Patent Description:
In order to optimize the investment in optical fiber links, it is desirable to increase the capacity of the optical fiber links. This can be achieved by increasing the spectral efficiency (SE) of the signals to be transmitted through the optical fiber links.

A common way to achieve the increase in the SE is to use more efficient modulation formats for the information to be transmitted. This can be used in conjunction with the wavelength division multiplexing (WDM) system.

Furthermore, the space division multiplexing (SDM) system is being used in order to increase the capacity of transmission through a single fiber maintaining the possibility of the transmission over a long distance.

The SDM system can be implemented with a multi core fiber (MCF) and a multicore (MC) erbium doped fiber amplifier (EDFA). The MCF includes several cores conducting optical signals within the same fiber. The MC-EDFA is a fiber amplifier with an MCF as a gain medium (see Patent Literature <NUM>, for example). The MC-EDFA consists in pumping individually the core of a single MCF gain medium with separate pumping lasers by a scheme of direct core pumping. The MC-EDFA system demonstrates the possibility to multiply the system capacity by the number of cores of the MCF. By using the MCF, it is possible to use the multiplicity of cores to spatially multiplex signals in addition to the WDM in each core and to increase the capacity to be transmitted through fibers without sacrificing the transmission distance.

In addition, a reduction in the power consumption of optical amplification is being pursued actively. The reduction in the power consumption at amplifiers is of primary importance for the system scalable to very high capacity and for the reduction in operation expenditure (OPEX) through the reduction in electricity consumed.

Several schemes have been proposed for standard WDM systems and even for systems using the SDM technology taking advantage of the spatial multiplicity.

Patent Literature <NUM> (PTL2) discloses an optical amplification system that includes an excitation light distribution unit, gain block units, and a node device control unit.

The excitation light distribution unit includes an excitation laser light source unit, a variable branching unit, and an excitation light distribution control unit. The variable branching unit branches excitation light that is outputted by the excitation laser light source unit and distributes the branched light to the gain block units.

Each of the gain block units combines the excitation light distributed from the variable branching unit and an optical signal inputted from the corresponding route by using a coupler, and then amplifies the optical signal with an amplification medium. Each of the gain block units includes an active fiber such as an Erbium doped fiber (EDF), isolators, photodetectors, a variable optical attenuator (VOA), an individual gain block control unit, and optical couplers.

The individual gain block control unit controls the VOA based on monitoring information of the photodetectors at respective sections. By controlling the VOA, the individual gain block control unit in each gain block can change excitation light power inputted to the active fiber.

The optical amplification system of PTL2 does not need to include an excitation light source for each gain block, and can reduce the number of arranged excitation light sources by integrating the excitation light sources. It is said that, as a result, the optical amplification system of PTL2 leads to the effect of reducing the cost and power consumption of an optical amplification system provided with optical amplifiers.

Another example of the SDM system is described in Patent Literature <NUM> (PTL3). A fiber type optical amplifier described in PTL3 has a redundant structure including two amplifying mediums and two excitation light sources.

In the fiber type optical amplifier described in PTL3, excitation light waves outputted by excitation light sources are polarized by wave polarization controllers, and they are inputted into a multi/demultiplexer with a polarization state normal to each other. After the polarization synthesis, they are distributed and inputted into rare-earth doped optical fibers through wavelength multiplexing multi/demultiplexers.

According to the fiber type optical amplifier described in PTL3, it is said that it becomes possible to constitute a fiber type optical amplifier without level variations caused by the synthesis of the two excitation-light beams.

<CIT> discloses an optical relay provided with: an excitation means which generates excitation light in a single wavelength range; a first light amplification means which is excited by the excitation light and the amplification band of which is a first wavelength range; and a second light amplification means which is excited by the excitation light and the amplification band of which is a second wavelength range different from the first wavelength range.

<CIT> discloses an optical amplifier arrangement for a WDM system using a common pump source connected to an input of a splitter deploying pump light via variable optical attenuators to a plurality of optical amplifiers.

<CIT> discloses an optical amplification system.

It is difficult to reduce the power consumption of the optical amplification by using common pumping among more than one EDFAs. The main difficulties consist in the structure of the amplifier and taking into account the difference in the required pumping power between EDFAs. For instance, the differences between EDFAs in the required pumping power may result from different device tolerances, or the aging of the amplifier or the transmission line to which the amplifier is connected. In addition, the differences between EDFAs in the required pumping power may originate from different optical bandwidth or the number of WDM channels amplified by the different EDFA. Furthermore, considering the phenomena associated with the aging of the dynamic bandwidth reconfiguration, the levels of the required pumping power between EDFAs will change during the lifetime of the amplifiers.

As mentioned above, the fiber type optical amplifier described in PTL3 uses pump-sharing techniques where the pumping power is split between EDFAs using optical couplers with fixed ratio. However, when the fiber amplification medium of the EDFA or the transmission fiber ages, or the transmission fiber is repaired in field by splicing, the required pumping power between EDFAs varies. Such a structure cannot change the distribution of the optical pumping power between EDFAs; therefore, the same power as the maximum required pumping power is distributed between EDFAs, and the output power of EDFA requiring less power has to be reduced afterwards. This causes inefficient power and additional power consumption. The same phenomenon occurs when one EDFA is amplifying significantly more WDM channels than another is. This will happen for different routes in traffic, through which optical signals amplified by EDFA are transmitted.

In addition, the fiber type optical amplifier described in PTL3 includes two amplifiers and two pump lasers. The output beams of the pump lasers are combined in polarization and then split between two fiber amplifiers. The output beams of the two pump lasers are controlled in polarization before being combined in order to minimize their interference. However, the polarization of the combined output beams cannot be controlled. Consequently, only the respective levels of the laser diodes can be controlled. The pumping power to be supplied to the fiber amplifiers cannot be adjusted independently; therefore, the power consumption of the amplifier cannot be reduced.

In the optical amplification system disclosed in PTL2, the excitation light power inputted into the active fiber can be changed in each gain block by controlling the variable optical attenuator (VOA), as mentioned above. However, the use of the VOA causes unnecessary light to be generated by the excitation laser light source unit.

As mentioned above, there has been the problem that it is necessary to reduce the power consumption of a plurality of optical amplifiers when there is a difference in the required pumping power between the plurality of optical amplifiers.

An exemplary object of the invention is to provide an optical amplifying apparatus and a method of amplifying an optical signal, which solve the above-mentioned problem that it is necessary to reduce the power consumption of a plurality of optical amplifiers when there is a difference in the required pumping power between the plurality of optical amplifiers.

An optical amplifying apparatus according to the invention is defined by claim <NUM>.

A method of amplifying an optical signal according to the invention is defined by claim <NUM>.

An exemplary advantage according to the present invention is that it is possible to reduce the power consumption of a plurality of optical amplifiers even when there is a difference in the required pumping power between the plurality of optical amplifiers.

The example embodiments of the present invention will be described with reference to drawings below. The arrow direction in the drawings denotes an example of direction and does not limit the direction of signals between blocks.

<FIG> is a block diagram illustrating the configuration of an optical amplifying apparatus in accordance with a first example embodiment of the present invention. The optical amplifying apparatus <NUM> includes a plurality of optical amplifiers (optical amplifying means) <NUM>, a plurality of laser light sources (laser light generating means) <NUM>, at least one optical coupling device (optical coupling means) <NUM>, and a controller (controlling means) <NUM>.

The plurality of optical amplifiers <NUM> amplifies a plurality of optical signals. Each of the plurality of optical amplifiers <NUM> includes a gain medium. The plurality of laser light sources <NUM> generates a plurality of laser beams.

At least one optical coupling device <NUM> couples the plurality of laser beams variably in accordance with a coupling factor and outputs a plurality of excitation light beams. Each of the plurality of excitation light beams excites the gain medium. The controller <NUM> controls the coupling factor and an output power of each of the plurality of laser light sources <NUM>.

As mentioned above, the optical amplifying apparatus <NUM> according to the present example embodiment includes the plurality of laser light sources <NUM>, and the optical coupling device <NUM> included in the optical amplifying apparatus <NUM> couples the plurality of laser beams variably in accordance with the coupling factor. As a result, it is possible to reduce the power consumption of a plurality of optical amplifiers even when there is a difference in the required pumping power between the plurality of optical amplifiers.

The at least one optical coupling device <NUM> may include a plurality of optical coupling devices. In this case, the controller <NUM> can control the coupling factor of each of the plurality of optical coupling devices.

The gain medium can be a core included in a multicore fiber. In this case, each of the plurality of laser light sources <NUM> can directly excite the gain medium by each of the plurality of excitation light beams.

Alternatively, the gain medium can be a multicore fiber. In this case, each of the plurality of laser light sources <NUM> can excite the gain medium by pumping a clad of the multicore fiber. Each of the plurality of excitation light beams can be a multimode light beam.

In addition, the number of the plurality of optical amplifiers <NUM> represented by positive integer N, the number of the at least one optical coupling device <NUM> represented by positive integer M, and the number of the plurality of laser light sources <NUM> represented by positive integer L can satisfy the following relational expressions: <MAT> <MAT> and <MAT>.

In this case, a relational expression of L = N = M<NUM>/<NUM> can be satisfied. Alternatively, a relational expression of L = N = M/<NUM> can be satisfied.

Next, a method of amplifying an optical signal in accordance with the present example embodiment will be described.

In the method of amplifying an optical signal, a plurality of laser beams is generated. The plurality of laser beams is coupled variably in accordance with a coupling factor, by which a plurality of excitation light beams is generated. By the plurality of excitation light beams, a plurality of gain mediums through which a plurality of optical signals pass is excited.

In addition, the coupling factor and an output power of each of the plurality of laser beams are controlled.

As mentioned above, according to the optical amplifying apparatus <NUM> and the method of amplifying an optical signal according to the present example embodiment, it is possible to reduce the power consumption of a plurality of optical amplifiers even when there is a difference in the required pumping power between the plurality of optical amplifiers.

Next, a second example embodiment of the present invention will be described.

<FIG> is a block diagram illustrating the configuration of an optical amplifying apparatus <NUM> in accordance with the second example embodiment of the present invention. The optical amplifying apparatus <NUM> includes two optical amplifiers. The two optical amplifiers are composed of respective gain mediums <NUM> and <NUM>. Each of the gain mediums <NUM> and <NUM> may be made of an Erbium doped single mode fiber. The optical amplifier may include an isolator, a WDM coupler, and an equalization filter.

The gain medium <NUM> amplifies an optical signal that is provided through an optical fiber <NUM> and that has passed through a power monitor <NUM>. The power monitor <NUM> taps a small portion of the light coming from the optical fiber <NUM> and converts it into an electrical monitor signal using a photodiode. Typically, the power monitor <NUM> may tap <NUM>% of the light to be inputted into the gain medium <NUM> in order to monitor the incoming optical power by the generated electrical monitor signal. Similarly, a power monitor <NUM> generates an electrical monitor signal that depends on the output optical power of the gain medium <NUM>. The optical signal amplified by the gain medium <NUM> and monitored by the monitor <NUM> is output to an optical fiber <NUM>.

Identically, the gain medium <NUM> amplifies an optical signal that is inputted from an optical fiber <NUM> and monitored by a power monitor <NUM>. The optical signal amplified by the gain medium <NUM> is monitored by a power monitor <NUM> and output to an optical fiber <NUM>.

The pumping light for optical amplification through the respective gain mediums <NUM> and <NUM> is provided through respective fibers <NUM> and <NUM>. The pumping light beams are output from a tunable coupling device <NUM>. The tunable coupling device <NUM> has two input ports to which respective fibers <NUM> and <NUM> are connected. The fibers <NUM> and <NUM> are connected to the outputs of respective pump lasers <NUM> and <NUM>.

The respective optical powers in the respective fibers <NUM>, <NUM>, <NUM>, and <NUM> are represented by P<NUM>, P<NUM>, P<NUM>, and P<NUM>. If the coupling factor of the tunable coupling device <NUM> is represented by C, and the transmission factor is represented by T, the relation between the powers becomes typically as follows: <MAT>.

The transmission factor T is high for low loss devices, typically above <NUM>. The coupling factor C is controlled for the tunable coupling device <NUM>, and it is tuned between <NUM> and <NUM>. The powers P<NUM> and P<NUM> are the respective output powers of the respective pump lasers <NUM> and <NUM>. The pumping power provided to the respective gain mediums <NUM> and <NUM> are the respective powers P<NUM> and P<NUM>.

A control unit <NUM> controls the pump lasers <NUM> and <NUM> as well as the coupling factor C of the tunable coupling device of <NUM>. The control is performed according to amplifier setting parameters provided by an interface <NUM>, according to operating information of the pump lasers <NUM> and <NUM>, and according to power monitor information provided by the power monitors <NUM>, <NUM>, <NUM>, and <NUM>.

The control according to the output powers of the monitors <NUM> and <NUM> is performed individually for the two gain mediums <NUM> and <NUM>. The control of the pump lasers <NUM> and <NUM> is performed individually depending on the monitored values and the amplifier setting parameters; consequently, one of the pump lasers may be turned off. In this case, the control unit <NUM> stores the updated operation values, and the control is performed according to the last stored values for the pump laser that is turned off.

Alternatively, the control unit <NUM> may perform the control uniquely according to the amplifier setting parameters provided by the interface <NUM>, according to the operating information of the pump lasers <NUM> and <NUM>, and according to power monitor information provided by the power monitors <NUM> and <NUM>. In this case, the information provided by the power monitors <NUM> and <NUM> is not used for the control.

The two amplifiers that include two gain mediums <NUM>, <NUM>, and are controlled by the control unit <NUM> are typically placed in the same location, so that the loss for transmitting the pumping power through short-distance fibers is negligible. The two fibers with respective optical fibers <NUM>, <NUM> for input and respective optical fibers <NUM>, <NUM> for output may be configured in unidirectional manner.

Alternatively, the fibers may be configured in bidirectional manner, in which respective optical fibers <NUM>, <NUM> are fibers for input, and respective optical fibers <NUM>, <NUM> are fibers for output. In this case, the power monitor <NUM> monitors the output power of the gain medium <NUM>, and the power monitor <NUM> monitors the input power to the same gain medium. The gain medium and components attached to it such as an isolator and a WDM coupler are configured according to the propagation direction of the optical signal.

According to the control by the control unit <NUM>, the pumping light beams of the pump lasers <NUM> and <NUM> are shared by the tunable coupling device <NUM>. The output of the pump lasers <NUM> and <NUM> is set for minimum power consumption of the pump lasers120 and <NUM>.

According to the control by the control unit <NUM>, the amplification by the gain medium <NUM> and <NUM> does not require more pumping power than necessary. The control is still valid when the input signals in the optical fibers <NUM> and <NUM> are reduced, or when the required output powers increase.

According to the present example embodiment, the power consumption of two optical amplifiers including the gain mediums <NUM>, <NUM> is reduced during the lifetime of the amplifiers. The reduction in power consumption is effective in the ageing of the system that causes the differences in the required optical pumping power, when the amplifiers are used for different bandwidths, or when the bandwidth changes over the lifetime of a network.

In a specific configuration of <FIG>, each of the pump lasers <NUM> and <NUM> is an identical type of laser. Their characteristics are similar. If the required total pumping power for the gain mediums <NUM> and <NUM> is moderate, the control unit <NUM> turns off either, uses the other one to generate the total pumping power, and configures the tunable coupling device <NUM> to share the pumping light adequately among the gain mediums <NUM> and <NUM>. For instance, the pump laser <NUM> will be on, and the pump laser <NUM> will be turned off. If the characteristics of the laser <NUM> degrade due to ageing, the efficiency of the laser <NUM> that is not used will be superior; consequently, the control unit <NUM> will gradually turn the laser <NUM> on and use more power from it. According to their relative efficiency and the required pumping power for the gain mediums <NUM> and <NUM>, the pump laser <NUM> may be turned off, and the pumping power may be generated by the laser <NUM> only. The control unit <NUM> controls adequately the tunable coupling device <NUM>.

Alternatively, the pump laser <NUM> is a laser capable of outputting very high pumping power, and the pump laser <NUM> is a laser capable of outputting moderate laser power only. That is, the pump laser <NUM> and the pump laser <NUM> are more than one type of lasers and have different specifications. For instance, the laser <NUM> may be a cooled pump laser, and the laser <NUM> may be an uncooled pump laser. Depending on the package and the chip structure, the laser <NUM> will have a better efficiency.

If the bandwidth of the optical signal to be amplified by the gain mediums <NUM> and <NUM> is limited, low pumping power only will be required. In this case, the control unit <NUM> will turn on the pump laser <NUM> only, and the pump laser <NUM> will be turned off. The tunable coupling device <NUM> will be configured adequately by the control unit <NUM>.

If the bandwidth of the optical signal to be amplified increases, more pumping power will be required. When the power limit of the pump laser <NUM> is reached and therefore the efficiency turns to zero, the control unit <NUM> turns the pump laser <NUM> on to provide more pumping power, sets the pump lasers <NUM> and <NUM> at the minimum power consumption setting, and adjusts the coupling factor of the tunable coupling device <NUM> adequately.

In a specific configuration of <FIG>, the amplifiers including the gain mediums <NUM> and <NUM> are used in auto power control manner. The control by the control unit <NUM> is performed according to the amplifier setting parameters provided by the interface <NUM>, according to the operating information of the pump lasers <NUM> and <NUM>, and according to the power monitor information provided by the power monitors <NUM> and <NUM>. The information provided by the power monitors <NUM> and <NUM> is not used.

Alternatively, the amplifiers including the gain mediums <NUM> and <NUM> are used in auto gain control manner. The control by the control unit <NUM> is performed according to the amplifier setting parameters provided by the interface <NUM>, according to the operating information of the pump lasers <NUM> and <NUM>, and according to the power monitor information provided by the power monitors <NUM>, <NUM>, <NUM>, and <NUM>. The information on the ratio of the power monitor <NUM> to the power monitor <NUM> provides the information on the gain by the gain medium <NUM>. Identically, the information on the ratio of the power monitor <NUM> to the power monitor <NUM> provides the information on the gain by the gain medium <NUM>.

Alternatively, the amplifiers including the gain mediums <NUM> and <NUM> are used in auto current control manner. The control by the control unit <NUM> is performed according to the amplifier setting parameters provided by the interface <NUM> and according to the operating information of the pump lasers <NUM> and <NUM>. The information provided by the power monitors <NUM>, <NUM>, <NUM>, and <NUM> is not used for the control. The target for the amplification is set using the sum of the driving currents of the pump lasers <NUM> and <NUM>.

Alternatively, the target for the amplification is set using the sum of the driving currents of the pump lasers <NUM> and <NUM> taking into consideration the coupling factor C and its complementary (<NUM>-C).

In the case of controlling 2N amplifiers, where N is a positive integer, the configuration illustrated in <FIG> can be obviously used in a parallel manner. In this case, N amplification systems will be employed each of which is similar to one described in <FIG>.

<FIG> is a block diagram illustrating the configuration of a tunable optical coupler <NUM>, which can be used as the tunable coupling device <NUM> illustrated in <FIG>. The tunable optical coupler <NUM> has two input fibers <NUM> and <NUM> and two output fibers <NUM> and <NUM>. The tunable optical coupler <NUM> includes two coupling fibers <NUM> and <NUM> whose cores can perform evanescent coupling.

The coupling factor is tuned by a controller <NUM>, which is actuated through an external signal. When the tunable optical coupler <NUM> is used as the tunable coupling device <NUM> illustrated in <FIG>, the controller <NUM> is activated by the control unit <NUM> in <FIG>. The controller <NUM> can be constituted by a step motor to displace the coupling fibers <NUM> and <NUM>. Typically, such fibers have very low insertion loss in the order of <NUM>. The control of the coupling factor requires very low power consumption only when the coupling factor is changed.

<FIG> is a block diagram illustrating the configuration of a tunable coupling apparatus <NUM>, which can be used as the tunable coupling device <NUM> illustrated in <FIG>. The tunable coupling apparatus <NUM> includes a polarization beam combiner <NUM>, a polarization controller <NUM>, and a polarization beam splitter <NUM>. The tunable coupling apparatus <NUM> has two input fibers <NUM> and <NUM> that are the inputs of the polarization beam combiner <NUM>. The tunable coupling apparatus <NUM> has two output fibers <NUM> and <NUM> that are the output fibers of the polarization beam splitter <NUM>.

Two linear polarizations are combined orthogonally by the polarization beam combiner <NUM>. When the tunable coupling apparatus <NUM> is used as the tunable coupling device <NUM> illustrated in <FIG>, the input fibers <NUM> and <NUM> are polarization maintaining fibers, so that the linear polarization outputs of the pump lasers <NUM> and <NUM> are maintained and then combined. The coupling factor of the tunable coupling apparatus <NUM> is tuned by the polarization controller <NUM>, which is actuated through an external signal. When the tunable coupling apparatus <NUM> is used as the tunable coupling device <NUM> illustrated in <FIG>, the polarization controller <NUM> is activated by the control unit <NUM> in <FIG>. The polarization controller <NUM> rotates the polarization of the light beam output by the polarization beam combiner <NUM> against the axes of the polarization beam splitter <NUM>. As a result, the light beam resulting from combining the two input light beams is mapped on the two orthogonal polarization axes of the polarization beam splitter <NUM>.

<FIG> is a block diagram illustrating the configuration of a Mach-Zehnder interferometer <NUM>, which can be used as the tunable coupling device <NUM> illustrated in <FIG>. The Mach-Zehnder interferometer <NUM> has two input fibers <NUM>, <NUM>, and two output fibers <NUM>, <NUM> through which the constructive and destructive interference outputs propagate. The Mach-Zehnder interferometer <NUM> includes an interferometer structure <NUM> and a heater <NUM> that is placed on one arm of the interferometer structure <NUM>. The interferometer structure <NUM> may be made of a fiber to reduce the insertion loss. The coupling factor is tuned by a controller <NUM>. The controller <NUM> is actuated through an external signal, and it controls the heater <NUM> accordingly. Depending on the setting of the heater <NUM>, the difference in phase appears between both arms of the interferometer structure <NUM>.

When the Mach-Zehnder interferometer <NUM> is used as the tunable coupling device <NUM> illustrated in <FIG>, the controller <NUM> is activated by the control unit <NUM> in <FIG>. In this case, the pump lasers <NUM> and <NUM> are chosen so that their wavelengths may be different from each other. For easier control, the wavelengths can be set so that the wavelength interval may be greater than a half of the free spectral range of the interferometer. According to the phase difference tuned by the heater <NUM>, the combination of the two input powers is output through the output fiber <NUM> as a constructive interference output, and its complementary part is output through the output fiber <NUM> as a destructive interference output.

<FIG> is a block diagram illustrating the configuration of a laser apparatus <NUM> that can be used as any one of the pump lasers <NUM> and <NUM> illustrated in <FIG>. The laser apparatus <NUM> includes two pump laser diodes <NUM> and <NUM>. The output beams of the pump laser diodes <NUM> and <NUM> are combined by a polarization beam combiner <NUM> and output through a fiber <NUM>. A controller <NUM> controls the output of both pump laser diodes <NUM> and <NUM> according to an external signal and returns monitor values of the pump laser diodes <NUM> and <NUM> to external control systems.

The pump laser diodes <NUM>, <NUM>, as well as the controller <NUM> and the polarization beam combiner <NUM> may be integrated in a package <NUM>. When the laser apparatus <NUM> is used as the pump laser <NUM> illustrated in <FIG>, the controller <NUM> drives both pump laser diodes <NUM> and <NUM> according to the control by the control unit <NUM> in <FIG>. In this manner, the redundant pump laser diodes can be controlled as a single laser in the present example embodiment, which also increases the reliability of the optical amplifying apparatus <NUM> in <FIG>.

<FIG> is a block diagram illustrating the configuration of an excitation apparatus <NUM> that can be used in place of the pump lasers <NUM> and <NUM> as well as the tunable coupling device <NUM> illustrated in <FIG>. The excitation apparatus <NUM> includes two laser diode chips <NUM>, <NUM> and a tunable coupling device <NUM>. The excitation apparatus <NUM> is mounted inside a package <NUM>. The excitation apparatus <NUM> has two outputs <NUM> and <NUM>, which are the outputs of the tunable coupling device <NUM>.

The driving of the laser diode chips <NUM> and <NUM> is performed by an external control system. The driving conditions of the laser diode chips <NUM> and <NUM> can be set by the external control system. The tunable coupling device can be also controlled from outside of the excitation apparatus <NUM>. For instance, the tunable coupling device <NUM> can be realized with a tunable coupler, which can be integrated with the laser diode chips <NUM> and <NUM>, and actuated by currents from the external control system.

When the excitation apparatus <NUM> is used in place of the pump lasers <NUM> and <NUM> as well as the tunable coupling device <NUM> in <FIG>, the laser diode chips <NUM> and <NUM> as well as the tunable coupling device <NUM> are controlled in the same manner by the control unit <NUM>. The driving conditions of the laser diode chips <NUM> and <NUM> are monitored by the control unit <NUM>. The use of the excitation apparatus <NUM> enables lower cost through the integration in addition to the power reduction due to controlling the amplifiers.

As mentioned above, according to the optical amplifying apparatus <NUM> of the present example embodiment, it is possible to reduce the power consumption of a plurality of optical amplifiers even when there is a difference in the required pumping power between the plurality of optical amplifiers.

Next, a third example embodiment of the present invention will be described. <FIG> is a block diagram illustrating the configuration of an optical amplifying apparatus <NUM> in accordance with the third example embodiment of the present invention.

The optical amplifying apparatus <NUM> includes four passive gain blocks denoted by <NUM>, <NUM>, <NUM>, and <NUM>. Each of these passive gain blocks <NUM> to <NUM> is similar to an apparatus in which the gain medium <NUM> is integrated with the power monitors <NUM> and <NUM> illustrated in <FIG>. Each of the passive gain blocks has a single mode fiber input and a single mode fiber output. The power monitors of each passive gain blocks are provided for a control unit <NUM>.

The control unit <NUM> performs a control in a similar manner to the control unit <NUM>; however, it is configured to operate with more passive gain blocks, pump lasers, and tunable coupling devices. The control unit <NUM> also contains the parameters for setting the amplifiers that use the passive gain blocks <NUM> to <NUM>.

The optical pumps for the passive gain blocks <NUM> and <NUM> are provided by the tunable coupling device <NUM> through its two outputs. The optical pumps for the passive gain blocks <NUM> and <NUM> are provided by the tunable coupling device <NUM> through its two outputs. The two inputs of the tunable coupling device <NUM> are connected to one output of the tunable coupling device <NUM> and one output of the tunable coupling device <NUM>. The two inputs of the tunable coupling device <NUM> are connected to one output of the tunable coupling device <NUM> and one output of the tunable coupling device <NUM>. The two inputs of the tunable coupling device <NUM> are connected to the pump lasers <NUM> and <NUM>. The two inputs of the tunable coupling device <NUM> are connected to the pump lasers <NUM> and <NUM>.

The pump lasers <NUM> to <NUM> are identical to the pump lasers <NUM> and <NUM> illustrated in <FIG>. The tunable coupling devices <NUM> to <NUM> are identical to the tunable coupling device <NUM> in <FIG>.

The control unit <NUM> controls the pump lasers <NUM> to <NUM> as well as the coupling factors C of the tunable coupling devices <NUM> to <NUM>. The control is performed according to the amplifier setting parameters internally provided for the control unit <NUM>, according to the operating information of the pump lasers <NUM> to <NUM>, and according to power monitor information provided by the passive gain blocks <NUM> to <NUM>. The control according to the output powers is performed individually for the four passive gain blocks <NUM> to <NUM>.

The control of the pump lasers <NUM> to <NUM> is performed individually depending on the monitored values and the amplifier setting parameters. At least one of the pump lasers may be turned off. In this case, the control unit <NUM> stores the updated operation values, and the control is performed according to the last stored values for the pump laser that is turned off.

The pumping light of the four pump lasers <NUM> to <NUM> can be distributed for each of the four passive gain blocks <NUM> to <NUM> through a 2x2-trellis structure of the tunable coupling devices. The same structure as that described on <FIG> can be adapted to 2N passive gain blocks, where N is a positive integer, with 2N laser and N<NUM> tunable coupling devices that are organized on N rows of N devices in a trellis manner.

Alternatively, up to three lasers from the pump lasers <NUM> to <NUM> may be removed from the structure illustrated in <FIG> for lower cost. In this case, this structure makes it possible to provide the pumping light correctly for the four passive gain blocks <NUM> to <NUM>.

According to the control by the control unit <NUM>, the pumping light of the pump lasers <NUM> to <NUM> is shared through the tunable coupling device. Each output of the pump lasers <NUM> to <NUM> is set to minimize the power consumption of the pump lasers.

According to the control by the control unit <NUM>, the amplification of the passive gain blocks <NUM> to <NUM> does not require more pump power than necessary. The control is still valid when the input signals in the input optical fibers are reduced, or when the required output powers increase.

According to the optical amplifying apparatus <NUM> of the present example embodiment, the power consumption of four optical amplifiers including the passive gain blocks is reduced during the lifetime of the amplifiers. The reduction of power consumption is effective in the ageing of the system that causes the differences in the required optical pumping power, when the amplifiers are used for different bandwidths, or when the bandwidth changes over the lifetime of a network.

Next, a fourth example embodiment of the present invention will be described. <FIG> is a block diagram illustrating the configuration of an optical amplifying apparatus <NUM> in accordance with the fourth example embodiment of the present invention.

The optical amplifying apparatus <NUM> includes a multicore erbium doped fiber (MC-EDF) <NUM> having seven cores, which are denoted by C01 to C07. Each of the cores is an individual gain medium and is included in the same fiber <NUM>. The input signals are denoted by <NUM> to <NUM> for the cores C01 to C07. The respective output signals are denoted by <NUM> to <NUM>.

A power monitor <NUM> having seven cores monitors individually the power of the input signal into the MC-EDF <NUM>. A monitor signal is generated individually for each input core by the power monitor <NUM>. A power monitor <NUM> having seven cores monitors individually the power of the output signal from the MC-EDF <NUM>. A monitor signal is generated individually for each output core by the power monitor <NUM>. The information of the power monitors is provided for a control unit <NUM>.

The control unit <NUM> performs a control in a similar manner to the control unit <NUM> illustrated in <FIG>, but it is configured to operate with more gain blocks, pump lasers, and tunable coupling devices. The control unit <NUM> also contains the parameters for setting the amplifiers including the cores C01 to C07.

The MC-EDF <NUM> is pumped in individual core pumping manner. The optical pumps of cores C01 to C07 are provided by tunable coupling devices <NUM>, <NUM>, <NUM>, and <NUM>. Three other tunable coupling devices <NUM> to <NUM> are used in conjunction with the tunable coupling devices <NUM> to <NUM> in order to couple and distribute the pumping light generated by three pump lasers denoted by <NUM> to <NUM>.

Each of the pump lasers is connected to the input of the tunable coupling device. Each of the tunable coupling devices <NUM>, <NUM> and <NUM> has two input ports that are connected either to the pump lasers or to the outputs of other tunable coupling devices. Each of the tunable coupling devices <NUM>, <NUM>, <NUM>, and <NUM> has one input port that is connected either to a pump laser or to the output of another tunable coupling device, whereas the other input port is left non-connected. Each of the output ports of the tunable coupling devices denoted by <NUM> to <NUM> is connected either to the core of the MC-EDF <NUM> or to the input port of another tunable coupling device.

Here, the number of the pump laser (<NUM>) is greater than one, but it is smaller than the number of individual gain mediums (<NUM>). The number of the tunable coupling devices (<NUM>) is greater than a half of the number of the gain mediums (<NUM>), but it is smaller than its square (<NUM>).

The control unit <NUM> controls the pump lasers denoted by <NUM> to <NUM> as well as the coupling factors C of the tunable coupling devices denoted by <NUM> to <NUM>. The control is performed according to the amplifier setting parameters provided internally to the control unit <NUM>, according to the operating information of the pump lasers <NUM> to <NUM>, and according to power monitor information provided by the power monitors <NUM> and <NUM>. The control according to the output powers is performed individually on the seven gain mediums C01 to C07 of the MC-EDF <NUM>. The control of the pump lasers denoted by <NUM> to <NUM> is performed individually depending on the monitored values and the amplifier setting parameters. At least one of the pump lasers may be turned off. In this case, the control unit <NUM> stores the updated operation values, and the control is performed according to the last stored values for the pump laser that is turned off.

According to the control by the control unit <NUM>, the pump light beams of the pump lasers denoted by <NUM> to <NUM> are shared through the tunable coupling device. The output of the pump lasers <NUM> to <NUM> is set for minimum power consumption of the pump lasers. According to the control by the control unit <NUM>, the amplification of the cores C01 to C07 of the MC-EDF <NUM> does not require more pumping power than necessary. The control is still valid when the input signals in the input fibers are reduced, or when the required output powers increase.

According to the present example embodiment, the power consumption of the MCF optical amplifier is reduced during the lifetime of the amplifier. The reduction of power consumption is effective in the ageing of the system that causes the differences in the required optical pumping power, when the amplifier is used for different bandwidths, or when the bandwidth changes over the lifetime of a network.

Next, a fifth example embodiment of the present invention will be described. <FIG> is a block diagram illustrating the configuration of an optical amplifying apparatus <NUM> in accordance with a fifth example embodiment of the present invention.

The optical amplifying apparatus <NUM> includes two multicore erbium doped fibers (MC-EDFs) denoted by <NUM> and <NUM>. For instance, the MC-EDF may have <NUM> cores. The MC-EDF <NUM> and <NUM> include multiple core power monitors to monitor individually the power of the input signals and the output signals of them. The information on the power monitors is provided for a control unit <NUM>. The control unit <NUM> performs a control in a similar manner to the control unit <NUM> in the fourth example embodiment.

The MC-EDFs <NUM> and <NUM> are pumped in common cladding pumping manner. The multimode optical pumps for cladding pumping are provided by a tunable coupling device <NUM>. The tunable coupling device <NUM> has two input ports that are connected to multimode pump lasers <NUM> and <NUM>.

The control unit <NUM> controls the multimode pump lasers <NUM> and <NUM> as well as the coupling factor C of the tunable coupling device <NUM>. The control is performed according to the amplifier setting parameters provided internally for the control unit <NUM>, according to the operating information on the multimode pump lasers <NUM> and <NUM>, and according to power monitor information provided by the power monitors integrated in the MC-EDF <NUM> and <NUM>. The control according to the output powers is performed on the MC-EDFs <NUM> and <NUM>.

The control of the multimode pump lasers <NUM> and <NUM> is performed individually depending on the monitored values and the amplifier setting parameters; consequently, one of the multimode pump lasers may be turned off. In this case, the control unit <NUM> stores the updated operation values, and the control is performed according to the last stored values for the multimode pump laser that is turned off. According to the control by the control unit <NUM>, the pump light beams of the multimode pump lasers <NUM> and <NUM> are shared through the tunable coupling device <NUM>, and the output of the multimode pump lasers <NUM> and <NUM> are set for minimum power consumption of the multimode pump lasers.

According to the optical amplifying apparatus <NUM> of the present example embodiment, as is the case with the above-mentioned example embodiments, it is possible to reduce the power consumption of a plurality of optical amplifiers even when there is a difference in the required pumping power between the plurality of optical amplifiers.

Next, a sixth example embodiment of the present invention will be described. In the present example embodiment, an example of the operation of the optical amplifying apparatus <NUM> in accordance with the second example embodiment in <FIG> is given with reference to a flowchart illustrated in <FIG>.

<FIG> is a flowchart to illustrate a method of amplifying an optical signal using the optical amplifying apparatus <NUM>.

First, the control unit <NUM> uses the monitor information that is provided by the power monitors <NUM>, <NUM> and compares them respectively with auto power control targets provided by the interface <NUM>. The control unit <NUM> calculates the ratio of the monitored power to the target power for both power monitors (step <NUM>).

If the two calculated ratios are equal (step <NUM>/Yes), the control unit <NUM> directly goes to step <NUM>. If the calculated ratios are not equal (step <NUM>/No), the control unit <NUM> controls the coupling factor C of the tunable coupling device <NUM> in order to make the ratios equal (step <NUM>). At this point, further checking the equality of the ratios is not required, and the control unit <NUM> can go directly to the step <NUM>. If one of the power control targets is null, the values for the coupling factor are only allowed to be C=<NUM> and C=<NUM> after the control, based on the power monitor that has a non-null target.

The control unit <NUM> ranks the pump lasers <NUM> and <NUM> according to their power consumption efficiency (step <NUM>). The efficiency may be calculated as the ratio of the monitored output power that is provided for the control unit <NUM> by monitor photodiodes integrated in the pump lasers <NUM> and <NUM> to the driving operation current provided for the pump lasers <NUM> and <NUM>. Alternatively, for pump lasers, the ratio may be calculated by dividing the monitored output power by the sum of the operation current and the pump current, or by the sum of their square values.

For pump lasers that have been turned off, the efficiency is calculated using stored value of the latest operation values of the pump lasers. As a result, one of the pump lasers <NUM> and <NUM> has the highest rank, and the other one has the lower rank.

Then the monitored power information provided by the power monitor <NUM> is compared to its target value provided by the interface <NUM> (step <NUM>). If the monitored value is greater than the target value (step <NUM>/Yes), the power output by the pump laser with lowest efficiency is reduced (step <NUM>). If the pump laser with lowest rank reaches an operation current of zero, it is turned off, and its last efficiency value is stored by the control unit <NUM>. If the pump laser with lowest rank has been already turned off, the laser with the next higher rank is controlled instead, to reduce its output power.

Then the monitored power information provided by the power monitor <NUM> is compared to its target value provided by the interface <NUM> again (step <NUM>). If the monitored value is smaller than the target value (step <NUM>/Yes), the power output by the pump laser with highest efficiency is increased (step <NUM>). If the pump laser with highest rank reaches to its above limit of the operation current, the laser with the next highest rank is controlled instead to increase its output power.

Then the monitored power information provided by the power monitor <NUM> is compared to its target value provided by the interface <NUM> again (step <NUM>). If the values are equal (step <NUM>/Yes), the control loop goes back to the step <NUM>. If the values are not equal or outside the tolerance range (step <NUM>/No), the control loop goes back to step <NUM>.

In this manner, the control unit <NUM> in <FIG> is able to control three parameters, that is, the coupling factor C of the tunable coupling device <NUM>, the output of the pump laser <NUM> and the output of the pump laser <NUM> according to two power monitors <NUM> and <NUM>. This makes it possible to achieve and maintain reduced power consumption despite the fact that only the target values and actual values of the outputs of the power monitors <NUM> and <NUM> are used.

In the above description, the step for reducing the output power of the pump laser (step <NUM>) and the step for increasing the output power of the pump laser (step <NUM>) are performed after the step for calculating the ratio of the monitored power to the target power (step <NUM>) and the step for controlling the coupling factor C in order to make the ratios equal (step <NUM>). However, the step for reducing the output power (step <NUM>) and the step for increasing the output power (step <NUM>) may be performed before the step for calculating the ratio (step <NUM>) and the step for controlling the coupling factor C (step <NUM>).

The same control method can be applied to the control unit <NUM> according to the third example embodiment illustrated in <FIG>. In this case, step <NUM> and step <NUM> are repeated for each tunable coupling devices <NUM> to <NUM>. For each tunable coupling device, the ratio between target value and monitor value is compared to a different pair of gain mediums to which the output of the controlled tunable coupling device is connected.

Because there are N<NUM> tunable coupling devices at maximum for N gain mediums, there are sufficient pairs of the tunable coupling devices for possible comparison. In the same case, all pump lasers <NUM> to <NUM> are ranked at step <NUM>; consequently, four ranks are created. The control of the highest rank and lowest rank LDs can be performed with the same monitor information.

An additional step for increasing the optical power output by the laser with highest-rank efficiency and reducing the optical power output by the laser with lowest-rank efficiency in the same proportion can be added between step <NUM> and step <NUM>. The effect of this additional step is to accelerate effectively the reduction in the power consumption of the optical amplifying apparatus.

Next, another example of the operation of the optical amplifying apparatus <NUM> in accordance with the second example embodiment in <FIG> is given with reference to a flowchart illustrated in <FIG>.

First, step <NUM> is identical to step <NUM> of <FIG>. If the two calculated ratios are equal (step <NUM>/Yes), the control unit <NUM> directly goes to step <NUM>. If the calculated ratios are not equal (step <NUM>/No), the control unit <NUM> controls the coupling factor C of the tunable coupling device <NUM> in order to make the ratios equal (step <NUM>). Step <NUM> and step <NUM> are repeated until the ratios calculated at step <NUM> becomes equal.

Steps <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are identical to respective steps <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> of <FIG>. However, after step <NUM> and step <NUM>, the control loop jumps to step <NUM>. After step <NUM>, the control loop jumps to step <NUM>.

In this manner, the control unit <NUM> in <FIG> is able to control three parameters, that is, the coupling factor C of the tunable coupling device <NUM>, the output of the pump laser <NUM> and the output of the pump laser <NUM> according to two power monitors <NUM> and <NUM>. This makes it possible to achieve and maintain reduced power consumption despite the fact that only the target values and actual values of the outputs of the power monitors <NUM> and <NUM>.

Identically to the control method illustrated in <FIG>, the control method illustrated in <FIG> can be applied to the control units <NUM>, <NUM>, and <NUM> illustrated in <FIG>, <FIG>, and <FIG>, respectively.

<FIG> is a diagram illustrating the simulation result of the power consumption of the pump laser <NUM> in <FIG> that is plotted against the output optical pump power. It is clear that there is a threshold effect. This implies that some amount of energy has to be consumed before high power required for optical pumping of a gain medium can be achieved. Therefore, sharing the same laser diode among several gain mediums leads to power reduction in optical amplification.

<FIG> is a diagram illustrating the simulation results of the traffic evolution in the network. In this simulation, the optical amplifying apparatus <NUM> according to the third example embodiment illustrated in <FIG> is used.

Each of the curves <NUM>, <NUM>, <NUM>, and <NUM> represents the traffic that passes through each of the passive gain blocks <NUM>, <NUM>, <NUM>, and <NUM> during the lifetime of the network. At first, the traffic is moderate; consequently, only a portion of the WDM wavelengths available passes through the passive gain block <NUM>. As time passes, the traffic increases, and more wavelengths are being used. That is to say, the wavelengths amplified through the passive gain blocks <NUM>, <NUM>, and <NUM> in addition to the passive gain block <NUM> are gradually used.

<FIG> is a diagram illustrating the simulation results of the power consumption of the optical amplifying apparatus <NUM> according to the third example embodiment illustrated in <FIG>. The simulation is performed under the traffic conditions illustrated in <FIG>.

The curve <NUM> shows the simulation results for the optical amplifying apparatus in <FIG> that is controlled according to the control method similar to that described in <FIG>. For comparison, the curve <NUM> is plotted on the same graph, and it represents the power consumption under the same conditions of the conventional amplification in which each passive gain block has its own dedicated pump laser. For comparison also, the curve <NUM> represents the power consumption under the same conditions of an optical amplifier using optical couplers with fixed ratio.

Because the curve <NUM> is always lower than the curves <NUM> and <NUM>, the optical amplifying apparatus according to the example embodiments makes it possible to reduce efficiently the power consumption of the optical amplification over the lifetime of the network, even when the traffic changes between fibers through which optical signals are amplified.

Next, a seventh example embodiment of the present invention will be described. <FIG> is a block diagram illustrating the configuration of a network system <NUM> using optical amplifying apparatuses in accordance with the seventh example embodiment of the present invention.

The network system <NUM> includes a node apparatus in which transponders are aggregated. The transponders in the node apparatus <NUM> multiplex optical signals in wavelength and also multiplex them spatially through four fiber lines, and links to another node apparatus <NUM> in which transponders are aggregated. Between the node apparatus <NUM> and the node apparatus <NUM>, two groups of erbium doped fiber amplifiers (EDFAs) <NUM> and <NUM> are placed between spans of optical fibers.

Each of the EDFAs <NUM> and <NUM> may be similar to the optical amplifying apparatus <NUM> illustrated in <FIG>. Alternatively, each of the EDFAs <NUM> and <NUM> may be similar to two sets of the optical amplifying apparatus <NUM> illustrated in <FIG> used in parallel. Alternatively, the optical fibers between the node apparatus <NUM> and the node apparatus <NUM> are multicore fibers, and the EDFAs <NUM> and <NUM> are similar to four sets of the optical amplifying apparatus <NUM> illustrated in <FIG> used in parallel. Alternatively, the optical fibers between the node apparatus <NUM> and the node apparatus <NUM> are multicore fibers, and the EDFAs <NUM> and <NUM> are similar to two sets of the optical amplifying apparatus <NUM> illustrated in <FIG> used in parallel.

Depending on variation in traffic carried by the transponders that are aggregated in the node apparatuses <NUM> and <NUM>, the setting of the EDFAs <NUM>, <NUM>, their pump lasers, and tunable coupling devices is changed. The setting may be changed according to the control method described in <FIG>. Alternatively, the setting may be changed according to the control method described in <FIG>. The power consumption of the optical amplification in the network system is reduced accordingly.

In addition, some of the optical fibers between the EDFA <NUM> and the EDFA <NUM> can be repaired and spliced, which causes an additional span loss. In this case, the setting of the EDFA may be changed again according to the control method described in <FIG> or <FIG>. The power consumption of the optical amplification in the network system is reduced accordingly.

<FIG> is a diagram illustrating the simulation results of the time required to setup the network system <NUM> in <FIG>. In this simulation, each of the EDFA <NUM> and the EDFA <NUM> is similar to the optical amplifying apparatus <NUM> in the third example embodiment illustrated in <FIG>. The setup time means the elapsed time from the introduction of the wavelength information of the transponders in the node apparatuses <NUM> and <NUM> to the stabilization of the EDFAs <NUM> and <NUM>.

The bar <NUM> represents the time required when the EDFAs <NUM> and <NUM> are controlled by the procedure described in <FIG>. For reference, the bar <NUM> represents the time required for the same conditions of the network system, but the setup values of the EDFAs <NUM> and <NUM> are determined by scanning the values for the tunable coupling devices and laser diodes in <FIG> over the possible values before starting the operation. It can be seen that the present example embodiment enables the amplifiers in the network system to be set more quickly.

As mentioned above, according to the network system <NUM> using optical amplifying apparatuses in accordance with the seventh example embodiment, it is possible to reduce the setting time of the network where several optical amplifiers are used. That is to say, it is possible to reduce the time between the installation of the network and the actual transmission of optical signals.

Claim 1:
An optical amplifying apparatus, comprising:
a plurality of optical amplifying means for amplifying a plurality of optical signals, each of the plurality of optical amplifying means including a gain medium;
a plurality of laser light generating means for generating a plurality of laser beams;
at least one optical coupling means for coupling the plurality of laser beams variably in accordance with a coupling factor and outputting a plurality of excitation light beams, each of the plurality of excitation light beams exciting the gain medium;
power monitoring means for monitoring an output level of optical signal having passed through each of the plurality of gain mediums; and
controlling means for controlling the coupling factor and an output power of each of the plurality of laser light generating means,
wherein the controlling means is configured, when a power limit of one of the plurality of laser light generating means is reached, to control each output power of the plurality of laser light generating means to minimize the power consumption of the plurality of laser light generating means, and to adjust the coupling factor of the optical coupling means
wherein the controlling means is configured to
calculate a ratio of the output level to a target value of non-null for auto power control,
evaluate efficiency for generating each of the plurality of laser beams,
decrease optical power of laser beam with the efficiency lowest among the plurality of laser beams when the output level being greater than the target value, and
increase optical power of laser beam with the efficiency highest among the plurality of laser beams when the output level being smaller than the target value.