This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2007-328592, filed on Dec. 20, 2007, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical transmission system that amplifies an optical signal using optical amplifiers and relays/transmits the optical signal. The present invention includes a control technique that allows the signal transmission to be maintained even when the output of an excitation LD in the optical transmission system decreases.
2. Description of the Related Art
Optical amplifiers are required in an optical transmission system that transmits/receives a wavelength division multiplexed (WDM) optical signal to combine a high power output property with a low-noise property successfully. The high power output property enables an amplification output to be increased in accordance with an increase in wavelength number of the WDM optical signal. The low-noise property enables a decrease in an optical signal-to-noise ratio (OSNR) of the optical signal to be inhibited after amplification.
As shown in
However, the excitation LD (expressed as LD 111 in a configuration example in
The factors contributing to the output power decrease in the 0.98 μm excitation LD include a failure due to wear (hereinafter, referred to as a “wear-out failure”), and an random failure. The wear-out failure is a slow degradation mode wherein the decrease of the output power in the excitation LD progresses in units of years. Specifically, the wear-out failure is a failure in which, out of injected current, current that does not contribute to light-emitting increases with time, so that the characteristic of an optical output with respect to the injected current gradually degrades. That is, the wear-out failure corresponds to a so-called “lifetime”.
On the other hand, the random failure is a fast degradation mode in which the decrease of the output power in the excitation LD progresses in a short time (specifically, in about 100 hours or less). Random failures include a failure due to a posteriori factor in which the neighborhood of an end face of the excitation LD, wherein the energy density is high, is melted due to momentary high optical output oscillation owing to an inflow of surge current or an overcurrent from the outside, to thereby form crystal defects; and a failure due to a priori factor in which there exist crystal defects in a semiconductor manufacturing process (i.e., during manufacturing). The above-described abrupt halt is subsumed under the random failure.
When the laser output power decreases due to crystal defects, since the crystal defects occur in a non-light-emitting area, the injected current changes into heat in this area. In addition, because the non-light-emitting area absorbs light, the non-light-emitting area also generates heat. These occurrences of heat lead to enlargement of crystal defects in a chain reaction manner, thereby causing a rapid decrease in the laser output as in the case of the above-described abrupt halt.
In the conventional optical amplifier, when an output power decrease in the 0.98 μm excitation LD occurs, control for compensating for the output power decrease has been performed within the pertinent optical amplifier in its closed state. Specifically, in the configuration example in
An optical apparatus connected to an up transmission line and a down transmission line, comprising an optical amplifier configured to amplify a light input from the up transmission line, including an optical amplification medium and an excitation light source, the excitation light source supplying an excitation light to the optical amplification medium; an optical supervisory channel (OSC) unit configured to receive OSC optical signals from, and transmit the OSC optical signals to, the up transmission line and the down transmission line; wherein, if an output power decrease of the excitation light source due to an random failure is detected, a driving condition of the excitation light is kept unchanged and the OSC unit transmits an information of the random failure of the excitation light source via the OSC optical signals, to the up transmission line and the down transmission line, and if the OSC unit receives an information of an random failure of an excitation light source in other optical apparatus connected to the up transmission line and the down transmission line, the output light power of the excitation light source is increased.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
The above-described embodiments of the present invention are intended as examples, and all embodiments of the present invention are not limited to including the features described above.
c are diagrams showing operations when a wear-out failure has occurred in the conventional optical amplifier, and
Reference may now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.
When the output power decrease in the excitation LD arises from the random failure as described above, the aforesaid control with respect to the output power decrease in the excitation LD in the conventional optical amplifier has a disadvantage of accelerating the progression of an random failure by keeping the output of the excitation LD constant. If the random failure progresses in a shorter time, it would be necessary to perform replacement of the excitation LD in which an output power decrease has occurred earlier, which raises a problem of increasing the burden on a maintainer. Such a problem occurs not only in the above-described two-stage optical amplifier shown in
As a related art, in order to avoid the above-described acceleration of the random failure, for example, there is control for keeping the entire optical amplifier constant by increasing excitation light power of the post-stage optical amplifier, while maintaining the drive condition of the excitation LD in which an output power decrease has occurred. Regarding operating characteristics of an optical amplifier when the above-described control is applied,
At the occurrence of a wear-out failure shown in
On the other hand, at the occurrence of an random failure shown in
In
Furthermore, the stations A to E exchange information among themselves using an optical supervisory channel (OSC) signal that is a signal of a different channel from that of the WDM optical signal (main optical signal). Information transmitted by the OSC signal includes output decrements in excitation LDs incorporated in optical amplifiers in each of the stations. If an random failure occurs in any excitation LD in the present optical transmission system, the present system shares the output power decrease in the excitation LD among all the stations and controls optical amplifiers by each of the stations on the basis of the output decrement, to thereby compensate for the above-described output power decrease in the excitation LD by the system in its entirety.
Specifically, the terminal station A wavelength-multiplexes respective optical signal lights that are mutually different in wavelength and that are outputted from a plurality of transmitters (TX) 11A, by an optical multiplexer 12A, and amplifies them up to a desired level by an optical amplifier 13A for transmission. Then, the terminal station A multiplexes the WDM optical signal outputted from the optical amplifier 13A for transmission with an OSC signal created by an OSC transmission section 33A, by an multiplexer 34A, and transmits the WDM optical signal and the OSC signal to the up line. Moreover, the terminal station A receives light transmitted through the down line, and separates the light into the WDM optical signal and the OSC signal by the demultiplexer 31B.
Then, after the terminal station A has amplified the separated WDM optical signal up to a desired level by an optical amplifier 21B for reception, separates the WDM optical signal into optical signal lights with respective wavelengths by the demultiplexer 22B, and receives them by receivers (RX) 23B corresponding to respective optical signal lights. The OSC signal separated by the demultiplexer 31B is received by an OSC reception section 32B, and supervisory control information is transmitted to a control section 35. On the basis of this information, the control section 35 controls the optical amplifier 13A on the up-line side and the optical amplifier 21B on the down-line side.
Each of the relay stations B to D receives light that has been outputted from the upstream station on the up-line side and that has propagated through the optical transmission line. Each of the relay stations B to D separates the light into WDM optical signal and an OSC signal by a demultiplexer 31A. Then, each of the relay stations B to D amplifies the WDM optical signal by the optical amplifier 40A to thereby compensate for loss on the optical transmission line, and outputs the WDM optical signal to the optical transmission line connected to the downstream station on the up-line side via a multiplexer 34A. The OSC signal separated by the above-described demultiplexer 31A is received by the OSC reception section 32A, and the supervisory control information is transmitted to the control section 35. Also, each of the relay stations B to D receives light that has outputted from the upstream station on the down-line side and that has propagated through the optical transmission line, and separates the light into the WDM optical signal and the OSC signal by the demultiplexer 31B. Then, each of the relay stations B to D amplifies the WDM optical signal by the optical amplifier 40B to thereby compensate for loss on the optical transmission line, and outputs the WDM optical signal to the optical transmission line connected to the downstream station on the down-line side via a multiplexer 34B.
The OSC signal separated by the above-described demultiplexer 31B is received by the OSC reception section 32B, and the supervisory control information is transmitted to the control section 35. The control section 35 controls the optical amplifiers 40A and 40B on the basis of the supervisory control information from the OSC reception sections 32A and 32B, respectively. Furthermore, upon receipt of monitor information on operation states of the optical amplifiers 40A and 40B in its own station, the control section 35 creates information to be transmitted to the downstream station on the up-line side and the down-line side, and outputs the information to the corresponding OSC transmission sections 33A and 33B. In response of the information from the control section 35, the OSC transmission sections 33A and 33B create OSC signals, and after having multiplexed them with the main optical signal by the multiplexers 34A and 34B, transmit the multiplexed signal to the optical transmission line.
The terminal station E receives light transmitted through the up line, and after having separated the light into the WDM optical signal and the OSC signal by the demultiplexer 31A, amplifies the separated WDM optical signal up to a desired level by an optical amplifier 21A for reception. Then, the terminal station E separates the WDM optical signal into optical signal lights with respective wavelengths by a demultiplexer 22A, and receives them by receivers (RX) 23A corresponding to respective optical signal lights. The OSC signal separated by the above-described demultiplexer 31A is received by the OSC reception section 32A, and supervisory control information is transmitted to the control section 35.
Also, the terminal station E wavelength-multiplexes respective optical signal lights that are mutually different in wavelength and that are outputted from a plurality of transmitters (TX) 11B, by an optical multiplexer 12B, and amplifies them up to a desired level by an optical amplifier 13B for transmission. Then, the terminal station E multiplexes the WDM optical signal outputted from the optical amplifier 13B with an OSC signal created by the OSC transmission section 33B, by the multiplexer 34B, and transmits the WDM optical signal and the OSC signal to the down line. On the basis of the supervisory control information from the OSC reception section 32A, the control section 35 controls the optical amplifier 21A on the up-line side and the optical amplifier 13B on the down-line side.
Here, configurations of optical amplifiers in the stations A to E: 13A, 13B, 21A, 21B, 40A, and 40B are described in detail.
The WDM optical signal amplified by the EDF 51 is sent to a post-stage optical amplification section, and a part thereof is provided to an optical detector (PD) 56 after have been branched by an optical coupler 55. Then, the signal output power of the pre-stage optical amplifier is monitored by the above-described optical detector 56, and the monitored result is sent to the drive control circuit 57. The drive control circuit 57 outputs the monitored results by the optical detectors 54 and 56 to the control section 35 (refer to
The post-stage optical amplification section amplifies the WDM optical signal to propagate through an EDF 61, up to a desired power level by supplying excitation light outputted from the a 1.48 μm excitation LD (LD) 62 to an EDF 61 into one end of which the WDM optical signal amplified in the pre-stage optical amplifier is inputted, via a multiplier 63. At this time, excitation light power outputted from the 1.48 μm excitation LD 62 is monitored by a photo detector (PD) 64, and the monitored result is sent to a drive control circuit 67. The WDM optical signal amplified by the EDF 61 is sent to an optical amplifier (PD) 66, and a part thereof is provided to an optical detector (PD) 66 after have been branched by an optical coupler 65. Then, the signal power of the post-stage optical amplification section is monitored by the optical detector 66, and the monitored result is sent to the drive control circuit 67. The drive control circuit 67 outputs the monitored results by the optical detectors 64 and 66 to the control section 35 (refer to
Here, detailed description is made of the transmission of the output decrement δ making use of the OSC signal between the stations A to E. In the optical transmission system performing relay/transmission of WDM optical signal, typically, information exchange is performed between stations on the system using the OSC signal set in a channel other than that of main optical signal. This OSC signal superimposes thereon control information for remotely controlling other stations from a terminal device connected to some station, or operation information for WDM optical signal (for example, information on number of a channel that is in the service-in, a signal output level set in each of the stations, status information at starting-up, etc).
In the present embodiment, when an output power decrease occurs in the 0.98 μm excitation LD 52 in any station (in the example in
Upon receipt of the above-described output power decrease information, the control sections 35 in the stations A to E transition into a control mode for boosting in unison outputs of excitation LDs that are normally operating in the optical amplifiers on the up-line side in accordance with the output decrement δ of the 0.98 μm excitation LD 52 in the optical amplifier 40A on the up-line side in the station B, so that the output power decrease in the 0.98 μm excitation LD 52 in the station B is compensated for by the system in its entirety.
Specifically, the control section 35 in the station B in which an output power decrease in the 0.98 μm excitation LD 52 has occurred, regarding the optical amplifier 40A on the up-line side, maintains the current drive condition of the 0.98 μm excitation LD 52 in the pre-stage EDFA (i.e., controls drive current to be constant), and creates an output control command to increase the output power of the 1.48 μm excitation LD 62 in the post-stage EDFA by a predetermined amount. The output control command is outputted to each of the drive control circuits 57 and 67 in the optical amplifier 40A, and thus the drive conditions of the excitation LDs 52 and 62 are controlled, respectively. Also, the control sections 35 in the other stations A, and C to E each creates an output control command to increase each of the output powers of the 0.98 μm excitation LD 52 and the 1.48 μm excitation LD 62 in the optical amplifier 40A on the up-line side by a predetermined amount. The output control command is outputted to each of the drive control circuits 57 and 67 in the optical amplifier 40A, and the drive conditions of the excitation LDs 52 and 62 are controlled, respectively.
Here, an example in which the drive condition of the 0.98 μm excitation LD 52 in which an output power decrease has occurred is maintained, has been shown. Alternatively, however, one may reduce the drive current of the 0.98 μm excitation LD 52 down to a preset level to retard the progression of the random failure.
In the control mode as described above, the increment in output power of an excitation LD that is normally operating can be determined, for example, in accordance with the following procedure. First, a table in which increments in output powers of excitation LDs for compensating for an output power decrease by the entire system have been calculated for each of the output decrements δ of the 0.98 μm excitation LDs 52 in which a failure has occurred, is created in advance. Then, the table is stored in a memory (not shown) provided in each of the control sections 35 in the stations A to E, and a value corresponding to the output decrement δ of the 0.98 μm excitation LD 52 transmitted by the OSC signal is read from the above-described memory table to determine the above-described increment in output power.
Here, a concrete example regarding calculation method in the above-described table is described.
In general, when optical signal with a power Pin [dBm] is inputted to an optical amplifier having a characteristic of a noise index NF [dB], the OSNR value [dB] of optical signal outputted from the pertinent optical amplifier is expressed by the following equation (1) where α is a constant.
OSNR=Pin−NF+α (1)
Also, the OSNR value [dB] of optical signal at a reception terminal in an optical transmission system that relays optical signal in multi-stages, as shown in
OSNRreceive=−10×log [Σ{10(−0.1×OSNR)}] (2)
Here, in order to distinguish from the OSNR value of an output signal of the optical amplifier alone shown by the above-described equation (1), the OSNR value of the optical signal at the reception terminal in the optical transmission system is denoted as OSNRreceive. As is evident from the equation (2), the OSNR value of optical signal at the reception terminal is a sum of OSNR values of output signals in all the optical amplifiers existing on the paths through which the optical signal is relayed/transmitted.
In the case in which the two-stage optical amplifiers as shown in
As described above, since the OSNR value at the reception terminal of the optical transmission system is a sum of the OSNR values of output signals of the optical amplifiers in all the stations, the OSNR value of optical signal at the reception terminal can be calculated in accordance with the output decrement, irrespective of which station on the system has been decreased in the output in its excitation LD. Therefore, if the output decrement in the excitation LD is shared among all the stations, then, in accordance with the output decrement, it is possible to determine drive conditions of optical amplifiers in each of the stations, such as to meet a lower limit value of the OSNR value of optical signal at the reception terminal (this lower limit corresponds to quality limit, for example), the lower limit value being defined by specifications or the like.
Specifically, during the output power decrease in the excitation LD, since the OSNR value of optical signal of the pertinent optical amplifier decreases, the value of the sum total (Σ) in the right side of the above-described equation (2) becomes smaller than during the normal state. At that time, regarding other excitation LDs that are normally operating, their drive condition are controlled so that their respective output power levels is boosted in unison so that the OSNR values of output signals in the optical amplifiers corresponding to the respective excitation LDs may be increased. For example, considering the case in
Regarding the number of excitation LDs that are normally operating, because the total number of excitation LDs on the system is known on the basis of design information, the number of excitation LDs that are normally operating can be obtained by utilizing the design information. Also, if an optical switch for switching the transmission line of optical signal is provided on the system and the total number of excitation LDs on the system changes, then, the number of the excitation LDs that are normally operating can be obtained by transmitting information on the total number of excitation LDs, being superimposed on the OSC signal, to each of the stations.
As a result, the decrement in the OSNR value in an optical amplifier in which an output power decrease in the excitation LD has occurred is compensated for by increments of the OSNR values of the other optical amplifiers. This allows the OSNR value of optical signal at the reception terminal to be kept at a level equal to that in a normal state, or at least at a level such as not to cause a signal transmission problem.
The curve corresponding to round symbols in
Assuming the lower limit of OSNR value (quality limit) at the reception terminal to be 14.0 [dB], the above-described output value (13.5 [dB]) falls short of the lower limit of OSNR value, which indicates that the optical transmission system is in a state of being incapable of normal signal transmission. So, the control by the present invention is applied as follows: the output power decrease in the 0.98 μm excitation LD in the second station is shared among all the stations, and the drive conditions of excitation LDs that are normally operating are controlled so that signal output levels of optical amplifiers corresponding to the respective excitation LDs are raised by, e.g., 2 dB. Thereupon, as shown in the curve corresponding to square symbols in
In a typical optical amplifier such as an EDFA, when the output power of excitation LD is increased by 10%, the signal output level of the optical amplifier rises by about 1 dB. When the output power of excitation LD is increased by 20%, the signal level of the optical amplifier rises by about 2 dB. When such a relationship holds, in order to implement the state exemplified in
As described above, according to the optical transmission system in the first embodiment, even when the output of an excitation LD rapidly decreases due to an random failure, it is possible to maintain the state of being capable of signal transmission for a longer time without accelerating the progression of the random failure, by sharing the output decrement among all the stations on the system by utilizing the OSC signal to maintain the current drive condition of the excitation LD in which the output power decrease has occurred, and regarding the other excitation LDs that are normally operating, by boosting in unison their respective output powers to compensate for the decrease in OSNR due to the output power decrease in the above-described excitation LD by the system in its entirety. This allows securing a sufficient time before the maintainer takes countermeasures against the failure, such as replacement of the excitation LD, thereby enabling a relief of burden on the maintainer.
In the above-described first embodiment, description has been made of the two-stage configuration in which, regarding the optical amplifiers in each of the stations, the EDFA by the 0.98 μm excitation method is used as the pre-stage optical amplifier, and the EDFA by the 1.48 μm excitation method is used as the post-stage optical amplifier. However, the configuration of the optical amplifiers in the present invention is not limited to the above-described configuration example. As shown in
Moreover, in the above-described first embodiment, explanation has been made of the case in which the output of the 0.98 μm excitation LD rapidly decreases due to the occurrence of an random failure. However, regarding an excitation LD other than the 0.98 μm excitation LD, the present invention is also applicable to the case in which an excitation LD which has any wavelength, and of which the output power can rapidly decrease in a progression in about 100 hours or less due to the occurrence of an random failure, is used in an optical amplifier.
In addition, in the above-described first embodiment, regarding the 1.48 μm excitation LD existing in the same station as that having the 0.98 μm excitation LD in which output power decrease has occurred, the example in which the output level is raised by the same control as that with respect to the excitation LDs in the other stations, has been shown. However, for example, it is also possible to control the drive condition of 1.48 μm excitation LD so that the signal output level of its own station is kept constant while maintaining the current drive condition of the 0.98 μm excitation LD in which an output power decrease has been occurred. In other words, it is also possible to maintain a constant-output control with respect to its own station up until the control of the 1.48 μm excitation LD reaches the upper limit even if the output power decrease in the 0.98 μm excitation LD has been occurred. In this case, the power of optical signal inputted to a downstream station keeps substantially the same level up until the control of the 1.48 μm excitation LD reaches the upper limit irrespective of the output decrement in the 0.98 μm excitation LD. However, because the OSNR value of optical signal during the time that intervenes decreases in response to an increase in the output power of the 1.48 μm excitation LD (refer to the right side in
Furthermore, in the above-described first embodiment, the output decrement δ in the 0.98 μm excitation LD has been transmitted to the other stations by superimposing the output decrement δ on the OSC signal. However, since the output decrement δ in the 0.98 μm excitation LD eventually leads to a decrease in the level of optical signal outputted from the pre-stage optical amplification section, the same control as the above-described control in the first embodiment can be performed also by determining the decrement in the signal output level of the pre-stage optical amplification section using a monitored value by the optical detector 56 (
Moreover, as shown in
Next, a second embodiment according to the present invention is described.
As shown in
Specifically, for example, in a system transmitting optical signal with a speed of 10 [Gbit/sec] or more, typically, error correction processing is performed at the reception terminal using an error correction code that is imparted to optical signal, and the number of error occurrences in received signals before and after the error correction processing can be counted. Accordingly, in the optical transmission system according to the present embodiment, in each of the terminal stations A and E, a signal ER that indicates the number of error occurrences before or after error correction and that is counted by the respective receiver 23B and 23A is provided to the respective control sections 35, so that an OSC signal including the above-described number of error occurrences is created by the respective transmission sections 33A and 33B, and is transmitted to each of the stations on the upstream side of the reception terminal via the opposed lines. In each of the stations, which have received the number of error occurrences at the reception terminal, the increment in output power of the excitation LD in its own station, set in accordance with the output decrement δ of the 0.98 μm excitation LD, is optimized so that the number of error occurrences becomes a minimum. This allows the decrease in OSNR caused by the rapid output power decrease in the excitation LD due to an random failure to be compensated for by the entirety of the system, with high accuracy.
In the above-described second embodiment, the example has been shown in which the control with respect to the excitation LD in each of the stations is optimized using the number of error occurrences counted during the error correction at the reception terminal. Alternatively, as shown in
According to the above-described embodiments, in the optical transmission system with optical amplifiers, even when a rapid output power decrease in the excitation LD due to an random failure occurs, it is possible to provide a control technique for maintaining the state of being capable of signal transmission for as long a time as possible without accelerating the degradation of the pertinent excitation LD, and securing a sufficient time before the maintainer takes countermeasures against a failure, thereby enabling a relief of burden on the maintainer.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
Number | Date | Country | Kind |
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2007-328592 | Dec 2007 | JP | national |