KR20120030725A - Bend insensitive optical fiber with large effective area - Google Patents

Abstract

PURPOSE: An optical fiber is provided to include a low refractive index trench layer, thereby forming lossless bending properties. CONSTITUTION: An inner layer is arranged outside of a core(110). The inner layer has a refractive index lower than the core. A trench layer(130) is arranged outside of the inner layer. An outer layer(140) is arranged outside of the trench layer. The outer layer has a refractive index lower than the core and higher than the minimum refractive index.

Description

BEND INSENSITIVE OPTICAL FIBER WITH LARGE EFFECTIVE AREA}

The present invention relates to an optical fiber, and more particularly, to a lossless bending characteristic optical fiber having a large effective area and at the same time having little loss due to bending.

With the recent expansion of the FTTH (Fiber To The Home) market, governments are actively investing in infrastructure to recover from the economic crisis.

A typical passive optical network (PON) for building FTTH is an optical line terminal (OLT) located at the central office (CO) side, and a user's premises or in the vicinity of the user premises. Optical network unit (ONU) or optical network termination (ONT), and a splitter or wavelength division multiplexer (WDM) for connecting them in a tree topology It includes a local base station (remote node: RN) having a. Between CO and RN and between RN and multiple ONUs are connected by low water peak fiber (LWPF), which is usually broadband fiber, and also feeder point such as telephone pole using incoming fiber cable ) To the entrance of the house. In this case, a lossless bending characteristic optical fiber (BIF) having a bending characteristic enhanced to be suitable for indoor / outdoor installation is mainly used as an incoming optical cable.

In addition, the increase in wireless data usage of smartphones, the expansion of 3D TVs, the introduction of TV portal services, and the growth of wireless Internet 4G (LTE, WiMAX) will soon lead to the development of optical backbone networks and indoor systems. Leads to increased capacity. As a result, compatibility with backbone networks using conventional single-mode fiber and lossless bendable fiber in the FTTH system, where fiber is connected to the inlet, is essential for efficient network construction.

To reduce bending losses caused by physical forces in the field, a lossless bending characteristic fiber designed to be suitable for indoor / outdoor installations includes a core having a diameter smaller than that of a normal single mode fiber. Lossless bending characteristics In optical fibers, optical loss and data loss due to external physical forces such as bending are minimized. However, when the lossless bending characteristics optical fiber is connected, the data may be lost due to the difference in core diameter.

As described above, in order to minimize the loss of the optical fiber due to the physical force, a lossless bending characteristic optical fiber having a small core diameter and a high core refractive index has been commercialized. In addition, by arranging a clad with a large difference in refractive index with the core on the outside of the core, an optical fiber having minimized optical loss due to physical force such as bending has been proposed.

However, since these conventional lossless bending characteristic optical fibers all have a small core diameter, a large loss occurs when connecting with a general single mode optical fiber which is a conventional backbone network. Further, when the light output from the subscriber light source enters the lossless bending characteristic optical fiber, the coupling efficiency is lower than that of the single mode optical fiber.

The present invention provides an optical fiber that satisfies a lossless bending characteristic while increasing connection compatibility with a conventional single mode optical fiber which is an existing line in an FTTH system connected to a home entrance.

Lossless bending characteristic optical fiber according to an aspect of the present invention, the core; An inner layer disposed outside the core and having a refractive index lower than that of the core; It is disposed outside the inner layer, and includes a trench layer having the lowest refractive index.

The optical fiber according to the present invention has a large diameter core, and the difference between a general single mode optical fiber and a mode field diameter is within 4.4%, and includes a low refractive index trench layer to simultaneously satisfy lossless bending characteristics, and general passive optical communication networks. It is a hybrid optical fiber that can be used over.

A lossless bending characteristic optical fiber having a large diameter core according to the present invention has the following additional advantages.

First, since it is insensitive to external force added during the cable process, fine diameter is possible, which increases cable productivity.

Second, the cost for a telecommunications operator to build a line is reduced.

Third, CCED (Compact Cabinet and Enclouse Designs) is possible, which saves track construction space, time and cost (CAPEX (Capital expenditures)).

Fourth, maintenance cost and installation time are reduced by increasing compatibility between different cables.

Fifth, it is possible to design insensitive to the strain added during the operation of the track, thereby reducing maintenance costs (CAPEX reduction).

Sixth, in the conventional lossless bending characteristic optical fiber, the bending loss characteristic is improved by reducing the core diameter, but in the present invention, the large diameter core is adopted together with the application of the trench structure, thereby increasing compatibility between heterogeneous lines.

Seventh, the present invention provides an optical fiber for WDM-PON that can be used in all bands of S (1460 nm to 1530 nm), C (1565 nm to 1625 nm), and L (1565 nm to 1625 nm).

Figure 1 (a) is a view showing a cross section of the optical fiber according to a preferred embodiment of the present invention,
1 (b) is a graph for explaining the refractive index distribution along the optical fiber cross section shown in (a) of FIG. 1,
2A is a diagram illustrating a case where high loss occurs in fusion splicing between optical fibers;
2 (b) is a diagram illustrating a case where a low loss occurs in the fusion connection between optical fibers,
3 is a diagram showing a splice loss occurring in fusion splicing between optical fibers;
4 is a diagram illustrating splice loss according to MFD difference in fusion splicing between optical fibers;
5 shows light intensity distribution in each of a conventional single mode optical fiber, a conventional lossless bending characteristic optical fiber and a lossless bending characteristic optical fiber according to the present invention.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention; In describing the present invention, detailed descriptions of related well-known functions or configurations are omitted in order not to obscure the subject matter of the present invention.

1 (a) is a view showing a cross section of the optical fiber according to a preferred embodiment of the present invention, Figure 1 (b) is for explaining the refractive index distribution along the optical fiber cross section shown in Figure 1 (a) It is a graph. In FIG. 1B, the horizontal axis represents the distance from the center of the optical fiber 100, and the vertical axis represents the refractive index difference Δn. The refractive index difference represents the difference between the refractive index of the corresponding position on the cross section of the optical fiber 100 and the refractive index of the outer layer 140.

The optical fiber 100 includes a core 110 and a clad 115.

The core 110 is positioned at the center of the optical fiber 100, has a constant refractive index higher than that of the clad 115, and transmits an optical signal through total internal reflection. The optical signal refers to light modulated by data, and the modulation scheme may be intensity modulation, frequency modulation, or the like. The optical signal in the core 110 totally reflects at the boundary between the core 110 and the clad 115 and travels along the length direction of the optical fiber 100. The core 110 has the form of a circular rod. Unlike the present embodiment, the core may have a refractive index distribution in which the refractive index increases linearly or nonlinearly from its outer circumference toward its center. The core 110 has the highest refractive index in the optical fiber 100. That is, the highest refractive index is located in the core 110 in the total refractive index distribution along the cross section of the optical fiber 100 (or the distance from the center or optical axis of the optical fiber 100).

The refractive index difference Δn of the core 110 may be in a range of 0.0040 to 0.0060. Preferably, in order to have a large mode field diameter (or effective area), a cutoff wavelength of 1260 nm or less and a zero-dispersion wavelength of 1340 nm or less, The refractive index difference Δn may be in the range of 0.004 to 0.005. The diameter 2a of the core 110 may be in the range of 6.0 μm to 10.0 μm. Preferably, in order to have a large mode field diameter and low bending loss, the diameter 2a of the core 110 may be 8.7 ± 0.4 μm.

The clad 115 is disposed outside the core 110 and has a lower refractive index than the core 110. The clad 115 is directly laminated (or coated) on the outer circumferential surface of the core 110 to completely surround the core 110 along the outer circumference of the core 110. The clad 115 is in the form of a circular tube, and the core 110 and the clad 115 are arranged concentrically.

The clad 115 includes an inner layer 120 having first and second inner sub layers 121 and 122, a trench layer 130, and an outer layer 140. The inner layer 120, the trench layer 130, and the outer layer 140 are sequentially stacked on the outer circumferential surface of the core 110, each having a form of a circular tube, and concentric with the core 110. Is placed.

The inner layer 120 has a refractive index distribution (or profile) that decreases from the inner circumference in contact with the core 110 to the outer circumference. The inner layer 120 has a circular tube shape, and includes first and second inner sub layers 121 and 122 disposed concentrically with the core 110.

The first inner sub layer 121 is disposed outside the core 110 and directly stacked on the outer circumferential surface of the core 110 to completely surround the core 110 along the outer circumference of the core 110. do. The first inner sub layer 121 has a refractive index distribution in which the refractive index decreases linearly from the inner circumference of the core 110 to the outer circumference thereof. Optionally, the first inner sub layer 121 may have a refractive index distribution that is convex upward or concave downward based on the linear refractive index distribution as shown in FIG. 1B. An inner circumferential refractive index of the first inner sub layer 121 is smaller than a refractive index of the core 110 and larger than a refractive index of the outer layer 140. An outer circumferential refractive index of the first inner sub layer 121 is substantially the same as a refractive index of the outer layer 140.

The second inner sublayer 122 is disposed outside the first inner sublayer 121 and completely surrounds the first inner sublayer 121 along the outer circumference of the first inner sublayer 121. So as to be directly stacked on the outer circumferential surface of the first inner sublayer 121. The second inner sublayer 122 has a constant refractive index equal to the outer circumferential refractive index of the first inner sublayer 121. Unlike the present embodiment, the second inner sublayer 122 may be omitted from the layers constituting the optical fiber 100. In this case, the trench layer 130 may be the first inner sublayer 121. It is directly stacked on the outer circumferential surface of the first inner sub-layer 121 so as to completely surround the first inner sub-layer 121 along the outer circumference.

The refractive index difference of the inner layer 120 may be in the range of -0.002 to 0.002. Preferably, in order to minimize light loss due to bending, the refractive index difference of the inner layer 120 may be in the range of 0.0000 to -0.0015. The thickness of the inner layer 120 from a point of contact with the core 110 may be greater than zero and less than or equal to 16 μm, 8.4 μm, or 1.78 × a (a is the radius of the core 110). Preferably, the inner layer 120 may have a thickness greater than 0 and less than or equal to 5.8 μm or 1.2 × a.

The trench layer 130 is disposed outside the second inner sublayer 122 and completely surrounds the second inner sublayer 122 along the outer circumference of the second inner sublayer 122. 2 is laminated directly on the outer peripheral surface of the inner sub-layer 122. The trench layer 130 has a constant refractive index lower than that of the outer layer 140. Unlike the present embodiment, the trench layer 130 has a refractive index distribution that decreases linearly or nonlinearly (concave or convex) from its inner circumference to its outer circumference, or increases from its inner circumference to its outer circumference. It may have a refractive index distribution that increases with redness (concave or convex shape). The trench layer 130 has the lowest refractive index in the optical fiber 100. That is, the lowest refractive index is located in the trench layer 130 in the entire refractive index distribution along the cross section of the optical fiber 100.

The refractive index difference of the trench layer 130 may be in the range of -0.005 to -0.001. The thickness of the trench layer 130 from the inner layer 120 may be 14 μm, or 2 × a (a is the radius of the core 110) or less, preferably 7.2 μm or 1.5 × a or less. .

The outer layer 140 is disposed outside the trench layer 130 and directly on the outer circumferential surface of the trench layer 130 to completely surround the trench layer 130 along the outer circumference of the trench layer 130. Are stacked. The outer layer 140 has a constant refractive index higher than the lowest refractive index of the trench layer 130 and lower than the highest refractive index of the core 110. The outer layer 140 may be made of pure silica glass, and may have a refractive index of 1.456.

FIG. 2A illustrates a case where a high loss occurs in fusion splices between optical fibers, and FIG. 2B illustrates a case where a low loss occurs in a fusion splice between optical fibers. do.

Referring to FIG. 2A, a conventional single mode optical fiber 210 having a core 212 and a clad 214, and a core 222 and clad 224 having a smaller diameter than the single mode optical fiber 210 are shown. A typical lossless bending characteristic optical fiber 220 having is shown. Because the waveguide mode 216 of the single mode optical fiber 210 and the waveguide mode 226 of the conventional lossless bending characteristic optical fiber 220 are inconsistent (measured by overlapping integration of the waveguide modes 216 and 226). , Connection loss occurs. At this time, the waveguide mode represents the intensity distribution of light propagating into the optical fiber.

Referring to FIG. 2B, a lossless bending characteristic optical fiber according to the present invention having a conventional single mode optical fiber 210, a core 232 and a clad 234 having the same diameter as the single mode optical fiber 210 ( 230 is shown.

Since the waveguide mode 216 of the single mode optical fiber 210 and the waveguide mode 236 of the lossless bending characteristic optical fiber 230 coincide, connection loss hardly occurs.

Conventional single mode optical fibers and conventional lossless bending characteristic optical fibers usually have a difference in Mode Field Diameter (MFD) of 5% or more because they have different diameters, effective refractive indices, and the like.

3 is a diagram showing a splice loss occurring in fusion splicing between optical fibers.

 The loss when the optical signal is coupled from the conventional single mode optical fiber to the lossless bending characteristic optical fiber according to the present invention is significantly lower than the loss when the optical signal is coupled from the conventional single mode optical fiber to the conventional lossless bending characteristic optical fiber. Able to know. Further, the loss when the optical signal is coupled from the lossless bending characteristic optical fiber to the conventional single mode optical fiber according to the present invention is significantly lower than the loss when the optical signal is coupled from the conventional lossless bending characteristic optical fiber to the conventional single mode optical fiber. It can be seen that.

Equation 1 shows _splice loss α _splice due to MFD difference.

In Equation 1, w ₁ represents the mode field radius (half of the mode field diameter) of the single mode optical fiber, w ₂ represents the mode field radius of the lossless bending characteristic optical fiber, and d represents the single mode optical fiber and the lossless bending. The distance between the characteristic optical fibers.

4 is a diagram illustrating splice loss according to MFD difference in fusion splicing between optical fibers. As shown, it can be seen that the connection loss increases as the MFD difference increases.

The mode field diameter 2W ₀ of the optical fiber according to the Petermann-II calculation method may be expressed by Equation 2 below.

In Equation 2, λ is the wavelength of the optical signal, 2θ is the divergence angle of the optical signal emitted from the cross section of the optical fiber, a (θ) is (1- (P (θ) / P _MAX )), P (θ ) Denotes the power of the optical signal emitted at an angle of 2θ, and P _MAX denotes the power of the entire optical signal emitted from the optical fiber. In this case, the divergence angle is measured based on the optical axis of the optical fiber (that is, the core center of the optical fiber).

In addition, the mode field diameter MFD of the optical fiber may be represented by Equation 3 below.

In Equation 3, r is the distance in the radial direction from the optical axis of the optical fiber, R (r) is the electric field distribution in the radial direction, 2φ represents the divergence angle of the optical signal emitted from the cross section of the optical fiber.

Referring back to FIG. 1, the MFD of the core 110 according to the Petermann-II calculation method is preferably in the range of 8.7 to 9.6 μm at a 1310 nm wavelength. The difference between the mode field diameter at 1310 nm wavelength by the Petermann-II calculation method of the single mode optical fiber having a refractive index difference of 0.5% or less and the lossless bending characteristic optical fiber 100 of the present invention is within 4.5%. The unidirectional splice loss of optical fibers is 0.2 dB or less. In this case, the specific refractive index difference refers to a value obtained by multiplying a value obtained by dividing the refractive index difference by the core refractive index and expressing it as a percentage.

5 is a diagram showing the light intensity distribution in each of a conventional single mode optical fiber, a conventional lossless bending characteristic optical fiber, and a lossless bending characteristic optical fiber according to the present invention. In Fig. 5, the horizontal axis represents radial distance and the vertical axis represents light intensity. In Fig. 5, the light intensity distribution 320 of a conventional single mode optical fiber with a specific refractive index difference of 0.5% or less and the light intensity distribution 320 of a lossless bending characteristic optical fiber according to the present invention are shown to be coincident, and a typical lossless bending The light intensity distribution 310 of the characteristic optical fiber is shown separately.

Table 1 shows a connection loss result between a conventional single mode optical fiber, a conventional lossless bending characteristic optical fiber and a lossless bending characteristic optical fiber according to the present invention. In Table 1 below, a typical single mode optical fiber is represented by optical fiber 1, a typical lossless bending characteristic optical fiber by optical fiber 2, and a lossless bending characteristic optical fiber according to the present invention as optical fiber 3. For example, when the MFD of the optical fiber 1 is 9.1 and the MFD of the optical fiber 3 is 8.6, it is expressed as (9.1-8.6). For an optical signal with a wavelength of 1310 nm, the forward direction (MFD small to large) represents the splice loss (unit is dB) when the optical signal is combined from the optical fiber 3 with small MFD to the optical fiber 1 with large MFD, and the reverse direction (MFD large to small). ) Represents the splice loss (unit is dB) when the optical signal is combined from the optical fiber 1 having the large MFD to the optical fiber 3 having the small MFD.

When bending is applied to the optical fiber, a leaky mode occurs due to a change in refractive index of the cladding. As a result, the optical loss is increased, and the long distance transmission of the optical signal becomes difficult. Accordingly, the present invention discloses an optical fiber structure that minimizes the change in refractive index of the cladding due to bending.

The optical fiber 100 according to the present invention is bent by including an inner layer 120 having a refractive index distribution whose refractive index decreases from its inner circumference to its outer circumference, and a clad 115 having a trench layer 130 having the lowest refractive index. It is possible to minimize the light loss due to such.

Tables 2 to 4 below illustrate the processes for producing lossless bending characteristic optical fibers according to the present invention. The processes utilize a Modified Chemical Vapor Deposition (MCVD) method based on a substrate tube of φ31 (inner diameter) × φ36 (outer diameter) × l1200 (length) (mm). Thus, these are processes for producing a core preform.

Table 2 below shows an example of a process for producing a core base material.

In Table 2 above, process steps proceed from top to bottom. That is, the polishing step, the outer layer forming step, the trench layer forming step, the sintering step, the inner layer forming step, the core forming step, the sintering step, the collapsing step and the close step are performed. do. The supply unit of raw materials is cc. In the MCVD method, a burner heats the substrate tube while reciprocating between positions corresponding to the first end and the second end of the substrate tube, wherein a pass causes the burner to cause the first tube of the substrate tube to pass through. It means moving once from the end to the second end (or vice versa). For example, 120 + 4.3 / p means that the supply starts at 120cc and increases by 4.3cc per pass. In addition, in the present example and other examples below, the supply amount of the corresponding raw material is increased or decreased in each of the core and trench layer forming steps, but the supply amount of the corresponding raw material may be kept constant.

Table 3 below shows another example of a process for producing the core base material.

Table 4 below shows another example of a process of manufacturing the core base material.

Since the core base material produced by Tables 2 to 4 above has a small diameter, an additional growth process is required to grow the core base material to a predetermined diameter. In a further growth process according to the Outside Vapor Phase Deposition (OVD) method, a soot is deposited around the outside of the core substrate using a deposition torch supplied with raw material and combustion gas. When the soot is deposited so that the core base material has a constant outer diameter and weight, the deposition is terminated and the core base material is slowly cooled and then sintered and vitrified.

The soot is vitrified through sintering and vitrification as an outer clad porous layer. Vitrification may take about 200 minutes at 1500 ° C. in a atmosphere of 0.375 slm of Cl ₂ and 15 slm of He. Sintering can be carried out under a temperature of 1550 ~ 1650 ℃, 1 × 10 ^-2 torr vacuum, it is preferable to add 0 ~ 15slpm of He in the sintering step. The finished final optical fiber base material may have an outer diameter of 90 mm and a length of 1200 mm.

In a further growth process according to the over jacketting method, the core base material is inserted into a glass tube prepared in advance, and the collapsing and closing steps are performed.

An anhydride process may be applied to the core base material and / or the optical fiber base material which has been subjected to the growth process according to the present invention. In this anhydration process, the core base material or the optical fiber base material is placed in a heating furnace and heated at a temperature of 1100 ° C. or higher under a chlorine gas and helium gas atmosphere. According to this anhydration process, the OH group which exists in the said core base material or the optical fiber base material is removed.

The optical fiber base material completed through the growth and anhydration process described above is withdrawn from the draw tower to the optical fiber, and the MFD (@ 1310 nm) of the lossless bending characteristic optical fiber manufactured by the above process is 9.1 ± 0.4 μm. The zero dispersion wavelength may have a value of 1340 nm or less.

When one bending is applied with a diameter of 20 mm, the lossless bending characteristic optical fiber has an optical loss value of 0.01 dB or less at 1310 nm, an optical loss value of 0.02 dB or less at 1380 nm, and 1550. It may have a light loss value of 0.05 dB or less at nm, and a light loss value of 0.14 dB or less at 1625 nm. In addition, when one bending is applied with a diameter of 7.5 mm, the lossless bending characteristic optical fiber may have a light loss value of 0.5 dB or less at 1550 nm, and a light loss value of 1.0 dB or less at 1625 nm. have. In addition, in the case where one rotation of a radius of 10 mm is applied, the lossless bending characteristic optical fiber may have a light loss value of 0.1 dB or less at a wavelength of 1550 nm, and a light loss value of 0.2 dB or less at a wavelength of 1625 nm.

Further, the anhydrous (or anhydrous) lossless bending characteristic optical fiber of the present invention according to the anhydration process may be an anhydrous lossless bending characteristic optical fiber whose light loss value at 1383 nm wavelength is smaller than the light loss value at 1310 nm wavelength. have. The lossless bending characteristic optical fiber may be an anhydrous lossless bending characteristic optical fiber having an optical loss value of 0.35 dB / km or less at a 1383 nm wavelength. The anhydrous lossless bending characteristic optical fiber permanently eliminates water peaks (ie, loss peaks due to moisture) due to light scattering of OH groups in the E-band (1360 nm to 1460 nm), thereby reducing overall optical loss. It has the advantage of guaranteeing and enabling additional bandwidth extension. The dehydrated lossless bending fiber has 33% channel count, 50% wavelength band, uniform low loss and dispersion optimization, enhanced PMD (Polarization Mode Dispersion) and dispersion with flexibility in network design changes, and Provides excellent compatibility with single mode fiber. In addition, the anhydrous lossless bending characteristics of the optical fiber can provide long-term reliability improvement through the hydrogen penetration prevention treatment and durability enhancement through a high performance double acrylic coating.

100: lossless bending characteristic optical fiber, 110: core, 120: clad, 121: first inner sublayer, 122: second inner sublayer, 130: trench layer, 140: outer layer

Claims (12)

In optical fiber,
A core;
An inner layer disposed outside the core and having a refractive index lower than that of the core;
And a trench layer disposed outside the inner layer, the trench layer having a lowest refractive index. The method of claim 1,
And an outer layer disposed outside the trench layer and having an index of refraction higher than the lowest index of refraction of the trench layer and lower than the index of refraction of the core. The method of claim 1,
And the refractive index of the inner layer decreases away from the core. The method of claim 1,
The mode field diameter of the core is a lossless bending characteristic optical fiber, characterized in that in the range of 8.7 ~ 9.6㎛ at 1310nm wavelength. The method of claim 1,
Lossless bending characteristic optical fiber, characterized in that the difference in refractive index of the core is in the range of 0.004 ~ 0.006. The method of claim 1,
Lossless bending characteristic optical fiber, characterized in that the diameter of the core is in the range of 6.0㎛ ~ 10.0㎛. The method of claim 1,
The lossless bending characteristic optical fiber, characterized in that the refractive index difference of the inner layer is in the range of -0.002 ~ 0.002. The method of claim 1,
The lossless bending characteristic optical fiber, characterized in that the thickness of the inner layer is greater than 0 and less than 16㎛. The method of claim 1,
The lossless bending characteristic optical fiber, characterized in that the difference in the refractive index of the trench layer is in the range of -0.005 ~ -0.001. The method of claim 1,
And the thickness of the trench layer is greater than zero and less than 14 μm. The method of claim 1,
In the case where one rotation of a radius of 10 mm is applied, the lossless bending characteristic optical fiber has a light loss value of 0.1 dB or less at a wavelength of 1550 nm, and a low bend characterized by a light loss value of 0.2 dB or less at a wavelength of 1625 nm. Loss of optical fiber. The method of claim 1,
The lossless bending characteristic optical fiber is a low bending loss optical fiber, characterized in that the optical loss value of less than 0.35 dB / km at 1383nm wavelength.

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