Patent Application
20100303429
The present invention relates to microstructured optical fibers and methods for making the same.
Optical fibers have found increasing uses in industrial, scientific, and military applications. Conventional optical fibers guide light passing through them using the principles of total internal reflection. Total internal reflection (TIR) occurs when light travels through a material having a high index of refraction n and strikes an interface between that material and a material having a lower value of n. If the angle of incidence of the light on the interface is greater than some angle, known as the “critical angle” θc, the light cannot pass through the interface into the lower-refractive material but instead is reflected back into the higher-refractive material. Thus, for optical glass fibers, the principle of total internal reflection requires that the inner core of the fiber have a higher index of refraction than the outer cladding. However, due to the nature of the materials used, such conventional fibers still exhibit some absorption and scattering of the light traveling through them and can therefore suffer some loss as the signal travels through the fiber.
More recently, microstructured optical fibers have been developed in an attempt to improve the transmission and reduce the leakage of light traveling therethrough. These microstructured optical fibers include solid core photonic crystal fibers (SC-PCF) and hollow core photonic band gap (HC-PBG) fibers. Like conventional optical fibers, both SC-PCF and HC-PBG fibers have a three-layer structure comprising a core area, an intermediate cladding surrounding the core area, and a jacket made of solid glass surrounding the cladding. However, in both SC-PCF and HC-PBG fibers, the cladding is not solid as in conventional optical fibers, but instead comprises a microstructured region having a periodic arrangement of glass and holes, which confines the light to the core of the fiber.
In SC-PCFs, the core area is solid, and the confinement mechanism is similar to that of conventional TIR fibers, in that the cladding has a lower average refractive index than the solid core due to the presence of air holes in the glass. One benefit SC-PCFs have when compared to conventional fibers is that single mode operation can readily be obtained simultaneously for a large range of wavelengths, rather than for a single wavelength (or very narrow band of wavelengths) as in conventional TIR fibers. This is primarily due to the wavelength dependence of the refractive effective index of the lowest order mode. See e.g., T. A. Birks et al., “Endlessly single-mode photonic crystal fiber,” Optics Letters, Vol. 22, pp. 961-963 (1997) (describing guidance in and design of PCF fibers). In addition to being “endlessly single-mode,” these fibers can also have very high nonlinearity and other useful properties.
In contrast, HC-PBG fibers have a hollow core, and operate on the principle of two-dimensional photonic bandgap confinement, a condition which prohibits the propagation of specific wavelengths within the photonic bandgap cladding region. The existence of a photonic bandgap is governed by the wavelength of interest, and the transverse dielectric function of the fiber. The transverse dielectric function describes the refractive index of a cross-section of the fiber and is governed by the refractive index of the glass, the shape and location of the holes, the hole diameter and pitch (the ratio of which governs the air fill fraction) and the lattice arrangement (i.e., triangular, square, etc.) Since the light in HC-PBG fibers is confined primarily to the air void and not the glass as in conventional TIR fibers, both signal loss and light-induced fiber damage are reduced. This enables HC-PBG fibers to transmit higher energy signals over longer distances.
Microstructured optical fibers have been fabricated from silica and other glasses, and their design and manufacture have been described in the literature. For example, see S. Barkou et al., “Silica-air photonic crystal fiber design that permits waveguiding by a true photonic bandgap effect,” Optics Letters, Vol. 24, No. 1, pp. 46-48 (1999) (describing silica glass fiber having air holes arranged in a honeycomb pattern with an additional central air hole in which light having specific wavelengths can be confined); N. Venkataraman, et al., “Low loss (13 dB/km) air core photonic band-gap fibre,” ECOC, Postdeadline Paper PD1.1, September, 2002 (describing low signal loss properties of silica glass HC-PBG fibers); and P. Russell, “Photonic Crystal Fibers,” Science, Vol. 299, No. 3, pp 358-362 (2003) (describing silica glass photonic crystal fibers in general).
Such microstructured optical fibers are typically made using a preform comprising an outer shell and a number of hollow tubes arranged in a periodic structure, with either a hollow (HC-PBG) or solid (SC-PCF) core. See e.g., R. F. Cregan, et al., “Single-mode photonic band gap guidance of light in air,” Science, Vol. 285, pp. 1537-1539 (1999) (describing photonic band gap (PBG) guidance of light through optical fiber comprising tubes of silica glass arranged in a periodic pattern); and U.S. Pat. No. 6,847,771 (describing microstructured optical fibers and fabrication of such fibers from optical fiber preforms).
Microstructured optical fibers also can be made from non-silica glass such as chalcogenide glasses. See, e.g., U.S. Patent Application Publication No. 2005/0074215; U.S. Patent Application Publication No. 2006/0230792; and U.S. Pat. No. 7,295,740, each of which shares at least one inventor in common with the present invention.
A microstructured optical fiber is typically made using a preform which is then drawn into the final fiber. In the preform, a number of glass microtubes or microcanes are placed in a periodic arrangement between the core and the outer jacket to form the cladding. Such microtubes are hollow tubes having an opening, i.e., a hole, extending through their entire length, while microcanes may be solid or hollow. The arrangement of the microtubes and/or microcanes creates a periodic structure of glass and holes in the cladding which affects the transmission of light therethrough. The preform is then drawn to create the optical fiber.
However, because the microtubes and/or microcanes comprising the cladding do not always fit together perfectly, there may be gaps, or voids, at the interfaces between the microtubes/microcanes or between the cladding area and the outer jacket. Such “interfacial voids” extend longitudinally through the entire length of the preform and are connected to the ambient atmosphere outside the preform via the preform ends. Many of these voids can be eliminated during the fiber drawing or other heat treatment step as the tubes are drawn closer together, but often some of these voids remain as “interstitial voids.” These interstitial voids are not connected to the atmosphere outside the fiber but are trapped within the fiber.
The presence of both the interfacial and interstitial voids is undesirable. The interfacial voids run the entire length of the preform and have a size similar to that of the intended holes in the structured region and so can make fiberization difficult. Furthermore, the accuracy of the periodicity and position of the intended holes is critical to the desired optical properties of the microstructured fiber, and the presence of such “stray” holes in the fiber can destroy the ability of the fiber to perform properly.
Conventional processes attempt to reduce or eliminate the number of such interstitial voids by using a two-step process, in which the tubes in the preform are consolidated prior to fiber drawing. However, this two-step process still leaves an undesirable number of interstitial voids in the finished fiber.
This summary is intended to introduce, in simplified form, a selection of concepts that are further described in the Detailed Description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. Instead, it is merely presented as a brief overview of the subject matter described and claimed herein.
The present invention provides a method and an apparatus for making a substantially void-free microstructured optical fiber using a one-step process. In the method of the present invention, a preform for the optical fiber is prepared, comprising an outer jacket made of solid glass, a cladding having a plurality of microtubes and/or microcanes arranged in a desired pattern within the jacket, and a core which may be solid or hollow, with the cladding and the core extending above the top of the outer jacket. The thus-prepared preform is placed into a fiber draw tower configured according to the present invention. As the fiber is drawn, negative gas pressure is applied to draw the canes together and consolidate the interfacial voids between the canes while positive gas pressure is applied to the preform to keep the holes of the microcanes open during the fiber drawing. Thus, the final microstructured fiber can be prepared in one step, with the consolidation of the interfacial voids being accomplished sequentially in-situ as the preform is drawn into the SC-PCF or HC-PBG fiber, thereby preventing the creation of interstitial voids in the drawn fiber.
An apparatus for use in the present invention includes a fiber draw tower having a jig comprising one or more support tubes that are connected to a vacuum pump for application of the negative gas pressure and a vent tube connected to a gas supply for application of the positive gas pressure. The interfaces between the support tube and the outer jacket and between the vent tube and the cladding are sealed to ensure that the appropriate application of negative or positive pressure during the draw step is obtained.
The preforms according to the present invention can include one or more components fabricated from specialty non-silica glass, such as chalcogenide and chalcohalide glasses and other oxide glasses including specialty silicates, germanates, phosphates, borates, gallates, tellurites, antimonates and their mixtures.
The aspects and features of the present invention summarized above can be embodied in several different forms. The following description shows, by way of illustration, various combinations and configurations in which aspects and features of the invention can be put into practice. It is understood that the aspects, features, and/or embodiments described herein are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or may make structural and functional modifications without departing from the scope of the present disclosure.
In describing optical fibers, the term “microstructured” is typically used to describe a structure with features on the micro scale (between approximately 1 μm and 1000 μm) and the term “structured” is typically used to describe features of any scale, including features smaller than, larger than, or the same size as “microstructured” features. In the present disclosure, the term “microstructured” is used in describing features of a “microstructured” optical fiber and the term “structured” is used in describing features of an optical fiber preform from which the “microstructured” optical fiber is drawn, regardless of the actual or approximate sizes of the features. This choice of language is for clarity only, and the terms “microstructured” and “structured” can be used interchangeably without departing from the scope of the present disclosure.
In addition, as used herein, a “tube” or “microtube” typically possesses one longitudinal capillary running through the entire length thereof A “cane” or “microcane” may possess no longitudinal capillaries, a single longitudinal capillary, or a plurality of longitudinal capillaries, and may also possibly possess other features such as a non-uniform refractive index profile or a solid or hollow core region. It may be noted that a “microtube” is by definition also a “microcane,” but a “microcane” is not required to also be a “microtube.” The tubes, microtubes, canes and microcanes may have arbitrary outer and inner transverse shapes and may be the product of a combination of various fabrication methods including extrusion, molding, rotational casting, stack and redraw, etc. For example, a “microtube” may be extruded and then stretched on a fiber draw tower and may possess a circular or hexagonal outer transverse shape, and a circular inner transverse shape. In some embodiments, a “cane” or “microcane” may itself be in essence a thin microstructured optical fiber, containing its own core, cladding, and jacket region, and may be fabricated using the method described in this disclosure. For simplicity, the method and apparatus of the present invention may often be described with respect to fibers constructed of preforms having a microstructured region comprising a plurality of “microcanes”; however, it will be appreciated by those skilled in the art that aspects of the invention described herein are equally applicable to fibers fabricated using one or more microcanes either alone or in combination with one or more microtubes.
As noted above, SC-PCF and HC-PBG microstructured optical fibers have been developed to improve the transmission and other properties of optical fibers, such as the transmission of specific desired wavelengths of light. These improved optical properties are the result of the specific structure of these fibers.
The cross-sections of the exemplary SC-PCF and HC-PBG microstructured optical fibers shown in
It is the distribution of glass and air (or, as noted above, other gases or vacuum) by the components of these regions that create the particular optical properties of each type of fiber.
In both SC-PCF and HC-PBG fibers, cladding 103 of the fibers is not solid as in conventional optical fibers, but is instead a microstructured region having a periodic arrangement of glass and air holes. Typically, the periodicity of the holes is on the scale of the wavelength of light. Because the cladding comprises both glass and air, the refractive index of the cladding region is different than it would be if the cladding were solid glass. In addition, by varying the number, size, and periodicity of the air holes, the refractive index of the cladding area can be tuned so that the fiber exhibits desired optical properties such as transmission of a desired wavelength of light.
As seen in
As seen in
In addition, in both SC-PCF and HC-PBG microstructured fibers, there can be many variations on the configuration of the core. For example, the fiber can have one single core or multiple distinct cores, for example, to encourage interaction between separate signals confined to separate cores. In addition, the transverse shape of the one or more of the cores can have a round, elliptical, hexagonal, or another shape, and the one or more cores can have either the same or different shapes, for example, to impart a birefringence condition for maintaining the polarization state of the propagating signal.
For both SC-PCF and HC-PBG microstructured optical fibers, the periodicity of the holes, the air fill fraction of the cladding and the refractive index of the glass dictate the optical properties of the fiber. As used in the art, the term “air fill fraction” refers to the ratio of the cross-sectional area of the capillaries to the combined area of the capillaries plus the solid material, or equivalently, the ratio of the volume of the capillaries to the total volume (volume of the capillaries plus volume of the solid material), in the microstructured region. More specifically, when the hole shape and arrangement is regular, the air fill fraction of a specific microstructured optical fiber design can be defined algebraically as a function of the ratio of the hole radius, r, to the hole pitch, Λ. For example, the air fill fraction for a microstructured optical fiber with round air holes arranged periodically in a triangular lattice, equals
Similarly, for a SC-PCF or HC-PBG fiber with round holes in a square lattice, the air fill fraction equals
If the air holes are not perfectly shaped or sized or are not arranged in a perfect lattice arrangement, the air fill fraction is not easily calculated but can be measured by computer.
In SC-PCF these parameters dictate the index contrast and therefore the allowed modes and their propagation constants. Some such fibers can be single-mode over a broad range of wavelengths, a property called “endlessly single-mode” that is unique to SC-PCF and not possible in conventional solid core fiber. In HC-PBG fibers, these parameters determine the position of the photonic band gap, i.e., namely the wavelengths of light that can be guided through the hollow core.
Thus, it is very important to maintain the intended glass-hole structure of the fiber, without the presence of unintended additional holes due to interstitial voids or the absence of intended holes due to collapse of one or more microcanes. The present invention provides a method and an apparatus that can achieve these results.
The method of the present invention starts with an assembled or “loose” structured preform described in more detail below, consisting of a jacket tube disposed around one or more glass microtubes or microcanes. As used herein, the term “loose” refers to the fact that the preform is assembled from inner and outer elements which may or may not be well fitting and the preform has not yet undergone a separate and subsequent heat treatment step so as to consolidate the loose-fitting elements of the preform into a consolidated preform in which the elements are bonded or fused to one another. The individual microcanes used in the preform may or may not be exactly or approximately the same size or possess the same inner and outer transverse shapes.
In some embodiments of the present invention, one or more of the jacket and the microcanes/microtubes may be made of a specialty non-silica glass. Suitable specialty glasses include chalcogenide glasses such as sulfides, selenides, tellurides and mixtures thereof, and chalcohalide glasses and other oxide glasses, including specialty silicates, germanates, phosphates, borates, gallates, tellurites, antimonates and mixtures thereof In addition, more than one glass may be used, for example, with the jacket being fabricated from one glass, one or more microcanes being fabricated from a second glass, and one or more other microcanes (and/or the core in the case of SC-PCFs) being fabricated from yet a third. One or more of these glasses may be a specialty glass or a non-specialty glass, and all of such combinations may be used to make microstructured optical fibers within the scope of the present disclosure.
An exemplary general form of a structured preform for a microstructured optical fiber is shown in
As shown in
The accuracy of the periodicity and position of the intended holes in the microstructured region created by the microcanes 203 is critical to preventing optical coupling between the core and cladding in SC-PCFs and in attaining bandgap guidance in the HC-PBG fiber. This precision is adversely affected by incorrect tube positioning and tube slippage during fiberization, which are common deficiencies of the tube stacking method.
In addition, as shown in
Conventional methods attempt to eliminate these voids through consolidation or some other heat treatment step before fiber drawing, wherein the space between the microcanes collapses thus eliminating the interfacial void. However, since the interfacial voids often have a size similar to those of the intended holes in the structured region of the preform, and run the entire length of the preform, it is difficult to eliminate such voids completely. This is especially true for specialty oxide and non-oxide glasses where the vapor pressure during fiberization may be sufficient to prevent collapse of these interstitial voids.
If the interfacial void does not collapse, it will become trapped in the final fiber, forming an “interstitial void” in the final fiber. Examples of optical fibers having such interstitial voids can be seen in
The presence of such interstitial voids can have significant adverse effects on the final fiber. For example, interstitial voids in an HC-PBG fiber can compromise the photonic bandgap and prevent the efficient transmission of light through the fiber core because all of the light will scatter through the cladding and/or the jacket, with none of the light passing through the fiber in its intended path. In SC-PCFs, interstitial voids can cause the average refractive index of the fiber to vary; in such a case, mode fields of different diameters can experience different average cladding indices, which in turn can narrow the wavelength region for single-mode operation, can prevent single-mode operation entirely or, through scattering, can permit coupling of the optical to the jacket region, thereby reducing or eliminating transmission of the signal through the fiber in its intended path.
Consequently, it is desirable to eliminate voids from the preform before they become trapped as interstitial voids in the final fiber.
As noted above, conventional methods attempt to consolidate the preform before the fiber drawing step. However, it often is not possible to fully eliminate the interfacial gaps in the preform by such a method, and interstitial voids may still remain, either in the consolidated preform or in the final fiber.
The present invention provides a method and apparatus for fabricating SC-PCFs and HC-PBG fibers to prevent the formation of interstitial voids. In accordance with the present invention, microstructured optical fibers can be fabricated from loosely assembled (non-consolidated) structured preforms which are sequentially consolidated in-situ during the fiber drawing step. A microstructured optical fiber fabricated in accordance with the present invention will be substantially void-free and so will exhibit improved optical performance.
As described in more detail below, in the method of the present invention, a non-consolidated structured preform is placed into a fiber draw tower for drawing into the final fiber. The assembled preform is stretched, for example, on a fiber draw tower at a temperature corresponding to a glass viscosity in the range of about 104 to 106 Poises, into microstructured optical fiber with considerably smaller dimensions than the preform. The fiber outer diameter is typically less than about 1 mm and more typically less than about 500 μm, although a microstructured cane, with an outer diameter typically greater than about 1 mm, and more typically between about 1.5 and 4 mm, may also be fabricated by this method.
In accordance with the invention, the intended void regions (i.e., the holes in any hollow microcanes present) are isolated from the interfacial voids via a jig. As the fiber is drawn, negative gas pressure is applied to consolidate the preform and remove the interfacial gaps and prevent the presence of undesired voids while positive gas pressure is simultaneously applied to prevent collapse of the microcanes and ensure the presence of the desired holes in the microstructured region of the fiber. In some embodiments, the top of the microcanes can be fused together using, for example, a low surface-tension glue to ensure that the positive gas pressure applied to the microcanes to keep them open does not prevent the negative gas pressure from closing the gaps between the tubes. In other embodiments, the microcanes can be held open, for example, by rigid inserts made from quartz, stainless steel, fluoropolymer, polyetheretherketone (PEEK), ceramic, other polymers, other metals, other glasses placed therein, so that the negative gas pressure applied to close the gaps between the tubes does not collapse the microcanes during the draw process. In either case, the preform can be consolidated in-situ in the fiber tower and the fiber drawn in one step.
Exemplary preforms for SC-PCF and HC-PBG microstructured fibers suitable for use in the present invention are shown in
Jacket 401 comprises a solid glass material, and as described above, a suitable glass may comprise a specialty non-silica glass such as a chalcogenide glass or a chalcohalide glass. Its outer shape can be round, elliptical, hexagonal, or any other suitable shape. In addition, as shown in
Similarly,
An exemplary embodiment of an apparatus for in-situ consolidation and drawing of a microstructured optical fiber according to the present invention is depicted in
As shown in
In the method for a one-step in-situ consolidation and drawing of a microstructured optical fiber of the present invention, negative gas pressure 607 and positive gas pressure 608 are applied to the outside and the inside, respectively, of the microstructured portion 604 of the preform. The negative gas pressure, i.e., vacuum, acts to draw the individual microtubes comprising the microstructured cladding together, thus eliminating the interfacial voids between the microtubes, while at the same time the positive gas pressure prevents the microtubes from collapsing due to the vacuum. While these negative and positive gas pressures are being applied, a radially compressive stress may also be applied to the jacket tube to further assist the in-situ consolidation. The pressures applied, whether positive, negative, or radial, can range from 0.03 to 10 psi.
As described in more detail below, in accordance with the present invention, as the preform is being consolidated, it is also drawn through the draw furnace 609 to produce the drawn fiber. The negative gauge pressure at the microtube-microtube interfaces and the microtube-jacket tube interfaces, combined with surface tension and positive gauge pressure inside each of the microtubes which may be applied using the jig, acts to sequentially consolidate the interfacial void region of the preform in-situ as it is drawn through draw furnace 609, thereby preventing the creation of interstitial voids in the drawn fiber.
The remainder of the drawing process is according to conventional methods, with the microstructured fiber 610 guided through LaserMike non-contact measurement system 611 and polymer coater 612, over capstan 613, and onto drum winder 614.
The resulting microstructured optical fiber prepared using the apparatus and method thus described is substantially free of interstitial voids and deformed micro-holes and therefore demonstrates lower transmission loss and better power handling than glass fibers made using conventional methods.
In a first exemplary embodiment shown in
Thus, as shown in
The improved microstructured optical fibers produced using the apparatus and method of the present invention will have an impact in both military and civilian applications.
For example, SC-PCFs can be used in a variety of non-linear optical devices including devices for wavelength translation, supercontinuum generation, etc. SC-PCF-based non-linear optical devices may replace crystal devices in some applications reducing cost, weight, and system complexity.
HC-PBG fibers can be used as sensors in facility clean up, biomedical analysis (e.g. glucose, blood, breath etc), CBW agent detection, toxic and hazardous chemical detection, and environmental pollution monitoring and process control, etc. In addition to chemical sensing, the HC-PBG fibers can be used for very high laser power delivery since the light is predominantly guided in the hollow core, unlike in traditional fibers which possess a solid core that will damage at high powers. In addition, HC-PBG fibers can also reduce system cost, weight, and complexity, and canenabe remoting of high power lasers for industrial applications such as cutting, welding, metrology and for biomedical applications such as laser surgery, cancer removal and glaucoma treatment.
In either case, the method and apparatus of the present invention will improve the performance and reliability of these fibers and reduce the difficulty of their fabrication, particularly in SC-PCF and HC-PBG fibers made from non-silica specialty glasses.
It is particularly anticipated that the method of fabricating microstructured optical fibers described herein will be used in the fabrication of fibers comprising one or more specialty non-silica glasses such as chalcogenide glasses, including sulfides, selenides, tellurides, and their mixtures, as well as chalcohalide glasses and other oxide glasses, such as specialty silicates, germanates, phosphates, borates, gallates, tellurites, antimonates and their mixtures. Chalcogenide glasses enable transmission from about 1 μm to 11 μm in microstructured optical fibers and so are particularly suitable to provide the optical properties desired for such fibers.
Although particular embodiments, aspects, and features have been described and illustrated, it should be noted that the invention described herein is not limited to only those embodiments, aspects, and features.
For example, the method of fabricating microstructured optical fibers by drawing assembled preforms with in-situ vacuum-assisted consolidation is not limited to the types of structures shown in the Figures, but can also be used for more complex structures. Thus, the method can also be applied to structures having microtubes with outer transverse shapes other than round or hexagonal or jacket tubes with different inner transverse shapes, for example, to microstructured fibers having holes in a square lattice arrangement.
Other alternative embodiments could include the use of solid micro-canes instead of micro-tubes to fabricate a solid fiber with multiple distinct cores. Furthermore, there is no constraint on uniformity in size or transverse shape of the individual micro-tubes or micro-canes, i.e. sizes and shapes can vary as appropriate for a desired arrangement of holes or features in a microstructured fiber.
It should be readily appreciated that these and other modifications may be made by persons skilled in the art, and the present application contemplates any and all modifications within the spirit and scope of the underlying invention described and claimed herein.
