Abstract:
A method is provided for making a photonic band gap fiber including the steps of etching a preform and then drawing the preform into a photonic band gap fiber. Glass tubes are bundled and then formed into a photonic crystal perform having a number of passageways by reducing the cross-section of the bundle. One of the passageways is enlarged by flowing an etchant through it. After cleaning, the band gap fiber is made from the etched photonic preform, for example, by drawing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to photonic band gap fibers, and particularly to a method of making photonic band gap fibers. 
     2. Technical Background 
     Traditionally optical waveguide fibers have used total internal reflection to guide the propagation of an optical signal. Optical waveguide fibers that rely upon total internal reflection for the transmission of optical signals typically have a core region and a cladding region. The core region is the portion of the optical waveguide fiber that the optical signal propagates within. Generally, the core region of an optical waveguide fiber relying on total internal reflection to guide the propagation of an optical signal has a higher index of refraction than surrounding cladding region. 
     Optical waveguide fibers that rely upon total internal reflection in order to guide the propagation of optical signals have of inherent limitations. Among these are relatively high dispersion and attenuation of the optical signal, and relatively low upper limits on the power of the optical signal. 
     Photonic band gap (PBG) fibers are photonic crystals that have a structure in which the refractive index varies periodically in 2 dimensions, (the x-y plane, where the z-coordinate is the longitudinal axis of the fiber), with a period of the order of an optical wavelength. Photonic band gap fibers may offer a better performance than total internal reflection optical waveguide fibers with regard to dispersion, attenuation and signal power. 
     A photonic crystal is a structure having a periodic variation in dielectric constant. The periodic structure of the crystal may be one, two or three-dimensional. A photonic crystal allows light of certain wavelengths to pass through it and prevents the passage of light having certain other wavelengths. Thus photonic crystals are said to have allowed light wavelength bands and band gaps that define the wavelength bands that are excluded from the crystal. A review of the structure and function of photonic crystals is found in, Joannopoulus et al., “Photonic Crystals: putting a new twist on light”,  Nature  vol. 386, Mar. 13, 1997, pp. 143-149. 
     A two-dimensional photonic crystal having certain geometries and effective indices of refraction may produce a photonic band gap fiber in which the optical signal propagates in either air or vacuum. Use of a 2 dimensional photonic crystal as an optical fiber is discussed in, Birks et al., “Full 2-D photonic band gaps in silica/air structures”,  Electronic Letters , Vol. 31 (22), Oct. 26, 1995, pp. 1941-1943. Through Bragg diffraction, these structures can support a series of optical resonances, band gaps and allowed states. 
     An optical waveguide fiber in which the optical signal propagates in air or vacuum is of great interest in the field of telecommunications. This interest arises because optical waveguide fibers in which the optical signal propagates in air or vacuum offer lower dispersion, lower attenuation of the optical signal being carried and have a near zero nonlinear refractive index. Compared to air guiding photonic band gap fibers, current total internal reflection fibers have a limited operating regime. 
     Recent theoretical work has indicated that large void-filling fractions are required for optical waveguide fibers to propagate light in a low index of refraction core utilizing the photonic band gap effect. The low index of refraction core typically includes an evacuated or air filled passageway in which the light is guided. The void-filling fraction is a function of the ratio of the diameter of the passageways to the center to center spacing, or pitch, of the passageways. Equation 1 is the mathematical expression for the void-filling fraction of a photonic band gap fiber, vf.              vf   =         π     2        3              [     d   Λ     ]       2             (   1   )                                
     where 
     vf is the void-filling fraction; 
     d is the diameter of internal passageways; and 
     Λ is the distance between the centers of adjacent passageways or pitch. 
     Photonic band gap air-guiding fibers with a void-filling fraction of 0.42 have been fabricated using a stack and draw process. A detailed description of the stack and draw process may be found in R. F. Cregan, Single-Mode Photonic Band Gap Guidance of Light in Air, SCIENCE, vol. 285, pp. 1537-39 (1999). 
     Optical waveguide fibers having large void-filling fractions are obtained by drawing photonic crystal preforms having large void-filling fractions into optical waveguide fibers using conventional optical waveguide fiber making techniques. 
     Photonic crystal preforms have been made using the stack and draw method and the extrusion method. The stack and draw method involves arranging glass capillary tubes into an array having desirable macroscopic cross-sectional properties and then reducing the cross section of the preform. Typically the preform is either forced through a die or drawn to reduce the cross section. Preforms made according to the stack and draw process are categorized as either close-packed arrays or non-closepacked arrays. A close-packed array is an array of capillary tubes where the capillary tubes support one another. A non-close-packed array is an array of capillary tube wherein spacers or jigs are placed in the array thus spacing the walls of the capillary tubes apart. 
     Making optical waveguide fibers with a high void-filling fraction with a small pitch is difficult. 
     There is a need for a method of making preforms for making photonic band gap fibers that is repeatable, versatile, and adaptable to a manufacturing environment. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is a method for making photonic band gap fibers including the step of making a photonic crystal preform having multiple longitudinal passageways. The photonic crystal preform is then etched and drawn into a photonic band gap fiber. In another aspect, the present invention includes an apparatus for etching a preform having a plurality of passageways. The apparatus includes a reservoir containing an etching agent. A heater is thermally coupled to the reservoir. A circulator having an input line and an delivery line is located so that the input line is connected to the reservoir and circulator draws etching from the reservoir and directs it to a nozzle connected to the delivery line of the circulator. The etching agent is directed by the nozzle into the passageways of the preform. The apparatus also includes a receptacle located to collect the etching agent as it exits the passageways. A return line is connected to the receptacle, and the etching agent flows through the return line and is returned to the reservoir. 
     In another aspect, the present invention includes a method for making photonic band gap fibers includes the steps of first assembling a number of glass tubes into a bundle. The bundle is then formed into a photonic crystal preform having a number of passageways by reducing the cross-section of the bundle. Next, one of the passageways of the photonic crystal preform is enlarged by flowing an etching agent through it. After a predetermined time has passed, the flow of the etching agent is stopped. After the etching agent is stopped flowing through the passageway, the photonic crystal preform is cleaned to remove any remaining liquid etching agent. A photonic band gap fiber is then made from the etched photonic preform. Typically, the photonic band gap fiber is made from the preform by traditional fiber drawing methods. 
     An advantage of the present invention is that preforms can be made which result in void-filling fractions on the order of 0.82 and greater. 
     Another advantage of one embodiment of the present invention is that special jigs are not required to make a preform having a large void-filling fraction. 
     Another advantage of the present invention is that it provides a relatively easy way to insert a large passageway in the structure of the preform and hence the resulting photonic band gap fiber. This follows from the observation that in an interior passageway or channel any surface with a positive radius of curvature, with respect to the wall of the passageway, e.g., a protrusion from the wall surface, has a greater etch rate than a flat surface. Furthermore, the etch rate of a flat surface is greater than that of a surface having negative curvature, e.g., a depression. Therefore, instead of making a preform with a large central passageway, a task that has proven difficult, a smaller passageway possessing wall of opposite curvature from the remaining passageways may be created in the preform. When the passageways are simultaneously exposed to an etching agent the desired cross-sectional shape of the passageway is realized while the void-filling fraction of the remaining passageways increases. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic depiction of a process in which the present invention is embodied; 
     FIG. 2 is a cross-section taken through the preform before etching; 
     FIG. 3 is a cross-sectional view of the preform shown in FIG. 2 after etching 
     FIG. 4 is a cross-section taken through the preform before etching; 
     FIG. 5 is a cross-sectional view of the preform shown in FIG. 4 after etching; 
     FIG. 6 is a diagrammatic depiction of an apparatus in which the present invention is embodied; 
     FIG. 7 is a chart showing the relationship between material removal and exposure to the etching agent; and 
     FIG. 8 is a diagramaitc depiction of an apparatus in whch the present invention is embodied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the apparatus for etching a photonic crystal preform, of the present invention is shown in FIG. 6, and is designated generally throughout by reference numeral  10 . 
     The present invention is directed to making photonic band gap fibers by etching a preform  12 . The etching of the preform  12  allows the preform  12  and thus the photonic band gap fiber made from it to have much larger void-filling fractions than are available with other photonic band gap fiber manufacturing techniques. FIGS. 2 and 4 are illustrative of possible partial cross-sections of the preform  12 . 
     The preform  12  is typically an elongated glass body having a number of parallel internal passageways  14  running the length of the preform  12 . The glass chosen for the preform  12  should have a high index of refraction. Examples of glasses that the preform may be made from are silica glasses, lead silicate glasses, germanium silicate glasses, emanate glasses, fluoride glasses and phosphate glasses. The preform  12  may be made in a number of ways including, for example, extruding a multicellular glass body and assembling capillary tubes  16  into multicell structures. The structure of the preform  12  depends upon the index of refraction of the selected glass and the wavelengths of the optical signals to be carried by the resulting photonic band gap fiber. 
     The preform  12  will have a lattice structure of internal passageways  14  having a certain diameter d and pitch a. Preferably the diameter d and pitch A of the internal passageways  14  are substantially uniform throughout the preform  12 . 
     FIG. 1 depicts an embodiment of a method  100  for making a photonic band gap fiber. The method  100  embodiment of the present invention will be described with reference to the apparatus  10  embodiment of the present invention depicted in FIG.  6 . 
     The method  100  includes the step  110  of determining the desired void-filling fraction for the etched preform  12 . The void-filling fraction is determined by the specific photonic band gap effects to be exhibited by the resulting photonic band gap fiber. 
     The method  100  of the present invention further includes the step  112  of stabilizing the temperature of the etching agent  18 . The stabilization temperature of the etching agent  18  depends upon the etching agent  18  and the composition of the preform  12 . For example, a heater  30  is used to stabilize the temperature of the etching agent  18  at about 58° C. when the etching agent  18  is NH 4 F.HF. 
     The method  100  of the present invention also includes the step  114  of placing the preform in an etching apparatus  10 . An example of an etching apparatus in which the present invention is embodied is shown in FIG.  6  and described in detail below. 
     The method  100  further includes the step  116  of connecting delivery and recovery lines  40 ,  50  to the preform  12 . FIG. 6 depicts an etching apparatus  10  in which the delivery line  40  in connected to a nozzle  44  and the recovery line  50  is attached to a receptacle  46 . The nozzle  44  and receptacle  46  are connected to opposite ends of the preform  12 . 
     The method  100  further includes the step  118  of flowing etching agent  18  through the internal passageways  14  of the preform  12  until the preform  12  has a desired void-filling fraction. Once the preform has the desired void-filling fraction and the flow of etching agent  18  through the preform is stopped. 
     The method  100  further includes the step  120  of flushing the etching agent  18  from the preform  12 . For example, a flushing agent  20 , such as water, is flowed through the internal passageways  14  of the perform  12  to remove any residual etching agent  18  and arrest the etching process. 
     In the next step  122  of the illustrated method  100 , the preform  12  is removed from the etching apparatus  10 . Finally, the preform  12  is made into an optical waveguide fiber using techniques known to those skilled in the art of making optical waveguide fibers from preforms. This is shown as step  124  in FIG.  1 . 
     FIG. 2 shows a partial cross-section of a preform  12  made from hexagonal capillary tubes  16  prior to the etching step  118  of the method  100  illustrated in FIG. 1. A void  60  is formed by the omission of the single hexagonal capillary tube  16 . The void  60  becomes a light guiding region in the photonic band gap fiber produced in step  124 . 
     FIG. 3 shows the same partial cross-section after the etching step  118  of the method  100 . The shape of the void  60  is a result of the difference in etching rates of surfaces having different curvatures. 
     FIG. 4 shows a partial cross-section of a preform  12  made by an extrusion process prior to the etching step  118  of the method embodiment  100  of the present invention described above. The preform has a void  60  having regions of negative curvature  62 . This void  60  will become a light guiding region in the photonic band gap fiber produced in step  124 . 
     FIG. 5 shows the same partial cross-section after the etching step  118 . The shape of the void  60  has changed as a result of the difference in etching rates of surfaces having different curvatures. 
     In an additional embodiment of the invention, as embodied herein and as shown in FIG. 6, the present invention includes an etching apparatus  10  for etching the internal passageways  14  of a preform  12 . The etching apparatus  10  includes a substantially closed loop delivery circuit  22  for passing an etching agent  18  through at least some of the internal passageways  14  of the preform  12 . The etching apparatus  10  also includes delivery and recovery systems  24 ,  26  for the flushing agent  20 . The flushing agent  20  delivery system  24  provides a flushing agent  20  to the preform  12 . The flushing agent  20  flows through the preform  12  and removes residual etching agent  18  after the etching of the preform  12  is completed. An example of a typical flushing agent  20  is water. 
     The etching apparatus  10  includes a reservoir  28  for holding an etching agent  18 . In this embodiment, the etching agent  18  is chosen to remove material from the internal passageways  14  of the preform  12  in an efficient and predictable manner. Typically the etching agent  18  is an acid; examples of acids suitable for use with a preform  12  made of silica glass include HF and NH 4 F.HF. The etching agent  18  selected for use with a particular preform  12  depends on chemical composition of the preform  12  to be etched. Those of ordinary skill in the art of etching glass may readily select a particular etching agent  18  for a preform  12  having a specific glass composition. 
     The reservoir  28  may be a commercially available reservoir for handling corrosive materials. Such reservoirs are commercially available. An example of such a reservoir  28  is a nalgene container. 
     The reservoir  28  is thermally coupled to a heater  30 . The heater  30  thermally stabilizes the etching agent  18  at a preselected temperature. The heater  30  may be, for example, a heat sink bath that surrounds a portion of the reservoir  28 . Such heat sink baths may be assembled from commercially available components. Examples of commercially available components that may be combined to form a heat sink bath are a CT 050 pump marketed by Schott Gerate and a GP- 100  bath temperature controller available from Neslab Instruments, Portsmouth, NH. In one embodiment when Ammonium Bifluoride (NH 4 F.HF) is used as the etching agent  18  and the preform  12  is made from silica glass the heater  30  maintains the etching agent  18  at a temperature of about 58° C. 
     The etching apparatus  10  further includes a circulator  32 . The circulator  32  includes an intake port  34  and an output port  36 . The input port  34  is connected to the reservoir  28  by an input line  38 . The circulator  32  draws etching agent  18  from the reservoir  28  through the input line  38 . Preferably the input line  38  is made of a material resistant to the corrosive effects of the etching agent  18 . Etching agent  18  entering the intake port  34  exits the circulator  32  through an output port  36 . The circulator  32  increases the dynamic pressure of the etching agent  18  so that the etching agent  18  may be circulated through the delivery circuit  22 . The output port  36  of the circulator is connected to a delivery line  40 . The other terminus of the delivery line  40  is connected to a valve  42 . 
     The valve  42  is configured to regulate the flow of etching agent  18  through the perform  12 . Preferably the delivery system  36  for the flushing agent  20  is also connected to the valve  42  and the valve  42  is configured to have at least three settings. The first setting directs etching agent  18  coming from the circulator  32  to a nozzle  44 . The second setting directs flushing agent  20  to the nozzle  44  and the third setting blocks both etching agent  18  and flushing agent  20  from entering the nozzle  44 . 
     The nozzle  44  engages one end of the preform  12  and is configured to direct any material flowing through it into the internal passageways  14  of the preform  12 . Using techniques known to those skilled in the art the nozzle may be configured to direct etching agent  18  and flushing agent  20  to all or some of the internal passageways  14  of the preform  12 . Preferably the nozzle  44  engages the preform  12  in such a manner as to preclude the etching agent  18  from acting on the external surface of the preform  12 . 
     Preferably the preform  12  is secured in near vertical position throughout the etching process by a support  46 . 
     A receptacle  46  engages the other end of the preform  12 . The receptacle  46  is configured to collect etching agent  18  and flushing agent  20  and direct the flow of such agents to a directional valve  48 . The directional valve  48  has at least two setting. When the receptacle  46  is collecting etching agent  18  the directional valve  48  is configured to direct the etching agent  18  into a return line  50 . The etching agent  18  flows through the return line  50  and is reintroduced into the reservoir  28 . When the receptacle  46  is collecting flushing agent  20  the directional valve  48  is configured to direct the flushing agent  18  into the recovery system  42  for the flushing agent  20 . 
     As described above the period of time that the etching agent  18  circulates through the preform  12  is determined by the pre-etch dimensions of the internal passageways  14  and the desired post-etching void-filling fraction of the preform. 
     FIG. 7 is an example of a chart that may be used to determine the etching period. The chart of FIG. 7 is used by determining the desired post-etch diameter of the internal passageways  14  and finding that value on the vertical axis. A horizontal line is drawn from that value until it intersects the line c. The corresponding time is then read from the horizontal axis. 
     Construction of such charts as shown in FIG. 7 is well known to those skilled in the art of etching glass. These charts may be constructed using mathematical models of the effect of etching agent  18  on a specific material. Such charts may also be constructed using empirical data gathered from etching preforms  12  having a specific glass composition and internal passageways  14  with a diameter d and a specific etching agent  18 . 
     With reference now to FIG. 8, an alternate embodiment of an etching apparatus  11  embodiment of the present invention is shown. 
     The etching apparatus  11  is a substantially closed loop delivery circuit for passing an etching agent  18  through at least some of the internal passageways  14  of the preform  12 . 
     The etching apparatus  11  includes a reservoir  28  for holding an etching agent  18 . In this embodiment, the etching agent  18  is chosen to remove material from the internal passageways of the preform  12  in an efficient and predictable manner. The etching agent  18  may be an acid; examples of acids suitable for use with a preform  12  made of silica glass include HF and NH 4 F.HF. The etching agent  18  selected for use with a particular preform  12  depends on chemical composition of the preform  12  to be etched. Those of ordinary skill in the art of etching glass may readily select a particular etching agent  18  for a preform  12  having a specific glass composition. The reservoir  28  is thermally coupled to a heater  30 . The heater  30  maintains the etching agent  18  in thermally equilibrium. The heater  30  may be a heat sink bath, as described above in connection with the etching apparatus  10  embodiment depicted in FIG. 6, that surrounds a portion of the reservoir  28 . When NH 4 F.HF is used as the etching agent  18  and the preform  12  is made from silica glass the heater  30  maintains the etching agent  18  at a temperature of about 58° C. 
     The etching apparatus  10  further includes a circulator  32 . The circulator  32  includes an intake port  34  and an output port  36 . The intake port  34  is connected to the reservoir  28  by an input line  38 . The circulator  32  draws etching agent  18  from the reservoir  28  through the input line  38 . Preferably the input line  38  is made of a material resistant to the corrosive effects of the etching agent  18 . Etching agent  18  entering the intake port  34  exits the circulator  32  through an output port  36 . The circulator  32  increases the dynamic pressure of the etching agent  18  so that the etching agent  18  may be circulated through the delivery circuit  52 . The output port  36  of the circulator is connected to a delivery line  40 . The other terminus of the delivery line  40  is connected to the perform  12 . 
     The delivery line  40  engages one end of the preform  12  and is configured to direct the etching agent  18  into the internal passageways  14  of the preform  12 . Preferably the delivery line  40  engages the preform  12  in such a manner as to preclude the etching agent  18  from acting on the external surface of the preform  12 . Preferably the preform  12  is secured in near vertical position throughout the etching process by a support  46 . 
     A return line  50  engages the other end of the preform  12 . The etching agent  18  flows through the return line  50  and is reintroduced into the reservoir  28 . 
     As described above the period of time that the etching agent  18  circulates through the preform  12  is determined by the pre-etch dimensions of the internal passageways  14  and the desired post-etching void-filling fraction of the preform. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.