Fiber optic amplifier

Abstract

The purpose of this disclosure is to increase amplification efficiency of a cladding pump optical fiber amplifier.　This disclosure pertains to an optical fiber amplifier provided with an optical fiber for amplification doped with a rare-earth element, a pump light combiner for causing multi-mode pump light to enter a cladding region of the optical fiber for amplification is connected to an input end or an output end of the optical fiber for amplification, and a reflection device which reflects the pump light and transmits a signal light is connected to the end portion to which the pump light combiner is not connected.

Description

[Technical Field]

[0001] This disclosure relates to an optical fiber amplifier. [Background technology]

[0002] In optical fiber communication systems, the loss of light propagating through the optical fiber is amplified by optical amplifiers at regular intervals and relayed to enable long-distance transmission. Amplification within the optical amplifier is performed by injecting signal light and excitation light (mainly 980 nm or 1480 nm light in the case of EDF) into an amplification optical fiber (mainly erbium-doped optical fiber: EDF) that has rare earth elements added to its core region, thereby amplifying the light without converting it into electricity.

[0003] In current single-mode optical fiber (SMF) communications, core-excited optical amplifiers are used to amplify the signal light propagating through the core by guiding excitation light to the core in a similar manner. On the other hand, in recent years, in order to increase the transmission capacity of optical fibers, spatial division multiplexing (SDM) optical fibers such as multicore fibers having multiple cores in the cross-section of the optical fiber, and multimode fibers having two or more modes propagating within the core, have been investigated, and optical fiber amplifiers applicable to these SDM optical fibers have been studied (for example, Non-Patent Document 1).

[0004] Furthermore, to simultaneously amplify multiple signal light waves propagating through an optical fiber for SDM, clad-pumped optical fiber amplifiers that guide excitation light into the cladding region of the optical fiber are being investigated (for example, Non-Patent Document 2). In the case of clad-pumped optical fiber amplifiers, a multimode light source can be used for the excitation light, and it has been shown that they have better power efficiency than single-mode light sources generally used in core-pumped types, and that temperature control by a Peltier element required for single-mode light sources is not necessarily required, and that they have excellent amplification efficiency (for example, Non-Patent Document 3).

[0005] However, a challenge with clad-pumped optical fiber amplifiers compared to core-pumped optical fiber amplifiers is that the overlap between the region where the excitation light propagates and the core region where the signal light propagates is low, resulting in less excitation light being absorbed within the amplification optical fiber.

[0006] To date, regenerative optical fiber amplifiers have been studied in which residual excitation light, which is transmitted to the output side without being absorbed by the amplifying optical fiber, is extracted by a regenerative device and re-injected into the amplifying optical fiber from the input side (for example, Non-Patent Document 4). However, in the regenerative optical fiber amplifier of Non-Patent Document 4, a large portion of the residual excitation light is lost when it is injected into the amplifying optical fiber, resulting in a problem of low amplification efficiency for the optical fiber amplifier. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Y. Tsuchida et al., “Amplification characteristics of a multi-core erbium-doped fiber amplifier,” in Proc. of OFC2012, paper OM3C.3 (2012) [Non-Patent Document 2] KS Abedin et al., “Clading-pumped erbium-doped multicore fiber amplifier,” Opt. Express, vol.20, No. 18, pp.20191-20200 (2012) [Non-Patent Document 3] Y. Jung et al., “High spatial density 6-mode 7-core fiber amplifier for L-band operation,” J. Lightw. Tecnol., vol. 38, No. 11, pp.2938-2943 (2020) [Non-Patent Document 4] H. Takeshita et al., “Configuration of pump injection and reinjection for improved amplification efficiency of turbo cladding pumped MC-EDFA,” in Proc. of ECOC2019, paper W.1.C.3 (2019). [Overview of the project] [Problems that the invention aims to solve]

[0008] This disclosure aims to improve the amplification efficiency of clad-excited optical fiber amplifiers. [Means for solving the problem]

[0009] This disclosure solves the above problems and improves the amplification efficiency of a clad-excited optical fiber amplifier by connecting a reflective device to the end face of the amplification optical fiber on the side other than the side to which the excitation optical combiner is connected.

[0010] Specifically, the optical fiber amplifier of this disclosure is Equipped with an amplification optical fiber doped with rare earth elements, An excitation light combiner for injecting multimode excitation light into the cladding region of the amplification optical fiber is connected to the input or output end of the amplification optical fiber. A reflective device that reflects the excitation light and transmits the signal light is connected to the end of the amplification optical fiber that is not connected to the excitation light combiner. [Effects of the Invention]

[0011] The optical fiber amplifier of this disclosure has a reflective device connected to the end of the amplification optical fiber, which improves the amplification efficiency of a clad-pumped optical fiber amplifier. [Brief explanation of the drawing]

[0012] [Figure 1A] This is a configuration example of an optical fiber amplifier. [Figure 1B] This is a configuration example of an optical fiber amplifier. [Figure 2] This is a configuration example of an optical fiber amplifier using the regeneration method of Non-Patent Document 4. [Figure 3A] An example of the configuration of the optical fiber amplifier of the present disclosure is shown. [Figure 3B] An example of the configuration of the optical fiber amplifier of the present disclosure is shown. [Figure 4] A configuration example of a reflection device is shown. [Figure 5] A configuration example of an excitation light combiner is shown. [Figure 6] This is an example of the absorption rate of the excitation light in the amplification optical fiber with respect to the residual coefficient s. [Figure 7] This is an example of the reflection efficiency R with respect to the residual coefficient s. [Figure 8] An example of the core number a and the cladding diameter when the residual coefficient s is 0.9 or less is shown.

Mode for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These examples are merely illustrative, and the present disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In the present specification and drawings, components having the same reference numerals indicate the same components.

[0014] (Embodiment Example 1) Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Figures 1A and 1B show an example of the configuration of a conventional clad-pumped optical fiber amplifier. In a conventional clad-pumped optical fiber amplifier, an excitation light combiner 13 is connected to either the input or output end of an amplification optical fiber 11 doped with rare earth elements, causing excitation light from an excitation light source 12 to be incident on the clad region of the amplification optical fiber 11, thereby amplifying the signal light guiding the core of the amplification optical fiber 11.

[0015] Figures 1A and 1B show a forward-excitation type, where the excitation light is incident from the input side of the signal light, and a backward-excitation type, where the excitation light is incident from the output side, respectively. Generally, in order to cause the multimode light from the excitation light source 12 to be incident on the cladding region of the amplification optical fiber 11, a multimode fiber 14 having a core with a diameter of 105 μm is connected between the excitation light source 12 and the excitation light combiner 13.

[0016] It is typical to connect an isolator to the input or output end of the amplification optical fiber 11 in accordance with the propagation direction of the signal light, but this is omitted in this disclosure. In addition, a residual excitation light remover may be installed to release excitation light that was not absorbed by the amplification optical fiber 11 to the outside of the amplification optical fiber 11.

[0017] Figure 2 shows an example configuration of a regenerative optical fiber amplifier described in Non-Patent Literature 4. For simplicity, only a forward-excited configuration is shown. A regenerative device 22 installed at the output end of the amplification optical fiber 11 separates the signal light from the residual excitation light, and a second excitation light combiner 21 couples it to the amplification optical fiber 11.

[0018] In this case, a multimode fiber (not shown) with a core diameter of 105 μm is generally used, and the residual excitation light propagating through the cladding of the amplification optical fiber 11 is once guided back to the core of the multimode fiber 23, and then re-input to the amplification optical fiber 11 via a second excitation light combiner 21 installed on the input side. However, it is difficult to inject the residual excitation light via the second excitation light combiner 21 without affecting the excitation light from the excitation light combiner 13, and in Non-Patent Literature 4, the regeneration efficiency of the regeneration device 22 and the second excitation light combiner 21 is 47.5% overall.

[0019] In Figure 2, the regeneration efficiency is the efficiency at which residual excitation light power that has passed through the amplification optical fiber 11 is injected back into the amplification optical fiber 11. Specifically, if x is the ratio of residual excitation light power extracted into the multimode fiber 23 by the regeneration device 22, and y is the ratio at which excitation light injected into the amplification optical fiber 11 via the second excitation light combiner 21 is injected back into the amplification optical fiber 11 via the multimode fiber 23, the regeneration efficiency can be calculated by multiplying x and y.

[0020] Figures 3A and 3B show an example of the configuration of the optical fiber amplifier of this disclosure. The optical fiber amplifier of this disclosure is An amplification optical fiber 11 doped with rare earth elements, An excitation light source 12 supplies excitation light to amplify signal light in the amplification optical fiber 11, An excitation light combiner 13 directs excitation light from the excitation light source 12 onto the cladding of the amplification optical fiber 11, A reflective device 15 that reflects excitation light and transmits signal light, It is equipped with. The reflective device 15 is connected to the unconnected end of the amplification optical fiber 11 of the excitation optical combiner 13.

[0021] Figure 3A shows an example of a forward-excited type where the excitation light is incident from the input side of the signal light, and the excitation light combiner 13 is connected to the input end of the amplification optical fiber 11. In this case, the reflection device 15 is connected to the output end of the amplification optical fiber 11, transmits the signal light amplified by the amplification optical fiber 11, and reflects the residual excitation light that has passed through the amplification optical fiber 11 back to the amplification optical fiber 11.

[0022] Figure 3B shows an example of a back-excitation type where the excitation light is incident from the output side of the signal light, and the excitation light combiner 13 is connected to the output end of the amplification optical fiber 11. In this case, the reflection device 15 is connected to the input end of the amplification optical fiber 11, allowing the signal light before amplification to pass through the amplification optical fiber 11, and reflecting the residual excitation light after it has passed through the amplification optical fiber 11 back to the amplification optical fiber 11.

[0023] Figure 4 shows an example configuration of the reflective device 15. This figure shows an example of a forward-excited type. The reflective device 15 comprises an excitation light reflection filter 151, lenses 152 and 153. The excitation light reflection filter 151 is a wavelength filter that reflects the excitation light wavelength and transmits the communication light wavelength. The excitation light reflection filter 151 reflects the excitation light emitted from the amplification optical fiber 11 to the cladding of the amplification optical fiber 11. In this disclosure, since the excitation light propagates through the cladding, residual excitation light spreads out and is emitted from the output end of the amplification optical fiber 11. Therefore, the excitation light reflection filter 151 is used which has a diameter larger than the cladding diameter of the amplification optical fiber 11.

[0024] In this disclosure, when the transmission optical fibers 51 and 52 are multicore fibers, the amplification optical fiber 11 also uses a multicore fiber having the same number of cores as the transmission optical fiber 51. The reflective device 15 is equipped with lenses 152 and 153, which allows each signal light propagated through each core 91 of the amplification optical fiber 11 to be coupled to the core 94 of the output transmission optical fiber 52.

[0025] In the case of a back-excited type, the same configuration as the forward-excited type reflective device 15 can be used by replacing the configuration of the transmission line optical fiber 52 with the configuration of the transmission line optical fiber 51. Furthermore, the reflective device 15 of this embodiment is not limited to a spatial reflective device 15, and any configuration can be adopted depending on the environment of the transmission line.

[0026] The configuration of this disclosure regenerates residual excitation light in the reverse propagation direction of the optical fiber having the same structure, thus eliminating the need for a multimode fiber to inject multimode light into the cladding region of the amplification optical fiber 11. Therefore, compared to the configuration shown in Figure 2, this disclosure does not have any factors that degrade the regeneration efficiency, and the residual excitation light can be re-input to the amplification optical fiber 11 with a regeneration efficiency close to 100% in the reflective device 15. For this reason, this embodiment can significantly improve the amplification efficiency with respect to the input excitation light power.

[0027] In this embodiment, the case where the amplification optical fiber 11 is a multi-core fiber has been described, but this disclosure can be applied to any SDM optical fiber. For example, the transmission optical fibers 51, 52 and the amplification optical fiber 11 may be multi-mode fibers having one core. For example, the transmission optical fibers 51, 52 and the amplification optical fiber 11 may be multi-core fibers with the same number of cores. For example, the transmission optical fibers 51, 52 and the amplification optical fiber 11 may be multi-core fibers with the same number of cores, and each core may be capable of propagating two or more modes.

[0028] (Example of Embodiment 2) As shown in Figure 4, the intercore distance Λ of the amplification optical fiber 11 A and the inter-core distance Λ of the optical fiber 52 for the transmission line S These may differ. In that case, by adjusting the focal length and placement of lenses 152 and 153, it is possible to connect amplification optical fiber 11 and transmission optical fiber 52 with different inter-core distances.

[0029] In the case of a back-excited type, the same configuration as the forward-excited type reflective device 15 can be used by replacing the configuration of the transmission line optical fiber 52 with the configuration of the transmission line optical fiber 51. Furthermore, the reflective device 15 of this embodiment is not limited to a spatial reflective device 15, and any configuration can be adopted depending on the environment of the transmission line.

[0030] Figure 5 shows an example configuration of the excitation light combiner 13. This figure shows an example of a forward-excitation type. The excitation light combiner 13 comprises a dichroic mirror 131, and lenses 132, 133, and 134. The dichroic mirror 131 is placed in the optical path between the output end of the transmission optical fiber 51 and the input end of the amplification optical fiber 11, and couples the excitation light from the multimode fiber 14 to the input end of the amplification optical fiber 11. As a result, the signal light from the transmission optical fiber 51 and the excitation light from the multimode fiber 14 are incident on the amplification optical fiber 11.

[0031] In this case, if the amplification optical fiber 11 is a multicore fiber, the inter-core distance of the amplification optical fiber 11 and the inter-core distance of the transmission optical fiber 51 may differ. In that case, by adjusting the focal length and placement of the lenses 132 and 133, the transmission optical fiber 51 and the amplification optical fiber 11 with different inter-core distances can be connected.

[0032] In the case of a back-excited type, the same configuration as the forward-excited type excitation optical combiner 13 can be used by replacing the configuration of the transmission line optical fiber 51 with the configuration of the transmission line optical fiber 52. Furthermore, the excitation optical combiner 13 of this embodiment is not limited to a spatial system excitation optical combiner 13, and any configuration can be adopted according to the environment of the transmission line.

[0033] (Example of Embodiment 3) Figure 6 shows the residual coefficient s = P p1 / P p0 The absorption rate of the excitation light in the amplification optical fiber 11 is calculated for the given value. The residual coefficient s is the input power P of the excitation light input from the excitation light source 12 to the amplification optical fiber 11. p0And the output power P of the residual excitation light after passing through the amplification optical fiber 11. p1 This is the ratio. In this disclosure, the input power P p0 This is referred to as the excitation light input power, and the output power P p1 This is sometimes referred to as residual excitation light output power.

[0034] Here, the excitation light absorption rate indicates how much of the excitation light was absorbed in total within the amplification optical fiber 11 by regeneration and reflection. In the case of the reflection device 15 of this embodiment, the absorption rate obtained by summing the absorption in the amplification optical fiber 11 in the forward and return paths was used. In the case of the regeneration method shown in Figure 2, assuming a configuration in which the multimode fiber 23 and amplification optical fiber 11 rotate in a loop, the sum of the regeneration efficiency and the absorption coefficient of the EDF was used.

[0035] In the case of the conventional regeneration method shown in Figure 2, the regeneration efficiency was estimated at a high of 50% based on the results of reports such as Non-Patent Literature 4, and calculations were performed accordingly. Furthermore, in the configuration of this disclosure, calculations were performed for the cases where the reflection efficiency R of the reflection device 15 is 0.7 and 0.9. In addition, the excitation light reflected by the reflection device 15 passes through the amplification optical fiber 11 again and is output to the excitation light source 12 side via the excitation light combiner 13, where it is removed by an isolator provided in the excitation light source 12, or by an excitation light isolator installed immediately after the excitation light combiner 13.

[0036] As shown in Figure 6, when comparing the conventional regeneration method with the configuration of this disclosure in which the reflectance efficiency R is 0.7, the superiority of the excitation light absorption rate reverses around a residual coefficient s of 0.55. Specifically, for s < 0.55, the excitation light absorption rate of this disclosure is superior.

[0037] Based on this calculation, the region in which the present disclosure is superior was calculated using the residual coefficient s of the amplification optical fiber 11 and the reflection efficiency R of the reflection device 15 of the present disclosure as parameters. The results are shown in Figure 7. The shaded region in the figure indicates the region in which the present disclosure has superior excitation light absorption compared to conventional regenerative methods. R>0.5+0.30126s-0.065695s2 +0.26456 s 3 was found to be the region of.

[0038] It was experimentally confirmed that the reflection efficiency R of the reflection device 15 is 0.9 or more. Therefore, by using the reflection device 15 having a reflection efficiency R of 0.9 or more, excellent amplification efficiency can be exhibited in the amplifying optical fiber 11 having a residual coefficient s of 0.9 or less.

[0039] For example, when a single-mode single-core fiber having a cladding diameter of 125 μm, a core radius of 4.5 μm, and an erbium doping amount of 6 × 10 24 ions / m 3 is used, assuming that the fiber length is adjusted to 8 m to amplify 1530 to 1565 nm, the excitation light is 980 nm, the excitation light input power is 10 W, and the signal light input power to the amplifying optical fiber 11 is -6 dBm / core, the residual coefficient s is 98%. When changing to a multi-core structure with the same core structure and erbium doping amount, Fig. 8 shows the calculation of the condition where the residual coefficient s becomes 0.9 with respect to the number of cores.

[0040] From the curve shown in Fig. 8, when the number of cores is a and the cladding diameter is b μm, b < -0.5653 × a 2 + 21.47 × a + 43.92 For a multi-core fiber that satisfies the condition of, even if the reflection efficiency R of the reflection device 15 is 0.9 or less, an amplification efficiency superior to the conventional regeneration method can be achieved.

[0041] Note that the black (× plot) in Fig. 6 indicates the absorption rate of the excitation light in the non-regeneration case without regeneration as shown in Figs. 1A and 1B. Comparing the absorption rate of the excitation light in the amplifying optical fiber 11 of the present disclosure with the conventional non-regeneration method, the absorption rate of the excitation light can be improved by 10% or more in the range of the residual coefficient s of 0.1 to 0.9, and by 18% or more in the range of 0.1 to 0.8.

Industrial Applicability

[0042] This disclosure can be applied to the information and communications industry. [Explanation of symbols]

[0043] 11: Optical fiber for amplification 12: Excitation light source 13: Excitation light combiner 131: Dichroic mirror 132, 133, 134: Lens 14: Multimode fiber 15: Reflective devices 151: Excitation light reflection filter 152, 153: Lens 21: Second excitation light combiner 22: Regenerative device 23: Multimode fiber 51, 52: Optical fibers for transmission lines

Claims

1. Equipped with multi-core amplification optical fibers doped with rare earth elements, An excitation light combiner for injecting multimode excitation light into the cladding region of the amplification optical fiber is connected to the input or output end of the amplification optical fiber. A reflective device that reflects the excitation light and transmits the signal light is connected to the end of the amplification optical fiber that is not connected to the excitation light combiner. The reflection efficiency of the aforementioned reflective device is less than 0.9, The amplification optical fiber has a number of cores a, b<-0.5653×a 2 +21.47×a+43.92 Having a cladding diameter b that satisfies the following conditions: Fiber optic amplifier.

2. Equipped with multi-core amplification optical fibers doped with rare earth elements, An excitation light combiner for injecting multimode excitation light into the cladding region of the amplification optical fiber is connected to the input or output end of the amplification optical fiber. A reflective device that reflects the excitation light and transmits the signal light is connected to the end of the amplification optical fiber that is not connected to the excitation light combiner. The excitation light input power input to the aforementioned amplification optical fiber is P ｐ０ The residual excitation light output power after passing through the amplification optical fiber is P ｐ１ In this case, the reflection efficiency R of the excitation light in the reflective device is such that the residual coefficient s = P ｐ１ / P ｐ０ In contrast, R>0.5+0.30126s-0.065695s ２ +0.26456s ３ Satisfying the conditions, The upper limit of the cladding diameter of the amplification optical fiber is determined such that the residual coefficient of the excitation light is high with respect to the number of cores of the amplification optical fiber. Fiber optic amplifier.

3. The reflective device includes an excitation light reflection filter that reflects the excitation light and transmits the signal light. The region in the excitation light reflection filter that reflects the excitation light is wider than the cladding region of the amplification optical fiber. The excitation light reflection filter reflects the excitation light emitted from the amplification optical fiber to the cladding region of the amplification optical fiber. The optical fiber amplifier according to claim 1 or 2.

4. The amplification optical fiber, with respect to the number of cores a of the amplification optical fiber, b<-0.5653×a 2 +21.47×a+43.92 Having a cladding diameter b that satisfies the following conditions: The optical fiber amplifier according to claim 2.

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