JP2011091099A - Fluorescent glass body and optical waveguide for amplification including the same as light guide section - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber for amplification further improved in gain characteristics. <P>SOLUTION: A fluorescent glass body contains an Er element and a specific element added to SiO<SB>2</SB>-based glass. The specific element may be a dyad separated away from an oxygen atom by 0.28 nm or longer in the SiO<SB>2</SB>glass, or a triad separated away from the oxygen atom by 0.18 nm or longer in the SiO<SB>2</SB>glass. It is preferable that additive concentrations of an Al element and a B element should be lower than that of the specific element. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a fluorescent glass body in which an Er element is added to a glass based on SiO ₂ and an optical waveguide for amplification having the same as a light guide section.

  In an optical communication system, in order to transmit a large amount of information over a long distance at high speed, multi-wavelength signal light is multiplexed and transmitted through an optical fiber transmission line, and the multi-wavelength signal light is collectively amplified at a relay station. To do. In general, a 1.55 μm band is used as a signal light wavelength band, and an Er element-doped optical fiber amplifier is used as an optical amplifier that optically amplifies signal light in this wavelength band.

The Er element-doped optical fiber amplifier includes an amplification optical fiber (EDF: Erbium-Doped Fiber) having a core in which Er element is added in SiO ₂ -based glass as an optical amplification medium. The Er element-doped optical fiber amplifier supplies pumping light having a wavelength (0.98 μm band or 1.48 μm band) capable of pumping Er element to the amplification optical fiber, and the signal light (1. 55 μm band) is optically amplified.

  It is known that an amplification optical fiber having an Er element-doped glass in a waveguide portion has improved gain characteristics when an Al element is co-doped with Er element-doped glass (Patent Document 1, Non-Patent Document 1). Reference 1). That is, it is known that co-addition of Al element broadens the Er fluorescence spectrum and improves gain flatness as an optical amplifier. This is believed to be due to the fact that the Er—O coordination number increases due to the co-addition of Al element, and the ligand field changes due to this structural change.

JP-A-5-279066

Tetsuya Haruna, et al., “Analysis of Er coordination structure in Er-doped fiber”, January 2008, SEI Technical Review, 172, pp. 125-129

  However, in order to transmit a large amount of information at a high speed over a long distance, it is necessary to further improve the gain characteristics of the optical fiber for amplification using Er element-doped glass (broadband by flattening the gain). In addition, there is a limit in optical fiber for amplification using Er element-doped glass co-doped with Al element.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a fluorescent glass body capable of further improving gain characteristics and an amplification optical waveguide having the same as a light guide section.

In the fluorescent glass body according to the present invention, an Er element and a specific element are added to a SiO ₂ -based glass, and the distance between the atom of the specific element and the O atom in the SiO ₂ glass is 0.28 nm or more. Or a trivalent element having a distance from an O atom of 0.18 nm or more in SiO ₂ glass.

  In the fluorescent glass body according to the present invention, it is preferable that the addition concentration of Al element and B element in the glass is smaller than the addition concentration of the specific element. It is preferable that the number of Er atoms in the glass is in the range of 1/100 to 1/6000 with respect to the number of Si atoms. Further, it is preferable that the number of atoms of the specific element in the glass is 7/20 or less with respect to the number of Si atoms.

  The fluorescent glass body according to the present invention is preferably added with a monovalent cation, and in this case, the number of cations is preferably 1/1000 or less with respect to the number of Si atoms.

  An amplification optical waveguide according to the present invention is characterized in that the above-described fluorescent glass body according to the present invention is included in a waveguide section.

  The fluorescent glass body according to the present invention and the amplification optical waveguide body having the same in the waveguide section can further improve the gain characteristics.

It is a figure which shows the energy diagram which contributes to light emission in the wavelength 1.55 micrometer band in EDF. It is a figure which compares and shows the fluorescence spectrum of Er element addition glass with the case where Al element is co-added, and the case where it is not so. It is a schematic diagram which compares and shows O ion coordination structure with respect to Er atom with a crystal | crystallization and glass. It is a figure explaining molecular dynamics simulation. It is a figure which shows an example of the Er element addition glass structure model obtained by molecular dynamics simulation. It is a graph which shows the coordination number of the oxygen which exists around the Er atom obtained by molecular dynamics simulation. It is a figure which shows typically the relationship between the number of Er-O pairs obtained by molecular dynamics simulation, and Er-O distance. It is a figure which shows the fluorescence spectrum calculated from the EDF structure obtained by molecular dynamics simulation. It is a figure which shows typically the relationship between the number of Er-O pairs and Er-O distance. It is a figure which shows typically the relationship between the force between each atom of Si-O, Al-O, and Er-O, and distance. The coordination around the Er atoms in SiO ₂ glass network is a diagram schematically illustrating. It is a graph which shows the relationship between Er-O integrated coordination number in case Er-O distance is less than 0.29 nm and MO distance at the time of adding monovalent element M together. It is a graph which shows the relationship between the Er-O integrated coordination number in case Er-O distance is less than 0.29 nm and MO distance at the time of co-adding the bivalent element M. It is a graph which shows the relationship between Er-O integrated coordination number in case Er-O distance is less than 0.29 nm and MO distance when trivalent element M is added together. It is a graph which shows the relationship between Er-O integrated coordination number in case Er-O distance is less than 0.29 nm and MO distance at the time of co-addition of the tetravalent element M. It is a graph which shows the relationship between the Er-O integrated coordination number in case Er-O distance is 0.24 nm or less and MO distance when the bivalent element M is added together. It is a graph which shows the relationship between the Er-O integrated coordination number in case Er-O distance is 0.24 nm or less and MO distance when trivalent element M is added together. It is a figure which shows the fluorescence spectrum of Er element addition glass in which Ba element was added together as an example of a specific element. It is a figure which shows the relationship between the Al element co-addition amount and Er-O integrated coordination number in Er element addition glass which co-added Al element in addition to the bivalent specific element. It is a figure which shows the relationship between the Al element co-addition amount and Er-O integrated coordination number in Er element addition glass which co-added Al element in addition to the trivalent specific element. The coordination around the Er atoms in SiO ₂ glass network is a diagram schematically illustrating. It is a figure which shows the structure of the optical fiber for amplification which concerns on this embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

The present inventor is conducting research and development on improvement of gain flatness of an amplification optical fiber (hereinafter referred to as “EDF”) having a waveguide portion in which Er element is added to SiO ₂ -based glass. Thus, the mechanism by which the Al element co-added into the Er element-doped glass contributes to the improvement of the gain characteristics was analyzed. In addition, based on the knowledge obtained by the analysis, the present inventor has newly found a co-added element capable of obtaining an effect of improving gain characteristics superior to that of the Al element.

This newly found co-added element is referred to as a “specific element”. Distance of a specific element is divalent element the distance between the O atoms is above 0.28nm in SiO ₂ in the glass (e.g., Ca, Sr, Ba, Ra, etc.), or an O atom in the SiO ₂ glass Is a trivalent element (for example, Ga, In, Y, La, or the like) having a thickness of 0.18 nm or more.

  Further, the present inventor can improve gain flatness by co-addition of the specific element described above when a certain element among other elements that may be added for various purposes is co-added to the Er element-added glass. It was found that functional expression is inhibited. An element that inhibits this function expression will be referred to as an “inhibiting element”. In the Er element-doped glass, it is preferable that the concentration of the inhibitory element is smaller than that of the specific element. The inhibitory element is an Al element or a B element.

  In the following, the process of finding the specific element and the inhibitory element will be described in more detail, and the fluorescent glass body and the amplification optical waveguide according to the present embodiment will be described in detail.

Emission at a wavelength 1.55μm band in Er-doped glass transition as shown in the energy diagram in FIG. 1, from ⁴ I _13/2 level of the 4f orbit of Er element to ⁴ I _15/2 level by. The energy of the 4f orbit of the Er element is split into several levels under the influence of the ligand field, that is, the electric field due to oxygen ions (O ²⁻ ) around the Er element. This phenomenon is known as the Stark effect. The Er—O distance and coordination number are factors that greatly affect the fluorescence spectrum. Therefore, as shown in FIG. 2, the Er-O distance and the coordination number are different and the fluorescence spectrum is different between the case where the Al element is co-added in the Er element-doped glass and the case where the Al element is not added.

The Er—O distance and coordination number are uniquely determined when the crystal is, for example, Er ₂ O ₃ (FIG. 3A). However, in the EDF that is glass (that is, amorphous), the structure is not as unique as the crystal, and the Er—O distance and coordination number exist in a certain range (FIG. 3B). )). The broadening of the fluorescence spectrum of EDF by Al element co-addition as described above was presumed to involve changes in the Er—O coordination structure, but the detailed mechanism has not been clarified. It was.

  The present inventor analyzed changes in the Er—O distance and coordination number due to the addition of an element to the Er element-doped glass using molecular dynamics simulation. In the molecular dynamics simulation, first, potential energy between atoms such as Er—O and Si—O is set. Thereafter, the step of differentiating the potential by the distance and calculating the force acting between the atoms (FIG. 4A) and the step of moving the atom for a short time according to the force (FIG. 4B) are repeated. . Thereby, a stable atomic arrangement in the composition is required.

  FIG. 5 is a diagram illustrating an example of an EDF structural model obtained by molecular dynamics simulation. This model includes about 3000 atoms in total such as Si element, O element and Er element. In this model, the vicinity of the Er atom is enlarged and shown in FIG.

  FIG. 6 is a graph showing the coordination number of oxygen existing around Er atoms obtained by molecular dynamics simulation. Here, the Er—O cumulative coordination number on the vertical axis corresponds to the number of O atoms existing in the spherical shell with the Er atom as the center and the radius of the Er—O distance on the horizontal axis as the radius. In the molecular dynamics simulation, the calculation was performed using a plurality of different random number patterns as the initial structure. The result shown in FIG. 6 is an average value of the Er—O cumulative coordination numbers obtained in a plurality of cases.

Here, assuming that the Er—O distance that is the first coordination with respect to the Er element is within 0.29 nm, and paying attention to the increase in the number of accumulated Er—O coordination within this range, co-addition of the Al element As an effect, the following features can be found from FIG. That is, with SiO ₂ alone, the Er—O cumulative coordination number rises steeply near the Er—O distance of 0.24 nm, and thereafter, the increase in the Er—O cumulative coordination number does not increase even if the Er—O distance increases. Few. On the other hand, in the SiO ₂ —Al ₂ O ₃ system, the rise of the Er—O integrated coordination number is moderate as the Er—O distance increases, but the Er—O integrated configuration reaches an Er—O distance of about 0.29 nm. The number of places continues to increase. Finally, the Er—O cumulative coordination number at the Er—O distance of 0.29 nm is larger in the SiO ₂ —Al ₂ O ₃ system.

FIG. 7 is a diagram schematically showing the relationship between the number of Er—O pairs and the Er—O distance obtained by molecular dynamics simulation. The results shown in FIG. 6 will be described schematically. As shown in FIG. 7, the total number of Er—O pairs in the SiO ₂ —Al ₂ O ₃ system up to the Er—O distance of 0.29 nm is shown. In addition, Er—O pairs are distributed over a wide distance range. Further, the center distance of the distribution is larger in the SiO ₂ —Al ₂ O ₃ system.

  FIG. 8 is a diagram showing a fluorescence spectrum calculated from an Er element-added glass structure obtained by molecular dynamics simulation. This method of calculating the fluorescence spectrum is called Judd-Ofelt analysis and is based on the perturbation theory of quantum mechanics. Since this calculation method is a kind of approximate calculation, it does not completely coincide with the actually measured fluorescence spectrum shown in FIG. 1, but the tendency of broadening the fluorescence spectrum of Er element-doped glass by Al element co-addition Can be reproduced qualitatively.

  As can be seen from the above description, the broadening of the fluorescence spectrum of the Er element-doped glass by the Al element co-addition is caused by an Er—O structural change. As shown in FIG. 7, when the Er—O coordination number is increased and the distribution range of the Er—O distance is expanded, variations of the Er—O coordination structure and the ligand field are increased. Thereby, the variation of the energy of the 4f orbit of Er element increases, and the fluorescence spectrum is broadened.

  The coordination structure of the Er element-added glass co-doped with a specific element capable of realizing further broadening of the fluorescence spectrum as compared with the case of Al element co-addition is as follows. Basically, when the number of coordination of Er—O is the same, the electric field felt by the 4f orbit of the Er element increases if the distribution of the Er—O distance spreads to the shorter side by co-addition of the specific element. This increases the effect of increasing the variation of the ligand field. Accordingly, as shown in FIG. 9, the specific element is “the Er—O cumulative coordination number is equal to or greater than that of the Al element co-added and the center of the Er—O distance distribution is short”. Is preferred. FIG. 9 is a diagram schematically showing the relationship between the number of Er—O pairs and the Er—O distance.

  FIG. 10 is a diagram schematically illustrating the relationship between the force and distance between Si—O, Al—O, and Er—O atoms. As shown in this figure, in general, in the case of a pair of positive ions (Si, Al, Er) and negative ions (O), there is a certain stable distance between the two ions, and both of the stable distances are The force between ions is minimal (ie, the attractive force is maximum), the repulsive force increases on the short distance side from the stable distance, and the attractive force gradually approaches zero on the long distance side from the stable distance.

  When comparing Si—O and Al—O, the stable distance of Al—O is slightly larger than that of Si—O, and the attractive force at the stable distance is smaller in Al—O. This is because Si has a valence electron number of 4, Al has a valence electron number of 3, and Al-O has a lower Coulomb attractive force than Si-O. In addition, since the Er ions are large in size, the distance between the minimum points of the Er—O force is extremely large compared to Si—O and Al—O.

The reason for the increase in the Er—O coordination number due to the co-addition of Al element is considered as follows. That is, when an Er element that protrudes in SiO ₂ glass and has a large size is coordinated, in the network composed only of rigid Si—O bonds, the periphery of the Er element has a coordination structure that is energetically unstable (see FIG. 11 (a)). However, when an Al element is co-added, an Al-O bond that tends to expand and contract exists in the vicinity of the Er element, and coordination that is not energetically impossible in the vicinity of the Er element becomes possible. 11 (b)). For this reason, the Er—O coordination number is increased by co-addition of Al element.

  From the above, the specific elements effective for improving the gain characteristics of the EDF are as follows. Basically, the specific element added to the Er element-added glass is preferably an element having a bond with the O element that is easier to expand and contract than Al—O. In order to realize this, the specific element is advantageous in that “the valence is small or the stable distance (MO distance) with oxygen is large”. In order to predict the effect, it was analyzed by molecular dynamics simulation.

  Specifically, four levels of +1, +2, +3, and +4 are set as the number of valence electrons of the element M that is a candidate for a specific element, and the distance between MO is in the range of 0.15 to 0.34 nm. Assuming that, the relationship between force and distance was calculated. Then, molecular dynamics simulation was performed brute force and Er-O cumulative coordination number was calculated. For the evaluation, the Er—O cumulative coordination number when the Er—O distance is 0.29 nm or less and 0.24 nm or less is used. Beyond the number was used as an indicator of effectiveness.

  12 to 15 are graphs showing the relationship between the Er—O cumulative coordination number and the MO distance when the Er—O distance is 0.29 nm or less. 12 shows a case where a monovalent element M is co-added, FIG. 13 shows a case where a divalent element M is co-added, FIG. 14 shows a case where a trivalent element M is co-added, FIG. 15 shows a case where a tetravalent element M is co-added. As can be seen from these figures, when the divalent element M is co-added and the MO distance is 0.28 nm or more, and when the trivalent element M is co-added and the MO distance is 0.2. In any case of 18 nm or more, the coordination number can be increased as compared with the case of Al element co-addition. On the other hand, when the monovalent or tetravalent element M is co-added, the MO distance needs to be 0.3 nm or more, but the element corresponding to this does not exist.

  FIG. 16 and FIG. 17 are graphs showing the relationship between the Er—O cumulative coordination number and the MO distance when the Er—O distance is 0.24 nm or less. FIG. 16 shows the case where the divalent element M is co-added, and FIG. 17 shows the case where the trivalent element M is co-added. As can be seen from these figures, when the divalent element M is co-added and the MO distance is 0.28 nm or more, and when the trivalent element M is co-added and the MO distance is 0.2. In any case of 18 nm or more, the coordination number can be increased as compared with the case of Al element co-addition.

From the above results, the co-added element (specific element) that has an effect of improving the gain characteristics superior to the Al element is a divalent element whose distance from the O atom is 0.28 nm or more in the SiO ₂ glass. (For example, Ca, Sr, Ba, Ra, etc.) or a trivalent element (for example, Ga, In, Y, La, etc.) whose distance from the O atom is 0.18 nm or more in the SiO ₂ glass. It is concluded that

  FIG. 18 is a diagram showing a fluorescence spectrum of Er element-added glass co-doped with Ba element as an example of the specific element. As shown in the simulation result in FIG. 6A, the fluorescence spectrum of Er element-added glass co-doped with Ba element is broadened compared to Er element-doped glass co-doped with Al element. Has been. This fluorescence spectrum was also calculated by Judd-Ofelt analysis.

Next, the result of creating a SiO ₂ glass sample to which Al element or Ba element is co-added will be shown. Powdery SiO ₂ , Er ₂ O ₃ , Al ₂ O ₃ or BaCO ₃ was mixed as a glass raw material, heated and melted at about 1700 degrees Celsius, and then rapidly cooled. The fluorescence spectrum of the glass sample thus obtained was measured. A spectroscope with a built-in InGaAs photodiode was used, and a laser with a wavelength of 0.98 μm was used as the excitation light source. The measurement results are shown in FIG. Compared to the SiO ₂ glass sample in which only the Al element was co-added, the Er fluorescence spectrum was broadened in all cases. From the above, it is expected that the amplification band can be expanded to a wavelength of 1565 nm to more than 1625 nm by using the Er element-doped glass.

  By the way, in an optical fiber (EDF) having an Er element-doped glass in a waveguide part, an element other than the Er element is often added to the waveguide part for various purposes. For example, in addition to the above Al element, Ge element or the like may be co-added for increasing the refractive index, and conversely, B element or F element may be added for decreasing the refractive index. However, the present inventor has found that when a certain element is added to the EDF waveguide (Er element-added glass), the function expression of the gain characteristic improvement by the co-addition of the specific element is inhibited. I found it. Specifically, the element (inhibiting element) that inhibits the function expression is an Al element or a B element. It is preferable that the additive concentration of the inhibitor element is smaller than the additive concentration of the specific element in the waveguide.

  FIG. 19 and FIG. 20 are diagrams showing the relationship between the Al element co-addition amount and the Er—O cumulative coordination number in the Er element-added glass in which Al element is added in addition to the specific element. FIG. 19 shows a case where a divalent specific element and an Al element are co-added, and FIG. 20 shows a case where a trivalent specific element and an Al element are co-added. As can be seen from these figures, when Al element as an inhibitory element is co-added in addition to the specific element, the decrease in the Er—O cumulative coordination number particularly when the Er—O distance is within 0.24 nm. It is remarkable.

  This phenomenon due to the co-addition of inhibitory elements is considered to occur by the following mechanism. As described with reference to FIGS. 11A and 11B, the Al—O bond easily expands and contracts, and is suitable for coordinating large atoms such as Er elements. On the other hand, specific elements are also relatively large in size. Therefore, it is considered that the Al element which is an inhibitory element is easily coordinated not only to the Er element but also to the specific element.

  In order for the specific element to exhibit the function of increasing the Er—O coordination number, the Er element and the specific element M1 need to be present at the second coordinate position via the O element (FIG. 21A )). However, if the inhibitory element M2 coexists, the inhibitory element M2 can easily enter the position of the second coordination via the O element with respect to both the Er element and the specific element M1 (FIG. 21B). For this reason, it is considered that the probability that the Er element and the specific element M1 enter the position of the second coordination with each other decreases.

  Similarly to the Al element, the B element has the property of being easily coordinated to a large atom and becomes an inhibitory element. In conclusion, it is preferable to control the concentration of the inhibitor element in a range lower than the concentration of the specific element in order to express the function of the specific element.

As shown in FIG. 22, the amplification optical waveguide 1 according to the present embodiment includes an Er element-added glass in which an Er element and the specific element are added to a SiO ₂ -based glass as described above. An optical fiber having a waveguide portion (core) 11 and a cladding 12 surrounding the core 11 and made of SiO ₂ -based glass, and can be manufactured by a manufacturing method similar to the conventional method. FIG. 22A shows a cross section perpendicular to the optical axis, and FIG. 22B shows a cross section including the optical axis. The amplification optical fiber according to the present embodiment can be suitably manufactured by, for example, an immersion method or a chemical vapor deposition (CVD) method.

  The amplification optical waveguide may be a so-called planar waveguide, and the waveguide section (core) may be made of the above Er element-doped glass.

  In the waveguide portion of the optical waveguide for amplification, the number of Er atoms is preferably in the range of 1/1000 to 1/6000 with respect to the number of Si atoms, and the number of atoms of the specific element is set to the number of Si atoms. On the other hand, it is preferably 3/10 or less. Further, a Ge element or F element used for adjusting the refractive index may be added to the waveguide.

Since the melting point of SiO ₂ to which Y element or La element is added is high, there is a possibility that cracking and residual distortion are likely to occur when the optical waveguide for amplification according to this embodiment is drawn from the base material. In order to suppress the occurrence of such a situation, it is preferable that a small amount of monovalent cation (for example, Na, K) is added to the waveguide portion. At this time, the number of cations in the waveguide is preferably 1/1000 or less with respect to the number of Si atoms.

DESCRIPTION OF SYMBOLS 1 ... Optical waveguide for amplification, 11 ... Core, 12 ... Cladding.

Claims (7)

Er elements and specific elements are added to the SiO ₂ -based glass,
The atoms of a particular element, divalent element the distance between the O atoms is above 0.28nm in SiO ₂ glass, or trivalent distance between the O atoms is above 0.18nm in SiO ₂ in the glass Elements of
A fluorescent glass body characterized by that.   2. The fluorescent glass body according to claim 1, wherein the additive concentration of Al element and B element in the glass is smaller than the additive concentration of the specific element.   The fluorescent glass body according to claim 1, wherein the number of Er atoms in the glass is in the range of 1/100 to 1/6000 with respect to the number of Si atoms.   2. The fluorescent glass body according to claim 1, wherein the number of atoms of the specific element in the glass is 7/20 or less with respect to the number of Si atoms.   The fluorescent glass body according to claim 1, wherein a monovalent cation is added to the glass.   6. The fluorescent glass body according to claim 5, wherein the number of cations in the glass is 1/1000 or less with respect to the number of Si atoms. An optical waveguide for amplification having the fluorescent glass body according to any one of claims 1 to 6 in a waveguide section.

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