Rare-earth-doped optical fiber and optical fiber amplifier

Abstract

Provided are a rare-earth-doped optical fiber and an optical fiber amplifier, which are aimed at achieving gain compensation with a simple structure without adding a new device externally. The present disclosure is a rare-earth-doped optical fiber having a core to which a rare earth is added and a cladding around the core, characterized in that the core has a hollow portion inside, and has at least one or more hollow portions in the long direction of the rare-earth-doped optical fiber.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a rare-earth-doped optical fiber and an optical fiber amplifier that amplify a plurality of mode signals in a mode multiplexing relay transmission path. BACKGROUND

[0002] In mode multiplexing relay transmission, intermodal gain difference in an optical fiber amplifier becomes a large problem. As a technique to compensate for this, a technique of converting the mode of excitation light into a high-order mode and a technique of optimally designing the refractive index profile and the rare-earth-doped region are proposed (for example, refer to Non-Patent Literatures 1 to 3).

[0003] However, for the former, a new device for mode conversion is required, and thus there is a problem of deterioration of amplification characteristics or upsizing due to additional loss, and for the latter, a complex design and fabrication are required, and thus there is a problem of deterioration of yield and increase in manufacturing cost.

[0004] PRIOR ART DOCUMENTS

[0005] Non-Patent Literature 1: M. Wada et al., “Modal Gain Controllable 2-LP-Mode Fiber Amplifier Using PLC Type Coupler and Long-Period Grating,” JLT, vol. 32, no. 24, pp. 4694-4700, 2014.

[0006] Non-Patent Literature 2: M. Wada et al., “L-band 2-LP mode EDFA with low modal dependent gain,” OFC 2015, Tu3C.3, 2015.

[0007] Non-Patent Literature 3: E. Ip et al., “Experimental characterization of a ring-profile few-mode Erbium-doped fiber amplifier enabling gain equalization,” OFC 2013, JTh2A.18, 2013. SUMMARY

[0008] Therefore, an object of the present disclosure is to enable gain compensation with a simple structure without adding a new device externally.

[0009] To achieve the above object, the present disclosure reduces the gain deviation between modes by providing a hollow region in the core of a rare-earth-doped optical fiber. The gain deviation between modes occurs because the loss generated in the hollow region differs depending on the mode. Thus, the present disclosure can achieve gain compensation with a simple structure without adding a new device externally.

[0010] Specifically, the rare-earth-doped optical fiber of the present disclosure has a core to which a rare earth is added and a cladding around the core, and is characterized in that,

[0011] the core has a hollow portion inside the core,

[0012] the rare-earth-doped optical fiber has at least one or more hollow portions in the long direction of the rare-earth-doped optical fiber.

[0013] Specifically, the optical fiber amplifier of the present disclosure is characterized in that it includes:

[0014] the rare-earth-doped optical fiber of the present disclosure;

[0015] a pump light input section that outputs pump light that amplifies signal light in the rare-earth-doped optical fiber; and

[0016] an optical coupling section that couples and inputs the pump light from the pump light input section to the rare-earth-doped optical fiber.

[0017] According to the present disclosure, gain compensation can be achieved with a simple structure without adding a new device externally. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 FIG. 1 is an example of a side view of a rare-earth-doped optical fiber of the present disclosure.

[0019] Figure 2 FIG. 2 is an example of a cross-sectional view of the rare-earth-doped optical fiber of the present disclosure.

[0020] Figure 3 FIG. 3 is an example of a system configuration of an optical fiber amplifier.

[0021] Figure 4 FIG. 4 is an example of a region of a core radius and a relative refractive index difference of a core that enables 2LP mode transmission in the C band.

[0022] Figure 5 FIG. 5 is an example of LP 01 mode and the loss of LP 11 mode when Δ1 and a1 are changed.

[0023] Figure 6 FIG. 6 is an example of the relationship between the intermodal loss difference Δ L oss and a2 / a1.

[0024] Figure 7 The definition of the eccentricity x of the cavity.

[0025] Figure 8 Indicates Δ L An example of the relationship between oss and x / a1.

[0026] Figure 9 Indicates the difference in loss between modes Δ L An example of the relationship between a2 / a1 and x / a1 when oss is below 0dB. Detailed Implementation

[0027] The embodiments of this disclosure will now be described in detail with reference to the accompanying drawings. However, this disclosure is not limited to the embodiments shown below. These embodiments are merely illustrative, and this disclosure can be implemented in various ways with modifications and alterations based on the knowledge of those skilled in the art. Furthermore, in this specification and the accompanying drawings, the same reference numerals denote identical constituent elements.

[0028] (Implementation Example 1)

[0029] Figure 1 , Figure 2 These show side and cross-sectional views of the rare-earth-based optical fiber assembly disclosed herein. (Example) Figure 1 As shown, the rare-earth-based fiber 14 of this disclosure is characterized by having one or more spherical voids 83 discontinuously along its long side. Here, the radius of the core 81, the radius of the void 83, the refractive index of the core 81, and the refractive index of the cladding 82 are respectively set as a1, a2, n1, and n2. Furthermore, the relative refractive index difference Δ1 of the core 81 is given by the following formula.

[0030] (Mathematical Formula 1)

[0031] Δ1=(n1 2 -n2 2 ) / 2n1 2

[0032] Furthermore, the implementation method is shown here as a 2LP mode, but the same can be considered when the modulus increases.

[0033] Generally, when using the fundamental mode for pump light, the gain of the 2LP mode rare-earth fiber 14 is higher for the fundamental mode that overlaps more with the pump light. Since the gain difference between these modes directly affects the overall mode-dependent loss of the system, it degrades the overall transmission characteristics of the system.

[0034] This technology involves creating a spherical cavity 83 in the center of a rare-earth-based optical fiber 14, and then facing the LP of the basic mode at the end. 01The mode is lossy, and the loss is given to the LP 11 The mode is not lossy, and thus the loss is given to only the fundamental mode, which enables the gain between modes to be flattened. For example, by making a hollow portion 83 from the end face or the side face of the optical fiber using a femtosecond laser, the center of the core 81 can be given the hollow portion 83.

[0035] Figure 3 A system configuration diagram of a fiber amplifier is shown. The fiber amplifier includes a signal input portion 11, a pump light input portion 12, an optical coupling portion 13, and a rare-earth-doped optical fiber 14. Signal light is input to the signal input portion 11. Pump light that amplifies the signal light in the rare-earth-doped optical fiber 14 is input to the pump light input portion 12. The optical coupling portion 13 couples the signal light from the signal input portion 11 and the pump light from the pump light input portion 12. The signal light and the pump light coupled by the optical coupling portion 13 are input to the rare-earth-doped optical fiber 14.

[0036] In addition, in Figure 3 an example in which pump light is input from one direction of the rare-earth-doped optical fiber 14 is shown, but the present technology is also effective for a bidirectional-pumping fiber amplifier in which pump light is input from both directions of the rare-earth-doped optical fiber 14.

[0037] Further, if the hollow portion 83 is given to the front stage of the rare-earth-doped optical fiber 14, the signal light is subjected to loss caused by the hollow portion 83 before being amplified, thereby causing degradation of the signal light and degradation of the noise figure (NF), and thus it is preferable that the hollow portion 83 be provided to the middle or the rear stage in the long direction of the rare-earth-doped optical fiber 14.

[0038] Further, in Figure 1 an example in which five hollow portions 83 are included is shown, but the effect of the present disclosure can be obtained by including at least one of the hollow portions 83-1 to 83-5. Here, the radii of the hollow portions 83-1 to 83-5 are arbitrary.

[0039] (Embodiment Example 2)

[0040] Utilization in the C band (wavelength 1530 nm-1565 nm) is assumed, Figure 4 An example of a region of the core radius a1 and the relative refractive index difference Δ1 of the core 81 in which 2LP mode transmission is possible in the C band is shown. The line L41 in the figure shows a theoretical boundary at which the light of the LP 21 mode of the wavelength 1530 nm is cut off. The line L42 shows a boundary at which the light of the LP 11 mode of the wavelength 1565 nm becomes a bend loss of 0.5 dB / 100 turns or less when the bend radius R is 30 mm. The line L43 shows the effective core cross-sectional area A of the LP01 modeeff The boundary is 80μm. 2LP mode transmission is possible in the region surrounded by lines L41, L42, and L43.

[0041] Figure 5 express Figure 4 within the range of P01 to P10, a2 / a1 and P 01 Patterns and LPs 11 An example of mode loss. P01 indicates Δ1 = 0.51% and a1 = 5.5 μm, P02 indicates Δ1 = 0.60% and a1 = 5.5 μm, P03 indicates Δ1 = 0.45% and a1 = 6.0 μm, P04 indicates Δ1 = 0.51% and a1 = 6.0 μm, P05 indicates Δ1 = 0.57% and a1 = 6.0 μm, P06 indicates Δ1 = 0.41% and a1 = 6.5 μm, P07 indicates Δ1 = 0.45% and a1 = 6.5 μm, P08 indicates Δ1 = 0.48% and a1 = 6.5 μm, P09 indicates Δ1 = 0.38% and a1 = 7.0 μm, and P10 indicates Δ1 = 0.48% and a1 = 7.0 μm. As shown in the figure, under the condition that 2LP mode transmission is possible, the loss of each propagation mode has little dependence on a1 and Δ1. Therefore, a structure with Δ1 = 0.45% and a1 = 6 μm will be used as a representative for calculations thereafter.

[0042] Figure 6 LP 01 Patterns and LPs 11 Inter-mode loss difference Δ L An example of the relationship between OSS and a2 / a1. Inter-mode loss difference Δ L oss is LP 01 Mode loss and LP 11 Difference in mode loss (LP) 01 -LP 11 It can be seen that by changing a2 / a1, the inter-mode loss difference Δ can be controlled. L OSS. (e.g.) Figure 5 As shown, the loss of each propagation mode has little dependence on a1. Therefore, by increasing the radius a2 of the cavity 83, LP can be reduced. 01 The loss of the mode is greater than that of LP 11 The model is large.

[0043] Furthermore, within the range where a2 / a1 is 0.48 or higher, due to the increased radius a2 of the cavity 83, the loss of each propagation mode is excessive, and the loss difference Δ between modes also increases. L The controllability of OSS is significantly reduced. Therefore, a2 / a1 is preferably set below 0.4.

[0044] In addition, the inter-mode loss difference Δ in a cavity 83L The maximum value of OSS is 2dB. Therefore, when it is necessary to minimize the loss difference Δ between modes... L When the OSS is above 2dB, by setting multiple voids 83 along the long side, the inter-mode loss difference Δ can be proportionally reduced to the number of voids 83. Loss Increase. For example, rare earth-based fiber optics 14 include... Figure 1 The cavities 83-1 and 83-2 shown can have their inter-mode loss difference Δ reduced by setting a2 / a1 of cavity 83-1 to 0.4 and a2 / a1 of cavity 83-2 to 0.2. L OSS is 3dB.

[0045] (Implementation Example 3)

[0046] To suppress LP 11 To mitigate the additional loss of the mode and control the loss difference between modes, the void portion 83 is preferably located at the center of the rare-earth-added fiber 14. Therefore, the eccentricity of the center of the void portion 83 relative to the center 84 of the core 11 is studied. Figure 7 This defines the eccentricity x of the cavity 83. Eccentricity x is the distance from the center 84 of the core 11 to the center of the cavity 83. If the eccentricity x of the center of the cavity 83 increases, then the cavity 83 and LP... 11 The strong electric fields of the modes overlap, therefore LP 11 The loss of the mode increases significantly, compared to LP. 01 The pattern suffers from significant losses.

[0047] Figure 8 Let Δ represent the values ​​of a2 / a1 when a2 / a1 = 0.1, 0.2, 0.3, and 0.4. L An example of the relationship between oss and x / a1. Similar to the previous calculations, with a1 = 6 μm and Δ1 = 0.45%, LP was calculated. 01 Patterns and LPs 11 Inter-mode loss difference Δ L oss. Here, LP 11 The mode loss is calculated using the average loss of four modes, including degeneracy and polarization. For each a2 / a1, in the region where x / a1 is above a certain value, the inter-mode loss difference Δ L If oss is negative, it cannot be used with LP. 01 Patterns impart losses. Focusing on Δ Loss For points below 0dB, (a2 / a1, x / a1) = (0.1, 0.46), (0.2, 0.38), (0.3, 0.3), (0.4, 0.18).

[0048] Figure 9 Indicates the difference in loss between modes Δ LFig. 1 is a graph showing the relationship between a2 / a1 and x / a1 when oss is 0 dB or less. From the graph, it is known that the relationship between a2 / a1 and x / a1 when oss is 0 dB or less is approximately as follows. Figure 9 It is known that the intermodal loss difference Δ L The boundary when oss is 0 dB or less can be approximated by the following relationship.

[0049] (Mathematical expression 2)

[0050] x / a1 = -0.92a2 / a1 + 0.6 (2)

[0051] Therefore, a2 / a1 and x / a1 are preferably set in a range smaller than the relationship of expression (2) in the region where the hollow is filled. Figure 9

[0052] In addition, the present technology is not dependent on the refractive index distribution, the rare earth addition distribution, and can be applied to a complex refractive index distribution, a rare earth addition distribution such as a step type or a ring type.

[0053] Further, the same effect can be obtained by using a rare earth addition optical fiber 14 in which different kinds of rare earths such as erbium, thulium, yttrium, praseodymium, and neodymium are added according to the wavelength of the signal light.

[0054] (Effects of the present disclosure)

[0055] By setting the hollow region by internal processing using a femtosecond laser without inserting a new device in the rare earth addition optical fiber 14, the NF characteristics are not deteriorated, and the intermodal gain deviation can be arbitrarily reduced.

[0056] By performing gain compensation with only a simple structure such as a rare earth addition optical fiber, and performing compensation at the center inside the optical fiber amplifier, deterioration of both the gain and the NF can be suppressed, and thus gain compensation with low loss and low NF can be achieved.

[0057] (Points of the present disclosure)

[0058] By making a hollow at the center in the long direction of the optical fiber, intermodal gain deviation compensation with low loss and low NF can be achieved.

[0059] Industrial applicability

[0060] The present disclosure can be applied to the information communication industry.

[0061] Explanation of reference numerals

[0062] 11: signal input unit

[0063] 12: pump light input unit

[0064] 13: optical coupling unit

[0065] 14: rare earth addition optical fiber ​

[0066] 81: core

[0067] 82: cladding

[0068] 83: hollow portion

Claims

1. A rare-earth-doped optical fiber having a core to which a rare earth is added and a cladding surrounding the core, the rare-earth-doped optical fiber characterized in that, a hollow portion is provided inside the core, there is at least one or more of the hollow portions along a long direction of the rare-earth-doped optical fiber, a center position of the hollow portion is arranged within a predetermined distance from a center of the core so that a loss of a fundamental mode is lower than a loss of a high-order mode.

2. The rare-earth-doped optical fiber according to claim 1, wherein a mode interval loss difference corresponding to a radius of the hollow portion is provided.

3. The rare-earth-doped optical fiber according to claim 1 or 2, characterized by, a ratio a2 / a1 of the radius a2 of the hollow portion to a radius a1 of the core is 0.4 or less.

4. The rare-earth-doped optical fiber according to claim 1 or 2, characterized in that, a ratio x / a1 of an eccentricity x of the hollow portion from the center of the core to the radius a1 of the core satisfies the following equation, (Mathematical Equation C1) x / a1 < -0.92a2 / a1 + 0.6 where a2 is the radius of the hollow portion. the rare earth is any one of erbium, thulium, yttrium, praseodymium, and neodymium. including:

5. The rare-earth-doped optical fiber according to claim 1 or 2, characterized by, the rare-earth-doped optical fiber according to any one of claims 1 to 5; 6. An optical fiber amplifier characterized by a pump light input portion that outputs pump light that amplifies signal light in the rare-earth-doped optical fiber; and an optical coupling portion that couples and inputs the pump light from the pump light input portion and the signal light to the rare-earth-doped optical fiber. ​ ​

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