Patent Description:
<FIG> is a diagram illustrating a configuration of a general optical amplification device <NUM> being used in a submarine cable system. The optical amplification device <NUM> is provided in a submarine repeater of the submarine cable system. In order to ensure high reliability required for the submarine repeater, a redundant configuration including four excitation LDs <NUM> to <NUM> (LD <NUM> to LD <NUM>) is employed. The LD means a laser diode. Beams of excitation light generated by the excitation LDs <NUM> to <NUM> are coupled by an optical coupler <NUM> in a preceding stage. Split excitation light excites erbium-doped fibers (EDFs) <NUM> and <NUM> arranged in a middle of each of optical fibers <NUM> and <NUM> via an optical coupler <NUM> in a subsequent stage. The EDFs <NUM> and <NUM> are gain media to be generally used in an optical amplification device of <NUM> band.

Intensity of the excitation light generated by the excitation LDs <NUM> to <NUM> is controlled by a control circuit <NUM>. Such a configuration in which a redundant function of excitation light source is achieved by four excitation LDs is hereinafter referred to as a "<NUM>-LDs redundant configuration".

In general, one optical fiber is assigned to each of optical fiber transmission lines of the submarine cable system for transmission in an upstream direction and transmission in a downstream direction. A pair of optical fibers in the upstream direction and the downstream direction is called a fiber pair (FP). Assuming that the optical fiber <NUM> is used for transmission in the upstream direction (Up) and the optical fiber <NUM> is used for transmission in the downstream direction (Down), the optical amplification device <NUM> amplifies an optical signal of one fiber pair (FP).

When the number of optical fibers connected to an optical repeater increases from one fiber pair (<NUM> FP) to two fiber pairs (<NUM> FPs), the number of EDFs requiring excitation also increases from two to four. In order to excite four EDFs by using the optical amplification device <NUM> in the configuration of <FIG>, two optical amplification devices <NUM> are required. In this case, eight excitation LDs are required. Further, when the number of optical fibers is increased to four fiber pairs (<NUM> FPs), <NUM> excitation LDs are required to excite eight EDFs.

<FIG> is a diagram illustrating a configuration of another general optical amplification device <NUM>. The optical amplification device <NUM> includes an excitation unit <NUM> having a control circuit <NUM> and four excitation LDs <NUM> to <NUM> (LD <NUM> to LD <NUM>), and an optical amplification unit <NUM> having optical couplers <NUM> to <NUM> and EDFs <NUM> to <NUM>. The optical amplification unit <NUM> excites the four EDFs <NUM> to <NUM> included in two fiber pairs by the <NUM>-LDs redundant configuration using the excitation LDs <NUM> to <NUM>. Therefore, the optical amplification unit <NUM> needs to include four input ports for inputting excitation light.

The optical amplification unit <NUM> includes four optical couplers <NUM> to <NUM> for distributing input four beams of excitation light to four EDFs. The optical coupler <NUM> couples the beams of excitation light generated by the excitation LDs <NUM> and <NUM>, and outputs the beams of coupled excitation light to the optical couplers <NUM> and <NUM>. The optical coupler <NUM> couples the beams of excitation light generated by the excitation LDs <NUM> and <NUM>, and outputs the beams of coupled excitation light to the optical couplers <NUM> and <NUM>. The optical coupler <NUM> couples and splits the beams of excitation light being input from the optical couplers <NUM> and <NUM>, and excites the EDFs <NUM> and <NUM>. The optical coupler <NUM> couples and splits the beams of excitation light being input from the optical couplers <NUM> and <NUM>, and excites the EDFs <NUM> and <NUM>.

By using the optical couplers <NUM> to <NUM>, the beams of excitation light generated by the excitation LDs <NUM> to <NUM> excite all of the EDFs <NUM> to <NUM>. In this manner, the optical amplification device <NUM> can excite four EDFs by the <NUM>-LDs redundant configuration, and the number of excitation LDs required for one EDF can be reduced by half as compared with the optical amplification device <NUM> in <FIG>.

In <FIG>, when four optical fiber transmission lines (FP <NUM> and FP <NUM>) are further increased, another set of optical amplification device having a similar configuration to that of the optical amplification device <NUM> needs to be prepared, as indicated by a broken line. The four EDFs of FP <NUM> and FP <NUM> are then excited by using four excitation LDs (LD <NUM> to LD <NUM>), and the four EDFs of the FP <NUM> and the FP <NUM> are excited by using other excitation LDs (LD <NUM> to LD <NUM>). When the configuration of <FIG> is extended to four fiber pairs, <NUM> excitation LDs are required, whereas in the configuration of <FIG>, eight excitation LDs (LD <NUM> to LD <NUM>) are required, and the number of excitation LDs can be reduced by half as compared with <FIG>.

In connection with the present invention, PTL <NUM> describes an optical signal repeater in which either one of two types of optical fiber amplifiers or a through fiber is selected in response to a failure state of an excitation LD in a redundant configuration of two excitation LDs.

PTL <NUM> relates to an optical repeater and a control method for an optical repeater, and relates particularly to an optical repeater including optical amplifiers different for respective wavelength ranges of signal light to be amplified, and a control method for an optical repeater.

PTL <NUM> relates to an optical amplification device used in an optical transmission system, and more particularly to techniques of supplying pump light for optical amplification.

In the optical amplification device <NUM> described with reference to <FIG>, since four excitation LDs and two EDFs are integrated, relationship between the number of excitation LDs and the number of EDFs cannot be changed. In the optical amplification device <NUM> described with reference to <FIG>, it is necessary to match the number (four) of the excitation LDs of the excitation unit <NUM> with the number (four) of the input ports for excitation light of the optical amplification unit <NUM>. As a result, in an optical amplification device using a general <NUM>-LDs redundant configuration, more than four EDFs cannot be excited by using four excitation LDs. Further, the configuration described in PTL <NUM> does not describe a technique for solving the problem of suppressing an increase in the number of excitation LDs for one EDF in an optical amplification device having a <NUM>-LDs redundant configuration.

An object of the present invention is to provide a technique being able to suppress the number of excitation LDs in an optical amplification device having a redundant configuration of an excitation LD.

The present invention is able to suppress the number of excitation LDs in an optical amplification device having a redundant configuration of an excitation LD.

<FIG> is a block diagram illustrating a configuration example of an optical amplification device <NUM> according to the present invention. The optical amplification device <NUM> includes an excitation unit <NUM>, a first distribution unit <NUM>, and optical amplification units <NUM> and <NUM>. The optical amplification unit <NUM> includes a second distribution unit <NUM> and a gain block <NUM>. The optical amplification unit <NUM> includes a second distribution unit <NUM> and a gain block <NUM>.

The excitation unit <NUM> includes excitation LDs <NUM> to <NUM> for outputting excitation light. The excitation LDs <NUM> to <NUM> are excitation light sources that generate excitation light that excites the gain blocks <NUM> and <NUM>. In other words, the excitation unit <NUM> has an excitation means for outputting a plurality of beams of excitation light. The first distribution unit <NUM> splits beams of excitation light being input from the excitation LDs <NUM> to <NUM>, and outputs a larger number of beams of excitation light (first distribution light) than the number of excitation LDs. In other words, the first distribution unit <NUM> has a first distribution means for splitting input light and outputting the split light as a plurality of beams of first distribution light. The second distribution units <NUM> and <NUM> couple and split the first distribution light being input from the first distribution unit, and output the split light (second distribution light). In other words, the second distribution units <NUM> and <NUM> have a second distribution means for coupling and splitting input light and outputting the split light as a plurality of beams of second distribution light.

The gain blocks <NUM> and <NUM> include EDFs <NUM> to <NUM> and EDFs <NUM> to <NUM>, respectively. Each of the plurality of beams of second split light being input to the gain blocks <NUM> and <NUM> excites the EDFs <NUM> to <NUM> and the EDFs <NUM> to <NUM>. The EDFs <NUM> to <NUM> and EDFs <NUM> to <NUM> are gain media for amplifying light. The EDFs <NUM> to <NUM> and the EDFs <NUM> to <NUM> excited by the second excitation light amplify an input optical signal. Since a basic configuration of an optical amplifier using an EDF is widely known, a description of input and output paths of optical signals amplified by the EDFs <NUM> to <NUM> and the EDFs <NUM> to <NUM> is omitted in the gain blocks <NUM> and <NUM> in <FIG>.

The first distribution unit <NUM> splits the excitation light, and thereby outputs a larger number of beams of excitation light (first distribution light) than the number of excitation LDs to the second distribution units <NUM> and <NUM>. For example, the first distribution unit <NUM> included in the optical amplification device <NUM> distributes four beams of input excitation light into eight beams of excitation light, and outputs the eight beams of excitation light. Each of the second distribution units <NUM> and <NUM> couples and splits the four beams of input excitation light, and generates and outputs four beams of excitation light.

The optical amplification device <NUM> having such a configuration has a redundant configuration including a plurality of excitation LDs, and can excite more EDFs than the number of excitation LDs. For example, the optical amplification device <NUM> can excite eight EDFs by providing a redundant configuration (<NUM>-LDs redundant configuration) including four excitation LDs. In other words, the optical amplification device <NUM> can suppress the number of excitation LDs in an optical amplification device having the redundant configuration of the excitation LDs. By providing such a redundant configuration, the optical amplification device <NUM> can maintain an amplification function of the optical amplification device by excitation light of another excitation LD even when one excitation LD is deteriorated.

<FIG> is a block diagram illustrating a configuration example of an optical transmission system <NUM> according to a second example embodiment of the present invention. In the present example embodiment and the subsequent example embodiments, the same reference signs are assigned to the already-mentioned elements, and a repetitive description thereof is omitted.

The optical transmission system <NUM> includes a terminal station <NUM>, a terminal station <NUM>, and an optical amplification device <NUM>. The terminal stations <NUM> and <NUM> are optical transceivers that transmit and receive an optical signal, to which four fiber pairs FP <NUM> to FP <NUM> are connected. The terminal station <NUM> and the terminal station <NUM> are connected to each other by an optical fiber transmission line including the FP <NUM> to the FP <NUM>. Each fiber pair includes an upstream line (Up) and a downstream line (Down). The optical amplification device <NUM> amplifies an upstream optical signal being input from the terminal station <NUM>, and outputs the amplified signal to the terminal station <NUM>. The optical amplification device <NUM> also amplifies a downstream optical signal being input from the terminal station <NUM>, and outputs the amplified signal to the terminal station <NUM>.

As described in the first example embodiment, the optical amplification device <NUM> includes an excitation unit <NUM>, a first distribution unit <NUM>, second distribution units <NUM> and <NUM>, EDFs <NUM> to <NUM>, and EDFs <NUM> to <NUM>. In <FIG>, the second distribution units <NUM> and <NUM> are illustrated as one block. The EDFs <NUM> to <NUM> and the EDFs <NUM> to <NUM> in <FIG> constitute a gain block <NUM> and a gain block <NUM> in <FIG>, respectively.

Four beams of excitation light generated by the excitation unit <NUM> are distributed to eight beams of excitation light by the first distribution unit <NUM> and the second distribution units <NUM> and <NUM>. A configuration and a procedure for the distribution are similar to those of the first example embodiment. Each of the eight beams of excitation light being output from the second distribution units <NUM> and <NUM> excites the EDFs <NUM> to <NUM> and the EDFs <NUM> to <NUM>. The EDF <NUM> amplifies an upstream optical signal propagating through the FP <NUM>, and the EDF <NUM> amplifies a downstream optical signal propagating through the FP <NUM>. Similarly, the EDFs <NUM> and <NUM> amplify an optical signal propagating through the FP <NUM>. Furthermore, the EDFs <NUM> and <NUM> amplify an optical signal propagating through the FP <NUM>, and the EDFs <NUM> and <NUM> amplify an optical signal propagating through the FP <NUM>.

<FIG> is a block diagram illustrating a detailed configuration example of the optical amplification device <NUM> according to the present example embodiment. The excitation unit <NUM> includes four excitation LDs <NUM> to <NUM> and control circuits <NUM> and <NUM> that control an optical output of the excitation LDs <NUM> to <NUM>. <FIG> illustrates an example in which each of the control circuits <NUM> and <NUM> controls two excitation LDs. However, the number of excitation LDs controlled by one control circuit is not limited. For example, one control circuit may control all excitation LDs. In the drawings of and after <FIG>, an arrow exemplifies a direction of a signal, and a direction of the signal is not limited.

The control circuits <NUM> and <NUM> control power of excitation light generated by each of the excitation LDs <NUM> to <NUM> in such a way that the excitation light of predetermined power is supplied to the EDFs <NUM> to <NUM> and the EDFs <NUM> to <NUM>. The power of the excitation light generated by the excitation LDs <NUM> to <NUM> is controlled by a drive current of each excitation LD. The control circuits <NUM> and <NUM> may detect output power of the excitation LD, based on a photocurrent of a monitor photodetector included in each excitation LD. Further, the control circuits <NUM> and <NUM> may store, in advance, a transmission loss for each optical path of the excitation light passing through the first distribution unit <NUM> and the second distribution units <NUM> and <NUM>. For example, by considering a loss of an optical path from the excitation unit <NUM> of the excitation LDs <NUM> and <NUM> to each EDF, the control circuit <NUM> can estimate excitation light power being supplied to each EDF and based on the excitation LDs <NUM> and <NUM>.

The control circuits <NUM> and <NUM> may be communicably connected to each other, and one control circuit may control any excitation LD, based on information acquired from the other control circuit. For example, the control circuit <NUM> may communicate with the control circuit <NUM> and acquire an operation state of the excitation LDs <NUM> and <NUM>, and adjust power of the excitation light of the excitation LDs <NUM> to <NUM> in response to the state.

With an increase in output of the excitation LD in recent years, optical power being able to excite five or more EDFs can be acquired even in the optical amplification device having the <NUM>-LDs redundant configuration. The first distribution unit <NUM> according to the present example embodiment is a <NUM>×<NUM> optical coupler that distributes four inputs to eight outputs, and is constituted of, for example, four <NUM>×<NUM> optical couplers (optical couplers <NUM> to <NUM>). Each of the inputs of the first distribution unit <NUM> is connected to one of the different excitation LDs <NUM> to <NUM>. The first distribution unit <NUM> splits the output of each of the four excitation LDs <NUM> to <NUM> included in the excitation unit <NUM> into two, and supplies four beams of the excitation light to each of the second distribution units <NUM> and <NUM>.

An optical amplification unit <NUM> includes four excitation light input ports for inputting four beams of excitation light. The optical amplification unit <NUM> includes four EDFs <NUM> to <NUM> for amplifying optical signals (the FP <NUM> and the FP <NUM>) of two sets of fiber pairs. Specifically, the optical amplification unit <NUM> includes the second distribution unit <NUM> and the EDFs <NUM> to <NUM>.

Each of the four excitation light input ports of the second distribution unit <NUM> is connected to a different output of the first distribution unit <NUM>. The second distribution unit <NUM> couples four beams of excitation light being input from the first distribution unit <NUM>, splits the coupled excitation light, and distributes the split excitation light to the four EDFs <NUM> to <NUM>. In the present example embodiment, the second distribution unit <NUM> is constituted of a <NUM>×<NUM> optical coupler by four <NUM>×<NUM> optical couplers (optical couplers <NUM> to <NUM>).

In <FIG>, fiber pairs through which optical signals amplified by each of the EDFs propagate are illustrated as the FP <NUM> to the FP <NUM>. A configuration of an optical amplification unit <NUM> including the second distribution unit <NUM> and the EDFs <NUM> to <NUM> is similar to that of the optical amplification unit <NUM>, and different in a point that fiber pairs through which optical signals propagate are the FP <NUM> and the FP <NUM>. Each of the EDFs <NUM> to <NUM> and EDFs <NUM> to <NUM> amplifies a C-band optical signal. As used herein, "C-band" indicates a wavelength band ranging generally from <NUM> to <NUM>.

An operation of the optical amplification device <NUM> according to the present example embodiment is described in more detail. Four beams of excitation light generated by the excitation unit <NUM> are distributed to eight beams of excitation light by the optical couplers <NUM> to <NUM> included in the first distribution unit <NUM>. Four beams of the distributed excitation light are supplied to the optical amplification unit <NUM>, and the remaining four beams of the distributed light are supplied to the optical amplification unit <NUM>. More specifically, one of beams of the excitation light split by the optical couplers <NUM> to <NUM> into two beams of light is output to the second distribution unit <NUM>, and the other is output to the second distribution unit <NUM>. As a result, the four beams of excitation light supplied to the optical amplification unit <NUM> include the beams of excitation light generated by the excitation LDs <NUM> to <NUM>. The four beams of excitation light supplied to the optical amplification unit <NUM> also include the beams of excitation light generated by the excitation LDs <NUM> to <NUM>. In other words, the four beams of excitation light generated by the excitation LDs <NUM> to <NUM> are input to both the second distribution unit <NUM> and the second distribution unit <NUM>.

Each of the optical couplers <NUM> to <NUM> included in the second distribution unit <NUM> couples two beams of input excitation light, splits each of the beams of coupled excitation light into two beams of light, and output each of the beams of split excitation light to the EDFs <NUM> to <NUM> as second distribution light. More specifically, the four beams of excitation light being input to the second distribution unit <NUM> are coupled in the optical coupler <NUM> or <NUM> and then split, and are output to the optical couplers <NUM> and <NUM>. As a result, the excitation light generated by the excitation LDs <NUM> to <NUM> is input to the optical couplers <NUM> and <NUM>. The optical couplers <NUM> and <NUM> couple the beams of excitation light being output from the optical couplers <NUM> and <NUM>, and then split the coupled excitation light. The optical coupler <NUM> supplies excitation light to the EDFs <NUM> and <NUM>. The optical coupler <NUM> supplies excitation light to the EDFs <NUM> and <NUM>.

Similarly, the four beams of excitation light being input to the second distribution unit <NUM> are coupled in the optical coupler <NUM> or <NUM> and then split, and are output to the optical couplers <NUM> and <NUM>. As a result, the excitation light generated by the excitation LDs <NUM> to <NUM> is also input to the optical couplers <NUM> and <NUM>. The optical coupler <NUM> supplies excitation light to the EDFs <NUM> and <NUM>. The optical coupler <NUM> supplies excitation light to the EDFs <NUM> and <NUM>.

With such a configuration, each of the EDFs <NUM> to <NUM> and the EDFs <NUM> to <NUM> is excited by the excitation light generated by the excitation LDs <NUM> to <NUM>. When power of the excitation light of any of the excitation LDs <NUM> to <NUM> fluctuates, the control circuits <NUM> and <NUM> may maintain the power of the excitation light supplied to the EDF within a predetermined range by controlling output power of other excitation LDs in such a way as to compensate for the fluctuation. For example, when power of the excitation light of the excitation LD <NUM> decreases, the control circuit <NUM> may suppress a decrease in the power of the excitation light being output from the second distribution units <NUM> and <NUM> by increasing a drive current of the excitation LD <NUM>. In a case where one control circuit <NUM> controls the excitation LDs <NUM> to <NUM>, the control circuit <NUM> may compensate for a decrease in power of the failed excitation LD by increasing output power of a plurality of normal excitation LDs. The control circuits <NUM> and <NUM> may adjust the drive current of the excitation LD in consideration of difference between a loss of an optical path from the excitation unit <NUM> of the excitation LD whose output power fluctuates to each EDF and a loss of an optical path from the excitation unit <NUM> of other excitation LD to each EDF. As a result, it is possible to more precisely suppress fluctuation of power of the excitation light supplied to the EDFs <NUM> to <NUM> and EDFs <NUM> to <NUM>.

As described above, the optical amplification device <NUM> can excite eight EDFs <NUM> to <NUM> and EDFs <NUM> to <NUM> in the optical amplification device <NUM> having a redundant configuration (<NUM>-LDs redundant configuration) using four excitation LDs <NUM> to <NUM>. Specifically, the optical amplification device <NUM> having the <NUM>-LDs redundant configuration can suppress an increase in the number of excitation LDs due to an increase in EDF. In other words, the optical amplification device <NUM> can suppress the number of excitation LDs of an optical amplification device having the redundant configuration of the excitation LDs.

The first distribution unit <NUM> is configured to be separable from the excitation unit <NUM> and the second distribution unit <NUM>. Therefore, in the optical amplification device <NUM>, by inserting the first distribution unit <NUM> between the excitation unit <NUM> and the optical amplification unit <NUM>, the optical amplification unit <NUM> can be added while maintaining the <NUM>-LDs redundant configuration without adding an excitation LD. An optical amplification device needs to mount an EDF associated with a wavelength band of an optical signal transmitted by an optical transmission system, as a gain block. In the present example embodiment, since the first distribution unit <NUM> and the second distribution units <NUM> and <NUM> are configured to be separable from each other, only the optical amplification unit <NUM> or <NUM> mounting the EDF <NUM> to <NUM> or the EDF <NUM> to <NUM> can be changed depending on a configuration of the optical transmission system <NUM>. Thus, for example, without designing a detailed configuration of the optical amplification device <NUM> for each system, it is possible to achieve an optical amplification device in association with a wavelength to be transmitted in the system by a simple design change. In the following modification example, an example in which the optical amplification units <NUM> and <NUM> are replaced with optical amplification units <NUM> and <NUM> or optical amplification units <NUM> and <NUM> having configurations different from those described above is described.

By changing the optical amplification units <NUM> and <NUM>, various optical amplification devices with high reliability by the <NUM>-LDs redundant configuration can be achieved. <FIG> is a block diagram illustrating a detailed configuration example of an optical amplification device <NUM> according to a first modification example of the second example embodiment.

The optical amplification device <NUM> in <FIG> includes optical amplification units <NUM> and <NUM> instead of the optical amplification units <NUM> and <NUM>. In the optical amplification device <NUM>, FP <NUM> and FP <NUM> transmit C-band optical signals, and FP <NUM> and FP <NUM> transmit L-band optical signals. As used herein, "L-band" indicates a wavelength band ranging generally from <NUM> to <NUM>. The EDFs <NUM> and <NUM> of the FP <NUM> and the EDFs <NUM> and <NUM> of the FP <NUM> may be the same EDF as the optical amplification device <NUM>, or may be EDFs designed for L-band. In other words, even when a wavelength of an optical signal is different for each fiber pair, the optical amplification device <NUM> can suppress the number of excitation LDs of an optical amplification device having the redundant configuration of the excitation LDs.

<FIG> is a block diagram illustrating a detailed configuration example of an optical amplification device <NUM> according to a second modification example of the second example embodiment.

The optical amplification device <NUM> in <FIG> includes optical amplification units <NUM> and <NUM>. The optical amplification device <NUM> differs, as compared with the optical amplification devices <NUM> and <NUM> in <FIG> and <FIG>, in a point that two EDFs (e.g., EDF <NUM> and EDF <NUM>) are connected in series and the fiber pairs are only FP <NUM> and FP <NUM>. In the optical amplification unit <NUM>, the EDFs <NUM> and <NUM> amplify an upstream optical signal of the FP <NUM>, and EDFs <NUM> and <NUM> amplify a downstream optical signal of the FP <NUM>. In the optical amplification unit <NUM>, EDFs <NUM> and <NUM> amplify an upstream optical signal of the FP <NUM>, and EDFs <NUM> and <NUM> amplify a downstream optical signal of the FP <NUM>. A wavelength band of the optical signals propagating through the FP <NUM> and the FP <NUM> may be C-band or L-band. The optical amplification device <NUM> can acquire a higher gain than the optical amplification devices <NUM> and <NUM> by connecting the EDFs in series, and can suppress the number of excitation LDs of an optical amplification device having the redundant configuration of the excitation LDs.

Description is made on still another configuration example of an optical amplification device that excites a plurality of EDFs by using a plurality of excitation LDs while maintaining high reliability by a redundant configuration of the excitation LDs. <FIG> is a block diagram illustrating a configuration example of an optical amplification device <NUM> according to a third example embodiment.

The optical amplification device <NUM> includes an excitation unit <NUM>, a first distribution unit <NUM>, and optical amplification units <NUM>, <NUM>, and <NUM>. The excitation unit <NUM> outputs six beams of excitation light. The first distribution unit <NUM> splits the six beams of input excitation light, and outputs <NUM> beams of excitation light. Four beams of excitation light being output from the first distribution unit <NUM> are supplied to each of the optical amplification units <NUM> to <NUM>.

<FIG> is a block diagram illustrating a detailed configuration example of the optical amplification device <NUM>. The excitation unit <NUM> includes excitation LDs <NUM> to <NUM> (LD <NUM> to LD <NUM>) and control circuits <NUM> to <NUM>. The control circuit <NUM> controls the excitation LDs <NUM> and <NUM>, the control circuit <NUM> controls the excitation LDs <NUM> and <NUM>, and the control circuit <NUM> controls the excitation LDs <NUM> and <NUM>. However, the number of excitation LDs controlled by a control circuit is not limited. For example, the control circuit <NUM> may control all of the excitation LDs <NUM> to <NUM>. The control circuits <NUM> to <NUM> are communicably connected to each other, and the control circuits <NUM> to <NUM> may control any excitation LD, based on information acquired from other control circuits. For example, the control circuit <NUM> may adjust power of excitation light of the excitation LDs <NUM> to <NUM> in response to a state of the control circuits <NUM> and <NUM> and the excitation LDs <NUM> to <NUM>.

The first distribution unit <NUM> includes six <NUM>×<NUM> optical couplers (optical couplers <NUM> to <NUM>). The first distribution unit <NUM> splits each of the six beams of excitation light being input from the excitation LDs <NUM> to <NUM> into two beams of light, and outputs <NUM> beams of excitation light. The first distribution unit <NUM> supplies four beams of the excitation light to each of the optical amplification units <NUM> to <NUM>. <FIG> illustrates an example in which the beams of excitation light generated by the excitation LDs <NUM> to <NUM> are supplied to the optical amplification unit <NUM>. The beams of excitation light generated by the excitation LDs <NUM> to <NUM> and <NUM> to <NUM> are supplied to the optical amplification unit <NUM>. The beams of excitation light generated by the excitation LDs <NUM> to <NUM> are supplied to the optical amplification unit <NUM>.

The optical amplification units <NUM> to <NUM> have a similar configuration to that of the optical amplification unit <NUM> according to the second example embodiment. Similarly to the second example embodiment, beams of excitation light from the four excitation LDs are input to the optical amplification units <NUM> to <NUM>. Each of the optical amplification units <NUM> to <NUM> amplifies an optical signal of the C-band propagating through two sets of fiber pairs by exciting four EDFs with excitation light coupled and split by the second distribution unit <NUM>. The optical amplification units <NUM> to <NUM> split and couple four beams of excitation light by the second distribution unit <NUM> (<NUM>×<NUM> optical coupler) illustrated in <FIG>. Therefore, the <NUM>-LDs redundant configuration is also achieved in the optical amplification device <NUM>. With such a configuration, the optical amplification device <NUM> can excite <NUM> EDFs while maintaining the <NUM>-LDs redundant configuration by using six excitation LDs. In other words, the optical amplification device <NUM> can also suppress the number of excitation LDs of an optical amplification device having the redundant configuration of the excitation LDs.

The configuration of the optical amplification device according to the second and third example embodiments can also be described as follows. Specifically, an excitation unit includes 2n excitation light sources (n is an integer of <NUM> or more), and a first distribution unit outputs 4n beams of first distribution light in response to excitation light being <NUM> or more beams and 2n or less beams. Each of n second distribution units generate four beams of second distribution light in response to the four beams of first distribution light. The n second distribution units that generate the four beams of second distribution light excite 4n gain media. The second example embodiment and the modification examples thereof are examples in a case of n=<NUM>, and the third example embodiment is an example in a case of n=<NUM>. In the case of n=<NUM>, the <NUM>-LDs redundant configuration is achieved by outputting beams of excitation light of four excitation LDs out of the six excitation LDs from the first distribution unit <NUM> to the optical amplification units <NUM> to <NUM>. Even when n is <NUM> or more, the optical amplification device can excite a greater number of the gain media while maintaining the <NUM>-LDs redundant configuration.

In the drawings of each of the example embodiments described above, an example has been described in which the first distribution units <NUM> and <NUM> include a plurality of <NUM>×<NUM> optical couplers, and the second distribution units <NUM> and <NUM> include a plurality of <NUM>×<NUM> optical couplers. However, a configuration of the first and second distribution units is not limited to the description in the drawings. For example, a <NUM>×<NUM> optical coupler may be used for the first distribution unit, instead of the <NUM>×<NUM> optical coupler. As a result, the first distribution unit can output more than 4n beams of the first distribution light in response to excitation light being one or more beams and 2n or less beams. Such a configuration may be used when there is a margin in power of the excitation LD, and more EDFs can be excited.

Further, the second distribution unit may couple and split five or more beams of the first distribution light generated by five or more different excitation LDs, and thereby generate the second distribution light. In this case, since a redundant configuration using five or more excitation LDs is achieved, further improvement in reliability is expected. Also, the number of beams of second distribution light being output in response to the number of beams of first distribution light is not limited to the number according to each example embodiment.

Note that, the example embodiments of the present invention may also be described as supplementary notes described below, but the present invention is not limited thereto.

An optical amplification device including:.

The optical amplification device according to supplementary note <NUM>, wherein each of inputs of the first distribution means is connected to mutually different one of the excitation light sources.

The optical amplification device according to supplementary note <NUM> or <NUM>, wherein the first distribution means includes a plurality of first optical couplers for splitting each of beams of input light into two beams of light.

The optical amplification device according to any one of supplementary notes <NUM> to <NUM>, wherein each of outputs of the first distribution means is connected to a different input of the different second distribution means.

The optical amplification device according to any one of supplementary notes <NUM> to <NUM>, wherein the second distribution means includes
a plurality of second optical couplers that each couple two beams of first distribution light, split each of beams of the coupled first distribution light into two beams of light, and output the split first distribution light as the second distribution light.

The optical amplification device according to any one of supplementary notes <NUM> to <NUM>, further including:.

The optical amplification device according to any one of supplementary notes <NUM> to <NUM>, wherein the first distribution means is configured to be separable from the excitation means and the second distribution means.

The optical amplification device according to any one of supplementary notes <NUM> to <NUM>, wherein one or more of the gain media for amplifying an optical signal propagating through an optical fiber are arranged in each of the two optical fibers constituting one fiber pair.

An optical transmission system including:.

An optical amplification method including:.

The optical amplification method according to supplementary note <NUM>, further including splitting light being input from the excitation light sources different from each other, and outputting split light as the first distribution light.

The optical amplification method according to supplementary note <NUM> or <NUM>, further including outputting the first distribution light by splitting each of beams of light being input from the excitation light source into two beams of light.

The optical amplification method according to any one of supplementary notes <NUM> to <NUM>, further including coupling and splitting beams of the first distribution light different from each other, and outputting split light as the second distribution light.

The optical amplification method according to any one of supplementary notes <NUM> to <NUM>, further including coupling two beams of the first distribution light, splitting each of beams of the coupled first distribution light into two beams of light, and outputting the split first distribution light as the second distribution light.

The optical amplification method according to any one of supplementary notes <NUM> to <NUM>, further including:.

While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the claims.

The configurations described in each of the example embodiments are not necessarily mutually exclusive. The functions and effects of the present invention may be achieved by a configuration in which all or part of the above-described example embodiments are combined.

Claim 1:
An optical amplification device (<NUM>) comprising:
excitation means (<NUM>) configured to output a plurality of beams of excitation light generated by a plurality of excitation light sources (<NUM> to <NUM>);
first distribution means (<NUM>) configured with input connected to the plurality of excitation light sources (<NUM> to <NUM>), and configured to split input light, and
configured to output split light as a plurality of beams of first distribution light;
a plurality of second distribution means (<NUM>, <NUM>) configured with input connected to the first distribution means (<NUM>), and configured to couple and split input light, and configured to output split light as a plurality of beams of second distribution light;
a plurality of gain media (<NUM> to <NUM>, <NUM> to <NUM>) configured to be excited by each of the plurality of beams of second distribution light;
the excitation means (<NUM>) including 2n of the excitation light sources (<NUM> to <NUM>), n being an integer of <NUM> or more;
n of the second distribution means (<NUM>, <NUM>) configured to generate four beams of the second distribution light in response to four beams of the first distribution light;
characterized in that:
the first distribution means (<NUM>) is configured to output 4n beams of the first distribution light in response to input from 2n of the excitation means (<NUM>); and in that the optical amplification device comprises 4n gain media (<NUM> to <NUM>, <NUM> to <NUM>).