JP5154484B2 - Optical fiber end structure - Google Patents

Description

  The present invention relates to an optical fiber end structure in which an end cap fiber is attached to one end of an optical fiber.

  In a fiber laser, an optical acceleration sensor, an optical add / drop device (OADM), etc., an optical fiber black grating (hereinafter referred to as FBG) having a diffraction grating in the core of an optical fiber is used as a mirror or an optical filter. It has been.

  The FBG is an optical fiber having a structure in which a quartz core at the center of the fiber is covered with a resin clad made of resin, and has a diffraction grating configuration in which a predetermined refractive index change is provided in the core in a predetermined region.

  This diffraction grating can be formed by exposing the core in a predetermined region and irradiating it with ultraviolet rays to increase the refractive index of the core in that portion. At this time, since the clad of the FBG is made of resin, the clad can be easily peeled to expose the quartz core.

  By the way, when light is incident on the core of the optical fiber, the incident light leaks to the clad and the light is transmitted through the clad, and there is a possibility that the clad and the coating provided around it may be damaged by the clad mode.

  As a measure for preventing the clad mode of the optical fiber, in Patent Document 1, a glass tube is provided at one end of an optical fiber in which a quartz core is coated with a quartz clad, and light refraction between the glass tube and the quartz clad is performed. A configuration is disclosed in which the rates are equal to each other and one end side of the inner peripheral surface of the glass tube is fused to the clad. According to this, it is described that the clad mode light propagates to the glass tube through the inner peripheral surface on one end side of the glass tube fused to the clad and is emitted to the outside through the glass tube.

Patent Document 2 discloses a configuration in which a light leakage member is provided on an outer periphery of a clad at an end portion of the fiber as an optical fiber terminal device in which a quartz core is coated with a quartz clad. And it is described that a cladding mode can be absorbed by this and damage to a coating layer can be suppressed.
JP 2008-158094 A JP 2001-66483 A

  However, the configurations of Patent Documents 1 and 2 are not effective measures against the cladding mode, for example, for an optical fiber having a configuration in which a quartz core is coated with a resin cladding. This is because according to the optical fibers having these configurations, the cladding mode can be absorbed by the glass tube or the light leakage member, but the core mode is also absorbed simultaneously with the cladding mode.

  An object of the present invention is to provide an optical fiber end structure capable of transmitting light to a core in a state where a cladding mode is removed in an optical fiber whose core is covered with a resin cladding.

An optical fiber end structure of the present invention includes an optical fiber having a core and a resin clad provided to cover the core,
An end cap fiber having a core and an air clad provided so as to cover the core and having holes formed therein;
A core connecting portion that connects the core at one end of the optical fiber and the core of the end cap fiber, while not connecting the cladding at the one end of the optical fiber and the cladding of the end cap fiber;
It has.

  According to said structure, the component of the light which exceeded the numerical aperture of an end cap fiber among the light which injected into the end cap fiber leaks from a core, and becomes a clad mode. The cladding mode is emitted from the side of the end cap fiber or scattered and absorbed by the air cladding. For this reason, light can be transmitted to the core of the optical fiber in a state where the cladding mode is removed, and damage to the cladding and other members due to the cladding mode can be suppressed.

  The numerical aperture of the end cap fiber is preferably less than or equal to the numerical aperture of the optical fiber.

  The air clad preferably has a length of T / tan (NA / n) or more, where n is the refractive index of the core of the end cap fiber, T is the core diameter, and NA is the numerical aperture.

  Moreover, it is preferable that a core connection part is 4 mm or more in length.

  According to the end structure of an optical fiber of the present invention, an optical fiber comprising a core and a resin cladding provided so as to cover the core, and an air provided with a core and a hole provided so as to cover the core. A core connection that connects an end cap fiber made of a clad, a core at one end of the optical fiber, and a core of the end cap fiber, but does not connect the clad at one end of the optical fiber and the clad of the end cap fiber Therefore, light that enters the end cap fiber and leaks from the core to become a cladding mode is emitted from the side of the end cap fiber or scattered and absorbed by the air cladding. For this reason, light can be transmitted to the core of the optical fiber in a state where the cladding mode is removed, and damage to the cladding and other members due to the cladding mode can be suppressed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  1 and 2 show an optical fiber end structure F according to this embodiment. In the optical fiber end structure F, an end cap fiber 20 is connected to one end of the optical fiber 10, and a core connecting portion C is formed at a connection portion between the optical fiber 10 and the end cap fiber 20.

(Optical fiber)
The optical fiber 10 has a configuration in which a core 11 at the center of the fiber is covered with a resin cladding 12. The optical fiber 10 has, for example, a numerical aperture NA1 of 0.2 to 0.5 and a fiber diameter of 250 to 600 μm. The optical fiber 10 is used as, for example, an FBG.

  The core 11 is made of, for example, quartz glass. When the core 11 is made of quartz glass, it may be made of pure quartz glass, and is made of quartz glass doped with germanium (Ge), aluminum (Al) or the like to have a high refractive index. Also good. For example, the core 11 has a core diameter of 125 to 400 μm. For example, the core 11 has a refractive index n1 of about 1.45 with respect to light having a wavelength of 1060 nm.

  The core 11 of the optical fiber 10 may have a configuration in which the second core surrounds the first core at the center of the fiber. In that case, the first core may be formed of, for example, quartz glass doped with germanium (Ge), aluminum (Al) or the like and having a high refractive index, such as ytterbium (Yb) or erbium (Eb). Rare earth elements may be doped. In that case, for example, the first core diameter is 30 to 80 μm and the second core diameter is 380 to 420 μm.

  The resin cladding 12 is formed of a material having a lower refractive index than that of the core 11. The resin cladding 12 has a function as a coating for protecting the core 11 from external force in addition to the function as a cladding of the optical fiber 10. The resin clad 12 is formed of a resin such as an ultraviolet curable resin, a silicone resin, or a nylon resin, for example. The resin cladding 12 has a refractive index of about 1.39, for example, with respect to light having a wavelength of 1060 nm. The resin clad 12 may be composed of a single layer or may be composed of a plurality of layers. The coating layer has a layer thickness of, for example, 50 to 100 μm.

  The optical fiber 10 has a portion where the core 11 is exposed by stripping the clad at one end. The core exposed portion has a length of 3 to 10 mm, for example. This core exposed portion constitutes the core connecting portion C together with the core exposed portion of the end cap fiber 20 described later.

(End cap fiber)
The end cap fiber 20 has a configuration in which a core 21 at the center of the fiber is covered with an air cladding 22. A support layer 23 is formed around the air cladding 22. The end cap fiber 20 has, for example, a numerical aperture NA2 of 0.2 to 0.5, a fiber diameter of 250 to 600 mm, and a length of 2 to 1000 mm.

  The core 21 of the end cap fiber 20 is made of, for example, quartz glass. When the core 21 is made of quartz glass, it may be made of pure quartz glass, and is made of quartz glass doped with germanium (Ge), aluminum (Al), or the like to have a high refractive index. Also good. For example, the core 21 has a core diameter of 125 to 400 μm. For example, the core 21 has a refractive index n2 of about 1.45 with respect to light having a wavelength of 1060 nm.

  As shown in FIG. 3, the core 21 of the end cap fiber 20 may have a configuration in which the first core 21a at the center of the fiber is surrounded by the second core 21b. In this case, the first core 21a has a configuration in which, for example, ytterbium (Yb), erbium (Eb), or the like is doped to increase the refractive index. In that case, for example, the first core diameter is 30 to 80 μm and the second core diameter is 380 to 420 μm.

  The air cladding 22 has a porous structure including a large number of pores (holes) extending in the fiber axis direction. Each of the plurality of pores is disposed substantially uniformly in the circumferential direction in the fiber cross section. The plurality of pores are, for example, formed with an arrangement pitch of 20 to 30 μm and 1 to 5 layers. Each of the plurality of pores has a pore diameter of, for example, 10 to 30 μm. The partition walls between mutually adjacent pores are formed with a wall thickness of, for example, 0.2 to 1 μm.

The air clad 22 has a length of 2 to 1000 mm, for example. The minimum length of the air cladding 22 is L _min , the core diameter of the end cap fiber 20 is T, the refractive index is n2, the numerical aperture is NA2, the maximum incident angle of the core mode is θ _max (that is, NA2), and the emission at that time Let the angle be Φ _max . At this time,
Φ _max = θ _max / n2 = NA2 / n2
It is. In order for the cladding mode to be reflected or refracted at the portion where the air cladding 22 is provided and enter the air cladding 22, as shown in FIG.
L _min = T / tanΦ _max
Holds. From the above two formulas,
L _min = T / tan (NA2 / n2)
It becomes.

For example, when the core diameter T of the end cap fiber 20 is 400 μm, the numerical aperture NA2 is 0.4, and the refractive index n2 of the core 21 is 1.44, L _min ≈1.4 mm. From the above, the length of the air clad 22 is preferably 2 mm or more.

  The support layer 23 has a function of protecting the air clad 22 having a porous structure and increasing the mechanical strength of the end cap fiber 20. The support layer 23 is made of, for example, quartz glass. The support layer 23 has a thickness of 80 to 120 μm, for example.

  The support layer 23 may be covered with a resin or the like. However, since the cladding mode is emitted from the side of the fiber, the coating may be damaged by the radiation. Preferably not.

  The numerical aperture NA2 of the end cap fiber 20 is preferably equal to or smaller than the numerical aperture NA1 of the optical fiber 10. Light incident on the core 21 of the end cap fiber 20 at an incident angle larger than the numerical aperture NA2 of the end cap fiber 20 enters the air cladding 22 without being reflected by the inner wall of the core 21, so that the incident angle is NA2 or less. Only light can be transmitted through the core 21 of the end cap fiber 20 and enter the core 11 of the optical fiber 10. That is, if NA2 is NA1 or less, all the light incident on the optical fiber 10 has an incident angle NA1 or less, so that the light incident on the core 11 of the optical fiber 10 does not leak into the resin cladding 12. 11 can be transmitted after being totally reflected by the inner wall.

  Further, the difference between the numerical aperture NA1 of the optical fiber 10 and the numerical aperture NA2 of the end cap fiber 20 is preferably small, and more preferably both are equal. If the incident angle is NA1 or less, the incident angle is greater than NA2 but less than NA1 even though light can enter the optical fiber 10 and transmit without being generated in the clad mode, even though the core 11 can be totally reflected. The light is removed by the air cladding 22 as a cladding mode during transmission of the end cap fiber 20. Therefore, if the difference between NA1 and NA2 is large, the transmission efficiency of the optical fiber 10 as a whole may be reduced.

  The numerical aperture NA2 of the end cap fiber 20 can be controlled by the diameter and density of the holes. When the diameter of the holes is large, that is, when there are many hole portions, the relative refractive index of the core 21 and the air clad 22 increases, so the numerical aperture NA2 also increases.

  The end cap fiber 20 has a portion where the core 21 where the air clad 22 is not provided is exposed at the connection end. The core exposed portion has a length of 2 to 20 mm, for example. The exposed core portion together with the exposed core portion of the optical fiber 10 constitutes the core connecting portion C.

(Core connecting part)
The optical fiber 10 and the end cap fiber 20 constitute a core coupling part C by causing the cores 11 and 21 to abut each other and fusion-bonded. In the core connecting part C, the resin cladding 12 and the air cladding 22 are not provided, and each core 21 is exposed on the surface. For example, the core connecting portion C has a length in the fiber axis direction of 4 to 20 mm.

  The core connecting portion C of the optical fiber 10 is set in a protective housing (not shown) for protection from external forces and for the operator's eye-safety. The protective housing is made of, for example, aluminum. The protective housing has, for example, a length of 30 to 60 mm, a width of 10 to 30 mm, a height of 10 to 20 mm, and a thickness of 3 to 10 mm.

  The light incident on the end cap fiber 20 and leaking into the air clad 22 is exposed to the air clad 22 because the cores 11 and 21 are exposed without the air clad 22 and the resin clad 12 being connected at the core connecting portion C. It is possible to prevent the absorbed cladding mode from entering the resin cladding 12.

  In the present embodiment, the exposed core portion of the optical fiber 10 and the exposed core portion of the end cap fiber 20 constitute the core connecting portion C, but the present invention is not limited to this. For example, even if the optical fiber is not provided with a core exposed portion and the core connecting portion is composed only of the core exposed portion of the end cap fiber, the air cladding and the resin cladding are connected at the core connecting portion. Therefore, the light leaking into the air clad of the end cap fiber is released into the air, and the entry into the resin clad can be prevented. Conversely, the end cap fiber is not provided with a core exposed portion, and the same applies to the case where the core connecting portion is constituted only by the core exposed portion of the optical fiber. Furthermore, the same effect can be obtained even when the core of the optical fiber and the core of the end cap fiber are connected via a bare fiber made of quartz or the like.

<Method for forming optical fiber end structure>
Next, a method for forming the optical fiber end structure F according to this embodiment will be described.

(Optical fiber manufacturing process)
First, a preform is formed by an OVD method, a VAD method, a rod-in-tube method or the like, and this is heated and drawn in a drawing furnace to obtain a fiber strand. At this time, the resin is coated simultaneously with the stretching, and the optical fiber 10 is manufactured.

  Next, the stretched fiber strand is coated with a resin. This coating constitutes the resin cladding 12. Thus, the optical fiber 10 can be manufactured.

(End cap fiber manufacturing process)
First, an optical fiber having an air cladding is manufactured. In manufacturing the air clad optical fiber, one or a plurality of core rods for forming the core 21, a plurality of capillaries for forming air holes of the air clad 22, and a support tube are prepared. These are all made of quartz glass. When the core 21 is constituted by the first core 21a and the second core 21b, one or a plurality of first core rods and a plurality of second core rods are prepared. Using these, a preform in which the core rod is arranged at the center position in the support tube and the capillary is arranged around it is obtained by the OVD method, the VAD method, the rod-in-tube method or the like.

  Subsequently, an air clad optical fiber is produced by heating and stretching the preform in a drawing furnace.

  Next, the air-clad optical fiber is cut into, for example, about 50 mm using a fiber cutter or the like to obtain the end cap fiber 20.

(Core connection process)
Next, a method for connecting the core 11 of the optical fiber 10 and the core 21 of the end cap fiber 20 will be described.

  First, at the end of the optical fiber 10, the resin clad 12 having a predetermined length is peeled off to expose the core 11. Specifically, about 20 mm from the end of the optical fiber 10 is peeled off using a dedicated tool for removing the coating.

  On the other hand, also at the connection end of the end cap fiber 20 with the optical fiber 10, the air clad 22 having a predetermined length is peeled off to expose the core 21. Specifically, the support layer is scratched by glass cutting or laser irradiation, and the support layer is pulled out with a finger. It should be noted that this step can be omitted when the core exposed portion is already formed on the cut surface when the long air-clad optical fiber is cut into the end cap fiber 20.

  Subsequently, the optical fiber 10 and the end cap fiber 20 are set in an optical fiber fusion splicer, aligned so that their optical axes coincide with each other, and fused and connected by arc discharge or the like. To do. Thereby, the cores of the optical fiber 10 and the end cap fiber 20 can be connected to each other.

  Finally, in order to protect the connection portion, the connection portion is accommodated in a protective housing made of, for example, aluminum. Thus, the optical fiber end structure F having the configuration in which the end cap fiber 20 is attached to the optical fiber 10 can be formed.

(Effect of this embodiment)
According to the optical fiber having the conventional configuration, when light is incident at an incident angle larger than the numerical aperture of the optical fiber, there is a problem that the resin clad is damaged because it cannot be totally reflected by the inner wall of the core and becomes a clad mode. In particular, when light is incident on the optical fiber of the present embodiment from an optical component having a tapered configuration whose diameter decreases from the incident end side toward the output end side, such as an optical combiner, reflection is performed on the tapered inner surface. Repeated transmission increases the incident angle of light, and the problem of damage to the coating due to the cladding mode becomes more serious.

  However, according to the optical fiber end structure F of the present embodiment, the optical fiber 10 including the core 11 and the resin cladding 12 provided so as to cover the core 11, the core 21 and the core 21 are provided so as to cover the core. The end cap fiber 20 formed of air clad 22 with holes formed therein is connected to the core 11 at one end of the optical fiber 10 and the core 21 of the end cap fiber 20, while at one end of the optical fiber 10. Since the core connecting portion C that does not connect the resin clad 12 and the air clad 22 of the end cap fiber 20 is provided, light that enters the end cap fiber 20 leaks from the core 21 and enters the clad mode. It is emitted from the side of the end cap fiber or is scattered and absorbed by the air cladding. For this reason, light can be transmitted to the core 11 of the optical fiber 10 with the cladding mode removed, and damage to the resin cladding 12 and other members due to the cladding mode can be suppressed.

  When the numerical aperture NA2 of the end cap fiber 20 is less than or equal to the numerical aperture NA1 of the optical fiber 10, light that enters the optical fiber 10 and enters a cladding mode (that is, light having an incident angle greater than NA1) has a numerical aperture of NA2. Is absorbed by the air cladding 22 of the end cap fiber 20, so that the cladding mode can be removed more efficiently.

  Further, when the length of the air clad 22 is T / tan (NA2 / n2) or more, the clad mode always collides with the inner wall of the core 21 at the portion where the air clad 22 is provided in the end cap fiber 20. The cladding mode can be removed more efficiently.

  Furthermore, since the length of the core connecting portion C is 4 mm or more, the entry of the clad mode optical fiber 10 absorbed by the air clad 22 into the resin clad 12 can be suppressed.

  As described above, the present invention is useful for an optical fiber end structure in which an end cap fiber is attached to one end of an optical fiber.

It is a figure which shows the optical fiber edge part structure of this embodiment. It is sectional drawing which shows the optical fiber edge part structure of this embodiment. It is a figure which shows the optical fiber edge part structure of the modification of this embodiment. It is explanatory drawing of the length of an air clad.

C Core connecting part F Optical fiber end structure 10 Optical fiber 11 Core 12 Resin cladding 20 End cap fiber 21 Core 22 Air cladding

Claims (4)

An optical fiber having a core and a resin cladding provided to cover the core;
An end cap fiber having a core and an air clad provided so as to cover the core and having holes formed therein;
A core connecting portion that connects the core at one end of the optical fiber and the core of the end cap fiber, while not connecting the cladding at the one end of the optical fiber and the cladding of the end cap fiber;
An optical fiber end structure comprising: In the end structure of the optical fiber according to claim 1,
An optical fiber end structure, wherein the numerical aperture of the end cap fiber is equal to or less than the numerical aperture of the optical fiber. In the optical fiber end structure according to claim 1 or 2,
An optical fiber characterized in that the air clad has a length of T / tan (NA / n) or more, where n is the refractive index of the core of the end cap fiber, T is the core diameter, and NA is the numerical aperture. End structure. In the optical fiber end structure according to any one of claims 1 to 3,
An optical fiber end structure, wherein the core connecting portion has a length of 4 mm or more.

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