Amplifying optical fiber and cladding-pumped optical fiber amplifier

Abstract

The purpose of the present invention is to improve the amplification factor of an L-band signal.　The present disclosure is an optical fiber for amplification to which a rare earth element has been added, said rare earth element–added optical fiber being characterized in that: two or more cores are comprised within the cladding cross section of the optical fiber for amplification; and the core density C obtained by dividing the core number by the cladding area is 0.0008 m2 or greater, and the core radius a is 1-3.5 μm.

Description

[Technical Field]

[0001] The present invention relates to an optical fiber amplifier. [Background technology]

[0002] In optical fiber communication systems, the loss of light propagating through an optical fiber is reduced by amplifying it at regular intervals using optical amplifiers and relaying it for long-distance transmission. Amplification within the optical amplifier is achieved by injecting signal light and pumping light (mainly 980nm or 1480nm light in the case of EDF) into an amplifying optical fiber with rare earth elements added to its core (mainly erbium-doped optical fiber: EDF), and amplifying the light without converting it to electricity.

[0003] In current communications using single-mode optical fibers (SMF), core-pumped optical amplifiers are used, which amplify signal light propagating through the core by guiding pumping light into the core in the same way.On the other hand, in recent years, in order to expand the transmission capacity of optical fibers, multicore fibers having multiple cores in the cross section of the optical fiber or optical fibers for space division multiplexing (SDM) using few-mode fibers with two or more modes propagating within the core have been considered, and amplifiers for these optical fibers in which multiple spatial modes propagate through a single optical fiber have been considered (for example, Non-Patent Document 1).

[0004] For these SDM optical fibers, SDM optical fiber amplifiers for simultaneously amplifying multiple spatial modes are being studied (for example, Non-Patent Document 2). Non-Patent Document 2 studies cladding-pumped optical fiber amplifiers, which, unlike core-pumped optical fiber amplifiers, guide pumping light in the cladding region of the optical fiber and amplify multiple cores or multiple modes simultaneously. Cladding-pumped optical fiber amplifiers can use a multimode light source for pumping light, which is superior in power efficiency to the single-mode light source generally used in core-pumped optical fiber amplifiers, and do not necessarily require temperature control using a Peltier element, which is required for single-mode light sources. Therefore, cladding-pumped optical fiber amplifiers are expected to exhibit superior amplification efficiency.

[0005] Compared with core pumping systems, cladding pumping optical fiber amplifiers have a low overlap between the region where the pump light propagates and the core region doped with rare earth elements, which has led to a problem of a small amount of pump light being absorbed in the amplification optical fiber. However, studies have been conducted to increase the amount of pump light absorbed in the optical fiber by increasing the core-cladding ratio Rcc, which is the ratio between the total area of ​​the cores in the optical fiber and the cladding area including the core region, and high amplification efficiency has been demonstrated (for example, Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0006] [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] T. Sakamoto et al., “Characteristics of Randomly Coupled 12-core Erbium-Doped Fiber Amplifier,” J. of Lightw. Technol., vol. 39, no. 4, pp. 1186-1193 (2021) [Non-patent document 4] S. Takasaka et al., “EDF length dependence of amplification characteristics of cladding pumped 19-core EDFA,” in Proc. of OFC2018, paper Th1K.2 (2018) Summary of the Invention [Problem to be solved by the invention]

[0007] However, efforts to increase the amplification efficiency by increasing the Rcc have only been considered for optical fiber amplifiers that amplify the C band (1530 to 1565 nm), and the structural conditions for the amplifying optical fiber required to achieve highly efficient amplification in optical fiber amplifiers that amplify the L band (1565 to 1610 nm), one of the low-loss communication wavelength bands for optical fiber, have not been clear.

[0008] In general, the length of the erbium-doped optical fiber (EDF) that amplifies the L band is longer than the length of the EDF in a C-band amplifier, and in studies focusing on uncoupled multicore fibers, it has been reported that the amount of pump light absorbed over the entire length of the EDF is larger than that in the C band due to the longer EDF, resulting in improved amplification efficiency (e.g., Non-Patent Document 4).

[0009] However, as described in Non-Patent Document 3, experimental results have been reported showing that the amplification efficiency of an L-band amplifier with a long EDF decreases even when the optical fiber structure is the same, and the structural conditions for an amplifying optical fiber to realize a highly efficient L-band optical amplifier have been unclear.

[0010] An object of the present invention is to improve the amplification factor of L-band signals. [Means for solving the problem]

[0011] The present disclosure is intended to solve the above-mentioned problems, and provides an optical fiber amplifier that amplifies L-band signals with high efficiency.

[0012] The amplification optical fiber of the present disclosure comprises: It is an amplifying optical fiber doped with rare earth elements, The amplification optical fiber has two or more cores in a cross section of the cladding, The core density C, calculated by dividing the number of cores by the cladding area, is 0.0008 μm -2 That's all, The core radius a is 1 μm or more and 3.5 μm or less.

[0013] The cladding-pumped optical fiber amplifier of the present disclosure comprises: an amplification optical fiber according to the present disclosure; a pump light combiner for injecting pump light into a cladding region of the amplification optical fiber; a pumping light source that supplies multimode pumping light to the pumping light combiner; Equipped with.

[0014] The cladding is a first cladding disposed around the core; a second cladding disposed around the first cladding; Equipped with The cladding area may be determined using the area of ​​the first cladding.

[0015] The length of the amplification optical fiber may be adjusted so as to amplify the L-band wavelength range of 1565 nm or more and 1610 nm or less.

[0016] The above disclosures can be combined as much as possible. [Effects of the Invention]

[0017] The amplification optical fiber of the present invention can improve the amplification efficiency of L-band signals. [Brief explanation of the drawings]

[0018] [Figure 1A] 1 illustrates an example of the configuration of a forward-pumped cladding-pumped optical fiber amplifier according to the present disclosure. [Figure 1B]1 shows an example of the configuration of a backward-pumped cladding-pumped optical fiber amplifier according to the present disclosure. [Figure 2] 1 shows an example of a cross-sectional structure of an amplification optical fiber in the present disclosure. [Figure 3] 1 shows an example of the amplification characteristics calculated according to a model of a multi-core fiber amplifier. [Figure 4] An example of the amplification characteristics when the cladding diameter is 80, 100, and 125 μm is shown. [Figure 5] An example of the calculation results of the PCE contours with respect to the core density and core radius a is shown. [Figure 6] The calculation results of PCE when the amount of erbium added is changed are shown. [Figure 7] This shows the calculation results of the core density contours with respect to the number of cores and the cladding diameter. DETAILED DESCRIPTION OF THE INVENTION

[0019] 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 implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0020] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. 1A and 1B show the configuration of a cladding-pumped optical fiber amplifier according to the present disclosure. A pumping light combiner 93 is connected to either the input or output terminal of an amplification optical fiber 91 doped with a rare earth element, and combines pumping light from a pumping light source 92 that supplies pumping light. The amplifier amplifies signal light guided through the core of the amplification optical fiber 91.

[0021] Typically, isolators are connected to the input and output ends in accordance with the propagation direction of the signal light, but this is omitted in this figure. A residual pump light remover may also be installed to emit pump light not absorbed in the amplification optical fiber 91 out of the optical fiber. Figures 1A and 1B show a forward pumping type in which pump light is incident from the input side of the signal light, and a backward pumping type in which pump light is incident from the output side, respectively. Generally, multimode pump light emitted from a pump light source 92 is coupled into an optical fiber with a core diameter of 105 μm.

[0022] FIG. 2 shows an example of the cross-sectional structure of an amplification optical fiber 91 according to the present disclosure. While the figure shows a cross-sectional view of a multi-core optical fiber with two cores 11, optical fibers with three or more cores in a square lattice, hexagonal close-packed, or annular core arrangement can also be used. There are core 11 regions with a refractive index of n1 and cladding 12 regions with a refractive index of n2, where n1 > n2. In the structure shown in the figure, the condition n1 > n2 can be achieved by using pure silica glass as the material for each region, or silica glass doped with impurities that increase the refractive index, such as germanium (Ge), aluminum (Al), or phosphorus (P), or impurities that decrease the refractive index, such as fluorine (F) or boron (B). Furthermore, the inter-core distance is denoted as Λ.

[0023] Moreover, the amplification optical fiber 91 according to the present disclosure has a second cladding 13 having a lower refractive index than the cladding 12. Hereinafter, the cladding 12 surrounding the core 11 may be referred to as the first cladding, and the cladding 13 surrounding the first cladding may be referred to as the second cladding.

[0024] The second cladding 13 is generally made of a resin having a lower refractive index than the first cladding 12, but may also be a glass cladding doped with fluorine or the like to have a refractive index lower than that of the first cladding 12. In the amplification optical fiber 91, a rare earth element is doped into a part or the entire core 11, or into the region around the core including the surrounding claddings 12 and 13.

[0025] Figure 3 shows the amplification characteristics calculated according to the model of the multi-core fiber amplifier described in Non-Patent Document 2. The vertical axis represents the photoconversion efficiency (PCE), and the pump light intensity is P p , the input signal intensity is P s0 , the output signal light intensity is P s1 When PCE=(P s1 -P s0 ) / P p In the figure, the above formula is multiplied by 100 and expressed in percentage units. The horizontal axis is the core-cladding ratio Rcc.

[0026] In this case, the cladding area indicates the area of ​​the cladding through which the pumping light is guided, and is defined by the area of ​​the first cladding 12, and the core area is defined by the sum of the areas of the individual cores 11 in a multicore fiber having two or more cores 11.

[0027] In this calculation, the input power per core 11 is set to -8 dBm, the diameter of the cladding 12 is fixed at 90 μm, and Rcc is changed by changing the core radius a of each core 11. The dashed line in the figure shows the calculation result when amplifying a C-band signal, and the solid line shows the calculation result when amplifying an L-band signal. The C-band is a four-wave WDM signal with signal light wavelengths of 1530, 1540, 1550, and 1565 nm, and the L-band is a four-wave WDM signal with signal wavelengths of 1570, 1580, 1590, and 1600 nm. In each case, the gain is set to 20 dB, and the EDF length and pump light intensity are adjusted so that the gain of the shortest and longest wavelength signals in the WDM signal is the same. The number of cores is 12, the pump light wavelength is 980 nm, and the amount of erbium doped into the cores is 6 × 10 24 ions / m 3 It was decided.

[0028] The figure shows that in cladding-pumped L-band optical fiber amplifiers, a specific Rcc range is required to achieve a high PCE, and unlike C-band optical fiber amplifiers, highly efficient amplification cannot be achieved by simply increasing Rcc.

[0029] Figure 4 shows the results of calculations similar to those in Figure 3, but for cladding diameters D of 80, 100, and 125 μm. It can be seen that the Rcc value at which PCE is maximized does not change much, independent of the cladding diameter D. On the other hand, when the cladding diameter D is changed with Rcc fixed, it can be seen that a smaller cladding diameter D results in a higher PCE.

[0030] Incidentally, calculations have confirmed that under conditions of a constant core diameter, if the core density (number of cores / cladding area) is the same, the Rcc will also be constant and the PCE characteristics will be the same. For example, when comparing a 12-core fiber with a cladding diameter D of 100 μm and a 3-core fiber with a quarter of the core count and a cladding diameter D of 50 μm (half the cladding area, i.e., a quarter of the cladding area), the Rcc vs. PCE curves will be exactly the same when compared at the same core diameter. In other words, Figure 4 is not limited to the case of a 12-core fiber, but can be said to generally compare the characteristics at three different core densities.

[0031] Figure 5 shows the results of calculations of the contours of PCE versus core density and core radius, calculated using the same conditions and procedures as in the calculations in Figures 3 and 4. According to Non-Patent Document 3, the highest PCE among the C-band amplifiers reported to date is 10%, and the conditions required to obtain characteristics equivalent to this are (i) Core density C>0.0008μm -2 (ii) 1 μm<core radius a<3.5 μm As a result, the amplification optical fiber of the present disclosure can improve the amplification efficiency of signals in the L band, which is equal to or greater than 1565 nm and equal to or less than 1610 nm.

[0032] In this calculation, the amount of erbium added, N0, is 6×10 24 ions / m 3The Rcc range above remains unchanged for other doping amounts. Figure 6 shows the calculation results for PCE when the erbium doping amount is changed. The number of cores is set to 12, and the core radius a is changed to 1.0, 2.5, and 5.5 μm. When the erbium doping amount increases or decreases, the EDF length must be changed to obtain the same amplification characteristics. In this calculation, the product of the erbium doping amount N0 and the EDF length L is 4.8 × 10 26 (ions / m 2 ) is kept constant. From the figure, it can be seen that even with an arbitrary erbium doping amount, the PCE characteristics remain unchanged by adjusting the EDF length to obtain equivalent amplification characteristics. In other words, the conditions for obtaining a high PCE in an L-band optical fiber amplifier do not depend on the erbium doping amount.

[0033] Figure 7 shows the calculated contours of the core density versus the number of cores and the cladding diameter D. The area surrounded by the dashed line represents the core density C>0.0008 μm mentioned earlier. -2 This is the region where the core density is larger than the core distance D. In the conventional MCF structures described in Non-Patent Documents 1 and 4, the core distance is 30 μm or more because the design is based on an uncoupled multi-core structure, and the cladding diameter D tends to be large and the core density small accordingly. Therefore, in such a design region, the core density is considered to be lower than the target of this disclosure. Therefore, it can be said that the present disclosure cannot be easily inferred from the results of previous studies.

[0034] For example, the optical fiber described in Non-Patent Document 4 has 19 cores and a cladding diameter D of 200 μm. According to FIG. 7, the core density is 0.0008 μm. -2 In the case of Non-Patent Document 3, the core density is 0.0008 μm -2 Although this is the case, the core radius is 5.5 μm, so this does not meet the conditions of the present disclosure.

[0035] Regarding adjustment of the amplification band of an optical fiber amplifier, for example, in the case of an erbium-doped optical fiber, as described in Non-Patent Document 4, a length of around 10 m generally provides the characteristic of amplifying the C band, and by increasing the length by several times that length (for example, 60 to 100 m), the amplification band shifts to the L band of 1565 nm or more and 1610 nm or less, making it possible to realize an L-band amplifier. A specific procedure involves lengthening the length of the amplification optical fiber 91 while checking the amplification band, or using a sufficiently long amplification optical fiber and shortening the fiber length while checking the amplification band, until the optimal fiber length is achieved when the desired amplification is obtained in the L-band wavelength band. [Explanation of symbols]

[0036] 11: Core 12: First Clad 13: Second Clad 91: Amplifying optical fiber 92: Excitation light source 93: Pump light combiner

Claims

1. An amplification optical fiber; a pumping light combiner for injecting pumping light into the clad of the amplification optical fiber; a pumping light source that supplies multimode pumping light to the pumping light combiner; Equipped with The amplification optical fiber is It is an amplifying optical fiber doped with rare earth elements, The amplification optical fiber has two or more cores in a cross section of the cladding, a core density C obtained by dividing the number of cores by the cladding area is 0.0008 μm −2 or more; The core radius a is 1 μm or more and 3.5 μm or less. Cladding-pumped optical fiber amplifier.

2. An amplification optical fiber; a pumping light combiner for injecting pumping light into the clad of the amplification optical fiber; a pumping light source that supplies multimode pumping light to the pumping light combiner; Equipped with The amplification optical fiber is It is an amplifying optical fiber doped with rare earth elements, The amplification optical fiber has two or more cores in a cross section of the cladding, a core density C obtained by dividing the number of cores by the cladding area is 0.0008 μm −2 or more; The core radius a is 1 μm or more and 3.5 μm or less, the length of the amplification optical fiber is adjusted so as to amplify a band of 1565 nm or more and 1610 nm or less; A cladding pumped optical fiber amplifier characterized by:

3. The cladding of the amplification optical fiber is a first cladding disposed around the core; a second cladding disposed around the first cladding; Equipped with The cladding area is determined using the area of ​​the first cladding.

3. A cladding-pumped optical fiber amplifier according to claim 1 or 2.

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