JP4046054B2 - Photonic crystal fiber and manufacturing method thereof - Google Patents

Description

  The present invention relates to a photonic crystal fiber having holes and a method for manufacturing the same.

  A photonic crystal fiber (PCF) 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. PCFs are roughly classified into two types based on the difference in wave guiding principle. One has a structure in which a photonic band gap is formed in the region corresponding to the clad and the light wave is confined in the core by black reflection, and the other has a refractive index of the core and the clad as in the conventional optical fiber. Using the difference, it is structured to confine the light wave in the core by total reflection. In contrast to the conventional optical fiber, which has a refractive index difference from that of the clad by doping the core with an additive in the core, the optical confinement is performed by lowering the effective refractive index using the clad holes. Realized.

  The PCF has characteristics that cannot be realized by a normal optical fiber, such as an ultra-wideband single mode transmission region, a large effective core area, a high relative refractive index difference, and a large structural dispersion due to the design of the holes in the PCF.

  The manufacturing method of the PCF preform in which the holes are formed is mainly a grinding method as shown in Patent Document 1 and a thin quartz tube array method as shown in Patent Document 2.

  The grinding method is to produce a normal preform by a known method such as VAD method or MCVD method, drill the preform with a drill or the like, and then draw the same as a normal optical fiber to obtain a PCF Is the method.

  In the small-diameter quartz tube array method, one end of each of a plurality of small-diameter quartz tubes is sealed, bundled in a high-density loading arrangement, the central thin-diameter quartz tube is replaced with a thin quartz rod, and then the quartz jacket tube is formed. Insert and draw one end of the small-diameter quartz tube from the non-sealed end by the same drawing process as a normal optical fiber while fusing and integrating the quartz jacket tube and the quartz tube in the quartz jacket tube. This is a method for obtaining PCF.

  In optical wavelength division multiplexing transmission, four-wave mixing (FWM: Four-Wave Mixing) in which light having a different wavelength is generated by mixing light having two or more different wavelengths near zero dispersion occurs. FWM is a phenomenon in which new interference light is generated due to nonlinear interaction between signal light, and it becomes crosstalk to the signal light and degrades transmission characteristics. Therefore, in order to suppress transmission quality degradation due to four-wave mixing, the absolute value of local chromatic dispersion is made larger than a predetermined value, and in order to suppress transmission quality degradation due to residual dispersion, chromatic dispersion over the entire transmission line Although a transmission line with a smaller absolute value is used, in order to construct such a transmission line, it is necessary to change the chromatic dispersion in the longitudinal direction. Therefore, in the drawing process disclosed in Patent Document 1, an optical fiber whose hole diameter is changed in the longitudinal direction can be manufactured by adjusting the pressure of the holes in the preform. The characteristics such as wavelength dispersion of the PCF are determined by the size and position of the holes, and when the hole diameter is changed in the longitudinal direction, the optical characteristics such as wavelength dispersion can be changed in the longitudinal direction. The wavelength dispersion of the pulse can be suppressed without tuning the phase of.

JP 2002-145634 A US Pat. No. 5,802,236

  However, in the manufacture of PCF in which the hole diameter changes in the longitudinal direction, when drawing the PCF preform, the diameter of the pores in the preform is changed by controlling the internal pressure, and at the same time the outer diameter of the optical fiber. There is also a problem that it changes.

  Further, when the outer diameter control of the fiber is performed, the drawing conditions such as the drawing tension and the drawing speed change, so that it is difficult to manufacture a PCF having a desired hole diameter and position.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-described problems, and provide a PCF in which the outer diameter does not vary and the chromatic dispersion characteristics change in the longitudinal direction, and a manufacturing method thereof.

  To achieve the above object, a first aspect of the present invention is a photonic crystal fiber comprising a core and a plurality of holes extending in the longitudinal direction at equal intervals on a circumference centered on the core. The position of the hole from the center of the fiber is changed along the longitudinal direction.

  According to a second aspect of the present invention, the central axis of the hole may be inclined at a predetermined angle with respect to the central axis of the photonic crystal fiber.

  According to a third aspect of the present invention, the photonic crystal fiber preform is formed by perforating the cladding so that the central axis of the air hole has a predetermined angle with respect to the central axis of the photonic crystal fiber preform. This is a method of drawing a line.

  The photonic crystal fiber according to the present invention exhibits excellent effects such as preventing a change in the outer diameter of the fiber and having a chromatic dispersion change in the longitudinal direction.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 shows a cross-sectional view of a PCF preform body 11 for producing the photonic crystal fiber of the present embodiment.

  The PCF preform body 11 is formed in a cylindrical shape by the VAD method, and includes a core 12 made of quartz to which germanium (Ge) is added and a cladding 13 made of pure quartz and having a slightly lower refractive index than the core 12. Have. Holes 14 are formed at equal intervals in the circumferential direction around the core 12 with the center of the core 12 as the central axis 15. The angle of the center axis (hole axis) 16 of the hole 14 extending in the longitudinal direction along the hole 14 is inclined 0.3 degrees with respect to the center axis 15. The PCF preform body 11 has a length of 200 mm and an outer diameter of 50 mm. The diameter of the core 12 of the PCF preform body 11 is 3.0 mm, and the relative refractive index difference Δ is 0.375%.

  As shown in FIG. 2A, a cross-sectional view of the preform main body 11 taken along line 2a-2a, the cross section of the air hole 14 is circular, and its diameter is 2 mm. On one end face of the PCF preform 11, the center of the hole 14 is located at the apex of a regular hexagon having an outer diameter of 4 mm with the core 12 as the center. As shown in FIG. 2 (b), which is a cross-sectional view taken along line 2b-2b of the preform body 11, the center of the hole 14 is located at the apex of a regular hexagon having an outer diameter of 6 mm with the core 12 as the center on the other end surface. Yes.

  Next, the manufacturing process of the PCF preform body 11 will be described.

  The angle between the central axis of the punching device (not shown) and the central axis 15 of the preform body 11 is set to 0.3 °, and drilling is performed. In the present embodiment, the angle between the PCF central axis 15 and the hole axis 16 of the hole is set to 0.3 °, and the hole 14 is formed in the preform body 11 by drilling.

  In addition, the number of holes 14 formed in the present embodiment is six. However, the number is not limited to this, and the number of holes 14 may be any number as long as it is three or more, preferably an even number of four or more. Individual is desirable. The holes 14 are formed so as to be arranged in a concentric circle centered on the core 12 and at equal intervals in the circumferential direction when viewed in the fiber cross section.

  After drilling, in order to remove dirt and debris adhering to the surface, after washing large debris such as fiber fragments with running water, ultrasonically wash with an organic solvent such as ethyl alcohol and trichlorethylene, Wash with 5% dilute hydrofluoric acid. The washed PCF preform body 11 is placed in a drying container and evaporated to dryness. The solution used in the cleaning step is not limited to these, and other solutions may be used, and the cleaning method is not limited to ultrasonic cleaning, and may be cleaned by other methods.

After drying, as shown in FIG. 3, a branch quartz tube 17 is attached to one end of the PCF preform body 11, and this is used as a PCF preform 10. At the time of attachment, a filter 19 is provided on the preform main body 11 at the end opposite to the end where the branch quartz tube 17 is attached so that dust and moisture are not mixed into the PCF preform 10 by the oxyhydrogen burner 18; Nitrogen gas that passed through the filter 19 was caused to flow through the holes 14. The gas flowing into the PCF preform 10 is not limited to N ₂ but may be Ar, Cl ₂ , He, or the like.

  FIG. 4 is a schematic diagram for explaining a process of drawing the preform 10 obtained by the above manufacturing method.

  The PCF preform 10 is fixed in a drawing furnace 25, and the drawing furnace 25 can control the furnace pressure, and the heating device 26 heats and melts the preform 10. A cap 21 for blocking the inside of the preform 10 from outside air is provided on the side of the branched quartz tube 17 (in the drawing) of the preform 10, and a pressure device 20 is connected to the cap 21 via a gas line 22. Has been. The pressure device 20 is for controlling the pressure difference because the hole diameter depends on the relationship between the drawing furnace internal pressure and the preform internal pressure.

  The preform 10 fixed to the drawing furnace is heated and melted while adjusting the pressure, and the preform 10 is drawn in the longitudinal direction (downward in FIG. 4) with a capstan or the like to draw the PCF 30.

  A PCF 30 having a PCF preform 10 drawn to an outer diameter of 125 μm by a drawing process was manufactured to 10 km, and the PCF 30 was divided into 5 parts every 2 km, and the transmission loss and dispersion characteristics of each PCF were examined. The results are shown in Table 1.

  As shown in Table 1, the transmission loss at a wavelength of 1.55 μm is 1.6 dB / km or less in every section, which is very low loss.

  The absolute value of the chromatic dispersion is monotonously increased or decreased monotonously in the longitudinal direction.

  The chromatic dispersion changing in the longitudinal direction can suppress four-wave mixing that degrades the transmission characteristics, and can cancel the accumulated dispersion remaining in the longitudinal direction. Therefore, the absolute value of the chromatic dispersion of the entire PCF becomes small.

  In the PCF 30 of this embodiment, when producing a preform, the holes 14 in the preform 10 were formed so as to have a predetermined angle with the core central axis 15. Thus, simply by drawing, a PCF 30 in which the center axis of the hole is changed in the longitudinal direction with respect to the fiber center axis can be obtained. As a result, there was obtained PCF30 in which the PCF outer diameter did not vary and the chromatic dispersion changed in the longitudinal direction of PCF30.

  Here, the PCF described in Patent Document 1 described above was produced as a comparative example of the PCF 30 of the present embodiment. The PCF manufacturing method is the same as that of the above-described embodiment, but the angle of the axis extending along the hole in the preform is 0 degree with respect to the fiber center axis, and the diameter of the hole is elongated. The difference is that the direction is changed.

  The produced preform was drawn so that the outer diameter was 125 μm. When the hole diameter is changed in the longitudinal direction of the PCF by controlling the internal pressure of the holes in the preform, the outer diameter of the PCF also changes. If the fiber outer diameter is controlled in order to suppress fluctuations in the outer diameter of the PCF, drawing conditions such as the drawing tension and the drawing speed change, and it is difficult to obtain a desired PCF. It was.

It is sectional drawing which shows the preform main body for producing the photonic crystal fiber based on this invention. It is sectional drawing of a photonic crystal fiber preform main body, (a) shows sectional drawing along the 2a-2a line, (b) shows sectional drawing along the 2b-2b line. It is the schematic explaining the fusion splicing of the preform for photonic crystal fibers of FIG. 1, and a branch quartz tube. It is the schematic which shows the drawing process of the photonic crystal fiber of FIG.

Explanation of symbols

10 Photonic crystal fiber preform 12 Core 14 Hole 15 Fiber center axis 16 Hole center axis 30 Photonic crystal fiber

Claims (3)

  In a photonic crystal fiber having a core and a plurality of holes extending in the longitudinal direction at equal intervals on the circumference centered on the core, the positions of the holes from the fiber center on the fiber cross section along the longitudinal direction Photonic crystal fiber characterized by changing   2. The photonic crystal fiber according to claim 1, wherein the hole has a central axis inclined at a predetermined angle with respect to the central axis of the photonic crystal fiber. The cladding is drilled so that the central axis of the hole has a predetermined angle with respect to the central axis of the preform for the photonic crystal fiber, and the preform for the photonic crystal fiber is drawn. A photonic crystal fiber manufacturing method.

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