WO2018138982A1

WO2018138982A1 - Optical fiber for amplification

Info

Publication number: WO2018138982A1
Application number: PCT/JP2017/037515
Authority: WO
Inventors: 淑通安間; 竹永　勝宏
Original assignee: 株式会社フジクラ
Priority date: 2017-01-27
Filing date: 2017-10-17
Publication date: 2018-08-02
Also published as: JP2018121033A; JP6668272B2

Abstract

An optical fiber (10) for amplification, in which, regarding light of a specific higher-order mode propagating through a core (11), the average concentration of an active element from the specific inside position (Ri) to the specific outside position (Ro) is higher than the average concentration of the active element from the center point (C) of a core (11) to the specific inside position (Ri) for light of any higher-order mode that is used for communication and that propagates through the core (11), where the specific inside position (Ri) is the innermost position at which the normalized intensity of light of the LP₀₁ mode and the normalized intensity of light of the specific higher-order mode match in the radial direction, and the specific outside position (Ro) is the outermost position at which the normalized intensity of light of the specific higher-order mode is 0.0001, and the amplification factor for light of all higher-order modes used for communication is greater than the amplification factor for light of the basic mode.

Description

Amplifying optical fiber

The present invention relates to an amplification optical fiber capable of amplifying higher-order mode light than fundamental mode light.

In optical communication using an optical fiber, information is superimposed on light in the LP ₀₁ mode (basic mode), and information communication is performed by superimposing information on light in an LP mode higher than the basic mode such as the LP ₁₁ mode. Multi-mode communication to perform is known. In optical communication, it is known that the intensity of light propagating through an optical fiber is attenuated by long-distance transmission, and the intensity of the attenuated light is amplified using an optical fiber amplifier.

For the optical fiber amplifier, an amplification optical fiber to which an active element such as a rare earth element is added is used. Non-Patent Document 1 below describes an amplification optical fiber in which an active element is added to a wide range of a core and a clad including a waveguide region for fundamental mode light and higher-order mode light. According to this amplification optical fiber, it is said that fundamental mode light and higher mode light can be amplified equally.

By the way, higher-order mode light propagating through an optical fiber tends to have a larger loss than fundamental mode light. The main cause of this is as follows. The mode field diameter (MFD) of the higher mode light is larger than the mode field diameter of the fundamental mode, and the light intensity at the interface between the core and the clad is higher in the higher mode light than in the fundamental mode light. Since impurities such as hydroxyl groups tend to remain at the interface between the core and the clad, light tends to be lost at the interface. Therefore, higher-order mode light having a higher intensity than fundamental mode light at the interface causes a greater loss.

For this reason, in the optical fiber amplifier, it is preferable that the light in the higher mode is amplified than the light in the fundamental mode. However, as described above, even if the amplification optical fiber of Non-Patent Document 1 is used, the light in the fundamental mode and the light in the higher order mode are equally amplified.

Therefore, an object of the present invention is to provide an amplification optical fiber capable of amplifying light of a higher mode than light of a fundamental mode.

In order to solve the above-mentioned problems, the present invention has a core and a clad, and light of a predetermined wavelength used for communication propagates through the core in at least two LP modes and forms at least a part of a light waveguide region. An amplification optical fiber to which an active element is added, focusing on the light of a specific higher-order mode propagating through the core, the normalized intensity of the LP ₀₁ mode light in the radial direction and the specific high-order light The innermost position where the normalized intensity of the light of the next mode matches the specific inner position, and the outermost position where the standardized intensity of the light of the specific higher order mode is 0.0001 is specified as the outer position. In the case of focusing on the light of any higher-order mode used for communication propagating through the core, the average concentration of the active element from the specific inner position to the specific outer position is It is higher than the average concentration of the active element from the center point to the specific inner position, and the amplification factor of all higher-order mode light used for communication is larger than the amplification factor of light in the fundamental mode It is.

In Non-Patent Document 1, an active element having a constant concentration is added to the light guiding region, and the fundamental mode light and the higher-order mode light are amplified with the same amplification factor. On the other hand, in the above invention, when focusing on each higher-order mode of light of a predetermined wavelength used for communication, the average of the average concentration of active elements from the specific inner position to the specific outer position is the basic mode. The light intensity is higher than the average concentration average of the active elements from the center point of the core, which is larger than the light intensity of the higher mode, to the specific inner position. Therefore, the amplification factors of all the higher-order mode light propagating through the core can be made larger than the amplification factors of the fundamental mode light.

Further, the higher order mode is preferably an LP ₁₁ mode and an LP ₂₁ mode.

Since the higher-order mode is limited to the above-mentioned mode, even higher-order mode light in the even mode does not enter, and higher-order mode light can be more easily amplified than in the fundamental-mode light.

It is preferable that the active element is not added from the center point of the core to the specific inner position in the LP ₁₁ mode light propagating through the core.

The specific inner position in the LP ₁₁ mode light is closer to the center point of the core than the specific inner position in any higher order mode light. Since the active element is not added to the region from the center point of the core to at least the specific inner position, the amplification factor of the higher mode light can be made larger than the amplification factor of the fundamental mode light.

Furthermore, it is preferable that the active element is not added from the center point of the core to the specific inner position in the highest order higher-order mode light used for communication propagating through the core.

Thus, by providing a region to which no active element is added, it is possible to further increase the amplification factor of the light in the higher mode than the amplification factor of the light in the fundamental mode.

Furthermore, it is preferable that the active element is not added to the core.

Since the active element is not added to the core, the amplification factor of the higher mode light can be made larger than the amplification factor of the fundamental mode light.

Also, from the specific inner position when focusing on the light of the highest order higher order mode used for communication propagating through the core to the specific outer position when focusing on the LP ₁₁ mode light propagating through the core. In all regions between the active element and the active element at a concentration higher than the average concentration of the active element up to the specific inner position when focusing on the LP ₁₁ mode light propagating through the core from the central point of the core Is preferably added.

In this case, pay attention to the light of the highest order higher order mode used for communication propagating through the core from the specific inner position when focusing on the light of the highest order higher order mode used for communication propagating through the core. From the average concentration of the active element to the specific inner position in the case where attention is paid to the light of the LP ₁₁ mode propagating through the core from the central point of the core in all regions between the specific outer position and the specific outer position. The active element is preferably added at a high concentration.

Further, in all of the region between the specific inner position definitive if focusing on light LP ₁₁ mode propagating through the core, until the specific outer position definitive if focusing on light LP ₁₁ mode propagating through the core The active element is preferably added at a concentration higher than the average concentration of the active element from the center point of the core to the specific inner position when paying attention to LP ₁₁ mode light propagating through the core. .

In this case, the specific outer position when focusing on the light of the highest order higher order mode used for communication propagating through the core from the specific inner position when focusing on the LP ₁₁ mode light propagating through the core. In all regions between the active element and the active element at a concentration higher than the average concentration of the active element up to the specific inner position when focusing on LP ₁₁ mode light propagating through the core from the central point of the core It is preferable that an element is added.

As described above, a region where the active element is added at a higher concentration than the average concentration of the active element up to a specific inner position when focusing on the LP ₁₁ mode light propagating through the core from the center point of the core is defined. Thus, the amplification factor of the higher-order mode light can be made larger than the amplification factor of the light in the fundamental mode.

Moreover, it is preferable to provide a plurality of the cores.

In this case, a multi-core amplification optical fiber can be realized. In each core, light propagates in two or more LP modes, and in the case of paying attention to any higher-order mode light propagating through the core, the average concentration of the active element from the specific inner position to the specific outer position is The higher than the average concentration of the active element from the center point of the core to the specific inner position, the amplification factors of all higher-order modes are larger than the amplification factors of the fundamental modes. Therefore, the amplification factors of all higher-order mode light propagating through the respective cores of the multi-core amplification optical fiber can be made larger than the amplification factors of the fundamental mode light.

Further, it is preferable to further include an outer cladding that covers the cladding and has a refractive index lower than that of the cladding.

With such a double clad structure, it is possible to employ clad pumping in which pumping light propagates through the clad, and it is possible to enter pumping light having a higher intensity than core pumping in which pumping light propagates through the core. Therefore, it is possible to increase the amplification factor of the fundamental mode light and the higher-order mode light.

In this case, it is preferable that the outer periphery of the clad has a polygonal shape.

In the case of a double clad structure, when the cross-sectional shape of the clad is circular, the excitation light continues to be reflected at a constant angle at the interface between the clad and the outer clad, and the skew light propagates through the clad without the excitation light entering the core. Is likely to occur. Therefore, by making the clad polygonal as described above, skew light can be suppressed, and the active element added to the core can be excited efficiently.

Further, the active element may be erbium.

In this case, the C-band and L-band light used for optical communication can be efficiently amplified.

As described above, according to the present invention, an optical fiber for amplification capable of amplifying higher-order mode light than fundamental mode light is provided.

It is a figure which shows the mode of a cross section perpendicular | vertical to the longitudinal direction of the optical fiber for amplification which concerns on 1st Embodiment of this invention. It is a figure which shows the mode of the refractive index of the optical fiber for amplification of FIG. 1, the intensity | strength of light, and distribution of an active element. It is a figure which shows the optical fiber amplifier using the optical fiber for amplification of FIG. It is a figure which shows the density | concentration distribution of the active element of the optical fiber for amplification in the modification of 1st Embodiment similarly to FIG.2 (D). Shows similar to FIG. 2 (C) the intensity distribution of light in the case of the amplification optical fiber 10 in FIG. 1 is light LP ₂₁ mode propagates. It is a figure which shows the density | concentration distribution of the active element of the optical fiber for amplification in 2nd Embodiment similarly to FIG.2 (D). It is a figure which shows the density | concentration distribution of the active element of the optical fiber for amplification in the 1st modification of 2nd Embodiment similarly to FIG.2 (D). It is a figure which shows the concentration distribution of the active element of the optical fiber for amplification in the 2nd modification of 2nd Embodiment similarly to FIG.2 (D). It is a figure which shows the concentration distribution of the active element of the optical fiber for amplification in the 3rd modification of 2nd Embodiment similarly to FIG.2 (D). It is a figure which shows the density | concentration distribution of the active element of the optical fiber for amplification in the 4th modification of 2nd Embodiment similarly to FIG.2 (D). It is a figure which shows the other example of the optical fiber for amplification. It is a figure which shows the example of the optical fiber for multi-core amplification. It is a figure which shows the other example of the optical fiber for multi-core amplification. And the effective core area of the LP ₀₁ mode of light in Example 1 is a diagram showing the relationship between the ratio of the LP ₀₁ optical amplification factor of the mode and the LP ₁₁ mode of the optical amplification factor of. And the effective core area of the LP ₀₁ mode of light in Example 2, a diagram showing the relationship between the ratio of the LP ₀₁ optical amplification factor of the mode and the LP ₁₁ mode of the optical amplification factor of.

Hereinafter, preferred embodiments of an optical fiber according to the present invention and an optical fiber amplifier using the same will be described in detail with reference to the drawings. For ease of understanding, the scale of each figure may be different from the scale described in the following description.

(First embodiment)
<Description of optical fiber for amplification>
FIG. 1 is a view showing a state of a cross section perpendicular to the longitudinal direction of an optical fiber according to a first embodiment of the present invention.

As shown in FIG. 1, an amplification optical fiber 10 according to this embodiment includes a core 11, a clad 12 that surrounds the outer peripheral surface of the core 11 without a gap, an outer clad 13 that covers the outer peripheral surface of the clad 12, and an outer clad 13 as a main component. The diameter of the core 11 is, for example, 11 μm. The outer diameter of the cladding 12 is, for example, 125 μm, and the outer diameter of the outer cladding 13 is, for example, 200 μm.

The amplification optical fiber 10 of the present embodiment amplifies light used for communication, and an active element is added to at least a part of a waveguide region of propagating light. The amplification optical fiber 10 is a multimode optical fiber that propagates light including LP ₁₁ mode higher-order mode light in addition to LP ₀₁ mode light, which is the fundamental mode of light propagating through the core 11. . Accordingly, when the LP ₀₁ mode light and the LP ₁₁ mode light are incident on the core 11 of the amplification optical fiber 10, the light of each mode can propagate through the amplification optical fiber 10. The predetermined wavelength is, for example, a C band band or an L band band. The amplification optical fiber 10 is intended to amplify the light used for communication as described above. Therefore, even when high-order mode light not related to communication is excited, this higher-order mode is used. There is no need to consider the light.

FIG. 2 is a diagram illustrating the state of the core 11 of the amplification optical fiber 10 of FIG. 1 and the surroundings thereof. Specifically, FIG. 2A shows the core 11 and the cladding 12 in the region indicated by the dotted line in FIG. 1, FIG. 2B shows the refractive index distribution in the region shown in FIG. FIG. 2 (C) amplification optical fiber 10 of the _{LP 01} mode when normalized by _{LP 01} mode of light and _{LP 11} mode power the light when the propagating light of a predetermined wavelength of light and _{LP 11} modes FIG. 2D shows the light intensity distribution, and FIG. 2D shows the concentration distribution of the active element in the region shown in FIG.

As shown in FIG. 2B, the refractive index of the cladding 12 is set lower than that of the core 11. Although not particularly illustrated, the refractive index of the outer cladding 13 is set lower than the refractive index of the cladding 12. Further, in the amplification optical fiber 10 of the present embodiment, the refractive index of the core 11 is substantially constant in the radial direction, and the refractive index of the cladding 12 is also substantially constant in the radial direction. The relative refractive index difference between the core 11 and the clad 12 is, for example, 0.6%. The core 11 is made of, for example, quartz to which a dopant such as germanium that increases the refractive index is added, and the cladding 12 is made of, for example, quartz that is not particularly added with a dopant that increases the refractive index. Alternatively, the core 11 may be made of, for example, quartz to which a dopant that increases the refractive index is not particularly added, and the cladding 12 may be made of, for example, quartz to which a dopant such as fluorine that lowers the refractive index is added. The core 11 and the clad 12 are added with an active element as necessary, as will be described later. The outer cladding 13 is made of, for example, an ultraviolet curable resin or quartz to which a dopant for lowering the refractive index is added. The covering layer 14 is made of, for example, an ultraviolet curable resin different from the outer cladding 13.

When the core 11 as described above light including light of the light and _{LP 11} modes _{LP 01} mode propagating light and the _{LP 01} mode when normalized with the power of light of the light and _{LP 11} modes _{LP 01} mode The LP ₁₁ mode light has an intensity distribution as shown in FIG. The LP ₀₁ mode light has the intensity distribution that is the strongest at the center point C of the core 11, and the LP ₁₁ mode light has an intensity of 0 at the center point C of the core 11, and the center point C of the core 11. And the outer peripheral surface IF of the core 11 have a peak intensity. Therefore, at a specific position between the center point C of the core 11 and the outer peripheral surface IF of the core 11, the light intensity of the LP ₁₁ mode matches the light intensity of the LP ₀₁ mode. This position is defined as a specific inner position Ri. The intensity of light in the LP ₁₁ mode is greater than the intensity of light in the LP ₀₁ mode on the outer peripheral side from the specific inner position Ri.

Further, light propagating through the amplification optical fiber 10 propagates so as to protrude from the outer peripheral surface IF of the core 11 to the cladding 12. Accordingly, the light waveguide region extends to a region adjacent to the core 11 in the clad 12, and the light that protrudes into the clad 12 is attenuated from the core 11 side toward the outer peripheral side of the clad 12 in the clad 12. Further, the LP ₁₁ mode light has a larger mode field diameter than the LP ₀₁ mode light, and the protrusion to the clad 12 is large. Therefore, on the outer peripheral surface IF of the core 11, the intensity of the LP ₁₁ mode light is greater than the intensity of the LP ₀₁ mode light. The normalized intensity of the LP ₁₁ mode light is 0.0001 at specific positions on the inner and outer peripheral surfaces of the clad 12. That is, the light intensity when the intensity distribution is normalized so that the integral value in the radial direction of the light intensity distribution of the LP ₁₁ mode is 1 becomes 0.0001. This position is defined as a specific outer position Ro. In this way, when the normalized intensity of light becomes 0.0001, it does not affect the propagation and amplification of light. Therefore, there is no need to consider light whose normalized intensity is less than 0.0001.

From the above description, in the region from the center point C of the core 11 to the specific inner position Ri, the light intensity of the LP ₀₁ mode is larger than the light intensity of the LP ₁₁ mode, and from the specific inner position Ri to the specific outer position Ro. In this region, the intensity of the LP ₁₁ mode light is larger than the intensity of the LP ₀₁ mode light. Therefore, on the outer peripheral surface IF of the core 11, the intensity of the LP ₁₁ mode light is greater than the intensity of the LP ₀₁ mode light.

In the amplification optical fiber 10 of the present embodiment, as shown in FIG. 2D, no active element is added to the region from the center point C of the core 11 to the specific inner position Ri, and the specific inner position Ri. The active element is added to the region from the specific outer position Ro. This active element is an element that is brought into an excited state by excitation light. When light in the C-band or L-band is propagated as described above, for example, erbium (Er) can be cited as the active element. Further, the active element to be added varies depending on the wavelength of the propagating light, and examples of other active elements include rare earth elements such as neodymium (Nd) and ytterbium (Yb). Furthermore, bismuth (Bi) can be cited as an active element in addition to rare earth elements.

As described above, when light having a predetermined wavelength is incident on the core 11 of the amplification optical fiber 10 and light having a wavelength for exciting the active element is incident on the cladding 12, the light incident on the core 11 is amplified as follows. . That is, the excitation light incident on the clad 12 propagates mainly through the clad 12, and the active element is excited when the excitation light passes through the region where the active element is added. The excited active element causes stimulated emission by light having a predetermined wavelength propagating through the amplification optical fiber 10, and the light having the predetermined wavelength is amplified by the stimulated emission. At this time, the active element is added to the region from the specific inner position Ri to the specific outer position Ro, and the active element is not added to the region from the center point C of the core 11 to the specific inner position Ri. It is amplified in the region from the inner position Ri to the specific outer position Ro, and is not amplified in the region from the center point C of the core 11 to the specific inner position Ri. Region where light is amplified is an area intensity of light of the light LP ₁₁ mode than the intensity of the large LP ₀₁ mode, no light is amplified region, than the intensity of light of the LP ₁₁ mode of the LP ₀₁ mode This is a region where the intensity of light is high. Therefore, when compared with the amplification optical fiber in which the active element is uniformly added to the entire core, according to the amplification optical fiber 10 of the present embodiment, the LP ₁₁ mode light is more efficient than the LP ₀₁ mode light. Amplified well.

As described above, according to the amplification optical fiber 10 of the present embodiment, the amplification factor of the LP ₁₁ mode light can be made larger than the amplification factor of the LP ₀₁ mode light.

<Description of optical fiber amplifier>
Next, an optical fiber amplifier using the amplification optical fiber 10 will be described with reference to FIG. FIG. 3 is a diagram showing the optical fiber amplifier of the present embodiment. As shown in FIG. 3, the optical fiber amplifier 1 in the present embodiment includes an incident end 20 on which signal light is incident, an excitation light source 30 that emits excitation light, an optical combiner 40 on which signal light and excitation light are input, The amplification optical fiber 10 of FIG. 1 into which signal light and pumping light emitted from the optical combiner 40 are input, and an emission end 50 from which the amplified signal light is emitted are mainly configured.

The incident end 20 is connected to, for example, an optical fiber that performs optical communication using at least LP ₀₁ mode light and LP ₁₁ mode light. Accordingly, light having a predetermined wavelength including light of LP ₀₁ mode and light of LP ₁₁ mode used for optical communication is incident from the incident end 20. The predetermined wavelength is not particularly limited, but is, for example, a C band band or an L band band as described above. The light incident from the incident end 20 is composed of a core and a clad covering the core, and propagates through the optical fiber 25 that propagates the light in at least the LP ₀₁ mode and the LP ₁₁ mode.

The excitation light source 30 is composed of a plurality of laser diodes 31. In the present embodiment, the laser diode 31 is, for example, a Fabry-Perot type semiconductor laser made of a GaAs-based semiconductor and emits light having a center wavelength of 980 nm. To do. Each laser diode 31 of the excitation light source 30 is connected to an optical fiber 35, and the excitation light emitted from the laser diode 31 propagates through the optical fiber 35 as multimode light.

The optical combiner 40 to which the optical fiber 35 and the optical fiber 25 are connected is configured, for example, by melting and extending a portion where the multimode optical fiber is disposed around the optical fiber 25 as a center. The core of the optical fiber 25 and the core 11 of the amplification optical fiber 10 are optically coupled, and the core of the optical fiber 35 and the cladding 12 of the amplification optical fiber 10 are optically coupled.

Next, the operation of the optical fiber amplifier 1 will be described.

First, signal light incident from the incident end 20 is emitted from the optical fiber 25. As described above, the wavelength of the signal light is, for example, C band or L band, and includes LP ₀₁ mode light and LP ₁₁ mode light. Then, the signal light propagating through the optical fiber 25 enters the optical combiner 40.

Also, excitation light for exciting the active element added to the core 11 of the amplification optical fiber 10 is emitted from the excitation light source 30. The wavelength at this time is, for example, 980 nm as described above. Then, the excitation light emitted from the excitation light source 30 propagates through the optical fiber 35 and enters the optical combiner 40.

The signal light incident on the core 11 of the amplification optical fiber 10 from the optical combiner 40 propagates through the core 11, and the excitation light incident on the cladding 12 of the amplification optical fiber 10 propagates mainly through the cladding 12. The active element added as described above is in an excited state. Then, the excited active element causes stimulated emission by the signal light, and the signal light is amplified. At this time, the signal light is amplified in the region from the specific inner position Ri to the specific outer position Ro as described above for the amplification optical fiber 10, and in the region from the center point C of the core 11 to the specific inner position Ri. Not amplified. Accordingly, the signal light obtained by amplifying the LP ₁₁ mode light with a higher amplification factor than the LP ₀₁ mode light is emitted from the amplification optical fiber 10, and the signal light is emitted from the emission end 50.

As described above, according to the optical fiber amplifier 1 of the present embodiment, it is possible to amplify the LP ₁₁ mode light with a higher amplification factor than the LP ₀₁ mode light. Accordingly, if the incident end 20 is a LP ₀₁ mode of the optical power of the optical power and the LP ₁₁ mode is the same in the light incident in the light emitted from the optical fiber amplifier 1 of LP ₁₁ mode the power of the light LP It can be made larger than the light power of the ₀₁ mode.

Next, a modification of this embodiment will be described.

FIG. 4 is a view showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modification as in FIG. 2D. As shown in FIG. 4, in this modification, the active element is not added to the core 11, and the active element is added to the region from the outer peripheral surface IF of the core 11 to the specific outer position Ro. According to such addition of the active element, the active element is not added to the region from the center point C to the specific inner position Ri as described above, and the active element is not added to the region from the specific inner position Ri to the specific outer position Ro. Compared with the case where it is added, the amplification factor of light in the LP ₁₁ mode can be made larger than the amplification factor of light in the LP ₀₁ mode.

(Second Embodiment)
Next, a second embodiment of the present invention will be described in detail with reference to FIGS. In addition, about the component which is the same as that of 1st Embodiment, or equivalent, except the case where it demonstrates especially, the same referential mark is attached | subjected and the overlapping description is abbreviate | omitted.

The amplification optical fiber 10 of the present embodiment differs from the amplification optical fiber 10 of the first embodiment in that LP ₂₁ mode light used for communication propagates.

FIG. 5 is a view showing the light intensity distribution in the case where the LP ₂₁ mode light propagates through the amplification optical fiber 10 of FIG. 1, as in FIG. As shown in FIG. 5, in this embodiment, when the LP ₀₁ mode light and the LP ₁₁ mode light are normalized by power, the intensity distribution of the LP ₀₁ mode light and the LP ₁₁ mode light is shown in FIG. Same as C). Therefore, in the present embodiment, the position at which the intensity of the LP ₁₁ mode light matches the intensity of the LP ₀₁ mode light is defined as the specific inner position Ri _1, and the normalized intensity of the LP ₁₁ mode light is 0.0001. the a position to identify the outer position Ro _1. Therefore, the specific inner position Ri ₁ of the present embodiment matches the specific inner position Ri of the first embodiment, and the specific outer position Ro ₁ of the present embodiment matches the specific outer position Ro of the first embodiment.

Further, the light intensity of the LP ₂₁ mode is 0 at the center point C of the core 11, and there are two positions where the intensity peaks between the center point C of the core 11 and the outer peripheral surface IF of the core 11. To do. Therefore, at a specific position between the center point C of the core 11 and the outer peripheral surface IF of the core 11, there are a plurality of positions where the light intensity of the LP ₂₁ mode coincides with the light intensity of the LP ₀₁ mode. Therefore, the innermost position among the positions where the light intensity of the LP ₂₁ mode matches the light intensity of the LP ₀₁ mode is set as the specific inner position Ri ₂ . Further, the LP ₂₁ mode light has a larger mode field diameter than the LP ₀₁ mode light and the LP ₁₁ mode light, and the protrusion to the clad 12 is large. Therefore, at the interface between the core 11 and the cladding 12, the intensity of the LP ₂₁ mode light is larger than the intensity of the LP ₀₁ mode light and the LP ₁₁ mode light. The position of this interface coincides with the position of the outer peripheral surface IF of the core 11. Further, the standardized intensity of the LP ₂₁ mode light is 0.0001 at a specific position on the outer peripheral surface side of the clad 12 with respect to the specific outer position Ro ₁ described above. This position is the specific outer position Ro _2.

FIG. 6 is a view showing the concentration distribution of the active element in the amplification optical fiber 10 of the present embodiment in the same manner as FIG. As shown in FIG. 6, in the amplification optical fiber 10 of the present embodiment, an active element is added to a region from the specific inner position Ri ₂ to the specific outer position Ro _1, and the specific inner position from the center point C of the core 11. No active element is added to the region up to Ri ₂ . In the present embodiment, an active element is added between the specific inner position Ri ₁ and the specific inner position Ri ₂ that are part of a region where the intensity of the LP ₁₁ mode light is greater than the intensity of the LP ₀₁ mode light. Absent. However, when attention is focused on LP ₁₁ mode light, which is higher-order mode light, the average concentration of the active element from the specific inner position Ri ₁ to the specific outer position Ro ₁ of the LP ₁₁ mode light is the center point of the core. It is higher than the average concentration of the active element from C to the specific inner position Ri ₁ . Further, in the present embodiment, an active element is added between the specific outer position Ro ₁ and the specific outer position Ro ₂ that are part of a region where the light intensity of the LP ₂₁ mode is larger than the light intensity of the LP ₀₁ mode. It has not been. However, when attention is focused on LP ₂₁ mode light, which is higher order mode light, the average concentration of the active element from the specific inner position Ri ₂ to the specific outer position Ro ₂ of the LP ₂₁ mode light is the center point of the core. higher than the average concentration of the active elements in the C to a certain inside position Ri _2. By having such an active element concentration distribution, the amplification factor of the LP ₁₁ mode light and the LP ₂₁ mode light can be made larger than the amplification factor of the LP ₀₁ mode light.

Next, a first modification of the present embodiment will be described.

FIG. 7 is a view showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modification as in FIG. 2D. In the above example, the active element is added to the region from the specific inner position Ri ₂ to the specific outer position Ro ₁ , whereas in the present modification, as illustrated in FIG. 7, the specific outer position Ri ₂ to the specific outer position An active element is added to the region up to Ro ₂ . Even in the same manner as in the embodiment in the present modification, when attention is paid to light LP ₁₁ mode, the average concentration of the active elements in the specified position inside Ri ₁ of LP ₁₁ mode of light to a certain position outside Ro ₁ is higher than the average concentration of the active element in the center point C of the core to a certain inside position Ri _1. Similarly, when focusing on the light of the LP ₂₁ mode, the average concentration of the active elements in the specified position inside Ri ₂ of LP ₂₁ mode of light to a certain position outside Ro ₂ are identified inside position Ri from the central point C of the core _It is higher than the average concentration of active elements up to ₂ . By adding the active element in this manner, the amplification factor of the LP ₂₁ mode light can be further increased.

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

FIG. 8 is a view showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modification as in FIG. 2D. In the above example, the active element is added to the region from the specific inner position Ri ₂ to the specific outer position Ro ₁ , whereas in this modification, as illustrated in FIG. 8, the specific outer position Ri ₁ to the specific outer position An active element is added to the region up to Ro ₁ . Even in the same manner as in the embodiment in the present modification, when attention is paid to light LP ₁₁ mode, the average concentration of the active elements in the specified position inside Ri ₁ of LP ₁₁ mode of light to a certain position outside Ro ₁ is higher than the average concentration of the active element in the center point C of the core to a certain inside position Ri _1. Similarly, when focusing on the light of the LP ₂₁ mode, the average concentration of the active elements in the specified position inside Ri ₂ of LP ₂₁ mode of light to a certain position outside Ro ₂ are identified inside position Ri from the central point C of the core _It is higher than the average concentration of active elements up to ₂ . By adding the active element in this way, the amplification factor of the LP ₁₁ mode light can be further increased.

Next, a third modification of the present embodiment will be described.

FIG. 9 is a view showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modification as in FIG. 2D. In the above example, the active element is added to the region from the specific inner position Ri ₂ to the specific outer position Ro ₁ , whereas in this modification, as illustrated in FIG. 9, the specific inner position Ri ₁ to the specific outer position An active element is added to the region up to Ro ₂ . Even in the same manner as in the embodiment in the present modification, when attention is paid to light LP ₁₁ mode, the average concentration of the active elements in the specified position inside Ri ₁ of LP ₁₁ mode of light to a certain position outside Ro ₁ is higher than the average concentration of the active element in the center point C of the core to a certain inside position Ri _1. Similarly, when focusing on the light of the LP ₂₁ mode, the average concentration of the active elements in the specified position inside Ri ₂ of LP ₂₁ mode of light to a certain position outside Ro ₂ are identified inside position Ri from the central point C of the core _It is higher than the average concentration of active elements up to ₂ . By adding the active element in this manner, the LP ₁₁ mode light amplification factor and the LP ₂₁ mode light amplification factor can be further increased.

Next, a fourth modification of the present embodiment will be described.

FIG. 10 is a view showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modification as in FIG. 2D. As shown in FIG. 10, in this modification, the active element is not added to the core 11, the active element is added to an area from the outer peripheral surface IF of the core 11 to a certain position outside Ro _2. Even in the same manner as in the embodiment in the present modification, when attention is paid to light LP ₁₁ mode, the average concentration of the active elements in the specified position inside Ri ₁ of LP ₁₁ mode of light to a certain position outside Ro ₁ is higher than the average concentration of the active element in the center point C of the core to a certain inside position Ri _1. Similarly, when focusing on the light of the LP ₂₁ mode, the average concentration of the active elements in the specified position inside Ri ₂ of LP ₂₁ mode of light to a certain position outside Ro ₂ are identified inside position Ri from the central point C of the core _It is higher than the average concentration of active elements up to ₂ . According to such addition of the active element, the active element is not added to the region from the center point C to the specific inner position Ri ₂ as described above, and the region from the specific inner position Ri ₂ to the specific outer position Ro ₁ is not added. Compared with the case where an active element is added, the amplification factor of light in the LP ₁₁ mode and the amplification factor of light in the LP ₂₁ mode can be made larger than the amplification factor of light in the LP ₀₁ mode.

When the amplification optical fiber 10 according to the present embodiment or the modification is used in the optical fiber amplifier shown in FIG. 3, LP ₀₁ mode light, LP ₁₁ mode light, and LP ₂₁ mode light are input to the incident end 20. In the optical fiber for amplification 10, the LP ₁₁ mode light and the LP ₂₁ mode light are amplified with a higher amplification factor than the LP ₀₁ mode light and are emitted from the emission end 50.

As mentioned above, although this invention was demonstrated to the example for embodiment, this invention is not limited to these.

For example, in the first embodiment, an active element having a low average concentration may be added to at least a part of the region from the center point C of the core 11 to the specific inner position Ri. However, the concentration distribution in which the average concentration of the active element in the region from the specific inner position Ri to the specific outer position Ro is higher than the average concentration of the active element from the center point C of the core 11 to the specific inner position Ri is satisfied. There is a need. Even in this case, the amplification factor of the LP ₁₁ mode light can be made larger than the amplification factor of the LP ₀₁ mode light. In the modification of the first embodiment, the active element is not added to the core 11, and the active element is added to the region from the outer peripheral surface IF of the core 11 to the specific outer position Ro. However, the above-described concentration distribution is satisfied in which the average concentration of the active element in the region from the specific inner position Ri to the specific outer position Ro is higher than the average concentration of the active element from the center point C of the core 11 to the specific inner position Ri. As long as it is possible, an active element having a low average concentration may be added to at least a partial region of the core 11.

In the second embodiment and the first to third modifications of the second embodiment, a low concentration active element is added to at least a part of the region from the center point C of the core 11 to the specific inner position Ri _1. May be. However, in this case, the following density distribution needs to be satisfied in the second embodiment and the first to third modifications of the second embodiment.

That is, in the second embodiment, the LP ₁₁ mode that propagates through the core 11 from the specific inner position Ri ₂ when focusing on the light of the highest order higher-order mode (LP ₂₁ mode) used for communication propagating through the core 11. In the entire region between the specific outer position Ro ₁ when focusing on the light of the center and the light from the center point C of the core 11 to the specific inner position Ri ₁ when focusing on the LP ₁₁ mode light propagating through the core 11. The concentration distribution in which the active element is added at a concentration higher than the average concentration of the active element is satisfied.

In the first modification of the second embodiment, the core is moved from the specific inner position Ri ₂ when focusing on the light of the highest order higher order mode (LP ₂₁ mode) used for communication propagating through the core 11. Focusing on LP ₁₁ mode light propagating through the core from the center point of the core 11 in all regions up to the specific outer position Ro ₂ when focusing on the light of the highest order higher order mode used for propagating communication In this case, the concentration distribution in which the active element is added at a concentration higher than the average concentration of the active element up to the specific inner position Ri ₁ is satisfied.

In the second modification of the second embodiment, a particular inner position Ri ₁ where definitive if attention is paid to the LP ₁₁ mode propagating through the core 11 of light, if attention is paid to the LP ₁₁ mode propagating through the core 11 of light From the average concentration of the active element up to the specific inner position Ri ₁ when focusing on the LP ₁₁ mode light propagating through the core 11 from the center point C of the core 11 in all regions between the specific outer position Ro ₁ The concentration distribution in which the active element is added at a high concentration is satisfied.

Further, in the third modification of the second embodiment, the highest order higher order used for communication propagating through the core 11 from the specific inner position Ri ₁ when focusing on the LP ₁₁ mode light propagating through the core 11. Identification when focusing on LP ₁₁ mode light propagating from the center point C of the core 11 to the core 11 in all regions up to the specific outer position Ro ₂ when focusing on mode (LP ₂₁ mode) light The concentration distribution in which the active element is added at a concentration higher than the average concentration of the active element up to the inner position Ri ₁ is satisfied.

Also in the fourth modified example of the second embodiment, a low concentration active element may be added to at least a partial region of the core 11. Also, part of the region to a specific position outside Ro from certain inside position Ri of the first embodiment, a part of the region from a particular inner position Ri ₂ to a certain position outside Ro ₁ of the second embodiment, the second embodiment A part of the region from the specific inner position Ri ₂ to the specific outer position Ro ₂ in the first modified example, and one region from the specific inner position Ri ₁ to the specific outer position Ro ₁ in the second modified example of the second embodiment. In some regions from the specific inner position Ri ₁ to the specific outer position Ro ₂ in the third region of the second modification of the second embodiment, no active element is added or a low concentration of the active element is added. May be. However, in these cases, in the case where attention is paid to any higher-order mode light used for communication propagating through the core 11, the average concentration of the active element from the specific inner position to the specific outer position is the center point C of the core 11. The concentration distribution that is higher than the average concentration of the active element from the position to the specific inner position is satisfied, and the amplification factors of all higher-order modes used for communication are higher than the amplification factors of the light in the fundamental mode (LP ₀₁ mode). Is also enlarged.

FIG. 11 is a diagram showing another example of the amplification optical fiber. In the embodiment and the modification thereof, the case where the outer periphery of the cladding 12 is circular has been described. However, as shown in FIG. 11, the outer periphery of the cladding 12 is preferably polygonal. When the cross-sectional shape of the clad 12 is circular, the excitation light continues to be reflected at a certain angle at the interface between the clad 12 and the outer clad 13, and the skew light propagating through the clad 12 without the excitation light entering the core 11 is generated. It is likely to occur. Therefore, as in the present example, the clad 12 is polygonal so that the skew light can be suppressed, and the active element added to the core 11 can be excited efficiently.

Moreover, in the said embodiment and its modification, the core 11 demonstrated one example, ie, the single core amplification optical fiber. However, the core 11 may be plural. FIG. 12 is a diagram illustrating a multi-core amplification optical fiber. In FIG. 12, the core 11 is disposed at the center of the cladding 12, and a plurality of cores 11 are disposed around the core 11. That is, the core 11 is arranged 1-6. However, this is an example in which a plurality of cores 11 are provided, and the arrangement of the plurality of cores 11 is not particularly limited. In this modification, at least the LP ₀₁ mode light and the LP ₁₁ mode light propagate through each core 11. In this case, the concentration distribution of the active element is the same as that of the first embodiment and the modification thereof in each core 11 and the cladding 12 in the vicinity of the core 11. In addition, LP ₂₁ mode light may further propagate through each core 11. In this case, the concentration distribution of the active element is the same as that in the second embodiment or the clad 12 in the vicinity of the core 11. This is the same as the modification. When the amplification optical fiber of this example is used for an optical fiber amplifier, pump light is incident on the clad 12 and signal light similar to that in the above embodiment is incident on each core 11.

FIG. 13 is a diagram showing another example of an optical fiber for multicore amplification. As shown in FIG. 13, it is preferable that the outer periphery of the cladding 12 is also a polygon in the multi-core amplification optical fiber. In this case, similarly to the description in FIG. 11, skew light can be suppressed, and the active element added to the core 11 can be excited efficiently.

In the examples of the above-described embodiment and modifications, the amplification optical fiber is a double-clad fiber having an outer cladding. However, the amplification optical fiber may not be a double clad fiber. In this case, a structure in which excitation light is incident on the core 11 may be used.

In the first embodiment, it is assumed that light of the light and LP ₁₁ modes LP ₀₁ mode used in communication is amplified by the amplification optical fiber 10, in the second embodiment, the LP ₀₁ mode used in communication The light, the LP ₁₁ mode light, and the LP ₂₁ mode light are amplified by the amplification optical fiber 10. However, the present invention is not limited to this, and higher-order mode light may be amplified by the amplification optical fiber. However, even in this case, when focusing on any higher-order mode light used for communication propagating through the core 11, the average concentration of the active element from the specific inner position to the specific outer position is The concentration distribution that is higher than the average concentration of the active element from the center point C to the specific inner position is satisfied, and the amplification factors of all the higher-order mode light used for communication are fundamental mode (LP ₀₁ mode) light. It is made larger than the amplification factor. Note that in the amplification optical fiber 10, light to be amplified is LP ₀₁ mode light and LP ₁₁ mode light used for communication, LP ₀₁ mode light used for optical communication, LP ₁₁ mode. And light in the LP ₂₁ mode are preferable because even mode higher order light, such as the LP ₀₂ mode, is not subject to amplification.

Hereinafter, the contents of the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these.

Example 1
The same amplification optical fiber as the amplification optical fiber shown in FIG. 2 was measured. Therefore, in this amplification optical fiber, the refractive index distribution of the core is constant in the radial direction, and the normalized intensity of the LP ₁₁ mode light is larger than the normalized intensity of the LP ₀₁ mode light only. An active element is added. In this optical fiber for amplification, when the theoretical cutoff wavelength λc of the LP ₂₁ mode light is 1.40 μm and 1.50 μm, the effective core area A _eff of the LP ₀₁ mode light in the light of wavelength 1.55 μm. When examined the relationship between the ratio gamma ₁₁ / gamma ₀₁ with the amplification factor gamma ₁₁ of light _{LP 01} mode optical amplification factor gamma ₀₁ and _{LP 11} modes in the optical wavelength 1.55 .mu.m. The result is shown in FIG.

As shown in FIG. 14, the ratio Γ is independent of the effective core area A _eff of the LP ₀₁ mode light, regardless of whether the theoretical cutoff wavelength λc of the LP ₂₁ mode light is 1.40 μm or 1.50 μm. ₁₁ / Γ ₀₁ was approximately 1.7. Therefore, in the amplification optical fiber 10 shown in FIG. 2, the amplification factor of the LP ₁₁ mode light is larger than the amplification factor of the LP ₀₁ mode light. Therefore, according to the amplification optical fiber 10 shown in FIG. 2, it was shown that the amplification factor of the LP ₁₁ mode light can be made larger than the amplification factor of the LP ₀₁ mode light.

(Example 2)
The same amplification optical fiber as the amplification optical fiber shown in FIG. 4 was measured. Therefore, in this amplification optical fiber, the refractive index distribution of the core is constant in the radial direction, and the active element is added only to the cladding. In this amplification optical fiber, in the same manner as in the first embodiment, when the theoretical cutoff wavelength λc of the LP ₂₁ mode light is 1.40 μm and 1.50 μm, the LP ₀₁ mode light in the light of 1.55 μm wavelength is obtained. The relationship between the effective core area A _eff and the ratio Γ ₁₁ / Γ ₀₁ between the LP ₀₁ mode light amplification factor Γ ₀₁ and the LP ₁₁ mode light amplification factor Γ ₁₁ in the light having a wavelength of 1.55 μm was examined. . The result is shown in FIG.

As shown in FIG. 15, the ratio Γ is independent of the effective core area A _eff of the LP ₀₁ mode light regardless of whether the theoretical cutoff wavelength λc of the LP ₂₁ mode light is 1.40 μm or 1.50 μm. ₁₁ / Γ ₀₁ was approximately 3. Therefore, in the example shown in FIG. 4, the amplification factor of the light in the LP ₁₁ mode is larger than the amplification factor of the light in the LP ₀₁ mode, compared to the example shown in FIG. Therefore, according to the amplification optical fiber 10 shown in FIG. 4, it is shown that the amplification factor of the LP ₁₁ mode light can be made larger than the amplification factor of the LP ₀₁ mode light.

As described above, according to the present invention, an optical fiber for amplification capable of amplifying higher-order mode light than fundamental mode light is provided, and is expected to be used in the field of optical communication.

DESCRIPTION OF SYMBOLS 1 ... Optical fiber amplifier 10 ... Amplifying optical fiber 11 ... Core 12 ... Cladding 13 ... Outer cladding 14 ... Covering layer 20 ... Incident end 30 ... Excitation light source 40・・・ Optical combiner Ri, Ri ₁ , Ri ₂ ... Specific inner position Ro, Ro ₁ , Ro ₂ ... Specific outer position

Claims

An amplifying optical fiber having a core and a cladding, wherein light of a predetermined wavelength used for communication propagates through the core in at least two LP modes, and an active element is added to at least a part of a light guiding region. There,
Focusing on the specific higher-order mode light propagating through the core, the normalized intensity of the LP 01 mode light in the radial direction matches the normalized intensity of the specific higher-order mode light. When the innermost position is the specific inner position and the outermost position where the standardized intensity of the light of the specific higher-order mode is 0.0001 is the specific outer position,
When focusing on any higher-order mode light used for communication propagating through the core, the average concentration of the active element from the specific inner position to the specific outer position is determined from the center point of the core to the specific inner side. Higher than the average concentration of the active element up to the position,
An amplification optical fiber characterized in that the amplification factors of all higher-order mode light used for communication are larger than the amplification factors of fundamental mode light.
The amplification optical fiber according to claim 1, wherein the higher-order modes are an LP 11 mode and an LP 21 mode.
3. The amplification optical fiber according to claim 1, wherein the active element is not added from the center point of the core to the specific inner position in LP 11 mode light propagating through the core. 4.
4. The amplification optical fiber according to claim 3, wherein the active element is not added from the center point of the core to the specific inner position in the highest-order higher-order mode light used for communication.
The amplification optical fiber according to claim 4, wherein the active element is not added to the core.
From the specific inner position when focusing on the light of the highest order higher order mode used for communication propagating through the core to the specific outer position when focusing on the light of LP 11 mode propagating through the core In all the regions, the active element is added at a concentration higher than the average concentration of the active element up to the specific inner position when focusing on the LP 11 mode light propagating through the core from the center point of the core The optical fiber for amplification according to any one of claims 1 to 4, wherein the optical fiber for amplification is used.
When paying attention to the light of the highest order higher order mode used for communication propagating through the core from the specific inner position when paying attention to the light of the highest order higher order mode used for communication propagating through the core Higher than the average concentration of the active element up to the specific inner position when focusing on the LP 11 mode light propagating through the core from the central point of the core in all regions up to the specific outer position The amplification optical fiber according to claim 6, wherein the active element is added at a concentration.
In all areas of between the specific inner position definitive if focusing on light LP 11 mode propagating through the core, until the specific outer position definitive if focusing on light LP 11 mode propagating through the core, the The active element is added at a concentration higher than the average concentration of the active element from the center point of the core to the specific inner position in the case of paying attention to LP 11 mode light propagating through the core. The amplification optical fiber according to any one of claims 1 to 3.
From the specific inner position when focusing on the LP 11 mode light propagating through the core to the specific outer position when focusing on the highest order higher mode light used for communication propagating through the core In all the regions, the active element is added at a concentration higher than the average concentration of the active element up to the specific inner position when focusing on the LP 11 mode light propagating through the core from the center point of the core The optical fiber for amplification according to claim 8, wherein the optical fiber for amplification is used.
The amplification optical fiber according to claim 1, comprising a plurality of the cores.
The amplification optical fiber according to any one of claims 1 to 10, further comprising an outer cladding that covers the cladding and has a lower refractive index than the cladding.
The amplification optical fiber according to claim 11, wherein a shape of an outer periphery of the clad is a polygon.
The optical fiber for amplification according to any one of claims 1 to 12, wherein the active element is erbium.