WO1996004580A1

WO1996004580A1 - Optical attenuator having a partially reflective surface

Info

Publication number: WO1996004580A1
Application number: PCT/US1995/009297
Authority: WO
Inventors: Warren H. Lewis
Original assignee: The Whitaker Corporation
Priority date: 1994-07-29
Filing date: 1995-07-18
Publication date: 1996-02-15

Abstract

An optical attenuator (10) uses a partially reflecting layer (30) interposed in the optical path. The layer is inclined relative to the fiber axis so that light traveling along the axis is reflected into the fiber's cladding (40) and does not propagate back down the fiber to the source. In a specific embodiment, an optical attenuator is provided having first and second optical fibers, each of which has a core axis, a cladding substantially concentric with the core axis, and a substantially planar endface at a common fixed angle to the core axis. The optical attenuator (10) also includes a partially reflective coating on the endface of at least one of the optical fibers (16, 18). The partially reflective coating (30) is for reflecting electromagnetic energy transmitted in the direction of the core axes into the cladding (40) of one of the optical fibers. The first and second optical fibers (16, 18) are held in a fixed relationship to each other with their respective endfaces substantially parallel to each other, and their respective core axes substantially collinear.

Description

OPTICAL ATTENUATOR HAVING A PARTIALLY REFLECTIVE SURFACE

BACKGROUND OF THE INVENTION The present invention relates generally to fiber optic devices. Specifically, the present invention relates to a fiber optic fixed attenuator.

Although a primary virtue of optical fibers as communication media is the low loss of the fibers, there are instances where it is necessary to provide attenuation in the optical path. U.S. Patent Nos. 4,557,556, 4,557,557, and 5,285,516 disclose methods of fabricating attenuators for optical fibers by fusion splicing. In each of these methods, a fiber is cleaved, the cleaved ends are brought into a desired relationship, and the ends are melted or fused together to provide the desired structure. During fabrication, light is injected by a transmitter into one of the fibers and received by a receiver coupled to the other fiber. When the fibers are being fused, the power level is monitored and relative movement of the fiber ends is effected to achieve a desired observed level of optical loss.

In the '556 patent, the axes of the cleaved ends are offset and butted against one another. The abutted offset ends of the fibers are melted and the axes of the cores of the melted abutted fiber ends are aligned by way of surface tension. This realigns the cores, except that the cleaved ends of the cores bend off the axis in opposite directions. In the '557 patent, the cleaved fiber ends are aligned and melted, and while the ends are in a plastic state, they are moved towards each other to distort the fiber ends. The cores bend slightly near their abutted ends and are partially or wholly misaligned to provide the attenuation. In the '516 patent, the cleaved fiber ends are offset and axially overlapped. The overlapping ends are melted, and moved axially. U.S. Patent No. 5,257,335 discloses a fixed attenuator wherein an attenuating (absorbing) membrane is sandwiched between the obliquely polished ends of a pair of fiber segments. This arrangement is presumably effective for its intended purpose, but the use of an attenuating membrane is subject to the possibility that at high optical power levels, the membrane could overheat and be damaged.

A further disadvantage suffered by the above described technologies is the dependency of the attenuation level on the application for which the respective attenuators are used. For example, an attenuator might be used in three different configurations. First, an attenuator could be interposed between a light source and an output fiber. Second, an attenuator could be interposed between an input fiber and a detector. Finally, an attenuator could be interposed between an input fiber and an output fiber. In each of the three configurations described, the attenuators described in the above-referenced patents yield different attenuation values. This is due to the fact that these attenuators transfer light from the fiber core into the cladding in the direction of propagation. If such an attenuator is coupled directly to a detector, a portion of the light energy which was transferred into the cladding may be detected by the detector because of its proximity, thus giving a lower attenuation value than for a configuration in which the attenuator is connected between two fibers. In this case, the light energy in the cladding will have dissipated before it reaches the next discontinuity.

From the foregoing, it is apparent that an optical attenuator is desired which may be inexpensively produced and which may provide a precise attenuation of light energy over a broad range, and for different configurations.

SUMMARY OF THE INVENTION The present invention provides an optical attenuator which is capable of being fabricated to provide any desired level of attenuation within a wide range of levels, and which is characterized by a high level of wavelength insensitivity. In brief, an optical attenuator according to the present invention uses a partially reflecting layer interposed in the optical path. The layer is inclined relative to the fiber axis so that the light traveling along the axis is reflected into the fiber's cladding and does not propagate back down the fiber to the source. This avoids the inconsistencies of previous attenuators which transfer light forward into the cladding. Thus, with attenuators designed according to the present invention, attenuation is constant whether light is being transmitted into a fiber or a detector.

In a specific embodiment, an optical attenuator is provided having first and second optical fibers, each of which has a core axis, a cladding substantially concentric with the core axis, and a substantially planar endface having a normal vector at a common fixed angle to the core axis. The optical attenuator also includes a partially reflective coating on the endface of at least one of the optical fibers. The partially reflective coating is for reflecting electromagnetic energy transmitted in the direction of the core axes into the cladding of one of the optical fibers. The first and second optical fibers are held in a fixed relationship to each other with their respective endfaces substantially parallel to each other, and their respective core axes substantially collinear In a more specific embodiment, the attenuator includes two assemblies, each having an optical fiber coaxially fixed within a cylindrical ferrule. Each assembly has a planar endface having a normal vector at a common fixed angle to the fiber axis, with the ferrule endface and its associated optical fiber endface being substantially coplanar. The two assemblies are held in a fixed relationship with respect to each other with the endfaces being substantially parallel to each other and the fiber cores substantially collinear as described above. For the present invention to operate properly, the endfaces of the optical fibers need not be in physical contact. They may be separated by optically transmissive material such as a dielectric. The attenuator of the present invention operates even with an air gap between the endfaces. The partially reflecting coating may be applied to one or both of the fiber endfaces. The coating is preferably largely wavelength insensitive in the relevant range.

The fixed angle is preferably large enough so that the portion of the light that is not transmitted is reflected outside the fiber's acceptance cone, but small enough so that the amount of light reflected does not depend significantly on the polarization of the incident light. Angles in the range of 8 to 12 degrees with respect to the normal to the ferrule endface are appropriate for normal single-mode fiber. Angles for multi-mode fibers may be chosen from a broader range because the reflection of light back to the source is not generally of concern. An angle of 15 degrees with respect to the normal to the ferrule endface, for example, would be appropriate for a multi-mode fiber.

The attenuator according to the present invention may be fabricated as follows. First and second ferrules having the desired endface angle are provided. Respective first and second optical fibers are inserted with their ends protruding slightly past the ferrule endfaces. The fibers are then fixed in place and the fiber endfaces are polished so that they are substantially coplanar with their respective ferrules' endfaces. In one embodiment, the partially reflecting coating is applied to one of the fiber endfaces. In another embodiment, the partially reflective coating is applied to both of the fiber endfaces. The assemblies are then registered to each other so that the fiber endfaces are substantially parallel to each other with the fiber cores being substantially collinear. Alignment may be effected by providing a sleeve that accommodates the assemblies and cementing the assemblies into the sleeve when the desired optical contact is achieved.

A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a representation of an optical attenuator assembly according to a particular embodiment of the present invention; Fig. 2 is a representation of a ferrule assembly employed by a particular embodiment of the present invention;

Fig. 3 is a longitudinal cross-section of an attenuator assembly according to a particular embodiment of the invention; Fig. 4 is a magnified view of a portion of the cross-section of Fig. 3 according to one embodiment;

Fig. 5 is a magnified view of a portion of Fig. 4 showing the path of transmitted light;

Fig. 6 is a magnified view of a portion of the cross-section of Fig. 3 according to another embodiment;

Fig. 7 is a magnified view of a portion of Fig. 6 showing the path of transmitted light;

Fig. 8 is magnified view of a portion of the cross- section of Fig. 3 according to a further embodiment of the invention;

Fig. 9 is a magnified view of a portion of Fig. 8 showing the path of transmitted light;

Figs. 10 and 11 show the present invention packaged for use with industry standard FC type optical fiber connectors; and

Figs. 12 and 13 show the present invention packaged for use with industry standard SC type optical fiber connectors.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. l is a representation of the operative portion of an optical attenuator assembly 10 designed according to a embodiment of the present invention. Assembly 10 includes two ferrule assemblies 12 and 14 having central bores which contain optical fiber segments 16 and 18, respectively.

Ferrule assemblies 12 and 14 are held together in a fixed relationship at an interface 20 inside an alignment sleeve 22, assembly 10 having mating faces 13 and 15 for optically mating fiber end faces 17 and 19 to other optical assemblies (not shown) . Sleeve 22 may be a split sleeve as shown in Fig. 1 exerting radial pressure on both ferrules, thereby centering and aligning them. Each ferrule assembly is similarly constructed and may be described with reference to ferrule assembly 12 as shown in Fig. 2. The corresponding reference numbers for ferrule 14 are shown in parentheses. Ferrule assembly 12 (14) is a length of capillary tubing having a central bore sized to accommodate optical fiber 16 (18) . The capillary tubing of ferrule assembly 12 (14) is typically constructed from ceramic, alumina, zirconia, or stainless steel.

Optical fiber 16 (18) may be either single-mode or multi-mode fiber. A single-mode fiber typically has a core diameter of 8.3-10 μm, a cladding diameter of 125 μm, and a buffer diameter of 250, 500, or 900 μm. Single-mode fibers typically operate at wavelengths between 1200 and 1600 nm. Multi-mode fibers with cladding diameters of 125 μm are available with core diameters of 50, 62, and 100 μm. Multi- mode fibers are also available with a core diameter of 100 μm and a cladding diameter of 140 μm. Multi-mode fibers typically operate at wavelengths between 700 and 1600 nm.

The outer diameter of ferrule assemblies 12 and 14 are not constrained by any particular fiber dimension, and may be, for example, on the order of 1.0-2.5 mm. One end of ferrule assembly 12 (14) is formed with an endface 24 (25) having a normal 26 (27) that is inclined from central bore axis 28 (29) by a desired fixed angle α. In specific embodiments, the angle ranges between 8 and 12 degrees. Ferrule assembly 12 (14) is fabricated as follows.

Optical fiber segment 16 (18) is cemented in the central bore of ferrule assembly 12 (14) . The end of fiber segment 16 (18) protrudes a short distance beyond the ferrule assembly's inclined endface 24 (25) . The protruding end of fiber 16 (18) is ground to approximately the angle of ferrule endface 24 (25) , and then polished so that endface of fiber 16 (18) is substantially coplanar with ferrule endface 24 (25) . The result is that both the normal to the fiber endface and the normal to ferrule endface 24 (25) are disposed at the same fixed angle from the central bore axis of fiber segment 16 (18) . A partially reflecting coating (see Figs. 3-9) may then be applied to the ferrule and fiber endfaces. While the optical performance of the attenuator only requires that the partially reflective coating be deposited on the endface of the fiber, it is easier and more practical to coat the entire ferrule endface. As indicated by the reference numbers in parentheses, ferrule assembly 14 is fabricated in a similar manner, although the endface of ferrule assembly 14 may or may not be coated with a partially reflective coating according to various embodiments, some of which are described below with reference to Figs. 3-9.

Fig. 3 shows a longitudinal cross-section of an attenuator assembly 10 designed according to a particular embodiment of the invention. As described above, attenuator assembly 10 is fabricated by aligning two ferrule assemblies 12 and 14 within alignment sleeve 22 so that fiber segments 16 and 18 have their endfaces in substantial optical contact, and the central axes of fiber segments 16 and 18 are substantially collinear. Several different embodiments of attenuator assembly 10 of Fig. 3 are possible, some of which are described below.

Fig. 4 is a magnified view of region A of Fig. 3 according to one embodiment of the invention. According to this embodiment, only the endfaces of ferrule assembly 12 and optical fiber 16 are coated with a partially reflective coating 30. Ferrule assemblies 12 and 14 are fixed together within sleeve 22 by means of epoxy 32. It will be understood that ferrule assemblies 12 and 14 may be coupled by a variety of means other than epoxy 32. Fig. 5 is a magnified view of region B from Fig. 4 and shows the optical path 34 of a light beam traveling down the optical fiber cores 36 and 38 of fiber segments 16 and 18, respectively. As described above, a portion of the light beam reflects from partially reflective coating 30 and into the cladding 40 of optical fiber 16 along path 42 which forms the angle with path 34. Substantially all of the rest of the light beam continues along path 34 through coating 30, epoxy 32, and core 38 of optical fiber 18. In actuality, while most of the light is either reflected specularly or transmitted through coating 30, some small amount of scattering is encountered. This scattering is unavoidable and does not occur by design. To avoid degradation of transmission performance, this scattering must be kept to a minimum. This may be accomplished in part by such strategies as carefully controlling the purity and/or thickness of coating 30 and epoxy 32. In specific embodiments, partially reflective material from Evaporated Coating, Inc. , of Willow Grove, Pennsylvania, is used for coating 30, and epoxy from Epoxy Technology, Inc., of Billerica, Massachusetts, is used as epoxy 32.

The angle α is chosen so that the light that is reflected by coating 30 enters cladding 40 and does not propagate back toward the source of the light beam. Thus the minimum value for the angle of inclination (i.e., a) is determined by the numerical aperture of fiber 16. That is, the angle α should be large enough so that the portion of the light that is not transmitted partially reflective coating 30 is reflected outside the fiber's acceptance cone. Angles in the range of 8 to 12 degrees are appropriate for normal single-mode fiber. The angle that will just prevent the reflected light from reentering the fiber core is given by sin^-1(NA/n) where NA is the numerical aperture in air of the fiber and n is the refractive index of the fiber core. For a numerical aperture of 0.26 and a refractive index of 1.5, the minimum angle is about 10 degrees. The angle α must also be small enough so that the amount of light reflected does not depend significantly on the polarization of the incident light, at least where it is desired to have the attenuation independent of polarization. In such a case, the angle should be as small as possible, that is, only slightly larger than the minimum value. Angles for multi-mode fibers may be chosen from a broader range because the reflection of light back to the source is not generally of concern.

Coating 30 may be metallic or a multi-layer dielectric coating. It is normally possible to have the coating reflectivity reasonably independent of wavelength for the range of operating wavelengths. As mentioned above, such coatings may be obtained from Evaporated Coatings Inc. Another source is Coherent Optical, Inc. The invention may also employ an epoxy such as Epo-

Tek 314 epoxy as epoxy 32 to hold ferrule assemblies 12 and 14 together. This same epoxy may be used to fix the optical fiber in the capillary bore. The 314 epoxy has the benefit of having an optical index of 1.49, which most nearly matches the optical index 1.46 of the glass fiber used in this embodiment while maintaining a strong and rugged bonding capability. This index match minimizes changes in optical index along the optical path of the device which degrade optical performance. Fig. 6 is a magnified view of region A of Fig. 3 according to another embodiment of the invention. In this embodiment, attenuator assembly 10 has partially reflective coatings 30 and 52 on ferrule assemblies 12 and 14, respectively. The partially reflective coatings are separated by epoxy 32. Like the previously described embodiment, this attenuator assembly attenuates light traveling in both directions down optical fibers 16 and 18. One advantage derived from having a coating on both ferrule assemblies, however, is that a smaller inventory of ferrule assemblies may be maintained for the manufacturing of a given number of attenuators having a wide variety of attenuation levels. For example, if only one ferrule assembly were coated in each assembly, one would need to coat ferrule assemblies differently for each desired level of attenuation. However, if two coated ferrule assemblies are used to construct one attenuator, and the assemblies have, for example, a 5 dB coating and a 10 dB coating, respectively, a 15 dB attenuator may be constructed without having to maintain an inventory with individual 15 dB ferrule assemblies.

Fig. 7 is a magnified view of region C from Fig. 6 and shows paths 54 and 56 of two light beams traveling in opposite directions. As with the embodiment described above, a portion of each light beam is reflected from coatings 30 and 52 into cladding 40 and 58 along reflection paths 60 and 62, respectively. Reflection paths 60 and 62 form the angle α with optical paths 54 and 56, respectively. As with the above-described embodiment, the angles α may range from 8 to 12 degrees. Coating 52 may be of the same material as coating 30, which may be the same as coating 30 of Figs. 4 and 5.

Fig. 8 is another magnified view of region A of Fig. 3 according to a further embodiment of the invention. In this embodiment, attenuator assembly 10 has a partially reflective coating 30 (similar to coating 30 of Figs. 4 and 5) on ferrule assembly 12, a layer of epoxy 32, and an anti-reflective coating 72 on ferrule assembly 14. In specific embodiments, anti-reflective coating 72 may comprise, for example, magnesium flouride (MgF₂) • Anti-reflective coating 72 reduces undesirable reflection from the endface of optical fiber 18, as well as reduces interference effects which result from gaps between the fiber endfaces. Such gaps may result from the polishing of the ferrule endfaces which may cause the fiber endfaces to be concave. Interference effects may also be reduced by filling such gaps with an epoxy or cement which improves the optical contact between the two optical fibers. Fig. 9 is a magnified view of region D from Fig. 8 and shows the optical path 74 of a light beam traveling down the optical fiber cores 36 and 38 of fiber segments 16 and 18, respectively. As with the embodiment of Figs. 4 and 5, a portion of the light beam is reflected from partially reflective coating 30 at an angle α along reflective path 76. Substantially all of the remaining light continues traveling along path 74, through epoxy 32, anti-reflective coating 72, and into core 38 of fiber 18. Figs. 10 and 11 show the present invention packaged for use with industry standard FC type optical fiber connectors. Fig. 10 shows a 2.5 mm threaded attenuator package 80 with dust caps 82 protecting connector mating interfaces 84 and 86. Fig. 11 shows a cross-section of attenuator package 80 in which attenuator 10 is shown. Figs.

12 and 13 show the present invention packaged for use with industry standard SC type optical fiber connectors. Fig. 12 shows a 3.25 mm attenuator package 90 with dust caps 92. Fig. 13 shows a cross-section of attenuator package 90 in which attenuator 10 is shown adapted for mating with conventional fiber optic connectors.

While the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in the form and details may be made therein without departing from the spirit or scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. An optical attenuator, comprising: first and second optical fibers, each optical fiber being characterized by a core axis, and having a cladding substantially concentric with the core axis, and a substantially planar endface having a direction normal thereto, the normal direction being at a common fixed angle to the core axis; and a partially reflective coating on the endface of at least one of said first and second optical fibers, said partially reflective coating for reflecting a portion of electromagnetic energy being transmitted in the direction of the core axes into the cladding of one of said first and second optical fibers; wherein said first and second optical fibers are held in a fixed relationship to each other with their respective endfaces substantially parallel to each other, and their respective core axes substantially collinear.

2. The optical attenuator of claim 1, further comprising a sleeve sized to accommodate the first and second assemblies.

3. The optical attenuator of claim 1 further comprising first and second cylindrical ferrules associated with said first and second optical fibers, each ferrule having a central bore sized to accommodate a respective associated optical fiber, wherein said ferrules are made of a material in the group comprising alumina, zirconia, stainless steel, and ceramic.

4. The optical attenuator of claim 1 wherein said common fixed angle is in the range of 8 to 12 degrees.

5. The optical attenuator of claim 1 wherein said endfaces of said first and second assemblies are in substantial physical contact.

6. The optical attenuator of claim 1 wherein said endfaces of said first and second assemblies are cemented to each other.

7. The optical attenuator of claim 1 wherein said first and second optical fibers comprise single-mode fiber.

8. The optical attenuator of claim 1 wherein said first and second optical fibers comprise multi-mode fiber.

9. The optical attenuator of claim 1 wherein said partially reflective coating is applied to the endface of both of said first and second optical fibers.

10. An optical attenuator, comprising: first and second optical fibers, each optical fiber being characterized by a core axis, and having a cladding substantially concentric with the core axis, and a substantially planar endface having a first direction normal thereto, the first normal direction being at a common fixed angle to the core axis; first and second cylindrical ferrules associated with said first and second optical fibers, each ferrule having a central bore sized to accommodate a respective associated optical fiber, and having a substantially planar endface having the first direction normal thereto; said first and second optical fibers being fixedly disposed in said first and second ferrules, respectively, said first optical fiber and said first ferrule defining a first assembly, and said second optical fiber and said second ferrule defining a second assembly, each of said first and second assemblies having its associated optical fiber's endface substantially coplanar with its associated ferrule's endface, thereby defining a substantially planar endface for each assembly; and a partially reflective coating on the endface of at least one of said first and second optical fibers, said partially reflective coating for reflecting a portion of electromagnetic energy being transmitted in the direction of the core axes into the cladding of one of said first and second optical fibers; wherein said first and second assemblies are held in a fixed relationship to each other with their respective endfaces substantially parallel to each other, and the core axes of their respective optical fibers substantially collinear.

11. The optical attenuator of claim 10, further comprising a sleeve sized to accommodate the first and second assemblies, the sleeve exerting radial pressure on the first and second optical fibers, thereby axially aligning the first and second optical fibers.

12. The optical attenuator of claim 10 wherein said ferrules are made of a material in the group comprising alumina, zirconia, stainless steel, and ceramic.

13. The optical attenuator of claim 10 wherein said common fixed angle is in the range of 8 to 12 degrees.

14. The optical attenuator of claim 10 wherein said endfaces of said first and second assemblies are in substantial physical contact.

15. The optical attenuator of claim 10 wherein said endfaces of said first and second assemblies are cemented to each other.

16. The optical attenuator of claim 10 wherein said first and second optical fibers comprise single-mode fiber.

17. The optical attenuator of claim 10 wherein said first and second optical fibers comprise multi-mode fiber.

18. The optical attenuator of claim 10 wherein said partially reflective coating is applied to the endface of both of said first and second optical fibers.

19. A method of fabricating an optical attenuator, comprising the steps of: providing first and second optical fibers, each being characterized by a core axis and having a cladding concentric with the core axis; providing first and second cylindrical ferrules, each ferrule having a central bore sized to accommodate a respective associated optical fiber, and having a planar endface having a first direction normal thereto, the first normal direction being at a common fixed angle to the central bore; fixedly disposing the first and second optical fibers in the first and second ferrules, respectively, with an end of each optical fiber protruding beyond the planar endface of the associated ferrule; polishing the end of each optical fiber, so protruding, so that the optical fiber end, so polished, has a planar endface which is substantially flush with the planar endface of the associated ferrule, the first optical fiber and the first ferrule defining a first assembly, and the second optical fiber and the second ferrule defining a second assembly, each assembly having a planar endface having the first direction normal thereto; depositing a partially reflective coating on the endface of at least one of the fiber segments; and orienting the first and second assemblies in a fixed relationship to each other with their respective planar endfaces being substantially parallel and the core axes of their respective optical fibers substantially collinear.

20. The method of claim 19 wherein said step of fixedly connecting further comprises the steps of: providing a sleeve sized to accommodate the first and second assemblies; and inserting the first and second assemblies into the sleeve with their respective planar endfaces in substantial optical contact.

21. The method of claim 19 wherein said step of fixedly connecting further comprises bringing the endfaces of said first and second assemblies into substantial physical contact.

22. The method of claim 19 wherein said step of fixedly connecting further comprises cementing said endfaces of said first and second assemblies to each other.