KR20020015395A - Process for the preparation of photonic crystal fiber - Google Patents

Abstract

PURPOSE: Provided is a method for producing parent material of photonic crystal fiber from which an elongated photonic crystal fiber can be drawn while maintaining the circular cross section. CONSTITUTION: The method comprises steps of (i) surrounding a quartz glass rod forming a core part of a photonic fiber with multi-layered glass tubes forming a clad part; (ii) inserting the structure from the step (i) into a mold; and (iii) collapsing the product from the step (ii) to form a parent material of photonic crystal fiber. In the step (i), silica glass forming material, silica glass fiber or mixtures thereof is filled in the space among the quartz glass rod of the core part and an adjacent quartz glass tube and the respective quartz glass tubes.

Description

PROCESS FOR THE PREPARATION OF PHOTONIC CRYSTAL FIBER}

The present invention relates to a method for producing a base material for photonic crystal fibers (PCF, hereinafter referred to as PCF) in which an air layer is formed in a honeycomb shape in a longitudinal direction in a core layer of an optical fiber.

A photonic crystal is a structure in which a photonic bandgap can be obtained by periodically arranging materials having different dielectric constants. The application of photonic crystals to optical fibers is made of pure quartz glass rods filled in the center part of the core of ordinary optical fibers, and the quartz glass tubes are surrounded by hexagonal shapes to form cladding. This periodic arrangement of quartz glass tubes creates a photon transition layer using the difference in dielectric constant between the air layer and the quartz glass layer.

This photon transfer layer serves to prevent the propagation of light in a specific direction, such as an electronic bandgap in a semiconductor. That is, only light that satisfies the conditions of the photon transfer layer can pass through the photon transfer layer. Using these characteristics, it is possible to fabricate optical fibers having a single mode for a wide variety of wavelengths.

In the manufacture of PCF-type optical fiber, in order to draw a sufficiently long length while maintaining a circular shape, the PCF base material must be manufactured in a circular shape. In general, the base material for photonic crystal fiber has a structure as schematically shown in FIG. In order to manufacture efficient optical fibers, it is important to maintain the spacing of the air layers in the base material for the PCF and its shape. Several methods have been developed for this.

First, there is a method of drilling a circular hole in a hexagonal quartz glass rod and drawing it out once through an optical fiber extractor, etc., and then stacking the rods having such circular holes in a layer and extracting three more times. JC Knight, TA Birks, P. St. J. Russell and DM Atkin, "All-silica single-mode optical fiber with photonic crystal cladding", Opt. Lett., 21 (19), 1996, 1547-1549). . In the case of using the above process, it is not economical because it requires a process of making a cylindrical hole in the quartz glass rod, a process of drawing it out once using an extractor, and a process of drawing out the drawn result again three times.

Another method is a method of stacking quartz glass tubes in a hexagonal layer and drawing them out once in an optical fiber extractor, and then attaching the borosilicate glass to the outer portion of the quartz glass tube bundle thus extracted (PJ Bennett, Tanya M. Monro, DJ Richardson, "Toward practical holey fiber technology: fabrication, splicing, modeling, and characterization", Opt. Lett., 24 (17), 1999, 1203-1205). The PCF made through this process is more economical than the first method, but has the disadvantage that the circular shape of the quartz glass tube cross section is not properly maintained as the space between the quartz glass tubes is deformed. This results in a difference in the thermal experience between the outermost cast quartz glass tube and the inner quartz glass tubes due to the thermal insulation effect by the air layer present in the PCF substrate, and the difference in the thermal experience results in an angle of quartz glass tubes. This is because the size of the air layer varies from layer to layer.

Accordingly, it is an object of the present invention to provide a base material for producing photonic crystal fibers and a photonic crystal fiber produced therefrom which can be pulled out to a long length without deforming the circular shape of the cross section.

1 is a schematic view showing the structure of a general photonic crystal fiber manufacturing base material;

2 shows a cross section of a base material for producing a photonic crystal fiber according to an aspect of the present invention;

Figure 3 shows a cross-section (draw temperature: 2050 ℃, extraction speed: 7m / min, outer diameter 140㎛) of the PCF made of a PCF base material prepared using a high viscosity silica colloid as a glass forming material.

According to the above object of the present invention, (i) surrounding a quartz glass rod forming a core portion of an optical fiber with a multilayer quartz glass tube forming a clad portion; A method of making a photonic crystal fiber matrix comprising (ii) placing the structure obtained in step (i) into a mold quartz glass tube and (iii) collapsing it; Provided is a method for producing a photonic crystal fiber base material characterized by filling a silica glass forming material, a silica glass fiber or a mixture thereof in the space between the quartz glass tubes and between each quartz glass tube.

Hereinafter, the present invention will be described in more detail.

When manufacturing a PCF-type optical fiber base material, as shown in FIG. 1, a pure quartz glass rod is placed in the center portion corresponding to the core of a general optical fiber, and surrounds the multi-layered quartz glass tube so that there is no empty space. To form a tube layer corresponding to the cladding portion of. For example, as shown in FIG. 2, after stacking a quartz glass tube (outer diameter: 3 mm) having a constant outer diameter and empty inside in a hexagonal shape around a quartz glass rod (3 mm (corresponding to a core of a general optical fiber), On top of this, the same shape of quartz glass tubes are stacked staggered layer by layer for maximum packing density (corresponding to the cladding of ordinary optical fibers). At this time, the quartz glass tube layer of the cladding portion for implementing the photon transfer layer should be a multilayer of two or more layers, and as the number of layers increases, it is easy to implement the photonic bandgap.

The quartz glass rods used in the present invention include Er ³⁺ , Pr ³⁺ , Sm ³⁺ , Co ³⁺ , Cr ³⁺ , Ce ³⁺ , Nd ³⁺ , Eu ³⁺ , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Tm ³⁺ , Yb ³⁺ , Lu ³⁺ , Th ³⁺ , Pa ³⁺ , U ³⁺ , V ³⁺ and the like may further be contained.

The outer diameter of the tube is 2 to 10 mm, the inner diameter is 0.5 to 2 mm and the length is 100 to 500 mm. Also, quartz glass rods are available with quartz glass tubes of diameter and length

The quartz glass tube used in the present invention has an outer diameter of 2 to 10 mm, an inner diameter of 0.5 to 2 mm and a length of 100 to 500 mm. In addition, the quartz glass rod uses the same diameter and length as a quartz glass tube.

Examples of the silica glass forming material that may be used in the present invention include silica powder, silica sol-gel forming solution, and highly viscous silica colloid, and the silica glass fiber may be 0.10 to 0.80 mm in diameter. Can be used. In the case of silica powder, a particle diameter of 10 to several hundred nm is used. The silica sol-gel forming solution may be prepared by a conventional method of dispersing an alkoxide compound such as TEOS (TetraEthylOrthoSilicate) (Si (OC ₂ H ₅ ) ₄ ) in distilled water, alcohol, or the like. The sol-gel forming solution may contain silica powder. The high viscosity silica colloid has a viscosity of 10 to 10,000 poise, and may be prepared by adding silica powder to distilled water, alcohol, or the like at a concentration of 30% by weight or more.

In addition, in order to control the physical properties such as silica glass forming material, silica glass fiber or mixtures thereof, glass transition temperature and melting point, glass structure components (Network Former, NWF) and glass structure modifications that can form glass in addition to silica Substances (Network Modifier, NWM), Network Intermediate (NWI), etc. may be additionally used. Typical glass tissue materials (Network Former, NWF) include SiO ₂ , GeO ₂ , B ₂ O _5, P ₂ O ₅ , As ₂ O ₅ , Sb ₂ O ₅ , V ₂ O _5, and the like. Examples of (Network Modifier, NWM) include Na ₂ O, Ca ₂ O, K ₂ O, and the like, and the glass intermediate (NWI) includes Al ₂ O ₃ , Ga ₂ O _3, ZnO, PbO, and the like. These may be used in combination with silica powder, or may be added in the preparation of silica sol-gel or high viscosity silica colloid. At this time, NWF is about 50wt% or more, and NWI and NWM use 10-50wt% (see Arun K. Varshneya, "Fundamentals of Inorganic Glasses", Academic press, 1994).

In the present invention, the silica glass fibers may be made of silica alone, or may further contain NWF, NWM, NWI, and the like together with silica.

The insulation effect of the air layer in the production of PCF results in a difference in thermal experience between the outermost quartz glass tube and the inner tubes, which results in a different air layer size for each layer of quartz glass tubes.

In order to overcome this problem, in the present invention, the inner diameter of the quartz glass tube is made smaller at the portion adjacent to the core of the optical fiber, and the inner diameter of the tube is made larger as it goes outside, so that the size of the air layer in the final drawn PCF is constant. By adjusting the inner diameter in this way it is possible to freely control the photonic bandgap of the PCF base material.

It is also desirable to seal the ends of the tubes before stacking them with the quartz glass rod to prevent the ingress of impurities into the quartz glass tube, so that the silica glass forming material is filled between the tubes.

For example, end-sealed quartz glass tubes are coated by immersion in a highly viscous silica colloid made from a mixture of silica powder and distilled water. At this time, the coating thickness varies depending on the speed of taking out the tube after immersion, in the present invention is preferably 7 to 10 mm / s. In the case of a solution for forming a silica sol-gel, the solution is poured into the space between the tubes, followed by drying at room temperature according to a conventional method to obtain a gel state, which is then heat-treated at a high temperature to form silica glass. When silica powder is used as the glass forming material, for example, vinyl or tissue is used to rub the quartz glass tube to generate an electrostatic force so that the powder is coated on the tube. In the case of fine powder, it can be coated without generating static electricity separately. In the present invention, the more the glass forming material is filled in the space between the quartz glass tubes, the better the circularity of the PCF base material can be maintained. In addition, the smaller the particle size, the higher the surface energy and vitrification easily occurs. In the case of silica glass fibers, when the tubes are stacked, glass fibers are sandwiched between the tubes to minimize voids between the tubes. In using silica glass fibers in the production of PCF base materials, silica powder, silica sol-gel, or silica colloid may be used alone or in combination with glass fibers.

The coated quartz glass tube is stacked hexagonally around the quartz glass rod for maximum packing density, and then the structure is placed in a mold quartz glass tube. At this time, a silica powder, a highly viscous silica colloid, a silica glass forming material such as a sol-gel forming solution, a silica glass fiber, or the like may be filled between the quartz glass tube and the mold quartz glass tube of the outermost layer to maintain circular shape.

The cast quartz glass tube, that is, the PCF base material precursor including the core and the cladding part thus manufactured is collapsed through a general MCVD process. The collapsing temperature is about 2,000 to 2,100 ° C., and the heat treatment fuses the outermost layer of quartz glass tubes and the mold quartz glass tube, and the silica powder, high viscosity silica colloid or sol-filled between the quartz glass tubes. Gel forming solutions and the like are vitrified. At this time, as the amount of silica powder, high viscosity silica colloid, and the like increases in the quartz glass tube spaces, the sintering temperature is lowered to allow complete vitrification. In addition, when using a sol-gel forming solution, the space between the tubes is vitrified through the sol-gel.

When silica glass fibers are used alone or in combination with silica powder, highly viscous silica colloids or sol-gel forming solutions, etc., the silica glass fibers alone or a mixture of silica glass fibers and silica forming materials together with It fills the space.

Since the temperature at the time of fiber drawing is usually 1,950 to 2,150 ° C., the glass forming material is more completely vitrified during the drawing out of the PCF using the base material thus prepared.

In the present invention, Er ³⁺ , Pr ³⁺ , Sm ³⁺ , together with silica glass forming materials such as silica powder, sol-gel, high viscosity silica colloid, silica glass fiber, etc., used to fill the space between the quartz glass tubes. Ce ³⁺ , Nd ³⁺ , Eu ³⁺ , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Tm ³⁺ , Yb ³⁺ , Lu ³⁺ , Th ³⁺ , Pa ³⁺ , U ³ Functionality is ^achieved by adding rare earth ions such as ⁺ ions and transition metal ions such as Co ³⁺ , Cr ³⁺ , V ³⁺ , or by using glass rods containing rare earth ions and transition metal ions as quartz glass rods for core formation. The PCF base material, the optical fiber amplifier and the active element using the same can be manufactured.

Although the present invention will be described in detail with reference to the following examples, the present invention is not limited to the following examples.

Example 1

Both ends of the quartz glass tube having an outer diameter of 3 mm, an inner diameter of 2 or 1 mm, and a length of 100 mm were melted and sealed at a high temperature of about 2000 ° C., and then coated with fumed silica using electrostatic force. As shown in Fig. 2, the layered layers were stacked around the silica rod having an outer diameter of 3 mm with a quartz glass tube. At this time, one layer was formed of a tube having an inner diameter of 1 mm, and two layers were formed of a tube having an inner diameter of 2 mm so that the cladding portions were all three layers. The silica powder used was made by Flame Hydrolysis Deposition (FHD) and had a particle size of 12 nm.

Hexagonal quartz glass tubes and quartz glass rods were inserted into a cast quartz glass tube with an inner diameter of 19 mm and an outer diameter of 25 mm, and then collapsed through an MCVD process (SG control, M754, temperature: about 2,000-2,200 ° C). PCF base material of about 25mm diameter was prepared.

The base material was used to withdraw PCF through an optical fiber extractor (SG control, M643).

Example 2

Instead of silica powder, the same procedure as in Example 1 was repeated except that the sol-gel method was used to gel at room temperature for at least 12 hours using a mixture of TEOS and ethyl alcohol (at least about 5 vol%) sol. Base materials and fibers were prepared (Refs. Hans Bach, Dieter Krause, "Thin Film on Glass", Springer, Heidelburg, 1997).

Example 3

Instead of silica powder, the silica glass was prepared by adding silica powder in an amount of 30% by weight to distilled water, except that the quartz glass tube was coated (7 mm / s of extraction rate after immersion). PCF base material and PCF were prepared in the same manner.

The cross-section of the prepared PCF is shown in Figure 3 (draw temperature 2,050 ℃, draw rate 7m / min, outer diameter 140㎛, length 10m). As shown in Fig. 3, it can be seen that the circular shape of the quartz glass tube is well maintained.

Example 4

Instead of silica powder, a PCF base material and PCF were prepared in the same manner as in Example 1 except that the silica glass fibers were used to fill the spaces between the quartz glass tubes.

Example 5

PCF base material and PCF were prepared in the same manner as in Example 1 except that the silica glass fibers were filled with the silica powder used in Example 1 to fill the space between the quartz glass tubes.

Example 6

PCF base material and PCF were prepared in the same manner as in Example 1 except that the silica glass fibers were filled with the space between the quartz glass tubes with the sol-gel method used in Example 2.

Example 7

PCF base material and PCF were prepared in the same manner as in Example 1 except that the silica glass fibers filled the space between the quartz glass tubes with the high viscosity silica colloid used in Example 3.

According to the method of the present invention, the long-distance withdrawal is possible while maintaining the circular shape of the cross section during the manufacture of the PCF. In addition, the composition filled between the quartz glass tubes includes Er ³⁺ , Pr ³⁺ , Sm ³⁺ , Ce ³⁺ , Nd ³⁺ , Eu ³⁺ , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Add rare earth ions such as Tm ³⁺ , Yb ³⁺ , Lu ³⁺ , Th ³⁺ , Pa ³⁺ , U ³⁺ ions and transition metal ions such as Co ³⁺ , Cr ³⁺ , V ³⁺ , or Instead of the quartz glass rod corresponding to the core of the glass rod, the glass rod containing the rare earth ions and transition metal ions may be used to implement an optical fiber amplifier or an active element as a PCF.

Claims (8)

(i) surrounding the quartz glass rod forming the core portion of the optical fiber with a multilayer quartz glass tube forming the clad portion; A method of making a photonic crystal fiber matrix comprising (ii) placing the structure obtained in step (i) into a mold quartz glass tube and (iii) collapsing it; A method for producing a photonic crystal fiber base material characterized by filling a silica glass forming material, a silica glass fiber or a mixture thereof in the space between the quartz glass tubes and between each quartz glass tube. The quartz glass rods Er ³⁺ , Pr ³⁺ , Sm ³⁺ , Co ³⁺ , Cr ³⁺ , Ce ³⁺ , Nd ³⁺ , Eu ³⁺ , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ^{3 +} , Tm ³⁺ , Yb ³⁺ , Lu ³⁺ , Th ³⁺ , Pa ³⁺ , U ³⁺ or V ³⁺ . The method of claim 1, The silica glass forming material is silica powder, silica sol-gel forming solution or high viscosity silica colloid. The method of claim 1, A method of making a photonic crystal fiber base material characterized by further using a glass tissue constituent material, a glass tissue modification material or a glass tissue intermediate material to fill a space between a quartz glass rod and an adjacent quartz glass tube and between each quartz glass tube. The method of claim 1, The silica glass fibers are made of silica alone, or together with silica, further contain a glass former (NWF), a network modifier (NWM), or a glass intermediate (NWI). Characterized in that. The method of claim 1, Together with the silica glass former, silica glass fiber or mixtures thereof Er ³⁺ , Pr ³⁺ , Sm ³⁺ , Co ³⁺ , Cr ³⁺ , Ce ³⁺ , Nd ³⁺ , Eu ³⁺ , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ , Tm ³⁺ , Yb ³⁺ , Lu ³⁺ , Th ³⁺ , Pa ³⁺ , U ³⁺ or V ³⁺ Way. The method of claim 1, Adjusting the inner diameter of the tube to increase from the core portion to the outer layer when forming the quartz glass tube layer in step (i). The method of claim 1, In step (ii) the silica glass forming material, the silica glass fibers or a mixture thereof is filled in the space between the quartz glass tube layer and the mold quartz glass tube. Way.