Ultra-low-loss optical fiber, and method and apparatus for producing same

Abstract

This ultra-low-loss optical fiber comprises a core having a higher relative refractive index difference than silica and a cladding having a lower relative refractive index difference than silica. The relative refractive index difference of the core with respect to the refractive index of silica is 0.0030 to 0.0055, for example, and the relative refractive index difference of the cladding with respect to the refractive index of silica is 0.0020 to 0.0003. The ultra-low-loss optical fiber has the loss characteristic of simultaneously having optical losses of at most 0.324 dB/km at a wavelength of 1310 nm, at most 0.320 dB/km at a wavelength of 1383 nm, at most 0.184 dB/km at a wavelength of 1550 nm, and at most 0.20 dB/km at a wavelength of 1625 nm. The ultra-low-loss optical fiber is supercooled when the surface temperature of the optical fiber has a temperature range in a glass transition section during drawing.

Claims

1. A method for manufacturing an ultra-low loss optical fiber, the method comprising the steps of: (a) drawing a fused optical fiber preform into an optical fiber; (b) supercooling the optical fiber drawn in the step (a) while maintaining a surface temperature of the optical fiber within a temperature range in a glass transition section; and (c) additionally cooling the optical fiber supercooled in the step (b), wherein, between the step (a) and the step (b), the surface temperature of the drawn optical fiber is maintained at a temperature of not less than an upper limit of the temperature range in the glass transition section.

2. The method of claim 1, wherein, in the step (b), the temperate range of the glass transition section is from 1000 C. to 1300 C.

3. The method of claim 2, wherein, in the step (b), the supercooling is performed at a cooling speed of not less than 3500 C./s.

4. An apparatus comprising: a furnace that fuses an optical fiber preform and draws the fused optical fiber preform into an optical fiber; a supercooling unit that supercools the optical fiber fused and drawn by the furnace while maintaining a surface temperature of the optical fiber within a temperature range in a glass transition section; a cooler that additionally cools the optical fiber cooled by the supercooling unit; and a unit that moves a location of the supercooling unit such that the surface temperature of the optical fiber introduced into the supercooling unit has a temperature higher than an upper limit of the temperature range of the glass transition section, according to a drawing speed of the optical fiber.

5. An ultra-low loss optical fiber manufactured through the method of claim 1, the ultra-low loss optical fiber comprising: a core having a relative refractivity difference of not less than 0.0030 and not more than 0.0055 with respect to silica; and a cladding arranged outside the core and having a low relative refractivity difference with respect to silica, wherein a relative refractivity difference of the cladding with respect to silica is not less than 0.0020 and not more than 0.0003.

6. An ultra-low loss optical fiber manufactured through the method of claim 1, the ultra-low loss optical fiber comprising: a core having a relative refractivity difference of not less than 0.0030 and not more than 0.0055 with respect to silica; and a cladding arranged outside the core and having a low relative refractivity difference with respect to silica, wherein a ratio (D/d) of a diameter (d) of the core to a diameter (D) of the cladding is not less than 3.0.

Description

DESCRIPTION OF THE INVENTION

(1) FIG. 1 is a graph depicting a relationship between a volume of glass and a temperature of the glass in a generally known process of cooling the glass;

(2) FIG. 2 is a view illustrating the concept of a method for manufacturing an optical fiber according to the present invention;

(3) FIG. 3 is a view schematically illustrating an apparatus for manufacturing an optical fiber according to the present invention; and

(4) FIG. 4 is a view illustrating a refractive index profile of an optical fiber according to the present invention.

BEST MODE

(5) Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(6) FIG. 1 is picture illustrating a change in a volume of glass according to a change in temperature in a process of rapidly cooling glass that has been sufficiently melt at a high temperature. As illustrated, the change rate of the volume rapidly changes at a specific point of inflection (a glass fictive temperature, T.sub.f). That is, the volume of the glass rapidly changes as the temperature of the glass decreases at a temperature higher than the glass fictive temperature T.sub.f, and the change rate significantly decreases at a temperature lower than the glass fictive temperature T.sub.f. In general, because a degree of density fluctuation does not change at a temperature of not more than the glass fictive temperature T.sub.f, this state is defined as a frozen-in state.

(7) The glass fictive temperature T.sub.f is closely relevant to the cooling speed of glass, and the glass fictive temperature T.sub.f increases as the cooling speed of the glass becomes higher. In FIG. 1, T.sub.f1 denotes a glass fictive temperature in the case of slow cooling, and T.sub.f2 is a glass fictive temperature in the case of fast cooling. The glass transition region is defined as a region between a glass fictive temperature T.sub.f,fast of the glass in the case of fast cooling and a glass fictive temperature T.sub.f,slow of the glass in the case of slow cooling, and in the region, the volume change rate according to a change in temperature is not strictly linear. It is known that the transition region of silica glass is approximately between 1000 C. and 1300 C.

(8) The present invention rapidly decreases the volume of glass by additionally rapidly cooling the glass in a glass transition region that is a step shortly before the glass structure reaches a completely frozen-in state, thereby shifting the phase of the glass such that the glass follows a volume change rate for the temperature change of a low cooling curve in a cooling curve. Accordingly, an effect that is achieved when slow cooling is performed due to low-speed drawing may be obtained while high-speed drawing is performed. As a result, the glass fictive temperature T.sub.f that is closely relevant to Rayleigh scattering is moved to a temperature T.sub.ff that is a temperature lower than the initial glass fictive temperature T.sub.fi so that Rayleigh scattering losses may be reduced and thus an ultra-low loss may be achieved. The concept view of a phase shift by supercooling is illustrated in FIG. 2. Although an optical fiber in state C1 illustrated in FIG. 2 is cooled into state C2 according to the related art, an optical fiber in state C2 is cooled into state C4 via state C3 corresponding to a supercooling process performed in a glass transition region.

(9) FIG. 3 schematically illustrates an apparatus for manufacturing an optical fiber according to the present invention. The supercooling apparatus 200 for supercooling the drawn optical fiber is situated immediately below an electric furnace 100. A temperature of the optical fiber that enters the supercooling apparatus 200 should be in a temperature range of the glass transition region. In the case of a general silica optical fiber, because the glass transition region is approximately from 1000 C. to 1300 C., the temperature of the optical fiber that enters the supercooling apparatus 200 falls within the temperature range by adjusting the location of the supercooling apparatus 200 according to the drawing speed of the optical fiber and the temperature of the optical fiber discharged from the electric furnace 100. When the temperature of the optical fiber that enters the supercooling apparatus 200 is lower than the temperature corresponding to the glass transition region, a phase shift in a cooling curve does not occur because the optical fiber enters the supercooling apparatus 200 while the glass structure does not have density fluctuation. That is, the glass fictive temperature T.sub.f cannot be lowered.

(10) The temperature of a general optical fiber furnace 100 varies according to the size and the drawing speed of the optical fiber preform 10, but falls within approximately 2000 C. to 2300 C. The optical fiber softened through sufficient dissipation of heat within the electric furnace 100 is extracted to the outside of the electric furnace 100 through driving and tensioning of a capstan 500. The optical fiber drawing tension is determined according to the temperature and the drawing speed of the electric furnace 100, and the value is approximately from 100 g to 200 g. The surface temperature of the optical fiber drawn to the outside of the electric furnace 100 varies according to the temperature of a heater in the electric furnace 100, the type and the flow rate of used gas, the internal structure of the electric furnace, and the drawing speed of the optical fiber. Generally, when the optical fiber of a diameter of 125 m is drawn, the surface temperature of the optical fiber is 1200 C. to 1400 C.

(11) The optical fiber drawn to the outside of the electric furnace 100 is immediately exposed to air to undergo a rapid cooling process, and enters a coater 400 that coats the outer side of the optical fiber for polymer coating while being sufficiently cooled (generally, not more than 60 C.) via a cooling process, after entering a cooler 300 at a lower end of the electric furnace 100.

(12) While the optical fiber enters an optical fiber coolera cooler generally used by the medium of a helium gasat a lower end of the electric furnace 100, the temperature of the optical fiber changes within a temperature range of the glass transition region. The present invention includes a process of supercooling the optical fiber by using a supercooling apparatus 200 that is a rapid cooling facility in the transition process. Then, the cooling speed of the optical fiber varies according to the drawing speed of the optical fiber, but should be a minimum of not less than 3500 C./s. When the cooling speed of the optical fiber is not sufficiently high, a phase shift by supercooling does not occur so that the glass fictive temperature cannot be lowered, making it impossible to lower the loss of the optical fiber. The supercooling apparatus 200 should be arranged at a location over the glass transition region or within the range, and it is suitable to adjust the location upwards and downwards according to the drawing speed of the optical fiber.

(13) The Rayleigh scattering loss .sub.d caused by density fluctuation of silica glass has the following relationship with the glass fictive temperature T.sub.f.

(14) $\begin{array}{cc}{a}_d=\frac{8{}^3}{3{}^4}{n}^8{p}^2{\mathrm{kt}}_f{}_T& \mathrm{Equation}1\end{array}$

(15) Here, denotes a wavelength, n denotes a refractive index, p denotes a photoelastic constant, and .sub.T is an equilibrium isothermal compressibility at a glass fictive temperature T.sub.f. As can be seen from the relationship, as refractive index increases (in a general optical fiber, as the amount of GeO.sub.2, which is a heterogeneous material, increases in silica glass) and the glass fictive temperature T.sub.f increases, Rayleigh scattering loss increases. Accordingly, in order to obtain a low loss optical fiber, it is necessary to lower a relative refractive index or a glass fictive temperature T.sub.f. S. Sakaguchi, S. Todoroki, T. Murata, J. Non-Cryst. solids 220 (1997), page 178 shows through an experimental result that the glass fictive temperature T.sub.f and the Rayleigh scattering loss have a linear relationship and asserts that Rayleigh scattering may be controlled by controlling the glass fictive temperature T.sub.f. Further, K. Tsujikawa, K. Tajima, M. Ohashi, J. Lightwave Technology Vol. 15 No. 11 (2000), page 1528 experimentally proves that the glass fictive temperature T.sub.f may be lowered by drawing an optical fiber at a low temperature and a low speed in an evaluation of a silica-based optical fiber and as a result, the Rayleigh scattering loss may be reduced. Furthermore, K. Saito, A. J. Ikushima, T. Ito, A. Ito J. Applied Physics Vol. 81 No. 11 (1997), page 7129 experimentally proves that a structural relaxation of silica glass may be induced by adding a fine amount of alkali or alkaline earth metal to silica glass, making it possible to lower the glass fictive glass T.sub.f, and asserts that an ultra-low loss optical fiber may be manufactured by using the result.

(16) In the drawing process, a tensile force is applied to the optical fiber, and the tensile force is a physical quantity that is inversely proportional to drawing temperature and proportional to speed. The tensile force applied to an optical fiber changes the refractive index of the optical fiber, and as the tensile force increases, the refractive index of a core having a glass composition of SiO.sub.2GeO.sub.2 and the refractive index of a cladding having a glass composition of SiO.sub.2F. Further, a rapid volume contraction occurring in the drawing process applies a compressive stress to the optical fiber, which also causes an increase in the relative refractive index with respect to silica of the core. Accordingly, when the relative refractive index of the optical fiber core with respect to silica, relative refractive index increases due to tension and rapid cooling in the drawing process, and as a result, an effect of reducing loss by supercooling is reduced by increasing a Rayleigh scattering loss due to a high refractive index. The present invention limits a suitable relative refractive index of the optical fiber core with respect to silica to a range of 0.0030 to 0.0055 with a positive relative refractive index difference (+).

(17) It is known that the refractive index of a cladding that is a layer immediately over the core that guides an input optical signal is effective to curving characteristics when the cladding has a relative refractive index that is lower than that of silica. However, when the cladding has a very low refractive index, it is difficult for optical characteristics, such as mode field diameter, zero dispersion, and cut-off wavelength, to satisfy all the optical fiber requirements (ITU-T requirements). Accordingly, it is necessary to design a refractive index of a cladding that satisfies curving characteristics while minimizing an influence on the optical characteristics. The present invention limits a relative refractive index with respect to silica of a cladding to a range of 0.0020 to 0.0003 with a negative relative refractive index ().

(18) In order to minimize an influence of an OH-absorption loss that is a loss of a specific wavelength band by curving characteristics of an optical fiber and a hydroxyl group, a ratio D/d of the diameter D of an optical fiber core and the diameter D of a cladding should exceed a predetermined level, and the present invention limits the radio D/d to not less than 3.0.

(19) FIG. 4 illustrates the aforementioned profile of a refractive index of an optical fiber according to the present invention.

(20) A result obtained by measuring optical characteristics of an ultra-low loss optical fiber manufactured according to the embodiment of the present invention.

(21) In relation to optical loss characteristics, the optical fiber loss was not more than 0.32 dB/km at a wavelength of 1310 nm, not more than 0.31 dB/km at a wavelength of 1383 nm, not more than 0.18 dB/km at a wavelength of 1550 nm, and not more than 0.20 dB/km at a wavelength of 1625 nm.

(22) In relation to the bending characteristics, when the optical fiber was wound on a diameter of 10 mm once, the bending loss was 0.75 dB at a wavelength of 1550 nm and 1.5 dB at a wavelength of 1625 nm, and when the optical fiber was wound on a diameter of 15 mm ten times, the bending loss was 0.25 dB at a wavelength of 1550 nm and 1.0 dB at a wavelength of 1625 nm.

(23) Further, the zero dispersion was not less than 1300 nm and not more than 1320 nm, the interruption wavelength was not less than 1150 nm and not more than 1330 nm, and the mode field diameter at a wavelength of 1310 nm was not less than 8.8 m and not more than 9.6 m.

(24) Further, the ultra-low loss optical fiber manufactured according to the embodiment of the present invention has an MAC value (MFD/cut-off wavelength) that is a ratio of a mode field diameter of 1310 nm to an interruption wavelength, of not more than 7.5.

(25) Table 1 compares an example of the present invention with an existing example.

(26) TABLE-US-00001 TABLE 1 Wave- length Example 1 Example 2 Example 3 Example 4 Optical 1310 nm 0.327 0.319 0.319 0.313 character- 1383 nm 0.291 0.27 0.269 0.271 istics for 1550 nm 0.189 0.182 0.181 0.177 wavelengths 1625 nm 0.203 0.192 0.193 0.19 (dB/km) Zero dispersion (nm) 1312 1309 1308 1311 Interruption wavelength 1228 1254 1255 1248 (nm) MFD (m) 9.23 9.26 9.43 9.4 Drawing speed (m/s) 30 30 30 15

(27) Table 1 compares a general condition with a result by the present invention. The optical fiber preforms used in the comparison were manufactured through the same method. In order to compare and verify the effects, the same drawing tower drew optical fibers while only coolers are different. Although four coolers using a helium gas as a process gas was used in an existing drawing condition, a cooler that may perform supercooling according to the present invention was used instead of one helium cooler on the upper side in a condition according to the present invention.

(28) In Table 1, Example 1 represents an optical fiber manufactured by drawing an optical fiber in an existing condition. Examples 2 and 3 illustrate optical fibers manufactured according to the present invention. Examples 2 and 3 apply the same furnace conditions (a temperature, a gas condition, and the like) as those of Example 1, and the drawing speed is 30 m/s. Example 4 corresponds to a result obtained by drawing an optical fiber while lowering the drawing speed to 15 m/s in the apparatus according to the present invention. In the optical characteristics shown in the optical fiber obtained by applying the same process conditions except for the supercooling equipment according to the present invention, Examples 2 and 3 showed remarkably low losses at all wavelength bands as compared with Example 1. As described in relation to FIG. 1, Example 4, in which the drawing speed is lowered, clearly shows that scattering loss decreases as a low glass fictive temperature becomes lower according to slow cooling.

(29) When the optical characteristics of the optical fibers manufactured through the examples are compared, in Examples 2 and 3 in which optical fibers are manufactured according to the present invention, a remarkably low optical loss value is shown at all the wavelength bands used in communication as compared with the existing condition. Further, a result that satisfies all the G657A1 standards was shown in the curving characteristic measurement result. That is, it was identified that the design of a refractive index of an optical fiber according to the present invention was suitable, and the it was identified that the optical fiber manufactured through a supercooling process according to the present invention has a low scattering loss and a low bending sensitivity.

(30) Further, in Example 4, in which an optical fiber is drawn after the drawing temperature and drawing speed thereof is lowered, a result in which the optical characteristic value is additionally lowered. The result shows that the example coincides with an existing theory on the relationship between a glass fictive temperature by an optical fiber cooling curve and a Rayleigh scattering loss.

(31) Although the present invention has been described with reference to the limited embodiments and drawings, the present invention is not limited thereto and it is apparent that the embodiments of the present invention may be variously changed and modified by those skilled in the art without departing from the technical spirit of the present invention and the equivalent scopes of the claims that will be described below.

DESCRIPTION OF REFERENCE NUMERALS

(32) 10: preform 100: electric furnace 200: supercooling apparatus 300: cooler 400: coater 500: capstan