CA2414795A1 - Fiber optic coupler - Google Patents
Fiber optic coupler Download PDFInfo
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- CA2414795A1 CA2414795A1 CA 2414795 CA2414795A CA2414795A1 CA 2414795 A1 CA2414795 A1 CA 2414795A1 CA 2414795 CA2414795 CA 2414795 CA 2414795 A CA2414795 A CA 2414795A CA 2414795 A1 CA2414795 A1 CA 2414795A1
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- fiber
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- coupling
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/32—Optical coupling means having lens focusing means positioned between opposed fibre ends
- G02B6/322—Optical coupling means having lens focusing means positioned between opposed fibre ends and having centering means being part of the lens for the self-positioning of the lightguide at the focal point, e.g. holes, wells, indents, nibs
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- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Optical Couplings Of Light Guides (AREA)
Abstract
An optical fiber tip is cleaved at an angle to its longitudinal axis. An optical element is disposed proximate to the fiber tip to convey a beam of light in either direction between the fiber and a target optical system. The space between the fiber tip and the optical element is filled with an index matching material to reduce back-reflection and athermalize the focal properties of the system.
Description
FIBER OPTIC COUPLER
TECHNICAL FIELD
The invention relates to the field of connections between fiber optic components and more particularly to coupling beams between a fiber and a target optical system.
BACKGROUND
The ever-increasing demand for faster transmission speeds on fiber optic networks requires that attention be paid to connections that are made in hooking up system components. Optical connections to may reduce the transmitted optical power, thus adding to the amplification requirements for a given transmission. Additionally back-reflected power from a connection is also problematic and may affect transmitting sources. Commonly used sources are sensitive to even low levels of optical feedback, which may cause power fluctuation or mode hopping, resulting in noise problems and/or corrupted transmissions.
Another effect known as "multipath interference" occurs when a back reflected signal is again reflected from another connection causing the reflection to travel back and forth between the reflective components. The re-reflected signal may cause ghost images of the signal to appear at the receiver.
TECHNICAL FIELD
The invention relates to the field of connections between fiber optic components and more particularly to coupling beams between a fiber and a target optical system.
BACKGROUND
The ever-increasing demand for faster transmission speeds on fiber optic networks requires that attention be paid to connections that are made in hooking up system components. Optical connections to may reduce the transmitted optical power, thus adding to the amplification requirements for a given transmission. Additionally back-reflected power from a connection is also problematic and may affect transmitting sources. Commonly used sources are sensitive to even low levels of optical feedback, which may cause power fluctuation or mode hopping, resulting in noise problems and/or corrupted transmissions.
Another effect known as "multipath interference" occurs when a back reflected signal is again reflected from another connection causing the reflection to travel back and forth between the reflective components. The re-reflected signal may cause ghost images of the signal to appear at the receiver.
In fiber-optic systems, reflections originate in the fiber material itself and at interconnections between fibers or components.
The Rayleigh backscatter occurring in the fiber core as a result of randomly distributed impurities is a characteristic of the fiber production process and is not further addressed in this application.
This application is primarily concerned with another class of reflections that occur at interconnection interfaces due to discontinuity in refractive index. These reflections are commonly known as "Fresnel reflections".
1o Consequently, it has become increasingly important to use low loss and low back reflection connections or spli<:es in constructing an optical fiber communication channel. The problem for general connections is largely addressed through use of connectors such as angle polished connectors that have reflectance of -65dB or better.
However, other potential sources of back-reflection from optical components and devices used in modern optical networks may also generate unacceptable levels of back reflection. The use of a low loss connection to a network component may be completely negated if the component itself has a substantial back-reflected signal.
2o In particular, many optical switches terminate the fiber channel into a lens whereafter the light from the lens is directed or launched via some steering system into a selected opposing fiber for further transmission. With the several optical interfaces involved in this operation substantial possibility of generating back-reflections exists.
The Rayleigh backscatter occurring in the fiber core as a result of randomly distributed impurities is a characteristic of the fiber production process and is not further addressed in this application.
This application is primarily concerned with another class of reflections that occur at interconnection interfaces due to discontinuity in refractive index. These reflections are commonly known as "Fresnel reflections".
1o Consequently, it has become increasingly important to use low loss and low back reflection connections or spli<:es in constructing an optical fiber communication channel. The problem for general connections is largely addressed through use of connectors such as angle polished connectors that have reflectance of -65dB or better.
However, other potential sources of back-reflection from optical components and devices used in modern optical networks may also generate unacceptable levels of back reflection. The use of a low loss connection to a network component may be completely negated if the component itself has a substantial back-reflected signal.
2o In particular, many optical switches terminate the fiber channel into a lens whereafter the light from the lens is directed or launched via some steering system into a selected opposing fiber for further transmission. With the several optical interfaces involved in this operation substantial possibility of generating back-reflections exists.
In US Patent 6,002,81 to Saito et al, an optical switch comprises a moveable fiber tip that is brought into alignment with one of an array of fibers. When in alignment a drop of index matching fluid is introduced in the gap between fibers to provide a matched transmission without significant back-reflection. The patent specifically addresses the degradation of index matching fluid with time, particularly where there may be dust or debris present from a mechanical system as is the case described in then patent.
There remains a need for better methods of reducing back to reflection at connection interfaces in fiber optical systems.
SU1~QARY OF THE INVENTION
In a first aspect of the present invention, an optical fiber tip is cleaved at an angle to its longitudinal axis. An optical element is disposed proximate to the fiber tip to convey a beam of light in either direction between the fiber and a target optical system. The space between the fiber tip and the optical element is filled with an index matching material to reduce back-reflection.
In another aspect of the present invention, an optical element is disposed proximate to a fiber tip to convey a beam of light in either direction between the fiber and a target optical system. The space between the fiber tip and the optical element is filled with an index matching material and the radius of a surface of the optical element proximate to the fiber tip is adjusted to athermalize the focal properties of the system.
There remains a need for better methods of reducing back to reflection at connection interfaces in fiber optical systems.
SU1~QARY OF THE INVENTION
In a first aspect of the present invention, an optical fiber tip is cleaved at an angle to its longitudinal axis. An optical element is disposed proximate to the fiber tip to convey a beam of light in either direction between the fiber and a target optical system. The space between the fiber tip and the optical element is filled with an index matching material to reduce back-reflection.
In another aspect of the present invention, an optical element is disposed proximate to a fiber tip to convey a beam of light in either direction between the fiber and a target optical system. The space between the fiber tip and the optical element is filled with an index matching material and the radius of a surface of the optical element proximate to the fiber tip is adjusted to athermalize the focal properties of the system.
For an understanding of the invention, reference will now be made by way of example to a following detailed description in conjunction by accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate by way of example only preferred embodiments of the invention:
FIG. 1 is a sectional view of a prior art fiber tip.
FIG. 1-B shows an enlarged view of core area of the sectional view of the fiber shown in FIG. 1-A.
FIG. 1-C is a sectional view of a prior art method of reducing the Fresnel back-reflection at a fiber tip.
FIG. 2 is a sectional view of an embodiment of the invention showing a beam launched from a fiber using a single lens element.
FIG. 3 is a sectional view of another embodiment of the invention showing a beam launched from a fiber using a combination of lens elements.
FIG. 4 is a sectional view of another embodiment of the invention wherein the fiber tip is moveable.
FIG. 5 is a sectional view of another embodiment of the invention wherein a beam is launched from a fiber tip via a gradient index lens element.
FIG. 6 is a sectional view of a fiber tip and associated optical elements for coupling a beam.
DESCRIPTION
Throughout the following description, specific details are set 5 forth in order to provide a more thorough understanding of the invention. However, the invention may be practi~~ed without these particulars. In other instances well known elema_nts have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and the drawings are to be 1o regarded in an illustrative rather than a restrictive sense.
The present invention is described in relation to a back-reflection treatment for a fiber tip that simultaneously provides a high level of attenuation without substantially diverting the beam launched from the fiber. This is achieved by cleaving the fiber at and angle while at the same time using an index matching material to further reduce the Fresnel reflection and prevent the beam from being angularly diverted from the fiber axis.
PRIOR ART
In FIG. 1-A an optical fiber 10 is shown comprising a core 12 and a cladding layer 14. A beam of light 16 is propagating in the direction shown by the arrow. The fiber is cleaved or polished at face 18. FIG. 1-8 shows an enlarged view of the interface 18 between the fiber core 12 and the surrounding air 20. An optical signal Pj is incident on surface 18 and a substantial portion of the signal is transmitted as Pt. The portion of the signal Prthat is reflected at normal incidence is given by the following relationship:
Pr = P ' ~1 n2 F..Q~I1 1 n1 + h2 where n1 and n2 are the refractive indices of the fiber core and the air respectively. Strictly speaking, Eqn l is only precise for normal incidence_but for illustrative purposes, the small error has 1o been neglected. For a typical situation where n1 is 1.47 (silica core) and n2 is 1.0, Pr will be 3.6~ of P;. Expressed i;n decibels this corresponds to reflected beam Pr that is only attenuated by 14.4dB
relative to Pi, which may cause a significant problem at the source.
In modern fiber optic systems, working at high b:it transmission rate, it is quite common to require reflection levels to be attenuated by more than 50dB relative to the transmitted power.
The most common prior art method of reducing reflection is to cleave the fiber tip at an angle as shown in FIG.. 1-C. The incident signal P~ is transmitted as Pt at an angle 6d to the axis of the fiber 2o for a cleave angle of 6~. 6d is given by:
B~ ~ p _ na . ~~ E~ 2 the approximation valid for small 6°. Pr is reflected by the angled face 22, the reflected beam being incident on the interface between core 12 and cladding layer 14 at a point 24. Cleave angle 6°
is chosen so that Pr is incident at point 24 at a sufficient angle to ensure that most of Pr is transmitted into the c:Ladding Z4 and is not guided in fiber core 12 thus attenuating the reflection. For a common fiber material like Corning SMF 28 a cleave angle of 5° at a wavelength of 1550 nm results in a reflected beam that is only attenuated by approximately ~30dB (14.5dB due to the cleave and 14.4dB due to the Fresnel reflection). For a 8° cleave angle the attenuation is around 5ldB. From Eqa 2 where n1 is 1.47, Ba is 3.8° fo:r the 8°
cleave. This deviation may not be very significant in connecting a pair of fibers where the fiber end tips are close together, however it is substantially more problematic in systems where the fiber tips may be separated by some distance. As an example where a l0um core fiber is separated from a target optical system by as little as 100 fiber diameters or lmm, the axial deviation of a beam would be ~66~Zm, ignoring the effect of any lenses in-between which may magnify this deviation. If the intention were to couple the beam back into another fiber the deviation would have to be accommodated in the design of the coupler.
The deviation of a beam is also significant when a lens must be spaced apart from the fiber tip. As an example, a common moulded aspheric microlens with a back focal length of 3mm would be placed approximately 3 mm from the fiber tip to optimally collect the beam of radiation from the fiber core. Here the axial deviation of the beam would be almost 200um. In a free space optical switching arrangement such as that described in commonly assigned US patent application 09/842225 to Laberge et al, the separation between adjacent fibers may be of the order of 1 meter. While the system may be offset to accommodate the deviation, this results in subsequent optical components working off axis. Lenses must be designed to minimise off axis aberrations and also need to be larger, thus increasing size and cost. In systems where a large number of channels must be switched, amplified, or otherwise operated on, additional complexity in each channel has a significant effect on the overall system complexity.
There is a considerable advantage to keeping optical systems for each channel compact and simple.
Another common method for reducing back reflection is to introduce a fluid or material of similar refractive index to the fiber core into the area of the interface between two fibers or a fiber tip and an optical element. From Eqn 1 it can be seen that. if n1 and n2 are the almost the same, the Fresnel reflection is very much reduced.
However for a -50dB attenuation of the reflected beam (100,000 times 2o reduction) n1 and n2 must be matched to within ~ 0.01. In practice, it is extremely difficult to achieve such a.precise match of refractive index, particularly over a range of temperatures where not only the index but also the change of index with temperature must be matched.
It is also difficult to achieve this over a range of wavelengths since the dispersion in most index matching materials is different to the dispersion in the fiber.
THE PRESENT INVENTION
FIG. 2 shows a first embodiment of the present invention. A
fiber 10 is cleaved at an angle to have a tilted face 30 at the fiber tip. A housing 32 locates both the fiber tip 30 and a lens 34 and forms a chamber that that i.s filled with an index matching material 36. The cleave angle in combination with the index matching material significantly reduces the back-reflection while at the same time beam 38 is not significantly deviated from the fiber axis 26 due to the presence of the index matching material.
As an example assuming a cleave angle of 8° the attenuation of the reflected beam due to the cleave alone will be approximately 37dB
for Corning SMF 28 at a wavelength of 1550 nm. In order to reach the 50dB attenuation of the back-reflected beam the index matching need only provide a further l.3dB of attenuation. Even for a poor index match where an = n1 - n2= 0.2 the additional attenuation due to the presence of index matching material is more than 20dB (calculated using Eqn 1).
2o The index matching material may be a fluid or a solid material or may be a resinous compound that is introduced as a fluid and then subsequently allowed to cure or harden in place. It may also be a fluid that cures or gels under application of light or heat radiation.
In practice the selection of a particular index matching material may involve a trade-off between the exactness of the match and the transmission characteristic. As an example in selecting a index matching material for a 1550nm light transmission in a silica glass fiber that has an index of refraction of ~ 1.47, a matching fluid with 5 an index of 1.449 Q 1505 nm is available from Cargille Laboratories of Cedar Grove, N.J. (supplier code 06350). However, at this wavelength, the transmission loss in tree fluid is ~7dB/cm of path length through the fluid. In a direct fiber-to-fiber coupling; this may not be of issue since the fiber tips will generally be in very close abutment-to giving negligible loss. However, in a situation where the beam from the fiber is being launched into a lens that is located 1 cm away the loss is much more significant. For the situation described a better choice may be Cargille index fluid code 3421 which has n = 1.39 ~ 1550 nm and 100 transmission over a lcm path. The an of 0.08 still gives 3ldB attenuation, which when added to the attenuation due to the cleave angle, is quite adequate. However, in this case the transmission loss is significantly less. The combination of angle cleaving and using a index matching material that at least partially matches the fiber index gives extremely low back reflection over a wide wavelength and temperature range without causing significant beam deviation. This is accomplished without requiring an exact index match thus greatly expands the available material choices.
In another embodiment shown in FIG. 3, a pair of lenses 40 and 42 are used to launch a beam 38 from a fiber 10. The shape of lens 40 as shown is particularly problematic for redirecting the beam back into the fiber since reflections are at least partially focussed back into the fiber core 12. In the worst case where the fiber tip is at the back focal length of lens 40, all back reflected light from the rear surface 44 of lens 40 is collected and directed back into the core 12 of fiber 10. Through a careful choice of material for lens 40 to at least approximately match the index of material 36, the back-reflection from surface 44 may be significantly reduced over the case where the chamber contains air. In practice, it may be difficult to exactly match commonly used lens materials with an index matching to material that is also matched to the silica fiber core but it is usually possible to at least obtain some improvement over the situation where no index matching material is used. The back reflections from surface 46 of lens 40 and surfaces 48 and 50 of lens 42 will not derive any attenuation effect from the index matching material 36 unless the chamber is extended to include these surfaces.
For these surfaces, the back reflection should be otherwise accounted for in the design of the optical system for launching beam 38.
In another embodiment shown in FIG. 4 the focussing system of FIG. 3 is applied to a situation where a fiber 10 has a fiber tip 30 2o that is moveable by some actuation means in a direction indicated by arrows 60 and 62. Such moveable fiber tips are used in optical switching to precisely align a fiber launching a beam into an opposing receiving fiber. In FIG. 4 the entire fiber tip and the actuation means are immersed in a fluid index matching material 36 contained in chamber. The chamber wall 64 locates lens 40 and also locates the fiber at a point 66 distal from fiber tip 30. The actuation may be magnetic, piezoelectric or any of a variety of other means. The configuration shown has the advantage that the beam 38 is only diverted in proportion to any slight mismatch between the fiber core index and the matching material index. The deviation is very much smaller than would be the case if the chamber were filled with air.
In another embodiment shown in FIG. 5 a fik>er 10 having fiber tip 30 cleaved at an angle is coupled into a Gradient index lens (grin lens) 70. Grin lens 70 has a similarly angled entrance surface 72 to allow abutment to angle cleaved fiber tip 30. The grin lens 70 may be bonded to the fiber tip 30 by filling the gap between the grin lens and the fiber tip 74 with an adhesive that is index matched to the fiber and lens material. Alternatively, fiber tip 30 and grin lens 70 may be held in location by mechanical means with a film of index matching material filling gap 74. The combination of an index matching material and an angled cleave again provides a high degree of back-reflection attenuation while not significantly deviating the launched beam 38.
The use of an index matching material between a fiber tip and an optical element such as a lens offers additional advantages. There are a number of factors that will change the focus of a lens system in response to a temperature change, including but not limited to;
changes in the radii of curvature of the optical surfaces, changes in the refractive index of various materials and positional shifts of optical elements due to expansion or contraction of mounting features and/or the optical elements themselves. In any circumstance where an optical system is required to work over a wide range of environmental conditions, the design may need to take into account effects such as de-focus with changing temperature. Typically, the change in refractive index of the index matching material with temperature is quite large compared to the same change for lens 80:
an » an~~ gqn 3 .
aT aT
This situation may be exploited to compensate the lens system f.or changes in focus due to temperature changes while at the same time providing low back-reflection of incident beams.
In FIG. 6 a fiber tip 10 is coupled into a lens 80 via an index matching material 36 in a chamber formed by walls 32. The optical system additionally comprises a second lens 82, which together with lens 80 direct and focus a beam of light 84 to a point 86. The material of lens 80 has a refractive index nlene and the index matching material has a refractive index n where the materials are selected such that nlens~ n. Because of this approximate index match the rear surface 88 of the lens 80. which has a radius of curvature R, contributes very little optical power. The radius R may be changed without significantly affecting the focal parameters of the lens system.
Lens 80 may be chosen to have a back focal length b = R. All light originating from a point 90 will be incident on lens 80 normal to surface 88 and will not be deviated regardless of the refractive index difference at surface 88. By making R either larger or smaller than b, it is possible to arrange for the temperature dependence of this interface to be negative or positive. The change in the back focal length b with temperature may be written as:
of _-b(b-R)__an aT ~ n~R aT Sqn 4 where small changes in nlens with temperature have been ignored.
Note from Sqrs 4 that for R = b, the change in back focal length b with temperature T is zero. In the case where the change in refractive 1o index of index matching material 36 has a negative coefficient of temperature (i.e. decreases with increasing temperature) then:
for R > b a~ is negative, and for R < b a~ is positive Eqn 5.
The size of db/dT is directly proportional to (b - R) and can thus be tuned as desired by changing R. Thus by changing R
temperature dependences of the focus in the optical system can be compensated, resulting in a substantially athermalized optical system.
While in the aforegoing the description has generally referred to the operation of launching a beam from a fiber tip the invention applies equally to the reverse operation of receiving a launched optical beam and coupling into a fiber for further transmission. In such a case, Fresnel reflections occur at surfaces in the same manner as for a launched beam and depending on the optical configuration of the system, may or may not be reflected back to the launching fiber.
The invention also applies to bi-directional fiber systems where the beam transmission may be in either direction depending on the network 5 configuration.
This invention is particularly relevant whenever a beam is launched across some distance and coupled into a target optical component and where it is necessary to significantly attenuate back-reflections. A variety of devices may benefit from the application of to this invention including but not limited to fiber amplifiers, attenuators, multiplexers, add-drop multiplexers and cross connect switches.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are 15 possible in the practice of this invention without departing from the spirit or scope thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate by way of example only preferred embodiments of the invention:
FIG. 1 is a sectional view of a prior art fiber tip.
FIG. 1-B shows an enlarged view of core area of the sectional view of the fiber shown in FIG. 1-A.
FIG. 1-C is a sectional view of a prior art method of reducing the Fresnel back-reflection at a fiber tip.
FIG. 2 is a sectional view of an embodiment of the invention showing a beam launched from a fiber using a single lens element.
FIG. 3 is a sectional view of another embodiment of the invention showing a beam launched from a fiber using a combination of lens elements.
FIG. 4 is a sectional view of another embodiment of the invention wherein the fiber tip is moveable.
FIG. 5 is a sectional view of another embodiment of the invention wherein a beam is launched from a fiber tip via a gradient index lens element.
FIG. 6 is a sectional view of a fiber tip and associated optical elements for coupling a beam.
DESCRIPTION
Throughout the following description, specific details are set 5 forth in order to provide a more thorough understanding of the invention. However, the invention may be practi~~ed without these particulars. In other instances well known elema_nts have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and the drawings are to be 1o regarded in an illustrative rather than a restrictive sense.
The present invention is described in relation to a back-reflection treatment for a fiber tip that simultaneously provides a high level of attenuation without substantially diverting the beam launched from the fiber. This is achieved by cleaving the fiber at and angle while at the same time using an index matching material to further reduce the Fresnel reflection and prevent the beam from being angularly diverted from the fiber axis.
PRIOR ART
In FIG. 1-A an optical fiber 10 is shown comprising a core 12 and a cladding layer 14. A beam of light 16 is propagating in the direction shown by the arrow. The fiber is cleaved or polished at face 18. FIG. 1-8 shows an enlarged view of the interface 18 between the fiber core 12 and the surrounding air 20. An optical signal Pj is incident on surface 18 and a substantial portion of the signal is transmitted as Pt. The portion of the signal Prthat is reflected at normal incidence is given by the following relationship:
Pr = P ' ~1 n2 F..Q~I1 1 n1 + h2 where n1 and n2 are the refractive indices of the fiber core and the air respectively. Strictly speaking, Eqn l is only precise for normal incidence_but for illustrative purposes, the small error has 1o been neglected. For a typical situation where n1 is 1.47 (silica core) and n2 is 1.0, Pr will be 3.6~ of P;. Expressed i;n decibels this corresponds to reflected beam Pr that is only attenuated by 14.4dB
relative to Pi, which may cause a significant problem at the source.
In modern fiber optic systems, working at high b:it transmission rate, it is quite common to require reflection levels to be attenuated by more than 50dB relative to the transmitted power.
The most common prior art method of reducing reflection is to cleave the fiber tip at an angle as shown in FIG.. 1-C. The incident signal P~ is transmitted as Pt at an angle 6d to the axis of the fiber 2o for a cleave angle of 6~. 6d is given by:
B~ ~ p _ na . ~~ E~ 2 the approximation valid for small 6°. Pr is reflected by the angled face 22, the reflected beam being incident on the interface between core 12 and cladding layer 14 at a point 24. Cleave angle 6°
is chosen so that Pr is incident at point 24 at a sufficient angle to ensure that most of Pr is transmitted into the c:Ladding Z4 and is not guided in fiber core 12 thus attenuating the reflection. For a common fiber material like Corning SMF 28 a cleave angle of 5° at a wavelength of 1550 nm results in a reflected beam that is only attenuated by approximately ~30dB (14.5dB due to the cleave and 14.4dB due to the Fresnel reflection). For a 8° cleave angle the attenuation is around 5ldB. From Eqa 2 where n1 is 1.47, Ba is 3.8° fo:r the 8°
cleave. This deviation may not be very significant in connecting a pair of fibers where the fiber end tips are close together, however it is substantially more problematic in systems where the fiber tips may be separated by some distance. As an example where a l0um core fiber is separated from a target optical system by as little as 100 fiber diameters or lmm, the axial deviation of a beam would be ~66~Zm, ignoring the effect of any lenses in-between which may magnify this deviation. If the intention were to couple the beam back into another fiber the deviation would have to be accommodated in the design of the coupler.
The deviation of a beam is also significant when a lens must be spaced apart from the fiber tip. As an example, a common moulded aspheric microlens with a back focal length of 3mm would be placed approximately 3 mm from the fiber tip to optimally collect the beam of radiation from the fiber core. Here the axial deviation of the beam would be almost 200um. In a free space optical switching arrangement such as that described in commonly assigned US patent application 09/842225 to Laberge et al, the separation between adjacent fibers may be of the order of 1 meter. While the system may be offset to accommodate the deviation, this results in subsequent optical components working off axis. Lenses must be designed to minimise off axis aberrations and also need to be larger, thus increasing size and cost. In systems where a large number of channels must be switched, amplified, or otherwise operated on, additional complexity in each channel has a significant effect on the overall system complexity.
There is a considerable advantage to keeping optical systems for each channel compact and simple.
Another common method for reducing back reflection is to introduce a fluid or material of similar refractive index to the fiber core into the area of the interface between two fibers or a fiber tip and an optical element. From Eqn 1 it can be seen that. if n1 and n2 are the almost the same, the Fresnel reflection is very much reduced.
However for a -50dB attenuation of the reflected beam (100,000 times 2o reduction) n1 and n2 must be matched to within ~ 0.01. In practice, it is extremely difficult to achieve such a.precise match of refractive index, particularly over a range of temperatures where not only the index but also the change of index with temperature must be matched.
It is also difficult to achieve this over a range of wavelengths since the dispersion in most index matching materials is different to the dispersion in the fiber.
THE PRESENT INVENTION
FIG. 2 shows a first embodiment of the present invention. A
fiber 10 is cleaved at an angle to have a tilted face 30 at the fiber tip. A housing 32 locates both the fiber tip 30 and a lens 34 and forms a chamber that that i.s filled with an index matching material 36. The cleave angle in combination with the index matching material significantly reduces the back-reflection while at the same time beam 38 is not significantly deviated from the fiber axis 26 due to the presence of the index matching material.
As an example assuming a cleave angle of 8° the attenuation of the reflected beam due to the cleave alone will be approximately 37dB
for Corning SMF 28 at a wavelength of 1550 nm. In order to reach the 50dB attenuation of the back-reflected beam the index matching need only provide a further l.3dB of attenuation. Even for a poor index match where an = n1 - n2= 0.2 the additional attenuation due to the presence of index matching material is more than 20dB (calculated using Eqn 1).
2o The index matching material may be a fluid or a solid material or may be a resinous compound that is introduced as a fluid and then subsequently allowed to cure or harden in place. It may also be a fluid that cures or gels under application of light or heat radiation.
In practice the selection of a particular index matching material may involve a trade-off between the exactness of the match and the transmission characteristic. As an example in selecting a index matching material for a 1550nm light transmission in a silica glass fiber that has an index of refraction of ~ 1.47, a matching fluid with 5 an index of 1.449 Q 1505 nm is available from Cargille Laboratories of Cedar Grove, N.J. (supplier code 06350). However, at this wavelength, the transmission loss in tree fluid is ~7dB/cm of path length through the fluid. In a direct fiber-to-fiber coupling; this may not be of issue since the fiber tips will generally be in very close abutment-to giving negligible loss. However, in a situation where the beam from the fiber is being launched into a lens that is located 1 cm away the loss is much more significant. For the situation described a better choice may be Cargille index fluid code 3421 which has n = 1.39 ~ 1550 nm and 100 transmission over a lcm path. The an of 0.08 still gives 3ldB attenuation, which when added to the attenuation due to the cleave angle, is quite adequate. However, in this case the transmission loss is significantly less. The combination of angle cleaving and using a index matching material that at least partially matches the fiber index gives extremely low back reflection over a wide wavelength and temperature range without causing significant beam deviation. This is accomplished without requiring an exact index match thus greatly expands the available material choices.
In another embodiment shown in FIG. 3, a pair of lenses 40 and 42 are used to launch a beam 38 from a fiber 10. The shape of lens 40 as shown is particularly problematic for redirecting the beam back into the fiber since reflections are at least partially focussed back into the fiber core 12. In the worst case where the fiber tip is at the back focal length of lens 40, all back reflected light from the rear surface 44 of lens 40 is collected and directed back into the core 12 of fiber 10. Through a careful choice of material for lens 40 to at least approximately match the index of material 36, the back-reflection from surface 44 may be significantly reduced over the case where the chamber contains air. In practice, it may be difficult to exactly match commonly used lens materials with an index matching to material that is also matched to the silica fiber core but it is usually possible to at least obtain some improvement over the situation where no index matching material is used. The back reflections from surface 46 of lens 40 and surfaces 48 and 50 of lens 42 will not derive any attenuation effect from the index matching material 36 unless the chamber is extended to include these surfaces.
For these surfaces, the back reflection should be otherwise accounted for in the design of the optical system for launching beam 38.
In another embodiment shown in FIG. 4 the focussing system of FIG. 3 is applied to a situation where a fiber 10 has a fiber tip 30 2o that is moveable by some actuation means in a direction indicated by arrows 60 and 62. Such moveable fiber tips are used in optical switching to precisely align a fiber launching a beam into an opposing receiving fiber. In FIG. 4 the entire fiber tip and the actuation means are immersed in a fluid index matching material 36 contained in chamber. The chamber wall 64 locates lens 40 and also locates the fiber at a point 66 distal from fiber tip 30. The actuation may be magnetic, piezoelectric or any of a variety of other means. The configuration shown has the advantage that the beam 38 is only diverted in proportion to any slight mismatch between the fiber core index and the matching material index. The deviation is very much smaller than would be the case if the chamber were filled with air.
In another embodiment shown in FIG. 5 a fik>er 10 having fiber tip 30 cleaved at an angle is coupled into a Gradient index lens (grin lens) 70. Grin lens 70 has a similarly angled entrance surface 72 to allow abutment to angle cleaved fiber tip 30. The grin lens 70 may be bonded to the fiber tip 30 by filling the gap between the grin lens and the fiber tip 74 with an adhesive that is index matched to the fiber and lens material. Alternatively, fiber tip 30 and grin lens 70 may be held in location by mechanical means with a film of index matching material filling gap 74. The combination of an index matching material and an angled cleave again provides a high degree of back-reflection attenuation while not significantly deviating the launched beam 38.
The use of an index matching material between a fiber tip and an optical element such as a lens offers additional advantages. There are a number of factors that will change the focus of a lens system in response to a temperature change, including but not limited to;
changes in the radii of curvature of the optical surfaces, changes in the refractive index of various materials and positional shifts of optical elements due to expansion or contraction of mounting features and/or the optical elements themselves. In any circumstance where an optical system is required to work over a wide range of environmental conditions, the design may need to take into account effects such as de-focus with changing temperature. Typically, the change in refractive index of the index matching material with temperature is quite large compared to the same change for lens 80:
an » an~~ gqn 3 .
aT aT
This situation may be exploited to compensate the lens system f.or changes in focus due to temperature changes while at the same time providing low back-reflection of incident beams.
In FIG. 6 a fiber tip 10 is coupled into a lens 80 via an index matching material 36 in a chamber formed by walls 32. The optical system additionally comprises a second lens 82, which together with lens 80 direct and focus a beam of light 84 to a point 86. The material of lens 80 has a refractive index nlene and the index matching material has a refractive index n where the materials are selected such that nlens~ n. Because of this approximate index match the rear surface 88 of the lens 80. which has a radius of curvature R, contributes very little optical power. The radius R may be changed without significantly affecting the focal parameters of the lens system.
Lens 80 may be chosen to have a back focal length b = R. All light originating from a point 90 will be incident on lens 80 normal to surface 88 and will not be deviated regardless of the refractive index difference at surface 88. By making R either larger or smaller than b, it is possible to arrange for the temperature dependence of this interface to be negative or positive. The change in the back focal length b with temperature may be written as:
of _-b(b-R)__an aT ~ n~R aT Sqn 4 where small changes in nlens with temperature have been ignored.
Note from Sqrs 4 that for R = b, the change in back focal length b with temperature T is zero. In the case where the change in refractive 1o index of index matching material 36 has a negative coefficient of temperature (i.e. decreases with increasing temperature) then:
for R > b a~ is negative, and for R < b a~ is positive Eqn 5.
The size of db/dT is directly proportional to (b - R) and can thus be tuned as desired by changing R. Thus by changing R
temperature dependences of the focus in the optical system can be compensated, resulting in a substantially athermalized optical system.
While in the aforegoing the description has generally referred to the operation of launching a beam from a fiber tip the invention applies equally to the reverse operation of receiving a launched optical beam and coupling into a fiber for further transmission. In such a case, Fresnel reflections occur at surfaces in the same manner as for a launched beam and depending on the optical configuration of the system, may or may not be reflected back to the launching fiber.
The invention also applies to bi-directional fiber systems where the beam transmission may be in either direction depending on the network 5 configuration.
This invention is particularly relevant whenever a beam is launched across some distance and coupled into a target optical component and where it is necessary to significantly attenuate back-reflections. A variety of devices may benefit from the application of to this invention including but not limited to fiber amplifiers, attenuators, multiplexers, add-drop multiplexers and cross connect switches.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are 15 possible in the practice of this invention without departing from the spirit or scope thereof.
Claims (26)
1. A coupling for reducing back-reflections at an interface between an optical fiber and an optical element the coupling comprising:
a) a fiber tip cleaved at an angle to the longitudinal fiber axis;
b) an index matching material disposed between the fiber tip and the optical element.
a) a fiber tip cleaved at an angle to the longitudinal fiber axis;
b) an index matching material disposed between the fiber tip and the optical element.
2. The coupling of claim 1 wherein the optical element comprises one or more lenses disposed to collect a beam of light originating at the core of the cleaved optical fiber and to direct the beam towards a target optical system.
3. The coupling of claim 1 wherein the optical element comprises one or more lenses disposed to focus and direct a beam received from a target optical system into the core of a optical fiber.
4. The coupling of claim 1 wherein the index matching material is an index matching fluid.
5. The apparatus of claim 1 wherein the optical element is a lens and wherein the first surface of the lens has a radius of curvature selected to athermalize the focal properties of the optical system.
6. The optical fiber coupling of claim 5 wherein the radius of curvature of the first surface of the lens is selected to be close to, but not equal to the back focal length of the lens.
7. A fiber optic coupling comprising:
a) a fiber tip cleaved at an angle to the longitudinal fiber axis;
b) an optical element disposed to convey a beam of light to or from the fiber optic, the optical element spaced apart from the fiber tip;
c) an index matching material disposed between the fiber tip and the optical element.
a) a fiber tip cleaved at an angle to the longitudinal fiber axis;
b) an optical element disposed to convey a beam of light to or from the fiber optic, the optical element spaced apart from the fiber tip;
c) an index matching material disposed between the fiber tip and the optical element.
8. The fiber optic coupling of claim 7 wherein the fiber tip and the optical element are spaced apart by at least 100 times the diameter of the fiber core.
9. The fiber optic coupling of claim 7 wherein the optical element comprises one or more lenses disposed to collect a beam of light originating at the core of the cleaved optical fiber and to direct the beam towards a target optical system.
10. The fiber optic coupling of claim 7 wherein the optical element comprises one or more lenses disposed to focus and direct a beam received from a target optical system into the core of a optical fiber.
11. The fiber optic coupling of claim 7 wherein the index matching material is an index matching fluid.
12. The apparatus of claim 7 wherein the optical element is a lens and wherein the first surface of the lens has a radius of curvature selected to athermalize the focal properties of the optical system.
13. The optical fiber coupling of claim 12 wherein the radius of curvature of the first surface of the lens is selected to be close to, but not equal to the back focal length of the lens.
14. An optical cross-connect switch comprising a pair of optical fibers, each with an optical element proximate to the fiber tips to convey beams of light in either direction, the pair of optical fibers brought into alignment for a data transmission, each fiber having a fiber tip cleaved at an angle to the longitudinal fiber axis and an index matching material disposed between the fiber tip and an optical element.
15. The optical cross-connect switch of claim 14 wherein the index matching material is a fluid and the fiber tips are free to move in the fluid in response to an actuation force to bring the opposing fibers into alignment for the data transmission.
16. The optical switch of claim 14 wherein each fiber tip is cleaved at an angle of greater than 5° to the longitudinal fiber axis.
17. An apparatus for coupling a beam of light from an optical fiber the apparatus comprising:
a) a fiber tip cleaved at an angle to the longitudinal fiber axis;
b) an optical element proximate to the fiber tip for conveying a beam of light;
c) an index matching material disposed between the fiber tip and the optical element;
d) a target optical system spaced apart from but substantially aligned with the longitudinal fiber axis of the fiber tip.
a) a fiber tip cleaved at an angle to the longitudinal fiber axis;
b) an optical element proximate to the fiber tip for conveying a beam of light;
c) an index matching material disposed between the fiber tip and the optical element;
d) a target optical system spaced apart from but substantially aligned with the longitudinal fiber axis of the fiber tip.
18. The apparatus of claim 17 wherein the optical element comprises one or more lenses disposed to collect a beam of light originating at the core of the cleaved optical fiber and to direct the beam towards a target optical system.
19. The apparatus of claim 17 wherein the optical element comprises one or more lenses disposed to focus and direct a beam received from a target optical system into the core of the optical fiber.
20 20. The apparatus of claim 17 wherein the optical element is a lens and wherein the first surface of the lens has a radius of curvature selected to athermalize the focal properties of the optical system.
21. The optical fiber coupling of claim 20 wherein the radius of curvature of the first surface of the lens is selected to be close to, but not equal to the back focal length of the lens.
22. An optical fiber coupling comprising:
a) an optical fiber tip;
b) a optical element disposed to convey beams of light to or from the fiber core, the optical system further comprising at least one lens having a first and second surface;
c) an index matching material disposed between the fiber tip and the first surface of the lens, the index matching material having a larger change of refractive index with temperature than the material of the lens;
wherein the first surface of the lens has a radius of curvature selected to athermalize the focal properties of the optical system.
a) an optical fiber tip;
b) a optical element disposed to convey beams of light to or from the fiber core, the optical system further comprising at least one lens having a first and second surface;
c) an index matching material disposed between the fiber tip and the first surface of the lens, the index matching material having a larger change of refractive index with temperature than the material of the lens;
wherein the first surface of the lens has a radius of curvature selected to athermalize the focal properties of the optical system.
23. The optical fiber coupling of claim 22 further comprising a fiber tip cleaved at an angle to the longitudinal fiber axis.
24. The optical fiber coupling of claim 22 wherein the radius of curvature of the first surface of the lens is selected to be close to, but not equal to the back focal length of the lens.
25. A method of coupling a beam from an optical fiber into one or more optical elements, the method comprising:
a) cleaving the tip of the optical fiber at an angle to the longitudinal fiber axis;
b) introducing an index matching material between the fiber tip and the optical element.
a) cleaving the tip of the optical fiber at an angle to the longitudinal fiber axis;
b) introducing an index matching material between the fiber tip and the optical element.
26. The method of claim 25 wherein the index matching material which is introduced in a fluid state and it subsequently cures to a solid or gel state either through passage of time or by an application of heat or light radiation.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2414795 CA2414795A1 (en) | 2002-12-18 | 2002-12-18 | Fiber optic coupler |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2414795 CA2414795A1 (en) | 2002-12-18 | 2002-12-18 | Fiber optic coupler |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2414795A1 true CA2414795A1 (en) | 2004-06-18 |
Family
ID=32514085
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2414795 Abandoned CA2414795A1 (en) | 2002-12-18 | 2002-12-18 | Fiber optic coupler |
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| Country | Link |
|---|---|
| CA (1) | CA2414795A1 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2013148758A1 (en) * | 2012-03-28 | 2013-10-03 | Corning Incorporated | Monolithic beam-shaping optical systems and methods for an oct probe |
| CN113359251A (en) * | 2021-06-23 | 2021-09-07 | 深圳市极致兴通科技有限公司 | Rapid coupling method of high-speed optical device |
| CN114609707A (en) * | 2020-12-09 | 2022-06-10 | 微凤凰有限公司 | Microlenses provided with opposite sides each having an aspherical shape |
-
2002
- 2002-12-18 CA CA 2414795 patent/CA2414795A1/en not_active Abandoned
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2013148758A1 (en) * | 2012-03-28 | 2013-10-03 | Corning Incorporated | Monolithic beam-shaping optical systems and methods for an oct probe |
| US9036966B2 (en) | 2012-03-28 | 2015-05-19 | Corning Incorporated | Monolithic beam-shaping optical systems and methods for an OCT probe |
| US9638862B2 (en) | 2012-03-28 | 2017-05-02 | Corning Incorporated | Monolithic beam-shaping optical systems and methods for an OCT probe |
| CN114609707A (en) * | 2020-12-09 | 2022-06-10 | 微凤凰有限公司 | Microlenses provided with opposite sides each having an aspherical shape |
| CN113359251A (en) * | 2021-06-23 | 2021-09-07 | 深圳市极致兴通科技有限公司 | Rapid coupling method of high-speed optical device |
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