Patent Description:
Optical fibers are waveguides that can transmit light, with minimal scattering and attenuation, between two locations. Optical fibers, and the associated fiber optics, are well known and used in applications such as, illumination, communications, information transfer, and sensors. Optical fibers are typically flexible and very thin, and have a transparent core surrounded by one or more transparent cladding layers. The core and the cladding layers are made of vitreous material, such as high quality glass (made from, for example, silica, fluoride, phosphates, etc.). Typically, the core material has a refractive index which is greater than the refractive index of the cladding material. These conditions enable internal reflection of light signals passing through the fiber, resulting in an efficient waveguide.

Optical fibers are generally manufactured by drawing the fiber from a preliminary article, also known as a preform, which is heated in a vertically oriented furnace with a radial heating element. The preform includes the core material and the cladding layers as described above in essentially the same cladding-to-core ratio and refractive index profile as the desired optical fiber product. When the preform is heated in the furnace, a drawing bulb or glass drop forms at the lower softened end of the preform. The component can then be drawn off from the softened end of the preform with a given geometry and desired dimensions. Importantly, the drawn fiber must maintain the ratio between the diameter of the core material and the diameter of the cladding layers which exist in the initial preform in order to have the correct waveguide properties. With a square-cut preform, however, material waste can occur due to at least two causes. First, the formation of the drawing bulb results in a substantial waste of good preform material because the drawing bulb or the glass drop itself does not yield optical fiber. Second, as the preform end is heated radially, the temperature distribution, and therefore the viscosity of the preform, is highly non-uniform and it is very difficult to prevent differential glass flow between the core material and the cladding layers. As a result, the cladding-to-core ratio may be distorted at the start of the preform tipping or fiber draw, resulting in unusable fiber there. Distortion in the cladding-to-core ratio negatively affects many waveguide properties of the fiber, such as cutoff wavelength, mode field diameter, dispersion, and core eccentricity. Accordingly, it is desirable to modify the square-cut preform in a way that causes the drawing bulb to form with less material waste and waveguide distortion.

One method of modifying the preform is tapering the end of the preform by machining or flame tipping. Machining a taper into the preform end, however, can destroy the correct cladding-to-core ratio, resulting in fiber failures in cutoff wavelength and other optical properties. Flame or furnace tipping on square cut preforms also wastes a significant amount of good preform material and cause waveguide distortion.

Other methods of modifying the preform, such as that disclosed in <CIT>, include attaching a cone-shaped piece to the machined and tapered end of the preform, such that the drawing bulb is formed from both the cone-shaped piece and the machined taper of the good preform material. However, the method disclosed in Peekhaus requires the preform to be machined to a taper prior to fiber draw, which increases the complexity and cost of the process as well as the waste of good preform material for the reasons described above.

<CIT> discloses a method of making an elongated glass component by drawing an assembly composed by a rod and a tube surrounding the rod. The rod is positioned in the tube such that an annular gap is defined between an outer surface of the rod and an inner surface of the tube. The upper end of the rod is provided with an increased diameter portion such that the increased diameter portion engages the tube and supports the rod with respect to the tube. During drawing the tube is collapsed onto the rod such that the tube portions fuse to the rod forming collapsed portions of the rod and the tube assembly. The collapsed portions are drawn to form an optical fiber or an optical fiber preform.

<CIT> discloses a method for forming a large optical glass tube that serves for jacketing a quartz glass rod according to the so-called "rod-in-tube technique". In order to comply with very precise dimension requirements the large quartz glass tube is obtained from an also large, thick-walled starting cylinder which dimensions and surface quality is set in several treatment steps. An obligatory forming step is a mechanical treatment of the <CIT> discloses a method for forming a large optical glass tube that serves for jacketing a quartz glass rod according to the so-called "rod-in-tube technique". In order to comply with very precise dimension requirements the large quartz glass tube is obtained from an also large, thick-walled starting cylinder which dimensions and surface quality is set in several treatment steps. An obligatory forming step is a mechanical treatment of the inner wall of the starting cylinder. As an optional step a subsequent elongation of the mechanically treated starting cylinder is mentioned. During that elongating step a dummy tube is fixed to the lower end of the large starting cylinder to be drawn.

<CIT> also discloses a method for elongating an optical preform wherein a dummy rod or a dummy tube is fixed to the lower end of the preform to be drawn. The adjacent end portions of the optical fiber preform and the dummy rod are machined in a convex-fashion and joint together by welding.

<CIT> discloses a method for making an optical fiber preform. A core glass rod is inserted into a cladding tube. The glass rod is fused to the cladding tube only at the ends, so that an annular gap is left between the rod and the tube. The annular gap is then evacuated so that it is closed and the cladding tube is collapsed onto the core glass rod.

<CIT> provides a method for stretching an optical fiber preform in order to remove non-uniformity on its surface. The preform consists of a core and a cladding made of vapor phase synthesized quartz glass. Dummy rods also made of vapor phase synthesized quartz are welded to both ends of the optical fiber preform. The dummy rods may consist of axial core parts which are aligned to the core of the preform and axial core coating parts which are aligned with the cladding.

Embodiments of the disclosure include glass preforms for producing elongated optical glass components. The preform has a bottom end and includes a primary rod having a constant outside diameter and a square bottom; and a cylindrical sacrificial tip having a first end attached to the bottom of the primary rod, a second end opposite the first end, and a hollow interior region extending from the first end to the second end. The primary rod includes a core rod surrounded by one or more outer cladding layers. The cylindrical sacrificial tip is circular in cross section and the first end of the cylindrical sacrificial tip has an outside diameter equal to the outside diameter of the primary rod. The cylindrical sacrificial tip is configured to form a drip at the bottom end of the preform when elongating the primary rod. The primary rod and the cylindrical sacrificial tip may both made of quartz glass, and the quartz glass of the primary rod may be of higher quality than the quartz glass of the cylindrical sacrificial tip. The cylindrical sacrificial tip may have a constant outside diameter equal to the outside diameter of the primary rod. The hollow interior region may have an inside diameter ranging from approximately <NUM>% to approximately <NUM>% of the outside diameter of the cylindrical sacrificial tip. The cylindrical sacrificial tip may have a length of approximately <NUM> to approximately <NUM>, preferably approximately <NUM> to approximately <NUM>, and most preferably approximately <NUM> to approximately <NUM>. The cylindrical sacrificial tip may be welded to the primary rod.

The glass preform is suitable for producing an elongated optical glass component. The method includes positioning the glass preform in a furnace, where the glass preform includes a primary rod having a constant outside diameter and a square bottom, and a cylindrical sacrificial tip having a first end attached to the bottom of the primary rod, a second end opposite the first end, and a hollow interior region extending from the first end to the second end; and heating the glass preform in the furnace to soften the cylindrical sacrificial tip. The cylindrical sacrificial tip is circular in cross section and the first end of the cylindrical sacrificial tip has an outside diameter equal to the outside diameter of the primary rod. Heating the glass preform in the furnace to soften the cylindrical sacrificial tip forms a drip at a bottom end of the preform and the drip pulls down on and elongates the primary rod. The primary rod and the cylindrical sacrificial tip may both made of quartz glass, and the quartz glass of the primary rod may be of higher quality than the quartz glass of the cylindrical sacrificial tip. The cylindrical sacrificial tip may have a constant outside diameter equal to the outside diameter of the primary rod. The hollow interior region may have an inside diameter ranging from approximately <NUM>% to approximately <NUM>% of the outside diameter of the cylindrical sacrificial tip. The cylindrical sacrificial tip may have a length of approximately <NUM> to approximately <NUM>, preferably approximately <NUM> to approximately <NUM>, and most preferably approximately <NUM> to approximately <NUM>. The cylindrical sacrificial tip may be welded to the primary rod. The primary rod includes a core rod surrounded by one or more outer cladding layers. The glass preform may be preheated at a height above the center of the furnace prior to positioning the glass preform in the furnace at an optimized location within the furnace. Preheating the glass preform outside of the furnace may include heating the furnace at a low power; positioning the glass preform at a first location above the center of the furnace at low power for a first period of time; raising the power of the furnace to a high operating power of the furnace; and lowering the preform into the furnace to an optimized hanging location above the center of the furnace. Lowering the preform into the oven to the optimized hanging location may include lowering the preform into the oven from the first location to a second location above the optimized hanging location; holding the preform at the second location for a period of time; and lowering the preform into the oven from the second location to the optimized hanging location. The drip formed at a bottom end of the preform may include substantially only material from the cylindrical sacrificial tip and not material from the primary rod. The primary rod comprises a core rod surrounded by an outer cladding layer having a constant cladding-to-core ratio. Due to the gravitational force acting on the glass at different radial positions with different temperatures and viscosities, the drip pulling down on and elongates the primary rod may pull on an outside portion of the cladding layer without pulling on the core rod, resulting in reduced differential clad and core glass flow and waveguide distortion. The elongated primary rod may have a cladding-to-core ratio which is substantially the same as the cladding-to-core ratio of the unelongated primary rod.

The disclosure is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. Included in the drawings are the following figures:.

Embodiments include a preform for fabricating a glass fiber. The preform includes a sacrificial tip welded to a primary rod made of high-quality material. When the preform is heated in a furnace, the sacrificial tip softens (i.e. the viscosity decreases) and collapses into a tapered tube which draws the primary rod into the glass fiber or results in a tipped preform. Embodiments of also include methods of using the preform to form the glass fiber or a tipped preform. Exemplary embodiments will now be described in conjunction with <FIG>, and <FIG>.

Referring to <FIG>, a preform <NUM> is provided according to exemplary embodiment. <FIG> is a cross-sectional view of the preform <NUM>. <FIG> is a bottom view of the preform <NUM>. The preform <NUM> includes a primary rod <NUM> and a sacrificial tip <NUM>.

The primary rod <NUM> may include a cladding layer <NUM> surrounding a core rod <NUM> in a coaxial arrangement aligned along a common center line CL. The cladding layer <NUM> and the core rod <NUM> may each be made of high-purity quartz glass formed by any suitable process, such as fused quartz or one or more types of chemical vapor deposition (CVD), including inside vapor deposition, outside vapor deposition and vapor axial deposition. The core material withinthe core rod <NUM> may have a refractive index which is greater than the refractive index of the material in the surrounding cladding layer <NUM> to enable internal reflection of light signals passing through a fiber drawn from the preform <NUM>, resulting in an efficient waveguide. In other embodiments, the primary rod <NUM> may include no cladding layers or two or more cladding layers, or may also include an uncollapsed rod-in-cylinder preform assembly with a core rod surrounded by one or more overclad tubes or cylinders. The primary rod <NUM> may have an essentially constant outside diameter. Although it will be understood that the primary rod <NUM> may have any outside diameter, in an exemplary embodiment may be up to <NUM> in some embodiments, but is not limited to this range. In other embodiments, the outside diameter of the primary rod <NUM> may be, for example, <NUM> to <NUM> or even larger.

In this exemplary embodiment, the sacrificial tip <NUM> is circular in cross section (measured perpendicular to the center line CL), and has a first end <NUM> attached to a bottom <NUM> of the primary rod <NUM> and a second end <NUM> opposite the first end <NUM>. The sacrificial tip <NUM> may be attached to the primary rod by thermal welding, for example. The primary rod <NUM> and the sacrificial tip <NUM> are aligned along the common center line CL. The sacrificial tip <NUM> further includes a hollow region <NUM> which is also circular in cross section and extends fully through the sacrificial tip <NUM> from the first end <NUM> to the second end <NUM>. To reduce the material cost of the preform <NUM>, the sacrificial tip <NUM> may be made of a lower quality material than the primary rod <NUM>. Like the primary rod <NUM>, the sacrificial tip <NUM> may be formed by any suitable process, such as, but not limited to, fused quartz or one or more types of chemical vapor deposition (CVD), including inside vapor deposition, outside vapor deposition and vapor axial deposition. The sacrificial tip <NUM> has an outside diameter at the first end <NUM> which is equal to the outside diameter of the primary rod <NUM> at the bottom <NUM>. In an exemplary embodiment, the sacrificial tip <NUM> has a constant outside diameter along its entire length equal to the outside diameter of the primary rod <NUM>. In other words, in the exemplary embodiment, the sacrificial tip <NUM> is a cylinder with a constant outside diameter equal to the outside diameter of the primary rod <NUM>. In other embodiments, the outside diameter of the sacrificial tip <NUM> may vary along with the length of the sacrificial tip <NUM>. As explained below in greater detail, the inside diameter of the sacrificial tip <NUM> (i.e., the diameter of the hollow region <NUM>) and the length (measured parallel to the center line CL) of the sacrificial tip <NUM> will vary based on the drawing conditions (e.g., the temperature distribution and dimensions of the draw furnace). In an exemplary embodiment, the optimized inside diameter ranges from approximately <NUM>% to approximately <NUM>% of the outside diameter of the sacrificial tip <NUM> and the length ranges from approximately <NUM> to approximately <NUM>, preferably <NUM> to <NUM>, and most preferably <NUM> to approximately <NUM>. The inside diameter may vary or be constant along the length of the sacrificial tip <NUM>. For example, the sacrificial tip <NUM> may have a constant inside diameter. In other words, the hollow region <NUM> may be cylindrical. In other embodiments where the outside diameter varies, the inside diameter may also vary by the same degree, such that the sacrificial tip has a constant wall thickness (i.e., the difference between the inside diameter and the outside diameter). In the exemplary embodiment depicted in <FIG>, both the inside diameter and outside diameter are constant such that the sacrificial tip is a hollow cylinder with a constant outside diameter equal to the outside diameter of the primary rod <NUM>.

By varying the dimensions of the sacrificial tip <NUM>, the preform <NUM> may be used in a method which draws an optical fiber from the preform <NUM> while minimizing material waste and waveguide distortion. As discussed in more detail below, the inside diameter and the length of the sacrificial tip <NUM> are optimized such that, when heated, the sacrificial tip <NUM> deforms and collapses into a tapered tube that is made primarily from material from the sacrificial tip <NUM> and minimizes the waste of material from the primary rod <NUM> in the initial glass drop. The sacrificial tip <NUM> also balances the gravitational and viscosity-related forces acting on the primary rod <NUM> in a radially-uniform manner that minimizes the distortion to the cladding-to-core ratio (i.e., by balancing the forces applied to various radial locations of the primary rod <NUM> to reduce or eliminate differential cladding and core glass flow).

Referring to <FIG>, the preform <NUM> described above may be used to form an elongated glass component by positioning the preform <NUM> in a furnace <NUM> and heating the preform <NUM> in the furnace <NUM>. The furnace <NUM> includes a heating element <NUM>, for example made of graphite or ceramic. The heating element <NUM> generates radiative heat, typically thorough electrical resistance or inductive heating, which increases the temperature of the furnace <NUM> and transfers thermal energy to the preform <NUM> through mutual radiation exchange. The available thermal energy is greatest horizontally in-line with the heating element <NUM>, and particularly adjacent to the center <NUM> of the heating element <NUM>. As the vertical distance from the center <NUM> increases, the available thermal energy in the furnace <NUM> decreases. As the preform <NUM> is heated, the primary rod <NUM> and the sacrificial tip <NUM> begin to soften according to the temperature, and therefore the viscosity, distribution. The sacrificial tip applies additional gravitational force to the outer part of the cladding layer <NUM> without pulling on the core rod <NUM>, which balances the glass flow between the cladding layer <NUM> and the core rod <NUM>. As a result, distortion to the cladding-to-core ratio is minimized and good waveguide or fiber yield is increased. Because the cladding-to-core ratio distortion is minimized, the waveguide properties of the resulting fiber, such as cutoff wavelength, mode field diameter, dispersion, and core eccentricity, are also improved. The sacrificial tip <NUM> also collapses to form the tapered tube at the bottom end of the preform <NUM> which is made essentially only material from the sacrificial tip. The formation of the tapered tube from the sacrificial tip <NUM> is best seen in <FIG>, discussed below in more detail along with Example <NUM>. The tapered tube is then able to pull down evenly on the remaining primary rod <NUM> and eliminates the formation of a bulb. Because the bulb is generally not usable as an optical fiber, eliminating the need to form the bulb from the primary rod <NUM> to draw the fiber reduces material waste. It was also discovered that, although the addition of the sacrificial tip <NUM> eliminates the formation of the bulb, the sacrificial tip <NUM> also reduces the drip time relative to a square cut preform with no sacrificial tip, as discussed below in more detail in conjunction with Example <NUM>.

In order to ensure maximum performance of the sacrificial tip <NUM> (i.e., minimize the amount of waste material from the primary rod <NUM> and the distortion of the cladding-to-core ratio), the positioning of the preform <NUM> and the way thermal energy is transferred to the preform <NUM> within the furnace are controlled. As explained above, because the radiative thermal energy in the furnace <NUM> varies with vertical position, the amount of thermal energy transferred to various parts of the preform <NUM> can be controlled by controlling the vertical position of the preform <NUM> in the furnace <NUM>. Therefore, the viscosity of the various parts of the preform <NUM> can also be controlled through the resulting temperature distribution. By controlling the relative viscosities of the sacrificial tip <NUM> and the primary rod <NUM>, the sacrificial tip <NUM> softens and begins to drip into the tapered tube before the primary rod <NUM> drips too much, eliminating the formation of a drawing bulb and balancing the forces applied to the core rod <NUM> and the cladding layer <NUM>. If the sacrificial tip <NUM> drips prematurely before the primary rod <NUM> is softened, the weight of the sacrificial tip <NUM> will not be able to pull the primary rod <NUM> into a fiber. If the primary rod <NUM> softens too quickly, a drawing bulb made of the primary rod <NUM> will form, resulting in increased waste.

As explained in greater detail in the Examples below, the joint between the primary rod <NUM> and the sacrificial tip <NUM> is preferably located above the center <NUM> of the heating element <NUM>. As a result, the sacrificial tip <NUM> is initially exposed to greater temperatures than the primary rod <NUM>. This temperature differential results in the sacrificial tip <NUM> softening prior to the primary rod <NUM> softening. As explained below in Examples <NUM> and <NUM>, positioning the preform <NUM> too high in the furnace <NUM> results in the primary rod <NUM> not softening enough to be pulled down by the sacrificial tip <NUM>, and positioning the preform <NUM> too low in the furnace <NUM> results in the primary rod <NUM> softening and dripping along with the sacrificial tip <NUM>. Each case results in wasted material of the primary rod <NUM> or an unacceptably long drip time. In some embodiments, the preform <NUM> may be lowered gradually into the furnace in order to further control heat transfer between the furnace <NUM> and the preform <NUM>. Gradually lowering the preform <NUM> into the furnace <NUM> prevents thermally induced cracking at the joint between the primary rod <NUM> and the sacrificial tip <NUM>. Generally, exposing the cold preform <NUM> to maximum oven temperature temperatures results in thermal shock which can crack the preform <NUM>. Heat transfer may also be controlled instead of, or in addition to, gradually lowering the preform <NUM> into the furnace <NUM> by ramping the temperature of the furnace <NUM> while the preform <NUM> is in the furnace <NUM>.

In an exemplary embodiment, the process includes initially positioning the joint between the primary rod <NUM> and the sacrificial tip <NUM> at a distance above the center <NUM> of the heating element <NUM> which is greater than the length of the heating element <NUM>, for example approximately <NUM>% of the length of the heating element <NUM>, while reduced power is applied to the heating element <NUM>. Power to the heating element <NUM> is then increased and the preform <NUM> is lowered into the furnace <NUM> once a desired temperature is reached inside the furnace <NUM>, for example <NUM>. The preform may then be lowered to the optimal position in which the joint between the primary rod <NUM> and the sacrificial tip <NUM> is located above the center <NUM> of the heating element <NUM>. In other embodiments, the preform <NUM> may first be lowered to a second position above the optimal position, held for a period of time, and then lowered the remaining distance to the optimal position. The second location may be approximately <NUM>% of the length of the heating element <NUM> below the initial position, and the preform <NUM> may be held at the second position for approximately <NUM> minutes.

The following examples are included to demonstrate the effects of changes in sacrificial tip thickness (i.e., difference between the outside diameter and the inside diameter), sacrificial tip length, and positioning of the preform in the draw furnace. In each example, finite element modeling (FEM) was used to simulate a primary rod having an outer diameter of <NUM> positioned in a draw furnace having an inner diameter of <NUM> and a graphite heating element <NUM> in length. The FEM model was able to accurately simulate the key radiation exchange mechanism between the furnace and the preform to capture the preform geometry and position inside the furnace during heating. The accuracy of the FEM model was confirmed by conducting experiments with actual preforms under the same conditions used in the model and comparing the results.

Examples <NUM>-<NUM> detail the impact of sacrificial tip geometry and preform <NUM> position on the change in shape of the preform over time. In each of <FIG>, the original position and geometry of the preform is indicated by the white outline. The position and shape of the preform <NUM> at the time of each figure is indicated by the shaded outline, with the shade corresponding to the temperature of the preform <NUM> according to the scale provided to the right of each figure. Example <NUM> depicts a model of a preform with a sacrificial tip having an optimized wall thickness, length, and furnace position. Example <NUM> depicts a model of a preform with a sacrificial tip which has a wall which is too thin. Example <NUM> depicts a model of a preform with a sacrificial tip which has a wall which is too thick. Example <NUM> depicts a model of a preform with a sacrificial tip which is too long. Example <NUM> depicts a model of a preform with a sacrificial tip which is too short. Example <NUM> depicts a model of a preform with a sacrificial tip which is positioned too high in the furnace. Example <NUM> depicts a model of a preform with a sacrificial tip which is positioned too low in the furnace.

Example <NUM>, described in conjunction with <FIG>, details the impact of the sacrificial tip on the glass drop waste of the drawing bulb at the bottom of the preform and the drip time of the preform.

Example <NUM>, described in conjunction with <FIG> and <FIG>, details the impact of the sacrificial tip on the cladding-to-core ratio of the resulting drawn glass strand in the glass drip.

Example <NUM>, described in conjunction with <FIG>, <FIG>, and <FIG>, details the impact on the position of the preform within the furnace on the cladding-to-core ratio of the resulting drawn glass strand in the glass drip.

In Example <NUM>, the model includes a hollow cylindrical sacrificial tip having an outside diameter of <NUM> (i.e., equal to the outside diameter of the primary rod), an inside diameter of <NUM>, and a length of <NUM>. The thickness (i.e., the difference between the outside diameter and the inside diameter) of the sacrificial tip is <NUM>. The preform is positioned in the draw furnace with the joint between the sacrificial tip and the primary rod positioned <NUM> above the center of the furnace. As can be seen from <FIG>, the sacrificial tip begins to drip so that it drags the preform bottom to form a narrow tip which consists almost entirely of material from the sacrificial tip. As a result, essentially no material of the primary rod (i.e., the higher quality preform material) is wasted to form a drawing bulb.

In Example <NUM>, the model of Example <NUM> was repeated with the sacrificial tip inside diameter increased to <NUM>, thereby reducing the sacrificial tip wall thickness to <NUM>. The remaining dimensions were kept constant from Example <NUM>. As can be seen from <FIG>, the reduced wall thickness results in a sacrificial tip that is too thin to drag a sufficient bottom area of the primary rod to draw a fiber. Accordingly, the tapered tube takes longer to develop and includes more material from the primary rod, resulting in material waste.

In Example <NUM>, the model of Example <NUM> was repeated with the sacrificial tip inside diameter reduced to <NUM>, thereby increasing the sacrificial tip wall thickness to <NUM>. The remaining dimensions were kept constant from Example <NUM>. As can be seen from <FIG>, when the sacrificial tip wall is too thick, the increased weight results in too much material from the primary rod being pulled into the drawing bulb, resulting in material waste. However, the waste is less than in Example <NUM> where the sacrificial tip wall is too thin. This suggests that there is greater tolerance toward thicker sacrificial tip walls.

In Example <NUM>, the model of Example <NUM> was repeated with the sacrificial tip length reduced to <NUM>. The remaining dimensions were kept constant from Example <NUM>. As can be seen from <FIG>, when the sacrificial tip is too short, the weight of the sacrificial tip is not sufficient to drag the bottom of the primary rod downward before the primary rod begins to drip by itself. As a result, a thicker than desired preform bottom drip develops and material is wasted.

In Example <NUM>, the model of Example <NUM> was repeated with the sacrificial tip length increased to <NUM>. The remaining dimensions were kept constant from Example <NUM>. As can be seen from <FIG>, when the sacrificial tip is too long, the weight makes the sacrificial tip drip easier and faster, and does not last long enough to drag the bottom of the primary rod downward. Instead, the sacrificial tip forms a very thin tube, and a drawing bulb forms from material from the primary rod, as if they sacrificial tip were not attached.

In Example <NUM>, the model of Example <NUM> was repeated with the joint between the sacrificial tip and the primary rod moved up to <NUM> above the center of the furnace. The remaining dimensions were kept constant from Example <NUM>. As can be seen from <FIG>, when the preform is positioned too high in the furnace, the sacrificial tip is heated more than the primary rod, and the sacrificial tip drips and forms a thin tube before the primary rod is sufficiently softened by the heat of the furnace to be drawn by the weight of the drip. A drawing bulb will instead form at the bottom of the primary rod once it is sufficiently hot, resulting a waste of material.

In Example <NUM>, the model of Example <NUM> was repeated with the joint between the sacrificial tip and the primary rod moved down to <NUM> above the center of the furnace. The remaining dimensions were kept constant from Example <NUM>. As can be seen from <FIG>, when the preform is positioned too low in the furnace, the primary rod is softened by the heat of the furnace prematurely and too much material from primary drips along with the sacrificial tip, resulting in wasted material.

In Example <NUM>, four different preforms were tested to determine the effect of a sacrificial tip on drawing bulb mass and drip time. The four preforms were a <NUM> primary rod with no sacrificial tip, a <NUM> primary rod with a solid <NUM> with an outside diameter of <NUM>, a <NUM> primary rod with a solid <NUM> stub with an outside diameter of <NUM>, and a <NUM> primary rod with a hollow cylindrical sacrificial tip with a length of <NUM>, an outside diameter of <NUM>, and an inside diameter of <NUM>. Each preform was tested with the preform bottom at various heats relative to the center of the heating element. As can be seen from <FIG>, as the preform bottom is moved up in the furnace, the mass of the drawing bulb decreases. In the case of the hollow cylinder sacrificial tip, the mass of the preform glass drop is goes to essentially zero as the preform bottom is moved at least <NUM> above the center of the furnace, indicating essentially no material waste. Furthermore, despite the reduced mass of the drawing bulb, the preform with the hollow cylinder sacrificial tip also demonstrated substantially reduced drip times, indicating a faster and more efficient draw process.

In Example <NUM>, the impact of the sacrificial tip on the cladding-to-core ratio of the resulting drawn fiber was measured by comparing a <NUM> preform with no sacrificial tip (<FIG>) to a <NUM> preform with a hollow cylindrical sacrificial tip with a length of <NUM>, an outside diameter of <NUM>, and an inside diameter of <NUM> (<FIG>). <FIG> and <FIG> depict the position, geometry, and temperature of the respective preforms after a tapered tube has formed at the bottom of the preform. <FIG> and <FIG> depict the respective preforms at the intersection of the tapered tube and the preform body, specifically detailing the presence of the core rod within the preform. <FIG> and <FIG> depict the cladding-to-core ratio along the length of the preform. As can be seen from <FIG>, without the sacrificial tip, the core rod is pulled down into a drawing bulb, resulting in large variations in cladding-to-core ratio. Such a distorted cladding-to-core ratio results in unusable fiber and the draw must continue until the cladding-to-core ratio stabilizes, resulting in material waste. In comparison, as can be seen from <FIG>, the addition of the hollow cylinder sacrificial tip forms a thin, tapered tube that includes essentially no material of the core rod and reduces the cladding-to-core ratio distortion in the neckdown and drip compared to the preform with no sacrificial tip.

In Example <NUM>, the impact on the position of the preform within the furnace on the cladding-to-core ratio of the resulting drawn fiber was measured by comparing the result of a <NUM> preform with a hollow cylinder sacrificial tip with a length of <NUM>, an outside diameter of <NUM>, and an inside diameter of <NUM> at various furnace positions, specifically at an optimized position (<FIG>), <NUM> below the optimized position (<FIG>), and <NUM> above the optimized position (<FIG>). <FIG>, <FIG>, and <FIG> depict the position, geometry, and temperature of the respective preforms after a tapered tube has formed at the bottom of the preform. <FIG>, <FIG>, and <FIG> depict the respective preforms at the intersection of the tapered tube and the preform body, specifically detailing the presence of the core rod within the preform. <FIG>, <FIG>, and <FIG> depict the cladding-to-core ratio along the length of the preform. As shown in <FIG>, when the preform with sacrificial tip is positioned at the optimized location, the preform bottom forms a tip with minimum glass waste, and a minimum portion of glass with an altered cladding-to-core ratio. As shown in <FIG>, when the preform is positioned too low, the drip from the sacrificial tip is much shorter and a drip of preform glass also forms. Material from the core rod can be observed in the drip, resulting in significant distortion to the cladding-to-core ratio. As shown in <FIG>, when the preform is positioned too high, a thin, hollow tube forms at the bottom of the preform which includes material from the cladding layer. Because of the dripping of the cladding glass, the cladding-to-core ratio is significantly distorted.

Claim 1:
A glass preform (<NUM>) for producing an elongated optical glass component, the preform (<NUM>) having a bottom end and comprising:
a primary rod (<NUM>) having a constant outside diameter and a square bottom (<NUM>), wherein the primary rod (<NUM>) includes a core rod (<NUM>) surrounded by one or more outer cladding layers (<NUM>); and
a cylindrical sacrificial tip (<NUM>) having a first end (<NUM>) attached to the bottom (<NUM>) of the primary rod (<NUM>), a second end (<NUM>) opposite the first end (<NUM>), and a hollow interior region (<NUM>) extending from the first end (<NUM>) to the second end (<NUM>);
wherein the cylindrical sacrificial tip (<NUM>) is circular in cross section and the first end (<NUM>) of the sacrificial tip (<NUM>) has an outside diameter equal to the outside diameter of the primary rod (<NUM>); and
said cylindrical sacrificial tip (<NUM>) is configured to form a drip at the bottom end of the preform (<NUM>) when elongating the primary rod (<NUM>).