JP5622297B2 - Optical fiber - Google Patents

Description

  The present invention relates to an optical fiber for optical communication, and in particular, a long-distance line transmitting a length of several tens of kilometers, and an indoor / outdoor in an optical fiber to the home (FTTH) or a local area network (LAN). The present invention relates to an optical fiber suitable for wiring.

Optical fibers are suitable for long-distance communications due to their broadband characteristics, and are widely used for communication on long-distance trunk lines of several tens of kilometers or more.
On the other hand, with the rapid spread of the Internet, the amount of information transmitted and received by individual personal computers has also increased dramatically. Here, a widely used technique is a copper electric cable such as a coaxial cable or an unshielded twisted pair (UTP) cable. However, since an electric cable has a narrow band and is easily affected by electromagnetic noise, it is difficult to transmit an enormous amount of information.
Therefore, FTTH is spreading as a technique for increasing transmission capacity by using optical fibers not only for long-distance communication between telephone stations but also for communication between the telephone station and each user. In FTTH, a single optical fiber is shared by multiple users up to the vicinity of the user group using the broadband characteristics of the optical fiber, and from there onward, the optical signal is branched for each user. The method of distributing the service lines to each user is adopted.

  Although there are various types of optical fibers, single-mode optical fibers of the ITU-T G.650 standard are mainly used for long-distance communication. This optical fiber has a small transmission loss and is relatively inexpensive. The cut-off wavelength is about 1300 nm or less so that the signal in the 1300 nm band becomes a single mode. The mode field diameter (MFD) is about 8 to 10 μm in terms of the standard, but is usually about 9 to 10 μm for the purpose of suppressing optical nonlinearity.

  In FTTH, the same kind of single mode optical fiber is used, and it is drawn from the branch point to the home of each user. This is because connecting optical fibers with greatly different MFDs at the branch point causes transmission loss. Here, bending loss is another important characteristic required for in-home wiring. This is because long-distance trunk cables are laid in places that are not easily affected by external forces such as underground ducts, so the bending applied to the optical fiber body is only assumed to be a 60mm diameter winding (at most 100 turns) in the terminal container. . On the other hand, wiring inside and outside the house is flexible and lightweight, so it is wired with a relatively thin cord (diameter of several millimeters), so it is easily affected by external forces and is subject to optical fibers. The bending radius is often 20 mm or less.

  In the first place, optical fiber propagates along the optical fiber core, so it can be transmitted even when the optical fiber is bent. However, as the bend radius becomes smaller, light that leaks from the core without being able to propagate. Increases exponentially, resulting in transmission loss. This is bending loss. In order to reduce the bending loss, it is effective to concentrate light more in the core, and it can be improved by reducing the MFD. For this reason, an MFD optical fiber of about 6 to 8 μm is usually used and, for example, the bending loss when the optical fiber is wound around a mandrel (cylinder) having a diameter of 20 mm is obtained at a wavelength of 1550 nm. 0.5dB / turn or less is achieved.

However, when an optical fiber with an MFD of about 6 μm and an optical fiber of about 10 μm are connected, there is a problem that the connection loss is large, and if this is the case, it can be used separately for a trunk single-mode optical fiber and a user single-mode optical fiber. Necessary.
One countermeasure is to increase the cutoff wavelength. For the same MFD, the bending loss decreases exponentially as the cutoff wavelength increases. Even if the fiber cutoff wavelength defined by ITU-T G.650 is 1350 nm, the cable cutoff wavelength is generally about 1250 nm, which is about 100 nm shorter. If used, there is no problem. At the same time, bending loss is improved by designing the MFD to about 8-9 μm near the lower limit of G652.

  Further, the double-clad optical fiber having a small inner cladding refractive index reported in Non-Patent Document 1 and the like can reduce bending loss while designing a large MFD. An optical fiber having such a structure with reduced impurity absorption loss and optimized zero dispersion wavelength is disclosed in Patent Document 1, and the relative refractive index difference Δ of the low refractive cladding is set to about −0.021 to −0.0007%. Therefore, the MFD is designed to be about 9.2 μm. Furthermore, although an optical fiber with improved bending characteristics is disclosed in Patent Document 2, the MFD is designed to be slightly smaller from 8.2 to 9.0 μm while the low refractive index cladding is further reduced to −0.08 to −0.02%. .

On the other hand, a technique for wiring between electronic devices using optical fibers is becoming widespread, and various optical fibers are listed in a typical LAN communication standard IEEE 802.3. In fact, since laser light sources and light receivers used in the 1300 nm wavelength band are relatively expensive, inexpensive 850 nm light emitting diodes (LEDs) and surface emitting lasers (VCSELs) are often used in this application. The optical fiber used at this time is a multimode optical fiber, and a typical core diameter is 50 μm. In the case of multimode optical fiber, the propagation speed of light is generally different for each mode. Therefore, the refractive index distribution is designed to be a parabolic shape (α type) instead of a simple step shape, and the difference in the propagation speed of light between modes. Is reduced. With such a device, the propagation speed is, for example, 500 MHz · km.
In order to cover the optical fiber with common specifications from long distance transmission lines to FTTH and LAN applications, Patent Document 3 designs the refractive index distribution of the core in a substantially α shape to reduce the mode delay in the 850 nm band. In addition, use in the 850nm band is guaranteed. However, there is no mention of bending loss at a bending diameter as small as 20 mm.

JP 2002-47027 A JP 2006-133496 A Special Table 2004-508600

Shojiro Kawakami and ShigeoNishida, “Characteristics of a Doubly Clad Optical Fiber with a Low-Index InnerCladding,” IEEE Jounal of Quantum Electronics, vol.QB-10, No.12, pp.879-887, Dec.1974)

  In view of the above prior art, the present invention provides an optical fiber having improved bending characteristics while maintaining MFD within the G652 standard, preferably 9 μm or more, and enabling single mode operation at 1310 nm, and transmission bandwidth. Is intended to provide a wide optical fiber.

The optical fiber of the present invention includes a first core in the center, a second core that covers the outer periphery adjacent to the first core, a third core that covers the outer periphery adjacent to the second core, and the third the outer periphery an optical fiber cladding covering adjacent the core, of when the reference to the refractive index of the cladding, the relative refractive index difference of the first core △ _1, the relative index second core the difference △ ₂ and was, when the third relative refractive index difference △ _{3 core, 0.28% ≦ △ 1 ≦ 0.4} %, - 1.0% ≦ △ is ₃ ≦ -0.5%, and the center of the first core As a reference, the radius of the boundary between the first core and the second core is a, the radius of the boundary between the second core and the third core is b, and the radius of the boundary between the third core and the clad is c. when the b is not less than 1.8 times the mode field diameter of the wavelength 1383 nm, LP01 mode of the mode field diameter of 8μm in further wavelength 1310nm And the upper 10μm or less, the fiber cutoff wavelength is less than or equal 1430 nm, is characterized in that the cable cutoff wavelength is less than 1300 nm.

The optical fiber of this configuration has a fiber cutoff wavelength of 1650 nm or less measured with a 2 m long optical fiber defined in the ITU-T G.650 standard , and a cable cutoff wavelength measured at a length of 22 m. Is 1450 nm or less.
Further, the difference between the fiber cutoff wavelength and the cable cutoff wavelength is 120 nm or more, and the signal propagated through the optical fiber having a length of 1 km is substantially single mode. Further, the transmission band measurement value at a wavelength of 1310 nm measured with an optical fiber having a length of 1 km is 2 GHz · km or more. The zero dispersion wavelength is 1290 nm or more and 1345 nm or less.
Furthermore, the transmission loss at a wavelength of 1383 nm is 0.4 dB / km or less.

  The optical fiber of the present invention has the above configuration, so that the MFD is within the G652 standard, particularly 9 μm or more, and at the same time, the bending loss is improved, and the single mode operation at 1310 nm is performed despite the large cutoff wavelength. It is possible to obtain an excellent effect such that an optical fiber having a wide transmission band and a desired zero dispersion wavelength can be easily obtained.

It is a figure explaining the refractive index distribution of the optical fiber produced in Example 1. FIG. It is a figure which shows the relationship between the cutoff wavelength of an optical fiber produced in Example 1, and fiber length. It is a figure which shows the relationship between the wavelength of the optical fiber produced in Example 1, and a transmission loss. It is a figure which shows the relationship between the cutoff wavelength of an optical fiber produced in Example 2, and fiber length. It is a figure which shows an impulse response. 6 is a view showing a refractive index distribution of an optical fiber preform manufactured in Example 3. FIG. 6 is a diagram showing a refractive index distribution of an optical fiber preform manufactured in Comparative Example 1. FIG.

  Hereinafter, although an example and a comparative example are given and an embodiment of the present invention is explained still in detail, the present invention is not limited to these.

[Example 1]
FIG. 1 shows the refractive index distribution of the optical fiber according to this embodiment.
First, the first core shown in the figure was produced by the VAD method, drawn to a desired diameter to obtain a core core material, and externally attached thereto to add a second core. Fluorine was added thereon to form a third core having a refractive index lower than the silica level, and a cladding was added thereon to produce an optical fiber preform. The base material was drawn to obtain an optical fiber having a cladding diameter of 125 μm and coated with urethane acrylate to obtain an optical fiber having a diameter of 250 μm.
This optical fiber has a first core radius a = 3.8 μm, a second core radius b = 18 μm, a low refractive index third core radius c = 27 μm, and a maximum relative refractive index difference of the first core. △ ₁ 0.39%, the maximum value of the relative refractive index difference of the second core △ ₂ 0.01%, and the minimum value △ ₃ of the specific refractive index difference of the third core was -0.75%.

An optical fiber with such a configuration has a fiber cutoff wavelength of 1417 nm and a cable cutoff wavelength of 1276 nm. Confirmed to do. The MFD at 1310 nm was 9.21 μm, the zero dispersion wavelength was 1321 nm, and the zero dispersion slope was 0.085 ps / nm ² · km. When this optical fiber was wound around a cylinder with a diameter of 20 mm, the loss increase was 0.05 dB / turn.

The relationship between the cutoff wavelength of this optical fiber and the fiber length is shown in FIG. From the figure, it can be seen that the cut-off wavelength becomes shorter as the fiber length increases, and that if it exceeds 10 m, it operates almost in single mode at a wavelength of 1310 nm.
Next, the transmission loss was measured, and the result is shown in FIG. The transmission loss at a wavelength of 1383 nm is 0.42 dB / km, and the second core layer (radius b) is 18 μm, which is about 1.9 times the MFD, so that the OH contained in the vicinity of the third core layer (radius c) It can be seen that the loss due to the group is reduced.

[Example 2]
The surface of the optical fiber preform produced in Example 1 was shaved and drawn to produce an optical fiber having a cladding diameter of 125 μm. Furthermore, it was coated with urethane acrylate to form an optical fiber having a diameter of 250 μm.
The relative refractive index differences Δ ₁ , Δ ₂ , Δ ₃ of each core of the manufactured optical fiber were the same as in Example 1, but the radius of each core was 1.15 times. That is, the first core radius a = 4.4 μm, the second core radius b = 21 μm, and the third core radius c = 31 μm.
FIG. 4 shows the relationship between the fiber length and the cutoff wavelength. From the figure, the fiber cutoff wavelength was 1625 nm, the cable cutoff wavelength was 1430 nm, and the difference between the fiber cutoff wavelength and the cable cutoff wavelength was 195 nm.

When the transmission band at a wavelength of 1310 nm was measured with an optical fiber having a length of 5 km, it was 3000 MHz · km, and the impulse response was as shown in FIG. This indicates that single mode operation is performed at a wavelength of 1310 nm. Further, the MFD at 1310 nm was 9.74 μm, the zero dispersion wavelength was 1301 nm, and the zero dispersion slope was 0.087 ps / nm ² · km. Further, the loss increase when this optical fiber was wound around a cylinder with a diameter of 20 mm was 0.01 dB / turn. The transmission loss at a wavelength of 1383 nm was 0.345 dB / km.
Note that the zero dispersion wavelength can be designed to be, for example, 1310 nm by adjusting the clad thickness more precisely to make it intermediate between the first and second embodiments.

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[Comparative Example 1]
A porous glass base material in which the first core and the second core were integrated was produced by the VAD method, which was made into transparent glass, and further stretched to a desired diameter to obtain a core core material. On top of this, porous glass was externally attached and fluorine was added to form a third core having a refractive index lower than the silica level. Further, a clad was added thereon to produce an optical fiber preform. FIG. 7 shows the result of measuring the refractive index distribution of the base material with a commercially available preform analyzer.
The base material was drawn to obtain an optical fiber having a cladding diameter of 125 μm and coated with urethane acrylate to obtain an optical fiber having a diameter of 250 μm. Since the optical fiber obtained in this manner is reduced in diameter while maintaining a similar shape to the base material, its refractive index distribution is the same as that of the base material except for the dimensions in the radial direction.

This optical fiber has a first core radius a = 3.5 μm, a second core radius b = 9.6 μm, a low refractive index third core radius c = 14.2 μm, and a relative refractive index difference of the first core. mean value △ ₁ 0.32%, the average value of the relative refractive index difference of the second core △ ₂ -0.05%, and the minimum value of the relative refractive index difference of the third core △ ₃ was -0.42%. Incidentally, △ ₁ was obtained by averaging the refractive indices inner than the radius position at which the refractive index of the first core is maximum, △ ₂ was obtained by averaging the refractive indices between ab of the second core, △ ₃ is The refractive index between bc of the third core was obtained by averaging in the radial direction. Also, a is the position of the half width with respect to △ ₁ of the first core, b is a position where the refractive index distribution of a boundary between the second core and the third core becomes the most precipitous, c is the boundary of the third core and the cladding The position where the refractive index is steepest was set (see FIG. 7).

This optical fiber has a fiber cutoff wavelength of 1205 nm, a cable cutoff wavelength of 1115 nm, and operates in a single mode at 1310 nm. Furthermore, the MFD at 1310 nm is 8.87 μm, the zero dispersion wavelength is 1327 nm, which is larger than the G652 standard. Zero-dispersion slope was ² · km ^0.087ps / nm. Furthermore, the increase in loss when this optical fiber is wound around a cylinder with a diameter of 20 mm is 0.15 dB / turn at a wavelength of 1625 nm, and it can be said that the bending loss is somewhat large. The transmission loss at a wavelength of 1383 nm was 0.964 dB / km.

Communication characteristics can be improved by using the optical fiber of the present invention.

Claims (6)

The first core in the center, the second core adjacent to the first core and covering the outer periphery, the third core adjacent to the second core and covering the outer periphery, and the outer periphery adjacent to the third core the an optical fiber cladding covering, when relative to the refractive index of the cladding, the relative refractive index difference of the first core △ ₁ and then, the relative refractive index difference of the second core △ ₂ and was, the when the relative refractive index difference △ ₃ of the third _{core, 0.28% ≦ △ 1 ≦ 0.4} %, - 1.0% ≦ △ is ₃ ≦ -0.5%, and with reference to the center of the first core, the first core Where b is the radius of the boundary between the second core and the third core, b is the radius of the boundary between the second core and the third core, and c is the radius of the boundary between the third core and the cladding. mode field is a diameter of 1.8 times or more and 10μm or less LP01 mode of the mode field diameter of 8μm or more in yet a wavelength 1310 nm, full Iba cutoff wavelength is less than or equal 1430 nm, the optical fiber cable cutoff wavelength is equal to or less than 1300 nm. The optical fiber according to claim 1 , wherein a difference between the fiber cutoff wavelength and the cable cutoff wavelength is 120 nm or more. The optical fiber according to claim 1 or 2 , wherein the signal propagated through the optical fiber having a length of 1 km is substantially single mode. The optical fiber according to claim 3 , wherein a transmission band measurement value at a wavelength of 1310 nm measured with an optical fiber having a length of 1 km is 2 GHz · km or more. The optical fiber according to any one of claims 1 to 4 , wherein a zero dispersion wavelength is 1290 nm or more and 1345 nm or less. The optical fiber according to any one of claims 1 to 5 , wherein a transmission loss at a wavelength of 1383 nm is 0.4 dB / km or less.

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