JP6668272B2 - Optical fiber for amplification - Google Patents

Description

  The present invention relates to an amplification optical fiber that can amplify higher-order mode light than fundamental mode light.

In optical communication using an optical fiber, LP ₀₁ mode with superimposes the information on light (fundamental mode), the superimposed information communicate information to higher order LP mode light than the fundamental mode, such as LP ₁₁ mode Performing multi-mode communication is known. In optical communication, it is known that the intensity of light propagating through an optical fiber is attenuated due to long-distance transmission, and the intensity of the attenuated light is amplified using an optical fiber amplifier.

  As 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 of light of a fundamental mode and light of a higher order mode. According to this amplifying optical fiber, light in the fundamental mode and light in the higher-order mode can be equally amplified.

Koji IGARASHI et al., “Cladding-pumped few-mode EDFA with wide Er + doping distribution for equalization of mode-dependent gain” IEICE Technical Report OCS2015-104, OPE2015-224 (2016-02)

  Incidentally, higher-order mode light propagating through an optical fiber tends to have a larger loss than fundamental mode light. The main causes are as follows. The mode field diameter (MFD) of the higher-order 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-order 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 is easily lost at the interface. Therefore, higher-order mode light having a higher intensity than the fundamental mode light at the interface causes a larger loss.

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

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

In order to solve the above 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 or more LP modes, and at least a part of a light waveguide region. An amplification optical fiber to which an active element is added, focusing on light of a specific higher-order mode propagating through the core, and standardizing the intensity of the _LP01 mode light in the radial direction and the specific high-order light. The innermost position where the standardized intensity of the light of the next mode matches the specified inner position, and the outermost position where the standardized intensity of the specific higher mode light is 0.0001 is the specified outer position. In the case of a position, even 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 inside position to the specific outside position is the average concentration of the core. Wherein the amplification factor of the light of the higher mode used for communication is higher than the amplification ratio of the light of the fundamental mode, which is higher than the average concentration of the active element from the center point to the specific inner position. It is.

  In Non-Patent Document 1, a certain concentration of an active element is added to a light guiding region, and light in a fundamental mode and light in a higher-order mode are amplified at 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 concentrations of the active elements from the specific inner position to the specific outer position is the basic mode. The light intensity is higher than the average average concentration of active elements from the center point of the core, which is higher than the light intensity of the higher-order mode, to a specific inner position. Therefore, the amplification factors of all higher-order mode light propagating through the core can be made higher than the amplification factors of the fundamental mode light.

Further, the higher order modes, _{LP 11} mode, and are preferably the _{LP 21} mode.

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

Wherein the the center point to the specific inside position in the light of the LP ₁₁ mode propagating through the core, it is preferable that the active element is not added.

Certain inside position in the LP ₁₁ mode of light is closer to the center point of the most core than a certain inside position in the light of any higher order modes. 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-order mode light can be made higher than that of the fundamental mode light.

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

  By providing the region to which the active element is not added as described above, the amplification factor of the higher-order mode light can be further increased than the amplification factor of the fundamental mode light.

  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-order mode light can be further increased than that of the fundamental mode light.

Further, from the particular inner position in the case of focusing on the highest order of higher-order mode light used for communication propagating through the core, until the specific outer position definitive if focusing on light LP ₁₁ mode propagating through the core in all of the region between the active element at a higher concentration than the average concentration of the active elements from the central point of the core to the specific inside position in the case of focusing on the light LP ₁₁ mode propagating through the core Preferably, it is added.

In this case, from the specific inside position when focusing on the light of the highest order higher mode used for communication propagating through the core, focus on the light of the highest order higher mode used for communication propagating through the core. than the average concentration of the active elements in all regions, up to the specific inside position in the case of focusing on the light LP ₁₁ mode propagating through the core from a center point of the core between the to a particular outer position in case of Preferably, the active element is 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 at a higher concentration than the average concentration of the active element of the up to a certain inside position in the case where the center point is focused on the light of the LP ₁₁ mode propagating through the core of the core are preferably added.

In this case, the specific outer position in a case where from the particular inner position definitive if focusing on light LP ₁₁ mode propagating through the core, focusing on the highest order of higher-order mode light used for communication propagating through the core the active element in all regions, at a concentration higher than the average concentration of the active element from the center point to the specific inside position in the case of focusing on the light LP ₁₁ mode propagating through the core of the core until Is preferably added.

As described above, than the average concentration of the active elements in to a particular inside position in the case of focusing on the LP ₁₁ mode of light propagating through the core from the center point of the core, a region in which the active element at a high concentration is added is defined Accordingly, the amplification factor of the higher-order mode light can be made larger than that of the fundamental mode light.

  Further, 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, when light propagates in two or more LP modes and focuses on 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 equal to , The higher than the average concentration of the active element from the center point of the core to the specific inner position, and the amplification factors of the light of all higher-order modes are higher than the amplification factors of light of the fundamental mode. Therefore, the amplification factors of all higher-order modes of light propagating through the respective cores of the multi-core amplification optical fiber can be made higher than the amplification factors of the fundamental mode light.

  Further, it is preferable that an outer clad that covers the clad and has a lower refractive index than the clad is further provided.

  With such a double clad structure, it is possible to employ cladding excitation in which the excitation light propagates through the clad, and to input excitation light having a higher intensity than core excitation in which the excitation light propagates through the core. Accordingly, it is possible to increase the amplification factors of the above-described fundamental mode light and higher-order mode light.

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

  In the case of a double clad structure, when the cross-sectional shape of the clad is circular, the skew light that continues to reflect at a certain angle at the interface between the clad and the outer clad, and the propagating light propagates through the clad without entering the core Tends to occur. Therefore, the skew light can be suppressed and the active element added to the core can be efficiently excited by the polygonal cladding as described above.

  Further, the active element may be erbium.

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

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

FIG. 2 is a diagram illustrating a cross section perpendicular to the longitudinal direction of the amplification optical fiber according to the first embodiment of the present invention. FIG. 2 is a diagram showing a state of the refractive index, light intensity, and distribution of active elements of the amplification optical fiber of FIG. 1. FIG. 2 is a diagram illustrating an optical fiber amplifier using the amplification optical fiber of FIG. 1. FIG. 4 is a diagram illustrating a concentration distribution of active elements in an amplification optical fiber according to a modification of the first embodiment, similarly to FIG. 2D. 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 concentration distribution of the active element of the optical fiber for amplification in 2nd Embodiment like FIG. 2 (D). It is a figure which shows the concentration distribution of the active element of the optical fiber for amplification in the 1st modification of 2nd Embodiment like 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 like 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 like FIG. 2 (D). It is a figure which shows the concentration distribution of the active element of the optical fiber for amplification in the 4th modification of 2nd Embodiment like FIG. 2 (D). FIG. 6 is a diagram illustrating another example of an amplification optical fiber. FIG. 2 is a diagram illustrating an example of an optical fiber for multicore 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. Note that, for easy understanding, the scale in each drawing may be different from the scale described in the following description.

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

  As shown in FIG. 1, the amplification optical fiber 10 of the present embodiment includes a core 11, a clad 12 surrounding the outer peripheral surface of the core 11 without any gap, an outer clad 13 covering the outer peripheral surface of the clad 12, and an outer clad 13. And a covering layer 14 covering the cover 13 as a main configuration. 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 amplifying 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. Further, the amplification optical fiber 10, the light propagating is a multimode optical fiber for propagating light comprising light of higher modes in addition to LP ₁₁ mode of the LP ₀₁ mode is the basic mode optical core 11 . Accordingly, when light of the light and LP ₁₁ modes LP ₀₁ mode into the core 11 of the amplification optical fiber 10 is incident, it is possible to light of each mode propagates through the amplification optical fiber 10. The predetermined wavelength is, for example, a C band band or an L band band. Since the amplification optical fiber 10 aims at amplifying the light used for communication as described above, even if light of a higher mode not related to communication is excited, You do not need to consider the light.

FIG. 2 is a diagram showing a state of the core 11 of the amplification optical fiber 10 of FIG. 1 and its surroundings. Specifically, FIG. 2A shows the core 11 and the clad 12 in the region shown 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 lower than the refractive index of the core 11. Although not shown, the refractive index of the outer cladding 13 is 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 for increasing the refractive index is added, and the clad 12 is made of, for example, quartz to which the dopant for increasing the refractive index is not particularly added. Alternatively, the core 11 may be made of, for example, quartz to which a dopant for increasing 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 for decreasing the refractive index is added. Note that an active element is added to the core 11 and the clad 12 as necessary, as 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 coating layer 14 is made of, for example, an ultraviolet curable resin different from the outer clad 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 light LP ₁₁ mode has an intensity distribution as shown in FIG. 2 (C). Light LP ₀₁ mode has a strongest consisting intensity distribution at the center point C of the core 11, light _{LP 11} mode becomes strength 0 at the center point C of the core 11, the center point C of the core 11 And the outer peripheral surface IF of the core 11 has a peak intensity. Therefore, in a particular position between the outer peripheral surface IF of the center point C and the core 11 of the core 11, the intensity of light LP ₁₁ mode match the intensity of the LP ₀₁ mode of light. This position is defined as a specific inside position Ri. The intensity of the light of LP ₁₁ mode, the outer peripheral side of the particular inner position Ri, larger than the intensity of light of _{LP 01} mode.

Light propagating through the amplification optical fiber 10 propagates out of 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 protruding into the clad 12 is attenuated from the core 11 side to the outer peripheral side of the clad 12 in the clad 12. Further, the light of the LP ₁₁ mode has a mode field diameter larger than that of the light of the LP ₀₁ mode. Thus, the outer peripheral surface IF of the core 11, the intensity of light _{LP 11} mode is greater than the intensity of light of _{LP 01} mode. Further, the normalized intensity of light LP ₁₁ mode, at a certain position between the inner and outer circumferential surfaces of the clad 12 becomes 0.0001. That is, the intensity of the light when the integrated value of the radial intensity distribution of LP ₁₁ mode of light intensity distribution is normalized to be 1 is 0.0001. This position is defined as a specific outer position Ro. When the normalized intensity of the light becomes 0.0001 in this way, it does not affect the propagation or amplification of the light. Therefore, it is not necessary to consider light whose normalized intensity is smaller than 0.0001.

From the above description, in the region from the center point C of the core 11 to a certain inside position Ri, the intensity of light LP ₀₁ mode is greater than the intensity of light of the LP ₁₁ mode, up to a certain position outside Ro from certain inside position Ri in the region, the intensity of light _{LP 11} mode is greater than the intensity of light of _{LP 01} mode. Thus, in the outer peripheral surface IF of the core 11, the intensity of light _{LP 11} mode is greater than the intensity of light of _{LP 01} mode.

  In the amplification optical fiber 10 of the present embodiment, as shown in FIG. 2D, the active element is not 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 to the specific outer position Ro. The active element is an element that is brought into an excited state by the excitation light. When light in the C band or the L band propagates as described above, for example, erbium (Er) can be used as the active element. The active element to be added varies depending on the wavelength of light to be propagated, and other active elements include, for example, rare earth elements such as neodymium (Nd) and ytterbium (Yb). Further, as the active element, bismuth (Bi) can be mentioned in addition to the rare earth element.

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 mainly propagates through the clad 12, and when the excitation light passes through the active element addition region, the active element is excited. The excited active element causes stimulated emission by light of a predetermined wavelength propagating through the amplification optical fiber 10, and the stimulated emission amplifies the light of the predetermined wavelength. 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 a region from the inner position Ri to the specific outer position Ro, and is not amplified in a 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 light intensity is high. Therefore, when compared with the amplification optical fiber in which the active element is uniformly added to the entire core, the light in the LP ₁₁ mode is more efficient than the light in the LP ₀₁ mode according to the amplification optical fiber 10 of the present embodiment. It is well amplified.

According to the amplification optical fiber 10 of the present embodiment as described above, the amplification factor of the light of LP ₁₁ mode can be larger than the gain of the LP ₀₁ mode of 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 illustrating the optical fiber amplifier according to the present embodiment. As shown in FIG. 3, the optical fiber amplifier 1 according to 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 that receives signal light and excitation light, The main components include the amplification optical fiber 10 of FIG. 1 to which the signal light and the pump light emitted from the optical combiner 40 are input, and the emission end 50 from which the amplified signal light is emitted.

Entrance end 20, for example, is connected to an optical fiber to perform optical communication with the light of at least LP ₀₁ mode light and LP ₁₁ modes. Therefore, from the entrance end 20, LP ₀₁ mode of light used in optical communications, light of a predetermined wavelength including light of LP ₁₁ mode is incident. The predetermined wavelength is not particularly limited, but is, for example, the C band and the L band as described above. Moreover, light incident from the incident end 20, a core, and is composed of a cladding covering the core, which propagates through the optical fiber 25 for propagating the light in at least LP ₀₁ mode and the LP ₁₁ mode.

  The pumping 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 semiconductor laser made of a GaAs-based semiconductor, and emits light having a center wavelength of 980 nm. I do. Each laser diode 31 of the pump light source 30 is connected to an optical fiber 35, and the pump 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 by, for example, melting and stretching a portion where the multi-mode optical fiber is arranged around the optical fiber 25 and integrated therewith. 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, the signal light incident from the incident end 20 exits from the optical fiber 25. The wavelength of the signal light is for example C-band and L-band, as described above, includes an optical light and LP ₁₁ modes LP ₀₁ mode. Then, the signal light propagating through the optical fiber 25 enters the optical combiner 40.

  Further, the excitation light source 30 emits excitation light for exciting the active element added to the core 11 of the amplification optical fiber 10. 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 entering the core 11 of the amplification optical fiber 10 from the optical combiner 40 propagates through the core 11, and the excitation light entering the cladding 12 of the amplification optical fiber 10 from the optical combiner 40 propagates mainly through the cladding 12. The active element added as described above is brought into an excited state. Then, the excited active element causes stimulated emission by the signal light, and the signal light is amplified. At this time, as described above for the amplification optical fiber 10, the signal light is amplified in the region from the specific inner position Ri to the specific outer position Ro, and in the region from the center point C of the core 11 to the specific inner position Ri. Not amplified. Therefore, the amplified signal light having high light amplification factor of LP ₁₁ mode than the light of the LP ₀₁ mode is emitted from the amplification optical fiber 10, the optical signal is emitted from the exit end 50.

According to the optical fiber amplifier 1 of the present embodiment, as described above, it can be amplified at a high amplification factor of light of LP ₁₁ mode than the light of the LP ₀₁ mode. 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 The power of the light in the ₀₁ mode can be made larger.

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

FIG. 4 is a diagram showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modified example, similarly to FIG. 2D. As shown in FIG. 4, in the present modification, the active element is not added to the core 11 but is added to the region from the outer peripheral surface IF of the core 11 to the specific outer position Ro. According to the addition of such an 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 added to the region from the specific inner position Ri to the specific outer position Ro. compared with when added, the amplification factor of the light of LP ₁₁ mode can be further increased than the amplification factor of the LP ₀₁ mode of light.

(2nd Embodiment)
Next, a second embodiment of the present invention will be described in detail with reference to FIGS. It should be noted that components that are the same as or equivalent to those of the first embodiment are denoted by the same reference numerals unless otherwise specified, and duplicate descriptions are omitted.

Amplification optical fiber 10 of this embodiment is in that light of LP ₂₁ mode used for communication to propagate different from the amplification optical fiber 10 of the first embodiment.

Figure 5 is a diagram showing the same manner as FIG. 2 (C) the intensity distribution of the light when the light of the LP ₂₁ mode to the amplification optical fiber 10 in FIG. 1 propagates. As shown in FIG. 5, in the present 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, a position where the intensity of light of _{LP 11} mode match the intensity of the _{LP 01} mode of light to a particular inner position Ri _1, normalized intensity of light _{LP 11} mode 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.

The intensity of light of LP ₂₁ mode, the location is two points as the peak intensity between the outer peripheral surface IF of the center point C and the core 11 of the intensity at the center point C of the core 11 becomes zero, the core 11 I do. Therefore, in a particular position between the outer peripheral surface IF of the center point C and the core 11 of the core 11, the position where the intensity of light LP ₂₁ mode match the intensity of the LP ₀₁ mode of light is present multiple locations. Therefore, the intensity of light LP ₂₁ mode is the specific inner position Ri ₂ the innermost position among the positions that match the intensity of the LP ₀₁ mode of light. Further, the light of _{LP 21} mode, _{LP 01} mode has a larger mode field diameter than the light of the light and _{LP 11} modes, a large way protrusion into the cladding 12. For this reason, at the interface between the core 11 and the clad 12, the light intensity of the LP ₂₁ mode is larger than the light intensity of the LP ₀₁ mode and the light intensity of the LP ₁₁ mode. The position of this interface coincides with the position of the outer peripheral surface IF of the core 11. Further, the normalized intensity of light _{LP 21} mode, at a certain position of the outer peripheral surface of the clad 12 than a certain position outside Ro ₁ above, becomes 0.0001. This position is the specific outer position Ro _2.

FIG. 6 is a diagram showing the concentration distribution of the active element in the amplification optical fiber 10 of the present embodiment, similarly to FIG. 2 (D). As shown in FIG. 6, the amplification optical fiber 10 of the present embodiment, the active element is added to an area from certain inside position Ri ₂ to a certain position outside Ro _1, particular inner position from the center point C of the core 11 the region up to ri _2, the active element is not added. In the present embodiment, the intensity of light LP ₁₁ mode has been added active elements between the particular inner position Ri ₁ is part of a larger area than the intensity of light of LP ₀₁ mode to a certain inside position Ri ₂ Absent. However, if attention is paid to LP ₁₁ mode of light is a light of high-order 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, the center point of the core higher than the average concentration of the active elements in the C to a certain inside position Ri _1. Further, in the present embodiment, the active element between the particular outer position Ro ₁ light intensity of LP ₂₁ mode is part of a larger area than the intensity of light of LP ₀₁ mode to a certain position outside Ro ₂ added It has not been. However, when focusing on the light of the LP ₂₁ mode is the light of the higher order 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, 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. Such concentration distribution of the active element to have a, the amplification factor of the light of the light and LP ₂₁ modes LP ₁₁ mode can be larger than the gain of the LP ₀₁ mode of light.

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

FIG. 7 is a diagram showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modified example, similarly to FIG. 2D. In the above example, while the area to the active element from a particular inner position Ri ₂ to a certain position outside Ro ₁ was added, in this modification, as shown in FIG. 7, a specific outer position from the particular inner position Ri ₂ active element is added to the area up 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 this active element as it is added, it is possible to further increase the amplification factor of the light LP ₂₁ mode.

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

FIG. 8 is a diagram showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modified example, similarly to FIG. 2D. In the above example, while the area to the active element from a particular inner position Ri ₂ to a certain position outside Ro ₁ was added, in this modification, as shown in FIG. 8, a specific outer position from the particular inner position Ri ₁ active element is added to the region of up 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 this active element as it is added, it is possible to further increase the amplification factor of the light LP ₁₁ mode.

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

FIG. 9 is a diagram showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modified example, similarly to FIG. 2D. In the above example, while the area to the active element from a particular inner position Ri ₂ to a certain position outside Ro ₁ was added, in this modification, as shown in FIG. 9, a specific outer position from the particular inner position Ri ₁ active element is added to the area up 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, it is possible to further increase the amplification factor of the LP ₁₁ mode light and the LP ₂₁ mode light.

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

FIG. 10 is a diagram showing the concentration distribution of the active element in the amplification optical fiber 10 of the present modified example, similarly to 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 the addition of such active element, the active element in the region of from the center point C to a specific position inside Ri ₂ as described above is not added, a region from a particular inner position Ri ₂ to a certain position outside Ro ₁ compared to the case where the active element is added, the amplification factor and LP ₂₁ modes amplification factor of light of light LP ₁₁ mode can be further increased than the amplification factor of the LP ₀₁ mode of light.

Incidentally, if the amplification optical fiber 10 of the present embodiment and the modifications is used for the optical fiber amplifier shown in FIG. 3, the light of LP ₀₁ mode is incident end 20, LP ₁₁ mode of light and LP ₂₁ mode light is incident, the amplification optical fiber 10, light of the light and _{LP 21} modes _{LP 11} mode is amplified at a higher gain than the light of the _{LP 01} mode, emitted from the emission end 50.

  As described above, the embodiments of the present invention have been described, but the present invention is not limited thereto.

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 that 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 light of LP ₁₁ mode can be larger than the gain of the LP ₀₁ mode of light. Further, in the modification of the first embodiment, the active element was not added to the core 11, but was added to a region from the outer peripheral surface IF of the core 11 to the specific outer position Ro. However, the above-mentioned concentration distribution that the average concentration of the active element in the region from the specific inner position Ri to the specific outer position Ro is made 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. As far as possible, an active element having a low average concentration may be added to at least a part of the region of the core 11.

In the first third modification from the second embodiment and the second embodiment, the low concentration of the active element is added to at least a region from the center point C of the core 11 to a certain position inside Ri ₁ May be. However, in this case, in the second embodiment and the first to third modifications of the second embodiment, the following density distribution needs to be satisfied.

That is, in the second embodiment, the LP ₁₁ mode propagating through the core 11 from the specific inner position Ri ₂ when focusing on the light of the highest order mode (LP ₂₁ mode) used for communication propagating through the core 11 of in all areas between to a specific position outside Ro ₁ of definitive if attention is focused on the light, specific to the inside position Ri ₁ in the case where attention is paid to the LP ₁₁ mode propagating through the core 11 from the center point C of the core 11 light The concentration distribution in which the active element is added at a higher concentration than the average concentration of the active element is satisfied.

In the first modification of the second embodiment, a specific position inside Ri ₂ in the case where attention is focused on the light of the highest order of the higher order mode used in communication propagating through the core 11 (LP ₂₁ mode), the core in all areas of until certain outer position Ro ₂ in the case of focusing on the highest order of higher-order mode light used for communications that propagate, paying attention to the LP ₁₁ mode propagating through the core from the center point of the core 11 light the concentration distribution of the active element is added at a concentration higher than the average concentration of the active element to a specific position inside Ri ₁ in the case where 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 In all the regions from the center point C of the core 11 to the LP ₁₁ mode light propagating through the core 11 in the entire region up to the specific outer position Ro _{1 in} the region, the average concentration of the active element up to the specific inner position Ri ₁ is calculated. The concentration distribution in which the active element is added at a high concentration is also satisfied.

Further, in the third 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, the highest order of high-order used in communication propagating through the core 11 mode in all areas between up to a certain position outside Ro ₂ in the case where attention is paid to light (LP ₂₁ mode), specific in the case of focusing on the LP ₁₁ mode propagating through the core 11 from the center point C of the core 11 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 modification of the second embodiment, a low-concentration active element may be added to at least a part 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 one from a particular inner position Ri ₂ in the first modification of the partial region to a specific position outside Ro _2, from a specific position inside Ri ₁ to a specific position outside Ro ₁ of the second modification of the second embodiment area parts, in some areas to a specific position outside Ro ₂ from a specific position inside Ri ₁ in the third modification of the second embodiment, or not added active element, low concentrations of the active element is added May be. However, in these cases, the average concentration of the active element from the specific inner position to the specific outer position is equal to the center point C of the core 11 regardless of which higher-order mode light used for communication propagating through the core 11. the concentration distribution that is higher than the average concentration of the active elements in to a specified position inside filled from, the optical amplification factor of all higher order modes to be used for communication from the optical amplification factor of the fundamental mode (LP ₀₁ mode) Is also increased.

  FIG. 11 is a diagram illustrating another example of the amplification optical fiber. In the above embodiment and its modifications, the case where the outer periphery of the clad 12 is circular is described. However, as shown in FIG. 11, the outer periphery of the cladding 12 is preferably polygonal. When the cross-sectional shape of the cladding 12 is circular, the pumping light continues to be reflected at a certain angle at the interface between the cladding 12 and the outer cladding 13, and the skew light that propagates through the cladding 12 without the pumping light entering the core 11 is generated. Easy to occur. Therefore, the skew light can be suppressed and the active element added to the core 11 can be efficiently excited by making the cladding 12 polygonal as in this example.

Further, in the above-described embodiment and its modified example, the example in which the core 11 is one, that is, the single-core amplifying optical fiber is described. However, a plurality of cores 11 may be provided. FIG. 12 is a diagram illustrating a multi-core amplification optical fiber. In FIG. 12, a core 11 is arranged at the center of the clad 12, and a plurality of cores 11 are arranged around the core 11. That is, the cores 11 are arranged 1-6. However, this is an example in the case where there are a plurality of cores 11, and the arrangement of the plurality of cores 11 is not particularly limited. In this modification, each of the core 11 _{LP 01} mode of light and _{LP 11} modes of the light is at least propagation. In this case, the concentration distribution of the active element in each core 11 and the clad 12 in the vicinity of the core 11 is the same as that in the first embodiment or its modification. Further, the light of the LP ₂₁ mode may further propagate through each of the cores 11. In this case, the concentration distribution of the active element is determined in each core 11 and the clad 12 in the vicinity of the core 11 according to the second embodiment or the like. This is the same as the modified example. When the amplifying optical fiber of the present example is used for an optical fiber amplifier, pump light is incident on the cladding 12 and signal light similar to that of the above embodiment is incident on each core 11.

  FIG. 13 is a diagram illustrating 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 polygonal in the multi-core amplification optical fiber. In this case, the skew light can be suppressed and the active element added to the core 11 can be efficiently excited, as in the description of FIG.

  In addition, in the above-described embodiments, the modifications, and the like, the amplification optical fiber is a double-clad fiber having an outer cladding. However, the amplification optical fiber need not be a double clad fiber. In this case, the structure may be such that the excitation light is incident on the core 11.

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 light of a higher mode may be amplified by the amplification optical fiber. However, even in this case, even 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 same as that of the core 11. filled density distribution that is higher than the average concentration of the active element in the center point C to a specific inside position, the optical amplification factor of all higher order modes used for communication of the light in the fundamental mode (LP ₀₁ mode) It is made larger than the amplification factor. Incidentally, the amplification optical fiber 10, the light of interest to be amplified, _{LP 01} mode of light and _{LP 11} modes of light used for communication, or the _{LP 01} mode used in the optical communication light, _{LP 11} mode if the light of the light and LP ₂₁ mode is preferable because the light of the higher order modes even mode such LP ₀₂ mode is not subject to amplification.

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

(Example 1)
The same amplification optical fiber as the amplification optical fiber shown in FIG. 2 was measured. Thus, in this amplification optical fiber, with a constant refractive index profile of the core in the radial direction, LP ₁₁ mode only area normalized intensity is greater than the normalized intensity of light LP ₀₁ mode of light of Active elements are added. In this amplification optical fiber, _{LP 21} mode when each theoretical cutoff wavelength λc of the light is 1.40μm and 1.50 .mu.m, _{LP 01} mode of the optical effective core area _{A eff} of the optical wavelength 1.55μm 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, when the theoretical cut-off wavelength λc of the light _{LP 21} mode is any of 1.40μm and 1.50μm also regardless of the effective core area _{A eff} of _{LP 01} mode of light, the ratio Γ ₁₁ / _{Γ 01} was roughly a 1.7. Therefore, the amplification optical fiber 10 shown in FIG. 2, the optical amplification factor of the LP ₁₁ mode is resulted greater than the amplification factor of the LP ₀₁ mode of light. Therefore, according to the amplification optical fiber 10 shown in FIG. 2, it can be larger than the amplification rate of the light in the LP ₁₁ mode optical amplification factor LP ₀₁ modes is shown.

(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, the theoretical cut-off wavelength λc of the light Likewise _{LP 21} mode as in Example 1 in each case of a 1.40μm and 1.50 .mu.m, the _{LP 01} mode in the light of the wavelength 1.55μm light and the effective core area _{a eff,} was examined a 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 1.55μm . The result is shown in FIG.

As shown in FIG. 15, when the theoretical cut-off wavelength λc of the light _{LP 21} mode is any of 1.40μm and 1.50μm also regardless of the effective core area _{A eff} of _{LP 01} mode of light, the ratio Γ ₁₁ / _{Γ 01} was roughly a 3. Thus, in the example shown in FIG. 4, further than the example shown in FIG. 2, the optical amplification factor of the LP ₁₁ mode it is resulted greater than the amplification factor of the LP ₀₁ mode of light. Therefore, according to the amplification optical fiber 10 shown in FIG. 4, it can be larger than the amplification rate of the light in the LP ₁₁ mode optical amplification factor LP ₀₁ modes is shown.

  As described above, according to the present invention, an amplifying optical fiber 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 and the like.

DESCRIPTION OF SYMBOLS 1 ... Optical fiber amplifier 10 ... Amplification optical fiber 11 ... Core 12 ... Clad 13 ... Outer clad 14 ... Coating layer 20 ... Incoming end 30 ... Excitation light source 40 ... light combiner _Ri, Ri _1, Ri 2 ··· certain inside position _Ro, Ro _1, Ro 2 ··· specific external position

Claims (11)

Having a core and a clad, light of a predetermined wavelength used for communication propagates in at least two or more LP modes in a region of the core and the clad adjacent to the core, and is activated in at least a part of a light waveguide region; An amplification optical fiber to which an element is added,
The focusing on light of a specific high-order mode of the predetermined wavelength of light, and a normalized intensity of the LP ₀₁ mode of light normalized intensity and the specific higher-order modes of light in the radial direction When the matching innermost position is the specific inner position and the outermost position where the standardized intensity of the light of the specific higher mode is 0.0001 is the specific outer position,
Even when focusing on light of any higher order mode of the light of the predetermined wavelength, the average concentration of the active element from the specific inside position to the specific outside position is from the center point of the core to the specific inside position. Higher than the average concentration of the active element in,
The optical amplification factor of all higher order modes of the predetermined wavelength of light much larger than the amplification factor of the fundamental mode light,
Between the from the specific inner position in the case where attention is focused on the light of the highest order of the higher order modes of the predetermined wavelength of light, until the specific outer position in the case of focusing on the light LP ₁₁ mode of the light of the predetermined wavelength in all areas of the active element at a higher concentration than the average concentration of the active elements from the central point of the core to the specific inside position in the case of focusing on the light LP ₁₁ mode of the light of the predetermined wavelength Added
The higher order modes, _{LP 11} mode and the amplifying optical fiber according to claim <br/> be a _{LP 21} mode. From the particular inner position in the case where attention is focused on the light of the highest order of the higher order mode, in all areas between to said specific outer position in the case of focusing on the light of the highest order of the higher order modes, the core claims, characterized in that the active element at a higher concentration than the average concentration of the active element of the up to a certain inside position in the case where the center point focusing on LP ₁₁ mode of light of the light of the predetermined wavelength is added Item 2. The amplification optical fiber according to Item 1. Wherein the the center point to the specific inside position in the light of the LP ₁₁ mode of the light of the predetermined wavelength, amplifying optical fiber according to claim 1 or 2, characterized in that said active element is not added. The optical fiber for amplification according to claim 3, wherein the active element is not added from the center point to the specific inner position in the light of the highest-order higher-order mode. Having a core and a clad, light of a predetermined wavelength used for communication propagates in at least two or more LP modes in a region of the core and the clad adjacent to the core, and is activated in at least a part of a light waveguide region; An amplification optical fiber to which an element is added,
The focusing on light of a specific high-order mode of the predetermined wavelength of light, and a normalized intensity of the LP ₀₁ mode of light normalized intensity and the specific higher-order modes of light in the radial direction When the matching innermost position is the specific inner position and the outermost position where the standardized intensity of the light of the specific higher mode is 0.0001 is the specific outer position,
Even when focusing on light of any higher order mode of the light of the predetermined wavelength, the average concentration of the active element from the specific inside position to the specific outside position is from the center point of the core to the specific inside position. Higher than the average concentration of the active element in,
The optical amplification factor of all higher order modes of the predetermined wavelength of light much larger than the amplification factor of the fundamental mode light,
From the particular inner position in the case of focusing on the LP ₁₁ mode of light of the predetermined wavelength light, all until the specific outer position in the case of focusing on the LP ₁₁ mode of light of the light of the predetermined wavelength in the region, the active element is added at a concentration higher than the average concentration of the active elements from the central point of the core to the specific inside position in the case of focusing on the light LP ₁₁ mode of the light of the predetermined wavelength An amplification optical fiber, characterized in that: From the particular inner position definitive when focusing on light LP ₁₁ mode of the light of the predetermined wavelength, to the specific outer position in the case of focusing on the light of the highest order of the higher order modes of the light of the predetermined wavelength in all of the region between the active element at a higher concentration than the average concentration of the active element of the up to a certain inside position in the case of focusing on the light LP ₁₁ mode of the light of the predetermined wavelength from the center point of the core Is added,
The higher order modes, _{LP 11} mode and the amplification optical fiber according to claim 5, characterized in <br/> be a _{LP 21} mode.   LP propagating through the core from the center point ₁₁ Up to the specific inner position in the light of the mode, the active element is not added
The amplification optical fiber according to claim 5 or 6, wherein: The amplification optical fiber according to any one of claims 1 to 7 , comprising a plurality of the cores. The amplification optical fiber according to any one of claims 1 to 8 , further comprising an outer cladding that covers the cladding and has a lower refractive index than the cladding. 10. The amplification optical fiber according to claim 9 , wherein the outer periphery of the cladding has a polygonal shape. The amplification optical fiber according to any one of claims 1 to 10, wherein the active element is erbium.

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