JP2010169965A - Photonic crystal fiber and manufacturing method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photonic crystal fiber which can control a GVD (Group Velocity Dispersion) with high accuracy. <P>SOLUTION: The photonic crystal fiber has a core 1 (core rod 202) which is formed with bismuth oxide glass, a hole forming clad 3 which has holes 2 of layers 6n and n layers (n≥3) in a concentric fashion around the core 1 so that layers are equally spaced, and a second clad 4 (300) which is provided around the hole forming clad 3. With this configuration, even when minimal size variation occurs, as each holes is small, it is possible to suppress characteristic change and to obtain a GVD with high accuracy. Further, when such a photonic crystal fiber and an optical transmission section such as a silica-based optical fiber are fusion spliced, as the holes are small, areas to be spliced are dispersed around the holes. Therefore, a splice strength upon fusion splicing is increased, and it is possible to obtain a reliable fusion-spliced part. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a photonic crystal fiber and a manufacturing method thereof, and more particularly to a high optical nonlinear device used in the fields of optical communication and optical information processing.

  In optical communication, a transmitted optical signal is once converted into an electrical signal, subjected to electrical signal processing, and then sent out again to the transmission line as an optical signal. However, the transmission capacity of optical communication is increasing year by year, and it is expected that the signal processing by electricity, which is a conventional method, will eventually reach the limit due to the essential upper limit of the operation speed of the electronic device.

  A nonlinear optical fiber is known as a device that can perform signal processing even faster than electrical signal processing.

Among these nonlinear optical fibers, a photonic crystal fiber (PCF) is drawing attention. A photonic crystal fiber is an optical fiber in which a photonic crystal having a uniform two-dimensional periodic structure in the longitudinal direction of an optical fiber is provided in a clad. In addition, the design of the holes in the PCF has characteristics that cannot be realized with ordinary optical fibers, such as ultra-wideband single mode transmission, large effective core area, high relative refractive index difference, and large structural dispersion. ing.
The PCF in the present invention has a structure in which a light wave is confined in the core by total reflection using the difference in refractive index between the core and the clad, as in the conventional optical fiber. While the conventional optical fiber has a refractive index difference from the cladding by a method such as doping the core with an additive, the PCF in the present invention is a method that lowers the effective refractive index using the holes in the cladding. Realizes optical confinement.

By the way, a method for producing a PCF preform is roughly classified into a grinding method and a small-diameter quartz tube array method.
Among these, the manufacture of PCF using a grinding method is performed as follows. That is, an ordinary preform is prepared by a known method such as a VAD (Vapor-phase Axial Deposition) method or an MCVD (Modified Chemical Vapor Deposition) method, and the preform is perforated with a drill or the like, and then the inside of the pores is washed. And PCF is obtained by performing the drawing process similar to a normal optical fiber.

  On the other hand, the small-diameter quartz tube array method is performed as follows. First, one end of the small-diameter quartz tube is sealed, and the small-diameter quartz tube is bundled in a densely packed arrangement around the small-diameter quartz rod and inserted into the quartz jacket tube. Next, one end of the thin quartz tube is heated from the non-sealed end by a normal optical fiber drawing process, and the fine quartz tube in the quartz jacket tube is fused and integrated to obtain a photonic crystal fiber.

Meanwhile, the nonlinear constant of the step-index fiber consisting of bismuth oxide-based glass is a γ = -1100km ^-1 W ^-1, compared with ^γ = -10km ^{-1 W} -1 of the silica-based nonlinear fiber widely used In other words, the glass highly nonlinear optical fiber formed of this bismuth oxide glass can exhibit the same nonlinearity as short as 1/110 of the conventional silica nonlinear fiber.

  However, high optical nonlinear glass such as bismuth oxide-based glass uses the high optical nonlinearity as a glass due to its high refractive index, and hence the group velocity due to the wavelength dependence of the refractive index. There is a tendency that the absolute value of dispersion (Group Velocity Dispersion, hereinafter referred to as GVD) becomes large.

  For example, in a bismuth oxide glass fiber having a normal step index type structure, the material dispersion caused by the material is about −210 ps / nm / km, and the waveguide dispersion caused by the structure is −90 ps / nm / km. It is known to exhibit a GVD of approximately -300 ps / nm / km.

  On the other hand, the GVD of the dispersion-shifted silica-based highly nonlinear optical fiber is typically GVD <| 1 | ps / nm / km.

  Therefore, even if the step index type bismuth oxide glass fiber can operate as a nonlinear device with a short length, the absolute value of the total dispersion value (GVD × length) corresponding to the group delay is high in the dispersion-shifted quartz type It becomes larger than the nonlinear optical fiber.

  GVD and group delay typically deform short pulse light having a pulse width of 10 ps or less, which is used for ultra-high-speed optical communication. Therefore, a step index type bismuth highly nonlinear optical fiber having a large GVD is It is not suitable for ultra-high speed optical signal processing.

  Therefore, a photonic crystal fiber in which a large hole portion is provided in a cladding portion and a reduction in GVD is measured in a bismuth oxide glass fiber has already been manufactured for use in ultrahigh-speed optical signal processing (Patent Document 1). ).

By the way, in the production of quartz-based photonic crystal fibers, a large number of glass tubes are prepared by mechanical molding, the outer periphery thereof is processed by grinding and polishing, and then they are bundled and heated and stretched. Obtaining crystal fiber.
However, as described above, since bismuth oxide glass is weaker than quartz, it is difficult to prepare many glass tubes by machining using bismuth oxide glass. In addition, unlike quartz that is produced at low cost and in large quantities by the VAD method, a glass base material is made only by a conventional melting method, and thus such a tube manufacturing method is expensive.

JP 2007-308323 A

  As described above, the glass fiber obtained by the glass fiber manufacturing method of Patent Document 1 is effective, but is not sufficient in workability, and further improvement in controllability of GVD is required.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a photonic crystal fiber having high controllability of GVD.
Another object of the present invention is to provide a method for producing a photonic crystal fiber having good processability.

The present invention comprises a core composed of bismuth oxide glass,
A photonic crystal fiber having a hole-forming clad covering the core and having n as a concentric circle with n being an integer of 3 or more,
The n-layer holes of the hole-forming clad are equidistant from each other, and the m-th layer is composed of 6m holes with the innermost layer as the first layer.
According to this configuration, a core made of bismuth oxide glass is used, and 6 m of each layer, n layers, and 3 fine pores are concentrically arranged around the core so that the respective layers are equally spaced. Since a vacancy-forming clad (hereinafter referred to as a first clad) provided with more than one layer is provided, a dispersion control effect by the vacancies can be obtained.
Further, since the diameter of the holes can be precisely controlled by the manufacturing process of the present invention described below, a highly accurate GVD can be obtained. That is, a desired GVD can be obtained by setting the ratio d / Δ of the hole diameter d to the closest hole interval Δ to a specific value. When the present inventor performed a simulation calculation calculated from the wavelength dependence of the effective refractive index obtained by the beam propagation method, if d / Δ is 0.5, GVD is −62 ps / nm / km, d / Δ. Was 0.56, the GVD was -55 ps / nm / km, and the dV was 0.63, the GVD was -45 ps / nm / km.
Further, when such a photonic crystal fiber and a light transmission unit such as a silica-based optical fiber are fusion-connected, since the holes are small, the region to be connected is dispersed around the holes. . For this reason, the connection strength at the time of fusion | melting increases, and it becomes possible to obtain a reliable fusion part.

In the photonic crystal fiber according to the present invention, a second cladding is provided outside the first cladding so as to surround the first cladding, and the nearest void interval (hereinafter simply referred to as a void interval). In other words, the ratio of the hole diameter to 0.2) or more is included.
According to this configuration, a sufficiently large air clad characteristic can be obtained, and an effect that a photonic crystal fiber having excellent transmission characteristics can be obtained can be achieved. The first clad and the second clad desirably have substantially the same thermal expansion coefficient and substantially the same glass transition point Tg.

In the photonic crystal fiber according to the present invention, the bismuth oxide glass typically contains 20 to 80 mol% of bismuth oxide.
According to this structure, the effect that the high nonlinearity of glass can be utilized succeeds.

In the photonic crystal fiber according to the present invention, the line segment connecting the three closest centers among the centers of the holes includes an equilateral triangle.
According to this configuration, a structure having a hexagonal close-packed structure and less deformation can be obtained.
In addition, such a line-up with good symmetry makes it possible to reduce or eliminate polarization dependence.

In the photonic crystal fiber according to the present invention, the refractive index of the bismuth oxide glass constituting the core is higher than the refractive index of the glass constituting the first cladding.
According to this configuration, since the core has a higher refractive index than that of the glass constituting the first clad, even if the voids disappear during fusion splicing, the light confinement effect can be exhibited.

In the photonic crystal fiber according to the present invention, the bismuth oxide glass constituting the core and the glass constituting the first clad are different.
According to this configuration, the degree of freedom in selecting a material for obtaining desired transmission characteristics increases, and the degree of freedom in design increases. Here, the bismuth oxide-based glass constituting the core and the glass constituting the first clad include cases where the compositions are different and cases where the vitrification state is different.

In the photonic crystal fiber according to the present invention, the bismuth oxide glass constituting the core and the glass constituting the first cladding are the same.
According to this configuration, the bondability between the core and the first cladding is high, and there is no occurrence of cracks even when the temperature changes, and the reliability is high.

The present invention also includes a core made of bismuth oxide glass, a first clad covering the periphery of the core and having n layers of concentric holes with n being an integer of 3 or more, and the first clad. A photonic crystal fiber having a surrounding second cladding, wherein the n-layer vacancies of the first cladding are equidistant from each other, and the m-th layer is the innermost layer as the first layer. A method of manufacturing a photonic crystal fiber having 6 m holes, a step of forming a capillary using molten glass, that is, a step of melting a glass to form a capillary, and a bismuth oxide glass constituting a core A glass rod is prepared by drawing a wire, and a plurality of capillaries each having 6 m layers and n layers are concentrically arranged around the glass rod at equal intervals between the layers. A method including a step of obtaining a fiber preform having a plurality of holes by inserting and heating and drawing into an outer tube constituting a cladding, and a drawing step of obtaining a photonic crystal fiber from the fiber preform. Including.
According to this configuration, since the photonic crystal fiber can be formed without using a cutting method, manufacturing is easy and control of GVD can be realized with extremely high accuracy.

In the photonic crystal fiber according to the present invention, the step of forming the capillary includes a step of forming a capillary having a diameter of 500 μm or less by 10 cm or more.
According to this configuration, n layers of capillaries can be arranged concentrically, and since the arrangement state is maintained as it is, high-precision GVD control that has not been obtained conventionally is possible. Become.

Further, in the photonic crystal fiber according to the present invention, in the step of forming the capillary, the step of forming molten glass made of bismuth oxide glass to form a glass tube base material, and the stretching of the glass tube base material And a drawing step of forming a glass tube and a drawing step of drawing the glass tube to obtain a capillary.
According to this configuration, a molten glass made of bismuth oxide glass is molded to form a glass tube base material, the glass tube base material is stretched to form a glass tube, and this is further drawn to draw a capillary. Since it forms, it becomes possible to provide a glass tube which is easy to manufacture and has high dimensional accuracy.

In the photonic crystal fiber according to the present invention, in the photonic crystal fiber, in the stretching step, the glass tube base material is stretched, and an outer diameter is less than 1 mm, a pore diameter is 100 μm or more, and a continuous length is 1 m or more. Including what is the method of forming.
According to this configuration, a desired glass tube can be obtained.

According to the present invention, in the photonic crystal fiber manufacturing method, the step of obtaining the fiber preform includes a step of binding a plurality of the glass tubes to form a glass bundle, and the glass bundle is second clad. And a step of forming a fiber preform by heating and drawing in the outer tube constituting the tube by a rod-in-tube method.
According to this configuration, GVD control that is easy to manufacture and highly accurate is possible.

  According to the present invention, a photonic crystal fiber having a regular periodic structure can be stably and easily manufactured using bismuth oxide glass, which is a highly nonlinear glass.

  The absolute value of GVD, which could not be obtained with the step index type bismuth glass fiber, can be reduced with good controllability. Thereby, application to ultrafast optical nonlinear signal processing becomes possible.

  In addition, in the fusion bonding, it is possible to perform bonding with high reliability and high yield.

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

The present invention provides a method for producing a photonic crystal fiber in which a large number of holes having a periodic structure are formed using a bismuth oxide glass and the GVD can be easily controlled and produced.
In production, first, a glass tube with a suitable inner diameter / outer diameter ratio is supplied in a length of several meters with a stable shape by, for example, a redraw molding method, and the glass tube is cut into an appropriate length. A short glass tube (capillary) is prepared. A plurality of capillaries are bundled and bound, and then heated and stretched using a method such as a rod-in-tube method to produce a fiber preform. A glass rod without holes is inserted in the center of the bundled capillaries and plays the role of a core. The obtained fiber preform is heated and drawn to obtain a photonic crystal fiber in which holes having appropriate intervals and hole diameters are periodically arranged. When the number of capillaries forming the hole array is 6 × mN where mN is a positive integer, a 6-fold symmetric periodic structure having an nN layer periodic structure is obtained. Here, the core may be bound together with the capillary, or may be inserted into the center after binding the capillary.

That is, the photonic crystal fiber according to the embodiment of the present invention includes a core 1 made of bismuth oxide-based glass and a periphery of the core 1 as shown in enlarged photographs in FIGS. , N is an integer of 3 or more, n layers having concentric holes arranged in a concentric manner, each layer is equally spaced, the innermost layer is the first layer, and the m-th layer is 6 m voids The first clad 3 having the hole 2 and the second clad 4 as a jacket tube surrounding the first clad 3 have a line segment connecting the centers of the nearest holes 2 forming an equilateral triangle. It is characterized by that. That is, 6 holes are regularly arranged in the first layer, 12 holes are arranged in the second layer, and 18 holes are regularly arranged in the third layer. Here, as shown in FIG. 5, the closest holes 2 are two holes that are closest to one hole, and form an equilateral triangle when these holes are connected.
It is to be noted that each layer 6m increases sequentially from the inside toward the outside so that the line segment connecting the centers of the nearest holes 2 forms an equilateral triangle and the respective layers are equally spaced. This is provided with the n-layer (n = 3) holes 2, but this is not necessarily the case, and the basic structure may be the structure shown in FIG. 1, with slight errors and deformations. Is also effective.
Here, the diameter R of the core was 5.4 μm, the interval Δ between the holes was 4.0 μm, and the diameter d of the holes was 2.5 μm.

Moreover, the bismuth oxide glass used in the present invention is typically expressed in mol%, Bi ₂ O ₃ is 10 to 45%, B ₂ O ₃ + SiO ₂ is 12 to 70%, Al ₂ O ₃ + Ga _2. A glass containing 7 to 25% of O ₃ and 0 to 3% of CeO ₂ and having a refractive index in the range of 1.9 <n <2.1. For example, “containing ₁₂ to 70% of B ₂ O ₃ + SiO ₂ ” means containing at least one of B ₂ O ₃ and SiO _{2 in} a total of 12 to 70%, and “CeO ₂ the may contain up to but 3% CeO ₂ is not essential to the "contains 0-3%, a meaning of.

  When the composition is out of the range, the optical nonlinearity is lowered, or the ability to stably form the glass is insufficient, which may make it difficult to produce a glass tube preform.

Further, when high optical nonlinearity is desired, Bi ₂ O ₃ is 45 to 70%, B ₂ O ₃ is 12 to 30%, Ga ₂ O ₃ is 7 to 20%, and In ₂ O ₃ is expressed in mol%. A glass containing ₂ to 15%, ZnO 0 to 15% and CeO ₂ 0 to 3% and having a refractive index N of 2.1 or more and less than 2.3 can be used.
When the composition is out of this range, the optical nonlinearity may be lowered or the preparation of the preform may be difficult as described above.

Next, a method for manufacturing this photonic crystal fiber will be described.
In this method, as shown in FIGS. 2 to 4, a capillary 102 (FIG. 2C) is formed through a glass tube base material 100 and a glass tube 101 using a molten glass made of bismuth oxide glass, A core rod 202 constituting a core made of bismuth oxide glass is prepared (FIG. 3 (c)), and there are 6 m pieces (m = 1,...) Around the core rod 202 at equal intervals in a concentric manner. N) and n layers (n ≧ 3), a plurality of the capillaries 102 are bundled and inserted into a jacket tube 300 as an outer tube constituting the second cladding (FIG. 4A), and heated and stretched. To obtain a fiber preform 41 having a plurality of holes (FIG. 4B), and a drawing process to obtain a photonic crystal fiber 42 from the fiber preform. Figure 4 and a (c)).

  That is, first, as shown in FIG. 2A, a cylindrical glass tube base material 100 made of bismuth oxide glass is prepared. The glass tube base material 100 typically has a length of 150 mm, a diameter of 15 mm, and an inner diameter of 2 mm to 10 mm. This cylindrical glass tube base material can be obtained by machining into molded glass. Alternatively, a cylindrical mold can be manufactured using centrifugal force by applying rotation to a main shaft using a method called a rotation method. At this time, the ratio between the diameter and the inner diameter is determined in accordance with the shape of the photonic crystal fiber to be finally obtained.

As shown in FIG. 2B, the glass tube base material 100 is heated to a temperature at which the viscosity is 10 ^4.5 to 10 ^9.5 poise, and stretched by mechanical tension. This is typically repeated once or a plurality of times to obtain a glass tube 101 having an outer diameter of 5 mm or less. The length of the glass tube 101 is typically 5 to 15 mm.

  Subsequently, the glass tube 101 is stretched into a thin glass tube having a diameter of 500 μm or less, that is, a capillary 102 by redraw molding (drawing process: FIG. 2C). The length of the capillary 102 obtained at this time depends on the length of the glass tube 101 before redraw molding and is typically 1 m or more. By redraw molding, a thin capillary 102 having a stable diameter and inner diameter in the longitudinal direction can be obtained in a very short time.

  Then, the capillary 102 is cut into an appropriate length to obtain a desired number of short glass tubes (capillaries 102). The length of the capillary depends on the length of a glass tube for a rod-in tube described later, and is typically 5 to 15 mm.

  Then, a core rod 202 corresponding to the core of the photonic crystal fiber is prepared so as to have the same diameter as the capillary 102. The length is preferably the same as or longer than the capillary 102 for ease of operation. The component of the core rod 202 is preferably the same as that of the capillary 102 or bismuth glass having a relative refractive index difference of 4% as an upper limit and a refractive index higher than that of the capillary 102 is used. When it is formed with a refractive index difference other than that, there is a possibility that the core does not guide light or multimode propagation occurs. That is, as shown in FIG. 3A, the core base material 200 is formed using the same glass as the glass constituting the capillary 102, and the core base material 200 is stretched as shown in FIG. 201 is formed and further drawn to form a core rod 202 as shown in FIG.

  Next, as shown in FIG. 4A, the core rod 202 is set as the center, the periphery is surrounded by the capillary 102, and they are bound so as to be closest packed, thereby obtaining the glass bundle 40. A cross-sectional view of the glass bundle 40 is shown in FIG. Due to the diameter stability of the capillary 102, the periodic array of capillaries 102 has a hexagonal close-packed structure. When the diameter stability of the capillary 102 is poor, a periodic structure cannot be taken.

  Then, as shown in FIG. 4 (b), these glass bundles 40 are inserted into the jacket tube 300, and then heated, and these are fused and integrated as shown in FIG. 5 (a). A reform 41 is obtained. The fiber preform 41 is typically 3-15 mm in diameter. In the rod-in tube process, it is preferable to reduce the pressure in the gap portion in order to reliably fill and fuse the gap between the capillary 102 and the jacket tube 300. In this case, the end of the capillary 102 is preferably sealed in order to seal the air in the capillary 102. In this way, as shown in FIG. 5B, after further heating and drawing to obtain a fiber preform 41, a final diameter photonic crystal fiber as shown in FIG. 5C is obtained through a drawing process. Can do. FIG. 7 is a schematic view showing a cross section of the photonic crystal fiber.

  The core diameter, hole diameter, and hole interval of the final photonic crystal fiber are determined depending on the outer diameter of the capillary 102 bound at the time of heating and stretching, and the outer diameter and inner diameter of the jacket tube 300. The When a desired structure cannot be obtained by a single rod-in-tube process, a plurality of rod-in-tube processes are performed.

The fiber preform 41 is stretched and then heated to a temperature at which redraw molding is possible, and the shape of the photonic crystal fiber 42 is drawn. At this time, since the holes typically have a small diameter of 10 μm or less, pressurization is performed from the upper end of the fiber preform 41 in order to prevent disappearance of the holes. The pressurizing pressure at this time is determined by the viscosity of glass, the drawing speed, and the hole diameter, and is 1 to 100 kPa. In this way, the photonic crystal fiber shown in FIG. 1 is formed.
In the above embodiment, the photonic crystal fiber having holes having a three-layer structure has been described. However, the photonic crystal fiber does not necessarily have to be three layers, and may be a multilayer.

In addition, the result of having measured GVD of the photonics crystal fiber formed in this way is shown in FIG. In the figure, S represents the calculated value, and a represents the GVD measurement result of this photonic crystal fiber. Here, r represents the result of measuring the GVD of a conventional step index type fiber as a comparative example. As can be seen from this figure, the GVD of the photonics crystal fiber was about -45 ps / nm / km. It can be seen that this value is significantly improved as compared to −300 ps / nm / km of the step index type fiber formed of the same bismuth oxide glass.
Here, the measurement error of the measuring instrument is ± 10 ps / nm / km, which is within the range.
As described above, according to the photonic crystal fiber of the first embodiment of the present invention, even if slight dimensional variation occurs, each of the holes is small, so that a change in characteristics can be suppressed, and high accuracy can be achieved. It is thought that GVD can be obtained.

  Furthermore, when fusion-connecting such a photonic crystal fiber and an optical transmission part such as a silica-based optical fiber, since the holes are small, the region to be connected is dispersed around the holes. Become. Accordingly, the connection strength at the time of fusion is increased, and a highly reliable fusion structure can be obtained.

  Next, in the photonic crystal fiber, the optical confinement was considered by changing the ratio of the hole diameter to the hole interval. Here, whether or not the hole diameter d and the hole interval Δ are appropriate is determined based on whether or not the fundamental mode does not diverge and propagates while confined in the fiber core. The basic mode is light having a light intensity distribution used in general optical communication, and a fiber that can propagate only the basic mode is referred to as a “single mode fiber”. Table 1 shows the calculation results of whether or not fundamental mode confinement is possible when d / Δ is changed, with “0” indicating that confinement is not possible and “1” indicating that confinement is possible. In general, when Δ (interval) is small, confinement is weak and the hole diameter must be increased (more dense structure). Here, a fiber with Δ = 4 μm was prepared. If d / Δ> 0.2, it is understood that confinement is OK. In addition, when Δ> 5 μm, the calculation can be confined even when d / Δ> 0.1 (perhaps the threshold value is d / Δ> 0.05, but the nonlinearity and GVD controllability may be deteriorated.

  Here, in the photonic crystal fiber, it is preferable that a ratio of the hole diameter to the hole interval is 0.2 or more. If it is less than 0.2, the effect as an air cladding cannot be achieved and light confinement is weakened, and a desired GVD cannot be obtained. However, if it is 0.2 or more, a sufficiently large air cladding characteristic is obtained. Therefore, it is possible to obtain a photonic crystal fiber having good transmission characteristics.

  Further, in the photonic crystal fiber, the glass had a bismuth oxide content in the range of 20 to 80 mol%. However, when the glass content was less than 20 mol%, the nonlinearity was insufficient. It becomes difficult to obtain.

  Further, a line segment connecting the three closest centers among the centers of the holes forms an equilateral triangle, so that a structure having a hexagonal close-packed structure and less deformation can be obtained.

In the photonic crystal fiber, the glass constituting the core and the glass constituting the first cladding may be the same.
By using the same glass, the bondability between the core and the first cladding is high, and cracks do not occur even when the temperature changes. Here, the glass composition and physical state are the same.
Furthermore, in the photonic crystal fiber according to the present invention, if the refractive index of the glass constituting the core is higher than the refractive index of the glass constituting the first cladding, the core has the first cladding. Since it is higher than the refractive index of the glass which comprises, even if a void | hole lose | disappears in the case of fusion splicing, the light confinement effect can be exhibited.

  Note that the glass constituting the core and the glass constituting the first cladding are not necessarily the same. According to this structure, the freedom degree of selection of the glass for obtaining a desired transmission characteristic becomes high, and a design freedom degree increases. The core, the first clad, and the second clad may be made of glass having substantially the same thermal expansion coefficient and substantially the same softening point, but more desirably, the core portion has a higher refractive index.

  Also, according to the photonic crystal fiber manufacturing method of the present invention, a photonic crystal fiber can be formed without using a cutting method, so that the reliability is high and GVD control can be realized with extremely high accuracy. It is.

  In the above embodiment, the step of forming the glass tube desirably forms a glass tube having a diameter of 500 μm or less by 10 cm or more. With this configuration, the n-layer glass tubes can be arranged in a regular and concentric manner with high accuracy, and the glass tube is stretched so that this arrangement state is maintained as it is. It becomes possible.

  Further, when forming the glass tube, it is desirable to stretch the glass tube base material so that the outer diameter is less than 1 mm, the pore diameter is 100 μm or more, and the continuous length is 1 m or more. When the continuous length is shorter than this, it is necessary to produce a capillary necessary for obtaining a desired bundle many times, and in that case, it may be difficult to stably align the outer diameter. .

(Embodiment 2)
Next, a second embodiment of the present invention will be described.
In the first embodiment, all the holes are formed so as to have the same size. However, in the present embodiment, as shown in a schematic cross-sectional view in FIG. The opposite holes 2L may be different in size.
In this way, polarization maintaining property can be obtained by changing the diameters of two opposed holes to other holes.

  Moreover, although the bismuth oxide glass has been described in the above embodiment, it can be applied to tellurite glass, chalcogen glass, and the like.

  Examples of the present invention will be described below.

Bi ₂ O ₃ 42.8%, SiO ₂ 34.2%, Ga ₂ O ₃ 14.3%, Al ₂ O ₃ 7.1%, La ₂ O ₃ 1.4%, CeO ₂ 0. The raw materials were prepared and melted to 2% to obtain bismuth glass. This glass had a refractive index N of 1.98 at a wavelength of 1.55 μm.

  The melted bismuth oxide glass was molded with a SUS cylindrical mold to obtain a core base material 200 having a length of 150 mm and a diameter of 15 mm. The core preform 200 was heated and stretched at 569 ° C. (stretched glass rod 201) to obtain a core rod 202 having a diameter of 500 μm.

  On the other hand, the melted bismuth oxide glass was molded by a rotational method to obtain a glass tube base material 100 having a length of 150 mm, a diameter of 15 mm, and an inner diameter of 6 mm.

  The glass tube base material 100 was heated and stretched at 569 ° C. to obtain a glass tube 101 having a diameter of 5 mm.

  The glass tube 101 was heated and redrawn at 585 ° C. to draw a capillary 102 as a thin glass tube having a length of 10 m. The average diameter of the thin capillary 102 was 500 μm, the maximum diameter was 504 μm, and the minimum diameter was 493 μm. FIG. 10 shows the location dependence of the diameter of the capillary 102 obtained. The horizontal axis indicates the glass length (m), and the vertical axis indicates the linearity (mm). As can be seen from this figure, there was no place dependency and it was uniform. Two more thin capillaries 102 having a length of 10 m were produced by the same method.

  36 capillaries 102 having a length of 100 mm were cut from the thin capillaries 102. Further, a portion having a length of 11 mm was cut out from the core rod 202. The glass bundle 40 was produced by binding them so that the capillary 102 had a close-packed structure around the core rod having a length of 11 mm (FIG. 4A).

  The edge part of the glass binding body 40 was sealed with the micro burner. Thereby, in the rod-in-tube process, it was possible to maintain the shape of the holes in the capillary.

As shown in FIG.
The glass bundle 40 was inserted into the jacket tube 300 and heated and stretched by the rod-in tube method at 569 ° C. to obtain a fiber preform 41 having a diameter of 5 mm (FIG. 5B). At this time, the space between the glass bundle 14 and the jacket tube 300 was a reduced pressure of 0.5 atm. As a result, it was possible to fuse both with no gap.

  The fiber preform 41 was stretched to 3 mm in diameter by redraw molding at 562 ° C., and then drawn to form a photonic crystal fiber 42 (FIG. 5C). At the time of this drawing, the inside of the air holes was pressurized with the pump pressure from the upper side of the fiber preform. The pressure at the time of this pressurization was 10 kPa, 20 kPa, 30 kPa, 40 kPa, 50 kPa, 60 kPa, and 70 kPa. At this time, by applying a pressure of 30 kPa, the holes could be drawn without changing the outer diameter / inner diameter ratio of the capillary. At lower pressures (10 kPa and 20 kPa9), the pores deformed and became smaller. At higher pressures (40 kPa, 50 kPa, 60 kPa, and 70 kPa), the pore diameters increased.

  The drawing speed was adjusted so that the outer diameter of the photonic crystal fiber was 125 μm. This was realized by the number of rotations of the roller of the winding device after drawing. FIG. 8 shows the length dependency of the diameter of the obtained photonic crystal fiber. The horizontal axis indicates the fiber length (m), and the vertical axis indicates the linearity (mm). As can be seen from the figure, the diameter was not dependent on the fiber length and was uniform. According to FIG. 11, the average value of the diameter was 125 μm, the maximum value was 126 μm, and the minimum value was 124 μm. At this time, the fiber length was 30 m.

  The obtained photonic crystal fiber 42 had a hole diameter of 2.5 μm, a hole interval of 4 μm, a core diameter of 5.4 μm, and a cross-sectional structure of several 36 holes arranged periodically. 1 and 2 show cross-sectional photographs of the photonic crystal fiber 42. FIG. FIG. 7 is an enlarged cross-sectional view of a main part.

  The GVD of the photonic crystal fiber 18 was measured by an Agilent 81910A measuring instrument, and D = −50 ps / nm / km. The GVD derived from the material of the bismuth oxide glass 8 used in the present invention is D = −133 ps / nm / km. Therefore, by introducing the photonic crystal structure, GVD can be shifted by +83 ps / nm / km.

  The photonic crystal fiber of the present invention can be applied to a device that is required to be short and compact in an ultrahigh-speed nonlinear optical signal processing system.

The photograph which shows the cross section of the photonic crystal fiber which took the close-packed structure in Embodiment 1 of this invention, (b) is a principal part enlarged view of (a). Manufacturing process diagram of capillary of embodiment 1 of the present invention Manufacturing process diagram of core rod of embodiment 1 of the present invention Manufacturing process diagram of the bundle of Embodiment 1 of the present invention Manufacturing process diagram of photonic crystal fiber according to Embodiment 1 of the present invention Cross-sectional enlarged schematic view of a bundle Schematic enlarged sectional view of the photonic crystal fiber according to the first embodiment of the present invention. The figure which shows the relationship between GVD of the photonic crystal fiber of Embodiment 1 of this invention, and a wavelength. The figure which shows the principal part expanded sectional view of the photonic crystal fiber of Embodiment 2 of this invention Diagram showing the relationship between the diameter and length of a thin glass tube of the same photonic crystal fiber The figure which shows the relationship between the diameter of the photonic crystal fiber produced in this invention, and its length

100 Preform for glass tube (Glass tube base material)
101 Glass tube 102 Capillary 200 Preform for core rod (core base material)
201 glass rod 202 core rod 300 jacket tube 40 united body 41 fiber preform 42 photonic crystal fiber

Claims (12)

A core made of bismuth oxide glass;
A photonic crystal fiber comprising a hole-forming clad covering the core and having n layers of concentric circles with n being an integer of 3 or more,
The n-layer holes of the hole-forming clad are equal intervals between layers, and the photonic crystal fiber is composed of 6m holes, with the m-th layer being the innermost layer as the first layer. The photonic crystal fiber according to claim 1,
Furthermore, it comprises a second clad formed outside the hole-forming clad so as to surround the hole-forming clad,
A photonic crystal fiber in which a ratio (d / Δ) of the hole diameter d to the closest hole interval Δ is 0.2 or more. The photonic crystal fiber according to claim 1 or 2,
The bismuth oxide-based glass is a photonic crystal fiber containing 20 to 80 mol% of bismuth oxide. The photonic crystal fiber according to claim 1,
A line segment connecting the three closest centers among the centers of the holes is a photonic crystal fiber constituting an equilateral triangle. The photonic crystal fiber according to any one of claims 1 to 4,
A photonic crystal fiber in which the bismuth oxide glass constituting the core has a refractive index higher than that of the bismuth oxide glass constituting the hole-forming clad. The photonic crystal fiber according to any one of claims 1 to 5,
The bismuth oxide glass constituting the core and the bismuth oxide glass constituting the hole forming clad are different photonic crystal fibers. The photonic crystal fiber according to any one of claims 1 to 5,
A photonic crystal fiber in which the bismuth oxide glass constituting the core and the bismuth oxide glass constituting the hole forming clad are the same. A core made of bismuth oxide glass;
A hole-forming clad covering the periphery of the core and having n layers of concentric holes with n being an integer of 3 or more;
A second clad formed so as to surround the hole forming clad,
The n-layer holes of the hole-forming clad are equally spaced from each other, and a photonic crystal fiber is manufactured in which the innermost layer is the first layer and the m-th layer is 6 m holes. A method,
Forming a capillary using molten glass;
A glass rod is prepared by drawing a bismuth oxide glass constituting the core, and a plurality of capillaries each having 6 m layers and n layers are concentrically arranged around the glass rod at equal intervals. , By inserting into the outer tube constituting the second clad and heating and stretching,
Obtaining a fiber preform having a plurality of pores;
A drawing method for obtaining a photonic crystal fiber from the fiber preform. A method for producing a photonic crystal fiber according to claim 8,
The step of forming the capillary is a method of manufacturing a photonic crystal fiber, which is a step of forming a capillary having a diameter of 500 μm or less by 10 cm or more. A method for producing a photonic crystal fiber according to claim 8 or 9,
The step of forming the capillary comprises:
Forming a glass tube base material by molding molten glass formed by melting bismuth oxide-based glass;
Stretching the glass tube base material to form a glass tube; and
And a drawing step of drawing the glass tube to form a capillary. It is a manufacturing method of the photonic crystal fiber according to claim 10,
The stretching step is a method for producing a photonic crystal fiber, which is a method of stretching the glass tube base material to form a glass tube having an outer diameter of less than 1 mm, a pore diameter of 100 μm or more, and a continuous length of 1 m or more. A method for producing a photonic crystal fiber according to any one of claims 8 to 11,
The step of obtaining the fiber preform comprises:
Bundling a plurality of the capillaries to form a glass bundle;
A method of manufacturing a photonic crystal fiber, comprising: inserting the glass bundle into an outer tube constituting a second clad and heating and stretching the rod by a rod-in-tube method to form a fiber preform.

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