JP3630767B2 - Rare earth doped polarization maintaining optical fiber - Google Patents

Description

【００３４】
尚、表１において、応力付与部の可視性の評価は、自動融着接続機で輝線Ｌを観察できたものを◎、輝線Ｌは観察できないが、作業者の目視により応力付与部の影から偏光軸を判別できたものを○、応力付与部を側面視できなかったものを×とした。また自動融着機の適用性の評価は、自動融着接続できたものを◎、できなかったものを×として示した。
【００３５】
表１の結果より、実施例１および３の希土類添加偏波保持光ファイバは、ファイバを側面視する方法による自動融着接続機の適用が可能であり、接続損失および接続後のクロストークも、光ファイバに直線偏波を入射して消光比をモニターしながら接続を行った場合と同程度の良好な値が得られた。またＭＦＤを拡大させたことによって、拡大前に比べて接続損失が充分に低減したことが認められた。
実施例２の希土類添加偏波保持光ファイバは、輝線Ｌの観察は不可能であったが応力付与部の影を用いて融着接続することが可能であり、接続後のクロストークも、実施例１および３には及ばないが良好であった。またＭＦＤを拡大させたことによって、拡大前に比べて接続損失が充分に低減したことが認められた。
尚、実施例１〜３、および比較例１の接続損失には若干のばらつきがあるが、これは測定再現性の低さによるばらつき、あるいはＭＦＤ拡大時の加熱条件が最適化されていないためのばらつきであって、構造の差異によるばらつきではないと考えられる。
【００３６】
【発明の効果】
以上説明したように本発明の希土類添加偏波保持光ファイバは、コアに希土類元素が添加され、コアの外方のクラッド内にコアに対して対称的に配された応力付与部を有する希土類添加偏波保持光ファイバであって、前記クラッドが、第１クラッドと、第１クラッドの周上に形成され第１クラッドよりも屈折率が高い第２クラッドとからなることを特徴とするものである。
したがって、ファイバを光学的に側面視する方法によって偏光軸の調心が可能であり、融着接続時には、精度よく軸合せを行うことができ、容易にかつ効率良く融着接続を行うことができる。
またコアがＧｅを含有し、その周上の第１クラッドがＦを含有する構成とすれば、接続端部を加熱することによってコア内のＧｅを容易に拡散させることができ、ＭＦＤを拡大させて他のＭＦＤが大きい光ファイバとの接続特性を向上させるのに好適である。
よって本発明の希土類添加偏波保持光ファイバを用いて、光ファイバ増幅器や光ファイバレーザを好適に構成することができる。
【図面の簡単な説明】
【図１】本発明の希土類添加偏波保持光ファイバの例を示したもので、（ａ）は断面図、（ｂ）はＸ−Ｘ’線で示した断面での屈折率分布、（ｃ）はＹ−Ｙ’線で示した断面での屈折率分布を示す。
【図２】本発明の希土類添加偏波保持光ファイバに側面から平行光を入射させたときの透過光線の軌跡の例を示した説明図である。
【図３】通常のＰＡＮＤＡファイバに側面から平行光を入射させたときの透過光線の軌跡の例を示した説明図である。
【図４】従来の希土類添加偏波保持光ファイバに側面から平行光を入射させたときの透過光線の軌跡の例を示した説明図である。
【図５】通常のＰＡＮＤＡファイバの例を示したもので、（ａ）は断面図、（ｂ）はＸ−Ｘ’線で示した断面での屈折率分布、（ｃ）はＹ−Ｙ’線で示した断面での屈折率分布を示す。
【図６】従来の希土類添加偏波保持光ファイバの例を示したもので、（ａ）は断面図、（ｂ）はＸ−Ｘ’線で示した断面での屈折率分布、（ｃ）はＹ−Ｙ’線で示した断面での屈折率分布を示す。
【符号の説明】
１…コア、２…第１クラッド、３…第２クラッド、４…応力付与部。[0001]
[Industrial application fields]
The present invention relates to a polarization maintaining optical fiber to which rare earth is added, and maintains a large relative refractive index difference between the core and the clad, so that even if there is almost no refractive index difference between the clad and the stress applying portion, the connection is achieved. In this case, the alignment of the polarization axis can be performed by a method of optically viewing the side of the fiber.
[0002]
[Prior art]
In recent years, an optical amplifier for a wavelength of 1.55 μm band has been constructed and attracted attention by utilizing the optical amplification function of a single mode optical fiber in which a rare earth such as erbium (Er) is added to the core. Such an optical amplifier is called an Er-doped optical fiber amplifier.
By the way, although it is known that the gain characteristic of the Er-doped optical fiber itself is hardly dependent on the polarization of the signal light or the pumping light, other optical components constituting the Er-doped optical fiber amplifier, such as an optical fiber, are known. Couplers and isolators have polarization dependency. For this reason, an Er-doped optical fiber having polarization maintaining characteristics is preferably used for the Er-doped optical fiber amplifier.
[0003]
The Er-doped optical fiber can also be used as a laser by forming a resonator using the Er-doped optical fiber. When an Er-doped optical fiber is used for a laser, it is indispensable to have polarization maintaining characteristics for the following reason.
That is, the oscillation frequency of the laser is determined by the wavelength of the standing wave that can exist in the resonator. That is, it is determined by the time required for one round trip of light in the resonator, that is, the group delay time of the transmission mode of the Er-doped optical fiber. However, in an actual single mode optical fiber, there are two orthogonal polarization modes due to birefringence caused by external force on the optical fiber and non-circularity of the core. Thus, oscillation occurs at two frequencies (longitudinal mode) corresponding to the group delay time of each of these two polarization modes, resulting in inconvenience that longitudinal single mode laser oscillation cannot be performed. On the other hand, it is possible to extract only one longitudinal mode by using appropriate polarization control, but the polarization state in the single-mode optical fiber is likely to fluctuate due to disturbance and oscillates stably. It's not easy. Therefore, when a laser is composed of an Er-doped optical fiber, it is necessary to stably maintain the polarization in the optical fiber by using one having polarization maintaining characteristics.
[0004]
On the other hand, optical fibers having polarization maintaining characteristics have been proposed in various structures (Juichi Noda, “Polarization-maintaining fibers and therer applications”, J. lightwave technology, Vol. , Pp. 1071-1089, 1986), a PANDA fiber in which two stress applying portions are arranged symmetrically with respect to the core in the outer cladding of the core has a large birefringence and a polarization maintaining characteristic. Widely used because of its superiority.
FIG. 5 shows an example of a conventional PANDA fiber, where (a) is a cross-sectional shape, (b) is a refractive index distribution at a cross section indicated by a line XX ′ in the figure, and (c) is Y in the figure. The refractive index distributions in the cross section indicated by the line −Y ′ are respectively shown. The PANDA fiber in this example is germanium oxide (GeO ₂ ) Added with a core 11 made of quartz glass, a clad 12 made of pure quartz glass, and boron oxide (B ₂ O ₃ ) Is provided with a stress applying portion 13 made of quartz glass to which a relatively large amount is added.
[0005]
Further, the alignment of the polarization axis when connecting the PANDA fiber is performed by optically viewing the fiber from the side and determining the position of the stress applying portion by utilizing the fact that the clad 12 and the stress applying portion 13 are different in refractive index. Is done by way.
This method performs image processing of the PANDA fiber side view and rotates the fiber around the fiber axis so that the polarization axes of the left and right fibers coincide with each other. A connecting device has also been put into practical use, and can be easily aligned in a short time (JP-A-1-147506, JP-A-1-225906, H. Taya, K. Ito, T. Yamada). and M. Yoshinuma: Proc. of OFC '89, THJ2, pp164, 1989).
[0006]
Since the PANDA fiber can perform a process of forming a desired core / cladding structure and a process of forming a stress applying portion separately, a polarization maintaining type is used by using a normal base material for an Er-doped optical fiber. This is suitable for obtaining an Er-doped optical fiber. Therefore, a PANDA fiber type structure is preferably used for the Er-doped polarization maintaining optical fiber.
FIG. 6 shows an example of a conventional Er-doped PANDA fiber, where (a) is a cross-sectional shape, (b) is a refractive index distribution in the cross-section indicated by line XX ′ in the figure, and (c) is a diagram. The refractive index distributions in the cross section indicated by the middle YY ′ line are shown. In this example PANDA fiber, the core 11 is composed of Er, aluminum (Al), and GeO. ₂ The cladding 12 is made of quartz glass to which fluorine (F) is added, and the stress applying portion 13 is boron oxide (B ₂ O ₃ ) Is made of quartz glass with a relatively large amount added.
[0007]
[Problems to be solved by the invention]
In an Er-doped optical fiber generally used for an optical amplifier, the core 11 has GeO. ₂ And F is added to the clad 12 to increase the relative refractive index difference between the core and the clad, and to reduce the mode field diameter, so that a highly efficient amplification effect can be obtained.
However, an Er-doped optical fiber having a PANDA fiber structure has a problem that the relative refractive index difference between the cladding 12 and the stress applying portion 13 becomes small, and the axis cannot be aligned by the method of viewing the fiber from the side. It was.
That is, in a normal PANDA fiber as shown in FIG. 5, the relative refractive index difference Δ between the cladding 12 and the stress applying portion 13. ₁ Is 0.5 to 0.8%, whereas in an Er-doped PANDA fiber as shown in FIG. ₁ Is 0.05 to 0.3%. Like this ₁ When the PANDA fiber is viewed from the side, it is difficult to determine the stress applying portion, and accurate polarization axis adjustment cannot be performed even when the side-view image is image-processed.
[0008]
On the other hand, it is possible to adjust the polarization axis while monitoring the extinction ratio by making linearly polarized light incident on the Er-doped PANDA fiber, but this is very time-consuming. In addition, when excitation light is not incident, unpolarized spontaneous emission light is generated in the operating wavelength (1.55 μm) band, thereby deteriorating the extinction ratio. Therefore, accurate polarization axis alignment at the operating wavelength cannot be performed. There was an inconvenience.
[0009]
The present invention has been made in view of the above circumstances, and provides a rare earth-doped polarization-maintaining optical fiber capable of applying a method of optically aligning a fiber in a side view at the time of fusion splicing. With the goal.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problems, the rare earth-doped polarization maintaining optical fiber of the present invention includes a stress applying portion in which a rare earth element is added to the core and is disposed symmetrically with respect to the core in the outer cladding of the core. A rare earth-doped polarization maintaining optical fiber, wherein the clad comprises a first clad and a second clad formed on the circumference of the first clad and having a higher refractive index than the first clad, The diameter of the first cladding is not less than three times the mode field diameter and smaller than the distance between the centers of the stress applying portions; The relative refractive index difference between the first cladding and the second cladding is -0.3 to -0.7%; The relative refractive index difference between the second cladding and the stress applying portion is -0.3 to -0.7%; and It is characterized in that it is used for the purpose of aligning the polarization axis optically when the fiber is viewed from the side at the time of fusion splicing.
Said For the core, germanium-added quartz glass is preferably used, and for the first cladding, fluorine-added quartz glass can be preferably used.
[0011]
The present invention will be described in detail below.
First, an outline of the principle of axial alignment by the method of viewing the PANDA fiber from the side will be described.
FIG. 3 is an explanatory view showing an example of the trajectory of transmitted light when parallel light is incident on the normal PANDA fiber shown in FIG. 5 from the side, and (a) shows two stress applying portions 13, 13B shows a state in which parallel light is incident from a direction perpendicular to the axis connecting the centers of 13 ', and (b) shows a state in which parallel light is incident from a direction parallel to the coaxial line.
In the PANDA fiber of this example, the clad 12 is made of pure quartz glass, and the stress applying portion 13 is B. ₂ O ₃ It is made of added silica glass and has a relative refractive index Δ (hereinafter simply referred to as Δ) = − 0.7% with respect to the refractive index of pure silica glass.
[0012]
In the state of FIG. 3A, first, the light rays incident on the clad 12 converge using the clad 12 as a convex lens. The light beam that has passed through the stress applying portion 13 having a lower refractive index than the clad 12 diverges using the stress applying portion 13 as a concave lens. Therefore, when the PANDA fiber is viewed from the side opposite to the light source, the stress applying portion 13 is seen through as a slightly dark portion. Further, since the light beam that has passed between the two stress applying portions 13 travels straight through the clad 12, there is a point L where this light beam and the light beam diverged by the stress applying portion 13 overlap. Further, even in the state of FIG. 3B, there is a point L where the light emitted by the stress applying portion 13 ′ overlaps the light beam that has passed through the cladding 12. When the plane including these points L is viewed from the side of the fiber as the focus plane F, these are observed as bright lines, and such bright lines are moved by the rotation of the PANDA fiber in the circumferential direction. Therefore, in the image processing of a fusion splicer or the like, it is possible to determine the position angle of the stress applying portion on the basis of the case where these bright lines L are equidistant from the center of the fiber.
For example, in the state of FIG. 3B, the bright line moves by a slight rotation in the circumferential direction of the PANDA fiber. Therefore, in the current automatic fusion device of the PANDA fiber, the state of FIG. The position angle of the stress applying portion is determined as a reference.
[0013]
FIG. 4 shows an example of the trajectory of transmitted light when parallel light is incident on the PANDA fiber from the side surface when the refractive indexes of the cladding 12 and the stress applying portion 13 are substantially equal as in the conventional rare earth-doped PANDA fiber. It is explanatory drawing shown. (A) is a state in which parallel light is incident from a direction perpendicular to the axis connecting the centers of the two stress applying portions 13, and (b) is a state in which parallel light is incident from a direction parallel to the axis. Each is shown.
In this example of the PANDA fiber, for example, the clad 12 is made of F-added quartz glass, and the stress applying portion 13 is B. ₂ O ₃ It consists of added quartz glass.
[0014]
In the state shown in FIG. 4A, in a state where parallel light is incident from a direction perpendicular to the axis connecting the centers of the two stress applying portions 13, the light incident on the clad 12 converges with the clad 12 as a convex lens. However, when the refractive indexes of the clad 12 and the stress applying portion 13 are the same or the difference between them is slight, the light beam goes straight without being refracted even when passing through the stress applying portion 13. Therefore, the stress applying part 13 is not visible as a dark part, and no bright line is observed, so the position angle of the stress applying part cannot be determined.
Also in the state of (b), the light ray shows the same locus as in (a). Therefore, it cannot be determined whether the state is (a) or (b), that is, whether the axis connecting the centers of the two stress applying portions 13 is parallel or perpendicular to the incident light beam.
[0015]
The rare earth-doped polarization maintaining optical fiber of the present invention will be described below. FIG. 1 shows an example of a rare earth-doped polarization maintaining optical fiber of the present invention, where (a) is a cross-sectional shape, (b) is a refractive index distribution at a cross-section indicated by line XX ′ in the figure, c) shows the refractive index distribution in the cross section indicated by the line YY 'in the figure. FIG. 2 is an explanatory view showing an example of the trajectory of transmitted light when parallel light is incident on the rare earth-doped polarization maintaining optical fiber shown in FIG. 1, and FIG. The state in which parallel light is incident from a direction perpendicular to the axis connecting the centers of the portions 4 is shown, and (b) shows the state in which parallel light is incident from a direction parallel to the coaxial line.
In this rare earth-doped polarization maintaining optical fiber, the first clad 2 is formed on the circumference of the core 1, the second clad 3 is formed on the circumference, and the two stress applying portions 4, 4 ′ are arranged on the outer side of the core 1. Are arranged symmetrically. The outer diameter is usually 125 μm in the state of a bare optical fiber.
The core 1 is formed with a high refractive index of Δ = 1.0 to 1.5% by adding a dopant that raises the refractive index to the silica glass, a rare earth element, and, if necessary, a metal. As the dopant, those which are easily diffused by heating are preferable. GeO ₂ Are preferably used. As the rare earth, erbium (Er), ytterbium (Yb), neodymium (Nd) or the like is preferably used, and as the metal, phosphorus (P), aluminum (Al), gallium (Ga) or the like is preferably used.
[0016]
The first cladding 2 is doped with a dopant that lowers the refractive index of quartz glass, the refractive index is in the range of Δ = −0.5 to −0.7%, and the mode field diameter (MFD) is 5 μm or less. Formed as follows. Use of fluorine (F) as a dopant is preferable because the diffusion coefficient of Ge in the core into the first cladding 2 is increased by heating.
[0017]
If the diameter of the first cladding 2 is too small, the waveguide light is coupled to the second cladding when the rare earth-doped polarization maintaining optical fiber is bent, and the conventional rare earth-doped polarization maintaining structure without the first cladding 2 is provided. There is an effect that the cutoff wavelength is smaller than that of the optical fiber or the bending loss is increased. Therefore, if the diameter of the first cladding 2 is set to 3 times or more of the MFD, it is considered that these effects do not appear in practice, and preferably 5 times or more can reliably eliminate these effects.
If the diameter of the first cladding 2 is too large, the stress applying portions 4 and 4 'are encased in the first cladding 2 having substantially the same refractive index, and as shown in FIG. It becomes difficult to identify 4, 4 ′ as a dark part. Also, as shown in FIG. 2A, when parallel light is incident from a direction perpendicular to the axis connecting the centers of the two stress applying portions 4 and 4 ′, the light beam diverged from the stress applying portion 4 or 4 ′ The light rays diverging in the first cladding 2 do not intersect, and no bright line can be obtained. It is considered that identification as a dark part becomes difficult when a region more than half of the stress applying portions 4 and 4 ′ is adjacent to the first cladding 2. Therefore, the diameter of the first cladding 2 is preferably set as appropriate in a range smaller than the distance between the centers of the stress applying portions 4 and 4 ′.
[0018]
The second cladding 3 is formed such that pure quartz glass or a dopant that lowers the refractive index is added, and the refractive index is about Δ = −0.0 to −0.4%. Any appropriate dopant can be used. For example, F can be preferably used and is added in a smaller amount than the first cladding 2.
The stress applying part 4 is B ₂ O ₃ It is preferably formed of quartz glass doped with.
[0019]
The rare earth-doped polarization maintaining optical fiber having such a structure can be manufactured as follows.
First, GeO ₂ A porous glass material made of quartz glass to which is added ₃ And AlCl ₃ Soak in an alcohol solution containing After being sufficiently impregnated, it is dried, further dehydrated and sintered to obtain a core base material. After silica glass soot is externally attached to the core base material and dehydrated, SiF ₄ A glass layer corresponding to the first cladding is formed by adding an appropriate amount of F by sintering in a gas atmosphere. After adding quartz glass soot and dehydrating, SiF ₄ A glass layer corresponding to the second cladding is formed by adding an appropriate amount of F by sintering in a gas atmosphere. A through-hole is drilled with an ultrasonic drill or the like at the position where the stress-applying portion of the core-clad base material obtained in this way is formed, and the inner surface of this hole is polished and flame-polished to form a PANDA fiber base material. To do.
Apart from this, B ₂ O ₃ A quartz glass rod to which is added is prepared, and the outer peripheral surface is polished to obtain a stress-applying base material.
After inserting the stress applying base material into the through hole of the PANDA fiber base material, drawing can be performed to obtain a rare earth-doped polarization maintaining optical fiber.
[0020]
Next, the operation of the rare earth-doped polarization maintaining optical fiber of the present invention will be described with reference to FIG.
In the state of FIG. 2A, first, the light rays incident on the second cladding 3 converge using the second cladding 3 as a convex lens. Then, the light beam that has passed through the stress applying portion 4 having a lower refractive index than the second cladding 3 diverges using the stress applying portion 4 as a concave lens. In addition, since the first cladding 2 having a lower refractive index than the second cladding 3 is formed between the two stress applying portions 4, the light beam that has passed through the first cladding 2 uses the first cladding 2 as a concave lens. Diverge.
As a result, there is a point where the light beam diverged by the first cladding 2 and the light beam diverged by the stress applying unit 4 overlap.
In the state of FIG. 2B, light that has entered the second cladding 3 that passes through the first cladding 2 diverges with the first cladding 2 and the stress applying portion 4 ′ as a concave lens. As a result, there is a point L where the light beam diverged from the stress applying portion 4 ′ and the light beam that has passed through the second cladding 3 overlap.
[0021]
When the plane including these overlapping points is viewed from the side surface of the fiber as the focus plane F, these are observed as bright lines, and such bright lines are moved by the rotation of the PANDA fiber in the circumferential direction. Therefore, in the image processing of a fusion splicer or the like, it is possible to determine the position angle of the stress applying portion on the basis of the case where these bright lines L are equidistant from the center of the fiber.
Accordingly, in the rare earth-doped polarization maintaining optical fiber of the present invention, in the image processing of a fusion splicer or the like, for example, in the case of image processing such as a fusion splicer, the two bright lines L by the two stress applying portions 4 are from the center of the fiber. The position angle of the stress applying portion 4 can be accurately determined based on the case where the distance is present.
[0022]
In addition, by adding Ge to the core and adding F to the cladding in the Er-doped optical fiber, when connecting to another optical fiber having a different MFD, the Ge in the core is thermally diffused, and the MFD of both fibers (A. Wada, T. Sakai, D. Tanaka, T. Nozawa and R. Yamauchi: Technical Digest of Optical Amplifiers and Thirp. -261, 1991), even when the rare earth-doped polarization maintaining optical fiber of the present invention is connected to another optical fiber constituting an optical fiber amplifier or an optical fiber laser, the fusion splice is heated by additional discharge or the like. By doing To diffuse the dopants in the 1 to the first cladding 2 can be enlarged MFD by. This makes it possible to easily achieve MFD consistency with other ordinary optical fibers having a large MFD, and to suppress connection loss.
Here, by enlarging the MFD, a part of the mode field guided through the core 1 may reach the second cladding 3 having a relatively high refractive index, and light may be coupled to the second cladding 3. Since the region to be heated for MFD expansion is a short distance from the end of the optical fiber, very little guided light is coupled to the second cladding 3 by this heating, and no problem occurs.
[0023]
Although the PANDA type is described here as an example of the polarization maintaining structure, the polarization maintaining structure of the rare earth-doped polarization maintaining optical fiber of the present invention is not limited to the PANDA type, and is symmetrically arranged with respect to the core. A structure having a stress applying portion, that is, a so-called bow tie type or an elliptical jacket type is also applicable.
[0024]
【Example】
As Examples 1 to 3 of the present invention, rare earth-doped polarization maintaining optical fibers having the structural parameters shown in Table 1 below were prepared. As Comparative Example 1, a conventional rare earth-doped polarization maintaining optical fiber having the structural parameters shown in Table 1 below was prepared.
That is, first, GeO is set so that Δ = 1.3% by the VAD method. ₂ A porous glass material made of quartz glass to which is added ₃ And AlCl ₃ Soaked in an alcohol solution containing After sufficiently impregnating, it was dried, further dehydrated and sintered to obtain a core base material. The addition concentration of Er was 850 ppm by weight, and the addition concentration of Al was 17500 ppm by weight.
After silica glass soot is externally attached to the core base material and dehydrated, SiF ₄ By sintering in a gas atmosphere, F was added so that Δ = −0.7% to form a glass layer corresponding to the first cladding. Let the glass rod obtained here be the rod A. Rod A was divided into two parts, one of which was further divided into two parts.
To the other rod A, a quartz glass soot is further externally attached and dehydrated. ₄ By sintering in a gas atmosphere, F was added so that Δ = −0.7% to form a glass layer corresponding to the first cladding. The glass rod in which the first cladding is formed thick in this way is referred to as rod B. Rod B was divided into two.
[0025]
(Example 1)
After silica glass soot is externally attached to the rod A and dehydrated, SiF ₄ By sintering in a gas atmosphere, F was added so that Δ = −0.4% to form a glass layer corresponding to the second cladding.
A through-hole was drilled with an ultrasonic drill at the stress-applying portion forming position of the core-clad base material thus obtained, and the inner surface of this hole was polished and flame-polished to obtain a PANDA fiber base material. .
Apart from this, B ₂ O ₃ A silica glass rod added with a refractive index equal to that of the first cladding (Δ = −0.7%) was prepared, and the outer peripheral surface was polished to obtain a stress-applying base material.
The stress-applying base material was inserted into the through hole of the PANDA fiber base material and then drawn to obtain a rare earth-doped polarization maintaining optical fiber. The MFD was 4.5 μm at a wavelength of 1.55 μm.
[0026]
The thus obtained rare-earth-doped polarization maintaining optical fiber is fusion spliced with a normal dispersion-shifted PANDA fiber by using the above-described polarization axis aligning automatic fusion splicer using a method of side-viewing the fiber. Went. The MFD of the dispersion shifted PANDA fiber used here was 8.2 μm at a wavelength of 1.55 μm. Therefore, after the fusion splicing, the connection point was heated to diffuse Ge in the core of the rare earth-doped polarization maintaining optical fiber, and the MFD was expanded to about 9 μm.
In this embodiment, the bright line L can be observed by adjusting the focus surface of the fusion splicer, and automatic fusion splicing can be easily performed in a short time.
After fusion splicing, the connection loss (1.55 μm) before MFD expansion and the connection loss after MFD expansion were measured. In order to evaluate the deviation of the polarization main axis, the crosstalk (1.55 μm) between the polarization modes was measured for the entire connected optical fiber.
These results are shown in Table 1 below.
[0027]
(Example 2)
After silica glass soot is externally attached to the rod B and dehydrated, SiF ₄ By sintering in a gas atmosphere, F was added so that Δ = −0.4% to form a glass layer corresponding to the second cladding.
In the same manner as in Example 1 above, a through-hole was formed at the stress applying portion forming position of the core-cladding base material, the stress applying base material was inserted into the PANDA fiber base material, drawing was performed, and a rare earth added A polarization maintaining optical fiber was obtained.
[0028]
The rare earth-doped polarization maintaining optical fiber thus obtained was fused and connected to an ordinary dispersion-shifted PANDA fiber using an automatic fusion splicer in the same manner as in Example 1.
In this example, the bright line L could not be found by the fusion splicer, but the polarization axis could be discriminated from the shadow of the stress applying portion by the operator's visual observation. Therefore, the fusion splicer was manually operated to perform the fusion splicing.
Further, similarly to Example 1, connection loss before and after MFD expansion and crosstalk after connection were measured. These results are shown in Table 1 below.
[0029]
(Example 3)
A quartz glass soot was externally attached to the rod B, dehydrated, and then sintered to form a glass layer corresponding to a second cladding made of pure quartz (Δ = 0.0%).
In the same manner as in Example 1 above, a through-hole was formed at the stress applying portion forming position of the core-cladding base material, the stress applying base material was inserted into the PANDA fiber base material, drawing was performed, and a rare earth added A polarization maintaining optical fiber was obtained.
[0030]
The rare earth-doped polarization maintaining optical fiber thus obtained was fused and connected to an ordinary dispersion-shifted PANDA fiber using an automatic fusion splicer in the same manner as in Example 1.
In this embodiment, the bright line L can be observed by adjusting the focus surface of the fusion splicer, and automatic fusion splicing can be easily performed in a short time.
Further, similarly to Example 1, connection loss before and after MFD expansion and crosstalk after connection were measured. These results are shown in Table 1 below.
[0031]
(Comparative Example 1)
On the circumference of the rod A, an F-added quartz glass layer having the same refractive index as that of the first cladding (Δ = −0.7%) was formed by an external method.
In the same manner as in Example 1, a through-hole was formed in the stress-applying portion forming position of the core-clad base material, the stress-applying base material was inserted into the PANDA fiber base material, and then drawing was performed. In this way, a conventional rare earth-doped polarization maintaining optical fiber having no first clad-second clad structure and uniform clad refractive index was obtained. The MFD was 4.5 μm at a wavelength of 1.55 μm.
[0032]
The rare earth-doped polarization maintaining optical fiber thus obtained was fusion spliced with an ordinary dispersion-shifted PANDA fiber. After the fusion splicing, the connection point was heated in the same manner as in Example 1 to diffuse the Ge in the core, and the MFD was expanded to about 9 μm.
In this comparative example, even when light was incident from the side surface of the optical fiber, the stress applying portion could not be viewed from the side. Therefore, the wavelength of 1.3 μm linearly polarized light previously passed through the polarizer was made incident on the optical fiber, and the two optical fibers to be connected and the analyzer were rotated to adjust the polarization extinction ratio to the maximum. Later, fusion splicing was performed.
Further, similarly to Example 1, connection loss before and after MFD expansion and crosstalk after connection were measured. These results are shown in Table 1 below.
[0033]
[Table 1]

[0034]
In Table 1, the evaluation of the visibility of the stressed portion is ◎ when the bright line L can be observed with an automatic fusion splicer, but the bright line L cannot be observed, but from the shadow of the stressed portion by the operator's visual observation. The case where the polarization axis could be discriminated was indicated by ◯, and the case where the stress application portion could not be viewed from the side was indicated by ×. In addition, the applicability of the automatic fusion machine was indicated as “◎” when the automatic fusion splicing was possible, and “x” when it was not possible.
[0035]
From the results in Table 1, the rare earth-doped polarization maintaining optical fibers of Examples 1 and 3 can be applied to an automatic fusion splicer by a method of viewing the fiber from the side, and connection loss and crosstalk after connection are also obtained. Good values comparable to those obtained when linear polarization was incident on the optical fiber and the connection was made while monitoring the extinction ratio were obtained. In addition, it was confirmed that the connection loss was sufficiently reduced by expanding the MFD as compared to before expansion.
Although the rare-earth-doped polarization maintaining optical fiber of Example 2 was not able to observe the bright line L, it could be fusion-bonded using the shadow of the stress applying portion, and crosstalk after connection was also performed. Although it did not reach Example 1 and 3, it was favorable. In addition, it was confirmed that the connection loss was sufficiently reduced by expanding the MFD as compared to before expansion.
The connection loss in Examples 1 to 3 and Comparative Example 1 has some variation, but this is due to variation due to low measurement reproducibility, or because the heating conditions during MFD expansion are not optimized. It is considered that the variation is not due to the difference in structure.
[0036]
【The invention's effect】
As described above, the rare earth-doped polarization maintaining optical fiber according to the present invention includes a rare earth element having a stress-applying portion in which a rare earth element is added to a core and symmetrically arranged with respect to the core in a cladding outside the core. A polarization-maintaining optical fiber, wherein the clad includes a first clad and a second clad formed on the circumference of the first clad and having a higher refractive index than the first clad. .
Therefore, it is possible to align the polarization axis by optically viewing the fiber side-by-side, and at the time of fusion splicing, it is possible to accurately align the axes and to perform fusion splicing easily and efficiently. .
If the core contains Ge and the first cladding on the circumference contains F, the Ge in the core can be easily diffused by heating the connection end, and the MFD can be expanded. It is suitable for improving the connection characteristics with other optical fibers having a large MFD.
Therefore, an optical fiber amplifier or an optical fiber laser can be suitably configured using the rare earth-doped polarization maintaining optical fiber of the present invention.
[Brief description of the drawings]
1A and 1B show an example of a rare earth-doped polarization maintaining optical fiber of the present invention, in which FIG. 1A is a cross-sectional view, FIG. 1B is a refractive index distribution in a cross section indicated by line XX ′, ) Shows the refractive index distribution in the cross section indicated by the YY ′ line.
FIG. 2 is an explanatory diagram showing an example of a trajectory of transmitted light when parallel light is incident from the side surface on the rare earth-doped polarization maintaining optical fiber of the present invention.
FIG. 3 is an explanatory diagram showing an example of a trajectory of transmitted light when parallel light is incident on a normal PANDA fiber from a side surface.
FIG. 4 is an explanatory diagram showing an example of a trajectory of transmitted light when parallel light is incident from a side surface on a conventional rare earth-doped polarization maintaining optical fiber.
FIGS. 5A and 5B show an example of a normal PANDA fiber, in which FIG. 5A is a cross-sectional view, FIG. 5B is a refractive index distribution in a cross section indicated by line XX ′, and FIG. 5C is YY ′. The refractive index distribution in the cross section shown by the line is shown.
6A and 6B show examples of a conventional rare earth-doped polarization maintaining optical fiber, in which FIG. 6A is a cross-sectional view, FIG. 6B is a refractive index distribution in a cross section indicated by line XX ′, and FIG. Indicates a refractive index distribution in a cross section indicated by a YY ′ line.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Core, 2 ... 1st clad, 3 ... 2nd clad, 4 ... Stress application part.

Claims (2)

A rare-earth-added polarization-maintaining optical fiber having a rare-earth element added to a core and having a stress-applying portion disposed symmetrically with respect to the core in a cladding outside the core, wherein the cladding includes a first cladding and A second clad formed on the circumference of the first clad and having a refractive index higher than that of the first clad , wherein the diameter of the first clad is not less than three times the mode field diameter and the center of the stress applying portion. smaller than the distance between, the difference in specific refractive index between the second cladding from the first cladding is the -0.3 0.7%, the relative refractive index of the second cladding and the stress applying section The rare earth addition is characterized in that the difference of −0.3 to −0.7% is used, and at the time of fusion splicing, the fiber is optically viewed from the side and used for the purpose of aligning the polarization axis. Polarization maintaining optical fiber. 2. The rare earth-doped polarization maintaining optical fiber according to claim 1, wherein the core is made of germanium-doped quartz glass, and the first cladding is made of fluorine-doped quartz glass.

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