Optical branching device, optical amplification apparatus, and optical amplification method

An optical branching device includes: a Faraday rotator capable of controlling polarized wave of input light based on a change of a magnetic flux density depending on a magnetic field to be provided; a magnet configured to provide the Faraday rotator with the magnetic field; a polarization beam splitter configured to branch, by a polarized wave component, the input light which passes through the Faraday rotator; a bimetal configured to deform depending on a temperature; and a controller configured to have a mechanism to use force accompanying with the deformation of the bimetal so as to control a relative positional relationship between the Faraday rotator and the magnet.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-078513, filed on Apr. 7, 2014, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical branching device, an optical amplification apparatus, and an optical amplification method.

BACKGROUND

As a conventional technology related to an optical amplifier, technologies described in Japanese Laid-open Patent Publication No. 2009-81473 and Japanese Laid-open Patent Publication No. 05-90671 are known.

In these Patent Documents, output light (excitation light) of one excitation light source is divided by an optical branching device, and the branched excitation light is entered into each of erbium-doped optical fibers (EDF) of a two-stage configuration.

A branching ratio of excitation light by an optical branching device may be variable. For example, Japanese Laid-open Patent Publication No. 2007-127988 describes a branch switching type optical splitter having a variable branching ratio of input light.

In the afore-mentioned technology, actively controlling (or adjusting) a branching ratio of excitation light is under study.

SUMMARY

According to an aspect of the invention, an optical branching device includes: a Faraday rotator capable of controlling polarized wave of input light based on a change of a magnetic flux density depending on a magnetic field to be provided; a magnet configured to provide the Faraday rotator with the magnetic field; a polarization beam splitter configured to branch, by a polarized wave component, the input light which passes through the Faraday rotator; a bimetal configured to deform depending on a temperature; and a controller configured to have a mechanism to use force accompanying with the deformation of the bimetal so as to control a relative positional relationship between the Faraday rotator and the magnet.

DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the present disclosure which allows a light branching ratio to be changed without using active control are described with reference to the drawings. However, the embodiments to be described hereinafter are simply exemplary, and not intended to preclude application of different variations or technologies which are not demonstrated hereinafter. Note that in the drawings using the following embodiments, a part to which a same reference numeral is assigned represents a same or similar part, unless otherwise stated.

FIG. 1is a block diagram illustrating a configuration example of an optical amplification apparatus according to an embodiment. The optical amplification apparatus1illustrated inFIG. 1exemplarily includes a first optical amplification medium11, a second optical amplification medium12, an excitation light source13, and an optical branching device14.

Signal light inputted to the optical amplification apparatus1through an optical transmission line5A on the input side is amplified at the first optical amplification medium11, is amplified at the second optical amplification medium12, and is outputted to an optical transmission line5B on the output side. Thus, the optical amplification apparatus1may be referred to as an optical amplification repeater1. A rare-earth doped optical fiber may be applied to the optical amplification media11and12. A non-limiting example of the rare-earth doped optical fiber includes an erbium-doped optical fiber (EDF).

Excitation light which is outputted from an excitation light source13being branched by the optical branching device14and the branched excitation light being inputted, EDFs11and12are each excited to amplify signal light. Signal light may be exemplarily WDM light which is multiplexed signal light having a plurality of wavelengths. A semiconductor laser diode (LD) may be applied to the excitation light source13. A non-limiting example of a semiconductor LD is a Fabry-Perot (FP) laser which emits light including a plurality of wavelengths. A FP laser is less expensive than a distributed feedback laser (DFB) capable of light-emitting a single wavelength in a stable manner.

The optical branching device14branches excitation light inputted from the excitation light source13to two branches of light, for example, and inputs one branched excitation light to the EDF11through a multiplexer17provided on a front stage of the one EDF11. In addition, the optical branching device14inputs the other branched excitation light to the EDF12through a multiplexer18provided on a front stage of the other EDF12.

Stated differently, the branched excitation light on the one side is multiplexed to signal light which is inputted to the EDF11at the multiplexer17and inputted to the EDF11together with the signal light. In addition, the branched excitation light on the other side is multiplexed to signal light which is inputted to the EDF12at the multiplexer18and inputted to the EDF12.

Thus, the EDFs11and12each have a so-called forward pumping configuration that excitation light is inputted from a same direction as a transmission direction of input signal light to be amplified. However, one or both of the EDFs11and12may have a backward pumping configuration or a two-way pumping configuration. In the case of the two-way pumping configuration, the number of branches of excitation light is larger than the case of the forward or backward pumping configuration.

In addition, as exemplarily illustrated inFIG. 1, a gain equalizer (GEQ) or a variable optical attenuator (VOA)16may be provided, as appropriate, between the EDF11and the EDF12. The EDFs11and12has a gain characteristic, referred to as a gain deviation or a gain tilt, which is wavelength dependent. Thus, when WDM light is amplified by the EDFs11and12, the optical amplification apparatus1may be provided with the GEQ15to compensate (which may also be referred to as “flattening”) power deviations among wavelengths due to a gain tilt.

For example, the GEQ15performs output power adjustment to offset any inter-wavelength power difference which occurs in WDM light due to a gain tilt which the EDF12on the subsequent stage has, on WDM light outputted from the EDF11on the front stage. A filter such as a dielectric multi-layer filter or an etalon filter may be applied to the GEQ15. Thus, the GEQ15may be referred to as a “gain equalization filter”. Note that the VOA16adjusts power of WDM light outputted from the GEQ15to adjust input light power to the EDF12.

A gain equalization characteristic of the GEQ15and an attenuation amount of the VOA16(which may be referred to as “VOA loss”) may be dynamically adjusted (controlled) based on input/output power of each of the EDFs11and12. Thus, as exemplarily illustrated inFIG. 1, the optical amplification apparatus1may be provided with an optical branching device19and a light receiving element20as an example of a monitor system on an input/output stage of the EDF11or an input/output stage of the EDF12.

The optical branching device19inputs branched WDM light to the light receiving element20as monitor light. The light receiving element20is exemplarily a photodiode (or a photo detector) (PD), and outputs an electric signal corresponding to light receiving power of the monitor light. Based on the electric signal, the gain equalization characteristic of the GEQ15and the attenuation amount of the VOA16may be controlled. Note that a control unit which performs the control is not illustrated inFIG. 1.

Note that of branched excitation light branched at the optical branching device14, branched excitation light which is not branched excitation light to be inputted to the EDF11may be inputted (which may be referred to “multicast”) to a plurality of the EDFs12.FIG. 2andFIG. 3illustrate one example thereof.FIG. 2andFIG. 3illustrate a configuration example of the optical amplification apparatus1wherein branched excitation light is introduced to four EDFs12. Note that the number of the EDFs12is not limited to four and may be two or more.

An optical amplification apparatus1exemplarily illustrated inFIG. 2includes, on the subsequent stage of the multiplexer18, an optical branching device21configured to branch output light of the multiplexer18to each EDF12. Thus, of branched excitation light branched at the optical branching device14, branched excitation light which is not branched excitation light to be inputted to an EDF11is inputted to the optical branching device21through the multiplexer18together with WDM light, branched at the optical branching device21together with the WDM light, and introduced to each of the EDFs12.

The optical branching device21may equally branch WDM light which is input light and branched excitation light and equally distribute the WDM light which is input light and the branched excitation light to each of the EDFs12, or unevenly distribute WDM light which is input light and branched excitation light at different branching ratios to each of the EDFs12. In addition, the branching ratio at the optical branching device21may be fixed or variable.

On the one hand, an optical amplification apparatus1illustrated inFIG. 3has such a configuration that each optical route (which may also be referred to as a “branched route”) from an optical branching device21to each EDF12is provided with a multiplexer18and branched excitation light is individually introduced to each EDF12through each multiplexer18. In this case, the number of excitation light branches at an optical branching device14is changed depending on the number of EDFs12to be multicast.

Note that while inFIG. 2andFIG. 3, one EDF12of the four EDFs12is provided with the optical branching device19and the light receiving element20which constitute one example of an output light monitor, each of two or more EDFs12may be provided with an output light monitor.

Incidentally, as described above, when excitation light is branched at the optical branching device14and supplied to each of the EDFs11and12, output light power of a common excitation light source13is desirable to be adjusted in order to adjust a gain corresponding to input light power to the EDFs11and12.

Here, if a branching ratio of the optical branching device14is fixed, the power of the branched excitation light fluctuates only uniformly even if the output light power of the excitation light source13is adjusted. Thus, the power of the branched excitation light may not be individually adjusted, which makes it difficult to compensate a gain tilt.

Note that a gain tilt which the EDFs11and12have may vary depending on not only fluctuations in input light power but also ambient temperatures. Stated differently, even when input light power to the EDFs11and12is uniform, the gain tilt may vary depending on a change in the ambient temperatures of the EDFs11and12.

Fluctuations of the temperature-dependent gain tilt may be compensated through uniform control of the ambient temperatures of the EDFs11and12by using a heater. In addition, fluctuations of temperature-dependent gain tilt may also be compensated through provision of a temperature sensor configured to sense the ambient temperatures of the EDFs11and12to adjust excitation light power depending on sensed temperatures.

If a branching ratio of the optical branching device14is fixed, compensation of a gain tilt as described above becomes difficult even if the excitation light power is adjusted depending on a temperature. Therefore, it is preferable that a branching ratio of excitation light at the optical branching device14is variable.

However, as schematically and exemplarily illustrated inFIG. 4, when an FP laser is used for the excitation light source13, excitation light includes a plurality of wavelengths and power and a polarized wave state (which may also be referred to as “polarization mode”) of each wavelength may also fluctuate easily.

Thus, when the FP laser is used for the excitation light source13, it is difficult to make a branching ratio of excitation light variable by using an interference method at the optical branching device14, unlike when a DFB is used.

As an example of a technique to make a branching ratio of excitation light variable, a technique to use polarized wave is possible, instead of the interference method. For example, it is envisioned that a laser by which polarized wave is uniform (exemplarily, linear polarized wave) and does not fluctuate even though wavelengths included in output light fluctuate is applied to the excitation light source13.

In this case, if a polarization beam splitter (PBS) having azimuth of 45 degrees to a polarization plane which does not fluctuate is used, excitation light may be branched at a branching ratio of 1:1. Stated differently, the branching ratio of the excitation light may be changed by causing the polarization plane of the excitation light to relatively change with respect to the azimuth of the PBS.

A Faraday rotator is a non-limiting example of an optical device capable of changing a polarization plane of excitation light.FIG. 5illustrates a configuration example of an optical branching device14using a Faraday rotator. The optical branching device14illustrated inFIG. 5exemplarily includes a Faraday rotator141and a PBS142.

The Faraday rotator141rotates a polarization plane of input light (here, exemplarily, excitation light) by utilizing a phenomenon referred to as the Faraday effect that a polarization plane rotates, when linear polarization parallel to a magnetic field is transmitted through a material. For example, the Faraday rotator141may control polarized wave of input light depending on a change in a magnetic field to be provided. A garnet single-crystal film may be used for the Faraday rotator141as a non-limiting example. Exemplarily, insertion loss of the Faraday rotator141is approximately 0.02 to 0.05 dB which corresponds to about one splice.

The PBS142separates by a polarized wave component the light which is transmitted through the Faraday rotator141and outputs branched excitation light. In response to changing of the magnetic field of the Faraday rotator141(stated differently, magnetic flux density applied to the Faraday rotator141), a polarization plane of excitation light, which is an example of input light, rotates. Thus, the polarization plane of the excitation light with respect to azimuth of the PBS142relatively changes, and the branching ratio of the excitation light at the PBS142changes.

Note that the configuration of the optical branching device14exemplarily illustrated inFIG. 5is useful when a polarization plane of each wavelength of excitation light, which is output light of the excitation light source13, is uniform without fluctuating. However, when the polarization plane of each wavelength of the excitation light temporally fluctuates, with a simple PBS utilizing the Brewster law (which may also be referred to as a “Brewster splitter”), the branching ratio changes depending on the fluctuation of the polarization plane.

Thus, a configuration exemplarily illustrated inFIG. 6may be applied to the optical branching device14when the polarization plane of each wavelength of the excitation light temporally fluctuates. An optical branching device14illustrated inFIG. 6exemplarily includes a PBS140, two Faraday rotators141-1and141-2, two PBSs142-1and142-2, and two multiplexers143-1and143-2.

The PBS140separates excitation light, which is output light of an excitation light source13, by a polarized wave component, and inputs light of one polarized wave component to one Faraday rotator141-1and light of the other polarized wave component to the other Faraday rotator141-2. In addition, it may be considered that the PBS140corresponds to a second PBS when the PBSs142-1and142-2are positioned as a plurality of first PBSs.

Similar to the Faraday rotator141exemplarily illustrated inFIG. 5, each of the Faraday rotators141-1and141-2utilizes the Faraday Effect to rotate a polarization plane of excitation light. Note that when the Faraday rotator exemplarily illustrated inFIG. 5is not distinguished from the Faraday rotators141-1and141-2exemplarily illustrated inFIG. 6, the Faraday rotator is simply described as a “Faraday rotator141”.

The light which is transmitted through the one Faraday rotator141-1enters the one PBS142-1, and the light which is transmitted through the other Faraday rotator141-2enters the other PBS142-2.

The one PBS142-1branches (which may also be referred to as “separates”) by a polarized wave component the light which entered from the Faraday rotator141-1. One (first) polarized wave component of the separated polarized wave components enters the one (first) multiplexer143-1and the other (second) polarized wave component enters the other (second) multiplexer143-2.

Similarly, the other PBS142-2separates by a polarized wave component the light which enters from the Faraday rotator141-2. The one (first) polarized wave component of the separated polarized wave components enters the one (first) multiplexer143-1and the other (second) polarized wave component enters the other (second) multiplexer143-2.

The first multiplexer143-1multiplexes and outputs light of the one polarized wave component which is entered from each of the PBSs142-1and142-2.

Similarly, the second multiplexer143-2multiplexes and outputs light of the other polarized wave component which is entered from each of the PBSs142-1and142-2.

As such, by separating excitation light by a polarized wave component, Faraday-rotating a polarization plane by the polarized wave component, and then further performing separation and multiplexing by the polarized wave component, even if the polarization plane or spectrum of the excitation light temporally fluctuates, the fluctuations may be averaged and minimized. Therefore, a stable branching ratio may be implemented at the optical branching device14.

In the optical branching device14of the configuration exemplarily illustrated inFIG. 5orFIG. 6, a branching ratio may be adjusted by adjusting a magnetic field to be provided to the Faraday rotator141. If the magnetic field of the Faraday rotator141may be caused to change passively in response to temperature in the magnetic field of the Faraday rotator141, the branching ratio of excitation light may be changed without active control of the optical branching device14. If the active control may be dispensed with, power feeding for control of the branching ratio may be dispensed with.

With application to the optical amplification repeater1of the optical branching device14for which power feeding is dispensed with and the branching ratio of excitation light is variable depending on a temperature, as exemplarily illustrated inFIG. 1toFIG. 3, temperature-dependent changes in the gain tilt of the EDFs11and12may be autonomously corrected (which may also be referred to as “compensated”).

Here, the excitation light source13exemplarily illustrated inFIG. 1toFIG. 3may be positioned in an end station and not in the optical amplification repeater1, and the EDFs11and12of the optical amplification repeater1may be excited by excitation light received remotely (from the end station, for example). Such an excitation may be referred to as “remote excitation” and excitation light used in the remote excitation may be referred to as “remote excitation light”.

With the remote excitation, power feeding to the excitation light source13may be dispensed with in the optical amplification repeater1, in addition to the power feeding for control of the branching ratio of the excitation light. Therefore, power feeding equipment or laying of a power cable and the like for the optical amplification repeater1may be dispensed with.

Stated differently, with the remote excitation, since an active component such as an excitation light source and the like may be disused in the optical amplification repeater1, a passive component may constitute the optical amplification repeater1, which may thus lead to disuse of the power feeding equipment. Since the power feeding equipment may be disused, at a way point where the optical amplification repeater1is installed, a space may be saved and maintenance work may be facilitated or simplified.

In addition, since the active component may be disused, increase in a failure ratio of the optical amplification repeater1and thus an optical transmission system using the optical amplification repeater1may be substantially reduced. Stated differently, stable operation (reliability) of the optical amplification repeater1and thus the optical transmission system may be considerably improved.

Accordingly, a degree of freedom for an installation place of the optical amplification repeater1improves, and the optical amplification repeater1may be laid under the ground, not in such a managed environment as in a building, for example.

Incidentally, an example of a controller having a structure (or a mechanism) which may passively change the magnetic field to be provided to the Faraday rotator141depending on a temperature may include a structure using a permanent magnet and a member configured to deform depending on a temperature. A member configured to deform depending on a temperature may be referred to as a “temperature-dependent deforming member”.

For example, a relative positional relationship of the Faraday rotator141and the magnet which provides the Faraday rotator141with the magnetic field is changed by using force accompanying deformation of the temperature-dependent deforming member depending on a temperature. With this, the magnetic field provided to the Faraday rotator141may be passively changed depending on a temperature.

A non-limiting example of the temperature-dependent deforming member is a bimetal. As schematically and exemplarily illustrated inFIG. 7, the bimetal is a member made by laminating metal plates31and32having different thermal expansion coefficients. As temperature rises higher, the metal plate31or32having a larger rate of thermal expansion expands more. Consequently, “bending” (or “warping”) occurs in the bimetal30. Such force accompanying the deformation in response to a temperature change may be used to change a relative position of the Faraday rotator141and the magnet.

When an electromagnet is used, power is desirable to be fed to the magnet. Thus, a “permanent magnet” capable of maintaining properties as a magnet even without receiving a magnetic field or supply of currents from outside may be used. A non-limiting example of a permanent magnet includes an alnico magnet, a ferrite magnet, a neodymium magnet and the like.

FIG. 8illustrates an example of arrangement of Faraday rotator141and permanent magnets145and146which are used in an optical branching device14. The arrangement illustrated inFIG. 8is an arrangement referred to forked arrangement in which a light travelling direction is orthogonal to a magnetic flux direction, and the Faraday rotator141having light being transmitted in a direction orthogonal to the magnetic flux is positioned between the two permanent magnets145and146.

Then, by using the force accompanying the deformation of the bimetal30in response to a temperature change and changing a position(s) of one or both of the permanent magnets145and146with respect to the Faraday rotator141, the magnetic field of the Faraday rotator141may be changed in response to the temperature change. Therefore, the optical branching device14to which no power is desirable to be fed and in which the branching ratio is variable in response to the temperature change may be implemented.

In addition, the angle of rotation a by the Faraday effect may be obtained with the following expression (1) where H represents intensity of a magnetic field, L length of a material through which polarized light is transmitted, V the Verdet's constant:
α=VHL  (1)

Note that the Verdet's constant V is a proportionality constant specific to a material through which polarized light is transmitted, and depends on a type of a material, a wavelength of polarized light, and a temperature.

Here, suppose that a terbium-gallium-garnet is used for the Faraday rotator141and the Verdet's constant of the terbium-gallium-garnet is 0.13 min/Oe/cm. In this case, the intensity of the magnetic field H sufficient to rotate a polarization plane by 45 degrees (α=45°) by the Faraday Effect of the Faraday rotator141having the length L=2 [cm] in a direction in which light is transmitted is H=45×60/0.13/2=10384G [gauss]=1.0384T [tesla].

If the length L of the Faraday rotator141is extended, the intensity of the magnetic field sufficient to obtain the same angle of rotation α may be reduced. For example, if the length L of the Faraday rotator141is doubled to 4 cm, the intensity of the magnetic field H sufficient to rotate the polarization plane by 45 degrees (α=45°) may be controlled to half of the above.

Controller Having a First Structure Example of Changing a Permanent Magnet Position

A controller having a specific example of a structure (or a mechanism) configured to change positions of permanent magnets145and146by using force accompanying deformation in response to a temperature change of bimetal30is described hereinafter with reference toFIG. 9.

A controller exemplarily illustrated inFIG. 9is a controller having a structure in which the bimetal30is attached to each of support members61and62which are fixed to the permanent magnets145and146.

The permanent magnets145and146are each arranged in an opposed manner so that a magnetic field is uniformly applied to the Faraday rotator141. The permanent magnets145and146may be placed in a movable mechanism40so that with the opposed arrangement being maintained, a position (distance) with a respect to the Faraday rotator141may change. An example of the movable mechanism40is a guide rail.

Stated differently, the permanent magnets145and146may be slidably placed in a direction along the guide rail40while maintaining uniformity of the magnetic field applied to the Faraday rotator141. A material which does not cause disturbance in the magnetic field such as a material having same magnetic permeability as air may be used.

Sliding of the permanent magnets145and146along the guide rail40is caused by transmission to the support members61and62of the force corresponding to the deformation of the bimetals30attached to the support members61and62. Thus, the bimetals30may be designed to have thickness which only generates force sufficient to slide the permanent magnets145and146due to the deformation.

One end of each bimetal30is fixed by a fixing member50and the other end of each bimetal30is connected to movable link mechanisms611and612provided at the support members61and62. The fixing member50may be exemplarily fixed to the bimetals30, the guide rail40, the support members61and62, and a housing80which houses the permanent magnets145and146. For example, as illustrated inFIG. 9, the fixing member50may be fixed onto an inner top surface of the housing80. The guide rail40may be fixed onto an inner bottom surface of the housing80.

The movable link mechanisms611and612are each an example of a mechanism capable of rotating at a free angle with a position where the bimetal30is attached as a supporting point, for example, so that the force corresponding to the deformation of the bimetal30may be converted into force in a direction along the guide rail40.

With the controller having the structure described above, the positions of the permanent magnets145and146with respect to the Faraday rotator141may be changed through the use of the force which is generated accompanying the deformation of the bimetal in response to a temperature change, as exemplarily illustrated inFIG. 8.

Controller Having a Second Structure Example of Changing a Permanent Magnet Position

Note that while the afore-mentioned example is an example using the bimetal30as a temperature-dependent deforming member, the temperature-dependent deforming member may be single metal or alloy. However, for the single metal or the alloy, since deformation in response to a temperature change is smaller than the bimetal30, amount of displacement to be obtained corresponding to the deformation is smaller than the bimetal30.

In order to obtain the amount of displacement corresponding to the deformation which is equivalent to the bimetal30, also with the single metal or the alloy, metal plates70of the single metal or the alloy may be treated to be spiral, as schematically illustrated inFIG. 10. The spiral metal plates70expand and shrink in a diametrical direction in response to a temperature change. Thus, the spiral metal plates70may ensure larger amount of displacement in the diametrical direction than metal plates70in the form of plates do.

If the displacement in the diametrical direction is converted to displacement in the direction along the guide rail40, for example, the permanent magnets145and146may be slid along the guide rail40, similar to the configuration exemplarily illustrated inFIG. 9. In order to convert the displacement in the diametrical direction to displacement in the direction along the guide rail40, crank members63and64may be used as exemplarily illustrated inFIG. 10.

The crank member63(64) includes movable link mechanisms631and634(641and642), and is rotatable with the movable link mechanisms631and634(641and642) as a supporting point. The support member61fixed to the one permanent magnet145is attached to the one movable link mechanism631(641). One end of the spiral metal plate70is attached to the other movable link mechanism632(642). The other end of the spiral metal plate70is fixed by a fixing member which is not illustrated. The fixing member may be exemplarily fixed to the housing80.

In the controller having the structure example described above, when the spiral metal plate70expands or shrinks to the diametrical direction in response to a temperature change, the crank member63(64) responds to the expansion or shrinkage and rotates with the movable link mechanism632(643) of the crank member63(64) as a supporting point. Force in the direction along the guide rail40acting on the support member61(62) in response to the rotation, the permanent magnet145(146) slides along the guide rail40.

As such, similar to the controller exemplarily illustrated inFIG. 9, the positions of the permanent magnets145and146with the respect to the Faraday rotator141may be changed through the use of the deformation of the metal plate70in response to a temperature change. Note that the spiral metal plate70exemplarily illustrated inFIG. 10may be replaced by the bimetal30. Stated differently, the bimetal30exemplarily illustrated inFIG. 9may be treated to be spiral and applied to the similar structure toFIG. 10.

Usage as a VOA of the Optical Branching Device14

In addition, the afore-mentioned temperature-dependent optical branching device14with the variable branching ratio may be used as a variable optical attenuator (VOA) having amount of loss which varies in response to a temperature change. For example, one of two beams of branched output light, which is obtained in the configuration illustrated inFIG. 5orFIG. 6, is used, while the other is not used.

For the branched output light which is not unused, the amount of loss varies since the branching ratio at the optical branching device14changes in response to a temperature change. Therefore, the optical branching device14may be used as a temperature-dependent VOA. Note that the optical branching device14which is used as a VOA may be hereinafter designated as a “VOA14a” for convenience.

The VOA14amay be used as a replacement for the VOA16exemplarily illustrated inFIG. 1toFIG. 3. If the VOA14ais used in place of the VOA16, a gain of the EDFs11and12may be autonomously adjusted in response to a temperature change.

Example of Application to an Optical Amplification Repeater1

FIG. 11illustrates a configuration example of an optical amplification repeater1using the optical branching device14and the VOA14adescribed above. The optical amplification repeater1illustrated inFIG. 11exemplarily has such a configuration that excitation light is received with signal light through an optical transmission line5A and that EDFs11and12are remotely excited by the excitation light. Note that the optical transmission line5A is an optical fiber transmission line, for example.

Thus, as illustrated inFIG. 11, for example, the optical amplification repeater1may not be provided with the excitation light source13exemplarily illustrated inFIG. 1toFIG. 3, and includes an optical filter25, an optical branching device26, and a multiplexer27. The optical amplification repeater1also includes the EDFs11and12which has already been described, a temperature-dependent branching-ratio variable optical branching device14, two temperature-dependent VOAs14a, a GEQ15, and multiplexers17and18.

The optical filter25separates signal light received from the optical transmission line5and excitation light, and outputs the signal light to the multiplexer17and the excitation light to the optical branching device26.

The optical branching device26branches the excitation light inputted from the optical filter25, and outputs the one branched excitation light to the temperature-dependent branching-ratio variable optical branching device14and the other branched excitation light to the multiplexer27.

The temperature-dependent branching-ratio variable optical branching device14branches the excitation light inputted from the optical branching device26at a branching ratio depending on a temperature, and outputs the one branched excitation light to the multiplexer17and the other branched excitation light to the multiplexer18.

The multiplexer17multiplexes the signal light inputted from the optical filter25and the excitation light inputted from the branching-ratio variable optical branching device14, and inputs the multiplexed light to the EDF11. With this, the signal light is amplified at the EDF11by the one branched excitation light branched at the branching-ratio variable optical branching device14.

After being subjected to gain and power control by the GEQ15and the first VOA14aprovided on the subsequent stage of the EDF11, the signal light amplified at the EDF11is inputted to the multiplexer18.

The multiplexer18multiplexes the signal light inputted from the first VOA14aand the other branched excitation light inputted from the branching-ratio variable optical branching device14, and inputs the multiplexed light to the EDF12. With this, the signal light is amplified at the EDF12by the other branched excitation light branched at the branching-ratio variable optical branching device14.

After being subjected to power control by the second VOA14aprovided on the subsequent stage of the EDF12, the signal light amplified at the EDF12is inputted to the multiplexer27.

The multiplexer27multiplexes the excitation light inputted from the optical branching device26and the signal light inputted from the second VOA14a, and outputs the multiplexed light to the optical transmission line5B. Stated differently, after being used for amplification of the signal light by the EDFs11and12, remote excitation light inputted to the optical amplification repeater1together with the signal light is transmitted to the downstream side through the multiplexer27, together with the amplified signal light. Note that the optical transmission line5B is an optical fiber transmission line, for example.

Therefore, when a plurality of optical amplification repeaters1are connected to an optical transmission system in multiple stages, remote excitation light may be shared by each optical amplification repeater1. Note that if transmission (which may also be referred to as “relay”) of the remote excitation light to the downstream side is dispensed with, the optical branching device26may be disused. For example, the excitation light separated by the optical filter25may be inputted to the branching-ratio variable optical branching device14without going through the optical branching device26.

As described above, by branching the excitation light received with the signal light at the optical branching devices26and14and introducing the branched excitation light to the EDFs11and12, the optical amplification repeater1may amplify the signal light with the remote excitation light.

Here, the excitation light is branched at a branching ratio depending on a temperature by the temperature-dependent branching-ratio variable optical branching device14. Stated differently, the branching ratio of excitation light changes, following a temperature change. Thus, a gain tilt of the EDFs11and12is autonomously adjusted individually following the temperature change and compensation of the gain tilt is allowed. In addition, output light power of the EDFs11and12also follows the temperature change and may be autonomously adjusted by the temperature-dependent VOA14a.

Furthermore, in the optical amplification repeater1exemplarily illustrated inFIG. 11, the EDFs11and12, the optical filter25, the optical branching devices14and26, the multiplexers17,18and27, the VOA14a, and a gain equalization filter15, which is an example of the GEQ, are all passive components. Therefore, as described above, the power feeding equipment may be dispensed with and the gain tilt or the output light power of the EDFs11and12may be autonomously adjusted without power feeding equipment.

However, when a sharp temperature change occurs in the optical amplification repeater1, autonomous adjustment may not follow. Thus, the optical amplification repeater1may be installed on a radiator (heat sink)2, so that no sharp temperature change occurs.

Furthermore, in order to shield any heat from outside air, as schematically and exemplarily illustrated inFIG. 12, the optical amplification repeater1may be laid under the ground. Depth of undergrounding may be such that heat exchange with the face of the earth is not substantially performed and a temperature is uniform throughout the year or loosely fluctuates even if it fluctuates. A non-limiting example of the depth of undergrounding is 10 m or more. In some cases, the thermal shield effect may be further improved by means of a heat insulating material3.

As described above, in the afore-mentioned optical amplification repeater1, since the power feeding equipment may be disused, an office provided with power feeding equipment may not be placed in the middle of an optical transmission section. Therefore, in a continent and the like where an optical transmission distance is long, the number of the optical amplification repeaters1to be installed may be reduced, which is highly useful.

Modification

In the optical amplification repeater1described above, temperature-dependent characteristic changes of the EDFs11and12may be compensated. However, if fluctuations in input/output level due to fluctuations in transmission line loss may be controlled as much as possible, a system capable of more stable optical transmission may be implemented.

The fluctuations in transmission line loss lead to power fluctuations of excitation light to be transmitted with signal light. For example, since excitation light power is stronger as the transmission line loss decreases, both signal light power and the excitation light power become strong. Thus, fluctuations in the output light power of the optical amplification repeater1easily becomes large, compared with a normal optical transmission system which does not use remote excitation light. In a system in which a plurality of optical amplification repeaters1are connected in multiple stages, fluctuations of the output light power easily increase in a cumulative manner.

In order to suppress the fluctuations in the excitation light power due to the fluctuations in the transmission line loss, an optical limiter having transmittance (stated differently, optical loss or optical reflectance) of light which varies depending on the input light power may be used.FIG. 13illustrates a configuration example of an optical limiter which uses a waveguide medium. The optical limiter28illustrated inFIG. 13is exemplarily a wavelength medium having a non-linear medium281and a linear medium282.

The non-linear medium281is a medium with a refractive index (which may be referred to as a “non-linear refractive index”) which varies depending on input light power and having input light transmitted at transmittance corresponding to the refractive index. The non-linear medium281is a medium having a greater non-linear effect (having a larger non-linear refractive index, stated differently) than the linear medium282.

The linear medium282is a medium having a smaller non-linear refractive index than the non-linear medium281and arranged so that an entrance plane of light is parallel to an exit plane of light transmitted through the non-linear medium281. The exit plane of the non-linear medium281and the entrance plane of the linear medium282constitute a boundary surface between the non-linear medium281and the linear medium282.

Here, the non-linear refractive index n1 of the non-linear medium281may be expressed by a sum of the refractive index n of the non-linear medium281and a fluctuation component dxP of the refractive index n which depends on the input light power P (n1=n+dxP). Thus, the non-linear refractive index n1 of the non-linear medium281increases in response to increase of the input light power P.

When the refractive index of the linear medium282is expressed as n2, the reflectance of the optical limiter28is expressed as R, and the transmittance of the optical limiter28is expressed as T, respectively, the reflectance R of the optical limiter28may be expressed by the following expression (2) and the transmittance T may be expressed by T=1−R:
R=(n1−n2)2/(n1+n2)2(2)

Thus, the transmittance T to the input light power of the optical limiter28may be expressed as a characteristic which has an extremal value (maximum value) shifting from increase to decrease in response to the increase of the input light power P, as illustrated inFIG. 14.

The optical limiter28having the characteristic of a region where the transmittance T decreases as the input light power P increases may be used for uniform control of output light (automatic level control (ALC)). Stated differently, the optical limiter28which adjusts the refractive indices n1 and n2 so that when the input light power P increases, the transmittance T decreases to offset the increase may be used for ALC of excitation light.

As illustrated inFIG. 15B, for example, if the optical limiter28has the characteristic of a region where when the input light power P increases by 1 dB, the insertion loss increases by 1 dB (a slope being+1 dB/1 dB), the transmittance T decreases by 1 dB when the input light power P increases by 1 dB. Thus, as exemplarily illustrated inFIG. 15A, the output light may be controlled to uniform level, irrespective of whether the input light power (excitation light, for example) increases or decreases.

On the one hand, the optical limiter28having a characteristic of a region where the transmittance T increases as the input light power P decreases may be used for uniform control of the excitation light power (automatic power control (APC)). Stated differently, the optical limiter28which adjusts the refractive indices n1 and n2 so that when the input light power decreases, the transmittance T increases to make up for the decrease may be used for APC of excitation light.

As illustrated inFIG. 16B, for example, if the optical limiter has a characteristic of a region where when the input light power P decreases by 1 dB, the insertion loss increases beyond 1 dB (a slope being sharper than −1 dB/dB), the transmittance T increases beyond 1 dB when the input light power P decreases by 1 dB.

Here, when the transmission line loss (which may also be referred to as “span loss”) increases by x(dB), a gain decreases only by “kx” where a dependence rate of the gain of the EDFs11and12on the excitation light is expressed by “k”, and the input light power also decreases only by “x”. Thus, the output light power of the optical amplification repeater1decreases only by (k+1) x. To compensate the decrease, inFIG. 14, for example, the characteristic of the optical limiter28is adjusted so that the slope is −(k+1) xdB/1 dB.

According to the optical limiter28having such a characteristic, as exemplarily illustrated inFIG. 16A, as the input light (excitation light, for example) power decreases accompanying span loss, the output light power may be increased to make up for the decrease.

In the following, for convenience, the optical limiter28available for ALC of excitation light is designated as the “optical limiter28ALC”, and the optical limiter28available for APC of excitation light is designated as “optical limiter28APC”.

As exemplarily illustrated inFIG. 17, provision of the optical limiter28ALC between the optical branching device26and the multiplexer27in the configuration exemplarily illustrated inFIG. 11allows ALC of remote excitation light to be relayed to the downstream side. Thus, power fluctuations of the remote excitation light to be relayed to the downstream side are inhibited.

In addition, as exemplarily illustrated inFIG. 17, provision of the optical limiter28APC between the optical branching device26and the branching-ratio variable optical branching device14in the configuration exemplarily illustrated inFIG. 11allows APC of excitation light power to be inputted to the branching-ratio variable optical branching device14. Thus, power fluctuations of the excitation light power to be inputted to the branching-ratio variable optical branching device14may be inhibited.

Consequently, power of the excitation light branched at the branching-ratio variable optical branching device14and supplied to the EDFs11and12is stable and amplification operation by the EDFs11and12is also stable. In addition, compared with power of each wavelength of WDM signal light, excitation light power is strong enough to cause the non-linear effect. Thus, as described above, the non-linear effect may be utilized. Even if an optical phase varies in the optical limiter28, it does not cause any problem to excitation of the EDFs11and12.

According to the optical amplification repeater1having the configuration exemplarily illustrated inFIG. 17, even if the optical amplification repeater1is connected in multiple stages in a WDM optical transmission system, stable no-power supply amplification relay may be implemented.

As described above, in the optical amplification repeater1, since excitation light is simply branched, the excitation light is branched without waste and with low loss. Yet, since a branching ratio may be made variable in response to a temperature change, autonomous compensation of a gain tilt may be implemented. In addition, no-power supply amplification relay which is also practically available in multiple stage relay is allowed.

Individual Reception Configuration of Signal Light and Excitation Light

While the optical amplification repeater1provided with the optical limiter28as exemplarily illustrated inFIG. 17is such configured that excitation light is received with signal light through the optical transmission line5A, the signal light and the excitation light may be received through individual optical transmission lines5A1and5A2as exemplarily illustrated inFIG. 18. In addition, the signal light amplified at the optical amplification repeater1(EDFs11and12) and the excitation light to be relayed to the downstream side may be transmitted through individual transmission lines5B1and5B2.

In the example ofFIG. 18, the excitation light is received through the optical transmission line5A1and the signal light is received through the optical transmission line5A2. In addition, the excitation light to be relayed to the downstream side is transmitted through the optical transmission line5B1and the signal light is transmitted through the optical transmission line5B2.

An optical fiber transmission line of a same type or a different type may be applied to the optical transmission lines5A1and5A2(5B1and5B2).

As a non-limiting example, a multi-mode (which may also be referred to as “multi-core”) fiber may be applied to the optical transmission lines5A1and5B1which transmit the excitation light, and a single-mode fiber may be applied to the optical transmission lines5A2and5B2which transmit the signal light. The larger the cross-sectional area of the optical transmission lines5A1and5B1which transmit the excitation light is, the more the excitation light power which may be transmitted increases.

Thus, excitation light having power which is desirable depending on a transmission distance of signal light or the number of stages of the optical amplification repeater1(number of spans) may be transmitted (relayed) by the optical transmission lines5A1and5B1. Stated differently, restrictions on the excitation light power may be alleviated compared with a case in which signal light and excitation light are transmitted by the same optical transmission line5A (5B), as in the configuration ofFIG. 17. Consequently, a degree of freedom as an optical transmission system or transmission performance may be improved.

As may be seen from a comparison ofFIG. 17andFIG. 18, the signal light and the excitation light are not desirable to be separated and multiplexed in the optical amplification repeater1exemplarily illustrated inFIG. 18. Thus, the optical filter25and the multiplexer27exemplarily illustrated inFIG. 17are disused.

Accordingly, inFIG. 18, one excitation light branched at the branching-ratio variable optical branching device14is multiplexed at the multiplexer17with the signal light received through the optical transmission line5A2and inputted to the EDF11. In addition, the other excitation light branched at the branching-ratio variable optical branching device14is multiplexed with the signal light after being amplified by the EDF11and inputted to the EDF12. The signal light amplified at the EDF12is transmitted to the optical transmission line5B2on the downstream side through the VOA14a.

Backward Excitation Configuration

While the configurations of the optical amplification repeaters1exemplarily illustrated inFIG. 11,FIG. 17, andFIG. 18correspond to so-called a “forward pumping configuration”, they may be a “backward pumping configuration” as exemplarily illustrated inFIG. 19andFIG. 20. The configuration exemplarily illustrated inFIG. 19corresponds to a configuration which is made by changing the “forward pumping configuration” exemplarily illustrated inFIG. 17to the “backward pumping configuration”. The configuration exemplarily illustrated inFIG. 20corresponds to a configuration which is made by changing the “forward pumping configuration” exemplarily illustrated inFIG. 18to the “backward pumping configuration”.

For example, in the optical amplification repeater1illustrated inFIG. 19, excitation light is introduced to the EDFs11and12in a direction opposite to a transmission direction of signal light, through the multiplexers17band18B which are each provided on the subsequent stage of the EDFs11and12.

For example, the multiplexer17bmay be an optical filter, outputs one excitation light branched at the branching-ratio variable optical branching device14to the EDF11, and outputs the output light of the EDF11to the side of the EDF12on the subsequent stage (GEQ15, for example).

The multiplexer18bmay also be an optical filter, outputs the other excitation light branched at the branching-ratio variable optical branching device14to the EDF12, and outputs the output light of the EDF12to the multiplexer27through the VOA14a.

In addition, an optical isolator (ISO)29configured to block propagation of the excitation light which passes through the EDF11in a direction opposite to the transmission direction of the signal light may be provided between the optical filter25and the EDF11, as exemplarily illustrated inFIG. 19. The optical isolator29lets the signal light separated by the optical filter25pass in one direction (input of the EDF11).

The optical amplification repeater1exemplarily illustrated inFIG. 20corresponds to a configuration in which, similar to the configuration exemplarily illustrated inFIG. 18, an aspect that signal light and excitation light are transmitted through the individual optical transmission lines5A1and5A2(5B1and5B2) is applied to the “backward pumping configuration” exemplarily illustrated inFIG. 19.

Thus, inFIG. 20, the optical filter25and the multiplexer27exemplarily illustrated inFIG. 19are disused. The configuration example ofFIG. 20is similar to the configuration example ofFIG. 19in that the excitation light branched at the branching-ratio variable optical branching device14is introduced to the EDFs11and12in the direction opposite to the transmission direction of the signal light, through the multiplexers17band18b.

Reverse Direction Excitation Configuration

While any of the optical amplification repeater1exemplarily illustrated inFIG. 11andFIG. 17toFIG. 20is such configured that remote excitation light is received from the upstream side, the optical amplification repeater1may be such configured that the remote excitation light is received from the reverse downstream side, as exemplarily illustrated inFIG. 21andFIG. 22.

FIG. 21illustrates a configuration example in which each of the EDFs11and12is “forward pumped” by using excitation light received from the downstream side (the optical transmission line5B).FIG. 22illustrates a configuration example in which each of the EDFs11and12is “backward pumped” by using excitation light received from the downstream side (the optical transmission line5B).

In any of the configurations exemplarily illustrated inFIG. 21andFIG. 22, the excitation light is transmitted in a direction opposite to the signal light in the optical transmission lines5A and5B. Thus, the optical filter25, the optical branching device26, and the multiplexer27exemplarily illustrated inFIG. 11,FIG. 17, andFIG. 19are each replaced by the optical filter25a, the optical branching device26a, and the multiplexer27a. In addition, inFIG. 21andFIG. 22, the optical limiter28ALC is provided between the optical filter25aand the optical branching device26since the excitation light is transmitted (relayed) to the upstream side. With the optical limiter28ALC, power fluctuations depending on transmission line loss of excitation light to be relayed to the upstream side may be inhibited as already described above. Note that arrangement and position of the optical limiter28APC remains same as the configuration examples which have already been described (seeFIG. 17toFIG. 20).

The optical filter25aoutputs to the multiplexer17signal light received from the optical transmission line5A on the upstream side and outputs to the optical transmission line5A excitation light received from the optical limiter28ALC. The optical filter25amay be implemented by means of an optical circulator.

The optical filter27aoutputs to the optical transmission line5B on the downstream side signal light inputted from the EDF12through the VOA14aand outputs to the optical branching device26aexcitation light received from the optical transmission line5B.

The optical branching device26abranches the excitation light which passes through the optical filter27a, outputs one branched excitation light to the side of the branching-ratio variable optical branching device14(optical limiter28APC), and outputs the other branched excitation light to the side of the optical transmission line5A (optical limiter28ALC).

The configuration ofFIG. 21is similar to the configuration example ofFIG. 17in that the excitation light branched at the branching-ratio variable optical branching device14is each introduced through the multiplexers17and18to the EDFs11and12in the same direction as the transmission direction of the signal light.

On the one hand, the configuration ofFIG. 22is similar to the configuration example ofFIG. 19in that the excitation light branched at the branching-ratio variable optical branching device14is each introduced through the multiplexers17band18bto the EDFs11and12in a direction opposite to the transmission direction of the signal light, and that the EDFs11and12are each backward pumped.

Note that in the configurations of the optical amplification repeaters1exemplarily illustrated inFIG. 21andFIG. 22, an aspect that signal light and excitation light are transmitted through the individual transmission lines5A1and5A2(5B1and5B2), as exemplarily illustrated inFIG. 18andFIG. 20may also be applied. In addition, in the optical amplification repeater1, remote excitation light may be received from both the upstream side and the downstream side.

In addition, one of the EDFs11and12of the optical amplification repeater1may be “forward pumped” and the other may be “backward pumped” by excitation light branched at the branching-ratio variable optical branching device14, or both may be pumped.

In any aspect, autonomous compensation of a gain tilt of the EDFs11and12by the temperature-dependent branching-ratio variable optical branching device14and fluctuation control of excitation light power by the optical limiter28may be implemented.

Other Applications of the Branching-Ratio Variable Optical Branching Device14

The temperature-dependent branching-ratio variable optical branching device14described above may be provided on the subsequent stage of a multiplexer41configured to multiplex output light (laser beam) of a plurality of light sources (LDs, for example)13-1to13-N (where N is an integer of 2 or higher). In this case, the branching-ratio variable optical branching device14may branch and output laser beam multiplexed at the multiplexer41at a branching ratio depending on a temperature.

Therefore, excitation light of desirable power may be multicast while extending life of each LD13-iby inhibiting output light power of each of the LD3-i(i=any of 1 to N).

Note that while in the embodiments described above, examples in which a target which makes a branching ratio variable depending on temperature is excitation light are described, the target is not limited to the excitation light. Other light such as signal light may be a temperature-dependent target with a variable branching ratio.