JPWO2004053547A1 - Optical fiber terminal, manufacturing method thereof, optical coupler and optical component - Google Patents

Abstract

Provided is a practical optical fiber terminal that can sufficiently satisfy the specifications of reflection loss and coupling efficiency required for ordinary optical components. Light obtained by joining one end faces of coreless fibers 102 and 103 made of a material having substantially the same refractive index as the core to the end face of the optical fiber 101 having the core 101a at the center and the clad 101b at the outer periphery thereof. At the fiber terminal, the optical path length of the coreless fiber is set so that the beam diameter when the light transmitted through the core of the optical fiber spreads in the coreless fiber and exits from the other end surface of the coreless fiber is within the outer diameter of the coreless fiber. The other end surfaces of the coreless fibers 102 and 103 were set to be perpendicular to the optical axis of the optical fiber 101.

Description

  The present invention relates to an optical fiber terminal used for optical communication and the like, a manufacturing method thereof, an optical coupler using the optical fiber terminal, and an optical component.

With the development of optical communication, miniaturization of optical devices and optical components to be used is desired. Of the methods of optical coupling between optical fibers, as an optical coupling method with low loss and the ability to install various optical elements between them, a lens is installed in the vicinity of the light beam exit end of the optical fiber. A method is generally used in which a light beam emitted from a fiber is converted into a parallel light beam, a similar optical system is assembled on the light receiving side, and light is coupled to the optical fiber again. An element having a function of converting a light beam emitted from an optical fiber into a parallel light beam is generally called a fiber collimator, and the above-described combination of an optical fiber and a lens is one type of fiber collimator.
In general, an optical coupling loss and a reflection loss are used as indexes representing the performance of this type of fiber collimator. The optical coupling loss reflects the luminous flux quality of the parallel light flux, and is affected by the numerical aperture (hereinafter abbreviated as NA) of the optical fiber used, the shape of the exit end face, the aberration at the exit end face, the lens performance, and the like. .
On the other hand, the reflected light represented by the reflection loss as an index brings about an effect that the loss of the output energy at the optical fiber output end or the stable operation of the light source is impaired. Of these influences of reflected light, particularly the influence on the stable operation of the light source is one of the serious problems in the optical communication field.
In recent years, in the optical communication field, a distributed feedback laser is generally used as a light source. This type of laser light source is characterized in that laser oscillation tends to become unstable due to so-called return light that travels backward in the optical fiber and reaches the light source, and as a result, output power fluctuations are likely to occur. That is, when the reflected light increases, in other words, when the reflection loss is small, it means that the return light is large, and the fluctuation of the output power is increased.
Generally, in a fiber collimator, in order to suppress the output fluctuation of the laser light source described above to a level that can be ignored, 50 dB or more is required as an end face reflection loss shown in the following equation (1).
End face reflection loss = −10 × log (IR / IO) (1)
Here, IR represents the amount of reflected light, and IO represents the amount of incident light.
At present, as a method for obtaining reflection loss, a method in which the end face of the optical fiber is inclined with respect to the optical axis is used. This type of optical fiber terminal can be obtained by inserting an optical fiber into a glass capillary and polishing the end face of each capillary with an angle of about 4 ° to 8 °. As a result, the reflected light at the end face attenuates in a clad mode, so that a large reflection loss can be obtained, and a reflection loss of 60 dB or more can be obtained together with the AR coating on the surface. Since this method is extremely simple, it has been the mainstream method so far.
By the way, when using a collimator using an optical fiber terminal having such an oblique end surface in an optical component, there is a particular point to be noted. This point will be described with reference to FIG.
A refractive index distribution lens (GRIN Lens) often used as a collimating lens of a fiber collimator depends on the lens length (Pitch) in an image formation state. Therefore, in the case of 0.25 Pitch, a refractive index distribution lens as shown in (a). When a light source is placed on the end face of (GRIN Lens) 1101, collimated light is emitted from the other end face. Actually, as shown in (b), the refractive index distribution lens (GRIN Lens) 1102 has a lens length of about 0.23 Pitch, so that the position adjustment of the light source has a degree of freedom.
An optical path when collimated light with this configuration is combined is shown in FIG. Due to the influence of the oblique end surface described above, the light emitted from the optical fiber terminal 1103 is inclined by about 3.8 ° and enters the refractive index distribution lens 1104, so that it is deviated from the optical axis of the optical fiber terminal 1103 by δ1. In addition, since the end face of the gradient index lens (GRIN Lens) 1104 is inclined in the same manner as the end face of the optical fiber terminal 1103, the exit beam is incident on the gradient index lens (GRIN Lens) 1104 at an angle. It has an angle (θ) with respect to the optical axis of the light emitted from the optical fiber terminal 1103. Therefore, in this combination of collimators, in order to make the optical axes coincide with each other, optical coupling cannot be performed unless it is shifted by δ2 with respect to the original optical axis. This is the reason why it is difficult to adjust the position when optical coupling is performed with a conventional collimator.
In order to eliminate the optical path deviation as described above, the optical fiber terminal and the lens end face may be all perpendicular to the optical axis. However, in this case, all end face reflections are reflected as return light. The reflection loss caused by the difference in refractive index between the glass end face and air is 14.7 dB. Even if a good AR coating (R <0.2%: 27 dB) is applied to this, the reflection loss at the end face is about 42 dB. Yes, the above required specification of 50 dB or more cannot be achieved.
As means for solving such a problem, a structure in which a coreless fiber is fused to an end face of an optical fiber and a necessary return loss is obtained by a diffusion action of a light beam in the coreless fiber is known (Patent Document). 1). This structure uses the principle that the reflection loss is increased by reducing the overlap integral between the reflected light and the mode field diameter (about 10 μm) at the end of the optical fiber by widening the outgoing beam diameter. According to this structure, when the end face of the optical fiber is 0 ° to 6 ° and the length of the coreless fiber portion is 1 to 4 mm, it is said that a reflection loss of 60 dB or more can be obtained together with the AR coat. .
Further, a structure using a graded index (GI) fiber has been proposed as a collimator configuration in which the end face of the optical fiber is perpendicular to the optical axis (see Patent Document 2). An example of the structure of this collimator and a state in which light is allowed to pass are shown in FIGS. First, in FIGS. 17A and 17B, reference numeral 1201 denotes an optical fiber (SMF), 1202 denotes a coreless fiber, 1204 denotes a graded index (GI) fiber, 1204C denotes an exit end, 1207 denotes a beam waist diameter, and 1208 denotes The end surface reflected light, 1211 indicates the beam waist distance. As shown in FIG. 17 (a), the structure is an optical fiber end structure having a condensing function, and a beam waist distance 1211 and a beam waist diameter 1207 of light from the output end 1204C are respectively set to desired values. The value, that is, an optical fiber end structure that can variably set them independently of each other is described.
Patent Document 1: Japanese Patent Laid-Open No. 7-281544 Patent Document 2: Japanese Patent Laid-Open No. 2003-437270

However, as a result of manufacturing and evaluating an optical fiber terminal with a coreless fiber using a standard fiber having an outer diameter of 125 μm, the present inventors showed a reflection loss of 60 dB or more. However, a lens was placed in front of the optical fiber. As a result of examining the coupling efficiency of a pair of collimators, it was found that the coupling efficiency greatly depends on the length of the coreless fiber, and that the coupling efficiency is dramatically deteriorated when the length of the coreless fiber is 1 mm or more.
In addition, it has been found that it is very difficult to accurately control the length of the coreless fiber when manufacturing an optical fiber terminal to which a coreless fiber having a desired length of less than 1 mm is connected.
Further, in the collimator having the optical fiber (SMF) 1201, the coreless fiber 1202, and the GI fiber 1204 described with reference to FIG. 17 (a), as shown in FIG. 17 (b), at the output end 1204C. The end surface reflected light 1208 converges and returns to the same position as the light emission position. Here, in order to reduce the end surface reflected light 1208, it is conceivable to apply an AR coating to the emission end 1204C. However, with the current ability of the AR coating film, the end-surface reflected light 1208 of about 42 dB returns even if AR coating is applied as described above, and a reflection loss of 50 dB or more that is generally required can be realized. Absent.
The present invention has been made in consideration of the above circumstances, and a practical optical fiber terminal capable of sufficiently satisfying the specifications of reflection loss and coupling efficiency required for ordinary optical components, and an optical device using the same. It is an object of the present invention to provide a component and an optical coupler, and to provide a manufacturing method capable of easily manufacturing the optical fiber terminal.
The optical fiber terminal of the invention of claim 1 is a coreless fiber made of a material having a uniform refractive index substantially the same as that of the core on the end face of the optical fiber having a core at the center and a clad at the outer periphery thereof. In the optical fiber terminal formed by joining one end faces, the beam diameter when the light transmitted through the core of the optical fiber spreads in the coreless fiber and exits from the other end face of the coreless fiber is the outer diameter of the coreless fiber. The optical path length of the coreless fiber is set so as to be within the range. By setting the outgoing beam diameter to be within the outer diameter of the coreless fiber, optical coupling exactly equivalent to that of a normal optical fiber can be performed. As a result, the straightness of the outgoing beam is excellent, and the reflection loss and the coupling efficiency that are required in practice can be obtained.
The optical fiber terminal of the invention of claim 2 is characterized in that, in claim 1, the optical path length of the coreless fiber is less than 1 mm. Even when the length of the coreless fiber is limited within this range, a reflection loss of 50 dB or more can be easily secured after the AR coating is applied. Therefore, there is no problem in practical use. In addition, features such as no axis deviation and easy assembly adjustment are not affected at all by limiting the length of the coreless fiber to less than 1 mm.
The optical fiber terminal of the invention of claim 3 is characterized in that, in claim 1 or 2, the outer diameter of the optical fiber and the outer diameter of the coreless fiber are different. In the range where there is no problem with the fusion, even if the diameter of the coreless fiber is different from the diameter of the optical fiber, the same performance as the inventions of the first and second claims can be obtained. Further, in this optical fiber terminal, since there is a difference in diameter between the optical fiber and the coreless fiber, there is an advantage that the position of the fusion point can be easily recognized and the length of the coreless fiber can be easily adjusted.
The optical fiber terminal of the invention of claim 4 is the optical fiber terminal of claim 1 or 2, wherein the optical fiber and the coreless fiber have an outer diameter of approximately the same diameter,
The center axis of the optical fiber and the center axis of the coreless fiber are bonded to each other while being shifted from each other. As long as there is no problem in the fusion, even if the central axis of the optical fiber is different from the central axis of the coreless fiber, the same performance as the inventions of the first and second claims can be obtained. Further, in this optical fiber terminal, since there is a step at the fusion point between the optical fiber and the coreless fiber, there is an advantage that the position of the fusion point can be easily recognized and the length of the coreless fiber can be easily adjusted.
The optical fiber terminal of the invention of claim 5 is the optical fiber terminal according to any one of claims 1 to 4, wherein the other end surface of the coreless fiber is formed on a surface perpendicular to the optical axis of the optical fiber. It is characterized by being. That is, the oblique angle of light incident / exiting is 0 ° within a processing tolerance range with respect to a plane perpendicular to the axis of light emitted from the fiber. Therefore, the light emitted from the optical fiber always coincides with the optical axis, thereby enabling optical coupling on a straight groove that has been impossible until now due to the positional deviation of the light beam by the oblique end surface.
The invention of claim 6 is characterized in that, in any one of claims 1 to 5, an antireflection film is provided on the other end face of the coreless fiber. In this way, by applying an antireflection film (AR coating) according to the intended wavelength to be used on the other end surface of the coreless fiber (the end surface opposite to the side bonded to the optical fiber), direct return light can be reduced, and other optical elements. It is possible to prevent optical interference and ghosting, and to obtain good optical coupling characteristics.
The invention of claim 7 is an optical coupler including the optical fiber terminal according to any one of claims 1 to 6,
An optical coupler comprising: at least one selected from an aspherical lens, a spherical lens, a spherical lens, and a drum lens on the optical axis of the optical fiber on the other end surface side of the coreless fiber. is there. The drum lens is, for example, a spherical lens having a cylindrical body obtained by centering a peripheral portion of a spherical lens by grinding or polishing.
For example, the optical fiber terminal and the collimator lens can be combined to enable optical coupling using collimator light. Also, a combination of an optical fiber terminal and a finite system lens is possible. By combining the optical fiber terminal and the lens in this manner, optical coupling having a low coupling loss equivalent to or less than that of a collimator manufactured using a normal optical fiber terminal can be achieved.
Further, due to the combination of the optical fiber terminal and the collimator lens, as shown in FIG. 17, the end face reflection from the lens is all diffused light, and the light does not return to the light emitting position. Therefore, there is an advantage that the reflection loss of the entire system is not reduced by arranging the lens. Further, since the beam diameter can be expanded to the range of the glass capillary or the lens outer diameter in mm, it is possible to increase the propagation distance.
An optical component according to an invention of claim 8 is a combination of the optical fiber terminal according to any one of claims 1 to 6 and an optical element having an optical multiplexing / demultiplexing function. It is said. For example, optical coupling by collimated light is realized using the above optical fiber terminal, and a dielectric multilayer filter having a characteristic of reflecting only a specific wavelength and transmitting other wavelengths is inserted between them. A multiplexing / demultiplexing function can be provided. In this case, by using the above-mentioned optical fiber terminal, optical coupling can be performed between a pair of collimators on a common V groove formed on the substrate, so that the number of parts can be reduced and the process can be greatly simplified. It becomes possible.
The invention of claim 9 is the method of manufacturing an optical fiber terminal according to any one of claims 1 to 6, wherein the first step of coupling the optical fiber and the coreless fiber. And a second step of polishing the other end surface of the coreless fiber to adjust the length of the coreless fiber to a desired value. In the second step, the amount of reflection loss of the joined body of the optical fiber and the coreless fiber is adjusted. While measuring, the length of the coreless fiber is adjusted to a desired value.
That is, in the present invention, in the second step of defining the length of the coreless fiber to be bonded to the optical fiber, the coreless fiber is ground and polished while monitoring the reflection loss of the optical fiber end after fusion. The coreless fiber can be adjusted to a desired length without directly observing the optical fiber and the coreless fiber. In this case, since there is a one-to-one relationship between the length of the coreless fiber and the reflection loss, by measuring the reflection loss on the finished polished surface of the optical fiber terminal with the coreless fiber being manufactured, the accuracy is 1 μm. It becomes possible to define the coreless fiber length. As a result, even when it is difficult to observe the fusion point between the optical fiber and the coreless fiber with an optical microscope, the fusion point can be reliably detected, and the length adjustment of the coreless fiber can be accurately performed. .
A tenth aspect of the invention is an optical fiber terminal manufacturing method according to the third aspect of the invention, wherein the optical fiber and the coreless fiber having different diameters are joined together, and the optical fiber. A second step of detecting a bonding point between the coreless fiber and a coreless fiber, and a third step of cutting the coreless fiber at a specified position set with reference to the bonding point. In the second step, The junction point is detected from a defocused microscope image using a microscope.
The invention of claim 11 is a method of manufacturing an optical fiber terminal according to claim 4, wherein the optical fibers having substantially the same diameter and the central axes of the coreless fibers are shifted from each other and joined. A first step of detecting, a second step of detecting a joint point between the optical fiber and the coreless fiber, and a third step of cutting the coreless fiber at a designated position set with reference to the joint point. In the second step, an optical microscope is used, and the junction point is detected from a defocused microscope image.
The length of the coreless fiber in the optical fiber terminal according to the present invention must be accurately formed. Therefore, in the invention of claim 10, the diameters of the optical fiber to be joined and the coreless fiber are made different, and in the invention of claim 11, the optical fiber to be joined and the coreless fiber are made different. By shifting the central axes from each other, it becomes possible to detect the joint point between the optical fiber and the coreless fiber, and the coreless fiber is cut at a designated position set based on the joint point.
In this case, the junction point is detected by observing the junction point in a defocused state using a general microscope. The present inventor found for the first time that the joint point can be determined even when the outer diameter of the optical fiber and the coreless fiber is as small as several μm by observing the joint point with a defocused microscope. It is. That is, according to the research of the present inventor, when the joint point between the optical fiber and the coreless fiber is determined by observing with a microscope (in a focused state), the conventional technique is used when the outer diameter difference is large. However, as the outer diameter difference becomes smaller, it becomes increasingly difficult, and when the outer diameter difference becomes about several μm, it has been found that it is impossible to distinguish at all.
However, in the course of this research, when the inventor accidentally observed a junction point having an outer diameter difference of several μm in a defocused state, it was found that the junction point could be identified. Thus, by using the defocused microscope image, the fusion point after the fusion can be identified very simply. Thereby, it becomes possible to easily manufacture an optical fiber terminal having an outer diameter almost the same as the outer diameter of the optical fiber and having a desired coreless fiber length. Further, by using this method, the fusion point can be accurately recognized, so that the cutting point of the coreless fiber can be determined with the fusion point as the origin, and by cutting at that point, 10 μm. The length of the coreless fiber can be adjusted with the accuracy of.

FIG. 1 is a schematic cross-sectional view of an optical fiber terminal according to an embodiment of the present invention, (a) is a configuration diagram of the optical fiber terminal of the first embodiment, and (b) is an optical fiber terminal of the second embodiment. The block diagram which shows the state in the middle of manufacture, (c) is a block diagram of the optical fiber terminal of 1st Embodiment of 2nd Embodiment.
FIG. 2 is a characteristic diagram showing the relationship between the length of the coreless fiber and the outgoing beam diameter.
FIG. 3 is a characteristic diagram showing the relationship between the coreless fiber length and the reflection loss.
FIG. 4 is a process diagram showing a procedure for manufacturing an optical fiber terminal.
FIG. 5 shows an observation image when the fusion point of the optical fiber and the coreless fiber is observed with an optical microscope, (a) is an observation image in a focused state, and (b) is an intentional removal. It is a figure which shows the observation image in a state (state defocused), respectively.
FIG. 6 is an explanatory view of a method for producing an optical fiber terminal of the present invention.
FIG. 7 is an explanatory diagram for measuring the reflection loss of the optical fiber terminal of the present invention.
FIG. 8 is an explanatory diagram for measuring the coupling loss of the optical fiber terminal of the present invention.
FIG. 9 is a configuration diagram of an optical multiplexer / demultiplexer to which the optical fiber terminal of the present invention is applied, in which (a) is a plan view and (b) is a side view.
FIG. 10 is an explanatory diagram of a problem in the optical fiber terminal in which the coreless fiber is bonded to the end face of the optical fiber.
FIG. 11 is an explanatory diagram of problems in conventional optical coupling.
FIG. 12 is a characteristic diagram showing the relationship between the coreless fiber length and the reflection loss.
FIG. 13 is a characteristic diagram showing the relationship between the coreless fiber length and the coupling loss.
FIG. 14 is a schematic cross-sectional view of an optical fiber terminal according to another embodiment of the present invention.
FIG. 15 is an explanatory diagram of a method of manufacturing the optical fiber terminal shown in FIG.
FIG. 16 is an explanatory diagram of a configuration of a collimator including an optical terminal and a collimating lens according to the present invention.
FIG. 17 is an explanatory diagram of a configuration of a collimator using a GI fiber according to the prior art.
DESCRIPTION OF SYMBOLS 101 Optical fiber 101a Core 101b Cladding 102 Coreless fiber 102a One end surface 102b Other end surface 103 Coreless fiber 103a One end surface 103b Other end surface 401 Fiber coating 402 Optical fiber 403 Coreless fiber 404 Objective lens 405 Fiber cutter blade 406 Glass capillary 407 Optical fiber terminal 502 Distortion Location 504 Fiber cutter blade 601 Optical fiber terminal 602 Fiber chuck 603 Objective lens 604 Fiber cleaver blade 605 Fiber fixing V-groove 606 Single axis stage with micrometer 701 Reflection loss measuring instrument 702 Patch cord 703 Optical fiber terminal 704 Connector 801 LD light source 802 Patch cord 803 Optical fiber terminal 804 Optical stage 805 Collimatorene 806 detector 807 connector 901 glass substrate 902 collimator 903 V groove 904 wavelength selection filter 905 correcting glass substrate 906 a reflecting mirror 1001 optical fiber (SMF)
1001a Core 1002 Coreless fiber 1003 Beam 1101 Gradient index lens (GRIN lens)
1102 Gradient Index Lens (GRIN lens)
1103 Optical fiber terminal 1104 Gradient index lens (GRIN lens)
1200 Optical fiber terminal 1201 Optical fiber (SMF)
1201a Core 201b Clad 202 Coreless fiber 1203 Optical fiber terminal 1204 GI fiber 1204C Emission end 1205 Collimating lens 1206 Capillary 1207 Beam waist diameter 1208 End face reflected light 1209 End face reflected light 1210 Collimated light

ＳａｍｐｌｅＡを出射ファイバとして、その他のサンプルを受光ファイバとし、第８図に示したように、両者の間に非球面コリメートレンズ（Ｆ＝３、ＮＡ＝０．２２）をおくことにより、結合損失を測定した。光源として、λ＝１．５５μｍのＬＤ光源、及び、同波長域に十分な感度を持つ光検出器を用いて、コリメートレンズ間距離を１００ｍｍとし、光結合損失量の測定を行った。その結果を表２に示す。

上の表に示した通り、コアレスファイバ付き光ファイバ端末同士の組合わせにより、約０．２ｄＢの結合損失を得ることが可能であることが実証できた。これは、通常の光ファイバ端末（ＳａｍｐｌｅＤ）と同様ないしはそれ以上の性能である。
以上のように、上記の作製方法によれば、目標仕様を満たし、実用可能なレベルの光ファイバ端末を容易に作製することができることが実証できた。また、コリメートビームの直線性は、結合損失測定の際に、サンプルを入れ替えても、光学ステージをほとんど動かすことなく、光結合を行うことができることからも、実証できた。
なお、コアレスファイバの長さは、請求の範囲第１項に記載した条件を満たす範囲で、反射損失の要求仕様に合わせて適当に調整すればよい。また、コアレスファイバの端面角度は、光線の直線性を持たせるためには０°であることが理想的であるが、研磨工程の公差範囲で若干の角度が付くことは実用上大きな問題はない。
コアレスファイバの長さについての計算上の好ましい範囲は、長い方では略９００μｍである。これは、ビーム径を光の強度が分布中心に対して１／ｅ^２となる長さとして定義した場合に、出射端におけるビーム径がほぼ１２０μｍとなって、標準ファイバの外径とほぼ等しくなる長さである。しかし、コアレスファイバが長いほど、ガラス媒質中を透過する際に損失が大きくなる可能性があり、また、光ファイバ端面の欠けが生じた場合、ビームの散乱要因となるので、長い方での好ましい範囲は９００μｍ以下で、より好ましくは５００μｍ以下と判断される。
一方、短い方の使用限界長は３００μｍである。これは、ＡＲコーティング後に光ファイバ端末単体で反射損失５０ｄＢを達成するために、ＡＲコーティング無しの状態の端面で得られる反射損失値を２３ｄＢ以上とするために必要な長さである。但し、ＡＲコーティングにより常に理想的な反射減衰量２７ｄＢが得られるとは限らない。実験的にはＡＲコーティング無しで２５ｄＢ以上となれば、ほぼ間違いなく５０ｄＢ以上を得ることができるので、短い方の好ましい範囲は、３００μｍ以上である。以上のことから、使用上最も好ましいコアレスファイバの長さは３００μｍ以上で５００μｍ未満である。
また、光ファイバに対してコアレスファイバの外径を異ならせる場合の好ましい範囲は次の通りである。外径を変える場合、標準ファイバより太くする方法と細くする方法とがある。通常、光ファイバ端末は、市販のガラスキャピラリに挿入し、固定して使用することが一般的である。従って、市販規格品の範囲内で使用する場合、接続するコアレスファイバは細くした方が有利である。径を変える主目的は、融着後の融着点を容易に観察するためであり、前述した融着点における歪みの観察を容易にするためには、少なくとも径にして２μｍ以上細くする必要がある。歪み点は径差が大きいほど明瞭となるが、径差が大きすぎると、キャピラリに固定する際に、キャピラリ内径に対して偏心が生じやすくなること、キャピラリとの固定で使用する接着剤の量が必然的に多くなること等から、耐候性が劣化する可能性がある。従って、接合するコアレスファイバの径差として好ましい範囲は、光ファイバ（ＳＭＦ）に対して２〜１０μｍ程度細いことが適当である。
以上、本発明に係る第１実施形態および第２実施形態の光ファイバ端末について説明した。これら第１実施形態および第２実施形態においては、コアレスファイバと光ファイバとの融着点を容易且つ正確に把握するため、コアレスファイバの径と光ファイバ（ＳＭＦ）の径とを異ならしめた例を掲げた。そして、本発明は、コアレスファイバと光ファイバとの融着点を容易且つ正確に把握するため、両者の径を異ならしめるかわりに、光ファイバ（ＳＭＦ）とコアレスファイバとの中心軸をずらして接合するようにしてもよい。このように、段差を故意に発生させることで、前記に示した両者の径を変えることと同様の効果を得ることが出来るので、光学顕微鏡を用いて、デフォーカスされた顕微鏡像により、融着点を極めて容易に判定することができる。そこで、この光ファイバ（ＳＭＦ）とコアレスファイバとの中心軸をずらして接合する構成を有する、本発明に係る第３実施形態の光ファイバ端末について、図面を参照しながら説明する。なお、この場合軸のずれ量は、１μｍ〜５μｍ程度が好ましい。ずれ量が１μｍ以上であれば、接合点の判別が容易になり、ずれ量が５μｍ以下であれば、ビーム出射位置の相対位置ずれが大きくならないので、減衰量が見込みから外れたり、接合点の強度に影響する可能性が無くなるためである。
第１４図は、本発明に係る第３実施形態の光ファイバ端末を示し、第１５図は、その製造工程の概略を示す。
まず、第１４図に示されるように、第３実施形態の光ファイバ端末１２００は、中心部のコア１２０１ａおよびその外周部のクラッド１２０１ｂをそれぞれ有する光ファイバ（ＳＭＦ）１２０１の端面に、前記コア１２０１ａと略同一で均一な屈折率を有する材料よりなり、かつ外径が光ファイバ（ＳＭＦ）と略同一であるコアレスファイバ（ＣＬＦ）１２０２を配置し、融着接合したものである。ここで重要なことは、光ファイバ（ＳＭＦ）１２０１とコアレスファイバ１２０２との相対的な中心位置をずらしてあることである。
次に、第１５図を参照しながら、第１４図に示した光ファイバ端末の製造方法の一例を説明する。なお、用いる光ファイバのファイバ径については特に制限されるものではないが、ここでは具体的な例として、光ファイバ（ＳＭＦ）１２０１（１２０１ａ，１２０１ｂ）とコアレスファイバ１２０２とが、それぞれ１２５μｍの標準外径である場合について述べる。
この光ファイバ端末１２００を製造する場合、まず、１２５μｍの標準外径の光ファイバ（ＳＭＦ）１２０１を用意する。ここでは、後に行う測定作業を簡便に行うため、一方の端部にコネクタがついた光ファイバ（ＳＭＦ）１２０１を任意長さだけ用意する。光ファイバ（ＳＭＦ）１２０１の他方の端部は融着可能な範囲で皮膜を除去し、ファイバクリーバを用いて切断して、接続端面を作っておく。次に同径の１２５μｍのコアレスファイバ１２０２を任意長さだけ用意し、同様に皮膜除去と端面切断を行う。
次に、第１５図（ａ）に示すように、光ファイバ（ＳＭＦ）１２０１の端面とコアレスファイバ１２０２の端面を対向させて、標準的な単芯用光ファイバ融着器に設置し、一度、外径基準調心を行う。外径基準調心が完了したら、第１５図（ｂ）に示すように、１．５μｍコアレスファイバの中心位置をずらす。そして第１５図（ｃ）に示すように、通常の石英ガラスシングルモードファイバ同士の外径基準融着条件（住友電気工業株式会社製光ファイバ融着接続機ＴＹＰＥ−８５ＳＥにおける条件設定で、ファイバ種類：ＳＭ ＦＩＢＥＲ ＳＴＤ、調芯方法：外径 外形：同一の条件）で融着作業を行い、接合する。この融着作業により、外見上光ファイバ（ＳＭＦ）１２０１とコアレスファイバ１２０２は一体化し、接合点の強度も、通常の同径ファイバを外径調心、もしくはコア調心によって光ファイバの中心を合わせて行った融着と同等となる。
この相対的なズレによって生じた微小な段差は、前述の径が異なった光ファイバ同士の融着点で発生した段差と同様な効果を発生させるので、融着点の判定について、全く同等の作業を応用することが出来る。したがって、融着点を判定する工程、およびコアレスファイバを指定長さで切断する工程について、上述した異なる径を有する光ファイバ同士の融着の場合とまったく同じ工程を経ることができるので、同様の効果、性能および特徴をもつ光ファイバ端末を、作製することが出来る。
この光ファイバ端末を使用した光部品の実施形態について以下に述べる。ここでは、第９図のような１枚の波長選択フィルタを用いて、４つの入出力用ポートを持つ光合分波器を作製した。
第９図（ａ）は上面から見た光合分波モジュールの概略図である。モジュール中の光路は図中に細線で示してある。このモジュールは、ガラス基板９０１上に、波長選択フィルタ９０４、補正用ガラス基板９０５、反射ミラー９０６を配置し、ガラス基板９０１に設けたＶ溝９０３にコリメータ９０２を配置している。
「Ｉｎ」から入射された波長多重光は、波長選択フィルタ９０４により、透過光と反射光に分波され、それぞれ「Ｄｒｏｐ Ｐｏｒｔ」と「Ｏｕｔ Ｐｏｒｔ」に出力される。また、外部挿入光は「Ａｄｄ Ｐｏｒｔ」から入射し、波長選択フィルタ９０４を通過して、「Ｏｕｔ Ｐｏｒｔ」へと合波される仕組みである。ここで用いたコリメータ９０２は、前述した光ファイバ端末と非球面レンズとを同一のガラス管内に接着して作製したものである。
機能としては、通常のＡＤＭと変わらないが、各々のコリメータ９０２はガラス基板９０１上のＶ溝９０３上に固定されており、特に「Ｉｎ」のコリメータ９０２と「Ｄｒｏｐ Ｐｏｒｔ」のコリメータ９０２は一直線上のＶ溝９０３上にある。従来方式のコリメータでは、光軸ずれが発生するため、Ｖ溝９０３を光軸のガイド溝として使用することが不可能であったが、本実施の形態の光ファイバ端末を用いたコリメータ９０２は、全く軸ずれが起きないため、コリメータ９０２間に波長選択フィルタ９０４などの光学素子が入らない状態では、ほとんど位置調整無しで、光結合が可能となる。
コリメータ光の間にガラス基板（波長選択フィルタ９０４）が斜めに入ると、光はガラス基板の厚みに依存して、元の光軸と平行に位置ずれが発生する。このずれは、図に示したように、同様のガラス基板（補正用のガラス基板９０５）を用いて補正することで、元の光軸は維持されるので、大きな問題とはならない。従って、この構成では、波長選択フィルタ９０４の両脇にある反射ミラー９０６の角度を調整するだけで、全ての光路を調整することができることになる。このように、本発明の光ファイバ端末を使用することで、Ｖ溝９０３がコリメータ光のガイド溝となるので、これまで実現不可能だった同一基板上でのコリメータの集積が技術的に可能となる。更に、組み立てが簡単にできることになり、調整時間も短縮できる。
なお、コリメータ９０２に用いた非球面レンズのかわりに、球面レンズ、球レンズ、ドラムレンズを用いても同様な結果が得られた。
以上、本発明に係る光ファイバ端末と、当該光ファイバ端末を用いた光部品の実施形態について説明した。さらに、本発明に係る光ファイバ端末とコリメータレンズとの組み合わせにより、コリメータ光を用いた光結合が可能になる。また、本発明に係る光ファイバ端末と有限系のレンズとの組み合わせによっても光結合が可能になる。このように本発明に係る光ファイバ端末とレンズとを組み合わせることで、通常の光ファイバ端末を用いて作製したコリメータと同等ないしはそれ以下の低結合損失を有する光結合が可能となる。そこで当該構成について、以下、第４の実施形態について図面を参照しながら説明する。
第１６図は本発明に係る光端末とコリメートレンズとによるコリメータの構成の説明図である。第１６図において、符号１２０１は光ファイバ（ＳＭＦ）を示し、符号１２０２はコアレスファイバを示し、符号１２０３は光ファイバ（ＳＭＦ）とコアレスファイバとを有する光ファイバ端末を示している。符号１２０５はコリメートレンズを示し、符号１２０６はガラス等のキャピラリを示している。また、符号１２０９は端面反射光を示し、符号１２１０はコリメート光を示す。
光ファイバ（ＳＭＦ）１２０１とコアレスファイバ１２０２とを有する本発明に係る光ファイバ端末１２０３から放出された光は、コリメートレンズ１２０５を通過してコリメート光１２１０となる。このとき、コリメートレンズレンズ１２０５からの端面反射光１２０９はすべて拡散光となり、光ファイバ端末１２０３の光出射位置に光が戻ることはない。この結果、コリメートレンズ１２０５を配置することで系全体の反射損失を低下させることはない、という利点を発揮する。またコリメート光１２１０のビーム径はｍｍ単位のキャピラリないしはレンズ外径の範囲まで広げることが出来るので、伝播距離を大きく取ることが可能となる。
当該第４の実施の形態に係る光ファイバ端末とコリメータレンズとの組み合わせを、上述した光部品に適用することは、好ましい構成である。
また、本発明に係る光ファイバ端末を用いて作製し得る光部品のバリエーションとしては、その他にコリメータアレイ、２ｐｏｒｔ ｍｏｄｕｌｅ（例：利得等価器）、アイソレータ、光スイッチ、光測距計、波長計、干渉計などがある。Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1A is a configuration diagram of the optical fiber terminal of the first embodiment, FIG. 1B is a configuration diagram showing a state in the middle of manufacturing the optical fiber terminal of the second embodiment, and FIG. 1C is the second embodiment. It is a block diagram of the optical fiber terminal.
The optical fiber terminal of FIG. 1 (a) is formed on the end face of a single-mode optical fiber (SMF) 101 having a standard outer diameter of 125 μm having a core 101a at the center and a clad 101b at the outer periphery thereof. One end face 102a of a coreless fiber (CLF) 102 made of a material having a uniform refractive index substantially the same as 101a is fusion bonded, and the length of the coreless fiber 102 is set to less than 1 mm. The other end surface 102b is ground and polished at 0 ° with respect to a surface perpendicular to the optical axis of the optical fiber 101.
Here, setting the length of the coreless fiber 102 to be less than 1 mm is an essential condition as an optical fiber terminal for the purpose of optical coupling. By defining the length of the coreless fiber 102 in this way, the beam diameter when the light transmitted through the core 101a of the optical fiber 101 spreads within the coreless fiber 102 and is emitted from the other end face 102b of the coreless fiber 102 to the outside. Within the outer diameter of the coreless fiber 102.
In this optical fiber terminal, the light emitted from the core 101a propagates while diffusing in the coreless fiber 102, so that the diameter of the outgoing beam from the other end face 102b of the coreless fiber 102 is enlarged. Since the reflection loss can be increased according to the length of the coreless fiber 102, the exit surface does not need to be inclined, and as a result, the exit beam goes straight. By setting the length L of the coreless fiber 102 to an appropriate value as described above, the outgoing beam diameter is within the outer diameter of the coreless fiber 102, so that optical coupling equivalent to that of a normal optical fiber can be performed. it can. Therefore, compared to a conventional optical fiber terminal having an oblique end face, the output beam is excellent in rectilinearity, and a reflection loss and a coupling loss at levels required in practice can be obtained. Further, by using this optical fiber terminal, it becomes possible to perform optical coupling between collimators on a straight line, so that the position adjustment becomes easy.
In order to obtain an optical fiber terminal having such a structure, first, an optical fiber 101 and a coreless fiber 102 are prepared, and the coatings of both are removed to a length that allows sufficient fusion. Subsequently, the coreless fiber 102 is cut using a fiber cleaver at a position 20 mm from the film removal position of the coreless fiber 102 to create a fused end face. Similarly, a fused end face is formed on the optical fiber 101 side. Then, both are installed in a standard single-core fiber connecting fuser, and the fusion work is performed under appropriate conditions. Normally, the end surfaces of both the fusion-spliced parts are integrated, so that the fusion-bonding point cannot be recognized by observation with an external appearance or a microscope.
For example, if the fiber core can be seen directly, such as in the Direct Core Monitoring method (DCM method), the fusion point can be determined accurately. However, when the length of the coreless fiber can be determined in units of approximately millimeters, the length of the coreless fiber 102 may be determined using the coating removal position of the coreless fiber as a guide.
Next, the significance of making the length of the coreless fiber 102 less than 1 mm will be described. Here, for comparison, the length of the coreless fiber 102 is 1 mm, 2 mm, 3 mm, by cutting the coreless fiber 102 at positions 19 mm, 18 mm, 17 mm, and 16 mm from the film removal position of the coreless fiber 102, respectively. The sample of the optical fiber terminal with a coreless fiber which became 4 mm was produced. The end face angle is almost 0 ° for all samples. When the optical characteristics of these four samples were evaluated, the following results were obtained.
First, with respect to the reflection loss, as shown in FIG. 12, it was confirmed that the reflection loss without AR coating was 37 dB or more. However, the reflection loss value of 0 mm in length (in the case of no coreless fiber) is 14.7 dB. Therefore, it can be seen that when the AR coating is applied, a reflection loss of 50 dB or more can be obtained in all samples.
Next, as shown in FIG. 8, the amount of coupling loss of a pair of optical fiber terminals with coreless fibers was measured using a collimating lens. In FIG. 8, 801 is an LD light source, 802 is a patch cord, 803 is an optical fiber terminal, 804 is an optical stage, 805 is a collimating lens, 806 is a detector, and 807 is a connector. Light emitted from the LD light source 801 enters the other optical fiber terminal 803 via the collimating lenses 805 and 805 from one optical fiber terminal 803 and is received by the detector 806.
As a result of measuring the coupling loss in such a configuration, as shown in FIG. 13, the coupling loss value was 1 dB or more in all samples. Even considering that the AR coating is not applied to the end face of the optical fiber, this coupling loss is a very large value as an optical fiber terminal component, and it is found that the collimator currently required cannot be manufactured without any change. It was.
The cause was considered as follows. That is, as shown in FIG. 10, when light is emitted from the core 1001a of the optical fiber 1001, the beam 1003 expands due to diffraction, so that the beam diameter r increases depending on the propagation distance. In an optical fiber terminal with a coreless fiber, the beam diameter r at the exit end surface depends on the length of the coreless fiber 1002. For this reason, if the length of the coreless fiber 1002 exceeds a certain length, the beam diameter r exceeds the optical fiber diameter R. For this reason, light leakage, diffraction due to the edge of the optical fiber, etc. occur. Then, it was thought that the uniformity of the emitted light was lost and the coupling loss increased.
Therefore, the relationship between the coreless fiber length and the outgoing beam diameter was examined. The results are shown in FIG. From the relationship shown in FIG. 2, if the length of the coreless fiber is determined, at the same time, the maximum optical fiber outer diameter that enables ideal lossless optical coupling is determined. Now, the beam diameter is defined as a length where the intensity of light is 1 / e ² with respect to the distribution center (hereinafter, the beam diameter follows this definition). When the coreless fiber length is 1 mm, the beam is already emitted. The beam diameter exceeds the standard fiber outer diameter of 125 μm. This is considered to be the reason why the coupling loss is increased in the optical fiber terminal with a coreless fiber manufactured as described above.
Therefore, in the optical fiber terminal having the structure of FIG. 1 (a), the length of the coreless fiber is limited in order to achieve both reflection loss and coupling loss under the assumption that the outer diameter of the optical fiber is fixed to a certain value. I know you need to do that. From the above, in order to produce an optical fiber terminal having the structure of FIG. 1 (a) and having the performance of achieving both reflection loss and coupling loss, the optical fiber 101 having a standard outer diameter (125 μm) is used. If used, it has been found that the length of the coreless fiber 102 must be less than 1 mm.
However, when actually manufacturing this optical fiber terminal, it was necessary to make a structure to which the coreless fiber 102 having such a short length was added, and thus it was found that the manufacturing operation becomes considerably difficult.
Therefore, as a result of intensive studies, the present inventors have found a solution to all the problems that have occurred in manufacturing the optical fiber terminal as described above. The contents will be described below.
FIG. 4 shows a flow of a typical method for manufacturing an optical fiber terminal. The left figure (A) is a flowchart, and the right figure (B) is a diagram schematically showing the contents of steps (a) to (d) in the flowchart. In this flow, first, (a) a fusion splicing process between the optical fiber (SMF) 402 and the coreless fiber (CLF) 403 is performed. The connection between optical fibers is not necessarily limited to fusion, but it can be easily fused with a commercially available fuser, and is the most excellent means of connection performance and reliability. Use connection. When connecting, in order to easily perform later measurement work, a standard optical fiber 402 with an outer diameter of 125 μm with a connector at one end is prepared in an arbitrary length, and the other end can be fused. Then, the coating is removed (fiber coating 401) and cut using a fiber cleaver to produce a connection end face. Next, a coreless fiber 403 having an outer diameter of 122 μm is prepared for an arbitrary length, and similarly, film removal and end face cutting are performed.
Chemical etching can be used as a method of reducing the outer diameter of the coreless fiber 403 from the standard diameter. Alternatively, the coreless fiber 403 may be manufactured with this outer diameter. These optical fiber 402 and coreless fiber 403 are installed in a standard single-core optical fiber fusion splicer, and fusion work is performed under an outer diameter reference fusion condition between normal quartz glass single mode fibers by outer diameter reference alignment. I do.
FIG. 1 (b) shows a state in which both (optical fiber and coreless fiber) are connected. In the drawing, the outer diameter difference between the two is exaggerated, but in fact, it is difficult to see the difference when the outer diameter difference is about 3 μm. The joining point maintains the same strength as that of fusion of ordinary fibers having the same diameter.
After fusing in this way, a step of cutting the coreless fiber 403 at a predetermined position is performed next. This process is divided into (a) a process for accurately determining the fusion point, and (b) a process for cutting the coreless fiber 403 at a position having an accurate length from the fusion point. When materials having substantially the same refractive index are fused and connected, they cannot be distinguished from each other, and it is very difficult to optically recognize the fusion point. If cutting with an approximate length is allowed, it is possible to measure the length based on the film removal point as described in the previous example, but it is an accurate length measurement of about 10 μm with a short length of less than 1 mm. This method is not sufficient when it is necessary.
If a fiber core can be directly observed by combining an optical system of the DCM method, it is possible in principle to determine the fusion point based on the presence or absence of the core. However, this method requires a highly accurate stage, a CCD camera, a laser light source, and the like, resulting in a very expensive system. In addition, it is difficult to combine such an optical system with a fiber cutter. Furthermore, when the magnification is large, there is a possibility that the cutting point may be out of view when the coreless fiber length is long. .
In order to avoid these problems, it is useful to provide an outer diameter difference between the optical fiber and the coreless fiber as described above. In other words, by providing the diameter difference, according to the method described below, the determination of the fusion point can be easily performed, and the coreless fiber length can be determined with an accuracy of 10 μm at an arbitrary length and cut. It becomes possible. The contents are described below.
The step of determining the fusion point and the step of cutting at the specified length are performed as follows. As shown in FIG. 6, a commercially available ultrasonic fiber cleaver having a fiber cleaver blade 604 and a uniaxial stage 606 with a micrometer capable of chucking an SMF (optical fiber terminal) 601 with a coreless fiber are prepared. In order to observe the cutting point, a fiber cleaver is placed under the objective lens 603 of the stereomicroscope. An observation magnification of about 10 to 20 times is sufficient.
The optical fiber end (SMF with coreless fiber) 601 fused with the above optical fiber and coreless fiber is semi-fixed on the fiber fixing V-groove 605 so that the fusion point comes near the fiber cleaver blade 604. Then, one end of the optical fiber terminal 601 is chucked to the uniaxial stage 606 with a micrometer using the fiber chuck 602. When the stage 606 is sent by a micrometer, the chucked optical fiber terminal 601 moves on the fiber fixing V groove 605 of the fiber cleaver by the amount of movement indicated by the scale.
By the way, as described above, the optical fiber (SMF) and the coreless fiber have a slight difference in diameter. However, when the difference in diameter is the same as that used in the present embodiment, the image focus is usually adjusted to be enlarged. Even if the image is observed, the fusion point (arrow position) cannot be recognized as shown in FIG. In the figure, the left side is an optical fiber (SMF) 402 and the right side is a coreless fiber (CLF) 403, and the junction point cannot be confirmed. However, when the focus is shifted slightly from the state where the image is in focus, as shown in FIG. 5 (b), a distorted portion 502 is observed in the defocused microscope image. . This distortion point 502 coincides with the fusion point between the coreless fiber 403 and the optical fiber (SMF) 402. When the diameters of the two coincide, this distortion is not observed, and the image is slightly deviated from the image focal position only when the diameter difference is about 2 μm (about 1.6% with respect to the diameter). It was confirmed that it was clearly observed at the position. The shifting direction may be either the near side or the far side. Note that reference numeral 504 in FIG. 5 denotes a cutter blade for reference.
Returning to FIG. The uniaxial stage 606 with a micrometer is moved by the fusion point found by this method, and placed at the tip of the fiber cleaver cutting blade 604. With this as the origin, the uniaxial stage 606 with a micrometer is again fed by the length of the necessary coreless fiber, and the coreless fiber is cut and fixed at the point where the feeding is completed. In this manner, an optical fiber terminal 601 in which a coreless fiber having a desired length is fused to the tip of the optical fiber (SMF) is completed. According to this method, the length of the coreless fiber portion can be controlled with an accuracy of 10 μm.
In this example, since the tip is ground / polished in the subsequent steps, an optical fiber terminal with a coreless fiber length of 1000 μm was fabricated in advance for the amount of grinding.
Next, returning to FIG. 4, the glass capillary bonding step (d) will be described. The function as an optical fiber is sufficient at the end of the above operation, but when performing optical evaluation or mounting on an optical component, the optical fiber terminal is often fixed to the glass capillary 406 for use.
In this example, this optical fiber terminal (SMF with coreless fiber) 407 is inserted into a glass capillary 406 having an outer diameter of φ1.8 mm / an inner diameter of 126 μm / a length of 6 mm, and a UV adhesive is applied at the time of insertion, followed by curing. Thus, the optical fiber terminal 407 and the glass capillary 406 were fixed. At the time of fixing, it is desirable to fix so that both end faces of the capillary 406 and the optical fiber terminal 407 coincide. Since the diameter of the coreless fiber 403 is 122 μm, the inner diameter difference from the capillary 406 is 4 μm. With this diameter difference, the eccentricity between the optical fiber terminal 407 and the capillary 406 and the increase in the adhesive layer are almost negligible. Therefore, there is no problem in practical use. A thermosetting adhesive may be used as the adhesive.
Next, the optical polishing step of the end face of the optical fiber (e) is performed as follows. Optical polishing is performed in order to obtain good and stable optical performance. Optical polishing can be easily performed by using a commercially available end-face polishing machine for optical fibers. Polishing is performed in the order of rough cutting / primary polishing / secondary polishing / finish polishing with the glass capillary 406 fixed to a polishing jig.
Naturally, the end surface is ground by this polishing, and the length of the coreless fiber at the tip of the glass capillary 406, that is, the optical fiber terminal 407 is shortened. Even if the polishing time is fixed due to the difference in load pressure on the polishing surface or the state of the polishing sheet, the actual grinding amount is often not constant, and it is difficult to specify the grinding amount only with the polishing time. .
In addition, if the adhesive is attached to the periphery of the fiber, even when observed with the above-described defocus microscope image, subtle distortion cannot be recognized and the fusion point may not be recognized accurately.
In such a case, the following method is adopted. In this method, as shown in FIG. 3, the relationship between the length of the coreless fiber and the reflection loss is experimentally obtained in advance. There is a one-to-one correspondence between the length of the coreless fiber and the reflection loss. Therefore, at the stage where finish polishing in the polishing process is completed, as shown in FIG. 7, the connector 704 is connected to the patch cord 702 of the reflection loss measuring device 701, and the output end face of the measurement sample (here, the optical fiber terminal 703) is connected. By monitoring the reflection loss, the length of the coreless fiber can be accurately known.
Therefore, even if the length of the coreless fiber is not directly observed, it is possible to easily know the length easily by monitoring the reflection loss. Since the grinding amount can be finely adjusted with the monitor numerical value as a guide, an optical fiber terminal with a coreless fiber whose length is controlled with a high accuracy of 1 μm can be produced. In this example, since data of about 600 μm was ground separately in one polishing operation, an optical fiber terminal having a coreless fiber length of 1000 μm is about 400 μm by the tip polishing operation. It is obtained as an optical fiber terminal having a coreless fiber length. In this way, the tip of the capillary is polished while measuring the reflection loss, and the polishing is terminated when the measured value of the reflection loss reaches the target value ((f) of the flow in FIG. 4).
Finally, by forming an antireflection film on the end face of the coreless fiber of the optical fiber terminal manufactured by the above process, an optical fiber terminal with reduced end face reflection, interference, or ghost can be obtained [first (G) in the flow of FIG.
With the above operations, the fabrication of an optical fiber terminal that has both high reflection loss and low coupling loss and that has the ability to be mounted on an optical component without causing an optical axis shift is completed.
Note that, as a matter of course, the above-described manufacturing method is an example, and the procedure and method are not limited thereto.
Three optical fiber terminal samples as shown in the table below are manufactured by the manufacturing method described above, and the sample names are Sample A to C, respectively. The coreless fiber length shown in this table is the length obtained from the reflection loss before AR coating. The reflection loss is an actual measurement value obtained by measuring with a reflection loss measuring instrument after AR coating. For comparison, a commercially available SMF (single mode optical fiber) with an end face of 0 ° and an AR coating was prepared, and this was designated as Sample D.

As shown in FIG. 8, an aspherical collimating lens (F = 3, NA = 0.22) is placed between the two as shown in FIG. It was measured. Using a LD light source of λ = 1.55 μm as a light source and a photodetector having sufficient sensitivity in the same wavelength region, the distance between collimating lenses was set to 100 mm, and the amount of optical coupling loss was measured. The results are shown in Table 2.

As shown in the above table, it was proved that a coupling loss of about 0.2 dB can be obtained by combining optical fiber terminals with coreless fibers. This is the same or better performance than a normal optical fiber terminal (SampleD).
As described above, according to the manufacturing method described above, it was proved that an optical fiber terminal of a practical level satisfying the target specification can be easily manufactured. In addition, the linearity of the collimated beam could be verified from the fact that optical coupling can be performed with little movement of the optical stage even if the sample is replaced during the coupling loss measurement.
The length of the coreless fiber may be appropriately adjusted in accordance with the required specification of the reflection loss within the range satisfying the condition described in claim 1. The endless angle of the coreless fiber is ideally 0 ° in order to provide linearity of the light beam. However, there is no practical problem that a slight angle is added within the tolerance range of the polishing process. .
The preferred calculation range for the length of the coreless fiber is approximately 900 μm for the longer one. This is because when the beam diameter is defined as a length at which the light intensity is 1 / e ² with respect to the distribution center, the beam diameter at the exit end is approximately 120 μm, which is substantially equal to the outer diameter of the standard fiber. Length. However, the longer the coreless fiber, the greater the loss when passing through the glass medium, and if the end face of the optical fiber is chipped, it becomes a factor for scattering the beam. The range is 900 μm or less, and more preferably 500 μm or less.
On the other hand, the shorter usable limit length is 300 μm. This is a length necessary to achieve a reflection loss value of 23 dB or more obtained at the end face without the AR coating in order to achieve a reflection loss of 50 dB with the optical fiber terminal alone after the AR coating. However, an ideal return loss of 27 dB is not always obtained by AR coating. Experimentally, if it becomes 25 dB or more without AR coating, 50 dB or more can be obtained almost certainly, so the shorter preferable range is 300 μm or more. From the above, the most preferable length of coreless fiber in use is 300 μm or more and less than 500 μm.
Moreover, the preferable range when making the outer diameter of a coreless fiber different with respect to an optical fiber is as follows. When changing the outer diameter, there are a method of making it thicker than a standard fiber and a method of making it thinner. Usually, an optical fiber terminal is generally inserted into a commercially available glass capillary and fixed for use. Therefore, when used within the range of commercial standard products, it is advantageous to make the coreless fiber to be connected thinner. The main purpose of changing the diameter is to easily observe the fusion point after the fusion. In order to facilitate the observation of the strain at the fusion point, it is necessary to make the diameter at least 2 μm thinner. is there. The strain point becomes clearer as the diameter difference increases, but if the diameter difference is too large, eccentricity tends to occur with respect to the capillary inner diameter when fixing to the capillary, and the amount of adhesive used for fixing to the capillary. As a result, the weather resistance may deteriorate. Accordingly, it is appropriate that the preferable range of the diameter difference of the coreless fibers to be joined is narrower by about 2 to 10 μm than the optical fiber (SMF).
The optical fiber terminals of the first embodiment and the second embodiment according to the present invention have been described above. In the first and second embodiments, the coreless fiber diameter and the optical fiber (SMF) diameter are made different in order to easily and accurately grasp the fusion point between the coreless fiber and the optical fiber. Was raised. In the present invention, in order to easily and accurately grasp the fusion point between the coreless fiber and the optical fiber, the optical fibers (SMF) and the coreless fiber are joined by shifting the central axes instead of making the diameters of the two different. You may make it do. In this way, by deliberately generating a step, it is possible to obtain the same effect as changing the diameters of the two as described above. Points can be determined very easily. Therefore, an optical fiber terminal according to a third embodiment of the present invention having a configuration in which the central axes of the optical fiber (SMF) and the coreless fiber are shifted and joined will be described with reference to the drawings. In this case, the amount of deviation of the axis is preferably about 1 μm to 5 μm. If the amount of deviation is 1 μm or more, it becomes easy to determine the junction point. If the amount of deviation is 5 μm or less, the relative positional deviation of the beam emission position does not increase. This is because there is no possibility of affecting the strength.
FIG. 14 shows an optical fiber terminal according to a third embodiment of the present invention, and FIG. 15 shows an outline of the manufacturing process.
First, as shown in FIG. 14, the optical fiber terminal 1200 according to the third embodiment has the core 1201a on the end face of an optical fiber (SMF) 1201 having a core 1201a at the center and a clad 1201b at the outer periphery thereof. And a coreless fiber (CLF) 1202 made of a material having the same and uniform refractive index and having an outer diameter that is substantially the same as that of the optical fiber (SMF). What is important here is that the relative center positions of the optical fiber (SMF) 1201 and the coreless fiber 1202 are shifted.
Next, an example of a method for manufacturing the optical fiber terminal shown in FIG. 14 will be described with reference to FIG. The fiber diameter of the optical fiber to be used is not particularly limited, but here, as a specific example, the optical fiber (SMF) 1201 (1201a, 1201b) and the coreless fiber 1202 are each non-standard of 125 μm. The case of a diameter will be described.
When manufacturing the optical fiber terminal 1200, first, an optical fiber (SMF) 1201 having a standard outer diameter of 125 μm is prepared. Here, an optical fiber (SMF) 1201 having a connector at one end is prepared in an arbitrary length in order to easily perform a measurement operation to be performed later. The other end of the optical fiber (SMF) 1201 is removed in a range where it can be fused and cut using a fiber cleaver to form a connection end face. Next, a coreless fiber 1202 having the same diameter of 125 μm is prepared for an arbitrary length, and the film removal and end face cutting are similarly performed.
Next, as shown in FIG. 15 (a), the end face of the optical fiber (SMF) 1201 and the end face of the coreless fiber 1202 are opposed to each other and installed in a standard single-core optical fiber fusion device. Perform outer diameter reference alignment. When the outer diameter reference alignment is completed, the center position of the 1.5 μm coreless fiber is shifted as shown in FIG. 15 (b). And as shown in FIG. 15 (c), the outer diameter reference fusion condition between ordinary silica glass single mode fibers (the fiber type is set by the condition setting in the optical fiber fusion splicer TYPE-85SE manufactured by Sumitomo Electric Industries, Ltd.). : SM FIBER STD, alignment method: outer diameter, outer shape: under the same conditions) and joining. By this fusion work, apparently the optical fiber (SMF) 1201 and the coreless fiber 1202 are integrated, and the strength of the joint is also adjusted to the center of the optical fiber by aligning the same diameter fiber with the outer diameter or core. This is equivalent to the fusion performed.
The minute step caused by this relative misalignment produces the same effect as the step generated at the fusion point between optical fibers having different diameters as described above. Can be applied. Therefore, since the process of determining the fusion point and the process of cutting the coreless fiber with a specified length can be performed through exactly the same process as the fusion of optical fibers having different diameters, the same as described above. An optical fiber terminal having effects, performance and characteristics can be manufactured.
An embodiment of an optical component using this optical fiber terminal will be described below. Here, an optical multiplexer / demultiplexer having four input / output ports was fabricated using a single wavelength selection filter as shown in FIG.
FIG. 9 (a) is a schematic view of the optical multiplexing / demultiplexing module viewed from above. The optical path in the module is indicated by a thin line in the figure. In this module, a wavelength selection filter 904, a correction glass substrate 905, and a reflection mirror 906 are disposed on a glass substrate 901, and a collimator 902 is disposed in a V groove 903 provided in the glass substrate 901.
The wavelength multiplexed light incident from “In” is demultiplexed into transmitted light and reflected light by the wavelength selection filter 904, and is output to “Drop Port” and “Out Port”, respectively. In addition, the externally inserted light enters from “Add Port”, passes through the wavelength selection filter 904, and is multiplexed to “Out Port”. The collimator 902 used here is manufactured by adhering the above-described optical fiber terminal and an aspherical lens in the same glass tube.
Although the function is not different from that of a normal ADM, each collimator 902 is fixed on a V-groove 903 on the glass substrate 901. In particular, the collimator 902 of “In” and the collimator 902 of “Drop Port” are in a straight line. On the V-groove 903. In the conventional collimator, since the optical axis shift occurs, it was impossible to use the V groove 903 as the guide groove of the optical axis. However, the collimator 902 using the optical fiber terminal of the present embodiment is Since no axial deviation occurs, optical coupling is possible with almost no position adjustment when no optical element such as the wavelength selection filter 904 enters between the collimators 902.
When the glass substrate (wavelength selection filter 904) enters obliquely between the collimator light, the light is displaced in parallel with the original optical axis depending on the thickness of the glass substrate. As shown in the figure, this shift is not a big problem because the original optical axis is maintained by correcting using the same glass substrate (correcting glass substrate 905). Therefore, in this configuration, all the optical paths can be adjusted only by adjusting the angles of the reflection mirrors 906 on both sides of the wavelength selection filter 904. In this way, by using the optical fiber terminal of the present invention, the V-groove 903 becomes a guide groove for collimator light, so that it is technically possible to integrate collimators on the same substrate, which could not be realized until now. Become. Further, the assembly can be simplified and the adjustment time can be shortened.
Similar results were obtained when a spherical lens, a spherical lens, and a drum lens were used instead of the aspherical lens used in the collimator 902.
The embodiments of the optical fiber terminal according to the present invention and the optical component using the optical fiber terminal have been described above. Further, the combination of the optical fiber terminal and the collimator lens according to the present invention enables optical coupling using collimator light. Further, optical coupling can be achieved by combining the optical fiber terminal according to the present invention and a finite lens. As described above, by combining the optical fiber terminal and the lens according to the present invention, optical coupling having a low coupling loss equivalent to or lower than that of a collimator manufactured using a normal optical fiber terminal is possible. Therefore, in the following, the fourth embodiment will be described with reference to the drawings.
FIG. 16 is an explanatory view of the configuration of a collimator comprising an optical terminal and a collimating lens according to the present invention. In FIG. 16, reference numeral 1201 indicates an optical fiber (SMF), reference numeral 1202 indicates a coreless fiber, and reference numeral 1203 indicates an optical fiber terminal having an optical fiber (SMF) and a coreless fiber. Reference numeral 1205 indicates a collimating lens, and reference numeral 1206 indicates a capillary such as glass. Reference numeral 1209 indicates end face reflected light, and reference numeral 1210 indicates collimated light.
Light emitted from the optical fiber terminal 1203 according to the present invention having the optical fiber (SMF) 1201 and the coreless fiber 1202 passes through the collimator lens 1205 and becomes collimated light 1210. At this time, all of the end surface reflected light 1209 from the collimating lens lens 1205 becomes diffused light, and the light does not return to the light emitting position of the optical fiber terminal 1203. As a result, there is an advantage that the reflection loss of the entire system is not reduced by arranging the collimating lens 1205. Further, since the beam diameter of the collimated light 1210 can be expanded to the range of the capillary in mm or the outer diameter of the lens, the propagation distance can be increased.
It is a preferable configuration to apply the combination of the optical fiber terminal and the collimator lens according to the fourth embodiment to the optical component described above.
In addition, as variations of optical components that can be manufactured using the optical fiber terminal according to the present invention, a collimator array, a 2 port module (eg, gain equalizer), an isolator, an optical switch, an optical distance meter, a wavelength meter, There are interferometers.

  As described above, according to the present invention, by setting the outgoing beam diameter to be within the outer diameter of the coreless fiber, it is possible to perform optical coupling equivalent to that of a normal optical fiber. As a result, it is possible to provide a practical optical fiber terminal that has excellent rectilinearity of the outgoing beam and that has the required level of reflection loss and coupling loss required for ordinary optical components, and an optical component using the optical fiber terminal. And an optical coupler can be provided.

Claims (11)

In an optical fiber end formed by joining one end face of a coreless fiber made of a material having a uniform refractive index substantially the same as the core to the end face of an optical fiber having a core at the center and a clad at the outer periphery thereof,
The optical path length of the coreless fiber is set so that the beam diameter when the light transmitted through the core of the optical fiber spreads in the coreless fiber and exits from the other end surface of the coreless fiber is within the outer diameter of the coreless fiber. Optical fiber terminal characterized by setting. The optical fiber terminal according to claim 1, wherein an optical path length of the coreless fiber is less than 1 mm. The optical fiber terminal according to claim 1 or 2, wherein an outer diameter of the optical fiber is different from an outer diameter of the coreless fiber. The optical fiber terminal according to claim 1 or 2, wherein the optical fiber and the coreless fiber have outer diameters that are approximately the same diameter.
An optical fiber terminal, wherein the central axis of the optical fiber and the central axis of the coreless fiber are joined with being shifted from each other. The optical fiber terminal according to any one of claims 1 to 4, wherein the other end surface of the coreless fiber is formed on a surface perpendicular to the optical axis of the optical fiber. The optical fiber terminal according to any one of claims 1 to 5, wherein an antireflection film is provided on the other end face of the coreless fiber. An optical coupler including the optical fiber terminal according to any one of claims 1 to 6,
An optical coupler comprising: at least one selected from an aspheric lens, a spherical lens, a spherical lens, and a drum lens on the other end surface side of the coreless fiber on the optical axis of the optical fiber. An optical component comprising a combination of the optical fiber terminal according to any one of claims 1 to 6 and an optical element having an optical multiplexing / demultiplexing function. A method for manufacturing an optical fiber terminal according to any one of claims 1 to 6, wherein the first step of coupling the optical fiber and the coreless fiber;
A second step of polishing the other end face of the coreless fiber and adjusting the length of the coreless fiber to a desired value,
In the second step, the length of the coreless fiber is adjusted to a desired value while measuring the reflection loss amount of the joined body of the optical fiber and the coreless fiber. A method of manufacturing an optical fiber terminal according to claim 3, wherein the first step of bonding the optical fiber and the coreless fiber having different diameters;
A second step of detecting a joint point between the optical fiber and the coreless fiber, and a third step of cutting the coreless fiber at a designated position set with reference to the joint point,
In the second step, an optical fiber is used and the junction point is detected from a defocused microscope image. A method of manufacturing an optical fiber terminal according to claim 4,
A first step of joining the center axes of the optical fiber and the coreless fiber, which are approximately the same diameter, while being shifted from each other;
A second step of detecting a junction between the optical fiber and the coreless fiber;
A third step of cutting the coreless fiber at a designated position set with respect to the joining point,
In the second step, an optical microscope is used, and the junction point is detected from a defocused microscope image.

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