JP4194974B2 - Photonic crystal fiber terminal processing method - Google Patents

Description

  The present invention relates to a photonic crystal fiber including a clad part in which a large number of pores extending in the axial direction of an optical fiber are arranged in a crystal form around a core, and an over clad part provided around the clad part It is about.

  In recent years, a photonic crystal fiber has attracted attention as an optical fiber that exhibits large chromatic dispersion that cannot be obtained with a normal optical fiber including a core and a clad. This photonic crystal fiber has a clad portion in which a large number of pores extending in the axial direction of the optical fiber are arranged in a crystal form around the core, and an overcoat provided around the clad portion to support the clad portion. And a cladding portion.

On the other hand, a polarization maintaining fiber having high polarization stability is used for an optical fiber sensor utilizing polarization or interference, coherent optical fiber communication, or the like. The photonic crystal fiber is also being considered for use as a polarization maintaining photonic crystal fiber by taking advantage of its wavelength dispersion characteristics. In order to make a photonic crystal fiber into a polarization maintaining fiber, the arrangement of pores in the core or in the vicinity of the core is devised. For example, the cross-sectional shape of the core is made elliptical or rectangular, or one of the pores adjacent to the core. The part may have a different diameter from the other pores, or the polarization maintaining function may be imparted by making the cross-sectional shape of the pores elliptical and aligning the major axis direction of the ellipses of each pore. (For example, see Patent Document 1 and Non-Patent Documents 1 and 2)
Of the above-mentioned methods for providing the polarization maintaining function, the method of making a polarization maintaining fiber by making a part of the pores adjacent to the core a diameter different from that of the other pores is the easiest to manufacture. It is also an easy method to set the wave holding characteristic to a predetermined value.

In order to use the photonic crystal fiber as described above, it is necessary to polish the end face of the photonic crystal fiber in order to connect or install to an optical input / output device or to connect the fibers. However, if the end face of the photonic crystal fiber is polished with the pores open at the end face, fiber fragments and abrasive will enter the pores and hinder transmission of light. The end surface is polished after the pores are melted to close the pores (after the terminal treatment is performed).
JP 2003-2222739 A JP 2002-236234 A SUZUKI, K. et al. High-speed bi-directional polarisation division multiplexed optical transmission in ultra low-loss (1.3dB / km) polarisation-maintaining photonic crystal fiber.In: Electronics Letters, (US), 8th November 2001, Vol .37, No.23, p.1399-1401 STEEL, MJ et al. Polarization and Dispersive Properties of Elliptical-Hole Photonic Crystal Fibers.In: Journal of Lightwave Technology, (US), April 2001, Vol.19, No.4, p.495-503

  However, if there are multiple types of pores with different diameters in a single fiber, such as a polarization maintaining photonic crystal fiber, when the end of the fiber is melted to close the pores, the pores from the end face collapse. A phenomenon occurs in which the distance between the closed portions varies depending on the diameter of the pores. In other words, if the diameter is small, the pores are easily crushed, so the distance that the pores are crushed becomes long, and if the diameter is large, the distance that the pores are crushed is short because the holes are difficult to crush. Depends on the size of the diameter.

  In the polarization-maintaining photonic crystal fiber, when the distance at which the pores are crushed is different as described above, there arises a problem that the mode of the signal light changes from the single mode to the multimode. In other words, the photonic crystal structure and the polarization maintaining structure are broken in a region where pores that are crushed and pores that are not crushed are mixed depending on the size of the diameter. This problem becomes more prominent as the wavelength of the signal light becomes longer. Note that the degree of such mode disturbance can be confirmed by Far Field Pattern (hereinafter referred to as FFP).

  The present invention has been made in view of the above points, and an object of the present invention is to provide a light propagation mode at an end portion subjected to terminal processing in an optical fiber having a plurality of types of pores having different diameters. It is to prevent disturbance.

  The photonic crystal fiber of the present invention is provided around a core, a clad portion provided around the core, and a plurality of pores extending in the axial direction of the optical fiber arranged in a crystal form, and around the clad portion A photonic crystal fiber having an overclad portion, wherein the pore includes a first pore and a second pore having a diameter larger than that of the first pore. The end of the pore is closed at at least one end of the nick crystal fiber, and the distance in the optical fiber axial direction between the closed end of the first pore and the closed end of the second pore is 50 μm or less. It is.

  The photonic crystal fiber may have a polarization maintaining function and may be a single mode fiber.

  The first photonic crystal fiber terminal processing method of the present invention includes a core, a clad portion provided around the core and having a plurality of pores extending in the axial direction of the optical fiber arranged in a crystal form, A photonic crystal fiber including an over clad portion provided around the clad portion, the pore including a first pore and a second pore having a diameter larger than the first pore. A method of treating the terminal of the method, comprising: heating at least one end of the photonic crystal fiber to melt and closing the end of the pore; and cutting the one end of the fiber to open only the second pore And an injection material having a refractive index substantially equal to the material constituting the fiber from the closed end of the first pore to a distance of 50 μm or less in the fiber axis direction. And a step of injecting into.

  The second photonic crystal fiber terminal processing method of the present invention includes a core, a clad portion provided around the core and having a plurality of pores extending in the axial direction of the optical fiber arranged in a crystal form, A photonic crystal fiber including an over clad portion provided around the clad portion, the pore including a first pore and a second pore having a diameter larger than the first pore. A method of treating an end of the photonic crystal fiber, the step of injecting an injection material having a refractive index substantially equal to a material constituting the fiber from at least one end of the photonic crystal fiber into the pore, and cutting one end of the fiber Opening only the second pore, and the injection material into the second pore from the closed end of the first pore to a distance of 50 μm or less in the fiber axis direction. And a step of entering.

  In the end of a photonic crystal fiber having a plurality of types of pores having different diameters, the distance in the fiber axis direction between the ends of the first pore having a smaller diameter and the second pore having a larger diameter is closed. Is 50 μm or less, the mode of propagation light in this terminal is prevented from being disturbed.

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

(Embodiment 1)
A cross section of a polarization maintaining photonic crystal fiber (hereinafter referred to as polarization maintaining PCF) according to Embodiment 1 is shown in FIG. 1 (hatching is omitted).

  In this polarization maintaining PCF, a core 1 made of quartz glass extending in the fiber axis direction is located at the center of the cross section, and a large number of pores 4a, 4b, 4c extending in the fiber axis direction around the core 1. However, the clad portion 2 is formed by arranging the minimum units in a crystal form of an equilateral triangular lattice. Here, the fact that the large number of pores 4a, 4b, and 4c are arranged in a crystal form means that the large number of pores 4a, 4b, and 4c are regularly arranged in the fiber cross section. Examples of such an array include a lattice array whose minimum unit is an equilateral triangle, a square, or a rectangle. The diameters of the pores 4a, 4b, and 4c are preferably 0.1 to 10 μm from the viewpoint of fiber characteristics. An over cladding 3 made of quartz glass is provided around the cladding 2. The over clad part 3 is a part that supports the clad part 2.

  The six pores 4a and 4b adjacent to the core 1 of the polarization maintaining PCF are composed of two types of pores 4a and 4b having different diameters. The diameters of the pair of first pores 4a facing each other with the core 1 interposed therebetween are smaller than the diameters of the other four second pores 4b. In addition, the pore 4c which comprises the clad part 2 is the same diameter as the 1st pore 4a. Therefore, the pore 4c of the clad part 2 can also be called the first pore in terms of the pore diameter. By arranging the two kinds of pores 4a and 4b having different diameters around the core 1 in this way, the polarization maintaining function is provided in the optical fiber. That is, the polarization plane perpendicular to the cross section of the fiber including a straight line connecting the centers of the pair of first pores 4a and 4a having a small diameter and the polarization plane perpendicular to the cross section propagate due to the pore arrangement adjacent to the core 1. Since there is a difference in the propagation constant between the two polarization modes, the polarization is maintained. Note that such a polarization maintaining PCF can be produced by forming a base material by filling a hollow portion of a large-diameter quartz cylinder with a quartz rod and a quartz capillary as described in Patent Document 1, for example. In addition, a method of drawing a base material after forming a base material by forming a base material using a drill or the like in a quartz rod as a base material can be mentioned.

  In the present embodiment, one end of the polarization maintaining PCF in which the pores 4a, 4b, and 4c are opened is melted by arc discharge using a fusion splicer to close the ends of the pores 4a, 4b, and 4c. . The reason for closing the ends of the pores 4a, 4b, and 4c is to prevent abrasives and glass fragments from entering the pores 4a, 4b, and 4c when the fiber end face is polished to attach the connector. is there. A cross-section along the fiber axis direction of the polarization-maintaining PCF in which one end is melted and the ends of the pores 4a, 4b, and 4c are closed in this manner (cross-sectional view taken along line XX in FIG. 1) is shown in FIG. Is shown schematically. Here, since the diameter of the first pore 4a and the pore 4c constituting the clad portion 2 is smaller than that of the second pore 4b, the range in which the pores are crushed by melting becomes wider than that of the second pore 4b. That is, the distance from the fiber end 8 on the closed side to the end of the closed hole is longer in the first pore 4a and the pore 4c in the cladding portion 2 than in the second pore 4b. The closed end positions of the first pore 4a and the pore 4c of the cladding portion 2 are substantially the same position B because the diameters of both are substantially the same. In such a state, only the second pore 4b exists around the core 1, and there is a portion where the first pore 4a has disappeared (closed). In such a state, the propagation mode that was originally in the single mode is disturbed, so that it becomes a multi-mode. Therefore, even if the fiber end in this state is attached to the connector, the light emitted from the fiber becomes light in which the propagation mode is disturbed, and an accurate optical signal (information) cannot be extracted.

  Therefore, in order to solve this problem, in this embodiment, the polarization maintaining PCF is first cut at the position of the one-dot chain line A in FIG. When cutting at the position of the alternate long and short dash line A, only the second pore 4b opens in the cut surface, and the first pore 4a and the pore 4c of the cladding portion 2 remain closed and do not open in the cut surface.

  Then, as shown in FIG. 3 (the hatching of the cut portion is omitted), an injection material 7 having a refractive index substantially equal to that of quartz constituting the fiber is injected into the open second pore 4b. The refractive index here is the refractive index at the wavelength of the signal light transmitted through the polarization maintaining PCF. In the injection material 7, the distance L1 in the fiber axis direction between the end of the injection material 7 in the second pore 4b and the closed ends of the first pore 4a and the pore 4c of the cladding portion 2 is 50 μm or less. Inject like so. The injection method may be injection using a syringe or the like, or the cut surface 10 may be immersed in the injection material 7 which is a liquid and injected using capillary action, and any method may be used. You may use. The injection material 7 may be any material as long as it has a refractive index substantially equal to that of quartz, but is preferably an ultraviolet curable acrylic resin or epoxy resin. This is because when such an ultraviolet curable resin is used as the injection substance 7, the injection end position can be fixed by irradiating ultraviolet rays after injection, and the distance L1 can be kept constant.

  Thereafter, the cut surface 10 is polished and attached to the connector.

  Next, the examination result about the preferable range of the distance L1 in the fiber axial direction between the closed end of the second pore 4b and the closed end of the first pore 4a and the pore 4c of the cladding portion 2 will be described.

  First, the distance L1 is set to three levels of 50 μm, 100 μm, and 500 μm. Assuming that light beams having wavelengths of 0.405 μm and 0.633 μm are propagated through the polarization-maintaining PCF, the FFP is calculated and the fiber crossing is calculated. A simulation to express the surface was performed. The results are shown in FIGS. In this simulation, the diameter of the second pore 4b is 3.6 μm, the diameter of the first pore 4a and the pore 4c of the cladding portion 2 is 1.3 μm, and the distance between adjacent pores is 3.5 μm. did.

  In FIGS. 6 to 8, the single red mode (the portion with high intensity) exists in the center is single mode, and if there are two or more red portions split, it is multi mode. . Looking at each figure from this point of view, when the distance L1 is 50 μm (FIG. 6), there is no mode disturbance in both wavelengths of 0.405 μm and 0.633 μm. Next, when the distance L1 is 100 μm (FIG. 7), the mode is not disturbed at the wavelength of 0.405 μm, but the mode is disturbed at the wavelength of 0.633 μm, and the red portion is divided into two. Further, when the distance L1 is 500 μm (FIG. 8), the modes are disturbed at both wavelengths of 0.405 μm and 0.633 μm, and the mode is almost multimode.

  Next, the actual polarization maintaining PCF (the diameter of the second pore 4b is 3.6 μm, the diameter of the first pore 4a and the pore 4c of the cladding part 2 is 1.3 μm, and the distance between adjacent pores is 3.5 μm), polarization maintaining PCF with a distance L1 of 35 μm and polarization maintaining PCF with 75 μm were prepared. FIG. 9 and FIG. 10 show the measured FFP of these polarization maintaining PCFs.

  When the distance L1 is 35 μm (FIG. 9), there is no mode disturbance in both wavelengths 0.405 μm and 0.633 μm. However, when the distance L1 is 75 μm (FIG. 10), mode disturbance occurs at both wavelengths of 0.405 μm and 0.633 μm. In the simulation, when the distance L1 is 100 μm, there is no mode disturbance at the wavelength of 0.405 μm, but when the distance L1 is 75 μm in the actual measurement, the mode disturbance is generated at the wavelength of 0.405 μm. This seems to be due to the fact that the hole structure and the structure of the hole prepared in the actual measurement (tapered by heat melting) differ from the hole structure in the simulation. It is done.

  From the above results, the mode disturbance that occurs when the distance L1 increases becomes larger as the wavelength of the signal light becomes longer. Considering the case of using signal light with a wavelength of 0.633 μm or less, if the distance L1 is 50 μm or less, There is virtually no mode disruption. It is preferable for the distance L1 to be 35 μm or less because mode disturbance can be reliably suppressed even when signal light having a longer wavelength is used. Note that even when visible signal light having a wavelength longer than 0.633 μm is used, if the distance L1 is 50 μm or less, the mode is hardly disturbed, and there is no practical problem.

(Embodiment 2)
The cross section of the polarization maintaining PCF according to the second embodiment is the same as the cross section of the polarization maintaining PCF according to the first embodiment (FIG. 1). In this embodiment, unlike the first embodiment in which the end of the fiber is first closed by melting, first, the quartz that constitutes the fiber is substantially formed from one end of the polarization maintaining PCF in which the pores 4a, 4b, and 4c are opened. An injection material having an equivalent refractive index is injected using capillary action. Specifically, for example, the injection is performed by immersing one end of the polarization maintaining PCF in an injection material that is a liquid. The refractive index here is the refractive index at the wavelength of the signal light transmitted through the polarization maintaining PCF. As the injection material, for example, those described in Embodiment 1 can be used.

  Then, after the injection into the pores 4a, 4b, 4c, the injection material is cured, and the injection material is fixed in the pores 4a, 4b, 4c. The curing method varies depending on the type of injection material, but is cured by, for example, irradiating with ultraviolet rays, applying heat, or reacting with moisture or oxygen in the air. A cross section of the polarization-maintaining PCF after curing (cross section including the fiber center axis) is schematically shown in FIG. As can be seen from FIG. 4, the first pore 4a having a smaller diameter than the second pore 4b having a larger diameter and the pore 4c of the cladding portion 2 are more distant from the fiber end face 8 by capillarity (FIG. 4). The injection material 7 is injected up to (position B). If the position of the injected substance 7 in the pores 4a, 4b, 4c does not change even if the polarization maintaining PCF is moved after the injection or various processes are performed, the injected substance 7 does not need to be cured. I do not care.

  In the present embodiment, in the state shown in FIG. 4, the closed end of the second pore 4b (the boundary between the hollow portion of the pore and the injection material 7), the first pore 4a, and the cladding portion 2 are used. Since the distance in the fiber axis direction between the closed end of the pore 4c (the boundary between the hollow portion of the pore and the injection material 7) in the fiber axis direction is larger than 50 μm, the mode described in the first embodiment is used. Disturbance will occur. Therefore, in order to make this distance 50 μm or less, in this embodiment, the injection material 7 is further injected only into the second pore 4b.

  The first step of this injection method is a step of cutting the fiber at the position of the alternate long and short dash line A in FIG. When the fiber is cut in this manner, only the second small pore 4b having a large diameter is opened on the cut surface, and the first small pore 4a having a small diameter and the pore 4c of the cladding portion 2 are closed by the injection material 7. It will be in a state that is not.

  Next, as shown in FIG. 5 (cross-sectional hatching omitted), the injection material 7 is injected from the cut surface 10 into the second pore 4b. Further, the injection material 7 is cured as necessary. By this injection, the end of the second pore 4b (the boundary between the hollow portion of the pore and the injection material 7) and the end of the first pore 4a and the pore 4c of the cladding portion 2 (the hollow portion of the pore and the injection) The distance L2 in the fiber axis direction from the boundary with the substance 7 is 50 μm or less. As described above, since the distance L2 is set to 50 μm or less, the signal light is not disturbed in the mode in the present embodiment as in the first embodiment. Similarly to the first embodiment, it is preferable that the distance L2 is 35 μm or less in this embodiment because the mode disorder can be reliably suppressed even when a longer wavelength signal light is used. The injected material injected from the cut surface 10 may be a material different from the injected material 7 initially injected into the pores 4a, 4b, and 4c.

  After processing the terminal of the polarization maintaining PCF in this way, the cut surface 10 is polished and attached to the connector.

  The two embodiments described so far are examples of the present invention, and the present invention is not limited to these embodiments. For example, the structure of the cross section of the polarization maintaining PCF is a structure in which the diameter of a pair of pores facing each other across the core is larger than the diameters of the other four pores among the six pores around the core. The core may have a hollow structure. Also, the arrangement structure of the pores in the cladding may be substantially circular or rectangular in the cross section of the fiber. Furthermore, the photonic crystal fiber is not limited to the polarization-maintaining PCF, and any photonic crystal fiber having a plurality of types of pores each having two or more different diameters may be used.

  Further, the closed end of the second pore having a large diameter may be located farther from the fiber end face than the closed end of the first pore having a small diameter.

  Then, the method of closing the ends of two or more kinds of pores having different diameters may be only melting or another method.

  Patent Document 2 discloses a technique in which a refractive index matching substance is placed in pores provided in an optical fiber, but this does not include an optical fiber having pores and no pores. This technique is limited to the technique related to the structure of the connection portion with the optical fiber and aims to reduce the connection loss. Therefore, the present invention, which aims at reducing the mode disturbance, is intended for a photonic crystal fiber having two or more kinds of pores having different diameters, and is completely different from the invention of Patent Document 2. is there.

  As described above, the photonic crystal fiber and the terminal processing method according to the present invention are useful as the polarization maintaining PCF and the terminal processing method, etc., by suppressing the disturbance of the mode of the signal light emitted from the fiber. .

It is a figure which shows the cross section of a polarization maintaining photonic crystal fiber (PCF). It is a schematic diagram of the XX line cross section of FIG. FIG. 3 is a schematic cross-sectional view of the polarization maintaining PCF according to the first embodiment along the fiber central axis. It is a cross-sectional schematic diagram along the fiber central axis in the middle of creation of the polarization maintaining PCF according to the second embodiment. 6 is a schematic cross-sectional view of the polarization maintaining PCF according to Embodiment 2 along the fiber center axis. FIG. It is a simulation figure of FFP when the distance between holes is 50 micrometers. It is a simulation figure of FFP when the distance between holes is 100 micrometers. It is a simulation figure of FFP when the distance between holes is 500 micrometers. It is an actual measurement figure of FFP when the distance between holes is 35 micrometers. It is an actual measurement figure of FFP when the distance between holes is 75 micrometers.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Core 2 Cladding part 3 Overclad part 4a 1st pore 4b 2nd pore 4c Pore 7 of clad part Injection material L1, L2 The closed end of the 1st pore, and the closed end of the 2nd pore In the fiber axis direction

Claims (2)

A core, a clad part provided around the core and extending in the optical fiber axis direction and arranged in a crystal form, and an overclad part provided around the clad part. A method for treating a terminal of a photonic crystal fiber, wherein the hole includes a first pore and a second pore having a diameter larger than that of the first pore,
Heating at least one end of the photonic crystal fiber to melt and close the ends of the pores;
Cutting one end of the fiber to open only the second pore;
Injecting into the second pore an injection material having a refractive index substantially equal to the material constituting the fiber from the closed end of the first pore to a distance of 50 μm or less in the fiber axis direction; A processing method for a terminal of a photonic crystal fiber including: A core, a clad part provided around the core and extending in the optical fiber axis direction and arranged in a crystal form, and an overclad part provided around the clad part. A method for treating a terminal of a photonic crystal fiber, wherein the hole includes a first pore and a second pore having a diameter larger than that of the first pore,
Injecting an injection material having a refractive index substantially equal to the material constituting the fiber into the pores from at least one end of the photonic crystal fiber;
Cutting one end of the fiber to open only the second pore;
And a step of injecting the injection material into the second pore from the closed end of the first pore to a distance of 50 μm or less in the fiber axis direction.

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