KR100554423B1 - method of controlling refractive index of optical fiber preform in MCVD and optical fiber made by the method - Google Patents

Abstract

The refractive index control method of the optical fiber base material in the MCVD method includes a method of manufacturing an optical fiber base material having a constant refractive index along the longitudinal direction of the optical fiber by controlling the amount of oxygen and helium in the reaction gas injected into the quartz tube.

Preferably, the flow rate ratio of the total reaction gas and oxygen in the quartz tube is 1.2 to 1.4, wherein the flow rate of the oxygen includes 1000 to 4000 sccm. In addition, the flow rate ratio of oxygen and helium in the quartz tube is 4.0 to 6.0, and the flow rate of the helium at this time includes 400 to 800 sccm.

In addition, according to another aspect of the present invention there is provided a single-mode or multi-mode optical fiber manufactured by the method defined above.

Reactive gas, oxygen, helium

Description

Method of controlling the refractive index of the optical fiber base material in the crystal chemical vapor deposition method and the optical fiber manufactured by the method {method of controlling refractive index of optical fiber preform in MCVD and optical fiber made by the method}

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 view showing a process of depositing particles in a quartz tube in the MCVD method.

2 is a view showing the collapse process of the optical fiber base material in the MCVD method.

3 is a view showing a cutting process of the optical fiber base material in the MCVD method.

4 is a graph showing the change of the refractive index along the longitudinal direction of the optical fiber in the MCVD method.

5 is a view showing the effect of the reaction gas POCl ₃ injected into the quartz tube in the MCVD method on the particles.

The present invention shows a method for adjusting the refractive index of the optical fiber base material in the crystal chemical vapor deposition method and the optical fiber manufactured by the method, more specifically, the reaction gas flowing into the quartz tube during the production of the single-mode or multi-mode fiber base material The present invention relates to a method for controlling the refractive index of an optical fiber base material in a modified chemical vapor deposition method which can minimize the change in the longitudinal refractive index of the base material by controlling the flow rate and the flow rate of the base material.

The most widely used method of manufacturing the optical fiber base material is the Modified Chemical Vapor Depositon (hereinafter referred to as MCVD) method. The MCVD process is largely divided into three stages: deposition, collapse and edge cutting.

1 to 3 sequentially show three steps of the MCVD method, and FIG. 1 shows a deposition process. The deposition process injects the chemical gas 20 into the quartz tube 10 rotating at a speed of 40 to 80 rpm, and slowly transfers the exterior of the quartz tube 10 in which the hydrogen / oxygen torch 30 rotates in the rotational axis direction. Heating to induce the reaction. Then, the chemical gas 20 flowing through the inside of the quartz tube 10 is heated near the front of the torch 30 to reach the reaction temperature, and is deposited on the inner wall of the quartz tube 10 while causing an oxidation reaction. . As the torch 30 is transferred once, a layer of particles 40 is formed, and in order to manufacture an optical fiber base material having a desired thickness, for example, an optical fiber base material having a desired refractive index, the process of depositing particles while changing the composition of a chemical gas in each layer. Will be repeated several times.

The quartz tube 10 having the core deposition layer manufactured through the above process is then subjected to a Collapse process. The collapse process is such that the torch 30, which is slowly transported in the axial direction of the quartz tube 10, as shown in FIG. 2, heats the quartz tube to a temperature of about 2000 to 2300 ° C. As a result, the inner and outer diameters of the quartz tube 10 are gradually reduced by the viscous flow in the quartz tube and the pressure difference and the surface tension of the inner and outer walls at high temperatures. When this process is repeated several times, the space existing inside the quartz tube 10 is completely removed to complete the final optical fiber base material in the form of a quartz rod, and the final device is provided with a heating apparatus having a heating furnace 32 as shown in FIG. The optical fiber is manufactured.

As described above, since the MCVD method is a process of depositing a reactant by a temperature difference in the quartz tube, the deposition efficiency in the longitudinal direction varies according to not only the temperature distribution inside the quartz tube but also the chemical gas flow rate inside the tube.

That is, as the length of the quartz tube increases, the deposition layer formed by the thermal reaction becomes nonuniform, and the nonuniformity of the deposition layer causes a problem of deteriorating optical characteristics of the optical fiber. For example, the change in refractive index with tube length affects the Mode Field Diameter (MFD) and Cutoff Vavelength (λc) for single mode, and the numerical aperture for multimode fiber. NA) and bandwidth are sensitive to changes in refractive index distribution, thereby degrading characteristics of the optical fiber.

 The present invention has been made to solve the above problems, by controlling the flow rate and the internal temperature of the reaction gas by adjusting the amount of oxygen and helium in accordance with the flow rate of the reaction gas injected into the quartz tube, the longitudinal direction of the optical fiber It is an object of the present invention to provide a refractive index control method of an optical fiber base material in an MCVD method capable of minimizing refractive index fluctuations.

In order to achieve the above object, the refractive index control method of the optical fiber base material in the MCVD method according to the present invention is to adjust the amount of oxygen and helium in the reaction gas injected into the quartz tube to the optical fiber base material having a constant refractive index along the longitudinal direction It includes a method of manufacturing.

Preferably, the flow rate ratio of the total reaction gas and oxygen in the quartz tube is 1.2 to 1.4, wherein the flow rate of the oxygen includes 1000 to 4000 sccm. In addition, the flow rate ratio of oxygen and helium in the quartz tube is 4.0 to 6.0, and the flow rate of the helium at this time includes 400 to 800 sccm.

According to another aspect of the present invention, the present invention is a method for producing an optical fiber base material having a constant refractive index in the longitudinal direction using the MCVD method is a flow rate ratio of the reaction gas and oxygen injected into the quartz tube is 1.2 ~ 1.4, oxygen and helium The ratio of the flow rate is adjusted to 4.0 to 6.0, including a method of manufacturing an optical fiber base material having a constant refractive index along the longitudinal direction.

Preferably, the flow rate of oxygen is 1000 to 4000 sccm, the flow rate of helium includes 400 to 800 sccm.

In addition, according to another aspect of the present invention there is provided a single-mode or multi-mode optical fiber manufactured by the method defined above.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms or words used in the present specification and claims should not be construed as being limited to the common or dictionary meanings, and the inventors should properly explain the concept of terms in order to best explain their own invention. Based on the principle that can be defined, it should be interpreted as meaning and concept corresponding to the technical idea of 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.

Looking at the deposition process of the MCVD method described in the prior art in more detail in conjunction with Figure 1, the chemical reaction of the SiCl ₄ , GeCl ₄ , POCl ₃ and O ₂ injected into the quartz tube 10 is represented by the following formula Same as 1.

SiCl ₄ (g) + O ₂ ↔ SiO ₂ (S) + 2Cl ₂

GeCl ₄ (g) + O ₂ ↔ GeO ₂ (S) + 2Cl ₂

2POCl ₃ (g) + 3 / 2O ₂ ↔ P ₂ O ₅ (S) + 3Cl ₂

In the above formula, SiO ₂ particles are an element that determines the diameter of the optical fiber core, GeO ₂ is an element added to adjust the refractive index of the optical fiber core. In addition, P ₂ O ₅ to lower the sintering temperature of the reaction particles by about 200 ~ 400 ℃ to facilitate the manufacture of the optical fiber base material. Although not shown in Chemical Formula 1, helium gas additionally added serves to radially diffuse the temperature distribution inside the quartz tube 10.

According to the present invention, the volume ratio of the reaction gas and oxygen injected into the quartz tube 10 may be expressed as q / Q. Where q is the total flow rate of the reaction gas flowing into the quartz tube 10 and Q is the flow rate of the oxygen gas flowing into the quartz tube 10. In addition, the ratio of oxygen and helium gas is represented as Q / α, where α represents thermal diffusivity of the reaction gases.

Preferably, the volume ratio (q / Q) of the reaction gas and oxygen of the multimode optical fiber in the preparation of the optical fiber base material using the MCVD process has a value of 1.2 to 1.4, and the ratio of oxygen to helium gas (Q / α) is determined and its value is 4.0 to 6.0. Because, the reaction gas injected into the quartz tube 10 is heated by the torch 30 installed outside the quartz tube 10 to generate the reaction particles 42, the movement speed of the generated reaction particles is oxygen This is because the temperature uniformity in the radial direction is ensured by the flow rate, and the helium flow rate. When q / Q, the flow rate ratio of the total reaction gas and oxygen, is 1.4 or more, that is, when the flow rate of oxygen is smaller than that of the total reaction gas, the oxidation reaction does not sufficiently occur inside the tube, thereby reducing the reaction efficiency. On the contrary, if the flow rate of oxygen is large, there is no problem in manufacturing, but the unevenness in the longitudinal direction of the optical fiber base material becomes severe. As a result, according to the present invention, the oxygen gas injected into the quartz tube is preferably 1000 to 4000 sccm, and the helium gas is preferably 400 to 800 sccm.

The influence of the α value representing the thermal diffusivity of the reaction gases on the optical properties of the optical fiber base material will be further discussed. In general, in the case of a single mode optical fiber, the mode field diameter is manufactured to have a value between 9.0 and 10.0 μm, and the blocking wavelength is 1310 nm or less. In the case of a multimode optical fiber, the core diameter is 60.0 to 65.0 µm and 47.5 to 52.5 µm, and the numerical aperture is 0.260 to 0.290 and 0.185 to 0.215.

Since the single mode optical fiber has a stepped refractive index distribution, optical properties such as mode field diameter and blocking wavelength are determined by the diameter of the core and the refractive index height. In the case of a multimode optical fiber, since it has a hill-shaped refractive index distribution in which the refractive index changes in the radial direction, the optical properties are greatly influenced by the diameter of the core and the gradient of the refractive index change in the center of the core. For example, the core refractive index structure of the multimode optical fiber is represented by Equation 1 below.

In the above formula, a represents a radius of the core and α represents a degree of inclination of the change in refractive index. When α is 1, it has a triangular refractive index distribution, and in infinity, it exhibits a stepped refractive index distribution like a single mode optical fiber. Here, Δ is a relative refractive index difference between the core and the clad, which is represented by Equation 2 below.

Therefore, in the multimode optical fiber, the bandwidth, which is a transmission characteristic of the optical fiber, is determined by the α value, and the α value changes in the longitudinal direction of the optical fiber because the deposition form of the MCVD method changes in the longitudinal direction of the quartz tube. In addition, the change of the bandwidth causes the gamma (γ) value, which is an index indicating the longitudinal reliability of the multimode optical fiber, to decrease.

Here, the gamma value indicates the dependence of the bandwidth characteristics in the multimode optical fiber, which is an index indicating the degree of nonuniformity in the longitudinal direction of the optical fiber, and is expressed by Equation 3 below.

As defined in the above equation, the bandwidth of the multimode optical fiber changes according to the length of the optical fiber, and in general, the longer the length of the optical fiber is, the better the bandwidth characteristic is due to the unevenness caused by the longitudinal direction of the optical fiber core portion. The dependence of the bandwidth on the optical fiber length is expressed by the gamma value of Equation 3, and in the case of optical fiber manufacturing by the MCVD method, the gamma value is generally preferably between 0.5 and 1.0. If the gamma value is 1.0, the bandwidth characteristic, which represents the transmission characteristic, becomes independent of the optical fiber length, which means a uniform optical fiber. In the present invention, when manufacturing a multi-mode optical fiber using the MCVD method, the gamma value can be manufactured to be close to 1.0 by controlling the nonuniformity in the longitudinal direction of the optical fiber core to the flow rate of the reaction gases in the quartz tube.

Figure 4 shows the change in the longitudinal direction of the α value which is the refractive index parameter of a general multimode optical fiber. Conventional multi-mode optical fibers exhibit a change in trajectory in the longitudinal direction as in the form of (A), (B) and (C), in which case the α value changes as shown in Equation 4 below.

In addition, in FIG. 4, the trajectory change (D) shows the longitudinal distribution of the optimized α value. When the multimode optical fiber has the α value changing in the longitudinal direction as shown in Equation 4, the average α value is It is calculated as in Equation 5.

When the average value α calculated in Equation 5 is equal to the optimized value α of FIG. 4, the bandwidth value is reduced to 2/3 of the bandwidth value when there is no change in refractive index in the longitudinal direction.

5 is a view showing the effect of the reaction gas POCl ₃ introduced into the quartz tube 10 in the MCVD method on the particles. Referring to this, reference numeral 50 denotes a reaction zone in which an oxidation reaction occurs in the quartz tube 10, and the particles 42 reacted in the reaction zone 50 draw various trajectories. It is deposited on the inner circumferential surface of the tube. For example, the particle trajectories 31 and 32 are when the reaction gas POCl ₃ is not injected into the tube, and the particle trajectory 33 is the trajectory of the particles when POCl ₃ participates. POCl ₃ is added to lower the sintering temperature of the reaction particles in the preparation of the multimode optical fiber base material, and has the effect of increasing the size of the particles and making the particle shape uniform.

Example

In this embodiment, when the flow rate of the reaction gas inside the quartz tube is constant when manufacturing the core of the optical fiber base material using the MCVD process, the amount of change in the alpha value, which is the height of the refractive index, the core diameter, and the refractive index gradient, depends on the amount of oxygen and helium. Was tested. When the inner diameter of the tube used as the base material is 27mm, the change according to the amount of oxygen and helium is shown in Table 1 below.

q / Q  Core diameter (㎛) Refractive Index (× 10 ^-3 ) α 1.2 0.263 1.6 0.8 1.3 0.028 0.6 0.8 1.4 0.056 1.1 1.7

In Table 1 q is the total volume of the reaction gas flowing into the quartz tube, Q is the flow rate of oxygen. As can be seen from the above table, it can be seen that the flow rate of the total gas in the quartz tube and the oxygen ratio are close to 1.0 × 10 ⁻³ , which is a preferable refractive index deviation when the ratio is 1.2 to 1.4. If the ratio of oxygen is 1.3, it can be seen that the diameter change width of the core is the smallest in the longitudinal direction.

In addition, Table 2 below shows the change in the core diameter and refractive index of the optical fiber according to the helium flow rate when the flow rate of oxygen is 3200sccm.

Q / α  Core diameter (㎛) Refractive Index (× 10 ^-3 ) α 2.13 0.580 1.40 0.12 5.12 0.410 1.00 0.10 10.67 0.650 0.80 0.13

In Table 2, Q is oxygen flow rate, and (alpha) shows the thermal diffusion degree inside a quartz tube. As can be seen from the above table, when the oxygen gas inside the quartz tube is 3200sccm, Q / α of 5.1 shows that the refractive index and the diameter of the core have the smallest change in the longitudinal direction.

Consequently, according to the embodiment of the present invention, when the ratio of the gas inside the quartz tube in the MCVD process is q / Q is 1.3 and Q / α is 5.1, the optical fiber core diameter and the refractive index change width in the longitudinal direction are the most wide. It can be seen that less.

As described above, the optical fiber base material manufactured by controlling the flow rate of the reaction gas, oxygen, and helium is manufactured into an optical fiber through a collapse process and a cutting process. Since the optical fiber manufacturing process is described in the related art, a separate description is omitted.

As mentioned above, although this invention was demonstrated by the limited embodiment and drawing, this invention is not limited by this and is within the equal range of a common technical idea in the technical field to which this invention belongs, and a claim to be described below. Of course, various modifications and variations are possible.

According to the refractive index control method of the optical fiber base material and the optical fiber manufactured by the method in the MCVD method of the present invention, the flow rate of oxygen and helium is adjusted according to the flow rate of the reaction gases injected into the quartz tube, By adjusting the flow rate and the internal temperature of the reaction gases according to the flow rate it can be made an optical fiber with the minimum refractive index variation in the longitudinal direction.

Claims (9)

delete In the method for producing an optical fiber base material having a constant refractive index in the longitudinal direction using the MCVD method, While controlling the amount of oxygen and helium in the reaction gas injected into the quartz tube, q / Q value (q: total flow rate of the reaction gas, Q: oxygen gas representing the ratio of the total reaction gas and oxygen flow rate inside the quartz tube) Method of manufacturing an optical fiber base material having a constant refractive index by adjusting the flow rate of the resin to 1.2 to 1.4. The method of claim 2, The flow rate of the oxygen is 1000 ~ 4000 sccm The method of manufacturing a fiber base material having a constant refractive index. The method of claim 2, A Q / α value (Q: flow rate of oxygen gas, α: thermal diffusivity of reactant gas) representing an oxygen-helium flow rate ratio in the quartz tube is 4.0 to 6.0. The method of claim 4, wherein The flow rate of the helium is 400 ~ 800 sccm The method of manufacturing a constant refractive index optical fiber base material. delete In the method for producing an optical fiber base material having a constant refractive index in the longitudinal direction using the MCVD method,  The amount of oxygen and helium in the reaction gas injected into the quartz tube is controlled, but the Q / α value (Q: oxygen gas flow rate, α: thermal diffusivity of the reaction gas) representing the flow rate ratio of oxygen and helium is 4.0 to 6.0. Adjust it, At this time, the flow rate of the oxygen is 1000 ~ 4000 sccm, the flow rate of helium is 400 ~ 800 sccm characterized in that the method for producing a constant optical fiber base material. An optical fiber made by the method as defined in any one of claims 2 to 5 and 7. The method of claim 8, The optical fiber has a gamma value of 0.5 to 1.0 indicating dependence of a bandwidth characteristic.

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