RU2530477C2 - Method of producing microstructured fibre light guides - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to fibre-optic engineering and can be used in producing microstructured fibre light guides used in optical amplifiers, lasers, spectral filters and telecommunication networks. A method of producing microstructured fibre light guides from a workpiece in which the upper end is sealed. The light guides are drawn from the lower heated end of the workpiece. To control and vary structural and physical parameters of the obtained microstructured fibre light guides, temperature of the upper part of the workpiece from the sealed end to the entrance of the furnace is controlled by an additional heater. To obtain microstructured fibre light guides with a constant external diameter, but with optical parameters which vary according to a given law along the light guide, temperature of the additional heater is varied according to a given law when drawing the light guide or a given constant temperature profile is created in the light guide.

EFFECT: wider field of use of the method owing to the possibility of producing long light guides with stable internal structural parameters, high reproducibility, easy monitoring and control of parameters of the light guides and producing a light guide structure with openings of different sizes.

3 cl, 6 dwg

Description

The invention relates to fiber optic technology and can be used in the production of microstructured fiber optical fibers used in optical amplifiers, lasers, spectral filters and telecommunication networks.

Known methods for the manufacture of microstructured optical fibers, in which the preform for the fiber has two groups of longitudinal holes, each of which is connected to a separate independent gas source, creating excess pressure in the holes. The excess pressures created in these groups of holes can be varied and controlled independently of each other, ultimately providing the desired geometry of the holes in the resulting microstructured fiber optical fibers (US Patent Nos. US6888992B2, G02B6 / 02, 2005 and No. US20030230118A1, C03B37 / 07, 2003) .

A disadvantage of the known methods is the high sensitivity of the dimensions of the openings obtained in the microstructured optical fiber from the temperature and the drawing speed of the optical fiber, which leads to low stability of the geometric and physical parameters of these optical fibers. Another disadvantage of the known methods is the high sensitivity of the sizes of the openings obtained in the microstructured fiber waveguide from the pressure created in them, which requires equal hole sizes within each group and limits the set of hole sizes to the number of pressure sources.

Known methods for the manufacture of microstructured optical fibers, in which the excess pressure created in the holes, can vary during the drawing of the fiber, which allows to obtain fibers with a constant outer diameter, but varying in length structure, i.e. with optical parameters that vary according to a given law (for example, the dispersion value) along the optical fibers (Japanese Patents No. JP2004238246A, G02B6 / 032, 2004 and No. JP2005084201A, G02B6 / 02, 2005). However, these methods have the same disadvantages as the above methods.

A known method of manufacturing microstructured optical fibers, in which the preform for the fiber is heated in the process of drawing two heaters, which allows you to control the temperature gradient and mechanical stresses in the zone of hauling the preform and thereby expand the range of materials used for the manufacture of optical fibers (German Patent No. DE102011103686A1, G02B6 / 00, 2011). However, this method does not eliminate the disadvantages inherent in the above methods.

There is also a known method of manufacturing microstructured optical fibers, selected as a prototype, in which the upper end of the preform is sealed and the fiber is drawn from the lower heated end of the preform, while the holes in the fiber can have different sizes that are established during the drawing of the fiber due to equilibrium between the surface tension forces of the molten workpiece material and the increase in internal pressure in the holes with a decrease in their diameter (US Patent A No. US005802236, G02B6 / 22, 1998).

The disadvantage of this method is the uncertainty in the degree of transformation of the geometric structure of the workpiece as it is pulled into the fiber (for example, a change in the relationship between the size of the holes and the distances between the holes) and the uncertainty depending on this degree of transformation on the length of the fiber. Given in a known method (US Patent No. US005802236, G02B6 / 22, 1998), examples of manufacturing a fiber show a practically proportional change in the size of the holes and the distance between them, which does not correspond to the actual fibers obtained using this method (SL Semjonov, A. N. Denisov , and E. M. Dianov, "Fabrication of Microstructured Fibers Using an Effect of Pressure Self-Regulation in Sealed Holes," in Specialty Optical Fibers, OSA Technical Digest (Optical Society of America, 2012), paper STu1 D.4). Due to this uncertainty, it is not possible to control and control the structural and physical parameters (for example, the dispersion value) of the fabricated microstructured optical fibers, which limits the practical application of the known method.

The task is to create a method of manufacturing microstructured fiber optical fibers, which provides the production of long optical fibers with stable internal structural parameters, with high reproducibility, ease of monitoring and control of optical fiber parameters, and allows fabrication of fiber structures with openings of different sizes.

The technical result is achieved by the fact that in the method of manufacturing microstructured optical fibers, in which the upper end of the preform is sealed, and the fiber is drawn from the lower heated end of the preform, an additional heater is used to maintain a certain controlled temperature of the upper part of the preform from the sealed end to the entrance to the furnace, providing the required degree of transformation of the geometric structure of the workpiece when it is pulled into the fiber.

Using a low-inertia additional heater, which allows you to change the temperature of the upper part of the preform from the sealed end to the entrance to the furnace during the drawing of the optical fiber, it is possible to obtain microstructured optical fibers with a constant external diameter, but a structure that varies in length, i.e. with varying optical parameters (for example, the magnitude of the dispersion) along the optical fibers.

Another way to change the temperature of the upper part of the preform from the sealed end to the entrance to the furnace using an additional heater is to create a temperature profile in it, which leads to a change in the average temperature of this part of the preform as it is lowered into the furnace during the drawing of the fiber. This path also allows one to obtain microstructured optical fibers with a constant external diameter and a structure varying in length, i.e. with changing parameters along the optical fibers.

The invention is illustrated by drawings.

Figure 1 shows the cross-section of a simple preform with one hole and elongated from it microstructured fiber.

Figure 2 shows an example of a cross-section of a workpiece for a microstructured fiber optic fiber with 60 holes.

Figure 3 shows the General drawing of the microstructured fiber optic fibers.

Figure 4 shows a graph of the change in the outer diameter of an elongated simple microstructured fiber with one hole as the workpiece is lowered into the furnace.

Figure 5 shows photographs of a cross section of an elongated microstructured optical fiber with 60 holes at different positions of the workpiece in the furnace.

Figure 6 shows a drawing scheme of the microstructured optical fibers with an additional heater to adjust the temperature of the part of the workpiece from the sealed end to the entrance to the furnace.

For simplicity, we consider the process of pulling a quartz glass preform (for specifying parameters; materials suitable for a particular task are used as material for the manufacture of optical fibers) with an initial external diameter D ₁ and one hole with a diameter of d ₁ into a microstructured optical fiber with an external diameter of D ₂ and an opening diameter d ₂ (see figure 1). We also assume that the feed rate of the preform into the furnace during drawing is ν ₁ , and the drawing speed of the fiber is ν ₂ . The general scheme of the case in question is depicted in FIG. 3. The upper end of the workpiece is sealed and part of the workpiece from this end to the entrance to the furnace is at a temperature T ₁ . Thus, the gas contained in the hole can only move towards the drawn fiber. The temperature in the center of the furnace (in the waist area) is equal to T ₂ . From the condition of continuity of the flow of quartz glass during stretching, it follows:

$$\frac{{\nu }_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="2"\; label="\nu "\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">\nu </figure-callout>}}_2}=\frac{S_2}{S_{\mathrm{one}}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="D"\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">D</figure-callout>}}_2^2-{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">d</figure-callout>}}_2^2}{D_{\mathrm{one}}^2-d_{\mathrm{one}}^2},\left(\mathrm{one}\right)$$

where S ₁ and S ₂ - the cross-sectional area of quartz glass in the workpiece and in the fiber.

It can be shown that with typical parameters of the process of drawing a microstructured fiber in the preform and up to the cone, the gas pressure P has time to establish the same throughout the volume of the preform ( P ₁ = P ₂ ). It can also be shown that the gas flow relative to the walls of the fiber in the direction of motion is negligible, so that in the elongated fiber, the gas substance is connected to the walls and motionless relative to them. Further, since different parts of the preform are maintained at different temperatures, the gas density ρ _i in different parts of the preform cavity will be different in accordance with the equation of state of an ideal gas:

$${\rho }_i=\frac{P_i\mu }{R{T}_i},\left(2\right)$$

where i = 1 or 2, μ is the molar mass of the gas substance, R is the universal gas constant.

In a stationary (steady state) case, from the condition of conservation of the amount of gas substance it follows:

$${\rho }_{\mathrm{one}}{s}_{\mathrm{one}}{\nu }_{\mathrm{one}}={\rho }_2{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">s</figure-callout>}}_2{\mathrm{<figure-callout\; id="2"\; label="\nu "\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">\nu </figure-callout>}}_2,\left(3\right)$$

where s ₁ and s ₂ are the cross-sectional area of the air holes in the preform and in the microstructured fiber, respectively. Then, taking into account relation (2), we obtain:

$$\frac{s_2}{s_{\mathrm{one}}}=\frac{{\nu }_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="2"\; label="\nu "\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">\nu </figure-callout>}}_2}\times \frac{T_2}{T_{\mathrm{one}}}.\left(\mathrm{four}\right)$$

In view of (1), this relation can be rewritten in the form:

$$\frac{s_2}{{\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">S</figure-callout>}}_2}=\frac{s_{\mathrm{one}}}{S_{\mathrm{one}}}\times \frac{T_2}{T_{\mathrm{one}}}.\left(5\right)$$

For a simple case with one hole, relation (4) is reduced to the form:

$$\frac{{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">d</figure-callout>}}_2^2}{d_{\mathrm{one}}^2}=\frac{{\nu }_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="2"\; label="\nu "\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">\nu </figure-callout>}}_2}\times \frac{T_2}{T_{\mathrm{one}}}.\left(6\right)$$

раз меньше, чем внешний диаметр световода (с учетом d ₁<<D ₁), что существенно отличается от приведенных в известном способе (Патент США № US005802236, G02В6/22, 1998) соотношений (~1,05). Это раздувание отверстий будет стабильным и постоянным до тех пор, пока поддерживается постоянной температура верхней части заготовки от ее запаянного конца до печи, т.е. пока запаянный конец заготовки не войдет в печь.In a typical case for quartz glass, T ₁ = 300 K and T ₂ 2100 K, then T ₂ / T ₁ = 7. Thus, from expression (6) we can conclude that, in comparison with proportional stretching, there is a significant relative inflation of the holes: the size of the holes when pulled into the fiber decreases $$\sqrt{7}\approx 2.65$$

times less than the outer diameter of the fiber (taking into account d ₁ << D ₁ ), which differs significantly from the ratios (~ 1.05) given in the known method (US Patent No. US005802236, G02B6 / 22, 1998). This inflating of the holes will be stable and constant as long as the temperature of the upper part of the workpiece is kept constant from its sealed end to the furnace, i.e. until the sealed end of the workpiece enters the oven.

By changing and controlling the temperature of a part of the preform from the sealed end to the entrance to the furnace with the help of an additional heater, it is easy to control and change the structures of the obtained microstructured optical fibers. For example, a change in T ₁ in the range from 300 K to 1200 K leads to a change in the degree of inflation of the holes ~ 2.65 to ~ 1.32.

It must be emphasized that if there are holes of different sizes ( d _1i ) in the preform, their degree of inflation in typical cases ( d _1i << D ₁ ) is practically equal, but in the general case they can be precisely taken into account, which makes it easy to fabricate fiber structures with different holes sizes.

Some refinement of the above relations, in particular, the expression (5), is required in the case of sufficiently large holes (more than 20 μm) in the fabricated microstructured optical fibers. In this case, it is necessary to take into account the cooling of the fiber after drawing to a temperature of T ₃ (about 300 K) and a corresponding decrease in the gas pressure in it, which leads to the formation of a gas flow and some decrease in the degree of inflation of the holes. In the general case, relation (5) can be represented as:

$$\frac{s_2}{{\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="00000012.png,00000013.png"\; state="\{\{state\}\}">S</figure-callout>}}_2}=\frac{s_{\mathrm{one}}}{S_{\mathrm{one}}}\times \frac{T_2}{T_{\mathrm{one}}}\times \left(\mathrm{one}-\theta \times \frac{T_2-T_3}{T_2}\right),\left(7\right)$$

where 0 < θ <1. The parameter θ depends on the diameter of the holes, on the drawing speed of the fiber (in particular, θ ~ 0 for d ₂ <20 μm and ν ₂ > 10 m / min) and can be estimated by a more detailed consideration of the drawing process or determined experimentally with specific drawing parameters.

In FIG. 1 shows cross-sections of a simple preform with one hole and a microstructured optical fiber elongated from it. The outer diameter of the workpiece is D ₁ , and the diameter of the hole in it is d ₁ . The microstructured optical fiber has an outer diameter D ₂ and a hole with a diameter of d ₂ . The scale of the images is significantly different: in practice, the diameter of the preform is usually about 1 - 2 cm, and the diameter of a standard fiber is 125 microns.

Figure 2 shows an example of a cross-section of a workpiece for a microstructured fiber optic fiber with 60 holes. All holes have a diameter d and are located at a distance Λ from the nearest holes. The hole missing in the center of the structure serves to form the light guide core of the microstructured fiber.

Figure 3 shows the General drawing of the microstructured fiber optic fibers. The upper end of the workpiece 1 is sealed and welded to the auxiliary rod 2, which is fixed in the feed system of the workpiece. Part of the workpiece from the welding area to the entrance to the furnace is at room temperature, that is, T ₁ ≈ 300 K. The feed system provides, when drawn, the workpiece is fed into the furnace at a speed of ν ₁ . The furnace 3 is used to heat the preform to the drawing temperature. The temperature in the center of the furnace in the waist zone for quartz glass is about 1800 ° C, i.e. T ₂ ≈ 2100 K. The microstructured fiber optic fiber 4 is drawn from the lower end of the preform using a receiving device, providing the extraction of the fiber with a speed of ν ₂ .

Figure 4 shows a graph of the change in the outer diameter of an elongated simple microstructured fiber with one hole as the workpiece is lowered into the furnace. The first section (the area marked by the number 1) corresponds to the process of establishing the degree of inflation of the fiber. At the initial moment, under the influence of surface tension forces in the molten billet, the hole completely collapses. Then, as the workpiece is immersed in the furnace, the volume of the cavity decreases and the gas pressure inside the workpiece begins to increase. This leads to the appearance of a small hole in the inner cavity of the drawn fiber. Then, when this inflation reaches the value determined by formula (6), the balance of the amount of gas substance begins to be fulfilled and a stationary state is reached, therefore, the pressure and diameter of the fiber does not increase further (the region marked by number 2). The constancy of geometry provides as long as the upper cold end of the workpiece has a constant temperature T ₁ . As the workpiece decreases, the cold end approaches the furnace and a rise in T ₁ begins from a certain distance. This leads to a decrease in inflation in accordance with the formula (6) (the area indicated by the number 3).

Figure 5 shows photographs of a cross section of an elongated microstructured optical fiber with 60 holes at different positions of the workpiece in the furnace. The top photo shows the cross section of the elongated fiber, related to the stationary mode, when the inflation reached the value determined by formula (4). The middle photo shows the cross section of the elongated fiber, referring to the beginning of the immersion of the workpiece in the furnace, when the inflation slightly decreased. The bottom photo shows the cross section of the elongated fiber, referring to the end of the workpiece when inflation is practically absent.

Figure 6 shows the drawing scheme of the microstructured optical fibers with an additional heater 5 for adjusting the temperature of the upper part of the workpiece from the sealed end to the entrance to the furnace (other designations are the same as in figure 3). By changing and controlling the temperature of this part of the preform with the help of an additional heater, it is easy to control and change the structures of the obtained microstructured optical fibers. For example, a change in T ₁ in the range from 300 K to 1200 K leads to a change in the degree of inflation of the holes from ~ 2.65 to ~ 1.32. By varying in some way the temperature of this part of the preform during the drawing of optical fibers, it is possible to obtain microstructured optical fibers with a constant external diameter but a structure that varies in length, i.e. with varying optical parameters (for example, the magnitude of the dispersion) along the optical fibers.

Claims (3)

1. A method of manufacturing a microstructured optical fiber, in which the upper end of the preform is sealed, and the optical fibers are drawn from the lower heated end of the preform, characterized in that for controlling and changing the structural and physical parameters of the obtained microstructured optical fiber, the temperature of the upper part of the preform from the sealed is used end to enter the furnace using an additional heater. 2. The method according to claim 1, characterized in that with the help of an additional heater during the drawing of the optical fibers, the temperature of the upper part of the preform changes from the sealed end to the entrance to the furnace, which makes it possible to obtain microstructured fiber optical fibers with a constant external diameter and a structure varying in length, t .e. with changing parameters along the optical fibers. 3. The method according to claim 1, characterized in that with the help of an additional heater a temperature profile is created, which leads to a change in the average temperature of the upper part of the workpiece from the sealed end to the entrance to the furnace as it is lowered into the furnace during the drawing of the fiber and allows you to get microstructured fiber optic fibers with a constant outer diameter and a varying length structure, i.e. with changing parameters along the optical fibers.

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