MXPA01003139A - An optical fiber having an expanded mode field diameter and method of expanding the modefield diameter of an optical fiber - Google Patents

Abstract

The present invention is directed to a generally small mode field diameter ("MFD") optical fiber having a core bounded by a cladding, a cleaved end, and an expanded mode field diameter. The expanded MFD is formed by thermally diffusing one or more dopants in the core of the small MFD optical fiber using a highly localized heat source.The resulting adiabatic taper has an expanded MFD that is optimized for connection to another optical fiber having a larger MFD. The adiabatic taper is formed in the smaller MFD optical fiber by aligning and abutting the cleaved ends of two fibers having different MFDs to form a splice seam. The splice seam is offset a predetermined distance from the center of the region heated by a heat source to splice the fibers and expand the mode field diameters. As the mode field diameters expand, the splice loss across the splice is monitored. When the splice loss is at or sufficiently close to a target loss, heating is terminated, and the spliced optical fiber is cleaved where the MFD of the smaller MFD fiber portion of the spliced optical fiber is optically expanded to match the MFD of another optical fiber.

Description

AN OPTICAL FIBER THAT HAS A MODEL FIELD DIAMETER EXPANDED AND A METHOD TO EXPAND THE DIAMETER OF FIELD OF MODE OF AN OPTICAL FIBER This application claims the benefit of the priority according to the C. E.U.A. 35 § 120 of the provisional patent application of E.U.A. No. 60/101888 filed September 25, 1999, the content of which is fully incorporated and incorporated in the present invention by reference.

FIELD OF THE INVENTION The present invention relates to the connection of optical fibers and other optical waveguides having different optical properties. Most particularly, the present invention relates to an optical fiber having an expanded Mode Field Diameter ("MFD"), and a method for expanding the optical fiber MFD for subsequent connection to optical fibers having higher MFDs. Although the invention is subject to a wide range of connectivity applications, it is well suited especially for the connection of fibers for specific use to standard single mode fibers, and particularly will be described in that regard.

BACKGROUND OF THE INVENTION As the fiber optic industry has matured, fibers for specific use such as erbium-impregnated fibers, dispersion-compensating fibers, Bragg's diffraction grating fibers as well as long-period diffraction grating fibers have been increasingly important in light wave photonic systems. To provide the necessary performance, these and other fibers for specific use need to be connected (or spliced) to other optical fibers or other optical devices without exhibiting excessive connection losses, or "splice losses" as are known to those skilled in the art. of light wave photonic systems. Invariably, these fibers for specific use have MFDs that differ in size and in other aspects of the MFDs of the fibers or devices to which the fibers will be connected for specific use. The connection of fibers that have such unequal MFDs generally cause excessive splice losses. Standard single-mode fiber, which is very commonly used today, is no exception. Numerous techniques have been developed in recent years to limit the adverse effect of splice losses that result from the MFD inequality. Therefore, for this purpose, methods such as physical tapering, in-line optical devices, and expanded core methods by thermal diffusion ("TEC") have been employed in an attempt to adequately match the fields so that the fibers and other devices that have different MFDs. The physical taper includes both the tapering taper and the tapering taper. Optical in-line devices include simple optical devices such as lenses, as well as beam expansion fibers combined with micro-optical devices such as insulators and modulators. TEC methods include those methods that are used to expand MFD through diffusion. In the descending taper method, the optical fiber is first spliced by conventional methods, and the spliced portion of the fiber is subsequently heated so that it can be pulled out by pulling. In this way, the softened spliced part develops a tapered shape. The lack of alignment of the reduced core due to the tapered shape and the expansion of the MFD in the smaller core diameter fiber typically result in smaller splice losses when compared to the original non-tapered splice. However, the tapers manufactured by this method are sensitive to physical disturbances or to the change of external refractive index because the mode field is not tightly attached to the core. In addition, the outer diameter of the tapered fiber changes during the stretching process, therefore special fiber connectors are typically required for any connection. Unlike the descending taper method, the ascending taper is manufactured in the drawing step of a preform and results in an elongated core. Lengthening the core results in an expanded MFD. This method is typically applicable for mechanical splicing, splicing by bonding, or connectors between erbium-doped fibers ("EDF") and a single-mode common ("SM") fiber. However, this method also requires special connections for the connectors, and also needs a special preform. Most optical in-line devices use lens elements that collimate a beam from a transmitting fiber, or focus the expanded beam on the core of the receiving fiber. Other combination devices such as laminated polarizers, micro-isolating integrated circuits, or modulators embedded within the fiber with thermally induced doping diffusion in certain fibers for specific use. However, both methods are complicated, unstable and expensive. In addition, for devices that use a lens, alignment is a matter of critical interest. The TEC method uses the phenomenon of doping diffusion in a heated fiber to expand the MFD. The general approach of the fusion connection of two fibers with different MFDs is to continuously or adiabatically vary the diameters of the core of one or both fibers so that the MFDs are matched in their contours. During the diffusion process of the impurifier, the core diameter expands locally, and the relative refractive index difference is locally smaller compared to the common fiber part. The result is a tapered core, and therefore a tapered MFD within the fiber. Therefore, the TEC method can be an effective method to locally expand the MFD of the fiber. However, as presented below, TEC methods known in the art are not effective for certain applications. The methods to carry out the TEC technique can generally be classified into two categories. The first is to heat-treat the lower MFD fiber in an oven or on a gas burner, and then melt-connect the expanded fiber with the larger MFD fiber. The second is to fuse the two fibers first, and then apply additional heat to diffuse the fused region. In the first method, ovens or microburners are generally used to supply heat for diffusion. Due to the temperature limits of most furnaces, the process typically takes hours to complete, and requires the application of a carbon coating once the primary coating has been stripped from the fiber. The application of a carbon coating is expensive and time-consuming, but it is necessary to reduce the exposure time to the heat required to properly diffuse the impurifier. Even when temperatures inside the oven are generally not considered extreme, long periods of exposure to a gas flame tend to make the fiber brittle. For this reason, open-ended furnaces with a maximum temperature of about 1, 300 ° are used to treat the fiber. Using said open-end furnace is generally required to expose a fiber having a delta of 1% greater than ten (10) hours. Due to the low temperature gradient in an open-end furnace, the core of the fiber slowly expands at least along 200 mm length of the fiber before reaching the maximum diameter. As a result, the longer heat treated section of the fiber has a relatively low mechanical strength and requires protection as well as extra packing before it can be effectively used in a photonic component. In addition, due to the large size of the furnace and microburner systems, the first method is not readily available for use in the field where different fiber splices must be realized. The second method works well only when the diffusion coefficient of the core dopant in the MDF fiber is much smaller than that of the larger MFD fiber. A minor MFD fiber doped with erbium is a typical example. For high delta ("HD") and single-mode ("SM") fibers, that both are contaminated by slow diffusion with germanium, the discontinuity of the nucleus can not be completely eliminated using this method. When the splice is fabricated using an arc fusion discharge, the resulting splice loss is typically around 0.3 dB which is still unacceptable, since there are typically numerous fusion connections of this kind in an optical network. Accordingly, an adiabatic coupling can not be achieved simply by heating the fused region after the connection. In view of the foregoing, there is a need for an optical fiber having an expanded MFD that enhances the MFD greater than an optical fiber or other optical waveguide device of a photonic component (or other photonic light wave systems) ) so that the fibers can be connected in a consistent manner with a minimum splice loss. In addition, there is a need for a method to expand the MFD of an optical fiber that can be easily repeated, that is consistent in its application, that consumes limited time and resources, and that results in a region of MFD expansion. compact with respect to the length of the optical fiber, which produces a minimum loss of splicing when the expanded MDF fiber is connected to another fiber, and which has the ability to be developed in the field.

BRIEF DESCRIPTION OF THE INVENTION Accordingly, the present invention relates to an optical fiber having an expanded MFD, and a method for expanding the MFD of an optical fiber that evidences the need for prolonged exposure to heat in an oven to facilitate diffusion of the core within the fiber, and therefore an expanded MFD. An advantage of said expanded MFD optical fiber is that it has a reduced expanded mode field region, which is easier to protect after splicing, and provides sufficient strength for the fiber. When an expanded MFD optical fiber of the present invention is connected to a standard SM optical fiber, the reduced expanded mode field region allows a photonic sub-assembly to be formed in that way to have a reduced packing size. Consequently, the costs of the subpacking are reduced, as well as the repair times of said subpacking. An advantage of the method for expanding the MFD of an optical fiber of the present invention is short duration heat treatment. The short duration of such treatment allows those skilled in the art to quickly determine the minimum splice loss that can be achieved for various combinations of fiber splices. Similarly, the target loss for various combinations of fibers can be easily determined. In contrast, those methods that require several hours of heat treatment will take a full day or more before they have the ability to determine if a single splice provides an acceptable splice loss. In addition, the method of the present invention does not require special treatment of the flexible connection cable of the fiber due to its short length. To achieve these and other advantages, an adiabatic taper is formed in an optical fiber by aligning and splicing a cut end of a lower MFD optical fiber with a cut end of a larger MFD optical fiber adjacent to the heat source to form a splice junction. The splice junction is deflected at a predetermined distance from the center of the heat region produced by the heat source and heat is applied in said heat region to split the fibers and expand the MFDs while the reduction in the splice loss When the observed splice loss is or is close enough to the target loss, the heat application is terminated and the lower MFD optical fiber is cut at the point where the heat is supplied from the heat source to the MFD optical fiber in the center of the heat region. In another aspect, the invention includes an optical fiber having a core bonded by a stainless steel liner, and a cut end having an adiabatic taper of less than 1 cm in length formed therein. The cut end is adapted so that it can be divided into a second optical fiber having a higher MFD, with a splice loss not less than 0.1 dB. In fact in another aspect, the invention includes a component for use with a wave division multiplexing ("WDM") system. The component includes an input fiber optic range with a higher MFD and an MFD optical fiber smaller than at least one Bragg diffraction grating of a fiber and an expanded MFD portion. The expanded MFD portion of the lower MFD optical fiber is spliced by fusion to the input optical fiber range according to the aspects of the present invention described above. Next, the additional features and advantages of the invention will be set forth in the detailed description, and in part will be apparent from the description, or may be learned by practicing the invention. Those skilled in the art will understand that both the foregoing general description and the following detailed description are by way of example and explanatory in nature and are intended to provide a further explanation of the invention as claimed.

The accompanying drawings are included to provide a better understanding of the invention, and are incorporated in this detailed description and constitute a part thereof. The drawings illustrate various embodiments of the invention, and together with the description are useful to explain the principles of said invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the splice loss in relation to the arc time during the expansion of MFD according to the present invention. Figure 2 is an enlarged partial cross-sectional view of the ends of a minor MFD optical fiber and a larger MFD optical fiber, each stripped of its primary coating according to the present invention. Figure 3 is an enlarged partial cross-sectional view of a minor MFD optical fiber within a cutter described schematically in accordance with the present invention. Figure 4 is an enlarged partial cross-sectional view of a minor MFD optical fiber and a larger MFD optical fiber that is shown schematically connected to a power meter and a laser source, respectively, and forming a splice junction within of a fusion splicer described schematically in accordance with the present invention. Figure 5 is an enlarged partial cross-sectional view showing the expansion of the MFDs of the smaller MFD optical fiber and the larger MFD optical fiber of Figure 4, according to the present invention. Figure 6 is an enlarged partial cross-sectional view of an expanded MFD optical fiber of the present invention shown within a schematically described cutting device. Figure 7 is an enlarged partial cross-sectional view of an expanded MFD optical fiber split to a higher MFD optical fiber to form an optical fiber component according to the present invention. Figure 8 is a schematic view of a fixed waveguide drop module incorporating a component according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In the field of photon light wave systems that are continuously increasing, fibers for specific use such as erbium doped fiber, compensatory dispersion fiber, Bragg grating fiber, and fiberglass diffraction patterns are playing an important role that has been increasing. Unfortunately, the inherent properties of these and other fibers have made the use of photonic light wave systems very difficult. Specifically, misalignment of the MFD between fibers for specific use and standard fibers or other optical waveguide components, have made the connection of fibers or splicing in these systems a very difficult task. In particular, fiber for printing Bragg diffraction gratings of the fibers, requires very high concentrations of germanium dopant in the core of the fiber, which results in fibers having lower MFDs. In these fibers, the Bragg diffraction grating couples the core mode to wavelengths slightly shorter than the Bragg wavelength to a reverse-propagated stainless steel coating mode. Because the diaphragm between the Bragg wavelength and the beginning of mode absorption of the stainless steel coating limits the total number of wave division multiplexed channels ("WDM") for which the grid device can be used fiber Bragg diffraction, there is an incentive to increase the size of the diaphragm between the Bragg waveguide and the start of the stainless steel coating mode. From the perspective of phase equalization, the diaphragm can be enlarged by increasing the delta, which is a relative difference between the refractive index of the core and that of the stainless steel cladding. In practice, a fiber Bragg diffraction grating written in a SMF 28 fiber having a delta of 0.36% exhibits a stainless steel coating mode start diaphragm of 2 nm, whereas in fiber with 2% HD, the diaphragm increases to 7 nm. Therefore, it is desirable to use fibers with a higher delta in WDM systems and in other components. In accordance with the present invention, a highly localized high temperature heat source, such as an arc fusion splicer, a tunsgtene filament or a C02 laser is used to expand the MFD and thus form the adiabatic taper. It is known to those skilled in the art that the discharge temperature of the arc of an arc-fusion splicer depends not only on the discharge current but also on the condition of the electrodes. Therefore, the MFD expansion is not adequately controlled by measuring the arc time against the current. Additionally, due to the narrow arc region provided by an arc fusion splicer, precise cutting with a position accuracy of 10 μm is required to consistently obtain the same expanded mode field at the end of the fiber. These drawbacks have previously prevented the use of arc fusion splicers for the expansion of MFD prior to fusion connection. Reference will now be made in greater detail to an exemplary embodiment of the invention, the example of which is illustrated by the accompanying drawings. Figure 1 graphically graphs splice loss over arc time during fusion splicing of a germanium doped optical fiber having a delta of 2%, and a standard single mode fiber, in this case an SMF fiber 28 manufactured by Corning Incorporated. The initial arc that fuses the two fibers having MFDs of different size is indicated by the reference number 12. As additional current is supplied to the fibers, the slope of the curve 10 decreases. During this time, splice loss is reducing as the germanium from the core of the HD fiber is diffused into the stainless steel coating region of the HD fiber. As shown in curve 10, this continues until the minimum loss of splice that can be achieved for these two fibers is reached as indicated by reference numeral 14 in FIG. 1. In this case, a minimum splice loss of 0.33 dB is achieved in approximately 105 seconds. This minimal loss of relatively high splice is due to the fact that the fiber HD and SM fiber MFDs do not equal to the splice junction. Subsequently, the splice loss increases as the additional current is applied to the heat region (indicated by reference number 16). During this period, excessive expansion of the fiber cores has occurred. Due to the high tolerances used during fiber production, splice loss versus arc time curve for the connection of any fiber with a 2% delta doped with germanium with any standard SM SMF 28 fiber manufactured by Corning Incorporated will be substantially similar to that shown as the curve 10 in fig. 1. Consequently, a minimum splice loss of approximately 0.33 dB will result. Although the time necessary to achieve this minimum loss will vary due to such variables as the condition of the electrodes. The minimum loss itself can be used to determine a target splice loss for this fiber combination. The target loss is always slightly greater than the minimum splice loss that can be achieved, because the core of the germanium-impurified HD fiber expands more in the center of the arc than in the melting contours. Therefore, the objective loss can be determined experimentally by means of several successive approximations. Through such experimentation, it has been determined that an objective loss of 0.45 dB is optimal for the MFD expansion of the present invention when a fiber of a 2% delta doped with germanium is supplied and connected to a standard SMF 28 fiber. Established differently, when the splice loss at the melting limit reaches 0.45 dB as indicated by the reference number 18 in fig. 1, the MFD of the 2% delta fiber impregnated with germanium will optimally expand to match the MFD of a standard SMF 28 fiber. Accordingly, the method of the present invention can be carried out as set forth below. The preferred embodiment of the method for expanding the MFD of an optical fiber of the present invention is illustrated in Figures 2-6. As shown in Figure 2, a fiber with a 2% delta doped with germanium 20 is stripped from its primary coating 22 over a portion of its length exposing a smaller diameter core 24 bonded by a coating of stainless steel 26. Likewise, a standard single-mode SMF 28 optical fiber is also stripped of its primary coating with stainless steel 32 over a portion of its length to expose a larger diameter core 34 joined by a 36 stainless steel coating. a high delta 20 is placed in a conventional cutter 40, such as an York EFC11 ultrasonic cutter such that the end of the primary coating 22 (or other reference point) is aligned with a reference mark 42 or other reference point in the cutter schematically described 40 as shown in fig. 3. To assist in this alignment, it is preferred that a low power (30X) microscope be used. When properly positioned within the cutter 40, the distance between the line mark 42, and therefore the end of the cover 22, and the cutting blade (not shown) is approximately 18 mm. The high delta fiber 20 is cut to provide a precise cut 28 at the unlined end of the high delta fiber 20. Although not shown in the figures of the drawings, a fiber of a larger MFD 30 is also placed in a cutter 40 and cut at its end that has no coating 38. As shown schematically in FIG. 4, the ends that have not been cut from the lower MFD fiber 20 and from the larger MFD fiber 30 are optically connected to a power meter 44 such as for example an HP8153A multimeter from Hewlett Packard and a laser source 46, respectively , to record the loss of connection during splicing. Due to optical reciprocity, loss of connection or loss of splice, it is independent of the direction of transmission of the laser beam supplied by the laser 46. Accordingly, a laser 46 can be connected to a smaller MFD fiber 20 and a power meter 44 connected to a larger MFD fiber 30 without effecting the method of the present invention. As shown further in fig. 4, the cut ends 28 and 38 of the minor MFD fiber 20 and the larger MFD fiber 30 are placed within a fusion splicer 48, such as an arc fusion splicer model No. FSU975 manufactured by Ericsson Inc. The fusion splicer 48 is programmed to join the ends 28 and 38 so that they abut one another and are in proper alignment. Another function of the fusion splicer 48 is to bias the fiber splice junction 50 to a known distance, preferably 100μm as shown in FIG. 5, so that a larger portion of the minor MFD fiber is placed in the arc region or in the heat region 52 different from the higher MFD fiber 30. Those skilled in the art will understand that the region of the arc can be deflected in location of the splice junction 50. The objective is to apply the higher intensity heat to the lower MFD fiber 20 instead of applying it to the splice junction 50. The fibers 20 and 30 are initially fused at the splice junction 50. by supplying an initial arc discharge current of approximately 15.5 mA for an arc time of approximately 2 seconds. While the laser beam light of the laser 46 passes through the fibers 30 and 20, an additional arc is applied intermittently over the arc region 52 to diffuse the dopant (s), in this case germanium, into the core of the minor MFD fiber 20 that resides in the region of the arc 52. Because the doping agent (s) also reside (s) in the core of the MFD major fiber 30, the expansion, although less penetrating, of the core of the MFD fiber 30 that resides within the arc region 52 also expands . In an example that has proven to be adequate, the arc current is applied repeatedly for periods of 10 seconds, while the reduction of the connection loss is recorded by a power meter 44. When the splice loss measured by the meter of power 44 is reduced to less than 0.8 dB, the arc time is gradually reduced for more precise control of the loss. The shorter arc times in the range of about 2 to 6 seconds are selectively applied until the splice loss measured by the power meter 44 is reduced to an objective loss of about 0.45 dB. As presented above, the target loss is slightly greater than the minimum splice loss that can be achieved as illustrated in the splice loss curve with the reference number 14 of figure 1. The result as illustrated in the figure 5 is a fiber 54 fused with a minor MFD fiber portion 20, which includes a portion of expanded MFD 56, and a larger MFD fiber portion 30 that includes a portion of expanded MFD 58. Depending on the condition of the electrodes, The total arc time required for this phase of the procedure is approximately one to two minutes. Referring now to Figure 6, the fused fiber 54 is placed within the cutter 40, and with the aid of a 30X microscope, the end of the stainless steel coating 22 (or other reference point) is deflected by the same distance as the splice junction 50 was deflected in the fusion splicer 48, preferably 100 μm, towards the cutting blade (not shown). The fused fiber 54 is then cut resulting in an expanded MFD fiber 80 having an adiabatic taper 60 at the second cut end 62. Because the fused fiber 54 was deflected during this cutting step, the cut is made at ! point in the expanded mode field region 56 exposed to the higher temperature heat supplied by the arc region center 52. Therefore, in the second cut 62, MFD 64 is optimized for connection to an SMF fiber 28 of a single standard mode. Referring now to another aspect of the present invention, the expanded MFD fiber 80 of the present invention is also described in Figure 6. The expanded MFD fiber 80 has a smaller diameter core 82 joined by a stainless steel coating 84. which has a refractive index higher than the core 82. The optical fiber 80 preferably is a high delta, 2% optical fiber, which contains germanium in the core 82. At least a portion of the fiber 80 has a coating primary 86, while the uncoated portion has an expanded core region or an adiabatic taper 60 at its cut end 62. The adiabatic taper occupies less than one (1) centimeter of the length of the uncoated portion of the fiber 80, and is preferably one (1) millimeter or less in length. Although fiber 80 is doped with germanium in the preferred embodiment, those skilled in the art will understand that optical fibers having cores containing other dopants, such as erbium, boron, fluorine, or other doping materials, can also form the fiber of the present invention. The MFD at the cut end 62 of the fiber 80 is optimized for connection to a standard single-mode optical fiber 70 as shown in Figure 7 to form an optical fiber component 72. Although not shown in the figure of FIG. the drawings, the splice 74 and the uncoated portions of the fibers 70 and 80 can be packaged and protected with a UV cured protective sheath and with a splice compound, or other protective sheath known in the art. The typical splice loss values for the optical component 72 formed using the fiber 80 of the present invention are typically less than 0.1 dB and have been recorded with a value less than 0.05 dB. The adiabatic taper of length 1 mm 60, within the optical fiber 80 of the present invention, is due in large part to the narrow high temperature region produced by the arc-fusion splicer used to diffuse the germanium in the core 82 of the fiber 80, as well as in the novel deflection step that is described in detail above. Other advantages provided by an arc fusion splicer is that the expansion of the MFD is adiabatic since the arc discharge has a smooth temperature profile. Additionally, the adiabatic region 60 is generally smaller than the adiabatic region of other expanded fibers by means of other methods known in the art with at least two orders of magnitude. The reduced adiabatic region of the present invention also allows the uncoated portion of the fiber 80 to be smaller (approximately 18 mm) than the uncoated portions of the fibers having expanded MFDs reduced by other methods known in the art. Accordingly, there is less polarization mode dispersion (PMD) experienced in photonic systems incorporating expanded MFD optical fiber 80 of the present invention. In addition, there is no special treatment of the uncoated portion of optical fiber 80 prior to heat treatment, and the overall strength of the splice, and therefore its component, is significantly greater than splices made with other TEC methods known in the art. . The results of the tensile or tensile strength test for splices made in accordance with the present invention have resulted as a measurement greater than 50 kpsi after packing, which is comparable to a mechanical stretch test result SM to SM furthermore, the temperature cycle period for a splice made in accordance with the present invention has been classified to -20 ° C to 80 ° C, once comparable with an SM to a SM classification.

In fact, in another aspect of the present invention, as shown in Figure 7, an expanded MFD fiber 80 having an adiabatic taper 60 can be spliced to an SMF 28 fiber of a single standard mode 70 to form a component 72. for a photonic light wave system or other device as briefly described in the previous bars. Using a SM to SM fusion program, optimized MFD 64 of adiabatic taper 60 is aligned with MFD 66 of core 68 of a SMF 28 fiber of a single standard mode 70 and fused to form a 64 splice. Due to the MFDs with an equality suitable at junction 74, the splice loss of component 72 is less than 0.1 dB. One embodiment of said component is described in Figure 8. Component 82 is shown forming a part of a major drop module 84 used in connection with a WDM system those skilled in the art will understand that component 82 may also be part of a addition module for a WDM system and can also be used in another photonic light wave system. The component 82 as shown in Fig. 8 is formed by an MFD fiber expanded by fusion splice 80 to a single standard mode SMF 28 fiber 70 to form the splice 74. The expanded MFD fiber 80 includes a variety of fibers. Bragg fiber-optic grating gratings 86 connected by a variety of fusion splice 88. Each of the fiber Bragg diffraction gratings 86 is printed on HD fibers, therefore the fusion splices 88 are made between the fibers that have the same MFD. Accordingly, fusion splices 88 can be made by means of those methods that are currently known in the art. As shown further in Figure 8, the SMF 28 single mode standard fiber 70 forms the flexible connection cable of an optical circulator 90 which in turn is connected to an input fiber optic range 92. The fiber interval exit optics 94 is also a standard single mode SMF 28 fiber and is therefore connected to the distal end of the expanded MFD fiber 80 with a fusion splice 96 made in accordance with the present invention. The Bragg fiber diffraction gratings 86 together with the optical circulator 90 operate to allow the WDM system to drop the selected channels corresponding to the diffraction gratings 86. In addition to developing this function, the 82 component provides an advantage distinct from other components known in the art since it reduces the overall insertion loss of the assembly. In yet another embodiment, the expanded MFD fiber 80 includes a variety of concatenated WDM add / drop filters (not shown) and is spliced to a standard SM optical fiber. Therefore the filters are connected directly to a range of SM fiber without the use of optical circulators. It will be apparent to those skilled in the art that various modifications and variations may be made in the optical fiber having an expanded MFD and a method for expanding the MFD of an optical fiber of the present invention without departing from the spirit or scope of the invention. Therefore, it is intended that the present invention cover the modifications and variations of this invention as they fall within the scope and spirit of the appended claims and their equivalents. In addition, the corresponding structures, materials, acts and equivalents of all means or steps plus the elements of function in the claims below are intended to include any structure, material, or acts to perform the function in combination with other elements claimed as it is specifically claimed in the present invention.

Claims (21)

NOVELTY OF THE INVENTION CLAIMS

1. - An optical fiber having an MFD and adapted to be connected to a second optical fiber with a higher MFD, said fiber comprising: a stainless steel coating; a core joined by said coating; and a cut end having an adiabatic taper of less than 1 cm in length formed therein, said cut end being adapted to be spliced to the second optic fiber with a splice loss less than 0.1 dB, wherein said adiabatic taper is formed by heating said fiber less than 3 minutes.

2. The optical fiber according to claim 1, further characterized in that said adiabatic taper is less than 2 mm in length.

3. The optical fiber according to claim 1, further characterized in that said adiabatic taper is less than 1 mm in length.

4. The optical fiber according to claim 1, further characterized in that said adiabatic taper is formed using an arc fusion splicer.

5. - The optical fiber according to claim 1, further characterized in that said adiabatic taper is formed using a C02 laser.

6. The optical fiber according to claim 1, further characterized in that said adiabatic taper is formed using a heat source of tungsten filament.

7. The optical fiber according to claim 1, further characterized in that said core includes germanium.

8. An optical fiber having a lower MFD and adapted to be connected to a second optical fiber having a higher MFD, said fiber comprising: a stainless steel coating; a core joined by said coating; and a cut end having an adiabatic taper of less than 1 cm in length formed therein, said cut end being adapted to be spliced to the second optic fiber with a splice loss of less than 0.1 dB and said fiber having a greater delta of 1 %.

9. The optical fiber according to claim 8, further characterized in that said delta is approximately 2%.

10. The optical fiber according to claim 8, wherein said fiber has a delta greater than 2%.

11. A method for forming an adiabatic taper in an optical fiber, said method comprises the following steps: aligning and supporting a cut end of a first optical fiber having a lower MFD and a cut end of a second optical fiber having a Major MFD adjacent to a heat source to form a splice junction; deflecting said junction from a predetermined distance from the center of the heat region of said heat source, applying heat in the heat region to splice the fibers and expand the MFDs; record the reduction in splice loss during the heating step; terminate the application of heat when the splice loss is or is close enough to an objective loss; cutting said first optical fiber wherein the heat of said heat source is supplied to said first optical fiber through the center of the heat region.

12. The method according to claim 11, further characterized in that the fiber has a primary coating and wherein the step to align and rest on further comprises the step of removing the first coating from at least a portion of the first fiber optics and the second optical fiber.

13. The method according to claim 11, further characterized in that the core of the first optical fiber includes germanium, and wherein the step to apply heat comprises the step of diffusing the germanium in the stainless steel coating of the first fiber optics.

14. The method according to claim 11, further characterized in that said heat source is an arc fusion splicer having electrodes, and wherein the step to apply heat comprises the step to intermittently apply an arc through the electrodes.

15. The method according to claim 11, further characterized in that said first optical fiber has a delta greater than 1% and wherein the step to apply heat comprises the step to optimize the MFD of the first optical fiber to equal the MFD of the second optical fiber.

16. The method according to claim 11, further comprising the step for splicing by fusion to the first optical fiber spliced another optical fiber, and wherein the resulting splice loss is less than 0.1 db.

17. The method according to claim 11, further characterized in that the core of the first optical fiber includes erbium and the second optical fiber is a single mode optical fiber, and wherein the step to apply heat comprises the step of diffuse the erbium inside the stainless steel coating of the first optical fiber.

18. The method according to claim 11, further characterized in that the core of the first optical fiber includes erbium and the second optical fiber is a single-mode optical fiber having a core doped with germanium, and wherein the step to apply heat comprises the step of spreading the erbium in the stainless steel coating of said first optical fiber and diffusing into the germanium in the stainless steel coating of said first optical fiber in a single mode.

19. - a component for use with a WDM system, wherein said component comprises: an input optical fiber interval having a higher MFD; a lower MFD optical fiber having at least one fiber Bragg diffraction grating and an expanded MFD portion, the expanded MFD portion is spliced to said input optical fiber range.

20. The component according to claim 19, further characterized in that said range of said input optical fiber includes an optical circulator wherein said lower MFD optical fiber includes a plurality of concatenated fiber optic Bragg diffraction gratings.

21. The component according to claim 20, further comprising a range of optical fiber output having a higher MFD and wherein said output fiber optic range is connected to said lower MFD optical fiber and to the WDM system .