CN1222128A - Manufacturing method of polarization maintaining optical fiber - Google Patents

Abstract

The glass preform (10) is drawn into an optical fiber. The holes (13) having a length equal to that of the preform (10) collapse during drawing, resulting in an elliptical cross-section of the core (11).

Description

Manufacturing method of polarization maintaining optical fiber

field of invention

The present invention relates to a method of manufacturing a polarization maintaining single mode (PRSM) optical fiber, in particular to a method of manufacturing a preform capable of drawing an elliptical core optical fiber.

Background technique

In many applications of single-mode fiber (eg, gyroscopes, sensors, etc.), it is important to preserve the input polarization properties of the propagated optical signal in the presence of external depolarization perturbations. This requires azimuthal asymmetry in the refractive index cross-section of the waveguide.

One of the first techniques to improve the polarization performance of single-mode fibers is to break the symmetry of the fiber core. A method of making a PRSM fiber is disclosed in US Patent No. 5,149,349, which is incorporated herein by reference. PRSM optical fibers are drawn from a drawn blank having a glass core surrounded by cladding glass containing apertures diametrically relative to the core. The draw rate and temperature of the fiber cause the aperture to close and the core to become elliptical. In the preferred drawn blank manufacturing method, longitudinal grooves are formed at both ends of the diameter of a cylindrical core preform in which the glass core is surrounded by cladding glass. Glass particles are deposited on the outer surface of the glass tube, and the core preform is inserted into the tube. The final assembly is heated to calcinate or solidify the glass particles so that the tube collapses and melts onto the grooved core preform to form an assembly with longitudinal apertures on opposite sides of the core.

When a blank containing apertures is drawn to form a PRSM fiber, the apertures are closed due to surface tension and molten glass flowing into the apertures. This flow causes the circular core of the blank to elongate in the direction of the aperture. The side ratio of an elliptical core is mainly determined by the spacing between the core and the aperture. As the distance between the core and the aperture in the drawn blank decreases, the core of the finished fiber optic is drawn longer in cross-section, but the core cross-section tends to have long, thin ends. If the pitch is too small, the core may break in the cladding region between the core and the aperture, creating a flared end on the core cross-section. "End" means the end of an elongated core along the major axis of the core viewed from a plane perpendicular to the longitudinal axis of the fiber.

Contents of the invention

It is therefore an object of the present invention to provide a PRSM optical fiber manufacturing method which overcomes the disadvantages of the prior art. A further object of the present invention is to provide a relatively simple implementation of a PRSM optical fiber manufacturing method. Another object of the present invention is to provide a method of manufacturing an elliptical PRSM fiber with an improved core cross-section.

According to the method, a polarization maintaining single mode optical fiber is formed by initially forming a fiber drawn blank with a glass core region having a refractive index _n1 surrounded by a cladding glass region having a refractive index _n2 . The cladding region includes apertures at diametrically opposite ends of the core region and separated by the core region. The drawn blank contains a low viscosity glass region with a refractive index _n3 between the core region and the aperture, and which has a lower viscosity than the cladding glass region. The rate at which the fiber is drawn from the blank is such that the aperture is closed and the core is elliptical in cross-section.

Another aspect of the invention relates to an optical fiber having an elliptical core with an aspect ratio _ρ1 , where _ρ1 is equal to _b1 / _a1 , where _b1 is the core major axis radius and _a1 is the minor axis radius. Surrounding the core is a low-viscosity region of elliptical cross-section with an aspect ratio of ρ ₂ equal to b ₂ /a ₂ , where b ₂ is the radius of the major axis of the region of low viscosity and a ₂ is the radius of the minor axis. The length ratio ρ ₂ of the low viscosity region is smaller than ρ ₁ . A region of cladding glass surrounds a region of low viscosity. The low viscosity region has a lower viscosity than the cladding glass region.

Brief description of the drawings

Figure 1 shows the cross-section of a preform from which an elliptical core PRSM fiber can be drawn.

FIG. 2 is a schematic diagram of drawing a PRSM fiber from the preform of FIG. 1 .

Fig. 3 is the sectional view of the PRSM optical fiber manufactured by this method.

Figures 4 and 5 show the major and minor radii, respectively, of the elliptical core and the low viscosity region surrounding it.

Figure 6 shows the process of coating glass particles onto a mandrel.

Fig. 7 is a cross-sectional graph of the refractive index of the fiber core.

Figure 8 is a schematic diagram of drawing a round rod from a cured core glass tube.

Fig. 9 is a schematic diagram of groove-shaped β blanks of various sizes.

Figure 10 is a schematic cross-sectional view of an assembly with a slotted core rod inside a dust-coated cladding glass tube.

Best Mode of Carrying Out the Invention

It should be pointed out that the drawings are only schematic, and their dimensions do not limit the present invention.

The drawn blank 10 of FIG. 1 from which a PRSM fiber can be drawn comprises core and cladding regions 11 and 12, respectively. The core and cladding regions can be constructed of common materials that form optical waveguides. The salient features of these materials are that the refractive index n ₁ of the core material must be greater than the refractive index n ₂ of the cladding material and that both materials have lower losses at the waveguide operating wavelength. Taking this example only as an example, the core region 11 may consist of pure silica or silica doped with one or more agents to increase the refractive index. Region 12 may be composed of pure silica, contain less index-raising dopants than core region 11, or contain one or more dopants (at least one of which is an index-lowering dopant, such as B ₂ O ₃ or fluoride) of silica composition. While silicon dioxide is the preferred matrix glass material due to its low loss at the operating wavelength, other matrix glass materials may also be used.

According to the invention, a region 14 of low viscosity relative to the cladding 12 is located between the core 11 and the aperture 13 . The viscosity of the low viscosity region 14 is preferably close to or slightly lower than that of the fiber core 11 . This is accomplished by forming the silicon dioxide region 14 doped with a suitable amount of _one or more viscosity-lowering dopants (eg, _B2O3 , fluoride, _P2O5 , _{GeO2 ,} etc.).

The refractive index n ₃ of region 14 should be equal to or smaller than n ₂ . By forming doped with suitable amounts of one or more refractive index lowering dopants (such as B ₂ O ₃ and fluoride) and one or more refractive index increasing dopants (such as P ₂ O ₅ , GeO ₂ , etc. ) of the silicon dioxide region 14 can make the refractive index of the region 14 equal to the refractive index of the cladding 12 .

By employing a cladding layer of silica 12 and forming a silica region 14 doped with B ₂ O ₃ or fluoride or doped with a refractive index increasing dopant (such as P ₂ O ₅ , GeO _{2 ,} etc.) and a sufficient amount of B ₂ O ₃ and/or fluoride (to make the composite material have a lower refractive index than silicon dioxide) can make the refractive index of region 14 smaller than that of cladding 12 . An example of an elliptical core fiber having such a refractive index profile is disclosed in US Patent No. 5,482,525.

Aperture 13 extends longitudinally through blank 10 parallel to core region 11 . Although the cross-section of the aperture 13 is shown here as D-shaped, the cross-section may also be half-moon, circular, or the like. Any desired cross-sectional elongated shape formed during fiber drawing is suitable.

Referring to FIG. 2, the drawn blank 10 is located in a conventional drawing furnace, and the guide rail 17 pulls the optical fiber 15 from the bottom of the blank 10 heated to the drawing temperature by the heating element. The closing tendency of the apertures 13 is a function of the amount drawn and the viscosity of the glass. The root viscosity of the drawing stock used for drawing optical fiber depends on the furnace temperature and glass composition. If the viscosity of the heated portion of the billet is sufficiently low and the drawing rate is sufficiently low, the apertures 13 will naturally close during drawing. Since the aperture is easier to close during vacuuming, the drawing speed can be increased by attaching a vacuum attachment 18 to the top of the blank.

When the apertures are closed, they are replaced by surrounding glass. When glass with a smaller diameter than the aperture rapidly flows into the aperture, the cross-section of the core region 11 is elongated. FIG. 3 shows the finished PRSM fiber 15 in cross-section comprising a cladding 22 , an oblong core 21 and a region of low viscosity 23 . The ellipticity, or aspect ratio, of an elliptical core is the ratio of the major axis to the minor axis in a plane perpendicular to the fiber axis (see Figure 4). The ellipticity of the core can be varied depending on the size of the apertures 13 and the spacing between those aperture spaces and the core.

The shape of the elliptical core 21 is also a function of the viscosity of the region 23 according to the method of the invention. By inserting the glass between the core region 11 and the aperture 13 when the glass from which the blank 10 is drawn begins to flow, the flow of the core glass to the aperture is less restricted than if the insert glass were silica. The core region 11 can thus flow faster towards the aperture before closing. Since the outer cladding 12 has a relatively high viscosity (eg, pure silica), the glass flows very little through the aperture, causing the core glass and low viscosity region 14 to flow faster.

Therefore, the cross-sectional shape of the core 21 is more like a rod shape ( FIG. 4 ), which is different from the common shape with small ends and a large middle. In the finished optical fiber produced by this process, the cross-sectional shape 23 (FIG. 3) of the low viscosity region 14 is such that the ends are more pointed than the core 21 and the middle is more bulging than the core. Figures 4 and 5 show more precisely the cross-sections of the core 21 and the region 23. As shown in the figures, the length ratio b ₁ /a ₁ of the elliptical fiber core 21 is greater than the length ratio b ₂ /a 1 of the low viscosity region 23 a ₂ .

The aperture 13 must be parallel to the core, and if the optical fiber 15 has uniform properties along its length, the aperture must be uniform across the cross-sectional area of the drawn blank 15 along its longitudinal axis. The apertures may be formed using any conventional technique meeting the above requirements. UK patent application 2,192,289 discloses two techniques for forming longitudinal holes in preforms on both sides of the fiber core:

(1) The hole can be drilled with a diamond drill.

(2) Place the core preform with flat sides in the center of the glass tube, and place two glass rods on the opposite sides of the core preform, leaving a relatively blank space between the core preform and the tube. fill area. The final assembly is drawn to reduce its diameter and the glass parts are fused together to form an object that has a solid cross-section except for two opposed axe-shaped holes in relatively unfilled regions.

Figures 6-10 illustrate elliptical core PRSM fibers fabricated according to the method. A cylindrical mandrel 25 (FIG. 6) rotates and translates relative to the flame hydrolysis burner 26 to form a glass particle vapor 27 or dust porous coating 28 on the mandrel.

The composition of the dust stream 27 is SiO ₂ doped with 37% (weight percent) GeO ₂ . As shown in FIG. 7 , the concentration of GeO ₂ decreases with increasing radius. The coating mandrel is removed from the lathe and out of the porous preform, leaving a longitudinal aperture within the porous preform. The porous preform is then dried and cured using the technique of US Patent 4,125,888. The final cured preform or core blank 30 is inserted into the drawing apparatus of FIG. 8, its tip portion heated to the drawing temperature by heating means 32, and a vacuum connection 34 attached to its upper portion. After stretching the preform ends so that the aperture 35 is very narrow or completely closed, the aperture is evacuated through the fixture 34 . As the lower end of the preform is drawn down and its diameter is reduced, the evacuated aperture 35 collapses. The radius r of the final alpha rod 31 is 6 mm. The germanium dioxide concentration cross-section is shown in Figure 7.

Cut multiple 90 cm sections from the alpha rod and insert one of the sections into the lathe and coat with _SiO2 dust as shown in Figure 6. The final synthetic preform was cured at 1450°C while a mixture of 94.3 vol. % Helium, 1.0 vol. % Chlorine, and 4.7 vol. % SiF ₄ flowed upward through the muffle furnace. In the final cured preform, the fluoride-doped silica layer is 13.4mm in diameter and the core diameter is around 6.2mm.

The cured preform is inserted into a lathe and coated with SiO ₂ dust which is cured in an atmosphere of chlorine and helium to form pure silica on top of the fluoride-doped silica layer.

The longitudinally extending slots are milled through the outer silica cladding on the opposite side of the core to extend into the fluoride-doped low viscosity region. The grooved β-blanks were washed and rinsed after the grinding operation (Fig. 9).

The grooved β-shaped billet was then inserted into a conventional drawing furnace as shown in Figure 8, where it was stretched to reduce the diameter to about 7.3 mm. The final β-rod 40 ( FIG. 10 ) includes a core region 41 , a silica cladding 42 and a low viscosity region 43 . The notches 44 extend longitudinally along the β-rod 40 on opposite sides of the core region 41 .

The inner and outer diameters of the silica-clad tube 47 were 7.5 mm and 9.5 mm, respectively. Tube 47 tapers inwardly at one end and is fused to a handle adapted to support assembly 52 within the curing oven. The slotted beta-rod 40 is inserted into the end of the tube 47 opposite the tapered end until it contacts the tapered end. The end of the tube 47 inserted into the preform 40 tapers inwards and fuses to the glass rod. Tube 47 is then secured to a lathe where it is rotated and translated relative to the dust deposition furnace to deposit silica dust particles thereon to form porous coating 48, thereby forming assembly 52.

Assembly 52 descends into a curing furnace where it is dried with a gas mixture of chlorine and helium and subsequently calcined to form optical fiber draw blank 10 of FIG. 1 . As the coating 48 cures, it exerts a force radially inwards on the tube 47 , forcing the tube inwardly against the preform 40 . The original cladding region 42 and the tube 47 are fully fused together, and the porous coating 48 is fully calcined and fused to the tube 47 , these layers constituting the cladding 12 .

The final drawn blank is inserted into a draw furnace where optical fiber is drawn from the preform.

Dimensions C, D, E and R of Figure 10 characterize the grooved β-blank produced by the process described above. Optical fibers 1-5 were drawn using five different grooved β-blanks manufactured by the above method, the characteristics of which are shown in Table 1. In each slotted β-blank, the radius R of the core region was around 3.5 mm.

Dimension D in Figure 9 is not measured but Table 1 lists the "dust weight". The soot weight is the weight of the silica glass particles deposited to form the low viscosity region 43 . A coating consisting of such particles is then doped with fluoride and cured. The greater the dust weight, the greater the thickness D.

The relatively high aspect ratios of fibers 3 and 4 indicate that a smaller size E is preferred.

Table 1 The three comparative fibers C1, C2 and C3 were manufactured in the same process as for fibers 1-5, but the low viscosity fluoride-doped silica region 43 was not used. That is, the entire cladding from the fiber core to the outer surface consists of silica. The aspect ratios of fibers C1, C2 and C3 are smaller than those of fibers 1-5.

In the embodiment of FIG. 1 , a region of low viscosity 14 extends from core 11 into aperture 13 . It is believed that if the region 14 does not extend very much into the aperture 13, some improvement in the aspect ratio can be obtained, but the effect of improving the aspect ratio is not as significant as that of the embodiment of FIG. 1 .

Claims (14)

1. methods for optical fiber manufacture is characterized in that may further comprise the steps:

Form the fibre-optical drawing blank, it comprises specific refractory power is n ₁And the rate that is refracted is n ₂The glass core zone that surrounds, cladding glass zone, described cladding glass zone comprises the aperture that is positioned at described core region diameter opposite end and is separated by by described core region, and described drawing blank comprises a specific refractory power between described core region and described aperture be n ₃The low viscosity glassy zone, and described viscosity is less than described cladding glass zone.And

Draw out optical fiber so that described aperture cross section closed and described fibre core is an ellipse with certain quantity from described drawing blank.

2. the method for claim 1 is characterized in that forming step and may further comprise the steps:

Form the cylindrical glass preform, wherein said glass core is surrounded by the first layer cladding glass, the first layer cladding glass is surrounded by second layer cladding glass, and the viscosity of described the first layer is less than the viscosity of the described second layer, and a pair of longitudinal slot extends along described glass preform diameter two ends;

Described glass preform is inserted Glass tubing to form the assembly that comprises longitudinal aperture at described core diameter two ends; And

Described pipe is contracted on the described fibre core preform.

3. method as claimed in claim 2, it is characterized in that described groove is whole extends in described second layer cladding glass.

4. method as claimed in claim 3 is characterized in that the step that further comprises is: the interface fusing forms curing assembly between described fibre core preform and the described pipe thereby make.

5. the method for claim 1 is characterized in that the described zone of low viscosity glass surrounds described core region fully.

6. the method for claim 1 is characterized in that n ₃≤ n ₂

7. the method for claim 1 is characterized in that described low viscosity zone is by comprising one or more doping agents B ₂O ₃, fluorochemical, P ₂O ₅And GeO ₂SiO ₂Form.

8. the method for claim 1 is characterized in that described cladding glass zone is by SiO ₂Form.

9. methods for optical fiber manufacture is characterized in that comprising following steps:

Form cylinder shape assembly, it comprises:

Central authorities' core region;

Surround the first layer cladding glass of described core region;

The second layer cladding glass that surrounds described the first layer surrounds, and the viscosity of the described second layer is greater than the viscosity of described the first layer,

A pair of longitudinal slot extends along described glass preform diameter two ends;

Surround the Glass tubing of the described second layer; And

Surround one deck glass particle of described pipe;

Heat described assembly so that described particle solidifies, thereby

Applying radially inner reactive force on the described pipe so that described heating pipe shrinks and is melted on the described second layer, thereby forming drawing blank with longitudinal aperture parallel with described fibre core; And

Draw final drawing blank has the entity cross section with formation optical fiber.

10. method as claimed in claim 10 is characterized in that n ₃≤ n ₂

11. an optical fiber is characterized in that comprising:

Having oval cross section structure and specific refractory power is n ₁Fibre core, the length of described fibre core is than being ρ ₁, ρ here ₁Equal b ₁/ a ₁, b ₁Be fibre core major axis radius a ₁Be minor axis radius;

Surround the low viscosity zone with oval cross section of described fibre core, the length in described low viscosity zone is than being ρ ₂Here ρ ₂Equal b ₂/ a ₂, b ₂Major axis radius a for the low viscosity zone ₂Be minor axis radius.The length in low viscosity zone compares ρ ₂Less than ρ ₁And

Specific refractory power is n ₂And the cladding glass zone in encirclement low viscosity zone, the viscosity in described low viscosity zone is less than the viscosity in described cladding glass zone.

12. optical fiber as claimed in claim 11 is characterized in that n ₃≤ n ₂

13. optical fiber as claimed in claim 11 is characterized in that described low viscosity zone is by comprising one or more doping agents B ₂O ₃, fluorochemical, P ₂O ₅And GeO ₂SiO ₂Form.

14. optical fiber as claimed in claim 11 is characterized in that described cladding glass zone is by pure SiO ₂Form.

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