WO2002033464A9

WO2002033464A9 - Low reflection optical fiber terminators

Info

Publication number: WO2002033464A9
Application number: PCT/US2001/032384
Authority: WO
Inventors: Scot K Ware; Brent J Ware
Original assignee: Amherst Holding Co
Priority date: 2000-10-18
Filing date: 2001-10-18
Publication date: 2003-02-20
Also published as: AU2002217762A1; WO2002033464A1

Abstract

An improved method for making an optical fiber terminator that may be effectively used both in production lines and in making stand-alone low-reflection optical terminators. The method has a high repeatability and reliability. The optical fiber terminator may have at least one, but preferably two, fused micro-bends in an optical fiber (102), and the optical fiber may have a cut end that terminates into a protective and/or light-absorbing covering (802). The resulting return loss may be significantly better than for conventional methods of optical fiber termination.

Description

LOW REFLECTION OPTICAL FIBER TERMINATORS

Inventors: Scot Ware and Brent Ware.

This application claims priority of U. S. Patent Application No. 09/690,439, filed October 18, 2000 entitled Low Reflection Optical Fiber Terminators.

FIELD OF THE INVENTION

The present invention relates to optical fiber terminators and methods for making the same, and more particularly to optical fiber terminators having a low reflectance and that are inexpensive and easy to manufacture.

BACKGROUND OF THE INVENTION

In fiber-optic communication systems, many fiber optic devices have redundant ports or redundant optical fiber pigtails. Such devices include couplers, combiners, splitters, Bragg grating selectors, and grating dispersion compensators. The redundant pigtails need to be properly terminated to avoid the back-reflection from the unconnected fiber end. Existing methods for optical fiber end termination may be divided into two categories: in-line termination (such as angle-cleaving) and splicing the optical fiber end with a stand-alone low reflection device such as a terminator.

An example of the angle cleave method is described in U.S. Patent No. 5,048,908, issued September 17, 1991 , to Blonder et al. The angle cleave method does not ensure a low return loss. The return loss depends greatly upon the way that the angle-cleaved end of the optical fiber is protected. The angle cleave method is normally used in laboratories but is not a preferred method in production lines.

An example of the other conventional method, the in-line termination method, is described in U.S. Patent No. 5,809,198, issued September 15, 1998, to Weber et al. In this method, the optical fiber end is heated with a flame to form a glass bead. Such a method normally requires lengthy manufacturing time and is difficult to accurately control in a repetitive process. Other similar methods require the use of a special optical fiber without a core (as described by U.S. Patent No. 5,619,610, issued April 8, 1997, to King et al.), or multiple V-grooves to bend the optical fiber (as described by U.S. Patent No. 5,933,564, issued August 3, 1999, to Pavlath).

What is needed is a method for providing an effective optical fiber terminator that may be used both in production lines and in making stand-alone low-reflection optical terminators. The method should preferably be easily implemented in most dense wavelength division multiplexing (DWDM) or erbium-doped fiber optic amplifier (EDFA) production lines, and should have a high repeatability and reliability.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a method for making an optical fiber terminator comprising the steps of applying shear stress to a portion of an optical fiber, and heating at least a sub-portion of the portion of said optical fiber, thereby creating a fused micro-bend in said optical fiber. Preferably, and to allow for reliable repetition in a manufacturing process, the optical fiber is heated and fused using an electrical arc generated between two electrodes. The electrodes may be part of a conventional optical fiber splicer.

Another aspect of the present invention is directed to an optical fiber terminator having an optical fiber with a fused micro-bend in the optical fiber disposed near the end of the optical fiber. The optical fiber terminator according to such an aspect of the present invention may be capable of producing attenuation at the fused micro-bend and reflection loss at the end of the optical fiber, thereby producing a total return loss that is significantly better than conventional optical fiber terminators.

Although the invention has been defined using the appended claims, these claims are exemplary and not limiting to the extent that the invention is meant to include one or more elements from the apparatus and methods described herein and in the applications incorporated by reference in any combination or subcombination. Accordingly, there are any number of alternative combinations for defining the invention, which incorporate one or more elements from the specification (including the drawings, claims, and applications incorporated by reference) in any combinations or subcombinations.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side view of an optical fiber.

Fig. 2 is a side of view of the optical fiber being misaligned by clamps.

Fig. 3 is a schematic diagram of an apparatus for performing at least some steps involved in making a terminator.

Fig. 4 is a display screen showing a portion of an optical fiber as positioned in an optical fiber splicer.

Fig. 5 is a view of the display shown in Fig. 4 after the optical fiber has been moved.

Fig. 6 is a side view of an electrical arc being applied to the optical fiber.

Fig. 7 is a side view of the optical fiber having a micro-bend in the optical fiber resulting from the steps shown in Figs. 1-6.

Fig. 8 is a side view of the optical fiber having the micro-bend and being covered by a covering at the end of the optical fiber. Fig. 9 is a side view of the optical fiber having a further light-absorbing layer over the covering.

Fig. 10 is a side close-up view of the optical fiber having the micro-bend.

Fig. 1 1 is a graph showing measured return loss of a terminator made according to aspects of the present invention.

Fig. 12 is a graph showing return loss versus bend radius of a micro-bend for an optical fiber terminator in accordance with aspects of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description and accompanying figures describe an exemplary method for making an optical fiber terminator. Such an optical fiber terminator according to aspects of the present invention may include at least one micro-bend and/or a protective covering around the end of the optical fiber and/or the micro-bend. As a result, an optical fiber terminator according to aspects of the present invention may have return losses of as much as 70 to 73 dB.

Referring to Fig. 1 , an optical cable 100, such as a single-mode fiber (SMF) cable, may include a coating 101 such as a light-absorbing insulator and an optical fiber 102 within the coating 101. The optical fiber 102 may typically have a light-conductive core (not shown) surrounded by a cladding (not shown) that together reflect the light so as to remain in the core. In this way, the optical fiber 102 can conduct light over a long distance, even where the optical fiber 102 is curved or has a reasonably shallow bend.

To make a terminator according to aspects of the present invention, one may strip the coating 101 back from the optical fiber 102 so that the bare optical fiber 102 may be worked with. The exposed optical fiber 102 may further be cleaned. It is preferable to provide enough bare optical fiber 102 to be convenient to work with, such as at least approximately 20 - 30 millimeters of bare optical fiber 102 as shown in Fig. 1.

Referring to Fig. 2, the optical cable 100 may next be placed in one or more clamps 201, 202. Preferably, one of the clamps 201 may be placed around the coating

101 and the other of the clamps 202 may be placed directly around the optical fiber 102 itself, as shown for example in Fig. 2. However, any reasonable combination and placement of clamps will work. For instance, both clamps 201 , 202 may be placed directly around the optical fiber 102. In preparation for creating a micro-bend in the optical fiber 102, the clamps 201, 202 may be misaligned with each other. Although the clamps 201, 202 may be of any desirable distance from each other, they are preferably spaced apart by a distance in the range of 2-30 millimeters, and more preferably by a distance in the range of 3-20 millimeters. In one preferred arrangement, the clamps are spaced apart by 1 1 millimeters.

The misalignment of the clamps 201 , 202 causes a shear stress to be applied to the optical fiber 102, thereby creating a slight bend in the optical fiber 102. The clamps 201, 202 may be misaligned horizontally and/or vertically relative to one another. With certain fiber types and sizes, it is believed that the amount of bending of the optical fiber

102 may be important. If the clamps 201, 202 are misaligned by too much, the resulting shear stress on the optical fiber 102 may damage the optical fiber 102 and/or may cause the bend radius of the micro-bend that will later result to be too small to be optimal or effective. On the other hand, where the clamps 201, 202 are not misaligned enough, the optical fiber 102 may not be bent enough, and the resulting micro-bend may have a bend radius that is too large to be optimal or effective.

For instance, in Fig. 2, the clamps 201 , 202 are shown as being misaligned an amount such that the clamped regions of the optical fiber 102 are offset by an amount preferably in the range of approximately 0.08 to 5.0 millimeters. In such an embodiment, the optical fiber 102 may have an S-shaped curve with a "bend offset" at the S-shaped curve of approximately 0.08 to 1.0 millimeters. It is also contemplated to offset the clamped regions of the fiber 102 by any and all amounts in range of 0.05 to 0.4 millimeters to obtain various results for different fiber types and sizes. In one preferred arrangement, the clamps 201 , 202 are offset by approximately 0.12 millimeters. Further, it is contemplated that the amount of offset between the clamps 201 and 202 is 50%- 200% of the diameter of the optical fiber 102. Any and all values in this range are contemplated in the present invention. In another preferred embodiment, the offset between the clamps 201 and 202 is approximately equal to two, three, or four times the diameter of the fiber 102.

Referring to Fig. 3, the clamps 201, 202 may be part of an apparatus such as an optical fiber splicer 300. One example of an optical fiber splicer 300 that may be used is the fusion splicer FSU 975 sold by Ericsson Cables AB, Stockholm, Sweden. Optical fiber splicers are typically used for splicing two optical fibers together at their respective ends. However, in embodiments of the present invention, the optical fiber splicer 300 may be used on the single optical fiber 102 to create a fused micro-bend in the optical fiber 102.

The optical fiber splicer 300 may include several clamps including the clamps 201, 202 previously discussed in connection with Fig. 2. The clamps 201 and/or 202 may be moveable as is conventionally known. While some clamps in the prior art have been designed to be movable relative to each other to prevent misalignment, at least one of the clamps 201, 202 in the preferred arrangement may be moved in order to misalign the optical fiber 102 as a prelude to creating the fused micro-bend in the optical fiber 102.

The optical fiber splicer 300 may further include one or more devices for heating the optical fiber 102. In the embodiment shown in Fig. 3, the optical fiber splicer 300 includes two opposing electrodes 301 , 302 for generating an electrical arc through the optical fiber 102. The electrodes 301, 302 may be coupled to a high voltage generator (not shown) as is conventionally known, and the resulting electrical arc acts to heat and fuse the optical fiber 102. In an alternative exemplary embodiment, the optical fiber splicer 300 may include other conventionally known devices for heating and fusing the optical fiber 102.

The optical fiber splicer 300 may further have a display 303 coupled to a camera (not shown) as is conventionally known that shows the area of the optical fiber 102 near and/or between the electrodes 301, 302. The display 303 may be any type of display such as an LCD screen or a CRT screen. The display 303 may be part of the optical fiber splicer 300 or may be physically separate (such as a separate television screen) coupled with a video output (not shown) of the optical fiber splicer 300. In one exemplary embodiment, the display 303 may be configured so as to show a length of approximately 1.5 to 2.0 millimeters along the optical fiber 102 between the electrodes 301, 302. An image of a portion of the optical fiber 102 can be seen as being shown on the display 303 in Fig. 3.

The optical fiber splicer 300 may further include one or more controls 304 coupled to the electrodes 301 , 302, the display 303, and/or the movement and positioning of the clamps 201 , 202 for controlling these devices. For instance, the controls 304 may include one or more buttons for moving the clamp 201 left, right, up, and/or down with respect to the longitudinal axis of the optical fiber 102. Referring to Fig. 4, the display 303 may show a view of at least a portion of the optical fiber 102, preferably a view of the optical fiber 102 between the electrodes 301,

302. In Fig.4, the optical fiber 102 is shown before the clamps 201, 202 are misaligned. To re-create the same fused micro-bend each time a new optical fiber is used, it may be desirable to accurately misalign the clamps 201 , 202 by approximately the same amount each time. To do this accurately, the user may wish to mark a position of the optical fiber 102 before the clamps 201, 202 are misaligned to act as a point of reference. Then, when the clamps 201, 202 are misaligned, the optical fiber 102 will be shown on the display 303 in a different position due to the bending of the optical fiber 102.

In one embodiment of the present invention, the user of the optical fiber splicer 300 may mark the points on an edge of the display 303 with marks 401 and/or 402 showing where the optical fiber 102 crosses the edge of the display 303. Although two marks 401, 402 are shown in Fig. 4, one for the top edge of the optical fiber 102 and one for the bottom edge of the optical fiber 102, any number of marks may be used. For instance, only the mark 401 might be used. The marks 401 , 402 may be drawn (e.g., with a pencil or pen) on the side of the display as shown in Fig.4, on the screen of the display

303, and/or the marks may be engraved and/or created by using stickers. Any type of mark may be used, as long as the mark indicates a position of the optical fiber 102 crossing an edge of the display. In an alternative embodiment, a mark may be made on the screen of the display 303 where another portion of the optical fiber 102 is shown on the display 303, such as where the optical fiber 102 crosses a vertical line (not shown) drawn down a portion of the display 303.

In an alternative embodiment of the present invention, the optical fiber splicer 300 may be configured to display one or more computer-generated marks on the screen of the display 303, such as computer-generated marks 403, 404, indicating where the optical fiber 102 crosses a certain portion of the display 303 such as an edge of the display 303.

Fig. 5 shows the display 303 after the clamps 201, 202 have been misaligned. In one preferred embodiment, the clamps 201, 202 may be misaligned so as to result in the optical fiber 102 as shown on the display 303 to be relocated by a distance of approximately the width of the optical fiber 102. Such a situation is shown in Fig. 5, where the optical fiber 102 is relocated on the display 303 in an upward direction by one width of the optical fiber 102. In other embodiments, the clamps 201, 202 may be misaligned so as to relocate the optical fiber 102 as shown on the display 303 by a repeatable fixed amount each time. In still a further embodiment, an edge of the display 303 may include a scale or ruler (either marked on the side of the display 303 or computer generated on the screen of the display 303) for use in measuring the amount that the optical fiber 102 is relocated on the display 303 when misaligning the clamps 201, 202.

Referring to Fig. 6, once the clamps 201, 202 are misaligned by the desired amount, the electrodes 301 , 302 (or other heating/fusing device) may be activated to heat or fuse the optical fiber 102 as a result of an electrical arc 601 between the two electrodes 301, 302. The term "fused" as used herein, means to heat at least an area of the optical fiber above its softening temperature. This causes the stress that has been previously applied to the optical fiber 102 due to the misalignment of the clamps 201 , 202 to become relieved when the optical fiber 102 is fused. Thus, a new "memory" is created in the optical fiber 102 so that the fiber 102 will have a new memory corresponding to its fused position with the micro-bends 602, 603. It is recognized that there are many other ways of heating or fusing the optical fiber 102, for instance using a laser beam or a flame from a micro-torch.

The micro-bends 602, 603 are relatively sharp bends in the optical fiber 102 that cause a significant portion of the light transmitted through the optical fiber 102 to be lost. The amount of the bending in the micro-bends 602, 603 determines how much of the transmitted light is lost. The amount of bend radius of the micro-bends 602, 603 may be controlled by the amount of current in the electrical arc 601 and the amount of time that the electrical arc 601 is applied to the optical fiber 102. If these two factors are kept constant, then the bend radii of the micro-bends 602, 603 may be accurately repeatable each time. In one preferred embodiment, an effective micro-bend may be created by applying an electrical arc of 15 milliamps for 2 seconds to the optical fiber 102. While other preferred ranges may be used, it is contemplated to apply an electrical arc of in the range of 8.5 to 17 milliamps for 0.5 to 10 seconds to the optical fiber 102.

In a preferred example, the clamps 201, 202 may be misaligned so as to cause bend 602, 603 in the optical fiber 102 having a bend radius R. Further, it is contemplated that the bend radius R of the microbend 602, 603 is preferably any amount in range of .15 to .45 millimeters to obtain various results for different fiber types and sizes. In one preferred arrangement, the bend radius R is approximately .3 millimeters. Further, the bend radius R is preferably, but not necessarily, 200%-400% of the diameter of the optical fiber 102.

Referring to Fig. 7, once the micro-bends 602, 603 have been created, the optical fiber 102 may be cut to remove redundant fiber. Although the amount of fiber removed is not critical, enough should be removed to make the remaining optical fiber portion convenient and portable. On the other hand, the cut should preferably not be too close to the micro-bends 602, 603 so that damage to the micro-bends 602, 603 may be avoided. In one embodiment, the optical fiber 102 may be cut so that approximately 2 to 4 millimeters are remaining after the last micro-bend 603. The cut may be accomplished using a scissors or any other shearing or cutting mechanism.

Referring to Fig. 8, at least a portion of the optical fiber 102 and/or the coating 101 may be covered with a material 801 such as ethylene vinyl acetate or mineral oil. The material may be light-absorbing at the frequency of light to be transmitted through the optical fiber 102 in order to prevent reflection of light existing at the end of the optical fiber 102 back into the optical fiber 102. Preferably, however, the material 801 may be light-transmissive and have an index of refraction higher than that of the optical fiber 102 in order to prevent reflection of the light. In a preferred embodiment, the relative refractive index of the material 801 is approximately 1.8 or higher.

The material 801 may further be partially or fully covered with a protective covering 802 such as a heat-shrink plastic, which may be made of, e.g., polyethylene, polyolefin, or polyvinyl chloride. In an exemplary embodiment, the protective covering 802 may be a sheathing enclosing fluid mineral oil as the material 801, wherein the mineral oil (i.e., the material 801) surrounds the end of the optical fiber 102. The protective covering 802 may preferably cover the micro-bends 602, 603 to not only protect the micro-bends 602, 603 from physical damage, but also to prevent unwanted receipt of external light into the micro-bends 602, 603.

Referring to Fig. 9, the protective covering 802 may be further covered and/or coated with a light-absorbing material 901. The light-absorbing material 901 may be used to not only prevent reflectance of light from the optical fiber 102 back into the optical fiber 102, but also to prevent light from a source external to the terminator from entering into the optical fiber 102. In a preferred embodiment, the light-absorbing material 901 may be an air-dryable flexible material into which the protective covering 802 may be dipped, such as PLASTI DIP, marketed by PDI Inc., Circle Pines, MN. In other embodiments, the protective covering 802 may be alternatively covered with electrical tape, paint, latex, and/or any other covering or coating that absorbs light.

Fig. 10 illustrates a close-up view of the two micro-bends 602, 603 in the optical fiber 102. A portion of the light that is guided by the optical fiber 102 is lost at the micro-bends 602, 603 by radiating into the surrounding environment. As the guided light travels through the optical fiber 102 from left to right across Fig. 10, the light is first attenuated by the first micro-bend 602, then again attenuated by the second micro-bend 603. At the end of the optical fiber, the light is then partially absorbed by the protective covering 801 and/or the light-absorbing material 901, and partially reflected due to the transition between the cut end of the optical fiber 102 and the protective covering 801. The partially reflected light then travels back through the optical fiber 102 from right to left across Fig. 10, again being attenuated by first the micro-bend 603 and then the micro- bend 602. Thus, each micro-bend 602, 603 performs a "double-duty" by twice attenuating the light guided by the optical fiber.

In the embodiment shown in Fig. 10 (not necessarily to scale), each of the micro- bends 602, 603 has a bend radius of approximately 0.3 millimeters as measured from the central axis of the optical fiber 102. Where the optical fiber 102 is an SMF optical fiber with a micro-bend bend radius of 0.3 millimeters, the guided light may typically be attenuated by at least 30 dB at the micro-bend. Further, it can be expected that the return loss of the light reflected back into the optical fiber 102 from the cut end of the optical fiber 102 may be on the order of 14.6 dB or more. All together, in theory about 74 dB or more total return loss may be expected from the entire optical fiber terminator comprising the two micro-bends 602, 603 and the cut end of the optical fiber 102 being covered by the protective covering 801 and the light-absorbing material 901. This expected return loss is much higher than the normal requirement by the fiber optic industry, which normally requires that the return loss of an optical fiber terminator be at least 55 to 60 dB.

Fig. 11 shows the results of an actual test that was performed by Applicant on a test optical fiber terminator built according to the present invention. As can be seen from Fig. 11 , the tested optical fiber terminator had a return loss ranging from about 70 dB to about 72 dB depending upon the wavelength of light guided by the optical fiber.

Referring to Fig. 12, Applicant has also found that the bend radius of a micro- bend largely determines the attenuation in guided light caused by the micro-bend. Applicant has further discovered that there is a non-proportional relationship between the bend radius of a micro-bend and the amount of return loss produced by the optical fiber terminator. It is expected that as the bend radius decreases, the amount of return loss increases. However, at a certain range of bend radii, the return loss peaks and then decreases again as the bend radius decreases further. Thus, there may be a certain range of bend radii that produces optimal retum loss in the optical fiber terminator.

Although particular embodiments of the present invention have been shown and described, it will be understood that it is not intended to limit the invention to the preferred embodiments and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the invention is intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the invention as defined by the claims.

Claims

What is claimed is:

1. A method for making an optical fiber terminator comprising the steps of: bending a portion of an optical fiber; and heating at least a section of the portion of said optical fiber while bent.

2. The method of claim 1, further including the steps of: clamping said optical fiber on a first side of said portion of said optical fiber; and clamping said optical fiber on a second side of said portion of said optical fiber, wherein the bending step includes misaligning the first side with the second side.

3. The method of claim 2, wherein the bending step further includes misaligning the first side with the second side by a distance approximately equal to a width of said optical fiber.

4. The method of claim 1, wherein the bending step includes: viewing at least some of the section of said optical fiber on a display, said optical fiber appearing to run between a first portion of said display and a second portion of said display; marking a location where said optical fiber appears to cross the first portion of said display; and moving said optical fiber based on the position of the marked location.

5. The method of claim 4, wherein the step of moving includes moving said optical fiber a distance approximately equal to a width of said optical fiber.

6. The method of claim 1, wherein the step of heating includes applying an electrical arc to the section of said optical fiber.

7. The method of claim 6, wherein the step of applying an electrical arc includes continuously applying the electrical arc for approximately 2 seconds with a current of approximately 15 milliamps.

8. The method of claim 1 , further including the step of cutting said optical fiber near said bent portion of said optical fiber.

9. The method of claim 8, wherein the step of cutting includes cutting said optical fiber at a distance of approximately between 2 and 4 millimeters from said bent portion.

10. The method of claim 1 , further including applying a covering to an end of said optical fiber.

11. The method of claim 1, further including applying a covering having an index of refraction higher than an index of refraction of said optical fiber to an end of said optical fiber.

12. The method of claim 10, wherein the step of applying includes applying a heat shrink plastic to the end of said optical fiber.

13. The method of claim 10, further including the step of applying a light- absorbing material to said covering.

14. The method of claim 13, wherein the step of applying said light-absorbing material includes dipping said covering into an air-dryable, light-absorbing material.

15. The method of claim 1 , wherein the bending and heating steps produce a micro-bend in the fiber having a bend radius in the range of 0.25-0.35 millimeters.

16. The method of claim 1, wherein the optical fiber has a diameter and the bending and heating steps produce a micro-bend in the fiber having a bend radius in the range of 200%-400% of the diameter of the optical fiber 102.

17. The method of claim 2, wherein the optical fiber has a diameter and the misaligning step includes offsetting the first and second sides of the fiber by an amount in the range of 50%-200% of the diameter of the optical fiber 102.

18. The method of claim 1 , wherein the heating step includes heating the fiber above a softening temperature of the fiber to reduce any stresses in the fiber caused by said bending step.

20. An optical fiber terminator, comprising: an optical fiber having: an end; and a fused micro-bend near the end of the optical fiber.

21. The optical fiber terminator of claim 20, wherein said optical fiber has an index of refraction, said optical fiber terminator further including a covering disposed at the end of said optical fiber, said covering having an index of refraction higher than the index of refraction of said optical fiber.

22. The optical fiber terminator of claim 21 , wherein said covering also covers said micro-bend.

23. The optical fiber terminator of claim 21 , wherein said covering comprises ethylene vinyl acetate.

24. The optical fiber terminator of claim 21, further including a light- absorbing material disposed around said covering.

25. The optical fiber terminator of claim 20, wherein said micro-bend has a bend radius of approximately 0.3 millimeters.