WO2010093187A2

WO2010093187A2 - Optical fiber having improved bending loss characteristics and method for manufacturing the same

Info

Publication number: WO2010093187A2
Application number: PCT/KR2010/000887
Authority: WO
Inventors: Ji-Sang Park; Lae-Hyuk Park; Soon-Il Sohn; Hyung-Soo Shin; Tae-Kyung Yook; Joong-Ho Pi
Original assignee: Ls Cable Ltd.
Priority date: 2009-02-11
Filing date: 2010-02-11
Publication date: 2010-08-19
Also published as: KR20100091710A; CN102317826A; WO2010093187A3

Abstract

A method for manufacturing a trench-type optical fiber with excellent bending loss characteristics includes: (a) forming a core and a first clad layer by means of deposition; (b) preparing a tube containing hydroxyl (OH ) impurities not exceeding a predetermined level, and joining the tube around the first clad layer by means of over-cladding to form a second clad layer; and (c) forming a third clad layer around the second clad layer.

Description

OPTICAL FIBER HAVING IMPROVED BENDING LOSS CHARACTERISTICS AND METHOD FOR MANUFACTURING THE SAME

The present invention relates to a method for manufacturing an optical fiber, and more particularly to a method for manufacturing a trench-type optical fiber having improved bending characteristics.

This application claims priority to Korean Patent Application No. 10-2009-0011034 filed in Republic of Korea on February 11, 2009, the entire contents of which are incorporated herein by reference.

As the demands on bandwidth are recently increased along with the development of information and communication techniques, optical fibers are widely used as wired communication media. Compared with other kinds of media, the optical fiber is advantageous in loss and bandwidth but disadvantageous in handling, particularly rather than POF (Polymer Optical Fiber).

In particular, in the FTTH (Fiber To The Home) service, various small bends occur when an optical fiber is housed or installed. Also, existing optical fibers have a great bending loss at a small curve, so it is not easy to install an optical fiber closely at a corner or to use an organizer with a small bending radius. Further, DWDM (Dense Wavelength Division Multiplexing) system or CWDM (Coarse Wavelength Division Multiplexing) system with an enhanced transmission capacity use 1600 nm wavelength band as well as existing 1550 nm wavelength band. However, in case existing optical fibers optimized to the 1550 nm wavelength band are used in the 1600 nm wavelength band, a mode field diameter is increased, which results in the increase of bending loss. Thus, in order to prevent the deterioration of system transmission characteristics caused by the increased loss, it is required to improve optical fibers such that a bending loss at 1600 nm wavelength band is not greater than that of 1550 nm wavelength band.

As the bending loss characteristics of an optical fiber becomes more important under the FTTH circumstance as mentioned above, a technical standardization institute, ITU-T, established G657 standards to handle optical fibers with decreased bending loss as an important issue. In particular, the standards are classified into A type and B type, where the A type standards deal compatibility with existing optical fibers, G652D, more significantly, but the B type standards treat bending losses more seriously rather than the compatibility with G652.

FIG. 1 is a table showing bending loss standards of optical fibers.

As shown in FIG. 1, though the standards are classified into A type and B type, along with recent technological innovations, optical fibers currently released on the market tend to have both compatibility of the type A and bending loss characteristics of the type B. In addition, ultra-low bending loss fibers (allowing R5mm bending) extremely advantageous in the FTTH circumstance are also commercialized.

As the bending loss characteristics of optical fibers are issued, there are proposed many schemes for implementing the same, as follows.

First, in order to make an improvement based on a SI (Step Index) structure that is an existing SMF (Single Mode Fiber) optical fiber structure, it is required to lower a MAC value (= MFD/CutOff). It allows limiting light into the center of an optical fiber to the maximum, thereby preventing the light from being leaked out when the optical fiber is bent. Though there may be any difference depending on the structure of an optical fiber, the Mac value generally has a relation with a bending loss as in the following equation 1.

Equation 1

Loss = exp[8.5-519*D_mm(1/λ*Mac)³]

λ: wavelength (nm), D_mm: bending diameter

When the SI structure is adopted, a bending loss is reduced by decreasing the Mac value, but it may cause a problem in the compatibility with existing optical fibers (due to the difference in MFD).

As an improvement of the existing SI structure, there is proposed a depressed structure (see FIG. 2). In the depressed structure, a clad 2-b adjacent to a core 2-a has a reduced refractive index rather than an existing one. If the depressed structure is applied, the compatibility with G652 optical fiber is improved rather than the SI structure, and also the bending loss is reduced. The depressed structure is generally implemented by means of a VAD process, one of outside deposition methods.

Along with the depressed structure, there is also proposed a trench structure as an improvement (see FIG. 3). In the trench structure, a clad 3-b adjacent to a core 3-a keeps its refractive index identical to an outermost clad 3-d, and a refractive index reduced portion 3-c similar to the depressed structure is located at a suitable distance from the core 3-a. This structure is somewhat complicated in comparison to the depressed structure, so inside deposition processes at which the refractive index may be easily adjusted are generally adopted for this structure, rather than the VAD process. However, this structure exhibits excellent bending loss characteristics, so many studies are in progress for implementing this structure through the VAD process.

The techniques for decreasing bending losses through the trench structure are disclosed in US Laid-open Patent Publication US20080056658, Korean Patent Registration No. 0820926 and Korean Laid-open Patent Publication No. 2007-0101145.

Such techniques suggest that the bending loss can be decreased through the trench structure. However, there is no suggestion to decrease hydroxyl (OH^-) loss, important in G657 standards, and productivity, besides the bending loss. In other words, the trench structure suggested for the decrease of bending loss may cause a problem in aspect of OH loss and productivity, but there is no substantial solution.

The optical fiber manufacturing technique is generally classified into an inside deposition process and an outside deposition process. In the inside deposition process, chemical components mixed at a suitable ratio are deposited in a tube such that an optical fiber has a desired refractive index. This process may reproduce a relatively complicated refractive index, but it is worse than the outside deposition process in aspect of OH loss characteristics and productivity.

The outside deposition process includes VAD (Vapor Axial Deposition) and OVD (Outside Vapor Deposition). In case of VAD, the inside of a core is not exposed to the outside during the process, so VAD is much better than OVD in aspect of OH loss. In this reason, when making SMF with a relatively simple structure, the VAD process is widely adopted by optical fiber manufacturers due to its excellent OH loss characteristics and excellent productivity.

However, when an optical fiber of a trench structure is manufactured using the VAD process, the following problems occur.

First, it is advantageous to locate a trench portion of an optical fiber adjacent to a core so as to decrease a bending loss in the trench structure of the optical fiber. At this time, in order to realize the trench structure when the VAD process is applied, a core and a clad layer contacting with the core are made, and then a next region (or, a trench portion) is formed by means of outside deposition. In this case, as the clad layer adjacent to the core has a smaller thickness, OH loss of the optical fiber is disadvantageously increased due to OH impurities when the next region (or, the trench portion) is formed by means of outside deposition.

In addition, in order to form the trench structure of the optical fiber, an existing VAD process is changed from two-stage deposition into three-stage deposition. In this reason, there occurs a serious problem in the productivity, which is one of important advantages of the VAD process.

The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a method for manufacturing an optical fiber with decreased OH loss and improved productivity in case a VAD process is applied to make an optical fiber having a trench structure with excellent bending characteristics. The present invention is also directed to providing an optical fiber manufactured by the above method.

Other objects and aspects of the present invention will be explained below and easily understood from the following description of embodiments. Also, the objects and advantages of the present invention can be implemented by means of components defined in the appended claims or their combinations.

In order to accomplish the above object, the present invention provides a method for manufacturing a trench-type optical fiber with excellent bending loss characteristics, which includes (a) forming a core and a first clad layer by means of deposition; (b) preparing a tube containing hydroxyl (OH^-) impurities not exceeding a predetermined level, and joining the tube around the first clad layer by means of over-cladding to form a second clad layer; and (c) forming a third clad layer around the second clad layer.

The prepared tube preferably has a predetermined refractive index, and the tube preferably has a fractional refractive index change (D) of -0.1% to -1.0%, based on a refractive index of the third clad layer, where the fractional refractive index change (D) of the tube is equal to (N3-N4)/N4, in which N3 is a refractive index of the second clad layer and N4 is a refractive index of the third clad layer.

In addition, the prepared tube preferably contains hydroxyl (OH^-) impurities of 10 ppm or less. Further, in the step (a), the core and the first clad layer are preferably formed by means of deposition.

Also, the step (a) may include (a-1) forming a core and a first clad layer by means of a sooting process; (a-2) removing hydroxyl impurities from the core and the first clad layer by means of a dehydration process; (a-3) sintering the core and the first clad layer to make a porous preform by means of a sintering process; and (a-4) elongating the porous preform by means of an elongation process.

Preferably, the step (b) includes (b-1) preparing an over-clad tube containing hydroxyl (OH^-) impurities not exceeding a predetermined level; (b-2) inserting a first-stage preform having the core and the first clad layer into the over-clad tube; and (b-3) melting the over-clad tube by applying heat thereto to join the over-clad tube to the first-stage preform.

In addition, in the step (c), the third clad layer is preferably formed around the second clad layer by means of outside deposition or over-cladding.

In another aspect of the present invention, there is also provided an optical fiber with improved bending loss characteristics, manufactured in accordance with the above method.

In still another aspect of the present invention, there is also provided an optical fiber with improved bending loss characteristics, which includes a core located at a center thereof, a first clad layer formed around the core, a second clad layer formed around the first clad layer, and a third clad layer formed around the second clad layer, wherein the core has a maximum refractive index greater than any of maximum refractive indexes of the first, second and third clad layers, and the maximum refractive index of the second clad layer is smaller than any of the maximum refractive indexes of the first and third refractive indexes, and wherein the second clad layer contains hydroxyl (OH^-) impurities of 10 ppm or less.

Preferably, the second clad layer is formed by joining a previously prepared tube to the first clad layer by means of over-cladding.

According to the present invention, when an optical fiber having a trench structure with excellent bending characteristics is manufactured, a second clad layer serving as a trench portion is formed by means of over-cladding using a previously prepared tube, so it is possible to save a production time and improve productivity.

In addition, when the second clad layer is formed by means of jacketing, an amount of hydroxyl (OH^-) included in the prepared tube is controlled, so it is possible to decrease an OH loss of a finally made optical fiber.

Other objects and aspects of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawing in which:

FIG. 1 is a table showing bending loss standards of an optical fiber;

FIG. 2 is a schematic view showing an optical fiber of a depressed type, obtained by improving an existing SI structure;

FIG. 3 is a schematic view showing an optical fiber of a trench type, obtained by improving an existing depressed structure;

FIG. 4 is a schematic view showing an optical fiber with improved bending loss characteristics, manufactured according to one embodiment of the present invention;

FIG. 5 is a flowchart illustrating a method for manufacturing an optical fiber with improved bending loss characteristics according to one embodiment of the present invention;

FIG. 6 is a schematic view showing an example of each process of the method for manufacturing an optical fiber with improved bending loss characteristics according to one embodiment of the present invention;

FIGs. 7, 8 and 9 are schematic views showing examples of the process for making a first-stage preform of the optical fiber according to one embodiment of the present invention;

FIG. 10 is a schematic view showing an example of an over-cladding process for making a second-stage preform of the optical fiber according to one embodiment of the present invention; and

FIG. 11 is a table showing measured characteristics of the optical fiber with improved bending loss characteristics, manufactured according to one embodiment of the present invention.

a: core b: first clad layer

c: second clad layer d: third clad layer

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

FIG. 4 is a schematic diagram showing an optical fiber with improved bending loss characteristics, manufactured according to one embodiment of the present invention.

Referring to FIG. 4, the optical fiber manufactured according to the present invention includes a core (a) at a center, a first clad layer (b) surrounding the core (a), a second clad layer (c) surrounding the first clad layer (b), and a third clad layer (d) surrounding the second clad layer (c). This optical fiber is a trench-type optical fiber having a trench portion like the second clad layer (c).

The core (a) has a maximum refractive index N1 greater than any of maximum refractive indexes N2, N3, N4 of the first to third clad layers (a)(b)(c). The maximum refractive index N3 of the second clad layer (c) is smaller than each maximum refractive index N2, N4 of the first and third clad layers (b)(d).

In addition, the second clad layer (c) includes hydroxyl (OH^-) impurities not exceeding 10 ppm. For this purpose, when the optical fiber according to the present invention is manufactured, the second clad layer (c) is made by joining a previously prepared tube containing hydroxyl impurities of 10 ppm or less to an outer periphery of the first clad layer (b) by means of over-cladding.

Now, each characteristic value in accordance with the structure of the optical fiber is explained. First, the core (a) has a radius R1 of 3.5 mm to 4.5 mm. Assuming that the first clad layer (b) has a radius R2, a value of R2/R1 is set to be 1.5 to 6.5. Also, assuming that the second clad layer (c) has a radius R3, a value of R3-R2 is set to be 1.0 mm to 10.0mm. A radius R4 of the third clad layer (d) is set to 62.5 mm.

In addition, among characteristics of the optical fiber structure, a fractional refractive index change based on the third clad layer is explained. Here, the fractional refractive index change based on the third clad layer is calculated using an equation Dn=(Nn-N4)/N4. First, the core (a) is configured to have a fractional refractive index change D1 of 0.3% to 0.5%. The first clad layer (b) is configured to have a fractional refractive index change D2 of -0.1% to 0.1%. The second clad layer (c) is configured to have a fractional refractive index change D3 of -1.0% to -0.1%.

Hereinafter, a method for manufacturing such an optical fiber is explained.

FIG. 5 is a flowchart illustrating a method for manufacturing an optical fiber with improved bending loss characteristics according to one embodiment of the present invention. FIG. 6 shows an example of each process in the method for manufacturing an optical fiber with improved bending loss characteristics according to one embodiment of the present invention.

Referring to FIGs. 5 and 6, in the method for manufacturing an optical fiber according to the present invention, first, the core (a) and the first clad layer (b) to be located at a core region of the optical fiber are formed by means of deposition, dehydration and sintering. At this time, the core (a) and the first clad layer (b) are made into suitable geometric structures to satisfy characteristic values (e.g., core and radius) of the optical fiber according to the present invention. In addition, the first clad layer (b) formed at this time has a relatively smaller refractive index than the core (a) (S10).

Through this process, a first-stage preform composed of a region of the core (a) and a region of the first clad layer (b) is made.

Then, a process of preparing a second clad layer (c) to be formed around an outer periphery of the first-stage preform is executed. In other words, a tube having predetermined characteristics to be used for making the second clad layer (c) is prepared in advance. This tube is a tube-shaped cylindrical preform to be joined to an outer periphery of the first clad layer (b) by means of over-cladding. The prepared tube is also called an over-clad tube. In addition, the prepared tube includes hydroxyl (OH^-) impurities not exceeding a predetermined level. In more detail, the hydroxyl impurities included in the prepared tube does not exceed 10 ppm. A tube including a smaller amount of hydroxyl impurities is preferred (S20).

If the over-clad tube is prepared as mentioned above, the prepared tube is located around the first clad layer (b), and then the tube is joined to the first clad layer (b) by means of over-cladding. Such an over-cladding process is also called 'jacketing'. The prepared tube is joined around the first clad layer (b) to form a second clad layer (c). If the region of the second clad layer (c) is formed as mentioned above, a second-stage preform is made. The second clad layer (c) formed at this time has a relatively smaller refractive index than the first clad layer (b). In the optical fiber structure, the second clad layer is called a trench portion (S30).

If the second-stage preform is prepared, a process of forming a third clad layer (d) around an outer periphery of the second clad layer (c) of the second-stage preform is executed. The third clad layer (d) is formed by means of deposition or over-cladding. In addition, the third clad layer (d) has a relatively greater refractive index than the second clad layer (c). If the region of the third clad layer (d) is formed as mentioned above, a third-stage preform is made (S40).

If the third-stage preform is made, a draw/PT process and also a process of measuring and evaluating characteristic values of each component of the optical fiber are executed.

In particular, the prepared tube to be joined by the over-cladding forms the second clad layer (c) surrounding the first clad layer (b), so the tube is configured to have a fractional refractive index change of -0.1% to -1.0%, based on the refractive index of the third clad layer (d).

Now, the outside deposition process for making the first-stage preform composed of the core (a) and the first clad layer (b) is explained below in more detail.

FIGs. 7, 8 and 9 show examples of the process for making the first-stage preform of the optical fiber according to one embodiment of the present invention.

First, FIG. 7 illustrates a sooting process in which glass particles generated from a burner by means of flame hydrolysis reaction are deposited to an outside of a preform.

Then, FIG. 8 illustrates a process of removing hydroxyl impurities by using Cl₂ gas from the porous preform obtained through the sooting process of FIG. 7. In other words, a Cl₂ circumstance is made in a chamber to remove hydroxyl groups included in the preform (a dehydration process). Along with it, the impurity-removed porous preform is sintered (a sintering process).

After the preform is sintered as mentioned above, the preform is elongated into a suitable thickness (an elongation process). FIG. 9 shows an example of the elongation process. As shown in FIG. 9, a heat source is located around the preform, and then the preform is elongated through an elongation unit to control a thickness. As mentioned above, desired thickness and geometric structure of the preform may be obtained by adjusting and controlling an elongation rate.

The over-cladding process for making the second-stage preform by forming the second clad layer on the first-stage preform is now explained with reference to FIG. 10.

FIG. 10 shows an example of the over-cladding process for making the second-stage preform of the optical fiber according to one embodiment of the present invention.

The over-cladding process shown in FIG. 10 is also called 'jacketing' and it is used for covering and joining a tube on the first-stage preform. In this process, an over-clad tube having a suitable refractive index and containing controlled impurities is prepared, and then the first-stage preform as made above is inserted into the tube. Heat is applied using a torch to the tube in which the first-stage preform is inserted, thereby melting the tube and joining the tube to the first-stage preform. At this time, the inside of the tube is kept at a negative pressure to induce joining. Here, since a tube having a suitable refractive index and containing controlled impurities is used to form the second clad layer serving as a trench portion, the degree of hydroxyl impurities contained therein may be lowered rather than the case of forming a second clad layer by means of outside deposition processes such as VAD. Thus, the OH loss of a finally made optical fiber may be resultantly decreased. In addition, in order to manufacture a trench-type optical fiber using VAD, both first and second clad layers should be formed by means of outside deposition, and for this purpose the manufacturing process should be changed from two-stage deposition into three-stage deposition. Thus, the productivity, which is an important advantage of VAD, is seriously deteriorated.

FIG. 11 is a table showing measured characteristic values of the optical fiber with improved bending loss characteristics, manufactured according to one embodiment of the present invention.

Seeing FIG. 11, eight embodiments are listed in the table in total with respect to the optical fiber according to the present invention.

An optical fiber (or, a trench-type optical fiber) according to each embodiment satisfies aforementioned structural characteristic values of the present invention. In other words, the radius R1 of the core, R2/R1, R3-R2 and R4 values as well as fractional refractive index change values are listed at an upper part of the table. All embodiments satisfy aforementioned characteristic values of the present invention.

Then, the tube used for configuring the region of the second clad layer by means of over-cladding has a refractive index of 1.45309 in all embodiments. Also, the degree of hydroxyl impurities contained in the tube is 1.5 to 1.9 ppm. In addition, the third clad layer (or, the outermost clad layer) was distinguishably made by means of deposition or jacketing. A production time might be saved as the second clad layer was formed by jacketing. When deposition was used for forming the third clad layer, a production time was saved as much as 20% in comparison to conventional cases. Meanwhile, when jacketing was used for forming the third clad layer, a production time was saved as much as 40%.

The characteristics of the optical fiber manufactured according to embodiments of the present invention as mentioned above are depicted in the table. First, MFD and cutoff characteristics are depicted. Together with them, loss characteristics at 1383 nm are also depicted, from which it could be understood that the optical fibers according to the embodiments of the present invention have smaller losses than existing ones (entirely manufactured by deposition).

Besides them, zero dispersion values and measured bending loss values are depicted. Here, the measured bending loss values of the optical fibers according to the present invention satisfy G657B standards at both 1550 nm and 1625 nm.

Evaluations through the characteristic values of the optical fibers according to the embodiments of the present invention are depicted at a lower part of the table. All of the eight embodiments satisfy G657A standards and also meet compatibility with G652 optical fibers. In addition, OH loss is 0.330 or less in all embodiments, satisfactorily. Also, considering the productivity of the embodiments of the present invention, it could be understood that a production time is saved as much as 20% to 40%.

The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Claims

A method for manufacturing a trench-type optical fiber with excellent bending loss characteristics, the method comprising:

(a) forming a core and a first clad layer by means of deposition;

(b) preparing a tube containing hydroxyl (OH^-) impurities not exceeding a predetermined level, and joining the tube around the first clad layer by means of over-cladding to form a second clad layer; and

(c) forming a third clad layer around the second clad layer.
The method for manufacturing a trench-type optical fiber with excellent bending loss characteristics according to claim 1,

wherein, in the step (b), the prepared tube has a predetermined refractive index.
The method for manufacturing a trench-type optical fiber with excellent bending loss characteristics according to claim 2,

wherein the tube has a fractional refractive index change (D) of -0.1% to -1.0%, based on a refractive index of the third clad layer,

where the fractional refractive index change (D) of the tube is equal to (N3-N4)/N4, in which N3 is a refractive index of the second clad layer and N4 is a refractive index of the third clad layer.
The method for manufacturing a trench-type optical fiber with excellent bending loss characteristics according to claim 1,

wherein, in the step (b), the prepared tube contains hydroxyl (OH^-) impurities of 10 ppm or less.
The method for manufacturing a trench-type optical fiber with excellent bending loss characteristics according to claim 1,

wherein, in the step (a), the core and the first clad layer are formed by means of deposition.
The method for manufacturing a trench-type optical fiber with excellent bending loss characteristics according to claim 1, wherein the step (a) includes:

(a-1) forming a core and a first clad layer by means of a sooting process;

(a-2) removing hydroxyl impurities from the core and the first clad layer by means of a dehydration process;

(a-3) sintering the core and the first clad layer to make a porous preform by means of a sintering process; and

(a-4) elongating the porous preform by means of an elongation process.
The method for manufacturing a trench-type optical fiber with excellent bending loss characteristics according to claim 1, wherein the step (b) includes:

(b-1) preparing an over-clad tube containing hydroxyl (OH^-) impurities not exceeding a predetermined level;

(b-2) inserting a first-stage preform having the core and the first clad layer into the over-clad tube; and

(b-3) melting the over-clad tube by applying heat thereto to join the over-clad tube to the first-stage preform.
The method for manufacturing a trench-type optical fiber with excellent bending loss characteristics according to claim 1,

wherein, in the step (c), the third clad layer is formed around the second clad layer by means of outside deposition or over-cladding.
An optical fiber with improved bending loss characteristics, manufactured in accordance with the method defined in any one of the claims 1 to 8.
An optical fiber with improved bending loss characteristics, which includes a core located at a center thereof, a first clad layer formed around the core, a second clad layer formed around the first clad layer, and a third clad layer formed around the second clad layer,

wherein the core has a maximum refractive index greater than any of maximum refractive indexes of the first, second and third clad layers, and the maximum refractive index of the second clad layer is smaller than any of the maximum refractive indexes of the first and third refractive indexes, and

wherein the second clad layer contains hydroxyl (OH^-) impurities of 10 ppm or less.
The optical fiber with improved bending loss characteristics according to claim 10,

wherein the second clad layer is formed by joining a previously prepared tube to the first clad layer by means of over-cladding.