JP2004177497A - Mode filtering in multiplex mode light guide, selection method therefor, light guide amplifier using the same, semiconductor laser and vcsel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide mode filtering in multiplex mode light guide, and to provide its selection method, a light guide amplifier using it, a semiconductor laser, and VCSEL. <P>SOLUTION: A mode filtering method has a structure, in which the refractive index to a lateral direction of the clad part of a multiplex mode light guide consisting of a core and a clad, and transmits only certain specific modes having a multiplex mode along a waveguide, by enlarging a reflectance only to a cross section directional wavelength of the certain specific modes, and lessening the reflectance to the other mode. If filtering or selection effect due to a periodical refractive index is applied to EDF, the semiconductor laser, VCSEL and the like, a high power single-mode amplifier that operates at a single mode and has high performance, a laser and the like can be manufactured while having a very large cross section, as compared with those of the conventional products. <P>COPYRIGHT: (C)2004,JPO

Description

【００１３】
ここで、 ｎ_ｅｆｆは光導波路モードの有効屈折率である。
上記断面方向伝播定数と断面方向波長（ｔｒａｎｓｖｅｒｓｅ ｗａｖｅｌｅｎｇｔｈ）は、数学式２に連関される。
【数２】

前記断面方向波長は、基底モード（１２）が最も長く、高次モード（ｈｉｇｈｅｒ
ｏｒｄｅｒ ｍｏｄｅ）に行くほど段々短くなる。
【００１４】
図２に示すように、コア径を大きくした１４マイクロメーターであり、１０マイクロメーターを周期として３マイクロメーターの空気層と、７マイクロメーターのシリカが周期的に繰り返されるように形成されたクラッドにより構成された、即ち、周期的な屈折率構造（１６）を有する光子格子構造の１次元光導波路のモードを計算して屈折率のサイズが大きい順序で羅列すると次の表１となる。
【００１５】
【表１】

【００１６】
上記数学式 ２を使用し、これらのモードに該当する断面方向波長を計算すると、基底状態である基底モード（１２；Ｆｕｎｄａｍｅｎｔａｌ ｍｏｄｅ）の場合のみが２９μｍ付近であり、残りは全部１５μｍ付近であることを分かる。
図３は、１次元上でクラッド部分に周期的屈折率構造（１６）を有するようにして形成することによって、有効屈折率効果によって導波されることができる多重モードのうち有効屈折率が２番目に高い最初に励起されたモード、第１励起モード（１４、ｆｉｒｓｔ ｅｘｃｉｔｅｄ ｍｏｄｅ）を示す図面である。
図４は、垂直方向の波長（ｖｅｒｔｉｃａｌ ｗａｖｅｌｅｎｇｔｈ）による周期的な屈折率構造の反射率を示した図面である。
図２又は図３に示した周期的な屈折率構造（１６）に対して導波路のコア部分からクラッドの方へ透過する波の断面方向波長に対する反射率を計算すれば図４の通りである。
【００１７】
図４で、２５μｍから３０μｍまでの横断方向波長でのみ反射率が１００％近くになり、残りの波長に対しては反射率が極小さい値を有することを分かる。
このために、残りの高次モードに対する周期的な屈折率構造（１６）を有するクラッドの反射率が非常に低くなり、クラッドの方へ光が漏れ出ることになる。
従って、図３のような高次モードは、クラッドの反射率が低いため長さ方向へ光が導波されることができなく、クラッド部分に漏れることになるが、図２の基底モード（１２）は、クラッドの反射率が１００％近くなるため導波路の長さ方向へ光の導波が可能になる。
【００１８】
即ち、図２は、光導波路のコアを大きくした多重モード光導波路でクラッド部分が周期的な屈折率構造（１６）を有するようにし、クラッドの反射率が基底モード（１２）に対してのみ高い反射率にならしめ、基底モード（１２）のみが透過できるようにすることで、端末モードのモードフィールドのサイズを大きく作製することができることを示している。
このように光導波路のクラッド部分を光子結晶構造に代置し、その周期と屈折率の差異を利用すると、多重モード光導波路のある特定モードのみが導波できるようにする光導波路を作製することができる。
【００１９】
このような原理は、半導体レーザーの内部に長さ方向で周期的な屈折率構造（１６）を使用することによって、長さ方向の各種の多重モード（１０）のうち一つのモードのみが共振するべくフィルターリングさせるＤＦＢ（Ｄｉｓｔｒｉｂｕｔｅｄ ＦｅｅｄＢａｃｋ： 分布帰還）レーザーやＤＢＲ（Ｄｉｓｔｒｉｂｕｔｅｄ Ｂｒａｇｇ Ｒｅｆｌｅｃｔｉｏｎ）レーザーの原理と非常に類似するが、長さ方向のモードではなく、導波路の断面方向へのモードをフィルターリングさせるという点が相違する。
【００２０】
図５は、周期的屈折率変化によるフィルターリング又は選択効果のＥＤＦ（Ｅｒ−ｄｏｐｅｄ ｆｉｂｅｒ）応用に関する図面である。
ＥＤＦのＥｒ添加コア（２０）のサイズを２０〜３０μｍ程度として、周囲に周期的な屈折率変化（２２）を与え、各種の多重モードのうち基底モード（Ｆｕｎｄａｍｅｎｔａｌ ｍｏｄｅ）の断面方向波長のみの反射率を高くしてＥＤＦ内に基底モードのみが導波できるようにしたものである。
このようにすることによって、単一モードのように作動しながらＥＤＦのＥｒ添加コア（２０）のサイズが非常に大きいので、ＥＤＦ内の光の密度が低くなることができるので、高い出力の増幅器に使用されることができる。
【００２１】
図６は、周期的屈折率変化によるフィルターリング又は選択効果の半導体レーザー応用に関する図面である。
半導体レーザーでは、レーザー利得物質内での過多の光密度がレーザーの寿命と発光特性に大く影響を及ぼす。
一般的に、レーザーの出力を高めるためには、半導体レーザーの導波路の断面積を大きくしなければならないが、 導波路の断面積を大きくすれば多重モード導波路になる。
このとき、導波路のクラッド（３２）部分に周期的な層（ｌａｙｅｒ）を積層し、基底モードの断面方向波長に対してのみ反射率を大きくすると、フィルターリング効果によって導波路の断面積は非常に大きいが、単一モードで作動されることになる。
【００２２】
即ち、図６は、光が通過するレーザー媒体の断面積のサイズを大きくして半導体レーザー上下のクラッド（３２）部分に、周期的な屈折率構造を有するようにしてモードフィールドのサイズを大きくしたものである。
これを利用し半導体レーザー左右のクラッド（３２）部分に、周期的な屈折率構造をさらに有するようにしてモードフィールドのサイズをさらに大きくすることもできる。
【００２３】
図７は、周期的な屈折率変化によるフィルターリング又は選択効果をＶＣＳＥＬ（Ｖｅｒｔｉｃａｌ−Ｃａｖｉｔｙ Ｓｕｒｆａｃｅ−Ｅｍｉｔｔｉｎｇ Ｌａｓｅｒ）に応用したものである。
ＶＣＳＥＬの周囲に周期的な屈折率変化を与え、表面から出る光が単一モードを形成するようにし、光が出る部分の面積もフィルターリング又は選択効果によって数１０μｍ程度のサイズで作製して、高い出力のレーザーにする。
光が出る部分の面積、即ち、レーザー発振領域を数１０μｍ程度のサイズで大きく作り、従来のＶＣＳＥＬより高い出力のレーザーになるように、ＶＣＳＥＬの周囲に周期的に屈折率変化を有するように配置されたエアホール（通気孔）を形成することによって、多重モードのうち基底モードのみが共振するようにする。
【００２４】
図８ａは、周期的な屈折率の変化をリング形態で与えることについて、導波路コア（３０）の外側のクラッド部分（３２）を屈折率が相違する物質を利用してリングの形態で配置し周期的な屈折率変化を与える方法を示す図面である。
図８ｂは、周期的な屈折率の変化を四角形態で与えることについて、導波路コア（３０）の外側のクラッド部分（３２）を屈折率が相違する物質を利用して四角形の形態で配置し周期的な屈折率変化を与える方法を示す図面である。
【００２５】
【発明の効果】
以上のように、本発明によれば、光導波路のクラッド部分に周期的な屈折率変化を与えることによって、得られるフィルターリング又は選択効果によって断面方向に対し単一モードのみが導波されることができる。
又、このような周期的な屈折率によるフィルターリング又は選択効果をＥＤＦ、半導体レーザー及びＶＣＳＥＬ等に適用すれば、従来の製品より断面積が非常に大きいながら、単一モードで作動する性能が格段と高い高出力（ｈｉｇｈ ｐｏｗｅｒ）単一モード増幅器、レーザーなどを作製することができる。
【図面の簡単な説明】
【図 １】１次元上で有効屈折率効果によって導波されることができるステップインデックス光導波路の多重モードを示す図面である。
【図 ２】本発明に係る多重モードのうち基底モードを示す図面である。
【図 ３】本発明に係る多重モードのうち第１励起モード（ｅｘｃｉｔｅｄ ｍｏｄｅ）を示す図面である。
【図 ４】本発明に係る垂直方向の波長に対する周期的な屈折率構造の反射率を示す図面である。
【図 ５】本発明の第１応用例としてＥＤＦに適用した図面である。
【図 ６】本発明の第２応用例として半導体レーザーに適用した図面である。
【図 ７】本発明の第３応用例としてＶＣＳＥＬに適用した図面である。
【図 ８ａ】本発明の実施例として周期的な屈折率の変化を与える例示図である。
【図 ８ｂ】本発明の実施例として周期的な屈折率の変化を与える例示図である。
【符号の説明】
１０；多重モード １２；基底モード
１４；第１励起モード １６；周期的屈折率構造
２０；Ｅｒ添加コア ２２；周期的屈折率変化
３０；コア ３２；クラッド[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a mode filtering and selection method in a multi-mode optical waveguide, and an optical waveguide amplifier, a semiconductor laser, and a VCSEL {Vertical Cavity Surface-Emitting Laser} using the same, and more particularly, to a lateral side of a cladding portion of an optical waveguide. Filtering or selective effect obtained by giving a refractive index that changes periodically in the direction can guide only a single mode in the cross-sectional direction, and this can be used as an optical element such as an EDF, a semiconductor laser, or a VCSEL. Related to applying to.
[0002]
[Prior art]
Recently, researches on a method of fabricating a photonic crystal fiber having a structure in which air holes are periodically arranged in a silica glass to form a clad and a light transmission characteristic thereof have been actively conducted. Have been.
It is known that such a photonic crystal optical fiber has a surprising effect that is inconsistent with the transmission characteristics of general optical fibers or that is difficult to explain.
[0003]
Two theories have been introduced to explain this effect.
One is an analysis based on a photonic bandgap effect, and the other is an analysis of a Helmholtz equation, which is an electromagnetic wave equation for a refractive index structure, and an effective refractive index (Effective Refractive Index) for this. ) Is an analysis method.
The one photon band gap effect analysis shows that the band gap, which is a non-transmissive region with respect to a propagation constant that satisfies the Bragg condition, is due to the fact that the cladding holes are arranged in a crystal structure. It is an explanation that it is formed.
[0004]
The above two methods using the effective refractive index have been reported by many researchers because numerical analysis is possible.
According to the description, the optical fiber has the effect that many holes forming the cladding lower the refractive index of the cladding on average to a value smaller than the silica refractive index value. Therefore, it is asserted that the incident light can be guided because the core without the hole has a relatively high refractive index.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to combine the effective refractive index theory and the photon band gap theory, and form a Bragg formed by a structure that is periodically arranged in a lateral direction of a clad among various modes capable of guiding waves. Based on the effective refractive index theory that only a single mode having a transversal propagation constant satisfying the condition can be present, a multi-mode waveguide is generally formed by filtering or selecting effects of such a mode. By forming a periodic refractive index structure in the clad portion with a waveguide considered to be a specific mode, only a specific mode can be selected, and the mode filtering effect of such a clad structure in the form of a lattice of photons can be reduced. Utilizing special optical waveguide and optical fiber, and light utilizing it An optical waveguide amplifier for use with multi-mode optical waveguide mode filtering and selection methods in which to have a variety of important properties in the child it is to provide a semiconductor laser, and VCSEL.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention has a periodic refractive index structure in the cladding portion of the multi-mode optical waveguide consisting of the core and the cladding, to increase the reflectance only for the cross-sectional wavelength of a specific mode, A mode filtering method for a multi-mode optical waveguide, wherein only a specific mode of the multi-mode is transmitted along the waveguide by reducing the reflectance for other modes.
[0007]
Further, the present invention provides a multi-mode optical waveguide having a core and a clad, in which a periodic mode change of the refractive index is given to allow only a specific mode of the multi-mode to pass along the waveguide to obtain a desired mode. And a method for selecting a mode in a multi-mode optical waveguide, wherein unwanted modes are removed.
[0008]
Further, the present invention increases the size of the core of the multi-mode optical waveguide composed of the core and the clad, so that the reflectivity of the clad due to the refractive index structure of the clad is high only for a single mode. An optical waveguide amplifier, a semiconductor laser, and a VCSEL are provided.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to the attached drawings.
First, the basic configuration of the present invention has a periodic refractive index structure in the clad portion of the multi-mode optical waveguide composed of the core and the clad, so that the reflectivity is increased only for the cross-sectional wavelength of a specific mode. By reducing the reflectivity of the other modes, only a specific mode of the multiple modes is transmitted along the waveguide.
[0010]
It is to be noted that the present invention provides a multi-mode optical waveguide composed of a core and a clad by providing a periodic refractive index change to transmit only a specific mode of the multi-mode along the waveguide. A mode is selected, and an undesired mode is removed.
FIG. 1 is a diagrammatic representation of a multiple mode (10) of a step index optical waveguide that can be guided in one dimension by an effective refractive index effect.
FIG. 2 illustrates a single-mode waveguide due to a filtering or a selection effect among multiple modes that can be guided by an effective refractive index theory in one dimension according to the present invention.
[0011]
That is, by forming the cladding portion on one dimension so as to have the periodic refractive index structure (16), the effective refractive index is the most effective among the multiple modes that can be guided by the effective refractive index effect. 4 is a diagram illustrating a high fundamental mode (12, fundamental mode).
[0012]
In general, by solving a Helmholtz equation, which is an electromagnetic wave equation for an optical waveguide having a refractive index structure in the form of a photon lattice, it is possible to obtain an effective refractive index solution corresponding to various multimodes (10). it can.
The transversal propagation constant for these solutions can be determined using Equation 1.
(Equation 1)

[0013]
Here, n _eff is the effective refractive index of the optical waveguide mode.
The cross-sectional propagation constant and the transverse wavelength are related to Equation 2.
(Equation 2)

The cross-sectional wavelength is the longest in the fundamental mode (12), and is higher in the higher mode (higher mode).
The order becomes shorter as it goes to (order mode).
[0014]
As shown in FIG. 2, the core diameter is 14 micrometers, the air layer of 3 micrometers with a cycle of 10 micrometers, and a clad formed so that silica of 7 micrometers is periodically repeated. The following Table 1 shows the calculated modes of the one-dimensional optical waveguide having a photon grating structure having a periodic refractive index structure (16), and enumerating the refractive index sizes in descending order.
[0015]
[Table 1]

[0016]
When the cross-sectional wavelengths corresponding to these modes are calculated using the mathematical formula 2, only the fundamental mode (12; Fundamental mode) is around 29 μm, and the rest is around 15 μm in all cases. I understand.
FIG. 3 shows that one of the multiple modes that can be guided by the effective refractive index effect has an effective refractive index of 2 by forming the periodic refractive index structure (16) in the clad portion on one dimension. FIG. 4 is a diagram showing a first excited mode (14, first excited mode), which is the first highest excited mode.
FIG. 4 is a graph showing the reflectivity of a periodic refractive index structure according to a vertical wavelength.
For the periodic refractive index structure (16) shown in FIG. 2 or FIG. 3, the reflectance with respect to the cross-sectional wavelength of the wave transmitted from the core portion of the waveguide toward the cladding is calculated as shown in FIG. .
[0017]
In FIG. 4, it can be seen that the reflectance is close to 100% only at the transverse wavelength of 25 μm to 30 μm, and that the reflectance has a very small value for the remaining wavelengths.
For this reason, the reflectivity of the cladding having the periodic refractive index structure (16) for the remaining higher-order modes becomes very low, and light leaks toward the cladding.
Therefore, in the higher-order mode as shown in FIG. 3, light cannot be guided in the length direction due to the low reflectivity of the clad, and leaks to the clad portion. In the case of (), since the reflectance of the cladding becomes close to 100%, light can be guided in the length direction of the waveguide.
[0018]
That is, FIG. 2 shows a multimode optical waveguide in which the core of the optical waveguide is enlarged, in which the cladding has a periodic refractive index structure (16), and the reflectance of the cladding is high only for the fundamental mode (12). This shows that the mode field size of the terminal mode can be made large by allowing only the fundamental mode (12) to pass through as well as the reflectance.
By replacing the cladding part of the optical waveguide with a photonic crystal structure and utilizing the difference between the period and the refractive index, an optical waveguide that can guide only a specific mode of a multimode optical waveguide is manufactured. Can be.
[0019]
This principle is based on the fact that a longitudinally periodic refractive index structure (16) is used inside a semiconductor laser, so that only one of the multiple modes (10) in the longitudinal direction resonates. Very similar to the principle of DFB (Distributed FeedBack: Distributed Feedback) laser or DBR (Distributed Bragg Reflection) laser to be filtered as much as possible, but to filter the mode not in the longitudinal direction but in the cross-sectional direction of the waveguide. Is different.
[0020]
FIG. 5 is a diagram illustrating an EDF (Er-doped fiber) application of a filtering or selection effect based on a periodic refractive index change.
The size of the Er-doped core (20) of the EDF is set to about 20 to 30 μm, and a periodic refractive index change (22) is given around the core, so that only the cross-sectional wavelength of the fundamental mode (fundamental mode) among various modes is reflected. The ratio is increased so that only the fundamental mode can be guided in the EDF.
In this way, the power density of the Er-doped core (20) of the EDF is very large while operating like a single mode, so that the light density in the EDF can be reduced, so that a high power amplifier Can be used for
[0021]
FIG. 6 is a diagram illustrating a semiconductor laser application of a filtering or selection effect based on a periodic refractive index change.
In semiconductor lasers, excessive light density in the laser gain material has a significant effect on laser life and emission characteristics.
Generally, in order to increase the output of a laser, the cross-sectional area of a semiconductor laser waveguide must be increased. However, if the cross-sectional area of the waveguide is increased, a multimode waveguide is obtained.
At this time, when a periodic layer is laminated on the cladding (32) portion of the waveguide and the reflectance is increased only for the cross-sectional wavelength of the fundamental mode, the cross-sectional area of the waveguide becomes extremely large due to the filtering effect. But would be operated in a single mode.
[0022]
That is, FIG. 6 shows that the size of the mode field is increased by increasing the size of the cross-sectional area of the laser medium through which light passes and by providing a periodic refractive index structure in the upper and lower claddings (32) of the semiconductor laser. Things.
By utilizing this, the size of the mode field can be further increased by further providing a periodic refractive index structure in the clad (32) portions on the left and right sides of the semiconductor laser.
[0023]
FIG. 7 shows an application of a filtering or selection effect due to a periodic change in refractive index to a VCSEL (Vertical-Cavity Surface-Emitting Laser).
Providing a periodic refractive index change around the VCSEL so that light emitted from the surface forms a single mode, and the area of the light emitting portion is also manufactured in a size of about several tens of μm by filtering or selecting effect, Use a high power laser.
The area of the part where light is emitted, that is, the laser oscillation region is made large with a size of about several tens of μm, and is arranged so as to have a periodically changing refractive index around the VCSEL so that the laser has a higher output than the conventional VCSEL. By forming a closed air hole (vent), only the fundamental mode of the multiple modes resonates.
[0024]
FIG. 8a shows that for providing a periodic refractive index change in the form of a ring, the cladding portion (32) outside the waveguide core (30) is arranged in the form of a ring using a material having a different refractive index. 5 is a diagram illustrating a method of giving a periodic change in refractive index.
FIG. 8b shows that the periodic cladding portion (32) outside of the waveguide core (30) is arranged in a square form by using a material having a different refractive index for providing a periodic change in the refractive index in a square form. 5 is a diagram illustrating a method of giving a periodic change in refractive index.
[0025]
【The invention's effect】
As described above, according to the present invention, by giving a periodic refractive index change to the clad portion of the optical waveguide, only a single mode is guided in the cross-sectional direction by the obtained filtering or selection effect. Can be.
In addition, if such a filtering or selection effect based on a periodic refractive index is applied to an EDF, a semiconductor laser, a VCSEL, or the like, the performance in a single mode is remarkably increased while the cross-sectional area is much larger than that of a conventional product. And high power single mode amplifiers, lasers, and the like.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a multiple mode of a step index optical waveguide that can be guided in one dimension by an effective refractive index effect.
FIG. 2 is a diagram illustrating a fundamental mode among multiple modes according to the present invention.
FIG. 3 is a diagram illustrating a first excitation mode among multiple modes according to the present invention;
FIG. 4 is a graph showing a reflectance of a periodic refractive index structure with respect to a vertical wavelength according to the present invention.
FIG. 5 is a drawing applied to an EDF as a first application example of the present invention.
FIG. 6 is a drawing applied to a semiconductor laser as a second application example of the present invention.
FIG. 7 is a drawing applied to a VCSEL as a third application example of the present invention.
FIG. 8a is an exemplary view showing a periodical change in refractive index as an embodiment of the present invention.
FIG. 8b is an exemplary view showing a periodic change in refractive index as an embodiment of the present invention.
[Explanation of symbols]
10; multiple mode 12; fundamental mode 14; first excitation mode 16; periodic refractive index structure 20; Er-doped core 22; periodic refractive index change 30; core 32;

Claims (9)

By having a structure in which the refractive index changes periodically in the lateral direction of the clad portion of the multimode optical waveguide consisting of the core and the clad, the reflectivity is increased only for the cross-sectional wavelength of a specific mode, and the other A mode filtering method in a multi-mode optical waveguide, characterized in that only a specific mode of the multi-mode is transmitted along the waveguide by reducing the reflectance for the mode. 2. The method according to claim 1, wherein the size of the core is increased so that the reflectivity of the clad due to the periodic refractive index is high only for the fundamental mode, so that only the fundamental mode is transmitted. A mode filtering method in a multimode optical waveguide. A material having a different refractive index is used for the cladding portion, and the material is arranged in one of a ring shape and a rectangular shape, and the refractive index is periodically changed in a lateral direction of the cladding portion. Item 4. A mode filtering method in a multimode optical waveguide according to Item 1. The desired mode is selected by periodically changing the refractive index with respect to the cross section of the multimode optical waveguide consisting of the core and the clad so that only a specific mode of the multimode is transmitted along the waveguide. And a mode selection method in a multi-mode optical waveguide, wherein unwanted modes are removed. By increasing the size of the core of the multimode optical waveguide consisting of the core and the clad, the reflectivity of the clad due to the refractive index structure of the clad is high only for the single mode, and only the single mode is An optical waveguide amplifier utilizing mode filtering in a multi-mode optical waveguide, wherein the mode mode is large while having a single mode for transmitting light. The size of the core of the multimode optical waveguide consisting of the core and the cladding is increased, and the refractive index structure of the cladding makes the reflectance of the cladding high only for a single mode, so that only the single mode is transmitted. A semiconductor laser utilizing mode filtering in a multimode optical waveguide, wherein the mode field is large, the mode field size is large, and the cross-sectional area of the laser medium is large. 7. The mode filtering in a multi-mode optical waveguide according to claim 6, wherein the upper and lower cladding portions of the semiconductor laser have a structure in which the refractive index changes periodically, and the size of the mode field is formed large. Semiconductor lasers that use. 8. The semiconductor laser using mode filtering in a multi-mode optical waveguide according to claim 7, further comprising a structure in which a refractive index periodically changes in right and left clad portions of the semiconductor laser. The size of the core of the multimode optical waveguide consisting of the core and the cladding is increased so that the refractive index structure of the cladding has a high reflectance only for a single mode, so that only the single mode is transmitted. A VCSEL utilizing mode filtering in a multi-mode optical waveguide, wherein the mode is a single mode, and the cross-sectional area of the laser medium is increased.

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