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    • 专利摘要:
    An optical amplifier is provided, in which adjacent cores of a plurality of cores which each contain a rare earth element and included in a multi-core amplifying optical fiber (MCF) serve as wavelength-coupled cores amplifier, a connection MCF is connected to the amplifying MCF, a pump light source is connected to the connection MCF, and the pump light source pumps the rare earth element into the MCF of amplification through the connecting MCF.
    公开号:FR3072511A1
    申请号:FR1859539
    申请日:2018-10-15
    公开日:2019-04-19
    发明作者:Takemi Hasegawa；Hirotaka Sakuma；Tetsuya Hayashi
    申请人:Sumitomo Electric Industries Ltd；
    IPC主号:
    专利说明:

    BACKGROUND OF THE INVENTION
    FIELD OF THE INVENTION The present invention relates to an optical amplifier and an optical fiber with several cores applied to it.
    Description of the Related Art [0002] A multi-core optical fiber (hereinafter abbreviated as "MCF") comprising a plurality of cores in a single cladding is a promising technology which increases spatial density, in terms of amount of information to transmit, to effectively use a limited section of a transmission channel such as an underground pipe or an undersea cable. In particular, a multi-core and coupled-core optical fiber (hereinafter abbreviated as "CC-MCF") comprising a plurality of cores, among which guided modes are connected to each other, is very effective in increasing the spatial density in terms of the amount of information to be transmitted, since the distance between the adjacent cores is short. To distinguish signals as a plurality of guided modes that propagate in coupled CC-MCF nuclei relative to each other, a signal processing technique technique (multi-input / multi-output) is required. The cost of signal processing ΜΙΜΟ increases with the difference in delay between the guided modes (ie the differential mode delay, hereinafter referred to as "DMD").
    There is a known technique for suppressing the increase in DMD, according to which the difference in group speed between the guided modes can be reduced by appropriately defining the coupling intensity between the nuclei. It is also known that DMD accumulation is randomized with mode coupling by bending or twisting an optical fiber during its use, whereby the DMD accumulation rate can go from fiber length to power 1 to a fiber length at the power Vz. This MCF is called “multi-core and kernel and coupled mode optical fiber (hereinafter abbreviated as“ CM-CC-MCF ”), and is described by Tetsuya Hayashi et al.,“ Coupled-Core Multi-Core Fibers: High-Density Space
    Optical Transmission Fibers with Low Differential Modal Properties ”, Proc. ECOC 2015, We. 1.4.1 (2015).
    In general, the CM-CC-MCF has a mode coupling coefficient between the cores of 1 [1 / m] or more, or a power coupling coefficient between the cores of 10 [1 / m] or more. The mode coupling coefficient designates the ratio between a complex amplitude in a component of a certain guided mode which is linked to another guided mode during its propagation and a unit of length. More specifically, according to Masanori Koshiba et al., “Multi-Core fiber design and analysis: coupled mode theory and coupled-power theory”, Optics Express Vol. 19, n ° 26, pp. B102-B111 (2011), the mode coupling coefficient is defined as the coefficient of a mode coupling equation. In the present application, for a brief description, the minimum value of the mode coupling coefficient between the fundamental modes which is defined for each pair of adjacent cores is designated “mode coupling coefficient between the cores”.
    The power coupling coefficient designates the ratio between the power in a component of a certain guided mode which is connected to another guided mode during its propagation and a unit of length. More specifically, according to Masanori Koshiba et al., “Multi-core fiber design and analysis: coupled mode theory and coipled-power theory”, Optics Express Vol. 19, n ° 26, pp. B102-B111 (2011), the power coupling coefficient is defined as the coefficient of a power coupling equation. In the present application, for a brief description, the minimum value of the power coupling coefficient between the fundamental modes which is defined for each pair of adjacent cores is designated “power coupling coefficient between the cores”. As soon as the mode coupling coefficient or the power coupling coefficient becomes higher, the effect of reducing the DMD accumulation rate relative to the propagation length becomes greater. Whether or not significant mode coupling occurred during propagation by a predetermined length can be assessed on the basis of crosstalk.
    Crosstalk refers to the ratio between a portion of the optical power which is supplied to a certain core but which is delivered by another core and the optical power which is originally supplied to the old core. In the present application, for a brief description, the maximum value of crosstalk which is defined for each pair of adjacent cores is designated “crosstalk between cores”. In general, if the crosstalk between cores is -20 dB or less, the mode coupling is considered less significant. If the crosstalk between cores is -20 dB or more, or preferably -17 dB or more, significant mode coupling is considered to occur.
    To transmit an optical signal over a long distance, an optical amplifier is necessary. It is known that the use of an optical fiber with an additional rare earth core in which a rare earth element, and more particularly erbium, is added to the nuclei which propagate an optical signal to be amplified constitutes a means of efficient optical amplification for the optical amplifier. However, to allow a rare earth element to exhibit an optical amplification characteristic, pumping light is required. It is important that the MCF efficiently couples the pumping light to a plurality of rare earth added nuclei.
    In an amplification MCF described by international application No. WO2011116075, a pumping core through which the pumping light is propagated is provided on a central axis of an amplification MCF, and several cores (designated below -after "amplifier cores") to each of which a rare earth element is added are provided around the pumping core. The amplification MCF amplifies the light signal by coupling the light signal emitted by each of a plurality of cores of an MCF transmission channel to one of the plurality of corresponding amplifier cores.
    SUMMARY OF THE INVENTION An object of the present invention is to provide an optical amplifier which is applicable to optical amplification in a system which uses multi-core and coupled-core optical fibers (CC-MCF) through which light is propagated while causing power coupling among a plurality of cores, the optical amplifier being configured to have a high gain uniformity among the cores, and to allow efficient use of the pumping light; and to provide an MCF applicable to an amplification MCF included in the optical amplifier.
    To solve the above problem, an optical amplifier is provided which comprises an optical fiber for amplification with several cores, an optical fiber for connection to several cores, and a source of pumping light. The multi-core amplification optical fiber comprises a plurality of first cores which each extend along a first central axis and composed of silica glass with a rare earth element added to the silica glass, and a first cladding which surrounds the first individual nuclei and is composed of silica glass which has a lower refractive index than that of all the first nuclei. Multicore amplification optical fiber has an absorption coefficient of 1 [dB / m] or more at a pumping wavelength at which the rare earth element is pumped, and crosstalk between cores of - 17 [dB] or more at an amplification wavelength at which the rare earth element amplifies the light. The optical fiber for connection to several cores comprises a plurality of second cores which each extend along a second central axis and are optically connected to one of the plurality of first corresponding cores, the second cores being composed of glass of silica; and a second cladding which surrounds the second individual cores and is composed of silica glass which has a lower refractive index than that of all the second cores. The pumping light source provides pumping light at the pumping wavelength to the rare earth element in the plurality of first cores through the optical fiber connecting to multiple cores.
    In the optical amplifier according to the present invention, the optical fiber for connection to several cores can have crosstalk between cores of -17 [dB] or more at the pumping wavelength. Additionally, the pumping light provided by the pumping light source can pump the rare earth element after the pumping light has been coupled to at least one of the plurality of second cores, is further coupled to the remaining cores of the plurality of second cores, and is further coupled between each of the plurality of second cores and one of the plurality of corresponding first cores which is optically connected to this second core.
    In the optical amplifier according to the present invention, in a section of the amplification optical fiber with several cores which is taken orthogonally to the first central axis, one of the plurality of first cores can be positioned on the first central axis. In addition, in a section of the optical fiber for connection to several cores which is taken orthogonally to the second central axis, one of the plurality of second cores can be positioned on the second central axis. In addition, the pump light source may include a single-mode single-core optical fiber which includes a third core which extends along a third central axis, and a third cladding which surrounds the third core. In addition, the single-mode single-core optical fiber and the multi-core connection optical fiber can be arranged so that the third core and the second core which is positioned on the second central axis are optically connected to each other. the other.
    As a variant, in the optical amplifier according to the present invention, the optical fiber for amplification with several cores may further comprise a first layer of resin which surrounds the first cladding and which has a refractive index lower than that of the first cladding. In addition, the optical fiber for connection to several cores can further comprise a second layer of resin which surrounds the second cladding and which has a refractive index lower than that of the second cladding, the second cladding being optically connected to the first cladding. In addition, the pumping light which is provided by the pumping light source can be coupled to the second cladding, be further coupled between the second cladding and the first cladding, and pump the element into rare earths in the plurality of first nuclei surrounded by the first cladding.
    In the optical amplifier according to the present invention, the rare earth element can comprise, for example, erbium. In addition, the pumping wavelength can be, for example, 980 nm, and the amplification wavelength can be, for example, 1550 nm.
    Another aspect of the present invention provides an optical fiber with several cores which comprises a plurality of cores which each extend along a predetermined central axis and composed of silica glass with a rare earth element added to the silica glass, a cladding which surrounds the individual cores and composed of silica glass which has a lower refractive index than that of all the cores, and a resin coating which surrounds an outer peripheral surface of the cladding. In multi-core optical fiber, crosstalk between cores in an index guide which represents a coupling state between adjacent cores of the plurality of cores is 17 [dB] or more at a wavelength of 1550 nm.
    In multi-core optical fiber according to the above aspect of the present invention, the plurality of cores may include a core which extends in a spiral around and along the central axis.
    According to each of the above aspects of the present invention, the gain variation among the cores is suppressed so as to become weak by means of the coupling between the cores of the optical fiber for amplification with several cores (MCF) . In addition, since a multi-core and coupled-core optical fiber (CC-MCF) which includes cores (amplification cores) to which a rare earth is added is used as the amplification optical fiber, the light from pumping and the rare earth element overlap over a large area. Therefore, the efficiency of using the pump light can be increased.
    BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of an optical transmission system to which an optical amplifier according to the present invention can be applied.
    Figure 2 is a sectional view of an optical fiber with several cores according to an embodiment of the present invention, taken along a plane which is orthogonal to a central axis thereof.
    Figure 3 is a sectional view of the optical fiber with several cores illustrated in Figure 2, taken along a plane on which extends the central axis thereof.
    Figure 4 is a diagram of an optical amplifier according to a first embodiment.
    Figure 5 is a diagram of an optical amplifier according to a second embodiment.
    Figure 6 is a diagram of an optical amplifier according to a third embodiment.
    Figure 7 is a diagram of an optical amplifier according to a fourth embodiment.
    DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have examined known amplification MCFs and have identified the following problems. When an optical signal which is propagated in a transmission channel formed by a multi-core and cores and coupled mode optical fiber (CM-CC-MCF) is amplified using a known amplification MCF , the gain tends to vary significantly among the nuclei. Therefore, the advantageous effect of signal processing multi (multi-input / multi-output), which is based on an assumption that the power is uniform between modes, can be reduced. In addition, the efficiency of use of the pump light is low, and it is difficult to couple the pump light to each of the CM-CC-MCF cores. The amplification MCF described by international application No. W02011-116075 is intended for an optical signal which is propagated in a transmission channel formed by a known uncoupled MCF. Thus, the crosstalk between cores in the amplification MCF is removed in order to become weak, and the variation in the characteristics of the cores which is attributed to a manufacturing error affects the gain variation relatively directly.
    In the amplification MCF described by international application No. WO2011116075, only part of the pumping light coupled to the pumping core is coupled to the amplifier cores and contributes to the amplification. The remaining part of the pumping light does not contribute to the amplification. Thus, the efficiency of use of the pumping light is low. In general, unless the pumping core in which the pumping light travels locally and the amplifier cores to which a rare earth element is spatially added coincide with each other, the efficiency of use of the pumping light is weak. This is due to the fact that the overlap area between the pump light and the rare earth element is reduced, and that a large part of the power of the pump light is therefore dissipated without being absorbed by the element in rare earth. When considering the efficiency of using pump light, it is beneficial to couple the pump light to each of the plurality of amplifier cores. In the known art, however, to couple the pump light to each of the plurality of amplifier cores, the optical system requires a larger number of components, which results in high cost and low reliability.
    Specific embodiments of the optical amplifier and of the multi-core optical fiber (MCF) according to the present invention will now be described in detail with reference to the attached diagrams. The present invention is not limited to the following embodiments. The scope of the present invention is defined by the appended claims and is intended to include all equivalents thereof and all modifications thereof made within the scope of its scope. In the following description given with reference to the diagrams, the same reference numbers indicate the identical elements, and any redundant description is omitted.
    Figure 1 is a diagram of an optical transmission system 100 to which an optical amplifier 2 according to an embodiment of the present invention can be applied. The optical transmission system 100 includes a plurality of repeater stations 100A to 100C, and transmission channels provided between the repeater station 100A and the repeater station 100B and between the repeater station 100B and the repeater station 100C, respectively. The transmission channels provided between the repeater stations are each a CM-CC-MCF (a transmission MCF) and each comprise the optical amplifier 2. In the CM-CC-MCF which forms the transmission channel, a mode coupling occurs. However, since a signal transmitted to the repeater station undergoes signal processing ΜΙΜΟ, the signal is correctly restored, despite the occurrence of mode coupling. However, to obtain the above advantageous effect from signal processing ΜΙΜΟ, the difference in insertion loss between the CM-CC-MCF cores must be negligibly small.
    Specifically, the transmission channel provided between the repeater station 100A and the repeater station 100B comprises 1 ”optical amplifier 2, a first transmission MCF (a CM-CC-MCF input side) 7 provided between the repeater station 100A and the optical amplifier 2, and a second transmission MCF (a CM-CC-MCF on the output side) 8 provided between the optical amplifier 2 and the repeater station 100B. In the optical transmission system 100, the transmission channel provided between the repeater station 100B and the repeater station 100C has the same configuration as the transmission channel provided between the repeater station 100A and the repeater station 100B. In the optical transmission system 100, the transmission channels between the repeater stations each comprise a single line. Alternatively, to ensure greater reliability, each transmission channel may include two or more lines.
    Figure 2 is a sectional view of an amplification MCF 1 (an MCF according to the embodiment of the present invention) which can be applied to the optical amplifier 2, taken along a plane which is orthogonal to a central axis 10 thereof. The amplification MCF 1 comprises seven cores 11, one of which is positioned on the central axis 10 and the others are provided around the central axis 10 in a substantially symmetrical manner. The amplification MCF 1 further comprises a cladding 12 which surrounds the individual cores 11, and a coating (resin coating) 13 provided above the external peripheral surface of the cladding 12.
    The cladding 12 and the coating 13 are substantially coaxial with each other. The cores 11 and the cladding 12 each contain silica glass as the base material. The nuclei 11 also each contain erbium (Er), a rare earth element, as a dopant for optical amplification. It is preferable that the core 11 also contains an element such as germanium (Ge) or aluminum (Al) as a dopant to optimize the amplification characteristic and the refractive index thereof. It is also preferable that the cladding 12 also contains fluorine as a dopant. In this case, the difference in refractive index between each core 11 and the cladding 12 increases, and the optical loss which occurs when the amplification MCF 1 is bent with a small radius of curvature can be reduced. Therefore, the size of the optical amplifier can be reduced. The cores 11 each have a refractive index greater than 0.3% to 1.5% than that of the cladding 12 in the context of a difference in relative refractive index. Thus, the light is confined in the nuclei 11.
    The coating 13 is composed of a resin which hardens under ultraviolet. Preferably, the coating 13 has a refractive index 1% to 5% lower than that of the cladding 12 in the context of a difference in relative refractive index. In this case, the rare earth element of the core 11 can be pumped with the pumping light which is propagated in the cladding 12, and the output power of the amplifier can therefore be increased. In addition, although not illustrated, the coating 13 may include a plurality of layers (resin layers). If the covering 13 comprises a plurality of layers of resin, it is preferable that one of the innermost layers joined to the cladding 12 has a Young's modulus lower than that of the outer layers which comprise the outer peripheral surface of the covering 13. With the configuration, a component of a random external force applied to the external peripheral surface of the amplification MCF 1 and which acts on the glass in order to cause micro-curvatures is attenuated.
    The amplification MCF 1 propagates pumping light which has a wavelength of 980 nm or 1480 nm to pump Er, and propagates a light signal which has a wavelength of 1530 nm to 1610 nm, which is typical of optical transmission, in order to amplify the light signal. The cores 11 each preferably have a diameter of 2 µm to 16 µm, or preferably 3 µm to 8 µm. In this case, the fundamental mode defined for each of the nuclei 11 is confined in this nucleus 11 at an adequate intensity (the propagation of the fundamental mode in each nucleus 11 is guaranteed), and the higher modes are attenuated by bending the MCF of amplification 1 (the propagation of the higher modes is suppressed). Thus, the increase in noise from higher modes can be suppressed. The cladding 12 has a diameter of 124 μm to 126 μm and can therefore be manufactured using a known assembly line of amplifiers, at low cost. The coating 13 has an external diameter of 240 µm to 260 µm. Therefore, the loss attributed to the slight intermittent curvature (micro-bends) which tends to occur when MCF amplification 1 is used can be reduced to a low level. In the present application, an optical characteristic assigned to a single core, assuming that there is no other core, is designated as an optical characteristic defined by the individual core.
    In the amplification MCF 1, it is preferable that the interval between the centers of the adjacent nuclei 11 is equal to 1.1 times the diameter of the nuclei 11, or to 30 μm or less, and that the crosstalk between nuclei at a wavelength of 1550 nm or 17 [dB] or more. In addition, as with the CM-CC-MCF which forms the transmission channel, it is preferable that the amplification MCF 1 has a mode coupling coefficient between the nuclei of 1 [1 / m] or more, or a power coupling coefficient between the cores of 10 [1 / m] or more.
    Figure 3 is a sectional view of the amplification MCF 1, taken along a plane on which extends the central axis 10 thereof. Preferably, the cores 11 of the amplification MCF 1 each extend in a spiral around and along a fiber axis (the central axis 10) with a spiral period of 0.5 [m] or less , whereby a mode coupling is caused. Thus, the difference in insertion loss between the cores 11 of the amplification MCF 1 can be reduced. Therefore, the reduction of the advantageous effect of signal processing ΜΙΜΟ which can be caused by the difference in insertion loss between the cores can be prevented. In addition, the requirement for the manufacturing tolerance of Amplification MCF 1 is made easier. Consequently, the manufacturing cost is reduced. In an amplifier, gain and loss occur. The gain can be considered a negative loss. Thus, gain and loss are generally referred to as "insertion loss".
    Figure 4 is a diagram of an optical amplifier 2a according to a first embodiment. In the optical transmission system 100, the optical amplifier 2a forms a part of each transmission channel which extends between the adjacent repeater stations. FIG. 4 illustrates only an optical configuration of the optical amplifier 2a, and the coatings of the respective optical fibers are not illustrated. However, it is desirable that each of the optical fibers is properly covered so that the mechanical strength and the ease of removal of the cladding modes are improved. The optical amplifier 2a is provided between the first transmission MCF 7 and the second transmission MCF 8 and comprises a multiplexer / demultiplexer 4, a pumping light source 5a, a first and a second connection MCF 3a and 3b, a Amplification MCF 1a, and an optical filter 6.
    The amplification MCF 1 comprises a plurality of cores (first nuclei) 11a and a cladding (first cladding) 12a which surrounds the individual cores 11a, and is provided between the multiplexer / demultiplexer 4 and the optical filter 6. The Amplification MCF 1a has substantially the same arrangement of nuclei (in terms of number and interval of nuclei) as amplification MCF 1 and propagates a light signal at a wavelength of 1550 nm while coupling the modes of it. The connection MCF comprises the first connection MCF 3 a provided between the multiplexer / demultiplexer 4 the amplification MCF la, and the second connection MCF 3 b provided between the multiplexer / demultiplexer 4 and the pumping light source 5a. A first mode field diameter (MDF) conversion MCF 3c is provided between the multiplexer / demultiplexer 4 and the first transmission MCF 7. A second MCF of 3d MFD conversion is provided between the optical filter 6 and the second MCF transmission 8. The pumping light source 5a comprises an electroluminescent part 51a, and a bundle of pumping light output fibers 52a provided between the electroluminescent part 51a and the second connecting MCF 3b. The pumping light output fiber bundle 52a is a single mode fiber bundle (SMF) which each comprise a single core.
    The output face of the first connection MCF 3a is connected to the input face of the amplification MCF 1a. The first connection MCF 3a has substantially the same arrangement of cores (in terms of number and interval of the cores) as the amplification MCF 1 and comprises a plurality of cores (second cores) 31a and a cladding (second cladding) 32a which surrounds the individual nuclei 31a. The first connection MCF 3a is composed of silica glass, like the amplification MCF 1, but does not contain any rare earth elements such as erbium as an additive, unlike the amplification MCF 1. If the first connection MCF 3a comprises cores composed of pure silica glass and a cladding containing fluorine as an additive, the boundary between each of the cores and the cladding becomes clear. This configuration is preferable, since alignment between the first connection MCF 3a and the amplification MCF becomes easy.
    As another preferable configuration, the first connection MCF 3a can comprise a plurality of nuclei containing Ge as an additive at the same concentration as in the amplification MCF 1a, and a cladding composed of glass silica with fluorine as an additive at the same concentration as in the amplification MCF 1a, or pure silica glass. An optical fiber having this configuration can be spliced by fusion with the amplification MCF 1a with low loss. In addition, it is preferable that the first connecting MCF 3a and the amplifying MCF have it substantially the same core diameter. In this case, the first connecting MCF 3a can be spliced by fusion with the amplification MCF 1a with an even lower loss.
    The input face of the first connection MCF 3a is connected to an output port 4c of the multiplexer / demultiplexer 4. The multiplexer / demultiplexer 4 comprises a multiplexing / demultiplexing device 4d, a signal input port light 4a to which the output face of the first conversion MCF of MFD 3c is connected, a pump light input port 4b to which the output face of the second connection MCF 3b is connected, and the output port 4c to which the input face of the first connection MCF 3a is connected. The light signal coupled to the light signal input port 4a through the first MFD conversion MCF 3c and the pump light coupled to the pump light input port 4b through the second connection MCF MCF 3b are multiplexed by the multiplexing / demultiplexing device 4d, and the multiplexed light is supplied by the output port 4c. The multiplexing / demultiplexing device 4d is an optical system which comprises optical elements such as a multilayer dielectric filter and a lens. It is preferable that the first connection MCF 3a, the second connection MCF 3b, and the first MFD conversion MCF 3c have substantially the same mode field diameter. Thus, the insertion loss of the multiplexer / demultiplexer 4 can be reduced.
    The first transmission MCF 7 has substantially the same arrangement of cores (in terms of number and interval of the nuclei) as the amplification MCF 1. The first transmission MCF 7 comprises a plurality of cores 71 and a cladding 72 which surrounds the individual cores 71. The cores 71 included in the first transmission MCF 7 and among which the modes are coupled are composed of silica glass, with no rare earth element added. Thus, the cores 71 are composed of pure silica glass. Conversely, sheathing 72 contains fluorine as an additive. Alternatively, the cores 71 may contain Ge as an additive, and the cladding 72 may be composed of pure silica glass. As described by Tetsuya Hayashi et al., "Coupled-Core Multi-core Fibers: High-Spatial-Density Optical Transmission Fibers with Low Differential Modal Properties", Proc. ECOC 2015, We. 1.4.1 (2015), the first transmission MCF 7 causes more efficient mode coupling for the optical transmission light signal at a wavelength of 1530 nm to 1610 nm and is designed to propagate the light signal with weak differential mode delay (DMD).
    The first MFD to convert MFD 3c includes a stable section 31c. The stable section 31c is connected to the multiplexer / demultiplexer 4. It is desirable that the first MFD conversion MCF 3c also comprise a transition section 32c provided between the first transmission MCF 7 and the stable section 31c. The transition section 32c has a continuously changing core diameter, whereby the core diameter changes continuously between the first transmission MCF 7 and the stable section 31c. Thus, even if the first transmission MCF 7 and the first MFD conversion MCF 3c have different mode field diameters, the optical loss attributed to the mode non-conformity can be reduced. The stable section 31c includes a plurality of cores 33c and a cladding 34c which surrounds the individual cores 33c. The transition section 32c includes a plurality of cores 35c and a cladding 36c which surrounds the individual cores 35c. The cores have different diameters between the two sections 31c and 32c, but are arranged as illustrated in FIG. 2, in the two sections 31c and 32c. In particular, in the stable section 31c, the first MFD for converting MFD 3c has substantially the same configuration as the first connecting MCF 3a. Consequently, the optical loss in the multiplexer / demultiplexer 4 is reduced, and the generation of noise in the entire optical amplifier 2a is reduced.
    The transition section 32c provided between the first transmission MCF 7 and the stable section 31c of the first conversion MCF from MFD 3c is obtained, for example, by heating one end of the stable section 31c with an arc discharge. or with a flame. The nuclei 35c and the cladding 36c are obtained by diffusing Ge or fluorine added to the nuclei 33c and the cladding 34c. During this process, it is desirable that the amount of heat is limited to a specific level or below, so that the outside diameter of the cladding 36c is constant. Thus, the reduction in mechanical strength can be avoided.
    The second connection MCF 3b has substantially the same configuration as the first connection MCF 3a. Thus, the optical loss of the pumping light in the multiplexer / demultiplexer 4 is reduced, and the energy efficiency of the optical amplifier 2a as a whole can be increased. Thus, the second connection MCF 3b has substantially the same arrangement of cores (in terms of number and interval of the cores) as the amplification MCF 1 and comprises a plurality of cores (second cores) 31b and a cladding (second cladding) 32b which surrounds the individual cores 31b.
    The pumping light source 5a is connected to the other end face of the second connection MCF 3b. The pumping light source 5a includes the light emitting part 51a in the form of a group of semiconductor laser diodes which emit pumping light, and the pumping light output fiber bundle 52a which transmits the pumping light . The pumping light output fiber bundle 52a is a conventional SMF bundle which each includes a core 53a (a single core) and a cladding 54a.
    In the first embodiment, the amplification MCF 1a and the first connection MCF 3a are configured as illustrated in FIG. 3 with the cores which extend in a spiral, and propagate a light signal during the coupling of their modes. At a wavelength of 1550 nm, which is a wavelength intended for optical transmission, the amplification MCF la and the first connection MCF 3a each have a crosstalk between cores of -17 [dB] or more . As with the CMCC-MCF which forms the transmission channel, it is preferable that the amplification MCF 1a and the first connection MCF 3a each have a mode coupling coefficient between nuclei of 1 [1 / m] or more, or a power coupling coefficient between cores of 10 [1 / m] or more. Thus, the modes of the light signal propagated through the cores of the optical amplifier 2a are coupled. This mode coupling harmonizes the insertion loss values which are different between the cores of the optical amplifier 2a, and the insertion loss difference is thus reduced. Crosstalk appears as noise in a signal that is propagated. The influence of this crosstalk can be eliminated by a signal processing ΜΙΜΟ or similar carried out after a coherent detection of the signal. However, for the signal processing ΜΙΜΟ to be effective, the gain variation among the nuclei must be small.
    It is preferable that at least one of the first connection MCF 3a and the second connection MCF 3b has crosstalk between cores of -17 [dB] or more at the pumping wavelength. In this case, in the connecting MCFs 3a and 3b which include the respective second cores 31a and 31b which are each optically connected to one of the plurality of first cores 11a of the amplification MCF la, before the light of pumping is provided to the plurality of cores 11a of the amplification MCF la, the pumping lumen is coupled in advance to at least one (a specific core) of the second cores, and is further coupled to the second cores remaining. Thus, the coupling of the pumping light between the plurality of second cores 31a and 31b of the connecting MCFs 3a and 3b and the plurality of cores 11a of the amplification MCF 1a (between two groups of cores which are optically connected to the one to the other) is realized. Therefore, the size and power consumption of the optical amplifier 2a which amplifies the light signal to be transmitted through the transmission channels which are each formed of a coupled core MCF (a transmitting MCF) can be effectively reduced.
    The difference in insertion loss between the cores of an optical amplifier can reduce the advantageous effect of the signal processing ΜΙΜΟ carried out in the optical transmission system 100. However, in the optical amplifier 2a according to the first mode embodiment, since the difference in insertion loss between the cores is small, the advantageous effect of the signal processing ΜΙΜΟ can be produced satisfactorily. In the prior art amplifier MCF which includes a pumping core and which is described by international application no. W02011-116075, the crosstalk between cores is -20 dB or less. Thus, to reduce the difference in insertion loss between cores, the manufacturing error of the amplification MCF must be reduced, causing a problem that the optical amplifier is expensive. Conversely, the optical amplifier 2a according to the first embodiment does not present said problem.
    In the optical amplifier 2a, the output face of the amplification MCF 1a is connected to an input port 6a of the optical filter 6. The optical filter 6 comprises the input port 6a connected to the face of the output of the amplification MCF 1a, and an output port 6b connected to the input face of the second MCF of conversion of MFD 3d. The optical filter 6 further contains an optical isolator 6c, a bandpass filter 6d, and other optical elements (not shown) such as a lens. Optical isolator 6c blocks noise light, such as Rayleigh scattered light and Fresnel reflection light, propagated in reverse by the second MCF converting from MFD 3d to the amplifying MCF la, to reduce the noise that occurs in the optical amplifier 2a. The bandpass filter 6d blocks the amplified spontaneous emission light (ASE light) and the residual pumping light supplied by the amplification MCF la, in order to reduce the noise that occurs in the optical amplifier 2a. It is preferable that the amplification MCF la and the second conversion MCF of MFD 3d have substantially the same field mode diameter. Thus, the insertion loss of the optical filter 6 can be reduced.
    The second MCF of conversion of MFD 3d is connected to the second MCF of transmission 8. The second MCF of conversion of MFD 3d and the second MCF of transmission 8 have substantially the same arrangement of cores (in terms of number and d 'nucleus interval) than the amplification MCF la. As with the first MFD to convert MFD 3c, the second MCF to convert MFD 3d includes a stable section 31d. The stable section 31d is connected to the output port 6b of the optical filter 6. It is desirable that the second MCF for conversion of MFD 3d further comprises a transition section 32d provided between the second transmission MCF 8 and the stable section 31d. The transition section 32d has a continuously changing core diameter, whereby the core diameter changes continuously between the second transmission MCF 8 and the stable section 31d. Thus, the optical loss attributed to the mode non-conformity can be reduced.
    The stable section 31d includes a plurality of cores 33d and a cladding 34d which surrounds the individual cores 33d. The transition section 32d includes a plurality of cores 35d and a cladding 36d which surrounds the individual cores 35d. The second transmission MCF 8 comprises a plurality of cores 81 among which the modes are coupled, and a cladding 82 which surrounds the individual cores 81. The second transmission MCF 8 propagates the light signal (output light) amplified by the amplifier optical 2a to a receiver or other optical amplifier provided near it. The second transmission MCF 8 has substantially the same configuration as the first transmission MCF 7.
    Figure 5 is a diagram of an optical amplifier 2b according to a second embodiment. The description of the characteristics which are identical to those of the first embodiment is omitted accordingly. The optical amplifier 2b is provided between the first transmission MCF 7 and the second transmission MCF 8 and comprises the multiplexer / demultiplexer 4, a pumping light source 5b, a first and a second connection MCF 3e and 3f, a MCF amplification lb, and the optical filter 6.
    The amplification MCF 1b comprises a plurality of cores (first cores) 11b and a cladding (first cladding) 12b which surrounds the individual cores 11b. The amplification MCF lb is provided between the multiplexer / demultiplexer 4 and the optical filter 6. The amplification MCF lb has substantially the same arrangement of nuclei (in terms of number and interval of nuclei) as the MCF of amplification 1 and propagates the light signal at a wavelength of 1550 nm while coupling their modes. The connection MCF comprises the first connection MCF 3e provided between the multiplexer / demultiplexer 4 and the amplification MCF 1b, and the second connection MCF 3f provided between the multiplexer / demultiplexer 4 and the pumping light source 5b. The pumping light source 5b comprises an electroluminescent part 51b, and a single core pumping light output fiber 52b provided between the electroluminescent part 51b and the second connecting MCF 3f.
    The output face of the first connection MCF 3e is connected to the input face of the amplification MCF 1b. The first connection MCF 3e has substantially the same arrangement of cores (in terms of number and interval of the cores) as the amplification MCF 1 and comprises a plurality of cores (second cores) 31e and a cladding (second cladding) 32e which surrounds the individual nuclei 31a. The first 3rd connection MCF propagates the light signal at a wavelength of 1550 nm and pumping light at a wavelength of 980 nm or 1480 nm during the coupling of their modes. The first connection MCF 3e is composed of silica glass, as with the amplification MCF 1, but does not contain any rare earth elements such as erbium as an additive, unlike the amplification MCF 1 If the first connection MCF 3e comprises cores composed of pure silica glass and a cladding containing fluorine as an additive, the boundary between each of the cores and the cladding becomes clear. This configuration is preferable, since alignment between the first connection MCF 3e and the amplification MCF 1b becomes easy.
    As another preferable configuration, the first connection MCF 3e can comprise a plurality of nuclei containing Ge as an additive at the same concentration as in the amplification MCF 1b, and a cladding composed of glass silica with fluorine as an additive at the same concentration as in the amplification MCF lb, or glass of pure silica. An optical fiber having this configuration can be spliced by fusion with the amplification MCF 1b with a low loss. In addition, it is preferable that the first connection MCF 3e and the amplification MCF 1b have substantially the same core diameter. In this case, the first connection MCF 3e can be spliced by fusion with the amplification MCF 1b with an even lower loss.
    The input face of the first connection MCF 3e is connected to the output port 4c of the multiplexer / demultiplexer 4. The multiplexer / demultiplexer 4 comprises the multiplexing / demultiplexing device 4d, the light signal input port 4a to which the output face of the first transmission MCF 7 is connected, the pumping light input port 4b to which the output face of the second connection MCF 3f is connected, and the output port 4c to which the face input of the first connection MCF 3e is connected. In the optical amplifier 2b, an end portion, which includes the output face, of the first transmission MCF 7 includes a stable section 7a, an increased core diameter section 7b, and a transition section 7c. The stable section 7a has the same configuration (core diameter) as the first transmission MCF 7 illustrated in FIG. 4 and includes a plurality of cores 71a and a cladding 72a which surrounds the individual cores 71a. The increased core diameter section 7b includes the outlet face of the first transmission MCF 7, and a plurality of cores 71b which each have an increased diameter. The transition section 7c is provided between the stable section 7a and the increased core diameter section 7b and includes a plurality of cores 71c which each have an increasing diameter between the stable section 7a and the increased core diameter section 7b.
    The light signal coupled to the light signal input port 4a through the increased core diameter section 7b and the pump light coupled to the pump light input port 4b through the second Connection MCFs 3f are multiplexed by the multiplexing / demultiplexing device 4d, and the multiplexed light is supplied by the output port 4c. The multiplexing / demultiplexing device 4d is an optical system which comprises optical elements such as a multilayer dielectric filter and a lens. It is preferable that the first and second connection MCF 3e and 3f and the increased core diameter section 7b have substantially the same mode field diameter. Thus, the insertion loss of the multiplexer / demultiplexer 4 can be reduced.
    The first transmission MCF 7 comprises the section with increased core diameter 7b at its end, connected to the multiplexer / demultiplexer 4. This configuration reduces the coupling loss attributed to the axial misalignment relative to the multiplexer / demultiplexer 4. At l 'inverse, in the stable section 7a of the first transmission MCF 7 which is not at the end of the first transmission MCF 7, if the core diameter is too large, the difference in propagation constant between the modes to propagate becomes too large, and makes coupling of modes difficult. Therefore, the differential mode delay (DMD) is increased. Thus, in the stable section 7a, it is desirable that the core diameter is small enough to have only negligible non-linearity. In addition, it is desirable that the transition section 7c which has a continuously changing core diameter is provided between the stable section 7a and the increased core diameter section 7b. Thus, the optical loss attributed to the mode non-conformity can be reduced.
    The increased core diameter section 7b includes the plurality of cores 71b and a cladding 72b which surrounds the individual cores 71b. The transition section 7c includes the plurality of cores 71c and a cladding 72c which surrounds the individual cores 71c. The cores have different diameters between the two sections 7b and 7c, but are arranged as illustrated in FIG. 2, in the two sections 7b and 7c.
    The increased core diameter section 7b and the transition section 7c are each obtained, for example, by heating an end portion, including the outlet face, of the first transmission MCF 7 with discharge by arc or with a flame. The nuclei 71b and 71c are obtained by diffusing Ge or fluorine added to the nuclei 71a and the cladding 72a. During this process, it is desirable that the amount of heat is limited to a specific level or below, so that the outside diameters of the cladding 72b and the cladding 72c are constant. Thus, the reduction in mechanical strength can be avoided.
    The second connection MCF 3f has substantially the same configuration as the first connection MCF 3e. Therefore, the optical loss of pumping light in the multiplexer / demultiplexer 4 is reduced, and the power utilization efficiency of the optical amplifier 2b as a whole can be increased. Thus, the second connection MCF 3f has substantially the same arrangement of cores (in terms of number and interval of cores) as the amplification MCF 1 and comprises a plurality of cores (second cores) 31f and a cladding (second cladding) 32f which surrounds the individual cores 31f.
    The second 3F connection MCF propagates the pumping light at a wavelength of 980 nm or 1480 nm, which is intended to pump Er and amplify the light signal, while coupling its modes. In particular, in comparison with the light signal at a wavelength of 1530 nm to 1610 nm, the pumping light at a wave light of 980 nm is more closely confined in the nuclei 31f and is therefore less likely to be subjected to a fashion coupling. Consequently, the degree of confinement of the light in the nuclei 31f is reduced by making the difference in refractive index between each core 31f and the cladding 32f of the second connecting MCF 3f lower than that of the first transmitting MCF 7 Thus, even pumping light at a wavelength of 980 nm can undergo mode coupling. This configuration also applies to the first 3rd connection MCF.
    The pumping light source 5b is connected to the other face of the second connection MCF 3f. The pump light source 5b includes the light emitting portion 51b formed by a semiconductor laser diode which emits pump light, and the pump light output fiber 52b which transmits pump light. The pumping light output fiber 52b is a conventional SMF which includes a single core 53b and a cladding 54b.
    In the second embodiment, it is desirable that the first and second connection MCF 3e and 3f have arrangements of respective cores in which one of the plurality of cores 31e and one of the plurality of cores 31f are positioned on the respective central axes. With this arrangement, the connection MCFs 3e and 3f, each of which is connected coaxially to the SME. Thus, the pumping light coming from the core 53b of the pumping light output fiber (SMF) 52b is coupled to one (a core specific) of the plurality of cores 31f which is positioned on the central axis of the second connecting MCF 3f. In addition, the pumping light is coupled to the remaining nuclei 31f, excluding the specific nucleus 31f, while it is propagated through the first and second connecting MCF 3e and 3f. With this connection method, the coupling of the pumping light between the pumping light source 5b and each of the cores 11b of the amplification MCF 1b is carried out in a space as small as for the known connection between the SMFs. Consequently, the size of the optical amplifier 2b which amplifies the light signal to be transmitted through the transmission channels formed from CC-MCF can also be reduced. In addition, with this desirable embodiment, very efficient coupling of the pumping light is achieved in each of the MCFs.
    In the second embodiment, the light emitted by a laser diode (the light-emitting part 51b) is distributed uniformly to a plurality of cores using the mode coupling between the connecting MCFs (the first and the second MCF of connection 3e and 3f). In addition, the first connection MCF 3e is connected to the amplification MCF lb (the cores 31e are optically connected to the cores 11b), whereby pumping light is supplied to each of the plurality of cores 11b of the MCF d amplification lb.
    In such a pumping light supply configuration, even if the number of cores included in each of the MCFs which form the transmission channels which comprise the amplifying MCF lb is increased, the pumping light can be effectively supplied to each of the cores 11b of the amplification MCF lb without increasing the number of laser diodes (light-emitting parts 51b). Therefore, the manufacturing cost, size and power consumption of the optical amplifier 2b are low. The reduction in size and energy consumption of an optical amplifier is particularly advantageous in submarine cable systems which are strictly limited in terms of the size of the repeaters provided on the transmission channels, and in energy of the system.
    In the second embodiment, the amplification MCF 1b and the first connection MCF 3e are each configured as illustrated in FIG. 3, with the cores 11b and 31e which extend in a spiral. In addition, the amplification MCF 1b and the first connection MCF 3 e each propagate a light signal during the coupling of their modes. At a wavelength of 1550 nm, which is a wavelength intended for optical transmission, the amplification MCF 1b and the first connection MCF 3e each have crosstalk between cores of -17 [dB] or more . As with the CM-CC-MCF which forms the transmission channel, it is preferable that the amplification MCF lb and the first connection MCF 3e each have a mode coupling coefficient between the cores of 1 [1 / m] or more, or a power coupling coefficient between the cores of 10 [1 / m] or more. Thus, the modes of the light signal propagated through the nuclei of the optical amplifier 2b are coupled. This mode coupling harmonizes the insertion loss values which are different between the cores of the optical amplifier 2b, and the insertion loss difference is thus reduced.
    The difference in insertion loss between the cores of an optical amplifier can reduce the advantageous effect of the signal processing ΜΙΜΟ carried out in the optical transmission system 100. However, in the optical amplifier 2b according to the second mode embodiment, since the difference in insertion loss between the cores is small, the advantageous effect of the signal processing ΜΙΜΟ can be produced satisfactorily. In the amplification MCF of the prior art which is described by international application No. W02011-116075, and which comprises a pumping enclosure, the pumping light is supplied by a single pumping core to a plurality of cores d amplification while undergoing mode coupling. This configuration causes a problem that the light signal coupled between the amplifier cores and the pumping core can cause loss. Conversely, the optical amplifier 2b according to the second embodiment does not present this problem.
    In the optical amplifier 2b, the output face of the amplification MCF 1b is connected to the input port 6a of the optical filter 6. The optical filter 6 comprises the input port 6a connected to the face of output of the amplification MCF lb, and the output port 6b connected to the input face of the second transmission MCF 8. In the optical amplifier 2b, an end part, which includes the input face , of the second transmission MCF 8 comprises a stable section 8a, a section with increased core diameter 8b, and a transition section 8c. The stable section 8a has the same configuration (core diameter) as the second transmission MCF 8 illustrated in FIG. 4 and includes a plurality of cores 81a and a cladding 82a which surrounds the individual cores 81a. The increased core diameter section 8b includes the input face of the second transmission MCF 8, and a plurality of cores 81b which each have an increased diameter. The transition section 8c is provided between the stable section 8a and the increased core diameter section 8b and includes a plurality of cores 81c which each have an increasing diameter between the stable section 8a and the increased core diameter section 8b.
    The optical filter 6 contains the optical isolator 6c, the bandpass filter 6d, and other optical elements (not shown) such as a lens. Optical isolator 6c blocks noise light, such as Rayleigh scattered light and Fresnel reflection light, propagated in the opposite direction between the increased core diameter section 8b and the amplification MCF lb, to reduce the noise that occurs in the optical amplifier 2b. The bandpass filter 6d blocks the amplified spontaneous emission light (ASE light) and the residual pumping light supplied by the amplifier MCF lb, in order to reduce the noise that occurs in the optical amplifier 2b. It is preferable that the amplification MCF lb and the second conversion MCF of MFD 8d have substantially the same field mode diameter. Thus, the insertion loss of the optical filter 6 can be reduced.
    The end part, which comprises the entry face, of the second transmission MCF 8 comprises the stable section 8a, the transition section 8c, and the section with increased core diameter 8b. The increased core diameter section 8b includes the plurality of cores 81b and a cladding 82b which surrounds the individual cores 81b. The transition section 8c includes a plurality of cores 81c which each have a continuously increasing diameter, and a cladding 82c which surrounds the individual cores 81c and which has a constant outside diameter.
    Figure 6 is a diagram of an optical amplifier 2c according to a third embodiment. The description of the features which are identical to those of the first or second embodiment is omitted accordingly. In the optical transmission system 100, the optical amplifier 2c is part of each transmission channel which extends between the adjacent repeater stations. FIG. 6 illustrates only an optical configuration of the optical amplifier 2c, and the coatings (resin coatings) of the respective optical fibers are not illustrated, except for certain coatings with a low refractive index (resin layers 13c, 33g, and 54c ) which are each part of a double-clad structure. However, it is desirable that each of the optical fibers is properly covered so that the mechanical strength and the ease of removal of the cladding are improved. The first transmission MCF 7 comprises the plurality of cores 71 and the cladding 72 which surrounds the individual cores 71. The second transmission MCF 8 comprises the plurality of cores 81 and the cladding 82 which surrounds the individual cores 81.
    The optical amplifier 2c is provided between the first transmission MCF 7 and the second transmission MCF 8 and comprises the multiplexer / demultiplexer 4, a pumping light source 5c, a first connection MCF 3g, an MCF d amplification le, and the optical filter 6. The amplification MCF le comprises a plurality of cores (first cores) 11c, a cladding (first cladding) 12c which surrounds the individual cores 11c, and the coatings, and is provided between the multiplexer / demultiplexer 4 and the optical filter 6. The amplification MCF has substantially the same arrangement of cores (in terms of number and interval of nuclei) as the amplification MCF 1 and propagates a light signal at a length wave of 1550 nm through the nuclei 11c while coupling their modes. The resin layer (a first resin layer) 13c is composed of a resin which hardens under ultraviolet rays and which has a refractive index 1% or more lower than that of the sheathing 12c in the context of a difference in refractive index. Therefore, the light coupled to the cladding 12c can also be transmitted.
    The connection MCF comprises only the first 3g connection MCF provided between the multiplexer / demultiplexer 4 and the amplification MCF on. The optical amplifier 2c further comprises a first MCF for converting MFD 3h provided between the multiplexer / demultiplexer 4 and the first transmission MCF 7, and a second MCF for converting MFD 3i provided between the optical filter 6 and the second MCF transmission 8. The pumping light source 5c comprises an electroluminescent part 51c and a multimode optical fiber 52c. Multimode optical fiber
    52c includes a glass core 53c, and the resin layer (a second resin layer) 54c which has a lower refractive index than that of the glass core 53c.
    The output face of the first 3g connection MCF is connected to the input face of the amplification MCF on. The first 3g connection MCF has the same core layout as the amplification MCF 1 and includes a plurality of 31g cores among which the modes are coupled. The first connection MCF 3g further comprises a cladding 32g which surrounds the individual cores 31g, and the resin layer (a second resin layer) 33g which surrounds the cladding (second cladding) 32g and which has a refractive index lower than that of 32g cladding. The 31g cores are capable of propagating a light signal which has undergone mode coupling. The 32g cladding is capable of propagating a pumping light. The 31g cores and the 32g cladding of the first 3g connection MCF are composed of silica glass, with no rare earth elements such as Er added to the 31g cores.
    If the first 3g connection MCF comprises cores composed of pure silica glass and a cladding which contains fluorine as an additive, the limit between each of the cores and the cladding becomes clear. This configuration is preferable, since alignment between the first 3g connection MCF and the amplification MCF becomes easy. As another preferable configuration, the first 3g connection MCF can comprise a plurality of nuclei containing Ge as an additive at the same concentration as in the amplification MCF 1c, and a cladding composed of silica glass with fluorine as an additive at the same concentration as in the amplification MCF le, or pure silica glass. An optical fiber having this configuration can be spliced by fusion with the amplification MCF with a low loss. In addition, it is preferable that the first 3g connecting MCF and the amplifying MCF have substantially the same core diameter. In this case, the first 3g connection MCF can be spliced by fusion with the amplification MCF le with an even lower loss.
    The input face of the first connection MCF 3g is connected to the output port 4c of the multiplexer / demultiplexer 4. The multiplexer / demultiplexer 4 comprises the multiplexing / demultiplexing device 4d, the light signal input port 4a to which the output face of the first MFD conversion MFD 3h is connected, the pumping light input port 4b to which the output face of the multimode optical fiber
    52c which is part of the pumping light source 5c is connected, and the output port 4c to which the input face of the first connection MCF 3g is connected. The light signal coupled to the light signal input port 4a through the first MFD conversion MCF 3h and the pump light coupled to the pump light input port 4b through the multimode fiber 52c are multiplexed by the multiplexing / demultiplexing device 4d, and the multiplexed light is supplied by the output port 4c. The multiplexing / demultiplexing device 4d is an optical system which comprises optical elements such as a multilayer dielectric filter and a lens.
    The first 3h MFD conversion MCF includes a stable 31h section. The stable section 31h is connected to the multiplexer / demultiplexer 4. It is desirable that the first MCF for converting MFD 3h further comprises a transition section 32h provided between the first transmission MCF 7 and the stable section 31h. The transition section 32h has a continuously changing core diameter, whereby the core diameter changes continuously between the first transmission MCF 7 and the stable section 31h. Thus, the optical loss attributed to the mode non-conformity can be reduced. The stable section 31h comprises a plurality of cores 33h and a cladding 34h which surrounds the individual cores 33h. The transition section 32h includes a plurality of cores 35h and a cladding 36h which surrounds the individual cores 35h. The cores have different diameters between the two sections 31h and 32h, but are arranged as illustrated in FIG. 2, in the two sections 31h and 32h. In particular, in the stable section 31h, the first MCF of conversion of MFD 3h has substantially the same configuration as the first MCF of connection 3g. Thus, the optical loss of the light signal in the multiplexer / demultiplexer 4 is reduced, and the generation of noise in the optical amplifier 2c as a whole is reduced.
    The 32h transition section provided between the first transmission MCF 7 and the stable section 31h of the first MFD conversion MCF 3h is obtained, for example, by heating one end of the stable section 31h with an arc discharge. or with a flame. The 35h cores and the 36h cladding are obtained by diffusing Ge or fluorine added to the 33h cores and the 34h cladding. During this process, it is desirable that the amount of heat is limited to a specific level or below, so that the outside diameter of the cladding 36h is constant. Thus, the reduction in mechanical strength can be avoided.
    The multimode optical fiber 52c comprises the core 53c composed of silica glass, and the resin layer 54c composed of a resin which hardens under ultraviolet rays and which has a refractive index lower by 1% or more than that of cladding 53c in the context of a difference in refractive index. The other side of the multimode optical fiber 52c is coupled to the light-emitting part 51c formed of a semiconductor laser diode which emits pumping light.
    In the third embodiment, the light emitted by a laser diode (the light-emitting part 51c) is propagated towards the sheathing 32g of the first connection MCF 3g and towards the sheathing 12c of the amplification MCF le, by means of whereby the rare earth element contained in the plurality of cores 11c in the cladding 12c of the amplification MCF is pumped uniformly. In such a pump light supply configuration, even if the number of nuclei included in each of the MCFs which form the transmission channels which comprise the boost MCF is increased, the pump light can be efficiently supplied to each of the 11c nuclei of the amplification MCF le without increasing the number of laser diodes (light-emitting parts 51c). Therefore, the manufacturing cost, size and power consumption of the optical amplifier 2c are low. The reduction in size and energy consumption of an optical amplifier is particularly advantageous in submarine cable systems which are strictly limited in terms of the size of the repeaters provided on the transmission channels, and in energy of the system.
    In the third embodiment, a multimode space laser can be used as a pumping light source 5c which couples the pumping light with the second cladding. The multimode space laser has a lower spatial power density than a single mode space laser. Consequently, the multimode space laser can emit pumping light at higher output power. Thus, the output power of the optical amplifier 2c can be increased.
    In the third embodiment, the amplification MCF 1c and the first connection MCF 3g are configured as illustrated in FIG. 3, with the cores 11c and 31g which extend in a spiral, and propagate a light signal while coupling their modes. At a wavelength of 1550 nm, which is a wavelength intended for optical transmission, the amplification MCF le and the first connection MCF 3g each have crosstalk between cores of -17 [dB] or more . As with the CM-CC-MCF which forms the transmission channel, it is preferable that the amplification MCF and the first connection MCF 3g each have a mode coupling coefficient between the nuclei of 1 [1 / m] or more, or a power coupling coefficient between the cores of 10 [1 / m] or more. Thus, the modes of the light signal propagated through the nuclei of the optical amplifier 2c are coupled. This mode coupling harmonizes the insertion loss values which are different between the cores of the optical amplifier 2c, and the difference in insertion loss is thus reduced. In addition, it is preferable that the first 3g connection MCF has a cross-talk between cores of -17 [dB] or more at the pumping wavelength.
    The difference in insertion loss between the cores of an optical amplifier can reduce the advantageous effect of the signal processing ΜΙΜΟ carried out in the optical transmission system 100. However, in the optical amplifier 2c according to the third mode embodiment, since the difference in insertion loss between the cores is small, the advantageous effect of the signal processing ΜΙΜΟ can be produced satisfactorily. In the amplification MCF of the prior art which is described by international application No. W02011-116075, and which comprises a pumping core, the pumping light is supplied by a single pumping core to a plurality of cores d amplification while undergoing mode coupling. This configuration causes a problem that the light signal coupled between the amplifier cores and the pumping core can cause loss. Conversely, the optical amplifier 2c according to the third embodiment does not present this problem.
    In the optical amplifier 2c, the output face of the amplification MCF 1c is connected to the input port 6a of the optical filter 6. The optical filter 6 comprises the input port 6a connected to the face of output of the amplification MCF le, and the output port 6b connected to the input face of the second conversion MCF of MFD 3i. In addition, the optical filter 6 contains the optical isolator 6c, the bandpass filter 6d, and other optical elements (not shown) such as a lens. The optical isolator 6c blocks noise light, such as Rayleigh scattered light and Fresnel reflection light, propagated in the opposite direction between the conversion MCF of MFD 3i and the amplification MCF le, in order to reduce the noise that occurs in the optical amplifier 2c. The bandpass filter 6d blocks the amplified spontaneous emission light (ASE light) and the residual pumping light supplied by the amplification MCF le, in order to reduce the noise that occurs in the optical amplifier 2c.
    The second MCF of conversion of MFD 3i is connected to the second MCF of transmission 8. The second MCF of conversion of MFD 3i and the second MCF of transmission 8 have substantially the same arrangement of cores (in terms of number and d (nucleus interval) than has amplification MCF 1. In addition, as with the first conversion MCF from MFD 3h, the second conversion MCF from MFD 3i comprises a stable section 31i. The stable section 31i is connected to the output port 6b of the optical filter 6. It is desirable that the second conversion MCF of MFD 3i also comprise a transition section 32i provided between the second transmission MCF 8 and the stable section 31i. The transition section 32i has a continuously changing core diameter, whereby the core diameter changes continuously between the second transmission MCF 8 and the stable section 31i. Thus, the optical loss attributed to the mode non-conformity can be reduced. The stable section 31i includes a plurality of cores 33i and a cladding 34i which surrounds the individual cores 33i. The transition section 32i includes a plurality of cores 35i and a cladding 36i which surrounds the individual cores 35i.
    Figure 7 is a diagram of an optical amplifier according to a fourth embodiment. The optical amplifier according to the fourth embodiment can be applied to a case in which the transmission channels between the repeater stations of the optical transmission system 100 each comprise two lines. The optical amplifier comprises two optical amplifiers 2d and 2e which are pumped by two respective pumping light sources 5d and 5e. With such a configuration, a light signal is propagated and amplified in each of the two lines of the transmission channel (the transmission line extending between a first transmission MCF 7A and a second transmission MCF 8A, and the transmission line s extending between a first transmission MCF 7B and a second transmission MCF 8B). Specifically, the light signal which is amplified by the optical amplifier 2d of the two amplification lines is supplied by an optical filter 6A to the second transmission MCF 8A, and the light signal which is amplified by the optical amplifier 2e is supplied by an optical filter 6B to the second transmission MCB 8B. In addition, the pump light supplied by the pump light source 5d and the pump light supplied by the pump light source 5e are multiplexed by an optical coupler 9 and then are coupled with two respective multiplexers / demultiplexers 4A and 4B.
    The two transmission MCFs 7A and 7B each correspond to the first transmission MCF 7 illustrated in any one of FIGS. 4 to 6. The two second transmission MCFs 8A and 8B each correspond to the second transmission MCF 8 illustrated in any one of FIGS. 4 to 6. The two multiplexers / demultiplexers 4A and 4B each correspond to the multiplexer / demultiplexer 4 illustrated in any one of FIGS. 4 to 6. The two optical filters 6A and 6B each correspond to the filter optical 6 illustrated in any one of FIGS. 4 to 6. The pumping light sources 5d and 5e each correspond to any of the pumping light sources 5a to 5c illustrated in FIGS. 4 to 6. The optical amplifiers 2d and 2e each correspond to any of the optical amplifiers 2a to 2c illustrated in FIGS. 4 to 6.
    In the optical amplifier according to the fourth embodiment, even if one of the pumping light sources 5d and 5e has a failure, the pumping light is supplied by the other. Therefore, a complete malfunction of one of the optical amplifiers 2d and 2e can be avoided. Thus, according to the fourth embodiment, the optical amplifier as a whole cannot stop, with a reduced number of components. Thus, greater reliability can be ensured with a smaller volume (a reduced optical amplifier adjustment capacity), this structural characteristic is particularly important in submarine cable systems.
    权利要求:
    Claims (7)
    [1" id="c-fr-0001]
    1. Optical amplifier which includes:
    a multi-core amplification optical fiber which includes a plurality of first cores which each extend along a first central axis and composed of silica glass with a rare earth element added to the silica glass, and a first cladding which surrounds the first individual cores and composed of silica glass which has a refractive index lower than that of all the first cores, the optical fiber amplification with several cores having an absorption coefficient of 1 dB / m or more at a pumping wavelength at which the rare earth element is pumped, and a crosstalk between nuclei of -17 dB or more at an amplification wavelength at which the rare earth element amplifies the light ;
    an optical fiber for connection to several cores which comprises a plurality of second cores which each extend along a second central axis and are optically connected to one of the plurality of corresponding first cores, the second cores being composed of glass silica, and a second cladding which surrounds the second individual cores and composed of silica glass which has a lower refractive index than that of all the second cores; and a pumping light source which provides pumping light at the pumping wavelength to the rare earth element in the plurality of first cores through the optical fiber connecting to multiple cores.
    [2" id="c-fr-0002]
    2. The optical amplifier according to claim 1, in which the optical fiber for connection to several cores has a crosstalk between cores of -17 dB or more at the pumping wavelength, and in which the pumping light supplied by the source of pumping light pumps the rare earth element after the pumping light has been coupled to at least one of the plurality of second cores, has been coupled to the remaining cores of the second plurality of cores, and has been coupled between each of the plurality of second cores and one of the plurality of corresponding first cores which is optically connected to this second core.
    [3" id="c-fr-0003]
    3. An optical amplifier according to claim 1 or 2, in which, in a section of the multi-core amplification optical fiber which is taken orthogonally to the first central axis, one of the plurality of first cores is positioned on the first central axis, in which, in a section of the optical fiber connecting to several cores which is taken orthogonally to the second central axis, one of the plurality of second cores is positioned on the second central axis, in which the pumping light source comprises a single mode single core optical fiber which includes a third core which contains and which extends along a third central axis, and a third cladding which surrounds the third core, and in which the single-mode single-core optical fiber and the multi-core connection optical fiber are arranged so that the third core and the second core which is positioned é on the second central axis are optically connected to each other.
    [4" id="c-fr-0004]
    4. The optical amplifier of claim 1, wherein the multi-core amplification optical fiber further comprises a first resin layer which surrounds the first cladding and which has a lower refractive index than that of the first cladding, in which the an optical fiber for connection to several cores further comprises a second layer of resin which surrounds the second cladding and which has a lower refractive index than that of the second cladding, the second cladding being optically connected to the first cladding; and wherein the pumping light which is provided by the pumping light source is coupled to the second cladding, is further coupled between the second cladding and the first cladding, and pumps the rare earth element into the plurality of first cores surrounded by the first cladding.
    [5" id="c-fr-0005]
    5. An optical amplifier according to any one of claims 1 to 4, in which the rare earth element comprises erbium, and in which the pumping wavelength is 980 nm, and the wavelength amplification is 1550 nm.
    [6" id="c-fr-0006]
    6. Multi-core optical fiber which includes:
    a plurality of cores which each extend along a predetermined central axis and composed of silica glass with a rare earth element added to the silica glass;
    a cladding which surrounds the individual cores and composed of silica glass which has a lower refractive index than that of all the cores; and a resin coating which surrounds an outer peripheral surface of the cladding, in which index crosstalk between cores which represents a state of coupling between adjacent cores of the plurality of cores is -17 dB or more at a length 1550 nm wave.
    [7" id="c-fr-0007]
    7. A multi-core optical fiber according to claim 6, wherein the cores comprise a core which extends in a spiral around and along the central axis.
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    同族专利:
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    GB201816538D0|2018-11-28|
    GB2569216B|2022-03-09|
    JP2019075450A|2019-05-16|
    CN109672074A|2019-04-23|
    GB2569216A|2019-06-12|
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    法律状态:
    2019-10-23| PLFP| Fee payment|Year of fee payment: 2 |
    2020-10-22| PLFP| Fee payment|Year of fee payment: 3 |
    2021-01-08| PLSC| Publication of the preliminary search report|Effective date: 20210108 |
    2021-10-22| PLFP| Fee payment|Year of fee payment: 4 |
    优先权:
    申请号 | 申请日 | 专利标题
    JP2017200294|2017-10-16|
    JP2017200294A|JP2019075450A|2017-10-16|2017-10-16|Optical amplifier and multi-core optical fiber|
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