Patent Application
20230411924
This application claims priority to European Patent Application No. 22180246.5, filed Jun. 21, 2022, the disclosure of which is hereby incorporated by reference in its entirety.
The present invention relates to a method for monitoring a pump laser of at least one optical amplifier in an optical transmission link in operation, wherein the optical output power of the pump laser to be monitored depends on an injection current and wherein the pump laser to be monitored is operated at an operating point defined by a given value of the injection current and a corresponding value of the optical output power. The present invention furthermore relates to a control device for controlling and monitoring a pump laser of at least one optical amplifier in an optical transmission link, the control device being configured to receive information about an operating point of the pump laser to be monitored, wherein the operating point is defined by a value of the injection current supplied to the pump laser of the at least one optical amplifier and a corresponding value of the optical output power created by the at least one pump laser, and to output control information to the at least one optical amplifier at least comprising information defining the operating point. The present invention furthermore relates to an optical amplifier comprising a control unit configured for monitoring a pump laser of at least one optical amplifier in an optical transmission link in operation and to an optical transmission link comprising at least one such optical amplifier.
Long-haul optical transmission links that span hundreds or even thousands of kilometers, for example transatlantic optical transmission links, comprise optical amplifiers in order to optically amplify the one or more optical signals to be transmitted over the optical transmission links. These signals may include wanted optical transmission signals carrying user data as well as optical management signals. Generally, an optical amplifier comprises at least one pump laser, especially a semiconductor pump laser diode, creating an optical pump signal having an optical spectrum that does not overlap with the optical spectrum covered by the optical transmission signals to be amplified. The optical pump signal at a predetermined optical pump power as well as the optical signals to be amplified are coupled to an optical amplifier medium such as an erbium-doped optical fiber in which the signals to be amplified are amplified by stimulated emission. The gain of the optical amplifier depends on the optical pump power and the width of the optical spectrum occupied by the optical signals to be amplified. The optical pump power depends on the injection current supplied to the pump laser. The injection current, however, is limited and must not exceed a maximum value in order not to damage or even destroy the pump laser, especially due to an unacceptably high heat dissipation.
The pump lasers are the parts of the amplifiers, which are most affected by aging. As time progresses, a pump laser requires an increasing injection current in order to maintain a constant optical output power, i.e. a constant optical pump power. Once the aging effects become too strong, the pump lasers need to be replaced as the maximum value of the optical pump power that must be guaranteed according to the specification of the optical amplifier cannot be reached with an admissible injection current. It is thus necessary to monitor the pump lasers with respect to their aging status.
Many known monitoring techniques for pump lasers of optical amplifiers, especially erbium—doped fiber amplifiers (EDFA), allow monitoring of amplifier quality only at operating points at high values of the optical output power from the pump laser diodes (especially at or near a specified target value of the optical output power, see below). An operating point is defined by a given value of the injection current and a resulting value of the optical output power, i.e. the optical pump power provided by a pump laser. With these known methods it is, in particular, checked whether the currently required injection current that must be supplied to the pump laser diode in order to create the specified target value of the optical output power exceeds a beginning-of-life (BOL) injection current that is required to create the target value of the optical output power by a certain percentage. This target value of the optical output power is chosen in such a way that it can be maintained for (at least) a given operating time during which the injection current must be increased due to aging effects in order to maintain the target value of the optical output power without exceeding a maximum (threshold) value of the injection current. Typically, the target value of the optical output power of a pump laser is specified in such a way that the injection current can be increased by up to 10% as compared with the BOL injection current until the maximum admissible (threshold) injection current is reached.
This technique comes with severe drawbacks and is in many cases not able to detect degradation of the pump laser diodes, as it is only capable of producing reliable results in cases in which the pump laser to be monitored is operated at or near the specified target value of the optical output power. However, pump lasers of optical amplifiers are often operated at a much lower optical pump power and thus at an injection current that is significantly smaller than the BOL injection current. In consequence, no alarm would be triggered although system operation, i.e. the whole transmission link, could fail if it becomes necessary to operate the respective optical amplifier (that includes the degraded pump laser) at the target optical output power. This would, for example, be the case if a wavelength division multiplex (WDM) transmission link that has been specified for a given maximum number of optical transmission channels and is currently operated with a lower number of optical transmission channels shall be upgraded to a higher, especially the maximum number of optical transmission channels. Using this known standard technique, it is, for example, impossible to acquire reliable information on whether it is still possible to expand the optical WDM transmission link to the given maximum number of channels without increasing the injection current beyond the maximum admissible injection current (e.g. 10% higher than the BOL current).
The dependency of the output power of a pump laser diode on the injection current is typically characterized by the so-called LI curve with a clear threshold behavior. This means, at low values of the injection current the optical output power rises very slowly. If the value of the injection current exceeds a threshold value, the LI curve becomes significantly steeper (almost linear) with a gradient depending on the stage of aging. If operated below the current threshold, the laser diode emits essentially incoherent light (like a light-emitting diode (LED)), whereas lasing (creation of coherent light) is achieved above the threshold.
A simple enhancement of the standard monitoring technique described above that allows monitoring of the aging status at power levels smaller than the maximum BOL output power is based on the assumption that the pump power is proportional to the injection current (DE 10 2007 019 345 B4). This method, however, neglects the impact of the threshold current, i.e. the actual course of the LI curve. Taking into account that the threshold current may amount to almost 10% of the BOL current, it becomes clear that this technique can provide usable results only if the output power of the operating point that is used to determine the slope of the proportional approximation of the actual LI curve is close to the specified target output power. It shall be mentioned at this point that the terms “optical power” and “injection current”, where applicable, are, as a short form, also used in order to designate respective values of these parameters.
A solution to overcome this problem would be to record the complete LI curve of the pump laser during manufacturing of the optical amplifier or during its first installation in a transmission system. With this known (BOL) LI curve it would be possible to compare the current injection current that is required to create a given optical pump power with the corresponding injection current providing the same optical output power at BOL conditions. However, this method increases production or installation time. Furthermore, amplifiers already installed in the field cannot be easily upgraded with this monitoring method as this would make it necessary to interrupt the transmission link and, as the case may be, to have physical access to the optical amplifiers in order to carry out the measurements and/or to install a new soft- or firmware.
Another approach for monitoring the quality of an amplifier in operation is described in Lutz Rapp, “Quality Surveillance Algorithm for Erbium—Doped Fiber Amplifiers”, Proc. DRCN, 2005 (corresponding patent: U.S. Pat. No. 7,379,234). This method allows to monitor the complete optical path within the amplifier and is—in contrast to the other techniques—not limited to monitoring the aging of the pump lasers. Furthermore, aging effects can be detected for arbitrary channel loads and power distributions at the input of the optical amplifier. However, the implementation of this method in an optical amplifier is complex and additional calibration data is required. Therefore, this method is not suitable for upgrading monitoring capabilities of an amplifier that is already installed in the field without interruption of the transmission link.
A monitoring method using high frequency modulation of the injection current for determining the slope of the LI curve is described in U.S. Pat. No. 7,038,769 B2. The modulation frequency must be higher than the cut-off frequency of the optical amplifier in order to avoid channel distortions. In this context, the term “cut-off frequency” means the frequency of the modulation component of the injection current above which the optical gain of the optical amplifier (created within the optically pumped medium, e.g. the erbium-doped optical fiber) shows a sufficiently reduced modulation component (e.g. defined by a quotient of the amplitude of the optical gain at the given modulation frequency and the amplitude obtained with the same amplitude of the modulation component of the injection current at a very low frequency, e.g. a few Hertz, wherein the cut-off frequency may be defined as the frequency as of which this quotient is lower than a given value, e.g. 0.5). In this publication, a modulation frequency of 2 MHz is proposed. This method, however, requires additional hardware components and is therefore not suitable for upgrading amplifiers that are already installed in the field. Furthermore, this method limits the maximum usable optical pump power.
Starting from this known prior art, it is an object of the present invention to provide a method for monitoring a pump laser of at least one optical amplifier in an optical transmission link in operation that can be carried out during operation of the optical amplifier, that does not require additional hardware or modification of the hardware, and that can easily be implemented in existing optical amplifiers, even during operation. It is a further object of the invention to provide a control device for controlling and monitoring a pump laser of at least one optical amplifier in an optical transmission link in operation that is configured to implement this method. It is a further object of the invention to provide an optical amplifier comprising such a control unit and an optical transmission link comprising at least one optical amplifier and such a control unit.
The present invention achieves these objects with the combinations of features as described herein.
The invention starts from the finding that the extent of aging at any arbitrary operating point can be determined by shifting the operating point of the pump laser to be monitored for a predetermined time interval to at least one further shifted operating point, and by using the information on the operating point and the at least one further shifted operating point in order to determine information on the stage of aging of the pump laser to be monitored, especially by using a mathematical regression method, i.e. by determining the current course of the LI curve from the measured operating points. According to the invention, shifting to the at least one further operating point is effected in such a way that the gain of the respective optical amplifier essentially reaches its steady state. That is, the change may either be effected by keeping the injection current and the pump power defining a given operating point constant until the optical gain reaches its steady state or by continuously changing the operating point (e.g. by slowly modulating the injection current) at a modulation frequency that is at least lower than the cut-off frequency of the optical amplifier, which is mainly determined by the characteristic of the optically pumped medium (e.g. an erbium-doped fiber).
As the driver components of pump lasers are in general designed in such a way that only slow changes of the injection current are possible, the method according to the invention allows accurate monitoring of the aging of pump lasers over a wide range of output powers and therefore at a large variety of operating conditions, without requiring any modification of the hardware or additional calibration data. It allows to upgrade optical amplifiers installed in the field via a simple software change. The respective software may be part of a control device that is included in the respective amplifier, i.e., the software change may be effected by uploading an upgraded software or an additional software module to the control device. The software may also be part of a higher instance control device (e.g., a management system that controls a whole transmission link including all optical amplifiers comprised by the transmission link).
According to an embodiment of the invention, the at least one shifted operating point can be created by controlling the injection current to a different, shifted value and the resulting optical power can be measured to obtain the full information on the shifted operating point. Alternatively, the at least one shifted operating point can be created by controlling the optical pump power to a different, shifted value and measuring the resulting injection current in order to obtain the information on the shifted operating point. As many optical amplifiers comprise a control device that is capable of controlling the optical power of a optical transmission signal at an output port of the optical amplifier to a predetermined value, it is also possible to control this optical power to a different, shifted value and to measure the resulting optical pump power and the resulting injection current to obtain the information on the shifted operating point.
The at least one shifted operating point can be determined in such a way that one or more parameters characterizing the transmission quality of the optical transmission link are not changed by more than a predetermined amount or do not exceed a predetermined threshold or do not fall below a predetermined threshold. This ensures that the live traffic over the optical transmission link is not interrupted while the amplifier is being monitored. As a relevant parameter, e.g., the bit error rate determined at the respective downstream end (with respect of the transmission direction comprising the optical amplifier(s)) of the transmission link may be used. It is also possible to set a maximum drop of the gain of the optical amplifier including the pump laser to be monitored and to allow only changes (especially reductions) of the optical pump power that create a corresponding acceptable change of the gain (or the optical output power of the optical transmission signal at the output port of the optical amplifier).
According to a further embodiment, the change in optical output power of a first pump laser between the operating point and the at least one further operating point can be compensated by a change of an injection current of at least one second pump laser.
In such embodiments, the second pump laser may be a component of the same optical amplifier as the first pump laser. In such a case, for example, if the first and second pump lasers are included in a two-stage amplifier, which comprises two pump lasers, monitoring can be carried out without changing the total gain of the optical amplifier. The injection current of the first pump laser can be reduced or increased which creates a reduction of or an increase in the pump power (and thus of the optical gain of the first amplifier stage), while the injection current of the second pump laser can be increased or reduced, which leads to an increase in or reduction of the optical pump power (and thus of the optical gain of the second amplifier stage), in order to compensate the shift of the operating point of the first pump laser. It is, of course, also possible that the compensation is not fully effected by a single second pump laser but by two or even more second pump lasers. A control device included in the optical amplifier that comprises the at least two stages (each of which comprises the first or a second pump laser) may be configured to carry out the monitoring method without creating additional traffic, e.g., in a management channel.
Alternatively, in such embodiments, the second pump laser may be a component of a further optical amplifier. This, however, leads to a change of the optical power of the optical transmission signal carrying used data between the respective two optical amplifiers that include the first and second optical amplifier, respectively. Further, this embodiment requires transmission of control or management information from and/or to the optical amplifiers comprising the respective first and (one or more) second pump lasers.
According to a further embodiment, the compensation of the variation of the optical power may be attained by measuring the optical power of an optical transmission signal at a predetermined position within the optical transmission link, which is located downstream of the at least one second pump laser, preferably in the region of an output port of a selected optical amplifier. This measured signal can be used as a target signal when controlling the pump power of the at least one second pump laser. Especially, the at least one second pump laser can be controlled in such a way that the optical power of the signal remains essentially constant. In this way, using the optical power of the signal at the respective position within the optical transmission link may be used to carry out the monitoring method according to the invention including compensation for all first and second pump lasers positioned upstream of the measurement position.
According to another embodiment, the information on the stage of aging of the pump laser to be monitored comprises a maximum value of the optical output power at a predetermined maximum value of the laser injection current of the amplifier in its current stage of aging or an information dependent on this maximum value of the optical pump power. This maximum optical pump power that is determined using the at least two operating points (i.e., the respective two pairs of injection current and optical pump power values) by applying a mathematical regression method, e.g., linear regression, cannot be exceeded. If, for example, an optical WDM transmission system shall be expanded by one or more optical (WDM) transmission channels, the pump power of the pump lasers of all optical amplifiers must be increased by a predetermined amount. This information may be provided or determined by a management or control device as known in the prior art. Further, each optical amplifier may provide information to the management or control device including the maximum optical pump power that can be created by each pump laser. From this information, the management or control device may, in advance, determine whether the intended increase in transmission channels is possible. In this way, it is, for example, possible to examine whether a transmission link currently still fulfills its specification requirements, e.g., its capability of transmitting a given number of optical WDM transmission channels.
Alternatively, the information on the stage of aging of the pump laser to be monitored can be a maximum number of channels by which the optical WDM transmission link can be expanded without exceeding a given maximum value of the injection current. This information, provided by each optical amplifier (for each of the pump lasers included therein) may be provided to a higher-level management system or device which may then determine the maximum number of channels that can be used for transmission by the whole transmission link (even if this number is smaller than the specification requirement).
According to another embodiment, the information on the stage of aging can be obtained at predetermined points in time or at given time intervals. From this information, the maximum period of time may be determined for which the pump laser that is monitored or the optical amplifier including this pump laser fulfills a predetermined specification requirement (e.g., a predetermined minimum value of the pump power that is reached at the maximum admissible value of the injection current). Of course, instead of the remaining time interval, the absolute point in time may be determined at which the pump laser monitored (or the optical amplifier) will (likely) fail.
The control device according to the invention is configured to carry out the invention according to the invention described above. It may be included in an optical amplifier, especially if the amplifier is a two or n-stage optical amplifier. Such a control device may be configured to monitor one or all of the pump lasers included in the respective amplifier. The control device may, however, also be provided as a separate control device that is configured either to monitor the pump lasers of a single or more dedicated optical amplifiers or even all optical amplifiers provided in an optical transmission link. The control device may be provided at an end of the optical transmission link. In this case, information between the control device and one or more optical amplifiers may be exchanged via a management channel that can be realized in any known manner, e.g., by amplitude-modulating a single optical transmission signal at a maximum modulation frequency that is at least one order (i.e., at least the factor 10) lower than the bit rate or minimum modulation frequency of the optical transmission signal. In case of an optical WDM signal, it is also possible to (correspondingly slowly) modulate the optical WDM signal. Of course, in this case, each optical amplifier must comprise a transceiver configured to bidirectionally communicate with the control device.
Further embodiments of the invention are apparent from the dependent claims.
In the following, the invention will be described in more detail with reference to the drawings. In the drawings,
The area on the right of the coordinate system defined by injection currents larger than the value Ial represents an “alarm-area”. If, during monitoring of the pump laser, the current value of the injection current exceeds this maximum value Ial, an alarm is created. Furthermore, an alarm is created if it is found, during monitoring the pump laser when operated at an operating point having an injection current value lower than the value Ial, that the value Ial would be exceeded if the operating point was shifted to a specified target value Ptgt of the optical pump power.
The target value Ptgt of the optical output power of the pump may, for example, be required in order to obtain a correspondingly high gain of an optical amplifier comprising the pump laser, wherein the gain is defined as the ratio of the optical power of the output signal amplified by the optical amplifier and the optical power of the optical input signal supplied to the optical amplifier. As already mentioned above, in an optical WDM transmission link, the pump power must be the higher the more optical channel signals are to be optically amplified in order to reach a given (minimum) gain for each of the optical channel signals. That is, the optical amplifiers in optical WDM transmission links must be specified in such a way that a predetermined gain can be reached for each transmission channel even if the maximum specified number of optical channels is used for transmitting a respective maximum number of optical channel signals.
As apparent from the LI curve for the aged pump laser in
This highlights the need for a monitoring method which makes it possible to assess whether the pump laser still matches its specification requirements in its current aging status. It would also be desirable to determine the remaining operational life span of a pump laser in an optical amplifier.
In
According to the invention, the course of the current LI curve (i.e., the course of the LI curve characterizing the pump laser in its current stage of aging) can be determined by shifting the pump laser from an initial operating point to at least two, preferably more than two, operating points (in the current stage of aging) and determining the respective values of the injection current and the optical pump power. Using these values, any suitable mathematical regression method can be used in order to reconstruct or approximate the actual course of the current LI curve by an analytical function. Especially linear regression may be used in order to approximate the actual LI curve. The corresponding analytical (or numerically described) function can then be used to determine the values defining any other operating point. For example, as explained above, this function may be used to determine whether an operating point at the specified target value Ptgt of the optical pump power can (still) be reached with an injection current below or at the maximum threshold value Ial.
It is further possible to determine a remaining time span for the pump laser monitored in which the pump laser is able to create the specified target pump power Ptgt with an admissible value of the injection current that is lower than Ial. For this, the method explained above can be carried out from time to time (at predetermined points in time, e.g., at equidistant points in time), wherein the injection current Itgt to is determined that is necessary to create the target pump power Ptgt. From these pairs of values, each comprising the respective point in time and the respective value Itgt to of the injection current, an aging function may be determined, e.g., using a mathematical regression method, especially linear regression. The aging function Itgt(time) may be used to calculate the point in time at which the injection current Itgt reaches the maximum admissible value Ial. In this way, the life-time of a pump laser may be assessed and the pump laser may be replaced in good time before its aging status reaches progresses beyond the border as of which the pump laser cannot match its specification requirements any more. Furthermore, quality of the pump laser can be indicated by relating the aging that has already happened to the maximum allowed aging, wherein aging is expressed in terms of the injection current Itgt to that is necessary to create the target pump power Ptgt
Next, the optical transmission signal passes through an optical isolator 110, which blocks reflected signal components and pump power components as well as backward propagating light consisting of ASE (Amplified Spontaneous Emission) created in the optically pumped media of the EDFA, In the further course of the optical signal path through the optical amplifier 100, a high-power pump light, created by a pump laser 116, is combined with the incoming optical transmission signal S1,opt using a first wavelength selective optical coupler (WSC) 114. The optical transmission signal S1,opt and the pump light reveal non-overlapping optical spectra. The combined light is then guided into a pump section realized by an erbium-doped fiber (EDF) 108. The optical pump light created by the pump laser excites the erbium ions to a higher-energy state. The photons of the optical transmission signal S1,opt at wavelengths differing from the wavelength of the pump light interact with the excited state of the erbium ions, wherein erbium ions are caused to drop from the excited state to a lower-energy state. In this way, additional photons are created having exactly the same wavelength, phase and direction as the photons of the optical transmission signal S1,opt to be amplified. The whole additional signal power is guided in the same fiber mode as the incoming optical transmission signal S1,opt.
The remaining portion of the pump light that has not been used in order to create the stimulated light is extracted from the main optical path through the optical amplifier 100 using a second (optional) WSC 115. The optical transmission signal S1,opt remaining within the optical path passes through a second optical isolator 110. In a next step, using a second tap coupler 112, the optical power (i.e., its absolute value or a (relative) value corresponding thereto) of the incoming optical transmission signal S1,opt is measured by a second optical sensor device comprising photo diode 102. The information about the amplified optical power of the optical transmission signal S1,opt may be transmitted to the control device 203 as indicated by the dashed arrow. The amplified optical signal S1,opt is output at an output port of the first stage 201 of the two-stage optical amplifier 100.
The optical transmission signal S1,opt that has been amplified by the first stage 201 passes through the VOA 106, which connects the two stages of the optical amplifier, and is then fed to an input port of the second stage 202 of the optical amplifier 100. The VOA 106 may be configured to level the gain of the optical amplifier 100 at differing signal wavelengths in order to achieve gain flatness, i.e., the VOA may be configured as a gain equalization filter. Alternatively, gain flattening may be achieved by a passive filter incorporated in the setup and called gain-flattening filter (GFF).
The second stage 202 of the two-stage optical amplifier 100 reveals the same structure as the first stage 201. Thus, corresponding components are designated by identical reference numbers. Also, the general functionality of the components of the second stage 202 is essentially identical to the functionality of the first stage 201.
As shown in
Even in an embodiment in which the control device 203 is comprised by the optical amplifier 100 or is located, as an external device, near the optical amplifier 100, further information may be exchanged between the control device 203 and a higher order management device or system which may be configured to control the whole optical transmission link comprising the optical amplifier 100 or even a plurality of optical transmission links. Also, for this purpose, a management channel may be used as explained above.
As explained above, the method for monitoring a pump laser according to the invention can be carried out during normal operation of the optical transmission link. It shall be assumed that the respective optical transmission link (not shown) is operated, during its normal operation, in such a way that the pump laser 116 of the optical amplifier 100 to be monitored is operated at an operating point OP0 shown in
However, as already explained above, in order to assess whether the respective pump laser 116 is still able to be operated at an operating point at the target value Ptgt of the optical pump power, it is necessary to gain sufficient information about the LI curve that characterizes the current status of the pump laser 116. For this purpose, the pump laser 116 to be monitored is operated at at least two different operating points. Using the pairs of values (in the LI curve) of the at least two operating points, the course of the LI curve in the current status of the pump laser 116 can be assessed by using a mathematical regression analysis.
In order to control the operating point of the pump laser 116 to be monitored, the control device 203 creates an interaction current control signal Sic, which is fed to the respective pump laser 116 or the respective driver circuit. In this way, the control device may shift the operating point to at least one further operating point, e.g., one of the operating points OP−2, OP−1 at a lower optical pump power or OP1, OP2, OP3 at a higher pump power (relative to the optical pump power of the operating point OP0) shown in
In one embodiment, one or more shifted operating points OPi (e.g., the operating points OPi (−2≤i≤3, i≠0) as shown in
In another embodiment, one or more shifted operating points OPi may be adjusted by controlling the optical pump power to a different, shifted value. In this case, the control device 203 outputs control information to the optical amplifier 100 comprising information about the desired optical output power. In this case, an already existing feedback control loop, implemented in the control device 203, may be used.
In a further embodiment, one or more shifted operating points OPi may be adjusted by controlling the optical power of an optical transmission signal at the output port of the optical amplifier 100 or, preferably, at the output port of the respective stage 201, 202 of the optical amplifier 100 comprising the pump laser 116 to be monitored to a different, shifted value. For this purpose, a feedback control loop might be used, which is usually present in an optical amplifier, wherein the feedback control loop may be implemented by the control device 203. In this case, the control device 203 adjusts the interaction current by monitoring the corresponding signal Swp of the second optical sensor device and creating and feeding an appropriate injection current control signal Sic to the respective pump laser 116 to be monitored.
As, in the simplest embodiment, shifting of the operating point (by statically adjusting one or more shifted operating points OPi or dynamically (slowly enough) shifting the operating point and measuring respective pairs of values of the current LI curve) is carried out so slowly that the gain of the optical amplifier (i.e., at least of the stage of the optical amplifier that includes the pump laser to be monitored if the amplifier comprises more than one stage) is correspondingly varied, a maximum shift as compared to the normal operating point during the current normal operation of the optical transmission link should not be exceeded. This maximum shift should be determined in such a way that the optical transmission link is still capable of receiving the amplified optical transmission signal S1,opf correctly, i.e., with a sufficient quality at the respective end of the optical transmission link.
In order to guarantee a sufficient transmission quality, one or more parameters characterizing the transmission quality of the optical transmission link can be monitored when carrying out the monitoring method for one or more pump lasers in one or more optical amplifiers. Likewise, it is also possible to determine a maximum shift of an operating point in advance so that it is not necessary to monitor the transmission quality of the optical transmission link while carrying out the monitoring method for the pump laser(s). In both cases, the one or more parameters characterizing the transmission quality should not be changed by more than a predetermined acceptable amount or should not exceed or not fall below a predetermined threshold, respectively.
The one or more parameters characterizing the transmission quality may comprise the optical power of the optical transmission signal S1,opt at an output port of the optical amplifier 100 including the pump laser to be monitored (or at any position within the transmission link downstream the point at which the pump power of the pump laser to be monitored is coupled to the transmission path) or the bit error rate (BER) at the end of the optical transmission link. According to a further alternative, the error vector magnitude (EVM) may be used as a parameter that characterizes the transmission quality. The optical power of the optical transmission signal carrying user data should not fall below a threshold value while the BER or EVM should not exceed a predetermined threshold in order to ensure a sufficient transmission quality. Especially, if the acceptable range of the shift of the operating point is low, the value pairs of more than two, preferably a plurality of at least five, most preferably a plurality of at least ten at least slightly shifted operating points should be determined in order to reach a sufficiently high approximation of the current LI curve by means of the regression analysis.
As explained above, the (values of the) one or more parameters characterizing the transmission quality may be supplied to the control device 203 via a management channel in case the control device 203 monitors whether the values of these parameters remain in an acceptable range (or whether given thresholds are exceeded).
As explained above, the calculations and application of the regression method may be carried out by the control device 203. It is, however, also possible to transmit the values defining the two or more operating points to a further device, e.g., a higher order management device or system, which is configured to carry out the calculations required and any process necessary in order to assess aging characteristics of the pump laser to be monitored.
As a simple example, in
It is, of course, also possible to acquire, as information on the stage of aging of the pump laser to be monitored, a maximum value of the optical output or pump power at the predetermined maximum value of the laser injection current Ial in its current stage of aging or information dependent on this maximum value of the optical output power.
In another embodiment, the optical transmission link is an optical WDM transmission link which, in the current operating mode, is operated with a number of optical channels smaller than a specified maximum number of channels. The information on the stage of aging of the pump laser to be monitored 116 may be the maximum number of channels or the maximum capacity by which the optical WDM transmission link can be expanded without exceeding a given maximum value of the injection current of the respective optical amplifier 100. Optionally, the information on the stage of aging of the pump laser 116 to be monitored comprises the information whether it is still possible to expand the optical WDM transmission link to the given maximum number of channels without exceeding a given maximum value of the injection current.
In the embodiments shown in
In this way, either one or, preferably, both of the pump lasers can be monitored with respect to their stage of aging as the values defining the operating points during normal operation as well as the values defining the changed operating points can be determined.
Compensating the change of the operating point of a pump laser in an optical amplifier that comprises two or more pump lasers (either in the same stage or in different stages) leads to the advantage that the gain of the amplifier can be kept constant so that the optical power of the optical transmission signal at the amplifier output port can be kept constant. In this way, a deterioration of the transmission quality can be minimized.
However, as will be explained with reference to
As shown in connection with this simple example, it is possible to essentially maintain the function of an optical transmission link comprising one or more optical amplifiers (which as a whole comprise two or more pump lasers) while varying the optical pump power, i.e. the operating point, of a first pump laser as this variation is compensated by correspondingly changing the optical pump power, i.e. the operating point, of a second pump laser. If the pump lasers are included in the same optical amplifier, the optical power of the optical transmission signal at the output port thereof can be maintained. Of course, the optical power of the two pump lasers can be varied in a predetermined range as long as the (total) gain of the optical amplifier can be maintained (and the noise properties of the amplifier are not deteriorated beyond a predetermined threshold value of a parameter that describes the noise properties, e.g. the noise figure). In this way, the transmission quality of the respective optical transmission link can be maintained.
The management system 205 may be configured to carry out the method described in connection with the embodiments shown in
The compensation of a pump power caused by the shift of the operating point of a respective pump laser through an “opposite” shift of the operating point of a further pump laser may be carried out analogously as previously described in connection with
As mentioned above, the method of monitoring the state of aging of at least one pump laser and a respective compensation may be carried out in a more complex way. Especially, the one or more first pump lasers may be shifted in their operating points according to predetermined specifications. In order to compensate these shifts, one or more second pump lasers may be shifted in the corresponding opposite direction along the LI curve. At least for all of the first pump lasers the method of monitoring the stage of aging is carried out by determining the values of the shifted operating points and approximating the actual current LI curve by carrying out a regression analysis. Of course, also for the second pump lasers that are involved in compensating the shift of the first pump lasers, the method of monitoring the stage of aging may be carried out.
The specifications for the shift of the operating points of the at least one first pump laser are communicated, in case of a structure according to
In general, such a generalized control device and its functionality may be distributed and realized in two or more devices as already indicated above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22180246.5 | Jun 2022 | EP | regional |
