hold-off time has elapsed.

Likewise, when an output level higher than Th1 is detected during the signal off condition, the target value control may be resumed immediately, and if the output level is lower than Th1 after the hold-off time has elapsed, the control may be switched back to the attenuation amount constant control.

FIG. 16is a flowchart showing a fifth example of the control performed in the control circuit30. In this example, a plurality of hold-off times HTm (m=1, 2, . . . ) are set in association of a plurality of threshold values Thm. InFIG. 16, steps1400,1402, and1404are the same as the corresponding steps1300,1302, and1304inFIG. 13. As described earlier, when it is detected that the output power is lower than Th-d, provisions may be made to retain the amount of attenuation at that instant in time. When the attenuation amount constant control is being performed (step1406), if it is detected that the output power is higher than Th1 (step1408), the process proceeds to the output constant control (step1409). If the output level drops below Th1 before the hold-off time HT1 associated with Th1 expires, the process returns to the attenuation amount constant control (step1406). If the output level continues to stay above Th1 until the hold-off time HT1 has elapsed, then the output level is checked against the next threshold value Thm (m=2, 3, . . . ) for the hold-off time HTm (steps1412and1414). When the processing is completed for all the threshold values, the process returns to the output constant control in step1400.

Alternatively, as shown inFIG. 17, when it is detected during the attenuation amount constant control that the output power is higher than the threshold value Th1, provision may be made not to switch the control to the output constant control until after the processing for all the threshold values is completed.

It is desirable that an upper limit value LOC1 and a lower limit value LOC2 be set for the target value L1 in the output constant control, and that an alarm be output to notify the apparatus or the administrator in the event that the output power level exceeds the upper limit value LOC1 or drops below the lower limit value LOC2 during the output constant control. When the output power level has dropped below the lower limit value LOC2, the alarm may be output only when the output level is higher than a signal off detection threshold value Th0 or Th-d that is set lower than the lower limit value LOC2.

Further, when the output power has exceeded the upper limit value LOC1 or has dropped below LOC2 but is higher than the threshold value Th0 or Th-d during the output constant control, the output may be shut down by setting the amount of attenuation in the variable optical attenuator to its maximum value.

It is desirable that the threshold value Th1 for detecting the restoration from the signal off condition be set as close as possible to the output level at the time of signal restoration in order to enhance the sensitivity of the detection. To achieve this, the attenuation amount A1 in the signal off condition must be made stable. For this purpose, data defining the relationship between the attenuation amount and a variation factor such as temperature variation that can cause variation in the attenuation amount is prestored, and monitoring is performed periodically or constantly for the occurrence of a variation or any change in the variation factor from the initial setting of A1; if there occurs a change in the variation factor, a correction value is derived from the prestored data and the amount of attenuation in the variable optical attenuator is controlled so that the amount of attenuation is brought back to A1. In this way, the amount of attenuation can be always set or maintained at A1.

For example, even when the driving current of the variable optical attenuator is constant, if the temperature changes, the attenuation value also changes as shown by a solid line inFIG. 18. In view of this, the characteristic such as shown by a dashed line inFIG. 18is prestored, and the driving current is changed in accordance with the change of the temperature to maintain the attenuation amount constant.

In the control method described so far, it is desirable that, when the signal restoration is detected, the determination by the threshold value Th-d or Th0 be masked until after a predetermined time elapses from the time a full transition is made to the output constant control. It is also desirable that, when it is detected during the output constant control that the output level is lower than Th-d or Th0, it be determined that the output level is lower than Th-d or Th0 only when the lower level condition has been detected a plurality of times in succession.

FIG. 19shows an example in which the present invention is applied to cope with level variations among channels or a fault condition in an optical ADM (OADM) or an optical cross-connect (OXC). A wavelength multiplexed signal is demultiplexed by an optical splitter10and input to an N×N optical switch40. Each signal light whose path is switched by the optical switch is input to a variable optical attenuator14and fed via an optical branching device16into an optical combiner18, where the signals are multiplexed again. The multiplexed signal is then amplified by an optical amplifier and transmitted out on a transmission line. It is shown here that the wavelength multiplexed signal from one path is demultiplexed and the demultiplexed signals are passed through the optical switch40and multiplexed again into one wavelength multiplexed signal, but actually, path switching is performed on wavelength multiplexed signals input from one or more paths, and the demultiplexed signals are combined into one or more wavelength multiplexed signals for transmission on one or more paths.

The level of the signal light input to each variable optical attenuator14differs from channel to channel because of components such as the optical switch; therefore, in the illustrated example, the output level is set to a uniform level before transmission to the next node. If the signal light level is not set to a uniform level, variation between channels increases, and the level may exceed the dynamic range of the receiver. Further, when there is no signal light due to a cut input fiber or the like, the amount of attenuation is set to zero in the output constant control. As a result, when the signal light is restored, the power level on any channel where the amount of attenuation is not to zero in the output constant control increases, causing a power surge; to prevent this, the amount of attenuation in VOA is automatically fixed to a certain predetermined value.

An apparatus control unit42changes the network configuration by switching the N×N optical switch40in accordance with network control information. Here, if the switch switching information is also input to each control circuit30so that the attenuation amount A1 in the signal off condition can be changed in accordance with use or nonuse of the corresponding channel, then even if the optical power at the time of signal restoration is small, the signal restoration can be reliably detected to switch the control to the output constant control.

FIG. 20is a diagram showing a second embodiment of the present invention. The same constituent elements as those inFIGS. 1 and 2are designated by the same reference numerals, and the description of those elements will be omitted.

In the embodiment ofFIG. 20, each control circuit42achieves both the optical power level constant control and the detection and control of a signal off condition and restoration from the signal off condition, based only on the result of the detection supplied from the optical branching device12and the photodetector26provided on the input side of the variable optical attenuator14.

If the signal power level detected by the photodetector26is lower than the threshold value Th0, the amount of attenuation in the variable optical attenuator14is set to maximum (or to a certain predetermined value), and when the power level is restored to a level higher than the threshold value Th0, the output of the variable optical attenuator14is controlled to the constant value L1.FIG. 21shows the relationship between the input power level to the variable optical attenuator14and the attenuation amount for controlling the output power level to the predetermined value L1, andFIG. 22shows the relationship between the attenuation amount and the driving current or voltage of the variable optical attenuator; the relationship between the input power and the driving current or voltage for controlling the output power to the predetermined value L1 is determined from the above relationships and is stored in advance. In the optical output constant control, based on these relationships, the driving current or voltage for controlling the output power level to the predetermined value is determined from the input power level and supplied to the variable optical attenuator14.

As in the first embodiment which performs control based on the level on the output side of the variable optical attenuator, modifications can also be made in the second embodiment; for example, the signal off detection threshold value Th-d and the signal restoration detection threshold value Th1 may be set to respectively different values, a hold-off time may be provided based on which to switch from one control mode to the other, and an alarm may be issued and/or the power may be shut down when the power level exceed the upper limit value or drops below the lower limit value.