KR100633918B1 - Optical fiber with low bending loss and improved induced Brillouin scattering threshold, optical transmission line and optical communication system using same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single mode optical fiber used in a signal transmission system. In particular, the present invention relates to an induction brillouin scattering and bending loss so that an analog signal having a high power can be transmitted with good characteristics even in a severe bending environment. The present invention relates to a reduced single mode optical fiber.

The optical fiber according to the present invention includes a core region having a radius r _core from the optical central axis and a clad region surrounding the core region. The core region is further divided into a first region and a second region, wherein the first region is located at the center of the core region, and the second region is located outside the first region. The first region has a refractive index distribution of a sine function, and the second region has a refractive index distribution of a Gaussian function.

Inductive Brillouin Scattering, Bending Loss, Singlemode Fiber

Description

FIBER WITH REDUCED STIMULATED BRILLOUIN SCATTERING AND LOW BENDING LOSS, AND OPTICAL TRANSMISSION LINE USING THE SAME, AND OPTICAL TRANSMISSION SYSTEM

The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to.

1 is a refractive index profile diagram of a conventional single mode optical fiber having a stepped refractive index structure.

2A and 2B are refractive index profiles of a single mode optical fiber according to the present invention.

3 is a graph showing the induced Brillouin scattering threshold of an optical fiber according to an embodiment of the present invention.

4 is a graph showing the induced Brillouin scattering threshold for the optical fiber of Comparative Example 1. FIG.

5 is a graph showing the induced Brillouin scattering threshold for the optical fiber of Comparative Example 2. FIG.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single mode optical fiber used in a signal transmission system. In particular, the present invention relates to an induction brillouin scattering and bending loss so that an analog signal having a high power can be transmitted with good characteristics even in a severe bending environment. The present invention relates to a reduced single mode optical fiber.

When photons enter the material, phonons are generated, which are classified into optical phonons and acoustic phonons. The photons are the vibrations of molecular stretching by the photons, and the acoustics are the collective vibrations such as the lattice of materials. Brillouin scattering is generated by the interaction of the photons with the acoustic sound.

When a high power optical signal is incident on a general single mode optical fiber, backscattering is generated by Stimulated Brillouin Scattering (SBS). This backscattering weakens the transmission signal and generates noise, which is particularly acute in Passive Optical Networks (PONs).

In analog signal transmission, the use of a PON element requires a higher power optical signal input to branch the optical signal into more branches. However, this increase in the power of the analog signal increases the induced Brillouin scattering in the optical fiber, which greatly degrades the signal quality. Therefore, it is very important to reduce the induced Brillouin scattering in the optical fiber to branch off and provide higher power analog signals to more users in a single optical fiber.

In an FTTx environment, an analog optical signal is transmitted through an optical fiber during a triple play system or a CATV signal that simultaneously transmits voice, video, and data through an optical fiber. When high, the signal is distorted by induced Brillouin scattering. In order to minimize this, an optical fiber having reduced induced Brillouin scattering, that is, an optical fiber having a high induced Brillouin scattering threshold is required.

Induced Brillouin scattering threshold (SBS _{threshold power} ) in the optical fiber is expressed by Equation 1 below.

_{SBS threshold power = (21A eff)} / (L eff · g B)

(Where A _eff : effective cross-sectional area of the optical fiber, L _eff : effective length of the optical fiber, g _B : Brillouin gain factor)

As can be seen from Equation 1, in order to increase the induced Brillouin scattering threshold (that is, to reduce the size of the induced Brillouin scattering), the effective cross-sectional area A _eff is increased or the effective length L _eff is shortened. Alternatively, the Brillouin gain factor (g _B ) must be reduced by changing the refractive index profile of the optical fiber.

However, in order to reduce induced Brillouin scattering over the same length while maintaining the effective area and loss level specified by ITU-T G.652 for the fiber produced, the method of reducing the Brillouin gain factor, ie the refractive index profile of the fiber Preference is given to a method of changing

An object of the present invention is to provide a single mode optical fiber which is suitable for high power analog signal transmission and has a small bending loss.

It is another object of the present invention to provide a single mode optical fiber which not only satisfies the specifications (effective cross-sectional area, bending loss) required for commercial single mode optical fibers, but also improves the threshold of induced Brillouin scattering.

Another object of the present invention is to provide an optical transmission line using the above-described optical fiber and an optical communication system employing the optical transmission line.

Other objects and advantages of the invention will be described below and will be appreciated by the embodiments of the invention. Furthermore, the objects and advantages of the present invention can be realized by means and combinations indicated in the appended claims.

In order to reduce the induced Brillouin scattering, the refractive index structure must be changed in the radial direction or the longitudinal direction so that the acoustic sound induced by the incident signal proceeds at the same speed in each part of the optical fiber and does not resonate.

However, since changing the refractive index in the longitudinal direction of the optical fiber causes problems of acidity in the manufacturing process, the present inventors propose a technique of keeping the refractive index structure the same in the longitudinal direction and changing only in the radial direction. In order to increase the induced Brillouin scattering threshold by effectively dispersing the speed of propagation of acoustic sound in the optical fiber, it is preferable to change the propagation speed of the sound through effective refractive index structure change according to the radius. Optical fibers having a parabolic refractive index profile or a triangular refractive index profile rather than the stepped refractive profile of the type shown in FIG. 1 are suitable for raising the induced Brillouin scattering threshold.

In addition, in the bending loss, when the optical fiber is bent, the field of the basic mode is slightly deviated from the core center of the optical fiber, so that a part of the power does not proceed through the optical fiber and the loss occurs. When the optical fiber is wound several times with a constant bending radius, the power is continuously discharged from the wound portion. In addition, when the basic mode of the bent optical fiber proceeds to the straight optical fiber again, the shape of the field is inconsistent at the interface, so that a part of the power that is in progress is lost. The field shape of the basic mode depends on the refractive index profile of the optical fiber. In the case of a normal single mode fiber, the radial refractive index has a constant stepped refractive index profile as shown in FIG. 1, and the field is close to Gaussian. On the other hand, if the core is formed in a shape similar to that of the field, the mode can be kept similar to the initial state even when the optical fiber is bent because the same field can be configured and maintained more stably.

The refractive index profile of the single mode optical fiber according to the present invention for implementing the above-described technical idea is shown in FIGS. 2A and 2B.

Referring to the drawings, the optical fiber according to the present invention includes a core region having a radius r _core from the optical central axis, and a clad region surrounding the core region.

The core region is further divided into a first region fa and a second region fb. The first region is located at the center of the core region, and the second region is located outside the first region. The first region has a refractive index distribution of a sine function fa represented by Equation 2 below, and the second region has a refractive index distribution of a Gaussian function fb represented by Equation 3 below.

f _a = n ₁ + (n ₂ -n ₁ ) sin [2π (rr ₁ ) / Λ]

f _b = n ₃ + (n ₂ -n ₃ ) exp [-(rr ₁ ) ² / ω]

Where n ₁ : refractive index at the optical central axis, n ₂ : refractive index at radius r ₁ , n ₃ : refractive index at radius r ₃ , Λ: period of the sine function, ω = r ₁ -r ₂ : Gaussian width)

From the optical central axis of the radius of the turning point to the first region and the second region referred to as r _1, and when the a distance of the radius of a width (ω) of a Gaussian from the r ₁ as r _2, wherein r _1, r ₂ And r ₃ satisfy the following relationship.

r ₁ <r ₂ <r ₃ ≤r _core

That is, the radius r ₁ is 1.5 to 2.0 µm, the radius r ₃ is 22.0 µm or more, and the width ω of the Gaussian is preferably 5.5 to 11.0 µm.

Further, when the refractive index at the optical center axis is n ₁ , the refractive index at the radius r ₁ is n ₂ , and the refractive index at the radius r ₃ is n ₃ , the specific refractive index Δ ₁ , Δ ₂ , Δ ₃ of the core region are Satisfies the expression.

Δ ₁ (%) = [(n ₁ ² -n _c ² ) / 2n ₁ ² ] × 100

Δ ₂ (%) = [(n ₂ ² -n _c ² ) / 2n ₂ ² ] × 100

Δ ₃ (%) = [(n ₃ ² -n _c ² ) / 2n ₃ ² ] × 100

In addition, the difference in specific refractive index of the core region has a relationship such that "Δ ₃ <Δ ₁ <Δ ₂ ". That is, the △ ₁ (%) has a value between 0.05 ~ 0.11, preferably has a value between the △ ₃ (%) is -0.02 ~ -0.03.

The actual refractive index profile of the optical fiber according to the present invention in which the center and the outer portion of the core region are represented by different refractive index distributions is shown in Fig. 2b. As can be seen from the figure, in the optical fiber of the present invention, a region having a refractive index distribution of sine function fa is disposed at the center of the core region, and a region having a refractive index distribution of Gaussian function fb is disposed at the outer portion thereof. As a result, it is possible to increase the induced Brillouin scattering threshold and reduce the bending loss.

In addition, the optical fiber of the present invention satisfies the specification of ITU-T G.652 required for commercial single mode optical fiber.

That is, in the optical fiber of the present invention, (1) the mode field diameter (MFD) at the wavelength 1310 nm is 8.6 to 9.5 µm, (2) the zero dispersion wavelength is present at 1300 to 1324 nm, and (3) the dispersion at the wavelength 1550 nm. The value is 18 ps / nm-km or less, and (4) the bending loss at a wavelength of 1550 nm has a characteristic of 0.1 dB or less when wound once with a bending radius of 16 mm.

Another aspect of the present invention relates to an optical transmission line using the above-described optical fiber and an optical communication system employing the optical transmission line as at least part of an optical transmission path.

Example

This embodiment relates to an optical fiber having a refractive index profile as shown in FIG. 2B.

(1) Radius: r ₁ = 1.2 um, r ₂ = 4.0 um, r ₃ = 20.0 um

(2) Specific refractive index difference: △ ₁ (%) = 0.35, △ ₂ (%) = 0.385, △ ₃ (%) = -0.04

(3) Zero Dispersion Wavelength: 1308nm

(4) Dispersion (1550nm): 18.0ps / nm-km

(5) Effective length (km): 25.1km

(6) Effective cross section (1550 nm): 65 um ²

(7) blocking wavelength: 1286nm

(8) Mode field diameter

1310nm: 9.14㎛, 1550nm: 8.9㎛

(9) Dispersion slope: 0.092ps / nm ² / km

Comparative example

The optical fiber of the comparative example relates to an optical fiber having a stepped refractive index profile as shown in FIG. 1. When the refractive index at the optical central axis is n ₁ and the refractive index of the clad region (having the same refractive index as that of the silica tube used as the base material) is n _c , the specific refractive index difference Δ ₁ (%) satisfies the following equation. The refractive index structure and optical properties are as follows.

Δ ₁ (%) = [(n ₁ ² -n _c ² ) / 2n ₁ ² ] × 100

Comparative example  One

(1) Radius: r _core = 4.8 um

(2) Specific refractive index difference: △ ₁ (%) = 0.35

(3) Zero Dispersion Wavelength: 1308nm

(4) Dispersion (1550nm): 16.8ps / nm-km

(5) Effective length (km): 25.3 km

(6) Effective cross section (1550 nm): 68 um ²

(7) blocking wavelength: 1230nm

(8) Mode field diameter

1310nm: 9.31㎛, 1550nm: 8.9㎛

(9) Dispersion slope: 0.088ps / nm ² / km

Comparative example  2

(1) Radius: r _core = 4.85 μm

(2) Specific refractive index difference: △ ₁ (%) = 0.356

(3) zero-dispersion wavelength: 1311 nm

(4) Dispersion (1550nm): 16.5ps / nm-km

(5) Effective length (km): 25.3 km

(6) Effective cross section (1550 nm): 70 um ²

(7) blocking wavelength: 1,220nm

(8) Mode field diameter

1310nm: 9.34㎛, 1550nm: 8.85㎛

(9) Dispersion slope: 0.087ps / nm ² / km

Test Example

For the optical fiber of the embodiment and the optical fiber of Comparative Example 1 and Comparative Example 2, respectively, ① radius 16mm, one bending loss (dB), and ② induced Brillouin scattering threshold value (mW) were measured, and the results are shown below. Table 1 shows.

Bending Loss (1550nm) [dB] Bending Loss (1625nm) [dB] Induced Brillouin Scattering Threshold (1550nm) [mW] Example 1 0.006 0.007 22.0 Comparative Example 1 0.014 0.054 7.7 Comparative Example 2 0.048 0.175 7.7

As can be seen from Table 1, although the mode field diameter (or effective cross-sectional area) of the optical fiber of the example is smaller than that of the optical fiber of the comparative example, the induced Brillouin scattering threshold is higher. This is due to the difference in Brillouin gain coefficient (g _B ) due to the difference in refractive index profile of the optical fiber.

3 to 5 show graphs in which the induced Brillouin scattering thresholds for Examples, Comparative Examples 1 and 2 were tested at a wavelength of 1550 nm, respectively.

As can be seen from this graph, the optical fiber of the example not only has a lower bending loss than the optical fiber of the comparative example but also has a higher induced Brillouin scattering threshold.

As described above, it can be seen that the induced Brillouin scattering threshold can be increased by changing the refractive index structure of the optical fiber in the radial direction rather than in the radial direction to disperse the propagation speed of the acoustic wave.

The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

Therefore, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiment of the present invention and do not represent all of the technical idea of the present invention, various modifications that can be replaced at the time of the present application It should be understood that there may be equivalents and variations.

The optical fiber of the present invention increases the induced Brillouin scattering threshold while keeping the bending loss small by changing the refractive index of the core region in the radial direction. Therefore, in the case of analog transmission, it is possible to install a higher power optical signal without sacrificing other transmission characteristics in a severe installation environment.

Claims (9)

A single mode optical fiber having a core region with a radius of r _core from an optical central axis and a clad region surrounding the core region, (1) The core region is located at the center of the core, and has a first region having a refractive index distribution of a sine function satisfying Equation 4 below, and a Gaussian function positioned outside the first region and satisfying Equation 5 below. A second region having a refractive index distribution; f _a = n ₁ + (n ₂ -n ₁ ) sin [2π (rr ₁ ) / Λ] f _b = n ₃ + (n ₂ -n ₃ ) exp [-(rr ₁ ) ² / ω] Where n ₁ : refractive index at the optical central axis, n ₂ : refractive index at radius r ₁ , n ₃ : refractive index at radius r ₃ , Λ: period of the sine function, ω = r ₁ -r ₂ : Gaussian Width, r ₁ <r ₂ <r ₃ ≤ r _core ) (2) the mode field diameter (MFD) at a wavelength of 1310 nm is 8.6 to 9.5 µm, (3) a zero dispersion wavelength exists between 1300 and 1324 nm, (4) the dispersion value at 1550 nm wavelength is 18 ps / nm-km or less; (5) Single mode optical fiber characterized in that the bending loss at a wavelength of 1550 nm is 0.1 dB or less when wound once with a bending radius of 16 mm. The method of claim 1, The relationship between the specific refractive index differences of the core regions is △ ₃ <△ ₁ <△ ₂ (where Δ ₁ (%) = [(n ₁ ^2- n _c ² ) / 2n ₁ ² ] × 100, Δ ₂ (%) = [ (n ₂ ² -n _c ² ) / 2 n ₂ ² ] × 100, Δ ₃ (%) = [(n ₃ ² -n _c ² ) / 2n ₃ ² ] × 100, n ₁ : refractive index in the optical central axis , n ₂ : refractive index at radius r ₁ , n ₃ : refractive index at radius r ₃ , n _c : refractive index at cladding area, r ₁ <r ₂ <r ₃ ≤r _core ) Characterized by single mode optical fiber. The method of claim 2, And a refractive index difference Δ ₁ (%) at the center of the core has a value between 0.05 and 0.11. The method of claim 2, The non-refractive index difference △ ₃ (%) has a value between -0.02 ~ -0.03. The method of claim 2, Single mode optical fiber, characterized in that the radius r ₁ is 1.5 ~ 2.0㎛. The method of claim 2, Single-mode optical fiber, characterized in that the radius r ₃ is 22.0㎛ or more. The method of claim 1, Single-mode optical fiber, characterized in that the Gaussian width ω is 5.5 ~ 11.0㎛. An optical transmission line employing the single mode optical fiber of claim 1. An optical communication system employing the optical transmission line of claim 8 as at least part of an optical transmission path.

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